Tailoring of graphene's electronic and magnetic properties via edge functionalization
Zusammenfassung der Projektergebnisse
In this project, graphene nanoribbons (GNRs) were prepared as a platform to test the influence of functional edge groups on the charge transport properties in graphene. The fabrication involved two different types of nanowires used as an etching mask during plasma etching of extended graphene sheets. GNRs obtained with the aid of CdSe nanowires as mask reached widths as small as 10 nm, and displayed a regular structure with only minor edge roughness, as concluding from atomic force, scanning tunneling and scanning electron microscopy inspection. Electrical transport studies revealed a charge transport gap whose size scaled inversely with the ribbon width. The GNRs were further characterized by Raman spectroscopy by decorating them with gold nanoparticles in order to achieve surface enhanced Raman scattering. In this manner, Raman spectra could be recorded from individual nanoribbons. However, no novel Raman features like the breathing-like mode predicted by theory could be identified. By implementing the GNRs into field-effect device configuration, fast and durable memory cells could be realized. The memory operation is based upon the hysteresis in the transfer curve of the devices. The hysteresis was found to originate from charge trapping by water molecules which presumably interact with the ribbon edges. Dynamic pulse response measurements revealed reliable switching between two conductivity states for clock frequencies of up to 1 7 kHz and pulse durations as short as 500 ns for >10 cycles. This performance is clearly superior to previously reported memory devices made of carbon nanotubes. Electrical transport studies of the GNRs at low temperatures revealed well-defined Coulomb charging and single-electron tunneling through discrete levels, demonstrating that single quantum dots could be realized. In some devices, a zero-bias conductance peak was observed, although measurements under applied magnetic field did not support the presence of the Kondo effect. Furthermore, the optoelectronic properties of individual GNRs were studied by scanning photocurrent microscopy. The pronounced photocurrent signal close to the nanoribbon/metal contacts was observed to be linearly proportional to the conductance of the devices, suggesting that a local voltage source is generated at the nanoribbon/metal interface by the photo-thermoelectric Seebeck effect. The dominance of this mechanism over charge separation induced by built-in electrical fields points toward notable local heating of the GNRs by the laser spot, due to a reduced thermal conduction capability of the nanoribbons in comparison to extended graphene sheets. Chemical functionalization of the edges of graphene and GNRs was attempted via different gas and liquid-phase approaches. While electrical transport and Raman measurements revealed pronounced doping effects, it was not possible to unequivocally prove the covalent attachment of atoms at the GNR edges, neither through changes in the charge transport characteristics, nor via scanning microscopy. As a major conclusion, there emerge two relevant hurdles regarding the chemical edge functionalization of graphene. The first one concerns the formation of quite rough edges during the plasma etching, which results in complex mixture of zigzag, armchair and chiral edges. The different edges not only exhibit different chemical reactivity but also affect by themselves the charge transport in the nanoribbons. The second issue involves a pronounced influence by the underlying substrate, which blocks access of the reactive chemical species and/or prevents the structural reorganization of the graphene lattice accompanying the covalent linkage of atoms/molecules. While these two complications are responsible for the very little progress that has been made toward graphene edge functionalization during the past three years, they may be overcome by intensified fabrication efforts such as rendering the GNRs free-standing or the development of etching procedures that are selective with respect to the crystallographic orientation of the graphene sheet.
Projektbezogene Publikationen (Auswahl)
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“A graphene nanoribbon memory cell”, Small 6 (2010), 2822
E.U. Stuetzel, M. Burghard, K. Kern, F. Traversi, F. Nichele, R. Sordan
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“Graphene nanoribbon memory cell”, Conference Graphene 2011, Bilbao, Spain, April 11-14, 2011
E.U. Stuetzel, F. Traversi, F. Nichele, R. Sordan, M. Burghard, K. Kern
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“Variable range hopping in graphene antidot lattices”, Phys. Stat. Sol. B 249 (2012) 2522
E.C. Peters, A.J.M. Giesbers, M. Burghard
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“Fabrication and optoelectronic properties of graphene nanoribbons”, PhD thesis (EPFL, Lausanne, 2013)
E.U. Stützel
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“Spatially resolved photocurrents in graphene nanoribbon devices”, Appl. Phys. Lett. 102 (2013) 043106
E.U. Stuetzel, T. Dufaux, A. Sagar, S. Rauschenbach, K. Balasubramanian, M. Burghard, K. Kern
