Superlong High-Quality Tellurium Nanotubes - American Chemical

ABSTRACT: Single-crystalline trigonal tellurium (t-Te) nanotubes with sloping cross-section and ... However, when tellurium dioxide in place of orthot...
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CRYSTAL GROWTH & DESIGN

Superlong High-Quality Tellurium Nanotubes: Synthesis, Characterization, and Optical Property Ji-Ming Song, Yun-Zhi Lin, Yong-Jie Zhan, Yang-Chao Tian, Gang Liu, and Shu-Hong Yu* DiVision of Nanomaterials & Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, Department of Chemistry, National Synchrotron Radiation Laboratory, UniVersity of Science and Technology of China, Hefei 230026, P. R. China

2008 VOL. 8, NO. 6 1902–1908

ReceiVed NoVember 14, 2007; ReVised Manuscript ReceiVed January 13, 2008

ABSTRACT: Single-crystalline trigonal tellurium (t-Te) nanotubes with sloping cross-section and hexagonal cross-section can be selectively synthesized on a large scale by a simple solvothermal reduction route, using tellurium dioxide (TeO2) as tellurium source and ethylene glycol (EG) as both a reducing agent and a solvent in the presence of cetyltrimethyl ammonium bromide (CTAB) and cellulose acetate (CA), respectively. The individual Te nanotubes with cylindrical morphology and open ends have outer diameters of 100–500 nm, wall thicknesses of 50–100 nm, and lengths of 150–200 µm. Both kinds of Te nanotubes grow along the [001] direction and have excellent crystallinity. The optical properties and the stability in ethanol of the t-Te nanotubes with sloping cross section have been investigated. 1. Introduction Recently, one-dimensional (1D) nanostructures, such as nanowires, nanorods, nanobelts, and nanotubes, have been the focus of current research on nanotechnology because of their unique applications in mesoscopic physics and fabrication of nanoscale devices.1 Among these 1D nanostructured materials, chalcogens (Se, Te) represent a class of interesting elements, in which anisotropic crystal structure gives a strong tendency toward 1D growth. Trigonal tellurium, as well as trigonal selenium (t-Se) and Se-Te alloys, have a highly anisotropic crystal structure consisting of helical chains of covalently bound atoms, which are in turn bound together through van der Waals interactions into a hexagonal lattice.2 This inherent anisotropy makes these materials ideal candidates for generating 1D nanostructures even though without the need for templates or surfactants to induce their anisotropic growth. Elemental tellurium is a narrow bandgap (direct bandgap energy of 0.35 eV) semiconducting material. As a p-type semiconductor, trigonal tellurium exhibits many useful and interesting properties, for example, photoconductivity, and catalytic activity toward some reactions, high piezoelectricity, thermoelectricity, and nonlinear optical responses.3 In addition, Te can react readily with the other elements to generate many functional materials such as ZnTe, CdTe, Nd3Te4, and Bi2Te3.4 1D tellurium nanocrystals, including Te nanotubes, nanorods, nanowires, and nanobelts, have been synthesized through different routes, such as refluxing process,6 solvothermal or hydrothermal methods,6 microwave-assisted method,7 biomolecule-assisted routes,8 chemical (physical) vapor deposition,9 visible-light-assisted technique,10 transformation of stabilizerdepleted CdTe nanoparticles in methanol,11 surfactant-assisted approach,12 and using hard templates.13 Stroeve and co-workers reported a novel route for fabricating Au-Te, with a radial metal-semiconductor heterostructure, by a slow electrodeposition process in which the Te semiconductor grew radially along the side of the nanotubes.14 Xia and co-workers first reported that Te nanotubes with blocking seeds (Te atom clusters) within, which could be obtained by adding orthotelluric acid to pure ethylene glycol refluxed at 197 °C.5a Two reaction steps were * Corresponding author. Fax: 86 551 3603040. E-mail: [email protected].

suggested, i.e., orthotelluric acid was first decomposed into tellurium dioxide at a temperature of 150-200 °C, and then TeO2 could be reduced into tellurium by ethylene glycol. However, when tellurium dioxide in place of orthotelluric acid was directly added to ethylene glycol refluxed under other identical experimental conditions, solid nanorods instead of hollow nanotubes were obtained.5a Mo et al. synthesized Te nanotubes by in situ disproportionation of sodium tellurite (Na2TeO3) in an aqueous ammonia system at 180 °C.6a Te nanowires and scrolled Te nanotubes can be synthesized by a wet chemical method using amino acids as a growth assisting agent.15 Te nanotubes with triangular cross-sections have been synthesized by a simple approach of vaporizing tellurium metal and condensing the vapor in an inert atmosphere onto a Si (100) substrate.9b A previous report has shown that only spherical particles can be obtained by reducing TeO2 using NaBH4 as a reducing agent with addition of a low amount of PVP via microwave heating.7b All these studies indicated that the growth processes of tellurium nanotubes were still diverse under different prepared conditions. Designing new approaches to synthesize Te nanotubes with special structural characteristics in a controlled manner under mild conditions has been of both fundamental and practical importance. In this paper, we report a facile route for selective synthesis of tellurium nanotubes with sloping cross-section and hexagonal cross-section by a surfactant-assisted solvothermal process under mild conditions. The synthesis of superlong and high-quality Te nanotubes with sloping cross-sections has been realized by use of precursor TeO2 being reduced by ethylene alcohol in the presence of cetyltrimethyl ammonium bromide (CTAB). When CTAB was substituted with cellulose acetate (CA), t-Te nanotubes with hexagonal cross sections were produced. To the best of our knowledge, synthesis of single-crystalline Te nanotubes with special cross-sections has been realized for the first time by directly reducing TeO2 precursor under mild solvothermal conditions. The optical properties of t-Te nanotubes with sloping cross-sections and their stability in ethanol have been also investigated. 2. Experimental Section Materials. All reagents in the experiment are commercially available from Shanghai Chemical Reagent Co. Ltd., and used in this study

10.1021/cg701125k CCC: $40.75  2008 American Chemical Society Published on Web 05/16/2008

Superlong High-Quality Tellurium Nanotubes without further purification. Tellurium dioxide (TeO2), Ethylene glycol (EG), and cetyltrimethyl ammonium bromide (CTAB) are analytical grade, whereas cellulose acetate (CA) is chemically pure. Preparation of Te Nanotubes with Sloping Cross-Sections in the Presence of CTAB. In a typical procedure, 0.1 g (ca. 0.6 mmol) of TeO2, and 0.23 g of CTAB (ca. 1 wt % in the total mass) were added into a 25 mL Teflon-lined stainless-steel autoclave. Then the autoclave was filled with EG up to 80% of the total volume and the reaction mixture formed a homogeneous white suspension under vigorous stirring. The autoclave was sealed and maintained at 180 °C for 24 h, and then cooled to room temperature naturally. The obtained silver-gray solids were collected by centrifuging the reaction mixture; the particles were then washed with distilled water and absolute ethanol several times each and dried in a vacuum at 60 °C for 6 h before further characterization. Preparation of Te Nanotubes with Hexagonal Cross-Sections in the Presence of CA. In a typical procedure, 0.05 g of CA was added to 20 mL of EG, and the mixture was then transferred into a 25 mL Teflon-lined autoclave (with a filling ratio of ca. 80%). The autoclave was closed and kept at 180 °C for 3 h until a homogeneous and clear solution was obtained. After the autoclave cooled to room temperature, 0.1 g of TeO2 was added to the clear solution under stirring. The autoclave was then closed again and kept at 180 °C for 24 h. After the autoclave was cooled to room temperature naturally, the obtained silvergray solid product was collected by centrifuging the reaction mixture; it was then washed with acetone and absolute ethanol several times and dried in a vacuum at 60 °C for 6 h for further characterization. Characterization. The phase purity of the as-prepared products was examined by X-ray diffraction (XRD) using a Philips X’Pert PRO SUPER X-ray diffractometer equipped with graphite monochromatized Cu KR radiation (λ ) 1.54178 Å). To obtain further evidence for the purities and compositions of the as-prepared products, the X-ray photoelectron spectra (XPS) were applied, which were recorded on an ESCALab MKII X-ray photoelectron spectrometer, using Mg Ka radiation as the exciting source. The Raman spectra were performed with 488 nm laser excitation with a micro-Raman system, which was modified by coupling an Olympus microscope to a Spex 1740 spectrometer with a CCD detector. The composition of the product was measured by a fully functional environmental scanning electron microscope (XL30ESEM) at an acceleration voltage of 20 kV. Scanning electron microscopy (SEM) and field emission scanning electron microscopy (FE-SEM) were applied to investigate the size and morphology and were carried out with a Hitachi X-650 scanning electron microanalyzer and a field-emission scanning electron microanalyzer (JEOL-6700F), respectively. Transmission electron microscope (TEM) images were taken with a Hitachi H-800 transmission electron microscope at an acceleration voltage of 200 kV. Highresolution transmission electron microscope (HRTEM) analysis and selected area electron diffraction (SAED) patterns were performed on a JEOL-2010 transmission electron microscope.

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Figure 1. XRD pattern of the as-prepared tellurium nanotubes by reducing TeO2 with EG in the presence of CTAB at 180 °C for 24 h.

3. Results and Discussion 3.1. Selective Synthesis and Characterization of Te Nanotubes with Different Cross-Sections. The X-ray diffraction (XRD) pattern shown in Figure 1 that all diffraction peaks can be indexed as hexagonal phase of tellurium with lattice constants a ) 0.446 nm and c ) 0.591 nm, which are in agreement with the reported values (JCPDS, 36–1452; space group: P3121 [152]). No other impurity was detected by the X-ray diffraction pattern. Compared with the standard pattern of hexagonal phase tellurium, unusually strong (h00) reflection peaks and weak (hkl) reflection peaks (l * 0) were observed in the XRD pattern. The SEM image in Figure 2a shows that the product obtained is composed of superlong wires of Te with lengths of several hundreds of micrometers, outer diameters of 100–500 nm and wall thicknesses of 50–100 nm as clearly observed in the high magnification SEM images (images b and c in Figure 2). The TEM image in Figure 2d shows that the Te nanostructures are tubelike, and the tubes are genuinely hollow and contain no blocking seeds within, showing that as-synthesized Te micro-

Figure 2. SEM, TEM, and SAED patterns of the tellurium nanostructures with sloping cross sections synthesized in the presence of CTAB (0.23 g) at 180 °C for 24 h. (a) Low-magnification FESEM image of as-prepared tellurium nanotubes. (b) FESEM image at high magnification; As indicated by an arrow in (b), some of the nanotubes aggregated into bundles in the solution or during the preparation of SEM sample. This may explain why some of the nanotubes look wider than the others. (c) FESEM image at higher magnification for Te nanotubes, revealing the sloping cross sections; (d, e) TEM images of middle part and tip section for an individual nanotube with sloping cross sections, respectively; (f) SAED pattern taken on the Te nanotube shown in (e).

tubes are different from the Te nanotubes obtained by the concentration depletion method on the surfaces of seeds.5a The SAED pattern in Figure 2e was taken on a typical individual

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Figure 4. FTIR spectra: (a) pure CTAB; (b) the as-prepared tellurium nanotubes by reducing TeO2 with EG in the presence of CTAB at 180 °C for 24 h.

Figure 3. (a) TEM image of Te nanotube tips. (b) SAED pattern taken along the [010] direction. (c, d) HRTEM images of different areas (indicated in (a)) of an individual tube. The HRTEM images show that lattice spacings of 5.95 and 2.23 Å are observed, respectively. Fringes with a spacing of 5.95 Å are observed, which are perpendicular to the axial direction, indicating that the nanotubes grew along [001].

nanotube, showing that the nanotube is single crystalline with growth direction along [001]. Figure 3 shows a typical transmission electron microscope (TEM) image and high-resolution TEM (HRTEM) image of the obtained sample, along with selected area electron diffraction (SAED) pattern. HRTEM images in images c and d in Figure 3 show lattice spacings of ca. 2.23, and 5.95 Å, respectively, corresponding to the lattice spacings of the (110) planes, and (001) planes for trigonal tellurium, respectively. The corresponding SAED pattern taken along the [110] direction on the Te tubes indicated that it is single crystalline (Figure 3b). The combination of the results by HRTEM and the SAED pattern confirmed that the axis of the nanotube is along the [001] direction, in agreement with the result reported previously,9a and the nanotube is structurally uniform, and no dislocation is observed in the examined area. This result confirms that highquality single-crystalline tellurium nanotubes with sloping cross sections can be synthesized on a large scale from commercial TeO2 precursor by using EG as reducing agent in the presence of CTAB. The IR spectra of the pure CTAB and the freshly prepared sample were shown in Figure 4. The results confirmed that the sample of the Te product does not contain CTAB (Figure 4). The influence of the reaction temperature and the amount of CTAB has been examined. When the temperature is kept at 160 °C, the product obtained still contains a little unreacted white TeO2 powder, which cannot be reduced by EG if other reaction conditions being kept as the same. The reaction cannot take place when the temperature is below 150 °C. If the temperature increased up to 200 °C, the uniformity of t-Te tubes became worse (Figure 5). To obtain long and uniform t-Te nanotubes, the most appropriate dosage range of CTAB is ca. 0.23 g in this reaction system. Figure 6 shows the FESEM images of tellurium nanotubes synthesized with sloping cross sections in the presence of

Figure 5. FESEM images of tellurium nanotubes synthesized with sloping cross-sections under different reaction times in the presence of CTAB: (a, b) 12 h, at 200 °C; (c, d) 24 h, at 200 °C.

different amount CTAB at 180 °C for 24 h. Only Te flakes or quasi-tetragonal tubes can be obtained in the absence of CTAB under other identical experimental conditions (images a and b Figure 6). When the amount of CTAB is 0.06 g, the Te nanotubes with sloping cross-sections are obtained, however, the length of the tubes is diverse in a wide range, and the diameter range of the tubes are also broad (images c and d in Figure 6). The uniformity of the particles gets improved when the amount of CTAB is changed from 0.06 to 0.12 g (images e and f in Figure 6). The above results indicated that the presence of CTAB is essential for the formation of Te nanotubes. Interestingly, when CA was used in this reaction system in place of CTAB, t-Te nanotubes with hexagonal cross sections were produced. Figure 7a showed the prismy rods produced in the presence of CA at 180 °C for 24 h. Compared with the Te tubes with sloping cross sections, the length is shorter, and uniformity is poorer for the rods with hexagonal cross sections (images a and b in Figure 7). Clearly, the Te tubes are common cylindrical morphology with six arrises (Figure 7b). Figure 7c shows an individual Te nanotube with well-defined tube-like morphology and the diameter is ca.150 nm. No obstruct is found within the tube. The crystal structure and phase composition of

Superlong High-Quality Tellurium Nanotubes

Figure 6. FESEM images of tellurium nanotubes synthesized with sloping cross-sections in the presence of different amounts of CTAB at 180 °C for 24 h: (a, b) 0, (c, d) 0.06, and (e, f) 0.12 g.

the obtained Te product were characterized further. Figure 7d shows a XRD pattern of the as-prepared Te nanotubes. The sharp and strong diffraction peaks can be readily indexed as hexagonal phase of Te (JCPDS, No. 36–1452). Herein, the diffraction peak of (101) is the strongest one, while the strongest peak is diffraction peak of (100) for the Te tubes with sloping cross-sections. The EDS pattern (Figure 7e) indicates that the nanotubes contain only Te element, the peaks of Cu, C arise from the copper grid and carbon film for sample preparation. Figure 7f shows the SAED pattern acquired by focusing the electron beam on these parallel nanotubes, demonstrating a single-crystalline nature of Te nanotubes with growth direction along [001]. It should be noted that the surfactant was not absolutely necessary for the generation of 1D t-Te, because of an inherent anisotropic growth of Te. However, the introduction of a suitable surfactant, such as CTAB and CA, seems to be essential for synthesis of tubelike particles. It is well-known that the presence of surfactant can prevent nanoparticles from aggregation, as well as influence the nanocrystal growth through chemically selective adsorption onto certain crystal faces of a target material.12a,16 Xia and co-workers have reported that Te nanotubes with blocking seeds within, which could be obtained by adding orthotelluric acid to pure ethylene glycol refluxed at 197 °C.5a On the basis of the experimental facts, it is believed that ethylene glycol (EG), which was used as both solvent and a reducing agent in the solvothermal synthesis of Te nanostructures, plays an important role. In addition, the suitable reaction temperature is required for the formation of the Te nanotubes. In fact, the formation of such elegant Te nanotubes is controlled by a synergistic effect, including solvent, surfactant and reaction temperature. Further study is required to understand the growth mechanism of Te nanotubes with sloping or hexagonal cross sections.

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Figure 7. (a, b) FESEM images of tellurium nanotubes synthesized with hexagonal cross-sections: (a) overall view of Te nanotubes; (b) FESEM image with higher-magnification, revealing the hexagonal cross sections; (c) TEM image for an individual nanotube with hexagonal cross sections; (d) XRD pattern of the as-synthesized tellurium nanostructures in the presence of CA. (e) EDAX spectrum of the asprepared Te nanotubes, confirming that the samples are composed of pure Te. (f) SAED pattern taken on the Te nanotube shown in (c).

3.2 Stability of Te Nanotubes. Very recently, the chemical stability of Te nanostructures in different solvents has been examined by our group.17 The results demonstrate that the freshly prepared Te nanowires or nanoribbons are not stable after being stored for a prolonged time in contact with air, ethanol, and water.17 The ultrathin nanowires prepared will disappear completely in solvent ethanol and change into spherical amorphous TeO2 nanoparticles over about 5 days.17b However, the t-Te nanotubes obtained by the present route can be stored for three months in ethanol. There are no obvious morphology changes, as shown in Figure 8, between the freshly prepared sample and the sample after being stored for 3 months in ethanol; however, high-magnification SEM images showed that there are certain changes for the samples after being stored. The freshly prepared Te tubes have smooth surfaces, while some tiny nanoparticles have been found on the their surfaces of the sample after being stored for three months in ethanol (images e and f in Figure 8). The tiny particles remaining on the surface of the tubes should be amorphous tellurium dioxide because of the oxidation of Te tubes during the storage period, as confirmed by the X-ray photoelectron spectra (XPS) analysis, which is also consistent with that reported previously.17 The X-ray photoelectron spectra (XPS) of the samples obtained in the presence of CTAB at 180 °C for different storing periods was measured to examine the composition of the surface. The XPS spectrum of the Te tubes stored in ethanol for 3 months indicated that the peak values at 284.85 and 532.05 eV can be readily assigned to the binding energies of C1s and O1s, respectively (Figure 9a). The Te 3d peak shift shown in Figure

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Figure 8. FESEM images of tellurium nanotubes synthesized with sloping cross sections in the presence of CTAB at 180 °C for 24 h. (a-c) Freshly prepared sample; (d- f) a sample after being stored in ethanol for 3 months.

Figure 9. XPS spectra for the obtained t-Te tubes in the presence of CTAB for different storage periods. The red line represents the XPS spectrum of the freshly prepared Te tubes; black line represents the XPS spectrum of the Te tubes after being stored in ethanol for 3 months. (a) Survey of the sample; (b) survey of the Te 3d region.

9b indicated that the different valence state of Te in the prepared Te nanostructures. Obviously, different signal peaks were observed for the freshly prepared sample and the sample after being stored in ethanol for 3 months. The freshly prepared Te tubes display two the strong peaks at 572.7 and 583.1 eV, corresponding to Te (0) 3d binding energy, while the samples being stored for 3 months shows four peaks. Besides above two

strong peaks at 572.7 and 583.1 eV, two new peaks appeared at 576.2 and 586.7 eV, which can be assigned as Te (IV) 3d binding energy. The spectrum in Figure 9b shows the presence of both elemental tellurium 3d5/2 peak (572.7 eV) and a small amount of oxidized tellurium (576.2 eV), which is consistent with that for TeO2 reported by Garbassi et al.18 The freshly prepared Te nanotubes by the present method show relatively high stability compared with the record reported previously.16 3.3. Optical Property of Te Nanotubes. Room-temperature photoluminescence spectroscopy of tellurium nanostructures has been reported previously.6d The excitation spectrum of the t-Te nanotubes show two peaks at 281 nm (weak peak) and 363 nm (strong peak) with existing some extent broadening because of two shoulders in both sides of the strong peak, respectively, under the wavelength of 458 nm (Figure 10a). Figure 10b shows the photoluminescence emission spectrum of the t-Te nanotubes with an excitation wavelength of 363 nm, which consists of a main intensive peak at 458 nm. The deconvolution of the strong, asymmetric and broad peak gives two Gaussian components with two peaks at 456 and 495 nm. Apparently different from the results obtained by Gautam et al.,5c the emission peak around 700 nm was not observed in our sample. The emission peaks observed in the present case are close to the results reported by our group previously on the ultrathin Te nanowires,6d even though there is ca. 30 nm red shift. The red shift of the emission peak might be associated with the thickness of the nanostructures and crystallization behavior of onedimensional nanostructures. To the best of our knowledge, it is the first observation that t-Te nanotubes synthesized by the present approach can also give blue-violet emissions after strong photoluminescence emission in blue-violet region (390–550 nm) by ultrathin Te nanowires under the excitation wavelength of 365 nm was reported by our group previously.6d The Raman spectrum of pure t-Te nanostructures has been rarely studied previously. In recent literature, Poborchii et al.19 has studied the Raman spectrum of tellurium clusters confined in nanocavities of zeolite A. The Raman scattering spectrum taken for the synthesized t-Te is depicted in Figure 11. The characteristic vibration peaks at 87.2, 114.8, and 134.4 cm-1 were observed at room temperature, which are apparently not associated with Te of which Raman peaks are known to be at 46, 62, and 182 cm-1 reported previously.18 This may be attributed to the accompanying allotropes of element Te with different Raman spectrum features or the interaction result

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cm-1 of t-Se.20 The peaks at 258.1 cm-1 can be assigned as the second-order spectra of t-Te. It is interesting to note that some wavenumber shifts of the Raman bands for the products with different crystallinity have been observed. 4. Conclusion In summary, selective synthesis of uniform single-crystalline Te nanotubes with either sloping cross-section or hexagonal cross section can be easily realized by a simple solvothermal reduction route. Especially, uniform superlong tellurium nanotubes with sloping cross sections can be synthesized in large quantity. The formation of superlong, high-quality Te nanotubes with sloping cross sections is strongly dependent on the presence of CTAB, the reaction temperature, and reaction time. Interestingly, these Te nanotubes obtained in this approach display strong luminescent emission in the blue-violet region and strong corrosion resistance in ethanol solvent. The obtained Te nanotubes with sloping cross sections can be potentially useful for the fabrication of nanodevices with novel property. Further studies on electrical and photoconductive properties of individual single-crystalline tellurium nanotube are underway. In addition, the as-prepared Te nanotubes may be used as templates for the preparation of tubular telluride or other materials.

Figure 10. (a) Photoluminescence excitation spectrum of the obtained superlong Te nanotubes with emission at 458 nm; (b) photoluminescence emission spectrum of the superlong Te nanotubes with an excitation wavelength of 363 nm. The deconvolution of the band gave two Gaussian components (blue lines) with peaks located at 456 and 495 nm.

Acknowledgment. This work is supported by the China Postdoctoral Science Foundation, the National Science Foundation of China (NSFC) (Grants 50732006, 20325104, 20621061, and 20671085), the 973 project (2005CB623601), Anhui Development Fund for Talent Personnel and Anhui Education Committee (2006Z027, ZD2007004-1), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, the Specialized Research Fund for the Doctoral Program (SRFDP) of Higher Education State Education Ministry, and the PartnerGroup of the Chinese Academy of Sciences-Max Planck Society.

References

Figure 11. Raman scattering spectrum of the superlong Te nanotubes. The resonance peak at 114.8 cm-1 is a characteristic stretching mode of t-Te.

between Te and zeolite A.18 Here, we try to distinguish the vibration modes according to t-Se, because of the similar crystal structure between t-Te and t-Se. It is reasonable to attribute these bands to the E bond-bending, A1 bond-bending and A1 bondstretching modes or E mode of the t-Te tubes, respectively, because of similarity of these bands to the 145, 233, and 237

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