CRYSTAL GROWTH & DESIGN
Template-free Synthesis and Transport Properties of Bi2Te3 Ordered Nanowire Arrays via a Physical Vapor Process
2009 VOL. 9, NO. 7 3079–3082
Yuan Deng,* Yan Xiang,* and Yuanzeng Song School of Chemistry & EnVironment, Beihang UniVersity, Beijing 100191, China ReceiVed July 24, 2008; ReVised Manuscript ReceiVed April 30, 2009
ABSTRACT: Bi2Te3 thin films, composed of ordered nanowire arrays, have been successfully fabricated by a convenient physical vapor deposition method without using any template. The composition and microstructure of these films were determined by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and high resolution transmission electron microscopy (HRTEM). The effects of deposition rate, substrate temperature, and deposition time on morphologies of Bi2Te3 films were investigated. The effects of morphology of film on the electrical conductivity and Seebeck coefficient were also studied. The results show that the nanowire arrays are composed of single-crystalline Bi2Te3 nanowires with diameters of about 18 nm. The nanowires are parallel to each other and uniformly distributed. The film of nanowire arrays shows good transport properties. The growth mechanism of such nanostructure was proposed.
1. Introduction During recent years, thermoelectric materials aiming at thermoelectric devices applications have attracted much attention, because the application of thermoelectric microcooling devices is very promising for thermal management of microelectronics and optoelectronics.1-4 Preparation of thermoelectric materials in thin film form is highly required to fabricate a microcooling device at the chip level. The low-dimensional systems offer remarkable advantages for the thermoelectric figure of merit (ZT) enhancement owing to the sharper density of states and increased phonon scattering.5,6 Until now, the highest ZT of 2.4 was reported by Venkatasubramanian et al. in Bi2Te3/Sb2Te3 superlattice films.7 Nanowires (one-dimensional) are predicted to exhibit a better thermoelectric performance than superlattice films.8-10 Therefore, Bi2Te3 in the form of nanowires is an excellent candidate material for thermoelectric applications. Pure Bi2Te3 nanomaterials with different shapes have been extensively developed by hard templates approach, kinetic control solution growth, and self-assembly process.11-18 In order to transfer meaningful amounts of thermal energy, a practical device must consist of an array of a large number of nanowires in parallel. It is a challenge to fabricate oriented nanowire arrays with a high density. One way to synthesize these nanowire arrays is to use the pores of microporous and mesoporous materials, which serve as molds to template the diameter and orientation of wires of suitable thermoelectric materials. Bi2Te3 nanowire arrays have been fabricated by electrochemical deposition11,13 and confined precipitation19 in porous alumina. However, it is still a challenge to find a simple and universal strategy with a high degree of control for fabricating the thermoelectric nanowire arrays. In this work, we report a novel and one-step route for the largescale formation of Bi2Te3 nanowire arrays by a convenient physical vapor process, and the transport properties of ordered nanowire arrays were investigated. This physical vapor phase growth technology would be a promising way to prepare various nanowire arrays of other metal chalcogenides. * To whom correspondence should be addressed. E-mail: dengyuan@buaa. edu.cn (Y.D.);
[email protected].
Figure 1. Schematic depiction of the measurement configuration.
2. Experimental Section Bismuth telluride films composed of nanowire arrays were successfully prepared by using a vacuum system consisting of single physical vapor deposition (PVD) chambers, evaporating dish, substrate holder, and relative pump parts. The bismuth telluride powders (99.99% in purity) were mounted on the evaporating dish which is connected to the alternating current (AC) power supplies. Common glass substrates (26 mm × 70 mm) were cleaned thoroughly by diluted nitric acid and acetone, and dried under the nitrogen airflow. After the substrate was loaded onto the substrate holder (parallel to the dish), N2 gas was introduced into the chamber and vacuumized three times to remove oxygen. All the working pressure was maintained at 2 × 10-6 Torr in the deposition process. Deposition rate was measured and controlled by adjusting the electrical current passing through the evaporation container. The distance between the evaporation source and the substrate was determined by changing the substrate location. Powder X-ray diffraction (XRD) data were collected on a Rigaku D/MAX 2200 PC automatic X-ray diffractometer with Cu Ka radiation (λ ) 0.154056 nm). The surface morphology and sectional structure of the films were observed by field emission scanning electron microscopy (FE-SEM, HITACHIH S-4800) and transmission electron microscopy (TEM, HITACHIH-8100). Further structural and elemental analyses were performed using high-resolution transmission electron microscopy (HRTEM, FEI Company, Tecnai G2 F20S-Twin FEG TEM at 300 kV) and selected area electron diffraction (SAED). Thermoelectric properties (electric conductivity properties and Seebeck coefficient) of the films were examined. Electric conductivity was characterized by a four-probe method by a PZ158 instrument. A measurement method similar to Purkayastha et al.20 was introduced to obtain the Seebeck coefficient (S) as seen in Figure 1. The S value was determined from the slope of the measured Seebeck voltage versus the temperature difference across the specimen. A temperature gradient was established by a WY-99 double-door (A and B) temperature-control device, in which branch range A was performed as the heater and B as a heat sink. Electrodes monitoring the Seebeck voltage drop between the two thermocouples were adhered to the substrate before deposition process, which can minimize the contact resistance.
10.1021/cg800808u CCC: $40.75 2009 American Chemical Society Published on Web 05/12/2009
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Figure 2. SEM images of formed ordered Bi2Te3 nanowire arrays (a) surface view; (b, c) oblique view; and (d) cross-sectional view.
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Figure 3. XRD pattern of the product prepared at different deposition rates. (a) 3.0 nm/min; (b) 17 nm/min; (c) 1.8 nm/min; (d) 0.7 nm/min.
3. Results and Discussion Bi2Te3 films composed of nanowire arrays were successfully prepared on a glass substrate with an evaporation-deposition of Bi2Te3 powders for 8 h at a deposition rate of 3.0 nm/min. Scanning electron microscopy (SEM) images of Bi2Te3 nanowire arrays formed on glass substrates are shown in Figure 2. Figure 2a shows the surface view of the Bi2Te3 nanowire arrays. It can be clearly seen that Bi2Te3 nanowires are of equal height and have an ordered array with a density of approximately 8 × 1010 tips/cm2. Figure 2b,c shows oblique views of the Bi2Te3 nanowire arrays. The Bi2Te3 nanowires are of uniform diameter of 18 nm and perpendicular to the glass substrate. Because of the interaction between the nanowires, some nanowires gathered to form clusters. Figure 2d shows the cross-sectional view of the Bi2Te3 nanowire arrays. The parallel cylindrical wires grew from glass substrate. Some of the Bi2Te3 nanowires were broken by bending the film. The density of Bi2Te3 nanowires will easily satisfy the request of nanowire density (5 × 1010) for realizing the potential benefits of nanowire components in electronic devices.11 By changing the deposition time in our experiment, the length of nanowires can be controlled to be above 5 µm. The oriented growth of nanowire arrays was also revealed in XRD analysis. An X-ray diffractometer was used to characterize the crystal structure of the products. All peaks in the patterns correspond to the reflections of rhombohedral phase R3jm (JCPDS 15-0863) (shown in Figure 3a). The peaks (015) and (1010) are ultrastrong as compared with other peaks, while the (110) peak disappears. In addition, the (1010) diffraction peak was more intense than expected for isotropically distributed crystallites relative to (015), indicating a highly preferential orientation of the nanowires, along the (015) and (1010) directions, respectively. Evidence for this “texturing” is also seen in the HRTEM images. The image, shown in Figure 4a, indicates that Bi2Te3 nanowire arrays are still stable even after a long ultrasonic treatment. The nanowires exhibit no split or fissure, and the nanowires stay together after ultrasonic treatment. The lattice fringe between two nearby nanowires is very similar, which indicates an oriented growth direction along (015). The two white lines in Figure 4b are nearly parallel. This phenomenon leads to a uniform growth direction of the nanowire arrays. Figure 4c is the enlarged image of selected area marked by a square in Figure 4b. The lattice spacing of 0.325 and 0.231 nm
Figure 4. TEM and HRTEM images of Bi2Te3 nanowire cluster. (a) General overview; (b) enlarged image of nanowires; (c) the image of selected area marked by the square; (d) the corresponding SAED of (b).
correspond to the lattice of (015) and (1010) crystal planes, respectively. This confirms that the nanowires grow along the 〈015〉 and 〈1010〉 directions. The selected area electron diffraction (SAED) pattern in Figure 4d indicates that the nanowires are single crystal and can be indexed as the rhombohedral Bi2Te3 phase, which is in accordance with the XRD results. The guiding principle in vapor phase growth is to control the supersaturation of the vapors. The degree of supersaturation plays an important role in the control over the morphology of the obtained nanostructures. Generally, high supersaturation leads to the growth of powders, while medium supersaturation is favorable for the preparation of whisker and nanowires, and low supersaturation for bulk single crystal growth. In our
Bi2Te3 Ordered Nanowire Arrays
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Figure 5. SEM images of the product prepared at different deposition rates: (a) 0.7 nm/min; (b, c) 1.8 nm/min; (d, e) 17 nm/min. Table 1. Transport Properties of Bismuth Telluride Films with Different Morphologies morphology flake multilayer nanowires ordered nanowires
growth rate electrical Seebeck coefficient (nm/min) conductivity (S/m) (µV K-1) 0.7 17 1.8 3
5732 6330 2264 2315
-65.7 -64.7 -118 -150
experiment, the degree of supersaturation is controlled by the deposition rate. The orderliness of the nanowires improved with the increasing of the deposition rate. When the deposition rate is low as 0.7 nm/min, the film is obviously stacked by uniform hexagonal flakes with a diameter of about 150 nm (shown in Figure 5a). When the deposition rate is about 1.8 nm/min, wires occur (shown in Figure 5b,c). With the increase of the deposition velocity from 1.8 to 3 nm/min, the growth of the nanowire arrays is better (as seen in Figure 2). The crystal growth in the radial direction is faster than that of the planar direction during this adjusting process. When the deposition rate further increases to 17 nm/min, a multilayer film rather than a single layer film was observed (shown in Figure 5d,e). In this condition, the nucleation rate of Bi2Te3 is faster than that of crystal growth, which results in an inevitable new nucleation and growth process on the early film. Then multilayer film forms. The intensity of the (1010) peak of the above product is enhanced with the increase of deposition rate (shown in Figure 3), which indicates that the growth balance between the (1010) and (015) direction is the essential reason for the formation of nanowire arrays. The effects of morphology of film on the electrical conductivity and Seebeck coefficient were studied. Table 1 shows the transport properties of Bi2Te3 films with different morphologies. The film of nanowire arrays exhibits a linear current-voltage (I-V) curve (see Figure 6a), indicating that the contacts are ohmic. The slope yields a resistance of ∼47 Ω. According to a one-dimensional (1D) electrical transport model, we obtained
an electrical conductivity σ ∼ 2300 S/m, a value approximately 1 order of magnitude smaller than bulk bismuth telluride materials. Figure 6b shows that the Seebeck voltage varies linearly as a function of the corresponding temperature difference. The Seebeck voltage is measured between different points on the film surface. Then Seebeck coefficient (S) is calculated to be about -150 µV K-1 from the slope, which means that the Bi2Te3 film is an n-type semiconductor. The Seebeck coefficient and electrical conductivity in the film of nanowire arrays are promisingly higher than those of films of disordered Bi2Te3 nanorods.20 An even higher S value of the films in the cross-sectional direction is expected due to the microstructure of nanowire arrays. Then those films are promising materials applied for cooler devices. The film of flakes exhibits a Seebeck coefficient of about -65.7 µV K-1 and an electrical conductivity of 5732 S/m. The multilayer film shows a high conductivity of 6330 S/m, while the Seebeck coefficient is down to -64.7 µV K-1. The Seebeck coefficient of nanowires is enhanced with the increase of the orderliness of arrays, and the electrical conductivity remains stable. The lower electrical conductivity of nanowires is probably due to numerous interspaces between the nanowire arrays in the film. The growth process of nanowire arrays was explored by varying the deposition time. When the deposition time is 20 min, only small hexagonal islands are obtained on the substrate (see Figure 7a). When deposition time increases to 40 min, crystal growth on hexagonal islands has preferential growth along the radial direction of the islands, which gives rise to nanorods (see Figure 7b). Then nanowire arrays formed with the deposition time increasing to 8 h. Although the detailed growth mechanism of oriented Bi2Te3 nanowire arrays is still not clear, the above results allow us to suggest a possible process to obtain Bi2Te3 nanowire arrays. It has weak van der Waals interactions between amorphous glass substrate and the additional Bi2Te3 atoms (ad-atoms), while the strain energy is strong between Bi2Te3 atoms. Then the driving force of the film would be the strain energy. The growth process of the film is illustrated in Figure 8, which is a Volme-Weber mode21 to give an island growth. First, Bi2Te3 atoms were evaporated and deposited on the surface of the substrate (see Figure 8a). Then, nucleation occurred and hexagonal islands formed due to the anisotropic diffusion (Figure 8b).22 Usually, the preferential growth of the (015) crystal plane makes it easy to form hexagonal flakes, while the preferential growth of (1010) and (015) crystal planes results in rods rather than hexagonal flakes. It is the key for fabricating Bi2Te3 nanowire arrays to keep a proper deposition rate. If the deposition rate of Bi2Te3 atoms was higher than that of the crystal growth rate, a new nucleation and new growth process occurred on the film and formed a multilayer film. The potential energy on the edge of islands is much higher than that of the center, so ad-atoms deposited on the islands were incorporated within the island itself.23 The elastic strain energy on the top parts of 3D islands is usually much lower than that at the bottom.21 As a result, the surface diffusion due to the strong uneven distribution of strain energy, drives ad-atoms to climb up to the tops of islands, and gives rise to the rapid radial growth of 3D islands (see Figure 8c). The adjacent islands blocked adatoms come from planar direction, and caught hold of ad-atoms deposited from the radial directions. Then the interspaces among islands formed and led to the growth of nanowire perpendicular to the glass substrate. Finally, nanowire arrays were obtained on the substrate (see Figure 8d).
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Figure 6. (a) I-V characteristics of the film of nanowire arrays; (b) dependence of the Seebeck voltage as a function of temperature difference along the thermoelectric film.
of other metal chalcogenides. The films of nanowire arrays also exhibit attractive thermoelectric properties, leading to novel thermoelectric materials and devices for applications. Acknowledgment. The work was supported by National Natural Science Foundation of China under Grant No. 50772005.
References Figure 7. SEM images of the initial sample: (a) deposited for 20 min; (b) deposited for 40 min.
Figure 8. The growth mechanism of the Bi2Te3 nanowire arrays.
4. Conclusions In summary, we have demonstrated a template-free approach to prepare ordered Bi2Te3 nanowire arrays by a convenient physical vapor process for the first time. The degree of supersaturation plays an important role in the control over the morphology of the obtained nanostructures. The formation of nanowire arrays is an island growth due to (1010) preferential growth matched with (015) spontaneous growth. This synthetic protocol could be extended for the synthesis of nanowire arrays
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