Native ligands have been exchanged with small organic or inorganic
linkers. The native ones had wide gaps between their highest occupied and
lowest unoccupied molecular orbital (HOMO and LUMO respectively) limited
effective interparticle coupling.
Physical size of band gap determines the photon’s emission
wavelength. Bandgap energy is inversely proportional to the square of the size
of the QD.
Quantum confinement effects: the bandgap (or HOMO-LUMO gap) of the
semiconductor nanocrystal increased with decreasing size, while discrete energy
levels arise at the band-edges. The energy separation between the band-edge
levels also increases with decreasing size. The colour of the luminescence
changes from red to blue as the QD diameter is reduced from 6 to 2nm.
Drummen, G., 2010: QD fluorescence: absorption of a photon higher in
energy than the spectral bandgap of the core semiconductor, resulting in
electron excitation to the conduction band, generating an electron-hole pair
(exciton). Broad absorption spectrum
achieved due to long lifetime (10-40ns), as it increases the probability of
absorption at shorter wavelengths.
Energy-band structure is what gives the different electrical
characteristics in different materials. Electrons can move from the valence to
the conduction band, but only if they can satisfy that minimum amount of energy
that they require (either by absorbing a phonon -heat- or a photon -light-).
Temperature dependence of the energy bandgap
The energy bandgap of semiconductors tends to decrease as the
temperature is increased. This is because: the interatomic spacing increases
when the amplitude of the atomic vibrations increases due to the increased
thermal energy. This effect is quantified by the linear expansion coefficient
of a material. An increased interatomic spacing decreases the potential seen by
the electrons in the material, which in turn reduces the size of the energy
bandgap. A direct modulation of the interatomic distance, such as by applying
high compressive (tensile) stress, also causes an increase (decrease) of the
Quantum dots are semiconductor
nanocrystals varying in size from 2-10nm (10-50 atoms) in diameter. They
exhibit quantum mechanical effects allowing
them to mimic atomic properties. Bandgap
is the energy that they require for their electrons
to become excited. Small dot (blue) =
larger bandgap (i.e. electrons require a lot of energy to enter the excited state) Big dot (red) = smaller bandgap.
High energy = high frequency = small wavelength.