A recent investigation led by Professor Gil-Ho Lee and Ph.D. candidate Hyeon-Woo Jeong from POSTECH’s Department of Physics, along with Dr. Kenji Watanabe and Dr. Takashi Taniguchi from Japan’s National Institute for Materials Science (NIMS), highlights that electron transport in bilayer graphene strongly relies on edge states and a nonlocal transport process. The results of this study were shared in the well-regarded international journal on nanotechnology, Nano Letters.
Bilayer graphene consists of two graphene layers stacked on top of each other. This structure can respond to external electric fields to adjust its electronic band gap, a crucial aspect for the movement of electrons. This unique capability has attracted significant interest due to its potential applications in “valleytronics,” a cutting-edge approach to future data processing. Valleytronics utilizes the “valley,” which is a distinct quantum state in the energy structure of an electron, acting as a separate unit of data storage. This allows for quicker and more efficient data operations compared to traditional electronics or spintronics. With the ability to adjust the band gap, bilayer graphene serves as a key platform for innovative research and development in valleytronics.
At the heart of valleytronics lies the ‘Valley Hall Effect (VHE),’ which captures how electron flow is guided through specific energy states, called “valleys,” within a material. This leads to the intriguing occurrence known as “nonlocal resistance,” where measurable resistance is detected in regions without any direct current flow, even when conduction paths are absent.
While many studies cite nonlocal resistance as clear evidence of the Valley Hall Effect (VHE), some researchers suggest that impurities at the edges of the material or factors related to the manufacturing process may also cause the detected signals, leaving the origin of VHE still debated.
To clarify the true source of nonlocal resistance in bilayer graphene, the combined team from POSCO and NIMS developed a dual-gate graphene device that allows for fine control of the band gap. They then analyzed the electrical properties of naturally formed graphene edges against those that were processed using Reactive Ion Etching.
The results showed that the nonlocal resistance observed in naturally formed edges matched theoretical predictions, while the edges processed through etching exhibited nonlocal resistance that was two orders of magnitude higher. This finding suggests that the etching process created additional conductive pathways that did not relate to the Valley Hall Effect, clarifying why a reduced band gap was noted in earlier studies of bilayer graphene.
Hyeon-Woo Jeong, the lead author of the study, remarked, “The etching process, which is a critical part of device manufacturing, has not been adequately explored, especially regarding its influence on nonlocal transport. Our results highlight the importance of reevaluating these factors and provide vital insights for improving the design and development of valleytronics devices.”
This study received support from multiple organizations, including the National Research Foundation of Korea (NRF), the Ministry of Science and ICT, the Institute for Information & Communications Technology Planning & Evaluation (IITP), the Air Force Office of Scientific Research (AFOSR), the Institute for Basic Science (IBS), the Samsung Science & Technology Foundation, Samsung Electronics Co., Ltd., the Japan Society for the Promotion of Science (JSPS KAKENHI), and the World Premier International Research Center Initiative (WPI).
