Progress in Surface Science最新文献

The emission of electron pairs from surfaces has the power to reveal details about the electron–electron interaction in condensed matter. This process, stimulated by a primary electron or photon beam, has been studied both in experiment and theory over the last two decades. An additional pathway, namely positron–electron pair emission, holds the promise to provide additional information. It is based on the notion that the Pauli exclusion principle does not need to be considered for this process.

We have commissioned a laboratory based positron source and performed a systematic study on a variety of solid surfaces. In a symmetric emission geometry we can explore the fact that positron and electron are distinguishable particles. Following fundamental symmetry arguments we have to expect that the available energy is shared unequally among positron and electron. Experimentally we observe such a behavior for all materials studied. We find an universal feature for all materials in the sense that on average the positron carries a larger fraction of the available energy. This is qualitatively accounted for by a simplified scattering model. Numerical results, which we obtained by a microscopic theory of positron–electron emission from surfaces, reveal however that there are also cases in which the electron carries more energy. Whether the positron or the electron is more energetic depends on details of the bound electron state and of the emission geometry. The coincidence intensity is strongly material dependent and there exists an almost monotonic relation between the singles and coincidence intensity. These results resemble the findings obtained in electron and photon stimulated electron pair emission. An additional reaction channel is the emission of an electron pair upon positron impact. We will discuss the energy distributions and the material dependence of the coincidence signal which shows similar features as those for positron–electron pairs.

In surface science, research traditionally employs macroscopically flat surfaces of single crystals. Curved surfaces have been applied more sporadically, but their history stretches back for many decades. Realization of the potential benefits and practical applications in surface physics and surface chemistry research progressed slowly in the 20th century. In more recent decades, research employing partial cylinders and dome-shaped crystals have found renewed interest. Modern surface sensitive techniques are being employed allowing the inherent large range of surface structures to reveal new insights. We briefly review the history, describe several types of surfaces and the range of structures they contain, suggest a notation for common types of curved surfaces, and discuss recent studies in more detail. We mainly focus on metal samples. We close with a short outlook.

The development of electron spin resonance (ESR) combined with scanning tunneling spectroscopy (STM) is undoubtedly one of the main experimental breakthroughs in surface science of the last decade thanks to joining the extraordinarily high energy resolution of ESR (nano-eV scale) with the single-atom spatial resolution of STM (sub-Ångström scale). While the experimental results have significantly grown with the number of groups that have succeeded in implementing the technique, the physical mechanism behind it is still unclear, with several different mechanisms proposed to explain it. Here, we start by revising the main characteristics of the experimental setups and observed features. Then, we review the main theoretical proposals, with both their strengths and weaknesses. One of our conclusions is that many of the proposed mechanisms share the same basic principles, the time-dependent electric field at the STM junction is modulating the coupling of the spin-polarized transport electrons with the local spin. This explains why these mechanims are essentially equivalent in a broad picture. We analyze the subtle differences between some of them and how they compare with the different experimental observations.

Ultra-wide bandgap materials show great promise as a solution to some of the limitations of current state of the art semiconductor technology. Among these, diamond has exhibited great potential for use in high-power, high-temperature electronics, as well as sensing and quantum applications. Yet, significant challenges associated with impurity doping of the constrained diamond lattice remain a primary impediment towards the development of diamond-based electronic devices. An alternative approach, used with continued success to unlock the use of diamond for semiconductor applications, has been that of ‘surface transfer doping’ - a process by which intrinsically insulating diamond surfaces can be made semiconducting without the need for traditional impurity doping. Here, we present a review of progress in surface transfer doping of diamond, both a history and current outlook of this highly exploitable attribute.

The emergence of local moments in graphene zigzag edges, grain boundaries, vacancies and sp³ defects has been widely studied theoretically. However, conclusive experimental evidence is scarce. Recent progress in on-surface synthesis has made it possible to create nanographenes, such as triangulenes, with local moments in their ground states, and to probe them using scanning tunneling microscope (STM) spectroscopy. Here we review the application of the theory of sequential and cotunneling transport to relate the $\mathrm{dI}/\mathrm{dV}$ spectra with the spin properties of nanographenes probed by STM. This approach permits us to connect the $\mathrm{dI}/\mathrm{dV}$ with the many-body energies and wavefunctions of the graphene nanostructures. We apply this method describing the electronic states of the nanographenes by means of exact diagonalization of the Hubbard model within a restricted Active Space. This permits us to provide a proper quantum description of the emergence of local moments in graphene and its interplay with transport. We discuss the results of this theory in the case of diradical nanographenes, such as triangulene, rectangular ribbons and the Clar’s goblet, that have been recently studied experimentally by means of STM spectroscopy. This approach permits us to calculate both the $\mathrm{dI}/\mathrm{dV}$ spectra, that yields excitation energies, as well as the atomically resolved conductivity maps, that provide information on the wavefunctions of the collective spin modes.

High-Resolution Neutron Diffraction (HRND), Electron Back-Scatter Diffraction (EBSD) and Scanning Kelvin Probe Force Microscopy (SKPFM) techniques were used to comparatively characterize the surface electrical properties of Inconel 690 and stainless steel 316L alloys in cold-rolled and unrolled (annealed) conditions. Results indicated that a direct relation exists between the density of lattice defects (measured by HRND and EBSD) and heterogeneity of surface potential (measured by SKPFM). Mapping of the Volta potential and deconvolution of the corresponding histogram plots of the acquired data were utilized to visualize and comparatively quantify crystal lattice defects and estimate the surface susceptibility to the formation of micro/nano-galvanic cells. SKPFM was found as a reliable alternative to electron and neutron scattering techniques for comparative evaluation of energy states on alloys’ surfaces.

Metasurfaces are nanopatterned structures of sub-wavelength thickness. Their effective refractive index and spectral characteristic can be tailored by material composition, intrinsic and extrinsic resonances, structure size, and ambient conditions. Consequently, they allow for phase, amplitude, polarisation, and spatial control of an optical field beyond what natural materials can offer. Dielectric metasurfaces with lower loss have opened a wide range of new applications such as enhanced imaging, structural colour, holography, and planar sensors. In particular, beam steering and control measures such as nonlinear optics, ultrafast optics, and quantum optics are of increasing importance for quantum communication, computation, and information processing. In this review, the recent progress on dielectric metasurfaces is summarised, including advanced fabrication technologies and novel applications from advanced wavefront shaping to quantum platforms. In addition, a perspective for the future development of the field is presented.

The recent development of double photoemission (DPE) spectroscopy at surfaces using laser-based high-order harmonic generation in combination with time-of-flight electron spectroscopy is reviewed. Relevant experimental conditions including the solid angle for collecting photoelectron pairs, the energy and angular resolutions, as well as the repetition rate and the photon energy range of light sources are introduced. As examples, we provide an overview of laser-based DPE results on the noble metals Ag and Cu as well as transition metal oxides NiO and CoO. The DPE energy and angular distributions of photoelectron pairs are compared with emphasis on the possible indications of electron-electron interaction. Potential further developments including femtosecond time-resolved DPE experiments are outlined.

Chemical reactions on surfaces play central roles in heterogeneous catalysis, and most reactions involve the formation and/or the cleavage of bonds. At present, density functional theory (DFT) has become the workhorse for computational investigation of reaction mechanisms, but its predictive power has been severely limited by the lack of appropriate exchange-correlation functionals. Here, we show that there are many cases where the chemical bonding and van der Waals (vdW) interactions both play a key role in chemical reactions on surfaces. After briefly introducing some DFT methods and basic theory in chemical reactions, we first demonstrate that DFT can help to understand the mechanisms of “classic” reactions that mainly dominated by covalent bonding and vdW forces, as exemplified in electrocatalytic reduction of CO₂ and the fabrication of 2D materials on metal substrates. We next show that DFT calculations can help to uncover the tautomerization reactions of molecules on metal surfaces, wherein the hydrogen bonding and vdW forces would largely affect the reaction process. More importantly, we show that in some cases, the vdW interactions can become the decisive effect that determines the adsorption configuration, energy hierarchy, and the potential-energy surface of chemical reactions, yielding distinct pathways and products. Additionally, we highlight the importance of more realistic conditions, such as surface defects, finite coverage, and temperature effects, in accurate modeling of chemical reactions. Finally, we summarize some challenges in modeling catalysis, which include many-body dispersive correction, strong correlation effect, and non-adiabatic approximations.

A ferromagnetic insulator (FI) attached to a conventional superconductor (S) changes drastically the properties of the latter. Specifically, the exchange field at the FI/S interface leads to a splitting of the superconducting density of states. If S is a superconducting film, thinner than the superconducting coherence length, the modification of the density of states occurs over the whole sample. The coexistence of the exchange splitting and superconducting correlations in S/FI structures leads to striking transport phenomena that are of interest for applications in thermoelectricity, superconducting spintronics and radiation sensors. Here we review the most recent progress in understanding the transport properties of FI/S structures by presenting a complete theoretical framework based on the quasiclassical kinetic equations. We discuss the coupling between the electronic degrees of freedom, charge, spin and energy, under non-equilibrium conditions and its manifestation in thermoelectricity and spin-dependent transport.