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  • Title: Structure and performance of dielectric films based on self-assembled nanocrystals with a high dielectric constant.
    Author: Huang L, Liu S, Van Tassell BJ, Liu X, Byro A, Zhang H, Leland ES, Akins DL, Steingart DA, Li J, O'Brien S.
    Journal: Nanotechnology; 2013 Oct 18; 24(41):415602. PubMed ID: 24060685.
    Abstract:
    Self-assembled films built from nanoparticles with a high dielectric constant are attractive as a foundation for new dielectric media with increased efficiency and range of operation, due to the ability to exploit nanofabrication techniques and emergent electrical properties originating from the nanoscale. However, because the building block is a discrete one-dimensional unit, it becomes a challenge to capture potential enhancements in dielectric performance in two or three dimensions, frequently due to surface effects or the presence of discontinuities. This is a recurring theme in nanoparticle film technology when applied to the realm of thin film semiconductor and device electronics. We present the use of chemically synthesized (Ba,Sr)TiO3 nanocrystals, and a novel deposition-polymerization technique, as a means to fabricate the dielectric layer. The effective dielectric constant of the film is tunable according to nanoparticle size, and effective film dielectric constants of up to 34 are enabled. Wide area and multilayer dielectrics of up to 8 cm(2) and 190 nF are reported, for which the building block is an 8 nm nanocrystal. We describe models for assessing dielectric performance, and distinct methods for improving the dielectric constant of a nanocrystal thin film. The approach relies on evaporatively driven assembly of perovskite nanocrystals with uniform size distributions in a tunable 7-30 nm size range, coupled with the use of low molecular weight monomer/polymer precursor chemistry that can infiltrate the porous nanocrystal thin film network post assembly. The intercrystal void space (low k dielectric volume fraction) is minimized, while simultaneously promoting intercrystal connectivity and maximizing volume fraction of the high k dielectric component. Furfuryl alcohol, which has good affinity to the surface of (Ba,Sr)TiO3 nanocrystals and miscibility with a range of solvents, is demonstrated to be ideal for the production of nanocomposites. The nanocrystal/furfuryl alcohol dispersions are suitable for the fabrication of thin films by chemical deposition techniques, including spin-coating, printing or a spraying process. To demonstrate the application of this technique to device fabrication, a multilayer capacitor with capacitance of 0.83 nF mm(-2) at 1 MHz is presented.
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