The coadsorption of CO and H2 on Fe(100)

The coadsorption of CO and hydrogen on an Fe(100) surface was studied by temperature programmed desorption and X-ray photoelectron spectroscopy. It was found that CO adsorption blocked the subsequent dissociative adsorption of H₂, although it did not seem to affect the hydrogen binding energy. Preadsorption of hydrogen was observed to reduce the binding energy of CO subsequently adsorbed and to inhibit the dissociation of CO. A new surface species was identified in a coadsorbed layer of CO and hydrogen. This species was evidenced by the formation of a desorption peak for H₂ at 475 K when CO was adsorbed subsequent to H₂ adsorption.     

The chemisorption of CO,CO2, C2H2, C2H4, H2 and NH3 on the clean Fe(100) and (111) crystal surfaces

The chemisorption of small molecules (CO, CO₂, C₂H₂, C₂H₄, H₂ and NH₃) has been studied on the clean Fe(110) and (111) crystal faces by low-energy electron diffraction (LEED) and thermal desorption. C₂H₄ and C₂H₂ yield the same sequence of surface structures that change with temperature and crystal orientation. CO and CO₂ chemisorption similarly results in the formation of the same types of surface structures that change with surface temperature and crystal orientation. Ammonia forms several ordered surface structures on both iron crystal faces. All of the molecules decompose as a function of temperature on the iron surfaces as indicated by the Auger and thermal desorption spectra.     

A thermal desorption study of thiophene adsorbed on the clean and sulfided Mo(100) crystal face

The adsorption of thiophene (C₄H₄S) on the clean and sulfided Mo(100) crystal surface has been studied. A fraction of the adsorbed thiophene desorbs molecularly while the remainder decomposes upon heating, evolving H₂ and leaving carbon and sulfur deposits on the surface. The reversibly adsorbed thiophene exhibits three distinct desorption peaks at 360, 230–290 and 163–174 K, corresponding to binding energies of 22, 13–16 and 7–9 kcal/mol respectively. Sulfur on the Mo(100) surface preferentially blocks the highest energy binding state and causes an increase in the amount of thiophene bound in the low binding energy, multilayer state. The thiophene decomposition reactions yield H₂ desorption peaks in the temperature range 300–700 K. We estimate that 50–66% of the thiophene adsorbed to the clean Mo(100) decomposes. The decomposition reaction is blocked by the presence of c(2 × 2) islands of sulfur and is blocked completely at θ_s = 0.5, at which point thiophene adsorption is entirely reversible.     

酞菁铜与MoS_2(0001)范德瓦耳斯异质结研究

利用光电子能谱、原子力显微镜以及低能电子衍射等表面研究手段系统研究了真空沉积生长的酞菁铜薄膜与衬底MoS2(0001)之间的范德瓦耳斯异质结界面电子结构和几何结构.角分辨光电子能谱清楚地再现了MoS2(0001)衬底在Γ点附近的能带结构.低能电子衍射结果表明,CuPc薄膜在MoS2(0001)表面沿着衬底表面[11ˉ20],[1ˉ210]和[ˉ2110]三个晶向有序生长,反映了衬底对CuPc的影响.原子力显微镜结果表明,CuPc在MoS2衬底上遵循层状-岛状生长模式:在低生长厚度下(单层薄膜厚度约为0.3 nm),CuPc分子平面平行于MoS2表面上形成均匀连续的薄膜;在较高的沉积厚度下,CuPc沿衬底晶向形成棒状晶粒,表现出明显的各向异性.光电子能谱显示界面偶极层为0.07 eV,而且能谱在膜厚1.2 nm饱和,揭示了酞菁铜与MoS2(0001)范德瓦耳斯异质结的能级结构.     

The CO + NH3 coadsorption over Re(0001) studied by optical second harmonic generation

The coadsorption of CO and NH₃ on Re(0001) was investigated utilizing the optical second harmonic generation (SHG) technique. The results were compared to temperature programmed desorption (TPD) and work function change measurements (Δ). It was found that the enhancement of the SHG due to ammonia is quenched very efficiently in the presence of small CO coverages. A quenching cross section by CO of the second harmonic response of the metal substrate due to ammonia adsorbates, is defined to be 100 ± 10 Ų. This extreme sensitivity of the SHG signal to the presence of CO adsorbed with NH₃ is in sharp contrast to both Δ and TPD measurements. These results present only a minor effect of CO on the Δ and TPD of ammonia on Re(0001). It is concluded that unlike the local contribution to the SHG signal generally assumed for separate adsorbates, the quenching by CO is a nonlocal effect. Moreover, the electronic interactions which govern the SHG from the NH₃-rhenium system, are very different from the interactions which lead to the TPD and Δ data. At ammonia preadsorption on the surface, the effect of the same amount of coadsorbed CO is largely reduced. This observation is discussed in terms of possible islanding mechanisms driven by the postadsorbed CO.     

Surface science studies of cobalt overlayers on clean and sulfur covered Mo(100) single crystal surfaces

The interaction of cobalt with clean and sulfur covered Mo(100) surfaces was investigated with Auger electron spectroscopy (AES), low energy electron diffraction (LEED) and temperature programmed desorption (TPD). On the clean surface, the deposition and subsequent annealing of one monolayer of cobalt resulted in the formation of an ordered overlayer with (1 × 1) surface structure. When cobalt was deposited on sulfur covered Mo(100) surfaces, after annealing the sulfur overlayer migrated on top of the cobalt layer. This topmost sulfur overlayer did not significantly affect the thermal desorption of cobalt from the Mo(100) surface. Various ordered structures of sulfur, cobalt and coadsorbed sulfur and cobalt were observed by LEED. A new surface structure showing (3 × 1) symmetry was observed when at least one monolayer of cobalt was deposited and annealed at 870 K on an ordered monolayer of sulfur on the Mo(100) surface. This surface structure was stable in ultrahigh vacuum up to 940 K.     

Modification of chemisorption properties by electronegative adatoms: H2 and CO on chlorided,sulfided, and phosphided Ni(100)

The effect of preadsorbed electronegative atoms Cl, S and P on the adsorption-desorption behavior of CO and H₂ on Ni(100) has been studied using thermal desorption, LEED and AES. It is found that the presence of the electronegative atoms causes a reduction of the adsorption rate, the adsorption bond strength and the capacity of the Ni(100) surface for CO and H₂ adsorption. The poisoning effect becomes stronger with increasing electronegativity of the preadsorbed atoms. In the case of CO adsorption the most tightly bound β₂ -state is suppressed most significantly. The variation of the initial sticking coefficient of CO as a function of the adlayer precoverage shows that at low Cl and S coverages the effective number of the influenced Ni surface atoms is more than four. The observed reduction of CO and H₂ adsorption and the difference in the poisoning effect of Cl, S and P is generally interpreted in terms of the changes in the surface electron density in the presence of electronegative atoms.     

The adsorption of CO,O2, and H2 on Pt(100)-(5×20)

The adsorption/desorption characteristics of CO, O₂, and H₂ on the Pt(100)-(5 × 20) surface were examined using flash desorption spectroscopy. Subsequent to adsorption at 300 K, CO desorbed from the (5×20) surface in three peaks with binding energies of 28, 31.6 and 33 kcal gmol^?1. These states formed differently from those following adsorption on the Pt(100)-(1 × 1) surface, suggesting structural effects on adsorption. Oxygen could be readily adsorbed on the (5×20) surface at temperatures above 500 K and high O₂ fluxes up to coverages of $\text{2}\text{3}$ of a monolayer with a net sticking probability to ssaturation of ? 10^?3. Oxygen adsorption reconstructed the (5 × 20) surface, and several ordered LEED patterns were observed. Upon heating, oxygen desorbed from the surface in two peaks at 676 and 709 K; the lower temperature peak exhibited atrractive lateral interactions evidenced by autocatalytic desorption kinetics. Hydrogen was also found to reconstruct the (5 × 20) surface to the (1 × 1) structure, provided adsorption was performed at 200 K. For all three species, CO, O₂, and H₂, the surface returned to the (5 × 20) structure only after the adsorbates were completely desorbed from the surface.     

The coadsorption of potassium and oxygen on Bi(0001): Accelerated oxidation

The effect of potassium adatoms upon the interaction of oxygen with Bi(0001) at room temperature has been studied using electron energy loss (ELS) and work function (Δφ) measurements, Auger electron spectroscopy (AES) and low energy electron diffraction (LEED). Submonolayer K coverages increase the rate of epitaxial BiO(0001) growth by orders of magnitude. Potassium stabilizes a new form of chemisorbed oxygen, which is thought to be a precursor in the accelerated oxidation mechanism. Growth of BiO terminates after three monolayers by diffusion limitation. The rate enhancement by potassium seems to involve localized, site specific dissociation of the O₂ molecule rather than a long-range, through-substrate electronic interaction. These results are discussed in light of similar results on transition metals and their differences in charge screening lengths.     

Effects of adsorbed carbon and oxygen on the chemisorption of H2 and Co on Mo(100)

The deposit of carbon and oxygen adatoms on Mo(100) was characterized by AES and LEED. Carbon was introduced by the thermal cracking of ethylene; several ordered structures were observed as a function of coverage with carbon atoms residing on four-fold sites. The Mo(100)—O system exhibited two different sequences of LEED patterns depending on the adsorption temperature of oxygen. The effects of adsorbed carbon and oxygen on the chemisorption properties of Mo(100) was investigated by FDS. The presence of either carbon or oxygen severely hindered the ability of Mo(100) to dissociatively adsorb hydrogen or carbon monoxide. The amount of CO dissociated was directly related to the available four-fold sites on the carbide surfaces. The molecular adsorption of CO was not significantly affected by the adlayers. It was found that one monolayer of adsorbed oxygen reduced the binding energy of molecular CO considerably more than the same amount of adsorbed carbon. A continuous shift in the binding energy of CO with the C/O ratio on the surface was observed.     

Electrical conductivity,thermoelectric power and hall effect in p-type molybdenite (MoS2) crystal

Principal electrical conductivities and seebeck coefficients, as well as Hall effect, in p-type single crystals of natural molybdenite (MoS₂) have been investigated within the temperature range 100°K to 850°K. The results have been discussed and the values of different parameters of the charge carriers obtained.     

Chemical effects of Ne bombardment on the MoS2(0001) surface studied by high-resolution photoelectron spectroscopy

The effect of 1 keV Ne⁺ bombardment on the clean MoS₂(0001)-1 × 1 surface with fluences between 4 × 10¹⁴ and 4 × 10¹⁶ Ne⁺/cm² was studied using high-resolution photoelectron spectroscopy excited with synchrotron radiation. Spectra of the Mo 3d and S 2p core levels were measured with photon energies that ensured that the kinetic energy of the photoelectrons was the same, resulting in the same depth being probed for both core levels. For lower fluences (i.e., 2 × 10¹⁵ Ne⁺/cm²), S vacancy defect formation occurs in the MoS₂ lattice, with the concurrent formation of a small amount (< 10%) of dispersed elemental molybdenum [Mo(0)]. For fluences greater than l × 10¹⁶ Ne⁺/cm², the Mo(0) is the predominant species in the surface region, while the remaining species consist of amorphous MoS_2−x and polysulfide species. Valence band spectra taken with photon energies of 152 and 225 eV were consistent with the core level results. The movement of the valence band maximum toward the Fermi level indicated the formation of a metallic surface region. Annealing the sample to temperatures up to 1000 K resulted in the formation of metallic Mo coexisting, in approximately equal amounts, with reformed MoS₂ in a surface with no long-range order as determined by LEED. Finally, a qualitative depth distribution of the chemical species present after Ne⁺ bombardment was determined by varying the photon energies used for the core level spectra. The results indicate that the preferential sputtering of sulfur over molybdenum occurs predominantly through a mechanism involving chemical bonding effects, specifically, through the preferential emission of polysulfide ions over other species in the bombarded region.     

High temperature coadsorption study of zirconium and oxygen on the W(100) crystal face

The coadsorption of zirconium and oxygen on W(100) has been studied by Auger electron spectroscopy, low energy electron diffraction, mass spectroscopy, ion sputtering, and work function measurement techniques. Adsorption of zirconium onto W(100) followed by heating in an oxygen partial pressure produces rapid diffusion of a ZrO complex into the bulk and the formation of a tungsten oxide layer. Heating in vacuum causes desorption of the tungsten oxide and segregation of the ZrO complex to the surface. The activation energy for the ZrO bulk-to-surface diffusion is 30 ± 2 kcal/mole. Upon heating in vacuum at 2000 K the composite surface exhibits predominantly a (1 × 1) LEED structure with a room temperature field emission retarding potential work function of 2.67 ± 0.05 eV. The Richardson work function for this unusually thermally stable surface is 2.56 ± 0.05 eV with a pre-exponential of 6 ± 2. The effects of carbon and nitrogen contamination on this low work function ZrOW composite surface are discussed and a structural model for the surface is presented.     

Characterization of clean and oxidized (100)LaB6

We have studied clean and oxidized (100)LaB₆ grown from aluminum melts by making Auger, LEED, evaporation and work function measurements on well-defined surfaces. The clean surface shows no La enrichment when initially heated as high as 1700°C. Its LEED pattern is 1 × 1, indicating no surface reconstruction. Langmuir evaporation studies up to temperatures of 1700°C show only La and B evaporating non-congruently, and LaO. The activation energy for B evaporation from LaB₆ (and from CeB₆ and EuB₆ also) is abot 5.5 eV, very close to that from elemental B. The rare-earth activation energies, however, vary, being highest for the rare-earth whose pure metal vapor pressure is lowest. Oxidation was carried out at room temperature using O₂ pressures up to 10^?7 Torr and at 1000°C using O₂ pressures up to 10^?4 Torr. At room temperature oxygen adsorption proceeeds to a saturated value indicated from LEED behavior to be about one monolayer. It produces a monotonic work function increase, which also saturates (at 1.40 V), varying linearly with the oxygen uptake. Oxidation at 1000°C is much more extensive than at room temperature, involving at least several monolayers, and results in a work function increase of 2.42 V. Results are discussed in terms of a terminal plane composed of La atoms, and adsorbed oxygen which, when given sufficient mobility, prefers bonding to La atoms at sites atop the B octahedra.     

Microstructure of sintered ND-Fe-Ga-B magnets with Mo and MoS2 addition

The effect of Mo and MoS₂ additions on the magnetic and microstructure properties has been investigated in Nd-Fe-Ga-B sintered magnets. Coercivity can be increased by both the additions, but the MoS₂ addition provides the larger increase per Mo atom for up to 0.6 at.% Mo. Microstructure investigation reveals a new amorphous intergranular Ga rich phase. This phase forms a thin layer in the grain boundaries and leads to a wetting behavior of the grain boundary phase, therefore increasing the coercivity. Molybdenum addition in the form of MoS₂ is found to modify the Nd₂Fe₁₄B phase, rather than form new minority phases, and the coercivity enhancement of the magnet is due to the increased anisotropy field of the hard magnetic phase.     

Adsorption of Cs and O2 on MoS2

Adsorption of Cs on basal planes of MoS₂ has been studied with LEED, Auger and work function measurements. LEED observations show that in the 200–300 K range Cs is adsorbed as amorphous layers on MoS₂. Correlation of Auger and work function measurements indicates that the work function, sticking coefficient and the maximum density of Cs that can be deposited on the MoS₂ surface depend strongly on substrate temperature. Cesium is deposited on MoS₂ in two adsorption states. Although MoS₂ is extremely inert to O₂ adsorption, the presence of Cs causes a drastic increase in the adsorption of oxygen which in turn increases the amount of Cs that can be deposited on the surface. Lastly, it has been found that part of the Cs adatoms are diffused into the bulk of MoS₂.     

Leed and thermal desorption studies of small molecules (H2, O2, CO,CO2, NO,C2H4, C2H2 AND C) chemisorbed on the rhodium (111) and (100) surfaces

The chemisorption of H₂, O₂, CO, CO₂, NO, C₂H₄, C₂H₂ and C has been studied on the clean Rh(111) and (100) surfaces. LEED, AES and thermal desorption were used to determine the surface structures, disordering and desorption temperatures, displacement and decomposition characteristics for each species. All of the molecules studied readily chemisorbed on both surfaces. A large variety of ordered structures was observed, especially on the (111) surface. The disordering temperatures of most ordered surface structures on the (111) surface were below 100°C. It was necessary to adsorb the gases at 25° C or below in order to obtain well-ordered surface structures. Chemisorbed oxygen was readily removed from the surface by H₂ or CO gas at crystal temperatures above 50°C. CO₂ appears to dissociate to CO upon adsorption on both rhodium surfaces as indicated by the identical ordering and desorption characteristics of these two molecules. C₂H₄ and C₂H₂ also had very similar ordering and desorption characteristics and it is likely that the adsorbed species formed by both molecules is the same. Decomposition of ethylene produced a sequence of ordered carbon surface structures on the (111) face as a result of a bulk-surface carbon equilibrium. The chemisorption properties of rhodium appear to be generally similar to those of iridium, nickel and palladium.     

Band structural properties of MoS2 (molybdenite)

Semiconductivity and superconductivity in MoS₂ (molybdenite) can be understood in terms of the band structure of MoS₂. We present here the band structural properties of MoS₂. The energy dependence of n_eff and ε_xeff is investigated. Using calculated values of n_eff and ε_xeff, the Penn gap has been determined. The value thus obtained is shown to be in good agreement with the reflectivity data and also with the value obtained from the band structure. The Ravindra and Srivastava formula has been shown to give values for the isobaric temperature gradient of $\text{E}{}_{\mathrm{G}}\text{[}\text{(?E}{}_{\mathrm{G}}\text{?T)}{}_{\mathrm{P}}\text{]}$, which are in agreement with the experimental data, and the contribution to $\text{(}\text{?E}{}_{\mathrm{G}}\text{?T)}{}_{\mathrm{P}}$ due to the electron lattice interaction has been evaluated. In addition, the electronic polarizability has been calculated using a modified Lorentz-Lorenz relation.     

Adsorption induced backscattering enhancement — CO and C2N2 on Pt(100)

Enhancement of the elastic backscattering coefficient is observed in the range 50–200 eV when CO or C₂N₂ are adsorbed on Pt(100). The general trends are insensitive to the state of order on the surface, and may be understood in terms of an incoherent scattering model, in which the backscattering power of the light elements is greater than that of the substrate at low energy. At higher energies bulk elastic scattering becomes relatively more prominent, the mean free path in the metal becomes greater and the enhancement is destroyed.