Vacuum-cleaved surfaces

Water adsorption on MgO(lOO) has been subject of two recent quantitative structural determinations [9,14]. The first of these is a LEED-IV study of a p(3x2) phase [9] formed following exposure of a vacuum-cleaved surface at 200 K to 5x10" mbar of water for 30 min, and then maintaining a water partial pressure of 1x10" mbar. This procedure resulted in a stable water overlayer of coverage 1 monolayer (ML) (where 1 ML is equal to the number of surface... 

These techniques have been applied to ultrahigh vacuum cleaved surfaces of several III-V materials (GaSb, GaAs, GaP,... 

The most important ZnS surface is the (110), which is the most common growth surface and is also the perfect cleavage surface. Therefore, the calculation is based on the ZnS (110) surface. The surfaces are cleaved from the bulk ZnS with the optimtun tinit cell volume determined using the GGA with CASTER The Cu and Fe doped surfaces are built by the substitution of Cu or Fe for Zn atom on the cleaved surface. A vacuum spacing of 1.5 nm is inserted in the z-direction to form a slab and mimic a 2D surface. In order to eliminate the interactions between mirror images in the z-direction due to the periodic boimdary conditions, in test calculations, we have done some total energy calculation to find a proper thickness of slab. The result shows that 1.5 run is a desirable thickness. 

In view of the complexity of real supported catalysts, consisting of randomly oriented and irregularly shaped metal particles on high surface area porous supports, well oriented and regularly shaped metal particles grown on planar thin supports are frequently used as model catalysts [19]. This facilitates the study by surface science and TEM techniques [11, 74, 75]. In the present work, Pt particles were grown at 623 K by electron beam evaporation of Pt at a pressure of 10 mbar on vacuum-cleaved (001) NaCl... 

The important role played by defect density in chemical reactivity can be seen in Figure 7.8, which shows Is photoemission spectra of a vacuum-cleaved, clean MgO (100) surface in comparison with (1) the Is spectrum of a vacuum-cleaved MgO (100) surface exposed to 2.3 x 10-8 torr p(H20) for 3 min, (2) the spectrum of an MgO (100) surface purposely cleaved off angle in vacuum then exposed to p(H20) = 3 x 10-8 torr for 3 minutes, and (3) the spectrum of an MgO (100) surface that was Ar+ ion sputtered and then exposed to p(H20) = 2.5 x 10 x torr... 

Figure 7.8. Is spectra of vacuum-cleaved, clean flat MgO(lOO) and various water-dosed MgO(lOO) surfaces (3 min exposures at the pressures indicated). One (clean surface) and two component (water-dosed surfaces) fits are shown, along with the fit residual. The incident photon energy was 620 eV. (from [144])...

Casey and Buehler have shown that the surface recombination velocity of n-InP ( 5xl017 carriers/cm3) is low, 103cm/sec.17 Suzuki and Ogawa have recently reported a sequence of surface treatments that cause substantial changes in the surface recombination velocity of InP.18 They found that in freshly vacuum cleaved (110) faces v, is much greater than at air exposed faces and that the quantum efficiency of band gap luminescence increases by an order of magnitude when the freshly cleaved face is exposed to air. This suggests that the surface recombination velocity is reduced when 02 is chemisorbed. 

Alumina, silica and many other metal oxides are insulators. However, recent experiments indicate that the surfaces of these insulators are mainly ionic (Masel, 1996). The pristine or freshly cleaved surfaces of single crystals of these oxides (cleaved under ultrahigh vacuum) are fairly inert and do not have significant adsorption capacities for even polar molecules such as CO and S02 (Masel, 1996 Henrich and Cox, 1994). However, the surface chemistry and adsorption properties are dominated by defects on real surfaces. For example, oxide vacancies on alumina expose the unsaturated aluminum atoms, which are electron acceptors, or Lewis acid sites. 

The work functions and ionization potentials of sputter-deposited ZnO and ZnO Al films are shown in Fig. 4.13. The different Fermi level positions of ZnO and ZnO Al for deposition at room temperature in pure Ar are also observed in the work function. The undoped films prepared under these conditions have a work function of 4.1eV, while the Al-doped films show values of 3.2eV. The difference is almost of the same magnitude as for the Fermi level position and, therefore, explained by the different doping level. Also the ionization potentials are almost the same under these preparation conditions. The work function of the undoped material is close to the value reported by Moormann et al. for the vacuum-cleaved Zn-terminated (0001) surface [20], The same authors report a work function of 4.95 eV for the oxygen terminated ZnO(OOOl) surface, which is in good agreement with the values obtained for films deposited with >5% oxygen in the sputter gas. Since the Fermi level position of the undoped ZnO films does not depend on the oxygen content in the sputter gas (Fig. 4.12), the different work functions correspond to different ionization potentials. 

Ohta, M., et al. (1995), Atomically resolved image of cleaved surfaces of compound semiconductors observed with an ultrahigh vacuum atomic force microscope,/. Vac. Sci. Technol., 13(3), 1265-1267. 

To shed light on this problem it was necessary to perform thermal desorption measurements on cleaved single crystal surfaces, being the surfaces with the least number of defects [64]. In Figs. 6 and 7 TDS data for CO and NO on vacuum-cleaved NiO(lOO) are compared with data for thin NiO(lOO) films grown... 

Liu P, Kendelewicz T, Brown GE Jr (1998b) Reaction of water with MgO (100) surfaces II. Synchrotron X-ray photoemission spectroscopy studies of defective surfaces. Surf Sci 412/413 315-332 Liu P, Kendelewicz T, Brown GE Jr, Parks GA, Pianetta P (1998c) Reaction of water with vacuum-cleaved CaO(lOO) surfaces An X-ray photoemission spectroscopy study. Surf Sci 416 326-340 Liu P, Kendelewicz T, Brown GE Jr, Nelson EJ, Chambers SA (1998d) Reaction of water with a-Al203 and a-Fe203 (0001) surfaces synchrotron X-ray photoemission studies and thermodynamic calculations. Surf Sci 417 53-65... 

Point defects are mainly involved in chemisorption on cleaved surfaces. On such surfaces the density of point defects is usually very small. They can be created by heating in vacuum and rapid quenching or by electron or ion bombardment. The predominant point defects are 0 vacancies (F centers) with two electrons at each vacancy in order to maintain the local electroneutrality. This localized charge partially overlaps with the adjacent cations, increasing die population of their lowest empty orbitals. The charge distribution does not affect signilicandy the anions near the vacancy defect because their already filled 2p shell cannot accept additional charge. The O vacancies affect predominandy die electronic structure in maximal... 

Each newly cleaved mica surface is very clean. Flowever, it is known that mica has a strong tendency to spontaneously adsorb particles [45] or organic contaminants [46], which may affect subsequent measurements. The mica sheets are cut into 10 nun x 10 nun sized samples using a hot platinum wire, then laid down onto a thick and clean 100 nun x 100 nun mica backing sheet for protection. On the backing sheet, the mica samples can be transferred into a vacuum chamber for themial evaporation of typically 50-55 mn thick silver mirrors. 

The best developed example of a material produced by VDP is poly(p-xylylene) designated as Parylene-N by the Union Carbide Corporation. Poly(/i-xylylene) was discovered by Szwarc12 in 1957 and then commercialized by Gorham at Union Carbide.13,14 (Scheme 1). Gorham has reported that di-p-xylylene is quantitatively cleaved by vacuum vapor-phase pyrolysis at 600°C to form two molecules of the reactive intermediate /i-xylylene, which subsequently polymerizes on the cold substrate. In a system maintained at less than 1 Torr, p-xylylene spontaneously polymerizes on surfaces below 30°C to form... 

The surface condition of a silicon crystal depends on the way the surface was prepared. Only a silicon crystal that is cleaved in ultra high vacuum (UHV) exhibits a surface free of other elements. However, on an atomistic scale this surface does not look like the surface of a diamond lattice as we might expect from macroscopic models. If such simple surfaces existed, each surface silicon atom would carry one or two free bonds. This high density of free bonds corresponds to a high surface energy and the surface relaxes to a thermodynamically more favorable state. Therefore, the surface of a real silicon crystal is either free of other elements but reconstructed, or a perfect crystal plane but passivated with other elements. The first case can be studied for silicon crystals cleaved in UHV [Sc4], while unreconstructed silicon (100) [Pi2, Ar5, Th9] or (111) [Hi9, Ha2, Bi5] surfaces have so far only been reported for a termination of surface bonds by hydrogen. 

An oxide layer of about 1 nm thickness is present on a silicon wafer as received from the supplier. This oxide is called a native oxide and forms on every bare silicon surface exposed to ambient air. A bare silicon surface can be generated, for example, by cleaving a silicon crystal in high vacuum. 

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