Graphene-based materials for antenna applications

Internet of Things (IoT) has become one of the trending technologies in recent years. With the wide applications and rapid development of wireless communication and IoT, there are growing number of requirements on novel device design, such as flexible antennas and tunable antennas. The demand of transparent and flexible wireless communicational devices is ever-increasing for a wide range of scenarios, such as noninvasive healthcare, real-time wearable electronics, etc. Besides, a highly integrated antenna with tunable bandwidth and low energy consumption is in pressing demands in order to reduce the number of antennas and achieve better signal strength and higher speed signal transmission. Graphene materials have drawn much attention due to its extraordinary optical properties, high electrical and thermal conductivity, and excellent mechanical properties. All of these properties underline that graphene can be considered as a highly competitive material for wearable communication devices. Moreover, the electrical resistance of graphene and graphene oxide could be simply changed by external electric fields, applied strain and environment, which alters the electrical characteristics of the graphene-based microwave devices. Accordingly, graphene materials have a great potential in various reconfigurable or tunable devices. Our group fabricated the first graphene-based flexible and transparent wireless antenna with 2D large single-crystalline graphene and 1D AgNW hybrid material based on the features of graphene, such as high carrier mobility, transparency and flexibility, etc. The antenna exhibited a 75% light transmittance and an ultrawide bandwidth ranging from 5.6 to 12.8 GHz, combining with outstanding durability and stability with an almost negligible variation of reflection coefficients over 1000 bending cycles. Further research confirmed that the existence of graphene harmonized the compatibility between antenna and integrated chips, hence improved the total performance of the radio frequency identification (RFID) tags. Graphene/AgNW-based RFID tag was fabricated with an operating UHF band of 850–960 MHz and a reading distance of 130 cm, showing a great commercial potential. The excellent performance and unique properties of graphene/AgNW-based antenna and RFID tag came from the synergistic effect between 1D and 2D nanomaterials, providing a new vision for the design of next generation wireless electronics. Meanwhile, a slot antenna with tunable bandwidth was proposed. The antenna consisted of a planar slot antenna with graphene ink printed at the gap between the ground and a small patch. The graphene ink was made from the mixture of graphene powder and graphene oxide aqueous dispersion. Graphene oxide sheets functioned as effective surfactants rendering assistance to form uniform printable inks, displayed alterable electrical resistance by optimization of mixing proportions between graphene and graphene oxide. Dried graphene inks displayed tunable electrical resistance under applied DC voltages. The variation in the resistance of graphene ink affected radiation of the slot antenna, resulting in a shift of operating bandwidth. The proposed antenna achieved a shifting bandwidth of 0.54 from 6 to 5.46 GHz after applying DC voltage less than 15 V. Such cost-effective, easily acquired, and printable graphene inks integrated the dual advantages of graphene and graphene oxide. It could be further applied in many other fields of printable electronic devices.