Novel Two-Dimensional Tetragonal Monolayer: Metal–TCNQ

Article pubs.acs.org/JPCA

Novel Two-Dimensional Tetragonal Monolayer: Metal−TCNQ Networks Yandong Ma, Ying Dai,* Wei Wei, Lin Yu, and Baibiao Huang School of Physics, State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, People’s Republic of China ABSTRACT: We present a systematic theoretical study on the structural, electronic, and magnetic properties of the novel tetragonal transition-metal-based 7,7,8,8-tetracyanoquinodimethane molecule coordination single sheets (referred to as TM@TCNQ, TM = Cr−Co). Our results unveil that, in TM@TCNQ, two valence electrons would transfer from one TM atom to TCNQ molecules, making them more stable. Among these structures, Cr@TCNQ, Mn@TCNQ, and Fe@ TCNQ exhibit long-range antiferromagnetic coupling while Co@TCNQ is paramagnetic; this dictates these sheets being ideal candidates for spintronic devices. Such long-range magnetic coupling in the studied systems is related to the modulation via the TCNQ ligands. Besides, to explain the magnetic moment qualitatively, we propose a model on d splitting named “4 + 1 splitting”. The possible underlying physical mechanisms are discussed in detail. In addition, TM@ TCNQ may conceal promising performance for hydrogen storage according to our results. These predications strongly revive these new synthesized systems as viable candidates for spintronics and hydrogen storage.

I. INTRODUCTION Two-dimensional (2D) nanosheets typically possess different properties from their three-dimensional (3D) bulk counterparts.1−5 Electronic and optical behaviors usually differ significantly arising from the 2D confinement effect and the absence of interlayer interactions, which, though generally weak, plays an important role in determining the phonon scatterings and single-particle excitations. Inspired by the intriguing properties of 2D materials, many efforts have been devoted to synthesizing free-floating 2D nanosheets of various materials in atomic thickness, and then corresponding electronic structures and physical properties have been primarily investigated.6−13 The first example is graphene.1,2 After that, layered materials, such as hexagonal BN, ZnO, transition metal oxides, transition metal dichalcogenides (TMDs), and complex oxides have also been isolated as 2D single sheets via mechanical cleavage and chemical exfoliation.14−19 While the reported 2D monolayers show different properties ranging from metals to insulators, strikingly, these 2D nanosheets are confined to a common featureas a hexagonal lattice structure. Whereupon, they share many similar properties, including isotropic properties, edge morphology, and symmetry-dependent band gap. Nevertheless, for familiar in human’s living, such as in coating meshes, pavements, and fences, tetragonal structures are more desirable and practical compared with a hexagon. However, searching for such a tetragonal 2D lattice is extremely challenging ascribing to the valence electron pair repulsion rule, in which one atom would favor a spatial arrangement of multiple bonds with its neighbor atoms.20 Up to now, only a few observations that such a tetragonal lattice can be produced in experiments have ever been reported. In this sense, recent progress in 2D metal− © 2013 American Chemical Society

7,7,8,8-tetracyanoquinodimethane (TCNQ) molecule coordination networks21−25 may pave a novel way for achieving the long-standing dream of 2D atomic sheets with tetragonal lattice for catalysis, hydrogen storage, and spintronics. For instance, Abdurakhmanova et al. synthesized 2D metal coordination networks self-assembled from TCNQ molecules.22 They produced 2D periodic covalent networks with a tetragonal lattice of atomic thickness through an on-surface synthesis approach and characterized the sheet employing scanning tunneling microscopy.22 Other TM-based 2D TCNQ monolayers have also been synthesized recently (i.e., Au-based TCNQ monolayers,23 Ni-based TCNQ monolayers,22,24 and Cs-based TCNQ monolayers25). For the further development of those tetragonal porous sheets, it is of both scientific and technological importance to understand their basic properties and potential applications well. On the basis of the first-principles calculations, in the present work, we systematically investigate the geometric, electronic and magnetic properties of free-standing 2D TM-based TCNQ monolayer (referred to as TM@TCNQ, TM = Cr−Co). We aim at providing theoretical insights leading to a better understanding of these novel frameworks and probing whether they are promising candidates for wide applications in nanoelectronics. Received: March 15, 2013 Revised: May 16, 2013 Published: May 17, 2013 5171

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II. COMPUTATIONAL METHODS The periodic slab calculations, including geometry relaxation and electronic structure calculation, are performed based on the spin-polarized density functional theory (DFT) as implemented in the Vienna ab Initio Simulation Package (VASP).26,27 The exchange-correlation interaction is treated within the generalized gradient approximation (GGA) in the parametrization of Perdew, Burke, and Ernzerhof (PBE).28 The electrons are divided into two classes: one is the delocalized s and p electrons, which are described by the GGA; the other is the localized d electrons, which are described by the Coulomb and exchange corrections (GGA+U).29 In our GGA+U calculations, correlation energy (U) of 4 eV and exchange energy (J) of 1 eV for TM d orbitals are adopted. The periodic TM@TCNQ is optimized within one repeating unit. The periodic boundary condition is applied along the xy plane, and the periodic boundary condition is applied with a vacuum space of about 17 Å along the z-directions (i.e., the direction perpendicular to the layer of the atoms) to avoid the interactions between two slabs in the nearest-neighbor unit cells. During the geometry optimization, a Monkhorst−Pack30 k-point grid with 9 × 5 (5 × 5) k-points in the xy plane and one k-point in the zdirection is chosen for the rectangle (square) unit of TM@ TCNQ, while for the static total energy calculations, a Monkhorst−Pack k-point grid with 13 × 9 (9 × 9) k-points in the xy plane and one k-point in the z-direction is adopted. Test calculations with denser meshes confirm that the results are fully converged. Geometries are optimized by relaxing both the unit cell and the position of the atoms within the unit cell until the force acting on each atom is less than 0.02 eV/Å. The kinetic energy cutoff of electron wave functions is 450 eV in the calculation, which is sufficient large for the systems considered here. The conjugated-gradient minimization scheme is used for the electronic structure calculations.

Figure 1. Equilibrium structure of TM@TCNQ views for (a and c) R-TM@TCNQ and (b and configurations. The H, C, N atoms of the TCNQ atoms are highlighted in blue, orange, red, respectively.

from top and side d) S-TM@TCNQ molecules and TM and dark yellow,

tional spintronics. It is also worth emphasizing that, according to previous works, the Jahn−Teller distortions are expected to occur in order to remove the degeneracy and lower the energy when the d bands are partially occupied, whereas our geometric calculations unveil that no Jahn−Teller distortions are observed for any of the studied systems. In detail, the optimized bond length between the central TM atoms and the host N atoms for all of the structures are listed in Table 1. As we can see, the calculated TM−N bond length decreases monotonically when the atomic number increases from Cr to Co. We also note that the same trend occurs for the relaxed values of the lattice constant. Such a trend can be understood as a result of the variation of the radii of the four-coordinated TM atoms in their divalent cation states, where two electrons of each TM atom form bonds with host TCNQ molecules. The likelihood of realizing TM@TCNQ is determined by its formation energy. The formation energy of TM@TCNQ, Hf, is obtained by combining the cohesive energies of TM@TCNQ and its constitutive elements. Here, the body-centered cubic (bbc) Cr and Fe, α-Mn, face-centered cubic (fcc) Co, and the isolated molecule of TCNQ have been chosen as the constitutive elements due to the experimental conditions which follows the reaction TM(s) + TCNQ(g) = TM@ TCNQ(s). After structure relaxation, the equilibrium lattice parameters of these constitutive systems are in fair agreement with experimental values. In crystalline solids, the formation energy represents the major part of the enthalpy of formation, ΔH. Indeed, the additional contributions, such as from the lattice and electronic excitation energies, are typically less than 2% of the enthalpy of formation at ambient temperature. Therefore, considering the experimental and theoretical accuracy in its determination, these additional contributions can be neglected. With this assumption in hand, one can express the formation energies per TM atoms, ΔH, of TM@ TCNQ as follows:

III. RESULTS AND DISCUSSION As an initial step for understanding the TM@TCNQ, the geometric properties of these frameworks are examined. Two typical structural configurations are considered for the TM@ TCNQ. One is the rectangle structure, in which the TCNQ molecules are arranged in parallel, as shown in Figure 1c (labeled as R-TM@TCNQ). The other is a square structure in which the TCNQ molecules are alternatively arranged, as shown in Figure 1d (labeled as S-TM@TCNQ). Our preliminary calculations for both R-TM@TCNQ and S-TM@ TCNQ structures suggest that, similar to pristine graphene, no buckling is found in these frameworks. As Figure 1 illustrates, each TCNQ molecule forms four bonds to the TM atoms via its cyano groups. As a result, the Cr−Co centers form a lattice with a spacing of 7.188 × 11.444 Å2 (12.209 × 12.209 Å2), 7.029 × 11.354 Å2 (12.072 × 12.072 Å2), 6.947 × 11.277 Å2 (11.932 × 11.932 Å2), 6.881 × 11.210 Å2 (11.834 × 11.834 Å2), respectively, in the frameworks of R-TM@TCNQ (S-TM@ TCNQ). This situation establishes that 2D TM@TCNQ porous sheets with regularly separated 3d TM atoms are produced when TCNQ molecules and TM atoms construct into 2D tetragonal frameworks. Such structures would display interesting magnetic properties as compared with conventional semiconductors, in which the doped TM atoms easily form clusters, thus resulting in nonintrinsic magnetism. Our results suggest that these studied frameworks could have strong potential to overcome the challenges encountered in conven-

ΔH ≈ Hf =

Etot − NTME TM − NTCNQ E TCNQ NTM

This expression requires the calculations of the total energy for the TM@TCNQ monolayer, Etot, the TM crystal [the bodycentered cubic (bbc) Cr and Fe, α-Mn, and face-centered cubic 5172

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Table 1. Geometric and Magnetic Properties of the TM@TCNQa R-TM@TCNQ

S-TM@TCNQ

atom

a/Å

L/Å

D/Å

Hf/eV

M0/μB

type

ΔE/meV

Cr (3d54s1) Mn (3d54s2) Fe (3d64s2) Co (3d74s2) Cr (3d54s1) Mn (3d54s2) Fe (3d64s2) Co (3d74s2)

7.188/11.444 7.029/11.354 6.947/11.277 6.881/11.210 12.209 12.072 11.932 11.834

7.188 7.029 6.947 6.881 11.290 11.178 11.595 11.648

2.028 1.936 1.881 1.842 2.032 1.968 1.894 1.850

−1.385 −0.764 0.068 0.621 −1.210 −0.509 0.230 0.662

4 3 2 1 8 6 4 2

AFM AFM AFM PM AFM AFM AFM PM

22 35 36 0 20 36 57 0

a

Terms: a is the lattice constant of the unit cell, L is the metal−metal distance, D is the metal−nitrogen distance. Hf is the formation energy per TM atoms. M0 is the magnetic moment per cell. AFM, antiferromagnetic; PM, paramagnetic. ΔE is the exchange energies.

(fcc) Co] energy per atom, ETM, and the isolated molecule TCNQ, ETCNQ. NTM and NTCNQ are the total number of TM atoms and TCNQ molecules involved in the total energy Etot calculation for the TM@TCNQ monolayer. The corresponding results are listed in Table 1. S-Cr@TCNQ, S-Mn@TCNQ, RCr@TCNQ, and R-Mn@TCNQ monolayers are shown to be thermodynamically stable with a negative Hf of −1.385, −0.764, −1.210, and −0.509 eV, respectively. On the other hand, Fe@ TCNQ and Co@TCNQ frameworks with positive Hf are metastable. However, it is important to notice that Hf for SFe@TCNQ, S-Co@TCNQ, R-Fe@TCNQ, and R-Co@ TCNQ frameworks are only 0.068, 0.621, 0.230, and 0.662 eV, respectively. Note that the magnitude of the calculated formation energy depends on the selected constitutive elements. Thus, the formation energy of R(S)-Co@TCNQ could be significantly reduced by modulating the selected constitutive elements, i.e., growth conditions. In particular, to further ensure the thermodynamically stability of these frameworks, we could grow the TM@TCNQ systems on suitable substrate as our calculations show that the van der Waals interactions between the TM@TCNQ frameworks and the substrate could reduce Hf significantly. The stability of the TM@TCNQ monolayer can also be understood by analyzing the cohesive energy of the TM@TCNQ monolayer with respect to the variation in the set of the lattice constants. The cohesive energy is found to have a single minimum at the lattice constants. From the formation energies listed in Table 1, one can also see that values of Hf depend on the gradual filling of the d orbitals of the TM atom. The strongest formation energy is realized for Cr@TCNQ, and the weakest formation energy is realized for Co@TCNQ. In detail, the calculated formation energy increases monotonically when the atomic number increases from Cr to Ni. Note that the unpaired electrons could contribute to the strength of binding between the TM atom and the TCNQ molecule, while the paired electrons have no contribution to the binding. The basic principle underlying this feature can be understood by considering the processes in which paired electrons are populated (i.e., unpaired electrons are decreased) in company of further filling of d orbital. Next, we investigate whether the TM@TCNQ can be stabilized by forming layered structures like in graphene. Because PBE fails to describe the van der Waals (vdW) interactions which cannot be ignored in layered structures, the calculations for layered structures are performed by a damped vdW correction (DFTD2). The results indicate that, similar to graphene,31 the layered structure is more stable than the single sheet. However, like in graphene, this does not mean that the single sheet could not exist in monolayer configuration. Besides, the TM@TCNQ monolayers have been synthesized in experiments. Therefore, it

is reasonable to study the electronic structures of the monolayer configuration. Since all the TM@TCNQ frameworks are clearly thermodynamically and kinetically stable, it is tempting to investigate their electronic structures and physical properties for corresponding applications in future. To this end, we discuss the electronic and magnetic properties of TM@TCNQ. We consider one unit cell to represent the TM@TCNQ. Our spinpolarized calculations reveal that all the studied structures energetically favor magnetic states; the energy differences between the magnetic and nonmagnetic states are very large, indicating that these sheets are robust magnets. The calculated magnetic moment per unit cell of R(S)-Cr@TCNQ, R(S)Mn@TCNQ, R(S)-Fe@TCNQ, and R(S)-Co@TCNQ is about 4.0 (8.0), 3.0 (6.0), 2.0 (4.0), and 1.0 (2.0) μB, respectively. In this situation, we can conclude that the magnetic properties can be controlled by employing different combinations of TM atoms; thus, this tunable magnetic behavior may offer impressive potential in the applications for future magnetic storage materials and spintronics. Using Mulliken population analysis, for all the structures, the magnetic moments are assigned mainly to TM atoms; the neighbor N and C atoms as well as the other atoms have only a little contribution to the total magnetic moments. Besides, it should be noted that the localized magnetic moment of one TM atom in R configuration is almost equal to that of one TM atom in S configuration. Explanation of this interesting phenomenon lies in the fact that the distance between the TM atoms is 7.188 (11.290), 7.029 (11.178), 6.947(11.595), and 6.881(11.648) Å, respectively, for the Cr@TCNQ, Mn@ TCNQ, Fe@TCNQ, and Co@TCNQ with R (S) configuration leading to weak interaction between the TM atoms induced by long distance. Nevertheless, for use in spintronics, a magnet with magnetic coupling is more desirable. Since all the studied frameworks are magnetic, it is of interest to study the preferred magnetic coupling between the TM atoms in TM@ TCNQ. In order to investigate the magnetic coupling between the magnetic moments, for R-TM@TCNQ, a supercell with two repeating units containing two TM atoms is used, while for S-TM@TCNQ, a unit cell containing two TM atoms is employed. Depending on the initial spin configurations, the energies of two stable magnetic configurations, ferromagnetic (FM) and antiferromanetic (AFM), are calculated. The exchange energy between the two phases, ΔE = EFM − EAFM, is used to evaluate the magnetic interactions, and the corresponding results are listed in Table 1. Positive (negative) exchange energy indicates that the ground state of the system is AFM (FM). As addressed in Table 1, our results indicate that the TM atoms favor AFM coupling for R(S)-Cr@TCNQ, 5173

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R(S)-Mn@TCNQ, and R(S)-Fe@TCNQ, and the energy of AFM states is 22 (20), 35 (36), and 36 (57) meV lower than that of the corresponding FM states, respectively. For both RCo@TCNQ and S-Co@TCNQ, however, the magnetic states are paramagnetic with the equivalent energies. Such long-range magnetic coupling obtained in these frameworks is very desirable, endowing these sheets with great potential in nanoelectronics. The specificity of the long-range magnetic coupling in these sheets is different from that obtained in TMatoms-doped conventional semiconductors in which the TM atoms only interact in their neighborhood. To understand the origin of this spin polarization, we analyze in detail the density of states (DOS) of these frameworks. Total DOS for all structures are illustrated in Figure 2. Increasing the

Figure 3. Spin-resolved charge density isosurface (isosurface value = 0.001 e/Å 3 ) of R-Fe@TCNQ and S-Fe@TCNQ sheets in antiferromagnetic state. Green indicates the positive values and pink the negative. The H, C, N atoms of the TCNQ molecules and TM atoms are highlighted in blue, orange, red, and dark yellow, respectively.

negligible, and the magnetic moments carried by nitrogen and carbon atoms are polarized antiferromagnetically with respect to the Fe atoms, endowing R(S)-Fe@TCNQ sheets with robust long-range magnetic coupling between the Fe atoms. Insets in Figure 3 zoom in the distribution of spin density on neighbor nitrogen atoms, showing that the nitrogen spin polarization does not display typical p character. These features imply that the spin polarization is not an intrinsic property, which may originate from the hybridization between Fe and other atoms. This pattern is common for all the studied frameworks. To investigate the effect of on-site interaction in such systems, some of our results are also tested with exclusion of on-site interaction. The results indicate that the exclusion of on-site interaction does not affect the main conclusions based on the GGA+U method. For example, there is little difference between the calculated magnetic moments with inclusion and exclusion of the on-site interactions; R-Co@TCNQ still shows half-metallic and S-Co@TCNQ still shows semiconducting properties when the on-site interaction is excluded. Consequently, the correlation effect is not critical in explaining the magnetic properties of such systems. We also note that, under the square-planar-coordinated crystal-field environment, the d orbitals of a TM atom usually split into four d orbitals close in energy and one high-lying orbital, which is named “4 + 1 splitting”.32 To explain the magnitude of the magnetic moments tabulated in Table 1, we hereby construct a simple model on d splitting in which there are four degenerate levels (g1) and one high-lying level (g2), as addressed in Figure 4. By such a splitting of the d orbital, minor spin orbitals can be filled only after filling four major g1 spin states. According to such a model, however, further investigation of the magnetic behaviors of these frameworks indicates that the magnetic moment of TM@TCNQ is strikingly different from what can be expected, if we were to base our understandings entirely on the valence electron configuration of the corresponding TM atoms. To illustrate this, we take the Cr@TCNQ sheet as an example. The valence electron configuration of Cr is 3d54s1, and in such a situation

Figure 2. Total DOS of computed TM@TCNQ sheets. All data correspond to one unit cell, with the left row corresponding to RTM@TCNQ and the right row corresponding to S-TM@TCNQ. The vertical dashed line represents the Fermi level.

number of the valence electrons from Cr to Co results in a gradual filling of the d bands; therefore, the Fermi level is expected to be shifted toward higher energies. As Figure 2 shows, for all the sheets, the total DOS indicates that the spinup states and spin-down states near the Fermi level are not symmetrical, implying that these systems should be magnetic, which is consistent with the above results. Besides, for Cr, Mn, and Fe, the TM@TCNQ sheets are found to be semiconductors irrespective of the lattice’s shape. On the other hand, it is important to notice that, different from the former three cases, the electronic behavior of Co@TCNQ depends on the lattice’s shape, i.e., R-Co@TCNQ displays half-metallic character while the S-Co@TCNQ is semiconducting. To study the distribution of spins on the TM@TCNQ sheets, taking R(S)-Fe@TCNQ sheet as an example, we calculate the spin density of R(S)-Fe@TCNQ in AFM states, as shown in Figure 3. Clearly, for both R-Fe@TCNQ and S-Fe@TCNQ, the magnetic moment mainly appears in Fe atoms, while neighbor nitrogen and carbon atoms only have a small contribution. The spin polarization of neighbor nitrogen and carbon is not 5174

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valence electrons to the TCNQ molecules, leading to four unpaired valence electrons in Cr. The Cr valence electron configuration of Cr@TCNQ is presented schematically in Figure 4. In such a situation, the Cr@TCNQ sheets have a magnetic moment of 2.0 μB per unit cell for the R configuration and 4.0 μB per unit cell for the S configuration, which agrees with our calculated results listed in Table 1. Quite naturally, as a consequence of the electron transfer between the TM atoms and TCNQ molecules, the electrostatic interactions within Cr@TCNQ are strengthened and the frameworks become more stable. As direct evidence to the model, the partial density of states (PDOS) of the d electrons of the TM atoms is presented in Figure 5. As mentioned in section II, the periodic boundary condition is applied along the xy plane; consequently, the dxy orbitals should be the high-lying unoccupied level, corresponding to g2 labeled in Figure 4. This result is in agreement with the result addressed in Figure 5. As Figure 5 illustrates, another important feature of the d orbitals for all the frameworks is that dz2 and dx2−y2 orbitals display narrow peaks, while the dxz and dyz orbitals display broad peaks. This is a consequence of their symmetries, according to which the dz2 and dx2−y2 orbitals cannot form π bonding with the TCNQ molecules, while the dxz and dyz orbitals can. At last, we wish to stress that all these frameworks may behave with promising performance for storing hydrogen, because all the sheets are high surface area materials with isolated and exposed TM ions. Note that transition metals dispersed atomistically can store hydrogen nondissociatively, but with binding energy intermediate between physisorption

Figure 4. Valence electron configurations for TM@TCNQ sheets. The circle represents a hole that derives from an electron transfer from TM atoms to the TCNQ molecules.

the six valence electrons originated from the filling of the 3d orbitals by the 4s electron occupy the d degenerate orbitals in Cr@TCNQ as follows. Four electrons from Cr occupy the major g1 orbitals, while the other two electrons occupy the minor g1 orbitals, resulting in two unpaired electrons in the fourfold degenerated g1 orbitals. Under this assumption, the Cr@TCNQ sheet should in principle have a magnetic moment of 2.0 μB per unit cell for the R configuration and 4.0 μB per unit cell for the S configuration. By contrast, our calculated magnetic moment is 4.0 μB per unit cell for the R configuration and 8.0 μB per unit cell for the S configuration. This discrepancy makes it reasonable to speculate that Cr@TCNQ behaves as if two electrons have been transferred from one Cr atom to TCNQ molecules. In fact, the Cr indeed denotes two

Figure 5. Partial density of states for (a) R-Cr@TCNQ, (b) S-Cr@TCNQ, (c) R-Mn@TCNQ, (d) S-Mn@TCNQ, (e) R-Fe@TCNQ, (f) S-Fe@ TCNQ, (g) R-Co@TCNQ, and (h) S-Co@TCNQ. The vertical dotted line indicates the Fermi level as zero energy reference. 5175

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and chemisorption; thus, they could be suitable for practical storage media operating at ambient conditions.33 However, the dispersed TM atoms in most systems are very unstable against cohesive interactions, and clusters formed by TM atoms do not contribute to hydrogen storage and are obstacles for their applications. Accordingly, the biggest challenge for fully utilizing the storage power of atomic TM is to disperse TM atoms stably against the strong cohesion tendency. Obviously, the present systems are expected to be applicable to improve TM dispersion characteristics; thus, our work also highlights a new direction for hydrogen storage and provides the necessary insight for the future studies.

IV. CONCLUSIONS In summary, on the basis of spin-polarized DFT, we have systematically studied the geometric, electronic, and magnetic properties of the TM@TCNQ monolayer sheets. In contrast to the currently realized monolayer which confined to a common feature, a hexagonal lattice, TM@TCNQ sheets exhibit tetragonal structure. We demonstrate that, in TM@TCNQ, two valence electrons would transfer from one TM atom to TCNQ molecules, making the two TM@TCNQ allotropes energetically stable. It is found that all the studied 2D tetragonal frameworks are magnetic, carrying magnetic moments of 4.0 (8.0), 3.0 (6.0), 2.0 (4.0), and 1.0 (2.0) μB for Cr− Co in the R (S) configuration, respectively. In this situation, we can conclude that the magnetic properties can be controlled by employing different combinations of the TM atoms; thus, this tunable magnetic behavior may offer more potential in the applications for future magnetic storage materials and spintronics. Further magnetic coupling calculations show that the free-standing TM@TCNQ covalent networks favor robust antiferromagnetic spin arrangement in spite of the large coupling distance, which depends crucially on the modulation via the TCNQ ligands. Additionally, to explain the magnitude of the magnetic moments, we construct a simple model, i.e., “4 + 1 splitting”. We also propose TM@TCNQ may behave with promising performance for hydrogen storage, because all the sheets are high surface area materials with isolated and exposed TM ions.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Basic Research Program of China (973 program, 2013CB632401), National Science foundation of China under Grant 11174180 and the Fund for Doctoral Program of National Education 20120131110066, Natural Science Foundation of Shandong Province under Grant number ZR2011AM009, and the Ministry of Education Academic award for postgraduates. We also thank the National Supercomputer Center in Jinan for providing high performance computation.

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