Top-down or destructive method refers to the reduction of a bulk material to nanometric scale particles. Mechanical milling, nanolithography, laser ablation, sputtering and thermal decomposition are among the most widely used nanoparticle synthesis methods. Unlike bottom-up method, top-down synthesis is a slow and expensive endeavor, and might not be suitable for large scale fabrication.

Among the various top-down methods, mechanical milling is most extensively used to produce various nanoparticles. Mechanical milling is used for milling and post annealing of nanoparticles during synthesis where different elements are milled in an inert atmosphere. The influencing factors in mechanical milling are: plastic deformation - determines particle shape, fracture and cold-welding - leads to decrease or increase in particle size, respectively.

Nanolithography is another commonly used top down method which has a long-term application in computer chip manufacturing and nanostructure engineering. Nanolithography is referred to as the study of fabricating nanometric scale structures with a minimum of one dimension in the size range of 1 to 100 nm. There are a number of different nanolithographic processes for instance optical, electron-beam, multiphoton, nanoimprint and scanning probe lithography. Generally, lithography is the process of printing a required shape or structure on a light sensitive material by selectively removing a portion of material to create the desired shape and structure. The main advantage of nanolithography is the ability to produce from a single nanoparticle to a cluster with desired shape and size. The disadvantages are the requirement of complex equipment and the high cost. Laser Ablation Synthesis in Solution (LASiS) is a common method for liquid-phase nanoparticle production. The irradiation of a solution-submerged metal by a laser beam condenses a plasma plume that produces nanoparticles. It is a reliable top-down method that provides an alternative solution to conventional chemical reduction of metals to synthesize metal-based nanoparticles. As LASiS provides a stable synthesis of nanoparticles in organic solvents and water without the need of stabilizing agent, it is considered as a “green” process.

Sputtering is the deposition of nanoparticles on a surface by ion bombardment. Sputtering of nanoparticles on the surface forms a thin layer, followed by annealing. The thickness of the layer, temperature and duration of annealing, substrate type, among other factors will eventually determine the shape and size of the nanoparticles. Thermal decomposition is an endothermic process that breaks the chemical bonds in the compound. The specific temperature at which an element chemically decomposes is the decomposition temperature. The nanoparticles are produced by decomposing the metal at specific temperatures, and then undergo a chemical reaction producing secondary products.

Reference:
Ealia, S. "A review on the classification, characterization, synthesis of nanoparticles and their application." IOP Conference Series: Materials Science and Engineering. IOP Publishing, 2017.

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