2024 Volume 92 Issue 7 Pages 077005
Ternary Ag8SnSe6 quantum dots (QDs) were synthesized via a heating-up method in which the reaction of corresponding metal acetates and selenourea was carried out at 250 °C in oleylamine containing 1-dodecanethiol (DDT) as a capping ligand. The obtained QDs were spherical particles with a cubic Ag8SnSe6 crystal structure. The size of Ag8SnSe6 QDs decreased from 7.8 to 4.9 nm as the amount of DDT in the reaction mixture was increased from 0 to 0.23 mmol. The absorption spectra of obtained QDs were broad, and the wavelength of the absorption onset was blue-shifted from ca. 1600 nm to ca. 1300 nm with an increase in the amount of DDT added. The energy gap determined from Tauc plots of the absorption spectra increased from 0.82 eV to 1.10 eV with a decrease in the QD size from 7.8 to 4.9 nm. Photoelectrochemical measurements revealed that Ag8SnSe6 QDs immobilized on ITO electrodes generated photocurrents under light irradiation. The action spectra of photocurrents roughly matched the corresponding absorption spectra, indicating that Ag8SnSe6 QDs photoexcited with near-IR light generated photocurrents. The onset potentials of photocurrent generation were located at 0.20–0.27 V vs. Ag/AgCl for the QDs. This suggested that intragap states, acting as trap sites for photogenerated carriers, were located above the valence band maximum (VBM) level of QDs and mediated the electron and hole transfers to ITO electrodes, generating anodic and cathodic photocurrents, respectively.
Increasing demand for energy from fossil fuels has led to environmental pollution issues and climate changes, making the exploration of clean energy resources an urgent research priority. Solar energy, being endless and abundant, offers vast opportunities for the development of unlimited clean energy solutions. For efficient solar energy conversion, semiconductor nanoparticles smaller than approximately 10 nm, known as quantum dots (QDs), have gained considerable attention. The confinement of charge carriers results in unique size-dependent properties of semiconductor particles when their size approaches twice the exciton Bohr radius.1–6 This feature makes the optical properties of QDs particularly well-suited for efficient light energy conversion. Since the early development, binary metal chalcogenide QDs, such as CdS,7,8 CdSe,9,10 PbS,11,12 and PbSe,13,14 have been intensively studied to develop their feasible synthesis methods and achieve high efficiency in solar cells or photocatalysts. However, the heavy metals contained in these QDs severely limit their practical and commercial use. Although binary QDs have been extensively studied, recent attention has shifted towards multinary QDs due to their wider absorption range and environmentally friendly characteristics.
In recent years, abundant and less toxic I–IV–VI-based semiconductors such as Cu2SnS3,15,16 Ag8GeS6,17,18 Ag6SnS6,19,20 and Ag8SnSe621 have been explored for their applications in solar cells, photovoltaics, and photocatalysts due to their band gaps (Egs) that are suitable for solar light absorption, high absorption coefficient, and significant carrier mobility. Among these semiconductors, Ag8SnSe6 is particularly noteworthy because it can absorb light across wide visible and near-IR wavelength ranges up to about 1500 nm due to its narrow band gap of 0.83 eV in bulk material.22–24 However, most previous studies have focused on their thermoelectric properties, with limited research on their photoelectrochemical performance. Cheng et al. reported the preparation of cubic AgSnSe2 thin film photoelectrodes via selenization of Ag–Sn metal precursors and investigated the impact of the silver fraction in AgSnSe2 films on their photoelectrochemical properties:25 The films had Eg values of 1.19–1.30 eV and exhibited an n-type semiconductor behavior, the optimum anodic photocurrent being observed for the AgSnSe2 film with an Ag/(Ag + Sn) ratio of 0.48. The same group also prepared ternary AgSnSe2 and Ag8SnSe6 semiconductor photoelectrodes in a similar manner, with increasing Ag/(Ag + Sn) ratios shifting the crystal phases from cubic AgSnSe2 to cubic Ag8SnSe6.26 The prepared Ag8SnSe6 films with Eg of 0.86–0.90 eV exhibited an anodic photocurrent in an aqueous solution containing Na2S and K2SO3 as hole scavengers. Despite these findings, there has been no report on the preparation of Ag8SnSe6 in nano-sized crystalline form using a feasible solution-phase method or on the effect of varying particle sizes on their electronic energy structure and photoelectrochemical performance.
Here, we report the synthesis of colloidal Ag8SnSe6 QDs through thermal reactions of corresponding metal acetates and selenourea in a hot oleylamine (OLA) solution. The QD size was controlled by varying the amount of 1-dodecanethiol (DDT) added as a capping ligand in OLA. The Eg of the QDs was tunable in the near-IR region by changing the particle size. The photoelectrochemical properties were analyzed on the basis of their experimentally evaluated electronic energy structure, demonstrating their potential as a light absorber in photovoltaics for near-IR light.
The following reagents were commercially available and were used without purification. Tin (IV) acetate (Sn(OAc)4) and selenourea were purchased from Sigma-Aldrich, and silver (I) acetate (Ag(OAc)) was obtained from Kishida Chemical. Oleylamine (OLA, Sigma-Aldrich) and 1-dodecanethiol (DDT, FUJIFILM Wako Pure Chemical Corporation) were used as solvents for the synthesis of QDs. 1,2-Ethanedithiol (EDT) was bought from Tokyo Chemical Industry. Acetonitrile (Kishida Chemical), triethanolamine (TEOA) (FUJIFILM Wako Pure Chemical Corporation), and lithium perchlorate (Kishida Chemical) were used as electrolytes for photoelectrochemical measurements. Other reagents (hexane, ethanol, and methanol) were purchased from Kishida Chemical.
2.2 Synthesis of Ag8SnSe6 QDs by a heating-up methodA typical synthesis of Ag8SnSe6 QDs with an Ag/Sn ratio of 4 was carried out by the following procedure. A mixture powder of Ag(OAc) (0.12 mmol) and Sn(OAc)4 (0.030 mmol) served as metal precursors, and selenourea (0.15 mmol) was used as an Se precursor. These precursors were added to test tubes with 3.0 cm3 volumes of OLA containing various amounts of DDT in the range between 0 and 0.23 mmol. The reaction suspension was degassed under vacuum and then heat-treated at 250 °C for 5 min with vigorous stirring under an N2 atmosphere, resulting in a black-colored suspension. After cooling to room temperature, the suspension was subjected to centrifugation at 4000 rpm for 5 min to remove large precipitates. By adding 4.0 cm3 ethanol to the thus-obtained supernatant, the target QDs were isolated as precipitates. The obtained QDs were washed three times with ethanol, followed by centrifugation. The precipitates were dispersed in 2.0 cm3 hexane for further experiments. The synthesis of Ag8SnSe6 QDs was also carried out with different Ag/Sn ratios, ranging from 0 to 8, and with fixed amounts of Sn(OAc)4 (0.030 mmol) and selenourea (0.15 mmol) to investigate the influence of the Ag/Sn ratio on the crystal structure of the resulting QDs. For comparison, the synthesis of Ag2Se QDs was carried out without addition of the Sn precursor in a similar way, in which 3.0 cm3 OLA containing Ag(OAc) (0.12 mmol) and selenourea (0.15 mmol) was heat-treated at 250 °C for 5 min with vigorous stirring under an N2 atmosphere.
2.3 Characterization of Ag8SnSe6 QDsAbsorption properties of QDs were evaluated using a JASCO V-770 UV-Visible/NIR spectrophotometer. Samples for transmission electron microscope (TEM) measurements were prepared by dropping a QD hexane solution onto a Cu TEM grid coated with amorphous carbon (Okenshoji Co., Ltd., ELS-C10 STEM Cu100P grid), followed by drying under vacuum. TEM images were acquired using a Hitachi H-7650 TEM operating at 100 kV, and they were used to analyze the morphology and size distribution of QDs. The chemical composition of QDs was investigated by an EDS analyzer (Horiba, Emax Energy EX-250) or an X-ray fluorescence spectrometer (Rigaku, NEX CG). The ionization energy was determined with photoemission yield spectroscopy in air (PYSA, Riken Keiki AC-2). A Rigaku SmartLab-3K X-ray diffractometer equipped with CuKα radiation was utilized to analyze the crystal structure of QDs.
2.4 Photoelectrochemical measurements of Ag8SnSe6 QDsAg8SnSe6 QDs were loaded on ITO substrates in a way similar to that previously reported in our paper.27 The ITO substrates were cleaned through sonication in acetone, followed by washing with ethanol and acetone before use. A portion of the Ag8SnSe6 QDs solution (40 mm3) was spin-coated on the ITO substrate (2.0 × 1.4 cm2) at 1000 rpm for 30 s. The resulting Ag8SnSe6 QDs film, the absorbance of which was ca. 0.3 at 600 nm, was immersed in an ethanol solution containing 10 wt% 1,2-ethanedithiol (EDT) for 24 h to perform ligand exchange on the QD surface, followed by washing with ethanol and drying under an N2 flow. The prepared QD-deposited ITO substrate was annealed at 150 °C for 30 min under vacuum to remove residual organic ligands on the surface of Ag8SnSe6 QDs and ITO substrates.
The photoelectrochemical properties of Ag8SnSe6 QDs deposited on ITO electrodes were evaluated in an acetonitrile solution containing 0.10 mol dm−3 LiClO4 and 0.10 mol dm−3 TEOA as a hole scavenger under an N2 atmosphere. The electrode potential was measured against an Ag/AgCl (saturated KCl) reference electrode, with a Pt wire used as the counter electrode. A potentiostat (Hokuto Denko, HAB-151A) was used to measure the photocurrent. Light irradiation was performed using a 300 W Xe lamp (λ > 350 nm) with an intensity of 0.10 W cm−2. For measuring the incident photon-to-current efficiency (IPCE), monochromatic lights were generated by passing the Xe lamp light through a monochromator (JASCO, CT-10T) and then irradiated onto the Ag8SnSe6 QD-deposited ITO electrodes in the solution, using a light chopper set at 7 Hz. The monochromatic lights intensities ranged from 5 to 25 mW cm−2. The photocurrent was recorded using a Hokuto Denko HAB-151A potentiostat and amplified with an NF circuit LI5640 lock-in amplifier. The IPCE values were determined by dividing the number of electrons detected in the photocurrent by the number of incident photons.
The syntheses of Ag8SnSe6 QDs were carried out at different temperatures, ranging from 100 to 250 °C, to determine the optimal reaction temperature. The reaction temperature for synthesis significantly affected the particle size and crystal structure of the resulting QDs. Figures 1a–1d show TEM images of particles obtained at different temperatures. Spherical particles were formed. Although the average size slightly increased from 10 nm to 12 nm with an increase in the reaction temperature from 100 °C to 150 °C, the QD size inversely decreased from 12 nm to 7.8 nm with an increase in the reaction temperature from 150 °C to 250 °C (Fig. S1). Figure 1e shows the absorption spectra of resulting QDs dispersed in hexane. The spike-like noises around 1700 nm in the absorption spectra originated from the absorption of probe light by the solvent, and the step changes observed in the absorption spectra at 755 nm were caused by the detector switching in the spectrophotometer. The QDs prepared at 250 °C were uniformly dissolved in hexane, and the absorption onset was observed at around 1600 nm. However, those prepared at 200 °C or a lower temperature produced solutions with an absorption onset wavelength longer than 2200 nm, probably due to the formation of Ag2Se particles as a by-product considering the bulk Eg of β-Ag2Se at 0.15 eV.28 Thus, in the following experiments, we chose the reaction temperature of 250 °C as the optimal condition in the present study.
(a–d) Representative TEM images of Ag8SnSe6 QDs prepared at reaction temperatures of 100 (a), 150 (b), 200 (c), and 250 °C (d). (e) Absorption spectra of Ag8SnSe6 QDs dispersed in hexane. The reaction temperatures for synthesis are represented in the panel.
We investigated the influence of the Ag/Sn ratio in precursors on the crystal structure of the resulting QDs. Figure 2 shows XRD patterns of QDs prepared with different Ag/Sn ratios. The QDs prepared with Ag/Sn ratios of 3–5 exhibited diffraction peaks assigned well to those of cubic Ag8SnSe6. However, the preparation with lower or higher Ag/Sn ratios resulted in the formation of by-products: In addition to the peaks originating from the Ag8SnSe6 crystal structure, QDs prepared with an Ag/Sn ratio of 2 or lower exhibited diffraction peaks at 30.70°, 40.04°, and 47.57°, assignable to a hexagonal SnSe crystal structure. In contrast, QDs prepared with an Ag/Sn ratio of 6 or higher showed peaks at 35.89° and 44.37°, assignable to Ag2Se with an orthorhombic crystal structure. The preparation without the addition of an Sn precursor, that is at the highest limit of Ag/Sn ratio, also produced Ag2Se QDs with an orthorhombic crystal structure. Furthermore, the decrease in the amount of selenourea in the precursors caused an increase in the content of Ag2Se crystal as a by-product in the resulting particles (Fig. 3), even when the amounts of Ag and Sn precursors were unchanged. The QDs prepared with Ag/Sn/Se = 4/1/3 exhibited XRD peaks only assignable to Ag2Se crystals of orthorhombic or cubic structures, even though the preparation was carried out under an Sn-rich condition. With an increase in the Se fraction in the precursors, the XRD peaks assignable to Ag8SnSe6 crystal were relatively enlarged. These phenomena are reasonably explained by the formation of Ag2Se nuclei at the initial stage of the reaction (Scheme 1). The heat treatment of the reaction solution at 250 °C caused rapid nucleation of Ag2Se nanocrystals, which then acted as nuclei for the subsequent growth of Ag8SnSe6 nanocrystals through co-doping with Sn4+ and Se2−. However, considering the absence of SnSe crystal phase in the QDs obtained with the precursor ratio of Ag/Sn/Se = 4/1/5, a sufficient amount of Se2− ions in the solution enabled the co-doping of Sn4+ and Se2− ions into Ag2Se nanocrystals more effectively, forming Ag8SnSe6 without producing SnSe a by-product. With a small amount of the Se precursor in the solution phase, it was difficult for Sn4+ ions to be incorporated into the lattice during synthesis, leading to the predominant formation of Ag2Se. Consequently, in the present study, we selected optimal reaction conditions for the formation of Ag8SnSe6 nanocrystals without by-products: precursor ratio of Ag/Sn/Se = 4/1/5 and reaction temperature of 250 °C.
XRD patterns of Ag8SnSe6 QDs prepared with different Ag/Sn ratios. The standard diffraction patterns of cubic Ag8SnSe6 (PDF card# 00-019-1133), orthorhombic Ag8SnSe6 (PDF card# 01-070-8907), orthorhombic Ag2Se (PDF card# 01-080-7685), and hexagonal SnSe (PDF card# 01-089-3197) are also shown as references.
XRD patterns of Ag8SnSe6 QDs prepared with different Se ratios. The numbers in the panel represent the Ag/Sn/Se ratios used for QD synthesis. The standard diffraction patterns of cubic Ag8SnSe6 (PDF card# 00-019-1133), cubic Ag2Se (PDF card# 00-027-0619), and orthorhombic Ag2Se (PDF card# 00-024-1041) are also shown as references.
Schematic illustration of the formation of Ag8SnSe6 QDs. (a) The heat treatment of the reaction solution caused rapid nucleation of Ag2Se nanocrystals, which then acted as nuclei for the subsequent growth of Ag8SnSe6 nanocrystals through co-doping with Sn4+ and Se2− in the presence of a large excess of Se2−. (b) It was difficult for Sn4+ ions to be doped into Ag2Se nanocrystals when only a small amount of Se2− ions was present in the solution phase.
The optical properties of QDs are tunable by their particle size due to the quantum size effect. We modulated the size of Ag8SnSe6 QDs by adding DDT as a surface capping ligand in the reaction solution. As shown in Figs. 4a–4c and S2, spherical particles were observed regardless of the amount of DDT added. The average size of QDs monotonously decreased from 7.8 nm to 4.9 nm with an increase in the amount of DDT added from 0 to 0.23 mmol (Figs. 4d and S3). The obtained QDs were not monodisperse regardless of the average size (Fig. S3), the standard deviations of size distributions being 8–14 % of the corresponding average sizes. XRD diffraction patterns of each type of QDs were attributed to a cubic Ag8SnSe6 crystal structure, and no peaks originating from the crystal phases of by-products, such as Ag2Se and SnSe, were observed (Fig. S4). The chemical composition of the QDs remained almost constant, close to the stoichiometric value of Ag8SnSe6, regardless of their particle size, although they exhibited Ag-deficient and slightly Sn-rich compositions (Table S1 and Fig. 4e). Figure 4f shows the absorption spectra of the obtained QDs dispersed in hexane. The outline of each spectrum was broad without showing any sharp exciton peaks. The wavelength of the absorption onset was blue-shifted from ca. 1600 nm to ca. 1300 nm with an increase in the amount of DDT added. It should be noted that the QDs did not exhibit a sharp absorption edge, regardless of their average sizes. This is probably because their size distribution was somewhat wide, and the presence of larger particles having smaller Eg significantly influenced the wavelength of absorption onset. No photoluminescence was observed for the present Ag8SnSe6 QDs.
(a–c) Representative TEM images of Ag8SnSe6 QDs prepared with DDT addition of 0 (a), 0.10 (b), and 0.23 mmol (c). (d) Change in the average size of QDs with an increase in the amount of DDT added. Error bars represent the standard deviation. (e) Chemical compositions of Ag8SnSe6 QDs prepared with different amounts of DDT. Dotted lines represent the stoichiometric values of Ag8SnSe6. (f) Absorption spectra of Ag8SnSe6 QDs dispersed in hexane, prepared with different amounts of DDT. (Inset) Relationship between Eg and average size of QDs.
The Eg of QDs was determined by making Tauc plots of absorption spectra (Fig. S5), considering that Ag8SnSe6 had a direct band gap.29,30 In the Tauc plot, a linear relationship between (αhν)2 and photon energy (hν) was observed in the energy region higher than the onset energy of each absorption spectrum. This straight line, when extrapolated to the abscissa, provided an intercept corresponding to the Eg of the QDs. Ag8SnSe6 QDs of 7.8 nm in size had an Eg of 0.82 eV, being in good agreement with that of bulk Ag8SnSe6, 0.83 eV.22–24 The Eg of QDs increased from 0.82 eV to 1.10 eV with a decrease in the average size from 7.8 nm to 4.9 nm (inset of Fig. 4f). The quantum size effect becomes remarkable for QDs with a diameter smaller than twice the exciton Bohr radius (aB). Although the dielectric constant of bulk Ag8SnSe6 has not been reported, we can roughly estimate aB of Ag8SnSe6 to be 2.9 nm by using the dielectric constant of εr = 10.531 for an Ag-doped SnSe semiconductor and effective masses of electron and hole (0.2m032 and 6.5m0,33 respectively) for Ag8SnSe6, where m0 is the free electron mass. Thus, since the average sizes of the Ag8SnSe6 QDs were comparable to or smaller than the estimated 2aB value of 5.8 nm for Ag8SnSe6, it was concluded that the changes in the absorption spectra and Eg values observed in the present study were attributed to the quantum size effect.
The energy structure of Ag8SnSe6 QDs is important information for fabricating QD-based optoelectronic devices such as LEDs and solar cells. The ionization energy of QDs was evaluated as the onset energy of PYSA spectra (Figs. 5a–5c and S6): The ionization energy of Ag8SnSe6 QDs remained almost constant, between 5.1 and 5.0 eV, as the QD size decreased from 7.8 nm to 4.9 nm. The opposite sign of ionization energy was assumed to correspond to the energy level of the valence band maximum (VBM), and the energy level of the conduction band minimum (CBM) was estimated by subtracting the Eg value of corresponding QDs from the VBM level. Figure 5d shows the energy structure of Ag8SnSe6 QDs as a function of the average size. The VBM level was almost constant at −5.0–−5.1 eV, while the CBM level shifted to a lower energy from −3.9 to −4.3 eV with an increase in the QD size. Considering that the effective mass of electron (0.2m0) was reported to be much lighter than that of hole (6.5m0) for Ag8SnSe6, it seems reasonable that the dependency of CBM level on QD size was greater than that of VBM level, because the shifts of CBM and VBM are assumed to be proportional to the reciprocals of effective masses of electron and hole, respectively.34
(a–c) Representative PYSA spectra of Ag8SnSe6 QDs with average sizes of 7.8 (a), 6.1 (b), and 4.9 nm (c), which were prepared with DDT addition of 0, 0.10, and 0.23 mmol, respectively. (d) Energy levels of the valence band maximum (VBM) (solid circles) and the conduction band minimum (CBM) (open circles) of Ag8SnSe6 QDs as a function of their particle size. The photocurrent onset potentials (solid squares) were also plotted.
Ag8SnSe6 QDs were loaded onto ITO substrates for photoelectrochemical measurements by spin coating, followed by cross-linking between QDs with EDT, and then the resulting films were annealed at 150 °C for 30 min. We also prepared QD films on quartz plates in a similar way to confirm the stability of the Ag8SnSe6 film during annealing. As shown in Fig. S7, the absorption spectra were almost the same before and after heat treatment, suggesting that the QDs immobilized on the substrates did not coalesce with each other to form larger particles with the 150 °C treatment. Figures 6a, 6c, and 6e show the photoelectrochemical response of Ag8SnSe6 QD-loaded ITO electrodes in an acetonitrile solution containing TEOA. The potentials of photocurrent onsets were observed at 0.27, 0.20, and 0.24 V vs. Ag/AgCl for the QDs with sizes of 7.8, 6.1 and 4.9 nm, respectively. Both cathodic and anodic photocurrents were observed, regardless of the particle sizes of QDs used, depending on the potential application. Irrespective of the QD size, the cathodic photocurrent was detected at potentials more negative than the onset potential, probably as a result of the reduction of H2O as an impurity in the solution by photogenerated electrons. In contrast, applying a potential more positive than the onset potential produced an anodic photocurrent, which was generated by the oxidation of TEOA with holes. The action spectra of cathodic photocurrents were measured under the potential application at −0.20 V vs. Ag/Ag/Cl, as shown in Figs. 6b, 6d, and 6f. In each case, the photoresponse was observed with light irradiation from visible to near-IR wavelength regions, and the profile of the action spectrum roughly agreed with that of the corresponding absorption spectrum. Since the intensities of monochromatic lights used for IPCE measurements were not constant and significantly varied with the wavelength of light (Fig. S8), it is possible that the observed action spectra were influenced by the spectral profile of the irradiation light source in the present study. That is, the irradiation intensities at around 680 nm, 770 nm, and 1000 nm or longer were lower than those of monochromatic lights at other wavelengths, and then the experimental errors of calculated IPCE values seemed to became large in these wavelength regions. This resulted in the disagreement of onset wavelength between absorption and photocurrent action spectra, as well as the appearance of peaks at approximately 680 and 770 nm in the action spectra.
(a, c, e) Photocurrent-potential curves of Ag8SnSe6 QD-immobilized ITO electrodes. The average sizes of QDs used were 7.8 (a), 6.1 (c), and 4.9 nm (e). The electrolyte used was an acetonitrile solution containing 0.10 mol dm−3 LiClO4 and 0.10 mol dm−3 TEOA. The arrows in the panels indicate the potentials of photocurrent onsets. (b, d, f) Action spectra of photocurrent (showed in blue circles) under potential application at −0.20 V vs. Ag/AgCl with the use of Ag8SnSe6 QD films of 7.8 (b), 6.1 (d), and 4.9 nm (f) in average size. Dashed lines are the absorption spectra of the QDs used. (g) Schematic illustration of the generation of both cathodic and anodic photocurrents with Ag8SnSe6 QDs.
The photocurrent onset potentials were also plotted in Fig. 5d as a function of the size of Ag8SnSe6 QDs. The onset potentials were located between the VBM and CBM levels, being close to the VBM level, regardless of the particle size. This suggested the presence of intragap states acting as trap sites for photogenerated carriers at the photocurrent onset potentials. As shown in Fig. 6g, when the potential is more negative than that of intragap states, photogenerated holes are trapped at these states, mediating the hole transfer to ITO electrodes to produce cathodic photocurrents. Conversely, when the ITO electrode is at a potential more positive than that of intragap states, photogenerated electrons trapped at these states can move to ITO electrodes, resulting in the generation of anodic photocurrents.
Ag8SnSe6 QDs with a cubic crystal structure were successfully prepared without any by-products by using a heating-up method with precisely controlled preparation conditions including reaction temperature and precursor ratio. The optimal conditions were determined to be precursor ratio of Ag/Sn/Se = 4/1/5 and reaction temperature of 250 °C. The particle size of the QDs was tunable by increasing the amount of DDT added during preparation. The Eg of Ag8SnSe6 QDs increased from 0.82 eV to 1.10 eV with a decrease in the average size from 7.8 nm to 4.9 nm due to the quantum size effect. We found that the VBM level was almost constant at −5.0–−5.1 eV, while the CBM level shifted to lower energy from −3.9 to −4.3 eV with an increase in the QD size. The Ag8SnSe6 QDs exhibited photoresponsivity in visible and near-IR regions and had intragap states close to the VBM level that acted as carrier trap sites to mediate charge transfer from QDs to ITO electrodes. These findings provide critical insights into the synthesis and optimization of environmentally friendly multinary metal chalcogenide QDs, paving the way for their potential use in advanced optoelectronic devices such as LEDs and solar cells.
The work was supported by JSPS KAKENHI Grant Numbers JP21K14703, JP22K19083, and JP22H00341. One of the authors (N.R.) acknowledges Grant-in-Aid for JSPS Fellows, Grant Number JP23KJ1087.
The data that support the findings of this study are openly available under the terms of the designated Creative Commons License in J-STAGE Data at https://doi.org/10.50892/data.electrochemistry.26075278.
Nurmanita Rismaningsih: Conceptualization (Lead), Data curation (Equal), Formal analysis (Lead), Investigation (Equal), Writing – original draft (Lead)
Takayuki Takiyama: Data curation (Equal), Investigation (Equal)
Kazutaka Akiyoshi: Data curation (Equal), Formal analysis (Equal)
Tatsuya Kameyama: Data curation (Equal), Formal analysis (Equal)
Tsukasa Torimoto: Conceptualization (Lead), Data curation (Equal), Formal analysis (Equal), Funding acquisition (Lead), Supervision (Lead), Writing – original draft (Lead)
The authors declare no conflict of interest.
Japan Society for the Promotion of Science: JP21K14703
Japan Society for the Promotion of Science: JP22K19083
Japan Society for the Promotion of Science: JP22H00341
Japan Society for the Promotion of Science: JP23KJ1087
N. Rismaningsih: ECSJ Student Member
K. Akiyoshi, T. Kameyama, and T. Torimoto: ECSJ Active Members
