Excited Surface States Research Articles

Relaxation of excited surface states of thin Ge-implanted silica films probed by OSEE spectroscopy

As an example of thin silica films, 30nm SiO2–Si heterostructures implanted with Ge+ ions (1016cm−2 fluence) and rapid thermally annealed (RTA) at 950°C are studied by means of optically stimulated electron emission (OSEE) in the spectral region of optical transparency for bulk silica. Quartz glass samples were used as references. Experimental data revealed a strong dependence between electron emission spectral features and RTA annealing time. The spectral contributions of both surface band tail states and interband transitions were clearly distinguished. The application of emission Urbach rule as well as Kane and Pässler equations allowed to analyze the OSEE spectra at different optical excitation energy ranges and to retrieve the important microstructural and energy parameters. The observed correlations between parameter values of Urbach- and Kane-related models suggest the implantation-induced conversion of both the vibrational subsystem and energy band of surface and interface electronic states.

Photoluminescence quenching in compressed MgO nanoparticle systems

Efficient use of highly dispersed metal oxides for lighting, energy conversion and catalysis requires knowledge about the impact of density and microstructure of the powders on the optical nanoparticle properties. For MgO nanocube powders we present a combined photoluminescence (PL) and electron paramagnetic resonance (EPR) approach which enables for samples of different aggregation states the quantification of the fractional powder volume that becomes illuminated with UV and visible light during the PL measurements. Using O2 as a PL emission quencher and - after light induced exciton separation and oxygen adsorption - as an EPR active adsorbate we observed clear aggregation dependent trends in PL emission quenching that originate from particle-particle contacts. Upon interaction of low coordinated surface elements with the surfaces of adjacent MgO nanocubes, which occurs even at powder consolidation levels that escape sorption analysis, the radiative decay of excited surface states becomes quenched down to 15% of the original intensity. Our results underline the critical role of microstructure and the aggregation state of a nanoparticle ensemble with respect to spectroscopic properties and related adsorption induced changes.

Communication: Momentum-resolved quantum interference in optically excited surface states

Surface states play essential roles in condensed matter physics, e.g., as model two-dimensional (2D) electron gases and as the basis for topological insulators. Here, we demonstrate quantum interference in the optical excitation of 2D surface states using the model system of C(60)/Au(111). These surface states are transiently populated and probed in a femtosecond time- and angle-resolved two-photon photoemission experiment. We observe quantum interference within the excited populations of these surface states as a function of parallel momentum vector. Such quantum interference in momentum space may allow one to control 2D transport properties by optical fields.

Photoelectron emission from implanted SiO2: Se+ films

Silica films implanted with selenium ions (330 keV, 5 × 1016 cm−2) were studied. The method of optically stimulated electron emission (OSEE) was used to analyze features specific to disordering of the film structure. Spectrum dependences of the OSEE are described in terms of the formalism of Urbach’s emission rule. It was found that the discrete-continuum disturbances arising upon implantation considerably distort the spectrum of the electronic and vibrational states of the SiO2 matrix. Results of the experiment and the numerical simulation provide evidence for the activation tunneling of electrons from optically excited surface states of SiO2 to a vacuum.

Ultrafast exciton relaxation dynamics in silicon quantum dots

AbstractThe photoluminescence properties of silicon quantum dots may be tailored by surface states via efficient coupling to resonant bulk states. Therefore various wet‐chemistry procedures were developed to fabricate silicon quantum dots with adjustable sizes and surface properties. While the energy gap of the Si core is tuned by the size, resonant electronic surface states may be attained by varying the structure and chemical composition of the grafting. The strength of electronic couplings between surface and bulk states determines the timescale of both photo‐induced electron transfer from excited surface states to conduction band states and capture of conduction band electrons by surface states. Whereas the origin of photoluminescence and therewith the quantum dot size distribution may be elucidated by stationary luminescence spectroscopy, ultrafast optical spectroscopy techniques, for instance the femtosecond transient absorption spectroscopy, allow to monitor photo‐induced electron transfer between surface and bulk states and carrier trapping on the subpicosecond timescale.

Electron–phonon coupling and lifetimes of excited surface states

Many important chemical and physical phenomena are influenced by inherent dissipative processes, which involve energy transfer between the electrons (electron–electron scattering) and between the electrons and the ionic motion (electron–phonon scattering). The non-adiabatic interaction between the valence electrons and the ion motion in a solid reveals the break down of the Born–Oppenheimer approximation. To pin down the influence of the electron and phonon structure on these scattering processes the two-dimensional surface states are ideal both from an experimental and theoretical point of view. Several experimental techniques presently in use are able to give information about the lifetime of an excited electron or hole in the surface state band. With help from advanced theoretical calculations it is possible to sort out the relative importance of the electron–electron and electron–phonon scattering processes responsible for the quenching of the excitation and to point out the key parameters of the electron and phonon structure.

Second-Harmonic Generation Spectroscopy on Thin Metal Films on Si(111)

Optical second-harmonic generation spectra have been recorded for various reconstructions of Si(111) surfaces as well as for thin metal films grown on these surfaces. Excitation of surface states gives rise to broad second harmonic spectra in the energy region below the Si direct band gap. Variations in the spectra with sample temperature and oxygen exposure have been used to identify resonances. Deposition of metal films leads to decay of surface state resonances and appearance of new resonances. In cases where the metal films grow epitaxially with a roughness of the order of a few atomic layers within the probed area, excitations of quantum well states can be observed. Quantization effects in second-harmonic generation are observed as oscillations in the signal as a function of film thickness with an oscillation period that depends on the pump frequency of the laser light.

Second‐harmonic generation spectroscopy on reconstructed Si(111) surfaces

Optical second harmonic generation spectra obtained from the clean 7 × 7 surface are compared to those of surfaces with Au and Ag induced (√3 × √3)-reconstructions. All three reconstructions lead to broad spectra in the region below the bulk direct interband transition energy due to excitation of surface states. Variations in the spectra with sample temperature and oxygen exposure have been used to identify resonances in the spectra. In this way several transitions consistent with energy separations between surface states known from photoemission have been identified. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Electrostatic response and surface electron states of a Ca(001) face-centered cubic film

The electron response of a Ca(001) face-centered cubic film to an external electrostatic field is calculated. The results of calculations are compared with the previously obtained data on the electron response of a Cu(001) film. The energy location of occupied and unoccupied excited surface states of the Ca(001) film is determined.

Dynamics of highly excited charge carriers in CdS nanocrystallites embedded into ZrO 2 films

Transient grating experiments were performed at room temperature in CdS nanocrystallites (nc) embedded into ZrO 2 sol–gel films. The dynamics of photo-generated carrier showed to depend on the excitation intensity. Relaxation of non-equilibrium charge carriers take place through two channels: electron–hole recombination in nanocrystallite volume and nanocrystallite/ZrO 2 matrix surface states. At higher intensities a saturation of the excited surface states occurs simultaneously with the carrier heating. Systematic increase of the carrier lifetime is observed with the nc size.

Binding of electrons to the surface of helium clusters.

A mean-field potential is derived for an excess electron near a ${(}^{4}$He${)}_{\mathit{N}}$ cluster, which incorporates a realistic description of the cluster surface profile. Calculations of the energies for the ground state and electronically excited surface states were performed. Over a broad cluster size domain (N=${10}^{7}$--${10}^{9}$) the binding energies are systematically lower (i.e., by 6--10 % for the ground state and by 10--20 % for electronically excited states) than those based on model potentials, which represented the interaction within the cluster interior in terms of the conduction-band energy of the liquid.

Pulsed laser-induced photochemical decomposition of GaAs(110) studied with time-resolved photoelectron spectroscopy using synchrotron radiation.

Cleaved GaAs surfaces decompose under weak pulsed-laser irradiation, leading to the formation of metallic Ga islands. The reduction of band bending nonuniformities in photoemission spectra acquired during the laser pulses, in concert with new core-level fitting techniques which treat broadened spectra, permits a detailed analysis. A decomposition rate consistent with a two-step photochemical excitation process is measured. The surface remains largely intact on an atomic scale, but is increasingly unable to support excited surface states as decomposition proceeds.

Laser-stimulated adspecies interaction with a semiconductor surface

Abstract : The surface electronic charge distributions of a one-dimensional semiconductor are compared for the ground state and the laser-excited surface states. A charged adspecies interaction potential with these excited surface states is examined for the case of silicon. The use of a laser to enhance desorption or adsorption by this process is discussed. (Author)