ISSN 1062�8738, Bulletin of the Russian Academy of Sciences. Physics, 2014, Vol. 78, No. 4, pp. 285–287. © Allerton Press, Inc., 2014.Original Russian Text © Ya.Yu. Volkova, P.S. Zelenovskiy, D.N. Sokolovskiy, A.N. Babushkin, 2014, published in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2014,Vol. 78, No. 4, pp. 430–432.

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INTRODUCTION

Carbon nanotubes have been the object of numer�ous experimental and theoretical studies because oftheir unique mechanical, chemical, electric and mag�netic properties. In addition to fundamental studies,carbon nanotubes turn out to be promising materialsfor the production of new nanomaterials and nanode�vices. Carbon nanotubes are extended cylindricalstructures with diameters of one to several tens ofnanometers and lengths of up to several microns, con�sisting of one or several graphene layers rolled into atube with the hexagonal organization of carbon atoms.The angle of the graphene plane’s rolling with respectto the nanotube axis determines its electronic charac�teristics; i.e., depending on its diameter, a carbon nan�otube can possess both metallic and semiconductorproperties. This allows us to vary the conductivity ofthe nanotube by changing its structure.

Recent experiments showed that under high non�hydrostatic pressure, single�wall carbon nanotubes(SWNTs) and SWNTs filled with fullerenes displayhigh structural stability up to 35 and 30 GPa, respec�tively [1, 2]. It is known from theoretical calculationsthat bundles [3] and individual nanotubes [4] are dis�torted under pressure. A structural transition is firstobserved in a nanotube due to distortion of the crosssection from the circular (cylindrical) to the oval; thecross section then assumes a ribbon shape, and thedestruction (collapse) of nanotubes occurs when acertain critical pressure is reached. The pressure atwhich the destruction of a nanotube occurs is inverselyproportional to its diameter. The possibility of thepolymerizing of nanotubes was predicted in [5] bymeans of molecular dynamics at pressures between10 and 30 GPa with the formation of graphite�like

phases, and polymerization could signal the start ofirreversible changes in the structure of nanotubes atpressures on the order of 35 GPa. The polymerizationof bundles of nanotubes should occur at pressures of42 and 62 GPa, respectively [6, 7]. The polymerizationof a sample of nanotubes will likely lead to the increasein resistance, and the possible graphitization (destruc�tion) will be accompanied by a drop in electroresis�tance.

EXPERIMENTAL

The investigated samples of single�wall carbonnanotubes were prepared by means of chemical vapor�phase deposition CVD and purified by the HiPCO(high pressure CO) method. The diameter of theSWNTs was estimated using an X�ray electron micro�scope and was 0.8–1.2 nm.

To obtain high pressures, we used the diamondanvil cell (DAC) with anvils of “rounded cone�plane”type (Vereschagin�Yakovlev DAC), made of syntheticpolycrystalline diamonds. The error in determiningthe pressure did not exceed 5%. The techniques forour electric measurements and estimating the pressurewere described in detail in [8].

The electroresistance was measured on an Agilent34970A instrument. The structure of SWNTs beforeand after pressure processing was analyzed using theAlpha 300 AR+ system of Raman reflection confocalmicroscopy. A He–Ne laser with a maximum power of37 mW and a wavelength of 632.8 nm was used as oursource of radiation. A lens with magnification of ×100and numerical aperture NA = 0.75 was used to focus thelaser beam. Averaging was performed over 10 spectra.

Structural Transformations in Single�Wall Carbon Nanotubes under High Pressure

Ya. Yu. Volkova, P. S. Zelenovskiy, D. N. Sokolovskiy, and A. N. BabushkinInstitute of Natural Sciences, Ural Federal University, Yekaterinburg, 620000 Russia

e�mail yana.volkova@usu.ru

Abstract—Single�wall carbon nanotubes (SWNTs) under high pressure exhibit high structural stability and aseries of structural transitions up to 35 GPa. As theoretically predicted, the irreversible transformation ofSWNTs in the pressure range of 10–30 GPa can be attributed to the polymerization of nanotubes. The elec�trical conductivity of SWNTs is studied at high pressures up to 35 GPa using a diamond anvil cell (DAC) withelectrically conductive anvils of the “rounded cone�plane” type made of synthetic carbonado�type dia�monds. SWNTs are studied before and after the application of high pressure using the Raman confocalmicroscopy technique. Analysis of Raman spectra and pressure dependences of the SWNT resistance showsthat the observed structural changes in SWNTs are reversible and no polymerization or collapse are observed.

DOI: 10.3103/S1062873814040327

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RESULTS AND DISCUSSION

Figure 1 shows the pressure dependence of theelectroresistance. A sharp drop in resistance isobserved at pressure about 2 GPa, and the resistanceremained virtually the same when the pressure wasraised to 7 GPa. Upon a further increase in pressure,the resistance continued to fall. When the load wasremoved, the resistance returned to its initial value.The pressure hysteresis of resistance was observed inthe pressure range to 25 GPa, due apparently to theemergence of metastable states.

The sharp drop in resistance at 2 GPa could be dueto the cross section of the tubes transitioning fromcylindrical to elliptical. The subsequent weak changein resistance in the pressure region of 2 to 7 GPa couldcorrespond to the preservation of the tubes’ ellipticalcross section. The further smooth decline in resistanceat pressures above 7 GPa can be considered the transi�tion from the elliptical to the ribbon shape and its pres�ervation in this interval. The slight smoothing of thecurve above 25 GPa could be due to the transition tothe fairly stable ribbon state. The return of the resis�tance to its initial value indicates that there is no flat�tened state (the total collapse of nanotubes) at pres�

sures on the order of 35 GPa. As confirmation of this,Fig. 2 shows Raman spectra of the initial SWNTs andsamples after compression at 35 GPa. The spectracontain three groups of lines: RBM (radial breathingmodes), D and G.

The RBM band lies in the region of low frequencies(150–300 cm–1) and is caused by radial vibrations ofcarbon atoms in the wall of a nanotube. The sharpnessof this band is a characteristic sign of the presence ofsingle�wall nanotubes in the studied sample, since theradial vibrations of carbon atoms in multi�wall nano�tubes are hampered by the walls of neighboring tubes;they are therefore quite weak, and this band in thespectrum is either not seen at all or is expressed muchmore weakly. The G band has the highest intensity andis found in the region of 1500–1600 cm–1; it is due toso�called tangential vibrations of the carbon atoms inthe plane of the graphene layer. The D band lies in thefrequency range of 1250–1450 cm–1, and it is seen inthe Raman spectra of all carbon materials. Its intensitycharacterizes the defect structure, i.e., the degree towhich the symmetry of the ideal graphite layer with thesp2�hybridiztion of carbon atoms is violated. The ratioof the intensities of bands D/G (table) characterizesthe relation between the amount of material with dis�ordered and ordered structures in a sample and can beeffectively used, particularly for determining thepurity of nanotubes [9].

The substantial rise in the intensity of the D bandafter the pressure processing of the SWNT samplecould be associated with the increase in the defectstructure of the sample, and the presence of the RBMband in the spectrum after the pressure is applied indi�cates that the main bulk of nanotubes is not distortedat 35 GPa.

500350 5 10 15 20 25 30

1000

1500

2000

2500

3000R, Ω

Р, GPa

Fig. 1. Pressure dependence of the resistance of SWNTbundles. The triangles show the pressure increasing, thesguares⎯pressure decreasing.

0 400 800 1200 1600

Initial materialMaterial after processing with pressure

Wawe number, cm–1

Inte

nsi

ty,

arb.

un

its

RBM

D

G

Fig. 2. Raman spectra of single�wall carbon nanotubesbefore and after exposure pressure of 35 GPa.

Ratio of the intensities of the bands D/G Raman spectra ofSWNTs before and after processing by pressures on the or�der of 35 GPa

Ratio D/G

Initial material 0.1

SWNTs after processing with pressure 0.8

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STRUCTURAL TRANSFORMATIONS IN SINGLE�WALL CARBON NANOTUBES 287

CONCLUSIONS

Our study reveals a strong dependence of SWNTsresistance on their structural state which evolves withpressure. Features found on the pressure dependencesof resistance at 2 and 7 GPa correspond to the phasetransitions associated with a change in the cross sec�tions of nanotubes. A pressure hysteresis in resistancewas observed for the sample of SWNT bundles, duepossibly to the existence of metastable states.

No polymerization of SWNTs under the conditionsof our experiment was observed, and there was nochange in the electron structure or resistance.

The pressure of 35 GPa was not critical for theinvestigated SWNT sample, and there was no totaldestruction of nanotubes.

ACKNOWLEDGMENTS

This work was supported by the Government ofSverdlovsk oblast; the Russian Foundation for BasicResearch, project no. 13�02_96039�r�ural; the RFMinistry of Education and Science; and the Programfor the Development of Ural Federal University.

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Translated by L. Mosina