9 research outputs found
Paraelectric KHPO Nanocrystals in Monolithic Mesoporous Silica: Structure and Lattice Dynamics
Combining dielectric crystals with mesoporous solids allows a versatile
design of functional nanomaterials, where the porous host provides a mechanical
rigid scaffold structure and the molecular filling adds the functionalization.
Here, we report a study of the complex lattice dynamics of a
SiO:KHPO nanocomposite consisting of a monolithic, mesoporous
silica glass host with KHPO nanocrystals embedded in its tubular
channels 12 nm across. A micro-Raman investigation performed in the
spectral range of 70-1600 cm reveals the complex lattice dynamics of the
confined crystals. Their Raman spectrum resembles the one taken from bulk
KHPO crystals and thus, along with X-ray diffraction experiments,
corroborates the successful solution-based synthesis of KHPO
nanocrystals with a structure analogous to the bulk material. We succeeded in
observing not only the high-frequency internal modes (900-1200
cm), typical of internal vibrations of the PO tetrahedra, but, more
importantly, also the lowest frequency modes typical of bulk KHPO
crystals. The experimental Raman spectrum was interpreted with a group theory
analysis and first-principle lattice dynamics calculations. The analysis of
calculated eigen-vectors indicates the involvement of hydrogen atoms in most
phonon modes corroborating the substantial significance of the hydrogen
subsystem in the lattice dynamics of paraelectric bulk and of KHPO
crystals in extreme spatial confinement. A marginal redistribution of relative
Raman intensities of the confined compared to unconfined crystals presumably
originates in slightly changed crystal fields and interatomic interactions, in
particular for the parts of the nanocrystals in close proximity to the silica
pore surfaces.Comment: 10 pages, 4 figures, in pres
Low-Temperature Reduction of Graphene Oxide: Electrical Conductance and Scanning Kelvin Probe Force Microscopy
Abstract Graphene oxide (GO) films were formed by drop-casting method and were studied by FTIR spectroscopy, micro-Raman spectroscopy (mRS), X-ray photoelectron spectroscopy (XPS), four-points probe method, atomic force microscopy (AFM), and scanning Kelvin probe force (SKPFM) microscopy after low-temperature annealing at ambient conditions. It was shown that in temperature range from 50 to 250 °C the electrical resistivity of the GO films decreases by seven orders of magnitude and is governed by two processes with activation energies of 6.22 and 1.65 eV, respectively. It was shown that the first process is mainly associated with water and OH groups desorption reducing the thickness of the film by 35% and causing the resistivity decrease by five orders of magnitude. The corresponding activation energy is the effective value determined by desorption and electrical connection of GO flakes from different layers. The second process is mainly associated with desorption of oxygen epoxy and alkoxy groups connected with carbon located in the basal plane of GO. AFM and SKPFM methods showed that during the second process, first, the surface of GO plane is destroyed forming nanostructured surface with low work function and then at higher temperature a flat carbon plane is formed that results in an increase of the work function of reduced GO
Infrared Reflectance Analysis of Epitaxial n-Type Doped GaN Layers Grown on Sapphire
Abstract Infrared (IR) reflectance spectroscopy is applied to study Si-doped multilayer n+/n0/n+-GaN structure grown on GaN buffer with GaN-template/sapphire substrate. Analysis of the investigated structure by photo-etching, SEM, and SIMS methods showed the existence of the additional layer with the drastic difference in Si and O doping levels and located between the epitaxial GaN buffer and template. Simulation of the experimental reflectivity spectra was performed in a wide frequency range. It is shown that the modeling of IR reflectance spectrum using 2 × 2 transfer matrix method and including into analysis the additional layer make it possible to obtain the best fitting of the experimental spectrum, which follows in the evaluation of GaN layer thicknesses which are in good agreement with the SEM and SIMS data. Spectral dependence of plasmon-LO-phonon coupled modes for each GaN layer is obtained from the spectral dependence of dielectric of Si doping impurity, which is attributed to compensation effects by the acceptor states