Raman Spectroscopy of Barium Titanate Nanocrystals
A variable-temperature Raman spectroscopy study of BaTiO3 nanocrystals of varied sizes (from 8 to 20 nm) will be presented. Highly uniform cube-shaped BaTiO3 nanocrystals have been prepared by solvothermal synthesis at temperatures below 140 degrees C and characterized by x-ray diffraction and transmission electron microscopy. Raman spectra (measured with ultraviolet and visible excitation) show that all nanocrystals studied are ferroelectric. The effect of nanocrystal size on the Curie temperature as well as on photoluminescence emission has been investigated. Temperature evolution of Raman spectra (10�600K) demonstrates that the ferroelectric phase of the nanocrystals is different from the bulk BaTiO3. The transitions from tetragonal to orthorhombic and from orthorhombic to rhombohedral phases, which are characteristic for bulk BaTiO3, have not been observed in the nanocrystals; the ferroelectric phase in the nanocrystals is the same in the entire temperature range below Tc, and is different from any of the bulk phases. The observed behavior may be explained by complex polarization patterns theoretically predicted for zero-dimensional ferroelectrics.
Supported in part by the National Science Foundation through the Grants DMR-0705127, DMR-1006136, EPS-1003897 and DMR-1004869, the Department of Defense through the DARPA Grant HR 0011-09-0047, and Research Corporation for Science Advancement Grant No. 7134.