Combinatorial Synthesis and Evaluation of Vanadium Oxide Films

Combinatorial technology is a powerful tool for new material exploration. La1-xCaxVO3 composition-spread films were fabricated by combinatorial pulsed laser deposition and their thermoelectric properties were evaluated paralelly by the multi-channel thermoelectric measurement system. Concurrent X-ray analysis verified the formation of solid soluted films in the full composition range (0≤x≤1) as judged from the linear variation of the lattice constants. Growth conditions of LaVO3 films were optimized. Good crystallinity of LaVO3 film was obtained at 800°C, and the power factor of 0.6 µW/cm K2 was achieved. The effects of oxygen content and the substitutions of Ca and Ce ions on TE properties of were also analysed respectively. Large TE properties in vanadium oxide system can be expected with the change of vanadium ion valence from 3+ to 2+. In La1-xCexVO3(0≤x≤1) system, Ce ion takes 3+ although Ce4+ is stable in theory.


Introduction
Combinatorial chemical technology started in drug industries for high throughput discovery of new drugs by the integrated organic synthesis [1] and now it is finding a wide possibility in solid state materials as a powerful tool for optimizing the compositions and fabrication conditions by systematically varying several process parameters [2][3][4][5][6]. A large number of samples (combinatorial library) can be fabricated and evaluated in a single run of experiments. So combinatorial technology is a powerful tool for new material exploration.
Thermoelectric (TE) materials attract much interest for such applications as to heat pumps and power generators. At present, the materials with highest TE performance are intermetallic compounds such as Bi 2 Te 3 [7][8][9]. However, their practical application to power generation from the waste heat has been drawn back by many material problems like a low melting point, poor oxidation resistance, and an insufficient conversion efficiency in the typical operating temperature range of the waste heat (300-800℃). The known material of NaCo 2 O 4 was recently discovered to be good TE material [10,11] to encourage researchers for exploring new TE materials in metal oxides. They are already in oxidized state and thermally stable, and therefore have the potential to overcome the problems mentioned above [12][13][14][15]. The large Seebeck coefficient in NaCo 2 O 4 is explained by the characteristic degeneracy due to spin and orbital degrees of freedom [16]. Thus, some 3d transitionmetal oxides are candidates for new thermoelectric materials. Vanadium oxides possess various interesting properties due to the multivalent nature of vanadium. In general, heavy electron systems generate large thermopower due to the large density of state at the Fermi level. Thus, vanadium oxides are good candidates of highly efficient TE materials. A perovskite vanadate LaVO 3 is wellknown as a typical Mott-insulator [17][18][19][20]. The substitution of divalent cations for La 3+ sites can cause the vanadium cation to change the valence state, leading to the enhancement of TE efficiency by the optimization of the vanadium valence state and the career density.
In this paper, combinatorial pulsed laser deposition (CPLD) was applied to fabricate La 1- Fig. 2. The structures of LaVO3 film grown at different temperatures (a) X-ray diffraction pattern (b) C-axis length

Results and discussions
In order to obtain good TE properties, the optimum crystal growth condition was investigated. The composition-spread films around LaVO 3 were fabricated at different temperatures. Fig.2 (a) shows the XRD patterns of LaVO 3 film grown at different temperatures (600-800℃). The (001) peak of LaVO 3 film grown at 800℃ was very sharp. On the other hand, there was another peak near the (001) peak of LaVO 3 film grown at 600℃, which meant there were two domains in the film grown due to bad crystallinity. Thus at 800℃, LaVO 3 film with good crystallinity was obtained. C-axis length became larger with the enhance of growth temperature, shown in Fig.2(b), which was attributed to the lack of oxygen. So exact formula for LaVO 3 film grown at higher temperature should be LaVO 3-δ , which caused the valence of vanadium ion change from 3+ to 2+. Fig.3 shows the TE properties of LaVO 3 films grown at various temperatures. There was little change in the Seebeck coefficients of LaVO 3 films grown at different growth temperatures, while the resistivity decreased with the enhance of the growth temperatures. The largest power factor (α=0.6 µW/cm K 2 ) of LaVO 3 film was obtained at 800℃. Fig.4 shows a part of CXRD image (21.6°≤2θ≤24.1°) for full composition area (0≤x≤1) of (La 1-x Ca x )VO 3 composition-spread films on a STO substrate. The diffraction angle 2θ in this image was calibrated by the STO (200) peak positions in order to remove the error due to the substrate setup in the CXRD. It is noted that the CXRD (200) peak in this range was separated into 10 pieces because the films were patterned by a shadow mask as shown in Fig. 1(a). The CXRD peak of the composition-spread films changed linearly, thus indicating that the composition-spread films formed solid solution in the whole composition range (0≤x≤1). Thus, the c-axis length obeyed Vegard's law. C-axis lengths were estimated to be 4.04Å (LaVO 3 side) and 3.74Å (CaVO 3 side) from the image, respectively. The crystal lattice of LaVO 3 films was elongated along the c axis, as the c-axis length of LaVO 3 films was larger than the data reported for the bulk sample [21]. A compressive strain from the substrate is presumed to elongate the c lattice normal to the substrate. Fig.5 shows the TE properties of La 1-x Ca x VO 3 (0≤x≤1) composition-spread films grown at 800℃. The Seebeck coefficients of CaVO 3 side films were close to zero(0～ -5 µV/K), while those of LaVO 3 side films were large negative values (～ -310 µV/K). The heavy electron in the Mottinsulator of LaVO 3 is considered to cause the large Seebeck coefficient of LaVO 3 films [25], while CaVO 3 film appears to behave like a normal metal. The substitution of La 3+ ions with Ca 2+ ions made the valence of vanadium ion change from 3+ to 4+, which was responsible for the change of TE properties. The transition point from large negative value to almost zero is around x=0.2, where the metal-insulator transitions was reported to occur [19], so resistivity of La 1-x Ca x VO 3 films decreased monotonically from x=0.2 to x =1.0. The Seebeck coefficient of La 1-x Ca x VO 3 close to CaVO 3 (0.6≤x≤1.0) was reported to be sensitive to the temperature and Ca content x, it could be positive and negative depending on these parameters [26]. This means that La 1-x Ca x VO 3 films have both electron-and hole-type carriers and the dominant carrier represents the polarity of Seebeck coefficient. In particular, the strong strain of LaVO 3 film on a STO may cause its large negative Seebeck coefficient opposite to the reported data of bulk samples [26].
Another work was done to change the valence of vanadium ion from 3+ to 2+ by doping Ce ion into LaVO 3 . Fig.6 shows the TE properties of La 1-x Ce x VO 3 (0≤x≤1) composition-spread films grown at 800℃.There were little change of TE properties of La 1-x Ce x VO 3 films in the range of 0≤x≤1.

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Mechatronics and Information Technology The Seebeck coefficient of La 1-x Ca x VO 3 close to CaVO 3 (0.6≤x≤1.0) was reported to be sensitive to the temperature and Ca content x, it could be positive and negative depending on these parameters [26]. This means that La 1-x Ca x VO 3 films have both electron-and hole-type carriers and the dominant carrier represents the polarity of Seebeck coefficient. In particular, the strong strain of LaVO 3 film on a STO may cause its large negative Seebeck coefficient opposite to the reported data of bulk samples [26].
Another work was done to change the valence of vanadium ion from 3+ to 2+ by doping Ce ion into LaVO 3 . Fig.6 shows the TE properties of La 1-x Ce x VO 3 (0≤x≤1) composition-spread films grown at 800℃.There were little change of TE properties of La 1-x Ce x VO 3 films in the range of 0≤x≤1. Fig.7 shows the XPS spectrums of (a) La ion of LaVO 3 side and (b) Ce of CeVO 3 side. In the system, Ce ion takes 3+ although Ce 4+ is stable in theory. The reason may be that the size of Ce 3+ ion is similar to that of La 3+ ion, so Ce takes 3+ easily in LaVO 3 .

Conclusion
La 1-x Ca x VO 3 composition-spread films were fabricated by combinatorial pulsed laser deposition. TE properties of La 1-x Ca x VO 3 composition-spread films were evaluated by the multi-channel thermoelectric measurement system. Growth conditions of LaVO 3 films were optimized. Good crystallinity of LaVO 3 film was obtained at 800℃, and the power factor of 0.6 µW/cm K 2 was achieved. The effects of oxygen content and the substitutions of Ca and Ce ions on TE properties of were also analysed respectively. Large TE properties in vanadium oxide system can be expected with the change of vanadium ion valence from 3+ to 2+. In La1-xCexVO3(0≤x≤1) system, Ce ion takes 3+ although Ce 4+ is stable in theory.