Geometric curvatures of plane symmetry black hole

In this paper, we study the properties and thermodynamic stability of the plane symmetry black hole from the viewpoint of geometry. Weinhold metric and Ruppeiner metric are obtained, respectively. The Weinhold curvature gives phase transition points, which correspond to the first-order phase transition only at N=1, where $N$ is a parameter in the plane symmetry black hole. While the Ruppeiner one shows first-order phase transition points for arbitrary $N\neq 1$. Both of which give no any information about the second-order phase transition. Considering the Legendre invariant proposed by Quevedo et. al., we obtain a unified geometry metric, which gives a correctly the behavior of the thermodynamic interactions and phase transitions. The geometry is also found to be curved and the scalar curvature goes to negative infinity at the Davies' phase transition points when the logarithmic correction is included.Comment: 16 pages, 6 figure

Atomistic Mechanisms Underlying Selectivities in C_1 and C_2 Products from Electrochemical Reduction of CO on Cu(111)

Practical environmental and energy applications of the electrochemical reduction of CO_2 to chemicals and fuels require far more efficient and selective electrocatalysts beyond the only working material Cu, but the wealth of experimental data on Cu can serve to validate any proposed mechanisms. To provide design guidelines, we use quantum mechanics to predict the detailed atomistic mechanisms responsible for C_1 and C_2 products on Cu. Thus, we report the pH dependent routes to the major products, methane and ethylene, and identify the key intermediates where branches to methanol, ketene, ethanol, acetylene and ethane are kinetically blocked. We discovered that surface water on Cu plays a key role in the selectivity for hydrocarbon products over the oxygen-containing alcohol products by serving as a strong proton donor for electrochemical dehydration reductions. We suggest new experiments to validate our predicted mechanisms

Nature of the active sites for CO reduction on copper nanoparticles; suggestions for optimizing performance

Recent experiments show that the grain boundaries (GBs) of copper nanoparticles (NP) lead to outstanding performance in reducing CO_2 and CO to alcohol products. We report here multiscale simulations that mimic experimental synthesis conditions to predict the structure of a 10nm Cu NP (158,555 atoms). To identify active sites, we first predict the CO binding at a large number of sites and select 4 exhibiting CO binding stronger than the (211) step surface. Then, we predict the formation energy of *OCCOH intermediate as a descriptor for C-C coupling, identifying two active sites, both of which have an undercoordinated surface square site adjacent to a subsurface stacking fault. We then propose a periodic Cu surface (4 by 4 supercell) with a similar site that substantially decreases the formation energy of *OCCOH, by 0.14 eV

Free-Energy Barriers and Reaction Mechanisms for the Electrochemical Reduction of CO on the Cu(100) Surface, Including Multiple Layers of Explicit Solvent at pH 0

The great interest in the photochemical reduction from CO_2 to fuels and chemicals has focused attention on Cu because of its unique ability to catalyze formation of carbon-containing fuels and chemicals. A particular goal is to learn how to modify the Cu catalysts to enhance the production selectivity while reducing the energy requirements (overpotential). To enable such developments, we report here the free-energy reaction barriers and mechanistic pathways on the Cu(100) surface, which produces only CH_4 (not C_2H_4 or CH_3OH) in acid (pH 0). We predict a threshold potential for CH_4 formation of −0.52 V, which compares well to experiments at low pH, −0.45 to −0.50 V. These quantum molecular dynamics simulations included ∼5 layers of explicit water at the water/electrode interface using enhanced sampling methodology to obtain the free energies. We find that that chemisorbed hydroxyl-methylene (CH–OH) is the key intermediate determining the selectivity for methane over methanol

Determining layer number of two dimensional flakes of transition-metal dichalcogenides by the Raman intensity from substrate

Transition-metal dichalcogenide (TMD) semiconductors have been widely studied due to their distinctive electronic and optical properties. The property of TMD flakes is a function of its thickness, or layer number (N). How to determine N of ultrathin TMDs materials is of primary importance for fundamental study and practical applications. Raman mode intensity from substrates has been used to identify N of intrinsic and defective multilayer graphenes up to N=100. However, such analysis is not applicable for ultrathin TMD flakes due to the lack of a unified complex refractive index ($\tilde{n}$) from monolayer to bulk TMDs. Here, we discuss the N identification of TMD flakes on the SiO$_2$/Si substrate by the intensity ratio between the Si peak from 100-nm (or 89-nm) SiO$_2$/Si substrates underneath TMD flakes and that from bare SiO$_2$/Si substrates. We assume the real part of $\tilde{n}$ of TMD flakes as that of monolayer TMD and treat the imaginary part of $\tilde{n}$ as a fitting parameter to fit the experimental intensity ratio. An empirical $\tilde{n}$, namely, $\tilde{n}_{eff}$, of ultrathin MoS$_{2}$, WS$_{2}$ and WSe$_{2}$ flakes from monolayer to multilayer is obtained for typical laser excitations (2.54 eV, 2.34 eV, or 2.09 eV). The fitted $\tilde{n}_{eff}$ of MoS$_{2}$ has been used to identify N of MoS$_{2}$ flakes deposited on 302-nm SiO$_2$/Si substrate, which agrees well with that determined from their shear and layer-breathing modes. This technique by measuring Raman intensity from the substrate can be extended to identify N of ultrathin 2D flakes with N-dependent $\tilde{n}$ . For the application purpose, the intensity ratio excited by specific laser excitations has been provided for MoS$_{2}$, WS$_{2}$ and WSe$_{2}$ flakes and multilayer graphene flakes deposited on Si substrates covered by 80-110 nm or 280-310 nm SiO$_2$ layer.Comment: 10 pages, 4 figures. Accepted by Nanotechnolog

Optical Fiber Harsh Environment Sensors

Various optical fiber harsh environment sensors were reported, including the miniaturized inline Fabry-Perot interferometer sensor by femtosecond laser micromachining, the long period fiber grating sensor and the inline core-cladding mode interferometer by CO2 laser irradiations