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Organometallic Hexahapto-Functionalized Graphene: Band Gap Engineering with Minute Distortion to the Planer Structure Jun Dai, Yu Zhao, Xiaojun Wu, Xiao Cheng Zeng, and Jinlong Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp408347w • Publication Date (Web): 26 Sep 2013 Downloaded from http://pubs.acs.org on October 1, 2013

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The Journal of Physical Chemistry

Organometallic Hexahapto-Functionalized Graphene: Band Gap Engineering with Minute Distortion to the Planer Structure Jun Dai1, Yu Zhao1, Xiaojun Wu2 *, Xiao Cheng Zeng1,3 *, and Jinlong Yang3 1

Department of Chemistry and Nebraska Center for Materials and Nanoscience, University of Nebraska-Lincoln, Lincoln, NE 68588, USA 2

CAS Key Lab of Materials for Energy Conversion, Department of Materials Science and Engineering and Hefei National Lab for Physical Science at Microscale, University of Science and Technology of China, Hefei, Anhui 230026, China 3

Department of Chemical Physics and Hefei National Lab for Physical Science at Microscale, University of Science and Technology of China, Hefei, Anhui 230026, China

ABSTRACT Motivated from experimentally realized Cr-chemisorbed graphene, we have systematically explored electronic properties of organometallic complexes of graphene with covalent mono-hexahapto-TM (TM=Cr, Fe, Ni) bonds using density-functional theory (DFT) calculations. We show that besides Cr, Fe and Ni can also bind strongly with the graphene. At the experimentally suggested coverage ratio (TM:C=1:18, TM=Cr, Fe, Ni), our calculations suggest that the computed band gap of perfectly arranged networks of (η6-graphene)-Cr(CO)3, (η6-graphene)-Fe(CO)2 and (η6-graphene)-NiCO can be enlarged to 1.08 eV, 0.61 eV and 0.29 eV, respectively. The inconsistence between the computed gap (1.08 eV) and the experimental gap (~10 meV) for the (η6-graphene)-Cr(CO)3 is explained, which is possibly due to the existence of regions with relatively lower coverage ratio, in view of the much smaller band gap for (η6-graphene)-Cr(CO)3 with Cr:C=1:32 (54 meV) and with Cr:C=1:50 (20 meV), respectively.

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Both band-gap values are much closer to the measured band gap (~10 meV). Yet, the functionalized graphene shows little structural distortion from its original planar structure. The notable features of the direct band gaps along with the planar structure render the TMfunctionalized graphenes quite appealing for applications not only in nanoelectronics but also in optoelectronics such as the infrared detector and solar cell photoanode.

INTRODUCTION Graphene1-4 has been a focus of intensive research in two-dimensional nanomaterials due to its many unique properties, such as the quantum Hall effect,5 the Dirac fermions behavior of carrier,6 high thermal conductivity,7,8 and ultra-high Young’s modulus.9 These superlative properties afford the graphene as a potential candidate for developing future generations of small, speedy and light electronic devices.10 However, graphene is a zero-gap semimetal, and the lack of a band gap limits its ability to switch current on and off to create logic circuits. Thus far, many research efforts have been devoted to the band-gap engineering of graphene. For example, atomic doping with either boron or nitrogen as used in traditional silicon-based semiconductors has been proven to be effective for band-gap opening, resulting in either p-type or n-type graphene.11-13 However, doping configuration in the graphene is difficult to control atomically and doping usually introduce various defects. Cutting graphene into narrow nanoribbons is another way to open the band gap,14-17 but large-scale production of uniform nanoribbons with well-controlled edges is still very challenging. It has been shown that a vertical external electric field can induce a band gap for the bilayer graphene, but this strategy offers limited tunability for the current on/off ratio.18-20 Chemical functionalization21,22,23 is another effective way to alter electronic structures of the graphene. However, chemical functionalization often leads to partial


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or full conversion of the sp2-hybridized carbon to sp3-hybridized carbon in the graphene, which not only can siginificantly reduce the carrier mobility of the graphene,24,25 but also can make the planar graphene highly buckled. Very recently, Sarkar et al. proposed a new approach by adding hexahapto (η6)-chromium bonds to the graphene surface.26,27 Their experiments show that the 2D structure shows little structural change with the formation of mono-hexahapto-metal bonds, and the in-plane transport properties are largely retained in the functionalized graphene with room-temperature field-effect mobilities ranging from 200 to 2000 cm2/Vs and with the on/off ratios of 5-13.27 This finding strongly supports the pursuit of this line of modifications and also calls for new ways to improve the efficiency of the device. Note that the stable organometallic complex ((η6-graphene)TM(CO)x) can be descibed by the 18-electron rule,28 in which the transition-metal ions are always considered to be zero-valent. Taking (η6-graphene)-Cr(CO)3 as an example, Cr(0) equals to d6, [η6]- equals to 6e and three [CO] ligands contains 6e, i.e., 18e in total. Based on this rule, we examine two other possible organometallic complexes of this kind, namely, (η6-graphene)Fe(CO)2 and (η6-graphene)-NiCO, where Fe(0) and Ni(0) equal to d8 and d10, respectively. In this letter, the electronic structure modification by the covalent mono-hexahapto-TM (TM=Cr, Fe, Ni) is investigated using DFT calculation. Our results show that all three transition metals can strongly bind with the graphene surface while retaining largely the integrity of the planar sp2 structure of the graphene. For (η6-graphene)-Cr(CO)3, the computed band gap at the coverage ratio of TM:C=1:18, 1:32 and 1:50 is 1.08 eV, 54 meV and 20 meV, respectively, suggesting possible existence of regions with relatively low Cr coverage ratio in the samples, which could result in a very narrow band gap around 10 meV in the experiment. Moreover, our calculations suggest that the perfect networks of (η6-graphene)-Fe(CO)2 and (η6-graphene)-NiCO with


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TM:C=1:18 are semiconductors with a direct band gap of 0.61 eV and 0.29 eV, respectively. The tunable band-gaps by this line of modification offer more options for the design of graphenebased electronic transistors.

COMPUTATIONL METHODS Geometrical optimizations and electronic structure calculations are performed using a DFT method implemented in VASP 5.3 software package.29 The exchange-correlation energy is treated using the Perdew-Burke-Ernzerhof (PBE) functional, and the Grimme’s DFT-D2 dispersion correction30 is applied to account for the long-range van der Waals interactions. Since the PBE functional tends to underestimate the band gap of semiconductors, the hybrid HSE06 functional31 is also adopted to check reliability of the computed band gaps based on PBE functional. We find that overall PBE gives reasonable results although the values of band gap are slightly smaller than those based on HSE06 calculations. A vacuum space of ~20 Å along the direction normal to the graphene plane is used so that the interlayer interaction generated by the periodic boundary condition can be neglected. Based on the reported optimal Cr:C ratio being about 1:18 from previous experiments,27 we adopt a model of one TM(CO)x on every graphene unit of 3×3 (18 C atoms), 4×4 (32 C atoms) and 5×5 (50 C atoms) supercell. For simplicity, if not mentioned, we use (η6-graphene)-Cr(CO)3, (η6-graphene)-Fe(CO)2 and (η6-graphene)-NiCO to refer to one-side adsorption of one TM(CO)x on a 3×3 graphene supercell. The ion-electron interaction is treated using the projector-augment-wave (PAW) technique. For geometric optimization, both lattice constants and atomic positions are relaxed until the forces on the atoms are less than 0.02 eV /Å and the total energy change is less than 1.0×10-5 eV. For the geometry


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optimizations, a12×12×1 Monkhorst-Pack32 grid and a kinetic energy cutoff of 500 eV are selected, while for density of states (DOS) computations, a finer 24×24×1 grid is chosen.

Figure 1. Top view of (η6-graphene)-Cr(CO)3 with Cr being chemisorbed on top sites (a) or bridge sites (b); top view of (η6-graphene)-Fe(CO)2 with Fe being chemisorbed on top sites (c) or bridge sites (d); top view of (η6-graphene)-NiCO (e). Dark blue, dark red, light blue spheres, and small yellow and red spheres denote Cr, Fe, Ni, C and O atoms, respectively. The organe triangle, rectangle and hexagon highlight the distribution of the CO ligands.

Table 1 Energetic and structural characteristics of various (η6-graphene)-TM(CO)x systems. Eint (eV)

dbuckle (Å)

dTM-Cring (Å)

dTM-Ccarbonyl (Å)

Cr-1

-3.416

0.0085

2.247-2.262

1.842

Cr-2

-3.583

0.0083

2.252

1.843

Fe-1

-3.535

0.0140

2.152-2.155

1.749

Fe-2

-3.523

0.0165

2.140-2.156

1.745

Ni

-2.928

0.0104

2.187

1.726


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RESULTS AND DISCUSSIONS The optimized structures of (η6-graphene)-Cr(CO)3, (η6-graphene)-Fe(CO)2 and (η6-graphene)NiCO are illustrated in Figure 1. Two highly symmetric chemisorption configurations are examined for (η6-graphene)-Cr(CO)3 and (η6-graphene)-Fe(CO)2, respectively,

in view of

possible self-rotation of the -Cr(CO)3 and -Fe(CO)2 ligands with respective to the surface normal of the graphene plane. We name the configuration shown in Figure 1(a) or (c) as the top-site configuration in which the projection of the CO ligands on the graphene plane is in line with a CC bond; we name the configuration shown in Figure 1(b) and (d) as the bridge-site configuration in which the projection of CO ligands on the graphene plane bisects a C-C bond. The top-site configuration and bridge-site configuration can superimpose with each other via a rotation of 30° with respect to the surface normal of the graphene. Table 1 summarizes the energetics and structural data of the three systems based on the PBE-D2 computation. For simplicity, we use Cr-1 and Fe-1 to represent the top-site configuration of (η6-graphene)-Cr(CO)3 and (η6graphene)-Fe(CO)2, respectively, and Cr-2 and Fe-2 to represent the corresponding bridge-site configuration, respectively. The chemisorption (or bonding) energy is defined as: Eint = E[(η6graphene)-TM(CO)x] - E(graphene) - E[TM(CO)x], where E[(η6-graphene)-TM(CO)x] refers to the total energy of (η6-graphene)-TM(CO)x, while E(graphene) and E[TM(CO)x] are the total energy of the graphene and TM(CO)x, respectively, whose corresponding atomic coordination is taken from that of the optimized complex. dbuckle is the standard deviation of the distance between carbon atoms and their mean in the z (surface normal) direction. The computed Cr-Cring distance agrees well with previous theoretical results on the corresponding distance in a molecular species (benzene)Cr(CO)332 (2.247 Å), but slightly longer than the solid-state experimental value (2.223 Å) for (benzene)Cr(CO)3.33 The computed distance of Cr-Ccarbonyl is in very good agreement with


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the solid-state experimental value (1.845 Å) for (benzene)Cr(CO)3.33 These results indicate that the optimized geometries based on the PBE functional with Grimme’s DFT-D2 dispersion correction are reliable. Importantly, as shown in Table 1, all systems exhibit strong adsorption interaction whereas the buckling of the graphene is negligible. Our computation confirms the experimental finding that the strong Cr-adsorption in the graphene complex has little effect on the planar structure of the graphene27. Note that the adsorption on the bridge site of (η6graphene)-Cr(CO)3 is 0.167 eV (per supercell) stronger than on the top site, which means the bridge-site configuration is energetically more favorable. For (η6-graphene)-Fe(CO)2, however, the top-site configuration is only slightly favored in energy compared to the bridge-site configuration as the difference in their adsorption energies is merely 0.012 eV per supercell. Next, we study effects of the mono-hexahapto-TM bonds on the electronic properties of the graphene. Note that the bis-hexahapto-TM bonding materials are believed to have the potential for applications in molecular spintronics.35-37 To examine whether mono-hexahapto-TM bonded organometallic complexes of graphene can be spin-polarized, we have performed spin-polarized DFT calculations. We find that all the five structures considered are non-magnetic. Hereafter, only spin-nonpolarized results are reported. We also find that the band structures for the top- and bridge-site configurations are nearly the same for the (η6-graphene)-Cr(CO)3 and (η6-graphene)Fe(CO)2 systems. Hence, we only present results for the bridge-site configuration. The PBE band structures of the three systems are plotted in Figure 2. All three systems, i.e., (η6-graphene)Cr(CO)3, (η6-graphene)-Fe(CO)2, and (η6-graphene)-NiCO, exhibit a semiconducting feature, that is, the d bands of the transition metals are located in the gap of graphene. To confirm the reliability of the band structures computed with the PBE functional, HSE06 functional is also adopted to compute the band gaps (the HSE06 band structures are shown in Supporting


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Information Figure S1). Comparing with the HSE06 results, the band gaps are slightly underestimated by the PBE functional. The PBE band gaps for (η6-graphene)-Cr(CO)3, (η6graphene)-Fe(CO)2, and (η6-graphene)-NiCO are 1.08 eV, 0.61 eV and 0.29 eV, respectively, while the corresponding HSE06 band gaps are 1.29 eV, 0.63 eV and 0.28 eV, respectively. We note that the HSE06 band structures for (η6-graphene)-Cr(CO)3 exhibit slightly different dispersion behavior of the localized Cr d states (see Supporting Information Figure S1), but overall, the PBE results are reasonable. There are three, four and five bands intercalated within the band gap of the graphene for (η6-graphene)-Cr(CO)3, (η6-graphene)-Fe(CO)2, and (η6graphene)-NiCO, respectively, consistent with the six, eight and ten valence electrons for Cr(0), Fe(0) and Ni(0). In addition, an evidence of the p-d hybridization can be seen in all three systems (see Supporting Information Figure S2, for exampke, for (η6-graphene)-NiCO). Near the G point, main contributions to the two highest valence bands are from the pz electrons of the graphene, while in other regions, they are dominated by the d electrons of the transition metals. The computed band gaps of η6-graphene)-Cr(CO)3 (1.08 eV) and (η6-graphene)-Fe(CO)2 (0.61 eV) are close to those of well-known semiconductors, Si (1.17 eV) and Ge (0.74 eV)38, and thus both graphene complexes may have the potential for applications in low-dimensional electronic devices. Moreover, the highest valence band of (η6-graphene)-Cr(CO)3 is quite narrow, which means the electrons are highly localized. However, for (η6-graphene)-Fe(CO)2 and (η6graphene)-NiCO, the bandwidth of the highest valence band is much larger compared to that of (η6-graphene)-Cr(CO)3, indicating much stronger electron delocalization. One can also see an obvious increase in the curvature of the highest valence band at the G point. As the effective mass is inversely proportional to the band curvature, a higher electron mobility can be expected for (η6-graphene)-NiCO and (η6-graphene)-Fe(CO)2 than (η6-graphene)-Cr(CO)3. As a result,


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(η6-graphene)-Fe(CO)2 and (η6-graphene)-NiCO can offer higher performance than (η6graphene)-Cr(CO)3 due to their higher carrier mobility and faster switching (lower values of band gap).

Figure 2: The PBE Band structures of (a) the bridge-site configuration of (η6-graphene)-Cr(CO)3, (b) the bridge-site configuration of (η6-graphene)-Fe(CO)2, (c) (η6-graphene)-NiCO. The size of


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the red circle and green square denote the contribution of the graphene’s pz states and the transition metals’ d states. G (0.0, 0.0, 0.0), K (1/3, 1/3, 0), M (0.5, 0.0, 0.0) refer to the special points in the first Brillouin zone. Note that the computed gap value of 1.08 eV for (η6-graphene)-Cr(CO)3 is notably larger than the experimentally measured energy gap, obtained from fitting to the temperature-dependent conductuance (3 - 14 meV).27 To explain this discrepancy, we compute the electronic structure of systems with several different Cr(CO)3 coverage ratios, doube-side adsorption in 3×3 graphene supercell (see Supporting Information Figure S3 for the three configurations), one-side adsorption on a 4×4 and 5×5 graphene supercell. For the double-side adsorption, we choose the one with the lowest energy, which is 298.4 meV and 679.4 meV lower in energy per supercell than other two configurations. These systems are shown in Figure 3 (d)-(f). The band structures of the three new systems with different coverage ratios are ploted in Figure 3. One can see that for the system with the double-side adsorption (see Figure 3 (a)), the computed band gap (1.11 eV) is almost the same as that with one-side adsorption (1.08 eV), while for one-side adsorption with a lower coverage ratio, a notable decrease of the band gap is seen. For the system with one Cr(CO)3 on a 4×4 graphene supercell (Cr:C=1:32), the PBE band gap is 44 meV (see the inset of Figure 3(b)), while for the one with 5×5 supercell (Cr:C=1:50), the band gap is 20 meV (see the inset of Figure 3(c)). Both values of the band gap are much closer to the measured band gap (314 meV)27, suggesting possible existence of regions with relatively lower coverage ratios of Cr(CO)3 than the average ratio reported (Cr:C=1:18)27. In addition, we compute band gaps of two systems, one with the one-side adsorption of Fe(CO)2 and another with the one-side adsorption of NiCO, using both 4×4 and 5×5 graphene supercells. For Fe(CO)2 with a 4×4 graphene supercell, the band gap is 110 meV, while with the 5×5 graphene supercell it is 73 meV.


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For NiCO with either 4×4 or 5×5 graphene supercell, the gap opening is negligible (< 1meV; see Supporting Information Figure S4). To understand mechanism of gap opening in these systems, we plot the charge density redistribution (Supporting Information Figure S5). The charge density redistribution is defined as the difference in charge density of (η6-graphene)-Cr(CO)3 and that of isolated Cr(CO)3 and graphene with the same structure as in adorbed systems. For the one with one Cr(CO)3 adsorbed on a 3×3 graphene supercell, we can see that all carbon atoms in the graphene network entail appearent charge density redistributions, which may weaken the electron transport in the graphene plane, resulting in a large band-gap opening. While for the system with one Cr(CO)3 adsorbed on a 4×4 graphene supercell, the charge density redistribution in the carbon atoms beyond the next-nearest carbon atoms of the adsorption center is neglegible (see the pink circle in Figure S5), indicating delocalization of the π electrons is well retained. As a result, only a tiny band gap is opened.


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Figure 3. The computed band structure of (a) double-side adsorption of Cr(CO)3 on a 3×3 graphene supercell, one-side adsorption of Cr(CO)3 on a 4×4 graphene supercell (b) and 4×4 graphene supercell (c). (d)-(f) are the corresponding systems whose band structures are on the left.


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Figure 4. The computed imaginary part of the frequency-dependent dielectric function for the bridge-site configuration of (η6-graphene)-Cr(CO)3, the bridge-site configuration of (η6graphene)-Fe(CO)2, and (η6-graphene)-NiCO. Compared to bulk materials, two-dimensional planar materials have a number of advantages for optical applications such as the small thickness and large surface area. The band gaps with values ranging from 0.29 to 1.08 eV grant these materials particular promising for applications in optical devices as well. For example, (η6-graphene)-Cr(CO)3 with a band gap of 1.08 eV might be a promising solar-cell photoanode material, while (η6-graphene)-Fe(CO)2 and (η6-graphene)NiCO with direct gaps of 0.61 eV and 0.29 eV might be useful as mid-infrared and far-infrared detectors. To expore these possibilites, we carried out calculations of the imaginary part of the


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frequency-dependent dielectric function via summation over pairs of occupied and empty states, but without considering the local field effects. Note that a dense k-point sampling is generally required to compute the dielectric function. Hence, covergence of the optical properties with the nubmer of k-points should be carefully examined. As shown in Supporting Information Figure S6, a large 24×24 k-point grid is required to give a reliable description of the properties of the system with one NiCO on a 3×3 graphene supercell. Figure 4 shows the computed results of (

) for one of the diagonal parts of the in-plane componets of (η6-graphene)-Cr(CO)3, (η6-

graphene)-Fe(CO)2 and (η6-graphene)-NiCO, respectively. For (η6-graphene)-Cr(CO)3, a peak around 1.5 eV is seen, which arises from the inter-band VBM-CBM transition. There are also peaks located around 2.3 eV and 3.6 eV due to other inter-band transitions. Overall, the absorption shows a good coverage of the wave-length range of visible light (1.6-3.2 eV). For (η6graphene)-Fe(CO)2 and (η6-graphene)-NiCO, we can see strong peaks located around 0.5 eV and 0.4 eV, respectively, due to the VBM-CBM transition. The peak heights are different for (η6graphene)-Cr(CO)3, (η6-graphene)-Fe(CO)2 and (η6-graphene)-NiCO because the states corresponding to CBM and VBM are mixtures of the pz states of carbon and the d states of transition metals, and the transition between different portions of them are prohibited by the optical selection rule. (η6-graphene)-Fe(CO)2 and (η6-graphene)-NiCO exhibit substantial light absorption approximately in the energy ranges of 0.3 - 0.7 eV, which means they might be good candidates as materials for infrared detector.

CONCLUSION In conclusion, we have shown that all Cr, Fe and Ni can bind strongly with the graphene in the forms of (η6-graphene)-Cr(CO)3, (η6-graphene)-Fe(CO)2 and (η6-graphene)-NiCO and yet the


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planar structure of the graphene is largely retained even with the strong binding interaction. At the coverage ratio of one TM(CO)x on a 3×3 graphene supercell, the band gap of the system can be opened up to 0.29 - 1.08 eV, depending on the transition metal. The much smaller experimental band gap (~10 meV) reported is possibly due to existence of regions with relatively lower coverage ratio of (CrCO)3 in the sample. Compared to (η6-graphene)-Cr(CO)3, the (η6graphene)-Fe(CO)2 and (η6-graphene)-NiCO are predicted to have smaller band gaps but higher carrier mobilities. Moreover, (η6-graphene)-Cr(CO)3 exhibits an optical absorption range in visible light, while (η6-graphene)-Fe(CO)2 and (η6-graphene)-NiCO may be promising for application in infrared detectors. These combined and novel structural, electronic, and optical properties render the functionallized graphene a promising candidate for either electronic or optoelectronic applications.

ASSOCIATED CONTENT Supporting Information. Computed band structures based on HSE06 functional, the iso-surface plot of the charge density of VBM and CBM for (η6-graphene)-NiCO, three configurations of double-side adsorption of Cr(CO)3 on a 3×3 graphene supercell, computed bans structures of Fe(CO)2 and NiCO on 4×4 and 5×5 graphene supercells, iso-surface plot of charge density redistribution for (η6-graphene)-Cr(CO)3 with different coverage ratio, Convergence test of the calculated imaginary part of the frequency-dependent dielectric function versus k-points for (η6graphene)-NiCO. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author


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* [email protected]; [email protected]. ACKNOWLEDGMENT USTC group is supported by the National Basic Research Programs of China (Nos. 2011CB921400, 2012CB 922001), NSFC (Grant Nos. 21121003, 11004180, 51172223), One Hundred Person Project of CAS, Shanghai Supercomputer Center, and Hefei Supercomputer Center. UNL group is supported by ARL (Grant No. W911NF1020099), NSF (Grant No. DMR0820521), and a grant from USTC for (1000 plan) Qianren-B summer research.

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