Promotie D. Davidovikj: membranen

23 februari 2018 15:00 - Locatie: Aula, TU Delft - Door: webredactie

Two-dimensional membranes in motion. Promotor 1: Prof.dr. P.G. Steeneken (TNW); Promotor 2: Prof.dr.ir. H.S.J. van der Zant (TNW). 

Not long ago the ultimate limit of how thin a material can be made was reached: a single layer of carbon atoms called graphene. Its discovery was followed by an entire zoo of other materials that can be thinned down to a single atomic layer, a family of so-called two-dimensional (2D) materials. Despite their sub-nanometre thickness, most of these materials have proven to be among the mechanically toughest ever tested.
Nowadays, many of the electronic appliances that we use, such as phones, tablets and laptops, employ one or more sensors that are based on tiny moving elements. Usually, these moving elements are brought into motion and/or their motion is read out electrically, hence their name micro-/nanoelectromechanical systems (MEMS/NEMS).  The displacement of the moving elements can be directly related to acceleration, orientation, ambient pressure, sound (microphones) and various kinds of external stimuli. This is where the mechanical strength, combined with the low mass and thickness of 2D materials can play a crucial role for further miniaturization of the mechanical elements in NEMS devices. This thesis explores the synergy between 2D materials and NEMS. As the title suggests, we fabricate two-dimensional membranes, tiny (nano)drums, by means of mechanical exfoliation and we set them in motion using electrical signals. The tiny displacements of the membranes are measured using an ultra-sensitive optical technique: laser interferometry. What we usually do is “flick” these drums and listen to the “sound” they make. The frequency of this sound and the duration of its ring-down tells us a lot about the mechanical properties of the material. Moreover, when subjected to external stimuli, such as heat, pressure or gas composition the frequency of the sound they make changes. In essence, the idea is to use this change in tone to tell something about the environment, i.e., to use the 2D membranes as sensing elements. The journey starts by exploring and characterizing the vibrations of the membranes in more detail: what exactly do the vibrational shapes look like and how can we tell more about their mechanical properties by listening to the sound they make? To this end, we imaged the vibrational shapes of the membranes with sub-micrometre resolution and managed to relate them to defects in the membrane. By driving their motion at high amplitudes, we developed a method to extract the elastic modulus of the membranes without making physical contact, just by listening to the sound they make. We continue by building a pressure sensor using graphene membranes, this time employing a fully electrical readout, which is an essential aspect of the integration of 2D materials into commercial sensing devices. Thereafter, we dwell on concepts where the membranes can instead be used as actuating elements, e.g. for pumping very small quantities of gases at the nanoscale. Finally, we explore a new family of materials that can be engineered in a layer-by-layer fashion and can combine many functionalities in a single membrane only a few nanometres thick.

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