Linear hydrogen adsorbate structures on graphite induced by self-assembled molecular monolayers

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1 2052 CARBON 50 (2012) 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 Department of Physics and Astronomy, and Interdisciplinary Nanoscience Centre, Aarhus University, DK-8000 Aarhus C, Denmark b Vinca Institute of Nuclear Sciences, University of Belgrade, RS Belgrade, Serbia ARTICLE INFO ABSTRACT Article history: Received 9 October 2011 Accepted 25 December 2011 Available online 59 January December Combined scanning tunnelling microscopy measurements and density functional theory calculations reveal a method to induce linear structures of hydrogen adsorbates on graphite 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. Ó 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(111) 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. address: Liv@phys.au.dk (L. Hornekær) /$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi: /j.carbon

2 CARBON 50 (2012) 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 = 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 = 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. Wehave 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))r30- reconstruction, 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(111)-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,

3 2054 CARBON 50 (2012) 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 = 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 = 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 4Å. 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 DFTresults, 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

4 CARBON 50 (2012) 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(111) 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: /j.carbon REFERENCES [1] Sofo JO, Chaudhari AS, Barber GD. Graphene: a twodimensional hydrocarbon. Phys Rev B 2007;75(15). [2] Cazaux S, Tielens A. Formation on grain surfaces. Astrophys J 2004;604(1): [3] Liu HT, Liu YQ, Zhu DB. Chemical doping of graphene. J Mater Chem 2011;21(10): [4] Duplock EJ, Scheffler M, Lindan PJD. Hallmark of perfect graphene. Phys Rev Lett 2004;92(22): [5] Balog R, Jorgensen B, Nilsson L, Andersen M, Rienks E, Bianchi M, et al. Bandgap opening in graphene induced by patterned hydrogen adsorption. Nat Mater 2010;9(4): [6] Chernozatonskii LA, Sorokin PB, Belova EE, Bruning J, Fedorov AS. Superlattices consisting of lines of adsorbed hydrogen atom pairs on graphene. Jetp Lett 2007;85(1): [7] Sljivancanin Z, Andersen M, Hornekaer L, Hammer B. Structure and stability of small H clusters on graphene. Phys Rev B 2011;83(20). [8] Balog R, Jorgensen B, Wells J, Laegsgaard E, Hofmann P, Besenbacher F, et al. Atomic hydrogen adsorbate

5 2056 CARBON 50 (2012) structures on graphene. J Am Chem Soc 2009;131(25): [9] Hornekaer L, Sljivancanin Z, Xu W, Otero R, Rauls E, Stensgaard I, et al. Metastable structures and recombination pathways for atomic hydrogen on the graphite (0001) surface. Phys Rev Lett 2006;96(15): [10] Sessi P, Guest JR, Bode M, Guisinger NP. Patterning graphene at the nanometer scale via hydrogen desorption. Nano Lett 2009;9(12): [11] Kannappan K, Werblowsky TL, Rim KT, Berne BJ, Flynn GW. An experimental and theoretical study of the formation of nanostructures of self-assembled cyanuric acid through hydrogen bond networks on graphite. J Phys Chem B 2007;111(24): [12] Ruffieux P, Groning O, Schwaller P, Schlapbach L, Groning P. Hydrogen atoms cause long-range electronic effects on graphite. Phys Rev Lett 2000;84(21): 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 , PR China b School of Materials Science and Engineering, Wuhan Textile University, Wuhan , PR China ARTICLE INFO ABSTRACT Article history: Received 29 October 2011 Accepted 24 December 2011 Available online 59 January December Carbon nanotube (CNT) fluids with liquid-like behavior were synthesized and incorporated into a polymeric matrix to investigate the processability and mechanical performance of 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. Ó 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 , PR China. Fax: address: cxx@live.whut.edu.cn (C. Xiong) /$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi: /j.carbon