-Maw Cheng

Observation of carbon-containing nanostructured mixed titania phases for visible-light photocatalysts

This research analyzed a visible-light-responsive carbon-containing titania photocatalysts prepared by sol-gel method using confocal Raman spectral mapping, transmission electron microscopy, and UV/visible spectroscopy. The incorporation of carbons in the initial titania powder of anatase and amorphous phases was found to facilitate the formation of rutile or brookite phases from the amorphous phase in the 150–250°C temperature range, resulting in nanostructured mixed titania phases which create interface states and effectively shift the band gap to ∼2.7eV. The further shift in the absorption tails up to 800nm was attributed to carbon species-covered and/or defected titania powders.

Growth and characterization of MBE-grown (Bi1-xSbx)2Se3 topological insulator

(Bi1− x Sb x )2Se3 thin films were prepared by molecular beam epitaxy (MBE). The existence of strong and robust topological surface states was demonstrated in the (Bi1− x Sb x )2Se3 ternary system by angle-resolved photoemission spectroscopy (ARPES). The sheet carrier density n 2D was found to be decreased by 75% by doping Sb into Bi2Se3, compared with that in the case of undoped Bi2Se3. The enhancement of the surface state transport due to Sb doping was also revealed via the high-field Hall effect and weak antilocalization measurement. Our results reveal the potential of this system for the study of tunable topological-insulator based device physics.

Dispersive resonance bands within the space-charge layer of a metal-semiconductor junction

Based on measurements of angle resolved photoemission, we report that in the Pb/Ge(111)- \sqrt{3}x\sqrt{3} R30^\circ structure, in addition to three bands resembling Ge heavy hole (HH), light hole (LH), and split off (SO) bulk band edges, a fourth dispersive band resembling the non split off (NSO) band is found near the surface zone center. While three Ge bulk-like bands get distorted due to strong coupling between Pb and Ge, the NSO-like band gets weaker and disappears for larger thickness of Pb, which, when combined with ab initio calculations, indicates its localized nature within space charge layer. Our results are clearly important for designing electronics involved with metal-semiconductor contacts.

Bilayer oscillation of subband effective masses in Pb/Ge(111) thin-film quantum wells

Subband dispersions of quantum-well states in Pb thin films on Ge(111) have been measured with angle-resolved photoemission spectroscopy. The effective masses at the surface zone center exhibit a bilayer oscillation with thickness, in both magnitude and sign. This behavior is attributed to a strong interaction between Pb quantum-well subbands and the Ge valence maximum near the Fermi level, which occurs about every two monolayers.

Controlling the surface photovoltage on WSe2 by surface chemical modification

The surface photovoltage (SPV) effect is key to the development of opto-electronic devices such as solar-cells and photo-detectors. For the prototypical transition metal dichalcogenide WSe2, core level and valence band photoemission measurements show that the surface band bending of pristine cleaved surfaces can be readily modified by adsorption with K (an electron donor) or C60 (an electron acceptor). Time-resolved pump-probe photoemission measurements reveal that the SPV for pristine cleaved surfaces is enhanced by K adsorption, but suppressed by C60 adsorption, and yet the SPV relaxation time is substantially shortened in both cases. Evidently, adsorbate-induced electronic states act as electron-hole recombination centers that shorten the carrier lifetime.The surface photovoltage (SPV) effect is key to the development of opto-electronic devices such as solar-cells and photo-detectors. For the prototypical transition metal dichalcogenide WSe2, core level and valence band photoemission measurements show that the surface band bending of pristine cleaved surfaces can be readily modified by adsorption with K (an electron donor) or C60 (an electron acceptor). Time-resolved pump-probe photoemission measurements reveal that the SPV for pristine cleaved surfaces is enhanced by K adsorption, but suppressed by C60 adsorption, and yet the SPV relaxation time is substantially shortened in both cases. Evidently, adsorbate-induced electronic states act as electron-hole recombination centers that shorten the carrier lifetime.