Cu2–xS1–ySey Alloy Nanocrystals with Broadly Tunable Near

Oct 3, 2013 - Liu , X.; Wang , X. L.; Zhou , B.; Law , W. C.; Cartwright , A. N.; Swihart , M. T. Size-Controlled Synthesis of Cu2–xE (E = S, Se) Na...
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Cu2−xS1−ySey Alloy Nanocrystals with Broadly Tunable Near-Infrared Localized Surface Plasmon Resonance Xin Liu, Xianliang Wang, and Mark T. Swihart* Department of Chemical and Biological Engineering, The University at Buffalo (SUNY), Buffalo, New York 14260-4200, USA S Supporting Information *

ABSTRACT: We report facile methods for synthesizing monodisperse Cu2−xS1−ySey alloy nanocrystals (NCs) with tunable composition and size. The near-infrared (NIR) localized surface plasmon resonance (LSPR) in these selfdoped Cu2−xS1−ySey alloy NCs can be tuned over a broad range from 975 to 1650 nm. The LSPR shifted to longer wavelength with increasing sulfur content and with increased concentration of oleic acid used in the synthesis. This method provides new possibilities for broadly tuning LSPR wavelength by controlling the anion composition, cationic vacancy concentration, and surface ligands in heavily doped Cu2−xS1−ySey alloy NCs. This opens up access to LSPR absorbance across a broad portion of the near-IR using small colloidal quasi-isotropic NCs. KEYWORDS: localized surface plasmon resonance, doped semiconductor nanocrystals, colloidal synthesis, ternary semiconductor nanocrystals

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INTRODUCTION Copper-containing semiconductors such as copper−indium− gallium sulfide/selenide (CIGS),1−6 copper−zinc−tin sulfide/ selenide (CZTS),7−11 and their related alloys12 are of great interest for use in optoelectronic devices due to their tailorable band gap, earth abundance, and low toxicity. Recently, localized surface plasmon resonance (LSPR), which is generally associated with metal particles or nanostructures due to the presence of free electrons in these materials, has been observed in cation-deficient copper(I) chalcogenide semiconductor nanocrystals (NCs), including Cu2−xS,13−15 Cu2−xSe,15−17 and Cu2−xTe.18−20 LSPR arises from the presence of free charge carriers in NCs that undergo collective oscillations when excited with an electromagnetic field of appropriate frequency. In contrast to the situation in metal nanoparticles, the free charge carriers in doped semiconductor NCs must be provided by cationic vacancies or extrinsic dopants. These new plasmonic nanomaterials have potential for use in nanomaterial-based theranostics,21−24 plasmonic optoelectronic devices,25,26 sensors,27 and other emerging applications. A key aspect of the study and use of these plasmonic nanomaterials is controlling and tuning the NIR LSPR wavelength by changing the concentration of cationic vacancies in them. Dorfs et al.16 proposed a reversible, stepwise oxidation/reduction method to change the charge density in Cu2−xSe NC by using NH4Ce(NO3)6 and Cu(CH3CN)4][PF6] as oxidizing and reducing reagents, respectively. Kriegel et al.18 employed a copper-free reducing agent, diisobutylaluminium hydride (DIBAH), to reversibly tune the LSPR in copper chalcogenide NCs. They also found that Cu(II) atoms did not © XXXX American Chemical Society

leave copper chalcogenide NCs during the oxidation process. Instead, they showed that Cu(II) ions that left the crystal lattice formed CuO or remained bound to ligands on the particle surface. These stepwise oxidizing/reducing methods provide the possibility of tuning the LSPR in copper chalogenide NCs after synthesis. However, the post-treatment approach may not be compatible with some ultimate uses of the NCs, such as bioimaging and theranostic applications, for which introduction of the strong oxidizing and reducing agents could be problematic. Our recent work demonstrated a method for in situ tuning of the NIR LSPR in Cu2−xS and Cu2−xSe NCs by controlling the ligands attached to the particle surface during synthesis.15 This method provides access to a range of LSPR wavelengths without requiring postsynthesis treatment of the NCs. However, the LSPR absorbance peak was tunable by only about 110 nm in Cu2−xS and by about 270 nm in Cu2−xSe. Although a few previous reports of Cu2−xS1−ySey alloy NCs28−30 have been presented, no general method for broad tuning of the LSPR by varying both S:Se ratio and ligand composition is available. The uniformity of the size and morphology of colloidal NCs is very important because of its impact on sizedependent physical and chemical properties of NCs. Herein, we present facile reaction models to synthesize nearly monodisperse Cu2−xS1−ySey alloy NCs. On the basis of these methods, we can obtain Cu2−xS1−ySey alloy NCs with strong Received: August 23, 2013 Revised: October 1, 2013

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chloroform. Ethanol was added to the resulting dispersion, and Cu2−xS1−ySey NCs were collected again by centrifugation. The procedure was repeated twice to adequately remove unreacted precursor and ligands. Characterization. Transmission Electron Microscopy (TEM). The size and morphology of Cu2−xS1−ySey NCs were characterized using a JEOL JEM-2010 microscope at a working voltage of 200 kV. Powder X-ray Diffraction. The crystal phases in Cu2−xS1−ySey NCs were determined using powder XRD (Bruker Ultima IV with Cu Kα X-ray source). Samples were prepared by drop-casting highconcentration Cu2−xS1−ySey NC dispersions onto glass. UV−Vis−NIR Spectroscopy. Optical absorption of Cu2−xS1−ySey NC dispersions was measured using a Shimadzu 3600 UV−visible− NIR scanning spectrophotometer. Energy Dispersive X-ray Spectrometry. Compositional analysis of Cu2−xS1−ySey 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). Fourier Transform Infrared Spectroscopy (FTIR). FTIR characterization was carried out using a Bruker Vertex 70 FTIR spectrometer in attenuated total reflectance (ATR) mode. Samples were prepared by drop-casting the Cu2−xS1−ySey NCs on the ZnSe crystal substrate.

LSPR and can tune the LSPR absorbance peak over a broad spectral region from 975 to 1650 nm, by controlling both the ratio of Se/S and the surface ligands on the NCs.

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EXPERIMENTAL SECTION

Chemicals. Copper(I) chloride (CuCl), oleylamine (OAm), sulfur (S) powder, and selenium (Se) powder were purchased from Sigma Aldrich. Oleic acid (OA) was purchased from Fisher Scientific. All chemicals were used as received. Preparation of OAm-S, OAm-Se, and OA-Se Precursors. OAm-S, OA-S, OAm-Se, and OA-Se precursors were synthesized following the methods we presented previously.15 Typically, OAm-S precursor was prepared by heating 1 mmol of S powder in 10 mL of OAm to 110 °C under nitrogen protection. The solution was held at this temperature for 20−30 min and then cooled to room temperature for further use. OA-S precursor was prepared by degassing a mixture containing 1 mmol of S powder in 10 mL of OA at 110 °C, then heating this mixture to 150 °C and holding it at this temperature for 5−10 min. OAm-Se precursor was prepared by dissolving 1 mmol of Se powder in 10 mL of oleylamine at 310 °C under nitrogen protection. As the Se powder was gradually dissolved, the solution became dark brown, indicating formation of Se-OAm. OA-Se precursor was prepared by dissolving 1 mmol of Se powder in 10 mL of OA at 330 °C under nitrogen protection. The solution gradually turned yellow and transparent indicating formation of Se-OA. Upon cooling to room temperature, the Se-OA precursor became a gel. It was warmed prior to its use, as a liquid, in synthetic steps described below. Method 1: Synthesis of Cu2−xS1−ySey Alloy NCs Using OA-Se/ OAm-S Precursors. In a typical synthesis of Cu2−xS1−ySey NCs, 0.5 mmol of CuCl powder was mixed with 10 mL of OAm, and the solution was degassed at 110 °C for 30 min. Then the solution was heated to 225 °C, and it turned transparent and deep yellow, indicating formation of organo-copper precursor complexes. The Se/S precursor was prepared by simply mixing the OA-Se and OAm-S precursors at a desired molar ratio (4:1, 2:1, 1:1, 1:2, or 1:4). A total of 5 mL of the resulting Se/S precursor solution was injected into the copper precursor solution at 225 °C. The temperature dropped to 205 °C, and the solution was held at this temperature for 2.5 min. After 2.5 min, the heating mantle was removed to allow the temperature to drop to 50 °C. Method 2: Effect of OA Addition to the Copper Precursor on the Synthesis of Cu2−xS1−ySey Alloy NCs. In this method, some of the OAm in the copper precursor solution was replaced with OA, as follows. First, 0.5 mmol of CuCl powder was mixed with 6.7 mL of OA, and the solution was degassed at 110 °C for 30 min and heated to 210 °C. Then the temperature was decreased to 100 °C, and 3.3 mL of OAm was added into the solution. The solution was heated to 225 °C, and 5 mL of OA-Se/OAm-S precursor, at a desired Se:S molar ratio (1:4, 1:2, 1:1, 2:1, and 4:1) was injected into the solution at this temperature. The solution temperature dropped to 205 °C, and the solution was kept at this temperature for 2.5 min. After 2.5 min, the heating mantle was removed to allow the temperature to drop to 50 °C. Method 3: Synthesis of Cu2−xS1−ySey Alloy NCs Using OAmSe/OA-S Precursors. In this method, we used the combination of OAm-Se and OA-S as the precursor for synthesis of Cu2−xS1−ySey NCs. First, 0.5 mmol of CuCl powder was mixed with 10 mL of OAm, and the solution was degassed at 110 °C for 30 min. Then the solution was heated to 225 °C to form organo-copper precursor complexes. A total of 5 mL of OAm-Se/OA-S precursor with a desired Se:S molar ratio (1:1, 2:1, and 4:1) was injected into the solution at this temperature. The solution temperature dropped to 205 °C, and the solution was kept at this temperature for 2.5 min. After 2.5 min, the heating mantle was removed to allow the temperature to drop to 50 °C. Separation and Purification of Cu2−xS1−ySey Alloy NCs. Ethanol was added into the solution containing Cu2−xS1−ySey NCs, and the NCs were collected by centrifuging at 8000 rpm (about 8000 G) for 1 min. The collected Cu2−xS1−ySey NCs were redispersed in

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RESULT AND DISCUSSION Synthesis of Cu2−xS1−ySey Alloy NCs Using OA-Se/ OAm-S. Initially, we attempted to use oleylamine to prepare both organo-sulfur and organo-selenium precursors. However, when we used an OAm-S/OAm-Se mixture as the S/Se precursor, the resulting NCs had a broad size distribution (Supporting Information Figure S1) and showed signs of aggregation. This behavior was observed for a wide range of reaction conditions. The polydispersity observed in this case may be attributed to differences in reactivity of OAm-S and OAm-Se precursors that induce inhomogeneous nucleation and nanocrystal growth. The typical synthesis temperature for copper chalcogenides using OAm-Se (∼220 °C) is about 100 °C higher than that employed when using OAm-S. Thus, if OAm-S and OAm-Se are used together at the same temperature, their reactivities will not be balanced. Sequential, rather than simultaneous, reaction of OAm-S and OAm-Se can lead not only to a broad size distribution but also to nonuniform composition in the alloy NCs and even formation of separate binary NPs (Cu2−xS and Cu2−xSe) rather than the desired alloy particles. To obtain monodisperse and highquality alloy NCs, we adopted the combination of OA-Se/ OAm-S as the chalcogenide precursors. In a typical synthetic procedure (method 1), CuCl was dissolved in pure OAm followed by injecting mixed organo-Se/S precursor at 225 °C. After injection, the reaction temperature was immediately dropped down to 205 °C. The reaction was held at 205 °C for 2.5 min for particle growth. Cu2−xS1−ySey alloy NCs with quasi-spherical morphology and relatively uniform size distribution were synthesized using the combination of OA-Se and OAm-S precursors. TEM revealed the monodispersity of Cu2−xS1−ySey NCs synthesized using 1:2, 1:1, 2:1, and 4:1 molar ratios of OA-Se/OAm-S (Figure 1). Energy dispersive X-ray spectroscopy (EDS) was used to determine the final elemental compositions in Cu2−xS1−ySey NCs synthesized using different molar ratio of OA-Se/OAm-S precursors. We found a critical minimum concentration of OASe for incorporating Se into the alloy NCs. When the molar ratio of OA-Se to OAm-S was below 1:4, only trace amounts of Se could be detected in the NCs (Table 1). EDS analysis showed the presence of minimal Se in the NCs synthesized at an OA-Se/OAm-S molar ratio of 1:4. TEM imaging revealed a B

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Figure 2. EDS analysis of elemental composition in Cu2−xS1−ySey alloy NCs synthesized using method 1. (A) copper content (2 − x) and selenium content (y) in Cu2−xS1−ySey NCs. (B) Linear fit of Se content to OA-Se precursor.

Figure 1. TEM images of Cu2−xS1−ySey alloy NCs, produced by method 1, with sizes of (A) 6.3 nm ± 0.8 nm, (B) 8.8 nm ± 0.8 nm, (C) 4.7 nm ± 0.8 nm, and (D) 5.4 nm ± 0.6 nm synthesized using OA-Se/OAm-S molar ratios of 1:2, 1:1, 2:1, and 4:1, respectively. Size distributions obtained by counting 120 particles of each sample are shown in Supporting Information Figure S2.

Table 1. EDS analysis of Cu2−xS1−ySey alloy NCs produced by method 1

Figure 3. (A) XRD patterns of Cu2−xS1−ySey alloy NCs synthesized using OA-Se/OAm-S with a molar ratio (from bottom to top) of 1:2 (blue curve), 1:1 (green curve), 2:1 (purple curve), and 4:1 (brown curve), respectively. (B) Expanded view of the (1,1,0) and (1,0,3) peaks, showing their shift to lower diffraction angles (larger lattice constant) with increasing Se content.

EDS analysis (mean atom percent) OA-Se/ OAm-S

Cu

S

Se

Cu2−xS1−ySey alloy NC stoichiometry

1:4 1:2 1:1 2:1 4:1

63.7 64.2 64.1 61.4 59.1

36.5 31.3 25.2 24.9 18.6

0.2 4.4 10.7 13.7 21.7

Cu1.73S0.995Se0.005 Cu1.8S0.88Se0.12 Cu1.79S0.7Se0.3 Cu1.59S0.64Se0.36 Cu1.48S0.46Se0.54

The diffraction peaks shifted to lower angles with increasing Se content, reflecting an increase in the lattice constant. The shift of diffraction peaks to lower diffraction angles can be more clearly seen in the expanded portion of the XRD pattern shown in Figure 3B. The increase in lattice parameter is expected, due to the larger size of the Se ion compared to S. Tunable LSPR in Cu2−xS1−ySey Alloy NCs. To study the LSPR in self-doped Cu2−xS 1−y Se y alloy NCs, we first investigated the NIR optical absorbance in solvents with varying refractive index. Figure 4A shows that the NIR absorbance continuously red-shifted with increasing refractive index of the solvent in which the Cu2−xS1−ySey NCs were dispersed, as expected for LSPR absorbance. Moreover, the LSPR absorbance was blue-shifted from 1260 nm to 1135 nm

bimodal size distribution of NCs synthesized at an OA-Se/ OAm-S molar ratio of 1:4 (Supporting Information Figure S6). The large NCs with hexagonal morphology result from the relatively high concentration of OAm-S inducing rapid nucleation and growth of large Cu2−xS NCs at the synthesis temperature above 200 °C, which is much higher than the temperature used to synthesize monodisperse pure Cu2−xS NCs from these precursors (140 °C or lower). The reaction temperature above 200 °C is required to activate the less reactive OA-Se precursor. Thus, when the OA-Se precursor concentration was low, Cu2−xS NCs nucleated and grew rapidly, without incorporating Se. Ostwald ripening was initiated due to the rapid depletion of monomers, leading to polydispersity of the resulting NCs. The Se content in the Cu2−xS1−ySey alloy NCs became significant when the OA-Se/ OAm-S ratio was increased to 1:2 and continued to increase almost linearly with increasing OA-Se precursor concentration (Figure 2B). Table 1 summarizes the mean elemental compositions in Cu2−xS1−ySey alloy NCs synthesized using OA-Se/OAm-S at different molar ratios (method 1). The values of x and y were calculated from EDS analysis. Note that the copper deficiency increases (x decreases) with increasing Se incorporation. Powder X-ray diffraction (XRD) showed that all of these Cu2−xS1−ySey NCs had a hexagonal crystal structure (Figure 3).

Figure 4. LSPR in Cu2−xS1−ySey NCs. (A) Dependence of LSPR absorbance peak wavelength on solvent refractive index and (B) absorbance spectra of NCs synthesized using different OA-Se to OAmS ratios. C

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with increasing Se content in the NCs (Figure 4B). This phenomenon should not be solely attributed to the change of copper vacancy concentration in Cu2−xS1−ySey NCs. When the molar ratio of OA-Se/OAm-S increased from 1:1 to 4:1, the cationic vacancy concentration (x) in Cu2−xS1−ySey alloy NCs did increase (Table 1) and the resulting increase in free carrier concentration would lead to a blue shift of the LSPR absorbance. However, the cationic vacancy concentration decreased very slightly when the OA-Se/OAm-S ratio was changed from 1:1 to 1:2. Moreover, the copper deficiency actually increased slightly again when the OA-Se/OAm-S ratio was changed from 1:2 to 1:4, which also led to little incorporation of Se in Cu2−xS1−ySey alloy NCs. Thus, the shift in the LSPR peak position with Se content cannot be attributed to the change of cationic vacancy concentration. For these concentrations a red-shift of plasmonic absorbance was observed even though the cationic vacancy concentration increased. Thus, we conclude that the Se content plays an important role in determining the LSPR peak position in Cu2−xS1−ySey alloy NCs. Actually, the plasmonic absorbance continuously shifted to shorter wavelength with increased Se content regardless of the change of the copper deficiency. This results from fundamental differences in material properties between Cu2−xS and Cu2−xSe. Some previous studies showed that the LSPR absorbance peak in Cu2−xSe NCs is at the same or higher energy than in Cu2−xS NCs even though the x value of Cu2−xSe NCs in those cases was much smaller than the value in Cu2−xS NCs.13,16,21 These studies imply that LSPR in Cu2−xSe NCs is generally stronger and at higher energy than in Cu2−xS NCs, for the same cationic vacancy concentration. In addition, in this synthesis, increasing the Se content in the precursors also increases the ratio of OA to OAm. However, for the pure Cu2−xS and Cu2−xSe NCs, we found that increasing the OA to OAm ratio actually results in a red-shift of the LSPR absorbance. Thus, we do not believe this change is attributable to the change in ligand mixture as the Se/S ratio is changed. Our previous report showed that the LSPR in pure Cu2−xS NCs synthesized using OA-S precursor could be tuned to wavelengths as low as 1050 nm, when the synthesis temperature was decreased to 140 °C. Recently, we modified the synthesis to adjust the LSPR to 975 nm by reducing the activation temperature (Supporting Information Part III). This further broadens the range over which the LSPR peak can be tuned using this general reaction model for synthesis of Cu2−xS1−ySey (0 ≤ y < 0.6) alloy NCs. Effect of OA on the LSPR in Cu2−xS1−ySey Alloy NCs. The LSPR in Cu2−xS and Cu2−xSe NCs can be tuned by addition of OA during synthesis, as shown in our recent work.15 This effect may be attributed to the deprotonated carboxylate (OA) trapping surface holes, thereby reducing effective carrier concentration. Here, to investigate the effect of OA on the LSPR in Cu2−xS1−ySey NCs, we used a mixture of OAm and OA to dissolve CuCl while keeping other reaction conditions as described above. For simplicity, we refer to this approach using OA in the copper precursor as method 2. TEM images (Figure 5) showed that this method produced Cu2−xS1−ySe NCs with mean sizes varying from 4.7 to 10.6 nm as the OA-Se/OAm-S ratio was varied from 1:2 to 4:1. EDS analysis was used to determine the elemental ratios among Cu, S, and Se in Cu2−xS1−ySe NCs (Table 2). Cu2−xS NCs were almost the only product when the OA-Se/OAm-S ratio was 1:4 or lower. This was consistent with our observations for Cu2−xS1−ySey NCs synthesized by method 1. The Se content in Cu2−xS1−ySe NCs

Figure 5. TEM images of the Cu2−xS1−ySe alloy NCs, produced by method 2, with sizes of (A) 4.7 nm ± 0.8 nm, (B) 6.3 nm ± 1.0 nm, (C) 7.6 nm ± 1.1 nm, and (D) 10.6 ± 1.0 nm synthesized using OASe/OAm-S molar ratios of 1:2, 1:1, 2:1, and 4:1. Size distributions obtained by counting 120 particles of each sample are shown in Supporting Information Figure S3.

Table 2. EDS Analysis of Cu2−xS1−ySey Alloy NCs Produced by Method 2 EDS analysis (mean atom percent) OA-Se/ OAm-S

Cu

S

Se

Cu2−xS1−ySey alloy NC stoichiometry

1:4 1:2 1:1 2:1 4:1

63.6 64.6 64.3 64.4 65.8

35.5 31.1 24.8 22.3 14.7

0.9 4.4 10.9 13.3 19.5

Cu1.75S0.98Se0.02 Cu1.81S0.88Se0.12 Cu1.8S0.7Se0.3 Cu1.81S0.63Se0.37 Cu1.92S0.43Se0.57

increased approximately linearly with increased Se precursor fraction (Figure 6B), with approximately the same slope as in method 1. EDS analysis showed little change in the copper vacancy concentration for precursor ratios of 1:2 ≤ OA-Se/ OAm-S ≤ 2:1 but decreased vacancy concentration for an OASe/OAm-S ratio of 4:1 (Figure 6A). This was opposite the trend observed for method 1, for which the vacancy concentration increased with increasing Se content. Powder XRD showed that the Cu2−xS1−ySey alloy NCs synthesized by method 2 had hexagonal crystal structure (Figure 7). The crystal structure is similar to the Cu2−xS1−ySey alloy NCs

Figure 6. EDS analysis of elemental composition in Cu2−xS1−ySey alloy NCs synthesized using method 2. (A) copper content (2 − x) and selenium content (y) in Cu2−xS1−ySey NCs. (B) Linear fit of Se content to OA-Se precursor. D

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Figure 7. XRD patterns of Cu2−xS1−ySey alloy NCs synthesized using method 2 with OA-Se/OAm-S molar ratios of 1:2 (blue curve), 1:1 (orange curve), 2:1 (green curve), and 4:1 (red curve), respectively.

synthesized by method 1. Peaks shifted to lower diffraction angle with increasing the Se content in the final Cu2−xS1−ySey alloy NCs. In our previous study, the cubic berzelianite crystal phase was obtained in the homogeneous Cu2−xSe NCs synthesized using OA-Se as the precursor. However, here, the hexagonal crystal structure was maintained, even when the fraction of OA-Se in the initial chalcogenide precursors was increased to 80%. EDS analysis (Table 2) showed that appreciable S was present in the final Cu2−xS1−ySey alloy NCs even when the 1:4 molar ratio of OAm-S and OA-Se was used. We attribute this to the high reactivity of OAm-S and the lower nucleation temperature for formation of Cu2−xS NCs compared to Cu2−xSe NCs. The crystal phase may be determined by the Cu2−xS nucleation even when the final NCs contain more Se than S. UV−vis−NIR spectrometry (Figure 8A) showed that the NIR LSPR in NCs synthesized by method 2 blue-shifted with increasing Se content, as observed for Cu2−xS1−ySey NCs synthesized by method 1. However, the LSPR in Cu2−xS1−ySey NCs synthesized by method 2 was significantly red-shifted relative to the LSPR of NCs prepared by method 1. The redshifts were 390, 320, 255, and 250 nm for OA-Se/OAm-S ratios of 1:2, 1:1, 2:1, and 4:1, respectively (Figure 8B−E). The LSPR wavelength depends on the concentration of cation vacancies and on the ratio of S:Se in the particles. However, as shown in Tables 1 and 2, the copper vacancy and Se:S ratio in Cu2−xS1−ySey NCs synthesized using 2:1 and 1:1 molar ratios of S/Se precursors by methods 1 and 2 are nearly identical. However, the LSPR of particles prepared by method 2 was redshifted by more than 300 nm relative to those prepared by method 1. The essential difference between these methods is the amount of oleic acid present during synthesis. NCs synthesized by method 2, in the presence of a higher concentration of oleic acid, may have more oleic acid ligands on their surface. The deprotonated carboxyl group on the NCs surface can potentially trap free holes, reducing the effective carrier concentration in the NCs. This is consistent with the behavior of Cu2−xS and Cu2−xSe NCs reported in our recent work.15 Fourier transform infrared spectroscopy (FTIR) revealed the ligands binding to the surface of Cu2−xS1−ySey NCs synthesized by methods 1 and 2 (Figure 9). The IR absorbance band at 1646 cm−1 corresponds to −NH 2 functional groups, indicating OAm binding to the surface of NCs.31 Two peaks at 1558 cm−1 and 1448 cm−1 are attributed to symmetric and asymmetric stretching modes of deprotonated carboxylate groups (−COO−) from OA.32,33 Absorbance

Figure 8. (A) Absorbance spectra showing that the LSPR in Cu2−xS1−ySey NCs synthesized by method 2 was red-shifted with decreasing OA-Se/OAm-S precursor ratio. (B−E) absorbance spectra of NCs synthesized by methods 1 and 2 at different precursor molar ratios of OA-Se/OAm-S.

Figure 9. FTIR of Cu2−xS1−ySey NCs synthesized by method 1 (blue curve) and method 2 (red curve), respectively.

at 1715−1730 cm−1 was not observed, indicating the absence of free OA.32 To compare the degree of OA binding to surface of Cu2−xS1−ySey NCs, we normalized the spectra to the IR absorbance intensity of −NH2 (1646 cm−1). Figure 9 clearly shows that the Cu2−xS1−ySey NCs synthesized by method 2, which used 6.7 mL more OA than method 1, had higher absorbance intensity of deprotonated OA indicating more OA ligands binding to the NP surface. We also found that copper deficiency in Cu2−xS1−ySey NCs synthesized by method 2 was increased at 2:1 and 4:1 molar ratio of OA-Se/OAm-S compared to the ones synthesized by method 1 (Tables 1 and 2). Moreover, a slight compositional increase of Se in Cu2−xS1−ySey NCs was observed at these two molar ratios. Thus, the red-shift of LSPR in these two cases was more complicated because all three factors impacting the LSPR, E

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including copper vacancy concentration, content of Se, and surface ligands were all changed. Synthesis of Cu2−xS1−ySey Alloy NCs Using OAm-Se/ OA-S. We also studied using the combination of OAm-Se and OA-S precursors for synthesizing Cu2−xS1−ySey NCs. The synthesis method (method 3) was the same as method 1 except that OAm-Se/OA-S was used instead of OA-Se/OAm-S (Experimental Section method 3). TEM revealed the size of Cu2−xS1−ySey NCs synthesized using 1:1, 2:1, and 4:1 molar ratios of OAm-Se and OA-S, respectively (Figure 10). It Figure 11. Plasmonic absorption of Cu2−xS1−ySey alloy NCs synthesized by method 3 with different OAm-Se/OA-S molar ratios of 1:1 (purple curve), 2:1 (orange curve) and 4:1(pink curve).

changing the OAm-Se/OA-S molar ratio from 2:1 to 4:1 should be attributed to the increased Se content in Cu2−xS1−ySey NCs, since the copper deficiency decreased in this case. As for the Cu2−xS1−ySey NCs synthesized by methods 1 and 2, the effect of increased Se content was more important than the change in cation deficiency.

Figure 10. TEM images of the Cu2−xS1−ySe alloy NCs, synthesized by method 3, with sizes of (A) 8.3 nm ± 1.0 nm, (B) 4.2 nm ± 0.8 nm, and (C) 4.5 nm ± 1.2 nm using OAm-Se/OA-S with the molar ratios of 1:1, 2:1, and 4:1. Size distributions obtained by counting 120 particles of each sample are shown in Supporting Information Figure S4.

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CONCLUSION In summary, we have reported facile methods for synthesis of nearly monodisperse Cu2−xS1−ySey NCs with tunable compositions and sizes. The Cu2−xS1−ySey NCs synthesized using these methods exhibit strong NIR LSPR due to the presence of cationic (copper) vacancies, which create free holes. We found that using the combination of OA-Se/OAm-S as the chalcogenide precursor resulted in narrow size distribution of the Cu2−xS1−ySey alloy NCs. The elemental composition of the Cu2−xS1−ySey NCs could be tuned over a relatively large range (0.5< y < 1). The LSPR was also tunable over a spectral range from 1135 to 1650 nm by controlling the molar ratios of OA-Se and OAm-S precursors as well as adjusting the molar fraction of OA in the reaction system. Both method 1 and method 2 rely on using a combination of OA-Se/OAm-S as the chalcogenide precursor. The only difference is that more OA was used in method 2, which allowed tuning the LSPR to longer wavlengths, from 1385 nm to 1650 nm. The combination of OAm-Se/OA-S was also investigated (method 3). This provided less flexibility in tuning elemental composition and LSPR than method 1 and method 2, indicating that the OA-Se/ OAm-S combination is more favorable for producing Cu2−xS1−ySey alloy NCs with broadly tunable LSPR. However, we need not be restricted to that combination in all cases, and we can select the appropriate synthesis method or materials (either binary Cu2−xS and Cu2−xSe or Cu2−xS1−ySey alloy NCs) for the specific applications. For example, if we need materials with relatively high plasmon resonance energy (wavelength < 1000 nm), binary Cu2−xS NCs are a good choice instead of Cu2−xS1−ySey alloy NCs. Complementary approaches to producing the binary and ternary NCs facilitate manipulating the LSPR in in situ-synthesized self-doped copper chalcogenide NCs over a broad spectrum from 975 to 1650 nm. This provides flexibility for utilizing plasmonic copper chalcogenide NCs in biological and optoeletronic applications, such as photothermal therapy, photoacoustic imaging, and plasmonic photodetectors.

showed that the Cu2−xS1−ySey NC size dramatically decreased from 8.3 to 4.2 nm when the molar ratio of OAm-Se/OA-S was increased from 1:1 to 2:1. The size of NCs changed little when the OAm-Se/OA-S molar ratio was further increased from 2:1 to 4:1. EDS analysis showed that relatively little Se was incorporated into the NCs even though the molar ratio between OAm-Se/OA-S reached 2:1 (Table 3). With the Table 3. EDS Analysis of Cu2−xS1−ySey Alloy NCs Produced by Method 3 EDS analysis (mean atom percent) OAm-Se/ OA-S

Cu

S

Se

Cu2−xS1−ySey alloy NC stoichiometry

1:1 2:1 4:1

65.3 63.1 63.4

33.0 33.3 26.3

1.7 3.5 10.2

Cu1.88S0.95Se0.05 Cu1.72S0.91Se0.09 Cu1.74S0.72Se0.28

increase of OAm-Se/OA-S molar ratio to 4:1, the elemental composition of Cu2−xS1−ySey NCs reached 28% Se, which is still low considering the initial high mole fraction of OAm-Se precursor. Overall, the Se content in Cu2−xS1−ySey NCs synthesized by method 3 was roughly half of the Se content in the Cu2−xS1−ySey NCs synthesized by method 1. Overall, the OAm-Se/OA-S combination was less effective for incorporating Se into the Cu2−xS1−ySey NCs compared to the OA-Se/OAm-S combination. This can be attributed to the differences in reactivity between the OAm-based precursors and the OAbased precursors. Nonetheless, this combination was still more effective than using OAm-based precursors for both S and Se. Absorbance spectra showing the LSPR in Cu2−xS1−ySey NCs synthesized by method 3 with different OAm-Se/OA-S molar ratios are shown in Figure 11. The LSPR peak again blueshifted with increasing Se content. The cationic vacancy concentration increased when the OAm-Se/OA-S molar ratio changed from 1:1 to 2:1. This enhances the blue shift of LSPR in Cu2−xS1−ySey NCs. The further blue shift of LSPR upon F

dx.doi.org/10.1021/cm402848k | Chem. Mater. XXXX, XXX, XXX−XXX

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ASSOCIATED CONTENT

S Supporting Information *

Additional TEM images, size distribution, and synthesis of Cu2−xS NCs with LSPR peak at 975 nm and a graphical summary of the LSPR frequency of Cu2−xS1−ySey alloy NCs corresponding to the different reaction methods and reaction parameters. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: swihart@buffalo.edu. Tel.: +1-716-645-1181. Notes

The authors declare no competing financial interest.

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REFERENCES

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