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Nanofabrication is a way of making devices, systems, and components on a nanoscale.
There are two main types of nanofabrication, top-down and bottom-up nanofabrication. Top-down nanofabrication miniaturizes blocks by cutting them down, whereas bottom-up builds up molecular blocks atom by atom. Top-down nanofabrication is described as being similar to carving a block of wood, whereas the bottom-up method is similar to building a house brick by brick
The bottom-up approach has a better chance of producing nanostructures that have fewer defects than they would with the top-down method, as well as having a more homogenous chemical composition and better short- and long-range ordering. Combing both top-down and bottom-up techniques provides the best possible tools for nanofabrication, but each technique has specific applications and uses.
How it Works
Bottom-up fabrication uses chemical and physical forces at a nanoscale level to assemble simple units into larger structures. The bottom-up approach mimics biological processes, where individual atoms pile up one after the other on a substrate to form molecules. Biological molecules then arrange themselves independently into their desired form for their required nanostructures.
The process starts with a short nucleation stage. Nuclei activation is achieved after supersaturation of reagents, with temperature application or external electric field application. Growth is then diffusion-controlled. There are numerous techniques that can be used for bottom-up fabrication, and these include vapor-phase deposition, atomic layer deposition, molecular self-assembly, and DNA-scaffolding for nanoelectronics
Vapor-phase techniques can be physical or chemical vapor deposition. With physical vapor, the active species is evaporated into the vapor phase, whereas with chemical vapor deposition a precursor is used which decomposes into the required species via a chemical reaction. Atomic layer deposition is based on successive, self-restricting reaction cycles to provide thickness adjustment and composition control at the nanometer level.
Molecular self-assembly uses the organizational ability of matter to form homogeneous monolayers. Physical and chemical vapor deposition are self-assembly in the gaseous phase. Liquid-phase techniques using inverse micelles produce size-selected nanoparticles for semiconductors and magnetic materials. Quantum dots are an example of self-assembly. Another example of self-assembly of an intricate structure is the formation of carbon nanotubes under the right set of chemical and temperature conditions.
DNA nanostructures are perfect templates for the fabrication of nanomaterials via bottom-up assembly. The structure of DNA can be altered to suit the required applications. When using DNA-like recognition, molecules on surfaces can directly attachment themselves between objects present in fluids. Polymers made with complementary DNA strands attach to other polymers only when the right pairing is present. When you have the correct complementary sequences at the end of the DNA molecules, faces of the small semiconductor building blocks can be made to adhere to or repel each other.
Uses
Bottom-up approaches are best suited for assembly and establishing short-range order at nanoscale dimensions.
Bottom-up nanofabrication has many healthcare and medical applications. Carbon nanotubes, nanotubes that are a macromolecular form of carbon, have a great potential in biology and medicine due to their desirable properties. Devices that can work on individual cells and provide treatments can be nanofabricated, providing a solution to the problems caused when trying to issue treatments in bulk throughout the body.
DNA assembly can be used to combine nanofabrication with electrical fields to assist in locating attachment sites and then more-permanent attachment approaches, such as electrodeposition and metallization.
Surface-enhanced Raman spectroscopy is a type of spectroscopy that uses nanoparticle substrates to enhance the signal. SERS substrates can be nanoparticles in colloidal solutions or roughened nanofabricated surfaces.
The structures and properties of materials can be improved using a nanofabrication process. The materials can end up stronger, lighter and more durable, as well as the possibility of other properties such as being antimicrobial, anti-fog, anti-reflective, water-repellent, electrically conductive, self-cleaning, and ultraviolet, infrared and scratch resistant. Products such as tennis rackets, baseball bats and crude oil refining catalysts can all be nanotechnology-enabled.
Future Research and Advancements
As the bottom-up approach creates products by building them up bit by bit it can be quite time-consuming. Scientists are researching the concept of self-assembly by placing certain molecular-scale components together spontaneously from the bottom-up into ordered structures.
Nanofabricated transistors have the possibility to lead to computers that are more powerful, faster and more energy efficient than those used today. There is also the potential to exponentially increase storage capacity by storing a computer’s entire memory on a single tiny chip. In the energy sector nanotechnology could enable high-efficiency, low-cost batteries, and solar cells.
Recent advances in nanofabrication have been particularly important for the progression of surface-enhanced Raman spectroscopy (SERS). To improve the enhancement of SERS signals, new substrates are being researched and fabricated.
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