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Electron beam nanosculpting of suspended graphene sheets
Michael D. Fischbein and Marija Drndić
a兲
Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
共Received 5 August 2008; accepted 21 August 2008; published online 16 September 2008兲
We demonstrate high-resolution modification of suspended multilayer graphene sheets by controlled
exposure to the focused electron beam of a transmission electron microscope. We show that this
technique can be used to realize, on time scales of a few seconds, a variety of features, including
nanometer-scale pores, slits, and gaps that are stable and do not evolve over time. Despite the
extreme thinness of the suspended graphene sheets, extensive removal of material to produce the
desired feature geometries is found not to introduce long-range distortion of the suspended sheet
structure. © 2008 American Institute of Physics. 关DOI: 10.1063/1.2980518兴
Graphene, a two-dimensional carbon crystal, has been
the focus of intense research since techniques were devel-
oped to extract it from graphite in the form of multilayers
1
and single layers.
2
Graphene-based devices measured on
substrates have revealed an impressive set of exotic elec-
tronic and optical properties with promising applications.
3–7
Furthermore, suspended graphene has been shown to have
exceptionally high electron mobilities
8
and high strength.
9,10
Due to its single-atom thickness and the relatively low
atomic number of carbon, suspended graphene is emerging
as powerful platform for transmission electron microscopy
共TEM兲.
10–12
In addition to serving as a nearly ideal substrate
for TEM analysis,
13
it has been shown that electron-beam-
induced deposition 共EBID兲 of carbon onto graphene can be
achieved with high accuracy in a TEM.
14
In this letter, we show that suspended multilayer
graphene sheets can be controllably nanosculpted with few-
nanometer precision by ablation via focused electron-beam
irradiation in a TEM at room temperature. We demonstrate
nanopores, nanobridges, and nanogaps. These examples and
other nanometer-scale patterns of arbitrary design may prove
useful in graphene-based electronic and mechanical applica-
tions. For instance, fabricating narrow constrictions in
graphene layers is of interest for electronic property
engineering.
15–23
Structures made by electron-beam irradia-
tion are stable and do not evolve over time. Furthermore, we
find that extensive removal of carbon does not introduce sig-
nificant long-range distortions of the graphene sheet. Specifi-
cally, the sheets do not begin to fold, wrinkle, curl, or warp
out of the focal plane during cutting.
Graphene sheets were extracted from graphite by me-
chanical exfoliation
2
on ⬃300 nm SiO
2
substrates coated
with ⬃100 nm of polymethyl methacrylate 共PMMA兲 and
then transferred to a ⬃50-nm-thick suspended SiN
x
mem-
brane substrate.
24
Prior to transfer, arrays of ⬃1
m square
holes were patterned into the SiN
x
membranes by exposing
the surface to a SF
6
reactive ion etch through a resist mask
made by electron beam lithography. In order to transfer
graphene sheets onto the SiN
x
membranes, we followed a
method used by Meyer et al.
14
for transferring graphene to
TEM-compatible holey carbon grids. After locating graphene
sheets on the PMMA surface with an optical microscope, a
drop of isopropanol is added to the surface.
A SiN
x
membrane substrate is then placed onto the drop over
a region containing graphene sheets, with its surface facing
the PMMA surface. As the isopropanol evaporates, its sur-
face tension brings the two surfaces into close contact, which
is further improved by heating at ⬃200° C for ⬃5 min. Fi-
nally, the PMMA is dissolved in acetone, which releases the
graphene sheets on the PMMA side and allows them to trans-
fer and stick to the SiN
x
membrane substrate.
Graphene sheets suspended over a hole in the SiN
x
membrane were identified in a TEM 共JEOL 2010F operating
at 200 kV兲. The number of graphene layers in a sheet could
often be determined by imaging the edge of a folded
region,
11
in a manner similar to counting the number of tubes
in a multiwalled nanotube. We have worked with samples
ranging in thickness roughly from 1–20 graphene layers,
though the majority of graphene sheets used in this work
were composed of approximately five layers. Using a method
described previously, arbitrary patterns were created in the
graphene sheets by increasing the TEM magnification to
⬃800 000⫻, condensing the imaging electron beam to its
minimum diameter, ⬃1 nm, and moving the beam position
with the condenser deflectors.
25
To avoid EBID of carbon,
likely to occur for a spot-mode beam setting, nanosculpting
was performed with the beam at crossover in a diffusive
mode. With the beam at crossover, the current density mea-
sured on the imaging screen was ⬃50 pA/ cm
2
which, after
accounting for magnification, corresponds to an estimated
⬃0.3 pA/ nm
2
at the sample position. The exposure of the
graphene sheets to the beam was ⬃1s/ nm
2
. All of the struc-
tures shown were made at room temperature.
Figures 1共a兲–1共c兲 show TEM images of a graphene sheet
before and after creating a ⬃3.5 nm diameter nanopore by
irradiating this spot with the condensed electron beam for
⬃5 s. We have also observed that very brief 共⬃500 ms兲 ex-
posure of graphene sheets to the condensed electron beam
can be used to create a partial nanopore by removing a frac-
tion of the graphene layers, while leaving other layers intact.
A single nanopore is the simplest structure that can be made
by ablation, yet nanopores have proven extremely valuable
in studies of molecular translocation, DNA in particular.
26
Given that graphene is the thinnest possible membrane while
at the same time structurally robust
9
and impermeable,
27
na-
nopores in graphene sheets may be useful for achieving sig-
nificant resolution enhancement in molecular translocation
measurements. As shown in Fig. 1共d兲, multiple nanopores
a兲
Electronic mail: drndic@physics.upenn.edu.
APPLIED PHYSICS LETTERS 93, 113107 共2008兲
0003-6951/2008/93共11兲/113107/3/$23.00 © 2008 American Institute of Physics93, 113107-1
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can be made in close proximity to each other, indicating that
large arrays of closely packed nanopore arrays can be
achieved. Since the average irradiation exposure time per
nanopore is on the order of seconds, serial processing is not
prohibitively time intensive and large arrays or more compli-
cated geometries can be made quickly. Moreover, parallel
fabrication with multiple electron beams would allow for
substantial scalability.
All of the nanopores that we have made have a concen-
tric ringlike structure extending several nanometers away
from their edges. This ringlike structure, evident in Figs. 1共c兲
and 1共d兲, bears a close resemblance to the dark lines often
observed at the edge of a folded graphene sheet, an example
of which is shown in Fig. 1共e兲. The orientation of a folded
graphene layer’s edge is locally parallel to the TEM beam
and consequently each layer in a folded graphene sheet in-
troduces a dark line along the edge of the fold,
11
similar to
what is seen at the radial edges of a multiwalled carbon
nanotube. Intensity cross sections 关Figs. 1共f兲 and 1共g兲兴 ob-
tained from the images of the folded graphene sheet 关Fig.
1共e兲兴 and nanopore 关Fig. 1共c兲兴 reveal an average spacing be-
tween dark lines of 0.38⫾ 0.02 and 0.39⫾ 0.02 nm, respec-
tively. These values are equivalent within the error intro-
duced by finite TEM resolution and are close to the interlayer
distance of highly oriented pyrolitic graphite 共⬃0.34 nm兲.
These observations suggest that irradiation can induce coor-
dinated interlayer bonding between freshly exposed layer
edges, leading in this case to an “inverted-onion-like” struc-
ture. Irradiation of carbon systems has been previously
shown to be capable of inducing a variety of structural
changes,
28
and our results demonstrate that graphene sheets
can provide a valuable initial system for deriving carbon
morphologies.
Figure 2共a兲 shows two parallel ⬃ 6-nm-wide lines, i.e.,
regions where graphene has been removed, separated by
⬃25 nm. Starting with these lines, additional focused irra-
diation was used to gradually increase the lines’ widths until
their separation was reduced to ⬃5 nm, resulting in a “nano-
bridge” 关Figs. 2共b兲 and 2共c兲兴. Although the final nanobridge
is highly crystalline 关Fig. 2共c兲兴, the extensive exposure to
irradiation may have induced significant interlayer rebonding
and atomic restructuring within individual layers. Nano-
bridges can be cut with the TEM beam to create a gap 关Fig.
2共d兲兴 with initial size less than a nanopore diameter but
quickly increasing with continued irradiation. In the regions
near the cut, irradiation induces morphological changes of
the crystalline structure and, in particular, we observe that
cut ends close completely, similar to fullerene capping ob-
served for irradiated nanotubes.
28
Such carbon-based point
contacts and nanobridges directly connected to a larger
graphene structure may find use in mechanical and electrical
applications.
In conclusion, we have demonstrated that suspended
graphene sheets can be controllably nanosculpted with
electron-beam irradiation. The ability to introduce features
into suspended graphene sheets by electron-beam-induced
FIG. 1. 共Color online兲 TEM images of a suspended graphene sheet 共a兲
before and 共b兲 after a nanopore is made by electron beam ablation. 共c兲
Higher magnification image of the nanopore. 共d兲 Multiple nanopores made
in close proximity to each other. 共e兲 Folded edge of a graphene sheet show-
ing lines corresponding to layer number. These lines are similar to those
seen around the nanopores 共Scale bars are 50, 50, 2, 10, and 5 nm兲. 共f兲
Average of intensity cross sections taken along six different radial directions
of the nanopore in 共c兲, each starting at the edge and proceeding radially
outward. 共g兲 Average of six intensity cross sections of the graphene sheet in
共e兲, each taken perpendicular to and starting at the sheet edge.
FIG. 2. 共a兲 Two ⬃6 nm lines cut into a graphene sheet. 共b兲 Electron irra-
diation is continued to create a ⬃5 nm wide bridge. 共c兲 Higher resolution of
the bridge shows clear atomic order. 共d兲 Small gap opened in the nanobridge
by additional electron irradiation. We note that the cut ends are closed.
共Scale bars are 20, 10, 5, 5 nm兲.
113107-2 M. D. Fischbein and M. Drndić Appl. Phys. Lett. 93, 113107 共2008兲
Downloaded 11 Jan 2011 to 130.91.117.41. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions
cutting and reshaping with high spatial resolution expands
their value as TEM compatible platforms and offers a route
to fabricating graphitic structures for potential use in electri-
cal, mechanical, and molecular translocation studies.
This work has been partially supported by NSF 共NSF
Career Award DMR-0449533 and MRSEC DMR05-20020兲,
ONR YIP N000140410489, the Penn Genome Frontiers In-
stitute and a grant with the Pennsylvania Department of
Health. The Department of Health specifically disclaims re-
sponsibility for any analyses, interpretations, or conclusions.
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