Johannes Reimann awarded dissertation prize of Philipps-Universität Marburg

/in /by

Congratulations to Dr. Johannes Reimann, doctoral student in SFB-project B6 (Höfer), for being awarded a prize by Philipps- Universität Marburg for his excellent dissertation presented in 2018.

In his thesis entitled “Charge carried dynamics and photocurrents in the Dirac cone of topological insulators” Johannes Reimann investigated a novel class of materials, topological insulators. These materials, discovered only a decade ago, are insulating in the volume, but conductive at their surfaces and at their interfaces with conventional materials.

In the framework of his thesis, Johannes Reimann advanced the development of time- and angle-resolved photoelectron spectroscopy within the group of Prof. Höfer. In particular, his work is the first to combine this powerful technique with Terahertz excitation and to achieve subcycle time resolution. In collaboration with the group of Prof. Rupert Huber in Regensburg, he succeeded in taking band structure movies of electrical currents carried by Dirac electrons as they are driven by an intense THz wave. First results were published in Nature in September 2018 (see also SFB news, university press release.

The results of Johannes Reimann’s work hold great promise to realize new lightwave-driven electronics, a concept to increase the clock rates of conventional semiconductor devices by a factor of 1000 and more. Moreover, the successful demonstration of the combination of intense THz pulses as pump and angle-resolved photoelectron spectroscopy (ARPES) as probe, has triggered worldwide experimental efforts to take advantage of THz-APRES for time-resolved investigation of a variety of solids, surfaces and interfaces.

See here for details of the event.

Novel single-atom sensitive imaging – Publication in Nature Materials by A12 (Tautz)

/in /by

The team of researchers from Jülich, supported by SFB 1083 together with external partners, has developed a new method to measure the electric potentials of a sample with atomic accuracy. Using conventional methods, it was virtually impossible until now to quantitatively record the electric potentials that occur in the immediate vicinity of individual molecules or atoms. The new scanning quantum dot microscopy method, presented in the journal Nature Materials, also opens new ways of characterizing internal interfaces, as they often involve charge transfer and therefore show unique signatures in their electric potential.

Image from a scanning tunnelling microscope (STM, left) and a scanning quantum dot microscope (SQDM, right). Using a scanning tunnelling microscope, the physical structure of a surface can be measured on the atomic level. Quantum dot microscopy can visualize the electric potentials on the surface at a similar level of detail – a perfect combination. (Copyright: FZ Jülich, Christian Wagner)

The positive atomic nuclei and negative electrons of which all matter consists, produce electric potential fields that superpose and compensate each other, even over very short distances. Conventional methods do not permit quantitative measurements of these microscopic fields, which are responsible for many material and interface properties and functions at the nanoscale. Almost all established methods capable of imaging such potentials are based on the measurement of forces that are caused by electric charges. Yet these forces are difficult to distinguish from other forces that occur on the nanoscale, which prevents quantitative measurements.

Four years ago, however, the scientists from Forschungszentrum Jülich discovered a method based on a completely different principle. Scanning quantum dot microscopy involves attaching a single organic molecule – the “quantum dot” – to the tip of an atomic force microscope. The molecule is so small that individual electrons from the tip of the atomic force microscope can be attached to the molecule in a controlled manner. With the new method it is not only possible to visualize the electric fields of individual atoms and molecules, it is also possible to quantify them precisely.

Finally, scanning quantum dot microscopy is particularly well-suited to study internal interfaces. This is illustrated, e.g., by its ability to clearly resolve sub-surface defects, as the team around Stefan Tautz has already demonstrated. For such investigations, the long-range nature of electrostatic potentials is an asset.

Publication

C. Wagner, M.F.B. Green, M. Maiworm, P. Leinen, T. Esat, N. Ferri, N. Friedrich, R. Findeisen, A. Tkatchenko, R. Temirov, and F.S. Tautz,
Quantitative imaging of electric surface potentials with single-atom sensitivity
Nature Materials (2019) DOI: 10.1038/s41563-019-0382-8

See also read-only access and German press release by FZ Jülich, as well as Nature Materials News & Views.

Contact

Prof. Dr. Stefan Tautz
Forschungszentrum Jülich
Peter Grünberg Institut
SFB 1083 project A12
Tel.: 02461 61 4561
EMAIL

Smaller, faster, more efficient? – Review by A5 (Volz)

/in /by

In their review paper, Andreas Beyer and Kerstin Volz describe in detail their investigation of novel composite materials, which may eventually replace today’s silicon-based electronic devices. As the latter increasingly reach their performance limit, one option to overcome these largely physics-based limitations is to cover silicon with a different material layer.

Experimentally measured and simulated structure of the galliumphospide/silicon-interface at atomic resolution. Electron diffraction patterns allow to determine the interfacial charge distribution.

However, covering silicon with different material layers like, for example, well-suited III/V semiconductors (containing elements of the 3rd and 5th group of the periodic system) is challenging. In joining different materials with their individual physico-chemical properties their interface may be marked by defects. Here, for example, “erroneous” attachments may lead to unwished-for local charges – rendering the combined material as unsuitable for application in devices.

The group of Kerstin Volz closely studied galliumphosphide on silicon as a model system of III/V semiconductors on silicon. In their invited review the authors now describe the various electron-microscopy-based approaches employed in the study of the internal interface between the two materials and its defects. By means of transmission-electron-microscopy the researchers were able to show that the interface between the two materials is far from smooth; in fact, it more resembles a pyramidal structure affecting several atomic layers. In addition, it was also possible to “see” the erroneous atomic attachments which cause the unwished-for charge effect and link the phenomenon directly to changes in preparatory procedures.

The insights gained will be applied to perfecting preparation methods in order to reduce the number of defects and to fine-tuning the interface with a focus on raising the efficiency of existing devices and encouraging the development of novel applications.

Publication

A. Beyer and K. Volz,
Quantitative Electron Microscopy for III/V on Silicon Integration
Adv. Mater. Interfaces (2019) DOI: 10.1002/admi.201801951

Contact

Prof. Dr. Kerstin Volz
Philipps-Universität Marburg
SFB 1083 project A5
Tel.: 06421 28 22297
EMAIL