Electronic Properties of Self-Assembled Trimesic ... - ACS Publications

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Electronic Properties of Self-Assembled Trimesic Acid Monolayer on Graphene F. Shayeganfar* and A. Rochefort* Engineering Physics Department and Regroupement québécois sur les matériaux de pointe (RQMP), Polytechnique Montréal, Montréal, Québec H3C 3A7, Canada ABSTRACT: The adsorption of trimesic acid (TMA) on a graphene surface has been studied with density functional theory. By considering the adsorption of a single TMA molecule on different sites on graphene, we have been able to perform a detailed analysis of the equilibrium geometry, charge transfer, electronic properties in terms of density of states and band structure, and finally scanning tunneling microscopy simulations on those simple systems. The results for isolated adsorption were then compared to the behavior of the TMA unit within two different self-assembled monolayers. Our results indicate that structural deformations of TMA may significantly contribute to the magnitude of p-doping and band gap opening in graphene. The formation of a hydrogen bonding network within the assembly improves the stability of the adlayer, but its adhesion on graphene is significantly reduced. The magnitude of p-doping in graphene per TMA unit remains nearly constant from the isolated to the assembled systems, but the magnitude of the band gap opening appears to be strongly correlated with the breaking of symmetry of π-states of graphene by the TMA patterning on the surface. Our results suggest that polymorphism in self-assembled adlayers could be used to tune and control the electronic properties of graphene. molecules on graphitic surfaces.13,21,22 On weakly reactive surfaces, most common bonding arrangements of carboxylic acids in extended network or large molecular crystals involve a periodic distribution of TMA dimers and trimers. Lackinger and co-workers13,21,22 synthesized, characterized, and revealed by scanning tunneling microscopy (STM) that TMA selfassembled into two different structures on graphite. First, a porous structure with hexagonal cavities (chicken-wire-like) made of six-fold rings of trimesic acid molecules with perfect hydrogen bond arrangement is shown at Figure 1a. The chicken-wire network is also the most common molecular arrangement observed on low-reactivity Cu, Au, and Ag surfaces.17−20,23 Second, TMA can form a close-packed structure named “flower” from a three-folded hydrogen bonding arrangement drawn in Figure 1b. A more densely packed structure called “super-flower structure”, such as the one in Figure 1c, was also observed on Au(111)19 but has surprisingly not been observed yet on graphitic surfaces.22 In general, the functional carboxylic acids groups are in equilibrium with their deprotonated state, and their chemical activity can also affect the self-assembling process.13,24 On a reactive substrate such as metal surfaces, carboxylic acid groups adsorb, sometimes react, but always create very strong interactions with the surface, and the resulting assemblies

I. INTRODUCTION The holistic understanding of molecular self-assemblies constitutes a major driving force in the design of new materials1−3 where the engineering arrangement of molecular blocks into complex nanoporous architectures remains the focus of numerous recent works.4−8 Examples of natural molecular self-assembly such as spider silk,9 DNA,10 and cellular membranes11 demonstrate the potential of encoding information1 to produce biological function as well as to create permanent nanoporosity.12 This can be usually exploited after a subsequent functionalization of the molecular blocks in the development of novel nanostructured materials dedicated to catalysis, gas storage, selective ion exchange, or high-density data storage.4 To this end, molecular self-assembly offers unique directions for the fabrication of two- and threedimensional organic nanoarchitectures using only a small number of relatively simple molecular building blocks.4 Trimesic acid (TMA, benzene-1,3,5-tricarboxylic acid) constitutes an interesting building block for hydrogen bonding architecture by providing reliable intermolecular cross-linking that facilitates self-assembly.13 TMA has three carboxylic groups that combine donor and acceptor character, and that can form several highly directional hydrogen bonds. It has often been used as a linker to build three-dimensional metal−organic coordination networks with copper.14−16 TMA has also demonstrated quite strong ability to form well-organized twodimensional networks on low-reactivity surfaces such as Ag,17 Cu,18 and Au.19,20 In addition, the carboxylic functional groups can play a significantly role in π−π interactions of adsorbed © 2014 American Chemical Society

Received: April 28, 2014 Revised: June 19, 2014 Published: July 29, 2014 9707

dx.doi.org/10.1021/la501619b | Langmuir 2014, 30, 9707−9716

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Figure 1. Representation of the most common (a) chicken-wire, (b) flower, and (c) super-flower network arrangement for trimesic acid on graphite.

in assembled TMA, where the COOH groups form highly directional hydrogen bonds among TMA units that flattened the TMA adlayer. Once self-assembled, the interaction of TMA with graphene becomes weaker but still has a profound influence on the electronic properties of the substrate, although the magnitude of the variation depends on the TMA patterned symmetry on graphene.

may strongly contrast with surfaces where TMA is more weakly adsorbed.25 We understand that the formation of a self-assembled overlayer with a given structure on surfaces, and its subsequent 3D growing, strongly depends on the magnitude of adsorbate− surface interaction. Beyond this interest for creating a wellorganized multidimensional network for catalysis or storage purposes, molecular networks can also be used to protect, modify, and control the electronic properties of surfaces. In a similar manner that passivation is used to eliminate strongly reactive sites, the presence of an adlayer, even weakly adsorbed, can deliberately be used to locally control the charge density of surface adsorption sites.26,27 The control of the electronic properties of graphene by such a noncovalent well-organized layer can be attempted by different approaches. The first one is based on the control of the interface between a graphene sheet and the substrate through the functionalization of the latter by a self-assembled monolayer.28,29 An alternative approach uses the effect of a single adsorbed layer on graphene which, depending on its chemical composition, can generate a p- or n-doped graphene.30−32 Finally, a mixture of these two approaches has also been used for the case of a bilayer graphene.33 Hence, a better understanding of the fundamental aspect of adsorption involving a single molecular unit and the surface constitutes a prerogative to exploit the benefit of self-assembling. The property of individual units could be conserved in the assembly or could also be completely different than the sum of individual units due to cooperative effects.34 In the present work, prior to the study of the adsorption of different TMA adlayers, we have carried out a systematic analysis of a single TMA molecule adsorption on high symmetry sites of graphene with the help of first-principles density functional theory (DFT) calculations. The relative stability of adsorption sites, the associated TMA geometry, charge transfer, band structures, and STM simulated images were analyzed in detail. For a single TMA molecule, the symmetry of the adsorption site and the magnitude of deformation of TMA to maximize its interaction with graphene constitute the major factors that affect the electronic properties of graphene. These deformations arise mainly from an attractive force of COOH groups to graphene that opposes a repulsive interaction of π-electron clouds between TMA and graphene. We show that the magnitude of the p-type doping and band gap opening in graphene strongly depends on the geometry of the COOH groups with respect to the surface plane of graphene. These structural deformations essentially disappeared

II. COMPUTATIONAL DETAILS The self-consistent DFT calculations were performed with the SIESTA software package35 that uses localized basis sets and a periodic supercell method. It also uses pseudopotentials that reproduce allelectron eigenvalues and excitation energies of multiple atomic configurations. The calculations were performed within the localdensity approximation (LDA) in conjunction with the double-ζ polarized orbital for the basis sets of carbon, hydrogen, and oxygen. We used a norm-conserving Troullier−Martins-type pseudopotential for carbon, hydrogen, and oxygen atoms. The size of the supercells was specifically chosen to study the isolated or self-assembled trimesic acid molecule: large enough to avoid any interactions between a neighboring adsorbate in the single molecule adsorption study, and small enough to reproduce relevant distances between adsorbed units in the network. The sampling of the Brillouin zone includes a fine 36 × 36 × 1 Monkhorst−Pack k-points grid for all adsorption sites studied. A similar fine grid was used to produce an accurate band structure. During the geometry optimization, the structure of graphene was kept frozen to the optimized gas phase geometry and the adsorbed TMA molecule was fully optimized. Since no covalent bonding between TMA and graphene is established for a physisorbed state, we can expect only minor structural deformation of graphene that would weakly increase the adsorption energy. Hence, the approximation of a rigid graphene sheet leads to a lower limit of the adsorption energy and should not drastically influence the conclusions of the present study. The geometry optimization was pursued until the convergence criterion was less than 10−5 eV for total energy and less than 0.01 eV/ Å for forces. Despite the weakness of the DFT method in describing dispersive forces such as van der Waals (vdW) interactions36,37 for which the adsorption energies are typically underestimated with LDA and generalized gradient approximation functional, LDA appears more successful in providing reasonable geometries for π-stacked systems.38−41 In contrast, LDA is well-known to overestimate adsorption energies when interactions are stronger than πdispersion.42 In fact, DFT-LDA can qualitatively and quantitatively give an accurate description of the electronic structure of graphitic and carbon nanotube structures where the band structures are dominated by the chemistry of sp2 orbitals and do not significantly depend on van der Waals forces.40,43,44 We have previously shown45 that adsorption energy of benzene on graphene, a pure dispersive system, is clearly underestimated at the LDA (0.20 eV) level, but the value obtained at 9708


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Figure 2. Representation of the unit cells used for the high symmetry configurations, (a) top, (b) cross, (c) center, and (d) bridge of adsorption sites of trimesic acid on graphene. the PBE+D level (0.5 eV) is in excellent agreement with the experimental value (0.5 eV). More importantly, the ordering in the relative stability of different adsorbed aromatic molecules on graphene at the LDA level was similar to the one obtained at the PBE+D level. Similar conclusions were drawn by Chang et al.46 (see their Supporting Information) for different aromatic molecules within a periodic framework on graphene studied with the VASP software. As well described by Kharche and Nayak,47 LDA provides electronic structures for graphene that are similar to the GW technique except for a small energy shift. From these facts, we are confident that our conclusions are not affected by our computational approach. The adsorption energy of TMA on graphene reported by the adsorbed unit is defined by Eads =

Table 1. Structural Analysis of Optimized Adsorbed Trimesic Acid on Graphene adsorption site Eads (eV) dC−C (Å) dO−C (Å) TMA net charge (|e|)

top

cross

center

bridge

−2.22 3.08 2.73 −0.11

−2.13 3.06 2.71 −0.14

−1.83 2.95 2.77 −0.18

−1.25 2.86 2.78 −0.17

oxygen atom from the CO moiety of the COOH group closest to a carbon atom of the graphene sheet (dO−C), and finally the net Mulliken charge on TMA. The trends observed in Table 1 are straightforward: (i) top and cross are the most stable adsorption sites, followed by center and bridge sites; (ii) the distance dC−C associated with the height of the benzene core of TMA where most of the π-electrons of carbon are concentrated followed a similar pattern; it decreases from top to cross, center and bridge; (iii) oxygen atoms from COOH groups are closer to the graphene sheet for top and cross sites than for center and bridge sites, and (iv) the net charge on TMA is more important for the less stable (center, bridge) adsorption sites. To summarize this, the most stable adsorption sites are those where the deformation of the functional groups is the more important and where the charge transfer is less important. Furthermore, such structural deformation is consistent with a minimization of π−π repulsion and an improving electrostatic attraction between O atoms of TMA and C atoms of graphene.56 As we have previously observed, the contribution of π−π repulsion to the adsorption energy becomes quite significant when the adsorbate adopts a face-to-face configuration, and the adsorption energy does not increase linearly with the number of attractive COOH groups within the adsorbate.56 Since a TMA molecule does not form covalent bonding with graphene, the calculated adsorption energy values reported in Table 1 could appear high at first glance, although the energy difference between them is quite reasonable. We need to emphasize that the experimental adsorption energy of benzene on graphene is already 0.5 eV for pure dispersion interaction. The interaction of TMA with the graphene surface is more complicated than benzene because the three COOH groups are participating quite significantly in the bonding with the surface. We have previously calculated an adsorption energy of 0.98 eV for a bridge site on a finite graphene model,56 and the higher value calculated here (1.25 eV) can be easily explained by the different basis set as well as the periodic boundary conditions

E(TMA/graphene) − [E(graphene) + nE(TMA)] n

where E(TMA/graphene) is the total energy for optimized TMA on a (fixed) graphene sheet, E(graphene) is the total energy for the relaxed pristine graphene, E(TMA) is the total energy of an optimized gas phase TMA molecule, and n is the number of TMA units in the unit cell. The size of the unit cell was especially chosen to simulate the adsorption of an isolated TMA molecule (n = 1) or a specific TMA self-assembly (n > 1). The STM simulations were carried out with the Flex-STM module52 now included in our in-house SPAGS-STM software.53 For the STM simulations, we used standard Hamiltonian derived from extended Hückel theory54 in conjunction the Tersoff−Hamann55 level of theory to compute the tunneling current. All of the simulated STM images were computed in topographic mode, or in other words in a constant current mode. The simulation parameters will be given with the presentation of the results.

III. RESULTS AND DISCUSSION 1. Adsorption of Isolated TMA Molecules. 1.1. Stability and Adsorption Geometry. Following our previous works on the adsorption of small aromatic molecules on graphene45,56 in which we have clearly demonstrated the influence of the chemical nature of functional groups on the ability of the molecule to accommodate a significant amount of additional charge, we are now focusing on a single TMA molecule but on different adsorption sites over graphene. Figure 2 shows the four highly symmetric adsorption sites considered where TMA is commensurate with the surface (top), top but followed by a TMA rotation by 30° (cross), or more simply centered on a single atom (center) or on dimer (bridge) carbon atoms. Our systematic study begins with a detailed description of the optimized geometries. The results of the DFT calculations are reported in Table 1 in terms of adsorption energy (Eads), the distance between the more central benzene ring of TMA and the graphene sheet (dC−C), the distance between the 9709


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used in the present work. Furthermore, as mentioned above, the adsorption energies including the dispersion contribution give higher values than with LDA.45,46 We can then anticipate that adsorption energy of TMA on graphene can be substantially high. For example, Iancu et al. have calculated a value of 3.6 eV on the low-reactivity Au(111) surface with a dispersion-corrected van der Waals density functional.20 Hence, a more direct comparison of calculated values to an accurate experimental measurement would be highly beneficial to validate our computational approach. The calculated Mulliken charges indicate that adsorbed TMA is negatively charged, hence the substrate is positively charged, or in other words, graphene is p-doped. This result is in agreement with the electron-withdrawing character of TMA species, which usually leave graphene positively charged.30,31,48−50 The magnitude of the charge transfer from graphene to TMA along with the structural deformation of TMA supports a more comprehensive description of the TMA/ graphene interaction. In a few words, the deformation of COOH groups facilitates an overlap (donation) between oxygen atoms and graphene that stabilizes the adsorbate, but the same deformation may also decrease the ability of the π*orbitals of TMA that are localized on the CO moiety to accommodate additional charges from graphene because the orientation of that deformed π*-orbital (CO) does not facilitate any charge transfer. Consequently, the net charges on the highly deformed TMA molecule is lower than that when TMA is kept planar due to a lack of back-donation. Figure 3

Figure 4. Density of states of TMA adsorbed on graphene (bridge site), as compared to an isolated graphene sheet, and gas phase trimesic acid.

graphene, with respect to the DOS of pristine graphene and gas phase TMA molecule. Except for a slight energy shift, the DOS of graphene is, as expected, not drastically affected by the presence of TMA to a point where the DOS(TMA/graphene) is more or less a direct superposition of DOS(graphene) + DOS(TMA). The states of graphene are slightly perturbed due to adsorption of TMA, and because of the existence of three electron-withdrawing COOH groups on TMA, some electrons from the valence band of graphene are transferred to TMA. Hence, the DOS of the TMA/graphene system is slightly shifted by 0.25 eV below the Fermi energy (and not at Fermi level) of isolated graphene, which is taken as a reference here. This Fermi energy shifting indicates p-doping of the graphene moiety, in agreement with values in Table 1 showing a charge transfer from graphene to TMA. A similar DOS description can be done for all adsorption sites, and no clear differences can be easily noticed in DOS among the four different adsorption sites. In order to clarify the influence of the different adsorption sites on the electronic structure of graphene, we are presenting differential projected density of states (PDOS) of carbon atoms from graphene, ΔPDOS = PDOS(graphene) − PDOS(graphene in TMA/graphene)) diagrams in Figure 5, where we distinguish the contribution of σ- (in-plane) from π- (outof-plane) electrons originating from graphene. First, the direct comparison of π (a) and σ (b) projections clearly shows that πelectrons (out-of-plane) are more affected by TMA adsorption around Fermi level than σ-electrons which are perturbed to a much lesser degree. The variation of PDOS observed in the regions below and above EF indicates a general shifting of the PDOS(TMA/graphene) curves toward high binding with respect to graphene, which is reminiscent of the p-doping of graphene. The more intense peaks below the Fermi energy are centered around −6.3 eV that, according to Figure 4, coincide with the energy position of the HOMO of TMA, while the peaks around −2.7 eV correspond in energy to the LUMO of TMA (see Figure 4). In addition, the most intense peaks (bridge, center) correspond also to the adsorption sites where the charge transfer is the more important and where the structural deformation of the COOH groups is less significant.

Figure 3. Influence of structural deformation on the charge transfer mechanism between TMA and graphene.

summarizes the influence of deformation on the calculated results; deformations of COOH groups decrease their abilities to accept additional electrons in π*-orbitals centered on the CO moiety. Nevertheless, we need to emphasize that the inverse mechanism, where the donation is larger for top and cross sites that are closer to the graphene surface than the other sites, but for a similar amount of back-donation in all sites, would also be consistent. We analyzed the electronic structure properties of adsorbed TMA on graphene in the following section. 1.2. Electronic Structure Properties. Although we calculated a significant charge transfer from graphene to TMA, there is no covalent bonding between the two species. TMA is physisorbed on graphene, and we could then anticipate that most of the physical properties of graphene will be conserved after TMA adsorption. In agreement with this statement, Figure 4a shows the total density of states (DOS) of TMA on a bridge site on 9710


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Figure 5. Differential projected density of states of carbon atoms from graphene (ΔPDOS = PDOS(graphene) − PDOS(graphene in TMA/ graphene)) of the four different adsorption sites of TMA/graphene emphasizing the contribution of (a) π-electrons (out-of-plane) and (b) σelectrons (in-plane) from graphene.

Figure 6. Projected contribution of (a) HOMO and (b) LUMO on the density of states of the different TMA/graphene complexes.

The projections of the contribution of HOMO and LUMO of TMA on the density of states of the TMA/graphene complexes are reported in Figure 6. As anticipated, the energy position of HOMO and LUMO fits relatively well with the large variation of ΔPDOS observed in Figure 5, but we can also notice that the ΔPDOS variations below and above Fermi energy are better described by a combined contribution of HOMO and LUMO states from TMA. It appears difficult to discriminate between the contribution of the HOMO and LUMO above Fermi energy for the adsorption sites investigated. Below EF, the most perceptible variation occurs for the center and bridge adsorption sites, where the projected HOMO is broader than for the top and cross sites. This energy broadening of states suggests an improved mixture of HOMO states of TMA with the states of graphene, which is also supported by the ΔPDOS observed in Figure 5 showing a much larger perturbation of π-states for bridge and center sites. Such improved state mixing with π-electrons of graphene would also contribute to facilitate the charge transfer between TMA and graphene, as reported in Table 1. It is also noticeable that

state mixing between the HOMO and LUMO of TMA is more significant for bridge and center sites, as revealed by the broadening of the HOMO at −6.0 eV. An analysis of PDOS intensities for the LUMO below the Fermi energy allowed us believe that the amount of charges returned to TMA could be more or less the same for every site, and that the lower depletion of electrons in graphene for top and cross sites could be simply caused by a higher HOMO donation to graphene, as suggested by the intensities of the HOMO below EF in Figure 6a. From a different perspective, Figure 7 compares the calculated band structures of the four adsorption sites with respect to pure graphene. We have deliberately chosen the same rectangular supercell for the graphene substrate to compare the different adsorption sites, and we used the same unit cell for pristine graphene. This supercell leads, according to the energy band-folding picture, to a four-fold degeneracy at the Γ point that coincides with Dirac points.51 Hence, we observed four lines (two sets of degenerate π-bands) describing the Dirac cone rather than two lines usually obtained with the 9711


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insets allow us to show that a small but systematic band gap opening in graphene is observed for all adsorption sites. The magnitude of the band gap is higher (EG = 47−48 meV) for the most stable adsorption site ((a) top, (b) cross) than the opening (EG = 17−19 meV) calculated for the two other sites ((c) center, (d) bridge). Since oxygen atoms are closer to the graphene sheet in the top and cross sites than in the center and bridge sites, this suggests that such proximity could improve the overlap and the mixing between O (TMA) and C atom (graphene) states, to finally introduce a band gap in graphene through a local electronic perturbation by O atoms.46 Recently, Chang et al.46 have shown that the adsorption of different aromatic molecules such as borazine (B3N3H6), triazine (C3N3H3), and benzene (C6H6) on graphene may provoke a band gap opening up to 63 meV. Moreover, they found that the magnitude of the opening strongly depends on the adsorption site. The mechanism proposed to explain the band gap opening is related to a redistribution of charges among the adsorption site and where the magnitude of the gap opening depends on how the charge distribution among C−C bonds or, in other words, the band-state symmetry of graphene can be locally broken by the adsorbate.46 In a different theoretical study, Tian et al.57 have also used DFT to investigate the adsorption of the very strong electron acceptor, F4-TCNQ, on a bilayer graphene. In this case, a large band gap opening of 253 meV was calculated. They attributed this opening to a built-in electric field between graphene layers that is promoted by the large charge transfer (0.45|e|) from the top graphene layer to F4-TCNQ. In brief, the proposed mechanism from these studies for the band gap opening involve a modification of the distribution of states and charge density near the adsorption site, and the magnitude of the band gap opening is somehow related to the importance of the charge transfer or state mixing.

Figure 7. Band structure of TMA/graphene near the Fermi energy for the (a) top, (b) cross, (c) center, and (d) bridge adsorption sites, as compared to (e) pristine graphene. An inset in each graph provides an expanded view of the bands in the vicinity of the Fermi level.

more standard unit cell. As a first remark, the band structures of the TMA/graphene systems are very similar to that of pure graphene; the two degenerate pz (π) bands with a linear dispersion near the Dirac point remain present. The small

Figure 8. (i) Simulated STM images from the top; (ii) simulated STM images from the bottom; and (iii) STM profiles along lines A and B of the bottom STM images for (a) top, (b) cross, (c) bridge, and (d) center adsorption sites of TMA on graphene. STM images and profiles for graphene are also shown for comparison. STM images were calculated with I = 0.1 nA and Vb = −0.5 V. 9712


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properties of graphene through molecular deformations of the functional groups on the adsorbate. Such structural deformations could be strongly hindered once the molecules that are forming the 2D network with highly directional hydrogen bonds between TMA units are formed. In the present section, we want to compare the variation of electronic properties induced by self-assembled TMA monolayers with respect to an ensemble of independent adsorbed species. We have investigated two network structures shown in Figure 1: the chicken-wire and the super-flower network. Figure 9 shows the supercells used to model the chicken-wire network that contains 42 atoms (12 O, 12 H, and 18 C) for the

From this, we could argue that our calculated band gap openings agree with previous works where a small charge transfer and state mixing of the adsorbate with graphene provokes a small band gap opening by breaking the local symmetry of the band states of graphene. However, in contrast to that general trend, we observed a larger gap opening for the sites (top, cross) where the charge transfer is weaker. In fact, the results presented at Figure 6a indicate a larger mixing of HOMO and LUMO states of TMA with graphene for center and bridge sites that provokes a blue shift of the PDOS of graphene which indicates a p-doping of graphene. The contribution of HOMO/LUMO states to TMA/graphene complexes for top and cross sites does not strongly contrast with the two other sites, but the PDOS of graphene is much less affected, indicating a lower p-doping. The origin of this lower p-doping could be explained by an increasing HOMO → graphene donation or by a decreasing graphene → π*-states of TMA. Although both mechanisms may contribute to lower the electron depletion in graphene, the PDOS of the LUMO shown in Figure 6b better agrees with the former mechanism. Finally, a series of simulated STM images were obtained for the different adsorption sites with the aim of using our numerical STM tool to probe charge transfer and state mixing in physisorbed states. Figure 8 gives different perspectives of the influence of different adsorption sites of TMA/graphene on STM images. The top panels show the topographic simulated STM images calculated above the adsorbate, while in the middle panels, STM images were computed from the bottom of the sample where we have simply flipped the TMA/graphene sample upside-down; finally, the bottom panels analyze the profiles of STM contrasts for those flipped samples along two different lines (A,B). First, the top STM images reproduce with a remarkable resolution the main features of the LUMO from gas phase TMA (not shown). This result supports the fact that TMA interacts weakly with the surface and where the main STM contrasts are localized near the COOH groups. Obviously, computing a STM image of a standard adsorbed phase could hardly reveal subtle information on the variation of electronic properties of graphene upon the adsorption of TMA. In contrast, computing STM images from the bottom of the sample could directly reveal the influence of the TMA adsorbate on the electronic properties of graphene. STM images computed from the bottom (middle panels) are much more informative; we observed that the adsorption of TMA introduces a significant decrease of STM contrast in the region of adsorption, which suggests that the electronic structure properties of graphene can be locally modified by weakly adsorbed species. In fact, the perturbation of the graphene states is nearly limited to the first carbon atoms in the vicinity of TMA. The STM profiles (bottom panels) reveal a more quantitative impact of the TMA adsorption on the local density of states (LDOS) of graphene. Along the lines A and B, a very regular pattern of the corrugation of the LDOS is obtained for pristine graphene. For TMA/graphene complexes, a depletion of LDOS is observed in the region where the TMA molecule is adsorbed, and this LDOS depletion appears more prononced for the top (a) and cross (b) sites. The improved LDOS depletion for the top and cross sites also supports an improving state mixing between TMA and graphene, as well as a larger band gap opening for these two sites. 2. Adsorption of a Self-Assembled TMA Layer. As shown in the previous section, the adsorption of a single TMA molecule may introduce significant variations of the electronic

Figure 9. Supercell used to model the formation of a self-assembled monolayer of TMA on graphene following (a) chicken-wire and (b) super-flower network structure.

adsorbate and 112 carbon atoms for graphene (Figure 9a), while the super-flower network model contains 63 atoms (27 C, 18 O, and 18 H) for TMA and 128 C atoms for graphene (see Figure 9b). In order to evaluate the influence of structural boundary condition constraints on the formation of the TMA networks, we have also considered the adsorption of an isolated dimer and trimer by increasing the size of the graphene substrate from 112 to 260 C for the dimer and from to 128 to 280 C atoms for the trimer; such a large substrate allows us to obtain a relevant optimized structure and to minimize the effect of neighboring cells. The first interesting property to compare is the adsorption energy (Eads) that corresponds, for the TMA monolayer, to the energy involved in the adsorbate−surface interaction (Esurf) and in the formation of the assembly through hydrogen bonding (EHB).56 Table 2 reports the adsorption energy per TMA molecule for the formation of a 2D network including the net Mulliken charges acquired per TMA unit. As mentioned, this Table 2. Adsorption Energy (Eads) Decomposed into the Contribution from Surface (Esurf) and from Hydrogen Bonding (EHB) and Net Mulliken Charge of Assembled TMA on Graphenea network structure Eads (eV/TMA) Esurf (eV/TMA) EHB (eV/H-bond) net charge (|e|/TMA)

chicken wire

flower-like

−2.19 (−2.47) −1.30 −0.89 −0.12

−2.03 (−2.44) −1.18 −0.85 −0.11

a

The values obtained for an isolated dimer and trimer are given in parentheses. All values are given per TMA molecule except EHB, which is given per hydrogen bond.

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adsorption energy is a combination of stabilization energy induced by the substrate and by the formation of hydrogen bonds between TMA units. A similar approach was also used by Iancu et al.20 to describe the formation of TMA networks on the Au(111) surface. The contribution from the surface (Esurf) can be directly compared with the energies reported in Table 1 for the adsorption of a single TMA molecule. In the network, the surface contributes −1.30 and −1.18 eV per TMA molecule to stabilize, respectively, the chicken-wire and the super-flower structure. These values are more consistent with the low TMA deformation sites (center, bridge) than for top and cross sites. The higher stability observed for an isolated dimer and trimer reflects the propensity of TMA to establish a hydrogen bonding network and also shows that these small molecular aggregates have an improved freedom to find more stable adsorption sites. It is also interesting to note that the hydrogen bonds (EHB) formed in the chicken-wire structure (−0.89 eV) are slightly stronger than those in the super-flower structure (−0.85 eV), which is probably due to a more significant energy strained in the TMA triad. This could also be a reason why this superflower structure has not been observed yet on a graphitic substrate.22 Consequently, both chicken-wire and super-flower networks give adsorption energies (Eads) in the range of a single TMA molecule adsorption. This clearly shows that a weakening of the adsorbate−surface interaction is largely compensated by the formation of a stable TMA network through hydrogen bonding. Adsorption energies obtained for an isolated dimer and trimer given in parentheses in Table 2 give an estimation of the influence of rigid boundary conditions on the final energy. The values obtained for nearly free dimers and trimers are not drastically different than those for the constrained molecules in the networks, and the energy variations are within the range of variations calculated for the different adsorption sites. The electronic properties of assembled TMA are also consistent with the results obtained for the adsorption of a single TMA molecule but where the formation of highly directional hydrogen bonds makes the molecule flatter in the assembly and reduced its interaction with graphene. From Table 2, the net charge on TMA in both network structures is essentially the same, an accumulation of 0.12|e| by the TMA unit. This value is also in the same range as those reported at Table 1 for top and cross sites in which the deformation of COOH groups was more important. In contrast, TMA remains quite planar within the networks, and the formation of directional intermolecular hydrogen bonds should decrease both the donation and the back-donation abilities of the COOH groups. Hence, the individual TMA−graphene interaction is weaker in the network than in single molecule adsorption. Although the magnitude of charge transfer calculated for the TMA adlayer is nearly equal to the sum of individual TMA contribution, the band gap opening does not follow a similar pattern for TMA networks. Figure 10 shows that the band gap strongly depends on the number of TMA molecules on the graphene surface. A gap opening of nearly 400 meV can even be obtained for isolated aggregates of TMA. The impact of the isolated aggregates is more significant because a good fraction of the terminal COOH groups can more freely interact with the surface. Nevertheless, the nonlinear trend observed with the number of TMA on the surface is quite similar for both isolated or networked TMA molecules. Since these two TMA networks show similar charge transfer, adsorption energy, and adlayer geometry over graphene, the large variation in band gap

Figure 10. Variation of the band gap of graphene along the adsorption of TMA molecules into isolated aggregates or forming a network.

opening calculated from chicken-wire to super-flower structures should have a different origin. One plausible explanation takes into account that TMA has different arrangements in these networks and where this variation of patterned TMA adsorption on graphene introduces a symmetry breaking of the π-states near the Fermi energy.46,58 We believe that the mechanism of the larger band gap opening is similar to the one propose by Chang et al.46 based on symmetry breaking but where we propose it is induced by an improved mixing of states with graphene that significantly affects the π-bands around the Fermi level. This suggestion is supported by the resulting band structure (not shown) where the degeneracy of the π-bands near the Fermi energy is partially maintained for the chickenwire but is totally lifted for the super-flower network. This result suggests that the band gap of graphene could be controlled by the chemical nature of the adsorbate and by the geometry adopted by the molecular self-assembly. In contrast to the single TMA adsorption, the analysis of simulated STM images of TMA networks does not provide any additional information on the adsorption process or the band gap opening that would be beneficial to show.

IV. CONCLUSIONS We have studied the adsorption of trimesic acid on graphene with the help of density functional theory from a single molecule to a more complete self-assembled monolayer. We showed that structural deformations of COOH groups from TMA can play a major role on the modulation of electronic properties (charge transfer, band gap) of graphene. The structural deformations of the COOH groups depend on the adsorption site and hence affect their abilities for electron donation and back-donation. The formation of a TMA adlayer decreases the magnitude of TMA deformation, and their adhesion on graphene becomes weaker. The stability of the selfassembled TMA network on graphene is largely compensated by the formation of multiple highly directional hydrogen bonds between TMA units. Our DFT results on assembled TMA on graphene suggest that the magnitude of p-doping of graphene depends on the number of TMA molecules on the substrate, and the band gap opening more strongly depends on the patterned adsorption on the surface, which suggests that polymorphism of an assembled layer could be used to modulate the electronic properties of graphene. It would be also quite interesting to validate this mechanism over a more extensive series of substrates. 9714


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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Ministère du Développement Économique, de l’Innovation et de l’Exportation (MDEIE) through the PSR-SIIRI program. Computational resources were provided by Calcul Québec and Compute Canada.

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