Theoretical discussion of the scanning tunneling microscope applied to a semiconductor surface

A free-electron model that we recently developed for a metal-metal microscope is extended to the case of a semiconducting sample. The probe, biased positive with respect to the sample, is taken to be a free-electron metal in the shape of a plane with a hemispherical projection. The sample, its surface being a parallel plane, is assumed to have spherical energy bands. The penetration probability is assumed to be the WKB result along the central axis using a potential energy which includes multiple image effects. We calculate current versus voltage curves at several electrode spacings for a tungsten/silicon combination.     

The characterization of oxygen surface arrangements on a heated model Pt3Pb bimetallic catalyst: A case study by SIMS

SIMS in the low damage mode is applied to characterize different surface arrangements of O on a heated polycrystalline Pt₃Pb sample which is exposed to several oxygen doses. Small oxygen doses at 600 K lead to a surface characterized by O in on-top positions principally. At higher doses oxygen saturation occurs which is probably accompanied by a transition to a reconstructed state of the treated surface. At 790 K immediately place-exchange between O and the substrate metal atoms happens after all doses supplied. The SIMS results are elucidated using recent presentations of the interaction of electronegative species with metallic surfaces. The connection between work function and oxygen surface arrangement is discussed in detail.     

Size effect contributions to the chemical potential of a film

The contributions of zero point and thermal excitations to the chemical potential of a liquid film are computed. The continuum approximation is made. Both bulk and surface waves are included. The effects are found to be small relative to the Van der Waals contribution in the case of a He film.     

Optical properties of Nd3+ pairs in LiYF4-existence of a short range interaction

Optical studies of Bridgman grown LiYF₄ crystals weakly doped with Nd³⁺ have been made in the vicinity of the ⁴I_9/2→⁴G_5/2 transition. By selective excitation with a narrow pulsed laser or with a CW monomode laser, excitation spectra and dynamical characteristics of infrared and ultraviolet (up-converted) fluorescences produced by satellites associated with Nd³⁺ pairs have been obtained. From the results, an assignment of the satellites to the various classes of pairs has been possible. The existence of a short range interaction, probably superexchange, has been proved which confirms previous observations in another fluoride compound, LaF₃.     

Comparative infrared study of silicon and germanium nitrides

Silicon and germanium nitride (Si₃N₄ and Ge₃N₄) are isomorphic compounds. They have been studied in the β-phase which crystallises in the hexagonal system. The space group is P6₃/m (C_6h²).The IR transmission spectra of these two nitrides are very similar but the absorption frequencies of germanium nitride are shifted to the lower values in comparison with silicon nitride. We noted that the atomic mass effect is the only cause of this shift for the streching modes but not for the bending modes.     

Isothermal transport properties of Fe1−δO from a polarization method

We report chemical diffusion coefficient D̃ and effective charge number Z¹ measurements for various deviations from stoichiometry in Fei_1−δO and at various temperatures. The results are compared with available data, and correlation factors ⨍ for Fe_1−δO are deduced from the comparison of our D̃ values with available self-diffusion coefficients of iron D¹_Fe ⨍ is shown to be mostly invariant with temperature and nonstoichiometry. Its value lies in the range 0.54–0.62. The charge of the moving cations is $\text{Z}{}^1\text{= 2}$. These two results indicate that iron diffusion in Fe_1−δO is mediated by a vacancy mechanism from octahedral to octahedral positions within regions where the number of first Fe neighbours of a vacancy is much lower than 12.     

On the mechanism of the rapid pyrolysis of cellulose

Experimental yields and liquid product analyses obtained from the fast pyrolysis of biomass and cellulose in a fluidized bed reactor have been used to demonstrate that the decomposition of cellulose to liquid products proceeds to a major degree by a single mechanism. At temperatures over about 450°C and at vapor residence times of one second or less, it is proposed that the monomer unit of cellulose decomposes preferentially to a two-carbon and a four-carbon fragment, with the two-carbon fragment rearranging to give a yield of hydroxyacetaldehyde (glycolaldehyde) which is 75% or more of that theoretically probable. The most likely route for decomposition or rearrangement of the four-carbon moiety is suggested. The formation of carbonyl or hydroxycarbonyl compounds with two to four carbon atoms is favored. Experimental results from the fast pyrolysis of poplar wood indicate that the cellulose decomposition in wood follows the same path as that of pure cellulose in the production of liquid products.     

The temperature dependence of the first,second and third harmonics of the non-linear pyroelectric current in monoaxial ferroelectrics having a second-order phase transition

The theoretical temperature dependences of the first, second and third harmonics of the non-linear pyroelectric current (NPC) are given. It has been established that the higher the order of the harmonic, the faster its magnitude rises, whereas the contribution of the appropriate harmonic to the total NPC decreases with rising harmonic order in all temperature ranges. The results obtained may provide useful data for the study of the phase transitions in ferroelectric media.