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1.
The growth and thermal stability of ultrathin ZrO2 films on the Si-rich SiC(0 0 0 1)-(3 × 3) surface have been explored using photoelectron spectroscopy (PES) and X-ray absorption spectroscopy (XAS). The films were grown in situ by chemical vapor deposition using the zirconium tetra tert-butoxide (ZTB) precursor. The O 1s XAS results show that growth at 400 °C yields tetragonal ZrO2. An interface is formed between the ZrO2 film and the SiC substrate. The interface contains Si in several chemically different states. This gives evidence for an interface that is much more complex than that formed upon oxidation with O2. Si in a 4+ oxidation state is detected in the near surface region. This shows that intermixing of SiO2 and ZrO2 occurs, possibly under the formation of silicate. The alignment of the ZrO2 and SiC band edges is discussed based on core level and valence PES spectra. Subsequent annealing of a deposited film was performed in order to study the thermal stability of the system. Annealing to 800 °C does not lead to decomposition of the tetragonal ZrO2 (t-ZrO2) but changes are observed within the interface region. After annealing to 1000 °C a laterally heterogeneous layer has formed. The decomposition of the film leads to regions with t-ZrO2 remnants, metallic Zr silicide and Si aggregates.  相似文献   

2.
We present a review on the formation of gold silicide nanostructures using in situ temperature dependent transmission electron microscopy (TEM) measurements. Thin Au films of two thicknesses (2.0 nm and 5.0 nm) were deposited on Si (1 1 0) substrate under ultra-high vacuum (UHV) conditions in a molecular beam epitaxy (MBE) system. Also a 2.0 nm thick Au film was deposited under high vacuum condition (with the native oxide at the interface of Au and Si) using thermal evaporation. In situ TEM measurements (for planar samples) were made at various temperatures (from room temperature, RT to 950 °C). We show that, in the presence of native oxide (UHV-MBE) at the interface, high aspect ratio (≈15.0) aligned gold silicide nanorods were observed. For the films that were grown with UHV conditions, a small aspect ratio (∼1.38) nanogold silicide was observed. For 5.0 nm thick gold thin film, thicker and lesser aspect ratio silicides were observed. Selected area diffraction pattern taken at RT after the sample for the case of 5.0 nm Au on Si (1 1 0)-MBE was annealed at 475 °C show the signature of gold silicide formation.  相似文献   

3.
A method for the formation of Au nanocrystal (nc) arrays embedded in an ultrathin SiO2 layer in one vacuum cycle is proposed. The method is based on the co-deposition in vacuum of ∼1 nm thick uniform Si-Au amorphous layer at a specific composition ratio by Pulsed Laser Deposition on the pre-oxidized Si(1 0 0) substrate, followed by its oxidation in the glow discharge oxygen plasma at room temperature, resulting in the precipitation of Au ncs at the bottom interface and/or at the surface of the forming SiO2 layer. The capping SiO2 layer is formed by the glow discharge plasma oxidation of further deposited ultrathin Si layer. Au ncs 2-5 nm in size and with the separation of ∼3-20 nm from each other segregate during the oxidation of Au-Si mixture as evidenced by transmission electron microscopy (TEM). The evolution of Au and Si chemical state upon each step of the SiO2:nc-Au nanocomposite structure formation is monitored in situ by X-ray photoelectron spectroscopy (XPS). The metrology of nanocomposite SiO2:nc-Au structures describing the space distribution of Au ncs as a function of Au/Si ratio is presented.  相似文献   

4.
Deposition and fabrication of films of Au nanoclusters protected by alkanethiolate ligands are attempted on a TiO2(1 1 0) surface and the structures of films are observed by a scanning tunneling microscope (STM). Effects of oxygen and hydrogen-plasma etching in addition to UV irradiation on the structure and chemical composition of the films are also investigated by using STM and X-ray photoelectron spectroscopy. Alkanethiolate Au nanoclusters are produced using a modified Brust synthesis method and their LB films are dip-coated on TiO2(1 1 0). Alkanethiolate Au nanoclusters are weakly bound to the substrate and can be manipulated with an STM tip. Net-like structures of alkanethiolate Au nanoclusters are formed by a strong blast of air. Oxygen-plasma etching removes alkanethiolate ligands and simultaneously oxidizes Au clusters. At room temperature, prolonged oxygen-plasma etching causes agglomeration of Au nanoclusters. UV irradiation removes ligands partly, which makes Au nanoclusters less mobile. The net-like structure of alkanethiolate Au clusters produced by a blast of air is retained after oxygen and hydrogen-plasma etching.  相似文献   

5.
We investigated the cleaning process of Si(1 0 0) surfaces by annealing in H2 gas ambient following chemical treatments by scanning tunneling microscopy. We observed the monohydride Si structure: Si(1 0 0):2 × 1-H on the surfaces annealed at 1000 °C in 2.5 × 104 Pa H2 gas ambient without conspicuous contaminants. On the sample annealed for 10 min or longer times, well-defined Si(1 0 0) structures with alternating SA and SB steps were observed, whereas the initial roughness still remained on the surfaces annealed for only 5 min.  相似文献   

6.
Infrared reflection absorption spectroscopy that used buried metal layer substrates (BML-IRRAS) and density functional cluster calculations were employed to investigate the water related oxidation reactions of 2H + H2O/Si(1 0 0)-(2 × 1), 2D + H2O/Si(1 0 0)-(2 × 1), and H2O + H/Si(1 0 0)-(2 × 1). In addition to the oxygen inserted coupled monohydrides, which were previously reported in the former reaction system, we report several other oxidized Si hydride species in our BML-IRRAS experiments. Three new pairs of vibrational bands are identified between 900 and 1000 cm−1. These vibrational frequencies were calculated using Si9 and Si10 cluster models that included all possible structures from zero to five oxygen insertions into the top layer silicon atoms using a B3LYP gradient corrected density functional method with a polarized 6-31G** basis set for all atoms. The three pairs of vibrational modes are assigned to the scissoring modes of adjacent and isolated SiH2 with zero, one, and two oxygen atoms inserted into the Si back bonds. All the other newly observed vibrational peaks related to Si oxidation are also assigned in this study. The Si-O stretching bands observed in the reaction 2D + H2O/Si(1 0 0)-(2 × 1) show an isotope effect, which suggests that in the system 2H + H2O/Si(1 0 0)-(2 × 1) also, hydrogen atom tunneling plays an important role for the insertion of oxygen atoms into Si back bonds that form oxidized adjacent dihydrides.  相似文献   

7.
A.M. Kiss  A. Berkó 《Surface science》2006,600(16):3352-3360
The effect of K on the morphology of Au nanoparticles deposited on TiO2(1 1 0) surface is investigated by STM-STS and AES methods. For comparison, the enhanced concentration of oxygen defect sites generated by Ar+ bombardment was also studied. It was found that both the K additive and the oxygen defect sites induce a pronounced decrease in the average size of the Au nanoparticles evolved at 320 K. On the clean TiO2(1 1 0) the average size of Au particles is 4.3 nm at approximately monolayer coverage of gold, while in the presence of K or oxygen vacancies this value decreased to 2.5 nm. In spite of the reduced average diameter detected at room temperature, the mean size of the Au nanoparticles increased significantly from 2.5 nm up to 7 nm on the effect of annealing at 500-700 K for K precoverages of 0.3-1 ML. For the clean and the Ar+ pretreated TiO2(1 1 0) surfaces the mean size of the Au particles changed only slightly on the effect of the same thermal treatments.  相似文献   

8.
Ab initio total energy methods are used to investigate the effects on a Ge(1 1 1)-5 × 5 surface of the lateral compressive stress that would be due to a Si substrate, and the effects of intermixing at the interface with the substrate. The effects of stress due to the lattice mismatch between Si and Ge are studied on a Ge slab by changing the lattice constant in the surface plane from that of experimental bulk diamond Ge to that of Si. When this is done the height difference of the Ge adatoms in the faulted half-cell from those in the unfaulted half is accentuated. Effects on the Ge surface due to the presence of the Si-Ge interface were studied using a thin Ge layer on a Si substrate. The presence of the substrate leads to corrugations with significant height differences appearing among the faulted adatoms. The energetics of intermixing were investigated for Si-Ge single atom interchanges. Additional corrugations resulted from the shortened bondlengths due to the Si impurity in the wetting layer.  相似文献   

9.
We have investigated the adsorption mechanism of SiO molecule incident on a clean Si(1 0 0) p(2 × 2) reconstructed surface using density functional theory based methods. Stable adsorption geometries of SiO on Si surface, as well as their corresponding activation and adsorption energies are identified. We found that the SiO molecule is adsorbed on the Si(1 0 0) surface with almost no activation energy. An adsorption configuration where the SiO binds on the channel separating the dimer rows, forming a Si-O-Si bridge on the surface, is the energetically most favourable geometry found. A substantial red-shift in the calculated vibrational frequencies of the adsorbed SiO molecule in the bridging configurations is observed. Comparison of adsorption energies shows that SiO adsorption on a Si(1 0 0) surface is energetically less favourable than the comparable O2 adsorption. However, the role of SiO in the growth of silicon sub-oxides during reactive magnetron plasma deposition is expected to be significant due to the relatively large amount of SiO molecules incident on the deposition surface and its considerable sticking probability. The stable adsorption geometries found here exhibit structural properties similar to the Si/SiO2 interface and may be used for studying SiOx growth.  相似文献   

10.
To reveal the influence of erbium interlayer on the formation of nickel silicide and its contact properties on Si substrate, Er(0.5-3.0 nm) and Ni(20 nm) are successively deposited onto Si(1 0 0) substrate and are treated by rapid thermal annealing in pure N2 ambient. The NiSi formation temperature is found to increase depending on the Er interlayer thickness. The formation temperature of NiSi2 (700 °C) is not influenced by Er addition. But with 2 nm Er interlayer, the formed NiSi2 is observed textured with preferred orientation of (1 0 0). During the formation of NiSi, Er segregates to the surface and little Er remains at the NiSi/Si(1 0 0) interface. Therefore, the Schottky barrier height of the formed NiSi/n-Si(1 0 0) contact is measured to be 0.635 ∼ 0.665 eV which is nearly invariable with different Er addition.  相似文献   

11.
A. Bahari  Z.S. Li 《Surface science》2006,600(15):2966-2971
The growth of ultrathin films of Si3N4 directly on Si surfaces is studied with valence band photoemission. The information from these studies about the growth mechanism and the changes of the electronic structure is enhanced by the use of various photon energies with synchrotron radiation. The silicon nitride films are grown isothermally on the Si(1 0 0) and Si(1 1 1) surfaces by reactions with atomic N. The atomic nitrogen is produced by using a remote, microwave excited nitrogen plasma. The growth under these conditions was earlier shown to be self limiting. The details in the valence band spectra are identified and resolved with numerical methods, and followed systematically during the growth. Thus the identification of Si surface states, Si-nitride interface states and bulk nitride states becomes possible. The previously obtained separation between amorphous and crystalline growth occurring around 500 °C is further supported in the present studies.  相似文献   

12.
Xueing Zhao 《Surface science》2007,601(12):2445-2452
This article reports photoemission and STM studies for the adsorption and dissociation of water on Ce-Au(1 1 1) alloys and CeOx/Au(1 1 1) surfaces. In general, the adsorption of water at 300 K on disordered Ce-Au(1 1 1) alloys led to O-H bond breaking and the formation of Ce(OH)x species. Heating to 500-600 K induced the decomposition or disproportionation of the adsorbed OH groups, with the evolution of H2 and H2O into gas phase and the formation of Ce2O3 islands on the gold substrate. The intrinsic Ce ↔ H2O interactions were explored by depositing Ce atoms on water multilayers supported on Au(1 1 1). After adsorbing Ce on ice layers at 100 K, the admetal was oxidized immediately to yield Ce3+. Heating to room temperature produced finger-like islands of Ce(OH)x on the gold substrate. The hydroxyl groups dissociated upon additional heating to 500-600 K, leaving Ce2O3 particles over the surface. On these systems, water was not able to fully oxidize Ce into CeO2 under UHV conditions. A complete Ce2O3 → CeO2 transformation was seen upon reaction with O2. The particles of CeO2 dispersed on Au(1 1 1) did not interact with water at 300 K or higher temperatures. In this respect, they exhibited the same reactivity as does a periodic CeO2(1 1 1) surface. On the other hand, the Ce2O3/Au(1 1 1) and CeO2−x/Au(1 1 1) surfaces readily dissociated H2O at 300-500 K. These systems showed an interesting reactivity for H2O decomposition. Water decomposed into OH groups on Ce2O3/Au(1 1 1) or CeO2−x/Au(1 1 1) without completely oxidizing Ce3+ into Ce4+. Annealing over 500 K removed the hydroxyl groups leaving behind CeO2−x/Au(1 1 1) surfaces. In other words, the activity of CeOx/Au(1 1 1) for water dissociation can be easily recovered. The behavior of gold-ceria catalysts during the water-gas shift reaction is discussed in light of these results.  相似文献   

13.
Au/TiO2/Ru(0 0 0 1) model catalysts and their interaction with CO were investigated by scanning tunneling microscopy and different surface spectroscopies. Thin titanium oxide films were prepared by Ti deposition on Ru(0 0 0 1) in an O2 atmosphere and subsequent annealing in O2. By optimizing the conditions for deposition and post-treatment, smooth films were obtained either as fully oxidized TiO2 or as partly reduced TiOx, depending on the preparation conditions. CO adsorbed molecularly on both oxidized and reduced TiO2, with slightly stronger bonding on the reduced films. Model catalyst surfaces were prepared by depositing submonolayer quantities of Au on the films and characterized by X-ray photoelectron spectroscopy and scanning tunneling microscopy. From X-ray photoelectron spectroscopy, a weak interaction between the Au and the TiO2 substrate was found. At 100 K CO adsorption occurred on both the TiO2 film and on the Au nanoparticles. CO desorbed from the Au particles with activation energies between 53 and 65 kJ/mol, depending on the Au coverage. If the Au deposit was annealed to 770 K prior to CO exposure, the CO adsorption energy decreased significantly. STM measurements revealed that the Au particles grow upon annealing, but are not encapsulated by TiOx suboxides. The higher CO adsorption energy observed for smaller Au coverages and before annealing is attributed to a significantly stronger interaction of CO with mono- and bilayer Au islands, while for higher particles, the adsorption energy becomes more bulk-like. The implications of these effects on the known particle size effects in CO oxidation over supported Au/TiO2 catalysts are discussed.  相似文献   

14.
T. Suzuki  J. Levy 《Surface science》2006,600(2):366-369
Two large and complex adsorbed organic molecules, coronene (C24H12) and C60, have been used to produce Si dimer vacancy defects on Si(0 0 1) by thermal decomposition. Studies by STM show that the aligned structural arrangement of the dimer vacancy defects produced is independent of the chemical structure of the organic molecules. This indicates that the chemistry of the thermally decomposed carbon species produced by decomposition of the organic molecule controls the organization of the Si dimer vacancy defects. It is found that ∼1 C atom is responsible for each dimer vacancy defect for both molecules in accordance with earlier studies of C2H2 decomposition on Si(0 0 1).  相似文献   

15.
The formation of ordered Sr overlayers on Si(1 0 0) by Atomic Layer Deposition (ALD) from bis(triisopropylcyclopentadienyl) Strontium (Sr(C5iPr3H2)2) and H2O has been investigated. SrO overlayers were deposited on a 1-2 nm SiO2/Si(1 0 0) substrate, followed by a deoxidation process to remove the SiO2 layer at high temperatures. Auger electron spectroscopy, Rutherford backscattering spectrometry, spectroscopic ellipsometry, and low-energy electron diffraction were used to investigate the progress of both ALD and deoxidation processes. Results show that an ordered Sr/Si(1 0 0) surface with 2 × 1 pattern can be obtained after depositing several monolayers of SrO on Si using ALD followed by an anneal at 800-850 °C. The (2 × 1) ordered Sr/Si(1 0 0) surface is known to be an excellent template for the epitaxial growth of SrTiO3 (STO) oxide. The present results demonstrate that ALD is a potential alternative to molecular beam epitaxy methods for the fabrication of epitaxial oxides on semiconductor substrates.  相似文献   

16.
The covalent attachment of alkyl groups to silicon surfaces, via carbon-silicon bond formation, has been attempted using gas-surface reactions starting from Cl-terminated Si(1 1 1) or H:Si(1 1 1) under ultraviolet light irradiation. The formation of Cl-terminated Si(1 1 1) and its resulting stability were examined prior to deposition of organic molecules. High-resolution electron energy loss spectroscopy (HREELS) was utilized for detecting surface-bound adsorbates. The detection of photo-deposited organic species on Cl:Si(1 1 1) from gas-phase CH4 or CH2CH2 was not significant. On H:Si(1 1 1), it was evident that after the photoreaction with gas-phase C2H5Cl, C2H5 groups were chemically bonded to the surface Si atoms through single covalent bonds. The C2H5 groups were thermally stable at temperatures below 600 K. Alkyl monolayers prepared on silicon surfaces by dry process will lead to a new prospective technology of nanoscale fabrication and biochemical applications.  相似文献   

17.
The growth mode and electronic structure of Au nano-clusters grown on NiO and TiO2 were analyzed by reflection high-energy electron diffraction, a field-emission type scanning electron microscope, medium energy ion scattering and photoelectron spectroscopy. Au was deposited on clean NiO(0 0 1)-1 × 1 and TiO2(1 1 0)-1 × 1 surfaces at room temperature with a Knudsen cell at a rate of 0.25-0.35 ML/min (1 ML = 1.39 × 1015 atoms/cm2:Au(1 1 1)). Initially two-dimensional (2D) islands with thickness of one Au-atom layer grow epitaxially on NiO(0 0 1) and then neighboring 2D-islands link each other to form three-dimensional (3D)-islands with the c-axis oriented to the [1 1 1] direction. The critical size to form 3D-islands is estimated to be about 5 nm2. The shape of the 3D-islands is well approximated by a partial sphere with a diameter d and height h ranging from 2.0 to 11.8 nm and from 0.95 to 4.2 nm, respectively for Au coverage from 0.13 to 4.6 ML. The valence band spectra show that the Au/NiO and Au/TiO2 surfaces have metallic characters for Au coverage above 0.9 ML. We observed Au 4f spectra and found no binding energy shift for Au/NiO but significant higher binding energy shifts for Au/TiO2 due to an electron charge transfer from Au to TiO2. The work function of Au/NiO(0 0 1) gradually increases with increase in Au coverage from 4.4 eV (NiO(0 0 1)) to 5.36 eV (Au(1 1 1)). In contrast, a small Au deposition(0.15 to 1.5 ML) on TiO2(1 1 0) leads to reduction of the work function, which is correlated with an electron charge transfer from Au to TiO2 substrate.  相似文献   

18.
In H2 and H2/CO oxidation, the H + O2 + M termination step is one of the most important reactions at elevated pressures. With the recent, increased interest in synthetic fuels, an accurate assessment of its rate coefficient becomes increasingly important, especially for real fuel/air mixtures. Ignition delay times in shock-tube experiments at the conditions selected in this study are only sensitive to the rates of the title reaction and the branching reaction H + O2 = OH + O, the rate of which is known to a high level of accuracy. The rate coefficient of the title reaction for M = N2, Ar, and H2O was determined by adjusting its value in a detailed chemical kinetics model to match ignition delay times for H2/CO/O2/N2, H2/CO/O2/Ar, and H2/CO/O2/N2/H2O mixtures with fuel/air equivalence ratios of ? = 0.5, 0.9, and 1.0. The rate of H + O2 + N2 = HO2 + N2 was measured to be 2.7 (−0.7/+0.8) × 1015 cm6/mol2 s for T = 916-1265 K and P = 1-17 atm. The present determination agrees well with the recent study of Bates et al. [R.W. Bates, D.M. Golden, R.K. Hanson, C.T. Bowman, Phys. Chem. Chem. Phys. 3 (2001) 2337-2342], whose rate expressions are suggested herein for modeling the falloff regime. The rate of H + O2 + Ar = HO2 + Ar was measured to be 1.9 × 1015 cm6/mol2 s for T = 932-965 K and P = 1.4 atm. The rate of H + O2 + H2O = HO2 + H2O was measured to be 3.3 × 1016 cm6/mol2 s for T = 1071-1161 K and P = 1.3 atm. These are the first experimental measurements of the rates of the title reactions in practical combustion fuel/air mixtures.  相似文献   

19.
Density functional theory is employed to investigate atomic layer deposition mechanism of HfO2 on Ge(1 0 0)-2 × 1 surface. Both the HfCl4 and H2O half-reactions proceed through an analogous trapping-mediated mechanism. The neighboring hydroxyl in the reaction of HfCl4 with two Ge-OH* sites has a major effect on the formation of HfCl4 adsorbed complex. In addition, both the Ge and Si reaction pathways are qualitatively similar, however, adsorption of HfCl4 is favorable on Ge than on Si surface hydroxyl sites. By comparison of the reactions of H2O on the different surfaces, the differences in energy are negligible to alter the reaction mechanism.  相似文献   

20.
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   

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