What is Quantum Dots?
Quantum dots (QD’s) are tiny particles or nanocrystals of a
semiconducting material with diameters in the range of 2-10 nanometers (10-50
atoms), having optical and electronic properties that
differ from larger particles due to quantum mechanics. It is one of the
central and most important topic in Nano-Technology. They
can emit light of various colors. These artificial semiconductor nanoparticles have found applications in composites, solar
cells and fluorescent biological labels.
Quantum dots have properties intermediate between bulk
semiconductors and discrete atoms or molecules. Their optoelectronic properties
change as a function of both size and shape. Larger QDs of 5–6 nm diameters
emit longer wavelengths, with colors such as orange or red. Smaller QDs
(2–3 nm) emit shorter wavelengths, yielding colors like blue and green.
However, the specific colors vary depending on the exact composition of the QD.
Due to their small size, the electrons in these
particles are confined in a small space (quantum box), and when the radii of
the semiconductor nanocrystal is smaller than the Exciton Bohr radius (Exciton
Bohr radius is the average distance between the electron in the conduction band
and the hole it leaves behind in the valence band), there is quantization of
the energy levels according to Pauli’s exclusion principle. The discrete,
quantized energy levels of these quantum particles relate them more
closely to atoms than bulk materials and have resulted in them being
nicknamed 'artificial atoms'. Generally, as the size of the crystal decreases,
the difference in energy between the highest valence band and the lowest
conduction band increases. More energy is then needed to excite the dot, and
concurrently, more energy is released when the crystal returns to its ground
state, resulting in a color shift from red to blue in the emitted light. As a
result of this phenomenon, these nanomaterials can emit any color of light
from the same material simply by changing the dot size. Additionally, because
of the high level of control possible over the size of the nanocrystals produced, these
semiconducting structures can be tuned during manufacturing to emit any color
of light.
The atom-like energy states of QDs furthermore contribute to
special optical properties, such as a particle-size dependent wavelength of
fluorescence; an effect which is used in fabricating optical probes for
biological and medical imaging.
So far, the use in bioanalytics and bio labeling has found the
widest range of applications for colloidal QDs. Though the first generation of
quantum dots already pointed out their potential, it took a lot of effort to
improve basic properties, in particular colloidal stability in salt-containing
solution. Initially, quantum dots have been used in very artificial
environments, and these particles would have simply precipitated in 'real'
samples, such as blood. These problems have been solved and QDs have found
numerous use in real applications.
Classification of Quantum Dots
Quantum dots
can be classified into different types based on their composition and
structure.
Core-Type Quantum Dots
These nanodots can be single component materials with
uniform internal compositions, such as chalcogenides (selenides, sulfides or
tellurides) of metals like cadmium, lead or zinc, example, CdTe or PbS. The
photo- and electroluminescence properties of core-type nanocrystals can be
fine-tuned by simply changing the crystallite size.
Core-Shell Quantum Dots
The luminescent properties of quantum dots arise
from recombination of electron-hole pairs (exciton decay) through
radiative pathways. However, the exciton decay can also occur through
nonradiative methods, reducing the fluorescence quantum yield. One of the
methods used to improve efficiency and brightness of semiconductor nanocrystals
is growing shells of another higher bandgap semiconducting material around
them. These particles with small regions of one material embedded in
another with a wider bandgap are known as core-shell quantum dots (CSQDs) or
core-shell semiconducting nanocrystals (CSSNCs). For example, quantum dots
with CdSe in the core and ZnS in the shell available from Sigma-Aldrich
Materials Science exhibit greater than 50% quantum yield. Coating quantum dots
with shells improves quantum yield by passivizing nonradiative recombination
sites and also makes them more robust to processing conditions for various
applications. This method has been widely explored as a way to adjust the
photophysical properties of quantum dots.
Alloyed Quantum Dots
The ability to tune optical and electronic properties by
changing the crystallite size has become a hallmark of quantum dots. However,
tuning the properties by changing the crystallite size could cause problems in
many applications with size restrictions. Multicomponent dots offer an
alternative method to tune properties without changing crystallite size.
Alloyed semiconductor nanodots with both homogeneous and gradient internal
structures allow tuning of the optical and electronic properties by merely
changing the composition and internal structure without changing the
crystallite size. For example, alloyed quantum dots of the compositions CdSxSe1-x/ZnS
of 6nm diameter emits light of different wavelengths by just changing the
composition. Alloyed semiconductor quantum dots formed by alloying together two
semiconductors with different bandgap energies exhibited interesting
properties distinct not only from the properties of their bulk counterparts but
also from those of their parent semiconductors. Thus, alloyed nanocrystals
possess novel and additional composition-tunable properties aside from the
properties that emerge due to quantum confinement effects.
Physicochemical
Properties of Quantum Dots
1. QDs are more photostable as compared to traditional dyes.
This is due to their inorganic composition and the fluorescence intensity they
work on.
2. QDs are 10–20 times brighter as compared to the conventional
organic dyes.
3. They are less prone to degradation as compared to other
optical imaging probes. This helps in tracking cellular processes effectively and
for long periods of time.
4. They have a higher signal to noise ratio than
organic dyes.
5. QDs have broader excitation spectra and a narrow, sharply
defined emission peak.
6. They have comparably longer fluorescence, high photo resistance, large stokes shift, and sharp emission spectra.
7. Molding of QDs in any shape is easy and they can be coated
with a range of biomaterials.
8. QDs give better contrast with the electron microscope.
9. QDs have novel optical and electronic properties because
of quantum confinement of electrons and photons in the nanostructure.
10. Confinement of quantum results in a widening of bandgap
which eventually increases when the size of the nanostructure is decreased further.
11. QDs
possess properties of luminescence: QDs due to their exceptional properties like high photostability, size-tunable narrow emission spectra, and broad range excitation
provides considerable value in various applications. Their size and composition
can be manipulated to get the desired emission properties, making them amenable
to simultaneous detection of multiple targets. The absorption of photons by QDs
takes place when excitation energy exceeds bandgap. Post absorption of this
energy, electrons shift from the ground state to the excited state. The energy
associated with the optical absorption and electronic structure of a material is
directly related to each other.
Application of Quantum Dots
Quantum dots in
medicine
Quantum dots enable researchers to
study cell processes at the level of a single molecule and may significantly
improve the diagnosis and treatment of diseases such as cancers. QDs are either
used as active sensor elements in high-resolution cellular imaging, where the
fluorescence properties of the quantum dots are changed upon reaction with the
analyte, or in passive label probes where selective receptor molecules such as
antibodies have been conjugated to the surface of the dots.
Quantum dots could revolutionize
medicine. Unfortunately, most of them are toxic. Ironically, the existence of
heavy metals in QDs such as cadmium, a well-established human toxicant and
carcinogen, poses potential dangers especially for future medical application,
where qdots are deliberately injected into the body.
As the use of nanomaterials for biomedical
applications is increasing, environmental pollution and toxicity have to be
addressed, and the development of a non-toxic and biocompatible nanomaterial is
becoming an important issue.
Quantum dots in Photovoltaic’s
The attractiveness of using quantum
dots for making solar cells lies in several advantages over other approaches:
They can be manufactured in an energy-saving room-temperature process; they can
be made from abundant, inexpensive materials that do not require extensive
purification, as silicon does; and they can be applied to a variety of
inexpensive and even flexible substrate materials, such as lightweight
plastics.
Although using quantum dots as the
basis for solar cells is not a new idea, attempts to make photovoltaic devices
have not yet achieved sufficiently high efficiency in converting sunlight to
power.
A promising route for quantum
dot solar cells is a semiconductor ink with the goal of enabling the
coating of large areas of solar cell substrates in a single deposition step and
thereby eliminating tens of deposition steps necessary with the previous
layer-by-layer method.
Graphene quantum
dots
Graphene, which basically is an
unrolled, planar form of a carbon nanotube therefore has become an extremely
interesting candidate material for nanoscale electronics. Researchers have
shown that it is possible to carve out nanoscale transistors from a single
graphene crystal (i.e. graphene quantum dots). Unlike all other known
materials, graphene remains highly stable and conductive even when it is cut
into devices one nanometer wide.
Graphene quantum dots (GQDs) also
show great potential in the fields of photo electronics, photovoltaics,
biosensing, and bioimaging owing to their unique photoluminescence (PL)
properties, including excellent biocompatibility, low toxicity, and high
stability against photobleaching and photoblinking.
Scientists still are working on
finding efficient and universal methods for the synthesis of GQDs with
high stability, controllable surface properties, and tunable PL emission
wavelength.
Perovskite quantum
dots
Luminescent quantum dots (LQDs),
which possess high photoluminescence quantum yields, flexible emission color
controlling, and solution processibility, are promising for applications in
lighting systems (warm white light without UV and infrared irradiation) and
high-quality displays.
However, the commercialization of
LQDs has been held back by the prohibitively high cost of their production.
Currently, LQDs are prepared by the HI method, requiring a high temperature
and tedious surface treating in order to improve both optical properties and
stability.
Although developed only recently,
inorganic halide perovskite quantum dot systems have exhibited comparable and
even better performances than traditional QDs in many fields.
By preparing highly emissive inorganic
perovskite quantum dots (IPQDs) at room temperature, IPQDs' superior
optical merits could lead to promising applications in lighting and displays.
Quantum dot TVs
and displays
The most commonly known use of
quantum dots nowadays maybe TV screens. Samsung and LG launched their QLED TVs
in 2015, and a few other companies followed not long after.
Quantum dots, because they are both
photo-active (photoluminescent) and electro-active (electroluminescent) and
have unique physical properties, will be at the core of next-generation
displays. Compared to organic luminescent materials used in organic light
emitting diodes (OLEDs), QD-based materials have purer colors, longer lifetime,
lower manufacturing cost, and lower power consumption. Another key advantage is
that, because QDs can be deposited on virtually any substrate, you can expect
printable and flexible – even rollable – quantum dot displays of all
sizes.
Quantum dots have found
applications in composites, solar cells (Grätzel cells) and fluorescent
biological labels (for example to trace a biological molecule) which use both
the small particle size and tunable energy levels.
Advances in chemistry have resulted
in the preparation of monolayer-protected, high-quality, monodispersed,
crystalline quantum dots as small as 2 nm in diameter, which can be
conveniently treated and processed as a typical chemical reagent.
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