Structural and Optical Study of Ga3+ Substitution in CuInS2

Article pubs.acs.org/JPCC

Structural and Optical Study of Ga3+ Substitution in CuInS2 Nanoparticles Synthesized by a One-Pot Facile Method Yaser Vahidshad,† Muhammad Nawaz Tahir,‡ Azam Iraji Zad,§ Seyed Mohammad Mirkazemi,*,† Reza Ghasemzadeh,† Hannah Huesmann,‡ and Wolfgang Tremel‡ †

School of Metallurgy and Material Engineering, Iran University of Science and Technology, Tehran 16844, Iran Institute of Inorganic Chemistry and Analytical Chemistry, Johannes Gutenberg University Mainz, Duesbergweg 10-14, D-55099, Mainz, Germany § Department of Physics, Sharif University of Technology, Tehran 11365-9161, Iran ‡

ABSTRACT: A one-pot method was used to synthesize CuInxGa1−xS2 nanoparticles by substituting In3+ with Ga3+. The samples with composition of gallium ranging from 0% to 100% were synthesized by solving copper chloride, indium trichloride, gallium acetylacetonate, and thiourea as precursors in 1-octadecene, oleylamine, and oleic acid as noncoordinating, coordinating, and capping agent solvents, respectively. Depending on the chemical composition and synthesis conditions, the morphology of the as-synthesized nanoparticles obtained was trigonal, semitrigonal, hexagonal, and quasispherical. X-ray photoelectron spectroscopy and X-ray diffraction confirmed that Ga3+ substituted In3+ without any segregation over a wide range. The as-synthesized CuInxGa1−xS2 nanoparticles showed narrow size distribution across the entire composition range (x = 0−1) and band gap tuned in the range from 1.44 to 2.28 eV. The morphology, structure, and optical properties of the synthesized nanoparticles were characterized using transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), UV− visible (UV−vis) spectroscopy, and X-ray photoelectron spectroscopy (XPS). The mechanism of complex formation up to nanoparticle synthesis was also discussed.

1. INTRODUCTION Increasing demands on clean energy result in development of photovoltaic products. Thin film solar cells are interesting because they are lightweight, have good photostability, are costeffective, and are able to be fabricated on flexible substrates.1−6 Thin films based on CuInxGa1−x(S1−ySey)2 (x, y = 0−1) (CIGSSe) compounds are attractive due to the high solar absorption coefficient (more than 105 cm−1). Also, band gap tuning of the CIGSSe compounds can be adjusted from 0.98 eV for CISe to 2.40 eV for CGS which is a wide spectral region from visible to near-infrared.7−10 Recent studies indicate that CIGSSe solar cells have exhibited the highest solar energy conversion efficiency (20.8%) among thin film solar cell groups.11−15 They are fabricated by rather expensive methods such as molecular beam epitaxy,16 coevaporation,17 sputtering,18 pulse laser deposition,19 and electrodeposition.20 One of the trends in this field is reducing the production cost by introducing low cost preparation protocols like printing methods. Quaternary chalcogenide thin films can be prepared using colloidal nanocrystals that are synthesized by simple solution-based methods21−25 such as © XXXX American Chemical Society

chemical bath deposition, microwave-assisted synthesis, spray pyrolysis, solvothermal, hot injection, thermolysis, and other chemical wet methods for printing applications.26−32 Controlling the stoichiometry and phase structure of quaternary CuInxGa1−xS2 is more difficult than ternary or binary compounds. Therefore, choosing a proper method for nanocolloid synthesis is really important to achieve an optimized morphology and structural and optical properties.33−38 The one-pot thermolysis method as a promising method is based on controlling the nucleation and growth of organometallic compounds (complex) by using proper solvents and capping agents. Various printing techniques are used, including dip coating,39 ink jet printing,40 drop casting,41 roll-to-roll42 spin, and spray coating methods.43,44 In this contribution, we report on the solution synthesis of quaternary phase copper indium gallium sulfide colloidal Received: July 2, 2014 Revised: September 20, 2014

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dx.doi.org/10.1021/jp506584a | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Figure 1. Various steps of complex formation (top row) and complex decomposition leading to nucleation and growth of nanoparticles (bottom row).

Figure 3. XPS spectra of CuInS2, CuIn0.5Ga0.5S2, and CuGaS2 compounds.

2. EXPERIMENTAL SECTION CuInxGa1−xS2 (x = 0−1) nanoparticles were synthesized by mixing copper chloride (CuCl, Sigma-Aldrich), indium trichloride (InCl3, Sigma-Aldrich), gallium acetylacetonate (Ga(acac)3, Sigma-Aldrich), and thiourea (CH4N2S, Fisher) as precursors and oleylamine (OLA) (C18H37N, Acros), oleic acid (OA) (C18H34O2, Fisher), and 1-octadecene (ODE) (C18H36, Acros) as coordinating solvent, capping agent, and noncoordinating solvent under Ar. The In:Ga ratio in the nanocrystals could be tuned systematically by changing the proportion of Ga and In reactants. In a typical synthesis, 0.5 mmol of CuCl, 0.5 mmol of InCl3, and 2 mmol of thiourea (TU) were dissolved in 20 mL of OLA, 4 mL of OA, and 6 mL of ODE under Ar at room temperature using the Schlenk

Figure 2. FT-IR spectra of pure oleylamine (black), oleic acid (red), and thiourea (blue) and Cu−In−Ga complexes with solvent molecules (green) and synthesized nanoparticles (purple).

nanoparticles with tunable band gap, which could find potential applications for printed thin film solar cells. The main goal is the substitution of In3+ with Ga3+ using a one-pot thermolysis method using solution synthesis that is simple, safe, low cost, and environmentally friendly. Our results indicate the synthesized materials are in the nano regime with size ranging from 10 to 20 nm and have narrow size distribution, good crystallinity, and tuned optical properties. B



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Figure 4. Scanning XPS spectra of CuInS2, CuIn0.5Ga0.5S2, and CuGaS2 compounding to Cu 2p, In 3d, Ga 2p, and S 2p.

technique. The contents of the flask were evacuated at 60 °C for 45 min. After that, the flask atmosphere was changed to argon, and the reaction mixture was heated to 210 °C with a rate of 18−20 °C/min. During the heating process, smoke began to evolve at temperature about 170 °C, and the color turned darker, indicating nucleation and growth phenomena. The flask was held at the above-mentioned temperature for 4 h before cooling it to room temperature (Figure 1). Then, 30 mL of ethanol was added to the reaction flask and centrifuged for 10 min (9000 rpm) to collect the product. For XRD and XPS analysis, the process of washing was repeated several times and finally dried at 80 °C for 5 h in air condition. The structure, morphology, and optical properties of the products were characterized by X-ray diffraction analysis (Mo Kα radiation, energy-dispersive detector SOLX), X-ray photoelectron spectroscopy (high-resolution excitation in the laboratory UV light and vacuum: 10−10 Pa), EDX-INCA350 Energy (FE-SEM/Quanta 200 FEG, 30 kV), the transmission electron microscope (Philips EM420 microscope operating at an acceleration voltage of 120 kV), FT-IR spectroscopy (acquired on a Mattson Instruments 2030 Galaxy spectrometer), UV−vis− NIR absorbance spectroscopy (Ocean Optics Ins. USB-4000), and photoluminescence spectroscopy (Bruins Omega-10 spectrometer).

Figure 5. XRD patterns of the CuInxGa1−xS2 nanoparticles (where x = 1−0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2, 0.1, and 0).

precursors and coordination solvents. According to Lewis principle, the hard acid with hard base or soft acid with soft base interactions are stronger than hard acid with soft base or soft acid with hard base interactions. On the basis of Lewis rule and coordinating properties of solvents, the thiourea (TU), oleic acid (OA), and oleylamine (OLA) can form appropriate complexes with metal elements.45−47 Copper (soft Lewis acid)

3. RESULT AND DISCUSSION To produce CuInxGa1−xS2 (x = 0−1) nanoparticles, the first step involves formation of multiple complexes between metal C



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Table 1. Experimental Results for the One-Pot Thermolysis Methoda molybdenum source

a

lattice constant

no.

sample name

Ga %

crystalline size avg. (nm)

particle size avg. (nm)

structure

angle (deg.)

d-spacing (Å)

c (Å)

a (Å)

1 2 3 4 5 6 7 8 9 10 11

CIS-WOA-M32 CIGS-WOA-M3 CIGS-WOA-M4 CIGS-WOA-M5 CIGS-WOA-M6 CIGS-WOA-M7 CIGS-WOA-M8 CIGS-WOA-M9 CIGS-WOA-M10 CIGS-WOA-M11 CGS-WOA-M4

0 10 20 30 40 50 60 70 80 90 100

23 ± 0.5 18 ± 0.5 13 ± 0.5 10 ± 0.5 8 ± 0.5 9 ± 0.5 8 ± 0.5 9 ± 0.5 9 ± 0.5 7 ± 0.5 6 ± 0.5

20 ± 3.1 21 ± 2.0 23 ± 2.0 11 ± 1.9 12 ± 1.8 29 ± 26.1 61 ± 38.0 56 ± 11.2 59 ± 10.9 11 ± 1.4 11 ± 2.9

CH CH CH CH + W CH + W CH + W CH + W CH + W CH + W CH + W CH + W

12.948 12.938 13.463 13.366 13.051 13.115 13.163 13.299 13.350 13.379 13.463

3.145 3.147 3.025 3.047 3.121 3.106 3.094 3.063 3.051 3.044 3.025

11.0803 11.0571 10.9970 10.8345 10.8225 10.9040 10.8966 10.4712 10.5162 10.3245 10.2260

5.5360 5.5080 5.5151 5.5072 5.5077 5.4657 5.4468 5.5301 5.4980 5.5439 5.4348

W: Wurtzite, Ch: Chalcopyrite.

Scheme 1. Schematic Representation of the Possible Nucleation and Growth Mechanism of CuInxGa1−xS2 (x = 0−1) Nanoparticles

Table 2. Molar Ratio of Elements in Precursors and Synthesized Samples (EDS Analysis)

a

sample no.

Ga %a

precursor (Cu:In:Ga:S)

Cu %b

In %b

Ga %b

S %b

EDS (Cu:In:Ga:S)

CIS-WOA-M32 CIGS-WOA-M3 CIGS-WOA-M4 CIGS-WOA-M6 CIGS-WOA-M7 CIGS-WOA-M9

0 10 20 40 50 70

1.0:1.0:0.0:4.0 1.0:0.9:0.1:4.0 1.0:0.8:0.2:4.0 1.0:0.6:0.4:4.0 1.0:0.5:0.5:4.0 1.0:0.3:0.7:4.0

0.254 0.214 0.236 0.229 0.213 0.213

0.249 0.216 0.209 0.161 0.127 0.072

0.000 0.0259 0.0480 0.093 0.098 0.138

0.497 0.543 0.507 0.516 0.562 0.578

1.0:1.0:0.0:2.0 1.1:1.1:0.1:2.7 0.9:0.8:0.2:2.0 0.9:0.6:0.4:2.1 1.1:0.6:0.5:2.8 1.1:0.4:0.7:2.9

Precursors. bSynthesized samples.

CuInxGa1−xS2 upon completion. On the basis of these facts we summarize the reaction mechanism with the following chemical equations.51−53

forms a complex with TU (soft Lewis base), and indium and gallium being borderline cases for Lewis acidity are prone to make a complex with oleylamine and oleic acid (hard Lewis base). The copper−thiourea (Cu−TU) complex is more reactive than indium and gallium complexes, and upon heating the reaction contents to ∼170 °C the Cu−TU complex (reaction 1) decomposes to form Cu2−xS (x = 0−1).48−50 Subsequently In−OLA−OA and Ga−OLA−OA complexes (reactions 2−5) decompose, and In3+ and Ga3+ ions incorporate into Cu2−xS lattices by ion exchange to produce CuInxGa1−xS2 and 3Cu+ (reaction 6). The released Cu+ reacts with extra thiourea to form more Cu−TU complex, and the reaction continues from stage number 7 and 2 to 6 until cation exchange finished in

Heating up

CuCl + CSHN2H3 → [Cu(TU)]1 + ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Cu 2 − xS (x = 0−1)

(1) Heating up

InCl3 + 3CH(CH 2)17 NH 2 → [In(OLA)3 ]3 + ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ In 3 + (2) Heating up

InCl3 + 3C(CH 2)17 O2 → [In(OA)3 ]3 + ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ In 3 + (3) D



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Figure 6. TEM images of CuInxGa1−xS2 (where x = 1, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2, 0.1, and 0) nanoparticles with different scale bar. Heating up

are formed by cation exchange completion. The surface atoms have a lower coordination number in comparison with the interior atoms of nanocrystals, so the extra OLA, OA, and thiourea bind to the surface of nanocrystals as a capping agent to control growth and stabilize the clusters by reacting amine NH2, carboxylic acid (COOH), and the thiol SH ligand with the cluster’s surface. To study the details of complex formation experimentally, we monitored the kinetics of reaction by taking aliquots at various stages (complexes formed observed by color change of reaction contents) at elevated temperatures (Figure 1D) and at the end of reactions (Figure 1H) using FT-IR. Figure 2 shows FT-IR data from the samples at elevated temperature (at a stage where the metal solvent complex is formed), at the end of the reaction, and pure OA, OLA, and TU as references. The various bands can be assigned to the stretching and bending modes of N−H, C−N,

GaCl3 + 3CH(CH 2)17 NH 2 → [Ga(OLA)3 ]3 + ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Ga 3 + (4) Heating up

GaCl3 + 3C(CH 2)17 O2 → [Ga(OA)3 ]3 + ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Ga 3 + (5)

2Cu 2 − xS + x In 3 + + (1 − x)Ga 3 + Annealing at 210 ° C

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ CuInxGa1 − xS2 + 3Cu+

(6)

Annealing at 210 ° C

Cu+ + CSHN2H3 → [Cu(TU)]1 + ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Cu 2 − xS (x = 0−1)

(7)

In fact the first nucleation during the reaction is Cu2−xS, and after that as growth continues the CuInxGa1−xS2 nanoparticles E



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Figure 7. UV−visible−NIR absorption spectra of CuInxGa1−xS2 (where x = 0.1−0.9) nanoparticles dispersed in n-hexane.

285.0 and 532.2 eV, respectively. For the CIS-WOA-M32 (0% Ga), CIGS-WOA-M7 (50% Ga), and CGS-WOA-M4 (100% Ga) samples, all the Cu-related, In-related, Ga-related, and S-related peaks appeared in the XPS spectrum (Figure 3). The Cu 2p splits into Cu 2p3 (931.4 eV) and Cu 2p1 (951.4 eV) peaks for CuInS2; Cu 2p splits into Cu 2p3 (931.4 eV) and Cu 2p1 (951.4 eV) peaks for CuIn0.5Ga0.5S2; and Cu 2p splits into Cu 2p3 (932.2 eV) and Cu 2p1 (951.4 eV) peaks for CuGaS2.61 They confirm that the valence states of Cu ions are +1 in all of the synthesized samples (Figure 4), due to the absence of the satellite peak at about 944.0 eV for Cu2+.62 Moreover, Figure 4 shows the binding energy of 445.0 and 452.2 eV corresponding to In 3d5 and In 3d for both CuInS2 and CuIn0.5Ga0.5S2 samples.63 The peaks at 1117.8 and 1144.2 eV correspond to Ga 2p3 and 2p1 for CuIn0.5Ga0.5S2, and 1117.8 and 1145.0 correspond to similar states for CuGaS2 samples. Lastly, the main peak at 162.6 eV corresponds to S 2p for all of three samples.41 To study the structure and phase purity of the CuInxGa1−xS2 compounds, we investigated XRD data of the synthesized samples across the entire composition range (x = 0−1) as indicated in Figure 5. The XRD patterns reveal that the main structure is chalcopyrite (JCPDS 27-0159) with broad peaks indicating the nanocrystalline nature of the product. Other impurities, such as binary sulfide or segregated phase (CuGaS2) related to reactants, were not detected up to 90% Ga substitution.

C−C, CC, C−H, C−O, CO, O−H, and CS groups present in oleylamine, oleic acid, and thiourea.54−60 However, FT-IR spectra corresponding to the aliquots taken at elevated temperature (complexes formed) indicate three characteristic peaks at 727, 1392, and 1624 cm−1 that can be assigned to δ(CS), υ(CS), and υ(−NH2) of the TU functional group. Also, two characteristic peaks at 1541 and 3298 cm−1 are related to δ(NH2) and υ(NH2) of OLA. The specific peak of the oleic acid spectrum is the carbonyl absorbance (υ(CO)) at 1707 cm−1. The existence of the low intensity peaks with small shifts indicates complex formation. The spectrum of the end product (Figure 2, purple line) indicates almost similar spectra with very small shifts as compared to the spectrum obtained from intermediate complexes because of the covalent binding of thiourea, oleylamine, and oleic acid with the cluster’s surface acting as a ligand to stabilize the surfaces. It shows peaks at 719, 1396, and 1619 cm−1 attributed to δ(CS), υ(CS), and δ(−NH2) for thiourea and at 1510 cm−1 attributed to δ(NH2) for oleylamine, and 1707 and 3420 cm−1 correspond to υ(CO) and υ(O−H) for oleic acid. Therefore, the OLA, OA, and TU played the principal role as a capping agent in the process of nanoparticle stabilization to avoid any agglomeration. To study the composition and chemical state of the compounds, three samples of CuInS2, CuIn0.5Ga0.5S2, and CuGaS2 were examined by X-ray photoelectron spectroscopy (XPS). The binding energies in the XPS analysis are corrected for specimen charging by referencing the C 1s and O 1s to binding energy of F



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The wurtzite structure can be observed by increasing gallium substitution from 30% along with chalcopyrite structure (polytypism).64 As the Ga content increases, the diffraction peak positions gradually shift toward larger angles. The main peaks of CuInxGa1−xS2 are observed at 2θ values of 12.938° (112), 21.136° (204) (220), and 24.790° (116) (312) for CuIn0.9Ga0.1S2 and up to 13.379° (112), 21.843° (204) (220), and 25.642° (116) (312) for CuIn0.1Ga0.9S2. The relationship between the shifted angle and composition is consistent with substitution of Ga3+ (radius = 0.62 Å) in place of In3+ (radius = 0.76 Å) according to Vegrad’s law.65,66 Table 1 presents details of average grain size, phase structure, and lattice constant for all obtained data. We observe lattice constants of c and a along with grain size decrease by increasing the gallium concentration ranging from CuInS2 to CuInxGa1−xS2 and finally CuGaS2. Lattice constant reduction is not linear due to the coexistence of chalcopyrite and wurtzite structures in nanocrystals (polytypism). Although the XRD data showed a grain size of about 10−20 nm, the details of the size, size distribution, and morphology of the nanoparticles were measured using transmission electron microscopy (TEM). Figure 6 shows images of the as-synthesized CuInxGa1−xS2 (x = 0−1) nanoparticles with size ranging from 10 to 100 nm diameter that were dispersed in n-hexane before dropping on the Cu grids. The shape of the nanoparticles varies from trigonal for CuInS2 to semitrigonal, hexagonal, and quasi-spheres when Ga3+ content increases to 80% and finally changes to hexagonal in CuGaS2. We observe that the particle size average for CuInxGa1−xS2 is about 20 nm for x ≤ 20% which is reduced to around 10 nm for x = 30 and 40%. The TEM data for 50 ≤ x ≤ 80 indicate aggregation and cluster formation of the particles. As is presented in Table 1, the particle size decreased to about 10 nm when samples are Ga3+ rich. By increasing the gallium the average particle size and standard deviation enhancement have been observed by a number of researchers.30,37,67−72 On the basis of the above data, the possible nucleation and growth mechanism for the synthesis of nanoparticles is shown in Scheme 1. The chemical compositions of CuInxGa1−xS2 nanoparticles were examined by energy-dispersive X-ray spectroscopy (EDS). The Cu:In:Ga:S precursor molar ratio in precursors and EDS-SEM obtained from synthesized samples are summarized in Table 2. In general, the mole ratios between Cu, In, and Ga of all the samples were close to the mole ratios in precursors. To study the effect of composition on optical transmission and band gap, UV−vis spectroscopy is used. The absorbance spectra of quaternary CuInxGa1−xS2 nanoparticles with various concentrations of Ga are shown in Figure 7. It can be seen that the absorption edge is tuned from the visible region for pure CuGaS2 to the near-infrared region for Ga3+ substitution (of CuInxGa1−xS2) and pure CuInS2. As a result, a significant backward shift of the band gap is clearly revealed by increasing the contents of Ga3+. The absorbance shoulder that appeared at 860 nm (1.44 eV) corresponds to the CuInS2 structure. The observed blue shift of the absorption edge by Ga3+ substitution is expected. This blue shift in band gap is not derived from a quantum effect because the particle size is much bigger than their exciton Bohr radius (around 5 nm for CuInS2).2,73−75 Substitution of Ga3+ with In3+ increases the band gap energy from 1.54 eV for CuIn0.9Ga0.1S2 (10% Ga) to 2.13 eV for CuIn0.1Ga0.9S2 (90% Ga) samples. This increase can be due to the shift of conduction and valence band to higher and lower energy levels, respectively. In fact, the substitution of In3+ with Ga3+ in CuInS2 brings Ga 4s states to its conduction band. Since the Ga 4s and S 3p states are somewhat higher in energy,76−79 the

Figure 8. Absorbance spectrum of CuInxGa1−xS2 NCs (a) where x = 0.9, 0.8, 0.7, 0.6 and (b) x = 0.6, 0.5, 0.4, 0.3, 0.2, 0.1 with an extrapolation of the spectra (dash line) to determine the band gap value. (c) Plot of the band gap of the nanocrystals as a function of the percent of Ga concentration.

conduction band of CuInxGa1−xS2 shifts to higher energies, and accordingly, their band gap increases. Also the valence-band offset is small because the valence band is primarily a bonding anion p state, and as a result the valence band offset should be small. Figure 8 shows the effects of Ga substitution with In on the transmission spectra and Tauc plots of the CIGS nanoparticles G



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Figure 9. PL spectra of CuInS2, CuIn0.5Ga0.5S2, and CuGaS2 nanocrystals recorded at room temperature.

4-. CONCLUSION The CuInxGa1−xS2 colloidal nanoparticles have successfully been prepared by a facile one-pot thermolysis method. The measured sample’s band gap from absorption spectroscopy was in the visible wavelengths for CuGaS2 to near-infrared for CuInS2. The absorption edge variation can be attributed to the substitution of Ga3+ instead of In3+ according to XRD and XPS results. Absorption spectroscopy results indicate substitution can be used in multijunction solar cells with the compounds that are highly environmentally friendly. The prepared CuInxGa1−xS2 colloidal nanoparticles were stable for several weeks which makes it a promising compound for low cost ink jet printing solar cells.

dispersed in n-hexane and a plot of the determined band gap versus percent of Ga3+ concentration for the nanocrystals synthesized in the ODE, OLA, and OA system and dispersed in n-hexane solvent. Scheme 2 and Figure 9 show the mechanism of excitation and emission light and the room-temperature photoluminescence Scheme 2. Schematic Diagram Showing a Possible Excitation, Relaxation, and Emission Pathway in CuInxGa1−xS2 Nanocrystals

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Iran University of Science and Technology and Institute of Inorganic Chemistry and Analytical Chemistry of Johannes Gutenberg University of Mainz. We are deeply grateful to Prof. Wolfgang Tremel for providing valuable advice.

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spectra of CuInS2, CuIn0.5Ga0.5S2, and CuGaS2 nanoparticles, respectively, under the excitation wavelength of Eex = 700, 450, and 350 nm, respectively, at room temperature. For CuInS2, the PL emission peak (assigned arrows) is observed at Eemiss = 823 and 760 nm (the 823 nm is stronger than 760 nm). The main emission peak is for the transition from the valence band to the conduction band (1.51 eV) of chalcopyrite nanocrystals, but the emission peak at 760 nm is for the wurtzite phase (polytypism).39,64,80,81 For the CuIn0.5Ga0.5S2, emission peaks (assigned arrows) at 516, 646, 760, and 823 nm are observed. These several emission peaks can be attributed to sub-bands appearing as a result of substitution of Ga3+ instead of In3+ that result in the hybridization of Ga 4s and S 3p which results in the formation of new bands with higher energy. Finally, for the CuGaS2 sample, the emission peak (assigned arrows) at Eemiss = 575 nm is observed which is attributed to the valence band to the conduction band. Other weak emission peaks may be related to a sublevel of the acceptor or donor or wurtzite structure.53,82,83

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