Theoretical Study of Carbon Clusters in Silicon Carbide Nanowires

Using first-principles methods we performed a theoretical study of carbon clusters in silicon carbide nanowires. We examined small clusters with carbon interstitials and antisites in hydrogen-passivated SiC nanowires growth along the [100] and [111] directions. The formation energies of these clusters were calculated as a function of the carbon concentration. We verified that the energetic stability of the carbon defects in SiC nanowires depends strongly on the composition of the nanowire surface: the energetically most favorable configuration in carbon-coated [100] SiC nanowire is not expected to occur in silicon-coated [100] SiC nanowire. The binding energies of some aggregates were also obtained, and they indicate that the formation of carbon clusters in SiC nanowires is energetically favored.Comment: 6 pages, 5 figures; 8 pages, http://www.hindawi.com/journals/jnt/2011/203423

Intrinsic Magnetism in Nanosheets of SnO2_{2}: A First-principles Study

We propose intrinsic magnetism in nanosheets of SnO$_{2}$, based on first-principles calculations. The electronic structure and spin density reveal that $p$ orbitals of the oxygen atoms, surrounding Sn vacancies, have a non itinerant nature which gives birth to localized magnetism. A giant decrease in defect formation energies of Sn vacancies in nanosheets is observed. We, therefore, believe that native defects can be stabilized without any chemical doping. Nanosheets of different thicknesses are also studied, and it is found that it is easier to create vacancies, which are magnetic, at the surface of the sheets. SnO$_{2}$ nanosheets can, therefore, open new opportunities in the field of spintronics.Comment: J. Magn. Magn. Mate. 2012 (Accepted

Exact and approximate relations for the spin-dependence of the exchange energy in high magnetic fields

The exchange energy of an arbitrary collinear-spin many-body system in an external magnetic field is a functional of the spin-resolved charge and current densities, $E_x[n_{\uparrow},n_{\downarrow},j_{\uparrow},j_{\downarrow}]$. Within the framework of density-functional theory (DFT), we show that the dependence of this functional on the four densities can be fully reconstructed from either of two extreme limits: a fully polarized system or a completely unpolarized system. Reconstruction from the limit of an unpolarized system yields a generalization of the Oliver-Perdew spin scaling relations from spin-DFT to current-DFT. Reconstruction from the limit of a fully polarized system is used to derive the high-field form of the local-spin-density approximation to current-DFT and to magnetic-field DFT.Comment: Int. J. Mod Phys. B, accepted, 2008 (Proceedings of the 18th International Conference on High Magnetic Fields in Semiconductor Physics and Nanotechnology). 5 page

Interaction between pentacene molecules and monolayer transition metal dichalcogenides

Using first-principles calculations based on density-functional theory, we investigated the adsorption of pentacene molecules on monolayer two-dimensional transition metal dichalcogenides (TMD). We considered the four most popular TMDs, namely, MoS$_2$, MoSe$_2$, WS$_2$ and WSe$_2$, and we examined the structural and electronic properties of pentacene/TMD systems. We discuss how monolayer pentacene interacts with the TMDs, and how this interaction affects the charge transfer and work function of the heterostructure. We also analyse the type of band alignment formed in the heterostructure and how it is affected by molecule-molecule and molecule-substrate interactions. Such analysis is valuable since pentacene/TMD heterostructures are considered to be promising for application in flexible, thin and lightweight photovoltaics and photodetectors.Comment: 13 pages, 4 figure

Contribution of the second Landau level to the exchange energy of the three-dimensional electron gas in a high magnetic field

We derive a closed analytical expression for the exchange energy of the three-dimensional interacting electron gas in strong magnetic fields, which goes beyond the quantum limit (L=0) by explicitly including the effect of the second, L=1, Landau level and arbitrary spin polarization. The inclusion of the L=1 level brings the fields to which the formula applies closer to the laboratory range, as compared to previous expressions, valid only for L=0 and complete spin polarization. We identify, and explain, two distinct regimes, separated by a critical density $n_c$. Below $n_c$, the per-particle exchange energy is lowered by the contribution of L=1, whereas above $n_c$ it is increased. As special cases of our general equation we recover various known, more limited, results for higher fields, and identify and correct a few inconsistencies in some of these earlier expressions.Comment: 7 pages, 2 figures, PRB accepte

Indium coverage of the Si(111)- 7×3 -in surface

The indium coverage of the Si(111)-√7 × √3-In surface is investigated by means of x-ray photoelectron spectroscopy and first-principles density functional theory calculations. Both experimental and theoretical results indicate that the In coverage is a double layer rather than a single layer. Moreover, the atomic structure of the Si(111)-√7 × √3-In surface is discussed by comparing experimental with simulated scanning tunneling microscopy (STM) images and scanning tunneling spectra with the calculated density of states. Our structural assignment agrees with previous studies, except for the interpretation of experimental STM images

Surface structural phase transition induced by the formation of metal-organic networks on Si(111)- √7×√3-In surface

We studied the adsorption of 7,7,8,8-tetracyanoquinodimethane (TCNQ) on the Si(111)- Image ID:c9nr07074e-t3.gif-In surface, a known surface superconductor. Scanning tunneling microscopy shows the development of a surface-confined metal–organic network (SMON) where TCNQ molecules coordinate with indium atoms from the underlying Image ID:c9nr07074e-t4.gif reconstruction. The formation of the SMON causes a surface structural phase transition from the Image ID:c9nr07074e-t5.gif reconstruction to a previously unknown 5 × 5 reconstruction of the Si(111)–In surface. Scanning tunneling spectroscopy measurements indicate that the 5 × 5 reconstruction has a stronger insulating character than the Image ID:c9nr07074e-t6.gif reconstruction. Density-functional-theory calculations are used to evaluate the atomic arrangement and stability of the 5 × 5 and Image ID:c9nr07074e-t7.gif reconstructions as a function of In coverage, and suggest that the structural phase transition is driven by a slight reduction of the In coverage, caused by the incorporation of indium atoms into the SMON

Indium coverage of the Si(111)- 7×3 -In surface

The indium coverage of the Si(111)-√7×√3-In surface is investigated by means of x-ray photoelectron spectroscopy and first-principles density functional theory calculations. Both experimental and theoretical results indicate that the In coverage is a double layer rather than a single layer. Moreover, the atomic structure of the Si(111)-√7×√3-In surface is discussed by comparing experimental with simulated scanning tunneling microscopy (STM) images and scanning tunneling spectra with the calculated density of states. Our structural assignment agrees with previous studies, except for the interpretation of experimental STM images