Department of Physics, University of Illinois at Chicago, Chicago, IL, USA.
J Phys Condens Matter. 2011 Aug 3;23(30):305005. doi: 10.1088/0953-8984/23/30/305005. Epub 2011 Jul 1.
The incommensurately layered cobalt oxide Ca(3)Co(4)O(9) exhibits an unusually high Seebeck coefficient as a polycrystalline bulk material, making it ideally suited for many high temperature thermoelectric applications. In this paper, we investigate properties of Ca(3)Co(4)O(9) thin films grown on cubic perovskite SrTiO(3), LaAlO(3), and (La(0.3)Sr(0.7))(Al(0.65)Ta(0.35))O(3) substrates and on hexagonal Al(2)O(3) (sapphire) substrates using the pulsed laser deposition technique. X-ray diffraction and transmission electron microscopy analysis indicate strain-free growth of films, irrespective of the substrate. However, depending on the lattice and symmetry mismatch, defect-free growth of the hexagonal CoO(2) layer is stabilized only after a critical thickness and, in general, we observe the formation of a stable Ca(2)CoO(3) buffer layer near the substrate-film interface. Beyond this critical thickness, a large concentration of CoO(2) stacking faults is observed, possibly due to weak interlayer interaction in this layered material. We propose that these stacking faults have a significant impact on the Seebeck coefficient and we report higher values in thinner Ca(3)Co(4)O(9) films due to additional phonon scattering sites, necessary for improved thermoelectric properties.
层状钴氧化物 Ca(3)Co(4)O(9) 在多晶块状材料中表现出异常高的 Seebeck 系数，非常适合许多高温热电应用。在本文中，我们研究了使用脉冲激光沉积技术在立方钙钛矿 SrTiO(3)、LaAlO(3)和(La(0.3)Sr(0.7))(Al(0.65)Ta(0.35))O(3)衬底以及六方 Al(2)O(3)（蓝宝石）衬底上生长的 Ca(3)Co(4)O(9) 薄膜的性能。X 射线衍射和透射电子显微镜分析表明，无论衬底如何，薄膜都可以无应变生长。然而，根据晶格和对称性失配的情况，只有在达到临界厚度后，才能稳定地生长无缺陷的六方 CoO(2)层，而且通常在衬底-薄膜界面附近会观察到形成稳定的 Ca(2)CoO(3)缓冲层。超过这个临界厚度，会观察到大量 CoO(2)堆垛层错，这可能是由于这种层状材料中层间相互作用较弱所致。我们提出这些堆垛层错对 Seebeck 系数有重大影响，并报告了在较薄的 Ca(3)Co(4)O(9) 薄膜中由于存在更多的声子散射位，Seebeck 系数更高，这对改善热电性能是必要的。
