Eric W. Bohannan

Enantiospecific electrodeposition of a chiral catalyst

Many biomolecules are chiral—they can exist in one of two enantiomeric forms that only differ in that their structures are mirror images of each other. Because only one enantiomer tends to be physiologically active while the other is inactive or even toxic, drug compounds are increasingly produced in an enantiomerically pure form using solution-phase homogeneous catalysts and enzymes. Chiral surfaces offer the possibility of developing heterogeneous enantioselective catalysts that can more readily be separated from the products and reused. In addition, such surfaces might serve as electrochemical sensors for chiral molecules. To date, chiral surfaces have been obtained by adsorbing chiral molecules or slicing single crystals so that they exhibit high-index faces, and some of these surfaces act as enantioselective heterogeneous catalysts. Here we show that chiral surfaces can also be produced through electrodeposition, a relatively simple solution-based process that resembles biomineralization in that organic molecules adsorbed on surfaces have profound effects on the morphology of the inorganic deposits. When electrodepositing a copper oxide film on an achiral gold surface in the presence of tartrate ion in the deposition solution, the chirality of the ion determines the chirality of the deposited film, which in turn determines the films enantiospecificity during subsequent electrochemical oxidation reactions.

Single source precursors for fabrication of I–III–VI2 thin-film solar cells via spray CVD

Abstract The development of thin-film solar cells on flexible, lightweight, space-qualified substrates provides an attractive cost solution to fabricating solar arrays with high specific power (W/kg). Thin-film fabrication studies demonstrate that ternary single source precursors can be used in either a hot, or cold-wall spray chemical vapour deposition reactor, for depositing CuInS2, CuGaS2 and CuGaInS2 at reduced temperatures (400–450 °C), which display good electrical and optical properties suitable for photovoltaic devices. X-ray diffraction studies, energy dispersive spectroscopy and scanning electron microscopy confirmed the formation of the single phase CIS, CGS, CIGS thin-films on various substrates at reduced temperatures.

POTENTIAL OSCILLATIONS DURING THE ELECTROCHEMICAL SELF-ASSEMBLY OF COPPER/CUPROUS OXIDE LAYERED NANOSTRUCTURES

Layered nanostructures of copper metal and cuprous oxide are electrodeposited from alkaline solutions of Cu(II) lactate at room temperature. No subsequent heat treatment is necessary to effect crystallization. The electrode potential spontaneously oscillates during constant-current deposition. At a fixed current density the oscillation period decreases as either the pH or temperature is increased. The oscillations are periodic in stirred solution, but show period doubling and evidence of quasi-periodic or chaotic behavior in unstirred solution. The phase composition and resistivity of the films can be controlled by varying the applied current density. The resistivity of the films can be varied over ten orders of magnitude. Scanning electron microscopy shows that the films are layered.

Low-temperature electrodeposition of the high-temperature cubic polymorph of bismuth(III) oxide

Abstract Nanocrystalline films of δ-Bi 2 O 3 were electrodeposited at 65°C directly from alkaline solutions of tartrate-complexed Bi(III). This face-centered-cubic polymorph of Bi 2 O 3 is normally only stable at high temperatures (729–825°C). The material has the highest known oxide ion mobility. We propose that the high temperature form of the oxide is stabilized due to the nanocrystalline (70 nm) size of the particles in the film. The oxide also deposits epitaxially onto a single-crystal Au(110) substrate with strong in-plane and out-of-plane orientation. The large lattice mismatch (35.4%) is accommodated by forming a coincidence lattice, in which the δ-Bi 2 O 3 is rotated 90° relative to the Au (110) substrate. The epitaxial relationship between film and substrate may also serve to stabilize the high-temperature structure.

An Electrochemical Method for CuO Thin Film Deposition from Aqueous Solution

An electrochemical procedure is described for the anodic deposition of CuO thin films from solution precursors at 25-30°C in an alkaline medium (pH > 13). The deposition bath was similar to Fehlings solution using tartrate ions as a complexing agent for Cu(II). Cupric oxide deposited onto a platinum substrate at an anodic current density of 5 mA cm - 2 has a preferred orientation of [010]. Rietveld refinement of the powder diffraction pattern reveals pure Cu(II) oxide with no trace of other copper oxides. The suggested mechanism involves the irreversible electrochemical oxidation of the tartrate ligand of the Cu(II) complex leading to the CuO precipitation. The same bath can also be used to deposit Cu 2 O films using a cathodic electrodeposition process. In this case, cuprous oxide deposited onto a platinum electrode has a [111] preferred orientation.

Epitaxial electrodeposition of Cu2O films onto InP(001)

Cu2O (cuprite) films were deposited electrochemically onto InP (001) single-crystal substrates from aqueous solutions at room temperature. X-ray diffraction indicates a unique epitaxial 45°[001] orientation relationship between Cu2O and InP. This reduces the mismatch between corresponding spacings to 2.9%, compared with a mismatch of 27.2% between the lattice parameters of Cu2O and InP. The morphology of the Cu2O film can be influenced via the electrolyte acidity. At a pH of 9.0, Cu2O forms pyramidal islands. At a pH of 12.0, on the other hand, cubelike morphologies of Cu2O are observed. Between a pH of 9.0 and 12.0, the direction of slowest growth changes from 〈111〉 to 〈100〉. In apparent contradiction to the observation of a unique epitaxial orientation relationship, transmission electron microscopy reveals an amorphous oxygen-rich interlayer between the Cu2O and the InP crystal.

Negative Differential Resistance in Electrochemically Self-assembled Layered Nanostructures

Resonant tunneling devices are used for ultrahigh-speed applications. In this work, tunnel junctions based on copper metal (Cu) and cuprous oxide (Cu2O) are electrochemically self-assembled from aqueous solution in an oscillating system. The Cu2O layer thickness (L) is tuned from 0.8 to 2.8 nm by simply changing the applied current density. The layered structures show sharp negative differential resistance (NDR) signatures at room temperature in perpendicular transport measurements, and the NDR maximum shifts to higher bias with a 1/L2 dependence as the Cu2O layer is made thinner. The results are consistent with resonant tunneling from Cu into hole states in the valence band of quantum-confined Cu2O through thin space−charge regions on each side of the Cu2O.

Superconducting Filaments Formed During Nonvolatile Resistance Switching in Electrodeposited δ-Bi2O3

We show that electrodeposited films of δ-Bi2O3 in a Pt/δ-Bi2O3/Au cell exhibit unipolar resistance switching. After being formed at a large electric field of 40 MV/m, the cell can be reversibly switched between a low resistance state (156 Ω) and a high resistance state (1.2 GΩ) by simply cycling between SET and RESET voltages of the same polarity. Because the high and low resistance states are persistent, the cell is a candidate for nonvolatile resistance random access memory (RRAM). A Bi nanofilament forms at the SET voltage, and it ruptures to form a 50 nm gap during the RESET step at a current density of 2 × 10(7) A/cm(2). The diameter of the Bi filament is a function of the compliance current, and can be tuned from 140 to 260 nm, but the current density in the RESET step is independent of the Bi diameter. An electromigration rupture mechanism is proposed. The Bi nanofilaments in the low resistance state are superconducting, with a Tc of 5.8 K and an Hc of 5 kOe. This is an unexpected result, because bulk Bi is not a superconductor.

Photolithographic Patterning and Doping of Silica Xerogel Films

This study shows that conventional photolithography can be applied for patterning native or organic dye-doped silica films (∼0.5 μm thick) obtained via a base-catalyzed sol-gel process. Photoresist was spin-coated onto high optical quality xerogel films, soft-baked, exposed to UV irradiation through a photomask, and developed with a commercial photoresist developing solution. Etching away of the photoresist-unprotected areas of the silica films was carried out with a dilute HF solution, while the remaining unexposed photoresist was removed with acetone. Interdigitated array patterns with features as small as 0.5 mm show a smooth surface and extremely sharp interfaces. Densification of the films at 550°C for 2 h decreases the film thickness by ∼11%, increases the refractive index from 1.420 to 1.456, and allows for well-defined patterning down to length scales of 10 μm. Since the densification conditions are incompatible with organic dopants, it is demonstrated that sol-gel films can be doped after pattering (post-doping) by adsorption of cationic dyes from solution. Scanning electron microscopy reveals that the microstructure of patterned sol-gel films is similar to that of bulk monoliths, indicating that the photolithographic procedure is not harmful to the film quality. All patterned films demonstrate highly regular light diffraction patterns.

Electrostatic force microscopy studies of boron-doped diamond films

Much has been learned from electrochemical properties of boron-doped diamond (BDD) thin films synthesized using microwave plasma-assisted chemical vapor deposition about the factors influencing electrochemical activity, but some characteristics are still not entirely understood, such as its electrical conductivity in relation with microscale structure. Therefore, to effectively utilize these materials, understanding both the microscopic structure and physical (electrical, in particular) properties becomes indispensable. In addition to topography using atomic force microscopy, electrostatic force microscopy (EFM) in phase mode measuring the long-range electrostatic force gradients, helps to map the electrical conductivity heterogeneity of boron-doped micro-/nanocrystalline diamond surfaces. The mapping of electrical conductivity on boron doping and bias voltage is investigated. Experimental results showed that the BDD films’ surfaces were partially rougher with contrast of conductive regions (areas much less than 1 μm2 in diameter), which were uniformly distributed. Usually, the EFM signal is a convolution of topography and electrostatic force, and the phase contrast was increased with boron doping. At the highest boron doping level, the conductive regions exhibited quasi-metallic electrical properties. Moreover, the presence of a “positive–negative–positive” phase shift along the line section indicates the presence of “insulating–conducting–insulating” phases, although qualitative. Furthermore, the electrical properties, such as capacitance and dielectric constants at operating frequency, were quantitatively evaluated through modeling the bias-dependent phase measurements using simple and approximate geometries. It was found that decreasing grain size (or increasing the boron concentration) lowers the dielectric constant, which is attributed to the change in the crystal field caused by surface bond contraction of the nanosized crystallites. These findings are complemented and validated with scanning electron microscopy, x-ray diffraction, and “visible” Raman spectroscopy revealing their morphology, structure, and carbon-bonding configuration (sp3 versus sp2), respectively. These results are significant in the development of electrochemical nano-/microelectrodes and diamond-based electronics.