Materials Science & Engineering

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    Directed Kinetic Self-Assembly of Mounds on Patterned GaAs (001): Tunable Arrangement, Pattern Amplification and Self-Limiting Growth
    (MDPI, 2014-05-12) Lin, Chuan-Fu; Kan, Hung-Chih; Kanakaraju, Subramaniam; Richardson, Christopher; Phaneuf, Raymond
    We present results demonstrating directed self-assembly of nanometer-scale mounds during molecular beam epitaxial growth on patterned GaAs (001) surfaces. The mound arrangement is tunable via the growth temperature, with an inverse spacing or spatial frequency which can exceed that of the features of the template. We find that the range of film thickness over which particular mound arrangements persist is finite, due to an evolution of the shape of the mounds which causes their growth to self-limit. A difference in the film thickness at which mounds at different sites self-limit provides a means by which different arrangements can be produced.
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    ACCELERATED SELF-ASSEMBLY OF PEPTIDE-BASED NANOFIBERS USING NANOMECHANICAL STIMULUS
    (2010) Chang, Jonathan Paul; Seog, Joonil; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    One-dimensional nanostructures are ideal building blocks for functional nanoscale assembly. Peptide-based nanofibers have great potential for building smart hierarchical structures due to their tunable structures at a single residue level and their ability to reconfigure themselves in response to environmental stimuli. In this study, it was observed that a pre-adsorbed silk-elastin-based protein polymer self-assembled into nanofibers through a conformational change on the mica substrate. Furthermore, using atomic force microscopy, it was shown that the rate of the self-assembling process was significantly enhanced by applying a nanomechanical stimulus. The orientation of the newly grown nanofiber was mostly perpendicular to the scanning direction, implying that the new nanofiber assembly was locally activated with a directional control. The method developed as a part of this study provides a novel way to prepare a nanofiber patterned substrate using a bottom-up approach.
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    Growth and Characterization of Multiferroic BaTiO3-CoFe2O4 Thin Film Nanostructures
    (2004-12-08) Zheng, Haimei; Salamanca-Riba, Lourdes; Ramesh, Ramamoorthy; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Multiferroic materials which display simultaneous ferroelectricity and magnetism have been stimulating significant interest both from the basic science and application point of view. It was proposed that composites with one piezoelectric phase and one magnetostrictive phase can be magnetoelectrically coupled via a stress mediation. The coexistence of magnetic and electric subsystems as well as the magnetoelectric effect of the material allows an additional degree of freedom in the design of actuators, transducers, and storage devices. Previous work on such materials has been focused on bulk ceramics. In the present work, we created vertically aligned multiferroic BaTiO3-CoFe2O4 thin film nanostructures using pulsed laser deposition. Spinel CoFe2O4 and perovskite BaTiO3 spontaneously separated during the film growth. CoFe2O4 forms nano-pillar arrays embedded in a BaTiO3 matrix, which show three-dimensional heteroepitaxy. CoFe2O4 pillars have uniform size and spacing. As the growth temperature increases the lateral size of the pillars also increases. The size of the CoFe2O4 pillars as a function of growth temperature at a constant growth rate follows an Arrhenius behaviour. The formation of the BaTiO3-CoFe2O4 nanostructures is a process directed by both thermodynamic equilibrium and kinetic diffusion. Lattice mismatch strain, interface energy, elastic moduli and molar ratio of the two phases, etc., are considered to play important roles in the growth dynamics leading to the nanoscale pattern formation of BaTiO3-CoFe2O4 nanostructures. Magnetic measurements exhibit that all the films have a large uniaxial magnetic anisotropy with an easy axis normal to the film plane. It was calculated that stress anisotropy is the main contribution to the anisotropy field. We measured the ferroelectric and piezoelectric properties of the films, which correspond to the present of BaTiO3 phase. The system shows a strong coupling of the two order parameters of polarization and magnetization through the coupled lattices. This approach to the formation of self-assembled ferroelectric/ferro(ferri-)magnetic nanostructures is generic. We have created similar nanostructures from other spinel-perovskite systems such as BiFeO3-CoFe2O4, BaTiO3-NiFe2O4, etc., thus making it of great interest and value to a broad materials community.