What is the difference between top down and bottom up methods for creating nano-structures?
Although there are many special techniques to fabricate nanosized structures,generally these methodscan be classified as top down and bottom up methods.Please give some examples and details?
1. Introduction
Nanoparticles and thin films are very common form of materials for utilization in different applications [1, 2, 3, 4]. Synthesis approaches play vital role to determine characteristics of nanoparticles [5] and thin films [6]. Thus, a number of methods are being developed to synthesize either nanoparticles [7, 8, 9] or thin films [10, 11, 12]. The motive behind to explore numerous methods is to look for reproducibility and cost effectiveness in terms of industrial utilization [13, 14]. Researchers are also working to get deep insights of involved phenomena during growth which persists a way to optimize for particular application [15, 16, 17, 18]. The factors, which are considered during nanoparticle growth, are size [19], shape [20, 21] and size distribution [22, 23]. In case of thin films, these factors are nature of growth, morphology, stress, strain developed across films substrate interface [24, 25, 26].
While growing nanoparticles, one need to take care annealing treatment [27, 28] and stoichiometry [29, 30], however, process is rather typical in case of thin film technology. Choice of substrate [31], annealing temperature [32, 33], base pressure [34], target to substrate distance [35], deposition pressure [36, 37] and nature of gas during growth determine the nature of film [38]. Textured of grown thin film [39], stoichiometry [40] and nature of surface [41, 42] are another important parameter, which are considered during deposition. Thus, keeping in mind the necessity and challenges in the synthesis, synthesis approaches for growing nanoparticles and thin films are discussed by taking a simple inorganic system. However, magnesium oxide is known from long time [43] but recent advances in application of this material motivated us to discuss these approaches for MgO [44]. In Table 1, a summary of properties of MgO are depicted [45, 46, 47].
| Crystallite structure | Rocksalt | Rocksalt | Rocksalt |
| Lattice parameter [Å] | 4.214 | 4.128 | 4.22 |
| Optical band-gap [eV] | 7.6 | 4–5 | 4–5 |
Table 1.
Properties and applications of MgO bulk, nanoparticles and thin films.
While keeping in mind the importance of this material, we attempt to give an overview of synthesis of MgO nanoparticle and thin film. To grow nanoparticles, two kinds of approaches are used: [1] bottom-up approach and [2] top-down approach [48, 49]. These approaches are explained on the basis of following schematic diagram. In general, bottom-up approach is meant by synthesis of nanoparticles by means of chemical reactions among the atoms/ions/molecules [Figure 1a]. Whereas top-down involves the mechanical methods to crush/breaking of bulk into several parts to form nanoparticles [Figure 1b]. In the next section both kind of approaches for growth of MgO nanoparticles and thin films are grown.
Figure 1.
Synthesis approaches for nanoparticles [a] bottom-up and [b] top-down approaches.
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Bottom-up approach
Bottom-up, or self-assembly, approaches to nanofabrication use chemical or physical forces operating at the nanoscale to assemble basic units into larger structures. As component size decreases in nanofabrication, bottom-up approaches provide an increasingly important complement to top-down techniques. Inspiration for bottom-up approaches comes from biological systems, where nature has harnessed chemical forces to create essentially all the structures needed by life. Researchers hope to replicate nature’s ability to produce small clusters of specific atoms, which can then self-assemble into more-elaborate structures.
Nanoparticles of a gold-palladium [yellow-blue] alloy supported on acid-treated carbon [gray] directly catalyzing hydrogen peroxide formation from hydrogen [white] and oxygen [red] while shutting off unwanted hydrogen peroxide decomposition.
Photo courtesy of Dr. David J. Willock, Cardiff UniversityA number of bottom-up approaches have been developed for producing nanoparticles, ranging from condensation of atomic vapours on surfaces to coalescence of atoms in liquids. For example, liquid-phase techniques based on inverse micelles [globules of lipid molecules floating in a nonaqueous solution in which their polar, or hydrophilic, ends point inward to form a hollow core, as shown in the figure] have been developed to produce size-selected nanoparticles of semiconductor, magnetic, and other materials. An example of self-assembly that achieves a limited degree of control over both formation and organization is the growth of quantum dots. Indium gallium arsenide [InGaAs] dots can be formed by growing thin layers of InGaAs on GaAs in such a manner that repulsive forces caused by compressive strain in the InGaAs layer results in the formation of isolated quantum dots. After the growth of multiple layer pairs, a fairly uniform spacing of the dots can be achieved. Another example of self-assembly of an intricate structure is the formation of carbon nanotubes under the right set of chemical and temperature conditions.
Phospholipid molecules, composed of fatty acid “tails” and a phosphate “head,” form an inverse micelle in a nonaqueous solution. The phosphate group converts one end of the lipid molecule into a polar, or hydrophilic, group, leaving the preferentially attracted nonpolar, or hydrophobic, end of the molecule to react with the nonaqueous solution.
Encyclopædia Britannica, Inc.DNA-assisted assembly may provide a method to integrate hybrid heterogeneous parts into a single device. Biology does this very well, combining self-assembly and self-organization in fluidic environments where weaker electrochemical forces play a significant role. By using DNA-like recognition, molecules on surfaces may be able to direct attachments between objects in fluids. In this approach, polymers made with complementary DNA strands would be used as intelligent “adhesive tape,” attaching between polymers only when the right pairing is present. Such assembly might be combined with electrical fields to assist in locating the attachment sites and then be followed by more-permanent attachment approaches, such as electrodeposition and metallization. There are several advantages of DNA-assisted approaches: DNA molecules can be sequenced and replicated in large quantities, DNA sequences act as codes that can be used to recognize complementary DNA strands, hybridized DNA strands form strong bonds to their complementary sequence, and DNA strands can be attached to different devices as labels. These properties are being explored for ways to self-assemble molecules into nanoscale units. For example, sequences of DNA have been fabricated that adhere only to particular crystal faces of compound semiconductors, providing a basis for self-assembly. By having the correct complementary sequences at the other end of the DNA molecule, certain faces of small semiconductor building blocks can be made that adhere to or repel each other. For example, thiol groups at the end of molecules cause them to attach to gold surfaces, while carboxyl groups can be used for attachment to silica surfaces. Directed assembly is an increasingly important variation of self-assembly where, in quasi-equilibrium environments, parts are moved mechanically, electrically, or magnetically and are placed precisely where they are intended to go.
S. Tom PicrauxNanofabrication
Two very different paths are pursued. One is a top-down strategy of miniaturizing current technologies, while the other is a bottom-up strategy of building ever-more-complex molecular devices atom by atom. Top-down approaches are good for producing structures with long-range order and for making macroscopic connections, while bottom-up approaches are best suited for assembly and establishing short-range order at nanoscale dimensions. The integration of top-down and bottom-up techniques is expected to eventually provide the best combination of tools for nanofabrication. Nanotechnology requires new tools for fabrication and measurement.
Top-down approaches have been developed for building structures at the scale of the micrometre [μm]. Bottom-up techniques have also been developed for assembling small groups of atoms or molecules at the scale of nanometres [nm]. The remaining task is to combine these approaches in order to create extended structures at the nanoscale.
Encyclopædia Britannica, Inc.Metallic Nanoparticles: Top-Down and Bottom-up Approaches
There are two different ways by which metallic nanoparticles can be formed. These are the “bottom-up" approach and the "top-down" approach. The two can be distinguished in the sense that while the bottom up method is sourced from scientific research including nanoscience, the top-down is not.
The bottom-up approach further comprises of creating nanomaterials and objects within the same nanosphere based on atoms, molecules, and aggregate grouping. This sort of grouping occurs in a clear and manageable manner, which allows for an increase in the functionality of the structure of such materials. The top-down approach which is sourced from microelectronics has to do with a clear reduction or breaking down of systems in their current state by making existing technologies more efficient. This results in a reduction in the size of the devices into nanoscale aspects.
In terms of the size of objects, both methods are very similar. Both approaches tend to converge in terms of the size range of objects. The former approach, however, tends to be more abundant based on the type of material, design varieties, and nanometric control, while the latter approach only makes the acquisition of materials of more importance, however, control may not be as strong.
Synthesis of Metallic Nanoparticles
When it comes to the synthesis of metallic Nanoparticles, two distinct approaches are utilized. The first is the top-down strategy and the second approach is referred to as the bottom-up strategy. While the former deals with the reduction in size of current technological devices, the latter performs an opposite role, which is building of even more complex molecular devices on an atomic arrangement.
While the top-down approach is beneficial in the production of technological structures in a far reached order and for connecting macroscopic devices, the bottom up is suitable for the production and arrangement of short-range order at the nanoscale aspect. The combination of both strategies is expected to form the best integration of equipment for nano-based fabrication.
Furthermore, the top-down technique is built up for architectural structures at the micrometer scale [um]. The bottom-up strategy is also built up for bringing small collections of atoms together measured in nanometers [nm]. What is left is to integrate both approaches to create elongated forms at the nanoscale.
The commonest form of the top-down approach of fabrication is the lithographic technique that utilizes enhanced visual sources of a short wavelength. One main benefit of the top-down technique in fabrication of joint circuits is the fact that all of its parts are created and structured in an orderly form so that no further assemblage is required. The high level of polishing makes visual lithography developed especially in the production of the micro-electric chip with the wavelength reaching a level below 100 nanometers [going by the traditional method].
On the other hand, the sources of shorter wavelengths like intense UV and X-ray, are created to permit the techniques for printing lithography to attain a level between 10-100 nanometers. Beams like the electron lithographic beam make provision for model reaching 20 nanometers. In this technique, the model is stated by flushing a finely patterned electron beam across the surface. Other mire concentrated ionized beams are utilized for the direct processing and modeling of wafers with a lesser effect compared to electron beam lithography.
Printing methods of a mechanical nature, also known as the nanoscale imprinting, stamping, and molding— expands to cover small measurements of 20 to 40 nanometers. Though the details differ, the main aim of this is to create a massive "stamp" by utilizing a high pixel method like the electron beam lithography thereafter adding the stamp or the following ones to the surface layer, thus, producing a model. Each variant comprises the coating of the surface layer of the stamp with the "ink" and then emptying directly on the surface of the stamp's model. Given an example, the model under control of a molecule monolayer can be obtained successfully by depositing the ink directly on the coated surface. Using another technique, the stamp is utilized for the purpose of mechanically pressing the model to the tiny layer of the element.
Typically, the surface layer is a polymeric element that has been patterned for molding by heating during the stamping process. Etching of plasma is then used for masking under the stamped layers; polymers are subsequently removed, while a nanoscale lithography model remains on the surface. Relief models are equally formed from photoresist on a wafer by visual or electro-beam lithography and then emptied on a watery precursor. The effect of this is a solid rubber-like substance that can be easily detached and utilized as a stamp. They can be utilized in any of the ways produced above. A distinguishing feature of the latter technique is the flexibility of the stamp.