Temperature programmed desorption studies of deuterium passivated silicon nanocrystals

As grown silicon (Si) surfaces are known to reconstruct in order to reduce the number of dangling bonds. Surface reconstructions of hydride-terminated Si(1 0 0) and Si(1 1 1) surfaces have already been extensively studied using temperature programmed desorption (TPD). The surfaces of nanocrystals, are yet to be probed using TPD. Si nanocrystals less than 8 nm and ranging from 50 to 200 nm in diameter are grown on SiO₂ surfaces in an ultra high vacuum chamber and the as grown surfaces are exposed to atomic deuterium. Desorption spectra are interpreted using analogies to Si(1 0 0). TPD spectra show that that the nanocrystals surfaces are covered by a mix of monodeuteride, dideuteride and trideuteride species. Monodeuteride species can be isolated by selectively annealing away the dideuteride and trideuteride, monodeuteride and dideuteride species can be isolated by annealing away the trideuteride. The relative populations of the deuterides depend on particle size, and their manner of filling on nanoparticles differs from that for extended surfaces. Etching of the nanocrystal surface is observed during TPD, which is a confirmation of the presence of trideuteride species on the nanocrystal surface.     

Temperature programmed desorption studies of CO on CdTe(110)

The thermal desorption of CO from the CdTe(110) surface was studied. The initial rate of desorption exhibits zero order kinetics with an activation energy of 2.8 kval/mol at low surface coverage rising to 4.4 kcal/mol at high surface coverage. The results may be explained in terms of the mechanism of island formation on the semiconductor surface.     

The temperature programmed desorption of hydrogen from platinum and platinum-gold films

The thermal desorption of hydrogen from Pt and PtAu films has been measured in an ultra-high vacuum system by means of a mass spectrometer. On the average, hydrogen is more loosely bound on the alloys than on pure Pt. About 50% of the adsorbate is desorbed by pumping at 78 K from the alloys while only a very small percentage is desorbed from Pt at this temperature. After maximum coverage of Pt films by hydrogen adsorption three desorption peaks have been observed: γ (120 K), β₁ (200 K) and β₂ (330 K). The same peaks have been found for the alloys as well but the relative population of the various adsorption types was different. The relative peak heights vary with the alloy composition.     

Adsorption and desorption of ammonia,hydrogen, and nitrogen on ruthenium (0001)

The adsorption of ammonia, hydrogen, and nitrogen on a Ru(0001) surface have been investigated by Auger electron spectroscopy, low-energy electron diffraction, and thermal flash desorption. The adsorption of ammonia on Ru(0001) can be divided into a low temperature mode (100 K) and a higher temperature mode (300–500 K). For a crystal temperature of 100 K the ammonia adsorbs into two weakly bound molecular γ states with s = 0.2. The ammonia desorbs as NH₃ molecules with desorption energies of 0.32 and 0.46 eV. At 300–500 K adsorption occurs via an activated process with a low sticking probability (s ? 2 × 10^?4).This adsorption is accompanied by dissociation and formation of an apparent (2 × 2) LEED pattern. Hydrogen adsorbs readily (s = 0.4) on Ru(0001) at 100 K and desorbs with 2nd order kinetics in the temperature range 350–450 K. Nitrogen does not appreciably adsorb on Ru(0001) even at 100 K; maximum nitrogen coverage obtained was estimated to be <2% of a monolayer. Changes in the ammonia flash desorption spectra after hydrogen preadsorption at 100 K will be discussed.     

Temperature programmed desorption study of acetylene on a clean,H-covered,and O-covered Pt(111) surface

The reactions of acetylene on a clean, a H-covered and an O-covered Pt(111) surface were studied by temperature programmed desorption for various coverages of acetylene, and acetylene to H or O ratios. The desorption products were quantitatively determined. On a clean surface, acetylene decomposes to hydrogen and surface carbon. A small amount of self-hydrogenation to ethylene also occurs during decomposition. On a H-covered surface, hydrogenation to CH₄, C₂H₆, and ethylene, and decomposition to hydrogen and surface carbon occur simultaneously. The reactions on these two surfaces can be explained by the presence of two sites. One site is a bare surface Pt atom on which decomposition is the primary reaction pathway. The other site is a Pt atom with adsorbed H on which hydrogenation is the primary reaction pathway. On the O-covered surface, the decomposition reaction takes place together with an oxidation reaction which yields CO, CO₂, and water. The oxidation reaction probably proceeds via an intermediate that has a stoichiometry of CH. Results on the O-covered surface are consistent with the model that oxygen absorbs in islands, and the oxidation reaction takes place at the perimeter of the islands. These results are compared with those of ethylene reaction on the same Pt surfaces.     

Separation of chiral molecules by temperature programmed desorption

The Monte Carlo simulation method was used to model thermal desorption of a pair of enantiomers from a solid surface with a chiral periodic pattern of active sites. The main objective of the study was to determine the optimal number of the active sites and their spatial distribution within the unit cell of the surface to achieve the most efficient separation of the enantiomers. For that purpose we tested the series of chiral patterns which were found previously for the equilibrium adsorption. Temperature programmed desorption spectra were calculated using a square lattice of adsorption sites in which the active sites were distributed spatially according to the candidate patterns. Additionally, influence of relaxation of the adsorbed layer on the relative shift of the TPD peaks of the enantiomers was assessed and the key factors affecting the chiral separation were identified.     

FTIR spectroscopic characterization of the adsorption and desorption of ammonia on MgO surfaces

In the present paper an attempt is made to disentangle the complex IR spectra obtained in the course of the adsorption of NH₃ on high surface area MgO. For this purpose two simplifications were introduced in comparison to previous studies: (a) Only uniformly hydroxylated MgO surfaces (degassing temperature: 200 °C) and completely dehydroxylated MgO surfaces (degassing temperature: 1000 °C) were used as adsorbent, (b) The equilibrium pressure of the adsorbate NH₃ was generally maintained below 200 Pa in all adsorption and desorption experiments. Under these conditions the position and the pressure dependence of the resulting bands may consistently be interpreted in terms of characteristic surface species. Only the completely dehydroxylated MgO surface gives rise to a heterolytic dissociation into neighbouring NH₂ and OH groups. In addition, NH₃ is physically adsorbed on less reactive sites via diverse types of hydrogen bonds. Less well defined hydroxylation states give rise to considerably more complicated spectra. In a first approximation they may be constructed by linearly combining two spectral patterns, one related to a uniformly hydroxylated and the other to a dehydroxylated surface. NH₃ pressures around and beyond 1 kPa give rise to significant changes of band positions and band shapes compared to those observed below 200 Pa. The interpretation of these effects which have previously been ignored must necessarily rely on more complicated models.     

Optical investigations of Zn,Hg and Li doped GaN

The use of infrared quenching of photoluminescence to study the spectral dependence of the photoionization cross-section in GaN doped with Zn, Hg, and Li is reported. It is shown that these impurities produce deep level centres 0.48 eV, 0.41 eV, and 0.75 eV, respectively, above the valence band. In addition, excitation spectra are investigated for Zn and Li doped GaN, giving values of 3.17 eV and 2.86 eV at 78 K for the energy distance of these levels from the conduction band. Finally, from the temperature dependence of the excitation spectra, it is concluded that the levels are probably pinned to the valence band.     

Temperature and pressure dependences of Zn-doped GaN

The electroluminescence spectrum of Zn-doped GaN is independent of uniaxial pressure up to kbars and of temperature in the range 100–300 K. This work was done during a one month visit by Dr. Pankove at UNICAMP, made possible through a grant from PNUD. Financial support from TELEBRAS and FAPESP is also acknowledged.     

Electron impact desorption of hydrogen on tungsten: I. Pure hydrogen lasers

The electron impact induced desorption of H⁺ ions from hydrogen layers on thermally annealed polycrystalline tungsten ribbons has been investigated mass spectrometrically. In agreement with Benninghoven et al. (1972) and Madey (1973) the more strongly bound β₂-state has been found to have a higher EID cross section than the more weakly bound β₁-state. Ionic and total desorption cross sections have been measured at 100 eV; the obtained values agree with the lower values published to data, which suggests that the much higher values reported by some authors do not apply to pure hydrogen layers. The contributions of the different states have also been investigated using partial coverage with deuterium. The implications of the findings for the understanding of the adsorption states of hydrogen on tungsten and for the mechanism of EID are discussed.     

Band structure investigations of GaN films using modulation spectroscopy

The paper presents investigation results concerning band structure of gallium nitride and position of intrinsic and associate defect levels. Main optical characteristics (transmission, reflection and luminescence) were measured in both ordinary and λ-modulation mode for epitaxy-grown GaN films, allowing to determine valence band splitting caused by spin-orbital interaction (48 meV) and crystalline field (10 meV). Analysis of photoluminescence spectra made it possible to identify main recombination mechanisms involving donor and acceptor levels formed by intrinsic point defects , and their associates.     

First-principles surface phase diagram for hydrogen on GaN surfaces

We discuss the derivation and interpretation of a generalized surface phase diagram, based on first-principles density-functional calculations. Applying the approach to hydrogenated GaN surfaces, we find that the Gibbs free energies of relevant reconstructions strongly depend on temperature and pressure. Choosing chemical potentials as variables results in a phase diagram that provides immediate insight into the relative stability of different structures. A comparison with recent experiments illustrates the power of the approach for interpreting and predicting energetic and structural properties of surfaces under realistic growth conditions.     

The influence of desorption kinetics on hydrogen permeation in iron

The common observation that the diffusivity of hydrogen in iron decreases anomalously at low temperatures is treated in terms of the barrier to adatom recombination and desorption that is known to be associated with the surfaces of this metal. Results obtained are in good agreement both with data on deuterium permeation through a polycrystalline iron sample, obtained in this study, and with the body of measurements for hydrogen diffusion through iron found in the literature.     

Tight-binding study of ammonia and hydrogen adsorption on magnetic cobalt systems

The semi-empirical self-consistent tight-binding model of ammonia and hydrogen adsorption at Co(0001) and small Co clusters is used to study the chemisorption role in surface magnetism. The adsorbate choice has been suggested by recent experiments. At the Co(0001) surface the atomic magnetization is predicted to diminish locally by 0.26 μ_B due to an isolated hydrogen atom adsorption; for Co₁₃ clusters the change is somewhat smaller but less localized. At H(1×1)–Co(0001) the magnetization of surface Co atoms drops to 0.88 μ_B. The hydrogen magnetic moment is very small and couples antiferromagnetically to Co. Ammonia adsorption is found to reduce the Co atom magnetization locally by 0.1 μ_B or less. We discuss the possibility of adsorbate–metal antiferromagnetic coupling in more detail.     

Characterization of hydrogen species on TiO2 by electron-stimulated desorption

The hydrogen species in TiO₂ have been characterized using the site specificity of Electron-Stimulated Desorption (ESD). Hydrogen is found to be bonded to surface Ti's in hydride-type bonds, to subsurface or bridgebonded O's in a hydroxyl-like bond, or be part of a surface hydroxyl. Hydrogen is found at high levels in the near-surface region as a contaminant and is intimately involved in the reaction of O₂ and H₂O with the defective TiO₂ surface. It is suggested that this phenomenon may occur in a wide range of compounds.     

A comparison of desorption rate parameters obtained by the variation of heating rate and the threshold temperature programmed desorption methods

Coverage dependent activation energies and preexponential factors for desorption of CO on Ni(110) have been obtained by the variation of heating rate and the threshold temperature programmed desorption (TTPD) methods. With the TTPD technique the initial coverage was varied by an adsorption and a desorption method. All three measurements yield nearly identical rate parameters at low coverages, but significant deviations are observed for higher coverages. In particular, abnormally low binding energies are observed in TTPD analysis of high coverages surfaces, where coverage is controlled by adsorption. These anamolous binding energies are attributed to desorption of CO which has not equilibrated to the tight binding sites on the surface. Better agreement is obtained between the variation of heating rate results and those obtained from TTPD analysis of surfaces where coverage is controlled by desorption. Remaining differences are considered to be a result of fundamental limitations in the TTPD technique as shown by comparison of the results with a calculated correction factor.     

EBIC investigations of GaN layers prepared by epitaxial lateral overgrowth

GaN films prepared by lateral overgrowth are investigated by scanning electron microscopy in the electron beam induced current (EBIC) mode. A comparison of experimental and simulated dependences of induced current on beam energy has allowed us to determine not only the diffusion length, but also the donor concentration in different areas of a film. It has been found that the donor distribution is inhomogeneous and this inhomogeneity increases under fast neutron irradiation. This is indicative of the significant influence of structural defects on the rate of radiation defect accumulation. An anomalously slow signal decay outside the Schottky barrier has been found, which can be determined by charged defects formed at the merger boundary.