Our research is focused on high-resolution local structure measurements on model systems for heterogeneous catalysis.
A detailed understanding of surface defects and adsorbates is highly desirable not least because of the suggested role as active sites in catalysis. Consequently we investigate localised electronic defects and ad-species on the surface. Most models are well defined metal supported oxide film systems or bulk crystal surfaces. The manifold of sites with different structures and various charge states often leads to highly convoluted signals in most averaging spectroscopy methods. Such results are difficult to disentangle. To gain both morphological and spectroscopic information on unit cell structure, defects, adsorption sites and ad-species at the atomic level, we employ local microscopic and spectroscopic characterization.
Our principal tool is non-contact atomic force microscopy (NC-AFM), also known as frequency modulation dynamic force microscopy (FM-DFM). We apply it in combination with scanning tunneling microscopy (STM). Both techniques are incorporated into a dual-mode nc-AFM/STM sensor operated in ultrahigh vacuum at low temperature (5 K).
This scientific approach opens up the way to analyse the local morphological nature of defects and adsorbates on thin oxide films as well as on thick films or bulk crystals. The charge state and the position of these defects and ad-species can be locally characterized as well as manipulated by interactions with the probing tip with high sensitivity and control. Our special dual-mode setup is capable of direct measurement and manipulation of local electronic properties and forces at the atomic level. The main goal for future experiments is to capture the dynamics of atomic scale processes.
Resolving Complex Oxide Surfaces
High resolution imaging and spectroscopy signifi cantly improves our understanding of surface structure and chemistry for complex materials. In this work low temperature ultrahigh vacuum noncontact atomic force and scanning tunneling microscopy has been employed to characterize surface structures ranging from periodic and ordered via defective towards amorphous networks. In the presented studies it has been shown how modern scanning probe microscopy techniques can complete and clarify the atomistic models, which have been derived so far mainly from diffraction experiments. The benefits of locally resolving complex surface structures are obvious. A direct atomic assignment from the obtained images is possible. Surface structures with increasing dimensionality from 0D to 2D are discussed. In this article three different sample systems are highlighted, namely, magnesia on Ag(001), alumina on NiAl(110) and silica on Ru(0001).
[M. Heyde, G. H. Simon, L. Lichtenstein, Phys. Status Solidi B 250, 895 (2013)]
Structure of Vitreous Silica
Silica glass has been an extensively studied material in science. Yet, we know little about its atomic-scale structure.
Here, the atomic structure of a metal supported vitreous thin silica film was resolved using low temperature STM and nc-AFM. Based on these image, we constructed a model and therewith evaluated the atomic arrangement of
the thin silica glass in detail. The total pair correlation function of our structural model shows good agreement with diffraction experiments performed on vitreous silica.
Recently, we analyzed the interface between a crystalline and a vitreous phase of a thin metal supported silica film. The locally resolved evolution of Si-Si nearest neighbor distances and characteristic angles was evaluated across the border. Furthermore, we investigated the behavior of the ring size distribution close to the crystalline-vitreous transition.
[L. Lichtenstein, C. Büchner, B. Yang, S. Shaikhutdinov, M. Heyde, M. Sierka, R. Włodarczyk, J. Sauer, H.-J. Freund, Angew. Chem. Int. Ed. 51, 404 (2012) /
L. Lichtenstein, M. Heyde, H.-J. Freund, J. Phys. Chem. C 116, 20426 (2012) /
L. Lichtenstein, M. Heyde, H.-J. Freund, Phys. Rev. Lett. 109, 106101 (2012)]
Measuring the Charge State of Point Defects on MgO/Ag(001)
A detailed understanding of surface defects is highly desirable, e.g. to clarify their role as active sites in catalysis. Here localized defects on the surface of MgO films deposited on Ag(001) are investigated. Since the electronic structure of color centers depends on their local position, spectroscopic signals are highly convoluted and often difficult to disentangle. In this study we aimed to obtain morphological and spectroscopic information on single color centers at a microscopic level with frequency modulated dynamic force microscopy (FM-DFM) and scanning tunneling microscopy (STM) in an ultrahigh vacuum and at low temperature. Four of the major and in literature mostly discussed defect types on MgO have been characterized by their charge state and finally identified by the complementary application of FM-DFM and STM in combination with density functional theory results.
[T. König, et al., J. Am. Chem. Soc. 131, 17544 (2009)]