Amorphous non-doped and Se-, Cu-, and Zn-doped Sb2S3 nanoparticles prepared by a hot-injection method: bandgap tuning and possible observation of the quantum size effect

Amorphous, non-doped, and copper- and selenium-doped Sb2S3 nanoparticles were synthesized by a hot-injection method. Zinc-doped Sb2S3 nanoparticles were prepared for the first time using the same approach. Electron microscopy revealed that spherical nanoparticles of 1–4 nanometers aggregated into larger spherical clusters. Introducing dopants into the Sb2S3 structure neither influenced the samples’ spherical morphology nor their sizes. The presence of the dopants (Cu, Se, or Zn) was confirmed by energy dispersive X-ray (EDX) and, in the case of Zn, also by inductively coupled plasma-mass spectrometry (ICP-MS). The X-ray powder diffraction (XRPD) patterns of the non-doped and doped samples imply an amorphous structure. Crystalline Zn-doped Sb2S3 revealed defined peaks from only the Sb2S3 phase, indicating successful doping. Diffuse reflectance spectroscopy (DRS) revealed high optical bandgap energies (2.03–2.12 eV) compared to the values (1.6–1.7 eV) for large non-doped and doped particles obtained at 240 °C, which might be attributed to a quantum size effect. X-ray photoelectron spectroscopy (XPS) revealed a phase without any impurities for the undoped and characteristic peaks for copper, selenium, and zinc Auger for the doped samples. XPS valence band confirm for the Zn-doped particles a shift towards lower binding energy compared to the non-doped samples, indicating successful doping. Photoluminescence (PL) measurements show that embedding Zn into the Sb2S3 host lattice suppresses the wide luminescence band related to intrinsic vacancy defects. Narrow peaks at 1.7–2.4 eV were found to be associated with singlet excitons. The energy dependence of the light emission on the synthesized nanoparticles’ size suggests quantum confinement.


Introduction
One of the most pressing issues in the materials science community is using a sustainable, renewable, and cost-effective energy source. The transition from fossil fuels to renewable technologies is an essential action that must be carried out as soon as possible. Solar energy is the most common renewable technology and requires capital investment, with zero carbon dioxide emissions. However, its use is still unsatisfactory [1][2][3][4][5]. Low solar cell power conversion efficiency (PCE) and high fabrication and installation costs limit solar energy applications. For these reasons, scientists are researching new low-cost materials, optimizing established device structures, and designing novel concepts [6][7][8][9][10][11][12][13].
Because of its abundant resources, non-toxicity, high stability, low cost, high absorption coefficient (1.5⋅10 5 cm −1 ) [14], and suitable bandgap, the antimony sulfide (Sb 2 S 3 ) semiconductor has piqued the interest of many researchers for use in solar cell devices. No impurity phase will be produced because this is a binary semiconductor compound with a single stable phase. The material is stable in its amorphous and crystalline form. This combination of favorable properties has driven an intensive effort to fabricate suitable materials or thin films for application in various solar devices [15][16][17].
In the last decade, we synthesized various nondoped and different-ion doped Sb 2 S 3 nanomaterials in the form of powders and applied them to different cheap solar device architectures [18][19][20][21][22][23][24][25][26][27]. In recent years, a wide range of doping elements and techniques have been experimentally applied to Sb 2 S 3 semiconductors for use in thin films and solar cells [28][29][30][31], although synthesized non-doped and doped Sb 2 S 3 nano-powders for solar device applications are still unknown to us. Doping strategies for the light-absorbing semiconductor Sb 2 S 3 are crucial for bandgap engineering, carrier transport, improving conductivity, regulating film morphology, and enhancing stability. Furthermore, in our case, films for use in solar cells were created through redispersion and deposition of synthesized nanoparticles. Because of this process, it is possible to combine modifications of the same material with different bandgaps and tunable electronic characteristics or combine this semiconductor with conductive polymers or dyes as absorption layer composites, thus extending the range of photon energies that are usable for absorbing layer [20,21,24,27,32,33]. Furthermore, a tunable bandgap can significantly expand the semiconductor's applicability. Sb 2 S 3 has been used in photovoltaic devices for a long time, but due to synthesis challenges, there was little knowledge on Sb 2 S 3 quantum dots. As far as we are aware, the power conversion efficiency of the prospective quantum dot solar cell configurations, made of synthesized Sb 2 S 3 quantum dots, has not yet been experimentally proven. The excitonic effect and quantum confinement have been found to play an important role in the electronic structures and optical properties of Sb 2 S 3 nanowires (NWs), indicating that optical anisotropy in Sb 2 S 3 NWs is more pronounced than in bulk. The optical absorption edge's blue-shift and red-shift indicate that these Sb 2 S 3 NWs could be used in nanoscale light-emitting devices [34].
Up to now, there have only been a few reports concerning Zn and Cu-doped Sb 2 S 3 , while Se-doped Sb 2 S 3 is more prevalent [17,28,[35][36][37][38][39]. It should be noted that all of those samples are doped in thinfilm form. However, colloidally synthesized Cu and Se-doped Sb 2 S 3 nanoparticles were reported rarely and mainly by our group [20,22], while Zn-doped Sb 2 S 3 nanoparticles are presented here for the first time. Zn could be a suitable p-dopant for the Sb 2 S 3 semiconductor due to their similar atomic radii and lower number of valence electrons. Earlier, Cu-doped Sb 2 S 3 was synthesized for the same reason. Therefore, we found this synthesis promising for possible applications.
In this manuscript, we present three new materials obtained by a modified hot-injection metallo-organic synthesis [18]: selenium-and copper-doped Sb 2 S 3 nanoparticles with significantly reduced nanoparticle sizes compared to earlier work, as well as amorphous Zndoped Sb 2 S 3 nanoparticles. The structural, morphological, electronic, optical, and photoluminescent properties of new materials based on non-doped and doped Sb 2 S 3 have been investigated, characterized, and presented here. Many characterizations were performed, such as high-resolution transmission electron microscopy (HRTEM), transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), diffuse reflectance spectroscopy (DRS), X-ray powder diffraction (XRPD), and X-ray photoelectron spectroscopy (XPS), as well as photoluminescence (PL) measurements, to show that the synthesis, especially the doping process was successful. It was demonstrated that the huge decrease in nanoparticle size observed compared to previous syntheses at higher temperatures (240 °C) [20,22] than the present ones (150 °C) is accompanied by a significant increase in the energy bandgap, indicating a strong quantum size effect. Bandgap values above 2.0 eV were already observed by several authors for larger undoped amorphous Sb 2 S 3 nanoparticles [40,41] and also found by us for undoped nanoparticles prepared by the same procedure as in this work [18]. So far, this was explained mainly by differences in the bond lengths and angles of the amorphous material leading to a different mobility gap, not by a quantum size effect. In addition, it was revealed that as particle size decreases, new peaks appear in the PL measurements that are shifted toward higher energies. The additional high exciton energy discovered in the PL measurements, in conjunction with a possible quantum size effect, could be associated with novel electronic properties of synthesized non-doped and differently doped Sb 2 S 3 nanoparticles.

Materials and methods
Materials and procedures for the synthesis of amorphous non-doped, Cu-doped, Se-doped, and Zn-doped Sb 2 S 3 nanoparticles All chemicals (antimony (III) chloride (SbCl 3 ) (99.0 % min), sulfur powder (S) (99.999 %), 2-ethylhexanoic acid (EHA) (99%) (available from Alfa Aesar), oleylamine (tech.70% Sigma-Aldrich), copper (I) acetate, Se pellet, and Zn acetate dihydrate as well as liquid paraffin (USP-NF, BP, Ph. Eur.) pure, PanReac, hexane, isopropyl alcohol, methanol, and benzene (all purchased from J. T. Baker)) were of the highest purity available and used without further purification. All experiments were conducted in an argon atmosphere using standard glass equipment, including a Schlenk line. Before use, the reaction vessels were cleaned with nitric acid and repeatedly rinsed with deionized water. Detailed experimental procedures, including the synthesis of non-doped and doped Sb 2 S 3 nanoparticles with different dopants, have been described in references [20,21]. Details are given in the Electronic supplementary material.

Characterization devices and measurements
The morphology of the samples was analyzed by high-resolution transmission electron microscopy (HRTEM) using a Talos F200X electron microscope from FEI Company, equipped with a CCD camera with a resolution of 4096×4096 pixels using the User Interface software package. The acceleration voltage was set at 200 kV. An energy-dispersive X-ray spectroscopy (EDs) system attached to the TEM operating in the scanning transmission (STEM) mode was used for elemental analysis, including surface chemical mapping of the samples. Additionally, a Zeiss EM 109 was used at an 80 kV acceleration voltage to record the TEM images.
Morphological and compositional properties were additionally investigated using field emission scanning electron microscopy (FESEM) on a FEI SCIOS 2 Dual Beam electron microscope with a 20 kV acceleration voltage. Energy dispersive spectrometry was used to determine the chemical composition (EDS).
The Zn:Se ratio was additionally determined by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS); for details, see the Electronic supplementary material.
Diffuse reflectance measurements (DRS) were taken using an integrating sphere and a Cary 5000 UV-vis-NIR spectrometer (Agilent Technologies) with an internal DRA 2500. Particles were measured in special quartz cuvettes (type 26.715/Q/10/Z20; Starna) in the 400-800 nm range as a dispersion in isopropyl alcohol.
The X-ray powder diffraction (XRPD) measurements were carried out on a PHILIPS 1050 X-ray diffractometer with Ni-filtered Cu K radiation and Bragg-Brentano focusing geometry. The amorphous samples' patterns were taken in the 5-90° 2θ range, with a step size of 0.05° 2θ and an exposure time of 5 s per step. The crystalline sample pattern was taken in the 10-90° 2θ range, with a step size of 0.02° 2θ and an exposure time of 12 s per step.
XPS spectra were recorded using a monochromatic Al K line (1486.74 eV) at 12.5 kV and 32 mA. The samples were analyzed using an XP50 M X-ray source for Focus 500 and a PHOIBOS 100/150 analyzer from SPECS Systems. The X-ray gun power was reduced to a minimum to minimize the binding energy shift, and a SPECS FG15/40 electron flood gun was used. All samples were pressed onto the copper foil to ensure a strong mechanical attachment and good electrical contact. Peak positions were calibrated using the C 1s line at 284.8 eV originating from adventitious carbon, according to the literature [42] and analyzed using the CASA XPS software package [43]. For further details, see the Electronic supplementary material.
Photoluminescence (PL) and excitation spectra were recorded using the Horiba Fluorolog 3 spectrometer (Japan). Excitation of luminescence was performed in a stationary mode employing a xenon lamp of 460-W power. The input monochromator provided an excitation wavelength accuracy of less than 0.1 nm. A Synapse CCD camera was used to record emission spectra, which provides a signalto-noise ratio of 20000/1. Excitation spectra were recorded based on registered 2D "excitation-luminescence" maps with a selection of the required luminescence wavelength. All measurements were carried out at room temperature.

Results and discussion
Non-doped and Cu-, Se-, and Zn-doped amorphous Sb 2 S 3 nanoparticle morphology and elemental analysis Figures 1, 2, 3, and 4 show HRTEM and TEM characterization of amorphous non-doped and Cu, Se, and Zn-doped Sb 2 S 3 nanoparticles. The same morphology was revealed for all samples: tiny nanoparticles, whose visibility depends on the sample, organized into larger nanoparticles. All the photographs of the synthesized non-doped and Cu, Se, and Zn-doped Sb 2 S 3 nanoparticles were observed at high magnification only, and some of the photographs confirm the presence of particles of a few nanometers, at least what is visible to us. It should be emphasized here that limited visibility at higher magnification was due to enhanced degradation of the samples because of the damage to the samples by the electron beam [44].
Furthermore, as seen in the photographs, tiny particles aggregate into larger, mostly spherical formations. We checked the diluted samples of the non-doped sample on atomic force microscopy (AFM) and found separate nanoparticles of 1-4 nm. The EDX spectra of all the synthesized samples, together with the EDX mapping of the amorphous Cu-doped Sb 2 S 3 sample, are presented in Fig. 5 a-d. It is evident that the spectra indicate the presence of S and Sb as well as of the dopants Se and Zn in the doped samples. Since C and Cu are detected in the Cu-doped Sb 2 S 3 sample because a carbon-covered Cu grid was used, an EDX mapping was performed. The homogenous distribution of Cu over all particles in the micrograph is evident (Fig. 5d). Furthermore, detectable oxygen (not shown in Fig. 5) in the EDX spectra of all investigated doped and non-doped nanoparticles could be attributed to impurities or oxidation of the nanoparticles during synthesis, cleaning, or preparation of the specimens [45]. Additional lower resolution FE-SEM micrographs were recorded only for the Zn-doped Sb 2 S 3 nanoparticles. These images as well as EDS and additional ICP-MS measurements are given in details in the Electronic supplementary materials.
XRPD structural analysis of undoped and Cu-, Se-, and Zn-doped amorphous Sb 2 S 3 samples and annealed crystalline Zn-doped Sb 2 S 3 sample XRPD is a technique used to characterize the samples' structural properties to prove their purity, desired composition, and doping [46]. The diffractograms of the prepared amorphous non-doped, Cu, Se, and Zn-doped Sb 2 S 3 samples, as well as  Fig. 6 a-e. All four non-doped and doped amorphous samples have similar broad and unstructured diffraction patterns demonstrating the amorphous structure of the measured samples [47,48]. The XRPD pattern of the annealed crystalline Zn-doped Sb 2 S 3 is shown in Fig. 6 e. The initially amorphous samples crystallize in the stibnite Sb 2 S 3 structure (COD 01-074-1046). No additional Zinc phases could be observed, indicating that the found Zinc is incorporated into the stibnite structure. However, the low Zn dopant concentration might also explain the absence of such phases. (See further in the text). Only a trace of the Sb 2 O 3 phase is present, which was formed during the heating process. Similar crystalline diffractograms were also obtained for the other non-doped and doped amorphous samples synthesized after a similar annealing process (see Electronic supplementary material).
DRS spectra of undoped, Cu-, Se-, and Zn-doped synthesized amorphous Sb 2 S 3 nanoparticles: Size dependences and bandgap values as potential consequences of the quantum size effect The bandgap (Eg) energies of the amorphous non-doped, Cu, Se, and Zn-doped Sb 2 S 3 synthesized samples were determined by performing DRS measurements and subsequent data analysis using the Tauc plot (see Fig. 7a, b). For this analysis, the absorption coefficient α is expressed by the Planck constant h, the photon's frequency ν, a constant B, which Davis and Mott described as the magnitude of the optical absorption constant [49], and a transition factor γ which equals 2 for an indirect allowed transition: We used the Kubelka-Munk function (Eq. 2) [50] to express α. F(R ∞ ) is the quotient of the absorption Fig. 3 HR-TEM micrographs of the synthesized amorphous, Se-doped Sb 2 S 3 nanoparticles. There are visible tiny nanoparticles between 1 and 4 nanometers coefficient k and the scattering coefficient s, which, in turn, is correlated to the reflectance of an infinitely thick specimen R.
We assume that the amorphous particles have an indirect bandgap, though several alternatives are available [51][52][53]. It should be emphasized that crystalline Sb 2 S 3 has a direct bandgap with a value between 1.6 and 1.7 eV [54].
This data analysis yields band gaps of 2.08 eV for the undoped, 2.07 eV for the Cu-doped, 2.03 eV for the Se-doped, and 2.12 eV for the Zn-doped Sb 2 S 3 . It should be emphasized that absorption measurements were also performed and the same values of the gaps were obtained (not shown). The results themselves can be used to draw two basic conclusions. One conclusion is that the differences in the bandgap values of non-doped and doped samples are insufficient to confirm doping. The second conclusion is that the bandgap energy values of the amorphous non-doped and doped samples are large compared to the values of large amorphous non-doped and doped synthesized particles obtained so far by the hot injection synthesis [20,22] as well as of crystalline Sb 2 S 3 . For undoped particles prepared by the same hot injection synthesis at a comparable lower temperature (150 °C), we found similar bandgap values after 30 min in earlier work [18] and even larger band gaps (2.18 eV and 2.12 eV) after shorter reaction times of 2 min and 5 min, respectively. Generally, the energy bandgap in amorphous semiconductors can vary [55,56]. Long-range disorder in amorphous nanoparticles disrupts the periodic arrangement of constituent atoms. Because atom-atom distances and binding energies vary depending on their location in amorphous semiconductors, it appears that the band edge of amorphous semiconductors becomes indistinct, Fig. 4 HR-TEM and TEM micrographs of the synthesized amorphous, Zn-doped Sb 2 S 3 nanoparticles. In the micrographs, there are also visible smaller nanoparticles with a diameter between 20 and 50 nm, which are also built of tiny nanoparticles between 1 and 4 nanometers diffusing into the bandgap. As a result of the calculated differences, we must demonstrate and prove the evidence of sample doping using additional characterization methods. SEM micrographs and DRS spectra with corresponding bandgap values for previously synthesized amorphous non-doped and Cu and Se-doped Sb 2 S 3 nanoparticles with larger particle sizes (not smaller than 100 nm) [20] are given in the Electronic Supplementary Materials. All the presented samples, non-doped, Cu and Se-doped Sb 2 S 3 nanoparticles, are, at least partially, amorphous structures. It is clear from the preceding claim that doping amorphous nanoparticles can change their electronic properties and, thus, their energy bandgap. There are also obvious differences in band gap values for different doped samples, which we do not see with the small nanoparticles obtained here (Fig. 7b). For a better understanding, Fig. 8 compares the SEM images of previous and current synthesis for a sample of Cu-doped Sb 2 S 3 amorphous nanoparticles, as well as bandgap values for all non-doped and doped samples with larger and reduced nanoparticle sizes.
It should be stressed that large reductions in the nanoparticle sizes are present in all the synthesized non-doped and Cu and Se-doped Sb 2 S 3 samples. The Zn-doped Sb 2 S 3 sample was synthesized for the first time, so we do not have comparison results. The large nanoparticle size reduction (from 100 nm to 1-4 nm) is followed by an increase in the values of the energy bandgap (~+0.4-0.6 eV), which is a strong characteristic of the quantum size effect [46,57,58]. For instance, it has been observed that Zndoping of Sb 2 S 3 thin films causes bandgap reduction [36], the same trend observed for Cu and Sedoped samples with larger nanoparticle sizes (see FESEM micrograms in the Electronic Supplementary Materials for details). In fact, the quantum size effect could be a reason for not observing a difference in the bandgap values of the non-doped and doped Sb 2 S 3 samples because the bandgap energy is also affected by the composition of nanoparticle semiconductors. These compositions, to which impurities known as dopants belong, disrupt the band structures by generating local quantum states within the band gaps. It is crucial to note that information about the electrical and optical characteristics of the Sb 2 S 3 semiconductor is lacking in the literature. We found several publications that assert that quantum size effects can be seen when a nanocrystal's one dimension is quite large (50-100 nm or even larger) and the band gap energy is above 1.7-1.8 eV. Several authors report on quantum size effects in one-dimensional (1D) Sb 2 S 3 nanowires [34,59,60] and two-dimensional (2D) Sb 2 S 3 films [61] but not yet for zero-dimensional (0D) Sb 2 S 3 nanoparticles. The Bohr radius of the excitons, which characterizes the distinct separation of a bonded electron-hole pair in a bulk semiconductor, is another significant length scale. The Coulomb potential causes bonded electron-hole pairs to be attracted to one another to form excitons, a type of quasiparticle. Since the exciton binding energy for the majority of bulk semiconductors is lower than kT at ambient temperature, electrons and holes are not bound. However, quantum size effects become significant when the produced semiconductor nanoparticles are shrunk to sizes lower than the Bohr radius of excitons. Using the density functional theory (DFT) with the generalized gradient approximation and also with an improved version of the exchange potential suggested by Tran and Blaha, we calculated that the excitons in our system should be about 1 nm in radius to observe the quantum confinement effect in one of our previous publications, where we combined experimental and theoretical results [54]. Additionally, the calculated size of excitons should be taken more as an order of particle sizes where the quantum size effect should be depicted and not the strict value due to the limitation of the single particle (one electron) approach in a many-particle system [54]. We are not aware of any work in which similar calculations have been made. Given the apparent size of nanoparticles of a few nanometers on HRTEM and the fact that even smaller particles are difficult to see for the reasons stated above, it is clear that we have a strong indication of the quantum nanoparticle size effect.  The effect of Cu, Se, and Zn ion incorporation into the Sb 2 S 3 lattice was studied by measuring the XPS spectra of the undoped and doped powders. Survey spectra of amorphous non-doped, Cu, Se, and Zn-doped Sb 2 S 3 samples, as well as the valence band spectra of non-doped and Zn-doped Sb 2 S 3 samples, are presented in Fig. 9 a-e. A spectrum of amorphous non-doped Sb 2 S 3 is provided for comparison, along with the doped samples' spectra, and a magnified section with the high-resolution characteristics of the Cu, Se, and Zn peaks are visible in the insets. It is possible to see the main photoelectron lines of Sb, S, Cu, Se, and Zn.
It should be mentioned that all the peaks of Sb and S identified in non-doped samples are present in all doped Sb 2 S 3 samples. Furthermore, because a carbon-based nanolayer is deposited on all types of samples, the C 1s peak is derived from the atmosphere [22]. As can be seen, copper is only detected in the powder of the Cu-doped Sb 2 S 3 sample (Fig. 9c) after the Cu-doping process (compared with the non-doped sample). An enlarged view of the Cu 2p lines taken from the sample is shown in the inset. Cu 2p 3/2 and 2p 1/2 lines at 932.1 eV and 951.9 eV can be attributed to the Cu ion [62,63]. It should be emphasized here that the wide band in the Cu 2p spectrum (see inset) at approximately 945.5 eV (near Cu 2p½ sub-band) is the Sb Auger line overlapped with the Sb 3s peak at 944 eV [64]. Furthermore, XPS survey spectra of Sedoped Sb 2 S 3 are shown in Fig. 9 b, with the characteristic line of Se shown in the inset at high resolution. Selenium in Se-doped samples is typically identified using Se 3d lines at 55.3 eV [65][66][67]. In our case, the Se 3d line coincides with an inelastic contribution of the Sb 4d line, preventing it from being used for Se quantification. A similar situation is observed with the Se L 3 M 4,5 M 4,5 Auger line, which overlaps with the inelastic contribution of the S 2p line and is the most intense line in the Se spectrum. Indeed, the only selenium line that does not overlap with any Sb or S line corresponds to its L 2 M 4,5 M 4,5 Auger transition (shown in the inset), which has a binding energy of 138.5 eV. Its position is slightly different in pure Se (137.8 eV [68]), most likely due to charging issues. In the spectrum of the Zn-doped Sb 2 S 3 (Fig. 9d), all zinc lines overlap with either the Sb or S lines. In one of the insets for the Zn-doped Sb 2 S 3 sample, a Zn LMM reference Auger spectrum and shape for different Zn-containing compounds [69] are given. The LMM spectrum of Zn has a very strong Poisson component (low statistics), and by using deconvolution, derivations of peak positions were made. In the second inset, a weak Zn LMM Auger peak at a kinetic energy value of 989 eV is visible, corresponding to the Zn-S connection. Given that Zn as an ion should replace Sb in the lattice in a Zn-doped Sb 2 S 3 sample, the Zn-S connection is something we expected. Due to the poor visibility of the Zn-LMM Auger line and thus the questioning of the identification of Zn in the sample, additional valence band measurements on the undoped and Zn-doped samples were performed and presented as the last spectra in Fig. 9 e. The shape of the valence band in the Zn-doped sample differs significantly from that in the non-doped sample, with the latter clearly wider. In the case of Zn doping, the valence band maximum shift with respect to the reference sample was approximately 0.55 eV towards lower binding energies. The positions of the valence band maxima were calculated by intersecting the background tangent and the steepest slope of the valence band edge. The filling of the conduction band is expected to cause the valence bands in n-type semiconductors to shift towards higher binding energy, whereas the filling of the acceptor level is expected to cause the valence bands in p-type semiconductors to shift towards lower binding energy [70]. Additionally, it should be mentioned that according to Crist [64], the main valence band of Sb 5p is located at approximately 2.5 eV and the weak sub-band of Zn 3d at 9.7 eV (it is evident that the broader and additional peak in the recorded valence band spectra of the Zn-doped Sb 2 S 3 sample appears). Also, no O 2p peak is seen in valence band measurements, nor an O 1s peak is seen in survey spectra, confirming that oxidation has not occurred in any of the synthesized samples.

Photoluminescence measurements
Photoluminescence measurements made earlier on crystalline Sb 2 S 3 particles and larger amorphized Sb 2 S 3 nanoparticles with a small crystalline fraction and a diameter around 100 nm [26], in the energy range of 2.4 to 1.5 eV, are shown in Fig. 10 a. The PL spectrum at room temperature is measured with an excitation of 2.48 eV (500 nm). The figure depicts the sharp and intense emission of the 1.65 eV peak in both samples. It should be noted that the photoluminescence and excitation spectra of crystalline and amorphous chalcogenides are nearly identical. It has been found that photoluminescence is caused by well-defined defects similar to those found in the corresponding crystals [71,72]. The bandgap energy determined by optical absorption measurements for amorphous and crystalline samples corresponds to the sharp and intense PL emission peaks ( Fig. 8 and FESEM images in the Electronic Supplementary Materials).
Emission spectra of the non-doped Sb 2 S 3 sample (Fig. 10b) obtained using stationary excitation (xenon lamp) are represented as a wide band covering the visible range of the spectrum (from green to red) and relatively narrow peaks at 741 nm (1.7 eV), 613 nm (2.0 eV), 559 nm (2.2 eV), and 507 nm (2.4 eV). Broad emission bands for Sb 2 S 3 nanoparticles, nanowires, and thin films were observed in some earlier works [36,73] and are presumably associated with intrinsic defects of the matrix. Embedding of Zn into the Sb 2 S 3 host leads to significant suppression of the wide luminescence band related to intrinsic defects. Zinc, penetrating into the cationic position of the matrix ( Zn ' Sb ), affects the defects of the anionic sublattice primarily, stimulating the formation of sulfur vacancies ( V •• S ): An increase in the concentration of vacancy defects induced by the embedding of zinc leads to the effect of concentration quenching of their emission (Fig. 10b). Although photoluminescence measurements of the Sb 2 S 3 nanoparticles are rarely reported, four narrow bands with spectral characteristics similar to our results were found in reference [74], which is devoted to the study of Sb 2 S 3 single crystals. The authors of this work have shown theoretically and experimentally that four types of excitons (conventionally designated as A, B, C, and D) with slightly different spectral parameters of luminescence can exist in Sb 2 S 3 . Four exciton types exist due to the complex band structure. These excitons are distinguished by their relatively high binding energy (100 meV), which is less than the thermal energy k B T = 26 meV at T = 300 K. The excitons are stable at room temperature, we can observe their emission response in room-temperature spectra. Previously, we found by combining experimental and theoretical calculations that an additional difference (a part of the temperature difference, 300 K at which experiments were performed and 0 K, which theory gives) can be explained only by excitonic effects that cannot be neglected here due to the high calculated energy of an exciton (0.1 eV) [54]. We found that photoluminescence studies reported, for instance, for Zn-doped nanostructured Sb 2 S 3 film [36] and reported emission peaks do not match our peaks, except for the peak at 715 nm (~1.7 eV) that we also observed in samples with larger and smaller non-doped and doped synthesized nanoparticles. The PL results confirming the possible quantum size effect should be highlighted here. If we compare Figs. 10a and b, we can see that with the reduction of particle size, there is a blue shift of the peaks, i.e., peaks at higher energies appear in accordance with the change and value of bandgap with the particle size [75]. Figure 10 c shows excitation spectra of narrow excitons-related emission bands at 1.67 eV (A-band), 2.02 eV (B-band), 2.22 eV (C-band), and 2.45 eV (D-band) for a Zn-doped Sb 2 S 3 sample. The observed excitation bands overlap with each other, and this overlapping indicates the similarity or very near origin of excitons. The maximum excitation band shifts towards higher energies with increasing luminescence energy (from A-band to D-band), indicating that the four types of excitons have slightly different energy level structures. The kinetics of emission for narrow bands was measured under pulsed laser excitation λ exc = 405 nm (E exc = 3 eV). The experimental results showed that the emission lifetime does not exceed the duration of the exciting laser pulse, which is about 100 ps. A very short lifetime is usually characteristic of mobile excitons, the relaxation of which is associated with singlet states. It can be assumed that the observed emission bands (A, B, C, and D) are associated with singlet excitons. Further, it was shown that the energy structure of excitons differs primarily in the position of the ground state level [74]. Based on the obtained spectroscopic data, we schematically depicted a system of configuration curves for the singlet ground states (S 0 ) and the excited states (S 1 ) of A, B, C, and D -excitons (Fig. 10d). From Fig. 10 d, one can see a tendency to narrow the configuration curve of the ground S 0 state from D-exciton to A-exciton. Such a system of configuration curves explains the slight differences in the maximum of the excitation band of different excitons along with a more significant change in the Stokes shift.

Conclusion
Amorphous non-doped and Cu, Se, and Zn-doped Sb 2 S 3 nanoparticles have been synthesized by a colloidal chemistry approach and characterized for potential use in solar cell devices. By using a hotinjection method at a lower temperature (150°C) with a reduced reaction time, we significantly reduce the sizes of the amorphous nanoparticles compared to those previously obtained at a similar approach at 240°C. Moreover, amorphous Zn-doped nanoparticles were obtained for the first time. HRTEM and TEM micrographs revealed nanoparticles of a few nanometers organized into larger nanoparticles, and EDX spectra and EDX mapping indicated the presence of the dopants Cu, Se, and Zn. ICP-MS confirmed the successful doping of the nanomaterial with Zn. DRS measurements and calculations were used to determine the values of the indirect bandgap of the amorphous non-doped and doped Sb 2 S 3 samples. The bandgap was determined using a Tauc plot and assuming an indirect bandgap. High bandgap values were obtained, which are not typical for this semiconductor. Compared to previously synthesized non-doped and doped amorphous nanoparticles of the same composition from a similar hot injection synthesis at a higher temperature (240 °C), the newly obtained, significantly smaller nanoparticles show a strong increase in the bandgap values for ~+0.4-0.6 eV, which could be attributed to the quantum size effect. XRPD measurements of amorphous samples produce broad, unstructured diffractograms, supporting the samples' amorphous nature. In addition, the Zn-doped sample was heated at 270 °C. For this sample, the XRPD measurements show a pure crystalline phase of Sb 2 S 3 with no other phases (except for a small amount of Sb 2 O 3 due to the heating in the presence of air) that could form the doping elements or foreign elements within the detection limit. XPS survey spectra of the amorphous non-doped and doped samples show characteristic peaks corresponding to the appropriate semiconductor and dopant elements. Because the corresponding Zn and Se peaks correspond to the S and Sb characteristic peaks, the Auger lines for Zn and Se were used as indications of doping. Photoluminescence measurements were performed on the non-doped and Zn-doped Sb 2 S 3 amorphous nanoparticles. The emission spectra of the Sb 2 S 3 nanoparticle samples show a wide band covering the visible range of the spectrum and relatively narrow peaks at 1.7 eV, 2.0 eV, 2.2 eV, and 2.4 eV, in contrast to previously synthesized larger nanoparticles where only one narrow band at 1.7 eV was observed. Embedding of Zn into the Sb 2 S 3 host leads to significant suppression of the wide luminescence band related to intrinsic defects.
Further steps will be the application of the synthesized non-doped and doped Sb 2 S 3 amorphous quantum dots nanomaterials in different solar device configurations. We have prior experience dealing with this topic, which involves fabricating and characterizing various novel types of solar cells based on the synthesized semiconductor. As far as we are aware, synthesized Sb 2 S 3 quantum dots were never applied in solar cells, and although the general fact that bulk Sb 2 S 3 have a more suitable gap for solar cell application, quantum dots can be found in applications with different semiconductors type solar cells. Also, different quantum dot solar cell configurations for enhancing the conversion efficiency were proposed earlier [76,77]. However, these potential high-efficiency configurations are mostly theoretical, and there are no experimental results so far. Perhaps these synthesized and designed quantum dots can be applied to some of those theoretically proposed designs.
Funding The research was funded by the Ministry of Science,Technological Development and Innovation of the Republic of Serbia. Yu. K. and D. Z. are grateful to the Ministry of Education and Science of Russian Federation (project no. FEUZ-2023-0014) for support. This work was also funded by the German Academic Exchange Service (DAAD) within the PPP Serbia program (grant 57447826). The work of M. J. was supported by a fellowship of the Platform for Ph. D. students of the Technical University of Darmstadt and the Darmstadt University of Applied Sciences. We thank Stefanie Schmidt from the Technical University of Darmstadt for the ICP-MS measurements.