Low-energy Electron Diffraction Measurements Research Articles

Aperiodic incommensurate phase of aC60monolayer on Ag(100)

Detailed combined scanning tunneling microscopy and low-energy electron diffraction measurements reveal that the structure of a ${\mathrm{C}}_{60}$ monolayer on Ag(100) is not the previously accepted commensurate $c(6\ifmmode\times\else\texttimes\fi{}4)$ phase but rather an incommensurate (111) close-packed phase. The film exhibits a characteristic molecular contrast pattern with merely short-range order, and room-temperature fluctuations of the contrast show that a thermal equilibrium state is reached. The nature of this controversial bright-dim ${\mathrm{C}}_{60}$ contrast is clarified as a topographic feature due to ${\mathrm{C}}_{60}$-induced reconstruction underneath the dim ${\mathrm{C}}_{60}$ molecules. Due to interactions between the incommensurate ${\mathrm{C}}_{60}$ adlayer and the reconstructed substrate, the (111) phase is distorted laterally, forming a novel ``tetramer'' configuration of specific contrast order. The spatial distribution of these tetramers is aperiodic; this has crucial implications for the peculiar short-range contrast order observed experimentally. A lattice gas model with anisotropic nearest-neighbor interactions and a configuration energy of the tetramer is developed. Quantitative agreements between observation and modeling are achieved with reasonable phenomenological parameters derived within experimental constraints.

The reaction of propene with oxygen-covered Pd(1 0 0)

The effect of oxygen coverage and surface structure on the total oxidation of propene over Pd(1 0 0) was studied using temperature-programmed reaction (TPR), isothermal kinetic measurements, low energy electron diffraction (LEED), and scanning tunneling microscopy (STM). TPR revealed CO 2 production peaks at 430 and 550 K. When the surface was covered by chemisorbed oxygen in (2 × 2) structures the 550 K peak dominated, while only the 430 K peak was seen at low propene doses and high oxygen coverages (∼0.8 ML) where a ( 5 × 5 ) R27° reconstruction covered the surface. Regardless of the oxygen coverage, water desorbed at 430 K indicating that the 550 K CO 2 peak was due to oxidation of C deposited by propene dissociation. The initial propene sticking coefficient at 330 K was a factor of five greater for (2 × 2)-O surfaces than for the ( 5 × 5 ) R27°-O surface, thus the lower activation energy pathway favored at high oxygen coverages did not necessarily translate into higher reaction rates. Above 450 K, the isothermal kinetic measurements showed that the reaction rate increases with decreasing oxygen coverage until the reaction becomes starved for oxygen. LEED measurements showed that the rate increases as the ( 5 × 5 ) R27° structure is replaced by the (2 × 2) structures. At lower temperatures, however, the oxidation rate of C deposited on the surface by propene dissociation at low oxygen coverages is slow and so higher rates were seen at high oxygen coverages. At room temperature, STM images showed that propene initially slowly randomly adsorbs atop the ( 5 × 5 ) R27° surface. As the propene coverage increased, however, adsorbates tended to cluster together forming disordered regions; as the surface disordered the adsorption rate increased. At 550 K, STM movies recorded during propene exposure to oxygen-covered surfaces showed the slow removal of ( 5 × 5 ) R27° domains followed by rapid dissolution of islands formed during oxygen adsorption. The results indicate that oxidizing the Pd surface affects hydrocarbon oxidation in two opposing ways: it decreases the hydrocarbon adsorption probability but also favors an easier oxidation pathway once the molecules adsorb.

A study of the surface structure and composition of annealed Ga0.96Mn0.04As(1 0 0)

The surface structure and chemical composition of annealed Ga0.96Mn0.04As(100) have been studied by scanning tunneling microscopy (STM), auger electron spectroscopy (AES) and low energy electron diffraction (LEED). The samples were As capped and subsequently transferred in-air from the MBE system to the STM chamber. After annealing to 600K it is found that the Mn segregates to the surface and forms a compound, which is stable up to annealing temperatures of 790K. For annealing temperatures above 825K a well-ordered phase exists signified by a LEED pattern consisting of a superposition of a (1×6) and a (4×2) pattern. LEED and STM measurements demonstrate that the surface is dominated by (1×6) domains coexisting with small patches of (4×2) domains. By comparing the STM images of the high temperature phase found on Ga0.96Mn0.04As(100) with the high temperature phases found on ordinary GaAs(100), we demonstrate differences between annealed Ga0.96Mn0.04As(100) and GaAs(100) in both surface morphology and atomic structure. We argue that the Ga0.96Mn0.04As surface is more As rich than the GaAs surface prepared in a similar fashion. Reasons for these differences are discussed.

Molecular-beam epitaxy of the half-Heusler alloy NiMnSb on (In,Ga)As/InP (001)

We report the growth of the half-Heusler alloy NiMnSb on InP (001) by molecular-beam epitaxy using a lattice-matched (In,Ga)As buffer. High-resolution x-ray diffraction confirms a high crystalline quality. Spot-profile analysis low-energy electron diffraction measurements show well-defined surface reconstructions. The samples show the expected high Curie temperature and an uniaxial anisotropy.

Study of the early stages of Cu/6H–SiC(0 0 0 -1) interface formation

The early stages of the Cu/SiC interface formation at room temperature and the influence of annealing were investigated on the carbon-face of the single crystal n-type 6H–SiC, by X-ray photoelectron and auger electron spectroscopy (XPS/XAES), low energy electron diffraction and work function (WF) measurements. Upon stepwise copper evaporation in UHV up to a thickness of 5–10 monolayers (ML) at RT, the binding energy of the XPS Cu2p 3/2 core level peak shifted from 933.6±0.1 eV, below 0.1 ML coverage, to 932.7 eV for the final metallic Cu deposit. The WF, following an initial steep decrease from the clean SiC substrate value of 4.4 down to 4.0 eV up to 0.1 ML, increased then gradually to the metallic Cu value of ∼4.75 eV after approximately 5 ML of Cu. The growth of the film was initially via 2D-cluster formation, and exhibited a 3D character beyond 1–2 ML of Cu. The height of the Schottky barrier for the Cu/6H–SiC(0 0 0 -1) contact was found by XPS to be 1.5±0.1 eV. Annealing of the contact up to 520 K caused small changes in the Cu and SiC XPS peak intensities accompanied by a 0.3 eV increase of the WF, indicating small structural changes within the film and gave rise to a slight change of the Schottky barrier height to 1.6±0.1 eV. The results do not indicate any significant silicide formation at 520 K.

ADSORPTION OF C60 ON Li-COVERED Ni(110) SURFACES

In this paper the adsorption of C60 on Li-covered Ni(110) surfaces is investigated by means of Auger electron spectroscopy, low-energy electron diffraction and work function measurements, in ultrahigh vacuum. Deposition of C60 on the 1 × 2 Li-induced Ni(110) surface at 650 K causes the formation of islands with a 4 × 2 structure, where the C60 molecules adsorb along neighboring troughs of the substrate. At higher C60 coverages, the Li-induced 1 × 2 reconstruction of the Ni(110) surface is lifted and a 9 × 3 structure is formed, which finally ends in a semihexagonal structure, as in the case of C60 adsorption on clean Ni(110) surfaces. AES and LEED measurements suggest that charge is transferred from Li to the C60 molecules, which in a rough approximation was estimated to be around one electron per C60 molecule. The above estimated charge transfer to the C60 molecules is substantially smaller than that we have calculated when Li is adsorbed on C60-covered Ni(110) surfaces. Apparently, the order of Li and C60 deposition is very important for the charge transfer and the deposition of Li on C60-covered surfaces provides a substantially greater amount of charge to the C60 molecules.

Adsorption of glycine on a NiAl(1 1 0) alloy surface

The adsorption and desorption of glycine (NH 2CH 2COOH), vacuum deposited on a NiAl(1 1 0) surface, were investigated by means of Auger electron spectroscopy (AES), low energy electron diffraction (LEED), temperature-programmed desorption, work function (Δ φ) measurements, and ultraviolet photoelectron spectroscopy (UPS). At 120 K, glycine adsorbs molecularly forming mono- and multilayers predominantly in the zwitterionic state, as evidenced by the UPS results. In contrast, the adsorption at room temperature (310 K) is mainly dissociative in the early stages of exposure, while molecular adsorption occurs only near saturation coverage. There is evidence that this molecularly adsorbed species is in the anionic form (NH 2CH 2COO −). Analysis of AES data reveals that upon adsorption glycine attacks the aluminium sites on the surface. On heating part of the monolayer adsorbed at 120 K is converted to the anionic form and at higher temperatures dissociates further before desorption. The temperature-induced dissociation of glycine (<400 K) leads to a series of similar reaction products irrespective of the initial adsorption step at 120 K or at 310 K, leaving finally oxygen, carbon and nitrogen at the surface. AES and LEED measurements indicate that oxygen interacts strongly with the Al component of the surface forming an “oxide”-like Al–O layer.

Surface properties of Sc–O/W( [formula omitted]) system as emitter at room and high temperatures

Surface characterization of an Sc–O/W(1 0 0) system as a Schottky emitter was carried out by low-energy electron diffraction (LEED) analysis, Auger electron spectroscopy (AES) and work function measurement. LEED and AES measurements were successfully performed at 1400 K, the operating temperature of the Sc–O/W(1 0 0) Schottky emitter. The results revealed that the atomic arrangement, composition and chemical bonding state of the Sc–O/W(1 0 0) surface show no change between room temperature (RT) and 1400 K when the Sc–O/W(1 0 0) surface is cooled to RT from 1400 K after oxygen processing. Investigation of the surfaces at RT as well as at 1400 K indicated that the reduced work function of 3.2 eV attained at 1400 K was maintained even when the sample temperature was lowered to RT. These properties of the Sc–O/W(1 0 0) system suggest the practical applicability of the Sc–O/W(1 0 0) emitter to a cold field emitter as well as to a Schottky emitter as reported.

Third-generation conical spot profile analyzing low-energy electron diffraction

A newly designed conical spot profile analyzing low-energy electron diffraction (LEED) allows optimized access to the sample up to an angle of 45° with respect to the surface plane. This allows in vivo LEED measurements during deposition or adsorption. The electron optics of the electrostatic deflection unit and the newly designed entrance lens were simulated and optimized using an electron-beam ray-tracing algorithm. Different sample distances can be accommodated by adjusting the deflection voltage between the front and rear section. A new “real-image” SPA-LEED operation mode is presented allowing simple control of the instrument and accurate electron-beam positioning on the sample.

Structure and formation of the Al( [formula omitted])- [formula omitted]R27°-Na phase: a LEED, DFT and HRCLS study

Adsorption of 0.2 ML Na on Al(1 0 0) at room temperature yields a disordered Al(1 0 0)-(1×1)-Na phase, which transforms reversibly to a well-ordered Al(1 0 0)- ( 5 × 5 )R27° -Na phase on cooling below 250 K. Based on low energy electron diffraction (LEED) and high resolution core-level spectroscopy (HRCLS) measurements, and on ab initio calculations, it is concluded that the structure of the Al(1 0 0)- ( 5 × 5 )R27° -Na phase consists of Na atoms occupying substitutional sites. The structural parameters obtained from LEED and density functional theory analyses are in quantitative agreement. Adsorption of 0.2 ML Na at 100 K yields an Al(1 0 0)- c(2×2)-Na island structure, which transforms irreversibly into the ( 5 × 5 )R27° -Na structure by annealing above 190 K. The nature of the reversible and irreversible phase transformations to the ( 5 × 5 )R27° structure is investigated by HRCLS and LEED.

Preferential Nucleation and Self-Limiting Growth of Cu Nanoclusters on S(4 × 4)/W(111)

The interaction of Cu with the highly ordered S(4 x 4)/W(111) surface has been studied by means of Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), and scanning tunneling microscopy (STM) at roomtemperature. The substrate is a sulfur-induced nanoscale reconstruction of W(111) with (4 x 4) periodicity characterized by broad, planar terraces (30 nm in width). We find that fractional monolayers of vapor-deposited Cu grow homogeneously as clusters on the S(4 x 4)/W(111) surface over a wide coverage range. At low Cu coverages (<0.1 ML), Cu nanoclusters are observed to nucleate preferentially at characteristic 3-fold hollow sites on the S(4 x 4)/W(111) surface; there is a clear energetic preference for one type of site over others. The formed Cu nanoclusters are uniform in size (0.6 nm) as coverage increases, indicating self-limiting growth. Evidence for long-range interactions that may affect the nucleation of nanoclusters is discussed. For coverages ≥0.1 ML, additional sites are populated, and the whole surface is decorated by Cu clusters. STM data are supported by LEED and AES measurements. The results are interpreted in terms of heat of formation, relative reactivity, surface corrugation, surface diffusion, lattice mismatch, and long-range interactions in the nanoscale regime.

Surface stabilization of organics on hematite by conversion from terminal to bridging adsorption structures

Insight into the complexation of organic molecules on hematite surfaces was obtained from molecular-level studies of a simple probe molecule (methanol) with the R-cut surface of hematite. The R-cut crystal orientation of hematite, designated in this paper as α-Fe 2O 3(012), has two stable surface structures under ultrahigh vacuum (UHV) conditions based on low-energy electron diffraction (LEED) measurements. These are a (1×1) structure consisting of a bulk terminated arrangement of undercoordinated Fe 3+ and O 2− surface sites and a (2×1) reconstructed structure with unknown atomic structure. Whereas the (1×1) surface is essentially free of Fe 2+, the (2×1) surface possesses a high surface concentration of Fe 2+ sites based on electronic structure measurements using electron energy loss spectroscopy (EELS). Methanol adsorbs dissociatively on the (1×1) surface by coordination of the molecule’s oxygen atom at a Fe 3+ site followed by transfer of the alcohol proton to a bridging O 2− surface site, resulting in terminal OCH 3 and bridging OH groups. Most of the dissociated methanol molecules recombine during heating and desorb in vacuum as methanol at 365 and 415 K for the (1×1) and (2×1) surfaces, respectively. However, a significant amount of the terminal OCH 3 and bridging OH groups interchange as the surface is heated above room temperature (RT), resulting in bridging OCH 3 and terminal OH groups. The bridging OCH 3 groups are retained on the surface to higher temperature than the terminal OCH 3 groups, but eventually decompose at about 550 K via a disproportionation reaction that forms gaseous CH 3OH and H 2CO. As a result of the disproportionation reaction, some surface Fe 3+ sites are reduced to Fe 2+ sites. The exchange process competes more successfully with recombinative desorption of methanol (from reaction of terminal OCH 3 and bridging OH groups) on the (2×1) surface, despite the fact that this surface is already partially reduced, because terminal OCH 3 groups are more stable on this surface than on the (1×1) surface. Based on these molecular-level findings, extensive exchange terminal organic ligands and bridging OH groups may play a significant role in stabilizing organics on hematite mineral surfaces. Such exchange processes may also play a role in destabilizing hematite surfaces toward reductive dissolution.

Islanding of Methyl Radicals: A Key Factor in Hydrocarbon Chain Propagation Reactions

A sequence of long-chain alkene products is found to evolve from chemisorbed methyl groups on Cu(111) in temperature-programmed reaction. The results of low-energy electron diffraction measurements suggest the formation of 2D islands for the chemisorbed radicals. The close-packed adsorption geometry and the reaction kinetics to generate high mass species are closely correlated. From this and other studies, we infer that the tendency of the methyl radicals to aggregate even at submonolayer coverages is crucial in promoting the chain growth, and the observed islanding effect may be significant in other hydrocarbon catalytical systems.

Structure, bonding, and anharmonic librational motion of CO on Ir{100}

A combined low energy electron diffraction (LEED)—density functional theory (DFT) study of the structure of the Ir{100}-c(2×2)-CO phase provides a comparison of the two techniques for a simple molecular adsorbate. Both studies clearly identify atop adsorption and agree on the key structural parameters: a strong buckling of the first Ir layer, a short Ir–C bond length, and a slight lengthening of the CO bond. The molecule is found to be adsorbed in an upright configuration, although an incomplete treatment of the correlated vibrational motion of the CO molecule across the surface in the LEED analysis results in an apparent tilt of 8° from the surface normal. The DFT study determines a high adsorption energy of 2.65 eV for the c(2×2) phase which can be associated with the relief of the high tensile stress of the metastable Ir{100}-(1×1) phase and can be correlated with the short Ir–C bond. The 0.25 ML p(2×2)-CO phase displays an almost identical local bonding geometry but has a slightly lower adsorption energy of 2.61 eV, indicative of an attractive nearest neighbor interaction in the c(2×2) phase. The potential-energy surface for displacement of the CO molecule away from the atop position is found to display quartic anharmonicity. The resulting vibrational amplitude of 0.19 Å can be associated with a harmonic frequency of 8 meV, in good agreement with previous EELS measurement. The level of agreement between the LEED and DFT determined structures is sufficiently good to demonstrate that the two techniques are capable of converging on very similar structures. Furthermore, this study clearly demonstrates the future role for low-temperature LEED measurements and DFT studies in achieving an understanding of the structure, bonding, and energetics of molecules adsorbed at surfaces.

Surface termination dependence of the reactivity of single crystal hematite with CCl4

We describe ultrahigh vacuum Auger electron spectrometric measurements of the uptake of chlorine following the room temperature exposure of single crystal hematite, α-Fe2O3, to CCl4. We compare the surface chemistry of two specific surface phases formed on the basal plane of α-Fe2O3: the Fe3O4(111)-(2×2) “selvedge” and the α-Fe2O3/Fe1−xO “biphase.” For Fe3O4(111)-(2×2) an estimated saturation level of Cl of ∼75% of a monolayer is readily attained. Carbon uptake is well below that expected for simple stoichiometric dissociative chemisorption, consistent with desorption of organic products during the surface reaction. Low energy electron diffraction measurements suggest that, dependent upon preparation procedures, at least two types of α-Fe2O3/Fe1−xO biphase structures can be formed. Surprisingly, upon exposure to CCl4, Cl uptake does not occur on either of these biphase surfaces, despite the fact that these surfaces are thought to have the same surface concentrations of iron and oxygen as Fe3O4(111). The dramatic difference between the reactivity of the Fe3O4 and biphase surfaces suggests that the active site for the dissociative adsorption of CCl4 on Fe3O4(111)-(2×2) comprises both an iron cation and an oxygen anion with a surface-normal-oriented dangling bond that is uncapped by iron cations. Electron stimulated and thermal desorption of Cl from the saturated Fe3O4(111)-(2×2) selvedge is also reported.

An experimental study on adsorption of benzene on Co(0 0 0 1)

The adsorption of benzene on Co(0 0 0 1) was studied by X-ray photoelectron spectroscopy, temperature programmed desorption, low energy electron diffraction (LEED) and work function measurements. The adsorption was found to be molecular at room temperature and to saturate at a fractional coverage of 0.125 ML. With LEED a c(2 3 ×4 ) overlayer structure was seen. Below 220 K at high exposures a p( 7 × 7 ) R19° LEED pattern was observed corresponding to a coverage of 0.143 ML. Temperature programmed desorption measurements stated that benzene starts to decompose around 340 K to hydrogen and a hydrocarbon fragment, most likely C 6H 5. While the hydrogen desorbed, the hydrocarbon stayed at the surface. The desorption of molecular benzene was negligible. The activation energy for the dehydrogenation was calculated to be about 102 kJ/mol. The work function of Co(0 0 0 1) decreased by 1.3 eV upon saturation with benzene. The induced dipole moment was calculated to be 1.9 Debye/molecule.

Structure of a p-Xylene Adlayer Formed on a Rh(111) Surface in Vacuum Studied by STM and LEED

The structure of a p-xylene adlayer formed on Rh(111) in ultrahigh vacuum (UHV) at room temperature was investigated by using scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). STM images revealed that p-xylene formed a highly ordered adlayer on Rh(111) in UHV. From the STM observation and the LEED measurement, the adlayer structure was determined to be c(2√3 × 4)rect, which was reported previously also for the adlayer on Rh(111) in HF solution. Time-dependent STM allowed direct observation of the dynamics of the self-ordering process. Small ordered domains of adsorbed p-xylene molecules appeared and gradually grew in size on terraces. The domain size was found to increase in a few minutes under the exposure to p-xylene. The average domain size was ca. 20 nm.

Organic monolayers with uniform domain orientation and reduced antiphase boundaries – MBE of perylene on Au(110)

Abstract The growth of epitaxial monolayers of perylene on Au(1 1 0) was investigated by means of scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). The preparation was done by molecular beam epitaxy (MBE). The experimental setup allowed us to perform LEED or STM measurements during evaporation. The pattern of the adsorbed perylene molecules shows several dosage-induced structural transitions, leading to a final coincident monolayer structure with uniform domain orientation and a low density of antiphase boundaries. The underlying Au(1 1 0) interface exhibits several different reconstructions, depending on the perylene coverage.

Multilayer relaxation of Cu(331)

Both quantitative low-energy electron diffraction (LEED) analysis and theoretical predictions based on the embedded-atom method suggested that Cu(331) has a sequence of interlayer spacing contractions and expansions dissimilar to that expected for a terrace with three atomic layers. To understand the anomalous behavior of Cu(331), we have performed a comparative study of Cu(331) and Cu(211) by using the first-principles full-potential linearized augmented plane-wave method within the generalized gradient approximation. For both surfaces, our calculations reproduce the multilayer relaxation sequences given by LEED measurements. The electronic structure analysis indicates that the seemingly anomalous multilayer relaxation in Cu(331) can also be understood in the charge smoothing picture. However, the trend of charge smoothing in such transition metal surfaces depends not only on how many atomic layers a terrace has, but also on the bond-length--bond-order interplays.

Adsorbate induced mesoscopic surface reconstruction of the system Te/Pd(1 0 2)

Low energy electron diffraction (LEED) measurements have been carried out on the clean and Te covered Pd(1 0 2) surface. This is the surface into which the Pd(1 0 0) surface is reconstructed by a concentration of more than 0.5 ML of Te. The Pd(1 0 2) surface was produced with a terrace width of more than 1000 Å. Up to 0.5 ML the adsorption of Te takes place on the flat (1 0 2) surface, on which islands with (2×1) structure are formed coupled with a reduction of the terrace widths to about half the initial value. Above 0.5 ML, and up to 1 ML, the adsorbed Te atoms produce a structural change of the surface, i.e. a terrace structure is formed with a clear average terrace length that depends on Te concentration and with step edges in [ 2 ̄ 0 1] direction. As concluded from the characteristic splitting of LEED spots as a function of energy, these terraces occupy only two height levels in an alternating sequence and are separated by steps of single atomic height. The terrace width of the alternating terraces increases with increasing Te-concentration. Again a flat surface is formed at 1 ML of Te. A geometrical model is established and discussed.