Jinyuan Yan

Monolayer behaviour in bulk ReS 2 due to electronic and vibrational decoupling

Semiconducting transition metal dichalcogenides consist of monolayers held together by weak forces where the layers are electronically and vibrationally coupled. Isolated monolayers show changes in electronic structure and lattice vibration energies, including a transition from indirect to direct bandgap. Here we present a new member of the family, rhenium disulphide (ReS2), where such variation is absent and bulk behaves as electronically and vibrationally decoupled monolayers stacked together. From bulk to monolayers, ReS2 remains direct bandgap and its Raman spectrum shows no dependence on the number of layers. Interlayer decoupling is further demonstrated by the insensitivity of the optical absorption and Raman spectrum to interlayer distance modulated by hydrostatic pressure. Theoretical calculations attribute the decoupling to Peierls distortion of the 1T structure of ReS2, which prevents ordered stacking and minimizes the interlayer overlap of wavefunctions. Such vanishing interlayer coupling enables probing of two-dimensional-like systems without the need for monolayers.

Texture of Nanocrystalline Nickel: Probing the Lower Size Limit of Dislocation Activity

Strength Under Pressure Above a lower cutoff value, shrinking the grain size of a metal tends to strengthen it because the overall increase in grain boundaries limits the activity of the dislocations as the material undergoes plastic deformation. Chen et al. (p. 1448) explore the question of whether this restriction in dislocation activity occurs when a metal is subjected to high pressures. Foils of nickel made from particles of different sizes were subjected to high pressures inside a diamond anvil cell. An increase in pressure extended dislocation activity to smaller grain sizes, indicating that pressure compensates for the inhibition of dislocation activity in small volumes. A metal subjected to high pressures retains the activity of dislocations, even for very small grain sizes. The size of nanocrystals provides a limitation on dislocation activity and associated stress-induced deformation. Dislocation-mediated plastic deformation is expected to become inactive below a critical particle size, which has been proposed to be between 10 and 30 nanometers according to computer simulations and transmission electron microscopy analysis. However, deformation experiments at high pressure on polycrystalline nickel suggest that dislocation activity is still operative in 3-nanometer crystals. Substantial texturing is observed at pressures above 3.0 gigapascals for 500-nanometer nickel and at greater than 11.0 gigapascals for 20-nanometer nickel. Surprisingly, texturing is also seen in 3-nanometer nickel when compressed above 18.5 gigapascals. The observations of pressure-promoted texturing indicate that under high external pressures, dislocation activity can be extended down to a few-nanometers-length scale.

Determination of the variation of the fluorescence line positions of ruby, strontium tetraborate, alexandrite, and samarium-doped yttrium aluminum garnet with pressure and temperature

The pressure and temperature dependent fluorescence line-shift of strontium tetraborate has been measured concurrently with x-ray diffraction from the pressure standards sodium chloride or gold. Temperature was found to have a small effect on the fluorescence line-shift under pressure. We found a maximum pressure uncertainty of ±1.8 GPa at 25 GPa (7.2%) and 857 K when making no temperature correction. The fluorescence line-shifts for ruby, Alexandrite, and samarium-doped yttrium aluminum garnet were also determined, using our strontium tetraborate calibration to determine pressure and a thermocouple to measure temperature. Fluorescence measurements were extended up to 800 K for ruby and Alexandrite. Temperature was found to have a small effect on the fluorescence line-shift of samarium-doped yittrium aluminum garnet. We found a maximum uncertainty of ±2.7 GPa at 25 GPa (11.1%) and 857 K when no temperature correction was applied. We determined equations relating to the fluorescence line position from thes...

Detecting grain rotation at the nanoscale

Significance The plastic deformation of nanomaterials has long been wrapped in mystery. Grain rotation is suggested to be a dominant mechanism of plastic deformation for ultrafine nanomaterials. However, the in situ observation of grain rotation has been made possible only for coarse-grained materials. Here we report the in situ high-pressure detection of grain rotation at the nanoscale. The surprising observation is that the texture strength of the same-sized platinum drops rapidly with decreasing grain size of the nickel medium, indicating that more active grain rotation occurs in the smaller nickel nanocrystals. Insight into these processes provides a better understanding of the plastic deformation of nanomaterials at a few-nanometer length scale. It is well-believed that below a certain particle size, grain boundary-mediated plastic deformation (e.g., grain rotation, grain boundary sliding and diffusion) substitutes for conventional dislocation nucleation and motion as the dominant deformation mechanism. However, in situ probing of grain boundary processes of ultrafine nanocrystals during plastic deformation has not been feasible, precluding the direct exploration of the nanomechanics. Here we present the in situ texturing observation of bulk-sized platinum in a nickel pressure medium of various particle sizes from 500 nm down to 3 nm. Surprisingly, the texture strength of the same-sized platinum drops rapidly with decreasing grain size of the nickel medium, indicating that more active grain rotation occurs in the smaller nickel nanocrystals. Insight into these processes provides a better understanding of the plastic deformation of nanomaterials in a few-nanometer length scale.

Fractal character of titania nanoparticles formed by laser ablation

Titania nanoparticles were fabricated by laser ablation of polycrystalline rutile in water at room temperature. The resulting nanoparticles were analyzed with x-ray diffraction, Raman spectroscopy, and transmission electron microscopy. The electron micrograph image of deposited nanoparticles demonstrates fractal properties.

High-pressure behavior of osmium : an analog for iron in Earth's core

High-resolution x-ray diffraction with diamond-anvil cells, using argon as a quasi-hydrostatic pressure medium, documents the crystal structure and equation of state of osmium to over 60 GPa at room temperature. We find the zero-pressure bulk modulus in fair agreement with other experiments as well as with relativistic electronic band-structure calculations: Osmium is the densest but not the most incompressible element at ambient conditions. We also find no evidence for anomalies in the ratio of unit-cell parameters, c/a, or in the compressibility of osmium as a function of pressure. This is in agreement with other experiments and quantum mechanical calculations but disagrees with recent claims that the electronic structure and equation of state of osmium exhibit anomalies at pressures of ∼15-25 GPa; the discrepancies are may be due to the effects of texturing.

Beamline 12.2.2: An Extreme Conditions Beamline at the Advanced Light Source

The Advanced Light Source (ALS) is a 1.9-GeV, third-generation synchrotron optimized for the production of VUV and soft X-rays from undulators. There is also a hard X-ray program at the ALS, which is based around three 6-T superconducting bending magnets [1] that shift the critical energy from 3 keV to 12 keV. The extreme conditions beamline at the ALS is situated on Beamline 12.2.2, which benefits from radiation produced by one of these superbend sources. The beamline is designed for X-ray diffraction, X-ray spectroscopy, and X-ray imaging of samples held in diamond-anvil high-pressure cells (DACs). In a DAC, samples are on the order of 10 to 50 μm in diameter and 10 to 30 μm thick and are contained in a metal gasket of typical inner diameters of 100 to 150 μm. For high-quality diffraction patterns with little or no contamination from diffraction from the gasket, the X-ray beam size needs to be on the order of 10 μm × 10 μm.

Pressure-induced structural transition of CdxZn1-xO alloys

CdxZn1−xO alloys, as a transparent conducting oxide, have recently attracted much attention for potential optoelectronic applications. In this letter, we report a hydrostatic pressure-induced phase transition of CdxZn1−xO alloys from the wurtzite to the rocksalt structure and its phase diagram probed using a diamond anvil cell. It is found that the transition pressure, determined by changes in optical and structural properties, depends sensitively on the composition. As the Cd content increases, the critical pressure decreases, until at x = 0.67 where the alloy is intrinsically stable in the rocksalt phase even at ambient pressure. The wurtzite phase is light emitting with a direct bandgap that slightly widens with increasing pressure, while the rocksalt phase has a much wider bandgap that is indirect. The pressure-sensitive light emission and phase transition may find potential applications in fields such as stress sensing and energy storage.

Investigation of phase transition of mercury decomposed from mercury oxide up to 20 GPa

The high pressure behavior of mercury decomposed from mercury oxide up to 20.4 GPa was investigated using angular-dispersive X-ray diffraction. The results showed that liquid mercury solidified at 2.0 GPa and was resolved as α hexagonal, R-3m, a=3.3743±0.0007 Å and c=6.8199±0.0013 Å. When compressed up to 5.7 GPa, α mercury transformed into orthorhombic γ phase directly, which is not the case of transforming from an α structure to a body-centered tetragonal structure (β). The space group of orthorhombic γ phase was interpreted successfully as Pmmn, with a=2.7722±0.0010 Å, b=4.0792±0.0028 Å and c=6.8285±0.0029 Å at 8.9 GPa.

The Development of an automated data analysis system for high-pressure powder diffraction data collected using an area detector

A program for the automation of the processing of powder diffraction data collected from an area detector is presented in this article. Encapsulation of existing software packages in wrapper programs can allow automation and linking of existing well-proven computer code. This eliminates the need to reinvest resources in developing new codes to meet evolving scientific needs. Here, we demonstrate this principle by automating the use of two programs commonly used in the processing of powder diffraction data from area detectors: fit2d and GSAS.