Photoluminescence and raman studies of Y2O3:Eu(3+) nanotubes under high pressure

In this work, the cubic compound Y2O3:Eu(3+) nanotubes with diameter of 70-90 nm and length of 2-3 microm are synthesized by a hydrothermal method. Photoluminescence and Raman spectra of Y2O3:Eu(3+) nanotubes in a diamond anvil cell under high pressure are measured at room temperature. The 5D0 --> 7F(0,1,2) transitions of the Eu(3+) ions exhibit red shifts to higher wavelength with pressure increasing. Above 13.4 GPa, all the Raman active modes disappear. When the pressure is released from 25.6 GPa to ambient pressure, these Raman peaks are not retrieved; this fact indicates that the nanotubes are transformed into amorphous from cubic phase at about 13.4 GPa. It may be related to the collapse of nanotube form under high pressure condition.     

Raman scattering spectroscopic study of n-pentane under high pressure

The Raman spectroscopy of n-pentane was investigated in a Moissanite anvil cell from 0.07 GPa to 4.77 GPa at ambient temperature. The result shows that the CH3 symmetric stretching vibration (2877 cm(-1)) and asymmetric stretching vibration (2964 cm(-1)), the CH2 symmetric stretching vibration (2856 cm(-1)) and asymmetric stretching vibration (2935 cm(-1)), and -(CH2)n- in-phase twist mode (1303 cm(-1)) shifted to higher wavenumbers almost linearly with increasing pressure. Around 2.4 GPa an abrupt visible change took place, indicating a sort of phenomenon of freezing due to over-pressurization. In the pressure range of 2.84 to 4.77 GPa a high-pressure phase transition may occur in the crystallized n-pentane. By determining pressure with the method of solid-liquid coexistence, we concluded that the equilibrium freezing pressure of n-pentane is 1.90 +/- 0.05 GPa at 27 degrees C.     

Structural transition in nanostructured Eu2O3 under high pressures

We report here studies on the effect of high pressure on the structural properties of nano-sized Europium sesquioxide (Eu2O3) up to a pressure of about 16.4 GPa. At ambient conditions, the starting sample was found to be predominantly cubic type Eu2O3 or in Eu3+ state with a trace of Eu2+. The presence of Eu2+ state is assumed to be arising due to the non-stoichiometric Eu(1-x)O phase which is obtained from XPS studies by the deconvolution of the Eu 3d-core levels. The Raman studies at ambient show a strong peak at about 333 cm(-1), which is known to occur due to the Fg mode of cubic Eu2O3 and in a similar way, the XRD data shows major peaks corresponding to the cubic phase of Eu2O3. A Mao-Bell type diamond anvil cell (DAC) was used to generate high pressures for XRD and Raman spectroscopy studies. It was observed that the material undergoes a structural change from cubic to monoclinic structure with an on set transition pressure at around 2 GPa and completes at around 8 GPa. This has been inferred from the fact that above about 2.0 GPa pressure, Raman studies show the emergence of a new peak corresponding to the monoclinic phase which increases in intensity and shifts further with increase in pressure, while the XRD studies show that above about 2.0 GPa, the peaks corresponding to monoclinic phase emerge, which show a slight increase in preferred orientation as the pressure is increased. A detailed discussion has been provided to explain this fact.     

High pressure Raman scattering of silicon nanowires

We study the high pressure response, up to 8 GPa, of silicon nanowires (SiNWs) with ～ 15 nm diameter, by Raman spectroscopy. The first order Raman peak shows a superlinear trend, more pronounced compared to bulk Si. Combining transmission electron microscopy and Raman measurements we estimate the SiNWs' bulk modulus and the Grüneisen parameters. We detect an increase of Raman linewidth at ～ 4 GPa, and assign it to pressure induced activation of a decay process into LO and TA phonons. This pressure is smaller compared to the ～ 7 GPa reported for bulk Si. We do not observe evidence of phase transitions, such as discontinuities or change in the pressure slopes, in the investigated pressure range.     

High pressure luminescence and Raman studies on the phase transition of Gd2O3:Eu3+ nanorods

The cubic Gd2O3:Eu3+ nanorods were synthesized by a hydrothermal method. The SEM image indicated the nanorods with diameter of 30-35 nm and length of 200-500 nm. The structural stability of Gd2O3:Eu3+ nanorods was investigated by in situ high pressure luminescence and Raman spectra up to 18.9 GPa at room temperature. The results reveals a pressure-induced phase transition from cubic to hexagonal structure at about 11.3 GPa. After releasing pressure, the part of hexagonal structure is retained and the other transfers to monoclinic phase.     

High pressure Raman spectroscopic study of structural phase transition in samarium oxide

High pressure Raman spectroscopic study of Sm₂O₃ poly crystal was performed up to 21.0 GPa and room temperature using a diamond anvil cell. Pressure induced phase transition was observed at 2.6 GPa in the pressure increasing process. This phase transition corresponds to the monoclinic B type phase → the hexagonal A type transformation. The A type phase was stable up to 21.0 GPa. In the pressure release process, the A type phase was stable above 1.8 GPa, and was completely reverted to the B type phase at 1.1 GPa. The phase transition was confirmed to be reversible with a hysteresis of approximately 1.0 GPa.     

Raman Spectroscopy under Extreme Conditions

We report the results of Raman measurements of various materials under simultaneous conditions of high temperature and high pressure in the diamond anvil cell (DAC). High temperatures are generated by laser heating or internal resistive (ohmic) heating or a combination of both. We present Raman spectra of cubic boron nitride (cBN) to 40 GPa and up to 2300 K that show a continuous pressure and temperature shift of the frequency of the transverse optical mode. We have also obtained high-pressure Raman spectra from a new noble metal nitride, which we synthesized at approximately 50 GPa and 2000 K. We have obtained high-temperature spectra from pure nitrogen to 39 GPa and up to 2000 K, which show the presence of a hot band that has previously been observed in CARS measurements. These data have also allowed us to constrain the melting curve and to examine changes in the intramolecular potential with pressure.     

Pressure-induced structural phase transformations in silicon nanowires

High-pressure structural behavior of silicon nanowires is investigated up to approximately 22 GPa using angle dispersive X-ray diffraction measurements. Silicon nanowires transform from the cubic to the beta-tin phase at 7.5-10.5 GPa, to the Imma phase at approximately 14 GPa, and to the primitive hexagonal structure at approximately 16.2 GPa. On complete release of pressure, it transforms to the metastable R8 phase. The observed sequence of phase transitions is the same as that of bulk silicon. Though the X-ray diffraction experiments do not reveal any size effect, the pressure dependence of Raman modes shows that the behavior of nanowires is in between that of the bulk crystal and porous Si.     

Thickness-dependent phase transition and optical behavior of MoS2 films under high pressure

Nano Research - We report the Raman and photoluminescence spectroscopic analysis of layered MoS2 under hydrostatic pressure up to ∼ 30 GPa. Unlike previous studies, throughout this work, a...     

Pressure dependence of the Raman-active modes in Bi4Ti3O12

Bismuth titanate (Bi₄Ti₃O₁₂), a layer-type ferroelectric material crystallizing in the monoclinic system, has been investigated by high-pressure Raman technique up to 17 GPa in a diamond anvil cell. A pressure-induced phase transition occurs near 3 GPa, and the softening of the lowest Raman mode near 31 cm⁻¹ seems to be associated with this transition. Furthermore, the Raman data as well as absorption measurements indicate that a second phase transition occurs near 11 GPa. This transition may be due to a change from the ferroelectric to the paraelectric phase under the influence of pressure.     

Materials dynamics under nanosecond pulsed pressure loading

A nanosecond pressure pulse is generated by focusing a nanosecond-pulsed laser onto an aluminum target with plasma confined geometry. A spatially uniform pressure pulse is generated by focusing laser beams with a flat-top spatial energy distribution. High-pressure pulse loading and recovery experiments were performed on yttria-doped (3 mol%) tetragonal zirconia polycrystals at 11 GPa. In the pressure-loaded region, the monoclinic phase was uniformely formed. The transition ratio was approximately 30%. Nanosecond time-resolved Raman spectroscopy was performed on polytetrafluoroethylene under high-pressure pulse loading at 1 GPa, and rapid structural phase transition within 10 ns was revealed.     

Synthesis and characterization of a binary noble metal nitride

There has been considerable interest in the synthesis of new nitrides because of their technological and fundamental importance. Although numerous metals react with nitrogen there are no known binary nitrides of the noble metals. We report the discovery and characterization of platinum nitride (PtN), the first binary nitride of the noble metals group. This compound can be formed above 45-50 GPa and temperatures exceeding 2,000 K, and is stable after quenching to room pressure and temperature. It is characterized by a very high Raman-scattering cross-section with easily observed second- and third-order Raman bands. Synchrotron X-ray diffraction shows that the new phase is cubic with a remarkably high bulk modulus of 372(+/-5) GPa.     

Pressure effect on optical properties and structure stability of LaPO4:Eu(3+) microspheres

Uniform monoclinic core-shell and hexagonal urchin-like LaPO4:Eu(3+) spheres are synthesized via an attractive hydrothermal method owing to the higher yield and simplicity. Photoluminescence and Raman spectra of two samples have been investigated under high pressure up to 28 GPa using diamond anvil cells. At ambient pressure, both samples exhibit same luminescent properties with that of bulk monazite LaPO4:Eu(3+). With the increase of pressure, the emission intensity of Eu(3+) decreases and the half-widths of transition lines increase for both samples, while emission peaks show a red shift toward longer wavelengths due to increase in both the crystal-field strength and the covalency. Monoclinic core-shell LaPO4:Eu(3+) becomes amorphous finally while hexagonal urchin-like one transforms to monoclinic structure at lower pressure of 3.2 GPa and turns into amorphous structure at higher pressures, which are presented based on the analysis of high pressure Raman spectra.     

Nanoscale uniaxial pressure effect of a carbon nanotube bundle on tip-enhanced near-field Raman spectra

In situ measurement of tip-enhanced near-field Raman spectra of an isolated single-wall carbon nanotube (SWNT) bundle has been demonstrated by applying a uniaxial pressure up to approximately 2 GPa to the bundle via a metal-coated atomic force microscope tip. We investigated the pressure dependences of Raman frequencies and the intensity of the radial breathing mode bands, the D-band and the G-band, which were related to deformation of SWNTs caused by the tip pressure.     

Raman studies of selenium nanowires under high pressure

The selenium nanowires with diameter of 70 nm and length of 40 μm were synthesized by a facile solution method. High-pressure behavior of Se nanowires has been investigated by in situ Raman scattering up to 20.2 GPa at room temperature. A reversible phase transition from hexagonal to monoclinic occurs at 18.1 GPa. This transition pressure is higher than that of 14.0 GPa for bulk Se. The intrinsic geometry and/or the increasing energy band gap of Se nanowires are considered to contribute to the increase of transition pressure.     

UV Raman studies on carbon nitride structures

Visible (514 nm) and deep UV (257 nm) Raman spectra of monoclinic tetracyanoethylene (tcne) are recorded at ambient conditions and also after laser heating at ambient pressure and at 40 GPa. Tetracyanoethylene (C₂(CN)₄) is a convenient precursor to synthesize hard C₃N₄ materials. At low incident laser powers the UV Raman spectra of virgin tcne resemble visible Raman spectra, and at higher powers there appear new, broad modes that increase in intensity as a function of laser power. When tcne is laser-heated at ambient pressure, there are two broad UV Raman peaks about 1,405 cm⁻¹ and 1,604 cm⁻¹ whereas visible laser Raman excitation results in too high a fluorescent background to show up any Raman modes. Raman spectrum of tcne laser heated at 40 GPa show broad peaks indicative of multiphase formation. The spectrum has additional modes at lower frequencies, and comparison with calculated Raman frequencies points to possible formation of α-C₃N₄.     

High-pressure phase transition in Ho2O3

High-pressure X-ray diffraction and Raman studies on holmium sesquioxide (Ho₂O₃) have been carried out up to a pressure of ∼17 GPa in a diamond-anvil cell at room temperature. Holmium oxide, which has a cubic or bixbyite structure under ambient conditions, undergoes an irreversible structural phase transition at around 9.5 GPa. The high-pressure phase has been identified to be low symmetry monoclinic type. The two phases coexist to up to about 16 GPa, above which the parent phase disappears. The high-pressure laser-Raman studies have revealed that the prominent Raman band ∼370 cm⁻¹ disappears around the similar transition pressure. The bulk modulus of the parent phase is reported.     

Spectroscopic Studies of p-H2 to Above 200 GPa

Raman and infrared vibrational spectra of H ₂ have been measured to pressures in excess of 200 GPa and at liquid helium temperatures using new high sensitivity techniques. Detailed study of the pressure dependence of o-p conversion rate reveals an initial increase followed by a decrease above 1 GPa. The conversion rate then increases dramatically with pressure, and this continues to above 50 GPa. New sets of vibron, phonon, roton, and libron excitations in converted para samples are documented as a function of pressure through phases I, II, and III. The results provide important information on the crystal structures, molecular orientational state, and vibrational dynamics of the high-pressure phases.     

Study the formation mechanism of silicon carbide polytype by silicon carbide nanobelts sintered under high pressure

In this paper, in order to reveal the formation mechanism of SiC polytype, four SiC specimens sintered under high pressure has been investigated, after being prepared from SiC nanobelts as initial powders. The structure and morphology variation dependence of SiC specimens with temperature and pressure was studied based on experimental data obtained by XRD, SEM, and Raman. The results show that SiC lattice structure and the crystallite size are greatly affected by pressure between 2 and 4 GPa under different sintering temperatures of 800 and 1200 degrees C. At the largest applied pressure and temperature, 4 GPa and 1200 degrees C, 3C-SiC crystal structure can be changed into to R-SiC due to the stress resulted in dislocations instead of planar defects. Based on our results, the multiquantum-well structure based a single one-dimensional nanostructure can be achieved by applying high pressure at certain sintered temperature.     

High pressure Raman spectroscopy of ferrite MgFe2O4

An in situ Raman spectroscopic study was conducted to explore the pressure-induced phase transformation of ferrite MgFe₂O₄ to 51.6 GPa. Results indicate that MgFe₂O₄ transforms to a high pressure polymorphism at a pressure of 27.7 GPa, which was assigned to an orthorhombic structure. Upon release of pressure to ambient conditions, this high pressure polymorphism of MgFe₂O₄ remains stable. The crystallization of high pressure phase of MgFe₂O₄ is dominated by a diffusionless crystallizing mechanism.