THE FORMATION OF SELF-ASSEMBLED STRUCTURES OF C₆₀ IN SOLUTION AND IN THE VOLUME OF AN EVAPORATING DROP OF A COLLOIDAL SOLUTION

1. Introduction

The fullerene C₆₀ molecule (icosahedral I_h symmetry), consisting of 60 sp²-bonded carbon atoms, is a  completely organic macromolecule of spherical shape with a diameter of d₀ ≈ 0.714 nm. High polarizability, strong electron-acceptor activity and hydrophobicity are main unique properties of fullerene C₆₀. Unlike other well-known allotropic forms of carbon, fullerene C₆₀ is well soluble in the vast majority of low-polarity organic solvents (for example, benzene, toluene, xylene, carbon disulfide, tetralin, and others), but practically insoluble in polar solvents such as alcohols [1, 2].

In a  number of pure and mixed organic solvents, fullerene С_n (n = 60, 70, 76, …) molecules show a pronounced tendency to self-assembly and the formation of fairly large functional fullerene aggregates of various shapes and sizes. The problem was fundamentally studied by different physical and chemical methods in [3–11] and the obtained results provided a  vector toward potential applications in material chemistry  [12, 13], biomedicine  [14–16], phototherapy  [17–19], molecular electronics [20], optoelectronics [21, 22] and solar energy [23–25].

In recent years, the  interest of researchers to the processes occurring during the drying of liquid droplets (in particular, nanoparticle solutions) on a flat substrate has increased significantly [26–30]. The  process of drying up a  drop of solutions attracts the attention of physicists and technologists as a natural model of a self-organizing system in which the variation in the type of a solute or solvent, in substrate type and its initial thermodynamic parameters leads to interesting physical phenomena taking place  [31–34]. The interest is determined also by the  need for improving the  technologies associated with these processes, for example, low-cost synthesis of organic solar cells [35, 36], as an auxiliary criterion in medical diagnostics [37–39], as a new direct-writing printing technique by applying paint coatings on various surfaces [40, 41], as a technique to fabricate ordered arrays of structures in nanosphere lithography [42, 43].

It should be noted that the physical foundations and mechanisms of self-aggregation processes of nanoparticles in solutions and in drying drops of nanoparticle solutions (in particular, solutions of fullerene C₆₀ in two-component organic solvents) on a  flat substrate are still not fully understood, which complicates the transition to effective management of nanomaterial production processes.

The purpose of this work is an experimental study of the self-aggregation of C₆₀ fullerene molecules both inside the solution and in the volume of an evaporating drop of C₆₀ colloidal solution sitting on the flat surface of a substrate.

2. Samples and techniques

To prepare initial molecular solutions of C₆₀, we used dry crystalline powders of fullerene C₆₀ of high purification (>99.8% of the  base material, manufacturer SES Research, USA) as well as organic solvents  –  xylene (C₈H₁₀) and tetrahydrofuran (С₄Н₈О) with 99.9% purity (Sigma-Aldrich, USA). The solvents were used as received. The maximum fullerene C₆₀ solubility at room temperature in pure xylene is about 7.2 mol/m³ and in pure tetrahydrofuran (THF) it is about 0.083  mol/m³. A  special standard cover glass (ISOLAB Laborgerate GmbH, Germany) was used as a substrate.

The initial fullerene working solutions were prepared in a dark room by the nonequilibrium method described in paper [44].

The structure of synthesized nanostructures of fullerene C₆₀ was characterized by a transmission electron microscope (TEM) LEO-912 AB (ZEISS, Germany) and a field emission scanning electron microscope (SEM) Hitachi S-4800 (Japan). In the experiments to study the evolution of distribution of fullerene C₆₀ molecules and the formation of ring structures of mC₆₀ nanoaggregates (where m is the number of C₆₀ molecules in a synthesized nanoaggregate) on the surface of a glass substrate, we used an optical binocular microscope of the brand Motic B1-220A (Germany) with a  digital camera for continuous recording of images.

The size distribution of C₆₀ fullerene nanoaggregates in the solutions was studied by dynamic light scattering (DLS). The DLS measurements were performed on a Zetasizer Nano ZEN3600 (Malvern Instruments Ltd.) equipped with a He-Ne laser (4 mW at 632.8 nm) at room temperature (T ≈ 24±1°C).

The electronic absorption spectra of C₆₀ solutions in two-component organic solvents (which were used in ‘drop drying’ experiments) were recorded on a Shimadzu UV-2700 UV–Vis recording spectrometer (Shimadzu, Japan) with a high spectral resolution (~0.1 nm) using a 1 cm thick quartz cuvette.

Before each series of experiments, the  surface of the used glass substrate was thoroughly plasma cleaned using a Plasma Cleaner device of the PDC-002 brand (Harrick Plasma Inc, USA). Droplets of the  colloidal solution of C₆₀ were placed using a  VITLAB piston-operated micropipette (VITLAB GmbH, Germany) on the previously cleaned surface of a strictly horizontal mounted flat glass substrate. The  shape of the  initial droplets of C₆₀ solutions on the flat substrate is approximately described by a spherical cap. Complete evaporation of the solvent from the droplet takes place in a laboratory box at a temperature of ~24±1°C and a relative humidity of ~40–45%. Drops of C₆₀ during the evaporation were protected against convective air flows.

3. Results and discussion

Figure 1 shows the DLS experiment of the distribution of light scattering particles according to their hydrodynamic diameters at two different concentrations of C₆₀ in a fresh solution prepared by the non-equilibrium method in a mixture of two organic solvents  –  xylene and THF at a  volume fraction of 0.95:0.05, respectively. At a C₆₀ concentration of ~0.312 mol/m³, the main fraction of light-scattering fullerene nanoparticles in the  solution is distributed in the diameter range ~3.5–63.2 nm and the maximum distribution of C₆₀ fullerene aggregates is localized in the  region of ~12.86  nm (a solid line). The  mean hydrodynamic diameter of light scattering fullerene particles at а relatively high concentration of C₆₀ (~0.468  mol/m³) shifts to ~41.9  nm and the  hydrodynamic size range of mC₆₀ nanoaggregates corresponds to ~11.7–135.0 nm (a dashed line). The obtained DLS results show that the used C₆₀ solutions belong to the dispersed colloidal system and the synthesis of mC₆₀ nanoaggregates in xylene/THF mixtures occurs almost immediately after the  preparation of the solution. During the self-assembly of fullerene molecules in a nonequilibrium solution, C₆₀ molecules form a nanostructure, finding the most advantageous combination of interactions between molecules with minimal free energy  [45, 46]. In this process, a higher initial solute concentration in the solution leads to a greater number of iterations (repetitions) of the self-assembly of C₆₀ molecules. The latter plays an important role in the synthesis of large mC₆₀ nanoaggregates in the solution (Fig. 1, a dashed line).

Figure 2 represents a TEM image of the nano-sized mC₆₀ aggregates synthesized in the  freshly prepared solution of C₆₀ in a xylene/THF mixture with a volume fraction of 0.95:0.05, respectively, at a solute concentration of ~0.468 mol/m³. The TEM image of the synthesized mC₆₀ nanoaggregates inside the C₆₀ solution is obtained by rapid freezing (~1270 K/s) of a thin layer of the drop of the working solution of C₆₀ fullerene with liquid nitrogen vapours using a special automated device Vitrification Robot FP 5350/62 (ZEISS, Germany). It can be seen that there are also small nanoaggregates with diameters up to ~30 nm, and large mC₆₀ nanoaggregates with diameters up to ~135 nm with porous structures containing discrete intermediate small nanoaggregates.

For the mC₆₀ nanoaggregates with a C₆₀ concentration of ~0.468  mol/m³ the  agreement between the TEM result and the DLS data is excellent. Note that earlier we studied  [44] the  self-aggregation of fullerene C₆₀ molecules in a  toluene/THF mixture with a volume fraction of 0.9:0.1, respectively. In [44], it was found that it was possible to synthesize larger porous spherical mC₆₀ aggregates with a diameter of ~700 nm in a nonequilibrium solution.

Figure 3 shows the UV–Vis absorption spectra of freshly prepared working solutions of fullerene C₆₀ at lower onset concentrations of C₆₀. With an increase in the  concentration of fullerene, the  intense absorption band for molecular C₆₀ with a  maximum at λ₁  ≈  336.1  nm, corresponding to the symmetry-allowed 1¹A_g → 3¹T_1u transition, expands and does exhibit a  small positive solvatochromism effect (~2  nm). This is due to the  processes of intermolecular dipole–dipole π~π* stacking interactions ‘C₆₀–solvents’ that control the formation and further growth of the mC₆₀ nanoaggregates [47]. It should be noted that in the experiments, an increasing concentration of C₆₀ monomers in the solution leads to overcome interactions between ‘C₆₀–solvents’ molecules and to increase the van der Waals interaction between ‘C₆₀–C₆₀’. The narrow absorption band of C₆₀ with a  maximum at λ₂  ≈  407.5  nm (corresponds to the symmetry-allowed 1¹A_g → 1¹T_1u transition) and the broad optical absorption bands with maxima at λ₃ ≈ 540.6 nm (S₁ → S₃ transition), λ₄ ≈ 598.4 nm (S₀ → S₁) and λ₅ ≈ 624.8 nm (h_u → t_1u + T_u) are also observed in the  spectrum. With increasing the used concentration of C₆₀ in the  fresh solution, the amplitudes of these characteristic optical absorption bands in the spectrum increase unevenly. The latter are attributed to the formation of mC₆₀ nanoaggregates in the solution due to the charge transfer between C₆₀ and C₆₀ resulting from electronic transitions HOMO–LUMO. Changes in the  optical absorption spectra of C₆₀ solutions indicate that the formation of mC₆₀ aggregates in the  solution begins directly in the  process of its preparation.

Figure 4 shows a HRTEM image of the porous mC₆₀ nanoaggregate that indeed suggests a fractal character. The self-aggregation of C₆₀ molecules and the formation of nanostructured porous fractal aggregates in the  initial solution of fullerene (with a solvent concentration of ~0.468 mol/m³), in our opinion, will occur according to the  following mechanism. It is known [45, 46, 48] that the  minimum free energy of the  C₆₀/‘low polar solvent’ system can only be achieved by forming the initial stable aggregate with a diameter of ~4.6 nm containing 55 molecules of C₆₀.

In our experiments, firstly, under the additional influence on C₆₀ molecules of the rotational diffusion of molecules in the initial non-equilibrium solution and their chaotic diffusion motion, primary spherical mC₆₀ clusters containing m = 55 fullerene molecules are synthesized. Then this procedure of self-aggregation in the  solution is repeated many times in accordance with the ‘cluster–cluster’ aggregation model, which over time increases the characteristic size of the clusters and reduces their number. This is possible to ensure the  predictability of the  dimensional, structural and weight characteristics of mC₆₀ nanoaggregates synthesized in experiments.

Under similarity conditions of the aggregation of C₆₀ particles, the fractal dimension of mC₆₀ nanoaggregate may be represented by the formula [46]

where m is the number of C₆₀ molecules in the synthesized nanoaggregate, d is the diameter of the nanoaggregate, and d₀ is the diameter of an individual C₆₀ macromolecule.

Using Eq. (1) and values of m = 55, d = 4.6 nm and d₀ = 0.714 nm, we found that the fractal dimension of the synthesized mC₆₀ aggregate in the solution is D ≈ 2.148. Our calculations allowed us to establish that one synthesized fractal nanoaggregate mC₆₀, in the centre of which one of the C₆₀ molecules is located, and having a diameter of ~100 nm can contain up to m ≈ 40762 individual C₆₀ molecules (see Fig. 4).

Figure  5 shows a  schematic representation of the circulation flows (a) leading to the self-assembly of fullerene C₆₀ particles in a variable volume of a  drop and a  photograph of an isolated drop of a fullerene C₆₀ solution lying on the surface of a glass substrate (b).

In the process of conducting experimental studies of the features of evaporation of droplets set on the surface of a horizontally installed flat glass substrate, and containing both pure organic solvent and solutions of fullerene C₆₀ in an organic solvent, we found the following basic patterns of the behaviour of a drop:

(i) Droplets taken from pure organic solvents (xylene or THF), throughout thermal evaporation, always keep the contact angle φ (see Fig. 5(a)) and their evaporation rate is not identical; however, there is a gradual narrowing of the base area of the ‘drop–glass substrate’ contact until the drop completely disappears;

(ii) If a  drop of a  mixture of organic solvents (xylene/THF) contains colloidal particles of a solute, for example, fullerene C₆₀ (see Fig. 5(b)), then a  fundamentally different picture is realized − as the thermal evaporation of solvents from the drop occurs, the base of the drop remains constant and the ‘pinning’ mode of the contact line is realized. The edge angle of the droplet gradually decreases up to φ ≈ 0°. In this case, due to the influence of Marangoni effects  [49] in near-surface layers of the evaporating drop of the C₆₀ solution, strong capillary flows arise (the so-called Marangoni flows). During evaporation, binary organic solvents with different volatility and surface tension from a sessile droplet cause radial convection inside the droplet. The latter directly initiates the mutual approach of C₆₀ colloidal particles and the synthesis of large mC₆₀ aggregates.

Figure 6 shows the evolution of ring formation during the thermal evaporation of a sessile C₆₀ drop on the surface of a glass substrate. In the figures, the evaporation process continues in the sequence a–b–c–d. The  time of complete evaporation of xylene and THF from the  volume of microdrop (~40 ml) at room temperature was ~2 h. Obviously, after the  completion of natural thermal evaporation of the organic solvents from the droplet of colloidal C₆₀ solution (initial fullerene concentration of ~0.468  mol/m³), non-concentric ring-shaped structures were found on the surface of the glass substrate (see Fig. 6), the occurrence of which can be explained only by self-assembly of colloidal fullerene particles in the process of evaporation of a solution drop and the formation of large mC₆₀ aggregates in different sizes.

One of the  main driving forces that initiate the self-aggregation of particles C₆₀ is the evaporation of solvents (xylene/THF) and the  associated change in the volume of the droplet. In this case, strong capillary forces in the  border (in the  near surface layers) of the droplet and the radial convection inside the droplet lead to motion and shifting of particles of fullerene C₆₀ from the  bulk of the droplet to the periphery. The key result is that part of the aggregated particles of C₆₀ is deposited and ultimately leads to the formation of a ring stain (see Fig. 6). Self-aggregation is a collective process in which the whole ensemble of fullerene C₆₀ nano-particles participates. It is not difficult to see that the C₆₀ nanoparticles, which did not participate in the synthesis of initial mC₆₀ nanoaggregates, as the droplets evaporated, moved further along the surface of the glass substrate together with the organic solvents.

Next, we investigated the location and distribution of the synthesized mC₆₀ aggregates inside and outside the ring by a scanning electron microscope. Figure 7 shows a two-dimensional SEM image of small arbitrarily selected areas inside the rings, shown in Fig. 6. It can be seen that after the complete evaporation of xylene and THF from a micro-droplet of the colloidal C₆₀ solution on the surface of the optical glass substrate large mC₆₀ aggregates of a quasispherical shape were aggregated. The beginning of each ring contains relatively large mC₆₀ aggregates. The space between the two rings is covered with layers of small mC₆₀ aggregates.

Figure 8 presents a SEM image of mC₆₀ aggregates synthesized during the thermal evaporation of organic solvents from the volume of microdrops of a C₆₀ colloidal solution on the surface of a substrate. It can be seen that the average geometric dimensions in the diameter of mС₆₀ aggregates vary in a range of ~380÷800 nm. The formed mC₆₀ aggregates consist of discrete intermediate nanoaggregates with sizes in diameter up to ~135 nm (see Fig. 8(a)). This size corresponds with the diameter of the aggregates that were synthesized inside the colloidal solutions of fullerene C₆₀. Other than that, most of the mC₆₀ structures formed have a pipe-like structure consisting of several layers (see Fig. 8(b)).

In our opinion, a droplet of the colloidal solution of C₆₀ in an organic solvent, as well as a drop of the solution of any other nanoparticles, always seek to minimize their total surface energy. The latter can be achieved, in particular, as a result of self-aggregation of solute particles. Suppose that two intermediate fullerene clusters with diameters d₁ and d₂ (where d₁ << d₂) are localized in the volume of an evaporating drop of the C₆₀ solution. Then, each of these nanoscale particles under consideration will tend to establish a thermodynamic equilibrium with the solution surrounding it. So, smaller particles with a diameter d₁ in a droplet will be deposited on the surface of clusters with a diameter d₂ that are larger in size to maintain the equilibrium in the system. As a result of this self-assembly of fullerene particles in the volume of evaporating droplets, large mC₆₀ nanoaggregates grow in size even more (up to ~800 nm in diameter).

Thus, our experimental results on the study of the evaporation of individual drops of a fullerene C₆₀ colloidal solution on the surface of a substrate will be very useful in solving the problems of evaporating limited volumes of nanoparticle liquids in various technological devices, for the further development of technologies for applying thin semi-conductor coatings, etc.

4. Conclusions

We have investigated the self-aggregation of C₆₀ fullerene molecules both inside the freshly prepared solution and in the volume of an evaporating drop of the C₆₀ colloidal solution. In the non-equilibrium solutions of fullerene C₆₀ in the xylene/THF mixture (prepared by continuous stirring of the solution on a magnetic stirrer) at room temperature, large quasispherical mC₆₀ nanoaggregates with a diameter of up to ~135 nm having a porous structure with fractal dimension D ≈ 2.148 were synthesized. The finite geometrical sizes of the mC₆₀ nanoaggregates are determined by the initial concentration of C₆₀ in the used solvent medium. The results obtained on the self-aggregation of C₆₀ molecules in the xylene/THF solution were confirmed by DLS, TEM and optical absorption studies.

After the completion of natural thermal evaporation of the organic solvent (xylene/THF) from a droplet of the colloidal C₆₀ solution, non-concentric ring-shaped structures were found on the standard substrate surface, the occurrence of which may be explained by the self-assembly of colloidal particles of fullerene. As a result, nanostructured and porous mC₆₀ aggregates of large geometrical sizes (up to ~800 nm in diameter) were synthesized on the substrate surface. In turn, finite mC₆₀ nanoaggregates consist of smaller intermediate discrete C₆₀ aggregates with geometrical sizes in diameter up to ~135 nm. Prolonged (within three months) microscopic observations of the state of the synthesized mC₆₀ aggregates on a glass surface allowed us to conclude that they have a high structural stability.

The experimentally obtained results on the formation of porous nanostructured mC₆₀ aggregates in a freshly prepared solution and from a drying microdroplet of the colloidal solution of C₆₀ apparently open up some new possibilities of producing new nanosized functional materials/thin films for micro- and optoelectronics, solar batteries, power electronics, biochips and other areas of semiconductor technology.

Aknowledgements

This research was supported by the Foundation for Basic Research of the Academy of Sciences of Uzbekistan: ‘Fundamental Foundations for the Synthesis of New Functional Nanomaterials for Opto-electronics and Solar Energy Based on Fullerenes and Their Derivatives’ (Project No. OT-F2-51). We are very grateful to the Government of Latvia for providing a research scholarship and assistance in completing a scientific internship at the Institute of Chemical Physics of the University of Latvia.

References

SAVITVARKIŲ C₆₀ DARINIŲ FORMAVIMASIS TIRPALE IR KOLOIDINIO TIRPALO GARUOJANČIO LAŠO TŪRYJE

U.K. Makhmanov ^a, A.M. Kokhkharov ^a, S.A. Bakhramov ^a, D. Erts ^b

^a Uzbekistano mokslų akademijos Jonų plazmos ir lazerinių technologijų institutas, Taškentas, Uzbekistanas

^b  Latvijos universiteto Cheminės fizikos institutas, Ryga, Latvija