An aging society has become a crucial expanding issue all over the world over continuously. An increasing number of elder people require 24-hour and high-quality nursing cares despite the shortage of caregivers. Therefore, it is indispensable for developing a wearable and smart health monitoring system. A highly sensitive strain sensor is one of the most important components of the monitoring devices for detecting the condition of the human body such as blood pressure, pulse, and so on. However, conventional strain sensors cannot fulfill the requirements, such as the full range of large deformation and the change of slight vibration of the heartbeat’s pulse. Therefore, it is very important to develop a flexible strain sensor with high sensitivity. From this point of view, graphene, a two-dimensional (2D) monolayer material is one of the most attractive candidate materials to substitute the sensing material such as silicon used in conventional electronic devices. It has preponderant mechanical and electronic transport properties for constructing flexible electronics, detectors, and biosensors. As for the application to a strain sensor, a high-quality graphene nanoribbon (GNR) with the width of tens of nanometer was found to show semiconductive properties and thus, high strain sensitivity of its resistivity.

Thus, a carbon-based strain gauge which consisted of a narrow GNR with metallic graphene at its both ends was proposed, and it was validated that a 40-nm wide GNR-base strain gauge showed a high gauge factor of about 100. The mechanism of the strain-induced change of the electronic properties of the dumbbell-shape GNR structure, however, was still unclear. Therefore, in this study, the electronic properties in the vicinity of the junction portion between a solo long semiconductive GNR segment and a large areal metallic graphene segment were investigated by using the first-principles calculation based on the density functional theory (DFT). Throughout these calculations, the electronic band structures, densities of states, and orbital distributions of the dumbbell-shape GNR were examined to describe the electronic properties of the dumbbell-shape GNRs and predict the performance as a strain sensor. Nonequilibrium Green’s function based on DFT was also implemented for illustrating the electron transport properties of the dumbbell-shape GNR. In order to elucidate the heterojunction-like electronic properties of dumbbell-shape GNR both analytically and numerically, the modeling was rebuilt by considering the phase-field method. The rebuilt model validated the gradient transition area of the electronic properties of the dumbbell-shape GNR in the vicinity of the junction portion. The coalition of a solo long semiconductive GNR and a large areal metallic graphene at its both ends gave rise to Schottky-barrier-like electronic properties even though the structure consisted of only carbon atoms. When the uniaxial strain was applied to the dumbbell-like GNR structure, the effective bandgap of the narrow GNR started to change almost linearly and thus, the effective resistivity changed drastically. There was a dead zone, however, depending on the magnitude of the Schottky-like barrier as a strong function of the width of the narrow GNR. The estimated gauge factor also varied a lot depending on the width. The maximum calculated gauge factor was about 53,000, and it was more 200 times higher than that of conventional silicon-based strain sensors. Therefore, the theoretical study furnished a feasible direction to assure low-cost, stable, and reliable industry-standard GNR-based highly sensitive strain sensors developed in the near future.  


Theoretical Study of Heterojunction-Like Electronic Properties Between a Semiconductive Graphene Nanoribbon and a Metallic Graphene for Highly Sensitive Strain Sensors

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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