Synthesis of Ultrathin and Thickness-Controlled Cu2–xSe Nanosheets

Letter pubs.acs.org/JPCL

Synthesis of Ultrathin and Thickness-Controlled Cu2−xSe Nanosheets via Cation Exchange Yuanxing Wang,† Maksym Zhukovskyi,*,† Pornthip Tongying,† Yang Tian,‡ and Masaru Kuno*,† †

Department of Chemistry and Biochemistry, University of Notre Dame, 251 Nieuwland Science Hall, Notre Dame, Indiana 46556, United States ‡ Department of Chemistry, Capital Normal University, Beijing 100048, People’s Republic of China S Supporting Information *

ABSTRACT: We demonstrate the use of cation exchange to synthesize ultrathin and thickness-controlled Cu2−xSe nanosheets (NSs) beginning with CdSe NSs. In this manner, extremely thin (i.e., 1.6 nm thickness) Cu2−xSe NSs, beyond which can be made directly, have been obtained. Furthermore, they represent the thinnest NSs produced via cation exchange. Notably, the exchange reaction preserves the starting morphology of the CdSe sheets and also retains their cubic crystal structure. The resulting nonstoichiometric and cubic Cu2−xSe NSs are stable and do not exhibit any signs of Cu or Se oxidation after exposure to air for 2 weeks. Resulting NSs also show the existence of a localized surface plasmon resonance in the infrared due to the presence of copper vacancies. Efforts to isolate intermediates during the cation exchange reaction show that it occurs via a mechanism where entire sheets are rapidly converted into the final product once the exchange reaction commences, precluding the isolation of alloyed species. SECTION: Plasmonics, Optical Materials, and Hard Matter

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to obtain different NSs as well as thicknesses, albeit without systematic control of the reaction parameter space. In this Letter, we illustrate the use of cation exchange to systematically produce ultrathin Cu2−xSe NSs difficult to synthesize directly. We further demonstrate that ultrathin 1.6 nm NSs can be produced via cation exchange with thicknesses below that achieved previously.23 We simultaneously demonstrate corresponding control over resulting NS thicknesses, yielding thickness-controlled ultrathin NSs. We have achieved this through the cation exchange of CdSe NSs, a wellestablished system with significant (recent) work on their colloidal growth.17,18,24,25 In this regard, much is currently understood about CdSe NS optical/electrical properties as well as their corresponding thickness dependencies.24 Furthermore, extensive work has been conducted on CdSe in quantum dot, nanorod, and nanowire forms over the last 2 decades,26,27 with much now known about the evolution of CdSe optical, electrical, and structural properties with dimensionality. Ultrathin 1.6 nm CdSe NSs were synthesized using a procedure adapted from Bouet et al.24 Briefly, a mixture of selenium powder dissolved in octadecene (Se-ODE) and oleic acid was injected into hot ODE at 220−250 °C containing cadmium acetate. CdSe NSs were isolated by centrifuging the resulting suspension, followed by several toluene/methanol washing steps to remove any excess surfactant. By varying the above reaction conditions, CdSe NSs with thicknesses of 1.8

opper (I) chalcogenide semiconductors are interesting materials that form nonstoichiometric (i.e., Cu2−xSe) as well as stoichiometric (i.e., Cu2Se) phases. They also adopt thermodynamically stable cubic1,2 as well as metastable hexagonal, orthorhombic, and monoclinic crystalline phases.1,2 Of these, nonstoichiometric, cubic, copper(I) chalcogenides are particularly interesting materials given their stability and their composition-dependent optical/electrical properties.3 One of their most prominent and interesting features is a localized surface plasmon resonance that occurs due to the presence of copper vacancies. This plasmon resonance is sensitive to various parameters such as stoichiometry and consequently can be tuned.3−6 As such, copper(I) chalcogenides are useful materials for photothermal therapy,7 photoacoustic imaging,8 field-effect transistors, and in microelectronic devices.9−11 Numerous reports describe the synthesis of size-tunable copper(I) chalcogenide nanostructures.11−13 All seek to combine the size-dependent properties of nanoscale materials with the inherent composition-dependent properties of copper(I) chalcogenides. Only a few, however, discuss the growth of nonstoichiometric, cubic Cu2−xSe nanosheets (NSs)14−16 whose optical and electrical properties are likely exquisitely sensitive to atomic level thickness changes.17,18 None describe work on thickness control or on obtaining ultrathin NSs. This stems from the fact that the direct bottom-up colloidal growth of NSs has only been extensively developed for II−VI cadmium chalcogenides (e.g., CdS, CdSe, CdTe).17 For other two-dimensional materials, their growth as well as their systematic thickness control remains a challenge.19−22 In these cases, empirical surfactant mixtures have been created © XXXX American Chemical Society

Received: September 11, 2014 Accepted: October 6, 2014

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single-crystal (0001) cut quartz substrates and allowing the mixture to dry. Additional details about these TEM and XRD measurements can be found in the SI. Figure 1c,d shows that cation exchange of 1.6 nm CdSe NSs leads to correspondingly thin ∼1.6 nm thick Cu2−xSe NSs. They also possess uniform rectangular shapes with comparable lateral dimensions. To the best of our knowledge, these sheets represent the thinnest Cu2−xSe NSs ever made using a wet chemical approach. They are also the thinnest NSs produced via cation exchange. Similar results have been obtained from thicker 1.8 and 2.2 nm CdSe NSs. Representative TEM images of resulting ∼1.8 and ∼2.2 nm as well as additional images of ∼1.6 nm Cu2−xSe sheets are shown in Figure S4 of the SI. Detailed TEM measurements establish the actual physical dimensions of the exchanged NSs. Namely, the thinnest Cu2−xSe NSs possess a thickness of 1.6 ± 0.1 nm. A corresponding long-axis lateral dimension is 137 ± 26 nm. For NSs obtained from 1.8 and 2.2 nm CdSe NSs, resulting thicknesses are 1.8 ± 0.1 and 2.2 ± 0.2 nm. Associated long-axis lateral dimensions are 15 ± 3 and 22 ± 4 nm. Detailed sizing histograms for all three samples can be found in the SI (Figure S5). In all cases, observed thicknesses and lateral dimensions are in excellent agreement with those of the starting CdSe sheets. TEM measurements show that resulting Cu2−xSe NSs are crystalline and possess the cubic form of the material. In particular, the top inset of Figure 1d shows lattice fringes stemming from the (111) planes of cubic Cu2−xSe. A corresponding lattice spacing is 0.333 nm and matches the (111) d spacing of bulk Cu2−xSe.30 Additional ensemble NS SAED measurements corroborate this conclusion wherein observed rings can be indexed to Cu2−xSe’s (200), (311), and (400) reflections (bottom inset, Figure 1d). Extracted SAED d spacings are 0.281 (200), 0.169 (311), and 0.143 nm (400) and again agree with bulk Cu2−xSe values.30 Additional SAED images of 1.8 and 2.2 nm thick Cu2−xSe NSs can be found in Figure S4 of the SI. Preservation of the parent material’s anion sublattice during cation exchange likely explains Cu2−xSe’s cubic lattice.2,31 In this regard, Figure 2a,b illustrates the common anion sublattice that exists between cubic CdSe and Cu2−xSe. From this, it is apparent that cation exchange would lead to interconversion between the two systems. To substantiate this hypothesis, analogous TEM and SAED measurements were conducted on CdSe NSs to verify their crystal structure (i.e., whether cubic or hexagonal). These measurements show that the starting CdSe NSs indeed possess a cubic lattice, consistent with their growth in the presence of fatty acid surfactants.32 In particular, the top inset of Figure 1b reveals characteristic (111) zinc blende lattice fringes with a corresponding d spacing of 0.349 nm. Accompanying ensemble SAED images (bottom inset, Figure 1b) show rings that can be indexed to zinc blende CdSe’s (200), (311), and (400) planes. Associated d spacings are 0.303, 0.183, and 0.155 nm and agree with those of bulk CdSe.33 Additional SAED images of 1.8 and 2.2 nm thick CdSe NSs can be found in Figure S2 of the SI. Note that the (111), (200), (311), and (400) d spacings of bulk CdSe and Cu2−xSe differ by only 5−6%, likely enhancing cation exchange between the two materials.34 To independently verify the crystal structures of both CdSe and Cu2−xSe NSs, powder XRD measurements were made on representative ensembles. Figure 2c illustrates the XRD pattern acquired from CdSe NSs and reveals characteristic zinc blende

and 2.2 nm were produced. Additional information about these syntheses can be found in the Supporting Information (SI). Figure 1a,b shows TEM images of starting 1.6 nm CdSe NSs. Detailed TEM measurements of edge-on-oriented NSs reveal a

Figure 1. Low- and high-magnification TEM images of 1.6 nm thick (a,b) CdSe and (c,d) Cu2−xSe NSs. Insets in (a) and (c): latticeresolved TEM images of edge-on CdSe and Cu2−xSe NSs. Insets in (b) and (d): lattice-resolved basal plane TEM (top) and corresponding ensemble selected area electron diffraction (SAED) (bottom) images.

thickness of 1.6 ± 0.1 nm. A representative image is provided in the inset of Figure 1a. Other images can be found in the SI (Figure S1). For thicker CdSe sheets, analogous TEM measurements show thicknesses of 1.8 ± 0.1 and 2.2 ± 0.1 nm. Corresponding edge-on images can be found in the SI (Figure S2). Measured lateral dimensions for all three samples along their long axes are 164 ± 30, 19 ± 4, and 25 ± 3 nm, respectively. In general, width estimates for the thinnest 1.6 nm sheets are complicated by their tendency to curl. Sizing histograms for all three samples can be found in the SI (Figure S3). Cu2−xSe NSs were subsequently obtained via cation exchange.28,29 In practice, this entailed slowly adding a solution of a copper(I) salt [tetrakis(acetonitrile)copper(I) hexafluorophosphate, [MeCN]4CuIPF6] in methanol (concentration = 7.2 × 10−3 M) to toluene suspensions of CdSe sheets. NS concentrations were estimated to be ∼7.0 × 10−11, ∼5.9 × 10−9, and ∼9.7 × 10−9 M for the 1.6, 1.8, and 2.2 nm thick samples, respectively, using inductively coupled plasma atomic emission spectroscopy (ICP-AES). Additional details about these concentration estimates can be found in the SI. During reactions, visual evidence for cation exchange was apparent through a color change of parent CdSe NS suspensions. Cu2−xSe NSs were subsequently isolated by centrifuging the resulting mixture followed by several toluene/methanol washing steps to remove any excess copper or cadmium ions. Additional details about the cation exchange reaction can be found in the SI. TEM imaging, TEM-based selected area electron diffraction (SAED), and ensemble powder X-ray diffraction (XRD) measurements were subsequently used to establish the morphology and crystal structure of resulting Cu2−xSe NSs. In the latter case, samples were prepared by drop-casting Cu2−xSe and CdSe NSs from viscous toluene suspensions onto 3609

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To independently verify this estimate, additional ICP-AES measurements were conducted. These ICP measurements yield an initial Cd-to-Se ratio of 1.54 (±0.08) for CdSe NSs (1.6 nm thick), in agreement with earlier EDXS results. The corresponding Cu-to-Se ratio is also consistent with a value of 2.58 (±0.27). Consequently, an estimate for the x value in nonstoichiometric Cu2−xSe NSs (1.6 nm thick) is again x1.6 nm = 0.42. Additional details about either EDXS or ICP-AES calculations can be found in the SI. At this point, X-ray photoelectron spectroscopy (XPS) was conducted on nonstoichiometric Cu2−xSe NSs to establish their Cu and Se oxidation states. Figure 3a shows results from these

Figure 2. Top view of the (111) faces of (a) CdSe and (b) Cu2−xSe, illustrating their common anion sublattice. Powder XRD patterns of as-synthesized (c) CdSe, (d) CdSe/Cu2−xSe, and (e) Cu2−xSe NSs. Included for reference purposes are JCPDS stick patterns for zinc blende CdSe (JCPDS No. 88-2346, red line) and cubic Cu2−xSe (JCPDS No. 06-0680, blue line). (Insets) Illustrations of the cubic unit cells of CdSe and Cu2−xSe; black dots and asterisks highlight the (220) reflections of CdSe and Cu2−xSe, respectively. Figure 3. XPS spectra of Cu2−xSe (a) Cu 2p, (b) Cu LMM, and (c) Se 3d core spectra. In (c), dashed lines represent fits to the data.

reflections at 25.4 (111), 42.0 (220), 49.7 (311), 60.9 (400), and 67.1° (331) (JCPDS no. 88-2346). Corresponding d spacings are 0.356, 0.219, 0.185, 0.154, and 0.141 nm. For Cu2−xSe (Figure 2e), obtained powder patterns show reflections at 26.7, 44.6, and 52.9° that match the (111), (220), and (311) reflections of cubic Cu2−xSe (JCPDS no. 06-0680). Corresponding d spacings are 0.335, 0.202, and 0.173 nm. Table S1 (SI) summarizes all experimental XRD d spacings along with corresponding bulk values for CdSe and Cu2−xSe. These XRD results therefore agree with the above TEM-based SAED measurements. In turn, both confirm that cation exchange preserves the initial cubic crystal structure of the parent material through retention of its anion sublattice.2,31 Having established the morphology and crystal structure of resulting Cu2−xSe NSs, energy dispersive X-ray spectroscopy (EDXS) measurements were conducted to determine their stoichiometry. Ultrathin 1.6 nm thick CdSe NSs were first examined to establish the starting cadmium-to-selenium ratio prior to cation exchange. A nonstoichiometric value of 1.67:1 (±0.09) was obtained. For 1.8 and 2.2 nm thick CdSe NSs, obtained Cd-to-Se ratios were 1.47 (±0.06) and 1.49 (±0.06), respectively. In all cases, nonstoichiometric Cd-to-Se ratios are not unexpected because it is well-known that colloidal zinc blende CdSe NSs exhibit excess Cd due to terminal cadmium layers present on their top and bottom {100}({111})faces.18 For 1.6 nm thick Cu2−xSe NSs, we find a copper-to-selenium ratio of 2.84 (±0.15). For the 1.8 and 2.2 nm Cu2−xSe NSs, we find Cu-to-Se ratios of 2.14 (±0.12) and 2.48 (±0.14). Tables S2−S4 in the SI summarize these EDXS results. Given the above Cd-to-Se ratios and a 2 to 1 exchange of Cu for Cd, we expect corresponding Cu-to-Se ratios of 3.34, 2.94, and 2.98 for the 1.6, 1.8, and 2.2 nm NSs. Observed differences therefore suggest that cation exchange produces nonstoichiometric Cu2−xSe NSs with corresponding x values of x1.6 nm = 0.42, x1.8 nm = 0.68, and x2.2 nm = 0.44.

measurements, revealing narrow, symmetric Cu 2p3/2 (932.7 eV) and Cu 2p1/2 (952.6 eV) peaks devoid of satellite lines at ∼942 and ∼962 eV, respectively.35 These energies and the absence of satellite lines indicate that Cu in the Cu2−xSe NSs resides as either Cu(I) or Cu(0). It is not oxidized Cu(II), however, given the absence of a 933.5 eV feature.35 Subsequent Cu LMM spectra (Figure 3b) refine the assignment wherein a peak, characteristic of Cu(I), is seen at 916.9 eV.36,37 From this, we conclude that copper in Cu2−xSe NSs exists as Cu(I). Finally, in accompanying Se XPS spectra (Figure 3c), we see no Se peak at ∼59 eV, which would indicate oxidized Se in the sheets.38 Only the typical Se 3d doublet at 55.4 and 54.6 eV is seen. Additional XPS spectra of 1.8 and 2.2 nm thick Cu2−xSe can be found in the SI, Figure S6. This latter observation as well as the fact that unoxidized Cu(I) exists in our Cu2−xSe NSs is important because both Cu and Se in nanostructures are known to oxidize upon prolonged exposure to air.3,4,38 Consequently, to further investigate the stability of Cu and Se in the exchanged Cu2−xSe NSs, a systematic time-dependent XPS study was conducted on the thinnest 1.6 nm sheets by continuously exposing them to air for a period of 2 weeks. At given intervals during this time (namely, 3 days, 1 week, 2 weeks), XPS measurements were conducted to look for evidence of Cu or Se oxidation. These results are summarized in Figure S7 (SI) and show no oxidation-induced shift of the Cu(I) 2p peaks as well as no growth of an oxidized Cu(II) 933.5 eV peak and no Se peak at ∼59 eV, which would indicate oxidized Se in the sheets.35,38 We therefore conclude that the Cu and Se in our Cu2−xSe NSs remain stable against oxidation, consistent with the empirically known properties of cubic Cu2−xSe when x > 0.2.3 Additional details about these XPS studies can be found in the SI. 3610

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Figure 4. UV/visible extinction (solid line) and PL (dashed line) spectra of 1.6, 1.8, and 2.2 nm CdSe NSs. Solid squares denote emission contributions from thicker sheet subpopulations within a given ensemble. (b) Extinction spectra of analogous Cu2−xSe NSs. (c) Corresponding localized surface plasmon resonances of Cu2−xSe NSs (*solvent-related features).

reported size-dependent behavior of nonstoichiometric Cu2−xS QDs4,5 wherein a blue shift is seen with increasing particle diameter. The shift is attributed to decreased free carrier scattering contributions to the surface plasmon frequency with increasing size.4,5 However, in the current Cu2−xSe NSs, additional shape-40 and stoichiometry-dependent3 sensitivities likely contribute to the observed plasmon shift, complicating any definite attribution to thickness variations at this time. Rudimentary estimates for carrier densities in Cu2−xSe NSs using their plasmon resonance40 suggest values on the order of ∼1021 cm−3. This is consistent with previously reported carrier densities for nonstoichiometric copper(I) selenide.5 Additional details about these carrier density estimates can be found in the SI. Having established the cation exchange of CdSe to nonstoichiometric, cubic Cu2−xSe as well as having demonstrated their corresponding surface plasmon resonances, we have conducted additional experiments to provide more insight into the cation exchange mechanism. Reactions were therefore stopped prior to completion whereupon intermediate products were isolated and analyzed to look for any evidence of partial exchange in the sheets. From powder XRD measurements (middle panel, Figure 2d), we find that XRD reflections of the parent CdSe NSs remain constant during the exchange reaction. Only their intensities change, decreasing with increasing Cu ion concentration. In parallel, characteristic reflections for copper selenide grow in intensity. This is illustrated in Figure 2d through the evolution of the (220) reflection, as depicted by black dots and asterisks. The lack of an apparent composition-dependent shift in the powder pattern thus suggests that cation exchange occurs without gradual alloying of the starting CdSe NSs. To further confirm this hypothesis, intermediate products of the reaction were isolated and analyzed using single-sheet highangle annular dark field scanning transmission electron microscopy (HAADF-STEM) along with EDXS measurements. As seen in Figure 5, EDXS measurements of individual sheets within a small ensemble show no evidence for alloying. Specifically, Figure 5b shows EDXS spectra acquired from two neighboring sheets. We find one to be exclusively CdSe while the other is exclusively Cu2−xSe. Analogous results are found from examining other individual NSs (Figure S9, SI), and

The optical properties of both starting CdSe and resulting Cu2−xSe NSs were subsequently investigated. Of interest in the latter case are the plasmonic properties of the sheets stemming from compositional nonstoichiometries.3,4,7,11 Figure 4a first shows extinction spectra for all three starting CdSe NS ensembles. In each case, two characteristic excitonic resonances are seen, corresponding to their light-hole/conduction band and heavy-hole/conduction band transitions.17 Effective mass estimates correlate excitonic energies of 2.68 (463 nm), 2.42 (513 nm), and 2.24 eV (553 nm) with the presence of 4−7 monolayers of CdSe, where a monolayer is defined as the distance between two cadmium planes. This corresponds to thicknesses of 1.22−2.13 nm,17 which agree with our TEM measurements. Room-temperature band edge emission accompanies the absorption with peaks at 2.66 (466 nm), 2.39 (518 nm), and 2.24 eV (554 nm). In all cases, additional band edge emission features at 2.34 (513 nm), 2.26 (549 nm), and 2.13 eV (581 nm) are seen and result from the presence of small subpopulations of thicker CdSe NSs within probed ensembles. In the 1.6 nm case, we additionally observe trap-related emission, centered about 600 nm. This likely stems from incomplete passivation of CdSe NS surface states. Additional details about these absorption and emission measurements can be found in the SI. Next, Figure 4b shows extinction spectra of resulting Cu2−xSe NSs. In each case, clear differences from their CdSe counterparts can be seen. Additionally, absorption edges for all three samples are blue shifted from Cu2−xSe’s bulk band gap of 2.2 eV39 (564 nm) and appear at successively larger energies between ∼2.4 (517 nm) and ∼2.9 eV (428 nm) (for a Tauc plot, see Figure S8, SI). These blue shifts stem from carrier confinement due to the narrow thicknesses of the NSs.5 Corresponding emission quantum yields are low and hence preclude acquisition of ensemble emission spectra. Most interesting, though, is the near-infrared plasmon resonance present in all Cu2−xSe samples. By contrast, prior work with Cu2−xS QDs shows complete damping of the plasmon resonance below a diameter of 3 nm.4,5 Figure 4c shows a plasmon resonance near 2594 nm for 1.6 nm Cu2−xSe NSs. With increasing thickness from 1.6 to 2.2 nm, we observe that the plasmon resonance blue shifts from 2594 to 2207 nm and becomes more distinct. This is in accord with previously 3611

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spacing obtained using XRD, TEM, and SAED; EDXS estimates of the Cd-to-Se and Cu-to-Se ratios in CdSe and Cu2−xSe NSs; details of EDXS stoichiometry estimates; XPS spectra of 1.8 and 2.2 nm thick Cu2−xSe NSs; stability test of Cu2−xSe NSs; details of XPS measurements; details of UV− visible extinction and emission measurements; Tauc plot corresponding to the optical absorbance spectra of 1.6, 1.8, and 2.2 nm Cu2−xSe NSs; carrier density estimates of Cu2−xSe NSs; single-sheet EDXS measurements on exchange intermediates; and UV−visible extinction and PL monitoring of the cation exchange reaction in solution. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 5. (a) HAADF-STEM image and (b) EDXS spectra of individual NSs in an intermediate sample. Numbered circles represent regions of individual sheets where EDXS spectra were acquired.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.Z.). *E-mail: [email protected] (M.K.).

corresponding EDXS line scans can be found in the SI (Figure S10). This illustrates that intermediates of the exchange reaction consist of a physical mixture of Cu2−xSe and CdSe NSs. They are not alloyed. By contrast, studies on thicker NS analogues show evidence for partial cation exchange, leading to alloyed specimens.23 Finally, to corroborate this and to probe the exchange reaction on faster time scales, extinction and emission spectroscopies were used to monitor the exchange reaction in solution upon adding Cu(I) to CdSe NSs. Results from these measurements (Figure S11, SI) show that the characteristic excitonic resonances of CdSe decrease immediately upon adding Cu(I). This is accompanied by a simultaneous increase of the Cu2−xSe absorption. More importantly, an apparent isosbestic point appears in the data, which clearly suggests that two distinct materials exist in the reaction mixture. This again supports conclusions from the above powder XRD and singlesheet EDXS results and suggests a mechanism wherein cation exchange proceeds quickly once initiated on a given sheet, being driven to completion due to a steady decrease of the exchange reaction activation energy.36,41 To summarize, we have synthesized via cation exchange ultrathin, air stable, and thickness-controlled Cu2−xSe NSs starting from CdSe NSs. The approach maintains the original morphology of the CdSe sheets and simultaneously preserves their crystal structure because the parent anion sublattice is maintained during cation exchange. What results are stable and nonstoichiometric ultrathin Cu2−xSe NSs with thicknesses below what have been made either directly through chemical synthesis or indirectly through cation exchange. The sheets exhibit confinement effects as well as the presence of a pronounced surface plasmon resonance in the infrared due to the presence of copper vacancies. Efforts to isolate intermediate products during the exchange reaction show that cation exchange occurs via a mechanism wherein entire sheets are rapidly converted into the final product once the exchange reaction commences.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This research is supported by the Army Research Office (W911NF-12-1-0578) and the Center for Sustainable Energy at Notre Dame (cSEND). We thank the Notre Dame Radiation Laboratory for partial financial support. We also thank the Beijing Municipal Commission of Education (CIT&TCD201404162). We additionally thank the Notre Dame Integrated Imaging Facility (NDIIF) as well as the Notre Dame Radiation Laboratory/Department of Energy (DOE), Office of Basic Energy Sciences for use of their facilities and equipment. P.T. thanks the Royal Thai Government Scholarships for partial financial support.

(1) Zhu, J.; Li, Q.; Bai, L.; Sun, Y.; Zhou, M.; Xie, Y. Metastable Tetragonal Cu2Se Hyperbranched Structures: Large-Scale Preparation and Tunable Electrical and Optical Response Regulated by Phase Conversion. Chem.Eur. J. 2012, 18, 13213−13221. (2) Li, H.; Zanella, M.; Genovese, A.; Povia, M.; Falqui, A.; Giannini, C.; Manna, L. Sequential Cation Exchange in Nanocrystals: Preservation of Crystal Phase and Formation of Metastable Phases. Nano Lett. 2011, 11, 4964−4970. (3) Dorfs, D.; Härtling, T.; Miszta, K.; Bigall, N. C.; Kim, M. R.; Genovese, A.; Falqui, A.; Povia, M.; Manna, L. Reversible Tunability of the Near-Infrared Valence Band Plasmon Resonance in Cu2−xSe Nanocrystals. J. Am. Chem. Soc. 2011, 133, 11175−11180. (4) Kriegel, I.; Jiang, C.; Rodríguez-Fernández, J.; Schaller, R. D.; Talapin, D. V.; da Como, E.; Feldmann, J. Tuning the Excitonic and Plasmonic Properties of Copper Chalcogenide Nanocrystals. J. Am. Chem. Soc. 2011, 134, 1583−1590. (5) Luther, J. M.; Jain, P. K.; Ewers, T.; Alivisatos, A. P. Localized Surface Plasmon Resonances Arising from Free Carriers in Doped Quantum Dots. Nat. Mater. 2011, 10, 361−366. (6) Hsu, S.-W.; Bryks, W.; Tao, A. R. Effects of Carrier Density and Shape on the Localized Surface Plasmon Resonances of Cu2−xS Nanodisks. Chem. Mater. 2012, 24, 3765−3771. (7) Hessel, C. M.; P. Pattani, V.; Rasch, M.; Panthani, M. G.; Koo, B.; Tunnell, J. W.; Korgel, B. A. Copper Selenide Nanocrystals for Photothermal Therapy. Nano Lett. 2011, 11, 2560−2566. (8) Liu, X.; Law, W.-C.; Jeon, M.; Wang, X.; Liu, M.; Kim, C.; Prasad, P. N.; Swihart, M. T. Cu2−xSe Nanocrystals with Localized Surface Plasmon Resonance as Sensitive Contrast Agents for In Vivo Photoacoustic Imaging: Demonstration of Sentinel Lymph Node Mapping. Adv. Healthcare Mater. 2013, 2, 952−957.

ASSOCIATED CONTENT

S Supporting Information *

Additional details about the synthesis of CdSe NSs; TEM images of 1.6, 1.8, and 2.2 nm CdSe NSs; sizing histograms for CdSe NSs; ICP-AES sample preparation details and CdSe NS concentration estimates; additional details about the synthesis of Cu2−xSe NSs; details of TEM and XRD measurements; TEM images of 1.6, 1.8, and 2.2 nm Cu2−xSe NSs; sizing histograms for Cu2−xSe NSs; comparison of CdSe and Cu2−xSe lattice d 3612

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The Journal of Physical Chemistry Letters

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dx.doi.org/10.1021/jz5019288 | J. Phys. Chem. Lett. 2014, 5, 3608−3613