How the Quantum dots special properties which are varying with size is explained

1. Quantum Dots and Their Applications
Submitted by -
Kadimi Vinay Kumar
208118009
NIT TRICHY

2. Presentation Content
• What is a Quantum Dot ?
• Why and how their optoeletronuc
properies chages with size and shape ?
• Overview of their fabrication techniques
and applications.

3. Basic Structure of Quantum Dots ?
• Quantum dots are semiconductor nanocrystals.
• The core of QDs is usually composed of
elements from groups II–VI such as CdSe, CdS
or CdTe, groups III–V such as InP or InAs, or
groups IV–VI such as PbSe. The shell is usually
composed of ZnS to improve the optical
properties.
• A cap to enable improved aqueous solubility
for biology-related applications.

4. Quantum Dots :Very small crystals with very
unsual properties
• Unlike ordinary bulk
semiconductors, which are
generally macroscopic objects,
quantum dots are extremely
small, on the order of a few
nanometres (2-10 nm, 10-50
atoms).
• Exhibit strongly size-
dependent optical and
electrical properties

5. HOW QDs different from bulk semiconductors:
The Energy band diagram of normal bulk semiconductor

6. The Band Gap variations with variation in QD size

8. WHY THESE VARIATIONS WITH SIZE ?
• Band Theory:
• Atomic orbital: a discrete set of energy levels.
• If several atoms are brought together into a molecule, their atomic
orbitals split, due to perturbation. • When a large number of atoms
(of order 1020 or more) are brought together to form a solid, the
number of orbitals becomes exceedingly large.
• Difference in energy between them becomes very small so the levels
may be considered to form continuous bands of energy rather than
the discrete energy levels of the atoms in isolation. Band Theory

9. EXCITON BOHR RADIUS:
• The bound electron- hole pair ( their lowest energy states) is called
Exciton.
• An exciton can form ,when a photon absorbed by a semiconductor.
• The average distance between an electron and hole is called “Exciton
bohr radius”, also referered to as de Broglie wavelength of
electron.

10. What is happening when we are reducing the size ?
• In a semiconductor crystallite whose size is smaller than twice the size
of its exciton Bohr radius, the excitons are squeezed, leading to
Quantum Confinement.
• A particle behaves as if it were free when the confining dimension is
large compared to the wavelength of the particle. During this state, the
bandgap remains at its original energy due to a continuous energy state.
• However, as the confining dimension decreases and reaches a certain
limit, typically in nanoscale, the energy spectrum becomes discrete. As a
result, the bandgap becomes size-dependent. This ultimately results in a
blueshift in light emission as the size of the particles decreases.

12. Quantum Confinement:
• The smaller you made the crystal the higher the energy of the electron will be
• The kinetic energy of that electron will get increased.
• The Uncertainty Principle states that the more precisely one knows the
position of a particle, the more uncertainty in its momentum (and vice versa).
• Therefore, the more spatially confined and localized a particle becomes, the
broader the range of its momentum/energy.
• This phenomena of increasing excitation energy with decreasing crystallite
size is known as quantum confinement.

13. ENERGY BAND GAPS BECAME SIZE DEPENDENT
• This is manifested as an
increase in the average
energy of electrons in
the conduction band =
increased energy level
spacing = larger
bandgap.
• The bandgap of a spherical
quantum dot is
proportional to 1/R^2,
where R is the particle
radius
• So the energy gap of
excitons in QDs is strongly
size dependent.

14. Quantum materials of other dimensions:

15. How QDs are getting fabricated:
• There are three main ways to confine excitons in semiconductors:
o Lithography
o Colloidal synthesis
o Epitaxy
• Patterned Growth
• Self-Organized Growth

16. Lithography
• Quantum wells are covered with a
polymer mask and exposed to an
electron or ion beam.
• The surface is covered with a thin
layer of metal, then cleaned and only
the exposed areas keep the metal layer.
• Pillars are etched into the entire
surface.
• Multiple layers are applied this way to
build up the properties and size
wanted.
• Disadvantages: slow, contamination,
low density, defect formation.

17. Colloidal Synthesis:
• Immersion of semiconductor
microcrystals in glass dielectric
matrices.
• Taking a silicate glass with 1%
semiconducting phase (CdS, CuCl,
CdSe, or CuBr).
• Heating for several hours at high
temperature.
• Formation of microcrystals of nearly
equal size.
• Typically group II-VI materials (e.g.
CdS, CdSe)
• Disadvantage:Size variations (“size
dispersion”).
• i.e: (PbS), (PbSe), (CdSe), (CdS),
(InAs), (InP)
•

18. Cadmium-free quantum dots
• In many regions of the world there is now, or soon to be, legislation to
restrict and in some cases ban heavy metals in many household
appliances such as IT & telecommunication equipment, Lighting
equipment , Electrical & electronic tools, Toys, leisure & sports
equipment.
• For QDs to be commercially viable in many applications they MUST
NOT CONTAIN cadmium or other restricted elements LIKE mercury,
lead, chromium.
• So research has been able to create non-toxic quantum dots using
silicon and carbon.

19. APPLICATIONS
• 1.QLED:
• Quantum dots may some day light
your homes, offices, streets, and entire
cities.
• Quantum dot LED’s can now produce
any colour of light, including white.
• Quantum dot LED’s are extremely
energy efficient. They use only a few
watts, while a regular incandescent
lamp uses 30 or more watts for the
same amount of light.

20. • The spectrum of photon emission is
narrow, characterized by its fullwidth
at half the maximum value-pure and
saturated emission colors with narrow
bandwidth- emission wavelength is
easily tuned by changing the size of the
quantum dots.
• Advantages : Low power consumption
– Ranger and accuracy color
• Brightness: 50~100 times brighter
than CRT and LCD displays ~40,000
cd/m2
• QLED screens are said to be twice as
power efficient as OLED screens,
and offer 30 to 40% improved
brightness.

22. How QLEDs better than LEDS
in Displays
1.Displays in any device generally uses
3 types of LEDs
1.RED
2.GREEN
3.BLUE
2.And rest of the colours will be
generated using the combinations of
thses 3 basic colours in various
proportions.

23. Problem in LED display:
1.Since LEDS emits these 3 colours with broad colour profiles they
will struggle to represent rest of the colours with high precision.

24. Spectrum of colors emitted by QDs:
QDs are capable of emiting the colours much more accuratley.
As the spectrum colours emitted by QDs are narrow compared
to that of LEDs

25. QD solar cells:
• Quantum dots may be able
to increase the efficiency
and reduce the cost of
today's typical silicon
photovoltaic cells.
• Quantum dot photovoltaic
would theoretically be
cheaper to manufacture, as
they can be made "using
simple chemical reactions.

26. Medical imaging:
• Quantum dots can be designed to bind to specific cell
receptors.
• The photo below shows human red blood cells, in which specific
membrane proteins are targeted and labeled with quantum dots.
• The number of purple features, which indicate the nuclei of malaria
parasites, increases as malaria development progresses.

27. LOCATING CANCER CELL & TARGETED DRUG
DELIVERY
• These quantum dots can be put into
single cells, or lots of cells, in the
tissue of living organisms.
• In future, it is planned to attach
specific antibodies to the quantum
dots – when injected into a body,
the quantum dots will find and bind
to cancer cells, and illuminate them
when they fluoresce.CdSe/ZnS QDs
used to image cancer cells in a live
mouse.
• Targeted Drug delivery can be done
by attach drug molecules to the
quantum dots, which will then be
able to deliver the drug just to the
cancer cells where it is needed

28. Programmable Matter
• It is possible to build an ensemble of elements which can be
programmed to change their physical properties in reality not just
in simulation.
• By the manipulation of quantum dots ,we can create materials that
have some of the properties of real elements
• Change the electron population in the quantum dots and you change
the material .
• EXAMPLE: A light weight breathable leather jacket that turns into
high carbon steel when you fall off your motorcycle.

29. References
• http://nextbigfuture.com/2006/05/quantum-dots-could-create-65-
efficient.html
• http://en.wikipedia.org/wiki/Quantum_dot
• https://www.youtube.com/watch?v=7_B3tPkQ3XU&pbjreload=10
• https://www.youtube.com/watch?v=e-32eGL5bns
• Google images

30. Thank you