Enhanced resistance of single-layer graphene to ion bombardment
J. J. Lopez,
1
F. Greer,
2
and J. R. Greer
3,
a
1
East Los Angeles College, California 91754, USA
2
Jet Propulsion Laboratory, California 91109, USA
3
Division of Engineering and Applied Science, California Institute of Technology, California 91125, USA
Received 17 January 2010; accepted 15 April 2010; published online 27 May 2010
We report that single-layer graphene on a SiO
2
/
Si substrate withstands ion bombardment up to
7
times longer than expected when exposed to focused Ga
+
ion beam. The exposure is performed in
a dual beam scanning electron microscope/focused ion beam system at 30 kV accelerating voltage
and 41 pA current. Ga
+
ion flux is determined by sputtering a known volume of hydrogenated
amorphous carbon film deposited via plasma-enhanced chemical vapor deposition. ©
2010
American Institute of Physics
.
doi:
10.1063/1.3428466
I. INTRODUCTION
Graphene, a one atom thick sheet of
sp
2
hybridized car-
bon atoms arranged in a honeycomb lattice, is highly re-
searched because of its unique and advantageous electronic
properties.
1
–
4
The combination of its ballistic electron
transport,
5
high elastic limit,
6
and the recent achievement of
producing macroscale conducting sheets
7
makes its use in
future nanoelectronic devices promising. Due to the chal-
lenges associated with manipulating and producing these
nanoscale specimens, multiple forms of characterization are
required, including but not limited to optical microscopy,
atomic force microscopy, Raman spectroscopy, scanning
electron microscopy
SEM
, and transmission electron mi-
croscopy. These techniques utilize different mechanisms to
determine material properties and, consequently, have differ-
ent interactions with the graphene lattice. Recent investiga-
tions demonstrate that defects can be introduced into the
graphene lattice via electron beam irradiation as manifested
by the appearance of a disorder-related D peak and a change
in height of both the D peak and zone-center G peak within
graphene’s Raman spectra.
8
It is, therefore, likely that par-
ticle bombardment—electronic or ionic—causes varying de-
grees of damage in graphene. Therefore, in order to design
and maintain graphene’s desired characteristics, it is critical
to understand its response to electron and ion bombardment.
Here we report that a single-layer graphene sheet is approxi-
mately seven times more resilient against ion bombardment
than expected based on sputtering mechanisms of other
carbon-based structures.
II. EXPERIMENT
Graphene samples were obtained by mechanical exfolia-
tion of highly oriented pyrolitic graphite
SPI Supplies
and
subsequently transferred onto Si substrates with a 300 nm
thick SiO
2
layer by using Nitto Denko tape.
9
,
10
Single-layer
flakes were first identified by optical microscopy
Nikon
Eclipse LV100D
using white light to distinguish the
graphene sheets via the added optical path that causes an
increase in contrast against the SiO
2
/
Si substrate.
9
,
11
Figure
1
shows optical and SEM images of such representative
single-layer sheets. Once the single-layer flakes were identi-
fied, Raman spectroscopy with a Reinshaw spectrometer, op-
erating at room temperature with a holographic notch filter
HNF
514 nm laser working at
1.25 mW, was used to
confirm their thickness. The Raman spectrum for the sample
shown in Fig.
1
with a G peak near 1580 cm
−1
and G
band
also known as 2D
near 2700 cm
−1
is provided in Fig.
2
.
The sample is defect-free as indicated by the lack of a
disorder-related D peak near 1350 cm
−1
. The symmetric
shape of the 2D peak confirms that the sample is indeed a
single sheet, as demonstrated by Ferrari
et al.
and Gupta
et
al.
,
12
,
13
Fig.
2
b
.
Ion and electron beam imaging was performed in a FEI
Nova 200 DualBeam System with a Ga
+
liquid metal ion
source. The e-beam was operated at an acceleration voltage
of 10 kV and beam current of 0.54 nA. During the initial
alignment, the graphene sample was exposed to the e-beam
for
20 minutes. The sample was then tilted to the eucentric
point at a 52° incline with respect to the e-beam at a working
distance of 5.1 mm to allow simultaneous imaging of the
region with both columns. Magnifications and beam posi-
tions were coupled to ensure that the graphene sample could
be reliably imaged without unnecessary exposure to either
beam. After taking the initial SEM image of the graphene
sheet, a single ion beam frame was taken at an accelerating
voltage of 30 kV and beam current of 41 pA
nominally 10
pA
to intentionally sputter away the single-layer region.
a
Author to whom correspondence should be addressed. Electronic mail:
jrgreer@caltech.edu.
FIG. 1.
Color
a
Optical image of single-layer graphene in the top left
corner of the sample indicated by an arrow. The dimensions of the single-
layer region are
3
2
m
2
.
b
SEM image of the same sample with
single-layer region depicted by the box, taken at 10 kV and 0.54 nA.
JOURNAL OF APPLIED PHYSICS
107
, 104326
2010
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SEM and focused ion beam
FIB
scans were iteratively re-
peated until the graphene sheet could no longer be identified
in subsequent SEM images, Fig.
3
.
We calculate the scan time per frame of 2.7 s by multi-
plying the total number of pixels in a single ion beam image
1024
884
by the beam dwell time per pixel
3
s
. The
graphene sheet was then iteratively sputtered away with each
scan. The final step, Figs.
3
d
and
3
i
, shows that the region
originally covered by single-layer graphene became the same
hue as the underlying SiO
2
. The total time to sputter away
this sheet was 57 s, a surprisingly high value for a single-
atom-thick material.
To determine whether this etch time was reasonable, we
performed a similar experiment on a hydrogenated amor-
phous carbon
a-C:H
film with an average thickness of 125
nm, Fig.
4
d
. Sputtering of this film was also used to calcu-
late the incident Ga
+
ion flux and subsequently determine the
sputtering time of these carbon-based structures. The a-C:H
was deposited on a Si substrate via plasma-enhanced chemi-
cal vapor deposition with a methane/hydrogen/argon contain-
ing plasma. The amorphous and hydrogenated nature of the
film was confirmed with Raman spectroscopy under the
same conditions as used for graphene. Using a two-peak
Gaussian fit for the deconvolution of the D and G peaks, we
determined their respective positions as 1371 and
1544 cm
−1
, as well as the areas under the fitted curves,
14
Fig.
4
b
. By using the ratio of these areas, I
D
/
I
G
=0.79, and specific position of the G peak we determined the
relative fractions of
sp
3
and
sp
2
hybridized carbon-carbon
bonds in the film to be
38%
sp
3
and 62%
sp
2
.
15
III. RESULTS AND DISCUSSION
We estimate the sputter yield of this a-C:H film to be
2.63 C atoms/incident Ga
+
by utilizing the determined
weighted average of the
sp
3
and
sp
2
bonds in the film and the
reported sputter yields for different carbon allotropes under
Ga
+
ion bombardment: 2.3, 2.55, and 2.7.
16
The sputter
yields at 30 kV are assumed to be the same as those experi-
mentally determined at 50 kV due to flattening out of the
yield curve as a function of accelerating voltage at energies
of 30 keV and higher.
17
Figure
4
a
shows the a-C:H film
before its exposure to the ion beam, and Fig.
4
c
shows the
selected 2.15
2.06
m
2
area
4.33
m
2
in the film that is
completely sputtered away after 45 s of exposure to the same
ion beam conditions as in the graphene experiments.
Generally, experimentally-determined values for a-C:H
sputter yield fall between 2.3 to 2.8 carbon atoms per inci-
dent ion at an acceleration voltage of 50 kV.
16
Theoretically
determined values for carbon sputter yield, however, range
from 1.2 to 1.7 at an acceleration voltage of 30 kV, with the
former obtained by transport of ions in matter
TRIM
simulations
18
and the latter through use of the linear cascade
collision
LCC
model developed by Sigmund.
17
We refrain
from using the computationally determined sputter yields
due to the limitations associated with these models. For ex-
ample, the TRIM simulations do not take into account any
orientation dependent phenomena such as the channeling of
Ga
+
ions through the graphene lattice. The LCC model,
whose underlying assumptions require ample material vol-
ume and an amorphous structure, is also unlikely applicable
in estimating the sputtering yield of graphene due to its 2D
nature and highly ordered lattice.
17
Furthermore, we find that
even if the theoretically determined sputter yields were taken
into account, they would have marginal influence on our sub-
sequent calculations for the estimated graphene sputter time.
FIG. 2.
a
Raman spectra at 514 nm for graphene region shown in Fig.
1
.
b
2D peak at
2684 cm
−1
.
FIG. 3.
a
–
e
SEM images of the graphene flake shown in Fig.
1
52°
tilt
.
f
–
j
FIB images of the same flake as a function of exposure
or-
thogonal
. Total area covered by the FIB image is 290.1
m
2
. Scale bar is
5
m. Contrast in images was digitally enhanced to accurately identify the
time of complete sample sputtering.
FIG. 4.
Color online
a
Initial SEM image of the a-C:H film prior to FIB
exposure.
b
Raman spectrum of the a-C:H film with D and G peak decon-
volution.
c
The darker region is the selected 4.33
m
2
area of the a-C:H
film after 45 s exposure to ion beam at 30 kV/41 pA. Wells in the middle of
the exposed area shows the SiO
2
underneath had been reached.
d
FIB-
machined cross-section of the a-C:H film taken in the SEM. Top
200 nm
of Pt
/middle
125 nm of a-C:H
/bottom
Si substrate
. SEM images 4
a
,
4
c
–4
d
are captured while the a-C:H film is at a 52° incline with respect
to the e-beam. All measurements are corrected for the 52° incline.
104326-2 Lopez, Greer, and Greer
J. Appl. Phys.
107
, 104326
2010
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We determine the Ga
+
ion flux incident on the a-C:H
film
and, therefore, on graphene
by calculating the total
number of sputtered carbon atoms from a selected area, and
then dividing it by both the sputter yield and the time re-
quired to fully etch away the film in that area. First, by as-
suming the mass density of the a-C:H film to be 2 g
/
cm
3
,as
widely reported for amorphous carbon films,
19
we determine
the number of sputtered C atoms, N
s
, from the 125 nm thick
a-C:H film to be 1.254
10
18
C atoms
/
cm
2
by utilizing Eq.
1
N
S
=
C
N
A
d
m
c
,
1
where
C
is the mass density of the a-C:H film, N
A
is
Avogadro’s Number, m
c
is atomic mass of carbon, and d is
the film thickness. Using the experimentally-determined etch
time, t
etch
, required to entirely remove the film in the
4.33
m
2
area, we calculate the sputter rate
R
S
to be
2.786
10
16
atoms
/
cm
2
s by using Eq.
2
R
S
=N
S
/
t
etch
.
2
We then determine the Ga
+
flux
J
Ga
+
to be 1.059
10
16
Ga
+
ions
/
cm
2
sthrough Eq.
3
J
Ga
+
=R
S
/
Y
a-C:H
.
3
We then use the carbon-carbon bond length of 1.42 Å and the
geometry of the graphene lattice, Fig.
5
, to calculate the
aerial density of graphene
A
to be 3.818
10
15
atoms
/
cm
2
. If the sputtering mechanism in graphene
is similar to that in other carbon-based structures, the theo-
retical time
t
theo
to remove a single graphene sheet of a
certain area in its entirety can be calculated by
t
theo
=
A
A
a−C:H
A
S
J
Ga
+
Y
a-C:H
,
4
here A
a-C:H
/
A
S
is the ratio between the exposed area of the
a-C:H film
A
a-C:H
=4.33
m
2
and the total area
A
S
of the
image depicting a graphene monolayer sample. This scaling
takes into account the frequency with which the ion beam
rasters over the exposed area. For the graphene monolayer
presented in Fig.
3
a
sample 1
,A
S1
=290.1
m
2
, and
t
theo
=8.96 s, resulting in the ratio between t
exp
and t
theo
to be
6.362. For the second graphene monolayer studied here
sample 2
,A
S2
=34.3
m
2
,t
theo
=1.06, and t
exp
=8.15 s. Us-
ing the t
exp
/
t
theo
ratios for these two samples, we calculate
that average t
exp
is
7.03 times longer than t
theo
for a single
sheet of graphene
Table
I
. This t
exp
surpasses any expecta-
tions for a carbon-based material under ion bombardment.
We believe that this resilience stems from the 2D nature
of graphene. Since graphene is only one atom thick, it elimi-
nates the volume in which sputtering cascade collisions oc-
cur in bulk materials and minimizes the interactions with
recoiling atoms from the underlying substrate and graphene
lattice.
17
,
20
,
21
The absence of cascade collisions near the sur-
face would greatly reduce the number of ejected carbon at-
oms from the graphene lattice. It is also possible that C at-
oms in graphene act as an intermediary in transferring the
kinetic energy to the underlying substrate without being
readily displaced from the lattice, acting like a system of
billiard balls where energy is completely transferred to the
substrate underneath. Graphene’s relatively open and ordered
atomic structure may also facilitate Ga
+
ion channeling
through its lattice.
17
,
22
This would limit the number of pri-
mary ion collisions, thus reducing the number of carbon at-
oms sputtered away. Ultimately, Ga
+
ions that are not back-
scattered by the graphene lattice come to a halt in the
underlying SiO
2
after losing their kinetic energy through
elastic and inelastic collisions. Since SiO
2
is an insulator,
Ga
+
ions may be implanted into the substrate surface, caus-
ing a net positive charge to build up in the region exposed to
ion bombardment, forcing the subsequent incoming incident
ions to be electrostatically repelled.
18
These considerations
should be taken into account when developing a more thor-
ough understanding of graphene’s ability to withstand a pro-
longed ion beam exposure.
Further investigations, involving the substitution and
elimination of the substrate, will help shed light onto the
specific mechanisms operating during sputtering of graphene
by removing any effects that the substrate has on the cascade
collision process. Elimination of the substrate will ensure
that mechanisms inducing the sputtering of graphene will be
solely from elastic and inelastic collisions with incident ions.
IV. CONCLUSION
In summary, we convincingly demonstrate the ability of
a single-atom-thick graphene layer to withstand approxi-
mately seven times higher ion bombardment than expected.
Our findings reaffirm the wide range of science that is yet to
be done on this unique single-atom thick material.
FIG. 5.
Color online
Unit cell of the graphene lattice outlined by the
dashed lines: two atoms and three bonds are included. a=1.42 Å.
TABLE I. Sputtering times for single-layer graphene.
The average of
t
exp
/
t
theo
is 7.03.
Sample
A
a-C:H
/
A
S
t
theo
s
t
exp
s
t
exp
/
t
theo
1
0.0153
8.96
57.0
6.362
2
0.1290
1.06
8.15
7.689
104326-3 Lopez, Greer, and Greer
J. Appl. Phys.
107
, 104326
2010
Downloaded 22 Jun 2010 to 131.215.220.165. Redistribution subject to AIP license or copyright; see http://jap.aip.org/jap/copyright.jsp
ACKNOWLEDGMENTS
We gratefully acknowledge financial support of the NRI
INDEX Center. We thank M. J. Burek for useful discussions
and assistance with the DualBeam system. Access to the
Dual-Beam System was provided by the Kavli Nanoscience
Institute
KNI
at Caltech. We also appreciate assistance
from E. Miura and G. R. Rossman with Raman Spectros-
copy, and we thank C. Daraio for access to the optical mi-
croscope.
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104326-4 Lopez, Greer, and Greer
J. Appl. Phys.
107
, 104326
2010
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