Peter Zeppenfeld

Registry effects in the thermodynamic quantities of Xe adsorption on Pt(111)

Abstract We report here on thermodynamic studies of Xe adsorption on Pt(111) using high resolution He scattering. From the data we extracted isosteric heats, partial molar entropies and lateral compressibilities in the adsorbed layer. Near completion of the commensurate ( 3 × 3 ) R30° phase, i.e. at the commensurate-incommensurate phase transition, we observe a substantial drop in the heat of adsorption of about 30 meV and an increase in the entropy of the adsorbed layer. The difference in adsorption heat Δq st ≈30 meV between the commensurate and the relaxed uniaxially compressed incommensurate phase reflects the binding energy difference between the lowest energy level occupied by the Xw atoms in the commensurate phase and the average of the energies of the sites occupied in the incommensurate phase; thus, Δ q st is a lower bound for the lateral corrugation of the substrate holding potential. The increase of the entropy of the adlayer in the incommensurate phase may be attributed to intrinsic properties of this phase. The results are discussed, and compared with recent Xe-adsorption studies on Ag(111) and Pd(100).

High‐resolution He‐scattering apparatus for gas–surface interaction studies

A high‐resolution apparatus designed for the study of elastic and inelastic scattering of thermal helium atoms from crystal surfaces is presented. The highly expanded He nozzle beam has an energy spread ΔE/E of about 1.4% and is collimated to 0.2°. The angle subtended by the detector opening as seen from the sample is also 0.2°. Beam intensities as low as 10−6 of the specular beam intensity from a low‐temperature clean Pt(111) surface are detectable. Pseudorandom chopping with a resolution of 2.5 μs (flight path 790 mm) is used for time‐of‐flight (TOF) analysis of the scattered helium. The base pressure in the sample chamber is in the low 10−11 mbar. The capabilities of the apparatus are demonstrated for physisorbed Xe adlayers on Pt(111). The results presented are obtained by using He scattering in various modes: coherent inelastic, coherent elastic, and incoherent (diffuse) elastic. This technique allows for a nondestructive nearly exhaustive characterization of the thermodynamics, structure, and dynami...

Symmetry breaking commensurate-incommensurate transition of monolayer Xe physisorbed on Pt(111)

Abstract We report a high resolution He-diffraction study of the commensurate-incommensurate transition of monolayer xenon physisorbed on Pt(111). The experimental results show that we have been able to observe for the first time a (√3x√3)R300 commensurate (C) striped incommensurate (SI) transition. The striped domain walls are found to run into the ΓK -direction, i.e. the uniaxial compression is in the ΓM -direction. The C-SI transition appears to be continuous within the experimental accuracy and the incommensurability in the weakly incommensurate phase follows a 1 2 power law versus reduced temperature.

Characterization by scanning tunneling microscopy of the oxygen induced restructuring of Au(111)

The morphology of the Au(111) surface exposed to oxygen under high pressure (po2 = 1 bar) and high temperature (500 ≤ T ≤ 800°C) during a prolonged period of time (between 12 and 24 h) was investigated by scanning tunneling microscopy (STM). At the atomic scale the surface exhibits a (√3 × √3)R30° structure which is ascribed to atomic oxygen being strongly chemisorbed at the gold surface. In addition, on a larger scale, a long-range superstructure with hexagonal symmetry and a periodicity varying between 60 and 80 A is observed. This superstructure is interpreted as a moire type pattern arising from the periodic height modulation induced at the surface by a small lattice mismatch and a slight rotation between the topmost two gold layers. The long-range superstructure exhibits local distortions which can be explained by the high sensitivity of moire patterns to the exact lattice mismatch and to the rotation angle between the two layers. The observation of small reconstructed areas forming on the Au(111) surface upon chemisorption of oxygen indicates that the restructuring proceeds via a nucleation and growth process.

An ultrahigh vacuum scanning tunneling microscope for use at variable temperature from 10 to 400 K

We report on the construction of an ultrahigh vacuum (UHV) scanning tunneling microscope (STM) specially designed for operation in the entire range of sample temperatures between 10 and 400 K. The sample is cooled by means of a liquid helium continuous‐flow cryostat, while the supporting manipulator and the surrounding devices remain at room temperature. This allows rapid variation of the sample temperature. The standard instruments for surface preparation and analysis and the STM are contained in a single UHV chamber. By rotation of the manipulator the sample can be positioned in front of any of these instruments without changing the sample temperature. The performance of the microscope is demonstrated by two examples of images of xenon adsorbed on platinum(111) showing: (a) the evolution of the morphology of a submonolayer of xenon from adsorption at 17K up to desorption at about 90 K and (b) atomically resolved images of the hexagonal incommensurate rotated phase for xenon at monolayer completion.

Xe monolayer adsorption on Cu(110): Experiments and interaction calculations

Abstract The isosteric heat of adsorption, the lateral energy, the frequency of the perpendicular monolayer vibrational mode and the sequential occurrence of metastable high order commensurate structures for the Xe monolayer adsorbed on the Cu(110) surface have been determined using helium atom scattering experiments. The same quantities are then calculated on the basis of simple expressions of semi-empirical interaction potentials and the pertinency of such expressions in describing the whole body of experimental data is discussed. It is shown that a fair agreement can be obtained in a first approximation with potentials exhibiting a minimum of three parameters.

A scanning tunneling microscopy study of the adsorption of Xe on Pt(111) up to one monolayer

The adsorption of Xe on Pt(111) has been investigated from the arrival of the very first atoms up to completion of the monolayer using a variable-temperature Scanning Tunneling Microscope (STM). Surprisingly, in the initial stages of the adsorption Xe preferentially binds to a low coordination site, theupper edge of the platinum steps. The strong binding to these sites leads to a local repulsive interaction with further Xe atoms. Therefore, the Xe atoms located at the upper edge of the steps do not serve as nuclei for 2D Xe islands, which, instead, form on the terraces and at thelower edges of the platinum steps. Only during completion of the monolayer do these islands make contact with the atoms adsorbed at the beginning in the upper-edge positions. The full monolayer exhibits the Hexagonal Incommensurate Rotated (HIR) phase already known from earlier helium-diffraction experiments.

DETERMINATION OF IODINE ADLAYER STRUCTURES ON AU(111) BY SCANNING TUNNELING MICROSCOPY

Chemisorbed iodine adlayers on Au(111) films on quartz were studied using scanning tunneling microscopy (STM) at room temperature in air. The iodine was adsorbed by successive deposition of droplets of a dilute solution of iodine in methanol. As a function of coverage, various adlayer structures were obtained. By changing the tunneling parameters, either the iodine adlayer or the Au(111) substrate can be imaged with atomic resolution. In this way, the adlattice properties such as periodicity, orientation and the local absolute coverage have been characterized with high accuracy. In the low coverage range (θ<0.33), due to the high mobility of iodine atoms, only the unreconstructed Au(111) substrate lattice could be imaged. At θ∼0.33, a (3×3)R30° structure is evident. With increasing coverage, a (p×3) structure of higher iodine packing density is observed, which can be described as a uniaxially compressed (striped) phase. Finally, near monolayer saturation coverage the iodine atoms form a hexagonal moire-li...

Adsorption and structure of N2 on copper(110)

The adsorption and structure of ultrathin physisorbed N2 films on Cu(110) has been investigated in a combined experimental and theoretical study. Using specular helium scattering the heat of adsorption for the unconstrained N2 monolayer was determined to qst = 88 meV per molecule and the 2D heat of condensation to q2D = 5.5 meV. In the coverage range up to one monolayer the nitrogen molecules form a complex (4113) high-order commensurate phase with an oblique, quasi-hexagonal unit cell. Increasing the coverage beyond completion of the unconstrained monolayer, the adlayer first undergoes a phase transition into a weakly compressed phase before the condensation of the bilayer sets in. In the bilayer the N2 molecules are arranged in a slightly distorted hexagonal structure. Using realistic interaction potentials, the structure of the N2 monolayer is investigated by detailed total energy calculations. As a result, the most stable structure is found to be the (4113) phase in agreement with experiment. In addition, the arrangement of the nitrogen molecules has been determined: the unit cell contains seven N2 molecules arranged in a novel type of 7-sublattice pinwheel structure. Molecular dynamics simulations corroborate this structure and further demonstrate its remarkable thermal stability.

Surface morphology of Au(111) after exposure to oxygen at high temperature and pressure

Abstract We report a scanning tunnelling microscopy (STM) study of the surface morphology of Au(111) exposed to oxygen at high temperature (700–800°C) and high pressure (1 bar). Two types of oxygen induced surface structures were observed: (i) a hexagonal long-range reconstruction [L. Huang et al., Appl. Phys. Lett. 66 (1995) 211] with a periodicity of ∼ 65 A and a corrugation of about 0.5 A. At the atomic scale this surface exhibits a (√3 × √3)R30° structure, which we attribute to atomic oxygen strongly chemisorbed at the Au(111) surface. (ii) a “modified” herringbone pattern related to the original herringbone reconstruction of the clean Au(111) surface, but with additional depressions in some of the fcc-regions between the reconstruction lines. At the atomic scale this surface exhibits a characteristic “row structure” with √3 periodicity. For both structures the dependence of the STM image contrast on the tunnelling parameters was investigated.