Convergence of multi-valley bands as the electronic origin of high thermoelectric performance in CoSb3 skutterudites.

Filled skutterudites R(x)Co4Sb12 are excellent n-type thermoelectric materials owing to their high electronic mobility and high effective mass, combined with low thermal conductivity associated with the addition of filler atoms into the void site. The favourable electronic band structure in n-type CoSb3 is typically attributed to threefold degeneracy at the conduction band minimum accompanied by linear band behaviour at higher carrier concentrations, which is thought to be related to the increase in effective mass as the doping level increases. Using combined experimental and computational studies, we show instead that a secondary conduction band with 12 conducting carrier pockets (which converges with the primary band at high temperatures) is responsible for the extraordinary thermoelectric performance of n-type CoSb3 skutterudites.

Novel family of chiral-based topological insulators: elemental tellurium under strain.

Employing ab initio electronic structure calculations, we predict that trigonal tellurium consisting of weakly interacting helical chains undergoes a trivial insulator to strong topological insulator (metal) transition under shear (hydrostatic or uniaxial) strain. The transition is demonstrated by examining the strain evolution of the band structure, the topological Z2 invariant and the concomitant band inversion. The underlying mechanism is the depopulation of the lone-pair orbitals associated with the valence band via proper strain engineering.

Aviram-Ratner rectifying mechanism for DNA base-pair sequencing through graphene nanogaps.

We demonstrate that biological molecules such as Watson-Crick DNA base pairs can behave as biological Aviram-Ratner electrical rectifiers because of the spatial separation and weak hydrogen bonding between the nucleobases. We have performed a parallel computational implementation of the ab initio non-equilibrium Green's function (NEGF) theory to determine the electrical response of graphene--base-pair--graphene junctions. The results show an asymmetric (rectifying) current-voltage response for the cytosine-guanine base pair adsorbed on a graphene nanogap.

Electric-field control of magnetism in graphene quantum dots: Ab initio calculations.

Employing ab initio calculations we predict that the magnetic states of hydrogenated diamond-shaped zigzag graphene quantum dots (GQDs), each exhibiting unique electronic structure, can be selectively tuned with gate voltage, through Stark or hybridization electric-field modulation of the spatial distribution and energy of the spin-polarized molecular orbitals, leading to transitions between these states. Electrical read-out of the GQD magnetic state can be accomplished by exploiting the distinctive electrical properties of the various magnetic configurations.

Room-temperature high on/off ratio in suspended graphene nanoribbon field-effect transistors.

We have fabricated suspended few-layer (1-3 layers) graphene nanoribbon field-effect transistors from unzipped multi-wall carbon nanotubes. Electrical transport measurements show that current annealing effectively removes the impurities on the suspended graphene nanoribbons, uncovering the intrinsic ambipolar transfer characteristic of graphene. Further increasing the annealing current creates a narrow constriction in the ribbon, leading to the formation of a large bandgap and subsequent high on/off ratio (which can exceed 10(4)).

"Seamless" graphene interconnects for the prospect of all-carbon spin-polarized field-effect transistors.

Magnetism in graphene nanofragments arises from the spin polarization of the edge-states; consequently, as the material inexorably shrinks, magnetism will become a dominant feature whereas the bulk carrier mobility will be less relevant. We have carried out an ab initio study of the role of graphene-ultra-nanofragment magnetism on electronic transport. We present, as a proof-of-concept, a nanoscopic spin-polarized field-effect transistor (FET) with the channel and metallic contacts carved from a single graphene sheet.

Performance of multiplicity-based energy correctors for molecules containing second-row elements.

We introduce a posteriori multiplicity-based corrections to ab initio energies in order to reproduce experimental atomization energies. This simple approach, as compared to the alternative ones to improve density functionals and standard correlated methods, requires less computational resources than higher levels of theory. We extend our approach to include molecules containing second-row elements.

Energy correctors for accurate prediction of molecular energies.

Energy correctors are introduced for the calculation of molecular energies of compounds containing first row atoms (Li-F) to modify ab initio molecular orbital calculations of energies to better reproduce experimental results. Four additive correctors are introduced to compensate for the differences in the treatment of molecules with different spin multiplicities and multiplicative correctors are also calculated for the electronic and zero-point vibrational energies. These correctors, individually and collectively yield striking improvements in the atomization energies for several ab initio methods.

Clustering effects on discontinuous gold film NanoCells.

Reproducible negative differential resistance (NDR)-like switching behavior is observed in NanoCells. This behavior is attributed to the formation of filaments and clusters between the discontinuous gold films. Control experiments are performed by self-assembly of insulating molecules between the gold islands and conducting molecules on these islands.