Search

A study of the changes of molecular packing in self-assembled monolayers of alkylthiols on Au(111) induced by external pressure is presented. Atomic force microscopy (AFM) is used to apply pressure and to measure the height of islands of alkanethiols partially covering the gold surface. The islands are made of ordered straight chain alkylthiol molecules tilted from the surface normal. Their height was found to decrease in a stepwise manner as a function of the load applied by the tip. Simultaneous stepwise increases in friction force were observed. A simple geometrical model involving the interlocking of alkyl chains at specific molecular tilt angles can explain the observations. According to the model, tilts in both the nearest neighbor and the next-nearest neighbor directions are necessary.

We present results of surface potential measurements of ice and water films condensed on mica surfaces in the temperature range of −26°C to 19°C. These results indicate the growth of dipole-oriented ice and water films giving rise to electrical potentials of up to −50 V at the surface. We propose that this is due to the growth of surface-induced ferroelectric ice. The results are significant because the films are in equilibrium with the vapor at temperatures that are commonly present on earth and in the atmosphere.

The adsorption of CO on Pd(111) has been studied with scanning tunneling microscopy in the very low coverage regime. Isolated CO molecules were imaged with two different corrugations of ∼0.25 A and ∼0.15 A, which are attributed to adsorption of the hcp and fcc hollow sites. Total energy calculations reveal that these two sites are the most favorable ones. Calculated image profiles using the ESQC method agree well with the experimental corrugations.

The growth of the first cobalt monolayer (ML) on the Cu(110)-(2×1)O surface was studied by scanning tunneling microscopy. Extensive exchange of Cu and Co atoms takes place in the first stages of the deposition. The displaced Cu atoms form new CuOCo mixed islands, with the same structure as those of the terrace surface. At ∼0.25 ML Co, a new structure nucleates, which contains three Cu atoms, four Co atoms and two O atoms per 2×2 cell. The structure consists of rows in the [110] direction with an internal periodicity of two lattice units. The rows are separated from one another by two lattice units along the [001] direction, and are found both in-phase and out-of-phase relative to one another. The result is a mixed p(2×2) and c(2×4) surface. The fraction of the surface covered by the new structure increases with Co coverage, and completely covers the surface at ∼1 ML Co.

The friction of nanometer thin ice films grown on mica substrates is investigated using atomic force microscopy (AFM). Friction was found to be of similar magnitude as the static friction of ice reported in macroscopic experiments. The possible existence of a lubricating film of water due to pressure melting, frictional heating, and surface premelting is discussed based on the experimental results using noncontact, contact, and lateral force microscopy. We conclude that AFM measures the dry friction of ice due to the low scan speed and the squeezing out of the water layer between the sharp AFM tip and the ice surface.

The wetting properties of 8CB ( 4(')-n-octyl-4-cyanobiphenyl) on silicon wafers have been studied with scanning polarization force microscopy (SPFM). Layer-by-layer spreading of 8CB droplets is observed. With the help of the surface potential mapping capability of SPFM, we found that the molecular dipole of the first monolayer of 8CB is parallel to the surface. A layer of nearly vertical molecular dimers on top of the monolayer has an associated surface potential of 40 mV, which is attributed to a distortion of the dimer. The dimer distortion propagates to the subsequent smectic bilayers, producing an additional 7 mV potential increase in the second layer, 2 mV on the third, and approximately 1 mV on the fourth.

The de-wetting process of Zdol-type lubricants on hard disks has been studied at the molecular scale with scanning polarization force microscopy (SPFM) and ellipsometry. It was found that de-wetting occurred over hours to days when the initial lubricant layer was thicker than one monolayer (∼20 A thick). De-wetting was found to take place in a layered fashion for Zdol-TX and Z-Tetraol, with weakly bound second layers accreting into droplets, however, no layering was seen for Zdol during de-wetting, only droplets. The droplets eventually grew to the point where they could be observed by optical microscopy. In all cases, the first monolayer remained bound to the surface. Although the atomic force microscopy (AFM) tip could easily penetrate this first layer, it did not remove it.

The electrical current through the point-contact junction of an AFM tip is used to image the surfaces of bulk graphite (HOPG) and the surface of a graphitized carbon monolayer on Pt(111) under ultra-high-vacuum (UHV) conditions. Lattice-resolved images are obtained simultaneously in topography, lateral friction, and contact current channels. Lattice resolution in current maps persisted up to 0.9 mA and pressures of up to 5 GPa. In both bulk graphite and the case of graphitized carbon monolayer on Pt(111), the current images show only one maximum per unit cell. In addition, the contact current images of the graphite monolayer reveal local conductivity variations. We observed local conductivity variations in the form of moire superstructures resulting from high order commensurability with the Pt lattice. (c) 1999 The American Physical Society.

We have constructed a scanning tunneling microscope with simultaneous light collection capabilities in order to investigate the opto-electronic properties of semiconductors. The microscope has in situ sample cleavage mechanism for cross-sectional sample. In order to reach low temperature (4 K), we used a specially designed cryostat. The efficiency of light collection generated in the tip-surface junction was greatly improved by use of a small parabolic mirror with the tip located at its focal point.

Atomic force microscopy (AFM) was used to study the growth and morphology of ice films on the cleavage surface of mica. Measurements performed in contact, as well as in noncontact operation modes of the microscope, allowed us to distinguish the solid and liquid parts of the film. At temperatures below −30 °C, supercooled water droplets formed on top of a thin (nanometer range) ice layer in contact with the substrate. After annealing, a contiguous flat film was formed. Between −20 and −10 °C and at a relative humidity of ∼83%, the film consisted of a solid ice layer ∼7 A thick, covered by a liquid-like layer 50±5 A thick. When the temperature was raised above 0 °C, droplets formed, which subsequently evaporated. Comparison of results obtained in the various AFM operation modes allowed us to conclude the existence of a liquid-like layer on the ice surface.

The surface forces apparatus (SFA) was used to study the lubricating properties of an 8CB liquid crystal film confined between mica surfaces. After displacement of most of the liquid crystal by compression, a 10.2 {angstrom} thick layer of 8CB was found between mica surfaces in the range of applied pressures from 1 to 20 MPa. Under these conditions, the authors found that friction depends linearly on contact area in the 1--1.5 MPa range. They found that the critical shear stress was anisotropic, with values of 0.28 MPa for shear along the 8CB molecular axis and 0.42 MPa for shear in the perpendicular direction. This anisotropy was observed through all the ranges of load where slip occurred. At higher loads (1.5--2.7 MPa), the critical shear stresses increase to 0.6 and 1.2 MPa for parallel and perpendicular shear, respectively.

The nanotribological properties of a hydrogen‐terminated diamond(111)/tungsten‐carbide interface have been studied using ultra‐high vacuum atomic force microscopy. Both friction and local contact conductance were measured as a function of applied load. The contact conductance experiments provide a direct and independent way of determining the contact area between the conductive tungsten‐carbide AFM tip and the doped diamond sample. We demonstrate that the friction force is directly proportional to the real area of contact at the nanometer‐scale. Furthermore, the relation between the contact area and load for this extremely hard heterocontact is found to be in excellent agreement with the Derjaguin–Muller–Toporov continuum mechanics model.

Heterogeneous catalysts that contain bimetallic nanoparticles may undergo segregation of the metals, driven by oxidizing and reducing environments. The structure and composition of core-shell Rh 0.5 Pd 0.5 and Pt 0.5 Pd 0.5 nanoparticle catalysts were studied in situ, during oxidizing, reducing, and catalytic reactions involving NO, O 2 , CO, and H 2 by x-ray photoelectron spectroscopy at near-ambient pressure. The Rh 0.5 Pd 0.5 nanoparticles underwent dramatic and reversible changes in composition and chemical state in response to oxidizing or reducing conditions. In contrast, no substantial segregation of Pd or Pt atoms was found in Pt 0.5 Pd 0.5 nanoparticles. The different behaviors in restructuring and chemical response of Rh 0.5 Pd 0.5 and Pt 0.5 Pd 0.5 nanoparticle catalysts under the same reaction conditions illustrates the flexibility and tunability of the structure of bimetallic nanoparticle catalysts during catalytic reactions.

We report different wetting properties of Cu(110) and Cu(111) at near-ambient conditions using in situ photoemission spectroscopy. At near-ambient conditions of pressure (1 Torr) and temperature (295 K), the Cu(110) surface is covered with a mixed OH and H2O layer, whereas the Cu(111) surface remains clean and adsorbate-free. We show that wetting is controlled by the presence of OH groups on the surface, acting as anchors for water adsorption. Hydroxylation of the Cu(110) surface is facilitated by a lower activation barrier for water dissociation compared to Cu(111).

The structure of a water film formed on mica at room temperature, in equilibrium with water vapor at various relative humidities (RH), was studied using sum-frequency-generation (SFG) vibrational spectroscopy and scanning polarization force microscopy (SPFM). Analysis of the O-D stretch modes in the SFG spectra of D{sub 2}O on mica indicates that as RH increases, the submonolayer water structure evolves into a more ordered hydrogen-bonding network. At full monolayer coverage ({approximately} 90{percent} RH) , the SFG spectrum suggests an icelike film with no dangling O-D groups, in agreement with a recent molecular dynamics simulation. {copyright} {ital 1998} {ital The American Physical Society}

The possibility of identifying the chemical nature of atomic adsorbates (C, N, O, F and S) on metal surfaces has been explored both theoretically and experimentally. It is shown that, although the tip and the surface may markedly influence the appearance of adatoms in STM images, atomic size and electronegativity are the dominant factors that determine the contrast. In this work, low-temperature (

Scanning tunneling microscopy (STM) was used to study the adsorption of para- and meta-xylene on the Rh(111) surface. Para-xylene appears in the STM images as a diamond-shaped feature with the long axis oriented along the [11 2 ] and equivalent directions. Ordered domains with three orientations differing by 120° are formed. The images of the meta-isomer have the appearance of triangular shapes and do not form any ordered structure. Individually, however, they are oriented with one side of the triangle parallel to the compact [1 1 0] directions. The internal structure in the molecular images can be discerned. By a combination of symmetry and packing considerations, it is possible to attribute the brightest part of the molecular image as due to the methyl groups. When the two isomers are coadsorbed, they exhibit complete mixing, and the individual components can be differentiated by their shape. At low coverage, both xylene molecules adsorb preferentially at the step edges, whereas on the terraces, they are mobile in the time scale of the STM imaging. Coadsorption with CO produces a mixed layer, and the xylene molecules become immobile.

The three complementary surface structure probes of X-ray photoelectron diffraction (XPD), scanning tunneling microscopy (STM), and low-energy electron diffraction (LEED) have been combined in a single instrument. This experimental system has been utilized to study the growth of iron oxide films on Pt(111) over the thickness range from 0.75 to 3.00 monolayers (ML). Each film was formed by first depositing an overlayer of pure Fe with a certain coverage in ML (ranging from 0.75 ML to 3.00 ML) and then thermally oxidizing the Fe at a temperature of 980 K and in an oxygen pressure of 4×10−6 Torr. For films up to ∼1 ML in thickness, a bilayer of Fe and O similar to those in bulk FeO parallel to a (111) plane formed. In agreement with a prior STM and LEED study by Galloway et al., we find this bilayer to be an incommensurate oxide film forming a lateral superlattice or Moire structure with short- and long-range periodicities of ∼3.1 and 26.0 A. From the XPD data, in addition, it can be concluded that the topmost oxygen layer is highly relaxed inward by ∼0.6 A as compared to the bulk FeO (111) interplanar spacing, and that the stacking of the topmost O atoms with respect to the underlying Pt is dominated by one of two structurally very similar possibilities. It is furthermore necessary to consider interactions over at least four atomic layers (O, Fe, and the first two Pt layers) to explain this dominance of one stacking type. For thicker iron oxide films from 1.25 to 3.0 ML, the growth mode is essentially Stranski–Krastanov: iron oxide islands form on top of the FeO(111) bilayer mentioned above. For iron oxide films of 3.0 ML thickness, X-ray photoelectron spectroscopy (XPS) yields an Fe 2p3/2 binding energy and an Fe:O stoichiometry consistent with the presence of Fe3O4. XPD data further prove this overlayer to be Fe3O4(111)-magnetite in two almost equally populated domains with a 180° rotation between them. The structural parameters for this Fe3O4 overlayer generally agree with those of a previous LEED study, except that we do not find a terminating partial monolayer of Fe and arrive at a significant difference in the first Fe–O interplanar spacing. Overall, this work demonstrates the considerable benefits to be derived by using this particular set of complementary surface structure probes in such epitaxial growth studies.

Potassium mica was treated with different ionic solutions to replace the naturally occurring K{sup +} on the surface by Ca{sup 2+}, Mg{sup 2+}, and H{sup +} ions. The extent of the exchange was monitored by variable emergence angle X-ray photoelectron spectroscopy (XPS). Scanning polarization force microscopy (SPFM) was used to measure the mobility of the surface ions as a result of water adsorption when the mica is exposed to different humidity levels.

The condensation of glycerol on mica surfaces in ambient air was studied with scanning polarization force microscopy (SPFM). Two different wetting regimes were found that depend on the state of the surface. On freshly cleaved mica, glycerol condenses, forming flat films. The films expand until a uniform layer is formed. The first molecular layer of glycerol was found to be more strongly bound to the surface than were subsequent layers. On contaminated mica (after 2 h of exposure to air) droplets in the shape of spherical caps form that partially wet the substrate. The droplets grow indefinitely in the saturated vapor. The contact angle of droplets with heights

The adsorption of water on several alkali halide surfaces was studied using scanning polarization force microscopy. Water adsorption leads to an overall increase of surface potential and ionic mobility. At a critical humidity that is characteristic of each salt (NaCl, KCl, KBr, and KI), important changes in the rate of increase of the surface potential and ionic mobility occurred. Topographical changes occurred as well, in the form of step motion. These were observed to occur at a fast rate above the critical point, while little step motion occurred at lower humidity. Dissolution of the crystal (deliquescence) is observed at high humidity. Below the critical humidity, contact potential images indicate that preferential solvation of cations occurs at steps producing a large enhancement of the step contrast in the images.

We apply our recent Green's function based STM formalism to study the adsorption of S monomers and clusters on Re(0001). Starting from previous STM experimental data and LEED results, we consider surface relaxations common to the different S structures observed on this system. The role of such relaxations on the final shape of the theoretical STM images is then examined, and we find that the sensitivity of the images to these structural parameters can be considerable in certain cases. The consistency of proposed relaxed models is also confirmed after a theory–experiment comparison of the STM images.

An adiabatic theory of spontaneous adatom transfer between a surface and an AFM/STM tip has been developed under the assumption that the dwell time of the adsorbate at the adsorption site is small compared to the phonon and electron-hole relaxation times. The adsorbate is assumed to be tunneling across the potential barrier from a ground state at the sample to that at the tip. A simple analytical expression has been obtained for the ground-level energy splitting, which governs the dynamics of the coherent tunneling. The transition rate is shown to depend strongly on the adatom mass, binding energy and separation distance between the adsorption sites. It is found that, at typical AFM/STM tip velocities and separation distances, an adatom may be involved in a process of consecutive transitions from the sample to the tip and vice versa. It is shown that coherent tunneling is a dominant mechanism of adatom transfer at temperatures below 10 K for insulators and semiconductors and below 0.1 K in the case of metals. The total transfer probability as a function of time may exhibit quantum oscillations whose period is very sensitive to variation of physical parameters.

Water molecules in the air play a very important role in the modification of the physical and chemical properties of NaCl surfaces. In this paper, we report on the dependence of the friction force between this alkali halide and a Pt-coated tip on the external load at different humidity levels. The results clearly demonstrate that the tribological properties of the surface are modified by water adsorption. We also found that at high humidity and below a certain threshold load, the water layer can act as a lubricant. (C) 1998 Elsevier Science B.V. All rights reserved.

Acetylene molecules adsorbed onto a Pd(111) crystal surface were imaged by scanning tunneling microscopy (STM) below 70 K. Single ${\mathrm{C}}_{2}{\mathrm{H}}_{2}$ molecules appeared as a combination of a protrusion and a depression 3.5 \AA{} apart. This peculiar shape is due to the molecular \ensuremath{\pi} orbital, which is oriented at a shallow angle out of the surface. Six orientations of the molecule with respect to the substrate were observed, indicating two different threefold hollow binding sites each with three possible rotational states. At 44 K, thermally activated rotation of the molecules between the three equivalent states on the same threefold hollow site occurs on the time scale of seconds. Diffusion of molecules between adjacent threefold sites began at $\ensuremath{\sim}70\mathrm{K}.$ The findings agree with the structure of the adsorption site determined by total-energy calculations and with the STM image calculated using the electron scattering quantum chemistry method.

Submicrometer size droplets of sulfuric acid on mica have been imaged using scanning polarization force microscopy (SPFM). Our study shows that, contrary to the macroscopic case, the drop profile depends on the size of the drops, with a decrease of the contact angle with decreasing size droplets in the present case. This dependence can be explained by taking into account long-range forces between the substrate and the fluid, which prove to have important effects for submicrometer size drops. Profile measurements at the submicron scale allow direct estimation of the interaction potential between a fluid and a substrate.

We present the design of a scanning force microscope and vacuum chamber for the growth and imaging of ice films in thermodynamic equilibrium and under controlled super or undersaturation. The apparatus allows measurements in the temperature range from −60 to +80 °C in a controlled water vapor atmosphere. First results on the morphology and the frictional properties of thin ice films on mica cleavage faces are presented. The films are found to grow in a two-dimensional manner, often exhibiting dendritic growth shapes. The lateral force measured on ice is higher than that observed on the surrounding substrate.

Friction Force Microscopy studies on ion-exchanged mica surfaces at different humidity levels were performed. Ion exchange was achieved by treating the potassium mica (muscovite) in different solutions to replace K+ surface ions by Ca2+, Mg2+, and H+. It is found that these different surface ions can significantly modify the tribological properties of mica.

An adiabatic theory has been developed to describe spontaneous transfer of adatoms from a surface to a tip in atomic force microscopy and scanning tunneling microscopy (AFM/STM). It is shown that, at sufficiently low temperatures, the influence of phonon and electron-hole excitations on the transfer process may become so small that coherent transitions of an adatom between the surface and the tip are possible. The adatom tunnels back and forth between the surface and the tip with a definite period that depends on the surface-tip separation. The effect is mainly due to a unique opportunity to vary the tunneling amplitude of the adsorbate by controlling the tip-surface separation distance. Two features contribute to the quantum coherence phenomenon being even more pronounced as compared to that of interstitial hopping in the bulk: (a) the electron density of states at the Fermi level, and (b) the lattice deformations associated with the presence of the adsorbate outside the surface are noticeably smaller than for an interstitial in the bulk. As a result, electron and phonon polaron effects, respectively, which reduce the mobility of the adatom with respect to surface-tip transition, are substantially suppressed at all temperatures. For typical AFM/STM tip velocities and separationmore » distances, the coherent transition rate of chemisorbed hydrogen atoms at temperatures below 10 K for insulators and semiconductors, and below 0.1 K for metals, may not only be comparable with but also exceed the thermally activated (incoherent) transfer rates at room temperature. {copyright} {ital 1998} {ital The American Physical Society}« less

Scanning tunneling microscopy was applied to studies of adsorbates on model catalysts at elevated pressures where dense overlayers in equilibrium with the gas phase can be formed. A structure due an incommensurate hexagonal overlayer is observed for CO on the Pt(111) surface in the 200{endash}750Torr pressure range. This structure has not been observed before at similar coverage under low pressure and temperature conditions. The results illustrate the inequivalence in this case between the study of surfaces in catalytic conditions of high pressure and the surface science studies carried out in high vacuum. {copyright} {ital 1998} {ital The American Physical Society}

The spatial distribution of sulfur and carbon monoxide coadsorbed on stepped Pt(111) surfaces was characterized by low-energy electron diffraction and scanning,tunneling microscopy. The formation of a p(2x 2) sulfur overlayer on the platinum substrate leads to the slow doubling of the original monatomic steps, but subsequent room temperature exposure of that surface to CO removes the reconstruction and induces the formation of approximately 50 A wide intermediate steps covered with carbon monoxide exclusively. Additional CO dosing causes the development of more complex structures at the edge of the steps, where the excess sulfur is pushed to the bottom of the step while the rest of the terrace becomes covered with CO + S intermixed layers. The implications of these results for catalysis are briefly discussed.

The structure of water films on mica was locally modified by contact with the tip of an atomic force microscope (AFM) in a humid environment. The subsequent evolution of the film was studied by noncontact scanning polarization force microscopy. At high relative humidity (>20%), capillary condensation caused water to form droplets and two-dimensional islands around the contact point. The droplets evaporated in a short period of time, but the islands remained for much longer periods (hours). At low relative humidity (

The structures formed by sulfur adsorbed on the Rh(111) surface have been studied with Auger electron spectroscopy (AES), low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). Five differently ordered overlayer structures were observed to form as a function of coverage. These are: (√3 × √3)R30°, c(√3 × 7)rect, c(4 × 2), (4 × 4) and (7 × 7). The first structure forms up to 0.33 monolayers, with S occupying fcc hollow sites. Above 0.33, hcp hollow sites are occupied as well. In the c(4 × 2) structure, at 0.5 monolayer coverage, there is equal occupation of fcc and hcp hollow sites. The transition from the (√3 × √3)R30° to c(4 × 2) overlayer occurs by formation of dense domain boundaries containing both fcc and hcp sites. The S in the hcp sites is metastable and, upon heating, it moves to nearest fcc sites to produce the (4 × 4) structure.

We present a theoretical formalism specially suited for the simulation of scanning tunneling microscopy (STM) images. The method allows for a realistic description of the STM system, taking fully into account its three-dimensional nature. Bias effects may also be considered since the theory is not restricted to the low-bias limit. The starting point is the previously applied Landauer-B{umlt u}ttiker formula, which expresses the current at the STM junction as a sum of transmission coefficients linking eigenstates at each electrode. The transmission coefficients are directly obtained from the scattering matrix which is, in our approach evaluated through Green-function techniques; in particular, we employ the surface Green-function matching (SGFM) method to find the Green function at the interface, and explicitly derive simple expressions for the current. Additionally, the formalism goes beyond the elastic-scattering limit by considering inelastic effects via an optical potential. We also present a method to analyze the current in terms of contributions arising from individual atomic orbital interactions and their interference with other interactions. To this end, the SGFM method is replaced by a first-order expansion of the interface Green function. {copyright} {ital 1997} {ital The American Physical Society}

We apply our recently developed Green-function formalism for the simulation of scanning tunneling microscopy (STM) images to the clean Rh(111) and the $\mathrm{Rh}(111)+c(4\ifmmode\times\else\texttimes\fi{}2)\ensuremath{-}2\mathrm{S}$ systems. The former represents a test case in order to study the adequacy of the extended H\"uckel theory (EHT) as an approximation to the Hamiltonian matrix elements of the entire system. It is shown that, via a suitable parametrization of the atomic-orbital basis set, the EHT provides a reasonable description of the electronic structure of the sample and the tip, together with the interactions at the STM interface. When the same parametrization scheme is applied to the $\mathrm{Rh}(111)+c(4\ifmmode\times\else\texttimes\fi{}2)\ensuremath{-}2\mathrm{S}$ system, we find a good agreement with the experimental images by considering a Pt tip with its apex terminated in a Pt or S atom. More specifically, the two inequivalent maxima per unit cell that appear in the images are unambiguously assigned to a S atom adsorbed at the hcp site (brightest maximum) and to another S atom at the fcc site. Furthermore, the simulations for each tip termination can be associated to two different types of experimental images which could be acquired in a reproducible way. At the same time, and after a detailed study on the relevance of most of the parameters involved in the calculation, some general criteria for the estimation of their value, prior to the STM calculation, are established. Finally, an analysis of the tunneling current on the Rh(111) system allows us to identify the $\mathrm{Rh}\ensuremath{-}{d}_{{z}^{2}}$ orbitals at the surface as the main states responsible for the atomic resolution. It is also found that the most relevant probing orbitals at the tip apex are those with axial symmetry along the surface normal; for the S ended tip, it is the S-${p}_{z}$ orbital which dominates by far the current, whereas, for the Pt-ended tip, first the Pt-$s$ and next the $\ensuremath{-}{d}_{{z}^{2}}$ provide the major contributions. The better resolution obtained with the former tip is a consequence of the different levels of localization of these probing orbitals. A similar analysis applied to the $\mathrm{Rh}(111)+c(4\ifmmode\times\else\texttimes\fi{}2)\ensuremath{-}2\mathrm{S}$ system explains the tip-dependent features in the simulated images, again through the higher localization of the tip $\mathrm{S}{\ensuremath{-}p}_{z}$ orbital with respect to the Pt-$s$ orbital, and the simultaneous interaction of these states with the ${p}_{z}$ orbitals of the S atoms at the surface. The inequivalence between the two S maxima in the unit cell, on the other hand, arises as a consequence of a better coupling of the substrate to the S-${p}_{z}$ orbitals when the adsorbate is at the hcp site than when it is at the fcc site.

The influence of a dissipative environment on quantum coherence in surface-tip adatom transfer in AFM/STM is studied within an adiabatic approach, taking into account the slow motion of the tip with respect to an adsorption site. It is shown that coherent tunneling is the dominant mechanism of adatom transfer at temperatures below 10 K for insulators and semiconductors, and below 0.1 K for metals. The negative influence of electron and phonon polaron effects, which manifests itself in the destruction of coherence, may be noticeably compensated by potential fluctuations due to atom vibrations. As a result, the renormalized coherent tunneling amplitude may be even greater than the ground energy level splitting in the absence of a dissipative environment. Prospective pratical applications of the quantum coherence in adatom transfer are discussed.

Polymerized thin films based on polydiacetylenes (PDAs) undergo distinct color transitions that lend themselves to applications in biosensing, surface modification, nonlinear optics, and molecular electronics. The mechanism of the thermochromic blue to red color transition of PDA thin films was investigated at the molecular level using atomic force microscopy and at the macroscopic level with visible absorption and Fourier transform infrared spectroscopy. The thermochromic transition temperature is found to be between 70 and 90 °C. At the molecular level, the ordering of the film increases at the thermochromic transition and remains ordered up to temperatures well above the transition (e.g., 130 °C). No evidence for previously suggested entanglement or disordering of the alkyl side chains is observed. The pendant side chains rearrange from a partially disordered configuration characteristic of the blue film,to a well-ordered close-packed hexagonal arrangement in the red form. The rearrangment of the pendant side chains is linked to the formation of the red phase PDA.

We performed capacitance measurements between silver layers on the back side of thin (

Scanning-probe microscopies (SPMs) have experienced an impressive development in the last 10 years, such that obtaining atomically resolved images of surfaces is now routine. The scanning tunneling and atomic force microscopes (STM and AFM) are the most widely known representatives of the growing family of SPMs. With these two instruments, which were invented more than 15 years ago, the atomic structure of the surfaces of solid materials can be imaged in real space and, very importantly, in virtually any environment: vacuum, air, reactive gas atmospheres, and under liquid. These two circumstances (atomic resolution and lack of environmental constraints) make SPM unique among microscopies. One frontier remains unconquered however: the imaging of liquid surfaces. Even though liquids constitute a very large fraction of all materials, the study of their microscopic morphology has been limited to the diffraction limit of visible optical microscopy, a situation that has remained unchanged for decades. Techniques capable of breaking this resolution limit are welcomed. One such technique was recently developed in our laboratory, which we call scanning-polarization-force microscopy (SPFM). It did not result from the discovery of any new physical principle but from the simple application of well-known methods and techniques, in combination with the AFM, to the field of liquid surface structures. In this article, we will first summarize the working principle of SPFM. Then we will show how we have applied it to the investigation of liquid films and droplets of nanometer dimensions and their interactions with solid supports.

We report studies of the effect of humidity and the chemical nature of the molecular end groups on the structure and spatial distribution of perfluorinated lubricants on hard carbon substrates. The studies are performed with a noncontact atomic force microscopy (AFM) technique that is capable of providing nanometer scale images of soft and liquid surfaces. Our results indicate that reactive end groups enhance the wear resistivity of the lubricants. ZDOL and ZDIAC, which have CH2OH and COOH end groups, respectively, could not be displaced by a sharp tip (≈500 A radius) under a load of 1 × 10-8 N while, under the same conditions, Z03, which has CF3 groups at both ends, and Demnum SA, which has a CF3 group and a CH2OH group at each end, could be easily displaced. We found that humidity enhances the mobility of the lubricants. Humidity also causes dramatic changes in the state of aggregation of ZDIAC and Demnum SA. The binding of these lubricants to the surface becomes unstable, and upon contact with the tip,...

The current status of photoelectron-diffraction studies of surface structures is briefly reviewed, and several recent developments and proposals for future areas of application are then discussed. The application of full-solid-angle diffraction data, together with simultaneous characterization by low-energy electron-diffraction and scanning-tunneling microscopy, to epitaxial growth is considered. Several new avenues that are being opened up by third-generation synchrotron-radiation sources are also considered. These include photoelectron diffraction from surface and interface atoms, the possibility of time-resolved measurements, and circular dichroism in photoelectron angular distributions. The addition of spin to the photoelectron-diffraction measurement is also considered, and can be achieved either through core-level multiplet splittings or by circular-polarized excitation of spin–orbit-split levels. This last development should make it possible to study short-range magnetic order, perhaps even in a holographic fashion.

Atomic force microscopy (AFM) has been used to study the effect of pressure on the structural and frictional properties of self-assembled monolayers of n-octadecanethiol on Au(111). Sharp microfabricated silicon nitride tips (tip radii 100−300 A) were used. At low load, the periodicity of the thiol layer is imaged. At higher load, the layer is observed to become disordered. At a critical contact pressure of ∼2.3 GPa, a transition from the thiol overlayer to the Au(111) substrate periodicity is observed in the lattice resolution images. This transition is gradual and reversible. During the transition, frictional forces first increase and then decrease as the tip−sample separation decreases by a distance approximately equivalent to the thickness of the thiol layer.

We have constructed an ultrahigh vacuum scanning tunneling microscope (STM) for operation in the temperature range 20–300 K. The design consists of a vibration isolated sample holder mounted on a continuous flow cryostat. By rotation and linear motion of the cryostat, the sample can be positioned in front of various surface preparation and analysis instruments contained in a single vacuum chamber. A lightweight beetle-type STM head is lowered from the top onto the sample by a linear manipulator. To minimize helium convection in the cryostat, the entire vacuum system, including a liquid helium storage Dewar, can be tilted by a few degrees perpendicular to the cryostat axis, which improves the operation. The performance of the instrument is demonstrated by atomically resolved images of the Pd(111) surface and adsorbed CO molecules.

We have studied the first monolayer of epitaxial iron oxide on Pt(111) with x-ray photoelectron diffraction and other methods. We confirm a previously proposed superlattice model for this monolayer, but also conclude that the oxide grows as a bilayer of FeO(111) with the oxygen layer outermost, the Fe-O interlayer distance is highly compressed relative to bulk FeO by about 50%, and only one of two possible domains forms due to interlayer interactions between oxygen and platinum.

A comparative study of the frictional properties of alkanethiols and alkylsilanes as a function of chain length is presented. The monolayers were produced by self-assembly on Au(111) and mica, respectively. The same tip was used for all the experiments, and freshly cleaved mica was used as a reference. For both types of films, the frictional forces depend strongly on the number of carbon atoms in the alkane chain (CH3−(CH2)n-1−R). Thiols and silanes give rise to similar frictional force for the same n when n > 11, while for n < 11 the behavior is markedly different; the silanes exhibit higher friction, larger than that for the thiols by a factor of ∼3 for n = 6. The increase in friction is attributed to the increased disorder that occurs when going from a thiol to a silane anchor or when decreasing n. It is proposed that disorder favors the increase of the number and type of low-energy modes (kinks, bending, distortions) that are available for excitation and energy dissipation.

The structures of sulfur at different coverages on the Pt(111) surface have been examined using low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). The following ordered structures were identified as a function of coverage: (2 × 2) at 1/4 ML (monolayer), (√3 × √3)R30° at 1/3 ML and c(7 × √3)rect. at 3/7 ML. Dynamical LEED calculations were used to determine the adsorption geometry of the sulfur atom and examine the adsorbate-induced surface relaxations of the (2 × 2) and (√3 × √3)R30° structures. The best agreement with calculated LEED I–V curves was obtained for sulfur adsorption in the fcc hollow sites for both overlayers. For the (2 × 2) structure, the first- and the second-layer Pt atoms are relaxed in the perpendicular and perhaps in the lateral directions. For the (√3 × √3)R30° structure, perpendicular relaxations are not allowed by symmetry, while small lateral relaxations are possible. The S Pt bond length is 2.24Afor the (2 × 2) and 2.25Afor the (√3 × √3)R30° structure. STM studies of the high-coverage c(7 × √3)rect.-3S structure indicate that there are three sulfur atoms per unit cell, with two sulfur atoms in fcc hollow sites and one in an hcp hollow site.

We consider studies of the atomic and magnetic structure near surfaces by photoelectron diffraction and by the holographic inversion of both photoelectron diffraction data and diffraction data involving the emission of fluorescent x-rays. The current status of photoelectron diffraction studies of surfaces, interfaces, and other nanostructures is first briefly reviewed, and then several recent developments and proposals for future areas of application are discussed. The application of full-solid-angle diffraction data, together with simultaneous characterization by low energy electron diffraction and scanning tunneling microscopy, to the epitaxial growth of oxides and metals is considered. Several new avenues that are being opened up by third-generation synchrotron radiation sources are also discussed. These include site-resolved photoelectron diffraction from surface and interface atoms, the possibility of time-resolved measurements of surface reactions with chemical-state resolution, and circular dichroism in photoelectron angular distributions from both non-magnetic and magnetic systems. The addition of spin to the photoelectron diffraction measurement is also considered as a method for studying short-range magnetic order, including the measurement of surface magnetic phase transitions. This spin sensitivity can be achieved through either core-level multiplet splittings or circular-polarized excitation of spin-orbit-split levels. The direct imaging of short-range atomic structure by both photoelectron holography and two distinct types of x-ray holography involving fluorescent emission is also discussed. Both photoelectron and x-ray holography have demonstrated the ability to directly determine at least approximate atomic structures in three dimensions. Photoelectron holography with spin resolution may make it possible also to study short-range magnetic order in a holographic fashion. Although much more recent in its first experimental demonstrations, x-ray fluorescence holography should permit deriving more accurate atomic images for a variety of materials, including both surface and bulk regions.

Using an atomic force microscope operated in contact and noncontact mode, we have studied the structures formed on the (100) cleavage surface of NaCl when exposed to water vapor. Above ∼35% relative humidity (RH), a uniform layer of water is formed and the surface steps are observed to evolve slowly. At ∼73% RH, the step structure becomes unstable and disappears abruptly because of dissolution (deliquescence) of the salt surface. Reversing the process by drying leads to the reappearance of new, more uniform monatomic steps. At humidity levels less than 30%, water adsorbs primarily at the step edges.