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There is ongoing interest in developing new types of memory that are less expensive. One prospect, ferroelectric random access memory (FRAM), is based on the prospect that the two stable polarization states of ferroelectric materials could be used to encode the 1 and 0 of memory and logic circuitry in computers. Technologies utilizing FRAM require the materials to be compatible with existing nanoscale components and address the issue of size dependent suppression of ferroelectricity. Most previous experimental and theoretical studies suggest the maximum temperature that a structure with nanometer dimensions can be ferroelectric (called the Curie temperature for bulk material) decreases with nanoscale dimensions and that nanometer BaTiO3 particles or films are not ferroelectric. Professor Stephen O'Brien has developed a novel hydrothermal synthesis route for BaTiO3 particles that makes ~ 5 nm diameter particles with relatively narrow size distribution. Professor Irving Herman has performed Raman measurements on these nanoparticles that have shown that they are in the tetragonal phase at room temperature, which could have important implications for ferroelectricity in small dimensions. The figure above plots the Raman frequency of one of the vibrational modes of these particles (in units of wavenumbers) as a function of temperature (in ?). The minimum at the highest temperature denotes the tetragonal-cubic phase transition. In bulk material (inset), barium titanate is ferroelectric in this tetragonal phase below 120-130?, and it is paraelectric in the cubic phase at higher temperature. In this nanoparticle (main part of figure), the tetragonal-cubic transition occurs at a lower temperature ~95?, so the particles have the tetragonal phase at room temperature. We believe that these are the smallest dimension BaTiO3 nanocrystals with a tetragonal structure at and well above room temperature. We are now determining whether these particles are ferroelectric at room temperature. Unlike bulk materials, it is possible that it is not, due to effects of particle size, morphology, surface termination, and carrier concentrations. The importance of some of these factors is still not well understood. Posted February 13, 2002. For more details contact Stephen O'Brien or Irving Herman . |
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