Stability and Electronic Properties of Biphenylene Based

Article pubs.acs.org/JPCC

Stability and Electronic Properties of Biphenylene Based Functionalized Nanoribbons and Sheets Pablo A. Denis* Computational Nanotechnology, DETEMA, Facultad de Química, University of Uraguay, UDELAR, CC 1157, Gral. Flores 2124, 11800 Montevideo, Uruguay

ABSTRACT: By means of first principle calculations, we have performed a theoretical characterization of the stability and electronic properties of sheets and nanoribbons that were recently synthesized employing octafunctionalized biphenylenes as building blocks. We found that the biphenylene sheet has a strong metallic character that is difficult to inhibit employing low levels of functionalization. Hydrogenation at full coverage induces a metal to insulator transition, but the band gap opened is very large, i.e., 6.6 eV. When other functional groups such as fluorine or chlorine are attached, the band gap can be regulated. The most effective chemical modification, in terms of gap opening, is the combination of hydrogen/chlorine or fluorine/chlorine. For the latter functional groups, band gaps similar to those of rutile were calculated at the HSEH1PBE/6-31G* level of theory. The biphenylene sheet functionalized on one side with fluorine and with chlorine on the other presented a CC bond length equal to 1.76 Å, one of the longest reported up to date. In contrast with recent claims, we found that, for armchair biphenylene nanoribbons, the twist induced by the functionalization of the edges does not increase the band gaps of the nanoribbons. Moreover, in some cases the gaps were reduced as we observed when the edges where saturated with hydrogen atoms. Finally, the high reactivity of the sheet indicated that it is may have promising applications in catalysis.

1. INTRODUCTION The large scale synthesis of nanomaterials with uniform properties represents a serious challenge.1 One of the most paradigmatic cases is the production of modified graphenes with identical electronic properties. For this reason, Drexler suggests in his book Engines of Creation,2 that the bottom up approach is the route that must be followed in order to obtain nanocomposites with uniform properties. A recent example of this procedure is the synthesis of carbon based nanoribbons which use biphenylene as building block.3 The new nanoribbons are composed by four-, six-, and eight-membered rings, and for this reason an optical band gap is expected to be opened. In the case of the nanoribbon formed by 24 biphenylene units joined using the pattern indicated in Figure 1, the measured optical band gap was 2.81 eV. This value did not show significant changes with the number of biphenylene molecules juxtaposed. In effect, the band gap determined for the flake composed by 8 biphenylene building blocks was only 0.15 eV larger. These findings are in agreement with the theoretical predictions made as early as 2010 by Hudspeth et al.4 With the aid of first principle calculations, these authors showed that the narrowest hydrogen terminated biphenylene armchair ribbon exhibits a band gap of 1.71 eV. For wider ribbons a © 2014 American Chemical Society

significant reduction of the gap was observed. The widest ribbon studied (2.14 nm wide) presented a band gap equal to 0.08 eV, at the HSE06 level. The agreement between the experimental results and the theoretical calculations is very satisfactory. However, it is worth mentioning that the ribbons synthesized are not terminated by H atoms, but functionalized with methoxy groups, which are supposed to introduce out-ofplane distortions, reduce the π-conjugation, and concomitantly increase the band gap.3 Considering the difference in the band gap determined experimentally, and that calculated, the latter statement seems to be true. Thus, the twist induced by the functionalization of the biphenylene based nanoribbons is expected to be of paramount importance to obtain tailor-made properties. Since the beginning of the nanotechnological revolution, theory has aided in the design and understanding of new nanomaterials.5−19 For this reason, and owing to the possibility of preparing biphenylene based nanoribbons via a bottom up approach, we have decided to study the effect of chemical functionalization on the stability and electronic properties of infinite sheets and armchair biphenylene based Received: July 13, 2014 Revised: September 11, 2014 Published: October 9, 2014 24976

dx.doi.org/10.1021/jp5069895 | J. Phys. Chem. C 2014, 118, 24976−24982

The Journal of Physical Chemistry C

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Figure 1. Optimized unit cells determined for the biphenyl based nanoribbons and sheet (arrow indicates the periodic direction and for n = 2 the ribbons is dubbed NR2 and so on).

along the Γ-M-K-Γ and Γ-X paths for the infinite sheet and nanoribbons, respectively. As for the M06-L and M06-2X27,28 calculations performed with Gaussian 2009,20 the basis set selected was the 6-31G*.29 In all cases calculations were run with the ultrafine grid. The M06-L functional was utilized in the periodic calculations for the aforementioned ribbons and sheet. In all cases, the structures were optimized with 50 k-points, and electronic properties were obtained with a higher density, namely 1000 kpoints. In addition to the M06-L/6-31G* periodic calculations, we performed additional HSEH1PBE/6-31G* calculations,30,31 because this functional corrects the underestimation of band gaps commonly observed in density functional theory. Finally, we note that internal parameters and the unit cell were optimized.

nanostructures. Our results indicated that the twist experienced when the edges nanoribbons are functionalized is not important for the opening of a band gap. However, it is possible to open a tunable band gap for the infinite sheet when it is subjected to chemical functionalization.

2. METHODS We carried out density functional theory calculations with Gaussian20 and SIESTA.21,22 For the VDW-DF23 calculations performed with SIESTA,21,22 we utilized the same procedure as in our previous works about doped,11−13 covalent,14−18 and noncovalent19 modified graphenes. In previous works,15−18,37 we have observed that the M06-L and VDW-DF density functionals yield similar energetics for covalent and noncovalent adsorptions. In this investigation we employed these functionals because the bulky methoxy groups as well as the chlorine atoms may interact between themselves or with the biphenylene structures via nonbonded interactions. The double-ζ basis set with polarization functions was selected for all the spin polarized VDW-DF calculations. We fixed the orbital confining cutoff to 0.01 Ry, and the split norm used was 0.15. The results obtained with SIESTA, in particular, the binding energies, can be prone to significant basis set superposition error (BSSE), even with relatively low degree of radial confinement.24 To avoid this problem, the counterpoise correction suggested by Boys and Bernardi25 was used. We utilized relaxed structures to estimate the BSSE corrected binding energies, and monomer deformation energies were taken into consideration. The interaction between ionic cores and valence electrons was described by the Troullier−Martins norm conserving pseudopotentials.26 We used the 2 × 2 unit cell shown in Figure 1 to model the infinite biphenylene sheet. The biphenylene armchair nanoribbons were studied employing two unit cells, and the width was varied from 1 to 8 benzene units as indicated in Figure 1. The narrowest ribbon is 6.19 Å wide (measured from the carbon atoms) while the widest one is 28.74 Å long. The lattice parameters were optimized for all structures. Monkhorst−Pack samplings of 30 × 30 × 1 and 500 × 1 × 1 were performed for the infinite sheet and nanoribbons, respectively. Band structures were calculated using 100 points

3. RESULTS AND DISCUSSION 3.1. Bare Sheet and Nanoribbons. Before studying the effect of chemical functionalization on the electronic properties, it is important to discuss the bare structures. In agreement with the work by Hudspeth et al.4 we found that the infinite sheet is metallic as can be seen in the density of states (DOS) presented in Figure 2. The armchair nanoribbons show a confinement effect that is translated in the appearance of a band gap for the narrowest members of the series. The results are listed in Table 1. The smallest ribbons studied, NR2, presented a band gap larger than 1 eV, at the M06-L/6-31G* level. However, the decay of the band gap upon nanoribbon widening is very fast. Indeed, the band gap determined for NR4 is 0.1 eV, and the NR5 is predicted to be semimetallic whereas the NR6 is metallic, at the M06-L/6-31G* level. The use of the more accurate HSEH1PBE/6-31G* method indicated that the semiconductor to metal transition occurs for ribbons wider that NR8. The band gaps determined for NR7 are very small, 0.087 and 0.065 eV, respectively. The latter two values clearly indicate the exponential decay of the gap upon ribbon widening. In effect, the gap decreases 0.022 eV when passing from NR6 to NR7, a value that is much smaller than the previous reduction when going from NR5 to NR6. 24977



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Table 2. Band Gaps (eV), Lattice Parameters, and Bond Distances (Å) Determined for the Addition of Different Functional Groups to the Infinite Biphenylene Sheet, at Full Coverage gap

M06-L/6-31G*

HSEH1PBE

VDW-DF

H F Cl H/F H−Cl F/Cl M06-L

5.8 4.1 0.6 4.9 2.2 1.8 a

6.6 5.1 1.8 5.8 3.0 2.6

5.2 3.5 0.6 4.3 1.8 1.3

bare H F Cl H/F H−Cl F/Cl VDW-DF

Figure 2. Optimized structure and density of states determined for the biphenylene sheet fully functionalized with hydrogen atoms on both sides of the sheet, at the VDW-DF/DZP level. (Distances in Å, C−H binding energy per H atom in kcal/mol.)

3.2. Effect of Covalent Functionalization on the Infinite Sheet. It is well-known that chemical functionalization can open a band gap in graphene. In particular, the fully hydrogenated graphene sheet (graphane)32 is an insulator with a band gap larger than 4 eV.32 We calculated that the fully hydrogenated biphenylene sheet has a band gap of 5.2 and 5.8 eV, at the VDW-DF/DZP and M06-L/6-31G* levels of theory, respectively. It is increased to 6.6 eV if the HSEH1PBE/6-31G* method is employed. Thus, hydrogenation turns the infinite biphenylene sheet from metal to a wide gap insulator. The optimized structure and DOS of the double side hydrogenated biphenylene sheet are presented in Figure 2. The addition of hydrogen is extremely favorable from an energetic stand point. At the M06-L/6-31G* and VDW-DF/DZP levels the binding energy (BE) per H atom is 66.3 and 67.6 kcal/mol, respectively. The nice agreement between the M06-L and VDW-DF results for the BE is in line with our previous findings about the performance of these two functionals. The addition of 2 H atoms to form the fully hydrogenated sheet is 23.3 kcal/ mol larger than the dissociation energy of the H2 molecule (103.1 kcal/mol at the M06-L/6-31G* level). This is an important result because only in a few cases it is energetically favorable to break the strong H−H bond, and thus, the biphenylene sheet may be a good candidate to replace metal catalysts in hydrogenation reactions. Although hydrogen opens a band gap in the infinite sheet, it is too large for electronics, so we investigated the addition of other groups at full coverage. The results are presented in Table 2.

bare H F Cl H/F H−Cl F/Cl

9.00 9.18 9.34 11.28 9.25 9.94 10.00 a 9.14 9.34 9.49 11.02 9.40 10.1 10.13

b

C−X/C−Yb

C−Ca

7.50 7.69 7.86 8.61 7.70 8.34 8.40 b

1.11 1.37 1.78 1.10/1.37 1.11/1.76 1.36/1.76 C−X/C−Yb

1.44 1.53 1.58 2.66 1.55 1.71 1.76 C−Ca

1.12 1.40 1.81 1.12/1.40 1.12/1.79 1.39/1.79

1.47 1.56 1.61 2.42 1.58 1.72 1.77

7.61 7.82 7.96 8.71 7..89 8.4 8.48

a

CC bond which connects the biphenylene units. bC−X and C−Y denote the distance between the carbon atoms and the heteroatoms.

The use of fluorine instead of hydrogen has similar consequences. The binding energy per fluorine atom is 68.7 and 67.5 kcal/mol, at the M06-L/6-31G* and VDW-DF/DZP levels, respectively. The band gap is 5.1 eV, at the HSEH1PBE/ 6-31G* level. The latter value is 1.7 eV smaller than that computed for the fully hydrogenated sheet, at the same level. Thus, it may indicate that at full coverage the gap may be tuned if the functional group is changed. To test this hypothesis further, we replaced fluorine with chlorine as functional group. The band gap obtained for the chlorinated nanocomposite was 0.6 eV, at the VDW-DF/DZP level, but the structure can no longer be considered a 2D sheet because most of the CC distances are abnormally large. As it can be appreciated in Table 2, the C−C distance which connects to biphenylene units is 2.42 Å, and thus, the C−C bond is broken. Also, the lattice parameters are elongated by 2.28 and 1.11 Å for axis a and b, respectively. The binding energy per chlorine atom is 10.1 kcal/ mol, at the M06-L/6-31G* level. This value is much smaller than those computed for hydrogen and fluorine and reflects the

Table 1. Band Gaps (eV) Determined for Biphenylene Based Nanoribbons and Sheet n

M06L/6-31G* H terminated

HSEH1PBE/6-31G* H terminated

VDW-DF/DZP H terminated

M06L/6-31G* OCH3 terminated

HSEH1PBE/6-31G* OCH3 terminated

2 3 4 5 6 7 8 ∞

1.2 0.6 0.1 0.0 metal metal metal metal

1.7 1.0 0.6 0.3 0.087 0.065 0.0 metal

1.1 0.5 0.1 0.0 metal metal metal metal

1.0 0.5 0.2 0.1 metal metal

1.5 0.9 0.6 0.4 metal metal

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low stability of the sheet against chlorination, probably because of the repulsion that exists between the large chlorine atoms. In order to avoid this problem we extended our investigations to assay the infinite sheet functionalized with different groups on opposite sides. The sheets which are functionalized with hydrogen on one side and with chlorine on the other present a band gap of 3.0 eV, at the HSEH1PBE/6-31G* level, close to that exhibited by rutile, and for this reason may be attractive for photovoltaic applications. When chlorine and fluorine are added on different sides we obtained the lowest band gap, i.e., 2.6 eV. The combination of fluorine and chlorine increases the lattice parameters by 1.00 and 0.90 Å for axis a and b, respectively. The CC distance between the biphenylene units is extremely large, 1.76 Å, but it is close to the long CC distances observed by Fokin et al. 33 for 2-(1-diamantyl)[121]tetramantane. The binding energy for addition of a F/Cl pair onto the biphenylene sheet is 71.10 kcal/mol, at the M06-L/631G* level of theory. Although this value is about half of the binding energy of a pair of fluorine atoms, it is significantly larger than that corresponding to a chlorine pair. Thus, the value is large enough to permit the addition of both groups. Since the unfunctionalized structures have been synthesized, it may be possible to prepare the F/Cl functionalized sheet by transferring the bare sheet to an inert substrate, functionalize one side, transfer again the sheet, and modify the side that has not reacted. Finally, the mixed functionalization with fluorine and hydrogen is not useful since the band gap is too large, namely, 5.8 eV at the HSEH1PBE/6-31G* level. Although it is important to show that full coverage can turn the biphenylene sheet into an insulator, it is also relevant to the study of the effect of lower levels of hydrogenation. In the first place, we analyzed where it is more convenient to attach the very first hydrogen atom. Two positions are available, a carbon atom which belongs to the square or another which connects two pentagons. These are sites 1 and 2, as can be appreciated in Figure 3. At the VDW-DF/DZP level, the addition to site 1 occurs with a BE of 56.3 kcal/mol, whereas onto site 2 the BE is lower, namely, 34.3 kcal/mol. The addition of one H atom to site 1 does not alter the metallic character of the biphenylene sheet. However, for site 2 we found that the electronic properties are modified since the system is a semimetal for the spin up channel and a semiconductor in the spin down channel (gap = 0.26 eV). The DOS can be appreciated in Figure 3. In both cases the addition of a single H atom induces a magnetic moment of 0.90 and 0.98 μB, for sites 1 and 2, respectively. After the study of the addition of one hydrogen atom, we focused on the addition of two H atoms, because in previous works we found that when functional groups are paired on graphene, the BE values are dramatically increased.24,34−36 The addition patterns considered are presented in Figure 4. A total of seven configurations were analyzed. The BE and band gaps are presented in Table 3. In line with the results obtained for the addition of one H atom, we found that the second hydrogen atom prefers to be attached onto the carbon atom which belongs to the same hexagon and is part of the square. In that case, the BE per H atom is 2 kcal/mol lower than the value obtained for the fully hydrogenated biphenylene sheet. The second most stable structure is pattern 5, which has two H atoms in para position on the same hexagon. The nonmagnetic solutions are more stable than the magnetic ones by 0.69 and 0.65 eV for sites 1 and 5, respectively. These results resemble graphene, because

Figure 3. Optimized structure and density of states (up = blue, down = red) determined for the biphenylene sheet functionalized with one hydrogen atom, at the VDW-DF/DZP level. (Distance in Å, C−H binding energy per H atom in kcal/mol.)

two H atoms prefer the nonmagnetic ortho or the para addition and the meta addition, which is site 4 in our case and is magnetic, being more stable than the nonmagnetic one by 0.1 eV.36 However, there is an important difference between graphene and the biphenylene sheet: in contrast with the result obtained for graphene, we found that for the ortho arrangement the structure which has the H atoms on opposite sides of the sheet is less stable than that with both groups on the same side by 11.9 kcal/mol. Among the other additions investigated, we found that for site 7 the magnetic and nonmagnetic solutions are degenerated whereas for site 6 the magnetic solution is preferred by 0.12 eV. We continued the addition of hydrogen atoms by attaching hydrogen onto the carbon atoms which belong to the squares. The addition of four H atoms on the same side of the sheet can be performed in two ways; the four H atoms can belong to the same square, or they can be paired 24979



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Figure 4. Addition sites considered for the attachment of two hydrogen atoms onto the biphenylene sheet. The first atom is attached to the rounded carbon and the second on sites 1−7.

Table 3. Binding Energies (kcal/mol per H Atom), Band Gaps (eV), and Magnetic Moments (μB) Determined for the Addition of Two, Four, and Six Hydrogen Atoms to the Biphenylene Sheet, at the VDW-DF/DZP Level of Theory

2H

4H 6H 8H

pattern

binding energy

band gap

1-SS 1-SS 1-OS 2-SS 2-SS 3-SS 3-SS 4-SS 4-SS 5-SS 5-SS 6-SS 6-SS 7-SS 7-SS 4-OS 4-SS 6-OS 6-SS 8-SS 8-OS

64.4 57.7 59.4 55.3 51.2 51.3 46.0 48.6 49.9 61.7 54.0 44.5 45.8 55.5 55.5 60.2 63.7 59.3 59.2 58.2 60.6

metal α = metal, β = metal metal semimetal metal α = 0.07, β = 0.07 metal α = semimetal, β = 0.20 α = metal, β = metal metal metal α = 0.12 β = 0.20 α = 0.35, β = metal metal α = metal β = metal metal metal metal metal 0.12 metal

Figure 5. Addition sites considered for the attachment of four, six, and eight hydrogen atoms onto the biphenylene sheet (same side), gaps in eV, and C−H binding energy per H atom in kcal/mol.

magnetic moment

these additions as indicated by the analysis of the values presented in Table 3. The addition of 4 and 6 H atoms do not open a gap. However, for the addition of 8 H atoms on the same side we found that a band gap of 0.2 eV is opened, but if the alternated pattern is employed the gap is not opened. Therefore, significant hydrogenation is needed to disrupt the metallic character of the biphenylene sheet, or it must be deposited on a suitable substrate and hydrogenated only on one side with H atoms only on the squares. 3.3. Effect of Covalent Functionalization on the Armchair Nanoribbons. We analyzed the effect of the chemical functionalization on the electronic properties of biphenylene based nanoribbons. In the first place, we replaced the hydrogen atoms of the edge by the more electronegative fluorine atoms. As we can appreciate in Figure 6, in order to avoid the repulsion between the F atoms, the nanoribbon adopts a twisted configuration. However, the consequences on the band gap are minimal since it is altered by 0.1 eV. When the much bulker methoxy groups employed experimentally are attached, the twist is more significant, but the band gap of the NR2 is reduced by 0.2 eV,

0 2.0 0 0 2.0 0 2.0 0 1.86 0 2.0 0.0 1.94 0 1.91

on different squares. The two structures considered are presented in Figure 5. The addition forming a line is more stable. The BE per H atom is 63.4 kcal/mol, slightly smaller than the one computed for the addition of two H atoms to the square. When six and eight H atoms are attached on the same side of the sheet, the BE decreases slightly; at the VDW-DF/DZP level the BE values per H atom are 59.0 and 58.2 kcal/mol, respectively. The decrease of the BE can be explained to the fact that we attached the H atoms to the same side of the sheet. If the atoms are alternated on different sides of the sheet the BE values do not decrease. In effect, the addition of 6H atoms is the limit at which the additions on the same side and opposite sides display similar energy. When 8 hydrogen atoms are attached following alternated pattern employed for the fully functionalized sheet, the BE is 61.1 kcal/mol, at the VDW-DF/DZP level. As regards the electronic properties, it is difficult to open a band gap for

Figure 6. Optimized unit cell determined for the functionalized biphenyl based nanoribbons, at the VDW-DF/DZP level. 24980



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with respect to the hydrogenated nanoribbons. For the sake of completeness we studied for all nanoribbons the termination of the edges with methoxy groups. The results presented in Table 1 indicate a similar trend. The metallic character is more readily seen when the ribbons present the methoxy group. Indeed, for NR6 the HSEH1PBE/6-31G* method indicated a clear metallic character while for the hydrogenated counterpart, a small band gap of 0.087 eV is observed with the same method. Thus, the twisted induced functionalization is not effective as expected as regards the band gap opening issue. Finally, because even the single hydrogenated edges are known to be very reactive,24,37 we studied the addition of a second hydrogen atom for each carbon at the edge, with the aim of introducing C sp3 centers and altering the electronic properties. Such structure can be appreciated in Figure 6. The second hydrogen atom induces a twist, but the deformation is smaller than that caused by fluorine. In terms of gap opening, the consequences were quite unexpected since the gap was reduced from 1.2 to 0.14 eV. The addition of the second H atom to each carbon at the edge is very favorable since the BE is 58.7 kcal/mol per H atom at the M06-L/6-31G* level of theory, so the reactivity of the edge is comparable to that exhibited by the sheet.

REFERENCES

(1) Service, R. F. Carbon Sheets an Atom Thick Give Rise to Graphene Dreams. Science 2009, 324, 875−877. (2) Drexler, E. K. Engines of Creation; Anchor Books: New York, 1986. (3) Schlutter, F.; Nishiuchi, T.; Enkelman, V.; Mullen, K. Octafunctionalized Biphenylenes: Molecular Precursors for Isomeric Graphene Nanostructures. Angew. Chem., Int. Ed. 2014, 53, 1538− 1542. (4) Hudspeth, M. A.; Whitman, B. E.; Barone, V.; Peralta, J. E. Electronic Properties of the Biphenylene Sheet and its Onedimensional Derivatives. ACS Nano 2010, 4, 4565−4570. (5) Samarkoon, D. K.; Chen, Z.; Nicolas, C.; Wang, X. Q. Structural and Electronic Properties of Fluorographene. Small 2011, 7, 965−969. (6) Bian, S.; Scott, A. M.; Cao, Y.; Liang, Y.; Osuna, S.; Houk, K. N.; Braunschweig, A. B. Covalently Patterned Graphene Surfaces by a Force-Accelerated Diels−Alder Reaction. J. Am. Chem. Soc. 2013, 135, 9240−9243. (7) Ghaderi, N.; Peressi, M. First-Principle Study of Hydroxyl Functional Groups on Pristine, Defected Graphene, and Graphene Epoxide. J. Phys. Chem. C 2010, 114, 21625−21630. (8) Al-Aqtash, N.; Vasiliev, I. Ab Initio Study of Carboxylated Graphene. J. Phys.Chem. C 2009, 113, 12970−12975. (9) Cabrera-Sanfelix, P.; Darling, G. R. Dissociative Adsorption of Water at Vacancy Defects in Graphite. J. Phys. Chem. C 2007, 111, 18258−18263. (10) Dai, J.; Yuan, J. Modulating the Electronic and Magnetic Structures of P-doped Graphene by Molecule Doping. J. Phys.: Condens. Matter 2010, 22, 225501. (11) Denis, P. A. Band Gap Opening of Monolayer and Bilayer Graphene Doped With Aluminium, Silicon, Phosphorus, and Sulphur. Chem. Phys. Lett. 2010, 492, 251−257. (12) Denis, P. A. When Noncovalent Interactions are Stronger Than Covalent Bonds: Bilayer Graphene Doped With Second Row Atoms, Aluminum, Silicon, Phosphorus and Sulphur. Chem. Phys. Lett. 2011, 508, 95−101. (13) Denis, P. A. Tuning the Electronic Properties of Doped Bilayer Graphene with Small Structural Changes. Comput. Theor. Chem. 2012, 974, 21−25. (14) Denis, P. A. Chemical Reactivity of Lithium Doped Monolayer and Bilayer Graphene. J. Phys. Chem. C 2011, 115, 13392−13398. (15) Denis, P. A. On the Addition of Aryl Radicals to Graphene: The Importance of Nonbonded Interactions. ChemPhysChem 2013, 14, 3271−3277. (16) Denis, P. A. Chemical Reactivity of Electron-Doped and HoleDoped Graphene. J. Phys. Chem. C 2013, 117, 3895−3902. (17) Denis, P. A. Organic Chemistry of Graphene: The Diels−Alder Reaction. Chem.Eur. J. 2013, 19, 15719−15725. (18) Denis, P. A.; Iribarne, F. A First-Principles Study on the Interaction between Alkyl Radicals and Graphene. Chem.Eur. J. 2012, 18, 7568−7574. (19) Denis, P. A.; Iribarne, F. Thiophene Adsorption on Single Wall Carbon Nanotubes and Graphene. THEOCHEM 2010, 957, 114−119. (20) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, Revision A.1; Gaussian, Inc.: Wallingford, CT, 2009. (21) 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. (22) 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. (23) 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. (24) Denis, P. A. Density Functional Investigation of Thioepoxidated and Thiolated Graphene. J. Phys. Chem. C 2009, 113, 5612−5619.

4. CONCLUSIONS We have employed density functional theory with periodic boundary conditions to study the stability of functionalized biphenylene 1D and 2D nanomaterials, and the effects of the groups covalently attached on the electronic properties. The following are considered to be the most relevant findings: (1) The biphenylene sheet has a metallic character that is difficult to inhibit employing low levels of functionalization. (2) Hydrogenation at full coverage induces a metal to insulator transition. However, the band gap opened is very large, i.e., 6.6 eV. The use of other functional groups such as fluorine or chlorine reduces the band gap and makes possible the regulation of the band gap. (3) The most useful chemical modification in terms of gap opening is the combination of hydrogen/chlorine or fluorine/chlorine. For the latter functional groups band gaps similar to those of rutile were calculated at the HSEH1PBE/6-31G* level of theory. (4) The biphenylene sheet functionalized on one side with fluorine and with chlorine on the other presented a CC bond equal to 1.76 Å, one of the longest reported up to date. (5) We found that, for armchair biphenylene nanoribbons, the twist induced by the functionalization of the edges does not increase the band gaps, but in some cases the gaps are reduced as we observed when the edges were saturated with 2 hydrogen atom on each carbon. (6) The high reactivity of the sheet suggests that it is may have promising applications in catalysis.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 0059899714280. Fax: 00589229241906. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The author thanks PEDECIBA Quimica, ANII-project FSE6160 and CSIC for financial support. 24981



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Stability and Electronic Properties of Biphenylene Based

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