What are top-down and bottom up approach for synthesis of nanomaterials?

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].

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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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.

Synthesis of nanomaterials: top-down and bottom-up approaches

There are two main approaches to synthesize nanomaterial or nanoparticle. One is top-down approach and another is the bottom-up approach.

Let us discuss these approaches one by one:

1. Top-down approach:

As the name suggests, the top-down approach means from top[larger] to bottom[smaller]. This approach is similar to making a statue made of stone. As in making of a statue, a bulk or big piece of stone is taken, similarly in top-down approach; a bulk piece of material is taken. Then carving and cutting is done until desired shape is achieved. Example: Different kinds of lithographic techniques cutting [such as electron beam, photo ion beam or X-ray lithograph cutting], etching, grinding, ball milling and sol gel technique.

2. Bottom-up approach:

As the name suggests, the bottom-up approach means from bottom[smaller] to top or up[larger]. In this technique, a nanometric structure is taken then using methods of assembly or self assembly, a mechanism is developed which is larger than where it is started.

Example: all cell use enzymes to create DNA by taking constituent molecules and binding them together to make the final component.

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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?

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