UC Berkeley

The creation of novel technologies based on semiconductor nanomaterials relies on new insights into the chemistry and physics of these confined systems. These insights will help propel chemically synthesized semiconductor nanomaterials from a class of materials that

have been promised to revolutionize industries ranging from solar energy and carbon capture to medicine and quantum computing to a commercially viable class of materials. A major roadblock impeding nanomaterial-based applications is the lack of a detailed understanding of the many–body interactions in nanomaterials, in particular the quasiparticle–quasiparticle

(e.g. quasielectron–hole, exciton–exciton) interactions are not well understood.

In this work, we enhance our understanding of quasiparticle–quasiparticle interactions in confined semiconductor nanomaterials through the development of theories and computational methods capable of calculating observables in nanomaterials with hundreds to tens of thousands of atoms. Furthermore, we apply these methods, often in close collaboration with experimentalist, to nanomaterials ranging from quasi–0D single material quantum dots to quasi–2D heterostructure nanoplatelets in order to test the accuracy of the formalisms and improve our understanding of semiconductor nanomaterials.

In the introduction, a brief overview of semiconductor nanomaterials is given with a focus on the impact that the size and shape have on the properties of the nanomaterial. The important timescales and decay channels of excitons (i.e. bound quasielectron–hole pairs)

and biexcitons are reviewed in the introduction. In Chapter 2, we develop a new formalism for calculating Auger recombination lifetimes, which is typically the dominant decay channel of biexcitons in semiconductor nanomaterials. And we show that the inclusion of quasielectron−hole correlations in the new, interacting formalism results in it being the first quantitatively accurate formalism for calculating Auger recombination lifetimes for quantum dots in both the strong and weak confinement regimes. Furthermore, we highlight how the interacting formalism is the first theoretical method to postdict the experimentally known “universal volume scaling law” for quantum dots in Chapter 2. In Chapter 3, we develop a low–scaling approach based on the stochastic resolution of identity. We then elucidate the shell thickness and band alignment dependencies of the biexciton Auger recombination lifetime for quasi–type–II CdSe/CdS and type–I CdSe/ZnS core/shell quantum dots. We find that the biexciton Auger recombination lifetime increases with the total nanocrystal volume for quasi–type–II CdSe/CdS core/shell quantum dots and is independent of the shell thickness for type–I CdSe/ZnS core/shell quantum dots. The impact that growing shells with shorter lattice constants (CdS and ZnS) has on the emission energies of CdSe cores is also discussed. In Chapter 4, we report on the size dependence of Auger recombination in CdSe nanorods and that noninteracting formalisms are incapable of accurately predicting Auger recombination lifetimes in nanorods. On the other hand, we show that our interacting formalism is accurate in nanorods. We then utilize kinetic modeling to uncover a competition between the kinetics of Auger recombination and charge separation in a hybrid semiconductor–metal nanoparticle. Next, we discuss how the alloying of Zn into the CdS shell can improve the optoelectronic properties of seeded nanorods. In Chapter 5, we apply the stochastic implementation of the interacting formalism to quasi–2D nanoplatelets. We find that the Auger recombination lifetimes depend nearly linearly on the lateral area and thickness of the nanoplatelet. We also connect these scalings to those of the area and thickness dependencies of single exciton radiative recombination lifetimes, exciton coherence areas, and exciton Bohr radii in these quasi–2D materials.