# Quantum dots-doped microlenses made by photolithography

Leila Issoufou Alfari, J. Houel, A. Belarouci, H. Sahib, S. Guy, F. Lerouge, M.

Leocmach, B. Mahler, A. Gassenq

► To cite this version:

Leila Issoufou Alfari, J. Houel, A. Belarouci, H. Sahib, S. Guy, et al. Quantum dots-doped microlenses made by photolithography. Optical Materials, 2025, 164, pp.117028. 10.1016/j.optmat.2025.117028. hal-05046244

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Submitted on 28 Apr 2025

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# Quantum dots-doped microlenses made by photolithography

L. Issoufou Alfari,<fnref n="1" /> J. Houel,<fnref n="1" /> A. Belarouci,<fnref n="2" /> H. Sahib,<fnref n="1" /> S. Guy,<fnref n="1" /> F. Lerouge,<fnref n="3" /> M. Leocmach<fnref n="1" />, B. Mahler<fnref n="1" />, A. Gassenq,<fnref n="1" />*

## Abstract

Microlenses doped with quantum dot emitters offer promising potential for photonics applications, but they require complex fabrication. In this work, we present a method to fabricate microlenses incorporating core/shell CdSe/CdₓZn₁-ₓS quantum dot red emitters using photolithography. The quantum dots were first synthesized and dispersed in propylene glycol monomethyl ether acetate solvent using suitable ligand exchange. This quantum dot-loaded solvent was then combined with a transparent commercial photoresist and patterned via ultraviolet lithography to create micro-disks with diameters ranging from 20 to 5 µm. The microstructures were then annealed, forming microlenses through the thermal reflow of the photoresist. Micro-photoluminescence and lifetime measurements confirm the homogeneous and efficient emission of the quantum dots within the microlenses. This work demonstrates the feasibility of fabricating microlenses doped with CdSe-based core/shell quantum dots through photolithography and thermal reflow, offering a straightforward approach for photonics applications.

**Keywords:** Photonics, Quantum dots, photoresist, photolithography, patterning, microlenses

**Highlights:**
- We incorporated CdSe/CdZnS core/shell quantum dots into a commercial photoresist.

- Microlenses doped with quantum dots were fabricated by photolithography.

- Quantum dot emission remains high in the microstructures due to the suitable ligand exchange.

## 1 - Introduction

Microlenses are ubiquitous in photonics applications. They are mainly used in microlenses arrays [1] for various applications such as imaging [2], sensing [3] and solar cell [4]. They can even improve the efficiency of light emitting diodes [5,6] by careful control of the microlenses shape [7], and they are also used for more fundamental studies with quantum dots [8] for single photon source applications [9]. Various methods have been developed for their fabrication, including etching [10], polydimethylsiloxane embossing [11–13], self-assembly [1], or 3D printing [14]. Among all these techniques, photoresist melting, studied since the 90's [15,16], appears as a very fast and cost-effective method [17] with a good control of the shape of the microlenses as well as their filling factor [1]. Microlenses are mainly made with polymer or SiO₂ materials [1] and the integration of active materials directly into the microlenses stays limited to few work like for magnetic applications [18]. Therefore, the integration of active emitters like quantum dots could be relevant for performing filtering function or wavelength conversion functionalities directly into the microlenses. Besides, photo-patterned Quantum Dots (QDs) [19] or QD inserted in photoresist have been also studied for multicolor emission and micro-display applications [20–23]. Note that QD layers can be also patterned using wet etching [24,25]. However, the fabrication of microlenses doped with quantum dots have never been reported to the best of our knowledge. Using photoresist thermal reflow [17], it requires careful control of the used chemistry to insert the QDs -that are both photo and thermal sensitive materials- into the microlenses using suitable ligand exchange [23,26,27].

In this work, we designed a process to integrate quantum dots into a commercial photosensitive resist for the fabrication of doped microlenses through photolithography and annealing. We

selected the AZ9260 photoresist because it enables the direct formation of microlenses via annealing after the lithography step [17] and exhibits transparency after UV exposure [28]. To mix the synthesized quantum dots with the photoresist, they were dispersed in Propylene Glycol Monomethyl Ether Acetate (PGMEA) solvent after phase transfer ligand exchange with mono-2-(methacryloyloxy) ethyl succinate (MES) ligand. Upon curing, the composite layers exhibit uniform emission without significant alterations to the quantum dots' optical properties. The doped photoresist was then patterned by UV lithography and annealing. The fluorescence and morphology of the microstructures were analyzed for different process conditions. Finally, photoluminescence mapping and lifetime measurements were conducted and compared with the initial quantum dots prior to ligand exchange.

## 2 - Material and methods

### 2.1 Investigation of the photoresist properties in the absence of quantum dots

The behavior of undoped AZ9260 photoresist [29] was initially investigated. To control the layer thickness, the photoresist was diluted in PGMEA solvent at volume ratios of 1:0, 4:1, and 2:1 (photoresist:solvent). These mixtures were spin-coated onto a 2 µm SiO₂ buffer layer on a silicon substrate at 3000 rpm and then annealed at 110°C for 2 minutes to evaporate the solvent (soft-bake). The samples were subsequently cleaved to observe their cross sections with a Nikon optical microscope. To evaluate the optical properties of the photoresist for photonics applications, light propagation within the pure layer was tested following UV exposure. This involved coupling light from a 532 nm laser diode into the cleaved layer with a microscope objective, focusing on one facet and collecting the guided light from the opposite facet to control that the light is well confined in the

<footnote n="1">¹ Université Claude Bernard Lyon 1, Institut Lumière-Matière UMR5306 CNRS, F-69622 Villeurbanne, France</footnote>

<footnote n="2">² Institut des Nanotechnologies de Lyon, UMR 5270, CNRS, ECL, INSA, UCBL, F-69134, LYON, France.</footnote>

<footnote n="3">³ Laboratoire de Chimie, Ecole Normale Supérieure de Lyon, CNRS, UCBL, UMR5182, F-69364 LYON, France</footnote>

<footnote>* Corresponding author: alban.gassenq@univ-lyon1.fr</footnote>
photoresist layers. A top-view image is then analyzed to evaluate light absorption along the pathway and calculate propagation losses using the ImageJ software. More details about such measurement setup is given in ref [30,31].

<figure><img src="image_2.png" /><figcaption>Figure 1: Process flow for the microlenses fabrication</figcaption></figure>

AZ9260 microstructures were first investigated without quantum dots. Figure 1 illustrates the process flow used for fabricating microlenses in AZ9260 photoresist. The process begins with the application of hexamethyldisilazane (HMDS) adhesion promoter, which is spin-coated at 3000 rpm onto a 2 µm SiO₂ layer on Si substrates to prepare the surface. The photoresist is then spin-coated at the same speed and annealed at 110°C for 2 minutes to evaporate the solvent. The layers are subsequently hydrated by immersing them in water for 5 minutes. The photoresist layer is exposed to 405 nm UV light using µPG101 laser lithography at 3 mW, with the light focused on a 1 µm² surface area at a speed of approximately 1-2 cm/s [32]. The exposed photoresist is then developed using AZ300K developer diluted 1:2 in deionized water for 2 minutes. Finally, the microstructures undergo a 1-minute annealing at 120°C to achieve thermal reflow of the microlenses. We chose at minimal temperature of 120 °C needed to get melting for one minute without flattening the microlenses at higher temperature [17] since, based on our experience, 110 °C was not hot enough to get melting.

## 2.2 Synthesis of quantum dots in hexane

CdSe-based coreshell quantum dots are among the most studied colloidal QD system since the seminal work of Murray, Norris and Bawendi [33]. CdSe/CdZnS were the first commercialized core/shell systems along with InP/ZnSe/ZnS [34]. We, used such CdSe/CdZnS core/shell QDs because they are extremely stable and robust even after chemical surface modification [27,35,36], exhibiting a high absorption and an efficient emission [35,37]. These red-emitting CdSe/CdₓZn₁₋S quantum dots (QDs) were synthesized using a two-step, one-pot method adapted from the procedure described in references [27,35]. The process involved preparing a CdSe core followed by the growth of a thick CdₓZn₁₋S shell, all carried out in a single flask without intermediate purification. To synthesize the CdSe cores, 35 mL of 1-octadecene (ODE), 321 mg of cadmium oxide, and 1712 mg of myristic acid were combined in a three-neck flask. The mixture was degassed to 60 °C under vacuum for 10 min. The reaction vessel was then heated to 300 °C under an argon atmosphere until a clear solution formed, to form cadmium myristate in situ. At this point, 0.63 mL of 2 M trioctylphosphine selenide was injected, and the mixture was annealed for 3 minutes to produce the CdSe cores. For the growth of the CdₓZn₁₋S shell, 7.5 mL of 0.5 M zinc oleate in ODE and 0.6 mL of dodecanethiol were added to the reaction vessel, and the solution was annealed at 300°C for 30 minutes. Then we

proceeded to 5 successive slow injections of a mixture composed of 5 mL cadmium oleate at a concentration of 0.5 M in ODE, 10 mL of zinc oleate at a concentration of 0.5 M in ODE and 3.8 mL of trioctylphosphine sulfide at a concentration of 2 M, followed by a holding time of 10 min. After the last annealing, the temperature of the reactor was lowered to room temperature. The resulting CdSe/CdxZnₓ₋S QDs were precipitated by adding 15 mL of ethanol and centrifugation at 10000 rpm for 10 minutes then washed 5 times with a solvent combination of 10 mL of hexane and 15 mL of ethanol by centrifugation at 10000 rpm for 10 min. The purified QDs were redispersed in hexane for ligand exchange and optical characterizations. The final QDs, consisting of a CdSe core and a gradient CdₓZn₁₋S shell, were initially capped with oleate ligands in the hexane solvent (see Figure 2-a). Transmission Electron Microscopy (TEM) images of the QDs were acquired on a JEOL JEM-2100 TEM equipped with a LaB6 electron source, operating at 200 kV. QD absorption evaluations were performed with an Avantes Avaspec fibered spectrometer coupled with an Avantes BAL-Avalight white light source.

## 2.3 Quantum dots insertion into the photoresist

As the oleate ligands capping the final nanoparticles did not allow proper dispersion in PGMEA solvent, a ligand exchange was necessary to first stabilize the QDs in the solvent and then further in the photoresist. The selected ligand for this exchange is mono-2-(Methacryloyloxy) Ethyl Succinate (MES), which contains a carboxylate head capable of replacing the oleate on the QD surface, as well as an acrylic tail that is compatible with the PGMEA solvent and polymerizable. The ligand exchange procedure follows the method outlined in [27]. This process involves transferring the QDs from a hexane phase, where they were initially stored, to a pure MES phase. First 1mL of the synthesized QDs stored in hexane were mixed with 1mL of pure MES solution. Given that MES is immiscible with hexane, this created a biphasic mixture. After 5 min of vigorous stirring with a vortex mixer the QDs slowly began to transfer into the MES phase. When the transfer was complete, 0.5 mL of ethanol was added to the mixture to reduce the viscosity of the MES solution. The mixture was then centrifuged at 10 000 rpm for 5 minutes to precipitate the ligand exchanged QDs which were subsequently redispersed in ethanol or in PGMEA (Figure 2-b). To assess the ligand exchange, infrared spectra were measured with Shimadzu Fourier Transform Infra-Red (FTIR) spectrometer using an Attenuated Total Reflectance (ATR). For that measurement, droplets of the QD dispersion was deposited on top of the ATR crystal, let it to dry, and the measurement was carried out after solvent evaporation. QDs dispersed in hexane with their native oleic acid ligands were compared to QDs in ethanol after the MES ligand exchange. Furthermore, before incorporating the QDs into the photoresist, Photo-Luminescence (PL), and lifetime measurements were conducted on QDs dispersed in both hexane and PGMEA using the PL setup described in [38]. For such measurements, a drop of each solution was deposited on a microscope coverslip. Finally, for fluorescence quantum yield evaluation, an Edinburgh Instruments FS5 spectrometer with an integration sphere (350 nm excitation wavelength) was used before and after ligand exchange.
<figure><img src="image_3.png" /><figcaption>Figure 2 : Ligand exchange in CdSe quantum dots from a) hexane to b) PGMEA solvent</figcaption></figure>

## 2.4 Fabrication of microstructures doped with quantum dots

For integrating the QDs in the photoresist, the photoresist was mixed with QD doped PGMEA at c<sub>QD</sub> = 8x10<sup>-8</sup> mol/L in a 1:9 volume ratio (PGMEA:AZ9260). The doped photoresists were spin-coated onto a SiO<sub>2</sub> on Si substrate giving thin films of 5µm thickness. This deposition was carried out either two hours after ligand exchange (referred to as "fresh" QDs in the following discussions) or two weeks later (referred to as "old" QDs). Quantum yield and macro PL measurements were conducted on the deposited layers to compare those without QD, with fresh QDs, and with old QDs, using a FL5 spectro-fluorometer equipped with an integration sphere. For the photolithography, we used the same exposition process, as described previously in Figure 1. Various recipes were tested, including (1) no QD, (2) old QDs, and (3) fresh QDs. For photoresists containing fresh QDs, a longer soaking time with AZ300K pure developer was necessary due to the altered chemistry of the doped layer. After lithography, microlenses were created through thermal reflow at 120 °C, following the same procedure shown in Figure 1 for cases without QD. By taking into account that the solvent makes about 50 % in the liquid state of the resist and is still present in the photoresist film with few percent after the soft-bake [29], we can evaluate the QD loading at x<sub>QD</sub> = 0.01% in weight in the photoresist. For this calculation we first used Equation 1 to determine n<sub>mCds</sub> the number of CdS atoms in a QD (as detailed in Figure 3), considering that the QDs primarily consist of CdS atoms [27].

$$n_{mCds} = n \cdot \frac{V_{QD}}{V_a} = n \cdot \left(\frac{4}{3} \cdot \pi \cdot \frac{r_{QD}^3}{a_{Cds}^3}\right) \qquad \text{Equation 1}$$

<figure><img src="image_4.png" /><figcaption>Figure 3 : Schematic representation of the QD for the calculation of the number of CdS atoms in a QD</figcaption></figure>

with n=4 the number of CdS per unit cell, V<sub>QD</sub>= 1767 nm<sup>3</sup> the volume of the quantum dots calculated with r<sub>QD</sub> the averaged radius of the QD (7.5 nm in Figure 4-a), and V<sub>a</sub> = 0.205 nm<sup>3</sup> the volume of the CdS unit cell crystal calculated with a<sub>Cds</sub>=0.59 nm

the lattice parameter for CdS crystal resulting in n<sub>mCds</sub> =49,916 the number of CdS atoms in the QD. We then calculated x<sub>QD</sub> the QD loading in weight in the photoresist with Equation 2.

$$x_{QD} = \frac{x_{mQD}}{\rho_{PGMEA}} = c_{QD \text{ after}} \cdot \frac{M_{QD}}{\rho_{PGMEA}} = x_{DP} \cdot c_{QD} \cdot \frac{n_{mCds} \cdot M_{Cds}}{\rho_{PGMEA}} \qquad \text{Equation 2}$$

with ρ<sub>PGMEA</sub>= 0.96 g/cm<sup>3</sup> the PGMEA density, x<sub>mQD</sub> the mass concentration calculated with c<sub>QD after</sub> = 1.6x10<sup>-8</sup> mol/L = 1.6x10<sup>-12</sup> mol/cm<sup>3</sup> the QD concentration after solvent evaporation and M<sub>QD</sub>= 4.9x10<sup>6</sup> g/mol the QD molar mass. M<sub>Cds</sub> = 144 g/mol the CdS molar mass, x<sub>DP</sub>= 20 % the doped solvent content in the doped photoresist before evaporation, c<sub>QD</sub> = 8x10<sup>-8</sup> mol/L the QD concentration in the solvent.

## 2.5 Characterization of the layers and the microstructures doped with quantum dots

To characterize the fabricated microstructures, we employed a standard Nikon microscope with 50x magnification in both bright field and fluorescence modes (using green light illumination), a TESCAN Mira3 Scanning Electron Microscope (SEM) and a Nanosurf-EasyScan2 Atomic Force Microscope (AFM) with a TAP190ALG-10 probe in tapping mode. For the macro-PL on the un-structured layers, FS5 spectrometer was used. To determine whether the optical properties of the QDs were degraded in the microstructures compared to the initial QDs diluted in hexane, micro-PL and lifetime measurements were conducted after annealing. PL mapping was performed using a custom-built confocal microscope configured in an inverted geometry, detailed in reference [38]. The QDs were excited using a supercontinuum pulsed laser (NKT superK Fianium). The first output of the superK is coupled to the wavelength selection module from NKT, the superK Varia, which enables wavelength selection from 400 to 800 nm, with a bandwidth ranging from 5 to 100 nm. The superK was used at a center wavelength of 450 nm and a 5 nm bandwidth, at an excitation power of 50 nW. We chose these excitation parameters for strong absorption in the QDs. The laser was focused onto the sample with an oil-immersion microscope objective (Olympus x60) with a numerical aperture (NA) of 1.35. The PL was collected through the same objective and, focused into a single-mode fiber, with 10% of the photons directed to a spectrometer (Andor Shamrock 163) equipped with a CCD camera (Andor iVac), and the remaining 90% guided to a single-photon avalanche diode (SPAD, MPD20-C) for confocal imaging and lifetime measurements. The laser repetition rate was 78 MHz for imaging and 1.22 MHz for lifetime measurements.

# 3 - Results and discussion

## 3.1 Photoresist properties without quantum dots

Figure 4-a presents the cross section of the photoresist designed with various volume ratios of PGMEA: 1:0, 4:1 and 2:1. The undiluted photoresist exhibits a thickness of 8 µm, whereas a 2:1 dilution leads to a reduced thickness of 1.5 µm. This indicates that the thickness of the layers can be effectively controlled by adjusting the solvent content and thus the liquid viscosity, giving
similar layer composition after spinning since the solvent evaporate during the process [39]. Figure 4-b displays the top-view of the scattered light after propagation through an 8 µm-thick layer deposited on a 2 µm SiO₂ buffer. For this measurement, a 532 nm laser beam was coupled to the layer using an 20x optical microscope objective focused on a cleaved facet of the chip [30,31]. Figure 4-c, which shows the extracted intensity profile from Figure 4-b along the propagation length, reveals multimode losses of 1.0 dB/mm, demonstrating that this photoresist is suitable for photonics applications in the visible wavelength range.

<figure><img src="image_5.png" /><figcaption>Figure 4: a) cross sectional view of the AZ9260 photoresist, diluted with different solvent ratio, deposited on a 2 µm SiO₂ layer on a silicon substrate; b) top view image of scattered light after propagation through the photoresist layer and c) Calculated optical losses from the propagation; d) SEM images showing the impact of annealing on AZ9260 microstructures, e) array of fabricated microlenses without quantum</figcaption></figure>

Figure 4-d illustrates the impact of the final processing step (as depicted in Figure 1), observed using a Scanning Electron Microscopy (SEM) on a sample tilted at 45°. Prior to annealing, the measured diameters closely match the dimensions specified in the GDS file used for lithography [32]. An aspect ratio of up to 1 and a minimum diameter of 8 µm were achieved. However, after annealing, the diameter increases while the thickness decreases due to the re-melting of the photoresist. Figure 4-e shows a SEM image of a microlenses matrix made on Si substrate, demonstrating the high reliability of the process in producing multiple microlenses with consistent diameters.

## 3.2 Synthesis of quantum dots in hexane

Figure 5-a shows the TEM image of the synthesized QDs. The size-distribution calculated over 100 QDs with image-J software is indicated in the inset of Figure 4-a, and lead to an average diameter of 15 nm. Figure 4-b displays the absorption and PL spectrum of the synthesized QDs in hexane. The QDs exhibit board absorption in the green and blue regions consistent with quantum-confined semiconducting materials [35]. They exhibit an emission peak at 640 nm with a Full Width at Half Maximum of 35 nm in good agreement with the absorption part, as

previously reported [27,35]. Additionally, the Quantum Yield (QY) of the QDs is found to be 65 %.

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /><figcaption>Figure 5: a) TEM image; b) absorbance and emission spectra of the CdSe/CdxZnₓS quantum dots in hexane</figcaption></figure>

## 3.3 Incorporation of quantum dots into the photoresist

Figure 6-a shows the FTIR spectra of the QDs before and after ligand exchange. Before ligand exchange, strong absorption peaks at 2900 cm⁻¹ (C-H stretching) are detected due to the presence of the long oleate carbon chain (see Figure 1). After ligand exchange, these peaks drastically decrease when C=O and C-O chemical bonds emerge, related to MES ligand (see Figure 1). Such measurements confirm that the ligand exchange was successful. Figure 7-b and c presents the PL (Figure 6-b) and lifetime measurements (Figure 6-c) conducted on QDs dispersed in hexane (before ligand exchange) or PGMEA (after ligand exchange). In Figure 6-b, the shape of the emission spectra remains unchanged despite the ligand exchange, indicating that the QD bandgap is not affected by the ligand exchange. Lifetimes experiments exhibit a complex, multi-exponential behavior, usual for these CdSe/CdZnS QDs [37], which changes, pointing to shorter decay times, on average. Complex dynamics arising from different highly emissive entities as the one presented in Figure 6-b can be described by an average lifetime, following the formula [40]:

$$ \tau > \frac{\int_{0}^{\infty} tI(t)dt}{\int_{0}^{\infty} I(t)dt} \qquad \text{Equation 3} $$

Where $I(t)$ is the lifetime curve. From this expression, we deduce an average lifetime for the QDs in hexane and in PMGEA of $<\tau>_{HX}$= 33.2 ns and $<\tau>_{PMGEA}$= 24.7 ns, respectively. This change in lifetime can be attributed to the refractive index
<figure><img src="image_8.png" /><figcaption>Figure 6 a) FTIR-ATR measurement before and after ligand exchange; b) Photoluminescence and c) lifetime measurements for QDs in hexane (before ligand exchange) or in PGMEA (after ligand exchange); d) images of the samples under visible or blue illumination</figcaption></figure>

change ($n_{hexane}$=1.375, $n_{PGMEA}$=1.402) and the introduction of additional non-radiative channels [37], consistent with the emission QY measured before ligand exchange (65%) in hexane and after ligand exchange in PGMEA (60%). Furthermore, we can compare the absorbance spectra of the quantum dot before and after ligand exchange (Figure 6-b). We notice that there is no changes in the absorbance after the ligand exchange which assures that the ligand exchange maintain the absorbance properties of the QD. The advantage of our system with a large shell is that even in harsh condition like the one used for the ligand exchange, the large shell of the QD preserve the luminescence properties. Figure 6-d presents a picture of a solution of the Oleate capped QD in hexane before ligand exchange and a solution of our ligand exchanged QD in the PEGMA solvent. Sample were illuminated under visible and blue light (Cômatec 395 nm wavelength lamp). Both solutions are transparent with a red emission witch assures of the good colloidal stability of the QD after ligand exchange in the PEGMA solvent.

The stability of the QDs in the PGMEA was evaluated at different aging times by imaging photoresist layers by imaging of the photoresists deposited on the substrate: Figure 7-a presents the top view of the photoresist layer without QD deposited on the substrate and a homogenous surface is observed. Figure 7-b and c show the layers formed by mixing the photoresist and loaded PGMEA either; two weeks after the ligand exchange (Figure 7-b) or only 2 hours afterwards (Figure 7-c). In the case of “Fresh” QDs (i.e. when the loaded solvent is mixed with the photoresist only 2 hours after the ligand exchange), a homogenous layer is observed comparable to the one without QDs. In the case of the “old” QDs (i.e. after 2 weeks), dark spots of various sizes are observed on the layer proving that the QDs are not well dispersed and tend to aggregate. This suggests that a long-term chemical reaction occurs when the functionalized QDs remain in the PGMEA solvent, destabilizing the dispersion. This long-term behavior could be possibly linked to a polymerization process appearing between the acrylates group present at the tail of the MES ligands. Such hypothesis was supported by the observation of the optical change (transparent to diffusing solution) after several days when the QD stays in PGMEA, indicating aggregation. However, further analysis is required to understand

the real long-term dynamic of the system. Anyway, we can note that such long term reaction is stopped when the QDs are embedded in the photoresist since the solvent have been evaporated and their mobility is restrained inside the photoresist. Lastly, Figure 7-d shows macro-PL in coated layers with or without fresh QDs incorporated into the photoresist and annealed at 120 °C for 1 minute. The QD emission is clearly observed peaking at 640 nm, as expected.

<figure><img src="image_9.png" /><figcaption>Figure 7: Impact of the quantum dots incorporation on top view imaging a) without quantum dots, with b) old or c) fresh quantum dots; d) macro-PL measurements in photoresist layers with and without quantum dots</figcaption></figure>

### 3.4 Characterization of the quantum dots doped microstructures

Figure 8 presents top view images for the patterns without QD (Figure 8-a), with old (Figure 8-b) or fresh QDs (Figure 8-c) in both bright field and fluorescence imaging. In the case of the old QDs, red aggregates are visible in both imaging modes, indicating poor dispersion and confirming that the QDs are unstable in PGMEA solvent after long-term storage (i.e., 2 weeks). In contrast, fluorescence imaging of the fresh QDs reveals a homogeneously dispersed pattern. Figure 7-d illustrates the effect of annealing on the morphology of the microstructures. Before annealing, the microstructures are 5 µm thick (compared to 8 µm in pure photoresist) due to the slight dilution of the photoresist in a 1:9 PGMEA solution for QD
doping. After annealing at 120 °C, micro-lenses are clearly visible with a final thickness of 3.5 µm. The top-view images show a grayscale gradient starting from the center to the border of the micro-lenses, corresponding to the out of plane reflectivity change caused by the spherical shape, as observed in the topography measured by AFM measurements.

<figure><img src="image_10.png" /><figcaption>Figure 8: Influence of QD insertion in the photoluminescence of the microstructure before annealing a) without QD, with b) old or c) fresh QDs; d) effect of the annealing on the microstructures with quantum dots.</figcaption></figure>

Figure 9-a presents the PL mapping for two microlenses and the un-patterned area adjacent to them, where a uniform emission is observed across the different structures. Figure 9-b provides a closer view of a 12 µm diameter microlens, confirming an even distribution of QDs within the micro-lenses. Indeed, the small fluorescence variation clearly show that we have no aggregation in the device since 0.01% QDs content by weight corresponds to an averaged distance between 2 QDs of about L<sub>QD</sub> = 500 nm giving only n<sub>QD</sub> = 20 QD/µm² calculated by Equation 4 and 5.

$$L_{QD} = V_{uc}^{\frac{1}{3}} = \left( V_{QD} \cdot \frac{\rho_{case}}{\rho_{PR}} \cdot \frac{1}{x_{QD}} \right)^{\frac{1}{3}} = \left( \frac{4}{3} \cdot \pi \cdot \left( \frac{d_{QD}}{2} \right)^{3} \cdot \frac{\rho_{case}}{\rho_{PR}} \cdot \frac{1}{x_{QD}} \right)^{\frac{1}{3}} \quad \text{Equation 4}$$

$$n_{QD} = \frac{d}{L_{QD}^{3}} \qquad \text{Equation 5}$$

With V<sub>uc</sub> = 5x10<sup>-6</sup> nm<sup>3</sup> the volume of the unit cell containing statistically only one QD, V<sub>QD</sub> = 276 nm<sup>3</sup> the QD volume, d<sub>QD</sub> = 15 nm the QD diameter (see Figure 4-a), ρ<sub>CDSe</sub> = 6 g/cm<sup>3</sup> the CdSe density and d = 2.5 µm the microlenses averaged thickness.

Figure 9-c displays the PL spectra at the center of the microlenses compared to the PL of QDs in hexane or PGMEA solvent, indicating no alteration in the emission profile confirmed by a stable device fluorescence after several months. Figure 9-d presents the lifetime measurements of the same samples, demonstrating that the incorporation of QDs into the photoresist does not affect the lifetime and thus the luminescence QY, despite the annealing at 120 °C. This points to the resin acting as a protective layer for the QDs.

## 4 – Conclusion

In conclusion, we have investigated doped microlenses incorporating CdSe-based core/shell quantum dots (QDs), fabricated via photolithography and annealing. The selected photoresist proved to be a suitable candidate due to its ability to control layer thickness through solvent dilution and its low propagation losses. Microlenses of various diameters were produced using thermal reflow post-lithography. Homogeneous incorporation of CdSe QDs was achieved by diluting the dispersed QDs in the photoresist immediately following ligand exchange. The annealing process was also examined, revealing no degradation in the optical properties of the QDs incorporated into the microlenses. This study demonstrates the feasibility of using photolithography to pattern CdSe-based core/shell QDs into transparent microstructures, such as microlenses for photonic applications. This work can be extended to other photoresist (like AZ10XT for 3D micro-structures, or Epoclad for low losses doped waveguides fabrications), multiple colors [41] and the full analysis of doped micro-lenses photoluminescence behavior as a function of the lenses curvature and size. More broadly, these QD in micro-lenses could enhance solar cell efficiency by improving the coupling of diffuse light and that of UV conversion through down-conversion schemes. In more speculative applications, single QDs coupled to micro-lenses could improve emission directivity for single-photon sources.

<figure><img src="image_11.png" /><figcaption>Figure 9: Micro photoluminescence: a) mapping of two microlenses, b) close-up of a 12 µm diameter lens, c) photoluminescence and d) lifetime and spectral measurements comparison between the microstructures and quantum dots embedded in the solvent</figcaption></figure>
## Acknowledgements

The authors would like to thank the NanoLyon platform and the Renatech network for the cleanroom facilities, and the Institute Light Matter for funding. L.Issoufou Alfari, B. Mahler and M. Leocmach acknowledge support from ANR Grant VitriPSA ANR-21-CE06-0025-02.

## Funding sources

This work was supported by the Institute Light Matter and the ANR Grant VitriPSA ANR-21-CE06-0025-02.

## CRediT authorship contribution statement

L. Issoufou Alfari : Investigation, Writing – review and editing,
J. Houel : Investigation, A. Bellaroussi : Conceptualization,
Investigation, H. Sahib : Investigation, S. Guy : Supervision, F.
Lerouge : Conceptualization, M. Leocmach Supervision, B.
Mahler, Supervision, Conceptualization, Resources, A.
Gassenq, Investigation, Supervision, Conceptualization,
Writing – original draft

## Data availability

Data will be made available on request.

## References

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