Dr. Hideo Miura
Photovoltaic Properties of Dumbbell-shape Graphene Nanoribbons for Solar Cells
Graphene has been found to show various electronic properties from super-metallic ones to semiconductive ones as a strong function of its size. When the width of graphene is thinner than about 70 nm, the graphene nano-ribbon (GNR) shows semiconductive properties, and the effective band gap of GNR increases monotonically with the decrease of the width. It is possible, therefore, to make various semiconductive structures by controlling the width of graphene. The authors have proposed a dumbbell-shape GNR structure in which the semiconductive GNR is jointed with metallic GNR by using, where the semiconductive GNR with the width thinner than 70 nm is terminated by metallic GNRs with the width wider than 70 nm at the both ends of the semiconductive GNR. The electronic properties of the dumbbell-shape GNRs can be controlled by changing the length of the semiconductive GNR.
In this study, the area-arrayed dumbbell-shape GNRs were fabricated by using chemical vapor deposition and photolithography technologies. High quality graphene was grown on a (111) Cu substrate by using acetylene gas. It was transferred to SiO2/Si substrate. Poly-methyl methacrylate (PMMA) resist was employed as an assistive material for the transfer process. Raman spectroscopy confirmed the quality of graphene to be mono/bi-layer graphene sheets by the I2D/IG intensity ratio, which was greater than 2. The graphene sheet transferred on the SiO2/Si substrate was further processed to fabricate area-arrayed GNRs structures. The fabricated area-arrayed GNRs structure was patterned using EB-lithography technique to make nano-ribbon structures with the width from 500 nm to 40 nm. These nano-ribbon patterned structures, protected the under-layer graphene while the exposed graphene sheet was etched using reactive ion etching method using oxygen plasma. The resist mask applied on the surface for patterning GNRs, also served as a protective coating for GNR.
The electronic behavior of the fabricated G-FET and GNR-FET was evaluated for its photonic properties with an incident light intensity of 1-mW using Agilent 4155C semiconductor parameter analyzer. The 200-nm wide GNRs structure showed metallic properties, while those with the width of 40 nm showed semiconductive properties as was expected. The light-induced photocurrent was observed in all the fabricated GNRs structures. The average photocurrent observed in the 2-m wide graphene structure was 3.3 A/m2 and that observed in the 40-nm wide area-arrayed GNRs structure was 261 A/m2 for G-FET and GNR-FET, respectively. Based on this photocurrent, the external photosensitivity of the 40-nm wide GNRs structure was about 2.6 x 105 A/Wm2 and this value was much larger than that of conventional Si-base solar cells. Finally, it was confirmed by experiments that GNRs with the width thinner than 70 nm showed semiconductive properties and they are strong candidates for highly-sensitive photovoltaic devices.
Graphene Nanoribbon, Dumbbell-Shape, Photosensitivity
Dr. Hideo Miura worked for Hitachi Ltd., Japan for 20 years as a chief researcher in the field of the reliability of electronic products. He is now Director and Professor of Fracture and Reliability Research Institute, Tohoku University, Japan. His main research topic is control of the order of atom alignment considering various atomic-scale defects for improving the performance and reliability of advanced materials.