An adaptive model for the optical properties of excited gold

We study the temperature-dependent optical properties of gold over a broad energy spectrum covering photon energies below and above the interband threshold. We apply a semi-analytical Drude-Lorentz model with temperature-dependent oscillator parameters. Our approximations are based on the distribution of electrons over the active bands with a density of states provided by density functional theory. This model can be easily adapted to other materials with similar band structures and can also be applied to the case of occupational nonequilibrium. Our calculations show a strong enhancement of the intraband response with increasing electron temperature while the interband component decreases. Moreover, our model compares well with density functional theory-based calculations for the reflectivity of highly excited gold and reproduces many of its key features. Applying our methods to thin films shows a sensitive nonlinear dependence of the reflection and absorption on the electron temperature. These features are more prominent at small photon energies and can be highlighted with polarized light. Our findings offer valuable insights for modeling ultrafast processes, in particular, the pathways of energy deposition in laser-excited samples

Optical properties of gold after intense short-pulse excitations

Intense ultrashort laser pulses can create highly excited matter with extraordinary properties. Experimental and theoretical investigations of these extreme conditions are very complex and usually intertwined. Here, we report on a theoretical approach for the electron scattering rates and the optical properties in gold at elevated temperatures. Our theory is based on the degree of occupancy of the conduction band as well as inputs from ab initio simulations and experimental data. After the electron system has reached a quasi-equilibrium, the occupancy is fully determined by the electron temperature. Thus, our approach covers the important relaxation stage after fast excitations when the two-temperature model can be applied. Being based on the electronic structure of solids, the model is valid for lattice temperatures up to melting but the electron temperature might exceed this limit by far. Our results agree well with recent experimental data for both the collision frequencies and the conductivity of highly excited gold. Scattering of sp-electrons by d-electrons is found to be the dominant damping mechanism at elevated electron temperatures and depends strongly on the number of conduction electrons, hence, revealing the microscopic origin of the conductivity change after heating. The supportive benchmarks with experiments are very valuable as the underlying scattering rates determine a number of other transport, optical and relaxation properties of laser-excited matter

Novel phase diagram behavior and materials design in heterostructural semiconductor alloys

Structure and composition control the behavior of materials. Isostructural alloying is historically an extremely successful approach for tuning materials properties, but it is often limited by binodal and spinodal decomposition, which correspond to the thermodynamic solubility limit and the stability against composition fluctuations, respectively. We show that heterostructural alloys can exhibit a markedly increased range of metastable alloy compositions between the binodal and spinodal lines, thereby opening up a vast phase space for novel homogeneous single-phase alloys. We distinguish two types of heterostructural alloys, that is, those between commensurate and incommensurate phases. Because of the structural transition around the critical composition, the properties change in a highly nonlinear or even discontinuous fashion, providing a mechanism for materials design that does not exist in conventional isostructural alloys. The novel phase diagram behavior follows from standard alloy models using mixing enthalpies from first-principles calculations. Thin-film deposition demonstrates the viability of the synthesis of these metastable single-phase domains and validates the computationally predicted phase separation mechanism above the upper temperature bound of the nonequilibrium single-phase region

Adaptive model for the optical properties of excited gold

We investigate the optical properties of laser-excited gold within the Drude-Lorentz framework. Our approach extends the Drude-Lorentz model, initially fitted to experimental data for ambient conditions, to accommodate higher temperatures. It covers a wide range of excitation strengths and frequencies, Fermi-distributed electrons, as well as nonequilibrium scenarios. It is in good qualitative agreement with density functional theory–based calculations capturing key features of the reflectivity and, thus, provides a very efficient alternative to these numerically expensive calculations. When applied to thin films, we find a nonlinear dependence of reflection and absorption on electron temperature, accentuated at lower photon energies and under polarized light. Our highly efficient model can be particularly valuable when analyzing experiments investigating ultrafast processes as the pathways of energy relaxation in laser-excited samples

Nickel Oxide Interlayer Films from Nickel Formate–ethylenediamine Precursor: Influence of Annealing on Thin Film Properties and Photovoltaic Device Performance

An organometallic ink based on the nickel formate–ethylenediamine (Ni(O2CH)2(en)2) complex forms high performance NiOx thin film hole transport layers (HTL) in organic photovoltaic (OPV) devices. Improved understanding of these HTLs functionality can be gained from temperature-dependent decomposition/oxidation chemistries during film formation and corresponding chemical structure-function relationships for energetics, charge selectivity, and transport in photovoltaic platforms. Investigations of as-cast films annealed in air (at 150 °C–350 °C), with and without subsequent O2-plasma treatment, were performed using thermogravimetric analysis, Fourier transform infrared spectroscopy, ultraviolet and X-ray photoelectron spectroscopy, and spectroscopic ellipsometry to elucidate the decomposition and oxidation of the complex to NiOx. Regardless of the anneal temperature, after exposure to O2-plasma, these HTLs exhibit work functions greater than the ionization potential of a prototype donor polymer poly(N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole) (PCDTBT), thereby meeting a primary requirement of energy level alignment. Thus, bulk-heterojunction (BHJ), OPV solar cells made on this series of NiOx HTLs all exhibit similar open circuit voltages (Voc). In contrast, the short circuit currents increase significantly from 1.7 to 11.2 mA cm−2 upon increasing the anneal temperature from 150 °C to 250 °C. Concomitantly, increased conductivity and electrical homogeneity of NiOx thin films are observed at the nanoscale using conductive tip-AFM. Similar Voc observed for all the O2-plasma treated NiOx interlayers and variations to nanoscale conductivity suggest that the HTLs all form charge selective contacts and that their carrier extraction efficiency is determined by the amount of precursor conversion to NiOx. The separation of these two properties: selectivity and conductivity, sheds further light on charge selective interlayer functionality

Increased PV Soiling from High Module Voltages

Natural soiling has reduced the energy output of PV systems since the technology was first used, and viable mitigation strategies have remained elusive ever since. With the ever-increasing deployments around the world, especially in dusty environments, soiling is becoming a billion-dollar problem, worldwide. Furthermore, as plant operators continue to look for ways to increase revenue, the PV operating voltages have increased to between 1000 V and 1500 V when the sun is shining. This has resulted in some unforeseen consequences nominally combined into what is termed Potential Induced Degradation.1 Recent work by Jiang et. al., 2 at NREL using Atomic Force Microscopy has demonstrated that these large potentials also affect soiling by substantially increasing the attraction of dust to the surface, but also by increasing the adhesion force. Jiang et. al.,have also shown that these higher soiling attraction and adhesion forces continue long into the night when the PV is no longer producing power. In this paper, we present a set of field results that demonstrate enhanced soiling rates that is due to the strong electric fields induced by these high voltage PV arrays. This includes observation of enhanced soiling rates measured in the field when amodule is held at ±1000 V. This is critical information for installation operators because soiling losses may be higher on some panels than what is measured by typical soiling stations, and because the high voltages are not uniform across an array, some modules may have more soiling than others, leading to potential issues with non-uniform soiling problems at the array level. We presentthis set of compelling electric field induced soiling results in this paper

Spatial epidemiology of urban health risks in select West African cities

West African cities face critical societal challenges that are linked to environmental and health changes. These challenges are further exacerbated by urbanization dynamics, climate change, socio-economic mutation and lack of capacity for sustainable urban development, governance and basic services delivery. The deficiency of environmental sanitation and ecosystem services have led to high complexity of urban health risks inequalities, resulting in the need for more research on efficient urban health policies. The purpose of this contribution is to present the main findings on the spatial epidemiology of diarrhaea and malaria, and their associated risks factors in the following select West African cities. Spatial variability of exposure to diarrhaea and malaria transmission is linked to several health risks such as lack of access to water and sanitation, solid wastes management, urban flooding

Structural, Optical, and Transport Properties of α- and β‑Ag₃VO₄

The structures of α- and β-Ag₃VO₄ were studied via single-crystal X-ray diffraction (XRD). The transition from α-phase to β-phase was found to occur at 110 °C. Single-crystal XRD revealed that the integrity of the single crystals was maintained as Ag₃VO₄ reversibly transitioned between α-Ag₃VO₄ and β-Ag₃VO₄. The optical and electrical properties of polycrystalline α-Ag₃VO₄ were studied by diffuse reflectance spectroscopy and impedance spectroscopy. In order to assess the optical and electrical properties of β-Ag₃VO₄, <i>in situ</i> measurements were performed above the phase-transition temperature. Thin films of α-Ag₃VO₄ were prepared by combinatorial sputtering and pulsed laser deposition (PLD). The crystallographic, optical, and electrical conductivity properties of the α-Ag₃VO₄ thin films were compared with the bulk properties