共查询到20条相似文献,搜索用时 46 毫秒
1.
Au island nucleation and growth on a Si(1 1 1) 7 × 7 vicinal surface was studied by means of scanning tunneling microscopy. The surface was prepared to have a regular array of step bunches. Growth temperature and Au coverage were varied in the 255-430 °C substrate temperature range and from 1 to 7 monolayers, respectively. Two kinds of islands are observed on the surface: Au-Si reconstructed islands on the terraces and three-dimensional (3D) islands along the step bunches. Focusing on the latter, the dependence of island density, size and position on substrate temperature and on Au coverage is investigated. At 340 °C and above, hemispherical 3D islands nucleate systematically on the step edges. 相似文献
2.
The structure of the ordered p(2 × 1) and p(3 × 1) phases of adsorbed oxygen as well as the formation of ultrathin NiO(1 0 0) layers on a Ni(1 1 0) single crystal are investigated by grazing scattering of fast hydrogen atoms. Via ion beam triangulation based on the detection of the number of emitted electrons, we obtain direct information on the structure of oxygen adsorbates and ultrathin nickel oxide layers. For oxidation using atomic instead of molecular oxygen, the gas exposure can be reduced by almost two orders of magnitude. We compare the experimental results with computer simulations based on classical projectile trajectories for grazing scattering of fast hydrogen atoms and test structure models for oxygen adsorbed on Ni(1 1 0) and NiO(1 0 0). 相似文献
3.
Ronnie T. Vang Søren Dahl Joachim Schnadt Bjerne S. Clausen Flemming Besenbacher 《Surface science》2006,600(1):66-77
The dissociative adsorption of ethylene (C2H4) on Ni(1 1 1) was studied by scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. The STM studies reveal that ethylene decomposes exclusively at the step edges at room temperature. However, the step edge sites are poisoned by the reaction products and thus only a small brim of decomposed ethylene is formed. At 500 K decomposition on the (1 1 1) facets leads to a continuous growth of carbidic islands, which nucleate along the step edges.DFT calculations were performed for several intermediate steps in the decomposition of ethylene on both Ni(1 1 1) and the stepped Ni(2 1 1) surface. In general the Ni(2 1 1) surface is found to have a higher reactivity than the Ni(1 1 1) surface. Furthermore, the calculations show that the influence of step edge atoms is very different for the different reaction pathways. In particular the barrier for dissociation is lowered significantly more than the barrier for dehydrogenation, and this is of great importance for the bond-breaking selectivity of Ni surfaces.The influence of step edges was also probed by evaporating Ag onto the Ni(1 1 1) surface. STM shows that the room temperature evaporation leads to a step flow growth of Ag islands, and a subsequent annealing at 800 K causes the Ag atoms to completely wet the step edges of Ni(1 1 1). The blocking of the step edges is shown to prevent all decomposition of ethylene at room temperature, whereas the terrace site decomposition at 500 K is confirmed to be unaffected by the Ag atoms.Finally a high surface area NiAg alloy catalyst supported on MgAl2O4 was synthesized and tested in flow reactor measurements. The NiAg catalyst has a much lower activity for ethane hydrogenolysis than a similar Ni catalyst, which can be rationalized by the STM and DFT results. 相似文献
4.
David N. McCarthy Daniel Robertson Pawel J. Kowalczyk Simon A. Brown 《Surface science》2010,604(15-16):1273-1282
We report on the growth of ultra-thin bismuth (Bi) films on the basal plane of highly ordered pyrolitic graphite (HOPG) substrates; we investigate the morphologies of films grown at room temperature and then annealed at high temperature, and the morphologies of Bi structures grown at high temperature. Films grown at room temperature nucleate flat islands on the HOPG terraces, and 1D nanorods from HOPG step edges, the islands and rods both have heights in the range 1–3 nm. During annealing, the flat islands break up into groups of aligned, ~ 2.5 nm tall rods. For films grown at high temperature, terrace nucleation is almost nonexistent, and 1D structures grow from step edges, with heights up to 30 nm. Finally, we observe rods with distinctively different morphologies corresponding to (110) and (111) orientations, and infer a surface energy driven Bi(110) to Bi(111) orientation transition. We speculate that the dominant mechanism for the reorientation is coalescence. 相似文献
5.
Several surface analysis techniques were combined to study the initial stages of oxidation of Cu(1 1 1) surfaces exposed to O2 at low pressure (<5 × 10−6 mbar) and room temperature. Scanning tunneling microscopy (STM) results show that the reactivity is governed by the restructuring of the Cu(1 1 1) surface. On the terraces, oxygen dissociative adsorption leads to the formation of isolated O adatoms and clusters weakly bound to the surface. The O adatoms are located in the fcc threefold hollow sites of the unrestructured terraces. Friedel oscillations with an amplitude lower than 5 pm have been measured around the adatoms. At step edges, surface restructuring is initiated and leads to the nucleation and growth of a two-dimensional disordered layer of oxide precursor. The electronic structure of this oxide layer is characterised by a band gap measured by scanning tunneling spectroscopy to be ∼1.5 eV wide. The growth of the oxide islands progresses by consumption of the upper metal terraces to form triangular indents. The extraction of the Cu atoms at this interface generates a preferential orientation of the interface along the close-packed directions of the metal. A second growth front corresponds to the step edges of the oxide islands and progresses above the lower metal terraces. This is where the excess Cu atoms extracted at the first growth front are incorporated. STM shows that the growing disordered oxide layer consists of units of hexagonal structure with a first nearest neighbour distance characteristic of a relaxed Cu-Cu distance (∼0.3 nm), consistent with local Cu2O(1 1 1)-like elements. Exposure at 300 °C is necessary to form an ordered two-dimensional layer of oxide precursor. It forms the so-called “29” superstructure assigned to a periodic distorted Cu2O(1 1 1)-like structure. 相似文献
6.
The formation of (1 1 1)-oriented Cu-clusters on ZnO(0 0 0 1)-Zn at room temperature is followed by in situ applied scanning tunneling microscopy. Kink-sites at step edges and especially the apexes of triangular ZnO-substrate terraces act as preferred nucleation sites. At room temperature the decay of small Cu-islands takes place on a time scale of minutes. Larger Cu-coverages lead to an ensemble of interconnected 3D-islands of uniform height separated by trenches down to the substrate. A disordered dislocation network is visible on top of the Cu-islands. Annealing leads to a piling up of the Cu-islands. An initially undisturbed ZnO-substrate in between the islands shows that there is no strong reaction between the Cu-clusters and the oxide at room temperature. A strong decrease of the adlayer coverage visible above the ZnO-substrate layer for annealing temperature above 570 K points to a partial entrenching of the islands into the oxide support and an alloy formation. 相似文献
7.
The deposition of ultrathin Fe films on the Mo(1 1 0) surface at elevated temperatures results in the formation of distinctive nanowedge islands. The model of island formation presented in this work is based on both experiment and DFT calculations of Fe adatom hopping barriers. Also, a number of classical molecular dynamics simulations were carried out to illustrate fragments of the model. The islands are formed during a transition from a nanostripe morphology at around 2 ML coverage through a Bales-Zangwill type instability. Islands nucleate when the meandering step fronts are sufficiently roughened to produce a substantial overlap between adjacent steps. The islands propagate along the substrate [0 0 1] direction due to anisotropic diffusion/capture processes along the island edges. It was found that the substrate steps limit adatom diffusion and provide heterogeneous nucleation sites, resulting in a higher density of islands on a vicinal surface. As the islands can be several layers thick at their thinnest end, we propose that adatoms entering the islands undertake a so-called “vertical climb” along the sides of the island. This is facilitated by the presence of mismatch-induced dislocations that thread to the sides of the islands and produce local maxima of compressive strain. Dislocation lines also trigger initial nucleation on the surface with 2-3 ML Fe coverage. The sides of the nanowedge islands typically form along low-index crystallographic directions but can also form along dislocation lines or the substrate miscut direction. 相似文献
8.
H. Hartmann T. Diemant A. Bergbreiter J. Bansmann H.E. Hoster R.J. Behm 《Surface science》2009,603(10-12):1439-1455
The formation and structure of monolayer PdRu/Ru(0 0 0 1) surface alloys and their adsorption properties with respect to deuterium adsorption were investigated by atomic resolution scanning tunneling microscopy and by temperature programmed desorption. Surface alloys, prepared by deposition of up to one monolayer of Pd and flash annealing to 1150 K show (i) negligible loss of Pd by desorption or diffusion into the Ru bulk during this procedure and (ii) dominant phase separation into 2D Pd and Ru islands, in contrast to the random distribution in PtRu/Ru(0 0 0 1) surface alloys [H.E. Hoster, A. Bergbreiter, P.M. Erne, T. Hager, H. Rauscher, R.J. Behm, Phys. Chem. Chem. Phys. 10 (2008) 3812]. 2D short-range order parameters and the abundance of specific adsorption ensembles were evaluated for different Pd contents, effective pair interaction (EPI) energies were derived from Monte Carlo simulations. Deuterium adsorption on Pd monolayer films shows a pronounced weakening of adsorption bond, which is attributed to compressive strain and metal–metal interactions between Pd and underlying Ru atoms (‘vertical ligand effect’). Mixed adsorption ensembles containing both Pd and Ru atoms give rise to D2 desorption in the intermediate temperature regime. The impact of the specific lateral distribution of the two metal species on the chemical surface properties is illustrated by comparison with deuterium adsorption on dispersed PtRu/Ru(0 0 0 1) surface alloys [T. Diemant, H. Rauscher, R.J. Behm, J. Phys. Chem. C 112 (2008) 8381]. 相似文献
9.
C. Gerl S. Schmult U. Wurstbauer H.-P. Tranitz C. Mitzkus W. Wegscheider 《Physica E: Low-dimensional Systems and Nanostructures》2006,32(1-2):258
Since Stormer and Tsang have introduced the first two-dimensional hole gas (2DHG) in the GaAs/AlGaAs heterosystem, the choice of suitable dopants was limited to beryllium and silicon over the last 20 years. Both acceptor atoms have significant disadvantages, i.e. either high-diffusion rates or a limitation to specific growth directions. Utilizing a carbon filament-doping source, we prepared high-quality 2DHGs in the (0 0 1) and the nonpolar (1 1 0) crystal plane with carrier mobilies beyond 106 cm2/Vs in quantum well and single interface structures. Low-temperature magnetoresistance measurements recover a large number of fractional QHE states and show a pronounced beating pattern from which the Rashba induced spin-splitting has been determined. In addition, 2DHGs have been grown on cleaved edges of (1 1 0) and (0 0 1) wafers with transport features in qualitative agreement to our findings on (1 1 0) substrates. 相似文献
10.
InN films with the wurtzite structure have been grown directly on YSZ (1 0 0) substrate by the RF-magnetron sputtering technique. Strongly (0 0 2) oriented films with smooth surfaces (0.7–2.9 nm surface roughness depending on substrate temperature), were grown within 30 min. Films deposited for 60 min developed three-dimensional (3D) pyramidal islands on top of their surfaces, which diminished the residual elastic strain. The optical absorption edge and PL peak energy around 1.7 eV were found to redshift with increasing film thickness and substrate temperature. 相似文献
11.
We have studied the initial stages of adsorption of C60 on the Pt (1 1 0)-(1 × 2) surface by means of STM. At room temperature, fullerene molecules adsorb in the troughs between two adjacent Pt rows of the missing row reconstruction. Mobility over the terraces is negligible, denoting strong bonding with the surface, also testified by a well-defined orientation of fullerene monomers with respect to the substrate. Upon annealing at 750 K, molecular migration towards kinks and step edges occurs, where small islands nucleation begins. A commensurate registry with the substrate is maintained by small (5-10 molecules) C60 aggregates, leading to expanded nearest-neighbour distances with respect to those found in hexagonal close packed fullerene ad-islands grown on other metallic substrates. 相似文献
12.
Atomic resolution scanning tunnelling microscopy (STM) has been used to study in situ the As-terminated reconstructions formed on GaAs(0 0 1) surfaces in the presence of an As4 flux. The relationship between the As-rich (2 × 4) and c(4 × 4) surfaces is observed throughout the gradual evolution of the reconstruction transformation. The results suggest that during the initial stage of the transformation, Ga-rich As-terminated variations of the c(4 × 4) form in order to accommodate excess mobile Ga produced by pit formation. These transient structures later planarize, as excess Ga is incorporated at step/island edges. Successive imaging of the same sample area during As4 irradiation allows point-by-point adatom binding to be analysed in a way inaccessible to MBE–STM systems relying on sample quenching and transfer. 相似文献
13.
The adsorption of H2S on Fe(1 0 0) is examined using ab initio molecular dynamics at 298 and 1808 K. Dissociation of H2S occurs at both temperatures simulated, to leave adsorbed S and two H atoms. The dissociation occurs via a two step process and the mechanism is found to be different depending on the temperature of the reaction. At 1808 K, diffusion of the dissociated H atoms into the subsurface region is also observed. 相似文献
14.
Nucleation of 2D islands in Si/Si(1 1 1)-7 × 7 molecular beam epitaxy is studied using scanning tunneling microscopy (STM). A detailed analysis of the population of small amorphous clusters coexisting on the surface with epitaxial 2D islands has been performed. It is shown that small clusters tend to form pairs. The pairs serve as precursors for 2D islands as confirmed by direct STM observations of the smallest 2D islands covering two adjacent half-unit cells of the 7 × 7 reconstruction. It is proved with scaling arguments that the critical nucleus for 2D island formation consists not only of the pair itself, but also includes additional adatoms not belonging to the stable clusters. 相似文献
15.
The growth of Pb films on the Si(1 0 0)-2 × 1 surface has been investigated at low temperature using scanning tunneling microscopy. Although the orientation of the substrate is (1 0 0), flat-top Pb islands with (1 1 1) surface can be observed. The island thickness is confined within four to nine atomic layers at low coverage. Among these islands, those with a thickness of six layers are most abundant. Quantum-well states in Pb(1 1 1) islands of different thickness are acquired by scanning tunneling spectroscopy. They are found to be identical to those taken on the Pb(1 1 1) islands grown on the Si(1 1 1)7 × 7 surface. Besides Pb(1 1 1) islands, two additional types of Pb islands are formed: rectangular flat-top Pb(1 0 0) islands and rectangular three-dimensional (3D) Pb islands, and both their orientations rotate by 90° from a terrace to the adjacent one. This phenomenon implies that the structures of Pb(1 0 0) and 3D islands are influenced by the Si(1 0 0)-2 × 1 substrate. 相似文献
16.
S. Nie C.D. Lee R.M. Feenstra Y. Ke R.P. Devaty W.J. Choyke C.K. Inoki T.S. Kuan Gong Gu 《Surface science》2008,602(17):2936-2942
The formation of step bunches and/or facets on hydrogen-etched 6H-SiC(0 0 0 1) and () surfaces has been studied, using both nominally on-axis and intentionally miscut (i.e. vicinal) substrates. It is found that small miscuts on the (0 0 0 1) surface produce full unit-cell high steps, while half unit-cell high steps are observed on the () surface. The observed step normal direction is found to be for both surfaces. Hence, for intentionally miscut material, a miscut oriented towards this direction produces much better order in the step array compared to a miscut oriented towards a direction. For (0 0 0 1) vicinal surfaces that are miscut towards the direction, the formation of surface ripples is observed for 3° miscut and the development of small facets (nanofacets) is found for higher miscut angles. Much less faceting is observed on miscut () surfaces. Additionally, the (0 0 01) surface is found to have a much larger spatial anisotropy in step energies than the () surface. 相似文献
17.
Masuaki Matsumoto Shohei Ogura Katsuyuki Fukutani Tatsuo Okano 《Surface science》2009,603(19):2928-2934
Room temperature (RT) adsorption of nitric oxide (NO) on Ir(1 1 1) was studied by scanning tunneling microscopy (STM). At low exposures, NO molecules can not be imaged by STM, because at RT the diffusion of NO is much faster than the STM scanning speed. At high exposures near the saturation coverage, however, a well-ordered 2 × 2 structure is observed. The coverage of the major 2 × 2 species is 0.25 and they can be assigned to the NO molecules adsorbed on the Ir ontop sites. A small number of less bright spots are assigned to nitrogen atoms produced by dissociation. Their number increases by annealing the NO-saturated surface at 380 K. A small number of another dissociation product, oxygen, are observed as black lines, indicating that the diffusion of oxygen atoms is fast. Scratch-like noise features were also detected by the STM, which suggests that a mobile precursor state exists, which was clearly shown by the effects of electron irradiation from the STM tip. These results are consistent with the previous molecular beam studies. Hopping of the 2 × 2 ordered NO species was frequently observed at the anti-phase domain boundaries and edges of the 2 × 2 islands. 相似文献
18.
Laterally resolved topography and Contact Potential Difference (CPD) images, acquired during the exposure of clean Si(1 1 1) 7 × 7 to molecular oxygen at room temperature, show a heterogeneous oxidation process, without preference for step edges. The increase of and lateral changes in work function variations show that the CPD variations of the final oxide film are related to the silicon/oxide interface. The molecular Höfer precursor has a pronounced influence on the development of the interface bonding. 相似文献
19.
Zhongliang Liu Jun Tang Chaoyang Kang Chongwen Zou Wensheng Yan Pengshou Xu 《Solid State Communications》2012,152(11):960-963
Few-layer graphene (FLG) was grown on Al2O3 (0 0 0 1) substrates at different temperatures via direct carbon atoms deposition by using solid source molecular beam epitaxy (SSMBE) method. The structural properties were characterized by reflection high energy electron diffraction (RHEED), Raman spectroscopy and near-edge X-ray absorption fine-structure (NEXAFS). The results showed that the FLG started to form at the substrate temperature of 700 °C. When the substrate temperature increased to 1300 °C, the quality of the FLG was the best and the layer number was estimated to be less than 5. At higher substrate temperature (1400 °C or above), the crystalline quality of the FLG would be deteriorated. Our experiment results demonstrated that the substrate temperature played an important role on the FLG layer formation on Al2O3 (0 0 0 1) substrates and the related growth mechanism was briefly discussed. 相似文献
20.
X-ray photoelectron spectroscopy was used to study the effect of atomic oxygen on Ru(0 0 0 1), and the effect of dissociated ammonia on RuO2/Ru(0 0 0 1), in UHV conditions at ambient temperature. The Ru(0 0 0 1) surface was exposed, at ambient temperature, to a mixed flux of atomic and molecular oxygen generated by dissociation of O2 in a thermal catalytic cracker, with 45% dissociation efficiency. The detailed study of the XPS spectra shows the formation of a disordered multilayer oxide (RuO2). No formation of higher oxides of Ru was observed. The formation of RuO2 proceeded without saturation for total oxygen exposures of up to 105 Langmuir, at which point an average oxide thickness of 68 Å was observed. RuO2 formed by the reaction with atomic oxygen was exposed to a flux of NHx (x = 1, 2) + H generated by the cracker. The reduction of RuO2 to Ru metal was observed by XPS. An exposure of 3.6 × 102 L of NHx + H, resulted in the observation of adsorbed H2O and OH, but no evidence of lattice oxide. The chemisorbed species were removed by additional NHx + H exposure. No nitrogen adsorption was observed. 相似文献