Chapter 2 : Manufacturing Process Bottom Up
Self-assembly is a method of integration in which the components spontaneously assemble, typically by bouncing around in a solution or gas phase until a stable structure of minimum energy is reached.
Self-assembly is important to the assembly of nanotechnology, and is thus a promising method for assembling atomically precise devices.
There are two basic methods for material fabrication:
Topics covered in this snack-sized chapter:
- One is to start with rough, large-scale material and carve a shape into it, often referred to as the “top-down” approach.
This methodology starts with atoms or molecules and builds up to nanostructures.
It is done using a surface to localize and stabilize them.
It is used to interconnect, assemble and test Nano-devices and Nano-machine starting from atomic or molecular parts.
The fabrication cost in a bottom up self-assembly is much less expensive.
Methods to produce nanoparticles from atoms are chemical processes based on transformations in solution.
- The other method is to create a construct from scratch by assembling simple building blocks according to a predesigned scheme, termed as “bottom-up” approach.
Chemical Vapor Deposition (CVD),
Atomic or molecular condensation.
The Sol-Gel Process, as the name implies, involves the evolution of inorganic networks through the formation of a colloidal suspension (sol) and gelation of the sol to form a network in a continuous liquid phase (gel).
The precursors for synthesizing these colloids consist usually of a metal or metalloid element surrounded by various reactive ligands.
The starting material is processed to form a dispersible oxide and forms a sol in contact with water or dilute acid.
Removal of the liquid from the sol yields the gel, and the sol/gel transition controls the particle size and shape.
Calcination of the gel produces the oxide.
Sol-gel processing refers to the hydrolysis and condensation of alkoxide-based precursors such as
(Tetraethyl Orthosilicate or ).
The reactions involved in the sol-gel chemistry based on the hydrolysis and condensation of metal alkoxides can be described as follows:
Sol-gel method of synthesizing Nano materials is very popular amongst chemists and is widely employed to prepare oxide materials.
The sol-gel process can be characterized by a series of distinct steps:
Formation of different stable solutions of the alkoxide or solvated metal precursor (Sol).
Gelation resulting from the formation of an oxide or alcohol bridged network (the gel) by a polycondensation or polyesterification reaction that results in a dramatic increase in the viscosity of the solution.
Aging of the gel (Syneresis), during which the polycondensation reactions continue until the gel transforms into a solid mass, accompanied by contraction of the gel network and expulsion of solvent from gel pores.
Ostwald ripening (also referred to as coarsening, is the phenomenon by which smaller particles are consumed by larger particles during the growth process) and phase transformations may occur concurrently with syneresis.
The aging process of gels can exceed 7 days and is critical to the prevention of cracks in gels that have been cast.
Drying of the gel, when water and other volatile liquids are removed from the gel network: this process is complicated due to fundamental changes in the structure of the gel.
The drying process has itself been broken into four distinct steps:
- The constant rate period,
If isolated by thermal evaporation, the resulting monolith is termed a xerogel. If the solvent (such as water) is extracted under supercritical or near super critical conditions, the product is an aerogel.
- The second falling rate period.
Dehydration, during which surface- bound M-OH groups are removed, there by stabilizing the gel against rehydration.
This is normally achieved by calcining the monolith at temperatures up to 8000°C.
Densification and decomposition of the gels at high temperature (T>8000°C).The pores of the gel network are collapsed, and remaining organic species are volatilized.
The typical steps that are involved in sol-gel processing are shown in the schematic diagram below:
CVD consists in activating a chemical reaction between the substrate surface and a gaseous precursor.
Activation can be achieved either with temperature or with plasma.
Plasma allows decreasing significantly the process temperature compared to the thermal CVD process.
CVD is widely used to produce carbon nanotubes.
Chemical Vapor Deposition:
This method is used mainly for metal containing nanoparticles.
A bulk material is heated in vacuum to produce a stream of vaporized and atomized matter, which is directed to a chamber containing either inert or reactive gas atmosphere.
Rapid cooling of the metal atoms due to their collision with the gas molecules results in the condensation and formation of nanoparticles.
If a reactive gas like oxygen is used, then metal oxide nanoparticles are produced.
Atomic or Molecular Condensation: