What is the basic difference (excluding the precursor's size) between top-down and bottom-up approaches in the synthesis of nanostructure materials?
In Bottom up method atoms (produced from reduction of ions) are assembled to generate nanostructures, and in top down method material is removed from the bulk material, leaving only the desired nanostructures. Common top down techniques are photolithography and electron beam lithography . Top down techniques suffer from the need to remove large amounts of material, while bottom up techniques suffer from poor monodispersity due to the need to arrest growth at the same point (by additing capping agents such as surfactant to avoid agglomeration of nanoparticles) for all the nanoparticles which is quite difficult .
In bottom up higher resolutions are obtained as compared to top down .
In the top to down approach you make a bulk material smaller and smaller to reach the nano size! But in bottom to up approach that it's name is nano chemistry you hold the particles or materials in nano size by some techniques like using surfactants or capping ligands like dendrimers and etc.
Bottom up approach is more precise and controllable than top down which is much prone to defects.
The difference is basically in the precursor size which you already know. Merits and demerits depends upon potential application.Interestingly, some scientists believe that only synthetic materials in the size range 1-100 nm may be called as nanomaterials while materials in this size range originating accidentally e.g. bruising, crushing and friction etc are not nanometerials. As accidental nano-material are an example of top-down process, you may see what the future holds for the scope of deliberate top-down processes.
In Bottom up method atoms (produced from reduction of ions) are assembled to generate nanostructures, and in top down method bulk material breaks continuously until the desired nanostructures is formed. Common top down techniques are photolithography and electron beam lithography. Top down techniques suffer from the need to remove large amounts of material, while bottom up techniques suffer from poor monodispersity due to the need to arrest growth at the same point (by additing capping agents such as surfactant to avoid agglomeration of nanoparticles) for all the nanoparticles which is quite difficult .
In bottom up higher resolutions are obtained as compared to top down.
In top down approach we are achieving bulk to nano structure generally by mechano-chemical process whereas in top down approach we achieve nano structure by the assembly of the atoms.
In the first we are removing atoms whereas in the second we are just adding and assembling atoms to get the nano structures.
In top down ,it is difficult to get monodispersibilty, it is time confusing, shape also may not be even and further time consuming. Again you need a good amount of the material. While in bottom up you synthesize the material and can control the size. Only point is that you have to ascertain its crystalinity by XRD etc.
In top down, you start with a bulk material (a "cunk2 steel, copper, aluminum or somethng else) and "carve, cut, machine etc. etc." to get the final shape or function. To even go back one more level, one could evens ay that top-down starts with mineral engineering or mining i.e. it even starts with the ore itself and its enrichment, purification etc.
Bottom its fundamentally different, in teh most extreme case one can at least imagine to start with individual atoms and then put them together into materials. To use supramolecules in the same manner is more realistic.
Moreover, there is an interesting discussion/debate about this in Scientific American (or a similar journal) between Smalley and Drexler. Aslo check out "On Love, Chemistry and Nanobots" by Smalley in Scientific American, published in 2001.
Hope this helps :)
Per
The similarity of the outcome nanostructures is one of the main difference, i.e. in bottom up methods, specially in hydrothermal methods (by autoclave reactor) shape and size of the nanoparticles are similar to each other which could be proved by DLS or even TEM imaging, but in top down methods, the shape and size of the nanoparticles might be completely different.
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