Graphene research highlights of 2018 included applications in chemical sensors, advanced uses of mechanical properties of graphene, and high frequency uses such as ultrafast graphene transistors and optical communications.
Trace detection of harmful chemicals has been a target application of graphene from the onset of applied graphene research. Last year, MIT and Graphenea have developed an array of graphene sensors for sensitive and selective detection of ammonia. The array consists of 160 graphene pixels, allowing large statistics that result in improved sensing performance. The sensors are extensively tested for various real-life operational conditions, which is a step forward to practical use. To make the sensors selective, i.e. sensitive only to ammonia, the graphene is functionalized by attaching porphyrin molecules. Functionalization has become an ubiquitous way of enforcing selectivity, for devices as advanced as an “electronic nose”, an array of graphene sensors that can “sniff” which chemicals are present in the environment. Another way of imposing selectivity is to employ machine learning on bare graphene sensors. Machine learning, which is based on self-refining algorithms that learn from optimizing for a certain outcome based on thousands of examples, is increasingly being used in many branches of science and technology. In the case of graphene sensors, the algorithm learns from the electronic response of hundreds of examples of graphene being exposed to various chemicals. In this way, a single sheet of unprocessed graphene can be used to sense and discriminate between the presence of compounds such as acetone, chloroform, toluene, hexane, acetic acid and others.
Mechanical properties of graphene are superb, although not yet extensively explored for applications. Last year marked some large steps forwards in putting to use those excellent mechanical properties, like large strength, light weight and thinness. A publication in Nature Communications highlighted the use of graphene in GIMOD – graphene interferometric modulator display. An interferometric modulator display uses an array of mechanical pixels that change color under applied voltage. The central component of mechanical pixels is a freestanding membrane that changes position when voltage is applied, scanning through optical interference fringes to create a specified color. IMODs made from other materials suffer from low frame rates and limited color gamut, which has hindered their use. Now, graphene IMODs are shown to operate at up to 400 Hz, cover the full visible spectrum with reduced flicker effect. The demonstrated GIMOD prototype has 2500 pixels per inch, equivalent to more than 12K resolution, which is higher than UHDTV.
A German-Spanish project NanoGraM that ended in 2018 explored the practical use of NEMS/MEMS devices based on graphene. Four companies and one university collaborated to produce graphene microphones, pressure sensors, and Hall sensors. Graphene microphones are more sensitive than traditional ones, across the audible and ultrasonic parts of the spectrum. Pressure and magnetic field sensors made of graphene have enhanced signal-to-noise ratios, sensitivity and robustness. The project delivered three patent applications and seven scientific publications.
The performance of graphene for high-frequency applications keeps improving with technological advances that yield higher quality graphene and better circuit integration. For example, contact engineering has brought down the contact resistance between graphene and metal to record low values of just 23 Wmm for gold contacts. Contact resistance below 50 Wmm is desired and is a typical value in high-frequency transistors used for computation. Improved integration of graphene in semiconductor devices has enabled record-breaking performance of graphene in optoelectronics and optical communication. In 2016, the bandwidth of graphene photodetectors reached 65 GHz, utilizing graphene/silicon pn junctions with potential bit rates of ~90 Gbit s-1. Already in 2017, graphene photodetectors with a bandwidth exceeding 75 GHz were fabricated in a 6” wafer process line. These record-breaking devices were showcased at the Mobile World Congress in Barcelona in 2018, where visitors could experience the world’s first all-graphene optical communication link operating at a data rate of 25 Gbit s-1 per channel. In this demonstration, all active electro-optic operations were performed on graphene devices. A graphene modulator processed the data on the transmitter side of the network, encoding an electronic data stream to an optical signal. On the receiver side, a graphene photodetector did the opposite, converting the optical modulation into an electronic signal. The devices were made with Graphenea CVD graphene and showcased at the Graphene Pavilion.
The examples of chemical sensors, high-frequency electronics and mechanical devices are only a select few of a vast array of applications of graphene that are ripening and enabling new technology. The Graphene Flagship midterm annual report is a good starting point for getting a comprehensive overview of the myriad of applications that are in development and about to come to markets in the next several years.