Archive:Energy transfer between semiconductor nanostructure

FRET(Foster or fluorescence resonant energy transfer) is a non-radiative dipole dipole coupling mechanism governed to a large extent by the spectral overlap between the emission spectrum of the donor and the absorption spectrum of the acceptor, as well as by the spatial distance between donor and acceptor. Electronic transition of the donor involves a larger energy state <math>\Delta E_D = E^1_D - E^0_D </math> Compared to those of acceptor <math>\Delta E_A = E^1_A - E^0_A </math>

Energy transfer from donor to the acceptor is resonant that a transfer from the excited donor level could be possible in the vibronically excited state of the acceptor

this excess vibronic energy is readily dissipated typically within less than a picoseconds, leaving the acceptor molecule in state . energy dissipation is much faster than the typical time scale of the origin resonant energy transfer. Energy transfer rate <math>R_{ET}</math> by foster

In components of <math>R_{ET}</math>, c denotes the speed of light, and n is the refractive index of the medium which hosts donor and acceptor molecules. k2 is a parameter given by the mutual orientation of the two dipoles of the acceptor and the donor. Energy Transfer depends on the distance between the donor and the acceptor and overlap integral which include the information about integral-normalized fluorescence spectrum of the donor and absorption cross section of the acceptor molecules. In the case of a point like acceptor molecules, Energy Transfer is affected on the inverse of the sixth power of d. When donor molecule resides in front of a bulk layer of acceptor, energy transfer rate is less sensitive to the distance so the inverse of the power of distance decreases to third power. Most of semiconductor nanocrystals which is considered as donor are placed in front of 2-dimesion assemblier of acceptor nanocrystals. Only under the condition is satisfied that a point like a molecule as donor and acceptor – the dimensions of the molecules are much smaller than the distance between them and both are much smaller that the wavelength of the associated transition energies). Nanocrystals (NCs) have been developed for their size dependent optical properties and synthesized with wet-chemistry method of highly emissive monodisperse NCs of variable size and compositions. NCs are considered as efficient inorganic dyes with enhanced photostability an tunable surface chemistry. Their absorption and emission spectra are controlled by chemical composition and by size due to quantum confinement effect.

FRET related studies of NCs in solution have capability of more efficient FRET based- sensor, while investigations on solid state structures may allow for the creation of novel light- harvesting devices.

Energy transfer form semiconductor to metal

Gold nanoparticle has generally been applied to be efficient quenchers of semiconductor nanocrystals emission in solution based assemblies and solid sample.

By means of the influence of plasmonic coupling on the exciton lifetime, the distance between a semiconductor CdTe nanowire and a gold nanoparticle could be detected. This method would be available to monitor the movement of biomolecules in living cells since the exciton lifetime in a semiconductor nanowire determines the emission wavelength. The longer the exciton lives, the lager the red shift will be

Fig.13 Exciton diffuses along a semiconductor nanowire toward the potential minima where exciton could be recombined. They will be trapped in the regions of lower band gap due to inhomogeneous thickness of the wire. Energy transfer of a nearby metal nanoparticle brings out the exciton lifetime shorten. This makes the emission wavelength blue-shifts as the metal nanoparticle approaches the wire. Otherwise red-shift arises when the metal nanoparticle moves away.

Energy transfer with semiconductor nanocrystals (NCs)

Foster model is valid under condition of point dipole (distance between the dipoles is much larger than the spatial extension of the dipoles). But in NCs case, energy transfer couldn`t be applied. The oscillating dipoles in NCs can`t be considering as point dipoles and local field coming from the surface charges can be affected. But the Forster formula can be used to give trends in comparing transfer rates between fluorophores with semiconductor NCs. the radiative lifetime of a fluorophore is of the order of 1 ns, while semiconductor NCs tend to have substantially larger radiative lifetimes which in turn tend to decrease the FRET rate in the case of semiconductor NCs compared to molecular fluorophores. This is compensated by the usually much larger absorption cross section of accepting semiconductor NCs compared to accepting fluorophores. Consequently the rate of Forster energy transfer is of the same order of magnitude for both semiconductor NCs and fluorophores.

Energy transfer in thin solid films containing semiconductor nanocrystals

Temperature dependence of the energy transfer rate and efficiency of the energy transfer process in CdTd NC solids shows that energy transfer process is relate to a dipole-dipole (or multipole-multipole interaction). The oscillator strengths of the transition between the donor and acceptor is proportional to the energy transfer rate increased in a dipole-dipole interaction.

Since both the transfer rate and the radiative decay of the donor are proportional to the oscillator strength of the transition on the donor, the transfer efficiency (or the ratio of the donor/ acceptor emission intensity) is expected to be independent of the oscillator strength of the transition on the donor. In the case of CdTe NCs the oscillator strength of the donor transition can be easily tuned by cooling the NCs and freezing the system partly in the dark (non-emitting) state. The spectral shift involved is small and will not influence the transitions involved on the (larger) acceptor NCs.

Fig.4 shows the temperature dependence of the donor emission decay curves for orange emitting CdTe NCs (3 nm) in chloroform (A, B) at low concentration (no energy transfer expected) and as a NC solid on quartz (C, D) (energy transfer possible). Decay curves were recorded as a function of temperature at 20K and 200K in the peak maximum (green) from the short wavelength side to long wavelength side of the inhomogeneously broadened emission band. The energy transfer rate can be observed in the range of the faster decay curves in the solid for the short wavelength side (donors) to the rise time on the long wavelength side (acceptors). The decay curves in the solvent (no energy transfer) serve as a reference. Fig.4 shows that the energy transfer rate is proportional to the radiative decay rate of the donor, satisfied with a dipole–dipole energy transfer mechanism.

The temperature has an effect on the transfer rate in mixed solids of donor (green) and acceptor (orange) NC systems. The transfer rates are observed to scale with the donor emission decay rate while the transfer efficiency is not influenced by temperature

See also

Quantum Dot Semiconductors Förster resonance energy transfer(FRET)

References

Andrey L. Rocache, Energy transfer with semiconductor nanocrystal, J.Mater.Chem.,2009,19,1208-1221

Aaron R. Clapp, Forster resonance Energy Transfer Investigations Using Quantum Dot Fluorophore, ChemPhysChem 2006,7,47-57

S.F.Wuister, Journal of Physical Chemistry B,2005,109,5504-5508