2009
Luican, Adina; Li, Guohong; Andrei, Eva Y.
Scanning tunneling microscopy and spectroscopy of graphene layers on graphite Journal Article
In: Solid State Communications, vol. 149, no. 27-28, pp. 1151–1156, 2009.
@article{luican2009scanning,
title = {Scanning tunneling microscopy and spectroscopy of graphene layers on graphite},
author = {Adina Luican and Guohong Li and Eva Y. Andrei},
url = {https://www.sciencedirect.com/science/article/abs/pii/S0038109809001446},
doi = {https://doi.org/10.1016/j.ssc.2009.02.059},
year = {2009},
date = {2009-03-29},
urldate = {2009-01-01},
journal = {Solid State Communications},
volume = {149},
number = {27-28},
pages = {1151--1156},
publisher = {Elsevier},
abstract = {We report low temperature scanning tunneling microscopy and spectroscopy on graphene flakes supported on a graphite substrate. The experiments demonstrate that graphite is exceptionally well suited as a substrate for graphene because it offers support without disturbing the intrinsic properties of the charge carriers. The degree of coupling of a graphene flake to the substrate was recognized and characterized from the appearance of an anomalous Landau level sequence in the presence of a perpendicular magnetic field. By following the evolution of the Landau level spectra along the surface, we identified graphene flakes that are decoupled or very weakly coupled to the substrate. From the Landau level sequence in this flake, we extract the local Fermi velocity and energy of the Dirac point and find extremely weak spatial variation of these quantities confirming the high quality and non invasive nature of the graphite substrate.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
We report low temperature scanning tunneling microscopy and spectroscopy on graphene flakes supported on a graphite substrate. The experiments demonstrate that graphite is exceptionally well suited as a substrate for graphene because it offers support without disturbing the intrinsic properties of the charge carriers. The degree of coupling of a graphene flake to the substrate was recognized and characterized from the appearance of an anomalous Landau level sequence in the presence of a perpendicular magnetic field. By following the evolution of the Landau level spectra along the surface, we identified graphene flakes that are decoupled or very weakly coupled to the substrate. From the Landau level sequence in this flake, we extract the local Fermi velocity and energy of the Dirac point and find extremely weak spatial variation of these quantities confirming the high quality and non invasive nature of the graphite substrate.
Li, Guohong; Luican, Adina; Andrei, Eva Y.
Electronic states on the surface of graphite Journal Article
In: Physica B: Condensed Matter, vol. 404, no. 18, pp. 2673–2677, 2009.
@article{li2009electronic,
title = {Electronic states on the surface of graphite},
author = {Guohong Li and Adina Luican and Eva Y. Andrei},
url = {https://www.sciencedirect.com/science/article/abs/pii/S0921452609004165},
doi = {https://doi.org/10.1016/j.physb.2009.06.071},
year = {2009},
date = {2009-01-01},
urldate = {2009-01-01},
journal = {Physica B: Condensed Matter},
volume = {404},
number = {18},
pages = {2673--2677},
publisher = {Elsevier},
abstract = {Graphite consists of graphene layers in an AB (Bernal) stacking arrangement. The introduction of defects can reduce the coupling between the top graphene layers and the bulk crystal producing new electronic states that reflect the degree of coupling. We employ low temperature high magnetic field scanning tunneling microscopy (STM) and spectroscopy (STS) to access these states and study their evolution with the degree of coupling. STS in magnetic field directly probes the dimensionality of electronic states. Thus two-dimensional states produce a discrete series of Landau levels while three-dimensional states form Landau bands providing a clear distinction between completely decoupled top layers and ones that are coupled to the substrate. We show that the completely decoupled layers are characterized by a single sequence of Landau levels with square-root dependence on field and level index indicative of massless Dirac fermions. In contrast weakly coupled bilayers produce special sequences reflecting the degree of coupling, and multilayers produce sequences reflecting the coexistence of massless and massive Dirac fermions. In addition we show that the graphite surface is soft and that an STM tip can be quite invasive when brought too close to the surface and that there is a characteristic tip–sample distance beyond which the effect of sample–tip interaction is negligible.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Graphite consists of graphene layers in an AB (Bernal) stacking arrangement. The introduction of defects can reduce the coupling between the top graphene layers and the bulk crystal producing new electronic states that reflect the degree of coupling. We employ low temperature high magnetic field scanning tunneling microscopy (STM) and spectroscopy (STS) to access these states and study their evolution with the degree of coupling. STS in magnetic field directly probes the dimensionality of electronic states. Thus two-dimensional states produce a discrete series of Landau levels while three-dimensional states form Landau bands providing a clear distinction between completely decoupled top layers and ones that are coupled to the substrate. We show that the completely decoupled layers are characterized by a single sequence of Landau levels with square-root dependence on field and level index indicative of massless Dirac fermions. In contrast weakly coupled bilayers produce special sequences reflecting the degree of coupling, and multilayers produce sequences reflecting the coexistence of massless and massive Dirac fermions. In addition we show that the graphite surface is soft and that an STM tip can be quite invasive when brought too close to the surface and that there is a characteristic tip–sample distance beyond which the effect of sample–tip interaction is negligible.
2006
Temirov, R; Soubatch, S; Luican, A; Tautz, FS
Free-electron-like dispersion in an organic monolayer film on a metal substrate Journal Article
In: Nature, vol. 444, no. 7117, pp. 350–353, 2006.
@article{temirov2006free,
title = {Free-electron-like dispersion in an organic monolayer film on a metal substrate},
author = {R Temirov and S Soubatch and A Luican and FS Tautz},
url = {https://www.nature.com/articles/nature05270},
doi = {https://doi.org/10.1038/nature05270},
year = {2006},
date = {2006-01-01},
urldate = {2006-01-01},
journal = {Nature},
volume = {444},
number = {7117},
pages = {350--353},
publisher = {Nature Publishing Group},
abstract = {Thin films of molecular organic semiconductors are attracting much interest for use in electronic and optoelectronic applications. The electronic properties of these materials and their interfaces are therefore worth investigating intensively1,2,3, particularly the degree of electron delocalization that can be achieved2,4. If the delocalization is appreciable, it should be accompanied by an observable electronic band dispersion. But so far only limited experimental data on the intermolecular dispersion of electronic states in molecular materials is available5,6,7,8, and the mechanism(s) of electron delocalization in molecular materials are also not well understood. Here we report scanning tunnelling spectroscopy observations of an organic monolayer film on a silver substrate, revealing a completely delocalized two-dimensional band state that is characterized by a metal-like parabolic dispersion with an effective mass of m* = 0.47me, where me is the bare electron mass. This dispersion is far stronger than expected for the organic film alone7, and arises as a result of strong substrate-mediated coupling between the molecules within the monolayer.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Thin films of molecular organic semiconductors are attracting much interest for use in electronic and optoelectronic applications. The electronic properties of these materials and their interfaces are therefore worth investigating intensively1,2,3, particularly the degree of electron delocalization that can be achieved2,4. If the delocalization is appreciable, it should be accompanied by an observable electronic band dispersion. But so far only limited experimental data on the intermolecular dispersion of electronic states in molecular materials is available5,6,7,8, and the mechanism(s) of electron delocalization in molecular materials are also not well understood. Here we report scanning tunnelling spectroscopy observations of an organic monolayer film on a silver substrate, revealing a completely delocalized two-dimensional band state that is characterized by a metal-like parabolic dispersion with an effective mass of m* = 0.47me, where me is the bare electron mass. This dispersion is far stronger than expected for the organic film alone7, and arises as a result of strong substrate-mediated coupling between the molecules within the monolayer.