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The
preparation of free-standing sheets of graphene via mechanical
exfoliation was first demonstrated in 2004 by Novoselov, et al. (Science
306, 666). The relative
ease in peeling off single or few atomic layers of graphite and the
possibility of locating them by optical microscope after they are
deposited onto an insulating substrate made this area of research
very accessible to a large community of researchers.
The thickness of graphene
layers can be identified by color and
confirmed via atomic force microscopy (AFM) and Raman spectroscopy
measurements.
Since its
discovery, there have been many experiments designed to measure the
electronic, magnetic, chemical, and mechanical properties of graphene.
The earliest reported charge carrier mobilities were
on the order of 10,000 cm2/Vs, high enough to measure
many phenomena such as an unconventional integer quantum Hall
effect, but not sufficiently high to host other phenomena such as
fractional quantum Hall effect.
Therefore improving the mobility of graphene has always been an
important goal of graphene research. Since most graphene devices are fabricated using multiple
lithographic steps that could introduce contamination and disorder
to the flake, we developed an all-dry, lithography-free technique of
fabrication (Appl. Phys.
Lett. 90,
143518 (2007)).
We place quartz filaments roughly one micron in
diameter onto the graphene and use it as a shadow mask during the
deposition of electrodes. Our
devices and experimental setup allow us to tune the Fermi level with
a back gate, temperature down to 30 mK, and magnetic field up to 8
T. Though the
lithography-free process did not result in generally higher
mobilities, the technique allows us to prepare graphene devices in a matter
of hours as opposed to days of lithographic procedures.
In addition, we have also developed a technique to produce planar tunnel
junctions and used them to measure directly the electronic density of states
at the Fermi level and the associated energy gap in graphene.
We used the technique described above to measure the electronic
transport properties of single (1LG) and multi-layered graphene
samples. We observed universal conductance fluctuations in bilayer (2LG) and
trilayer (3LG) devices as a function of gate voltage and magnetic
field, and their unexpected suppression close to the undoped Fermi
level in graphene (Phys.
Rev. B 77,
155429 (2008)).
We have also studied the electrical transport properties of
graphene hybrid structures consisting of 1LG, 2LG, and the interface
between them on one contiguous flake (arxiv:0809.1312)
under magnetic fields.
In these graphene hybrid structures, we find evidence for charge transfer between 1LG and 2LG as a result of
dramatically different electron energy scales of Landau levels
emerge with the application of a modest magnetic field and anomalous
quantum oscilations.
More research
is currently underway in an effort to study more
closely the physics at the interface.
Graphene’s
mechanical strength and high mobility, even at room temperature,
make it a hopeful candidate for technological applications. However, if graphene is to be considered for potential use,
it must be produced in wafer size (exfoliation
produces micron-scale flakes) and be engineered to feature an
electron band gap (in its pure form, both 1LG and 2LG are
semimetals). The most
promising method of large-scale graphene growth is by sublimation of
silicon on the Si-face of silicon carbide.
When controlled, the carbon on the heated surface rearranges
into single or multiple layers of graphene.
This film growth is commonly called epitaxial graphene.
We are working to characterize the properties of epitaxial graphene
using a variety of means, with
Raman spectroscopy (arxiv:0809.1616) we
measure the strain uniformity, we
measure the electronic transport properties of devices prepared on patterned
films, and developing methods to induce a bandgap in both epitaxial
1LG and 2LG.
Contacts:
Conor Puls, cpp116 @ psu.edu
Neal Staley, nes151 @ psu.edu
Publications:
N.
E. Staley et al., “Lithography-free fabrication of graphene
devices,” Appl. Phys. Lett. 90,
143518 (2007).
N.
E. Staley, C. P. Puls, and Y. Liu, “Suppression of
conductance fluctuation in weakly disordered mesoscopic graphene
samples near the charge neutral point”,
Phys. Rev. B 77,
155429 (2008).
C.
P. Puls, N. E. Staley, and Y. Liu , “Interface states and
anomalous quantum oscillations in graphene hybrid structures”, arxiv:0809:1312.
J.
A. Robinson et al., “Raman Topography and Strain
Uniformity of Large-Area Epitaxial Graphene”,
arxiv:0809.1616.
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Graphene bilayer lattice structure.
"Graphene staircase" distinguishes number of layers by color.
Quartz filament laid atop a graphene hybrid.
Oscillations in resistance arising from the emergence of Landau levels in
graphene hybrids.
Hall bar device made of epitaxially grown graphene film on SiC.
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