ARTICLE
pubs.acs.org/JPCC
Surface Diffusion of Simple Organic Molecules on Graphene on
Pt(111)
Antonio J. Martínez-Galera* and Jose M. Gomez-Rodríguez
Departamento de Física de la Materia Condensada, Universidad Autonoma de Madrid, E-28049 Madrid, Spain
ABSTRACT: Studying the behavior of organic molecules
adsorbed on graphene is an issue of special importance to
exploit the full potential of graphene in future technology. Here
we report on a scanning tunneling microscopy study in ultrahigh vacuum of the surface diffusion and initial stages of growth
of a simple organic molecule, the azabenzene 1,3,5-triazine, on
epitaxial graphene on Pt(111) at low temperatures. Our study
reveals the formation of fractal shape islands below 100 K;
inside the islands, the molecules are arranged in a well-ordered hexagonal structure with the molecular plane parallel to the surface.
The orientation of the molecules in this lattice is determined by the interpretation of STM images exhibiting intramolecular features
via the electrostatic potential map of the 1,3,5-triazine molecule. From nucleation experiments we have measured, for the first time,
the diffusion barrier for single molecules adsorbed on the graphene/Pt(111) surface. This energy barrier (68 ( 9 meV) is higher
than that previously found for 1,3,5-triazine on graphite surfaces. This important finding shows that even on the graphene/Pt(111)
system, which is one of the most weakly coupled graphene-metal systems, dynamic processes such as surface diffusion, which is a
fundamental process involved in the growth of monolayers of organic molecules, is affected by the interaction of graphene with the
underlying metal.
’ INTRODUCTION
Graphene presents an unconventional electronic structure
that is responsible for the extraordinary properties observed in
this material in the last years.13 Some of these properties such as
the high mobility of the charge carriers make graphene a promising candidate to become integral part of future devices.
However, to fulfill all requirements of a forthcoming graphenebased technology, incorporating other materials in such devices
will be essential. For this reason, understanding the interaction of
graphene with different materials is of vital importance. On this
subject, graphene grown on transition metals has served as a
platform for the study of the graphene-metal contact. This
interaction was found to be strongly dependent on the metal.46
Whereas metals as Ir7 and Pt8 present a weak interaction with the
graphene sheet, which preserves an electronic structure similar to
that of free-standing graphene, for other metals as Ru9 or Ni,10
the strong graphene-metal interaction results on a dramatic effect
on the electronic properties of graphene. The adsorption of
organic molecules on graphene, where its low reactivity promotes the formation of well-ordered organic layers of selfassembled molecules,1116 is also an issue of special importance.
Despite this low reactivity, in some cases, a charge transfer between molecule and graphene has been observed.1720 This
molecular doping of graphene opens the door to tuning in a
controlled way the electronic properties of graphene. The interplay between graphene epitaxially grown on metals, whose properties can be set by using the appropriate metallic substrate, and
the huge range of possible organic adsorbates could be the
starting point of a next coming technology.
r 2011 American Chemical Society
In this work, we present a study by scanning tunneling microscopy (STM) in ultrahigh vacuum (UHV) at sample temperatures below 100 K in the initial stages of growth of the organic
molecule 1,3,5-triazine (also denoted sym-triazine or s-triazine)
on graphene epitaxially grown on Pt(111) substrates. This
molecule and some similar azabenzenes have been studied on
other substrates where they form ordered monolayers.21 The
binding of this molecule to the substrate could be either through
the nitrogen lone pair or through the π orbitals in the molecular
ring, and the structure of the monolayers of the adsorbed molecules is determined by the kind of interaction between molecule
and substrate. Accordingly, whereas binding through the lone
pair of the nitrogen atoms gives rise to structures where the molecules are tilted with respect to the substrate, binding through the
π orbitals results in structures where the molecules are adsorbed
with the molecular plane parallel to the surface. As a consequence, surface diffusion, which is a dynamic process involved in
the growth of the organic layer, could be influenced also by the
kind of bonding between molecule and substrate. However,
studies relative to dynamic processes for this kind of azabenzenes
are scarce. Here we present a quantitative study of nucleation
based on STM measurements to extract the surface diffusion
barrier of single molecules on a graphene/Pt(111) substrate and
the critical nucleus size. A higher diffusion barrier is found with
respect to the system 1,3,5-triazine/HOPG (highly oriented
pyrolytic graphite),22 which provides an evidence of the influence
Received: August 20, 2011
Published: October 14, 2011
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Figure 1. (a) STM image of graphene epitaxially grown on Pt(111). Tunneling parameters: IT = 0.1 nA, Vs = 1.02 V, size: 100 100 nm2. (b) LEED
pattern of a graphene/Pt(111) sample acquired at 62 eV. The (1 1) spots of the Pt(111) surface as well as some ring-like features related to different
graphene moire patterns are schematically outlined. (c) Atomically resolved STM image of a region of graphene exhibiting a moire pattern with unit cell
of 19.2 Å. It is the result of a rotation of 3.7° of the graphene lattice with respect to the Pt(111) surface. Tunneling parameters: IT = 0.2 nA, Vs = 50 mV,
size: 5.0 5.0 nm2. The STM image is accompanied by a schematic model of this moire pattern. (d) Atomically resolved STM image of a region of
graphene with a moire pattern with unit cell of 7.38 Å. It is a consequence of a rotation of 19.1° between the graphene and Pt(111) lattices. Tunneling
parameters: IT = 3 nA, Vs = 150 mV, size: 5.0 5.0 nm2. A schematic model of this moire pattern is included.
of the underlying metal on the diffusion of organic molecules on
graphene.
The Article presents the following structure. After this Introduction, the morphology and structure of epitaxial graphene
grown on Pt(111) are described. Then, the adsorption of 1,3,
5-triazine on this surface is addressed and the self-assembled
molecular layer is characterized. The nucleation experiments as a
function of temperature and deposition rate are presented, and
the energy barrier for diffusion of the single 1,3,5-triazine molecules on graphene is extracted. Finally, the influence of the
rotational orientation of the graphene layer with respect to the
Pt(111) lattice in these results is discussed. All of these findings
are discussed in relation to surface diffusion of single molecules
on HOPG surfaces.
’ EXPERIMENTAL METHODS
The experiments were carried out in an UHV system with a
base pressure below 1 1010 Torr. This system is equipped
with a homemade variable temperature scanning tunneling
microscope (VT-STM)23 and a four-grid LEED/Auger optics for
the characterization of surfaces as well as all of the necessary
devices for preparing samples under the UHV environment.
A clean Pt(111) surface was obtained by repeated cycles of Ar+
bombardment at 1 kV and annealing at 600 °C in an oxygen
atmosphere with a partial pressure of 106 Torr. After each annealing step, the sample was flashed at 1000 °C under the same partial
pressure of oxygen. The growth of graphene was performed by
chemical vapor deposition of ethylene (C2H4) at a pressure of
2 107 Torr during 1 min with the Pt(111) substrate at
1000 °C. Such a procedure is known to result on large areas of
very perfect graphene that present, however, some wrinkles on a
micrometer scale.24 Afterward, the quality of the sample was
checked by LEED and transferred to the VT-STM.
This VT-STM is attached to a continuous flow cryostat that
allows us to change the temperature of the sample in a controlled
way. Once the sample has reached the thermodynamic equilibrium at the desired temperature in the range 40100 K, it is
exposed to 3 langmuir of 1,3,5-triazine at a partial pressure of
108 Torr. Previously to the exposure of the graphene/Pt(111)
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surface to 1,3,5-triazine, the glass container with the 1,3,5triazine, separated from the main UHV chamber by a leak valve,
was cleaned by repeated freezepumpthaw cycles.
After the exposure to 1,3,5-triazine at the selected temperature
in the range 40100K, the temperature of the sample is
quenched at 40 K to have a better scenario for the STM
measurements avoiding further motion of the molecules. Data
acquisition as well as image processing were performed by using
the software WSxM from Nanotec Electr
onica S.L.25
’ RESULTS AND DISCUSSION
Graphene Epitaxially Grown on Pt(111). Graphene epitaxially grown on Pt(111) is one of the most weakly coupled systems
because as reported previously,8 it preserves the linear π-band
dispersion similar to that of free-standing graphene with Fermi
velocity close to 106 m/s. As a result of this weak graphene-metal
interaction, the graphene lattice can adopt several orientations
with respect to the underlying Pt(111) substrate giving rise to
different so-called moire patterns.8,2631 As a straightforward
consequence, in low-energy electron diffraction (LEED) measurements one can observe some segments at different angles
related to the atomic periodicity of graphene together with the
spots corresponding to the hexagonal lattice of the Pt(111)
surface. This is shown in Figure 1b, where a typical LEED pattern
measured on our graphene/Pt(111) surfaces is displayed. Despite this macroscopic orientational disorder, large areas of graphene with a single orientation with respect to the Pt(111)
surface can be found. An STM image over an area of 100 100 nm2
Figure 2. Schematic drawing of the 1,3,5-triazine molecule.
ARTICLE
exhibiting a single orientation of graphene, where a moire pattern
is observed, is shown in Figure 1a. Two STM images acquired
over areas of graphene/Pt(111) exhibiting two different moire
patterns are observed in Figures 1c,d. The moire pattern in
Figure 1c with a unit cell of 19.2 Å is the result of a rotation of 3.7°
of the graphene lattice with respect to the Pt(111)
surface.
This
√
√
pattern
√ can√be identified by the notation (4 3 4 3)R30° or
[( 61 61)R26.3°]G. The moire pattern in Figure 1d with a
periodicity of 7.38 Å corresponds to a rotation of 19.1° between
the graphene
√
√ and the Pt(111) lattices. This moire can be denoted
by ( 7 7)R19.1°, with respect to Pt(111), or alternatively as
(3 3)G, relative to the graphene overlayer. As previously
reported, nucleation of none of the moire pattern is preferred
over the others, but those with smaller unit cells grow faster than
those with larger ones.8 As a result, moire patterns with smaller
unit cells cover larger areas of the surface with respect to the others.
In particular, the (3 3)G is the most frequently found moire on
Pt(111).30,31 For this reason, most of our measurements have
been performed on the small (3 3)G moire.
Adsorption of 1,3,5-Triazine on Graphene/Pt(111). The
1,3,5-triazine is an aromatic molecule belonging to the family of
the azabenzenes. It consists of a benzenic ring where three carbon
atoms placed in alternated sites of the molecular ring are replaced
by three nitrogen atoms. A schematic drawing of the 1,3,5triazine molecule is shown in Figure 2.
After the exposure of the graphene/Pt(111) surface to 1,3,5triazine while keeping the surface temperature below 100 K,
fractal shape islands of 1,3,5-triazine molecules are formed. In
Figure 3a one can see a 300 300 nm2 STM image of the
graphene/Pt(111) surface where some of such islands have been
formed upon its exposure to 3 L of 1,3,5-triazine with the sample
kept at 44 K. These ramified 2D islands are usually associated
with the diffusion-limited aggregation (DLA) model proposed by
Witten and Sander.32 The STM image in Figure 3b has been
acquired on the area marked in Figure 3a, and it is observed that
the islands are composed of an ordered array with 6.25 Å periodicity of identical bright protrusions. This hexagonal structure is
similar to that observed on graphite surfaces22 but in that case
with a shorter lattice constant (6.1 Å). These protrusions can be
associated to 1,3,5-triazine molecules adopting a flat-lying configuration, which is characteristic of physisorption via weak van
Figure 3. STM images of consecutive zooms of the same region of a surface of graphene epitaxially grown on Pt(111) after the exposure of 1,3,5-triazine
at 44 K. (a) Size: 300 300 nm2, Vs = 1.2 V, IT = 0.29 nA. (b) Size: 34 34 nm2, Vs = 0.5 V, IT = 0.29 nA. (c) Size: 2.9 2.9 nm2, Vs = 0.27 V, IT = 0.29 nA.
The images were measured at 44 K.
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Figure 4. (a) Geometry of a 1,3,5-triazine molecule as optimized by
ArgusLab,34 (b) electrostatic potential map (ESP) also calculated by
ArgusLab, and (c) 2 2 nm2 STM image (Vs = 0.270 V, IT = 0.29 nA)
showing 1,3,5-triazine molecules with intramolecular resolution.
A lattice of molecules with 6.25 Å periodicity has been overlaid.
The orientation of the molecules has been determined by means of
the ESP map in panel b. The distance predicted by our model
between the hydrogen atom of one molecule and the nitrogen atom
of the adjacent one is 2.49 Å, which is consistent with the formation of
CH 3 3 3 N bonds.
der Waals interactions between the π orbitals in the molecular
ring and the substrate. A high-resolution STM image, acquired
over the area indicated by the white square in Figure 3b, exhibiting the molecular structure inside of such fractal islands is shown
in Figure 3c. One can observe intramolecular resolution consisting of three lobes presenting the three-fold symmetry characteristic of the 1,3,5-triazine molecule. It is consistent with a flat-lying
configuration of the 1,3,5-triazine molecules with respect to the
graphene substrate.
The optimized geometry as well as the calculated electrostatic
potential (ESP) mapped on the electron density contour of gasphase molecules are shown in Figure 4a. Regions of higher
negative ESP are displayed in red, whereas those of higher positive ESP are shown in white. The ESP map presents the threefold symmetry characteristic of the molecule. ESP maps are very
useful tools for describing the electron distribution on molecules,
and, in particular, it has been used for gas-phase azabenzenes.33
A negative value of the ESP in a point is related to an excess of
electrons, whereas a positive value is indicative of an electronpoor region where the nuclei charge is predominant.
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Figure 5. STM images acquired over areas of 100 100 nm2 of
graphene/Pt(111) after the exposure to 1,3,5-triazine. The temperature of the graphene/Pt(111) sample during the exposure was (a) 45 K.
Tunneling parameters: Vs = 2.9 V, IT = 60 pA. (b) 59 K. Tunneling
parameters: Vs = 2.0 V, IT = 90 pA. (c) 68 K. Tunneling parameters:
Vs = 2.0 V, IT = 130 pA. (d) 89 K. Tunneling parameters: Vs = 2.6 V,
IT = 70 pA.
STM images as the one in Figure 3c, presenting intramolecular
resolution and acquired at low tunneling bias (inside the molecular HOMOLUMO gap), are difficult to interpret. However,
it has been recently proposed that ESP maps can be very useful to
interpret STM images acquired under those conditions.35,36
In particular, ESP maps were used for the determination, from
STM images exhibiting intramolecular features, of the orientation of molecules physisorbed on surfaces of low reactive metals.35
These authors reported that areas with higher negative ESP are
imaged in STM with larger apparent height. Then, according to
this argument, the three lobes with three-fold symmetry observed in STM images as the one in Figure 3c should be related to
the red areas displayed in Figure 4a, which are found on the
surroundings of the nitrogen atoms. This has allowed us to model
in Figure 4b the molecular array with a lattice constant of 6.25 Å
by superposing it on top of an STM image with intramolecular resolution. A distance of 2.49 Å between N and H atoms
of adjacent molecules is determined by our model. This distance is consistent with the formation of CH 3 3 3 N bonds
between neighbor molecules, as indicated by the dashed lines
in Figure 4b.
Nucleation Experiments: Surface Diffusion Barrier.
Figure 5 shows a set of STM images acquired on 100 100 nm2
areas of graphene/Pt(111) surfaces after 1/3 monolayers of
1,3,5-triazine have been deposited while keeping the graphene
substrate at different temperatures. On them it is visible that the
island density decreases by increasing the temperature, whereas
the size of the islands increases by increasing the temperature.
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Figure 7. STM images measured on 100 100 nm2 areas of graphene/
Pt(111) after the exposure to 1,3,5-triazine. The temperature of the
graphene/Pt(111) sample during the exposure was 68 K for both√
images.
(a) Image acquired over an area exhibiting a large periodicity [( 61 √
61)R26.3°]G moire, as the one shown in Figure 1c. Tunneling parameters: Vs = 2.4 V, IT = 80 pA. (b) Image acquired over a zone with the
small periodicity (3 3)G moire pattern as the one shown in Figure 1d.
Tunneling parameters: Vs = 2.0 V, IT = 130 pA.
Figure 6. (a) Plot of the island density versus the deposition rate F.
From the slope, we obtain χ = 0.3 ( 0.2, in reasonable agreement with
0.33 corresponding to a critical nucleus equal to 1. (b) Arrhenius plot of
the island density versus the inverse temperature, 1/T. From the slope, it
is extracted a value of 68 ( 9 meV for the diffusion barrier of single 1,3,5triazine molecules on graphene/Pt(111).
Nucleation theory establishes the following relationship between the island density and the temperature of the surface
during the exposure37,38
"
#χ
4Ω2 F
<N > ¼ η 2
exp½Ei þ iEd =ði þ 2ÞkT
ð1Þ
a ν0
where i is the critical nucleus defined as the minimum number of
molecules in a stable cluster, <N> is the islands average number
per adsorption site, F is the deposition rate, ν0 is the attempt
frequency, Ei is the binding energy of the cluster corresponding
to the critical nucleus, Ed is the barrier for single molecules
diffusion, T is the temperature of the substrate, Ω is the area of
the molecular unit cell, a is the lattice parameter of the substrate
unit cell, and χ = i/(i+2). The dimensionless parameter η
depends on the coverage and is usually taken as a constant equal
to 0.25. (See refs 37 and 38.)
The first step to apply the nucleation theory is to determine
the critical nucleus size. The power law dependence between the
islands density <N> and the deposition rate F allows the determination of i by means of experiments consisting of maintaining
the same exposure of 1,3,5-triazine on the graphene/Pt(111)
surface but at different deposition rates F. Then, the plot resulting
from such experiments is shown in Figure 6a. From this plot,
it is extracted that χ = 0.3 ( 0.2, which means i = 1. Therefore,
dimers of 1,3,5-triazine are stable, and they do not diffuse. It is
interesting to compare this fact with the result reported by
Kim et al.39 that dimers of s-triazine molecules in the gas
phase are very stable via the formation of two hydrogen bonds
CH 3 3 3 N between them.
Because of the exponential dependence indicated on [1] between the island density and the substrate temperature during its
exposure to 1,3,5-triazine, the diffusion barrier of a single 1,3,
5-triazine molecule on the graphene/Pt(111) surface can be
extracted from the Arrhenius plot shown in Figure 5d. The value
obtained for the diffusion barrier is 68 ( 9 meV and for the
attempt frequency 1 1012(1 Hz. This energy barrier is low
enough as to be considered the consequence of physisorption.
Very low diffusion barriers have been measured very recently on
HOPG surfaces for benzene40 and for 1,3,5-triazine.22 Moreover,
theoretical calculations of the adsorption energy of 1,3,5-triazine
on free-standing graphene have revealed very low values,41 and
this weak interaction of the molecule with the graphene substrate
is consistent with the orientation of the 1,3,5-triazine molecules
parallel to the graphene surface. However, although small, this
diffusion barrier is higher than that of 55 ( 8 meV previously
reported by us on HOPG substrates.22 This increase of 20% of
the barrier on graphene indicates that the metallic substrate
below the graphene surface has an important influence on the
diffusion and hence in the molecule-graphene interaction of the
azabenzene 1,3,5-triazine. Very recently it has been predicted by
first-principles calculations that the diffusion barrier of oxygen on
graphene can be varied very substantially by controlling the surface carrier density through a gate voltage.42 The enhancement of
the barrier diffusion of 1,3,5-triazine molecules on graphene on
Pt(111) with respect to graphite could be the first experimental
indication of such doping effect as it has been shown that the
Dirac point in this system is located at ∼300 meV above the
Fermi level;4,8,31 the corresponding p-doping of graphene would
be consistent with an increase in the barrier at least in the case of
oxygen diffusion. Another aspect that could result on the enhancement of the diffusion barrier in the present case could be related
not only to a doping effect but also to a different moleculegraphene bonding due to the platinum substrate. In this sense, it
has been shown very recently that defects as vacancies on the
graphene layer can alter very substantially the reactivity of a graphene layer epitaxially grown on top of a Pt(111) substrate.31
Pattern. A final important issue
Dependence with the Moire
we have analyzed is the possible influence of the rotational
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orientation of the graphene layer with respect to the metal on the
adsorption and diffusion of the 1,3,5-triazine molecules. As previously mentioned, most of the measurements presented in this
work have been performed on the small (3 3)G moire. However,
we have not found any evidence of a substantial influence of the
moire pattern on the nucleation of 1,3,5 triazine islands on graphene/Pt(111) in the range of temperatures 40100 K. As an
example, two STM images acquired after the adsorption of 1,3,5triazine at 68 K, over areas of the graphene/Pt(111) sample
exhibiting large and small moire patterns as those presented in
Figure 1c,d are, respectively, shown in Figure 7a,b. The island
density in Figure 7a is 2.3 104 islands/site and in Figure 7b is
2.0 104 islands/site, both reasonably close to the average
island density of 1.6 104 islands/site obtained at 68 K. It is
possible to observe that the difference between both values and
between each one and the average value lies inside the value of
0.8 104 islands/site obtained for the standard deviation of the
island density at this temperature. This lack of a significant dependence (within our experimental error) of the diffusion barrier
on the moire pattern is consistent with the observation by Sutter
et al.8 that no significant changes in energy of the Dirac point of
the graphene/Pt(111) on the specific moire has been detected by
photoemission experiments. This is also consistent with the low
influence (if any) of the moire periodicity on the electronic
structure of single vacancy-defects on graphene/Pt(111) recently
reported.31
’ CONCLUSIONS
In summary, the early stages of growth of the organic molecule
1,3,5-triazine have been studied by STM. The molecules are
adsorbed on the graphene/Pt(111) substrate with their molecular ring parallel to the surface giving rise to a well-ordered
hexagonal structure with a lattice parameter of 6.25 Å. Highresolution STM images have allowed us to observe intramolecular features with three-fold symmetry that are compatible with
a flat-lying configuration with respect to the substrate. The
interpretation, by means of the ESP map calculated for the
1,3,5-triazine molecule, of these intramolecular features has
allowed us to determine the orientation of the molecule inside
the hexagonal lattice. The resulting distance of 2.49 Å between
the nitrogen atom of one molecule and the hydrogen atom of the
adjacent one is consistent with the formation of CH 3 3 3 N
bonds. Through nucleation experiments it has been obtained that
the critical nucleus size for 1,3,5-triazine molecules adsorbed on
graphene/Pt(111) surfaces is 1. Therefore, dimers of molecules
are stable and they do not diffuse. The diffusion barrier, also
obtained by nucleation experiments, for a single molecule of
1,3,5-triazine on the graphene/Pt(111) surface is 68 ( 9 meV. A
comparison with the results previously reported on the system
1,3,5-triazine/HOPG, where a diffusion barrier of 55 ( 8 meV
was obtained, suggests that the underlying metallic substrate has
a significant influence on the diffusion of 1,3,5-triazine molecules
on graphene. Finally, nucleation of 1,3,5-triazine islands seems
not be significantly affected by the moire pattern in the temperature range 40100 K.
’ AUTHOR INFORMATION
Corresponding Author
*Tel: +34 91 497 56 06. Fax: +34 91 497 39 61. E-mail: antonio.
[email protected].
ARTICLE
’ ACKNOWLEDGMENT
Financial support from the Spain’s MICINN under grant nos.
MAT2010-14902 and CSD2010-00024 and from CAM under
grant nos. S2009/MAT-1467 and CPI/0256/2007 is gratefully
acknowledged.
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