My primary research interests lie in the investigation and control of individual atoms and molecules on surfaces. I am motivated both by fundamental science and by the possibilities of creating novel nano-scale devices. I have recently initiated a programme of research to measure the conductance of individual organic and magnetic molecules on semiconductor surfaces. This is specifically aimed at addressing the molecule-electrode contact problem to produce quantitative and reproducible conductance measurements of individual molecules. Other research areas that I am active in include the electrical and magnetic properties of individual dopant atoms in semiconductors, fundamental studies of surfaces and molecular adsorption to surfaces, and the characterization of nanowires.
There are currently openings for Ph.D. candidates. For more information, please contact Dr. Schofield directly.
Figure 1: This figure gives an introduction to some of the measurement capabilities of scanning tunnelling spectroscopy (STS). In particular, shown are STS measurements of the clean, low defect density silicon (001) surface. Panels A – B show conductance (dI/dV) maps of the same area of a Si(001) surface at the energies shown (0.8, 1.14 and 1.46 eV), which correspond to the peaks seen in the single dI/dV plot in panel D. Since the conductance is an approximate measure of the surface local density of states (LDOS), these images provide a direct real space measure of the electronic configuration of the surface. The contrast in panel A arises due to the surface pi* antibonding state, while the contrast in panel C comes from antibonding states between the surface layer atoms and the second layer atoms. Panel B reveals contrast that is not intrinsic to the properties of the surface, but arises due to a nearby charged surface defect.
Panels E – G show how STS can be used to measure electron dispersion of a surface state. Panel E shows dI/dV conductance maps (top) where electron scattering from a line defect and the resulting interference is evident in the form of an electron standing wave pattern. The bottom part of panel E shows the Fourier transform of the conductance maps, used to measure the wave vector of the standing waves. The resulting dispersion measurement (electron energy from bias versus the measured wave vector) is plotted in panel F and found to be a good match to the dispersion calculated using density functional theory (DFT), which can be seen in the circled region of panel G.
Figure 2: The envisioned prospects for electronic devices that function on the molecular scale has directed considerable attention at organic molecules and their reactions with the silicon surface. The scanning tunnelling microscopy (STM) images above show two acetaldehyde molecules adsorbed to a Si(001) surface. In the gas phase, the acetaldehyde molecule is not chiral, but the molecule becomes chiral upon adsorption to the surface. In each of the two STM images are shown two molecules (bright protrusions) adsorbed on top of a row of silicon dimer units. The two adsorbed molecules are left and right handed enantiomers of one another, and are separated by 5 (left image) and 4 (right image) surface atomic spacings, respectively. The schematic illustrates the case for the left image. Each of the two enantiomers induces buckling of the surface silicon dimers on the side away from its methyl group. The corresponding in-phase dimer pinning for the 5 dimer separation results in the zig-zagged appearance of the dimers between the two molecules as seen in the left hand image. When the molecules are separated by an even number of dimers, the buckling is out of phase, and the dimers appear symmetric (right hand image). These images demonstrate the high degree of precision in the determination of the atomic configuration of adsorbed molecules that is possible using STM and semiconducting substrates. In conjunction with scanning tunnelling spectroscopy (see Fig. 1), the silicon (001) surface has great potential to become an expedient test bed on which to evaluate single organic molecules as potential circuit elements.
- Rahnejat, K.C. et al. Charge density waves in the graphene sheets of the superconductor CaC6. Nat. Commun. 2:558 doi: 10.1038/ncomms1574 (2011).
Carbonyl Mediated Attachment to Silicon: Acetaldehyde on Si(001), D. R. Belcher, S. R. Schofield, O.Warschkow, M.W. Radny and P. V. Smith, J. Chem. Phys. 131 (2009) 104707 [Link]
- Valence surface electronic states on Ge(001), M.W. Radny, G. A. Shah, S. R. Schofield, P. V. Smith and N. J. Curson, Phys. Rev. Lett. 100 (2008) 246807 [Link]
- Water on silicon (001): C defects and initial steps of surface oxidation, O.Warschkow, S. R. Schofield, N. A. Marks, M.W. Radny, P. V. Smith, and D. R. McKenzie, Phys. Rev. B 77 (2008) 201305 (2008) [Link]
- Organic bonding to Si(001) via a carbonyl group: New insights from atomic-scale images, S. R. Schofield, S. A. Saraireh, P. V. Smith, M.W. Radny and B. V. King, J. Am. Chem. Soc., 129 (2007) 11402 [Link]
- Atomically precise placement of single dopants in Si, S. R. Schofield, N. J. Curson, M. Y. Simmons, F. J. Ruess, T. Hallam, & R. G. Clark, Physical Review Letters 91 (2003) 136104; [Link]
- The above article was covered by Science magazines online daily news web site, Science Now - [Link]