Single Particle Spectroscopy of Quantum Dots and Energy Transfer from Quantum Dots into Silicon and TMDCs
Semiconductor nanocrystal quantum dots (NQDs) have long demonstrated potential in a broad range of optoelectronics applications such as lasers, light-emitting diodes (LEDs), photovoltaic solar cells, photo- and biosensors, etc. Exhibiting pronounced quantum-confinement effects (hence the popular name “quantum dots”), these nanocrystals feature discrete absorption signatures, tunable photophysical properties along with high photoluminescence (PL) emission quantum yield (PLQY) and the ability of easy chemical manipulation and processing. The ability to solution-process and assemble them on a variety of substrates and in various architectures enables their use in emerging functional materials. As confinement “squeezes” charge carriers into a shrinking volume, spatial co-location is influenced by strong Coulombic interactions and facilitates the formation of bound carrier states, the excitons. This induces size-dependent changes in the density of the electronic states, strongly affecting band-edge emission energy. Single e-h pair (exciton, X) occupies lowest bound state and typically undergoes radiative recombination (for high PLQY samples). Higher-order excitonic states such as charged excitons (trions) and neutral multiexcitons (MX) may be formed as well. Their behavior is, however, more complex as trions and MXs are typically affected by non-radiative Auger recombination that dramatically reduces their emission efficiency. Nevertheless, MX states are extremely important for lasing and solar conversion applications where they are thought to amend the Shockley – Queisser (SQ) limit of single-junction cell. Although a simple confinement picture allows capturing a number of salient features in the NQD’s behavior, the synthesized nanocrystals represent much more complex entities. In particular, synthetic nuances and surface chemistry play crucial part affecting PL properties and charge transfer in nanocrystal systems. The majority of NQD-based solid-state architectures are based on charge-transfer schemes where the quality and nature of surface ligands plays a defining role. Nevertheless, the emergence of hybrid systems based on non-radiative (near-field, NRET) and radiative (far-field, RET) energy transfer between the components has gained research attention. In particular, non-contact ET between NQDs and semiconductor substrates (Si, GaAs) have been demonstrated as a viable approach to harvest solar energy by avoiding some pitfalls of charge transfer based systems, such as low charge carrier mobility and interfacial trapping in NQD systems. In this dissertation, we start with a comparative experimental study of excitons and multiexcitons in large core/shell, “giant” CdSe/CdS NQDs. By using single particle PL spectroscopy, we observed the appearance of up to 8 well-defined intensity levels with discrete emission lifetimes at various excitation powers. By using scaling approach to describe Auger and radiative recombination rates, we assigned these states to singly and doubly charged exciton, biexciton and triexciton states. We have confirmed that Auger decay of the biexciton state is comprised of a superposition of the Auger decays of the negatively (X-) and positively (X+) charged excitons. Next, we employed such “giant” NQDs to study the energy transfer of trions and biexcitons into underlying crystalline Si. Using statistical analysis of thousands of PL lifetime traces at different emission wavelengths, we confirmed that the radiative decay rates for trions and biexcitons on all substrates scale according to the number of available recombination channels while Auger rates remain unchanged. We obtained ET transfer efficiencies of ca. 55% and 45% for biexcitons and trions, indicating the feasibility of multiexciton harvesting through energy transfer approach. To expand the range of possible interacting substrates, we studied ET from NQDs into 2D atomic monolayers of transition metal dichalcogenides (TMDC). Monolayer TMDCs have been recently discovered as a new class of semiconductor materials with high carrier mobility, direct optical bandgaps and large excitonic binding energies due to the reduced dielectric screening. As a result, such strong optical responses make TMDCs great candidates for novel types of optoelectronics devices with enhanced functionalities. Using both time-resolved PL and femtosecond transient pump-probe optical techniques, we studied near-field ET from CdSe NQDs into MoS2 monolayers. By monitoring PL lifetime quenching of NQD emission we recorded over 95% transfer efficiency into MoS2 flakes. Concurrently, we observed an order of magnitude enhancement of the PL emission from MoS2 monolayer by the energy influx from proximal NQDs. By directly monitoring femtosecond transient absorption dynamics of excitons in MoS2, we estimated 10 times increase of MoS2 PLQY by slow, ns-scale “feeding” of excitons from nearby NQDs, thus avoiding detrimental Auger recombination of primary excitons in MoS2. These observations confirm ET methods to be applicable to the design of novel optoelectronics devices based on 2D atomic layers.