Photoinduced Carrier Dynamics of Nearly Stoichiometric Oleylamine

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Photoinduced Carrier Dynamics of Nearly Stoichiometric Oleylamine-Protected Copper Indium Sulfide Nanoparticles and Nanodisks Masanori Sakamoto,*,†,‡ Lihui Chen,§ Makoto Okano,† David M. Tex,†,∥ Yoshihiko Kanemitsu,†,∥ and Toshiharu Teranishi*,† †

Institute for Chemical Research and §Department of Chemistry, Graduate School of Science, Kyoto University, Gokasho Uji, Kyoto 611-0011, Japan ‡ Japan Science and Technology Agency (JST), PRESTO, Gokasho Uji, Kyoto 611-0011, Japan ∥ Japan Science and Technology Agency (JST), CREST, Gokasho Uji, Kyoto 611-0011, Japan S Supporting Information *

ABSTRACT: Copper indium sulfide (CuInS2) nanocrystals (NCs) are nontoxic and cost-effective, making them attractive for use in highly efficient solar cells. However, direct observations of the photoinduced carrier dynamics of CuInS2 NCs to examine the relationship between carrier dynamics and structural parameters with the aim of optimizing NC properties for specific applications are limited. Here, we synthesized nearly stoichiometric oleylamine-protected CuInS2 NCs using two convenient onepot methods to produce CuInS2 nanoparticles and nanodisks. Transient absorption measurements using femtosecond laser flash photolysis revealed that the use of oleylamine as a protecting ligand leads to the formation of vacancy-doped NCs. The rapid decay of the photogenerated excitons in the oleylamine-protected NCs was not sensitive to the crystal structure or shape of NCs, and could be explained by defect trapping and energy transfer to the hole-based localized surface plasmon resonance in the oleylamine-protected NCs. Our results confirm that careful selection of a protecting ligand is important to obtain CuInS2 NCs with optimized properties for particular applications.

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INTRODUCTION Semiconductor nanocrystals (NCs) have attracted substantial attention in a variety of fields ranging from fluorescent probes for bioimaging to photoenergy conversion.1−4 Because the optical and electronic properties of NCs can be finely tuned by tailoring their size, composition, structure, and surface functionalization, the search for NCs that suit the demands for specific applications from a practically infinite number of combinations has been an intense focus for a decade. General I−III−VI2 ternary semiconductor NCs are an important class of materials with a wide range of applications in quantum dot (QD) solar cells, light-emitting devices, and fluorescent probes for bioimaging because of their wide coverage of the solar spectrum, high absorption coefficient, optical anisotropy, and nonlinear optical properties.5−10 Among various I−III−VI2 ternary semiconductors, copper indium sulfide (CuInS2) has © XXXX American Chemical Society

the advantages of being nontoxic, cost-effective, and composed of abundant elements, making it a promising candidate as a QD ink for solar cells.8,10−13 CuInS2 has a narrow band gap of 1.5 eV, which is close to the ideal bandgap for a maximum conversion efficiency under one sun.14 Recently, Liu et al. reported that CuInS2 solar cell with glass/Mo/CIS/CdS/iZnO/ITO structure exhibited air mass 1.5G power conversion efficiencies of as high as 12.2%.5 Special Issue: Current Trends in Clusters and Nanoparticles Conference Received: November 27, 2014 Revised: January 16, 2015

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Figure 1. TEM images of (a) CuInS2 NPs and (b) CuInS2 NDs. (c) XRD patterns of chalcopyrite CuInS2 NPs and wurtzite CuInS2 NDs.

at 110 °C was rapidly added to the above solution. The mixture was heated up to 180 °C for 1 h and then cooled down to room temperature (RT). The obtained NPs were precipitated by addition of ethanol, centrifuged, and then redispersed in chloroform. To investigate the relationship between the concentration of precursors and diameter and composition of the NPs, we fixed the amounts of o-dichlorobenzene (4.9 mL for the solution containing the metal precursor and 2.1 mL for the injection solution) and OAm (4.3 mmol) and varied the amounts of the precursors Cu(acac)2, In(acac)3, and S. Synthesis of CuInS2 NDs. OAm-protected CuInS2 NDs were synthesized according to previous research.19 A mixture of CuCl (0.5 mmol), InCl3 (0.5 mmol), thiourea (1.0 mmol), and OAm (46 mmol) was degassed in a three-necked flask for 30 min at 60 °C under vacuum. The mixture was heated at 240 °C for 1 h under nitrogen and then cooled down to RT. Ethanol was added to precipitate the NDs, which were centrifuged at 7000 rpm for 3 min, and then redispersed in chloroform. The dispersion was centrifuged again at 6000 rpm for 1 min to remove inadequately capped NDs. Characterization. TEM observations were carried out using a TEM (JEM1011, JEOL) at 100 kV. XRD patterns were obtained on a diffractometer (X’Pert Pro MPD, PANalytical) with Cu Kα radiation (λ = 1.542 Å) at 45 kV and 40 mA. UV− vis−NIR absorption spectroscopy was conducted using a spectrophotometer (U-4100, Hitachi). XRF elemental analysis was carried out using an elemental analyzer (JSX-3202C, JEOL). TA Measurements. The fs TA spectroscopy setup used in this study was based on a regenerative amplifier Yb:KGW laser system (pulse width: 300 fs, repetition rate: 50 kHz). The TA spectra were measured using white-light probe pulses (470− 1000 nm) generated by illumination of a sapphire crystal with 1030 nm fs pulses from the regenerative amplifier. The remaining power of the regenerative amplifier was used to excite an optical parametric amplifier to generate a pump pulse with a wavelength of 650 nm. To suppress many-body effects such as Auger recombination, the intensity of the pump pulses was adjusted so that the signal intensities were proportional to the pump intensities. In all measurements, CuInS2 NCs were dispersed in CHCl3 solutions in a 1 mm-thick quartz cell at RT. The recovery time of the bleaching signals of CuInS2 was evaluated by fitting of the temporal evolution at 797 or 874 nm using the following fitting function.

Numerous methods to synthesize CuInS2 NCs with a variety of shapes and crystal structures have been developed because of their potential applications as a QD ink for solar cells.2,15−19 For example, Li et al. synthesized CuInS2 nanorods through the direct reaction of copper nitrate, indium nitrate, and dodecanethiol.15 The size and shape of the NCs could be elaborately controlled by varying the Cu/In ratio and the ligands other than dodecanethiol, such as oleylamine and oleic acid. Recently, Rosenthal et al. synthesized CuxInyS2 NCs that exhibited localized surface plasmon resonance (LSPR) in the near-infrared (NIR) region,20 and claimed that plasmonic CuxInyS2 NCs are better candidates for QD solar cells than those without LSPR because the strong LSPR in the NIR region supports the injection of photoexcited electrons from the NCs to the substrate.21 Transient absorption (TA) measurements of CuInS2 NCs using femtosecond (fs) laser flash photolysis (LFP) should provide better understanding of the photoinduced carrier dynamics in CuInS2 NCs with different structures and surface conditions. Furthermore, the investigation of carrier dynamics could provide important guidelines for developing specific structures suited for applications such as QD inks for solar cell and fluorescent probes. However, direct observations of the photoinduced carrier dynamics of CuInS2 NCs using LFP are still rare in contrast to their rapidly evolving synthetic technologies.12,17,22 In this study, we synthesize nearly stoichiometric oleylamineprotected CuInS2 NCs (CuInS2 nanoparticles (NPs) and nanodisks (NDs)) by a convenient one-pot method. The synthesized NCs are characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray fluorescence (XRF). The photoinduced carrier dynamics of oleylamine-protected CuInS2 NPs and NDs are investigated through TA measurements using fs-LFP. The structure and ligand dependence of the carrier dynamics of CuInS2 NCs are discussed.

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EXPERIMENTAL SECTION

Reagents. Oleylamine (OAm, 80−90%; Acros Organics and Aldrich), N,N-dibutylthiourea (DBTU, 97% Acros Organics), as well as Cu(acac)2, CuCl, and sulfur purchased from Kanto Chemicals, In(acac)3 and InCl3 purchased from Aldrich, and ethanol (99.5%), toluene, and chloroform from Wako Chemicals were used as received. Synthesis of CuInS2 NPs. OAm-protected CuInS2 NPs were synthesized according to a reported method.16 An odichlorobenzene solution of OAm (8.6 mmol), Cu(acac)2 (1.4 mmol), and In(acac)3 (1.4 mmol) was heated in a three-necked flask at 110 °C under N 2 atmosphere. Then, an odichlorobenzene solution containing sulfur (2.4 mmol) heated B

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Table 1. Relationship between the Ratio of Precursors and the Diameter and Composition of NPs Obtained under Four Different Synthetic Conditions entry

Cu(acac)2 (mmol)

In(acac)3 (mmol)

S (mmol)

OAm (mmol)

1 2 3 4

1.05 0.70 0.35 0.18

1.05 0.70 0.35 0.18

2.1 1.4 0.70 0.35

4.3 4.3 4.3 4.3

diameter (nm)

Cu/In/S (molar ratio)

Cu/In

± ± ± ±

26.8:19.4:53.8 26.7:23.8:49.5 9.96:27.2:62.9 8.79:26.9:64.3

1.38 1.12 0.370 0.327

8.6 7.4 9.1 8.3

0.9 1.0 1.3 1.2

Figure 2. (a) Absorption spectra of (black) CuInS2 NPs and (red) NDs. Time evolution of absorption spectra of CuInS2 (b) NPs and (c) NDs in the NIR region. Broken lines are baselines.

⎡ ⎛ t⎞ ⎛ t ⎞⎤ −Δα(t ) = A1⎢exp⎜ − ⎟ − exp⎜ − ⎟⎥ ⎢⎣ ⎝ τ1 ⎠ ⎝ τr ⎠⎥⎦

To investigate the shape- and crystal structure-dependent carrier dynamics of the CuInS2 NCs, wurtzite CuInS2 NDs were synthesized from a mixture of CuCl, InCl3, thiourea, and OAm following a reported procedure.19 The TEM image of the obtained NCs indicated that they were hexagonal disks with a diameter of 14.6 ± 1.8 nm and thickness of 10.1 ± 1.3 nm (Figure 1b). An XRD pattern of the NDs (Figure 1c) indicated that they consist of wurtzite domains with minor chalcopyrite domains. Korgels et al. revealed that CuInS2 NDs contain both wurtzite and chalcopyrite domains interfaced epitaxially with the wurtzite domains across wurtzite (002) and chalcopyrite (112) facets through stacking faults.19 We consider that our CuInS2 NDs possess a similar crystal structure. XRF analysis of the NDs revealed a molar ratio of Cu/In/S of 27.5:24.4:48.1, indicating the formation of nearly stoichiometric CuInS2 NDs. For the spectroscopic analysis, we selected the nearly stoichiometric CuInS2 NPs (Cu/In/S = 26.7:23.8:49.5) and NDs (Cu/In/S = 27.5:24.4:48.1). UV−vis−NIR absorption spectra of the CuInS2 NPs and NDs are shown in Figure 2. The optical absorption edges of both types of NCs were observed at 880 nm, consistent with the typical band gap of bulk CuInS2 (1.5 eV). Because the CuInS2 NCs are about twice the size of the excitonic Bohr radius of CuInS2 (4.1 nm), the quantum size effect was not observed in the present NCs.2 The nondistinct excitonic peaks and tails extending to long wavelength in the absorption spectra may reflect the defects in the NCs. In addition, weak LSPR bands were observed in the NIR region for both types of NCs. The appearance of LSPR indicates the existence of free holes generated by vacancies in the CuInS2 NCs. Generally, cation-deficient p-type semiconducting materials like CuInS2 exhibit a free hole-based LSPR peak in the NIR region.20 The intensity of the LSPR peak from the CuInS2 NCs increased considerably with time in ambient atmosphere (Figure 2b,c). This indicates that the free holes were formed by aerobic oxidation of CuInS2. The presence of cation defects was also confirmed by fluorescence measurements of the CuInS2 NCs. It is wellknown that stoichiometric CuInS2 NCs exhibit fluorescence in the NIR region.12,17 However, our CuInS2 NCs did not show any fluorescence; that is, the quantum yield of fluorescence was lower than the detection limit of our instrument. This result

⎡ ⎛ t ⎞ ⎛ t ⎞⎤ + A 2 ⎢exp⎜ − ⎟ − exp⎜ − ⎟⎥ ⎢⎣ ⎝ τ2 ⎠ ⎝ τr ⎠⎥⎦

Here, τ1 and τ2 are the decay times of CuInS2 bleaching signals, τr is the rise time of the CuInS2 bleaching signals, and A1 and A2 are amplitudes. Considering the instrument response function, we fitted the TA decay curves with a fitting function convoluted with a Gaussian function.

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RESULTS AND DISCUSSION The CuInS2 NPs synthesized according to the reported method16 were spherical with a diameter of 7.4 ± 1.0 nm (Figure 1a). The XRD pattern of the NPs is characteristic for the chalcopyrite structure of CuInS2 (Figure 1c). XRF analysis of the CuInS2 NPs revealed a molar ratio of Cu/In/S of 26.7:23.8:49.5, which is almost identical to the stoichiometry of CuInS2. Interestingly, we discovered that the stoichiometry of CuInS2 NPs deviated considerably when an excess amount of OAm relative to the cationic species was added to the reaction system. The diameter and composition of NPs synthesized under four different synthetic conditions are summarized in Table 1. The diameter of NCs did not change much with precursor concentration (see Supporting Information). In contrast, the content of cationic species (Cu+ and In3+) in the NCs decreased remarkably when the concentrations of precursors in the reaction solution were decreased, which is unexpected considering the precise precursor stoichiometry (Cu/In/S = 1:1:2). This could be explained by the coppervacancy-doping mechanism reported for copper sulfide NCs.23 It is suggested from this mechanism that the copper vacancies are created via an equilibrium process between NCs and Cu0. The OAm molecules, which bind strongly to copper, should assist in extracting copper from the CuInS2 lattices.23 Consequently, the equilibrium is shifted to the direction of creating copper vacancies. This indicates that a suitable ratio of ligand to precursor is essential to synthesize stoichiometric CuInS2 NPs. C

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Figure 3. TA absorption spectra of CuInS2 (a) NPs and (b) NDs upon excitation with a 650 nm laser pulse.

electron−hole recombination. The different ratios between NPs and NDs suggested that the amount of defect sites in NPs was larger than that of NDs. Although the NPs and NDs have the distinct crystal structure, size, and shapes, their excited state lifetimes were not much different. Interestingly, the decay rates of both NCs were faster than those reported for dodecanethiol-protected CuInS2 NCs synthesized by using dodecanethiol as a sulfur source. Recently, Klimov and co-workers reported that the fluorescent relaxation of dodecanethiol-protected CuInS2 NCs is biexponential with time constants of 6.5 and 190 ns.17 The dependence of the decay rates of CuInS2 NCs on the protecting ligand could help us to understand the photoinduced carrier dynamics in the present NCs. Klimov et al. used the dodecanethiol for both sulfur source and protecting ligand.17 It has been proposed that the use of dodecanethiol as a sulfur source leads to defect-less NCs. Macdonald pointed out that when thiols are used as a sulfur source, “crystal-bound” thiols sit on high coordination sites and become the terminal S layer of the crystal, resulting in the formation of defectless high-quality NCs.24 However, we used OAm as a protecting ligand in our NCs. OAm tends to bind to copper, which leads to the extraction of the cations from the NC surface.23 This dopes CuInS2 NCs with cationic vacancies. The cation shortage depending on the molar ratios of precursors to OAm is consistent with the explanation that the formation of cation-OAm complexes induces vacancies in the NCs. In addition, Zerger and co-workers discovered that technical-grade OAm (i.e., commercial OAm without further purification) tends to produce CuInS2 NCs with defects because it contains impurities.25 Although the stoichiometry of our CuInS2 NDs is close to ideal, the use of OAm induces the formation of cation vacancies. The appearance of LSPR supports the presence of cation vacancies in the NCs, because LSPR of CuInS 2 arises from the doping of cationic vacancies.20,21 It is suggested that the cationic vacancies in the present OAm-protected CuInS2 NCs work as a trapping site, which dramatically decreases the lifetime of excitons in the NCs. In addition, it should be noted that the energy transfer to LSPR in plasmonic NCs is generally fast compared with electron−hole recombination.26,27 Therefore, we concluded that the rapid decay process of excitons in our NCs could be explained by defect trapping and/or energy transfer to the holebased LSPR (Scheme 1). Surface passivation is an important factor determining the lifetimes of excitons in CuInS2 NCs. The OAm-induced

indicates that nonradiative decay, which is related to cation defects, is faster than the luminescent charge recombination. Finally, to clarify the photoinduced carrier dynamics in the CuInS2 NCs, we conducted TA measurements using fs-LFP. To suppress many-particle effects such as Auger recombination, relatively low excitation intensities corresponding to the linear power dependence regime were used (15−112 μJ cm−1 per pulse). Upon excitation of the CuInS2 NCs with a 650 nm laser pulse, bleaching was observed around the exciton peak of CuInS2 NCs (Figure 3). For both NCs, the bleaching spectra exhibited a red shift from 760 to 800 nm up to 0.5 ps after excitation. Then, the position of the bleaching peaks remained constant and the bleaching recovered monotonically over time. The rapid red shift of the bleaching spectra corresponds to the relaxation of excited electrons from conduction band to trap states. As shown in Figure 4, the decay profiles of bleaching were fitted by biexponential decay functions with time constants of

Figure 4. Decay profiled of bleaching of CuInS2 NPs at 874 nm (black) and NDs at 797 nm (blue) upon excitation at 650 nm. Red lines show the best fits.

1.6 ± 0.1 and 6.6 ± 0.9 ps for NPs and 0.98 ± 0.1 and 4.0 ± 0.2 ps for NDs. Ratios of the fast and slow two components of the decay fit (fast component/slow component, A1/A2) were estimated to be 2.9 and 1.6 for NPs and NDs, respectively. The fast dynamics in semiconductor NCs are commonly attributed to electron relaxation from conduction band to trap states, whereas the slow dynamics are assigned to electron−hole recombination. However, because of the lack of fluorescence from these NCs, we could not assign the slow decay process to D

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(4) Kamat, P. V. Quantum Dot Solar Cells. The Next Big Thing in Photovoltaics. J. Phys. Chem. Lett. 2013, 4, 908−918. (5) Liu, W.; Mitzi, D. B.; Yuan, M.; Kellock, A. J.; Chey, S. J.; Gunawan, O. 12% Efficiency CuIn(Se,S)2 Photovoltaic Device Prepared Using a Hydrazine Solution Process. Chem. Mater. 2009, 22, 1010−1014. (6) Jao, M.-H.; Liao, H.-C.; Wu, M.-C.; Su, W.-F. Synthesis and Characterization of Wurtzite Cu2ZnSnS4 Nanocrystals. Jpn. J. Appl. Phys. 2012, 51, 10NC30. (7) Panthani, M. G.; Khan, T. A.; Reid, D. K.; Hellebusch, D. J.; Rasch, M. R.; Maynard, J. A.; Korgel, B. A. In Vivo Whole Animal Fluorescence Imaging of a Microparticle-Based Oral Vaccine Containing (CuInSexS2−x)/ZnS Core/Shell Quantum Dots. Nano Lett. 2013, 13, 4294−4298. (8) Santra, P. K.; Nair, P. V.; George Thomas, K.; Kamat, P. V. CuInS2-Sensitized Quantum Dot Solar Cell. Electrophoretic Deposition, Excited-State Dynamics, and Photovoltaic Performance. J. Phys. Chem. Lett. 2013, 4, 722−729. (9) Torimoto, T.; Tada, M.; Dai, M.; Kameyama, T.; Suzuki, S.; Kuwabata, S. Tunable Photoelectrochemical Properties of Chalcopyrite AgInS2 Nanoparticles Size-Controlled with a Photoetching Technique. J. Phys. Chem. C 2012, 116, 21895−21902. (10) Speranskaya, E. S.; Beloglazova, N. V.; Abé, S.; Aubert, T.; Smet, P. F.; Poelman, D.; Goryacheva, I. Y.; De Saeger, S.; Hens, Z. Hydrophilic, Bright CuInS2 Quantum Dots as Cd-Free Fluorescent Labels in Quantitative Immunoassay. Langmuir 2014, 30, 7567−7575. (11) Weil, B. D.; Connor, S. T.; Cui, Y. CuInS2 Solar Cells by AirStable Ink Rolling. J. Am. Chem. Soc. 2010, 132, 6642−6643. (12) Venkatram, N.; Batabyal, S. K.; Tian, L.; Vittal, J. J.; Ji, W. Shape-Dependent Nonlinear Absorption and Relaxation in CuInS Nanocrystals. Appl. Phys. Lett. 2009, 95, 201109. (13) Booth, M.; Brown, A. P.; Evans, S. D.; Critchley, K. Determining the Concentration of CuInS2 Quantum Dots from the Size-Dependent Molar Extinction Coefficient. Chem. Mater. 2012, 24, 2064−2070. (14) Shockley, W.; Queisser, H. J. Detailed Balance Limit of Efficiency of p−n Junction Solar Cells. J. Appl. Phys. 1961, 32, 510− 519. (15) Lu, X.; Zhuang, Z.; Peng, Q.; Li, Y. Controlled Synthesis of Wurtzite CuInS2 Nanocrystals and Their Side-by-Side Nanorod Assemblies. CrystEngComm 2011, 13, 4039−4045. (16) Panthani, M. G.; Akhavan, V.; Goodfellow, B.; Schmidtke, J. P.; Dunn, L.; Dodabalapur, A.; Barbara, P. F.; Korgel, B. A. Synthesis of CuInS2, CuInSe2, and Cu(InxGa1−x)Se2 (CIGS) Nanocrystal “Inks” for Printable Photovoltaics. J. Am. Chem. Soc. 2008, 130, 16770−16777. (17) Li, L.; Pandey, A.; Werder, D. J.; Khanal, B. P.; Pietryga, J. M.; Klimov, V. I. Efficient Synthesis of Highly Luminescent Copper Indium Sulfide-Based Core/Shell Nanocrystals with Surprisingly Long-Lived Emission. J. Am. Chem. Soc. 2011, 133, 1176−1179. (18) Lei, S.; Wang, C.; Liu, L.; Guo, D.; Wang, C.; Tang, Q.; Cheng, B.; Xiao, Y.; Zhou, L. Spinel Indium Sulfide Precursor for the PhaseSelective Synthesis of Cu−In−S Nanocrystals with Zinc-Blende, Wurtzite, and Spinel Structures. Chem. Mater. 2013, 25, 2991−2997. (19) Koo, B.; Patel, R. N.; Korgel, B. A. Wurtzite−Chalcopyrite Polytypism in CuInS2 Nanodisks. Chem. Mater. 2009, 21, 1962−1966. (20) Niezgoda, J. S.; Harrison, M. A.; McBride, J. R.; Rosenthal, S. J. Novel Synthesis of Chalcopyrite CuxInyS2 Quantum Dots with Tunable Localized Surface Plasmon Resonances. Chem. Mater. 2012, 24, 3294−3298. (21) Niezgoda, J. S.; Yap, E.; Keene, J. D.; McBride, J. R.; Rosenthal, S. J. Plasmonic CuxInyS2 Quantum Dots Make Better Photovoltaics Than Their Nonplasmonic Counterparts. Nano Lett. 2014, 14, 3262− 3269. (22) Yumashev, K. V.; Malyarevich, A. M.; Prokoshin, P. V.; Posnov, N. N.; Mikhailov, V. P.; Gurin, V. S.; Artemyev, M. V. Optical Transient Bleaching/absorption of Surface-Oxidized CuInS2 Nanocrystals. Appl. Phys. B: Laser Opt. 1997, 65, 545−548. (23) Jain, P. K.; Manthiram, K.; Engel, J. H.; White, S. L.; Faucheaux, J. A.; Alivisatos, A. P. Doped Nanocrystals as Plasmonic Probes of Redox Chemistry. Angew. Chem., Int. Ed. 2013, 52, 13671−13675.

Scheme 1. Schematic Illustration of Photo-Induced Carrier Dynamics of CuInS2 NCs

appearance of LSPR may be potentially important for the fabrication of plasmonic semiconductor NCs, while it is obviously unfavorable for fluorescent probes or light-emitting devices. Careful choice of the protecting ligand is needed to design optimized CuInS2 NCs for specific applications.

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CONCLUSIONS We synthesized nearly stoichiometric OAm-protected CuInS2 NCs and investigated their photoinduced carrier dynamics using fs-LFP. The OAm-protected NCs exhibited vacancyinduced weak LSPR. The decay rates of excitons in the NCs were faster than those in stoichiometric dodecanethiolprotected NCs. The rapid decay of the photogenerated excitons in the OAm-protected NCs was not sensitive to the crystal structure or shape of the NCs, and could be explained by defect trapping and/or energy transfer to the hole-based LSPR. The choice of the protecting ligand is an important factor determining the optical properties of CuInS2 NCs.

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ASSOCIATED CONTENT

S Supporting Information *

TEM images of nonstoichiometric CuInS2 shown in Table 1. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Artificial Photosynthesis Project (ARPChem) of the New Energy and Industrial Technology Development Organization (NEDO) of Japan and KAKENHI Grant-in-Aid for Scientific Research C (No. 25390017; M.S.), Grant-in-Aid for Scientific Research A (No. 25247052; Y.K.).

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