V.B. Aleskovskii. St. Petersburg University
V.I. Gubaidullin. St. Petersburg Scientific-Production and Commercial Firm "Svet"
St. Petersburg Street Himicov 28
St. Petersburg 195030, Russia
fax: (812) 428-6939
Summarizing the 1994 discussion at the NATO Advanced Research Workshop on the Ultimate Limits of Fabrication and Measurement, the writer of an article in Nature noted, "There is a sharp contrast between the current rapid progress in the nanometer scale and the longer team aim of converting newly discovered quantum devices and nanometer materials into useful engineering nanotechnology...A search for new functionalities and new types of processes, and not just improvements in known devices, might provide the innovative thinking required. There is an increasing, worldwide effort [in this direction]" (Brus 1994).
From the theory of chemical-information synthesis, it follows that such processes produce not only a product but also a sufficient amount of information to recreate the product's structural organization (Aleskovskii 1996, Lehn 1989, Aleskovskii 1997b). The natural prototype of these processes is biosynthesis-a programmable, irreversible, multi-stage process proceeding by replication-a matrix buildup of the high-molecular complementary structural units (CSU) out of low-molecular structural units (SU)-and a self-buildup. In addition, it is mainly at the CSU-self-buildup (SB) stage that the amount of information necessary and sufficient for the structural organization of a biosynthesis product is produced. Naturally, the synthesis of such highly organized structures as nanostructures is connected with the production of a very large amount of information. So the synthesis of such structures is possible only in processes similar to biosynthesis-namely, in chemical-informational synthesis processes being created by chemically designing the biosynthesis (Aleskovskii 1992, Aleskovskii 1994, Aleskovskii 1997a, Aleskovskii 1997b).
The idea of chemically designed biosynthesis was realized in the 1960s while scientists were creating the process of chemical buildup (CB) modeling the replication of structural units, not on a one-dimensional matrix, like biosynthesis, but on a two-dimensional matrix or substrate (Aleskovskii 1974, Aleskovskii 1975, Aleskovskii 1990b). The process is organized as follows: Irreversible chemisorption is carried out in turnsfirst, of methane, then of Teterachlormethaneon the prechlorinated diamond surface [C]a Cl at 300-600°C and atmospheric pressure as in formula 1:
[C]aCl + CH4 > [C]aCH3 + HCl
[C]aCH3 + C Cl4> [C]a CCCl + 3HCl
By repeating the cycle A-B n-fold, a diamond film can be created, consisting of 2n carbon monolayers. Using this CB procedure, the synthesis of superlattices is carried out, alternating cycles of corresponding complementary reactions under a certain program.
Recently this group succeeded in creating a CSU-SB chemical model, when it appeared that nanoglobules, produced by different methods, might be used as CSU in this process. Iler's investigation demonstrated that sol-nanoparticles, polysiliconic acid (silicasol) for example, could be used in a process like the CB-process (Iler 1979). Hence it follows that sol-particles will come into the SB process under certain conditions. At the same time the group considered structurizing a nanoglobule by depositing a system of concentric shells of different compositions on it. This information allowed us to synthesize quantum devices and quantum substances, i.e., chemical compounds of artificial elements, where superatoms represent atom-like quantum nanostructures (Watanabe 1986).
The scheme of quantum devices (Fig. 4.1), "being a zero-dimensional (Z-D) structure system, for example, quantum dots of one or different kinds," was thus developed (Aleskovskii 1990a).
Fig. 4.1. Scheme of structured quantum dots.
The idea that structurized nanoglobules could be produced by the CB-method was proved when this method was used to encapsulate Aerosil A-380 nanoglobules (Æ~7nm) in a CdS-shell with a thickness of from one to several monolayers, the last thickness being 1 nm (Romanichev 1996).
The CdS-shells' synthesis is carried out in a vacuum installation, in turn providing the reagents' supply to the reactor and evacuating the gaseous products. With the aid of a quartz vibrator (f 0 = 10 MHz) placed in the reactor, the group determined the regimes of the reagents' supply and the evacuation that would provide the necessary conditions for a course of CB-reactions. The linear relationship between the change in oscillation frequency of the quartz vibrator and the number of CB-reaction cycles confirms the monolayer character of the CdS-film's growth.
Synthesized samples, consisting of SiO 2 nanoglobules encapsulated in CdS shells of different thickness, were dispersed in glycerol and were investigated on the "Specord" spectrophotometer 40M over the range of 200 to 900 nm. When the CdS shells' thickness of 12 is decreased to 1 monolayer, a shifting of the border of the fundamental absorption band occurred, moving to the region of higher energies with a value shifting to 0.4 eV, thus revealing the quantum-dimensional effect. The group thus created structurized nanoglobulesquantum nanostructures [CdS]n /SiO 2 , where n = 1 to 12.
In order to approach biosynthesis conditions, the development of a group fluid-phase chemical-informational synthesis (FCIS), including a modified CB-process, was begun. The group created the principal FCIS' scheme (see Fig. 4.2) and determined the next step for this synthesis.
Fig. 4.2. Scheme of the liquid phase synthesis of quantum structures.
Nanoparticles (Æ 1-3 nm) of sol, produced by one of the known proceduresthe fluid-phase CBmethod-are also encapsulated in sol by a system of concentric shells of different composition and thickness, the shells being alternated in a predetermined order.
Also in that sol or in the mixture of different sols, taken in definite proportion, the group ensures the conditions necessary for realizing the process of sol particle CB, resulting in a thoroughly programmed regular structure, orin another variantfor the CB of these globules on the substrate, prepared correspondingly. Additional operations (in particular, thermal treatment) are carried out.
The creation of a programmed structure can be illustrated by a scheme for producing quantum nanostructures of a quantum dot type on a base of a CdS-sol, (ZnSn /CdS, for example. For this purpose, either a Zn-salt solution or a H 2 S solution is introduced n-times in turn into the CdS-sol, with the excess reagents and byproducts being evacuated out of the electrodialyser. Thus the reaction proceeds cycle by cycle (Fig. 4.3):
CdS + Zn2+> CdSZn2+
CdSZn2++ S2-> CdSZnS
Perhaps the group could produce more complicated nanostructures, for example (ZnS)n (CdS)m /Ag (or Cu) on the base of an Ag (or Cu)-sol, by carrying out m cycles of CdS-nanolayerCB first, but after that n cycles of ZnS-CB.
Fig. 4.3. Synthesis of structured nanoglobules (SNG).
The structurized nanoglobules' self-buildup is carried out by complementing the nanoglobules one to the other beforehand, for example, as in the scheme in Fig. 4.4.
It is possible to produce an Unusual Supergreat Integral Scheme (USIS) model, a buildup of original elements of the QD-type by depositing the mixture SNG-1, SNG-2, SNG-3, taken in the necessary proportions, on the armed substrate.
Fig. 4.4. Complementary structural units of SNG.
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