A major research theme within the Nanoscience Group is the exploitation of dynamic force microscopy for the imaging, spectroscopy, and manipulation of individual atoms and molecules.
Dynamic force microscopy is an exceptionally powerful technique which exploits shifts in the resonant frequency of a vibrating cantilever to map the variation in the interaction between a sharp tip (at the end of the cantilever) and an underlying sample. If that tip is atomically sharp then not only is it possible to image individual atoms but the force between two atoms can be mapped out with resolution on length scales comparable to the width of individual electron orbitals.
Our force microscopy - and, more generally, scanning probe microscopy - research falls into a few broad categories, which we outline below. The Nanoscience Group currently coordinates the multi-partner ACRITAS European training network. ACRITAS focuses on actuation and characterisation at the single bond limit using different types of atomic force microscopy. All of our force microscopy work relies on strong collaborations with a variety of groups both here at Nottingham (including those of Janette Dunn, Physics and Astronomy; Neil Champness, Chemistry; and Natalio Krasnogor, Computer Science, in the UK, and internationally.
An exciting and rapidly evolving sub-field of nanoscientific research is the use of dynamic force microscopy to manipulate atoms through (chemo)mechanical force alone. A number of groups worldwide, including a number of members of the ACRITAS network, have made pioneering in-roads in this direction. Our focus in Nottingham is on the manipulation of atoms at silicon surfaces using the qPlus technique pioneered by Franz Giessibl.
This video describes the trials and tribulations associated with our initial experiments in this field. Our aim is now to extend this type of approach to a broader range of Group XIV elements (Ge, Sn, Pb) on the Si(100) surface in order to build three-dimensional nanostructures, atom-by-atom and under computer control.
Mapping Molecular Force Fields
Transferring a single molecule from a surface onto a force microscope tip can not only facilitate extremely high resolution imaging (as shown by Leo Gross and co-workers at IBM Zurich, it also enables the measurement of intermolecular force-fields and potential energy landscapes. We have recently carried out the first direct measurement of the intermolecular potential for two C60 molecules using this strategy. Atomic resolution force microscopy of the fullerene cage adsorbed on the force microscope tip (using the electron orbitals of the underlying surface as a 'probe') enabled us to determine the orientation of the C60 molecule. This ability to modify molecular orientation with atomic precision has particular potential in the measurement of the force-fields for more complex molecules (hydrogen-bonding species, porpyhrins, phthalocyanines, etc…) and this will form the basis of our future work in this area.
Images obtained using scanning probe microscopies are usually explained with the aid of density functional theory (DFT) calculations. In our case, we collaborate closely with Lev Kantorovich on DFT calculations. DFT can be very time-consuming, however, especially when a functionalised tip is used to image complex molecules. Janette Dunn's group is developing computationally-fast methods of interpreting STM and AFM images based on Hückel and extended Hückel methods. These are not ab initio methods so can't be used as predictive tools. However, they provide very fast means of simulating images for a given situation. This approach has been used to interpret experimental images of C60 fullerene molecules on Si surfaces. It has been particularly successful at explaining images obtained using a C60-functionalised tip to image C60 surface molecules. Simulated images for given orientations of the tip and sample molecules can be obtained in a matter of minutes on a desktop computer. The orientations that provide the best matches to experimental images agree with those postulated theoretically elsewhere.