The Si(100)(2 × 1) surface serves as a template for the formation of Mn wires at room temperature, which are the starting point for the annealing experiments discussed here. The evolution of the Mn surface structures as a function of annealing temperature was observed with scanning tunneling microscopy for the temperature range between 115 and 600 °C and establishes a surface phase diagram for the Mn−Si(100)(2 × 1) system. The stability of the Mn nanowires is limited; they break up below 115 °C (the lowest annealing temperature studied), and ultrasmall clusters are formed. These clusters are initially positioned on the terraces but migrate to the step edges at around 250 °C, which sets the limit for the Mn surface diffusion length. At around 300 °C the Mn adatoms move into subsurface sites, and the empty and filled states images strongly indicate that Mn acts as an acceptor in this near-surface region. A further increase in temperature leads to the formation of large crystallites (several tens of nanometers), which exhibit the characteristic shape associated with Mn silicides. The modification of the Si surface with temperature is characterized by the dramatic increase in the defect population (defect density and size and step edge roughness). The condensation of dimer vacancies begins around 200 °C and progresses to the formation of long dimer vacancy lines at elevated temperatures. The step edge roughness and the step edge formation energies were calculated for SA and SB steps, and their modulation with annealing temperature illustrates the impact of Mn on the defect and kink stabilities. These data will be used to perform a thermodynamic and kinetic modeling of defect population in the presence of Mn at moderate substrate temperatures. This study presents a surface phase diagram, which includes the evolution of the Mn nanostructures and the modification of the Si surface. The identification of near-surface layers of Mn acceptors is particularly relevant for the design of basic building blocks for future Si-based spintronics.

Heteroepitaxial Ge1-xMnx quantum dots (QDs) were grown on Si (001) by molecular beam epitaxial co-deposition, with x = 0 to 0.10, in order to explore the interaction between Mn content, surface morphological evolution, and magnetism. Morphological evolution typical of the Ge/Si (001) system was observed, where the effect of Mn on surface morphology is surprisingly minimal at low Mn content, with no obvious surface morphological indicators of second phase formation. As the Mn content increases, secondary phase formation becomes evident, appearing to heterogeneously nucleate on or within Ge QDs. Still higher Mn concentrations lead to extensive second phase formation interspersed with an array of Ge QDs. Although ferromagnetism up to 220 K is observed, likely arising from intermetallic precipitates, there is no clear evidence for room-temperature ferromagnetism associated with a dilute magnetic solution phase.

Heteroepitaxial Ge 0.98 Mn 0.02 quantum dots on Si (001) were grown by molecular beam epitaxy. The standard Ge wetting layer-hut-dome-superdome sequence was observed, with no indicators of second phase formation in the surface morphology. We show that Mn forms a dilute solid solution in the Ge quantum dot layer, and a significant fraction of the Mn partitions into a sparse array of buried, Mn-enriched silicide precipitates directly underneath a fraction of the Ge superdomes. The magnetic response from the ultra-thin film indicates the absence of robust room temperature ferromagnetism, perhaps due to anomalous intermixing of Si into the Ge quantum dots.

Nanocrystalline titania of different phases were produced by ambient condition sol process with phase control originating from alterations in experimental variables. The produced titania photocatalysts were characterized by use of x-ray diffraction, BET surface area, transmission electron microscopy and related to methyl orange degradation. The results showed that the photocatalytic activity of brookite and anatase phase titania samples to be greater than that of Degussa P-25 and rutile phase titania sample. In addition, brookite, due to surface area considerations, appears to be the most photocatalytically active phase of titania.