A. James Clark School of Engineering

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The collections in this community comprise faculty research works, as well as graduate theses and dissertations.

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Subject: search.filters.subject.nanostructures

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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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    Effect of Extended Extinction from Gold Nanopillar Arrays on the Absorbance Spectrum of a Bulk Heterojunction Organic Solar Cell
    (MDPI, 2015-02-18) Tsai, Shu-Ju; Ballarotto, Mihaela; Kan, Hung-Chih; Phaneuf, Raymond J.
    We report on the effects of enhanced absorption/scattering from arrays of Au nanopillars of varied size and spacing on the spectral response of a P3HT:PCBM bulk heterojunction solar cell. Nanopillar array-patterned devices do show increased optical extinction within a narrow range of wavelengths compared to control samples without such arrays. The measured external quantum efficiency and calculated absorbance, however, both show a decrease near the corresponding wavelengths. Numerical simulations indicate that for relatively narrow nanopillars, the increased optical extinction is dominated by absorption within the nanopillars, rather than scattering, and is likely dissipated by Joule heating.
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    3D ENGINEERING OF VIRUS-BASED PROTEIN NANOTUBES AND RODS: A TOOLKIT FOR GENERATING NOVEL NANOSTRUCTURED MATERIALS
    (2018) Brown, Adam Degen; Culver, James N; Bioengineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Technological innovation at the nanometer scale has the potential to improve a wide range of applications, including energy storage, sensing of environmental and medical signals, and targeted drug delivery. A key challenge in this area is the ability to create complex structures at the nanometer scale. Difficulties in meeting this challenge using traditional fabrication methods have prompted interest in biological processes, which provide inspiration for complex structural organization at nanometer to micrometer length scales from self-assembling components produced inexpensively from common materials. From that perspective, a system of targeted modifications to the primary amino acid structure of Tobacco mosaic virus (TMV) capsid protein (CP) has been developed that induces new self-assembling behaviors to produce nanometer-scale particles with novel architectures. TMV CPs contain several negatively charged carboxylate residues which interact repulsively with those of adjacent CP subunits to destabilize the assembled TMV particle. Here, the replacement of these negatively charged carboxylate residues with neutrally charged or positively charged residues results in the spontaneous assembly of bacterially expressed CP into TMV virus-like particles (VLPs) with a range of environmental stabilities and morphologies and which can be engineered to attach perpendicularly to surfaces and to display functional molecular patterns such as target-binding peptide chains or chemical groups for attachment of functional targets. In addition, the distinct electrostatic surface charges of these CP variants enable the higher-level coassembly of TMV and VLP into continuous rod-shaped nanoparticles with longitudinally segregated distribution of functionalities and surface properties. Furthermore, the unique, novel, environmentally responsive assembly and disassembly behaviors of the modified CPs are shown to act as simple mechanisms to control the fabrication of these hierarchically structured functional nanoparticles.
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    ALD PROCESSES AND APPLICATIONS TO NANOSTRUCTURED ELECTROCHEMICAL ENERGY STORAGE DEVECES
    (2013) Chen, Xinyi; Rubloff, Gary W; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Next generation Li-ion batteries (LIB) are expected to display high power densities (i.e. high rate performance, or fast energy storage) while maintaining high energy densities and stable cycling performance. The key to fast energy storage is the efficient management of electron conduction, Li diffusion, and Li-ion migration in the electrode systems, which requires tailored material and structural engineering in nanometer scale. Atomic layer deposition (ALD) is a unique technique for nanostructure fabrications due to its precise thickness control, unprecedented conformality, and wide variety of available materials. This research aims at using ALD to fabricate materials, electrodes, and devices for fast electrochemical energy storage. First, we performed a detailed study of ALD V2O5 as a high capacity cathode material, using vanadium tri-isopropoxide (VTOP) precursor with both O3 and H2O as oxidant. The new O3-based process produces polycrystalline films with generally higher storage capacity than the amorphous films resulting from the traditional H2O-based process. We identified the crucial tradeoff between higher gravimetric capacity with thinner films and higher material mass with thicker films. For the thickness regime 10-120 nm, we chose areal energy and power density as a useful metric for this tradeoff and found that it is optimized at 60 nm for the O3-VTOP ALD V2O5 films. In order to increase material loading on fixed footprint area, we explored various 3-dimentional (3D) substrates. In the first example, we used multiwall carbon nanotube (MWCNT) sponge as scaffold and current collector. The core/shell MWCNT/V2O5 sponge delivers a stable high areal capacity of 816 μAh/cm2 for 2 Li/V2O5 voltage range (4.0-2.1 V) at 1C rate (nC means charge/discharge in 1/n hour), 450 times that of a planar V2O5 thin film cathode. Due to low density of MWCNT and thin V2O5 layer, the sponge cathode also delivers high gravimetric power density in device level that shows 5X higher power density than commercial LIBs. In the other example, Li-storage paper cathodes, functionalized of conductivity from CNT and Li-storage capability from V2O5¬, presented remarkably high rate performance due to the hierarchical porosity in paper for Li+ migration. The specific capacity of V2O5 is as high as 410 mAh/g at 1C rate, and retained 116 mAh/g at high rate of 100C. We found V2O5 capacities decreased by about 30% at high rates of 5C-100C after blocking the mesopores in cellulose fiber, which serves to be the first confirmative evidence of the critical role of mesoporosity in paper fibers for high-rate electrochemical devices. Finally, we made high density well-aligned nanoporous electrodes (2 billion/cm2) using anodic alumina template (AAO). ALD materials were deposited into the nanopores sequentially - Ru or TiN for current collection, and V2O5 for Li-storage. Ru metal by ALD shows high conductivity and conformality, and serves best as the current collector for V2O5. The capacity of V2O5 reaches about 88% of its theoretic value at high rate of 50C. Such electrodes can be cycled for 1000 times with 78% capacity retention.
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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.