46323-AC5
Identifying the Driving Forces for Alloying in Ultra-Thin Films
The goal of our project is to determine surface structure and composition, and to explore kinetic and dynamical surface processes at a nanometer scale. The approach used is our newly developed LEEM-IV (low-energy electron microscopy – intensity vs voltage) technique, which is based on conventional LEED-IV (low-energy electron diffraction – intensity vs voltage) performed on a LEEM system and delivers an in-plane resolution of about 8 nm techniques. This new method allows therefore for the first time the truly three-dimensional compositional analysis of heterogeneous nanostructured surfaces. Several projects have been performed and accomplishments have been achieved as described below. We have made significant progress toward the ability of engineering high-performance surface alloys for catalytic and electro-magnetic applications.
Pd-Cu(100) surface alloys are interesting as model systems for metal/metal epitaxy, as well as for their catalytic properties, and as coatings, e.g. for electromigration resistance. We employ the LEEM-IV technique, with 8.5 nm spatial resolution and submonolayer chemical sensitivity, to investigate Pd interdiffusion into the Cu(100) surface. The LEEM-IV technique is sensitive to the layer-by-layer composition down to the fourth subsurface layer. After annealing a 0.4 ML Pd surface alloy at around 540 K, some regions of the surface develop a Cu3Pd structure, a familiar bulk alloy phase. In other regions, the surface Pd concentration becomes dilute due to Pd diffusion into the bulk. We estimate the thermal activation barrier to Pd diffusion from the surface alloy into Cu bulk to be 1.7±0.15 eV. The LEEM allows real-time, real-space, observation of the interdiffusion process, and the concurrent evolution of the surface structure, at the nanometer scale.
SiC surface is a good platform to make an atom layer of graphite – graphene. In our study, the surface phase transition of Si-terminated 6H-SiC(0001) upon heat treatment is studied by low energy electron microscopy (LEEM). Bright and dark field imaging demonstrates a direct in situ observation of the surface phase evolution, transitions in a sequence from 1×1, 3×3, √3×√3, 6√3×6√3 to the graphene phase due to gradually increasing the temperature. Intensity vs. voltage (IV) spectra extracted from single domain diffraction images is used to determine the local surface structure and chemical stoichiometry. Results from a quantitative dynamical analysis of the LEEM-IV curves show a Si-depleted 1×1 structure and an adatom-trimer-adlayer structure on 3×3 reconstruction. We have learned how to make cubic structure on the √3×√3 structure in this investigation. Ongoing work on the structure of the √3×√3 and 6√3 × 6√3 phases is aimed to unraveling the initial growth mechanism of graphene on SiC.
The Bi(001) surface is unique in its surface electronic property. While all Bi surface states studied are spatially confined to the first layer, Bi(001) is a notable exception with deeply penetrating states, which could have a significant effect on the bulk properties of nanostructures. This work concerns surface morphology observation by STM and atomic structure determination by LEED, which are expected to be closely related to the electronic properties. STM shows an unreconstructed surface and wide terraces with double-layer step heights of about 3.76 ± 0.02 Å. We also identify the short termination by obtaining unstable single step heights via special sputtering operations. In the LEED analysis, the termination with an intact bilayer also results in a much better agreement between calculated and measured intensities than the broken bilayer. Strong multilayer oscillatory relaxations (about 10%) are found to reach deep into the fifth layer, which can be seen as the structural response to the unusually deep surface state penetration at this surface. The measured relaxations agree well with those from first-principles calculations.
