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Strong N-doped Graphene: The Case of 4-(1,3-dimethyl-2,3dihydro-1 H-benzoimidazol-2-yl)phenyl)dimethylamine (N-DMBI) Pablo A. Denis, and Federico Iribarne J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b01683 • Publication Date (Web): 08 Jun 2015 Downloaded from http://pubs.acs.org on June 15, 2015

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The Journal of Physical Chemistry

Strong N-doped Graphene: The Case of 4-(1,3-dimethyl-2,3-dihydro-1 Hbenzoimidazol-2-yl)phenyl)dimethylamine (N-DMBI) Pablo A. Denis* and Federico Iribarne a- Computational Nanotechnology, DETEMA, Facultad de Química, UDELAR, CC 1157, 11800 Montevideo, Uruguay. * e-mail: [email protected] Tel: 0059899714280, Fax: 00589229241906

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Abstract By means of first principle calculations, we investigated the effects of the adsorption of the strong ntype dopant 4-(1,3-dimethyl-2,3-dihydro-1 H-benzoimidazol-2-yl)phenyl)dimethylamine (N-DMBI) onto graphene. The adsorption of radical N-DMBI (R-N-DMBI) occurs with large adsorption energy (Eads) of 44.4 kcal/mol and induces a metallic character on graphene by shifting the Dirac point 0.6 eV below the Fermi level. The charge received by graphene can be almost completely removed and thus the metallic character may be inhibited, if 2,3,5,6-tetrafluoro-7,7,8,8-tetra-cyanoquinodimethane (F4TCNQ) is simultaneously adsorbed on any side of graphene. In effect, by doing so, the semimetallic character is recovered or a tiny gap is opened. When F4-TCNQ and radical N-DMBI are adsorbed on the same side, the molecules are strongly held together thanks to the hydrogen bond formed between the fluorine atoms of F4-TCNQ and the H of R-N-DMBI. In this case, the Eads is over 120 kcal/mol, per F4TCNQ/R-N-DMBI pair. When the latter two molecules are adsorbed on opposite sides, the Eads is 87.6 kcal/mol. This value is more than 30 kcal/mol smaller than that computed when they are adsorbed on the same side, even though it is nearly 10 kcal/mol larger than the sum of the Eads of isolated F4-TCNQ and R-N-DMBI. In this case, graphene behaves as a medium which transfers electrons from R-N-DMBI to F4-TCNQ, without having the molecules in direct contact. We see a strong synergic interaction that increases the Eads when an electron donating molecule is combined with an electron withdrawing one. RN-DMBI has a profound influence on the reactivity of graphene. Bond energies between the graphene surface and H, F and OH radicals increased when R-N-DMBI is adsorbed, but the addition of these radicals destroys the metallic character of the graphene-R-N-DMBI complex and band gaps in the range of 0.2-0.4 eV are opened. These results show that the radical N-DMBI alters the chemistry and electronic properties of graphene to an extent which rivals the most popular molecules and substrates used in the non covalent chemistry of graphene. Keywords: carbon nanomaterials, adsorption, graphene, density functional calculations, doping.

1. Introduction ACS Paragon Plus Environment

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Journal of Physical Chemistry Molecules with strong electronThe withdrawing or donating characteristics have been used to modify

the electronic structure1-19 and chemical reactivity of graphene.20 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) is one of the first examples that can be found in the literature of a chemical species that induces p-type doping of graphene.1 The interaction between F4-TCNQ and graphene has been well characterized by experimental and theoretical techniques.1-5,21 In particular, it was shown that the n-type doping of graphene induced by the substrate SiC(0001) can be compensated by the strong electron acceptor character of F4-TCNQ.2 By means of first principle calculations, Pinto et al.3 showed that 0.3 e- are transferred from graphene to F4-TCNQ. In the same vein, the related electron acceptors tetracyanoethylene (TCNE) and tetracyanoquinodimethane (TCNQ) have also been used to p-type dope graphene.5-11 Particularly, the coverage with TCNE molecules was able to control the carrier concentration and band gap.6 Substitutional doping with N or B can be used to enhance the interaction between graphene and TCNE.11 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ),12 has been reported to accept more e- than F4-TCNQ. In effect, Zhang et al.,12 reported that 0.51 e- are donated by graphene to DDQ. This molecule gives metallic properties to graphene, even though the modifications in the electronic structure are not enough for sensing applications. Electron donating molecules are used to induce n-type doping of graphene. Among them, we can highlight tetrathiafulvalene (TTF),8-10-12-14 perylene-3,4,9,10-tetracarboxylic-3,4,9,10-diimide (PTCDI) and

perylene-3,4,9,10-tetracarboxylic-

3,4,9,10-dianhydride (PTCDA),14-15 which interact very strongly with graphene via π stacking, and in some cases can open a 0.1 eV band gap in its electronic structure.14 Although the optical band gap is small, these molecules modify the carrier type of graphene.14 Notwithstanding the fact that n or p-type doping is a useful approach to produce modified graphenes with tailor made properties, a significantly higher degree of adjustment can be obtained if both options are included simultaneously. Park et al.16 reported that a tunable band gap can be opened when bilayer graphene is over a NH2-functionalied monolayer and F4-TCNQ is adsorbed on the top layer. The estimated optical band gap was between 160 and 220 meV. The single gate field effect transistors constructed using this nanocomposite showed a high on/off current ratios. A similar approach


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amorphous SiO2 to induce a p-type doping and N,N-dimethyl-p-phenylenediamine (DMPD) or TCNE to achieve a n-type doping. The combination of both dopants increased the band gap from 106 to 253 meV when using DMPD and from 98 to 211 meV for TCNE. Also, the carrier mobility remained high. In the second approach, a larger gap was obtained for bilayer graphene, but in this case the n-type dopant was decamethylcobaltocene (DMC). Recently, the strong electron donor molecule 4-(1,3-dimethyl-2,3-dihydro-1 H-benzoimidazol-2yl)phenyl)dimethylamine (N-DMBI) was thermal annealed onto graphene20 and the radical of N-DMBI (R-N-DMBI) was adsorbed onto it. As a consequence of the adsorption of R-N-DMBI, Xu et al.20 found very interesting modifications of the electronic properties of graphene. For instance, graphene was heavily n-doped and the Dirac point was at the negative voltage of -140 V, while the electron mobility increased. Interestingly, for practical applications, stable electrical properties were observed for more than one month. In previous investigations, we have studied the alteration of the reactivity of graphene when different species are adsorbed. We considered the adsorption of 2,3,5,6-tetrafluoro-7,7,8,8tetracyanoquinodimethane (F4-TCNQ, electron acceptor) and tetrathiafulvalene (TTF, electron donor) and lithium atom.22 In all cases, we found that the reactivity is enhanced but the effect of lithium is higher than the one found for F4-TCNQ or TTF. These results are also supported by experimental23 and theoretical works24-27 that studied the consequences of electron transfer in graphene nanocomposites. Taking into consideration that R-N-DMBI is one of the best n-type dopants available, we decided to perform a theoretical investigation on the electronic properties and reactivity of graphene when R-NDMBI is adsorbed. In addition, we also studied the effect of the simultaneous adsorption of R-N-DMBI and F4-TCNQ onto graphene. Our results confirm the experimental findings by Xu et al,20 but also evidence a tremendous increase in the reactivity of graphene towards the adsorption of free radicals, being much stronger than those found for F4-TCNQ or TTF. Finally, we show that the adsorption of F4TCNQ quenches the n-type doping induced by R-N-DMBI. We expect that this work can stimulate new investigations about the chemistry and electronic properties of non covalently modified graphene.


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2. Methods

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We carried out density functional theory (DFT) calculations as implemented in Gaussian 200928 and SIESTA.29-30 For the M06-L31-32 exchange correlation functional and the screened HSEH1PBE33-34 functional, we performed geometry optimizations with Gaussian, using 1000 k-points (Monkhorst-Pack sampling) and the ultrafine (99,590 point) grid. The basis set selected was the 6-31G*.35 Basis set superposition error (BSSE) was not considered because we have showed that the omission of BSSE cancels the underestimation of dispersion interaction found with M06-L and M06-2X.36-38 To simulate infinite graphene sheets, we selected 7×7 and 8×8 supercells comprising 72 and 98 atoms, respectively. We optimized the unit cells along the a and b axes while the c axis was large enough to prevent interaction between adjacent sheets (25Å). The spin polarized VDW-DF39 calculations were undertaken with SIESTA which performs selfconsistent field (SCF) calculations using numerical basis sets. For comparative purposes selective calculations were performed using a fixed spin. We selected the double-zeta basis set29-30 with polarization functions and fixed the orbital confining cut-off to 0.01 Ry. The split norm used was the default value of 0.15; this value defines sensible default radii for the split-valence type of basis. The DFT implementation in SIESTA can be prone to significant basis set superposition error (BSSE), even with relatively low degree of radial confinement.40 To avoid this problem, we used the counterpoise correction suggested by Boys and Bernardi.41 We utilized relaxed structures to estimate the BSSE corrected binding energies and took monomer deformation energies into account. The interaction between ionic cores and valence electrons was described by the Troullier−Martins norm conserving pseudopotentials.42 Geometry optimizations were carried out using the conjugate gradient algorithm until all residual forces were smaller than 0.01 eV/Å. The unit cells were optimized and they were sampled using a 40×40×1 (about 900 k-points gamma centered) Monkhorst-Pack sampling,43 similar to the one used in our previous work about graphene interacting with F4-TCNQ and TTF.21 A small broadening of 0.005 was used to plot the density of states, and band structures were plotted along the Γ−Μ−Κ−Γ path using ACS Paragon Plus Environment

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50 points for each direction (Γ(0,0,0) Μ(0.5,0.0) Κ(0.33,0.33,0.0). The adsorption energies of the 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

molecules considered were calculated as: Eads = E(graphene) + E(adsorbed-molecule) - E(graphenemolecule-ads).

3. Results and Discussion 3.1 Adsorption of N-DMBI and R-N-DMBI onto graphene: Xu et al.20 showed that the N-DMBI dopant is not useful unless it loses a hydrogen atom, something that can be achieved by thermal annealing. This is not an unfeasible scenario given that the C-H bond energy is 78.4 kcal/mol, about 27 kcal/mol lower than the C-H bond energy in methane, at the M06-L/6-31G* level. The carbon atom that loses an electron has sp3 hybridization and is the one which connects the pentagonal and benzene rings. Once this atom is lost the molecule adopts a significantly more planar structure which facilitates pi stacking with graphene. In Figure 1a, we present the structure of the graphene-N-DMBI complex and in Table 1, the adsorption energies (Eads). One of the hexagonal rings of N-DMBI tries to adopt a position parallel to graphene, while the other ring is perpendicular to it. The interaction between graphene and NDMBI is dominated by five CH:::π (H atom over the hexagonal ring) interactions and the π-stacking with one hexagon of N-DMBI. As can be seen in Figure 1a, five H atoms point to the graphene plane, being placed 2.2-2.4Å above it. As regards the benzene ring parallel to the surface, it is 3.3-3.4 Å above the sheet, within the range of distances typically observed for stacked aromatic molecules. For 7×7 and 8×8 graphene unit cells, the Eads are 15.7 and 16.0 kcal/mol, at the M06-L/6-31G* level. Thus, we cannot expect huge changes if the size of the graphene model is increased. The Eads energy computed at the VDW-DF/DZP level for a 7×7 graphene unit cell is 19.3 kcal/mol, in agreement with the M06-L/631G* value. As for the effect of N-DMBI on graphene, we found that charge transfer is almost inexistent, since 0.03 e- are transferred from graphene to N-DMBI. The band structure and density of states (DOS) are presented in Figure 1b. The adsorption of N-DMBI breaks the degeneracy of the π and π* bands at the k-point. The conduction band of graphene touches the Fermi level at the k-point but the valence band now is located 0.09 eV below. However, a gap is not opened since there is an occupied state at the Fermi level belonging to N-DMBI which maintains the semimetallic character of graphene as ACS Paragon Plus Environment

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The Journal of Physical Chemistry can be seen in Figure 1b. This finding is supported by the M06-L/6-31G* results which also indicated

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that when N-DMBI is adsorbed, the system remains with a zero band gap. When N-DMBI loses an H atom forming R-N-DMBI, the picture is completely modified. The structure of this system is presented in Figure 2a. In first place, the Eads is dramatically increased to 39.5, 40.6 and 42.1 kcal/mol, for 7×7, 8×8 and 9×9 graphene unit cells, at the M06-L/6-31G* level, respectively. In the same vein, the Eads is 44.4 kcal/mol, at the VDW-DF/DZP level, for a 7×7 unit cell, supporting the enhancement observed at the M06-L/6-31G* level. This change in the Eads is accompanied by a charge transfer from R-N-DMBI to graphene of 0.69 e-, as indicated by Mulliken analysis at the M06-L/6-31G* level. The amount of charge transfer observed from R-N-DMBI is one of the largest that has been reported for an adsorbed molecule and it can be related to the low ionization potential of 3.74 eV and small electron affinity at 0.22 eV of R-N-DMBI , both values computed, at the M06-2X/6-311++G(3df,3pd) level. For example, the strong electron withdrawing molecule F4-TCNQ accepts 0.40 e- from graphene and even when F4-TCNQ and TTF are simultaneously adsorbed, 0.47 eare transferred to F4-TCNQ.21 We also studied the adsorption of two R-N-DMBI molecules onto graphene, one on each side of the sheet. For this system, each adsorbed molecule donates 0.71 e- to graphene and thus the total charge accepted by graphene is 1.42 e- for a 7×7 unit cell. This value is twice the charge transfer observed when one R-N-DMBI molecule is adsorbed. Eads is 40.4 kcal/mol per R-NDMBI molecule, nearly the same as calculated above for the graphene-R-N-DMBI complex. Finally, we investigated the interaction between R-N-DMBI and 555-777 defective graphene. The reason for selecting this type of defective graphene is that it can act as a p-type semiconductor.44 Interestingly, we found that the Eads at the M06-L/6-31G* level is 57.4 kcal/mol for a 7×7 555-777 defective graphene unit cell. This value is 16.0 kcal/mol larger than the value computed for perfect graphene and confirms the strong effect that defects have on the interaction between graphene and adsorbed molecules as recently shown by Li et al.45 for 585 defective graphene. The increase in Eads observed for the adsorption of R-DMBI onto 555-777 defective graphene is accompanied by a larger charge transfer. In this sense,


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spin densities revealed that 1.0 e- is donated by R-N-DMBI. In relation to the electronic structure of the system, we found conflicting results between Siesta (VDW-DF) and Gaussian (M06-L) because the former predicted a metallic character and M06-L a halfmetallic behavior. We investigated the source of the discrepancy in detail and found that the reason behind the inconsistency was that Gaussian performs, by default, a calculation with a fixed magnetic moment, maintaining an unpaired electron. On the contrary, Siesta allows the magnetic moment to vary and at the end of the optimization the net magnetic moment at the VDW-DF/DZP level is zero since Qup = Qdown= 247.5. To confirm this hypothesis, we performed a VDW-DF geometry optimization with a fixed spin. As expected, the outcome was similar to the one obtained with Gaussian, thus the halfmetallic behavior was verified. Interestingly, the difference in energy between the spin free and spin fixed calculations performed at the VDW-DF/DZP level is very small: 0.12 eV. In Figure 2b, the band structure and DOS are presented. For the VDW-DF/DZP calculation with variable magnetic moment only the spin up bands and DOS are presented since they are identical to the spin down. In this case, the Dirac cone moves 0.60 eV below the Fermi level as expected for n-type doped graphene. The system displays metallic character as the DOS is non zero at the Fermi level. Finally, when two R-N-DMBI molecules are adsorbed, we also found that the graphene sheet turns into metallic at the M06-L/6-31G*, HSEH1PBE/6-31G* and VDW-DF/DZP levels. 3.2 Dual doping of graphene with R-N-DMBI and F4-TCNQ: taking into consideration the results obtained in the previous section, we were interested in measuring the effect of attaching a strong electron acceptor to the 7×7 graphene-R-N-DMBI complex. To this end we considered one F4-TCNQ molecule adsorbed: a) on the opposite side of R-N-DMBI and b) on the same side. We recall that F4TCNQ is a strong e- acceptor,3,21 which is adsorbed with a Eads of 32.1 kcal/mol, onto a 7×7 graphene unit cell, at the M06-L/6-31G* level. In Figure 3a we present the optimized structure of the F4-TCNQgraphene-R-NDMBI complex, its band structure and density of states in Figure 3b. Both molecules are adsorbed with an Eads of 85.6 kcal/mol, at the M06-L/6-31G* level. This value is 14 kcal/mol larger that


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The Journal of Physical Chemistry the sum of the Eads calculated for the separated graphene-F4-TCNQ and graphene-R-N-DMBI

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complexes. Therefore, as we observed for the F4-TCNQ-graphene-TTF complex, there is a synergic effect which increases the Eads when electron donor and electron acceptor molecules are combined. Yet, it is important to note that in the F4-TCNQ-graphene-TTF complex, the synergic effect is much weaker given that the Eads is 1.1 kcal/mol larger than the sum of the separated Eads.21 The analysis of the electronic properties is challenging. The band structure presented in Figure 3b shows that for the spin up channel, a small gap of 0.047 eV is opened at the VDW-DF/DZP level, while for the spin down channel a weak metallic character is observed since the Dirac cone is 0.1 eV below the Fermi level. As it is well known that most density functionals underestimate band gaps, we evaluated the electronic properties of this system employing the HSEH1PBE method, which has been developed to accurately predict band gaps.33-34 At the HSEH1PBE/6-31G* level we found that for the spin up channel the gap becomes zero when 500 k-points gamma centered are used. However, for the spin down channel the convergence with respect to the number of k-points was slower. For 1000 and 2000 k-points gamma centered the band gaps were 0.04 and 0.03 eV, respectively. On the basis of the HSEH1PBE results, we consider that the adsorption of one F4-TCNQ molecule restores the semimetallic character of graphene and all the charge transferred to graphene by R-N-DMBI is now located on F4-TCNQ. Moreover, the presence of F4TCNQ increases the charge donated by R-N-DMBI from 0.67 to 0.83 e-. Analysis of Mulliken spin densities support this result since the spin density located in F4-TCNQ is 0.87. It is worth mentioning that in the absence of graphene, when R-N-DMBI and F4-TCNQ interact, Mulliken spin densities revealed that 1.0 e- is transferred from R-DMBI to F4-TCNQ, at the M06-L/6-31G* level. This situation contrasts with the results obtained by Nistor et al.46 which proved that for intercalates like SbCl5 and HNO3 graphene works as a charge reservoir donating electrons to the adsorbed species and facilitating their dissociation.46 In the second case assayed (b), we considered the adsorption of F4-TCNQ on the same side of R-N-DMBI. The reason for doing so is that in the gas phase these two molecules interact via hydrogen bonding with interaction energy of 63.1 kcal/mol. Here we employed the 7×7 and 6×7 unit cells. In both cases we observed that the hydrogen atoms of R-N-DMBI and the fluorine of F4-TCNQ


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kcal/mol per F4-TCNQ/R-N-DMBI pair, respectively. These values are more than 30 kcal/mol above the Eads computed for the adsorption on opposite sides. Also, it is important to remark that the hydrogen bond between F4-TCNQ and R-DMBI is weakened to a lesser extent (with respect to the gas phase situation) when these two molecules are supported over graphene. In effect, the Eads determined for the 7×7 unit cell is only 14.5 kcal/mol smaller than the sum of the Eads of F4-TCNQ (32.1 kcal/mol) , R-NDMBI (39.5 kcal/mol) and the hydrogen bond energy of the F4-TCNQ/R-N-DMBI pair (63.1 kcal/mol). The structure of the complex is presented in Figure 4a and the band structure in Figure 4b. Finally, as regards the electronic structure of the system we found that in contrast with the results obtained for a), a small gap is opened at the VDW-DF/DZP level, when both molecules are adsorbed on the same side. For the spin up channel the gap is 0.15 eV while for the spin down is a bit smaller, 0.03 eV. This finding is supported by HSEH1PBE/6-31G* calculations. 3.3 Reactivity of graphene with radical N-DMBI (R-N-DMBI) adsorbed: in previous investigations we showed that the adsorption electron donating molecules increase the reactivity of graphene as it becomes electron or hole doped.21 Considering that R-N-DMBI donates 0.69 e- to graphene and that it is one of the highest values reported for charge transfer between this material and an adsorbed molecule, we can expect a significant enhancement of reactivity. To test this hypothesis, we considered the addition of H, OH and F onto graphene doped with N-DMBI. First, we studied the addition of hydrogen to the two carbon atoms bearing the largest spin densities as indicated in Figure 5a; these are C1 and C2. The structure of the graphene sheet functionalized with a hydrogen atom and one R-N-DMBI adsorbed molecule is presented in Figure 5b. At the M06-L/6-31G* level, we found that the C-H binding energies for both carbon atoms are similar: 26.7 and 25.9 kcal/mol, for C1 and C2, respectively. These values are nearly 10 kcal/mol larger than the C-H binding energy determined for pristine graphene at the same level, which is 16.7 kcal/mol. The effect produced by the donation of 0.69 e- from R-N-DMBI to graphene is an increase of the C-H binding energy of 10.4 kcal/mol (0.45 eV). We note that the donation of 0.69 e- corresponds to a charge density of 3.1×1013 e-/cm2 which is accepted by graphene. The latter


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Journal of Physical value, is larger than that reported byThe Huang et al.25 using Chemistry artificial charges. At the PBE level, these

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authors found that for a doping charge of 5.0×1013 cm-2, the C-H binding energy increases less than 0.2 eV. Yet, enhancement of reactivity is comparable to what we found for Li-doped graphene. For the sake of completeness we computed the C-H binding energy for C3 which is a carbon atom located away from the absorption site, about 13 and 14 Å from C1 and C2. At the M06-L/6-31G* level, we found that the presence of R-N-DMBI increases the C-H binding energy by 7.5 kcal/mol. Although the latter value is 2.9 kcal/mol smaller than the increase observed for C1, it represents an augmentation of 45% with respect to the bare sheet, showing that the effect of R-N-DMBI is long ranged. In the case of HO and F, when they are placed on the opposite side to R-N-DMBI, the binding energies for the addition onto C1 are 26.7 and 47.9 kcal/mol, respectively, at the M06-L/6-31G* level. These values are 15.7 and 17.6 kcal/mol larger than those determined for the addition of HO and F onto pristine graphene, respectively. Thus, an astonishing increase of reactivity is obtained when R-N-DMBI interacts with graphene. This finding is supported by the results calculated at the VDW-DF/DZP level, which are listed in Table 2. The C-F and C-OH bond distances are 1.491 and 1.475 Å, about 0.005 Å longer than those found when R-N-DMBI is not present. R-N-DMBI is much more effective than F4-TCNQ and TTF in augmenting the reactivity of graphene since these two molecules increase the C-OH and C-F binding energies by 2-3 kcal/mol, at the same level.21 The reason for such dramatic difference maybe not only related to the larger change transfer that exists for R-N-DMBI but to the fact that R-N-DMBI is a free radical. Therefore, when another radical is attached to graphene, two electrons become paired and the energy of the system becomes stabilized. This statement is supported by the Mulliken charge analysis. In particular, the charge donated by R-N-DMBI climbs from 0.69 e- to 0.84 e- when H, OH or F are attached to graphene. Besides, it is important to remember that the large electronegativity of the adsorbed radicals also contributes to an enhancement of the charge transfer from R-N-DMBI to graphene. In this way, R-N-DMBI loses almost one electron when a radical is attached to graphene and thus it becomes a closed-shell molecule (no unpaired electrons). Regarding the impact of the covalent functionalization on the electronic properties of the graphene-R-N-DMBI complex, the addition of H,


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The Journal of Physical Chemistry Page 12 of 27 OH and F opens band gaps of 0.38, 0.24 and 0.20 eV, at the VDW-DF/DZP level, respectively. The use 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

of the more accurate HSEH1PBE method produced negligible changes in the computed band gaps; the estimated values being 0.48, 0.35 and 0.30 eV for H, OH and F, respectively. The band structures computed at the VDW-DF/DZP level are presented in Figure 6a, 6b and 6c. In the case of hydrogen, the adsorption of R-N-DMBI does not significantly alter the electronic properties as the size of the bands gap is maintained, the major change being the appearance of an unoccupied band belonging to R-NDMBI. On the contrary, important variations occur for OH and F. In effect, graphene functionalized with fluorine and hydroxyl radicals exhibit metallic properties (as can be seen in Figure 6). The metallic character is not surprising. Wang et al.47 also observed metallic properties in fluorinated graphene. However, this character disappears when R-N-DMBI is adsorbed and a band gap is opened. In both cases, the adsorption of the electron donor molecule shifts the Fermi level in the gap opened at the K point.

4. Conclusions We have applied the M06-L and VDW-DF density functionals to study the adsorption of 4-(1,3dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl)dimethylamine (N-DMBI) and its neutral radical onto graphene. The following are the most relevant findings of the work: 1- R-N-DMBI interacts very strongly with graphene as evidenced by adsorption energy of 44.4 kcal/mol, at the VDW-DF/DZP level. Also, this molecule donates 0.69 e- to graphene, shifting the Dirac point 0.6 eV below the Fermi level and the doped sheet becomes metallic. 2- The n-type doping of graphene induced by R-N-DMBI can be cancelled if a strong electron acceptor molecule

is

simultaneously

adsorbed.

We

found

that

2,3,5,6-tetrafluoro-7,7,8,8-tetra-

cyanoquinodimethane (F4-TCNQ) recovers the semimetallic character of graphene or a tiny gap is opened. When F4-TCNQ and R-N-DMBI are adsorbed on opposite sides, graphene acts as medium which transfers electrons from R-N-DMBI to F4-TCNQ without having the molecules in direct contact. 3- If F4-TCNQ and the R-N-DMBI are adsorbed on the same side, strong hydrogen bonds are formed between the fluorine atoms of F4-TCNQ and the hydrogen atoms of R-N-DMBI. In this case, the Eads is


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The Journal of Physical Chemistry over 120 kcal/mol per F4-TCNQ/R-N-DMBI pair. For the adsorption on opposite sides, the Eads is 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

kcal/mol smaller than the one computed when they are adsorbed on the same side, but it is 10 kcal/mol larger than the sum of the Eads of isolated F4-TCNQ and R-N-DMBI. 4- R-N-DMBI strongly influences the covalent chemistry of graphene. In the case of the addition of H, OH and F, the C-H, C-OH and C-F bond energies are increased by 10.4, 15.7 and 17.4 kcal/mol, at the M06-L/6-31G* level, respectively, upon adsorption of R-N-DMBI. The addition of these radicals turns the metallic graphene-R-N-DMBI complex into a small gap semiconductor (0.2-0.4 eV). Acknowledgments The authors thank PEDECIBA Quimica, ANII and CSIC for financial support. References 1) Chen, W.; Chen, S.; Qi, D. C.; Gao, X. Y.; Wee, A. T. S. Surface Transfer p-Type Doping of Epitaxial Graphene. J. Am. Chem. Soc. 2007, 129, 10418-10422. 2) Coletti, C.; Riedl, C.; Lee, D.S.; Krauss, B.; Patthey, L.; von Klitzing, K.; Smet, J.H.; Starke, U. Band Structure Engineering of Epitaxial Graphene on SiC by Molecular Doping. Phys. Rev. B 2010, 81, 235401. 3) Pinto, H.; Jones, R.; Goss, J. P.; Briddon, P. R. p-type Doping of Graphene With F4-TCNQ. J. Phys.: Condens. Matter. 2009, 21, 402001. 4) Barja, S.; Garnica, M.; Hinarejos, J. J.; Vazquez de Parga, A. L.; Martın, N.; Miranda, R. Selforganization of Electron Acceptor Molecules on Graphene. Chem. Commun. 2010, 46, 81988200. 5) Chi, M.; Zhao, Y.-P. First Principle Study of the Interaction and Charge Transfer Between Graphene and Organic Molecules. Comput. Mater. Sci. 2012, 56, 79-84. 6) Lu, Y. H.; Chen, W.; Feng, Y. P.; He, P. M. Tuning the Electronic Structure of Graphene by an Organic Molecule. J. Phys. Chem. B 2009, 113, 2-5. 7) Chen, L.; Wang, L.; Shuai, Z.; Beljonne, D. Energy Level Alignment and Charge Carrier Mobility in Noncovalently Functionalized Graphene. J. Phys. Chem. Lett. 2013, 4, 2158−2165.


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The Journal of Physical Chemistry Page 14 of 27 8) Varghese, N.; Ghosh, A.; Voggu, R.; Ghosh, S.; Rao, C. N. R. Selectivity in the Interaction of 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Electron Donor and Acceptor Molecules with Graphene and Single-Walled Carbon Nanotubes. J. Phys. Chem. C 2009, 113, 16855-16859. 9) Subrahmanyam, K.S.; Voggu, R.; Govindaraj, A.; Rao, C.N.R. A Comparative Raman Study of the Interaction of Electron Donor and Acceptor Molecules with Graphene Prepared by Different Methods. Chem. Phys. Lett. 2009, 472, 96-98. 10) Manna, A. K.; Pati, S. K. Tuning the Electronic Structure of Graphene by Molecular Charge Transfer: A Computational Study. Chem. Asian J. 2009, 4, 855-860. 11) Yong, Y.; Song, B.; He, P. Stability and Magnetism of Tetracyanoethylene Adsorbed on Substitutionally Doped Graphene. J. Appl. Phys. 2012, 111, 083713. 12) Zhang, Y.-H.; Zhou, K.-G.; Xie, K.-F.; Zeng, J.; Zhang, H.-L.; Peng, Y. Tuning The Electronic Structure and Transport Properties of Graphene by Noncovalent Functionalization: Effects of Organic Donor, Acceptor and Metal Atoms. Nanotechnology 2010, 21, 065201 13) Kaminska, I.; Das, M. R.; Coffinier, Y.; Niedziolka-Jonsson, J.; Woisel, P.; Opallo, M.; Szunerits, S.; Boukherroub, R. Preparation of Graphene/Tetrathiafulvalene Nanocomposite Switchable Surfaces. Chem. Commun. 2012, 48, 1221-1223. 14) Zhang, Z.; Huang, H.; Yang, X.; Zang, L. Tailoring Electronic Properties of Graphene by ππ Stacking with Aromatic Molecules. J. Phys. Chem. Lett. 2011, 2, 2897-2905. 15) Lauffer, P.; Emtsev, K. V.; Graupner, R.; Seyller, T.; Ley, L. Molecular and Electronic Structure of PTCDA on Bilayer Graphene on SiC(0001) Studied With Scanning Tunneling Microscopy. Phys. Status Solidi B 2008, 245, 2064–2067. 16) Park, J.; Jo, S. B.; Yu, Y.-J.; Kim, Y.; Yang, J.W.; Lee, W.H.; Kim, H.H.; Hong, B.H.; Kim, P.; Cho, K.; et al. Bandgap Opening of Bilayer Graphene by Dual Molecular Doping. Adv. Mater. 2012, 24, 407-411. 17) Wang, T. H.; Zhu, Y. F.; Jiang, Q. Bandgap Opening of Bilayer Graphene by Dual Doping from Organic Molecule and Substrate. J. Phys. Chem. C, 2013, 117, 12873–12881.


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The Journal of Physical Chemistry 18) Wang, T.H.; Zhu, Y.F.; Jiang, Q. Towards Single-gate Field Effect Transistor Utilizing Dual-

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Doped Bilayer Graphene. Carbon, 2014, 77, 431. 19) de Oliveira, I.S.S.; Miwa, R.H. Organic Molecules Deposited on Graphene: A Computational Investigation of Self-assembly and Electronic Structure. J. Chem. Phys., 2015, 142, 044301. 20) Xu, W.; Lim, T.-S.; Seo, H.-K.; Min, S.-Y.; Cho, H.; Park, M.-H.; Kim, Y.-H.; Lee, T.-W. NDoped Graphene Field-Effect Transistors with Enhanced Electron Mobility and Air-Stability. Small, 2014, 10, 1999. 21) Denis, P.A.; Chemical Reactivity of Electron-Doped and Hole-Doped Graphene. J. Phys.Chem. C 2013, 117, 3895-3902. 22) Denis, P. A. Chemical Reactivity of Lithium Doped Monolayer and Bilayer Graphene. J. Phys.Chem. C 2011, 115, 13392-13398. 23) Fan, X.; Nouchi, R.; Tanigaki, K. Effect of Charge Puddles and Ripples on the Chemical Reactivity of Single Layer Graphene Supported by SiO2/Si Substrate. J. Phys. Chem. C 2011, 115, 12960-12964. 24) Tapia, A.; Acosta, C.; Medina-Esquivel, R. A.; Canto, G. Potassium Influence in the Adsorption of Hydrogen on Graphene: A Density Functional Theory Study. Comput. Mater. Sci. 2011, 50, 2427-2432. 25) Huang, L. F.; Ni, M. Y.; Zhang, G. R.; Zhou, W. H.; Li, Y. G.; Zheng, X. H.; Zeng, Z. Modulation of the Thermodynamic, Kinetic, and Magnetic Properties of the Hydrogen Monomer on Graphene by Charge Doping. J. Chem. Phys. 2011, 135, 064705. 26) Huang, L.F.; Cao, T.F.; Gong, P.L.; Zeng, A.; Zhang, C. Tuning the Adatom-surface and Interadatom Interactions in Hydrogenated Graphene by Charge Doping. Phys. Rev. B 2012, 86, 125433. 27) Logsdail, A. J.; Johnston, R. L.; Akola, J. Improving the Adsorption of Au Atoms and Nanoparticles on Graphite via Li Intercalation. J. Phys. Chem. C 2013, 117, 22683-22695.


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The Journal of Physical Chemistry Page 16 of 27 28) Gaussian 09, Revision D.1, Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. D. J. Gaussian, Inc., 29) Soler, J. M.; Artacho, E.; Gale, J. D.; Garcia, A.; Junquera, J.; Ordejon, P.; Sanchez-Portal, D. The SIESTA Method for Ab Initio Order-N Materials Simulation. J. Phys.: Condens. Matter, 2002, 14, 2745-2779. 30) Ordejon, P.; Artacho, E.; Soler, J. M. Self-consistent order-N Density-functional Calculations for Very Large Systems. Phys. Rev. B 1996, 53, R10441-R10444. 31) Zhao, Y.; Truhlar, D. G. A New Local Density Functional for Main-group Thermochemistry, Transition Metal Bonding, Thermochemical Kinetics, and Noncovalent Interactions J. Chem. Phys. 2006, 125, 194101. 32) Zhao, Y.; Truhlar, D.G. Density Functionals with Broad Applicability in Chemistry. Theor. Chem. Account. 2008, 120, 215-241. 33) Heyd J.; Scuseria, G. E. Assessment and Validation of a Screened Coulomb Hybrid Density Functional. J. Chem. Phys., 2004, 120, 7274. 34) Barone V.; Scuseria, G. E. Theoretical Study of the Electronic Properties of Narrow Singlewalled Carbon Nanotubes: Beyond the Local Density Approximation. J. Chem. Phys. 2004, 121, 10376. 35) Hehre, W.; Radom, L.; Schleyer, P. v. R.; Pople J. A. Ab initio Molecular Orbital Theory, Wiley, New Work (1986). 36) Denis, P.A. Theoretical Characterization of Existing and New Fullerene Receptors. RSC Adv. 2013, 3, 25296. 37) Denis, P.A. Design and Characterization of Two Strong Fullerene Receptors Based on Ball–socket Interactions. Chem. Phys. Lett. 2014, 591, 323.


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Conjugated Systems and Their Interaction with Fullerenes. Chem. Phys. Lett. 2011, 516, 82. 39) Dion, M.; Rydberg, H.; Schroder, E.; Langreth, D.C.; Lundqvist, B.I. Van der Waals Density Functional for General Geometries. Phys. Rev. Lett. 2004, 92, 246401. 40) P.A. Denis, Density Functional Investigation of Thioepoxidated and Thiolated Graphene. J. Phys.Chem. C 2009, 113, 5612-5619. 41) Boys, F. S.; F. Bernardi, F. The Calculation of Small Molecular Interactions by the Differences of Separate Total Energies. Mol. Phys. 1970, 19, 553-566. 42) Troullier, N.; Martins, J.L. Efficient Pseudopotentials for Plane-Wave Calculations. Phys. Rev. B 1991, 43, 1993-2006. 43) Monkhorst, H.J.; Pack, J.D. Special points for Brillouin-zone integrations. Phys. Rev. B 1976, 13, 5188. 44) Chakrabarty, S.; Wasey, A.H.M.A.; Thapa, R.; Das, T.G.P. First Principles Design of Divacancy Defected Graphene Nanoribbon Based Rectifying and Negative Differential Resistance Device. arXiv:1502.07465v1. 45) Li, J.-W.; L. Y.-Y.; Xie, K.-H.; Shang, J.-Z.; Qian, Y.; Yi, M.-D.; Yu, T.; Huang, W. Revealing the Interactions Between Pentagon–Octagon–Pentagon Defect Graphene and Organic Donor/Acceptor Molecules: a Theoretical Study. Phys. Chem. Chem. Phys., 2015,17, 4919-4925. 46) Nistor, R.A.; Newns, D.M.; Martyna, G.J. The Role of Chemistry in Graphene Doping for Carbon-Based Electronics. ACS Nano, 2011, 5, 3096–3103. 47) Wang, Z.; Qin, S.; Wang, C.; Hui, Q. Fluorine adsorption on the graphene films: From metal to insulator. Comput. Mater. Sci., 2015, 97, 14-19.


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Page 18 of 27

Table 1. Adsorption energies (kcal/mol) determined for N-DMBI and R-N-DMBI, at different levels of theory.

unit cell

N-DMBI

N-DMBI

R-N-DMBI

R-N-DMBI

VDW-DF/DZP M06-L/6-31G* VDW-DF/DZP M06-L/6-31G* 7×7 8×8

19.3

15.7

44.4

16.0

9×9

39.5 40.6 42.1


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Page 19 of 27

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Table 2. Bond energies (kcal/mol) determined for the addition of H, OH and F to 7×7 graphene with an adsorbed R-N-DMBI molecule, at different levels of theory.

M06-L/6-31G*

H

OH

F

graphene

16.7

11.0

30.3

graphene+R-N-DMBI 27.1

26.7

47.9

Extra:

10.4

15.7

17.5

VDW-DF/DZP

H

OH

F

graphene

25.2

9.7

30.3

graphene+R-N-DMBI 33.3

19.4

45.7

Extra:

9.7

15.4

8.1


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Page 20 of 27

Figure 1. a) Optimized structure determined at the M06-L/6-31G* level for N-DMBI adsorbed onto a 7×7 graphene unit cell. b) Band structures and density of states computed at the VDWDF/DZP are shown on the right. (Grey: carbon atoms of graphene, pink: carbon atoms of NDMBI, blue: nitrogen, White: hydrogen). Fermi level is at 0 eV.


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Figure 2. a) Optimized structure determined at the M06-L/6-31G* level for radical N-DMBI adsorbed onto a 7×7 graphene unit cell. b) Band structures and density of states computed at the VDW-DF/DZP are shown on the right. (Grey: carbon atoms of graphene, pink: carbon atoms of N-DMBI, blue: nitrogen, White: hydrogen). Fermi level is at 0 eV, E (energy) in eV.


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Figure 3. a) Optimized unit cell and b) band structure determined at the VDW-DF /DZP level for 7×7 graphene with one radical N-DMBI molecule adsorbed on one side and a F4-TCNQ molecule adsorbed on the opposite side. Fermi level is at 0 eV, E (energy) in eV.


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Figure 4. a) Optimized structure and b) band structure determined at the VDW-DF /DZP level for 6×7 graphene with one radical N-DMBI and a F4-TCNQ molecule adsorbed on the same side. Fermi level is at 0 eV. (The carbon atoms of radical N-DMBI and F4-TCNQ are in pink to differentiate them from those corresponding to graphene). Fermi level is at 0 eV, E (energy) in eV.


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Figure 5. a) Optimized structure determined at the M06-L/6-31G* level for radical N-DMBI adsorbed onto a 7×7 graphene unit cell functionalized with a hydrogen atom at carbon 1. b) Carbon atoms selected to study the reactivity of n-doped graphene by R-N-DMBI adsorption. (Grey: carbon atoms of graphene, pink: carbon atoms of N-DMBI, blue: nitrogen, White: hydrogen).


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment

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The Journal of Physical Chemistry Page 26 of 27 Figure 6. Band structures determined at the VDW-DF/DZP level for functionalized graphene 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

with and without adsorbed radical N-DMBI: a) functionalization with hydrogen, b) functionalization with hydroxyl c) functionalization with fluorine. G77 denotes a 7×7 graphene unit cell. Fermi level is at 0 eV, E (energy) in eV.


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232x127mm (72 x 72 DPI)


Strong N-Doped Graphene - American Chemical Society

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