A Δ -self-consistent-field study of the nitrogen 1s binding energies in carbon nitridesÅsa Johansson and Sven Stafström Citation: The Journal of Chemical Physics 111, 3203 (1999); doi: 10.1063/1.479662 View online: http://dx.doi.org/10.1063/1.479662 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/111/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in A tiered approach to Monte Carlo sampling with self-consistent field potentials J. Chem. Phys. 135, 184107 (2011); 10.1063/1.3660224 Self-consistent-field calculations of core excited states J. Chem. Phys. 130, 124308 (2009); 10.1063/1.3092928 The convergence of complete active space self-consistent-field energies to the complete basis set limit J. Chem. Phys. 123, 074111 (2005); 10.1063/1.1999630 Self-consistent field, ab initio molecular orbital and three-dimensional reference interaction site model study forsolvation effect on carbon monoxide in aqueous solution J. Chem. Phys. 112, 9463 (2000); 10.1063/1.481564 Core ionization energies of carbon–nitrogen molecules and solids J. Chem. Phys. 111, 9678 (1999); 10.1063/1.480300

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A D-self-consistent-field study of the nitrogen 1 s binding energiesin carbon nitrides

Åsa Johanssona) and Sven Stafstromb)

Department of Physics and Measurement Technology, IFM, Linko¨ping University,S-581 83 Linko¨ping, Sweden

~Received 14 December 1998; accepted 24 May 1999!

Binding energies of the N 1s level in hard and elastic CNx films are investigated by means oftheoretical studies of model molecules. Results for the model systems are obtained fromab initioHartree–Fock calculations, where the core electron binding energies are determined using the deltaself-consistent-field method. The theoretical results are compared with experimental x-rayphotoelectron spectroscopy data in order to understand the microstructure of the CNx films. Bothsp2 and sp3 bonded nitrogen have been identified. The presence of nitrogen bonded to graphiteedges is also likely. ©1999 American Institute of Physics.@S0021-9606~99!30931-4#

I. INTRODUCTION

Carbon based materials have been shown to exist in anumber of forms in addition to the well known graphite anddiamond phases. The most intensively studied examples ofthese novel forms are fullerenes and carbon nanotubes. Ma-terials such as amorphous carbon, hydrogenated amorphouscarbon and nitrogenated carbon have attracted a great deal ofattention recently since they have shown very interesting me-chanical properties. In particular, it was shown that hardnessand elasticity can be combined in nonstoichiometric CNx

thin films prepared from sputtering of a graphite target in anitrogen atmosphere.1 High resolution transmission electronmicrographs~HRTEM! of this material show a graphitelikematerial with a large content of curved graphite layers andinterlayer crossings. The elasticity and hardness of the mate-rial could be rationalized as originating from the curvedgraphite layers and the crosslinkings, respectively.

Even though the overall structural properties of the CNx

material can be observed in HRTEM studies, we still lack theknowledge about the nature of the chemical bonds in thematerial and the role played by nitrogen for the structure ofthe material. Useful information concerning the differentchemical environments to a particular type of atom can beobtained from x-ray photoelectron spectroscopy~XPS!. In-dependent XPS studies of the N 1s core level report twomajor peaks at 398.1–398.4 eV and 400.3–400.7 eV.2–6 Thesmall difference between these data could be attributed todifferent growing temperatures and different nitrogen con-tent in the films. The interpretation of these peaks are inaccord, the low binding energy peak originates from nitrogenin an environment ofsp3 carbons and the high binding en-ergy peak corresponds to nitrogen in asp2 carbon environ-ment. Here the chemical shift is defined as the difference inbinding energy of two such peaks. In general, binding ener-gies depend upon both initial and final state effects. Initialstate effects include the charge on the atom from which the

core electron is ejected and the charge on the other atoms,while final state effects result from the electron redistributionafter ionization. Thus the chemical shift reflects differencesin these effects due to differences in the chemical surround-ing of the particular atom. In the case of thesp2 and sp3

peaks discussed above, the chemical shift is partially due todifferent charges on the nitrogens in the two systems. Theshift may also depend on the fact that thesp3 system is moreclosely packed which gives rise to a stronger relaxation andconsequently a lower binding energy in that case.

In two recent reports, high resolution N 1s XPS spectraof the same CNx material as introduced in Ref. 1 werepresented.5,6 In addition to the two N 1s peaks discussedabove, this spectrum showed a high binding energy peak at402.6 eV. This peak has also been observed before2 in asimilar type of material and was in that case assigned tonitrogen bound to oxygen. In recent experiments, however,very little oxygen is present in the sample and it might bethat other chemical environments to nitrogen also contributeto the peak in this energy region.6 Another new peak is ob-served at 399.0 eV. This feature is much weaker than thepreviously reportedsp2 and sp3 peaks but becomes moredominating in the spectrum as the take off angle of the pho-toelectrons was changed from normal exit to almost grazingdirections. This is a clear indication that the origin of thispeak can be associated with nitrogen present at the surface ofthe material. Likewise, the low binding energy peak corre-sponding to nitrogen in asp3 carbon environment is hardlypresent at grazing take off angles. Thus, thesp3 content isvery low at the surface, instead there are other types of bondsformed between carbon and nitrogen.

As a complement to the XPS results, Raman spectros-copy studies have also been performed on the samples dis-cussed in the previous paragraph.5 These data show clearindications of C[N triple bonds. Thus, it is important tolocate the contribution from this type of nitrogen in the XPSspectrum. Together with the somewhat unclear interpretationof the high binding energy peak in the N 1s spectrum, this

a!Electronic mail: asajo@ifm.liu.seb!Electronic mail: sst@ifm.liu.se

JOURNAL OF CHEMICAL PHYSICS VOLUME 111, NUMBER 7 15 AUGUST 1999

32030021-9606/99/111(7)/3203/6/$15.00 © 1999 American Institute of Physics

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calls for a systematic study of the chemical shifts of nitrogenin various carbon environments.

In this article we present results from calculations of theN 1s binding energies. Based on these results we can per-form a direct analysis of the XPS results discussed above, inparticular the origin of the N 1s peaks at 399.0 and 402.6 eV,by direct comparison of the experimental and calculatedchemical shifts. The nitrogen atoms were studied in carbonenvironments of different chemical nature and different size.The calculations were performed at theab initio Hartree–Fock level of theory and using the delta self-consistent-field(D-SCF! technique7 to obtain the chemical shifts betweenthe different systems. The methodology is presented brieflyin Sec. II followed by a presentation and analysis of theresults in Sec. III.

II. METHODOLOGY

The nitrogen containing clusters~molecules! that wehave studied are shown in Figs. 1–4 below. The systems aregrouped together according to the chemical nature of thecarbon atom~s! attached to nitrogen, i.e.,sp3, sp2 and sp1

type of carbon~s!. For the graphitelike systems (sp2), wehave also studied the binding energies of nitrogen located atthe edge of the cluster. The varying size of the clusters ineach group were introduced to obtain information about thesize dependence of the binding energies. Indeed, this is a bigeffect and it must be stressed that the calculations were per-

formed on model systems. However, as shown below thebasis set used allows for accurate calculations on fairly largemolecules and calculated chemical shifts are in close agree-ment with known experimental data.

The idea of studying a series of structures of varying sizeis twofold. First, by careful extrapolation of the results forthe finite systems we can obtain a good estimate of the N 1sbinding energies in the type of solids we are interested in.Second, the evolution of the binding energies with increasingsize of the system provides a very clear separation of initialand final state contributions to the chemical shifts.8 Thisseparation allows us to identify how one or the other of thesetwo contributions is affected by the different chemical envi-ronment.

The calculations were performed at theab initioHartree–Fock level, using a split valence basis set of con-tracted Gaussians~6-31G!.9 To test the influence of the basisset, some calculations were also performed using a largerbasis set~see below!.

The DSCF method7 is very well known and has alreadysince the 1970s been an effective approach to the determina-tion for XPS binding energies. The core electron bindingenergy is defined as the energy difference between theground state of the neutral system and the core hole state ofthe ionic system. In theD-SCF method, both the initial andfinal states are determined as SCF wave functions

Eb5E@CSCF(n21)#2E@CSCF

(n) #. ~1!

For CSCF(n21) , i.e., then21 electron wave function of the

final state, the electrons are attracted by the core hole. Theattraction energy as well as the relaxation of the final state,i.e., screening of the core hole, must be included to get rea-

FIG. 1. Structures with nitrogen bonded tosp3 hybridized carbon~left! andsp2 hybridized carbon~right!.

FIG. 2. Structures with nitrogen bonded to two carbons~nitrogen bonded tosp2 hybridized carbon!.

3204 J. Chem. Phys., Vol. 111, No. 7, 15 August 1999 Å. Johansson and S. Stafstrom

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sonable values for the binding energies. TheD-SCF methodtakes this into account in contrast to calculations based uponKoopmans’ theorem where only the initial state contributionsare considered.

Due to the use of relatively small model systems of theactual material and since the calculations were performedusing a fairly small basis set, the values of the binding ener-gies cannot be compared directly with the experimental bind-ing energies. However, since the discussion below is based

on differences in binding energies, the errors introduced byperforming calculations with small basis sets and on modelsystems are systematic and thus cancel to a large extent.

As a test of the accuracy of the model we have per-formed calculations of binding energies of small systems forwhich it is possible to extend the basis set and for whichexperimental data are available. In Table I are shown the N1s binding energies of pyrrole and pyridine. In the first twocolumns are the values obtained in this study with two dif-ferent basis sets, but with the same geometry. The third andfourths columns show experimental and calculated valuesfrom other people’s work. The calculated N 1s chemicalshift between these two systems are in very close agreementwith the experimental value for all types of calculations. Inparticular, it seems as if the 6-31G basis set is particularlygood, a result that justifies the use of small basis sets incalculations of chemical shifts. Consequently, accurateD-SCF calculations of fairly large molecules can be per-formed, which is the approach taken in the present study.

III. RESULTS AND DISCUSSION

Focusing first on the two major features in the N 1sspectrum, namely the peaks assigned to nitrogen bound tosp3 and sp2 hybridized carbon with experimental bindingenergies of 398.1 and 400.6 eV, respectively.6 Since thebinding energy is strongly size dependent~see below!, it isimportant to consider systems of similar size in the calcula-tion of the difference in the 1s binding energies of these twonitrogens. The model system for nitrogen in asp3 carbonenvironment is N~C~CH3)3)3 , see Fig. 1~a!. We obtain abinding energy of 405.2 eV for the N 1s level in this system.For nitrogen bound tosp2 hybridized carbon we use themodel system NC15H10, see Fig. 1~b!. The N 1s bindingenergy in this system is 407.7 eV.~Note that the nitrogen inthis case is situated in the interior of a graphitelike systemand is threefold coordinated tosp2 carbon atoms.! Thesemodel systems are of the same size and represent the correctnearest and second nearest neighbor environment to nitro-gen. Using the binding energy of the nitrogen in an environ-ment ofsp3 carbons as reference, we obtain a shift of 2.5 eVtowards higher binding energies for nitrogen in asp2 envi-ronment. The corresponding experimental shift given in Ref.6 is 1.9 eV for samples prepared at 100 °C and 2.5 eV forsamples prepared at 350– 500°C. Thus, our calculatedchemical shift is in close comparison for this type of struc-ture and gives full support for the assignments of the twomajor features in the N 1s spectrum.

TABLE I. N(1s) binding energies~eV! for pyridine, Fig. 2~a!, and pyrrole,Fig. 2~a!, DSCF calculated with different basis sets.

Structure

Binding energy

6-31G/6-31G 6-31G/TZP Exp. Papera

pyridine 406.81 404.21 404.94b 405.09pyrrole 407.96 405.68 406.15c 406.26

aSee Ref. 11.bSee Ref. 12.cSee Ref. 13.

FIG. 3. Structures nitrogen bonded to one carbon~nitrogen bonded tosp1

hybridized carbon!.

FIG. 4. Coupled system; triple bonded nitrogen surrounded bytert-butyl.

3205J. Chem. Phys., Vol. 111, No. 7, 15 August 1999 1s binding energy in nitrides

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The additional peaks that are observed in the core levelspectrum of nitrogen have been shown to result from struc-tures near or at the surface. Two possibilities for such struc-tures involving nitrogen are shown in Figs. 2 and 3, i.e.,nitrogen in an environment ofsp2 andsp1 hybridized carbonatoms, respectively. In contrast to nitrogen incorporated in agraphite layer, the type ofsp2 environment discussed hereresults in a twofold coordination of nitrogen to neighboringcarbons atoms~amide nitrogen!. This turns out to have aprofound effect on the binding energy of the N 1s core level.

Table II shows the N 1s binding energies of the twofoldcoordinated nitrogen insp2 hybridized systems. The labelingof the structure refers to Fig. 2. The overall shift in the bind-ing energy in going from pyridine (C5H5N) to C15H9N is20.53 eV. Compared to the case of nitrogen bound to threeneighboring carbons, this level is relatively insensitive tochanges in the size of the system, which indicates that thevalue for the infinite system should not be very differentfrom that of the largest system included in this study. Inrelation to the reference binding energy of thesp3 hybrid-ized system this corresponds to a chemical shift of about 1.1eV towards higher binding energies. This is in very closeagreement with the experimentally observed chemical shiftof approximately 0.9 eV between thesp3 peak and the newfeature at 399.0 eV.6

The calculated N 1s binding energies for nitrogen in acarbonsp1 environment (N[C, see Fig. 3! are shown inTable III. The binding energy is in this case strongly depen-dent on the system size. For the largest system included inour study, N[C–C14H9 @Fig. 3~e!#, the binding energy is407.16 eV, which is a 2.70 eV shift towards lower bindingenergies as compared to HCN and 1.43 eV compared toCH3CN. The two medium sized systems shown in Figs. 3~c!and 3~d! both give approximately the same binding energy.This indicates that for this system, it is the number of carbonatoms that is important and not if the second nearest neigh-bor atoms aresp2 or sp3 hybridized. The N[C–C14H9 sys-tem is comparable in size to the model systems discussedabove, which allows for a direct comparison between theirbinding energies. The calculated chemical shift is 2.0 eVtowards higher binding energies with respect to the corre-

sponding peak of nitrogen in a carbonsp3 environment. Thisis not in agreement with any of the observed peaks in theXPS core level spectrum of nitrogen. However, it is only 0.5eV above the N 1s binding energy of nitrogen in asp2 car-bon environment. Possibly, this peak could be hidden in thelow binding energy tail of the strong feature correspondingto this type of nitrogen.

To further investigate thesp1 structure we have per-formed calculations of the binding energy in the presence ofsurrounding molecules. For this study we have chosentert-butyl as representing the surrounding. The binding energiesfor various intermolecular ([N– – – C) distances are pre-sented in Table IV. For a distance of 8.25 Å, the effect is asexpected negligible. When the distance is reduced, the bind-ing energy of the N 1s electron is reduced as a result of theincreased possibility of screening of the core hole. However,the effect is quite small, a total shift of 0.5 eV towards lowerbinding energies was obtained in going from the isolatedmolecule to a distance of 4.0 Å. Thus, in the context of theassignment of the peaks we are discussing here, this effectcan be neglected.

We conclude from our calculated values that the experi-mentally observed peak at 399.0 eV cannot be assigned tothe nitrogen in the C[N group since the calculated bindingenergy for this type of N 1s electron is about 1 eV too largeas compared to the experimental value. Instead, we havefound that the 1s binding energy of nitrogen situated in apyridine like environment gives a much better agreementwith the experimental data. This interpretation supports asurface structure of terminated graphitelike sheets with somenitrogen incorporated in the carbonsp2 structure at thegraphite boundaries.10

As discussed above, Raman spectroscopy has shownclear indications of C[N triple bonds5 in the CNx samples.The presence of this type of nitrogen environment is consis-tent with our data in that the calculated binding energies ofthe sp1 ~see Fig. 3 and Tables III and IV! andsp2 @see Fig.1~b!# nitrogens are quite similar and thesp1 peak could behidden under the dominatingsp2 peak. It could also be thatthesp1 is not present in the XPS spectrum due to the differ-ent probing depths of XPS and Raman spectroscopy.

For a graphite edge~zig-zag and armchair!, there are twodifferent sites: one threefold coordinated and one twofoldcoordinated. The later type is the one discussed above. Wehave also calculated the N 1s binding energy for a nitrogensituated in threefold connected site at the boundary~see Fig.5!. This binding energy is 409.74 eV, i.e., considerablyhigher than that of the twofold coordinated nitrogen. Thesame observation was made by Casanovaset al.11 on consid-

TABLE II. N(1 s) binding energies~eV! for structures of Fig. 2.

No. Molecule DSCF-BE KT-BE

a C5H5N 406.81 423.77b C12H8N 406.50 open shellc C12H8N-m 405.79 open shelld C15H9N 406.28 423.78

TABLE III. N(1 s) binding energies~eV! for structures of Fig. 3.

No. Molecule DSCF-BE KT-BE

a HCN 409.86 425.09b CH3CN 408.59 424.32c C(CH3)3CN 407.97 424.09d C6H5CN 407.86 424.39e C14H9CN 407.16 424.29

TABLE IV. N(1 s) binding energies~eV! for different molecular distances~Å! of coupled system in Fig. 4.

Distance DSCF-BE

4.0 407.445.0 407.766.0 407.927.0 407.988.25 407.99

3206 J. Chem. Phys., Vol. 111, No. 7, 15 August 1999 Å. Johansson and S. Stafstrom

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erably larger model systems of graphite. With nitrogen in-corporated in the carbon matrix of graphite edge structuresformed in the growth of the CNx material, the predicted corelevel binding energy of the nitrogen at the threefold con-nected site at the boundary is about 2 eV below that of ni-trogen situated in the interior of a graphite sheet~see above!.Thus, there is a possibility that this kind of structure couldcontribute to the experimental peak which appears about 2.0eV below the peak assigned tosp2 coordinated nitrogen.

The calculatedD-SCF binding energies show clearly theeffect of increasing the size of the molecules. For a givennearest neighbor chemical environment the binding energyshifts to a lower value when the size of the system is in-creased. This behavior can be understood from the fact thatwith an increasing number of electrons, the screening of thecore hole is more efficient. The energy gained in the screen-ing process is transferred to the photoemitted electron duringthe emission process and this electron comes out with higherkinetic energy the more efficient the screening is. In additionto this process, there are also initial state effects that influ-ence the binding energy. In the present model, these effectsare included in the Koopmans’ binding energies of the N 1slevel. Thus a comparison of theD-SCF and the Koopmans’binding energies could provide useful information regardingthe importance of initial and final state effects as the size ofthe systems is increased.8

Tables III and II show both theD-SCF and the Koop-mans’ binding energies. For both types of nitrogen, the de-crease in binding energy~BE! with increasing size of thesystem is considerably larger in theD-SCF calculations thanin the Koopmans’ energies. For the twofold connected nitro-gen at a graphite edge, the shift is20.53 eV in theD-SCFBE compared to 0.01 eV in the Koopmans’ BE. The sametrend but considerably stronger is observed for thesp1 struc-ture: 22.7 eV in theD-SCF-BE and20.7 eV in the Koop-mans’ BE. It should be noted that the net atomic chargesremain essentially constant, which further proves that theshift is mainly a result of final state effects as discussedabove. Thus, calculating the chemical shift between thesetwo features with these two techniques results in very differ-ent numbers depending on the size of the system. This gives

a clear indication of the importance of including both initialand final state effects when comparing binding energies ofmolecules of different size.

IV. CONCLUSIONS

TheD-SCF binding energies of the N 1s level have beencalculated for nitrogen in different chemical environmentsthat are relevant for the CNx material. These environmentsincludesp3, sp2 andsp1 hybridized carbons. The chemicalshifts caused by these different types of structures are used toanalyze experimental core level XPS data. The four peaksthat have been observed experimentally can be assigned tothe following structures: the two main peaks at 398.1–398.4eV and 400.3–400.7 eV are due to nitrogen in an environ-ment ofsp3 andsp2 carbons, respectively. The peak at 399.0eV, which originates from the surface of the CNx material, ismost probably due to nitrogen atoms incorporated in a graph-ite edge structure. Previously, this peak was assigned to ni-trogen bonded tosp1 hybridized carbons. Our results are notconsistent with this interpretation. Finally, the high bindingenergy peak at 402.6 eV could also be assigned to nitrogenin graphite edges, in this case nitrogens that are connected tothree neighboring carbon atoms at the edge. In this case wecannot rule out the possibility of a contribution to this peakfrom nitrogen bound to oxygen. However, since recent ex-perimental data seem to indicate that this peak exists even inessentially oxygen free samples, there are reasons to believethat even in a pure carbon environment, the nitrogen 1s levelcould shift to this very high binding energy.

The assignments that are made here are fully consistentwith the structure of the CNx material as observed fromHRTEM micrographs. Very recent investigations report a tu-bular morphology of the material.10 The tubes themselvescorrespond to thesp2 system, whereas interconnects be-tween the tubes are a buildup of moresp3 like materials. Thetube ends, in which nitrogen is incorporated, form the sur-face of the material. These nitrogens give rise to the addi-tional weak signals in the N 1s core level spectrum.

ACKNOWLEDGMENTS

The authors would like to thank Niklas Hellgren andJan-Eric Sundgren at Linko¨ping University for providing theexperimental input to this work. Financial support from theSwedish Research Council for Engineering Science~TFR!and the Swedish Natural Science Research Council~NFR! isgratefully acknowledged as well as computational supportfrom the National Supercomputer Center~NSC! in Linkop-ing Sweden.

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FIG. 5. Structure with nitrogen bonded to three carbons at a graphite edge~nitrogen bonded tosp2 hybridized carbon!.

3207J. Chem. Phys., Vol. 111, No. 7, 15 August 1999 1s binding energy in nitrides

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3208 J. Chem. Phys., Vol. 111, No. 7, 15 August 1999 Å. Johansson and S. Stafstrom

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