Organic-free suspension of large-area graphene

Article abstract of DOI:10.1063/1.4737415

We report an entirely organic-free method to suspend monolayer graphene grown by chemical vapour deposition over 10-20 μm apertures in a Cu substrate. Auger electron spectroscopy, Raman spectroscopy, scanning electron microscope, and transmission electron microscope measurements confirm high quality graphene with no measurable contamination beyond that resulting from air exposure. This method can be used to prepare graphene for fundamental studies and applications where the utmost cleanliness and structural integrity are required.

Full text of DOI:10.1063/1.4737415

Organic-free suspension of large-area graphene
E. Ledwosinska, P. Gaskell, A. Guermoune, M. Siaj, and T. Szkopek
Citation: Appl. Phys. Lett. 101, 033104 (2012); doi: 10.1063/1.4737415
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APPLIED PHYSICS LETTERS 101, 033104 (2012)
Organic-free suspension of large-area graphene
1
,2
1,2
E. Ledwosinska, P. Gaskell, A. Guermoune, M. Siaj, and T. Szkopek
3,4
3,4
1,2
1
Regroupement Qu e´ b e´ cois sur les Mat e´ riaux de Pointe, Montr e´ al, Qu e´ bec H3C 3J7, Canada
2
Department of Electrical and Computer Engineering, McGill University, Montr e´ al, Qu e´ bec H3A 0E9,
Canada
Centre Qu e´ becois sur les Mat e´ riaux Fonctionnels, Qu e´ bec, Qu e´ bec G1V 0A6, Canada
3
4
D e´ partment de Chimie, Universit e´ du Qu e´ bec a` Montr e´ al, Montr e´ al, Qu e´ bec H3C 3P8, Canada
(Received 14 May 2012; accepted 1 July 2012; published online 17 July 2012)
We report an entirely organic-free method to suspend monolayer graphene grown by chemical
vapour deposition over 10–20 lm apertures in a Cu substrate. Auger electron spectroscopy, Raman
spectroscopy, scanning electron microscope, and transmission electron microscope measurements
confirm high quality graphene with no measurable contamination beyond that resulting from air
exposure. This method can be used to prepare graphene for fundamental studies and applications
where the utmost cleanliness and structural integrity are required. VC 2012 American Institute of
Physics. [http://dx.doi.org/10.1063/1.4737415]
The criterion of greatest importance in fabricating sam-
ples for probing graphenes intrinsic properties is cleanliness.
Owing to graphene’s monolayer thickness, even a single mo-
lecular layer of foreign material on a graphene membrane
can lead to contaminant species outnumbering the constitu-
ent carbon atoms of the graphene itself. Research on sus-
pended graphene has attracted attention for a variety of
substrate. There is no step of the process where the graphene
is in physical contact with any chemical apart from the inor-
ganic etchant used to etch apertures in the Cu growth sub-
strate and water. To date, experimentalists interested in
probing graphene’s intrinsic properties faithfully rely on
9,20,21
exfoliated graphene
to ensure clean samples despite
modern advances in large-scale, monolayer chemical vapour
deposition (CVD) growth of graphene. Moreover, the study
of the intrinsic properties of CVD-grown graphene has been
inhibited by interaction with the substrate and/or contamina-
tion arising from the transfer processes involving organics.
Our organic-free method enables large-scale synthesis of
clean, suspended graphene of high quality suitable for
experiments that demand graphene free of contamination.
Our technique for suspending CVD-graphene may find
use in the manufacture of TEM grids. Graphene has long
been eyed as the ideal candidate for a TEM grid support due
1
reasons, owing to graphene’s unique electrical, mechani-
2,3
4
5
cal, thermal, and optical properties. Surface contamina-
tion resulting from the various transfer methods in common
use has long been a critical problem for transmission electron
6
7
microscopy (TEM), Raman spectroscopy, scanning tunnel-
8
ling microscopy (STM), mechanical studies, and gas per-
9
10
meability studies of graphene. Various methods have been
reported for large-scale transfer and/or suspension of gra-
phene, with all but one method including a critical step
where graphene is coated with a polymer handle, typically
polymethyl methacrylate (PMMA) or polydimethyl siloxane
2
2–24
to its low TEM background signal,
conductivity that minimizes charging effects from the elec-
its high electrical
11–16
(PDMS),
to ensure structural rigidity for subsequent re-
22
2
tron beam, and its superior strength allowing the support
moval of the sacrificial growth substrate. While a method
has been developed to transfer graphene without a handle
25
of large molecules. Unlike previous efforts to employ
13,14
CVD-grown graphene as a TEM grid support,
1
7
onto a flat and continuous polymer substrate, handle-free
suspension of clean graphene over an aperture has proved
elusive.
our
method ensures a substrate free of residue that may have a
larger electron scattering cross-section than graphene itself.
Other applications of our suspended membranes include
pressure sensors, capacitors, and other NEMS (nanoelectro-
mechanical system) devices.
Efforts to remove the polymer residue following transfer
have included various solvent treatments as well as thermal
annealing in an effort to decompose the PMMA or other or-
ganic residue. Exhaustive solvent treatments leave polymer
Large-area graphene was grown by CVD on 25 lm-thick
Cu foils in a split-tube furnace with H and CH precursors
15,18
residue.
Thermal annealing of the residue not only
2
4
ꢀ
26
results in thermal stress that causes breakage of graphene
without ever completely removing the polymer from all
areas but also changes graphene’s electronic properties by
at 850 C, with the growth procedure detailed elsewhere.
The device fabrication process is outlined in Fig. 1. The
graphene-coated Cu foils were floated in an etchant of 0.1 M
(NH ) S O and monitored over the course of a few hours
2
3
rehybridizing carbon ortibals from sp to sp and modulating
4
2 2 8
7
graphene’s band structure. To date, the only polymer-free
(varying from sample to sample) to observe the first forma-
tion of etch pits (Fig. 1(a)). After the etch, the sample is
placed in a water bath for rinsing and allowed to dry in air.
A powerful tool in the study of plastic deformation of metals
is observation of the selective attack of etchants to reveal
19
method of suspension has been developed by Regan et al.,
with 1.2 lm diameter graphene membranes suspended on
TEM grids using the surface tension of isopropyl alcohol to
effect transfer between handles.
27
We report here an entirely organic-free method to sus-
pend graphene over 10–20 lm apertures formed in a Cu
dislocation sites in metal crystals by surface pitting.
Because defects in copper’s fcc crystal structure are more
0003-6951/2012/101(3)/033104/4/$30.00
101, 033104-1
VC 2012 American Institute of Physics
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033104-2
Ledwosinska et al.
Appl. Phys. Lett. 101, 033104 (2012)
indicating CVD graphene’s supreme strength and potential
for use as a TEM grid support.
Fig. 2(a) shows the Raman spectrum of the suspended
membrane in (b) (optical) and (c) (SEM) measured with a
diffraction limited spot at the marked location. A pump
wavelength of 633 nm and a pump power of 10 mW were
used. The spectrum confirms monolayer graphene and the
integrated intensity ratio I /I ¼ 0.05, indicating high struc-
D
G
tural quality of our suspended graphene. The defect density
2
8
10
ꢃ2
was estimated from I /I to be 5 ꢂ 10 cm . We attribute
D
G
the low disorder to the quality of the growth and to the gentle
nature of the suspension process, where the graphene never
leaves its original substrate. The G peak is located at
–1
1584 cm , as is expected for clean graphene where the
Fermi level lies at the charge neutrality point. It has previ-
29
7
,15
ously been found
that residual PMMA on suspended gra-
ꢃ1
phene introduces a low frequency (1100–1600 cm ) broad
background signal, indicative of amorphous carbon. Since
the graphene membranes fabricated with our method have
never contacted any large hydrocarbon molecules, the
Raman spectra show no such background.
A Philips CM200 TEM was used to verify the structural
integrity and cleanliness of the graphene membranes. In
Fig. 3(a), monolayer graphene is seen under bright-field
TEM (200 kV) as a homogeneous region, with the edge of
the ruptured membrane scrolling within the field of view for
contrast purposes. Fig. 3(b) was taken in the region of a
wrinkle, where parallel edges indicate graphene scrolling.
Electron diffraction patterns were measured on regions free
of wrinkles (Fig. 3(a), inset), producing the expected hexago-
nal diffraction pattern for crystalline graphene. The diffrac-
tion patterns do not exhibit diffuse inner circular halos,
which would indicate presence of amorphous material.
Auger electron spectroscopy (AES) was performed at
FIG. 1. (a) The fabrication process harnesses the selective attack of the
etchant to defects, suspending graphene membranes over etch pits. (b) Gra-
phene suspended over a triangular etch pit in the Cu surface, with the h110i
edges forming the outline of the (111) surface. (c) Typical etched region of
Cu depicting yield of suspended (grey) to non-suspended (black) openings.
(
d) Optical (left) and SEM (right) image of a Cu flake suspended on a gra-
phene membrane, illustrating graphene’s suitability for an extremely strong
TEM grid support.
prone to chemical attack, etch pits most likely form along
slip dislocations through to the graphene surface before the
Cu foil loses significant overall thickness. Thus, graphene
membranes become suspended over these apertures, with the
remaining Cu serving as a supporting substrate. Etch pits are
either triangular or irregular in shape, and no preference for
graphene suspension over either shape of etch pit has been
observed. The triangular etch pits have edges parallel to the
h110i directions, with graphene suspended over the (111)
face. The other (111) planes intersect along h110i directions,
so that slip planes are parallel to the edges of the triangular
pits as illustrated in Fig. 1(b). However, if the crystallo-
graphic orientation of the Cu surface is tilted by a few
degrees from the (111) plane, the pits become irregular in
shape, as seen in Fig. 1(c). As etching time is increased, the
pits widen until they coalesce with neighbouring pits, and
new pits form with dimensions suitable for graphene suspen-
sion. Preferential etching also occurs along grain boundaries.
The largest membranes that we observed had side lengths of
2
5 kV with a 3 ꢂ 3 lm spot size on multiple graphene mem-
branes to verify the absence of contamination. AES is a
standard surface analysis technique, allowing one to probe
the elemental distribution of the first few atomic layers of a
sample with high spatial resolution and precise chemical sen-
30
sitivity. In Fig. 4(a), we see an Auger element map, where
lighter regions indicate higher fractional presence of Cu.
(
a) 1500 (b)
G’
(
c)
1000
1
0 µm
ꢁ
20 lm, and Fig. 1(c) provides an indication of typical yield
of suspended (grey) to non-suspended (black) openings.
The relatively high yield of the simple process suggests
graphene’s mechanical strength plays an important role in
the process. Fig. 1(d) shows an optical transmission (left)
and SEM image (right) of a region with a small Cu crystallite
suspended on a graphene membrane. The suspended Cu
mass is approximately 5 pg, which, using the areal mass den-
G
G*
G’’
500
D
1000 1500 2000 25001 3000
Raman shift (cm
)
10
ꢃ8
2
FIG. 2. (a) Raman spectrum of suspended graphene membrane exhibiting
I
sity of graphene ¼ 7:4 ꢂ 10 g=cm , is a Cu/C mass ratio
of 35:1. Other such suspended regions revealed graphene
supporting Cu flakes up to 3000 times the membrane weight,
D
/I
c) SEM image of the measuring location on the suspended membrane. The
G
¼ 0.05, indicating graphene of high structural integrity. (b) Optical and
(
etch pit formed along a Cu grain boundary.
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033104-3
Ledwosinska et al.
Appl. Phys. Lett. 101, 033104 (2012)
32
phene A sweep out to 2000 eV confirmed the absence of
further peaks, indicating contamination of no more than 1%
from foreign species introduced by the etchant or any other
source.
(a)
(b)
In conclusion, we have developed an entirely organic-
free method to suspend CVD-grown graphene membranes
on their original growth substrate. AES, TEM, SEM, and
Raman spectroscopy confirm high-quality, uncontaminated
graphene, unlike other suspension methods involving poly-
mer handles. Such high quality graphene membranes are not
only desired but absolutely necessary for fundamental stud-
ies of graphene’s mechanical, optical, thermal, and chemical
properties where only the cleanest samples suffice.
0
-110
1
-210
200 nm
5 nm
FIG. 3. (a) TEM image of ruptured membrane scrolling. Inset: electron dif-
fraction pattern from a monolayer region, with Miller-Bravais indices (hkil)
˚
labeled as (0-110) (2.5 A lattice constant) for the innermost hexagon and
This work was made possible by support from the Natu-
ral Science and Engineering Research Council, Le Fonds de
Recherche du Qu e´ bec—Nature et Technologies, the Canada
Research Chairs program and the Canadian Institute for
Advanced Research. We thank Josianne Lefebvre and David
Liu for assistance with Auger spectroscopy and transmission
electron microscopy, and Marta Cerruti for access to her
Raman spectrometer.
˚
1-210) (1.42 A interatomic spacing) for the outer. (b) A wrinkled region
(
revealing multiple lines due to numerous graphene layers.
A thicker graphene layer is evidenced by a dark line on the
Cu surface. Fig. 4(b) shows the same region but probed for C
KLL electrons, with the suspended region showing the high-
est signal strength. Again, the thicker layer of graphene at
the boundary is evident now as a bright line on the Cu sur-
3
1
face. Previous work suggests that variations in graphene
thickness occur at graphene grain boundaries.
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