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Linear hydrogen adsorbate structures on graphite induced
by self-assembled molecular monolayers
Louis Nilsson a, Željko Šljivančanin b, Richard Balog a, Wei Xu a, Trolle R. Linderoth a,
Erik Lægsgaard a, Ivan Stensgaard a, Bjørk Hammer a, Flemming Besenbacher a,
Liv Hornekær a,*
a
b
Department of Physics and Astronomy, and Interdisciplinary Nanoscience Centre, Aarhus University, DK-8000 Aarhus C, Denmark
Vinca Institute of Nuclear Sciences, University of Belgrade, RS-11001 Belgrade, Serbia
A R T I C L E I N F O
A B S T R A C T
Article history:
Combined scanning tunnelling microscopy measurements and density functional theory
Received 9 October 2011
calculations reveal a method to induce linear structures of hydrogen adsorbates on graph-
Accepted 25 December 2011
Available
2012
Available online
online 59 January
December
2011
ite by covering the surface with a self-assembled molecular monolayer of cyanuric acid and
exposing it to atomic hydrogen. The method can in principle be applied to obtain nanopatterned hydrogen structures on free standing graphene and graphene laid down on insulating substrates, hereby opening up for the possibility of substrate independent bandgap
engineering of graphene.
2012 Elsevier Ltd. All rights reserved.
The interaction of hydrogen with graphite has been studied
extensively due to its broad relevance within fields as diverse
as hydrogen storage [1] and interstellar catalysis [2]. These
investigations have within the last years been extended to
hydrogen–graphene interactions with particular emphasis
on engineering of graphene’s electronic properties by hydrogen functionalization [3]. Different approaches to reach this
goal include: creation of periodic hydrogen adsorbate patterns
[4,5] to achieve global band gap engineering; and creation of
linear structures of hydrogen dimers to achieve local confinement induced band gap opening in between the hydrogen dimer rows [6]. However, calculations [7] and experiments [8–10]
show that such structures will not occur spontaneously by
exposure of graphite or graphene on a weakly-interacting substrate to atomic (or molecular) hydrogen and neither through
subsequent thermal annealing [9]. In fact, ordered H adsorbate
structures have so far only been observed on graphene on an
Ir(1 1 1) substrate [5]. In that case the origin of the order is a
hydrogen induced reactivity between carbon atoms in graphene and iridium atoms in the substrate on some parts of the
Moire pattern, caused by the lattice mismatch between the
graphene and the iridium lattices.
In this letter we demonstrate, from combined scanning
tunnelling microscopy (STM) measurements and density
functional theory (DFT) studies, that a self-assembled monolayer of cyanuric acid (CyA) can induce the formation of linear
hydrogen adsorbate structures on graphite. Despite the fundamental difference between graphite and graphene from
an electronic point of view, DFT calculations predict no difference in the chemical activity of the top layer of graphite and
single layer graphene with hydrogen [7,9]. Hence, the method
presented here for inducing specific hydrogen adsorbate
structures on graphite should also be applicable for graphene
independently of the substrate, hence, providing a generally
applicable tool for local and possibly also global band gap
engineering in graphene.
Fig. 1a depicts an STM image of the HOPG surface after
deposition of CyA (see Supporting information for further details). Several regions with different self-assembled molecular
structures are observed. The structure in the region marked A
in Fig. 1a has been observed before and is referred to as the
flower structure [11]. A high resolution STM picture of this
structure is shown in Fig. 1b. In our experiments the flower
structure was observed to be the dominant structure on the
surface, even though it has been reported theoretically elsewhere [11] that the heptamer structure, visible in region B
in Fig. 1a, is the most stable one. However, the calculated energy difference between these two structures is very small
(0.11 eV per molecule) [11] and the coexistence of various
CyA structures has been reported previously [11]. Minor parts
of the surface were covered with ordered structures with larger periodicities (e.g. area C), however, the molecular structure was not identified for these areas. When the coverage
was below one monolayer, a 2D liquid-like molecular phase
(disordered) with very mobile molecules was observed in between the ordered structures (area D).
* Corresponding author.
E-mail address: [email protected] (L. Hornekær).
0008-6223/$ - see front matter 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carbon.2011.12.050
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Fig. 1 – STM image of self-assembled CyA structures on a graphite surface. (a) Domains with different self-assembly
structures: flower structure (A), heptamer structure (B), a structure with a bigger periodicity (C) and a 2D liquid-like phase (D)
(imaging parameters: Vt = 1.250 V, It = 0.39 nA). Insert shows the keto-form of the CyA molecule (black = carbon;
blue = nitrogen; red = oxygen; white = hydrogen). (b) High-resolution STM picture of the flower structure. (Imaging
parameters: Vt = 1.250 V, It = 0.06 nA). (For interpretation of the references to colour in this figure legend, the reader is referred
to the web version of this article.)
The graphite surface covered by a monolayer of CyA was
kept at room temperature and subsequently exposed to atomic hydrogen. The exposure to hot atomic hydrogen is observed
to destroy the ordered layer gradually. However, on some
areas of the surface hydrogen exposure was observed to result in the formation of bright linear structures, Fig. 2a. These
stripes are observed only outside the remaining ordered parts
of the CyA, which indicates that the molecular monolayer
initiates line formation in the process of being destroyed.
The linear hydrogen related structures do not cross graphite
step edges, but they do form kinks with 60 or 120. We have
found that they are always aligned along one of the three
directions defined by the nearest C–C direction of the graphite
surface. A zoom-in on a linear structure is depicted in Fig. 2b.
The linear structures are observed to induce the well-known
(sqrt(3) · sqrt(3))R30-reconstruction [12] in the top graphite
layer in their vicinity. The fact that the sqrt(3)-reconstruction
is observed indicates that there is a strong binding between
the linear structures and the graphite, i.e. a chemical binding.
The numbered white lines in Fig. 2b mark positions at
which line-profiles, shown in Fig. 2c and d, have been obtained. The width of the individual linear structures is found
to be approximately 4 Å. The intensity profile along the linear
structure, depicted in Fig. 2d, reveals a modulation with a
4.3 Å periodicity. Linear structures with a maximum length
of 65 Å have been observed.
The observations of the linear H related structures may
have two possible origins:
(a) The linear structures may consist of atomic hydrogen on
the clean graphite surface. Several different arrangements of
adsorbed hydrogen with respect to the graphite lattice were
investigated theoretically (see Supporting information for further details), see Fig. 3. Of these, the most stable structure is the
straight dimer line in Fig. 3a, displayed together with the corresponding simulated STM image, and line scans along two
directions indicated by blue lines. This H configuration has a
binding energy of 1.71 eV per hydrogen atom, making it a more
stable structure than ortho- and para-dimers previously observed on hydrogen exposed graphite surfaces [9]. As can be
seen in Fig. 3a, the distance between intensity maxima within
the straight lines is 4.26 Å and is thus in good agreement with
the measured value. Moreover, the predicted structure can
form only along the three different nearest neighbour carbon
directions, similar to the directions of the observed linear
structures. Also, the strong binding of the H dimer line to the
substrate is expected to give rise to a (sqrt(3) · sqrt(3))R30reconstruction, as observed. Hence, the predicted hydrogen dimer line structure in Fig. 3a is in excellent agreement with all
experimentally obtained characteristics of the linear adsorbate
structures. The simulated STM images of two other less stable,
but still favourable, H structures are not in accordance with the
experimental results, see Fig. 3b and c.
(b) The second tentative option for the H related structure
is that the linear structures consist of CyA molecules or
hydrogen-induced derivatives of CyA molecules. However,
DFT calculations show that the CyA molecule binds weakly
to graphite via van der Waals interactions and hence cannot
give rise to strongly bound structures like those observed.
The removal of a single H atom from the molecule enhances
the reactivity, but still does not result in strong binding of the
molecule to the graphite substrate. Smaller fragments of the
CyA molecule were not investigated.
Hence, based on the experimental STM data and the DFT
calculations we propose that the observed linear structures
are composed of lines of hydrogen dimers. Such hydrogen
arrangements have not previously been observed on graphite
or graphene. Indeed, the only ordered hydrogen structures
on graphene and graphite previously reported are the periodic
nanopatterns on the graphene/Ir(1 1 1)-system [5]. In that
work, the formation of ordered adsorbate structures is mediated by the Moire superlattice, strictly defined by the mismatch between the substrate and the graphene overlayer,
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5 0 ( 2 0 1 2 ) 2 0 4 5 –2 0 6 0
Fig. 2 – (a) STM image of the graphite surface after hydrogen exposure on the self-assembled monolayer structure of CyA
molecules. The bright linear protrusions are identified as rows of hydrogen dimers. Ordered self-assembled CyA molecules
are still present in the top right corner and the right side of the picture. A step edge is visible in the upper part of the picture
(imaging parameters: Vt = 1.250 V, It = 0.37 nA). (b) Section of (a) imaged at high-resolution. The sqrt(3)-reconstruction of the
graphite is clearly seen around the hydrogen stripes. The areas partly confined by stripes of hydrogen are depicted dark
indicating limited or vanishing conductivity. Two perpendicular line scans, marked 1 and 2, are seen in figures (c) and (d),
respectively (imaging parameters: Vt = 1.250 V, It = 0.37 nA). (c) Intensity profile across the two stripes of hydrogen dimers.
The apparent height of the stripes according to the surrounding areas is 1.2 Å, and the width of the stripes is approximately
4 Å. The distance between the two stripes is 14.8 Å. (d) Intensity profile along the length of one of the stripes. A periodicity of
4.3 Å is observed.
and is thus not readily generalizable to graphene on other substrates. In contrast, the linear hydrogen adsorbate structures
observed here are induced by the self-assembled molecular
monolayer. A complete understanding of the role of the CyA
monolayer is not obtained at present. However, control experiments have verified that the linear hydrogen adsorbate structures are exclusively produced by the unique combination of
the self-assembled molecular monolayer and atomic hydrogen dosing: the substrate covered with the self-assembled
molecular monolayer was placed in front of the hot hydrogen
doser without letting in any gas, as well as exposed to hot helium and no changes were observed. The experiments clearly
demonstrate that the exposure to atomic hydrogen gradually
destroys the self-assembled monolayer structure. Whether
the linear hydrogen adsorbates are formed by the ordered
phase, the unordered phase or at the interface between the
two faces, is still to be determined. Hence, the extent to which
the geometry and structure of the hydrogen induced lines can
be controlled by the choice of self assembly molecule awaits
further investigation.
From high-resolution STM images in Fig. 2b it is seen that
some areas between the bright hydrogen related linear protrusions appear dark in the STM image, indicating a reduction
in the surface conductivity. Chernozatonskii and co-workers
showed theoretically that areas of graphene confined between two hydrogen lines will exhibit a local bandgap opening and that the size of the gap depends on the width and
edge geometry of the enclosed area [6], thus resembling the
situation with graphene nanoribbons. The induced linear
structures therefore offer the possibility to implement local
engineering of the band structure of graphene. Furthermore,
self-assembled molecular adsorbate systems which result in
globally ordered monolayers can potentially act as templates
for global patterning with nanoscale resolution of e.g. hydrogen structures on graphene on insulating substrates.
In conclusion, from an interplay of STM and DFT results, we
have shown that a self-assembled molecular monolayer of
CyA molecules on graphite can induce the formation of very
stable linear hydrogen adsorbate structures. This scheme
opens up for the possibility of forming nanoscale integrated
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Fig. 3 – Simulated STM images of three linear H structures and the corresponding H binding energies: (a) straight dimer lines,
(b) alternating dimer lines and (c) sideways dimer lines. The H atoms are represented by small blue spheres. For straight
dimer lines the line-scans along directions 1 and 2 are also depicted.
circuits and thereby molecular electronics on graphene by
locally adjusting the size of the bandgap. Furthermore, the
approach presented here could in principle extend the proven
method of bandgap engineering in graphene on Ir(1 1 1) by
chemical functionalization to free standing graphene and
graphene on substrates, more suitable for production of electronic devices.
Acknowledgements
We would like to acknowledge the financial support from the
Danish Research Agency, from the Villum Kahn Rasmussen
and the Carlsberg Foundation and from the European Research Council for an Early Starting Grant (LH) and for an Advanced Grant (FB).
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at doi:10.1016/j.carbon.2011.12.050.
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The effect of the addition of carbon nanotube fluids
to a polymeric matrix to produce simultaneous reinforcement
and plasticization
Qi Li a, Lijie Dong a, Liubin Li a, Xiaohong Su a, Haian Xie a, Chuanxi Xiong
a,b,*
a
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, and School of Materials Science and Engineering,
Wuhan University of Technology, Wuhan 430070, PR China
b
School of Materials Science and Engineering, Wuhan Textile University, Wuhan 430073, PR China
A R T I C L E I N F O
A B S T R A C T
Article history:
Carbon nanotube (CNT) fluids with liquid-like behavior were synthesized and incorporated
Received 29 October 2011
into a polymeric matrix to investigate the processability and mechanical performance of
Accepted 24 December 2011
Available online
2012
online 59 January
December
2011
the resulting composite, as well as the distribution of CNTs in the host material. We demonstrated that the simultaneous reinforcement and plasticization effect on the polymeric
matrix by the novel multifunctional component should be ascribed to the soft organic coating and the unique flowability of such surface-functionalized CNTs. The solvent- and
plasticizer-free nature along with the above-mentioned advantages provides a green and
efficient route to fabricate high performance composite materials.
2012 Elsevier Ltd. All rights reserved.
Composites have been recognized as radical alternative to
conventional particle-filled polymer materials because the
performance of polymer composites will increase dramatically
as the dimension of the filler particles decreases to the nanometer-scale [1]. Particularly, carbon nanotube (CNT)/polymer
composites have attracted much attention [2] due to the unique and remarkable properties of CNTs, including extraordinary low density, excellent mechanical properties and high
electrical and thermal conductivity [3,4]. However, the homogeneous dispersion of nanostructures in a polymeric matrix
remains a challenge task because they are easy to agglomerate
spontaneously. As with the case of CNTs, the 1-dimensional
nature facilitating a heavy entanglement of these flexible
tubes with ultrahigh aspect ratio will also hamper their dispersion in polymeric matrix [5]. This can not only deteriorate the
overall performance of a composite system but also significantly hinder its processing. Besides, as for most polymer
composites, plasticizers are used to improve the processability
and flexibility of polymeric materials. Unfortunately, there are
still some great disadvantages of conventional plasticizers in
practical applications, such as ease of evaporation, high toxicity, poor thermal stability and reduction of the mechanical
properties of composite materials [6]. Therefore, to develop a
green yet efficient method to achieve a homogeneous dispersion of nanostructures in polymer matrix without sacrificing
its processability is of great importance in polymer science.
In this paper, we report a simultaneous reinforcement
and plasticization on polymer materials by using surfacefunctionalized CNTs as a novel multifunctional component.
Since these surface-functionalized CNTs present as sticky flu-
* Corresponding author at: State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, and School of Materials
Science and Engineering, Wuhan University of Technology, Wuhan 430070, PR China. Fax: +86 27 87652879.
E-mail address: [email protected] (C. Xiong).
0008-6223/$ - see front matter 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carbon.2011.12.051