Controllable Transformation from Rhombohedral Cu1.8S Nanocrystals

Article pubs.acs.org/cm

Controllable Transformation from Rhombohedral Cu1.8S Nanocrystals to Hexagonal CuS Clusters: Phase- and CompositionDependent Plasmonic Properties Lige Liu,†,‡ Haizheng Zhong,*,† Zelong Bai,† Teng Zhang,†,§ Wenping Fu,† Lijie Shi,‡ Haiyan Xie,*,§ Luogen Deng,*,‡ and Bingsuo Zou‡ †

Beijing Key Laboratory of Nanophotonics and Ultrafine Optoelectronic Systems, School of Materials Science & Engineering, ‡School of Physics, §School of Life Science, Beijing Institute of Technology, 5 Zhongguancun South Street, Haidian District, Beijing 100081, China S Supporting Information *

ABSTRACT: Because of the rich polymorphs and lower diffusion energy barriers of copper chalcogenide systems, the phase transformation of colloidal Cu2−xS (0 ≤ x ≤ 1) nanocrystals is critical for understanding their fundamental properties and designing convenient synthetic routes. In this work, high quality digenite Cu1.8S nanocrystals with rhombohedral structure were synthesized at gram-scale. The as-prepared colloidal nanocrystals undergo an in situ phase transformation from rhombohedral Cu1.8S nanocrystals to hexagonal CuS clusters upon keeping the resulting colloidal solution for a few days. The observed transformation was explored by a combination of structural and spectroscopic analyses, including powder X-ray diffraction, transmission electron microscopy, energy dispersive spectroscopy, and X-ray photoelectron spectroscopy characterizations. A possible mechanism is proposed and thoroughly discussed. We further determined the evolution of plasmonic absorption spectra during the transformation. The Cu1.8S nanocrystals and CuS clusters exhibit composition-dependent local surface plasmon resonance absorption (LSPR) in the near-infrared region, which are in good agreement with calculated extinction spectra based on Mie-Drude model. Combined experimental and theoretical analyses demonstrated that both the phase induced dielectric constant change and the composition induced carrier concentration variation account for the spectroscopic evolution. KEYWORDS: colloidal nanocrystals, Cu1.8S, CuS, clusters, plasmonics, phase transformation

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INTRODUCTION Due to the localized surface plasmon resonances (LSPR) arising from copper deficiency, colloidal Cu2−xE (E = S, Se, Te, P) nanocrystals (NCs) exhibit well-defined plasmonic absorption in the near-infrared (NIR) region,1−7 which enable them to be potential candidates for many applications including solar cells,8 thermoelectronics,9 gas sensing,10 and photothermal therapy.3,11,12 To realize flexible manipulation of plasmonic frequency and further explore these applications, it has been of great interest to understand their size-, geometry-, phase-, and composition- dependence of LSPR related absorption properties in Cu2−xE NCs.1,13−17 Primary studies have demonstrated that the plasmonic absorption peak of colloidal Cu2−xS NCs is mainly determined by their copper deficiency induced self-doping.2 Because of the rich polymorphs and lower diffusion energy barriers of Cu2−xE systems, the copper deficiency of colloidal Cu2−xE NCs can be well tuned by controlling their crystal phase and composition through chemical- or thermal- induced transformation. This provides a good platform to study the influence of phase and composition on LSPR related physical properties. For example, Alivisatos’ group demonstrated the temperature-induced © 2013 American Chemical Society

structural transformations of Cu2S nanorods from a low to a high chalcocite structure and investigated their size dependence of phase transformation.18,19 Plass et al. reported the iron induced phase selective synthesis of Cu2S NCs and primarily studied their phase related absorption properties.20 In Kriegel’s recent work, a phase transformation of hexagonal Cu2Se NCs into cubic Cu1.8Se NCs was observed upon oxygen oxidation.21 Here, we report a controllable in situ transformation from rhombohedral Cu1.8S NCs to hexagonal CuS clusters as well as the corresponding phase- and compositiondependent plasmonic absorption properties. To our best knowledge, CuS clusters have not been reported in the previous works. Nonstoichiometric Cu2−xS compounds including djurleite Cu1.97S (x = 0.03),22 digenite Cu1.8S (x = 0.2),23,24 anilite Cu1.75S (x = 0.25),25,26 and covellite CuS (x = 1),27,28 have been the focus in the family of copper chalcogenides. Among them, Cu1.8S and CuS are p-type semiconductors with indirect bandgaps of 1.5−1.6 eV and 2.0−2.5 eV.29,30 Although Received: October 16, 2013 Revised: November 11, 2013 Published: November 14, 2013 4828

dx.doi.org/10.1021/cm403420u | Chem. Mater. 2013, 25, 4828−4834

Chemistry of Materials

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Characterizations. Powder X-ray diffraction (XRD) measurements were carried out at a Rigaku D/max 2500 PC diffractometer using a Cu Kα radiation source (wavelength at 1.5405 Å). Energy dispersive X-ray spectroscopy (EDS) was obtained using an energy dispersive X-ray spectrometer attached to the S-4800 scanning electron microscopy. A JEM-2100F transmission electron microscope (TEM) machine operating at an acceleration voltage of 200 kV was employed to determine the size and morphology of resulting NCs as well as finish the selected area electron diffraction (SAED) characterizations. X-ray photoelectron spectroscopy (XPS) measurements were performed at a ULVAC-PHI machine (PHIQUANTERAII SXM). The absorption spectra of NCs dissolved in TCE were determined at a Shimadzu UV−vis−NIR-3600 spectrophotometer. Fourier transform infrared spectroscopy (FTIR, TENSOR 27) was used for characterizing the ligands binding to NCs surface.

synthesis of colloidal Cu2−xS NCs has been widely explored, there is limited literature on the synthesis of high quality monodisperse Cu1.8S NCs and the preparation of CuS clusters has been rarely reported. Moreover, bulk Cu1.8S has three polymorphs, including high-temperature phase with cubic symmetry, which is stable above 73 ± 3 °C, low-temperature phase also with cubic symmetry but different lattice constants and metastable phase with rhomohedral symmetry obtained by cooling from 73 ± 3 °C.31 These three phases are close related to each other.32 In considering of this, phase identification of Cu1.8S NCs remains a challenge. Moreover, the relationship between carrier density and composition of Cu2−xE NCs varies with the structural phase.6 The LSPR properties of heavily doped Cu1.8S NCs and CuS clusters with higher copper deficiency larger than 10% (x ≥ 0.2) were observed but rarely investigated.33−35 Experimental measurements and theoretical modeling of these heavily doped NCs are very necessary to gain further insight into the LSPR related properties. In this work, we developed a hot-injection method to prepare high quality colloidal Cu1.8S NCs with tunable size of 6 to 20 nm at gram scale. The as-prepared digenite Cu1.8S NCs have been well characterized and identified to have a metastable phase of rhomohedral structure. When the resulting colloidal NCs solution was kept at room temperature for a few days without washing, the digenite Cu1.8 S NCs undergo a composition evolution from 1.8 to 1.4 and then digested into hexagonal CuS clusters. We further monitored their LSPR related absorption spectra during above transformation process. The spectra evolution can be explained by applying the electrostatic approximation calculations.

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RESULTS AND DISCUSSION Synthesis and Characterization of Colloidal Cu1.8S NCs. High quality digenite Cu1.8S nanocrystals were synthesized through a typical hot-injection method using CuCl and S powder as precursors in the presence of OLA and OA as ligands and ODE as solvent. The procedure is described in the Experimental Section. By varying the reaction time and temperature, we synthesized a series of samples with average diameter ranging from ∼6 to ∼20 nm. Figure 1 presents the

EXPERIMENTAL SECTION

Materials. Sulfur (99.98%), oleylamine (OLA, 70%,), and oleic acid (OA, 90%) were purchased from Sigma-Aldrich. Copper(I) chloride (CuCl, 97%), 1-octadecene (ODE, 90%), and tetrachlorethylene (TCE, 99%) were purchased from Alfa Aesar. Toluene (A.R., >99.5%), acetone (A.R., >99.5%) and methanol (A.R., >99.5%) were purchased from Beijing Fine Chemical Company. All the chemicals were used as received. Synthesis of Cu1.8S Nanocrystals. Synthesis of Cu1.8S NCs was accomplished following a typical hot-injection method by injecting copper precursor into sulfur precursor. All the syntheses were carried out under air-free conditions using a standard Schlenk-line setup. A typical synthesis is as follow. Copper precursor was prepared by mixing CuCl (1g, 0.01 mol) with a mixture of 4 mL OLA and 5 mL OA at 130 °C under continuous mechanical stirring. Then, the as-prepared copper precursor was cooled to room temperature. Sulfur precursor was prepared in a three necked bottle by dissolving sulfur powder (0.32 g, 0.01 mol) into 40 mL ODE at 200 °C under mechanical stirring. Subsequently, the sulfur solution was then set to 180 °C, followed by a swift injection of copper precursor and kept at this temperature for 5 to 15 min, resulting in a black colloidal solution. After that, the heating mental was removed and cooled to room temperature. The obtained colloidal solution was then precipitated using excess acetone and recovered by centrifuging the suspension and discarding the supernatant. The precipitate was redispersed in organic solvent such as chloroform, toluene, or hexane. The precipitation and redispersion was repeated twice. The purified samples were redispersed in toluene or dried into powder for further characterizations. Phase Transformation. Phase transformation was accomplished by keeping the obtained colloidal Cu1.8S NCs solution for a few days. Aliquots S2 to S6 were taken at indicated times after keeping the colloidal solution for 1, 2, 3, 4, and 5 days. The aliquots were purified following the purification procedures in above description.

Figure 1. TEM images of Cu1.8S NCs with average diameters of (a) 6.6 ± 0.4 nm, (b) 8.0 ± 0.7 nm, (c) 14.6 ± 1.3 nm, and (d) 17.3 ± 2.2 nm. The red dashed line highlights the geometry of particles.

TEM images of some representative samples with average diameter of 6.6 nm, 8.0 nm, 14.6 nm, and 17.3 nm. It is noted that the resulting sample with average diameter of ∼6.6 nm are monodisperse spherical particles. With the diameter increased, some nonspherical particles with specific geometry (triangle and quadrilateral shapes) were observed among larger size particles (see Figure 1c and d). This can be explained as follows. The surface energy varies with crystal facets and results in different growth rate. The facets with fast growth rate diminished with NC growth, producing nonspherical NCs.36 In addition, our synthetic method is also suitable for large scale noninjection production. One batch of enlarged noninjection synthesis can produce over 6 g samples (see Supporting Information (SI) Figure S1). The elemental compositions of resulting samples were determined by EDS analysis (see Figure S2 in the Supporting Information). The atomic ratio of Cu and S is close to ∼1.8 for 4829



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all the samples, confirming the formation of Cu1.8S compounds. As mentioned above, bulk digenite Cu1.8S have three polymorphs. It has been learned that many nanoscale materials exhibit different phase diagram.37 The crystal structure of resulting NCs were studied using XRD measurements. Figure 2

Figure 3. (a−c) HRTEM images of the as-prepared Cu1.8S NCs with average diameter of 6.6 nm. (d−f) FFT patterns generated from the corresponding square in parts a−c. (g) Illustration of rhombohedral crystal structure with cell constants a = b = 3.93 Å and c = 48.14 Å.

well with the (0,0,15), (1,0,7), (1,0,10), and (0,1,17) lattices distance of rhombohedral Cu1.8S phase. The phase structure was further confirmed by analyzing angles between two intersection planes. The obtained intersection angle between (1,0,10) and (0,1,17) lattice fringes, and angle between (0,0,15) and (1,0,7) planes are 45° and 62°, respectively, which is consistent with the calculated values of 45.2° and 63.7°. The (1,0,10) plane has an inclination angle of 59° to (0,0,15) plane, which is close to the ideal value of 54.7°. Based on the above analysis, the as-produced Cu1.8S NCs are confirmed to have a rhombohedral structure (as shown in Figure 3g). The rhombohedral Cu1.8S phase can be considered as Cu-deficient hexagonal Cu2S phase, where every 1.0 Cu atom distributes statistically over four equivalent positions in each sulfur tetrahedron.39 Phase Transformation from Rhombohedral Cu1.8S NCs to Hexagonal CuS Clusters. Previous works have demonstrated that the oxidation and reduction reaction can control the composition of Cu2−xS NCs.21 Cu1.8S has a formula of [(Cu+)8Cu2+(S2−)5] with the atomic ratio of Cu(II):Cu(I) = 1:8.35 The FTIR spectra (see SI Figure S3) indicate that the surface of colloidal Cu1.8S NCs is capped with OLA as surface ligands. Therefore, OLA may act as a surfactant, solvent, and oxidation catalystic agent in the synthetic process of Cu1.8S NCs.40 Especially, OLA should play an important role in controlling Cu1.8S NC growth due to the well-known strong coordinated ability between OLA and copper ions.41 We first studied the influence of OLA on preformed colloidal Cu1.8S NCs in toluene under the atmosphere of open air. The results show that the purified Cu1.8S NCs capped with OLA are stable in the open air for more than 10 days (see SI Figure S5). However, the as-synthesized samples quickly degrade in the presence of excess OLA in the open air. This phenomenon has been also observed in CuInSe2 and CuInS2 NCs.42,43 Therefore, it is deduced that OLA has potential to control the composition of Cu2−xS NCs. Occasionally, we observed that the as-synthesized Cu1.8S NCs solutions undergo an in situ color transformation upon keeping the resulting colloidal solution for a few days before purification. The elemental and structural evolution during color transformation was monitored by analyzing the original sample (S1) and other samples that purified from the aliquots after aging for 1, 2, 3, 4, and 5 days (S2, S3, S4, S5, S6). Quantitative EDS results indicated that the composition of these samples gradually changed from Cu:S = 1.77 of S1 to 1.63 of S2, 1.43 of S3 and 1.14 of S4, respectively.44 Figure 4a shows the XRD patterns of S1−S6. It shows that S2 (aging for 1 day) and S3 (aging for and 2 days)

Figure 2. (a) XRD patterns of the as-synthesized NCs that synthesized at 110 °C, 150 °C, and 180 °C. The vertical bars indicate the standard diffraction peaks of rhombohedral and cubic Cu1.8S crystals. (b) The electron diffraction rings of the as-synthesized NCs with average diameter of 6.6 nm.

shows the XRD patterns of some typical samples. All the XRD patterns have four obvious diffraction peaks at 2θ degree of 27.8°, 32.2°, 46.2°, and 54.7°. Upon carefully analyzing the data and critically reviewing the previous database, these diffraction peaks can be either indexed to (0,0,15), (1,0,10), (0,1,20), and (1,1,15) planes of rhombohedral Cu1.8S with space group of R3m ̅ (JCPDS No. 47-1748) or assigned to (1,1,1), (2,0,0), (2,2,0), and (3,1,1) planes of cubic Cu1.8S with space group of Fm3̅m (JCPDS No. 24-0061). According to the literature, the rhombohedral phase and cubic phase of Cu1.8S are closely related.38 The cubic phase of digenite Cu1.8S phase has a facecentered cubic (fcc) structure (with a lattice parameter of a = 5.57 Å). The fcc structure can, in general, also be described by a rhombohedral (rh) unit cell (with a = b ≠ c, α = β = 90°, γ = 120°). Although the observation of a less intense diffraction peak at 2θ degree of 29.7 and 42.3 for larger sized samples supports the existence of rhombohedral phase, the coexistence of cubic Cu1.8S phase should be further clarified. Electron diffraction (ED) performed on a TEM instrument was further applied to determine the crystal structure of resulting NCs. Figure 2b shows a typical ED pattern of Cu1.8S NCs with average diameter of 6.6 nm. The type of crystal structure and the “lattice parameters” can be determined by analyzing the observed diffraction rings. Five strong characteristic diffraction rings can be attributed to the (1,1,15), (0,1,20), (0,1,17), (1,0,10), as well as (0,0,15) lattice fringes from hkl Miller analysis. This confirmed that resulting Cu1.8S NCs with smaller size also have a rhombohedral phase. The formation of metastable rhombohedral phase is most likely due to the coordinate between OLA and copper ions, which need further study. Figure 3 shows three typical HRTEM images of Cu1.8S NCs with average diameter of 6.6 nm and their corresponding Fast Fourier transformation (FFT) patterns generated from the crystalline regions outlined by the squares (also see SI Figure S3). The HRTEM images indicated that Cu1.8S NCs are singlecrystalline with lattice plane spacing distances of ∼0.328, ∼0.310, ∼0.280, and ∼0.211 nm. These four distances match 4830



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Figure 4. (a) Powder XRD patterns of samples that obtained during phase transformation. The green bar represents the standard diffraction peaks of Cu1.8S powder belonging to JCPDS card no. 471748, and the black bar is the standard card no. 65-3561 of powder CuS. (b) Enlargement picture of diffraction peak of (0,1,20) plane of S1, S2, and S3.

Figure 5. TEM images of the aliquots during the transformation (a) S1, (b) S2, (c) S3, (d) S4, (e) S5, and (f) S6. (g,h) HRTEM of a typical hollow particle of S4. (i) HRTEM of a typical CuS cluster.

also exhibit the diffraction feature of rhombohedral phase. Compared with original Cu1.8S NCs, the diffraction peaks of S2 and S3 slightly shifted toward high 2θ angles with time prolonged, indicating a relative decrease of the lattice parameters, which was confirmed by the calculations (see Supporting Information). It is noted that the lattice parameter a reduced by only 0.005 Å, and the lattice constants c decreased from 48.15 Å (sample 1) to 46.87 Å (sample 3). This means that the crystals evolution with the transformation has a preferred direction along c axis. After the colloidal solution was kept for more than two days, a different phase was observed in the purified samples. By carefully analyzing the data, the diffraction peaks match well with hexagonal covellite CuS phase with P 63/mmc space group (JCPDS No.65-3561). The (102), (103), and (110) interplanar d-spacing were calculated to be 0.309 nm, 0.278 nm, and 0.190 nm. In addition, the diffraction peaks of hexagonal CuS samples are much broader in compassion to the original rhombohedral Cu1.8S NCs, implying a size decrease. Figure 5 presents the TEM images of the aliquots during the transformation and Figure 6 plotted the size and phase evolution of samples with the aging time prolonged. The average diameter of rhombohedral NCs first decreased and abruptly changed to CuS clusters. It is worth to note that the CuS samples have a small fraction of ring-shaped NCs along with the clusters (see Figure 5d−f and SI Figure S5). In addition, there are also many 2−4 nm clusters along with the ring-shaped particles. HRTEM images (Figure 5g, h, i) show that these ring-shaped particles are composed by small crushes and the CuS cluster also has a crystalline structure. Although, understanding the phase transformation process of NCs was hindered by difficulties in directly monitoring the nanoscale systems. The transformation from larger size Cu1.8S NCs to smaller CuS clusters is similar with the observed digestive ripening in the preparation of monodisperse gold NCs.45,46 Hence, the transformation from Cu1.8S NCs to CuS clusters may also experience an OLA assisted process. Cu1.8S can be written as the formula of [(Cu+)8Cu2+(S2−)5] with the atomic ratio of Cu(II):Cu(I) of 1:8, while the Cu(II):Cu(I) change to 1:2 of CuS with the formula of [Cu2+Cu2+(S2)2−(S2−)].47,48 Therefore, the transformation process should involve an oxidation assisted digestive process. The XPS spectra (see

Figure 6. Plot of the size and phase evolution of the samples during the transformation.

Figure S6 in the Suporrting Information) shows that the sulfion (S2−) ions on the NC surface are easily oxided into SO32− along with the transformation. To understand the role of OLA and O2 moleculars in the oxidation process, several controlled experiments were performed. No obvious phase transformation was observed when adding OLA into Cu1.8S NCs solutions without oxygen assistance or only exposing the purified Cu1.8S NCs in the open air for a few days (see Figure S7 in the Supporting Information). A fast degradation is observed when adding OLA into Cu1.8S NCs solutions in the open air. Based on the above results, the transformation mechanism is proposed as follow. OLA mainly controls the copper oxidation for their high chelating ability to copper ions, while O2 controls the oxidation of S2−. The oxidation of Cu+ and S2− corroded the sulfur and copper atoms on the particle’s surface and controls the transformation process. In addition, Cu2−xS is known for their “liquid-like” ionic mobility.49 More likely, the oxidation induced corrosion penetrate into the inner of big particles due to the presence of relative high concentration Cu(II) in the inner of big NCs, results in the morphology evolution from spherical shape to ring-shaped particles along 4831



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with the digestion into smaller CuS clusters when the copper ions reach a critical content. Phase- and Composition- Dependent LSPR Related Properties. The resulting Cu1.8S NCs undergo a composition evolution from Cu1.8S to Cu1.4S as well as a phase transition from rhombohedral phase to hexagonal CuS clusters. This provides a chance to explore the influences of phase transformation on the plasmonic behavior. Figure 7a shows

Table 1. Important Parameters for the Theoretical Calculationsa S1

S2

S3

rhombohedral Cu1.8S NCs Cu/S ratio ωsp (eV) ωp (eV) ε∞b Nh (× 1021 cm−3)b

1.77 1.06 2.83 2.6 4.7

1.63 1.11 2.91 2.3 4.98

1.43 1.25 3.19 2.0 5.99

S4

S5

S6

hexagonal CuS clusters 1.14 1.09 3.21 4.2 6.08

∼ 1.03 3.05 4.4 5.46

∼ 1.03 2.94 3.8 4.65

a

Free carrier densities were calculated from UV-vis-NIR spectrum according to Mie theory,2 and Cu and S atoms ratios were obtained from EDS spectra. While, the high frequency dielectric constants were derived from the best fitted spectra. bε∞ is high-frequency dielectric constant, and Nh represents free carrier density.

where e is the elemental charge, ε0 is the free space permittivity, mh is the hole effective mass, approximated to be 0.8. The hole carrier concentration of these samples is calculated to be 4.7−6.1 × 1021 cm−3, which is in consistent with the previous prediction.2 To understand the factors that govern the frequency evolution of plasmonic resonance during phase transformation, we compared the experimental spectra with theoretical extinction spectra that calculated based on electrostatic approximation modeling.15,53−55 Within the electrostatic approximation for spherical particles, the extinction cross section σext is given by

Figure 7. (a) Evolution of the plasmon absorption spectra of the samples that dissolved in TCE. (b) Calculated extinction cross section spectrum of single particle.

the evolution of absorption spectra during the transformation. It is noticed that the LSPR related absorption spectra exhibit an obvious transition with the transformation from rhombohedral Cu1.8S NCs to hexagonal CuS clusters. The plasmonic absorption band exhibits a blue-shift from 1180 nm (sample 1) to 990 nm (sample 3) for the samples with rhombohedral phase and then a red-shift for the samples with hexagonal structure. CuS is known to be a stoichiometric compound.50 According to Cozzoli’s recent report, the observed plasmonic absorption of CuS clusters in the near-infrared wavelength is attributed to the significant density of lattice-constitutional valence-band free holes.35 Moreover, CuS clusters have a broad absorption peak with a full width at half height of 1.1 eV, in comparison to 0.54 eV of Cu1.8S NCs. This can be explained to the broaden size and/or composition distribution of CuS clusters along with the enhanced surface scattering of carriers due to smaller size.2,35 The LSPR can be described by the interaction of free carriers on NC surface with external electric field. Based on the Mie− Drude theory performed on the Cu2−xE NCs,2,51,52 the relationship between LSPR absorption frequency ω with the bulk plasma oscillation frequency ωp arising from free carriers is described by the equation. ωsp =

ωp2 1 + 2εm

where R is the diameter of nanoparticle, c is the propagation speed of light, ε1 and ε2 are the real part and imaginary part of the complex dielectric function, respectively, which can be expressed as follow. ε1 = ε∞ −

ε2 =

ωp2 ω2 + γ 2

ωp 2γ ω(ω 2 + γ 2)

where ε∞ is the high-frequency dielectric constant. Based on the above discussion, three related parameters including free carrier density, high frequency dielectric constant and average diameter need to be considered to interpret the plasmonic behavior. Figure 7b shows the best fitted spectra and Table 1 summarizes the parameters that used for best fits. The samples that obtained during transformation have various size, carrier density and dielectric constants. To determine the critical factor among these three parameters, a controlling variable method was adopted. By comparing the experimental absorption spectra of rhombohedral Cu1.4−1.8S NCs in the NIR region with the calculated spectra with various size, dielectric constant and carrier concentrations, we draw the following conclusions. (i) From Figure 8a, we can conclude that the plasmonic absorption peak of Cu2−xS NCs show weak size dependence, which is different with the plasmonic behavior of gold and silver NCs.56−58 The best fitted parameters indicate that the transformation process from Cu1.8S to CuS includes an obvious transition of the dielectric constant from 2.0−2.6 for rhombohedral NCs to 3.8−4.4. For hexagonal NCs of LSPR related absorption peak in both phases, the dielectric constant

− γ2

where εm represents the dielectric constant of the environmental medium. γ is the full-width half at maximum (fwhm) of the LSPR related absorption peak. TCE has a dielectric constant εm of 2.28. ωp can be determined from the LSPR related absorption spectra (see Table 1). From this equation, ωp can be calculated by using the parameters that derived from the experimental spectra (see table 1). To obtain the free carrier density Nh, the following equation is applied: ωp =

12πεm 3/2 3 ωε2 R c (ε1 + 2εm)2 + ε22

σext(ω) =

Nhe 2 ε0mh 4832



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analyses, it was demonstrated that both the phase induced dielectric constant change and the composition induced carrier concentration variation account for the spectra evolution. Moreover, this work also provides a series of Cu2−xS NCs with broad tunable NIR absorption properties, which could be potential candidates for optoelectronics and bioapplications.

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ASSOCIATED CONTENT

* Supporting Information S

Optical image of the sample that prepared in large scale synthesis, EDS and FTIR spectra of as-synthesized Cu1.8S NCs, calculation of lattice parameters, TEM images and XPS spectra of the samples that taken from the transformation, absorption spectra of the samples dissolved in TCE under open air and N2 atmosphere. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Prof. Jiatao Zhang for the UV−vis−NIR absorption spectroscopic measurements and Prof. Jianhua Liang for helpful assistance. This work is supported under National Basic Research Program of China (No. 2011CB933600), NSFC Grant (No. 21343005), and Shanghai Aerospace Science and Technology Fund (SAST201267).

Figure 8. (a−c) Left: Experimental and fitted curves using the parameters in the right table. Right: fitting parameters.

that changed with the phase transformation contributes to the observed transition of LSPR related spectra from blue shift to red shift. (ii) From figure 8b, it is noted that the free carrier density of Cu2−xS NCs plays an important role in determining the LSPR related absorption peak. With the composition of the sample evolution from 1.8 to 1.4, the carrier density increased from 4.7 to 5.99 × 1021cm−3. (iii) From Figure 8c, the dielectric constant may be also related to the composition of NCs, which account for the blue-shift of plasmon peak during the transformation process. Similar phenomenon has been observed in Cu2−xSe NCs.21

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CONCLUSION In summary, a facile hot-injection method was developed to synthesize high quality Cu1.8S NCs at gram-scale, and their metastable rhombohedral Cu1.8S crystal structure was carefully identified. The colloidal Cu1.8S NCs undergo an in situ phase transformation from rhombohedral Cu1.8S NCs to hexagonal CuS clusters upon keeping the resulting colloidal solution for a few days. To illustrate the transformation mechanism, the evolution of their morphology and structure during phase transformation process was monitored using various characterization techniques. It was deduced that the OLA assisted oxidation are involved in the transformation. The spectroscopic study with the phase transformation allowed us to investigate the phase- and compositiondependent plasmonic properties of Cu2−xS NCs. Combining the experimental and theoretical 4833



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Controllable Transformation from Rhombohedral Cu1.8S Nanocrystals

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