Bottom-Up Nanotechnology

Figure 1.5 displays the two general methods available for producing nanosize materials. The first (the top-down approach) starts with a bulk material and then break it into smaller pieces using mechanical, chemical, or other forms of energy. The opposite, bottom-up approach is to synthesise the material from atomic or molecular species through chemical reactions that allow the precursor particles to grow in size.

Both approaches can be performed in the gas or liquid phases, supercritical fluids, the solid state, or under vacuum.

Bulk nano particle, rod, wire, tube…etc.

Figure 1.5 Two basic approaches toward the fabrication of nanomaterials: top-down (from left to right) and bottom-up (from right to left).

Section 1.2 introduced some of the fabrication methods that may be used to shrink the sizes of electronic devices. In contrast, we can also fabricate nano-electronic devices by using the bottom-up method. The term “bottom-up” means that the nano-components (particles, nanotubes, and nanowires) are synthesized from single molecules, whereas the top-down process fabricates these nano-components from bulk materials.

Figure 1.6 System model for nanocomposites produced by the sol–gel process (source: Fraunhofer IST).

Methods for producing nanoscale materials from atoms are chemical processes based on transformations that occur in solution; e.g., sol–gel processing, chemical vapor deposition (CVD), plasma or flame spraying synthesis, laser pyrolysis, and atomic or molecular condensation. These chemical processes rely on the availability of appropriate “metal–organic” molecules as precursors. Sol–gel processing is different from other chemical processes because it requires a relatively low processing

temperature, which makes it a cost-effective and versatile technique (Figure 1.6). In spraying processes, the flow of reactants (gas or liquid in the form of aerosols, or mixtures of both) is introduced to a high-energy flame produced, for example, by a plasma spraying apparatus or a carbon dioxide laser. The reactants decompose and particles are formed in a flame by homogeneous nucleation and growth. Rapid cooling results in formation of nanoscale particles.

Chemical processes—that are based on transformations in solution—toward materials include sol–gel processing, hydro- or solvo-thermal syntheses, metal organic decomposition (MOD), and vapor phase chemical vapor deposition (CVD).

Most chemical routes rely on the availability of appropriate “metal–organic”

molecules as precursors. Of the various precursors of metal oxides, the metal β-diketonates, metal carboxylates, and metal alkoxides are the most versatile. They are available for nearly all elements; cost-effective syntheses from cheap feedstocks have been developed for some of these materials.

Two general methods are available for controlling the formation and growth of these nanoscale materials. One, “arrested precipitation,” depends on either the exhaustion of one of the reactants or the introduction of a chemical that would block the reaction. The other method uses templates to physically restrict the volume available for growth of the individual nanoscale materials.

Figure 1.7 displays the principle of atomic or molecular condensation, which is used primarily for metal-containing nanoparticles. A bulk material is heated under vacuum to produce a stream of vaporized and atomized matter that is directed to a chamber containing either an inert or reactive gas atmosphere. Rapid cooling of the metal atoms—through collisions with gas molecules—results in the condensation and formation of nanoparticles. Metal oxide nanoparticles are produced if a reactive gas, such as oxygen, is used.

The theory of gas phase condensation for the production of metal nanopowders is well known; it was first reported in 1930.^[20] Gas phase condensation uses a vacuum chamber comprising a heating element, the metal to be transformed into the nano-powder, powder collection equipment, and vacuum hardware.

Figure 1.7 The inert gas condensation method for producing nanoparticulate material (source: FHG-IFAM, Bremen).

The process utilizes a gas, which is typically inert, at pressures high enough to promote particle formation, but also low enough to allow the production of spherical particles. The metal is introduced onto a heated element and is rapidly melted. The metal is quickly heated to a temperature far above its melting point, but less than its boiling point, so that an adequate vapor pressure is achieved. Gas is introduced into the chamber continuously and removed by the pumps so that the gas transfers the evaporated metal away from the hot element. Nanometer-sized particles form as the gas cools the metal vapor. These particles are in liquid form because they are still too hot to solidify. The liquid particles collide and coalesce in a controlled environment so that the particles grow to specific size and remain spherical and have smooth surfaces. When the liquid particles are further cooled under controlled conditions, they become solid and no longer grow. At this point the nanoparticles are very reactive; they must be coated with a material that prevents further interaction with

other particles (agglomeration) or with other materials.

Nanoparticles of a wide range of materials—including a variety of organic and biological compounds, inorganic oxides, metals, and semiconductors—can be processed using chemical self-assembly techniques (Meier et al. 2000, Zhang et al.

2002, Shimizu et al. 2003, Shimomura et al. 2000, Tomalia et al. 1999, Fendler et al.

2001). These techniques exploit the selective attachment of molecules to specific surfaces, biomolecular recognition, and self-ordering principles (e.g., the preferential docking of DNA strands with their complementary base pairs) in addition to the well-developed chemistry for attaching molecules onto clusters and substrates [e.g., thiol (–SH) end groups]; other techniques include reverse micelle, sonochemical, and photochemical syntheses to realize 1D, 2D, and 3D self-assembled nanostructures.

The molecular building blocks act as parts of a jigsaw puzzle that join together in perfect order without the presence of an obvious driving force. Long-term and visionary nanotechnological concepts go far beyond these first approaches, particularly for the development—by means of molecular nanotechnology—of biomimetic materials that have the ability to self-organize, self-heal, and self-replicate.

One objective here is to prepare combinations of synthetic and biological materials, architectures, and systems to imitate biological processes in technological applications.

At present, the field of nanobiotechnology remains at the stage of basic research, but it is regarded as one of the most promising future research fields (European Commission 2001).