Concepts,instruments, and model systems that enabled the rapid evolution of surface science

Over the past forty years, surface science has evolved to become both an atomic scale and a molecular scale science. Gerhard Ertl’s group has made major contributions in the field of molecular scale surface science, focusing on vacuum studies of adsorption chemistry on single crystal surfaces. In this review, we outline three important aspects which have led to recent advances in surface chemistry: the development of new concepts, in situ instruments for molecular scale surface studies at buried interfaces (solid–gas and solid–liquid), and new model nanoparticle surface systems, in addition to single crystals. Combined molecular beam surface scattering and low energy electron diffraction (LEED)- surface structure studies on metal single crystal surfaces revealed concepts, including adsorbate-induced surface restructuring and the unique activity of defects, atomic steps, and kinks on metal surfaces. We have combined high pressure catalytic reaction studies with ultra high vacuum (UHV) surface characterization techniques using a UHV chamber equipped with a high pressure reaction cell. New instruments, such as high pressure sum frequency generation (SFG) vibrational spectroscopy and scanning tunneling microscopy (STM) which permit molecular-level surface studies have been developed. Tools that access broad ranges of pressures can be used for both the in situ characterization of solid–gas and solid–liquid buried interfaces and the study of catalytic reaction intermediates. The model systems for the study of molecular surface chemistry have evolved from single crystals to nanoparticles in the 1–10 nm size range, which are currently the preferred media in catalytic reaction studies.     

Surface science of complex environmental interfaces: Oxide and carbonate surfaces in dynamic equilibrium with water vapor

Surface scientists are dealing more and more with complex systems that are challenging to investigate from both experimental and theoretical perspectives. The surface science of complex interfaces, such as environmental interfaces under ambient conditions of temperature and relative humidity, requires both advances in experimental and theoretical methods in order for conceptual insights to emerge. In this prospective, several aspects of environmental interfaces and the field of environmental surface science are discussed. These include: (i) adsorbed water on oxide and carbonate interfaces; (ii) surface chemistry of oxide and carbonate interfaces in the presence of co-adsorbed water; (iii) solvation of ions by co-adsorbed water on environmental interfaces; and (iv) research needs and challenges in environmental surface science.     

Field ion and field desorption mass spectrometry of inorganic compounds

The review on recent developments in field ionization mass spectrometry of inorganic compounds concerns the different mechanisms of ion formation at surfaces and under the influence of extremely high electric fields, field dependent chemical reactivity at surfaces and ion desorption from surfaces for mass spectrometric analysis. Applications in various areas are discussed, where this method has been used to identify surface compounds or to study kinetic phenomena at interfaces.     

The interaction of water with solid surfaces: fundamental aspects revisited

Water is perhaps the most important and most pervasive chemical on our planet. The influence of water permeates virtually all areas of biochemical, chemical and physical importance, and is especially evident in phenomena occurring at the interfaces of solid surfaces. Since 1987, when Thiel and Madey (TM) published their review titled ‘The interaction of water with solid surfaces: fundamental aspects’ in Surface Science Reports, there has been considerable progress made in further understanding the fundamental interactions of water with solid surfaces. In the decade and a half, the increased capability of surface scientists to probe at the molecular-level has resulted in more detailed information of the properties of water on progressively more complicated materials and under more stringent conditions. This progress in understanding the properties of water on solid surfaces is evident both in areas for which surface science methodology has traditionally been strong (catalysis and electronic materials) and also in new areas not traditionally studied by surface scientists such as electrochemistry, photoconversion, mineralogy, adhesion, sensors, atmospheric chemistry and tribology. Researchers in all these fields grapple with very basic questions regarding the interactions of water with solid surfaces such as how is water adsorbed, what are the chemical and electrostatic forces that constitute the adsorbed layer, how is water thermally or non-thermally activated and how do coadsorbates influence these properties of water. The attention paid to these and other fundamental questions in the past decade and a half has been immense. In this review, experimental studies published since the TM review are assimilated with those covered by TM to provide a current picture of the fundamental interactions of water with solid surfaces.     

Density functional theory simulations of water–metal interfaces: waltzing waters, a novel 2D ice phase, and more

There are few molecules, if any, more important than water. Yet remarkably little is known about how it interacts with solid surfaces, particularly at the all important atomic level. This is true despite widespread general interest and compelling environmental and economic incentives. Here, I will discuss detailed density-functional theory studies aimed at putting our understanding of water–solid interfaces, specifically water–metal interfaces, on a much firmer footing. In this paper, I will attempt to answer some key questions: Where do isolated water monomers adsorb on flat metal surfaces? How do water monomers diffuse across metal surfaces? How do water dimers adsorb and diffuse across metal surfaces? What factors control the structure and stability of water bilayers on metal surfaces?     

Current investigations in theoretical studies of nanostructure–liquid interfaces

Wettability of surfaces is a significant factor affecting properties like water dispersion, spreading, evaporation, dissociation and etc. Surface wettability and wetting behavior of a surface are a subject of broad interest, there is then a great interest to understand better liquid–solid interfaces and water contact angle, in addition to the potential applications in micro- and nano fluidics. This subject is interesting as the growing attractions on the wetting and dynamical properties of water on 2D materials. Also, two clearly defined rigid water layers on solid surfaces are a well-known phenomenon and have been described on several surfaces. Detailed molecular dynamic simulation studies on the origin of this phenomenon are also of general interest. In this current review, recent attempts concerning to the wettability of graphene, graphene oxide and also some metal surfaces obtained by theoretical are presented. Their result contents, therefore, is of interest in order to understand the behavior of water nano-droplets when physisorbed on different substrates. The information is relevant for experimental teams working in this subject, with application in areas as catalysis, friction, surface chemistry, adsorption, etc.     

Recent advances in imaging of properties and growth of low dimensional structures for photonics and electronics by XPEEM

Spectroscopic Photoemission and Low Energy Electron Microscopy (SPELEEM) is a very powerful and diverse microscopy technique for the investigation of surfaces, interfaces, buried layers and nanoscale objects like nanoparticles and nanowires. The many significant results from photoemission Electron microscopy (PEEM) in recent years are linked with the exploitation of advanced light sources such as synchrotrons and new advanced laser systems. Combined also with low energy electron microscopy (LEEM) it allows a complementary chemical and structural analysis making LEEM/PEEM a versatile multitechnique instrument. To illustrate the extreme diversity, we give a summary of the recent studies with the SPELEEM installed at the soft X-ray beamline I311 at the MAXII synchrotron storage ring and a portable electrostatic PEEM used with ultra-fast XUV laser technology. The examples cover topics such as full-cone 3D band mapping by using the photoelectron diffraction mode of the microscope, growth mechanism and detailed doping profile of III–V nanowires, growth and intercalation of graphene on SiC substrates, droplet dynamics on GaP(1 1 1) surface, surface chemistry and control of nanostructure fabrication. Moreover, the first results of PEEM experiments using extreme ultraviolet attosecond pulse trains are discussed.     

Interactions of incident H atoms with metal surfaces

Atomic hydrogen is a highly reactive species of interest because of its role in a wide range of applications and technologies. Knowledge about the interactions of incident H atoms on metal surfaces is important for our understanding of many processes such as those occurring in plasma-enhanced catalysis and nuclear fusion in tokamak reactors. Herein we review some of the numerous experimental surface science studies that have focused on the interactions of H atoms that are incident on low-Miller index metal single-crystal surfaces. We briefly summarize the different incident H atom reaction mechanisms and several of the available methods to create H atoms in UHV environments before addressing the key thermodynamic and kinetic data available on metal and modified metal surfaces. Generally, H atoms are very reactive and exhibit high sticking coefficients even on metals where H₂ molecules do not dissociate under UHV conditions. This reactivity is often reduced by adsorbates on the surface, which also create new reaction pathways. Abstraction of surface-bound D(H) adatoms by incident H(D) atoms often occurs by an Eley-Rideal mechanism, while a hot atom mechanism produces structural effects in the abstraction rates and forms homonuclear products. Additionally, incident H atoms can often induce surface reconstructions and populate subsurface and bulk absorption sites. The absorbed H atoms recombine to desorb H₂ at lower temperature and can also exhibit higher subsequent reactivity with adsorbates than surface-bound H adatoms. Incident H atoms, either directly or via sorbed hydrogen species, hydrogenate adsorbed hydrocarbons, sulfur, alkali metals, oxygen, halogens, and other adatoms and small molecules. Thus, H atoms from the gas phase incident on surfaces and adsorbed layers create new reaction channels and products beyond those found from interactions of H₂ molecules. Detailed aspects of the dynamics and energy transfer associated with these interactions and the important applications of hydrogen in plasma processing of semiconductors are beyond the scope of this review.     

Surface chemistry at the liquid/solid interface

In this Prospective, a critical overview is provided on the status and future of the analytical techniques available for the study of chemistry at liquid/solid interfaces. A number of spectroscopies already available are identified, including infrared absorption, surface-enhanced Raman (SERS) and sum frequency generation (SFG) to obtain vibrational information, and second harmonic generation (SHG) and X-ray absorption (XAS) to provide electronic details of surfaces and adsorbates. X-ray scattering and X-ray diffraction techniques are also used for structural characterization, and surface plasmon resonance (SPR) and quartz crystal microbalance (QCM) to follow adsorption uptakes and kinetics. Finally, optical and scanning microscopies add a spatial dimension to these studies. Overall, a number of surface-sensitive techniques do already exist to address chemical issues at liquid/solid interfaces, but those are still limited, and have perhaps not been exploited to their fullest yet. There is also a need for more cross collaboration among different research communities, and for new and clever developments to augment the toolbox of liquid/solid interface characterization.     

Chemical and physical interactions at metal/self-assembled organic monolayer interfaces

The purpose of research on metals (M) deposited onto self-assembled monolayers (SAMs) is to understand the interactions between metal (M) and eventually metal oxide overlayers on well-ordered organic substrates. Application of M/SAM and inorganic/SAM research results to the understanding of real inorganic/ organic interfaces in vacuum and under environmental conditions can potentially play a key role in the development of advanced devices with stable interfacial properties. The M/SAM approach to interface research is delineated as a new subfield in surface science in the context of other approaches to inorganic/organic interface research. Current issues in M/SAM research are outlined, including chemical compound formation, the morphology (spreading, clustering, or penetration) of the metal species, the kinetics of the metal morphology, the effect of the metal on the degree of order in the SAM, and the rate of metal penetration into the SAM. Probes are recommended that are suitable for M/SAM research. The results of M/SAM studies to date are reviewed, and M/SAM combinations are ranked according to reactivity and penetration. Key probes for addressing gaps in the research results are identified. The effects of defects, disordering, air exposure, and X-ray and electron beam exposure on the experimental results to date are evaluated. Thus far, the results have successfully revealed qualitative relationships of M/SAM chemistry, temperature, and penetration. The chemical interactions that have been found are applicable to real M/polymer interfaces as formed in vacuum. It has yet to be shown that M/SAM research will yield quantitative understanding of interface formation or that M/SAM interfaces are entirely analogous to M/polymer interfaces in the details of interface formation. The future of this subfield of surface science lies in its expansion from M/SAM interfaces in vacuum to other inorganic/SAM interfaces in vacuum and, eventually, under environmental conditions.     

Biomedical surface science: Foundations to frontiers

Surfaces play a vial role in biology and medicine with most biological reactions occurring at surfaces and interfaces. The foundations, evolution, and impact of biomedical surface science are discussed. In the 19th century, the first observations were made that surfaces control biological reactions. The advancements in surface science instrumentation that have occurred in the past quarter of a century have significantly increased our ability to characterize the surface composition and molecular structure of biomaterials. Similar advancements have occurred in material science and molecular biology. The combination of these advances have allowed the development of the biological model for surface science, where the ultimate goal is to gain a detailed understanding of how the surface properties of a material control the biological reactivity of a cell interacting with that surface. Numerous examples show that the surface properties of a material are directly related to in vitro biological performance such as protein adsorption and cell growth. The challenge is to fully develop the biological model for surface science in the highly complex and interactive in vivo biological environment. Examples of state-of-the-art biomedical surface science studies on surface chemical state imaging, molecular recognition surfaces, adsorbed protein films, and hydrated surfaces are presented. Future directions and opportunities for surface scientists working in biomedical research include exploiting biological knowledge, biomimetics, precision immobilization, self-assembly, nanofabrication, smart surfaces, and control of non-specific reactions.     

Optically and thermally induced molecular switching processes at metal surfaces

Using light to control the switching of functional properties of surface-bound species is an attractive strategy for the development of new technologies with possible applications in molecular electronics and functional surfaces and interfaces. Molecular switches are promising systems for such a route, since they possess the ability to undergo reversible changes between different molecular states and accordingly molecular properties by excitation with light or other external stimuli. In this review, recent experiments on photo- and thermally induced molecular switching processes at noble metal surfaces utilizing two-photon photoemission and surface vibrational spectroscopies are reported. The investigated molecular switches can either undergo a trans-cis?isomerization or a ring opening-closure reaction. Two approaches concerning the connection of the switches to the surface are applied: physisorbed switches, i.e.?molecules in direct contact with the substrate, and surface-decoupled switches incorporated in self-assembled monolayers. Elementary processes in molecular switches at surfaces, such as excitation mechanisms in photoisomerization and kinetic parameters for thermally driven reactions, which are essential for a microscopic understanding of molecular switching at surfaces, are presented. This in turn is needed for designing an appropriate adsorbate-substrate system with the desired switchable functionality controlled by external stimuli.     

Fluorescent Probe Studies of Polarity and Solvation within Room Temperature Ionic Liquids: A Review

Ionic liquids display an array of useful and sometimes unconventional, solvent features and have attracted considerable interest in the field of green chemistry for the potential they hold to significantly reduce environmental emissions. Some of these points have a bearing on the chemical reactivity of these systems and have also generated interest in the physical and theoretical aspects of solvation in ionic liquids. This review presents an introduction to the field of ionic liquids, followed by discussion of investigations into the solvation properties of neat ionic liquids or mixed systems including ionic liquids as a major or minor component. The ionic liquid based multicomponent systems discussed are composed of other solvents, other ionic liquids, carbon dioxide, surfactants or surfactant solutions. Although we clearly focus on fluorescence spectroscopy as a tool to illuminate ionic liquid systems, the issues discussed herein are of general relevance to discussions of polarity and solvent effects in ionic liquids. Transient solvation measurements carried out by means of time-resolved fluorescence measurements are particularly powerful for their ability to parameterize the kinetics of the solvation process in ionic liquids and are discussed as well.     

Solvent effects on catalytic reactions and related phenomena at liquid-solid interfaces

Catalytic reactions involve the direct interaction of reactants, intermediates and products with the catalyst surface. We not only need to control the atomic structure and electronic properties of the active site, but also explore the multiple molecular interactions that occur beyond the active site; they play an essential role in altering the binding and reactivity of surface species. In liquid-phase catalysis, solvents provide additional degrees of freedom in the design of the catalytic process for desirable activity and selectivity. The multi-faceted effects of solvents have a profound impact on the catalyst performance by restricting the mass transfer to the site, tuning the chemical potential of the surface species, competing for active sites, stabilizing the initial and transition states, and causing mechanistic changes by participating in the kinetically relevant elementary steps. This review addresses the different aspects of solvent effects, using a few prototype solid-liquid interfaces to illustrate these fundamental features. Recent experimental and computational studies that provide new insight at the molecular level are examined. Solvent structures in the proximity of the catalyst surface are discussed along with their influence in molecular binding and reaction at the solid-liquid interfaces. Furthermore, opportunities to alter such a solid-liquid interaction by tuning the wettability of the catalyst surfaces are explored.     

The surface science of semiconductor processing: gate oxides in the ever-shrinking transistor

Due to the extreme dimensional scaling required by Moore's law, Si device technology is increasingly subject to the limitations imposed by the intrinsic physics and chemistry of surfaces and interfaces. In this review we outline ways in which fundamental surface science has contributed an understanding to the microelectronics community and discuss areas where surface science may impact future development. We focus on the example of silicon dioxide (SiO₂) on silicon, since this interface lies at the heart of modern transistor technology and has therefore received a great deal of attention in recent years. We highlight a number of experimental and theoretical approaches that have elucidated the fundamental phenomena associated with the formation and evolution of this critical technological interface, revealing the remarkable interdependence of science and technology that now characterizes this rapidly evolving industry.     

Synchrotron X‐Ray scattering of polymer nanocomposites

Because of the importance of probing molecular-scale chemical and physical structure of environmental samples in their natural and often hydrated state, synchrotron radiation has been a powerful tool for environmental scientists for decades. Thus, the crucial role that a highly coherent and high-brightness hard X-ray source such as the Advance Photon Source (APS) can play in addressing many of the outstanding questions in molecular environmental science (MES) was recognized even before “first light” at the facility. No single synchrotron-based technique or experimental approach can adequately address the tremendous temporal and spatial heterogeneities of the chemistry, physics, and biology of natural environmental samples. Thus, it is common at the APS that multiple X-ray techniques and experimental systems are employed to investigate environmental samples, often chosen for their ability to focus on solute species, plants, microbes, organics, interfacial species, or solids.     

Electronic,structural and chemical effects of charge-transfer at organic/inorganic interfaces

During the last decade, interest on the growth and self-assembly of organic molecular species on solid surfaces spread over the scientific community, largely motivated by the promise of cheap, flexible and tunable organic electronic and optoelectronic devices. These efforts lead to important advances in our understanding of the nature and strength of the non-bonding intermolecular interactions that control the assembly of the organic building blocks on solid surfaces, which have been recently reviewed in a number of excellent papers. To a large extent, such studies were possible because of a smart choice of model substrate-adsorbate systems where the molecule-substrate interactions were purposefully kept low, so that most of the observed supramolecular structures could be understood simply by considering intermolecular interactions, keeping the role of the surface always relatively small (although not completely negligible). On the other hand, the systems which are more relevant for the development of organic electronic devices include molecular species which are electron donors, acceptors or blends of donors and acceptors. Adsorption of such organic species on solid surfaces is bound to be accompanied by charge-transfer processes between the substrate and the adsorbates, and the physical and chemical properties of the molecules cannot be expected any longer to be the same as in solution phase. In recent years, a number of groups around the world have started tackling the problem of the adsorption, self- assembly and electronic and chemical properties of organic species which interact rather strongly with the surface, and for which charge-transfer must be considered. The picture that is emerging shows that charge transfer can lead to a plethora of new phenomena, from the development of delocalized band-like electron states at molecular overlayers, to the existence of new substrate-mediated intermolecular interactions or the strong modification of the chemical reactivity of the adsorbates. The aim of this review is to start drawing general conclusions and developing new concepts which will help the scientific community to proceed more efficiently towards the understanding of organic/inorganic interfaces in the strong interaction limit, where charge-transfer effects must be taken into consideration.     

Small,but Perfectly Formed: The Microstructure of Nanocrystalline Oxides

There is considerable interest in nanocrystalline materials due to their unusual properties that offer the possibility of exciting technological applications. This paper concentrates on the microstructure of nanocrystalline binary oxides as revealed by X-ray absorption studies. It will be shown that these experiments yield a picture of the materials in which, even when the particles are only a few nanometres in size, the crystallites are highly ordered and the interfaces are similar to grain boundaries in normal bulk solids. This is in conflict with earlier ideas where it was often assumed the surfaces of nanocrystals and the interfaces between them were very disordered.     

Organic functionalization of group IV semiconductor surfaces: principles, examples, applications, and prospects

Organic functionalization is emerging as an important area in the development of new semiconductor-based materials and devices. Direct, covalent attachment of organic layers to a semiconductor interface provides for the incorporation of many new properties, including lubrication, optical response, chemical sensing, or biocompatibility. Methods by which to incorporate organic functionality to the surfaces of semiconductors have seen immense progress in recent years, and in this article several of these approaches are reviewed. Examples are included from both dry and wet processing environments. The focus of the article is on attachment strategies that demonstrate the molecular nature of the semiconductor surface. In many cases, the surfaces mimic the reactivity of their molecular carbon or organosilane counterparts, and examples of functionalization reactions are described in which direct analogies to textbook organic and inorganic chemistry can be applied. This article addresses the expected impact of these functionalization strategies on emerging technologies in nanotechnology, sensing, and bioengineering.