**Figure 23. continued**

above the CM threshold of $2E_g$. The dashed gray line represents the ideal SE-CM scenario, where the threshold is defined by $E_{Mn}$. Adapted with permission from ref 714. Copyright © 2025 Noh et al.

II transitions (Figure 22F). Though precisely controlled coupling between the two transitions, these QDs support “hybrid” direct/indirect biexcitons with slow, charged-exciton-like Auger dynamics. As a result, type-(I+II) QDs exhibit long optical gain lifetimes (up to ~3 ns), enabling lasing even in low-concentration solution samples.

The demonstrated devices exhibited dye-laser-like characteristics but with several notable advantages. In particular, they featured an extended optical gain bandwidth, enabling wide-range spectral tunability of the laser line from a single QD sample (Figure 22G). Additionally, they demonstrated excellent operational stability even without the need for circulating the QD solution, whereas dye lasers rely on continuous circulation for stable operation. Elimination of circulation is an attractive feature of liquid-state QD lasers as it significantly simplifies their design, reduces costs, and enhances their suitability for miniaturization and integration with other devices.

**Manipulating Hot Carriers via Ultrafast Spin-Exchange (SE) Interactions.** Hot, nonequilibrated electrons offer immense potential for advanced photoconversion and photochemistry. Beyond their enhanced reduction capabilities, these electrons exhibit expanded electronic wave functions, greater mobility, and the potential for extended transport ranges when stabilized in excited states. However, their practical utilization is limited by significant energy losses due to phonon emission, resulting in rapid carrier cooling.

Recent studies have demonstrated that hot electrons can be effectively harnessed through ultrafast SE interactions in manganese (Mn)-doped QDs.⁷¹¹⁻⁷¹³ These interactions enable exceptionally rapid energy transfer ($>10$ eV ps⁻¹),⁷¹¹ potentially allowing for the capture, stabilization, and utilization of hot electrons before they dissipate energy via phonon emission. This strategy presents a promising avenue for advancing photoconversion and photochemical applications.

In ref 712, the SE energy transfer rate was directly quantified through comparative studies of Auger recombination in the Mn-doped CdSe QDs and reference undoped samples (both samples contained a thin protective CdS shell).⁷¹² In the undoped sample, biexciton (XX) Auger decay proceeds via the recombination of an electron-hole pair, with the released energy transferred to a third carrier (electron or hole). As shown in Figure 23A (left), the biexciton Auger recombination is characterized by a time constant of $\tau_{XX} = 30$ ps for a CdSe core radius of ~2 nm (Figure 23B, black), consistent with the universal “volume scaling” of Auger lifetimes.⁶⁹⁹ In this process, the acceptor carrier gains energy equivalent to the QD bandgap ($E_g$), allowing the energy-transfer (or energy-gain) rate to be estimated as $r_{A,gain} = E_g/\tau_{XX}$, yielding approximately 0.06 eV ps⁻¹.

The Mn-doped sample also exhibited a fast signal at comparable excitation levels; however, its time constant was almost two-orders of magnitude shorter ($\tau_{XX,Mn} = 360$ fs; Figure 23B, orange). This striking reduction suggests a dramatic enhancement in the rate of Auger interactions, attributed to the involvement of a magnetic impurity. In this case, Auger recombination involved a hybrid biexciton,

consisting of the intrinsic QD exciton and an excited Mn ion in its spin-3/2, $^4$T₁ configuration. This process proceeds through two corelated SE steps (Figure 23A, right): (1) spin-down electron transfer from the 3d Mn shell to the excited (hot) electron state in the QD, accompanied by (2) spin-up electron transfer from the QD conduction band to the Mn ion. As a result, a hot exciton is generated in the QD, while the Mn ion relaxes to its ground-state spin-5/2, $^6$A₁ configuration.

Based on these measurements, the SE-assisted energy transfer rate can be estimated as $r_{SE-A,gain} = E_{Mn}/\tau_{XX,Mn}$, where $E_{Mn} = 2.1$ eV represents the energy of the excited Mn ion. This yields a transfer rate of 8.4 eV ps⁻¹. Not only is this value approximately 100 times higher than that of the conventional Auger process, but it also surpasses the energy-loss rate due to photon emission ($r_{loss}$, generally less than ~1 eV ps⁻¹)⁷¹⁵ by at least an order of magnitude.

These results provide direct experimental evidence that SE interactions enable efficient manipulation of hot carriers before they lose their kinetic energy to phonons.

The above assessment was recently confirmed by the demonstration of highly efficient SE-assisted photoemission driven by visible light pulses. This process was realized through a two-step SE-Auger re-excitation mechanism, in which a band-edge electron was excited to the vacuum state outside the QD via successive energy transfers from two excited Mn ions (Figure 23C).⁷¹² In undoped QDs, the second step of Auger re-excitation would typically be hindered by hot-electron relaxation through phonon emission. However, in Mn-doped structures, the exceptionally high SE energy transfer rate allows the hot electron to be efficiently excited to the vacuum state before undergoing phonon-assisted intraband cooling.

Yet another effect enabled by SE-Auger interactions is the high-yield production of solvated electrons.⁷¹² This process was realized with both UV and visible photons, with photon energies $E_{phot} = 2.4$ eV and 3.6 eV, respectively. In the UV-excitation case, generation of solvated electrons occurred via single-step Auger ionization, where a hot electron was ejected from the QD directly into the surrounding medium (water). In the visible-excitation case, the process proceeded through two-step SE-Auger ionization (Figure 23C). The maximum internal quantum efficiency ($\eta_{sol-e}$) reached 11% and 3.5% for $E_{phot} = 3.6$ eV and 2.4 eV, respectively (Figure 23D). These values compare favorably with other electron emitters, despite being achieved with significantly lower photon energies. For instance, studies of hydrogen-terminated diamond surfaces reported a $\eta_{sol-e}$ of ~0.6% for deep-UV photons with $E_{phot} = 5.86$ eV.⁷¹⁶

Ultrafast SE interactions can also facilitate the conversion of hot-carrier kinetic energy into additional electron-hole pairs, enabling high-efficiency carrier multiplication (CM). To achieve SE-driven CM (SE-CM), Jin et al.⁷¹³ designed Mn-doped PbSe/CdSe core/shell QDs with a bandgap smaller than half of $E_{Mn}$. In these structures, CM proceeded through two SE-mediated steps (Figure 23E): (1) SE energy transfer from a hot exciton delocalized throughout the QD to a Mn ion at the core/shell interface, followed by (2) energy- and spin-conserving relaxation of the excited Mn ion, generating two excitons (one bright and one dark) in the PbSe core. Due to the extremely short SE time scales, both SE steps occurred
<figure><img src="image_68.png" /><figcaption>**Figure 24.** (A) Strain engineering of Wurtzite Cd<sub>x</sub>Zn<sub>1-x</sub>Se. (B) The highly strained thick-shell particles exhibit substantially narrower single-dot line width compared to state-of-the-art CdSe. Panels (A) and (B) reproduced with permission from ref 730. Copyright © 2019 Park et al. under exclusive license to Springer Nature Limited. (C) Antibunching signature and emission intensity trace of a single InP/ZnSe QDs. The emission is remarkably stable even in near-saturation conditions. Reproduced from ref 254. Copyright © 2017 American Chemical Society. (D) Proposed dual-cavity architecture for the coupling of colloidal QDs to SiN waveguides. The rational cavity design may yield on-chip single-photon sources with engineered performance. Reproduced from ref 748. Copyright © 2019 American Chemical Society. (E) The time-resolved PL of a single CsPbBr<sub>3</sub> QD at 4K reveals purely radiative lifetimes of around 200 ps. Commensurate optical coherence times allow the amplitude interference of two indistinguishable single photons emitted from the same QD, manifested as a dip in the coincidence center peak recorded after a beam splitter (Hong-Ou-Mandel dip). Reproduced with permission from ref 736. Copyright © 2023 Kaplan et al. under exclusive license to Springer Nature Limited.</figcaption></figure>

without significant interference from phonon emission, resulting in a high SE-CM efficiency. Notably, at $E_{phot} = 2.4$ eV—just 0.3 eV above the nominal SE-CM threshold defined by $E_{Mn} = 2.1$ eV—the multiexciton yield (the probability of generating a biexciton or higher-order multiexciton per absorbed photon) reached ~50%, marking a more than 2-fold enhancement compared to undoped PbSe/CdSe QD reference samples.

In a more recent advancement, SE-CM was employed to enhance the photocurrent of a real-world photoconductive device.⁷¹⁴ However, the Mn-doped PbSe/CdSe QDs studied

in ref 713 were not suitable for this demonstration, as the charge carriers generated through SE-CM remained confined within the PbSe core, effectively isolated from the external circuit by the wide-bandgap CdSe shell.

To address this limitation, Noh et al.⁷¹⁴ developed Mn-doped inverted CdSe/HgSe QDs, where the lower-bandgap material (HgSe) was positioned in the shell region (Figure 23F, left inset). This design allowed both electrons and holes to be efficiently extracted into the external circuit. The QDs were characterized through TA spectroscopy in solution and photocurrent measurements in solid-state films (Figure 23F,
top right insets). The results from both methods showed excellent agreement (Figure 23F, compare data shown by circles and triangles), clearly demonstrating the impact of SE-CM, as evidenced by a sharp increase in both the TA signal and the photocurrent just above the energy of the Mn spin-flip transition.

Although only recently discovered, ultrafast SE-type energy transfer has already demonstrated significant potential for advanced photoconversion applications. These include the generation of hot, free and solvated electrons by low-energy photons via SE-assisted photoemission, as well as enhanced carrier production through SE-CM. Such effects hold great promise for electro-optical devices and photochemistry. In the latter, the enhanced reductive power of hot or solvated electrons can be leveraged to drive challenging chemical reactions requiring a high reduction potential. Simultaneously, the pairwise generation of electron-hole pairs via SE-CM can improve the efficiency of multistep, multielectron/hole chemical reactions by alleviating bottlenecks associated with the waiting time between successive reduction/oxidation steps.

**Quantum Light Sources.** An emerging part of nano-science with NCs is quantum light generation. Conversely to classical sources, quantum light sources prepare states with defined correlations between photons, with single-photon states being the canonical example. Photonic quantum states exist as Eigen or superposition states within the vector space spanned by the photon's spatiotemporal mode, frequency, and polarization. Entangled photons are then superposition states of a group of photons, where the state of each photon depends on the state of other photons, even if separated by large distances. The preparation of such quantum states is an outstanding problem, particularly when using device-compatible solid-state systems. Material systems are needed that allow the faithful, i.e., coherent transcription between electronic excitations and photons and with a defined number of quanta.⁷¹⁷ Single photons that are indistinguishable in all degrees of freedom can be interfered with to build up higher-order quantum states and are, therefore, versatile building blocks and a desirable output for quantum light sources. Colloidal QDs have long been proposed for the emission of single photons owing to their size-confinement restricting the number of electronic excitations,⁷¹⁸ effectively acting as a two-level system (TLS). QDs have indeed served as model systems in studies of single-emitter photophysics even before quantum light sources moved into the spotlight of engineering efforts. Key milestones included the demonstration of single-photon generation,⁷¹⁹ Purcell enhancement of the emission,⁷²⁰ and studies of discrete spectral jitter,⁷²¹ and decoherence.⁷²² Years of continuous improvements in materials and measurement techniques, largely motivated by fundamental science questions, have recently led to renewed interest in QDs as single-photon emitters (SPEs), partially spurred by accelerating interest in practical quantum technologies. Several review articles have specifically addressed the application of QDs in quantum science more broadly, including applications in coupled excitonic arrays.⁷²³,⁷²⁴ Here, we discuss the most recent advances made in QDs as emitters of single photons in the context of specific performance metrics of future single-photon sources.

**Single-Photon Purity.** The single-photon purity of a quantum emitter describes the ratio between one-photon to multiphoton emission events, typically measured under pulsed laser excitation with a second-order intensity correlation

measurement $g^{(2)}(\tau = 0)$. An ideal SPE does not permit simultaneous emission of multiple photons. As QDs can host multiple excitations, single-photon emission requires targeted optimization, either via spectral rejection of multiexcitons or increasing their nonradiative Auger rate. The former strategy relies on relatively large biexciton binding energies and high spectral stability with narrow emission lines, but is in principle straightforward to implement in analogy to epitaxially grown quantum wells.⁷²⁵ The latter strategy benefits from long radiative lifetimes of the emitter, which is detrimental for many applications. The Auger rate in II-VI QDs is increased with quantum confinement, which typically degrades the stability of the QD and induces spectral jitter at low temperatures.

Recent work on InP and lead-halide perovskite QDs has helped overcome these limitations. InP/ZnSe QDs have shown improved single-photon purity $g^{(2)}(\tau = 0) = 0.03$ (average 0.19) compared to CdSe/CdS QDs with simultaneously improved spectral stability and reduced blinking, even at high excitation fluence close to saturation (Figure 24C).²⁵⁴ This remarkable behavior of near absence of blinking in the presence of fast (70 ps) Auger recombination—causing charge-induced intermittency in II-VI QDs—justifies further investigation. For applications not requiring a high degree of optical coherence at low temperatures, InP-based materials might provide the needed stability.

With the advent of perovskite QDs, several studies specifically addressed the single-photon purity. Initial experiments on CsPbX₃ X = Br, I, showed room-temperature purities of ~6%, but also revealed problems of particle instabilities and blinking.³³¹ Quantum confinement has improved the best values to around 2%³³¹ with a distinct increase in purity with decreasing size. Complementary work has further shown that single particles in solution can have average single-photon purities of 2% in the strong confinement regime, even though the stability of these particles did not permit extended single-particle spectroscopy.⁷²⁶ Single-photon emission is further retained for different stoichiometries of A-site cations from FA to cesium in APbBr₃ QDs⁷²⁷ and with changing halide composition,⁷²⁶ furthering the prospect of color-multiplexed single-photon emitting devices using such QDs. Remarkably high count rates up to 9 × 10⁶ with purities around 2% at room temperature have enabled quantum random number generation conforming to National Institute of Standards and Technology standards.⁷²⁸ The critical challenge in increasing the single-photon purity of these QDs at room temperature via confinement will be to increase the long-term stability.

**Optical Coherence, Coherent Fraction, and Spectral Stability.** An ideal SPE behaves akin to a quasi TLS with a single Lorentzian line, the width of which is only determined by the spontaneous emission lifetimes without further decoherence. Solid-state SPEs are limited in the fraction of such coherently emitted photons by phonon side-bands. The coherence time is further reduced owing to spin- and phonon-bath interactions, and charge- and dielectric fluctuations further induce spectral jitter i.e., spectral diffusion. To render colloidal QDs suitable as coherent SPEs, the processes need to be minimized, which has also been summarized in terms of design rules for minimizing the optical emission line width.⁷²⁹

Decades of work in II-VI QDs have provided the microscopic origin to these physics. We have reached a good understanding of how lattice strain and spin-orbit terms induce fine-structure splitting, how resonant phonon-mediated exchange between fine-structure states leads to pure dephasing
of the optical transition, and how exciton-phonon coupling defines the broad room-temperature line shapes and causes phonon side-bands at low temperatures.

Synthetic strategies to improve the room temperature color-pure emitters in LEDs can similarly be translated into color-multiplexed single-photon sources. CdSe/ZnSe QDs have reached a remarkably narrow line width of around 20 meV, achieved via strain engineering (Figure 24A,B).<sup>730,731</sup> For perovskite QDs, low energy surface phonon modes have been removed from the spectral density to achieve down to 35 meV line width.<sup>329</sup> The line width, or inversely the coherence time, is more critical at low temperatures, where the generation of indistinguishable single-photons becomes feasible. For II-VI QDs, the degree of optical coherence $T_2/2T_1$ is insufficient regardless of the applied technique, and pronounced charging-induced blinking and spectral diffusion of II-VI QDs are at odds with the requirements of indistinguishable single-photon generation. InP/ZnSe/ZnS QDs have been demonstrated as remarkable alternatives with 250 ps coherence times at low temperatures, substantially reduced spectral jitter, and high photostability.<sup>732</sup> In this study, the observed single-photon purity was 0.07, which may be improved with spectral filtering in practical applications. As an impediment to applications in quantum optics, the lowest-lying exciton fine-structure state in InP is nominally dark, leading to delayed emission with lifetimes on the order of microseconds, which can reduce the single-photon purity under high repetition rate excitation. Even the bright-state lifetime of 15 ns is still orders of magnitude longer than the coherence time, which will require substantial radiative enhancement with photonic structures in future sources of indistinguishable single photons. Nevertheless, the high intensity and spectral stability of InP QDs render them a promising platform for further optimization.

The challenge of incoherent emission of QDs is dismissed in perovskite QDs, which show a remarkable combination of unusually fast (200 ps) radiative lifetimes,<sup>332,333</sup> long 80–200 ps optical coherence times, and minimal phonon side-bands.<sup>733–735</sup> Reduced spectral jitter, likely owing to the relative absence of surface trap states, makes spectral filtering of multiexciton emission straightforward to achieve high single-photon purity $g^{(2)}(\tau = 0) < 0.05$. This remarkable combination of properties led to the recent observation of Hong-Ou-Mandel interference in perovskite QDs,<sup>736</sup> a hallmark demonstration of quantum optical phenomena in SPEs previously reserved to epitaxial QDs and defects in diamond. Optical coherences of $T_2/2T_1$ of up to 0.55, absent any cavity integration (Figure 24E). The further optimization of perovskite QDs as quantum emitters will rely on the in-depth understanding of the relationships between the morphology of nanosized perovskites and the electronic structure and optical response. Control over the fine-structure splitting may reduce optical dephasing and allow the generation of entangled photon pairs via the biexciton–exciton cascade, but is complicated by recent findings pointing to an avoid crossing of fine-structure splitting intrinsic to the polaronic lattice distortion, thus complicating the control of the splitting with synthetic targeting of the shape and lattice anisotropy.<sup>737</sup>

Harnessing charged perovskite QDs with faster lifetimes may be a promising approach to increasing the optical coherence, although initial work showed that the trion is more strongly coupled to optical lattice modes, which may reduce the optical coherence.<sup>738</sup> Perovskite NPLs might have higher optical coherence owing to their faster radiative decay times.<sup>739</sup>

However, very few studies have addressed the properties of individual platelets, and the fine-structure splitting is generally found to be larger than in cubes.<sup>740</sup> The criticality of surface phonons has also been confirmed at low temperatures, where rational ligand design improved the coherence time and stability. Remarkably, the same work identified inhomogeneous broadening at low temperatures that approaches typical levels of epitaxial QDs.<sup>741</sup> The application of electric fields is a particularly promising direction for tuning the electronic structure and spectral stability in perovskite QDs. As such, static electric fields have been shown to remove the fine-structure splitting in CsPbI<sub>3</sub> QDs and mitigate spectral diffusion.<sup>742,743</sup> The relative simplicity of static E-field imposition compared to magnetic fields paired with the ever-improving synthetic procedures foretell a bright future for perovskite QDs as a source of indistinguishable single-photons, especially as efforts for cavity-and device integration are just beginning.

**Device Integration.** Besides the materials science aspects of the material generating single-photons, the integration of QDs into devices is equally important for applications.<sup>744,745</sup> Two broad challenges are to be addressed: the coupling to nanophotonic architectures and the electrical addressability. The former serves to increase the emission rate to achieve higher bit-rates in single-photon devices<sup>746,747</sup> and the indistinguishability<sup>748</sup> of single photons (Figure 24D). This integration is discussed in the context of ‘hybrid-integration’ of disparately grown emitters with on-chip photonic circuits.<sup>749</sup> Several avenues have addressed the main challenge of deterministic placement of QDs in on-chip devices, including template-assisted self-assembly,<sup>750</sup> contact printing,<sup>751</sup> and DNA-origami strategies.<sup>752</sup>

The electrical addressability of single QDs will benefit from the knowledge base developed for classical QD LEDs, but it is not without challenges. Nonbalanced charge injection can cause instabilities in the emission.<sup>753</sup> On the other hand, the single-photon purity of electrically excited single QDs is consistently higher than under optical excitation, owing to a reduction in the spurious background emission under optical excitation. On the other hand, the sequential carrier injection can slow the formation of multiple excited states, in principle improving the single-photon purity, as long as simultaneous device instabilities can be avoided.<sup>754</sup>

**Collective QD Multiphoton Emission.** Artificial atoms, like colloidal or epitaxially grown QDs, are extensively explored for the generation of multiphoton emission. Epitaxially grown QDs are indeed capable of generating entangled photon pairs via the biexciton (XX) to exciton (X) radiative cascade process.<sup>755,756</sup> Alternatively, by properly adjusting the X and XX energy levels, twin photons could be generated despite the very low yield of particles satisfying the rigid constraint of having degenerate X and XX excitonic states.<sup>757</sup> Colloidal QDs have been notoriously affected by the fast nonradiative Auger process which renders multiexciton states poorly emissive. Therefore, the schemes explored by employing epitaxially grown QDs remained elusive for the generation of more complex quantum light fields.

The advent of perovskite QDs operating in the weak confinement regime, with strongly reduced Auger recombination rates,<sup>332</sup> could bring colloidal QDs on par with what has been so far achieved employing epitaxially grown QDs. In fact, very bright emission from XX exciton states has been reported by several groups,<sup>758–760</sup> but the current limited control over
<figure><img src="image_69.png" /><figcaption>Figure 25. Coupled QD emission. (A) Schematic of the build-up process of SF: an initially uncorrelated ensemble of TLS (randomly oriented green arrows) is excited by a light pulse (blue arrow, top left). After time $τ_D$, their phases are synchronized (aligned green arrows) such that they cooperatively emit a SF light pulse (red arrow at right) with a characteristic decay time $τ_{SF}$. Reproduced with permission from ref 511. Copyright © 2018 Springer Nature Limited. (B) Time-resolved decay traces for the two emitting bands, showing a strongly accelerated decay for the SF band. The presence of oscillations in the time domain are a very peculiar feature of superfluorescent emission of multiphotons burst. Inset: an example of superbunching with $g^{(2)}(0) > 2$ from a single SL. Reproduced with permission from ref 773. Copyright © 2020 The Materials Research Society. (C) Echo-like SF behavior under a controllable disturbance, highlighting the collapse and the revival of the collective state after hot dipoles injection. Reproduced with permission under CC BY 4.0 from ref 767. (Copyright © 2023 Wang et al. Open access). (D) Comparison of the robustness of the superradiant enhancement factor to static disorder in SLs of different NC aspect ratios. Reproduced with permission under CC BY 4.0 from ref 770. (Copyright ©2023 Ghonge et al. Open access). (E) Scheme for fabrication of coupled CdSe/CdS colloidal QD molecule and a exemplary TEM image of a QD dimer. Reproduced with permission under CC BY 4.0 from ref 771. (Copyright ©2019 Cui et al.open access).</figcaption></figure>

the fine structure sublevel exciton states,³³⁴,⁷⁴²,⁷⁶¹ renders the generation of entangled photon pairs not feasible. Alternative strategies in addition to electrical and strain engineering could be devised to reach this goal.

Beyond single QDs, coupled QD systems are an elegant and alternative avenue for the generation of multipartite, N-photon states. Very recently, a solid-state source of photon triplets has been demonstrated employing an epitaxial QD molecule.⁷⁶² The main challenge with epitaxially grown QDs is the limitation in scaling up the number of coherently coupled QDs, which remains limited to only a few emitters.⁷⁶³

As outlined above, perovskite QDs are excellent sources of coherent single photons,⁷³³,⁷⁶⁴ with record fast radiative lifetime.³³²,³³⁴ In addition, such emitters can be assembled in very well-defined 3D SLs,³³⁹ by a self-drying mediated process. Contrary to the spontaneous emission of photons, coupling among several excited QDs, mediated by the common vacuum modes, could become strong enough to enable the formation of a giant dipole and the radiation of a burst of photons.

In 1954, Dicke predicted that an ensemble of N identical TLS, here exemplified by photoexcited excitons in QD SLs, confined in a volume smaller than about $λ^3$ (where $λ$ is the corresponding emission wavelength of the TLS) can exhibit coherent and cooperative emission. If the excited TLS are initially fully uncorrelated, the coherence can be established through spontaneously triggered correlations due to quantum fluctuations (Figure 25A). When this occurs, a so-called SF pulse is emitted (Figure 25B). Coherent SF bursts of photons

are characterized by an accelerated radiative decay time $τ_{SF} ∝ τ_{SE}/N$, where the exponential decay time $τ_{SE}$ of spontaneous emission from the uncoupled TLS is shortened by the number of coupled emitters $N$. In addition, SF exhibits the following fundamental signatures, the magnitudes of which are also dependent on the excitation density (or number of coupled QDs): (i) a delay or build-up time $τ_{build-up} ∝ \log(N)/N$ during which the emitters couple and phase-synchronize to each other, and which corresponds to the time delay between the excitation and onset of the cooperative emission (Figure 25A); and (ii) coherent Rabi-type oscillations in the time domain due to the strong light-matter interaction, known as Burnham-Chiao ringing (Figure 25B).

Time-resolved PL decay measurements reported in Figure 25B revealed an accelerated PL decay of the SF emission peak compared to the PL decay of uncoupled QDs, as predicted by Dicke for superradiant emission. Considering the change in radiative rates, it was possible to estimate that more than 20 QDs contribute collectively to the emission process.⁵¹¹ This is an order of magnitude more than what has been achieved with any other solid-state quantum emitters.⁷⁶³ Interestingly, the coherent coupling also affects the second-order coherence of the emitted light, as evinced by the photon statistics of the arrival time on a detector. It is well known that coherent light has a random distribution (Poissonian) of the photon arrival times while a TLS shows photon antibunching (sub-Poissonian distribution). In contrast, the cooperative emission from the coupled QDs leads to coherent multiphoton emission bursts
which manifest itself in a bunching peak in the second-order correlation function (Figure 25B).

These peculiar features of SF were first observed in perovskite monocomponent QD SLs,⁵¹¹ attesting the capabilities of time-correlated multiphotons via the SF process. Subsequent studies have then explored the rich physics behind this rather unconventional process. By exploring binary QD SLs,⁵¹⁵ it has been observed that the onset of SF is crucially dependent on the volume fraction of the excited QDs. ABO₃-like SLs exhibit the lowest threshold while NaCl-like SLs were unable to sustain collective emission despite very good structural homogeneity. In fact, the remaining energetic disorder among the QDs and the finite dephasing time, require a sizable number of coupled QDs, and consequently a strong coupling strength, for SF to occur. This explains why SLs with low NC fraction were unable to sustain collective emission.

Similarly, strong exciton-phonon interactions, inducing a net acceleration of the exciton dephasing time, render the collective coupling less efficient. In fact, a much larger $\tau_{\text{build-up}}$ and a higher threshold are found at higher temperatures, with typical SF features persisting up to 105 K,⁵¹⁵ but eventually vanishing entirely at even higher temperatures. Employing bulk-like perovskite compounds, the SF regime has been pushed to room temperature,⁷⁶⁵,⁷⁶⁶ despite a clear consensus on the underlying photophysics (e.g. whatever or not the polaron formation is responsible for an elongated coherence) has not emerged yet.

The delicate coherent coupling responsible for collective emission, has been very recently further exploited by a two-pump, time-resolved PL spectroscopy.⁷⁶⁷ The collective response of macroscopic quantum states under perturbation is widely used to study quantum correlations and cooperative properties, such as defect-induced quantum vortices in Bose-Einstein condensates and the nondestructive scattering of impurities in superfluids. Similarly, the SF effect, enabled by dipole-dipole coupling through virtual photon exchange, leads to the macroscopic, giant dipole moment which can be perturbed by the injection of uncorrelated dipoles. As shown in Figure 25C, echo-like behavior is observed in a cooperative exciton ensemble under a controllable perturbation, corresponding to an initial collapse followed by a revival of the SF collective emission. Such a dynamic response could refer to a phase transition between the macroscopic coherence regime and the incoherent classical state on a time scale of 10 ps. The echo-like behavior is absent above 100 K due to the instability of the photogenerated giant dipole, as a result of phonon-induced exciton dephasing. Experimentally, the SF response to perturbations is shown to be controlled by the amplitude and injection time of the perturbations. Such a phase transition, and the occurrence of collective emission has been recently postulated to serve as a platform to simulate and investigate the physics of correlated quantum materials.⁷⁶⁸ The ultrafast optical injection of quantum confined excitons plays the role of doping in real materials, and, at large photodoping level, the exciton gas undergoes an excitonic Mott transition, which fully realizes the magnetic-field-driven insulator-to-metal transition described by the Hubbard model. At lower photodoping, the long-range interactions drive the formation of a collective superradiant state, in which the phases of the excitons generated in each single perovskite QDs are coherently locked.

While synthetic efforts resolved in a vast library of SLs,⁵¹⁶,⁵¹⁷ currently more than 20 different crystals have been realized

including lamellar 1D linear chains of perovskite QDs, theoretical modeling with progressively higher complexity has elucidated the occurrence of SF emission and critically highlighted the role of thermal dephasing and energetic disorder.⁷⁶⁹

Recently, a comprehensive theoretical work⁷⁷⁰ has modeled the onset of collective QD response in SLs of different dimensionalities (1D, 2D, and 3D) with variable QD aspect ratios (Figure 25D). They predicted as much as a 15-fold enhancement in robustness against realistic values of energetic disorder in 3D SLs composed of cuboid-shaped, as opposed to cube-shaped NCs. Superradiance from small ($N \lesssim 10^3$) 2D SLs is up to ten times more robust to static disorder and up to twice as robust to thermal decoherence than 3D SLs with the same $N$. As the number of $N$ increases, a crossover in the robustness of superradiance occurs from 2D to 3D SLs. For large $N$ ($>10^3$), the robustness in 3D SLs increases with $N$, showing cooperative robustness to disorder. Despite this theoretical effort still considers a low number of coherently excited QDs (single exciton regime), it helps rationalizing the critical role played by several material parameters (mainly exciton coherence time and energetic disorder in the ensemble) and could guide the design of SLs which sustain SF and coherent multiphoton emission even at room temperature.

So far, collective coupling occurs in SLs which are significantly larger than the wavelength of the emitted photons, and the number of coherently coupled QDs is not well defined but self-limited by the optical properties of constituent build block, e.g. through their mutual energetic disorder and characteristic dephasing time. To control the number of emitted photons, a possible solution could be to control, via bottom-up assembly strategies, the number of coupled QDs. Recently, a QD molecule consisting of CdSe/CdS core/shell QDs has been realized with a very high yield of dimers formation (Figure 25E).⁷⁷¹ Coherent coupling and wave function hybridization were manifested by a redshift of the band gap, in agreement with quantum mechanical simulations. Similar to atomic systems which could form molecules and polymer chains once brought into vicinity, the close proximity of two (or more) QDs lead to the formation of hybrid states with electron wave function spreading over the entire QD molecule.

Interestingly, QD molecules were found to reversibly switch between two emission colors, characteristic of the individual QD building block, without intensity loss.⁷⁷² Because the electronic wave functions of both QDs are delocalized over the entire QD molecule, an applied electric field can shuffle electrons between the two QDs thus switching the emission color. Appealingly, the concept is highly engineerable: the specific emission centers of the QD molecule can be easily adjusted by the size, composition and possibly even the number and shape of the constituent QDs. This could realize electrical multiplexing color switches at the single photon level. The technology seems to be scalable and a higher number of QD assemblies could be engineered toward more complex quantum light sources. The grand challenge is to now couple two QDs with very similar, likely degenerate exciton states, thus favoring the formation of SF and coherent multiphoton emission in a control manner.

**Quantum Information Science.** Colloidal semiconductor NCs present an excellent platform to explore the fundamental physics and chemistry of optical materials for quantum
<figure><img src="image_70.png" /><figcaption>Figure 26. (A) TEM image and (B) energy level diagram of low-fluorescence, colloidal, milled nanodiamonds doped with NV-centers. Inset (A): schematic of the NV-center in diamond.⁷⁹⁶ (C) Schematic and AFM image of the template-assisted self-assembly of colloidal nanodiamonds. (D) PL map of assembled fluorescent nanodiamonds. (E) Statistical characterization of the number of NV-centers ($N_{\text{eff}}$) in individual nanodiamonds characterized from autocorrelation measurements, where $N_{\text{eff}} = 0$ (purple), $0 < N_{\text{eff}} < 1.5$ (yellow-green), $1.5 < N_{\text{eff}} < 2.5$ (green), and $N_{\text{eff}} > 2.5$ (blue). (Inset) AFM height distribution for the nonfluorescent nanodiamonds. (F) TEM image and (G) schematic energy diagram of colloidal, wet-chemically synthesized ZnS:Cu NCs. Inset (F): schematic of the $Cu_{2}Zn_{1-x}V_{x}$ center in ZnS. (H) Temperature-dependent PL spectra of ZnS:Cu NCs. (I) Integrated PL intensity (black symbols) and peak energy (colored symbols) as a function of temperature, extracted from Gaussian fits of data in (H). The solid black curve is a fit to the intensity data and the dashed colored curve is a sum of the emissions, weighted by their corresponding best-fit emission intensities, consistent with (J) an energy level diagram of two manifolds of states created by the defect, inside the ZnS bandgap, with coupled relaxation processes. Red and blue shaded regions in (I) represent the relative temperature-dependent intensities $I_{\text{A}}(T)$ and $I_{\text{B}}(T)$ from the best-fit model. Panels (A–E) reproduced from ref 796. Copyright © 2022 American Chemical Society. Panels (F–H) reproduced from ref 806. Copyright © 2023 American Chemical Society.</figcaption></figure>

information science and to realize solid-state technologies for applications in quantum computation, sensing, simulation, and communication.⁷²³ Colloidal semiconductor NCs are prized for their optical properties as excitons are spatially confined within, and for particles smaller than or comparable in size to the excitonic Bohr radius, delocalized over the NC volume.⁴⁴⁴,⁷⁷⁴⁻⁷⁷⁸ Semiconductor NCs can also host quantum point defects, localized optically active impurity or vacancy centers and their complexes, known as color centers or phosphors, with charge or spin degrees of freedom.⁷⁷⁹⁻⁷⁸² These excitonic, charge, or spin-states can be isolated within individual NCs creating the requisite TLSs for quantum bits, known as qubits. These qubits can be optically initialized to set the quantum state, manipulated through interactions to control the quantum dynamics to process information, and read out optically or electrically to measure the output.

The required characteristics of the NCs are different for various applications. For example, excitons with short lifetimes, and even when transform limited, with similar, but short optical coherence times are suitable as bright sources of indistinguishable single photons needed for use in quantum photonic circuits.⁷³³ Spin qubits, with longer lifetimes, and thus coherence times, are required for quantum storage and manipulation.⁷⁸³⁻⁷⁸⁵ Regardless of the application, colloidal NCs present a number of advantages, in comparison to bulk materials, in studying photon and spin qubits and integrating these qubits for use,⁷²³ as dispersions in fluids for sensing or as single NCs or NC arrays on surfaces and in optical cavities for computation and communication.

The negatively charged, nitrogen-vacancy (NV-) center (Figure 26A, inset) in the ultrawide bandgap semiconductor diamond is a well-studied quantum point defect and a

prototypical spin qubit.⁷⁸⁶,⁷⁸⁷ Diamond has a low nuclear spin bath and low spin-orbit coupling, limiting interaction with phonons and magnetic and electronic noise. The NV-center introduces dipole-allowed, molecular-like s- and p-states, deep within the diamond bandgap (Figure 26B), creating spin-triplet and spin-singlet manifolds that are coupled by intersystem crossing (dashed arrows). Preferential intersystem crossing allows nonresonant optical excitation (green arrow) to preferentially populate the $m_{s} = 0$ sublevel, initializing the spin state. The spin-state dynamics can be manipulated by external magnetic and electric fields, strain, temperature, and pressure. Differences in the spin-state energy and lifetime create contrast in the visible fluorescence (green arrows), providing a scheme for *readout*.

While the best NV-center spin coherence is seen in bulk diamond crystals, < 100 nm nanodiamond particles can be dispersed in solvents to form colloids, particularly interesting for *in vivo* and *in vitro* sensing.⁷⁸²,⁷⁸⁷ However, chemically pure, nanodiamonds with stable NV-centers are created by milling bulk diamond crystals, yielding particles varying in size, shape, surface chemistry, and the number of NV-centers, and thus with inhomogeneity in their optical and quantum optical properties (Figure 26A). Template-assisted self-assembly⁷⁸⁸⁻⁷⁹¹ and optical trapping⁷⁹²,⁷⁹³ techniques have been used to deterministically and scalably position arrays of single and countable numbers of NCs on surfaces and in optical cavities.⁷⁹⁴,⁷⁹⁵ The assembly of NCs on surfaces, in addition to allowing the study of single NC photophysics, reduces the number of defects within the beam spot and therefore the purity requirements from <100 ppt in bulk crystals to <100 ppm in NCs.⁷²³ Figure 26C shows a schematic of the template-assisted self-assembly process, in which topographical trap sites
<figure><img src="image_71.png" /><figcaption>Figure 27. (A) Definition of the ensemble of sites on Pd/Pt NCs composed of 1500 atoms (~4 nm) with color varying from yellow to red, indicating coordination numbers from low (6) to high (9). (B) Correlation between the simulated fractions of ensemble of sites and experimental turnover frequency at 124 °C with different NC size. (C–F) High-resolution TEM images of alumina-supported postcatalysis (C and D) 2.3 nm Pd/Pt NC and (E and F) 10.2 nm Pd/Pt NC. The boxes in (C) and (E) are used to highlight the high-resolution particles presented in (D) and (F), respectively. (G–I) Representative high-angle annular dark field scanning transmission electron microscopy images of dense (0.659 wt %) (G), intermediate (0.067 wt %) (H) and sparse (0.007 wt %) (I) Pd/Al₂O₃ samples where different loadings are used to change the Pd interparticle distance in the catalysts. (J) CH₄ conversion profiles in methane oxidation for Pd/Al₂O₃ catalysts with different nanoparticle loadings following the temperature profile (black line and right axis). (K) Averaged CH₄ conversion values at 460 °C for the Pd/Al₂O₃ catalysts before ('Fresh') and after ('Aged') aging. Error bars represent the minimum and maximum results of at least three repeat experiments. (L) Schematic illustration of nanoparticle/ordered-ligand interlayer (NOLI) formation and its effect in CO₂ electrocatalysis. Surface ligands (tetradecylphosphonic acid) are initially covalently bonded to the nanoparticle (NP) surface. Upon biasing under CO₂-reducing conditions, the ligands collectively detach and form a structurally ordered ligand layer. The starting small NCs also fuse into a larger NC during the process. The initial interaction between ligands induced by NC assembly is considered crucial for the NOLI formation. Panels (A–F) were adapted with permission from ref 817. Copyright © 2020 National Academy of Sciences. Panels (G–K) were adapted with permission from ref 838. Copyright © 2019 Goodman et al. under exclusive license to Springer Nature Limited. Panel (L) was adapted from ref 847. Copyright © 2021 American Chemical Society.</figcaption></figure>

created in resist templates allow capillary-driven assembly of commercial, ~40 nm diameter nanodiamonds (Figure 26A).⁷⁹⁶ Atomic force microscope (AFM) measurements (Figure 26C), after the resist is removed, shows an example assembly of an array of single nanodiamonds, achieved with ~80% yield. Spatially resolved PL (Figure 26D), autocorrelation, power-dependent intensity, and spin lifetime measurements of these samples allow statistical characterization of the nanodiamonds. For example, measurements of >200 commercial, low-fluorescence, milled nanodiamonds show that 31% of nanodiamonds have no NV-centers, 12% have single emitters, and the remainder have multiple NV-centers. The non-Poissonian distribution of emitters is hypothesized to arise from inhomogeneity in the nitrogen incorporation during the parent single crystal growth⁷⁹⁷,⁷⁹⁸ and the stochastic creation of NV-centers. Most studied milled nanodiamonds have surface carboxyl groups with patchy coverage, reported to reside at undercoordinated carbon sites between crystal facets.⁷⁹⁹,⁸⁰⁰ While sufficient to allow their aqueous dispersion, it is also difficult to functionalize nanodiamonds to both stabilize the particles in fluids and maintain their quantum optical properties, prompting the development of surface coatings, important for their use as sensors.⁸⁰¹

While nanodiamonds are a relatively mature technology with desirable quantum optical properties, the discovery of new materials and spin qubits is needed for applications.⁷⁸⁴,⁸⁰² In contrast to milled nanodiamonds, the wet-chemical synthesis of colloidal NCs allows the preparation of a wide range of semiconductor NC compositions (e.g., II–VI, III–V, IV–VI, and metal-halide perovskite NCs) with near-atomic precision in control over size and shape. The surface chemistry of these NCs is better understood and tailorable using known organic and inorganic ligand libraries.⁴¹⁵ Advantageously, wet-chemical synthesis also allows the more rapid preparation of NCs with different defects, e.g., in comparison to implantation in bulk crystals, to facilitate quantum point defect discovery. With control over NC size and shape and the achievement of complex core–shell and Janus structures, NCs also allow wave function engineering to sculpt the spatial distribution of charge and spin states.⁷²³

Like diamond, ZnS has a dilute nuclear spin bath and low spin–orbit coupling. Impurity-doped ZnS is among the best-known phosphor materials and ZnS has recently been explored to host quantum point defects.⁸⁰³,⁸⁰⁴ Cu-doped ZnS (ZnS:Cu) emits in the visible red, green, and blue light from color centers known as R-Cu, G-Cu, and B-Cu.⁷⁸¹,⁸⁰⁵ The R-Cu emission is reported to originate from a Cu<sub>Zn</sub>-V<sub>S</sub> defect complex, which is
a more dipole-allowed radiative transition (unlike ZnS:Mn that arises from intra-d-shell, dipole forbidden transitions) with the same symmetry as that of the NV-center in diamond (Figure 26A–F). Recently, red-emitting, colloidal ZnS:Cu NCs were reported (Figure 26F).<sup>806</sup> The red emission is consistent with that seen in bulk materials arising from localized electronic transitions associated with the Cu<sub>Zn</sub>-V<sub>S</sub> defect complex (Figure 26G). Temperature- and time-dependent optical spectroscopy (Figure 26H) has been used to map the peak energy, intensity, and lifetime of the red emission. A blue shift and a plateau in the intensity dependence (Figure 26I) of the red emission with increasing temperature is empirically modeled by the thermally activated carrier transfer between two manifolds of radiative states (Figure 26J). Room temperature quantum emission has now been observed from Cu<sub>Zn</sub>-V<sub>S</sub> quantum point defects in single ZnS NCs.<sup>807</sup> Understanding the luminescence characteristics of the defects is an important first step. Further studies, using electron spin resonance and optically detected magnetic resonance are important to understanding the spin-dependent optical properties of the defects, toward developing protocols for quantum point defect initialization, control, and readout. Studies of single quantum point defects in NCs are important to learning their location within the NC, concentration, and charge state, which can be manipulated by engineering the NC surface chemistry, exploiting their large surface-to-volume ratio to passivate surface states and remotely dope the NCs. The quick, controlled, and scalable synthesis of quantum point defects in colloidal NCs and the assembly of individual NCs on surface and in cavities make NCs excellent materials as light-matter interfaces to explore fundamentally and for applications in quantum technologies.<sup>796</sup>

**NCs for Catalysis.** Colloidal NCs have been used as precursors of active and selective heterogeneous catalysts for over two decades now. There are multiple reasons that make colloidal NCs very relevant in advancing catalytic applications: (1) they can be used as model systems where several crucial parameters for catalytic activity (exposed facets, composition, metal-support interactions, etc.) can be tuned and utilized in advancing fundamental understanding of active sites and motifs under catalytically relevant conditions of temperature and pressure; (2) they can be engineered to display the largest fraction of active sites, when active site motifs are known, for a specific application or reaction; (3) they can be utilized to follow and observe processes of catalysts restructuring, activation and deactivation, given the uniform nature of surface structure and composition; (4) the NC-ligand interface can be engineered and utilized to display catalytic properties that differ from classic systems, such as supported catalyst. For the above reasons, research in colloidaly prepared catalysts has been expanding in the recent years.<sup>808,809</sup> Industrial uses of these catalysts have also been realized, although only for small-scale applications so far.<sup>810</sup> Continued research in this field is expected to expand application areas and deepen fundamental knowledge, while also enhancing their impact in practical applications. A few case examples from recent years illustrate the potential and promise of this approach.

The use of colloidal NCs as precursors for supported catalysts has been one of the first applications of these controlled materials in the field.<sup>811,812</sup> In analogy to single-crystal surfaces, NCs with well-defined sizes and facets allow to control the type and number of active sites exposed to the reaction (unless restructuring occurs, see below).<sup>813,814</sup> The fraction of sites can then be studied as a function of catalytic

performance to evaluate whether certain sites are majorly responsible for the reactivity patterns.<sup>815,816</sup> The development of theoretical models, especially using advanced computational tools, to explain the reactivity patterns is equally useful. As an example of the latter, composition- and size-controlled alloyed Pd/Pt NC catalysts are useful to show the importance of step sites on the reactivity for propene combustion.<sup>817</sup> As NCs vary in size between ~2 and ~10 nm, the fraction of terrace, edge and kink sites change accordingly, and comparing accurate turnover frequency values with the computationally modeled fraction of exposed sites allow proposing the hypothesis that pairs of atoms with specific coordination numbers (7–7 step sites, i.e., coordinated to 7 near neighbors) are the most active sites for propene combustion under reaction conditions (Figure 27A–F). Similar strategies have been used by de Jong and co-workers to understand performance trends in Fischer–Tropsch synthesis over Co- and Fe-based catalysts.<sup>818–820</sup> These catalysts not only provide useful information on the most active and selective sites toward hydrocarbon and olefin formation, but also on structural evolution of the materials, as well as metal-support interactions.<sup>821</sup>

This latter topic is one where colloidal NCs excel given the opportunity to utilize the same NC precursors for the preparation of samples on different supports can easily be used as a strategy to identify the influence of the support on the catalytic performance.<sup>822</sup> Finally, the use of colloidal NCs with specific defective structures (e.g., grain boundaries, point defects) can also help elucidate the role of these sites on activity, thus continuing to expand the reach of these materials in the fundamental understanding of catalytic performance.<sup>823</sup>

The past few years have witnessed the use of engineered colloidal catalysts to deliberately improve catalytic performance in several fields. The tunable nature of colloidal NCs endows them with the potential to achieve the largest fraction of active site motifs through tailoring their structure in solution. Alloyed and intermetallic NCs are clear examples of this approach, whereby mixing elements with complementary properties, i.e., one reactive element and one modulating its reactivity, catalysts with improved rates and selectivity can be obtained. Exemplary cases in electroreduction of CO<sub>2</sub> to CO on ordered AuCu NCs<sup>824</sup> and Pt/Sn intermetallic catalysts for propane dehydrogenation<sup>825</sup> highlight this aspect, where the deliberate preparation of NCs with appropriate compositions allow to overcome activity and stability challenges in the reaction of interest. Pd-based bimetallic NCs have been utilized for the catalytic oxidation of methane. The study systematically shows the effects of adding a second metal to increase catalytic activity (for PdZn and PdNi) or to inhibit the sintering process (for PdSn, PdFe, and PdCo).<sup>137</sup> In another paper, Ni–In NCs have been studied for catalytic hydrogenation of unsaturated aldehydes. Tuning the ratio between Ni:In as well as size of NCs, high catalytic activity and selectivity can be achieved. Eventually, 5 nm in size Ni<sub>2</sub>In NCs exhibit optimal performance.<sup>826</sup> Intermetallic NCs offer opportunities for exploring alternative catalytic concepts, such as adsorption of homogeneous catalysts on the surface of NC,<sup>827</sup> plasmonic catalysis,<sup>828</sup> and electrocatalysis.<sup>138</sup>

Recent cases of more complex architectures built from colloidal NCs have been reported for the chemical upcycling of waste plastics. Perras and colleagues have shown that by astutely sandwiching Pt NCs between a nonporous silica core and a porous silica shell, it is possible to induce polymer melts to react with the Pt surface in specific conformations, leading
to narrower distributions of hydrocarbon products in polyethylene hydrogenolysis.<sup>829</sup>

Another relevant example is the engineering of NC shapes to
induce the formation of specific surface site motifs of relevance
in several electrochemical transformations.<sup>830,831</sup> The use of
overlayers of oxides or other materials to cap and protect
colloidal NCs has also emerged as an engineering tool to
dictate selectivity and stability in heterogeneous catalysts, such
as in the example of thermally stable catalysts for emission
control applications using encapsulated Pt and Pd NCs in
alumina,<sup>832</sup> or in hybrid oxide coatings prepared in solution to
enhance the stability of Cu-based catalysts for CO<sub>2</sub> electro-
reduction.<sup>833</sup> It is expected that this field of application of
colloidal catalysts will continue to grow in the future.

Catalyst restructuring and activation/deactivation processes are somewhat unavoidable and often affect catalyst performance in crucial ways, and understanding these dynamic processes is an important step toward the identification of active sites and motifs in heterogeneous catalysts.<sup>834,835</sup> Gaseous and liquid environments provide challenges for probing the most important factors responsible for processes like leaching, sintering, agglomeration, decomposition, and poisoning of active sites. Using uniform and well-defined colloidal catalysts therefore simplifies the study of catalyst dynamics because they reduce the number of variables that need to be considered when identifying their origin.

Harsh temperature and pressure conditions are usually associated with restructuring by ripening and particle migration that induce deactivation of supported catalysts. Using narrow size distributions allows to ascertain the contribution of particle size to the ripening/agglomeration phenomena, and colloidal catalysts can even allow to engineer these distributions and probe their involvement in deactivation processes.<sup>836,837</sup> Controlling the dispersion and positioning of supported metal NCs allow demonstrating how the interparticle distance can affect particle decomposition processes that are responsible for deactivation of methane combustion catalysts through decomposition into single atoms (Figure 27G–K).<sup>838</sup> The decomposition was found to be dependent on the NC density on the support, with denser (higher loading) catalysts being more stable than sparse (lower loading) catalysts, contrarily to what sintering deactivation phenomena would predict. In analogy to this work, more recent research prove how interparticle separation is crucial for enhancing catalytic selectivity in liquid-phase reactions by appropriately positioning supported metal particles within the catalyst layers.<sup>839</sup>

The advantage of colloidal NC approaches is that
interparticle distance, metal loading, composition, can be
independently tuned while guaranteeing similar catalytic
properties of individual supported particles because those are
encoded in the synthesis process. Changes occurring to the
structure as well as the catalytic performance can easily be
followed at the same time, and the changes can be very
dramatic, as in the case of supported Ru NCs that go from
producing methane to producing almost exclusively CO as a
function of catalyst pretreatments in CO₂ hydrogenation.<sup>840</sup>
The visualization of restructuring processes in real time can
now be obtained using a combination of spectroscopic and
microscopic measurements. Cu NCs used as electrocatalysts
for CO₂ reduction have been recently found to undergo
redissolution and reprecipitation processes during activation,
leading to changes in rates and selectivity for reduced products

during operation and as a function of voltage cycles/
operation.<sup>841</sup> These examples highlight how colloidal NCs
help identify and explain restructuring phenomena much more
readily than with other catalyst synthesis techniques and
provide a way to understand catalyst activation/deactivation
phenomena.

One element that clearly distinguishes colloidal catalysts
from other catalysts is the potential for taking advantage of the
ligands/surfactants for imparting properties that would not
otherwise be possible with other methods.<sup>842</sup> In general,
ligands/surfactants on the surface of colloidal NCs need to be
removed after synthesis and before catalytic applications in
order to allow the reactive surface of the NCs to be fully
exposed to gaseous and liquid reactants.<sup>843-845</sup> However, in
certain cases, ligands can actually be utilized to direct the
activity and selectivity of catalytic processes. In a particularly
exciting example of utilizing ligand layer for reactivity, Kim,
Yang, and co-workers demonstrate that a detached layer of
ligands on the surface of Ag, Au, and Pd NCs can create an
interlayer that can host cations and reactive species responsible
for increased and highly selective CO<sub>2</sub> electroreduction.<sup>846</sup>
Later on, it was found that ordered assemblies of NCs can also
enhance these ligand interactions responsible for the improved
performance, including the electrochemical stabilization of
intermediates within the ordered organic ligand inter-
layers,<sup>846,847</sup> leading to the formation of reservoirs of
intermediates and reactive species (in this latter case mostly
CO) that can facilitate the formation of higher carbon number
products.<sup>848</sup>

The colloidal NC environment can be further engineered
postsynthesis to drive the catalytic activity of the formed metal
surfaces. Riscoe et al. hypothesized that coating NCs with
polymer ligands can modify the reactivity of metal surfaces,
and the development of NC-porous polymer hybrid materials
first demonstrate that the polymer layers can dictate transition
state stability in Pd-catalyzed CO oxidation reaction.<sup>849</sup>
Further engineering of the polymer layers and NC composition
in Ru/TiO<sub>2</sub> supported catalysts also demonstrated how the
modification of the NC environment using functional groups
in polymer layers can dictate the C–C coupling probability,
largely increasing by few orders of magnitude the rate of higher
hydrocarbon formation.<sup>850,851</sup> The exploitation of surface
ligands is an area of nascent exploration in the community
that promises to further increase the utilization of engineered
NCs in several catalysis application areas.

NC-Based Precursors for Thermoelectric Materials.

The conversion of heat into electricity and vice versa, known as thermoelectricity, presents a promising method for utilizing waste heat and optimizing thermal management.<sup>852,853</sup> However, the high costs associated with expensive raw materials, energy-intensive manufacturing processes, and low efficiency have impeded the large-scale implementation of thermoelectric devices.<sup>854</sup>

Traditionally, thermoelectric materials are either done in
single crystal form, high-temperature reactions, or by
consolidating powders to produce dense ingots. Alternatively,
powders can be synthesized in solution and constituted of
NCs.<sup>855</sup> Compared with solid-state methods, solution-
processed thermoelectric materials demand shorter reaction
times and lower temperatures, reducing energy consumption.
Furthermore, NC reactions are intended to include self-
purifying processes that should yield defect-free NCs of the
targeted compound. Therefore, reagent purity requirements is
<figure><img src="image_72.png" /><figcaption>Figure 28. (A) Steps involved in the solution-processing of thermoelectric materials. Reproduced with permission from ref 856. Copyright © 2024 Fiedler et al. (B) Example of the role of NC surface adsorbates into the thermoelectric material microstructure. Reproduced with permission under CC BY 4.0 from ref 864. (Copyright © 2021 Liu et al. open access). (C) Example of grain growth control through NC surface ligands. Reproduced from ref 874. Copyright © 2021 American Chemical Society.</figcaption></figure>

less demanding as side products, unreacted species, and solvents can be separated after the synthesis.⁸⁵⁶ After the purification process, the NC-based powders are then consolidated into dense inorganic solids by applying pressure and temperature (Figure 28A).

Beyond the potential reduced powder cost, solution synthesis offers pathways to create powders from precisely engineered NCs that can yield dense materials with features not achievable by other synthetic approaches.⁸⁵⁷ The reason is that by adjusting various controllable parameters when designing the powder (such as NC size, shape, crystal structure, composition, surface species, and organization), we can determine the final material features (such as crystal structure, defects, grain size, grain orientation, and interfaces), and as a result, the thermoelectric performance.⁸⁵⁵ In this scenario, NCs can act as versatile precursors that, when subject to the right reaction conditions, can develop into bulk materials with specific structural characteristics in a similar fashion as metal complexes are converted into well-defined NCs through colloidal synthesis.⁸⁵⁸,⁸⁵⁹

The development of this approach is still in its early stages, mainly due to the complex processes involved in transforming NCs into dense solids (e.g., sintering, decomposition of surface species, solid-state reactions, melting) and the extensive range of parameters that determine the final outcome. One of the most significant challenges is establishing clear correlations between the properties of the NCs and the structural characteristics of the consolidated material. A key issue arises from the requirement for comprehensive and accurate structural and compositional information about both the NC-based precursors and the resulting solid material, which is not only difficult but often time-consuming and requires specialized equipment not readily available in many laboratories.⁸⁵⁵

For example, NC composition usually differs from the nominal one, unlike traditional solid-state methods. Yet, minor changes in NC stoichiometry can massively impact the properties of the final solid. NC shape determines the surface atoms and their atomic environment, altering the particle's surface energy and reactivity,⁸⁶⁰ both crucial parameters for the transformation that occurs during consolidation. Moreover, the
surface species connected to the under-coordinated termination atoms can vary from molecules with long aliphatic chains to molecular or ionic species, each of them having very different roles during the transformation into dense solids.<sup>861-864</sup>

Typically, colloidal syntheses yield NCs capped with long hydrocarbon chains,<sup>863</sup> that form composites with a carbon-containing phase<sup>843,865</sup> even after calcination of the ligands,<sup>866,867</sup> potentially resulting in barriers for atomic diffusion<sup>868</sup> and blocking charge transport.<sup>869</sup> Alternatively, inorganic ligands can be used not only to avoid any carbon presence but also as a tool to further guide sintering and solid-state reactions during consolidation. Where the ligand could (i) become a crystalline inorganic matrix with the NCs<sup>870,871</sup> embedded, (ii) react with the NCs to yield a new phase,<sup>872</sup> (iii) control crystal growth<sup>873-875</sup> and coarseing, (iv) transform into secondary phases,<sup>874</sup> and (v) introduce atomic impurities.<sup>866,876</sup>

Functionalizing NC surface using inorganic molecules has proven significantly powerful in providing nanocomposites with features that cannot be achieved using the most established spinodal decomposition in high-temperature reactions.<sup>866,868,877-879</sup> The inorganic molecules can be selected based on their role during sintering, which allows the control of the final materials' structure and composition and, therefore, provides an additional tool to optimize thermoelectric performance. Recent examples have shown the possibility of controlling grain growth during sintering depending on the inorganic ligand. If the inorganic ligand decomposes into a compound that is miscible with the matrix phase, grain growth will be promoted, while if it is immiscible, it will be hindered (Figure 28C).<sup>874,880</sup> Through hindering grain growth, SnSe-CdSe composites were produced with structural features that yield one of the highest-performance thermoelectric materials.<sup>874</sup>

Despite the importance of identifying surface species for rationalizing the process and the different methods to study the NC surface chemistry,<sup>881</sup> some surface species can remain elusive. For example, observing ionic groups intercalating between organic molecules is complicated.<sup>459</sup> Previously overlooked ionic adsorbates in SnSe NCs were unveiled by studying the resulting solid through atom probe tomography.<sup>864</sup> When using Na salts in the reaction mixture, Na<sup>+</sup> ions are electrostatically adsorbed on the NC surface and stay there after the NCs are purified to maintain charge neutrality.<sup>864</sup> Na is present within the matrix in the sintered pellets, acting as a dopant in dislocations, precipitates, and forming grain boundary complexions. Moreover, it was proven that they play a crucial role not only in directing the material nano/microstructure during thermal processing but also in determining the transport properties of the consolidated material (Figure 28B).<sup>864</sup> The interfaces created between SnSe and Na-rich phases lead to energy filtering, enhancing the Seebeck coefficient. These findings highlight the importance of a holistic analysis of the NC precursors and resulting materials to establish synthetic rules and have correct structure–property relationships.

In addition to the possibilities of tailoring the NC characteristics and their surface species, NC-based powders can be created using different types of NCs to form composites and even with a certain degree of ordering to direct the sintering process. Blending different types of NCs has been one of the most effective strategies for obtaining materials with

exceptional properties that cannot be achieved through other methods. For instance, by blending Ag or Cu NCs with PbS, Ag/Cu-PbS nanocomposites could be produced with thermoelectric properties that surpassed any previously reported PbS-based material.<sup>882,883</sup> This achievement was made possible thanks to the presence of long organic chains as ligands, which, upon annealing, converted into graphitic carbon, preventing the diffusion of Ag or Cu into the PbS matrix. The presence of metallic Ag/Cu, with a work function close to the conduction band of PbS, provided a way to control carrier concentration through modulation doping, allowing much higher mobility than the ionic doping method, resulting in high electrical conductivities throughout the entire temperature range.<sup>882</sup>

NC blends can also be conceived to induce reactions that yield different materials from the original NCs. An example is the combination of CsPbBr<sub>3</sub> and PbS NCs.<sup>884</sup> During the consolidation process under temperature, PbBr<sub>2</sub> is extracted from CsPbBr<sub>3</sub> and diffuses into the PbS matrix, controlling the material doping level, as Br<sup>-</sup> acts as an electron donor, leaving behind Cs<sub>4</sub>PbBr<sub>6</sub> nanodomains.<sup>884</sup>

In the past decade, significant efforts have been made not only to unravel the unknowns of NC-based precursors to produce dense inorganic material<sup>856,864,885,886</sup> but also to use them to reach record thermoelectric performance while trying to minimize cost.<sup>857,874,887,888</sup> In this line, NC-based precursors have also gained lots of traction as active materials for the production of thermoelectric materials through additive manufacturing techniques.<sup>889,890</sup>

### Printed Thermoelectric Devices from NC-Dispersed inks.
Conventional bulk-scale thermoelectric modules consist of *n*-type and *p*-type cuboid-shaped thermoelectric semiconductor legs connected electrically in series and thermally in parallel. They are typically manufactured through bulk-scale processing techniques that involve multistep processes such as synthesis of ingots, dicing, metallization, and chipping.<sup>891</sup> However, these traditional methods are not only energy-intensive and expensive but also limit the flexibility in module design. This issue becomes particularly critical when fabricating emerging thermoelectric devices such as cylindrical, miniatured, and wearable devices. Microscale miniatured devices offer the potential to be used as auxiliary power supply or local thermal management devices for electronic systems such as the Internet of Things, wireless sensor networks, and lab-on-a-chip devices.<sup>892</sup>

Recently, various printing techniques have gained attention for fabricating thermoelectric devices with customized designs by patterning *n*-type and *p*-type thermoelectric semiconductors and metal electrodes.<sup>893,894</sup> Compared to conventional manufacturing techniques, printing techniques offer several advantages, including enhanced design flexibility in device structures and cost-effectiveness in processing. Colloidal NCs have emerged as a highly effective ink for these printing processes. They can be synthesized via well-established methods, and their surface chemistry enables the optimization of ink printability. Additionally, the intrinsic thermoelectric properties of the printed materials can be enhanced by controlling the size, composition, and surface ligands of the NC building blocks.

For printing with thermoelectric NC inks, the colloidal stability of the NC inks must be secured, and their rheological properties adjusted to conform with the selected printing method. The colloidal stability of NCs is typically achieved through the utilization of surface ligands. For instance, long-
<figure><img src="image_73.png" /><figcaption>Figure 29. (A) Scheme of the formulation of all-inorganic thermoelectric-NC-based inks. (B) Scheme of the direct 3D writing process of thermoelectric NC-based inks with high viscoelasticity. (C) Cylindrical power generators that consist of 3D-printed *p*-type and *n*-type thermoelectric half-rings and its output voltage and power depending on temperature differences. (D) Microscale thermoelectric devices fabricated by the direct 3D writing process. (E) Schematic diagram of the used 3D printer with a picture of the printed thermoelectric legs and their assembly into the device. (F) Comparison of the maximum cooling gradients achieved for thermoelectric coolers fabricated by different methods. Panels (A) and (C) were reproduced with permission from ref 899. Copyright © 2018 Kim et al. Panels (B) and (D) were reproduced with permission from ref 904. Copyright © 2021 Kim et al. under exclusive license to Springer Nature Limited. Panels (E) and (F) were reproduced with permission from ref 908. Copyright © 2025, The American Association for the Advancement of Science.</figcaption></figure>

chained organic surfactants or polymers are frequently employed for enhancing the colloidal stability of NCs through steric stabilization. However, these organic stabilizers, which are generally electrical insulators, can potentially introduce impurities that can degrade the thermoelectric properties of the final product.⁸⁹⁵,⁸⁹⁶ Consequently, post-thermal or chemical processing steps may become necessary to eliminate these organic impurities. Alternatively, electrostatic stabilization of NCs presents an effective approach to securing colloidal

stability while minimizing the presence of undesired impurities. For instance, all-inorganic NCs coated with molecular metal chalcogenide complexes, also known as chalcogenidometal-lates, are highly soluble in polar solvents. Furthermore, upon heating, they can induce phase transformations in semi-conducting metal chalcogenides.⁵⁵⁴,⁸⁶⁷,⁸⁹⁷,⁸⁹⁸ This feature can be leveraged to control their composition and doping.

The rheological properties of inks are of paramount importance in ensuring the printing processability. For
<figure><img src="image_74.png" /><figcaption>A PCM nanoparticles</figcaption></figure>

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<figure><img src="image_76.png" /><figcaption>D PCM molecular inks</figcaption></figure>

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<figure><img src="image_78.png" /></figure>

<figure><img src="image_79.png" /></figure>

<figure><img src="image_80.png" /></figure>

Figure 30. (A) Schematics of reversible phase transitions between amorphous and crystalline PCM nanomaterials. Note the chain ordering of cations (blue) in the amorphous structure. (B) Size-dependent crystallization temperature of GeTe nanoparticles and a Lindemann criterion fit of the dependence. (C) Crystallization mechanism bulk vs nanoscale GeTe PCM material. (D) Schematics for the thin film deposition from telluride molecular inks. (E) Structural properties of ink-based Ge–Sb–Te (GST) thin films. (F, G) Switching and cycling of a PCM device with solution-engineered GST memory layer by tuning the amplitude and duration of the voltage pulses. Note the voltage window in F and the resistivity contrast in G for reliable switching and reading of a memory cell, respectively. Panels (A) and (C) were reproduced with permission under CC BY 4.0 from ref 929. (Copyright © 2024 Wintersteller et al. open access). Panel (B) was reproduced with permission from ref 917. Copyright © 2018 American Chemical Society. Panels (D)-(E) were reproduced from ref 923. Copyright © 2023 American Chemical Society.

instance, the success of screen printing relies on highly viscous inks, whereas inkjet printing necessitates inks with low viscosity.⁸⁹³ Aerosol jet printing accepts a wide range of ink viscosities.⁸⁹³ However, Newtonian rheological properties are often required to achieve a uniform deposition of inks. Optimizing these rheological properties for inks containing NCs is generally achievable by adjusting the volume fraction of NCs within the medium. However, enhancing the viscosity of colloidal NC inks can sometimes be challenging due to solubility limitations. One approach to address this challenge is to introduce surface charges on the NC, which creates an electric field near the charged surface. This induced electric field alters the structure of the surrounding fluids and the rheological properties of the NC-dispersed inks via the electroviscous effect.⁸⁹⁹⁻⁹⁰³ Kim et al. developed all-inorganic viscoelastic thermoelectric inks for dispensing-based 3D printing by using Sb₂Te₄²⁻ chalcogenidometallate anions as inorganic additives for Bi₂Te₃-based NCs (Figure 29A). Employing compositionally similar anions has proven to be beneficial in enhancing the viscoelasticity of the NC-dispersed inks and maintaining the compositional integrity of the final printed product.

Another key parameter is the characteristics of NCs. Highly viscoelastic thermoelectric inks have been developed by optimizing the particle size, size distribution, and surface states of the inks, facilitating direct 3D printing using the inks

(Figure 29B).⁹⁰⁴ This process can produce single filaments with lateral dimensions on the order of hundreds of micrometers, which can be used as thermoelectric legs in microscale thermoelectric devices.

Ink printing has been used to manufacture devices using well-known thermoelectric materials of Bi₂Te₃, Bi₂ₓSbₓTe₃, PbTe, and Cu₂Se as inks. The NC surfaces in the Bi₂Te₃ and Bi₂ₓSbₓTe₃ inks were engineered using Sb₂Te₄²⁻-based ions, while those in the Cu₂Se ink were modified using Se₈²⁻ polyanion as a surface capping agent. Lee et al. enhanced the viscoelasticity of PbTe inks by electronically doping them with Na and Sb, which induced surface charges in the inks through surface-charge imbalance. Efficient particle sintering was achieved using these inks, which increased the peak ZT values of the Bi₂ₓSbₓTe₃, PbTe, and Cu₂Se products up to 1.1, 1.4, and 2.0, respectively. These ZT values are comparable to those obtained for typical ingots with same compositions.⁹⁰⁰,⁹⁰²,⁹⁰³,⁹⁰⁵

Ink printing technology uniquely enables the fabrication of thermoelectric devices with customized shapes, intricate structures, and microscale dimensions (Figure 29C,D), offering a level of flexibility not attainable through traditional manufacturing processes. For instance, studies have demonstrated the efficiency of dispensing-based 3D printing and aerosol jet printing technologies in fabricating heat-source conformable thermoelectric devices.⁸⁹⁹,⁹⁰⁰,⁹⁰⁶ These methods
enable the production of complex architectural designs and the precise deposition of inks on curved surfaces. Another distinct advantage of printing technology is its versatility: materials can be printed on a diverse range of substrates. Moreover, the thickness of the printed thermoelectric materials can be controlled using this technology. This adaptability allows for the efficient and straightforward manufacturing of flexible thermoelectric devices catering to wearable applications.⁹⁰⁷ The streamlined printing process and meticulous control over material utilization significantly enhance production efficiency and cost-effectiveness, ultimately expanding the market for thermoelectric devices. Very recently, Xu et al.⁹⁰⁸ reported the fabrication of high-performance thermoelectric materials (p-type bismuth antimony telluride [(Bi, Sb)₂Te₃] and n-type silver selenide (Ag₂Se)) with a record-high cooling temperature gradient of 50 °C in an ambient environment by using an extrusion-based 3D printing technique (Figure 29E,F). By tailoring the ink formulation, Xu et al. demonstrate the formation of interfacially bonded grains, allowing high mobilities despite the presence of a large number of pores. This resulted in 3D-printed materials with excellent thermoelectric properties.

## Phase Change Memory (PCM) at the Nanoscale.

Currently, our computers are built by combining several silicon technologies to reach an intricate optimum for the cost, speed, and density of microchips.⁹⁰⁹ One problem of such a computer architecture is the pronounced mismatch between the speed of slow but nonvolatile storage-class memory (e.g., NAND Flash SSD) and fast but volatile operating memory (e.g., Dynamic RAM). This speed gap can extend beyond 1–2 orders of magnitude, rendering operating memory idle upon slow data transfer from the memory-storage segment of the computer and thus limiting the computer performance, power efficiency, and upscaling of computer clusters.⁹¹⁰ PCM devices are ideal for closing the speed gap as their performance can be tuned between the DRAM and NAND characteristics by the choice of PCM material.⁹¹¹,⁹¹² Besides solving one of the most pressing challenges in computer architecture, PCM technology features favorable and well-differentiated performance parameters, including a bandwidth up to 5 GB/s, subns switching rate, and improved data retention at ambient and elevated temperatures.⁹¹³

The two main challenges of PCM technology are the high price per bit and the further miniaturization of memory devices. The future development of the PCM field thus belongs to nanoscience, and specifically, liquid-based chemistry offers an all-embracing approach to improve PCM technology.⁹¹⁴,⁹¹⁵ Monodisperse colloidal nanoparticles are ideal for studying scaling rules for ultrasmall PCM devices, disentangling intertwined effects of structure at the nanoscale and a contribution of surface atoms.⁹¹⁶⁻⁹¹⁹ Furthermore, liquid-phase clusters and complexes⁹²⁰,⁹²¹ unlock a convenient materials platform for screening PCM materials and enabling alternative fabrication approaches for PCM devices, such as spin-coating and printing, which are less expensive, more material-efficient, and template-independent.⁹²²⁻⁹²⁴ Figure 30 summarizes the status of liquid-phase PCM nanomaterials.

**PCM Nanoparticles.** PCM technology is based on crystallization and melting phase transitions (Figure 30A), switching the memory material between amorphous and crystalline phases with distinctly different electrical (i.e., resistivity) and optical (i.e., reflectivity) properties.⁹¹² In contrast to bulk materials, however, nanoparticles attain a

size dependency for both phase transitions.⁹¹⁵ For example, it has been consistently observed by several groups that the crystallization temperature of GeTe nanoparticles increases as their size decreases (Figure 30B).⁹¹⁶,⁹¹⁷,⁹²⁵,⁹²⁶ This phenomenon has been explained by the effect of surface atoms, holding excessive energy per atom and thus rendering the structure of the nanoparticle more disorganized compared to the bulk. The analogy can be taken from the melting point depression phenomenon,⁹²⁷ except that the crystallization phase transition is characterized by increased ordering (i.e., negative change of entropy), which is the opposite to the melting phase transition.⁹¹⁷ Therefore, the surface plays the opposite effect too, requiring higher crystallization temperatures for materials with larger fraction of surface atoms (i.e., smaller sizes). In common, both phase transitions can be quantified by the Lindemann criterion, as shown in the case of GeTe crystallization in Figure 30B.⁹¹⁷

The size-dependent phase transitions are the game changers for the ultrasmall PCM devices, affecting all phase change properties. For example, higher crystallization temperatures lead to better data retention properties, while low melting temperature improves the power consumption characteristics of PCM devices.⁹¹⁷ On the flip side, however, the bonding and structural dynamics are also affected.⁹²⁸ Specifically, PCM NCs exhibit stronger glass-forming properties, leading to slower crystallization kinetics and stronger covalent bonding.⁹²⁹ Therefore, the crystallization mechanism of GeTe nanoparticles includes an additional transition state for Ge atoms with the coordination number of 5 as they are changing the amorphous tetrahedral environment to the rock-salt-type octahedral coordination (Figure 30C).⁹²⁹ Although this nanoscale effect leads to slower switching, it appears beneficial in suppressing the aging of PCM devices, which is particularly useful for multibit data storage and analog-type computing applications. In summary, while only several telluride PCM compositions have been so far developed in the form of size-uniform colloidal nanoparticles,⁹¹⁸,⁹¹⁹ it will be important to extend this library of PCM nanomaterials in order to test the generalizability of reported nanoscale phase-change effects.⁹¹⁶,⁹¹⁷,⁹²⁵,⁹³⁰

**PCM Molecular Inks.** Bulk tellurides can be dissolved in several solvents, including hydrazine chemistry and, more recently, a cosolvent formulation of diamine and dithiol.⁹²⁰,⁹²¹ Upon reduction and purification steps, bulk telluride powders are broken down to molecular complexes and small clusters,⁹³¹ which are easy to spin coat on a variety of substrates (Figure 30D). Such thin films show excellent structural characteristics, including small roughness, thickness tunability, compactness, and high crystallinity. Furthermore, telluride inks can be deposited on prepatterned substrates, filling grooves and vias with small lateral dimensions (Figure 30E).⁹²³,⁹²⁴ Therefore, molecular telluride inks are a convenient materials platform for PCM technology (Figure 30D).

Recently, a proof-of-concept memory device with an ink-based PCM layer has been demonstrated.⁹²³ The telluride ink can be drop-casted or spin-coated to infill the via opening and thus connect two buried planar electrodes. This device is then switched by tuning the amplitude and the length of the voltage pulse (Figure 30F), and the switching can be repeated many times while both resistivity states remain nonvolatile (Figure 30G). While this memory device shows several key properties, such as distinct resistivity contrast, cyclability, and low power consumption, further improvement of endurance and aging
properties of liquid-borne PCM devices are necessary to reach the standards of sputtered PCM chips.⁹¹³ In addition, the possibility to achieve sub-10 nm memory cell dimensions will become the key for widespread integration of ink-based PCM layers in the fabrication processes of memory chips.

## CONCLUSIONS AND OUTLOOK

In the past decade, the landscape of colloidal NC research has undergone a transformative evolution, pushing the boundaries of nanoscience and technology with breakthroughs that seemed unthinkable just a few years ago. These tiny structures have transcended their origins as lab curiosities to become pivotal components in fields as diverse as energy harvesting, optoelectronics, catalysis, and quantum information science.

Exquisite control over the size and shape of NCs of various metals, semiconductors, magnetic, and upconverting materials has been achieved. Some of these materials, such as luminescent QDs, are nowadays synthesized on an industrial kg-scale for display applications. At the same time, oxide nanoparticles are used in sunscreens, composites, and other products. NC syntheses have been extended to material systems with ever-increasing complexity. The latter includes randomized complex compositions, yielding high configurational entropy in high-entropy materials (HEMs), chiefly metals and metal oxides. The other exciting avenue toward highly entropic materials is to leverage extreme structural dynamics, as in metal-halide perovskite NCs. These QDs have challenged the ethos of the field, whereby the antibonding valence band and overall structural softness give rise to an unexpectedly clean electronic behavior, coined as defect tolerance.

One major focus has been on unveiling mechanisms that underpin the NC formation, with the ultimate goal of devising universal synthesis approaches. Adapting the concept of retrosynthesis from organic chemistry, the idea is to decompose the target NCs into simpler building blocks, mapping out the chemical pathways, reaction conditions, and reagents required. Improved *in situ* characterization techniques, providing real-time information on the NC formation process, emerge as crucial tools to obtain the much sought-after molecular-level understanding. Such studies are currently challenging established models of the NC growth and increasingly suggest that nonclassical nucleation is a rather common NC formation path.

NC self-assembly continues to trigger the imagination as a powerful, yet delicate, path to devise advanced materials. Notable strides have been made in the shape-controlled NC assembly and the development of *in situ* electron microscopy and X-ray scattering methods as accompanying diagnostic tools. Liquid-phase TEM, allowing real-time observation of NC dynamics at the nanoscale, has revealed complex, nonclassical crystallization pathways and intermediate phases during NC self-assembly. Despite the progress, challenges remain in minimizing artifacts from electron-beam interactions, better understanding the hydrodynamic properties of liquid-cell TEM systems, and replicating *ex situ* self-assembly conditions.

The remarkable innovations in surface chemistry have been the gateway to the development of NC-based gels, 2D patterning, and 3D printing of NCs. The assembly of NC gels has enabled the construction of porous networks with remarkable versatility in applications. Dynamic bonding between the particles renders the gels reconfigurable and

responsive to diverse stimuli, such as temperature or chemical changes. There are abundant possibilities when fine-tuning NC gel characteristics and harnessing their capabilities. However, creating spatially uniform, stable equilibrium gels with NC building blocks still presents a major challenge.

High-fidelity direct optical lithography of NCs into high-resolution 2D patterns has been accomplished through photochemically active ligands or linkers. This advance has enabled integration of NCs into complex and multilayered device architectures. The major undertakings for the years to come are to further improve spatial resolution, especially below the 100 nm scale, optimize NC stability under various processing conditions, and expand the range of materials that can be patterned with these techniques.

Beyond 2D patterning, the programmable assembly of 3D structures with nanoscale resolution has been accomplished by using photosensitive ligands or molecular additives. Semiconductor, metal, and metal-oxide NCs have been printed into 3D functional structures through light-induced chemical reactions, forming bonds between the NCs through their capping ligands. NC 3D printing has brought the vision of fully 3D printing all the device components closer to reality. However, there are still considerable hurdles in achieving high material purity, strong mechanical integrity, and efficient charge and exciton transport in printed NC structures.

In the last four decades since the conception of colloidal QDs, optoelectronic applications have been at the forefront of QD research, including displays, LEDs, lasers, photovoltaics, and IR imaging and sensing. Successful large-scale commercialization has already been achieved for InP-based QDs (e.g., by Samsung), CdSe-based QDs (e.g., by TCL), and lead-halide-perovskite-based QDs (e.g., by Avantama), particularly in the context of display applications, where such QDs enable wide color gamut, high brightness, and high energy efficiency. Presently, increasing research activities are foreshadowing near-term market entry also for several other NC material platforms, such as Zn-chalcogenides QDs (as blue emitters, e.g., for LEDs). However, several challenges remain in advancing the commercial competitiveness of QD-based light-emitting applications, including addressing the toxicity of some QD materials, improving their long-term device stability and efficiency, especially for blue-emitting QDs and when incorporated in LEDs and lasers, and scaling up production while maintaining quality.

In the wake of the prominent QD display and LED developments, QD lasing has also been blessed with substantial progress in the past decade. Among the noteworthy recent developments is ASE in electrically stimulated colloidal QDs, bringing the field closer to realizing electrically pumped laser diodes. Future work should focus on achieving stable and efficient electrically driven QD lasing for practical applications, particularly in the NIR spectral region for telecommunications and integrated photonics.

More limited progress has been made with QD-based solar cells, which were once at the epicenter of NC research as a potential cost-effective alternative to the established semiconductor thin-film technology. However, the slow progress in efficiency, problems with stability, and concerns regarding production scalability, all accompanied by strong competition from emerging technologies, have diluted researchers' efforts and turned the attention to alternative photovoltaic materials like (bulk) perovskite thin films, providing comparable benefits with fewer limitations.
Emerging as a more promising light-absorbing technology are colloidal QD photodetectors, especially in the field of IR imaging and sensing, with lead and mercury chalcogenide QDs as the main contender in industry, commercialized, e.g., by Emberion⁹³² and Quantum Solutions.⁹³² These detectors now cover a broad spectral range, extending from the visible to the SWIR and mid-IR regions, with enhanced quantum efficiency and detectivity, rendering them competitive with traditional semiconductor devices regarding sensitivity and noise performance. Combined with the ability to produce large-area sensors at low cost, QDs hold the promise to induce a paradigm shift in SWIR detection. Future research will focus on ensuring long-term stability, reducing noise levels, developing scalable fabrication techniques, and minimizing the adverse environmental impact of heavy-metal-based QDs.

Transcending the many classical optoelectronic applications studied early on, the past years have also seen colloidal QDs emerge as a promising candidate for quantum-light applications, either as a solution-processed quantum-light sources on their own, or as a controllable platform for hosting quantum-point defects with exciton and spin qubits. Similar to epitaxial QDs, the size-confinement effects in colloidal QDs facilitate single-photon emission, while the mature and versatile wet-chemical fabrication methods also allow fine control over the emission color and bestow solution processability. Single-photon purity at room temperature and high emission rates render InP and lead-halide perovskite QDs suitable for practical quantum applications. However, to successfully compete with various alternative quantum-light material platforms, massive efforts will be required. The community may further enhance the single-photon purity, spectral stability, and coherence times, as well as fully leverage their solution-processability as a powerful feature enabling versatile approaches for QD integration into nanophotonic device schemes.

In the realm of quantum science, colloidal NCs, with their ability to host quantum point defects, such as NV-centers, may enable the creation of qubits with both short-lived excitons for photon generation and long-lived spin states for quantum memory. However, several questions require attention; *inter alia*, the impact of surface states on quantum properties and the integration of NCs with existing technologies.

NCs have been pivotal players in catalytic applications due to their large surface-to-volume ratio and have gained significant attention due to their tunable size, shape, composition, and surface properties. These precise features present opportunities to control catalytic activity, selectivity, and stability, and to understand catalytic mechanisms at a fundamental level. Despite their potential, the stability of these materials under harsh catalytic conditions, such as high temperatures and pressures, is a concern. Engineering the NC-ligand interface and developing robust surface chemistries are critical areas for future research. Such efforts, both for metal- and semiconductor-NC catalysis platforms, may profit from developing suitable *in-operando* and single-particle-level characterization methods to obtain the mechanistic insights needed to formulate rational-design strategies.

Using NCs as precursors presents a promising alternative to traditional synthetic methods of thermoelectric materials by leveraging precise control over the NC's properties, such as size, shape, and composition, to tailor the resulting material composition and microstructure. However, converting NC-based powders into dense, high-performance thermoelectric

solids is challenging. The transformation involves complex processes requiring further research to understand and rationalize fully. While these significant questions are being answered, the ability to produce cost-effective, high-performance thermoelectric materials with customizable shapes and structures, especially through advanced techniques like 3D printing, makes NCs a compelling option for next-generation thermoelectric devices.

Integrating colloidal NCs into PCM technology allows for precise control over the size and distribution of memory materials. Size-dependent phase transitions can improve the energy efficiency and switching speed of PCM devices, making them suitable for applications requiring rapid data access.

Although this perspective has not delved into biomedical applications of colloidal NCs, it is important to acknowledge their significant impact in this field. NCs have been employed in drug-delivery systems to improve the solubility and bioavailability of poorly soluble drugs, allowing for controlled and targeted release. In imaging, QDs and other luminescent NCs have facilitated high-resolution cellular and *in vivo* imaging, providing deeper insights into biological processes at the molecular level. Additionally, NCs have found applications in biosensing, enabling ultrasensitive detection and theranostics, combining diagnosis and treatment in a single platform. For those interested in exploring the biological applications of colloidal NCs, we defer them to more bio-focused perspectives and reviews.

Finally, we suggest that the coming decade of nanoscience with NCs will see a further surge in computational tools for accelerated materials discovery, advanced materials characterization, and device optimization. While currently not yet as established as in fields such as protein design and protein structure prediction,⁹³³ an increasing number of NC researchers incorporate artificial intelligence (AI) tools into their everyday research endeavors, such as machine-learning (ML) algorithms, and increasingly also deep-learning methods. In the field of NC synthesis, ML methods may accelerate the tedious exploration of vast and often still poorly explored synthesis parameter spaces to achieve specific NC properties and functionalities.⁹³⁴ Such efforts are underway at various levels: many traditional human-centered research laboratories are incorporating simple tools to harness the existing low-data-volume data sets from either existing literature⁹³⁵ or their own research laboratories;²² simultaneously, few research groups are also starting to build and operate full-fledged robotic synthesis-and-characterization platforms for colloidal NCs, leveraging high-throughput experiments and deep-learning algorithms based on multilayered neural networks.⁹³⁶,⁹³⁷ In the field of NC characterization, an advantage is particularly expected for methods handling huge data volumes from multidimensional data sets, such as those generated from *in situ* or *operando* TEM, synchrotron-based studies, shot-to-shot analyses in laser spectroscopy, or real-time monitoring of device performance. In the field of computational chemistry, ML will bring a long-term target closer to reality: the accurate simulation of NCs with realistic size, shape, and ligand passivation over sufficiently long time scales, as required, e.g., to quantify and rationalize charge/energy transport, charge/energy recombination, or chemical reactions. For example, machine-learned force fields may achieve the high computational accuracy of quantum-chemical methods at the much-reduced computational cost of classical methods. Given the pace at which AI is entering our everyday life inside and
outside research laboratories, it may not be easy to predict precisely how, but certainly that, AI will impact the coming decade of nanoscience with NCs.

In conclusion, the progress made in colloidal NC research over the past decade has been truly remarkable, paving the way for innovative applications across various fields. The success of NC research is a testament not only to the perseverance of individual researchers but also to the vibrant, interdisciplinary collaborations that have driven (and will continue to shape) this field. These cross-disciplinary efforts, along with computational aid from techniques such as AI, are the only way to continuously stream breakthroughs and innovations in nanoscience with NCs.

## AUTHOR INFORMATION

### Corresponding Authors

Maria Ibáñez – Institute of Science and Technology Austria (ISTA), 3400 Klosterneuburg, Austria; ● orcid.org/0000-0001-5013-2843; Email: mibanez@ista.ac.at

Maksym V. Kovalenko – Institute of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zürich, 8093 Zürich, Switzerland; Laboratory of Thin Films and Photovoltaics, Empa-Swiss Federal Laboratories for Materials Science and Technology, 8600 Dübendorf, Switzerland; SKKU Institute of Energy Science and Technology (SIEST), Sungkyunkwan University (SKKU), Suwon, Gyeonggi-do 16419, Republic of Korea; ● orcid.org/0000-0002-6396-8938; Email: mvkovalenko@ethz.ch

### Authors

Simon C. Boehme – Institute of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zürich, 8093 Zürich, Switzerland; Laboratory of Thin Films and Photovoltaics, Empa-Swiss Federal Laboratories for Materials Science and Technology, 8600 Dübendorf, Switzerland; ● orcid.org/0000-0002-8399-5773

Raffaella Buonsanti – Laboratory of Nanochemistry for Energy (LNCE), Department of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne, CH-1950 Sion, Switzerland; ● orcid.org/0000-0002-6592-1869

Jonathan De Roo – Department of Chemistry, University of Basel, 4058 Basel, Switzerland; ● orcid.org/0000-0002-1264-9312

Delia J. Milliron – McKetta Department of Chemical Engineering, The University of Texas at Austin, Austin, Texas 78712, United States; Department of Chemistry, University of Texas at Austin, Austin, Texas 78712, United States; ● orcid.org/0000-0002-8737-451X

Sandrine Ithurria – Laboratoire de Physique et d'Etude des Matériaux, ESPCI-Paris, PSL Research University, 75005 Paris, France; ● orcid.org/0000-0002-4733-9883

Andrey L. Rogach – Department of Materials Science and Engineering, and Centre for Functional Photonics (CFP), City University of Hong Kong, Hong Kong, SAR 999077, P. R. China; ● orcid.org/0000-0002-8263-8141

Andreu Cabot – Catalonia Institute for Energy Research – IREC, Barcelona 08930, Spain; ICREA, 08010 Barcelona, Spain; ● orcid.org/0000-0002-7533-3251

Maksym Yarema – Chemistry and Materials Desing Group, Institute for Electronics, Department of Information Technology and Electrical Engineering, ETH Zurich, CH-

8092 Zürich, Switzerland; ● orcid.org/0000-0002-2006-2466

Brandi M. Cossairt – University of Washington, Seattle, Washington 98195-1700, United States; ● orcid.org/0000-0002-9891-3259

Peter Reiss – Univ. Grenoble Alpes, CEA, CNRS, IRIG-SyMMES, STEP, 38000 Grenoble, France; ● orcid.org/0000-0002-9563-238X

Dmitri V. Talapin – Department of Chemistry, James Franck Institute, and Pritzker School of Molecular Engineering, University of Chicago, Chicago, Illinois 60637, United States; ● orcid.org/0000-0002-6414-8587

Loredana Protesescu – Zernike Institute for Advanced Materials, University of Groningen, Groningen 9747AG, The Netherlands; ● orcid.org/0000-0002-9776-9881

Zeger Hens – Physics and Chemistry of Nanostructures, Department of Chemistry, Ghent University, B-9000 Gent, Belgium; Center for Nano- and Biophotonics, Ghent University, B-9052 Gent, Belgium; ● orcid.org/0000-0002-7041-3375

Ivan Infante – BCMaterials, Basque Center for Materials, Applications, and Nanostructures, Leioa 48940, Spain; Ikerbasque Basque Foundation for Science, Bilbao 48009, Spain; ● orcid.org/0000-0003-3467-9376

Maryna I. Bodnarchuk – Institute of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zürich, 8093 Zürich, Switzerland; Laboratory of Thin Films and Photovoltaics, Empa-Swiss Federal Laboratories for Materials Science and Technology, 8600 Dübendorf, Switzerland; ● orcid.org/0000-0001-6597-3266

Xingchen Ye – Department of Chemistry, Indiana University, Bloomington, Indiana 47405, United States; ● orcid.org/0000-0001-6851-2721

Yuanyuan Wang – State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, China; ● orcid.org/0000-0003-3971-8362

Hao Zhang – Department of Chemistry, Center for BioAnalytical Chemistry, Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical Biology (Ministry of Education), Tsinghua University, Beijing 100084, China; ● orcid.org/0000-0003-4513-0813

Emmanuel Lhuillier – Sorbonne Université, CNRS, Institut des NanoSciences de Paris, 75005 Paris, France; ● orcid.org/0000-0003-2582-1422

Victor I. Klimov – Nanotechnology and Advanced Spectroscopy Team, C-PCS, Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States; ● orcid.org/0000-0003-1158-3179

Hendrik Utzat – Department of Materials Science and Engineering, Stanford University, Palo Alto, California 94305, United States; ● orcid.org/0000-0003-2901-0141

Gabriele Rainò – Institute of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zürich, 8093 Zürich, Switzerland; Laboratory of Thin Films and Photovoltaics, Empa-Swiss Federal Laboratories for Materials Science and Technology, 8600 Dübendorf, Switzerland; ● orcid.org/0000-0002-2395-4937

Cherie R. Kagan – Electrical and Systems Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States; Department of Chemistry and Department of Materials Science and Engineering, University
of Pennsylvania, Philadelphia, Pennsylvania 19104, United States; ● orcid.org/0000-0001-6540-2009

**Matteo Cargnello** — Department of Chemical Engineering and SUNCAT Center for Interface Science and Catalysis, Stanford University, Stanford, California 94305, United States

**Jae Sung Son** — Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Gyeongsangbuk-do 37673, Republic of Korea; ● orcid.org/0000-0003-3498-9761

Complete contact information is available at:
https://pubs.acs.org/10.1021/acsnano.5c07838

### Notes

The authors declare no competing financial interest.

### ACKNOWLEDGMENTS

This article was inspired by the discussions and presentations at the NaNaX10 (Nanoscience with Nanocrystals) conference held in the Institute of Science and Technology of Austria (ISTA), July 3–7, 2023. M.I. acknowledges financial support from the Werner Siemens Foundation (WSS) and Abayomi Lawal, Christine Fiedler, Ihor Cherniukh, Francesco Milillo, Navita Jakhar, and Magali Lorion for all their help in editing this manuscript. M.I. would also like to acknowledge Christine Fiedler for the design of the TOC. S.C.B. acknowledges Dr. Dmitry Dirin for proofreading and the Weizmann-ETH Zurich Bridge Program for financial support. A.C. thanks Linlin Yang for drafting Figure 6 and acknowledges support from the project Sydecat with reference PID2022-136883OB-C22 under MCIN/AEI/10.13039/501100011033/FEDER, UE, and to the Departament de Recerca i Universitats of the Generalitat de Catalunya (2021 SGR 01581). M.C. acknowledges support from the Sloan Foundation, BASF Corporation, the Novo Nordisk Foundation CO₂ Research Center (CORC), and the US Department of Energy, Chemical Sciences, Geosciences and Biosciences Division of the Office of Basic Energy Sciences, via the SUNCAT Center for Interface Science and Catalysis. D.V.T. acknowledges support from the U.S. National Science Foundation under Grant Number CHE-2404291. V.I.K. acknowledges support by the Solar Photochemistry Program of the Chemical Sciences, Biosciences and Geosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy (overview of studies of spin-exchange interactions in Mn-doped QDs) and the Laboratory Directed Research and Development (LDRD) program at Los Alamos National Laboratory under project 20250443ER (overview of QD optical gain and lasing studies). E.L. acknowledges financial from the ERC grant blackQD (grant no. 756225) and AQDtive (grant no. 101086358), and from French state funds managed by the ANR through the grants Bright (ANR-21-CE24-0012-02), MixDferro (ANR-21-CE09-0029), Quicktera (ANR-22-CE09-0018), E-map (ANR-23-CE50-0025), DIRAC (ANR-24-ASM1-0001), camIR (ANR-24-CE42-2757), and Piquant (ANR-24-CE09-0786). L.P. acknowledges financial support from SOLAR NL, funded by the National Growth Fund in The Netherlands. G.R. acknowledges funding from the Swiss National Science Foundation (Grant No. 200021_192308, “Q-Light—Engineered Quantum Light Sources with Nanocrystal Assemblies”). P.R. acknowledges funding from European Union’s Horizon research and innovation program under grant

agreement 101135704 (HortiQD project) and from the French Research Agency ANR (grant ANR-24-CE09-0786-01 PIQUANT). A.L.R. acknowledges financial support from the Innovation and Technology Commission of Hong Kong (ITS/027/22MX), and from the Research Grant Council of Hong Kong SAR through the RGC Senior Research Fellow Scheme (SRFS 2324-1S04). J.S.S. acknowledges financial support from the National Research Foundation of Korea (NRF) grant funded by the Ministry of Science and ICT (2022R1A2C3009129). X.Y. acknowledges support from the U.S. National Science Foundation under awards DMR-2102526 and CBET-2223453. Y.W. acknowledges the support from the Science and Technology Program in Jiangsu Province (BK20232041) and the National Natural Science Foundation of China (22171132 and 52472165). M.Y. acknowledges funding by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme, grant agreement No. 852751. I.I., Z.H. and M.K. acknowledge the European Commission for funding (MSCAN Track The Twin, grant agreement 101168820). Z.H. acknowledges funding from the FWO-Vlaanderen (research projects G0B2921N and G0C5723N) and Ghent University (BOF-GOA 01G02124). H.Z. acknowledges W. Liu for editing Figure 19 and the financial support from Beijing Natural Science Foundation (JQ24003).

### ABBREVIATIONS

NC, nanocrystal; QD, quantum dot; PL, photoluminiscence; QY, quantum yield; TOPO, trioctylphosphine oxide; TOP, trioctylphosphine; NMR, nuclear magnetic resonance; XRD, X-ray diffraction; CNT, classical nucleation theory; NPL, nanoplatelet; NR, nanorod; UV-vis, ultraviolet visible; UV, ultraviolet; SAXS, Small-angle X-ray scattering; 2D, two-dimensional; ZB, zinc blende; ML, monolayer; 1D, one dimension; 3D, three-dimensional; IR, infrared; NIR, near-infrared; SWIR, short-wave infrared; LED, light emitting diode; MWIR, midwave infrared; ITO, indium tin oxide; BIC, bound states in the continuum; HEM, high-entropy material; HEA, high-entropy alloy; MR, metal-rich; $M_xB_y$, metal boride; B, boron; FA, formamidinium; MA, methylammonium; AZ, aziridinium; RP, Ruddlesden-Popper; SEM, scanning electron microscope; TB, tight binding; FF, force field; ML-FFs, machine learning-based force fields; SL, superlattices; DOLFIN, direct optical lithography of functional inorganic nanomaterials; PEB, photoexcitation-induced chemical bonding; MPA, 3-mercaptopropionic acid; ROIC, read-out integrated circuit; CMOS, complementary metal-oxide-semiconductor; ASE, amplified spontaneous emission; QLD, QD laser diodes; EL, electroluminescent; *j*, high current densities; BRW, Bragg reflection waveguide; DBR, distributed Bragg reflector; TLS, two-level system; SPE, single-photon emitter; SF, superfluorescence; NV-, nitrogen vacancy; AFM, atomic force microscope; PCM, phase-change memory; CM, carrier multiplication; SE, spin exchange; TA, transient absorption; AI, artificial intelligence; ML, machine learning

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