Research

Local Probes of Surfaces

Shortly after its discovery, in 1979, the scanning tunneling microscope (STM) provided atomic-resolution images of the long-range 7x7 reconstruction on Si(111) and the missing row 1x2 reconstruction on Au(110). The images of the silicon surface comprised twelve bright features, due to silicon atoms located in an elevated 'adatom' configuration and dark features at the corners of the unit cell, arising from the 'corner hole'. The images provided direct 'visual' confirmation of structural models that had been painstakingly developed over many years using electron diffraction and Patterson maps. In the case of Si(111)-7x7, this was the Takanagi dimer-adatom-stacking fault reconstruction - one of the most complex and beautiful surface reconstructions ever discovered. In the three decades that have passed since its discovery, the STM has been used to solve many other surface structures. It has also been applied to other areas that include: homo and heteroepitiaxial growth, diffusion, adsorption, molecular self-assembly and catalysis. The STM, and other local probes, can, of course, be used to study the assembly of materials and nanostructures from their components. This approach, using local probes to study growth processes and pattern formation, at the atomic and molecular level, represents a new paradigm in materials science. Of course, it is still possible to perform diffraction studies of fully formed surface structures. However, having the ability to follow the growth process from its initial stages provides new information that was hitherto hidden from view.

Self-organization, Growth Processes and Pattern Formation

Our group has used a diverse range of growth methodologies to tailor the electronic and optical properties of surfaces at the nanometer length scale. Much or our research has involved elements from group IV of the period table (Si, Ge and more recently C). Surface processes are modified to generate stucture on the nanometer length scale. This structure is obtained by using far from equilibrium growth, careful control of growth kinetics and self-assembly. For example, we have grown metallic nanowires and semiconducting nanolines on low index and vicinal silicon surfaces and magic clusters (see right), clusters with the same number of atoms, on Si(111) surfaces. Each of the triangular shapes that you can see in the image (right) contains six indium atoms. Our studies of nanowire systems were motivated by the fact that they are model one-dimensional systems that possess interesting symmetry breaking phase transitions as they are cooled. The Si(111)-In(4x1) system, that we have studied, is an example of a quasi-one dimensional system with narrow bands that has fascinating properties at low temperature.

Our experimental studies are complemented by ab initio theoretical studies performed by an international group of collaborators who are located in Brazil, England and the Czech Republic. Theoretical methods, such as density functional theory, and modern parallel computer architectures now allow the properties of nanometer-scale structures to be calculated from first principles. Our experimental studies are performed concurrently with the theoretical studies and insights gained from the theoretical studies frequently guide the experimental program.

Nanoprobe @ Queen's, Scanning Tunneling Microscopy, Non-Contact Atomic Force Microscopy and Electron-excited Luminescence.

In parallel with the research described above, we design and build our own microscopes keeping the long tradition of instrument building in physics alive. The design is performed using 3D modelling software. We used home-built beetle-type scanning probes for almost one decade to study a variety of systems on silicon surfaces. However, we are currently testing a low and variable temperature scanning tunneling microscope, of our own design, for microscopic studies down to 8 K. This instrument has the capability to collect light that is excited by hot electrons (or holes) in the tunnel junction. It will also be possible to couple light in/out of the tunnel junction.