„Microscopic understanding“ of a phenomenon is a synonym for the ability to describe it by first principles. The notion mirrors the outstanding importance of truly microscopic techniques for the investigation of matter. Here we use electron microscopy in various forms to study novel materials and combine it with laterally resolved and integrated spectroscopy. The range of problems that can be addressed comprises e.g. the investigation of the microstructure and the distribution of electric, magnetic and strain fields of solids, their chemical in homogeneities, the characterization (and even manipulation) of nano-objects, down to the direct observation of the atomic arrangement of a material. This information is naturally complemented by electronic structure investigations for which electron spectroscopy is the leading tool. We focus on photoemission and (in)elastic electron scattering using state-of-the art laboratory equipment and we develop new microscopy instrumentation and methods, e.g., for electron tomography or cryogenic microscopy.
The fundamental properties and the application of organic materials in electronic devices is an issue of intensive research. Potential devices are anticipated to enable flexible, low cost electronics. We carry out experimental studies of charge transfer processes, charge transfer states and charge transfer excitations in order to determine many relevant properties and to provide a comprehensive knowledge of these issues.
The material class of transition metal dichalcogenides (TMD) comprises a number of fascinating physical phenomena. For instance, various competing ground states – such as superconductivity and charge density waves – can be found in some of its representatives. Moreover, these materials have a two-dimensional crystal structure with weak bonding along the crystal c axis, which is reminiscent of graphene. In this regard, TMD materials are also considered to be representatives for future electronics beyond graphene. We investigate the electronic properties, in particular the dynamic response, which e.g. reveals intriguing dispersion relations for plasmons and excitons in some of these materials.
Fascinating and counterintuitive phenomena have beenobserved at the interface of certain complex oxides. The mostimportant is the appearance of metallic conductivity betweentwo firm insulators such as SrTiO3 and LaAlO3 reported by Ohtomo and Hwang in 2004. But also superconductivity and magnetism have been found. We investigate the electronic and chemical structure of these new interfaces which hold prospects for both, fruitful fundamental research and the implementation into devices.
The properties of heavy fermion materials sharply deviatefrom conventional metals, most notably in the occurrenceof very large effective masses at low temperatures. Thisis often accompanied by rich phase diagrams consisting ofmagnetic order and unconventional superconductivity. The electronic structure is determined by a subtle interplay between f-electrons and conduction electrons which can be analyzed by means of electron spectroscopy
Recently, an emergent class of magnetic inhomogeneities and textures, such as skyrmions and spin polarons opened up new avenues to a wide range of fundamental problems in spintronics, eventually enabling novel magneto-electronic applications such as storage devices. Substantial scientific progress on nanoscale magnetic inhomogeneities depends on qualitative improvements of the capabilities and detection limits of the current magnetic imaging and characterisation techniques. Here, we develop new techniques for high resolution imaging of magnetic textures using advanced electron holographic techniques. We particularly elaborate on the tomographic reconstruction of magnetic fields in 3D and the mapping of these fields at cryogenic temperatures under the application of external electric and magnetic fields.
Plasmon resonances are collective excitations of the conduction electrons in solids. In particular at surfaces of, e.g., metallic nanoparticles, spatially confined resonances referred to as localised surface plasmon resonances (SPR) or surface plasmon polaritons (SPP) lead to a range of extraordinary properties, such as strong and locally tunable transient electrical fields, which are very sensitive to nanometer scale environmental changes. Emerging opto-electronic devices exploiting SPRs comprise on-chip light spectrometers and linear accelerators, increased efficiency LED and photovoltaics, and metamaterials with properties such as negative refractive index and slow-light propagation and flat metalenses. Here we develop and apply advanced electron microscopy techniques such as SPR mapping to characterize the resonant modes in terms of energy spectrum and spatial distribution.
Mesocrystals are built from nanocrystals, which self-assemble into a crystalline superlattice, maintaining a specific crystallographic orientation across the individual nanocrystal building blocks. Different non spherical nanocrystals allow generating a variety of complex superlattices depending on the particular type of faceting. Mesocrystals are a new class of materials, allowing to extend properties of nanoparticles, such as superparamagnetism, to mesoscopic and macroscopic length scales. Here, we plan to investigate the structure of such crystals with advanced transmission electron microscopy methods (e.g., holography).
T. Li, V.K. Bandari, M. Hantusch, J. Xin, R. Kuhrt, R. Ravishankar, L. Xu, J. Zhang, M. Knupfer, F. Zhu, D. Yan and O.G. Schmidt
Fully integrated high-frequency molecular scale rectifiers based on organic nanostructure heterojunctions
Nature Communications (2020), im Druck
O. Kataeva,K. Ivshin, K. Metlushka, S. Latypov, K. Nikitina, D. Zakharychev, A. Laskin, V. Alfonsov, O. Sinyashin, E. Mgeladze, A. Jäger, Y. Krupskaya, B. Büchner, and M. Knupfer
Charge transfer complexes of linear acenes with a new acceptor perfluoroanthraquinone. The interplay of charge transfer and F···F interactions
C. Habenicht, J. Simon, M. Richter, R. Schuster, M. Knupfer and B. Büchner
Potassium-intercalated bulk HfS2 and HfSe2: Phase stability, structure, and electronic structure
R. Schuster, C. Habenicht, M. Ahmad, M. Knupfer and B. Büchner
Direct observation of the lowest indirect exciton state in the bulk of hexagonal boron nitride
Nonlocal dielectric function and nested dark excitons in MoS2, A. Koitzsch, A.-S. Pawlik, C. Habenicht, T. Klaproth, R. Schuster, B. Büchner, and M. Knupfer, npj 2D Materials and Applications 3, 41 (2019)
Thickness dependent electronic structure of exfoliated mono- and few-layer 1T′−MoTe2 , A.-S. Pawlik, S. Aswartham, I. Morozov, M. Knupfer, B. Büchner, D. V. Efremov, and A. Koitzsch, Phys. Rev. Materials 2, 104004 (2018)
Nearest-neighbor Kitaev exchange blocked by charge order in electron-doped alpha-RuCl3, A. Koitzsch, C. Habenicht, E. Müller, M. Knupfer, B. Büchner, S. Kretschmer, M. Richter, J. van den Brink, F. Börrnert, D. Nowak, A. Isaeva, and Th. Doert, Phys. Rev. Materials 1, 052001 (2017)
Polarization driven conductance variations at charged ferroelectric domain walls, A.-S. Pawlik, T. Kämpfe, A. Haussmann, T. Woike, U. Treske, M. Knupfer, B. Büchner, E. Soergel, R. Streubel, A. Koitzsch, L. M. Eng, Nanoscale 9, 10933 (2017)
Many solid state phenomena in modern materials are not well understood at the microscopic scale. For example, topological effects like Skyrmions have been observed as a lattice-like or diluted assemblies of spin vortices, however, their inner structure and interaction with with geometric constraints is frequently unknown. Another example pertains to the spatial and momentum distribution of the wide class of electronic excitations in geometrically confined structures (e.g., heterostructures, low dimensional materials) such as plasmons and excitons in the presence of boundaries and other spatial inhomogeneities.
To solve these questions we explore electron-matter interaction at the nanoscale in the electron microscope by developing and applying novel in-situ, holography, tomography, spectroscopy techniques as well as electron optical setups in the TEM. The range of questions includes the solution of phase problems by means of electron holography, the investigation of collective electronic excitations (e.g. plasmons, excitons), the analysis of local electrical, magnetic and mechanical fields in, e.g., semiconductor devices, nanomagnets, superconductors, organic materials, 2D materials and interfaces thereof.
Our specialities is high-resolution electron holography for atomic structure analysis, electron holography and tomography with nanometer resolution range for the measurement of three-dimensional electrical, magnetic and mechanical fields, in-situ experiments in the electron microscope, spatially resolved electron energy loss spectroscopy (EELS) as well as the theoretical modeling of the electron-object interaction.
Our tasks can be divided into two parts: the first is the methodical development of electron microscopy. These include new optical setups, holographic and tomographic procedures, construction of in-situ experiments, novel EELS setups and the accompanying theoretical modeling. The second area comprises materials science studies. Our focus is on semiconductor technology, nanomagnetism, plasmonic strutures and the study of complex oxides. A great variety of other solid-state physics problems result from the cooperation with external users. Our microscopes have special equipment for high-resolution and diffraction (structure analysis), holography and tomography (3D fields), electron energy loss spectroscopy and energy-filtered imaging (chemical element analysis).
Our experiments are carried out in close collaboration with a large number of researchers from all over the world. Amongst others we are supported by the ERC Starting Gratn AToM. Close collaborations exist among others with the TU Dresden, Max Planck Institute for Chemical Physics of Solids, and the Leibniz Institute of Polymer Research.
Group leader: Dr. Axel Lubk
Phone: +49 351 4659 302
Project duration: 2017 - 2021
The ongoing miniaturization in nanotechnology and functional materials puts an ever increasing focus on the development of three-dimensional (3D) nanostructures, such as quantum dot arrays, structured nanowires, or non-trivial topological magnetic textures such as skyrmions, which permit a better performance of logical or memory devices in terms of speed and energy efficiency. To develop and advance such technologies and to improve the understanding of the underlying fundamental solid state physics effects, the nondestructive and quantitative 3D characterization of physical, e.g., electric or magnetic, fields down to atomic resolution is indispensable. Current nanoscale metrology methods only inadequately convey this information, e.g., because they probe surfaces, record projections, or lack resolution. AToM will provide a ground-breaking tomographic methodology for current nanotechnology by mapping electric and magnetic fields as well as crucial properties of the underlying atomic structure in solids, such as the chemical composition, mechanical strain or spin configuration in 3D down to atomic resolution. To achieve that goal, advanced holographic and tomographic setups in the Transmission Electron Microscope (TEM) are combined with novel computational methods, e.g., taking into account the ramifications of electron diffraction. Moreover, fundamental application limits are overcome (A) by extending the holographic principle, requiring coherent electron beams, to quantum state reconstructions applicable to electrons of any (in)coherence; and (B) by adapting a unique in-situ TEM with a very large sample chamber to facilitate holographic field sensing down to very low temperatures (6 K) under application of external, e.g., electric, stimuli. The joint development of AToM in response to current problems of nanotechnology, including the previously mentioned ones, is anticipated to immediately and sustainably advance nanotechnology in its various aspects.
DFG SPP 2137 LU 2261/2-1
Project duration: 2018 - 2021
DFG LU 2261/2-1
Project duration: 2019 - 2021
This Independent Junior Research Group lead by Dr. Aliaksei Charnukha is funded by an Emmy Noether Starting Grant from the German Research Foundation (DFG). The group investigates equilibrium and dynamic emergent phenomena in complex correlated-electron materials by means of terahertz near-field microscopy and spectroscopy at cryogenic temperatures.
Strongly correlated electron systems and the diverse emergent phenomena they host are one of the cornerstones of contemporary condensed-matter physics research. The intimate interplay of lattice, orbital, spin, and charge degrees of freedom emphasized by strong electronic interactions has been found to give rise to an enormous complexity of correlated phases and physical properties. Due to the strong coupling between the aforementioned degrees of freedom, it is possible to navigate between different phases by means of easily accessible external parameters such as static or transient lattice strain, static or transient electric/magnetic fields, and temperature. In many cases the transition between phases is of first order, entailing a phase separation in real space and the occurrence of substantial hysteresis in physical properties. The latter is of substantial technological importance because it provides persistent electronic states that can be switched at will: an essential property of the next generation transistor. Such an implementation is particularly appealing as it minimizes leakage current, which limits further miniaturization and reduction in power consumption of modern electronic devices. It is, therefore, of both fundamental and technological importance to achieve a good understanding of the formation, dynamics, and interplay between various correlated phases. Given that the phase separation typically occurs on the nano-/mesoscale and that the phases often exhibit dramatically different transport properties (such as a metal-insulator transition in VO2), a spectroscopic, low-energy and -temperature, ultrafast optical probe with nanometer spatial resolution is indispensable. Our group is currently constructing such an instrument based on the unique combination of already existing and well-established techniques: scattering-type near-field optical spectroscopy affording spectroscopic investigation of quasiparticle optical response at sub-10 nm spatial resolution, coupled with ultrafast time-domain terahertz spectroscopy providing access to the static and transient lowest-energy material optical response, all at cryogenic temperatures down to 1.6 K required for state-of-the-art condensed-matter research.