Figure 1. Laser synthesis of carbon nanostructures in a uniform electric field:
1 — the direction of the laser radiation, 2 — metal plates under a dc voltage, 3 — coordinate table with a sample of graphite on it
Use as a controlling factor of the external electric and magnetic fields can control the structure of the synthesized nanoparticles. Details of research on this issue will be published in the May issue of the journal "Russian Nanotechnology". The work was conducted under the Federal Program "Research and development on priority directions of scientific-technological complex of Russia for 2007-2012".
There are many methods for the preparation of nanoparticles. However, these methods have one serious drawback: it is often a result of heterogeneous synthesis obtained by its structure and constitution of the particle. At the same time, the production of knowledge-intensive industries, such as electronics, a large number of absolutely identical and standardized nanoparticles. Therefore, researchers have to learn to effectively select the desired nanoparticles from the crowd and invent new methods of synthesis, allowing to increase their percentage.
In order to yield the required percentage of particles was high, we must be able to manage the process of synthesis. Scientists of Vladimir State University and the NT-MDT Company found a way to control the structure of the nanoparticles obtained by evaporation of the target under the laser. To do this, the scientists placed the vaporized sample of pure graphite in a constant electric and magnetic field inhomogeneities.
In the first case (Fig. 1) the synthesis proceeded as follows. Under the influence of the laser is evaporated and ionized atoms of the graphite target. Due to the action of an external electric field ionized atoms deflected from the main flow toward the negatively charged plate.
Study using an electron microscope the layer deposited on the wafer enabled scientists confirmed that the carbon nanofiber is formed. By reducing the voltage across the plates managed to control the deposition process. The result is a change in the structure of deposited particles — obtained nanoclusters having the structure of fractals, which have been characterized using fractal geometry.
Fig. 2. Laser synthesis of carbon nanostructures in an inhomogeneous magnetic field:
1 — the direction of the laser radiation, 2 — cylindrical magnets, 3 — cold substrate (glass) 4 — laying, 5 — graphite target
In the second case, the synthesis of a non-uniform magnetic field (Fig. 2), the same pattern of pure graphite was placed in a uniform field between the two magnets. Only in this case vaporized under the laser target atoms deposited on the substrate disposed above it. To create a gap for distribution to the plasma produced by the laser beam on the target, between the substrate and the target was placed ceramic washer, gasket.
The study of the deposited layer has allowed scientists to argue that the formation of nanostructured carbon layer was similar to the growth of dendrites. In the first phase were formed on the substrate "germ", small aggregates of atoms, from which, like a tree branch and a dendrite structure was increasing.
By changing the magnetic field, according to the researchers, it is possible to change the mode of deposition, which leads to the possibility of building the necessary "germs." Structure "nuclei", in turn, is responsible for the further growth of dendrites.
Analyzing the results, the researchers concluded that the electric field is necessarily directs the flow of particles propagating along their lines. As a result of this orientation, it is possible the formation of chemical bonds between the atoms and the formation of nanofibers is directed. Similar arguments hold for the propagation of particles in a magnetic field, however due to the fact that the field is essentially uniform, observed the formation of dendritic structures.
According to scientists, the described techniques for fabricating nanostructures appear quite promising. With the change in the electric or magnetic field can be specifically regulate the formation of nanostructures.