Deep-ultraviolet near band-edge emissions from nano-polycrystalline diamond

ABSTRACT

Deep-ultraviolet continuous-wave photoluminescence spectroscopy is performed for nano-polycrystalline diamond (NPD) synthesized by a high pressure high-temperature technique. NPD exhibits clear deep-ultraviolet emissions, which originate from intrinsic excitonic transitions assisted by a momentum-conserving phonon with a photon. Surprisingly, the peak emission energy is about 30?meV higher than that of the single-crystalline diamond. Raman scattering spectroscopy indicates that the energy difference should originate from the excitonic properties of the NPD and not the phonon. Hence, NPD has a large bandgap compared to single-crystalline diamond.     

Resonance Raman spectroscopy of carbon nanotubes: pressure effects on G-mode

We use 488 and 568 nm laser Raman spectroscopy under high pressure to selectively follow evolution of Raman G-mode signals of single-walled carbon nanotubes (SWCNTs) of selected diameters and chiralities ((6, 5) and (6, 4)). The G-mode pressure coefficients of tubes from our previous work are consistent with the thick-wall tube model. Here we report the observation of well-resolved G-minus peaks in the Raman spectrum of SWCNTs in a diamond-anvil cell. The pressure coefficients of these identified tubes in water, however, are unexpected, having the high value of over 9 cm^?1 GPa^?1 for the G-plus and the G-minus, and surprisingly the shift rates of the same tubes in hexane have clearly lower values. We also report an abrupt increase of G-minus peak width at about 4 GPa superposed on a continuous peak broadening with pressure.     

Nano-polycrystalline diamond synthesized through the decomposition of stearic acid

ABSTRACT

Here we report a novel route for synthesizing nano-polycrystalline diamond (NPD) using stearic acid (C₁₈H₃₆O₂) as a starting material under high pressure and high temperature. The obtained NPD shows a transparent dark-yellowish color similar to the standard NPD synthesized from graphite and consists of extremely fine diamond grains (～10?nm). The temperature required for the present synthesis of pure transparent NPD is as low as 1000°C at 13 and 17?GPa, which is surprisingly lower than that for conventional NPD synthesis (1800–2000°C). The amorphous-like, extremely poorly crystalline graphite produced by the thermal decomposition of stearic acid likely provides preferential nucleation sites for diamond and significantly lower the activation energy. The removal of volatile components such as H₂O generated through the decomposition from the system is a key to obtain pore-free transparent NPD. Magnesite, MgCO₃ and periclase, MgO can be used as an efficient H₂O remover through the hydration reaction.     

Raman spectroscopic and optical absorption study of the high pressure behaviour and polymorphism in KO2

Abstract

Raman scattering, visible absorption, and optical observation studies have been made on polycrystalline potassium superoxide (KO₂) in a diamond anvil cell as a function of pressure and temperature. Three new phases are observed. With increasing pressure at 298 K, KO₂ transforms from the well known modified CaC₂ structure (Phase II), to two new phases (VII, and VIII). The transformation from III to VII occurs at about 3.2GPa. Phase VII transforms to phase VIII at about 4.4GPa. However, in some samples phase VII does not occur and phase II transforms directly into phase VIII at about 4.2 GPa. These structural transformations are indicated by marked changes in the Raman spectrum. The transitions out of phase II are also marked by a discontinuous red shift in the optical absorption edge. From optical observations we have also determined the pressure and temperature dependence of the transitions from phase II to the high temperature cubic (B1) phase I as well as from the high pressure phases VII and VIII to a new nonbirefringent phase IX. This new phase IX has the cubic B2 (CsCl) structure as is shown by our recent X-ray synchrotron experiments.     

0.1~800 MPa压力下方解石拉曼光谱的实验研究

 利用石英的拉曼谱峰与温度和压力的关系，检验了在金刚石压腔中用方解石拉曼谱峰确定体系压力的可行性，并初步确定了在常温下方解石的拉曼谱峰与压力的关系。实验研究结果表明：在实验的压力范围内方解石稳定，且其1 085 cm^-1 谱峰约为石英464 cm^-1谱峰的3倍强度，因此非常适合作为热液金刚石压腔的压力标定物。在温度26 ℃、压力0.1~800 MPa条件下，方解石的拉曼谱峰（1 085 cm^-1）随着压力的增加，呈线形增大，其关系式为：p(MPa)=192×(ν_p－1 085)－21.8，1 085 cm^-1<ν_p<1 090 cm^-1。     

Applications of nano-polycrystalline diamond anvils to X-ray absorption spectroscopy under high pressure

ABSTRACT

Recently, nano-polycrystalline diamond (NPD) anvils have been widely applied in high pressure research using X-ray absorption spectroscopy (XAS). The nanometer-sized polycrystallization in NPD anvils enables us to obtain glitch-free X-ray absorption spectra. This advantage of NPD anvils drastically improves the experimental conditions of XAS, which has previously used conventional single-crystal diamond (SCD) anvils. Distorted spectra due to the glitches from the SCD anvils have been an inevitable problem of XAS. This paper reviews recent studies of XAS and related spectroscopic techniques using the NPD anvils, which have mainly been performed on BL39XU of SPring-8. We demonstrate how NPD anvils are useful when using XAS for high pressure research.     

Optical pressure sensors for high-pressure–high-temperature studies in a diamond anvil cell

We review the various optical pressure sensors that are suitable for high-pressure and high-temperature studies in a diamond anvil cell. Two different kinds of sensors are considered: those based on the pressure shift of a fluorescence line (ruby, SrB₄O₇:Sm²⁺) and those based on the pressure shift of a Raman line (c-BN, diamond). The calibration of those sensors are presented in detail, and discussion is made on their useful pressure and temperature ranges.     

Synthesis of nano-polycrystalline diamond from glassy carbon at pressures up to 25 GPa

ABSTRACT

Nano-polycrystalline diamond (NPD) with various grain sizes has been synthesized from glassy carbon at pressures 15–25?GPa and temperatures 1700–2300°C using multianvil apparatus. The minimum temperature for the synthesis of pure NPD, below which a small amount of compressed graphite was formed, significantly increased with pressure from ～1700°C at 15?GPa to ～1900°C at 25?GPa. The NPD having grain sizes less than ～50?nm was synthesized at temperatures below ～2000°C at 15?GPa and ～2300°C at 25?GPa, above which significant grain growth was observed. The grain size of NPD decreases with increasing pressure and decreasing temperature, and the pure NPD with grain sizes less than 10?nm is obtained in a limited temperature range around 1800–2000°C, depending on pressure. The pure NPD from glassy carbon is highly transparent and exhibits a granular nano-texture, whose grain size is tunable by selecting adequate pressure and temperature conditions.     

High pressure behavior of nano-crystalline CeO2 up to 35 GPa: a Raman investigation

The present paper reports the results of in situ Raman studies carried out on nano-crystalline CeO₂ up to a pressure of 35 GPa at room temperature. The material was characterized at ambient conditions using X-ray diffraction and Raman spectroscopy and was found to have a cubic structure. We observed the Raman peak at ambient at 465 cm^?1, which is characteristic of the cubic structure of the material. The sample was pressurized using a diamond anvil cell using ruby fluorescence as the pressure monitor, and the phase evolution was tracked by Raman spectroscopy. With an increase in the applied pressure, the cubic band was seen to steadily shift to higher wavenumbers. However, we observed the appearance of a number of new peaks around a pressure of about 34.7 GPa. CeO₂ was found to undergo a phase transition to an orthorhombic α -PbCl₂-type structure at this pressure. With the release of the applied pressure, the observed peaks steadily shift to lower wavenumbers. On decompression, the high pressure phase existed down to a total release of pressure.     

正辛烷的高压拉曼光谱研究

 采用金刚石对顶砧高压装置,在室温下对正辛烷（C₈H₁₈）进行了原位高压拉曼光谱研究,实验的最高压力为13GPa。在实验的压力范围内,正辛烷的拉曼峰位随压力的升高均向高频移动,峰强逐渐减弱,峰形变宽。常态为液态的正辛烷在0.8 GPa时,拉曼频移随压力的变化曲线出现了拐点,发生了液－固相变;在6.8 GPa时,伴随着原拉曼峰的消失或劈裂,以及新拉曼峰的出现,此时正辛烷可能发生了固－固相变。该相变压力低于已有的低碳数正烃烷的压致相变结果。正烃烷的压致相变压力点,具有随着结构链长的增加,其相变压力降低的规律。     

Raman spectroscopy of solid nitrogen up to 374 kbar

We measured the Raman spectrum of room-temperature solid nitrogen in a diamond cell at pressures up to 374 kbar. Only the molecular stretching mode was observed. The frequency of this mode appears to increase smoothly from 2340 cm^-1 at 39 kbar to 2394 cm^-1 at 374 kbar. Between about 50 and 120 kbar, the Raman line is split into a second, higher-frequency peak, which may be associated with molecular ordering in the solid. Measurements of broadening and splitting in fluorescence lines from the ruby pressure gauge indicate that solid nitrogen is reasonably plastic up to 130 kbar. We suggest that nitrogen might serve as a useful pressure medium.     

Raman scattering in hydrogenated amorphous silicon under high pressure

The first studies on the pressure dependences of the first-order Raman spectra in plasma-deposited a-Si:H films are reported. With increasing pressure up to 25 kbar the TO optical phonon band shows a shift in peak to higher frequencies with a sharpening of width while the TA acoustic phonon band shows a shift in peak to lower frequencies with a broadening of width. The LO optical phonon band shows a shift in peak to higher frequencies whereas the LA acoustic phonon band remains unchanged. These pressure effects are discussed with changes in force constant and structural disorder. The alloying effect of H atoms on the Raman spectra is also discussed while comparing the pressure effects.     

A high pressure raman study of ThO2 to 40 GPa and pressure-induced phase transition from fluorite structure

The pressure dependence of the first-order Raman peak and two second-order Raman features of ThO₂ crystallizing in the fluorite-type structure is investigated using a diamond anvil cell, up to 40GPa. A phase transition from the fluorite phase is observed near 30 GPa as evidenced by the appearance of seven new Raman peaks. The high pressure phases of ThO₂ and CeO₂ exhibit similar Raman features and from this it is believed that the two structures are the same, and have the PbCl₂-type structure. The pressure dependence dω/dP of the observed phonons and their mode Grüneisen parameters are similar to the isostructural CeO₂. The observed second-order Raman features are also identified from the calculated phonon dispersion curves for ThO₂.     

High pressure raman spectra of crystalline sulfur

Abstract

Well—resolved Raman spectra of crystalline sulfur have been recorded in a diamond anvil cell (DAC) in the pressure range from atmospheric pressure to 50 GPa at room temperature, using an 0.6 m triple spectrograph and a CCD multichannel detector. The spectra indicate two phase transitions in the pressure region between 10 and 15 GPa.     

Growth and annealing study of hydrogen-doped single diamond crystals under high pressure and high temperature

A series of diamond crystals doped with hydrogen is successfully synthesized using LiH as the hydrogen source in a catalyst-carbon system at a pressure of 6.0 GPa and temperature ranging from 1255 C to 1350 C.It is shown that the high temperature plays a key role in the incorporation of hydrogen atoms during diamond crystallization.Fourier transform infrared micro-spectroscopy reveals that most of the hydrogen atoms in the synthesized diamond are incorporated into the crystal structure as sp 3-CH 2-symmetric(2850 cm-1) and sp 3 CH 2-antisymmetric vibrations(2920 cm-1).The intensities of these peaks increase gradually with an increase in the content of the hydrogen source in the catalyst.The incorporation of hydrogen impurity leads to a significant shift towards higher frequencies of the Raman peak from 1332.06 cm-1 to 1333.05 cm-1 and gives rise to some compressive stress in the diamond crystal lattice.Furthermore,hydrogen to carbon bonds are evident in the annealed diamond,indicating that the bonds that remain throughout the annealing process and the vibration frequencies centred at 2850 and 2920 cm-1 have no observable shift.Therefore,we suggest that the sp 3 C-H bond is rather stable in diamond crystals.     

Effects of high pressure on the Raman and fluorescence emission spectra of two novel 1,3,4-oxadiazole derivatives

The effects of pressure on the fluorescence emission and Raman spectra of 1,4-bis[（4-methyloxyphenyl）-1,3,4-oxadiazolyl]- 2,5-bisheptyloxyphenylene （OXD-2） and on the fluorescence emission spectra of 1,4-bis[（4-methylphenyl）- 1,3,4-oxadiazolyl]phenylene （OXD-1） are investigated using a diamond anvil cell. With the increase of pressure, the intensity of the fluorescence emission increases and reaches maxima at 13GPa for OXD-1 and at 9.6GPa for OXD-2. The effect of pressure on the peak position of the emission shows a similar trend, red shift with the increase of pressure. But at higher pressures, the intensity of emission drops down dramatically. The Raman spectra of OXD-2 indicate that there appears a structural change at ca 3GPa.     

常温和100~800 MPa压力下石膏的Raman光谱研究

 利用金刚石压腔装置测量了高压下石膏中S—O键的4种振动模式和结晶水中羟基伸缩振动Raman位移，研究结果表明：在常温（25 ℃）和100~800 MPa压力范围内，石膏中S—O键的Raman谱峰的位移随压力的增加而向高波数方向移动，结晶水中羟基的两个伸缩谱峰随着压力的增加而向低波数方向移动，同时得到了各个谱峰与压力之间的关系式，其中结晶水中羟基的两个伸缩谱峰的dν/dp值有较大不同，是由于结晶水中含有两个强度不同的氢键所致。     

Conical support for double-stage diamond anvil apparatus

ABSTRACT

Both micro-paired and conical support type double-stage diamond anvil cells (ds-DAC) were tested using a newly synthesized ultra-fine nano-polycrystalline diamond (NPD). Well-focused X-ray sub-micron beam and the conically supported 2nd stage anvils (micro-anvils) with 10?μm culet enable us to obtain good quality X-ray diffraction peaks from the sample at around 400?GPa. The relationship between confining pressure and sample pressure depends heavily on the initial height (thickness) of micro-anvils, the difference of a few micrometers leads to a quite different compression path. The conical support type is a solution to retain both enough thickness and strength of micro-anvils at higher confining pressure conditions. All conical support ds-DAC experiments terminated by the failure of the 1st stage anvil instead of 2nd one. The combination of ultra-fine NPD 2nd stage anvil and NPD 1st stage anvil opens a new frontier for measurement of the X-ray absorption spectrum above 300?GPa.     

Ionic liquids based on the bis(trifluoromethylsulfonyl)imide anion for high‐pressure Raman spectroscopy measurements

Assembling a diamond anvil cell for high‐pressure measurements involves placing in a gasket hole the sample of interest, a pressure transmitting fluid, and a material for pressure calibration. In this communication, we propose the use of ionic liquids containing the bis(trifluoromethylsulfonyl)imide anion ([Tf₂N]^‐), [(CF₃SO₂)₂ N]^‐, as a simultaneous pressure transmitting and calibrant material for high‐pressure Raman spectroscopy measurements of solid samples that are not soluble in ionic liquids. The position of the characteristic Raman band of the [Tf₂N]^‐ anion at 740 cm⁻¹ exhibits linear frequency shift for pressures up to 2.5 GPa. High‐pressure Raman spectra of different ionic liquids containing the same anion indicate that the actual magnitude of the pressure‐induced frequency shift of the [Tf₂N]^‐ normal mode depends on the counterion, the typical shift being 4.2 cm⁻¹/GPa. Ionic liquids based on the [Tf₂N]^‐ anion are also good pressure transmitting mediums because hydrostatic condition is kept at high pressure, and no crystallization is observed up to 4.0 GPa. Copyright © 2012 John Wiley & Sons, Ltd.