Introduction & Background

Why Study Surfaces?

Science

unique material properties at boundary because surface terminates solid, breaks existing symmetry

Ž different structural and chemical properties

Practical importance $$$

surface is where the material meets the outside world.

Basic understanding needed for better products and processes:

Catalysis for accelerating chemical reactions in chemical industries

Oxidation & corrosion produce enormous corrosion damage in metals

Friction beneficial and wasteful in machinery and transportation

Adhesion often needs to be enhanced or avoided

Wear, a main cause of limited life of products

Condensation and nucleation (snow, icing of airplanes)

Growth of thin films and crystals (e.g., semiconductor and coating industries)

Desorption and sublimation (e.g., freeze drying industry, evaporation cooling)

Electrostatic charging in the industry of xerography and a source of electronic failure of materials

Production of vacuum like in TV tubes, lamps, particle accelerators

Behavior of surfaces in vacuum like in space applications, nuclear fusion machines

Materials analysis methods with distinct surface phenomena

A surface is not a mathematical plane

Extent of the surface region depends on the phenomena considered, but is at least the outermost atomic layer of the solid and the electron selvage extending outside it. This includes any foreign material, if present.

Advances in science directly related to what can be measured.

What do we want to know?

Usually the same as what we want to know about the bulk.

what atoms are present

how they are arranged in space

how they vibrate.

These in turn determine the extent and energies of the electronic states which are responsible for bonding, chemical activity, magnetic properties, etc.

Need for UHV (< 10^-9Torr)

Advance in science requires

ability to measure

reproducibility

Ultra-high vacuum achieved in the 1920s (now we know). Took off after invention of Bayard-Alpert pressure gauge in 1950

Strategy of surface science

simplify the problems so they can be understood in detail

construct a base of knowledge about mechanisms and phenomena

apply to the most complex ‘real’ situations.

Works in some cases not in others.

E. g., catalysis sees little benefit from the vast effort in understanding basic, simple physical systems.

Real system: rough surfaces or powders at high pressures

Basic research: flat surfaces in vacuum.

To simplify:

study ‘perfect’ surfaces. Needs careful preparation and keeping surfaces in a know state for the duration of the experiment. This requires ultra high vacuum (UHV).

Study effect of adsorbates, defects, etc. on properties.

Why UHV?

in normal gaseous environments like the atmosphere, surfaces are bombarded by gas phase molecules at an enormous rate.

If these molecules stick to the surface, they will alter its properties.

A surface is not a mathematical plane

What do we want to know?

Need for UHV (< 10-9 Torr)

Why UHV?

Need for UHV (< 10^-9Torr)