Goals and Methods of Surface Analysis
The ultimate goal of surface analysis is to determine the structure and properties of an arbitrary surface with resolution at the level of atomic/molecular layers (or even down to individual atoms/molecules). In practice, surfaces are often disordered at the atomic level, so the structural information has to be interpreted in terms of some averaged and simplified model, which may not represent any specific structure on a real surface. Similarly, the sheer variety of surface properties that may be of interest (biological, chemical, electronic, optical, magnetic, etc.) requires one to use multiple techniques to study the same surface in detail.
A more realistic goal of surface analysis then is to identify all the atoms and molecules that are present on a surface at concentration above about 0.1–1% and to gain a general understanding of how these atoms are arranged. For example, one may be interested to find out not only that both silicon and oxygen are present on the surface, but also whether they are chemically bonded to each other (as silicon oxide) or to other surface elements. Another type of a structural question involves asking if the topmost atomic layer is dominated by one element—important, for example, when designing surfaces having a specific type of chemical reactivity, or a property such as wettability.
Surface Properties of Nanoscale Objects
Surfaces and surface properties are important across a wide range of scales, because surfaces largely determine how objects interact with each other and with their environment. For example, such effects as friction and corrosion—which need to be controlled in something as large as a train engine and as small as an edge of a razor blade—are related to surface properties.
For nanoscale objects such as nanostructures and biomolecules, surface properties are even more important, both because nanoscale objects are essentially "all surface" (i.e., there is much less material "inside" a nanostructure or molecule than on its surface) and because surface forces are typically the strongest forces that act on nanoscale objects. For example, a spherical nanoparticle is not going to roll down an inclined surface or fall off an inverted surface, because the downward pull of gravity on an object that small is much weaker than the physical or chemical forces holding it in place.
Most surface properties are determined by atoms located within the top few atomic layers, i.e., within a few nanometers from the surface. Accordingly, it is very important to analyze these top few atomic layers. Historically, surface analysis methods have been first developed to study atomically-clean surfaces of pure materials (single-crystal metals or semiconductors) under ultra-high vacuum (i.e., vacuum comparable to that in interplanetary space) conditions. The draconian requirements on surface cleanliness and quality in such work are meant to ensure that many samples with essentially identical surface properties can be prepared. Models of the ideal surface structures for many materials can be found in the SSD Gallery.
SPM: Touching the Surface Atoms
Scanning Probe Microscopy (SPM) tools offer the possibility to image and manipulate individual atoms and molecules on surfaces. Scanning Tunneling Microscopy (STM) was the first SPM technique to produce images of surface structures with atomic resolution. A complex and particularly stable atomic arrangement on a silicon surface—Si(111)7×7 reconstruction—was one of the first STM images that captured the imagination of surface scientists and sparked a wide interest in using the STM technique.
The ability of SPM techniques to obtain atomic-resolution images of generic surfaces is limited because an atomically-clean surface is typically required for such imaging. In addition, the high-resolution scanning process is rather slow and thus only a very small fraction of any realistic surface can be sampled using SPM tools. Finally, while some variations of SPM allow to identify the chemical nature of atoms and molecules that are imaged, in general such chemical assignments remain ambiguous and require independent confirmation using other experimental and theoretical methods.
Spectroscopy: Counting the Surface Atoms
The topmost atomic layer of a typical solid surface contains around 1015 atoms per cm2—a number both too large and too small. The number of atoms is too large to count or otherwise analyze them all individually, e.g., using an STM. On the other hand, the number is much smaller than, for example, a typical number of atoms (1017–1023) involved in chemical reactions in solutions, which means that surfaces present a challenge for even the most sensitive traditional analytical techniques.
In a typical surface analysis experiment, a "probe" beam of photons or charged particles (electrons, ions, clusters of atoms) is directed at a surface. The "probe" beam interacts with surface atoms and this interaction either modifies the incoming particles (by absorption, diffraction, reflection, scattering, attenuation, etc.) or extracts particles (electrons, ions, photons, clusters of atoms) from the surface. The resulting outgoing beams or particles are collected and analyzed by energy, atomic/molecular mass, direction, etc.
The interpretation of a surface analysis experiment typically relies on known (i.e., calculated from theory or previously measured from a standard sample) signatures of the outgoing particles that are characteristic of a particular surface composition and/or structure. For example, illuminating a surface with x-rays produces electrons (called photoelectrons), which have different kinetic energies that depend on the chemical composition of the surface. In another type of such an experiment, the directions of escaping photoelectrons can be analyzed either to produce an image of the surface or to obtain information about the surface structure.