11. Nanomaterials (incl. Quantum dots) Flashcards
What is a quantum dot?
Quantum dots (QDs) are tiny semiconductor particles a few nanometres in size, having optical and electronic properties that differ from larger LED particles.
They are small in respect to how far the electrons move, with the ‘quantum’ referring to quantum confinement. They have dimensions comparable to the exciton-Bohr radius (<10 nm)
They are a realisation of the ideal 1D potential well but in 3D, where the size D of the well dictates the allowed, discrete energy levels.
What is an exciton?
It is an electron-hole pair created when an electron leaves the valence band and enters the conduction band
What is the exiton Bohr radius?
The average distance between the electron and the hole in an exciton
How does the equation for the ground state energy level equation of a quantum dot vary from the ground state energy of a potential well?
Just multiply it by 3/m*
What 3 factors can be changed to vary the excitation and emission bands of a quantum dot?
Size
Shape
Composition
How can the energy of an emitted photon be calculated for a quantum dot?
Work out the energy difference of the transition between the two energy levels defined by their n value
Which has a smaller quantum dot dimension, red or blue emitted light?
Blue
What are the 3 main geometries/allotropes of sp2 hybridised carbon?
Fullerene - spherical/like a football
Carbon nanotube - tubular
Graphene - 2d plane / single sheet
What benefit does graphene have over 3d bonded materials when it comes to conductivity
The flat, anisotropic structure means that it has no dangling bonds at its surface.
What is the key feature of the band structure of graphene?
That the bands touch at the edges, meaning there is no bandgap (therefore graphene isn’t a semiconductor
What feature of carbon nanotubes is integral to their electronic properties?
The orientation of the hexagonal unit cell relative to the axis of the nanotube.
