Principle Scientists: Erik Menke
E.J. Menke, Q. Li, and R.M. Penner*, Bismuth Telluride (Bi2Te3) Nanowires Synthesized by Cyclic Electrodeposition/Stripping Coupled with Step Edge Decoration, NanoLetters 4 (2004) 2009[PDF], and,
E.J. Menke, M.A. Brown, Q. Li, J.C. Hemminger, and R.M. Penner*, Bismuth Telluride (Bi2Te3) Nanowires: Synthesis by Cyclic Electrodeposition/Stripping, Thinning by Electrooxidation, and Electrical Power Generation, Langmuir 22 (2006) 10564.[PDF]
Thermoelectric (TE) generators produce electrical power from a temperature gradient. Efficient thermoelectric devices are likely to require nanowires that are not only extremely narrow, but also extremely long - in excess of 50 microns. Min and Rowe have derived equations that allow performance metrics (power, efficiency, etc.) to be calculated for two-element Peltier devices, like that shown in Fig. 1 (inset). These equations, include a consideration of the properties of the electrical and thermal contacts. One important metric is the electrical power generated by such a device:
To make a long story short, when realistic values are substituted for these parameters, the effect of nanowire length can be assessed. For example, the dependence of Lo on P is plotted in Figure 1 for three different values of the ratio rho/rhoc (0.05, 0.1, and 0.2).
Figure 1. (inset) Schematic diagram of a two-element thermoelectric device showing parameters contained in Eq. 1. Plot of total power generated, P, from the device shown in (a) as a function of the nanowire length, Lo. Shown are plots for three values of the ratio rho/rhoc as indicated. Arrows mark the optimum nanowire length for each value of rho/rhoc. The values of the other parameters indicated in Eq. 2 are as follows: r = 10 x 10-6 mm, S = 2.00 x 10-4 microV/K, deltaT = 25 K, deltac = 7 kOhms, and Lc = 0.5 mm.
Fig 1 shows that P is maximized for nanowires that are 50 microns in length (for rho/rhoc = 0.05) or longer (rho/rhoc > 0.05), and that P falls rapidly with decreasing nanowire length below these optimum values. The plots of Fig. 1 were calculated for nanowires with a diameter of 10 nm, but the optimum nanowire lengths, indicated by the arrows, are identical across the diameter range from 1-100 nm. Since a value of rho/rhoc = 0.10 is typical for state-of-the-art devices, we conclude that nanowires longer than any synthesized to date may be required for device applications. In this Letter, we describe a method for preparing arrays of hundreds of Bi2Te3 nanowires that are 100 - 300 nm in diameter and up to 1 mm in length. This method, a variant of the electrochemical step edge decoration (ESED) developed in our lab, involves the electrodeposition of Bi2Te3 selectively at the step edges present on a graphite surface, as shown schematically in Figure 2.
Figure 2.Schematic diagram of the three-step method employed here for synthesizing Bi2Te3 nanowires by cyclic electrodeposition/stripping coupled with ESED.
The potential program for our method consists of three steps: 1) Mild oxidation of the basal plane step edges on a piece of highly oriented pyrolytic graphite (HOPG) at +0.80V for 5 s; 2) Nucleation of nanoscopic Bi2Te3 particles along the oxidized step edges at -0.60 V for 5 ms; 3) Codeposition of Bi2Te3 and excess bismuth during a negative-going potential scan from +0.3 to -0.05 V and subsequent anodic stripping of excess bismuth - producing a stoichiometric Bi2Te3 deposit - during a positive-going potential scan from -0.05 V to 0.30 V. Step 3 is then repeated a number of times until the Bi2Te3 particles grow large enough to coalesce, forming continuous Bi2Te3 nanowires. This cyclic electrodeposition/stripping strategy is identical in concept to the method described by Mike Sailor for the electrodeposition of stoichiometric thin films of CdSe. Actual voltammetric data for the growth of an ensemble of nanowires is shown in Figure 3.
Figure 3.Cyclic voltammograms, CVs, for the HOPG basal plane in contact with the plating solution employed for Bi2Te3 nanowire growth (blue line). This solution contained: 1.5 mM Bi(NO3)3 and 1.0 mM TeO, in 1 M HNO3.. Also shown are CVs acquired in plating solutions containing just Bi3+ (red line) and just TeO2+ (green line).
Here are what the resulting nanowires look like in the SEM:
Figure 3.a. Low magnification scanning electron microscope (SEM) image of Bi2Te3 nanowires prepared by cyclic electrodeposition/stripping. Bi2Te3 nanowires that nucleate and grow on step edges are arranged in parallel arrays. These nanowires are up to 1 mm in length. b. High magnification SEM image of Bi2Te3 nanowires prepared using 10 electrodeposition/stripping cycles. The mean, outermost diameter of these nanowires is 115 nm, however such wires are characterized by periodic constrictions - indicated by circles - that are 20 nm in diameter or smaller. c. SEM image of Bi2Te3 nanowires prepared using 90 electrodeposition/stripping cycles.
