Amar Nath Yadav07/09/2016

Quantum Dots

What are they?• Quantum dots are semiconductor nanocrystals.

• They are made of many of the same materials as ordinary semiconductors, Ex.- CdSe, CdS, GaAs,GaP (mainly combinations of transition metals and/or metalloids).

• Unlike ordinary bulk semiconductors, which are generally macroscopic objects, quantum dots are extremely small, on the order of a few nanometers (2-10 nm, 10-50 atoms).

• They are very nearly zero-dimensional in comparison to

bulk semiconductor.

• 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

• However, some intervals of energy contain no orbitals, called band gap.

• 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”.

Excitons

Exciton…

Quantum Confinement 3-D

All carriers act as free carriers in all three directions

2-D or Quantum WellsThe carriers act as free carriers in a planeFirst observed in semiconductor systems

1-D or Quantum WiresThe carriers are free to move down the direction of the wire

0-D or Quantum DotsSystems in which carriers are confined in all directions (no free carriers).

• The reduction in the number of atoms in a material results in the confinement of normally delocalized energy states.

• Electron-hole pairs become spatially confined when the diameter of a particle approaches the de Broglie wavelength of electrons in the conduction band(means excitons bohr radius).

• As a result the energy difference between energy bands is increased with decreasing particle size.

• 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 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/R2, where R is the particle radius.

• So the energy gap of excitons in QDs is strongly size dependent.

• This size dependent phenomenon is due to the effect of confinement: The smaller QDs have stronger confinement making the energy gap larger. Similarly, a larger size gives a smaller energy gap.

• Hence, QDs with different emission colors can be made from the same material by changing their size.

Eg

Absorption and emission occur at specific wavelengths, which are related to QD size.

Experimental Observation of Confinement-

Just imaging a small dot is not enough to say it is confined.. Optical data allows insight into confinement..

Optical AbsorptionRaman Vibration SpectroscopyPhotoluminescence Spectroscopy

Optical Absorption Optical Absorption is a technique that allows one to

directly probe the band gap. The band gap edge of a material should be blue shifted if

the material is confined. Here I present the optical absorption of Ge quantum dots

in a SiO2 matrix. As the dot decreases in size there is a systematic shift of

the band gap edge toward shorter wavelengths.

Raman Vibrational Spectroscopy

Raman vibrational spectroscopy probes the vibrational modes of a sample using a laser.

As the nanocrystal becomes more confined the peak will broaden and shrink.

Here we see a peak shift toward the laser line Various Ge dots of different sizes on an Alumina film.

• Photoluminescence spectroscopy is a technique to probe the quantum levels of quantum dots.• Here we see dots of various size in a quantum well

(a) is quantum well spectrum(d) is smallest particles 80 nm

Photoluminescence Spectroscopy

Practical Applications:

• Optical Storage

• LEDs

• Organic Dyes

• Quantum Computing

• Security

• Solar Power

Optical Storage

• Quantum dots have been an enabling technology for the manufacture of blue lasers. • The high energy in a blue laser allows for as much as 35 times as much data storage than conventional optical storage media.• Less affected by temperature fluctuations, which reduces data errors. • This technology is currently available in new high- definition DVD players, and will also be used in the new Sony PlayStation.

Light Emitting Diodes

Light Emitting Diodes..

• Quantum Light Emitting Diodes (QLEDs) are superior to standard LEDs in the same ways the quantum dots are superior to bulk semiconductors.

• The tunability of QDs gives them the ability to emit nearly any frequency of light - a traditional LED lacks this ability.

• Traditional bulbs may be replaced using QLED technology, since QLEDs can provide a low-heat, full-spectrum source of light.

Organic Dyes• In vivo cell imaging of biological specimens.

• Possible uses for tumor detection in fluorescence spectroscopy.

▶ Red Quantum Dot lo-cating a tumor in a live mouse

Solar Power• The adjustable band gap of quantum dots allow the construction of advanced solar cells.

• Quantum dots have been found to emit up to three electrons per photon of sunlight, as opposed to only one for standard photovoltaic panels.

• Theoretically, this could boost solar power efficiency from 20-30% to as high as 65%.

Thank You for your attention..