Highly efficient with excellent stability electron field emitters based on monolayer graphene coated on wellaligned Si tip (graphene/SiT) arrays fabricated by a simple transfer method are demonstrated. The graphene monolayer is coated on the Si tip array using a chemical vapor deposition process, while the SiTs are prepared through the etching process of a Si substrate. The novel heterostructure field emitter enhanced electron tunneling, as a result, exhibits a better emission property. In addition, the fabricated microplasma devices based on the graphene/SiT heterostructure are closely correlated to the field emission properties of the graphene/SiT materials. The monolayer graphene supported on vertical SiTs provides the protruded nanostructure, which locally enhances the electric field and thus improves the field emission and the plasma illumination characteristics. Experimental Preparation of graphene/SiT arraysA schematic description of the fabrication process of graphene/ SiT array eld emitters is illustrated in Fig. 1. A photoresist (PR, EPG512) lm of thickness 1.0 mm is spin-coated on a Si substrate. This is then patterned by a conventional photolithographic process to form a square periodic pattern of size

The unmanned aerial vehicle (UAV) is a promising enabler of Internet of Things (IoT) owing to its highly flexible features. Combined with wireless power transfer (WPT) techniques, a UAV can provide energy for IoT nodes, which can extend the lifetime of energy constrained communication systems. This paper studies resource and trajectory optimization in UAV-powered wireless communication systems, which consists of two UAVs and two ground nodes (GNs). The system works in a way that the two UAVs alternately charge the two GNs through wireless power transfer and two GNs also alternately send their information to the corresponding UAV with the harvested energy, which can effectively reduce the interference while receiving the information of GNs. Aiming to maximize the minimum throughput of two GNs, wireless resource and UAVs' trajectories are jointly optimized with the constraints of UAV collision avoidance, flying speed, and transmit power. Successive convex programming (SCP) and block coordinate descent (BCD) are utilized to solve the optimization problem. Simulation results show that the proposed scheme achieves larger minimum throughput than the benchmark scheme.