Wavelength-Tunable Band-Edge Photoluminescence of

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Wavelength-Tunable Band-Edge Photoluminescence of Nonstoichiometric Ag−In−S Nanoparticles via Ga3+ Doping Tatsuya Kameyama,† Marino Kishi,† Chie Miyamae,† Dharmendar Kumar Sharma,‡ Shuzo Hirata,‡ Takahisa Yamamoto,† Taro Uematsu,§ Martin Vacha,‡ Susumu Kuwabata,*,§ and Tsukasa Torimoto*,† †

Graduate School of Engineering, Nagoya University, Chikusa-ku, Nagoya 464-8603, Japan Department of Materials Science and Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo 152-8552, Japan § Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan

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S Supporting Information *

ABSTRACT: The nonstoichiometry of I−III−VI semiconductor nanoparticles, especially the ratio of group I to group III elements, has been utilized to control their physicochemical properties. We report the solution-phase synthesis of nonstoichiometric Ag−In−S and Ag−In−Ga−S nanoparticles and results of the investigation of their photoluminescence (PL) properties in relation to their chemical compositions. While stoichiometric AgInS2 nanoparticles simply exhibited only a broad PL band originating from defect sites in the particles, a narrow band edge PL peak newly appeared with a decrease in the Ag fraction in the nonstoichiometric Ag−In−S nanoparticles. The relative PL intensity of this band edge emission with respect to the defect-site emission was optimal at a Ag/(Ag + In) value of ca. 0.4. The peak wavelength of the band edge emission was tunable from 610 to 500 nm by increased doping with Ga3+ into Ag−In−S nanoparticles due to an increase of the energy gap. Furthermore, surface coating of Ga3+-doped Ag−In−S nanoparticles, that is, Ag−In−Ga−S nanoparticles, with a GaSx shell drastically and selectively suppressed the broad defect-site PL peak and, at the same time, led to an increase in the PL quantum yield (QY) of the band edge emission peak. The optimal PL QY was 28% for Ag−In−Ga−S@GaSx core−shell particles, with green band-edge emission at 530 nm and a full width at half-maximum of 181 meV (41 nm). The observed wavelength tunability of the band-edge PL peak will facilitate possible use of these toxic-element-free I−III−VI-based nanoparticles in a wide area of applications. KEYWORDS: semiconductor nanocrystals, quantum dots, I−III−VI2 semiconductor, multinary semiconductor, nonstoichiometry, band-edge photoluminescence, visible photoluminescence, wavelength tunability

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INTRODUCTION Multinary I−III−VI semiconductors and related materials exhibit unique composition-dependent physicochemical properties due to their structural tolerance to large nonstoichiometry,1−4 and they have attracted much attention for possible applications in various photofunctional devices such as solar cells5,6 and photocatalyts.7−12 In addition to the tunability of their properties through chemical composition, controlling the size of these materials in a size-quantized regime is another strategy for obtaining the desired optical properties. Since nanoparticles of CuInS2,13 AgInS2,14 and their solid solution with ZnS14,15 were reported to exhibit strong photoluminescence (PL) in the visible-light wavelength region, I−III−VI-based nanoparticles with well-controlled size, structure, and chemical composition have been intensively developed as novel quantum dots for practical use.16−31 Advantages of these materials include a direct band gap, nontoxic composition, and strong absorption coefficients in the © XXXX American Chemical Society

visible to near-IR wavelength regions. For example, solidsolution nanoparticles of AgInS2 −ZnS, 32−41 AgInSe 2 − ZnSe,42,43 AgGaS2−ZnS,44 and CuInS2−ZnS45 have tunable energy gaps (Egs) depending on the particle size and Zn content in the particles and they exhibit a broad PL peak assignable to defect-site emissions, the peak wavelength of which decreases with an increase in the Eg. The optical properties of these nanoparticles were varied by different ratios of group I to group III elements:38,46−48 the Eg was enlarged with a decrease in the Ag/In ratio in Ag−In−S nanoparticles, resulting in large variations of the wavelength of the broad PL peak and its quantum yield (QY).49,50 Wood and co-workers reported that with a decrease in the particle size, the Eg of Ag− In−Se nanoparticles increased and their PL peak wavelength Received: September 3, 2018 Accepted: November 21, 2018

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DOI: 10.1021/acsami.8b15222 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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the defect-site emission was considerably suppressed by the surface coating of Ag−In−S and Ag−In−Ga−S cores with GaSx shells. The wavelength of the band-edge emission peak was controllable in the range between ca. 610 and 500 nm by changing the In3+/Ga3+ ratio in the nonstoichiometric Ag−In− Ga−S core particles.

was blue-shifted due to the quantum size effect, the degree of which was dependent on the Ag/In ratio.51 Zhong and coworkers successfully prepared PL-color-tunable CuInS2 nanoparticles with a high PL QY by tuning their nonstoichiometry.47 Cu-deficient CuInS2 nanoparticles prepared by Kamat and co-workers showed excited-state charge carrier dynamics that were dependent on the Cu/In ratio.48 To date, reports of narrow band-edge emission for I−III−VI-based nanoparticles have been scarce, unlike those of conventional binary nanoparticles such as CdSe, CdTe, and PbS. Thus, although little experimental evidence and theoretical evidence have been obtained, a general consensus seems to be that the optical properties of I−III−VI semiconductor nanoparticles are characterized both by a lack of exciton peaks in absorption spectra and by the exhibition of a broad defect-site PL peak with a large Stokes shift, which may limit large-scale commercial applications. In contrast, we have observed by single-particle spectroscopy that the large width of the defect PL emission of (AgIn)xZn2(1‑x)S2 nanoparticles is largely due to inhomogeneous broadening of the size distribution and to spectral diffusion in single nanocrystals.52 Similarly, Klimov and coworkers later found by single-particle spectroscopy that the single dot of a CuInS2 nanoparticle covered with a thick ZnS shell exhibited a much smaller PL emission peak width (ca. 60 meV) than those obtained with ensemble measurements.53 Recently, we have successfully prepared I−III−VI-based nanoparticles that showed only a narrow band-edge emission peak in their PL spectra with high QYs.54,55 Rod-shaped AgInTe2 nanoparticles with almost stoichiometric composition exhibited a clear exciton peak in the absorption spectra and then showed a narrow band-edge PL peak at 1010 nm with a PL QY of 47%. The wavelength of the band-edge PL peak was further blue-shifted to 809 nm by making a solid solution with ZnTe.55 Although stoichiometric AgInS2 nanoparticles exhibited a broad PL peak, surface coating with a GaSx shell could produce AgInS2@GaSx core−shell particles exhibiting an intense band-edge PL peak at 585 nm with a considerably high PL QY of 56%.56 Furthermore, it was reported in our previous paper57 that the Eg of AgInS2 was controllable by making a solid solution with AgGaS2, though the resulting Ag−In−Ga− S nanoparticles exhibited a broad PL peak assignable to defectsite emission. These results suggest that precise composition control of Ag−In−Ga−S nanoparticles enhances their PL properties and then the band-edge emission peak can be controlled in the visible-light wavelength region by changing their Eg. However, the preparation of band-edge-emissive Ag− In−Ga−S nanoparticles remains a challenge. Furthermore, there has been no report on the investigation of the influence of the nonstoichiometry of AgInS2 nanoparticles on their bandedge emission, even though tuning the chemical composition of I−III−VI nanoparticles may provide a route to optimize their optical properties. Here, we report the solution-phase synthesis of nonstoichiometric Ag−In−S (AIS) and Ag−In−Ga−S (AIGS) nanoparticles and an investigation of their PL properties in relation to their chemical compositions. The as-obtained nanoparticles exhibited an intense narrow band-edge emission in addition to a broad defect-site emission on the longer wavelength side of the band-edge PL peak. The relative peak intensity of the band-edge emission to the defect-site emission was optimal for Ag−In−S particles with significant nonstoichiometric composition. Furthermore, the PL intensity of

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EXPERIMENTAL SECTION

Materials. Indium(III) acetate [In(OAc)3], indium(III) acetylacetonate [In(acac)3], gallium(III) acetylacetonate [Ga(acac)3], and oleylamine (OLA) were purchased from Sigma-Aldrich. Elemental sulfur and 1-dodecanethiol (DDT) were supplied by Wako Chemicals. Silver acetate [Ag(OAc)], thiourea, and other chemicals were purchased from Kishida Reagents Chemicals. All reagents were used as received. Preparation of Nonstoichiometric Ternary Ag−In−S Nanoparticles via Two-Step Heating. A 0.25 mmol portion of thiourea was used as an S2− precursor, and a mixture of Ag(OAc) and In(OAc)3 was used as a metal ion precursor. The mixing ratio of Ag/ (Ag + In)prep was varied between 0.30 and 0.60. Metal sources were mixed with the 0.25 mmol of thiourea in such a way that the total charge of Ag+ and In3+ became 10% larger than that of S2−. Both the S2− precursor and the metal ion precursor were put into a test tube with a 2.90 cm3 portion of OLA and a 0.10 cm3 portion of DDT. Two-step heat treatment of the thus-prepared solution was carried out with vigorous stirring under a N2 atmosphere: The mixture was heated at 150 °C for 10 min, immediately followed by heating at 250 °C for 3−30 min, typically 10 min. The thus-obtained suspension was cooled to room temperature and subjected to centrifugation to remove large precipitates. By adding methanol to the supernatant, the target AIS nanoparticles were isolated as wet precipitates. Thusobtained nanoparticles were washed several times with methanol and ethanol and then uniformly dissolved in chloroform. Synthesis of Quaternary Ag−In−Ga−S Nanoparticles via Single-Step Heating. AIGS nanoparticles were synthesized by single-step heat treatment of the precursor mixture at a target temperature. Powder of elemental sulfur was used as an S2− precursor, and a mixture of Ag(OAc), Ga(acac)3, and In(acac)3 was used as a metal ion precursor. The ratio of Ag+ to total metal ions in the metal ion precursor, Ag/(Ag + In + Ga)prep, was fixed to 0.40, while the ratio of In3+ to total group III elements in the metal ion precursor, In/(In + Ga)prep, was varied from 1.0 to 0.20. A 0.23 mmol portion of elemental sulfur was put into a test tube together with the metal ion precursor in such a way that the total charge of Ag+, In3+, and Ga3+ was equal to that of S2−. After the addition of a mixture of OLA (2.75 cm3) and DDT (0.25 cm3), the solution was heated at 300 °C for 10 min with vigorous stirring under a N2 atmosphere. The resulting suspension was cooled to room temperature and then large precipitates were removed by centrifugation. By adding methanol to the thus-obtained supernatant, the target AIGS nanoparticles were isolated as wet precipitates. The AIGS nanoparticles were washed several times with methanol and ethanol and then uniformly dissolved in chloroform. Surface Coating of Ag−In−S and Ag−In−Ga−S Nanoparticles with GaSx Shells. GaSx shell coating was carried out by a procedure modified from that described in our previous paper.56 A 10 nmol (particles) portion of AIS nanoparticles, a 53 μmol portion of Ga(acac)3, and a 53 μmol portion of thiourea were suspended in a mixture of OLA (2.90 cm3) and DDT (0.10 cm3), followed by heat treatment at 300 °C typically for 60 min with vigorous stirring under a N2 atmosphere. The thus-obtained precipitates were isolated by centrifugation and washed several times with methanol and ethanol. The precipitates of AIS nanoparticles coated with a GaSx shell, AIS@ GaSx, were uniformly dissolved in chloroform. On the other hand, the surface coating of AIGS nanoparticles was carried out by adding AIGS nanoparticles (10 nmol of particles) to OLA (3.0 cm3) containing Ga(acac)3 (53 μmol) and thiourea (53 μmol), followed by heating at 300 °C for 15 min in a similar manner. Since a large amount of target particles was uniformly dissolved in the B

DOI: 10.1021/acsami.8b15222 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces solution, the suspension was subjected to centrifugation to remove large aggregates. By adding a large portion of methanol to the thusobtained supernatant, AIGS nanoparticles covered with GaSx, AIGS@ GaSx, were isolated. The AIGS@GaSx nanoparticles were washed several times with methanol and ethanol and uniformly dissolved in chloroform. Characterization of Nanoparticles. The size distribution of obtained nanoparticles was evaluated by using transmission electron microscopy (TEM) (Hitachi, H-7650) at an operation voltage of 100 kV. A copper TEM grid covered with an amorphous carbon overlayer (Okenshoji Co., Ltd., ELS-C10 STEM Cu100P grid) was used for preparing TEM samples. A Cs-corrected HR-STEM (JEOL Co. Ltd., ARM-200F) with an acceleration voltage of 200 kV was used to acquire images of high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Energy-dispersive X-ray spectroscopy (EDS) analysis was simultaneously carried out during the HAADF-STEM measurements. X-ray diffraction (XRD) patterns were obtained with an X-ray diffractometer (Rigaku, SmartLab-3K) using Cu Kα radiation. X-ray fluorescence spectroscopy (Rigaku, EDXL-300) or energy dispersive X-ray spectroscopy (Horiba, Emax Energy EX-250) were used for evaluation of the chemical composition of nanoparticles. The absolute PL quantum yield (QY) was determined by using an absolute PL QY measurement system (HAMAMATSU C9920-03) with an excitation wavelength of 365 nm. PL decay profiles were recorded with 470 nm excitation at room temperature by a time-correlated single-photon counting apparatus (HAMAMATSU, Quantaurus-Tau). Single-particle PL imaging and spectroscopy were performed using an inverted fluorescence optical microscope (Olympus, IX71) equipped with a high numerical aperture objective (Olympus, UPlan FLN 100×/1.3 NA, oil). Samples were excited with a 375 nm (∼13 W cm−2) diode laser (PicoQuant, LDH-PC-375), and the PL signal was detected using an electron-multiplying CCD (EMCCD) camera (Andor Technology, iXon). A 400 nm long-pass filter (Thorlabs, FELH0400) was used to suppress unwanted excitation and scattered light in the detection path. The PL spectra were measured using an imaging spectrograph (Bunkou Keiki, CLP-50, 0.5 nm resolution) placed between the microscope and the EM-CCD camera. Data were recorded as image sequences with acquisition times of 0.05 s (20 Hz) and 1s (1 Hz) for imaging and spectroscopy, respectively. All single-particle measurements were carried out at room temperature in a nitrogen gas environment. For sample preparation, the AIS and AIS@GaSx nanoparticles were dispersed in toluene and spincoated onto precleaned quartz coverslips (1 × 0.2, Technical Glass Products) at 2000 rpm. The concentration of nanoparticles was optimized to obtain well-separated (typically with a 1-μm interval) and randomly distributed diffraction-limited emission spots. Characteristic single-step PL blinking and photobleaching observed from individual spots supported the assignment of PL from such spots to isolated single nanoparticles.

Figure 1. Absorption spectra (a) and photoluminescence spectra (b) of AIS nanoparticles with various reaction times in the second heat treatment at 250 °C. (c) Change in the chemical composition of AIS nanoparticles as a function of heating time at 250 °C. The ratio of Ag/ (Ag + In)prep in the metal ion precursor was fixed to 0.40. The reaction time just after the first heat treatment at 150 °C is represented as 0 min in the figures.

pair recombination (DAP) emission. With the elapse of time during the second heat treatment at 250 °C, a clear narrow PL peak assignable to band-edge emission emerged at 580 nm on the shorter wavelength side of the broad peak, accompanied by a blue shift of the broad PL peak from 790 to 700 nm. Figure 1c shows the change in the chemical composition of AIS nanoparticles during the second heat treatment. Although the synthesis was carried out under an Ag-deficient condition of Ag/(Ag + In)prep = 0.40, the AIS nanoparticles obtained just after the first heat treatment, that is, at 0 min in Figure 1c, had the chemical composition of Ag:In:S = 22:23:55, which is almost the correct stoichiometric composition of AgInS2. It should be noted that the fraction of S was slightly larger than the stoichiometric value due to the surface modification with DDT. The Ag fraction in particles decreased with the elapse of time during heating at 250 °C and finally reached a plateau at ca. 16 atom % after heating for 10 min or longer, accompanied by a gradual increase of the In fraction to ca. 27 atom % for 10 min heating. The S fraction was almost constant at 55−58 atom %, regardless of the reaction time. The relative PL intensity of the band-edge peak to the broad peak, PL(b.e.)/ PL(defect), was dependent on the reaction time in the second step and became optimal at 10 min with a value of 1.8. These AIS nanoparticles had a Ag-deficient composition, that is, an experimentally determined Ag/(Ag + In) of 0.37, which was

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RESULTS AND DISCUSSION Optical Properties of Nonstoichiometric Ag−In−S Nanoparticles Prepared via Two-Step Heating. Figure 1 shows the changes in optical properties and chemical composition of AIS nanoparticles synthesized under an Agdeficient condition, Ag/(Ag + In)prep = 0.40, during the second heat treatment at 250 °C. The absorption and PL spectra of AIS nanoparticles varied depending on the reaction time. The absorption spectra were slightly blue-shifted from ca. 650 to 600 nm due to an increase in the nonstoichiometry of particles, as discussed below, and an absorption shoulder, assigned to an exciton peak, developed at ca. 530 nm with elapse of heating time (Figure 1a). The PL spectra were also dependent on the reaction time in the second step, as shown in Figure 1b. The nanoparticles formed after the first heat treatment at 150 °C exhibited only a broad PL peak at 790 nm, being assignable to the emission originating from defect sites or donor−acceptor C

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nanoparticles decreased from a nearly stoichiometric composition [Ag/(Ag + In) = 0.53] to an Ag-deficient one [Ag/(Ag + In) = 0.37] [Table S1, Supporting Information (SI)] due to the increase in the amount of cation-exchanged In3+ during the second step. As shown in Figure 3a, the absorption spectra of

almost equal to that used in the preparation, Ag/(Ag + In)prep = 0.40. As observed in TEM images (Figure 2a,b), spherical or polygonal particles of 4.3 ± 0.82 nm in size were formed by the

Figure 2. TEM images of AIS nanoparticles obtained after the first heat treatment at 150 °C (a) and after the second heat treatment at 250 °C for 10 min (b). (c) XRD patterns of particles in panels a and b.

Figure 3. Absorption spectra (a) and photoluminescence spectra (b) of AIS nanoparticles prepared with various ratios of Ag/(Ag + In)prep in the metal ion precursor via two-step heating. The numbers in the figure represent the ratios of Ag/(Ag + In)prep. The inset in panel a shows the average size (dav) (solid circles) with the standard deviation (error bars) as a function of Ag/(Ag + In)prep.

first heating at 150 °C, and the following second heating at 250 °C for 10 min produced nanoparticles with a similar size of 4.1 ± 0.53 nm with a slightly narrowed distribution. Both kinds of particles exhibited broad XRD peaks, as shown in Figure 2c, the diffraction patterns of which were well-assigned to the orthorhombic crystal structure of AgInS2. Since a decrease of nanoparticle size can be expected by the cation exchange reaction, in which three Ag+ ions are replaced with one In3+ ion, these results suggested that the second heat treatment at 250 °C accelerated the cation exchange of Ag+ in the crystals for In3+ in the solution to form nonstoichiometric AIS nanoparticles. It was reported for CdSe nanoparticles that heat treatment at 220 °C induced the crystallization of amorphous CdSe nanoparticles ca. 2 nm in diameter, in which the exciton peak became more prominent and the band-edge emission intensity was enlarged with the elapse of heating time, due to the removal of deep surface defects.58,59 Considering these results, the improvement of crystallinity of AIS nanoparticles during heat treatment, as well as the change in the particle composition, could enlarge the relative PL intensity of band-edge emission, as shown in Figure 1. It should be noted that the temperature in the first step influenced the band-edge PL peak intensity: when the second heat treatment at 250 °C for 10 min was carried out after the first heating step at 50 °C, the resulting AIS nanoparticles had a smaller average size of 3.7 nm and exhibited a relative band-edge PL intensity of PL(b.e.)/PL(defect) = 1.0, the value of which is smaller than that of 1.8 obtained with the first heat temperature at 150 °C in Figure 1b. The Ag+ fraction in the metal ion precursor, Ag/(Ag + In)prep, greatly influenced the PL spectra of AIS nanoparticles. With a decrease in the ratio of Ag/(Ag + In)prep from 0.60 to 0.40, the experimentally determined Ag fraction in AIS

AIS nanoparticles were blue-shifted with a decrease in the ratio of Ag/(Ag + In)prep, being similar to the behavior observed in Figure 1a. It was reported in our previous paper49 that the Eg of AIS nanoparticles increased with a decrease in the Ag+ fraction in the particles because of the lowering of the valence band maximum composed of hybrid orbitals of S 3p and Ag 4d, resulting in a blue-shift of the absorption spectra. Therefore, since the average particle size was roughly constant at 3.6−4.3 nm, regardless of the Ag/(Ag + In)prep ratio (inset of Figure 3a), the observed spectral shift was caused by the Eg increase due to the decrease in the Ag+ fraction, not due to the quantum size effect. The relative band-edge PL intensity, PL(b.e.)/PL(defect), was enhanced from 0 to 1.8 with a decrease in Ag/(Ag + In)prep from 0.60 to 0.40, as shown in Figure 3b. This indicated that nonstoichiometric AIS nanoparticles with the composition of Ag/(Ag + In) = ca. 0.37 exhibited the most intense bandedge emission peak, even though a large amount of defect sites, such as Ag vacancies and antisites of In on Ag sites, can be expected to form in the nanocrystals. It should be noted that a further decrease of Ag/(Ag + In)prep from 0.40 to 0.30 caused a decrease of PL(b.e.)/PL(defect) from 1.8 to 1.0, probably owing to the change in the surface condition of particles, but the experimentally obtained Ag fraction was similar to Ag/(Ag + In) = 0.37 for AIS nanoparticles between Ag/(Ag + In)prep = 0.30 and 0.40. Enhancement of the Band-Edge Emission of Nonstoichiometric AIS Nanoparticles by GaSx Shell Coating. Coating of semiconductor nanoparticles with other semiD

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ACS Applied Materials & Interfaces conductors of larger Eg is well-known as a useful strategy for eliminating the recombination sites on the particle surface, resulting in enhancement of PL QY and a decrease of defectsite PL peak intensity. In our previous study,56 surface coating with GaSx caused the appearance of a band-edge PL peak in stoichiometric AgInS2 nanoparticles. Figure 4a shows the

the defect sites on the particle surface and that appropriate surface coating with GaSx could effectively remove most of them. Furthermore, although the peak wavelength of the bandedge emission was constant at ca. 580 nm, its peak width became larger, especially on the shorter wavelength side, with the elapse of time during heat treatment for GaSx shell coating. This suggested that the prolonged heating at 300 °C caused broadening of the size distribution due to Ostwald ripening and/or Ga3+ doping in the AIS core surface to form a solid solution between AgInS2 and AgGaS2 with a larger Eg. Figure 4c shows the changes of PL QY and the full width at half-maximum (fwhm) of the band-edge PL peak as a function of the reaction time used for GaSx shell coating. The fwhm was roughly constant at ca. 160−190 meV at the early stage of the heat treatment but drastically increased to ca. 300 meV with a coating reaction time of 120 min or longer. On the other hand, the quantum yield of whole photoluminescence, including both band-edge and defect-site emissions, increased with an increase in the reaction time, except for an initial decrease from 11% to 3.6%, which was due to a drastic decrease of broad defect-site peak intensity. Considering the intense band-edge emission at 580 nm with a narrow fwhm of 186 meV (50 nm) as well as removal of most of the defect-site emission, we conclude that the GaSx shell coating condition to obtain AIS nanoparticles showing optimal monochromaticity is heat treatment at 300 °C for 60 min, though the PL QY of 5.8% was rather moderate. The difference between the Eg of AIS@ GaSx prepared with this optimal condition and the peak energy of band-edge emission, that is, Stokes shift, was calculated to be 0.01 eV, being comparable to that obtained in a similar way for stoichiometric AgInS2@GaSx particles, 0.02 eV, in our previous study,56 though the fwhm of the particles in the present study was larger than that reported for stoichiometric AgInS2@GaSx, 103 meV (29 nm).56 By TEM measurement (Figure S2, SI), we found that the diameter of AIS@GaSx increased to 6.5 ± 1.26 nm from that of AIS nanoparticles used as the core, 4.1 ± 0.53 nm, suggesting that the surface of AIS nanoparticles was covered by a GaSx shell of ca. 1.2 nm in thickness. The estimated shell thickness was smaller than the expected thickness, ca. 1.7 nm, calculated on the basis of the amount of precursors added. The presence of a GaSx shell layer was confirmed by HAADF-STEM measurement. As shown in Figure 5a,b, original AIS nanoparticles were polygonal and exhibited clear lattice fringes, the interlattice spacing of which was 0.35 nm, assignable to the (120) plane of the orthorhombic structure, 0.356 nm. On the other hand, the heat treatment of AIS nanoparticles in the presence of the shell precursors caused formation of an amorphous layer on each core surface, accompanied by a slight change of the core particle shape from polygonal to a rounded shape (Figure 5c,d), being similar to our previous results for stoichiometric AgInS2 particles coated with GaSx.56 Nanoscale EDS analysis during TEM measurement revealed that the composition of the shell layer was Ag:In:Ga = 0.85:1.0:3.0, indicating that the shell was mostly composed of amorphous GaSx. The thicknesses of the GaSx shells on AIS cores were estimated from Figure 5d to be ca. 1 nm, being in good agreement with the value calculated from the change in the size distribution of particles with GaSx shell deposition (Figure S2, SI). The spacing between lattice fringes of the core in AIS@ GaSx nanoparticles was estimated to be ca. 0.33 nm (Figure 5d), being assignable to the (002) plane of the orthorhombic AIS, 0.335 nm. This suggested that the amount of Ga3+ doping

Figure 4. Absorption spectra (a) and photoluminescence spectra (b) of AIS nanoparticles prepared with various reaction times at 300 °C for surface coating with a GaSx shell. (c) Plots of total PL QY and peak width (fwhm) of band-edge PL as a function of the time used for GaSx shell coating. The absorption spectra were normalized at the exciton peak, 550 nm. The PL spectra were normalized at the bandedge PL peak. The inset of panel b shows a photograph of solutions containing AIS nanoparticles before (right) and after GaSx coating for 60 min (left) under a UV light (λ = 365 nm).

change in the absorption spectra of nonstoichiometric AIS nanoparticles prepared with Ag/(Ag + In)prep = 0.40 as a function of heating time at 300 °C for GaSx shell coating. Although the AIS nanoparticles have an absorption onset at 600 nm, regardless of the reaction time, absorbance at wavelengths shorter than ca. 500 nm was enhanced with the elapse of heating time, indicating the formation of a GaSx shell on AIS nanoparticles without a significant change in the Eg of the AIS core. In fact, the AIS nanoparticles had similar Eg values before and after GaSx coating for 60 min. The values were determined to be 2.13 and 2.15 eV, respectively, by Tauc plots (Figure S1, SI) for direct transition semiconductors using the absorption spectra shown in Figure 4a. In contrast, the PL spectra were significantly varied by the GaSx deposition, as shown in Figure 4b. The broad peak of the defect-site emission, observed at around 700 nm, rapidly decreased with the elapse of heating time, its intensity almost disappearing at 60 min or longer. This indicated that the broad PL emission, observed for AIS nanoparticles, originated from E

DOI: 10.1021/acsami.8b15222 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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I(t ) = ∑n = 1 A n exp( −t /τn), where τn represents the decay lifetime of the PL emission and An represents the amplitude corresponding to the lifetimes. The fitting results are summarized in Table 1. The second component (26.4 ns) and the third component (100 ns) of the band-edge emission of AIS nanoparticles at 580 nm contributed ca. 49% and ca. 36%, respectively, of the total emission intensity when the comparison was made for the components, f i(Aτ), described in terms of the product of amplitude and lifetime (A × τ). On the other hand, the surface coating with GaSx increased the lifetimes of the second and third components, in which the contribution of the second component (53.1 ns) slightly decreased to ca. 42% and that of the third component (260 ns) increased to ca. 47%, probably due to the formation of shallow trap sites on the AIS core surface by covering with the GaSx layer. Similar elongation of PL lifetimes has been reported for the surface coating of CdSe nanoparticles with a CdS shell:60 the PL lifetimes of CdSe nanoparticles were increased by surface coating with a CdS thin layer because of the formation of strain at the interface between the CdSe core and CdS shell. The observed lifetimes and their contribution to the total emission intensity were comparable to those of band-edge emission from stoichiometric AgInS2 nanoparticles surface coated with GaSx in our previous study.56 In contrast, the lifetimes of defect-site emission at 700 nm were much longer than those at 580 nm, regardless of the presence of a GaSx shell layer, and the third components (852 ns for AIS and 1105 ns for AIS@GaSx) contributed more than 72% of the total emission intensity. It should be noted that a small residual photoluminescence at 700 nm in Figure 4b, observed for AIS@ GaSx nanoparticles with 60 min heat treatment, was attributed to remaining defect-site emission rather than to the tail of band-edge emission, although the defect-site emission mostly disappeared with GaSx coating. It was reported for stoichiometric AgInS2@GaSx nanoparticles in our previous paper56 that fwhm values of the bandedge PL peak from individual nanoparticles had a mean value as small as 80 meV, thus being significantly narrower than that of the ensemble spectra (fwhm of 103 meV). The ensemble PL spectra were inhomogeneously broadened due to the wavelength distribution of PL peaks of individual particles. Thus, single-particle spectroscopy is an advantageous strategy for clarifying the origins of peak broadening in the ensemble PL spectra and for determining whether the band-edge emission peak of single AIS nanoparticles can be broadened with nonstoichiometry. Figure 7 shows representative single-particle PL spectra of individual AIS@GaSx particles with nonstoichiometric AIS cores. The single-particle nature of the emission was confirmed by observing the PL intermittency (blinking) phenomenon from individual nanoparticles (not shown). It was clarified by statistical analyses that 53% of all of the AIS@GaS x nanoparticles showed only a narrow band-edge PL peak (Figure 7a), 9% of them exhibited only a broad defect-site emission (Figure 7b), and the remaining 38% interestingly showed both peaks. These fractions are comparable to those of previously reported stoichiometric AgInS2@GaSx nanoparticles.56 These results not only indicate that the GaSx coating succeeded in fully passivating the surface defects in the majority of AIS particles but also indicate that some of the GaSx-coated particles still have purely defect-type emission, either due to an imperfect GaSx shell or due to the presence of

Figure 5. High-resolution HAADF-STEM images of AIS nanoparticles prepared with Ag/(Ag + In)prep = 0.40 before (a, b) and after surface coating with GaSx (c, d). Images b and d are highmagnification images of the corresponding images a and c, respectively. GaSx shell coating was carried out on AIS nanoparticles with heat treatment for 60 min at 300 °C.

into the AIS core, if any, was negligibly small for the particles with 60 min heating. The PL decay behavior depended strongly on the type of emission. The band-edge emission of both kinds of nanoparticles decayed more rapidly than the corresponding defectsite emission, as shown in Figure 6. The PL decay profiles fitted well with a three-component exponential equation,

Figure 6. Decay profiles of band-edge emission at 580 nm and defectsite emission at 700 nm for (a) AIS nanoparticles prepared with Ag/ (Ag + In)prep = 0.40 used as a core and (b) AIS@GaSx nanoparticles. GaSx shell coating was carried out with heating for 60 min at 300 °C. The experimentally obtained curves are fitted by three-component exponential decay curves (solid lines) with the parameters listed in Table 1. F

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Table 1. Fitting Results for PL Decay Profiles of Band-Edge Emission at 580 nm and Defect-Site Emission at 700 nm with Three-Component Exponential Equations sample AIS AIS@GaSx a

λPL/nm

τ1/ns

A1/%

f1(Aτ)a/%

τ2/ns

A2/%

f 2(Aτ)a/%

τ3/ns

A3/%

f 3(Aτ)a/%

χ2

580 700 580 700

5.77 18.5 5.49 12.3

53.6 49.0 68.5 50.4

14.8 3.97 11.5 2.47

26.4 173 53.1 152

38.9 31.7 25.6 31.8

49.0 24.0 41.6 19.2

100 852 260 1105

7.6 19.3 5.9 17.8

36.3 72.0 46.9 78.3

0.763 1.10 1.09 1.03

3

fi (Aτ ) = 100 × Ai τi/∑n = 1 A nτn .

Figure 7. Representative single-particle PL spectra of AIS@GaSx nanoparticles recorded for eight different particles on a quartz substrate. Narrow band-edge emission peaks (a) and broad defect-site emission peaks (b) were observed. Each spectrum was fitted with a single Gaussian function (orange line). Peak wavelengths (fwhm’s) are displayed beside the corresponding peaks in the unit of nanometers. GaSx shell coating was carried out on AIS nanoparticles with heat treatment for 60 min at 300 °C.

Figure 8. Histograms of peak energy (a, c) and fwhm (b, d) of PL peaks recorded during single-particle analyses. The numbers of samples measured were 280 for AIS nanoparticles (a, b) and 199 for AIS@GaSx nanoparticles (c, d). GaSx shell coating was carried out with heat treatment for 60 min at 300 °C. The regions marked as i and ii in the panels correspond to band-edge emission and defect-site emission, respectively, and are fitted with Gaussian functions (red lines).

intrinsic-type defects. The observation of a dual band-edge and defect emission in a considerable fraction of the coated particles is very interesting. This reflects the fact that the passivation is not complete and defects remain in the particles and that the population and emission rates of the band edge and defects states are comparable. The peak widths (fwhm) of the band-edge emission and of the defect-site emission were much smaller than those of the corresponding peaks, ca. 50 and >150 nm, respectively, observed in the ensemble PL spectra (Figure 4b). Furthermore, the peak positions and widths of band-edge emission, as well as those of the defect-site emission peak, fluctuated greatly from particle to particle, resulting in inhomogeneous broadening of the ensemble PL peaks. On the other hand, nonstoichiometric AIS nanoparticles also exhibited similar single-particle PL spectra without GaSx coating (Figure S3, SI). Here, the fraction of nanoparticles showing only band-edge emission, 7%, was much smaller than that of the AIS@GaSx nanoparticles, while a much larger fraction of particles, 61%, exhibited only a defect-site PL peak. The remaining particles simultaneously showed both peaks. Figure 8 shows the distributions of peak energy and fwhm of PL peaks observed in single-particle spectra obtained for the AIS and AIS@GaSx nanoparticles. The profiles of the peak energy distribution (Figure 8a,c) resembled the corresponding ensemble PL spectra shown in Figure 4b, as expected from the inhomogeneous broadening. The frequency of a broad defectsite emission peak was greatly reduced by surface coating of the AIS nanoparticles with the GaSx shell. This was accompanied by blue shifts of individual PL peaks from 2.07 eV (600 nm) to 2.14 eV (580 nm) for the band-edge emission and from 1.77 eV (700 nm) to 1.82 eV (680 nm) for the

defect-site emission. Moreover, as shown in Figure 8b,d, the peak widths also fluctuated. By surface coating with a GaSx layer, the mean fwhm value of the band-edge emission peak, which was much smaller than the value of 186 meV for the ensemble PL spectra (Figure 4c), became further narrowed from 90 meV (26 nm) to 73 meV (20 nm) due to the removal of shallow trap sites on the AIS surface. It should be noted that the mean peak width of the band-edge emission of the nonstoichiometric AIS@GaSx nanoparticles, 73 meV (20 nm), was comparable to that of stoichiometric AgInS2@GaSx nanoparticles, fwhm = 80 meV (24 nm), reported in our previous paper.56 This indicates that the nonstoichiometry of AIS particles used as a core did not significantly affect the PL peak width of individual particles. Thus, we conclude that the band-edge PL peaks observed for AIS and AIS@GaSx nanoparticles by the ensemble measurement are inhomogeneously broadened and then their widths can be potentially narrowed at least to the same level as that obtained with singleparticle PL measurement, 73 meV, both by precisely controlling the particle size and size distribution and by reducing the fluctuation of chemical composition of individual AIS cores. Tuning the Wavelength of the Band Edge PL Peak by Controlling the Chemical Composition of Ag−In−Ga−S Nanoparticles. We previously reported57 that thermal decomposition of a metal diethyldithiocarbamate complex successfully produced Ga3+-doped AIS nanoparticles, that is, G

DOI: 10.1021/acsami.8b15222 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces AIGS nanoparticles, the Eg of which was enlarged with an increase in the Ga3+/In3+ ratio in AIGS, though only a broad defect-site emission was observed in each PL spectrum regardless of the chemical composition of the AIGS particles. Thus, we developed a strategy for Ga3+ doping into AIS nanoparticles showing a band-edge emission and tuned the PL peak wavelength of the resulting AIGS nanoparticles by controlling their Ga3+/In3+ ratio. The two-step heating method for AIS nanoparticles did not successfully produce Ga3+-doped AIS nanoparticles because of the relatively low reaction temperatures, 150 °C followed by 250 °C. As a result, there was almost no shift in the wavelength of the band-edge PL peak. Thus, the reaction temperature for quaternary AIGS nanoparticles was increased to 300 °C and then the precursor mixture was heat-treated via single-step heating. The ratio of Ag/(Ag + In + Ga)prep in the metal ion precursor was fixed to a nonstoichiometric value, 0.40, because the relative band-edge PL intensity, PL(b.e.)/PL(defect), was highest at Ag/(Ag + In + Ga)prep = 0.40 for AIGS particles prepared in the range of Ag/(Ag + In + Ga)prep ratios from 0.60 to 0.30 (Figure S4, SI), being similar to the optimal condition for the band-edge PL-emissive AIS nanoparticles (Figure 3). It should be noted that even when the two-step heating, 150 °C followed by 300 °C, was used for the preparation, the AIGS nanoparticles obtained exhibited properties similar to those of AIGS nanoparticles prepared via single-step heating at 300 °C. The AIGS nanoparticles obtained in the present study were spherical or polygonal with the average diameter (dav) being dependent on the In/(In + Ga)prep ratio used (Figure S5, SI): nanoparticles prepared in the range of In/(In + Ga)prep = 0.50−1.0 had almost the same dav of ca. 4.5 nm, while dav decreased to 2.9−3.6 nm for those with In/(In + Ga)prep of 0.40 or smaller. The core−shell-structured AIGS@GaSx nanoparticles had larger dav values by 0.4−1 nm, suggesting that AIGS cores were covered with GaSx shells 0.2−0.5 nm in thickness, though the actual thickness was difficult to evaluate due to the polydispersibility of the core particles used. XRD analysis (Figure S6, SI) revealed that AIGS nanoparticles prepared with In/(In + Ga)prep = 0.40 exhibited a diffraction pattern assignable to a tetragonal crystal structure in which each diffraction peak was located between the corresponding peaks of tetragonal AgInS2 and tetragonal AgGaS2 crystals. This proves that the AIGS nanoparticles obtained were composed of the solid solution between tetragonal AgInS2 and AgGaS2. The XRD pattern of AIGS@GaSx nanoparticles also exhibited diffraction peaks at the same positions as those corresponding to AIGS particles used as a core, indicating that the crystal structure and solid-solution composition of AIGS cores remained almost unchanged after the amorphous GaSx shell coating. Figure 9a shows absorption spectra of AIGS nanoparticles prepared with various ratios of In/(In + Ga)prep. The absorption onset was blue-shifted from ca. 630 to 510 nm with a decrease in In/(In + Ga)prep from 1.0 to 0.20, indicating that the Eg of AIGS particles was enlarged with an increase in the Ga3+ fraction. A band-edge emission peak was observed in each PL spectrum of AIGS nanoparticles in addition to a broad defect-site PL peak regardless of the value of In/(In + Ga)prep (Figure 9c). It should be noted that the single-step heating at 300 °C could produce AIS nanoparticles, but the observed relative PL intensity of the band-edge emission, PL(b.e.)/ PL(defect) = 0.81, was much smaller than that of 1.8 for

Figure 9. Absorption spectra (a, b) and photoluminescence spectra (c, d) of AIGS nanoparticles with and without GaSx surface coating. Samples were AIGS nanoparticles (a, c) and AIGS@GaSx nanoparticles (b, d). The ratios of In/(In + Ga)prep used for AIGS cores are indicated in each panel. The inset of panel d shows a photograph of solutions containing AIGS nanoparticles prepared with In/(In + Ga)prep = 0.40 before (right) and after GaSx coating (left) under a UV light (λ = 365 nm).

particles prepared by the optimized two-step heating method (Figure 1b). This was probably caused by an increase of AIS particle size from 4.1 to 4.5 nm due to an increase in the reaction temperature. The wavelengths of both the band-edge and defect-site emission peaks were blue-shifted with a decrease in In/(In + Ga)prep. Being similar to the case of AIS@GaSx nanoparticles as described above, the band-edge PL peak became prominent by surface-coating with a GaSx shell: the intensity of the defect-site emission was remarkably reduced to less than 15% of the intensity of the corresponding band gap emission, as shown in Figures 9d and S7 (SI). An exception was the case of using AIGS cores with In/(In + Ga)prep = 0.20, in which the defect-site emission had ca. 35% of the intensity of the band-edge emission. The absorption onsets of the AIGS@GaSx particles shown in Figure 9b were slightly blue-shifted from those of the corresponding cores (Figure 9a), indicating that a small amount of Ga3+ was further doped into the AIGS cores to enlarge their Eg during the heat treatment at 300 °C for GaSx shell formation. This was supported by the results of elemental analysis of particles: except for the cases of In/(In + Ga)prep = 0.30 or smaller, the ratio of Ag/(Ag + In) in particles was slightly enlarged by the surface coating of AIGS nanoparticles with a GaSx shell (Table S2, SI), probably due to the partial cation exchange of In3+ in the cores for Ga3+ in solutions. Figure 10a shows the experimentally determined chemical composition of AIGS nanoparticles. The ratio of Ag+ to total metal ions, Ag/(Ag + In + Ga), in the nanoparticles was almost constant at ca. 0.42, being comparable to the ratio used in the preparation, Ag/(Ag + In + Ga)prep = 0.40, but slightly larger than that of AIS particles prepared via the optimized two-step heating method (Figure 1b), 0.37. The ratios of In/(In + Ga) detected in the particles also roughly agreed with those of the corresponding metal ion precursors, In/(In + Ga)prep. These results indicated that the thus-obtained AIGS nanoparticles had a nonstoichiometric Ag-deficient composition, regardless of the In/(In + Ga)prep ratios in the precursors. The Eg values H

DOI: 10.1021/acsami.8b15222 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 11. (a) Plots of the wavelengths of band-edge PL peaks and their fwhm’s as a function of the experimentally determined In/(In + Ga) ratio in AIGS cores. (b) Changes in the quantum yields of whole PL with changes in the chemical composition of AIGS cores used. Samples were AIGS and AIGS@GaSx nanoparticles. The results for particles prepared via the optimized two-step heating are also shown with red symbols.

Figure 10. (a) Changes in the chemical composition of AIGS nanoparticles as a function of the ratio of In/(In + Ga)prep. The result for AIS particles prepared via the optimized two-step heating is also shown (red triangle). (b) Eg values of AIGS and AIGS@GaSx nanoparticles as a function of the experimentally determined In/(In + Ga) ratio in AIGS nanoparticles used as cores. The values of AIS (red cross) and AIS@GaSx (red triangle) particles prepared via the optimized two-step heating are also shown.

+ Ga) ratio in the core particles. Although a slight increase of Eg was observed with the GaSx coating, as shown in Figure 10b, the peak position of the band-edge emission was almost unchanged, except for the nanoparticles with a ratio of In/(In + Ga) larger than ca. 0.8. It should be noted that the bandedge emission peak wavelengths of AIS and AIS@GaSx particles obtained by the two-step heating method as mentioned above were shorter than those of corresponding particles prepared by the single-step heating at 300 °C, mainly due to their larger Eg values. The fwhm values of AIGS@GaSx nanoparticles, also shown in Figure 11a, were slightly increased from 137 to 193 meV with a decrease in In/(In + Ga). These values were similar to those of AIS@GaSx prepared via the two-step method. Figure 11b shows the PL QYs of the whole PL spectra, including the band-edge and defect-site emissions, as a function of In/(In + Ga) in the cores. A volcano-type dependence was observed, with the PL QY being optimal at In/(In + Ga) = 0.43 for both kinds of particles. A relatively high PL QY of 28% was obtained for AIGS@GaSx with an intense green band-edge emission at 530 nm and with a fwhm of 181 meV (41 nm).

of AIGS and AIGS@GaSx nanoparticles were estimated from the absorption spectra with Tauc plots (Figure S8, SI) for direct transition semiconductors and they are plotted in Figure 10b as a function of the experimentally determined In/(In + Ga) ratio of the AIGS nanoparticles, which were used as cores for AIGS@GaSx preparation. With a decrease of the In/(In + Ga) ratio in AIGS nanoparticles from 1.0 to 0.20, their Eg values increased linearly from 2.07 to 2.54 eV. This trend agrees with theoretical expectation.61−63 It was reported on the basis of the density functional theory61 that the conduction band minimum of an AgIn(1‑y)GayS2 semiconductor was shifted to a higher level (a more negative potential) with an increase in the ratio of Ga to In, accompanied by an increase in Eg, because the orbitals of Ga 4s4p and In 5s5p mainly made the conduction band minimum, in which the Ga orbitals formed higher levels than those of the In orbitals, and because the valence band was composed of S 3p and Ag 4d. AIGS@GaSx nanoparticles exhibited a similar linear relation between the Eg and the In content in cores. Here, the Eg values obtained were approximately 0.06 eV larger than those of the corresponding core AIGS nanoparticles used, being consistent with the expectation from the blue-shift of the absorption spectra with GaSx coating, as shown in Figure 9b. It should be noted that the Eg values of AIS and AIS@GaSx particles prepared via single-step heating at 300 °C were slightly smaller than those of corresponding particles prepared via the above-mentioned two-step heating (150 and 250 °C), due to an increase of the AIS core size from 4.1 to 4.5 nm. Figure 11a shows plots of the peak wavelength of the bandedge emission of AIGS and AIGS@GaSx nanoparticles as a function of the chemical composition of the nanoparticles used as cores. The peak wavelengths were tunable between 610 and 500 nm for the AIGS nanoparticles and between 590 and 500 nm for the AIGS@GaSx nanoparticles by changing the In/(In

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CONCLUSION This work demonstrated that Ag-deficient Ag−In−S nanoparticles could exhibit strong and narrow band-edge emission, despite the fact that a large amount of defect sites, such as Ag vacancies and antisites of In on Ag sites, can be expected to form in the nanocrystals. The relative intensity of the bandedge PL peak was optimal at the ratio of Ag to group III elements of ca. 0.4. The wavelength of the band-edge emission was tunable from 610 to 500 nm with an increase in the degree of Ga3+ doping into Ag−In−S nanoparticles, which caused an increase of Eg. Surface coating of Ag−In−S and Ag−In−Ga−S nanoparticles with GaSx shells remarkably decreased the broad defect-site PL peak intensities, causing at the same time I

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(5) Azimi, H.; Hou, Y.; Brabec, C. J. Towards Low-cost, Environmentally Friendly Printed Chalcopyrite and Kesterite Solar Cells. Energy Environ. Sci. 2014, 7, 1829−1849. (6) Oba, F.; Kumagai, Y. Design and Exploration of Semiconductors from First Principles: A Review of Recent Advances. Appl. Phys. Express 2018, 11, 060101. (7) Kudo, A.; Miseki, Y. Heterogeneous Photocatalyst Materials for Water Splitting. Chem. Soc. Rev. 2009, 38, 253−278. (8) Kaneko, H.; Minegishi, T.; Domen, K. Recent Progress in the Surface Modification of Photoelectrodes toward Efficient and Stable Overall Water Splitting. Chem. - Eur. J. 2018, 24, 5697−5706. (9) Zhao, J.; Minegishi, T.; Zhang, L.; Zhong, M.; Gunawan; Nakabayashi, M.; Ma, G. J.; Hisatomi, T.; Katayama, M.; Ikeda, S.; Shibata, N.; Yamada, T.; Domen, K. Enhancement of Solar Hydrogen Evolution from Water by Surface Modification with CdS and TiO2 on Porous CuInS2 Photocathodes Prepared by an ElectrodepositionSulfurization Method. Angew. Chem., Int. Ed. 2014, 53, 11808−11812. (10) Regulacio, M. D.; Han, M.-Y. Multinary I-III-VI2 and I2-II-IVVI4 Semiconductor Nanostructures for Photocatalytic Applications. Acc. Chem. Res. 2016, 49, 511−519. (11) Muzzillo, C. P.; Klein, W. E.; Li, Z.; DeAngelis, A. D.; Horsley, K.; Zhu, K.; Gaillard, N. Low-Cost, Efficient, and Durable H2 Production by Photoelectrochemical Water Splitting with CuGa3Se5 Photocathodes. ACS Appl. Mater. Interfaces 2018, 10, 19573−19579. (12) DeAngelis, A. D.; Horsley, K.; Gaillard, N. Wide Band Gap CuGa(S,Se)2 Thin Films on Transparent Conductive Fluorinated Tin Oxide Substrates as Photocathode Candidates for Tandem Water Splitting Devices. J. Phys. Chem. C 2018, 122, 14304−14312. (13) Castro, S. L.; Bailey, S. G.; Raffaelle, R. P.; Banger, K. K.; Hepp, A. F. Synthesis and Characterization of Colloidal CuInS2 Nanoparticles from a Molecular Single-source Precursor. J. Phys. Chem. B 2004, 108, 12429−12435. (14) Torimoto, T.; Adachi, T.; Okazaki, K.; Sakuraoka, M.; Shibayama, T.; Ohtani, B.; Kudo, A.; Kuwabata, S. Facile Synthesis of ZnS-AgInS2 Solid Solution Nanoparticles for a Color-adjustable Luminophore. J. Am. Chem. Soc. 2007, 129, 12388−12389. (15) Nakamura, H.; Kato, W.; Uehara, M.; Nose, K.; Omata, T.; Otsuka-Yao-Matsuo, S.; Miyazaki, M.; Maeda, H. Tunable Photoluminescence Wavelength of Chalcopyrite CuInS2-based Semiconductor Nanocrystals Synthesized in a Colloidal System. Chem. Mater. 2006, 18, 3330−3335. (16) Zhong, H.; Bai, Z.; Zou, B. Tuning the Luminescence Properties of Colloidal I-III-VI Semiconductor Nanocrystals for Optoelectronics and Biotechnology Applications. J. Phys. Chem. Lett. 2012, 3, 3167−3175. (17) Chen, B. K.; Pradhan, N.; Zhong, H. Z. From Large-Scale Synthesis to Lighting Device Applications of Ternary I-III-VI Semiconductor Nanocrystals: Inspiring Greener Material Emitters. J. Phys. Chem. Lett. 2018, 9, 435−445. (18) Kolny-Olesiak, J.; Weller, H. Synthesis and Application of Colloidal CuInS2 Semiconductor Nanocrystals. ACS Appl. Mater. Interfaces 2013, 5, 12221−12237. (19) Torimoto, T.; Kameyama, T.; Kuwabata, S. Photofunctional Materials Fabricated with Chalcopyrite-Type Semiconductor Nanoparticles Composed of AgInS2 and Its Solid Solutions. J. Phys. Chem. Lett. 2014, 5, 336−347. (20) Torimoto, T. Nanostructure Engineering of Size-Quantized Semiconductor Particles for Photoelectrochemical Applications. Electrochemistry 2017, 85, 534−542. (21) Hamanaka, Y.; Ozawa, K.; Kuzuya, T. Enhancement of DonorAcceptor Pair Emissions in Colloidal AgInS2 Quantum Dots with High Concentrations of Defects. J. Phys. Chem. C 2014, 118, 14562− 14568. (22) van der Stam, W.; Berends, A. C.; de Mello Donega, C. Prospects of Colloidal Copper Chalcogenide Nanocrystals. ChemPhysChem 2016, 17, 559−581. (23) Pietryga, J. M.; Park, Y.-S.; Lim, J.; Fidler, A. F.; Bae, W. K.; Brovelli, S.; Klimov, V. I. Spectroscopic and Device Aspects of Nanocrystal Quantum Dots. Chem. Rev. 2016, 116, 10513−10622.

enhancement of PL QY of the band-edge emission peaks. Single-particle spectroscopy revealed that the homogeneous broadening of the band-edge emission peak of single particles was not significantly influenced by the nonstoichiometric composition of Ag−In−S particles. Our findings clearly reveal the fascinating properties of Ag−III−VI-based multinary nanoparticles composed of less-toxic elements. In particular, the controllability of the optical properties with the composition of multinary particles is not expected for conventional binary nanoparticles. These multinary nanoparticles will not only act as alternative quantum dots to highly toxic binary particles but also allow us to design and fabricate novel luminescent materials and quantum dot-based optoelectronic devices for a wide range of practical applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b15222.

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The chemical compositions, Tauc plots, TEM images, PL spectra, and XRD patterns of AIS, AIS@GaSx, AIGS, or AIGS@GaSx nanoparticles (PDF)

AUTHOR INFORMATION

Corresponding Authors

*S.K. e-mail: [email protected]. *T.T. e-mail: [email protected]. ORCID

Tatsuya Kameyama: 0000-0002-9860-6662 Shuzo Hirata: 0000-0003-2591-7678 Martin Vacha: 0000-0002-5729-9774 Tsukasa Torimoto: 0000-0003-0069-1916 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported financially by JSPS KAKENHI Grant Numbers JP26107014 and JP17H05254 in Scientific Research on Innovative Areas “Photosynergetics”, Grant Number JP16H06507 in Scientific Research on Innovative Areas “Nano-Material Optical-Manipulation”, Grant Numbers JP16H06052, JP18H03927, JP18K19128, and JP18H03863, and by Nichia Corp. Part of this work (HRTEM and STEM observations) was supported by MEXT “Nanotechnology Platform” (project No. 12024046).

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REFERENCES

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DOI: 10.1021/acsami.8b15222 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsami.8b15222 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsami.8b15222 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX