The structural and electronic properties of Au induced self-organized nanowires on Ge(001), prepared by different procedures, are investigated with scanning tunnelling microscopy and spectroscopy at room temperature and 77 K. We find that the Au induced nanostructures are identical for the different preparation procedures. The Au induced nanowires are comprised of dimers that have their bond aligned in a direction perpendicular to the nanowire. The dimers are buckled, leading to a 2× periodicity along the nanowires. Dimers located at antiphase boundaries are dynamic and flip back and forth between two buckled configurations. The troughs between the nanowires have a depth of several atomic layers. Finally, the differential conductivities of the nanowires and the troughs are very comparable in magnitude.

Summary Background: Helium ion microscopy is a new high-performance alternative to classical scanning electron microscopy. It provides superior resolution and high surface sensitivity by using secondary electrons. Results: We report on a new contrast mechanism that extends the high surface sensitivity that is usually achieved in secondary electron images, to backscattered helium images. We demonstrate how thin organic and inorganic layers as well as self-assembled monolayers can be visualized on heavier element substrates by changes in the backscatter yield. Thin layers of light elements on heavy substrates should have a negligible direct influence on backscatter yields. However, using simple geometric calculations of the opaque crystal fraction, the contrast that is observed in the images can be interpreted in terms of changes in the channeling probability. Conclusion: The suppression of ion channeling into crystalline matter by adsorbed thin films provides a new contrast mechanism for HIM. This dechanneling contrast is particularly well suited for the visualization of ultrathin layers of light elements on heavier substrates. Our results also highlight the importance of proper vacuum conditions for channeling-based experimental methods.

Metallic nanowires show unique physical properties owing to their one-dimensional nature. Many of these unique properties are intimately related to electron-electron interactions, which have a much more prominent role in one dimension than in two or three dimensions. Here we report the direct visualization of quantum size effects responsible for preferred lengths of self-assembled metallic iridium nanowires grown on a germanium (001) surface. The nanowire length distribution shows a strong preference for nanowire lengths that are an integer multiple of 4.8 nm. Spatially resolved scanning tunneling spectroscopic measurements reveal the presence of electron standing waves patterns in the nanowires. These standing waves are caused by conduction electrons, that is the electrons near the Fermi level, which are scattered at the ends of the nanowire.

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The adsorption of Cu-phthalocyanine (CuPc) molecules on Au-modified Ge(001) surfaces has been studied with low-temperature scanning tunneling microscopy. The Au-modified Ge(001) surface consists of well-ordered arrays of perfectly straight nanowires, which are separated by 1.6 nm wide and about 0.6 nm deep trenches. Six different adsorption configurations for CuPc are identified. Four of these configurations are “molecular bridge” configurations where the molecule bridges two adjacent nanowires. The core of the CuPc molecule, i.e., the Cu atom, is fully decoupled from the underlying substrate. For sufficiently high sample biases (>1 V), rotation and diffusion events of the CuPc molecules are observed.