Chem. Mater. 2003, 15, 335-339
335
Structure of Bismuth Telluride Nanowire Arrays Fabricated by Electrodeposition into Porous Anodic Alumina Templates M. S. Sander,†,§ R. Gronsky,*,‡ T. Sands,‡ and A. M. Stacy† Department of Chemistry and Department of Materials Science & Engineering, University of California, Berkeley, California 94720 Received July 30, 2002. Revised Manuscript Received November 4, 2002
Arrays of bismuth telluride (Bi2Te3) nanowires with diameters of ∼25, ∼50, and ∼75 nm have been produced by electrochemical deposition into porous anodic alumina templates. Scanning electron microscopy confirms that the nanowire arrays are dense with a narrow distribution of nanowire diameters. The structure of the nanowires was assessed immediately after deposition, after annealing to ∼80% of the melting point, and after melting/ recrystallization. As determined by XRD analysis, there is strong fiber texture in the arrays that depends on both the nanowire diameter and the postdeposition processing conditions. Bright-field/dark-field imaging and diffraction in the transmission electron microscope reveal that the as-deposited nanowires are polycrystalline with a bamboo-type grain structure that does not change significantly upon annealing, and a similar grain structure is obtained after melting and resolidification.
Introduction Nanowires have a variety of potential applications based on their unique properties relative to the bulk, which result from confinement effects and/or significant surface effects due to their small size. Arrays of nanowires are suitable for practical applications where high signal and high density are necessary. The properties of nanowire array composites are strongly dependent on the distribution of nanowire sizes in the array as well as the nanostructure and orientation of the individual wires. Arrays of bismuth telluride nanowires are of special interest as thermoelectric materials. Bulk Bi2Te3 has a high thermoelectric figure-of-merit and is widely used in commercial applications. Superlattices of Bi2Te3/Sb2Te3 have recently been shown to have an enhanced thermoelectric figure-of-merit relative to bulk Bi2Te3,1 and theoretical studies suggest that 1D structures (nanowires) may have an even higher figure-of-merit.2-4 In previous work, both ∼200 nm5-8 and ∼40 nm9 diameter nanowire arrays have been fabricated by deposition of Bi2Te3 into porous anodic alumina templates. * To whom correspondence should be addressed. Phone: 510-6439708. Fax: 510-643-5792. E-mail:
[email protected]. † Department of Chemistry. § Current address: Institute of Materials Research and Engineering, 3 Research Link, Singapore, 117602. ‡ Department of Materials Science & Engineering. (1) Venkatasubramanian, R.; Siivola, E.; Colpitts, T.; O’Quinn, B. Nature 2001, 413, 597. (2) Hicks, L. D.; Dresselhaus, M. S. Phys. Rev. B 1993, 47, 16631. (3) Dresselhaus, M. S.; Dresselhaus, G.; Sun, X.; Zhang, Z.; Cronin, S. B.; Koga, T. Phys. Solid State 1999, 41, 679. (4) Dresselhaus, M. S.; Dresselhaus, G.; Sun, X.; Zhang, Z.; Cronin, S. B.; Koga, T.; Ying, J. Y.; Chen, G. Microscale Thermophys. Eng. 1999, 3, 89. (5) Sapp, S. A.; Lakshmi, B. B.; Martin, C. R. Adv. Mater. 1999, 11, 402. (6) Huber, T. E.; Calcao, R. Thermoelectric properties of Bi And Bi2Te3 Composites. In 16th International Conference on Thermoelectrics, Dresden, Germany, 1997; American Institute of Physics Press: New York, 1997.
These templates have good characteristics for nanowire array fabrication because they are thermally and mechanically stable and can be produced with a high density of high-aspect-ratio, parallel, nearly uniform pores with diameters ranging from ∼10 nm to several hundred nanometers.10-13 Although a variety of methods have been reported for depositing Bi2Te3,14-18 we have chosen electrodeposition because it enables good control over stoichiometry, can be used to deposit highaspect-ratio structures, and results in electrically continuous wires. In this work, we have fabricated porous anodic alumina array composites with nanowire diameters of ∼25, 50, and 75 nm using previously established electrodeposition parameters for Bi2Te3 films.19-21 (7) Huber, C.; Sadoqi, M.; Huber, T.; Chacko, D. Adv. Mater. 1995, 7, 316. (8) Huber, C. A.; Huber, T. E.; Sadoqi, M.; Lubin, J. A.; Manalis, S.; Prater, C. B. Science 1994, 263, 800. (9) Prieto, A. L.; Sander, M. S.; Martin-Gonzalez, M. S.; Gronsky, R.; Sands, T.; Stacy, A. M. J. Am. Chem. Soc. 2001, 123, 7160. (10) Huczko, A. Appl. Phys. A 2000, 70, 365. (11) Martin, C. R. Chem. Mater. 1996, 8, 1739. (12) Martin, C. R. Science 1994, 266, 1961. (13) Routkevitch, D.; Tager, A. A.; Haruyama, J.; Almawlawi, D.; Moskovits, M.; Xu, J. M. IEEE Trans. Electron Devices 1996, 43, 1646. (14) Ding, Z. F.; Viculis, L.; Nakawatase, J.; Kaner, R. B. Adv. Mater. 2001, 13, 797. (15) Cho, S. L.; Kim, Y.; DiVenere, A.; Wong, G. K.; Ketterson, J. B.; Meyer, J. R. Appl. Phys. Lett. 1999, 75, 1401. (16) Boulouz, A.; Giani, A.; Pascal-Delannoy, F.; Boulouz, M.; Foucaran, A.; Boyer, A. J. Cryst. Growth 1998, 194, 336. (17) Ferhat, M.; Liautard, B.; Brun, G.; Tedenac, J. C.; Nouaoura, M.; Lassabatere, L. J. Cryst. Growth 1996, 167, 122. (18) Dauscher, A.; Thomy, A.; Scherrer, H. Thin Solid Films 1996, 280, 61. (19) Magri, P.; Boulanger, C.; Lecuire, J. M. J. Mater. Chem. 1996, 6, 773. (20) Magri, P.; Boulanger, C.; Lecuire, J. M. Electrodeposition of Bi2Te3 Films. In 13th International Conference on Thermoelectrics, Kansas City, MO, 1994; American Institute of Physics: New York, 1994. (21) Fleurial, J. P.; Borshchevsky, A.; Ryan, M. A.; Phillips, W.; Kolawa, E.; Kacisch, T.; Ewell, R. Thermoelectric Microcoolers for Thermal Management Applications. In 16th International Conference on Thermoelectrics, Dresden, Germany, 1997; American Institute of Physics: New York, 1994.
10.1021/cm0207604 CCC: $25.00 © 2003 American Chemical Society Published on Web 12/17/2002
336
Chem. Mater., Vol. 15, No. 1, 2003
Sander et al.
Figure 1. SEM cross-section images (emissive mode) of empty porous anodic alumina templates and Bi2Te3 nanowire array/ template composites. (a) Empty template and (b) array composite with average wire diameter of 75 nm and (c) empty template and (d) array composite with average wire diameter of 25 nm. The insets show pore diameter distribution for large (a) and small (c) diameter templates. Dark areas in these images are due to empty pores, whereas bright spots are due to the Bi2Te3 nanowires.
We have studied both the composite structure of nanowires arrayed in alumina templates as well as the structure of individual nanowires after releasing the wires from the alumina. SEM has been employed to assess the nanowire array composite morphology, and XRD reveals information about the nanowire structure and texturing. Bright-field and dark-field imaging in the TEM have been used to assess characteristic grain size, morphologies, crystallinity, and grain boundaries within individual polycrystalline nanowires. Experimental Section Bi2Te3 nanowire arrays were fabricated using a procedure described previously.22 A silver film (∼1 µm thick) was sputtered onto one side of a piece of aluminum foil (130 µm thick × ∼2 cm2 with a long strip at one corner to contact during anodization, 99.9995% Alfa Aesar) to serve as an electrode for electrodeposition. After mechanically and electrochemically polishing the top surface of the aluminum, the aluminum/Ag coupon was affixed to a glass microscope slide using CrystalBond adhesive, and the edges of the Al were coated for protection from etching by nail polish (Revlon TopSpeed). Then the aluminum foil was anodized using a Pt mesh counter electrode and previously established conditions of 40 V in 0.3 M oxalic acid for larger (∼75 nm) diameter pores, 30 V in 0.3 M oxalic acid for medium (∼50 nm) diameter pores, and 25 V in 20 wt % H2SO4 for smaller (∼25 nm) diameter pores. The uniformity of the anodization reaction was assessed visually. The template/Ag assembly appears dark in patches (>1 mm2) where the aluminum has fully anodized. Each template assembly was then transferred to the electrodeposition solution, 0.075 M Bi and 0.1 M Te in 1 M HNO3. The assembly was submerged in this solution at room temperature for a period of 30 min to 2 h depending on the desired amount of pore widening, after which the solution was cooled in an ice bath. Electrodeposition was conducted at -0.46 V vs Hg/Hg2SO4 (sat. K2SO4) in a three-electrode configuration with the template assembly as the working electrode, Pt mesh as the counter electrode, and the reference solution in a separate (22) Sander, M. S.; Prieto, A. L.; Gronsky, R.; Sands, T.; Stacy, A. M. Adv. Mater. 2002, 14, 665.
beaker of 1 M KNO3 connected to the deposition solution via a KNO3/agar salt bridge. After several hours, the Bi2Te3 deposit filled the nanopores and began growing across the top surface of the template, causing the template assembly color to change from black to gray. To prepare the templates for study by XRD, the Bi2Te3 that overfilled the pores and the Ag film were mechanically polished away. For SEM imaging, the bottom surfaces of the arrays were polished using a colloidal silica (∼50 nm) suspension, then rinsed thoroughly with water and dilute HNO3 followed by sputter deposition of a thin carbon layer to avoid charging. To study individual nanowires by TEM, the alumina template was dissolved in a solution of 3.5 vol % H3PO4 and 45 g/L CrO3 at room temperature for ∼24 h, then sonicated briefly, and the resulting nanowire solution was diluted in several steps with water and finally ethanol. The nanowires were then dispersed from the ethanol suspension onto a holey carbon grid. The SEM images were obtained using a JEOL 6340F, under typical working conditions of 5 kV. The pore-size distribution in the templates was assessed using AnalySIS software (Soft Imaging System). TEM studies were performed using Philips CM200 and CM300 instruments at 200 kV. Dark-field images were obtained using a displaced objective aperture. Energydispersive spectroscopy was performed on both nanowire arrays and individual nanowires in the SEM and TEM, respectively, and results confirmed the composition of the nanowires to be approximately 40:60 Bi/Te within experimental error.
Results and Discussion Array Structure. SEM images of empty nanoporous alumina templates and filled arrays of Bi2Te3 nanowires within the templates are shown in Figure 1. The pores are dense and hexagonally ordered over small regions. The distribution of pore diameters in the templates is shown within insets of the SEM images of the empty pores (Figure 1a and c). The mean pore diameters and standard deviations for the two samples shown and one representative medium-diameter pore sample (not shown) are 74.6 ( 3.1 nm, 47.3 ( 7.5 nm, and 24.5 ( 2.7 nm for the large, medium, and small diameter pores,
Structure of Bismuth Telluride Nanowire Arrays
respectively. Although the pore-diameter distribution is relatively narrow in each case, it is possible to produce an even narrower distribution with longer range ordering by using a longer, 2-step anodization procedure,23 or by patterning the initial Al surface using a mold.24-27 In this case, we have chosen to use anodization conditions that have been demonstrated to produce hexagonal pore ordering.28-30 With a short pore widening step for the smaller and medium diameter pores and a longer step for the larger pores, these three cases represent the middle and near extremes for small and large pore diameters that are possible using these anodization conditions. By tuning the pore widening time, it is possible to produce well-ordered, hexagonal pore arrays with diameters anywhere between these two extremes (∼20-80 nm). It is also possible to produce hexagonal pore arrays with larger diameters (∼200 nm) using a phosphoric acid solution for anodization,31 or to produce dense, smaller diameter, but not hexagonally ordered arrays using smaller potentials in a H2SO4 electrolyte. From the images of Figure 1b and d it is apparent that there is a high density of nanowires within the template structure. These images are of the bottom surface of the nanowire array composites after polishing off the Ag electrode layer. Recently we reported that by employing the fabrication process described here, we were able to fabricate dense, thick, large-area ∼45 nm diameter nanowire arrays in templates. The amount of pore filling is limited by defects in the alumina template, which may block or restrict diffusion and solution penetration in the very narrow, high-aspect-ratio pores. To improve pore filling, a mold may be used to pretexture the aluminum surface before anodization. This method has been employed to produce nearly perfectly cylindrical pores, with very few defects even for thick templates (tens of µm).24 XRD was employed to assess the structure in the nanowires of the array composites. XRD patterns are shown in Figure 2 for the as-deposited nanowire arrays and for the arrays after annealing at 400 °C for 3 h. The XRD peaks all indexed to Bi2Te3, except for the small, broad peak at ∼23° which is attributed to the alumina template. For the larger diameter wires (∼75 nm), the dominant 110 peak indicates that the wires are strongly textured with the {110} planes perpendicular to the wire axis. Upon annealing, the pattern changes very little, with the 110 peak still dominant. The same behavior was observed in arrays with an average wire diameter of ∼50 nm. The strong 〈110〉 texturing is consistent with results of XRD analysis on thin films produced by potentiostatic electrochemical (23) Masuda, H.; Fukuda, K. Science 1995, 268, 1466. (24) Masuda, H.; Yamada, H.; Satoh, M.; Asoh, H.; Nakao, M.; Tamamura, T. Appl. Phys. Lett. 1997, 71, 2770. (25) Asoh, H.; Nishio, K.; Nakao, M.; Tamamura, T.; Masuda, H. J. Electrochem. Soc. 2001, 148, B152. (26) Asoh, H.; Nishio, K.; Nakao, M.; Yokoo, A.; Tamamura, T.; Masuda, H. J. Vac. Sci. Technol. B 2001, 19, 569. (27) Masuda, H.; Asoh, H.; Watanabe, M.; Nishio, K.; Nakao, M.; Tamamura, T. Adv. Mater. 2001, 13, 189. (28) Jessensky, O.; Muller, F.; Gosele, U. Appl. Phys. Lett. 1998, 72, 1173. (29) Li, A. P.; Muller, F.; Birner, A.; Nielsch, K.; Gosele, U. J. Appl. Phys. 1998, 84, 6023. (30) Masuda, H.; Hasegwa, F.; Ono, S. J. Electrochem. Soc. 1997, 144, L127. (31) Masuda, H.; Yada, K.; Osaka, A. Jpn. J. Appl. Phys. 1998, 37, L1340.
Chem. Mater., Vol. 15, No. 1, 2003 337
Figure 2. XRD spectra of Bi2Te3 nanowire array composites. (a) As-deposited and (b) annealed (400 °C for 3 h) 75-nm-diam Bi2Te3 nanowire arrays; (c) as-deposited and (d) annealed (400 °C for 3 h) 25-nm-diam Bi2Te3 nanowire arrays.
deposition using the same deposition conditions.21,32 In the as-deposited smaller diameter wires (∼25 nm), both the 110 and 300 peaks are strong, with a weaker contribution from the 101 peak. After annealing, the 300 and 101 peak intensities are significantly reduced, while the 110 peak remains strong, and a strong 015 peak appears. On the basis of these results, it appears that fiber texture development in the arrays depends on the initial pore diameter, and can be changed by postdeposition annealing. Additional information on the grain structure may be obtained by assessing the peak width, which is related to average grain size and/or lattice strain; however, in this case the nanowire diameter is on the order of the grain size (see below), which makes such an analysis difficult.33 Nevertheless, in the spectra from the smaller diameter wires, the 300 peak is clearly more broad than the 110 and other peaks, suggesting that the grains oriented with the 300 planes perpendicular to the wire axis are on average smaller, or subject to greater lattice strain, than grains with other orientations. This information could perhaps be used to gain insight into the electrochemical deposition mechanism within the pores. However, to draw quantitative conclusions about the grain size and orientation in the arrays from XRD analysis, a more thorough study of these parameters as a function of wire diameter and processing conditions is required. In this study we employed electron microscopy for complementary data in the internal structure of individual nanowires. Nanowire Structure. A representative phase contrast high-resolution transmission electron microscope image of the edge of a nanowire (∼50 nm diam) is shown in Figure 3. The lattice fringes apparent in this image indicate perfect crystallinity across this section of the nanowire. A thin amorphous layer is evident at the (32) Fleurial, J. P.; Borshchevsky, A.; Ryan, M. A.; Phillips, W. M.; Snyder, J. G.; Caillat, T.; Kolawa, E. A.; Herman, J. A.; Mueller, P. Development of thick-film thermoelectric microcoolers using electrochemical deposition. Mater. Res. Soc. Symp. Proc. 1999. (33) Routkevitch, D.; Bigioni, T.; Moskovits, M.; Xu, J. M. J. Phys. Chem. 1996, 100, 14037.
338
Chem. Mater., Vol. 15, No. 1, 2003
Sander et al.
Figure 3. High-resolution TEM image of the edge of ∼50nm Bi2Te3 nanowire. The thin (∼2 nm) amorphous region at the top is attributed to a thin surface oxide layer.
outermost edge of the wire (∼2 nm thick), which is ascribed to a small amount of surface oxide. Typical selected-area diffraction and BF/DF images of as-deposited and annealed (400 °C for 3 h) nanowires are shown in Figure 4. These wires are ∼50 nm in diameter, and their internal structure is similar to that of other wires with different diameters. The nanowires are dense, continuous, and of uniform diameter along the wire length. The selected-area diffraction patterns (inset Figure 4a and c) taken from as-deposited and annealed wires show overlapping crystalline patterns, indicating that several grains are present within each of the wires, that is, they are polycrystalline. In the bright-field images, variation in diffraction contrast is apparent along the length of the wires. Using +g/-g dark-field imaging analysis, it was determined that this contrast arises from segments of individual grains oriented for strong Bragg scattering. In the darkfield images, several grains exhibiting strong diffraction contrast are simultaneously visible as small, bright regions, a consequence of the strong fiber texture in these wires. As shown in Figure 4d, we found that there is little increase in the average grain size even upon annealing the templates to ∼80% of the Bi2Te3 melting point (400 °C, mp 580 °C). The grain structure in the nanowires can be described as bamboo-type, where the grain widths are typically on the order of the wire diameter and the grain boundaries are mostly oriented perpendicular or nearly perpendicular to the wire axis. With this type of grain structure, the total grain boundary area is minimized, and there is very little driving force for grain growth.34 Coalescence of adjacent bambootype grains is not favored because the bamboo-type structure represents a very stable local free energy minimum. Therefore, when the nanowires are annealed, the grain boundaries may propagate slightly along the length of the wire (as we have observed occasionally under electron beam irradiation in the TEM), but significant further grain growth does not occur. Grain growth in narrow-diameter quasi-1D structures such as the nanowires described here has been studied extensively in order to understand failure modes due to electromigration in interconnects for microelectronics applications.35 (34) Mullins, W. W. Acta Metallogr. 1958, 6, 414.
Figure 4. Bright-field (BF) and dark-field (DF) TEM images of individual Bi2Te3 nanowires with an average wire diameter of ∼50 nm. (a) BF and (b) DF of an as-deposited wire; (c) BF and (d) DF of an annealed wire (400 °C for 3 h). The insets in (a) and (c) are selected-area diffraction patterns from the individual wires.
Melting/Recrystallization. As shown above, the asdeposited nanowires are polycrystalline with a small average grain size, and annealing does not promote significant grain growth because of their highly stable bamboo-type grain structure. Therefore, we studied the effect of melting and resolidifying the nanowires under an applied temperature gradient, in an attempt to promote single-crystal wire growth during the solidification reaction. Bi2Te3 is a congruently melting compound within a narrow composition range, which allows for melt synthesis of nanowires with a homogeneous structure and composition. To melt the wires in the array without evaporating the wire material, an encapsulating oxide layer was deposited onto both sides of a nanowire array/template composite. The template was then heated to 650 °C, which is significantly above the melting temperature of Bi2Te3 (∼580 °C), and the composite was allowed to cool to room temperature over ∼20 h. By keeping one side of the composite flush with the surface of (35) Fayad, W. R.; Kobrinsky, M. J.; Thompson, C. V. Phys. Rev. B 2000, 62, 5221.
Structure of Bismuth Telluride Nanowire Arrays
Figure 5. XRD spectra of an (a) as-deposited and (b) recrystallized Bi2Te3 nanowire array composite with an average wire diameter of ∼75 nm. The nanowire arrays were melted by heating to ∼650 °C and resolidified by allowing the composite to cool over a period of ∼20 h. (The peak at ∼45° is attributed to crystallization of the alumina template.)
Figure 6. BF (a) and DF (b) images of several Bi2Te3 nanowires after melting/recrystallization.
a ceramic crucible and flowing N2 over the other side of the array, we attempted to establish a temperature gradient across the ∼50-µm-thick structure during recrystallization. This experiment was conducted three times on templates with the largest diameter (∼75 nm) wires. The XRD spectrum of the recrystallized nanowire array, shown in Figure 5b, is dominated again by the 110 peak; the peak at ∼45° is attributed to crystallization in the alumina template.36 The grain orientation in the recrystallized nanowires is therefore similar to that of the as-deposited and annealed arrays, and this same texturing was also observed in ∼200-nm-diameter Bi2Te3 nanowire arrays produced by a pressure injection process.7 Representative bright-field and dark-field images of the recrystallized wires are shown in Figure 6. It is clear
Chem. Mater., Vol. 15, No. 1, 2003 339
from these images that melting and recrystallization of the wires resulted in a grain structure that is very similar to the as-deposited structure with small, bambootype grains. A polycrystalline grain structure was also reported for ∼200 nm wires produced by pressure injection, where the average grain size was determined to be ∼40 nm using Scherrer’s equation to evaluate the peak width in XRD spectra.6 Although the experimental control of a temperature gradient to achieve single crystals is well-established for larger structures (for example zone-melting in semiconductor growth), this approach is more difficult for dimensionally restricted structures such as the templates employed in this work (thickness ∼50 µm). It may be possible to produce single-crystalline wires using this method if a large temperature gradient could be established over such small distances. Alternatively, it may be possible to control the electrodeposition conditions during initial nanowire growth to produce single-crystal Bi2Te3 nanowires, as has been reported for the fabrication of single-crystal CdS37 and Cu38 nanowires using DC electrodeposition and fabrication of Pb nanowires by pulse electrodeposition.39 Conclusions We have studied the nanowire diameter distribution, density, texturing, and grain structure in Bi2Te3 nanowire arrays produced by electrodeposition into porous anodic alumina templates. The arrays are dense with a narrow distribution of nanowire diameters, which is important for potential applications as the nanowire properties depend on the diameter due to quantum confinement and a strong contribution from interface effects. The nanowires show a strong 〈110〉 fiber texture, with the texture orientation in the ∼25-nm-diameter wires differing from that of the larger diameter wires and changing upon annealing. The texture variation with annealing conditions for the narrower diameter wires suggests the ability to control grain orientation, and therefore properties, in the nanowire arrays. The grain structure in the as-deposited, annealed, and recrystallized wires is similar, and can be described as a bamboo-type structure. This type of grain structure with minimum grain boundary area is thermodynamically stable, and nanowires with this structure are not subject to failure due to electromigration at high currents. However, due to the stability of the grain structure, it is difficult to increase the average grain size as may be desirable for some potential applications. Acknowledgment. This work was supported by a DoD MURI grant (N00014-97-1-0516). Electron microscopy was performed at the National Center for Electron Microscopy (NCEM), Lawrence Berkeley National Laboratory, Berkeley, CA. Note Added after ASAP Posting. Due to a production error, incorrect versions of the figure captions were posted ASAP on 12/17/2002. The correct version of the article was posted on 12/18/2002. CM0207604 (36) Li, Y.; Li, G. H.; Meng, G. W.; Zhang, L. D.; Phillipp, F. J. Phys. - Condens. Mater. 2001, 13, 2691. (37) Xu, D. S.; Xu, Y. J.; Chen, D. P.; Guo, G. L.; Gui, L. L.; Tang, Y. Q. Adv. Mater. 2000, 12, 520. (38) Molares, M. E. T.; Buschmann, V.; Dobrev, D.; Neumann, R.; Scholz, R.; Schuchert, I. U.; Vetter, J. Adv. Mater. 2001, 13, 62. (39) Yi, G.; Schwarzacher, W. Appl. Phys. Lett. 1999, 74, 1746.
