University of Groningen
Atomically Thin Mica Flakes and Their Application as Ultrathin Insulating Substrates for
Graphene
Castellanos-Gomez, Andres; Wojtaszek, Magdalena; Tombros, Nikolaos; Agrait, Nicolas; van
Wees, Bart J.; Rubio-Bollinger, Gabino; Agraït, Nicolás
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DOI:
10.1002/smll.201100733
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Castellanos-Gomez, A., Wojtaszek, M., Tombros, N., Agrait, N., van Wees, B. J., Rubio-Bollinger, G., &
Agraït, N. (2011). Atomically Thin Mica Flakes and Their Application as Ultrathin Insulating Substrates for
Graphene.
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,
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small 2011, 7, No. 17, 2491–2497
Atomically Thin Mica Flakes and Their Application
as Ultrathin Insulating Substrates for Graphene
Andres Castellanos-Gomez , * Magdalena Wojtaszek , Nikolaos Tombros ,
Nicolás Agraït , Bart J. van Wees , and Gabino Rubio-Bollinger *
B y mechanical exfoliation, it is possible to deposit atomically thin mica fl akes down
to single-monolayer thickness on SiO
2
/Si wafers. The optical contrast of these mica
fl akes on top of a SiO
2
/Si substrate depends on their thickness, the illumination
wavelength, and the SiO
2
substrate thickness, and can be quantitatively accounted
for by a Fresnel-law-based model. The preparation of atomically thin insulating
crystalline sheets will enable the fabrication of ultrathin, defect-free insulating
substrates, dielectric barriers, or planar electron-tunneling junctions. Additionally,
it is shown that few-layer graphene fl akes can be deposited on top of a previously
transferred mica fl ake. Our transfer method relies on viscoelastic stamps, as used for
soft lithography. A Raman spectroscopy study shows that such an all-dry deposition
technique yields cleaner and higher-quality fl akes than conventional wet-transfer
procedures based on lithographic resists.
Graphene Transfer
DOI: 10.1002/smll.201100733
Dr. A. Castellanos-Gomez ,
[+]
Prof
. N. Agraït ,
Prof
. G. Rubio-Bollinger
Departamento de Física de la Materia Condensada
Universidad Autónoma de Madrid
Campus de Cantoblanco, E-28049 Madrid, Spain
E-mail: a.castellanosgomez@tudelft.nl; gabino.rubio@uam.es
Dr. A. Castellanos-Gomez ,
[+]
M. Wojtaszek , Dr. N. Tombros ,
Prof. B. J. van Wees
Physics of Nanodevices, Zernike Institute for Advanced Materials
University of Groningen, The Netherlands
Dr. N. Tombros
Molecular Electronics
Zernike Institute for Advanced Materials
University of Groningen, The Netherlands
Prof. N. Agraït , Prof. G. Rubio-Bollinger
Instituto Universitario de Ciencia de Materiales “Nicolás Cabrera”
Universidad Autónoma de Madrid
Campus de Cantoblanco, E-28049 Madrid, Spain
Prof. N. Agraït
Instituto Madrileño de Estudios Avanzados en Nanociencia
IMDEA-Nanociencia, E-28049 Madrid, Spain
[ +] Current address: Kavli Institute of Nanoscience, Delft University of
Technology, Lorentzweg 1, 2628 CJ Delft, The Netherlands
1. Introduction
The experimental realization of graphene, just a single
atomic layer of graphite, by mechanical exfoliation of
graphite on SiO
2
[
1,2
]
surfaces has paved the way to study a
very interesting family of 2D crystals almost unexplored so
far. Apart from graphene, mechanical exfoliation has been
used to prepare other atomically thin crystals
[
1
]
such as
MoS
2
,
[
3–6
]
a semiconductor. However, apart from conducting
and semiconducting materials, the microelectronic industry
also needs insulators that can be used as substrates, dielec-
trics, or electron-tunnelling barriers. Mechanical exfoliation
also enables the production of atomically thin, insulating
crystals but, up to now, the fabrication has been restricted
to a few insulating 2D crystals such as hexagonal boron
nitride.
[
7,8
]
The layered structure of muscovite mica, a phyllosilicate
mineral of aluminum and potassium with chemical formula
(KF)
2
(Al
2
O
3
)
3
(SiO
2
)
6
(H
2
O), makes this material a promising
candidate to produce atomically thin insulating crystals by
mechanical exfoliation. Moreover, due to its high resistance
to heat, water, and chemical agents, its mechanical properties,
and to its high dielectric constant, bulk mica has already been
A. Castellanos-Gomez et al.
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extensively used in the electronic industry in many applica-
tions as a substrate, heat and electrical insulator, or dielec-
tric barrier. Recently, bulk mica has been also employed as
a substrate to fabricate ultrafl at graphene samples.
[
9
]
It has
been observed that graphene deposited on mica adheres to
the atomically fl at terraces of mica without noticeable rip-
pling. Note that the ripples in graphene, unavoidable when it
is either deposited on SiO
2
[
10
]
or suspended,
[
11
]
can modify its
electronic properties and induce charge inhomogeneities.
[
12,13
]
It would be therefore very interesting to study how the atomi-
cally fl at topography of graphene on mica affects the elec-
tronic properties of graphene. The use of a bulk mica substrate
for graphene-based devices, however, hampers the ability to
electrostatically doping graphene with a backgate. This limita-
tion was overcome by employing 10–50 nm thick mica fl akes
deposited on SiO
2
/Si substrates.
[
14
]
The 2D nature of these
atomically thin crystalline insulator sheets makes them very
interesting candidates in applications such as insulating bar-
riers in planar tunnel junctions or as fl exible substrates for
graphene or molecular electronic devices. However, despite
the large number of potential applications of these crystalline
insulator nanosheets, the details about the fabrication, identi-
fi cation, and characterization of these ultrathin mica fl akes are
missing in the literature.
In this work the fabrication of atomically thin mica fl akes
as thin as just one layer thick (1 nm) on SiO
2
/Si substrates
is reported. Also presented is a combined characterization of
these fl akes by quantitative optical microscopy and atomic
force microscopy (AFM). From this study, one can determine
the optimal SiO
2
substrate thickness and illumination wave-
length to reliably identify atomically thin crystals of mica
by optical microscopy. In addition, an all-dry procedure to
transfer few-layer graphene (FLG) fl akes on top of atomi-
cally thin mica fl akes is demonstrated, based on viscoelastic
stamps like the ones used in soft lithography. From Raman
spectroscopy measurements, it is concluded that this transfer
technique produces cleaner and higher-quality fl akes than
conventional wet-transfer procedures based on lithographic
resists.
[
8
,
15
]
2. Results
2.1. Sample Fabrication
In order to produce atomically thin muscovite mica fl akes,
the micromechanical cleavage technique was employed,
widely known from the fabrication of graphene fl akes
[
2
]
and
other materials.
[
16
]
To cleave the starting material, this tech-
nique usually employs an adhesive tape, which can leave
traces of glue on the surface that contaminate the fabri-
cated sample.
[
17
]
This problem can be avoided by replacing
the adhesive tape by a viscoelastic stamp, similar to the ones
used in soft-lithography.
[
18
]
In previous works, we successfully
used stamps of poly(dimethyl)-siloxane (PDMS) to fabricate
graphene,
[
19
]
NbSe
2
, and MoS
2
[
3
]
atomically thin crystals. To
produce atomically thin mica fl akes, fi rst a bulk muscovite
mica sample is cleaved by pressing the surface of the stamp
against the bulk mica and peeling it off suddenly. After this
step of the process, part of the mica surface is cleaved and
transferred to the stamp surface. Next, to transfer the mica
fl akes to an arbitrary surface, one presses the surface of the
stamp against the receptor surface and peels it off slowly (5 s).
Using this technique, mica fl akes with thicknesses ranging
from hundreds of nanometers down to just one nanometer
have been produced, which is the thickness of a single layer
of mica. While the chemical structure of mica is different from
graphene (and so their chemical, electrical, and mechanical
properties), the transfer rate and the area of atomically thin
fl akes is comparable to graphene prepared by mechanical
exfoliation of highly oriented pyrolitic graphite (HOPG).
Figure 1 a shows some mica fl akes transferred onto a
silicon substrate with a 300 nm SiO
2
capping layer. It was
observed that the apparent color of a fl ake depends on its
thickness. Figure 1 b presents an optical micrograph under
white illumination of a mica fl ake, with thickness ranging
from 2 to 20 nm. The top insert in Figure 1 b shows a contact-
mode AFM topographic profi le measured through the
boundary between the SiO
2
surface and the mica fl ake. The
Figure 1 . a) Optical micrograph of several mica fl akes deposited on a 300 nm SiO
2
/Si substrate by mechanical exfoliation. The different colors
of the fl akes correspond to different thicknesses. b) Optical micrograph of an ultrathin muscovite mica fl ake on a 300 nm SiO
2
/Si substrate. Top
insert in (b): contact-mode AFM topographic line profi le measured across the interface between the mica fl ake and the SiO
2
substrate (dotted
red line). The thickness of the fl ake in this region is 2 nm, which corresponds to the thickness of two single mica layers. Bottom insert in (b): the
topographic profi le (along the dashed blue line) showing the boundary between a region 2 nm thick and another 3 nm thick. c) Contact-mode
AFM topographic image of a mica fl ake with a thickness of 1.2 nm, corresponding to a single layer of mica. A topographic profi le along the green
dashed line is shown below panel (c).
Atomically Thin Mica Flakes as Insulating Substrates for Graphene
2493
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small 2011, 7, No. 17, 2491–2497
height difference between the SiO
2
substrate and the mica
fl ake in this region is 2 nm, which corresponds to the thick-
ness of two single layers of muscovite mica. The bottom
insert in Figure 1 b shows the contact-mode AFM topographic
profi le measured across the boundary between a region two
layers thick and another three layers thick (3 nm thickness).
Although it was found that single-layer fl akes are usually not
much larger than 1 μ m × 1 μ m (Figure 1 c), the typical area
of thin mica fl akes (2 or 3 layers) can be up to 5 μ m × 5 μ m,
but some of them can be signifi cantly larger (Figure 1 b). The
large area of these nanometer-thick mica sheets, comparable
to the one of exfoliated graphene fl akes, makes them prom-
ising candidates for use not only as substrates for graphene-
based devices but also as insulator barriers in planar tunnel
junctions or as dielectrics in nanocapacitors.
2.2. Optical Characterization
The physics behind the optical visibility of these atomically
thin crystals can be illustrated with the example of the interfer-
ence colors in SiO
2
thin fi lms. It is well known that the thick-
ness of thermally grown SiO
2
layers on Si wafers can be readily
determined with ≈ 5 nm accuracy from their color under white-
light illumination.
[
20,21
]
This apparent color is due to the interfer-
ence of the paths refl ected at the air/SiO
2
and SiO
2
/Si interfaces
(similar to a Fabry–Perot interferometer). The interfering paths
will have a relative phase shift, which depends on the illumina-
tion wavelength and the SiO
2
layer thickness, hence the SiO
2
thickness dependence of the color of the wafers. If one deposits
a thin mica sheet instead of a SiO
2
layer the effect should be
similar. If the mica sheet is atomically thin, the optical path dif-
ference could be small and the color difference between the
mica sheet and the bare Si wafer could thus be imperceptible.
The optical visibility of atomically thin crystals, however, can be
enhanced by depositing them on a multilayered medium, typi-
cally a Si wafer with a thermally grown SiO
2
capping layer.
[
22
]
To interpret the observed optical contrast one can employ
a simple model based on the Fresnel law.
[
22,23
]
This approach
has been successfully employed to study the optical contrast
of different 2D crystals such as graphene,
[
22–24
]
transition
metal dichalcogenides MoS
2
and NbSe
2
,
[
3
,
25
]
or hexagonal
boron nitride.
[
26
]
The subscripts 0, 1, 2, and 3 will label here-
after the air, mica fl ake, SiO
2
, and Si media, respectively.
Normal incidence of the light through the trilayer structure
is considered formed by the mica fl ake, the SiO
2
, and the
Si. The optical properties of the Si layer, considered semi-
infi nite, are given by its complex refractive index ñ
3
(
λ
) =
n
3
– i
κ
3
, which strongly depends on the illumination wave-
length (
λ
) in the visible range of the spectrum.
[
27
]
The SiO
2
layer, with a thickness d
2
, is described by its refractive index
ñ
2
(
λ
) which also depends on the illumination wavelength.
[
27
]
Note that, using the refractive indices of SiO
2
and Si, one
can accurately account for the interference colors of the
oxidized wafers with a Fresnel law-based model.
[
20,21
]
The
refl ected intensity for a SiO
2
/Si wafer ( I
0
) can be expressed
in terms of the phase shift produced by the SiO
2
layer (
Φ
2
=
2
π
ñ
2
d
2
/
λ
) and the amplitudes of the paths refl ected at the
air/SiO
2
and SiO
2
/Si interfaces ( r
02
and r
23
respectively),
I
0
(
8
) =
r
02
+ r
23
e
−2i
2
1 + r
02
r
23
e
−2i
2
2
(1)
where the amplitude of the refl ected path in the interface
between the media i and j is r
ij
= ( ñ
i
− ñ
j
)/( ñ
i
+ ñ
j
) with ñ
j
being
the refractive index of medium j . The atomically thin mica crystal
is taken into account as a layer of thickness d
1
on the SiO
2
medium, whose refractive index is ñ
1
(
λ
) = n
1
– i
κ
1
. The refl ected
intensity from the mica fl ake ( I
0
) can be written as
[
22,23
]
I
1
(
)
r
01
e
i(
8
1
8
2
)
r
12
e
i(
8
1
8
2
)
r
23
e
i(
8
1
8
2
)
r
01
r
12
r
23
e
i(
8
1
8
2
)
e
i(
8
1
8
2
)
r
01
r
12
e
i(
8
1
8
2
)
r
01
r
23
e
i(
8
1
8
2
)
r
12
r
23
e
i(
8
1
8
2
)
2
(2)
Φ
1
= 2
π
ñ
1
d
1
/
λ
is the phase shift of the path produced by
the presence of the mica fl ake. Using the expressions (1)
and (2), one can obtain the optical contrast ( C ) which is
defi ned as
C =
I
1
− I
0
I
1
+ I
0
.
(3)
Figure 2 a shows the optical contrast, measured at dif-
ferent illumination wavelengths, for fl akes 2–10 layers thick.
Figure 2 . a) Optical contrast measured at different illumination
wavelengths on mica fl akes from 2 to 10 nm thick. The contrast-versus-
wavelength dependence calculated with the Fresnel law-based model is
also shown (solid lines). The refractive index of mica fl akes is n = 1.55
(
κ
= 0) and an uncertainty in its value of Δ n ± 0.1 has been considered
and is displayed as the grayed region enveloping the solid lines. Note that
the data for the 3, 4, 8, and 10 nm have been vertically displaced for clarity
by 0.15, 0.3, 0.45, and 0.6. b–e) Optical contrast maps measured, in the
mica fl ake shown in Figure 1 , at different illumination wavelengths.
A. Castellanos-Gomez et al.
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The solid lines in the plot are the contrast versus wavelength
dependencies according to expressions (1), (2), and (3). The
thickness of the fl ake has been determined by contact-mode
AFM and the refractive index of the mica fl ake is taken to be
ñ
1
= 1.55–0 i , that is independent from the illumination wave-
length and similar to the one of bulk muscovite mica. The
grayed areas enveloping the solid lines mark the calculated
contrast versus wavelength dependencies considering an
uncertainty of the real part of the refractive index Δ n
1
= ± 0.1.
The measured optical contrast values are in good agreement
with the calculated data indicating, within experimental reso-
lution, that the refractive index of the atomically thin mica
fl akes can be considered close to its bulk value. Notice that in
Figure 2 a the experimental contrast is systematically slightly
lower than the calculated one. This was observed in similar
experiments
[
23
]
and was attributed to the fi nite numerical
aperture of the microscope objective.
[
28
]
Figure 2 b shows the
optical contrast maps for the same mica fl ake shown in Figure
1 b at 500, 546, 568, and 632 nm illumination wavelengths. In
agreement with the data shown in Figure 2 a, at
λ
= 568 nm
the optical contrast of the thinnest part of the fl ake vanishes
while it shows a maximum (minimum) at
λ
= 520 (630) nm.
Expressions (1), (2), and (3), can also be used to calcu-
late the optical contrast yielded by a single layer of mica for
different illumination wavelengths and SiO
2
capping-layer
thicknesses. Figure 3 shows the result of this calculation,
which can be used as a guide to fi nd the appropriate condi-
tions to identify ultrathin mica fl akes atop SiO
2
/Si substrates.
SiO
2
thicknesses were selected that optimize the contrast for
λ
= 550 nm, which is the illumination wavelength to which
the human eye attains maximum sensitivity.
[
29
]
The fi rst four
different SiO
2
thicknesses which optimize the optical con-
trast for a single layer of mica are: 55 nm ( − 1.5% contrast,
nearly
λ
-independent), 100 nm ( + 1.5% contrast, also nearly
λ
-independent), 260 nm (–1.5% at
λ
= 550 nm and 0% at
λ
=
500 nm) and 305 nm ( + 1.5% at
λ
= 550 nm and 0% at
λ
=
580 nm). Note that 1.5% contrast is roughly the detection
limit of the human eye and thus special care has to be taken
to optimize both illumination wavelength and SiO
2
thickness
in order to identify these ultrathin mica fl akes by eye. The
source of such a low optical contrast is that the absorption of
mica in the visible spectrum is very low and thus the optical
contrast is mainly due to the optical path difference due to
the presence of the atomically thin mica fl ake. On the con-
trary, Roddaro et al.
[
22
]
reported that the opacity of graphene
is the key element to explain the optical contrast of graphene
layers and the interference color only plays a marginal role in
the visibility.
It is common practice to assist the optical characteriza-
tion of layered materials with their Raman spectra. Such
spectra turned out to be especially informative in the case
of graphene, where the precise distinction between single,
double, and multilayer samples is possible.
[
30,31
]
We measured
the Raman spectrum for the mica fl akes of different thick-
nesses, ranging from 2 nm (2 layers) to 10 nm (10 layers)
deposited on 300 nm SiO
2
and for bulk mica. None of the
spectra of thin mica fl akes showed typical Raman bands of
bulk mica, even after long acquisition times (5 min), pre-
senting only the spectral features of its substrate: silicon
(a strong Lorentzian peak at ≈ 520 cm
− 1
) and a broad fl at band
between 930 and 1000 cm
− 1
(see Figure 4 a). For comparison,
Figure 3 . Color map of the calculated optical contrast for a monolayer
mica fl ake as a function of the illumination wavelength and the SiO
2
thickness. The color bar inserted in the plot shows the opitcal contrast
while the one at the right indicates the color of the illumination light.
Figure 4 . a) Raman spectra of a thin mica fl ake (less than 12 nm thick)
on SiO
2
/Si (black) and bulk mica (blue), a parent material for analyzed
fl akes. For mica fl akes from 2 to 60 nm thick, Raman spectra showed
only features of the Si underneath. No peaks corresponding to mica
vibrational modes can be resolved. b) Raman spectrum of multilayer
graphene after PDMS-based transfer (black) and the spectrum of single
layer graphene after poly(methyl methacrylate) (PMMA)-based transfer
(blue). No baseline corrections were applied to the spectra. The strong
linear increase of the background, emphasized by a red dashed line as
an eye-guide, for the case of PMMA-based transfer is apparent.
