Dielectric Properties of Nanoconfined Water: A Canonical Thermopotentiostat Approach.

We introduce a novel approach to sample the canonical ensemble at constant temperature and applied electric potential. Our approach can be straightforwardly implemented into any density-functional theory code. Using thermopotentiostat molecular dynamics simulations allows us to compute the dielectric constant of nanoconfined water without any assumptions for the dielectric volume. Compared to the commonly used approach of calculating dielectric properties from polarization fluctuations, our thermopotentiostat technique reduces the required computational time by 2 orders of magnitude.

Optically excited structural transition in atomic wires on surfaces at the quantum limit.

Transient control over the atomic potential-energy landscapes of solids could lead to new states of matter and to quantum control of nuclear motion on the timescale of lattice vibrations. Recently developed ultrafast time-resolved diffraction techniques combine ultrafast temporal manipulation with atomic-scale spatial resolution and femtosecond temporal resolution. These advances have enabled investigations of photo-induced structural changes in bulk solids that often occur on timescales as short as a few hundred femtoseconds. In contrast, experiments at surfaces and on single atomic layers such as graphene report timescales of structural changes that are orders of magnitude longer. This raises the question of whether the structural response of low-dimensional materials to femtosecond laser excitation is, in general, limited. Here we show that a photo-induced transition from the low- to high-symmetry state of a charge density wave in atomic indium (In) wires supported by a silicon (Si) surface takes place within 350 femtoseconds. The optical excitation breaks and creates In-In bonds, leading to the non-thermal excitation of soft phonon modes, and drives the structural transition in the limit of critically damped nuclear motion through coupling of these soft phonon modes to a manifold of surface and interface phonons that arise from the symmetry breaking at the silicon surface. This finding demonstrates that carefully tuned electronic excitations can create non-equilibrium potential energy surfaces that drive structural dynamics at interfaces in the quantum limit (that is, in a regime in which the nuclear motion is directed and deterministic). This technique could potentially be used to tune the dynamic response of a solid to optical excitation, and has widespread potential application, for example in ultrafast detectors.

High-pressure core structures of Si nanoparticles for solar energy conversion.

We present density functional and many body perturbation theory calculations of the electronic, optical, and impact ionization properties of Si nanoparticles (NPs) with core structures based on high-pressure bulk Si phases. Si particles with a BC8 core structure exhibit significantly lower optical gaps and multiple exciton generation (MEG) thresholds, and an order of magnitude higher MEG rate than diamondlike ones of the same size. Several mechanisms are discussed to further reduce the gap, including surface reconstruction and chemistry, excitonic effects, and embedding pressure. Experiments reported the formation of BC8 NPs embedded in amorphous Si and in amorphous regions of femtosecond-laser doped "black silicon." For all these reasons, BC8 nanoparticles may be promising candidates for MEG-based solar energy conversion.

Entropy explains metal-insulator transition of the Si(111)-In nanowire array.

Density functional theory calculations are performed to determine the mechanism and origin of the intensively debated (4 x 1)-(8 x 2) phase transition of the Si(111)-In nanowire array. The calculations (i) show the existence of soft phonon modes that transform the nanowire structure between the metallic In zigzag chains of the room-temperature phase and the insulating In hexagons formed at low temperature and (ii) demonstrate that the subtle balance between the energy lowering due to the hexagon formation and the larger vibrational entropy of the zigzag chains causes the phase transition.

Structure of si(111)-in nanowires determined from the midinfrared optical response.

The anisotropic optical response of Si(111)-(4x1)/(8x2)-In in the midinfrared, where ab initio studies predict significant changes in the band structure between competing models of this important quasi-1D system, has been measured using infrared spectroscopic ellipsometry (IRSE) and reflection anisotropy spectroscopy (RAS). Both IRSE and RAS of the (8x2) phase show that the anisotropic Drude tail of the (4x1) phase is replaced by two peaks at 0.50 and 0.72 eV, which appear in ab initio optical response calculations for the hexagon model of the (8x2) structure, but not the trimer model.

Adatom-induced conductance modification of in nanowires: potential-well scattering and structural effects.

First-principles calculations on the influence of adatoms (In, Pb, H, O) on the Landauer conductance of Si substrate-supported atomic-scale In nanowires are performed. Despite the increase of the total (and partially even local) density of states at the Fermi level due to the adsorption, all adatom species lower the nanowire conductance. Apart from hydrogen, which barely changes the transport properties, the conductance drop is pronounced, ranging from 17% for Pb to 38% for In. It is related to potential-well scattering and/or structural deformations of the nanowires.

Hexagon versus trimer formation in in nanowires on Si(111): energetics and quantum conductance.

The structural and electronic properties of the quasi-one-dimensional In/Si(111) surface system are calculated from first principles. It is found that the symmetry lowering of the In chains is energetically favorable, provided neighboring nanowires are correlated, giving rise to a doubling of the surface unit cell both along and perpendicular to the chain direction. The recently suggested formation of hexagons within the In nanowires [C. González, F. Flores, and J. Ortega, Phys. Rev. Lett. 96, 136101 (2006)]--in clear contrast to the trimer formation proposed earlier-drastically modifies the electron transport along the In chains, in agreement with experiment.