Although there are many special techniques to fabricate nanosized structures, generally these methods can be classified as top down and bottom up methods. Please give some examples and details?
Top-down and bottom-up methods are two types of approaches used in nanofabrication. The bottom-up approach is more advantageous than the top-down approach because the former has a better chance of producing nanostructures with less defects, more homogenous chemical composition, and better short- and long-range ordering.
A bottom up synthesis method implies that the nanostructures are synthesized onto the substrate by stacking atoms onto each other, which gives rise to crystal planes, crystal planes further stack onto each other, resulting in the synthesis of the nanostructures. A bottom-up approach can thus be viewed as an synthesis approach where the building blocks are added onto the substrate to form the nanostructures.
A top down synthesis method implies that the nanostructures are synthesized by etching out crystals planes (removing crystal planes) which are already present on the substrate. A top-down approach can thus be viewed as an approach where the building blocks are removed from the substrate to form the nanostructure.
Top-down and bottom-up methods are two types of approaches used in nanofabrication. The bottom-up approach is more advantageous than the top-down approach because the former has a better chance of producing nanostructures with less defects, more homogenous chemical composition, and better short- and long-range ordering.
A bottom up synthesis method implies that the nanostructures are synthesized onto the substrate by stacking atoms onto each other, which gives rise to crystal planes, crystal planes further stack onto each other, resulting in the synthesis of the nanostructures. A bottom-up approach can thus be viewed as an synthesis approach where the building blocks are added onto the substrate to form the nanostructures.
A top down synthesis method implies that the nanostructures are synthesized by etching out crystals planes (removing crystal planes) which are already present on the substrate. A top-down approach can thus be viewed as an approach where the building blocks are removed from the substrate to form the nanostructure.
Very briefly: bottom up is chemistry (synthesis), while top down is nano-fabrication ("milling"). It does not mean that top down may not involve chemistry (frequently it does), but you start with bulk (or micro) structures and decrease the size, while in bottom up you start form homogeneous solution or gas and build up the nano-particle or nano-layer, nano-wire or whatever.
This is the unique case, which is solely related to the structure of Carborundum, SiC, because it crystallizes in a large number of polymorphic forms. The cubic form (beta- SiC) has the zinc-blende structure. The atomic positions are the same as in diamond, alternate atoms being Si and C. Even though it is not the layer structure in the sense in which we use this term, but we may dissect these structures into layers of types a) and b) . For the case of beta- SiC one layer sequence is a a... ; Wurtzide a b a. ; but for the alpha-SiC , which has a hexagonal unit cell, there are at least eighteen different forms are found, where the number of layers in the unit cell are ranging 4-594 (WELLS) .
If you have a beta- SiC single crystal film, one side of which is always decorated by Si layer (a), and the other side is carbon decorated layer(a). Therefore, it is very clear that if one uses beta-SiC crystalline film as a temp plate having carbon decorated layer of which as a top surface then the attachment (the chemical adsorption) of the incoming carbon atomic species would be much easier than the any other surface including Si decorated layer of beta-SiC (i.e. bottom is called?) regardless the source for the 'carbon' supply.
In the case of bottom-top where the surface of beta-SiC exposed to impinging carbon atoms is made out Si decorated layer. Firstly this layer should be dis-integrated chemically or thermo-mechanically in order to adsoption of the incoming carbon atomic species could take place. The later steps would be then similar to the regular top-down process. But these two alternative temp plate surfaces they have not only different compositions but they have also behave differently when it comes to activation energy barrier for the adsorption of incoming C species.
I would make it more general: bottom-up is when the structure (not necessary on the surface and not necessary crystalline) is created from smaller building blocks, chemical synthesis of nanoparticles is a typical example of it; top-down is when the structure is cut out from a bigger piece "manually" or by a kind of self-structuring process, in some sense all current microelectronics is fabricated using this approach.
I would say that the advantage of bottom-up is the cost, scalability and in general better uniformity of the product. Top-down typically provides better control, but is limited to "countable" number of structures, though this number may be billions and billions.
As an example, which I personally was involved in, you may consider the mechanism of synthesis of fullerene molecule. It was considered to be bottom-up mechnism - molecule is assembled from C2 fragments, while some years ago we have shown that a top-down mechanism is also feasible, i.e. a ball can roll up from a flat flake of graphene.
Dear Dr. Neveling, you have done excellent job in describing the procedures, which are adopted for the production line by keeping the productivity in mind. But as a material scientist, I am not satisfied these ad hoc explanations of the top-down or bottom-up scenarios to build up structures of real materials without touching the energetics and kinetics of the growth processes. Those comments you have made just tells me that how a given micro structure can be put together in a production line of a factory in economically most efficient way by relying on the know -how, which coasted for the companies sometimes millions of dollars to get them. I recalled how IBM labs spent more than few billion dollars during the last quarter of the Twenty Century on the research & development to understand and control the catastrophic failure of aluminum interconnect lines induced by electromigration stressing in microelectronic devices. Regards.
Nanostructures can be made in numerous ways. ِA broad classification divides methods into either those which build from the bottom up, atom by atom, or those which construct from the top down using processes that involve the removal or reformation of atoms to create the desired structure.
In the bottom-up approach, atoms, molecules and even nanoparticles themselves can be used as the building blocks for the creation of complex nanostructures; the useful size of the building blocks depends on the properties to be engineered.
On the other hand, top-down approaches are inherently simpler and rely either on the removal or division of bulk material, or on the miniaturization of bulk fabrication processes to produce the desired structure with the appropriate properties.
Dear Ashraf
All the panel members give their valuable suggestions. I just add a paper and you can see it. I think it is useful to you.
The top-down approach is a process of miniaturizing or breaking down bulk materials (macro-crystalline) structures while retaining the original integrity. The bottom-up approach involves building of nanomaterials from the atomic scale (assembling materials from atoms/molecules). For synthesis of nanomaterials attrition or ballmilling is a typical example of top-down method and colloidal dispersion is a good example of bottom-up approach.
1- Top-down approach refers to slicing or successive cutting of a bulk material to get nano sized particle.
2- Bottom-up approach refers to the build up of a material from the bottom: atom by atom, molecule by molecule or cluster by cluster.
Both approaches play very important role in modern industry and most likely in nanotechnology as well. There are advantages and disadvantages in both approaches.
Top-Down
Attrition or Milling is a typical top-down method in making nano particles, whereas the colloidal dispersion is a good example of bottom-up approach in the synthesis of nano particles.
The biggest problem with top-down approach is the imperfection of surface structure and significant crystallographic damage to the processed patterns.
These imperfections which in turn leads to extra challenges in the device design and fabrication. But this approach leads to the bulk production of nano material. Regardless of the defects produced by top-down approach, they will continue to play an important role in the synthesis of nano structures.
Bottom-up
Though the bottom-up approach oftenly referred in nanotechnology, it is not a newer concept. All the living beings in nature observe growth by this approach only and also it has been in industrial use for over a century. Examples include the production of salt and nitrate in chemical industry.
Although the bottom-up approach is nothing new, it plays an important role in the fabrication and processing of nano structures.
Recently we have published a paper which uses a cylindrical shape sample that is formed out of thin flat copper film exposed to elastic deformation during shaping procedure. Both inner concave and outer convex surfaces are exposed to CVD annealing treatment in order to obtain graphene thin film over the copper substrate at 1283 K. The main reason to use this configuration is to see effects of the residual uni- or biaxial compressive (concave inner face) and tension (convex outer face) stresses on the morphological evolution of graphene thin film during cooling state down to the room temperature. Especially as one expects from the theoretical consideration the inner surface should be rather very smooth due to the negative effect of the compressive stresses on the rate of evaporation of copper surface via negative elastic dipole tensor interaction on Mobility. That should be just the opposite on the convex side where dipole tensor interaction due to tension becomes positive in sign that give enhancement on the evaporation process as observed experimentally by others. The results are very smooth inner surface compared to the rather rough outer surface which are all covered by graphene layers having few mono layer thicknesses. SEE our recent experimental work in RG, and another paper which presents some analytical results in press, which will appear in RG soon!
Generally, The initial state of the material is solid in a top down approach, it is liquid or gas in case of a bottom-up approach.
Bottom-up approach analogous to building a house, however, top-down is analogous to forming a statue from stone.
Thanks for the valuable insight to all researchers.
Techniques such as grinding, for example, make graphene from graphite powder top to bottom, but methods like pcvd, for example, make the graphene layer using the carbon atoms layer on the bottom-up method.
In summery bottom-up approach yield a well organized nano particles comparing with Top-Down approach
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