Temperature micro-Raman Research Articles

Influence of phosphorus ion implantation on the optical properties of CdTe bulk crystal

Bulk CdTe crystals were implanted with 400 keV phosphorus ions to investigate the dopability of the host material. Induced devastation in crystal structure related to changes in optical constants of the material was examined by UV–Vis-IR transmittance measurements and spectrometric ellipsometry in a wide range of energies, corresponding to 200–2500 nm for transmittance and 191–2000 nm for ellipsometry. We used phosphorus particle fluences in a range of 5⋅1013 to 5⋅1016 P/cm2. Such fluences should provide p-type dopants concentration at a range of 1018 to 1021 P/cm3 assuming an average range of implanted ions (R) and longitudinal straggling (SR) as R ± SR = 403 ± 166 nm as estimated by SRIM software. The highest doping level was achieved with minor damages in the host lattice. Charged phosphorus particles implanted into bulk CdTe produce sublayer of CdTe:P, indicated by changes in the optical constants and luminescence. It was also observed that the doped layer of CdTe:P affects the transmittance of the crystals. Room temperature micro-Raman spectra of implanted samples, obtained with 532 nm and 785 nm lasers, do not differ from those for an undoped CdTe crystal. Luminescence measurements performed at temperature range down to 11 K show great improvement in luminescence emission, strong band-acceptor excitonic emission (A,X), exciton binding energy EXB = 8.7 meV and shallow acceptor level established by P-ion at about 19.1 meV above valence band maximum (VBM). We concluded that bulk crystals can be successfully doped with phosphorus ion of 400 keV energy up to ion fluences at the range of 5∙1016 P/cm2.

In-situ high temperature micro-Raman investigation of annealing behavior of high-pressure phases of Si

Among the 13 polymorphic phases of Si, the ambient temperature stable body-centered cubic (bc8) and rhombohedral (r8) polymorphs have gained significant interest due to their attractive optical and electronic properties suitable for photovoltaic applications. Though ex situ methods were extensively employed previously to understand the pressure-induced phase transformation kinetics of Si, the limited number of available in situ studies has significantly improved the knowledge in this field and clarified uncertainties. Similarly, in this article, we attempt to understand the thermal annealing behavior of nanoindentation-induced r8 and bc8 phases of Si and their volume dependence using in situ high temperature micro-Raman spectroscopy and cross-sectional transmission electron microscopy (XTEM). A spherical diamond indenter of ∼20-μm radius was chosen to indent diamond cubic (dc) Si (100) at different peak loads (Pmax) to create different volumes of high-pressure phases. The Raman spectra, Raman imaging, and XTEM of the pre- and postannealed indents confirm complete annealing of r8/bc8 phases at 200 ± 10 °C, irrespective of the volume of indents. In contrast to the previous ex situ studies, no signature of the presence of the hexagonal diamond (hd)-Si phase was found at elevated temperatures and the overall transformation observed is directly from r8 → polycrystalline dc-Si and bc8 → polycrystalline dc-Si rather than through other metastable phases such as Si-XIII/hd-Si. The present systematic in situ study provides evidence for a few earlier predictions and clarifies ambiguities involved in understanding the annealing behavior and transformation pathways of two high-pressure phases of Si at elevated temperatures.

Luminescence properties of α-Ag2WO4 nanorods co-doped with Li+ and Eu3+ cations and their effects on its structure

Nanorods of the formula α-(Ag2–3×-yEuxLiy)WO4 (where x = 0.01, y = 0.01–0.03 mol) were efficiently synthesized via a low temperature (90 °C) co-precipitation (CP) method. They were structurally characterised using both: X-ray powder diffraction (Rietveld refinement) and low temperature (20 °C to −190 °C) micro-Raman spectroscopy. The morphology of the nanorods was confirmed with field emission scanning electron microscopy. The peaks in the XRD patterns of α-(Ag2–3×-yEuxLiy)WO4 samples were all indexed to the pure orthorhombic phase of α-Ag2WO4 and are in good accordance with ICSD n° 4165. These results indicate that Eu3+ and Li+ ions have both been introduced efficiently into the α-Ag2WO4 lattice. Moreover, the Li+ co-dopant facilitated smoother incorporation of Eu3+ concentration into the α-Ag2WO4 lattice with less distortion of the lattice. In addition the presence of the Li+ co-dopant eases strain in the lattice and this allows the incorporated Eu3+ cations to emit red light much more efficiently. This is seen for α-(Ag2–3×-yEuxLiy)WO4 (where x = 0.01, y = 0.02 mol) in which the optimum red emission is observed. Analysis of the emission spectra in respect to the known metal coordination sites in the structure suggest the Eu3+ cations are only in Ag1 and Ag2 sites which are coordinated by seven O atoms (AgO7 deltahedral polyhedrons). This finding is also confirmed by the photoluminescent decay curves which can be analysed in terms of one depopulation mechanism is in accord with the occurrence of a single coordination environment for the Eu3+ cations. Although the co-doped α-(Ag2–3×-yEuxLiy)WO4 phosphors are much brighter than those containing no Li+ cations they are not bright enough (as yet) to be used in commercial devices.

Effect of La doping on the structural, optical and electrical properties of spray pyrolytically deposited CdO thin films

Abstract Solution based chemical spray pyrolysis technique was employed to deposit lanthanum (La) doped cadmium oxide (CdO) thin films on soda-lime microscope glass slides. X-ray diffraction (XRD) analysis revealed that deposited films are polycrystalline having cubic crystal system. The undoped and 0.25 wt % La doped CdO films show relatively intense (111) plane, which is shifted to (200) plane for doping concentrations greater than 0.50 wt %. The parameters such as lattice constants, crystallite size, micro-strain, dislocation density and texture coefficient are extracted from XRD data. Field emission scanning electron microscopy analysis confirmed the smooth and uniform surface. Energy dispersive spectroscopy analysis was used to estimate the preliminary information on elemental composition. The deposited undoped and La doped films possess optical transmittance ranging 78–82% in the visible and near infrared (NIR) region. The estimated direct optical band gap of undoped and La doped CdO thin films is varied between 2.38 eV and 2.46 eV. Hall measurements confirmed the n-type conductivity in the films. The doping-modulated mobility and carrier concentration are ranging 68 cm2/V s–78 cm2/V s, and 1.0 × 1020 cm−3–4.06 × 1020 cm−3, respectively. Room temperature micro-Raman studies confirmed the metal oxide (Cd O) bond vibrations in deposited films. A high figure of merit (11.45 × 10−3 Ω−1) is obtained from 1.0 wt % La doped CdO film.

Low temperature micro Raman and laser induced upconversion and downconversion spectra of europium doped silver tungstate Ag2\u22123xEuxWO4 nanorods

All the red emitting europium-doped silver tungstate α-Ag2−3xEuxWO4 (x = 0.01–0.08 mol) phosphor reported herein crystalise as nanorods. The nanorods were structurally characterised by X-ray powder diffraction, Rietveld refinement, low temperature (20 to −190 °C) micro-Raman spectroscopy . The morphology of the nanorods reported herein was confirmed by field emission scanning electron microscopy. The optical properties were studied using low temperature (20 to −190 °C) high resolution laser excited upconversion and downconversion luminescence.

Determination of band offsets at GaN/single-layer MoS2 heterojunction

We report the band alignment parameters of the GaN/single-layer (SL) MoS2 heterostructure where the GaN thin layer is grown by molecular beam epitaxy on CVD deposited SL-MoS2/c-sapphire. We confirm that the MoS2 is an SL by measuring the separation and position of room temperature micro-Raman E12g and A1g modes, absorbance, and micro-photoluminescence bandgap studies. This is in good agreement with HRTEM cross-sectional analysis. The determination of band offset parameters at the GaN/SL-MoS2 heterojunction is carried out by high-resolution X-ray photoelectron spectroscopy accompanying with electronic bandgap values of SL-MoS2 and GaN. The valence band and conduction band offset values are, respectively, measured to be 1.86 ± 0.08 and 0.56 ± 0.1 eV with type II band alignment. The determination of these unprecedented band offset parameters opens up a way to integrate 3D group III nitride materials with 2D transition metal dichalcogenide layers for designing and modeling of their heterojunction based electronic and photonic devices.

Local lattice distortions vs. structural phase transition in NdFeAsO1−xFx

The lattice properties at low temperatures of two samples of NdFeAsO1−xFx (x=0.05 and 0.25) have been examined in order to investigate possible structural phase transition that may occur in the optimally doped superconducting sample with respect to the non-superconducting low-F concentration compound. In order to detect small modifications in the ion displacements with temperature micro-Raman and high resolution synchrotron powder diffraction measurements were carried out. No increase of the width of the (220) or (322) tetragonal diffraction peaks and microstrains could be found in the superconducting sample from synchrotron XRD measurements. On the other hand, the atomic displacement parameters deviate from the expected behavior, in agreement with modifications in the phonon width, as obtained by Raman scattering. These deviations occur around 150K for both F dopings, with distinct differences among the two compounds, i.e., they decrease at low doping and increase for the superconducting sample. The data do not support a hidden phase transition to an orthorhombic phase in the superconducting compound, but point to an isostructural lattice deformation. Based on the absence of magnetic effects in this temperature range for the superconducting sample, we attribute the observed lattice anomalies to the formation of local lattice distortions that, being screened by the carriers, can only acquire long-range coherence by means of a structural phase transition at low doping levels.

Micro Raman, Mossbauer and magnetic studies of manganese substituted zinc ferrite nanoparticles: Role of Mn

A series of Mn–Zn Ferrite nanoparticles (<15nm) with formula MnxZn1−xFe2O4 (where x=0.00, 0.35, 0.50, 0.65) were successfully prepared by citrate-gel method at low temperature (400°C). X-ray diffraction analysis confirmed the formation of single cubic spinel phase in these nanoparticles. The FESEM and TEM micrographs revealed the nanoparticles to be nearly spherical in shape and of fairly uniform size. The fractions of Mn2+, Zn2+ and Fe3+ cations occupying tetrahedral sites along with Fe occupying octahedral sites within the unit cell of different ferrite samples are estimated by room temperature micro-Raman spectroscopy. Low temperature Mossbauer measurement on Mn0.5Zn0.5Fe2O4 has reconfirmed the mixed spinel phase of these nanoparticles. Room temperature magnetization studies (PPMS) of Mn substituted samples showed superparamagnetic behavior. Manganese substitution for Zn in the ferrite caused the magnetization to increase from 04 to18emu/g and Lande's g factor (estimated from ferromagnetic resonance measurement) from 2.02 to 2.12 when x was increased up to 0.50. The FMR has shown that higher Mn cationic substitution leads to increase in dipolar interaction and decrease in super exchange interaction. Thermomagnetic (M–T) and magnetization (M–H) measurements have shown that the increase in Mn concentration (up to x=0.50) enhances the spin ordering temperature up to 150K (blocking temperature). Magnetocrystalline anisotropy in the nanoparticles was established by Mossbauer, ferromagnetic resonance and thermomagnetic measurements. The optimized substitution of manganese for zinc improves the magnetic properties and makes these nanoparticles a potential candidate for their applications in microwave region and biomedical field.

The influence of substrate temperature on the optical and micro structural properties of cerium oxide thin films deposited by RF sputtering

In this investigation, CeO2 thin films were deposited on glass substrates with different substrate temperatures by using RF Magnetron sputtering technique. The deposited films were characterized by X-ray diffraction (XRD), UV–visible spectroscopy, room temperature photoluminescence spectroscopy, micro-Raman and X-ray photoelectron spectroscopy. The thicknesses of the film were determined by using profilometer and it was varied from 0.70μm to 0.87μm. From the XRD results, it is evident that the film deposited at room temperature (RT) is amorphous in nature. The remarkable change in character from amorphous to crystalline is observed by increasing the substrate temperature from RT to 300°C, and the crystallites were found to be in cubic phase with preferred orientation along (220). Calculated crystallite sizes were in the range of ∼8.6nm. Optical characteristics were studied as a function of change in substrate temperature and thickness in air. Overall transmittance percentages of the films (85–60%) were found to decrease with increase in substrate temperatures. The results show that the energy band gap was found to be decreased (3.95–3.75eV) with the increase of substrate temperature as well as thickness of the film. Optical constants like refractive index (n), extinction coefficient (k), optical conductivity (σ), real and imaginary part of dielectric constant (ε1 &ε2), volume energy loss function (VELF) and surface energy loss function (SELF) were evaluated by using UV spectra. It is found from PL spectra that the decreased defects for the films prepared at a higher substrate temperature. A strong near band edge emission and weak green emission were observed at ∼360 and ∼519nm respectively. The micro-Raman results show the characteristic peak of CeO2F2g at ∼465cm−1. It is found from the spectrum that the peak intensity of the films increased with increase in substrate temperature. XPS analysis confirms the highly non-stoichiometric nature of the films with dominant occurrence of Ce4+ (CeO2) and subsidiary occurrence of Ce3+ (Ce2O3).

Fabrication Gallium Nitride (GaN) Nanowires by Thermal Chemical Vapor Deposition (TCVD) Technique

In this paper, low-dimensional gallium nitride (GaN) nanowires have been successfully grown on silicon substrate through thermal chemical vapor deposition (TCVD); no metal catalyst was used to assist growth of nanostructure. A high purity of gallium nitride powder was used as a starting material, evaporated at 1150 OC for 2 hour and then annealing at temperature 1000 OC under stable flow of ammonia (NH3) gas in horizontal quartz tube. The morphological investigation and crystalline and orientations growth of GaN nanostructure were carried out by employing scanning electron microscopy (SEM), high resolution X-ray diffractmeter (HRXRD). A room temperature micro-Raman spectrum were employed to study the optical properties and crystalline defects. XRD shows the diffraction peaks located at 2θ= 32.43, 34.57, 36.89, 48.05, 57.83, 63.62, 69.02, and 70.470 corresponding to the (100 ), (002), (101), (102), (110) , (103),(112 ) and (201) plane diffraction of GaN structure. These results revealed that the diffraction peaks can be attributed to hexagonal GaN phase with lattice constant of a = 3.190 A° and c = 5.1890 A°. Here we report on the growth of GaN nanowires on Si (111) substrate by CVD . This technique is much simpler and cheaper than such techniques as MBE, MOCVD and HVPE.

Penetration depth profiling of proton-irradiated 4H-SiC at 6 MeV and 8 MeV by micro-Raman spectroscopy

4H-SiC samples irradiated with high energy protons were probed by low temperature photoluminescence (PL) and room temperature micro-Raman scattering spectroscopies. The quench of the near band-edge emission and the presence of a number of new sharp lines in the luminescence spectra of the proton-irradiated samples confirm the formation of various new defects. The changes of the line-shape and peak position of the longitudinal optical phonon-plasmon coupled (LOPC) mode in 4H-SiC are consistent with the decrease in the free carrier concentrations due to the introduction of carrier traps induced by the high energy proton irradiation. The estimated penetration depths for 6 and 8MeV energy proton were 180μm and 300μm, respectively, which are in good agreement with the Monte Carlo numerical simulation results. At the 180μm and 300μm depths, the carrier concentrations were reduced by approximately 34% and 21%, respectively.

Improving Structural and Micro-Raman Properties of Camphor-Grown Pristine Carbon Nanotubes with Special Focus on Single-Stage Thermal Annealing System

The camphor-grown pristine carbon nanotubes (CNT) was annealed by a single-stage thermal annealing system through controlled ambient in air, argon and nitrogen, are reported. Field emission scanning electron microscopy images confirmed that the heat treatment process gives a place to re-ordering carbon nanostructures which involves: (i) an elimination of structural defects and (ii) better graphitization of the amorphous carbon phase without damaging CNT structure. The room temperature micro-Raman measurements showed that no significant changes on the D and G-line position. However, different annealing gas ambient could give different values degree of graphitization (ID/IG ratio) due to the nature of gas itself. It reveals that single-stage thermal annealing system is relatively simple and effective to obtain an ideal CNT.

Nanocrystalline diamond on SiO 2 fiber: A new class of hybrid material

We report a template technique for the fabrication of high density nanocrystalline diamond (NCD)-coated silica ( a-SiO 2) nanofibers with diameters of 1–5 μm. This method includes the synthesis of templates ( a-SiO 2 nanofibers) by conventional Vapor–Liquid–Solid method and the conformal coating of the nanofibers with nanodiamond by Microwave Plasma Chemical Vapor Deposition technique in hydrogen-deficient conditions. A detailed micro-structural analysis was performed to probe the interaction of the NCD grains with a-SiO 2 nanofibers. The specimen for Transmission Electron Microscopy was prepared using Focused Ion Beam lift-out method. Room temperature micro-Raman was performed to study the crystalline quality of the NCD-coated silica nanofibers. Field electron emission of as-synthesized NCD-coated silica nanofibers was observed with a threshold field of ~ 3 V/µm.

Impedance spectroscopy of multiferroicPbZrxTi1−xO3∕CoFe2O4layered thin films

The electrical properties of ferroelectric $\mathrm{Pb}(\mathrm{Zr},\mathrm{Ti}){\mathrm{O}}_{3}$ (PZT) and ferromagnetic $\mathrm{Co}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$ (CFO) thin film multilayers (MLs) fabricated by pulsed laser deposition technique has been studied by impedance and modulus spectroscopy. The effect of various PZT/CFO configurations having three, five, and nine layers has been systematically investigated. The transmission electron microscopy images revealed that the ML structures were at least partially diffused near the interface. Diffraction patterns indicate clear PZT and CFO crystal structures in the interior and at the interface of the ML structure. Room temperature micro-Raman spectra indicate separate PZT and CFO phases in ML structure without any impurity phase. We studied frequency and temperature dependencies of impedance, electric modulus, and ac conductivity of ML thin films in the ranges of $100\phantom{\rule{0.3em}{0ex}}\mathrm{Hz}--1\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$ and $200--650\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, respectively. We observed two distinct electrical responses in all the investigated ML films at low temperature $(&lt;400\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ and at elevated temperature $(&gt;400\phantom{\rule{0.3em}{0ex}}\mathrm{K})$. We attributed these contributions to the grain effects at low temperature and grain boundary effects at high temperature. We explained this electrical behavior by Maxwell-Wagner-type contributions arising from the interfacial charge at the interface of the ML structure. Master modulus spectra indicate that the magnitude of grain boundary compared to grain becomes more prominent with the increase in the number of layer. The frequency dependent conductivity results well fitted with the double power law, $\ensuremath{\sigma}(\ensuremath{\omega})=\ensuremath{\sigma}(0)+{A}_{1}{\ensuremath{\omega}}^{{n}_{1}}+{A}_{2}{\ensuremath{\omega}}^{{n}_{2}}$, and the results showed evidence of three types of conduction process at elevated temperature: (i) low frequency $(&lt;1\phantom{\rule{0.3em}{0ex}}\mathrm{kHz})$ conductivity is due to long-range ordering (frequency independent), (ii) midfrequency conductivity $(&lt;10\phantom{\rule{0.3em}{0ex}}\mathrm{kHz})$ may be due to the short-range hopping, and (iii) high frequency $(&lt;1\phantom{\rule{0.3em}{0ex}}\mathrm{MHz})$ conduction is due to the localized relaxation hopping mechanism.

An induction heating diamond anvil cell for high pressure and temperature micro-Raman spectroscopic measurements

A new external heating configuration is presented for high-temperature diamond anvil cell instruments. The supporting rockers are thermally excited by induction from an externally mounted copper coil passing a 30 kHz alternating current. The inductive heating configuration therefore avoids the use of breakable wires, yet is capable of cell temperatures of 1100 K or higher. The diamond anvil cell has no resistive heaters, but uses a single-turn induction coil for elevating the temperature. The induction coil is placed near the diamonds and directly heats the tungsten carbide rockers that support the diamond. The temperature in the cell is determined from a temperature-power curve calibrated by the ratio between the intensities of the Stokes and anti-Stokes Raman lines of silicon. The high-pressure transformation of quartz to coesite is successfully observed by micro-Raman spectroscopy using this apparatus. The induction heating diamond anvil cell is thus a useful alternative to resistively heated diamond anvil cells.

Structural and morphological characterization of Pb 1− xBa xTiO 3 thin films prepared by chemical route: An investigation of phase transition

Detailed room temperature micro-Raman scattering, X-ray diffraction, atomic force microscopy and specular reflectance infrared Fourier transform spectroscopy studies were carried out on soft chemical by processed Pb 1− x Ba x TiO 3 thin films. The micro-Raman spectra pointed the existence of a stable tetragonal ferroelectric phase in the entire composition range (0 < x ≤ 1). The infrared reflectance spectra showed that the frequency of several peaks decreases as the Ba 2+ concentration increases. These features are correlated to a decrease in the tetragonal distortion of the TiO 6 octahedra as the Ba 2+ concentration increases. Furthermore, as x increases from 0.70 to 1.0, the Raman spectrum shows an evolution towards the well-known Raman spectrum of the tetragonal BaTiO 3. Therefore, we demonstrated that the combination of solid solution PbTiO 3–BaTiO 3 with a grain size in the order of 30–40 nm supports the tetragonal ferroelectric phase at room temperature.

GaN materials growth by MOVPE in a new-design reactor using DMHy and NH 3

Thin films of GaN were grown on template substrates of 4-μm-thick GaN layers on sapphire substrates by low-pressure metal organic vapour-phase epitaxy (LP-MOVPE) in a new-design reactor with the shape T. Wide range of growth temperature from 520 to 1100 °C was explored. At low temperature growth between 550 and 690 °C, dimethylhydrazine (DMHy) was used as source of atomic nitrogen while ammonia (NH 3) was used at high temperature growth (1000–1100 °C). At low temperature micro-Raman spectroscopy revealed a significant relaxation of the selection rules for the scattering by the optical phonons in the films grown at lower temperatures. Variation of the intensity ratio for E 2 H and E 1 phonon modes has been attributed to changes in the structural quality of the films grown at different temperatures. At high temperature, the quality of GaN layers were comparable to that of the substrate before growth.

Low temperature micro-Raman measurements under magnetic field of [formula omitted] thin films

We have investigated the temperature ( 78 – 295 K ) and the magnetic field dependence of the Raman active phonons of Pr 1 - x Ca x MnO 3 thin films deposited on LaAlO 3 substrates for two different Ca concentrations ( x = 0.5 and 0.6) and for SrTiO 3 substrates with x = 0.6 at room temperature. The comparison between the two substrates did not show any effect from the strains. At low temperatures close to the phase transitions certain modes are affected by the application of the field ( ∼ 1 T ) produced by permanent magnets, indicating possible spin coupling with the lattice.

Structure and optical properties of polyvinyle alcohol capped GaAs nanocrystals

Polyvinyl alcohol capped GaAs nanocrystals, assembled in thin film form, have been synthesized by a simple electrochemical technique. Transmission electron microscopy (TEM) study reveals the presence of 20–50 nm size GaAs nanoparticles embedded in the polymer host. Selected area diffraction pattern contains a series of spotted rings indicative of textured nature of the thin film. The excitonic peak broadening and the blue shift of its spectral position with respect to the bulk band gap depend upon the concentration of the polyvinyle alcohol. The X-ray photoelectron spectrum shows C(1s) peak corresponding to the surface bonded polyvinyl alcohol along with the Ga3d and As3d peaks of the GaAs core. Room temperature micro-Raman spectrum shows one surface phonon mode that shifts to the lower energy side with decrease in the crystallite size.

GaAs nanocrystals: Structure and vibrational properties

GaAs nanocrystals were grown on indium tin oxide substrate by an electrodeposition technique. Atomic force microscopic measurement indicates an increase in the size of the nanocrystal with decrease in the electrolysis current density accompanied by the change in the shape of the crystallite. Transmission electron microscopic measurements identify the crystallite sizes to be in the range of 10–15 nm and the crystal structure to be orthorhombic. On account of the quantum size effect, the first optical transition was blue shifted with respect to the band gap of the bulk GaAs and the excitonic peak appeared prominent. A localized phonon mode ascribed to certain point defect occurred in the room temperature micro-Raman spectrum.