Decomposition of thin titanium deuteride films; thermal desorption kinetics studies combined with microstructure analysis

The thermal evolution of deuterium from thin titanium films, prepared under UHV conditions and deuterated in situ at room temperature, has been studied by means of thermal desorption mass spectrometry (TDMS) and a combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). The observed Ti film thickness dependent morphology was found to play a crucial role in the titanium deuteride (TiD_y) film formation and its decomposition at elevated temperatures. TDMS heating induced decomposition of fine-grained thin Ti films, of 10-20 nm thickness, proceeds at low temperature (maximum peak temperature T_m about 500 K) and its kinetics is dominated by a low energy desorption (E_D = 0.61 eV) of deuterium from surface and subsurface areas of the Ti film. The origin of this process is discussed as an intermediate decomposition state towards recombinative desorption of molecular deuterium. The TiD_y bulk phase decomposition becomes dominant in the kinetics of deuterium evolution from thicker TiD_y films. The dominant TDMS peak at approx. T_m = 670 K, attributed to this process, is characterized by E_D = 1.49 eV.     

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.     

Interaction of hydrogen with palladium surface: FIM and FEM studies

In the present paper, we focus on the geometrical and electronic changes in palladium surface structure which appeared during its interaction with hydrogen in the presence of an external electric field. The interaction process was examined by using the field ion microscopy (FIM) as well as the field emission microscopy (FEM) techniques. In order to study the geometrical changes in substrate surface structure, the distance distribution function (DDF) was constructed on the basis of FIM patterns of both a clean and hydrogen-covered palladium surface. The electronic changes were examined by the measurement of the total energy distribution (TED) of electrons emitted from the palladium tip surface. The most pronounce examples of such changes are an expansion of the equilibrium interatomic distance in palladium surface and a shift of the Fermi level of the metal. These changes may be explained among others by palladium hydrides formation. This process is the most efficient if the field strength exceeds 23 V/nm.     

Computer modeling of dissociative gas adsorption on laser-roughened surfaces

A simple computer model of dissociative adsorption of diatomic molecules on a solid surface with laser-induced defects was proposed. The defects (ablation craters) were assumed to have either cubicoid or pyramidal shape, depending on the approximation level. Special attention was paid to the influence of a degree of structural disorder on the adsorptive properties of the surface. In particular, both equilibrium adsorption isotherms and temperature programmed desorption (TPD) spectra of non-interacting diatomic molecules from the surfaces subjected to a different number of laser pulses were simulated. The observed changes in the adsorptive properties of the surface were explained using simple geometric arguments linking the adsorption probability for a single molecule with the topography of the surface. For example, it was demonstrated that, for a sufficiently large number of laser pulses (N), the adsorption probability scales with , regardless of the assumed crater shape. The obtained results also indicate that, in general, the surface roughness greatly affects the TPD spectra while it has minor influence on the shape of the adsorption isotherms.     

Diffusion of rhodium layers on the stepped surface of a tungsten micro-crystal

We have investigated the spreading of rhodium at coverages of 0.25, 0.5, 1, 2 and 3 ML over the curved surface of a field emitter tip using field electron microscopy. We have found that the activation energy of spreading as well as the prefactor for the diffusivity depend strongly on the thickness of the layer diffusing, due to a change in interactions in the adsorbate-substrate system. The derived average activation energy for spreading first decreases from E_dif = 1.32 eV/atom at Θ = 0.25 ML to E_dif = 0.71 eV at Θ ≈ 2 ML and than rises again to E_dif = 1.20 eV at Θ ≈ 3 ML. The prefactor for the diffusivity D₀ also decreases with increasing coverage from 0.5 to 1 ML, and stays almost constant for multilayer diffusion in a range of few orders of magnitude lower than for single atom diffusion. We register typical spreading behavior with a sharp moving boundary in the (0 1 1)-(0 0 1) zone of the tip and an unusual picture of diffusion in the (0 1 1)-(1 1 2) region of the tip. In the second region diffusion proceeds without a sharp boundary, independent of the thickness of the moving layer. We think that such an unusual picture can be caused by the change in composition of the second and next layers of adsorbing material due to the early stage of faceting observed in this region of the tip at higher temperature. The results are compared with data for diffusion of individual Rh atoms and small clusters; to understand the observed diffusion we propose taking account of the atomic surface structure of the substrate, modified by strong interactions of the Rh adsorbate with the W micro-crystal surface.     

Surface treatments toward obtaining clean GaN(0 0 0 1) from commercial hydride vapor phase epitaxy and metal-organic chemical vapor deposition substrates in ultrahigh vacuum

We studied processes of cleaning GaN(0 0 0 1) surfaces on four different types of wafers: two types were hydride vapor phase epitaxy (HVPE) free-standing substrates and two types were metal-organic chemical vapor deposition (MOCVD) films grown on these HVPE substrates and prepared by annealing and/or Ar ion sputtering in ultra high vacuum. We observed the surfaces through treatments using in situ low-energy electron diffraction (LEED), reflection high-energy electron diffraction (RHEED), scanning tunneling microscopy (STM), and Auger electron spectroscopy, and also using ex situ temperature programmed desorption, X-ray photoelectron spectroscopy, X-ray diffraction, and secondary ion mass spectrometry. For HVPE samples, we obtained relatively clean surfaces under optimized three-step annealing conditions (200 °C for 12 h + 400 °C for 1 h + 500 °C for 5 min) without sputtering, after which the surface contamination of oxide and carbide was reduced to ∼20% of that before annealing. Clear GaN(0 0 0 1)1×1 patterns were obtained by LEED and RHEED. STM images showed flat terraces of ∼10 nm size and steps of ∼0.5 nm height. Upon annealing the HVPE-GaN samples at a much higher temperature (C), three-dimensional (3D) islands with facets were formed and the surface stoichiometry was broken down with the desorption of nitrogen in the form of ammonia, since the samples include hydrogen as an impurity. Ar⁺ sputtering was effective for removing surface contamination, however, postannealing could not recover the surface roughness but promoted the formation of 3D islands on the surface. For MOCVD/HVPE homoepitaxial samples, the surfaces are terminated by hydrogen and the as-introduced samples showed a clear 1×1 structure. Upon annealing at 500-600 °C, the surface hydrogen was removed and a 3×3 reconstruction structure partially appeared, although a 1×1 structure was dominant. We summarize the structure differences among the samples under the same treatment and clarify the effect of crystal quality, such as dislocations, the concentration of hydrogen impurities, and the residual reactant molecules in GaN films, on the surface structure.     

Adsorption of cobalt phthalocyanine on a Si(1 0 0) surface with Bi-line structures as evaluated by scanning tunneling microscopy

We report the reaction dynamics of cobalt phthalocyanine (CoPc) molecules with Bi-line structures (BLSs) on a Si(1 0 0) surface, investigated using scanning tunneling microscopy (STM). When CoPc molecules were deposited on a Si(1 0 0) surface with BLSs at room temperature, single-spot protrusions were observed in the STM image instead of four-spot images corresponding to CoPcs flat molecular structure. Moreover, domains with a c(4 × 4) periodicity appeared on the terraces of the Si(1 0 0) surface. This indicates that CoPc molecules may have decomposed on the surface by catalytic reaction with Bi atoms.     

Behaviour of discontinuous gold films on SrTiO3 substrates under annealing

Morphological changes of thin, discontinuous gold films on SrTiO₃ substrates, resulting from evaporation in the temperature range of 1143-1278 K, have been investigated by means of scanning electron microscopy (SEM) and atomic force microscopy (AFM). If the gold covered fraction of the surface is small, the evaporation kinetics can be related to the desorption of adatoms. Measuring the density of the gold beads and the time dependence of the effective thickness of the film as calculated from the diameter of the beads, the following parameters have been determined: the surface diffusion length of the gold adatoms, , the mass transfer surface diffusion coefficient, and the evaporation flux, .     

Dynamics of molybdenum nano structure formation on the TiO2(1 1 0) surface: A kinetic Monte Carlo approach

The rutile TiO₂(1 1 0) surface is a highly anisotropic surface exhibiting “channels” delimited by oxygen rows. In previous experimental and theoretical DFT works we could identify the molybdenum adsorption sites. They are located inside the channels. Moreover, experimental studies have shown that during subsequent annealing after deposition, special molybdenum nano structures can be formed, especially two monolayer high pyramidal chains of atoms.In order to better understand the dynamics of nano structure formation, we present a kinetic Monte Carlo study on diffusion and adsorption of molybdenum atoms on a TiO₂(1 1 0) surface. A quasi one-dimensional lattice gas model has been used which describes the possible adsorption sites of a Mo atom in a single channel of the surface. The atomic positions of a 1.5 monolayer thick Mo film formed of pyramidal chains define the lattice sites of the model. Thereby the formation of three-dimensional clusters could be studied. Here we have studied the cluster formation as a function of parameters that can be controlled in a growth experiment by physical vapor deposition: deposition and annealing temperature, flux and total amount of deposited Mo. Good qualitative agreement with recent experiments is obtained.     

Isotope effects in the kinetics of simultaneous H and D thermal desorption from Pd

The kinetics of simultaneous hydrogen and deuterium thermal desorption from PdH_xD_y has been investigated. A novel experimental approach for the study of the transition state (TS) characteristics of the surface recombination reaction is proposed based on the analysis of the H and D partitioning into H₂, HD and D₂ molecules. It has been found that the hydrogen molecular isotopes distribution is determined by the energy differences of the corresponding TS of the atom-atom recombination reactions. On the other hand, the mechanisms and activation energies of the desorption process have been obtained. At 420 K, the desorption reaction changes from a surface recombination limiting mechanism during desorption from β-PdH_xD_y to a reaction limited by the rate of β to α phase transformation during the two phase coexistence. Surface recombination reaction becomes again rate limiting above 480 K, due to a change in the catalytic properties of the Pd surface. TS energies obtained from the kinetic analysis of the thermal desorption spectra are in good accordance with those obtained from the analysis of the H₂, HD and D₂ distributions. Anomalous TS energies have been observed for the H-D recombination reaction, which may be related to the heteronuclear character of this molecule.     

Ordered thin films of alkyloxy-substituted benzaldehydes on a graphite surface studied by scanning tunneling microscopy

The molecular assembly of three different trialkyloxy-substituted benzaldehydes on graphite has been studied using scanning tunneling microscopy (STM). It is found that these benzaldehydes, which do not have intermolecular hydrogen bonding, could form a lamella arrangement in which the headgroups are aligned side by side. The effect due to the alkyl chain length on the lamella structure is also presented. Received: 21 January 2002 / Accepted: 22 March 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +86-10/6255-7908     

AFM investigation on surface evolution of amorphous carbon during ion-beam-assisted deposition

Hydrogen-free amorphous carbons (a-C) have been prepared on mirror-polished Si(1 1 1) wafers through thermally evaporated C₆₀ with simultaneous bombardments of Ne⁺ ions. The time evolution of film surfaces has been characterized by atomic force microscopy (AFM) at two temperatures of 400 and 700 °C, respectively. Based on the topography images and the root-mean-square (rms) roughness analysis, it is found that the a-C surfaces present roughening growth at the initial stage. With increasing growth time, the cooperative nucleation of the islands and pits appears on the surfaces, suggesting three-dimensional growth, and then they continue to evolve to irregular mounds at 400 °C, and elongated mounds at 700 °C. At the steady growth stage, these surfaces further develop to the structures of bamboo joints and ripples corresponding to these two temperatures, respectively. It is believed that besides ion sputtering effect, the chemical bonding configurations in the amorphous carbon films should be taken into considerations for elucidating the surface evolutions.     

Natural variability in the surface roughness of combinatorial libraries of materials

Using a conventional method for the fabrication of solid-state combinatorial libraries, we have produced the binary libraries Fe_1−xCr_x and Mo_1−xSn_x (0 < x < 1) and found that their surface roughness is highly dependent on chemical composition. Roughness enhancements of up to 20-fold have been observed on the nanometer scale and were manifested in the boundaries between structurally dissimilar phases. It is anticipated that this is a common occurrence in thin-film libraries of metals and an obstacle in separating the influence of roughness and composition on the performance of library materials in biomaterial studies.     

Substrate-step-induced effects on the growth of CaF2 on Si (111)

The growth of CaF₂ on vicinal Si (111) surfaces was studied by scanning tunneling microscopy (STM) and atomic force microscopy (AFM) for the temperature range from 300 to 750 °C. In the submonolayer range a transition from terrace to step nucleation is observed for increasing temperature. The first monolayer grows in the step-flow growth mode since second layer islands are not nucleated before completion of the wetting layer. For the subsequent growth of CaF₂ on the CaF interface layer, substrate-induced steps do not act as preferential nucleation sites, but rather as growth barriers. Thus CaF₂ films grow very inhomogeneously at high temperatures. At lower deposition temperatures, the film homogeneity increases due to the increased (homogeneous) nucleation rate on terraces. The lattice mismatch leads to (lateral) relaxation of thicker CaF₂ film close to substrate steps. In addition, CaF₂ self-decoration effects are caused by the relaxed regions close to the film steps at temperatures above 500 °C. Received: 7 August 2001 / Accepted: 23 October 2001 / Published online: 3 April 2002     

Effect of thermal annealing on microstructural properties of Ti/Ge2Sb2Te5/Ti thin films deposited on SiO2/Si substrates by a sputtering method

Ti/Ge₂Sb₂Te₅/Ti thin films deposited by a sputtering method on SiO₂/Si substrates were annealed at 400 °C in N₂ atmosphere and characterized by using transmission electron microscopy (TEM) and Auger electron spectroscopy (AES) in order to investigate the inter-diffusion of the Ti/Ge₂Sb₂Te₅/Ti system due to annealing. The TEM and AES results showed that the interface between the Ti and the Ge₂Sb₂Te₅ layers was unstable and Ti atoms were incorporated into the Ge₂Sb₂Te₅ thin film upon annealing. The Te and Sb atoms of the Ge₂Sb₂Te₅ layer diffused into the Ti layer. The intermixing layers between the Ge₂Sb₂Te₅ layer and two Ti layers were formed. These results indicate that the microstructural properties of the Ti/Ge₂Sb₂Te₅/Ti systems can be degraded by the postgrowth thermal annealing.     

Anisotropic diffusion of Cu adatoms on strained Cu (1 1 1) surface

Diffusion of Cu atoms on a strained Cu (1 1 1) surface was studied by molecular dynamic simulation using an EAM potential. The anisotropic diffusion behaviour is found when the uniaxial strain is imposed on the surface, which does not exist under the biaxial strain. The migration of the adatom is suppressed along the tensile strain direction. The results suggest that different island morphology can be obtained by controlling anisotropic diffusion of adatoms on the strained surfaces during film growth.     

A Diffusion Model of Field-Induced Aggregation in Ferrofluid Film

By introducing Arrhenius behaviour to the ferroparticles on the surface of the aggregated columnar structure in a diffusion model, equilibrium equations are set up. The solution of the equations shows that to keep the aggregated structures stable, a characteristic field is needed. The aggregation is enhanced by magnetic fields, yet suppressed as the temperature increases. Analysing the influence of the magnetic field on the interaction energy between the dipolar particles, we estimate the portion of the diffusing particles, and provide the agreeable ratio of the column radius over the centre-to-centre spacing between columns in a hexagonal columnar structure formed under a perpendicular magnetic field.     

Y2O3 stabilized ZrO2 thin films deposited by electron beam evaporation: Structural, morphological characterization and laser induced damage threshold

Four kinds of Y₂O₃ stabilized ZrO₂ (YSZ) thin films with different Y₂O₃ content have been prepared on BK7 substrates by electron-beam evaporation method. Structural properties and surface morphology of thin films were investigated by X-ray diffraction (XRD) spectra and scanning probe microscope. Laser induced damage threshold (LIDT) was determined. It was found that crystalline phase and microstructure of YSZ thin films was dependent on Y₂O₃ molar content. YSZ thin films changed from monoclinic phase to high temperature phase (tetragonal and cubic) with the increase of Y₂O₃ content. The LIDT of stabilized thin film is more than that of unstabilized thin films. The reason is that ZrO₂ material undergoes phase transition during the course of e-beam evaporation resulting in more numbers of defects compared to that of YSZ thin films. These defects act as absorptive center and the original breakdown points.     

Thickness influence on surface morphology and ozone sensing properties of nanostructured ZnO transparent thin films grown by PLD

Transparent zinc oxide (ZnO) thin films with a thickness from 10 to 200 nm were prepared by the PLD technique onto silicon and Corning glass substrates at 350 °C, using an Excimer Laser XeCl (308 nm). Surface investigations carried out by atomic force microscopy (AFM) and X-ray diffraction (XRD) revealed a strong influence of thickness on film surface topography. Film roughness (RMS), grain shape and dimensions correlate with film thickness. For the 200 nm thick film, the RMS shows a maximum (13.9 nm) due to the presence of hexagonal shaped nanorods on the surface. XRD measurements proved that the films grown by PLD are c-axis textured. It was demonstrated that the gas sensing characteristics of ZnO films are strongly influenced and may be enhanced significantly by the control of film deposition parameters and surface characteristics, i.e. thickness and RMS, grain shape and dimension.     

The structural formation of methylthiolate SAMs on Au(1 1 1) for short deposition times from solution

Scanning tunnelling microscopy was used to investigate the structural formation of methylthiolate self-assembled monolayers on Au(1 1 1). SAMs were prepared by exposing the gold crystal to an ethanol-dimethyldisulfide solution for immersion times of 12 min, 12 h, and 24 h. After preparation the formation of a () rect. striped phase was found. For this phase, the immersion time is the key parameter determining the size and number of ordered domains. Annealing induced a phase transition leading to large domains in a (3×4) structure. The annealing temperature determines whether a mixed phase of both structures or only the (3×4) structure are formed. We find no influence of the immersion time on the formation of the second structure. A structure model is presented for both phases on the basis on the same building block containing two methylthiolate molecules and a gold ad-atom.