Apr 25, 2014
Watching phase transitions in 2D semiconductors
Researchers in Japan say that they have clearly visualized the semiconducting-to-metallic phase transformation in a technologically important single-layer material called molybdenum disulphide for the first time thanks to a state-of-the art scanning transmission electron microscope. Until now, such phase transitions were studied as collective motions of atoms but the new observations show that atom-by-atom movements are at play. The result might be important for "in-layer" assembly of electronics devices in the future.
Molybdenum sulphide, or molybdenite (MoS2) is a direct bandgap 2D semiconductor that might be ideal for making electrical and optoelectronics devices. The material boasts good charge mobilities of greater than 100 cm2/Vs (and perhaps even up to 500 cm2/Vs) – values that compare well to silicon. And being a “van der Waals solid” (made up of 2D sheets of Mo and S that are weakly bonded to each other), it is compatible with a variety of substrates – even transparent or plastic ones. Single-layer molybdenite is only about 0.65 nm thick, which means that devices made from it can suppress so-called short-channel effects, so allowing for very thin transistors.
MoS2 is unique in that it is polymorphic – that is, it has different electronic characteristics depending on how the S atoms are arranged in the structure. The material is either semiconducting (with a trigonal prismatic structure) or metallic (with an octahedral structure). The two phases can interconvert thanks to intralayer atomic plane gliding but such a transformation had never been directly observed in an experiment until now.
Gliding atomic planes
A team led by Kazu Suenaga at AIST in Tsukuba has seen that gliding atomic planes in MoS2 cause a new phase to nucleate in the material. Intermediate interfacial phases come out too. These transformations are triggered by the heat from the electron beam in the STEM the researchers employed to observe their sample.
In fact, the electron beam irradiation introduces a very small metallic domain in the MoS2 semiconductor matrix, which kick-starts the phase transition, explained Suenaga. Such a phenomenon could be used to intentionally induce the phase transformation in MoS2, and other such single-layered materials, in a controllable way.
“Our result implies that electronic devices, such as nanodiodes, might be made ‘in-layer’ rather than via layer-by-layer bottom-up assembly of layers with distinct properties, which is the conventional way of going about such fabrication,” he told nanotechweb.org. “Making nanodevices using bottom-up processes is no easy task but we show here that simple e-beam patterning can introduce nanoscale domains with distinct (metallic) electronic properties within a single-layer (semiconducting) matrix. We can make the structures with atomic-scale precision and even monitor how the device grows in situ thanks to observations in the STEM.”
Indeed, the researchers say that they have already produced prototypes of several nanodevices using their technique. For example, they made a serial junction of semiconductor and metallic phases, which is to all intents and purposes a Schottky diode. They also managed to produce a local semiconductor region sandwiched between two metallic electrodes to form a nanoscale transistor. And that is not all: they also found that they could embed a wire-shaped metallic region in the semiconducting matrix and use this as a quantum lead. Encouraging results for starters.
The research is detailed in Nature Nanotechnology doi:10.1038/nnano.2014.64.
About the author
Belle Dumé is contributing editor at nanotechweb.org