Hierarchical Self-Assembly of Cu7Te5 Nanorods into Superstructures

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Hierarchical Self-Assembly of Cu7Te5 Nanorods into Superstructures with Enhanced SERS Performance Jiaojiao Zheng,†,‡ Baosong Dai,†,§ Jia Liu,† Jialong Liu,∥ muwei Ji,† Jiajia Liu,† Yuanmin Zhou,† Meng Xu,† and Jiatao Zhang*,† †

Beijing Key Laboratory of Construction-Tailorable Advanced Functional Materials and Green Applications, School of Materials Science & Engineering, Beijing institute of Technology, Beijing 10081, China ‡ State Key Laboratory of Low-Dimensional Quantum Physics, Tsinghua University, Beijing 100084, China § Patent Examination Cooperation Hubei Center of The Patent Office, Wuhan 430205, China ∥ Department of Physics, Beihang University, Beijing 100191, China S Supporting Information *

ABSTRACT: This paper reports a strategy to get selfassembly of Cu7Te5 nanorods into hierarchical superstructures: the side-by-side self-assembly of nanorods into microscale onedimensional (1D) nanowires (primary structure), the side-byside alignments of the 1D nanowires into two-dimensional (2D) nanowire bundles (secondary structure), and the further rolling up of the 2D bundles into three-dimensional (3D) microtubes (tertiary structure). It was found that the oleylamine (OLA)/n-dodecanethiol (DDT) mixture as a binary capping agent was key to produce Cu7Te5 nanorods in the quantum size regime with high monodispersity, and this was a prerequisite for their hierarchical self-assembly based on elaborate control of the solvent evaporation process. The obtained Cu7Te5 microtube superstructures were used as SERS substrate and showed much stronger SERS enhancement than the as-prepared Cu7Te5 nanorods before assembly. This was probably ascribed to the remarkably enhanced local electromagnetic field arising from the plasmon coupling of Cu7Te5 nanorods in the well-assembled superstructures. KEYWORDS: hierarchical superstructures, self-assembly, Cu7Te5 nanorods, plasmon coupling, local electromagnetic field, SERS

1. INTRODUCTION A growing demand for understanding and controlling the selfassembly of anisotropic colloidal nanocrystals (NCs) is fueled by their significance in the area of biomineralization and the exploration into their distinctive optical, electronic properties.1−9 Self-assembly is a natural and spontaneous process occurring mainly through noncovalent interactions such as van der Waals force, hydrogen bonding, hydrophilic/hydrophobic interaction, electrostatic interaction, and metal ligand coordination.4−11 It is well-known that nature systems are capable of synthesizing structures with impressive complexity and functionality, typically represented by the four distinct levels of protein structure.7,8,12−14 Self-assembly of nanoscale building blocks, such as clusters, quantum-sized NCs, and ultrathin nanostructures, into two-dimensional (2D) and three-dimensional (3D) superstructures has attracted considerable attention.15−24 The obtained 2D and 3D superstructures have shown promising potentials in bionic, optical,and electronic applications.25−29 For example, Jiang et al. reported that a highly ordered assembly of nanoparticles could realize biomimetic functionalities, such as the superwettability of © XXXX American Chemical Society

opal made with periodic arrangements of silica nanoparticles.6,25 Acharya and co-workers have demonstrated a surfactant-driven assembly route of Au nanoparticles from aqueous solution phase into dimension controlled 1D, 2D, and 3D structures.23 In addition to the researches carried out on the assembly of colloidal particles with isotropic morphology, they also developed a bottom-up approach to fabricate high quality ultrathin 2D single-crystalline sheet based on collective coalescence of PbS nanowires.24 Moreover, regarding the nanorods (NRs) or nanoplates as anisotropic building blocks, the primary structure constructed by side-by-side assembly of NRs or face-to-face assembly of nanoplates into 1D patterns has been realized.30−33 However, a further high-level assembly of anisotropic colloidal NCs into 3D superstructures is still challenging because of the high entropy-induced driving forces required for such assembly process. Based on the previous works, here we present our recent study on the hierarchical Received: September 1, 2016 Accepted: December 1, 2016 Published: December 1, 2016 A

DOI: 10.1021/acsami.6b11058 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. (A) TEM image and (B) XRD pattern of as-prepared typical Cu7Te5 NRs with ∼12 nm in length and ∼2 nm in diameter; (C) schematic of the crystal cell structure of Cu2−xTe NRs drawn by Diamond 3.2 software; (D) UV−vis−NIR absorption spectra of Cu2−xTe NRs with varied lengths.

2. EXPERIMENTAL SECTION

assembly of semiconductor NCs with anisotropic morphology, in particular the ones featuring plasmonic properties, into multidimensional (1D, 2D, and 3D) superstructures. Copper telluride (Cu2−xTe) NCs with copper vacancies can exhibit near-infrared (NIR) localized surface Plasmon resonance (LSPR), unique thermoelectric and ionic conductivity properties,34−37 thus have a good potential in developing surface enhanced Raman scattering (SERS) spectroscopy, photothermal conversion transducer, as well as photovoltaic and nanoscale electronic devices.38−40 For actualizing their practical applications in above areas, the hierarchical selfassembly of Cu2−xTe NCs is highly desirable in addition to their shape-controlled synthesis and optical characterization. Here, by taking advantage of their different coordination abilities to Cu cations 41,42 oleylamine (OLA) and ndodecanethiol (DDT) were used as binary capping agents to produce Cu7Te5 NRs with adjustable sizes (typically in the quantum-sized range) and high monodispersity via the lowtemperature solvothermal process. The size and uniformity of Cu7Te5 NRs could be easily modulated by regulating the concentration of OLA and DDT capping ligands. More importantly, it was found that elaborate control of the solvent evaporation process enabled a hierarchical self-assembly of the monodisperse Cu7Te5 NRs into superstructures, namely the side-by-side self-assembly of Cu7Te5 NRs into 1D nanowires (primary structure), the side-by-side alignments of the nanowires into 2D nanowire bundles (secondary structure), and the rolling up of these 2D alignments into 3D microtubes (tertiary structure). Furthermore, we demonstrated that the hierarchical Cu7Te5 microstructures were much favored candidates for achieving significantly enhanced SERS signals compared with the isolated Cu7Te5 NRs.

2.1. Materials. All chemicals were used without further purification. Copric chloride dihydrate (CuCl2·2H2O, >99% purity), Tellurium powder (Te, >99% purity) and sodium stearate (C18H35NaO2, >96% purity) were purchased from Beijing Chemical Works. Oleylamine (OLA, >80%), oleic acid (OA, >99%), trioctylphosphine (TOP, 90% purity), and n-dodecanethiol (DDT, 98% purity) were purchased from Aladdin chemistry Co. Ltd. Shanghai, China. 2.2. Copper Stearate (CuSt2) and TOP-Te Organic Precursor. The synthesis of CuSt2 and TOP-Te were conducted following our previous work.43 2.3. Synthesis of Cu7Te5 Nanorods. Typically, 0.25 mmol of CuSt2 was dissolved in 5 mL of toluene to form a clear solution. Then 6 mL of OLA and 2 mL of TOP-Te precursor were added into the above solution. After stirring for 5 min, 6 mL of DDT was introduced to get a homogeneous dispersion. The mixture was then transferred into a 25 mL Teflon-lined autoclave. The autoclave was sealed and heated at 150 °C for 1 h. The product was washed by ethanol for 2 times. After these procedures, Cu7Te5 nanorods with 9−10 nm in length and 2 nm in diameter could be obtained. In this procedure, OLA and DDT not only act as the capping agents but also can make some contribution to the reduction of Cu2+ to Cu+. 2.4. Self-Assembly of Cu7Te5 Nanorods. After the solvothermal process as described in Experimental Section 2.3, the obtained Cu7Te5 nanorods could easily assemble into the primary structure, the secondary structure, and the tertiary structure on the TEM grids based on manipulating the conditions of the solvent evaporation process. Generally, the obtained Cu7Te5 nanorods were redispersed in toluene at different concentrations, where the concentration of the NCs suspension was found played a key role in determining the ultimate assembled morphologies (0.6 mg/mL for the primary structure, 1.0 mg/mL for the secondary structure, and 3 mg/mL for the tertiary structure, respectively). Then 10 μL of these Cu7Te5 suspensions were deposited on the TEM grids, allowing a spontaneous evaporation under room temperature (28 °C) and humidity (40%) to generate the hierarchical structures. As for the 3D microtubes (tertiary structure) B


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Figure 2. Angle-dependent-TEM characterization of obtained Cu7Te5 nanorods tilting along the Y axis.

Figure 3. (A, B, D−F) TEM and (C, G−I) HRTEM images of as-prepared Cu7Te5 (A−E) NRs and (F) nanowires with different diameters and lengths. support film. Low-resolution transmission electron microscopic (LRTEM), JEOL JEM-1200EX working at 100 kV, and highresolution transmission electron microscopic (HRTEM), FEI Tecnai G2 F20 S-Twin working at 200 kV, were utilized to characterize the morphology, monodispersity, crystallization details and hierarchical self-assembly of the Cu7Te5 colloids. The TECNAI G20 working at 200 kV, was utilized to obtain the angle-dependent TEM characterization to confirm the nanorods morphology of as prepared Cu7Te5, as well as to obtain the energy-dispersive spectrometry (EDS) analysis for the nanowires appeared on the TEM grid. The EDS analysis obtained

formed on the silicon wafer, carefully controlling the speed of solvent evaporation was required. Typically, 20 μL of the Cu7Te5 suspension in toluene (3 mg/mL) was deposited on a cleaned Si wafer (The ptype single-crystalline (100) silicon wafer with resistivity of 0.03−0.01 Ω.cm was purchased from Semiconductor Research Institute of Tianjin, China), and was quickly transferred into a drying oven at 50 °C, with an optimal solvent evaporation speed of 0.2 mL/h. 2.5. Characterization Techniques. The samples for TEM characterizations were prepared by adding one drop of Cu7Te5 colloids in toluene solution onto a 300 mesh nickel grid with carbon C


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Figure 4. FT-IR spectrum of quantum-sized Cu7Te5 nanorods capped with DDT and OLA. from the sample on silicon substrate was through Hitachi FE-SEM 4800 instrument. The samples for XRD measurement were prepared by adding several drops of concentrated product onto silicon (100) wafers and dried at room temperature. The phases of products were determined by XRD on a Bruker D8 Advance X-ray power diffractometer with Cu Kα radiation (λ= 1.5418 Å). The ultraviolet−visible−near-infrared (UV−vis−NIR) absorption spectra of Cu7Te5 colloids in toluene solution were recorded on a Shimadzu UV3600 UV−vis−NIR spectrophotometer. The surface-enhanced Raman scattering (SERS) measurements were carried out by using a Renishaw InVia micro-Raman spectrometer with an excitation wavelength of 785 nm. The laser power at the sample location was 1 mW. The spectra were recorded with an exposure time of 30 s and the number of accumulations was 3. Thermogravimetric Analysis (TGA) was carried out using a Mettler-Toledo analyzer in the temperature range of 50−800 °C, at a heating rate of 10 °C/min under a nitrogen (N2) atmosphere.

provided in Figure S5. On the basis of these statistical analysis, one can identify that the Cu7Te5 NRs shown in Figure 3A possess a diameter of 2.56 ± 0.42 nm (in quantum size regime) and a length of 9.06 ± 0.82 nm, with an averaged aspect ratio of 3.54. Through decreasing DDT concentration, the Cu7Te5 NRs displayed in Figure 3B gained a larger size with a diameter of 2.72 ± 0.32 nm, a length of 14.24 ± 2.34 nm, and an averaged aspect ratio of 5.23. The HRTEM image in Figure 3C indicated the good crystallinity of the obtained Cu7Te5 NRs (∼3 nm in diameter and ∼11 nm in length for the selected NR). The distances measured between two adjacent fringes were 0.27 and 0.31 nm, which should be respectively assigned to the (152) plane and (052) plane of Rickardite phase (JCPDS # 26−1117) Cu7Te5. From Figure 3C, the angle between (152) plane and the NCs surface can be evaluated at about 25°. By analyzing the crystal data of Cu7Te5, the angle between (152) and (010) planes was nearly 25°, in a good agreement with the electron diffraction pattern analysis.45 Therefore, we deduce that the Cu7Te5 NRs grew along the [010] direction. Further reducing the DDT concentration resulted in the generation of even larger-sized NRs with increased polydispersity (Figure 3D, E), and finally a mixture of nanorods and nanowires (Figure 3F). The corresponding histograms provided in Figure S5 clearly denote the grown particle size (both diameters and lengths) accompanied by decreased monodispersity as the DDT concentration reduced gradually. From the XRD patterns in Figure S6, the obtained larger-sized NRs and nanowires were all pure Cu7Te5 with Rickardite phase (JCPDS # 26−1117) independent of varied aspect ratios. The EDS analysis performed over the nanowires provided further evidence that they were composed of Cu7Te5 with an atomic ratio very close to 7:5 (Figure S7). The HRTEM images in Figure 3G, H (corresponding to the Cu7Te5 NRs presented in Figure 3D, E, respectively) suggested the good crystallinity of the larger-sized Cu7Te5 NRs. The distance between two adjacent fringes was 0.33 nm, corresponding to the (052) plane of Rickardite phase (JCPDS # 26−1117) Cu7Te5. The HRTEM image of the Cu7Te5 nanowires in Figure 3F (TEM image shown in Figure 3I) also demonstrated a good crystallization. The distance between two adjacent fringes was 0.38 nm, which should be assigned to the (051) plane of Rickardite phase (JCPDS # 26− 1117) Cu7Te5. As demonstrated by Figure 3A, B, the side-byside assembly of Cu7Te5 NRs with smaller sizes can proceed easily in large scale. It was noticed that both the OLA and DDT were the key to controlling the size, shape and monodispersity of the Cu7Te5 nanorods. Without DDT addition, the exclusive use of OLA or the coaddition of OLA and oleic acid lead to a mixture of Cu7Te5 nanorods and nanocuboids (Figure S8A− C). On the other hand, the exclusive use of DDT resulted in the production of Cu7Te5 nanocuboids (Figure S8D). More-

3. RESULTS AND DISCUSSION Using OLA and DDT with appropriate relative ratios as surface stabilizers, monodispersed Cu7Te5 NRs could be prepared in large scale, as shown in Figure 1A and Figure S1. The XRD pattern in Figure 1B demonstrates the Rickardite phase (JCPDS # 26−1117) of as prepared Cu7Te5 NRs with ∼12 nm in length and ∼2 nm in diameter. Figure 1C displays the schematic visualization of the crystal structure of Cu7Te5 NRs (drawn by Diamond 3.2 software), which illustrates that different from the single unit cell structure of Cu1.4Te (Figure S2) (a = 0.398 nm, b = 0.398 nm, c = 0.621 nm), Cu7Te5 has a superlattice structure of (a, 5a, 2c).44 In such a unit cell, its outside is mainly occupied by Cu atoms, thus favoring further coordination with −NH2 and −SH functional groups in OLA and DDT. The UV−vis−NIR absorption spectra of Cu7Te5 colloids with different lengths in toluene solution were presented in Figure 1D. One can see that the as-prepared Cu7Te5 NRs exhibited good LSPR properties in NIR region (800−1400 nm) and shorter NRs displayed stronger SPR peaks due to the better colloidal dispersion, similar to previously reported observations over Cu2−xTe nanostructures.34−38 To confirm that the as prepared Cu7Te5 actually existed as nanorods instead of nanodisks vertically standing on the TEM grid, the angle-dependent-TEM characterizations along the X axis (Figure S3) and Y axis (Figure 2) were conducted. The results strongly suggested that the as prepared Cu7Te5 have a nanorod morphology. Additionally, the HRTEM image of Cu7Te5 with random sizes as shown in Figure S4 also reflected the nanorod morphology of the Cu7Te5 crystals. By finely tuning the concentration and relative ratio of OLA and DDT, the size (length and diameter) of Cu7Te5 NRs could be adjusted flexibly. The histograms of the length and diameter for the nanocrystals shown in Figure 3 A-B and D-F were D


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Figure 5. Schematic growth mechanism of Cu7Te5 NRs. (A) Crystal structure of Cu7Te5 NRs without ligands; (B) crystal structure of Cu7Te5 NRs capped with OLA and DDT; (C) schematic Cu7Te5 NRs with OLA and DDT as ligands; (D) schematic mechanism of side-by-side self-assembly of Cu7Te5 NRs into 1D nanowires; (E) side-by-side alignments of nanowires into 2D nanowire bundles.

manipulation of the self-assembled monolayer (SAM) on metal (Au, Ag, Cu etc.) substrates.49 Moreover, the −SH group has a stronger coordination ability than the − NH2 group.50 Therefore, the optimal DDT concentration was significant not only for achieving synergistically self-assembled layers of OLA and DDT on NRs surface, but also for preferential ligands capping on the Cu-rich longitude side of Cu7Te5 NRs rather than the tips (Figure 5B). With appropriate DDT ligands confinement, Cu7Te5 was able to grow into quantum-sized NRs along the [010] direction (Figure 5C). The resultant monodisperse ultrathin structures were an important foundation for the following 2D and 3D superstructures formation.51 Interestingly, different from the reported close packing of isotropic NCs,17,19,26 we observed that hierarchical selfassembly of Cu7Te5 NRs could take place unprecedentedly when carefully manipulating the solvent evaporation process. In specific, the Cu7Te5 NRs here could spontaneously selfassemble into primary, secondary, and tertiary superstructures step-by-step. On the basis of the above results and analysis, it is reasonable to conclude that because of the synergistic capping of OLA and DDT, we obtained the quantum-sized Cu7Te5 NRs with high monodispersity. During the solvent evaporation process, largescale side-by-side alignments of quantum-sized NRs (∼2 nm in diameter, 9−10 nm in length) into micrometer-scaled 1D assemblies (primary structure) can proceed easily. The uniform nanogaps (∼3 nm) between NRs in these 1D assemblies could be attributed to the passivated monolayer of OLA (single molecule length of ∼2.2 nm) and DDT (single molecule length of ∼1.5 nm) as illustrated in Figure 5D. Furthermore, after the carefully implemented solvent evaporation procedure, the 1D “nanowires” (Figure 6a) could further side-by-side assemble into large-scale “nanowires” bundles (secondary structure,

over, the relative ratio between OLA and DDT keenly determined the size and uniformity of the nanorods. Only at an appropriate ratio (1:1), the quantum-sized nanorods with high uniformity (Figure S8F) can be obtained. Further increasing or decreasing the relative ratio of OLA to DDT failed to obtain the nanorods with small size and uniform morphology suitable to serve as building blocks for side-by-side self-assembly (Figure S8E, G). This is reasonable in virtue of the theoretical simulations proposed by Zeravcic et al.46 That is, there are size limits for colloidal nanocrystals to get selfassembly by using specific stabilization interactions. FT-IR spectroscopy was utilized to investigate the surface passivation of Cu7Te5 NRs with DDT and OLA (Figure 4). The absence of stretching vibration mode of S−H bond at ∼2566 cm−1 (according to the report by Bai et al.47) and the extremely strong bending vibration mode of C−S bond at ∼1385 cm−1 (very low ratio due to the bending vibration mode of CH3 group) suggested the strong coordination between the NRs surface and DDT.48 The peak observed at 1595 cm−1 was originated from the bending vibration of N−H in OLA, implying a weak coordination of NRs with OLA. As shown in the schematic crystal structure of Cu7Te5 NRs (Figure 5A), along [010] direction, Cu atoms instead of Te atoms locate at the surface dominantly. This enabled the preferential coordination of DDT and OLA ligands with Cu ions and the consequent anisotropic growth of NRs. The fewer Cu atoms at the end of NRs induced a less extent of capping with DDT and OLA, where the (010) atomic planes were active to receive more Cu7Te5 species from the solution. Therefore, the preferential growth along the [010] direction eventually resulted in a 1D structure, as illustrated in Figure S9. It is known that OLA and DDT ligands with − SH or − NH2 head groups and (C−C)n chain tails are efficient candidates for E


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Figure 6. Hierarchical self-assembly of Cu7Te5 NRs studied by TEM and STEM characterizations: (a) side-by-side assembly of Cu7Te5 NRs into 1D “nanowire” structures; (b) side-by-side alignments of 1D structures into 1D bundles; (c, d) side-by-side assembly of 1D “bundles” to be larger 2D “bundles”; (e−h) rolling up of “nanowire” bundles into 3D “microtube” superstructure; inseted cartoons in a, b, d, and e illustrate the step-by-step assembly of NRs into hierarchical superstructures.

illustrated by the SEM, TEM and STEM images in Figure 6e−h and Figures S11−S13. It should be mentioned that the speed of solvent evaporation played a crucial role in getting a high yield of 3D “microtubes” structures. The hollow “tube” feature was clearly demonstrated by the SEM images in Figure 6e. The SEM and STEM images

Figure 6b−d). From the STEM images in Figure 6c, d and TEM image in Figure S10, it can be seen that these 2D “nanowires” bundles formed in large scale can have a width and length in the range of micrometers. With precise control of solvent evaporation, such artificial lamellar “nanowires” bundles could roll up into “microtubes” (tertiary structure), as F


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ACS Applied Materials & Interfaces of the “microtubes” in Figure 6f−h further corroborated that the “microtubes” were constituted by hierarchical assembly of the primary “nanowires” structures with Cu7Te5 NRs as the building blocks. The composition of the eventually obtained “microtubes” was consistent with the Cu7Te5 building blocks as indicated by the energy-dispersive spectrometry (EDS) analysis (Figure S12). Figure 6g shows that the rolling of the 2D “nanowires” bundles into 3D “microtubes” can occur in a large scale. A unique four-way structured “microtube” was also observed (Figure S14). The prepared Cu7Te5 structures before and after assembly were used as substrates for SERS measurement at an excitation wavelength of 785 nm. Figure 7 shows the SERS spectra of

the possibility of electromagnetic enhancement mechanism.61 However, in this work, the SPR peak of the Cu7Te5 NRs was in the near-infrared region and close to the excitation wavelength of 785 nm. Thus, it is reasonable to conjecture that the electromagnetic field enhancement mechanism can make a good contribution to the strong SERS enhancement over the “microtube” structure, which is similar to the cases over CuTe particles demonstrated by Cabot and co-workers.62 As reported before, the assemblies or aggregates of Au nanorods showed stronger SERS enhancement than the isolated nanocrystals due to the so-called “hot spots” (specific sites where the electric field is greatly enhanced).63,64 Such “hot spots” usually exist in the gaps or the junctions between nanocrystals after assembly or aggeration.65−67 Similarly, because of the numerous nanogap regions generated between the Cu7Te5 nanorods after assembly, the plasmon coupling of Cu7Te5 nanorods at these gap regions could produce an intense local electromagnetic field. In contrast, the nanorods structure before assembly with few gap regions lead to a weak local electromagnetic field and accordingly much reduced signals. On the basis of the above discussion, one can confer that the self-assembly of Cu7Te5 nanorods is of importance for achieving significantly enhanced SERS signals relying on the LSPR effect of Cu7Te5 NCs. On the other hand, considering the stronger SERS enhancement of DDT than OLA over the “microtube” structure, the chemical enhancement should also be taken into account. The stronger interaction of the “microtube” surface with −SH than that with −NH2 was considered to be the main reason for the much stronger SERS enhancement of DDT relative to OLA.50,68

Figure 7. SERS spectra of DDT and OLA over the as-prepared nanorods structure (before assembly) and the tertiary “microtube” structure (after assembly) using an excitation wavelength of 785 nm.

4. CONCLUSION In summary, resting on binary capping with OLA and DDT, we synthesized the quantum-sized Cu7Te5 NRs with high uniformity to be suitable building blocks for large-scale selfassembly. Both the OLA and DDT concentrations played a significant role in regulating the size and shape of the Cu7Te5 colloids. By carefully controlling the condition of solvent evaporation, hierarchical self-assembly behaviors can be observed for the as-prepared Cu7Te5 NRs on versatile substrates, such as copper grids and Si wafer. The mechanism for such self-assembly process was investigated in detail through studying every assembly step from the formation of primary structure to tertiary structure. Furthermore, the tertiary “microtube” structure after hierarchical assembly showed much stronger SERS signals than the as-prepared Cu7Te5 NRs before assembly, which was presumed mainly due to the enhanced local electromagnetic field arising from the plasmon coupling of Cu7Te5 NRs in the well-assembled superstructures. We believe that this facile one-pot synthesis and self-assembly strategy could be extended to prepare other novel superstructures to achieve enhanced performance in versatile fields.

DDT and OLA cappings on the as-prepared nanorods (before assembly) and the tertiary “microtube” structure (after assembly). Before assembly, DDT on the Cu7Te5 nanorods showed very weak signals. Interestingly, after assembly, DDT on the Cu7Te5 “microtube” structure yielded a much stronger SERS activity with a SERS enhancement factor calculated higher than 1 × 104 (the details for determining the SERS enhancement factor are in the Supporting Information). The peaks at 652 and 723 cm−1 are ascribed to the ν (C−S) bands. Two weak peaks at 892 and 934 cm−1 are assigned to the ρ (CH3) bands. Peaks at 1063, 1084, and 1129 cm−1 are attributed to the ν (C−C) bands. The peaks at 1302 and 1446 cm−1 are ascribed to the ω (CH2) band and the δ (CH3) band, respectively.52,53 As for OLA, relative weaker enhancement was observed even after assembly. Peaks at 874, 1031, and 1245 cm−1 are attributed to the τ (NH2) band, ν (C−N) band, and δ (NH2) band, respectively.54,55 Two main mechanisms are believed to be responsible for the SERS enhancement: the electromagnetic field enhancement and the chemical enhancement effect.56,57 The electromagnetic enhancement is due to the local surface plasmon resonance induced by laser irradiation, while the chemical enhancement is mainly related to the charge transfer effect between the adsorbate and the substrate. In the previous work, SERS enhancement over semiconductor nanocrystals such as ZnO58 and CdTe59 was mostly due to the chemical enhancement mechanism. This is because the SPR peaks of these semiconductor nanocrystals usually fall into the infrared region and are far away from the excitation wavelength,60 excluding

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b11058. TEM, HRTEM, and STEM images; schematic illustrations; XRD patterns of larger-sized NRs and nanowires with different aspect ratios; EDS spectra; measurement procedure for determining the SERS enhancement factor; Raman spectrum of DDT on Si wafer; and TGA curve of Cu7Te5 microtube (PDF) G


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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jiatao Zhang: 0000-0001-7414-4902 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (51372025, 91323301, and 21322105), and the Open Research Fund Program of the State Key Laboratory of LowDimensional Quantum Physics.

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