Dual-Band Infrared PbS Colloidal Quantum Dot Focal Plane Array

Domain: neuroscience
Published: 2026-02-03
Editorial outcome: major_revision

Abstract
Near-infrared and short-wave infrared dual-band detection has emerged as a pivotal enabling technology in across diverse applications spanning material identification, biological diagnostics, and machine vision. Current dual-band device architectures based on vertically stacked photodetectors such as those employing two-dimensional materials or back-illuminated colloidal quantum dots remain constrained by limited large-area manufacturability and incompatibility with standard readout integrated circuits. Here, we report a top-illuminated p-i-n-i-p dual-band photodetector using two distinct sizes of solution-processed PbS colloidal quantum dots, which enables bias-switchable spectral response between near-infrared and short-wave infrared regimes. The device achieves a specific detectivity exceeding 1×1011 cm·Hz1/2·W−1 in both bands, with short-wave infrared crosstalk of 0.5% and near-infrared crosstalk of 7.7%. The successful fabrication of a monolithic integrated 128×128 dual-band focal plane array showcases a functional dual-band infrared imager. This work establishes a scalable and silicon-compatible platform toward high-performance, low-cost dual-band infrared imagers.

Introduction
Near-infrared (NIR, 780–1,100 nm) and short-wave infrared (SWIR, 1,100–2,500 nm) imaging technologies are garnering significant interest for applications spanning machine vision1, material identification2, and biological imaging3. Their utility in non-destructive analysis stems from the sensitivity of these spectral bands to characteristic overtone and combination vibrations of molecular bonds such as C–H, O–H, N–H, and S–H4, enabling the discrimination of materials based on their distinct chemical fingerprints. Furthermore, the differing scattering and absorption characteristics of materials in the NIR and SWIR regions result in distinct penetration depths for these two spectral bands5. This complementary behavior makes NIR/SWIR dual-band imaging a highly effectively approach for comprehensive material characterization. A representative example can be found in agricultural quality control, where the technique allows simultaneous evaluation of surface texture and internal defects—such as bruising and non-uniform ripening in apples6. As illustrated in Figure 1a, integrating dual-band imaging into optical sorting systems enhances machine vision for intelligent production, leading to more accurate and automated quality grading. By routing only premium produce to the fresh market and diverting substandard items to processed products (e.g., jam production), this technology helps minimize food waste and strengthen supply chain sustainability.

Conventional dual-band NIR/SWIR imaging systems typically employ hybrid configurations of silicon (Si)-based NIR and indium gallium arsenide (InGaAs)-based SWIR detectors, requiring complex optical alignment and post-acquisition image fusion7. Although such systems achieve useful performance, their large footprint and high cost restrict deployment in resource-constrained or portable settings. Monolithic integration of dual-band photodetectors with readout integrated circuits (ROICs) on a single substrate represents a promising alternative, enabling substantial miniaturization and system-level cost reduction. Among integrated device architectures, vertically stacked photodiodes with back-to-back junction design allow bias-selectable spectral response while maintaining high pixel utilization, an essential feature for high-resolution dual-band imaging8.

Dual-band NIR and SWIR photodetectors based on back-to-back junction architectures have been demonstrated using a variety of material systems, including inorganic thin films (e.g., Si, Ge, GeSn)9,10, two-dimensional (2D) materials (e.g., MoS , black phosphorus)11,12, and colloidal quantum dots (CQDs) (e.g., PbS, HgTe)13–15. However, Ge-based photodetectors suffer from relatively low detectivity in the SWIR range. Meanwhile, the practical deployment of 2D materials is hampered by the absence of scalable synthesis and reliable heterointegration strategies, which limits their compatibility with focal plane arrays (FPAs) manufacturing workflows9.

In contrast, CQDs, particularly those based on PbS, have emerged as a versatile material platform for both NIR and SWIR detection, owing to their bandgap tunability, solution processability, and inner compatibility with RONICs16. Pejović et al. pioneered a vertically stacked n-p-n PbS CQDs photodiode architecture that enables bias-selectable dual-band detection across the NIR and SWIR regimes14. Nevertheless, this device was implemented on a glass substrate using a back-illuminated configuration, rendering it incompatible with monolithic integration onto ROICs. Furthermore, CQDs-based dual-band photodetectors have so far been largely confined to single-pixel demonstrations, where performance is compromised by high dark current and non-negligible spectral crosstalk, there impeding their adoption in practical imaging application17.

Here, we report a top-illuminated, p-i-n-i-p structured dual-band photodetector based on two distinct sizes of PbS CQDs, enabling bias-switchable spectral response between the NIR and SWIR ranges. Through band engineering, the device achieves a specific detectivity exceeding 1×1011 cm·Hz1/2·W−1 in both bands, along with SWIR spectral crosstalk of 0.5% and NIR crosstalk of 7.7%. By monolithically integrating the CQDs photodetector array with a custom-designed ROIC featuring bidirectional current sensing, we demonstrate a 128×128 dual-band infrared FPA capable of NIR and SWIR imaging. This work establishes a scalable, Si-compatible route to high-performance dual-band infrared FPAs using solution-processed semiconductors.

Results
Device design and working principle
We have engineered a novel dual-band photodetector employing two designed PbS CQDs absorption layers with controlled size distributions, achieving bias-selective wavelength detection. The dual-band photodetector adopts a vertically stacked p-i-n-i-p architecture comprising two back-to-back photodiodes (Figure 1b). The structure integrates a p-type PbS CQDs layer functionalized with 1,2-ethanedithiol (denoted as EDT) via solid-state ligand exchange, an intrinsic PbS CQD layer treated with halide ions via liquid-phase ligand exchange, and an n-type ZnO layer deposited by sputtering. To avoid damage to the CQDs layer, a protective fullerene (C ) layer is incorporated at the CQDs/ZnO interface. The electrical performance of the C film was evaluated via field-effect transistor measurements, as detailed in Supplementary Figure 1. The NIR photodiode, which utilizes smaller PbS CQDs (denoted as PbS-I), is positioned above the SWIR photodiode containing larger PbS CQDs (denoted as PbS-II), optimizing light absorption across both layers under top-side illumination. Optical transparency was achieved through implementation of a 100 nm indium tin oxide (ITO) top electrode, demonstrating broadband transmittance exceeding 70% across the spectral range 400–2,000 nm (Supplementary Figure 2), enabling efficient light absorption in the dual-layer structure.

Through optimized synthetic protocols, we obtained different-sized PbS CQDs with absorption peaks tailored to the NIR and SWIR spectral windows, respectively (Figure 1c). Transmission electron microscopy (TEM) confirmed uniform size distributions, with mean particle diameters of 2.9 nm and 7.5 nm (Supplementary Figure 3). The batch-to-batch reproducibility of the SWIR PbS CQD synthesis was illustrated in Supplementary Figure 4. Cross-sectional TEM image of the device (Figure 1d) demonstrates that the spin-coated PbS-I and PbS-II layers exhibit good uniformity and surface smoothness. Energy dispersive X-ray spectroscopy (EDS) analysis further confirms the presence of sharp interfacial transitions between adjacent functional layers.

The dual-band photodetector operates under a back-to-back diode configuration, as schematically depicted in Figure 1e. By reversing the bias direction across the device terminals, the NIR/SWIR photodiode can be selectively switched to a reverse-biased state, enabling spectral response to corresponding wavelength bands (Supplementary Figure 5). Additonally, the energy band alignment of the heterostructure before contact formation (Figures 1f and S6) shows a small conduction band offset (<0.4 eV), enabling efficient extraction of photogenerated electrons, while the significant valence band offset (>1.7 eV) creates a unipolar barrier that effectively blocks hole transport.

Under forward bias, the built-in electric field in the PbS-I layer sweeps out NIR-generated carriers, whereas those produced in the PbS-II layer are obstructed by the valence-band barrier (Figure 1g). Conversely, under reverse bias, SWIR photocurrent generated in the PbS-II layer is effectively collected, while NIR-generated carriers in PbS-I layer are confined (Figure 1h). By exploiting a large valence band offset as a unipolar barrier, this design not only curbs dark current by blocking majority carriers without sacrificing minority-carrier photocurrent, but also spatially confines photogenerated carriers collection to respective layers, thereby minimizing spectral crosstalk.

Dual-band performance characterization
The dark current density-voltage (J-V) characteristics (Figure 2a) exhibits typical dual-band operation, with measured dark current densities as low as 3.9 μAcm−2 at −1 V and 0.69 μAcm−2 at +1 V, indicating effective suppression of dark current. To elucidate the underlying transport mechanisms, we conducted a detailed simulation analysis of the dark current components. Under reverse bias, the dark current in the SWIR PbS diode is dominated by generation-recombination (G-R) processes. In contrast, under forward bias, trap-assisted tunneling (TAT) constitutes the dominant contribution to the dark current in the NIR PbS diode, which may be attributed to defect states introduced during ITO sputtering18, as corroborated by Supplementary Figure 7. Notably, the dark current levels attained in this dual-band photodetector are comparable to those reported for single-junction devices under reverse bias (Supplementary Table 1). To furhter evaluate the role of the barrier layer in dark current suppression, we performed a comparative analysis of devices incorporating ZnO and tetrabutylammonium iodide (TBAI)-treated PbS CQDs barrier layers (Supplementary Figure 8). The ZnO barrier layer reduces dark current by three orders of magnitude relative to the TBAI-based counterpart, identifying barrier height modulation as the principal mechanism for majority carrier confinement.

The logarithmic J-V curves under 940 nm and 1650 nm laser illumination at varying power intensities are presented in Figures 2b and 2c, while the corresponding linear J-V curves are shown in Supplementary Figure 9. The device demonstrates a pronounced photoresponse to 940 nm illumination under forward bias, while the response to 1,650 nm illumination emerges exclusively under reverse bias (Supplementary Figure 10). The dependence of photocurrent on incident light power at these wavelengths is summarized in Supplementary Figures 11 and 12. The calculated linear dynamic range (LDR) reaches 80 dB at 940 nm and 48 dB at 1,650 nm, underscoring the device’s capacity for dual-band discrimination while maintaining consistent responsivity over a broad intensity range. The calculation methods are provided in Supplementary Note 1.

To quantitatively assess the photodetection performance of the dual-band photodetector, we systematically measured both responsivity and external quantum efficiency (EQE) across the wavelength ranges of 500–1,000 nm and 1,100–2,000 nm, as shown in Figures 2d, 2e and Supplementary Figure 13. Under biases of +1 V and −1 V, the detector achieves peak responsivities of 0.13 AW−1 at 920 nm and 0.38 AW−1 at 1,765 nm, corresponding to EQE values of 17% and 26%, respectively. These results reflect efficient photon-to-electron conversion capabilities in both the NIR and SWIR regimes. Notably, a slight redshift (~30 nm) is observed between the solution absorption peak and the device EQE peak (Supplementary Figure 14). This spectral shift aligns closely with the redshift induced by ligand exchange (Supplementary Figure 15), attributed to ligand-related effects18.

To validate the reproducibility of the fabrication process, we characterized a set of 25 devices by measuring their dark currents and responsivities under ±1 V bias at 940 nm and 1,650 nm illumination (Supplementary Figure 16). The statistical analysis demonstrates good uniformity and device-to-device consistency, confirming the robustness of the fabrication methodology. Furthermore, the temporal photoresponse of the device were systematically evaluated, as shown in Supplementary Figure 17. Under 940 nm illumination, the photoresponse exhibited rise and fall times of 12.7 µs and 85.5 µs, respectively. In comparison, under 1,650 nm illumination, the device demonstrated faster response dynamics, with corrsesponding rise and fall times of 9.1 µs and 67.3 µs, respectively. These dynamic performance metric were further supported by frequency-domain measurements showing a −3 dB bandwidth of approximately 10 kHz, which closely aligns with the time-domain reponse data and demonstrates the device’s capacity for high-speed optical signals detection (Supplementary Figure 18).

Specific detectivity (D*), a critical figure of merit for evaluating photodetector performance, is defined as:

𝐷∗ = 𝑅√𝐴 / 𝑖𝑛 (1)

where R is the responsivity, i is the root mean square noise current within a 1 Hz bandwidth, and A is the device area (9.18×10−3 cm²)19. To determine 𝑖 , the current noise spectra of the dual-band device were measured at ±1 V (Figure 2f). The results show that the noise current decreases with increasing frequency, following a i ∝f −1/2 across the measured range. Importantly, at 10 kHz the noise current (0.35 pA·Hz−1/2 at −1 V and 41 fA·Hz−1/2 at +1 V) remains above the fundamental shot noise limit (96 fA·Hz−1/2 at −1 V and 20 fA·Hz−1/2 at +1 V). This behavior indicates that 1/f noise is the dominant noise source in the device within the operational frequency band. Using these values, the device achieves peak detectivities of 3.04×1011 cm·W−1·Hz1/2 for NIR and 1.04×1011 cm·W−1·Hz1/2 for SWIR, demonstrating high sensitivity across both spectral ranges.

In addition, we fabricated n-i-p-i-n structured devices (Supplementary Figure 19) and characterized their photoresponse under NIR and SWIR illumination over a range of optical power intensities (Supplementary Figure 20). While these devices retained dual-band detection functionality, both their responsivity and specitic detectivity were lower than those of the p-i-n-i-p structured counterparts. This performance difference is likely attributed to the bidirectional potential barrier in the valence band, which may hinder the efficient collection of photogenerated holes. These results underscore the critical role of precise energy band engineering in high-performance dual-band detection, as the band alignment directly governs the efficiency of carrier separation and collection in multi-junction photodetectors.

Optimization of spectral crosstalk
Spectral crosstalk represents a critical performance parameter for dual-band photodetectors, as it directly impacts their ability to accurately distinguish signals from distinct spectral bands20. In the current device architecture, the incomplete absorption of NIR light in the relatively thin PbS-I layer leads to measurable NIR-to-SWIR crosstalk under reverse bias. Although increasing the PbS-I layer thickness can enhance NIR absorption and reduce spectral crosstalk, this approach entails significant trade-offs. Owing to the inherently low carrier mobility and high density of surface trap states in CQDs films, excessive thickening PbS-I layer promotes carrier recombination during transport, thereby degrading responsivity in both the NIR and SWIR bands (Supplementary Figure 21). Given this constraint, we exploited bias-dependent spectral response as an alternative strategy to mitigate spectral crosstalk and achieve selective band detection. To quantify the effect of bias modulation, we conducted calibration experiments to establish a comprehensive responsivity matrix spanning various biases and wavelengths (Figures 3a and Supplementary Figure 22).

The responsivity matrix reveals four distinct operational regions defined by bias ranges. In Region I (0.1 V to 1 V), the device exhibits a strong response in the NIR band with minimal SWIR contribution. Region II (−0.25 V to 0.1 V) is marked by weak photoresponses across the entire spectrum. Region III (−0.25 V to −0.5 V) yields a strong response combined with a weak NIR signal, whereas Region IV (−0.5 V to −1 V) produces a broad spectral response covering both NIR and SWIR bands. In Region IV, the bottom diode (PbS-II) is reverse-biased, and the top diode (PbS-I) is forward-biased (Figure 1h). Under illumination, the resulting photocurrent is primarily contributed by electron-hole pairs generated in PbS-II due to its broadband absorption characteristics from NIR to SWIR. As the applied voltage decreases into Region III, electron-hole pairs are simultaneously generated in both diodes. However, under NIR illumination, opposing photocurrent components cancel out, preserving the SWIR selective detection (Figure 3b). Region II, the bias is insufficient to efficiently extract carriers across the inter-layer energy barriers, leading to minimal responses in both bands. Under positive voltage in Region I, only the top diode operates in reverse bias, enabling selective NIR detection (Figure 1g). Thus, spectral photoresponse can be finely tailored through external bias tuning.

To further quantify spectral crosstalk, we selected three representative operating voltages (1 V, −0.32 V, and −1 V) from regions I, III, and IV, respectively, and analyzed their normalized spectra response profiles, as shown in Figure 3c. The relative spectral crosstalk was quantified by integrating the overlap area between the NIR and SWIR response curves, normalized to the total response areas of each spectral band. The analysis yielded spectral crosstalk of 7.7% for 𝐶 and 0.5% for 𝐶 , with the detailed methodology provided in Supplementary Figure 23. These results underscore the efficacy of bias modulation in minimizing spectral crosstalk and optimizing dual-band detection performance.

Supplementary Figure 24 presents the J-V curves under 940 nm NIR illumination and dark conditions at varying optical power densities. The intersection points of the photocurrent and dark current curves define the optimum bias voltage for spectral crosstalk suppression. Remarkably, the bias voltage exhibits minimal variation (5%) across illumination intensities, demonstrating its effectiveness in suppressing spectral crosstalk over a broad range of illumination conditions. Meanwhile, the device performance was further evaluated at −0.32 V (Supplementary Figure 25), yielding a noise current density of 1.1×10−13 A·Hz1/2, and a specific detectivity of 7.6×1010 cm·Hz1/2·W−1 in the SWIR band under low incident optical power.

To systematically assess the spectral disrimination capability of the dual-band photodetector, we measured its response to 120 Hz modulated NIR and SWIR signals under controlled bias conditions, as depicted in Figures 3d and 3e. The NIR signal was generated using a xenon lamp coupled with a monochromator to produce 920 nm monochromatic light, while the SWIR signal was derived from an 800°C blackbody source filtered through a long-pass filter with a 1 µm cutoff wavelength. At +1 V, the device exhibited strong responsivity to the NIR signal but negligible response to the SWIR signal. Conversely, at −0.32 V, the device showed pronounced SWIR detection while effectively suppressing the NIR signal. At −1 V, the device demonstrated broadband responsivity, enabling simultaneous detection of both spectra bands. These results confirm that bias voltage modulation effectively minimizes spectral crosstalk between NIR and SWIR signals. In addition, the feasibility of sequential NIR and SWIR signals readout was demonstrated, as illustrated in Supplementary Figures 26 and 27.

Subsequently, the device was subjected to extended cycling tests under 940 nm and 1650 nm illumination to evaluate its reliability in dual-band detection. As shown in Supplementary Figure 28, both photocurrent and dark current remained stable over 1,000 continuous cycles. To rigorously assess the operational stability, we further subjected the device to a continuous bias stress test under constant illumination. As presented in Supplementary Figure 29, the device exhibited negligible degradation in photocurrent over a 10,000 s period under both +1 V (NIR mode) and -1 V (SWIR mode) biases, confirming robust operation under realistic working conditions. We further characterized the shelf-life stability of PbS CQDs films and devices by monitoring their optical and electrical properties under different storage conditions. As shown in Supplementary Figure 30, the absorption spectra of ligand-exchanged films exhibit negligible shifts after 20 days in both air and nitrogen (N2), indicating robust optical stability. Electrically, the field-effect transistors based on PbS CQDs fabricated on Si/SiO2 substrates with Ti/Au electrodes retained nearly identical transfer characteristics after 20 days of N2 storage (Supplementary Figure 31a), confirming excellent electrical stability in inert environments. In contrast, devices stored in ambient air exhibited a gradual decline in conductivity and carrier mobility, dropping to 50% and 40% of initial values after 20 days, respectively (Supplementary Figure 31b). This degradation is attributed to co-oxidation by atmospheric O2 and moisture, leading to the formation of insulating PbSO3 or PbSO4 species at dot surfaces or interfaces21, which increases charge hopping barriers and impair transport performance. Consistent with these findings, device-level performance showed mild degradation after two weeks in air, with slight reductions in both dark and photocurrents in the NIR and SWIR bands (Supplementary Figure 32a). Crucially, devices stored in a N2-purged environment for 15 months retain nearly all initial performance, with no significant change in dark current or responsivity (Supplementary Figure 32b). These results affirm the intrinsic stability of the material system when isolated from the ambient atmosphere. However, achieving practical long-term reliability requires further comprehensive studies, particularly regarding the development and assessment of effective encapsulation strategies.

A comprehensive benchmarking analysis against state-of-the-art dual-band photodetectors reported9–13,17,20,22–24 is systematically summarized in Supplementary Table 2 and Figure 3f. Our device achieves a room-temperature D* exceeding 1×1011 cm·Hz1/2·W−1 in both NIR and SWIR bands, with spectral crosstalk below 1%, outperforming most existing dual-band photodetectors. These combined characteristics enable high-fidelity dual-band infrared imaging, positioning the device as a promising solution candidate for multispectral sensing applications.

Dual-band infrared imaging
A custom ROIC was designed to operate under both positive and negative bias conditions, with its schematic diagram illustrated in Supplementary Figure 33. Under reverse bias, the photodetector generates a cathodic photocurrent that is injected into the capacitive trans-impedance amplifier (CTIA) input node, leading to a linear decrease in output voltage during integration. Conversely, under forward bias, the anodic photocurrent flows out of the CTIA input, resulting in a rising output voltage. By carefully tuning the DC bias levels of the reference voltage and the internal operational amplifiers, the output swing range is symmetrically configured to accommodate both polarities of photocurrent integration. The fabrication process of the dual-band FPA is illustrated in Supplementary Figure 34. The resulting dual-band FPA features an array size of 128×128 with a 15 μm pixel pitch, as shown in Figure 4a. The dual-band imaging system, depicted in Figure 4b, utilizes a lens to focus tungsten light reflected from the target onto the dual-band FPA. The ROIC controls the bias voltage of the FPA, while a dedicated circuit board collects and processes the signals, enabling image reconstruction through custom software. The system achieves a frame rate of 333 Hz with a 2 ms integration time and 1 ms readout time. The complete experimental setup is presented in Supplementary Figure 35.

As shown in Supplementary Figures 36 and 37, the dual-band FPA exhibits effective pixel rates of 87.99% and 90.76% functional elements in NIR and SWIR bands, respectively. This non-uniformity primarily arises from surface unevenness of the ROIC (Supplementary Figure 38), which introduces variations in pixel response. In comparison, the PbS CQDs film spin-coated on the flat substrate exhibit a markedly lower roughness of only 3 nm (Supplementary Figure 39). Adopting planarization processes in future ROIC is expected to reduce defective pixel rates and improve spatial uniformity. Furthermore, the image data processing workflow is illustrated in Supplementary Figure 40.

To demonstrate the dual-band imaging capability, we constructed a test scene comprising the Fudan university emblem printed on a white paper, partially overlaid with acrylic paint and a Si wafer. Figure 4c compares images captured using a standard complementary metal-oxide-semiconductor (CMOS) image sensor (CIS) with those capatured by the dual-band FPA operating in NIR and SWIR modes. In the NIR mode, the dual-band FPA clearly resolved the emblem pattern beneath the acrylic paint, leveraging the strong NIR transmission through organic materials. Conversely, in the SWIR mode, the FPA successfully imaged the emblem pattern obscured by the Si wafer, ultilizing the characteristic of SWIR light to transmit through Si. These results highlight the complementary nature of NIR and SWIR imaging modalities, achieving precise target identification in complex, layered scenarios through spectral discrimination. As shown in Figure 4d, different colored inks exhibit distinct absorption behaviors across the NIR and SWIR bands. Red ink remains undetectable in both NIR and SWIR images, whereas green ink is clearly visible under NIR illumination but becomes faint in the SWIR. In contrast, black ink is discernible in both spectral bands. These marked differences in spectral response enable the design of effective anti-counterfeiting markers. Figure 4e further illustrates the differential absorption properties of water, acetone, polyethylene (PE), and ink in the NIR and SWIR regions. When contained in white PE bottles, water and acetone are not observable under visible light. The NIR image reveals the meniscus of water but not that of acetone, whereas both menisci are clearly observed in the SWIR image, with water appearing darker due to stronger absorption. The ink markings on the bottle surface are prominent in the NIR image but fade in the SWIR image. Additionally, Supplementary Figure 41 shows imaging results of the NIR/SWIR dual-band FPA in other scenarios. Collectively, these results confirm that the distinct NIR and SWIR absorption can be effectively utilized for spectral identification.

Methods
Synthesis of PbS colloidal quantum dots
NIR-absorbing PbS CQDs were synthesized using a hot-injection approach. A mixture of PbO (0.45 g), oleic acid (OA, 1.5 mL), and octadecene (ODE, 22.5 mL) was degassed under vacuum (10−3 mbar) at 100°C for 1 hour until a clear solution formed. Under N2 atmosphere, bis(trimethylsilyl) sulfide (TMS2S, 180 µL in 3 mL octadecene) was swiftly injected, leading to rapid nucleation and a color change to dark brown. The resulting PbS CQDs were purified with hexane and acetone, then redispersed in octane at a concentration of 50 mg·mL−1.

SWIR-absorbing PbS CQDs were synthesized using a two-step hot-injection process. Lead oleate was first formed by dissolving PbO (0.446 g) in a mixture of ODE (22.5 mL) and OA (2.5 mL) under N2 at 130°C for 1 hour. The first sulfur precursor (53 µL TMS2S in 3 mL ODE) was injected and reacted for 20 minutes, followed by injection of the second sulfur precursor (30 µL TMS2S in 3 mL ODE) and further reaction for 15 minutes. The resulting PbS CQDs were purified with hexane and acetone, then redispersed in octane at a concentration of 50 mg·mL−1.

Ligand exchange
The solution-phase ligand-exchange process was conducted in a N2-filled glovebox. For NIR-absorbing PbS CQDs, a ligand solution containing PbI2 (0.01 M), PbBr2 (0.04 M), and ammonium acetate (0.06 M) in N,N-dimethylformamide (DMF). An octane solution of OA-capped PbS CQDs (20 mg·mL−1) was added, and the mixture was vigorously shaken for 5 minutes to introduce phase transfer into DMF. After three washes with hexane, the PbS CQDs were precipitated with toluene, isolated by centrifugation, vacuum-dried for 20 minutes, and redispersed in a butylamine (TBA)/DMF mixture (1:1 by volume) at 400 mg·mL−1.

For SWIR-absorbing PbS CQDs, a mixed halide-based ligand solution was prepared by dissolving PbI2 (0.05 M), PbBr2 (0.04 M), PbCl2 (0.05 M), and sodium acetate (0.044 M) in DMF. A PbS CQDs solution (~6 mg·mL−1 in octane) was added, and the mixture was vigorously shaken to transfer the CQDs into the DMF phase. After removing octane, the solution was washed three times with octane. The PbS CQDs were then precipitated by adding toluene, centrifuged, vacuum-dried for 20 minutes, and redispersed in a DMF/TBA mixture (1:1 by volume) at 300 mg·mL-1.

Device fabrication
The dual-band PbS CQDs photodetectors were fabricated on Si/SiO2 substrates pre-patterned with 5 nm Ti/10 nm Au bottom contacts. A 50 nm PbS-EDT layer was first deposited via a layer-by-layer method. The OA-capped PbS-CQDs were spin-coated at 2,000 r.p.m. for 40 s, treated with a 0.01% volume ratio of EDT in acetonitrile for 20 s and then washed by acetonitrile, and this process was repeated twice. Subsequently, the SWIR-absorbing layer (PbS-II, ~110 nm) was spin-coated at 4,000 r.p.m. for 60 s from a 300 mg·mL−1 CQDs inks. Next, a 50 nm C60 layer was thermally evaporated, followed by the deposition of a 50 nm ZnO layer via magnetron sputtering, serving as the electron transport layers. Furthermore, the NIR-absorbing layer (PbS-I, ~140 nm) was then spin-coated at 4,000 r.p.m. for 60 s using 400 mg·mL−1 CQDs ink and capped with two additional PbS-EDT layers. Finally, a transparent ITO top electrode was deposited by magnetron sputtering at room temperature.

Dual-band FPA fabrication
The ROIC for the dual-band FPA was fabricated in a 0.18 μm CMOS process. After cleaning the chip, a Ti/Au (150/50 nm) bottom electrode was patterned by UV lithography and electron beam evaporation, expanding the pixel size to 12×12 μm. Two PbS-EDT layers were deposited via spin-coated and solid-state ligand exchange. Subsequently, SWIR-absorbing PbS CQDs (PbS-II) were spin-coated from solution using a liquid-phase ligand exchange process and annealed at 85°C for 15 minutes. Then, a 50 nm C60 layer was thermally evaporated, followed by a 50 nm ZnO layer deposited as electron transport layers by magnetron sputtering. NIR-absorbing PbS CQDs (PbS-I) were spin-coated, annealed under the same conditions, and covered with two additional PbS-EDT layers via solid-state ligand exchange as hole transport layers. Finally, a 100 nm ITO top electrode was sputtered to complete the device structure. The completed dual-band FPA was wire-bonded to a testing circuit board for performance evaluation.

Material characterization
Optical absorption spectra were recorded using a Hitachi U-4100 Plus spectrophotometer. Cross-sectional TEM samples of PbS CQDs dual-band photodetectors were prepared using the focused ion beam lift-out technique on a Tescan GAIA3 system. TEM imaging and energy dispersive spectroscopy (EDS) mapping were collected using a Thermo Fisher TALOS F200X system. Ultraviolet photoelectron spectroscopy (UPS) was performed under ultrahigh vacuum (~10−10 mbar) using a KRATOS AXIS Ultra DLD system with a monochromatic HeI (21.22 eV) excitation source.

Device characterization
The J-V curves and time-dependent photocurrent under dark conditions and 940 nm illumination were performed using an MS200 system equipped with a Keithley 6482 semiconductor analyzer. The transient response and −3 dB bandwidth were recorded using the same setup together with a PicoScope 4262 oscilloscope and an SR570 low-noise current preamplifier (Stanford Research Systems). Noise currents of the PbS CQDs photodiode were measured using a semiconductor parameter analyzer (FS-Pro, Primarius Technologies). The EQE spectral at each wavelength was determined using a lock-in amplifier (Stanford Research Systems, SR865A), a chopper, an SR570 low-noise current preamplifier, and a Horiba iHR350 spectrometer equipped with a xenon lamp and a tungsten lamp.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript presents a highly significant and innovative work on the development of a monolithically integrated, dual-band (NIR/SWIR) infrared focal plane array (FPA) using solution-processed PbS colloidal quantum dots (CQDs). The novel p-i-n-i-p unipolar barrier heterostructure design, achieving bias-switchable spectral discrimination with exceptionally low crosstalk and high detectivity, represents a major advancement in low-cost, high-performance multispectral infrared imaging. The successful demonstration of a functional 128×128 FPA prototype with real-world imaging applications underscores the technological impact of this work. The manuscript is well-written, logically structured, and supported by comprehensive data. It is suitable for publication after addressing the points below.\n1.While operability is mentioned (Fig. S23), quantitative data on pixel-to-pixel uniformity (e.g., histogram of responsivity or dark current across the array) for both bands should be provided, which would strengthen the assessment of FPA manufacturability.\n2.The ROIC readout speed (frame rate) for the demonstrated FPA is not stated. Providing this would help contextualize its suitability for real-time applications.\n3.Page 6 mentions the PbS-I layer thickness limits NIR absorption, contributing to residual crosstalk. Was thickness optimization performed? Could a slight increase further reduce NIR→SWIR crosstalk without compromising SWIR response or speed?\n4.The ligand exchange process is critical for performance. Briefly commenting on the reproducibility and stability of the exchanged films (e.g., shelf-life, environmental stability) would be beneficial.\n5.The energy band diagrams (Fig. 1f, S4) are schematic. Adding estimated values (e.g., from UPS/IPES or simulation) for the conduction/valence band offsets and Fermi level positions would provide deeper insight into the carrier blocking/extraction mechanisms.\n6.How do the reported detectivity and dark current values for each band in the dual-band device compare to state-of-the-art single-band PbS CQD detectors? This would highlight the efficiency of the integrated design.\n7.The 1000-cycle test (Fig. S19) shows excellent short-term stability. Commenting on potential degradation mechanisms (e.g., oxidation, ion migration) and any preliminary data on longer-term stability under ambient or operational conditions would be useful for assessing practical viability.\n8.The ROIC schematic (Fig. S20) is mentioned but not described. A brief explanation of its key features enabling dual-bias operation would be helpful.\n9.The methodology in Fig. S15 should be briefly summarized in the main text or caption of Fig. 3b for clarity.\n10. Define \"LDR\" (Linear Dynamic Range) upon first use in the main text (Page 9).", "reviewer_2": "In this work, Di et al reported a dual-band heterojunction photodetector with PbS colloidal quantum dots in a unipolar barrier configuration. The optimized device achieves a peak detectivities of 3.7×1011 cm·W−1·Hz1/2 in near-infrared and 6.6×1011 cm·W−1·Hz1/2 in short-wavelength infrared region, with crosstalk effectively suppressed below 0.5% through bias-switching modulation. They also integrated this photodetector with a readout integrated circuit, developing a 128×128 dual-band focal plane array featuring a 15 μm pixel pitch. This is solid work. However, I have some concerns on novelty and device performance. Personally, I think the authors should address those concerns, or the manuscript may not be published in this journal.\nComments:\nEDT treated PbS as p type layer and ZnO as n type layer are very common in PbS CQD photodetectors, while vertically stacked p-i-n-i-p configuration is also very typical in CQD PDs in other references. The authors may need to illustrate their new development on the material side and device side.\nAs Figure 3b shows, the lowest cross talk below 0.5% was achieved at -0.32V. However, good device performance is achieved at +1V and -1V. As a result, the cross talk is not small in real working mode.\nBesides, I have a question on the detectivity at NIR and SWIR. The authors claim a peak detectivities of 3.7×1011 cm·W−1·Hz1/2 in near-infrared and 6.6×1011 cm·W−1·Hz1/2 in short-wavelength infrared region， where the device shows slightly higher detectivity at SWIR. However, if one looks at Figure S18b&c, the noise fluctuation is obviously larger for SWIR compared with NIR. The NIR part shows a higher signal to noise ratio.\nAnother question about the device performance on noise is the noise power density as a function of frequency. The authors may discuss the noise origin (like 1/f noise, shot noise…)\nFigure 4& Figure S25. The authors try to demonstrate the advantages of dual band Imaging in complex scenes. Unfortunately, it seems very hard to get the idea that this dual band PD could figure out item difference in NIR mode. As Figure 4d/4e show the difference between NIR and SWIR is quite small. Could the authors establish a different scene with better contraction.\nThe uniformity is not very good compared to reported CQD broadband PD or dual band bulk material PD. Is it come from the uniform doping? What is the roughness of each material layer? How to solve this problem?\nFigure 1c and Figure 2e. The cut off wavelength of absorption and photoresponse spectra of SWIR do not agree with each other. Explain it.\nSome suggestions:\nThey may avoid using words like innovative (line 34), first prototype (line 40) in the main text.\nThe transmission lines in the FPA imaging system are quite long, which may induce extra electric noise in the imaging.", "reviewer_3": "In this paper Di et al build a dual band detector from a nanocrystal stack and study its operation going up to an imager demonstration\nI have no doubt that this paper should be published after some edits, main question is if it fits nature com.. :i am open with a good edit\nInb the introduction, the message is sometime a but confused. The goal of this paper is NIR and SWIR, while some argument and technology are actually MWIR based. Quantum well and superlattice technologies are mostly MWIR/LWIR and type II superlattioce barely start in the SWIR\nYou entertain me claiming that Quantum well, superlattice and HgCdTe are emerging. HgCdte is from the 60’s, quantum well in the sense of qwip is from the 90’s and actually more collapsing than emerging\nYou say being lattice matched is a chalenge ; not much InGaAs on InP (or HgCdTe on CdZnTe or quantum well on GaAs….) are lattice matched…the cost of it is that it becomes more difficult to tune composition/band gap (because then it turns non lattice matches)\nRegarding applications : is swir that much driven by defense…not sure\nAbout multiband device it actually started before the cited reference see work dedicated to visible /SWIR, Nano Lett. 14, 2715(2014).\nDetail but page 6 about « ultimately minimizing waste at the consumer level. », i am not sure that more sorting will generate less waste, likely i twill turn to premium selection nd more waste\nHow thick is c60 layer\nIs there any damage induced by ITO deposition\nWhy the detectivity of swir diode larger ? sure absorption from NIR device is sub optimal, but dark current in the swir one should be much larger due to weaker band gap, this brings back to my previous question can ITO may have damage the NIR diode\nDo not provide QD size with a 10 pm resolution while atomic lattice is 0.3 nm\nI have feeling that the word « crosstalk » is used in two contexts the spatial crosstalk between pixel and spectral crosstalk between diode. Go through the text to always be clear about which one you speak about\nAbout linearity, a lot is said in the littérature, to be linear the log log plot of figure s7 not only needs to be a line, but the slope MUST be one, make a power law fit and provide exponent to confirm your actual linearity and therfore the LDR values\nI have a major concern about your noise measurement. I acknowledge that it is great that measurements have been done while most of the litterature papesr just make claim based on white noise which is highly misleading. But you use an amplifier with a noise of 5fA.Hz-1/2, so can you explian how you measure noise that is lower than your setup noise….This point needs to be genuinely adressed and is a clear bottleneck to match nature comm standard. Especially because your value of detectivity will be reduced and therfore the final perf might be less appealing\nThe back to back diode discussion is poor, but on same time one of the highest value of this paper is figure 3a. in the Si there is a mathematical discussion about the concept but the main text misses a discussion explaining the concept. There are two diodes back to back and therefore the current from one diode can cancel the one from the other, so by choosing carefully the bias value, the NIR response of the SWIR diode can be mostly supress : say it as simply. Then graph 3a must be better discussed as well, i see what look like some different area and somehow some kind of phase diagram of the device operation , discuss it, because in most previous paper this was not shown\nIn method « Ac » in NH4Ac or NaAc is not defined, is it acetate ?\nAxis ultra DLD is XPS from kratos say it", "reviewer_4": "Submitted paper entitled “Dual-Band Infrared PbS Colloidal Quantum Dot Focal Plane Array” presents a dual-band infrared imaging system based on a vertically stacked configuration of oppositely oriented near-infrared (NIR) and short-wave infrared (SWIR) absorbing PbS CQD layers. The authors report dark current densities on the order of 10⁻⁶ A and EQE of ~25%, followed by dual-wavelength imaging demonstrations. While the experimental realization is technically competent, the manuscript, in its current form, falls short of the scientific depth expected for a publication in Nature Communications. The work reads more as a mid-level engineering report/demonstration than a contribution of fundamental scientific insight. I recommend either submitting this manuscript to a more engineering- or imaging-focused journal, or doing more experimental designs and substantially revising it to clearly articulate and support a meaningful scientific question or hypothesis. Below, I outline several concerns and suggestions:\n1. The use of PbS CQDs for infrared detection and imaging has been extensively explored and commercialized. The device architecture, including the hole/electron transport layers and charge blocking strategies, appears to rely on well-established approaches that have been in the literature for over a decade.\n2. While PbS CQDs are widely considered the gold standard among CQD materials, their intrinsic ionic character leads to well-known issues with operational and environmental instability. The device architecture employed here has previously been shown to suffer from performance degradation under electrical bias and prolonged operation. However, the manuscript does not address these stability concerns, either through experimental investigation or discussion.\n3. The authors state that the concept of dual-band CQD-based imaging has been previously demonstrated across several architectures. It is unclear what the central innovation of this work is. If the novelty lies in the monolithic integration on a focal plane array (FPA), this aspect is confined to a brief discussion centered around Figure 4 and does not sufficiently elevate the scientific significance.\n4. The manuscript does not clearly articulate the scientific challenge being addressed or the fundamental insight offered by this study. As a result, the broader implications for the CQD or imaging community remain unclear.\n5. The proposed architecture may face limitations for real-world imaging applications. Notably, the system appears to require relatively high bias voltages to achieve meaningful contrast, which can negatively affect device stability, induce ghost imaging artifacts, and slow response times.\n6. Due to the opposing orientation of the stacked junctions, the internal electric fields generated upon illumination in each spectral band are in opposite directions. This raises the possibility of electrostatic coupling or cancellation effects, whereby excitation in one junction perturbs the band alignment and detection fidelity in the other. This undermines the assumption of independent dual-band detection and should be thoroughly investigated.\n7. Several figures contain data that appear inconsistent. The authors are encouraged to carefully review their measurements, ensure statistical significance, and include reproducibility assessments.\n8. The images presented in Figure 4 are difficult to interpret. Aside from panel c, the differences among the panels are subtle or unclear.\n9. The experimental section lacks sufficient detail on the fabrication of both the devices and the integrated FPA. More comprehensive information is needed to ensure reproducibility and to assess the scalability or practicality of the proposed approach."}, "author_response": "Below are our point-to-point responses to the reviewers’ comments. We have uploaded our manuscript with the revised parts marked red in the main text and Supporting Information file.\nReviewer #1:\nThis manuscript presents a highly significant and innovative work on the development of a monolithically integrated, dual-band (NIR/SWIR) infrared focal plane array (FPA) using solution-processed PbS colloidal quantum dots (CQDs). The novel p-i-n-i-p unipolar barrier heterostructure design, achieving bias-switchable spectral discrimination with exceptionally low crosstalk and high detectivity, represents a major advancement in low-cost, high-performance multispectral infrared imaging. The successful demonstration of a functional 128×128 FPA prototype with real-world imaging applications underscores the technological impact of this work. The manuscript is well-written, logically structured, and supported by comprehensive data. It is suitable for publication after addressing the points below.\nWe appreciate the favorable and helpful comments on our work.\nComment 1-1. While operability is mentioned (Fig. S23), quantitative data on pixel-to-pixel uniformity (e.g., histogram of responsivity or dark current across the array) for both bands should be provided, which would strengthen the assessment of FPA manufacturability.\nResponse 1-1: Thank the reviewer for this insightful suggestion.\nTo quantitatively assess pixel-to-pixel uniformity and manufacturability of the FPA, we have performed a statistical analysis of the dark and photocurrent (converted to voltage via the readout circuit) across the entire array in both NIR and SWIR modes, as shown in the updated Figure S33.\nThe results demonstrate good uniformity across the array. Specifically, in the NIR mode, the dark signal shows an average voltage of –9.0 mV (σ = 20.4 mV), while the photoresponse under illumination averaged –136.4 mV (σ = 20.4 mV). In the SWIR mode, the average dark voltage was 47.2 mV (σ = 44.1 mV), and the average photoresponse reached 188 mV (σ = 84.4 mV).\nIn the main text on page 15, we added:\n“As shown in Figure S32 and S33, the dual-band FPA exhibits effective pixel rates of 87.99% and 90.76% functional elements in NIR and SWIR bands, respectively.”\nMeanwhile, we added the Figure S33 and the corresponding description in the Supporting Information on page S24 and page S25:\n“Figure S33. Statistical distribution of the dark noise voltage and the signal voltage under uniform tungsten lamp illumination for the dual-band FPA in the NIR and SWIR operating modes.\n“The performance of the FPA was characterized by analyzing dark-current and flat-field response images in both the NIR and SWIR spectral bands to quantify the defective pixel ratio. Pixels with responses exceeding twice the mean response value are defined as overheated pixels, while those below half the mean response value are classified as dead pixels11. Based on analysis of dark and uniform illumination images, the defective pixel rates for NIR and SWIR modes were calculated to be 12.01% and 9.24%, respectively. Furthermore, the dark signal in the NIR mode averaged -9.0 mV (σ = 20.4 mV), while the photoresponse under illumination averaged -136.4 mV (σ = 78.7 mV). Corresponding values for the SWIR mode were 47.2 mV (σ = 44.1 mV) for the dark signal and 188 mV (σ = 84.4 mV) for the photoresponse.”\nReference:\n11. Mu, G. et al. Visible to mid-wave infrared PbS/HgTe colloidal quantum dot imagers. Nat. Photonics 18, 1147–1154 (2024).\nComment 1-2. The ROIC readout speed (frame rate) for the demonstrated FPA is not stated. Providing this would help contextualize its suitability for real-time applications.\nResponse 1-2: Thank the reviewer for such professional question.\nThe demonstrated FPA achieves a frame rate of 333 Hz, corresponding to an integration time of 2 ms and a readout time of 1ms.\nIn the main text on page 15, we added:\n“The system achieves a frame rate of 333 Hz with a 2 ms integration time and 1 ms readout time.”\nComment 1-3. Page 6 mentions the PbS-I layer thickness limits NIR absorption, contributing to residual crosstalk. Was thickness optimization performed? Could a slight increase further reduce NIR→SWIR crosstalk without compromising SWIR response or speed?\nResponse1-3: Thank the reviewer for such professional question.\nWe have systematically investigated the effect of PbS-I (NIR absorption layer) thickness on device performance. As suggested, increasing the thickness of the PbS-I layer does indeed help reduce NIR→SWIR spectral crosstalk due to enhanced NIR absorption. However, we observed a concomitant decrease in SWIR responsivity. This trade-off is likely attributed to increased carrier recombination losses for SWIR-generated photocurrent as the carriers travel a longer path through the thicker layer.\nIn the main text on page 10 and page 11, we added:\n“Although increasing the PbS-I layer thickness can enhance NIR absorption and reduce spectral crosstalk, this approach entails significant trade-offs. Owing to the inherently low carrier mobility and high density of surface trap states in CQDs films, excessive thickening PbS-I layer promotes carrier recombination during transport, thereby degrading responsivity in both the NIR and SWIR bands, (Figure S18). Given this constraint, we exploited bias-dependent spectral response as an alternative strategy to mitigate spectral crosstalk and achieve selective band detection.”\nMeanwhile, we added the Figure S18 in the Supporting Information on page S13.\n“Figure S18. Dependence of responsivity at 940 nm and 1650 nm, and spectral crosstalk, on the thickness of the PbS-I film in the dual-band device.”\nComment 1-4: The ligand exchange process is critical for performance. Briefly commenting on the reproducibility and stability of the exchanged films (e.g., shelf-life, environmental stability) would be beneficial.\nResponse 1-4: Thank the reviewer for such professional question.\nTo evaluate reproducibility, we fabricated three independent batches of PbS CQD films using the solution-phase ligand exchange process. As shown in Figure 1, the absorption peaks of these films exhibit a variation of less than 10 nm, indicating good batch-to-batch consistency and minimal aggregation during the ligand exchange process.\nFigure 1. Absorption spectra of films after ligand exchange performed in three different batches.\nRegarding environmental stability, we investigated both optical and electrical properties under different storage conditions. As presented in Figure S26, the absorption spectra of the ligand-exchanged films show negligible spectral shift after 20 days of storage in both air and nitrogen atmospheres, demonstrating good optical stability.\nFor electrical stability, we fabricated field-effect transistors with Ti/Au electrodes on Si/SiO substrates to monitor the transfer characteristics and conductivity of the PbS CQD films. As shown in Figure S27a, the transfer curves remain nearly unchanged after 20 days of storage in N , confirming excellent electrical stability under inert conditions. However, as illustrated in Figure S27b, the conductivity and carrier mobility of films stored in ambient air gradually decrease over time. After 20 days, the conductivity drops to 50% and the mobility to 40% of their initial values. This degradation is likely caused by co-oxidation from atmospheric oxygen and moisture, leading to the formation of insulating PbSO andPbSO species at the quantum dot surfaces or interfaces[1]. This increases the energy barrier for carrier hopping and reduces charge transport efficiency.\nIn summary, the ligand exchange process is reproducible, and the resulting films exhibit robust optical stability. However, long-term preservation of optimal electrical performance requires storage in an inert atmosphere. We have added this discussion along with the relevant data in the revised manuscript and Supplementary Information, to better address the stability aspect.\nReference:\n[1] Tang, J. et al. Quantum dot photovoltaics in the extreme quantum confinement regime: The surface-chemical origins of exceptional air- and light-stability. ACS Nano 4, 869–878 (2010).\nIn the main text on page 13, we added:\n“We further characterized the environmental stability of PbS CQDs films and devices by monitoring their optical and electrical properties under different storage conditions. As shown in Figure S26, the absorption spectra of ligand-exchanged films exhibit negligible shifts after 20 days in both air and nitrogen (N ), indicating robutst optical stability. Electrically, the field-effect transistors based on PbS CQDs fabricated on Si/SiO substrates with Ti/Au electrodes retained nearly identical transfer characteristics after 20 days of N storage (Figure S27a), confirming excellent electrical stability in inert environments. In contrast, devices stored in ambient air exhibited a gradual decline in conductivity and carrier mobility, dropping to 50% and 40% of initial values after 20 days, respectively (Figure S27b). This degradation is attributed to co-oxidation by atmospheric O and moisture, leading to the formation of insulating PbSO or PbSO species at dot surfaces or interfaces21, which increases charge hopping barriers and impair transport performance.”\nMeanwhile, we added the Figures S26 & S27 in the Supporting Information on page S18 and page S19.\n“Figure S26. Comparison of absorption spectra for ligand-exchanged PbS films after being stored for 20 days in (a) air and (b) nitrogen atmosphere.”\n“Figure S27. Environmental stability of ligand-exchanged PbS films. (a) Transfer characteristics after 20 days of storage in N atmosphere. (b) Evolution of conductivity and charge carrier mobility of the ligand-exchanged PbS films as a function of storage time in air.”\nReference:\n[21] Tang, J. et al. Quantum dot photovoltaics in the extreme quantum confinement regime: The surface-chemical origins of exceptional air- and light-stability. ACS Nano 4, 869–878 (2010).\nComment 1-5: The energy band diagrams (Fig. 1f, S4) are schematic. Adding estimated values (e.g., from UPS/IPES or simulation) for the conduction/valence band offsets and Fermi level positions would provide deeper insight into the carrier blocking/extraction mechanisms.\nResponse 1-5: Thank the reviewer for such professional question.\nWe have performed UPS and UV-Vis absorption measurements to experimentally determine the valence band maximum, conduction band minimum, and Fermi level positions of the key materials: PbS-II, ZnO, and C . These results are now included in 60 Figure S4 of the revised Supporting Information. The band energy levels of PbS-I and EDT were referenced from our previous report[1].\nReference:\n[1] Di, Y. et al. Interface engineering to drive high‐performance MXene/PbS quantum dot NIR photodiode. Adv. Sci. 11, 2307169 (2024).\nIn the main text on page 6, we added:\n“Additionally, the energy band alignment of the heterostructure before contact formation (Figures 1f and S4) shows a small conduction band offset (<0.4 eV), enabling efficient extraction of photogenerated electrons, while the significant valence band offset (>1.7 eV) creates a unipolar barrier that effectively blocks hole transport.”\nMeanwhile, we added the Figure S4 in the Supporting Information on page S7 and page S8:\n“Figure S4. (a-d) UPS analysis of PbS-Ⅱ, ITO, ZnO, and C60 films. (e-f) UV-Vis absorption spectra of ZnO and C60 films. (g) Schematic of the energy level alignment, showing the conduction band, valence band, and Fermi level positions of the analyzed films.\nTo elucidate the energy landscape governing charge transport, we systematically characterized the band alignment architecture of the multilayer device through complementary spectroscopic analyses. The Fermi levels positions and valence band maxima were UPS directly measured by UPS. These results were combined the optical bandgaps, obtained from UV-Vis absorption spectroscopy, to locate the conduction band minima. The band energy levels of PbS-I and EDT were referenced from our previous report2.”\nComment 1-6: How do the reported detectivity and dark current values for each band in the dual-band device compare to state-of-the-art single-band PbS CQD detectors? This would highlight the efficiency of the integrated design.\nResponse 1-6: Thank the reviewer for such professional question.\nTo provide a comprehensive assessment, we have summarized the performance metrics of PbS photodiodes in Table S1.\nSingle-band PbS CQD photodiodes typically operate near 0 V bias, resulting in detectivity values that can exceed 1×1012 cm·Hz1/2·W−1. In contrast, our dual-band device requires an applied bias to enable efficient charge separation and collection across both NIR and SWIR spectral channels. Consequently, the detectivity of our dual-band device is relatively lower compared to these optimal single-band detectors due to the inherent increase in dark current at non-zero biases.\nHowever, it is important to note that the dark current levels at operating bias in our dual-band device are comparable to those reported for state-of-the-art single-band PbS CQD detectors. This indicates that our integrated design maintains competitive noise performance, which is crucial for practical dual-band imaging applications.\nIn the main text on page 7, we added:\n“Notably, the dark current levels attained in this dual-band photodetector are comparable to those reported for single-junction devices under reverse bias (Table S1).”\nMeanwhile, we add the Table S1 in the Supporting Information on page S30:\n“Table S1. Summary of recently reports of PbS photodiodes.”\nDark\nPeak Detectivity\ncurrent\nLight\nDevice structure wavelength (cm·Hz1/2· Year\ndensity\ndirection\n(nm) W-1)\n(μA·cm-2)\nITO/PbS/Al 1450 1×1012 1 @-0.5V Back 2009\nITO/ZnO/PbS\n1300 2.2×1012 0.1 @-0.4V Back 2023\nCQD/EDT/Au\nITO/ZnO/PbS\n1000 1.4×1012 0.25 @-1V Back 2019\nCQD/Spiro/Au\nContact/TL/PbS\n1450 9.5×1011 1 @-1V Top 2021\nCQD/TL/ITO\nITO/ZnO/PbS\n1550 8.0×1011 20 @-1V Back 2021\nCQD/EDT/Au\nAu/NiO/PbS\n1000 2.1×1012 0.02 @-1V Top 2022\n/C /ZnO/ITO\n2\n900 7×1013 30 @-1V Back 2017\nPbS/EDT/Au\nAl/Si/EDT/ITO 1500 1.5×1011 1 @-1V Top 2020\nAl/Si/ZnO/PbS/\n1490 4.0×1011 6 @-1V Top 2020\nEDT/ITO\nAu/NiO/EDT/PbS/\n1550 4.1×1012 1 @-0.4V Top 2023\nC /SnO /ITO\n60 2\nAu/ZnO/PbS/EDT/\n940 5.5×1012 2 @-1V Top 2023\nMXene\nAu/EDT/PbS/C /\n60\n920 3.0×1011 0.69 @1V Top This work\nZnO/PbS/EDT/ITO\nAu/EDT/PbS/C /\n60\n1750 1.0×1011 3.9 @-1V Top This work\nZnO/PbS/EDT/ITO\nComment 1-7: The 1000-cycle test (Fig. S19) shows excellent short-term stability. Commenting on potential degradation mechanisms (e.g., oxidation, ion migration) and any preliminary data on longer-term stability under ambient or operational conditions would be useful for assessing practical viability.\nResponse 1-7: Thank the reviewer for such professional question.\nTo evaluate environmental stability, we monitored device performance after storage in ambient air for two weeks. As shown in Figure S28a, the dark current and photocurrent in both the NIR and SWIR bands exhibit a slight degradation. This mild performance decline is likely attributed to oxidation-induced reduction in charge carrier mobility within the quantum dot film, possibly due to the formation of surface oxides or trap states that hinder carrier transport. Interestingly, the device-level degradation is significantly slower than that observed in bare quantum dot films, which we attribute to the partial environmental protection provided by the top-layer ITO electrode.\nAs shown in Figure S28b, the devices stored in a nitrogen-purged environment for 15 months retained nearly all of its initial performance, with no significant change in dark current or responsivity, confirming the intrinsic stability of the materials when shielded from ambient atmosphere.\nThese results suggest that the primary degradation mechanisms—likely involving oxygen and moisture-induced oxidation and possibly ion migration—are effectively mitigated by proper encapsulation. Therefore, for practical applications, we recommend hermetic packaging using metal or ceramic housings with inert gas sealing to ensure long-term reliability of the quantum dot image sensors.\nIn the main text on page 14, we added:\n“Consistent with these findings, device-level performance showed mild degradation after two weeks in air, with slight reductions in both dark and photocurrents in the NIR and SWIR bands (Figure S28a). Crucially, devices stored in a N2-purged environment for 15 months retain nearly all initial performance, with no significant change in dark current or responsivity (Figure S28b), affirming the intrinsic stability of the materials system when isolated from ambient atmosphere. For practical applications, hermetic packaging using metal or ceramic housings with inert gas filling is necessary to ensure long-term reliability of CQD-based image sensors.”\nMeanwhile, we added the Figure S28 in the Supporting Information on page S19:\n“Figure S28. Comparison of dark current and photocurrent (under 0.1 μW 1650 nm and 1 μW 940 nm illumination) of the devices after storage for (a) 14 days in air and (b) 15 months in nitrogen, measured at −1 V and +1 V bias.”\nComment 1-8:The ROIC schematic (Fig. S20) is mentioned but not described. A brief explanation of its key features enabling dual-bias operation would be helpful.\nResponse 1-8: Thank the reviewer for such professional question.\nThe circuit, as shown in Figure S29, enables dual-band detection by supporting the integration of bidirectional photocurrents. Under reverse bias, the photodetector generates a cathodic photocurrent that is injected into the CTIA input node, causing the output voltage to decrease linearly during integration. Conversely, under forward bias, the anodic photocurrent flows out of the CTIA input, resulting in a rising output voltage. By carefully tuning the DC bias levels of the reference voltage (V ) and the operational ref amplifiers within the CTIA, the output swing range is symmetrically optimized for both upward and downward integration. This dual-mode operation is essential for independent readout of NIR and SWIR signals in our dual-band detector.\nWe have added this explanation to the caption and discussion of Figure S29 in the Supplementary Information to provide clearer insight into the circuit’s key features.\nIn the main text on page 15, we added:\n“Under reverse bias, the photodetector generates a cathodic photocurrent that is injected into the capacitive trans-impedance amplifier (CTIA) input node, leading to a linear decrease in output voltage during integration. Conversely, under forward bias, the anodic photocurrent flows out of the CTIA input, resulting in a rising output voltage. By carefully tuning the DC bias levels of the reference voltage and the internal operational amplifiers, the output swing range is symmetrically configured to accommodate both polarities of photocurrent integration.”\nMeanwhile, the related context was added in the Supporting Information on page S20 and page S21:\n“Figure S29. Detailed pixel circuit diagram.\nThe photocurrent from the PbS CQD photodetector is integrated using a CTIA architecture, selected for its high injection efficiency and input linearity10. The CTIA converts the photocurrent into a voltage signal and accumulates charge onto the integration capacitor, stabilizing the input node voltage and thereby minimizing dark current fluctuation. Additionally, the circuit supports both forward and reverse bias operation by maintaining a constant reference voltage (V ) while adjusting the bias at the photodetector cathode.\nref\nUnder reverse bias, a cathodic photocurrent that is injected into the CTIA input node, causing a linear decrease in the output voltage during integration. Conversely, under forward bias, an anodic photocurrent flows out of the input node, resulting in an increasing output voltage. By carefully tuning the DC bias levels of the reference voltage and the internal operational amplifiers are carefully tuned to symmetrically accommodate both photocurrent polarities within the output swing range.\nThe two-stage integrating capacitance, 20.3 fF (C1) and 58.9 fF (C1+C2) is switched via C2_EN. Capacitor C3 provides low-frequency noise filtering after CTIA integration. Correlated double sampling (CDS) is implemented at the pixel level to suppress reset noise and low-frequency noise in CTIA and detectors. In the CDS phase, SH1 and SH2 are sequentially enabled at the start and end of integration to sample the reset and signal voltage onto capacitors C4 and C5, respectively. The sampled signals are then read out row-by-row through a source follower (SF) to the column circuit.\nBased on the maximum integration capacitance and the dynamic range of CTIA output voltage, the full well capacity of a single pixel is calculated to be approximately 551 Ke-. Based on the measured results, the 128×128 readout circuit operates at a frame rate of 333 Hz with a total power consumption of 25.5 mW, dominated by the CTIA operational amplifier.”\nReference:\n[10] Huang, Z. Noise model of large-format readout integrated circuit for infrared focal plane array. IEEE Trans. Circuits Systems I Fund. Theory Appl. 70, 142–153 (2023).\nComment 1-9:The methodology in Fig. S15 should be briefly summarized in the main text or caption of Fig. 3b for clarity.\nResponse 1-9: Thank the reviewer for such professional question.\nIn the previous revision, Figure S15 presented a schematic diagram of crosstalk in the normalized spectral response of dual-band photodetectors, which resembled the representation in Figure 3b. In the revised version, however, these figures have been updated: Figure S15 is now Figure S20, and Figure 3b has been renumbered as Figure3c.\nFigure 3. Bias-switchable NIR/SWIR sensing with the dual-band PbS CQDs photodetector.\nIn the main text on page 12, we update the sentences as follows:\n“To further quantify spectral crosstalk, we selected three representative operating voltages (1 V, −0.32 V, and −1 V) from regions I, III, and IV, respectively, and analyzed their normalized spectra response profiles, as shown in Figure 3c. The relative spectral crosstalk was quantified by integrating the overlap area between the NIR and SWIR response curves, normalized to the total response areas of each spectral band. The analysis yielded spectral crosstalk of 7.7% for and 0.5% for , with the detailed methodology provided in Fig 𝑪 u𝐒r𝐖e 𝐈S𝐑→20𝐍.𝐈 𝐑These results und 𝑪 e𝐍r𝐈s𝐑c→o𝐒r𝐖e𝐈 𝐑the efficacy of bias modulation in minimizing spectral crosstalk and optimizing dual-band detection performance.”\nIn the Figure 3c caption on page 27, we update the sentence as follows:\n“(c) Spectral response of the dual-band detector under bias voltages of +1 V, −1 V, and −0.32 V. (The spectral crosstalk between the two bands of a dual-band detector is quantified by integrating the relative spectral response curves of each detector within specific wavelength ranges.)”\nComment 1-10:Define \"LDR\" (Linear Dynamic Range) upon first use in the main text (Page 9).\nResponse 1-10: Thank the reviewer for such professional question.\nThe abbreviation “LDR” has now been defined as “Linear Dynamic Range” upon its initial use in the main text on page 8:\n“The calculated linear dynamic range (LDR) reaches 80 dB at 940 nm and 48 dB at 1650 nm, underscoring the device’s capacity for dual-band discrimination while maintaining consistent responsivity over a broad intensity range.”\nIn this work, Di et al reported a dual-band heterojunction photodetector with PbS colloidal quantum dots in a unipolar barrier configuration. The optimized device achieves a peak detectivities of 3.7×1011 cm·Hz1/2·W−1 in near-infrared and 6.6×1011 cm·Hz1/2·W−1 in short-wavelength infrared region, with crosstalk effectively suppressed below 0.5% through bias-switching modulation. They also integrated this photodetector with a readout integrated circuit, developing a 128×128 dual-band focal plane array featuring a 15 μm pixel pitch. This is solid work. However, I have some concerns on novelty and device performance. Personally, I think the authors should address those concerns, or the manuscript may not be published in this journal.\nWe appreciate favorable and helpful comments on our work.\nComment 2-1: EDT treated PbS as p type layer and ZnO as n type layer are very common in PbS CQD photodetectors, while vertically stacked p-i-n-i-p configuration is also very typical in CQD PDs in other references. The authors may need to illustrate their new development on the material side and device side.\nResponse 2-1: We fully agree that both EDT-treated PbS as a p-type layer and ZnO as an n-type layer are well-established components in PbS CQD photodetectors. Furthermore, vertically stacked back-to-back diode configurations are indeed a common architecture for dual-band detection in quantum dot systems, as exemplified by prior work on PbS or HgTe CQD dual-band phototectors (Figure 1)[1-3].\nFigure 1. Architectures of previously reported CQD-based dual-band photodetectors.\n[1] Zhao, P. et al. Band-engineered dual-band visible and short-wave infrared photodetector with metal chalcogenide colloidal quantum dots. J. Mater. Chem. C 11, 2842–2850 (2023).\n[2] Pejović, V. et al. Photodetectors based on lead sulfide quantum dot and organic absorbers for multispectral sensing in the visible to short-wave infrared range. Adv. Funct. Mater. 32, 2201424 (2022).\n[3] Tang, X., Ackerman, M. M., Chen, M. & Guyot-Sionnest, P. Dual-band infrared imaging using stacked colloidal quantum dot photodiodes. Nat. Photonics13, 277–282 (2019).\nHowever, these previous dual-band devices were typically built on ITO glass with bottom illumination through the substrate, which is fundamentally incompatible with monolithic integration onto silicon-based ROICs. This back-illuminated architecture prevents direct pixel-to-pixel coupling with ROICs and has thus limited dual-band CQD imaging to single-pixel or scanning-based systems.\nThe key innovation of our work lies in the first design and realization of a top-illuminated dual-band CQD photodetector based on a vertically stacked architecture. By engineering the device stack for top-side illumination, we enable direct integration with ROICs in a FPA configuration. Moreover, we optimized the device structure to achieve high detectivity, low spectral crosstalk and developed a bidirectional readout circuit to independently address the NIR and SWIR channels.\nThis work represents the first demonstration of NIR/SWIR dual-band imaging using a CQD FPA. This advancement provides a scalable and practical pathway toward low-cost, high-performance multispectral imaging systems based on solution-processed quantum dots.\nWe have revised the Introduction part to better highlight the innovation of this work in the main text on page 4 and page 5:\n“In contrast, CQDs, particularly those based on PbS, have emerged as a versatile 19 material platform for both NIR and SWIR detection, owing to their bandgap tunability, solution processability, and inner compatibility with ROICs16. Vladimir et al. pioneered a vertically stacked n-p-n PbS CQDs photodiode architecture that enables bias-selectable dual-band detection across the NIR and SWIR regimes. Nevertheless, this device was implemented on a glass substrate using a back-illuminated configuration, rendering it incompatible with monolithic integration onto ROICs. Furthermore, CQDs-based dual-band photodetectors have so far been largely confined to single-pixel demonstrations, where performance is compromised by high dark current and non-negligible spectral crosstalk, there impeding their adoption in practical imaging application17.\nHere, we report a top-illuminated, p-i-n-i-p structured dual-band photodetector based on two distinct sizes of PbS CQDs, enabling bias-switchable spectral response between the NIR and SWIR ranges. Through band engineering, the device achieves a specific detectivity exceeding 1×1011 cm·Hz1/2·W−1 in both bands, along with spectral crosstalk as low as 0.5%. By monolithically integrating the CQDs photodetector array with a custom-designed ROIC featuring bidirectional current sensing, we demonstrate a 128×128 dual-band infrared FPA capable of NIR and SWIR imaging. This work establishes a scalable, Si-compatible route to high-performance dual-band infrared FPAs using solution-processed semiconductors.”\nReference:\n[16] Ba, K. & Wang, J. Advances in solution-processed quantum dots based hybrid structures for infrared photodetector. Mater. Today 58, 119–134 (2022).\n[17] Tang, X., Ackerman, M. M., Chen, M. & Guyot-Sionnest, P. Dual-band infrared imaging using stacked colloidal quantum dot photodiodes. Nat. Photonics13, 277–282 (2019).\nComment 2-2: As Figure 3b shows, the lowest cross talk below 0.5% was achieved at -0.32V. However, good device performance is achieved at +1V and -1V. As a result, the cross talk is not small in real working mode.\nResponse 2-3: Thank the reviewer for such professional question.\nWe have measured the noise performance of the dual-band device at −0.32 V (area: 9.18 × 10−3 cm²), as shown in Figure S22. The measured noise current density is 1.1 × 10−13 A·Hz−1/2, which enables accurate evaluation of detectivity at this low-bias condition.\nUsing this noise value, we calculated the responsivity and specific detectivity at −0.32 V under varying 1650 nm illumination intensities (Figure S22). Notably, at low incident optical power, the device achieves a detectivity of 7.6 × 1010 cm·Hz1/2·W−1 in the SWIR band. This value is comparable to the detectivity measured at −1 V, indicating that high detection sensitivity can still be maintained at −0.32 V under weak-light conditions.\nIn the main text on page 12, we added:\n“Meanwhile, the device performance was further evaluated at −0.32 V (Figure S22), yielding a noise current density of 1.1×10−13 A·Hz1/2, and a specific detectivity of 7.6×1010 cm·Hz1/2·W−1 in the SWIR band under low incident optical power.”\nMeanwhile, we added the Figure S22 in the Supporting Information on page S15:\n“Figure S22. (a) Measured noise current of the device at −0.3 V bias. (b) Responsivity and detectivity of the device at −0.32 V under different incident light power of 1650 nm illumination.”\nComment 2-3: Besides, I have a question on the detectivity at NIR and SWIR. The 21 authors claim a peak detectivities of 3.7×1011 cm·Hz1/2·W−1 in near-infrared and 6.6×1011 cm·Hz1/2·W−1 in short-wavelength infrared region，where the device shows slightly higher detectivity at SWIR. However, if one looks at Figure S18b&c, the noise fluctuation is obviously larger for SWIR compared with NIR. The NIR part shows a higher signal to noise ratio.\nResponse 2-3: Thank the reviewer for such professional question.\nIn the original Figure S18b&c, the noise fluctuations appear more pronounced in the SWIR channel compared to the NIR channel. This observation, however, does not reflect an intrinsically lower signal-to-noise ratio of the SWIR photodetector itself. Rather, it stems from the difference in incident optical power during the measurement. The SWIR signal in those plots was acquired using an 800°C blackbody filtered by a double-polished single-crystalline silicon wafer, which resulted in a substantially lower incident optical power compared to the NIR signal generated by a 920 nm monochromatic light from a xenon lamp.\nTo enable a direct and equitable comparison of the temporal response, we re-measured the on-off switching dynamics under controlled conditions with well-defined, low-intensity illumination at both wavelengths. As shown in Figure S8, the measurements were performed with identical incident optical power (10 nW) of 940 nm (NIR) and 1650 nm (SWIR), respectively. Under these balanced conditions, the SWIR channel exhibits a higher responsivity.\nFurthermore, we have recalculated the specific detectivity based on these revised measurements, and the updated values confirm that the peak detectivities in the NIR (3.04×1011 cm·Hz1/2·W−1) and SWIR (1.04×1011cm·Hz1/2·W−1) are comparable.\nIn the main text on page 8, we added:\n“The device demonstrates a pronounced photoresponse to 940 nm illumination under forward bias, while the response to 1650 nm illumination emerges exclusively under reverse bias (Figure S8).”\nMeanwhile, we added the Figure S8 in the Supporting Information on page S9:\n22\n“Figure S8. Time-dependent photoresponse of the dual-band device under 10 nW illumination at 940 nm and 1650 nm, measured at +1 V and −1 V bias.”\nComment 2-4: Another question about the device performance on noise is the noise power density as a function of frequency. The authors may discuss the noise origin (like 1/f noise, shot noise…)\nResponse 2-4: Thank the reviewer for such professional question.\nWe have performed noise characterization and analysis to investigate the noise origin in our device. Key noise sources in photodiode systems encompass thermal noise, shot noise and 1/f noise[1]:\n2 2 2 2\nAmong these, 1/f noise de𝐼c 𝑛 rea=ses𝐼1 / w 𝑓 it+h 𝐼𝐽 inc+rea𝐼𝑠 s ℎ in 𝑜𝑡 g frequency and follows the relationship[1,2]:\n𝛼\nwhere K is the proportionality facto𝐼1r,/ 𝑓I i=s the 𝛽bia∆s𝑓 current, α is a constant whose value d\n𝑓\nis about 2, and β is a constant whose value is about 1.\nThermal noise is associated with the finite resistance of the device[1]:\n2 4𝑘𝑇\nwhere R is the zero-bias dynamic re𝐼s𝐽ist=ance. ∆𝑓\n0 𝑅0\nShot noise is a result of the statistical process of carriers flowing over a potential barrier.\n𝐼\nc𝑛on\n=\ndi\n2\nti\n𝑞\no\n𝐼\nn𝑑s\n∆\n, 𝑓\nthe shot noise is much greater than the thermal noise; therefore, the noise current is primarily composed of shot noise and 1/f noise.\nAs shown in the Figure 2f, we measured the noise current power spectral density as a function of frequency. The results show that the noise current decreases with increasing frequency, following a In∝f −1/2 across the measured range. Importantly, at 10 kHz—the 3 dB bandwidth of the device—the noise level (0.35 pA·Hz−1/2 at−1 V and 41 fA·Hz−1/2 at +1 V) remains above the fundamental shot noise limit (96 fA·Hz−1/2 at −1 V and 20 fA·Hz−1/2 at +1 V).\nThis behavior is consistent with 1/f noise being the dominant noise source within the device’s the operational bandwidth. The significant 1/f noise is characteristic of charge trapping/detrapping processes, typically occurring at material interfaces, grain boundaries, and surface states, as well as imperfect contacts[2].\nReference:\n[1] Rogalski, A. Infrared and Terahertz Detectors. (CRC press, 2019).\n[2] Hooge, F. N. 1/f noise sources. IEEE Trans. Electron Devices41, 1926–1935 (1994).\nIn the main text on page 10, we added:\n“The results show that the noise current decreases with increasing frequency, following a i ∝f −1/2 across the measured range. Importantly, at 10 kHz the noise n level (0.35 pA·Hz−1/2 at −1 V and 41 fA·Hz−1/2 at +1 V) remains above the fundamental shot noise limit (96 fA·Hz-1/2 at −1 V and 20 fA·Hz−1/2 at +1 V). This behavior indicates that 1/f noise is the dominant noise source in the device within the operational frequency band. Using these values, the device achieves peak detectivities of 3.04×1011 cm·Hz1/2·W−1 for NIR and 1.04×1011 cm·Hz1/2·W−1 for SWIR, demonstrating high sensitivity across both spectral ranges.”\n“Figure 2f. Measured current noise of the dual-band photodetector at ±1 V bias.”\nComment 2-5: Figure 4& Figure S25. The authors try to demonstrate the advantages of dual band Imaging in complex scenes. Unfortunately, it seems very hard to get the idea that this dual band PD could figure out item difference in NIR mode. As Figure 4d/4e show the difference between NIR and SWIR is quite small. Could the authors establish a different scene with better contraction.\nResponse 2-5: We thank the reviewer for pointing out the need for clearer image interpretation in Figure 4. We agree that some differences between panels may appear subtle, as the dual-band imaging is designed to resolve fine spectral variations rather than large morphological contrasts. To better highlight the detector’s spectral discrimination capability, we have redesigned several test scenes with enhanced color contrast, as shown in Figures 4d and 4e.\nIn Figure 4d, different inks exhibit distinct absorption in the NIR and SWIR bands: red ink is invisible in both; green ink is visible in NIR but fades in SWIR; black ink is clearly visible in both. These differences are promising for anti-counterfeiting applications. In Figure 4e, water and acetone in polyethylene bottles are indistinguishable under visible light. In the NIR image, only water shows a visible meniscus; in SWIR, both liquids are clearly resolved (water darker due to stronger absorption), while surface ink markings remain visible in NIR but vanish in SWIR—demonstrating effective material identification.\nFurthermore, we have added additional dual-band imaging examples in the Supporting Information (Figure S37). In FigureS37a, salt and sugar are hard to distinguish under visible or NIR light, but show clear contrast in SWIR (sugar appears darker). In Figure S37b, NIR and SWIR PbS CQD solutions look identical in visible/NIR images, but the NIR CQD solution becomes transparent in SWIR. In Figure S37c, bruising on a plum is revealed in both NIR and SWIR; plums and pears show similar contrast in NIR, but differ clearly in SWIR due to skin reflectance differences.\nThese results demonstrate that the implementation of the NIR/SWIR dual-band FPA holds great promise for applications in material identification and food safety monitoring.\nIn the main text on page 16 and page 17, we updated Figure 4d and 4f and added:\n“As shown in Figure 4d, different colored inks exhibit distinct absorption behaviors across the NIR and SWIR bands. Red ink remains undetectable in both NIR and SWIR images, whereas green ink is clearly visible under NIR illumination but becomes faint in the SWIR. In contrast, black ink is discernible in both spectral bands. These marked differences in spectral response enable the design of effective anti-counterfeiting markers. Figure 4e further illustrates the differential absorption properties of water, acetone, polyethylene (PE), and ink in the NIR and SWIR regions. When contained in white PE bottles, water and acetone are not observable under visible light. The NIR image reveals the meniscus of water but not that of acetone, whereas both menisci are clearly observed in the SWIR image, with water appearing darker due to stronger absorption. The ink markings on the bottle surface are prominent in the NIR image but fade in the SWIR image. Collectively, these results confirm that the distinct NIR and SWIR absorption can be effectively utilized for spectral identification.”\nMeanwhile, Figure 4 is updated in the main text on page 28:\n26\n“Figure 4. NIR/SWIR dual-band imaging. (a) Optical image and magnified view of the dual-band PbS CQDs FPA. (b) Schematic diagram of the experimental setup for dual-band imaging. NIR and SWIR images are presented for the following: (c) the emblem pattern covered with paint and shielded by a silicon wafer, (d) patterns drawn with red, green, and black ink, and (e) plastic bottles containing water and acetone.”\nAdditionally, we also updated the Figure S37 in the Supporting Information on page S28 and S29 with the following text:\n27\n“Figure S37. Application of the dual-band FPA for spectral discrimination. (a) Sugar and salt sealed in brown glass bottles. (b) PbS CQDs solutions with distinct NIR and SWIR absorption peaks. (c) A comparison of plums and pears.\nThe NIR/SWIR dual-band FPA demonstrates strong potential for material identification and food inspection by leveraging distinct spectral signatures. In Figure S37a, while salt and sugar within brown glass bottles are indistinguishable under both visible and NIR illumination, they exhibit a clear grayscale contrast in the SWIR band, with sugar appearing darker due to its higher SWIR absorption. 28 Conversely, printed ink labels visible in NIR are absent in SWIR. Figure S37b highlights differential spectral transparency: solutions of NIR- and SWIR-absorbing CQDs appear opaque and similar in the NIR image, yet the NIR-CQD solution becomes transparent in SWIR, enabling clear differentiation. Figure S37c further reveals subsurface bruising on a plum, which is invisible in visible light but clearly detected in both NIR and SWIR. Moreover, while plums and pears show similar intensity in NIR, the pear appears significantly brighter in SWIR, likely due to differing skin reflectance properties in this spectral region.”\nComment 2-6:The uniformity is not very good compared to reported CQD broadband PD or dual band bulk material PD. Is it come from the uniform doping? What is the roughness of each material layer? How to solve this problem?\nResponse 2-6: Thank the reviewer for such professional question.\nOur analysis attributes the primary source of non-uniformity to the inherent topographical variations of the ROIC surface. As evidenced by Figure S34, the fabricated ROIC exhibits micrometer-scale height variations across the pixel array. The solution-processed PbS CQD film, with the thickness of less than 500 nm, is highly susceptible to such substrate topography during spin-coating. This conformity to the underlying surface directly leads to pixel-to-pixel variations in film thickness, morphology, and consequently, photoresponse.\nTo isolate the influence of substrate topography, we fabricated control devices on flat Si/SiO substrates. Atomic force microscope (AFM) characterization of these control 2 devices reveals a substantially lower surface roughness of only 3.0 nm RMS (Figure S35). The significantly improved uniformity achieved on these flat substrates provide compelling evidence that substrate planarity is a critical factor for the performance homogeneity of solution-processed CQD FPAs.\nTo address the fundamental issue in future iterations, we propose a co-design and optimization framework between the ROIC and the photodetector stack. From a fabrication perspective, this will involve the introduction of an additional chemical-mechanical polishing (CMP) step within the back-end-of-line process of the ROIC. This targeted planarization protocol is designed to achieve sub-5 nm RMS surface topography prior to CQD deposition, thereby ensuring a uniform substrate for subsequent solution-processing[1].\nReference:\n[1] Klem, E. J. D., Gregory, C. W., Temple, D. S. & Lewis, J. S. Colloidal quantum dot 29 vis-SWIR imaging: Demonstration of a focal plane array and camera prototype (presentation recording). in Optical Sensing, Imaging, and Photon Counting: Nanostructured Devices and Applications vol. 9555 28–28 (SPIE, 2015).\nIn the main text on page 15, we added:\n“This non-uniformity primarily arises from surface unevenness of the ROIC (Figure S34), which introduces variations in pixel response. In comparison, the PbS CQDs film spin-coated on the flat substrate exhibit a markedly lower roughness of only 3 nm (Figure S35). Adopting planarization processes in future ROIC is expected to reduce defective pixel rates and improve spatial uniformity.”\nMeanwhile, we added the Figures S34 & S35 in the Supporting Information on page S25 and page S26:\n“Figure S34. Surface morphology imaged by confocal microscopy: (a) The pristine ROIC surface, and (b) the same surface after deposition of the PbS CQDs film, demonstrating the uniformity and coverage of the solution-processed layer.”\n30\n“Figure S35. AFM morphology and roughness of PbS CQD spin-coated on Si/SiO 2 substrates.”\nComment 2-7: Figure 1c and Figure 2e. The cut off wavelength of absorption and photoresponse spectra of SWIR do not agree with each other. Explain it.\nResponse 2-7: Thank the reviewer for such professional question.\nThe observed discrepancy in the cutoff wavelength between the absorption spectrum (Figure 1c) and the photoresponse spectrum (Figure 2e) arises primarily from the intrinsic batch-to-batch variation inherent in our quantum dot synthesis. These characterizations were performed on samples fabricated from two different CQD batches. To quantitatively assess this variation, we synthesized and characterized nine independent batches of SWIR PbS CQDs under nominally identical conditions. The absorption peak positions, as summarized in Figure S12, follow a distribution centered at 1650 nm with a standard deviation of 96.3 nm. The spectral difference between the devices in Figure 1c and 2e falls well within this statistical distribution. We attributed this batch-to-batch variation to the two-injection synthesis protocol, where minor fluctuations in critical parameters—such as the injection or timing of sulfur precursor—are known to exert a significant impact on the final quantum dot size.\nTherefore, the observed offset between the absorption and photoresponse cutoff wavelengths is mainly due to synthesis batch variability, rather than measurement error or fundamental device physics. We have added a discussion of this point in the revised manuscript, to clarify the origin of the spectral shift.\nReference:\n[1] Kirmani, A. R., Walters, G., Kim, T., Sargent, E. H. & Amassian, A. Optimizing solid-state ligand exchange for colloidal quantum dot optoelectronics: How much is enough? ACS Appl. Energy Mater. 3, 5385–5392 (2020).\nIn the main text on page 8, we added:\n“Notably, the spectral positions of the responsivity peaks in the NIR and SWIR regions are red-shifted by 11.1 meV and 35.6 meV, respectively, relative to the absorption peak of the PbS CQDs (Figure 1c). These energy shifts can be attributed to ligand-related effects (Figure S11) as well as variations introduced during the synthesis process (Figure S12).”\nMeanwhile, we added the Figures S11 & S12 in the Supporting Information on page S10 and page S11:\n32\n“Figure S11. Absorption spectra of PbS CQDs before and after liquid-phase ligand exchange for (a) NIR and (b) SWIR bands.”\n“Figure S12. (a) Absorption spectra of PbS CQDs synthesized in nine batches under identical conditions, and (b) statistical distribution of the peak wavelengths.”\nComment 2-8: They may avoid using words like innovative (line 34), first prototype (line 40) in the main text.\nResponse 2-8: Thank the reviewer for such professional question.\nWe thank the reviewer for the suggestion. We agree that terms such as \"innovative\" and \"first prototype\" can be subjective. As suggested, we have thoroughly revised the manuscript to replace all such terms with more objective and descriptive language.\n“Here, we report a top-illuminated, p-i-n-i-p structured dual-band photodetector based on two distinct sizes of PbS CQDs, enabling bias-switchable spectral response between the NIR and SWIR ranges. Through band engineering, the device achieves a specific detectivity exceeding 1×1011 cm·Hz1/2·W−1 in both bands, along with spectral crosstalk as low as 0.5%. By monolithically integrating the CQDs photodetector array with a custom-designed ROIC featuring bidirectional current sensing, we demonstrate a 128×128 dual-band infrared FPA capable of NIR and SWIR imaging. This work establishes a scalable, Si-compatible route to high-performance dual-band infrared FPAs using solution-processed semiconductors.”\nComment 2-9:The transmission lines in the FPA imaging system are quite long, which may induce extra electric noise in the imaging.\nResponse 2-9: Thank the reviewer for such professional question.\nWe thank the reviewer for this insightful comment. To address this concern, we have optimized the imaging measurement configuration by minimizing cable lengths.\nIn this paper Di et al build a dual band detector from a nanocrystal stack and study its operation going up to an imager demonstration. I have no doubt that this paper should be published after some edits, main question is if it fits Nature Communications, I am open with a good edit.\nWe appreciate the constructive comments and suggestions of the reviewer.\nComment 3-1: In the Introduction, the message is sometime a but confused. The goal of this paper is NIR and SWIR, while some argument and technology are actually MWIR based. Quantum well and superlattice technologies are mostly MWIR/LWIR and type II superlattice barely start in the SWIR.\nResponse 3-1: We thank the reviewer for this valuable feedback.\nWe have carefully revised the Introduction to better align with the scope of our study. The updated version now clearly emphasizes the development of solution-processed PbS CQD photodetectors for NIR and SWIR imaging applications. We have refocused the background discussion on existing NIR/SWIR technologies, including commercial InGaAs detectors and emerging CQD-based systems, while de-emphasizing or removing less relevant discussions on MWIR/LWIR-specific platforms such as quantum well infrared photodetectors (QWIPs) and long-wave type-II superlattices. This revision ensures a more coherent narrative that accurately reflects the goals and context of our work. The manuscript now better highlights the advantages of CQD technology—such as tunable bandgap, solution processability, and compatibility with silicon readout circuits—specifically for high-performance, low-cost NIR and SWIR imaging.\nIn the main text on page 2−4, We have revised the Introduction part as follows:\n“Near-infrared (NIR, 780–1100 nm) and short-wave infrared (SWIR, 1100– 2500 nm) imaging technologies are garnering significant interest for applications 35 spanning machine vision1, material identification2, and biological imaging3. Their utility in non-destructive analysis stems from the sensitivity of these spectral bands to characteristic overtone and combination vibrations of molecular bonds such as C–H, O–H, N–H, and S–H4, enabling the discrimination of materials based on their distinct chemical fingerprints. Furthermore, the differing scattering and absorption characteristics of materials in the NIR and SWIR regions result in distinct penetration depths for these two spectral bands5. This complementary behavior makes NIR/SWIR dual-band imaging a highly effectively approach for comprehensive material characterization. A representative example can be found in agricultural quality control, where the technique allows simultaneous evaluation of surface texture and internal defects—such as bruising and non-uniform ripening in apples6. As illustrated in Figure 1a, integrating dual-band imaging into optical sorting systems enhances machine vision for intelligent production, leading to more accurate and automated quality grading. By routing only premium produce to the fresh market and diverting substandard items to processed products (e. g., jam production), this technology helps minimizes food waste and strengthen supply chain sustainability.\nConventional dual-band NIR/SWIR imaging systems typically employ hybrid configurations of silicon (Si)-based NIR and indium gallium arsenide (InGaAs)-based SWIR detectors, requiring complex optical alignment and post-acquisition image fusion7. Although such systems achieve useful performance, their large footprint and high cost restrict deployment in resource-constrained or portable settings. Monolithic integration of dual-band photodetectors with readout integrated circuits (ROICs) on a single substrate represents a promising alternative, enabling substantial miniaturization and system-level cost reduction. Among integrated device architectures, vertically stacked photodiodes with back-to-back junction design allow bias-selectable spectral response while maintaining high pixel utilization — an essential feature for high-resolution dual-band imaging8.”\nResponse 3-2: We appreciate the reviewer’s expertise. We have revised the manuscript accordingly, removing the misleading characterization.\nWe update the sentence in the main text on page 3 and page 4:\n“Conventional dual-band NIR/SWIR imaging systems typically employ hybrid configurations of silicon (Si)-based NIR and indium gallium arsenide (InGaAs)-based SWIR detectors, requiring complex optical alignment and post-acquisition image fusion7. Although such systems achieve useful performance, their large footprint and high cost restrict deployment in resource-constrained or portable settings. Monolithic integration of dual-band photodetectors with readout integrated circuits (ROICs) on a single substrate represents a promising alternative, enabling substantial miniaturization and system-level cost reduction. Among integrated device architectures, vertically stacked photodiodes with back-to-back junction design allow bias-selectable spectral response while maintaining high pixel utilization — an essential feature for high-resolution dual-band imaging8.”\nComment 3-3: You say being lattice matched is a challenge; not much InGaAs on InP (or HgCdTe on CdZnTe or quantum well on GaAs….) are lattice matched…the cost of it is that it becomes more difficult to tune composition/band gap (because then it turns non lattice matches).\nResponse 3-3: We appreciate the reviewer’s expertise. We have revised the manuscript accordingly in the main text on page 3 and page 4:\n“Conventional dual-band NIR/SWIR imaging systems typically employ hybrid 37 configurations of silicon (Si)-based NIR and indium gallium arsenide (InGaAs)-based SWIR detectors, requiring complex optical alignment and post-acquisition image fusion7. Although such systems achieve useful performance, their large footprint and high cost restrict deployment in resource-constrained or portable settings. Monolithic integration of dual-band photodetectors with readout integrated circuits (ROICs) on a single substrate represents a promising alternative, enabling substantial miniaturization and system-level cost reduction. Among integrated device architectures, vertically stacked photodiodes with back-to-back junction design allow bias-selectable spectral response while maintaining high pixel utilization — an essential feature for high-resolution dual-band imaging8.”\nComment 3-4: Regarding applications : is SWIR that much driven by defense…not sure.\nResponse 3-4: We thank the reviewer for this thoughtful comment.\nWhile defense and surveillance provided the initial impetus for SWIR technology, the field is now increasing by non-defense applications, as the reviewer rightly notes. To reflect this shift, we have revised the manuscript to place greater emphasis on burgeoning industrial domains, including machine vision and substance detection. This provides a more balanced perspective on the current technology drivers. We have revised the manuscript accordingly.\nIn the main text on page 2, we added:\n“Near-infrared (NIR, 780–1100 nm) and short-wave infrared (SWIR, 1100– 2500 nm) imaging technologies are garnering significant interest for applications spanning machine vision1, material identification2, and biological imaging3.”\nResponse 3-5: We are grateful to the reviewer for bringing this important reference to our attention.The reference has accordingly been incorporated into in the Introduction, thereby providing a more comprehensive and accurate foundation for our discussion.\nIn the main text on page 4, we added:\n“Dual-band NIR and SWIR photodetectors based on back-to-back junction architectures have been demonstrated using a variety of material systems, including inorganic thin films (e.g., Si, Ge, GeSn)9,10, two-dimensional (2D) materials (e.g., MoS , black phosphorus)11,12, and colloidal quantum dots (CQDs) 2 (e.g., PbS, HgTe)13–15.”\nReference:\n15. Lhuillier, E., Robin, A., Ithurria, S., Aubin, H. & Dubertret, B. Electrolyte-gated colloidal nanoplatelets-based phototransistor and its use for bicolor detection. Nano Lett. 14, 2715–2719 (2014).\nComment 3-6: Detail but page 6 about « ultimately minimizing waste at the consumer level. », I am not sure that more sorting will generate less waste, likely it will turn to premium selection and more waste.\nResponse 3-6: Thank the reviewer for such professional question.\nOn page 6, we aim to convey that advanced NIR/SWIR-based sorting facilitates more intelligent and value-preserving utilization of agricultural produce, moving beyond the mere rejection of substandard items. For instance, fruits with cosmetic defects (e.g., bruising, irregular shape) but intact internal quality can be diverted from fresh markets to alternative uses such as juice, jam, or puree production, rather than being discarded at the consumer or retail level.\nBy enabling precise, non-destructive assessment of both external and internal fruit quality (e.g., sugar content, water distribution, early rot), NIR/SWIR sorting allows for targeted routing of produce into appropriate supply chains. This reduces over-rejection in retail and minimizes consumer-level waste due to unexpected spoilage or dissatisfaction.\n“As illustrated in Figure 1a, integrating dual-band imaging into optical sorting systems enhances machine vision for intelligent production, leading to more accurate and automated quality grading. By routing only premium produce to the fresh market and diverting substandard items to processed products (e.g., jam production), this technology helps minimizes food waste and strengthen supply chain sustainability.”\nComment 3-7: How thick is C layer.\nWe performed FIB sectioning and cross-sectional TEM characterization of the dual-band photodetector to verify its layer structure. The resulting TEM image in Figure 1(d) shows a C layer with a thickness of approximately 50 nm, thereby confirming the 60 target dimension.\nFigure 1 (d). Cross-sectional TEM image of the dual-band photodetector.\nIn the Methods part on page 19, we added:\n“Next, a 50 nm C layer was thermally evaporated, followed by the deposition of 60 a 50 nm ZnO layer via magnetron sputtering, serving as the electron transport layers.”\nResponse 3-8: Thank the reviewer for such professional question.\nOur results affirm that ITO sputtering—due to its inherent energetic particle bombardment—causes interfacial damage and increased defect densities in PbS CQD films (Figure 17), consistent with our prior findings (Adv. Sci. 2024, 11, 2307169). Recognizing that this sputter-induced damage is fundamentally incompatible with delicate CQD layers, we engineered a solution-based alternative using Ti C Tx MXene 3 2 as the transparent electrode.\nFigure 2. Schematic diagram and interface defects characterization of PbS photodiodes with ITO as the top electrode.\nAfter careful consideration of the trade-offs, we selected ITO as the transparent top electrode for our dual-band architecture. This decision was driven by its optimal balance of high transparency in the NIR-to-SWIR spectrum, excellent conductivity, and exceptional film uniformity, which are critical for the performance of our specific device.\nIn the main text on page 7, we added:\n“To elucidate the underlying transport mechanisms, we conducted a detailed simulation analysis of the dark current components. Under reverse bias, the dark current in the SWIR PbS diode is dominated by generation-recombination (G-R) processes. In contrast, under forward bias, trap-assisted tunneling (TAT) constitutes the dominant contribution to the dark current in the NIR PbS diode, 41 which may be attributed to defect states introduced during ITO sputtering18, as corroborated by Figure S5.”\nMeanwhile, we added Figure S5 in the Supporting Information on page S8:\n“Figure S5. (a) Comparison of measured and simulated dark current density versus voltage curves for the PbS CQDs dual-band device, and (b) simulation analysis of the components contributing to the dark current.”\nComment 3-9: Why the detectivity of SWIR diode larger? sure absorption from NIR device is sub optimal, but dark current in the SWIR one should be much larger due to weaker band gap, this brings back to my previous question can ITO may have damage the NIR diode.\nResponse 3-9: Thank the reviewer for such professional question.\nAs the reviewer correctly hypothesizes, we believe this is likely due to interface or bulk defects introduced during the ITO deposition process. Indeed, the energetic particle bombardment inherent to the sputtering process can damage the underlying NIR CQDs layer, increasing its trap state density. These defects, in turn, can strongly enhance Shockley-Read-Hall (SRH) recombination and facilitate tap-assisted tunneling (TAT).\nTo elucidate the origin of this asymmetry, we performed TCAD simulations of the dark current mechanisms in the dual-band device. Given the inherently low carrier mobility in CQDs films, we find that the dark current is dominated by generation-recombination (G-R) and TAT, with diffusion current being negligible. As shown in the Figure S5, the 42 simulated tunneling current is significantly higher in the NIR diode than in the SWIR diode, consistent with the mechanism of ITO-induced defects.\nIn the main text on page 7, we added:\n“To elucidate the underlying transport mechanisms, we conducted a detailed simulation analysis of the dark current components. Under reverse bias, the dark current in the SWIR PbS diode is dominated by generation-recombination (G-R) processes. In contrast, under forward bias, trap-assisted tunneling (TAT) constitutes the dominant contribution to the dark current in the NIR PbS diode, which may be attributed to defect states introduced during ITO sputtering18, as corroborated by Figure S5.”\nMeanwhile, we added the Figure S5 in the Supporting Information on page S8:\n“Figure S5. (a) Comparison of measured and simulated dark current density versus voltage curves for the PbS CQDs dual-band device, and (b) simulation analysis of the components contributing to the dark current.”\nComment 3-10: Do not provide QD size with a 10 pm resolution while atomic lattice is 0.3 nm.\nResponse 3-10: Thank the reviewer for such professional question.\nWe have revised the manuscript to present quantum dot diameters with realistic 43 precision.\nIn the main text on page 6, we revised\n“Transmission electron microscopy (TEM) confirmed highly uniform size distributions, with mean particle diameters of 2.9 nm and 7.5 nm (Figure S2).”\nMeanwhile, we updated Figure S2 in the Supporting Information on page S4:\n“Figure S2. Representative TEM image and corresponding particle size distribution histograms of (a, b) NIR- and (c, d) SWIR-absorbing PbS CQDs in ink.”\nComment 3-11: I have feeling that the word « crosstalk » is used in two contexts the spatial crosstalk between pixel and spectral crosstalk between diode. Go through the text to always be clear about which one you speak about.\nResponse 3-11: We thank the reviewer for this critical clarification.\nTo address this point, the crosstalk mechanism discussed in our manuscript has been clarified and precisely defined as spectral crosstalk—which refers specifically to the undesired photoresponse overlap between the NIR and SWIR diodes co-integrated 44 within a single pixel. This overlap thereby degrades the spectral isolation essential for high-fidelity dual-band detection. To prevent any potential misunderstanding, the relevant text has been systematically revised throughout the manuscript.\nComment 3-12: About linearity, a lot is said in the littérature, to be linear the log log plot of figure s7 not only needs to be a line, but the slope MUST be one, make a power law fit and provide exponent to confirm your actual linearity and therefore the LDR values.\nResponse 3-12: Thank the reviewer for such professional question.\nIn accordance with the suggestion, we performed a power-law fit on the responsivity data (previously Figure S7) to extract the exponent for both the NIR and SWIR channels. The fitted slopes are 0.98 for the NIR diode and 0.93 for the SWIR diode, which are close to 1. Based on this more rigorous analysis, we have updated Figure S9 to include the fitted slopes and explicitly annotate the power-law exponents. Furthermore, we have recalculated the linear LDR using the region, yielding revised LDR values of ＞80 dB for NIR and＞48 dB for SWIR.\nIn the main text on page 8, we added:\n“The calculated linear dynamic range (LDR) reaches 80 dB at 940 nm and 48 dB at 1650 nm, underscoring the device’s capacity for dual-band discrimination while maintaining consistent responsivity over a broad intensity range.”\nMeanwhile, we updated Figure S9 in the Supporting Information on page S9.\n45\n“Figure S9. LDRs of the dual-band photodetector under (a) +1 V and (b) −1 V bias.”\nComment 3-13:I have a major concern about your noise measurement. I acknowledge that it is great that measurements have been done while most of the literature paper just make claim based on white noise which is highly misleading. But you use an amplifier with a noise of 5fA.Hz−1/2, so can you explain how you measure noise that is lower than your setup noise….This point needs to be genuinely addressed and is a clear bottleneck to match nature comm standard. Especially because your value of detectivity will be reduced and therefore the final perf might be less appealing.\nResponse 3-13: We sincerely thank the reviewer for this extremely important and technically critical comment.\nThe characterization of our device’s noise performance was initially constrained by the inherent bandwidth and noise floor of the low-noise amplifier. To overcome the limitation and ensure accurate noise characterization, we conducted additional measurements using a semiconductor parameter analyzer (FS-Pro, Primarius Technologies), which offers a significantly lower current noise floor compared to amplifier-based setups. To further enhance the signal-to-noise ratio relative to the instrument noise, larger-area devices (9.18 × 10−3 cm2) were fabricated to amplify the absolute noise signal, as illustrated in Figure S20.\nThe FS-Pro system has a specified current noise floor of 10 fA·Hz−1/2. Using this setup, 46 the measured noise current of the device at ±1 V bias was 41 fA·Hz−1/2 at +1 V and 0.35 pA·Hz−1/2 at −1 V, respectively. Based on these direct noise measurements, the specific detectivity for both NIR and SWIR diodes was recalculated, yielding updated values of 3.04 × 1011 cm·Hz1/2·W−1 at +1 V and 1.04 × 1011 cm·Hz1/2·W−1 at −1 V, which remain competitive within the field.\nThe revised noise and detectivity results have replaced the previous data in both the main text and Supporting Information. A detailed description of the measurement methodology has also been included to ensure reproducibility.\nWe revised the Figure 2f and added the following text in the main text on page 9:\n“The results show that the noise current decreases with increasing frequency, following a i ∝ f−1/2 across the measured range. Importantly, at 10 kHz the noise n current (0.35 pA·Hz−1/2 at −1 V and 41 fA·Hz−1/2 at +1 V) remains above the fundamental shot noise limit (96 fA·Hz−1/2 at −1 V and 20 fA·Hz−1/2 at +1 V). This behavior indicates that 1/f noise is the dominant noise source in the device within the operational frequency band. Using these values, the device achieves peak detectivities of 3.04×1011 cm·W−1·Hz1/2 for NIR and 1.04×1011 cm·W−1·Hz1/2 for SWIR, demonstrating high sensitivity across both spectral ranges.”\n“Figure 2f. Measured current noise of the dual-band photodetector at ±1 V bias.”\nIn the Methods part on page 20, we added:\n“Noise currents of the PbS CQDs photodiode were measured usinga 47 semiconductor parameter analyzer (FS-Pro, Primarius Technologies).”\nComment 3-14:The back to back diode discussion is poor, but on same time one of the highest value of this paper is figure 3a. in the Si there is a mathematical discussion about the concept but the main text misses a discussion explaining the concept. There are two diodes back to back and therefore the current from one diode can cancel the one from the other, so by choosing carefully the bias value, the NIR response of the SWIR diode can be mostly suppress: say it as simply. Then graph 3a must be better discussed as well, i see what look like some different area and somehow some kind of phase diagram of the device operation, discuss it, because in most previous paper this was not shown.\nResponse 3-14: Thank the reviewer for such professional question.\nRegarding Figure 3a, we have expanded our discussion to better explain it. Specifically, in region IV, the bottom diode (PbS-II) is reverse biased, whereas the top diode (PbS-I) is forward biased (Figure 1g). Consequently, when exposed to light, the output photocurrent is primarily contributed by electron-hole pairs generated in PbS-II due to its broadband absorption characteristics from NIR to SWIR. As the applied voltage decreases into region III, electron-hole pairs are simultaneously generated in both diodes, but under NIR illumination, these pairs tend to cancel out, preserving the SWIR response. Further lowering the bias into region II results in insufficient energy for carriers to overcome barriers between absorption and transport layers, leading to weak responses in both bands. When applying positive voltage in region I, only the top diode operates in reverse bias, focusing the response solely on the NIR band. Thus, wavelength-dependent photoresponse can be finely tuned by adjusting the external bias.\nIn the main text on page 11, we added:\n“The responsivity matrix reveals four distinct operational regions defined by bias ranges. In Region I (0.1 V to 1 V), the device exhibits a strong response in the NIR band with minimal SWIR contribution. Region II (−0.25 V to 0.1 V) is marked by weak photoresponses across the entire spectrum. Region III (−0.25 V to −0.5 V) yields a strong response combined with a weak NIR signal, whereas Region IV (−0.5 V to −1 V) produces a broad spectral response covering both NIR and SWIR bands. In Region IV, the bottom diode (PbS-II) is reverse-biased, and the top diode (PbS-I) is forward-biased (Figure 1h). Under illumination, the resulting photocurrent is primarily contributed by electron-hole pairs generated in PbS-II due to its broadband absorption characteristics from NIR to SWIR. As the applied voltage decreases into Region III, electron-hole pairs are simultaneously generated in both diodes. However, under NIR illumination, opposing photocurrent components cancel out, preserving the SWIR selective detection (Figure 3b). Region II, the bias is insufficient to efficiently extract carriers across the inter-layer energy barriers, leading to minimal responses in both bands. Under positive voltage in Region I, only the top diode operates in reverse bias, enabling selective NIR detection (Figure 1g). Thus, spectral photoresponse can be finely tailored through external bias tuning.”\nComment 3-15: In method « Ac » in NH Ac or NaAc is not defined, is it acetate ?\n“Ac” refers to acetate, and we have clarified this in the Methods part on page 18:\n“For NIR-absorbing PbS CQDs, a ligand solution containing PbI (0.01 M), PbBr 2 2 (0.04 M), and ammonium acetate (0.06 M) in N,N-dimethylformamide (DMF). An octane solution of OA-capped PbS CQDs (20 mg·mL−1) was added, and the mixture was vigorously shaken for 5 minutes to introduce phase transfer into DMF.”\nComment 3-16:Axis ultra DLD is XPS from kratos say it\nResponse 3-16: Thank the reviewer for such professional question.\nWe have updated the Methods part to clarify this on page 20:\n“Ultraviolet photoelectron spectroscopy (UPS) was performed under ultrahigh vacuum (~10−10 mbar) using a KRATOS AXIS Ultra DLD system with a monochromatic HeI (21.22 eV) excitation source.”\nSubmitted paper entitled “Dual-Band Infrared PbS Colloidal Quantum Dot Focal Plane Array” presents a dual-band infrared imaging system based on a vertically stacked configuration of oppositely oriented near-infrared (NIR) and short-wave infrared (SWIR) absorbing PbS CQD layers. The authors report dark current densities on the order of 10⁻⁶ A and EQE of ~25%, followed by dual-wavelength imaging demonstrations. While the experimental realization is technically competent, the manuscript, in its current form, falls short of the scientific depth expected for a publication in Nature Communications. The work reads more as a mid-level engineering report/demonstration than a contribution of fundamental scientific insight. I recommend either submitting this manuscript to a more engineering- or imaging-focused journal, or doing more experimental designs and substantially revising it to clearly articulate and support a meaningful scientific question or hypothesis. Below, I outline several concerns and suggestions:\nWe appreciate the constructive comments and suggestions of the reviewer.\nComment 4-1: The use of PbS CQDs for infrared detection and imaging has been extensively explored and commercialized. The device architecture, including the hole/electron transport layers and charge blocking strategies, appears to rely on well-established approaches that have been in the literature for over a decade.\nResponse 4-1: We thank the reviewer for this insightful comment regarding the maturity of PbS CQDs technology. The true novelty of this work, however, lies not in the individual layer constituents, but in the first dual-band photodetector stack engineered for top-side illumination and direct FPA integration.\nWhile employing established single-band concepts, we tackled unique challenges in spectral crosstalk suppression—issues inherent to our dual-band architecture but absent in single-junction devices. Our key innovations form a cohesive development pipeline: first, we re-engineered the device stack to facilitate top-side illumination while maintaining high-detectivity–a configuration that is critical for direct coupling with 50 ROICs in an FPA. Building upon this foundation, we developed a tailored optimization protocol to achieve the essential low spectral crosstalk between NIR and SWIR. Finally, we conceived a novel bidirectional readout circuit to independently access the two photodiodes, thereby enabling NIR/SWIR dual-band data acquisition.\nThis work thereby provides the first experimental demonstration of a solution-processed, dual-band CQD-FPA capable of simultaneous NIR/SWIR imaging. We have revised the manuscript to more explicitly frame our work within this context of integration science, clarifying that its primary contribution is a scalable architecture for on-chip spectral discrimination, advancing CQD technology from single-band detection toward functional multispectral imaging.\nIn the main text on page 4, we added:\n“Here, we report a top-illuminated, p-i-n-i-p structured dual-band photodetector based on two distinct sizes of PbS CQDs, enabling bias-switchable spectral response between the NIR and SWIR ranges. Through band engineering, the device achieves a specific detectivity exceeding 1×1011 cm·Hz1/2·W−1 in both bands, along with spectral crosstalk as low as 0.5%. By monolithically integrating the CQDs photodetector array with a custom-designed ROIC featuring bidirectional current sensing, we demonstrate a 128×128 dual-band infrared FPA capable of NIR and SWIR imaging. This work establishes a scalable, Si-compatible route to high-performance dual-band infrared FPAs using solution-processed semiconductors.”\nComment 4-2: While PbS CQDs are widely considered the gold standard among CQD materials, their intrinsic ionic character leads to well-known issues with operational and environmental instability. The device architecture employed here has previously been shown to suffer from performance degradation under electrical bias and prolonged operation. However, the manuscript does not address these stability concerns, either through experimental investigation or discussion.\nResponse 4-2: We thank the reviewer for raising such critical points regarding device 51 stability, which we have now addressed with systematic experimental evidence.\nOperational stability was evaluated under continuous electrical bias, with Figure S25 demonstrating that the photoresponse remain stable over 1,000 consecutive measurement cycles at both +1 V and –1 V, confirming the robustness of the stack under repeated electrical stress.\nFigure S25. Transient photoresponse of the dual-band photodetector during operation about 1,000 cycles at (a) +1 V and (b) −1 V bias.\nTo evaluate operational longevity, we conducted a controlled aging study to assess the environmental stability. As shown in Figure S28a, both the dark current and photocurrent in the NIR and SWIR bands exhibit a slight yet gradual degradation over time. This mild performance decline is likely attributed to oxidation-induced reduction in charge carrier mobility within the CQDs film, possibly due to the formation of surface oxides or trap states that hinder carrier transport.Notably, the degradation at the device level is substantially slower than that observed in bare CQDs films, underscoring the partial protective role of the top-layer ITO electrode. Consistent with this interpretation, devices stored in a N glovebox retained initial responsivity after 15 2 months (Figure S28b), confirming their inherent stability in inert atmospheres.\nThese results suggest that the primary degradation mechanisms—likely involving oxygen and moisture-induced oxidation and possibly ion migration—are effectively mitigated by proper encapsulation. Therefore, for practical applications, we recommend 52 hermetic packaging using metal or ceramic housings with inert gas sealing to ensure long-term reliability of the CQDs image sensors.\nIn the main text on page 13, we added:\n“Subsequently, the device was subjected to extended cycling tests under 940 nm and 1650 nm illumination to evaluate its reliability in dual-band detection. As shown in Figure S25, both photocurrent and dark current remained stable over 1,000 continuous cycles, demonstrating exceptional operational stability in dual-band detection scenarios. We further characteristiced the environmental stability of PbS CQDs films and devices by monitoring their optical and electrical properties under different storage conditions. As shown in Figure S26, the absorption spectra of ligand-exchanged films exhibit negligible shifts after 20 days in both air and nitrogen (N ), indicating robust optical stability. Electrically, the 2 field-effect transistors based on PbS CQDs fabricated on Si/SiO substrates with 2 Ti/Au electrodes retained nearly identical transfer characteristics after 20 days of N storage (Figure S27a), confirming excellent electrical stability in inert 2 environments. In contrast, devices stored in ambient air exhibited a gradual decline in conductivity and carrier mobility, dropping to 50% and 40% of initial values after 20 days, respectively (Figure S27b). This degradation is attributed to co-oxidation by atmospheric O and moisture, leading to the formation of 2 insulating PbSO or PbSO species at dot surfaces or interfaces21, which increases 3 4 charge hopping barriers and impair transport performance. Consistent with these findings, device-level performance showed mild degradation after two weeks in air, with slight reductions in both dark and photocurrents in the NIR and SWIR bands (Figure S28a). Crucially, devices stored in a N -purged environment for 15 months 2 retain nearly all initial performance, with no significant change in dark current or responsivity (Figure S28b), affirming the intrinsic stability of the materials system when isolated from ambient atmosphere. For practical applications, hermetic packaging using metal or ceramic housings with inert gas filling is necessary to ensure long-term reliability of CQDs-based image sensors.”\n“Figure S27. Environmental stability of ligand-exchanged PbS films. (a) Transfer characteristics after 20 days of storage in N atmosphere. (b) Evolution of 2 conductivity and charge carrier mobility of the ligand-exchanged PbS films as a function of storage time in air.”\n“Figure S28. Comparison of dark current and photocurrent (under 0.1 μW 1650 nm and 1 μW 940 nm illumination) of the devices after storage for (a) 14 days in air and (b) 15 months in N , measured at −1 V and +1 V bias.”\nResponse 4-3: We sincerely thank the reviewer for raising this important comment, which provides us with the opportunity to better articulate the fundamental advance of our work.\nWe fully acknowledge that various configurations of CQDs-based dual-band detection have indeed been reported. However, it is critical to distinguish that all previous systems relied on mechanical pixel-by-pixel scanning–an approach inherently limited by low imaging speed, mechanically complexity, and impracticality for large-scale or real-time applications[1-3]. This fundamental constraint stemmed from the back-illuminated device architectures, fabricated on ITO-glass substrates (Figure 1), which are structurally and electrically incompatible with direct monolithic integration onto silicon ROICs.\nFigure 1. Architectures of previously reported CQD-based dual-band photodetectors.\nReference:\n[1] Zhao, P. et al. Band-engineered dual-band visible and short-wave infrared photodetector with metal chalcogenide colloidal quantum dots. J. Mater. Chem. C 11, 2842–2850 (2023).\n[2] Pejović, V. et al. Photodetectors based on lead sulfide quantum dot and organic absorbers for multispectral sensing in the visible to short-wave infrared range. Adv. Funct. Mater. 32, 2201424 (2022).\n[3] Tang, X., Ackerman, M. M., Chen, M. & Guyot-Sionnest, P. Dual-band infrared imaging using stacked colloidal quantum dot photodiodes. Nat. Photonics13, 277–282 55 (2019).\nThe central advance of our work is therefore not merely another demonstration of dual-band detection, but the creation of a novel top-illuminated device architecture specifically engineered for monolithic integration into an FPA. This architectureal breakthrough overcomes a long-standing materials-processing integration challenge and was realized through several co-optimized innovations. Notably, we developed integration-aware band engineering that not only ensures compatibility with the ROIC fabrication process but also effectively suppresses dark current, thereby enabling high specific detectivity (>1011 cm·Hz1/2·W-1) across both the NIR and SWIR bands—performance that surpasses previously reported CQDs-based dual-color detectors.\nFurthermore, we implemented active crosstalk suppression via a bias-tunable spectral response strategy, which minimizes inter-band interference and enables spectral discrimination during independent readout.\nBy combining this ROIC-compatible device architecture with a custom-designed readout system, we successfully demonstrate–for the first time–dual-band imaging using a monolithic CQDs FPA. This achievement marks a critical transition from single-pixel or scanned prototypes toward practical, scalable platform for multispectral imaging based on solution-processed quantum dot materials.\nIn the main text on page 3 and page 4, we have revised the Introduction part to better highlight the innovation of this work.\n“Conventional dual-band NIR/SWIR imaging systems typically employ hybrid configurations of silicon (Si)-based NIR and indium gallium arsenide (InGaAs)-based SWIR detectors, requiring complex optical alignment and post-acquisition image fusion7. Although such systems achieve useful performance, their large footprint and high cost restrict deployment in resource-constrained or portable settings. Monolithic integration of dual-band photodetectors with readout integrated circuits (ROICs) on a single substrate represents a promising alternative, enabling substantial miniaturization and system-level cost reduction. Among integrated device architectures, vertically stacked photodiodes with back-to-back junction design allow bias-selectable spectral response while maintaining high pixel utilization—an essential feature for high-resolution dual-band imaging8.\nDual-band NIR and SWIR photodetectors based on back-to-back junction architectures have been demonstrated using a variety of material systems, including inorganic thin films (e.g., Si, Ge, GeSn)9,10, two-dimensional (2D) materials (e.g., MoS , black phosphorus)11,12, and colloidal quantum dots (CQDs) 2 (e.g., PbS, HgTe)13–15. However, Ge-based photodetectors suffer from relatively low detectivity in the SWIR range. Meanwhile, the practical deployment of 2D materials is hampered by the absence of scalable wafer-level synthesis and reliable heterointegration strategies, which limits their compatibility with FPAs manufacturing workflows9.\nIn contrast, CQDs, particularly those based on PbS, have emerged as a versatile material platform for both NIR and SWIR detection, owing to their bandgap tunability, solution processability, and inner compatibility with ROICs16. Vladimir et al. pioneered a vertically stacked n-p-n PbS CQDs photodiode architecture that enables bias-selectable dual-band detection across the NIR and SWIR regimes. Nevertheless, this device was implemented on a glass substrate using a back-illuminated configuration, rendering it incompatible with monolithic integration onto ROICs. Furthermore, CQDs-based dual-band photodetectors have so far been largely confined to single-pixel demonstrations, where performance is compromised by high dark current and non-negligible spectral crosstalk, there impeding their adoption in practical imaging application17.\nHere, we report a top-illuminated, p-i-n-i-p structured dual-band photodetector based on two distinct sizes of PbS CQDs, enabling bias-switchable spectral response between the NIR and SWIR ranges. Through band engineering, the device achieves a specific detectivity exceeding 1×1011 cm·Hz1/2·W−1 in both bands, along with spectral crosstalk as low as 0.5%. By monolithically integrating the CQDs photodetector array with a custom-designed ROIC featuring bidirectional current sensing, we demonstrate a 128×128 dual-band infrared FPA capable of 57 NIR and SWIR imaging. This work establishes a scalable, Si-compatible route to high-performance dual-band infrared FPAs using solution-processed semiconductors.”\nComment 4-4:The manuscript does not clearly articulate the scientific challenge being addressed or the fundamental insight offered by this study. As a result, the broader implications for the CQD or imaging community remain unclear.\nResponse 4-4: We thank the reviewer for the opportunity to clarify the scientific significance of this work. In response, we have comprehensively restructured the manuscript to clearly define the fundamental challenge addressed and the conceptual advances offered by our study.\nThe core scientific challenge we tackle is the fundamental incompatibility between conventional CQDs device architectures—typically fabricated on ITO-glass substrates—and the structural requirements for direct monolithic integration with silicon-based ROICs. This mismatch has persistently hindered the transition of CQDs technology from single-pixel demonstrations to practical, scalable FPAs.\nOur work provides three foundational insights that collectively address this challenge:\n(1) A shift in device architecture: We present a top-illuminated dual-band CQDs photodetector architecture designed to enable monolithic integration with ROICs. This design addresses the inherent limitations of conventional back-illuminated devices fabricated on ITO/glass substrates, which are incompatible with front-side-coupled ROICs and thus hinder scalable FPA development. The proposed architecture provides a viable pathway toward integrated, large-format CQDs-based imaging systems.\n(2) Investigation of band alignment: We elucidate the critical relationship between band alignment engineering and dark current suppression in vertically stacked CQDs heterojunctions, providing a design principle for managing charge transport in complex multi-junction devices.\n(3) An operational strategy for spectral discrimination: We demonstrate a bias-tunable operational mode that actively mitigates inter-band crosstalk, thereby addressing a pervasive physical limitation in integrated CQDs dual-band photodetectors.\nCollectively, these advances establish a scalable pathway for developing solution-processed, multi-spectral imaging systems. We have substantially revised the Introduction part to better highlight the scientific significance.\nComment 4-5: The proposed architecture may face limitations for real-world imaging applications. Notably, the system appears to require relatively high bias voltages to achieve meaningful contrast, which can negatively affect device stability, induce ghost imaging artifacts, and slow response times.\nResponse 4-5: Thank the reviewer for such professional question.\nWe have evaluated the device’s dynamic performance under typical operating conditions. As shown in Figure S14, the rise/decay response times are ~86 μs for the NIR channel and ~67 μs for the SWIR channel, respectively. Furthermore, the 3 dB bandwidth measurement confirms that the device maintains a modulation bandwidth of up to 10 kHz (Figure S15), well exceeding the requirements for standard video-rate imaging.\nFigure S14. Transient response of the dual-band photodetector at (a) +1 V and (b) −1 V bias.\nIn the main text on page 9, we add:\n“Furthermore, the temporal photoresponse of the device were systematically evaluated, as shown in Figure S14. Under 940 nm illumination, the photoresponse exhibited rise and fall times of 12.7 µs and 85.5 µs, respectively. In comparison, under 1650 nm illumination, the device demonstrated faster response dynamics, with corrsesponding rise and fall times of 9.1 µs and 67.3 µs, respectively. These dynamic performance metric were further supported by frequency-domain measurements showing a −3 dB bandwidth of approximately 10 kHz, which closely aligns with the time-domain reponse data and demonstrates the device’s capacity for high-speed optical signals detection (Figure S15).”\nComment 4-6: Due to the opposing orientation of the stacked junctions, the internal electric fields generated upon illumination in each spectral band are in opposite directions. This raises the possibility of electrostatic coupling or cancellation effects, whereby excitation in one junction perturbs the band alignment and detection fidelity in the other. This undermines the assumption of independent dual-band detection and should be thoroughly investigated.\nResponse 4-6: Thank the reviewer for such professional question.\nWe have conducted both theoretical and experimental investigations. In the Supporting Information Figure S3, we derive, based on the Shockley equation of a diode, that two back-to-back photodiodes can achieve dual-band independent detection.\nFigure 2. Comparison of J–V characteristics of the device.\nWe added the relative discussion and Figure S3 in the Supporting Information on page S5 and page S6:\n“Figure S3. (a) Equivalent circuit diagram and (b) J-V characteristics of the dual-band photodetector under dark and illuminated conditions.\nThe dual-band detector is composed of a NIR photodiode and a SWIR photodiode configured in a back-to-back arrangement (Figure S3a). The anode of 61 the NIR photodiode is grounded, while a working voltage is applied to the anode of the SWIR photodiode. Under dark conditions, the J-V characteristics of an ideal diode follows the Shockley equation:\n𝒒𝑽\nwhere is the reverse satur𝑱a=tio𝑱n𝒔 [c𝐞u𝐱r𝐩re(nt d)en−si𝟏ty], k is the Boltzmann consta(n𝟏t), 𝒌𝑻\nand q is\n𝑱\n𝒔the elementary charge. For the dual-band device formed by connecting two diodes back-to-back, the following equations hold:\n𝒒𝑽𝟐 𝒒𝑽𝟏\n𝑱 = 𝑱𝒔𝟐[𝐞𝐱𝐩( )−𝟏] = −𝑱𝒔𝟏[𝐞𝐱𝐩( )−𝟏] (𝟐)\n𝒌𝑻 𝒌𝑻\nwhere and are the operati\n𝑽\nng\n=\nvo\n𝑽\nl𝟐ta\n−\nge\n𝑽\ns 𝟏of the near-infrared and short-wa(𝟑 ve)\ninfrared\n𝑽\n𝟏photod\n𝑽\nio𝟐des, respectively, while and represent their respective\nreverse saturation current densities. So\n𝑱\nlv𝒔𝟏ing the\n𝑱\ns𝒔e𝟐 equations yields the J-V\nrelationship of the dual-band device:\n𝑱𝒔𝟏 +𝑱𝒔𝟐\n𝑱 = 𝑱𝒔𝟏 − (𝟒)\n𝑱𝒔𝟐 𝒒𝑽\nUnder illumination, the J-V relations\n𝑱\nh𝒔i𝟏p𝒆 f𝒙o𝒑r (a\n𝒌\np\n𝑻\nh)ot+od𝟏iode is described by:\n𝒒𝑽\nBy analogy, upon NIR 𝑱lig=ht𝑱 𝒔a[𝐞b𝐱so𝐩r(ptio)n −b𝟏y] −th𝑱e𝒑 𝒉NIR photodiode, the J(𝟓-V)\n𝒌𝑻\nrelationship of the dual-band device becomes:\n𝑱𝒔𝟏 +𝑱𝒔𝟐 +𝑱𝒑𝒉𝟏\n𝑱 = 𝑱𝒔𝟏 +𝑱𝒑𝒉𝟏 − (𝟔)\n𝑱𝒔𝟐 𝒒𝑽\nSimilarly, upon SWIR light absorption b𝒆y𝒙 𝒑th(e SW)+IR𝟏 photodiode, the device\n𝑱𝒔𝟏 𝒌𝑻\nsatisfies the following equation:\n𝑱𝒔𝟏 +𝑱𝒔𝟐 +𝑱𝒑𝒉𝟐\n𝑱 = 𝑱𝒔𝟏 − (𝟕)\n𝑱𝒔𝟐 𝒒𝑽\nThe J-V curves corresponding to Equat𝒆io𝒙n𝒑s ((4), )(6+), 𝟏and (7) are plotted in Figure\n𝑱𝒔𝟏 𝒌𝑻\nS3b. Under reverse bias, the device operates as a SWIR photodetector, while under forward bias, it functions as an NIR photodetector.”\nResponse 4-7: Thank the reviewer for such professional question.\nWe have carefully reviewed all measurements and data presented in the figures. To demonstrate the reproducibility and statistical reliability of our device fabrication and performance, we have included a statistical analysis in the Figure S13, where responsivity and dark current—are plotted for 25 individual devices fabricated under identical conditions.\nFigure S13. Statistical distribution of 25 dual-band photodetector devices. (a) dark current and (b) responsivity at 940 nm under +1 V bias, and (c) dark current and (d) responsivity at 1650 nm under −1 V bias.\nThe results show consistent performance with a narrow distribution, confirming the robustness and repeatability of our process. We have also ensured that all representative data in the figures are aligned with this statistical dataset.\n“To validate the reproducibility of the fabrication process, we characterized a set of 25 devices by measuring their dark currents and responsivities under ±1 V bias at 940 nm and 1650 nm illumination (Figure S13). The statistical analysis demonstrates good uniformity and device-to-device consistency, confirming the robustness of the fabrication methodology.”\nComment 4-8: The images presented in Figure 4 are difficult to interpret. Aside from panel c, the differences among the panels are subtle or unclear.\nResponse 4-8: We thank the reviewer for their valuable feedback regarding the image interpretation in Figure 4. We agree that certain distinctions between panels could benefit from clearer presentation. To more effectively demonstrate the spectral discrimination capability of our dual-band detector, we have redesigned several imaging scenes with enhanced color contrast, as shown in Figures 4d and 4e.\nIn Figure 4d, different inks exhibit distinct spectral behaviors: red ink is invisible in both NIR and SWIR; green ink is visible in NIR but attenuates in the SWIR; black ink remains clearly observable across both bands. These characteristics suggest potential utility in anti-counterfeiting applications. In Figure 4e, water and acetone in polyethylene bottles are indistinguishable under visible light. In the NIR image, only water shows a visible meniscus; in SWIR, both liquids are clearly resolved (water darker due to stronger absorption), while surface ink markings remain visible in NIR but vanish in SWIR—demonstrating effective material identification.\nAdditional dual-band imaging examples are provided in the Supporting Information (Figure S37). In Figure S37a, salt and sugar are hard to distinguish under visible or NIR light, but show clear contrast in SWIR (sugar appears darker). In Figure S37b, NIR and SWIR CQD solutions look identical in visible/NIR images, but the NIR CQD solution becomes transparent in SWIR. In FigureS37c, bruising on a plum is revealed in both NIR and SWIR; plums and pears show similar contrast in NIR, but differ clearly in SWIR due to skin reflectance differences.\nCollectively, these results demonstrate that the implementation of the NIR/SWIR dual-band FPA holds great promise for applications in material identification and food safety monitoring.\nIn the main text on page 16 and page 17, we updated Figure 4d and 4f and added:\n“As shown in Figure 4d, different colored inks exhibit distinct absorption behaviors across the NIR and SWIR bands. Red ink remains undetectable in both NIR and SWIR images, whereas green ink is clearly visible under NIR illumination but becomes faint in the SWIR. In contrast, black ink is discernible in both spectral bands. These marked differences in spectral response enable the design of effective anti-counterfeiting markers. Figure 4e further illustrates the differential absorption properties of water, acetone, polyethylene (PE), and ink in the NIR and SWIR regions. When contained in white PE bottles, water and acetone are not observable under visible light. The NIR image reveals the meniscus of water but not that of acetone, whereas both menisci are clearly observed in the SWIR image, with water appearing darker due to stronger absorption. The ink markings on the bottle surface are prominent in the NIR image but fade in the SWIR image. Collectively, these results confirm that the distinct NIR and SWIR absorption can be effectively utilized for spectral identification.”\nMeanwhile, Figure 4 is updated in the main text on page 28:\n65\n“Figure 4. NIR/SWIR dual-band imaging. (a) Optical image and magnified view of the dual-band PbS CQDs FPA. (b) Schematic diagram of the experimental setup for dual-band imaging. NIR and SWIR images are presented for the following: (c) the emblem pattern covered with paint and shielded by a silicon wafer, (d) patterns drawn with red, green, and black ink, and (e) plastic bottles containing water and acetone.”\nAdditionally, we also updated the Figure S37 in the Supporting Information on page S28 and S29 with the following text:\n“Figure S37. Application of the dual-band FPA for spectral discrimination. (a) Sugar and salt sealed in brown glass bottles. (b) PbS CQDs solutions with distinct NIR and SWIR absorption peaks. (c) A comparison of plums and pears.\nThe NIR/SWIR dual-band FPA demonstrates strong potential for material identification and food inspection by leveraging distinct spectral signatures. In Figure S37a, while salt and sugar within brown glass bottles are indistinguishable under both visible and NIR illumination, they exhibit a clear grayscale contrast in the SWIR band, with sugar appearing darker due to its higher SWIR absorption. 66 Conversely, printed ink labels visible in NIR are absent in SWIR. Figure S37b highlights differential spectral transparency: solutions of NIR- and SWIR-absorbing CQDs appear opaque and similar in the NIR image, yet the NIR-CQD solution becomes transparent in SWIR, enabling clear differentiation. Figure S37c further reveals subsurface bruising on a plum, which is invisible in visible light but clearly detected in both NIR and SWIR. Moreover, while plums and pears show similar intensity in NIR, the pear appears significantly brighter in SWIR, likely due to differing skin reflectance properties in this spectral region.”\nComment 4-9: The experimental section lacks sufficient detail on the fabrication of both the devices and the integrated FPA. More comprehensive information is needed to ensure reproducibility and to assess the scalability or practicality of the proposed approach.\nResponse 4-9: Thank the reviewer for such professional question.\nThe complete fabrication process of the PbS CQDs dual-band FPA is illustrated in Figure S30, with the detailed procedure described as follows. The process begins with cleaning the ROIC chip, followed by photolithography and electron-beam evaporation to deposit Ti/Au layers. These layers are patterned to extend the pixel-sized bottom electrodes to 12×12 μm. Two layers of solid-state ligand-exchanged PbS-EDT CQDs are then spin-coated onto the ROIC. Next, the SWIR PbS CQDs layer (PbS- II) is deposited via spin-coating of a solution containing liquid-phase ligand-exchanged PbS CQDs, followed by thermal drying. A 50 nm-thick C electron transport layer is then 60 deposited by thermal evaporation, followed by a 50 nm ZnO layer deposited via magnetron sputtering. Afterward, the NIR PbS CQDs layer (PbS-I) is spin-coated from a liquid-phase ligand-exchanged solution and thermally annealed to form a uniform film. Two additional layers of PbS-EDT are then spin-coated to serve as the hole transport layer. Finally, a 100 nm ITO transparent top electrode is deposited using sputtering. For device testing, the fabricated FPA chip is connected to a custom test board via wire bonding to access individual pixel signals.\nThe relevant content has been added in the Methods part on page 19 and page 20:\n“Device fabrication.\nThe dual-band PbS CQDs photodetectors were fabricated on Si/SiO substrates 2 pre-patterned with 5 nm Ti/10 nm Au bottom contacts. A 50 nm PbS-EDT layer was first deposited via a layer-by-layer method. The OA-capped PbS-CQDs were spin-coated at 2,000 r.p.m. for 40 s, treated with a 0.01% volume ratio of EDT in acetonitrile for 20 s and then washed by acetonitrile, and this process was repeated twice. Subsequently, the SWIR-absorbing layer (PbS-II, ~110 nm) was spin-coated at 4,000 r.p.m. for 60 s from a 300 mg·mL−1 CQDs inks. Next, a 50 nm C layer 60 was thermally evaporated, followed by the deposition of a 50 nm ZnO layer via magnetron sputtering, serving as the electron transport layers. Furthermore, the NIR-absorbing layer (PbS-I, ~140 nm) was then spin-coated at 4,000 r.p.m. for 60 s using 400 mg·mL−1 CQDs ink and capped with two additional PbS-EDT layers.\nFinally, a transparent ITO top electrode was deposited by magnetron sputtering at room temperature.\nPbS CQDs dual-band FPA fabrication.\nThe ROIC for the dual-band FPA was fabricated in a 0.18 μm CMOS process.\nAfter cleaning the chip, a Ti/Au (150/50 nm) bottom electrode was patterned by UV lithography and electron beam evaporation, expanding the pixel size to 12×12 μm. Two PbS-EDT layers were deposited via spin-coated and solid-state ligand exchange. Subsequently, SWIR-absorbing PbS CQDs (PbS-II) were spin-coated from solution using a liquid-phase ligand exchange process and annealed at 85°C for 15 minutes. Then, a 50 nm C layer was thermally evaporated, followed by a 60 50 nm ZnO layer deposited as electron transport layers by magnetron sputtering.\nNIR-absorbing PbS CQDs (PbS-I) were spin-coated, annealed under the same conditions, and covered with two additional PbS-EDT layers via solid-state ligand exchange as hole transport layers. Finally, a 100 nm ITO top electrode was sputtered to complete the device structure. The completed dual-band FPA was wire-bonded to a testing circuit board for performance evaluation.”\nReviewer #1:\nThe authors have addressed these points and made the corresponding revisions. The quality of the manuscript has been significantly improved, and I recommend accepting it in its current version.\nResponse: We thank the reviewer for the positive assessment of our work.\nThe authors have addressed all of my concerns in detail. The revised manuscript is now acceptable for publication.\nResponse: We thank the reviewer for the positive assessment of our work.\nI appreciate overall the response of the authors and the push of bicolor detection on a ROIC is probably matching the Nature comm. standard, I have few residual questions.\nComment 3-1: About spectral cross talk, lets be clear it is 0.5% in the SWIR, but only in the SWIR, in the NIR its value is massive just be clear about this claim.\nResponse 3-1: We thank the reviewer for pointing out the need to further refine precision of the statements regarding the spectral crosstalk claim, and we fully agree with the reviewer’s observations. It is important to clarify that ultra-low crosstalk of 0.5% is indeed specific to the SWIR channel, whereas the crosstalk in the NIR region is more significant. To ensure clarity and avoid any potential misunderstanding, we have carefully revised the manuscript to explicitly state that the 0.5% crosstalk value refers strictly to the SWIR band. We have also added a clearer description of the NIR cross-talk behavior in the revised text.\nIn the main text on page 2, we revised:\n“The device achieves a specific detectivity exceeding 1×1011 cm·Hz1/2·W−1 in both bands, with SWIR crosstalk of 0.5% and NIR crosstalk of 7.7%.”\nIn the main text on page 5, we revised:\n“Through band engineering, the device achieves a specific detectivity exceeding 1×1011 cm·Hz1/2·W−1 in both bands, along with SWIR spectral crosstalk of 0.5% and NIR crosstalk of 7.7%.”\nIn the main text on page 17, we revised:\n“Through systematic optimization of the operating voltage, spectral crosstalk was suppressed to 0.5% for SWIR and 7.7% for NIR.”\nComment 3-2: When reading « Vladimir et al. pioneereda vertically stacked n-p-n PbS CQDs photodiode architecture that enables bias-selectable dual-band detection across the NIR and SWIR regimes. » i do expect a ref from Vladimir et al\nResponse 3-2: We sincerely apologize for this oversight. We regret that the citation was accidentally omitted in the original manuscript. We have now inserted the correct 3 reference in the revised text.\nIn the main text on page 4, we revised:\n“Pejović et al. pioneered a vertically stacked n-p-n PbS CQDs photodiode architecture that enables bias-selectable dual-band detection across the NIR and SWIR regimes14.”\nReference:\n14. Pejović, V. et al. Photodetectors based on lead sulfide quantum dot and organic absorbers for multispectral sensing in the visible to short-wave infrared range. Adv. Funct. Mater. 32, 2201424 (2022).\nComment 3-3: Clarify how intrinsic is C layer\nIn the main text on page 5, we added:\n“The electrical performance of the C film was evaluated via field-effect transistor 60 measurements, as detailed in Figure S1.”\nMeanwhile, we added the Figure S1 and the corresponding description in the Supporting Information on page S4:\n4\n“Figure S1.Electrical characterization of the C organic field-effect transistor.(a) 60 Output characteristics and (b) transfer characteristics measured at a constant drain voltage (10 V), plotted on linear (black, left axis) and logarithmic (red, right axis) scales.\nThe field-effect electron mobility (μ ) was extracted from the linear region of e the transfer characteristics using the following equation:\n𝑳 𝝏𝑰𝒅𝒔\n𝝁𝒆 = ( ) (𝟏𝟎)\nwhere L and W are the channel leng𝑾th𝑪 (𝒊1𝑽0𝒅0𝒔 μ𝝏m𝑽) 𝒈a𝒔nd width (27 mm) of the device, respectively, C is the specific capacitance of the SiO (1.2×10-8 F/cm2), V is the i 2 ds drain-source voltage (10 V) applied during the measurement, and represents 𝝏𝑰𝒅𝒔\nthe transconductance extracted from the linear slope of the transfe𝝏r𝑽 𝒈c𝒔urve.\nThe carrier concentration (n) of the C film was estimated from the 60 conductivity (σ) utilizing the extracted mobility value using the following equation:\n𝝈 𝑰𝒅𝒔𝑳\nwhere q is the elementary cha𝒏rg=e, t is t=he thickness of the C film (75 nm). (𝟏𝟏)\n𝒒𝝁𝒆 𝒒𝝁𝒆𝑾 𝒕𝑽𝒅𝒔 60\nThe transfer characteristics explicitly exhibit typical n-type electron transport behavior. The field-effect electron mobility and carrier concentration extracted from the curves are 8.8×10-2 cm2·V-1·s-1 and 1.2×1012 cm-3, respectively. These values indicate the high mobility and low unintentional doping level of the thermally evaporated C active layer, confirming its suitability as a protective 60 electron-transporting layer.”\nResponse 3-4: We appreciate the reviewer's scrutiny regarding the spectral shift. The large shift observed in Figure 2e and Figure 1c arises because the absorption spectrum and the EQE spectrum were sourced from different batches. The absorption spectrum (Figure 1c) shown was intended to illustrate the typical excitonic features of the SWIR PbS CQDs used in this work, while the EQE spectrum represents the performance of our champion device. Since batch-to-batch variations in synthesis (Figure S4) are unavoidable, the \"best-performing\" device batch had a slightly larger average CQD diameter than the \"typical\" absorption sample plotted. To strictly address the reviewer's question regarding the physical shift from solution to film: We have performed a control experiment tracking the spectral evolution of a single CQD batch from solution to the final device as shown in Figure S14. The plot compares the normalized absorption spectrum of PbS CQDs in solution with the normalized photocurrent response of the device fabricated using the same batch of quantum dots. A bathochromic shift of approximately 33 nm of NIR CQD and 27 nm of SWIR CQD is observed, which is close to the absorption spectra shift of PbS CQDs (26 nm for NIR and 10 nm for SWIR) before and after liquid-phase ligand exchange (Figure S15). This shift is attributed to the combined effects of the change in the dielectric environment and the enhanced inter-dot electronic coupling.\nIn the main text on page 6, we revised:\n“The batch-to-batch reproducibility of the SWIR PbS CQD synthesis was 6 illustrated in Figure S4.”\nIn the main text on page 8, we revised:\n“Notably, a slight redshift (~30 nm) is observed between the solution absorption peak and the device EQE peak (Figure S14). This spectral shift aligns closely with the redshift induced by ligand exchange (Figure S15), attributed to ligand-related effects18.”\nMeanwhile, we added the Figure S14 and the corresponding description in the Supporting Information on page S11:\n“Figure S14. Spectral evolution of (a) NIR and (b) SWIR PbS CQDs from oleic acid-capped solution to device.”\nThe revised manuscript shows clear improvement, and the authors have made a substantial effort in addressing the reviewers’ comments. Still, I have a few concerns that should be resolved before publication:\nComment 4-1: The manuscript conflates optical stability, electrical stability, and shelf-life stability. These are distinct concepts and should be treated separately.\n• Optical stability must be evaluated under continuous illumination, not by measuring absorption after storage under a particular environment.\n• Electrical stability requires extended operation under applied bias, not short experiments involving chopped illumination.\nThe current ~2 h measurement with intermittent light exposure is not representative of realistic detector operation. Moreover, the manuscript does not provide experiments that identify the origin of instability, and several interpretations appear speculative or unsupported by systematic data. The suggestion that encapsulation would resolve the instability is an unverified extrapolation and should be avoided unless backed by dedicated experiments.\nI recommend revising the entire stability discussion to remove speculation, avoid misinterpretation of limited data, and clearly state that stability assessment and mitigation require further comprehensive studies.\nResponse 4-1: We thank the reviewer for this constructive suggestion. We agree that distinct types of stability were conflated in the original manuscript, and the initial stability assessment was limited. Addressing the reviewer's concern that \"Electrical stability requires extended operation under applied bias,\" we have performed a new, extended stability test under continuous bias and illumination for 10,000 seconds.\nAs shown in the Figure S4, the device was subjected to a constant bias of +1 V (NIR mode) and −1 V (SWIR mode) under continuous illumination. The device exhibited remarkable stability with negligible photocurrent degradation over the testing period. 8 This confirms the operational stability of the detector under realistic continuous working conditions, replacing the previous chopped-light data. Additionally, we have removed the unverified claim that encapsulation would definitively resolve the long-term instability and claimed that stability require further studies.\nIn the main text on page 13, we added:\n“As presented in Figure S29, the device exhibited negligible degradation in photocurrent over a 10,000 s period under both +1 V (NIR mode) and -1 V (SWIR mode) biases, confirming robust operation under realistic working conditions.”\nIn the main text on page 14, we revised:\n“These results affirm the intrinsic stability of the material system when isolated from the ambient atmosphere. However, achieving practical long-term reliability requires further comprehensive studies, particularly regarding the development and assessment of effective encapsulation strategies.”\nMeanwhile, we added the Figure S29 and the corresponding description in the Supporting Information on page S19:\n“Figure S29. Operational stability test under continuous bias. Current-time characteristics of the device measured under continuous operation for 10,000 s.\nThe device was biased at +1 V (red line) and −1 V (blue line) under continuous illumination, and at corresponding voltages in the dark (grey and black lines).”\nComment 4-2: Figure 2 remains unclear. From panels 2b and 2c, the EQE of the 9xx nm band under positive bias appears higher than the 16xx nm band under negative bias, yet this trend is not reflected in panels 2d and 2e. To clarify the device behavior, the authors should report EQE spectra across the full wavelength range (e.g., 500–2000 nm) under both positive and negative biases (for example, ±0.5 V and ±1 V). Additionally, providing EQE measurements under broadband white-light illumination would help contextualize the device performance for more realistic operating conditions.\nResponse 4-2: We thank the reviewer for this detailed observation. The apparent discrepancy arises from the difference in incident light intensity between the measurements. As shown in the linearity analysis in Figures S11 and S12, the photoresponse of the device is intensity-dependent. At low light intensities (< 0.01 W·cm−2), the responsivity at 1650 nm is essentially higher than that at 940 nm. Since photodetectors are primarily designed for weak signal detection, the EQE measurements (Panels 2d and 2e), which are performed under low-intensity monochromatic light, reflect the intrinsic, unsaturated performance of the device. At high light intensities (> 0.01 W·cm−2), The photocurrent in the 1650 nm band tends to saturate earlier than the 940 nm band. The data in Panels 2b/2c were acquired at higher intensities where this saturation occurs, making the 940 nm response appear relatively stronger.\nFigure S11. LDRs of the dual-band photodetector measured under a bias of (a) +1 V with 940 nm illumination and (b) −1 V with 1650 nm illumination.\nIt is also important to note the relationship EQE = R×1240/λ (where R is responsivity). Even though the responsivity at 1650 nm is higher than at 940 nm in the linear regime, the larger denominator at 1650 nm reduces the calculated EQE value. Consequently, the peak EQE values for both bands appear comparable in Panels 2d/2e, despite the difference in responsivity. Following the reviewer’s suggestion, we have measured and included the full-wavelength EQE spectra (500–2000 nm) under both positive and negative biases. These results are now presented in Figure S13. They clearly show the bias-selectable spectral response across the entire range.\nRegarding the suggestion for white-light EQE measurements, we respectfully note that standard EQE measurements utilizing monochromatic light are the established method for quantifying spectral cross-talk and specific detectivity. Configuring our setup for calibrated spectral extraction under broadband white-light illumination would require significant modification to the optical path. We believe the current monochromatic EQE data, combined with the crosstalk analysis provided, sufficiently characterizes the device’s spectral selectivity for realistic operating conditions.\nIn the main text on page 8, we revised:\n“To quantitatively assess the photodetection performance of the dual-band photodetector, we systematically measured both responsivity and external quantum efficiency (EQE) across the wavelength ranges of 500–1000 nm and 1100–2000 nm, as shown in Figures 2d, 2e and S13.”\n“Figure S13. EQE spectra of the dual-band photodetector at ±1 V bias.”\nComment 4-3: The figure captions remain incomplete and do not provide sufficient context for readers. Please revise all captions to include essential experimental details, such as applied voltage, device architecture under test, illumination intensity, and measurement conditions. These elements are necessary for reproducibility and proper interpretation.\nResponse 4-3: We thank the reviewer for this constructive suggestion. We have meticulously revised figure captions in both the main manuscript and the Supplementary Information.\nIn the main text on page 27, we revised:\n“Figure 2. Performance of the NIR/SWIR dual-band PbS CQDs photodetector. (a) Dark current characteristics of the single-junction and dual-band photodetector (PbS-Ⅰ: Au/ZnO/PbS-Ⅰ/EDT/ITO, PbS-ⅠⅠ: Au/EDT/PbS-ⅠⅠ/C /ZnO/ITO, Dual- 60 band: Au/EDT/PbS-ⅠⅠ/C /ZnO/PbS-Ⅰ/EDT/ITO). J-V characteristics under (b) 60 940 nm and (c) 1650 nm illumination at varied power densities. EQE and responsivity spectra of the device in the (d) NIR regime at +1 V and (e) SWIR regime at −1 V. (f) Measured current noise of the dual-band photodetector at ±1 V bias.”\n“Figure 3. Bias-switchable NIR/SWIR sensing with the dual-band PbS CQDs photodetector. (a) Responsivity matrix of the device across various biases and wavelengths. (b) Spectral crosstalk suppression mechanism of the dual-band photodetector. (c) Spectral response of the dual-band detector under bias voltages of +1 V, −1 V, and −0.32 V. (The spectral crosstalk between the two bands of a dual-band detector is quantified by integrating the relative spectral response curves of each detector within specific wavelength ranges.) Photocurrent response to chopped (d) NIR (920 nm monochromatic light ~1 μW) and (e) SWIR light signals (800°C blackbody source filtered through a long-pass filter with a 1 µm cutoff wavelength) under bias voltages of +1 V, −1 V, and −0.32 V. (f) Detectivity and spectral crosstalk performance of the dual-band PbS CQDs photodetector compared to various other reported dual-band photodetectors.”\nIn the main text on page 29, we revised:\n“Figure 4. NIR/SWIR dual-band imaging. (a) Optical image and magnified view of the 128×128 dual-band PbS CQDs FPA. (b) Schematic diagram of the experimental setup for dual-band imaging. NIR and SWIR images are presented for the following: (c) the emblem pattern covered with paint and shielded by a silicon wafer, (d) patterns drawn with red, green, and black ink, and (e) plastic bottles containing water and acetone.”\nIn the supporting information on page S11, we revised:\n“Figure S12. Responsivity of the dual-band photodetector under a bias of +1 V with 940 nm illumination and −1 V with 1650 nm illumination.”\n13"}, {"round": 2, "reviewer_comments": {"reviewer_3": "Authors have genuinely adress my (sometime picky) comments, thus i see no reason to not let this paper getting published now", "reviewer_4": "The reviewers have satisfactorily addressed all of my concerns, and I recommend the manuscript for publication without further comments."}, "author_response": ""}]

Decision letter