Dry transfer technology for two-dimensional materials applied to various platforms"In the short term, graphene can be applied to existing devices to dramatically improve their performances. Such devices include secondary cells such as lithium-ion batteries, touch panels with which the user can interact with screen directly, and the self-luminous displays using organic light-emitting diodes.In the long term, the application of graphene can be extended to cutting-edge fields such as the flexible displays that can be bent or twisted with no damage by building display components on paper-thin, flexible substrates, printed electronics, and even the bioelectronics. Recent research statistics show that Korea has become the leading country in the fields of the transparent electrodes and optoelectronic devices with graphene."Next-generation technology to develop nano-scale devicesGraphene, known as a dream material for next-generation electronics, is one-atom-thick carbon layer in which carbon atoms are connected with one another in the shape of a honeycomb structure. It has outstanding thermal and electrical conductivity, as well as excellent mechanical strength; graphene can conduct heat twice as well as diamond and is 100 times stronger than steel. The charge carrier mobility of graphene is even superior to that of silicon semiconductor, which enables 100 times faster data processing as compared to current semiconductor devices. The fascinating point is that those outstanding properties are manifested in ultra-thin, one-atom-thick graphene whose thickness is only 0.4 nanometers, 10 million times thinner than a human hair. Hence, graphene can be used in various fields, such as displays, energy devices, transparent electrodes, composite materials, and next-generation semiconductor devices once the application technology is developed.According to the current graphene R&D trend, the commercial applications based on graphene will begin to emerge in the market in 2015 and the total revenue will amount to 600 billion dollars in 2030. Currently, in terms of the number of academic papers and core patents on graphene, Korea is in forth and second (just behind the US) place in the world, respectively. Korea also boasts a world-class level in the commercialization of graphene. In particular, Korea is leading the world in research on graphene applications (i.e., transparent electrodes and electronic devices), and at the heart of that trend is the research group led by Professor Sung-Yool Choi (Graphene Research Center, KINC)Prof. Choi’s research group has focused on the development of a facile transfer technique for graphene and 2D materials to apply those to a variety of platforms since the foundation of the Graphene Research Center (September, 2012). In principle, graphene shows metal-like properties because it is a one-atom-thick sheet of carbon where carbon atoms are structured as a honeycomb. The electronic properties of graphene can be tuned by reducing the lateral size of the graphene sheet or modifying its chemical structure, which produces graphenes with semiconducting or insulating properties. At the same time, graphene is transparent in every wavelength of light, and mechanically flexible (even stretchable). Therefore, we can consider graphene as a flexible and “invisible metal”. This makes graphene a good candidate for transparent electrodes in manufacturing flexible displays. It has been expected that graphene, the thinnest, man-made electronic material on the planet, will play a key role in the development of next-generation nano-scale devices or large-area displays. However, industries have had difficulties in handling graphene due to its extremely thin nature when it is applied to the industrial process. Currently, chemical vapor deposition (CVD) is the most popular method for the mass production of large-area graphene. With this method, graphene is grown on metal thin films or foils (also called the growth substrate) at high temperatures of ~1000oC. Graphene grown by CVD should be transferred to a silicon wafer or plastic sheet (also called the target substrate) to fabricate the electronic devices. In general, graphene transfer is carried out using the wet transfer technique, in which graphene is separated from the growth substrate by etching metals away with chemical solution (metal etchant) and the transfer is completed by scooping up the floating graphene on the water with a target substrate. The major drawback of this method includes the loss of the graphene’s original properties due to contamination by ionic and metallic residues originated from the metal etching step. The use of hazardous metal etchant also causes environmental issues and is not cost-effective. From an industrial perspective, the scale-up of the transfer method is not an effective way to produce large-area, transferred graphene due to the difficulty in handling and the process-induced damage issues.To address these issues, Prof. Choi’s research group has developed a novel way to separate the graphene from growth substrate mechanically without metal etching and to transfer it to a target substrate. The separation of graphene became possible by applying a functional carrier layer on the graphene growth substrate and retrieving the carrier/graphene stack with an elastomeric stamp. This retrieved graphene stack was successfully transferred to the target substrates without causing serious damage to the graphene. The carrier layer was easily removed by water. The damage and contamination of graphene can be avoided by the developed transfer method and graphene’s original properties can be maintained during the transfer process. It is also possible to transfer graphene to various platforms from rigid to flexible target substrates. The size of the transferred graphene can be easily scaled up or down. No use of hazardous chemicals in the process make this method more eco-friendly.Due to the fact that the feasibility of this dry transfer method can be extended to other 2D materials, it is expected that the developed dry transfer method will pave the way for the realization of nano-scale devices based on graphene and 2D materials.
Prof. Choi, Sung-Yool 2013 Annual Report