The surface structure of W{100} (2 × 2)R45°

In a recent paper Tung, Graham and Melmed [Surface Sci. 115 (1982) 576] have suggested, on the basis of field ion and field evaporation microscopy, that an important ingredient of the W{100} $\text{(}\text{2}\text{×}\text{2}\text{)R45°}$ structure seen in LEED is distortion of the top W atom layer perpendicular to the surface. It is pointed out that the previously ascribed space group symmetry of p2mg rules out this possibility. A reassessment of the qualitative LEED data leading to this space group assignment indicates that while some small element of perpendicular distortion cannot be totally excluded, the amplitude of this differential distortion must be very much less than 0.1 Å.     

The surface structure of Si(100) surfaces using averaged LEED: II. The (2×1) clean surface structure

Constant momentum transfer averaged LEED data from a Si(100) (2 × 1) clean surface structure have been produced for the $\text{(00), (10), (11), (}\text{1}\text{2}\text{0)}$ and $\text{(1}\text{1}\text{2}\text{)}$ beams. All averages show strong features in addition to those attributable to single scattering from the bulk. If this structure is assumed to also originate from single scattering, the surface reconstruction must be deep (>^～4 layers). Alternatively this structure can be ascribed to multiple scattering. As data from the Si(100) (1 × 1)H surface structure produced “good” averaging over an identical range of data, this latter conclusion has considerable bearing on the future usefulness of this averaging approach to surface structure analysis by LEED.     

Composition,structure, surface states,and O2 sticking coefficient for differently prepared GaAs(1̄1̄1̄)As surfaces

The polar GaAs(1̄1̄1̄)As surface can be prepared in three stable and ordered states: two by molecular beam epitaxy (MBE), namely the As-stabilized and the Ga-stabilized states and one simply by ion bombardment and annealing at 770 K. The respective LEED structures are $\text{(2 × 2), (}\text{19}\text{×}\text{19}\text{)}\text{R}\text{23.4°}$, and (1 × 1) with a diffuse faint (3 × 3) superstructure. Auger measurements and the comparison with the stoichiometric cleaved (110) surface show that there are different As concentrations in the first atomic layer associated with each of these three surfaces. Whereas about 10 to 15% of the first As layer appears to be missing on the (2 × 2) surface, about 50% is missing on the $\text{19}$ surface. On the (1 × 1) surface the first As layer is removed completely. The intensity of emission from the surface sensitive states between 1 and 4 eV below the valence band edge, as seen by angular resolved UPS, roughly corresponds to the amount of As at the surface thus confirming their interpretation as As-derived surface states. The inital sticking coefficent for oxygen depends strongly on the surface structure: ～10^?8 for the (2 × 2), ～10^?7 for the $\text{19}$, and ～10^?4 for the (1 × 1) surface. The sticking coefficient does not depend on the surface concentration of As but rather on the degree of saturation of dangling bonds on Ga atoms.     

A LEED and AES study of the adsorption of bromine on W(100) at room temperature

Bromine gas adsorbs atomically on W(100) at room temperature to a saturation concentration of θ = 0.88 relative to the surface tungsten atom density (10¹⁹ m^?2). Below θ ～ 0.4, a c(2 × 2) overlayer is formed. Beyond this a ($\text{3}\text{4}$√2 × √2)R45° structure is preferred and this saturates at θ = 0.67. Higher surface bromine concentrations result in hexagonal variable compression structures on W(100). The sequence begins w structures on W(100). The sequence begins with a c(4 × 2) coincidence mesh which at higher coverages is compressed in one 〈0,1〉 substrate direction. At certain compressions the overlayer achieves p(5 × 2), c(6 × 2), p(7 × 2) coincident configurations and perhaps c(8 × 2) at saturation. This would correspond to θ = 0.875 and is the closest coincidence structure to a perfect hcp overlayer. Bromine prefers a rectangular overlayer geometry on W(100) and compression into an hexagonal array greatly reduces the overlayer stability. The nn repulsions incurred limit room temperature adsorption as the overlayer compresses to perfect hep. Halogen behaviour on W(100) is compared with that on Fe(100). Most differences can be explained in terms of geometrical and bond strength differences but chlorine on W(100) appears to be an exception to this rule.     

Halogen adsorption on Fe(100): I. The adsorption of Cl2 studied by AES,LEED, work function change and thermal desorption

The initial sticking probability of chlorine on Fe(100) at room temperature is calculated to be 0.13, and there is evidence to suggest that the chlorine adsorbs into a short lived mobile precursor state above the surface. The work function change, Δφ, is proportional to coverage and reaches a maximum value of 1.43 eV at saturation. At this coverage a c(2 × 4) LEED pattern is formed. On heating, chlorine is lost from the surface, but the mechanism is such that no detectable loss is incurred at a constant elevated temperature. The c(2 × 4) pattern is shown to be a coincidence structure formed from a $\text{(}\text{1}\text{2}\text{3}\text{?1}\text{2}\text{3}\text{)}$ net of chlorine atoms on the Fe(100) substrate. This structure is a special case of the more general $\text{(}\text{1}\text{2}\text{tan}\text{α}\text{?1}\text{2}\text{tan}\text{α}\text{)}$ structure formed at lower concentrations of chlorine. The c(2 × 4) is formed when α = 56.31°, which gives the chlorine atoms a hard sphere diameter of 0.345 nm and a concentration of 0.75 atoms per four-fold site.     

Subject index

Field-emission energy distributions from the (100) facet of Ge exhibit a double peak. Comparison of the measured distributions with theory shows that the lower energy peak arises from valence band emission while the higher energy peak represents emission from a band of surface states overlapping the valence band. The field-emission energy distribution from the surface states is a maximum at 0.18 eV above the valence band edge. The surface of the emitter is found to be 4kT degenerate n-type with an applied field of $\text{3 × 10}{}^7\text{V}\text{cm}$. This implies 6.3 × 10¹² surface states/cm² at the center of the clean, annealed (100) facet. The effect of the applied field is to broaden the surface state distribution. The degree of broadening can be accounted for by the Stark effect. Adsorption of contamination from the vacuum system ambient or geometric alteration of the surface from the annealed end form reduces the number of surface states.     

Electronic structure of the Si (111) reconstructed surface in the vacancy model

The self-consistent pseudopotential method is applied to the Si (111) 7 × 7 reconstructed surface in the vacancy model with a simplified $\text{3}\text{×}\text{3}$ superlattice structure. Numerical results with and without relaxation of surface atoms are presented. It is concluded that the relaxation, if any, is to be much smaller than the atomic distance to explain the photoemission spectrum of the 7 × 7 surface. The importance of the many-body effect is suggested in the photoemission process associated with the dangling bond surface states of Si.     

The clean thermally induced W {001} (1 × 1) → (√2 × √2) R 45° surface structure transition and its crystallography

A (√2 × √2)R45° surface structure on W {001} produced only by cooling below ~370 K, first reported by Yonehara and Schmidt, has been investigated by LEED, AES, work function change, characteristic loss and low energy Auger fine structure measurements. No significant changes at any energy up to 520 eV occur in the standard Auger spectrum upon cooling to 220 K for as long as 30 min after a flash to >2 500 K. The work function of the (√2 × √2) R45° at 210 K is 20 ± 10 mV below that of the (1 × 1) surface, and a sensitive feature in the fine structure of the N₇VV AES transition shows approximately 60% attenuation. Unlike for H₂ adsorption, the “surface plasmon” loss peak exhibits little if any measurable attenuation and no measurable shift in energy as the crystal cools to form the (√2 × √2)R45°. The rate of intensity buildup in the $\text{1}\text{2}$-order LEED beams is strictly temperature dependent, and significant differences exist between the $\text{1}\text{2}$-order LEED spectra produced by cooling and those produced by H₂ adsorption. Only 2-fold symmetry was observed in the LEED beam intensities at exactly normal incidence, rather than 4-fold as expected for statistically equal numbers of rotationally equivalent domains. The LEED I-V spectra for 24 fractional order beams and 12 integral order beams, taken over large energy ranges at normal incidence, clearly establish that the beam intensities display 2 mm point group symmetry, and hence a preference of one domain orientation over the other. No beam broadening or splitting effects were apparent, implying only incoherent scattering from the various domains. The half-order beam spectra (±h/2, ±h/2) are identical in relative intensity to the (±h/2, ±h/2) spectra but different in absolute intensity by a constant factor, which can be explained only by domains with p2mg space group symmetry rather than just p2mm. Adsorption of H₂ onto the cooled (√2 × √2)R45° structure restores the 4-fold symmetry in the LEED beam intensities at normal incidence, giving a c(2 × 2) hydrogen structure, the same as when adsorbing H₂ onto the above room temperature (1 × 1) crystal. This strongly supports the observed p2mg symmetry as being a true property of the cooled (√2 × √2)R45° surface structure. These results show that the (1 × 1) → (√2 × √2) R45° transition produced by cooling is a transition involving displacement of surface W atoms, and that it apparently can be characterized as an order-order, second degree, homogeneous nucleation process, which is strongly prohibited by the presence of impurities or defects.     

The adsorption of benzene and naphthalene on the Rh(111) surface: A LEED,AES and TDS study

The adsorption of benzene and naphthalene on the Rh(111) single-crystal surface has been studied by low-energy electron diffraction (LEED), Auger electron spectroscopy (AES) and thermal desorption spectroscopy (TDS). Both benzene and naphthalene form two different ordered surface structures separated by temperature-induced phase transitions: benzene transforms from a ($\text{3}\text{1}\text{1}\text{3}$) structure, which can also be labelled c($\text{2}\text{3}\text{× 4}$)rect, to a (3 × 3) structure in the range of 363–395 K, while naphthalene transforms from a ($\text{3}\text{3}\text{× 3}\text{3}$)R30° structure to a (3 × 3) structure in the range 398–423 K. Increasing the temperature further, these structures are found to disorder at about 393 K for benzene and about 448 K for naphthalene. Then, a first H₂ desorption peak appears at about 413 K for benzene and 578 K for naphthalene and is interpreted as due to the occurrence of molecular dissociation. All these phase transitions are irreversible. The ordered structures are interpreted as due to flat-lying or nearly flat-lying intact molecules on the rhodium surface, and they are compared with similar structures found on other metal surfaces. Structural models and phase transition mechanisms are proposed.     

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.     

Adsorption of sulfur dioxide and the interaction of coadsorbed oxygen and sulfur on Pt(111)

The adsorption of sulfur dioxide and the interaction of adsorbed oxygen and sulfur on Pt(111) have been studied using flash desorption mass spectrometry and LEED. The reactivity of adsorbed sulfur towards oxygen depends strongly on the sulfur surface concentration. At a sulfur concentration of 5 × 10¹⁴ S atoms cm^?2 ($\text{(}\text{3}\text{×}\text{3}\text{)R30°}$ structure) oxygen exposures of 5 × 10^?5 Torr s do not result in the adsorption of oxygen nor in the formation of SO₂. At concentrations lower than 3.8 × 10¹⁴ S stoms cm^?2 ((2 × 2) structure) the thermal desorption following oxygen dosing at 320 K yields SO₂ and O₂. With decreasing sulfur concentration the amount of desorbing O₂ increases and that of SO₂ passes a maximum. This indicates that sulfur free surface regions, i.e. holes or defects in the (2 × 2) S structure, are required for the adsorption of oxygen and for the reaction of adsorbed sulfur with oxygen. SO₂ is adsorbed with high sticking probability and can be desorbed nearly completely as SO₂ with desorption maxima occurring at 400, 480 and 580 K. The adsorbed SO₂ is highly sensitive to hydrogen. Small H₂ doses remove most of the oxygen and leave adsorbed sulfur on the surface. After adsorption of SO₂ on an oxygen predosed surface small amounts of SO₃ were desorbed in addition to SO₂ and O₂ during heating. Preadsorbed oxygen produces variations of the SO₂ peak intensities which indicate stabilization of an adsorbed species by coadsorbed oxygen.     

Adsorption of CO on clean and oxygen covered Ni(111) surfaces

Adsorption of CO on Ni(111) surfaces was studied by means of LEED, UPS and thermal desorption spectroscopy. On an initially clean surface adsorbed CO forms a √3 × $\text{√3}\text{R30°}$ structure at θ = 0.33 whose unit cell is continuously compressed with increasing coverage leading to a c4 × 2-structure at θ = 0.5. Beyond this coverage a more weakly bound phase characterized by a $\text{√7}\text{2}$ × $\text{√7}\text{2R19°}$ LEED pattern is formed which is interpreted with a hexagonal close-packed arrangement (θ = 0.57) where all CO molecules are either in “bridge” or in single-site positions with a mutual distance of 3.3 Å. If CO is adsorbed on a surface precovered by oxygen (exhibiting an O 2 × 2 structure) a partially disordered coadsorbate 2 × 2 structure with θ_o = θ_co = 0.25 is formed where the CO adsorption energy is lowered by about 4 kcal/mole due to repulsive interactions. In this case the photoemission spectrum exhibits not a simple superposition of the features arising from the single-component adsorbates (i.e. maxima at 5.5 eV below the Fermi level with O_ad, and at 7.8 (5σ + 1π) and 10.6 eV (4σ) with CO_ad, respectively), but the peak derived from the CO 4σ level is shifted by about 0.3 eV towards higher ionization energies.     

A combined LEED,AES and XPS study of the ZnO {0001} polar surfaces: I. LEED,AES and XPS of contaminated surfaces

The {0001} polar surfaces of ZnO single crystals have first been examined after a chemical treatment involving HCl and H₃PO₄ and a 24 hr bakeout at 250 °C. The impurities detected on the (000$\text{1}$)-O surface with AES were carbon, chlorine, phosphorus and to a lesser extent sulphur. On the (0001)-Zn surface, carbon, chlorine and sulphur were the dominant impurities, while the phosphorus signal was less important. These results were confirmed by XPS measurements on frehsly etched surfaces. The AES spectra were recorded as distribution curves N(E). Averaging, curve-fitting and related numerical techniques were used to obtain high resolution spectra, enabling the identification of the phosphorus L₁-transitions. The etched surfaces were cleaned progressively using argon ion bombardment and ohmic heating. It has been consistently observed that the clean surfaces exhibit primitive (1 × 1) structures. Superstructures such as $\text{(}\text{3}\text{×}\text{3}\text{)}$ on the (000$\text{1}$)-O surface, and $\text{(4}\text{3}\text{× 4}\text{3}\text{)}$ and (3 × 3) on the (0001)-Zn surface, were repeatedly observed at discrete spots of contaminated surfaces. A clear correlation with impurities as observed by AES however could not be found. Facetting was observed after prolonged heating.     

Hydrogen-induced reconstruction on a high step density W(001) surface

The hydrogen-induced reconstruction on a high step density W(001) crystal, ($\text{2}\text{×}\text{2}$)R45°-H, with steps oriented parallel to the [110] and ～ 28 Å average terrace width has been investigated using LEED symmetry, beam shape analyses, and EELS. The symmetry of the LEED pattern is observed to change from p2mg for the ($\text{2}\text{×}\text{2}$)R45° clean surface reconstruction to c2mm for the commensurate phase ($\text{2}\text{×}\text{2}$)R45°-H reconstruction. Correspondingly, the shapes of the half-order beams indicate that the hydrogen-induced reconstruction domains are much less elongated than the clean surface domains. A splitting of each half-order beam into four beams at higher exposures indicates the existence of two domains of the incommensurate phase. A commensurate phase v₁ vibrational loss peak centered at 160 meV in the EELS spectrum broadens on the low-energy side during the incommensurate phase and then shifts toward 130 meV and narrows as the (1×1)-H saturation structure develops. These observations imply that there is no long-range inhibition ( ～ 20 Å) to the formation of either commensurate or incommensurate phase; hydrogen induces a switching of the atomic displacements from 〈110〉 directions on a clean surface to 〈100〉 directions, even with steps oriented parallel to the [110]; and in the incommensurate phase there is a distribution of hydrogen site geometries with the most probable geometry more like the commensurate phase geometry than the saturation phase geometry.     

Determination of the atomic arrangement of the unreconstructed Ir (110) surface by low-energy electron diffraction a

An Ir(110)-(1 × 1) surface structure has been prepared by adsorbing $\text{1}\text{4}$ monolayer of oxygen at 850 K on a clean, reconstructed (1 × 2) surface. Results of the low-energy electron diffraction structure analysis reveal that the oxygen is probably distributed randomly over the crystal surface, and the (1 × 1) structure is the same as a clean unreconstructed (1 × 1) structure, with a topmost interlayer Ir spacing of 1.26 ± 0.05 Å. This is equivalent to a contraction of approximately 7.5% of the bulk interlayer spacing of 1.36 Å.     

Effect of Bragg-Williams disorder on reconstructed polar surfaces of tetrahedrally coordinated compound semiconductors by optically simulated leed patterns

Laser generated optical diffraction patterns obtained from two-dimensional model gratings have been used to simulate the surface reconstructions observed by LEED with the polar surfaces of tetrahedrally coordinated compound semiconductors. The 2 × 2 and $\text{4}\text{3}\text{× 4}\text{3}\text{?30°}$ reconstructions conform ideally with the $\text{1}\text{4}$-monolayer criterion dictated by bonding ionicity [Nosker, Mark and Levine, Surface Science 19 (1970) 291] while the 3 × 3, $\text{3}\text{×}\text{3}\text{?30°}$ and $\text{19}\text{×}\text{19}\text{?23.4°}$ reconstructions do not. It is shown that the introduction of Bragg-Williams disorder that preserves long-range order into the latter structures does permit the achievement of conformity with the $\text{1}\text{4}$-monolayer criterion without altering the symmetry of the diffraction pattern. Specifically the intensities of the fractional order beams are reduced relative to those of the integral order beams. Thus all the observed reconstruction LEED patterns can be consistent with the $\text{1}\text{4}$-monolayer criterion, provided account is taken of the insensitivity of LEED to surface defect structure.     

Characterization of oxygen,carbon, and sulfur adlayers on W(211)

Adlayers of oxygen, carbon, and sulfur on W(211) have been characterized by LEED, AES, TPD, and CO adsorption. Oxygen initially adsorbs on the W(211) surface forming p(2 × 1)O and p(1 × 1)O structures. Atomic oxygen is the only desorption product from these surfaces. This initial adsorption selectively inhibits CO dissociation in the CO(β₁) state. Increased oxidation leads to a p(1 × 1)O structure which totally inhibits CO dissociation. Volatile metal oxides desorb from the p(1 × 1)O surface at 1850 K. Oxidation of W(211) at 1200 K leads to reconstruction of the surface and formation of p(1 × n)O LEED patterns, 3 ? n ? 7. The reconstructed surface also inhibits CO dissociation and volatile metal oxides are observed to desorb at 1700 K, as well as at 1850 K. Carburization of the W(211) surface below 1000 K produced no ordered structures. Above 1000 K carburization produces a c(6 × 4)C which is suggested to result from a hexagonal tungsten carbide overlayer. CO dissociation is inhibited on the W(211)?c(6×4)C surface. Sulfur initially orders into a c(2 × 2)S structure on W(211). Increased coverage leads to a c(2×6)S structure and then a complex structure. Adsorbed sulfur reduces CO dissociation on W(211), but even at the highest sulfur coverages CO dissociation was observed. Sulfur was found to desorb as atomic S at 1850 K for sulfur coverages less than $\text{7}\text{6}$ monolayers. At higher sulfur coverages the dimer, S₂, was observed to desorb at 1700 K in addition to atomic sulfur desorption.     

Angle-resolved photoemission of the c(2 × 2) and c(3 × 1) oxygen overlayers on Fe(110)

Angle-resolved photoemission spectroscopy utilizing synchrotron radiation has been used to study the band structure of the c(2×2) and (3×1) oxygen overlayers on Fe(110). The symmetries of the O-2p-derived states at the center of the surface Brillouin zone $\text{(}\text{Γ}\text{)}$ were identified using polarized light. At $\text{Γ}$ the p_xp_y- and p_z-derived levels are at about 5.5 and 7.0 eV below the Fermi level, respectively, for both ordered overlayers. The p-states of the c(2×2)-O structure show very little dispersion (?0.1 eV) with $\text{k}{}_{\mathrm{}}$. On the other hand, the c(3×1)-O overlayer exhibits considerable dispersion of ～1.6 eV. The essential features of the measured dispersion are reproduced well by the dispersion predicted in a qualitative way for an isolated c(3×1) oxygen monolayer.     

The structure of epitaxially grown metal films on single crystal surfaces of other metals: Gold on Pt(100) and platinum on Au(100)

Gold was evaporated onto Pt(100) and platinum was evaporated onto a Au(100) single crystal surface. Deposition of gold onto Pt(100) removed the $\text{(}\text{14}\text{1}\text{1}\text{5}\text{)}$ reconstructed surface structure and at a coverage of about 0.5 monolayer a (1 × 1) pattern fully developed. This pattern remained unchanged up to 2 gold monolayers. Multilayers of gold produced (1 × 5) and (1 × 7) surface structures after annealing. These observations imply variable interatomic distances in the gold layers. The (1 × 5) and (1 × 7) surface structures can be explained by the formation of a hexagonal top atomic layer on a substrate that retains a square lattice. The well known structure of clean Au(100) did not form, even at 32 layers of gold on Pt(100). Platinum deposited onto Au(100) removed its surface reconstruction yielding a fully developed (1 × 1) pattern at about one-half layer. This pattern remained unchanged upon further platinum deposition. The absence of new reconstructions in this case may be linked with the growth mechanism that is inferred from the variation of the Auger signal intensities of the substrate and adsorbate metals with coverage of the adsorbate. It was found that platinum on Au(100) forms microcrystallites (Volmer-Weber type growth), while gold on Pt(100) grows layer-by-layer (Frank-van der Merwe growth mechanism).