Size-, Shape-, and Composition-Controlled Synthesis and Localized

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Size‑, Shape‑, and Composition-Controlled Synthesis and Localized Surface Plasmon Resonance of Copper Tin Selenide Nanocrystals Xianliang Wang,† Xin Liu,†,‡ Deqiang Yin, Yujie Ke, and Mark T. Swihart* Department of Chemical and Biological Engineering, The University at Buffalo (SUNY), Buffalo, New York 14260-4200, United States S Supporting Information *

ABSTRACT: We report a robust methodology for synthesizing monodisperse copper-tin-selenide (CTSe) nanocrystals (NCs) of tunable size, shape, and composition including single-crystalline nanosheets, nanoplates, spheres, and tetrahedra. Both the identity and concentration of the selenium precursor play important roles in determining the morphology and crystal structure of the CTSe NCs. In contrast to previous studies, we demonstrated broad tunability of the Cu to Sn ratio. The size of CTSe NCs continuously decreased with increasing Sn incorporation. Moreover, the near-infrared (NIR) localized surface plasmon resonance (LSPR) in CTSe alloy NCs was tuned over a broad range by varying the Cu:Sn ratio. The LSPR red-shifted and decreased in intensity with increasing Sn content. This indicates that the free charge carrier concentration can be manipulated by varying the cation ratio. The cation deficiency responsible for self-doping in these NCs decreases with increasing Sn content. The resulting CTSe NCs and related materials with tunable size, shape, band gap, and doping level provide new opportunities in solution-processed optoelectronic devices.

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INTRODUCTION In recent years, Cu-based ternary I−IV−VI and quaternary I− II−IV−VI colloidal nanocrystals (NCs) have attracted significant interest because of their tunable bandgap, potential low cost, low toxicity, and anticipated applications in photovoltaics, electronics, and thermoelectrics.1−12 The most heavily studied materials in this system include Cu2ZnSnS4 (CZTS),13−17 Cu2ZnSn(S,Se) (CZTSSe),3,5,18−22 Cu2Zn(Sn,Ge)S 4 (CZTGS), 2 3 , 2 4 and Cu 2 Zn(Sn,Ge)(S,Se) 4 (CZTGSSe).4,25,26 These prior investigations demonstrate that precise control of reaction conditions (temperature, solvents, ligands and precursors) is critical for controlling NC growth, crystal phase, shape evolution, and optoelectronic properties.8,14,27−35 The morphology and crystal structure may influence the device fabrication and ultimately limit the performance of devices.1,13,19,36 More recently, much research effort has been devoted to preparing I−IV−VI and I−II−IV− VI NCs with desired crystal structure, morphology and stoichiometry. For example, Norako et al. produced metastable wurtzite CTSe using 1-dodecanethiol capping ligands and ditert-butyl diselenide as selenium source.37 Wang et al. generated colloidal CTSe NCs with varying shapes from dots (0D) to rods (1D) to tetrapods (3D).38 Zou et al. observed the crystal structure evolution of CZTS using different sulfur precursors.39 Nonetheless, the ability to produce monodisperse I−IV−VI or I−II−IV−VI colloidal NCs of specific controlled crystalline phase with tunable composition remains limited. In particular, preparing compositions away from the stoichio© XXXX American Chemical Society

metric compounds (i.e., CuxSnySez with x, y, and z values other than 2, 1, and 3) provides new opportunities to tune the charge-carrier concentration in self-doped ternary and quaternary semiconductor NCs. Compared with binary NCs, nucleation and growth of ternary and quaternary colloidal NCs involve more possible mechanisms, due to reactivity differences between precursors for different elements and differences in binding affinities of ligands for passivating NCs of varying composition. Often, this complexity leads to size polydispersity, morphology inhomogeneity, and inability to smoothly vary the NC stoichiometry. As a result, few reports of compositionally tunable synthesis of I−(II)−IV−VI colloidal NCs have been published. To meet this need for expanded compositional control of this family of NCs, we employed inexpensive CuCl and SnCl2 as cation sources, with several different Se sources, for controllable synthesis of CTSe (I−IV−VI) NCs with morphologies including nanosheets (from nanosize to microsize), dots and tetrahedra. For the Se precursors, we focused on welldeveloped, accessible Se complexes. We found that trioctylphosphine-Se (TOP-Se) was effective for preparing CTSe nanosheets with tunable size and low Sn content. Oleylamine/ sodium borohydride-Se (OAm/NaBH4-Se) and dodecanethiol/ oleylamine-Se (DDT/OAm-Se) were both viable options for Received: February 16, 2015 Revised: April 9, 2015

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DOI: 10.1021/acs.chemmater.5b00618 Chem. Mater. XXXX, XXX, XXX−XXX

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mixture was held at 305 °C (rather than 300 °C) for 10 min. To terminate the reaction, we removed the heating mantle, and when the temperature dropped below 200 °C, we added ethanol as described below. Synthesis of CTSe with Tunable Size and Composition Using OAm/NaBH4-Se Precursor. In a typical synthesis, a total of 0.75 mmol of CuCl and SnCl2 powder at a specified molar ratio (2:1, 1:1, 1:2, or 1:5) was mixed with 10 mL of OAm and 4 g of TOPO, and the solution was degassed at 110 °C for 30 min under nitrogen protection. Then the solution was heated to 200 °C and becamse transparent and deep yellow, indicating formation of organo-copper and organo-tin precursor complexes. After reducing the temperature to 180 °C, 3 mL of OAm/NaBH4-Se precursor was rapidly injected. The temperature was then reduced to 150 °C and held there for 4 min. Synthesis of CTSe with Tunable Size and Composition Using of DDT/OAm-Se Precursor. A total of 1.5 mmol CuCl and SnCl2 powder at a specified molar ratio (4:1, 2:1, 1:1, or 1:2) was mixed with 10 mL OAm and 4 g TOPO, and the solution was degassed at 110 °C for 30 min under nitrogen protection. Then the solution was heated to 200 °C and turned transparent and deep yellow, indicating formation of organo-copper and organo-tin precursor complexes. After increasing the solution temperature to 230 °C, 4 mL of DDT/OAm-Se precursor solution was rapidly injected. The temperature was then reduced to 200 °C and held there for 90 s. Synthesis of Tetrahedral CTSe NCs Using TOP/OA-Se Precursor. In a typical synthesis, 1 mmol of CuCl and 0.5 mmol of SnCl2 powder were mixed with 10 mL of OAm and 4 g of TOPO. The mixture was degassed at 110 °C for 30 min under nitrogen protection, then heated to 265 °C followed by injection of 4 mL of TOP/OA-Se precursor. The temperature was then reduced to 205 °C and held there for 5 min. Synthesis of Copper Tin Selenide/Copper Selenide NCs Using OA-Se or OAm-Se Precursor. In a typical synthesis, 1 mmol of CuCl and 0.5 mmol of SnCl2 powder were mixed with 10 mL of OAm and 4 g TOPO. The mixture was degassed at 110 °C for 30 min under nitrogen protection, then heated to 200 °C. It turned transparent and deep yellow, indicating formation of organo-copper and organo-tin precursor complexes. The temperature was increased to 220 °C followed by injection of 20 mL of 0.1 M OA-Se or OAm-Se precursor solution. After injection, the temperature was reduced to 180 °C and held there for 90 s. Separation and Purification of Product NCs. Ethanol (20−30 mL) was added into the colloidal suspension of product NCs, to quench the reaction and promote aggregation to enable separation. The NCs were collected by centrifuging at 9000 rpm (about 9000 G) for 1 min. The collected NCs were redispersed in chloroform. Ethanol was added to the resulting dispersion and product NCs were collected again by centrifugation. The final products were dispersed in chloroform and stored for future characterization. Characterization. Transmission Electron Microscopy (TEM). The size and morphology of product NCs were characterized using a JEOL JEM-2010 microscope at a working voltage of 200 kV. Samples for TEM imaging were prepared by drop-casting one or two drops of a dilute NC dispersion onto a carbon-coated TEM grid, which was then allowed to dry at ambient conditions. UV−Vis−NIR Spectroscopy. Optical absorbance spectra of product NCs dispersions were measured using a Shimadzu 3600 UV− visible−NIR scanning spectrophotometer. Spectra were collected from dispersions in chloroform, which produces a band of interference from the solvent near 1700 nm. Powder X-ray Diffraction. The crystal phases of product NCs were determined using powder XRD (Rigaku Ultima IV with Cu K α X-ray source). Samples were prepared by drop-casting high-concentration NC dispersions onto glass slides. Energy-Dispersive X-ray Spectrometry. Compositional analysis of product 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). Atomic Force Microscopy (AFM). Surface topology was measured using an Asylum Research MFP-3D microscope with Asylum Research

composition and size tunable preparation of CTSe NCs. With trioctylphosphine/oleic acid-Se (TOP/OA-Se) precursors, monodisperse tetrahedral CTSe were obtained. Building upon our previous work on Cu2−xS and Cu2−xSe,40 we also studied OA-Se and OAm-Se as selenium precursors for CTSe alloy NCs. Although these precursors are very effective for Cu2−xSe synthesis, when used in CTSe synthesis, they produced very little, if any, Sn incorporation, reflecting the lower reactivity of Se with Sn compared to its reactivity with Cu. Compared to the binary Cu2−xSe NCs, the corresponding ternary copperdeficient CTSe NCs have greater potential in optoelectronic applications because of the possibility of compositional tuning of both their doping level and band gap.

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EXPERIMENTAL SECTION

Chemicals. Copper(I) chloride (CuCl, 99.995%), tin chloride (SnCl2, reagent grade 98%), oleylamine (OAm, technical grade 70%), oleic acid (OA, technical grade 90%), trioctylphosphine (TOP, technical grade 90%), dodecanethiol (DDT, ≥98%), selenium powder (Se, 99.99%), sodium borohydride (NaBH4, 99.99%), and trioctylphosphine oxide (TOPO, technical grade 90%) were purchased from Sigma-Aldrich and were used as received. Synthesis Apparatus. All of the NC syntheses were carried out under nitrogen protection, in 100 mL 3-neck flasks equipped with a condenser and heated with a hemispherical mantle that was controlled with a J-KEM precision temperature controller. Reaction mixtures were stirred using a magnetic stir plate and Teflon-coated stir bar. Preparation of TOP-Se Precursor. Two millimoles of Se powder was dissolved in 2 mL of TOP by stirring at room temperature in a capped 20 mL sample vial until the solution became clear. Preparation of OAm/NaBH4-Se Precursor. Se powder (0.5 mmol) was dissolved in a mixture of 3 mL of OAm and 10 mg of NaBH4 via sonication for 2−3 h in a capped 20 mL sample vial placed in a bath sonicator. The NaBH4-containing mixture is air-sensitive. Thus, the mixture was prepared under an inert atmosphere and the vial was capped before removing it from the glove box. Preparation of DDT/OAm-Se Precursor. Se powder (1.5 mmol) was dissolved in a mixture of 2 mL of DDT and 2 mL of OAm in a capped 20 mL sample vial via vigorous stirring for up to 30 min using a magnetic stir plate. Preparation of TOP/OA-Se Precursor. Se powder (2 mmol) was dissolved in 1 mL of TOP via stirring to form a transparent solution, then 3 mL of OA was added and the solution was stirred for another 5 min. Preparation of OA-Se Precursor. Se powder (2 mmol) was dissolved in 20 mL of OA. The mixture was heated at 110 °C for 30 min under nitrogen protection to remove dissolved oxygen. Then the solution was rapidly heated to 320−330 °C and kept at this temperature until a clear and transparent solution formed. The solution was cooled to room temperature and kept in an inert atmosphere for further use. Upon cooling to room temperature, the solution became a gel with light yellow color. The gel was easily melted by gentle heating for subsequent use as a liquid. Preparation of OAm-Se Precursor. Similar to preparation of OA-Se, 2 mmol Se powder was dissolved in 20 mL of OAm. The mixture was degassed at 110 °C for 30 min under nitrogen protection. The solution was then rapidly heated to 300−310 °C and kept at this temperature until a clear and transparent solution formed. The solution was cooled to room temperature and was stored under an inert atmosphere for further use. Synthesis of CTSe Nanosheets Using Top-Se Precursor. In a typical synthesis, 1 mmol of CuCl and 0.5 mmol SnCl2 powder were mixed with 10 mL of OAm and 4 g of TOPO. The mixture was degassed at 110 °C for 30 min under nitrogen protection, then heated to 240 °C followed by injection of 2 mL of 1 M TOP-Se solution. After injection, the solution was heated to 300 °C and kept at this temperature for 10 min. Micron-size nanosheets of CTSe were obtained using the same procedure, except that after injection the B

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Chemistry of Materials model AC160TS probe. The spring constant (k) of the cantilever was 42 N/m with a resonance frequency ( f) of 300 kHz. The images were obtained using a silicon tip. The measurements were carried out in tapping mode. The scan rate was varied between 0.6 and 1 Hz. Scan points and scan lines were varied between 256 and 512. Samples for AFM imaging were prepared by drop-casting from a dilute chloroform dispersion onto a mica substrate.

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RESULTS AND DISCUSSION Synthesis of Single-Crystal CTSe Nanosheets Using TOP-Se (Method 1). Along with graphene, transition-metal chalcogenide two-dimensional (2D) nanomaterials have recently generated great research interest due to their unique properties and potential applications in field-effect transistors, gas sensors, lithium-ion batteries and electrocatalysis.41−46 Recently, thin copper chalcogenide (e.g., CuS and CuSe) nanosheets of small thickness with 2D morphology have been synthesized by using conventional wet-chemical methods.47,48 Here, we demonstrate, for the first time, a facile and robust method for the preparation of size controllable phase-pure copper tin selenide (CTSe) alloy nanosheets. Use of TOP-Se, as described in the Experimental Section, with a mixture of Cu and Sn precursors led to formation of these nanosheets, with relatively low Sn content. Figure 1 A&B provide representative TEM images of CTSe nanosheets with an average lateral dimension near 200 nm, prepared at 300 °C. At 305 °C, these hexagonal 2D nanosheets grew to the micrometer scale (Figure 1C, D and Figure S1 in the Supporting Information) with most sheets exceeding 0.5 μm in lateral dimensions. The uniform hexagonal overall shape was retained in these larger 2D nanosheets. Selected area election diffraction (SAED) combined with TEM imaging proved the single-crystalline nature of these CTSe hexagonal nanosheets. EDS analysis (Figure 1E, inset) confirmed the incorporation of ∼3% Sn into these structures. The powder XRD pattern (Figure 1E) showed that the CTSe nanosheets were in the hexagonal klockmannite phase (PDF Card No. 00−034−0171). The thickness of hexagonal nanosheets was measured by AFM (Figure 2), which showed that both the smaller and larger nanosheets were typically 9−15 nm in thickness. The representative AFM images in Figure 2 show a smaller nanosheet of 14 nm thickness and a larger one with 12 nm thickness. Other nearby sheets in the sample of smaller nanosheets measured 11, 9, 15, and 9 nm in thickness. Nearby nanosheets in the sample of larger lateral size were 12, 10, 14, and 9 nm in thickness. In the absence of tin precursor or with hot-injection at the reaction temperature (300 °C), these uniform hexagonal nanosheets were not obtained. Sphere-like copper selenide NCs were produced without SnCl2 addition (see the Supporting Information, Figure S2A, B). This demonstrates the important role of Sn in controlling the morphology of the NCs. Injection of the Se precursor at the final reaction temperature (300 °C) produced a mixture of less uniform nanosheets and quasispherical NCs (see the Supporting Information, Figure S2C, D). The formation of nanosheets upon injection at lower temperature is consistent with a mechanism in which relatively few NCs nucleate during heating from the injection temperature (240 °C) to the final growth temperature (300 or 305 °C), without secondary nucleation, and these nuclei grow selectively along directions perpendicular to the [001] direction of the hexagonal crystal lattice. This implies that the (001) facets were relatively unreactive, presumably because of strong passivation by TOP. This may also reflect low reactivity

Figure 1. CTSe nanosheets synthesized using TOP-Se as selenium precursor (A, C) TEM and (B, D) HRTEM images for samples produced at (A, B) 300 °C and (C, D) 305 °C. (E) Powder XRD pattern and EDS analysis of CTSe nanosheets produced at 305 °C. Insets in panels (B) and (D) show selected area electron diffraction patterns.

between TOP-Se and SnCl2 or between TOP-Se and Snterminated (001) facets. Note that in our previous study of SnSe NC synthesis,49 we similarly found that the reaction of SnCl2 with TOP-Se produced nanosheets. Here, a similar nanosheet morphology is observed, even though the total amount of Sn incorporated in the NCs is small (only about 5% of the cations are Sn). Passivation of the (001) facets with Sn would be consistent with such a dramatic effect on morphology by incorporation of only a small amount of Sn cations. This is also consistent with prior literature reports of strong passivation of specific facets of transition metal chalcogenide NCs by TOP, guiding the growth of NCs of specific nonspherical shapes.49−51 Synthesis of Monodisperse CTSe NCs with Tunable Size and Composition Using OAm/NaBH4-Se Precursor (method 2). Previous studies have demonstrated synthesis of high-quality CTSe NCs using ditert-butyl diselenide37 and diphenyl diselenide38 as selenium precursors. CTSe NCs of various morphologies can be produced by adjusting concenC

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Figure 2. AFM analysis of CTSe nanosheets produced at 300 °C (left) and 305 °C (right). Insets are at higher magnification. Topological profiles (bottom panels) taken along the solid lines drawn on the inset images show typical thicknesses of 14 nm (left, small sheet) and 12 nm (right, large sheet).

trations of those Se precursors and modifying the reactivity of the copper and tin precursors. However, monodisperse CTSe NCs of controlled and varied composition have rarely been produced using that strategy. OAm/NaBH4−Se is a highly reactive Se precursor, which was developed by Ying’s group for synthesis of CdSe.52 On the basis of its high reactivity, we can expect it to increase the amount of Sn incorporation relative to a lower-reactivity precursor such as TOP-Se. Here we modified the previous protocol for preparing OAm/NaBH4-Se and use it for the first time to synthesize monodisperse CTSe NCs with a wide range of Sn incorporation. CTSe alloy NCs with relatively uniform size distribution and quasi-spherical morphology were synthesized by hot injection of the OAm/NaBH4-Se precursor at 180 °C, followed by aging at a lower temperature, near 150 °C. Compared with TOP-Se, OAm/NaBH4-Se is more reactive with the Cu and Sn precursors and generated a high density of crystal nuclei. TEM imaging (Figure 3) showed the spherical morphology, size, and monodispersity of CTSe NCs prepared using this method. CTSe NCs synthesized using Cu:Sn precursor ratios of 2:1, 1:1, 1:2, and 1:5 produced NCs with average diameters of 15.8, 8.0, 4.7, and 3.6 nm, respectively. The size of CTSe NCs continuously decreased with increasing amount of tin precursor used (Figure 4A). EDS was used to quantify the elemental composition of CTSe prepared using different Cu:Sn ratios. We found a critical minimum Sn precursor content was required to observe incorporation of Sn into the NCs. For Cu:Sn precursor ratios above 2:1, tin content in the NCs was minimal (Table 1). Thus, when the organo-Sn concentration was low enough, copper selenide nucleated and grew rapidly without incorporating Sn. XRD of NCs produces with a 5:1 Cu:Sn precursor ratio showed the pure orthorhombic copper selenide crystal structure (see the Supporting Information, Figure S4A). For Cu:Sn ratios of 2:1 or lower, the Sn content increased linearly with increasing Sn fraction in the precursors (Figure 4B). Powder X-ray diffraction (XRD) patterns (Figure 4C) showed that the CTSe NCs synthesized by method 2 with Cu:Sn precursor ratios of 2:1 to 1:5 adopted a cubic crystal phase corresponding to PDF card No. 01−089−2879 for copper tin selenide. The diffraction peaks shifted to lower angles with increasing Sn content. This is

Figure 3. TEM images of CTSe alloy NCs, produced by method 2, with sizes of (A, B) 15.8 ± 1.2 nm, (C, D) 8.0 ± 1.4 nm, (E, F) 4.7 ± 0.8 nm, and (G, H) 3.6 ± 0.5 nm synthesized using Cu:Sn precursor molar ratios of 2:1, 1:1, 1:2, and 1:5, respectively. Size distributions are shown in the Supporting Information, Figure S3.

easily understood on the basis of the fact that the ionic radius of Sn is larger than that of Cu, so increasing Sn content increases the lattice constant. Synthesis of Monodisperse CTSe NCs with Tunable Size and Composition Using DDT/OAm-Se Precursor (Method 3). Dodecanethiol/oleylamine (DDT/OAm) mixtures have recently proven useful for dissolving selenium powder to produce Se precursors for semiconductor nanocrystal syntheses.53 To extend this reaction model to synthesize CTSe NCs of tunable composition, we tested the feasibility of using DDT/OAm-Se as a reactive selenium source for preparing size tunable CTSe NCs. DDT/OAm-Se was prepared by dissolving selenium powder in a 1/1 v/v mixture D

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Figure 4. Dependence of (A) particle size and (B) Sn content, from EDS analysis, on the Sn precursor fraction. (C) Powder XRD pattern of CTSe NCs produced by method 2, using Cu:Sn precursor ratios, from top to bottom, of 1:5 (green), 1:2 (blue), 1:1 (red), and 2:1 (black).

Table 1. EDS Analysis of CTSe NCs Produced by Method 2 EDS analysis (mean atom percent) precursor ratio (Cu:Sn)

Cu

Sn

Se

alloy NC stoichiometry CuxSnySe

5:1 2:1 1:1 1:2 1:5

53.4 52.4 44.1 28.5 20.1

0.3 1.5 9.7 25.1 33.9

46.3 46.1 46.2 46.4 46.0

Cu1.15Sn0.006Se Cu1.14Sn0.03Se Cu0.95Sn0.21Se Cu0.61Sn0.54Se Cu0.44Sn0.74Se

of DDT/OAm at room temperature. High-quality CTSe NCs were obtained for injection temperatures of 230 °C or higher. TEM images (Figure 5) show that this method produced CTSe NCs with mean size varying from 16.9 to 3.4 nm. Particles size decreased with increasing Sn precursor fraction (Figure 6A), similar to our observations for method 2. EDS analysis was used to determine the composition of the alloy nanocrystals (Table 2). A small amount of S (∼2%) was detected for all of these samples. Thus, DDT is serving here as a sulfur source to produce CTSSe NCs with low S content. The Se content in alloy NCs also remained nearly constant. The Sn content in the NCs increased almost linearly with increasing Sn content in the precursors, with the Sn cation fraction approaching 50% when a 1:1 ratio of SnCl2 to CuCl was used for the synthesis. However, when the SnCl2 precursor fraction was increased further, the Sn incorporation did not increase proportionally (Figure 6B). Figure 6C compares EDS analysis of Sn content in alloy NCs prepared by method 2 and method 3. For Cu:Sn precursor molar ratios of 1:2 or higher, the Sn content in the NCs was higher for method 3 than for method 2, and thus in this range the DDT/OAm-Se precursor favors Sn incorporation to an even greater extent than the OAm/NaBH4−Se precursor. The reactivity of organo-Sn precursor relative to the organo-Cu precursor was higher for DDT/OAm-Se than for OAm/ NaBH4-Se for Cu:Sn precursor ratios of 1:2 or higher. However, when a Cu:Sn molar ratio less than 1:2 to 1:4 was employed in method 3, large orthorhombic tin selenide crystals were formed (see the Supporting Information, Figure S6B, D). Thus, when using the DDT/OAm-Se precursor as Se donor, the range of precursor ratios for producing alloy NCs is limited by segregation into two different phases (small copper-rich NCs and large orthorhombic Sn-rich NCs). In contrast, using OAm/NaBH4-Se at a Cu:Sn molar ratio of 1:5 still produced further incorporation of Sn into the CTSe NCs, reaching a Sn cation fraction above 60%. Thus, OAm/NaBH4-Se provided the ability to achieve higher Sn cation fraction than the DDT/

Figure 5. TEM images of CTSe alloy NCs with size of (A, B) 16.9 ± 1.6 nm, (C, D) 8.1 ± 1.1 nm, (E, F) 5.0 ± 0.7 nm, and (G, H) 3.4 ± 0.4 nm synthesized by method 3 using Cu:Sn precursor ratios of 4:1, 2:1, 1:1, and 1:2, respectively. Size distributions are shown in the Supporting Information, Figure S3.

E

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Figure 6. (A) Dependence of particle size on Sn precursor fraction used in method 3. (B) EDS analysis showing the increase in Sn content in CTSSe NCs with increasing Sn precursor fraction. (C) Comparison of Sn content in NCs vs Sn content in precursors for method 2 and method 3. (D) Powder XRD pattern of CTSe NCs produced by method 3, using Cu:Sn precursor ratios, from top to bottom, of 1:2 (yellow), 1:1 (red), 2:1 (green), and 4:1 (blue).

Table 2. EDS Analysis of CTSSe NCs Produced by Method 3 EDS analysis (mean atom percent) precursor ratio (Cu:Sn)

Cu

Sn

Se

S

alloy NC stoichiometry CuxSnySezS1−z

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

39.9 33.6 27.0 23.6

12.5 18.5 25.6 28.2

45.9 46.3 45.6 46.1

1.7 1.6 1.8 2.1

Cu0.84Sn0.26Se0.96S0.04 Cu0.70Sn0.39Se0.97S0.03 Cu0.57Sn0.54Se0.96S0.04 Cu0.49Sn0.59Se0.96S0.04

OAm-Se precursor. Overall, we show that the Sn cation fraction can be tuned from 3 to 63% using method 2. Note that both precursors allowed incorporation of Sn at fractions greater than the 2:1 Cu:Sn ratio found in the stoichiometric Cu2SnSe3 compound. Powder XRD (Figure 6D) showed that all alloy NCs prepared by method 3 had the cubic copper tin selenide crystal structure, as also found for method 2. Again, the diffraction peaks ((111), (220), and (311) planes) shifted to lower angles with increasing Sn content because of the larger radius of Sn (Figure 6D). Synthesis of CTSe NCs with Tetrahedral Morphology (Method 4). Choices of reactants and synthesis temperature significantly affect the NC morphology by changing the relative stability and reactivity of different crystal facets. By using the TOP/OA-Se precursor and varying the temperature, tetrahedral CTSe NCs, which have not previously been reported, were obtained. The synthesis method (method 4) was identical to method 1, which produced hexagonal nanosheets, except that OA was added to the selenium precursor and the reaction temperature was reduced. The selenium precursor was prepared by dissolving Se powder in TOP, then adding OA. TEM imaging (Figure 7A, B) showed that most of the CTSe NCs were triangular in 2D projection. Although these could, in

Figure 7. (A) TEM and (B) HRTEM images of CTSe NCs synthesized using TOP/OA-Se as selenium precursor at 265 °C. The inset in A shows a ball-and-stick model of a tetrahedral CTSe NC sitting on its base, viewed from above. (C) Powder XRD pattern and EDS analysis of NCs produced by method 4.

principle, be triangular plates, we believe they are in fact tetrahedral nanocrystals that appear triangular in projection. F

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Chemistry of Materials Comparing TEM images of CTSe NCs synthesized using method 1 and 4, the morphology changed from large hexagonal nanosheets to much smaller tetrahedral NCs at the same Cu:Sn precursor ratios. EDS analysis showed that only a small amount of Sn was incorporated into the NCs (Figure 7C, inset). Although the Cu:Sn ratio in the precursors was only 2:1, in the NCs, it was nearly 20:1. Powder XRD (Figure 7C) shows that the CTSe alloy NCs had cubic crystal symmetry, in contrast to the hexagonal phase observed for the hexagonal nanosheets formed in the absence of OA. The observed peaks corresponded to (111), (220), (311), and (400) planes of the cubic phase. The triangular shape, in 2D projection and the cubic crystal structure together strongly suggest that these are tetrahedral NCs capped by (111) planes. Reducing the amount of TOP in the selenium precursor and adding OA allowed reaction to occur at much lower temperature than with only TOP in the Se precursor. The change in both reaction temperature and capping ligands allowed stabilization of a different crystal phase and NC morphology. We have observed similar changes in crystal phase, from hexagonal wurtzite to tetragonal kesterite, in copper tin sulfide NCs in previous studies, producing different crystal structures with different combinations of surface ligands at the same level of tin incorporation.35 In general, changing the mixture of ligands as well as the precursor activity, by changing reaction temperature, can induce changes in crystal phase in systems that exhibit polymorphism. The best known example of this is perhaps the growth of CdSe tetrapods with a zincblende core and wurtzite arms using mixtures of TOPO and hexylphosphonic acid (HPA) ligands.54 In copper chalcogenides, several crystal phases are possible, making the situation more complex. Here, these factors interact with the effect of the level of Sn incorporation to determine the crystal phase that ultimately nucleates and grows, but the detailed mechanism by which this occurs remains uncertain. Roles of OA-Se and OAm-Se Precursors in the Synthesis. OAm-Se and OA-Se were also studied for synthesis of CTSe NCs. Here, OA and OAm were employed as coordinating solvents to dissolve Se powder. For these experiments, all synthesis parameters were held constant except the amount of Sn precursor (i.e., the Cu precursor concentration was kept constant). Figure 8A, B shows representative TEM images of NCs obtained using the OASe precursor with the organo-copper precursor in the absence of Sn. Disklike NCs, as evident from their stacking in columns, with narrow size distributions were obtained. When the synthesis was repeated using 0.5 mmol of SnCl2 along with 1.0 mmol of CuCl, the NC morphology changed slightly, but the NCs were still basically disklike (Figure 8C, D). When the amount of SnCl2 was increased to 1 mmol, the morphology changed from disklike to tetrahedral (Figure 8E, F). The presence of highly concentrated Sn2+ in solution may influence the growth of CuSe NCs by passivating particular crystal facets. Passivation of the surface of semiconductor NCs by atomic ligands and metal chalcogenide organometallics both demonstrate the high affinity and interaction between charged metal ions or chalcogenide atoms and the chalcogenide or metal terminated surface of colloidal NCs.55 Here, we suggest that Sn2+ may bind with Se2−-terminated surfaces and confine the growth of the nanocrystals, by being unreactive with subsequent Se precursors in solution. More detailed studies of the mechanisms of anisotropic growth of NCs in the presence of metal halide precursors are a promising area for

Figure 8. TEM images of copper selenide and copper tin selenide NCs produced using OA-Se. (A, B) without SnCl2, (C, D) with 1 mmol CuCl, 0.5 mmol SnCl2, and (E, F) with 1 mmol CuCl, 1 mmol SnCl2.

further study. EDS analysis showed that very little Sn was incorporated in these NCs prepared using OA-Se. For the Cu:Sn precursor ratio of 2:1 Sn was not detected by EDS, and even for the Cu:Sn ratio of 1:1, the Cu:Sn ratio in the NCs was ∼50:1 (Table 3). The Se content remained approximately Table 3. EDS analysis of NCs prepared using OA-Se EDS Analysis (mean atom percent)

a

Cu precursor/Sn precursor (mmol)

Cu

Sna

Se

1/0 1/0.5 1/1

65.3 65.0 64.6

ND ND 1.4

34.7 35.0 34.0

ND = not detected by EDS.

constant with Sn addition to the precursors. Figure 9 shows the XRD patterns of the NCs prepared with and without Sn precursor. They were consistent with PDF Card No. 04−007− 1966 for cubic berzelianite. The XRD peaks shifted to slightly lower diffraction angle with Sn incorporation, reflecting the larger radius of Sn compared with Cu (Figure 9B−D). We also tested the use of OAm-Se as the Se source under the same conditions. Figure 10 shows TEM images of NCs produced using OAm-Se without (A) and with (B) addition of Sn precursor. No obvious morphology change was observed upon increasing the amount of SnCl2 (Figure 10). The crystal G

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strating a new pathway for morphology-controlled synthesis of semiconductor nanocrystals. Comparison of Results using Different Se Precursors. Considering all of the different Se precursors studied here, we see that the nature of the Se precursor affects both the ability to incorporate Sn and the crystal structure and overall morphology of the NCs. We also see that high-quality ternary plasmonic NCs can be produced using more than one phosphine-free precursor chemistry, i.e., using DDT/OAm-Se or OAm/NaBH4-Se. For TOP-Se, OAm-Se, and OA-Se, the reactivity of the Se precursor with copper appears to be much higher than its reactivity with tin, and thus limited Sn incorporation is observed. In each of these cases, the same molecule (TOP, OAm, or OA) serves both as a coordinating solvent to dissolve the Se and as a ligand that passivates the surface of the growing NCs. Differences in the affinities of these molecules for specific crystal facets of the NCs can induce changes in both overall morphology and the crystal phase of the product nanocrystals, from the thin plates with hexagonal crystal symmetry produced using TOP-Se to disklike, quasispherical, and tetrahedral NCs with cubic crystal symmetry produced using OA-Se and OAm-Se. Both the DDT/OAm-Se and the OAm/NaBH4-Se precursors allowed incorporation of substantial amounts of Sn into the NCs, showing that their reactivity with copper and tin precursors was more nearly equal. Although the Cu:Sn ratio in the product NCs remained higher than the Cu:Sn ratio in the precursor mixture in most cases, indicating somewhat higher reactivity toward Cu compared to Sn, these precursors allowed substantial Sn incorporation. DDT/OAm-Se exhibited the highest relative reactivity toward tin. However, this high reactivity eventually led to nucleation of separate tin selenide NCs, limiting the maximum amount of Sn that could be incorporated into alloy NCs. Although the relative reactivity of NaBH4/Se toward Sn was somewhat lower, as reflected by lower Sn incorporation for the same Cu:Sn precursor ratios, compared to DDT/OAm-Se, ultimately, OAm/NaBH4-Se provided the highest level of Sn incorporation observed in this study. Its more moderate reactivity toward Sn prevented nucleation of separate tin selenide NCs, and thus it could be used with much lower Cu:Sn ratios (higher Sn:Cu ratios) in the precursor mixture. Optical Properties of Colloidal CTSe NCs. Copper chalcogenide-based NCs have attracted much interest in recent years because of their potential to have high concentrations of cation vacancies, which produces high concentrations of free holes.56−58 The high free carrier concentration produces localized surface plasmon resonance (LSPR) at near-infrared wavelengths in these materials, analogous to the visible LSPR observed in noble metal NCs. Several methods of tuning the LSPR in these plasmonic semiconductor NCs have been reported, including tuning the aspect ratio of anisotropic nanostructures,59−61 cation exchange,62 stepwise oxidizationreduction,63,64 and manipulating the surface ligands40,65 of the NCs. These NCs also exhibit excitonic absorbance via band-toband transitions across their fundamental band gap at visible wavelengths. The presence of both plasmonic and excitonic absorbance, both of which can depend upon size, shape, and composition, provides interesting opportunities to tailor optical properties. Here, we first compare the optical absorbance of small and large nanosheets. The excitonic, band-to-band transition at visible wavelengths is somewhat red-shifted in the large nanosheets compared to the smaller ones (Figure 11C). Both

Figure 9. (A) XRD patterns of NCs synthesized using different amounts of tin precursor with OA-Se (copper precursor amount held constant): 0 mmol SnCl2 (black curve), 1 mmol SnCl2 (red curve). (B−D) show the shift of XRD peaks corresponding to the (1,1,1), (2,2,0), and (3,1,1) crystal planes with increasing Sn content.

Figure 10. TEM images of NCs produced using OAm-Se: (A) 1 mmol CuCl, 0 mmol SnCl2, and (B) 1 mmol CuCl, 1 mmol SnCl2.

structure and composition also remained unchanged when Sn precursor was added (Supporting Information, Figure S7 and Table S1). Sn incorporation was not observed using the OAmSe precursor. Thus, neither OA-Se nor OAm-Se was an effective precursor for producing CTSe alloy NCs with high Sn content, although some Sn doping was possible using OA-Se. In addition, when using the OA-Se precursor, Sn addition strongly affected the NC morphology, suggesting that use of metal halides can be an interesting means of controlling NC shape, even when the metal cation is not incorporated into the NCs. Similar methods were used to passivate the surface of semiconductor nanocrystals in prior studies, demonstrating the chemical interaction between metal halides and the surface of chalcogenide semiconductor nanocrystals.55 In the present study, the SnCl2 precursor is present during the synthesis of Cu2−xSe nanocrystals, in contrast to previous reports in which CdCl2 was added at the end of the synthesis of PbS NCs, as a source of halogen atoms. The results show obvious evolution of morphology with changing concentration of SnCl2, demonH

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Figure 11. Absorbance spectra of (A) CTSe alloy NCs prepared using OAm/NaBH4−Se; (B) CTSe alloy NCs prepared using DDT/OAm-Se; (C) CTSe nanosheets prepared using TOP-Se; (D) NCs prepared using OAm-Se and OA-Se.

exhibit a very broad LSPR absorbance that extends past 2500 nm. They show some slight structure in their absorbance spectrum at near-IR wavelengths that might be attributed to a weak out-of-plane LSPR, whereas the broad absorbance is attributed to the dominant in-plane LSPR mode of the nanosheets. For the CTSe NCs produced by method 2 and 3, the LSPR energy and intensity could be tuned by controlling the Cu:Sn ratio. Figure 11A shows the evolution of LSPR absorbance of CTSe NCs synthesized using OAm/NaBH4-Se precursor. A strong LSPR absorbance peak was observed at 1235 nm for NCs prepared using a 2:1 Cu:Sn precursor ratio. With increasing Sn content (produced by decreasing Cu:Sn precursor ratio), the LSPR peak was red-shifted by 205 to 1440 nm for a 1:1 Cu:Sn precursor ratio. The LSPR was further dampened and red-shifted to 1700 nm for a 1:2 Cu:Sn ratio and vanished for the NCs prepared using a 1:5 Cu:Sn ratio. The dampening of LSPR is attributed to the dramatic decrease in the concentration of free holes as the cation vacancies were partially filled by Sn ions in the NCs. Similar phenomena were observed in CTSe NCs produced using DDT/OAm-Se as the selenium precursor (Figure 11B). The copper-rich alloy CTSe NCs prepared using a 4:1 Cu:Sn precursor ratio showed LSPR absorbance peaking at 1150 nm. Upon decreasing the Cu:Sn precursor ratio to 2:1, the LSPR peak red-shifted by 250 nm, to 1400 nm. The LSPR peak was significantly dampened in NCs prepared at 1:1 Cu:Sn precursor ratio. Hence both methods provided a means of synthesizing alloy NCs of tunable composition and LSPR energy. Figure 11D shows the LSPR absorbance spectra of NCs produced using OAm-Se or OA-Se. A strong plasmonic peak at 990 nm was observed for NCs prepared using OAm-Se. When OA-Se was used as selenium precursor, the LSPR peak red-shifted to 1080 nm. This red-shift

can be attributed to deprotonated carboxyl groups of OA interacting with the NCs, where they may trap free holes at the NC surface and thereby reduce the effective free carrier concentration.40 For the tetrahedral NCs, the LSPR peaked around 1200 nm. This further red-shift may arise from the Sn ions reducing the concentration of free holes.

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CONCLUSION

In summary, we demonstrated facile methods for synthesizing 2D CTSe nanosheets, tetrahedral CTSe NCs and monodisperse quasi-spherical CTSe alloy NCs with tunable composition and size. We showed that the widely used TOPSe precursor generally induced the formation of large CTSe nanosheets with low Sn content, when used in combination with CuCl and SnCl2. In contrast to previous methods for preparing monodisperse colloidal CTSe NCs, the Cu:Sn ratio in the final products could be broadly varied by varying the Cu:Sn precursor ratio when using appropriate selenium precursors. The Cu:Sn ratio in the precursors also plays an important role in determining the size of the NCs, along with the reactivity of Se precursors and the presence of ligands. The OAm-Se and OA-Se precursors had low reactivity with SnCl2 compared with CuCl, and were not suitable for producing CTSe NCs with high Sn content. However, tin addition altered the morphology of NCs produced using OA-Se, suggesting a possible new route for tuning NC morphology. Alloying Cu2−xSe NCs with Sn allows broad tuning of the LSPR energy, which implies broad tuning of the free carrier concentration in these NCs. I

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(16) Shavel, A.; Cadavid, D.; Ibanez, M.; Carrete, A.; Cabot, A. J. Am. Chem. Soc. 2012, 134, 1438. (17) Singh, A.; Geaney, H.; Laffir, F.; Ryan, K. M. J. Am. Chem. Soc. 2012, 134, 2910−2913. (18) Ghorpade, U.; Suryawanshi, M.; Shin, S. W.; Gurav, K.; Patil, P.; Pawar, S.; Hong, C. W.; Kim, J. H.; Kolekar, S. Chem. Commun. 2014, 50, 11258−11273. (19) Riha, S. C.; Parkinson, B. A.; Prieto, A. L. J. Am. Chem. Soc. 2011, 133, 15272−15275. (20) Shavel, A.; Arbiol, J.; Cabot, A. J. Am. Chem. Soc. 2010, 132, 4514−4515. (21) Zhong, J.; Xia, Z.; Zhang, C.; Li, B.; Liu, X.; Cheng, Y. B.; Tang, J. Chem. Mater. 2014, 26, 3573−3578. (22) Lesnyak, V.; George, C.; Genovese, A.; Prato, M.; Casu, A.; Ayyappan, S.; Scarpellini, A.; Manna, L. ACS Nano 2014, 8, 8407− 8418. (23) Chesman, A. S. R.; van Embden, J.; Gaspera, E. D.; Duffy, N. W.; Webster, N. A. S.; Jasieniak, J. J. Chem. Mater. 2014, 26, 5482− 5491. (24) Ford, G. M.; Guo, Q.; Agrawal, R.; Hillhouse, H. W. Chem. Mater. 2011, 23, 2626−2629. (25) Zeier, W. G.; LaLonde, A.; Gibbs, Z. M.; Heinrich, C. P.; Panthofer, M.; Snyder, G. J.; Tremel, W. J. Am. Chem. Soc. 2012, 134, 7147−7154. (26) Shi, L.; Yin, P. Dalton Trans. 2013, 42, 13607−13611. (27) Wu, X. J.; Huang, X.; Qi, X.; Li, H.; Li, B.; Zhang, H. Angew. Chem. 2014, 126, 9075−9079. (28) Ahmadi, M.; Pramana, S. S.; Batabyal, S. K.; Boothroyd, C.; Mhaisalkar, S. G.; Lam, Y. M. Inorg. Chem. 2013, 52, 1722−1728. (29) Wang, X.; Swihart, M. T. Chem. Mater. 2015, 27, 1786−1791. (30) Wang, X.; Ke, Y.; Pan, H.; Ma, K.; Xiao, Q.; Yin, D.; Wu, G.; Swihart, M. ACS Catal. 2015, 5, 2534−2540. (31) Xiao, G.; Ning, J.; Liu, Z.; Sui, Y.; Wang, Y.; Dong, Q.; Tian, W.; Liu, B.; Zou, G.; Zou, B. CrystEngComm 2012, 14, 2139−2144. (32) Xiao, G.; Zeng, Y.; Jiang, Y.; Ning, J.; Zheng, W.; Liu, B.; Chen, X.; Zou, G.; Zou, B. Small 2013, 9, 793−799. (33) Ning, J.; Xiao, G.; Jiang, T.; Wang, L.; Dai, Q.; Zou, B.; Liu, B.; Wei, Y.; Chen, G.; Chen, G.; Zou, G. CrystEngComm 2011, 13, 4161− 4166. (34) Yi, L.; Wang, D.; Gao, M. CrystEngComm 2012, 14, 401−404. (35) Liu, X.; Wang, X.; Swihart, M. T. Chem. Mater. 2015, 27, 1342− 1348. (36) Ibáñez, M.; Zamani, R.; Li, W.; Cadavid, D.; Gorsse, S.; Katcho, N. A.; Shavel, A.; López, A. M.; Morante, J. R.; Arbiol, J.; Cabot, A. Chem. Mater. 2012, 24, 4615−4622. (37) Norako, M. E.; Greaney, M. J.; Brutchey, R. L. J. Am. Chem. Soc. 2012, 134, 23−26. (38) Wang, J. J.; Liu, P.; Seaton, C. C.; Ryan, K. M. J. Am. Chem. Soc. 2014, 136, 7954−7960. (39) Zou, Y.; Su, X.; Jiang, J. J. Am. Chem. Soc. 2013, 135, 18377− 18384. (40) Liu, X.; Wang, X. L.; Zhou, B.; Law, W. C.; Cartwright, A. N.; Swihart, M. T. Adv. Funct. Mater. 2013, 23, 1256−1264. (41) Kang, J.; Seo, J. W.; Alducin, D.; Ponce, A.; Yacaman, M. J.; Hersam, M. C. Nat. Commun. 2014, 5, 5478. (42) Huang, X.; Tan, C.; Yin, Z.; Zhang, H. Adv. Mater. 2014, 26, 2185−2204. (43) Schliehe, C.; Juarez, B. H.; Pelletier, M.; Jander, S.; Greshnykh, D.; Nagel, M.; Meyer, A.; Foerster, S.; Kornowski, A.; Klinke, C.; Weller, H. Science 2010, 329, 550−553. (44) Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A. Nat. Nanotechnol. 2011, 6, 147−150. (45) Zeng, Z.; Tan, C.; Huang, X.; Bao, S.; Zhang, H. Energy Environ. Sci. 2014, 7, 797−803. (46) Ithurria, S.; Tessier, M. D.; Mahler, B.; Lobo, R.; Dubertret, B.; Efros, A. L. Nat. Mater. 2011, 10, 936−941. (47) Du, Y.; Yin, Z.; Zhu, J.; Huang, X.; Wu, X. J.; Zeng, Z.; Yan, Q.; Zhang, H. Nat. Commun. 2012, 3, 1177.

ASSOCIATED CONTENT

S Supporting Information *

Additional TEM images, size distributions, XRD, and EDS analysis of copper selenide and copper tin selenide alloy NCs corresponding to the different reaction methods. 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. Present Address ‡

X.L. is currently at Lawrence Berkeley National Laboratory, One Cyclotron Road Berkeley, CA 94720, USA. Author Contributions

† X.W. and X.L. contributed equally. The manuscript was written through contributions of all authors.

Funding

This work was supported, in part, by the New York State Center of Excellence in Materials Informatics. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Professor Marina Tsianou and Mr. Kun Yu for help with AFM imaging and for valuable discussions of the results. REFERENCES

(1) Cao, Y.; Denny, M. S.; Caspar, J. V.; Farneth, W. E.; Guo, Q.; Ionkin, A. S.; Johnson, L. K.; Lu, M.; Malajovich, I.; Radu, D.; Rosenfeld, H. D.; Choudhury, K. R.; Wu, W. J. Am. Chem. Soc. 2012, 134, 15644−15647. (2) Carrete, A.; Shavel, A.; Fontané, X.; Montserrat, J.; Fan, J.; Ibáñez, M.; Saucedo, E.; Pérez-Rodríguez, A.; Cabot, A. J. Am. Chem. Soc. 2013, 135, 15982−15985. (3) Fan, F. J.; Wang, Y. X.; Liu, X. J.; Wu, L.; Yu, S. H. Adv. Mater. 2012, 24, 6158−6163. (4) Ibanez, M.; Zamani, R.; LaLonde, A.; Cadavid, D.; Li, W.; Shavel, A.; Arbiol, J.; Morante, J. R.; Gorsse, S.; Snyder, G. J.; Cabot, A. J. Am. Chem. Soc. 2012, 134, 4060−4063. (5) Jiang, C.; Lee, J.-S. S.; Talapin, D. V. J. Am. Chem. Soc. 2012, 134, 5010−5013. (6) Steinhagen, C.; Panthani, M. G.; Akhavan, V.; Goodfellow, B.; Koo, B.; Korgel, B. A. J. Am. Chem. Soc. 2009, 131, 12554−12555. (7) Tan, J. M.; Lee, Y. H.; Pedireddy, S.; Baikie, T.; Ling, X. Y.; Wong, L. H. J. Am. Chem. Soc. 2014, 136, 6684−6692. (8) Yang, C.; Zhou, B.; Miao, S.; Yang, C.; Cai, B.; Zhang, W.-H. H.; Xu, X. J. Am. Chem. Soc. 2013, 135, 5958−5961. (9) Yang, H.; Jauregui, L. A.; Zhang, G.; Chen, Y. P.; Wu, Y. Nano Lett. 2012, 12, 540−545. (10) Yang, W.; Duan, H.-S. S.; Cha, K. C.; Hsu, C.-J. J.; Hsu, W.-C. C.; Zhou, H.; Bob, B.; Yang, Y. J. Am. Chem. Soc. 2013, 135, 6915− 6920. (11) Zhou, H.; Song, T. B.; Hsu, W. C.; Luo, S.; Ye, S.; Duan, H. S.; Hsu, C. J.; Yang, W.; Yang, Y. J. Am. Chem. Soc. 2013, 135, 15998− 16001. (12) van Embden, J.; Chesman, A. S.; Della Gaspera, E.; Duffy, N. W.; Watkins, S. E.; Jasieniak, J. J. J. Am. Chem. Soc. 2014, 136, 5237− 5240. (13) Guo, Q.; Hillhouse, H. W.; Agrawal, R. J. Am. Chem. Soc. 2009, 131, 11672−11673. (14) Jiang, C.; Liu, W.; Talapin, D. V. Chem. Mater. 2014, 26, 4038− 4043. (15) Riha, S. C.; Parkinson, B. A.; Prieto, A. L. J. Am. Chem. Soc. 2009, 131, 12054−12055. J

DOI: 10.1021/acs.chemmater.5b00618 Chem. Mater. XXXX, XXX, XXX−XXX

Article

Chemistry of Materials (48) Wu, X. J.; Huang, X.; Liu, J.; Li, H.; Yang, J.; Li, B.; Huang, W.; Zhang, H. Angew. Chem., Int. Ed. 2014, 53, 5083−5087. (49) Liu, X.; Li, Y.; Zhou, B.; Wang, X.; Cartwright, A. N.; Swihart, M. T. Chem. Mater. 2014, 26, 3515−3521. (50) Vaughn, D. D., II; In, S.-I; Schaak, R. E. ACS Nano 2011, 5, 8852−8860. (51) Li, H.; Brescia, R.; Povia, M.; Prato, M.; Bertoni, G.; Manna, L.; Moreels, I. J. Am. Chem. Soc. 2013, 135, 12270−12278. (52) Wei, Y.; Yang, J.; Lin, A. W. H.; Ying, J. Y. Chem. Mater. 2010, 22, 5672−5677. (53) Liu, Y.; Yao, D.; Shen, L.; Zhang, H.; Zhang, X.; Yang, B. J. Am. Chem. Soc. 2012, 134, 7207−7210. (54) Manna, L.; Scher, E. C.; Alivisatos, A. P. J. Am. Chem. Soc. 2000, 122, 12700−12706. (55) Ip, A. H.; Thon, S. M.; Hoogland, S.; Voznyy, O.; Zhitomirsky, D.; Debnath, R.; Levina, L.; Rollny, L. R.; Carey, G. H.; Fischer, A.; Kemp, K. W.; Kramer, I. J.; Ning, Z.; Labelle, A. J.; Chou, K. W.; Amassian, A.; Sargent, E. H. Nat. Nanotechnol. 2012, 7, 577−582. (56) Comin, A.; Manna, L. Chem. Soc. Rev. 2014, 43, 3957−3975. (57) Liu, X.; Swihart, M. T. Chem. Soc. Rev. 2014, 43, 3908−3920. (58) Luther, J. M.; Jain, P. K.; Ewers, T.; Alivisatos, A. P. Nat. Mater. 2011, 10, 361−366. (59) Xie, Y.; Carbone, L.; Nobile, C.; Grillo, V.; D’Agostino, S.; Della Sala, F.; Giannini, C.; Altamura, D.; Oelsner, C.; Kryschi, C.; Cozzoli, P. D. ACS Nano 2013, 7, 7352−7369. (60) Hsu, S.-W. W.; On, K.; Tao, A. R. J. Am. Chem. Soc. 2011, 133, 19072−19075. (61) Li, W.; Zamani, R.; Rivera Gil, P.; Pelaz, B.; Ibáñez, M.; Cadavid, D.; Shavel, A.; Alvarez-Puebla, R. A.; Parak, W. J.; Arbiol, J.; Cabot, A. J. Am. Chem. Soc. 2013, 135, 7098−7101. (62) Wang, X.; Liu, X.; Zhu, D.; Swihart, M. T. Nanoscale 2014, 6, 8852−8857. (63) Dorfs, D.; Härtling, T.; Miszta, K.; Bigall, N. C.; Kim, M. R.; Genovese, A.; Falqui, A.; Povia, M.; Manna, L. J. Am. Chem. Soc. 2011, 133, 11175−11180. (64) Kriegel, I.; Jiang, C.; Rodriguez-Fernandez, J.; Schaller, R. D.; Talapin, D. V.; da Como, E.; Feldmann, J. J. Am. Chem. Soc. 2012, 134, 1583−1590. (65) Liu, X.; Wang, X.; Swihart, M. T. Chem. Mater. 2013, 25, 4402− 4408.

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