Using image resonances to probe molecular conduction at the n-heptane/Au(111) interface

The binding energies and lifetimes of the n=1 image resonance on Au(111) are measured as a function of n-heptane layer thickness by femtosecond time-resolved two-photon photoemission (TR-2PPE) spectroscopy. The lifetime of the image resonance dramatically increases from approximately 4 fs on clean Au(111) to 1.6 ps with three layers of n-heptane. Because the image resonance is above the L1 band edge of Au, this increase in lifetime is attributed to the tunneling barrier presented by the sigma-sigma* band gap of the n-heptane film. We use the one-dimensional dielectric continuum model (DCM) to approximate the surface potential and to determine the binding energies and the lifetimes of the image resonances. The exact solution of the DCM potential is determined in two ways: the first by wave-packet propagation and the second by using a tight-binding Green's function approach. The first approach allows band-edge effects to be treated. The latter approach is particularly useful in illustrating the similarity between TR-2PPE and conductance measurements.     

Spanning lengths of percolation clusters

The spanning length of a percolation cluster is defined as the difference between the maximum and minimum coordinates of the cluster with respect to some chosen direction. It is statistically related to the number size of the cluster by an exponent that differs from the iriverse dimension that would characterize a compact cluster. This exponent for large percolation clusters in simple cubic lattice sites was studied by the Monte Carlo technique, and results are presented. Previous theoretical treatments of this exponent and its relationship with other critical exponents are discussed.In the present paper we shall refer exclusively to the site percolation problem, and all our definitions will be within that context.     

Electrical and structural properties of annealed AsTe and AsTeI semiconducting glasses: surface and bulk effects

The effects of thermal annealing, above the glass transition temperature, on the electrical conductivity and structural properties of bulk As₅₀Te_50?xI_x (x = 0, 2, 5, 20) glasses have been studied. Upon devitrification, an electrically conductive layer is formed on the sample surface The conductivity of the surface layer is nearly temperature independent and has an activation energy of ≈10^?2 eV. X-ray diffraction experiments show that an fcc crystalline phase is associated with the surface layer. Helium ion backscattering experiments indicate that on a sample with a surface layer the average atomic composition does not change significantly with depth (probed to a depth of ≈2 μm) and is approximately the same as that of the as-prepared material. Results suggest that the conductive regions may be composed of a matrix of interconnected crystallites (fcc phase) with interdispersed non-crystalline material. As annealing progresses, crystallization extends throughout the bulk material with a corresponding increase in sample conductance. The electrical changes are considerably more pronounced in the x = 0 and x = 2 compositions. The degree of crystallization and the crystalline phases induced in the bulk material by annealing were determined using differential scanning calorimetry (DSC) and X-ray diffraction. These experiments show that following the anneal series (130, 155, 170, and 190°C; each for 5 in vacuum) the more conductive (x = 0 and 2) samples contain monoclinic As₂Te₃, whereas the fcc phase is observed in the x = 5 and 20 samples. Results also suggest that the first crystallization exotherm, T_X1, observed in the DSC data is associated with the formation of the fcc phase, whereas the second crystallization exotherm, T_X2, is associated with the conversion of the fcc phase to the high-temperature stable monoclinic As₂Te₃ phase. From all of these results a model is formulated for the amorphous-to-crystalline transformation and its effects on electrical transport in these semiconducting alloys.