Electronic and structural properties of amorphous carbon–nitrogen alloys

Section 14. Amorphous carbon alloys

Electronic and structural properties of amorphouscarbon±nitrogen alloys

F. Alvarez *, M.C. dos Santos

Instituto de Fisica Gleb Wataghin, Universidade Estadual de Campinas, UNICAMP, 13083-970 Campinas, Brazil

Abstract

The main interest in carbon±nitrogen alloys (a-CNx) stems from theoretical predictions of a metastable silicon-

nitride-like phase, i.e. b-C3N4: This phase is expected to have insulating properties, hardness, and thermal conductivity

comparable to those of a diamond. Also, the amorphous phase of the material has interesting electronic and structural

properties. We report the evolution of the electronic and structural properties of the amorphous phase (a-CNx) on

nitrogen content prepared by dual-ion beam-assisted-deposition. Some properties of a material obtained by arc dis-

charge are also reported and discussed. Ó 2000 Elsevier Science B.V. All rights reserved.

1. Introduction

Considerable interest has been devoted to car-bon±nitrogen alloys (a-CNx) since Liu and Cohen[1] predicted the existence of a metastable covalentcarbon±nitrogen compound. Although problemshave emerged in producing the predicted crystal-line phase (b-C3N4), amorphous CNx ®lms haveuseful optical and mechanical properties [2]. Re-cently a super hard C±N alloy with hardness aslarge as �50 Gpa was reported [3,4]. Besides thetechnological potential, the alloy is an interestingsystem per se. Furthermore, theoretical calcula-tions predict the existence of closed molecularstructures such as `fullerene-like' and `nano' tubes[5]. Indeed, tubes formed in some non-stoichio-

metric C±N alloys were hypothesized to be thecause of the hardness of the material [3].

In this paper, we present theoretical calcula-tions and experimental results of a-CNx ®lms. Thelocal structure and the shape of the top of thevalence band as function of the nitrogen contentare reported. The role of N in the formation ofmolecular structures is discussed. The e�ect ofhydrogen on the properties of carbon±nitrogenalloy is also reported.

2. Theory

Our theoretical approach relies on calculationsof the density of states (DOS) and core levelbinding energies of model molecules containing Cand N atoms. Geometry optimizations of themolecular structures were performed based on thesemi-empirical AM1 and PM3 quantum chemicaltechniques [6].

Journal of Non-Crystalline Solids 266±269 (2000) 808±814

www.elsevier.com/locate/jnoncrysol

* Corresponding author. Fax: +55-192 393 127.

E-mail address: alvarez@i®.unicamp.br (F. Alvarez).

0022-3093/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved.

PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 8 2 0 - 0

2.1. Core-level binding energy

Small molecules representative of the chemicalbond in C±N alloys are chosen for evaluation ofthe chemical shifts of the N1s electron bindingenergy [7]. The electronic structures of the opti-mized molecular conformations were obtainedthrough ab initio calculations. The molecules in-clude sp, sp2 and sp3 carbon and nitrogen hy-bridizations and fragments of the theoreticallypredicted a and b carbon-nitride (C3N4) com-pound [1,8]. The numerical values associated witheach structure are grouped as follows: (a) sp3 Nbonded to sp3 C in `open' structures; (b) sp2 Nsubstituting sp2 carbon in aromatic structures; (c)sp N in nitrile compounds and sp2 N in pyridine;(d) sp3 N bonded to sp3 C in a closed structure inwhich C±N±C bonds are stressed. These calcula-tions found four typical binding energies: (a)�398.4 eV for threefold coordinated N bonded tofourfold coordinated C; (b) �401.5 eV for substi-tutional sp2 N in graphite-like structures; (c)�399.2 eV for sp2 N (pyridine-like) or �399.4 eVfor sp N (nitrile-like); and (d) �399.0 eV for astressed structure containing sp3 N.

2.2. Valence-band electronic structure

Large clusters including 54±96 carbon atoms inthe graphite symmetry were adopted to study thevalence-band (VB) electronic structure by ran-domly replacing C with N atoms [7]. The elec-tronic structure was obtained by the valencee�ective Hamiltonian (VEH) method [9]. Bonds atthe boundary were saturated with hydrogen. Sys-tems containing [N]/[C]� 7, 12, 14, 17, 20, 23, 29,and 37% (or [N]/[N + C]� 6.5, 10.7, 12.3, 14.5,16.7, 18.7, 22.5, and 27 at.%) were studied. Twomain sets were optimized: (1) one set imposing Cs

symmetry to maintain planar geometry; and (2) asecond set allowing full relaxation of the atomiccoordinates. As before, the electronic structures ofthe optimized molecules were obtained by theVEH method. For convenience, the Fermi energyof the pure graphite cluster was shifted to zero.Since the clusters have a di�erent number ofatoms, the DOS were normalized [5,7].

3. Experiments

Two types of samples were prepared: amor-phous carbon±nitrogen (a-CNx) ®lms, andgraphite powder containing nitrogen. The powderwas produced under conditions similar to thoseused to obtain fullerene. In general, fullerenes aredeposited in a conventional bell jar where twohigh purity graphite electrodes are vaporized byan arc discharge between them. In fullerene pro-duction, He promotes the nucleation [10]. Wequenched the reaction by using a suitable atmo-sphere of N2 and He at 100±200 mb. The resultingsoot was collected from the chamber and analyzedex situ by mass spectrometry. The a-CNx:H ®lmswere prepared by ion beam assisted deposition(IBAD) system attached to an ultra high vacuumchamber for photoelectron spectroscopy analysis(PES). In the IBAD method, a graphite target(99.99 at.%) is sputtered by an N ion beam. Si-multaneously, a suitable mixture of N, Ar and Hions bombards the growing ®lm [11]. The sampleswere deposited on polished Si wafers (1 0 0) andglass substrates, maintained at a temperature of150°C. The base pressure of the chamber was�2� 10ÿ5 Pa. Immediately after deposition, the®lms were transferred to the UHV chamber(<2.0 ´ 10ÿ9 mbar) and measured without furthertreatment. Some samples were prepared ex situ byradio frequency sputtering of a graphite target in agaseous mixture of Ar and N2 [8,13]. No impor-tant di�erences were found between ®lms grownby IBAD or by sputtering. Hydrogenation has ane�ect on the material [12]. The evolution of thecore level electron energies and the top of thevalence band on nitrogen incorporation were an-alyzed by X-ray photoelectron spectroscopy(XPS) and ultra photon spectroscopy (UPS), re-spectively. For the XPS analysis the Al Ka linewas used (hm � 1486:6 eV, width 0.85 eV). ForUPS the HeII (40.8 eV) line from a resonant dis-charge lamp was used. The total spectrometerresolution is 0.3 eV. The samples were furtheranalyzed by infra-red spectroscopy. Elastic recoildetection analysis (ERDA) was used to measurehydrogen (deuterium) concentrations [13]. Mea-surements of stress, electrical conductivity andhardness were also performed.

F. Alvarez, M.C. dos Santos / Journal of Non-Crystalline Solids 266±269 (2000) 808±814 809

4. Results

4.1. Core-level results

Fig. 1 shows the experimental changes of theN1s core level spectra as a function of N concen-tration. The curves show two structures at 398.2and 400.5 eV (from now on peaks P1 and P2) as-sociated with N atoms in two di�erent con®gura-tions [13]. Peak P1 becomes largest in the materialcontaining the larger N concentrations. Therefore,depending on the amount of N incorporated in thesample, a particular structure is favored (curve a,Fig. 1). The ab initio calculation shows that theN1s spectra of con®gurations containing Nbonded to C sp3 (i.e., group a, Section 2.1) andsubstitutional N sp2 (i.e., group b, Section 2.1) areconsistently near the position of peaks P1 and P2,respectively. Therefore, based on these results wesuggest that the coordination of N goes from aplanar structure to a three-dimensional structurewhen the N concentration is raised.

4.2. Valence-band results

Fig. 2(a) shows the UPS spectra of the samplesin function of the [N]/[N + C] at.% ratio. Thespectrum of pure a-C has two bands located atbinding energies �7.7 and �3.6 eV, respectively.These bands are associated with electrons fromC2p r and p bonds. On increasing N content,three new features emerge at energies of �9.5,

�7.1, and �4.5 eV. For [N]/[C + N] ratios largerthat 20 at.%, the bands at 7.7 and 3.6 eV present inpure carbon are not detected. Also, the band at 7.1eV is the largest in the spectrum for intermediate Ncontent (10% < [N]/[C + N] < 25%). The bands at�9.5 and �4.5 eV are wider and increase for larger[N]/[C + N] ratios (>25 at.%). These bands are thelargest in the spectra for the sample with highestnitrogen content. Finally, the leading edge of theVB changes to lower energy on increasing Ncontent. The simulated DOS for N-substitutedgraphite clusters are displayed in Fig. 2(b). Curve a

in this ®gure corresponds to the pure graphitecluster. It presents two well-de®ned structures at

Fig. 1. Photoemission spectra of the N1s core-level of the

a-CNx alloy.

Fig. 2. (a) Valence band photoelectron spectra; (b) simulated

DOS for N-substituted graphite clusters; (c) simulated DOS for

N-substituted graphite cluster (17%, curve a); and experimental

UPS spectra (curve c).

810 F. Alvarez, M.C. dos Santos / Journal of Non-Crystalline Solids 266±269 (2000) 808±814

binding energies of �4 and �8 eV, associated withp- and r-bonds, respectively. As N is incorporatedin the cluster, these two bands move to largerbinding energies (curve b to d, Fig. 2(b)). At thelargest N concentration of [N]/[C + N]� 22.5% (N/C� 29%), the band associated with p bonds hasshifted to �7.1 eV while the one associated with rbonds has broadened and shifted to �11 eV. Anew feature appears close to the Fermi energy dueto the fact that sp2 N in a planar graphite structurecontributes with two electrons to the p-system.This contribution is probably the reason for thesystem to change into a three-dimensional fully sp3

structure, in which the N p-electrons turn intonon-bonding lone-pairs (Fig. 2(c)). Apart fromoscillations that are due to the ®nite cluster size,the calculated DOS for the b-C3N4 structure(Fig. 2(c), curve b) presents two bands located at�5 and �10 eV. The former is related to N lone-pair electrons while the latter is akin to r orbital ofC±N bonds. The structure in the 0±5 eV bindingenergy region corresponds to N lone-pair electronslocalized at the cluster boundaries. Therefore, weexpect then to move toward the peak located at 5eV on increasing cluster size. For comparisonpurposes, the calculated DOS of the graphite-likecluster for [N]/[C + N]� 14.5 at.% (N/C� 17%ratio in the plot) is also shown as curve a, Fig. 2(c).In this curve the absence of the structure located at�5 eV associated with N lone-pair electrons isapparent. The simulation allows us to identifythree regions: (1) a band located at �5 eV that isassociated with N lone-pairs; i.e., N and C bondedin a structure similar to crystalline silicon-nitridewhere C is sp3 hybridized; (2) a band located at�7.1 eV associated with electrons occupying p-orbitals of C±N bonds; (3) a band located at �9.5±11 eV associated with r-orbitals of C±N bonds.

Next we compare the experimental spectra withthe simulation results. For the sake of clarity, inFig. 2(c) we plotted only the He II spectrum of thelargest nitrogenated sample, [N]/[N + C]� 30% (N/C� 43%). To reproduce the total DOS of sampleswith [N]/[N + C] > 17 at.% (N/C� 20%), it is nec-essary to assume a partial contribution of DOSdue to N lone-pair electrons as in the b-C3N4

phase (Figs. 2(c)). Below [N]/[N + C]� 17 at.%(N/C� 20%), the lack of structure at �4.5 eV in

Fig. 2(a) indicates that N is occupying sites ingraphite-like structures. In conclusion, below the[N]/[N + C]� 17 at.%, N is occupying a C site ingraphite-like structures. Above [N]/[N + C]� 17at.%, the alloy tends to form a three-dimensionalstructure with C and N atoms fourfold andthreefold coordinated, respectively.

4.3. Molecular closed forms

In the theoretical study of random a-C1ÿxNx

clusters, we calculated the enthalpy of formation(DHf ) of the alloy for di�erent nitrogen concen-trations. Fig. 3(a) shows the quantity DH � f�DHf

�CnÿxNx� ÿ DHf�Cn��=xg as a function of the [N]/[N + C] at.% concentration. Here, n is the totalnumber of N and C atoms and x, as before, is thefraction of N atoms in the alloy. DH represents theaverage energy necessary to incorporate one ni-trogen atom either in the planar (curve a) or in the

Fig. 3. (a) Square of the plasmon energies vs. N concentration.

(h) Samples deposited using pure N2 in the ion gun. (s)

Samples deposited using N2 plus Ar in the ion gun. (´) (+)

Samples deposited using N2 + H2 and N2 + D2, respectively in

the ion gun. The theoretical curves (solid) represent the relative

enthalpies of formation of N-substituted clusters, (DH), for

planar (a) and corrugated (b) structures. (b) Integrated intensity

of the absorption band associated with ±C¹N vs. N concen-

tration. The error bars are of the order of the symbol size.


corrugated structures (curve b). The optimizedgeometry of both sets of clusters is similar forsmallest nitrogen concentrations, i.e., even a fullrelaxation calculation results in a planar geometry.Buckling of the structure develops only for [N]/[C + N] J 20%. These calculations show two im-portant trends: (1) Both theoretical curves have abarrier for nitrogen incorporation at [N]/[N + C]� 20%; and (2) above this threshold, thecorrugated structure collapses to a state of muchsmaller energy than the planar structure.

To understand this change, we note that sub-stitutional N in graphite-like sheets contributestwo electrons to the graphite conduction band,which is partially anti-bonding. Therefore, the in-crease in N concentration will produce instabilitiesdue to the augmented electronic energy, distortingthe system locally. Due to this distortion the hy-bridizations of carbon and nitrogen change fromsp2 to sp3, localizing electrons in nitrogen lone-pairs. This change is consistent with the fact thatfor [N]/[N + C] > 17 at.% a band associated withlocalized lone-pair electrons emerges at the top ofthe valence band (Fig. 2(a)) [8]. The square of theplasmon energy ��hxm�2 associated with the C1selectron is a measure of the density of the material[14]. In Fig. 3(b) we have plotted ��hxm�2 obtainedfrom XPS spectra [15]. The square of the plasmonenergy of the hydrogen (deuterium) free samples(h and s) goes through a maximum around [N]/[C + N]� 20%. For [N]/[C + N] J 20% the datareasonably follow the `corrugated' theoreticalenthalpy curve. Based on this result we suggest aspontaneous buckling of the structure with a de-creasing density of the material. The change of theplasmon energy is also accompanied by the emer-gence of a-C¹N absorption band at �2159 cmÿ1

in the transmission IR spectra (not shown). Ab-sorption by the ±C¹N stretching mode (�2159cmÿ1) sets in at [N]/[N + C]� 20%, indicating thatonly at large concentrations nitrile groups areformed (Fig. 3(b)). This formation makes thenetwork less dense, interrupting the continuity ofthe network and inducing the existence of danglingbonds and voids, as obtained in the numericalcalculation. The stress of the ®lms as a function ofthe nitrogen content shows the same trend as theplasmon energy (Fig. 3(b)). For smaller nitrogen

concentrations, the increasing stress is consistentwith an increasing density of the material onnitrogen substitution for carbon. For [N]/[N + C] J 20%, the decreasing stress is a result ofthe buckling of the structure, i.e., a decreasingdensity of the material. Finally, the plasmon en-ergy of hydrogenated and deuterated samples in-creases monotonically with composition (´ and +in Fig. 3(b)). Therefore, hydrogen promotes theformation of a soft, polymeric stress-free materialwith a smaller relative density. Furthermore, deu-terated samples show the existence of primaryamines, and when exposed to air, the material in-corporates hydroxyls by extensive hydrogen-bondformation [13].

In pioneering work Pradeep et al. [16] reportedN substitution in C60. Among the several possi-bilities of new systems that could grow fromN-doped graphite, we investigated molecularforms. A natural consequence of the curvatureinduced by N is the rolling of the cluster to form atubule. Carbon nitride and CxNyBz family tubuleshave already been proposed [17±20]. The di�erentclosures of the sheets produce cylindrical mole-cules having a range of electric dipole moments,varying from zero to same maximum. We foundthe most stable tubule structure to be the one inwhich C±N bonds point along the tube axis di-rection, i.e., the molecule with largest dipole mo-ment. Geometry optimizations based on the PM3technique were performed on this polar tube. Theend bonds were saturated with hydrogen. Analternating pattern of longer (0.146 nm) andshorter (0.135 nm) C±N bonds resulted, and thisalternation is coupled to a modulation of the tubediameter, from 0.776 to 0.834 nm. This seems to berelated to the Peierls instability [21], which isusually observed in quasi-one-dimensional sys-tems. Other molecular forms of CNx could exist inreal samples allowing variation of the relative [N]/[N + C] concentration. CN analogs of fullerenecages are not possible because of the ®ve-mem-bered rings in the ball. However, we numericallyfound a high symmetry molecular cage with thecomposition C24N32, a compound belonging to theC3N4 family (Fig. 4, inset). This molecule hasC4h symmetry. Its building blocks are eight-mem-bered connected rings resembling the structure of


b-C3N4. The nitrogen occupies no equivalent sites:eight N have coordination 3 and the remaining 24N have coordination 2. The bonds around a N ofcoordination 3 are 0.147±0.148 nm in length andare not all in the same plane. Bond angles varyfrom 117° to 119°. Coordination 2 atoms, on theother hand, show typical aromatic bonds length of1.36 nm and bond angle of 120°. All carbons are inequivalent sites and bonds in the same plane.Computer calculations and experimental resultssuggest that other structures with di�erent com-positions are also possible. Fig. 4 shows the massspectrum of the soot collected in the arc experi-ments. The spectrum shows the relative intensityof the species present in the soot as a function ofthe m/z ratio, where m and z are the total mass andcharge of the species, respectively. Assuming singleionization �z � 1�, the dominant peak at m/z� 368corresponds to a hypothetical molecule of massequivalent to 4 ´ (C3N4). Of course, assumingdouble ionization (z� 2) this mass correspondsexactly to the mass of the ball displayed in Fig. 4.Several molecules could give formed 368 masspeak. Therefore, more experimental results arenecessary prior to concluding the existence of thecage shown in Fig. 4, inset. The smaller side peaksaround the dominant ones in Fig. 4 correspond to

nitrogen substitution of carbon atoms and will notbe discussed here. Another interesting feature inFig. 4 is the species grouped around the ratiom=z � 523. Starting at m=z � 481 and up to 537,the peaks are sequentially obtained by adding themass of one N atom. The sequence starting at m/z� 552 is obtained by adding to the 537 species themasses of a N and a H. The source of H is prob-ably residual water in the deposition chamber.

5. Conclusions

The electronic structure of CNx ®lms was ex-perimentally determined by photoemission elec-tron spectroscopy. The spectra were comparedwith results from numerically determined modelmolecules containing C and N atoms. The incor-poration of N up to �20 at.% produces hard,dense and stressed materials having a planarstructure. A large concentration, the extra elec-trons introduced by N destabilize the material andthe structure curls and bends. A band of localizedstates rises at the top of the valence band, as in b-Si3N4. Numerical simulations show that severalclosed molecular structures are possible in hydro-gen-free alloys containing more than 20% at. N. Inparticular, a closed ball having the 8 ´ (C3N4)�C24N32 stoichiometry was predicted [5]. The sootobtained by vaporizing carbon in an arc dischargewas studied by mass spectrometry. The massspectra show species having a mass compatiblewith the 4 ´ C3N4 stoichiometry (i.e., half of theatomic mass predicted for C24N32). Other combi-nations giving the same mass and di�erent stoi-chiometry are also possible and more work isnecessary to identify the species present in thesoot. Based on the evolution from stressed nitro-gen containing nitrogen structure to a more stable,softer and distorted structure we suggest thatstandard deposition methods are not suitable toobtain the crystalline phase. Therefore, other de-position techniques such as arc discharge couldhelp in the attempts to produce the material. Fi-nally, e�orts to concentrate the hypothetical4 ´ (C3N4) phase are underway with the aim ofusing it as precursor for synthesizing the bulkcrystalline b-C3N4 phase.

Fig. 4. Mass spectrum of the C±N soot material. The masses to

charge ratios (m/z) of the more relevant peaks are indicated.

Inset: ball-and-stick model of the molecular cage

(C24N32)� 8 ´ (C3N4). C (gray) and N (black).


Acknowledgements

We are indebted to the members of the PVGroup, to C. Luengo and M. Eberlin for the fabri-cation and mass spectrometry measurements of thesoot, respectively. This work was partially spon-sored by Fapesp. The authors are CNPq fellows.

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Electronic and structural properties of amorphous carbon–nitrogen alloys