Electronic and Structural Properties of Two-Dimensional Carbon Nitride Graphenes

Malek Deifallah,† Paul F. McMillan, †,‡ and Furio Cora* ,†,‡,§

DaVy Faraday Research Laboratory, 3rd Floor, Kathleen Lonsdale Building, UniVersity College London,Gower Street, London, WC1E 6BT, United Kingdom, Department of Chemistry and Materials ChemistryCentre, Christopher Ingold Laboratory, UniVersity College London, 20 Gordon Street, London, WC1H 0AJ,United Kingdom, and Materials Simulation Laboratory, UniVersity College London, Gower Street, London,WC1E 6BT, United Kingdom

ReceiVed: December 5, 2007; In Final Form: January 16, 2008

The structure and electronic properties of single-layered carbon nitride graphenes are examined computationallywith hybrid-exchange functionals in periodic density functional theory calculations. Unlike pure carbon graphenethat provides a metallic nanomaterial, carbon nitride graphenes form semiconductors with band gaps rangingup to 5 eV. The band gap is sensitive to external perturbations that can be introduced chemically by adatomadsorption or physically by constraining the lattice parameter. Carbon nitride graphenes could possibly pavethe way for a new range of smaller and faster transistors, as well as have useful sensing and actuating properties.

1. Introduction

There is currently renewed interest in the remarkable elec-tronic and mechanical properties of single graphitic layers ofcarbon known as “graphene”. Theoretical studies beginning over60 years ago indicated that the unusual two-dimensional (2D)crystal formed by a single graphite layer would have remarkableelectronic properties that are dominated by relativistic quantumeffects.1,2 From the earliest days, theoretical approaches havebeen used to predict whether the planar 2D crystal structurewould remain stable or if it would distort spontaneously or undercompressive stress by “buckling” into the three-dimensional(3D) regime.3,4 Graphene sheets have only recently been realizedin the laboratory, and the experimentally determined propertiesmatch the theoretical predictions.5 Introducing curvature intothe flat sheets leads to carbon nanotubes or closed fullerenestructures highlighting the special properties of this 2Dstructure.6-8 It is likely to play a major role in the developmentof future nanoscale devices and quantum electronics.

It is well-known that nitrogen can replace carbon in sp2-bonded molecular systems to lead to heterocyclic aromaticcompounds with resulting systematic changes in the chemicalproperties. Just as graphene can be considered as the ultimatecondensation of the aromatic benzene (C6H6) molecule in twodimensions, C- and N-containing systems can be formed fromheterocyclic benzene derivatives such as triazine (C3N3H3) thatcan be condensed into planar structures,9 with correspondingchanges in the electronic structure due to the valency changeof the layers upon replacing C with N. Certain molecular crystalssuch as Liebig’s melem (C6N7(NH2)(NH))n phase10 contain largeplanar units with alternating C and N atoms. The existence offully condensed graphitic C3N4 structures has been predictedby theory,11,12 and a nanocrystalline structure of this type hasbeen prepared by chemical vapor deposition.13,14CxNy nanotubeshave also been reported from a solvothermal synthesis.15,16High-pressure-high-temperature syntheses from melamine (C3N3-

(NH2)3) and cyanuric chloride (C3N3Cl3) have resulted in layeredgraphitic structures with a C/N ratio close to 3:4.17 Interestingly,the stoichiometry causes hexagonal or larger voids to appear inthe layers. The existence of such solids indicates that it may bepossible to produce single CxNy layers that might provide usefultuning of the electronic and chemical properties of graphenesheets.

In this work we investigate computationally the structure,stability, and electronic properties of single layers of carbonnitride graphenes at, or near, the C3N4 composition, usingquantum chemical methods to gauge their suitability to provideroutes to new electronic, sensing, and actuating devices.

2. Methods

Computational results are based on periodic density functionaltheory (DFT) calculations, performed using the CRYSTAL06code.18 Full geometry optimizations and band structure calcula-tions were carried out using the hybrid B3LYP functional,19

which is known to provide accurate estimates of band gaps incrystalline solids.20,21 All the calculations were repeated at thelocal density approximation (LDA) level using the Dirac-Slaterexchange22 and the Vosko Wilk Nusair parametrization of theCeperley-Alder free electron gas correlation,23 producingqualitatively similar results. The DFT exchange-correlationcontribution is evaluated by numerical integration over the unitcell volume. Radial and angular points of the grid are generatedthrough Gauss-Legendre radial quadrature and Lebedev 2Dangular point distributions. We have used a pruned grid with75 radial and 974 angular points for our calculations. The basissets employed are expressed in a series of Gaussian-typefunctions and are of double-valence plus polarization qualityfor each atomic species; all elements are treated at theall-electron level. C, N, and H atoms are described with a6-21G* basis set and Cl ions with 86-311G contraction; thesebasis sets are available from the online library of the CRYSTALcode.24

We employed the standard truncation thresholds of (6 6 6 612) for the selection of integrals, while SCF convergencethresholds were set to 1.0× 10-9 hartree for both eigenvalues

* Corresponding author. E-mail: f.cora@ucl.ac.uk.† Davy Faraday Research Laboratory.‡ Department of Chemistry and Materials Chemistry Centre.§ Materials Simulation Laboratory.

5447J. Phys. Chem. C2008,112,5447-5453

10.1021/jp711483t CCC: $40.75 © 2008 American Chemical SocietyPublished on Web 03/18/2008

and total energies. We used converged Pack-Monkhorst gridsfor integration in reciprocal space, withk-point nets rangingfrom 3× 3 for the insulating layers with the largest unit cell to13 × 13 for metallic graphite. Geometry optimizations wererepeated until internal consistency was achieved upon restartand have been checked against the root-mean-square (rms) andabsolute value of the largest component for both gradients andnuclear displacements. Optimization was considered completewhen the four conditions were simultaneously satisfied for bothfractional coordinates and unit cell parameters, using the tightvalues of 1.2× 10-4 for the maximum gradient and 1.8× 10-4

for the maximum displacement (all in au).For all the structures examined, we have calculated theΓ

point phonons to verify that the optimized structures are trueminima with respect to the internal coordinates. The mass-weighted Hessian matrix was obtained by numerical differentia-tion of the analytical first derivatives. These are calculated atgeometries obtained by incrementing each of the nuclearcoordinates by a small amount with respect to the equilibriumgeometry. All the parameters employed in the phonon calcula-tions are the standard values of the CRYSTAL code.18

3. Results

3.1. C3N4 Graphene.We start our survey of CxNy grapheneswith those of composition C3N4; this is the only C/N ratio thatyields long-range ordered crystalline 2D materials, in which sp2-bonded C and N atoms are strictly alternating and the chemicalvalences of 4 for C and 3 for N are simultaneously satisfied.The structures examined will be referred to in this paper withthe short-hand notation of “CNG”, standing for “carbon nitridegraphene”.

Three layered C3N4 phases are examined; of these, two arebuilt on bridged triazine rings, one suggested by Kouvetakis etal. for nanocrystalline C3N4 materials prepared by chemicalvapor deposition (CVD),14 referred to as CNG-1 in this paper(Figure 1a), and an orthorhombic phase, CNG-2 (Figure 1b).25

The structure of CNG-1 consists of triazine rings connectedsolely through three-coordinated bridging nitrogens; in CNG-2instead, direct bonds between triazine rings are present, origi-nating 1D chains bridged by two-coordinated nitrogens. CNG-1and CNG-2 can also be described as formed by 1D chains oftriazine rings bridged by sp2-hybridized nitrogen atoms anddiffering in the connection between adjacent chains; therelationship between the two phases is similar to stacking faultsbetween the 1D chains. This feature bears an analogy withcarbon nanotubes, in which different ring closures lead to either

chiral or nonchiral structures of different electronic properties,and suggests that structures CNG-1 and CNG-2 may be expectedto show different electronic behavior. The third C3N4 phaseexamined differs from the two structures mentioned above sinceit originates from the condensation of tricyanomelaminates andcontains bridged melem units. This structure is labeled as CNG-3and contains larger void sizes (Figure 1c).26 Finally, and forcomparison, we also re-examine the electronic behavior ofpure C graphene.27

The first question that we address computationally in thiswork is whether the 2D C3N4 graphene sheets, of atomicthickness, are stable in a planar configuration like pure Cgraphene, or if they spontaneously distort along a directionperpendicular to the layers leading to buckled structures.Geometry optimizations using both the B3LYP and LDAfunctionals show that all three C3N4 single-layer structuresexamined are most stable in a buckled conformation. Thisstructural feature of the CN graphenes contrasts therefore withthat for pure C graphene. Under periodic boundary conditions,the buckling distortion of the CN graphenes is expressed as awavelike conformation of the layers. In Figure 2 we show thestable optimized conformations for CNG-2 and CNG-3 underno confining compressional stress.

The relative stability of the three 2D phases in buckledconformation is reported in Table 1. Results indicate that therelative stability of the C3N4 layers decreases in the order ofCNG-3 > CNG-1 > CNG-2, with energy differences at theB3LYP level of 0.112 eV/f.u. (f.u.) 1 C3N4 formula unit) forCNG-1 and 0.284 eV/f.u. for CNG-2. All three materials arecalculated to be wide band gap semiconductors, with estimatedvalues of the gap ranging between 3.82 and 4.10 eV (see Table1). For all three layers, the fundamental transition is from theunshared electron pairs on the nitrogen atoms within the layersto the π* levels of the condensed rings. Figure 3 shows theelectronic density of states (DOS) calculated for CNG-1 andCNG-3. Projections of the DOS onto atomic orbitals of differentsymmetry are also reported for CNG-1, to characterize theatomic contributions to valence and conduction bands. Thedirections labeled asx andy are defined within the plane of thelayer, whereasz is perpendicular to the layer orientation. Bandswith contributions from the pz atomic orbitals identify thereforethe set of π and π* crystalline orbitals; those with majorcontributions from orbitals ofx andy symmetry comprise theσ andσ* C-N bonds and include the lone pairs on the nitrogenatoms. In the DOS for CNG-1, it is clearly visible that the topof the valence band is given by the lone pair orbitals on the

Figure 1. Single-layer materials of C3N4 composition investigated in this study: (a) the model of a single layer proposed by Kouvetakis et al. (ref14), termed CNG-1 here, (b) one layer within a proposed orthorhombic structure for C3N4 (CNG-2) (ref 25), and (c) one layer of the melemstructure studied by Ju¨rgens et al. (ref 10) (CNG-3). Here and in the following figures, N, C, and H atoms are shown as blue, gray, and whitespheres, respectively.

5448 J. Phys. Chem. C, Vol. 112, No. 14, 2008 Deifallah et al.

two-coordinated nitrogens of the triazine rings (labeled as N2).The bottom of the conduction band is instead formed by pz

contributions on C and N atoms and is therefore a level ofπ*character.

Although the C3N4 graphenes are found to spontaneouslyadopt a stable buckled geometry, the constraint of planarity canbe artificially imposed computationally by constraining all atomsto lie in the planez ) 0 and imposing this plane to be asymmetry operation in the calculations. We have therefore away of estimating any effects that achieving structural planaritymight have on the electronic properties. All three C3N4 layersprovide convergent solutions with a planar geometry in ourcalculations. The relative energies of the planar configurationsare reported in Table 1. The energetic cost of “flattening” thelayer is of only 0.0128 eV/atom in CNG-3 and is increased to0.0255 and 0.0277 eV/atom in CNG-1 and CNG-2, respectively.This condition could be induced experimentally by interactionof the graphene layers with the environment, such as depositionon a suitable substrate, and it could lead to useful sensing/actuating applications of the CN graphenes. Our calculationsalso indicate that the main effect of planarity in the layers is aclosure of the band gap; this effect is most pronounced forCNG-2 and CNG-3, where the band gap varies by over 0.8 eV.

Our calculations have therefore shown that planar sheets withC3N4 composition are unstable. The stability of the buckledconfigurations stems from the repulsion experienced betweenthe lone pairs on the two-coordinated nitrogen atoms, i.e., theN atoms belonging to the triazine rings, which in the planargeometry point directly against each other across the triangularvoids. Buckling causes these lone pairs to reorient and becomemisaligned, thus reducing their steric repulsion; this, however,occurs at the expense of a decrease in theπ electron delocal-ization energy across the layer. Buckling thus confers partialsp3 character to the overall bonding, reduces the overlap betweenπ orbitals, and opens up the band gap.

The range of band gap values calculated for C3N4 graphenessuggests them to be suitable wide band gap semiconductors for

future application in nano- and optoelectronics; the fine detailsof the electronic properties appear to depend on both the sizeof the internal voids within the layers and on the nature of thecondensed aromatic rings that make up the layer.

3.2. C6N9H3 Graphene. We now examine a new CNgraphene sheet that contains larger voids. It can be obtainedfrom CNG-1 by cleaving a regular array of C3N3 rings andsaturating the dangling bonds with hydrogens for valencycompletion, resulting in a composition C6N9H3; this structureis termed CNG-4, and it is shown in Figure 4a. A bulk 3Dversion of this layered material, consisting of infinite stackingC6N9H3 sheets, can be prepared by condensation of melamineand cyanuric chloride (C3N3H3Cl3)28 via solvothermal reactionscarried out at high pressure (P ) 1-1.5 GPa) or underhydrothermal conditions. Such layered bulk phases have inter-esting luminescent properties, and they might exhibit graphite-like intercalation reactions.

Figure 2. Geometry-optimized structures for C3N4 graphenes: (a)CNG-2 and (b) CNG-3. Both show buckled conformations in theiroptimized geometry.

TABLE 1: Structural and Electronic Parameters for AllSingle-Layer Systems Investigated

structurea exptl

(Å)a calcd

(Å)∆E(eV)

band gap(eV)

CNG-1: planar 9.544 0.041a 3.67CNG-1: buckled 9.315 0.016a 4.10CNG-2: planar 9.580 0.068a 2.93CNG-2: buckled 9.322 0.041a 3.84CNG-3: planar 7.11 0.013a 3.01CNG-3: buckled 7.109 0.000a 3.82CNG-4 8.575 5.16CNG-5-1 8.438b 8.495 0.000c 4.12CNG-5-2 8.438b 8.467 1.970c 4.45pure carbon graphene 2.464d 2.469 0.00

a For the C3N4 structures, CNG-1, CNG-2, and CNG-3,∆E is therelative energy per atom with respect to the stable CNG-3 structure inbuckled conformation.b Obtained from ref 9.c For the two layers ofcomposition C6N9H3‚HCl, ∆E is the relative energy per HCl molecule.d Obtained from ref 27.

Figure 3. Total and projected electronic density of state (DOS) plotsfor (a) CNG-1 and (b) CNG-3. The labeling of atoms in (a) is asfollows: N1 is the three-coordinated nitrogen, N2 is the two-coordinatednitrogen, and C are the carbons in the layer;x, y, andz indicate thesymmetry of the atomic orbitals on which the projections are made.The labeling of atoms in (b) is as follows: N-inner is the nitrogen inthe center of the melem units, N-bridge is the three-coordinated nitrogenconnecting separate melem units, N-edge is the two-coordinatednitrogen, C-bridge is the carbon connecting the melem unit to aN-bridge, and C-inner is the internal carbon of the melem unit. Hereand in the following DOS pictures, the vertical dashed lines representthe Fermi energy.

Two-Dimensional Carbon Nitride Graphenes J. Phys. Chem. C, Vol. 112, No. 14, 20085449

Geometry optimization on a single-layer C6N9H3 graphenesheet results in a final conformation that is stable as planar andshows no obvious tendency to buckle. The absence of spontane-ous buckling distortions has been verified via phonon calcula-tions in the flat geometry. The larger void sizes in C6N9H3

graphene, in combination with the bridging NH groups, thereforeallow it to accommodate the lone pairs of the nitrogens causingthe layer to remain stable in flat geometry. It appears thereforethat such low-density structures containing larger voids withinthe layers are required to stabilize planar configurations of CN-based graphenes.

The large hole sizes found in CNG-4 affect not only thestability and configuration of the layer but also its electronicproperties. The CNG-4 structure has a much larger band gapthan the C3N4 systems, calculated as 5.16 eV with the B3LYPfunctional, see Table 1. The main band gap corresponds oncemore to a transition from the unshielded electron pairs on thenitrogens (top of the valence band) to theπ* levels of thetriazine rings (bottom of the conduction band) as shown inFigure 4b. The increased value of the band gap is a direct resultof the reduced repulsion among the lone pairs on the nitrogenatoms, which stabilizes the orbitals at the top of the valenceband.

The large holes present in CNG-4 enable the possibleoccurrence of host/guest interactions within the bulk 3Dmaterial, or chemisorption processes on the layer surface, thatmight be exploited to alter the electronic properties. Here wechose to examine the adsorption of an HCl component, with Clatoms located within or above the voids in the layer and H

species attached to N atoms; this choice was motivated by thefact that HCl is incorporated in the experimentally derived solidproduced by condensation of melamine and cyanuric chlorideinto a 3D version of the CNG-4 structure.9

It is believed that HCl incorporation into CNG-4 structuresoccurs via a heterolytic process that yields H+ bound to N atomsand Cl- ions located within the voids. The stability of thiselectronic state compared to a molecular HCl adsorption isconfirmed by our calculations. There are two symmetry uniqueproton acceptor sites in the layer that can in principle accom-modate protons: the first is the lone pair of the triazinenitrogens, resulting in CNG-5-1 (Figure 5a). In this configurationthe layer remains planar since there is no disruption of electrondelocalization, and the proton may receive extra stabilizationby H-bonding to adjacent nitrogen atoms, as highlighted inFigure 5a. The second potential proton acceptor site is the lonepair of the (bridging) N atoms that link consecutive triazinerings. Proton docking at these sites causes a disruption to thedelocalized electron nature of the layer since protonation causesthe formation of a local structure resembling an ammonium ionspecies. This latter protonated CN graphene is referred to asstructure CNG-5-2 in this paper (Figure 5b).

The energy difference between the two protonated layers(CNG-5-1 vs CNG-5-2) obtained upon geometry optimizationis calculated as 1.97 eV/proton in favor of the planar layerstructure CNG-5-1. The change in the electronic property ofthe layer upon HCl adsorption is pronounced: addition of Cl-

causes a closure of the band gap from 5.16 to 4.12 eV. Thischange is not due to a reduction of then f π* gap (n ) lone

Figure 4. Structure (a) and electronic density of states (DOS) (b) for the C6N9H3 graphene denoted as CNG-4.

Figure 5. (a) CNG-5-1 structure obtained from CNG-4 by protonation at N of the triazine ring. Dashed lines show H-bonding between an anchoredproton and close-lying nitrogen on an adjacent triazine ring, (b) CNG-5-2 structure protonated at the bridging NH group, and (c) electronic densityof states diagram (DOS) for the CNG-5-1 structure. In parts a and b Cl- ions are shown as green spheres.

5450 J. Phys. Chem. C, Vol. 112, No. 14, 2008 Deifallah et al.

pair of electrons on N atoms) in the C6N9H4+ backbone but,

rather, to the presence of an additional band due to Cl- locatedin the gap just above the valence band, as can be observed inthe DOS for the CNG-5-1 layer in Figure 5c. This observationsuggests that CN graphenes might have useful applications aschemical sensors.

3.3. Response of C6N9H3 Graphene to External Perturba-tions. As a possible way to alter the electronic properties ofC6N9H3 (CNG-4), or its C6N9H3‚HCl derivative, we examinethe effects of changing its lattice parameters. This feature couldbe achieved experimentally by growing the CN graphenes onsubstrates of different periodicity or subjecting the material tocompressional strain. We examine the effect obtained both byexpanding and constraining the lattice parameters in CNG-4,CNG-5-1, and CNG-5-2.

Response of CNG-4 to compression was investigated bycarrying out geometry optimization of the internal coordinatesat fixed values of the lattice parameters in the range of 7.50 Åe (a ) b) e 8.80 Å. This method mimics the effects of 2Dcompression and expansion. We considered planar structures;however, to permit structural distortions to occur, we alsogenerated “buckled” starting configurations in which the planar-ity constraint was broken prior to geometry optimization byshifting all atoms randomly along the direction perpendicularto the layer.

Compression of CNG-4 in the range of 8.00 Åe a e 8.80Å results in planar geometries, consistently with the chemicalpicture of completeπ delocalization. Conversely, constrainingthe layer toa < 8.00 Å results in sp2 f sp3 rehybridization,and buckled conformations become energetically favored result-ing in a reduction ofπ delocalization. This process is controlledby the interplay between two competing contributions: the lossof aromaticity and the relief of compressional strain at the locallybonded atom sites. As a consequence of this, an ordered“wavelike” rumpling of the layer occurs (Figure 6a), in whichlargely planar rings are connected through bent NH groups, thatdevelops an increased amplitude and moves to shorter wave-length under further compression (Figure 6b).

The relative contributions of the structural groups to thedistortion have been monitored by plotting a number of dihedral

angles as a function of the lattice parameters: changes in thedihedral internal to the triazine ring (A-B-C-D) and therelative change in the dihedral angle across the bridging NHgroup (A-B-E-F) are shown in Figure 6c. Structural distor-tions in the layer occur simultaneously at values ofa < 8.00Å, showing that bending modes internal to the triazine ringsand through the bridging NH groups are correlated. The biggestchange of 31° at a ) 7.50 Å is observed in the dihedral angleacross the bridging NH group, whereas a smaller change of 18°occurs for the dihedral angle internal to the triazine ring. Thelatter indicates that buckling of the triazine rings is a secondaryprocess compared to the buckling of bridging NH groups, andtherefore, upon compression, the layer attempts to preserve thelocal aromaticity of the C3N3 rings. The bridging NH groupsare instead a softer structural unit in the layer. Bending of theNH bridges explains the observed “wavelike” equilibriumconfiguration of the layer highlighted in Figure 6a, whichbecomes more pronounced at larger compression (Figure 6b).

If we now examine the change in the band gap as a functionof compression for the C6N9H3 graphene (CNG-4 structure),we find that compression leads to a closure of the band gap by1.51 eV from the unconstrained lattice parameter to the valueof a ) 7.50 Å. Although the energy required to achieve such alarge change in the lattice parameter appears to be excessivelyhigh to be achieved experimentally, compressing the layer to avalue for which buckling is first observed (a ) 8.0 Å) incursan energetic cost of only 0.068 eV/atom, making it feasible viaexisting grafting techniques.

An analogous examination of the derivatized C6N9H3‚HClgraphenes (CNG-5-1 and CNG-5-2) shows that for all magni-tudes of compression investigated, CNG-5-1 is stable withrespect to CNG-5-2; compression does not therefore alter therelative stability of the two protonation sites (N atom of triazinering vs bridging N atoms). We will therefore concentrate onlyon CNG-5-1 in our discussion of the type of structural distortionsexperienced by the derivatized layer.

As discussed earlier, the unconstrained CNG-5-1 structureis stable as planar; this planar structure becomes unstable withrespect to a buckled conformation for values ofa e 8.30 Å.The onset of buckling in the CNG-4 structure occurred at the

Figure 6. (a) Geometry-optimized CNG-4 structure ata ) 7.95 Å and (b) ata ) 7.50 Å viewed perpendicular to the layer direction; (c) relativechange of the dihedral angles in the CNG-4 structure as a function of thea lattice parameter.

Two-Dimensional Carbon Nitride Graphenes J. Phys. Chem. C, Vol. 112, No. 14, 20085451

smaller value ofa < 8.00 Å, indicating that the presence ofadsorbed species facilitates the occurrence of layer distortions.To understand in more detail the buckling processes in the CNG-5-1 layer, we monitored the relative changes in a number ofintralayer parameters, shown in Figure 7a: (1) the internaldihedral angle of the protonated triazine ring (A-B-C-D);(2) the dihedral angle including the proton anchoring to an Natom of a triazine ring (A-B-C-E); (3) the dihedral angleacross a bridging NH group (A-B-F-G); (4) the internaldihedral angle of the nonprotonated C3N3 ring (H-I-J-K);(5) the dihedral angle between consecutive triazine rings (A-B-F-H). All intralayer parameters begin to deviate fromplanarity at values ofa e 8.30 Å; this is also the threshold atwhich buckled conformations of the slab become stablecompared to the planar one indicating a clear link betweenstructural conformation and energetic stability. The change ofthe dihedral angle across a bridging NH group of the protonatedlayer (A-B-F-G) shows a 43° change ata ) 7.50 Åhighlighting significant compressional relaxation. As for theCNG-4 structure, the main relaxation process is a bending ofthe NH bridges, confirming that bridging NH groups are softdegrees of freedom in layered CxNy materials.

The nature of the rumpling process in CNG-4 and CNG-5-1is, however, different: in CNG-4 we observed 1D rumpling,whereby NH bridges were bent only along the directionperpendicular to the layer. In CNG-5-1, however, consecutivetriazine rings are twisted with respect to each other forminghemispherical cages around the guest Cl ions, see Figure 7b.The twist-distortion of the triazine rings is represented by thedihedral angle A-B-F-H, which undergoes the most signifi-cant change of 52° between 7.50 Åe a e 8.30 Å. Theoccurrence of such a twist relaxation in the derivatized C6N9H3‚HCl layer (CNG-5-1), as opposed to the C6N9H3 case (CNG-4), is linked with the presence of the Cl- ions: the extent bywhich the triazine rings twist with respect to each other iscontrolled by the electrostatic interaction between the protonsanchored to the layer and the negatively charged chloride ions.Upon compression, the three bridging NH groups in each cavityare oriented to point toward the Cl- ion. The proton bonded to

the nitrogen of the triazine ring, however, does not point towardthe chloride ion and instead lies approximately in the plane ofthe triazine ring it is bonded to (this is deduced by the smallrelative change of 7° in its dihedral angle A-B-C-E), whereit is stabilized by H-bonding with a nitrogen on an adjacenttriazine ring (labeled as L in Figure 7a). The internal dihedralangles of the protonated and unprotonated triazine rings (A-B-C-D and H-I-J-K) undergo similar changes of 19° inthe range of 7.50 Åe a e 8.30 Å. This value is similar to thatmeasured for the CNG-4 structure which experienced an 18°change in the same range of compression. The result showstherefore that internal distortions of the triazine rings, and hencea disruption of the local aromaticity present in CN graphenes,are energetically unfavorable processes, irrespective of thepresence of adatoms, and that these adatoms play a critical rolein the distortion process.

Finally, compression of the derivatized CNG-5-1 layer causesan opening of the band gap, as expected during sp2 f sp3

rehybridization. The band gap increases by 0.25 eV when thestructure is compressed from its equilibrium lattice parameterto the value ofa ) 7.5 Å. Although this extreme compressionwould be difficult to achieve experimentally by grafting CNmaterials onto a surface, a reduction of the lattice parameter to8.0 Å appears feasible considering that the layer would incuran energetic cost of only 0.032 eV/atom. The dependence ofthe band gap on compression and lattice spacing thus providesthe possibility of fine-tuning the electronic properties as afunction of both external strain and adsorption of adatom specieson to CN graphene layers.

4. Conclusion

In conclusion, our calculations show that depending on theprecise chemical composition and void structure within thelayers, 2D CxNy graphenes form a new family of wide-gapsemiconducting materials, in which the gap can be tuned bysmall variations in stress applied to the layers or by the presenceof adatoms. These novel materials may find applications assensors/actuators and in nanoelectronic/optoelectronic circuits.

Figure 7. (a) Relative change of the dihedral angles in CNG-5-1 as a function of thea lattice parameter and (b) geometry-optimized structure ofCNG-5-1 ata ) 7.50 Å viewed perpendicular to the layer direction, highlighting the regular array of “bowl” structures that encapsulate the Cl-

adatoms at high compressive stress (i.e., small values of the lattice parameter).

5452 J. Phys. Chem. C, Vol. 112, No. 14, 2008 Deifallah et al.

Acknowledgment. We thank EPSRC (U.K.) for funding andDrs. A.A. Sokol and M. Zwijnenburg for useful discussions.

Note Added after ASAP Publication. This article waspublished ASAP on March 18, 2008. A minor text change hasbeen made in the fifth paragraph of the Results Section. Thecorrect version was published on April 3, 2008.

References and Notes

(1) Wallace, P. R.Phys. ReV. 1947, 71, 622.(2) Slonczewski, J. C.; Weiss, P. R.Phys. ReV. 1958, 109, 272.(3) Mermin, M. D.Phys. ReV. 1968, 176, 250.(4) Venables, J. A.; Spiller, G. D. T.; Hanbucken, M.Rep. Prog. Phys.

1984, 47, 399.(5) Geim, A. K.; Novoselov, K. S.Nat. Mater. 2007, 6, 183.(6) Falvo, M. R.; Clary, G. J.; Taylor, R. M.; Chi, V.; Brooks, F. P.;

Washburn, S.; Superfine, R.Nature1997, 389, 582.(7) Maiti, A.; Brabec, C. J.; Bernholc, J.Phys. ReV. Lett. 1993, 70,

3023.(8) Hiura, H.; Ebbesen, T. W.; Fujita, J.; Tanigaki, K.; Takada, T.

Nature1994, 367, 148.(9) Zhang, Z.; Leinenweber, K.; Bauer, M.; Garvie, L. A. J.; McMillan,

P. F; Wolf, G. H.J. Am. Chem. Soc. 2001, 123, 7788.(10) Jurgens, B.; Irran, E.; Senker, J.; Kroll, P.; Mu¨ller, H.; Schnick,

W. J. Am. Chem. Soc. 2003, 125, 10288.(11) Ortega, J.; Sankey, O. F.Phys. ReV. B 1995, 51, 2624.(12) Teter, D. M.; Hemley, R. J.Science1996, 271, 53.

(13) Todd, M.; Kouvetakis, J.; Groy, T. L.; Chandrasekhar, D.; Smith,D. J.; Deal, P. W.Chem. Mater. 1995, 7, 1422.

(14) Kouvetakis, J.; Bandari, A.; Todd, M.; Wilkens, B.; Cave, N.Chem.Mater. 1994, 6, 811.

(15) Guo, Q.; Xie, Y.; Wang, X.; Zhang, S.; Hou, T.; Lv, S.Chem.Commun.2004, 26.

(16) Guo, Q.; Yang, Q.; Zhu, L.; Yi, C.; Zhang, S.; Xie, Y.Solid StateCommun. 2004, 132, 369.

(17) Montigaud, H.; Tanguy, B.; Demazeau, G.; Alves, I.; Courjault, S.J. Mater. Sci. 2000, 35, 2547.

(18) Dovesi, R.; Saunders, V. R.; Roetti, C.; Orlando, R.; Zicovich-Wilson, C. M.; Pascale, F.; Civalleri, B.; Doll, K.; Harrison, N. M.; Bush,I. J.; D’Arco, Ph.; Llunell M.CRYSTAL 2006 User’s Manual; Universityof Torino: Torino, Italy, 2006.

(19) Becke, A. D.J. Chem. Phys. 1993, 98, 5648.(20) Cora, F.; Alfredsson, M.; Mallia, G.; Middlemiss, D. S.; Mackrodt,

W. C.; Dovesi, R.; Orlando, R.Struct. Bonding2004, 113, 171.(21) Muscat, J.; Wander, A.; Harrison, N. M.Chem. Phys. Lett. 2001,

342, 397.(22) Dirac, P. A. M.Proc. Cambridge Philos. Soc. 1930, 26, 376.(23) Vosko, S. H.; Wilk, W.; Nusair, M.Can. J. Phys. 1980, 58, 1200.(24) http://www.crystal.unito.it.(25) Alves, I.; Demazeau, G.; Tanguy, B.; Weill, F.Solid State Commun.

1999, 109, 697.(26) Lotsch, B. V.; Schnick, W.Chem. Mater. 2006, 18, 1891.(27) Trucano, P.; Chen, R.Nature1975, 258, 136.(28) Vodak, D. T.; Kim, K.; Iordanidis, L.; Rasmussen, P. G.; Matzger,

A. J.; Yaghi, O. M.Chem. Eur. J. 2003, 9, 4197.

Two-Dimensional Carbon Nitride Graphenes J. Phys. Chem. C, Vol. 112, No. 14, 20085453