Thermoelectric properties of epitaxial and topotaxial Na x CoO 2 thin films
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0886-F02-02.1
Thermoelectric properties of epitaxial and topotaxial NaxCoO2 thin films
A. Venimadhav1, Z. Ma1, Qi Li1, A. Soukiassian2 X. X. Xi1,2, D. G. Schlom2, R. Arroyave2, Z. K. Liu2, Minhyea Lee3 and N. P. Ong3 1
Department of Physics, The Pennsylvania State University, University Park, PA 16802,
2
Department of Materials Science and Engineering, The Pennsylvania State University,
University Park, PA 16802 3
Department of Physics, Princeton University, NJ 08544
ABSTRACT
We report structural, electrical, and thermopower properties of epitaxial and topotaxial NaxCoO2 thin films on (0001) sapphire substrate. Topotaxial NaxCoO2 films were prepared by converting an epitaxial Co3O4 film to NaxCoO2 by annealing in Na vapor and epitaxial NaxCoO2 films were obtained by pulsed laser deposition. All the films are c-axis oriented. For topotaxial films, annealing in different Na vapor pressures resulted in films with different Na concentrations, which showed distinct transport properties. For directly deposited epitaxial films by pulsed laser deposition, deposition parameters are found to control the Na concentration and hence the film properties. The largest thermoelectric power of the samples made by different methods is found to be similar in the range of 70-100 µV/K at room temperature.
0886-F02-02.2
INTRODUCTION Layered cobaltate NaxCoO2 has attracted much attention recently due to its exceptional properties [1]. It has an unusually high thermoelectric power with low mobility, low resistivity, and high carrier density.[1] The Fermi surface [2] and electrical properties [3] of NaxCoO2 depend on the Na concentration and show variety of properties depending on the doping level [6] [3,7]. The triangular structure of the CoO2 planes and the strong electron correlation effect has been recognized as sources of rich properties of NaxCoO2[8]. For example, the large thermo-power in NaxCoO2 has been attributed to the spin entropy due to the strong electron correlation effects [9].
NaxCoO2 has been
prepared in polycrystalline and single crystalline forms, but there are very few reports on NaxCoO2 thin films. Recently, growth of epitaxial NaxCoO2 films were reported by reactive solid phase epitaxy [10,11] and Pulsed laser deposition (PLD) [12,13]. However thermopower and temperature dependence of thermopower properties of those films were not reported except that Venimadhav et al showed that the thermopower behavior in thin films of x=0.7 follows that of the bulk samples. In this paper, we describe the growth of the epitaxial NaxCoO2 thin films using different methods and compare their thermopower and electrical transport properties.
EXPERIMENT We have prepared NaxCoO2 films in two different ways. In the first method, NaxCoO2 films were fabricated by a reactive solid phase epitaxial technique that included a two-step process. First, an epitaxial Co3O4 film was grown by pulsed laser deposition (PLD) on a (0001) sapphire substrate [11]. A KrF excimer laser was used with an energy density of 3.7 J/cm2 on a CoO target.
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