Watching Atoms Move
Researchers usually compare “before” and “after” pictures of typical regions of a material to see how it changes during a phase transition. But now a team has watched a specific set of atoms continuously during a surface phase transition to see exactly how it occurs, and has made atomic-scale movies. In the 4 February PRL they describe their ultrastable scanning-tunneling microscope, or STM, that can remain fixed on the same set of surface atoms, even as the temperature rises by 100 Kelvin–a huge warming that would cause other microscopes to “lose their place.” By studying one especially pristine region of the surface, the researchers were able to learn how the transition occurs, atom-by-atom, in a perfectly clean system.
Materials scientists like to exploit the regular array of atoms on the face of a crystal as a template, or substrate, for growing other crystals or for forming nanostructures. To understand such processes in detail, they need clean surfaces. But even in the ultraclean lab environment, defects such as extra atoms from the substrate crystal can confuse experimental results. Traditional probes of the crystalline ordering, which scatter electrons or light off of the surface, don’t provide images directly, but instead give “global” data that doesn’t distinguish between regions with and without defects.
One case where defects matter is the behavior of surface phase transitions. A layer of tin atoms on a germanium surface forms a flat lattice, but at temperatures below about 150 Kelvin–for reasons that aren’t entirely clear–the tin layer spontaneously corrugates to form a new structure, or phase, with one atom out of every three moving away from the substrate. Researchers have always studied tin surfaces that included some extra substrate atoms (defects), and they have disagreed on how the defects influence the transition. No one has ever zeroed in on a single, completely defect-free region of a sample and continuously imaged it during the transition to see a pristine version of the process. The problem is that the parts of the microscope expand or contract as they change temperature, which makes it hard to remain aligned with atomic-scale accuracy.
So José Gómez-Rodríguez and his group from the Madrid Autonomous University built a variable temperature scanning-tunneling microscope using the best stabilizing technology they could devise. To improve beyond it, they also included software that compares subsequent scans and corrects for drifts.
The team found that a lead layer on silicon undergoes a similar phase transition to tin on germanium, but with much larger defect-free regions and a transition temperature of just 86 Kelvin. With their ultrastable microscope, they were able to watch a single such region, 20 by 20 nanometers, as the temperature climbed from 40 to 136 Kelvin and see the atoms rearrange. For the first time, they continuously observed atoms as they went through a phase transition. They found that the degree of corrugation in the lead layer changes with temperature in precisely the way expected for an ideal system with no defects.
Anatoli Melechko, of the University of Tennessee at Knoxville and Oak Ridge National Labs, says the Spanish team’s ability to track a chosen region as they vary the temperature gives them a “really true measurement” of the properties of the ideal system. “I wish I had this tool when I was doing my Ph.D. work,” Malechko adds.
Don Monroe is a freelance science writer in Murray Hill, New Jersey.