Objects viewed with scanning tunneling microscopes (STMs) may appear rougher than they really are according to a new computer simulation described in this week's Physical Review Letters. Although scientists have used STMs for more than two decades to create images of just about everything from cells to silicon, they have not understood in full how the instruments work. The new model fills in some missing information¿and shows that better image processing algorithms may improve the accuracy of STMs in general.
Werner Hofer of the University College in London worked with British and Canadian colleagues to study the interactions between an atomic surface and an STM tip. To create images, scientists apply voltage to this tip¿in essence a fine probe topped with a single atom¿such that a small tunneling current arises between the tip and surface. As the tip moves over the surface like a finger reading braille, the current fluctuates, revealing the topography.
That much the scientists knew. What Hofer and his crew didn't realize until they completed their new simulation¿which modeled the outer electrons of the atoms involved in great detail¿was that the tip and surface experience a strong attraction. This attraction is so strong, in fact, that the tip actually pulls some of the atoms up from the surface by as much as one atomic radius. The shift in distance changes the tunneling current and hence, the STM image. Says Hofer, the revelation may, among other things, explain why gold sheets appear coarser than theory would predict.