QUANTUM SIZE EFFECT AND NANOCRYSTAL COLORS
When materials are subdivided into sufficiently small fragments, quantum effects begin to influence their physical properties. The fluorescent nanoparticles discussed earlier are in fact semiconductors that are small enough to show such quantum effects.
Semiconductors are substances that conduct electricity under some conditions but not others. In N-type semiconductors (as in normal electric wires) the current consists of negatively charged electrons. In P-type semiconductors the current consists of holes.
A hole is the absence of an electron from an atom. Although not physical particles, holes can move from atom to atom. Electrons and holes may combine and cancel out, a process that releases energy. Conversely, energy absorbed by certain semiconductors may generate an electron-hole pair whose two components may then move off in different directions.
Nanoparticle labels can be made with different emission wavelengths, covering the UV, visible spectrum, and near infrared. Emission wavelengths obviously vary depending on the semiconductor material. However, in addition, the quantum size effect (Fig. 7.10) allows the same semiconductor to emit at different wavelengths, depending on the size of the nanoparticle. The smaller the nanoparticle the shorter the wavelength (i.e., the higher the energy).
Fluorescent nanoparticles may be regarded as miniaturized light-emitting diodes (LEDs). These are semiconductors that work by absorbing energy (either electrical or light) and creating electron-hole pairs. When the electrons and holes recombine, light is emitted. For bulk material, the energy, and hence the wavelength, of the emitted light depends on the chemical composition of the semiconductor. However, at nanoscale dimensions, quantum effects become significant.
If the physical size of the semiconductor is smaller than the natural radius (the Bohr radius) of the electron-hole pair, extra energy is needed to confine the electron-hole pair. This is referred to as quantum confinement and occurs with nanocrystals of around 20 nm or less.
The smaller the semiconductor crystal, the more energy is needed and the more energetic (shorter in wavelength) is the light released.