Clean Single Crystal Surfaces Research Articles

An improved single crystal adsorption calorimeter for determining gas adsorption and reaction energies on complex model catalysts

A new ultrahigh vacuum microcalorimeter for measuring heats of adsorption and adsorption-induced surface reactions on complex single crystal-based model surfaces is described. It has been specifically designed to study the interaction of gaseous molecules with well-defined model catalysts consisting of metal nanoparticles supported on single crystal surfaces or epitaxial thin oxide films grown on single crystals. The detection principle is based on the previously described measurement of the temperature rise upon adsorption of gaseous molecules by use of a pyroelectric polymer ribbon, which is brought into mechanical∕thermal contact with the back side of the thin single crystal. The instrument includes (i) a preparation chamber providing the required equipment to prepare supported model catalysts involving well-defined nanoparticles on clean single crystal surfaces and to characterize them using surface analysis techniques and in situ reflectivity measurements and (ii) the adsorption∕reaction chamber containing a molecular beam, a pyroelectric heat detector, and calibration tools for determining the absolute reactant fluxes and adsorption heats. The molecular beam is produced by a differentially pumped source based on a multichannel array capable of providing variable fluxes of both high and low vapor pressure gaseous molecules in the range of 0.005-1.5 × 10(15) molecules cm(-2) s(-1) and is modulated by means of the computer-controlled chopper with the shortest pulse length of 150 ms. The calorimetric measurements of adsorption and reaction heats can be performed in a broad temperature range from 100 to 300 K. A novel vibrational isolation method for the pyroelectric detector is introduced for the reduction of acoustic noise. The detector shows a pulse-to-pulse standard deviation ≤15 nJ when heat pulses in the range of 190-3600 nJ are applied to the sample surface with a chopped laser. Particularly for CO adsorption on Pt(111), the energy input of 15 nJ (or 120 nJ cm(-2)) corresponds to the detection limit for adsorption of less than 1.5 × 10(12) CO molecules cm(-2) or less than 0.1% of the monolayer coverage (with respect to the 1.5 × 10(15) surface Pt atoms cm(-2)). The absolute accuracy in energy is within ∼7%-9%. As a test of the new calorimeter, the adsorption heats of CO on Pt(111) at different temperatures were measured and compared to previously obtained calorimetric data at 300 K.

Adsorption of cytosine, thymine, guanine and adenine on Cu(110) studied by infrared reflection absorption spectroscopy

The molecular internal vibration modes and orientation of the DNA base molecules (cytosine, thymine, guanine and adenine) adsorbed on copper(1 1 0) clean single crystal surface were investigated by infrared reflection adsorption spectroscopy in ultrahigh vacuum. The adsorption was performed by thermal evaporation of powder of each species onto Cu(1 1 0) at room temperature. The surface uptakes of the molecules were determined by Auger electron spectroscopy. Cytosine exhibited characteristic asymmetric and symmetric NH2 stretching signal. The asymmetric stretching signal was missing in the submonolayer region, indicating that the whole molecule was standing upright, with the NH2 group sticking out of the surface. Thymine was also proved to stand upright at surface. Guanine exhibited weakened infrared signals, indicating that the core purine ring was tilted on the surface, with an interaction of NHCO side of the molecule. Adenine at coverages below one monolayer gave no absorption signals, evidencing the molecules laid flat along the surface. The vibrational information obtained will be helpful in developing the future methods of surface DNA sequencing.

Studies of step stiffnesses and relaxation on Pt(1 1 1), Pd(1 1 1) and Mo(0 1 1)

We describe investigations of equilibrium step structure on Pt(1 1 1), Pd(1 1 1) and Mo(0 1 1), using low energy electron microscopy. From capillary wave analyses we obtained fluctuation amplitudes and relaxation times for Fourier components of the step edge displacement as functions of wave number q. The τ q −1 at ∼ T m/2 vary quite accurately with wavevector q as q 3 and show that surface diffusion is responsible for the relaxation at these lower temperatures for all clean single crystal surfaces studied. The temperature dependence of the step stiffnesses is very weak in the intervals 1190–1520 K for Pt, 990–1300 K for Pd and 1350–1680 K for Mo.

Molecular-beam/surface-science apparatus for state-resolved chemisorption studies using pulsed-laser preparation

We describe a new apparatus that combines pulsed laser excitation in a molecular beam with surface-science methods for preparation of clean single-crystal surfaces and detection of adsorbates to enable state-selected studies of gas–surface reaction dynamics. Reactant molecules are prepared in specific vibrationally excited states via overtone pumping using tunable, narrow-band laser radiation. The collision-free environment of the molecular beam prevents relaxation of the prepared molecules before impact on the target surface and enables complete control over the collision energy and incidence angle. Chemisorption products are detected after a given deposition time by Auger electron spectroscopy. To achieve sufficient beam flux of state-selected reactant molecules for product detection by standard surface-science techniques, we use a high-intensity, short-pulse molecular-beam source matched to the low duty cycle of the pulsed lasers used in our experiments. We present the design and characterization of this new apparatus together with a scheme for generating infrared laser pulses of high spectral brightness for saturating weak vibrational overtone transitions within a significant volume of the molecular beam. The effectiveness of our approach is demonstrated by state-resolved sticking coefficient measurements for overtone-excited (2ν3) CH4 on Ni(100) for a range of impact energies.

Holography with photoelectrons: a direct approach

This article reviews the work performed by the authors regarding photoelectron holography on clean metal surfaces and simple adsorbate arrangements. The following systems were investigated: Pt(1 1 0)-1 × 2, Ni(1 1 0), Ni(1 1 1), Ni(1 1 0)-c(2 × 2) S, Ni(1 1 1)-p(2 × 2) S and Ni(1 1 1)-(5√3 × 2) S. The photoelectron diffraction patterns, which are interpreted as holograms, were recorded using synchrotron radiation at the German synchrotron facility BESSY-I in Berlin. For most systems these holograms were acquired at various kinetic energies. The real space structures were reconstructed from these data using the single wave number as well as the multiple wave number algorithm, without any prior assumptions. It was demonstrated that the local geometrical structure of clean single crystal surfaces and of adsorbate systems can be successfully reconstructed by using photoelectron holography with some restrictions.

Calculations of the energy accommodation coefficient using classical scattering theory

The theory of the energy accommodation coefficient for exchange of energy between a rarefied gas and a clean surface is developed in terms of the differential reflection coefficient for state-to-state scattering of an incoming atomic projectile and a surface. This theory is applied to classical models which have been shown to accurately predict the measured scattered distributions in numerous state-to-state experiments of monoenergetic beams of atoms scattering from clean single crystal surfaces and from clean liquid surfaces. Full three-dimensional calculations are carried out and compared with available experimental data for the accommodation of rare gases at a clean tungsten surface. Good agreement with the experimental measurements is obtained for the heavier mass rare gases where classical theory is expected to be most valid at all measured temperatures.

Pyroelectric detector for single-crystal adsorption microcalorimetry: analysis of pulse shape and intensity

In microcalorimetry for measuring heats of adsorption on clean single-crystal surfaces, a pulse of gas from a molecular beam adsorbs on an ultrathin single crystal's surface, causing a measurable transient heat input and temperature rise. One new and sensitive method of heat detection uses a 9 μm pyroelectric polymer ribbon, which is mechanically driven to make a gentle mechanical/thermal contact to the back of the single-crystal sample during measurement (J.T. Stuckless, N.A. Frei, C.T. Campbell, A novel single-crystal adsorption calorimeter and additions for determining metal adsorption and adhesion energies, Rev. Sci. Instr. 69, 1998, 2427–2438). Here we describe simulations of the signal pulse shape and an analysis of absolute signal intensities, based on system parameters such as the detector pyroelectric coefficient and thermal conductivities of the detector and sample, in order to understand heat collection efficiency in this and related calorimetric techniques.

A novel single-crystal adsorption calorimeter and additions for determining metal adsorption and adhesion energies

A new microcalorimeter for measuring heats of adsorption on clean single-crystal surfaces is described, and its operational characteristics are presented. The principle is similar to that pioneered by David King’s group: A pulse of gas from a molecular beam adsorbs on an ultrathin single crystal’s surface, causing a measurable transient heat input and temperature rise. Our novel heat detector is a 9 μm thick pyroelectric polymer ribbon, which is mechanically driven to make a gentle mechanical/thermal contact to the back of the single-crystal sample during measurements. Advantages include use of thicker samples (1 μm), sample preparation at very high temperatures, and potential measurements at cryogenic temperatures. A novel chopped molecular beam of metal vapor and a method of correcting for absorbed radiation from the hot effusion cell are also described. This system is applied to study the heats of adsorption of metals on clean, well-defined and single-crystalline surfaces as a detailed function of coverage, from which metal/substrate adhesion energies can also be extracted. We obtain pulse-to-pulse standard deviations of <2% in the heat of adsorption for pulses containing <0.03 ML of Cu and Pb, and absolute accuracy to within a few percent.

A New Single-Crystal Adsorption Calorimeter for Determining Metal Adsorption and Adhesion Energies

AbstractA microcalorimeter for measuring adsorption heats on clean single-crystal surfaces is described. The principle is similar to that pioneered by David King's group: A pulse of gas from a molecular beam adsorbs on an ultrathin single crystal's surface, causing a measurable transient heat input and temperature rise. Our novel heat detector is a 9 μm pyroelectric polymer ribbon, which is mechanically driven to make a gentle mechanical/thermal contact to the back of the singlecrystal sample during measurements. Advantages include use of thicker samples (>3 microns), sample preparation at very high temperatures, and measurements at 100 K. We have applied this to study the heats of adsorption of metals on clean, well-defined and single-crystalline oxide surfaces as a detailed function of coverage, from which we also extract the metal/oxide adhesion energy. We obtain pulse-to-pulse standard deviations of >2% for pulses containing >0.03 ML of Cu, and absolute accuracy within a few percent.

Adsorption kinetics involving precursor states and promoters

If the sticking coefficient for adsorption on a perfect clean single-crystal surface is low, the adsorption process on a real surface may primarily occur via trapping of molecules into precursor states, random walks in these states, and then chemisorption at e.g. step sites, defects or other adsorption promoters. This scheme of adsorption is analyzed in detail, and general expressions describing the coverage dependence of the sticking coefficients are derived for different cases. The latter include intrinsic and/or extrinsic precursors, combined with step sites, randomly distributed promoters, and self-promoted adsorption. Some of the results are briefly used to interpret or discuss experimental data for sticking of O 2 on a partly potassium-covered graphite surface, sticking of CO on Pt(111), and D 2 on Cu(100), respectively.

Core-level shift spectroscopy on tungsten surfaces. Overlayer and underlayer adsorption

Core-level binding energy shifts in the 4f7/2 level of W previously reported from a range of clean single-crystal surfaces are consistent with a simple linear relationship to an effective coordination number. Here we report data for the adsorption of H2, O2 and CO on W{110}, where conventional overlayers are formed, and for N2 on W{110}, where an underlayer is formed. The observed adsorbate-induced shifts are consistent with a model which accounts for changes in the coordination of surface W atoms due to adsorption, invariably leading to an increase in binding energy, and also changes due to charge transfer between adsorbate and substrate. It is pointed out that these two effects are additive for electron-withdrawing adsorbates, but tend to cancel each other with electron-donating adsorbates. Recent observations for alkali-metal atom adsorption are consistent with this model, and do not imply non-ionic adsorption.

Secondary electron emission due to MeV He ions at glancing incidence angles on clean single crystal surfaces

The total secondary electron yields per ion, γ, are measured for MeV He + ions incident on clean (001) surfaces of SnTe and PbSe single crystals. The dependence of γ on the incidence angle of the ions is well fitted by 1/sin θ at larger θ. (θ is the angle of incidence with respect to the surface plane.) However, at θ ~ ψ a an abrupt decrease of γ is observed, where ψ a is the characteristic angle of planer channeling. The decrease is due to the specular reflection of ions at the single crystal surface.

Scanning kinetic spectroscopy (SKS): A new method for investigation of surface reaction processes

Multiple reaction pathways are available to a polyatomic molecule interacting with a solid surface. Delineation of exact temperature regions in which the various pathways are either active or inactive is accomplished using a new method, Scanning Kinetic Spectroscopy (SKS). SKS uses a calibrated and collimated beam of reactant molecules incident upon a clean single crystal surface in UHV. A multiplexed quadrupole mass spectrometer (QMS) is enclosed inside a differentially pumped random flux shield, in line of sight to the crystal surface. The crystal temperature is programmed with a linear ramp ( dT/ dt = 2 K/ s.) and reactant consumption, product evolution, and desorption of stable surface species are simultaneously measured in one experiment. SKS data are presented here which characterize the reactions of methanol with the single crystal surfaces Ni(111), Cu(111), and Cu(111) plus preadsorbed oxygen. Application of the SKS method as an efficient probe of surface reaction pathways is illustrated by the contrasting chemistry of these surfaces. The methanol plus Ni(111) system is examined in detail in order to relate the observed SKS features to specific molecular reaction pathways on the Ni(111) surface.

Core level photoelectron spectroscopy for surface crystallography

Inner shell photoionisation as a basis for surface structure determination is reviewed. During the recent decade, this field has received tremendous impetus from the advent of synchrotron radiation. Core level photoelectron spectroscopy allows the detection at submonolayer levels of foreign adsorbate atoms and the resolution of the first layers from the bulk for a clean single crystal surface as well. The surface-bulk core level binding energy shift is shown to be surface geometry dependent and may be employed to monitor clean surface reconstruction. Shifting following chemisorption, surface core levels deliver information on adsorption binding site geometry. The emission intensity is modulated by photoelectron diffraction effects along k-vector variation obtained by varying photon energy or in measurements resolved in azimuthal and polar angles. Interference patterns are sensitive to bond lengths. Finally correlation with surface extended X-ray fine structure achieved by detecting photon stimulated Auger excitation or ion emission is discussed.

Semiempirical theory for surface core-level shifts

Numerical results are given for the surface core-level shifts at clean single crystal surfaces, stepped and reconstructed surfaces and surfaces with chemisorbed atoms. These results demonstrate that surface core-level shifts can be used as a diagnostic tool to study various surface effects.

Particle ejection from ion-bombarded clean and reacted single-crystal surfaces

A model based on a classical dynamical calculation of the impact of a 600-eV Ar+ ion into a single crystal has been developed to the point where it compares quantitatively to a wide variety of experimental observables. Here, we make direct comparisons between experiment and theory for relative sputtering yields, energy and angular distributions of the ejected atoms, and multimer yields. The model is also extended from clean single-crystal surfaces to include adsorbate atoms and molecules. These results are compared to experimental SIMS studies aimed at sorting out the structure-sensitive factors that contribute to the cluster ion yield. In oxygen chemisorption on Ni(100), for example, the O2−/O− yield is four times larger for a c (2×2) coverage than for a p (2×2) coverage, a value also predicted by the calculations. In molecular CO chemisorption, the ejection of CO is found to occur molecularly in both the experiments and in the calculations. In comparison to the calculations, a high value of Ni2CO+/NiCO+ is consistent with a bridge-bonded CO structure at low coverage.

Study of valence mixing inSmB6by x-ray photoelectron spectroscopy

Clean single-crystal surfaces of samarium hexaboride (${\mathrm{SmB}}_{6}$) have been investigated by x-ray photoelectron spectroscopy. The results confirm the existence of a mixed configuration of Sm in this compound. Comparison of the valence-band spectrum with that of ${\mathrm{LaB}}_{6}$ allows an accurate determination of the Sm $4f$ contribution, and a direct comparison with theoretical ${\mathrm{Sm}}^{2+}$ and ${\mathrm{Sm}}^{3+}$ $4f$ spectra. The effective Coulomb interaction is found to be 7.0 \ifmmode\pm\else\textpm\fi{} 0.2 eV; the lifetime broadening of the ${\mathrm{Sm}}^{3+}$ $4f$ spectrum is as large as 1.0 eV, which is one order of magnitude larger than that of the ${\mathrm{Sm}}^{2+}$ $4f$ spectrum. The ratio of ${\mathrm{Sm}}^{2+}$ to ${\mathrm{Sm}}^{3+}$ from the $4f$ photoemission intensities is 0.4 in good agreement with the value deduced from earlier susceptibility and M\ossbauer experiments. The investigation of the Sm $4d$ core levels provides additional evidence for the mixed valence state. The background contribution to the Sm $4d$ spectrum is determined accurately by comparison with the tail associated with the narrow B $1s$ line; after background correction, a quantitative comparison with ${\mathrm{Sm}}^{2+}$ and ${\mathrm{Sm}}^{3+}$ $4d$ spectra is performed; good agreement can be obtained by including a large lifetime broadening (0.8 eV) for the ${\mathrm{Sm}}^{3+}$ $4d$ spectrum. Our results are discussed in connection with other data on more complex mixed-configuration systems.

Plasmon excitation energies in ZnO, CdO, and MgO

A study has been made to experimentally determine the plasmon excitation energies in the oxides of Zn, Cd, and Mg using the reflection electron energy loss technique. The measurements were made on clean single-crystal surfaces using electrons of 250–1000-eV energy. Distinction was made between surface- and bulk-plasmon excitation by recording the changes which occurred in the energy loss spectrum as the surface was selectively contaminated. Surface-plasmon excitation energies of 17.0, 15.9, and 14.7 eV and bulk-plasmon excitation energies of 18.8, 22.8, and 22.0 eV were measured for ZnO, CdO, and MgO, respectively. Comparison is made with similar data obtained from optical reflectivity and transmission electron energy loss measurements.

Kinetics of the reaction of oxygen with clean nickel single crystal surfaces: II. Ni(111) surface

Previous studies of the initial interaction of oxygen with clean single crystal surfaces of nickel, using AES, LEED, and work function change measurements, have been extended to the case of the Ni(111) surface. Results indicate that the course of the interaction in this case is very similar to that reported previously for the Ni (100) surface. The reaction proceeds by the initial formation of chemisorbed structures having a periodicity based on the nickel lattice. At a coverage of 0.34 monolayer, NiO islands nucleate and grow until the surface reaches a saturation coverage of 1.8 monolayer of NiO. The kinetics of this growth process are well explained by an island nucleation and growth model in which growth occurs by the dissociation of a molecularly adsorbed oxygen species at island perimeter sites.

A pulsed-molecular-beam mass spectrometer for studies of atomic and ionic impact phenomena on metal surfaces

A pulsed-molecular-beam mass spectrometer has been developed, which offers a new approach in the investigations of the kinetics of gas/solid interactions and in research on molecular beams. Since the construction of the major portion of the apparatus has been completed, it seems appropriate at this time to describe the design features of the machine and to discuss briefly some particular research objectives which will be made possible by the special design features of the apparatus. It is intended to limit this discussion mainly to the types of investigations of atomic impact phenomena on metal surfaces made possible by the new design and refer to other reports [1] for discussion of ionic impact phenomena (such as inelastic and elastic reflection of ions from clean single-crystal surfaces, resonance neutralization of ions, etc.).