Inductive Detection of Magnetic Beads in a Microfuidic Channel

1
Electrical Characterization of Graphite Oxide
by Ira Jewell
To an engineer, this is interesting because the
number and type of carriers (electrons or holes)
can be tuned by application of an electric field.
Moreover, both carrier types have quite high
mobilities (µ 10,000 cm2/V-s) even with
carriers approaching concentrations as high as
the theoretical limit 71012 cm-2. Graphenes
high mobililty, coupled with the fact that its
conductivity can be smoothly modulated by
application of an electric field through a gate
electrode, make it an attractive material for FET
applications.
Results
Abstract
Unfortunately, results from this current design
have not yet been obtained (see Future Work
below). We have, however, shown with two-point
measurements of rather thick samples of graphite
oxide that the resistance of the sample can be
significantly reduced by simple exposure to a
forming gas ambient at an elevated temperature
(300C)
Since the discovery in 2004 that single atomic
planes of graphite could exist individually 1,
there has been much interest in this
two-dimensional material known as graphene. The
original technique for isolating graphene was
simply micro-mechanical cleavage from bulk HOPG
(highly-oriented pyrolitic graphite) with the
sticky-tape method being among the most
successful innovations. With this technique, the
experimenter repeatedly divides a piece of HOPG
using regular Scotch tape and presses it
against a substrate (usually Si oxidized to a
specific thickness to ensure maximal visibility
of graphenes) and transfers it with gentle
rubbing. While this method does transfer single
and few-layer graphenes, it also transfers many
larger graphitic particles (as seen in this
slides background) and finding the desired
material is much like finding the needle in the
haystack. Alternative, chemical, methods for
isolating graphene have been pursued and this
work intends to follow one of the most popular.
In this method, graphite oxide (GO) is first
synthesized. For this we used the technique of
Hummers as modified by Kovtyukhova 2. This
graphite oxide is hydrophilic and readily
disperses in water with ultrasonic agitation.
Single-layer platelets can then be isolated by
centrifugation. The resulting product is sprayed
onto a heated substrate so that it flash
evaporates, leaving a distribution of platelets.
These GO platelets are non-conductive and not of
much use, but under certain conditions they can
be made to conduct. We propose to use
direct-write laser lithography to define
four-point resistivity structures atop individual
single-layer GO platelets. The number of atomic
layers in the samples will be characterized using
both atomic force microscopy (AFM) and Raman
spectroscopy. Resistivity measurements will be
taken in situ as the GO sample is exposed to a
variety of reducing gasses over a range of
temperatures. We have already shown the
possibility of increasing the conductivity of a
sample by several orders of magnitude (using
two-point contact measurements on a multi-layered
sample) and hope to refine our technique to
produce device-quality graphenes by this method
in the near future.
To a magnetician, graphene is interesting because
spins can remain coherent over long distances
because of the long times between scattering
events. This makes graphene a good candidate
material for spintronics applications. One group
of researchers have demonstrated the spin valve
effect with a sample of several-layer graphene
forming the spacer layer between a pair of
Cobalt contacts. The magnetic electrodes were
separated by 500nm and the experiment can be
taken as evidence that the spinformation
contained in the injected electron survives at
least that distance.
The resistance (inverse slope of plots shown) is
seen to decrease dramatically after reduction.
Spin valve effect showing lower resistance state
when the direction of magnetization of the two
electrodes are aligned. 6
Experimental Design
Future Work
A solution of graphite oxide was first prepared
according to the method of Hummers as modified by
Kovtyukhova 2. The solution was filtered and
allowed to dry, forming GO paper. The graphite
oxide was then resuspended in solution and
exfoliated by stirring for several days.
The next step in the project is to characterize
each of the potential samples of GO, keeping only
those with one or two layers. We will use
atomic force microscopy, as step height profiles
can reveal the number of layers in the sample.
Why is Graphene Interesting?
Single-layer graphene is interesting for a large
number of reasons, both theoretical and
practical. Perhaps the most unique feature of
graphene lies in its bandstructure. At certain
points in reciprocal space the conduction band
and the valence band actually touch, making
graphene a zero-bandgap semiconductor. Even
stranger, in the region of k-space near these
points the dispersion is linear.
AFM image of single folded sheet of GO 8
A Heidelberg DWL66fs laser lithography system was
used to pattern photoresist atop a Si wafer
substrate (with 300nm thermally grown SiO2 to
enhance visibility of graphenes). The pattern is
a repeating grid of four-point measurement
structures consisting of 1µm wide lines
connecting to larger (90µm x 90µm) pads for
probing or wirebonding.
Once characterized by AFM, the next step is to
connect each of the 90µm pads (of the structure
with the correctly situated GO sample) to larger
contact pads via wirebonding. We will use a
Mech-El 827 manual bonder.
Finally, the appropriate electrical connections
will be made, and the entire substrate placed in
a ProboStat measurement cell so that
resistivity can be monitored in situ as reducing
gasses are flowed and temperature is varied.
Gold (100nm) and a thin chrome adhesion layer
(5nm) was then deposited using thermal
evaporation. Liftoff was performed using acetone
and ultrasonic agitation to remove the
undeveloped photoresist, thus defining the metal
contact structures.
Bandstructure of single-layer graphene 3
To a physicist, this is interesting because it
forces the (conduction) electrons in graphene to
behave as massless Dirac fermions and not like
an ordinary electron at all. These electrons
possess zero rest mass and travel with a constant
velocity 106 m/s 4. In addition, graphene
displays such exotic effects as the anomalous
(half-integer) quantum Hall effect and a nonzero
Berrys phase 5.
Acknowledgements
I would like to thank Dr. Albrecht Jander, Chris
Tasker, Sean Smith, and Chien-Chih Huang. This
project is funded by the Army Research
Labscontract /280710A/
The substrate was then heated on a hotplate and
the solution of GO spray deposited as a fine mist
so that the droplets flash-evaporate. This is
necessary because the GO is hydrophilic and would
agglomerate if larger drops were used 7. The
substrates were then analyzed by optical
microscope to find samples that have by chance
landed in the correct place (spanning all four
probes of a measurement structure)
References
1 Novoselov. Science. 306, 666 (2004) 2
Kovtyukhova. Chemistry of Materials. 11, 771
(1999) 3 Geim. Physics Today. 60 (Issue 8), 35
(2007) 4 Novoselov. Nature. 438, 197 (2005) 5
Zhang. Nature. 438, 201 (2005) 6 Nishioka.
Applied Physics Letters. 90, 252505 (2007) 7
Gilje. Nano Letters. 7, 3394 (2007) 8
Gomez-Navarro. Nano Letters. 7, 3499 (2007)
Hall coefficient (which is inversely proportional
to carrier concentration) as function of applied
field 1