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

Nanographenes with zigzag edges are predicted to manifest non-trivial π-magnetism resulting from the interplay of concurrent electronic effects, such as hybridization of localized frontier states and Coulomb repulsion between valence electrons. This provides a chemically tunable platform to explore quantum magnetism at the nanoscale and opens avenues towards organic spintronics. The magnetic stability in nanographenes is thus far greatly limited by the weak magnetic exchange coupling, which remains below the room-temperature thermal energy. Here, we report the synthesis of large rhombus-shaped nanographenes with zigzag peripheries on gold and copper surfaces. Single-molecule scanning probe measurements show an emergent magnetic spin singlet ground state with increasing nanographene size. The magnetic exchange coupling in the largest nanographene (C70H22, containing five benzenoid rings along each edge), determined by inelastic electron tunnelling spectroscopy, exceeds 100 meV or 1,160 K, which outclasses most inorganic nanomaterials and survives on a metal electrode. ; This work was supported by the Swiss National Science Foundation (grant nos. 200020-182015 and IZLCZ2-170184), NCCR MARVEL funded by the Swiss National Science Foundation (grant no. 51NF40-182892), the European Union's Horizon 2020 research and innovation programme (grant no. 785219, Graphene Flagship Core 2), the Office of Naval Research (N00014-18-1-2708), the Max Planck Society, Ministry of Science and Innovation of Spain (grant nos. PID2019-106114GB-I00 and PID2019-109539GB), Generalitat Valenciana and Fondo Social Europeo (grant no. ACIF/2018/175), MINECO-Spain (grant no. MAT2016-78625) and Portuguese FCT (grant no. UTAPEXPL/ NTec/0046/2017). Computational support from the Swiss Supercomputing Center (CSCS) under project ID s904 is gratefully acknowledged.