Universal law for hydrogen discovered
Researchers of the Fritz Haber Institute in Berlin and of the Palo Alto Research Center (PARC) in California discover a universal rule for predicting how hydrogen determines the properties of semiconductors, insulators, and solutions (Nature, 5 June 2003).
Hydrogen is the most abundant element in the universe. On earth, we mostly know it as a component of water (H2O), but because it is such a small atom, hydrogen is usually present inside most materials. It can, in fact, play a critical role in determining the properties of those materials. Knowing or even predicting how it behaves in different environments is thus crucial in developing and improving materials.
A key property of hydrogen is its ability to remove or add electrons to a material. In fact, in many materials it acts like a sponge it effectively soaks up excess electrons or holes (missing electrons). Semiconductor industry relies heavily on this effect. For instance, even under clean-room conditions the formation of defects imperfections which often have adverse effects on the device performance is unavoidable. Acting like a sponge, hydrogen efficiently mops up these defects. If it werent for hydrogen, silicon integrated circuits or silicon-based solar cells would simply not work with the required degree of perfection and reliability.
The ability of hydrogen to give or take electrons from its surrounding is not only important for the semiconductor industry but it is one of the key mechanisms in many chemical and biological processes. Examples can be found in hydrogen storage systems, fuel cells, catalysts, or the activity of bio-molecules in solutions.
The key quantity to describe this behavior of hydrogen is the so-called transition (crossover) energy: If this energy is above the electron reservoir (chemical potential) of the environment hydrogen donates (adds) electrons, while if it is below hydrogen accepts (removes) electrons. So far this transition energy had been assumed to be highly material dependent, and therefore elaborate calculations or experiments had to be performed for every potentially interesting material. A simple rule to predict the behavior of hydrogen for novel materials was lacking.
Dr. Chris G. Van de Walle of the Palo Alto Research Center (PARC) in Silicon Valley and Dr. Jörg Neugebauer of the Fritz Haber Institute in Berlin (Max Planck Society) used powerful computer simulations to attack this problem. Their so called ab initio -- "from first principles" -- simulations are based on the fundamental physical laws of quantum mechanics, and are completely free of any adjustable parameters. They discovered that the hydrogen transition energy exhibits a universal alignment. The alignment is not restricted to a certain class of materials but applies to materials as different as semiconductors, insulators, and even liquids. It thereby provides unexpected insight into the way hydrogen interacts with liquids and solutions as well as solids.
Now that the universal alignment rule has been established, it will be a lot easier to predict how hydrogen will behave in new materials that are being evaluated or developed, for instance for applications in ultraviolet lasers (needed for the next generation of DVD players), wireless communication, fuel cells, or hydrogen storage systems.