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V. Venugopal 《Thin solid films》2009,517(18):5482-5488
Thermal helium desorption spectrometry has been used to study the interaction of helium with defects in Cu films (5-300 Å) deposited on Ta(110) and Ta(100) single crystals by ultrahigh vacuum electron beam evaporation. The thermal stability of the Cu films was also investigated. Cu films on Ta(110) and Ta(100) at room temperature are metastable and on heating, the films transform into islands. The temperature at which this takes place is strongly dependent on the Cu film thickness and for a given thickness (> 40 Å) occurs at a lower temperature on Ta(100) than on Ta(110). The activation energy for island formation is 1.6 ± 0.4 eV for 50 Å Cu/Ta(110) and 0.8 ± 0.1 eV for 100 Å Cu/Ta(100) obtained by Kissinger analysis. The geometry of the Cu islands resulting from annealing 50 Å Cu films at 1000 K for 10 s depends strongly on the Ta substrate orientation. There is evidence for the stressed states of both the Cu films and the Ta substrates. Helium release from monovacancies and vacancy clusters in Cu films (> 75 Å) on Ta(110) and Ta(100) was detected at ~ 750 K and ~ 800-1000 K respectively. The sublimation of the Cu films from the Ta substrates could be observed by the release of retained helium at ~ 1300 K.  相似文献   

4.
We report studies of the interaction of atoms and molecules with solid molecular surfaces, water in particular, by combining photoelectron spectroscopy, UPS with HeI and II, and metastable impact electron spectroscopy (MIES). In MIES charge exchange processes of the Auger-type taking place between metastable He atoms and the surface under study are utilized to gain information on their electronic structure. The MIES spectra give a rather direct image of the surface DOS. We concentrate on the following processes taking place on water films produced at 80 K:(1) Interaction of Na Atoms with Amorphous Solid H2O Films: emphasis was on the role of the 3sNa electrons in the water dissociation process. In order to make a detailed comparison with density functional theory (DFT), DOS (density of states) information is compared with the MIES spectra. Our results are consistent with the theoretical prediction that the 3s-electron is delocalized from the Na-core and trapped (solvated) between the Na-core and water molecules of the surrounding water shell.(2) Ionization and Solvation of NaCl Interacting with Amorphous Solid Water: at 90 K there is no interpenetration of H2O and NaCl. However, ionic dissociation of NaCl takes place when H2O and NaCl are in direct contact. At 105 K the solvation of the ionic species Cl and Na+ becomes significant. The desorption of H2O from the mixed film takes place between 145 and 170 K; those species bound ionically to Na+ and Cl are removed last.(3) The Interaction of PBTs (persistent, bio-accumulative, and toxic substances), chlorobenzene and chlorophenyl, with amorphous solid water: the organic layers produced at 80 K were annealed up to 200 K under in situ control of MIES and UPS. The different behaviour of the interfaces for the three studied cases is traced back to the different mobilities of the molecules with respect to that of water. The interaction between H2O and the benzene derivatives is discussed on the basis of qualitative free energy profiles.  相似文献   

5.
Imre Kovács  János Kiss 《Vacuum》2007,82(2):182-185
The formation of PdZnx alloy on Pd(1 0 0) and its characteristics were investigated by various methods, such as photoelectron, auger-electron, electron energy loss, thermal desorption spectroscopic methods and work-function measurement. The alloy was produced by the decomposition of diethyl zinc on Pd(1 0 0). The alloy surface reacts with O2 and ZnOx is formed. The reactivity of alloy to hydrogen is similar to that of K/Pd. The stability of adsorbed CO is lower than on clean Pd(1 0 0).  相似文献   

6.
The behavior of Co atoms on Si (111)-7 × 7 surfaces at low temperatures was studied by using a variable-temperature scanning tunneling microscopy (VT-STM). Co atoms deposited on Si (111)-7 × 7 surfaces are randomly adsorbed at 100 K. Co atoms start to react with adatoms of Si (111)-7 × 7 surfaces at temperatures between 126 K and 130 K. The reaction transfers the bright dots of Co atoms to dark dots under the STM observation of negative bias. Analysis of the reaction occurrence sites and comparing with the results of room temperature deposition shows that the Co atoms tend to diffuse and react with the adatoms of Si (111)-7 × 7 surfaces at the center sites of unfaulted half unit cell (UHUC) at higher temperatures.  相似文献   

7.
Self-assembled nanostructures on a periodic template are fundamentally and technologically important as they put forward the possibility to fabricate and pattern micro/nano-electronics for sensors, ultra high-density memories and nanocatalysts. Alkali-metal (AM) nanostructure grown on a semiconductor surface has received considerable attention because of their simple hydrogen like electronic structure. However, little efforts have been made to understand the fundamental aspects of the growth mechanism of self-assembled nanostructures of AM on semiconductor surfaces. In this paper, we report organized investigation of kinetically controlled room-temperature (RT) adsorption/desorption of sodium (Na) metal atoms on clean reconstructed Si (111)-7 × 7 surface, by X-ray photoelectron spectroscopy (XPS). The RT uptake curve shows a layer-by-layer growth (Frank-vander Merve growth) mode of Na on Si (111)-7 × 7 surfaces and a shift is observed in the binding energy position of Na (1s) spectra. The thermal stability of the Na/Si (111) system was inspected by annealing the system to higher substrate temperatures. Within a temperature range from RT to 350 °C, the temperature induced mobility to the excess Na atoms sitting on top of the bilayer, allowing to arrange themselves. Na atoms desorbed over a wide temperature range of 370 °C, before depleting the Si (111) surface at temperature 720 °C. The acquired valence-band (VB) spectra during Na growth revealed the development of new electronic-states near the Fermi level and desorption leads the termination of these. For Na adsorption up to 2 monolayers, decrease in work function (−1.35 eV) was observed, whereas work function of the system monotonically increases with Na desorption from the Si surface as observed by other studies also. This kinetic and thermodynamic study of Na adsorbed Si (111)-7 × 7 system can be utilized in fabrication of sensors used in night vision devices.  相似文献   

8.
Adsorption and desorption of CO on Ni(100) and Pt(111) surfaces are presented. At the thermodynamic equilibrium, the site occupation between the terminal and the bridged sites are described with the free energy of the system, including the vibrational entropy. Adsorption of CO onto a cold surface as 20 K has also been studied by infrared reflection absorption spectroscopy (IRAS). The occupation ratio of bridged CO to terminal CO species on Ni(100) at 20 K ranges from ~ 2·8 to 0·7 at the total coverage from 0·003 to 0·15 ML. Such strong coverage dependence of the occupation ratio even at small coverages suggests that the interaction between CO molecules operates at relatively long range (> 10 Å). The isotope experiments suggest that there is substantial interaction between preadsorbed (accommodated) CO species and incoming (mobile) CO species. Desorption process is also affected by the interaction between the adsorbed CO and the incoming species. The effect of temporal bimolecular CO interaction on the desorption kinetics is also discussed.  相似文献   

9.
The density of neutral oxygen atoms was determined in a plasma reactor for surface functionalization of polymer materials. Plasma was created in a stainless steel chamber by capacitively coupled RF generator at 13.56 MHz and adjustable forward power up to 100 W. Measurements were performed with a classical nickel catalytic probe. Systematic measurements were performed at a constant pumping speed, different flow rates from 15 to 100 sccm corresponding to pressures between 30 and 110 Pa, different powers between 40 and 100 W and different probe positions in the discharge chamber. The results showed that the O atom density did not depend much on probe position as long as it was between the powered electrode and grounded housing facing the electrode. The O density depended rather linearly with power at fixed pressure. At low power, the O density did not depend much on pressure, but at high power, it was increasing with increasing pressure. The O density was of the order of 1019 m−3 and increased slightly over 1020 m−3 at the highest power and pressure. The results were explained by gas phase and surface reactions.  相似文献   

10.
Z.H. Gan  L.M. Qiu  X.B. Zhang  Y.L. He 《低温学》2009,49(5):198-201
In order to explore the lowest attainable refrigeration temperature and improve cooling performance at temperatures around 20 K for a single-stage G-M type pulse tube cryocooler (PTC), numerical and experimental studies were performed. The National Institute of Standards and Technology (NIST) numerical model known as REGEN was applied to the simulation of a G-M type PTC for the first time. Based on the calculation results, a single-stage G-M type PTC was designed, fabricated and tested. The performance improvement of the regenerator in the temperature range of 10-80 K was investigated. The calculations predicted a lowest temperature of 10 K. A lowest temperature of 10.6 K was achieved experimentally with an input power of 7.5 kW, which is the lowest temperature ever achieved by a single-stage PTC. Further more, the cryocooler can provide a cooling power of 20 W at 20.6 K and 39.5 W at 30 K, respectively.  相似文献   

11.
Thin single-crystal yttrium-stabilized zirconia (YSZ) substrate was prepared by indentation fracture and mechanical polishing. The specimen was analyzed in detail by transmission electron microscopy (TEM). The (110) edge surface was faceted, in contrast to the smooth (001) edge surface, and the facet surfaces were identified as {111}-type planes. Good cross-sectional TEM specimens comprised of crystalline Ni and NiO layers deposited on YSZ edge surface could be prepared by sputtering of a Ni support grid using Ar+ ion milling and subsequent re-deposiotion on the smooth (001) fracture surface of the YSZ specimen. The epitaxial growth of a pure Ni layer on the YSZ edge planes occurred during ion milling in vacuum. However, subsequent ion milling of the specimen after exposure in air for several minutes resulted in the formation of a NiO layer on top of the first Ni layer. Reduction of the NiO layer was confirmed by electron energy-loss spectroscopy after annealing at 973 K in a vacuum of 1.2 × 10−5 Pa. This Ni layer was re-oxidized upon annealing in air at 1073 K for 1 h. The deposition behavior of the Ni and NiO layers was discussed on the basis of the surface oxidation of Ni layer.  相似文献   

12.
In order to improve the cooling performance of pulse tube cooler (PTC) at 20-40 K, hybrid regenerators are often employed. In this paper a three-layer regenerator, which consists of woven wire screen, lead sphere and Er3Ni is optimized to enhance the cooling performance and explore the lowest attainable refrigeration temperature for a single-stage PTC. The efforts focus on the temperature range of 80-300 K, where woven wire screens are used. Theoretical and experimental studies are carried out to study the metal material and the mesh size effect of woven wire screens on the performance of the single-stage G-M type PTC. A lowest no-load refrigeration temperature of 11.1 K was obtained with an input power of 6 kW. The PTC can supply 17.8 W at 20 K and 39.4 W at 30 K, respectively.  相似文献   

13.
A Kutana  T Ito  B Makarenko 《Vacuum》2004,73(1):73-78
The kinetics of atomic hydrogen isothermal adsorption and desorption on a Si(1 0 0) surface was studied using the time-of-flight scattering and recoiling spectrometry technique at temperatures below and above the thermal desorption threshold. A continuous decrease in saturation coverage with temperature under constant atomic hydrogen exposure has been observed in both regions for temperatures in the range 325-820 K. For TS=500-650 K, the decrease is described by a kinetic model where Eley-Rideal (ER) abstraction is responsible for hydrogen removal from the surface and hydrogen coverage depends on the temperature due to the changing rate of migration from precursor to primary monohydride sites. For TS=650 K and higher, in addition to the ER abstraction, the thermal desorption from primary monohydride sites leads to a further decrease of the saturation coverage. The first-order desorption rates after source shut-off have been measured and an activation barrier of 1.89 eV has been obtained.  相似文献   

14.
M. Balden 《Thin solid films》2011,519(12):4032-4036
Bonding structure of carbon and metal as well as nanostructural changes of metal-doped amorphous carbon films (a-C:Me) were investigated depending on metal type (W, Ti, V, and Zr), concentration (<25 at.%) and annealing temperature (< 1300 K, except W: < 2800 K). Pure C films exhibit ~ 2 nm distorted aromatic and graphene-like regions. Both increase in size with annealing. After deposition the metals have carbide-like bonding and are mainly distributed atomically disperse in an amorphous environment. Annealing leads to the formation of carbide crystallites (TiC, VC, ZrC, WC, W2C, and WC1 − x) of several nanometers. The VC particles reach the largest size up to 1300 K. All metal dopings reduce the erosion rate against oxidation (expect V) and hydrogen impact.  相似文献   

15.
The growth of In onto W(1 1 0) was observed at room temperature by means of low-energy electron diffraction and photoemission from core levels and valence levels. Surface diffusion of In onto the W(1 1 0) surface was studied by using ESCA imaging property of SCIENTA ESCA200 instrument at the temperature range of 400-550 K.  相似文献   

16.
A new material constituted by cerium dioxide highly dispersed on activated carbon (CeO2/AC) was prepared by an impregnation method using cerium(III) nitrate as CeO2 precursor. In order to evaluate the degree of ceria dispersion on the carbon support, CeO2/AC was characterized by a number of techniques: thermogravimetry coupled with a mass spectrometer (TG-MS), N2 adsorption at 77 K, temperature-programmed desorption (TPD), temperature-programmed reduction (TPR) and transmission electron microscopy (TEM). The analysis of the decomposition process under inert atmosphere indicated that cerium nitrate decomposes at 440-460 K, with the evolution of NO. Furthermore, this process produces an additional oxidation of the carbon surface (with evolution of N2O) and the subsequent onset of new oxygen surface groups, detected by means of temperature-programmed desorption. The ceria deposition process takes place with a decrease in the N2 adsorption capacity of the starting carbon support, and the analysis of the pore size distribution showed that the majority of ceria particles are situated at the most internal part of the carbon porosity. The temperature-programmed reduction profile of CeO2/AC was very different to that shown by unsupported CeO2, with only one continuous reduction process at low temperatures (800-900 K). Finally, TEM pictures gave direct evidence that ceria is highly dispersed on the carbon surface, with a narrow CeO2 particle distribution centred around 3 nm.  相似文献   

17.
The growth and thermal conductivity of InAs quantum dot (QD) stacks embedded in GaInAs matrix with AlAs compensating layers deposited on (1 1 3)B InP substrate are presented. The effect of the strain compensating AlAs layer is demonstrated through Atomic Force Microscopy (AFM) and X-ray diffraction structural analysis. The thermal conductivity (2.7 W/m K at 300 K) measured by the 3ω method reveals to be clearly reduced in comparison with a bulk InGaAs layer (5 W/m K). In addition, the thermal conductivity measurements of S doped InP substrates and the SiN insulating layer used in the 3ω method in the 20–200 °C range are also presented. An empirical law is proposed for the S doped InP substrate, which slightly differs from previously presented results.  相似文献   

18.
In this paper we report experimental results on investigation of Mo (110) single crystals surface topology carried out using AFM technique after high-temperature homoepitaxy within the frame of deposition rates 1-40 ML/s and in the interval of substrate temperatures 1900 °C-2300 °C. The systems of atomically flat steps on the surface of single crystal samples were observed. Typical height of the steps was in the range 1-4 of (110) parallel lattice plane spacing, with width being about 50-70 nm. High temperature homoepitaxy of Mo leads to formation of micro-crystals with dimensions up to 150 μ in width and few micrometres in height. Huge monatomic steps up to 5-10 μ in width were observed on flat (110) Mo micro-crystal planes slightly declined to the substrate surface. The root-mean-square (RMS) roughness of the micro-crystal surfaces achieves the values of 0.2 nm which is determined by the presence of monoatomic steps. The residual pattern of spiral growth was also observed on the top of (110) micro-crystal surfaces.  相似文献   

19.
The purpose of this study was to prepare an activated carbon (AC)-supported copper catalyst by a simple method - the microwave polyol process - and to evaluate the effects of microwave power and protecting agent on the resulting catalyst activity. The catalysts were characterized by Brunauer-Emmett-Teller (BET) surface area, X-ray diffraction (XRD), and field-emission scanning electron microscopy (FESEM). Experimental results indicated that a microwave power of 700 W was highly effective, and that copper particles (60 (±18) nm) dispersed well on the AC support, even in the absence of the protecting agent poly-vinyl pyrrolidone (PVP). The AC-supported Cu0 catalyst generated high catalytic activity for CO oxidation (90% CO conversion at 175 °C).  相似文献   

20.
Low temperature (115-160 K) adsorption of CO on clean Cu(0 0 1), CO2 on potassium modified Cu(0 0 1) and surface reactions between H2 and CO2 were studied. Pre-adsorption of K leads to strong interaction of the interface with arriving carbon dioxide molecules and linear CO2(phys), bent , CO and are observed as adsorbed species. Annealing of the CO2/K/Cu(0 0 1) interface causes, successively, desorption of the reaction products. Pre-adsorbed hydrogen on the alkali-modified substrate changes the surface reaction channels, mainly with no presence of CO and creation of surface formate species. Details of surface reactions are compared with results for thick potassium film on a copper substrate.  相似文献   

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