Deconvoluting Energy Transport Mechanisms in Metal Halide Perovskites Using CsPbBr3 Nanowires as a Model System

Understanding energy transport in metal halide perovskites is essential to effectively guide further optimization of materials and device designs. However, difficulties to disentangle charge carrier diffusion, photon recycling, and photon transport have led to contradicting reports and uncertainty regarding which mechanism dominates. In this study, monocrystalline CsPbBr₃ nanowires serve as 1D model systems to help unravel the respective contribution of energy transport processes in metal-halide perovskites. Spatially, temporally, and spectrally resolved photoluminescence (PL) microscopy reveals characteristic signatures of each transport mechanism from which a robust model describing the PL signal accounting for carrier diffusion, photon propagation, and photon recycling is developed. For the investigated CsPbBr₃ nanowires, an ambipolar carrier mobility of μ = 35 cm² V⁻¹ s⁻¹ is determined, and is found that charge carrier diffusion dominates the energy transport process over photon recycling. Moreover, the general applicability of the developed model is demonstrated on different perovskite compounds by applying it to data provided in previous related reports, from which clarity is gained as to why conflicting reports exist. These findings, therefore, serve as a useful tool to assist future studies aimed at characterizing energy transport mechanisms in semiconductor nanowires using PL.     

Automated contact tracing: a game of big numbers in the time of COVID-19

One of the more widely advocated solutions for slowing down the spread of COVID-19 has been automated contact tracing. Since proximity data can be collected by personal mobile devices, the natural proposal has been to use this for automated contact tracing providing a major gain over a manual implementation. In this work, we study the characteristics of voluntary and automated contact tracing and its effectiveness for mapping the spread of a pandemic due to the spread of SARS-CoV-2. We highlight the infrastructure and social structures required for automated contact tracing to work. We display the vulnerabilities of the strategy to inadequate sampling of the population, which results in the inability to sufficiently determine significant contact with infected individuals. Of crucial importance will be the participation of a significant fraction of the population for which we derive a minimum threshold. We conclude that relying largely on automated contact tracing without population-wide participation to contain the spread of the SARS-CoV-2 pandemic can be counterproductive and allow the pandemic to spread unchecked. The simultaneous implementation of various mitigation methods along with automated contact tracing is necessary for reaching an optimal solution to contain the pandemic.     

Data scattering in scanning tunneling spectroscopy

We investigated the scattering of current-voltage data obtained with scanning tunneling spectroscopy (STS) at room temperature at a solid-liquid interface on highly oriented pyrolytic graphite (HOPG) and in ultrahigh vacuum on HOPG and Au(111). For both experimental conditions, the data scattering can be described by a lognormal function for a moderate number of subsequent measurements. The lognormal distribution of the current can be explained by a normal distribution of the tip-surface distance. We give a simple empirical rule for STS data sorting.     

Multilayer Fresnel zone plate for soft X-ray microscopy resolves sub-39nm structures

Best resolutions in X-ray focusing are obtained to date by using diffractive lenses called Fresnel zone plates (FZPs). Their further improvement is nevertheless hindered by fundamental limitations in the employed manufacturing techniques. Here, we show a novel method to fabricate FZPs based on multilayer deposition with atomic layer deposition (ALD) and subsequent sectioning with focused ion beam (FIB). For the first time a multilayer FZP working in the soft X-ray range was prepared and could achieve the best resolution obtained so far for multilayer FZPs by resolving features below 39 nm in size in a scanning soft X-ray microscope. The new technique presents high potential for high resolution microscopy in both the soft and hard X-ray range.     

Chemical Shift and Exchange Interaction Energy of the 1s States of Magnesium Donors in Silicon. The Possibility of Stimulated Emission

Semiconductors - The results of experiments aimed at the observation of split 1s states in Mg-doped Si are reported. From the results, it is possible to determine the chemical shift and exchange...     

Decomposition of a Solid Solution of Interstitial Magnesium in Silicon

Semiconductors - The decomposition of a solid solution of interstitial magnesium Mgi in silicon is studied. Float-Zone dislocation-free single-crystal n-Si with a resistivity of ~8 × 103...     

Relaxation of the Excited States of Arsenic in Strained Germanium

Semiconductors - The relaxation times of the lower p states of the arsenic donor in a germanium crystal strained along the [111] crystallographic direction are studied. Measurements are performed...     

Frequency Tuning of Terahertz Stimulated Emission under the Intracenter Optical Excitation of Uniaxially Stressed Si:Bi

Semiconductors - The results of experimental and theoretical investigations of terahertz-emission-spectrum tuning by means of uniaxial stress of a silicon crystal doped with shallow bismuth donors...