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Final Report Summary - ORBITAL IMAGING (Electron orbital resolution in scanning tunneling microscopy)

Scanning Tunneling Microscopy (STM) is one of the most powerful tools for investigating the atomic and electronic structure of conductive surfaces with extremely high spatial resolution. The technique uses a sharp tip or probe that ideally ends in a single atom, which is brought close to the surface being measured, a voltage is placed on the surface and a current flows. The value of this current is determined by the makeup of the sample, the tip and the distance between them. By scanning the tip over the surface, a map is produced which reflects the local atomic and electronic structure.
The major objectives of this Marie Curie project (“Orbital Imaging”) were: the reliable fabrication of functionalized STM probes with well-defined structures for use in high-resolution STM studies; investigation of the atom-atom interaction at small tip-surface distances; and selective imaging of electron orbitals with different angular momentum projections in distance-dependent STM experiments. These goals were achieved, and in some cases exceeded, and the main results of the project are summarized below.

1. Orbital Imaging
Procedures for fabricating sharp tungsten probes with well-defined structures have been developed. The structure of the chemically etched, [001]-oriented single crystalline tungsten probes sharpened in ultra-high vacuum using electron beam heating and ion sputtering has been studied using electron microscopy (EM). EM proves that tips with a single nanoscale pyramid apex grained by the {011} planes can be reproducibly fabricated. These sharp, [001]-oriented tungsten tips can be utilized in high-resolution STM studies of various complex surface atomic structures.
The electronic structure of the interacting W tip and graphite surface atoms has been studied experimentally and theoretically using STM and density functional theory (DFT) calculations. It is demonstrated that at small tunneling gaps (2–3 Å) the overlap of the tip and sample wave functions is responsible for the suppression of further extended orbitals with lower momentum projections on the z-axis (pz, dz 2 , dxz), leading to the direct visualization of the electron states in STM experiments (Fig. 1) The EM characterization performed before and after these STM experiments demonstrates the high stability of the W[001] tips and provides a direct correlation between the tip structure and picometer-scale (1 pm = 0.001 nm) spatial resolution achieved in the experiments.
This result is very important for STM analysis of multi-component systems. In particular, it shows that atomically resolved, chemically selective imaging at different gap resistances can be controlled using a general knowledge of the surface electron orbitals’ shapes.

2. Nanoscale Lithography
Single crystalline W[001] probes have been successfully utilized in STM studies of: the Si(557) atomic structure (in preparation); the dynamics of C60 molecules on the oxidized W(110) surface (Nanoscale 5 (2013) 3380); and the atomic structure of an oxygenrich MoO2+x/Mo(110) surface (Nano Res. 6 (2013) 929). The atomic structure of MoO2(010)/Mo(110) has been determined from STM data and DFT calculations. It is demonstrated that stable W[001] probes can be used for the controllable desorption of oxygen adatoms from the surface at positive sample biases greater than 1.5 V. Tip movement along the surface was used for controlled lithography (writing) at the nanoscale, with a minimum feature size of just 3 nm (Fig. 2).
This result paves the way to use W[001] probes for atomic scale lithography utilizing the highly stable tip apex.

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