Doctor of Philosophy

dissertationThe forefront of current nanoscience initiatives includes the investigation and development of semiconducting colloidal nanocrystals for optoelectronic device concepts. Being highly facile in their synthesis, a wide range of sizes, morphologies, materials, interactions, and effects can easily be engineered by current synthetic chemists. Their solution-processability also makes available the use of long established industrial fabrication techniques such as reel-to-reel processing or even simple inkjet printing, offering the prospect of extremely cheap device manufacturing. Aside from anticipated technologies, this material class also makes available a type of "playground" for generating and observing novel quantum effects within reduced dimensions. Since the surface-to-volume ratio is very large in these systems, unsatisfied surface states are able to dominate the energetics of these particles. Although simple methods for satisfying such states are usually employed, they have proven to be only semieffective, often due to a significant change in surface stoichiometry caused by complex atomic reorganization. Serving as charge "trap" states, their effect on observables is readily seen, for instance, in single particle photoluminescence (PL) blinking. Unfortunately, most methods used to observe their influence are inherently blind to the chemical identity of these sites. In absence of such structural information, systematically engineering a robust passivation system becomes problematic. The development of pulsed optically detected magnetic resonance (pODMR) as a method for directly addressing the chemical nature of optically active charges while under trapping conditions is the primary tenet of this thesis. By taking advantage of this technique, a great wealth of knowledge becomes immediately accessible to the researcher. The first chapter of this work imparts the relevant background needed to pursue spin resonance studies in colloidal nanocrystals; the second chapter addresses technical aspects of these studies. In Chapter 3, pODMR is used to explore shallow trap states that dominate the charge transfer process in CdSe/CdS heterostructure nanocrystals. Several trapping channels are observed, while two in particular are correlated, demonstrating for the first time that both electrons and holes are able to be trapped within the same nanoparticle at the same time. The intrinsically long spin coherence lifetime for these states allows for the spin multiplicity and degree of isolation to be explored. Demonstration of novel effects is also performed, such as coherent control of the light-harvesting process and remote readout of spin information. The study presented in Chapter 4 focuses on the spin-dependencies observed in the historically ill-described emissive CdS defect. By monitoring deep-level emission from nanorods of this material, it is shown that the cluster defect can ultimately be fed by the same shallow trap states explored in Chapter 3. The degree of interaction between trap states and the cluster defect is probed. Also, a surprisingly long spin coherence lifetime (T2 « 1.6 /is) for the defect itself is observed, which opens the possibility of highly precise chemical fingerprinting through electron spin echo envelop modulation (ESEEM). This dissertation lays the groundwork for further use of these, and more powerful magnetic resonance probes of the states that fundamentally limit the practical utility of colloidal nanocrystal optoelectronics devices. Furthermore, by gaining access to these optically active electronic states, novel methods of coherent quantum control may be exerted on the energetics of this material system

Magnetic resonance of paramagnetically doped materials

Colloidal quantum dots (QDs) allow for the tuning of dopant concentration as well as flexibility in the engineering of the surrounding medium. This thesis explores the use of magnetic resonance techniques and the development of hardware in order to characterize paramagnetically doped materials, in particular Mn-doped PbS colloidal QDs, and assess their potential for applications in quantum technologies such as quantum information processing (QIP). Colloidal PbS:Mn QDs capped with thioglycerol/dithiolglycerol ligands were synthesised in aqueous solution. Methods of tailoring the Mn-Mn and Mn-1H interactions, with the aim of maximizing phase memory times, were investigated. The distance between spins was optimized by initially, overgrowing the QDs with an undoped shell and secondly, by dispersing the QDs in solution. The use of a deuterated solution was found to further reduce the dephasing effects of Mn-1H interactions. This resulted in unprecedentedly long phase memory (TM ~ 8 μs) and spin–lattice relaxation (T1 ~ 10 ms) time constants for Mn2+ ions at T= 4.5 K, and in the observation of electron spin coherence (TM ~ 1 μs) near room temperature. Further improvements to relaxation times, as well as enhanced optical properties useful for the initialization and readout of spin qubits, were also studied by embedding the QDs in photonic crystals. Magnetic resonance techniques combined with paramagnetic Mn-impurities in PbS QDs are used for sensitive probing of the QD surface and environment. We report inequivalent proton spin relaxations of the capping ligands and solvent molecules. We determine the strengths and anisotropies of the Mn-1H spin interactions, and establish Mn-1H distances with ~1 Å sensitivity. These findings demonstrate the potential of magnetically doped QDs as sensitive magnetic nano-probes and the use of electron spins for surface sensing. We explore a means of characterizing mechanisms responsible for the functionality of paramagnetically doped materials. The development of instrumentation to identify and quantify interactions between paramagnetic and ordered magnetic phases is described. A probe was designed and built with a fast response time and with the aim of facilitating fast field jump experiments to identifying interactions between the different magnetic phases by correlating the response of a sample to mw irradiation with its response to a field jump

Magnetic resonance of paramagnetically doped materials

Colloidal quantum dots (QDs) allow for the tuning of dopant concentration as well as flexibility in the engineering of the surrounding medium. This thesis explores the use of magnetic resonance techniques and the development of hardware in order to characterize paramagnetically doped materials, in particular Mn-doped PbS colloidal QDs, and assess their potential for applications in quantum technologies such as quantum information processing (QIP). Colloidal PbS:Mn QDs capped with thioglycerol/dithiolglycerol ligands were synthesised in aqueous solution. Methods of tailoring the Mn-Mn and Mn-1H interactions, with the aim of maximizing phase memory times, were investigated. The distance between spins was optimized by initially, overgrowing the QDs with an undoped shell and secondly, by dispersing the QDs in solution. The use of a deuterated solution was found to further reduce the dephasing effects of Mn-1H interactions. This resulted in unprecedentedly long phase memory (TM ~ 8 μs) and spin–lattice relaxation (T1 ~ 10 ms) time constants for Mn2+ ions at T= 4.5 K, and in the observation of electron spin coherence (TM ~ 1 μs) near room temperature. Further improvements to relaxation times, as well as enhanced optical properties useful for the initialization and readout of spin qubits, were also studied by embedding the QDs in photonic crystals. Magnetic resonance techniques combined with paramagnetic Mn-impurities in PbS QDs are used for sensitive probing of the QD surface and environment. We report inequivalent proton spin relaxations of the capping ligands and solvent molecules. We determine the strengths and anisotropies of the Mn-1H spin interactions, and establish Mn-1H distances with ~1 Å sensitivity. These findings demonstrate the potential of magnetically doped QDs as sensitive magnetic nano-probes and the use of electron spins for surface sensing. We explore a means of characterizing mechanisms responsible for the functionality of paramagnetically doped materials. The development of instrumentation to identify and quantify interactions between paramagnetic and ordered magnetic phases is described. A probe was designed and built with a fast response time and with the aim of facilitating fast field jump experiments to identifying interactions between the different magnetic phases by correlating the response of a sample to mw irradiation with its response to a field jump

Photon statistics and power-law blinking of single semiconductor nanocrystals

Al voordat het grote publiek kennismaakte met nanotechnologie, besteedde de wetenschap aandacht aan steeds kleinere objecten. Sinds 15 jaar is het bijvoorbeeld mogelijk om met speciale microscopen naar individuele moleculen te kijken! Deze techniek hebben we nu gebruikt om halfgeleider nanokristallen te bestuderen. Halfgeleiders zijn materialen die worden gebruikt in computers. Afhankelijk van hun bewerking en toepassing kunnen zij stroom goed of slecht geleiden. Maar deze nanokristallen zijn zo klein, slechts 5 nanometer (1 miljoen nm = 1 mm) en dus veel kleiner dan de golflengte van licht, dat ze zich heel anders gedragen. Ze kunnen bijvoorbeeld licht absorberen en uitzenden (fluoresceren), maar omdat ze zo klein zijn altijd slechts __n lichtdeeltje (foton) tegelijk, net als een molecuul. Ook het ritme (de statistiek) waarmee dit gebeurt, is heel bijzonder en vergelijkbaar met de statistiek waarmee aardbevingen voorkomen. De pauze tussen het uitzenden van twee fotonen varieert bijvoorbeeld zo sterk, dat we de gemiddelde pauze niet kunnen uitrekenen! Dankzij diverse wiskundige trucs kunnen we dit ritme nu toch beschrijven. Deze wiskunde is ook handig om verschillende experimenten te kunnen vergelijken, zowel voor nanokristallen als voor moleculen. Vervolgens hebben we een model opgesteld dat beschrijft wat er in nanokristallen gebeurt, als een foton wordt geabsorbeerd of uitgezonden.FOMUBL - phd migration 201

Revealing the Surface Structure of CdSe Nanocrystals by Dynamic Nuclear Polarization-Enhanced 77Se and 113Cd Solid-State NMR Spectroscopy

Dynamic nuclear polarization (DNP) solid-state NMR (SSNMR) spectroscopy was used to obtain detailed surface structures of zinc blende CdSe nanocrystals (NCs) with plate or spheroidal morphologies and which are capped by carboxylic acid ligands. 1D 113Cd and 77Se cross-polarization magic angle spinning (CPMAS) NMR spectra revealed distinct signals from Cd and Se atoms on the surface of the NCs, and those residing in bulk-like environments below the surface. 113Cd cross-polarization magic-angle-turning (CP-MAT) experiments identified CdSe3O, CdSe2O2, and CdSeO3 Cd coordination environments on the surface of the NCs, where the oxygen atoms are presumably from coordinated carboxylate ligands. The sensitivity gain from DNP enabled natural isotopic abundance 2D homonuclear 113Cd-113Cd and 77Se-77Se and heteronuclear 113Cd-77Se scalar correlation solid-state NMR experiments that reveal the connectivity of the Cd and Se atoms. Importantly, 77Se{113Cd} scalar heteronuclear multiple quantum coherence (J-HMQC) experiments were used to selectively measure one-bond 77Se-113Cd scalar coupling constants (1J(77Se, 113Cd)). With knowledge of 1J(77Se, 113Cd), heteronuclear 77Se{113Cd} spin echo (J-resolved) NMR experiments were then used to determine the number of Cd atoms bonded to Se atoms and vice versa. The J-resolved experiments directly confirmed that major Cd and Se surface species have CdSe2O2 and SeCd4 stoichiometries, respectively. Considering the crystal structure of zinc blende CdSe, and the similarity of the solid-state NMR data for the platelets and spheroids, we conclude that the surface of the spheroidal CdSe NCs is primarily composed of {100} facets. The methods outlined here will generally be applicable to obtain detailed surface structures of various main group semiconductors

Investigation of the interactions between selected nanoparticles and human lung carcinoma cells at the single cell and single particle level

The recent advances in nanomaterials development and applications have sparked concerns regarding the safety of these materials in living organisms. This body of work investigates specific interactions between chosen nanoparticles and living human lung carcinoma (A549) cells --Abstract, page iv

Theoretical characterisation of spheroidal PbSe/PbS Core/Shell colloidal quantum dot heterostructures

Nanocrystal quantum dots (NQDs) show great promise in the advancement of the field of photovoltaics. While the maximum efficiency of conventional solar cell (SC) devices is limited to ∼ 31% (Shockley-Queisser limit), devices based on NQDs may attain a maximal thermodynamic efficiency of 42% through the exploitation of multiple exciton generation (MEG). In this process, several electron-hole pairs are created by the absorption of a single high energy photon, as opposed to the single excitons created in conventional solar cell devices. IV-VI semiconductor nanocrystals (PbS, PbSe) are of particular interest as candidates for the exploitation of MEG due to the narrow band gap, high confinement energies, and long radiative carrier lifetimes observed in these systems. In order to realise the full potential of MEG devices, full characterisation of the optoelectronic properties of the underlying nanoparticles is desirable. While the size-dependent properties of NQDs are well understood, the effects of NQD shape are less so. This thesis investigates the effect of ellipticity on the optoelectronic properties associated with spheroidal NQDs. To this end, a four-band, anisotropic, and radially variant k · p system Hamiltonian is expanded in a planewave basis in order to calculate single-particle eigenenergies and eigenfunctions of colloidal PbSe/PbS core/shell heterogeneous NQDs of varying ellipticity. Many-body effects are accounted for via a full configuration interaction (CI) Hamiltonian, the basis of which is comprised of the single-particle states. Exci-tonic and bi-excitonic corrections are then found by mixing of the basis states. In this manner, such diverse electronic and optical properties as quasi-particle binding energies, momentum matrix elements, and charge carrier lifetimes, both radiative and non-radiative, may be predicted