Bulk acoustic wave (BAW) filters are ubiquitous in our every day lives. They are the devices enabling the reliable 4G and 5G telecom technologies. BAW resonators, or thin film bulk acoustic wave resonators (FBAR) are composed of a piezoelectric material sandwiched between electrodes, where the thickness of the piezoelectric greatly determines the resonance (filtration) frequency of the resulting device. In order to reach the next bandwidth, the piezoelectric layer must be further thinned down. However, currently used polycrystalline piezoelectric technology and the impedance of the electrode material poses a physical limit on the maximum frequency which can be realized. 

In this work, we combine the best qualities of 2D materials and single crystal free-standing complex oxides into the FBAR device geometry to produce the world's thinnest and fastest FBAR resonator. Graphene is used as the electrode since it can sustain lossless transmission beyond 300 GHz, and single crystal free-standing BaTiO3 is used due to its high piezoelectric properties - similar to that of PZT but in the absence of lead allowing future scalability and RoHS compatibility. In the membrane mode, the graphene-BaTiO3-graphene heterostructure demonstrates ferroelectric switching, and mechanical memory effect. In the breathing mode, the device resonates at 233 GHz, which is sitting comfortably in the 6G telecom band.

2D materials such as graphene have been predicted as the next platform to make MEMS sensors. However, the very fact that 2D materials have weak van der Waals gap is prohibiting them from producing the next generation pressure sensors, which need air tight clamping of the membrane to separate the reference pressure from the outside. In our earlier work (Nano Lett. 2019) we successfully demonstrated that the gases leak through the van der Waals interface in 2D materials, and that further sealing procedure is necessary to hermetically seal the 2D materials before they can be used as pressure sensors. In this work, we demonstrate that free-standing complex oxides - 3D analogs of 2D materials with similarly impressive mechanical properties and gas impermeability - can have much stronger adhesion to the substrate due to the thermally activated bonding to the substrate thanks to their 3D nature. 

At the time of writing in 2023, there is a global chip shortage. Chips are made on Si substrates which require sophisticated foundries and CMOS compatible cleanroom procedures. In this work, we make highly sensitive thermal sensors fabricated on paper substrates which are significantly cheaper than Si. The sensors are fabricated by by rubbing a 2D semiconductor WS2 onto the paper and creating electrical contacts via graphite pencil. Resulting device is sensitive enough to measure the temperature difference between the inhaling and exhaling of a human. 

Thermodynamic properties can shed a lot of information about a material. In particular, the anomaly in the specific heat and the thermal expansion coefficient at the phase transition temperature is a smoking gun signature of all phase transitions. However, traditional methods of measuring the thermodynamic properties are incompatible with 2D flakes which are ~ microns wide and ~ nanometer thick. This is unfortunate, because the electronic and phononic properties of many 2D materials drastically change with the number of layers in the few-layer-limit. In this work, we show a new method of extracting the thermodynamic properties of 2D flakes by measuring the nanomechanical resonance as a function of temperature. By analysing the resonance frequency we extract the specific heat and the thermal expansion coefficients, both of which show sharp anomalies at the phase transitions of the materials.

Graphene is the strongest, lightest and electrically conductive material that is also impermeable to gases. These qualities make graphene an ideal material platform for next generation MEMS gas sensors. Unfortunately every study so far has surprisingly shown that graphene membrane devices always have small - but non-zero - gas leakages. In this work, we investigate the possible permeation pathways and provide a way to completely shut off the gas permeation. To our surprise, we discovered that majority of the big gases such as the diatomic nitrogen leak through the van der Waals interface while the small gases such as helium permeate through the substrate itself.