|Posted on May 9, 2016 at 6:30 PM|
Graphene caused a lot of excitement with its one atomic layer-thick form and extraordinary properties. But this just opened the door to a whole load of other new 2D materials. Some of the more successful ones so far have been transition metal dichalcogenides (TMDs) like sulfides, selenides and tellurides of molybdenum and tungsten (MoS2, WS2, WSe2, WTe2, MoTe2 and MoSe2. These ultimately thin sheets of atoms are held together by relatively weak van der Waals forces, but have interesting hold optical, electronic, and mechanical properties.
Unlike graphene sheets (which are metallic in bulk), these TMDs have a direct band gap in the visible-near IR range, like silicon, which is the heart of modern electronic devices. They also have high carrier mobilities and large on/off ratios, like Si, but their nanoscale dimensions offer new approaches for electronics, integration with photonics, even quantum electronics. TMDs also tolerate the mechanical flexion needed if deposited on flexible supports.
The critical need in this area is to be able to synthesize TMDs as highly crystalline bulk and thin films. Cecilia Mattevi (Imperial College London) has recently published a high profile review that summarizes progress towards making high quality TMDs more attainable, even at wafer scale. [Reale et al., Applied Materials Today 3 (2016) 11].
She and her colleagues describe how well established chemical vapor transport methods developed in the 1970s and 1980s are now being extended to produce bulk single crystals of group VI chalcogenides, like the ones listed above. CVD growth of TMD thin films, like MoS2 and WS2, has been achieved using volatile chemical intermediates at lower temperatures. If you want very thin layers, ultrathin transition metal or metal oxide films can be made into TMDs simply by furnace heating with chalcogen vapors or using physical vapor deposition. They even detail a one-step method that creates atomically thin TMDs by evaporating metal oxide and chalcogen powers simultaneously. This approach produces high-quality TMD monolayers with grain sizes up to the millimeter scale.
(Cecilia Mattevi's research group at Imperial College.)
Cecilia Mattevi is a Royal Society Research Fellow at Imperial College’s Materials department. She received her PhD in Materials Science in 2008 at the European Synchrotron Facility Elettra, Trieste in Italy. She was a post-doc with Prof. Manish Chhowalla at Rutgers University, NJ, USA, working on graphene for large-area optoelectronic applications, before joining Imperial College in 2010.
- Written by Paulette Clancy
(Photo credit: Used with permission, credit to: Imperial College London).