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Mol. Cryst. Liq. Cryst. 1994, Vol. 256, pp. 915-920 Reprints available directly from the publisher Photocopying permitted by license only

0 1994 OPA (Overseas Publishers Association)

Amsterdam B.V. Published under license by Gordon and Breach Science Publishers S.A. Printed in the United States of America

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GAP REDUCTION OF C6o AND C7o AT HIGH PRESSURE.

K.P.MELETOV, V.K.DOLGANOV, AND YU.A.OSSIPYAN Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Moscow distr.,142432,Russia Abstract The absorption spectra of the C6o and C7o are measured at pressures up to 19 GPa. The pressure dependence of the fundamental absorption edge position E(P) is determined for both materials. The initial value of dE/dP=-0.15 eV/GPa for the stronger-absorption region of C6o decreases up to -0.019 eV/GPa at 12 GPa. The weaker-absorption region located near the fundamental absorption edge shifts slower dE/dP=-0.05 eV/GPa. For the C7o the initial value of dE/dP=-0.1 eV/GPa decreases up to -0.029 eV/GPa at 10 GPa. All pressure induced changes are reversible in this pressure range.

INTRODUCTION The measurements and calculations of the energy specgave the basic features trum of the fullerite C6o and lC;o of the crystal band structure For the improvement of the calculations one has to know the band gap behavior at high pressure, which makes experimental study in this field quite essential.

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EXPERIMENT The initial matfrial of C6o and C7o was prepared using Kratschmer's method A mass-spectral analysis has shown that the purity was better than 99% for C6o and 97% for C7o. The measurements were performed for the single crystals of C6o grown from the supersaturated solution in benzene. The grown crystals wfre platelets with thickness from 0.5 to 5 wm and 200x300 wm dimensions. The measurements of the absorption spectra of the c70 were performed for the pellets of (270. Pellets were made using a high pressure diamond anvil cell (DAC). The high-pressure measurements were carried out in a DAC; a mixture of alcohols was used as the pressure-transmitting medium. The pressure was determined from the luminesfence in the Ri line of a ruby crystal with accuracy 0.1 GPa

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FIGURE 1 Absorption spectra of 0.7 pm thick Cbo crystal at pressures up to 19 GPa. Curve 1 corresponds to normal pressure, curves 2, 3, 4, 5, and 6 to pressures 0.9, 3.1, 9.5, 14 and 19 GPa. RESULTS Fig. 1 depicts the absorption spectra of C6o crystal 0.7 I.tm in thickness at 300K and pressure up to 19 GPa. Curve l corresponds to the normal pressure, curves 2, 3, 4, 5, and 6 correspond to pressures of 0.9, 3.1, 9.5, 14 and 19 GPa, respectively. The growing of pressure gives rise to a strong red shift of the absorption spectrum. The spectrum form is invariable in pressure range from 2.0 GPa up to 12 GPa. One can observe a spectrum broadening connected with solidification of the alcohol mixture at pressure exceeding 12 GPa. Fig. 2 depicts the absorption spectra of 2.8 pm thick C6o crystal at ambient pressure (curve 1) and pressures of 0.9, 1.4, and 2.4 GPa (curves 2, 3, and 4, respectively). A pressure growth leads to a rapid decrease of the


GAP REDUCTION OF C," AND C7o

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width of the weaker-absorption region. It is connected with the difference in the pressure-induced shifts of the wea-

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FIGURE 2 Absorption spectra of 2.8 Mum thick C6o crystal at pressures 0.0001, 0.9, 1.4, and 2.4 GPa - curves 1, 2, 3, and 4, respectively. Dashed lines cut off on the energy axis the absorption edge position. ker- and stronger-absorption regions. The dashed lines in Fig. 2 cut off on the energy axis the values corresponding approximately to the absorption edge position for weakerabsorption (1.83 eV) and stronger-absorption (2.04 eV) regions at ambient pressure. In Fig.3 the absorption spectra of C7o pellets are shown in solid lines at pressures of 10.1 GPa (far left) and, accordingly, 8.0, 4.7, 1.4 and 0.2 GPa. The lover dashed curve corresponds to the absorption spectrum of the C7o solution in toluene and is given for comparison with the absorption spectrum of thin pellet of C7o (the upper


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dashed curve). Measurements on pellets of various thickness yield the value of 1.78+0.005 eV for the fundamental absorption edge position of solid C7o.

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FIGURE 3 Absorption spectra of fullerite C7o pellets at pressures of 10.1, 8.0, 4.7, 1.4 and 0.2 GPa from left to right, respectively (solid lines). The upper dashed curve stands for thin pellet, the lower for C7o solution. Fig.4 shows the pressure dependence of the absorption edge position of the C6o for the stronger-absorption (filled circles) and weaker-absorption (open circles) regions. As a whole, this dependence is reminiscent of the pressure dependence of a relative change of t2e volume V/Vo, obtained by Duclos et al. for C6o crystal The initial value of dE/dP=-0.15 eV/GPa for the stronger-absorption region decreases in absolute value up to -0.019 eV/GPa at 12 GPa. For the weaker-absorption region dE/dP=-0.055 eV/GPa. The absorption edge position of the C7o is shown in Fig.4 by triangles. For the C7o the initial value of dE/dP=-0.1

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GAP REDUCTION OF C,, AND C70

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eV/GPa at ambient pressure decreases in absolute value up to -0.029 eV/GPa at 10 GPa.

PRESSURE GPa
FIGURE 4 Absorption edge position vs the pressure for the C6o and C7o crystals. Filled and open circles correspond to the stronger- and weaker-absorption regions of (260. Triangles correspond to the (270.
DISCUSSION

The negative pressure shift of the absorption edge of and C7o is characteristic also for molecular crystals of hydrocarbon compounds whose optical absorption iq also For governed by n-electron shells of carbon skeletons these crystals the behavior of the absorption spectra at high pressure is connected with the specificity of Van der Waals intermolecular interaction and is markedly different
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from those of traditional inorganic semiconductors*. The basic difference is that the pressure shift in C6o and C7o crystals is negative and its dependence is nonlinear not only on the pressure but on intermolecular distances as well. The latter circumstance complicates estimations of the pressure at which the band-gap may go to zero. The obtained data suggest that the pressure region >50 GPa is most interesting from this point of view. Measurements in this region are of particular interest since the distances between carbon atoms from neighboring fullerene molecules are comparable with intramolecular bond length. The interest to such a measurements is also related to the stability of fullerene molecule at hiqhlyressure since the results in this area are contradictory ACKNOWLEDGMENTS We are grateful to 1.Kremenskaya and 0.Zharikov for supplying fullerene. K.P.M. thanks Soros International Science Foundation for support under conference travel grant N 1121/2. REFERENCES

1. 2. 3. 4. 5. 6. 7.

J.P.Hare et al., Chem.Phvs.Letters, (1991), 394. C.Reber et al., J.Phvs.Chem., 95, (1991), 2127. S.Saito et al., Phvs.Rev.Letters, 66, (1991), 2637. W.Kratschmer et al., Nature, 347, (1990), 354. G.J.Piermarini et al., J.Auul.Phvsics, 46, (1975), 2774. S.J.Duclos et al., Nature, 351, (1991), 380. R.Sonnenschein et al., J.Chem.Phvsics, 74, (1961), 4315. 8. A.Jayaraman, Reviews of Modern Phvsics, 55, (1983), 65. 9. D.W.Snoke et al., Phvs.Rev. B, 47, (1993), 4146. 10. M.Nunez Regueiro et al., -s.Rev. B, 46, (1992), 9903. 11. F.Moshary et al., Phvs.Rev.Letters, 69, (1992), 466.

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