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Disputation Jan Niklas Kirchhof

May 03, 2024 | 03:30 PM
Thema der Dissertation:
Nanomechanics of two-dimensional materials: From tuning phonons to probing excitons
Abstract: Nanoelectromechanical systems (NEMS) based on two-dimensional (2D) materials represent the ultimate – atomic-scale – size limit for the miniaturization of mechanical devices. When employed as resonators, these devices bring a range of remarkable features. Due to their small effective mass, they oscillate at very high frequencies (up to gigahertz). Furthermore, it is easy to drive them in their non-linear regime. Additionally, they come with the exceptional ability to broadly tune their resonance frequencies. In this thesis, I realize two unique systems capitalizing on these properties.
First, I conceptualize a tunable phononic crystal made from graphene. By periodically patterning a suspended graphene membrane, I transform it into the world’s thinnest possible phononic crystal. This device features a phononic band gap in the megahertz range, which we can broadly tune using an electrostatic gating approach. In the next step, we demonstrate full control of the band gap size. By carefully tension engineering our suspended phononic crystal, we are able to dynamically reduce the size of the phononic band gap – down to completely closing it. This change in hierarchy/topology in the phononic band structure can be seen as the mechanical simulation of a metal-insulator transition. Upon placing an artificial irregularity in our phononic lattice, we can spatially localize a tunable mechanical ‘defect mode’. This mode is mechanically isolated from its environment, which makes it a highly coherent mechanical oscillator and a potential reservoir for storing quantum information.
Second, I develop a platform for nanomechanical spectroscopy of 2D materials. Specifically, I show that a purely mechanical measurement can be used as an ultrasensitive spectroscopic probe for transition metal dichalcogenides (TMDs) and plasmonic nanostructures. We extract the optical absorption of a 2D material from frequency shifts of a hybrid NEMS resonator vs. the wavelength of incoming light. In combination with optical reflectivity data, we derive — without any further assumptions — the full dielectric function of the material under study. Our measurement is fast and sensitive, and we can characterize 2D materials in a broad spectral range.
Summarizing, I developed a concept to add tunability to the field of phononics and an approach to spectroscopically characterize 2D materials based on mechanical measurements. These are powerful tools to realize tunable condensed matter physics analogs in phononic systems and to observe hitherto undetected phenomena in 2D materials.

Time & Location

May 03, 2024 | 03:30 PM

Hörsaal B (0.1.01)
Fachbereich Physik, Arnimallee 14, 14195 Berlin