Suchergebnisse

Due to its outstanding electrical properties and chemical stability, graphene finds widespread use in various electrochemical applications. Although the presence of electrolytes strongly affects its electrical conductivity, the underlying mechanism has remained elusive. Here, we employ terahertz spectroscopy as a contact-free means to investigate the impact of ubiquitous cations (Li+, Na+, K+, and Ca2+) in aqueous solution on the electronic properties of SiO2-supported graphene. We find that, without applying any external potential, cations can shift the Fermi energy of initially hole-doped graphene by ∼200 meV up to the Dirac point, thus counteracting the initial substrate-induced hole doping. Remarkably, the cation concentration and cation hydration complex size determine the kinetics and magnitude of this shift in the Fermi level. Combined with theoretical calculations, we show that the ion-induced Fermi level shift of graphene involves cationic permeation through graphene. The interfacial cations located between graphene and SiO2 electrostatically counteract the substrate-induced hole doping effect in graphene. These insights are crucial for graphene device processing and further developing graphene as an ion-sensing material. ; X.J., Z.L., Z.C., A.N., K.M., M.B., and H.I.W. acknowledge the financial support by the Max Planck Society. X.J. acknowledges support by a Deutsche Forschungsgemeinschaft-funded position through the Excellence Initiative by the Graduate School of Excellence Materials Science in Mainz (MAINZ) (GSC 266) and support from the Max Planck Graduate Center mit der Johannes Gutenberg-Universität Mainz (MPGC). K.-J.T. acknowledges financial support through the MAINZ Visiting Professorship. X.Y. acknowledges the fellowship support from Chinese Scholarship Council (CSC). This project has received funding from the European Union's Horizon 2020 research and innovation program under Grant Agreement No. 804349. ICN2 is supported by the Severo Ochoa program from Spanish MINECO (Grant No. SEV-2017-0706). F.H.L.K. acknowledges financial support by Fundacio Cellex Barcelona, support from the Spanish Ministry of Economy and Competitiveness, through the "Severo Ochoa" Programme for Centres of Excellence in R&D (SEV-2015-0522), and support by the Generalitat de Catalunya through the CERCA program. ; Peer reviewed

Van der Waals heterostructures consisting of graphene and transition metal dichalcogenides have shown great promise for optoelectronic applications. However, an in-depth understanding of the critical processes for device operation, namely, interfacial charge transfer (CT) and recombination, has so far remained elusive. Here, we investigate these processes in graphene-WS heterostructures by complementarily probing the ultrafast terahertz photoconductivity in graphene and the transient absorption dynamics in WS following photoexcitation. We observe that separated charges in the heterostructure following CT live extremely long: beyond 1 ns, in contrast to ~1 ps charge separation reported in previous studies. This leads to efficient photogating of graphene. Furthermore, for the CT process across graphene-WS interfaces, we find that it occurs via photo-thermionic emission for sub-A-exciton excitations and direct hole transfer from WS to the valence band of graphene for above-A-exciton excitations. These findings provide insights to further optimize the performance of optoelectronic devices, in particular photodetection. ; S.F. acknowledges fellowship support from Chinese Scholarship Council (CSC). X.J. acknowledges financial support by DFG through the Excellence Initiative by the Graduate School of Excellence Materials Science in Mainz (MAINZ) (GSC 266) and support from the Max Planck Graduate Center mit der Johannes Gutenberg-Universität Mainz (MPGC). A.J.H. acknowledges support from the European Research Council Horizon 2020 ERC grant no. 678004 (Doping on Demand). ICN2 was supported by the Severo Ochoa program from Spanish MINECO (grant no. SEV-2017-0706). K.-J.T. acknowledges funding from the European Union's Horizon 2020 Research and Innovation Programme under grant agreement no. 804349 (ERC StG CUHL) and financial support through the MAINZ Visiting Professorship.