Surface and bulk investigations of fullerene-based materials under high pressure and temperature by means of vibrational spectroscopy

SURFACE AND INTERFACE ANALYSISSurf. Interface Anal. 27, 213È220 (1999)

Surface and Bulk Investigations of Fullerene-basedMaterials under High Pressure and Temperatureby Means of Vibrational Spectroscopy

N. I. Afanasyeva* and R. F. BruchDepartment of Physics, University of Nevada Reno, Reno, NV 89557, USA

Metastable materials have been formed under high pressure and temperature. These carbon structures haveC60been developed and studied in a wide spectral range by Fourier transform infrared spectroscopy (FTIR) and

micro-Raman spectroscopy. The FTIR spectra of carbon-based polymer materials (pristine fullerenes, carbonnanostructures, diamond-like forms) were recorded in the 4000–10 cm—1 region. Our focus lies on low-C

60frequency infrared (IR) spectra of fullerene and metastable forms of obtained under high pressures and tem-C60peratures as well as the spectra of natural diamond and graphite in the 8–170 cm—1 region. Several new bands of

diamond-like phases arise in the high pressure–temperature phase of reÑection IR spectra. The band structuresconnected with diamond-like structures in sp3 hydridization of the bonds observed were identiÐed as phases of highpressure and temperature. It is our understanding that these low-frequency modes can be assigned to zone- andspace-folded phonons. In addition, our observation on micro-Raman spectra of these carbon-based materials andtheir new forms allowed us to determine the level of disorder in these types of polymer forms of carbon, as well asto study the formation of unusual nanostructures. Copyright 1999 John Wiley & Sons, Ltd.(

FTIR; Raman spectroscopy ; far infrared ; surface spectraKEYWORDS: C60 ;

INTRODUCTION

Recent advances in understanding fullerene-basedmaterials lead to a new class of fullerene nanotechnol-ogy.1 In particular, the rapid progress in the prep-aration of chemically inert, mechanically hard andthermally stable materials, such as polycrystallinediamond Ðlms2 and other hard and super-hardmaterials, has attracted much attention. The fabricationof hard and super-hard materials is based on toC60enhance the diamond synthesis leading to new indus-trial applications, speciÐcally for micromachining.2,3The amorphous carbons can consist of sp3-, sp2- andsp-bonded carbon with medium- to short-range order-ing. The classiÐcation and properties of such carbonsdepend on the sp3/sp2/sp bond ratio, on the hydrogencontent and on the speciÐc production process.1,3

Another remarkable property of carbon-basedmaterials is the formation of molecules with highC60stability and crystallinity and a wide bandgap (2.5 eV)in a crystal, characterized by D 0.28 eV perC60bonds. Special attention has been given to newC60ÈC60metastable forms obtained under high pressure andC60temperature. These new hard and super-hard carbonstates have been described as diamond-like structures inseveral cases.3,4 Such hard and super-hard materialsobtained from pure pristine fullerene show face-C60entered cubic (fcc) structure and have been createdunder high pressure in the range 9.5È13 GPa and in the

* Correspondence to : I. Department of Physics,N. Afanasyeva,University of Nevada Reno, Reno, NV 89557, USA.

temperature interval from 650 to 1830 K.5h8 Thesematerials have speciÐc unique optical properties thatdi†er from diamond. In addition, they exhibit hardnessnear or above that of diamond.8,9

In this work, we have developed a special technologyfor high-pressure and high-temperature treatment ofpure pristine fullerene fcc structures in the pressureC60range 9.5È13 GPa and in the temperature interval from650 to 1830 K.6h8 These hard and sometimes super-hard materials were created in a high pressureÈtemperature regime, where shear deformations areimportant. Using this technique, several unusual three-dimensional polymers with mixed crystalline andamorphous structures have been obtained. Thesematerials could be characterized as sp2- and sp3-hybridized carbon bonds. Some of these new materialshave also shown nanostructure-type phenomena. Suchmaterials have speciÐc optical properties and unusualhardness close to diamond, in several cases the hardnesswas even superior to diamond.

Detailed spectroscopic studies of the optical absorp-tion and reÑection spectra of these super-hard samplesallowed us to determine the structural features of thesenew materials.7 SpeciÐcally, optical absorption andreÑection spectra were used to determine the opticaland electronic properties of these new materials and todeÐne their structures more precisely. In this work weprimarily utilized Fourier transform infrared spectros-copy (FTIR) and micro-Raman spectroscopy fordetailed studies of “orderÈdisorderÏ phenomena in pris-tine and new states obtained under high pres-C60 C60sures and temperatures. For comparison, diamond,graphite, fullerene and our new hard carbon materialswere investigated via far-infrared (FIR) spectroscopic

CCC 0142È2421/99/040213È08 $17.50 Received 17 June 1998Copyright ( 1999 John Wiley & Sons, Ltd. Accepted 23 December 1998

214 N. I. AFANASYEVA AND R. F. BRUCH

techniques. The observed structures are discussed interms of sp2 and sp3 hybridization. To our knowledgewe have obtained, for the Ðrst time, detailed informa-tion on low-frequency infrared vibrational modes of ful-lerene and metastable forms of obtained under highC60pressures and temperatures. In particular, the IRspectra of natural diamond and graphite in the 8È170cm~1 region have been analyzed and classiÐed.

In this work we report on reÑection spectra in theFIR region. These spectra were recorded directly fromthe surfaces of our hard materials, as well as fromgraphite and diamond. The surface analysis from FIRreÑection spectra allowed us to investigate the surfacestructure as a function of pressure and temperature. Inaddition, we present here micro-Raman spectra of thesame hard and super-hard materials for detailedC60studies of “orderÈdisorderÏ phenomena in pristine andintercalated carbon Ðbers,10 nanotubes and new C60states obtained under pressure and temperature com-posed of crystalline and amorphous diamond, fullereneand graphiteÈlike forms. These structures can beobserved via Raman scattering and are discussed interms of sp2 and sp3 hybridization of the bonds.

EXPERIMENTAL

Characterization of samples

We have investigated the IR absorption and reÑectionspectra of our samples obtained from pure (99.8%)C60containing 0.1% in di†erent pressure and tem-C70perature ranges. The Ðrst set of samples were preparedunder a pressure of 13 GPa in the temperature range1200È1800 K, whereas the second set of samples wereobtained under a pressure of 11 GPa and at tem-peratures of 703, 823 and 1637 K, respectively. SpeciÐ-cally, we produced super-hard samples of with aC60smaller lattice parameter, i.e. a \ 12.8 (sample 1) andÓa \ 12.9 (sample 2). The lattice parameters and typeÓof structure were characterized by x-ray di†raction.These samples were prepared under the following con-ditions : P\ 13 GPa and T \ 1000 K for sample 1 ;P\ 9.5 GPa and T \ 750 K for sample 2. Two othersamples were also created from at the same pres-C60sure but at temperatures of 650 K and 1800 K (samples3 and 4). The fcc structure of sample 3 is observed tohave a lattice parameter of a \ 16.6 whereas sampleÓ,4 is closely related to turbostratic graphite. We alsoinvestigated IR absorption and reÑection spectra ofsolids formed from the powder of pressed tabletsC60and the natural diamond IV group. These samples havea cross-sectional area of D1 or 2 mm2. The surfaces ofthe samples were polished by diamond powder and thencleaned carefully using special solvents.

Measurements of spectra

Absorption and reÑection spectra of the samples wererecorded at room temperature, mainly in the usualrange of 4000È10 cm~1, using a commercially availableIFS 113 V Bruker Fourier Transform spectrometer withan MCT (HgCdTe) nitrogen-cooled detector. In theFIR, the spectrometer was evacuated. Special acces-

sories were used for investigations of very smallsamples. The spectral resolution used at all wavelengthswas \1 cm~1. To enhance the signal-to-noise ratio, alarge number of scans (D1256) was necessary. Thesignal-to-noise ratio obtained was within standardlimits for this type of instrument. For the evaluation ofreÑectance spectra, a harmonic oscillator model9 of thealloy lattice dynamics was used. We further note thatseveral experiments were performed in the spectralregion below 10 cm~1 with special FIR optics.

Moreover, the hard and super-hard samples of atC60pressures up to 13 GPa and temperatures up to 1673 Khave been investigated by micro-Raman spectroscopyby utilizing a Jobin-Yvon T64000 spectrometer in con-nection with an optical microscope with a spot size of\2 lm. In the case of Raman spectroscopy, the charge-coupled device (CCD) detector was cooled to 130 K andan argon ion laser was used to produce 457.9 nm lineradiation. Standard deconvolution techniques havebeen applied to analyze the spectral features obtained.

RESULTS AND DISCUSSION

The behavior of fullerene under high pressure is ofC60great interest due to the possibility of direct transform-ation to diamond. This type of transformation wasobserved using the shock compression and rapid quen-ching technique at pressures of 16È69 GPa.8,11 In ourstudy we have used fullerene material because thisC60transformation mechanism to a diamond-like structurecan be achieved more easily. The bonding in mol-C60ecules has an aromatic character and is di†erent frompure sp2 (graphite) or sp3 (diamond) hybridization. Asreported by Sandar et al.,12 the decomposition of inC60a vacuum begins at D973 K and becomes completeD 1173 K. Therefore, the transformation of into aC60new phase with diamond-like properties under moder-ate pressure is expected to occur at D 973 K. For thisreason we were interested in the comparison of opticalproperties for pure and the new samples.C60In this study we concentrated on the observation oflow-frequency modes, particularly the frequency ofvibrational and translational modes. In the wavelengthregion 15È150 cm~1 we investigated the di†erences inphonon band positions between the and theC60samples prepared under di†erent pressure and tem-perature. These additional structures have beenexplained within the surface-phonon approach.13,14Thus, we can conclude that pressure exerts a profoundinÑuence on the phonon spectra of pure as well asC60 ,new forms of solid C60 .

In Fig. 1 the reÑectance spectra of samples 1 and 2, aswell as pristine are presented. As can be seen fromC60 ,Fig. 1, the spectra of samples 1 and 2 are similar butdi†er substantially from the spectra of pristine inC60the range 20È160 cm~1. The spectra of our coincideC60with the data.4,12h17 In this range we registered the fol-lowing phonon modes of our sample (see Table 1).C60The main low-frequency modes of are centered atC608.5, 17.4 and 25 cm~1, respectively, and are assigned tovibrational modes of whereas the bands occurringC60 ,at 34.7 and 42.4 cm~1 originate from translationalmodes.11,12 We note that the bands in the 17È41 cm~1region have been obtained at 2 K.15 In our experiments

Surf. Interface Anal. 27, 213È220 (1999) Copyright ( 1999 John Wiley & Sons, Ltd.

FTIR OF FULLERENE-BASED MATERIALS 215

Figure 1. Low-frequency IR reflection spectra in the 20–160 cmÉ1 region: (1) sample 1; (2) sample 2; (3) pristine C60

.

we observed the same modes at room temperature inthe spectra of our samples. The peak position of thebands at 42.4 and 56.0 cm~1 in the spectra of coin-C60cide with the frequencies of two bands, i.e. 40.2 and 54.3cm~1 and 2 K,15 and belong to translational modes ofF-symmetry.3

Our method of FIR reÑectivity measurementsallowed us to receive data in both bulk and molecularlayer structures.3,4,18h30 SpeciÐcally, we have foundthat the spectra of super-hard samples di†er from thoseof pristine composition. Table 2 lists the phononC60modes obtained in this work of pristine high-C60 ,pressure samples 1 and 2, diamond30 and graphite. It isevident that the main bands of are at 42.4, 44.4,C6056.0, 65.6, 75.3, 87.0, 100.3, 115.3, 121.5, 150.4 and 158.2cm~1. As can be seen, these bands are also present inthe spectra of dense fullerenes21 (samples 1 and 2). Partof these bands can be attributed to diamond-like peaks,e.g. structures at 57, 64.5, 90.6, 102.2, 115.7, 121.7 and150.4 cm~1. From these results we conclude that spe-ciÐc fullerene modes26 (arising from pristine in theC60)spectra of dense fcc phases (samples 1 and 2) are 42.4,44.4 and 75.3 cm~1, respectively. We further note thatthe band centered at 76.0 cm~1 was observed in thespectra of graphite3 and of sample 4 obtained from C60in the high-temperature synthesis.

A comparison of the spectra of super-hard metastablefcc-phase fullerene with those of diamond andC60 21graphite in the low-frequency IR region is presented inFig. 2 and Table 2. From Fig. 2 we have deduced thefollowing pronounced band structures : 37È39 ; 42È44 ;50 ; 65 ; 75 ; 100 ; 110È115 ; 120È122 cm~1. According toFig. 2 and Table 2, the super-hard carbon states basedon fullerene exhibit many reÑection bands in theC60FIR region that coincide with the pristine as wellC60 17as with diamond30 and graphite.3

When comparing our results from Figs 1 and 2 andTables 1 and 2 we found that fullerene bands in the

spectral range 42È44 cm~1 (samples 1 and 2) were pre-served in these dense fcc structures. On the other hand,we obtained distinct lines mainly at 90.6 and 110 cm~1for samples 1 and 2 that are absent in the spectra of

and graphite,3 although they are present in theC60 18diamond spectra. In our view these lines correspond tomodes of covalent intermolecular bonds in the dense fccstructures of interatomic-bonded diamond. Addi-tionally, in the spectra of natural diamond we observedfor the Ðrst time very-low-frequency bands 1.9, 3.9, 7.7and 13.5 cm~1. These modes can be assigned to thevery low vibrations. In addition, in the diamond spectrawe have measured intense well-known bands13,14 at21.2 and 28.0 cm~1. Furthermore, we have found thatthe solid samples (after compression) show band struc-tures that are absent in fullerene and diamond spectra.These bands are located at D50 cm~1 (samples 1È4)and D68 cm~1 (samples 1 and 3) and are named“hybridÏ bands.16 A detailed comparison of our fullereneand diamond results is summarized in Table 2. Forexample, the doublet of the new “diamond-likeÏ bands at91.2 and 96.6 cm~1 in the spectrum of sample 1 can beassigned to the di†erent orientation of this new phase. Itcan be seen in the spectra of all samples (from the very-low-temperature sample 3 to the spectra of the highpressureÈtemperature phase sample 1). In this contextwe note that only the band shape varies. The fullereneand diamond spectra observed in the vicinity of thebands at 115.7, 121.7, 125.4, 131.3, 150.4 and 164.0cm~1 in the higher frequency region indicate similarity.For instance, the structure at 115.7 cm~1 coincides withthe Raman-active mode at 115.5 cm~1 in the spectra ofthe high-pressure phase, and corresponds to polymerstates.17

Another interesting structure (see Table 2) at 100.3cm~1 for pristine is responsible for a dumb-bellC60structure with asymmetric distortion.16 This mode isabsent in the phonon spectra of the very dense sample 1

Copyright ( 1999 John Wiley & Sons, Ltd. Surf. Interface Anal. 27, 213È220 (1999)


Table 1. Comparison of phonon modes (cm~1) for C60diamond, graphite and high-pressure and high-temperature sample 1, with band positionsdeduced from far-infrared reÑection spectra, andthe assignment of speciÐc modes in the FTIRreÑection spectra

Phonon modes (cmÉ1)a Assignment of mode

C60

Diamond Graphite Sample 1 structures

1.9 Vibrational

3.9 Vibrational

7.7 Vibrational

8.5 Vibrational

13.5 Vibrational

17.4 Vibrational

21.2 Vibrational

25.1 Vibrational

28.0 Vibrational

34.7 Translational

38.6 Translational

40.5 40.5 40.5 Translational

42.4 42.4 Translational



52.1 52.1 52.1 Translational

56.0 Translational

57.0 Hybrid

57.5 Hybrid

57.9 57.8

64.5 Phonon

65.6 65.6 65.8 Phonon

69.4 69.3 Phonon

71.4 Phonon

73.3 Phonon

75.5 75.3 Phonon

77.1 84.0 76.0 Phonon

86.8 Phonon

90.6 91.2 Phonon

98.4 96.6 Phonon

99.3 Phonon

100.3 102.2 100.5 Dumb-bell

110.0

115.7 115.7 115.0 Polymer state

119.8

121.5 121.7 121.5 F1 Cubic lattice

125.4 Intermolecular

131.2 131.3 Intermolecular


150.4 150.4 Intermolecular




a See also Refs 14, 16 and 18.

and the high-temperature sample 4. It can be connectedwith the increased rigid coordinated structure associ-ated with sp3 hybridization of the bonds in the fullerenemolecules (for the dense samples). The IR-active modeat 121.5 cm~1 occurs in the spectra of diamondC60 ,and all of our samples. This mode is assigned to a cubiclattice of F1 symmetry.18 We also observed an intermo-lecular mode22,24,25 at D 127 cm~1, which is upshiftedby 131 cm~1 under high pressure. Furthermore, latticemodes14,18 at 137 and 145 cm~1 are present in thespectra of samples 1 and 2, where the mode located at145 cm~1 is shifted under pressure from 144 to 147cm~1. We found evidence that this mode occurs at144.7 cm~1 in the diamond structure but for fullerenesthe closest peak is at 140.8 cm~1 in the spectra andC60at 144.7 cm~1 in diamond. The modes with peaks at150.4, 158.2 and 165.1 cm~1 were measured for the Ðrsttime in the spectra of pristine diamond and sampleC60 ,1 (with some upshift).

Phototransformation of fullerene was achieved atroom temperature.21,22 In this process a weakly bondedVan der Waals solid transforms to a polymeric (C60)nphase in which molecules are cross-linked via covalentbonds. In the polymeric phase, a new Raman-activemode was observed at D 118 cm~1. Several scientistsobserved this mode and interpreted it as the stretchingof cross-linked bonds in the case of covalent bondsbetween adjacent fullerene molecules. This wavenumberlies in the gap between the high-frequency lattice (orintermolecular) modes (60 cm~1) and the low-wavenumber intra-molecular mode observed at 272cm~1 (i.e. Hg(1) mode). The cross-links also provide amechanism to lower the icosahedral molecular sym-metry of thereby activating numerous new RamanC60 ,and infrared modes. We ensured that the intermolecularmodes higher than 100È200 cm~1 can be assigned topolymer structures in with new cross-linking cova-C60lent bonds. In particular, the modes centered at 127, 137and 145 demonstrate upshifts under pressure and tem-perature and they are responsible for the creation ofnew cross-linking of polymer-type and possiblydiamond-like solid. The polymerization process of C60takes place in the above interval. Thus, we observedtransformations from weakly interacting moleculesC60to strongly interacting agglomerated or new net-C60works under high pressure and temperature.25

The lowering of the perfect icosahedral symmetry(Ih)for the dimer (polymer) gives rise to the removal ofdegeneracies of vibrational modes present in the iso-

Table 2. Main peak positions (cm~1) in the far infrared range for high-pressure and high-C60 ,temperature samples 1 and 2, diamond and graphite

Sample Main peak positions (cmÉ1)a

C60

38.6 42.4 44.4 56.0 65.6 71.4 75.3 86.8 100.3

Sample 1 38.0 42.4 44.4 50.1 52.1 57.8 65.8 69.3 91.2 96.6

Sample 2 44.4 50.1 59.7 65.6 73.4 90.5 100.3

Diamond 40.5 50.6 52.1 57.0 64.5 69.4 73.3 90.6 99.3

Graphite 40.5 50.1 52.1 57.5 65.6 75.5 84.0 100.5

a The peak positions have been deduced from the corresponding FTIR spectra. See also Refs 14, 16 and 29.



Figure 2. Low-frequency IR reflection spectra in the 20–160 cmÉ1 region: (1) sample 1; (2) diamond; (3) graphite.

lated resulting in many additional modes in theC60spectra obtained. The intermolecular stretch and vibra-tional modes dominate in our spectra. The linkagebetween monomer units in the polymer takes placeC60across parallel double bonds of the fullerene molecule,transforming it into a four-sided ring with a dimerresembling a dumb-bell structure.22

We also note that the mode at 137 cm~1 wasobserved by inelastic neutron scattering29,31 at 25 Kand was assigned to lattice mode. Hence, lattice defor-mation, partial disordering and the process of poly-merization inÑuence the low frequency modes andsurface modes of carbon-based structures.

To summarize our investigations in the 20È170 cm~1region we have discovered several new band structurescentered at 40.5, 52.1, 57.8, 65.8, 69.3, 91.2, 96.6, 110.0,155 and 121.5 cm~1 for the super-hard samples in theIR reÑectance spectra. These bands are associated withthe state of covalent intermolecular bonds. In particularthese bands are very stable in the new dense fcc phase.19This process is conÐrmed by our spectra, i.e. theyexhibit no wavenumber shifts in the range of pressurefrom 9.5 to 13 GPa. These phonon modes may beassigned to zone- and space-folded phonons.3 Conse-quently, by IR spectral analysis the IR spectra of thehard and super-hard carbon phases allowed us to eluci-date the structure and role of di†erent phonon statesduring phase transformation.

MICRO-RAMAN SPECTROSCOPY FOR HARDMATERIALS

In Fig. 3, Raman spectra of hard and super-hardsamples are presented. It is evident that the position ofthe dominant peak shifts from 1565 to 1590 cm~1 withincreasing temperature during the synthesis. All threelines are asymmetric in the range 1000È1400 cm~1. Thelow-temperature spectrum is more asymmetric, whereas

the corresponding spectra of the other two samplesobtained at 1473 and 1673 K are nearly identical. Inthese spectra we can observed another less pronouncedstructure near 500 cm~1. In the case of lower tem-perature (1223 K) samples this line is upshifted towards600È700 cm~1. The intensity ratio of the twoI1590/I520major peaks is increased 1.3 times for the high-temperature case (1673 K). Therefore we conclude thatthe degree of order is higher in the two high-temperature phases. We further note that the half-widthof the central line 1590 cm~1 decreases from 178 to 142cm~1 when the temperature increases from 1223 to1637 K. The observed di†erences of the full width athalf-maximum (FWHM) are D 40 cm~1. It can be seenthat the highest temperature sample exhibits the narro-west central peak and is also associated with the high-temperature and high-pressure samples.

In addition to the above Ðrst-order Raman results wehave also obtained second-order Raman spectra.32,33One of our prominent second-order lines appears atD 2960 cm~1 and stems from high-pressure and high-temperature samples. This second-order Raman peakprovides additional information about the orderÈdisorder phenomena in such high-temperature/high-pressure samples. The strong asymmetry observed inthe strongest peak in Fig. 3 at D 1400 cm~1 may beused as a Ðngerprint for “orderÈdisorderÏ phenomena ingraphite and carbon structures. As indicated in Fig. 3,the spectrum arising from the lowest temperature ischaracterized by a signiÐcant shoulder of the main peakat D 1400 cm~1.

This region represents high-density phonon states.The asymmetry of the main line and the appearance ofweaker structures below 1200 cm~1 may allow us tostudy also the di†erent states of disordering in thesenew types of structures. It is evident that the averageposition of these orderÈdisorder structures at D1333cm~1 coincides with the position of the diamond peakat 1332 cm~1 in the Ðrst-order Raman spectra.30 The



Figure 3. Micro-Raman spectra of samples obtained under a pressure of 14 GPa and different temperatures : (1) 1673 K; (2) 1473 K;C60

(3) 1223 K.

sp3 bonding predicts scattering peaks5,20 ranging from1355 to 1600 cm~1. The obtained close agreement ofour main peak position with this sp~3-bonding modelconÐrms the observation of diamond-like structures inour samples.

In Fig. 4 we have displayed another series of micro-Raman spectra arising from as a function of tem-C60perature for a pressure of 11 GPa. As representativeresults we show three typical curves. These curves di†erdramatically from previous spectra indicated in Fig. 3.

Figure 4. Micro-Raman spectra of samples obtained under a pressure of 11 GPa and different temperatures : (1) 823 K; (2) 703 K; (3)C60

1673 K.



The main peak structure arises at D1600 cm~1.Another prominent spectral feature on the low-wavenumber side is shown D1450 cm~1. A broad linelocated in the region between 1580 and 1600 cm~1 isascribed to regions of pyrolytic graphite3 or highly dis-ordered amorphous carbon.4 An important parameterto evaluate the degree of disorder is the intensity ratioof these two characteristic bands, i.e. ThisI1600/I1400 .ratio decreases from D2 to 1.5 with increasing tem-perature (703È1673 K) at constant pressure (11 GPa).The Ðrst-order Raman spectra presented in Fig. 4 canbe correlated with our second-order micro-Ramanspectra. In these spectra we have found the followingpeaks at 2954, 2750 and 2921 cm~1 corresponding tothe low-temperature cases (703 and 973 K, respectively).In this connection we note that at pressures \11 GPawe have found more evidence for disordering processes.However, these processes do not depend on the tem-perature in the 823È1023 K range.

In addition the second-order micro-Raman spectra ofhard materials show that “silent modesÏ are activatedunder the inÑuence of high pressure and temperature. Inparticular, the band structure at 2750 cm~1 in ourspectra appeared as a result of pressure/temperaturee†ects. This induced silent mode was assigned to a com-bination of (7) symmetry in combinationF1u (3) ] Huwith the two additional modes with (6)Gg (6) ] Hgsymmetry, where only (6) is Raman active.31 WeHgobserved this mode for the Ðrst time in our Ramanspectra. Another mode at 2954 cm~1 was associatedwith 2 This mode was found also in the spectra ofAg .

single crystals at 40 K.35 The third mode seen inC60our second-order Raman spectra centered at 2920 cm~1is a combination of two other silent modes, i.e. (3)F1g(7). Such modes were observed also in the IR] Hutransmission spectra of Ðlms deposited on KBr sub-C60strates by Wang and co-workers.22 Therefore, thesecond-order Raman spectra might be very useful forfuture studies to elucidate orderÈdisorder phenomena incarbon-based materials.

As illustrated in Fig. 4, speciÐc temperature regionscan be selected to optimize orderÈdisorder processes.SpeciÐcally, these types of samples are dominated bysp2-bonded microcrystalline graphite structures.Regions of sp3 bonding may exist in our samples withsome Ðnite probability, but were not detected directlyvia Raman scattering. In addition, the spectra indicatedin Fig. 4 are associated with sp2 bonding in the vicinity

of 1400 cm~1. These samples have structures verysimilar to graphite, with a large degree of disorder. Theanalysis of Figs 1È4 clearly indicates that FTIR andmicro-Raman methods may be utilized for designing,testing and selecting speciÐc new fullerene materials,including nanostructures.

CONCLUSION

In this work we have investigated unusual physicalproperties of new metastable carbon structures basedon molecules. The FTIR and micro-Raman tech-C60niques have been used to examine di†erent series ofsamples for the creation of new hard and super-hardmaterials under high-energy conditions as a function ofpressure and temperature. Such high-pressure and high-temperature regimes allowed us to produce diamond-like structures. From the spectral analysis we concludethat our samples exhibit sp2 type of bonding. In thesenew solid phases the spectra indicate only a few pro-nounced peaks that do not shift under di†erent pressureconditions. The associated bands are connected withdiamond-like structures, with sp3 hybridization of thebonds. Such structures appeared only in the samplesobtained under high pressure and temperature. Wesuggest here that low-frequency modes can be assignedto zone- and space-folded phonons. Finally, our investi-gations have shown that new structures can be manu-factured more efficiently under detailed spectroscopiccontrol. Furthermore, the FTIR and Ðrst- and second-order micro-Raman methods can reveal the detailedstructure, order and disorder and composition ofcomplex composite materials.

Acknowledgement

This work was performed with support from the International Scienceand Technology Centre (ISTC), Moscow, Russia (grant no. 015) andthe Russian Foundation for Fundamental Investigations (RFFI)(project no. 96-02-018015). The authors are indebted to Professor V.Blank from the Institute of Spectroscopy, Troitsk, Moscow Region,Russian Academy of Sciences. In addition we would like to thankProfessor Ovchinikov from the Institute of Biochemical Physics,Moscow, Russian Academy of Sciences. We are also grateful to DrBelogorokhov from the Institute of Rare Metals, Moscow Russia andDr T. Jawhari, University of Barcelona, Spain for assistance in themeasurements.

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Surface and bulk investigations of fullerene-based materials under high pressure and temperature by means of vibrational spectroscopy