Electrical Conductivity of 3DPolymerized Fullerites C6o Obtained by HPHT Treatment

The initial fullerite C60 has band-gap of 1.7 eV and possesses a high degree of localization of charge carriers on molecules due to the van der Waals character of intermolecular interaction in crystals.

In polymerization of pure fullerenes a-orbitals are partially overlapped and strong cage nanostructures are synthesized based on molecules and clusters formed between molecules. Linear and planar polymers of fullerites obtained at a pressure of up to 8 GPa possess a strong anisotropy of electrical properties due to the presence of covalent interatomic bonds in the chains and planes of the polymer and van der Waals bonds between chains (planes) [28].

At pressures of 9.5-13 GPa and temperatures of synthesis Ts within the range of 300-800 K, 3-D-polymerized crystalline structures with densities within the range of 2.2-2.6 g/cm3 were obtained as described in Section 16.3. At higher temperatures there occurs a disordering of the polymer to form new cluster structures with the maximal density within the range of 2.7-3.3 g/cm3, depending on the actual values of pressure and temperature. All these structures have mixed interatomic bonds of sp2 and sp3 type.

Studies of the temperature dependence of the electrical resistance of 3-D-polymerized fullerites with various structures within the range of 2.5-300 K allow one to determine the activation energy of charge carriers and to draw conclusions on the degree of ordering of the structure. Measurements of the dependence of the electrical resistance on temperature were carried out by a four-contact method in a helium cryostat Oxford Instruments MagLab 2000 Cryostat System [29]. The results of measurements are shown in Figures 16.18 and 16.19 in Arrhenius coordinates and as power-law dependences. The characteristic values of conductivity at room temperature and activation energy of charge carriers are presented in Table 16.6.

SE 4-cc

10 20 30 1000/7", K-1

SE 4-cc

Figure 16.18. A temperature-dependence of the electrical resistance of samples in Ar-rhenius coordinates. The numbers of the curves correspond to those of the samples in Table 16.1.

Electrical conductivity of 3-D-polymerized fullerenes C60 strongly decreases at low temperatures which indicates its semiconductor character.

The characteristic Boltzmann exponential dependence of conductivity on temperature was observed only on samples with crystal 3-D-polymerized structure as, for example, in sample No. 2 (Figure 16.18a). As seen in Figure 16.18a, the curve

Figure 16.19. Power-law temperature dependences of the conductance of C60 samples with crystal and disordered 3-D-polymerized structures obtained at P = 11.5-15 GPa and various temperatures of synthesis.
Table 16.6 The values of specific electrical resistance p300K and charge-carrier activation energy Ea at room temperature of 3-D-polymerized specimens of C60, obtained at pressures of 11.5-12.5 GPa and temperatures of 820-1500 K.

No.

P, GPa

^synthesis? K

n, g cm

P300K, Scm

Ea ,eV

1

12.5

820

2.55

102

0.2

2

11.5

850

2.45

103

0.18

3

12

1000

2.7

103

0.2

4

11.5

1300

2.5

0.1

0.06

5

12.5

1500

3.05

104

0.18

of ln R(T-1) has two well-defined linear regions, giving two different values Ea of the activation energy of charge carriers. At T > 130 K Ea « 0.18eV, and at T < 130 K Ea = 2.3 meV. The value of 0.18 eV can correspond to the band-gap width, as interatomic bonds in the model of this structure are mainly of sp2 type as in graphite. This suggests that the band-gap in this intrinsic carbon semiconductor should not be large.

The dependences of ln R(T-1) for samples with disordered structure have almost no linear regions. The deviations from the Boltzmann law of activation of charge carriers in semiconductors are observed in the presence of a complex energy spectrum of carriers with various energy levels, in the presence of various types of doping impurities, and also due to various dimensional effects, such as quasi-1-D or 2-D conductivity channels. At low temperatures the conductivity is provided due to shallow impurity levels and defect states. With the rise in temperature the charge carriers occupying deeper energy levels become more active. In most samples at temperatures lower than 100 K the dependence of the resistance on temperature is proportional to exp(T0/T)1/4, which testifies to the localization of charge carriers in the approximation of the 3-D system of charge carriers.

At the same time it was found that the dependences of conductivity on temperature had a clearcut power character. For the chosen samples 4 and 1,3,5 the conductivity was proportional to T3/2 and T4, respectively (Figure 16.19).

The experimentally observed dependence (a ~ T3/2) in a wide temperature range (4-300 K) may indicate that the temperature dependence of mobility is of exponential character and compensates the Boltzmann exponential component. It is known, for example, that in diamonds the mobility of holes depends as T-s on temperature, where the parameter s smoothly changes from 1.5 below 400 K up to 3 above 400 K. In the assumption of two types of charge carriers the total conductivity is their superposition:

111 3/2 - = — + — , au = e^T)No1,2T3/2 exp(-EaU2/2kT), a a1 a2

where e is the charge of electron, ^1>2(T) is the Hall mobility of respective charge carriers as a function of temperature, and N01,2 is the density of states in a valent zone.

Figure 16.19b is a plot of the dependence of electrical conductivity of samples 1, 3, 5 with activation energy of 0.17-0.2 eV, showing the linear dependence on T4 in a broad range of temperatures. This regularity represents a certain system and is characteristic of the samples obtained both from C60 and from C70. Further studies are required to explain such power-law dependence of electrical conductivity on temperature at higher temperatures.

In fact, 3-D-polymerized fullerites are new unique intrinsic carbon semiconductors. Their uniqueness is that unlike initial fullerite C60 which is also a semiconductor, 3-D polymers are a covalent crystal, not a van der Waals crystal, as the initial C60. This is the basic difference for applications of these new materials in solid-state electronics.

In general, crystalline 3-D polymerized fullerites are a new type of intrinsic carbon semiconductor and can be used for development of novel semiconductor devices.

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