Very interesting answers is here:
https://www.researchgate.net/post/What_is_the_difference_between_top_down_and_bottom_up_methods_for_creating_nano-structures
In general: Chemistry synthesis.
Chemical Bath Deposition or Chemical precipitation method is one of the low cost, easily affordable method to grow the nanostructure in ambient atmosphere. In this method the solutions of two different reactant materials are mixed together to form compound nanomaterials using suitable capping agent which control the growth parameter. However various physical conditions also influence the nucleation growth of the nanostructure particle. Therefore controlling the size of the as-prepared particle is also a challenging task and it invites tremendous research works.
Self-assembly is common in nature, and can happen to particles of all sizes, including atomic, nano-, meso- and macroparticles. Different disciplines have had varying definitions, a general definition is: ‘‘self-assembly is the autonomous organization of components into patterns or structures without human intervention’’. Or “self-assembly is a spontaneous process by which molecules to macroscopic entities assemble into one-, two- and three-dimensional ordered arrays”, in other words a structure that builds itself.
self-assembly is important for at least four reasons. The first is that it is centrally important in life; living cells contain a range of complex structures that form by self-assembly. The second is it provides a route to a range of materials with regular structures. Third, self-assembly of components larger than molecules, provides great potential for use in materials science. Fourth, self-assembly offers a general strategy for generating nanostructures. Thus self-assembly has become important in a range of fields: chemistry, physics, biology, materials science, nano-science, and manufacturing. The potential of self-assembly as a strategy for forming interesting structures now extends beyond molecules, and provides a way of assembling electrically or optically functional components, photonic crystals, 3D microelectronic systems, displays, and sensors.
wet self-assembly or “bottom up” methods commonly begin with the deposition or adsorption of particles from the solution to a substrate. “Bottom-up” self-assembly has the advantage over mechanically directed assembly in that it is simple and requires no machinery to move and locate particles, letting random, Brownian motion and other forces do the job instead.
Self-assembly on the sub-micron scale occurs in nature e.g. precious opals are self-assembled sub-micron silica particles, and some butterfly wings have grown repeating structures on the same scale. Natural opals are an example of a colloidal crystal with photonic properties. Self-assembled natural opals have ordered arrays of silica colloidal particles that produce a “play of colours” based on Bragg diffraction
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