Progress in Surface Science最新文献

Macroscopic friction is the result of the interplay of several processes occurring at different scales; an atom-scale description of the tribological interactions is then paramount for the explanation of the elementary phenomena at the basis of such processes, and finds immediate application in technological fields involving nanostructured devices. At the moment, there is no theory which tells us what is the friction coefficient given the atomic description of two surfaces in contact: it is measured experimentally or computationally case by case at specific environmental parameters and chemical composition of the moving surfaces. A general theory describing nanoscale friction is then desirable to reduce human effort, search time and material costs necessary to design new tribological materials with target response. We here provide a selective overview of theoretical and computational models which, from our perspective, may pave the avenue towards a unified theoretical framework of nanofriction. In this respect, we believe that the key aspect is to identify a novel mathematical formulation of friction based on its energetic aspects, i.e. energy dissipation, rather than its dynamical effects, i.e. hindering the relative motion of interacting surfaces. Ultimately, such avenue might lead to a way to predict the value of the friction coefficient of two surfaces in contact from the sole knowledge of the atom types and their arrangement, without the need to measure it in operative conditions: one of the biggest challenges in the field of nanotribology.

Semiconductor interfaces are at the heart of the functionality of many devices for opto-electronic applications. At these interfaces, the importance of ultrafast dynamics – processes that occur on sub-nanosecond timescales – has been long understood. While these ultrafast spectroscopic studies have revealed important information, there remains a rich array of physics that is hidden within sub-micrometer length scales when using spatially-averaged techniques. However, powerful tools that could access material dynamics in semiconductors simultaneously at ultrafast time- and sub-micrometer length scales are challenging to implement. Here, we review recent developments in time-resolved photoemission electron microscopy as a technique to study ultrafast electron dynamics at semiconductor interfaces at the nanoscale. In particular, we review recent work in traditional semiconductor interfaces and heterojunctions, low-dimensional materials, and semiconductors for photovoltaic applications.

Light-control of structural dynamics at surfaces promises switching of chemical and physical functionality at rates limited only by the velocity of directed atomic motion. Following optical stimulus by femtosecond light pulses (1 fs = 10^-15 s), transient electronic and lattice excitations can drive phase transitions in solids. Coherent control schemes facilitate a selective transfer of optical energy to specific electronic or vibrational degrees of freedom, as exemplified by the steering of molecular reactions via optical pulse sequences in femtochemistry. However, a transfer of this concept from molecules to solids requires coupling of few decisive phonons to optical transitions in the electronic band structure, and a weak coupling to other lattice modes to maximize coherence times. In this respect, atomic indium wires on the (111) surface of silicon represent a highly attractive model system, with a Peierls-like phase transition between insulating (8×2) and metallic (4×1) structures, governed by shear and rotation phonons. This review provides a survey on our advances in the time-resolved probing and coherent vibrational control of the In/Si(111) surface. In particular, we discuss how coherent atomic motion can be harnessed to affect the efficiency and threshold of the phase transition. Starting from a description of the (8×2) and (4×1) equilibrium structures and key vibrational modes, we study the structural dynamics following single-pulse optical excitation of the (8×2) phase. Our results highlight the ballistic order-parameter motion in the nonequilibrium transition as well as the impact of microscopic heterogeneity on the excitation and subsequent relaxation of the metastable photo-induced (4×1) phase. Furthermore, we discuss our results on the combination of ultrafast low-energy electron diffraction (ULEED) with optical pulse sequences to investigate the coherent control over the transition, mode-selective excitation and the location of the transition state.

The study of processes concerning adsorption, diffusion and reaction of atoms and molecules on surfaces is one of the core areas of surface science research. Resolving these dynamic processes with atomic resolution in real space and at real time is of great significance for the understanding of catalytic reaction mechanism and the development of new materials. A scanning tunneling microscope with fast imaging function, a so-called “high-speed scanning tunneling microscope” combining both high temporal and high spatial resolution, is an ideal instrument to characterize processes within this area. This review aims to highlight some recent developments of high-speed scanning tunneling microscope technique and its application to study the structural dynamics on surfaces. Firstly, factors that limit the time resolution of scanning tunneling microscope are analyzed from the aspects of both hardware and software. Secondly, strategies and instrument designs enabling imaging rate up to 100 frames per second are introduced. Then, recent breakthroughs on resolving surface structural dynamics, such as atom diffusion, on-surface synthesis of low-dimensional materials and chemical reaction, by high-speed scanning tunneling microscope are highlighted. Finally, the challenges and opportunities of high-speed scanning tunneling microscope technique are outlined and a perspective is provided.

Many fundamental processes in nature occur on ultrashort time scales within picoseconds to attoseconds, and on intrinsic length scales from nanometers to picometers. The structure of crystalline solids is dictated by long range order and the periodic arrangement of atoms, but the elementary excitations that define its interaction with the environment may vary locally at the atomic scale. Multiple domains and phases can coexist on length scales down to a few nanometer, and impurities and defects can influence the collective many-body response of solids at the single-atom level. Ultrafast pump–probe techniques provide valuable information about fundamental many-body interactions in solids and at surfaces, but spatially average over macroscopic spot sizes such that the influence of local order or disorder at angstrom scales is not directly accessible. Therefore, real-space observation of ultrafast dynamics with atomic spatial resolution is highly desirable, and motivates the development of time-resolved ultrafast scanning tunneling microscopy (USTM) since the early 1990’s. Tremendous progress has been made in this field in the past decade, and a number of breakthrough achievements have significantly advanced our possibilities to add ultrafast time resolution to the angstrom spatial resolution of STM. This article reviews new technical approaches and developments in the field of USTM. A particular focus will be the classification of light-matter interaction in tunnel junctions, based on the criteria for adiabatic tunneling from Keldysh's theory of strong-field ionization and a tunneling time as defined by Büttiker and Landauer, and on Tucker's definition of quantum detection in a tunnel junction mixer. Moreover, various mechanisms to generate an ultrafast tunneling current in USTM are discussed and are to some extent related to those from other techniques such as optical spectroscopy or photoemission spectroscopy. The resulting new possibilities for imaging the ultrafast dynamics of electronic and vibrational excitations at surfaces with USTM will be highlighted. Finally, the article outlines possible future directions of USTM for studying ultrafast processes and light-induced phenomena at surfaces and in quantum materials.