Richard J. Phillips

Electrochemical and photoelectrochemical deposition of thallium(III) oxide thin films

Thallium (III) oxide is a degenerate n -type semiconductor with high optical transparency and electrical conductivity. Films of thallium(III) oxide can be electrochemically deposited onto conducting and p -type semiconducting substrates, and photoelectrochemically deposited onto n -type semiconducting substrates. Films deposited at currents below the mass transport limit onto platinum or stainless steel were columnar, and the current efficiency on stainless steel was 103 ±2%. Dendritic films were deposited at mass-transport-limited currents. Films were deposited with thicknesses ranging from 0.1 μm on n -silicon, to 170 μm on stainless steel. The photoelectrochemically deposited films were “direct-written” onto n -silicon, since the material was deposited only at irradiated portions of the electrode. Thin films were grown by irradiating the n -silicon with 450 nm monochromatic light, since the light was strongly absorbed by the thallium(III) oxide. The most uniform thin films were deposited when the n -silicon was initially irradiated with a short pulse of high intensity light. The pulse apparently promoted instantaneous nucleation of a high density of thallium(III) oxide nuclei.

Scanning tunneling microscopy of electrodeposited ceramic superlattices

Cleaved cross sections of nanometer-scale ceramic superlattices fabricated from materials of the lead-thallium-oxygen system were imaged in the scanning tunneling microscope (STM). The apparent height differences between the layers were attributed to composition-dependent variations in local electrical properties. For a typical superlattice, the measured modulation wavelength was 10.6 nanometers by STM and 10.8 nanometers by x-ray diffraction. The apparent height profile for potentiostatically deposited superlattices was more square than that for galvanostatically deposited samples. These results suggest that the composition follows the applied potential more closely than it follows the applied current. The x-ray diffraction pattern of a superlattice produced under potential control had satellites out to the fourth order around the (420) Bragg reflection.

Composition profiles in electrodeposited ceramic superlattices

Superlattices in the Pb–Tl–O system with layer thicknesses in the 4–6 nm range were electrodeposited from a single aqueous solution by pulsing the applied potential during deposition. The current‐time transients that resulted from the potential steps were monitored to both calculate and tailor the composition profiles of the superlattices during growth. The Cottrell method was used to determine that Tl(l) oxidation was diffusion limited at high potentials. The diffusion limitation resulted in a composition profile that was graded throughout the layer with a t−1/2 dependence. Superlattices grown at lower potentials in which both reactants were under kinetic control had square composition profiles.

Electrochemical Synthesis of Zirconia

Zirconia powder was produced in aqueous solution from zirconyl nitrate using electrogenerated base. Both divided and undivided electrochemical cells were used. In the divided cell, hydroxide ion was discharged in the cathode compartment, and hydrated zirconia was produced. The as-produced material was weakly agglomerated, amorphous, and had a surface area of up to 316 m 2 /g. The surface area of the powder did not vary systematically with the electrosynthesis current density, but did depend on subsequent processing. Crystalline zirconia was produced by calcining in air at temperatures above 400°C. Tetragonal zirconia was the only phase observed until about 800°C. After calcining at 800°C the crystallite size increased to about 20nm and about 34% monoclinic zirconia was produced. When an undivided cell was used, the pH remained constant (1–1.5) throughout the electrosynthesis, and amorphous zirconia deposited on the cathode.

Determination of modulation wavelengths in nanoscale electrodeposited superlattices using the scanning tunneling microscope

Abstract We have determined modulation wavelengths of electrochemically deposited metal oxide superlattices using a scanning tunneling microscope (STM). Alternating layers of Pb 0.74 Tl 0.26 0 1.9 and Pb 0.46 Tl 0.54 0 1.7 were deposited from a single aqueous solution under galvanostatic control. The sum of the thickness of the alternating layers equals the modulation wavelength. The concentration of lead and thallium within the layers was controlled by varying the current, which gave each alternating layer different conductivities. STM was used to image these nanoscale materials in real space, by exploiting the electronic differences of the individual superlattice layers. Also, the alternating layers were grown to equal thicknesses by varying the dwell times for the applied current density. The modulation wavelengths were designed electrochemically to be in the 6 to SO nm range. The modulation wavelength was determined using Fourier analysis of the STM images and found to be in good agreement with those calculated from x-ray diffraction results. This demonstrates the ability of the STM as a real space characterization tool for determining distances in the nanoscale regime of conductive electronic structures and devices.

Real-Space Imaging of Nanoscale Electrodeposited Ceramic Superlattices in the Scanning Tunneling Microscope

Superlattices in the Pb-Tl-O system were electrodeposited with layer thicknesses ranging from 1.5 to 25 nm. Both compositional and defect-chemistry superlattices were grown. In defect-chemistry superlattices based on thallium(III) oxide, it was found that thallium interstitials were favored at low overpotentials and oxygen vacancies were favored at high overpotentials. The superlattices were studied in reciprocal space by x-ray diffraction, and in real space by scanning tunneling microscopy (STM). The x-ray pattern of a compositionally modulated sample grown under potentiostatic control exhibited superlattice satellites out to fourth order. STM images were obtained in air on cleaved cross-sections. The composition profile in the superlattices was shown to manifest itself as an apparent height profile in the STM image.. The STM helped us engineer “better” superlattices, since it was found that compositional superlattices grown under potential control had apparent height profiles which were more square than samples grown under current control.

Potential Step Probes of Epitaxial Growth in Electrodeposited BCC Tl 2 O 3 Films onto FCC Conducting Metal Oxides

Potential step transients were investigated as an in-situ probe of epitaxial growth for electrodeposited conducting metal oxides of the Tl 2 O 3 and Pb a Tl b O c systems. Changes in the induction time and growth type were observed for Tl 2 O 3 deposition as a function of the substrate. The substrates studied were glassy carbon and [210]-textured Pb 0.8 Tl 0.2 O 1.9 · T1 2 O 3 deposited onto the glassy carbon electrode showed a distinct induction time, the magnitude of which was potential dependent. Also the type of growth was characterized as three dimensional with instantaneous nucleation. X-ray diffraction of this film shows a nearly random pattern. The potential step transient for Tl 2 O 3 deposited onto the [210]-textured Pb 0.8 Tl 0.2 O 1.9 , showed the absence of an induction time and elimination of the growth segment. The type of growth was characterized as two dimensional. X-ray diffraction indicated epitaxy was obtained for the Tl 2 O 3 films which grew two dimensionally onto the [210]-textured mixed oxide.