Graphite's lubricating properties due to the “weak” interactions between individual layers have long been known. However, these interactions are not weak enough to allow graphite to readily exfoliate ...into graphene on a large scale. Separating graphite layers down to a single sheet is an intense area of research as scientists attempt to utilize graphene's superlative properties. The exfoliation and processing of layered materials is governed by the friction between layers. Friction on the macroscale can be intuitively understood, but there is little understanding of the mechanisms involved in nanolayered materials. Using molecular dynamics and a new forcefield, graphene's unusual behavior in a superlubric state is examined, and the energy dissipated between two such surfaces sliding past each other is shown. The dependence of friction on temperature and surface roughness is described, and agreement with experiment is reported. The accuracy of the simulated behavior enables the processes that drive exfoliation of graphite into individual graphene sheets to be described. Taking into account the friction between layers, a peeling mechanism of exfoliation is predicted to be of lower energy cost than shearing.
Graphite's lubricating properties due to the “weak” interactions between individual layers, have long been known. However, graphene still cannot be exfoliated on a large scale. A new classical molecular dynamics forcefield is applied to unravel the remarkable interactions between graphene sheets. It is found that peeling graphene sheets apart, rather than shearing, is an easier route to exfoliation.
A review is presented of the development of in situ high-resolution transmission electron microscopy (HRTEM) and its application to directly study the atomic behavior in thermally activated material ...reactions. Not only are the atomic re-arrangements continuously recorded, but kinetic measurements can be made at controlled elevated temperatures. Examples include work on the atomic motion on CdTe surface ledges, solid phase epitaxial regrowth of silicon, crystallization of amorphous silicon and of amorphous tantalum oxide thin films, solid-state amorphization at metal-silicon interfaces, metal-induced crystallization of amorphous silicon, germanium and carbon, phase separation and crystallization in hafnium silicate thin films, and “spiking” across thin gate oxides separating nickel silicide from a monocrystalline silicon substrate. The future prospects of in situ HRTEM are discussed, and the increasing breadth of application of this approach is recognized, especially in light of the advances in HRTEM capabilities.
Controlling the structure of graphene and graphene oxide (GO) phases is vitally important for any of its widespread intended applications: highly ordered arrangements of nanoparticles are needed for ...thin‐film or membrane applications of GO, dispersed nanoparticles for composite materials, and 3D porous arrangements for hydrogels. By combining coarse‐grained molecular dynamics and newly developed accurate models of GO, the driving forces that lead to the various morphologies are resolved. Two hydrophilic polymers, poly(ethylene glycol) (PEG) and poly(vinyl alcohol) (PVA), are used to illustrate the thermodynamically stable morphologies of GO and relevant dispersion mechanisms. GO self‐assembly can be controlled by changing the degree of oxidation, varying from fully aggregated over graphitic domains to intercalated assemblies with polymer bilayers between sheets. The long‐term stability of a dispersion is extremely important for many commercial applications of GO composites. For any degree of oxidation, GO does not disperse in PVA as a thermodynamic equilibrium product, whereas in PEG dispersions are only thermodynamically stable for highly oxidized GO. These findings—validated against the extensive literature on GO systems in organic solvents—furnish quantitative explanations for the empirically unpredictable aggregation characteristics of GO and provide computational methods to design directed synthesis routes for diverse self‐assemblies and applications.
Controlling the dispersion and aggregation of graphene oxide (GO) is critical for harnessing its potential in many applications. This can be challenging as many competing factors contribute to its behavior in solution, and they are poorly understood. Using multiscale simulation, the key factors governing this behavior are mapped out and a framework for controlling the morphology of GO is proposed.
Graphene oxide (GO) is an amorphous 2D material, which has found widespread use in the fields of chemistry, physics, and materials science due to its similarity to graphene with the benefit of being ...far easier to synthesize and process. However, the standard of GO characterization is very poor because its structure is irregular, being sensitive to the preparation method, and it has a propensity to transform due to its reactive nature. Atomistic simulations of GO are common, but the nanostructure in these simulations is often based on little evidence or thought. We have written a computer program to generate graphene oxide nanostructures for general purpose atomistic simulation based on theoretical and experimental evidence. The structures generated offer a significant improvement to the current standard of randomly placed oxidized functional groups and successfully recreate the two-phase nature of oxidized and unoxidized graphene domains observed in microscopy experiments. Using this model, we reveal new features of GO structure and predict that a critical point in the oxidation reaction exists as the oxidized region reaches a percolation threshold. Even by a conservative estimate, we show that, if the carbon to oxygen ratio is kept above 6, a continuous aromatic network will remain, preserving many of graphene’s desirable properties, irrespective of the oxidation method or the size distribution of graphene sheets. This is an experimentally achievable degree of oxidation and should aid better GO synthesis for many applications.
Purpose
Amazon Mechanical Turk is an increasingly popular data source in the organizational psychology research community. This paper presents an evaluation of MTurk and provides a set of practical ...recommendations for researchers using MTurk.
Design/Methodology/Approach
We present an evaluation of methodological concerns related to the use of MTurk and potential threats to validity inferences. Based on our evaluation, we also provide a set of recommendations to strengthen validity inferences using MTurk samples.
Findings
Although MTurk samples can overcome some important validity concerns, there are other limitations researchers must consider in light of their research objectives. Researchers should carefully evaluate the appropriateness and quality of MTurk samples based on the different issues we discuss in our evaluation.
Implications
There is not a one-size-fits-all answer to whether MTurk is appropriate for a research study. The answer depends on the research questions and the data collection and analytic procedures adopted. The quality of the data is not defined by the data source per se, but rather the decisions researchers make during the stages of study design, data collection, and data analysis.
Originality/Value
The current paper extends the literature by evaluating MTurk in a more comprehensive manner than in prior reviews. Past review papers focused primarily on internal and external validity, with less attention paid to statistical conclusion and construct validity—which are equally important in making accurate inferences about research findings. This paper also provides a set of practical recommendations in addressing validity concerns when using MTurk.
Whole-genome duplication (WGD) is believed to be a significant source of major evolutionary innovation. Redundant genes resulting from WGD are thought to be lost or acquire new functions. However, ...the rates of gene loss and thus temporal process of genome reshaping after WGD remain unclear. The WGD shared by all teleost fish, one-half of all jawed vertebrates, was more recent than the two ancient WGDs that occurred before the origin of jawed vertebrates, and thus lends itself to analysis of gene loss and genome reshaping. Using a newly developed orthology identification pipeline, we inferred the post–teleost-specific WGD evolutionary histories of 6,892 protein-coding genes from nine phylogenetically representative teleost genomes on a time-calibrated tree. We found that rapid gene loss did occur in the first 60 My, with a loss of more than 70–80% of duplicated genes, and produced similar genomic gene arrangements within teleosts in that relatively short time. Mathematical modeling suggests that rapid gene loss occurred mainly by events involving simultaneous loss of multiple genes. We found that the subsequent 250 My were characterized by slow and steady loss of individual genes. Our pipeline also identified about 1,100 shared single-copy genes that are inferred to have become singletons before the divergence of clupeocephalan teleosts. Therefore, our comparative genome analysis suggests that rapid gene loss just after the WGD reshaped teleost genomes before the major divergence, and provides a useful set of marker genes for future phylogenetic analysis.
Social resource crafting is a job crafting strategy in which employees proactively increase their social resources (e.g., feedback from coworkers, coaching from supervisor). We examined social ...resource crafting's relationships with work‐related social support (perceived supervisor and coworker support) and work engagement. Specifically, as there has been growing interest in understanding resource crafting and work engagement, we investigated the directionality of this relationship. Using a two‐wave study and structural equation modelling, we tested the dual pathways between resource crafting and work engagement as well as resource crafting's relationship with each type of support. The results suggested social resource crafting was associated with higher levels of coworker support—but not supervisor support ‐ over time and supported a unidirectional path from social resource crafting to work engagement. Our findings provide valuable insight into the power employees have to craft their own positive experiences at work.
Workers and their families bear much of the economic burden of COVID‐19. Even though they have declined somewhat, unemployment rates are considerably higher than before the start of the pandemic. ...Many workers also face uncertainty about their future employment prospects and increasing financial strain. At the same time, the workplace is a common source of transmission of COVID‐19 and many jobs previously seen as relatively safe are now viewed as potentially hazardous. Thus, many workers face dual threats of economic stress and COVID‐19 exposure. This paper develops a model of workers’ responses to these dual threats, including risk perception and resource depletion as mediating factors that influence the relationship of economic stress and occupational risk factors with COVID‐19 compliance‐related attitudes, safe behavior at work, and physical and mental health outcomes. The paper also describes contextual moderators of these relationships at the individual, unit, and regional level. Directions for future research are discussed.
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