Shuangyou Zhang

Optical properties of the ZnO nanotubes synthesized via vapor phase growth

A large quantity of nanosized ZnO tubular structures was prepared using a very simple thermal evaporation of mixed Zn–ZnO powders under a wet oxidation condition. The ZnO nanotubes have a hollow core with crystalline wall of 8–20 nm in thickness. Optical properties of ZnO nanotubes were studied at room temperature. Raman peaks arising from the ZnO nanotubes were analyzed, which correspond well to that of the bulk ZnO sample. The photoluminescence measurements of ZnO nanotubes revealed an intensive UV peak at 377 nm corresponding to the free exciton emission, and a broad peak at about 500 nm arising from defect-related emission.

Temperature dependence of the Raman spectra of single-wall carbon nanotubes

Raman spectra of single-wall carbon nanotubes (SWCNTs) were measured at different temperatures by varying the incident laser power. The elevated temperature of the SWCNTs and multiwall carbon nanotubes (MWCNTs) is confirmed to be due to the presence of impurities, defects, and disorder. The temperature coefficient of the frequency of the C–C stretching mode E2g (GM) and that of the radial breathing mode in the SWCNT were determined to be ∼−0.038 and ∼−0.013 cm−1/K, respectively. It is found that the temperature coefficient of the GM in the SWCNT is larger than that of the MWCNT, highly oriented pyrolytic graphite, and the graphite. This is attributed to the structural characteristic of the SWCNT—a single tubular carbon sheet with smaller diameter.

Study on the quantum confinement effect on ultraviolet photoluminescence of crystalline ZnO nanoparticles with nearly uniform size

Crystalline ZnO nanoparticles with nearly uniform size were studied using photoluminescence and Raman spectroscopy. The main spectral feature of ultraviolet photoluminescence is attributed to the recombination of free exciton (FX) and the first surface phonon replica of FX emission. The energy of FX emission shows a clear blueshift as the size of nanoparticles decreases, indicating that the quantum confinement effect exists in the electronic structure of nano-ZnO, although no confinement effect on the vibrational modes has been found in the same series of samples.

Strain-induced formation of K2Ti6O13 nanowires via ion exchange

Nanowires with 10 nm in diameter and several microns in length have been synthesized by ion exchange reaction of Na2Ti3O7 in KOH aqueous solution, and the structure of these nanowires has been determined to be K2Ti6O13. It was shown that the nanowires were formed as a result of strain induced phase transformation from the Na2Ti3O7 structure to K2Ti6O13. The length of the nanowires was determined largely by the size of the raw Na2Ti3O7 crystal along its [010] direction, and the cross section was determined mainly by the competition between the lattice misfit resulted strain energy and the bonding characteristics of the Na2Ti3O7 structure.

Raman spectroscopy of nanocrystalline GaN synthesized by arc plasma

We report on a Raman study of nanocrystalline GaN with the wurtzite structure synthesized by arc plasma method. Resonant Raman scattering is observed using 514.5 nm (2.41 eV) laser excitation, which is near the band gap of the “yellow band” (2.2–2.3 eV). Under such near-resonant excitation, new Raman bands unexpected in an ideal wurtzite GaN crystal were found. The transverse optical modes of A1 (531 cm−1) and E1 (560 cm−1), and the nonpolar modes of E2 [567 cm−1 (high) and 143 cm−1 (low)] normally observed in bulk crystals, were recorded and were observed to be resonantly enhanced. Two new bands (680 and 344 cm−1) were assigned to the inactive optical phonon modes B1 (high) and B1 (low), respectively. A broadband centered at 710 cm−1 was attributed to surface modes of the nanocrystals, providing good agreement with the calculated result based on Frohlich theory. As a result of this study, Raman scattering of GaN nanocrystals has been characterized.

Study of the size effect on the optical mode frequencies of ZnO nanoparticles with nearly uniform size

It was found in ZnO nanoparticles of nearly uniform size that the Raman frequencies of both the polar A1(LO) and nonpolar E2(H) modes do not shift with particle sizes. This observation is in perfect agreement with previous theoretical prediction. The observed result is consistent with the prediction that the size insensitivity of optic vibration frequency in polar nanosemiconductors originates from the Frohlich interaction, which exists only in polar nanosemiconductors.

Imaging helical potassium hexaniobate nanotubes

Potassium hexaniobate nanotubes have been synthesized at room temperature and characterized by high-resolution transmission electron microscopy (HRTEM) and energy-dispersive x-ray analysis. It is shown that HRTEM images may be used effectively to determine the helicity of the nanotubes, and, in particular, it is found that almost all nanotubes have their axes pointing within a few degrees from the [100] direction of the K4Nb6O17 structure.

Observation of the photoinduced anomalous Hall effect in GaN-based heterostructures

The photocurrent has been measured in Al0.25Ga0.75N/GaN heterostructures at room temperature, and the photoinduced anomalous Hall effect (AHE) was observed. The AHE current changes linearly with the varied longitudinal electric fields. Due to the strong Rashba spin–orbit coupling of the two-dimensional electron gas in Al0.25Ga0.75N/GaN heterostructures, the intrinsic anomalous Hall mechanism is supposed to contribute to the photoinduced AHE. The photoinduced AHE measurement proposed in this study could be used to other spin related measurements at room temperature.

A stable frequency comb directly referenced to rubidium electromagnetically induced transparency and two-photon transitions

We demonstrate an approach to create a stable erbium-fiber-based frequency comb at communication band by directly locking the combs to two rubidium atomic transitions resonances (electromagnetically induced transparency absorption and two-photon absorption), respectively. This approach directly transfers the precision and stability of the atomic transitions to the comb. With its distinguishing feature of compactness by removing the conventional octave-spanning spectrum and f-to-2f beating facilities and the ability to directly control the combs frequency at the atomic transition frequency, this stable optical comb can be widely used in optical communication, frequency standard, and optical spectroscopy and microscopy.

PHOTOLUMINESCENCE OF OZONE OXIDIZED AND HF ETCHED POROUS SILICON AND THE MULTIPLE SOURCE QUANTUM WELL MODEL

Porous silicon (PS) samples were alternately oxidized in an ozone atmosphere and etched in HF solution. The variation of the corresponding photoluminescence (PL) implies that there is more than one origin responsible for the PL of PS. These results are explained by a multiple source quantum well model, where the nanoscale Si units and their covering oxide layers both contribute to the PL in different situations.