The opening of an electrical band gap in graphene is crucial for its application for logic circuits. Recent studies have shown that an energy gap in Bernal-stacked bilayer graphene can be generated by applying an electric displacement field. Molecular doping has also been proposed to open the electrical gap of bilayer graphene by breaking either in-plane symmetry or inversion symmetry; however, no direct observation of an electrical gap has been reported. Here we discover that the organic molecule triazine is able to form a uniform thin coating on the top surface of a bilayer graphene, which efficiently blocks the accessible doping sites and prevents ambient p-doping on the top layer. The charge distribution asymmetry between the top and bottom layers can then be enhanced simply by increasing the p-doping from oxygen/moisture to the bottom layer. The on/off current ratio for a bottom-gated bilayer transistor operated in ambient condition is improved by at least 1 order of magnitude. The estimated electrical band gap is up to ∼111 meV at room temperature. The observed electrical band gap dependence on the hole-carrier density increase agrees well with the recent density-functional theory calculations. This research provides a simple method to obtain a graphene bilayer transistor with a moderate on/off current ratio, which can be stably operated in air without the need to use an additional top gate.

The orientation dependence of Br-atom formation in the reaction of the oriented OH radical with the HBr molecule using the hexapole electrostatic field was studied. Experimental results for the orientation dependence in the reaction were analyzed using a Legendre polynomial fit. The results show two reactive sites. It was found that O-end attack is most favored for this reaction, and that H-end attack also shows a pronounced reactivity. The reactivity of the side-ways attack was found to be small. By comparing the results of the orientation dependence in the reaction with studies of inelastic collisions and theoretical calculations, two reaction pathways are proposed. Reaction by O-end attack is followed by a direct abstraction of the H-atom from the HBr molecule. The mechanism for H-end attack may have H-atom migration from HBr to form the water molecule. © 2011 the Owner Societies.

The adsorption mechanism of water on the GaN (0001) polar surface is investigated via both the Density Functional Theory (DFT) method and its derived classical force field. The physisorption binding energy and the adsorption geometry of the water molecule on the clean Ga-terminated surface are analyzed via the first-principle static calculations. The adsorption energy hypersurfaces are then extracted to be used in the fitting of the interaction potentials between water and GaN. Classical molecular dynamics (MD) simulations based on the developed force field are performed for the interfacial system of liquid water and the GaN surface slab. From our computations, the interfacial water exhibits significant oscillatory profiles for the atomic densities and the molecular orientations. Further data analysis suggests a highly confined first layer with the O being locked right upon the surface Ga atoms and the H pointing toward the neighboring O to form the weakened hydrogen bonds. A bilayer configuration with opposite dipole orientations is consequently characterized as the wetting structure on the GaN polar surface and is explained by the anisotropic perturbations from the surface polar sites. Our simulations would be helpful to provide an atomistic picture for the water adsorption configuration on this semiconductor surface and would be useful in the relevant nanofluidic and nanoengineering applications.

Room temperature ferromagnetism in both transition-metals doped and undoped semiconductor thin films and nanostructures challenges our understanding of the magnetism in solids. In this report, we performed the magnetic measurement and Andreev reflection spectroscopy study on undoped Indium–Tin oxide (ITO) thin films and bulk samples. The magnetic measurement results of thin films show that the total magnetization/cm2 is thickness independent. Prominent ferromagnetism signal was also discovered in bulk samples. Spin polarized electron transports were probed on İTO\} thin film/superconductor interface and bulk samples surface/superconductor interface. Based on the magnetic measurement results and spin polarization measurement data, we propose that the ferromagnetism in this material originates from the surface spin polarization and this surface polarization may also explain the room temperature ferromagnetism discovered in other undoped oxide semiconductor thin films and nanostructures.

The structural and dynamic properties of water on the GaN(0001) polar surface are investigated via classical molecular dynamics simulations. The interfacial molecules are observed to have enhanced structural ordering and slowed-down dynamics compared to the liquid bulk; these unique properties are evidenced in the slower reorientational relaxation, smaller diffusion constant, and longer residence lifetime for water located at the surface region up to ∼7 Å from the substrate. Further analysis of the vibrational spectra at low frequencies shows that both the hydrogen bond network bending and the hydrogen bond stretching bands at the interface shift to the blue compared to those in the bulk, due to the strong coupling between the O atom of water and the Ga sites. The distinct spectral features along with the anisotropy of the hydrogen bond distributions of the interfacial water are complex results determined by both the substrate–water and water–water interactions.

The nature of water networks exposed to ionizing radiation is important in various radiation-related chemistry and biology. To understand structural evolution of ionized water networks at the molecular level{,} we report here infrared spectra of water cluster radical cations (H2O)n+ (n = 3 - 11) in the gas phase. Spectral features of free OH stretch modes are quite similar to those of protonated water clusters H+(H2O)n{,} of which the hydrogen-bond network structures have been revealed. In addition{,} we observed an extra band attributed to the stretch of an OH radical in (H2O)n+. These results indicate that nominal (H2O)n+ should be regarded as H+(H2O)n-1(OH) motifs having similar network shapes to those of H+(H2O)n. We also analyzed hydrogen-bonded OH stretch bands and found that hydrogen-bond strength is a key factor to determine the position of the OH radical relative to the protonated site (H3O+/H5O2+). Because an OH radical is a weaker hydrogen bond acceptor than water{,} the first solvation shell of the protonated site is preferentially filled with water. As a result{,} the OH radical is separated from the protonated (charged) site by at least one water molecule in n [greater-than-or-equal] 5 clusters. This result shows the instability of the H3O+-OH ion-radical contact pair in water networks{,} and implies the higher mobility of the OH radical due to its release from the charged site. Observed structural preferences are confirmed both in cold and warm cluster ion sources.

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In this paper, we present the direct observation of quantum size effects (QSE) on the work function in ultrathin Pb films. By using scanning tunneling microscopy and spectroscopy, we show that the very existence of quantum well states (QWS) in these ultrathin films profoundly affects the measured tunneling decay constant kappa, resulting in a very rich phenomenon of "quantum oscillations" in kappa as a function of thickness, L, and bias voltage, V(s). More specifically, we find that the phase of the quantum oscillations in kappa vs. L depends sensitively upon the bias voltage, which often results in a total phase reversal at different biases. On the other hand, at very low sample bias (vertical bar V(s)vertical bar < 0.03 V) the measurement of kappa vs. L accurately reflects the quantum size effect on the work function. In particular, the minima in the quantum oscillations of kappa vs. L occur at the locations where QWS cross the Fermi energy, thus directly unraveling the QSE on the work function in ultrathin films, which was predicted more than three decades ago. This further clarifies several contradictions regarding the relationship between the QWS locations and the work function.

We report on a first-principles study of the conductance through graphene suspended between Al contacts as a function of junction length, width, and orientation. The charge transfer at the leads and into the freestanding section gives rise to an electron-hole asymmetry in the conductance and in sufficiently long junctions induces two conductance minima at the energies of the Dirac points for suspended and clamped regions, respectively. We obtain the potential profile along a junction caused by doping and provide parameters for effective model calculations of the junction conductance with weakly interacting metallic leads.

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The orientation dependence for the Br atom formation in the reaction of the oriented OH radicals with HBr molecules at 0.26 eV collision energy has been observed for the first time using the hexapole electric field, and we found that the reaction cross-section for O-end attack is more favorable than that for H-end attack by a factor of 3.4 ± 2.3. © the Owner Societies.

Basing on the results of the scanning tunneling microscopy (STM) observations and density functional theory (DFT) calculations, the structural model for the Cu magic clusters formed on Si(1 1 1)7 x 7 surface has been proposed. Using STM, composition of the Cu magic clusters has been evaluated from the quantitative analysis of the Cu and Si mass transport occurring during magic cluster converting into the Si(1 1 1)’5.5 x 5.5’-Cu reconstruction upon annealing. Evaluation yields that Cu magic cluster accommodates similar to 20 Cu atoms with similar to 20 Si atoms being expelled from the corresponding 7 x 7 half unit cell (HUC). In order to fit these values, it has been suggested that the Cu magic clusters resemble fragments of the Cu2Si-silicide monolayer incorporated into the rest-atom layer of the Si(1 1 1)7 x 7 HUCs. Using DFT calculations, stability of the nineteen models has been tested of which five models appeared to have formation energies lower than that of the original Si(1 1 1)7 x 7 surface. The three of five models having the lowest formation energies have been concluded to be the most plausible ones. They resemble well the evaluated composition and their counterparts are found in the experimental STM images. (C) 2009 Elsevier B.V. All rights reserved.