Luminescence and Exciton Dynamics in Semiconductor Nanocrystals: from Single Particles to Organized Ensembles
Embargoed until 2024-06-24
- Doctoral Thesis
Semiconductor nanocrystals (NCs) are particles on the nanometer scale that are made of crystalline semiconducting material. Chemical composition, size, and shape dictate their optical properties, making them technologically relevant for current and next-generation optoelectronic devices. Their application requires a fundamental understanding of their optical behavior. In this thesis, we contribute to the knowledge behind light generation in semiconductor NCs. We provide methodologies to study the processes that drive the fluorescence of well-established semiconductor NCs and characterize newly introduced nanomaterials. Furthermore, we investigate the optical behavior of solids made of three-dimensional lattices of NCs. First, we demonstrate an experimental method to study complex excited-state dynamics of semiconductor NCs. Our approach is based on a modification of the local NC photonic environment, which controls the NC spontaneous emission rate. This is obtained by placing NCs on top of a layer of variable thickness that lies on a reflective surface. Depending on the distance of the NCs from the reflector, the emission rate is enhanced or inhibited. By collecting and modeling the time-dependent NCs emission, we then extrapolate the mechanisms and parameters that control it. We study two classes of semiconductor NCs that feature complex light-generation dynamics. First, we analyze the low-temperature emission of CdSe quantum dots (QDs) and obtain information on the efficiency and polarization properties of their fine-structure dynamics. Second, we clarify the charge-carrier trapping mechanisms that cause slow emission in CdSe nanoplatelets (NPLs). Our findings highlight how modification of photonic environments can augment conventional time-resolved experiments. Thus, it adds another parameter like temperature and magnetic field to disentangle complex dynamics in NCs. Second, we investigate the origin of the slower fraction of emission of individual CsPbBr3 NCs at low temperatures. Time-dependent spectra allowed us to discern the moments when the exciton is neutral or charged (trion). Simultaneous measure-ments of emission spectra and decay curves show fast emission dynamics during both exciton and trion moments. However, an additional unexpected longer decay component characterizes the exciton emission. To understand its origin, we measure the relative polarization properties of the fast and slow NC intensity by sorting the emitted photons based on their emission time scales. We find that early and late emissions do not always share the same polarization, suggesting that the late intensity might have contributions from photons originating from a long-lived dark state of CsPbBr3 NCs. Our approach, based on discerning the emission polarization at different time scales, could be generalized to other NCs that present puzzling decay components. Third, we study the emission of structured solids obtained by spontaneous self-assembly of NCs into three-dimensional lattices, i.e., supercrystals (SCs). By mapping the photoluminescence (PL) across the SC surface, we observe an unusual distribution of the spectrum peak wavelength. We find that the emission spectrum of the NCs located at the center of the SC is redshifted compared to the SC edge. Moreover, this shift is gradual across the SC surface. Performing additional experiments, including measurements of the spatially dependent PL spectrum on the SC surface in contact with the substrate and on mechanically separated SCs, we attribute the spectral shifts to differences in the optical gap of the NCs composing individual SCs. We further propose that the differences in emission wavelength arise from the size segregation of the NCs within the SC. Namely, larger- and smaller-sized NCs are located at the center and edge regions of the SCs, respectively. These findings might be the starting point to develop an understanding of the mechanisms that drive the assembly of CsPbBr3 NCs into SCs. Fourth, we consider the low-temperature emission of individual SCs of CsPbBr3 NCs. In particular, we investigate the origin of a puzzling emission spectrum located at lower energies than the exciton peak. We observe that this spectral feature is localized in certain regions of the SCs and the areas surrounding the SCs. Moreover, the spectra of SCs and of aged disordered films of NCs share similar features. Thus, we conjecture that damaged and coalesced NCs present in our SCs might cause the low-energy emission peak. To corroborate our hypothesis, we produce SCs made of smaller-sized NCs and measure their low-temperature PL spectrum. Here, we find a low-energy emission peak similar to that measured in SCs of larger-sized NCs, localized on micrometer-sized particles present on the SCs and in the regions surrounding them. Thus, we conclude that the spontaneous formation of bulk particles of CsPbBr3 causes the secondary low-energy peak in the PL spectrum of the SCs. Our findings provide a simple answer to explain the enigmatic low-temperature emission spectrum of SCs of CsPbBr3 NCs and highlight the problem of the chemical instability of such NCs. In summary, this thesis investigates the collective and individual optical properties of different types of NCs and provides an experimental approach to studying NC emission dynamics. Show more
External linksSearch print copy at ETH Library
ContributorsExaminer: Norris, David J.
Examiner: Meijerink, Andries
Examiner: Wood, Vanessa
Examiner: Urban, Alexander
SubjectSpectroscopy; Optical materials; Nanocrystals; Nanotechnology; Nanooptics; Quantum dots; Excited-state dynamics; Fluorescence; Supercrystals; Cryogenics; Self-assembly; Excitons
Organisational unit03875 - Norris, David J. / Norris, David J.
03875 - Norris, David J. / Norris, David J.
MoreShow all metadata