Stable cation inversion at the MgAl2O4(100) surface

From an interplay of atom-resolved noncontact atomic force microscopy, surface x-ray diffraction experiments, and density functional theory calculations, we reveal the detailed atomic-scale structure of the (100) surface of an insulating ternary metal oxide, MgAl2O4 (spinel). We surprisingly find that the MgAl2O4(100) surface is terminated by an Al and O-rich structure with a thermodynamically favored amount of Al atoms interchanged with Mg. This finding implies that so-called Mg-Al antisites, which are defects in the bulk of MgAl2O4, become a thermodynamically stable and integral part of the surface.     

Mechanisms and energetic of atomic processes in surface diffusion

Surface diffusion is a subject of basic importance for understanding mass transport phenomena in surface and nano science. In the particle aspect of surface diffusion of single atoms and simple molecules, information of interest is the detail atomic mechanisms and the activation energy of various atomic processes, and also the binding energy of atoms at different surface sites. In the absence of an external force, atoms will perform random walk without a preferred direction. When an atom is subjected to an external force, or when a chemical potential gradient exists, it will move preferentially in the direction of the force, or in the direction of decreasing chemical potential, thus the random walk becomes directional. Using atomic resolution microscopy, it is now possible to observe random walk diffusion of atoms, molecules and atomic clusters directly as well as to study the dynamic behavior of atoms as perturbed by the electronic interactions of the surface in great detail. Here, methods of studying quantitatively the particle aspect of surface diffusion and how it affects the dynamic behavior of the surface are very briefly reviewed.     

Investigation of local electronic transport and surface potential distribution of Cu(In,Ga)Se2 thin-films

Local current mapping and surface potential distributions on polycrystalline Cu(In,Ga)Se₂ (CIGS) films are investigated by conductive atomic force microscopy and Kelvin probe force microscopy. The two kinds of samples fabricated by co-evaporation had extremely different conversion efficiencies of 10% and 0.2% for stoichiometric and Cu- and Se-deficient compositions, respectively. We examined the microscopic reasons for the differences in the local electrical properties. Current mapping and current–voltage behaviors were measured at intragrain regions (IGs) and grain boundaries (GBs). Electronic transport between a Pt scanning probe and the CIGS layer is explained by the Schottky conduction mechanism. The surface potential distribution shows an intriguing relation with topological variation, inferring that a local built-in potential is possibly formed on positively charged GBs. The surface potential is about 100 mV, which shows energy band bending near GBs in the films. Exciton separation near GBs is explained by the bending of the conduction and valence bands, which is sensitive to compositional and structural inhomogeneities.     

True atomic resolution imaging of surface structure and surface charge on the GaAs(110)

We demonstrated a novel method to detect the van der Waals and the electrostatic force interactions simultaneously on an atomic scale, which is based on frequency modulation detection method. For the first time, the surface structure and the surface charge at atomic-scale point defects on the GaAs(110) surface have been simultaneously resolved with true atomic resolution under ultra-high vacuum condition. From the bias voltage dependence of the image contrast, we can verify that the sign of the atomically resolved surface charge at the point defect was positive.     

Local surface cleaning and cluster assembly using contact mode atomic force microscopy

Conventional contact mode atomic force microscopy (AFM) has been used for local surface cleaning and cluster alignment. By using the AFM tip to sweep and push in contact mode, we have demonstrated that Cu clusters, prepared by vacuum evaporation onto Dow Cyclotene 3022 polymer and subsequent exposure to atmosphere, can easily be moved by the AFM tip, and assembled at the outer edge of the scanned region to form a line of clusters. We have found that the force applied by the tip plays an important role in the ease of cluster motion. Cyclotene surface treatment that enhances cluster adhesion hinders this ability, and may be used as a method of nanofabrication.     

The surface morphology of pyrolytic graphite irradiated by hydrogen atoms

Interaction between atomic hydrogen and pyrolytic graphite is investigated by thermal desorption spectroscopy, atomic force microscopy, and scanning tunneling microscopy. After exposure in an atomic hydrogen flow, the initially smooth graphite surface becomes rough, with a height difference of several nanometers. When heated, the samples release hydrogen and their surface is smoothed out, showing monolayer-deep etch pits. After multiple sorption-desorption cycles, both the linear sizes and the depth of the pits increase.     

General theory of microscopic dynamical response in surface probe microscopy: from imaging to dissipation

We present a general theory of atomistic dynamical response in surface probe microscopy when two solid surfaces move with respect to each other in close proximity, when atomic instabilities are likely to occur. These instabilities result in a bistable potential energy surface, leading to temperature dependent atomic scale topography and damping (dissipation) images. The theory is illustrated on noncontact atomic force microscopy and enables us to calculate, on the same footing, both the frequency shift and the excitation signal amplitude for tip oscillations. We show, using atomistic simulations, how dissipation occurs through reversible jumps of a surface atom between the minima when a tip is close to the surface, resulting in dissipated energies of 1.6 eV. We also demonstrate that atomic instabilities lead to jumps in the frequency shift that are smoothed out with increasing temperature.     

Statistical characterization of surface morphologies

We implement and assess several statistical methods for rough surface morphology characterization, which can largely be divided into two classes. The first class is based on estimation of two-point, quadratic statistical functions and includes: estimates of the sample autocovariance function, sample height–height correlation (also, structure) function, and periodogram estimate of the surface power spectrum. The second class incorporates estimation of up to the fourth statistical moments of the local curvature on a fixed scale.We apply these methods first on computer-simulated epitaxial surfaces, which permits the characterization using large sets of “data” and rich statistics. Then we deal with real surfaces, whose roughness profiles are measured using atomic force microscopy (AFM). In both cases we infer and discuss scaling properties, degree of anisotropy and deviation from Gaussian distribution of surface heights.     

Structural changes on the surface of tungsten foils under uniaxial tension

A change in the surface morphology of recrystallized tungsten foil under the effect of uniaxial tension in ultrahigh vacuum is studied by low-energy electron diffraction and atomic force microscopy. It is found by using low-energy electron diffraction that on the foil surface consisting of separate blocks with dominant face (112), there is a turn in orientation of the structural blocks. The analysis of the topograms of different areas of the side surface of a broken sample, obtained by atomic force microscopy, enabled the association of changes in the atomic structure of the surface layers of foil with a change in its relief by mechanical action.     

Atomic force microscopy characterization of the surface wettability of natural fibres

Natural fibres represent a readily available source of ecologically friendly and inexpensive reinforcement in composites with degradable thermoplastics, however chemical treatments of fibres are required to prepare feasible composites. It is desirable to characterize the surface wettability of fibres after chemical treatment as the polarity of cellulose-based fibres influences compatibility with a polymer matrix. Assessment of the surface wettability of natural fibres using conventional methods presents a challenge as the surfaces are morphologically and chemically heterogeneous, rough, and can be strongly wicking. In this work it is shown that under atmospheric conditions the adhesion force between an atomic force microscopy (AFM) tip and the fibre surface can estimate the water contact angle and surface wettability of the fibre. AFM adhesion force measurements are suitable for the more difficult surfaces of natural fibres and in addition allow for correlations between microstructural features and surface wettability characteristics.     

Shear modulation force microscopy study of near surface glass transition temperatures

We report results of glass transition (T(g)) measurements for polymer thin films using atomic force microscopy (AFM). The AFM mode, shear modulation force microscopy (SMFM), involves measuring the temperature-dependent shear force on a tip modulated parallel to the sample surface. Using this method we have measured the surface T(g) of thin (17-500 nm) polymer films and found that T(g) is independent of film thickness (t>17 nm), strength of substrate interactions, or even presence of substrate.     

Ultrasonic radiation in dynamic force microscopy

In dynamic force microscopy the cantilever of an atomic force microscope is vibrated at ultrasonic frequencies in the range of several 10 kHz up to several MHz while scanning a sample surface. The amplitude and phase of the cantilever vibration as well as the shift of the cantilever resonance frequencies provide information about local sample surface properties. In several operation modes of dynamic force microscopy, for example force modulation microscopy, tapping mode or atomic force acoustic microscopy, the sensor tip is in contact with the sample at least during a fraction of its vibration cycle. The periodic indentation of the tip with the sample surface generates ultrasonic waves. In this paper, the ultrasonic radiation of a vibrating cantilever into a sample and its contribution to the damping of the cantilever vibration are calculated. The theoretical results are compared to experiments.     

Effect of electric fields on the surface of nanostructured electrodes on charge formation

The processing of the atomic force microscopy images of specific nanostructured electrodes made of different metals shows that the shape of a nanostructure’s peak can be well approximated using a parabolic dependence. The local electric fields and injection currents are calculated using the data on the nanostructured surface of electrodes with the aid of the Fowler-Nordheim dependence. A qualitative analysis is performed and the applicability of the proposed approach in the construction of the electroconvection mass- and heat-exchange devices is demonstrated.     

Thermostimulated surface autosegregation in bismuth titanate single crystals

High resolution scanning electron microscopy (HRSEM), atomic force microscopy (AFM), and electron microprobe X-ray analysis (EPMA) were used to study the morphology and local phase composition of (001) faces in single crystals of bismuth titanate Bi₄Ti₃O₁₂ formed as a result of thermostimulated surface segregation (TSAS). One possible mechanism for this phenomenon, generated by the selective internal mass transfer of the matrix’s own atoms to the surface in competition with the processes of selective component evaporation, is discussed.     

Nanoscale imaging of surface piezoresponse on GaN epitaxial layers

Surfaces of GaN films were investigated by atomic force microscopy (AFM) with implemented piezoelectric force microscopy technique. A model of PFM based on the surface depletion region in GaN films is discussed. The local piezoelectric effect of the low frequency regime was found to be in phase with the applied voltage on large domains, corresponding to a Ga-face of the GaN layer. Low piezoresponse is obtained within the inter-domain regions. The use of frequencies near a resonance frequency enhances very much the resolution of piezo-imaging, but only for very low scanning speed the piezo-imaging can follow the local piezoelectric effect. An inversion of the PFM image contrast is obtained for frequencies higher than the resonance frequencies. The effect of a chemical surface treatment on the topography and the piezoresponse of the GaN films was also investigated. Textured surfaces with very small domains were observed after the chemical treatment. For this kind of surfaces, piezo-induced torsion rather than bending of the AFM cantilever dominates the contrast of the PFM images. A small memory effect was observed, and explained by surface charging and confinement of the piezoelectric effect within the carrier depletion region at the GaN surface.     

A search for high frequency vibrational modes at a stepped surface

In this paper we present a theory of the vibrations of atoms in the vicinity of a stepped surface on a Bravais crystal. The static relaxations in the positions of the atoms in the crystal are determined, and the atomic force constants are then calculated in the relaxed atomic configuration. The general theory is applied to a simple stepped surface, and the local phonon density of states is carried out for atoms at several points on the stepped surface by the real space continued fraction recursion method. No evidence is found for high frequency surface phonons.     

Photochemical surface modification of PET by excimer UV lamp irradiation

UV irradiation has interesting potential for the photochemical modification of polymers. In order to study cross-linking effects and/or thin-layer deposition following a treatment in the presence of bi-functional media or in inert atmosphere, irradiation of PET in various atmospheres was performed using a KrCl excimer lamp. Surface properties were investigated by atomic force microscopy, nanoindentation, micro-thermal analysis, and X-ray photo-electron spectroscopy. The studies reveal that surface chemical composition, morphology, adhesion, thermomechanics, and stiffness/modulus are strongly affected by UV irradiation in the presence of bi-functional media. Films treated in octadiene and argon show an increase of surface modulus, much less expansion, and lower soft/melt temperatures, which is an indication of the surface cross-linking effect and a decrease of crystallinity within the near-surface layer. In the case of a diallylphthalate-treated film, depending on the local structure, either a strong decrease of melting temperature or no melting point is found, which is attributed to the irregular cross linking and thickness of the modified layer associated with a decrease of surface modulus. A significant increase of the alkali resistance is found after irradiation, as a result of both wetting and cross-linking effects on the polymer surface.     

Self-organization of structures on a bismuth single-crystal surface in an atmosphere of atomic hydrogen

The morphology of a cleavage surface of single-crystal bismuth has been studied by atomic force microscopy after treatment in atomic hydrogen. It has been ascertained that the surface relief changes due to the formation of micro- and nanocrystalline structures.     

Dependence of the atomic structure and surface relief of platinum foils on the annealing and rolling conditions

The effect of cold rolling, polishing, and thermal annealing conditions on the atomic structure and surface geometry of platinum foils has been studied. The surface morphology has been analyzed using low-energy electron diffraction, atomic force microscopy, and scanning tunneling microscopy. The chemical composition of the surface has been evaluated by Auger electron spectroscopy. It has been demonstrated that a variation in the conditions used for the preparation of the samples makes it possible to produce surfaces with different degrees of perfection from atomically smooth to rippled, fractal, and diffraction-disordered surfaces.     

Single-atom contact mechanics: from atomic scale energy barrier to mechanical relaxation hysteresis

The potential energy landscape of surfaces governs the dynamics of adsorbed molecules, as well as atomic scale friction processes. We measure the potential energy landscape of a single-atom tip interacting with a vicinal nonconducting NaCl(100) surface in real space using noncontact atomic force microscopy. We find that the shape of the potential energy profile is of sinusoidal form with a barrier height of 48 meV. Furthermore, we observe a discontinuity in the force curves at specific lattice sites, indicating the onset of reversible yet hysteretic mechanical relaxations.