Two researchers from the US and Germany have calculated a ‘universal alignment’ effect for the electronic energy levels in hydrogen. This effect could allow scientists to predict how hydrogen behaves in materials used for applications as diverse as integrated circuits, fuel cells and hydrogen storage materials (C G Van de Walle and J Neugebauer 2003 Nature 423 626).
Hydrogen is widely used in the processing of electrical materials and strongly affects their electronic and structural properties. When hydrogen is added to a material – such as a semiconductor – it can bind to defects or other impurities, which prevents them from destroying the electronic properties of the material. This ‘passivation’ works by reducing the electrical conductivity of the impurities and is crucial to the performance of many devices. In 2000 Chris Van de Walle at the Palo Alto Research Center in California showed that hydrogen could also increase the electrical conductivity of certain other types of materials.
The behaviour of hydrogen, whether as a passivating agent or as a source of conductivity depends on its ‘transition energy’. Hydrogen donates electrons below this energy, and accepts electrons above it. It was thought that the transition energy depended on the host material but now, Van de Walle and a colleague, Jörg Neugebauer from the Fritz-Haber Institute der Max Planck in Germany, have calculated that it has the same value – about –4.5 electron volts – for a wide variety of different materials.
“This ‘universal alignment’ effect is not restricted to a certain class of materials but applies to materials as different as semiconductors, insulators and even liquids,” Van de Walle told PhysicsWeb. “It will allow researchers to predict the electrical behaviour of hydrogen in these different materials rather than them having to perform elaborate calculations or experiments for every possible candidate material.”
