May 31, 2013
MoS2 transistors get bendy
Researchers at the University of Texas at Austin and the University of Notre Dame, Indiana, both in the US, are the first to have succeeded in making high-performance molybdenite transistors on plastic substrates. The feat, hitherto deemed too difficult, means that the material might be ideal for making high-speed and low-power flexible electronics devices.
Made of molybdenum and sulphur, molybdenite (MoS2) – thought by many to be the semiconducting 2D crystal of the moment – has a direct bandgap (of 1.8 eV). This means that the material might be better than indirect bandgap silicon for making certain photonic devices. Indeed, some scientists are even starting to believe that it might be a real rival to "wonder material" graphene (which does not have a bandgap at all in its pristine state) for use in future electronic circuits.
A direct bandgap is important when it comes to making devices like LEDs, solar cells and photodetectors, and any other photonic devices that exploit electron–hole pair excitation, because devices made with direct rather than indirect gap semiconductors are more efficient. Having a bandgap also means that a device can be more easily switched on and off – crucial for making devices like transistors, for example.
The material also has good charge mobilities of greater than 100 cm2/Vs – and perhaps even up to 500 cm2/Vs – values that compare well to state-of-the-art silicon. And because it is a van der Waals solid (made up of 2D sheets that are weakly bonded to each other), it should be compatible with a variety of substrates – even transparent or plastic ones. However, previous attempts to mount MoS2 on plastics have not been very promising, with finished devices functioning quite poorly compared with those placed on conventional, hard, oxide substrates.
High on/off current ratios
Using standard lithography techniques, a team led by Deji Akinwande and Nanshu Lu at Austin and Debdeep Jena at Notre Dame has now succeeded in making multilayer MoS2 transistors on the flexible plastic Kapton that have high on/off current ratios – comparable to those seen in devices made on silicon-based substrates. The devices also appear to work using very little power and can be bent at will without suffering any damage to either their mechanical or electronic properties. "Indeed, the bending radius of 1 mm that we observed in our experiments is similar to that possible in graphene – the best-known flexible crystalline material," said Akinwande.
"We expect these transistors to find use in flexible and smart electronics," he told nanotechweb.org. "We now know that such 2D materials are suitable for unconventional substrates like plastics, glass and even fabrics, thanks to their thin nature and lack of 'dangling bonds' at the interface. Future electronics are likely to be increasingly flexible – for example in displays and 'roll-up' computers – so MoS2 could come into its own here."
The researchers, who have published their results in ACS Nano, are now looking at making flexible p-type transistors based on these 2D semiconducting crystals. "In our present work, we have fabricated flexible n-type transistors but to realize complementary digital systems, both n- and p-type transistors will be required," said Akinwande.
About the author
Belle Dumé is contributing editor at nanotechweb.org.