Quantum Dot Features
Quantum Dots- Tunable Absorption Pattern
In addition to emissive advantages, quantum dots display advantages in their absorptive properties. In contrast to bulk semiconductors, which display a rather uniform absorption spectrum, the absorption spectrum for quantum dots appears as a series of overlapping peaks that get larger at shorter wavelengths. Owing once more to the discrete nature of electron energy levels in quantum dots, each peak corresponds to an energy transition between discrete electron-hole (exciton) energy levels. The quantum dots will not absorb light that has a wavelength longer than that of the first exciton peak, also referred to as the absorption onset. Like all other optical and electronic properties, the wavelength of the first exciton peak (and all subsequent peaks) is a function of the composition and size of the quantum dot. Smaller quantum dots result in a first exciton peak at shorter wavelengths.
Quantum Dots - Tunable Emission Pattern
The peak emission wavelength is bell-shaped (Gaussian) and occurs at a slightly longer wavelength than the lowest energy exciton peak (the absorption onset). This energy separation is what is referred to as the Stoke's Shift. An interesting property of quantum dots is that the peak emission wavelength is independent of the wavelength of the excitation light, assuming that it is shorter than the wavelength of the absorption onset. The bandwidth of the emission spectra, denoted as the Full Width at Half Maximum (FWHM) stems from the temperature, natural spectral line width of the quantum dots, and the size distribution of the population of quantum dots within a solution or matrix material. Spectral emission broadening due to size distribution is known as inhomogeneous broadening and is the largest contributor to the FWHM. Narrower size distributions yield smaller FWHM. For CdSe, a 5% size distribution corresponds to ~ 30nm FWHM.
Quantum Dots - Molecular Coupling
Colloidally prepared quantum dots are free floating and can be attached to a variety of molecules via metal coordinating functional groups. These groups include but are not limited to thiol, amine, nitrile, phosphine, phosphine oxide, phosphonic acid, carboxylic acid or others ligands. This ability greatly increases the flexibility of quantum dots with respect to the types of environments in which they can be applied. By bonding appropriate molecules to the surface, the quantum dots can be dispersed or dissolved in nearly any solvent or incorporated into a variety of inorganic and organic films. In addition, the surface chemistry can be used to effectively alter the properties of the quantum dot, including brightness and electronic lifetime.
Quantum Dots - Quantum Yield
The percentage of absorbed photons that result in an emitted photon is called Quantum Yield (QY). QY is controlled by the existence of nonradiative transition of electrons and holes between energy levels- transitions that produce no electromagnetic radiation. Nonradiative recombination largely occurs at the dot's surface and is therefore greatly influenced by the surface chemistry.
Adding Shells to Quantum Dots
It is established that capping a core quantum dot with a shell (several atomic layers of an inorganic wide band semiconductor) reduces nonradiative recombination and results in brighter emission, provided the shell is of a different semiconductor material with a wider bandgap than the Core semiconductor material.
The higher QY of Core-Shell quantum dots comes about due to changes in the surface chemistry of the core quantum dot. The surface of quantum dots that lack a shell has both free (unbonded) electrons, in addition to crystal defects. Both of these characteristics tend to reduce QY by allowing for nonradiative electron energy transitions at the surface. The addition of a shell reduces the opportunities for these nonradiative transitions by giving conduction band electrons an increased probability of directly relaxing to the valence band. The shell also neutralizes the effects of many types of surface defects.
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