Tribology at the nanoscale – fundamental mechanisms
Tribology is the science and technology of interacting surfaces in relative motion to each other. The interactions at a moving interface control its friction, wear, and lubrication behavior. The control of friction and wear is highly important for the long-term reliability of mechanical components in practically every industrial application. In that context, environmentally friendly tribology also gains importance, which aims at saving energy and material, optimizing the product usage and reducing the energy consumption. To reach these goals the tribological mechanisms have to be understood.
Ultimately, friction and wear phenomena originate from interatomic and intermolecular interactions. At the nanoscale, simple empirical friction laws can lose their validity. This is due to the large surface-to-volume ratio and the greater importance of surface chemistry, adhesion and roughness. Adhesion as well as friction and wear at the nanoscale are governed by non-equilibrium processes. For example, between two shearing surfaces the local pressure can vary between 100 kPa and several GPa within microseconds. Hence, conventional techniques utilized for examining the tribology of large objects are often ineffective at the nanometer scale and, thus, new monitoring methods are necessary.
The field of nanotribology is an emerging and a highly innovative topic at the interface of mechanical engineering, chemistry and physics. It involves experimental and theoretical studies of adhesion, wear, friction and lubrication at the atomic and molecular level. Their nanoscale description, in particular the processes of energy dissipation, are among the open aspects of the nanotribology. Phonons, electron-hole pairs and electronic excitations, which can decay via emission of electrons or photons, contribute to nanoscale friction. Determining the significance of the different contributions in dependence on external (nonlinear) conditions, such as pressure and temperature, is challenging and far from being understood. We aim at describing the physical-chemical behavior of the tribological surface material at the atomic/molecular level using optical spectroscopy. We analyze phase transformations, mixing processes between the solid surface and the lubricant, particle adsorption, oxidation or, for example, the impact of the structural orientation of molecular bonds on friction. The central methods of choice are confocal Raman scattering with tunable excitation wavelength, Brillouin scattering and Fourier-transformed infrared spectroscopy. The energies and dispersions measured as well as efficiencies of the interatomic vibrations allow for revealing the forces responsible for nanoscale interactions.
Based on the spectroscopic data and their correlations with conventional macro- and microscopic parameters, a model shall be developed to predict the behavior of the tribological surface material so that wear, fatigue or, for example, crack initiation can be identified by changes already at the atomic and molecular level.
In-situ and operando sensing with optical spectroscopy
Besides addressing fundamental mechanisms of nanotribological processes, as described above, we aim at improving and developing laser spectroscopic techniques to characterize surfaces of, e.g., mechanical components on site (“in-situ”) and during the application of load (“operando”). By achieving a spatial resolution of 1 μm and a temporal resolution in the sub-μs range, physical and chemical processes at the surface can be identified and underlying mechanisms at the molecular scale can be understood. To follow changes in surface properties, light scattering methods combined with imaging techniques will be exploited, even across extensive spatial areas. It is the aim to describe the functionality of surfaces of mechanical components, to assess the surface quality in a non-destructive and contactless way and, ultimately, to realize real-time sensing.
Spins and pseudospins of excitons in tailored MoWSe alloys
Transition metal dichalcogenides (TMDCs) are typically binary semiconductors of the type MX2, where M is a transition metal atom, such as Mo or W, and X is a chalcogen atom, such as S or Se. Like graphene, they are very popular due to their fascinating properties. They combine atomic-scale thickness, a direct band gap in the visible spectrum and strong spin-orbit coupling with promising electronic and mechanical properties. This unique combination of properties makes them attractive to fundamental studies and applications in, for example, energy harvesting, DNA sequencing and opto-electronics [Nature Reviews Materials 2, 17033 (2017)].
A trend in future electronics is to utilize internal degrees of freedom of the electron, in addition to its charge, for nonvolatile information processing. Suitable candidates, like TMDCs, include the electron spin and the valley pseudospin; TMDCs offer an exciting platform to explore valley- and spintronics. However, a great number of questions about the excitonic spins and pseudospins is still waiting for answers.
Why ternary Mo1-xWxSe2 alloys? Depending on the composition (x value) of these TMDCs, the energies of the exciton spin states can be tuned in a sophisticated manner. Such engineering of the exciton energies will gain novel and thorough insights into the spin and valley pseudospin properties and dynamics. For fine tuning of the spin state energies and manipulating the spin-orbit coupling of the electrons and particularly holes, strain will additionally be applied to the substrate material of the MoWSe alloys. Using these energy tuning possibilities, magnetic resonance imaging of the exciton spins shall be realized to make a step towards spin-based quantum information devices and sensor technologies.
Interactions between magnetic ions and carriers confined in doped semiconductor structures
Dilute magnetic semiconductors offer a great potential for spin-based applications due to the existence of giant magneto-optical effects. Traditionally, transition metals (TMs), like Mn2+ ions, are implemented in semiconductor structures. Now, semiconductors, which contain both Mn2+ ions as well as rare-earth (RE) ions with particularly large magnetic moments, will be in the focus of stationary and time-resolved spectroscopic studies. The interactions between these ions and carriers, mainly s/p-d and s/p-f exchange interactions, will be investigated on time scales ranging from picoseconds to minutes, addressing phenomena such as spin precession, spin memory, and spin diffusion. Not only individual spin interactions, but also collective phenomena, like magnetic polaron formation and the interaction of magnetic ions with a 2D carrier gas, resulting in Kondo physics, are of decisive interest.
The coexistence of 4f and 3d electrons in mixed RE-TM semiconductor structures is expected to create novel magneto-optical phenomena with interesting implications for spin manipulation and quantum information processing. Complexes made from RE ions exhibit a very strong ionic anisotropy, while they usually display a weak exchange coupling. However, in such mixed structures the exchange splitting can be enhanced remarkably. Thus, a strong magnetic anisotropy is combined with a strong exchange interaction which could realize a long-standing goal in the design of quantum magnets.
Current offers for Bachelor-, Master- or PHD-theses
Please contact Jörg Debus if you are interested in thesis work on these topics.
- Dr. Jörg Debus
- Hendrik Bökenkamp
- Gabriel Brune
- Nils Denkmann
- Janina Schindler
- Carl Arne Thomann
- Adrian Wittrock