Nature of Carbon-Carbon Bonding in Evaporated and Ion-Sputtered (Diamondlike) Amorphous Carbon from (e, 2e) Spectroscopy

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1 Utah State University A Physics Facuty Pubications Physics Nature of Carbon-Carbon Bonding in Evaporated and Ion-Sputtered (Diamondike) Amorphous Carbon from (e, 2e) Spectroscopy Chao Gao Yun Yu Wang A. L. Ritter JR Dennison Utah State University Foow this and additiona works at: Part of the Physics Commons Recommended Citation Chao Gao, Yun Yu Wang, A. L. Ritter, and JR Dennison, "Nature of Carbon-Carbon Bonding in Evaporated and Ion Sputtered ("Diamond-ike") Amorphous Carbon Fims from (e,2e) Spectroscopy," Phys. Rev. Lett., 62, (1989). This Artice is brought to you for free and open access by the Physics at It has been accepted for incusion in A Physics Facuty Pubications by an authorized administrator of DigitaCommons@USU. For more information, pease contact dyan.burns@usu.edu.

2 Nature of Carbon-Carbon Bonding in Evaporated and Ion-Sputtered (Diamondike) Amorphous Carbon from (e, 2e) Spectroscopy Chao Gao, Yun Yu Wang, and A. L. Ritter Virginia Poytechnic Institute, Backsburg, Virginia 2406I J. R. Dennison Utah State University, Logan, Utah {Received 1 December 1988) The 1& ca carbon-carbon bonding in evaporated (e-c) and ion-sputtered (i-c) amorphous carbon has been inv..stigated by transmission eectron-energy-oss spectroscopy (EELS) and by (e, 2e) spectroscopy. The EELS data indicate that the e-c is graphicike and the i Cis -diamondike. The (e, 2e) data demonstrate that the concentration of sp bonding is 100% in both materias (0.85~ c, z~ 1.0 for e-c and 0.75 ~ c, ~~ 1.0 for i-c). There is significant rehybridization of the n orbita in i C- PACS numbers: Mg, Fy, Kz The properties of amorphous carbon (a-c) range from back, soft "graphitic" fims to hard, transparent "diamondike" fims depending on the preparation method and on the concentration of hydrogen in the sampe. Evaporated amorphous carbon (e-c) tends to be graphitic whie ion-sputtered amorphous carbon (i-c) prepared with ow pasma power is more diamondike. The presence of hydrogen in the sampe aso produces a diamondike fim. A natura mode to describe this extreme range of optica and structura variation is to assume the oca carbon-carbon bond. ng varies from predominanty sp in graphiticike fims t.i a high proportion of sp bonds in the diamondike fims. The vaidity of this mode has been discussed exter. sivey, but concusive evidence for or against it does rot exist. Some authors inferred a arge fraction of sp bonding in ion-sputtered carbon fims (hydrogenated and nonhydrogenated) from an anaysis of optica and transmission eectron-energy-oss spectroscopy (EELS) measurements. Their arguments hinged on an earier measurement of the z- eectron osciator strength in crystaine graphite. An error in this anaysis has recenty been discovered and it now appears that the reative fraction of sp and sp bonds cannot be determined from the compex dieectric function of e-c. We have investigated graphitic e-c and diamondike i Cby EELS a-nd (e, 2e) spectroscopy and find that the bonding is predominanty sp in both materias, but the eectronic structure of the z eectrons in e-c and i-c is significanty different. The x-eectron spectra density in e-c is simiar to an anguar average of the z-eectron spectra density in crystaine graphite. However, in diamondike i-c, the spectra density of the "x eectrons" shows significant s-p hybridization. We suggest that this rehybridization of the z orbita in i-c arises from the oss of mirror pane symmetry when the sp bonded sheets are twisted and wrinked. The admixture of 5 character into the x orbita as a function of the oca radius of the sp bonded sheets has been discussed extensivey by Haddon, Brus, and Raghavachari. Our mode is consistent with a detaied computer simuation of a-c by Gai, Martin, and Car. We present EELS and (e, 2e) data to support this mode. The singe-crysta graphite sampe was prepared by the "scotch tape" method from natura graphite taken from mines at Ticonderoga, New York. The evaporated fims were made by Arizona Carbon foi by arc evaporation from high-purity carbon rods. The conductivity of the e-c fims was 0.5/0 cm. The ion-sputtered fims were prepared by dc magnetron sputtering using a Soan Orbitorr source which has a base pressure in the ow 10 -Torr range (cryopumping). The argon pressure for sputtering was 36 mtorr and the sputtering power was 200 W. The conductivity of these fims was ~ 10 /0 cm. The diffraction pattern for the e-c and i-c fims exhibited diffusive rings which were quaitativey simiar to pubished measurements. The oss function, Im( I/e), of crystaine graphite, e-c, and i-c was measured from 0 to 80 ev by EELS. The incident beam energy was 80 kev and the energy resoution was -0.2 ev. A the data were taken at a momentum transfer of 0. 1 A. The momentum vector was in the basa pane of the graphite sampe. The oss function was extended to infinite energy by assuming that it was proportiona to I/E for E) 80 ev. In order to scae the data, we assumed that Re(1/e) =0 for graphite and e-c. The i-c sampe was not a conductor so the data for it were scaed from an eipsometry measurement of e~ and eq at 2 ev (e~ =5.8 and ez=4.2). For a three sampes the compex dieectric function, et(ro) +iez(ro), was determined by a Kramers-Kronig anaysis of the oss function. The rea and imaginary part of the graphite dieectric function agreed with previous measurements. The osciator strength was cacu The American Physica Society 945

3 .O O Energy Loss (ev) FIG. 1. The effective number of eectrons (osciator strength) as a function of energy oss for graphite (dash-dotted ine), diamond (dotted ine), evaporated carbon (soid ine), and ion-sputtered carbon (dashed ii&e)..25, 75 ated from The resuts are shown in Fig. 1 for A. co~ 10 ev. The dash-dotted ine is our resut for graphite. The soid and dashed ines are for e-c and i-c, respectivey. In a three cases the osciator strength saturates at four eectrons for co=~. Our resuts for graphite are consistent with the correction of the Taft-Phiipp data noted by Sonnenschein, HanAand, and Syassen. The osciator strength of diamond, the dotted ine in Fig. 1, is zero beow the band gap, 7 ev, then rises sharpy. If the osciator strength for the z eectrons in graphite were neary saturated at one eectron by 7 ev, as suggested by the earier measurement, then the fraction of sp bonding in a-c woud be simpy N, c(e =7 ev). The fraction of sp bonds woud be 1 N c(e =7 ev). By this argument it was inferred that the oca bonding in e-c is 100% sp, whie the bonding in an i-c sampe, such as ours, woud be 50% sp. Since the z-eectron osciator strength is not saturated at the diamond band gap, the fraction of sp bonds cannot be determined from N ff(7 ev). We find the same N, ff(e ) for e-c and i Caspreviousy measured, but with the corrected resut for graphite it is no onger possibe to separate the z from o. eectron transitions to determine the fraction of sp and sp bonding. We wi argue, based on the (e, 2e) data, that the fraction of sp bonding is neary the same in e-c and i C( 100%) -but that the eectronic structure of the x eectrons is quite diaerent in the two materias. The spectra momentum density, p(e, q), of crystaine and amorphous soids can be measured by (e, 2e) spectroscopy. In the independent-eectron approximation the spectra density is p(e, q) = ti, (q) 8(E E,), where for crystaine soids a is a wave vector, k, in the Briouin zone and q=k+g (G is a reciprocaattice vector). The spectra densities of crystaine I Binding Energy ( ev) I" IG. 2. The spectra density of ion-sputtered carbon as a function of binding energy (referenced to the Fermi energy) for four different momenta. The soid ines are a fit of two Lorentzians with the data. graphite and evaporated carbon have been pubished. Here we report measurements on ion-sputtered fims. Some of our (e, 2e) data for i Care sh-own in Fig. 2. The spectra density as a function of energy (with respect to the Fermi energy) for four diferent momenta is potted. The data are corrected for mutipe scattering. The smaer peak at E =8 ev we associate with the graphite z band and the stronger peak which disperses with momentum may be either the graphite o. bands or a mixture of diamond and graphite o bands. The energy dependence of the spectra (constant momentum) has been fitted by two Lorentzian functions. The widths of the peaks are independent of q, within our experimenta resoution, and equa to 6.60 ~ 0.5 and ev for the z and o. peaks, respectivey. The widths are primariy from incompete deconvoution of the instrumenta resoution function. We estimate the intrinsic widths of the peaks to be ~ 2 ev. In Fig. 3 the integrated intensities of the two Lorentzian functions are represented by arrows on the energy-momentum surface. The existence of energy-momentum correations in the

4 ~t Spectt o intensity {Arb. units) O Momentum ( A ) I 0 ~M P 0 5,0 b ( sp = 1.00 Csp = 0 Csp =0 50 FIG. 3. The spectra intensities of ion-sputtered carbon as functions of momentum and energy (referenced to the Fermi energy). The height of the arrows is proportiona to p(q, E~,k)* peak width. The peak widths (FWHM) of the "«r" and "(T" bands are 6.6 and 8 ev, respectivey. Finay, we address the question of why e-c is soft and back whie i -C is hard and transparent if the oca bonding in both cases is 100% sp. Part of the difference, undoubtedy, is due to subte dia erences in the eectronic structure of the z band near the Fermi energy which we N c(q) e ~N.,(q ) cannot resove. But there is a major difference between the z-band spectra densities of e-c and i-c which may ( (q) c p2n G(q)+ ( c~2)n D(q) be an important factor expaining the disparate properspectra momentum density of a disordered soid is a chaenging conceptua probem. Some theoretica studies of simpified modes for disordered soids do find that energy-momentum correations exist in the amorphous phase. H aydock finds these correations are simiar to an anguar average of the crystaine phase band structure. A rough anguar average of the rr (dashed ine) and cr (soid ines) bands of graphite are shown in the figure. An anguar average of the diamond o bands is quaitativey simiar to the averaged o. bands in graphite except that the bandwidth for diamond is 2 1 ev whie the width of the o. bands in graphite is 1 8 ev. It can be seen that the "band structu re" of i -C is very simiar to an anguar average of the graphite band structure. The band structure of e-c is aso much ike graphite. We can estimate the fraction of sp and sp bonding in e-c and i-c from the normaization condition on the spectra momentum density as is discussed next. The integra of the spectra densities for the o. bands in graphite gives three eectrons per carbon whie the integra over the n band gives one eectron. The integra of the spectra densities for the four o bands in diamond woud give four eectrons. If we assume the band in e-c at - 8 ev arises from the z -eectron system and assume the dispersing band is associated with the o. bands of either graphite or diamond (or some mixture of the two), then according to the mode proposed above in which ony pure sp and sp bonds exist, the concentration, c p 2 of sp bonds in a-c can be estimated from the ratio I Momentum ( A ) FIG. 4. The ratio of the integrated spectra intensities, V; (q ) = fdef/4«rq dqp; (E, q), for the "«r" and "a" bands of evaporated carbon (open circes) and ion-sputtered carbon (soid circes) as a function of momentum. The soid ines are theoretica predictions of this ratio. where oo f q Np «(q ) = de d A dqq pp, «(E, q), p = «r or o., y = tr-c (e-c or i-c), G denotes graphite, and D denotes diamond. In the imit q ~, this ratio is c, 2/(4 c, 2). Unfortunatey, we are unabe to integrate the spectra density of i Cbeyond-q = 3.5 A. (q = 2.5 for e-c) because the intensity is ost in the statistica noise. The spectra density of graphite and diamond has been cacuated for certain high-symmetry directions by a first-principes, den sity-function a technique. We estimated Np o (q ) and N D (q ) by smoothy interpoating pp (E,q) between the symmetry directions. The experimenta and theoretica resuts for the ratio of z -to- o. intensity are shown in Fig. 4. The open and soid circes are tne experimenta resuts for e-c and i -C, respectivey. The soid ines are the theoretica ratios for the three diae rent con centration s of sp bonding given in the figure. Note the break in the horizonta axis and the exact vaues of the ratios for q =~. The experimenta ratio is very sensitive to statistica Auctuations at ow momentum, but for q ~ 2.0 A the ratio for both e-c and i -C have stabiized to vaues consistent with 1.0. Given the statistica uncertainty, we estimate c, 2 = that 0.85 ~ c,p2 1.0 for e-c and 0.75 ~ c p2 1.0 for i-c.

5 VOLUME 62) NUMBER 8 PHYSICAL REVIEW LETTERS 20 FEBRUARY 1989 ties of the two materias. We find that the spectra density of the z band in e-c goes to zero as q 0 and has neary the same momentum dependence as an anguar average of the spectra density of the graphite z orbita. In contrast, the momentum density of the z orbita in i C-(see Fig. 3) is finite at q =0 and has the characteristics of a hybridized s-p wave function. In i-c a distribution of radii for the curvature of the sp bonded system presumaby exists so that the z-band spectra density is some average over diaerent degrees of rehybridization. A quantitative anaysis of this rehybridization wi require a more detaied mode for the structure of i-c. In summary, we have argued that the sum rue on the compex dieectric function of e-c does not give a reiabe estimate of the reative proportion of sp and sp bonding. We have measured the spectra density of diamondike i Cby -(e, 2e) spectroscopy and find that it has band structure which is quaitativey simiar to graphite. From the normaization condition on the spectra density, we estimate that the oca bonding in e-c and i-c is neary 100% sp. In e-c the momentum density of the z orbita is in agreement with an anguar average of the graphite z-orbita momentum density. The momentum density of the z orbita in i-c dispays significant rehybridization which, we suggest, is due to wrinking and twisting of the sp bonded sheets. We gratefuy acknowedge the preparation and characterization of the ion-sputtered sampes by T. J. Fabish and J. A. Nemetz-Keso at Acoa Technica Center. Theoretica cacuations of the spectra density of graphite and diamond were done by N. A. W. Hozwarth at Wake Forest University. This research was supported in part by grants from the Auminum Company of America and the Virginia Center for Innovative Technoogy and by Grant No. DMR from the Nationa Science Foundation. J. Robertson, Adv. Phys. 35, 317 (1986), and references therein. ~J. C. Angus and C. C. Hayman, Science 241, 913 (1988). N. Wada, P. J. Gaczi, and S. A. Soin, J. Non-Cryst. Soids 35, 543 (1980). 4N. Savvides, J. App. Phys. 59, 4133 (1986). sj. Fink et a., Phys. Rev. B 30, 4713 (1984). se. A. Taft and H. R. Phiipp, Phys. Rev. 138, A197 (1965). R. Sonnenschein, M. Hanfand, and K. Syassen, Phys. Rev. B 38, 3152 (1988). See footnote 30. 8R. C. Haddon, L. E. Brus, and Krishnan Raghavachari, Chem. Phys. Lett. 131, 165 (1986). Giuia Gai, Richard M. Martin, Roberto Car, and Michee Parrineo, Phys. Rev. Lett. 62, 555 (1989). oh. Venghaus, Phys. Status Soidi (b) 71, 609 (1975). Chao Gao et a., Phys. Rev. B 37, 3914 (1988). A. L. Ritter, J. R. Dennison, 53, 2054 (1984). and R. Jones, Phys. Rev. Lett. R. Jones and A. L. Ritter, J ~ Eectron Spectros. Reat. Phenom. 40, 285 (1986). 4J. L. Beeby and T. M. Hayes, (1985). Phys. Rev. B 32, 6464 sroger Haydock, in Current Trends in the Physics of Materias, edited by M. Yussouff (Word Scientific, Singapore, 1987), p We have taken more data on evaporated carbon than are presented in Ref. 12. It wi be pubished in a comprehensive artice.