New nonhydrolytic route to synthesize crystalline BaTiO 3 nanocrystals with surface capping ligands
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Jiaqing He and Yimei Zhu Department of Materials Science, Brookhaven National Laboratory, Upton, New York 11973
Stephen O’Briena) Department of Applied Physics and Applied Mathematics, the Columbia Materials Research Science and Engineering Center (MRSEC), and the Columbia Nanocenter (NSEC), Columbia University, New York, New York 10027 (Received 2 June 2006; accepted 24 August 2006)
A new nonhydrolytic route for the preparation of well-crystallized size-tunable barium titanate (BaTiO3) nanocrystals capped with surface ligands is reported. Our approach involves: (i) synthesizing a “pseudo” bimetallic precursor, and (ii) combining the as-synthesized bimetallic precursor with a mixture of oleylamine with different surface coordinating ligands at 320 °C for crystallization and crystal growth. Different alcohols in the precursor synthesis and different carboxylic acids were used to study the effect of size and morphological control over the nanocrystals. Nanocrystals of barium titanate with diameters of 6–10 nm (capped with decanoic acid), 3–5 nm (capped with oleic acid), 10–20 nm (a nanoparticle and nanorod mixture capped with oleyl alcohol), and 2–3 nm (capped with oleyl alcohol) were synthesized, and can be easily dispersed into nonpolar solvents such as hexane or toluene. Techniques including x-ray diffraction, transmission electron microscopy, selected area electron diffraction, and high-resolution electron microscopy confirm the crystallinity and morphology of these as-synthesized nanocrystals.
I. INTRODUCTION
Complex oxide perovskites have been of interest for more than half a century due to their ferroelectric, pyroelectric, piezoelectric, and dielectric properties.1,2 Their applications in the electronics industry include transducers and actuators,3 high-K dielectric capacitors,4 and memory applications [ferroelectric random access memory (FRAM)], which rely on the existence of a spontaneous polarization in the crystal unit cell.5–7 Because the physical properties of materials in the nanoscale regime (1–100 nm) can be quite different from the bulk8–10 and the precise nature of ferroelectricity at the nanoscale is still debated,11 interest has been stimulated over the preparation and study of complex oxide perovskite nanocrystals. Of particular interest is the nature of the phase transition temperature (TC, called the Curie temperature) that marks the transition between the ferroelectric
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0389 J. Mater. Res., Vol. 21, No. 12, Dec 2006
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and paraelectric phase, respectively.12,13 This transition is known to be size dependent for the ferroelectric perovskites at the nanoscale.12,13 Uniform, monodisperse, and highly crystalline nanoparticles with tunable sizes and morphologies are desired in order for a consensus to be reached on the exact nature of critical size suppression of ferroelectricity. As a prototype and model system of ferroelectric perovskite crystal
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