The Role for Graphene in Electronics Outside of Digital Logic
While the role for graphene in digital logic applications appears to be limited—despite grand efforts to engineer a band gap into it—this doesn’t mean that graphene won’t have an important role to play in electronic applications.
In this last quarter, since our previous newsletter, we have seen some pretty significant developments in using graphene for sensor and display applications that could enable new approaches to all sorts of electronic devices from our mobile devices to even future devices, like quantum computers.
Banging the Graphene Drumhead
In research that builds on work that was done originally at the National Institute of Standards and Technology (NIST) and the University of Maryland in 2012, researchers TU Delft’s Kavli Institute of Nanoscience in the Netherlands has demonstrated that using this drumhead principal for graphene could lead to new types of sensors for mobile phones, or even quantum memory used in quantum computing.
The Dutch research team, which published their results in the journal Nature Nanotechnology, used the graphene as a mirror in optomechanical cavity and shot microwave-frequency light at it to essentially bang the graphene drumhead.
“In optomechanics you use the interference pattern of light to detect tiny changes in the position of an object, explains Dr. Vibhor Singh, one of the researchers, in a news release. “In this experiment, we shot microwave photons at a tiny graphene drum. The drum acts as a mirror: by looking at the interference of the microwave photons bouncing off the drum, we are able to sense minute changes in the position of the graphene sheet of only 17 femtometers, nearly 1/10000th of the diameter of an atom.”
The pressure that is created from the photons striking the graphene drumhead creates a kind of amplifier. In a mobile phone application, the mechanical motion of the drum would amplify the microwave signals employed by the phone.
While highlighting the mechanism’s potential in mobile phones is a smart move on the part of the researchers, they also suggest that the technology could be used for more far off technologies, such as quantum memory for quantum computers, which are computers that exploit the laws of quantum mechanics to solve complex problems much faster than conventional computers do.
“One of the long-term goals of the project is explore 2D crystal drums to study quantum motion,” said research group leader Dr. Gary Steele in the release. “If you hit a classical drum with a stick, the drumhead will start oscillating, shaking up and down. With a quantum drum, however, you cannot only make the drumhead move up and then down, but also make it into a ‘quantum superposition’, in which the drum head is both moving up and moving down at the same time.” Steele added: “This ‘strange’ quantum motion is not only of scientific relevance, but also could have very practical applications in a quantum computer as a quantum ‘memory chip’.”
First Graphene-enabled Flexible Display Demonstrated
Another area of electronics that may not be in the area of digital processing but is no less important for most of the devices we use every day is digital displays.
Now a research partnership between the Cambridge Graphene Centre, located at the University of Cambridge, and Plastic Logic have demonstrated what they claim is the first graphene-based flexible display ever produced.
For years now, researchers around the world have been looking to graphene to replace the expensive and brittle indium-tin oxide (ITO) that is used as a transparent conductor to control display pixels.
The Cambridge/Plastic Logic team has in this case developed an electrode for the pixel electronics made from solution-processed graphene. Plastic Logic has basically replaced the sputtered metal electrode they typically used in their devices with the graphene-based electrode.
The prototype the team built was a electrophoretic film display, which is the kind you see on an e-reader that only displays black and white images. However, the research team plans in the future to use the graphene-based electrode in combination with liquid crystal (LCD) and organic light emitting diodes (OLED) technology to produce color images.
New Graphene–based Sensor Could Improve Night-vision Goggles and Airport Body Scanners
In collaborative research, a team from the University of Maryland and Monash University in Australia has built a sensor for detecting long wavelength (far infrared or terahertz) light that is as sensitive as any existing detector, but far smaller and more than a million times faster.
The research team played to graphene’s strengths, which, in this case, is its ability to absorb radiation, from the ultraviolet to the terahertz regions. The sensor works on the principle of the photothermoelectric effect in which photons striking the graphene cause electrons in the material to jump to a higher energy level, or heat up. In most other materials this energy would quickly dissipate because of the vibrations in the atoms, which we sense as heat. However, in graphene that process is very inefficient and it gives up this energy as heat very slowly.
The researchers were able to exploit this by placing metal contacts on the graphene, which allows the material to shed excess energy by pushing electrons to the metal. By making the contacts from two different metals you can produce a current and this current reveals how much terahertz power is being absorbed by the graphene.
The prototype device they developed is remarkably faster than the state-of-the-art terahertz sensors available today. Whereas a current detector takes a second to make a measurement, the graphene-based sensor can make a measurement in 0.1 nanosecond.
Detecting Impurities in Graphene Using Terahertz Waves
Strictly speaking this technology out of Rice and Osaka universities is not a graphene-based sensor, but it is a sensor technology that could provide vast improvements to the manufacturing of graphene for electronic applications.
The international team has developed a method using terahertz waves that can detect impurities in graphene during the manufacturing process.
The new technique can detect a single foreign molecule on the surface of graphene and do so without attaching contacts to the material, which in itself can introduce impurities onto the material.
The basis of the sensing technique is the material indium phosphide (InP), which when it is excited by light emits terahertz signals. Graphene is first layered on the substrate of the InP using chemical vapor deposition and then the combined material is hit with femtosecond laser pulses. This excites the InP to emit the terahertz signals back through the graphene, which are being monitored by a spectrometer.
"The change in the terahertz signal due to adsorption of molecules is remarkable," said Junichiro Kono of Rice University, in a news release. "Not just the intensity but also the waveform of emitted terahertz radiation totally and dynamically changes in response to molecular adsorption and desorption. The next step is to explore the ultimate sensitivity of this unique technique for gas sensing."
While the researchers believe that this technique could benefit anyone wishing to use graphene for electronic applications, they also present the work as a cautionary tale.
"For any future device designs using graphene, we have to take into account the influence of the surroundings," said Kono. He added that graphene in a vacuum or sandwiched between noncontaminating layers would probably be stable, but exposure to air would contaminate it.