This presentation discusses quantum dots, which are nanoparticles that exhibit quantum confinement. Quantum dots are usually made of semiconductors and their optical and electrical properties depend on their size due to quantum confinement effects. They can be made through lithography, colloidal synthesis, or epitaxial growth methods. The presenter notes that quantum dots made of heavy metals like cadmium may not be commercially viable due to legislation, so silicon quantum dots are being researched as a non-toxic alternative. Potential applications of quantum dots include solar cells, biosensing, LEDs, displays, and lasers due to their size-dependent properties.

1. PRESENTED BY:
ASHRAF ALI
DEPARTMENT OF NANOTECHNOLOGY
NEHU,SHILLONG

2.  Quantum dots (QD) are nanoparticles/structures that
exhibit 3 dimensional quantum confinement, which
leads to many unique optical and transport properties.
 Quantum dots are usually regarded as semiconductors
by definition
 Similar behavior is observed in some metals.
Therefore, in some cases it may be acceptable to speak
about metal quantum dots.
 QDs are band gap tunable by size
which means their optical and electrical
properties can be engineered to meet
specific applications

3.  Quantum Confinement is the spatial confinement of
electron-hole pairs (excisions) in one or more
dimensions within a material.
◦ 1D confinement: Quantum Wells
◦ 2D confinement: Quantum Wire
◦ 3D confinement: Quantum Dot
 Quantum confinement is more prominent in
semiconductors because they have an energy gap in
their electronic band structure.
 Metals do not have a band gap, so quantum size
effects are less prevalent. Quantum confinement is
only observed at dimensions below 2 nm

4.  Nanocrystals (2-10 nm) of semiconductor
compounds
 Small size leads to confinement of excitons
(electron-hole pairs)
 Quantized energy levels and altered relaxation
dynamics
 Examples: CdSe, PbSe, PbTe, InP
 Absorption and emission occur at specific
wavelengths, which are related to QD size

5.  Electrons in conduction band (and holes in
the valence
 band) are free to move in all three
dimensions of space

6.  Electrons in conduction band (and holes in
the valenceband) are free to move in two
dimensions.
 Confined in one dimension by a potential
well.
 Potential well created due to a larger
bandgap of thesemiconductors on either side
of the thin film.
 Thinner films lead to higher energy levels.

7.  Thin semiconductor wire surrounded by a
material with a larger bandgap.
 Surrounding material confines electrons and
holes in two dimensions (carriers can only
move in one dimension) due to its larger
bandgap.
 Quantum wire acts as a
potential well.

8.  Electrons and holes are confined in all three
dimensions of space by a surrounding
material with a larger bandgap.
 Discrete energy levels (artificial atom).
 A quantum dot has a larger bandgap.
 Like bulk semiconductor, electrons tend to
make transitions near the edges of the
bandgap in quantum dots.

9.  Very small semiconductor particles with a
size comparable to the Bohr radius of the
excitons (separation of electron and hole).
 Typical dimensions: 1 – 10 nm
 Can be as large as several μm.
 Different shapes (cubes, spheres, pyramids,
etc.)

10.  The energy levels depend on the size, and also the
shape, of the quantum dot.
 Smaller quantum dot:
1. Higher energy required to confine excitons to a
smaller volume.
2. Energy levels increase in energy and spread out
more.
3. Higher band gap energy.

11.  5 nm dots: red
 1.5 nm dots: violet

12.  There are three main ways to confine
excitons in semiconductors:
1. Lithography
2. Colloidal synthesis
3. Epitaxy:
a) Patterned Growth
b) Self-Organized Growth

13.  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.

14.  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)
 Size variations (“size dispersion”).

15.  Semiconducting compounds with a smaller
bandgap (GaAs) are grown on the surface of a
compoundwith a larger bandgap (AlGaAs).
 Growth is restricted by coating it with a
masking compound (SiO2) and etching that
mask with the shape of the required crystal cell
wall shape.
 Disadvantage: density
Of quantum dots limited
bymask pattern.

16.  Uses a large difference in the lattice constants
of the substrate and the crystallizing material.
 When the crystallized layer is thicker than the
critical thickness, there is a strong strain on
the layers.
 The breakdown results in
randomly distributed islets
of regular shape and size.
 Disadvantages: size and shape
fluctuations, ordering

17.  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.

18.  Very narrow spectral line width, depending on
the quantum dot’s size.
 Multiplexed detection
 Large absorption coefficients across a wide
spectral range.
 Small size / high surface-to-volume ratio.
 Very high levels of brightness.
 Blinking.

19.  Photovoltaic devices: solar cells
 Biology : biosensors, imaging
 Light emitting diodes: LEDs
 Quantum computation
 Flat-panel displays
 Memory elements
 Photodetectors
 Lasers

20. THANK YOU