Tip–surface interactions in noncontact atomic force microscopy on reactive surfaces

The imaging process in noncontact atomic force microscopy (AFM) is studied on a number of reactive surfaces, namely, the Takayanagi reconstructed Si(111), InP(110), and GaAs(110). We show that on these surfaces, the short-range dangling-bond type of interaction between the tip and the surface is decisive in achieving atomic resolution. The short-range tip–surface interaction is modeled in the density functional theory within the GGA approximation. We show that we can achieve quantitative agreement with the experimental data in the commonly used frequency modulation technique for AFM surface corrugation with a very simple model for the tip geometry treating the tip–surface interaction in the perturbation theory. The nature of the short-range tip–surface interaction on the three surfaces is considered and the consequences thereof for the experiments is discussed.     

Surface holography with LEED electrons

The present status of surface holography using low energy electron diffraction intensities is described. It is shown that diffuse intensity distributions appearing with disordered adsorption on a crystalline substrate can be interpreted in a holographic sense by the single adatom acting as a beam splitter for the primary electron beam. We demonstrate that intensities taken at many energies need to enter the reconstruction integral in order to retrieve well resolved atomic images. We also show that the method can be extended to use also discrete superstructure spot intensities instead of diffuse maps, so opening the field to ordered superstructures. The power and applicability of the method is discussed.     

Surface cosegregationp phenomena on alloys and steels: Structural features and phase transitions

In recent years surface cosegregation phenomena have been studied on various alloy and steel surfaces using surface sensitive techniques such as Auger electron spectroscopy (AES), x-ray photoelectron spectroscopy (XPS), x-ray photoelectron diffraction (XPD) and low energy electron diffraction (LEED). Surface cosegregation causes the formation of two-dimensional surface compounds which may be stabilized by epitaxy on substrate surfaces of suitable structure and orientation. It has been found that in many cases surface compounds undergo phase transitions which are reviewed in this short report.     

Angle-resolved x-ray photoelectron spectroscopy

In this review, various aspects of angle-resolved x-ray photoelectron spectroscopy (ARXPS) as applied to solid state- and surface chemical- studies are discussed. Special requirements for $\text{instrumentation}$ are first considered. The use of $\text{grazing-emission}$ angles to enhance surface sensitivity and study surface concentration profiles of various types is then discussed. Various effects that may limit the accuracy of such measurements such as surface roughness, electron refraction, and elastic scattering are considered. Several examples of surface-specific electronic structure changes as studied by grazing-emission ARXPS (e.g., valence-band narrowing and core-level shifts) are also reviewed. The use of $\text{grazing-incidence}$ geometries for surface enhancement is also briefly considered. Single-crystal studies providing additional types of information via ARXPS are next discussed. For core-level emission from single-crystal substrates or adsorbed overlayers, $\text{x-ray photoelectron diffraction (XPD)}$ is found to produce considerable fine structure in polar- or azimuthal- scans of intensity. Such XPD effects can be very directly related to the atomic geometry near a surface, for example, through simple intramolecular or intermolecular scattering processes. A straightforward single scattering or kinematical theory also appears to describe such effects rather well, thus far permitting several structures to be solved by analyses of azimuthal intensity scans. Likely future developments and possible limitations of such XPD structure studies are also discussed. Finally, $\text{valence-band ARXPS}$ is considered, and it is shown that pronounced direct-transition effects can be observed provided that the specimen Debye-Waller factor is not too small. A simple free-electron final-state model is found to predict these direct-transition effects very well, and future studies at low temperatures and with higher angular resolution seem promising.     

Multidimensional spatial-spectral holographic interpretation of NMR photography

A spectral holographic interpretation arises naturally in nuclear magnetic resonance (NMR) photography from either the intrinsic chemical shift anisotropy of the spin system or the field inhomogeneity due to the applied spatial encoding gradients. We can thus think of NMR photography as arising from a "diffraction" off a spatial-spectral holographic grating. The spatial holographic component arises from a high dielectric constant (>50) of the NMR medium at high field strength (>4 T) when the excitation wavelength is commensurate with the size of the NMR sample; otherwise, it is a volume spectral holographic grating. In this paper, the NMR localized spectroscopy (imaging) equation is derived from the principles of spatial-spectral holography. Holographic properties of storage and programmable time delay and time reversal are shown to follow naturally from this viewpoint and are experimentally demonstrated in an inhomogeneously broadened NMR sample. These ideas are shown to be extendable to complex signal processing functions such as recognition, correlations, and triple products.     

Excitation of local magnetic moments by tunneling electrons

The advent of milli-kelvin scanning tunneling microscopes (STM) with inbuilt magnetic fields has opened access to the study of magnetic phenomena with atomic resolution at surfaces. In the case of single atoms adsorbed on a surface, the existence of different magnetic energy levels localized on the adsorbate is due to the breaking of the rotational invariance of the adsorbate spin by the interaction with its environment, leading to energy terms in the meV range. These structures were revealed by STM experiments in IBM Almaden in the early 2000s for atomic adsorbates on CuN surfaces. The experiments consisted in the study of the changes in conductance caused by inelastic tunneling of electrons (IETS, inelastic electron tunneling spectroscopy). Manganese and Iron adatoms were shown to have different magnetic anisotropies induced by the substrate. More experiments by other groups followed up, showing that magnetic excitations could be detected in a variety of systems: e.g. complex organic molecules showed that their magnetic anisotropy was dependent on the molecular environment, piles of magnetic molecules showed that they interact via intermolecular exchange interaction, spin waves were excited on ferromagnetic surfaces and in Mn chains, and magnetic impurities have been analyzed on semiconductors. These experiments brought up some intriguing questions: the efficiency of magnetic excitations was very high, the excitations could or could not involve spin flip of the exciting electron and singular-like behavior was sometimes found at the excitation thresholds. These facts called for extended theoretical analysis; perturbation theories, sudden-approximation approaches and a strong coupling scheme successfully explained most of the magnetic inelastic processes. In addition, many-body approaches were also used to decipher the interplay between inelastic processes and the Kondo effect. Spin torque transfer has been shown to be effective in changing spin orientations of an adsorbate in theoretical works, and soon after it was shown experimentally. More recently, the previously mentioned strong coupling approach was extended to treat the excitation of spin waves in atomic chains and the ubiquitous role of electron–hole pair creation in de-exciting spins on surfaces has been analyzed. This review article expounds these works, presenting the theoretical approach by the authors while trying to thoroughly review parallel theoretical and experimental works.     

Investigation of antirelaxation coatings for alkali-metal vapor cells using surface science techniques

Many technologies based on cells containing alkali-metal atomic vapor benefit from the use of antirelaxation surface coatings in order to preserve atomic spin polarization. In particular, paraffin has been used for this purpose for several decades and has been demonstrated to allow an atom to experience up to 10?000 collisions with the walls of its container without depolarizing, but the details of its operation remain poorly understood. We apply modern surface and bulk techniques to the study of paraffin coatings in order to characterize the properties that enable the effective preservation of alkali spin polarization. These methods include Fourier transform infrared spectroscopy, differential scanning calorimetry, atomic force microscopy, near-edge x-ray absorption fine structure spectroscopy, and x-ray photoelectron spectroscopy. We also compare the light-induced atomic desorption yields of several different paraffin materials. Experimental results include the determination that crystallinity of the coating material is unnecessary, and the detection of C[Double Bond]C double bonds present within a particular class of effective paraffin coatings. Further study should lead to the development of more robust paraffin antirelaxation coatings, as well as the design and synthesis of new classes of coating materials.     

Energetics driving the short-range order in CuxPd1-x/Ru(0001) monolayer surface alloys

The energetics determining the distinct short-range order in two-dimensional (2D) monolayer Cu(x)Pd(1-x) surface alloys on a Ru(0001) substrate were investigated by Monte Carlo simulations and density functional theory calculations. Using a 2D lattice gas Hamiltonian based on effective pair interaction (EPI) parameters, the EPIs were derived for different Cu concentrations with Monte Carlo (MC) simulations by comparing with the atomic distributions obtained from atomic resolution STM images and the related Warren-Cowley short-range order parameters (Hoster et al., Phys. Rev. B, 2006, 73 165413). The ground state structures and mixing energies at 0 K derived from these EPIs agree well with mixing energies determined from DFT calculations of different ordered surface alloys. Additional MC simulations yield rather low transition temperatures which explain the absence of ordered 2D phases in the experiments. The consequences of our findings for the use of alloy surfaces and surface alloys as model systems for adsorption and catalytic reaction studies are discussed.     

"Click-functional" block copolymers provide precise surface functionality via spin coating

There are few existing methods for the quantitative functionalization of surfaces, especially for polymeric substrates. We demonstrate that alkyne end-functional diblock copolymers can be used to provide precise areal densities of reactive functionality on both hard (e.g., glass and silicon oxide) and soft (i.e., polymeric) substrates. Alkyne functionality is extremely versatile because the resultant functional surfaces are reactive toward azide functional molecules by Sharpless click chemistry. Spin-coated films of alpha-alkyne-omega-Br-poly( tert-butylacrylate- b-methylmethacrylate) (poly( tBA-MMA)) spontaneously self-assemble on the aforementioned substrates to present a surface monolayer of PtBA with a thickness in the range of 1 to 9 nm. The PMMA block physisorbs to provide multivalent anchoring onto hard substrates and is fixed onto polymer surfaces by interpenetration with the substrate polymer. The areal density of alkyne functional groups is precisely controlled by adjusting the thickness of the block copolymer monolayer, which is accomplished by changing either the spin coating conditions (i.e., rotational speed and solution concentration) or the copolymer molecular weight. The reactivity of surface-bound alkynes, in 1,3-dipolar cycloaddition reactions or by so-called "click chemistry", is demonstrated by covalent surface immobilization of fluorescently labeled azides. The modificed surfaces are characterized by atomic force microscopy (AFM), contact angle, ellipsometry, fluorescent imaging and angle-dependent X-ray photoelectron spectroscopy (ADXPS) measurements. Microarrays of covalently bound fluorescent molecules are created to demonstrate the approach and their performance is evaluated by determining their fluorescence signal-to-noise ratios.     

Modification of catheter materials by coating with polymer layers containing silver

The surface of polyurethane based catheter material or of silicon wafers as model surfaces were modified by spin coating of solutions of poly(ethylene oxide) or poly(vinyl alcohol) in water. For the incorporation of silver ions, silver nitrate was added to some of the solutions or the as-cast surfaces were dipped into AgNO₃ solution. Furthermore, samples coated with a thin layer of metallic silver were prepared by deposition of silver vapor in vacuum. The as-prepared surfaces were studied by atomic force microscopy and X-ray photoelectron spectroscopy. During the spin coating of the solutions containing AgNO₃, clusters of the silver component were formed. They were well dispersed in a poly(vinyl alcohol) matrix but act as nucleation agents in poly(ethylene oxide) where then large spherulites are formed. The surface compositions of coated samples and the depth profiling were carried out by angle dependent X-ray photoelectron spectroscopy.     

Electron holography with a Cs-corrected transmission electron microscope.

Cs correctors have revolutionized transmission electron microscopy (TEM) in that they substantially improve point resolution and information limit. The object information is found sharply localized within 0.1 nm, and the intensity image can therefore be interpreted reliably on an atomic scale. However, for a conventional intensity image, the object exit wave can still not be detected completely in that the phase, and hence indispensable object information is missing. Therefore, for example, atomic electric-field distributions or magnetic domain structures cannot be accessed. Off-axis electron holography offers unique possibilities to recover completely the aberration-corrected object wave with uncorrected microscopes and hence we would not need a Cs-corrected microscope for improved lateral resolution. However, the performance of holography is affected by aberrations of the recording TEM in that the signal/noise properties ("phase detection limit") of the reconstructed wave are degraded. Therefore, we have realized off-axis electron holography with a Cs-corrected TEM. The phase detection limit improves by a factor of four. A further advantage is the possibility of fine-tuning the residual aberrations by a posteriori correction. Therefore, a combination of both methods, that is, Cs correction and off-axis electron holography, opens new perspectives for complete TEM analysis on an atomic scale.     

原子分辨显微分析技术研究进展

非接触原子力显微技术(NC-AFM)近年来发展迅速. NC-AFM对单个分子的成像和谱学实现了原子分辨和单个化学键分辨. NC-AFM自身功能的拓展及其与不同探针技术的联用将为材料、物理、化学和生命科学有关的研究提供崭新的思路. 本文首先介绍NC-AFM和qPlus 传感器的基本原理, 然后讨论原子尺度的相互作用力和短程力的精确测量, 总结近年来NC-AFM在原子尺度的化学结构成像、化学识别、电子结构性质分析以及原子操纵技术中的研究进展, 并讨论了开尔文探针力显微技术(KPFM)在局域接触势差(LCPD)测量方面的应用. 最后展望了NC-AFM面临的挑战和发展机遇.     

Recent advances in surface x-ray diffraction and the potential for determining structure-sensitivity relations in single-crystal electrocatalysis

Determining how electrode structure governs the performance of an electrocatalyst requires techniques capable of probing structure at the atomic scale, often in situ and operando. In recent years, there have been numerous advances in the main experimental techniques for determining the structure of the electrochemical interface. In situ/operando synchrotron surface x-ray diffraction measurements are key to investigate the atomic structure of the electrode surfaces as well as understand the structure-reactivity relations in electrocatalysis. Here we discuss some recent improvements that have taken place in surface x-ray diffraction and how we expect them to lead to an enhanced understanding of electrocatalysis.     

Capture of polarized electrons into excited terms of fast atoms during grazing surface collisions

Fast ions are scattered from magnetized surfaces under grazing angles of incidence. During the interaction with the surface charge exchange is effective and results in a population of stable and excited atomic terms. This capture of electrons is characterized by anisotropic distributions of atomic orbital angular momenta and in addition — for magnetized targets — by anisotropic distributions of electronic spins. We will discuss in some detail, how these anisotropic distributions can be studied via the analysis of the state of polarization of the fluorescent light, emitted in electronic transitions from excited terms of free atoms after the impact with the surface. We show that a defined variation of the magnetization of the target affects the polarization of the emitted light in a characteristic way, which allows to deduce the electronic spin polarization of the atoms. The method implies some perspectives with respect to the study of magnetic properties of the vacuum-solid interface well above the topmost layer of surface atoms.     

Adsorption of carbon monoxide and oxygen on transition metal surfaces: A comparative study of the results from electron spectroscopy and theoretical calculations

Results of investigations on the adsorption of CO andO ₂ on transition metal surfaces by employinguv and x-ray photoelectron spectroscopy and electron energy loss spectroscopy (eels) are presented. Results of molecular orbital calculations on adsorbed CO and O₂ are also discussed. Some of the interesting aspects discussed are, satellites in the O(ls) region due to adsorbed CO, vibrationaleels of adsorbed O₂ and dissociation energy profiles of adsorbed O₂ on clean surfaces as well as surfaces covered with potassium or presorbed atomic oxygen. Contribution No 245 from the Solid State and Structural Chemistry Unit.     

Atomic hydrogen interaction with Ru(1010)

The interaction of atomic hydrogen with clean and deuterium precovered Ru(1010) was studied by means of temperature-programmed desorption (TPD) spectroscopy. Compared to molecular hydrogen experiments, after exposure of the clean surface to gas-phase atomic hydrogen at 90 K, two additional peaks grow in the desorption spectra at 115 and 150 K. The surface saturation coverage, determined by equilibrium between abstraction and adsorption reactions, is 2.5 monolayers. Preadsorbed deuterium abstraction experiments with gas-phase atomic hydrogen show that a pure Eley-Rideal mechanism is not involved in the process, while a hot atom (HA) kinetics describes well the reaction. By least-squares fitting of the experimental data, a simplified HA kinetic model yields an abstraction cross section value of 0.5 +/- 0.2 angstroms2. The atomic hydrogen interaction with an oxygen precovered surface was also studied by means of both TPD and x-ray photoelectron spectroscopy: oxygen hydrogenation and water production take place already at very low temperature (90 K).     

Effect of applied curent on copper sulphide-based ion-selective electrodes

The properties of precipitate-based copper sulphide electrodes are investigated. Solid-phase studies wre done by x-ray photoelectron spectroscopy and solution-phase studies by combined potentiometric and atomic absorptioni spectrometric techniques. The predominant valence state of copper in the copper sulphide samples is shown to be Cu(I), but Cu(II) can also be identified at the surfaces. The oxidation of the membrane surface results in dissolution of copper ion and a decrease in the Cu/S ratio in the solid phase; reduction of the surface causes sulphide dissolution and an increase in the Cu/S ratio. Application of anodic or cathodic curents was used to study the redox behaviour of the copper suphide memebrane.     

The Rh oxide ultrathin film on Rh(100): an x-ray photoelectron diffraction study

The surface and interface structure of the RhO(2) ultrathin film grown on Rh(100) is investigated by means of x-ray photoelectron diffraction. Experimental and simulated one- and two-dimensional angular distribution intensities of the O1s and Rh3d(5/2) chemically shifted core levels are quantitatively analyzed. The previously proposed O-Rh-O trilayer model is independently confirmed. A rippled buckling of the metal surface is observed at the oxide-metal interface, with a mean interfacial Rh-O distance which is 0.2 A? larger with respect to previous findings. The link between the local atomic rearrangement and the overall geometric and electronic properties of the oxide is discussed on the basis of a thorough comparison with the corresponding RhO(2) rutile structure.     

Activation of PET Using an RF Atmospheric Plasma System

The plasma treatment of polymer surfaces is routinely used to enhance surface properties prior to adhesive bonding or biomolecule interaction. This study investigates the influence of plasma treatment conditions on the surface activation of polyethylene terephthalate (PET) using the SurFx Atomflo? 400L plasma source. In this study the effect of applied plasma power, processing speed, gas composition and plasma applicator nozzle to substrate distance were examined. The level of polymer surface activation was evaluated based on changes to the water contact angle (WCA) of PET samples after plasma treatment. PET surface properties were also monitored using surface energy and X-ray photoelectron spectroscopy (XPS) analysis. The heating effect of the plasma was monitored using thermal imaging and optical emission spectroscopy (OES) techniques. OES was also used as a diagnostic tool to monitor the change in atomic and molecular species intensity with changes in experimental conditions in both time and space. XPS analysis of the PET samples treated at different plasma powers indicated that increased oxygen content on samples surfaces accounted for the decreases observed in WCAs. For the first time a direct correlation was obtained between polymer WCA changes and the OES measurement of the atomic hydrogen Balmer H_α and molecular OH line emission intensities.