By G.P. Thomas
Introduction to Graphene
New Research into 2D Materials
Introduction to Graphene
Graphene was first identified in 2004 and for the last few years has been the focus of much scientific research. Graphene is a very flat sheet of carbon (just one-atom thick) with its atoms arranged into a 2D honeycomb structure, making it the thinnest man-made material ever.
The unique properties of this innovative material have gained widespread interest, and it is hoped that it can soon be utilized in the manufacturing industry, sparking somewhat of a global material revolution. Graphene is stronger than diamond, 100 times stronger than steel, and as flexible as rubber. It is also a good conductor of electricity, even better than copper.
However, the 2D materials world does not begin and end with graphene, and advancements in this field are being made all the time. Below, we look at some of the 2D materials that are currently being researched and how they may support and enhance graphene - or even compete against it.
New Research into 2D Materials
Until 2011 graphene was the only 2D material that could be easily and consistently synthesized in large quantities for device manufacture. A research team from Singapore created a new process to create a large number of single-layer nanosheets in a quick and efficient manner wherein they can be applied to any suitable bulk material. The process requires inducing lithiation with an electrochemical base where the materials can be carefully monitored until they reach a state that is perfect for ultrasonication and exfoliation. The process resulted in a molybdenum disulfide (MoS2) nanosheet yield of 92% within 6h at room temperature conditions.
To emphasis the relevance and importance of these MoS2 sheets, the researchers created a proof-of-concept nitric oxide detector using the sheets. As the sheets are electrically conductive over large areas, and operate as p-type semiconductors, they tend to respond electronically to different nitric oxide concentrations. With the addition of gold electrodes to this setup, the response could measured, thus this detector device showed great potential with a detection limit of 190 ppt, which is ideal for a promising sensing platform.
Further experiments yielded tungsten disulfide (WS2) and titanium sulfide (TiS2) sheets. The success of this innovative process made the researchers realize that it could be adapted to produce a range of 2D nanosheets that can be used in areas such as catalysis, electronics, and sensing.
Scientists from the Commonwealth Scientific and Industrial Research Organisation (CSIRO), Australia's national science agency, and RMIT University have created a new 2D nano-material made up of layers of crystals of molybdenum oxides, which is likely to revolutionise the electronics sector. The distinctive properties of the new material allow free flow of electrons at ultra-high speeds without scattering.
One of the key achievements was that the researchers were able to remove the usual obstacles that hindered the free flow of the electrons, which is a crucial step in high-speed electronics. The opportunities for the use of this material are numerous. Electronic devices can be built smaller and can transfer data at much higher speeds. However, more research is required to study the potential of this material, before it is moved from the lab to the field.
Although graphene is an exceptional electronic material, conductors alone will not suffice to build a working device. An insulator would also be required. Studies revealed that hexagonal boron nitride (h-BN) works well as an insulator. In February 2013, scientists at Rice University revealed that they were able to combine graphene and hexagonal boron nitride into sheets which were built into a variety of patterns at nanoscale dimensions. This was an essential step towards creation of 2D electronics with a process to make patterns in atom-thick layers (that combined a conductor and an insulator).
The technical capabilities at Rice allowed them to create materials with a resolution of about 100 nm; however, the scientists believe that it would be possible to make fully operational 2D devices with circuits that are 30-20 nm wide.
H-BN resembles graphene with the same atomic array. Finely detailed patterns of graphene could thus be laced into gaps created in sheets of h-BN. It is possible to imprint combs, bars, concentric rings and even microscopic Rice Owls via a lithographic process. The interface between the materials shows a razor-sharp transition from graphene to h-BN along a sub-nanometer line. The process is well engineered and enables control of the domain sizes and domain shapes, which are important to manufacture electronic devices.
Another Rice researcher Li Song completed an experiment wherein pure h-BN sheets (one to five atoms thick) could be deposited onto a copper substrate. The sheets could then be transferred to other substrates. His research team used a chemical vapor deposition process to deposit the h-BN sheets onto a 5 x 5 cm copper substrate at temperatures of about 1000° C (1832°F). With the aid of these techniques graphene can very well replace silicon.
Researchers are looking into the possibility of placing a third element - a semiconductor - along with the insulator and conductor combination into a 2D fabric, which would help in manufacturing flexible electronics.
The new 2D materials can be applied to creating nanoscale field-effect transistors, biosensors or quantum capacitors. They could also be widely used in graphene-based electronics.