Search

Group-IV monochalcogenides have recently shown great potential for their thermoelectric, ferroelectric, and other intriguing properties. The electrical properties of group-IV monochalcogenides exhibit a strong dependence on the chalcogen type. For example, GeTe exhibits high doping concentration, whereas S/Se-based chalcogenides are semiconductors with sizable bandgaps. Here, we investigate the electrical and thermoelectric properties of gamma-GeSe, a recently identified polymorph of GeSe. gamma-GeSe exhibits high electrical conductivity (~106 S/m) and a relatively low Seebeck coefficient (9.4 uV/K at room temperature) owing to its high p-doping level (5x1021 cm-3), which is in stark contrast to other known GeSe polymorphs. Elemental analysis and first-principles calculations confirm that the abundant formation of Ge vacancies leads to the high p-doping concentration. The magnetoresistance measurements also reveal weak-antilocalization because of spin-orbit coupling in the crystal. Our results demonstrate that gamma-GeSe is a unique polymorph in which the modified local bonding configuration leads to substantially different physical properties.

Using density functional theory calculations, we have studied the edge-functionalization of armchair graphene nanoribbons (AGNRs) with pentagonal-hexagonal edge structures. While the AGNRs with pentagonal-hexagonal edge structures (labeled (5,6)-AGNRs) are metallic, the edge-functionalized (5,6)-AGNRs with substitutional atoms opens a band gap. We find that the band structures of edge-functionalized (5,6)-N-AGNRs by substitution resemble those of defect-free (N-1)-AGNR at the {\Gamma} point, whereas those at the X point show the original ones of the defect-free N-AGNR. The overall electronic structures of edge-functionalized (5,6)-AGNRs depend on the number of electrons, supplied by substitutional atoms, at the edges of functionalized (5,6)-AGNRs., Comment: 12 pages, 5 figures

Black Phosphorus (BP) is an excellent material for post graphene era due to its layer dependent band gap, high mobility and high Ion/Ioff. However, its poor stability in ambient poses a great challenge in its practical and long-term usage. Optical visualization of oxidized BP is the key and foremost step for its successful passivation from the ambience. Here, we have done a systematic study of the oxidation of BP and developed a technique to optically identify the oxidation of BP using Liquid Crystal (LC). Interestingly we found that rapid oxidation of thin layers of BP makes them disappear and can be envisaged by using the alignment of LC. The molecular dynamics simulations also proved the preferential alignment of LC on oxidized BP. We believe that this simple technique will be effective in passivation efforts of BP and will enable it for exploitation of its properties in the field of electronics.

Electrochemical intercalation is a powerful method for tuning the electronic properties of layered solids. In this work, we report an electro-chemical strategy to controllably intercalate lithium ions into a series of van der Waals (vdW) heterostructures built by sandwiching graphene between hexagonal boron nitride (h-BN). We demonstrate that encapsulating graphene with h-BN eliminates parasitic surface side reactions while simultaneously creating a new hetero-interface that permits intercalation between the atomically thin layers. To monitor the electrochemical process, we employ the Hall effect to precisely monitor the intercalation reaction. We also simultaneously probe the spectroscopic and electrical transport properties of the resulting intercalation compounds at different stages of intercalation. We achieve the highest carrier density $> 5 \times 10^{13} cm^{-2}$ with mobility $> 10^3 cm^2/(Vs)$ in the most heavily intercalated samples, where Shubnikov-de Haas quantum oscillations are observed at low temperatures. These results set the stage for further studies that employ intercalation in modifying properties of vdW heterostructures.

Nano-materials, such as metal-organic frameworks, have been considered to capture CO$_2$. However, their application has been limited largely because they exhibit poor selectivity for flue gases and low capture capacity under low pressures. We perform a high-throughput screening for selective CO$_2$ capture from flue gases by using first principles thermodynamics. We find that elements with empty d orbitals selectively attract CO$_2$ from gaseous mixtures under low CO$_2$ pressures at 300 K and release it at ~450 K. CO$_2$ binding to elements involves hybridization of the metal d orbitals with the CO$_2$ $\pi$ orbitals and CO$_2$-transition metal complexes were observed in experiments. This result allows us to perform high-throughput screening to discover novel promising CO$_2$ capture materials with empty d orbitals and predict their capture performance under various conditions. Moreover, these findings provide physical insights into selective CO$_2$ capture and open a new path to explore CO$_2$ capture materials., Comment: 17 pages, 6 figures

Bilayer graphene (BLG) with a tunable bandgap appears interesting as an alternative to graphene for practical applications, thus its transport properties are being actively pursued. Using density functional theory and perturbation analysis, we investigated, under an external electric field, the electronic properties of BLGs in various stackings relevant to recently observed complex structures. We established the first phase diagram summarizing the stacking-dependent gap openings of BLGs for a given field. We further identified high-density midgap states, localized on grain boundaries, even under a strong field, which can considerably reduce overall transport gap., Comment: 6page in two column format, 3 figures

Proper inclusion of van der Waals (vdW) interactions in theoretical simulations based on standard density functional theory (DFT) is crucial to describe the physics and chemistry of systems such as organic and layered materials. Many encouraging approaches have been proposed to combine vdW interactions with standard approximate DFT calculations. Despite many vdW studies, there is no consensus on the reliability of vdW methods. To help further development of vdW methods, we have assessed various vdW functionals through the calculation of structural prop- erties at equilibrium, such as lattice constants, bulk moduli, and cohesive energies, for bulk solids, including alkali, alkali-earth, and transition metals, with BCC, FCC, and diamond structures as the ground state structure. These results provide important information for the vdW-related materials research, which is essential for designing and optimizing materials systems for desired physical and chemical properties., Comment: 10 pages, 6 Figures, 3 Tables

Using density functional theory, we study physical properties of boron nitride nanotubes (BNNTs) with the substitutional carbon pair defect. We also consider the Stone-Wales (SW) rearrangement of the C-C pair defect in the BNNT. The formation energy of an SW defect of the carbon dimer is approximately 3.1 eV lower than that of the SW-transformed B-N pair in the undoped BNNT. The activation energies show that the SW defect in the C-doped BNNT may be experimentally observed with a higher probability than in the undoped BNNT. Finally, we discuss the localized states originating from the carbon pair impurities., Comment: 5 pages, 4 figures, to appear in Chemical Physics Letters

Using density functional theory calculations, we have investigated the interactions between hydrogen molecules and metalloporphyrins. A metal atom such as Ca or Ti is introduced for incorporation in the central N$_4$ cavity. Within local density approximation (generalized gradient approximation), we find that the average binding energy of H$_2$ to the Ca atom is about 0.25 (0.1) eV/H$_2$ up to four H$_2$ molecules, whereas that to the Ti atom is about 0.6 (0.3) eV per H$_2$ up to two H$_2$. Our analysis of orbital hybridization between the inserted metal atom and molecular hydrogen shows that H$_2$ binds weakly to Ca-porphyrin through a weak electric polarization in dihydrogen, but is strongly hybridized with Ti-porphyrin through the Kubas interaction. The presence of $d$ orbitals in Ti may explain the difference in the interaction types., Comment: 12 pages, 5 figures. Selected as a front cover of the Journal of Chemical Physics

We present a first-principles study of geometrical structure and energetics of hydrogen adsorbed on hexagonal single-walled silicon nanotubes (SiNTs). The adsorption behaviors of hydrogen molecules in SiNTs are investigated. The binding energies for the most stable physisorbed configurations are calculated to be less than 0.1 eV. The energy barriers are also investigated for dissociation of H2 molecules. Finally, we consider encapsulation of H2 molecules in SiNTs. The possibility of SiNTs as hydrogen storage materials is discussed.

We present a first-principle study of geometrical and electronic structure of hexagonal single-walled silicon nanotubes with a monovacancy or a substitutional defect. The C, Al or P atoms are chosen as substitutional impurities. It is found that the defect such as a monovacancy or a substitutional impurity results in deformation of the hexagonal single-walled silicon nanotube. In both cases, a relatively localized unoccupied state near the Fermi level occurs due to this local deformation. The difference in geometrical and electronic properties of different substitutional impurities is discussed., Comment: 5 figures

The bare and hydrogen-covered diamond (100) surfaces were investigated through pseudopotential density-functional calculations within the local-density approximation. Different coverages, ranging from one to two, were considered. These corresponded to different structures including 1x1, 2x1, and 3x1, and different hydrogen-carbon arrangements including monohydride, dihydride, and configurations in between. The formation energy of each phase was expressed as a function of hydrogen chemical potential. As the chemical potential increased, the stable phase successively changed from bare 2x1 to (2x1):H, to (3x1):1.33H, and finally to the canted (1x1):2H. Setting the chemical potential at the energy per hydrogen in H$_2$ and in a free atom gave the (3x1):1.33H and the canted (1x1):2H phase as the most stable one, respectively. However, after comparing with the formation energy of CH$_4$, only the (2x1):H and (3x1):1.33H phases were stable against spontaneous formation of CH$_4$. The former existed over a chemical potential range ten times larger than the latter, which may explain why the latter, despite of having a low energy, has not been observed so far. Finally, the vibrational energies of the C-H stretch mode were calculated for the (2x1):H phase., Comment: 8 pages, single uuencoded postscript file, submitted to Phys.Rev.B