Photo-emission Electron Microscopy
Contact Dr. K.M. Pavlov
Surface electron microscopy utilizing low energy electrons has made appreciable contributions to our general understanding of surfaces. Of particular importance has been the capacity to study dynamic events at video rates, often during growth or annealing. Various complementary imaging modes are readily available including diffraction and phase-contrast low energy electron microscopy (LEEM), photoemission electron microscopy (PEEM), and mirror electron microscopy (MEM) making surface electron microscopy a versatile technique of wide applicability. However, a major challenge common to all of these imaging modes is the extraction of quantitative surface topographical data. Indeed, it is well appreciated that images from three-dimensional (3D) surface objects can be significantly distorted by the cathode immersion lens where the sample surface forms an integral part of the electron optics. In the article (Jesson et al), we combined PEEM with the classic Lloyd's mirror optical geometry (see Fig.3) in a new approach to image surface topography. Lloyd's fringes in the incident UV radiation (see Fig.4) were used to modulate electron photoemission from 3D surface objects which forms the basis of an iterative reconstruction scheme to correct for distortions in the PEEM fringe pattern due to the cathode immersion lens. This facilitates a quantitative determination of surface shape directly from PEEM images. The technique is of sufficient intensity and contrast to observe real-time changes in surface topography and we applied the method to study contact-line dynamics during the reactive wetting of liquid Ga droplets on GaAs (001).
(a) Lloyd's mirror PEEM geometry for imaging a surface cluster.
(b) Schematic of photoelectrons emitted at various points on the surface cluster, which are accelerated by the electric field E0. The electrons are focused by the lens L onto the PEEM screen D. Electric field perturbations, arising from surface roughness h_x; y_, affect the photoelectron trajectories (1), (2), and (3), giving rise to fringe shifts.
(a) Enlargement of the Ga droplet region.
(b) Intensity line trace IF measured along the solid white line.
(c) An interpolated first approximation to the quasi-1D shape function h(x).
(d) Reconstructed shape function obtained by an iterative method based on the inverse Hilbert transform. The shaded portion denotes the range of reconstructions based on successively removing bright Lloyd fringes (7) and (8), respectively.
Reference:D. E. Jesson, K. M. Pavlov, M. J. Morgan, and B. F. Usher. Imaging surface topography using Lloyd's mirror in Photo-Emission Electron Microscopy. Phys. Rev. Lett. 99, 016103 (2007).PhD Project :
An experimental research project will aim to further develop this new technique (in collaboration with the Monash University Physics team) to make possible nanoscale surface topography imaging using synchrotron sources.