Electronic Transport Behavior of Bismuth Nanotubes with a

May 14, 2008 - with thinner walls shows a larger temperature dependence than that of BiNTs .... the wall thickness of the BiNTs reduces abruptly from ...
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J. Phys. Chem. C 2008, 112, 8614–8616

Electronic Transport Behavior of Bismuth Nanotubes with a Predesigned Wall Thickness Dachi Yang, Guowen Meng,* Qiaoling Xu, Fangming Han, Mingguang Kong, and Lide Zhang Key Laboratory of Materials Physics, and Anhui Key Laboratory of Nanomaterials and Nanostructures, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China ReceiVed: January 28, 2008; ReVised Manuscript ReceiVed: March 24, 2008

Bismuth nanotubes (BiNTs) were synthesized by electrodeposition inside the nanochannels of an anodic aluminum oxide template coated with a thin mesh-like Au layer onto one planar surface side. By tuning the Au layer thickness and current density during electrodeposition, BiNTs with a predesigned wall thickness and with a wall thickness variation along the axis were achieved. Measurements of resistance-temperature demonstrate that BiNTs show a semiconducting electronic transport behavior, and the resistance of BiNTs with thinner walls shows a larger temperature dependence than that of BiNTs with thick walls. Our approach could be used to build other materials that can be obtained via electrodeposition into nanotubes with a designed wall thickness that might have potential in future nanotechnology. Introduction Semimetallic bismuth (Bi) with a rhombohedral structure exhibits unique physical properties with potential applications due to its small effective mass, low density, and long mean free path of carriers.1 For example, both Bi thin films1 and Bi nanowires (BiNWs)2,3 show large magneto resistances and are good candidates for magnetic sensors. In recent years, BiNWs with various morphologies, such as linear BiNWs with a uniform diameter along the axis4–6 and step-7 and Y-shaped8 BiNWs, were synthesized via electrodeposition inside the corresponding shaped nanochannels of an anodic aluminum oxide (AAO) template. These Bi nanostructures can be either metallic or semiconducting depending on their diameters because of the semimetal-semiconductor transition induced by its quantum confinement effect,9 showing a unique electrical conductivity,7,8,10,11 size-dependent optical absorption,12 and thermoelectric6,13 and superconductivity properties.14 Recently, Bi nanotubes (BiNTs) were synthesized inside the nanochannels of an AAO template by electrodeposition.15 However, the synthesis of BiNTs with a predesigned wall thickness and BiNTs with a wall thickness variation along the axis is still a challenge. Herein, we demonstrate the synthesis of BiNTs with a predesigned wall thickness by modulating the sputtered Au layer thickness and the current density during electrodeposition; the schematic procedure is depicted in Figure 1.16 The as-prepared BiNTs show a semiconducting electronic transport behavior, and the resistance of BiNTs with thinner walls shows a larger temperature dependence than that of BiNTs with thick walls. Experimental Procedures AAO Template Fabrication and Au Sputtering. AAO templates were fabricated in a 0.3 M oxalic acid solution similar to work reported elsewhere.16,17 A thin mesh-like Au layer was sputtered onto one planar surface side of the AAO templates to serve as a working electrode in the electrodeposition.16 Electrodeposition and Microstructural Characterization. The electrolyte for BiNTs8,16 contained 75 g/L Bi(NO3)3 · 5H2O, 125 g/L C3H5(OH)3(glycerol), 50 g/L C4H6O6, and 65 g/L KOH * Corresponding author. E-mail: [email protected].

Figure 1. Schematic procedure for the synthesis of BiNT arrays. (a) Sputtering a mesh-like Au layer on one planar surface side of the AAO template and (b) electrodeposition of BiNTs.

and was buffered to pH 0.9 with nitric acid. BiNTs with a predesigned wall thickness and with a wall thickness variation along the axis were electrodeposited inside the nanochannels of the AAO template by tuning the thickness of the coated Au layer and parameters of the current densities; details will be described next. After the BiNTs were released from the template in a 5% NaOH solution and rinsed in deionized water thoroughly, the BiNTs were characterized by using fieldemission scanning electron microscopy (FE-SEM, Sirion 200, at 5 kV) with energy dispersive X-ray spectroscopy (EDS, Oxford), transmission electron microscopy (TEM, Hitachi 800, at 200 kV), and high resolution TEM (HRTEM, JEOL-2010, at 200 kV). R-T Curve Measurements on BiNT Arrays Embedded in AAO Template. A DC four-probe method was used in R-T curve measurements. Prior to the measurements, Au layers were sputtered on the dual planar surface sides of the AAO template embedded with BiNTs as conducting electrodes. Several thin copper wires were stuck on the sputtered Au layer with colloidal silver. The measured temperatures were performed in the range of 100 to 350 K. Results and Discussion Synthesis and Characterization of BiNTs with Predesigned Wall Thickness. Figure 2 shows the morphology of BiNT arrays achieved inside an AAO template with a Au layer of ∼25 nm in thickness and an electrodepositing current density of 1.5 mA/ cm2 for 30 min. Figure 2a reveals a top-view SEM image of the BiNT arrays after template removal. The closeup view (inset in Figure 2a) taken from the dashed rectangle in Figure 2a shows

10.1021/jp8008892 CCC: $40.75  2008 American Chemical Society Published on Web 05/14/2008

Electronic Transport Behavior of Bi Nanotubes

J. Phys. Chem. C, Vol. 112, No. 23, 2008 8615

Figure 4. Temperature dependence of the resistance of BiNT arrays with a designed wall thickness (normalized to resistance at 300 K). Curves correspond to BiNTs with wall thicknesses of 10 nm (I) and 20 nm (II) and wall thickness reductions from 20 to 10 nm abruptly (III) and uniformly (IV), respectively.

Figure 2. (a) Top-view SEM image of BiNT arrays; inset is a closeup view of the dashed rectangle with a scale bar of 100 nm. (b) Sideview SEM image. (c) Bottom-view SEM image; inset is an enlarged image with a scale bar of 100 nm. (d) Typical TEM image of BiNT roots on a sputtered Au layer after template removal.

Figure 3. TEM images of BiNTs with different wall thicknesses. (a and b) Uniform wall thicknesses of ∼10 and 20 nm, respectively. (c and d) Wall thickness reduction from 20 to 10 nm abruptly and uniformly, respectively. Scale bars are all 100 nm.

that the BiNTs are closed on top, similar to our previous result.16 Figure 2b is a side-view SEM image of a bundle of the BiNTs with diameters of ∼75 nm, in agreement with the pore diameter of the AAO template used. The bottom-view (Figure 2c) reveals that the bottom ends of the BiNTs are open after manual removal of the Au layer with sandpaper. Figure 2d is a TEM image of BiNT roots on the mesh-like Au layer separated from the AAO template after electrodeposition, indicating that BiNTs initiate from the thin Au layer edge around the nanochannels. Our experiments show that BiNTs with a designed wall thickness can be achieved by tuning the thickness of the sputtered Au layer and the current density during electrodeposition. Under the same applied current density, the thicker the Au layer sputtered on the AAO template (but keeping pores open), the thicker the walls of the BiNTs. For example, under the same current density of 0.7 mA/cm2, BiNTs with a wall thickness of ∼10 nm (Figure 3a) and 20 nm (Figure 3b) were obtained inside the channels (75 nm in diameter) of AAO templates coated with Au layers of ∼20 and 35 nm in thickness,

respectively. Selected-area electron diffraction (SAED) patterns taken from individual BiNTs (Figure 3a) reveal that BiNTs with thinner walls are polycrystalline, while those with thicker walls are single-crystalline (Figure 3b), similar to our previous work.16 As mentioned previously, BiNTs initiate from the circular edges of the mesh-like Au layer around the nanochannels of the AAO template; therefore, a thinner Au layer with shorter circular edges usually leads to nanotubes with thinner walls. Further experiments demonstrated that under a fixed thickness of the Au layer, the variation of current density during electrodeposition can result in a corresponding variation tendency of wall thickness along the BiNT axis. For example, with the same thickness of the Au layer of ∼35 nm and the same deposition conditions (constant current), when the applied current density is abruptly decreased from 0.7 to 0.45 mA/cm2, the wall thickness of the BiNTs reduces abruptly from 20 to 10 nm, as shown in Figure 3c (marked with arrows). In contrast, at the same Au layer thickness and deposition conditions mentioned previously, when the applied current density gradually reduces from 0.7 to 0.45 mA/cm2, the wall thickness of the BiNTs reduces uniformly along the growth direction, as shown in Figure 3d (marked with arrows). These results suggest that the wall thickness along the BiNTs axis can be tuned by modulating the current density during electrodeposition. The pH value of the electrolyte, hydrogen generation, and electrolyte characteristics also may affect the wall thickness of nanotubes, as was shown elsewhere.18 A microstructure of BiNTs with wall thickness variation can be found in the Supporting Information. Electronic Transport Properties of BiNTs. To understand the electronic transport properties of BiNTs with a designed wall thickness as shown in Figure 3, resistance-temperature (R-T) curves were measured. Figure 4 shows the temperature dependence of the resistances of BiNT arrays embedded in an AAO template, which were normalized to the resistance at 300 K.15,19 For all samples, the resistance decreases with an increase in temperature, suggesting that our BiNTs take on semiconducting electronic transport behavior. For BiNTs with a uniform wall thickness, the resistance of BiNTs with a thinner wall of ∼10 nm (curve I in Figure 4, corresponding to BiNT shown in Figure 3a) shows a larger temperature dependence than that of BiNTs with a thick wall of ∼20 nm (curve II in Figure 4, corresponding to BiNT shown in Figure 3b). For BiNTs with wall thickness variations from 20 to 10 nm along the axis, the resistance of BiNTs with an abrupt wall thickness reduction (curve III in Figure 4, corresponding to BiNT shown in Figure 3c) shows a larger temperature dependence than that of BiNTs with a uniform wall thickness reduction (curve IV in Figure 4, corresponding to BiNT shown in Figure 3d).

8616 J. Phys. Chem. C, Vol. 112, No. 23, 2008 Previous studies reveal that the electronic transport properties of BiNT arrays depend on the wall thickness and that the quantum confinement effect of Bi leads to the semimetalsemiconductor transition if the wall thickness of the nanotubes decreases to a certain value,15 thus showing semiconducting behavior. In our cases mentioned previously, the carrier mobility of BiNTs with various wall thicknesses is mainly dependent on the photoelectron scattering, nanotube boundary scattering, and grain boundary scattering.10,15 For BiNTs with a thinner wall of 10 nm (Figure 3a), the carriers undergo a strong quantum confinement effect, and the dominant scattering mechanisms for carriers are nanotube boundary and grain boundary scattering due to its polycrystalline nature. Comparatively, for BiNTs with a thick wall of 20 nm (Figure 3b), the carriers undergo a weak quantum confinement effect, and the dominant scattering mechanisms for carriers are mainly nanotube boundary scattering without grain boundary scattering due to their single crystal nature. Therefore, the resistance of BiNTs with a thinner wall of 10 nm (corresponding to Figure 3a) in curve I of Figure 4 is more sensitive to temperature than that of the BiNT with a thick wall of 20 nm (corresponding to Figure 3b) in curve II of Figure 4. For BiNTs with an abrupt wall thickness decrease from 20 to 10 nm (Figure 3c), the carriers undergo a transition from a weak to a strong quantum confinement effect abruptly, and the dominant scattering mechanisms for carriers change from nanotube boundary scattering to a combination of nanotube boundary and grain boundary scattering quickly. Similarly, for BiNTs with a uniform decrease in wall thickness from 20 to 10 nm (Figure 3d), the dominant scattering mechanisms for carriers change from nanotube boundary scattering to a combination of nanotube boundary and grain boundary scattering slowly. In contrast, the resistance of the BiNT with an abrupt wall thickness decrease (corresponding to Figure 3c) in curve III of Figure 4 is a little more sensitive to temperature than that of the BiNT with a uniform wall thickness decrease (corresponding to Figure 3d) in curve IV of Figure 4. Conclusion In summary, BiNTs with predesigned wall thicknesses were achieved by tuning the thickness of the Au layer sputtered on the planar surface side of the AAO template and the applied current density during the nanochannel confined electrodeposition. The BiNTs show semiconducting behavior. The resistances of BiNTs with thinner walls (10 nm) are more sensitive to temperature than that of BiNTs with thick walls (20 nm). Our approach could be used to build other semimetals that can be obtained via electrodeposition into nanotubes with a designed

Yang et al. wall thickness and different semiconducting behavior, which might have potential in future nanotechnology. Acknowledgment. This work was financially supported by the National Science Fund for Distinguished Young Scholars (Grant 50525207), the National Basic Research Program of China (Grant 2007CB936601), and the NSF of China (Grant 10374092). Supporting Information Available: Microstructures of BiNTs with wall thickness uniform reduction. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Yang, F. Y.; Liu, K.; Hong, K. M.; Reich, D. H.; Searson, P. C.; Chien, C. L. Science (Washington, DC, U.S.) 1999, 284, 1335. (2) Liu, K.; Chien, C. L.; Searson, P. C.; -Zhang, K. Y. Appl. Phys. Lett. 1998, 73, 1436. (3) Zhang, Z. B.; Sun, X. Z.; Dresselhaus, M. S.; Ying, J. Y.; Heremans, J. P. Appl. Phys. Lett. 1998, 73, 1589. (4) Huber, T. E.; Nikolaeva, A.; Gitsu, D.; Konopko, L.; Foss, C. A., Jr.; Graf, M. J. Appl. Phys. Lett. 2004, 84, 1326. (5) Wang, X. F.; Zhang, J.; Shi, H. Z.; Wang, Y. W.; Meng, G. W.; Peng, X. S.; Zhang, L. D.; Fang, J. J. Appl. Phys. 2001, 89, 3847. (6) Gitsu, D.; Konopko, L.; Nikolaeva, A.; Huber, T. E. Appl. Phys. Lett. 2005, 86, 102105. (7) Tian, Y. T.; Meng, G. M.; Wang, G. Z.; Phillipp, F.; Sun, S. H.; Zhang, L. D. Nanotechnology 2006, 17, 1041. (8) Tian, Y. T.; Meng, G. W.; Biswas, S. K.; Ajayan, P. M.; Sun, S. H.; Zhang, L. D. Appl. Phys. Lett. 2004, 85, 967. (9) Hoffman, C. A.; Meyer, J. R.; Bartoli, F. J.; Venere, A. D.; Yi, X. J.; Hou, C. L.; Wang, H. C.; Ketterson, J. B.; Wong, G. K. Phys. ReV. B: Condens. Matter Mater. Phys. 1993, 48, 11431. (10) Lin, Y.-M.; Cronin, S. B.; Ying, J. Y.; Dresselhaus, M. S.; Heremans, J. P. Appl. Phys. Lett. 2000, 76, 3944. (11) Zhang, Z. B.; Sun, X. Z.; Dresselhaus, M. S.; Ying, J. Y.; Heremans, J. P. Phys. ReV. B: Condens. Matter Mater. Phys. 2000, 61, 4850. (12) Black, M. R.; Hagelstein, P. L.; Cronin, S. B.; Lin, Y. M.; Dresselhaus, M. S. Phys. ReV. B: Condens. Matter Mater. Phys. 1998, 68, 235417. (13) Heremans, J.; Thrush, C. M. Phys. ReV. B: Condens. Matter Mater. Phys. 1999, 59, 12579. (14) Tian, M. L.; Wang, J. G.; Kumar, N.; Han, T. H.; Kobayashi, Y.; Liu, Y.; Mallouk, T. E.; Chan, M. H. W. Nano Lett. 2006, 6, 2773. (15) Li, L.; Yang, Y. W.; Huang, X. H.; Li, G. H.; Ang, R.; Zhang, L. D. Appl. Phys. Lett. 2006, 88, 103119. (16) Yang, D. C.; Meng, G. W.; Zhang, S. Y.; Hao, Y. F.; An, X. H.; Wei, Q.; Ye, M.; Zhang, L. D. Chem. Commun. (Cambridge, U.K.) 2007, 17, 1733. (17) Masuda, H.; Fukuda, K. Science (Washington, DC, U.S.) 1995, 268, 1466. (18) Fukunaka, Y.; Motoyama, M.; Konishi, Y.; Ishii, R. Electrochem. Solid-State Lett. 2006, 9, 62. (19) Yang, Y. W.; Li, L.; Huang, X. H.; Ye, M.; Wu, Y. C.; Li, G. H. Appl. Phys. A: Mater. Sci. Process. 2006, 84, 7.

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