Synthesis of Lanthanum Telluride Thin Films By Electrodeposition from Ionic Liquids

Thermoelectric devices allow for conversion between electrical and thermal energy and have a great deal of potential as both cooling devices and alternative energy sources. Currently they are limited by their low efficiency, and so are only used in very specialized applications. Some potential materials for use in high temperature thermoelectric devices include rare earth telluride compounds. One such material is bulk lanthanum telluride, La_3-xTe₄, 0 ≤ x ≤ 1/3, which was found to have a ZT greater than unity at temperatures near 1000 C [1]. Lanthanum is already commonly used in many industrial applications, including as lanthanum hydride in the cathode of nickel metal hydride batteries [2].

Nanostructured materials have been shown to have a higher thermoelectric efficiency relative to bulk materials due to the enhancement of phonon scattering through quantum confinement [3]. This has generated a great deal of interest in low-dimensional and nanostructured thermoelectric devices. Since lanthanum telluride is usually made by ball milling, the creation of complex nanostructures or thin films with traditional methods is difficult.

We have developed a technique to electrochemically deposit lanthanum telluride thin films from an ionic liquid at low temperatures (approximately 70 C). The films were deposited on noble metal substrates from 1-ethyl-3-methylimidazolium bromide ionic liquid based electrolytes containing to 0.05 M to 0.25 M lanthanum nitrate and 0.025M (HTeO2)⁺. The ratios of lanthanum to tellurium in the deposited films were determined by relative concentrations of the anions and cations in the electrolyte. The technique used for the deposition of  these thin films could ultimately be used in the synthesis of nanostructured lanthanum telluride. This electrochemical method promises to be simpler, less expensive, and more efficient than current techniques.

1. A. May, J-P. Fleurial and G. J. Snyder, Phys. Rev. B, 78, 125205 (2008).

2. T. Sakai et al., J. Less-Common Met., 161, 193-202 (1990).

3. L. D. Hicks and M. S. Dresselhaus, Phys. Rev. B, 47, 12727 (1993).