Spectroscopic Studies of Semiconductor Nanocrystals: From Core, to Surface, and Beyond
Bibliographic record
Abstract
Semiconductor nanocrystals (NCs) have been actively investigated for the past 40 years due to their easily-tunable optical properties and rich, size-dependent photophysics. While technologies using NCs as the active material have already been commercialized, fundamental photophysical puzzles endure. Particularly, their optical properties often diverge from simple theoretical predictions. In this thesis, we explore the fundamental photophysics of two NC materials, CdSe and PbS. We begin by briefly reviewing canonical models for the electronic structure and optical properties of CdSe NCs. Then, we explore single-particle photophysics by developing a novel spectroscopic technique to interact with NC excited states. We show that all-optical modulation can perturb the ubiquitous ‘blinking’ phenomenon in CdSe/ZnS core-shell NCs. We demonstrate that this perturbation is timescale-free, characteristic, and selective for the ON-state. Next, we probe the kinetics of triplet energy transfer from core-only CdSe NCs to surface-bound anthracene moieties. Tailoring the energy level alignment between NC band-edges and frontier molecular orbitals, we find that sequential, hole-first carrier transfer outpaces correlated, Dexter-like exciton transfer when the barrier to the former is comparable to a few kBT. In tandem, measurements of energy transfer from surface-oriented trap states in the same NCs support a hypothesis where band-edge and trap states remain in dynamic equilibrium. From the known energy of the lowest-lying spin-triplet exciton, we infer a lower bound on the chemical potential of the surface state. Finally, we investigate the fundamental photophysics of ultrasmall PbS NCs, motivated by recent synthetic advances that have yielded smaller ensemble absorption linewidths. Our spectroscopic studies provide tests of models constructed from observations on larger PbS NCs. We discuss our observations in the context of prior descriptions for PbS electronic structure and propose that direct extrapolation from previous models is unsatisfactory—leading to a call for dedicated theoretical calculations in this size regime. We conclude by looking forward to studies which extend the present work and discuss key experiments to continue shedding light on the photophysics of semiconductor nanocrystals.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.001 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".