Graphene has long been regarded as a “wonder material”, but its use in a number of interesting practical applications has been held back by its poor performance as a semiconductor. This is because graphene does not have an energy band gap, unlike molybdenum disulfide (MoS2), another monolayer material.
As previously reported on New Materials News, research into this new material could pave the way to the construction of one-atomic layer thick semiconductors, important for the development of various consumer electronics, including wearable intelligent devices.
In this two-part interview, New Materials News speaks to Professor A.T. Charlie Johnson about his research on molybdenum disulfide; firstly about how the material fills in the gaps left by graphene and secondly, how his research has implications for the mobile health and military industries.
Professor Johnson is a Professor of Physics and Director of the Nano/Bio Interface Center at the University of Pennsylvania. He completed his Ph.D at Harvard University before joining the University of Pennsylvania in 1994.
In layman’s terms, what is molybdenum disulphide and why is it worth researching?
The story really starts with graphene. Graphene and molybdenum disulphide (MoS2) are both “monolayer materials”. Monolayer materials are one atomic layer thick and they’re interesting as they can be very high performance and could be useful in advanced computer chips or even in flexible wearable systems.
Graphene started the ball rolling but it only has one set of properties and we want materials with many types of properties. Particularly important are semiconducting behavior and light emission. The nice thing about MoS2 compared to graphene is that MoS2 is really a semiconductor, it has an energy band gap whereas graphene is between being a metal and a semiconductor.
Also, MoS2 will emit light and absorb light very efficiently although only when it’s one atom layer thick. Interestingly, if you get thicker MoS2, say two, three or four layers-thick, it doesn’t emit light efficiently. In technical speak, you’d say single layer MoS2 is a direct band gap semiconductor but multi-layer MoS2 is indirect band gap.
What got you interested in researching MoS2?
I have been interested in one layer thick materials for some time. I started working on carbon nanotubes in 1998, and then moved into graphene around 2008. We have also worked on monolayer boron nitride. So, it was very natural for my interests to turn to monolayer MoS2.
Many of the experimental methods and approaches to scientific understanding are common among all the monolayer materials. This is one reason why progress in this field is so rapid. There is a very well developed infrastructure of measurement techniques and understanding.
Will MoS2 replace graphene as the newest ‘wonder material’?
I think they’re complementary materials that have different attributes. Graphene is a wonder material but it can’t do everything. It’s interesting to think about how you could make progress by combining graphene with other very lightweight flexible materials that have other properties, boron nitride, for example.
If you’re going to make flexible systems you would want something like graphene but you’d also want an insulator like boron nitride. MoS2 is a nice pure semiconductor and it also has interesting photonic properties, it can absorb/emit light which graphene doesn’t do as easily.
Although we’re not working in it, it might be interesting to ask can we make magnetic materials? Maybe we could do spintronic-type applications? I would say we’re expanding the tool set for monolayer materials and it’s a huge field.
Tell us about the new process you have developed for creating MoS2.
We use a seeding process. It turns out when you grow a large area of MoS2, if you want to grow a continuous sheet over a large region, the properties tend to vary greatly from location to location. That has to do with the details of the way the material grows, using the methods that are currently being used.
Why is ‘seeding’ better than the old process?
The process that we demonstrated gives us the ability to grow the material in desired locations which is definitely an advance over the current process because we could rationally make large numbers of devices that we could expect to work. Previously, scientists had grown small areas of high quality MoS2 in random locations and that makes it difficult to make devices in some rational way. You could make the devices, you could go look and find some of the MoS2 and connect to it but with our process, if I make MoS2 in known locations then I can make devices in those locations and then it’s guaranteed to work.