AVS2010 Session GR+TF-TuM: Characterization, Properties, and Applications

Near-field microwave microscopy can be used as an alternative to atomic-force microscopy or Raman microscopy in determination of graphene thickness. We evaluated the values of AC impedance for few layer graphene. The impedance of mono and few-layer graphene at 4GHz was found predominantly active. Near-field microwave microscopy allows simultaneous imaging of location, geometry, thickness, and distribution of electrical properties of graphene without device fabrication. Our results may be useful for design of future graphene-based microwave devices.

Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the

United States Department of Energy's National Nuclear Security Administration under contract DE-AC04 94AL85000.

We report on the fabrication and mechanical characterization of novel graphenoid nanomembranes with a thickness of approximately 1 nm. The nanomembranes are prepared from electron cross-linked aromatic self-assembled monolayers (SAMs). The membranes are then transferred to window-substrates (Si) for mechanical characterization. Bulge testing of such freestanding nanomembranes within an atomic force microscope is utilized to investigate their mechanical properties.

A series of biphenyl-based molecules were used to prepare the nanomembranes, such as carbonitrile-biphenyl-trimethoxysilyl (CBPS), biphenyl-thiol (BPT) and nitro-biphenyl-thiol (NBPT). Biphenyl-based nanomembranes have elastic moduli ranging from 6 to 12 GPa. They display outstanding performance in the ultimate tensile strength with values of 400 to 500 MPa, which is several times higher than the values of other carbon based membranes. Furthermore, annealing of the cross-linked nanomembranes in ultra high vacuum systematically increase of the Young’s moduli from 10 GPa to ~45 GPa for an annealing temperature of ~1000 K. Strain relaxation lowers the residual strain from 0.9 % to ~0.35 % for temperatures of 800 K and above. This is caused by a structural transformation in which the nanomembrane is converted into nanocrystalline graphene.

We have performed scanning tunneling spectroscopy (STS) measurements to investigate Dirac particle interactions and localization by local impurities in a gated single-layer exfoliated graphene device in the quantum Hall regime at a temperature of 4.3 K. At the Dirac point, electron-hole puddles created by the disorder potential in SiO₂ substrate are observed at zero magnetic field. In an applied magnetic field, the carriers are condensed into well-resolved Landau levels (LLs), whose general evolution as a function of both charge density and magnetic field is well described by the context of ‘massless’ Dirac particles. Tunneling spectroscopy measurements as a function of magnetic field and applied gate potential are shown to give insight into the localization of carriers and their relation to the disorder potential. At low magnetic fields, tunneling spectra display long-range scattering features related to the graphene disorder potential variation. The disorder potential also determines the spatial distribution of LLs in higher magnetic fields. We observe that isolated compressible LL regions surrounded by incompressible strips behave like graphene quantum dots (QDs). Single-electron charging of the QDs is seen as four-fold Coulomb oscillations in individual dI/dV curves. These results show that the plane of the graphene 2DEG breaks into a checkerboard pattern of electron- or hole-rich QDs localized at either maxima or minima of the disorder potential.

It has been shown recently, both theoretically [1] and experimentally [2-3], that a bandgap can be opened and even tuned continuously in bilayer graphene (BLG) in the presence of a strong electrical field, which induces asymmetry of the electrostatic potential of the two graphene layers. This makes BLG an attractive material for future digital electronic applications, infrared nanophotonics, pseudospintronics, and terahertz technology. On the other hand a complete understanding of the physics of BLG and the effect of disorder on a microscopic scale is missing. In this work, we present local tunneling measurements of bilayer graphene exfoliated on SiO₂/Si using scanning tunneling spectroscopy at a temperature of 4.3 K. The graphene bilayer is probed with both the application of a perpendicular magnetic field and with an external gate voltage applied to the Si substrate. We study the effect of disorder potential induced by SiO₂/Si on the electronic properties of bilayer graphene. We find that disorder potential causes a bandgap opening in BLG, while a backgate voltage has a secondary effect on the density of states. We also find that the magnetic quantization of the carriers, evidenced by the formation of Landau levels (LL), does not obey the simple scaling of LL energy versus magnetic field for an ideal graphene bilayer.[4] The LL spectra are seen to vary greatly depending on the local potential variation and associated charge density. We have investigated these variations with detailed spectroscopic maps of the LL spectra as a function of energy, gate voltage, and local potential variation. We find the assignment of the spectral features to be much more complex than expected, and may require the introduction of an intrinsic electrical bias in the bilayer system. In this talk, we will discuss the possible theoretical models that may account for our observations.

[1] E. McCann, V. I. Fal’ko, Phys. Rev. Lett. 96, 086805 (2006).

[2] J. B. Oostinga, et. al., Nature Mater. 7, 1510157 (2008).

[3] E. V. Castro Phys. Rev. Lett. 99, 216802 (2007).