Earlier this spring two separate research projects were building transistors made solely from two-dimensional (2-D) materials. Argonne National Laboratory researchers described a transparent thin-film transistor (TFT) that they had created in the Nano Letters journal. They used tungsten diselenide (WSe2) as the semiconducting layer, graphene for the electrodes and hexagonal boron nitride as the insulator. A week later the ACS Nano journal published that researchers from the Lawrence Berkeley National Laboratory had also built an all 2-D transistor that took the shape of a field emissions transistor (FET). The Berkeley Lab FET used the same materials for their electrode and insulator layers as Argonne’s TFT, but used molybdenum disulfide (MoS2) as the semiconducting layer.
While the fabrication of transparent TFTs made from 2-D materials could lead to flexible displays with super-high density pixels, the impact of an all 2-D FET could potentially have a broader impact. FETs are nearly omnipresent, being used in computers, mobile devices, and many other electronic devices.
Issues with FETs prior to Berkeley Lab’s work has been that their charge-carrier mobility degrades because of mismatches between the crystal structure and the atomic lattices of the individual components, namely the gate, source and drain electrodes. These mismatches result in rough surfaces and in some cases dangling chemical bonds. The completely 2-D FET developed at Berkeley Lab eliminates this issue by creating an electronic device in which the interfaces are based on van der Waals interactions. These interactions represent all the attractive or repulsive forces between molecules that are not covalent bonds, instead of covalent bonding. “In constructing our 2D FETs so that each component is made from layered materials with van der Waals interfaces, we provide a unique device structure in which the thickness of each component is well-defined without any surface roughness, not even at the atomic level,” said Ali Javey, a faculty scientist in Berkeley Lab’s Materials Sciences Division. He also said that the approach “represents an important stepping stone towards the realization of a new class of electronic devices.” By having interfaces based on van der Waals interactions instead of covalent bonding, it will be possible to reach a degree of control in material engineering and device exploration that has yet to be seen.