The pressure-induced metallization of monoclinic vanadium dioxide
The structure transition sequence follow M 1 → M' 1 → X, the PIM in M' 1 phase occurs at 43.2 GPa and mainly driven by electron-correlation.
36 MATERIALS SCIENCE↗
Engineering topics
Liu, Bo
Publications and source records attributed to Liu, Bo.
Cr-doped Ge{sub 2}Sb{sub 2}Te{sub 5} for ultra-long data retention phase change memory
Phase change memory is regarded as one of the most promising candidates for the next-generation non-volatile memory. Its storage medium, phase change material, has attracted continuous exploration. Ge{sub 2}Sb{sub 2}Te{sub 5} (GST) is the most popular phase change material, but its thermal stability needs to be improved when used in some fields at high temperature (more than 120 °C). In this paper, we doped Cr atoms into GST and obtained Cr{sub 10}(Ge{sub 2}Sb{sub 2}Te{sub 5}){sub 90} (labeled as Cr-GST) with high thermal stability. For Cr-GST film, the sheet resistance ratio between amorphous and crystalline states is high up to 3 orders of magnitude. The crystalline Cr-GST film inherits the phase structure of GST, with metastable face-centered cubic phase and/or stable hexagonal phase. The doped Cr atoms not only bond with other atoms but also help to improve the anti-oxidation property of Cr-GST. As for the amorphous thermal stability, the calculated temperature for 10-year-data-retention of Cr-GST film, based on the Arrhenius equation, is about 180 °C. The threshold current and threshold voltage of a cell based on Cr-GST are about 6 μA and 2.7 V. The cell could be operated by suitable voltages for more than 40 000 cycles. Thus, Cr-GST is proved to be a promising phase change material with ultra-long data retention.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DETECTION OF THE INTERMEDIATE-WIDTH EMISSION LINE REGION IN QUASAR OI 287 WITH THE BROAD EMISSION LINE REGION OBSCURED BY THE DUSTY TORUS
The existence of intermediate-width emission line regions (IELRs) in active galactic nuclei has been discussed for over two decades. A consensus, however, is yet to be arrived at due to the lack of convincing evidence for their detection. We present a detailed analysis of the broadband spectrophotometry of the partially obscured quasar OI 287. The ultraviolet intermediate-width emission lines (IELs) are very prominent, in high contrast to the corresponding broad emission lines (BELs) which are heavily suppressed by dust reddening. Assuming that the IELR is virialized, we estimated its distance to the central black hole to be ∼2.9 pc, similar to the dust sublimation radius of ∼1.3 pc. Photo-ionization calculations suggest that the IELR has a hydrogen density of ∼10{sup 8.8}–10{sup 9.4} cm{sup −3}, within the range of values quoted for the dusty torus near the sublimation radius. Both its inferred location and physical conditions suggest that the IELR originates from the inner surface of the dusty torus. In the spectrum of this quasar, we identified only one narrow absorption-line system associated with the dusty material. With the aid of photo-ionization model calculations, we found that the obscuring material might originate from an outer region of the dusty torus. We speculate that the dusty torus, which is exposed to the central ionizing source, may produce IELs through photo-ionization processes, as well as obscure BELs as a natural “coronagraph.” Such a “coronagraph” could be found in a large number of partially obscured quasars and may be a useful tool to study IELRs.
79 ASTRONOMY AND ASTROPHYSICS↗
Modal reduction in single crystal sapphire optical fiber
A new type of single crystal sapphire optical fiber (SCSF) design is proposed to reduce the number of guided modes via a highly dispersive cladding with a periodic array of high and low index regions in the azimuthal direction. The structure retains a “core” region of pure single crystal (SC) sapphire in the center of the fiber and a “cladding” region of alternating layers of air and SC sapphire in the azimuthal direction that is uniform in the radial direction. The modal characteristics and confinement losses of the fundamental mode were analyzed via the finite element method by varying the effective core diameter and the dimensions of the “windmill” shaped cladding. The simulation results showed that the number of guided modes were significantly reduced in the “windmill” fiber design, as the radial dimension of the air and SC sapphire cladding regions increase with corresponding decrease in the azimuthal dimension. It is anticipated that the “windmill” SCSF will readily improve the performance of current fiber optic sensors in the harsh environment and potentially enable those that were limited by the extremely large modal volume of unclad SCSF.
36 MATERIALS SCIENCE↗
High pressure synthesis of amorphous TiO{sub 2} nanotubes
Amorphous TiO{sub 2} nanotubes with diameters of 8-10 nm and length of several nanometers were synthesized by high pressure treatment of anatase TiO{sub 2} nanotubes. The structural phase transitions of anatase TiO{sub 2} nanotubes were investigated by using in-situ high-pressure synchrotron X-ray diffraction (XRD) method. The starting anatase structure is stable up to ∼20GPa, and transforms into a high-density amorphous (HDA) form at higher pressure. Pressure-modified high- to low-density transition was observed in the amorphous form upon decompression. The pressure-induced amorphization and polyamorphism are in good agreement with the previous results in ultrafine TiO{sub 2} nanoparticles and nanoribbons. The relationship between the LDA form and α-PbO{sub 2} phase was revealed by high-resolution transmission electron microscopy (HRTEM) study. In addition, the bulk modulus (B{sub 0} = 158 GPa) of the anatase TiO{sub 2} nanotubes is smaller than those of the corresponding bulks and nanoparticles (180-240 GPa). We suggest that the unique open-ended nanotube morphology and nanosize play important roles in the high pressure phase transition of TiO{sub 2} nanotubes.
77 NANOSCIENCE AND NANOTECHNOLOGY↗
Lithium and lithium ion batteries for applications in microelectronic devices: A review
Not Available
Wang, Yuxing↗
Enhanced light extraction of scintillator using large-area photonic crystal structures fabricated by soft-X-ray interference lithography
Soft-X-ray interference lithography is utilized in combination with atomic layer deposition to prepare photonic crystal structures on the surface of Bi{sub 4}Ge{sub 3}O{sub 12} (BGO) scintillator in order to extract the light otherwise trapped in the internal of scintillator due to total internal reflection. An enhancement with wavelength- and emergence angle-integration by 95.1% has been achieved. This method is advantageous to fabricate photonic crystal structures with large-area and high-index-contrast which enable a high-efficient coupling of evanescent field and the photonic crystal structures. Generally, the method demonstrated in this work is also suitable for many other light emitting devices where a large-area is required in the practical applications.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
An embryo of protocells: The capsule of graphene with selective ion channels
In this study, the synthesis of artificial cell is a route for searching the origin of protocell. Here, we create a novel cell model of graphene capsules with selective ion channels, indicating that graphene might be an embryo of protocell membrane. Firstly, we found that the highly oxidized graphene and phospholipid-graphene oxide composite would curl into capsules under a strongly acidic saturated solution of heavy metallic salt solution at low temperature. Secondly, L-amino acids exhibited higher reactivity than D-amino acids on graphene oxides to form peptides, and the formed peptides in the influence of graphene would be transformed into a secondary structure, promoting the formation of left-handed proteins. Lastly, monolayer nanoporous graphene, prepared by unfocused 84 Kr 25+ , has a high selectivity for permeation of the monovalent metal ions ( Rb + > K + > Cs + > Na + > Li + , based on permeation concentration), but does not allow Cl- go through. It is similar to K+ channels, which would cause an influx of K + into capsule of graphene with the increase of pH in the primitive ocean, creating a suitable inner condition for the origin of life. Therefore, we built a model cell of graphene, which would provide a route for reproducing the origin of life.
59 BASIC BIOLOGICAL SCIENCES↗
Temperature dependence of sapphire fiber Raman scattering
Anti-Stokes Raman scattering in sapphire fiber has been observed for the first time. Temperature dependence of Raman peaks’ intensity, frequency shift, and linewidth were also measured. Three anti-Stokes Raman peaks were observed at temperatures higher than 300°C in a 0.72-m-long sapphire fiber excited by a second-harmonic Nd YAG laser. The intensity of anti-Stokes peaks are comparable to that of Stokes peaks when the temperature increases to 1033°C. We foresee the combination of sapphire fiber Stokes and anti-Stokes measurement in use as a mechanism for ultrahigh temperature sensing.
47 OTHER INSTRUMENTATION↗
Strong variability of overlapping iron broad absorption lines in five radio-selected quasars
We present a study of the variability of broad absorption lines (BALs) in a uniformly radio-selected sample of 28 BAL quasars using the archival data from the FIRST Bright Quasar Survey and the Sloan Digital Sky Survey as well as data we obtained ourselves, covering timescales of ∼1−10 yr in the quasar rest frame. To our surprise, five quasars exhibiting strong variations all belong to a special subclass of “overlapping-trough” iron low-ionization BAL (OFeLoBAL) quasars; however, four other non-overlapping FeLoBALs (non-OFeLoBALs) are invariant, except in one case with a weak change in optical depth. Meanwhile, we also identify six typical variations of high-ionization and low-ionization BALs in this BAL quasar sample. Photoionization models suggest that OFeLoBALs are formed in relatively dense (n{sub e} > 10{sup 6} cm{sup −3}) outflows at distances from the continuum source that range from the subparsec scale to dozens of parseconds. These formation conditions differ from those of non-OFeLoBALs, which are likely produced by low-density gas located at distances of hundreds to thousands of parseconds. Thus, OFeLoBALs and non-OFeLoBALs, i.e., FeLoBALs with/without strong BAL variations, may represent the bimodality of Fe ii absorption; the former are located in active galactic nucleus environments rather than in the host galaxy. We suggest that a high density and a small distance are the necessary conditions for the creation of OFeLoBALs. As suggested in the literature, strong BAL variability is possibly attributable to variability in the covering factor of the BAL regions caused by clouds transiting across the line of sight rather than to variations in ionization.
79 ASTRONOMY AND ASTROPHYSICS↗
Molecular dynamics simulations of large macromolecular complexes
Connecting dynamics to structural data from diverse experimental sources, molecular dynamics simulations permit the exploration of biological phenomena in unparalleled detail. Advances in simulations are moving the atomic resolution descriptions of biological systems into the million-to-billion atom regime, in which numerous cell functions reside. In this opinion, we review the progress, driven by large-scale molecular dynamics simulations, in the study of viruses, ribosomes, bioenergetic systems, and other diverse applications. Finally, these examples highlight the utility of molecular dynamics simulations in the critical task of relating atomic detail to the function of supramolecular complexes, a task that cannot be achieved by smaller-scale simulations or existing experimental approaches alone.
59 BASIC BIOLOGICAL SCIENCES↗
A comprehensive study of broad absorption line quasars. i. Prevalence of He i* absorption line multiplets in low-ionization objects
Neutral helium multiplets, He i* λλ3189,3889,10830, are very useful diagnostics for the geometry and physical conditions of the absorbing gas in quasars. So far only a handful of He i* detections have been reported. Using a newly developed method, we detected the He i*λ3889 absorption line in 101 sources of a well-defined sample of 285 Mg ii broad absorption line (BAL) quasars selected from SDSS DR5. This has increased the number of He i* BAL quasars by more than one order of magnitude. We further detected He i*λ3189 in 50% (52/101) of the quasars in the sample. The detection fraction of He i* BALs in Mg ii BAL quasars is ∼35% as a whole, and it increases dramatically with increasing spectral signal-to-noise ratio (S/N), from ∼18% at S/N ⩽ 10 to ∼93% at S/N ⩾ 35. This suggests that He i* BALs could be detected in most Mg ii LoBAL quasars, provided the spectra S/N is high enough. Such a surprisingly high He i* BAL fraction is actually predicted from photoionization calculations based on a simple BAL model. The result indicates that He i* absorption lines can be used to search for BAL quasars at low z, which cannot be identified by ground-based optical spectroscopic surveys with commonly seen UV absorption lines. Using He i* λ3889, we discovered 19 BAL quasars at z<0.3 from the available SDSS spectral database. The fraction of He i* BAL quasars is similar to that of LoBAL objects.
79 ASTRONOMY AND ASTROPHYSICS↗
Characterization and luminescence properties of sol–gel derived M′-type LuTaO{sub 4}:Ln{sup 3+} (Ln = Pr, Sm, Dy) phosphors
Graphical abstract: Emission spectra of LuTaO{sub 4}:Ln (Ln = Pr, Sm and Dy) phosphors under X-ray excitation. The insets illustrate their SEM micrographs. - Highlights: • M′-type LuTaO{sub 4}:Ln{sup 3+} (Ln = Pr, Sm, Dy) phosphors were synthesized by sol–gel technique. • The phosphors exhibited an efficient energy transfer from the host to activators. • High intensity of activator emission was achieved under X-ray excitation. • The phosphors are encouraging for application in high-spatial-resolution X-ray CT imaging. - Abstract: M′-type Lu{sub 1−x}Ln{sub x}TaO{sub 4} (Ln = Pr, Sm, Dy) phosphors have been successfully synthesized by sol–gel technique, their crystallization, morphology, photoluminescence and X-ray excited luminescence properties were investigated in detail. The phosphors had good crystallization behavior. The optimum doping concentrations of Pr{sup 3+}, Sm{sup 3+} and Dy{sup 3+} in LuTaO{sub 4} were at x = 0.003, 0.025, 0.02, respectively. They exhibited a more efficient host excitation relative to the 4f–4f excitations of the rare-earth ions, and a dominant {sup 1}D{sub 2} → {sup 3}H{sub 4}, {sup 4}G{sub 5/2} → {sup 6}H{sub 7/2} or {sup 4}F{sub 9/2} → {sup 6}H{sub 13/2} emission for Pr{sup 3+}, Sm{sup 3+} or Dy{sup 3+}, respectively, which corresponds to the average decay time of 21.7, 745.7 or 10.0 μs, respectively. It is expected that Pr{sup 3+}- or Dy{sup 3+}-doped LuTaO{sub 4} phosphors with a microsecond level decay time are very encouraging for applications in X-ray computerized tomographic imaging with high spatial resolution.
36 MATERIALS SCIENCE↗
High thermal stability and low density variation of carbon-doped Ge{sub 2}Sb{sub 2}Te{sub 5} for phase-change memory application
Carbon-doped Ge{sub 2}Sb{sub 2}Te{sub 5} (GSTC) film has been experimentally studied as a thermal stable material for high temperature applications. The 10-yr data retention temperature is remarkably increased through C doping. Furthermore, GSTC films have better interface properties after annealing at 410 °C for 30 min. The density variation of GSTC film is significantly improved, which is very important to device reliability. X-ray photoelectron spectroscopy results reveal that the thermal stability enhancement of GSTC film attributes to the forming of C-Ge, C-Sb, and C-Te bonds. The perfect thermal stability makes GSTC materials a good candidate in the actual production of phase-change memory.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Two new helical compounds based on Keggin clusters and N-donor multidentate ligand: Syntheses, structures and properties
Two isostructural polyoxometalate-based inorganic–organic hybrids with 1D helical chain, [CuH{sub 3}L{sub 2}(GeMo{sub 12}O{sub 40})]·2H{sub 2}O (1) and [CuH{sub 3}L{sub 2}(SiMo{sub 12}O{sub 40})]·2H{sub 2}O (2), where L=2,4,5-tri(4-pyridyl)-imidazole have been synthesized under hydrothermal conditions. It is the first time to use the L ligand to synthesis the Keggin-type polyoxometalate-based inorganic–organic hybrids. The two compounds possess the left- and right-handed helical chains and the POMs as pendants attach in the helical chains through Cu–O bonds. The two compounds have been characterized by elemental analyses, infrared (IR) spectroscopy, powder X-ray diffraction (PXRD), X-ray photoelectron spectra (XPS), thermogravimetric analysis (TGA) and photoluminescent spectroscopy. Moreover, nitrogen adsorption−desorption measurement, electrochemical and photocatalysis properties for degradation of methylene blue (MB) upon a UV irradiation of compound 1 have been examined. - Graphical abstract: Two new compounds, [CuH{sub 3}L{sub 2}(GeMo{sub 12}O{sub 40})]·2H{sub 2}O (1) and [CuH{sub 3}L{sub 2}(SiMo{sub 12}O{sub 40})]·2H{sub 2}O (2) have been synthesized under hydrothermal conditions. The two compounds possess the left- and right-handed helical chains. - Highlights: • The tridentate L ligand is first used to synthesis Keggin-type polyoxometalates. • The two compounds possess the left- and right-handed Cu–L helical chains. • Relationship between surface properties and photocatalytic activity was studied. • Two compounds exhibit photoluminescence of ligand-to-metal charge transfer.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗