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A quantum dot is a nanoparticle which is so small that the movement of electrons are confined to all physical dimensions of the particle. These crystals are normally made up of elements from the periodic groups II-VI, III-V, or IV-VI.
These QDs are semi-conductors and the conducting characteristics are related to the shape and size of the individual crystal. Their sizes range between 2 - 10 nm (~ 10 - 50 atoms) in diameter. The smaller the crystal, the larger the band gap (energy difference between valence band and conduction band) becomes. For this reason more energy is required to excite the QD and as a result, more energy is released once the crystal returns to the resting state.
The Exciton Bohr Radius (EBR) is the physical difference between the electron and the hole (different for each material). In a bulk material the dimensions of a semi-conductor are larger than the EBR so that the exciton goes beyond its natural limit. But if the semiconductor crystal is so small that its size is similar to the material's EBR, then there is a small and finite separation between the electron energy levels i.e. quantum confinement. The semiconductor is no longer similar to the bulk material and is called a quantum dot.
It is possible to maintain a high level of control over the size of the QD crystal produced as control over the size results in having precise control over the conductive properties of the material. These particles are so small (they behave differently than bulk material) giving them unparalleled variability and enabling innovative applications.
The energies of a QD can be varied of a large range. By adding or removing a few atoms from the QD, the boundaries of the band gap can be changed. As the surface of the QD is changed, the band gap is changed again because of the size of the QD and the quantum confinement. The band gap in a QD will always be larger in energy. The electrons must fall a greater distance in energy, will emit shorter wavelengths, which results in radiation emitted being "blue shifted".
This principle is illustrated through fluorescent dye applications: as the crystal becomes smaller, higher frequencies of light is emitted after the excitation of the QD. This results in the light being emitted to change in colour from red to blue.
Properties of Quantum Dots
QDs of the same material emit light of different colours as the particle size changes because more energy is required to confine the semiconductor excitation in a small volume (i.e. quantum confinement).
There is an inverse relationship between the bandgap energy and the size of the QD. Large QDs have a lower energy fluorescent spectrum (emits red light) and smaller dots have higher energy spectrums (emits blue light). The lifetime of a fluorescence spectrum is also determined by the size of the QD: the energy levels of larger QDs are closely spaced so that an electron-hole pair can get trapped and thus, live longer. There are recent studies that suggest that the shape of the QD could also have an affect on the colouration of the QD.
Variable absorption pattern
Bulk semiconductors have an even absorption spectrum. But the absorption spectrum of QD semiconductors is made up of overlapping peaks which increase in size at shorter wavelengths. Each peak is a result of an energy transition between exciton energy levels. The QD will not absorb light at a wavelength longer than the absorption onset (first excitation peak). The latter is determined by the composition and size of the QD i.e. small QDs will have an absorption onset at shorter wavelengths.
Variable emission pattern
The peak emission wavelength does not depend on the wavelength of the excited light if it's shorter than the wavelength of the absorption onset. The bandwidth of the emission spectra (FWHM) relies on the temperature, natural spectral line width of the QDs and the particle size distribution (PSD) of the QDs within a solution. Spectral emission broadening because of PSD contributes the most to the FWHM and is known as inhomogeneous broadening. A small FWHM is the result of a narrow PSD.
QDs can be coupled to a variety of functional groups (e.g. amine, phosphine, nitrile and other ligands) which increases their applications in a variety of environments. If a suitable molecule is bound to the surface of a QD, the latter can be dispersed or dissolved in any solvent or it can be used in many organic and inorganic films. The surface chemistry of the QD can also be changed such as the brightness and electronic lifetime.
Quantum yield is the percentage of absorbed photons that result in an emitted photon. This property is controlled by non-radiative transitions of excitons at the QD's surface and is thus affected by the surface chemistry of the QD.
Variation of size of the particles
Electrons make transitions near the edges of the band gap in semiconductor material. With QDs the size of the band gap is controlled by adjusting the size of the QD. The emission frequency depends on the band gap of the QD and for this reason one can precisely control the output wavelength.
Applications of Quantum Dots
An exciting application for QDs are in quantum computing where the flow of electrons can be managed and precise measurements of the spin can be made. This makes quantum calculations and quantum computers possible.
QDs are favoured for use in organic dyes due to its brightness (high extinction coefficient) and stability (less photobleaching).
The excellent photostability of QDs make them useful as real-time tracking probes for molecules and cells
Semiconductor QDs have been used for in-vitro imaging of cells in order to image single-cell migration in real-time
QDs are used to deliver a gene-silencing tool into cells (siRNA)
Tumour targeting under in-vivo conditions enables QDs to bind selectively to tumours
QDs could increase the efficiency of photovoltaic cells because it can produce seven excitons (compared to one from conventional cells) from one high energy photon of sunlight. It should also be more cost-effective to manufacture QD photovoltaics as they are made using simple chemical reactions.
Light emitting devices (LED's)
QDs are used in the manufacturing of displays so that the colours more closely resemble what the human eye can observe. Displays which essentially produce monochromatic light can also be more efficient since more light reaches the eye.
Fabrication to point of use:
QDs are grown in general by advanced epitaxial techniques (bottom up), by ion implantation (bottom up) or by using lithographic (top down) techniques. QDs are made of binary alloys such as cadmium selenide, cadmium sulfide, indium arsenide, and indium phosphide. They could contain ~ 100 - 100Â 000 atoms within the QD volume corresponding to about 2 - 10 nm.
With the use of colloidal synthesis, large batches of QDs can be manufactured. This makes it ideal for commercial applications because it can be scaled up and is more convenient to use.
Self-assembly through electrochemical techniques produces highly ordered arrays of QDs (bottom up). An ionic reaction at an electrolyte-metal interface results in the assembly of nanostructures (such as QDs) onto a metal. This is then used as a mask for mesa-etching.
In order to produce large quantities of consistent, high-quality QDs, one can create nanoparticles from chemical predecessors in the presence of a molecular cluster. The individual molecules of the cluster compound act as a nucleation point where crystal growth starts (bottom up). Nanoparticle growth can start at lower temperature because a suitable nucleation site is already available. This technique can be scaled up.
Calculation that depicts the properties and size of particles:
The equation below relates some of the properties of nanomaterials to the size of the particle.
âˆ†Î¸ = 2T0Ïƒ/ ÏLr
âˆ†Î¸ = Deviation of melting point from the bulk value
T0 = Bulk melting point
Ïƒ = Surface tension coefficient for a liquid-solid interface
Ï = Particle density
L = Latent heat of fusion
r = Particle radius
The equation above illustrates that as the radius of the particle decreases (gets smaller), the deviation of the meting point from the bulk value increases.
Nanotech solutions to South Africa's grand challenges:
The Farmer to Pharma value chain to strengthen the bio-economy
Drug delivery for optimal usage of medicines e.g current research in nano-TB drugs
Treatment of cancer, tumours etc.
Sensors for early detection and prevention
Space science and technology
Nanotechnology has enabled the production of smaller components and devices which could be used in space
Ultra small sensors, communication and navigation systems need to have a low mass, low volume and very low power consumption for use in space.
Improved solar cells as well as cleaner, cost-effective energy sources
More efficient and long-lasting lighting
Solid state lighting could reduce electricity consumption and reduce carbon emissions
Construction materials changing their inner structure depending on the environmental conditions
Global-change science with a focus on climate change
Due to the large surface area of nanomaterials, more pollutants can be removed from the environment (gases, water etc)
Desalination of sea water
Catalysts are redesigned in order to reduce exhaust emissions
Human and social dynamics
Getting more school children interested in science!
Homogeneous nucleation: The reactants and catalysts exist as a vapour (gas phase)
Heterogeneous nucleation: Within the same sphere of a reaction one will find vapour-liquid or vapour-solid phases
In heterogeneous processes an elemental alloy is deposited onto a bulk substrate. Two broad classifications:
Physical vapour deposition (PVD): Growth species are transferred from a source/target onto a substrate to form a film. The transfer process involves a high energy tool such as a laser, plasma, ion beam or electron beam etc.
Chemical vapour deposition (CVD): A volatile compound is mixed with other gases to produce a non-volatile solid which is then deposited as atoms onto a substrate. Homogeneous (gas phase) and heterogeneous (solid phase) reactions are mixed.
There are three nucleation modes in the vapour deposition processes:
Island growth: Growth species are more attracted to each other than to the substrate. Thus, small islands form which later coalesce to form a continuous film
Layer growth: Surface attraction is greater than the attraction between the growth species. Thus, the first layer forms, then another layer forms on top of the first etc.
Island-layer growth: Combination of island and layer growth. The first layer forms and then the attraction amongst the growth species are greater than with the first layer.
The layer growth nucleation provides the best uniformity. This process would be considered a Bottom up method for growing films because small building blocks (atoms or molecules) are deposited to build a thin film layer.