Infrared near-field microscopy with the Vanderbilt free electron laser: overview and perspectives

Scanning near-field optical microscopy (SNOM) makes it routinely possible to overcome the fundamental diffraction limit of standard (far-field) microscopy. Recently, aperture-based infrared SNOM performed in the spectroscopic mode, using the Vanderbilt University free electron laser, started delivering spatially-resolved information on the distribution of chemical species and on other laterally-fluctuating properties. The practical examples presented here show the great potential of this new technique both in materials science and in life sciences.     

Strain and high temperature superconductivity: unexpected results from direct electronic structure measurements in thin films

Angle-resolved photoemission spectroscopy reveals very surprising strain-induced effects on the electronic band dispersion of epitaxial La(2-x)Sr(x)CuO(4-delta) thin films. In strained films we measure a band that crosses the Fermi level (E(F)) well before the Brillouin zone boundary. This is in contrast to the flat band reported in unstrained single crystals and in our unstrained films, as well as in contrast to the band flattening predicted by band structure calculations for in-plane compressive strain. In spite of the density of states reduction near E(F), the critical temperature increases in strained films with respect to unstrained samples. These results require a radical departure from commonly accepted notions about strain effects on high temperature superconductors, with possible general repercussions on superconductivity theory.     

Electronic states of CdIn2S4 and of other II–III2–VI4 compounds: How sensitive are they to the crystal structure?

The valence band states of the spinel semiconductor CdIn₂S₄ have been studied by UV photoemission spectroscopy. Contrary to what structural considerations would suggest the measured spectra closely resemble those of defect-zincblend CdIn₂Se₄ and of other II–III₂–VI₄ compounds. The likelihood of structural effects in the electronic states of this family is discussed in light of the above results.     

A direct comparison between gas-phase and solid-state Hg 5d photoionization processes

We present an experimental study of the Hg 5d spin-orbit intensity branching ratio in the photon energy range 18–100 eV. The results obtained on cleaved α-HgS exhibit strong differences with respect to the recent gas-phase data by Shannon and Codling. This is the first time that $\text{experimental}$ results on gas-phase photoionization and $\text{experimental}$ results on solid-state photoionization of the same core levels are directly compared to each other. The evidence we provide for solid-state effects in the photoionization process is, therefore, independent of any particular theoretical interpretation of the data.     

Oxidation of silicon: New electron spectroscopy results

New X-ray photoemission spectroscopy data and high resolution electron scattering spectroscopy data are presented which indicate that a peroxide-like model of the oxygen chemisorption on silicon surface is correct. These results are discussed in light of a recent double-bonded oxygen atom model due to Ludeke and Koma.     

Samarium valence changes and reactive interdiffusion at the Si(111)-Sm interface

Synchrotron radiation photoemission studies show that the Si(111) 2×1-Sm interface has a complex morphology involving sequential formation stages with different Sm valence states. For metal coverages up to 2–3 Å, Sm atoms appear only in a divalent state. At higher coverages (up to 10–15 Å) the trivalent Sm configuration dominates and large chemical shifts of Si 2p and Sm 4f and 5p levels are observed. The Sm valence change is related to the onset of the reactive interdiffusion of Sm and Si.     

Fabrication of high‐aspect‐ratio Fresnel zone plates by e‐beam lithography and electroplating

The fabrication of gold Fresnel zone plates, by a combination of e‐beam lithography and electrodeposition, with a 30 nm outermost zone width and a 450 nm‐thick structure is described. The e‐beam lithography process was implemented with a careful evaluation of applied dosage, tests of different bake‐out temperatures and durations for the photoresist, and the use of a developer without methylisobutylketone. Electrodeposition with a pulsed current mode and with a specially designed apparatus produced the desired high‐aspect‐ratio nanostructures. The fabricated zone plates were examined by electron microscopy and their performances were assessed using a transmission X‐ray microscope. The results specifically demonstrated an image resolution of 40 nm.     

A solution of the core-exciton issue for silicon and germanium

The large discrepancies among the Si L_2,3 core-excitonic shifts measured by different techniques can be explained by the recently discovered surface shifts of the Si 2p level. New, accurate photoemission measurements of both the L_2,3 edge and of the 2p binding energy with equal surface sensitivity have been performed. Our present results and those of previous experiments are consistent with a single value 0.3^-00_+0.15 eV for the Si L_2,3 core excitonic shift. Preliminary results for the 3d core exciton in Ge give a shift of 0.35 ± 0.25 eV.     

High-resolution resonant photoemission study of the 4f states in a typical Kondo system: CePd3

We exploited resonant photoemission at the Ce absorption edge to investigate the Ce 4f states in . High resolution spectra reveal, near the Fermi level, the characteristic fine structure of intermediate valence Ce compounds. The spectral lineshape is consistent with the typical “Kondo” character of CePd, but the prominent ionization peak is found at the unusually low binding energy of 1 eV. We briefly discuss the implications of these observations. Received: 13 October 1997 / Accepted: 21 January 1998