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Controlling the Size, Shape, Phase, Band Gap, and LSPR of Cu S and CuInS Nanocrystals 2-x
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Xianliang Wang, and Mark T. Swihart Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm504626u • Publication Date (Web): 04 Feb 2015 Downloaded from http://pubs.acs.org on February 11, 2015
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Chemistry of Materials
Controlling the Size, Shape, Phase, Band Gap, and LSPR of Cu2-xS and CuxInyS Nanocrystals Xianliang Wang and Mark T. Swihart* Department of Chemical and Biological Engineering, University at Buffalo (SUNY), Buffalo, New York 14260-4200, USA KEYWORDS: colloidal synthesis, localized surface plasmon resonance, semiconductor nanocrystals, bandgap ABSTRACT: We show how indium incorporation can effect size, morphology, crystal structure and optical properties in CIS nanocrystals (NCs) and demonstrate a robust methodology for preparing monodisperse plasmonic and nonplasmonic CIS NCs. In contrast with previous methods of producing CIS nanocrystals exhibiting localized surface plasmon resonance (LSPR), which employed expensive and hazardous bis(trimethylsilyl) sulfide (TMS2S), we employ sulfur dissolved in oleic acid (OA)-S as the sulfur donor in combination with appropriate cation precursors to enable production of monodisperse CIS NCs with broad variation of the Cu:In ratio in the products. The crystal phase of the CIS NCs is not only determined by the reactivity of the ligands, but also by the cation composition. The LSPR shifted to longer wavelength with increasing In content until it vanished, while the bandgap of the CIS NCs decreased linearly from 2.1 eV to 1.2 eV. The manipulation of optical properties of CIS NCs with controlled and well-defined size, shape, and phase may open up new possibilities for applying I-III-VI materials in solution-processed optoelectronic devices.
Introduction Colloidal routes to preparing semiconductor nanocrystals (NCs) are of great interest because of their potential for reducing the cost of electronics1-3, optoelectronics2, 4-10, energy storage11, 12 devices and related1, 5, 13-17 applications, particularly large area or large volume devices such as solar cells4-9 and thermoelectric1, 5, 13-16 devices. This cost reduction is expected to result from replacement of hightemperature high-vacuum processing steps with lower cost printing, coating, and atmospheric-pressure annealing processes.18, 19 This potential has helped to generate tremendous research interest in solutionphase synthesis of colloidal semiconductor NCs of controlled size, shape, phase, and composition over the past two decades. Recently, copper chalcogenide nanocrystals20-23 and related alloys24-26 have attracted increased interest due to their potential low cost, tunable bandgap, and changeable carrier concentration. A range of protocols for producing colloidal Cu-based ternary (I-III-VI, I-IV-VI)27-32 or quaternary (I-III-VI and I-II-IV-VI)5, 33-37 NCs of controllable size, shape and crystal structure have been presented. These ternary and quaternary materials present an enormous range of possible band structures and related optical and electronic properties. For example, Norako and coworkers
produced metastable wurtzite-phase CTSe (I-IV-VI) NCs using di-tert-butyl diselenide as the selenium precursor in the presence of dodecanethiol.29 Zou and coworkers found the sulfur donor affected the shape and crystalline phase of CZTS (I-II-IV-VI) NCs.38 Dilena and coworkers produced CIGS NCs with tunable band gap from 1.48 to 2.10 eV by tailoring the In:Ga cation ratio. The resulting NCs exhibited high conductivity after ligand exchange and were suitable for use in photovoltaics.37 Numerous previous studies demonstrated the use of organic molecules, (i.e., aliphatic amines, phosphonic acids and carboxylic acids) and appropriate precursors to prepare high quality ternary or quaternary Cu-based chalcogenide NCs with fixed stoichiometry.5, 30 Another exciting recent development has been the observation of localized surface plasmon resonance (LSPR) in copper-deficient copper chalcogenide NCs.39, 40 The LSPR energy in these materials can be tuned by several means, including oxidation/reduction41, 42 or use of different surface ligands20, 24 to vary the effective free carrier concentration, as well as by variation of aspect ratio in anisotropic structures.43, 44 However, there have been relatively few reports of varying the cation composition to tune the LSPR energy (i.e. producing I-III-VI or I-IV-VI NCs exhibiting LSPR). A few
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reports of I-IV-VI NCs (CuxSnyS and CuxSnySe) with LSPR have been presented, and one group has previously studied CuxInyS NCs (CIS NCs) with LSPR and shown that these NCs can potentially outperform their non-plasmonic counterparts in PV applications.45 The size of stoichiometric CuInS2 nanoparticles (without LSPR) can be tuned by varying the reaction time and temperature, with the resulting size-dependent absorption and photoluminescence spectra showing the effects of quantum confinement.46 In plasmonic CuxInyS NCs, we therefore expect to be able to vary both the band gap and the LSPR through both size and composition effects. Here, we show how indium incorporation can effect crystal size, shape, and optical properties in plasmonic CIS NCs. We present a precursor and ligand combination that leads to In incorporation in plasmonic Cu2-xS NCs in linear proportion to the In content of the precursor mixture, allowing production of NCs with In cation fraction up to almost 60% (i.e. beyond the nominal CuInS2 composition). We show that size, morphology, phase, band gap, and LSPR of CIS NCs are strongly affected by In content. Moreover, plasmonic Cu2-xS and CIS NCs with In cation fractions of up to 15% exhibit the roxbyite crystal phase, which has not previously been reported for CIS NCs. We demonstrate production of both plasmonic and non-plasmonic CIS NCs without using the relatively hazardous and expensive bis(trimethylsilyl) sulfide (TMS2S) precursor employed in the only previously-reported method for producing plasmonic CIS NCs. The methods presented here allow broad tuning of the band-gap, LSPR energy, and carrier concentration in these I-IIIVI materials and therefore open up new possibilities for optoelectronic devices based upon these colloidal NCs, while also providing new insights into their formation.
Experimental Section Chemicals. Copper(II) acetate (Cu(ac)2, 98.0%), Indium(III) chloride (InCl3, 98%), In(III) acetate (In(ac)3, 99.99%), oleylamine (OAm,70%), sulfur (S) powder (99.98%) and trioctylphosphine oxide (TOPO, technical grade 90%) were purchased from Sigma Aldrich and were used as received. Oleic acid (OA) was purchased from Fisher Scientific and used as received. Preparation of OA-S precursors. The OA-S precursor was prepared by heating 5 mmol S powder in 5 mL OA to 110 ºC. The solution was held at this
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temperature for 20-30 min, then heated to about 150°C for another 10-20 min. Synthesis of copper indium sulfide alloy NCs using OA-S precursor. In a typical synthesis of CIS NCs, a total of 1.5 mmol of cation precursor powder, consisting of Cu(ac)2 and InCl3 at a specified molar ratio (7:1, 4:1, 2:1, 1:1,1:2 or 1:4), was mixed with 4 g TOPO and 10 mL OAm. The solution was degassed at 110 ºC for 30 min under nitrogen protection. Then the solution was heated to 205 ºC and it became transparent and deep yellow, indicating formation of organo-Cu and organo-In precursor complexes. After reducing the temperature to 145 ºC, 2 mL of the 1M OA-S precursor was rapidly injected, and the mixture was held at this temperature for 30 min. The heating mantle was then removed and 20-30 mL ethanol was injected, reducing the temperature to ~50°C. CIS NCs were collected by centrifugation at 9000 rpm (about 9000 G) for 1 min. The collected CIS NCs were redispersed in chloroform. Ethanol was added to the resulting dispersion and CIS NCs were collected again by centrifugation. The resulting products were dispersed in chloroform and stored at ambient conditions for future characterization. Characterization. Transmission Electron Microscopy (TEM). The size and morphology of CIS NCs were characterized using a JEOL JEM-2010 microscope at a working voltage of 200 kV. Samples were prepared for analysis by drop casting from a dilute dispersion onto a carboncoated copper TEM grid. Powder X-Ray Diffraction (XRD). The crystal phases in CIS NCs were determined using powder XRD (Bruker Ultima IV with Cu K-α X-rays). Samples were prepared by drop-casting high-concentration CIS NC dispersions onto glass. Average crystallite size was estimated by application of the Scherrer equation to an appropriate peak for each crystal phase. Energy Dispersive X-Ray Spectrometry. Compositional analysis of CIS NCs was obtained using an Oxford Instruments X-Max 20 mm2 energy dispersive X-ray spectroscopy (EDS) detector within a Zeiss Auriga scanning electron microscope (SEM) UV-Vis-NIR Spectroscopy. Optical absorption spectra of CIS NCs dispersions were measured using a Shimadzu 3600 UV–visible-NIR scanning spectrophotometer. All spectra were measured under ambient conditions in air. Results and Discussion Sulfur powder readily dissolves in oleic acid upon moderate heating to produce an OA-S precursor with
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Figure 1. TEM images of Cu(In)S NCs synthesized using OA-S as sulfur donor and Cu:In precursor ratios of (A&B) 7:1; (C&D) 4:1; (E&F) 2:1; (G&H) 1:1; (I&J) 1:2; and (K&L) 1:4. The In cation fraction increases from ~0% to ~60% from panels (A&B) to panels (K&L).
very good stability. However, there have been few reports of the use of OA-S for synthesis of colloidal metal-sulfide NCs. Previously, bis(trimethylsilyl) sulfide (TMS2S) was used in CIS NC synthesis to produce NCs that exhibited LSPR.31 Compared to TMS2S, OA-S is safe, environmentally-friendly, stable, and low cost. Here we demonstrate that it can be used to synthesize CIS NCs with broadly tunable In content and LSPR energy. Initially, we attempted to prepare CIS NCs using the OA-S sulfur precursor with Cu(II) acetate (Cu(ac)2) and In(III) acetate as cation precursors. However, this (In(ac)3) produced NCs with a broad size distribution. This was the case for a wide range of Cu:In precursor ratios. The resulting NCs also showed signs of phase separation (Supporting Information, Figure S1), suggesting that copper-rich and indium-rich NPs nucleated and grew separately in this case. However, when we employed InCl3 instead of In(ac)3 as the indium precursor, we obtained high-quality monodisperse NCs with tunable In content. This difference may be attributable to the lower reactivity of InCl3 with the OA-S precursor, compared to Table 1. EDS analysis of CIS NCs produced by OA-S. Cu precursor/ In precursor (mmol) 1.31/0.19 1.20/0.30 1.00/0.50 0.75/0.75 0.50/1.00 0.30/1.20
Precursor ratio (Cu:In)
7:1 4:1 2:1 1:1 1:2 1:4
ND = not detected by EDS
EDS Analysis (mean atom percent) Cu
In
S
58.7 54.6 47.7 38.4 28.0 21.1
ND 3.2 8.7 16.9 24.9 30.4
41.3 42.3 43.6 44.7 47.1 48.5
alloy NC stoichiometry CuxInyS
Cu1.42S Cu1.29In0.08S Cu1.09In0.20S Cu0.86In0.38S Cu0.59In0.53S Cu0.44In0.63S
In(ac)3. This moderately lower reactivity can suppress separate nucleation of an indium-rich phase. Instead, only the copper-rich phase nucleates, and In is incorporated into the single-phase NPs. This is consistent with the higher total In content observed using In(ac)3 compared to identical synthesis conditions using InCl3 (Supporting Information, Table S1). TEM imaging (Figure 1) revealed that the C(I)S NCs were quasi-spherical. NCs synthesized using Cu:In precursor molar ratios of 7:1, 4:1, 2:1, 1:1, 1:2, and 1:4 had average diameters of 10.5 nm, 3.6 nm, 6.9 nm, 9.1 nm, 11.7 nm and 14.5 nm, respectively (Figure 1). The size and monodispersity of C(I)S NCs varied with the Cu:In precursor ratio, while the NC morphology did not change significantly. Energy dispersive X-ray spectroscopy (EDS) provided elemental concentrations of Cu, In and S in the alloyed CIS NCs (Table 1) for each precursor ratio. For the Cu:In ratio of 7:1, no indium was detected in the NCs. Decreasing the precursor molar ratio to 4:1 led to detectable incorporation of In (3.2%) accompanied by a substantial decrease in NC size, from 10.5 nm to 3.6 nm diameter. The indium cation fraction in the CIS NCs increased linearly with increasing indium fraction in the precursors (Figure 2), accompanied by a monotonic increase in NC size. Extrapolating the results in Figure 2A to zero In fraction in the NCs suggests that a minimum InCl3 precursor fraction (InCl3/(InCl3 + Cu(ac)2)) near 15% is required to achieve In incorporation in the NCs. For lower InCl3 concentrations, Cu2-xS NCs were obtained. The highest level of In content reached a cation fraction of 60%, obtained using a 1:4 Cu;In precursor molar ratio. The size of CIS NCs increased continuously with increasing In content, from 3.6 to 14.5 nm as In cation fraction increased from 5% to 60% for NCs prepared using Cu:In precursor ratios
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from 4:1 to 1:4. (Figure 2B). At the highest In concentrations, both TEM and XRD, discussed below, suggest that the morphology is evolving from quasi-spherical to plate-like. As noted above, NCs produced at a 7:1 Cu:In precursor ratio, for which In incorporation was not observed, were significantly larger than those with low In content. Thus, the trend of increasing size with increasing In content only holds for NCs with significant In content. Incorporation of a small amount of In decreases the NC size compared to pure Cu2-xS NCs prepared under the same conditions. The reaction temperature and time of 145°C and 30 min are slightly higher and longer, respectively, than the typical synthesis conditions for Cu2-xS NCs using the OA-S precursor (≤ 140°C and ≤ 10 min). This suggests that In incorporation decreases the NC surface reactivity, reducing their growth rate and delaying the onset of Ostwald ripening. The relationship between the crystal phase and In content was investigated by powder X-ray diffraction (XRD). The crystal phase underwent clear changes as the Cu:In precursor ratio was increased while keeping
Figure 2. (A) Dependence of In content in alloy NCs upon the In content of the precursor mixture used in reaction. (B) Dependence of NC size on the In cation fraction.
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other reaction conditions constant. High quality monodisperse Cu2-xS NCs prepared using a 7:1 Cu:In ratio were in the monoclinic roxbyite crystal phase (Figure 3A, black curve). This roxbyite crystal structure persisted for NCs with In cation fraction up to 15%, prepared using a 2:1 Cu:In ratio. The diffraction peaks shifted to lower angles with increasing indium content, reflecting an increase in the lattice constant due to the larger radius of In compared to Cu. To the best of our knowledge, this is the first report of the CIS NCs with the roxbyite structure; prior reports demonstrated NCs with the chalcopyrite, zincblende and wurtzite crystal structures.30 For Cu:In precursor ratios of 1:1 or lower, the CIS NCs tended to adopt the hexagonal wurtzite structure (Figure 3B, PDF card No. 04-016-2016 for copper indium sulfide). Thus, in this synthesis, CIS NCs with low In content (cation fraction ≤15%) maintain the roxbyite copper sulfide phase, while higher In content promoted formation of the wurtzite CIS phase. Meanwhile, with increasing indium
Figure 3. (A) XRD patterns of Cu2-xS and CuxInyS NCs synthesized by Cu:In precursor ratios of 7:1 (black curve), 4:1 (red curve), 2:1 (green curve). (B) XRD patterns of CuxInyS NCs synthesized by Cu:In precursor ratios of 1:1 (green curve), 1:2 (orange curve), 1:3 (pink curve).
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content, from Cu:In 1:1 to Cu:In 1:4 precursor ratio, all of the wurtzite diffraction peaks became sharper, consistent with the overall increase in crystal size (Figure 5). The (1,0,0) and (1,1,0) peaks become significantly narrower than the other peaks for high In content, suggesting anisotropic growth of oriented disk- or plate-like structures with their short dimension parallel to the (0,0,2) planes. Prior reports have shown that crystal phase could be changed by
C
Figure 4. Optical properties of CIS NCs. (A) Dependence of LSPR absorbance peak wavelength on varying refractive index of solvent for CIS NCs synthesized using a 2:1 Cu:In ratio (In cation fraction of 15.5%). (B) Absorbance spectra of Cu2-xS NCs and alloy CIS NCs synthesized using Cu:In precursor ratios of 7:1(black curve), 4:1 (red curve), 2:1 (green curve), 1:1 (blue curve), 1:2 (orange curve) and 1:4 (pink curve). (C) Relationship between In content in alloy NCs or Cu:In precursor ratio with bandgaps of the CIS NCs.
appropriate choice of the capping agent, temperature and precursors.5, 28, 29 Here, we show that crystal phase and growth habit also depend critically on the In content in CIS NCs. To understand the effects of In content in CIS alloy NCs on their optical properties, we first investigated the LSPR absorbance. Figure 4A shows the red-shift of NIR absorbance with increasing refractive index of the solvent in which CIS NCs were dispersed. The peak red-shifts by nearly 120 nm from hexane to carbon disulfide. The CIS NCs with the lowest detectable level of In incorporation exhibited a NIR LSPR peak centered at 1355 nm (Figure 4B, red curve). With increasing In cation fractions of 15%, 31% and 47%, the absorbance red-shifted to 1419, 1456, and 1533 nm, respectively. This red-shift was accompanied by significant broadening of the LSPR peak and decrease in LSPR absorbance (Figure 4B). Negligible LSPR absorbance was observed in the CIS NCs prepared using a Cu:In precursor ratio of 1:4. The red-shift and eventual disappearance of the LSPR is attributed to a decrease in free carrier concentration with increased In content, implying a decrease in the concentration of cation vacancies with increasing In content. Changes in LSPR could, in general, also result from changes in size or aspect ratio. However, in the small size regime (
