|
Quick Links Related Websites |
December 5, 2007 Spectroscopic Imaging of Heterogeneous Nanomaterials – X-Ray Photoemission Microscopy at the NSLSJ.I. Flege1, E. Vescovo2, G. Nintzel2, S. Hulbert2, P. Sutter1 We report the present status of a newly installed low-energy electron microscopy and photoelectron microscopy
end station at NSLS beamline U5UA. In the first test experiments on sub-monolayer Au coverages deposited on Ru(0001), we demonstrate
core-level and valence band photoelectron imaging with a lateral resolution of about 65 nm at a field of view of 10
Low-energy electron microscopy (LEEM) has contributed considerably to the understanding of dynamic surface processes such as
adsorption, thin film growth, or chemical reactions under ultra-high vacuum conditions. The contrast observed in LEEM arises
from local changes in atomic surface structure, resulting in pronounced modulations of the local reflectivity for electrons
impinging on the sample with kinetic energies of typically a few electron volts. In general, these variations may reflect atomic
steps, co-existing surface phases, or chemical inhomogeneities. While the biggest advantages of LEEM are its in-situ, video-rate
imaging capability of sample areas of about 2-50 Au/Ru(0001) was chosen as a model system for initial LEEM/XPEEM imaging because it constitutes a heteroepitaxial system in which the film nearly perfectly wets the substrate, and in which there is negligible intermixing between film and substrate materials. Hence, sharply delineated two-dimensional (2D) Au islands on Ru can be grown at submonolayer coverage, providing an ideal test structure for element-specific surface imaging. In Figure 2, bright-field LEEM micrographs are displayed that show the evolution of the surface morphology upon Au deposition onto a Ru(0001) single crystal. Starting from the clean surface on which only atomic steps and step bunches are visible as dark bands (Figure 2a), the heterogeneous nucleation of Au islands (dark patches) is observed, which starts at the lower side of the substrate step edges (Figure 2b). Upon further deposition, dendritic 2D island growth by Au adatom capture, i.e. adsorption of Au atoms and subsequent diffusion to the edges of the growing islands, and the nucleation of additional islands at substrate steps are competing processes. Finally, the growth is stopped at about a half-monolayer Au coverage (Figure 2(c)) to obtain a patterned surface for the photoemission electron microscopy (PEEM) experiments.
In the following, we will provide examples for three different modes of photoemission microscopy based on the kinetic energy of the detected electrons: In PEEM, the slow inelastically scattered electrons, which exhibit kinetic energies close to 0 eV, are employed, generally providing chemical image contrast due to local work function variations at high overall intensity. Such an image is shown in Figure 3a, in which the Au-covered regions appear dark due to an increase of the work function as compared to the clean Ru surface.
In contrast, XPEEM makes use of photoelectrons that can be attributed to specific atomic core-levels, thereby providing direct access to the spatial distribution of the corresponding elements or chemical states. This elemental or chemical specificity is demonstrated in Figure 3b, which shows an image that has been acquired using Au 4f7/2 core-level photoelectrons. This can be interpreted as an intensity map of the elemental Au distribution, thus confirming the previous identification of monolayer gold islands. Finally, a different type of contrast may be achieved in valence band photoelectron emission microscopy (VPEEM) when the electron energy analyzer is tuned onto distinct features about 4.5 eV below the Fermi level in the valence band structure of the sample (Figure 3c). Since all bright regions in this image also show up bright in Figure 3b, this electronic state can be localized at the Au atoms. Hence, if the lateral element distribution is already known, e.g. as a result of preceding XPEEM investigations, then it is possible to identify contributions to the electronic structure from VPEEM micrographs. Future research will comprise in-situ studies of oxidation and catalysis on transition metals, growth of organic thin-film semiconductors, and characterization of magnetic materials. BEAMLINE FUNDING PUBLICATION FOR MORE INFORMATION |
The National Synchrotron Light Source at Brookhaven National Laboratory in New York, is a national user research
facility funded by the U.S. Department of Energy's Office of Basic Energy Science. The NSLS operates two electron
storage rings: an X-Ray ring and a Vacuum UltraViolet (VUV) ring which provide intense light spanning the electromagnetic
spectrum from the infrared through x-rays. Each year over 2300 scientists from universities, industries and government
labs perform research at the NSLS.
Privacy and Security Notice | Contact Webmaster
m.
In contrast to other installations for photoelectron microscopy, the new NSLS end station uses illumination under normal incidence,
maximizing the photon flux and enhancing the sensitivity for future in-plane characterization of magnetic materials.
