Manipulation Of Structure And Electro-optical Properties Of Colloidal Semiconductor Nanocrystals

When the size of the semiconductor crystal is smaller than its exciton bohr radius in three dimensions,both the conduction and the valance band of the crystal evolve into atom-like discrete band structures due to the quantum confinement effect.The band gap energy is thus size-dependent.Since this kind of semiconductor crystals are usually of the size of several nanometers,they are also called semiconductor nanocrystals.The nanometer-sized boundaries enhance the overlap of the carrier wave function in the nanocrystal,so that the coulomb interactions between carries are tremendously strengthened,and the discrete energy level breaks the conservation of momentum in the nanocrystal,resulting in strong Auger effect in the nanocrystals.Fast Auger nonradiative process significantly reduces the possibility of multi-exciton emission,making nanocrystal an ideal single photon source.The size-tunable band gap energy makes semiconductor nanocrystals promising materials for LED,lasers,solar cells etc.The stable single photon source is the basic unit needed in the fields of quantum information processing and quantum communication.With the emergence of new semiconductor nanocrystalline materials and structures,the opto-electronic properties of semiconductor nanocrystals have also been continuously improved.In-depth understanding of the carrier dynamics process in semiconductor nanocrystals not only provides technical support for nanocrystals in the application field,but also provides guidance for the synthesis and structural design of nanocrystals.This paper mainly studies the interaction between nanocrystals,the fluorescence characteristics of single nanocrystals under two-photon excitation and the nanocrystalline optical properties under electric field control.Interactions between nanocrystals significantly influence the performance of the devices like LED and solar cells,in which nanocrystals need to be closely packed.In chapter ?,we introduced the studies of interactions between CdSe/CdS nanocrystals.In the previous works,it was difficult to extract fluoresce information of a single nanocrystal due to the average effect of the ensemble.We chemically synthesized nanocrystal clusters containing 3-5 giant CdSe/CdS core-shell nanocrystals.The coreto-core distance is about 18 nm.Nanocrystal clusters show non-blinking fluorescence emission at room temperature.Even though each cluster contains more than one nanocrystal,it still exhibits single photon emission.We can deduce from the fluorescence saturation curve that every single nanocrystal could absorb photons by its own,so there must exit some exciton-exciton annihilating effects between the nanocrystal to make sure only one exciton is eventually recombined by emitting a photon.Calculation results imply an inefficient fluorescence resonant energy transferring process to guarantee single photon emission in our CdSe/CdS clusters.Single photon emission is well maintained at cryogenic temperature.In a single cluster,3-5 spectrum peaks could be clearly distinct,each of which have a similar fluorescence life time,indicating the absence of fluorescence resonant energy transferring.We attribute this single photon emission phenomenon to inter-dot Auger process.When used as single photon source,nanocrystals are required to maintain the coherent properties of the exactions.This makes resonant excitation of the nanocrystals a certain way to avoid the phonon induced decoherence during the thermal relaxation process.Two photon excitation of the nanocrystals can efficiently cancel the background scattering due to the large wavelength difference between the laser and the fluorescence,offering a possibility for background free resonant excitation of nanocrystals.In chapter ?,we studied optical properties of single CsPbI3 nanocrystal under non-resonant and resonant two-photon excitation.Two-photon absorption crosssection(ACSTP)of single nanocrystal is estimated from the fluorescence saturation curve excluding the influence of inaccuracy in single photon absorption cross section which is a key parameter in traditional ACSTP measurement.The ACSTP of single CsPbI3 is as large as 9.2 × 105 GM,one or two magnitude higher than traditional nanocrystals like CdSe.No significant difference is observed in fluorescence blinking and exciton lifetime between single-photon and two-photon excitation measurement.Single photon emission under two-photon excitation is fulfilled.At cryogenic temperature,the spectrum of single nanocrystal under two photon excitation contain two fine fluorescence peaks.We mainly studied the PLE of the nanocrystals under twophoton excitation.When two-photon energy is resonant with the nanocrystal,excitonenhanced second harmonic generation is observed.The influence of electric field on nanocrystals are of importance in integrated optoelectronic device.Meanwhile,electric field offers a powerful tool to manipulate optical properties of nanocrystals.In chapter ?,we studied the influence of electric field on optical properties of single CsPbI3 nanocrystal at cryogenic temperature.Due to the quantum confined stark effect,the energy of fluorescence photon varies as the electric field changes from negative side to positive side.The monotonous spectra shifting indicates a permanent built-in dipole moment in CsPbI3 nanocrystals.Electric field also eliminates the fine structure splitting which is caused by electron-hole anisotropic exchange interaction.Since the electric field changes the interaction between carriers,the biexciton binding energy can also be effectively manipulated.