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Miracle material graphene has no band gap, meaning it cannot stop conducting electricity and thus is not as useful as it could be in making transistors. Researchers are trying to change that.
Graphene’s innumerable uses have been tirelessly touted since its discovery in 2004. This wonder material, derived from graphite, is seen as a game changer for various sectors, including telecommunications, health and engineering. But this one-atom-thick sheet of carbon, which is many times stronger than steel and is a good conductor of heat and electricity, has no band gap, meaning that it cannot stop conducting electricity.
What that means is that unlike silicon, graphene’s electrons are always on the move. And that is a drawback for making transistors, which need to be able to switch on and off. The lack of a band gap “poses a problem for controlling the flow of electrons through the material, which is a requirement for any electronic device,” Chemistry World reported last month.
Why are we talking about molybdenum disulphide?
As researchers search for ways to add a band gap to graphene, a new material — molybdenum disulphide (MoS2) — that naturally possesses a band gap has garnered some attention. The Institute of Electrical and Electronics Engineers (IEEE) said last month that because of MoS2 “graphene is losing favor as the two-dimensional material of the future.”
Quoting research from Switzerland, the IEEE said that MoS2 may become preferable to graphene in a post-silicon world. The article said that MoS2 has become so attractive that “even the discoverers of graphene are now focusing much of their research into using MoS2 … researchers at MIT, who have struggled to get graphene to do anything in electronics except for some radio-frequency applications, have turned to MoS2 and have quickly managed to get the one-atom-thick material to serve as the basis for a variety of electronic components.”
Using large sheets of MoS2, MIT researchers have produced an inverter, a Negated AND gate, a memory device and a ring oscillator. The MIT researchers believe that this list of electronic components is only the beginning of what is possible, the IEEE reported, adding that one of the researchers believes that the material could find early applications in large-screen displays in which a separate transistor would control each pixel of the display.
Keeping graphene in the race
With options like MoS2 being looked into, the race to solve graphene’s band gap problem is heating up. Researchers are busy drawing up ways to make hybrid graphene materials that will be suitable for electronic devices.
Researchers at Cornell University have come up with a way to combine graphene with boron nitride (BN) to create a two-dimensional hybrid structure that could solve the band gap problem.
“Nobody knows yet which horse will win the race as there are a few proposals for graphene-based electronic devices,” Antonio Castro Neto, a graphene expert at the National University of Singapore, was quoted as saying by Chemistry World.
Neto said the research at Cornell is “very promising … [i]t proves what we have been saying for quite sometime, namely that graphene alone will have limited impact in electronics but when it is combined with other two-dimensional crystals, such as BN, the possibilities are only limited by our own imagination.”
Another possible solution comes from researchers at IBM and the University of California Riverside. That team has succeeded in making “the narrowest ever nanoribbon arrays of epitaxial graphene on a silicon carbide wafer,” Nanotechweb.org reported.
One method of introducing a band gap into graphene is to make extremely narrow ribbons of the material, the researchers said. Christos Dimitrakopoulos of IBM told Nanotechweb that by fabricating dense arrays of graphene nanoribbons that are 10 nanometers wide, the team has “taken an important step towards addressing this shortcoming of graphene.”
Although much work still needs to be done before GNRs (graphene nanoribbons) find their way into applications, the next logical step is to try to make GNR devices, such as field-effect transistors, Dimitrakopoulos added. “It will also be important to study the structural quality of GNR edges and experiment with different edge-terminating atoms. Increasing the band gap by further reducing the width of the GNRs would also be good thing to aim for.”
