SMF - Surfaces at Physics at Work 2018
SMF - Surfaces
Why study surfaces?
Every solid has a surface, and they play a huge role in physics. A few reasons why we are interested in studying surfaces
- Catalysis
Some surfaces act as catalysts, speeding up or enabling reactions when the molecules involved adsorb (stick) on them. These catalysts are very important in industrial processes, such as the manufacture of chemicals. Another important example is the catalytic converters found in cars, which contain platinum, to catalyse the conversion of the toxic components of exhaust. Understanding how reactions proceed at surfaces is crucial to designing effective catalysts.
- Friction
Friction affects our everyday lives, but what are its origins at the atomic level? How do the forces between surfaces at an atomic level affect the larger scale behaviour of a system? As advances in technology allow electronic and mechanical components to become ever smaller, these become important questions, and understanding how friction occurs between surfaces at a molecular level is essential.
- Material properties
Often, the properties of a material, such as its electrical or thermal conductivity, change dramatically between the bulk (inside the material) and the surface. To properly understand the behaviour of materials, the surface properties, as well as the bulk, must be studied.
Studying surfaces
A variety of techniques are used to study surfaces at the atomic level, making use of electrons, atoms, neutrons and infra-red light as probes. Here we touch upon two important ones.
Scanning tunnelling microscopy – seeing surfaces at the atomic level
In a scanning tunnelling microscopy experiment, a metal tip is held above a conducting surface, with a gap (the size of a few atoms) between them. When a voltage is applied to the probe an electrical current flows between the tip and sample. This is surprising - how can a current flow when there is a gap? To understand this, we invoke quantum mechanics, an area of physics which becomes important when considering things on atomic scales or even smaller. In quantum mechanics, it is possible for a particle to penetrate, or 'tunnel' through, a barrier which, according to classical, 'everyday', physics, should be impossible. In this case, electrons are tunnelling and the 'barrier' is the vacuum between the tip and sample. The size of the tunnelling current depends on the number of electrons at the surface, so by scanning the tip across the surface and recording the current at each point, we can build up a map of surface electron density. Electrons tend to form a 'cloud' around atoms, so the electron density maps can be interpreted to determine the structures formed by atoms at surfaces.
Helium atom diffraction - using atoms as diffracting waves
Most people are familiar with the diffraction of light when it bends around obstacles. Significant diffraction occurs when the obstacle encountered is similar in size to the light wavelength.
A consequence of quantum mechanics is that particles can exhibit wave-like properties. The discovery of this wave-particle duality was an important milestone in the development of quantum mechanics over a hundred years ago. The wavelength of matter is called the de Broglie wavelength, after one of the scientists behind the discovery.
Helium atoms have a de Broglie wavelength of about 1 angstrom (10-10m, a ten thousand millionth of a metre), which is close to the spacing between atoms on a surface. Therefore, a helium atom can be thought of as a wave, which has the right wavelength to undergo diffraction when it hits a surface of atoms. In a typical helium scattering experiment, a beam of helium atoms travelling at about 2000 km per second is scattered from a surface and detected using a mass spectrometer. From the way the helium atoms diffract, we can determine the structure of the surface at an atomic scale.
Atoms or molecules with a similar mass to helium can also undergo diffraction from surfaces, but helium is a particularly useful and widely used probe since it is inert (unreactive) and neutral (no net electronic charge) and therefore scatters from the surface without damaging or affecting it in any way.