J. Lüning

Soft X-ray spectroscopy of single sized CdS nanocrystals: size confinement and electronic structure

Abstract Soft X-ray spectroscopy of CdS nanocrystals within their crystalline superlattice allows to relate unambiguously, changes of the electronic structure with the particle size and shape which is known from single crystal X-ray diffraction. We find that the valence and conduction band edge shift contribute to the same extent to the total band gap opening of about 2xa0eV with respect to CdS bulk as the particle size decreases to 10xa0A. Taking a finite height of the potential walls into account, we can reproduce these results within an effective mass approximation model.

Quantum confinement effects in the soft X-ray fluorescence spectra of porous silicon nanostructures

Abstract The electronic structure of porous Si is investigated using soft x-ray emission and absorption spectroscopies. Porous Si prepared under different wavelengths of illumination was studied along with porous Si that was purposely oxidized. In the x-ray fluorescence measurements we are able to selectively probe the silicon core of oxidized nanostructures by suitable choice of the excitation wavelength. The observed changes in the valence and conduction bands are consistent with electronic structure changes associated with quantum confinement in silicon nanostructures and are similar for the oxidized and unoxidized porous silicon.

Resonant inelastic soft X-ray scattering at the Si L3 edge: experiment and theory

Abstract We present experimental data on the resonant inelastic soft X-ray scattering at the Si L 3 edge in crystalline silicon. The experimental results are compared with scattering calculations based on LCAO and KKR band-structure calculations. We discuss the origin of the spectral features including the role of Si d -states in the scattering.

Dichroism and spin information in soft X-ray emission

Abstract We demonstrate that soft X-ray emission excited at the L 3 threshold of Fe metal maps the spin-resolved local partial density of occupied states. In contrast to earlier predictions, the spin resolution is achieved primarily by selective excitation. Circular magnetic dichroism in the X-ray emission process is also shown to have an influence on the final state spin contribution to the spectra. The importance of taking reflection, self-absorption of the incoming and outgoing radiation and the Auger decay channels into consideration in the data analysis is emphasized.

Core level photoionization—decay coincidence spectroscopy of free molecules

Abstract Coincidence measurements of the outgoing particles in resonant soft X-ray scattering on molecules are presented. In a simplifying picture the first method measures the photoelectron and the soft X-ray photon, whereas the second method measures the photoelectron and the Auger electron in coincidence. In the first experiment the zero-kinetic energy photoelectrons are detected in a time-of-flight electron spectrometer, and photons are collected in a large solid angle by a detector situated close to the interaction region. The spectrum of N 2 shows an adiabatic 1 s line, free from electron-electron post collision interaction effects, together with structures below the ionization limit, associated with Rydberg electron shake-off during the radiative decay. Above the ionization limit the latter process is no longer possible, resulting in a drop at threshold for the total intensity. For the O 2 molecule we find an anomalous quartet/doublet intensity ratio, which we tentatively ascribe to a resonance just below the 4 Σ − threshold. The O 2 data also reveal an inconsistency in the literature values regarding the calibration of the X-ray photoelectron and the soft X-ray absorption energy scales. In the second experiment two cylindrical mirror analyzers are used to measure electron-electron coincidences. The interpretation of several features in the conventional Auger spectrum follows directly from the results, e.g. the decay of the pure 1 s hole almost is entirely contained in the main structures, the additional structure and the spread-out low-energy background being associated with the decay of multiply excited states. Fine structure in the decay of the singlet coupled 1 s −1 π u −1 π g shake-up state is well reproduced by calculations from the literature. The methods open up new perspectives on core hole excitation-emission dynamics.

Soft X-ray emission of porous silicon nanostructures

Abstract The electronic structure of Si nanostructures in porous Si and purposely oxidized porous Si is investigated using soft x-ray emission spectroscopy. Significant changes as compared to bulk Si are observed. We interpret the spectral changes as due to an altered electronic structure in the Si nanostructures. By imposing standing wave boundary conditions to the valence band wavefunctions we calculate the emission spectrum for thin Si sheets, in good agreement with the experimental spectra.