Evolution of anatase surface active sites probed by in situ sum-frequency phonon spectroscopy

Surface active sites of crystals often govern their relevant surface chemistry, yet to monitor them in situ in real atmosphere remains a challenge. Using surface-specific sum-frequency spectroscopy, we identified the surface phonon mode associated with the active sites of undercoordinated titanium ions and conjoint oxygen vacancies, and used it to monitor them on anatase (TiO2) (101) under ambient conditions. In conjunction with theory, we determined related surface structure around the active sites and tracked the evolution of oxygen vacancies under ultraviolet irradiation. We further found that unlike in vacuum, the surface oxygen vacancies, which dominate the surface reactivity, are strongly regulated by ambient gas molecules, including methanol and water, as well as weakly associated species, such as nitrogen and hydrogen. The result revealed a rich interplay between prevailing ambient species and surface reactivity, which can be omnipresent in environmental and catalytic applications of titanium dioxides.

Influence of Mn Doping on Structural and Vibrational Properties of Self-Assembled Mn Doped ZnO Nanocrystals

Nanocrystallites (15–25 nm) of Mn doped ZnO prepared by refluxing their acetate precursors in the presence of diethylene glycol, self-assemble into polydisperse spheres of broad size distributation (100–400 nm). The center of X-ray diffraction peaks shifts towards lower angle and the line gets broadened on increasing Mn concentration. Compared to the vibration modes of wurtzite ZnO, one additional vibration mode is observed in the range of 522–518 cm−1 in Raman spectra of Mn doped ZnO whose intensity increases on increasing the Mn concentration. The origin of this peak could be related to the incorporation of Mn2+ in Zn2+ lattice site. Further, on increasing Mn concentration, infrared band red shifted and surface phonon mode absorption get pronounced due to the incorporation of Mn2+ in Zn2+ lattice site resulting changes in the local structure parameters (effective mass, force constant, bond length).

Raman studies of optical phonons in vertical cadmium sulfide nanorod arrays

Electrochemically grown cadmium sulfide (CdS) nanorod arrays were studied with Raman spectroscopy. The resonant Raman spectroscopy unravels the enhanced electron-phonon interaction up to the fifth-order multiphonon process in the vertically aligned CdS nanorods after annealing. Resonant Raman scattering at room temperature reveals a surface phonon mode at 253 cm−1 in the annealed nanorod. This unprecedented observation is accounted for by the lateral confinement in the nanorod whose average aspect ratio is approximately 5. An intersubband transition near 3000 cm−1 is also observed. These results point to important optoelectronic applications of this material.

Growth and Optical Properties of GaP, GaP@GaN and GaN@GaP Core-shell Nanowires

High-quality GaP, GaP@GaN and GaN@GaP nanowires were grown by a convenient vapor deposition technique. The wire-like and two-layers structures of GaP@GaN and GaN@GaP core-shell nanowires were clearly resolved using X-ray powder diffraction and high-resolution transmission electron microscopy (HRTEM) and their growth directions were identified. Photoluminescence intensity of GaP@GaN nanowires increased as temperature increased. The result was interpreted by the piezoelectric effect induced from lattice mismatch between two semiconductor layers. An unexpected peak at 386 cm-1 was found in the Raman spectra of GaN@GaP and assigned to a surface phonon mode due to the interface. Detailed synthetic conditions and possible growth mechanisms of those nanowires were proposed.

QUANTUM CONFINEMENT OF EMBEDDED COPPER CLUSTER SANDWICHED BY LITHIUM FLUORIDE

Optical absorption and laser Raman scattering spectroscopies as well as EXAFS are used to investigate the copper clusters embedded in the lithium fluoride, and the results indicate that (i) the metal clusters of about 2 nm in diameter have the bulk-like fcc structure with a contracted interatomic distance and lower mean coordination number, (ii) the embedded metal clusters show a surface-plasmon resonance, (iii) for clusters smaller than certain size the surface-phonon mode becomes important, and the high-order longitudinal phonon vibrational modes emerge.