# Key Roles of Plasmonics in Wireless THz Nanocommunications--A Survey
## 3 Wireless Nano Sensor Networks
The latter subtype working principle is based on the use of noble metal nanoparticles and the excitation using optical waves of surface plasmons. More specifically, a typical generic architecture of a WNSN node as seen in Figure 3[35], would be consisted of: (i) Sensing unit: graphene material and its derivatives, namely, graphene nanoribbons (GNRs) and carbon nanotubes (CNTs) [56], provide outstanding sensing capabilities and they are the basis for implementing many types of sensors [57]. (ii) Actuation unit: an actuation unit will allow nanosensors to interact with their close environment. Several nanoactuators have also been designed and implemented so far [58]. (iii) Processing unit: nanoscale processors are being enabled by the development of different forms of miniature FET transistors in the nanometer scale. They were mostly implemented via CNTs and GNRs nanomaterials. (iv) Storage unit: graphene has shown excellent performance in a number of applications from supercapacitors [59] to photomechanical actuators [60], however, so far, its potential in nanomemory construction has not been adequately explored. (v) Power Unit: there are two types of nanobatteries [61] for feeding nanomachines: (a) harvesting the energy from the environment via nanoscale energy harvesting systems [62] and (b) wireless energy induced from an external power source [63]. (vi) Communication unit: this consists of nanoantennas and transceivers for guaranteeing EM communication between nanosensors. The working principle of energy harvesting is based on the conversion of mechanical or vibrational or hydraulic energy into electrical energy. The mechanical energy is produced by the human body movements, or muscle stretching, the vibrational energy is generated by acoustic waves or structural vibrations of buildings, and finally the hydraulic energy is produced by body fluids, or the blood flow. This energy conversion is achieved by the piezoelectric effect seen in zinc oxide (ZnO) nanowires, as they are bent, when a voltage appears in the nanowires (Figure 4) [35]. Figure 3: Wireless nanosensor networks (WNSN) node architecture. As mentioned, nanoantennas are mostly implemented via CNTs and GNRs nanomaterials. Concerning the latter case, the propagation of EM waves on a graphene sheet was first analyzed in [64] while in [34], nano-patch antennas based on GNRs and nano-dipole antennas based on CNTs were quantitatively compared. EM transceivers of nanosensors are embedded systems that include all the necessary circuit equipment which processes the transmitted or received signals from the free space through the nanoantenna with proper functioning such as frequency conversion, filtering and power amplification. Several GFET transistors capable of such functioning and operating in the sub-THz or THz band have been demonstrated so far [65]. Other materials, such as Au and Ag have been successfully used for plasmonic sensors in the visual [66,67] and near infrared [68], as well. Ge plasmonic material in the mid-IR, has been also proposed as a promising material for replacing silicon, as a substrate for MOS devices [68]. According to [35], the applications of WNSNs can be classified in four main groups: biomedical, environmental, industrial, and military applications. As far as concerns in the environmental and industrial application domains, various THz nanosensors have been used for the detection of pollutants and empowering the technology of food preservation and food processing [69]. Nanowire based nanosensors are suggested for sensing ambient intelligence, such as vertically bridged nanowires, laterally bridged nanowires, ultra-sharp Ga2O3 nanowire, the nanowire FET chem-biosensor, and the CNT biosensor [70]. As far as concerns military applications domain, various THz nanosensors, detectors and cameras have been suggested for security applications, and specifically for the detection of weapons, explosives, as well as chemical and biological agents [71].