Biomedical subjects

RS Williams

Publications and source records attributed to RS Williams.

16 recordsLinked to original sources

PubMed2000

Strain evolution in coherent Ge/Si islands

Strain evolution of coherent Ge islands on Si(001) is measured using a newly developed transmission electron microscopy technique based on two-beam dark-field strain imaging. The strain measurements show that a metastable Ge island shape is involved in the shape transition between pyramids and domes; this shape is more readily observed for growth at 550 than 600 degrees C because of the slower rate at which islands cross the kinetic barrier between shapes. The strain relaxation changes discontinuously between pyramids and domes, indicating that the underlying shape transition is first order.

Journal Article↗

PubMed1999

Electronically configurable molecular-based logic gates

Logic gates were fabricated from an array of configurable switches, each consisting of a monolayer of redox-active rotaxanes sandwiched between metal electrodes. The switches were read by monitoring current flow at reducing voltages. In the "closed" state, current flow was dominated by resonant tunneling through the electronic states of the molecules. The switches were irreversibly opened by applying an oxidizing voltage across the device. Several devices were configured together to produce AND and OR logic gates. The high and low current levels of those gates were separated by factors of 15 and 30, respectively, which is a significant enhancement over that expected for wired-logic gates.

Journal Article↗

PubMed1998

Shape transition of germanium nanocrystals on a silicon (001) surface from pyramids to domes

Chemical vapor deposition of germanium onto the silicon (001) surface at atmospheric pressure and 600 degrees Celsius has previously been shown to produce distinct families of smaller (up to 6 nanometers high) and larger (all approximately 15 nanometers high) nanocrystals. Under ultrahigh-vacuum conditions, physical vapor deposition at approximately the same substrate temperature and growth rate produced a similar bimodal size distribution. In situ scanning tunneling microscopy revealed that the smaller square-based pyramids transform abruptly during growth to significantly larger multifaceted domes, and that few structures with intermediate size and shape remain. Both nanocrystal shapes have size-dependent energy minima that result from the interplay between strain relaxation at the facets and stress concentration at the edges. A thermodynamic model similar to a phase transition accounts for this abrupt morphology change.

Journal Article↗

PubMed1994