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1.  Closed-loop conductance scanning tunneling spectroscopy: demonstrating the equivalence to the open-loop alternative 
We demonstrate the validity of using closed-loop z(V) conductance scanning tunneling spectroscopy (STS) measurements for the determination of the effective tunneling barrier by comparing them to more conventional open-loop I(z) measurements. Through the development of a numerical model, the individual contributions to the effective tunneling barrier present in these experiments, such as the work function and the presence of an image charge, are determined quantitatively. This opens up the possibility of determining tunneling barriers of both vacuum and molecular systems in an alternative and more detailed manner.
PMCID: PMC4464298  PMID: 26171288
image charge; scanning tunneling spectroscopy (STS); tunneling barrier; work function; z(V)
2.  Digging gold: keV He+ ion interaction with Au 
Helium ion microscopy (HIM) was used to investigate the interaction of a focused He+ ion beam with energies of several tens of kiloelectronvolts with metals. HIM is usually applied for the visualization of materials with extreme surface sensitivity and resolution. However, the use of high ion fluences can lead to significant sample modifications. We have characterized the changes caused by a focused He+ ion beam at normal incidence to the Au{111} surface as a function of ion fluence and energy. Under the influence of the beam a periodic surface nanopattern develops. The periodicity of the pattern shows a power-law dependence on the ion fluence. Simultaneously, helium implantation occurs. Depending on the fluence and primary energy, porous nanostructures or large blisters form on the sample surface. The growth of the helium bubbles responsible for this effect is discussed.
PMCID: PMC3740815  PMID: 23946914
formation and healing of defects in crystals; helium ion microscopy; ion beam/solid interactions; vacancies in crystals
3.  Imaging ultra thin layers with helium ion microscopy: Utilizing the channeling contrast mechanism 
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.
PMCID: PMC3458595  PMID: 23019545
channeling; contrast mechanism; helium ion microscopy; ion scattering; thin layers
4.  Channeling in helium ion microscopy: Mapping of crystal orientation 
Background: The unique surface sensitivity and the high resolution that can be achieved with helium ion microscopy make it a competitive technique for modern materials characterization. As in other techniques that make use of a charged particle beam, channeling through the crystal structure of the bulk of the material can occur.
Results: Here, we demonstrate how this bulk phenomenon affects secondary electron images that predominantly contain surface information. In addition, we will show how it can be used to obtain crystallographic information. We will discuss the origin of channeling contrast in secondary electron images, illustrate this with experiments, and develop a simple geometric model to predict channeling maxima.
Conclusion: Channeling plays an important role in helium ion microscopy and has to be taken into account when trying to achieve maximum image quality in backscattered helium images as well as secondary electron images. Secondary electron images can be used to extract crystallographic information from bulk samples as well as from thin surface layers, in a straightforward manner.
PMCID: PMC3458594  PMID: 23019544
channeling; crystallography; helium ion microscopy; ion scattering
5.  The influence of substrate temperature on growth of para-sexiphenyl thin films on Ir{111} supported graphene studied by LEEM 
Surface Science  2012;606(3-4):475-480.
The growth of para-sexiphenyl (6P) thin films as a function of substrate temperature on Ir{111} supported graphene flakes has been studied in real-time with Low Energy Electron Microscopy (LEEM). Micro Low Energy Electron Diffraction (μLEED) has been used to determine the structure of the different 6P features formed on the surface. We observe the nucleation and growth of a wetting layer consisting of lying molecules in the initial stages of growth. Graphene defects – wrinkles – are found to be preferential sites for the nucleation of the wetting layer and of the 6P needles that grow on top of the wetting layer in the later stages of deposition. The molecular structure of the wetting layer and needles is found to be similar. As a result, only a limited number of growth directions are observed for the needles. In contrast, on the bare Ir{111} surface 6P molecules assume an upright orientation. The formation of ramified islands is observed on the bare Ir{111} surface at 320 K and 352 K, whereas at 405 K the formation of a continuous layer of upright standing molecules growing in a step flow like manner is observed.
► Para-sexiphenyl growth on graphene and Ir was observed in real time using LEEM. ► The structure of individual 6P features was analyzed using μLEED. ► At 320 K and 352 K Stranski–Krastanov growth of flat lying 6P is found on graphene. ► Islands formed by upright standing molecules are found on Ir(111). ► At 405 K only propagation of upright 6P islands, nucleated at graphene rims on Ir.
PMCID: PMC3267044  PMID: 22308005
Graphene; Low energy electron microscope (LEEM); Self-assembly; Organic thin film; Sexiphenyl(6P); LEED; Iridium

Results 1-5 (5)