Preparation And Characterizations Of PbTiO₃Nano-crystals

Bistable memories based on switching of a ferroelectric polarization are considered as promising candidates in the future due to their nonvolatile nature, high speed operation, high irradiation resistance and etc. Ferroelectrics in nanoscale are essential to the fabrication of high density ferroelectric memories. To date, various approaches have been proposed to prepare well-defined ferroelectric nanostructures. These include atomic force microscope tip-assisted methods, hard-mask-based approaches and micelle-based self assembly. On the other hand, it is also fundamentally important to understand the size-effect in ferroelectrics and its relation with microstructures such as surface properties, strain effects and crystal lattice symmetries. In this paper, we focus on two aspects:(1) preparation of mono-dispersed PbTiO3(PTO) nano-crystals by a micelle-based self-assembly approach; and (2) the thickness dependent polarization retention characteristics of Bi3.15Nd0.85Ti3O12(BNdT) ferroelectric thin films. The main achievements are summarized below.1. PTO nano-crystals in uniform size have been prepared on silicon substrate over a large area (1×1cm2) by utilizing polystyrene-block-poly(4-vinylpridine) copolymer (PS-b-P4VP) micelles. The nano-crystals are10-12nm in height and80-100nm in diameter. The dependence of PTO nano-crystal morphology on the block copolymer concentration, incubation time, annealing temperature and substrate surface characteristics was studied. The results indicate that the copplymer concentration and substrate surface characteristics have important effects on the morphology of nano-crystals. The best PS-b-P4VP concentration are0.1%or0.085%. 2. The PTO nano-crystals on Si annealed at550,600and650℃were characterized using various methods including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), piezoelectric force microscopy (PFM) and conductive atomic force microscopy (CAFM). XRD and XPS results show that the nano-crystals are tetragonal PTO. PFM measurements indicate the ferroelectric nature of these PTO nano-crystals. Local current-voltage measurements on individual nano-crystals exhibits an obvious bipolar resistance switching phenomena.3. BNdT thin films were deposited on Pt/TiO2/SiO2/Si substrates by chemical solution deposition.1-6depositions were repeated to obtain films with various thickness from90to540nm. Polarization retention of BNdT thin films has been studied as a function of film thickness by PFM. The polarization pointing to substrate is found unstable when film thickness is below180nm, while the opposite polarization is stable in all the samples measured. This observation is discussed in terms of opposite domain nucleation assisted by the built-in field in the films.