Progress in Surface Science最新文献

One of the most universal physical processes shared by all matter at finite temperature is the emission of thermal radiation. The experimental characterization and theoretical description of far-field black-body radiation was a cornerstone in the development of modern physics with the groundbreaking contributions from Gustav Kirchhoff and Max Planck. With its origin in thermally driven fluctuations of the charge carriers, thermal radiation reflects the resonant and non-resonant dielectric properties of media, which is the basis for far-field thermal emission spectroscopy. However, associated with the underlying fluctuating optical source polarization are fundamentally distinct spectral, spatial, resonant, and coherence properties of the evanescent thermal near-field. These properties have been recently predicted theoretically and characterized experimentally for systems with thermally excited molecular, surface plasmon polariton (SPP), and surface phonon polariton (SPhP) resonances.

We review, starting with the early historical developments, the emergence of theoretical models, and the description of the thermal near-field based on the fluctuation–dissipation theory and in terms of the electromagnetic local density of states (EM-LDOS). We discuss the optical and spectroscopic characterization of distance dependence, magnitude, spectral distribution, and coherence of evanescent thermal fields. Scattering scanning near-field microscopy proved instrumental as an enabling technique for the investigations of several of these fundamental thermal near-field properties. We then discuss the role of thermal fields in nano-scale heat transfer and optical forces, and the correlation to the van der Waals, Casimir, and Casimir–Polder forces. We conclude with an outlook on the possibility of intrinsic and extrinsic resonant manipulation of optical forces, control of nano-scale radiative heat transfer with optical antennas and metamaterials, and the use of thermal infrared near-field spectroscopy (TINS) for broadband chemical nano-spectroscopic imaging, where the thermally driven vibrational optical dipoles provide their own intrinsic light source.

Molecular hydrogen exists in nuclear-spin isomers of ortho and para species according to the total nuclear spin. These species are correlated to the rotational states with even and odd rotational quantum numbers because of the symmetry of the total wavefunction with respect to the permutation of the two nuclei. Although interconversion between the ortho and para states is extremely slow in an isolated state, the conversion is promoted in a physisorption state via interaction with surfaces of not only magnetic but also diamagnetic materials. In a physisorption state, the rotational motion of hydrogen molecules is modified due to the potential anisotropy. The physisorption properties and interconversion rate of the ortho and para hydrogen have recently been investigated on well-defined surfaces, which allow detailed comparison with theory. Furthermore, relative abundance of the ortho and para hydrogen in astronomical circumstances has been reported in recent years, which often shows a value out of equilibrium with the environment temperature. Physisorption and ortho–para conversion on the surfaces of interstellar media are expected to enable deeper understanding of astronomical phenomena. In this article, we review recent progress of experimental and theoretical studies on the physisorption and ortho–para conversion of molecular hydrogen and its relevance to the recent astronomical observation.

The control over biointerfacial interactions is the key to a broad range of biomedical applications, ranging from implantable devices to drug delivery and nanomedicine. In many of these applications, coatings are required that reduce or prevent non-specific interactions with the biological environment, while at the same time presenting specific bioactive signals. Whilst surface coatings based on polymers such as poly(ethylene glycol) (PEG) have been used successfully, many limitations persist in regard to the biocompatibility, stability and functionality of state-of-the-art polymer coatings. Most of these limitations are related to the fact that, typically, linear polymers are used with associated limited chemical functionality. Here, we examine the development of hyperbranched polyglycerols (HPGs) as promising candidates for the replacement of traditional linear polymers, such as the chemically analogous PEG, for the control of biointerfacial interactions. HPGs are highly branched globular molecules that exhibit a high valency, allow easy access to a variety of functionalities and can present biologically active signals. In this review, a comprehensive overview is provided with respect to the history, synthetic strategies, modifications and applications of HPGs.

The guiding of charged particles through microscopic and, more recently, also macroscopic capillaries is a remarkable effect discovered in 2002 by Stolterfoht and coworkers. After an initial charge-up phase, a beam of charged particles entering an insulating capillary can be effectively steered along the tilted capillary axis. The effect results from self-organized charge-up of the capillary wall which subsequently deflects ions electrostatically thereby inhibiting close collisions with the capillary walls. Indeed, in the case of multiply charged projectile ions the projectiles transmitted through the capillary keep their initial charge state indicating that the ions never touched the inner walls. We will review both the existing experimental data as well as theoretical models for this phenomenon and similar guiding processes for energetic charged particles collected over the past 10 years.

Graphene, a single atomic layer of sp²-hybridized carbon atoms arranged in a hexagonal structure and the Nobel winning material in 2010, has attracted extensive research attention in the last few years due to its outstanding physical, chemical, electrical, optical and mechanical properties. To further extend its potential applications, intensive research efforts have been devoted to the functionalization of graphene. Examples include improving graphene solubility by attaching different chemical functional groups to its basal plane, modulating the charge carrier type and concentration via surface transfer doping by coating it with various metals films or organic molecules, improving the bio-selectivity by decorating it with different π-conjugated organic molecules, and so on. Different methods have been developed to functionalize graphene. Among them, non-covalent molecular functionalization represents one of the most effective and promising methods. The extended π-conjugation is largely preserved without creating extensive structural defects on the graphene sheet, thereby retaining the high charge carrier mobility. In this review, a brief summary about different functionalization methods of graphene and its derivatives by covalent and non-covalent interactions will be presented, with particular focus on the non-covalent molecular functionalization. A broad review of the applications of non-covalently functionalized graphene and its derivatives will be presented in detail, including field-effect-transistors, organic optoelectronics, and molecular sensing.

Sn whiskers are thin filaments that grow spontaneously out of the surface of coatings on Cu and have become a critical reliability problem in Pb-free electronics. In this review, we focus on what creates the driving force for whiskers (or more rounded “hillocks”), and what determines where on the surface they will form. Experimental studies are reviewed that quantify the relationship between the Cu–Sn intermetallic (IMC) formation, stress in the layer and whisker/hillock density. Measurements of the mechanical properties show how stress relaxation in the Sn layer is intimately related to how much stress develops due to the IMC formation. Real-time scanning electron microscope (SEM)/focused ion beam (FIB) studies are described that illustrate the whisker/hillock growth process in detail. Whiskers are found to grow out of a single grain on the surface with little lateral growth while hillock growth is accompanied by extensive grain growth and crystallite rotation. Electron-backscattering detection (EBSD) shows the grain structure around where the whiskers/hillocks form, indicating that whiskers can grow out of pre-existing grains and do not require the nucleation of new grains. This has led to a picture in which stress builds up due to IMC growth and causes whiskers/hillocks to form at “weak grains”, i.e., grains that have a stress relaxation mechanism that becomes active at a lower stress than its neighbors. FEA (finite element analysis) calculations are used to simulate the evolving stress and whisker growth for several different mechanisms that may lead to “weak” grains.

The interaction of alkali atoms with metal surfaces is reviewed. The peculiar electronic configuration of such atoms, with only one valence electron participating in the bond formation, suggested simple pictures to describe their interaction with a metal surface. But it was early evident that the adsorption properties depend on many aspects, related to the electronic structure of constituents, leading, for example, to different degrees of ionicity/covalency of the alkali atom-metal bond. Sophisticated theoretical modeling tried to shed light on this aspect. The spectral properties are the ultimate features in determining how the systems interact with each other. In this review the electronic and spectral properties are discussed focusing on different theoretical representations of the physical system and on their consequences. Surface projected energy gaps of the substrate as well as the substrate continuous spectrum are key aspects in determining the nature of the interaction and bonding with alkali adsorbates.

Micro- and Nano-Electro-Mechanical Systems (MEMS and NEMS) represent existing (MEMS) and emerging (NEMS) technologies based on microfabrication of micron to nanometer scale miniature mechanical components (gears, latches, mirrors, etc.) that are integrated with electrical elements to allow for electro-mechanical actuation and/or capacitive displacement detection. One common aspect of MEMS and NEMS devices is that they have mechanical functionality that may include moveable parts whose motion is controlled by external electrical connections. Current fabrication methods, along with high surface to volume ratios, make MEMS and NEMS devices highly susceptible to surface forces and adsorbed surface species, to the point where the devices are now being increasingly utilized as sensitive probes in fundamental surface science studies. This sensitivity can potentially be used to great advantage if the devices can be made to operate reproducibly in well controlled environments. This review highlights a number of such recent studies, beginning with an overview of the fabrication processes employed for silicon, metal, diamond, graphene and carbon nanotube – based device technologies. A discussion of how traditional surface science studies on passive two-dimensional substrates compare to and contrast with studies performed on, or by, MEMS and/or NEMS devices, is also included. The overall goal is to highlight areas of current opportunity for surface scientists in the flourishing arena of micro- and nano-device fabrication and technology.

Diffusion of atomic hydrogen on silicon serves as a model system for the investigation of thermally activated diffusion processes of covalently bound adsorbates on semiconductor surfaces. Over the past two decades, a detailed understanding of the hopping mechanisms for H/Si(0 0 1) and H/Si(1 1 1) has been obtained using a variety of experimental and theoretical methods. Hydrogen diffusion on silicon is in general characterized by energy barriers that are substantially larger than for adsorbate diffusion on metal surfaces, by the occurrence of different pathways on one surface, as well as by a strong participation of the underlying lattice in the hopping process.

In the case of the flat Si(0 0 1) surface, three diffusion pathways were identified: site exchange within one Si dimer, hopping along dimer rows, and hopping across dimer rows, with barriers of 1.4, 1.7 and 2.4 eV, respectively. These barriers correlate with the distances of the involved adsorption sites of 2.4, 3.8 and 5.2 Å. While hydrogen diffusion on Si(0 0 1) is strongly anisotropic at surface temperatures below 700 K, the measurement of high hopping rates by means of a combination of pulsed laser heating and scanning tunneling microscopy reveals similar jump frequencies around 10⁸ s⁻¹ at 1400 K. Diffusion across steps is found to occur with similar speed as diffusion along dimer rows.

Hydrogen diffusion on Si(1 1 1) 7 × 7 involves 4.4-Å-long jumps between restatom and adatom sites, accompanied by strong distortions of the adatom backbonds. Crossing the unit-cell boundaries via a 6.7-Å-long migration pathway between two adatoms is the rate limiting process for diffusion on macroscopic length scales, which has an activation energy of 1.5 eV.