S1
Atomic Force Microscopy Characterization of Room-
Temperature Adlayers of Small Organic Molecules through
Graphene Templating
Peigen Cao
1
, Ke Xu
1,2
, Joseph O. Varghese
1
, and James R. Heath
1
*
1. Kavli Nanoscience Institute and Division of Chemistry and Chemical Engineering,
California Institute of Technology,
MC 127-72, Pasadena, CA 91125
2. Department of Chemistry and Chemical Bi
ology, Harvard University, 12 Oxford St.,
Cambridge, MA 02138
Supporting Information
Materials and Methods
Materials.
Anhydrous inhibitor-free tetrahydrofuran (THF,
≥
99.9%, water content <0.002%) and
anhydrous cyclohexane (99.5%, water content <0
.001%) were purchased from Sigma-Aldrich. These
reagents were used as supplied and stored in a gl
ove-box purged with nitrogen. Muscovite mica (Grade
V1; round disks of diameter 10 mm) was obtained
from Ted Pella. Kish graphite was obtained from
Covalent Materials US Inc.
Sample preparation.
Samples were prepared in a glove-bag
(Sigma-Aldrich AtmosBag) that was purged
and protected under a continuous flow
of ultra-high purity argon, in which the relative humidity (RH) was
controlled to be <2%. Humidity was monitored us
ing a Fluke 971 temperature humidity meter. All
experiments were performed at room temperature (22±2
°C). Mica disks were first heated in air at 200 °C
for 10 min to remove absorbed moisture, and then tr
ansferred into the glove-bag. The mica surface was
cleaved in the glove-bag and exposed to organic va
pors for ~10 s to ~1 min. The partial pressure of
organic molecules at the mica surface, which dete
rmines the surface coverage at equilibrium, was
adjusted by varying the distance between the vapo
r source and the mica surface. Graphene sheets were
deposited onto the mica surface through the standard
method of mechanical exfoliation (Novoselov
et al.
2005; Lui
et al.
2009) of Kish graphite, thus sealing and preserving the adlayers of organic molecules.
Identification of graphene layers.
Monolayer graphene sheets were identified through optical
microscopy and confirmed by spatia
lly resolved Raman spectroscopy (Xu
et al.
2010). Raman spectra
were recorded with a Renishaw M1000 Micro Raman
spectrometer system using a 514.5 nm laser beam
and a 2400 lines per mm grating. A confocal optical
microscope with a ×100 objective lens was used to
record spectra with a
spatial resolution of 2
μ
m. No noticeable D peak was
observed in the Raman spectra
(Fig. S1), indicating high-crystalline order of our samples.
Atomic Force Microscopy.
All AFM images were acquired under ta
pping mode on a Digital Instrument
Nanoscope IIIA at ambient conditions
. A sharp TESP tip (Veeco) with a radius of end of 8 nm was used.
Typical values for the force constant and resonan
ce frequency were 42 N/m and 320 kHz respectively.
Height calibrations were performed using the step hei
ghts of freshly cleaved gra
phite samples. Due to the
super-flatness of the samples, some
times the laser interfere
nce pattern along the slow-scan axis was hard
to avoid, which is more noticeable in large-area sca
nning and have a period of
twice the wavelength of
the laser. This is caused by the constructive interfe
rence of laser reflected from the sample surface and
S2
that reflected from the cantilever. The broad stripe-like
features seen in Fig. 2a and Fig S7ab were due to
this effect.
References for Materials and Methods
Lui, C. H., Liu, L., Mak, K. F., Flynn, G. W.
and Heinz, T. F. (2009)
. "Ultraflat graphene."
Nature
462
, 339.
Novoselov, K. S., Jiang, D., Schedin, F., Booth, T. J., Khotkevich, V. V., Morozov, S. V. and Geim, A. K. (2005).
"Two-dimensional atomic crystals."
Proc. Natl. Acad. Sci. U. S. A.
102
, 10451.
Xu, K., Cao, P. G. and Heath, J. R. (2010). "Graphene
visualizes the first water adlayers on mica at ambient
conditions."
Science
329
, 1188.
S3
Supplementary Figures, with Addi
tional Discussion in the Captions
10
μ
m
1500
2000
2500
3000
3500
4000
0
1
2
3
4
5
6
7
Mica OH
mode
G
Counts (x10
3
)
Wavenumbers (cm
-1
)
2D
Fig. S1.
Raman spectrum of a monolayer graphene sheet deposited on a mica surface that was in
equilibrium with a THF vapor. Inset: transmission optical
image of the graphene sheet at the center. The
2D and G bands of graphene and the OH mode of mi
ca are labeled. Similar Raman spectra were also
observed for monolayer graphene sheets deposited on
mica surfaces that were in equilibrium with
cyclohexane vapors.
1 μm
500 nm
0
1
M
(b)
(a)
0
1
M
M
Fig. S2.
Graphene on top of a near-complete monolayer of THF adlayer on mica (a), and a close up of the
edge (b), where the adlayer is missing. M labels
the mica surface, while 0 and 1 label regions where
monolayer graphene is on top of 0 and 1 adlayers
of THF on mica, respectively. No second adlayer is
observed before the first adlayer is completed.
S4
Height
Phase
500 nm
M
01
(a)
(b)
Fig. S3.
AFM height (a) and phase (b) images for monolay
er graphene sheets deposited on a mica surface
that was in equilibrium with a cyclohexane vapor, corr
esponding to Fig. 3a in the main text. M labels the
mica surface, while 0 and 1 label regions where monolay
er graphene is on top of 0 and 1 adlayers of
cyclohexane on mica, respectively. Significant pha
se difference is observed between the mica and
graphene surfaces, reflecting the difference in surf
ace properties. By comparison, the same phase is
observed for the flat islands and other parts of gra
phene, indicating the AFM tip is interacting with the
same surface (graphene), and the islands ar
e cyclohexane adlayers underneath graphene.
500 nm
500 nm
M
0
1
0
1
1
μ
m
500 nm
0
0
0
1
ML
Gr
0
1
(a)
(b)
(c)
Fig. S4.
Cyclohexane adlayers tend to form large, con
tinuous islands on the mica surface. M labels the
mica surface, while 0 and 1 label regions where monolay
er graphene is on top of 0 and 1 adlayers of
cyclohexane on mica, respectively.
“ML Gr” labels multilayer graphene.
(a,b):
At reduced surface
coverage, the adlayers tend to form narrow “necks”
as opposed to isolated small islands, suggestive of
weak molecule-substrate interactions.
(c):
The same area as (b), but imaged
after the sample was stored at
ambient conditions for 2 months.
All adlayer structures, including th
e narrow necks, remain unchanged.
S5
Starting pattern 25 days later 80 days later
500 nm
M
0
1
Fig. S5.
Graphene-templated water adlayers are stable
for months under ambient conditions without
significant structural changes. Left: AFM image of
a monolayer graphene sheet deposited on mica at
ambient conditions (~40% RH). Center: The same samp
le, after being kept at ambient conditions for 25
days. Right: The same sample, after being kept at
ambient conditions for 80 days. M labels the mica
surface, while 0 and 1 label regions where monolayer gr
aphene is on top of 0 and
1 adlayers of water on
mica, respectively.
S6
-0.2
-0.1
0.0
0.1
0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Mica,
= 27 pm
TMCS-Mica
= 60 pm
Frequency (a. u.)
Height (nm)
200 nm
100 nm
0
200
400
600
800
1000
-1
0
1
2
z (nm)
x (nm)
(a)
(c)
Mica
TMCS-mica
(b)
100 nm
T-M
0
D
Mica
CH
3
H
3
C-Si-CH
3
O
Fig. S6.
Water adsorption on TMCS-functionalized
mica surfaces at ambient conditions.
As a control experiment, we utilized graphene temp
lating to characterize the water adsorption on mica
surfaces that were rendered somewhat hydrophobic
(and slightly rougher) via chemical functionalization
with trimethylchlorosilane (TMCS). The surface
functionalization was carried out through vapor
deposition. Freshly prepared mica surfaces were first
exposed to an environment with controlled RH of
35±2% for about 5-10 min. The mica substrates were th
en quickly transferred into a sealed reaction vessel
containing a beaker filled with about 5 ml of purifi
ed TMCS liquid and allowed to react for about 30-60
min. The TMCS-functionalized mica surface was then
equilibrated with ambient air (~40% RH) for ~10
min before graphene was deposited.
The water contact angles of fresh mica and TMCS-functionalized
mica were measured, via contact-angle goniometry, to be ~0
◦
and 40
◦
, respectively.
(a):
AFM images indicate a uniform surface passivation of mica by TMCS. However, the functionalized
surface, although still very flat, is rougher that
what is observed for freshly cleaved mica.
(b):
Height
histograms of a fresh mica surface and a TMCS-f
unctionalized mica surface. The measured RMS
roughness of fresh mica (27 pm) is likely limited by the noise of AFM, whereas the roughness of TMCS-
functionalized mica surface is significantly higher
(60 pm). There is, however, no evidence that TMCS
functionalization introduces etch pits
or other large defects into the mica.
The inset is a simple molecular
drawing of TMCS-functionalized mica.
(c):
Graphene-templating reveals that under ambient conditions
(~40% RH), water adsorbs as nanometer-sized
droplets on the relatively hydrophobic TMCS-
functionalized mica surface. “T-M” labels TMCS
-functionalized mica surface, “0” labels where
monolayer graphene is in direct contact with th
e TMCS-functionalized mica surface, and “D” points to
two droplets. A cross-sectional profile is given for th
e blue line, which indicates the droplets have varying
heights on the order of ~1-2 nm. These results contr
ast with the flat 2D islands typically observed for
adlayers on fresh (hydrophilic) mica surfaces.
S7
Figure S7.
AFM images of graphene-templated THF adlaye
rs reveal both structural and dynamical
information. These images represent a more complete ser
ies of what is presented in Figure 2 in the main
text. M labels the mica surface, and 0, 1, and 2 label
regions where monolayer graphene is on top of 0, 1,
and 2 adlayers of THF, respectively. Yellow arrows poi
nt to droplets. (a)-(e) were
taken within a few hours
after graphene sheets were deposited.
(a)
: The case in which the trapped adlayer is a submonolayer.
(b):
Another sample with very low surface coverage of THF.
(c):
Zoom-in of the square in (a).
(d):
Another
sample showing the second THF adlayer on top of the first.
(e):
Zoom-in of the second adlayer in (d).
(f-h):
The same areas as (c-e), after
the samples were kept at ambient conditions for 2 months.
(i-k):
Another
sample freshly prepared (i), and after being kept at
ambient conditions for 60 days (j) and 68 days (k),
illustrating processes in which droplets revert back to
islands. The green circles mark a defect on the
graphene edge, which served as a reference point for aligning the images.
S8
Figure S8.
AFM images of a graphene-templated cyclohexane
adlayer. The labels 0, 1, and 2 indicate
regions where monolayer graphene is on top of 0,
1, and 2 adlayers of cyclohexane, respectively.
Yellow
arrows point to droplets.
This particular image reveals the second cyclohexane adlayers.