The desire to build devices and structures at nanometre scales poses challenges not only for technology but also for our understanding of the behaviour of matter at the atomic level. In a novel approach to clarifying the properties of nanostructures, Taiwanese researchers have obtained real time photoelectron spectra of the structural changes of an ultrathin metal film undergoing a transition from disorder to order during annealing. Their experiments provide a deeper understanding into the quantum mechanical processes that govern this behaviour.

Nanometer sized materials systems exhibit quantum effects. For example, when a metal or semiconductor is grown sufficiently thin, its usually wide electronic energy bands transform into a collection of just a few discrete levels. Quantization effects are crucial for the operation of nanoscale devices. But for practical applications, films showing quantum effects must be well-ordered and atomically flat.

A two-step process in which films are deposited at low temperature and then annealed is an increasingly popular means of achieving this. Low-temperature growth ensures well controlled quasi-layer-by-layer growth, but creates films that are disordered. Annealing removes this disorder. Yet little is known about the details of the transition from disorder to order.

Fig. 1: Angle resolved photoelectron spectrum of an annealed silver film. The shape of the ridges in the plot of photoemission intensity versus binding energy and emissions angle enable researchers to monitor the development of order in a film in real-time as it is annealed.

To gain insight into this process, Dah-An Luh of the National Central University in Taiwan, and colleagues used angle-resolved photoelectron spectroscopy to monitor the effect of annealing on the quantized states of silver films just 20 atomic layers thick.1 They found regions of high atomic order to arise as small isolated patches scattered across a film at a certain transition temperature. As the annealing temperature was gradually increased, these patches grew and merged with one another, until eventually the whole film crystallized (Fig. 1).

The strength of this procedure lies in the ability to observe transitional states during the process of film ordering. “It is very surprising that these ordered patches are fully crystallized along the direction normal to the film surface, even in the early stage of the ordering process,” says Luh. “The structure of ordered patches on a partially ordered film had never before been investigated and documented in such great detail.”