This course consists of about 14 lectures on the subject of surface science. The scope of the series is to give a brief introduction into some important concepts in the physics and chemistry of solid surfaces. It is probably appropriate to start with a few warnings / disclaimers:
I gratefully acknowledge the help from my colleagues David Adams and Flemming Besenbacher. I have used a lot of electronic pictures and lecture notes from David in this document. Many of the other images are made by Erik Holst Mortensen. I am very grateful for this professional help.
This course deals with some aspects of the physics and chemistry of surfaces and interfaces. Surfaces and interfaces are everywhere and many related phenomena are common in daily life (texture, friction, surface-tension, corrosion, heterogeneous catalysis). We are here concerned with understanding the microscopic properties of surfaces, asking questions like: what is the atomic structure of the surface compared to that of the bulk? What happens to the electronic properties and vibrational properties upon creating a surface? What happens in detail when we adsorb an atom or a molecule on a surface? In some cases, establishing a connection to the macroscopic surface phenomena is possible in others the microscopic origin of these phenomena is still completely unclear. We will mostly concentrate on simple model systems like the clean and defect-free surface of a single-crystal substrate. Such things do of course only exist in theory but the technological progress in the last 30 years (see below) has made it possible to get close to this ideal. This together with the progress in surface science theory makes it meaningful to compare experimental results to first-principles calculations. Why does one want to study surfaces and interfaces in the first place? Here are some motivating aspects.
One of the most important motivations in surface science is the understanding of heterogeneous catalysis . The fact that the presence of a solid could accelerate a chemical reaction without modifying the solid was first discovered in the early 19th century. Knowledge about catalysis has then rapidly grown and been the basis of the developing chemical industry. In the beginning, the microscopic mechanism of the catalytic process was, of course, unknown. Much was tried and “good” catalysts were made from experience. A typical surface science experiment on an “ideal” single crystal surface in ultra-high vacuum (UHV) is rather far away from the conditions a real catalyst is working in: the catalyst may be made of small metal particles dispersed on an inert substrate in a high pressure of gas at elevated temperature. Nevertheless, the surface science approach can give important information about many fundamental processes in catalysis. But there are of course situations where this is not enough. Therefore one tries to move into a direction where one is closer to the real catalyst but still very controlled. One can, for example, study the catalytic properties of well-defined metal clusters on a well-defined surface. The ultimate goal is of course to really understand the catalytic reaction in all steps and to improve the catalyst (make it cheaper or more efficient).
Closely related to this is the issue of corrosion. Questions are: What are the chemical reactions leading to corrosion? How do they take place on the surface and what can we do to prevent them?
Another reason for the growing interest in surfaces is related to the semiconductor industry. There is a strong need to build smaller and smaller structures to get higher integration on computer chips. One consequence of small structures is that the relative importance of the surfaces is increasing. Another, more practical, consequence is the need to build these structures with high precision and to have flat interfaces between them. This is also an issue in the growth of thin and ultra-thin films and multilayers needed for semiconductors, magnetic storage, coatings and so on. One wishes to learn how to grow thin and uniform films of material A on material B. Even if such a film growth is not possible (for example because A forms drops and clusters on B) one might still find a way to grow the film adding by using so-called “surfactants”. Surface Science research on semiconductor surfaces is much closer to the real technological world than the research in heterogeneous catalysis. Most semiconductor devices are made of Si, grown on the surface of a single-crystal waver.
Related to the increased importance of surfaces in connection to smaller semiconductor structures is the field of nano technology. The importance of surface (or even edge and kink) effects is obvious. In addition to this, the surface is the ideal starting point for building very small structures. A promising current research field is for example the study of structural formation by self-organization.
A more fundamental issue is that surfaces and interfaces provide a unique opportunity to study (nearly) two-dimensional electronic systems. The most famous examples for this are the integer and the fractional quantum hall effect where a two-dimensional electron gas is generated in a semiconductor heterostructure (we will later see how) and studied at very low temperature and in high magnetic fields. Another example might be the electronic structure of the new high TC superconductors and related compounds where much of the interesting physics goes on in the two-dimensional copper oxide planes. Yet another example are quasi two-dimensional electronic states on surfaces which can by used to study a lot of interesting many-body effects.
Historically, the interest in surfaces is an old one [6]. People where always fascinated by the more macroscopic phenomena of surface colour and texture. It was also realized early that very little oil spreads over enormous areas on the surface of water. Catalysis was discovered, as mentioned above, in the first half of the 19th century. In the 1870ies the theory of surface thermodynamics was essentially completed by J.W. Gibbs. Systematic experimental studies where pioneered in the beginning of last century by I. Langmuir. His studies of gas-surface interactions where even closely related to technical progress in electrical lamps. Around the same time, the basis of two other very important surface science experimental techniques was discovered: Einstein developed the theory of the photoelectric effect and Davisson and Germer showed that electrons behave like “matter waves” and can be diffracted from a crystal. We will come back to these two types of experiments later. In the 30s and 40s of last century tremendous theoretical progress about the surface and interface electronic structure was made (electronic surface states and semiconductor junctions) and the transistor was invented. Well-controlled experimental work first took off in the 60s when it became possible to generate vacuum conditions which were so good that surfaces could be cleaned and kept clean for a time long enough to do some useful experiments with them. Many experimental techniques were introduced, in particular the use of electron spectroscopy turned out to be essential for the progress in surface science. In the 80s the scanning tunnelling microscope was invented. It lead to a wide range of real-space studies on a truly atomic scale.
Here is a short list of some books on surface science. When you compare the contents of certain books to these notes, you will find striking similarities because I have stolen a lot of the material I liked.
A lot of very interesting teaching material can be found on the www.