Role of ripples in altering the electronic and chemical properties of graphene.

Ripples of graphene are known to manipulate electronic and hydrogenation properties of graphitic materials. More detailed work is needed to elucidate the structure-property relationship of these systems. In this work, the density functional theory is used to compute the energy and electronic structure of the graphene models with respect to variable curvatures and hydrogen adsorption sites. The magnitude of finite bandgap opening depends on the orientation of ripples, and the hydrogen adsorption energy depends on the local curvature of graphene. An adsorbed hydrogen alters the local curvature, resulting in relatively weakened adsorption on the neighboring three sites, which gives a rationale to experimentally observed dynamic equilibrium stoichiometry (H:C = 1:4) of hydrogenated graphene. The surface diffusion transition state energy of adsorbed hydrogen is computed, which suggests that the Eley-Rideal surface recombination mechanism may be important to establish the dynamic equilibrium, instead of the commonly assumed Langmuir-Hinshelwood mechanism.

Multi-Scale Analyses and Modeling of Metallic Nano-Layers.

The current work centers on multi-scale approaches to simulate and predict metallic nano-layers' thermomechanical responses in crystal plasticity large deformation finite element platforms. The study is divided into two major scales: nano- and homogenized levels where Cu/Nb nano-layers are designated as case studies. At the nano-scale, a size-dependent constitutive model based on entropic kinetics is developed. A deep-learning adaptive boosting technique named single layer calibration is established to acquire associated constitutive parameters through a single process applicable to a broad range of setups entirely different from those of the calibration. The model is validated through experimental data with solid agreement followed by the behavioral predictions of multiple cases regarding size, loading pattern, layer type, and geometrical combination effects for which the performances are discussed. At the homogenized scale, founded on statistical analyses of microcanonical ensembles, a homogenized crystal plasticity-based constitutive model is developed with the aim of expediting while retaining the accuracy of computational processes. Accordingly, effective constitutive functionals are realized where the associated constants are obtained via metaheuristic genetic algorithms. The model is favorably verified with nano-scale data while accelerating the computational processes by several orders of magnitude. Ultimately, a temperature-dependent homogenized constitutive model is developed where the effective constitutive functionals along with the associated constants are determined. The model is validated by experimental data with which multiple demonstrations of temperature effects are assessed and analyzed.

Well-Adhered Copper Nanocubes on Electrospun Polymeric Fibers.

Electrospun polymer fibers can be used as templates for the stabilization of metallic nanostructures, but metallic species and polymer macromolecules generally exhibit weak interfacial adhesion. We have investigated the adhesion of model copper nanocubes on chemically treated aligned electrospun polyacrylonitrile (PAN) fibers based on the introduction of interfacial shear strains through mechanical deformation. The composite structures were subjected to distinct macroscopic tensile strain levels of 7%, 11%, and 14%. The fibers exhibited peculiar deformation behaviors that underscored their disparate strain transfer mechanisms depending on fiber size; nanofibers exhibited multiple necking phenomena, while microfiber deformation proceeded through localized dilatation that resulted in craze (and microcrack) formation. The copper nanocubes exhibited strong adhesion on both fibrous structures at all strain levels tested. Raman spectroscopy suggests chemisorption as the main adhesion mechanism. The interfacial adhesion energy of Cu on these treated PAN nanofibers was estimated using the Gibbs-Wulff-Kaischew shape theory giving a first order approximation of about 1 J/m2. A lower bound for the system's adhesion strength, based on limited measurements of interfacial separation between PAN and Cu using mechanically applied strain, is 0.48 J/m2.

Nisin infusion into surface cracks in oxide coatings to create an antibacterial metallic surface.

The efficacy of surface topology and chemistry on the ability for a surface to retain antimicrobial performance via the immobilization of a peptide was evaluated. A nanosecond pulsed laser was used to create oxide films on Ti-6Al-4V and 304L stainless steel. The laser conditions employed created a mudflat cracked surface on titanium, but no cracks on the steel. An antimicrobial peptide, nisin, was infused into the cracked and uncracked oxide surfaces to provide antimicrobial activity against Gram-positive bacteria; Listeria monocytogenes was chosen as the model microorganism. Release tests in distilled water at room temperature show that nisin is slowly liberated from the uncracked stainless steel surface, while there was no evidence of nisin liberation from the cracked titanium alloy surfaces, likely due to immobilization of the peptide into the artificially created micro-cracks on the surface of this alloy. Surfaces treated with nisin became active and exhibit anti-microbial performance against L. monocytogenes; this behavior is mostly retained after scrubbing/washing and simple immersion in water.

Uncovering the Thermo-Kinetic Origins of Phase Ordering in Mixed-Valence Antimony Tetroxide by First-Principles Modeling.

Phase ordering in the mixed-valence oxide Sb2O4 has been examined by density functional theory (DFT) calculations. We find that the ground-state total energies of the two phases (α and ß) are almost degenerate and are highly sensitive to the choice of the approximation to the exchange correlation (xc) functional used in our calculations. Interestingly, with the inclusion of the zero-point energy corrections, the α phase is predicted to be the ground state polymorph for most xc functionals used. We also illustrate the pronounced stereochemical activity of Sb in these polymorphs of Sb2O4, setting an exception to the Keve and Skapski rule. Here, we find that the actual bonding in the α phase is more asymmetric, while the anomalous stability of the ß phase could be rationalized from kinetic considerations. We find a non-negligible activation barrier for this α-ß phase transition, and the presence of a saddle point (ß phase) supports the separation of Sb(III) over a continuous phase transition, as observed in experiments.

Effect of accelerated aging on dental zirconia-based materials.

OBJECTIVE: To investigate the effect of aging on phase transformation and mechanical properties of yttria-tetragonal zirconia polycrystal (Y-TZP). MATERIALS AND METHODS: Fully-sintered Y-TZP slabs, IPS E-max ZirCAD (ZC - Ivoclar) and Z-5 ceramic (Z5 - C5 Medical Werks), were artificially aged in autoclave for: 0, 30, 60 or 90min. Flexural strength (FS), crystalline changes (X-ray diffraction analysis - XRD) and surface topography were analyzed. 0 and 90min-aged samples were evaluated by nanoindentation to measure hardness and modulus, and results were compared using Wilcoxan Mann Whitney rank sum test (p≤0.05). FS results were compared using two-way ANOVA and Tukey HSD (α=0.05). RESULTS: Material factor had significant effect (p=0.001) on flexural strength (Z5=966.95MPa; ZC=847.82MPa), but aging did not. Nanoindentation showed incidence of typical load/depth curves combined with some exhibiting features compatible with cracking. When typical curves were considered, aging had no effect on the modulus and hardness, but hardness was dependent on material type. A steady increase in the m phase related to aging time was observed for ZC samples. The maximum incidence of m phase was 6.56% for Z5/60min. SIGNIFICANCE: Flexural strength is not affected by surface transformation in dental Y-TZP. Hydrothermal aging has an effect on m content and surface topography of different zirconia brands, but mechanical tests that can precisely characterize surface changes in aged Y-TZP are still missing.

Improved electro-mechanical performance of gold films on polyimide without adhesion layers.

Thin metal films on polymer substrates are of interest for flexible electronic applications and often utilize a thin interlayer to improve adhesion of metal films on flexible substrates. This work investigates the effect of a 10 nm Cr interlayer on the electro-mechanical properties of 50 nm Au films on polyimide substrates. Ex situ and in situ fragmentation experiments reveal the Cr interlayer causes brittle electro-mechanical behaviour, and thin Au films without an interlayer can support strains up to 15% without significantly degrading electrical conductivity.

Nanomechanical testing technique for millimeter-sized and smaller molecular crystals.

Large crystals are used as a control for the development of a mounting and nanoindentation testing technique for millimeter-sized and smaller molecular crystals. Indentation techniques causing either only elastic or elastic-plastic deformation produce similar results in assessing elastic modulus, however, the elastic indents are susceptible to surface angle and roughness effects necessitating larger sample sizes for similar confidence bounds. Elastic-plastic indentations give the most accurate results and could be used to determine the different elastic constants for anisotropic materials by indenting different crystal faces, but not by rotating the indenter about its axis and indenting the same face in a different location. The hardness of small and large crystals is similar, suggesting that defect content probed in this study is similar, and that small crystals can be compared directly to larger ones. The Young's modulus and hardness of the model test material, griseofulvin, are given for the first time to be 11.5GPa and 0.4GPa respectively.

Characterization of flexible ECoG electrode arrays for chronic recording in awake rats.

We developed a 64-channel flexible polyimide ECoG electrode array and characterized its performance for long-term implantation, chronic cortical recording and high resolution mapping of surface-evoked potentials in awake rats. To achieve the longest possible recording periods, the flexibility of the electrode array, adhesion between the metals and carrier substrate, and biocompatibility were critical for maintaining the signal integrity. Experimental testing of thin film adhesion was applied to a gold-polyimide system in order to characterize relative interfacial fracture energies for several different adhesion layers, yielding an increase in overall device reliability. We tested several different adhesion techniques including the following: gold alone without an adhesion layer, titanium-tungsten, tantalum and chromium. We found titanium-tungsten to be a suitable adhesion layer considering the biocompatibility requirements as well as stability and delamination resistance. While chromium and tantalum produced stronger gold adhesion, concerns over biocompatibility of these materials require further testing. We implanted the polyimide ECoG electrode arrays through a slit made in the skull of rats and recorded cortical surface evoked responses. The arrays performed reliably over a period of at least 100 days and signals compared well with traditional screw electrodes, with better high frequency response characteristics. Since the ultimate goal of chronically implanted electrode arrays is for neural prosthetic devices that need to last many decades, other adhesion layers that would prove safe for implantation may be tested in the same way in order to improve the device reliability.

A MEMS fabricated flexible electrode array for recording surface field potentials.

We developed a method to microfabricate flexible electrode arrays on a thin Kapton substrate, which was engineered to minimize trauma when inserted between the dura and skull to obtain surface EEG recordings. The array consisted of 64 gold electrodes, each 150microm in diameter on a 750microm spaced 8x8 grid. Using photolithographic procedures, any arrangement of electrodes can be implemented. We used the electrode array to record evoked response signals to create topographical maps of the whisker barrels on the cortical surface with excellent signal stability over a period of 8h. The materials used for this fabrication are potentially biologically inert and, with some additional modifications to the design, can be chronically implanted with minimal side effects. Retinal prosthesis, human neurosurgery, and neurological research are all limited to some degree by the resolution and biological compatibility of the implants used. This type of array could greatly enhance the spatial resolution, signal quality, and stability of implantable surface electrode arrays.