1.8.2 Electrochemical Assembly
Highly ordered arrays of quantum
dots may also be self-assembled by electrochemical techniques. A
template is created by causing and ionic reaction at an
electrolyte-metal interface which results in the spontaneous assembly of
nanostructures, including quantum dots, onto the metal which is then
used as a mask for mesa-etching these nanostructures on a chosen
substrate.
1.8.3 Bulk-manufacture
Conventional, small-scale
quantum dot manufacturing relies on a process called “high temperature
dual injection†which is impractical for most commercial applications
that require large quantities of quantum dots.
A reproducible method
for creating larger quantities of consistent, high-quality quantum dots
involves producing nano-particles from chemical precursors in the
presence of a molecular clusters compound under conditions whereby the
integrity of the molecular clusters is maintained and acts as a
prefabricated seed template. Individual molecules of a clusters compound
act as a seed or nucleation point upon which nano- particle growth can
be initiated. In this way, a high temperature nucleation step is not
necessary to initiate nano-particle growth because suitable nucleation
sites are already provided in the system by the molecular clusters. A
significant advantage of this method is that it is highly scalable.
Recently
a consortium of U.S. and Dutch companies reported a “milestone†in high
volume quantum dot manufacturing by applying the traditional high
temperature dual injection method to a flow system. However as of 2011,
applications using bulk- manufactured quantum dots are scarcely
available.
1.8.4 Cadmium-free quantum dots
Cadmium-free quantum
dots are also called “CFQDâ€. In many regions of the world there is now a
a restriction or ban on the use of heavy metals in many household goods
which means that most cadmium based quantum. Dots are unusable for
consumer- goods applications.
For commercial viability, a range of
restricted, heavy metal-free quantum dots has been developed showing
bright emissions in the visible and near infra-red region of the
spectrum and have similar optical properties to those of CdSe quantum
dots.
Cadmium and other restricted heavy metals used in conventional
quantum dots are of a major concern in commercial applications. For
Quantum Dots to be commercially viable in many applications they must
not contain cadmium or other restricted metal elements.
A new type of
CFQD can be made from rare earth (RE) doped oxide colloidal phosphor
nano-particles. Unlike semiconductor nano-particles, excitation was due
to UV absorption of hopst material, which is same for different RE doped
materials using same host. Multiplexing applications can be thus
realized. The emission depends on the type of RE, which enables very
large stokes shift and is narrower than CdSe QDs. The synthesis is
aqueous based, which eliminated issues of water solubility for
biological applications. The oxide surface might be easier for chemical
fictionalization more and chemically stable in various environments.
Some reports exist concerning the use of such phosphor nano-particles on
biological targeting and imaging.
1.9 Optical properties of quantum dots
An immediate optical feature of colloidal qantum dots is their coloration. While the material which makes up a quantum dot defines its intrinsic energy signature, the nanocrystalâ€s quantum confined size is more significant at energies near the band gap. Thus quantum dots of the same material, but with different sizes, camemit light of different colors. The physical reason is the quantum confinement effect.
The lager the dot, the redder (lower energy) its fluorescence spectrum.
Conversely, smaller dots emit bluer (higher energy) light. The coloration is directly related to the energy levels of the quantum dot. Quantitatively speaking, the band gap energy that determines the energy (and hence color) of the fluoresencnt light is inversely proportional to the size of quantum dot. Lager quantum dots have more energy level which also more closely spaced. This allows the quantum dot to absorb photons containing less energy, i.e those closer to the red end of the spectrum. Recent articles in nanotechnology and in other journals have begun to suggest that the shape of the quantum dot may be a factor in the coloration as well, but as yet not enough information is available. Furthermore, it was shown that the lifetime of fluorescence is determined by the size of the quantum dot. Lager dots have more closely spaced energy level in which the electron-hole pair can be trapped. Therefore, electron-hole pairs in larger dots live longer causing larger dots to show a longer lifetime
As with any crystalline semiconductors , a quantum dots electronic wave function extend over the crystal lattice similar to a molecule, a quantum dot has both a quantize energy spectrum and a quantized density of electronics state near the edge of the band gap.
Quantum dots can be synthesize with larger (thicker) shells (CdSe qdots with CdS shells.) the shell thickness has shown direct correlation to the lifetime and emission intensity.
1.10 Application of quantum dots
Quantum dots are particularly significant for optical applications due to their high extension co-efficient. In electronics application they have been proven to operate like a single-electron transistors and show the Coulomb blockade effect.
Quantum dot have also been suggested as implementations of qubits for quantum information processing.
The ability to tune the size of quantum dots have advantageous for many applications. For instance, larger quantum dots have a greater spectrum-shift towards red compared to smaller dots , and exhibit less pronounced quantum properties. Conversely, the smaller particle allows one to take advantage of more subtle quantum effects.
Researchers at Los Alamos National Laboratory have developed a wireless device that efficiently produces visible light, through energy transfer from thin layers of quantum wells to crystals above the layers.
Being zero dimensional, quantum dots have a shaper density of state than higher- dimensional structures. As a result, they have superior transport and optical properties, and are being researched for use in diode lasers, amplifiers, and biological sensors. Quantum dots may be excited within a locally enhanced electromagnetic field produced by gold nano- particles, which can then be observed from the surface Plasmon resonance in the photo luminescent excitation spectrum of (CdSe) ZnS nanocrystal. High –quality quantum dots are well suited for optical encoding and multiplexing applications due to their broad excitation profiles and narrow/symmetrical emission spectra. The new generations of quantum dots have far-reaching potential for the study of intracellular processes at the single-molecule level, high-resolution cellular imaging, long-term in vivo observation of cell trafficking, tumor targeting, and diagnostics. Measurement of the spin and other properties therein can be made. With several entangle quantum dots or qubits plus a way of performing operations; quantum calculations and the computers that would perform them might be possible.
