Nanocrystals: What are these?

The prefix "nano" stands for 10^-9. Particles with dimensions in the nanometer regime are more commonly called nanoparticles in general, nanocrystals in case they are crystalline. Other names such as "quantum dots" have also been suggested for these tiny materials. Typically particles in the size range 2-20 nm exhibit interesting properties and hence it is appropriate to limit the nano nomenclature to such entities. The transmission electron micrograph (TEM) shows an arrangement of self-assembled, 7.7 nm diameter PbSe nanocrystals. PbSe is a semiconductor material. It is possible to synthesize nanoparticles of other types of materials such as metals. Semiconductors are different from metals due to the fact that they have a band gap between the valence band (VB, occupied by electrons) and the conduction band (CB, occupied by holes or in other words empty). Electrons (holes) can be excited from the VB (CB) to the CB (VB) by pumping in appropriate amount of energy into the system. Here, we shall only describe the semiconductor nanocrystals since they exhibit the "quantum confinement effect".

Quantum confinement

In order to understand quantum confinement, we need to go back to the very basics of quantum mechanics; namely the particle-in-a-box. All we need to worry about is, that the spacings between the energy levels increase as the length of the box decreases. Quantitatively, E_n = n2h2/8mL2. In the case of semiconductors this simply means that the band gap, starting from the bulk value, increases as the size of the nanocrystal decreases. In bulk solids the energy levels are closely spaced and thus form quasi-continuous bands. Going to the nano-regime the energy level separation increases and discrete energy levels are observed. Calculations on different systems show that quantum confinement effects are observable at sizes below 10 nm for most materials (~20 nm for Pb chalcogenides). Onset of confinement depends on a number of parameters such as the dielectric constant of the semiconductor and effective masses of the charge carriers.

Consequences of quantum confinement

Since the VB-CB transitions are electronic transitions, they occur in the near UV, visible or the near IR region of the electromagnetic spectrum. By choosing different semiconductors, which have different band gaps, it is possible to tune the band gap to be at any energy in this range. The electron that has been excited from the VB to the CB must relax back. If the relaxation occurs radiatively then we have emitters in the entire UV-vis-IR range. One such example is shown in the logo picture. The vials containing solutions of CdSe nanocrystals of different sizes (size increasing on going from left to right) are illuminated by a UV lamp. The fluorescence from these nanocrystals can be tuned all the way between the blue and red regions of the visible spectrum.

Applications

Fluorescent materials find a large number of applications. A simple example is that of light emitting devices (LEDs). Since nanocrystals absorb all energies higher than their band gap, they can also be used as color converters. Sizes of biological molecules are also on the order of a few nanometers. Tagging nanocrystals to such proteins can help in tracing these biomolecules.