AVS - American Vacuum Society - New England Section - 41st Annual Symposium, Burlington, MA, 6/6/2011

3D Surface Compositional Mapping with Nanometer Resolution

Karsten Pohl

Department of Physics and Materials Science Program, University of New Hampshire, Durham, NH, U.S.A.

The novel surface analysis technique of LEEM-IV combines low energy electron microscopy (LEEM) data with the formalism of multiple electron scattering calculations, as done in LEED-IV, to generate time-resolved 3D surface compositional maps. We are able to determine the local stoichiometry and structure with 8.5 nm lateral spatial resolution in the top 4 surface layers to address important questions of controlling compositional heterogeneity in ultrathin films. Detailed information of this type has proven difficult to obtain because techniques that are capable of accurately determining subsurface structure and composition -- electron and x-ray diffraction -- implicitly assume a laterally homogeneous film, while techniques with high spatial resolution -- scanning probe microscopies -- typically have poor chemical and subsurface sensitivity.

By measuring e.g. the evolution of the 3D composition of ultrathin Pd films on Cu(001) in real time using LEEM, we have shown that a simple step-overgrowth mechanism, potentially relevant in many systems, is responsible for the heterogeneity we observe [1]. This novel technique can also be extended to locally resolve the electronic surface structure, such as the plasmon losses in mono- and bi-layer graphene grown on polycrystalline Ni. Our recent results show that the pi-band structure of free-standing graphene appears only in films with a thickness of at least 2 layers and demonstrate the sensitivity of the 6.5 eV plasmon loss to the electronic structure [2].

[1] J.B. Hannon, J. Sun, K. Pohl, and G.L. Kellogg - Phys. Rev. Lett. 96, 246103 (2006) & J. Sun, J.B. Hannon, G.L. Kellogg, and K. Pohl - Phys. Rev. B 76, 205414 (2007).

[2] J. Sun, J.B. Hannon, R.M. Tromp, P. Johari, A.A. Bol, V.B. Shenoy, and K. Pohl - ACS Nano 4, 7073 (2010).