An Introduction to the Quantum Mechanics of Nanoparticles

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Particles that are so small they are invisible to the naked eye behave very differently to those that we can see. The behavior of these nanoparticles is ruled by quantum effects; their fluorescence, electrical conductivity, and chemical reactivity change as a function of their size.

Take the example of nanoscale gold: usually, we think of gold as having a shimmering yellow color, but at the nanoscale, it appears red or purple because the motion of the particles is confined and it reacts differently with light compared to larger gold particles. This behavior can be put to practical use, for example, to selectively accumulate in cancerous tumors allowing for precise imaging and targeted laser destruction while also avoiding healthy cells.

Nanoparticles are characterized as being less than 100 nm in size and are frequently synthesized. The properties of bulk materials change dramatically when shrunk to nanoscale dimensions; below 100 nm the materials break the size barrier below which quantization of energy for electrons in solids become relevant.

This effect – the quantum size effect – describes the physics of electron properties in solids with a great reduction in particle size. It becomes dominant when nanometer scales are reached and affects the optical, electrical, and magnetic behavior of materials, especially at the lower end of the scale.

Bulk properties of any material are essentially the average of all the quantum forces affecting all the atoms making up a material. As things become smaller, eventually a point is reached where this averaging no longer works and the specific behavior of individual atoms and molecules comes into play. Materials suddenly exhibit very different properties: opaque substances like copper become transparent; stable materials such as aluminum turn out to be combustible; solids like gold become liquid at room temperature; insulators such as silicon become conductors. The high surface area of nanomaterials also makes them more chemically reactive and can affect their strength or electrical properties.

A powerful and fascinating result of quantum effects on the nanoscale is the concept of ‘tunability’. By changing particle size, one can fine-tune a material’s property of interest, such as changing the fluorescence color, which can then be used to identify particles and label them with markers for various purposes.

Quantum dots exhibit such tunability: larger dots exhibit less pronounced quantum properties, while smaller dots allow you to take advantage of quantum properties. Such dots are promising for the development of solid-state quantum computers.

Quantum dots are nanoparticles – below 10 nm – made of semiconductor materials that have fluorescent properties. Their electronic properties are closely related to their size and shape, and they lie between those of bulk semiconductors and those of discrete molecules.

The color of the light emitted from quantum dots depends on the size of the dot. Generally, smaller dots have a larger bandgap and greater energy difference between the highest valence band and the lowest conduction band; this means more energy is needed to excite the dot and more energy is released when it returns to its resting state.

Take the example of fluorescent dyes, the smaller the dot, the higher the frequency of light emitted after excitation, resulting in a shift from red to blue. The larger the dot the redder – and lower energy – its fluorescence spectrum; the smaller the dot the bluer and higher energy emitted. Quantum dots are up to a thousand times brighter and glow longer than conventional fluorescent dyes.

Quantum nanoscience research is at the cross-section of quantum science and nanoscience; it enables scientists to develop nanotechnologies utilizing quantum mechanics to explore and use quantum effects in engineered nanostructures. This may aid the design of novel types of nanodevices and nanoscopic scale materials where functionality and structure of quantum nanodevices are described through superposition and entanglement.

Nanotechnology is ever-changing, and this evolutionary nanotechnology aims to improve existing processes, materials, and applications by scaling materials down to the nano realm to fully exploit their unique quantum and surface phenomena. In computing and electronics, microchips have already reached sub-100 nm, and the semiconductor industry is well on its way to becoming a nanotech industry.

Sources and Further Reading

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Kerry Taylor-Smith

Written by

Kerry Taylor-Smith

Kerry has been a freelance writer, editor, and proofreader since 2016, specializing in science and health-related subjects. She has a degree in Natural Sciences at the University of Bath and is based in the UK.

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