Nanocrystals of CdSe Formed by the Pressure Cycle Method

CdSe nanocrystals were prepared by the pressure cycle method using the Paris-Edinburgh cell up to 7 GPa. The recovered samples are nanocrystals in the cubic phase (zinc-blende structure) and were characterized using transmission electron microscopy (TEM), electron diffraction and Raman scattering. Transmission electron micrographs shows that these nanocrystals are nearly spherical with diameters ranging from 20 to 50 nm. The Raman scattering measurement confirm the existence of CdSe nanocrystals in the cubic phase (ZB).     

High pressure study of the 2D polymeric phase of C60 by means of raman spectroscopy

Abstract

The effect of high hydrostatic pressure, up to 12GPa, on the intramolecular phonon frequencies and the material stability of the two-dimensional tetragonal Cm polymer has been studied by means of Raman spectroscopy in the spectral range of the radial intramolecular modes (200-800cm^?1). A number of new Raman modes appear in the spectrum for pressures ～ 1.4 and ～ 5.0 GPa. The pressure coefficients for the majority of the phonon modes exhibit changes to lower values at P=4.0 GPa, which may be related to a structural modification of the 2D polymer to a more isotropic phase. The peculiarities observed in the Raman spectra are reversible and the material is stable in the pressure region investigated.     

Pressure Response of the Higher Fullerene C 84 Studied by Raman and X-ray Scattering

The pressure behavior of the intramolecular phonon modes of the fullerene C 84 and its structural stability have been studied for the first time by means of Raman spectroscopy and synchrotron X-ray diffraction at high pressure up to ～10 GPa. The volume of the cubic unit cell has been measured as a function of pressure. The experimental data fitted by the Murnaghan equation-of-state (EOS) gave K_{0}=19.5\pm 0.9\,\hbox{GPa} and K_{0}^{\prime}=16.4\pm 0.6 . The pressure coefficients and Grüneisen parameters of the intramolecular phonon modes of C 84 have been determined and compared with those of other fullerenes. The data obtained do not show any phase transition and the pressure behavior of the material is entirely reversible in the pressure region investigated.     

Raman phonon spectra and lattice dynamics of 9,10-dinitroanthracene under pressure

Abstract

Raman phonon spectra of 9, 10-dinitroanthracene have been recorded in the pressure range 0-6GPa. No phase transition is detected up to the maximum pressure studied. Quasi Harmonic Lattice Dynamics calculations, based on an atom-atom potential previously modeled on homologous 9,10-disubstituted anthracenes, have been performed. The optimized potential was used to calculate the equilibrium geometry and the lattice phonon frequencies as a function of pressure. The calculated structure at ambient conditions closely resembles the experimental one. The calculated phonon frequencies show a good agreement with the experimental values at all pressures measured.     

Energy spectrum and phase transitions in C70 fullerite crystals at high pressure

Measurements have been made of the Raman, optical absorption, and luminescence spectra of single crystals and pellets of the fullerite C₇₀ at T=300 K and at pressures up to 12 GPa. The baric shift dω/dP and the Grüneisen parameters of the Raman-active intramolecular phonon modes have been determined. It has been established that the d ω/dP value for certain phonon modes abruptly changes at pressures of P ₁≈2 GPa and P ₂≈5.5 GPa, as do the half-widths of the Raman lines. These features in the Raman spectrum are associated with phase transitions at high pressure. The baric shifts of the absorption and luminescence edges of C₇₀ crystals have been determined and are −0.12 eV/GPa and −0.11 eV/GPa, respectively, for absorption and luminescence. The baric shift of the absorption edge decreases significantly with increasing pressure and is −0.03 eV/GPa at 10 GPa. These data have been used to determine the deformation potential of the fullerite C₇₀, which is about 2.1±0.1 eV. Zh. éksp. Teor. Fiz. 111, 262–273 (January 1997)     

Pressure dependence of zone-boundary phonons in AlSb

Abstract

We have investigated the effect of hydrostatic pressure on zone-boundary and other critical-point phonon frequencies of AlSb by second-order Raman scattering. A softening of the TA(X), TA(L) and (L/T)A([Sgrave]) modes has been observed for pressures up to the first phase transition at 7.7 GPa. The LA(L) as well as the optical TO at X-, L-, and LO at [Sgrave]-, X-points harden with increasing pressure. Mode Griineisen parameters of all the resolved modes were calculated. Reflectivity measurements indicate that the high pressure phase above 7.7 GPa is metallic.     

Effect of Pressure on Phonon Modes in Wurtzite Zinc Oxide

The pressure dependence of first- and second-order Raman frequencies of wurtzite ZnO has been measured up to the wurtzite-rocksalt phase transition pressure of 8.3 GPa. A small increase of the LO-TO splitting with increasing pressure is observed. This effect is related to the combined pressure dependences of the electronic dielectric constant in the phonon region and Born's transverse dynamic effective charge. Our results indicate a rather weak dependence of the dynamic charge on pressure, a behavior which is similar to that found for GaN, AlN, and SiC and different from that of other polar tetrahedral semiconductors.     

Phonon deformation potentials of CdTe

Abstract

Single and multiple quantum wells (QWs) of CdTe/Cd_1?xZn_xTe, of different composition (x), grown on a Cd_1?xZn_xTe (001) substrate are studied by Raman spectroscopy at low temperature, under resonance conditions. Two of the three phonon deformation potentials (PDPs) of the LO phonon of CdTe are calculated using the observed phonon frequency shifts due to a lattice-misfit bisotropic strain.     

Raman and IR study of high-pressure atomic phase of nitrogen

Atomic phase of nitrogen has been studied up to pressure 250 GPa and temperature 3300 K using a shear diamond anvil cell. This phase was synthesized both from azide NaN₃ and molecular N₂. The atomic phase has been interpreted as a cubic gauche (CG) structure by means of Raman and IR absorption spectroscopy procedures. The phase transition to CG begins at pressure 50 GPa and room temperature for NaN₃ and at 127 GPa for N₂. Observed pressure dependencies and degeneration of phonon modes, the selection rules for IR and Raman spectra, as well equilibrium pressure between molecular N₂ and atomic phase of nitrogen agree well with theoretical predictions for CG.     

Phonon spectrum of a naphthalene crystal at a high pressure: Influence of shortened distances on the lattice and intramolecular vibrations

The Raman spectra of a naphthalene crystal have been measured at room temperature in the pressure range up to 20 GPa. The pressure shift and Grüneisen parameters for intermolecular and intramolecular phonons have been determined. The maximum rate of the pressure shift for intermolecular phonons is 44 cm^?1/GPa, and the rate of the pressure shift for intramolecular phonons lies in the range from 1 to 11 cm^?1/GPa for different modes. The pressure dependence of the phonon frequencies for direct and inverse pressure variations has a hysteresis in the pressure range from 2.5 to 16.5 GPa. It has been shown that the linear dependence of the intermolecular phonon frequency on the crystal density has a peculiarity, which indicates a possible phase transition at a pressure of 3.5 GPa. The pressure dependence of intramolecular phonons related to the stretching vibrations of hydrogen atoms exhibits features that are characteristic of intermolecular phonons, which is associated with the influence of shortened distances between the hydrogen atoms of the neighboring molecules on the intermolecular interaction potential.     

Dynamics of the BiTeI lattice at high pressures

Raman measurements of the phonon spectrum of BiTeI at pressures of up to 20 GPa have been performed. A decrease in the linewidth of E² vibration by almost a factor of 2 with an increase in the pressure to 3 GPa has been detected. The frequencies of all four Raman active modes increase monotonically with the pressure. These lines are observed in spectra up to ～8 GPa. Sharp change in the spectrum occurs at pressures of 8–9 GPa, indicating a transition to the high-pressure phase, which holds up to 20 GPa. This transition is reversible and hardly has any hysteresis. A sample in the high-pressure phase is single crystal.     

Magneto-optical studies of CdTe/CdMnTe semimagnetic semiconductor superlattices under high pressure

Abstract

The photoluminescence (PL) of a CdTe/CdMnTe superlattice has been studied at pressures up to 4.1GPa, where the phase transition occurs. PL is observed up to this pressure, and it moves to higher energy with pressure at 66meV/GPa. This result is consistent with theory. Magnetic fields decrease the band-gap of the semimagnetic CdMnTe barriers and this reduces the PL energy. The pressure dependence of this effect is expected to provide a stringent test of the theory of semimagnetic materials and of superlattices.     

镶嵌在SiO2薄膜中InAs纳米颗粒的Raman散射

对镶嵌在SiO₂薄膜中纳米InAs颗粒的Raman散射谱进行了研究.与大块InAs晶体相比,InAs纳米颗粒的Raman散射谱具有相似的特征,即由纵光学声子模和横光学声子模组成,但是散射峰宽化并红移.用声子限域效应解释了散射峰的红移现象,并结合InAs纳米颗粒的应力效应解释了红移量与理论值的差异. 关键词： 2薄膜')" href="#">SiO₂薄膜 InAs量子点 Raman散射     

Brillouin scattering study of methanol at high pressure

Abstract

Brillouin scattering has been used to study methanol as a function of pressure up to 8.4 GPa. Our near-forward-scattering experiments yield the pressure dependence of the velocity of longitudinal acoustic (LA) phonons, while we determine the pressure dependence of the product of the index of refraction and the LA phonon velocity from our backscattering experiments. The pressure dependence of the LA phonon lifetimes is obtained from the linewidth of the measured Brillouin peaks. From our backscattering experiments we find that the normalized phonon attenuation is a decreasing function of pressure.     

Resonant Raman scattering from CdTe/ZnTe self-assembled quantum dot structures

We report resonant Raman scattering results of CdTe/ZnTe self-assembled quantum dot (QD) structures. Photoluminescence spectra reveal that the band gap energies of the CdTe QDs decrease with the increase of CdTe thickness from 2.0 to 3.5 monolayers, which indicates that the size of the QDs increases. When the CdTe/ZnTe QD structures are excited by non-resonant excitation, a longitudinal optical (LO) phonon response from the ZnTe barrier material is observed at 206 cm⁻¹. In contrast, when the CdTe/ZnTe QD structures are resonantly excited near the band gap energy of the QDs, additional phonon modes emerge at 167 and 200 cm⁻¹, while the ZnTe LO phonon response completely disappears. The 167 cm⁻¹ mode corresponds to the LO phonon of the CdTe QDs. A spatially resolved Raman scattering from the cleaved edge of the QD sample reveals that the 200 cm⁻¹ mode is strongly localized at the interface between the CdTe QDs and ZnTe cap layer. This phonon mode is attributed to the interface optical (IO) phonon. The analytically calculated value of the IO phonon energy using a dielectric continuum approach, assuming a spherical dot boundary, agrees well with the experimental value.     

A high pressure Raman study of KTP and pressure induced phase transitions

Potassium titanate orthrophosphate KTiOPO₄ (KTP) has been studied by high pressure Raman technique to 17 GPa using a diamond cell. The Raman data reveal that two phase transitions occur in the system: one near 5.5 GPa and another near 10 GPa. The Lower-pressure transition is definitely first-order but appears to be driven by the phonon mode near 56 cm⁻¹, which exhibits marked softening. A mean field-like behavior is observed. It is hypothesized that this transition is likely to be from ferroelectric (FE) to an antiferroelectric (AF) phase. The 10 GPa transition may be due to AF-PE (paraelectric) transition driven by pressure from high temperature to room temperature. From the larger pressure responce of the PO₄ vibrational modes it is believed that the PO₄ polyhedral compression is larger than the TiO₆ polyhedral compression. leading to polyhedral tilt transitions.     

Reconstruction of the conduction band in metallic hydrogen sulfide

The theory of the normal properties of a metal generalized to the case of particular properties of an electron band with a finite width for electron–phonon systems with a varying electron density of states has been used to study the normal state of the SH₃ phase of hydrogen sulfide at a pressure of 225 GPa and a temperature of 200 K. The frequency dependences of the real, ReΣ(ω), and imaginary, ImΣ(ω), parts of the selfenergy part of the Green’s function of the electron Σ(ω), as well as the electron density of states N(ε) of the Im–3m stable orthorhombic structure of SH₃ hydrogen sulfide at a pressure of P = 225 GPa, which is renormalized by the strong electron–phonon coupling, have been calculated. It has been established that a part of the electron conduction band of the SH₃ phase of hydrogen sulfide adjacent to the Fermi level undergoes renormalization-induced reconstruction in the form of a number of energy pockets with the widths equal to fractions of the characteristic phonon energies of the system.     

静压下(CdTe)_2(ZnTe)_4-ZnTe多量子阱结构的多声子共振拉曼散射

用加静高压的方法改变光学能隙来实现共振条件。在以(CdTe)_2(ZnTe)_4短周期超晶格为阱层,(ZnTe)_(4)为垒层的多量子阱结构中观察到高达四阶的类 ZnTe 纵光学声子模的多声子共振拉曼散射。通过对拉曼位移随压力变化的分析,发现在与(CdTe)_2(ZnTe)_4短周期超晶格共振时测得的类ZnTe 纵光学声子模的频率比与 ZnTe 势垒层共振时测得的 ZnTe 纵光学声子模的频率低4cm~(-1)。并将它归结为在短周期超品格中纵光学声子模的限制效应。在与短周期超品格严格的2LO 声子出射共振条件下观察到了类 CdTe 的2LO 声子的共振拉曼峰。     

A high pressure Raman study of terbium molybdate

Abstract

Tb₂(MoO₄)₃ has been studied by Raman spectroscopy under hydrostatic pressure up to 9 GPa at room temperature. The measurements reveal two phase transitions, one at around 2 GPa and another one above 5 GPa. The first phase transition is associated with an increase in the coordination number of Mo while the second is probably a transition to an amorphous phase in which only a wide band originating from Mo-O vibrations remains. This behaviour is irreversible as the Raman spectrum of the initial structure is not recovered at atmospheric pressure.     

Pressure and temperature dependence of optical phonons in La0.75Ca0.25MnO3

We present the results of extensive studies on Raman and infrared active phonons in the La_0.75Ca_0.25MnO₃ manganite over a wide temperature (100–320 K) and pressure (0–14 GPa) range. The analysis of the temperature dependent data allows to identify a clear spectroscopic signature of the insulator to metal transition. Indeed the abrupt reduction of the effective electron–phonon interaction on entering the metallic phase determines a change in slope in the temperature dependence of the Jahn–Teller phonon line width. The analysis of the pressure dependent data shows that the octahedral Jahn–Teller distortion, and consequently the electron–phonon interaction, is strongly reduced only in the low-pressure regime. At very high pressure, the onset of a pressure-activated localizing mechanism efficiently contrasts the natural delocalizing tendency of pressure. We finally guess that this effect could be attributed to charge-localizing antiferromagnetic interactions activated by the strong lattice compression.