Surface-state models

The selection on an empirical basis of collective-electron factors or active-site concepts or some combination thereof in order to account for the activity of surfaces in catalysing various processes has obvious disadvantages. Possibilities for a more systematic approach to the integration of collective-electron and localised-state aspects of surface structure have developed from theoretical treatments of intrinsic and extrinsic surface states, respectively. Models based on such developments, by reason of their relative novelty, have not yet been as widely applied as collective electron or active-site models to interpret catalytic activity of various surfaces and still less to considerations of sensitivity to irradiation. However, an abbreviated consideration of such surface state models is deemed essential here both as a basis for assessing their possible relevance in the explanation of radiation-induced effects and as an illustration of the integration of electronic and localised state aspects into a common framework. 

Various theoretical approaches with differing degrees of rigour are emerging for the characterisation of extrinsic surface states and their effects upon surface properties. Thus the successful application of lattice 

Several possibilities for modifying the electronic and catalytic properties of surfaces by irradiation follow from the surface state approach. Direct modification by irradiation should occur in cases where high energy radiations create additional surface defects with an associated increase, A[SS], in the density of surface states, as in eqn. (7), or in cases where incident radiation interacts directly with pre-existing surface states to ionise them, as in eqn. (8), or to promote electrons from full to empty surface states as in eqn. (9), viz. 

Incident radiations used and information on gas/solid interfaces obtainable by surface spectroscopies Adapted from ref. 71(a). 

Incident radiation Surface spectroscopy Type of interaction Output(s) Information on adsorbed species 

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The surface-state model. The absorption of a photon generates an exciton within a quantum-confined silicon crystallite but its radiative recombination occurs at localized electtonic states on the surface of the crystallites [Ko5] or in defects in the oxide coverage of the crystallites [Pr5]. 

The surface-state model, in which the luminescent recombination occurs via surface states, was proposed to explain certain properties of the PL from PS, for example long decay times or sensitivity of the PL on chemical environment. In the frame of this model the long decay times are a consequence of trapping of free carriers in localized states a few hundred meV below the bandgap of the confined crystallite. The sensitivity of the PL to the chemical environment is interpreted as formation of a trap or change of a trap level by a molecule bonding to the surface of a PS crystallite. The surface-state model suffers from the fact that most known traps, e.g. the Pb center, quench the PL [Me9], while the kinds of surface state proposed to cause the PL could not be identified. 

Andersson, A. (1982). An oxidized surface state model of vanadium oxides and its application to catalysis. /. Solid State Chem. 42, 263-75. 

The radiative decay time of the red PL is of the order of several tens of microseconds in all cases. The lack of the blue shift in the compact films has been explained in terms of the surface state model [19]. The theory predicts an increase of the transition probability for absorption and PL with decreasing crystallite size [9, 10, 13]. Fig. 5 shows the measured dependence of the PL intensity on the average crystallite size in the compact ncSi/SiO films [19,25]. 

Figure A3.10.23 Schematic diagram of molecular CO chemisorption on a metal surface. The model is based on a donor-acceptor scheme where the CO 5 a FIOMO donates charge to surface unoccupied states and the surface back-donates charge to the CO 2 71 LUMO [58].

By using this approach, it is possible to calculate vibrational state-selected cross-sections from minimal END trajectories obtained with a classical description of the nuclei. We have studied vibrationally excited H2(v) molecules produced in collisions with 30-eV protons [42,43]. The relevant experiments were performed by Toennies et al. [46] with comparisons to theoretical studies using the trajectory surface hopping model [11,47] fTSHM). This system has also stimulated a quantum mechanical study [48] using diatomics-in-molecule (DIM) surfaces [49] and invoicing the infinite-onler sudden approximation (lOSA). 

The excellence of a properly formed Si02—Si interface and the difficulty of passivating other semiconductor surfaces has been one of the most important factors in the development of the worldwide market for siUcon-based semiconductors. MOSFETs are typically produced on (100) siUcon surfaces. Fewer surface states appear at this Si—Si02 interface, which has the fewest broken bonds. A widely used model for the thermal oxidation of sihcon has been developed (31). Nevertheless, despite many years of extensive research, the Si—Si02 interface is not yet fully understood. 

Fig. 8. Steady-state model for the earth s surface geochemical system. The kiteraction of water with rocks ki the presence of photosynthesized organic matter contkiuously produces reactive material of high surface area. This process provides nutrient supply to the biosphere and, along with biota, forms the array of small particles (sods). Weatheriag imparts solutes to the water, and erosion brings particles kito surface waters and oceans.

The results for the reaction of (S)-2-phenylpropanal and (Z)-2-butenylboronate may be reconciled with this transition state model if it is assumed that the phenyl substituent is smaller than methyl in the pair of transition states 6 and 7. analogous to 4 and 5. This is possible if the lowest energy transition state is one in which the phenyl group eclipses C(cc)-H, such that a flat, sterically undemanding surface is presented to the incoming (Z)-2-butenylboronate. Similar... 

Although liquid Hg would never be used as a reference (model) surface in surface physics because its liquid state and high vapor pressure do not allow appropriate UHV conditions, this metal turns out to be a reference surface in electrochemistry for precisely the same reasons reproducibility of the surface state, easy cleaning of its surface, and the possibility of measuring the surface tension (surface thermodynamic conditions). In particular, the establishment of a UHV scale for potentials is at present based on data obtained for Hg. 

The Hg/V-methylformamide (NMF) interface has been studied by the capacitance method as a function of temperature.108,294,303 The potential of Hg was measured with respect to the reference electrode Ag/0.05 M AgC104 + 0.05 M NaC104 in water. The specific adsorption of C104 was found to be negligible at a < 6 /iC cm"2. The experimental capacitance data have been discussed in terms of the four-state model,121,291,294 which assumes the presence of both monomers and clusters in the surface layer of the solvent. The model has been found to describe the experimental picture qualitatively but not quantitatively. This is related to the fact that NMF is a strongly associated solvent.108,109,294,303... 

Kolb and Franke have demonstrated how surface reconstruction phenomena can be studied in situ with the help of potential-induced surface states using electroreflectance (ER) spectroscopy.449,488,543,544 The optical properties of reconstructed and unreconstructed Au(100) have been found to be remarkably different. In recent model calculations it was shown that the accumulation of negative charges at a metal surface favors surface reconstruction because the increased sp-electron density at the surface gives rise to an increased compressive stress between surface atoms, forcing them into a densely packed structure.532... 

A controversy exists over the interpretation of such a correlation. According to the simple two-state model for water at interfaces, the higher the preferential orientation of one of the states, the higher the value of BEa=Q/BT. If the preferentially oriented state is that with the negative end of the dipole down to the surface, the temperature coefficient of Ev is positive (and vice versa). Thus, in a simple picture, the more positive BEa=0/BTt the higher the orientation of water, i.e., the higher the hydro-philicity of the surface. On this basis, Silva et al.446 have proposed the... 

Coulomb blockade effects have been observed in a tunnel diode architectme consisting of an aluminum electrode covered by a six-layer LB film of eicosanoic acid, a layer of 3.8-nm CdSe nanoparticles capped with hexanethiol, and a gold electrode [166]. The LB film serves as a tunneling barrier between aluminum and the conduction band of the CdSe particles. The conductance versus applied voltage showed an onset of current flow near 0.7 V. The curve shows some small peaks as the current first rises that were attributed to surface states. The data could be fit using a tunneling model integrated between the bottom of the conduction band of the particles and the Fermi level of the aluminum electrode. 

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