Wanyin Ge

Physisorption-Based Charge Transfer in Two-Dimensional SnS2 for Selective and Reversible NO2 Gas Sensing

Nitrogen dioxide (NO2) is a gas species that plays an important role in certain industrial, farming, and healthcare sectors. However, there are still significant challenges for NO2 sensing at low detection limits, especially in the presence of other interfering gases. The NO2 selectivity of current gas-sensing technologies is significantly traded-off with their sensitivity and reversibility as well as fabrication and operating costs. In this work, we present an important progress for selective and reversible NO2 sensing by demonstrating an economical sensing platform based on the charge transfer between physisorbed NO2 gas molecules and two-dimensional (2D) tin disulfide (SnS2) flakes at low operating temperatures. The device shows high sensitivity and superior selectivity to NO2 at operating temperatures of less than 160 °C, which are well below those of chemisorptive and ion conductive NO2 sensors with much poorer selectivity. At the same time, excellent reversibility of the sensor is demonstrated, which has rarely been observed in other 2D material counterparts. Such impressive features originate from the planar morphology of 2D SnS2 as well as unique physical affinity and favorable electronic band positions of this material that facilitate the NO2 physisorption and charge transfer at parts per billion levels. The 2D SnS2-based sensor provides a real solution for low-cost and selective NO2 gas sensing.

Stress Perturbation Method for the Assessment of Cathodoluminescence Probe Response Functions

A method to determine the in-plane cathodoluminescence (CL) probe response function (PRF) (i.e., the function characterizing the in-plane luminescence intensity distribution within the electron probe volume) is proposed, which is based on “perturbing” the spectral position of a selected luminescence band using a highly graded stress field. The method is applied to the stress field developed ahead of the tip of an equilibrium crack in three different cases of CL bands, which arise from different structural phenomena: (i) the F+ (oxygen excess) defect band in a nominally stoichiometric sapphire (α-Al2O3) single crystal; (ii) the chromophoric R-line in ruby lattice (α-Al2–xCrxO3); and (iii) the near band-gap line in n-type GaN semiconductor crystal. A computer-aided data restoration procedure was applied to rationalize data retrieved from crack-tip line scans performed at different acceleration voltages. For the excitonic band-gap in GaN and for F+ emission in sapphire the CL probe in the electron focal plane was found to be comparable, but not necessarily coincident, in size to the electron probe. On the other hand, the occurrence of self-absorption in the case of R-line photons in ruby resulted in a significantly broadened CL probe with respect to the average scattering length of electrons.

A photo-stimulated spectroscopic method for spatially resolved stress analysis in hetero-epitaxial films

In this paper, we have developed a laser-stimulated piezo-spectroscopic method for high-resolution stress analysis in ceramic thin films and coatings, with emphasis placed on correcting the convoluting effect arising from the finite size of the laser probe. A series of 3C–SiC/Si and Al2O3/Si3N4 samples were employed for this purpose, with various film thicknesses and substrate orientation as well. In-depth defocusing scans of selected spectral bands arising from both Raman scattering and fluorescence emission were collected. According to a quantitative measurement of the luminescence/Raman probe response function for each material investigated, a spatial probe deconvolution was carried out, from which the actual residual stress distributions could be retrieved by using a computer-aided data restoration procedure. The validity of the proposed methodology was confirmed by using different spectral stress sensors and by altering the collection configuration. The results show that with the aid of a probe deconvolution procedure, spatially resolved stress analyses can be experimentally carried out, thus greatly reducing the error involved in the averaging effect of the laser probe.

Quantitative evaluation of local domain patterns in [110] poled Pb(Mg1/3Nb2/3)O3–0.35PbTiO3 single crystal using a polarized Raman microprobe

An experimental/computational procedure was presented to quantitatively analyze by polarized Raman spectroscopy (PRS) unknown domain textures in [110] poled Pb(Mg1/3Nb2/3)O3–0.35PbTiO3 single crystal. The three-dimensional domain orientation was described in terms of three Euler angles in space, and an Euler matrix was employed to transform the axial coordinates detected at the molecular scale into the laboratory coordinate frame. Raman spectral intensity of selected bands belonging to the Ag and Eg vibrational modes varied as a function of polarization geometry and rotation angle about the [110] direction. Periodic functions, which precisely fit experimental data, could be located to describe such dependences, according to theoretical considerations. The reliability and the efficiency of PRS characterization were demonstrated and the angles of domain orientation were observed and mapped at the microscopic scale.

Spectrally resolved microprobe cathodoluminescence of intergrowth Bi5−xLaxTiNbWO15 ferroelectrics

This work was supported by the Ministry of Sciences and Technology of China through 973-project 2002CB613307 and National Natural Science Foundation of China NSFC No. 50572113.

Microscopic scale evaluation of the anisotropic Young's modulus in PbWO4 single crystal using Raman microprobe spectroscopy

In this research, we attempted to calibrate Youngs modulus of lead tungstate (PWO) single crystals by combing the piezo-spectroscopic (PS) method and the indentation technique. A series of Vickers indentation cracks were generated at the corners of the indenter print on the surface of a-plane and c-plane PWO single crystals, respectively. According to microscopic assessments of crack opening displacements (COD) for cracks propagated along different crystallographic directions, we first determined the stress intensity factor associated with the tensile stress field developed ahead of the indentation crack tip. Then, upon clarifying both the PS algorithm for spectral shift analysis and the convolution effect of the laser probe, the related plane Youngs modulus could be retrieved from Raman line-scans ahead of the crack tip. Youngs modulus values of 70 GPa and 89 GPa were determined for the a-plane and c-plane of PWO single crystal, respectively. The Raman PS technique applied to controlled microscopic stress fields represents a convenient method for assessing the mechanical properties in small portions of materials embedded in devices.

Raman piezospectroscopic evaluation of intergrowth ferroelectric polycrystalline ceramic in biaxial bending configuration

The piezospectroscopic (PS) effect was studied in an intergrowth bismuth layer-structure ferroelectric ceramic Bi5TiNbWO15 according to a micro-Raman spectroscopic evaluation. By using a ball-on-ring flexure configuration, a biaxial stress was generated in a Bi5TiNbWO15 plate-like specimen and in situ collected Raman spectra were acquired and analyzed under several loading conditions. As the observed spectral line contained signals arising from the whole illuminated in-depth region, the laser probe information was deconvoluted (by means of an in-depth probe response function obtained according to the defocusing method) in order to deduce biaxial PS coefficients for the three Raman bands of Bi5TiNbWO15 located at 763, 857, and 886 cm−1, respectively. The biaxial PS coefficients of these bands were derived to be −1.74±0.16, −2.51±0.16, and -2.64±0.31 cm−1/GPa, respectively, and should be referred to the c axis of the Bi5TiNbWO15 crystal.