The Synthesis and Surface Chemistry of Colloidal Quantum Dots

Colloidal semiconductor nanocrystals, also known as quantum dots, are an extraordinary class of material, combining many of the most attractive properties of semiconductors with the practicality of solution chemistry. As such, they lie at a unique interface between inorganic chemistry, organic chemistry, solid-state physics, and colloidal chemistry. The rapid advance in knowledge of quantum dots over the past 30 years has largely been driven by interest in their fundamental physical properties and their broad applicability to challenges in nanoscience. However, much less attention has been paid to the chemistry underlying these features. In this dissertation, we discuss the state of nanocrystal chemistry and new insights we have unlocked by taking a bottom-up, chemistry-based approach to nanocrystal synthesis. We will cover these in a case-by-case fashion in the context of four chapters. Chapter 1 covers our CdTe nanocrystal synthesis surface chemistry studies with an eye toward CdTe photovoltaic technology, in which the role of CdTe surfaces is poorly understood. CdTe nanocrystals are traditionally a difficult material to synthesize, particularly with well-defined surface chemistry. In order to enable quantitative surface studies, we looked upstream and re-evaluated CdTe synthesis from the ground up. We identified a CdTe precursor largely overlooked since 1990, cadmium bis(phenyltellurolate) (Cd(TePh)2), and harnessed its excellent reactivity toward a synthesis of CdTe nanocrystals solely bound by cadmium carboxylate (Cd(O2CR)2) ligands. We then use this well-defined material to show that Cd(O2CR)2 ligands bind less tightly to CdTe nanocrystals than CdSe nanocrystals. This finding holds promise for the development of photovoltaics from colloidal CdTe feedstocks. Chapter 2 covers a tunable library of substituted thiourea precursors to metal sulfide nanocrystals. Controlling the size of nanocrystals produced in a given reaction is paramount to their use in opto-electronic devices, but the most widely used technique to control size is prematurely arresting crystal growth. We introduce a library of thiourea precursors whose organic substituents tune the rate of precursor conversion, which dictates the number of nanocrystals formed and the final nanocrystal size following complete precursor conversion. We use PbS as a model system to 1) demonstrate the concept of kinetically controlled nanocrystal size, 2) quantify substituent trends, and 3) optimize multigram scale syntheses. We then expand the thiourea methodology to a broad range of materials and nanocrystal morphologies. This work represents a paradigm shift that will greatly accelerate the pace of progress in nanocrystal science as it transitions from academia to a multibillion-dollar industry. Chapter 3 covers an analogously tunable library of substituted selenourea precursors, but focuses on the synthesis of PbSe nanocrystals. PbSe nanocrystal synthesis is notoriously low-yielding and poorly tunable, but the remarkable properties of PbSe nanocrystals in photovoltaics and electrical transport have driven interest in the material for decades. We develop a library of N,N,N’-trisubstituted selenourea precursors and leverage their fine conversion rate tunability to synthesize PbSe nanocrystals of many sizes in quantitative yields. Interestingly, the nanocrystals produced in this reaction are demonstrably less polydisperse than literature samples, exhibiting absorption linewidths approaching the single-particle limit. We quantify this narrowness using a transient absorption spectroscopy technique called spectral hole burning. Chapter 4 covers our efforts to dig deeper into nanocrystal nucleation and growth and use that new knowledge to develop luminescent downconverters ready for on-chip integration into LED lighting. By studying early time points in PbS and PbSe nanocrystal synthesis, we estimate solute concentrations, nucleation thresholds, and nanocrystal growth rates. In particular, we find that metal selenides and sulfides have very different nucleation and growth behavior, as well as that PbS nucleation is a surprisingly slow process. The lessons learned from these fundamental experiments have enabled us to rapidly develop red-emitting CdS/CdSe/CdS “spherical quantum well” emitters whose photoluminescence quantum yields are 90 – 95%

Layers, resources and property templates in the specification and analysis of two interactive systems

The paper briefly explores a layered approach to the analysis of two interactive systems (Nuclear Control and Air Traffic Control), indicating how the analysis enables exploration of the particular features emphasised by the use cases relating to the examples. These features relate to the interactive behaviour of the systems. To facilitate the analysis, property templates are proposed as heuristics for developing appropriate requirements for the respective user interfaces.Jose Creissac Campos and Michael Harrison were funded by ´ project ref. NORTE-07-0124-FEDER-000062, co-financed by the North Portugal Regional Operational Programme (ON.2 O Novo Norte), under the National Strategic Reference Framework (NSRF), through the European Regional Development Fund (ERDF), and by national funds, through the Portuguese foundation for science and technology (FCT). Paul Curzon, Michael Harrison and Paolo Masci were funded by the CHI+MED project: Multidisciplinary Computer Human Interaction Research for the design and safe use of interactive medical devices project, UK EPSRC Grant Number EP/G059063/1.info:eu-repo/semantics/publishedVersio

Automated theorem proving for the systematic analysis of an infusion pump

This paper describes the use of an automated theorem prover to analyse properties of interactive behaviour. It offers an alternative to model checking for the analysis of interactive systems. There are situations, for example when demonstrating safety, in which alternative complementary analyses provide assurance to the regulator. The rigour and detail offered by theorem proving makes it possible to explore features of the design of the interactive system, as modelled, beyond those that would be revealed using model checking. Theorem proving can also speed up proof in some circumstances. The paper illustrates how a theory generated as a basis for theorem proving (using PVS) was developed systematically from a MAL model used to model check the same properties. It also shows how the CTL properties used to check the original model can be translated into theorems

Subducting slab ultra-slow velocity layer coincident with silent earthquakes in southern Mexico

Great earthquakes have repeatedly occurred on the plate interface in a few shallow-dipping subduction zones where the subducting and overriding plates are strongly locked. Silent earthquakes (or slow slip events) were recently discovered at the down-dip extension of the locked zone and interact with the earthquake cycle. Here, we show that locally observed converted SP arrivals and teleseismic underside reflections that sample the top of the subducting plate in southern Mexico reveal that the ultra-slow velocity layer (USL) varies spatially (3 to 5 kilometers, with an S-wave velocity of ~2.0 to 2.7 kilometers per second). Most slow slip patches coincide with the presence of the USL, and they are bounded by the absence of the USL. The extent of the USL delineates the zone of transitional frictional behavior

Working hard on the outside: a multimodal critical discourse analysis of The Biggest Loser Australia

The Biggest Loser (TBL) is a reality television weight-loss programme that positions itself as a response to the so-called “obesity crisis”. Research on TBL has thus far focussed on audience responses and its effect on viewers’ beliefs about weight loss. This article focuses instead on how meaning is constructed in TBL. We conducted a multimodal critical discourse analysis of a key episode of TBL (the 2012 Australian season finale) to examine how the textual, visual and auditory elements combine to construct meanings beyond the ostensible health messages. Although the overt message is that all contestants have worked hard, turned their lives around and been “successful”, examination of editing choices, lighting and colour, clothing and time spent on contestants allows us to see that the programme constructs varying degrees of success between contestants and provides accounts for these differences in outcomes. In this way the programme is able to present itself as a putative celebration of all contestants while prescribing narrow limits around what constitutes success. TBL reinforces an ideology in which “success” is a direct result of “the work” of weight loss (both physical and emotional), which can apparently be read straightforwardly off the body. TBL’s “celebration” of weight loss thus reproduces and strengthens the widespread view of fat bodies as physical manifestations of individual (ir)responsibility and psychological dysfunction, and contributes to the ongoing stigmatisation of obesity

Horizontal subduction and truncation of the Cocos Plate beneath central Mexico

Based on analysis of data from a trans-Mexico temporary broadband seismic network centered on Mexico City, we report that the subducting Cocos Plate beneath central Mexico is horizontal, and tectonically underplates the base of the crust for a distance of 250 km from the trench. It is decoupled from the crust by a very thin low viscosity zone. The plate plunges into the mantle near Mexico City but is truncated at a depth of 500 km, probably due to an E-W propagating tear in the Cocos slab. Unlike the shallow slab subduction in Peru and Chile, there is active volcanism along the Trans Mexican Volcanic Belt (TMVB) that lies much further inland than regions to either side where subduction dip is not horizontal. Geodynamical modeling indicates that a thin weak layer such as imaged by the seismic experiment can explain the flat subduction geometry

Inhibition of αvβ5 Integrin Attenuates Vascular Permeability and Protects against Renal Ischemia-Reperfusion Injury

Ischemia-reperfusion injury (IRI) is a leading cause of AKI. This common clinical complication lacks effective therapies and can lead to the development of CKD. The αvβ5 integrin may have an important role in acute injury, including septic shock and acute lung injury. To examine its function in AKI, we utilized a specific function-blocking antibody to inhibit αvβ5 in a rat model of renal IRI. Pretreatment with this anti-αvβ5 antibody significantly reduced serum creatinine levels, diminished renal damage detected by histopathologic evaluation, and decreased levels of injury biomarkers. Notably, therapeutic treatment with the αvβ5 antibody 8 hours after IRI also provided protection from injury. Global gene expression profiling of post-ischemic kidneys showed that αvβ5 inhibition affected established injury markers and induced pathway alterations previously shown to be protective. Intravital imaging of post-ischemic kidneys revealed reduced vascular leak with αvβ5 antibody treatment. Immunostaining for αvβ5 in the kidney detected evident expression in perivascular cells, with negligible expression in the endothelium. Studies in a three-dimensional microfluidics system identified a pericyte-dependent role for αvβ5 in modulating vascular leak. Additional studies showed αvβ5 functions in the adhesion and migration of kidney pericytes in vitro Initial studies monitoring renal blood flow after IRI did not find significant effects with αvβ5 inhibition; however, future studies should explore the contribution of vasomotor effects. These studies identify a role for αvβ5 in modulating injury-induced renal vascular leak, possibly through effects on pericyte adhesion and migration, and reveal αvβ5 inhibition as a promising therapeutic strategy for AKI

Heterogeneous N2O5 Uptake During Winter: Aircraft Measurements During the 2015 WINTER Campaign and Critical Evaluation of Current Parameterizations

Nocturnal dinitrogen pentoxide (N2O5) heterogeneous chemistry impacts regional air quality and the distribution and lifetime of tropospheric oxidants. Formed from the oxidation of nitrogen oxides, N2O5 is heterogeneously lost to aerosol with a highly variable reaction probability, γ(N2O5), dependent on aerosol composition and ambient conditions. Reaction products include soluble nitrate (HNO3 or NO3−) and nitryl chloride (ClNO2). We report the first‐ever derivations of γ(N2O5) from ambient wintertime aircraft measurements in the critically important nocturnal residual boundary layer. Box modeling of the 2015 Wintertime INvestigation of Transport, Emissions, and Reactivity (WINTER) campaign over the eastern United States derived 2,876 individual γ(N2O5) values with a median value of 0.0143 and range of 2 × 10−5 to 0.1751. WINTER γ(N2O5) values exhibited the strongest correlation with aerosol water content, but weak correlations with other variables, such as aerosol nitrate and organics, suggesting a complex, nonlinear dependence on multiple factors, or an additional dependence on a nonobserved factor. This factor may be related to aerosol phase, morphology (i.e., core shell), or mixing state, none of which are commonly measured during aircraft field studies. Despite general agreement with previous laboratory observations, comparison of WINTER data with 14 literature parameterizations (used to predict γ(N2O5) in chemical transport models) confirms that none of the current methods reproduce the full range of γ(N2O5) values. Nine reproduce the WINTER median within a factor of 2. Presented here is the first field‐based, empirical parameterization of γ(N2O5), fit to WINTER data, based on the functional form of previous parameterizations

Forests for the New Millennium - MAKING FORESTS WORK FOR PEOPLE AND NATURE

THE WAYS IN WHICH FORESTS ARE PERCEIVED AND USED HAVE CHANGED DRAMATICALLY OVER RECENT YEARS. FORESTS ARE NO LONGER SEEN SIMPLY AS A SOURCE OF TIMBER, BUT AS COMPLEX ECOSYSTEMS WHICH SUSTAIN LIVELIHOODS AND PROVIDE A RANGE OF PRODUCTS AND ENVIRONMENTAL SERVICES. IT IS NOW WIDELY RECOGNISED THAT FORESTS CAN CONTRIBUTE TO RURAL DEVELOPMENT AND POVERTY ALLEVIATION.Forest, economics, livelihoods

Ultralong spin lifetime in one-dimensional semiconductor nanowires

We experimentally demonstrate ultralong spin lifetimes of electrons in the one-dimensional (1D) quantum limit of semiconductor nanowires. Optical probing of single wires of different diameters reveals an increase in the spin relaxation time by orders of magnitude as the electrons become increasingly confined until only a single 1D sub-band is populated after thermalization. We find the observed spin lifetimes of more than 200ns to result from the robustness of 1D electrons against major spin relaxation mechanisms, highlighting the promising potential of these wires for long-range transport of coherent spin information