A Study of Dissipative Phenomena in Semiconductor Nanocrystals
Quantum dots (QDs) are semiconductor nanoparticles, where carriers are confined in regions smaller than a few tens of nanometers. The physics governing the behavior of these nano structures are fundamentally different from their bulk counterpart. This thesis studies the dissipative phenomena in QDs. In chapter 1, I give a brief introduction of QDs and their carrier dissipation dynamics. In chapter 2, I show that in CuInS2/CdS QDs, the spontaneous emission (SE) lifetime evolves from 46 ns to ~ 300 ns over a 15 ps time scale due to the collapse of the hole to the intragap states through dissipation. This is also observed in other chalcopyrite QDs. The results are obtained employing upconversion photoluminescence (UPL) measurements. In chapter 3, I try to understand the dissipation dynamics in chalcopyrite QDs by theoretical modelling. The study confirms the ultrafast hole localization in the system due to strong electron-phonon coupling as observed experimentally. However, the system possesses a very high defect-assisted SE lifetime which suggests that along with the vibrational coupling, fine-structure participation also needs to be considered which arises due to the involvement of copper d-orbitals to the valence band of the QDs. In chapter 4, I try to regulate dissipation through controlling SE rate by activating alternative radiative channels. For that, I consider CuCdZnSe QD alloys. From the UPL measurement, I find that this scheme enables us to tune SE lifetimes by three orders of magnitude, from ~ 15 ns to over ~ 7 μs. In chapter 5, I probe the ultrafast carrier dynamics of CuAlS2/ZnS QDs, which directly convert aqueous solutions of bicarbonate ions to formate with remarkable efficiency (~ 20 %). Here I show that it is essentially dominated by ultrafast electron transfer (560 fs) to the surface. In addition, I observe that the electron dwell time in the conduction band increases with the excitation fluence which is reverse of the auger recombination. I further investigate this system through two-pump transient absorption which show that the electron dynamics are governed by the temporal evolution of the hole wave function. In chapter 6, I utilize the dissipation in QDs and build all-optical switching and all-optical logic gates implementing microbubble. The experiments are done using low power continuous-wave laser. In conclusion, I have studied the carrier dissipation dynamics in QDs and built all-optical switching and universal all-optical logic gates which paves the way for the design of photonic circuits.