Mehrnaz Modaresialam

Gabrielle Bordelet

Zeinab Chehadi

Martin O'byrnen

Luc Favre

Magali Putero

Marco Abbarchi

Sol-gel TiO 2 nanoimprinted metasurface combined to hybrid-silica sensitive layers for selective VOC detection with high refractive index sensitivity

TOC VOC detection at sub-ppm levels, associated to a high RI variation sensitivity, was achieved by direct reflection on a surface composed of sol-gel organic-inorganic microporous sensitive layers embedding an array of 140 nm TiO2 pillar antennas prepared by sol-gel nanoimprint. Such a performance was attributed to the electromagnetic confinement within the Titania pillars, enhancing the interaction of the radiation with the sensitive media. The system also shows a relatively good selectivity by comparing the adsorption of five different VOCs with various polarity indexes within six different sensitive sol-gel layers.

Introduction

Refractive Index (RI) sensitive materials are critical elements for environmental analysis through optical transduction in chemical, biological, and physical sensors. They rely on variations of electromagnetic confinement in metals (Localized Surface Plasmonic Resonances, LSPR) or dielectrics (Fabry-Perot interferences, Mie resonances) upon small changes of RI in the confinement volume of the electromagnetic waves. [1], [2] Size and geometry of micro-and nanostructures are thus designed to enhance the electromagnetic resonances in the regions where the RI changes. In chemical sensing, such a variation is induced at the surface or inside the material (if porous) through the modification of the chemical environment triggered by combination of a target molecule with a specific host chemical group or simply by adsorption. [3] Gas sensing is a category of detector that suffers from a high competitivity of adsorption between the molecules composing the air. For instance, Volatile Organic Compounds (VOC) are more or less toxic molecules produced by different human activities or nature. There are many of them and they cohabit in the air at low concentrations (e.g. ppm levels) [4] with water that is the most critical interferent due to its high concentration. These small molecules tend to interact easily with surfaces, rendering them easy to quantify as this phenomenon is governed by thermodynamic equilibrium. However, designing a sensor that is selective to one VOC is extremely challenging. [4] Chemical sensors based on optical transduction through RI variations rely on measuring either wavelength shifts or intensity variations (Reflectance R, Transmittance T, or Scattering S). Many different metasurfaces or metamaterials have been designed and studied to enhance the optical signal and increase the performance. Sensitivity S (𝑆 = 𝛥 𝜆 𝛥 ⁄ 𝑛 and Figure of Merit FOM (𝐹𝑂𝑀 = 𝑆/FWHM with FWHM being the Full Width at Half Maximum of the resonance) are indicators of performance considering both resonance sharpness and wavelength shift upon RI change. Alternatively, detection can be based on R, S or T intensity variation that are usually compared through the "alternative" Figure of Merit R* (𝑅 = (𝛥 𝐼 𝐼 ⁄ ) 𝛥 ⁄ 𝑛) given as R, S or T / RIU. Due to their relatively sharp LSPR and narrow wavelength-shift, gold or silver nanostructures (core-shell) nanoparticles [5] , pillars [6] , disks [7] , rods [8] , pyramids [9] , stars, etc., are the most attractive metasurfaces for RI sensing: sensitivity up to 2.10 3 nm / RIU (Refractive Index Unit), with a FOM of 100 [6] , have been experimentally evidenced with gold nanopillars, while simulations with gold nano shell systems revealed a potential sensitivity of 3.10 3 with a FOM around 10 [10] .

More recently, dielectric (Si, Ge, Metal oxides (MOn)) nanostructures have been considered as alternative RI sensitive materials due to their low toxicity, low cost and fabrication easiness. Interestingly, the possibility to select the intrinsic RI of some dielectrics, such as MOn, enables optimizing sensing within specific RI spans. The electromagnetic confinement in dielectric nano objects leads to a strong modifications of R, T and S intensity and can be highly sensitive to the surrounding RI [2] , [11] . First examples concerned combined dielectrics and metals [12,13] in complex architectures. For instance, FOM values up 6.7 x 10 3 are deduced from simulations with Au/SiO2-Al2O3 pillar systems. [14] Finally, Al/Al2O3 ring systems provided an experimental FOM of 6 for Δn = 0.6. [13] For pure dielectric systems, a R* of 6 was experimentally demonstrated for SiGeTe nano spheres [11] , whereas the highest value simulated for metal oxide (e.g., V2O5, TiO2) patterns reaches 13 [15] . The following Table 1 reports state-of-the-art FOM, R*, Q-factors and sensitivity for these classes of materials. [16][17][18][19][20] It reveals that high performances are achieved with complex metals or semiconductor (SC) architectures, while the more attractive ones have only being modelled without experimental corroborations. Detecting gas by optical transduction often requires a porous sensitive material with inner pore surface being covered with a chemical group that has a good affinity with the target molecules. These materials need to be highly porous, with a high specific surface area (micro-or meso-pores with high accessibility), robust and stable, easy to process as optical elements. [26][27][28] Among them, sol-gel based materials gather these features providing an excellent versatility to tune porosity, chemical composition and RI as thin coatings [29] .

Silica-based layers have been exploited in this work as sensitive materials with refractive index around RI0 ~ 1.5. Since VOC need to be detected at ppm levels (0 to 100 ppm) through adsorption, the porosity needs to be adapted so that the sensitivity is maximal, and ideally with a linear dependence on concentration, for extremely small RI increases from RI0 of the desorbed material.

As such, microporous materials with pore dimension < 2 nm are ideal since they exhibit a high specific surface area (high density of adsorption sites) enabling high adsorption at low relative vapor pressures (P/P0). Such pores can be further chemically adapted to tune the adsorption affinity with specific types of VOC molecules exhibiting different polarity index. Owing to the small size of VOC, forbidding an individual molecular identification, it is thus only possible to modulate the chemical affinity during adsorption by changing the polarity of the pore surface with organic functions that exhibit more or less polar characteristics.

Such materials are elaborated as thin films using simple high throughput chemical liquid deposition processes such as dip-coating [30] . They require further thermal curing, typically around 400°C to stabilize the inorganic backbone, but this treatment should be adapted to prevent decomposition of the organic function. Since such materials are never sensitive to a single VOC molecule and are also relatively insensitive to humidity, one may combine several of them in a single "artificial nose" device to provide several signals that would be analyzed using Machine-Learning technology.

The aim of this work is to demonstrate that the combination of all-dielectric metal oxides sensitive materials and metasurfaces, all prepared by simple high throughput sol-gel methods, can lead to relatively high sensitivity (1 ppm) and relatively good chemical selectivity for VOC sensing, using simple reflectivity transduction at visible frequency. First, several microporous hybrid silica (H-SiO2) layers have been elaborated with different organic contents and tested as adsorbents with common VOC, such as isopropanol, acetone, chloroform, hexane, toluene and water as adsorbates.

1 ppm of adsorbate in air can increase the layer RI by 10 -4 . Besides, adsorbent/adsorbate pairs showed relatively different affinities one to the other, which could be exploited in artificial noses to address the chemical selectivity. One of these pairs has then been selected and combined with a TiO2 metasurface to prepare a nanostructured composite sensitive layer. The metasurface is composed of TiO2 (Anatase) nano-pillars, and is obtained by Nano Imprint Lithography [31][32][33] and organized in a 2D hexagonal pattern on a Si wafer. It is then capped by the microporous Hybrid silica (H-SiO2) layer. The reflectivity of the layer has then been measured in the visible range with increasing vapor pressure. The measured sensing performances are: sensitivity S up to 4500 nm / RIU, reflectivity variation up to 15 R / RIU, FOM up to 12, with a Q-Factor of 4 for specific wavelength, which is compatible with sub-ppm gas detection by simple specular reflection. These performances have been compared to the theoretical responses of the corresponding Bruggeman Effective Medium Approximation (BEMA) model of the system, revealing the critical role of the pillar in exalting the signal. Structural characterizations and spectroscopy were performed by FTIR, HR-SEM, TEM, AFM, XRD, and ellipsometry.

Experimental section

Preparation of hybrid-silica sensitive plain coatings

All chemicals were purchased from Aldrich. In a typical procedure, TetraEthylOrthoSilicate (TEOS) and hybrid silanes precursors were dissolved in a mixture of 25 isopropanol: 11 water: 3 hydrochloric acid (molar ratio), with molar ratios of precursors provided in table 2. Solutions were aged 1 day at room temperature before using for film deposition (shelf-life between 2 and 10 weeks).

Table 2. Molar ratios of the chemical solutions precursor

Silicon wafer substrates were first cleaned with detergent, acetone and ethanol in a ultra-sound bath. Depositions were performed by dip-coating using an ACE-dip from Solgelway between 1 and 6 mm.s - 1 at low humidity (< 10%) to adjust the final thickness between 300 and 600 nm, depending on the solution viscosity. As-prepared coatings were then calcined at 400 °C for 10 min. Final refractive indices were between 1.42 and 1.57, depending on the film composition.

Preparation of Titanium dioxide plain film

Titania solution was prepared by mixing molar ratios of 1 TiCl4; 40 EtOH; 7 H2O; 2.10 -5 F127 (Pluronics F127: PEO-PPO-PEO) all purchased from Aldrich. After 24 h aging at room temperature, the solution was dip coated on Si wafer at 2 mm.s -1 withdrawal speed under relative humidity HR < 10%. The as-prepared film was either used directly for nanoimprint (see below) or calcined at 450 °C for 10 min under infrared lamps to yield a final thickness of 75 nm and a RI of n = 2.252 at 633 nm.

Preparation of S3M3P3/TiO2 nanostructured composite coating.

A commercial perfluoropolyether Fluorolink (MD700) master was first replicated in Perfluoropolyether (PFPE) to obtained the inversed mold. To do so, a few droplets of PFPE precursors were applied on the center of the master and spread homogenously using a solid roller. The mold was peeled off from the master after 10 min of curing under nitrogen and ultra-violet LED (365 nm, 4 W). The mold was then degassed for 10 min in primary vacuum. As-prepared titania sol-gel films were directly transferred from the dip-coater into a nano imprint chamber in which a 22 °C temperature and a 50% relative humidity were maintained using a constant 5L.min - 1 flux from a Solgelway humidity controller. The degassed PFPE mold was applied onto the solgel film and maintained at 60 °C for 15 min before demolding. then annealed at 450 °C for 10 min to ensure complete removal of the F127 polymer and crystallization. The microporous organic-inorganic S3M3P3 silica layer was coated on top of the TiO2 pattern by dip-coating at RT using 4 mm.s -1 . Then, the final nanostructured composite coating was annealed at 400 °C for 10 min for stiffening.

Structural and chemical analysis of the nanocomposite coating.

Structural investigation of the nanoimprinted samples was performed by atomic force microscopy (AFM, PSIA XE-100) and high-resolution scanning electron microscopy (HR-SEM) after Focused Ion Beam (FIB) milling (Dual-beam FIB FEI HELIOS 600 NanoLab). Focused Ion Beam etching was performed at 30 keV with Ga + ions. HR-SEM images were collected using secondary electrons with the in-lens detector at 5 keV. X-ray Diffraction (XRD) investigation was performed in order to check the crystalline phases of TiO2 plain films and metasurfaces. XRD patterns were recorded on a conventional diffractometer (PANalytical Empyrean) using Cu radiation ( = 0.154 nm), and a rapid detector (PANalytical PIXcel) in Bragg-Brentano geometry. Fourier Transform InfraRed spectroscopy (FTIR) spectra were collected in transmission mode directly on the sensitive layer deposited on Si wafers using a PerkinElmer (Sprectrum Two) spectrometer.

Spectroscopic investigations.

Thicknesses and refractive index of pure plain (non-imprinted) TiO2 and S3M3P3 coatings were determined with a Spectroscopic Ellipsometry (Woollan M2000V) instrument and a Cauchy model for transparent materials. The variation of RI of the S3M3P3 sensitive coatings with respect to gas adsorption was followed in situ using an environmental chamber. The gas was selected to be iPrOH for its high volatility and its good affinity with the layer. Its relative pressure (P/P0) was adjusted using an air-dilutor system, mixing both iPrOH saturated and pure airs. Normalized reflected intensity spectra on the nanostructure composite layer were collected at 45° incident angle, between 380 and 1000 nm for different values of P/P0 (1.45 < nS3M3P3 < 1.5). The theoretical reflectivity of the Bruggeman Effective Medium Approximation (BEMA) model (see Figure 1), corresponding to the nanocomposite system, was generated from the CompleteEASE software of the ellipsometer using experimental structural data and 45° incident angle.

Results and discussion

Investigation of gas adsorption in plain sensitive layers.

First, plain layers of sensitive H-SiO2 materials were tested in different VOC vapor environments at 0% humidity to avoid perturbation due to adsorbed water. The refractive index variations 𝛿n were measured using spectroscopic ellipsometry. Figure 2a) provides the adsorption isotherm of isopropanol in the S3M3P3 sensitive layer, confirming the typical Langmuir behavior for microporous materials, following equation ( 1) for which the fit parameters were found to be 𝛿 𝑛𝑚𝑎𝑥 = 0.044 and the affinity constant b = 22. Using PIPrOHsat = 2 kPa, one can then deduce equation ( 2) that links the measured 𝛿 𝑛 to the concentration in ppm of adsorbate in the environment. To detect 2 ppm of iPrOH in air at 1 Atm and 20 °C, one needs to be able to detect a 𝛿 𝑛 = 10 -4 . Figure 2b) shows the evolution of 𝛿 𝑛 measured by ellipsometry on a S4T6 and a S3M3P3 layers in presence of small concentrations of n-hexane at 0% humidity. One remarks first that 𝛿 𝑛 = 10 -4 is the resolution of the ellipsometer. It will be demonstrated in the second part of the article that subppm detection is achievable with a much simpler method than ellipsometry. Both films do not show the same RI variation for the same concentration of vapor. Such significant differences in adsorbate/adsorbent chemical affinity is attributed to the different chemical composition and porosity of the two layers. The porosity of the films cannot be directly compared, but one can verify the presence of the introduced chemical functions from FTIR spectroscopy as in Figure 3A). It is clear that, even if calcined at 400 °C, the films still embed the methyl and the aromatic functions introduced as precursors. They should thus be accessible as adsorption sites to the VOC adsorbates through the porosity and must play a crucial role in the chemical affinity difference. To go further, when testing the SiO2 reference (REF) and the five different H-SiO2 layers with the five different VOCs at a concentration of 600 ppm, and with 100% humidity (saturation), one observes that each pair present a specific affinity that is due to the difference in the layer chemical and porosity characteristics and in the VOC index of polarity (see Figure 3B)). As expected, the REF layer mainly adsorbs water since it contains no organics, but only highly polar Si-OH groups on the pore walls. Such reference layer acts thus as a water-selective sensitive layer, that is used to assess humidity between 0 and 100% and to correct the water up-take into the other hydrophobic layers. Indeed, the other H-SiO2 layers are more prone to interact with VOCs, since their adsorption of water at 100% RH is much lower than the adsorption of VOCs at 600ppm. This suggests that the less polar methyl and unsaturated phenyl, bi-phenyl, tolyl and naphthyl groups are pendent and occupy most accessible adsorption sites at the pore wall surface. It has been demonstrated that when methyl groups are attached to a Si-O-Si backbone, they tend to position at the interface with the pore nanocavities upon thermal annealing without decomposing due to the stability of the Si-C bond below 450 °C in air [34] . The same behavior is verified for the unsaturated rings by FTIR investigations.

Since all H-SiO2 layers do not adsorb water with the same affinity, it occurs that porosity and distribution of polar OH and less-polar P, T, M, PP, Np groups are different for each layer, certainly due to the influence of the organics on the condensation of the silica network during solution ageing, film drying, and thermal treatment. S4T6 layer adsorbs water and the five VOCs with relatively similar affinity. It thus acts as a "universal" adsorber in the present work. The four remaining H-SiO2 shows relative different affinities with the VOC and will serve for differencing adsorbates (VOC with low polarity index being favorably adsorbed when high concentration of hydrophobic functions are present). However, to do so, competitive adsorptions of VOCs in H-SiO2 layer must be as low as possible, especially with water. When two of the selected vapors are injected in the chamber at the same time, such as for example iPrOH and water, they must not interfere, at least at low pressures. This has been confirmed in Figure 4 that shows the evolution of 𝛿 𝑛 in the S3M3P3 layer when submitted successively to 100, 220, and 350 ppm of isopropanol, together with 0, 30, 60 and 90% of relative humidity. It is clear from the results that whatever the relative humidity, the RI increasing for a given VOC concentration is constant, and add to the 𝛿 𝑛 induced by the water uptake. One may carefully mention at this stage that adsorption sites are relatively specific in such H-SiO2 microporous materials. These results confirm that they are partially selective to specific types of vapors, and can be combined together into an optical sensor to design an artificial nose. A dip study of selectivity in such materials is somewhat extremely interesting but is out of the scope of the paper, that will concentrate in the following second part on sensitivity enhancement.

Time / s

Performances of nanostructured sensitive layer assessed in direct specular reflection

While ellipsometry has been used in the previous part to assess RI variations, the following par will focus on a simpler method based on specular reflectivity to asses VOC concentration in atmosphere. To do so, only one adsorbate/adsorbent pair will be used to investigate performances in sensitivity, since the transduction exploits RI variations. The S3M3P3/isopropanol pair has been selected as representative of all other possible pairs (see Figure 3). Experiments were conducted on a sensitive coating composed of a high index TiO2 metasurface, elaborated on Si wafer and embedded into a layer of the S3M3P3 material. The role of the metasurface is to confine the visible electromagnetic radiations inside the high index nanofeatures to enhance their interaction with the surrounding sensitive material. The incident and collection angles were fixed at 45° as this configuration provided a good penetration of the mixed SP polarized light into the nanostructure coating (Brewster angle ≈ 55°), while collecting high enough reflected intensity. To assess the effective influence of the metasurface, the experimental reflectance was then compared to the simulated reflectance of a corresponding 3 layers stack coating for which the nanostructured composite thickness of the experimental sample was replaced by a Bruggeman Effective Medium Approximation layer (see Figure 1). with 270 nm height and 140 nm width. A residual layer of about 10 to 20 nm thickness was spotted all over the Si wafer. After applying the S3M3P3 layer, the interpillar voids were totally filled up with the sensitive microporous material, while a plain layer of 190 nm thick extends above of the pillars. XRD investigations (Figure 5 d)) reveals that TiO2 plain layer and pillar array were composed of Tetragonal Anatase (see PDF) Titania with the characteristic 101, 004, 200, 105 and 211 diffraction peaks of poly-oriented crystal. Both diagrams show very similar peak features, peak width (FWHM) and intensity, suggesting that the nanoimprinting does not change significantly the overall structuration induced by the thermal annealing. Fitting the diffraction peaks allows to deduce FWHM which is inversely related to the crystal size (along the direction of the scattering vector i.e. the direction normal to the film, and neglecting microstrain contributions). Using the Scherrer formula reveals nanocrystals of 16±1 nm for the plain layer and 18±4nm for the pillars. This is an important statement since the refractive index of the pillars, used in the BEMA model (see below), is extracted from the ellipsometry analysis of the plain layer. The fact that the Anatase crystals have similar average dimension around 20 nm suggests that RI are significantly close in both geometries.

A simple geometrical calculation based on the pillar dimensions and distribution (assuming a cylindrical shape) provides a composite thickness of 20% Vol TiO2 and 80% Vol S3M3P3. The roughness of the top S3M3P3 layer was about 0.8 nm (RMS) from AFM analysis (not shown).

Refractive indices of TiO2 and S3M3P3 materials measured by ellipsometry on plain layers are 2.252 and 1.457 at 633 nm in dry air, respectively.

As studied in the first part, the refractive index of the sensitive layer increases with vapor pressure of isopropanol being introduced into the chamber, following a classical adsorption/desorption equilibrium behavior in microporous materials (Figure 2a). The vapor up-take is very large at lowpressure, going from 1.4570 to 1.4696 ± 10 -4 (RI) between P/P0 = 0 and 0.02. In the following investigations, only this range will be explored since for gas detection, sensitivity at low concentrations must be as high as possible (i.e. P/P0 = 0.02 corresponds to 850 ppm in iPrOH at 20°C). The variation of Titania refractive index for the same vapor pressure is insignificant. As for pure Titania system, the RI of the sensitive material in the nanostructured composite system cannot be measured and is thus assumed to be that one of the plain layer counterpart.

Specular reflectivity spectra for the nanostructured composite system between P/P0 = 0 and P/P0 = 0.02 (nS3M3P3 = 1.4570 and 1.4696, respectively) are plotted in Figure 6a). These are compared to the simulated reflection spectra of the corresponding theoretical BEMA system calculated for the same 𝛿 𝑛 increments, in Figure 6b). Both show a typical etalon interference trend that are relatively similar, especially at long wavelength. The lower intensity of the experimental signal at short wavelength (see Figure 6c)) is likely due to scattering effect of the pillars, this effect being attenuated at lower energy.

While the BEMA spectra undergo a global red shift of 6 nm around 670 nm, attributed to the S3M3P3 RI increase of 0.0126, a red shift of around 60 nm for the same interference is observed for the nanostructured system for the same 𝛿 𝑛 increment. The corresponding Q-factor is 4 at (𝜆 𝑚𝑎𝑥 = 675 nm) for both systems. This difference is attributed to the TiO2 metasurface antenna effect. Figure 6d) shows the plot of the wavelength of the interference maxima versus RI for the nanostructured system. A linear RI sensitivity of around 4500 nm / RIU is found up to RI = 1.463, which is extremely high and corresponds to a sensitivity of 0.2 nm/ppm of iPrOH vapor in the atmosphere. The corresponding FOM = 12. Such a good sensitivity corresponds to the S3M3P3/iPrOH pair. Higher sensitivity should be found for pairs with higher chemical affinity. Using optical transduction through spectral shift requires spectral analysis of the signal. More conveniently, RI variation detection can also be performed at a single wavelength using the reflected intensity collected by a photodiode. [35] In such ultimate simplified methods, the sensitivity if provided by the R* (equation ( 3)) for a given wavelength, and for a given RI span.

𝑅 (𝜆,𝑛 𝑖 ,𝛿𝑛) = (𝑅 𝜆,(𝑛 𝑖 +𝛿𝑛) -𝑅 𝜆,𝑛 𝑖 ) 𝛿𝑛

The dispersion of R* has been determined for both the experimental and simulated reflectivity variation for different incremental 𝛿n and for different initial RI comprised between 1.4570 and Nano structured composite d)

1.4665, which corresponds to the iPrOH concentration range 0 -250 ppm. Figure 7 provides different plots of the results. For the BEMA model, for which the Nano-structuration of the TiO2/S3M3P3 composite layer is not considered, R* has maximal absolute values between 1.5 and 3, resulting from the local reflected intensity variation induced by the red shift of the interference fringes. In the same conditions, the nanocomposite R* can achieve absolute values from 8 to 17, which is almost one order of magnitude higher than the R* of the BEMA model. These mapping clearly reveals that the R* can take values with drastic differences depending on the wavelength, but also on the initial RI (ni). Thus, in order to maximize the response, we select a wavelength of 408 nm for the BEMA model, and at 450 nm for the nanostructured system (in principle, narrow band illumination at about 450 nm can be provided with blue LEDs).

We now compare the maximal reflectance sensitivity provided in Figure 7a). The highest R* = 17 / RIU is found at 450 nm for ni = 1.4607 and nf = 1.4619. It drops to 8 between ni = 1.4562 and nf = 1.4578. For these two conditions the sensitivity corresponds to a 0.36 10 -3 R / ppm between 0 and

a) b) c) d)
35 ppm and 0.55 10 -3 R / ppm between 98 and 135 ppm. This suggests that if the analysis is to be performed at a specific wavelength such as 450 nm of a blue LED, the variation of R* has to be implemented in the signal treatment for the whole RI detection span through a careful calibration.

Conclusion

We reported that the presence of a dielectric metasurface (TiO2 pillar arrays) can significantly increase the sensitivity to RI variations in the surrounding medium in direct reflection up to 4500 nm / RIU. However, such high sensitivity can only be found in narrow conditions including wavelength, incident angle, RI and RI span. The metasurface has also been combined with microporous sol-gel layers to detect VOC by adsorption. Optimal sub-ppm sensitivity of 0.2 nm/ppm (spectral shift) and 0.55 10 -3 R / ppm (single wavelength reflectivity) were demonstrated. Even if Q-factor and FOM were relatively low, the sensitivities were sensibly higher than those reported for plasmon-based system and those simulated for pure metal oxide metasurfaces (see Table 1). We also showed that robust and stable silica-based hybrid layers composed of different pendent organic moieties on the inorganic backbone have different chemical affinity with different VOC. The combination of a TiO2 metasurface and sol-gel sensitive materials provide a good sensitivity and selectivity that constitutes critical transduction elements to design and elaborate a cost-effective "artificial nose" for VOC sensing. The present performances could be further increased through designing a more appropriated metasurface.

Figure 1 .

 1 Figure 1. Scheme representing the experimental nanostructured composite system (experimental) and the corresponding Bruggeman Effective Medium Approximation BEMA multilayer stack system (model) used as sensitive coatings for RI variation and VOC concentration detection. The variation of RI is induced by the controlled adsorption of isopropanol into the H-SiO2 material (layer).

Figure 2 .

 2 Figure 2. a) Adsorption isotherm of isopropanol in S3M3P3 sensitive plain layer, plotted as 𝛿 𝑛 , measured by spectroscopic ellipsometry, vs. iPrOH concentration. b) Variation of refractive index of S4T6 and S3M3P3 plain layers, measured by ellipsometry through exposition to different concentrations of isopropanol in dry air.

Figure 3 .

 3 Figure 3. A) FTIR spectra of sensitive layer showing regions of C-H stretching from methyl and aromatic functions, and regions of O-H stretching from water (High Density (liq.); Low Density (sol.)), and silanol. B) Comparison of chemical affinity measured on a set of sensitive layer adsorbent / VOC adsorbate pairs when submitted to 600 ppm of VOC or to 100% humidity (10 3 𝛿 𝑛 ).

Figure 4 .

 4 Figure 4. Variation of 𝛿 𝑛 measured by ellipsometry on the S3M3P3 plain sensitive layer when submitted to 100, 220 and 350 ppm of isopropanol and 0, 30, 60, and 90 % humidity.

Figure 5 .

 5 Figure 5. SEM images of a) and b) H-SiO2-S3M3P3 / TiO2 nanostructured composite, and c) of TiO2 nanostructured metasurface. d) XRD patterns of the TiO2 plain layer, the array of TiO2 pillars, and the H-SiO2-S3M3P3 / TiO2 nanostructured composite.

Figure

  Figure 5a-c) shows the SEM images of the nanostructured composite coating, revealing the array of TiO2 pillars homogeneously organized in a 2D hexagonal pattern. The pillar aspect ratio is ~ 2

Figure 6 .

 6 Figure 6. Reflectivity spectra measured at 45° incidence for the S3M3P3/TiO2 nanostructured composite, a) and c), and simulated for the corresponding BEMA model, b) and c), for different increments of 𝛿 𝑛 between 1.4570 and 1.4696. d) 𝜆 𝑚𝑎𝑥 vs. RI (sensitivity) extracted from a).

Figure 7 .

 7 Figure 7. a) and b) 2D mapping of R* dispersion values for different RI measured for the S3M3P3/TiO2 nanostructured composite system and simulated for the BEMA system, respectively. c) and d) R* dispersion and values extracted from a) and b) for both systems at an optimal wavelength of 450 and 408 nm.

Table 1 .

 1 Comparison of state-of-the-art sensitivities, FOM, R* and Q factor of different refractive indexbased sensors from the literature with those reported in this current work (Exp. and Sim. stand for experimental and simulated values, respectively).

	Materials	S nm/RIU	FOM	S,T,R / RIU Q-factor	References
	Metals (Au, Al)	Exp : up to 10 3	Exp: 10 to 110	-	-	[1,6,7,10,12,21,22]
		Sim. : up to 3.10 3	-	-	-
	SC (Si, Ge, Te)	Exp : 200 to 920	Exp: 2 to 120	Exp : 4 to 9	Exp : up to 10 3	[2,11,16,18,19,23]
		Sim. : 350 to 2.10 3	Sim. : 80 to 2.10 3	-	Sim. :up to 10 5
	Metal/MOn	Exp : up to 10 3	Exp : 6	-	Exp : 26-27	[14,22,25]
		Sim. : up to 2.10 3	Sim. : 500 to 6.10 3	-	-
	MOn (V2O5, TiO2)	Sim. : 10 to 800	Sim. : 732	Sim. : 13	-	[15,24]
	TiO2	Exp : 4500	Exp : 12	Exp : 17	Exp : 4	This work

  The imprinted sol-gel patterns were

	Solutions	Silane precursors
		(molar ratio Precursor / Si)
	REF	1 Tetraethoxysilane (S)
	S3M3P3 0.3 (S)
		0.33 Methyl triethoxysilane (M)
		0.34 Phenyltriethoxysilane (P)
	S4T6	0.4 (S)
		0.6 p-Tolyltrimethoxysilane (T)
	S3M3T4 0.3 (S)
		0.3 (M)
		0.4 (T)
	S4M3Np3 0.4 (S)
		0.3 (M)
		0.3 1-Naphtyl tri-methoxysilane (NP)
	S3M3PP3 0.3 (S)
		0.3 (M)
		0.3 4-bi-Phenylyl tri-ethoxysilane (PP)

Mis en forme : Anglais (États-Unis)

Acknowledgements

This material is based on the work supported in part by the National Science Foundation under the International Research Fellowship Program (grant OISE-0505786). The authors also acknowledge funding provided by CNRS ,UPMC and IUF (Institut Universitaire de France) , the PRCI network ULYSSES (no. ANR-15-CE24-0027-01) funded by the French ANR agency, the A*MIDEX foundation (ANR-11-IDEX-0001-02), the A*MIDEX project TITANIDE (no. A-M-AAP-EI-17-58-170228-16.21-ABBARCHI-SAT), and the FET-OPEN project NARCISO (no. 828890). We acknowledge the facilities of the NANOTECMAT platform at the IM2NP Institute. HR-SEM High Resolution Scanning Electron Microscopy TEM Transmission Electron Microscopy AFM Atomic Force Microscopy XRD X-Ray Diffraction RMS Roughness Mean Square

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Abbreviation


Marco Abbarchi
email: marco.abbarchi@im2np.fr

Takaaki Mano

Takashi Kuroda

Kazuaki Sakoda

Exciton dynamics in droplet epitaxial quantum dots grown on (311)A oriented substrates

Keywords: III-V Quantum dots, Droplet Epitaxy, exciton dynamics, (311)A oriented substrate

Introduction

The continuous development of droplet epitaxy (DE) [START_REF] Mantovani | Low density GaAs/ AlGaAs quantum dots grown by modified droplet epitaxy[END_REF][START_REF] Wu | Droplet epitaxy for advanced optoelectronic materials and devices[END_REF][START_REF] Gurioli | Droplet epitaxy of semiconductor nanostructures for quantum photonic devices[END_REF] as growth protocol for III-V-based semiconductor nanostructures enabled the fabrication of state-of-the-art devices such as lasers [START_REF] Mano | Lasing in Ga As/ Al Ga As self-assembled quantum dots[END_REF][START_REF] Mano | Ring-shaped GaAs quantum dot laser grown by droplet epitaxy: effects of post-growth annealing on structural and optical properties[END_REF][START_REF] Mano | As/ Al Ga As quantum dot laser fabricated on GaAs (311) A substrate by droplet epitaxy[END_REF][START_REF] Jo | Self-assembly of GaAs quantum wires grown on (311) A substrates by droplet epitaxy[END_REF][START_REF] Jo | Electrical Lasing in GaAs Quantum Dots Grown by Droplet Epitaxy[END_REF] and quantum emitters, including single photon sources [START_REF] Kuroda | Photon correlation in GaAs self-assembled quantum dots[END_REF][START_REF] Abbarchi | Photon antibunching in double quantum ring structures[END_REF][START_REF] Abbarchi | Bunched photon statistics of the spectrally diffusive photoluminescence of single self-assembled GaAs quantum dots[END_REF][START_REF] Benyoucef | Single-photon emission from single InGaAs/GaAs quantum dots grown by droplet epitaxy at high substrate temperature[END_REF][START_REF] Kumano | Stable and efficient collection of single photons emitted from a semiconductor quantum dot into a single-mode optical fiber[END_REF] and entangled photons [START_REF] Kuroda | Symmetric quantum dots as efficient sources of highly entangled photons: Violation of Bell's inequality without spectral and temporal filtering[END_REF][START_REF] Kumano | Nonlocal biphoton generation in a Werner state from a single semiconductor quantum dot[END_REF][START_REF] Basso | High-yield fabrication of entangled photon emitters for hybrid quantum networking using high-temperature droplet epitaxy[END_REF][START_REF] Ramírez | Effects of electrostatic environment on the electrically triggered production of entangled photon pairs from droplet epitaxial quantum dots[END_REF] with electrical injection [START_REF] Ha | Current-injection quantum-entangled-pair emitter using droplet epitaxial quantum dots on GaAs (111) A[END_REF]. The versatility of this method allowed to grow many different semiconductor alloys (GaInSb [START_REF] Kawazu | Effects of antimony flux on morphology and photoluminescence spectra of GaSb quantum dots formed on GaAs by droplet epitaxy[END_REF], AlGaAs [START_REF] Abbarchi | Exciton fine structure in strain-free GaAs/Al 0.3 Ga 0.7 As quantum dots: Extrinsic effects[END_REF][START_REF] Plumhof | Experimental investigation and modeling of the fine structure splitting of neutral excitons in strain-free GaAs/Al x Ga 1x As quantum dots[END_REF][START_REF] Mano | Self-assembly of symmetric GaAs quantum dots on (111) A substrates: Suppression of fine-structure splitting[END_REF][START_REF] Bouet | Charge tuning in [111] grown GaAs droplet quantum dots[END_REF][START_REF] Yeo | Comparative Chemico-Physical Analyses of Strain-Free GaAs/Al0. 3Ga0. 7As Quantum Dots Grown by Droplet Epitaxy[END_REF], InGaAs [START_REF] Saidi | Growth conditions effects on optical properties of InAs quantum dots grown by molecular beam epitaxy on GaAs (1 1 3) A substrate[END_REF][START_REF] Zuerbig | Growth-Temperature Dependence of Wetting Layer Formation in High Density InGaAs/GaAs Quantum Dot Structures Grown by Droplet Epitaxy[END_REF][START_REF] Skiba-Szymanska | Universal growth scheme for quantum dots with low fine-structure splitting at various emission wavelengths[END_REF][START_REF] Mano | Growth of metamorphic InGaAs on GaAs (111) a: counteracting lattice mismatch by inserting a thin InAs interlayer[END_REF][START_REF] Ha | Wavelength extension beyond 1.5 µm in symmetric InAs quantum dots grown on InP (111) A using droplet epitaxy[END_REF][START_REF] Fuster | InAs nanostructures grown by droplet epitaxy directly on InP (001) substrates[END_REF][START_REF] Sala | InAs/InP Quantum Dots in Etched Pits by Droplet Epitaxy in Metalorganic Vapor Phase Epitaxy[END_REF] InGaP [START_REF] Kumano | Stable and efficient collection of single photons emitted from a semiconductor quantum dot into a single-mode optical fiber[END_REF][START_REF] Prongjit | Type-II recombination dynamics of tensile-strained GaP quantum dots in GaAs grown by droplet epitaxy[END_REF][START_REF] Ha | Single photon emission from droplet epitaxial quantum dots in the standard telecom window around a wavelength of 1.55 µm[END_REF]), forming a plethora of nanostructures [START_REF] Bietti | Precise shape engineering of epitaxial quantum dots by growth kinetics[END_REF] such as quantum dots (QDs), multiple-concentric quantum rings [START_REF] Mano | Ring-shaped GaAs quantum dot laser grown by droplet epitaxy: effects of post-growth annealing on structural and optical properties[END_REF][START_REF] Abbarchi | Photon antibunching in double quantum ring structures[END_REF][START_REF] Mano | Nanometer-scale GaAs ring structure grown by droplet epitaxy[END_REF][START_REF] Mano | Self-assembly of concentric quantum double rings[END_REF][START_REF] Somaschini | Fabrication of multiple concentric nanoring structures[END_REF][START_REF] Shwartz | Concentric GaAs nanorings formation by droplet epitaxy-Monte Carlo simulation[END_REF][START_REF] Sanguinetti | Self-assembled semiconductor quantum ring complexes by droplet epitaxy: growth and physical properties[END_REF][START_REF] Heyn | Functionalization of Droplet Etching for Quantum Rings[END_REF], coupled structures such as ring-on-a-disk [START_REF] Somaschini | Self-assembled GaAs/AlGaAs coupled quantum ring-disk structures by droplet epitaxy[END_REF], dot in-a-ring [START_REF] Somaschini | Coupled quantum dot-ring structures by droplet epitaxy[END_REF] or dot-on-a-disk [START_REF] Elborg | Self-assembly of vertically aligned quantum ring-dot structure by Multiple Droplet Epitaxy[END_REF], as well as elongated structures such as nanowires [START_REF] Jo | Self-assembly of GaAs quantum wires grown on (311) A substrates by droplet epitaxy[END_REF]. This technique allows to independently tune size and density of the nanostructures [START_REF] Ohtake | Extremely high-and low-density of Ga droplets on GaAs {111} A, B: Surface-polarity dependence[END_REF] and to grow them with or without a wetting layer [START_REF] Jo | Self-assembly of GaAs quantum wires grown on (311) A substrates by droplet epitaxy[END_REF][START_REF] Mano | Self-assembly of symmetric GaAs quantum dots on (111) A substrates: Suppression of fine-structure splitting[END_REF][START_REF] Zuerbig | Growth-Temperature Dependence of Wetting Layer Formation in High Density InGaAs/GaAs Quantum Dot Structures Grown by Droplet Epitaxy[END_REF][START_REF] Skiba-Szymanska | Universal growth scheme for quantum dots with low fine-structure splitting at various emission wavelengths[END_REF][START_REF] Sanguinetti | Role of the wetting layer in the carrier relaxation in quantum dots[END_REF][START_REF] Mano | Self-assembled GaAs quantum dots coupled with GaAs wetting layer grown on GaAs (311) A by droplet epitaxy[END_REF][START_REF] Keizer | Structural atomic-scale analysis of GaAs/AlGaAs quantum wires and quantum dots grown by droplet epitaxy on a (311) A substrate[END_REF][START_REF] Keizer | Atomic-scale analysis of self-assembled quantum dots by cross-sectionalscanning, tunneling microscopy, and atom probe tomography[END_REF][START_REF] Shahzadeh | Wetting layer-assisted modification of in-plane-polarized transitions in strain-free GaAs/AlGaAs quantum dots[END_REF], aspects that are not matched by the conventional Stranski-Krastanov approach based on strain [START_REF] Sautter | Strain-driven quantum dot self-assembly by molecular beam epitaxy[END_REF].

Another aspect relevant for applications is the possibility to grow high-quality nanostructures on different substrate orientation, providing the ground for highly symmetric QDs on (111)A surfaces [START_REF] Kumano | Stable and efficient collection of single photons emitted from a semiconductor quantum dot into a single-mode optical fiber[END_REF][START_REF] Kuroda | Symmetric quantum dots as efficient sources of highly entangled photons: Violation of Bell's inequality without spectral and temporal filtering[END_REF][START_REF] Mano | Self-assembly of symmetric GaAs quantum dots on (111) A substrates: Suppression of fine-structure splitting[END_REF][START_REF] Bouet | Charge tuning in [111] grown GaAs droplet quantum dots[END_REF][START_REF] Mano | Growth of metamorphic InGaAs on GaAs (111) a: counteracting lattice mismatch by inserting a thin InAs interlayer[END_REF][START_REF] Ha | Wavelength extension beyond 1.5 µm in symmetric InAs quantum dots grown on InP (111) A using droplet epitaxy[END_REF][START_REF] Ha | Single photon emission from droplet epitaxial quantum dots in the standard telecom window around a wavelength of 1.55 µm[END_REF][START_REF] Ohtake | Extremely high-and low-density of Ga droplets on GaAs {111} A, B: Surface-polarity dependence[END_REF][START_REF] Jo | Self-limiting growth of hexagonal and triangular quantum dots on (111) A[END_REF][START_REF] Liu | Vanishing fine-structure splittings in telecommunication-wavelength quantum dots grown on (111) A surfaces by droplet epitaxy[END_REF][START_REF] Trapp | Formation of self-assembled GaAs quantum dots via droplet epitaxy on misoriented GaAs (111) B substrates[END_REF][START_REF] Bietti | High-temperature droplet epitaxy of symmetric GaAs/AlGaAs quantum dots[END_REF]] (e.g. for entangled photon pairs generation) and for ultra-high density quantum wires [START_REF] Jo | Self-assembly of GaAs quantum wires grown on (311) A substrates by droplet epitaxy[END_REF] and QDs [START_REF] Mano | As/ Al Ga As quantum dot laser fabricated on GaAs (311) A substrate by droplet epitaxy[END_REF][START_REF] Kawazu | Effects of antimony flux on morphology and photoluminescence spectra of GaSb quantum dots formed on GaAs by droplet epitaxy[END_REF][START_REF] Saidi | Growth conditions effects on optical properties of InAs quantum dots grown by molecular beam epitaxy on GaAs (1 1 3) A substrate[END_REF][START_REF] Mano | Self-assembled GaAs quantum dots coupled with GaAs wetting layer grown on GaAs (311) A by droplet epitaxy[END_REF][START_REF] Keizer | Structural atomic-scale analysis of GaAs/AlGaAs quantum wires and quantum dots grown by droplet epitaxy on a (311) A substrate[END_REF][START_REF] Keizer | Atomic-scale analysis of self-assembled quantum dots by cross-sectionalscanning, tunneling microscopy, and atom probe tomography[END_REF][START_REF] Mano | High-density GaAs/AlGaAs quantum dots formed on GaAs (3 1 1) A substrates by droplet epitaxy[END_REF][START_REF] Abbarchi | Energy renormalization of exciton complexes in GaAs quantum dots[END_REF] formation on the highly anisotropic (311)A surface (e.g. for laser emission). This latter class of nanostructures grown on (311)A surface has not yet been thoroughly investigated and a clear assessment of the corresponding excitonic dynamics has not yet been reported.

In this paper we show a detailed structural and optical characterization of individual QDs grown on the (311)A surface. We provide a clear-cut attribution of the main recombination lines observed in the photoluminescence (PL) spectrum to the s-shell excitons [START_REF] Abbarchi | Exciton fine structure in strain-free GaAs/Al 0.3 Ga 0.7 As quantum dots: Extrinsic effects[END_REF][START_REF] Abbarchi | Energy renormalization of exciton complexes in GaAs quantum dots[END_REF][START_REF] Accanto | Kinetics of multiexciton complex in gaas quantum dots on si[END_REF] based on polarization resolved PL measurements, power dependence and line broadening measurements. Neutral exciton X and biexciton XX, positive X + and negative charged excitons X -are characterised by the presence of fine interactions [START_REF] Abbarchi | Exciton fine structure in strain-free GaAs/Al 0.3 Ga 0.7 As quantum dots: Extrinsic effects[END_REF][START_REF] Plumhof | Experimental investigation and modeling of the fine structure splitting of neutral excitons in strain-free GaAs/Al x Ga 1x As quantum dots[END_REF][START_REF] Skiba-Szymanska | Universal growth scheme for quantum dots with low fine-structure splitting at various emission wavelengths[END_REF][START_REF] Tong | Theory of excitons in cubic III-V semiconductor GaAs, InAs and GaN quantum dots: fine structure and spin relaxation[END_REF] as well as by heavy-hole/light-hole mixing [START_REF] Belhadj | Impact of heavy hole-light hole coupling on optical selection rules in GaAs quantum dots[END_REF][START_REF] Lin | Anticorrelation between the splitting and polarization of the exciton fine structure in single self-assembled InAs/GaAs quantum dots[END_REF][START_REF] Liao | Geometrical impact on the optical polarization of droplet epitaxial quantum dots[END_REF][START_REF] Plumhof | Tuning of the valence band mixing of excitons confined in GaAs/AlGaAs quantum dots via piezoelectric-induced anisotropic strain[END_REF][START_REF] Luo | Supercoupling between heavy-hole and light-hole states in nanostructures[END_REF]. Their power dependence under laser excitation above barrier is well reproduced by a simple Poissonian model that precisely accounts for the main features and allows to estimate the excitonic capture volume [START_REF] Abbarchi | Poissonian statistics of excitonic complexes in quantum dots[END_REF]. Inhomogeneous line broadening at low temperature is ascribed to spectral diffusion [START_REF] Abbarchi | Bunched photon statistics of the spectrally diffusive photoluminescence of single self-assembled GaAs quantum dots[END_REF][START_REF] Ramírez | Effects of electrostatic environment on the electrically triggered production of entangled photon pairs from droplet epitaxial quantum dots[END_REF][START_REF] Abbarchi | Spectral diffusion and line broadening in single self-assembled Ga As/ Al Ga As quantum dot photoluminescence[END_REF][START_REF] Mano | Ultra-narrow emission from single GaAs self-assembled quantum dots grown by droplet epitaxy[END_REF][START_REF] Kuroda | Distribution of exciton emission linewidth observed for GaAs quantum dots grown by droplet epitaxy[END_REF][START_REF] Nguyen | Photoneutralization and slow capture of carriers in quantum dots probed by resonant excitation spectroscopy[END_REF]induced by the presence of charged defects nearby the QDs [START_REF] Ha | Size-dependent line broadening in the emission spectra of single GaAs quantum dots: Impact of surface charge on spectral diffusion[END_REF] and is specific of the excitonic complex in study [START_REF] Schimpf | Resolving the temporal evolution of line broadening in single quantum emitters[END_REF][START_REF] Reigue | Resonance fluorescence of a single semiconductor quantum dot: the impact of a fluctuating electrostatic environment[END_REF].

Finally, at larger temperature, the photoluminescence of individual QDs broadens and quenches owing to phonon interactions [START_REF] Sanguinetti | Temperature dependence of the photoluminescence of InGaAs/GaAs quantum dot structures without wetting layer[END_REF][START_REF] Abbarchi | Phonon sideband recombination kinetics in single quantum dots[END_REF][START_REF] Lee | Exciton-phonon coupling channels in a 'strain-free'GaAs droplet epitaxy single quantum dot[END_REF].

Materials and Methods

Sample fabrication

The sample was grown on a semi-insulating GaAs (311)A substrate by conventional solid-source molecular-beam epitaxy system. After the growth of a 2 µm-thick Al 0.55 Ga 0.45 As layer, a 136 nm-thick

Optical spectroscopy

The photoluminescence (PL) of individual QDs was collected with a confocal-spectroscopic setup (lateral resolution about 1 µm) CW laser excitation was performed above-barrier, at 532 nm (about 2.3 eV). The PL is then fed into a spectrometer and detected by a Si-based CCD camera, allowing for a spectral resolution better than 50 µeV in full width at half maximum (FWHM). All experiments were performed in a liquid-helium cryostat between 10 and 100 K. Well-defined QDs are present with a density of about 5×10 9 /cm 2 . The QD morphology is highly asymmetric, with a U-shape (Figure 1 B). We attribute the formation mechanism of these asymmetric QDs to the low-intensity As 4 supply for the crystallization of the droplets and surface asymmetry of the (311)A surface [START_REF] Mano | As/ Al Ga As quantum dot laser fabricated on GaAs (311) A substrate by droplet epitaxy[END_REF][START_REF] Jo | Self-assembly of GaAs quantum wires grown on (311) A substrates by droplet epitaxy[END_REF][START_REF] Kawazu | Effects of antimony flux on morphology and photoluminescence spectra of GaSb quantum dots formed on GaAs by droplet epitaxy[END_REF][START_REF] Saidi | Growth conditions effects on optical properties of InAs quantum dots grown by molecular beam epitaxy on GaAs (1 1 3) A substrate[END_REF][START_REF] Mano | Self-assembled GaAs quantum dots coupled with GaAs wetting layer grown on GaAs (311) A by droplet epitaxy[END_REF][START_REF] Keizer | Structural atomic-scale analysis of GaAs/AlGaAs quantum wires and quantum dots grown by droplet epitaxy on a (311) A substrate[END_REF][START_REF] Keizer | Atomic-scale analysis of self-assembled quantum dots by cross-sectionalscanning, tunneling microscopy, and atom probe tomography[END_REF][START_REF] Mano | High-density GaAs/AlGaAs quantum dots formed on GaAs (3 1 1) A substrates by droplet epitaxy[END_REF]. In this growth conditions the Ga droplets crystallization into GaAs is enhanced around the edge of the droplets. In the case of standard (100) surface, in which the oppositely oriented directions of [011] and [0-11] are equivalent, ring-like structures with central holes are formed [START_REF] Mano | Nanometer-scale GaAs ring structure grown by droplet epitaxy[END_REF][START_REF] Mano | Self-assembly of concentric quantum double rings[END_REF]. On the (311)A surface however, the oppositely oriented directions of [-233] and [2-3-3] are not equivalent while the directions of [01-1] and [0-11] are equivalent. Thus, the crystallization is enhanced only in a particular direction ([-233]). As the results, U-shaped QDs are formed.

Results and discussion

Macro PL on the QDs ensemble taken at low temperature shows the different contributions from the GaAs substrate (at about 1.48 eV), the Al 0.26 Ga 0.74 As barriers (at about 1.85 eV) and the QDs in between, extending over a broad band from about 1.55 eV up to 1.75 eV. Typical spectra of individual QDs on (311)A substrate appear structured in several sharp lines (Figure 2 A). In analogy to most III-V epitaxial QDs including those fabricated via DE [START_REF] Kuroda | Photon correlation in GaAs self-assembled quantum dots[END_REF][START_REF] Abbarchi | Exciton fine structure in strain-free GaAs/Al 0.3 Ga 0.7 As quantum dots: Extrinsic effects[END_REF][START_REF] Abbarchi | Scanning Fabry-Pérot interferometer with largely tuneable free spectral range for high resolution spectroscopy of single quantum dots[END_REF], in the low excitation power regime the s-shell excitons dominate the PL spectrum. We ascribe the brightest lines to the recombination of the neutral exciton X (one electron, e, and a heavy-hole, hh), neutral biexciton (XX, two e and two hh), positive charged exciton (one e and two hh), and to the negative charged exciton (two e and a hh). This attribution is based on the study of the electron-hole fine interaction [START_REF] Abbarchi | Exciton fine structure in strain-free GaAs/Al 0.3 Ga 0.7 As quantum dots: Extrinsic effects[END_REF] (fine structure splitting), line broadening [START_REF] Abbarchi | Spectral diffusion and line broadening in single self-assembled Ga As/ Al Ga As quantum dot photoluminescence[END_REF] (spectral diffusion), and level filling [START_REF] Abbarchi | Poissonian statistics of excitonic complexes in quantum dots[END_REF], as discussed in the following sections. Seldom, we also observe other sharp lines, as M0 and M1 (Figure 2 A) that likely involve carrier recombination from the p-shell. However, a precise attribution of these lines is not possible with this set of data and goes beyond the aim of this paper.

s-shell excitons.

e-h spin fine interaction: fine structure splitting.

A first identification of the s-shell excitons is based on the study of the fine interaction between electron and hh spin [START_REF] Abbarchi | Exciton fine structure in strain-free GaAs/Al 0.3 Ga 0.7 As quantum dots: Extrinsic effects[END_REF][START_REF] Plumhof | Experimental investigation and modeling of the fine structure splitting of neutral excitons in strain-free GaAs/Al x Ga 1x As quantum dots[END_REF][START_REF] Tong | Theory of excitons in cubic III-V semiconductor GaAs, InAs and GaN quantum dots: fine structure and spin relaxation[END_REF][START_REF] Trotta | Universal recovery of the energy-level degeneracy of bright excitons in InGaAs quantum dots without a structure symmetry[END_REF] (the fine structure splitting, FSS). At intermediate laser excitation power (at about 420 nW) all the PL components in the spectrum are well visible (Figure 2 A). At this power, by changing the polarization angle of the detected light we monitor the emission energy of the PL. We observe for X and XX a mirror-symmetric energy splitting (Figure 2 C). X + and X -instead, do not feature any splitting. This picture corresponds very well to what is commonly observed in strain-free, III-V QDs: the presence of geometrical asymmetries in the confining potential breaks the invariance of the hamiltonian for rotations around the vertical growth axis [START_REF] Abbarchi | Exciton fine structure in strain-free GaAs/Al 0.3 Ga 0.7 As quantum dots: Extrinsic effects[END_REF][START_REF] Plumhof | Experimental investigation and modeling of the fine structure splitting of neutral excitons in strain-free GaAs/Al x Ga 1x As quantum dots[END_REF][START_REF] Yeo | Comparative Chemico-Physical Analyses of Strain-Free GaAs/Al0. 3Ga0. 7As Quantum Dots Grown by Droplet Epitaxy[END_REF][START_REF] Tong | Theory of excitons in cubic III-V semiconductor GaAs, InAs and GaN quantum dots: fine structure and spin relaxation[END_REF][START_REF] Liao | Geometrical impact on the optical polarization of droplet epitaxial quantum dots[END_REF][START_REF] Trotta | Universal recovery of the energy-level degeneracy of bright excitons in InGaAs quantum dots without a structure symmetry[END_REF][START_REF] Adorno | Annealing induced anisotropy in GaAs/AlGaAs quantum dots grown by droplet epitaxy[END_REF]. More precisely, e-h spin-spin interaction for the X state lifts the degeneracy of the corresponding energy level and leads to two linearly polarised PL components depending on which recombination path is radiating. The XX state has overall zero spin for e and hh in the initial state and its FSS is completely determined by that of the X state, towards which it relaxes. X + and X -can recombine with two possible energy equivalent paths and have no FSS.

The measured FSS in this QD is of about 100 µeV. In other QDs we measured values in between 30 and 140 µeV (not shown). These values are very large if compared with QDs grown on (111) substrates, where the three-fold symmetry of the crystal provides a more isotropic surface diffusion and a corresponding triangular or hexagonal nanostructure shape [START_REF] Kumano | Stable and efficient collection of single photons emitted from a semiconductor quantum dot into a single-mode optical fiber[END_REF][START_REF] Kuroda | Symmetric quantum dots as efficient sources of highly entangled photons: Violation of Bell's inequality without spectral and temporal filtering[END_REF][START_REF] Mano | Self-assembly of symmetric GaAs quantum dots on (111) A substrates: Suppression of fine-structure splitting[END_REF][START_REF] Bouet | Charge tuning in [111] grown GaAs droplet quantum dots[END_REF][START_REF] Mano | Growth of metamorphic InGaAs on GaAs (111) a: counteracting lattice mismatch by inserting a thin InAs interlayer[END_REF][START_REF] Ha | Wavelength extension beyond 1.5 µm in symmetric InAs quantum dots grown on InP (111) A using droplet epitaxy[END_REF][START_REF] Ha | Single photon emission from droplet epitaxial quantum dots in the standard telecom window around a wavelength of 1.55 µm[END_REF][START_REF] Ohtake | Extremely high-and low-density of Ga droplets on GaAs {111} A, B: Surface-polarity dependence[END_REF][START_REF] Jo | Self-limiting growth of hexagonal and triangular quantum dots on (111) A[END_REF][START_REF] Liu | Vanishing fine-structure splittings in telecommunication-wavelength quantum dots grown on (111) A surfaces by droplet epitaxy[END_REF][START_REF] Trapp | Formation of self-assembled GaAs quantum dots via droplet epitaxy on misoriented GaAs (111) B substrates[END_REF][START_REF] Bietti | High-temperature droplet epitaxy of symmetric GaAs/AlGaAs quantum dots[END_REF]. The origin of this splitting in (311)A QDs is attributed to asymmetries in the QDs shape that is affected by the anisotropy of the underlying crystal. The measured values of FSS are in line with those observed in (001) QDs in spite of the larger anisotropy of the (311)A surface [START_REF] Abbarchi | Exciton fine structure in strain-free GaAs/Al 0.3 Ga 0.7 As quantum dots: Extrinsic effects[END_REF][START_REF] Plumhof | Experimental investigation and modeling of the fine structure splitting of neutral excitons in strain-free GaAs/Al x Ga 1x As quantum dots[END_REF][START_REF] Tong | Theory of excitons in cubic III-V semiconductor GaAs, InAs and GaN quantum dots: fine structure and spin relaxation[END_REF][START_REF] Trotta | Universal recovery of the energy-level degeneracy of bright excitons in InGaAs quantum dots without a structure symmetry[END_REF]. Owing to the growth method in use, that allows for 3D growth of lattice-matched materials such as GaAs on AlGaAs, we exclude the presence of any strain in the structure that is a major origin of symmetry breaking (and thus FSS) in conventional Stranski-Krastanov III-V QDs [START_REF] Trotta | Universal recovery of the energy-level degeneracy of bright excitons in InGaAs quantum dots without a structure symmetry[END_REF].

Polarization intensity anisotropy: heavy-hole light-hole mixing.

A remarkable intensity change is visible for the two orthogonally polarized components (between 1/3 and 1/2, depending on the observed line, Figure 2 C)) of X and XX as well as in the polarization intensity of X + and X -. We ascribe this difference to heavy-hole/light-hole (hh-lh) mixing [START_REF] Belhadj | Impact of heavy hole-light hole coupling on optical selection rules in GaAs quantum dots[END_REF][START_REF] Lin | Anticorrelation between the splitting and polarization of the exciton fine structure in single self-assembled InAs/GaAs quantum dots[END_REF][START_REF] Liao | Geometrical impact on the optical polarization of droplet epitaxial quantum dots[END_REF][START_REF] Plumhof | Tuning of the valence band mixing of excitons confined in GaAs/AlGaAs quantum dots via piezoelectric-induced anisotropic strain[END_REF][START_REF] Luo | Supercoupling between heavy-hole and light-hole states in nanostructures[END_REF][START_REF] Fras | The role of heavy-light-hole mixing on the optical initialization of hole spin in InAs quantum dots[END_REF]. This feature was first highlighted in DE QDs grown on (001)-oriented substrates and was explained as an effect of the different hh and lh bands dispersion in the different crystallographic directions [START_REF] Belhadj | Impact of heavy hole-light hole coupling on optical selection rules in GaAs quantum dots[END_REF].

While e states can be approximately described as isotropic and parabolic bands, valence hh and lh bands are affected by strong anisotropy and different curvatures. In spite of the large energy splitting between hh and lh bands in GaAs (tens of meV), strain and corresponding piezoelectricity [START_REF] Lin | Anticorrelation between the splitting and polarization of the exciton fine structure in single self-assembled InAs/GaAs quantum dots[END_REF][START_REF] Plumhof | Tuning of the valence band mixing of excitons confined in GaAs/AlGaAs quantum dots via piezoelectric-induced anisotropic strain[END_REF][START_REF] Luo | Supercoupling between heavy-hole and light-hole states in nanostructures[END_REF][START_REF] Fras | The role of heavy-light-hole mixing on the optical initialization of hole spin in InAs quantum dots[END_REF] (in SK QDs) or shape asymmetries [START_REF] Belhadj | Impact of heavy hole-light hole coupling on optical selection rules in GaAs quantum dots[END_REF][START_REF] Liao | Geometrical impact on the optical polarization of droplet epitaxial quantum dots[END_REF] (in DE QDs) lead to a substantial mixing of the corresponding hh and lh states, providing different selection rules (uneven PL intensity) for the two recombination paths. Here we observe differences in the PL intensity that are of the same order of those observed in the (001) counterpart [START_REF] Belhadj | Impact of heavy hole-light hole coupling on optical selection rules in GaAs quantum dots[END_REF][START_REF] Lin | Anticorrelation between the splitting and polarization of the exciton fine structure in single self-assembled InAs/GaAs quantum dots[END_REF][START_REF] Plumhof | Tuning of the valence band mixing of excitons confined in GaAs/AlGaAs quantum dots via piezoelectric-induced anisotropic strain[END_REF][START_REF] Luo | Supercoupling between heavy-hole and light-hole states in nanostructures[END_REF][START_REF] Fras | The role of heavy-light-hole mixing on the optical initialization of hole spin in InAs quantum dots[END_REF].

Line broadening: spectral diffusion.

By monitoring the line-shape and corresponding broadening of each PL component in the s-shell we observe respectively, a common Gaussian envelope and a remarkably different full width at half maximum (FWHM, Figure 2 B). The common origin of the line broadening in this class of QDs is spectral diffusion: in spite of the expected natural linewidth that should be of the order of a few µeV with a corresponding Lorentian lineshape [START_REF] Abbarchi | Spectral diffusion and line broadening in single self-assembled Ga As/ Al Ga As quantum dot photoluminescence[END_REF][START_REF] Kuroda | Distribution of exciton emission linewidth observed for GaAs quantum dots grown by droplet epitaxy[END_REF][START_REF] Nguyen | Photoneutralization and slow capture of carriers in quantum dots probed by resonant excitation spectroscopy[END_REF][START_REF] Abbarchi | Scanning Fabry-Pérot interferometer with largely tuneable free spectral range for high resolution spectroscopy of single quantum dots[END_REF], the measured broadening values (tens to hundreds of µeV) and Gaussian envelopes are the fingerprints of a fluctuating charged environment. Charging and un-charging of electron and holes traps nearby the QDs (within a few tens of nm), produces a variable quantum confined Stark-shift of the excitonic energy levels [START_REF] Abbarchi | Bunched photon statistics of the spectrally diffusive photoluminescence of single self-assembled GaAs quantum dots[END_REF][START_REF] Ramírez | Effects of electrostatic environment on the electrically triggered production of entangled photon pairs from droplet epitaxial quantum dots[END_REF][START_REF] Abbarchi | Spectral diffusion and line broadening in single self-assembled Ga As/ Al Ga As quantum dot photoluminescence[END_REF][START_REF] Mano | Ultra-narrow emission from single GaAs self-assembled quantum dots grown by droplet epitaxy[END_REF][START_REF] Kuroda | Distribution of exciton emission linewidth observed for GaAs quantum dots grown by droplet epitaxy[END_REF][START_REF] Nguyen | Photoneutralization and slow capture of carriers in quantum dots probed by resonant excitation spectroscopy[END_REF][START_REF] Ha | Size-dependent line broadening in the emission spectra of single GaAs quantum dots: Impact of surface charge on spectral diffusion[END_REF].

A PL measurement lasting for a few seconds acquires many photons emitted at slightly different energy, thus providing a Gaussian lineshape. However, owing to the different hh and e confinement (larger penetration in the barrier for e with respect to hh) and a screening of external electric field for many-particles states, the corresponding Stark-shift can be very different. It has been rutinely reported a larger broadening of X with respect to the other s-shell excitons, whereas the relative broadening of XX, X + and X -can be very different, depending on the carrier trapped and its position with respect to the QD [START_REF] Abbarchi | Bunched photon statistics of the spectrally diffusive photoluminescence of single self-assembled GaAs quantum dots[END_REF][START_REF] Abbarchi | Spectral diffusion and line broadening in single self-assembled Ga As/ Al Ga As quantum dot photoluminescence[END_REF][START_REF] Dotti | Germanium-based quantum emitters towards a time-reordering entanglement scheme with degenerate exciton and biexciton states[END_REF]. As an example, the case in Figure 2 B seems to correspond to electrons confined in the plane of the QD [START_REF] Abbarchi | Spectral diffusion and line broadening in single self-assembled Ga As/ Al Ga As quantum dot photoluminescence[END_REF]. However, this picture can be quite different from a QD to another, owing to the randomness and local changes of the extrinsic disorder.

In this samples the presence of defects in the QDs environment is likely due to the rather low temperature used to form 3D nanostructures. This is a well-known issue affecting QDs grown by droplet epitaxy. However, in principle it could be overcome as it has been shown for droplet epitaxial QDs grown on other surface orientations [START_REF] Kuroda | Symmetric quantum dots as efficient sources of highly entangled photons: Violation of Bell's inequality without spectral and temporal filtering[END_REF][START_REF] Mano | Ultra-narrow emission from single GaAs self-assembled quantum dots grown by droplet epitaxy[END_REF]. A typical evolution of the PL spectrum of an individual QD as a function of the incident power is provided in Figure 3 A. At low power only two lines, X and X + are visible with a dominant intensity of X, while when increasing the power also X -and, soon after, XX appear. At larger power X and X + swap their relative intensity and then quench. XX and X -reach their maximum and quench at very large power. M0 and M1 follows a similar dynamics of X -and XX, respectively. At lower energy, other peaks appear together with a broad background that then extends to all the spectrum. These latter features are interpreted as the effect of multi-excitons recombining from the p-shell and the coupling with the continuum of states above the confined energy levels. For the sake of thoroughness, we note a slight red-shift of the PL intensity at large excitation power. This effect is interpreted as a heating of the sample. It is also worth noting that the PL intensity of the s-shell excitons can be followed over more than 3 order of magnitude of the excitation power, meaning that the capture and recombination processes are very efficient in this sample.

A simple but meaningful description of the level filling of individual QDs can be provided by a model based on the assumption that carrier capture and recombination are random processes [START_REF] Grundmann | Theory of random population for quantum dots[END_REF].

By assuming an infinite set of confined levels in the nanostructure, that obviously constitutes a rough approximation, the model can be further simplified to a Poissonian dependence of the PL intensity [START_REF] Abbarchi | Poissonian statistics of excitonic complexes in quantum dots[END_REF][START_REF] Grundmann | Theory of random population for quantum dots[END_REF]. In spite of its simplicity, this modeling accounts extremely well for power dependence [START_REF] Abbarchi | Poissonian statistics of excitonic complexes in quantum dots[END_REF] and recombination dynamics [START_REF] Ha | Single photon emission from droplet epitaxial quantum dots in the standard telecom window around a wavelength of 1.55 µm[END_REF][START_REF] Kuroda | Bunching visibility for correlated photons from single GaAs quantum dots[END_REF] of the s-shell excitons. In this model we assume that the PL intensity I PL of a given energy level is proportional to the occupation probability N n of that level:

I PL ∝ N n (1)
The Poissonian model for level filling predicts that the occupation of a certain level with n excitons N n is provided by:

N n = < n > n n! e -<n> (2)
Thus, for n = 1 N 1 describes the level filling of X, for n = 2 N 2 describes the level filling of XX, etc. < n > is the average number of exciton created in the QD. This number is obtained averaging over many capture/recombination cycles and under steady state excitation and is defined as < n >= N R τ r /τ c , with N R is the number of e-h pairs created in the reservoir, τ r is the exciton recombination rate and τ c the exciton capture rate. Provided that τ r and τ c lie in the ps to ns range and the integration time for a PL measurement last about one second, this hypothesis is well satisfied.

A final phenomenological assumption for < n > that allows to link the experimental data to this Poissonian model is that

< n >= βP α exc (3)
where α and β are constants characterising the capture mechanism. Note that in the vast majority of works on QDs, the level filling of the s-shell excitons is simply provided by equation 3, with α(X) ∼ 1 for X, and α(XX) ∼ 2 for XX. However, this over-simplified assumption does not account for saturation and quenching of the PL lines under CW excitation [START_REF] Ha | Current-injection quantum-entangled-pair emitter using droplet epitaxial quantum dots on GaAs (111) A[END_REF][START_REF] Abbarchi | Exciton fine structure in strain-free GaAs/Al 0.3 Ga 0.7 As quantum dots: Extrinsic effects[END_REF][START_REF] Liu | Vanishing fine-structure splittings in telecommunication-wavelength quantum dots grown on (111) A surfaces by droplet epitaxy[END_REF][START_REF] Tighineanu | Decay dynamics and exciton localization in large GaAs quantum dots grown by droplet epitaxy[END_REF].

The Poissonian model well reproduces the power dependence of X and XX that are extracted by integrating in energy their PL line up to 1600 nW after background subtraction (3 B). Fitting the data using Equation 2 and 3 we obtain α X = 1.1 (very close to 1 as expected) and a simultaneous fit of X and XX with the same parameters. Saturation power of X is about 1500 nW and about two times larger for XX.

Although the simple Poissonian model allows only for neutral excitons capture (n = 1, 2, ....)

and in principle cannot account for charged complexes, a nice fitting of the data can be recovered by assuming half-integer numbers [START_REF] Abbarchi | Poissonian statistics of excitonic complexes in quantum dots[END_REF]. In this case the model is modified by adding a phenomenological coefficient γ to α X modifying as follows:

N n = (< n > n ) γ n! e -<n> (4)
thus, for instance γ = 1 corresponds to the neutral exciton X state whereas γ = 1.5 is one and an half exciton (between X and XX), that is a charged exciton. By using Equation 4we can nicely fit the power dependence of X + and X -by keeping unchanged the parameters α and β that were used to fit X and XX (3 B). We obtain α(X + ) = 1.3α(X) and α(X -) = 1.6α(X) that are very close to 1.5. Finally, we note that also the dynamics of M0 and M1 with incident power can be nicely accounted for by this pseudo-Poissonian model providing α(M0) = 1.5α(X) and α(M1) = 2.1α(X). These values of γ larger than 1 account for the super-linear dependence on the incident power and suggest a multi-exciton nature of these energy levels.

To conclude this section we estimate the QD capture volume for excitons [START_REF] Abbarchi | Poissonian statistics of excitonic complexes in quantum dots[END_REF]. This can be deduced from the saturation power of the X level: at that power (of about 1500 nW distributed over a diameter of 1 µm) the number of carriers created in the host matrix around the QD provides a steady occupation of the X level by one exciton. Thus, the inverse of the carrier density injected in the semiconductor at that saturation power represents the capture volume. Assuming a penetration depth of the laser light of about 1 µm, an excitonic recombination lifetime of about 1 ns and considering the QD as a sphere, we can roughly deduce a capture radius of the order of 50 nm. This value is rather larger than the physical size of the QD, that is of the order of a few nm. We also observe that this value is perfectly in line with those measured in the (001) DE QDs counterpart [START_REF] Abbarchi | Poissonian statistics of excitonic complexes in quantum dots[END_REF] as well as in other kind of quantum emitters in III-V [START_REF] Dotti | Germanium-based quantum emitters towards a time-reordering entanglement scheme with degenerate exciton and biexciton states[END_REF] and in group IV materials [START_REF] Beaufils | Optical properties of an ensemble of G-centers in silicon[END_REF]. . Bright PL emission can be observed well above liquid nitrogen temperature (Figure 4). A quadratic red-shift of the exciton emission is observed, as expected for this class of nanostructures that follows the empirical Varshini law (Figure 4 A). At the same time, a strong quenching of the PL intensity of about two orders of magnitude is observed when passing from 10 to 90 K.

The low-temperature spectrum (up to about 30 K), as discussed in the previous sections, shows sharp lines attributed to the s-shell excitons and is well described by a Gaussian envelope (Figure 2) [START_REF] Abbarchi | Spectral diffusion and line broadening in single self-assembled Ga As/ Al Ga As quantum dot photoluminescence[END_REF].

At higher temperature instead, when the phonon population plays an important role, we observe the onset of a broad Lorentzian-shaped pedestal below the original sharp line that can be now well approximated with a Lorentzian envelope (Figure 4 B) [START_REF] Sanguinetti | Temperature dependence of the photoluminescence of InGaAs/GaAs quantum dot structures without wetting layer[END_REF][START_REF] Abbarchi | Phonon sideband recombination kinetics in single quantum dots[END_REF][START_REF] Favero | Acoustic phonon sidebands in the emission line of single InAs/GaAs quantum dots[END_REF]. The relative intensity of this broad pedestal increases up to dominate over the sharp central line. This phenomenology corresponds well to what has been thoroughly explained by using the Huang-Rhys formalism for (001) DE QDs as well as for SK QDs [START_REF] Sanguinetti | Temperature dependence of the photoluminescence of InGaAs/GaAs quantum dot structures without wetting layer[END_REF][START_REF] Abbarchi | Phonon sideband recombination kinetics in single quantum dots[END_REF][START_REF] Favero | Acoustic phonon sidebands in the emission line of single InAs/GaAs quantum dots[END_REF]: the broad band is interpreted as a superposition of acoustic phonon replicas (polaron) whereas the sharp central line emission is assigned to the zero phonon line (ZPL).

Conclusions

In conclusion we showed that droplet epitaxial QDs grown on the highly anisotropic (311)A surface are characterised by bright PL lines emitting at visible frequency. These lines correspond well to the picture of s-shell excitons recombination in III-V nanostructures. We measured a relatively large fine structure splitting (10-100 µeV) of the neutral exciton X, that is justified by the anisotropic shape of these QDs. Inhomogeneus line broadening lies in the 100µeV and is ascribed to spectral diffusion originating from the fluctuating charged environment. This broadening is specific of each excitonic species and reflects the nature and position of the trapped carriers. Level filling of the s-shell excitons is well accounted for by a simple Poissonian model that allows to describe the power dependence of the main PL lines and to estimate an excitonic capture volume much larger than the physical size of the QD. Finally, a bright PL emission is still visible well above liquid nitrogen temperature.

Overall, these results account for the high quality and brightness of this class of QDs where X, XX and X + emission can be followed for about 3 orders of magnitude of excitation power below saturation and up to relatively large temperature that are appealing features for applications as single photon sources. The low-temperature linewidth is rather large with respect to state-of-the-art droplet epitaxial quantum dots and in principle it could be improved by using alternative fabrication processes at higher temperature.

  Al 0.26 Ga 0.74 As core-layer was grown at 610 • C. At the core-layer center GaAs QDs were formed by droplet epitaxy. On the Al 0.26 Ga 0.74 As surface, nominally 1.5 monolayers of Ga (at a growth speed of about 0.1 monolayers per second) was supplied without As 4 flux at 275 • C for the droplets formation. These droplets were then crystallized into GaAs QDs by supplying an As 4 flux (2 × 10 -6 Torr beam equivalent pressure) at 200 • C. The QDs were annealed at 400 • C for 10 min under As 4 flux supply without capping in order to improve the crystal quality. After annealing, the QDs were covered with a 30 nm-thick Al 0.26 Ga 0.74 As capping layer at 400 • C and the rest of the Al 0.26 Ga 0.74 As (38 nm) layer was grown at 625 • C. Once the entire growth sequence was completed, a rapid thermal annealing process was performed at 785 • C for 4 min in an As 4 atmosphere to improve the optical quality.

Figure 1 .

 1 Figure 1. (A) Atomic force microscope micrograph representing a 3D view of GaAs QDs grown on a (311)A oriented Al 0.26 Ga 0.74 As surface. The main chrstallographic axes are highlighted. (B) Blow-up of a single QD from A. The arrow highlights a hole in the QD side. (C) Photoluminescence (PL) spectrum (in semi-log scale) at 5 K showing the emission of (respectively from the low to the high energy side) the GaAs, the GaAs QDs and the Al 0.26 Ga 0.74 As barrier layers.

Figure 1 A

 1 Figure 1 A and B show atomic force micrographs (AFM) of the sample after annealing at 400 • C.

Figure 2 .

 2 Figure 2. (A) PL spectrum of a single GaAs QD sandwiched between (311)A Al 0.35 Ga 0.65 As barrier layers. The labels on the most intense lines X, X + , XX and X -highlight respectively the emission from the neutral exciton, the positive charged exciton, the neutral biexciton and the negative charged exciton. M0 and M1 highlight other non-attributed multiexciton complexes. (B) Symbols: PL spectra of X, X + , XX and X -(respectively from the bottom to the top panel). Red lines are Gaussian fit to the data. The corresponding full width at half maximum (FWHM) is reported on each panel. (C) Linearly polarized components of X, X + , XX and X -PL (respectively from the right to the left panel). Red and black symbols indicate orthogonal polarization. The continuous lines are Gaussians fits. The fine structure splitting (FSS) measured from the X and XX components, is about 100 µeV.

6 .

 6 Power dependence: level filling.

Figure 3 .

 3 Figure 3. (A) PL spectra from low to high excitation power for the same QD shown in Fig. 2. The incident power is highlighted on each panel. The vertical dashed lines are guides to the eyes. (B) Evolution of the six main PL lines as a function of the incident excitation power. The PL intensity has been normalized to the maximum for each component. Each panel refers to a specific exciton complex recombination as highlighted by the corresponding labels. Symbols are the experimental data whereas lines are Poissonian fits.

6. 1 .

 1 Temperature dependence.

Figure 4 .

 4 Figure 4. (A) Selected PL spectra from 10 to 90 K of a single QD with a bright X + emission. (B) Bottom panel: detail of the spectrum from A at 60 K with Lorentzian fits of the zero phonon line and the polaron band. The inset shows a blow up of the ZPL. Top panell: detail of the spectrum at 90 K with Lorentzian fits.

Funding:

Acknowledgments:

Conflicts of Interest:

The authors declare no conflict of interest.

Abbreviations

The following abbreviations are used in this manuscript: PL photoluminescence QD quantum dot FSS fine structure splitting X neutral exciton X + positive charged exciton XX neutral biexciton X - negative charged exciton FWHM full width at half maximum