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Resolving individual atoms has always been the ultimate goal of surface microscopy. The scanning tunneling microscope images atomic-scale features on surfaces, but resolving single atoms within an adsorbed molecule remains a great challenge because the tunneling current is primarily sensitive to the local electron density of states close to the Fermi level. We demonstrate imaging of molecules with unprecedented atomic resolution by probing the short-range chemical forces with use of noncontact atomic force microscopy. The key step is functionalizing the microscope's tip apex with suitable, atomically well-defined terminations, such as CO molecules. Our experimental findings are corroborated by ab initio density functional theory calculations. Comparison with theory shows that Pauli repulsion is the source of the atomic resolution, whereas van der Waals and electrostatic forces only add a diffuse attractive background.

Graphene: new process yields ultraflat form Graphene is the subject of intense research thanks to its novel fundamental properties and its potential for possible electronics applications. Though graphene is essentially two-dimensional, a layer of carbon atoms just one atom thick, it is in fact always slightly crumpled. Whether laying on a substrate or suspended, it always presents ripples, which are thought to define a remarkably diverse set of the observed properties of graphene. Now a team from Columbia University has developed a simple, but effective method of producing ultraflat graphene by deposition on an atomically flat mica surface that tightly binds to the carbon atoms. Thus ripple formation is not an essential feature of high-quality graphene. The availability of ultraflat samples will facilitate studies of the effect of ripples on the physical and electronic properties of graphene. Graphene, an atom-thin carbon sheet is interesting for its fundamental properties as well as for its possible applications in electronics, is not strictly two-dimensional. Microscopic corrugations, or ripples, have been observed in all graphene sheets so far. Direct experimental study of the physics of such ripples has been hindered by the lack of flat graphene layers. Ultraflat graphene is now achieved through its deposition on the atomically flat terraces of cleaved mica surfaces. Graphene, a single atomic layer of carbon connected by sp 2 hybridized bonds, has attracted intense scientific interest since its recent discovery 1 . Much of the research on graphene has been directed towards exploration of its novel electronic properties, but the structural aspects of this model two-dimensional system are also of great interest and importance. In particular, microscopic corrugations have been observed on all suspended 2 and supported 3 , 4 , 5 , 6 , 7 , 8 graphene sheets studied so far. This rippling has been invoked to explain the thermodynamic stability of free-standing graphene sheets 9 . Many distinctive electronic 10 , 11 , 12 and chemical 13 , 14 , 15 properties of graphene have been attributed to the presence of ripples, which are also predicted to give rise to new physical phenomena 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 that would be absent in a planar two-dimensional material. Direct experimental study of such novel ripple physics has, however, been hindered by the lack of flat graphene layers. Here we demonstrate the fabrication of graphene monolayers that are flat down to the atomic level. These samples are produced by deposition on the atomically flat terraces of cleaved mica surfaces. The apparent height variation in the graphene layers observed by high-resolution atomic force microscopy (AFM) is less than 25 picometres, indicating the suppression of any existing intrinsic ripples in graphene. The availability of such ultraflat samples will permit rigorous testing of the impact of ripples on various physical and chemical properties of graphene.

Surface activation: Changing facets Single crystals of titanium dioxide (TiO 2 ), with highly reactive surfaces, show promise for energy and environmental applications. Unfortunately, the highly reactive surfaces tend to disappear during crystal growth as a result of the minimization of surface energy. Most available samples of anatase, a naturally occurring crystalline form of TiO2, are therefore dominated (to over 90%) by thermodynamically stable {101} facets, rather than the more reactive {001} type. Hua Gui Yang et al . use hydrofluoric acid treatment of anatase TiO 2 to remedy this situation. Based on theoretical predictions, they synthesized uniform anatase TiO 2 single crystals containing 47% of the reactive {001} facets. This work may pave the way for the more general use of non-metallic atoms as surface controlling agents. Extensive first principles calculations carried out show that the relative stability of facets of anatase can be switched by terminating the surfaces with fluorine. It is then demonstrated that uniform anatase single crystals with a high percentage of {001} facets can be generated using hydrofluoric acid as a structure directing agent. Subsequently, surfaces can be freed of fluorine using a simple heat treatment. Owing to their scientific and technological importance, inorganic single crystals with highly reactive surfaces have long been studied 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 . Unfortunately, surfaces with high reactivity usually diminish rapidly during the crystal growth process as a result of the minimization of surface energy. A typical example is titanium dioxide (TiO 2 ), which has promising energy and environmental applications 14 , 15 , 16 , 17 . Most available anatase TiO 2 crystals are dominated by the thermodynamically stable {101} facets (more than 94 per cent, according to the Wulff construction 10 ), rather than the much more reactive {001} facets 8 , 9 , 10 , 11 , 12 , 13 , 18 , 19 , 20 . Here we demonstrate that for fluorine-terminated surfaces this relative stability is reversed: {001} is energetically preferable to {101}. We explored this effect systematically for a range of non-metallic adsorbate atoms by first-principle quantum chemical calculations. On the basis of theoretical predictions, we have synthesized uniform anatase TiO 2 single crystals with a high percentage (47 per cent) of {001} facets using hydrofluoric acid as a morphology controlling agent. Moreover, the fluorated surface of anatase single crystals can easily be cleaned using heat treatment to render a fluorine-free surface without altering the crystal structure and morphology.

Friction at the nanoscale For large objects sliding over one another, the friction force is proportional to the true contact area between the two bodies — which is smaller than the apparent contact area because the surfaces are rough, consisting of a large number of smaller features (asperities) that actually make the contact. The situation for nanomaterials, however, has been unclear, since the continuum contact theory that can account for macroscale effects has been predicted to break down at the nanoscale. Using large-scale molecular dynamics simulations of scanning force microscopy experiments, Yifei Mo et al . show that, despite this, simple friction laws do apply at the nanoscale: the friction force depends linearly on the number of atoms, rather than the number of asperities, that are chemically interacting across the sliding interfaces. For large objects sliding over one another, the friction force is proportional to the true contact area between the two bodies — this is smaller than the apparent contact area as the surfaces are rough, consisting of a large number of smaller features (asperities) that actually make contact. Here a related idea holds for contacts at the nanoscale: the friction force depends linearly on the number of atoms (rather than asperities) chemically interacting across the sliding interfaces. Macroscopic laws of friction do not generally apply to nanoscale contacts. Although continuum mechanics models have been predicted to break down at the nanoscale 1 , they continue to be applied for lack of a better theory. An understanding of how friction force depends on applied load and contact area at these scales is essential for the design of miniaturized devices with optimal mechanical performance 2 , 3 . Here we use large-scale molecular dynamics simulations with realistic force fields to establish friction laws in dry nanoscale contacts. We show that friction force depends linearly on the number of atoms that chemically interact across the contact. By defining the contact area as being proportional to this number of interacting atoms, we show that the macroscopically observed linear relationship between friction force and contact area can be extended to the nanoscale. Our model predicts that as the adhesion between the contacting surfaces is reduced, a transition takes place from nonlinear to linear dependence of friction force on load. This transition is consistent with the results of several nanoscale friction experiments 4 , 5 , 6 , 7 . We demonstrate that the breakdown of continuum mechanics can be understood as a result of the rough (multi-asperity) nature of the contact, and show that roughness theories 8 , 9 , 10 of friction can be applied at the nanoscale.

The diffusion of hydrogen atoms across solid oxide surfaces is often assumed to be accelerated by the presence of water molecules. Here we present a high-resolution, high-speed scanning tunneling microscopy (STM) study of the diffusion of H atoms on an FeO thin film. STM movies directly reveal a water-mediated hydrogen diffusion mechanism on the oxide surface at temperatures between 100 and 300 kelvin. Density functional theory calculations and isotope-exchange experiments confirm the STM observations, and a proton-transfer mechanism that proceeds via an H₃O⁺-like transition state is revealed. This mechanism differs from that observed previously for rutile TiO₂(110), where water dissociation is a key step in proton diffusion.

Pleating crystals A hexagonal lattice can be persuaded to tile a curved surface by the introduction of alternative shapes or 'topological defects' — such as heptagons and pentagons — as in the well-known 'buckyball' with its 20 hexagons and 12 pentagons. This paper reports a previously unknown type of defect that accommodates curvature in the same way as fabric pleats. Defects of this type, uncharged grain boundaries that vanish on the surface, can be observed on the negatively curved surfaces of stretched colloidal crystals. These findings will facilitate the exploration of general theories of defects in curved spaces, the engineering of curved structures and novel methods for soft lithography and directed self-assembly. Hexagons can easily tile a flat surface, but not a curved one. Defects with topological charge (such as heptagons and pentagons) make it easier to tile curved surfaces, such as soccer balls. Here, a new type of defect is reported that accommodates curvature in the same way as fabric pleats. The appearance of such defects on the negatively curved surfaces of stretched colloidal crystals are observed. The results will facilitate the exploration of general theories of defects in curved spaces, the engineering of curved structures and novel methods for soft lithography and directed self-assembly. Hexagons can easily tile a flat surface, but not a curved one. Introducing heptagons and pentagons (defects with topological charge) makes it easier to tile curved surfaces; for example, soccer balls based on the geodesic domes 1 of Buckminster Fuller have exactly 12 pentagons (positive charges). Interacting particles that invariably form hexagonal crystals on a plane exhibit fascinating scarred defect patterns on a sphere 2 , 3 , 4 . Here we show that, for more general curved surfaces, curvature may be relaxed by pleats: uncharged lines of dislocations (topological dipoles) that vanish on the surface and play the same role as fabric pleats. We experimentally investigate crystal order on surfaces with spatially varying positive and negative curvature. On cylindrical capillary bridges, stretched to produce negative curvature, we observe a sequence of transitions—consistent with our energetic calculations—from no defects to isolated dislocations, which subsequently proliferate and organize into pleats; finally, scars and isolated heptagons (previously unseen) appear. This fine control of crystal order with curvature will enable explorations of general theories of defects in curved spaces 5 , 6 , 7 , 8 , 9 , 10 , 11 . From a practical viewpoint, it may be possible to engineer structures with curvature (such as waisted nanotubes and vaulted architecture) and to develop novel methods for soft lithography 12 and directed self-assembly 13 .

Breaking down barriers Topological states have become the subject of much attention from condensed-matter physicists, as evidence accumulates to show that these states can be found on the surface of certain materials — in particular, bulk compounds called topological insulators. As a product of their topological nature, topological surface states are predicted to have the remarkable property of being robust against imperfections. This can allow, for example, the conduction of electronic currents without dissipation. Ali Yazdani and his team now report a tantalizing finding from scanning tunnelling microscope measurements — that topological surface states on antimony can be transmitted with high probability through naturally occurring barriers that stop other conventional surface states of common metals. The authors suggest that their findings indicate that topological surface states could be exploited in novel applications of nanoscale electronic devices. Topological surface states are a class of electronic states that might be of interest in quantum computing or spintronic applications. They are predicted to be robust against imperfections, but so far there has been no evidence that these states do transmit through naturally occurring surface defects. Here, scanning tunnelling microscopy has been used to show that topological surface states of antimony can be transmitted through naturally occurring barriers that block non-topological surface states of common metals. Topological surface states are a class of novel electronic states that are of potential interest in quantum computing or spintronic applications 1 , 2 , 3 , 4 , 5 , 6 , 7 . Unlike conventional two-dimensional electron states, these surface states are expected to be immune to localization and to overcome barriers caused by material imperfection 8 , 9 , 10 , 11 , 12 , 13 , 14 . Previous experiments have demonstrated that topological surface states do not backscatter between equal and opposite momentum states, owing to their chiral spin texture 15 , 16 , 17 , 18 . However, so far there is no evidence that these states in fact transmit through naturally occurring surface defects. Here we use a scanning tunnelling microscope to measure the transmission and reflection probabilities of topological surface states of antimony through naturally occurring crystalline steps separating atomic terraces. In contrast to non-topological surface states of common metals (copper, silver and gold) 19 , 20 , 21 , 22 , 23 , which are either reflected or absorbed by atomic steps, we show that topological surface states of antimony penetrate such barriers with high probability. This demonstration of the extended nature of antimony’s topological surface states suggests that such states may be useful for high current transmission even in the presence of atomic-scale irregularities—an electronic feature sought to efficiently interconnect nanoscale devices.

Nanotechnology: surface patterns to order Nanotechnologists generally turn to one of two methods when engineering a pattern onto a surface: supramolecular self-assembly of molecules into hydrogen-bonded surface networks, or the deposition of self-assembled monolayers (SAMs). Supramolecular assembly is attractive because it yields patterns that are exactly defined at the nanometre scale, whereas self-assembly offers unprecedented flexibility for creating functionalized surfaces. Madueno et al . have now harnessed the advantages of both methods, by first creating self-assembled surface networks and then depositing SAMs into the network pores. The approach is expected to serve as a versatile platform for the production of a wide range of tailored surface structures. The patterning of surfaces is often accomplished by either supramolecular self-assembly of molecules into surface networks, or the deposition of self-assembled monolayers (SAMs). Supramolecular assembly is attractive because it yields patterns that are exactly defined at the nanometre scale, whereas SAMs offer unprecedented flexibility for creating functionalized surfaces. Now the advantages of both methods have been harnessed by first creating self-assembled surface networks and then depositing SAMs into the network pores. One of the central challenges in nanotechnology is the development of flexible and efficient methods for creating ordered structures with nanometre precision over an extended length scale. Supramolecular self-assembly on surfaces offers attractive features in this regard: it is a ‘bottom-up’ approach and thus allows the simple and rapid creation of surface assemblies 1 , 2 , which are readily tuned through the choice of molecular building blocks used and stabilized by hydrogen bonding 3 , 4 , 5 , 6 , 7 , 8 , van der Waals interactions 9 , π–π bonding 10 , 11 or metal coordination 12 , 13 between the blocks. Assemblies in the form of two-dimensional open networks 3 , 9 , 10 , 13 , 14 , 15 , 16 , 17 are of particular interest for possible applications because well-defined pores can be used for the precise localization and confinement of guest entities such as molecules or clusters, which can add functionality to the supramolecular network. Another widely used method for producing surface structures involves self-assembled monolayers (SAMs) 18 , which have introduced unprecedented flexibility in our ability to tailor interfaces and generate patterned surfaces 19 , 20 , 21 , 22 . But SAMs are part of a top-down technology that is limited in terms of the spatial resolution that can be achieved. We therefore rationalized that a particularly powerful fabrication platform might be realized by combining non-covalent self-assembly of porous networks and SAMs, with the former providing nanometre-scale precision and the latter allowing versatile functionalization. Here we show that the two strategies can indeed be combined to create integrated network–SAM hybrid systems that are sufficiently robust for further processing. We show that the supramolecular network and the SAM can both be deposited from solution, which should enable the widespread and flexible use of this combined fabrication method.

Electron mobility within iron (oxyhydr)oxides enables charge transfer between widely separated surface sites. There is increasing evidence that this internal conduction influences the rates of interfacial reactions and the outcomes of redox-driven phase transformations of environmental interest. To determine the links between crystal structure and charge-transport efficiency, we used pump-probe spectroscopy to study the dynamics of electrons introduced into iron(III) (oxyhydr)oxide nanoparticles via ultrafast interfacial electron transfer. Using time-resolved x-ray spectroscopy and ab initio calculations, we observed the formation of reduced and structurally distorted metal sites consistent with small polarons. Comparisons between different phases (hematite, maghemite, and ferrihydrite) revealed that short-range structural topology, not long-range order, dominates the electron-hopping rate.

Diffusion of atoms in a crystalline lattice is a thermally activated process that can be strongly accelerated by defects such as grain boundaries or dislocations. When carried by dislocations, this elemental mechanism is known as \"pipe diffusion.\" Pipe diffusion has been used to explain abnormal diffusion, Cottrell atmospheres, and dislocation-precipitate interactions during creep, although this rests more on conjecture than on direct demonstration. The motion of dislocations between silicon nanoprecipitates in an aluminum thin film was recently observed and controlled via in situ transmission electron microscopy. We observed the pipe diffusion phenomenon and measured the diffusivity along a single dislocation line. It is found that dislocations accelerate the diffusion of impurities by almost three orders of magnitude as compared with bulk diffusion.