Showing papers by "Clemson University published in 2011"

Ancestral polyploidy in seed plants and angiosperms

Abstract: Whole-genome duplication (WGD), or polyploidy, followed by gene loss and diploidization has long been recognized as an important evolutionary force in animals, fungi and other organisms, especially plants. The success of angiosperms has been attributed, in part, to innovations associated with gene or whole-genome duplications, but evidence for proposed ancient genome duplications pre-dating the divergence of monocots and eudicots remains equivocal in analyses of conserved gene order. Here we use comprehensive phylogenomic analyses of sequenced plant genomes and more than 12.6 million new expressed-sequence-tag sequences from phylogenetically pivotal lineages to elucidate two groups of ancient gene duplications-one in the common ancestor of extant seed plants and the other in the common ancestor of extant angiosperms. Gene duplication events were intensely concentrated around 319 and 192 million years ago, implicating two WGDs in ancestral lineages shortly before the diversification of extant seed plants and extant angiosperms, respectively. Significantly, these ancestral WGDs resulted in the diversification of regulatory genes important to seed and flower development, suggesting that they were involved in major innovations that ultimately contributed to the rise and eventual dominance of seed plants and angiosperms.

A Major Constituent of Brown Algae for Use in High-Capacity Li-Ion Batteries

Abstract: The identification of similarities in the material requirements for applications of interest and those of living organisms provides opportunities to use renewable natural resources to develop better materials and design better devices. In our work, we harness this strategy to build high-capacity silicon (Si) nanopowder–based lithium (Li)–ion batteries with improved performance characteristics. Si offers more than one order of magnitude higher capacity than graphite, but it exhibits dramatic volume changes during electrochemical alloying and de-alloying with Li, which typically leads to rapid anode degradation. We show that mixing Si nanopowder with alginate, a natural polysaccharide extracted from brown algae, yields a stable battery anode possessing reversible capacity eight times higher than that of the state-of-the-art graphitic anodes.

Technostress: technological antecedents and implications

Abstract: With the proliferation and ubiquity of information and communication technologies (ICTs), it is becoming imperative for individuals to constantly engage with these technologies in order to get work accomplished. Academic literature, popular press, and anecdotal evidence suggest that ICTs are responsible for increased stress levels in individuals (known as technostress). However, despite the influence of stress on health costs and productivity, it is not very clear which characteristics of ICTs create stress. We draw from IS and stress research to build and test a model of technostress. The person-environment fit model is used as a theoretical lens. The research model proposes that certain technology characteristics--like usability (usefulness, complexity, and reliability), intrusiveness (presenteeism, anonymity), and dynamism (pace of change)--are related to stressors (work overload, role ambiguity, invasion of privacy, work-home conflict, and job insecurity). Field data from 661 working professionals was obtained and analyzed. The results clearly suggest the prevalence of technostress and the hypotheses from the model are generally supported. Work overload and role ambiguity are found to be the two most dominant stressors, whereas intrusive technology characteristics are found to be the dominant predictors of stressors. The results open up new avenues for research by highlighting the incidence of technostress in organizations and possible interventions to alleviate it.

Phylogenetic Molecular Ecological Network of Soil Microbial Communities in Response to Elevated CO2

Abstract: Understanding the interactions among different species and their responses to environmental changes, such as ele- vated atmospheric concentrations of CO2, is a central goal in ecology but is poorly understood in microbial ecology. Here we describe a novel random matrix theory (RMT)-based conceptual framework to discern phylogenetic molecular ecological net- works using metagenomic sequencing data of 16S rRNA genes from grassland soil microbial communities, which were sampled from a long-term free-air CO2enrichment experimental facility at the Cedar Creek Ecosystem Science Reserve in Minnesota. Our experimental results demonstrated that an RMT-based network approach is very useful in delineating phylogenetic molecu- lar ecological networks of microbial communities based on high-throughput metagenomic sequencing data. The structure of the identified networks under ambient and elevated CO 2levels was substantially different in terms of overall network topology, net- work composition, node overlap, module preservation, module-based higher-order organization, topological roles of individual nodes, and network hubs, suggesting that the network interactions among different phylogenetic groups/populations were markedly changed. Also, the changes in network structure were significantly correlated with soil carbon and nitrogen contents, indicating the potential importance of network interactions in ecosystem functioning. In addition, based on network topology, microbial populations potentially most important to community structure and ecosystem functioning can be discerned. The novel approach described in this study is important not only for research on biodiversity, microbial ecology, and systems micro- biology but also for microbial community studies in human health, global change, and environmental management. IMPORTANCE The interactions among different microbial populations in a community play critical roles in determining ecosys- tem functioning, but very little is known about the network interactions in a microbial community, owing to the lack of appro- priate experimental data and computational analytic tools. High-throughput metagenomic technologies can rapidly produce a massive amount of data, but one of the greatest difficulties is deciding how to extract, analyze, synthesize, and transform such a vast amount of information into biological knowledge. This study provides a novel conceptual framework to identify microbial interactions and key populations based on high-throughput metagenomic sequencing data. This study is among thefirst to doc- ument that the network interactions among different phylogenetic populations in soil microbial communities were substantially changed by a global change such as an elevated CO2level. The framework developed will allow microbiologists to address re- search questions which could not be approached previously, and hence, it could represent a new direction in microbial ecology research.

Predictive Cruise Control: Utilizing Upcoming Traffic Signal Information for Improving Fuel Economy and Reducing Trip Time

Abstract: This brief proposes the use of upcoming traffic signal information within the vehicle's adaptive cruise control system to reduce idle time at stop lights and fuel consumption. To achieve this goal an optimization-based control algorithm is formulated that uses short range radar and traffic signal information predictively to schedule an optimum velocity trajectory for the vehicle. The control objectives are: timely arrival at green light with minimal use of braking, maintaining safe distance between vehicles, and cruising at or near set speed. Three example simulation case studies are presented to demonstrate the potential impact on fuel economy, emission levels, and trip time.

Trust in a specific technology: An investigation of its components and measures

Abstract: Trust plays an important role in many Information Systems (IS)-enabled situations. Most IS research employs trust as a measure of interpersonal or person-to-firm relations, such as trust in a Web vendor or a virtual team member. Although trust in other people is important, this article suggests that trust in the Information Technology (IT) itself also plays a role in shaping IT-related beliefs and behavior. To advance trust and technology research, this article presents a set of trust in technology construct definitions and measures. We also empirically examine these construct measures using tests of convergent, discriminant, and nomological validity. This study contributes to the literature by providing: (a) a framework that differentiates trust in technology from trust in people, (b) a theory-based set of definitions necessary for investigating different kinds of trust in technology, and (c) validated trust in technology measures useful to research and practice.

Graphene oxide: a nonspecific enhancer of cellular growth.

Abstract: There have been multiple conflicting reports about the biocompatibility and antimicrobial activity of graphene oxide. To address this, we conducted a study to characterize the antimicrobial properties of graphene oxide (GO) and its biocompatibility with mammalian cells. When GO was added to a bacterial culture at 25 μg/mL, the results showed that bacteria grew faster and to a higher optical density than cultures without GO. Scanning electron microscopy indicated that bacteria formed dense biofilms in the presence of GO. This was shown by a large mass of aggregated cells and extracellular polymeric material. Bacterial growth on filters coated with 25 and 75 μg of GO grew 2 and 3 times better than on filters without GO. Closer analysis showed that bacteria were able to attach and proliferate preferentially in areas containing the highest GO levels. Graphene oxide films failed to produce growth inhibition zones around them, indicating a lack of antibacterial properties. Also, bacteria were able to grow on GO films to 9.5 × 10(9) cells from an initial inoculation of 1.0 × 10(6), indicating that it also lacks bacteriostatic activity. Thus, silver-coated GO films were able to produce clearing zones and cell death. Also, graphene oxide was shown to greatly enhance the attachment and proliferation of mammalian cells. This study conclusively demonstrates that graphene oxide does not have intrinsic antibacterial, bacteriostatic, and cytotoxic properties in both bacteria and mammalian cells. Furthermore, graphene oxide acts as a general enhancer of cellular growth by increasing cell attachment and proliferation.

Catalytic reduction of 4-nitrophenol using biogenic gold and silver nanoparticles derived from Breynia rhamnoides.

Abstract: A simple, green method is described for the synthesis of Gold (Au) and Silver (Ag) nanoparticles (NPs) from the stem extract of Breynia rhamnoides. Unlike other biological methods for NP synthesis, the uniqueness of our method lies in its fast synthesis rates (~7 min for AuNPs) and the ability to tune the nanoparticle size (and subsequently their catalytic activity) via the extract concentration used in the experiment. The phenolic glycosides and reducing sugars present in the extract are largely responsible for the rapid reduction rates of Au(3+) ions to AuNPs. Efficient reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in the presence of AuNPs (or AgNPs) and NaBH(4) was observed and was found to depend upon the nanoparticle size or the stem extract concentration used for synthesis.

Sustainable Consumption: Opportunities for Consumer Research and Public Policy:

Abstract: This essay explores sustainable consumption and considers possible roles for marketing and consumer researchers and public policy makers in addressing the many sustainability challenges that pervade the planet. Future research approaches to this interdisciplinary topic must be comprehensive and systematic and would benefit from a variety of different perspectives. There are several opportunities for further research; the authors explore three areas in detail. First, they consider the inconsistency between the attitudes and behaviors of consumers with respect to sustainability. Second, they broaden the agenda to explore the role of individual citizens in society. Third, they propose a macroinstitutional approach to fostering sustainability. For each of these separate, but interrelated, opportunities, the authors examine the area in detail and consider possible research avenues and public policy initiatives.

Two-Stage DEA: Caveat Emptor

Abstract: This paper examines the wide-spread practice where data envelopment analysis (DEA) efficiency estimates are regressed on some environmental variables in a second-stage analysis. In the literature, only two statistical models have been proposed in which second-stage regressions are well-defined and meaningful. In the model considered by Simar and Wilson (J Prod Anal 13:49–78, 2007), truncated regression provides consistent estimation in the second stage, where as in the model proposed by Banker and Natarajan (Oper Res 56: 48–58, 2008a), ordinary least squares (OLS) provides consistent estimation. This paper examines, compares, and contrasts the very different assumptions underlying these two models, and makes clear that second-stage OLS estimation is consistent only under very peculiar and unusual assumptions on the data-generating process that limit its applicability. In addition, we show that in either case, bootstrap methods provide the only feasible means for inference in the second stage. We also comment on ad hoc specifications of second-stage regression equations that ignore the part of the data-generating process that yields data used to obtain the initial DEA estimates.

Carbon Nanoparticles as Visible-Light Photocatalysts for Efficient CO2 Conversion and Beyond

Abstract: Increasing atmospheric CO2 levels have generated much concern, driving the ongoing carbon sequestration effort. A compelling CO2 sequestration option is its photocatalytic conversion to hydrocarbons, for which the use of solar irradiation represents an ultimate solution. Here we report a new strategy of using surface-functionalized small carbon nanoparticles to harvest visible photons for subsequent charge separation on the particle surface in order to drive the efficient photocatalytic process. The aqueous solubility of the catalysts enables photoreduction under more desirable homogeneous reaction conditions. Beyond CO2 conversion, the nanoscale carbon-based photocatalysts are also useful for the photogeneration of H2 from water under similar conditions.

Neowise observations of near-earth objects: preliminary results

Abstract: With the NEOWISE portion of the Wide-field Infrared Survey Explorer (WISE) project, we have carried out a highly uniform survey of the near-Earth object (NEO) population at thermal infrared wavelengths ranging from 3 to 22 μm, allowing us to refine estimates of their numbers, sizes, and albedos. The NEOWISE survey detected NEOs the same way whether they were previously known or not, subject to the availability of ground-based follow-up observations, resulting in the discovery of more than 130 new NEOs. The survey's uniform sensitivity, observing cadence, and image quality have permitted extrapolation of the 428 near-Earth asteroids (NEAs) detected by NEOWISE during the fully cryogenic portion of the WISE mission to the larger population. We find that there are 981 ± 19 NEAs larger than 1 km and 20,500 ± 3000 NEAs larger than 100 m. We show that the Spaceguard goal of detecting 90% of all 1 km NEAs has been met, and that the cumulative size distribution is best represented by a broken power law with a slope of 1.32 ± 0.14 below 1.5 km. This power-law slope produces ~13,200 ± 1900 NEAs with D > 140 m. Although previous studies predict another break in the cumulative size distribution below D ~ 50-100 m, resulting in an increase in the number of NEOs in this size range and smaller, we did not detect enough objects to comment on this increase. The overall number for the NEA population between 100 and 1000 m is lower than previous estimates. The numbers of near-Earth comets and potentially hazardous NEOs will be the subject of future work.

A survey of problems, solution techniques, and future challenges in scheduling semiconductor manufacturing operations

Abstract: In this paper, we discuss scheduling problems in semiconductor manufacturing. Starting from describing the manufacturing process, we identify typical scheduling problems found in semiconductor manufacturing systems. We describe batch scheduling problems, parallel machine scheduling problems, job shop scheduling problems, scheduling problems with auxiliary resources, multiple orders per job scheduling problems, and scheduling problems related to cluster tools. We also present important solution techniques that are used to solve these scheduling problems by means of specific examples, and report on known implementations. Finally, we summarize some of the challenges in scheduling semiconductor manufacturing operations.

Design of Highly Emissive Polymer Dot Bioconjugates for In Vivo Tumor Targeting

Abstract: Nanoparticle-based diagnostic and therapeutic agents have attracted considerable interest because of their potential for clinical oncology and other biomedical research.[1] Versatile nanostructures have been demonstrated for in vivo applications, such as lipid and polymeric nanocapsules for drug delivery,[2] iron oxide nanoparticles for magnetic resonance imaging,[3] gold nanoparticles for X-ray computed tomography,[4] and quantum dots (Qdots) for fluorescence imaging.[5] Qdots represent one of the exciting nanotechnologies translated to biology in the past decade. The size-tunable luminescence makes them appealing as multicolor fluorophores for biological labelling, imaging, and sensing.[6,7] For in vivo applications, however, the intrinsic toxicity of Qdots is of critical concern,[8] which may impede their final clinical translation. Therefore, the design of bright probes with biologically benign materials is highly desirable for many in vivo clinical applications.

Teachers' Perceptions of Integrating Information and Communication Technologies into Literacy Instruction: A National Survey in the United States

Abstract: This research explores literacy teachers’ perceptions of integrating information communication technologies (ICTs) into literacy instruction. To this end, a national survey of 1,441 literacy teachers in the United States was conducted. The survey provided data concerning the types and levels of reported availability and use of ICTs, beliefs about the importance of integrating ICTs into literacy instruction, and perceived obstacles to doing so. The analysis of data included descriptive statistics, an exploratory factor analysis, and a path analysis used to test a model hypothesizing a relation between teachers’ perceived importance of technology and reported levels of integration. Results revealed relatively low levels of curricular integration, consistent perceptions about obstacles to integration, and technological rather than curricular definitions of ICTs and of integration. The path analysis suggested several characteristics and influences associated with higher levels of integration and use. The findings advance understanding of the extent to which ICTs are being integrated into literacy instruction and what factors should be considered toward profitably increasing integration consistent with expanding definitions of literacy.

Attitudes Toward Online Social Connection and Self-Disclosure as Predictors of Facebook Communication and Relational Closeness

Abstract: This investigation tested a theoretical model of communication behavior with specific Facebook friends, such that attitudes toward (a) online self-disclosure, and (b) online social connection, predict Facebook communication frequency and, in turn, relational closeness. Participants included both undergraduates and older adults. Results generally supported the model, with the interaction effect between self-disclosure and social connection directly predicting Facebook communication and indirectly predicting relational closeness. For both dependent variables, online social connection was a positive predictor at low and moderate levels of online self-disclosure, but high levels reduced the association to nonsignificance. One implication of these results was that high-warrant information may discourage those with social anxiety from social network site communication.

Stimulus responsive nanogels for drug delivery

Abstract: Stimulus responsive nanogels are polymeric nanoparticles which are capable of responding to external stimuli by changing their physico-chemical properties, such as volume, water content, refractive index, permeability, and hydrophilicity–hydrophobicity. Compared with other polymer nanoparticles used for drug delivery, stimulus responsive nanogels are noted for their ability to encapsulate bioactive drugs, their high stability for prolonged circulation in the blood stream, and their controlled release and site-specific targeting of loaded drugs modulated by environment stimuli. Particularly, the application of stimulus responsive nanogels provides an interesting opportunity for drug delivery in which the delivery system becomes an active participant, rather than a passive carrier, in the optimization of disease therapy. In this article, the authors review the recent developments in the preparation and application in drug delivery of stimulus responsive nanogels which can respond to small temperature and pH changes, light, magnetic field, biomolecule recognition (specifically glucose responsive nanogels for insulin delivery), and multi-responsive nanogels. The limitations and future improvements of stimulus responsive nanogels are also discussed.

Rheology and Phase Behavior of Lyotropic Cellulose Nanocrystal Suspensions

Abstract: The dispersion microstructure and rheological properties of aqueous sulfonated cellulose nanocrystal (CNC) suspensions have been investigated. Between 3.07 and 10.4 vol % the suspensions phase separated into liquid crystalline and isotropic domains. At 12.1 vol %, no isotropic phase was visible, and the samples had the fingerprint texture characteristic of a cholesteric liquid crystal. Below 35 °C, temperature had little influence on rheology and phase behavior. However, between 35 and 40 °C there was a significant change in both the fraction of isotropic phase and the rheological properties. In contrast to many lyotropic suspensions, the steady shear viscosity did not go through a maximum with increasing concentration. Maxima were observed for complex viscosity, storage modulus, and loss modulus at concentrations that appeared fully liquid crystalline. Time–concentration superposition was successful for the loss modulus but not the storage modulus. This suggests that the interface in biphasic samples aff...

Stigma, Negative Attitudes About Treatment, and Utilization of Mental Health Care Among Soldiers

Abstract: Stigma and organizational barriers have been identified as factors for why a small proportion of soldiers with psychological problems seek professional help. In this article, we examine the impact of negative attitudes toward treatment on treatment seeking among soldiers previously deployed to Afghanistan or Iraq (n = 2,623). We asked soldiers with psychological problems questions about stigma, organizational barriers, negative attitudes toward treatment, and whether they sought treatment for their psychological problems. We found that negative attitudes about treatment inversely predicted treatment seeking. These results provide a more comprehensive examination of reasons that soldiers do not seek needed treatment and highlight the need for policy aimed at reducing negative attitudes toward mental health treatment.

When distress hits home: the role of contextual factors and psychological distress in predicting employees' responses to abusive supervision.

Abstract: We developed a model of the relationships among aggressive norms, abusive supervision, psychological distress, family undermining, and supervisor-directed deviance. We tested the model in 2 studies using multisource data: a 3-wave investigation of 184 full-time employees (Study 1) and a 2-wave investigation of 188 restaurant workers (Study 2). Results revealed that (a) abusive supervision mediated the relationship between aggressive norms and psychological distress, (b) psychological distress mediated the effects of abusive supervision on spouse undermining, (c) abusive supervision had a direct positive relationship with supervisor-directed deviance, (d) the positive relationship between psychological distress and spouse undermining was stronger for men as opposed to women, and (e) employees engaged in relationship-oriented occupations reported greater levels of abusive supervision and psychological distress. Implications for theory and practice are discussed.

More Oil, Less Democracy: Evidence from Worldwide Crude Oil Discoveries

Abstract: This article exploits variations in the timing and size of oil discoveries to identify the impact of oil wealth on democracy. I find that discovering 100 billion barrels of oil (approximately the initial endowment of Iraq) pushes a country's democracy level almost 20 percentage points below trend after three decades. The estimated effect is larger for oilfields with higher-quality oil and lower exploration and extraction costs. However, the estimates become less precise when oil abundance is measured by oil discovery per capita, suggesting politicians may care about the level instead of the per capita value of oil wealth.

On the electrical conductivity of microbial nanowires and biofilms

Abstract: Dissimilatory metal-reducing bacteria (DMRB), such as Geobacter and Shewanella spp., occupy a distinct metabolic niche in which they acquire energy by coupling oxidation of organic fuels with reduction of insoluble extracellular electron acceptors (i.e., minerals). Their unique extracellular electron transfer (EET) capabilities extend to reduction of anodes (electrodes maintained at sufficiently positive potentials) on which they form persistent, electric current generating biofilms. One hypothesis describing the mechanism of EET by Geobacter and Shewanella spp. involves superexchange in which electrons are conducted by a succession of electron transfer reactions among redox proteins associated with the outer cell membranes, aligned along pilus-like filaments (e.g.pili), and/or throughout the extracellular matrix. Here we present theory, previously developed to describe superexchange within abiotic redox polymers, to describe superexchange within DMRB biofilms grown on anodes. We show that this theory appears to apply to recent ex situ measurements of electrical conductivity by individual pilus-like filaments of S. oneidensis MR-1 and G. sulfurreducensDL1, referred to as microbial nanowires. Microbial nanowires have received much recent attention because they are thought by some to impart electrical conductivity to DMRB biofilms and because of the prospect of microbe-produced conductive nanomaterials. We also show that this theory appears to apply to preliminary in situ demonstration of electrical conductivity of an anode-grown G. sulfurreducensDL1 biofilm. Based on these results we suggest a role for nanowires of S. oneidensis and G. sulfurreducens in biofilm conductivity.

The Role of a Bilayer Interfacial Phase on Liquid Metal Embrittlement

Abstract: Intrinsically ductile metals are prone to catastrophic failure when exposed to certain liquid metals, but the atomic-level mechanism for this effect is not fully understood. We characterized a model system, a nickel sample infused with bismuth atoms, by using aberration-corrected scanning transmission electron microscopy and observed a bilayer interfacial phase that is the underlying cause of embrittlement. This finding provides a new perspective for understanding the atomic-scale embrittlement mechanism and for developing strategies to control the practically important liquid metal embrittlement and the more general grain boundary embrittlement phenomena in alloys. This study further demonstrates that adsorption can induce a coupled grain boundary structural and chemical phase transition that causes drastic changes in properties.

Oxide thermoelectrics: The challenges, progress, and outlook

Abstract: Most state-of-the-art thermoelectric (TE) materials contain heavy elements Bi, Pb, Sb, or Te and exhibit maximum figure of merit, ZT∼1–2. On the other hand, oxides were believed to make poor TEs because of the low carrier mobility and high lattice thermal conductivity. That is why the discoveries of good p-type TE properties in layered cobaltites NaxCoO2, Ca4Co3O9, and Bi2Sr2Co2O9, and promising n-type TE properties in CaMnO3- and SrTiO3-based perovskites and doped ZnO, broke new ground in thermoelectrics study. The past two decades have witnessed more than an order of magnitude enhancement in ZT of oxides. In this article, we briefly review the challenges, progress, and outlook of oxide TE materials in their different forms (bulk, epitaxial film, superlattice, and nanocomposites), with a greater focus on the nanostructuring approach and the late development of the oxide-based TE module.

Polymer brushes by the "grafting to" method.

Abstract: This paper focuses on the attachment of densely grafted polymer layers (polymer brushes) to various inorganic and polymeric substrates by the "grafting to" method A brief overview of synthesis of polymer brushes by the method is first provided, with emphasis on chemical approaches to polymer attachment The second part of the paper covers the synthesis of polymer layers via a recently developed macromolecular anchoring layer approach Several examples of application of the grafting technique are presented for generation of hydrophobic, hydrophilic, gradient, and switchable surfaces

The nature of "dark" gamma-ray bursts

Abstract: Context. Thirteen years after the discovery of the first afterglows, the nature of dark gamma-ray bursts (GRB) still eludes explanation: while each long-duration GRB typically has an X-ray afterglow, optical/NIR emission is only seen for 40–60% of them. Aims. Here we use the afterglow detection statistics of the systematic follow-up observations performed with GROND since mid-2007 in order to derive the fraction of “dark bursts” according to different methods, and to distinguish between various scenarios for “dark bursts”. Methods. Observations were performed with the 7-channel “Gamma-Ray Optical and Near-infrared Detector” (GROND) at the 2.2 m MPI/ESO telescope. We used the afterglow detection rate in dependence on the delay time between GRB and the first GROND exposure. Results. For long-duration Swift bursts with a detected X-ray afterglow, we achieve a 90% (35/39) detection rate of optical/NIR afterglows whenever our observations started within less than 240 min after the burst. Complementing our GROND data with Swift/XRT spectra we construct broad-band spectral energy distributions and derive rest-frame extinctions.Conclusions. We detect 25–40% “dark bursts”, depending on the definition used. The faint optical afterglow emission of “dark bursts” is mainly due to a combination of two contributing factors: (i) moderate intrinsic extinction at moderate redshifts, and (ii) about 22% of “dark” bursts at redshift >5.

Toward quantitatively fluorescent carbon-based "quantum" dots.

Abstract: Carbon-based “quantum” dots (or “carbon dots”) are generally defined as surface-passivated small carbon nanoparticles that are brightly fluorescent. Apparently, the carbon particle surface passivation in carbon dots is critical to their fluorescence performance. An effective way to improve the surface passivation is to dope the surface of the precursor carbon nanoparticles with an inorganic salt, followed by the typical functionalization with organic molecules. In this work we passivated small carbon nanoparticles by a combination of the surface-doping with nanoscale semiconductors and the organic functionalization, coupled with gel column fractionation to harvest the most fluorescent carbon dots, which exhibited fluorescence emission quantum yields of up to 78%. Experimental and mechanistic issues relevant to potentially further improve the performance of carbon dots toward their being quantitatively fluorescent are discussed.

Dreams Fulfilled, Dreams Shattered: Determinants of Segmented Assimilation in the Second Generation

Abstract: We summarize prior theories on the adaptation process of the contemporary immigrant second generation as a prelude to presenting additive and interactive models showing the impact of family variables, school contexts and academic outcomes on the process. For this purpose, we regress indicators of educational and occupational achievement in early adulthood on predictors measured three and six years earlier. The Children of Immigrants Longitudinal Study, used for the analysis, allows us to establish a clear temporal order among exogenous predictors and the two dependent variables. We also construct a Downward Assimilation Index, based on six indicators and regress it on the same set of predictors. Results confirm a pattern of segmented assimilation in the second generation, with a significant proportion of the sample experiencing downward assimilation. Predictors of the latter are the obverse of those of educational and occupational achievement. Significant interaction effects emerge between these predictors and early school contexts, defined by different class and racial compositions. Implications of these results for theory and policy are examined.

Electromechanical Modeling and Nonlinear Analysis of Axially Loaded Energy Harvesters

Abstract: To maximize the electromechanical transduction of vibratory energy harvesters, the resonance frequency of the harvesting device is usually tuned to the excitation frequency. To achieve this goal, some concepts call for utilizing an axial static preload to soften or stiffen the structure (Leland and Wright, 2006, "Resonance Tuning of Piezoelectric Vibration Energy Scavenging Generators Using Compressive Axial Preload, " Smart Mater. Struct., 15, pp. 1413-1420; Morris et al., 2008, "A Resonant Frequency Tunable, Extensional Mode Piezoelectric Vibration Harvesting Mechanism, " Smart Mater. Struct., 17, p. 065021). For the most part, however, models used to describe the effect of the axial preload on the harvester's response are linear lumped-parameter models that can hide some of the essential features of the dynamics and, sometimes, oppose the experimental trends. To resolve this issue, this study aims to develop a comprehensive understanding of energy harvesting using axially loaded beams. Specifically, using nonlinear Euler-Bernoulli beam theory, an electromechanical model of a clamped-clamped energy harvester subjected to transversal excitations and static axial loading is developed and discretized using a Galerkin expansion. Using the method of multiple scales, the general nonlinear physics of the system is investigated by obtaining analytical expressions for the steady-state response amplitude, the voltage drop across a resistive load, and the output power. These theoretical expressions are then validated against experimental data. It is demonstrated that in addition to the ability of tuning the harvester to the excitation frequency via axial load variations, the axial load aids in (i) increasing the electric damping in the system, thereby enhancing the energy transfer from the beam to the electric load, (ii) amplifying the effect of the external excitation on the structure, and (iii) enhancing the effective nonlinearity of the device. These factors combined can increase the steady-state response amplitude, output power, and bandwidth of the harvester.