Design of van der Waals Two-Dimensional Heterostructures from

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Design of van der Waals Two-Dimensional Heterostructures from Facially Polarized Janus all-cis 1,2,3,4,5,6-hexafluorocyclohexane (CHF) 6

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Saied Md Pratik, Abdulrahiman Nijamudheen, and Ayan Datta J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 23 Dec 2016 Downloaded from http://pubs.acs.org on December 26, 2016

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The Journal of Physical Chemistry

Design of van der Waals Two-Dimensional Heterostructures from Facially Polarized Janus all-cis 1,2,3,4,5,6-hexafluorocyclohexane (C6H6F6) Saied Md Pratik,1 A. Nijamudheen,2 Ayan Datta1* 1

Department of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur-700032,

West Bengal (India), 2

Department of Chemistry, University at Buffalo, The State University of New York, Buffalo, NY

14260-3000 *Email: [email protected] Abstract Recently synthesized all-cis-1,2,3,4,5,6-hexafluorocyclohexane (1) has a large dipole moment (6.2 Debye) and a uniaxial facial polarization. Based on density functional theory (DFT) calculations, it is shown that both the “positive” and “negative” surfaces of 1 can recognize flat aromatic molecules (X, X= benzene, pyrene, and coronene), 2D materials (graphene), and their fully hydrogenated analogues via attractive noncovalent interactions. 1 can be sandwiched in between graphene and graphane layer where the enhanced polarity of the axial C-H and C-F bonds leads formation of an unusual ‘triple decker’ complex. On adsorption with 1, the band gap of graphane reduces from 3.40 eV to 2.05 eV which may be useful for visible light energy conversion applications. We have shown the controlled tuning of structural and electronic properties in 1-benzene, 1-cyclohexane, and benzene-1-cyclohexane complexes by the application of an external electric field along the polarization axis. The extended analogue of 1, hydroflourinated graphene (HFG) with semiconducting properties (band gap ~3.0 eV)

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can form strong C-H···F-C interlayer interactions with graphane to form a stable, metallic bilayer. Our calculations show that the two-dimensional HFG can be realized by high pressure topochemical condensation of monofluoroacetylene (C2HF) for which the barrier of activation is 18.7 kcal/mol.

Introduction Non-covalent interactions have profound importance in natural science and play a significant role in structural chemistry, biology as well as in material science.1–7An atomistic level understanding of weak dispersion interactions can assist crystal engineering and the controlled synthesis of functional polymers, supramolecular assemblies, and heterostructured nanomaterials.8–16 These forces also play an important role in vital biological processes such as protein folding and enzyme catalysis.4,17 Tailoring the mechanical, chemical, and opto-electronic properties of two-dimensional materials through noncovalent interactions with suitable functional molecules and other layered materials has been recently proposed as a design principle.18–29 Experimental fabrications of hydrogen and fluorine terminated graphene (graphane, and fluorogrphane, respectively) have revealed exciting electronic and mechanical properties for them.30–34 Graphane (CH)n, has a large direct band gap (>3.5 eV) that can exhibit diverse mechanical, electronic and magnetic properties while fully fluorinated graphene, fluorographene (CF)n (band gap =3.43 eV) is well explored for its exceptional stability, ease of synthesis, and interesting electronic and mechano-chemical properties.32,35–38 Recently, Singh and Bester proposed that hydrofluorinated graphene (HFG), a 2D analogue of polyvinyllidene fluoride is a wide band gap (~3.3 eV) semiconductor.39 However, for majority of electronic applications and solar light energy harvesting the band gap in H or F functionalized graphene has to be reduced to < 2.5 eV. The band gap in these systems can be tuned by interfacing them with other layered materials or small molecules through charge transfer mechanisms. Among the various kind of noncovalent interactions, C—H···π, π···π, and C−H···F−C interactions are particularly interesting to pursue the properties of interest in graphene, ACS Paragon Plus Environment

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graphane and flurographane based materials that are different from their original counterparts.18,20,27,40 Similarly, application of external electric field in bi-, tri-, and few-layers of 2D materials is a promising approach to manipulate and control their electronic structures.20,27,28,41 Recently developed van der Waals heterostructures where two or three different monolayers of materials such as graphene, 2D h-BN, 2D MoS2, and 2D WS2 are stacked through van der Waals interactions, have been shown to play a crucial role to establish novel optoelectronic properties.13–16 Hereof, based on density functional theory (DFT) calculations, Fokin et. al. have elucidated the existence of strong C-H···σ interactions between the multilayered graphanes.42 The heterolayers formed between graphene and hydrogenated or fluorinated graphene via

C ̶ H···π attractive

interactions can effectively modify the electronic properties of individual layers.18,19 Chen and coworkers have predicted a novel graphane/fluorographene heterolayer, which is stabilized via weak CH···F-C noncovalent bonding between graphane and fluorographene wherein, the band gap in heterolayer is substantially lowered (0.5 eV) compared to either of the graphane or fluorographene monolayers.27 Li et. al. showed that hydrofluorinated graphene monolayer is a semiconductor (bandgap = 2.82 eV) which becomes metallic (zero band gap) in its bilayer.28 Similarly, a number of experimental and theoretical studies have shown that the deposition of the small molecules on the graphene and graphene-like materials is a potential strategy to control their optoelectronic properties.19,21,43,44 Recently, Wong et al. and Chen. et al. have theoretically predicted that the noncovalently deposited dipolar molecules like mechanochromic polymers and ladder-like polydiacetylene derivatives over graphene nanoribbons (GNRs) can significantly change the electronic properties.44,45 Manna et al. have demonstrated that the noncovalent deposition of organic donor or acceptor molecules over graphene can alter its electronic, optical and CT properties.46


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Scheme 1: Structure of all-cis-1,2,3,4,5,6-hexafluorocyclohexane (1). The external electric field (FZ) was applied either along positive to negative face (+Z direction) or along the reverse direction (-Z direction). Recently, the O’Hagan group have reported the synthesis of a Janus type molecule all-cis1,2,3,4,5,6-hexafluorocyclohexane (1), which has an unusually large facial polarization and a large dipole moment of 6.2 D.47,48 Because of the large polarity, 1 exhibits strong C-H···π interaction with benzene with a binding energy of -7.9 kcal/mol at MP2 and -6.4 kcal/mol calculated at SCS-MP2 levels of theory.49 In another study, it was demonstrated that 1 can form exceptionally stable complexes through simultaneous binding of Na+ and Cl- ions with its negative and positive face, respectively.50 Our previous computational study showed that 1 can interact with cyclohexane via C-H···F-C interactions.51 It is expected that such a large dipole moment and facial polarization could be useful to design supramolecular assemblies and aggregates with novel optoelectronic properties. The structure of 1 is interesting because it can be considered as a small molecular level analogue for 2D material, fluorographene and more closely to hydrofluorinated graphene (HFG).33,39 Therefore, understanding the mechanisms and binding affinities of 1 with aromatic molecules and their hydrogenated analogues visa-vis in their solid state analogues could provide insights to engineer van der Waals heterolayers. In this article, we have performed dispersion corrected DFT calculations following a bottom up approach to demonstrate the importance of noncovalent interactions of 1 with small planar/non-planar ACS Paragon Plus Environment

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molecules and materials namely benzene, pyrene, coronene, graphene and their fully saturated counterparts. 1 forms strong C—H···π interactions with benzene, pyrene, coronene, and their higher 2D analogue graphene. Similar attractive interactions are preserved when the aromatic molecules are replaced by their corresponding hydrogenated derivatives such as cyclohexane, perhydropyrene, perhydrocoronene and 2D graphane where the C—F···H—C interactions are important. We have shown that because of the separate positive and negative facial polarity, 1 can bind through both C— H···π and C ̶ H···F ̶ C interactions when sandwiched in between benzene and cyclohexane as well as in their extended molecular and 2D analogues. The binding energies for these systems are calculated and different stabilizing/de-stabilizing forces that contribute to the overall stability are critically analyzed. The dipole moments (µ) and HOMO-LUMO gaps can be altered by applying external electric field along a particular direction. Periodic dispersion corrected DFT, DFT-D2 calculations revealed that noncovalently deposited 1 over graphane reduce its band gap by 1.35 eV. The structural and electronic properties for heterolayers and sandwiched structures of extended 2D analogue of 1, namely hydrofluorinatedgraphene (HFG or (C2HF)n) with graphene and graphane are studied. While previous theoretical studies have shown that HFG has interesting electronic and mechanical properties, it has not been synthesized experimentally.39 The present calculations predict that the formations of layered HFG monolayers might be plausible from the topochemical condensation of C2HF molecules under pressure. Computational Details Based on our previous benchmark study on 1, calculations for the finite molecular models were carried out at the M06/6-31+G(d,p) level using Gaussian 09 package.51–53 Additionally, harmonic vibrational frequencies were calculated on energy minimized structures to ensure that the obtained geometry was either a minimum or a first-order saddle point. The basis set superposition error (BSSE) corrected binding energies (∆E) for the stacked as well as the sandwich conformers were calculated as ∆E = (Ecomplex – (E1 + E2 + …)) (where, Ecomplex, E1, E2 are the energy of the complex and individual ACS Paragon Plus Environment

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entities, respectively) by applying a counterpoise correction (CP) scheme.54 The effects of external electrical field was studied by applying an external electric field (FZ) along the perpendicular direction to the molecular plane where a +Z direction is assumed when the FZ is applied from positive to negative face and hence, the reverse as –Z direction. (see Scheme 1). The strength of FZ varied from 0.0 to 0.8 V/Å along both directions with a step size of 0.1 V/Å. Energy decomposition analysis (EDA) was performed at the BLYP-D3(BJ)/TZ2P level using the Amsterdam density functional (ADF 2008) program to dissect and quantify different components that contribute to the total interaction energy.55–57 The noncovalent interaction regions were calculated with NCIPLOT program (see supporting Info. File) and visualized with VMD.58,59 For the extended 2D systems, DFT calculations were performed using the projected augmented wave (PAW) approach. The exchange-correlation potential was incorporated by a generalized gradient approximation (GGA) with Perdew–Burke–Ernzerhof (PBE) functional as implemented in the Vienna ab initio simulation package (VASP 5.3).60,61 The ionic core and electron interactions were taken into account using ultrasoft pseudopotential methods. During geometry optimizations, all atomic positions were fully relaxed until all the forces acting on the atoms were less than 10-3 eV/Å. The Brillouin zone was sampled by 9×9×3 Monkhorst-Pack k-point mesh for optimization and 12×12×1 (or, 11×11×1) for the density of states (DOS) and band structure calculations. Noncovalent van der Waals interactions were taken into account using the DFT-D2 empirical formalism of Grimme and co-workers.62 Spurious interactions between the neighboring cells were avoided by considering a large vacuum region (15Å20Å).


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Results and Discussion The positive face of 1 can form C-H···π interactions with the flat aromatic molecules (X) benzene, pyrene, and coronene as shown in Figure 1(a)–1(c). The axial H-atoms are directed towards the Catoms of the benzene through weak C-H···π bonding of distances ~2.70 Å that results in complete parallel stacked arrangement (Figure 1(a)) with the BSSE corrected binding energy (∆E) -5.7 kcal/mol calculated at M06/6-31+G(d,p) level of theory. As reported by O'Hagan et al., C-H···π distance between benzene and 1 are 2.69 Å at B3LYP-D3/def2-TZVP and 2.71 Å at MP2/aug-cc-pVDZ level of calculations, for this completely parallel stacked arrangement and our results at M06/6-31+G(d,p) level of theory are in fair agreement.49 Further increasing the π-surface from benzene to pyrene, the binding energy increases by ~4 kcal/mol (Figure 1(b)). However, this interaction is almost saturated in 1— coronene complex (Figure 1(c)) as the ∆E only increase by 0.9 kcal/mol with respect to the 1—pyrene complex (see Table 1). In both, 1—pyrene and 1—coronene complexes, the C-H···π distances are in the range 2.58 Å to 2.70 Å.


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Figure 1: The optimized geometries for 1(a) 1-benzene, 1(b) 1-pyrene, 1(c) 1-coronene, 1(d) 1cyclohexane, 1(e) 1-perhydropyrene, 1(f) 1-perhydrocoronene, 1(g) benzene-1-cyclohexane, 1(h) pyrene-1-perhydropyrene and 1(i) coronene-1-perhydrocoronene. The important C-H···π and C-H···FC interactions are shown.


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Table 1: BSSE-corrected binding energies (ΔE) for 1-X (X = benzene, pyrene, and coronene), 1-Y (Y = cyclohexane, perhydropyrene, perhydrocoronene), and X-1-Y complexes calculated at the M06/631+G(d,p) level of theory. Complex

∆E (kcal/mol)

(1a) 1-benzene

-5.7

(1b) 1-pyrene

-10.0

(1c) 1-coronene

-10.9

(1d) 1-cyclohexane

-1.5

(1e) 1-perhydropyrene

-2.4

(1f) 1-perhydrocoronene

-2.5

(1g) benzene-1-cyclohexane

-7.9

(1h) pyrene-1-perhydropyrene

-13.1

(1i) coronene-1-perhydrocoronene

-14.0

In addition to the CH···π interactions with the positive face of 1, the negative face of 1 can form hydrogen bonding with the fully hydrogenated derivatives of the benzene and their higher order analogues (Y). The axial F atoms in 1 facilitate sufficiently strong intermolecular C-H···F-C interactions with the average distance of 2.59 Å toward cyclohexane in 1-cyclohexane complex (Figure 1(d)). When considering cyclohexane, perhydropyrene, and perhydrocoronene as the hydrogen bond donors, the binding energy with 1 varies as -1.5 kcal/mol in 1-cyclohexane, -2.4 kcal/mol in 1perhydropyrene, and -2.5 kcal/mol in 1-perhydrocoronene. In both 1—perhydropyrene and 1— perhydrocoronene, C-H···F-C interacting distances are ~2.50 Å (Figure 1(e) and 1(f)). Since both the positive and negative faces of 1 are simultaneously available for selective


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binding, we have investigated the effect of C-H···π and C-H···F-C intermolecular interactions when 1 is sandwiched between one aromatic surface and its corresponding fully hydrogenated system (X-1-Y) as shown in Figure 1(g)-1(i). The sandwiched complex of benzene-1-cyclohexane is found to be stable (∆E = -7.9 kcal/mol) with respect to 1-benzene and 1-cyclohexane complexes and the resulting ∆E is almost equal to the sum of binding energy (∆E) of 1-benzene and 1-cyclohexane complexes (i.e, ∆Ebenzene-1-cyclohexane ≈ ∆E1-benzene + ∆E1-cyclohexane, Table 1). Herein, the C-H···π and C ̶ H···F ̶ C bond distances are ~2.35 Å and ~2.76 Å (Figure 1(g)). For pyrene-1-perhydropyrene complex the important C-H···π distances are 2.37 Å, 2.37 Å and 2.52 Å, respectively, where the axial hydrogens are pointed towards center of the pyrene rings, while the C ̶ H···F ̶ C networks arise at the distances of 2.45 Å to 2.79 Å (Figure 1(h)). In coronene-1-perhydrocoronene complex C-H···π interactions are in the range of ~2.49 Å and C-H···F-C interacting networks are at 2.46 Å -2.76 Å (Figure 1(i)). These short CH···π and C-H···F-C bonding distances in X-1-Y complexes reveal that simultaneous binding of the positive and negative faces of 1 with flat molecules as well as their hydrogenated analogues would form stable stacked complexes. Calculated binding energy for pyrene-1-perhydropyrene complex is 13.1 kcal/mol, larger than the sum of binding energies of 1-pyrene and 1-perhydropyrene by -0.7 kcal/mol. Similarly, in coronene-1-perhydrocoronene, binding energy is computed to be -14.0 kcal/mol which is -0.6 kcal/mol more than the sum of individual binding energies of 1-coronene and 1perhydrocoronene complexes. An enhanced binding energy for the sandwich systems is predicted to be due to the increased polarity of the axial C-F and C-H bonds in 1.


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Figure 2: HOMO and LUMO plots for 1-coronene, 1-perhyhrocorone, and coronene-1perhydrocoronene. The HOMO-LUMO gaps (∆H-L) in benzene, pyrene and coronene are calculated to be 6.98 eV, 4.17 eV and 4.35 eV, respectively. Upon interaction with 1, the HOMO-LUMO gap for these molecules remains unperturbed as the HOMO/LUMO is stabilized by equivalent extents. As shown in Figure S1 (in supporting Info. File) and Figure 2, both the HOMO and LUMO are positioned on benzene, pyrene and coronene, respectively in 1-benzene, 1-pyrene and 1-coronene complexes and hence, their electronic properties are controlled by the corresponding π-conjugated systems (X). However, in 1-Y complexes, the ∆H-L is found to decrease from 6.14 eV to 5.14 eV as Y varies from cyclohexane to perhydrocoronene, respectively. In stacked 1-Y complexes, the HOMO and LUMO states are delocalized between Y and 1, respectively (Figure 2). Such kind of charge redistribution in these complexes result in significant reduction in ∆H-L as previously reported in charge transfer and hydrogen bonded complexes.63,64 However, for the sandwiched complexes (X-1-Y), both the HOMO and LUMO are delocalized over X and hence controlled by X as found in 1-X complexes. Due to the absence of significant charge redistribution in 1-X and X-1-Y, the ∆H-L remains almost unchanged. ACS Paragon Plus Environment

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Figure 3: NCIPLOT showing attractive CH···π and CH···F-C interactions in 3(a) 1-benzene, 3(b) 1pyrene, 3(c) 1-coronene, 3(d) 1-cyclohexane, 3(e) 1-perhydropyrene, 3(f) 1-perhydrocoronene, 3(g) benzene-1-cyclohexane, 3(h) pyrene-1-perhydropyrene and 3(i) coronene-1-perhydrocoronene.


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Table 2: Total interaction energy (Etotal) and different energy components contributing to the Etotal value for stacked complexes with 1, calculated at the BLYP-D3(BJ)/TZ2P level. Energies are reported in kcal/mol. Conformation

Epau

Eelec

Eorb

Edis

Etotal

1 (a)

9.20

-6.90

-3.31

-7.01

-7.20

1 (b)

13.39

-7.06

-5.25

-13.36

-12.29

1 (c)

14.85

-6.64

-5.90

-15.91

-13.61

1 (d)

6.64

-3.27

-1.17

-4.49

-2.28

1 (e)

7.81

-4.21

-1.30

-6.34

-4.05

1 (f)

8.47

-4.56

-1.38

-7.23

-4.71

1 (g)

14.00

-7.97

-4.26

-11.30

-9.54

1 (h)

24.02

-12.61

-7.57

-21.57

-17.73

1 (i)

26.52

-12.88

-8.40

-25.00

-19.76

In order to gain quantitative understanding for the noncovalent interactions, we have plotted the noncovalent interactions regions by using NCIPLOT program for 1-X, 1-Y and X-1-Y complexes and the total interaction energy (Etotal) is deciphered by energy decomposition analysis (EDA). NCIPLOT demonstrates that substantial stabilization due to C-H···π and C-H···F-C interactions for 1-X and 1-Y complexes, respectively, and both of them are available in X-1-Y complexes (Figure 3). From the reduced density gradient (s) vs sign(λ2)ρ plot for all these complexes (Figure S2 in Supporting Info. File) it is found that sign(λ2)ρ are ~0.02 a.u and ~-0.01 a.u respectively, at s < 0.5 a.u which indeed confirms that the interactions are noncovalent and are enumerated as van der Waals interactions. Further analyses are carried out to assign the origin of binding energy in each complexes by means of


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EDA at the BLYP-D3(BJ)/TZ2P level. In an EDA scheme, the total interaction energy (Etotal) is separated into its constituent energy terms shown as: 𝐸!"!#$ =𝐸!"# + 𝐸!"!# + 𝐸!"# + 𝐸!"# where, Epau, Eelec, Eorb, and Edis are the energies due to Pauli repulsion, electrostatic interaction, orbital interaction and dispersion interaction, respectively. As reported in Table 2, in 1-X and X-1-Y, the Pauli repulsion is almost balanced by dispersion interactions, whereas in 1-Y orbital and dispersion interactions overcompensate Pauli repulsion. Hence, in the former case, both the electrostatic and orbital interactions are of equal importance but in the latter, electrostatic interactions are major stabilizing forces, rendering 1-X more stable with respect to the 1-Y. In all stacked complexes, dispersion energy plays a critical role in stabilizing C-H···π interactions in 1-X, C-H···F-C interactions in 1-Y and both of these interactions simultaneously in X-1-Y complexes.

Figure 4: Relative change in (a) dipole moment (µ) in Debye and (b) Relative HOMO-LUMO gaps (∆H-L) in eV on application of external electric field (FZ) in V/Å for 1, 1-benzene, 1-cyclohexane, and benzene-1-cyclohexane complexes.


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We have studied the effect of external electric field on 1, 1-benzene, 1-cyclohexane and benzene-1-cyclohexane complexes. Application of an electric field has been shown to modulate the electronic and transport properties in 2D materials and flat molecules.65,66 Compared to the intrinsic dipole moment in 1 (µ= 6.27 D), its complexes 1-benzene, 1-cyclohexane and benzene-1-cyclohexane complexes have enhanced permanent dipole moments of 7.56 D, 7.03 D, and 8.01 D, respectively. Under the influence of an external electric field along +Z direction (F+Z, see Scheme 1) charge separation is enhanced in these polar complexes while the opposite trend is observed when field is applied along opposite direction (F-Z). As a consequence, µ in these complexes are found to increase at F+Z and decrease at F-Z with respect to FZ = 0 V/Å, as shown in Figure 4(a). The calculated µ at M06/631G+(d,p) level of theory varies from 3.6 D to 8.8 D for 1, from 2.7 D to 12.4 D for 1-benzene, from 1.2 D to 12.7 D for 1-cyclohexane, and from 2.1 D to 16.4 D for benzene-1-cyclohexane, as the field varies from -0.8 V/Å to +0.8 V/Å. The slope of ∆µ {µ (at FZ) – µ (at F0)} is maximum in benzene-1cyclohexane and minimum in 1 amongst these complexes. Clearly, in the presence of both C-H···π and C-H···F-C interactions, 1 becomes significantly polarized in benzene-1-cyclohexane complex and hence, the change in ∆µ is maximum, such field induced polarization is minimum in 1 due to the absence of such interactions. The relative variation in HOMO-LUMO gaps (∆H-L) are plotted as a function of FZ as shown in Figure 4(b). For all systems, relative ∆H-L decreases upon increasing the strength of electric fields along -Z directions, however it is initially increases and then falls back as field is applied along +Z directions, except for the case of 1-benzene. On increasing the strength of the applied electric field, particularly along -Z direction, LUMO is more stabilized compared to HOMO for 1 and 1-benzene whereas, HOMO is destabilized while LUMO is stabilized for 1-cyclohexane and benzene-1-cyclohexane. Hence the slope in relative ∆H-L is larger in 1-cyclohexane and benzene-1cyclohexane compared to that of 1 and 1-benzene. Based on our model calculations, it is expected that the macroscopic polarization as well as the electronic properties of all 1-X, 1-Y and X-1-Y complexes can be tuned by applying external electric field. ACS Paragon Plus Environment

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Table 3: The binding energies for the complexes of 1 (and HFG) with graphene and graphane calculated at plane wave GGA-PBE level of theory. Complex

∆E (kcal/mol)

1-Graphene

-10.9

1-Graphane

-9.3

Graphene-1-Graphane

-11.7

HFG-Graphene

-1.9

HFG-Graphane

-7.9

Graphene-HFG-Graphane

-17.1

Figure 5: Optimized geometries of (a) 1-graphene, (b) 1-graphane and (c) graphene-1-graphane


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calculated at plane wave GGA-PBE level of theory. The important C-H···π and C-H···F-C interactions are shown. Subsequent to the study of the nature of the interaction of 1 with various small molecules in different configurations, we have explored the binding ability of 1 with 2D analogues of the aforesaid molecular entities. Stacked complexes of 1 with graphene and graphane as well as its sandwiched form are considered (Figure 5). At DFT-D2 level, the calculated binding energy of 1-graphene, 1-garphane, graphene-1-graphane are found to be -10.9 kcal/mol, -9.3 kcal/mol and -11.7 kcal/mol, respectively (See Table 3). This stabilization energy is anticipated form C-H···π and C-H···F-C interacting networks between 1 and graphene or/and graphane as shown in their molecular analogues. The calculated C-H···π distances are 3.14 Å in 1-graphene and 2.40-2.47 Å in graphene-1-graphane. The CH···F-C distances of 2.27-2.60 Å for 1-graphane and 2.23-2.42 Å for graphene-1-graphane are comparable to the molecular levels calculations reported in Figure 1. However, because of the increased polarity in both axial C-F and C-H bonds of 1, the C-H···π and C-H···F-C interactions becomes more prominent in graphene-1-graphane system resulting in increased binding energy with respect to 1-graphene and 1-graphane.


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Figure 6: (a) Band structure of 1-graphane. Total density of states (TDOS) (black line), partial density of states (PDOS) of graphane (red line) and 1 (green line) are shown. The energy is scaled with respect to Fermi level. The partial charge densities of (b) VBM (c) CBM at the Γ-point. Our molecular level calculations demonstrated that orbital interactions strongly modulate the electronic properties in 1-Y stacked complexes. Therefore, we have performed similar calculations to study the electronic properties of 1-graphane. The band structure, total density of states (TDOS) and the partial density of states (PDOS) calculated for 1-graphane are presented in Figure 6 (a). The GGAPBE level of calculation predicts a large band gap in graphane (Eg ~3.4 eV) which is in nice agreement with the previous report.35 Interestingly, in 1-graphane, the band gap is reduced to ~2.05 eV (Figure ACS Paragon Plus Environment

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6(a)). The PDOS analysis show that both 1 and graphane (represented with green and red line, respectively) have large band gaps. Notably, the TDOS of the composite system namely, 1 and graphane shows significant reduction in energy gap that is smaller than both 1 and graphane. To obtain further insight, we have analyzed the valance band maxima (VBM) and conduction band minima (CBM) for this system. The VBM (Figure 6(b)) is localized over graphane whereas the CBM (Figure 6(c)) is populated over 1 which is consistent with the observed charge redistribution in molecular level calculations. Hence, as a consequence of the sufficiently stronger C-H···F-C intermolecular interactions between 1 and graphane, the band gap of the overall system is reduced.

Figure 7: The optimized geometries of (a) HFG-graphene, (b) HFG-graphane and (c) graphene-HFGgraphane calculated at plane wave GGA-PBE level of theory. The important C-H···π and C-H···F-C interactions are shown. Hydroflourinated graphene (HFG or (C2HF)n), having Janus type geometry, might be considered as 2D extension of 1 and hence we are interested to explore the structural and electronic properties of HFG when assembled with graphene/graphane and sandwiched between graphene and


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graphane (Figure 7). The calculated lattice constants for 2×2 supercell of graphene, graphane and HFG are 4.94 Å, 5.06 Å and 5.13 Å, respectively, which are in good agreement with previous reports.18,28 Our DFT-D2 calculations reveal that HFG can be interfaced with graphene through C-H···π interactions of 2.53 Å where the hydrogen atoms are pointed towards the ring center of the graphene with a binding energy of -1.9 kcal/mol. HFG and graphane can be interfaced together via significantly stronger C-H···F-C interactions of 2.55 Å that leads to a binding energy of -7.9 kcal/mol. It is important to mention here that unlike 1-graphene and 1-graphane, the binding energy of HFG-graphene is smaller compared to HFG-graphane, which might be a consequence of larger lattice mismatch in former than the latter one. However, both the C-H···π and C ̶ H···F ̶ C interactions play crucial role when HFG is sandwiched between graphene and graphane. Herein, comparatively smaller C-H···π of 2.43 Å and C-H···F-C of 2.49 Å and 2.52 Å leads to a significantly stronger binding energy of -17.1 kcal/mol, in graphene- HFG-graphane which is higher than that of HFG-graphene and HFG-graphane. Bader charge population analysis is performed on these systems to understand the nature of charge transfer interactions. Only 0.01 |e| per unit cell charge transfers from HFG to graphene, which clearly indicates that C-H···π interactions are dispersive in nature and contribution from CT is insignificant.18 But interestingly, 0.03 |e| per unit cell charge transfer from graphane to HFG which ensures spontaneous interlayer polarization from HFG to graphane by means of sufficiently stronger interlayer C ̶ H···F ̶ C interactions. Because of enhanced polarity of axial C-F and C-H bonds in sandwiched form, there is significant charge transfer of ~0.10 |e| per unit cell from graphane to HFG and 0.07 |e| per unit cell from HFG to graphene, respectively which makes this complex particularly stable with respect to individual HFG-graphene and HFG-graphane heterostructures. The consequence of weak interactions in HFG-graphene, HFG-graphane, and graphene-HFGgraphane on the electronic properties are investigated by means of band structure and density of state (DOS) calculations. Based on GGA-PBE calculations the band gap of monolayer HFG is found to be


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~3.0 eV (Figure S3 in supporting Info. File) which is in good agreement with the previous report.24,28 Notably, significant alteration occurs in the band structure of the HFG-graphane bilayer. The computed band structure of this bilayer exhibits metallic behavior (Figure 8(a)) which is in sharp contrast to the individual monolayers. The partial DOS analysis (Figure S4 in supporting Info. File) demonstrates the VBM near to Fermi level is mainly dominated by graphane whereas, that for the CBM arises from the HFG, which is further justified from the charge density distribution of VBM and CBM. Calculated partial charge densities of the VBM (Figure 8(b)) and CBM (Figure 8(c)) shows that the VBM has large contribution from graphane while the CBM is strongly delocalized on the HFG which furnishes an overall metallic nature HFG-graphane bilayer.

Similarly, HFG-graphene and graphene-HFG-

graphane also exhibit the metallic nature (Figure S5). While preserving their zero-gap, the Dirac cones gets shifted downward by 0.45 eV and 0.96 eV with respect to the Fermi level for HFG-graphene and graphene-HFG-graphane, respectively, because of the enhanced charge transfer.

Figure 8: (a) Band structure of HFG-graphane bilayer. The energy is scaled with respect to Fermi level. The partial charge densities of (b) VBM (c) CBM for HFG-graphane bilayer at Γ point. Recently, we have shown that acetylene molecules (C2H2) might undergo topochemical transformation into layered graphane (C2H2)n by surmounting a barrier of ~1.2 eV.67 Here, we have ACS Paragon Plus Environment

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investigated possible routes to synthesize HFG by the condensation of C2HF molecules into layer 2D systems, nC2HF → (C2HF)n. To model this topochemical reaction, the lengths of the unit cell along the lattice plane were increased from 2.3 Å – 8.0 Å with step sizes of 0.05 Å, 0.10 Å and 0.25 Å and corresponding geometries were optimized at each unit cell length. From the potential energy surface obtained by this method, the activation barrier for the condensation of the C2HF molecules into the lowest energy 2D form, nC2HF → (C2HF)n, (Figure 9(a)) was calculated to be ~0.81eV. The overall process is controlled by two opposing forces. The C2HF molecule needs to adapt a conformation transformation from linear (C∞v symmetry) to trans-bent (Cs symmetry) to form layered structure. For distorting the geometry of C2HF molecule (θ1 = θ2 = 0°) to trans-bent (θ1 = θ2 = 107.65°, within layered C2HF) within a 10×10×10 cubic box requires a distortion energy ∆Edistort = +87.2 kcal/mol which is an energetically uphill process. This large distortion energy is compensated by the intermolecular interactions during the condensation of C2HF molecules. The interaction energy (∆Einteract) is calculated as -163.1 kcal/mol, by increasing the unit cell length from the optimal C2HF layered structure upto a non-interacting limit of 10 Å in trans-bent geometry. At the large separation only the weak van der Waals interactions predominate in C2HF···C2HF, while in the interacting region, d (unit vector) ≤ 3.5 Å, additional covalent bonds start to form which eventually furnished to 2D layered (C2HF)n structures. The energy profile for this distortions and interactions are represented in Figure 9(b).


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Figure 9: (a) Variation of the electronic energy as a function of length of the lattice vectors for the formation of layered two-dimensional materials (nC2HF → (C2HF)n) and (b) The energy diagram for distortion in C2HF during C∞v → Cs (bottom left panel) and for interaction of C2HF molecules in Cs geometry during condensation with a rigid θ1 = -θ2 = 107.65° (bottom right panel).

Conclusion From first principle calculations, we have shown that the facially polarized all-cis-1,2,3,4,5,6hexafluorocyclohexane (1) can provide attractive C-H···π interactions towards flat aromatic molecules and 2D graphene, as well as C-H···F-C interactions towards their fully hydrogenated analogues. The enhanced polarity of the axial C-F and C-H bonds of 1 can facilitate the formation of both the C-H···π and C-H···F-C interactions simultaneously when sandwiched between benzene and cyclohexane or for ACS Paragon Plus Environment

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their corresponding extended 2D system. The attractive interactions are identified as noncovalent which mainly arise from van der Waals interactions. Based on the energy decomposition analysis (EDA), it is showed that the dispersion interactions play a crucial role in stabilizing the complexes formed between 1 and X (X = benzene, pyrene, coronene) or Y (Y = cyclohexane, perhydropyrene, perhydrocoronene) or X-1-Y. There is a sharp decrease in HOMO-LUMO gaps for 1-Y complexes with respect to their original counterpart because of the charge redistribution which is absent in 1-X and X-1-Y complexes. The structural and electronic properties of these complexes could be tuned by applying an external electrical field. By means of plane wave dispersion corrected DFT, DFT-D2, we find that the band gap of 1-graphane gets reduced significantly with respect to pristine graphane. The hydroflourinatedgraphene (HFG) which resembles the 2D extended analogue of 1, can substantially interact with graphene, graphane and simultaneously with both of them in a sandwiched form. Remarkably, the stacked HFG-graphane is metallic because of the strong interlayer C-H···F-C interactions though individual HFG and graphane monolayers are semiconductors. Calculations predict that a high pressure topochemical condensation of C2HF molecules can lead to the formation of HFG layered structure as the barrier for nC2HF → (C2HF)n is only ~19.0 kcal/mol. The present study advocates the utilization of weak non-covalent interactions like C-H···π and C-H···F-C interactions for bestowing new electronic properties in 2D-materials. Interestingly, because of the intrinsic polarization, van der Waals complexes of 1 and its 2D analogue, HFG might be exploited to trigger macroscopic polarization in the bulk which can be further tuned by the application of an external electric field. Our prediction of a semiconductor → metal quantum phase transition in bilayers of HFG-graphane exemplifies the strong electronic effects of such seemingly weak dispersion interactions.


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Supporting Information HOMO-LUMO plots, details of NCIPLOT and diagrams, band structure of HFG monolayer, TDOS and PDOS of HFG-graphane bilayer, band structure of HFG-graphene and graphene-HFG-graphane heterostructures, optimized Cartesian coordinates and complete reference of G09 and ADF. This information is available free of charge on the Internet. Acknowledgements A.D. and S.M.P. thank CSIR India for financial assistance. A.D. thanks DST, BRNS, and INSA for partial funding.

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Design of van der Waals Two-Dimensional Heterostructures from

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