Real-space Surface Research Articles

Transition from reciprocal-space to real-space surface science—advent of the scanning tunneling microscope

The emergence of surface science as an identifiable field of research depended to a large measure on structural determinations, which were dominated in the early days by diffraction methods. The scanning tunneling microscope enabled a transition to real-space imaging, making surface science visual and thus much more accessible. The evolution of surface structural determination is roughly traced from its inception to the present, where both diffraction and scanning tunneling microscopy have become commodities: Must haves for the serious surface scientist.

Tunneling decay in a magnetic field

We provide a semiclassical theory of tunneling decay in a magnetic field and a three-dimensional potential of a general form. Because of broken time-reversal symmetry, the standard WKB technique has to be modified. The decay rate is found from the analysis of the set of the particle Hamiltonian trajectories in complex phase space and time. In a magnetic field, the tunneling particle comes out from the barrier with a finite velocity and behind the boundary of the classically allowed region. The exit location is obtained by matching the decaying and outgoing WKB waves at a caustic in complex configuration space. Different branches of the WKB wave function match on the switching surface in real space, where the slope of the wave function sharply changes. The theory is not limited to tunneling from potential wells which are parabolic near the minimum. For parabolic wells, we provide a bounce-type formulation in a magnetic field. The theory is applied to specific models which are relevant to tunneling from correlated two-dimensional electron systems in a magnetic field parallel to the electron layer.

An experimental method to investigate the structure and kinetics of patterned surfaces using laser light diffraction

We describe a novel experimental method using the diffraction of a He–Ne laser beam to study surfaces patterned with structures on mesoscopic to macroscopic length scales. The technique provides high spatial and temporal resolution; it is not limited to periodic, artificial structures, but is also well suited to study the development of self-organized surface relief. Measurements can be performed under in situ conditions in a diffraction mode or an imaging mode, providing (1) qualitative and quantitative information on the surface structures, (2) information on time-dependent surface changes with a resolution of 10 μs or better, (3) observation of incubation processes (including determination of incubation time) in first-order, displacive phase transformations, and (4) observation of the surface in real space, in particular, the pattern evolution as a function of temperature or other parameters. As an example we show results of the application of our method to a Ni0.63Al0.37 single crystal undergoing a martensitic transformation.

Vacancy-enhanced submonolayer nucleation of Si on Si( [formula omitted

Scanning tunneling microscopy (STM) and high-resolution low-energy electron diffraction techniques are used to determine the characteristics of the two-dimensional Si island distribution on defect-free and imperfect Si(1 1 1) surfaces in real space and reciprocal space. The surface power spectra of STM and modeling of diffraction profiles give consistent island separation. Unlike the submonolayer nucleation of silicon islands on a defect-free Si(1 1 1) surface, it is shown that the island size distribution of Si, nucleated on a Si(1 1 1) surface having a high density of vacancy defects, does not exhibit a preferred characteristic island size. The process is interpreted as defect-induced nucleation with the critical island size equal to zero, for which there is no free energy barrier for nucleation. The results are qualitatively consistent with recent computer simulations, and are compared with other recent experimental work.

Real-space surface crystallography: Experimental stereographic projections from ion scattering

Scattering and recoiling imaging spectrometry (SARIS) in the blocking configuration is used to obtain experimental two-dimensional stereographic projections of the Ni(110) and Pt(111) surfaces. The development of this technique as an element-specific real-space surface crystallography that is sensitive to interatomic spacings in the surface and subsurface layers of a crystal is described. This projection imaging method is based on the blocking of atomic trajectories scattered from subsurface layers by atoms in layers nearer to the surface. The resulting magnification of the blocking cones, as captured by a gated position-sensitive microchannel plate detector, is similar to 10(9). The images of the blocking patterns provide direct information on interatomic spacings and surface symmetry and structure. Classical ion trajectory simulations using the three-dimensional scattering and recoiling imaging code (SARIC) are used to simulate the stereographic projections and blocking patterns and to provide quantitative interpretations. The method is sensitive to interatomic spacings in the surface and subsurface layers. The physical properties of the blocking process are derived from analysis of the data and a simplified, approximate, two-atom model of the scattering/blocking process is developed. (C) 2000 American Institute of Physics. [S0021-9606(00)71216-5].

Fluid bilayer vesicles Statistical physics of soft two-dimensional surfaces

The combination of length-xales of several microns, time scales for dynamic fluctuations accessible to video frequency, and energy scales of a few kbT all contribute to make vesicles a unique model system to study the statistical physics of soft surfaces in real space.

Low-Energy Electron Diffraction and Scanning Tunneling Microscopy Study on the Reconstruction of the Vanadium (111) Surface

In a previous paper, we proposed a model of a reconstructed vanadium single-crystal (111) surface on the basis of our low-energy electron diffraction (LEED) observations of the clean surface formed in the (√ 3×√ 3) R30° structure [Surf. Sci. 288 (1993) 355.]. In this paper we report the observation of the reconstructed V(111) surface in real space by use of a ultrahigh vacuum (UHV)-scanning tunneling microscopy (STM) (JSTM-4500VT) with which it is possible to precisely control the temperature of a sample even at high temperatures up to 1500 K. A main cleaning procedure was to anneal the sample for 20 h at over 1300 K in UHV and to flash it at 1500 K many times immediately prior to the STM measurement. It was possible to interpret the observed STM images reasonably using our previously proposed model for the reconstructed surface. In the STM images, however, we observed unexpected step structures on the (111) surface that could not be found in the LEED measurement. We have proposed a model of this step structure on a V(111) face based on the above model of the reconstructed surface.

Some electrochemical consequences of potential-induced surface reconstruction on Au(100): Double-layer nonuniformity and electrode kinetics

Some consequences of potential-induced surface reconstruction upon the double-layer structure and electrocatalytic properties of Au(100) in aqueous perchloric acid have been explored by means of cyclic voltammetry and differential capacitance measurements, together with in-situ scanning tunneling microscopy (STM). Initially unreconstructed (i.e. (1 × 1)) Au(100) yields capacitance-potential ( C-E) profiles in dilute (1 mM) perchloric acid in the vicinity of the potential of zero charge E pzc which are consistent with the presence of an essentially uniform distribution of electronic charge across the surface. Upon electrochemically inducing hexagonal surface reconstruction by holding the potential below −0.2 V for periods of up to ca. 20 min, these C-E minima are substantially broadened. The latter observation implicates the presence of electrostatically distinct surface domains with “local” E pzc values that are significantly (0.1–0.2 V) lower as well as higher than that for unreconstructed Au(100), having dimensions at least comparable with the Debye length of the diffuse layer (ca. 9.5 mn) in 1 mM HClO 4. This finding is consistent with the real-space surface morphology as obtained by in-situ STM under these conditions, which shows the common presence of corrugated features, such as edges of reconstructed strands, in addition to domains featuring quasi-hexagonal atomic packing. The sharp potential-induced removal of the reconstruction inferred from the appearance of a voltammetric feature at 0.6 V/SCE in perchloric acid is confirmed by STM data acquired during potential sweeps, which also show that the 24% excess gold atoms released form arrays of metal clusters. The rates of proton electroreduction are significantly (1.3 to 1.7-fold) accelerated by Au(100) reconstruction. These effects appear to be due to the involvement of gold atomic sites featuring lower coordination numbers which are seen to be formed upon surface reconstruction.

Non-contact atomic-force microscopy for soft surfaces

The atomic force microscope (AFM) was invented in the mid-1980s, in response to strong interest in the high resolution, real-space surface imaging capabilities of the scanning tunneling microscope (STM). The AFM provides one real benefit that the STM cannot: it is able to image insulating surfaces. As a result, the AFM can operate on a wider variety of samples; it also can image samples in air, where many conductors oxidize rapidly, and in solution. Essentially no surface preparation is necessary. Historically, however, even the AFM has had limitations. Until recently, the contact forces exerted by the AFM tip on the sample surface meant that AFM was limited to surfaces of substantial rigidity. Noncontact AFM removes that barrier, opening up the possibility of AFM imaging of very soft surfaces, or of surfaces that cannot be contaminated by contact with the tip.An AFM uses a piezoelectric transducer to scan the sample beneath a sharp probe.

Nuclear tracks and analogue Fermi surfaces

Ranges of 1.65 GeV Xenon ions in muscovite mica vary strongly with incident angle. This anomalous effect is due to a new kind of ‘governed’ particle motion - ridging — which is caused by, and exploits the stark anisotropy of the basic crystal lattice. Substantial angular differences in ridging orbits (refractive pathways) at molecular layers are the cause of a corresponding angular dependence in the effective ‘non-linear’ electronic stopping power S e , and hence in the vector range R (θ,φ,E). In a 4π solid angle the surface described by range vectors R of common origin and energy - spherical for amorphous condensed matter - is characteristically topographic for a crystal, due to orbital ridging, bridging, channelling, quasichannelling, blocking, and other consequences of atomic order. It is shown that this classical mechanical surface in real space, not considered previously, is the direct analogue of the quantum mechanical Fermi surface in reciprocal space for conduction electrons. Transformation of the Fermi into this classical C-surface with increasingly relativistic electronic mass is described geometrically. Two-beam (Bloch wave) dispersive surfaces (Fermi) become a single quasi-classical surface in the many-beam (wave) limit. Kikuchi patterns become classical star patterns. The Maxwell-Boltzmann statistics overwhelm the Fermi-Dirac. The stable ‘fundamental’ particles - leptons, mesons, baryons etc. - exhibit equi-energy surfaces of one kind or the other, sometimes both, depending on mass, energy, spin, charge, and strength of interaction with crystalline condensed matter. For photons (massless Bosons) and Bose-Einstein statistics a duality in scattering is also evident - from the visible to hard γ-rays. For diffractive photon scattering in a crystal field a wave equation for the classic electric and magnetic field vectors now describes a corresponding structure in reciprocal space. We describe this as a Maxwell surface. The ‘old’ Fermi surface, and the new classical C- and Maxwell surfaces, round off our basic understanding of the scattering of radiation generally by crystals, and symmetry is restored to the physics. There are also ramifications for solid state nuclear tracks.

High precision temperature- and energy-dependent refractive index of GaAs determined from excitation of optical waveguide eigenmodes

A GaAs-Al0.22Ga0.78As heterostructure was prepared and used as a multimode optical waveguide. Propagation constants for individual modes were measured by exciting one mode at a time via real-space surface grating couplers, and the resulting eigenmode distributions were used to obtain the refractive index of GaAs at a matrix of temperatures and photon energies spanning 40 K<T<300 K and 1.40 eV<hν<1.50 eV. Values for dn/dT and the extrapolated refractive index at T=0 K were also obtained. The dominant error source in these measurements was uncertainty in the angular placement of the sample. These measurements agree well with the few pre-existing temperature-dependent measurements, but are an order of magnitude more precise.

Surface step and defect structure of Cr(001) studied by scanning tunneling microscopy

The real-space surface structure of a Cr(001) single crystal has been examined by scanning tunneling microscopy (STM). Terraces separated predominantly by monatomic steps have been found consistent with a recent microscopic model of this surface by Blügel et al. [Phys. Rev. B 39 (1989) 1392] which has been developed on the basis of self-consistent total-energy calculations. We have also obtained clear images of screw dislocations on the Cr(001) surface by using STM.

Clean and metal‐contaminated Si(110) surfaces studied by RHEED, XPS and STM

SUMMARYWe have studied the atomic structure of clean and metal‐contaminated Si(110) surfaces in real space with Scanning Tunnelling Microscopy (STM) and compared the results with Reflection High Energy Electron Diffraction (RHEED) data taken on samples from the same wafer.On clean Si(110) surfaces flatreconstructed regions coexist with (15 17 1), (15 173) and related facets. Thesurface consists of periodically‐alternating high and low terraces, with a height difference corresponding to the interlayer spacing (0·19 nm). This structure is found in two domains, with terrace edges aligned along the and directions respectively. Atomic scale zig‐zag chains can be recognized on the terraces.Contamination of the surface with Ni or Cu results in the formation of reconstructions with drastically different unit cells, all orientated along [001] and . The transition proceeds via a disordered 5×1‐like intermediate phase for Ni coverages around 0·007 ML.