Constantine Douketis

Synthesis and resonance Raman spectroscopy of CdS nano-wire arrays

Abstract Highly anisotropic arrays of aligned semiconductor nano-wires with diameters in the range 9–22 nm and lengths up to 1 μm were fabricated by electrodepositing CdS into the pores of anodic aluminum oxide. The value of the exciton energy of the semiconductor nano-wires was determined as a function of wire diameter from the excitation wavelength dependence of their polarized resonance Raman spectra in the vicinity of the CdS absorption edge. The resonance Raman (RR) spectra were dominated by a progression in the longitudinal optical phonon mode. The relative intensities of the members of the progression are both excitation-wavelength and particle-size dependent. The LO phonon frequency was also found to be wire-diameter dependent. The RR spectra were found to be almost independent of the polarization of the light exciting them. The optical exciton energy was determined to vary from 2.36 eV for the largest diameter wires to 2.42 eV for the smallest. The absence of polarization dependence is interpreted to be in part due to the fact that the nano-wires consist of strings of semiconductor crystallites so that the local aspect ratio of the crystallites is both not very great and not uniform in the axial direction. For the smallest nano-wires the radial size uniformity is, however, very high. The dependence of the band-gap energy on the diameter of the nano-wires is in good agreement with theory.

Self-affine silver films and surface-enhanced Raman scattering: Linking spectroscopy to morphology

The relationship between the self-affine structure of cold-deposited films and the surface-enhanced Raman (SERS) intensity of benzene adsorbed on the films is examined. Based on variable temperature STM studies the structure of cold-deposited silver films is shown to be self-affine with a fractal dimension ∼2.6, more or less independent of temperature for T less than ∼270 K. The fractal structure is shown to collapse to a more or less compact structure when the films are annealed to ∼280 K. SERS activity ceases at a somewhat lower temperature (∼250 K) for all the films examined. SERS enhancements rise by factors as great as 8 as a function of annealing temperature over and above their values at the lowest deposition temperatures used (24 K). The rise reaches a maximum at a temperature that depends both on the deposition temperature of the films and the excitation wavelength. (Such an annealing effect on the SERS intensity has been known for many years.) We suggest that the observations are consistent with...

Rough silver films studied by surface enhanced raman spectroscopy and low temperature scanning tunnelling microscopy

The surface topography of Ag films and surface enhanced Raman scattering (SERS) from benzene on Ag films have been simultaneously recorded. The Ag films were formed by vacuum deposition at temperatures ranging from 100 K to 500 K. Analysis of scanning tunnelling microscopy (STM) images shows that films formed below 250 K are fractal structures with Hausdorff-Besicovitch dimension 2.55 < D < 2.72, while for those formed above 250 K, D≈2. The lower temperature, rough films exhibit strong surface enhanced Raman scattering but the higher temperature, smooth films do not. We consider the consequences of fractal character and the possible correlation between this and the SERS activity of these films.

Fractal character and direct and indirect transitions in photoemission from silver films

We have examined the linear and nonlinear photoelectric response as well as low-temperature scanning tunnelling microscope (STM) images of rough (coldly deposited) and smooth (annealed) Ag films. Our one-photon and two-photon optical excitation wavelengths were chosen to induce photoemission with common total photon energy, polarization and light penetration depth. Direct optical transitions dominate one-photon and two-photon photoemission from smooth films and one-photon photoemission from rough films; this leads to energy distribution curves that are adequately modelled using well established theory. In contrast, photoelectron emission from rough films induced by two-photon absorption is mediated by the excitation of localized surface-plasmon (LSP) modes centred on roughness features. This photoelectric response is strongly enhanced and it involves indirect, momentum nonconserving transitions. STM images of rough films show an apparently fractal arrangement of structures which support the plasmon activity.

Toward a universal extinction spectrum of self-affine silver colloid clusters: Experiment and simulation

Polarized extinction spectra of large fractal aggregates of colloidal silver deposited gravitationally onto a quartz substrate were measured by transmission as a function of angle of incidence. The spectral components tangential and normal to the average plane of the clusters compacted along the direction of deposition were extracted from these and compared with spectra calculated from simulated fractal aggregates that were projected onto a plane. The two sets agree acceptably, suggesting that the normal and tangential spectral components represent “universal,” i.e., size-independent extinction spectra characterizing all (compacted) cluster–cluster aggregates of colloidal silver.

Direct and roughness-induced indirect transitions in photoemission from silver films

Abstract One-photon and two-photon electron emission spectra from vapour deposited rough and smooth Ag films are compared. Photoemission from rough films is strongly enhanced due to the participation of localized surface plasmons (LSP). A theory is outlined that successfully estimates the enhancement and predicts that LSP-initiated photoemission would involve momentum non-conserving transitions.

Light-induced drift of electrons in metals

Abstract Photoexcitation of electrons in solids is, in general, velocity selective. It is accompanied by a net current provided that the electron mobilities differ for excited and ground states. This effect considered earlier for intersubband transitions in semiconductors is extended to metals. Photoinduced currents in the range 0.1–100 μA are predicted for intensities of order 1 MW cm 2 . Experimental evidence for this effect is presented in the form of spatially asymmetric photoemission from rough silver films.

The role of localized surface plasmons in photoemission from silver films: Direct and indirect transition channels

Abstract We have examined electron photoemission spectra and scanning tunnelling microscopy images from rough (coldly deposited) and smooth (annealed) Ag films. Photoemission is induced by 1-photon and 2-photon processes under conditions of common total photon energy, polarization and light penetration depth. For smooth films we show that photoemission involves direct optical transitions and that the 2-photon excitation is a simultaneous absorption rather than a 2-step process. On rough films the 2-photon process is mediated by the excitation of localized surface plasmon modes, it is strongly enhanced, and it involves indirect, momentum non-conserving transitions. One-photon excitation on rough films is not induced by localized surface plasmons since the frequency of the light falls outside their resonance band. Low temperature scanning tunnelling microscope images of rough films directly show the roughness features on which localized surface plasmon activity is centred.