Tuesday, 30 May 2017: 10:35
Chequers (Hilton New Orleans Riverside)
As the first issue, transition-metal dichalcogenides (TMDs) have reported on the piezoresponse of the materials as an external force is applied due to their broken inversion symmetry. TMD materials exhibit a strong piezoelectricity in their monolayer configuration, but the piezoelectric effect disappears or is significantly reduced when more than two layers are present. Piezoelectric devices employing monolayer TMD materials are not practically feasible because they lack the mechanical durability that is needed, and such mechanical robustness is crucial to develop useful piezoelectric devices that can be engineered using a multilayered structure. However, multilayered TMDs have a greatly reduced or absent piezoelectricity since the continuous growth of multilayered TMD leads to a stable stacking structure with alternating polarization directions in neighboring layers. We report on a simulation and experimental observation of piezoelectricity in mono/bilayer WSe2 synthesized via CVD and turbostratic stacking. The piezoelectricity and mechanical durability of the mono/bilayer WSe2 were assessed by conducting PFM measurements, a DFT simulation, and fabricating and characterizing the piezoelectric energy harvesters. As the second issue, I will present about the demonstration of large electric power generation from a single moving water droplet on a monolayer graphene, producing an output power of about 1.9 μW, which is about 100 times larger than the power output achieved in previous reports. This result is explained to be a result of the change in triboelectrification-induced pseudocapacitance between a water droplet and the monolayer graphene on polytetrafluoroethylene (PTFE). Positive and negative charges were found to respectively accumulate at the bottom and top surfaces of graphene on PTFE by the triboelectric potential generated during the graphene transfer process. The negative charges accumulate onto the top surface of the graphene and are driven forward by the moving droplet, charging and discharging at the front and rear of the droplet. As the third topic, I am going to introduce a graphene tribotronic touch sensor which is based on coplanar coupling of a single electrode mode triboelectric nanogenerator (S-TENG) and a graphene FET. When any object (e.g. human finger) comes into contact with friction layer of the S-TENG, the charges are produced due to well-known triboelectric effect. The triboelectric charges act as a gate bias to the graphene FET and modulate its current transport. The tribotronic transistor therefore does not require any external gate voltage as in traditional metal oxide field effect transistors. The tribotronic sensors have displayed a sensitivity of ~2% kPa-1, a limit of detection <1 kPa, and a response time of ~30 ms.