Effects of Carrier Density and Shape on the Localized Surface

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Effects of Carrier Density and Shape on the Localized Surface Plasmon Resonances of Cu2−xS Nanodisks Su-Wen Hsu, Whitney Bryks, and Andrea R. Tao* Department of NanoEngineering, University of California, San Diego, 9500 Gilman Dr. MC 0448, La Jolla, California 92039-0448, United States S Supporting Information *

ABSTRACT: A major challenge in the synthesis of plasmonic semiconductor nanocrystals is the ability to control localized surface plasmon resonance (LSPR) properties by varying the size, shape, and carrier density of the nanocrystal. For example, copper sulfide (Cu2−xS) nanodisks possess two distinct LSPR modes that occur in the infrared range. Here, we demonstrate that the wavelengths of these LSPR modes can be modulated by independently varying the aspect ratio of the disk and the overall carrier density of Cu2−xS. These variables can be controlled during nanocrystal growth by carrying out thermolysis of a copper-thiolate precursor under a specific gas environment. Our results show that during thermolysis, the presence of oxygen enhances the growth rate of crystalline Cu2−xS nanodisks and the formation of Cu vacancies that contribute to free carrier concentration. By carrying out thermolysis under a nitrogen environment, we are able to tune the aspect ratio of nanodisks independent of Cu vacancy formation. Using these methods to carefully control nanodisk size and carrier density, we demonstrate that nanodisks achieve a critical carrier density beyond which the nanocrystals undergo an irreversible phase change, placing a limit on LSPR wavelength tuning in these doped semiconductor nanocrystals. KEYWORDS: surface plasmon, nanodisk, nanocrystal, semiconductor, copper sulfide

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INTRODUCTION Copper sulfide (Cu2−xS) is a self-doped p-type semiconductor that is composed of highly earth-abundant elements and has attracted significant interest for application in photovoltaics, photocatalysis, and chemical sensing.1−6 More recently, Cu2S nanoparticlesalong with nanoparticles composed of other doped semiconductors such as Cu2−xSe, Cu2−xTe and Sb-doped SnO2have attracted interest as plasmonic materials that support localized surface plasmon resonances (LSPRs).7−10 Unlike metal nanoparticles, nanoparticles composed of doped semiconductors possess LSPRs whose wavelengths can be dynamically tuned by controlling free carrier density.8,10−12 For example, Cu2−xS nanoparticles possess an LSPR mode in the near-infrared (NIR) range.7,10,11 Introducing Cu vacancies into the Cu2−xS lattice generates hole carriers which cause the nanoparticle LSPR band to blue-shift as dopant concentration is increased. These self-doped semiconductor nanoparticles have the potential as actively tunable plasmonic materials capable of response to chemical or electrical stimuli. Further, these semiconductor materials have the potential to exhibit new optical or electronic properties that stem from coupled plasmonic and excitonic bands. The effect of shape on the plasmonic properties of these doped semiconductor nanocrystals is largely unexplored. The synthesis of high aspect ratio nanostructures composed of doped semiconductors that support LSPRs is of particular interest. In metal nanoparticles, anisotropic nanoparticle shape can result in large electromagnetic field enhancements localized © XXXX American Chemical Society

near the corners and edges of the nanoparticles and provide a convenient strategy for tuning the LSPR frequency and amplitude.13,14 Recently, we reported the observation of shape-dependent LSPRs for Cu2−xS nanodisks that possess in-plane and out-of-plane dipoles associated with the nanoscale disk geometry.11 We observed that nanodisks possess two distinct LSPR modes and demonstrated that these LSPR modes can be modulated by varying both the aspect ratio of the disk and the overall dopant concentration of the Cu2−xS. The fabrication of an active or tailored plasmonic material could capitalize on the ability to tune both of these parameters independently. A major difficulty in the synthesis of these doped semiconductor nanoparticles (Cu2−xS and Cu2−xSe in particular) is the ability to predictably change nanoparticle size or shape without modulating dopant concentration. These nanoparticles are typically fabricated using colloidal synthesis, where nanoparticle size or aspect ratio (in the case of nanodisks) is readily increased by prolonging the reaction time.15,16 For example, the Korgel group previously synthesized Cu2−xS nanodisks with high aspect ratios by controlling the synthetic temperature and duration, whereby longer reactions at higher temperature tend to produce higher aspect ratios.10 Other examples of anisotropic Cu2S nanostructures with large Received: July 26, 2012 Revised: September 5, 2012

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aspect ratios include bicones17 and nanorods.18 However, in each of these earlier studies, the observed absorption peaks in the infrared were attributed to excitonic absorption instead of LSPR excitation. Thus, the relationships between nanostructure aspect ratio, LSPR wavelength, and carrier density have remained largely unresolved. Previously, we synthesized Cu2−xS nanodisks using a solventless thermolysis method adapted from the Korgel group.15 We observed that characteristic LSPR wavelengths of the nanodisks vary with aspect ratio, and we were able to apply scattering theory and the Drude approximation to calculate the average carrier densities of the nanodisks. We found that aspect ratio and carrier density are positively correlated, which we attribute to a surface oxidation mechanism: a longer reaction time leads to larger nanodisks with greater aspect ratios, but the increase in nanodisk surface area promotes the formation of copper vacancies at the Cu2−xS surface. Thus, longer reaction times yielding high aspect ratio nanodisks inevitably lead to higher charge carrier densities. This correlates with the observed LSPR properties of spherical semiconductor nanoparticles,7,15,19 where the carrier density of Cu2−xS quantum dots was found to correlate with quantum dot radius and quantum dot size was increased by prolonging the synthetic reaction time. A major difficulty encountered in our previous work was the ability to independently address carrier density and nanodisk aspect ratio. Figure 1 depicts the effects of each property on the in-plane and out-of-plane LSPR wavelengths for Cu2−xS nanodisks. On the basis of Mie scattering theory, λout is expected to blue-shift and λin is expected to red-shift when only disk diameter is increased.13 When only hole carrier density (Nh) is increased, Drude theory can be used to predict that both λout and λin are expected to blue-shift.10 However, the effects of carrier density were difficult to characterize due to the variability of the chemical synthesis, which was performed under a redox atmosphere. Controlled oxidation of the Cu2−xS nanodisks is further complicated by the polymorphic nature of copper sulfide compounds, whose crystallographic structures are typically determined by compound stoichiometry. The ability to independently modulate the oxidation of Cu2−xS would further the plasmonic characterization of these selfdoped semiconductor nanocrystals and would also provide new strategies for tuning LSPR wavelengths of high aspect ratio structures. In this study, we demonstrate that the aspect ratio and carrier density of Cu2−xS nanodisks can be independently controlled by carrying out the thermolysis reaction in a closed vessel with a controlled gas environment. Our results show that the kinetics of crystalline Cu2−xS nanodisk growth are highly dependent on the presence of oxygen. We also demonstrate that independent of changes in nanocrystal size or shape, a maximum carrier density can be accommodated by the Cu2−xS nanodisks before the nanocrystals undergo a catastrophic phase change that promotes particle agglomeration and fusion.

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Figure 1. Schematic of how LSPRs are expected to change with respect to changes in (a) aspect ratio and (b) free carrier density. precipitate formed. The solid precipitate was collected by centrifugation, washed two times with deionized water and ethanol (volume ratio of 1:2) to remove the excess reagents, and then dried in a vacuum desiccator overnight to remove residual solvent and to obtain a yellow, waxy powder. Synthesis of Cu2−xS Nanodisks. To synthesize nanodisks under air, the powder Cu-alkanthiolate precursor was placed in a glass vial. To synthesize nanodisks under a N2 environment, the powder precursor was placed in a round-bottom flask capped with a rubber stopper. The flask volume was purged with N2 a total of four times before carrying out thermolysis. For thermolysis, the glass vial or flask containing the powder precursor was placed into an oil bath with stirring at a temperature between 190 and 200 °C. The powder transformed into a dark-brown liquid which was then cooled to room temperature, dispersed in chloroform, and centrifuged at 7500 rpm for 7.5 min to remove any unwanted byproduct. This purification process was repeated three times. The purified product was dried under vacuum to obtain a brown powder for characterization. To carry out thermal oxidation of the nanodisks, the purified and dried Cu2−xS powder was placed into an uncovered glass vial and heated in an oven at a temperature of 80−120 °C. Sample Characterization. To prepare samples for transmission electron microscopy (TEM, FEI Tecnai Sphera), the nanodisks undergo solvent transfer followed by sample deposition. Initially, the

EXPERIMENTAL SECTION

Synthesis of Cu−Alkanethiolate Precursor. A 2.0 M solution of 1-dodecanethiol in ethanol was prepared by mixing 4.83 mL of 1dodecanethiol (20 mmol) with 10 mL of ethanol. An aqueous 1.0 M copper nitrate (Cu(NO3)2) solution was prepared by dissolving 1.16 g of copper nitrate (5 mmol) in 5 mL of deionized water. The dodecanethiol solution was added to the copper nitrate solution with vigorous stirring until the solution became colorless and a yellow-white B

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Figure 2. XRD spectra and TEM images of nanodisks synthesized under air and N2. (a) XRD spectra show that both nanocrystal powders possess a crystal structure closely resembling the hexagonal lattice of stoichiometric chalcocite (Cu2S). (b) The morphology obtained for both samples are disk-shaped nanocrystals, seen here in stacked pancake-like structures that occur during evaporative assembly at an air−water interface. nanodisks are dispersed in chloroform, which is known to dissolve the carbon support film on commercial TEM grids. To avoid this, we dropcast the colloidal solution onto an air−water interface and allow the chloroform solvent to evaporate prior to nanodisk transfer to TEM grid. Because the as-made nanodisks are highly hydrophobic, ordered stacks of nanodisks are generated by this dispersion at the air−water interface. UV−vis−NIR spectroscopy (Shimadzu UV-3600) was used to characterize the surface plasmon resonance peaks of the Cu2−xS products. For LSPR measurements, the powder product was dispersed in carbon tetrachloride and NIR transmission was obtained in the range of 800 to 3200 nm.

Figure 3a shows a graph of nanodisk aspect ratio as a function of thermolysis time for two different temperatures (190 and 200 °C) and for air and N2 gas environments. At lower temperatures, we observe that thermal decomposition of the Cu−alkanethiolate precursor does not occur. This is consistent with thermogravimetric analysis (TGA) measurements indicating that decomposition of the Cu-dodecanethiol complex occurs at T = 178 °C (Figure 3b), which we attribute to cleavage of the C−S bond. Higher reaction temperatures (>210 °C) proceed quickly under air and make it difficult to observe the time-dependent nanodisk growth by removing aliquots for TEM imaging. Both air and N2 reactions are sensitive to the reaction temperature, and carrying out the thermolysis reaction at a temperature of 200 °C results in a significantly faster growth rate than a slightly lower reaction temperature of 190 °C. We observe that the nucleation and growth of Cu2−xS nanodisks under air occurs much faster than when the reaction is carried out under N2. Under air, high aspect ratio (∼7) nanodisks can be attained within one hour of the thermolysis reaction. In contrast, carrying out the thermolysis reaction under N2 requires upward of 6 h to achieve the same aspect ratios. Figure 3c shows a TEM image of the nanodisks synthesized under N2, which possess an aspect ratio of 4.65 ± 0.29 (D = 18.9 ± 3.6 nm, T = 3.94 ± 1.01 nm) after 4 h of thermolysis. The aspect ratio can be increased to 6.60 ± 0.35 (D = 36.5 ± 6.4 nm, T = 5.53 ± 1.30 nm) by extending the thermolysis reaction time to 6 h, as shown in the TEM image in Figure 3d. The different growth rate of nanodisks synthesized under N2 versus air is due to oxygen, which plays an important role in nanodisk nucleation and growth. Oxygen is known to promote the formation of copper vacancies in Cu2S, and others have hypothesized that this is largely a surface effect where small copper oxide particles are formed at the particle−liquid interface.8 On the basis of our data, we propose that copper vacancies formed near the nanodisk surface encourage precipitation of the copper alkanethiolate precursor onto the growing nucleation site. The slower growth rate of nanodisks observed under N2 can thus be attributed to a lower occurrence of these nucleation sites that facilitate Cu2S precipitation on the nanodisk edges.

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RESULTS AND DISCUSSION In order to tune nanodisk aspect ratio (which we define here as the diameter, D, divided by the nanodisk thickness, T) without simultaneously increasing carrier density, we synthesized Cu2−xS nanocrystals by carrying out the thermolysis of Cu− alkanethiolate under an N2 purged environment. Figure 2 compares the XRD spectra and representative transmission electron microscope (TEM) images obtained for nanodisks synthesized under N2 and air. Both methods produce colloidal nanodisk dispersions composed of a solid-state Cu2−xS phase that possesses a crystal structure closely related to the hexagonal lattice of chalcocite (Cu2S). The nanodisks are oriented with a (002) basal plane and undergo radial growth in the direction parallel to this plane. The TEM images correspond to nanodisks with aspect ratios of 3.87 ± 0.25 (D = 14.7 ± 4.3 nm, T = 3.80 ± 1.31 nm) and 4.65 ± 0.29 (D = 18.9 ± 3.6 nm, T = 3.94 ± 1.01 nm), synthesized under air and N2 environments, respectively. While the two different gas environments do not prohibit nanodisk growth and can both form nanodisks with varying aspect ratios, comparison of the relative rates of nanodisk growth shows a significant difference and indicates two different modes of nanodisk growth. The thermolysis reaction that leads to the growth of anisotropic Cu2−xS nanodisks is characterized by two steps: (i) decomposition of the Cu-alkanethiolate precursor and (ii) solution-phase precipitation of Cu2S. These two steps favor a growth rate faster along the ⟨100⟩ direction of the hexagonal disk than along the ⟨001⟩ direction, resulting in the formation of the anisotropic disk structure.20 C

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To measure the effect of nanodisk shape (independent of carrier density) on LSPR wavelength, we synthesized Cu2−xS nanodisks under N2 to obtain dispersions composed of nanodisks with varying aspect ratios, ranging from 4.65 ± 0.29 to 7.10 ± 0.53 (for exact dimensions, see Table S3 in the Supporting Information). Figure 4a shows the extinction of these nanodisks in the NIR range as measured by UV−vis− NIR transmission spectroscopy. The spectra for the colloidal dispersions (in CCl4) show a peak wavelength near 1800 nm corresponding to the out-of-plane LSPR mode, as previously assigned by Mie scattering calculations.11 For a nanodisk aspect ratio of 4.65 ± 0.29, λout = 1860 nm, which undergoes a blue shift as the nanodisks grow larger and the overall aspect ratio is increased. This result is consistent with Mie scattering theory and with previously observed shifts in LSPR wavelengths for metal nanodisks.13 The in-plane LSPR mode (which appears at λin = 3100 nm for nanodisks synthesized under air) of the N2synthesized nanodisks undergoes a red-shift beyond the measurable wavelength range of the spectrometer due to the low carrier densities of these samples. Carrier density can be calculated by using the Drude relationship between the bulk plasmon frequency (ωp) and hole carrier density (Nh). This relationship can be expressed as ωp =

Nhe 2 ε0mh

where mh is the effective hole mass (approximated as 0.8mo where mo is electron mass) and e is electron charge.10 A more detailed explanation of these calculations can be found in S4 in the Supporting Information. Figure 4b shows the calculated carrier densities for nanodisks synthesized under air and N2. On the basis of our calculations, the carrier density of airsynthesized nanodisks increased from Nh = 8.9 × 1020 cm−3 to Nh = 12.3 × 1020 cm−3 as the nanodisks grew in aspect ratio from 3.87 ± 0.25 to 6.78 ± 0.52, respectively. (See Table S3 in the Supporting Information for exact nanodisk dimensions.) In comparison, we calculate that the carrier density of the N2synthesized nanodisks is approximately Nh = 7.7 × 1020 cm−3 for all of the observed aspect ratios, remaining nearly constant as nanodisks grow in size. This indicates that carrying out thermolysis of the Cu−alkanethiolate under N2 can suppress Cu oxidation during nanodisk formation, enabling nanodisk growth without increasing the carrier density during an extended reaction time. The lower carrier density for the N2synthesized nanodisks also explains the absence of the

Figure 3. Nanocrystal growth rates obtained air and N2. (a) A comparison of nanocrystal aspect ratio (measured by TEM image analysis) and thermolysis time shows that the kinetics of nanocrystal growth are drastically different when thermolysis is carried out under different gas environments. (b) TGA of the copper−alkanethiol precursor indicates that decomposition of the precursor begins to occur at 178 °C (c,d) Under N2, aspect ratio nanodisks can be controlled in the range of 3.0−7.0 by tuning thermolysis time.

Figure 4. LSPR spectra and carrier density with varying nanodisk aspect ratios. (a) IR extinction spectra for colloidal Cu2−xS nanodisks synthesized under N2 with different aspect ratios. The out-of-plane LSPR mode is observed to blue-shift while the in-plane LSPR mode lies beyond the measured wavelength range (>3200 nm). (b) The calculated carrier densities of nanodisks synthesized under air and N2. D

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measurements on nanodisk samples dispersed in chloroform (see Supporting Information, S1). These measurements indicate that the average hydrodynamic radius of the nanodisks (approximately 28 nm) and the size dispersity (given by the full-width half-maximum) remains unchanged even after 135 h of thermal treatment. These data indicate that the observed shift in LSPR wavelength is not due to a change in nanodisk geometry and results only from a change in the carrier density of the Cu2−xS. Nanodisks treated at 100 °C display similar LSPR behavior; thus, the extinction spectra are not shown in Figure 5. Figure 5b shows the NIR extinction spectra for Cu2−xS nanodisks after thermal oxidation at 120 °C. Between oxidation times of 0 to 40 h, the LSPR wavelengths λin and λout blue-shift as heat treatment time is increased. Using Mie scattering theory, we used values for λout obtained by UV−vis−NIR measurements to calculate the carrier density for each nanodisk sample. Figure 6a shows a plot of carrier density as a function of thermal oxidation time for nanodisks heated at the three different temperatures. As temperature is increased, the rate of formation of Cu vacancies follows an Arrhenius rate law (Figure 6a, inset), as expected for a thermally activated process. The rate constant, k, for each oxidation temperature was calculated by fitting the data in Figure 6a to the exponential rate law for a first-order Arrhenius reaction. On the basis of this fit, the activation energy for Cu vacancy formation in the nanodisks by thermal oxidation is calculated to be Ea = 14.6 kJ/mol. For comparison, the activation energy for Cu vacancy formation in a CuGaS2 single crystal which has an ordered packing arrangement with alternating layers of I−VI(Cu2S) and III−VI (Ga2S3) is Ea = 55.5 kJ/mol.21 The low activation energy for vacancy formation in our nanodisks is attributed to the high surface energy associated with the nanoscale surfaces, particularly along the disk edges and other regions of high curvature where step edges and defects are expected to be prominent.

extinction peaks associated with the in-plane LSPR mode. Given a carrier density of Nh = 7.7 × 1020 cm−3, we expect λin > 3200 nm (see Table 1) which is beyond the detectable wavelength range of our instrumentation. Table 1 aspect ratio λout (nm) λin (nm) carrier density (× 1020 cm−3)

4.65

5.50

6.60

7.10

1861 3285 7.72

1762 3500 7.75

1679 3760 7.70

1650 3860 7.75

In order to study the effect of carrier density on the LSPR properties of the Cu2−xS nanodisks, we carried out thermal oxidation to tune the carrier density of the nanodisks, independent of nanodisk shape. Nanodisks with an aspect ratio of 4.65 ± 0.29 were heated in an ambient environment for various times ranging between 0 and 160 h and at three different temperatures of 80, 100, and 120 °C. Figure 5a,b shows the change in the NIR extinction spectra for nanodisks heated at 80 and 120 °C, respectively. For nanodisks oxidized at 80 °C, two peaks are observed corresponding to the in-plane (λin ≈ 1700 nm) and out-of-plane (λout ≈ 3100 nm) LSPR modes. With longer heat treatment, we observe that both peaks blue-shift. TEM images show that the average size and shape of the nanodisks remain unchanged up to 135 h of thermal treatment. TEM images obtained for samples subjected to different thermal oxidation times show that the nanodisks retain their shape during heat treatment at 80 °C. A change in the polydispersity of the colloidal dispersion also has the potential to result in a broadening of the LSPR peak, even if the average particle size remains the same. To confirm that a change in the size or shape polydispersity does not occur during the annealing process, we carried out dynamic light scattering

Figure 5. Tuning nanodisk carrier densities by thermal oxidation. (a) IR extinction spectra of Cu2−xS nanodisks oxidized at 80 °C under air. Both inand out-of-plane LSPRs are observed to blue-shift with increasing thermal oxidation time. (b) IR extinction spectra of nanodisks oxidized at 120 °C under air. After extended oxidation times, the LSPR spectrum indicates a change in nanodisk morphology. (c−e) TEM images corresponding to nanodisk samples (c) before oxidation, (d) after oxidation for 135 h at 80 °C, and (e) after oxidation for 60 h at 120 °C upon shape change. E

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Figure 6. Change in the carrier densities with oxidation at different temperatures. (a) The maximum carrier density obtained for the nanodisks is approximately 1.36 × 1021 cm−3 which corresponds to a stoichiometry of Cu1.94S. Inset figure is the rate of formation of Cu vacancies expects by an Arrhenius rate law. (b) XRD spectra of nanodisks before and after deformation indicating a change in crystal structure from chalcocite (hexagonal, Cu2S) to digenite (rhombohedral, Cu9S5).

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CONCLUSIONS In this work, we demonstrate that the aspect ratio and carrier density of Cu2−xS nanodisks can be independently tuned by carrying out thermolysis of a Cu−alkanethiolate precursor under a N2 gas environment and by thermal oxidation, respectively. The nanodisks synthesized under N2 exhibit low carrier densities (Nh = 7.7 × 1020 cm−3) and slower growth rates than nanodisks synthesized under air. We find that the presence of oxygen can enhance surface reactivity through Cu oxidation, which increases the rate of thermolysis and the generation of Cu vacancies. For N2-synthesized nanodisks, varying the aspect ratio of the nanodisks independent of carrier density produces changes in the LSPR wavelength that can be predicted by Mie scattering. Nanodisks that are subjected to postsynthetic thermal oxidation in air exhibit changes in carrier density that are independent of nanodisk shape. We observe that beyond a critical carrier density of Nh = 1.36 × 1021 cm−3, the nanodisks undergo an irreversible phase transformation that promotes particle agglomeration and fusion. Carrier density is thus limited to a specific range, placing an upper limit on LSPR wavelength tuning from the infrared wavelength range. However, the simple strategies presented here provide experimental methods to independently control the carrier density and aspect ratio of self-doped Cu2−xS nanodisks. This should enable future application of these nanomaterials in plasmonic thin films or dispersions by allowing reasonable LSPR tuning through synthetic or postsynthetic control over nanocrystal size, shape, and carrier density.

We also observe that the carrier density within the Cu2−xS nanodisks does not increase beyond a critical value of Nh = 1.36 × 1021 cm−3 regardless of thermal oxidation temperature or oxidation time. This critical carrier density corresponds to a stoichiometry of Cu1.94S. For samples oxidized beyond this point, we observe that the Cu2−xS nanodisks undergo an irreversible phase transformation that is characterized by a loss of nanodisk shape (as evidenced by TEM) and a change in the nanocrystal structure (as evidenced by XRD). Figure 5d,e shows TEM images of nanodisk powders that have been heat treated at 80 and 120 °C, respectively, until reaching this critical carrier density. In Figure 5d, it is apparent that the nanodisks begin to experience reshaping. In Figure 5e, the phase transformation is characterized by agglomeration and fusion of the nanodisks. We believe that heavy oxidation at the nanodisk surface disrupts the passivating dodecanethiol layer and promotes nanodisk sintering. Figure 6b shows the XRD spectrum obtained for the heat-treated Cu2−xS nanodisk powder at 120 °C for 60 h. With comparison to the unheated nanodisk powder, the XRD spectrum indicates that the nanodisks undergo a change in crystal structure from chalcocite (hexagonal unit cell, Cu2S) to a crystal structure with a lower copper-to-sulfur ratio, such as digenite (rhombohedral unit cell, Cu9S5). To confirm that this structural change results from the formation of Cu vacancies and does not result from the extended heating time alone, we carried out thermal annealing of a powder nanodisk sample under N2 at 120 °C for 60 h. On the basis of TEM images (see Figure S2 in the Supporting Information), we observe that the nanodisks do not change size or shape after thermal treatment under N2. This irreversible morphological and structural change observed for the Cu2−xS nanodisks at high carrier densities contrasts with other studies indicating that the carrier densities of semiconductor quantum dots can be fully reversible when subjected to oxidizing or reducing environments.8 For example, previous reports for Cu2−xSe quantum dots observe that spherical quantum dots with a carrier density corresponding to a stoichiometry of Cu1.81Se retain their β-phase Cu2Se crystal structure and do not undergo transformation to α-phase Cu1.78Se.22 This suggests that utilizing carrier density as a tunable parameter for adjusting LSPR properties is limited by the structural stability of the semiconductor.

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ASSOCIATED CONTENT

* Supporting Information S

Dynamic light scattering measurements, mean hydrodynamic diameters and full-width at half maximum values, TEM images, average nanodisk dimensions, and bulk plasmon frequency calculations. This information is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*Email: [email protected]. Notes

The authors declare no competing financial interest. F

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ACKNOWLEDGMENTS Financial support was provided by NSF (ECCS-1125789) and ONR (N000141210574). REFERENCES

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