Causal Inference in Accounting Research GOW, IAN D.; LARCKER, DAVID F.; REISS, PETER C.
Journal of accounting research,
20/May , Letnik:
54, Številka:
2
Journal Article
Recenzirano
Odprti dostop
This paper examines the approaches accounting researchers adopt to draw causal inferences using observational (or nonexperimental) data. The vast majority of accounting research papers draw causal ...inferences notwithstanding the well-known difficulties in doing so. While some recent papers seek to use quasi-experimental methods to improve causal inferences, these methods also make strong assumptions that are not always fully appreciated. We believe that accounting research would benefit from more in-depth descriptive research, including a greater focus on the study of causal mechanisms (or causal pathways) and increased emphasis on the structural modeling of the phenomena of interest. We argue these changes offer a practical path forward for rigorous accounting research.
InP/ZnS core/shell nanocrystals are prepared using a single-step heating-up method relying on the difference in reactivity of the applied InP and ZnS precursors. The obtained particles exhibit ...size-dependent emission in the range of 480−590 nm, a fluorescence quantum yield of 50−70%, and high photostability.
We review the synthesis of semiconductor nanocrystals/colloidal quantum dots in organic solvents with special emphasis on earth-abundant and toxic heavy metal free compounds. Following the ...Introduction, section 2 defines the terms related to the toxicity of nanocrystals and gives a comprehensive overview on toxicity studies concerning all types of quantum dots. Section 3 aims at providing the reader with the basic concepts of nanocrystal synthesis. It starts with the concepts currently used to describe the nucleation and growth of monodisperse particles and next takes a closer look at the chemistry of the inorganic core and its interactions with surface ligands. Section 4 reviews in more detail the synthesis of different families of semiconductor nanocrystals, namely elemental group IV compounds (carbon nanodots, Si, Ge), III–V compounds (e.g., InP, InAs), and binary and multinary metal chalcogenides. Finally, the authors’ view on the perspectives in this field is given.
Core/Shell Semiconductor Nanocrystals Reiss, Peter; Protière, Myriam; Li, Liang
Small (Weinheim an der Bergstrasse, Germany),
January 19, 2009, Letnik:
5, Številka:
2
Journal Article
Recenzirano
Colloidal core/shell nanocrystals contain at least two semiconductor materials in an onionlike structure. The possibility to tune the basic optical properties of the core nanocrystals, for example, ...their fluorescence wavelength, quantum yield, and lifetime, by growing an epitaxial‐type shell of another semiconductor has fueled significant progress on the chemical synthesis of these systems. In such core/shell nanocrystals, the shell provides a physical barrier between the optically active core and the surrounding medium, thus making the nanocrystals less sensitive to environmental changes, surface chemistry, and photo‐oxidation. The shell further provides an efficient passivation of the surface trap states, giving rise to a strongly enhanced fluorescence quantum yield. This effect is a fundamental prerequisite for the use of nanocrystals in applications such as biological labeling and light‐emitting devices, which rely on their emission properties. Focusing on recent advances, this Review discusses the fundamental properties and synthesis methods of core/shell and core/multiple shell structures of II–VI, IV–VI, and III–V semiconductors.
Quantum Dots: This Review gives a survey of the synthesis and properties of colloidal core/shell nanocrystals, combining two or more semiconductors in an onionlike structure (see image). The epitaxial‐type shell enhances the fluorescence quantum yield and photostability in type‐I structures. In cases of different band alignment, the shell(s) strongly influence the emission wavelength and excited‐state lifetime.
All-inorganic α-CsPbI3 perovskite quantum dots (QDs) are attracting great interest as solar cell absorbers due to their appealing light harvesting properties and enhanced stability due to the absence ...of volatile organic constituents. Moreover, ex situ synthesized QDs significantly reduce the variability of the perovskite layer deposition process. However, the incorporation of α-CsPbI3 QDs into mesoporous TiO2 (m-TiO2) is highly challenging, but these constitute the best performing electron transport materials in state-of-the-art perovskite solar cells. Herein, the m-TiO2 surface is engineered using an electron-rich cesium-ion containing methyl acetate solution. As one effect of this treatment, the solid–liquid interfacial tension at the TiO2 surface is reduced and the wettability is improved, facilitating the migration of the QDs into m-TiO2. As a second effect, Cs+ ions passivate the QD surface and promote the charge transfer at the m-TiO2/QD interface, leading to an enhancement of the electron injection rate by a factor of 3. In combination with an ethanol-environment smoothing route that significantly reduces the surface roughness of the m-TiO2/QD layer, optimized devices exhibit highly reproducible power conversion efficiencies exceeding 13%. The best cell with an efficiency of 14.32% (reverse scan) reaches a short-circuit current density of 17.77 mA cm–2, which is an outstanding value for QD-based perovskite solar cells.
Abstract
The precise construction of photocatalysts with diatomic sites that simultaneously foster light absorption and catalytic activity is a formidable challenge, as both processes follow distinct ...pathways. Herein, an electrostatically driven self-assembly approach is used, where phenanthroline is used to synthesize bifunctional LaNi sites within covalent organic framework. The La and Ni site acts as optically and catalytically active center for photocarriers generation and highly selective CO
2
-to-CO reduction, respectively. Theory calculations and in-situ characterization reveal the directional charge transfer between La-Ni double-atomic sites, leading to decreased reaction energy barriers of *COOH intermediate and enhanced CO
2
-to-CO conversion. As a result, without any additional photosensitizers, a 15.2 times enhancement of the CO
2
reduction rate (605.8 μmol·g
−1
·h
−1
) over that of a benchmark covalent organic framework colloid (39.9 μmol·g
−1
·h
−1
) and improved CO selectivity (98.2%) are achieved. This work presents a potential strategy for integrating optically and catalytically active centers to enhance photocatalytic CO
2
reduction.
This critical review discusses specific preparation and characterization methods applied to hybrid materials consisting of π-conjugated polymers (or oligomers) and semiconductor nanocrystals. These ...materials are of great importance in the quickly growing field of hybrid organic/inorganic electronics since they can serve as active components of photovoltaic cells, light emitting diodes, photodetectors and other devices. The electronic energy levels of the organic and inorganic components of the hybrid can be tuned individually and thin hybrid films can be processed using low cost solution based techniques. However, the interface between the hybrid components and the morphology of the hybrid directly influences the generation, separation and transport of charge carriers and those parameters are not easy to control. Therefore a large variety of different approaches for assembling the building blocks--conjugated polymers and semiconductor nanocrystals--has been developed. They range from their simple blending through various grafting procedures to methods exploiting specific non-covalent interactions between both components, induced by their tailor-made functionalization. In the first part of this review, we discuss the preparation of the building blocks (nanocrystals and polymers) and the strategies for their assembly into hybrid materials' thin films. In the second part, we focus on the charge carriers' generation and their transport within the hybrids. Finally, we summarize the performances of solar cells using conjugated polymer/semiconductor nanocrystals hybrids and give perspectives for future developments.
Small thiol-containing amino acids such as cysteine are appealing surface ligands for transferring semiconductor quantum dots (QDs) from organic solvents to the aqueous phase. They provide a compact ...hydrodynamic diameter and low nonspecific binding in biological environment. However, cysteine-capped QDs generally exhibit modest colloidal stability in water and their fluorescence quantum yield (QY) is significantly reduced as compared to organics. We demonstrate that during phase transfer the deprotonation of the thiol group by carefully adjusting the pH is of crucial importance for increasing the binding strength of cysteine to the QD surface. As a result, the colloidal stability of cysteine-capped InP/ZnS core/shell QDs is extended from less than one day to several months. The developed method is of very general character and can be used also with other hydrophilic thiols and various other types of QDs, e.g., CdSe/CdS/ZnS and CuInS2/ZnS QDs as well as CdSe and CdSe/CdS nanorods. We show that the observed decrease of QY upon phase transfer with cysteine is related to the generation of cysteine dimer, cystine. This side-reaction implies the formation of disulfide bonds, which efficiently trap photogenerated holes and inhibit radiative recombination. On the other hand, this process is not irreversible. By addition of an appropriate reducing agent, tris(2-carboxyethyl)phosphine hydrochloride (TCEP), the QY can be partially recovered. When TCEP is already added during the phase transfer, the QY of cysteine-capped InP/ZnS QDs can be maintained almost quantitatively. Finally, we show that penicillamine is a promising alternative to cysteine for the phase transfer of QDs, as it is much less prone to disulfide formation.
With the introduction of combination antiretroviral therapy AIDS dementia complex or HIV-associated dementia, as it was termed later, largely disappeared in clinical practice. However, in the past ...few years, patients, long-term infected and treated, including those with systemically well controlled infection, started to complain about milder memory problems and slowness, difficulties in concentration, planning, and multitasking. Neuropsychological studies have confirmed that cognitive impairment occurs in a substantial (15-50%) proportion of patients. Among HIV-1-infected patients cognitive impairment was and is one of the most feared complications of HIV-1 infection. In addition, neurocognitive impairment may affect adherence to treatment and ultimately result in increased morbidity for systemic disease. So what may be going on in the CNS after so many years of apparently controlled HIV-1 infection is an urgent and important challenge in the field of HIV medicine. In this review we summarize the key currently available data. We describe the clinical neurological and neuropsychological findings, the preferred diagnostic approach with new imaging techniques and cerebrospinal fluid analysis. We try to integrate data on pathogenesis and finally discuss possible therapeutic interventions.
Strongly luminescent CuInS2/ZnS core/shell nanocrystals were synthesized from copper iodide, indium acetate, zinc stearate, and dodecanethiol as starting compounds in octadecene solvent. The ...as-prepared core/shell nanocrystals exhibit a low size distribution (<10%), and present photoluminescence in the range of 550−815 nm with a maximum fluorescence quantum yield (QY) of 60%. Time-resolved fluorescence spectroscopy revealed that the lifetimes of the different spectral components are on the order of hundreds of nanoseconds, indicating that donor−acceptor pair recombinations are at the origin of the observed emission bands. The CuInS2/ZnS nanocrystals were subsequently transferred to the aqueous phase via surface ligand exchange with dihydrolipoic acid and used as fluorescent labels for in vivo imaging. After tail vein injection into nude mice, the biodistribution of the quantum dots was monitored during 24 h using fluorescence reflectance imaging.
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