Fundamentally, synthesis of nanomaterials can be classified as either top-down, or bottom-up. In top-down synthesis, bulk material is either carved-out, or selectively removed using a nanostructured template. Carving-out is a sequential process that is inherently slow, yet offers a higher degree of control over the final size and shape. The more efficient templating process involves using a premade mask in combination with a removal agent, such as acid or light, to reveal the desired nanostructure.
In bottom-up synthesis, the opposite is true – whereby nanostructures are made from the ground-up using the constituent atoms as the building blocks. Although top-down approaches have been successfully used – as in the semiconductor industry – there are limitations. The potential of the bottom-up approach is that much smaller structures can be grown, which are truly nanoscale in size. Also, complex structures can more easily be realized, for example through modulation of different materials. This is all possible notwithstanding that the rates of production are faster – therefore rendering certain technological concepts commercially viable.
However, there are some setbacks which need to be overcome using the bottom-up synthesis paradigm. The four main issues to address – in order to raise the commercial viability of associated state-of the art technologies – comprise: (i) mass production rates, (ii) ease of self-assembly, (iii) fine-tuning nanostructure morphology, and (iv) good dispersion.
Gas-phase synthesis offers many advantages. We can synthesize a variety of nanostructures and compositions in minutes versus hours or even days – with growth rates typically exceeding 1 μm/min. It also allows for high-purity and phase-specific materials – not always available via wet-phase chemistry methods. The widespread application of flame synthesis or CVD is based on simplicity – being single-step processes that do not require time consuming drying or separation from the liquid phase.
With the abovementioned advantages of gas-phase synthesis, the following comprise the main techniques – in approximate order of growth rate.
Using Flame Synthesis:
This is primarily useful for synthesizing solid oxides with growth rates of at the very least ~1000 nm / min – which is fast. This is 2-3 orders of magnitude faster than liquid-based methods. The primary cause is the high temperature – and being gas-phase means it is not limited by diffusion.
Using Chemical Vapour Deposition:
CVD is similar to flame synthesis, but operates under oxygen deficient conditions. An example would be synthesis of silicon nanowires using a gold catalyst – showing growth rates of around 100 nm/min. On the other hand, single-walled carbon nanotubes hold the record for growth rates of up to ~ 5,000,000 nm/min! It should be noted that CNTs naturally grow faster owing to their hollow tube-like structure.
Using Physical Vapour Deposition:
PVD is similar to CVD, yet operates in the absence of chemical reactions. We can directly heat samples such as metals, either through electric-arc or through flame-heating, to redeposit nanostructures on a neighbouring cooler substrate.
Using Pulsed Laser Ablation :
PLA is useful for growing extremely high crystalline quality materials from substrates with high melting points – such as ceramics. This requires a high-power pulsed laser, and the yield of material is very low.
Using Molecular Beam Epitaxy:
MBE is the slowest of them all due to being performed at very low pressure. However, the advantage of such conditions is the resulting lower rate of interdiffusion within nanosized heterostructures. In situ analysis tools can also be used simultaneously to see how the structures grow in real time.
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