Strong Hot Carrier Effects in Single Nanowire Heterostructures.

We use transient Rayleigh scattering to study the thermalization of hot photoexcited carriers in single GaAs0.7Sb0.3/InP nanowire heterostructures. By comparing the energy loss rate in single core-only GaAs0.7Sb0.3 nanowires which do not show substantial hot carrier effects with the core-shell nanowires, we show that the presence of an InP shell substantially suppresses the longitudinal optical phonon emission rate at low temperatures which then leads to strong hot carrier effects.

Clastic cells in orthodontic treatment: Translational challenges and recent advances.

OBJECTIVES: Orthodontic treatment consists of numerous appliance activations that rely on stimulation of osteoclasts at alveolar bone sites. However, the action of osteoclast-like cells on dentin ("odontoclasts") is a pathological side effect of orthodontic treatment. The aim of this article is twofold: (a) To report preliminary results from ongoing cell culture experiments to identify unique markers of dentin resorption, and (b) To discuss our work using nanoparticle tracking analysis (NTA) and exosomes for developing biological fluid-based biopsies to monitor clastic cell activity. SETTING AND SAMPLE POPULATION: Twelve healthy volunteers in permanent dentition. MATERIAL AND METHODS: For the in vitro experiments, murine clastic cell precursors were cultured on dentin or bone slices for 7 days and phage-display biopanning was used to identify molecular surface differences between osteoclasts and odontoclasts. In the human study, gingival crevicular fluid (GCF) samples were collected using different tools and analysed for protein and exosome recovery. RESULTS: Biopanning generated antibody fragments that were uniquely reactive to odontoclasts. Numerous nanoparticles in the size range of exosomes were detected in all of the human GCF samples. CONCLUSIONS: Our results support that there are molecular differences between osteoclasts and odontoclasts. Emerging technologies may allow the use of exosomes in GCF as a clinical tool to detect markers of root resorption.

Revealing Optical Transitions and Carrier Recombination Dynamics within the Bulk Band Structure of Bi2Se3.

Bismuth selenide (Bi2Se3) is a prototypical 3D topological insulator whose Dirac surface states have been extensively studied theoretically and experimentally. Surprisingly little, however, is known about the energetics and dynamics of electrons and holes within the bulk band structure of the semiconductor. We use mid-infrared femtosecond transient reflectance measurements on a single nanoflake to study the ultrafast thermalization and recombination dynamics of photoexcited electrons and holes within the extended bulk band structure over a wide energy range (0.3 to 1.2 eV). Theoretical modeling of the reflectivity spectral line shapes at 10 K demonstrates that the electrons and holes are photoexcited within a dense and cold electron gas with a Fermi level positioned well above the bottom of the lowest conduction band. Direct optical transitions from the first and the second spin-orbit split valence bands to the Fermi level above the lowest conduction band minimum are identified. The photoexcited carriers thermalize rapidly to the lattice temperature within a couple of picoseconds due to optical phonon emission and scattering with the cold electron gas. The minority carrier holes recombine with the dense electron gas within 150 ps at 10 K and 50 ps at 300 K. Such knowledge of interaction of electrons and holes within the bulk band structure provides a foundation for understanding how such states interact dynamically with the topologically protected Dirac surface states.

Toward an Integrative Framework for Studying Human Evaluation: Attitudes Toward Objects and Attributes.

Evaluation is central to human experience, and multiple literatures have studied it. This article pulls from research on attitudes, human and nonhuman mating preferences, consumer behavior, and beyond to build a more comprehensive framework for studying evaluation. First, we distinguish between evaluations of objects (persons, places, things) and evaluations of attributes (dimensions, traits, characteristics). Then, we further distinguish between summarized attribute preferences (a valenced response to a direction on a dimension, such as liking sweetness in desserts) and functional attribute preferences (a valenced response to increasing levels of a dimension in a set of targets, such as the extent to which sweetness predicts liking for desserts). We situate these constructs with respect to existing distinctions in the attitude literature (e.g., specific/general, indirect/direct). Finally, new models address how people translate functional into summarized preferences, as well as how attribute preferences affect (a) subsequent evaluations of objects and (b) situation selection.

Thermal Delocalization of Excitons in GaAs/AlGaAs Quantum Well Tube Nanowires.

We use temperature-dependent photoluminescence (PL), photoluminescence imaging, and time-resolved photoluminescence measurements to gain insights into the localization of excitons in single 2 nm GaAs/AlGaAs quantum well tube nanowires. PL spectra reveal the coexistence of localized and delocalized states at low temperatures, with narrow quantum dot-like emission lines on the high energy side of a broad emission band, and delocalized states on the low energy side. We find that the high energy QD-like emissions are metastable, disappearing at higher temperatures with only delocalized states (quantum well tube ground states) surviving. By comparing temperature- and time-dependent PL, we develop a theoretical model which provides insights into the confinement potentials and relaxation dynamics which localize the excitons in these quantum well tube nanowires.

Titi Monkeys as a Novel Non-Human Primate Model for the Neurobiology of Pair Bonding
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It is now widely recognized that social bonds are critical to human health and well-being. One of the most important social bonds is the attachment relationship between two adults, known as the pair bond. The pair bond involves many characteristics that are inextricably linked to quality of health, including providing a secure psychological base and acting as a social buffer against stress. The majority of our knowledge about the neurobiology of pair bonding comes from studies of a socially monogamous rodent, the prairie vole (Microtus ochrogaster), and from human imaging studies, which inherently lack control. Here, we first review what is known of the neurobiology of pair bonding from humans and prairie voles. We then present a summary of the studies we have conducted in titi monkeys (Callicebus cupreus)-a species of socially monogamous New World primates. Finally, we construct a neural model based on the location of neuropeptide receptors in the titi monkey brain, as well as the location of neural changes in our imaging studies, with some basic assumptions based on the prairie vole model. In this model, we emphasize the role of visual mating stimuli as well as contributions of the dopaminergic reward system and a strong role for the lateral septum. This model represents an important step in understanding the neurobiology of social bonds in non-human primates, which will in turn facilitate a better understanding of these mechanisms in humans.

Quantum Confined Stark Effect in a GaAs/AlGaAs Nanowire Quantum Well Tube Device: Probing Exciton Localization.

In this Letter, we explore the nature of exciton localization in single GaAs/AlGaAs nanowire quantum well tube (QWT) devices using photocurrent (PC) spectroscopy combined with simultaneous photoluminescence (PL) and photoluminescence excitation (PLE) measurements. Excitons confined to GaAs quantum well tubes of 8 and 4 nm widths embedded into an AlGaAs barrier are seen to ionize at high bias levels. Spectroscopic signatures of the ground and excited states confined to the QWT seen in PL, PLE, and PC data are consistent with simple numerical calculations. The demonstration of good electrical contact with the QWTs enables the study of Stark effect shifts in the sharp emission lines of excitons localized to quantum dot-like states within the QWT. Atomic resolution cross-sectional TEM measurements and an analysis of the quantum confined Stark effect of these dots provide insights into the nature of the exciton localization in these nanostructures.

Polarized light absorption in wurtzite InP nanowire ensembles.

We investigate the absorption properties of ensembles of wurtzite (WZ) InP nanowires (NWs) by high-resolution polarization-resolved photoluminescence excitation (PLE) spectroscopy at T = 10 K. The degree of linear polarization of absorbed light, ρ(abs), resulting from the PLE spectra is governed by a competition between the dielectric mismatch effect and the WZ selection rules acting differently on different optical transitions. These two contributions are deconvoluted with the help of finite-difference time-domain simulations, thus providing information about the symmetry of the three highest valence bands (A, B, and C) of WZ InP and the extent of the spin-orbit interaction on these states. Moreover, ρ(abs) shows two characteristic dips corresponding to the two sharp A and B exciton resonances in the PLE spectra. A model developed for the dip in A provides the first experimental evidence of an enhancement in the dielectric mismatch effect originating from the Coulomb interaction between electron and hole.

Zn3As2 nanowires and nanoplatelets: highly efficient infrared emission and photodetection by an earth abundant material.

The development of earth abundant materials for optoelectronics and photovoltaics promises improvements in sustainability and scalability. Recent studies have further demonstrated enhanced material efficiency through the superior light management of novel nanoscale geometries such as the nanowire. Here we show that an industry standard epitaxy technique can be used to fabricate high quality II-V nanowires (1D) and nanoplatelets (2D) of the earth abundant semiconductor Zn3As2. We go on to establish the optoelectronic potential of this material by demonstrating efficient photoemission and detection at 1.0 eV, an energy which is significant to the fields of both photovoltaics and optical telecommunications. Through dynamical spectroscopy this superior performance is found to arise from a low rate of surface recombination combined with a high rate of radiative recombination. These results introduce nanostructured Zn3As2 as a high quality optoelectronic material ready for device exploration.

Spatially Resolved Doping Concentration and Nonradiative Lifetime Profiles in Single Si-Doped InP Nanowires Using Photoluminescence Mapping.

We report an analysis method that combines microphotoluminescence mapping and lifetime mapping data of single semiconductor nanowires to extract the doping concentration, nonradiative lifetime, and internal quantum efficiency along the length of the nanowires. Using this method, the doping concentration of single Si-doped wurtzite InP nanowires are mapped out and confirmed by the electrical measurements of single nanowire devices. Our method has important implication for single nanowire detectors and LEDs and nanowire solar cells applications.

Emergence of localized states in narrow GaAs/AlGaAs nanowire quantum well tubes.

We use low-temperature photoluminescence, photoluminescence excitation, and photoluminescence imaging spectroscopy to explore the optical and electronic properties of GaAs/AlGaAs quantum well tube (QWT) heterostructured nanowires (NWs). We find that GaAs QWTs with widths >5 nm have electronic states which are delocalized and continuous along the length of the NW. As the NW QWT width decreases from 5 to 1.5 nm, only a single electron state is bound to the well, and no optical excitations to a confined excited state are present. Simultaneously, narrow emission lines (fwhm < 600 µeV) appear which are localized to single spatial points along the length of the NW. We find that these quantum-dot-like states broaden at higher temperatures and quench at temperatures above 80 K. The lifetimes of these localized states are observed to vary from dot to dot from 160 to 400 ps. The presence of delocalized states and then localized states as the QWTs become more confined suggests both opportunities and challenges for possible incorporation into quantum-confined device structures.

Carrier thermalization dynamics in single zincblende and wurtzite InP Nanowires.

Using transient Rayleigh scattering (TRS) measurements, we obtain photoexcited carrier thermalization dynamics for both zincblende (ZB) and wurtzite (WZ) InP single nanowires (NW) with picosecond resolution. A phenomenological fitting model based on direct band-to-band transition theory is developed to extract the electron-hole-plasma density and temperature as a function of time from TRS measurements of single nanowires, which have complex valence band structures. We find that the thermalization dynamics of hot carriers depends strongly on material (GaAs NW vs InP NW) and less strongly on crystal structure (ZB vs WZ). The thermalization dynamics of ZB and WZ InP NWs are similar. But a comparison of the thermalization dynamics in ZB and WZ InP NWs with ZB GaAs NWs reveals more than an order of magnitude slower relaxation for the InP NWs. We interpret these results as reflecting their distinctive phonon band structures that lead to different hot phonon effects. Knowledge of hot carrier thermalization dynamics is an essential component for effective incorporation of nanowire materials into electronic devices.

Optical, structural, and numerical investigations of GaAs/AlGaAs core-multishell nanowire quantum well tubes.

The electronic properties of thin, nanometer scale GaAs quantum well tubes embedded inside the AlGaAs shell of a GaAs core-multishell nanowire are investigated using optical spectroscopies. Using numerical simulations to model cylindrically and hexagonally symmetric systems, we correlate these electronic properties with structural characterization by aberration-corrected scanning transmission electron microscopy of nanowire cross sections. These tubular quantum wells exhibit extremely high quantum efficiency and intense emission for extremely low submicrowatt excitation powers in both photoluminescence and photoluminescence excitation measurements. Numerical calculations of the confined eigenstates suggest that the electrons and holes in their ground states are confined to extremely localized one-dimensional filaments at the corners of the hexagonal structure which extend along the length of the nanowire.

Illuminating the second conduction band and spin-orbit energy in single wurtzite InP nanowires.

We use polarized photoluminescence excitation spectroscopy to observe the energy and symmetry of the predicted second conduction band in 130 nm diameter wurtzite InP nanowires. We find direct spectroscopic signatures for optical transitions among the A, B, and C hole bands and both the first and the second conduction bands. We determine that the splitting between the first and second conduction bands is 228 ± 7 meV in excellent agreement with theory. From these energies we show that the spin-orbit energy changes substantially between zinc blende and wurtzite InP. We discuss the two quite different solutions within the quasi-cubic approximation and the implications for these measurements. Finally, the observation of well-defined optical transitions between the B- and C-hole bands and the second conduction band suggests that either the theoretical description of the second conduction band as possessing Γ8 symmetry is incomplete, or other interactions are enabling these forbidden transitions.

Transient Rayleigh scattering: a new probe of picosecond carrier dynamics in a single semiconductor nanowire.

Using a new technique, transient Rayleigh scattering, we show that measurements from a single GaAs/AlGaAs core-shell semiconductor nanowire provide sensitive and detailed information on the time evolution of the density and temperature of the electrons and holes after photoexcitation by an intense laser pulse. Through band filling, band gap renormalization, and plasma screening, the presence of a dense and hot electron-hole plasma directly influences the real and imaginary parts of the complex index of refraction that in turn affects the spectral dependence of the Rayleigh scattering cross-section in well-defined ways. By measuring this spectral dependence as a function of time, we directly determine the thermodynamically independent density and temperature of the electrons and holes as a function of time after pulsed excitation as the carriers thermalize to the lattice temperature. We successfully model the results by including ambipolar transport, recombination, and cooling through optic and acoustic phonon emission that quantify the hole mobility at ∼68,000 cm(2)/V·s, linear decay constant at 380 ps, bimolecular recombination rate at 4.8 × 10(-9) cm(3)/s and the energy-loss rate of plasma due to optical and acoustic phonon emission.

Impact of weekly asymptomatic testing for severe acute respiratory coronavirus virus 2 (SARS-CoV-2) in inpatients at an academic hospital.

We analyzed the impact of a 7-day recurring asymptomatic SARS-CoV-2 testing protocol for all patients hospitalized at a large academic center. Overall, 40 new cases were identified, and 1 of 3 occurred after 14 days of hospitalization. Recurring testing can identify unrecognized infections, especially during periods of elevated community transmission.

Healthcare personnel interactive pathogen exposure response system.

Exposure investigations are labor intensive and vulnerable to recall bias. We developed an algorithm to identify healthcare personnel (HCP) interactions from the electronic health record (EHR), and we evaluated its accuracy against conventional exposure investigations. The EHR algorithm identified every known transmission and used ranking to produce a manageable contact list.

Severe acute respiratory coronavirus virus 2 (SARS-CoV-2) exposure investigations using genomic sequencing among healthcare workers and patients in a large academic center.

Severe acute respiratory coronavirus virus 2 (SARS-CoV-2) transmissions among healthcare workers and hospitalized patients are challenging to confirm. Investigation of infected persons often reveals multiple potential risk factors for viral acquisition. We combined exposure investigation with genomic analysis confirming 2 hospital-based clusters. Prolonged close contact with unmasked, unrecognized infectious, individuals was a common risk.

Imaging and Molecular Annotation of Xenographs and Tumours (IMAXT): High throughput data and analysis infrastructure.

With the aim of producing a 3D representation of tumors, imaging and molecular annotation of xenografts and tumors (IMAXT) uses a large variety of modalities in order to acquire tumor samples and produce a map of every cell in the tumor and its host environment. With the large volume and variety of data produced in the project, we developed automatic data workflows and analysis pipelines. We introduce a research methodology where scientists connect to a cloud environment to perform analysis close to where data are located, instead of bringing data to their local computers. Here, we present the data and analysis infrastructure, discuss the unique computational challenges and describe the analysis chains developed and deployed to generate molecularly annotated tumor models. Registration is achieved by use of a novel technique involving spherical fiducial marks that are visible in all imaging modalities used within IMAXT. The automatic pipelines are highly optimized and allow to obtain processed datasets several times quicker than current solutions narrowing the gap between data acquisition and scientific exploitation.

Bypass of a psoralen DNA interstrand cross-link by DNA polymerases ß, ι, and κ in vitro.

Repair of DNA interstrand cross-links in mammalian cells involves several biochemically distinctive processes, including the release of one of the cross-linked strands and translesion DNA synthesis (TLS). In this report, we investigated the in vitro TLS activity of a psoralen DNA interstrand cross-link by three DNA repair polymerases, DNA polymerases ß, κ, and ι. DNA polymerase ß is capable of bypassing a psoralen cross-link with a low efficiency. Cell extracts prepared from DNA polymerase ß knockout mouse embryonic fibroblasts showed a reduced bypass activity of the psoralen cross-link, and purified DNA polymerase ß restored the bypass activity. In addition, DNA polymerase ι misincorporated thymine across the psoralen cross-link and DNA polymerase κ extended these mispaired primer ends, suggesting that DNA polymerase ι may serve as an inserter and DNA polymerase κ may play a role as an extender in the repair of psoralen DNA interstrand cross-links. The results demonstrated here indicate that multiple DNA polymerases could participate in TLS steps in mammalian DNA interstrand cross-link repair.