Disorder-induced vibration-mode coupling in SiO2 films observed under normal-incidence infrared radiation

Symmetry-Controlled Electron-Phonon Interactions in van der Waals Heterostructures

Light-matter interactions in the van der Waals (vdWs) heterostructures exhibit many fascinating properties which can be harnessed to realize optoelectronic applications and probe fundamental physics. Moreover, the electron-phonon interaction in the vdWs heterostructures can have a profound impact on light-matter interaction properties because light excited electrons can strongly couple with phonons in heterostructures. Here, we report symmetry-controlled electron-phonon interactions in engineered two-dimensional (2D) material/silicon dioxide (SiO₂) vdWs heterostructures. We observe two Raman modes arising from originally Raman-silent phonon modes in SiO₂. The Raman modes have fixed peak positions regardless of the type of 2D materials in the heterostructures. Interestingly, such Raman emissions exhibit various symmetry properties in heterostructures with 2D materials of different crystalline structures, controlled by their intrinsic electronic band properties. In particular, we reveal chiral Raman emissions with reversed helicity in contrast to that of typical valley polarization in honeycomb 2D materials due to the phonon-assisted excitonic intervalley scattering process induced by electron-hole exchange interaction. The observation of the symmetry-controlled Raman scattering process not only provides a deep insight into the microscopic mechanisms of electron-phonon interactions in vdWs heterostructures but also may lead to the realization of valley-phononic devices.

Determination of the network structure of sensor materials prepared by three different sol-gel routes using Fourier transform infrared spectroscopy (FT-IR)

Solid acid-base sensor materials were prepared by encapsulating three pH indicators (alizarin red, brilliant yellow, and acridine) within a silica matrix using a sol-gel approach through three different routes: (1) non-hydrolytic, (2) acid-catalyzed, and (3) base-catalyzed. Raman and Fourier transform infrared spectroscopies were used to evaluate the silica-indicator interactions. Because vibrational bands assigned to functional groups present in the indicator molecules were not detected, the main silica stretching mode νSi-O between approximately 1300 and 1000 cm(-1) was used to detect the presence of our indicators within the silica matrix. The large band centered at 1100 cm(-1) was deconvoluted into four components corresponding to the longitudinal optic and transversal optic modes of the silicon monoxide (SiO)4 and (SiO)6 siloxane rings. Using the component area of each mode, it was possible to calculate the percentage of each structure. Such percentages ranged from 49% to 70% (SiO)6 for the analyzed samples, within a confidence level of 95% (p = 0.05). (The confidence limits were 53-62%.) These results could be related to the pH indicator content, indicating that the quantity of the encapsulated molecule affects the (SiO)6 percentage values. In addition, a comparison with the radius of gyration obtained by small angle X-ray scattering was done. These results indicate that the analyte accesses the receptor elements through the passages between the siloxane rings but not through the siloxane rings themselves.

Thermally Induced Nano-Structural and Optical Changes of nc-Si:H Deposited by Hot-Wire CVD

We report on the thermally induced changes of the nano-structural and optical properties of hydrogenated nanocrystalline silicon in the temperature range 200-700 degrees C. The as-deposited sample has a high crystalline volume fraction of 53% with an average crystallite size of ~3.9 nm, where 66% of the total hydrogen is bonded as identical withSi-H monohydrides on the nano-crystallite surface. A growth in the native crystallite size and crystalline volume fraction occurs at annealing temperatures >/=400 degrees C, where hydrogen is initially removed from the crystallite grain boundaries followed by its removal from the amorphous network. The nucleation of smaller nano-crystallites at higher temperatures accounts for the enhanced porous structure and the increase in the optical band gap and average gap.