A Novel Approach to Synthesize Lanthanum Telluride Thermoelectric Thin Films in Ambient Conditions
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A Novel Approach to Synthesize Lanthanum Telluride Thermoelectric Thin Films in Ambient Conditions Su (Ike) Chih Chi1, Stephen L. Farias1, and Robert C. Cammarata1-2 1 Department of Materials Science and Engineering, Johns Hopkins University, 3400 N. Charles Street, Baltimore, Maryland 21218, USA 2 Department of Mechanical Engineering, Johns Hopkins University, 3400 N. Charles Street, Baltimore, Maryland 21218, USA
ABSTRACT Rare-earth telluride compounds are characterized by their high performance thermoelectric properties that have been applied to the development of functional materials [1]. Recently, May and co-workers reported that nanostructured bulk lanthanum telluride (La3-xTe4, 0 x 1/3) by mechanical ball-milling exceeded the figure of merit (ZT) of 1 at high temperatures near 1300K [2-3]. Since the increased thermoelectric efficiency of nanostructured materials is due to the enhancement of phonon scattering introduced by quantum confinement, thin films have also generated significant scientific and technological interest [4-6]. Here, we report on the electrodepostion of lanthanum telluride and lanthanum thin films in ionic liquids in ambient conditions. Surface morphologies varied from needle-like to granular structures and depend on deposition conditions. This novel electrochemical synthesis approach is a simple, inexpensive and laboratory-environment friendly method of synthesizing nanostructured thermoelectric materials. INTRODUCTION Rare-earth elements and their telluride have attracted significant attentions because of their potential applications in thermoelectrics, electronics and optoelectronics [7]. In particular, lanthanum and lanthanum telluride alloys are of interest because lanthanum is used in large quantities in nickel metal hydride (NiMH) rechargeable batteries for hybrid automobiles. The negative electrode (cathode) in NiMH batteries is a mixture of metal hydrides, one of which is typically lanthanum hydride. The active material at the cathode is hydrogen, which is stored in the metal hydride structure [8]. A Toyota Prius battery requires 10 to 15 kg (22 to 33 lb) of lanthanum [9-10]. Lanthanum can also be used to make the infrared-absorbing glass in night vision goggles. High quality camera and telescope lenses contain lanthanum oxide (La2O3) making use of its high refractive index and low dispersion [11]. In addition, May and co-workers reported that nanostructured bulk lanthanum telluride (La3-xTe4, 0 x 1/3) produced by mechanical ball-milling exceeded the figure of merit (ZT) of 1 at high temperatures near 1300K [2-3]. LaTe and La2Te3 are good candidates for photoconducting and photovoltaic cells applications [12]. LaTe2 and LaTe3 are n-type semiconductors having unique electronic properties [13].
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Electrochemical deposition is a very attractive process for synthesizing thick films of compound semiconductors on metallic surfaces [14]. An advantage of this technique is that it is a simple, inexpensive, and laboratory-environment friendly synthesis methods for telluride base
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