Michael T. Brumbach

Lithographically defined three-dimensional graphene structures.

A simple and facile method to fabricate 3D graphene architectures is presented. Pyrolyzed photoresist films (PPF) can easily be patterned into a variety of 2D and 3D structures. We demonstrate how prestructured PPF can be chemically converted into hollow, interconnected 3D multilayered graphene structures having pore sizes around 500 nm. Electrodes formed from these structures exhibit excellent electrochemical properties including high surface area and steady-state mass transport profiles due to a unique combination of 3D pore structure and the intrinsic advantages of electron transport in graphene, which makes this material a promising candidate for microbattery and sensing applications.

Three dimensional nickel–graphene core–shell electrodes

The annealing of nickel-coated porous carbon structures results in a new three dimensional nanostructured graphene encapsulated nickel core–shell electrode. A highly interdependent and dynamic process is observed that results in the complete reversal of the spatial orientations of the two component system after the annealing process. We examine the mechanism of carbon diffusion and observe unexpected morphological changes of the nickel in response to carbon crystallization. The new nickel–graphene core–shell electrode demonstrates excellent electrochemical properties with promising applications in micro-batteries and biosensors.

Metalloporphyrin Assemblies on Pyridine-Functionalized Titanium Dioxide

Porphyrin adsorption on TiO2 nanoparticles has been achieved for multiple porphyrins, and in mixed porphyrin assemblies, via axial ligation to surface-bound pyridine anchored by either para carboxylic or phosphonic functionalizations. Homogenous assemblies were prepared and characterized, while mixed metalloporphyrin assemblies were demonstrated by controlling the concentration ratios of respective porphyrins in the modifying solution. Evaluation of the assemblies using spectroscopic techniques and electrochemistry confirms high porphyrin retention, while exhibiting their surface bound optical and electrochemical properties. A thorough study is discussed where several metalloporphyrins have been evaluated (Ru(CO)OEP, Ru(CO)TPP, and ZnTPP) for relative comparisons and relationships to pyridyl axial binding strengths. The systematic study evaluates multiple background cases using either H2TPP, TiO2 modification with benzoic acid, or unmodified TiO2 to confirm the high affinity of Ru and Zn porphyrins for surface-anchored pyridyl sites. The simple method of step-by-step coordinative anchoring of porphyrins to TiO2 using small, commercially available molecules is highly adaptable for use in dye-sensitized solar cells (DSSC) where intimate contact between the absorbing dye and the semiconductor is required. DSSC devices with novel mixed porphyrin assemblies were shown to give higher power performance than DSSCs utilizing sensitization with only one type of porphyrin.

Nanostructured Ruthenium Oxide Electrodes via High-Temperature Molecular Templating for Use in Electrochemical Capacitors

Ruthenium oxide is a model pseudocapacitive materials exhibiting good electronic and protonic conduction and has been shown to achieve very high gravimetric capacitances. However, the capacitance of thermally prepared ruthenium oxide is generally low because of low protonic conductivity resulting from dehydration of the oxide upon annealing. High-temperature processing, however also produces the electrically conducting ruthenium oxide rutile phase, which is of great interest for electrochemical capacitors. Here, unusual electrochemical characteristics were obtained for thermally prepared ruthenium oxide when fabricated in the presence of alkyl-thiols at high temperature. The performance characteristics have been attributed to enhanced multifunctional properties of the material resulting from the novel processing. The processing method relies on a simple, solution-based strategy that utilizes a sacrificial organic template to sterically direct hierarchical architecture formation in electro-active ruthenium oxide. Thin films of the templated RuO(2) exhibit energy storage characteristics comparable to hydrous ruthenium oxide materials formed under dramatically different conditions. Extensive materials characterization has revealed that these property enhancements are associated with the retention of molecular-sized metal oxide clusters, high hydroxyl concentrations, and formation of hierarchical porosity in the ruthenium oxide thin films.

Evaluating tantalum oxide stoichiometry and oxidation states for optimal memristor performance

Tantalum oxide has shown promising electrical switching characteristics for memristor devices. Consequently, a number of reports have investigated the electrical behavior of TaOx thin films. Some effort has been made to characterize the composition of the TaOx films and it is known that there must be an optimal stoichiometry of TaOx where forming and switching behavior are optimized. However, many previous reports lack details on the methodology used for identifying the chemistry of the films. X-ray photoelectron spectroscopy has been the most commonly used technique; however, peak fitting routines vary widely among reports and a native surface oxide of Ta2O5 often confounds the analysis. In this report a series of large area TaOx films were deposited via sputtering with controlled O2 partial pressures in the sputtering gas, resulting in tunable oxide compositions. Spectra from numerous samples from each wafer spanning a range of oxide stoichiometries were used to develop a highly constrained peak fitting...

Pyroelectric response in crystalline hafnium zirconium oxide (Hf1-xZrxO2) thin films

Pyroelectric coefficients were measured for 20 nm thick crystalline hafnium zirconium oxide (Hf1-xZrxO2) thin films across a composition range of 0 ≤ x ≤ 1. Pyroelectric currents were collected near room temperature under zero applied bias and a sinusoidal oscillating temperature profile to separate the influence of non-pyroelectric currents. The pyroelectric coefficient was observed to correlate with zirconium content, increased orthorhombic/tetragonal phase content, and maximum polarization response. The largest measured absolute value was 48 μCm−2 K−1 for a composition with x = 0.64, while no pyroelectric response was measured for compositions which displayed no remanent polarization (x = 0, 0.91, and 1).

Oxidation of ultrathin GaSe

Oxidation of exfoliated gallium selenide (GaSe) is investigated through Raman, photoluminescence, Auger, and X-ray photoelectron spectroscopies. Photoluminescence and Raman intensity reductions associated with spectral features of GaSe are shown to coincide with the emergence of signatures emanating from the by-products of the oxidation reaction, namely, Ga2Se3 and amorphous Se. Photoinduced oxidation is initiated over a portion of a flake highlighting the potential for laser based patterning of two-dimensional heterostructures via selective oxidation.

Controlled Nucleation and Growth of Pillared Paddlewheel Framework Nanostacks onto Chemically Modified Surfaces

The nucleation and growth of metal-organic frameworks onto functional surfaces stands to facilitate the utility of these supramolecular crystalline materials across a wide range of applications. Here, we demonstrate the solvothermal nucleation and growth of a pillared paddlewheel porphyrin framework 5 (PPF-5) onto semiconductor surfaces modified with carboxylic acids. Using versatile diazonium and catechol chemistries to modify silicon and titania surface chemistries, we show that solvothermally grown PPF-5 selectively nucleates and grows as stacked crystalline sheets with preferential (001), (111), and (110) crystallographic orientations. Furthermore, variations in the synthesis temperature produce modified stack morphologies that correlate with changes in the surface-nucleated PPF-5 photoluminescence.

Kinetics of chemi-ionization reactions of lanthanide metals (Nd, Sm) from 150 to 450 K

The kinetics of chemi-ionization reactions of neodymium and samarium atoms with an oxygen atom to yield a metal monoxide cation and electron were studied using a flow tube apparatus over a temperature range of 150-450 K. Nd reacts efficiently with O, near the hard-sphere collision limit at all temperatures, with a rate constant of 3 × 10(-10) cm(3) s(-1) at 300 K and a slight positive temperature dependence. No chemi-ionization of Nd with N2O was observed, despite the reaction being exothermic. Chemi-ionization of Sm with O is slow, with a rate constant at 300 K determined to be 7 × 10(-12) cm(3) s(-1), although with large uncertainty. The Sm reaction also shows a slightly positive temperature dependence, described by a small activation energy of 60 meV. Although not definitive, the data suggest that excited states of Sm react efficiently whereas ground state Sm reacts inefficiently.

Grain boundary diffusivity of Ni in Au thin films and the associated degradation in electrical contact resistance due to surface oxide film formation

The low temperature diffusion from a nickel strike layer through varying thickness films of gold, a common electrical contact overlayer, was characterized and correlated to changes in electrical contact resistance (ECR). The diffusivity of Ni in Au was determined by the surface accumulation method (type C kinetics) for Au film thicknesses of 280 and 994 nm at an annealing temperature of 150 °C over a 32 day period. X-ray photoelectron spectroscopy (XPS) was used to determine the rate of Ni surface accumulation. The average product of grain boundary width and diffusivity for Ni in polycrystalline Au was calculated to be δbDb≅2.0 × 10−22cm3s using the Hwang-Balluffi model, and δbDb≅2.5×10−22cm3s using the Ma-Balluffi model. ECR measurements were made in parallel to surface accumulation measurements, revealing a correlation between ECR and Ni-O surface concentration; ECR values for both film thicknesses increased by 1 to 2 orders of magnitude at saturation coverage. Film cross-sections were extracted using f...