Fully coordinated silica nanoclusters: (SiO2)(N) molecular rings

A new form of finite silica with edge-sharing SiO2 units connected in a ring is proposed. High-level density-functional calculations for (SiO2)(N), N = 4-14, show the rings to be energetically more stable than the corresponding (SiO2)(N) linear chains for N > 11. The rings display frequency modes in remarkable agreement with infrared bands measured on dehydrated silica surfaces indicating their potential as models of strained extended silica systems. Silica rings, if synthesized, may also be useful precursors for new bulk-silica polymorphs with tubular or porous morphologies

Apparent Scarcity of Low-Density Polymorphs of Inorganic Solids

For most inorganic solids, very few dense polymorphs and no low-density polymorphs are observed. Taking a wide range of tetrahedrally-coordinated binary solids (e.g., ZnO, GaN) as a prototypical system, we show that the apparent scarcity of low-density polymorphs is not due to significant structural or energetic limitations. Using databases of periodic networks as sources of novel crystal structures, followed by ab initio energy minimization, we predict a dense spectrum of low-density low-energy polymorphs. The diverse range of materials considered indicates that this is likely to be a general phenomenon

The effect of particle size on the optical and electronic properties of magnesium oxide nanoparticles

The quasiparticle states, fundamental gaps, optical gaps, exciton binding energies and UV-vis spectra for a series of cuboidal nanoparticles of the prototypical oxide magnesium oxide (MgO), the largest of which has 216 atoms and edges of 1 nm, were predicted using many-body perturbation theory (evGW-BSE). The evolution of the properties with the particle size was explicitly studied. It was found that, while the highest occupied and lowest unoccupied quasiparticle states and fundamental gap change with the particle size, the optical gap remains essentially fixed for all but the smallest nanoparticles, in line with what was previously observed experimentally. The explanation for these observations is demonstrated to be that, while the optical gap is associated with an exciton that is highly localised around the particle's corner atoms, the highest occupied and lowest unoccupied quasiparticle states, while primarily localised on the oxygen corner atoms (hole) and magnesium corner atoms (electron), show significant delocalisation along the edges. The strong localisation of the exciton associated with the optical gap on the corner atoms is argued to also explain why the nanoparticles have much smaller optical gaps and red-shifted spectra compared to bulk MgO. Finally, it is discussed how this non-quantum confinement behaviour, where the properties of the nanoparticles arise from surface defects rather than differences in localisation of quasiparticle or exciton states, appears typical of alkaline earth oxide nanoparticles, and that the true optical gap of bulk crystals of such materials is also probably the result of surface defects, even if unobservable experimentally

The Effect of Particle Size and Composition on the Optical and Electronic Properties of CdO and CdS Rocksalt Nanoparticles

Quantum confinement like behaviour in CdO and CdS nanoparticles is demonstrated through explicit evGW-BSE many-body perturbation theory calculations on 0.6-1.4 nanometre particles of these materials. However, while the lowest optical excited-state, exciton, and the highest occupied and lowest unoccupied quasiparticle states in such nanoparticles are predicted to be delocalised, they are found to be delocalised over the surface of the particle only and not the whole particle volume. The electronic and optical properties of CdO and CdS rocksalt nanoparticles are predicted to differ dramatically from their structurally analogous MgO counterparts, where the lowest exciton and highest occupied and lowest unoccupied quasiparticle states are strongly localised, in contrast. This difference in behaviour between MgO and CdO/CdS is explained in terms of the more polarisable, less ionic, bonding in CdO and CdS. The effect on the optical and fundamental gaps of the particles due to the presence of amine capping agents on the particles’ surface is explored and predicted to be relatively small. However, the highest occupied and lowest unoccupied quasiparticle states are found to consistently shift to more shallow values when increasing the surface density of capping agents. An explanation of this shift, finally, is proposed in terms of the dipole field induced by the aligned dipoles of the capping agents

Excited state localisation cascades in inorganic semiconductor nanoparticles

Excited state relaxation in zinc sulfide (ZnS) nanoparticles is studied as a model for the fate of the excited state in inorganic nanoparticles in general. A series of time-dependent density functional theory optimisations on the S1 and T1 excited states predict the existence of not merely isolated minima, as found before, but rather a connected cascade of excited state minima ending up in a conical intersection between the excited state energy surface and the ground state. The localisation of the excited state in the different minima increases down the cascade, while the barriers separating these minima, studied here for the first time for nanoparticles, are predicted to be in some cases electronic (strongly avoided crossing) in origin. The cartoon picture of excited state relaxation in inorganic nanoparticles that involves relaxation to the bottom of only one approximately harmonic well followed by photoluminescence appears for the ZnS nanoparticles studied here to be at best rather simplistic. The localisation cascade is finally found to strongly affect the excited state properties of nanoparticles and predicted to lead to the formation of defected nanoparticles after de-excitation in selected cases

The dimensions of human and murine CD8 T lymphocyte diversity

The adaptive immune system generates and maintains a pool of CD8+ T cells with an almost limitless specificity with the purpose of maintaining homeostasis. Upon antigen encounter, CD8+ T cells undergo functional diversification and give rise to daughter cells. The daughter cells adopt one of many distinct states along a functional gradient from stem-like towards highly cytotoxic. After antigen clearance, a small fraction of antigen-experienced CD8+ T cells survives and constitutes a long-lived memory population that conveys long-term protection. The likelihood of a T cell acquiring or possessing the ability to maintain a long-term presence is related to the functional characteristics of that T cell during the active response phase. This thesis aims to unravel the biological dimensions underlying CD8+ T cell diversity and fate determination. Recent developments in high dimensional proteome and transcriptome analysis revealed that antigen- experienced T cells form a continuum of functional states. This continuum is in contrast with the established subsetting into discrete subpopulations. We identified the chemokine receptor CX3CR1 as a graded, cross-species, pan T cell differentiation marker. CX3CR1 can be used to, in a convenient and simple manner, capture a principal component of T cell diversity. Furthermore, the CX3CR1 expression levels reflected similar functional states across species, enabling cross-species comparison of T cell properties. The enhanced insight into differentiation revealed that CX3CR1high CD8+ T cells uniquely patrol the luminal arteriolar surface. These CD8+ T cells scan the arteriolar surface for antigen, which supported their long-term survival. Finally, we unraveled T cell activation and proliferation as principal aspects of the T cell diversity in addition to differentiation. From this data, we constructed transcriptomic- and protein-based scores to provide a precise yet practical tool to identify and isolate differentiation and phase-specific CD8+ T cells. In this thesis, we moved beyond binary classification of CD8+ T cell states and identified transcriptomic and functional gradients of CD8+ T cell diversity. These axes of diversity precisely described the position of individual CD8+ T cells within the T cell heterogeneity found in the effector and memory phase. By leveraging these axes, we identified markers and marker panels that easily, practically and precisely characterized CD8+ T cells

Leveraging emerging technologies to enable environmental monitoring and accountability in conflict zones

The growth of access to the internet, wide availability of smart phones and increased public access to remote sensing data from hundreds of satellite systems have spurred a revolution in tracking the linkages between armed conflict and environmental damage. Over the last decade, a growing community of open-source investigative experts, environmentalists, academics and civil society groups have applied these methods to document war crimes, human rights violations and environmental degradation. These developments have created new opportunities for building accountability and transparency. The wealth of data on conflict-linked environmental damage has already been successfully leveraged to address acute and long-term environmental health risks and inform humanitarian response and post-conflict environmental assessments in Iraq, Syria and Ukraine. There are, however, larger questions on how to best make use of these data streams and information layers, and how to navigate the opportunities and limitations of these developments. This article will outline the new developments in this field and provide recommendations to ensure that data is used responsibly and effectively to strengthen accountability for environmental damages as a result of armed conflict

Exploration of the Photocatalytic Cycle for Sacrificial Hydrogen Evolution by Conjugated Polymers Containing Heteroatoms

We analyze the photocatalytic activity of heteroatom containing linear conjugated polymers for sacrificial hydrogen evolution using a recently proposed photocatalytic cycle. We find that the thermodynamic barrier to electron transfer, relevant both in the presence and absence of noble metal co-catalysts, changes with polymer composition, reducing upon going from electron-rich to electron-poor polymers, and disappearing completely for the most electron-poor polymers in a water rich environment. We discuss how the latter is probably the reason why electron-poor polymers are generally more active for sacrificial hydrogen evolution than their electron-rich counterparts. We also study the barrier to hydrogen-hydrogen bond formation on the polymer rather than the co-catalyst and find that it too changes with composition but is always, at least for the polymer studied here, much larger than that experimentally reported for platinum. Therefore, it is expected that in the presence of any noble metal particles these will act as the site of hydrogen evolution

Chemical trends in the optical properties of rocksalt nanoparticles

The nature and magnitude of the optical gaps of rocksalt alkaline earth (MgO, CaO, SrO, MgS, MgSe) and transition metal chalcogenide (CdO, PbS) nanoparticles are studied using time-dependent density functional theory (TD-DFT) calculations on (MX)32 nanoparticles. We demonstrate, just as we previously showed for MgO, that TD-DFT calculations on rocksalt nanoparticles require the use of hybrid exchange–correlation (XC-)functionals with a high percentage of Hartree–Fock like exchange (e.g. BHLYP) or range-separated XC-functionals to circumvent problems related to the description of charge-transfer excitations. Concentrating on the results obtained with TD-BHLYP we show that the optical gap in rocksalt nanoparticles displays a wide range of behavior; ranging from large optical gaps stemming from a localized excitation involving corner atoms in alkaline earth oxides to a delocalized excitation and small optical gaps in the transition metal chalcogenides. Finally, we rationalize this wide range of behaviour in terms of differences in the degree to which the Coulombic interaction between the excited electron and hole is screened in the different nanoparticles, and relate it to the optical dielectric constants of the bulk materials the nanoparticles are made from