Scanning tunneling microscopy

Gold nanoparticles protected by a binary mixture of thiolate molecules have a ligand shell that can spontaneously separate into nanoscale domains. Complex morphologies arise in such ligand shells, including striped, patchy, and Janus domains. Characterization of these morphologies remains a challenge. Scanning tunneling microscopy (STM) imaging has been one of the key approaches to determine these structures, yet the imaging of nanoparticles' surfaces faces difficulty stemming from steep surface curvature, complex molecular structures, and the possibility of imaging artifacts in the same size range. Images obtained to date have lacked molecular resolution, and only domains have been resolved. There is a clear need for images that resolve the molecular arrangement that leads to domain formation on the ligand shell of these particles. Herein we report an advance in the STM imaging of gold nanoparticles, revealing some of the molecules that constitute the domains in striped and Janus gold nanoparticles. We analyze the images to determine molecular arrangements on parts of the particles, highlight molecular "defects" present in the ligand shell, show persistence of the features across subsequent images, and observe the transition from quasimolecular to domain resolution. The ability to resolve single molecules in the ligand shell of nanoparticles could lead to a more comprehensive understanding of the role of the ligand structure in determining the properties of mixed-monolayer-protected gold nanoparticles.

The preparation and characterization of Langmulr-Blodgett (LB) films of mesoporphyrin IX dimethylester indium chloride are reported. The utility of complementary experimental techniques such as electron diffraction, polarized visible spectroscopy and cyclic voltammetry in yielding information about molecular orientatton and interactions is highlighted. Our results are consistent w~th verhcal packing of the porphyrin ring on the aqueous subphase and transfer of the material in a similar orientation onto a number of different substrates. From polarized UV-visible spectroscopic measurements an average tilt angle 0 between molecular planes and substrate of 64 ° _+ 15 ° is calculated. An area per molecule on the subphase and m the LB films of 0.55 nm 2 ts obtained from pressure-area curves and coulometrtc measurements respectively. A high degree of crystallimty in the layers is apparent from selected area electron diffraction patterns and the films degrade rapidly on exposure to the electron beam. There is considerable interaction between molecules m the films and this leads to peak broadening in both visible absorption spectra and cyclic voltammograms. An underpotential of 700 mV for the reduction of the porphyrin is consistent with surface potential measurements reported earlier.

InAs/GaAs quantum dot heterostructures grown by molecular-beam epitaxy are studied using cross-sectional scanning tunneling microscopy and spectroscopy. The images reveal individual InAs quantum dots (QDs) having a lens shape with maximum base diameter of 10.5 nm and height of 2.9 nm. Analysis of strain relaxation of the QDs reveals an indium composition varying from 65% at the base of the QD, to 95% at its center, and back to 65% at its apex. Room-temperature tunneling spectra acquired 3-4 nm from the center of a dot show a peak located in the upper part of the GaAs bandgap originating from the lowest electron confined state of the QD, along with a tail in the conductance extending out from the valence band and originating from QD hole states. A computational method is developed for simulating the tunneling spectra using effectivemass bands treated in an envelope-function approximation. By comparison of the computations to low-current spectra, the energy of the lowest electron and highest hole QD states are determined. These energies are found to be in reasonably good agreement both with optical measurements and prior theoretical predictions of Wang et al. [Phys. Rev. B 59, 5678 (1999)].

In situ scanning tunneling microscopy (STM) and reflection high energy electron diffraction (RHEED) were used to characterize mercury film electrodeposited onto a Pt(1 1 1) electrode at room temperature. Depending on the amount of Hg deposit, two different growth modes were observed. At low Hg coverage, crystalline (0 0 0 1)Hg adlayer accompanied by 30 •-rotated (1 1 1)-Pt patches was found on Pt(1 1 1). Deposition of multilayer Hg resulted in layered PtHg 2 and PtHg 4 amalgams, which grew epitaxially by aligning their (2 0 1) and (11 0) planes, respectively, parallel to the Pt(1 1 1) substrate. The preference of these epitaxial relationships for the electrochemically formed Pt-Hg intermetallic compounds on Pt(1 1 1) could result from minimization of the surface energy.

Wheat germ agglutinin (WGA) lectin is a model compound for the interaction between viruses and cells during infection events and thus an interesting analyte for mass-sensitive sensing to study these interaction phenomena. Scanning tunneling microscopy studies reveal that surface molecular imprinting leads to cavities having the dimensions of WGA dimers. These reincorporate WGA from phosphatebuffered saline between 1 and 160 μg/ml. Whereas the quartz crystal microbalance (QCM) frequency for molecularly imprinted polymer (MIP)-coated electrodes decreases, indicating uptake of the analyte, their nonimprinted counterparts yield positive, concentration-dependent frequency shifts characteristic for slip of the analyte on the QCM surface. The MIPs achieve selectivity factors towards bovine serum albumin of roughly 4 at higher protein concentrations. Brunauer-Emmett-Teller analysis reveals that binding is favored by 29 kJ/mol until the adsorption of up to ten monolayers on the MIP, whereas above this range the value is lower. Together with the binding behavior of MIP and nonimprinted polymers, this indicates that the MIP acts as a nucleus for multilayer deposition onto the surface.

Physical electrochemistry has undergone a remarkable evolution over the last few decades, integrating advanced techniques and theory from solid state and surface physics. Single-crystal electrode surfaces have been a core notion, opening for scanning tunnelling microscopy directly in aqueous electrolyte (in situ STM). Interfacial electrochemistry of metalloproteins is presently going through a similar transition. Electrochemical surfaces with thiol-based promoter molecular monolayers (SAMs) as biomolecular electrochemical environments and the biomolecules themselves have been mapped with unprecedented resolution, opening a new area of single-molecule bioelectrochemistry. We consider first in situ STM of small redox molecules, followed by in situ STM of thiol-based SAMs as molecular views of bioelectrochemical environments. We then address electron transfer metalloproteins, and multi-centre metalloenzymes including applied single-biomolecular perspectives based on metalloprotein/metallic nanoparticle hybrids.

Molecular resolution scanning tunneling microscopy (STM) imaging and high sensitivity infrared reflection spectroscopy (IRS) measurements have been combined to characterize the structures of mixed self-assembled monolayers (SAMs) of fully conjugated, linear thiolate-terminated molecules and short chain (8-12) n-alkanethiolates on Au{111}. Immersion of preformed, ordered alkanethiolate SAMs into dilute solutions of the conjugated molecules results in two-dimensional matrix isolation of conjugated adsorbates in the host SAM. The post-immersion host SAM matrix shows retained alkanethiolate ordering with the guest molecules inserted both singly into boundaries between SAM structural domains and in bundles at substrate step edges. Inserted molecules of lengths in the ∼15 Å range adopt surface orientations similar to those of alkanethiolate molecules at all compositions, including the pure conjugated SAMs. In contrast, the configuration of an inserted, long conjugated molecule (∼40 Å), varies monotonically with the final SAM composition. When inserted at decreasing fractions into an n-octanethiolate matrix, the average tilt of the long molecular axis decreases and approaches alignment with the host SAM. Combined quantum chemical calculations and IRS data for the short guest-host SAMs support a picture of a dense local environment of the host SAM around the guest molecules, despite their insertion at host defects. These results have important implications for designing electronic devices based on the addressing of individual, fully conjugated molecules self-assembled at gold electrodes.

Fe epitaxy on Cu( 100) is investigated for Fe coverages 8<:3.0 ML. The layer filling statistics are quantitatively related to an evolving growth process, which includes intermixing at the substrate overlayer interface. The resulting inhomogeneous substrate surface and first layer affect the processes by which arriving Fe atoms add to the growth front. Our results explain the previously reported covering of initial Fe not as bilayer growth, but instead as the result of island growth on top of Fe incorporated in the top substrate layer. First layer composition and structure influence the nucleation and growth of the second layer. Island coalescence and formation of a first layer percolation network change the connected first layer area thereby changing the nucleation and growth behavior of the second layer. After both first and second layer growth are completed, images show additional growth is much more layer-by-layer in nature. Oxygen exposure after Fe deposition changes the layer filling by both promotion of step crossing and expansion of partially filled Fe layers.

The structure and formation of an ultrathin hexagonal boron nitride (h-BN) film on Pt(111) has been studied by a combination of scanning tunneling microscopy, low energy electron diffraction, low energy electron microscopy, x-ray absorption and high resolution core level spectroscopy. The study shows that a single boron nitride layer is formed on Pt(111), resulting in a coincidence structure. High resolution scanning tunneling microscopy (STM) images of the h-BN ultrathin film display only one of the atomic species in the unit cell. Probing the boron and nitrogen related local density of states by near edge x-ray adsorption fine structure measurements we conclude that the nitrogen sublattice is visible in STM images. The growth of the single hexagonal boron nitride layer by vapourized borazine in the pressure range of 1 × 10 −6 to 1 × 10 −8 at 800 • C is further studied by low energy electron microscopy, and reveals that the number of nucleation sites and the perfection of the growth is strongly pressure dependent. A model for the single, hexagonal, boron nitride layer on Pt(111)is proposed.

Epitaxial Ag ultra-thin films grown on Au(1 1 1) have been studied by scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES). In the submonolayer range, the spectroscopic measurements reveal the existence of two parabolic dispersive surface Shockley bands associated with the Au substrate and the Ag islands, respectively. The surface state in the Ag region is shifted by about 160 meV towards the Fermi level. High resolution measurements on Au(1 1 1) reveal the splitting of each band in two spin-orbit contributions. A reduction of this spin-orbit splitting is observed for a 1 ML Ag film. Moreover, an annealing of the interface leads to an interdiffusion and to an additional shift of the surface state.

Over the last decade, electrochemists have made increasingly the use of an STM as a tool for nanostructuring electrode surfaces. The various approaches include: (i) a mechanical impact of the tip on the surface, e.g., creating reaction centres by a tip crash or by locally removing tarnishing overlayers, (ii) the spatially confined metal deposition or dissolution via the tip and (iii) down-scaling of the SECM to nanometer dimensions. Examples for each of the different strategies are given. Nanostructuring electrode surfaces by metal clusters, which are generated by a jump-to-contact between tip and substrate is described in more detail with Cu and Pd on Au as examples. Preliminary results of tunneling spectroscopy are also presented.

Electrochemical properties of glassy carbon electrodes of two types were examined, one thermally treated at 10008C (sample K) and another thermally treated at 25008 (sample G). Mechanically polished or electrochemically polarized electrodes were characterized in NaOH, HClO and H SO solutions by cyclic voltammetry (cv) at different sweep rates in 4 2 4 the potential range between the hydrogen and oxygen evolution. The activity of the electrodes depended on the properties of the glassy carbon examined, as determined by both the temperature of thermal treatment and the mechanical or electrochemical pretreatment of the sample. It was noticed that both types of electrodes, when polished exhibited an increase in the double layer charge upon increasing the pH value of the solution. The cv charges, for both types of samples, increase upon anodic polarization. The higher the potential of oxidation, the more pronounced is the increase in charge, particularly in 22 acidic solution. The increase in charge amounts from below 1 mC cm for polished glassy carbon up to few hundreds of 22 mC cm for surfaces anodically polarized in acidic solution. Analysis of the dependence of voltammetric charge, as well as morphological changes of the electrode surface, on the time of oxidation suggests the existence of three stages in the electrochemical activation process. The first one occurs only once at the beginning of the activation, while the other two repeat themselves, reflecting a periodical activation and deactivation process. These stages were discussed and ascribed to a surface layer oxidation, graphite oxide layer growth and mechanical destruction of the surface. Independent surface analysis by AES, XPS and STM confirms the results obtained by electrochemical methods.

Thin films of vapor-deposited Ni(II) and Co(II) complexes of tetraphenylporphyrin (NiTPP and CoTPP) were studied supported on gold and embedded in Al-Al 2 O 3 -MTPP-Pb tunnel diodes, where M ) Ni or Co. Thin films deposited onto polycrystalline gold were analyzed by ultraviolet photoelectron spectroscopy (UPS) using He I radiation. Scanning tunneling microscopy (STM) and orbital-mediated tunneling spectroscopy (STM-OMTS) were performed on submonolayer films of CoTPP and NiTPP supported on Au(111). Inelastic electron tunneling spectroscopy (IETS) and OMTS were measured in conventional tunnel diode structures. The highest occupied π molecular orbital of the porphyrin ring was seen in both STM-OMTS and UPS at about 6.4 eV below the vacuum level. The lowest unoccupied π* molecular orbital of the porphyrin ring was observed by STM-OMTS and by IETS-OMTS to be located near 3.4 eV below the vacuum level. The OMTS spectra of CoTPP had a band near 5.2 eV (below the vacuum level) that was attributed to transient oxidation of the central Co(II) ion. That is, it is due to electron OMT via the half-filled d z 2 orbital present in Co(II) of CoTPP. The NiTPP OMTS spectra show no such band, consistent with the known difficulty of oxidation of the Ni(II) ion. The STM-based OMTS allowed these two porphyrin complexes to be easily distinguished. The present work is the first report of the observation of STM-OMTS, tunnel junction OMTS, and UPS of the same compounds. Scanning tunneling microscope-based orbital-mediated tunneling provides more information than UPS or tunnel junction-based OMTS and does so with molecular-scale resolution.

The high speed performance of a scanning probe microscope ͑SPM͒ is improved if a microelectromechanical systems ͑MEMS͒ device is employed for the out-of-plane scanning motion. We have carried out experiments with MEMS high-speed z-scanners ͑189 kHz fundamental resonance frequency͒ in both atomic force microscope and scanning tunneling microscope modes. The experiments show that with the current MEMS z-scanner, lateral tip speeds of 5 mm/s can be achieved with full feedback on surfaces with significant roughness. The improvement in scan speed, obtained with MEMS scanners, increases the possibilities for SPM observations of dynamic processes. Even higher speed MEMS scanners with fundamental resonance frequencies in excess of a megahertz are currently under development.

This paper describes the growth and characterization of large-scale ultra-smooth metal surfaces produced by an adapted replica technique. Making use of this method, either amorphous or crystalline masters of different materials with ultra-flat surfaces, e.g. mica, glass or polymer coatings on silicon, were coated by a physical vapor deposition (PVD) process with a thin precious-metal layer. On the top of this layer a thick Ni surface was grown by electroplating. Both the precious-metal layer and the nickel reinforcement can be stripped off from the master, and the free metal surface that is made to appear can be used as a substrate for the self-assembly of molecules, mostly via chemisorption of thiol-functionalized moieties. The use of either gold or silver layers led to films exhibiting different morphologies and roughnesses, which are all strongly influenced by the structure of the master's surface and by the conditions during the PVD-coating procedure. Utilizing mica as a master it was possible to grow Ag and Au surfaces made of ultra-smooth well-defined 111 -oriented crystals. A root mean square roughness down to 0.2 nm was measured over micrometersized areas by scanning tunneling microscopy. Very flat Au and Ag films have been also produced using the amorphous masters.

Functionality of biological and inorganic systems ranging from nonvolatile computer memories and microelectromechanical systems to electromotor proteins and cellular membranes is ultimately based on the intricate coupling between electrical and mechanical ...

Photochromic systems can convert light energy into mechanical energy, thus they can be used as building blocks for the fabrication of prototypes of molecular devices that are based on the photomechanical effect. Hitherto a controlled photochromic switch on surfaces has been achieved either on isolated chromophores or within assemblies of randomly arranged molecules. Here we show by scanning tunneling microscopy imaging the photochemical switching of a new terminally thiolated azobiphenyl rigid rod molecule. Interestingly, the switching of entire molecular 2D crystalline domains is observed, which is ruled by the interactions between nearest neighbors. This observation of azobenzenebased systems displaying collective switching might be of interest for applications in high-density data storage.

An isotropic carbon fibre was surface-treated by microwave oxygen plasma at different conditions and characterised by scanning electron microscopy (SEM), scanning tunneling microscopy (STM), N / CO adsorption, Raman spectrometry, 2 2 X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD). It is shown that the structure of the fibre suffers only limited alterations upon plasma treatment in such a way that the local disorder on its surface, which was already large in the fresh material, barely increases after the plasma exposure, as detected by Raman measurements. At the nanometre scale, STM images revealed a moderate increase in surface roughness. Evidence for chemical changes undergone by the fibre following the etching was provided by XPS and TPD, showing that stable oxygen functionalities were introduced by the plasma exposure, a result of practical importance for the application of this treatment not only to this type of carbon fibre, but to carbon materials in general. It was also observed that very gentle plasma exposures were generally sufficient to provide the fibre surface with a large amount of oxygen functional groups and that more intense treatments had a negative effect in this respect (i.e. they were not able to supply oxygen to the surface in larger amounts than the softer treatments did). 

Nanotechnology has already started to significantly impact many industries and scientific fields including biotechnology, pharmaceutics, food technology and semiconductors. Nanotechnology-based tools and devices, including high-resolution imaging techniques, enable characterization and manipulation of materials at the nanolevel and further elucidate nanoscale phenomena and equip us with the ability to fabricate novel materials and structures. One of the most promising impacts of nanotechnology is in the area of nanotherapy. Employing nanosystems such as dendrimers, nanoliposomes, niosomes, nanotubes, emulsions and quantum dots, nanotherapy leads toward the concept of personalized medicine and the potential for early diagnoses coupled with efficient targeted therapy. The development of smart targeted nanocarriers that can deliver bioactives at a controlled rate directly to the designated cells and tissues will provide better efficacy and reduced side effects. Nanocarriers improve the solubility of bioactives and allow for the delivery of not only small-molecule drugs but also the delivery of nucleic acids and proteins. This review will focus on nanoscale bioactive delivery and targeting mechanisms and the role of high-resolution imaging techniques in the evaluation and development of nanocarriers. #

ZnO is a strategic material for various photonic applications. We present our results on characterization of thin films of ZnO grown by sol-gel spin and RF sputtering methods. The characterization techniques involved ellipsometry, scanning electron microscopy with energy dispersive analysis, X-ray diffraction and scanning tunneling microscopy. Microstructural characterization of the films using zinc nitrate precursor has delineated many interesting features

Qingkai Yu 1,2,$,* , Luis A. Jauregui 3,4,$ , Wei Wu 1,2,# , Robert Colby 3,5,# , Jifa Tian 3,6,# , ... Zhihua Su 2 , Helin Cao 3,6 , Zhihong Liu 2 , Deepak Pandey 3,6 , Dongguang Wei 7 , Ting Fung ... Chung 3,6 , Peng Peng 1,2 , Nathan Guisinger 8 , Eric A. Stach 3,5,9 , Jiming ...

Polar surfaces are known to be unstable due to the divergence of the surface electrostatic energy. Here we report on the experimental determination, by grazing incidence x-ray diffraction, of the surface structure of polar Ti-terminated (111) SrTiO 3 single crystals. We find that the polar instability of the 1 × 1 surface is solved by a pure electronic reconstruction mechanism, which induces out-of-plane ionic displacements typical of the polar response of SrTiO 3 layers to an electron confining potential. On the other hand, the surface instability can be also eliminated by a structural reconstruction driven by a change in the surface stoichiometry, which induces a variety of 3 × 3 (111) SrTiO 3 surfaces consisting in an incomplete Ti (surface)-O 2 (subsurface) layer covering the 1 × 1 Ti-terminated (111) SrTiO 3 truncated crystal. In both cases, the TiO 6 octahedra are characterized by trigonal distortions affecting the structural and the electronic symmetry of several unit cells from the surface. These findings show that the stabilization of the polar (111) SrTiO 3 surface can lead to the formation of quasi-two-dimensional electron systems characterized by radically different ground states which depend on the surface reconstructions.

In a 1959 speech entitled There's Plenty of Room at the Bottom [1], Richard Feynman invited scientists to a new field of research: to see individual atoms distinctly, and to arrange the atoms the way we want. Feynman envisioned that, by achieving those goals, one could synthesize any chemical substance that the chemist writes down, resolve many central and fundamental problems in biology at the molecular level, and dramatically increase the density of information storage. Some 20 years later, those goals began to be achieved through the invention and application of the scanning tunneling microscope (STM) [2, 3] and the atomic force microscope (AFM) . The inventors of STM, two physicists at IBM Research Division, Gerd Binnig and Heinrich Rohrer, shared the 1986 Nobel Prize in physics .

Scanning tunneling microscopy ͑STM͒ and ab initio calculations based on density functional theory ͑DFT͒ were used to study the self-aligned silicon nanoribbons on Ag͑110͒ with honeycomb, graphene-like structure. The silicon honeycombs structure on top of the silver substrate is clearly observed by STM, while the DFT calculations confirm that the Si atoms adopt spontaneously this new silicon structure.

Several technical modifications related to the fabrication and ultra-high vacuum (UHV) treatments of the scanning tunneling microscope (STM) tips have been implemented to improve a reliability of the tip preparation for high-resolution STM. Widely used electrochemical etching drop-off technique has been further refined to enable a reproducible fabrication of the tips with a radius ⩽3 nm. For tip cleaning by a controllable UHV annealing, simple and flexible setup has been developed. Proper W tip preparation has been demonstrated via an imaging of the TiO2 (1 1 0) surface atomic structure.

Advanced Study Institutes are high-level tutorial courses offering in-depth study of latest advances in a field. Advanced Research Workshops are expert meetings aimed at critical assessment of a field, and identification of directions for future action.

Ultrathin ordered titanium oxide films on a Pt(111) surface have been prepared by reactive deposition and characterized by low-energy electron diffraction and scanning tunneling microscopy (STM). According to the postdeposition annealing condition, three different phases have been prepared which show a wagonwheel-like (hereafter ww) morphological pattern. Two of them can be prepared as single phases (w-and w′-TiO x ) and one (w int -TiO x ) as a mixed phase which always coexists with at least one of the other two phases. All of them are formed by a Ti-O bilayer, where the Ti atoms are located at the interface with the substrate, but they show a rather distinct STM ww pattern. The experimental STM contrast has been discussed on the basis of a Moiré-like model, i.e., as deriving from a modulation of the Ti occupancy of the different substrate sites (i.e., hollow, bridge and on-top sites). The major part of the STM data can be easily interpreted on the basis of this simplified model.

A term first coined by Mott back in 1968 a “pseudogap” is the depletion of the electronic density of states at the Fermi level, and pseudogaps have been observed in many systems. However, since the discovery of the high-temperature superconductors (HTSC) in 1986, the central role attributed to the pseudogap in these systems has meant that by many researchers now associate the term pseudogap exclusively with the HTSC phenomenon. Recently, the problem has got a lot of new attention with the rediscovery of two distinct energy scales (“two-gap scenario”) and charge density waves patterns in the cuprates. Despite many excellent reviews on the pseudogap phenomenon in HTSC, published from its very discovery up to now, the mechanism of the pseudogap and its relation to superconductivity are still open questions. The present review represents a contribution dealing with the pseudogap, focusing on results from angle resolved photoemission spectroscopy (ARPES) and ends up with the conclusion that the pseudogap in cuprates is a complex phenomenon which includes at least three different “intertwined” orders: spin and charge density waves and preformed pairs, which appears in different parts of the phase diagram. The density waves in cuprates are competing to superconductivity for the electronic states but, on the other hand, should drive the electronic structure to vicinity of Lifshitz transition, that could be a key similarity between the superconducting cuprates and iron-based superconductors. One may also note that since the pseudogap in cuprates has multiple origins there is no need to recoin the term suggested by Mott.

We discuss fluctuating order in a quantum disordered phase proximate to a quantum critical point, with particular emphasis on fluctuating stripe order. Optimal strategies for extracting information concerning such local order from experiments are derived with emphasis on neutron scattering and scanning tunneling microscopy. These ideas are tested by application to two model systemsthe exactly solvable one dimensional electron gas with an impurity, and a weakly-interacting 2D electron gas. We extensively review experiments on the cuprate high-temperature superconductors which can be analyzed using these strategies. We adduce evidence that stripe correlations are widespread in the cuprates. Finally, we compare and contrast the advantages of two limiting perspectives on the high-temperature superconductor: weak coupling, in which correlation effects are treated as a perturbation on an underlying metallic (although renormalized) Fermi liquid state, and strong coupling, in which the magnetism is associated with well defined localized spins, and stripes are viewed as a form of micro-phase separation. We present quantitative indicators that the latter view better accounts for the observed stripe phenomena in the cuprates.

Silicene -the silicon-based counterpart of graphene -has a two dimensional structure that is responsible for the variety of potentially useful chemical and physical properties. The existence of silicene has been achieved recently owing to experiments involving epitaxial growth of silicon as stripes on Ag(001), ribbons on Ag(110), and sheets on Ag(111). The nano-ribbons observed on Ag(110) were found -by both high definition experimental scanning tunneling microscopy images and density functional theory calculations -to consist of an arched honeycomb structure. Angle resolved photo-emission experiments on these silicene nano-ribbons on Ag(110), along the direction of the ribbons, showed a band structure which is analogous to the Dirac cones of graphene. Unlike silicon surfaces, which are highly reactive to oxygen, the silicene nano-ribbons were found to be resistant to oxygen reactivity.

Fire opal, a transparent orange variety of opal, which does not diffract visible light, is built from the random accumulation of granular particles of hydrated silica, about 20 nm in size. This opal variety does not present the structure most commonly associated with precious opal, that is, a regular three-dimensional network of amorphous silica spheres about 200 nm in diameter. About 60 samples (from Mexico, Brazil, Kazakhstan, Ethiopia, Tanzania, Slovakia and USA) were documented using scanning electron and atomic force microscopy. This work demonstrates that nanograins are the elementary building blocks of this variety of opal.

Angle-dependent studies of the gap function provide evidence for the coexistence of two distinct gaps in hole doped cuprates, where the gap near the nodal direction scales with the superconducting transition temperature $T_c$, while that in the antinodal direction scales with the pseudogap temperature. We present model calculations which show that most of the characteristic features observed in the recent angle-resolved photoemission spectroscopy (ARPES) as well as scanning tunneling microscopy (STM) two-gap studies are consistent with a scenario in which the pseudogap has a non-superconducting origin in a competing phase. Our analysis indicates that, near optimal doping, superconductivity can quench the competing order at low temperatures, and that some of the key differences observed between the STM and ARPES results can give insight into the superlattice symmetry of the competing order.

Microscopic structures for molecular monolayers of L-cysteine and L-cystine assembled on Au(111) have been disclosed by employing electrochemistry and in situ scanning tunneling microscopy (STM). Highresolution STM images show that the adlayers of both cyteine and cystine exhibit highly-ordered networklike clusters with (3 3 × 6)R30°structure. By combining the surface coverage estimated from voltammetric data, each cluster is demonstrated to include six individual cysteine molecules or three cystine molecules. As a comparison, no cluster structure is observed for the 1-butanethiol adlayer prepared and examined under the same conditions as those for cysteine and cystine. This suggests that intermolecular and intramolecular hydrogen bonds among adsorbed cysteine or cystine molecules could be responsible for the origin of the cluster-network structures for the adlayers. Several models are proposed and used to explain the in situ STM observations in detail.