Tunneling spectroscopy has played a central role in the experimental verification of the microscopic theory of superconductivity in classical superconductors. Initial attempts to apply the same approach to high-temperature superconductors were hampered by various problems related to the complexity of these materials. The use of scanning tunneling microscopy and spectroscopy (STM and STS) on these compounds allowed the main difficulties to be overcome. This success motivated a rapidly growing scientific community to apply this technique to high-temperature superconductors. This paper reviews the experimental highlights obtained over the last decade. The crucial efforts to gain control over the technique and to obtain reproducible results are first recalled. Then a discussion on how the STM and STS techniques have contributed to the study of some of the most unusual and remarkable properties of high-temperature superconductors is presented: the unusually large gap values and the absence of scaling with the critical temperature, the pseudogap and its relation to superconductivity, the unprecedented small size of the vortex cores and its influence on vortex matter, the unexpected electronic properties of the vortex cores, and the combination of atomic resolution and spectroscopy leading to the observation of periodic local density of states modulations in the superconducting and pseudogap states and in the vortex cores.

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Scanning tunneling spectroscopy of high-temperature superconductors

Spin mapping at the nanoscale and atomic scale

Electrocatalysts with single metal atoms as active sites have received increasing attention owing to their high atomic utilization efficiency and exotic catalytic activity and selectivity. This review aims to provide a comprehensive summary on the recent development of such single-atom electrocatalysts (SAECs) for various energy-conversion reactions. The discussion starts with an introduction of the different types of SAECs, followed by an overview of the synthetic methodologies to control the atomic dispersion of metal sites and atomically resolved characterization using state-of-the-art microscopic and spectroscopic techniques. In recognition of the extensive applications of SAECs, the electrocatalytic studies are dissected in terms of various important electrochemical reactions, including hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), carbon dioxide reduction reaction (CO2RR), and nitrogen reduction reaction (NRR). Examples of SAECs are deliberated in each case in terms of their catalytic performance, structure-property relationships, and catalytic enhancement mechanisms. A perspective is provided at the end of each section about remaining challenges and opportunities for the development of SAECs for the targeted reaction.

Advanced Electrocatalysts with Single-Metal-Atom Active Sites.

Significant progress has been made both in experimentation and in theoretical modeling of scanning probe microscopy. The theoretical models used to analyze and interpret experimental scanning probe microscope (SPM) images and spectroscopic data now provide information not only about the surface, but also the probe tip and physical changes occurring during the scanning process. The aim of this review is to discuss and compare the present status of computational modeling of two of the most popular SPM methods---scanning tunneling microscopy and scanning force microscopy---in conjunction with their applications to studies of surface structure and properties with atomic resolution. In the context of these atomic-scale applications, for the scanning force microscope (SFM), this review focuses primarily on recent noncontact SFM (NC-SFM) results. After a brief introduction to the experimental techniques and the main factors determining image formation, the authors consider the theoretical models developed for the scanning tunneling microscope (STM) and the SFM. Both techniques are treated from the same general perspective of a sharp tip interacting with the surface---the only difference being that the control parameter in the STM is the tunneling current and in the SFM it is the force. The existing methods for calculating STM and SFM images are described and illustrated using numerous examples, primarily from the authors' own simulations, but also from the literature. Theoretical and practical aspects of the techniques applied in STM and SFM modeling are compared. Finally, the authors discuss modeling as it relates to SPM applications in studying surface properties, such as adsorption, point defects, spin manipulation, and phonon excitation.

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Theories of scanning probe microscopes at the atomic scale

Although extensive knowledge has rapidly developed in the synthesis and processing of silicon quantum dots (zero-dimen-sional) and silicon nanowires (one-dimensional), the two-dimensional (2D) allotropic form of silicon has been hardly accessed and only recently few attempts have targeted 2D sil-icon nanosheets (NSs) akin to graphene. In this context, a 2D honeycomb crystalline form of silicon, the so-called “silicene”, has recently afforded great appeal as a novel graphene-like material.

Two-dimensional Si nanosheets with local hexagonal structure on a MoS(2) surface.

Scanning tunneling microscopy has repeatedly resolved individual atoms on a number of metal surfaces with atomic distances 2.5--3 \AA{}. This is in sharp contradiction to the resolution limits previously predicted, 6--9 \AA{}. We present a theory of such atomic resolution in terms of actual tip states, for example, ${\mathit{d}}_{\mathit{z}}^{2}$ tip states on tungsten tips. Quantitative interpretation of the observed images is obtained with no adjustable parameters. We predict that to achieve atomic resolution, the tip material should be either a d-band metal or certain semiconductor.

Origin of atomic resolution on metal surfaces in scanning tunneling microscopy.

We analyze the effects of m\ensuremath{\ne}0 tip states in scanning tunneling microscopy (STM). If near the Fermi level, an m\ensuremath{\ne}0 tip state dominates, images with large but inverted atomic corrugation are expected. This provides an explanation of the STM images with inverted corrugations, and the sudden reversal of the STM corrugation from inverted to noninverted. An analysis of the effects of various combinations of m\ensuremath{\ne}0 and m=0, d-type tip states is presented. We show that in most cases, a stable and large corrugation enhancement, either positive or negative, is expected, with predicted values in agreement with experimental observations.

Chen Cj

Papers

Origin of atomic resolution on metal surfaces in scanning tunneling microscopy.

Effects of m≠0 tip states in scanning tunneling microscopy: The explanations of corrugation reversal