Charge-Carrier Recombination in Halide Perovskites.

The success of halide perovskites in a host of optoelectronic applications is often attributed to their long photoexcited carrier lifetimes, which has led to charge-carrier recombination processes being described as unique compared to other semiconductors. Here, we integrate recent literature findings to provide a critical assessment of the factors we believe are most likely controlling recombination in the most widely studied halide perovskite systems. We focus on four mechanisms that have been proposed to affect measured charge carrier recombination lifetimes, namely: (1) recombination via trap states, (2) polaron formation, (3) the indirect nature of the bandgap (e.g., Rashba effect), and (4) photon recycling. We scrutinize the evidence for each case and the implications of each process on carrier recombination dynamics. Although they have attracted considerable speculation, we conclude that multiple trapping or hopping in shallow trap states, and the possible indirect nature of the bandgap (e.g., Rashba effect), seem to be less likely given the combined evidence, at least in high-quality samples most relevant to solar cells and light-emitting diodes. On the other hand, photon recycling appears to play a clear role in increasing apparent lifetime for samples with high photoluminescence quantum yields. We conclude that polaron dynamics are intriguing and deserving of further study. We highlight potential interdependencies of these processes and suggest future experiments to better decouple their relative contributions. A more complete understanding of the recombination processes could allow us to rationally tailor the properties of these fascinating semiconductors and will aid the discovery of other materials exhibiting similarly exceptional optoelectronic properties.EPSRC DTP Studentshi

Plexciton Dirac points and topological modes

Plexcitons are polaritonic modes that result from the strong coupling between excitons and plasmons. Here, we consider plexcitons emerging from the interaction of excitons in an organic molecular layer with surface plasmons in a metallic film. We predict the emergence of Dirac cones in the two-dimensional band-structure of plexcitons due to the inherent alignment of the excitonic transitions in the organic layer. An external magnetic field opens a gap between the Dirac cones if the plexciton system is interfaced with a magneto-optical layer. The resulting energy gap becomes populated with topologically protected one-way modes, which travel at the interface of this plexcitonic system. Our theoretical proposal suggests that plexcitons are a convenient and simple platform for the exploration of exotic phases of matter and for the control of energy flow at the nanoscale

Surface Doping Quantum Dots with Chemically Active Native Ligands: Controlling Valence without Ligand Exchange

One remaining challenge in the field of colloidal semiconductor nanocrystal quantum dots is learning to control the degree of functionalization or valence per nanocrystal. Current quantum dot surface modification strategies rely heavily on ligand exchange, which consists of replacing the nanocrystal\u27s native ligands with carboxylate- or amine-terminated thiols, usually added in excess. Removing the nanocrystal\u27s native ligands can cause etching and introduce surface defects, thus affecting the nanocrystal\u27s optical properties. More importantly, ligand exchange methods fail to control the extent of surface modification or number of functional groups introduced per nanocrystal. Here, we report a fundamentally new surface ligand modification or doping approach aimed at controlling the degree of functionalization or valence per nanocrystal while retaining the nanocrystal\u27s original colloidal and photostability. We show that surface-doped quantum dots capped with chemically active native ligands can be prepared directly from a mixture of ligands with similar chain lengths. Specifically, vinyl and azide-terminated carboxylic acid ligands survive the high temperatures needed for nanocrystal synthesis. The ratio between chemically active and inactive-terminated ligands is maintained on the nanocrystal surface, allowing to control the extent of surface modification by straightforward organic reactions. Using a combination of optical and structural characterization tools, including IR and 2D NMR, we show that carboxylates bind in a bidentate chelate fashion, forming a single monolayer of ligands that are perpendicular to the nanocrystal surface. Moreover, we show that mixtures of ligands with similar chain lengths homogeneously distribute themselves on the nanocrystal surface. We expect this new surface doping approach will be widely applicable to other nanocrystal compositions and morphologies, as well as to many specific applications in biology and materials science

Molecular Chemistry to the Fore: New Insights into the Fascinating World of Photoactive Colloidal Semiconductor Nanocrystals

Colloidal semiconductor nanocrystals possess unique properties that are unmatched by other chromophores such as organic dyes or transition-metal complexes. These versatile building blocks have generated much scientific interest and found applications in bioimaging, tracking, lighting, lasing, photovoltaics, photocatalysis, thermoelectrics, and spintronics. Despite these advances, important challenges remain, notably how to produce semiconductor nanostructures with predetermined architecture, how to produce metastable semiconductor nanostructures that are hard to isolate by conventional syntheses, and how to control the degree of surface loading or valence per nanocrystal. Molecular chemists are very familiar with these issues and can use their expertise to help solve these challenges. In this Perspective, we present our group\u27s recent work on bottom-up molecular control of nanoscale composition and morphology, low-temperature photochemical routes to semiconductor heterostructures and metastable phases, solar-to-chemical energy conversion with semiconductor-based photocatalysts, and controlled surface modification of colloidal semiconductors that bypasses ligand exchange

Quantifying water volumes of ungauged lakes using optical satellite imagery and high-resolution lidar dems

Surface water bodies such as lakes, rivers and wetlands are critical water resources to both human and ecological systems, but are under increasing pressure from competing users. Monitoring of the location, quantity and movement of water is crucial in effectively managing these resources and implementing a sustainable water management strategy for the future. However, continuous information on the quantity and distribution of water across the landscape is limited in some regions because of the high cost of traditional in-situ monitoring. As such, global remote sensing datasets are being used more frequently to complement these sparse networks. This paper aims to develop new methods to estimate the quantity of water (volume) in open water storages such as lakes, using remote sensing data. Lake Menindee, part of the greater Menindee Lakes complex in the Murray-Darling Basin, was selected as the case study of this research because of its geographic location and data availability. Water management in the Murray-Darling Basin has been under increasing scrutiny partly due to exposure of water theft by irrigators. As such, there is a pressing need for large scale monitoring of water resources in the region using novel data and methods. This paper developed three methods to estimate water volumes in a lake, all of which only used a high-resolution (5m) LiDAR DEM in conjunction with optical imagery. As an initial preprocessing step, the water observations from space (WOfS) algorithm (Mueller et al. 2016) was applied to Landsat optical imagery to detect areas of surface water in the lake which was used as an input to all the methods. The first method derived a relationship between lake inundated surface area and volume using the DEM. Subsequently, this relationship was used to convert WOfS-derived surface areas to volumes. The second method evaluated the quality of match between the WOfS spatial inundation pattern and DEM-modelled inundation patterns at 0.1m water level increments, from which an optimal match and the respective DEM-derived volume was picked. Quality of match was quantified with three metrics commonly used in weather forecasting. In the third method, the elevation of the WOfS water body edge was derived from the DEM, and a volume was estimated by "filling" the lake DEM to this height. Water volumes by all three methods were estimated using 19 years of high-quality Landsat data equivalent to 209 scenes, and daily gauged measurements were used for validation. A combination of scatterplots and statistical metrics were used for evaluation. Initial findings show that all methods have reasonable skill in estimating water volumes with high Pearson correlation coefficients, and estimates from methods 2 and 3 have relative biases of less than 10 percent. No single method performed consistently better across all ranges of volumes, with method 3 having poorest performance for low volumes while method 1 substantially overestimated high volumes. Additionally, estimation errors were volume-dependent, with medium-range of volumes having highest accuracy estimates while prediction skill consistently worsened at higher volumes across all methods. Future research should further investigate drivers of the volume-dependent errors, expand the evaluations to multiple case studies, including the large on-farm water storages across the Murray-Darling Basin, and test other remote sensing data sources such as radar altimetry. These results clearly demonstrate the potential of remote sensing based methods for lake volume estimation

Coarsening and solidification via solvent-annealing in thin liquid films

We examine solidification in thin liquid films produced by annealing amorphous Alq[subscript 3] (tris-(8-hydroxyquinoline) aluminium) in methanol vapour. Micrographs acquired during annealing capture the evolution of the film: the initially-uniform film breaks up into drops that coarsen, and single crystals of Alq[subscript 3] nucleate randomly on the substrate and grow as slender ‘needles’. The growth of these needles appears to follow power-law behaviour, where the growth exponent, γ, depends on the thickness of the deposited Alq[subscript 3] film. The evolution of the thin film is modelled by a lubrication equation, and an advection–diffusion equation captures the transport of Alq[subscript 3] and methanol within the film. We define a dimensionless transport parameter, α, which is analogous to an inverse Sherwood number and quantifies the relative effects of diffusion- and coarsening-driven advection. For large α-values, the model recovers the theory of one-dimensional, diffusion-driven solidification, such that γ→1/2. For low α-values, the collapse of drops, i.e. coarsening, drives flow and regulates the growth of needles. Within this regime, we identify two relevant limits: needles that are small compared to the typical drop size, and those that are large. Both scaling analysis and simulations of the full model reveal that γ→2/5 for small needles and γ→0.29 for large needles