Raktim Baruah

• CdSe quantum dots (QDs), owing to their size tunable optoelectronic properties, narrow photoluminescence linewidth, high photoluminescence quantum yields etc., have been a prime interest of intensive research for numerous applications, including light-emitting diodes, solar concentrators, photocatalysis, photovoltaics, and nanocrystal lasers. Upon photoexcitation, QDs generate a pair of an electron and a hole, referred as exciton, and under high excitation density photoexcitation, multiexcitons (multiple pairs of interacting excitons) are formed. The dynamics of excitons and multiexcitons govern the photophysics behind QD-based applications. For example, effective separation of electron and hole enhances photocatalytic efficiency, while multiexcitons with long lifetimes enhance the optical gain, thereby increasing laser performance. In recent years, it has been demonstrated that the properties of excitons and multiexcitons can be modified by altering the size and the surface properties of QDs. To develop a suitable material for optoelectronic applications, it is essential to gain a comprehensive understanding of the mechanisms underlying the size and surface modification induced changes in exciton and multiexciton properties. With this motivation, the thesis ‘Exciton and Multiple Exciton Dynamics in Colloidal Semiconductor Nanocrystals’, investigates the influence of QD size, surface modification, and inter-QD coupling on exciton and multiexciton properties in solution and thin film. To achieve this, colloidal tri-octylphosphine oxide (TOPO)-capped CdSe QDs were synthesized via the hot injection method, which provided the advantages of size tunability with high precision, surface modification, and solution processing. Surface modification of the TOPO capped QDs were performed via ligand exchange method optimized in this work to prepare mercaptoacids and sulfide (S2−) capped QDs. Strong photoluminescence quenching and enhanced trap-state emission were observed with these surface ligands, having the most pronounced effect with S2−, attributed to hole trapping at surface sites introduced by the ligands, which is evaluated by the extent of luminescence quenching. Additionally, the acoustic phonon modes of the CdSe QDs with these surface ligands were monitored using Raman spectroscopy, as the acoustic phonon vibrations influence both trapping processes and photoluminescence linewidth. A mass loading effect was observed in the acoustic phonon mode frequencies, with lower phonon frequencies noted in heavier surface ligands due to the mechanical strain caused by the surface ligands. The S2− capped QDs, exhibiting the strongest effect on photoluminescence and phonon properties, were subjected to further investigation of exciton and multiexciton dynamics in comparison to the TOPO capped QDs. Intensity-dependent transient absorption (TA) spectroscopy was performed to evaluate the spectral and dynamical features of excitons and multiexcitons using a global fitting method based on Markov Chain Monte Carlo (MCMC) sampling. Through this approach, multiexcitons up to tetraexcitons were examined by fitting the Auger recombination model. The multiexciton lifetimes in both TOPO and S2− capped QDs exhibit a cubic relationship with QD radius, consistent with previous literature. However, longer multiexciton lifetimes were observed in S2− capped QDs compared to TOPO capped QDs, attributed to suppression of Auger recombination due to reduced carrier wavefunction overlap, originating from strong surface hole trapping in the S2− capped QDs. Due to this, the multiexciton binding energies also were reduced in the S2− capped QDs resulting in significant spectral difference compared to the respective TOPO capped QDs. Moreover, the exciton and multiexciton properties of TOPO and S2− capped QD thin films were also investigated through intensity-dependent TA spectroscopy. Thin films were fabricated by drop casting the colloidal QDs. In S2− capped QD thin films, a strong electronic coupling between the close packed QDs were observed resulting in a significant red shift in the photoluminescence peak position, while this effect is minimal in the TOPO capped QDs as prevented by the steric hindrance caused by the long-chained TOPO ligands keeping the QDs apart. The intensity-dependent TA spectra and dynamics of the TOPO capped QD thin films follow a similar multiexciton recombination model as compared to the respective QDs in solution. However, in the S2− capped QD thin films, the typical spectral signatures of multiexciton recombination were not observed in contrast to the corresponding solutions. Rather, the unique spectral shapes and kinetic traces in the S2− capped QD thin films suggest that the multiexcitons undergo exciton delocalization in the ultrafast time regime dominating over the Auger recombination process. Furthermore, the fabrication of QD thin films embedded in a porous silica matrix is demonstrated as a novel approach for the development of thin film materials containing immobilized QDs for optoelectronic applications. Two methods were employed for this purpose: in-situ growth of CdSe NCs in a porous silica matrix and infiltration of pre-synthesized CdSe QDs into the porous matrix. The Successive Ionic Layer Adsorption and Reaction (SILAR) method was employed for the in-situ growth of CdSe NCs within the porous silica matrix. However, the uncontrolled growth observed in the SILAR method results in a broad distribution of NC sizes, which in turn gives rise to broad absorption features and the absence of photoluminescence. This is attributed to the high density of defect states present in the material. To address this challenge, QDs were synthesized via the hot-injection method and infiltrated into the porous matrix through the pores of the silica matrix. The infiltration was achieved in two ways: by drop casting a QD solution on the porous layers or by soaking the porous layers in a QD solution. The QDs infiltrated via these two methods retain their luminescent properties, resulting in thin films of high quality and well-defined immobilized QDs suitable for integration into optoelectronic devices. The findings of this thesis contribute to the advancement of QD-based optoelectronic applications by providing insights and avenues for the development of novel materials.