D. Davidović

Patterned growth of vertically aligned ZnO nanowire arrays on inorganic substrates at low temperature without catalyst.

We report an approach for growing aligned ZnO nanowire arrays with a high degree control over size, orientation, dimensionality, uniformity, and possibly shape. Our method combines e-beam lithography and a low temperature hydrothermal method to achieve patterned and aligned growth of ZnO NWs at <100degreesC on general inorganic substrates, such as Si and GaN, without using catalyst. This approach opens up the possibility of applying ZnO nanowires as sensor arrays, piezoelectric antenna arrays, two-dimensional photonic crystals, IC interconnects, and nanogenerators.

Coulomb blockade and discrete energy levels in Au nanoparticles

We describe a fabrication technique in which a nanoscale Au grain is connected to two Cr electrodes. Electron transport in our samples can be fitted to orthodox theory of single electron tunneling on a single particle. In one sample, we can resolve additional steps in current, which are interpreted as discrete quantum states of electrons added to the grain.

Spectroscopy, Interactions, and Level Splittings in Au Nanoparticles

We have measured the electronic energy spectra of nm-scale Au particles using a new tunneling spectroscopy configuration. The particle diameters ranged from 5nm to 9nm, and at low energies the spectrum is discrete, as expected by the electron-in-a-box model. The density of tunneling resonances increases rapidly with energy, and at higher energies the resonances overlap forming broad resonances. Near the Thouless energy, the broad resonances merge into a continuum. The tunneling resonances display Zeeman splitting in a magnetic field. Surprisingly, the g-factors (~0.3) of energy levels in Au nano-particles are much smaller than the g-factor (2.1) in bulk gold.

Transport in graphene tunnel junctions

We present a technique to fabricate tunnel junctions between graphene and Al and Cu, with a Si backgate, as well as a simple theory of tunneling between a metal and graphene. We map the differential conductance of our junctions versus probe and backgate voltage, and observe fluctuations in the conductance that are directly related to the graphene density of states. The conventional strong-suppression of the conductance at the graphene Dirac point cannot be clearly demonstrated, but a more robust signature of the Dirac point is found: the inflection in the conductance map caused by the electrostatic gating of graphene by the tunnel probe. We present numerical simulations of our conductance maps, confirming the measurement results. In addition, Al causes strong n doping of graphene, Cu causes a moderate p doping, and in high resistance junctions, phonon resonances are observed, as in STM studies.

Spin-Polarized Electron Transport through Nanometer-Scale Al Grains

We investigate spin-polarized electron tunneling through ensembles of nanometer-scale Al grains embedded between two Co reservoirs at 4.2 K, and observe tunneling-magnetoresistance (TMR) and effects from spin precession in the perpendicular applied magnetic field (the Hanle effect). The spin-coherence time

Nanometer-scale metallic grains connected with atomic-scale conductors

({T}_{2}^{\ensuremath{\star}})

Proximity and coupling effects in superconductor/ferromagnet multilayers (invited)

measured using the Hanle effect is of order ns. The dephasing is attributed to electron spin precession in local magnetic fields. Dephasing process does not destroy TMR, which is strongly asymmetric with bias voltage. The asymmetric TMR is explained by spin relaxation in the Al grains and asymmetric electron dwell times.

Saturation of spin-polarized current in nanometer scale aluminum grains

We describe a technique for connecting a nanometer-scale gold grain to leads by atomic-scale gold point contacts. These devices differ from previous metallic quantum dots in that the conducting channels are relatively well transmitting. We investigate the dependence of the Coulomb blockade on contact resistance. The high-resistance devices display Coulomb blockade and the low-resistance devices display a zero-bias conductance dip, both in quantitative agreement with theory. We find that in the intermediate regime, where the sample resistance is close to h/e2, the I–V curve displays a Coulomb staircase with symmetric contact capacitances.

Tunneling Through Metallic Quantum Dots

New features in superconductor/ferromagnet multilayers have been observed. A nonmonotonic dependence of the superconducting Tc on the ferromagnetic layer thickness, due to the coupling across a thin magnetic layer, has been observed in both Nb/Gd multilayers and trilayers. The results are consistent with those of the predicted π-phase coupling. Effects on superconductivity due to insulating ferromagnetic layers have been observed in NbN/GdN multilayers, where the main pair-breaking effect is that of the ferromagnetic walls. The insulating ferromagnetic layers also give rise to very large superconducting critical fields.

Fine structure in the energy spectra of ultrasmall Au nanoparticles

We describe measurements of spin-polarized tunnelling via discrete energy levels of single Aluminum grains. In high resistance samples (