, 2002, 44, . 3 Electron density as the main parameter influencing the formation of fullerenes in a carbon plasma G.N. Churilov, P.V. Novikov, V.A. Lopatin, N.G. Vnukova, N.V. Bulina, S.M. Bachilo, D. Tsyboulski, R.B. Weisman Kirensky Institute of Physics, Siberian Branch of Russian Academy of Sciences 660036 Krasnoyarsk, Russia E-mail: churilov@iph.krasn.ru Krasnoyarsk State Technical University, 660074 Krasnoyarsk, Russia Rice University, TX 77005 Houston, USA Thermodynamic estimates are presented for the formation of spheroidal and flat carbon clusters from reactant species of different charges. Charge is shown to strongly influence the geometry and stability of flat clusters.

Changes in the charge of flat clusters can promote both their folding to spheroidal structures and their dissociation.

It is cuncluded that the fluctuations of electron concentration in carbon plasmas can result in the accumulation of fullerene clusters and the dissociation of flat clusters. The research described in this publication was made possible in part by Award RE1-2231 of the U.S. Civilian Research & Development Foundation for the Independent States of the Former Soviet Union (CRDF). Any opinions, findings and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect those of the CRDF. The work is also partially supported by the Russian State Program Fullerenes and Atomic Clusters (N 5-3-00) and RF Education Ministry program Sceintific research of high school in priority directions of science and technique (N

The carbon-helium plasma at a pressure of 100 Torr is discharge was 44 kHz and the current was 10 to 15 A. It the optimal environment for synthesizing fullerenes, as was was found that the discharge at atmospheric pressure is first demonstrated in Krtschmers method [1]. Different stratified [6]. It has been known that such strata are the modifications of this method now exist [2]. Usually, at these visual result of ionization waves (ionization instability). Until pressures and especially in a rare gas atmosphere, ionization recently, however, strata were normally observed only at low waves can be observed [3]. pressure in regions restricted by glass tube walls.

The method used in our laboratory can be considered Fig. 2 shows the discharge in the argon stream and a highto be a modification of Krtschmers method [2,4,5]. We speed photograph of this discharge. Here, the presence of designed and successfully used plasma-chemical reactor running ionization waves is easily visible. Thus, we observe based on thermal graphite evaporation with formation of that the discharges at atmospheric pressure can be stratified, a carbon plasma jet, which is combined with helium too. Ionization waves arise when the discharge is driven by flow at atmospheric pressure in a water-cooled chamber. the alternating current.

A transformer matches the amplifier impedance with that In reference [7], the equilibrium states of rare gas plasmas of the plasmatron. The distinctive feature of our setup is were calculated by the method of level kinetics. According that the synthesis is conducted at atmospheric pressure in to these results, more than one value of the electron the stream of carbon-helium plasma. The arc is fed by concentration can exist for definite values of gas density and an alternating current at the frequency of 66 or 44 kHz.

Carbon evaporated from the central electrode acts as a plasma-forming gas. The temperature of this carbon plasma jet was measured both by the relative intensity technique and by a pyrometer. It was found to vary from 5000 K close to the outer electrode, to 2000 K in the tail part.

Our latest measurements have shown that the fullerene mixture synthesized in our setup contains approximately 60% of C60, 25% C70 and 15% of higher fullerenes (Fig. 1).

The total yield of fullerene from our setup is within the same range as obtained with other generally used methods.

However, we are not aware of other reports of effective fullerene synthesis at atmospheric pressure, so in this rspect our experimental setup is unique.

We carried out investigations of a discharge in an argon stream between the water-cooled coil of copper tubing and Figure 1. Typical HPLC chromatogram obtained from cona water-cooled copper electrode containing an axial hole centrated extract of fullerene mixture, using toluene eluent and for introducing the argon. The frequency of the current Cosmosil Buckyprep column.

Electron density as the main parameter influencing the formation of fullerenes in a carbon plasma Figure 2. The principal scheme of the discharge in an argon flow at atmospheric pressure and a photo-registration of the irradiation intensity of the plasma discharge revealing forced running ionization waves. The current of the discharge arc is 7 A, the frequency is 44 kHz, and the linear rate of argon flow is 42 m/s. The hole of the central electrode is about 2.0 mm in diameter.

electron temperature. This effect of ionization instability calculating the total energy of different clusters, the energy is usually observed in experiments on the generation and of formation was estimated using the following relation:

study of ionization waves in rare gases, at pressures ranging Cn + Cm Cn+m, from fractions of 1 up to 200 Torr. In the well known and popular experimental setup of Krtschmer, fullerene E = Et(Cn+m) - Et(Cn) - Et(Cm), synthesis is usually carried out at pressures between 100 and 200 Torr. At these pressures the local electron concentration where E is the energy of reaction and Et(Ci) is the in a carbon-helium plasma can vary over a wide range calculated total energy of cluster Ci.

because of the presence of spontaneous ionization waves.

Our estimations showed that spheroidal cluster formation The above considerations suggest that electron concentraat 1000 K is more favorable than the formation of flat clusters tion pulsations are also present in our atmospheric pressure having the same number of atoms (Fig. 3, a,b and Table 1).

carbon-helium plasma arc. The commom feature for This result can be explained by the increased number of effective fullerene synthesis in the experimental setups at carbon atoms with non-saturated bonds in the flat clusters.

low and atmosperic pressures is the plasma instability related The energy of the reaction Cn + Cm Cn+m for the to electron concentration fluctuations. So, it is possible to formation of a cluster Cn+m depends on the charges of the deduce that electron concentration (and especially the varireacting clusters. Table 2 shows the calculated energies of ations in electron concentration) may be a major parameter formation of fullerene C60 from the clusters C20 and Cthat influences the production of carbon clusters in the form with different charges, as well as the energies of formation of fullerene molecules.

of the flat cluster C60 from the flat clusters C20 and C40.

Many publications have now appeared concerning the The most favorable are the neutralion and the anion local redistribution of electrons in plasmas caused by the cation reactions. The least favorable are reactions between injection of dust particles. As the electrons are condensed ions with the same charge. Reactions between neutral on the particles of dust [8], so they will also be condensed on clusters are intermediate in energy. Formation reactions for carbon clusters during their formation. Thus, in reviewing small-sized clusters follow the same pattern as described the formation of fullerene molecules from carbon clusters, it above for fullerene C60.

is necessary to take into account the charge of these clusters.

The most intersting results were obtained when analyzing the influence of charge on the geometry and stability of the clusters. In Table 2, missing data indicate that the final 1. Calculations cluster does not exist. The charge of a cluster influences We carried out computer simulations of fullerene C60 its geometry and stability significantly (Fig. 2, c). Although the spherically symmetric molecule fullerene C60 keeps its formation from carbon clusters having different charges. The structure regardless of its charge, the flat cluster C60 behaves simulations were carried out using the program HyperChem differently depending on charge. The neutral cluster C5 to calculate the optimal geometry of molecules and their molecular dynamics at different temperatures. All of the calculations were performed with the PM3 semi-empirical Table 1. Reaction energy for forming flat and spheroidal carbon quantum chemical method.

clusters Estimations of the formation energies of different carbon Number Spheroidal clusters, Flat clusters, clusters were made at a temperature of 1000 K because Reaction in Fig. 3 E, kJ/mol E, kJ/mol fullerene formation occurs at about 1000 to 2000 K. The influence of the charge of clusters on the process of their I C14 + C4 C18 -1484 -formation was investigated. We considered the formation II C18 + C2 C20 -1237 -of flat clusters consisting only of hexagons, as well as III C20 + C20 C40 -2337 -IV C20 + C40 C60 -3290 -non-planar ones, containing at least one pentagon. After , 2002, 44, . 408 G.N. Churilov, P.V. Novikov, V.A. Lopatin, N.G. Vnukova, N.V. Bulina, S.M. Bachilo Figure 3. Formation reactions of spheroidal (a) and flat (b) carbon clusters (the calculated reaction energies are presented in Table 1) and energy minimization of a large flat cluster C60 with different charges (c).

and singly-charged anion C- are folded into a portion of The problem of pentagon formation in flat clusters is a spherical surface and remain stable at 1000 K. The folding very important. Using the example of cluster C18 (Fig. 3, a) happens in the places where two hexagons are divided by with one incomplete pentagon, it is possible to observe an unfinished hexagon having four bonds. In this place the that the changes in its geometry depend on its charge. A fifth bond appears and the cluster becomes curved due to pentagon forms during the geometry optimization of the the appearance of pentagon. In our calculations, the flat singly-charged ions C- and C+ and the doubly-charged 18 singly-charged cation C+ and the doubly-charged ions C2- cation C2+. The ab length (Fig. 3, a) decreases to a 60 and C2+ dissociate in the process of geometry optimization. typical CC bond length of about 1.4. And with geometry Table 2. Dependence of the reaction energy on the charges of reacting clusters Reaction Fullerene C60, Flat cluster C60, Type of reaction n = 40, m = 20 E, kJ/mol E, kJ/mol Neutralion Cn + C+ C+ -m n+m Cn + C- C- -3423 -m n+m Anioncation C- + C+ Cn+m -3302 -n m Neutralneutral Cn + Cm Cn+m -3290 -Anionanion C- + C- Cn+m -2n m Cationcation C+ + C+ C2+ -n m n+m cluster is unstable.

, 2002, 44, . Electron density as the main parameter influencing the formation of fullerenes in a carbon plasma optimization of the neutral cluster and doubly-charged anion [5] G.N. Churilov, L.A. Solovyov, Y.N. Churilova, O.V. Chupina, S.S. Malcieva. Carbon 37, 3, 427 (1999).

C2-, the ab length between outer hexagons increases. This [6] G.N. Churilov, V.A. Lopatin, P.V. Novikov, N.G. Vnukova. The example cleary suggests that a lower electron concentration proceedings of the 1st International Congress on Radiation in the plasma is necessary for the formation of spheroidal Physics and Chemistry of Condensed Matter, High Current clusters containing pentagons.

Electronics, and Modification of Materials with Particle Beams and Plasma Flows. Tomsk, 2, 223 (2000).

2. Conclusions [7] A.Yu. Gavrilova, A.G. Kiselyov, E.P. Skorokhod, M.E. Stanishevskaya. Matematicheskoe Modelirovanie (Math. Modelling) In the phase of an ionization wave with a low electron 8, 6, 103 (1996).

[8] V.I. Molotkov, M.Yu. Nefedov, M.Yu. Pustylnik, V.M. Torchinconcentration, the formation of clusters containing pentagons sky, V.E. Fortov, A.G. Khrapak, K. Yoshino. JETP Letters 71, is favored. Further decrease of the electron concentration 3, 102 (2000).

to a minimum reduces the effeciency of cluster formation because of higher energy of cationcation reactions.

As the electron concentration increases in the opposite phase of the ionization wave, the large flat clusters acquire negative charge and dissociate into smaller clusters or separate atoms. Because the time of elementary reactions is about 10-12 s, while the period of the electron concentration wave in the plasma is 10-310-5 s, the cluster distributions can stay near equilibrium as the electron concentration varies. Therefore small-sized clusters, including the spheroidal ones, have time to be generated from separate atoms. With the increase of the electron concentration, the efficiency of their formation decreases due to the higher energy of anionanion reactions. Because the electron concentration does not have such a strong effect on the stability of small spheroidal clusters and fullerene shells, the clusters and fullerene molecules already generated are not destroyed.

The large flat clusters tend to dissociate into smaller sized clusters during oscillations of the electron concentration. As the energies of formation of small-sized clusters with and without pentagons are similar, there are always a number of clusters in the plasma suitable for forming fullerenes. These clusters remain stable once they have been formed.

Thus, the ionization wave executes two functions during the synthesis of fullerenes. At low electron concentrations, it favors the formation of clucters, especially spheroidal ones, whereas at high electron concentrations it tends to preferentially destroy the flat clusters.

We note that the proposed mechanism does not consider statistical processes, but only the driving role of electron concentration variations. Recognition of the importance of electron concentration variations may provide an essential step on the way to the controlled synthesis of fullerenes and, possibly, fullerene derivatives.

References [1] W. Krtschmer, K. Fostiropoulos, D.R. Huffman. Chem. Phys.

Lett. 170, 167 (1990).

[2] G.N. Churilov. Instruments and Experimental Techniques 43, 1, 1 (2000).

[3] P.S. Landa, N.A. Miskinova, Yu.V. Ponomarev. Soviet PhysicsUspekhi 132, 4, 601 (1980).

[4] G.N. Churilov. Int. Winterschool on Electronic Properties of Novel Materials Progress in Fullerene Research (Kirchberg, Tyrol, Austria) World Scientific (1994). P. 135.

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