Nanowire growth has generally relied on an initial particle of a catalyst such as Au to define the wire diameter and stabilize the growth. Self-catalyzed growth of III–V nanowires avoids the need for ...a foreign element, with the nanowire growing from the vapor via a droplet of the native group-III liquid. However, as suggested by Gibbs’ phase rule, the absence of third element has a destabilizing effect. Here we analyze this system theoretically, finding that growth can be dynamically stable at pressures far above the equilibrium vapor pressure. Steady-state growth occurs via kinetic self-regulation of the droplet volume and wire diameter. In particular, for a given temperature and source-gas pressures there is a unique stable wire diameter and droplet volume, both of which decrease with increasing V/III ratio. We also examine the evolution of the droplet size and wire diameter toward the steady state as the wire grows and discuss implications for structural control.
Conduction between graphene layers is suppressed by momentum conservation whenever the layer stacking has a rotation. Here we show that phonon scattering plays a crucial role in facilitating ...interlayer conduction. The resulting dependence on orientation is radically different than previously expected, and far more favorable for device applications. At low temperatures, we predict diode-like current-voltage characteristics due to a phonon bottleneck. Simple scaling relationships give a good description of the conductance as a function of temperature, doping, rotation angle, and bias voltage, reflecting the dominant role of the interlayer beating phonon mode.
Starting with a liquid eutectic droplet on a surface, we calculate its dynamical evolution into an epitaxial nanowire via the vapor-liquid-solid growth process. Our continuum approach incorporates ...kinetic effects and crystalline anisotropy in a natural way. Some realistic features appear automatically even for an isotropic solid, e.g., the tapered wire base. Crystal anisotropy leads to a richer variety of morphologies. For example, sixfold anisotropy leads to a wire shape having broken symmetry and an intriguing resemblance to the 110-oriented Si wires seen in Au-catalyzed growth on Si (111), while higher symmetry leads to a shape more like Si wires.
Nanowires are conventionally assumed to grow via the vapor-liquid-solid process, in which material from the vapor is incorporated into the growing nanowire via a liquid catalyst, commonly a ...low--melting point eutectic alloy. However, nanowires have been observed to grow below the eutectic temperature, and the state of the catalyst remains controversial. Using in situ microscopy, we showed that, for the classic Ge/Au system, nanowire growth can occur below the eutectic temperature with either liquid or solid catalysts at the same temperature. We found, unexpectedly, that the catalyst state depends on the growth pressure and thermal history. We suggest that these phenomena may be due to kinetic enrichment of the eutectic alloy composition and expect these results to be relevant for other nanowire systems.
During heteroepitaxy, misfit strain causes nanoscale islands to form spontaneously, as "self-assembled quantum dots." The growth and evolution of these islands are remarkably complex. We show that ...continuum modeling reproduces and explains many of the surprising phenomena observed experimentally. The free energy is reduced by both morphological change and alloy intermixing. However, because diffusion occurs only at the surface, the morphological and compositional evolution are strongly coupled. This leads to a complex dynamical response to the rather simple thermodynamic driving forces.
We calculate the radiative lifetime and energy bandstructure of excitons in semiconducting carbon nanotubes within a tight-binding approach including the electron−hole correlations via the ...Bethe-Salpeter equation. In the limit of rapid interband thermalization, the radiative decay rate is maximized at intermediate temperatures and decreases at low temperature because the lowest-energy excitons are optically forbidden. The intrinsic phonons cannot scatter excitons between optically active and forbidden bands, so sample-dependent extrinsic effects that break the symmetries can play a central role. We calculate the diameter-dependent energy splittings between singlet and triplet excitons of different symmetries and the resulting dependence of radiative lifetime on temperature and tube diameter.
The electrical properties of graphene depend sensitively on the substrate. For example, recent measurements of epitaxial graphene on SiC show resistance arising from steps on the substrate. Here we ...calculate the deformation of graphene at substrate steps, and the resulting electrical resistance, over a wide range of step heights. The elastic deformations contribute only a very small resistance at the step. However, for graphene on SiC(0001) there is strong substrate-induced doping, and this is substantially reduced on the lower side of the step where graphene pulls away from the substrate. The resulting resistance explains the experimental measurements.
We calculate the electron-phonon scattering and binding in semiconducting carbon nanotubes, within a tight-binding model. The mobility is derived using a multiband Boltzmann treatment. At high ...fields, the dominant scattering is interband scattering by LO phonons corresponding to the corners K of the graphene Brillouin zone. The drift velocity saturates at approximately half the graphene Fermi velocity. The calculated mobility as a function of temperature, electric field, and nanotube chirality are well reproduced by a simple interpolation formula. Polaronic binding give a band-gap renormalization of approximately 70 meV, an order of magnitude larger than expected. Coherence lengths can be quite long but are strongly energy dependent.
Low-frequency “1/f ” noise is a major issue for nanoscale devices such at carbon nanotube transistors. We show that nanoscale ballistic transistors give voltage-dependent sensitivity to the intrinsic ...potential fluctuations from nearby charge traps. A distinctive dependence on gate voltage is predicted, without reference to the number of carriers. This dependence is confirmed by comparison with recent measurements of nanotube transistors. Possible ways of decreasing the noise are discussed.
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