Graphene: from research to applications

A successful series of conferences tracking the progress of applications of graphene continued this month at the National Physical Laboratory (NPL) in the UK. In this year’s edition Alba Centeno held an invited talk, highlighting some of the applications of our high-quality graphene thin films.

Graphenea’s monolayer, multilayer and suspended graphene has been applied to photodetectors, biosensors, touch panels, UV LEDs, Hall (magnetic) sensors, microphones, pressure sensors, and graphene mechanical pixels (GIMODs), among other applications.

A trending application in recent years has been photodetection. In particular, graphene combined with quantum dots (QDs) makes for a very sensitive detector of light. Such detectors can be flexible and transparent for wearable applications, for example wristbands that measure blood volume and heart rate. The advantage of graphene-QD sensors compared to other solutions is their high sensitivity, wide spectral coverage (across the visible and infrared parts of the spectrum) and lack of cooling requirements. Arrays of such sensors were utilized as advanced CMOS camera chips that are sensitive from 300 nm to 2000 nm wavelengths. The chips are so sensitive that they can operate as infrared cameras under partial-moon and twilight conditions.

Photonic devices enabled by graphene have numerous advantages over standard technology. A collaboration within the Graphene Flagship demonstrated an optical communication link with graphene modulators and receivers operating at a data rate of 25 Gb/s per channel. Other optoelectronic applications of graphene include flexible touch panels and OLEDs. For these applications, graphene is integrated with flexible substrates, such as PEN polymer, on which graphene serves as a transparent electrode. For such applications, few-layer graphene made with our patented transfer method can be very useful, because it has lower sheet resistance than single-layer graphene while maintaining high optical transparency.

UV LEDs, which have applications in water treatment, air purification and food processing, can be manufactured by growing semiconducting nanowires on graphene. Nanowires grown on graphene have extremely high crystal quality and efficiency of light extraction, and the graphene layer further serves as transparent electrode in the final device. Graphene-enhanced UV LEDs will have improved efficiency, heat and cost reduction.

Another interesting application of graphene is GIMOD – Graphene interferometric modulator display. This novel type of display consists of graphene “mechanical pixels” that change color depending on an applied voltage. The color of each pixel is fully tunable and the small pixel size allows for 12K resolution, which is beyond the reach of other technology of today. Compared to mechanical pixel displays made of other materials, that made of graphene has fast response, allowing for operation up to 400 Hz and real life use.

Alba also spoke about challenges to improving the performance of graphene and integration in technological processes: surface cleanliness, encapsulating materials, and device fabrication. These inter-related properties are geared towards use of graphene in actual technology. The surface of graphene can get contaminated during transfer from the growth substrate to the use substrate, however it can be cleaned again with argon cluster ion beam cleaning to retain original performance. Due to the ultimate thinness of graphene, the substrate that the material lies on has a large effect on performance. Even minute substrate roughness, doping inhomogeneity, lattice mismatch and dangling bonds can have a detrimental effect on carrier mobility and other parameters of graphene. Graphene works better when encapsulated with atomically thin layers, because it is then protected from environmental effects. Overall the best performance is achieved with hexagonal boron nitride (hBN) substrates and encapsulation, but much work remains to be done on obtaining wafer-scale quality hBN. Current state-of-the-art for wafer-scale manufacture is graphene on SiO2 passivated with an Al2O3 protection layer. Such devices exhibit field effect mobility up to 6900 cm2/Vs, which is excellent and good for many applications, but still not as high as the 19000 cm2/Vs obtained with hBN.

Care must also be taken to minimize contact resistance between graphene and metal contacts. This is done by reducing metal thickness, but also by perforating holes in graphene by electron beam lithography  prior to depositing contacts to achieve atomically thin line contacts between the graphene and the metal.

Figure: Graphene devices with metal contacts (Luca Anzi et al 2018 2D Mater. 5 025014)

The two-day conference addressed new concepts of graphene applications, metrology and technology through talks given by high-ranking speakers from the academia and industry. Apart from displaying the latest technology built from graphene films and flakes, speakers addressed the important issues in characterization, standardization, and quality control of graphene.