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AbstractChitosan, a biopolymer known for its biocompatibility, biodegradability, and chemical adaptability, has attracted significant attention in scientific research. Chitosan-metal nanocomposites represent an emerging area of exploration. Among these, Chitosan-Fe nanocomposite has gained prominence due to its various applications, including photoremediation, bioimaging, and drug delivery systems. In this study, we delved into the synthesis and physicochemical properties of Chitosan-Fe nanocomposites. These nanocomposites exhibited a spherical structure with active binding sites that allowed for the functionalization of Fe3+ions on their surface. Density functional theory simulations corroborated this alteration, demonstrating changes in surface charge properties resulting in increased mechanical strength. A key finding of this study is the enhanced antibacterial activity exhibited by the Cs-Fe nanocomposites against both Escherichia coli (62.5 µg/mL)and Staphylococcus aureus (125.25 µg/mL), surpassing that of bare Chitosan nanoparticles. Furthermore, our investigation confirms the therapeutic safety of the Chitosan-Fe nanocomposite. The cell viability calculated by MTT assay and accepted therapeutic dose concentration for CS-Fe NC was 125 ug/mL whereas for CSNP < 15.62 µg/mL. This underscores the nanocomposite’s potential in biomedical applications. In addition to its biomedical promise, the Chitosan-Fe nanocomposite also demonstrates remarkable potential in environmental remediation, particularly in the removal of hexavalent chromium.

Sodium (Na)-based batteries are being explored as a sustainable and cost-effective alternative to Lithium (Li)-ion batteries. In particular, an “anode-free” Na metal battery offers the possibility to match or even exceed the energy density of the incumbent Li-ion technology. Nevertheless, the present lifespan of these batteries is insufficient to render them suitable as an energy source for current electronic devices and grid systems. The main reason for this is the evolution and growth of Na metal dendrites during the charge-discharge process. In this study, we report a “nano-silica modified suspension electrolyte” that improves the average coulombic efficiency and cycling performance of anode-free Na metal batteries. The nano-silica additives increase the Na+diffusion coefficient in the electrolyte by ∼1000-fold, thereby decreasing the nucleation overpotential and inhibiting the formation of Na metal dendrites. We demonstrate that a Na|Cu half-cell with the suspension electrolyte can cycle stably for over 500 cycles at ∼1 mA cm−2current density and an aerial capacity of ∼2 mAh cm−2. When paired with an Na3V2(PO4)3(NVP) cathode, the anode-free NVP|Cu full-cell device with the nano-silica infused electrolyte drastically outperformed the conventional electrolyte in terms of specific capacity retention and coulombic efficiency.

Pyroelectricity describes the generation of electricity by temporal temperature change in polar materials1–3. When free-standing pyroelectric materials approach the 2D crystalline limit, how pyroelectricity behaves remained largely unknown. Here, using three model pyroelectric materials whose bonding characters along the out-of-plane direction vary from van der Waals (In2Se3), quasi-van der Waals (CsBiNb2O7) to ionic/covalent (ZnO), we experimentally show the dimensionality effect on pyroelectricity and the relation between lattice dynamics and pyroelectricity. We find that, for all three materials, when the thickness of free-standing sheets becomes small, their pyroelectric coefficients increase rapidly. We show that the material with chemical bonds along the out-of-plane direction exhibits the greatest dimensionality effect. Experimental observations evidence the possible influence of changed phonon dynamics in crystals with reduced thickness on their pyroelectricity. Our findings should stimulate fundamental study on pyroelectricity in ultra-thin materials and inspire technological development for potential pyroelectric applications in thermal imaging and energy harvesting.

Hotspot engineering has the potential to transform the field of surface-enhanced Raman spectroscopy (SERS) by enabling ultrasensitive and reproducible detection of analytes. However, the ability to controllably generate SERS hotspots, with desired location and geometry, over large-area substrates, has remained elusive. In this study, we sculpt artificial edges in monolayer molybdenum disulfide (MoS2) by low-power focused laser-cutting. We find that when gold nanoparticles (AuNPs) are deposited on MoS2by drop-casting, the AuNPs tend to accumulate predominantly along the artificial edges. First-principles density functional theory (DFT) calculations indicate strong binding of AuNPs with the artificial edges due to dangling bonds that are ubiquitous on the unpassivated (laser-cut) edges. The dense accumulation of AuNPs along the artificial edges intensifies plasmonic effects in these regions, creating hotspots exclusively along the artificial edges. DFT further indicates that adsorption of AuNPs along the artificial edges prompts a transition from semiconducting to metallic behavior, which can further intensify the plasmonic effect along the artificial edges. These effects are observed exclusively for the sculpted (i.e., cut) edges and not observed for the MoS2surface (away from the cut edges) or along the natural (passivated) edges of the MoS2sheet. To demonstrate the practical utility of this concept, we use our substrate to detect Rhodamine B (RhB) with a large SERS enhancement (∼104) at the hotspots for RhB concentrations as low as ∼10–10M. The single-step laser-etching process reported here can be used to controllably generate arrays of SERS hotspots. As such, this concept offers several advantages over previously reported SERS substrates that rely on electrochemical deposition, e-beam lithography, nanoimprinting, or photolithography. Whereas we have focused our study on MoS2, this concept could, in principle, be extended to a variety of 2D material platforms.

Exploring efficient electrocatalysts for lithium–sulfur (Li–S) batteries is of great significance for the sulfur/polysulfide/sulfide multiphase conversion. Herein, we report nickel–iron intermetallic (Ni3Fe) as a novel electrocatalyst to trigger the highly efficient polysulfide-involving surface reactions. The incorporation of iron into the cubic nickel phase can induce strong electronic interaction and lattice distortion, thereby activating the inferior Ni phase to catalytically active Ni3Fe phase. Kinetics investigations reveal that the Ni3Fe phase promotes the redox kinetics of the multiphase conversion of Li–S electrochemistry. As a result, the Li–S cells assembled with a 70 wt % sulfur cathode and a Ni3Fe-modified separator deliver initial capacities of 1310.3 mA h g–1at 0.1 C and 598 mA h g–1at 4 C with excellent rate capability and a long cycle life of 1000 cycles at 1 C with a low capacity fading rate of ∼0.034 per cycle. More impressively, the Ni3Fe-catalyzed cells exhibit outstanding performance even at harsh working conditions, such as high sulfur loading (7.7 mg cm–2) or lean electrolyte/sulfur ratio (∼6 μL mg–1). This work provides a new concept on exploring advanced intermetallic catalysts for high-rate and long-life Li–S batteries.

Solar-driven water evaporation has been proposed as a renewable and sustainable strategy for the generation of clean water from seawater or wastewater. To enable such technologies, development of photothermal materials that enable efficient solar steam generation is essential. The current challenge is to manufacture such photothermal materials cost-effectively and at scale. Furthermore, the photothermal materials should be strongly hydrophilic and environmentally stable. Herein, we demonstrate facile and scalable fabrication of carbon nanotube (CNT)-based photothermal nanocomposite foam by igniting an ethanol solution of ferric acetylacetonate [Fe(acac)3] absorbed within nickel (Ni) foam under ambient conditions. The Fe(acac)3precursor provides carbon and the zero-valent iron catalyst for growing CNTs on the Ni foam, while ethanol facilitates the dispersion of Fe(acac)3on the Ni foam and supplies heat energy for the growth of CNTs by its burning. A forest of dense and uniform CNTs decorated with Fe2O3nanoparticles is generated within seconds. The resultant Fe2O3/CNT/Ni nanocomposite foam exhibits “superhydrophilicity” and high light absorption capacity, ensuring rapid transport and fast evaporation of water within the entire foam. Efficient light-to-heat conversion causes the surface temperature of the foam to reach ∼83.1 °C under 1 sun irradiation. The average water evaporation rates of such foam are as high as ∼1.48 and ∼4.27 kg m–2h–1with light-to-heat conversion efficiencies of ∼81.3 and ∼93.8% under 1 sun and 3 sun irradiation, respectively. Moreover, the versatile and scalable combustion synthesis strategy presented here can be realized on various substrates, exhibiting high adaptability for different applications.

A new modifier locus of adenomatous polyposis coli (APC)-Induced Intestinal tumorigenesis called Modifier of Min2 (Mom2) is developed to study the embryonic lethality of a mutation in the ATP synthase gene (Atp5a1) that is found to follow a novel polyp suppression mechanism. The results suggest that the sequence of events leading to tumors is the same in mice, as well as humans due to the absence of loss of heterozygosity at the Apc locus.

High specific capacity materials that can store potassium (K) are essential for next-generation K-ion batteries. One such candidate material is phosphorene (the 2D allotrope of phosphorus (P)), but the potassiation capability of phosphorene has not yet been established. Here we systematically investigate the alloying of few-layer phosphorene (FLP) with K. Unlike lithium (Li) and sodium (Na), which form Li3P and Na3P, FLP alloys with K to form K4P3, which was confirmed by ex situX-ray characterization as well as density functional theory calculations. The formation of K4P3results in high specific capacity (∼1200 mAh g–1) but poor cyclic stability (only ∼9% capacity retention in subsequent cycles). We show that this capacity fade can be successfully mitigated by the use of reduced graphene oxide (rGO) as buffer layers to suppress the pulverization of FLP. We studied the performance of rGO and single-walled carbon nanotubes (sCNTs) as buffer materials and found that rGO being a 2D material can better encapsulate and protect FLP relative to 1D sCNTs. The half-cell performance of FLP/rGO could also be successfully reproduced in a full-cell configuration, indicating the possibility of high-performance K-ion batteries that could offer a sustainable and low-cost alternative to Li-ion technology.

Graphene is the thinnest known two-dimensional (2D) material. This thinness is responsible for graphene’s well-known optical transparency. In addition to being transparent to light, its extreme thinness and nonpolar nature also render graphene partially transparent to van der Waals and electrostatic interactions. This enables media present on opposite sides of a graphene sheet to sense or feel each other and be influenced by each other. Such crosstalk between materials separated by an impermeable barrier is impossible for typical barrier or coating materials that are usually thick enough to completely screen out such interactions. In this article, we review graphene’s partial transparency to atomic interactions at the liquid–solid, solid–solid, and liquid–liquid interfaces. We compare graphene with other 2D materials such as hexagonal boron nitride and show that the extent of graphene’s transparency is strongly dependent on the nature and interaction range of the materials placed on opposite sides of the graphene layer. We end with recommendations for future research to better understand the underlying science and to develop practical applications of this exciting phenomena.

There has been enormous curiosity in the development of alternative plant based medicines to control diabetes, oxidative stress and related disorders. One of the therapeutic approaches is to reduce postprandial release of glucose in the blood. Two key enzymes that are involved in reducing postprandial glucose are α-amylase and α-glucosidase. Mentha arvensisL. has been traditionally used by several tribes as a medicinal plant to treat various disorders.

The identification of a second modifier of Min 2 (Mom2) locus that is the result of spontaneous mutation has been reported. An analysis revealed that the lifespan of Apc Min mice is increased by the presence of one resistant Mom2 allele.

High specific capacity anode materials such as silicon (Si) are increasingly being explored for next-generation, high performance lithium (Li)-ion batteries. In this context, Si films are advantageous compared to Si nanoparticle based anodes since in films the free volume between nanoparticles is eliminated, resulting in very high volumetric energy density. However, Si undergoes volume expansion (contraction) under lithiation (delithiation) of up to 300%. This large volume expansion leads to stress build-up at the interface between the Si film and the current collector, leading to delamination of Si from the surface of the current collector. To prevent this, adhesion promotors (such as chromium interlayers) are often used to strengthen the interface between the Si and the current collector. Here, we show that such approaches are in fact counter-productive and that far better electrochemical stability can be obtained by engineering a van der Waals “slippery” interface between the Si film and the current collector. This can be accomplished by simply coating the current collector surface with graphene sheets. For such an interface, the Si film slips with respect to the current collector under lithiation/delithiation, while retaining electrical contact with the current collector. Molecular dynamics simulations indicate (i) less stress build-up and (ii) less stress “cycling” on a van der Waals slippery substrate as opposed to a fixed interface. Electrochemical testing confirms more stable performance and much higher Coulombic efficiency for Si films deposited on graphene-coated nickel (i.e., slippery interface) as compared to conventional nickel current collectors.

Urolithiasis is the third common disorder of the urinary system affecting 10–15% of the general population. In recent years, search for new antilithiatic drugs from natural sources has assumed greater importance.

Single-layer rhenium disulfide (ReS2) is a unique material with distinctive, anisotropic electronic, mechanical, and optical properties and has the potential to be used as an anode in alkali-metal-ion batteries. In this work, first principles calculations were performed to systematically evaluate the potential of monolayer pristine and defective ReS2as anodes in lithium (Li)- and sodium (Na)-ion batteries. Our calculations suggest that there are several potential adsorption sites for Li and Na on pristine ReS2, owing to its low-symmetry structure. Additionally, the adsorption of Li and Na over pristine ReS2is very strong with adsorption energies of −2.28 and −1.71 eV, respectively. Interestingly, the presence of point defects causes significantly stronger binding of the alkali-metal atoms with adsorption energies in the range −2.98 to −3.17 eV for Li and −2.66 to −2.92 eV for Na. Re single vacancy was found to be the strongest binding defect for Li adsorption, whereas S single vacancy was found to be the strongest for Na. The diffusion of these two alkali atoms over pristine ReS2is anisotropic, with an energy barrier of 0.33 eV for Li and 0.16 eV for Na. The energy barriers associated with escaping a double vacancy and single vacancy for Li atoms are significantly large at 0.60 eV for the double-vacancy case and 0.51 eV for the single-vacancy case. Similarly, for Na, they are 0.59 and 0.47 eV, respectively, which indicates slower migration and sluggish charging/discharging. However, the diffusion energy barrier over a Re single vacancy is found to be merely 0.42 eV for a Li atom and 0.28 eV for Na. Overall, S single and double vacancies can reduce the diffusion rate by 103–105times for Li and Na ions, respectively. These results suggest that monolayer ReS2with a Re single vacancy adsorbs Li and Na stronger than pristine ReS2, with negligible negotiation with the charging/discharging rate of the battery, and therefore they can be used as an anode in Li- and Na-ion batteries.

We report the use of a single layer of two-dimensional hexagonal boron nitride (SL-hBN) as the thinnest insulating barrier to microbial corrosion induced by the sulfate-reducing bacteria Desulfovibrio alaskensisG20. We used electrochemical methods to assess the corrosion resistance of SL-hBN on copper against the effects of both the planktonic and sessile forms of the sulfate-reducing bacteria. Cyclic voltammetry results show that SL-hBN-Cu is effective in suppressing corrosion effects of the planktonic cells at potentials as high as 0.2 V (vsAg/AgCl). The peak anodic current for the SL-hBN coatings is ∼36 times lower than that of bare Cu. Linear polarization resistance tests confirm that the SL-hBN coatings serve as a barrier against corrosive effects of the G20 biofilm when compared to bare Cu. The SL-hBN serves as an impermeable barrier to aggressive metabolites and offers ∼91% corrosion inhibition efficiency, which is comparable to much thicker commercial coatings such as polyaniline. In addition to impermeability, the insulating nature of SL-hBN suppresses galvanic effects and improves its ability to combat microbial corrosion.

Silicon (Si) shows promise as an anode material in lithium-ion batteries due to its very high specific capacity. However, Si is highly brittle, and in an effort to prevent Si from fracturing, the research community has migrated from the use of Si films to Si nanoparticle based electrodes. However, such a strategy significantly reduces volumetric energy density due to the porosity of Si nanoparticle electrodes. Here we show that contrary to conventional wisdom, Si films can be stabilized by two strategies: (a) anchoring the Si films to a carbon nanotube macrofilm (CNM) current collector and (b) draping the films with a graphene monolayer. After electrochemical cycling, the graphene-coated Si films on CNM resembled a tough mud-cracked surface in which the graphene capping layer suppresses delamination and stabilizes the solid electrolyte interface. The graphene-draped Si films on CNM exhibit long cycle life (>1000 charge/discharge steps) with an average specific capacity of ∼806 mAh g–1. The volumetric capacity averaged over 1000 cycles of charge/discharge is ∼2821 mAh cm–3, which is 2 to 5 times higher than what is reported in the literature for Si nanoparticle based electrodes. The graphene-draped Si anode could also be successfully cycled against commercial cathodes in a full-cell configuration.

The non-toxic and stable chalcogenide perovskite BaZrS3fulfills many key optoelectronic properties for a high-efficiency photovoltaic material. It has been shown to possess a direct band gap with a large absorption coefficient and good carrier mobility values. With a reported band gap of 1.7–1.8 eV, BaZrS3is a good candidate for tandem solar cell materials; however, its band gap is significantly larger than the optimal value for a high-efficiency single-junction solar cell (∼1.3 eV, Shockley–Queisser limit)─thus doping is required to lower the band gap. By combining first-principles calculations and machine learning algorithms, we are able to identify and predict the best dopants for the BaZrS3perovskites for potential future photovoltaic devices with a band gap within the Shockley–Queisser limit. It is found that the Ca dopant at the Ba site or Ti dopant at the Zr site is the best candidate dopant. Based on this information, we report for the first time partial doping at the Ba site in BaZrS3with Ca (i.e., Ba1–xCaxZrS3) and compare its photoluminescence with Ti-doped perovskites [i.e., Ba(Zr1–xTix)S3]. Synthesized (Ba,Ca)ZrS3perovskites show a reduction in the band gap from ∼1.75 to ∼1.26 eV with <2 atom % Ca doping. Our results indicate that for the purpose of band gap tuning for photovoltaic applications, Ca-doping at the Ba-site is superior to Ti-doping at the Zr-site reported previously.

Two-dimensional sheets of transition metal dichalcogenides are an emerging class of atomically thin semiconductors that are considered to be “air-stable”, similar to graphene. Here we report that, contrary to current understanding, chemical vapor deposited transition metal dichalcogenide monolayers exhibit poor long-term stability in air. After room-temperature exposure to the environment for several months, monolayers of molybdenum disulfide and tungsten disulfide undergo dramatic aging effects including extensive cracking, changes in morphology, and severe quenching of the direct gap photoluminescence. X-ray photoelectron and Auger electron spectroscopy reveal that this effect is related to gradual oxidation along the grain boundaries and the adsorption of organic contaminants. These results highlight important challenges associated with the utilization of transition metal dichalcogenide monolayers in electronic and optoelectronic devices. We also demonstrate a potential solution to this problem, featuring encapsulation of the monolayer sheet by a 10–20 nm thick optically transparent polymer (parylene C). This strategy is shown to successfully prevent the degradation of the monolayer material under accelerated aging (i.e., high-temperature, oxygen-rich) conditions.

The next generation of deformable and shape-conformable electronics devices will need to be powered by batteries that are not only flexible but also foldable. Here we report a foldable lithium–sulfur (Li–S) rechargeable battery, with the highest areal capacity (∼3 mAh cm–2) reported to date among all types of foldable energy-storage devices. The key to this result lies in the use of fully foldable and superelastic carbon nanotube current-collector films and impregnation of the active materials (S and Li) into the current-collectors in a checkerboard pattern, enabling the battery to be folded along two mutually orthogonal directions. The carbon nanotube films also serve as the sulfur entrapment layer in the Li–S battery. The foldable battery showed <12% loss in specific capacity over 100 continuous folding and unfolding cycles. Such shape-conformable Li–S batteries with significantly greater energy density than traditional lithium-ion batteries could power the flexible and foldable devices of the future including laptops, cell phones, tablet computers, surgical tools, and implantable biomedical devices.

Smart energy storage materials, which not only store electrochemical energy but also exhibit sensitivity to environmental stimuli, can provide novel multi-functionalities. Here, we report a smart shape memory fiber supercapacitor constituted with a highly stretchable shape memory polymer nanocomposite fiber substrate, multi-walled carbon nanotubes (MWCNT) conductive layer and pesudocapacitive polyaniline layer. Using layer by layer (LBL) technology, we first produce a densely packed MWCNT layer that serves as a well-developed conductive current collector network. Then polyaniline, as an active material, was deposited on the surface of the MWCNT layer to further promote the energy storage capability of the fibers. After the infiltration of a polyvinyl alchohol (PVA) based polymer electrolyte into the porous MWCNT and PANI layer, conductivity of ∼50S/cm and stretchability of more than 400% are obtained. The resultant shape memory polymer supercapacitors (SMPFSCs) not only possess shape memory properties, but also exhibit outstanding energy storage performance, with the best pseudo-capacitance exceeding ∼427F/cm3. Such design strategies can also be extended to other polymer substrates, and could open the door to smart (multi-functional) supercapacitor technologies.

The recent interest and excitement in graphene has also opened up a pandora’s box of other two-dimensional (2D) materials and material combinations which are now beginning to come to the fore. One family of these emerging 2D materials is transition metal dichalcogenides (TMDs). So far there is very limited understanding on the wetting behavior of “monolayer” TMD materials. In this study, we synthesized large-area, continuous monolayer tungsten disulfide (WS2) and molybdenum disulfide (MoS2) films on SiO2/Si substrates by the thermal reduction and sulfurization of WO3and MO3thin films. The monolayer TMD films displayed an advancing water contact angle of ∼83° as compared to ∼90° for the bulk material. We also prepared bilayer and trilayer WS2films and studied the transition of the water contact angle with increasing number of layers. The advancing water contact angle increased to ∼85° for the bilayer and then to ∼90° for the trilayer film. Beyond three layers, there was no significant change in the measured water contact angle. This type of wetting transition indicates that water interacts to some extent with the underlying silica substrate through the monolayer TMD sheet. The experimentally observed wetting transition with numbers of TMD layers lies in-between the predictions of one continuum model that considers only van der Waals attractions and another model that considers only dipole–dipole interactions. We also explored wetting as a function of aging. A clean single-layer WS2film (without airborne contaminants) was shown to be strongly hydrophilic with an advancing water contact angle of ∼70°. However, over time, the sample ages as hydrocarbons and water present in air adsorb onto the clean WS2sheet. After ∼7 days, the aging process is completed and the advancing water contact angle of the aged single-layer WS2film stabilizes at ∼83°. These results suggest that clean (i.e., nonaged) monolayer TMDs are hydrophilic materials. We further show that substitution of sulfur atoms by oxygen in the lattice of aged monolayer WS2and MoS2films can be used to generate well-defined ‘hydrophobic–hydrophilic’ patterns that preferentially accumulate and create microdrop arrays on the surface during water condensation and evaporation experiments.

It is well established that defects strongly influence properties in two-dimensional materials. For graphene, atomic defects activate the Raman-active centrosymmetric A1gring-breathing mode known as the D-peak. The relative intensity of this D-peak compared to the G-band peak is the most widely accepted measure of the quality of graphene films. However, no such metric exists for monolayer semiconducting transition metal dichalcogenides such as WS2or MoS2. Here we intentionally create atomic-scale defects in the hexagonal lattice of pristine WS2and MoS2monolayers using plasma treatments and study the evolution of their Raman and photoluminescence spectra. High-resolution transmission electron microscopy confirms plasma-induced creation of atomic-scale point defects in the monolayer sheets. We find that while the Raman spectra of semiconducting transition metal dichalcogenides (at 532 nm excitation) are insensitive to defects, their photoluminescence reveals a distinct defect-related spectral feature located ∼0.1 eV below the neutral free A-exciton peak. This peak originates from defect-bound neutral excitons and intensifies as the two-dimensional (2D) sheet is made more defective. This spectral feature is observable in air under ambient conditions (room temperature and atmospheric pressure), which allows for a relatively simple way to determine the defectiveness of 2D semiconducting nanosheets. Controlled defect creation could also enable tailoring of the optical properties of these materials in optoelectronic device applications.

Lithium-sulfur (Li-S) batteries have attracted great attention as next-generation high specific energy density storage devices. However, the low sulfur loading in the cathode for Li-S battery greatly offsets its advantage in high energy density and limits the practical applications of such battery concepts. Flexible energy storage devices are also becoming increasingly important for future applications but are limited by the lack of suitable lightweight electrode materials with robust electrochemical performance under cyclic mechanical strain. Here, we proposed an effective strategy to obtain flexible Li-S battery electrodes with high energy density, high power density, and long cyclic life by adopting graphene foam-based electrodes. Graphene foam can provide a highly electrically conductive network, robust mechanical support and sufficient space for a high sulfur loading. The sulfur loading in graphene foam-based electrodes can be tuned from 3.3 to 10.1mgcm−2. The electrode with 10.1mgcm−2sulfur loading could deliver an extremely high areal capacity of 13.4mAhcm−2, much higher than the commonly reported Li-S electrodes and commercially used lithium cobalt oxide cathode with a value of ~3–4mAhcm−2. Meanwhile, the high sulfur-loaded electrodes retain a high rate performance with reversible capacities higher than 450mAhg−1under a large current density of 6Ag−1and preserve stable cycling performance with ~0.07% capacity decay per cycle over 1000 cycles. These impressive results indicate that such electrodes could enable high performance, fast-charging, and flexible Li-S batteries that show stable performance over extended charge/discharge cycling.

Diamond-based cutting tools are preferred by the manufacturing community to machine high-value, hard-to-cut materials, especially for applications with stringent surface roughness requirements. However, these tools are not an economically viable option to machine any of the transition metals or their alloys. This is due to the extreme rates of tool wear caused by chemical diffusion of carbon from the diamond tool into the transition metal/alloy workpiece. In this paper we report the use of carbon-rich, graphene oxide colloidal suspensions, as a cutting fluid, to address this critical diamond tool wear issue. Our machining tests conducted on a low-carbon steel alloy reveal that the use of graphene oxide colloidal suspension results in a ∼74% reduction in the tool wear over that seen under the dry machining condition. It also results in a ∼50% reduction in the cutting temperatures and a ∼20 to 30% reduction in the cutting forces, when compared against the dry machining condition. The trends seen in the cutting temperatures, cutting forces and the X-ray photoelectron spectroscopy (XPS) data collectively point to the possibility of the graphene oxide platelets serving as a barrier to the carbon diffusion reaction. These findings have the potential to make diamond a cost-effective tool material for machining transition metal-based materials such as ferrous and titanium alloys that have wide-spread engineering applications.

Emerging two-dimensional (2D) materials such as transition metal dichalcogenides offer unique and hitherto unavailable opportunities to tailor the mechanical, thermal, electronic, and optical properties of polymer nanocomposites. In this study, we exfoliated bulk molybdenum disulfide (MoS2) into nanoplatelets, which were then dispersed in epoxy polymers at loading fractions of up to 1% by weight. We characterized the tensile and fracture properties of the composite and show that MoS2nanoplatelets are highly effective at enhancing the mechanical properties of the epoxy at very low nanofiller loading fractions (below 0.2% by weight). Our results show the potential of 2D sheets of transition metal dichalcogenides as reinforcing additives in polymeric composites. Unlike graphene, transition metal dichalcogenides such as MoS2are high band gap semiconductors and do not impart significant electrical conductivity to the epoxy matrix. For many applications, it is essential to enhance mechanical properties while also maintaining the electrical insulation properties and the high dielectric constant of the polymer material. In such applications, conductive carbon based fillers such as graphene cannot be utilized. This study demonstrates that 2D transition metal dichalcogenide additives offer an elegant solution to such class of problems.

We explored supercapacitor devices by utilizing 3-D highly porous (with average pore size of ∼80nm) carbon nanotube sponges as a conductive substrate for polyaniline deposition. The porous structure of the sponge is beneficial for precursor penetration and uniform deposition of polyaniline on to the nanotubes. The self-supported, free-standing and flexible carbon nanotube polyaniline composite structure does not require any conductive additives or mechanical binders and delivers excellent areal capacitance (1.85–1.62F/cm2in the 4.9–49mA/cm2current density range) coupled with high rate capability and cycle stability.

In this study, we report a novel route viamicrowave irradiation to synthesize a bio-inspired hierarchical graphene–nanotube–iron three-dimensional nanostructure as an anode material in lithium-ion batteries. The nanostructure comprises vertically aligned carbon nanotubes grown directly on graphene sheets along with shorter branches of carbon nanotubes stemming out from both the graphene sheets and the vertically aligned carbon nanotubes. This bio-inspired hierarchical structure provides a three-dimensional conductive network for efficient charge-transfer and prevents the agglomeration and restacking of the graphene sheets enabling Li-ions to have greater access to the electrode material. In addition, functional iron-oxide nanoparticles decorated within the three-dimensional hierarchical structure provides outstanding lithium storage characteristics, resulting in very high specific capacities. The anode material delivers a reversible capacity of ∼1024 mA·h·g–1even after prolonged cycling along with a Coulombic efficiency in excess of 99%, which reflects the ability of the hierarchical network to prevent agglomeration of the iron-oxide nanoparticles.

We demonstrate here the effect of electron beam induced deposited platinum on the electrical transport through multilayer graphene sheets. Platinum metal is deposited at different positions on the graphene multilayers, i.e., including as well as excluding the bottom contact sites and the change in electrical conductance of the same multilayer graphene sheets before and after platinum deposition is segregated. An improvement in electrical conductance is observed even if the metal is deposited at the part of the graphene sheets that does not touch the bottom gold electrodes, and hence this experimental approach directly demonstrates that the contact improvement is not the sole reason for the improved electrical conduction. The improvement in electrical performance of the graphene sheets is explained in terms of the doping of graphene sheets caused by the charge transfer between the deposited metal and the graphene and thereby modified density of states for electrical conduction. Metal deposition also leads to the increased interlayer interaction of the graphene sheets as revealed by the transmission electron microscopy analysis. Further, two types of breakdown behaviors viz. sharp and stepped breakdowns observed for these graphene devices are explained in terms of the effective graphene–metal contact area. These studies reveal the implications of top metal contact fabrication on graphene for electronic devices.

Previous studies of the interaction of water with graphene-coated surfaces have been limited to flat (smooth) surfaces. Here we created a rough surface by nanopatterning and then draped the surface with a single-layer graphene sheet. We found that the ultrasheer graphene drape prevents the penetration of water into the textured surface thereby drastically reducing the contact angle hysteresis (which is a measure of frictional energy dissipation) and preventing the liquid contact line from getting pinned to the substrate. This has important technological implications since the main obstacle to the motion of liquid drops on rough surfaces is contact angle hysteresis and contact line pinning. Graphene drapes could therefore enable enhanced droplet mobility which is required in a wide range of applications in micro and nanofluidics. Compared to polymer coatings that could fill the cavities between the nano/micropores or significantly alter the roughness profile of the substrate, graphene provides the thinnest (i.e., most sheer) and most conformal drape that is imaginable. Despite its extreme thinness, the graphene drape is mechanically robust, chemically stable, and offers high flexibility and resilience which can enable it to reliably drape arbitrarily complex surface topologies. Graphene drapes may therefore provide a hitherto unavailable ability to tailor the dynamic wettability of surfaces for a variety of applications.

Conventional graphitic anodes in lithium-ion batteries cannot provide high-power densities due to slow diffusivity of lithium ions in the bulk electrode material. Here we report photoflash and laser-reduced free-standing graphene paper as high-rate capable anodes for lithium-ion batteries. Photothermal reduction of graphene oxide yields an expanded structure with micrometer-scale pores, cracks, and intersheet voids. This open-pore structure enables access to the underlying sheets of graphene for lithium ions and facilitates efficient intercalation kinetics even at ultrafast charge/discharge rates of >100 C. Importantly, photothermally reduced graphene anodes are structurally robust and display outstanding stability and cycling ability. At charge/discharge rates of ∼40 C, photoreduced graphene anodes delivered a steady capacity of ∼156 mAh/ganodecontinuously over 1000 charge/discharge cycles, providing a stable power density of ∼10 kW/kganode. Such electrodes are envisioned to be mass scalable with relatively simple and low-cost fabrication procedures, thereby providing a clear pathway toward commercialization.

Engineering the dropwise condensation of water on surfaces is critical in a wide range of applications from thermal management (e.g. heat pipes, chip cooling etc.) to water harvesting technologies. Surfaces that enable both efficient droplet nucleation and droplet self‐removal (i.e. droplet departure) are essential to accomplish successful dropwise condensation. However it is extremely challenging to design such surfaces. This is because droplet nucleation requires a wettable surface while droplet departure necessitates a super‐hydrophobic surface. Here we report that these conflicting requirements can be satisfied using a hierarchical (multiscale) nanograssed micropyramid architecture that yield a gobal superhydrophobicity as well as locally wettable nucleation sites, allowing for ˜65% increase in the drop number density and ˜450% increase in the drop self‐removal volume as compared to a superhydrophobic surface with nanostructures alone. Further we find that synergistic co‐operation between the hierarchical structures contributes directly to a continuous process of nucleation, coalescence, departure, and re‐nucleation enabling sustained dropwise condensation over prolonged periods. Exploiting such multiscale coupling effects can open up novel and exciting vistas in surface engineering leading to optimal condensation surfaces for high performance electronics cooling and water condenser systems.

Composites of few layered graphene (G) and platinum (Pt) nanoparticles (NP) with different loadings of Pt were used as counter electrode (CE) in dye-sensitized solar cell (DSSC). NPs were deposited directly on to G using pulsed laser ablation method (PLD). DSSCs formed using the composite CEs show improved performance compared to conventional Pt thin film electrode (Std Pt) and unsupported Pt NPs. Composite with 27% loading of Pt shows 45% higher efficiency (η = 2.9%), greater short circuit current (Jsc= 6.67 mA cm–2), and open circuit voltage (Voc= 0.74 V) without any loss of the fill factor (FF = 58%) as compared to the cells fabricated using Std Pt electrodes. Values of η, Jscand Vocfor DSSC using Std Pt CE were 2%, 5.05 mA cm–2and 0.68 V, respectively. Electrochemical impedance spectroscopy using I–3/I–redox couple confirm lower values of charge transfer resistance for the composite electrodes, e.g., 2.36 Ω cm2as opposed to 7.73 Ω cm2of Std Pt. The better catalytic activity of these composite materials is also reflected in the stronger I–3reduction peaks in cyclic voltammetry scans.

We compared the electrical conductivity of multiwalled-carbon-nanotube/polystyrene and graphene/polystyrene composites. The conductivity of polystyrene increases from ∼6.7 × 10–14to ∼3.49 S/m, with an increase in graphene content from ∼0.11 to ∼1.1 vol %. This is ∼2–4 orders of magnitude higher than for multiwalled-carbon-nanotube/polystyrene composites. Furthermore, we show that the conductivity of the graphene/polystyrene system can be significantly enhanced by incorporation of polylactic acid. The volume-exclusion principle forces graphene into the polystyrene-rich regions (selective localization) and generates ∼4.5-fold decrease in its percolation threshold from ∼0.33 to ∼0.075 vol %.

The liquid−solid phase change enthalpy, crystallization, and thermal conductivity of graphene/1-octadecanol (stearyl alcohol) composite, a nanostructured phase change material, was investigated as a function of graphene content. The thermal conductivity (κ) of the nanocomposite increased by nearly 2.5-fold (∼140% increase) upon ∼4% (by weight) graphene addition while the drop in the heat of fusion (i.e., storage capacity) was only ∼15.4%. The enhancement in thermal properties of 1-octadecanol obtained with the addition of graphene is markedly superior to the effect of other nanofillers such as silver nanowires and carbon nanotubes reported previously in the literature. Boosting the thermal conductivity of organic phase change materials without incurring a significant loss in the heat of fusion is one of the key issues in enabling their practical application as latent heat storage/release units for thermal management and thermal protection.

The majority of work in graphene nanocomposites has focused on polymer matrices. Here we report for the first time the use of graphene to enhance the toughness of bulk silicon nitride ceramics. Ceramics are ideally suited for high-temperature applications but suffer from poor toughness. Our approach uses graphene platelets (GPL) that are homogeneously dispersed with silicon nitride particles and densified, at ∼1650 °C, using spark plasma sintering. The sintering parameters are selected to enable the GPL to survive the harsh processing environment, as confirmed by Raman spectroscopy. We find that the ceramic’s fracture toughness increases by up to ∼235% (from ∼2.8 to ∼6.6 MPa·m1/2) at ∼1.5% GPL volume fraction. Most interestingly, novel toughening mechanisms were observed that show GPL wrapping and anchoring themselves around individual ceramic grains to resist sheet pullout. The resulting cage-like graphene structures that encapsulate the individual grains were observed to deflect propagating cracks in not just two but three dimensions.

We report the use of graphene as an additive to improve the lubrication and cooling performance of semisynthetic metal-working fluids (MWFs) used in micromachining operations. Microturning experiments were conducted in the presence of MWFs containing varying concentrations of graphene platelets. Graphene-based MWF formulations performed significantly better as compared to conventional MWFs. Moreover, an analysis of the trends in the cutting forces and cutting temperatures, taken in conjunction with the trends in the wetting ability, thermal conductivity, and kinematic viscosity of the modified MWFs, establishes graphene as a superior additive over both single and multiwalled carbon nanotubes. The superior performance of graphene is attributed to the increased wettability of the cutting fluid that allows for penetration of the graphene platelets into the tool−workpiece interface. Once in that interface, the graphene platelets provide efficient lubrication because of the relative sliding of graphene layers within the platelet while they also conduct heat away from the cutting zone.

It is well established that pristine multiwalled carbon nanotubes offer poor structural reinforcement in epoxy-based composites. There are several reasons for this which include reduced interfacial contact area since the outermost nanotube shields the internal tubes from the matrix, poor wetting and interfacial adhesion with the heavily cross-linked epoxy chains, and intertube slip within the concentric nanotube cylinders leading to a sword-in-sheath type failure. Here we demonstrate that unzipping such multiwalled carbon nanotubes into graphene nanoribbons results in a significant improvement in load transfer effectiveness. For example, at ∼0.3% weight fraction of nanofillers, the Young’s modulus of the epoxy composite with graphene nanoribbons shows ∼30% increase compared to its multiwalled carbon nanotube counterpart. Similarly the ultimate tensile strength for graphene nanoribbons at ∼0.3% weight fraction showed ∼22% improvement compared to multiwalled carbon nanotubes at the same weight fraction of nanofillers in the composite. These results demonstrate that unzipping multiwalled carbon nanotubes into graphene nanoribbons can enable their utilization as high-performance additives for mechanical properties enhancement in composites that rival the properties of singlewalled carbon nanotube composites yet at an order of magnitude lower cost.

We report the synthesis and fatigue characterization of fiberglass/epoxy composites with various weight fractions of graphene platelets infiltrated into the epoxy resin as well as directly spray-coated on to the glass microfibers. Remarkably only ∼0.2% (with respect to the epoxy resin weight and ∼0.02% with respect to the entire laminate weight) of graphene additives enhanced the fatigue life of the composite in the flexural bending mode by up to 1200-fold. By contrast, under uniaxial tensile fatigue conditions, the graphene fillers resulted in ∼3−5-fold increase in fatigue life. The fatigue life increase (in the flexural bending mode) with graphene additives was ∼1−2 orders of magnitude superior to those obtained using carbon nanotubes. In situ ultrasound analysis of the nanocomposite during the cyclic fatigue test suggests that the graphene network toughens the fiberglass/epoxy-matrix interface and prevents the delamination/buckling of the glass microfibers under compressive stress. Such fatigue-resistant hierarchical materials show potential to improve the safety, reliability, and cost effectiveness of fiber-reinforced composites that are increasingly the material of choice in the aerospace, automotive, marine, sports, biomedical, and wind energy industries.

In this study, the mechanical properties of epoxy nanocomposites with graphene platelets, single-walled carbon nanotubes, and multi-walled carbon nanotube additives were compared at a nanofiller weight fraction of 0.1 ± 0.002%. The mechanical properties measured were the Young’s modulus, ultimate tensile strength, fracture toughness, fracture energy, and the material’s resistance to fatigue crack propagation. The results indicate that graphene platelets significantly out-perform carbon nanotube additives. The Young’s modulus of the graphene nanocomposite was ∼31% greater than the pristine epoxy as compared to ∼3% increase for single-walled carbon nanotubes. The tensile strength of the baseline epoxy was enhanced by ∼40% with graphene platelets compared to ∼14% improvement for multi-walled carbon nanotubes. The mode I fracture toughness of the nanocomposite with graphene platelets showed ∼53% increase over the epoxy compared to ∼20% improvement for multi-walled carbon nanotubes. The fatigue resistance results also showed significantly different trends. While the fatigue suppression response of nanotube/epoxy composites degrades dramatically as the stress intensity factor amplitude is increased, the reverse effect is seen for graphene-based nanocomposites. The superiority of graphene platelets over carbon nanotubes in terms of mechanical properties enhancement may be related to their high specific surface area, enhanced nanofiller−matrix adhesion/interlocking arising from their wrinkled (rough) surface, as well as the two-dimensional (planar) geometry of graphene platelets.

The scientific need for a standard protocol permitting the exchange of generic observing services is rapidly escalating as more observatories adopt service observing as a standard operating mode and as more remote or robotic telescopes are brought on-line. To respond to this need, we present the results of the first interoperability workshop for Heterogeneous Telescope Networks (HTN) held in Exeter. We present a draft protocol, designed to be independent of the specific instrumentation and software that controls the remote and/or robotic telescopes, allowing these telescopes to appear to the user with a unified interface despite any underlying architectural differences. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report results from XMM-Newton observations of 13 X-ray bright BL Lacertae objects, selected from the Einstein Slew Survey sample (SSS). The survey was designed to look for evidence of departures of the X-ray spectra from a simple power-law shape (i.e., curvature and/or line features) and to find objects worthy of deeper study. Our data are generally well fit by power-law models, with three cases having hard (G < 2; dN/dE [?] E-G) spectra that indicate synchrotron peaks at E [?] 5 keV. Previous data had suggested a presence of absorption features in the X-ray spectra of some BL Lac objects. In contrast, none of these spectra show convincing examples of line features in either absorption or emission, suggesting that such features are rare among BL Lac objects, or, more likely, are artifacts caused by instrumental effects. We find significant evidence for intrinsic curvature [steepening by dG/d(log E) = 0.4 +- 0.15] in 14 of the 17 X-ray spectra. This cannot be explained satisfactorily via excess absorption, since the curvature is essentially constant from 0.5-6 keV, an observation that is inconsistent with the modest amounts of absorption that would be required. We use the XMM-Newton Optical Monitor data with concurrent radio monitoring to derive broadband spectral energy distributions and peak frequency estimates. From these, we examine models of synchrotron emission and model the spectral curvature we see as the result of episodic particle acceleration.

We investigate the ultraviolet-to-optical spectral energy distributions of 17 active galactic nuclei (AGNs) using quasi-simultaneous spectrophotometry spanning 900-9000 A (rest frame). We employ data from the Far Ultraviolet Spectroscopic Explorer, the Hubble Space Telescope, and the 2.1 m telescope at Kitt Peak National Observatory. Taking advantage of the short-wavelength coverage, we are able to study the so-called big blue bump, the region in which the energy output peaks, in detail. Most objects exhibit a spectral break around 1100 A. Although this result is formally associated with large uncertainty for some objects, there is strong evidence in the data that the far-ultraviolet spectral region is below the extrapolation of the near-ultraviolet-optical slope, indicating a spectral break around 1100 A. We compare the behavior of our sample to those of non-LTE thin-disk models covering a range in black hole mass, Eddington ratio, disk inclination, and other parameters. The distribution of ultraviolet-optical spectral indices redward of the break and far-ultraviolet indices shortward of the break are in rough agreement with the models. However, we do not see a correlation between the far-ultraviolet spectral index and the black hole mass, as seen in some accretion disk models. We argue that the observed spectral break is intrinsic to AGNs, although intrinsic reddening as well as Comptonization can strongly affect the far-ultraviolet spectral index. We make our data available online in digital format.

We have obtained Chandra High Resolution Camera (0.1-10 keV) X-ray imaging of the inner optical filaments of Cen A. Faint X-ray emission is observed associated with the filaments with significance levels of 2-3.5 s when the data are adaptively smoothed. Comparison with optical [O III] l5007 emission-line imaging suggests that the X-ray-emitting gas is associated with the northwest edge of the optical line-emitting gas. This region of the filaments is closest to the radio jet and is therefore a candidate for shock heating induced by turbulent instabilities in the interstellar medium. The observed X-ray luminosity is significantly smaller than that predicted by purely radiative shock models of the filaments, implying that mechanical energy input does not provide 100% of the ionizing energy budget for the filaments.

We present results from a 900 ks exposure of NGC 3783 with the High-Energy Transmission Grating Spectrometer on board the Chandra X-Ray Observatory. The resulting X-ray spectrum, which covers the 0.5-10 keV energy range, has the best combination of signal-to-noise ratio and resolution ever obtained for an AGN. This spectrum reveals absorption lines from H-like and He-like ions of N, O, Ne, Mg, Al, Si, and S. There are also possible absorption lines from H-like and He-like Ar and Ca as well as H-like C. We also identify inner-shell absorption from lower ionization ions such as Si VII-Si XII and S XII-S XIV. The iron absorption spectrum is very rich; L-shell lines of Fe XVII-Fe XXIV are detected, as well as probable resonance lines from Fe XXV. A strong complex of M-shell lines from iron ions is also detected in the spectrum. The absorption lines are blueshifted relative to the systemic velocity by a mean velocity of -590 +- 150 km s-1. We resolve many of the absorption lines, and their mean FWHM is 820 +- 280 km s-1. We do not find correlations between the velocity shifts or the FWHMs with the ionization potentials of the ions. Most absorption lines show asymmetry, having more extended blue wings than red wings. In O VII we have resolved this asymmetry to be from an additional absorption system at approximately -1300 km s-1. The two X-ray absorption systems are consistent in velocity shift and FWHM with the ones identified in the UV lines of C IV, N V, and H I. Equivalent width measurements for all absorption and emission lines are given and column densities are calculated for several ions. We resolve the narrow Fe Ka line at 6398.2 +- 3.3 eV to have an FWHM of 1720 +- 360 km s-1, which suggests that this narrow line may be emitted from the outer part of the broad-line region or the inner part of the torus. We also detect a "Compton shoulder" redward of the narrow Fe Ka line, which indicates that it arises in cold, Compton-thick gas.

Min (Multiple intestinal neoplasia) mice carry a dominant mutation in the adenomatous polyposis coli (Apc) gene and develop multiple adenomas throughout their intestinal tract (Moser et al. 1990; Su et al 1992). Polyp multiplicity in Min mice is greatly influenced by genetic background. A modifier locus, Mom1 (Modifier of Min 1), was identified and localized to distal mouse chromosome 4 (Moser et al. 1992; Dietrich et al. 1993), and accounts for some of the genetic variance in polyp multiplicity. Mom1 is a semidominant modifier of polyp size and multiplicity in Min mice (Gould and Dove 1997), and encodes the secretory type II nonpancreatic phospholipase A2 (Pla2g2a) gene (MacPhee et al. 1995; Cornier et al. 1997, 2000). We now report the identification of a second Modifier of Min 2 (Mom2) locus that is the result of a spontaneous mutation. One resistant Mom2 allele can suppress 88%-95% of polyps detected in Apc(Min)/+ mice, indicating that Mom2 acts in a dominant fashion. Linkage analysis has localized Mom2 to distal mouse chromosome 18. The effects of the Mom2 locus on reducing polyp multiplicity are stronger than the effects of the Mom1 locus, in both the small and large intestines. Some Apc(Min)/+ mice that carried one resistant Mom2 allele were tumor-free at 21 weeks of age, even in the absence of a resistant Mom1 allele. Thus, the resistant Mom2 allele can, in some cases, completely suppress the penetrance of the Apc(Min) mutation.

We describe deep (40 orbits) Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph observations of the broad absorption line (BAL) quasi-stellar object (QSO) PG 0946+301 and make them available to the community. These observations are the major part of a multiwavelength campaign on this object aimed at determining the ionization equilibrium and abundances (IEAs) in BALQSOs. We present simple template fits to the entire data set, which yield firm identifications for more than two dozen BALs from 18 ions and give lower limits for the ionic column densities. We find that the outflow's metallicity is consistent with being solar, while the abundance ratio of phosphorus to other metals is at least 10 times solar. These findings are based on diagnostics that are not sensitive to saturation and partial covering effects in the BALs, which considerably weakened previous claims for enhanced metallicity. Ample evidence for these effects is seen in the spectrum. We also discuss several options for extracting tighter IEA constraints in future analyses and present the significant temporal changes that are detected between these spectra and those taken by the HST Faint Object Spectrograph in 1992.

AbstractA new and unique approach to human development and transformation through “Sahajyoga” has been discussed. In several thousand human being spread over 75 million of the world, this transformation and its beneficial effects have been noticed. Application of Sahajyoga techniques in medical and agricultural research and their positive effects have also been presented in the paper. The paper highlights the need of adoption of Sahajyoga, for an overall desirable changes in the present day situation.

During 1998 April 13-16, the bright, strongly variable Seyfert 1 galaxy NGC 3516 was monitored almost continuously with HST for 10.3 hr at ultraviolet wavelengths and 2.8 days at optical wavelengths, and simultaneous RXTE and ASCA monitoring covered the same period. The X-ray fluxes were strongly variable with the soft (0.5-2 keV) X-rays showing stronger variations (~65% peak to peak) than the hard (2-10 keV) X-rays (~50% peak to peak). The optical continuum showed much smaller but still highly significant variations: a slow ~2.5% rise followed by a faster ~3.5% decline. The short ultraviolet observation did not show significant variability. The soft and hard X-ray light curves were strongly correlated, with no evidence for a significant interband lag. Likewise, the optical continuum bands (3590 and 5510 A) were also strongly correlated, with no measurable lag, to 3 s limits of [?]0.15 day. However, the optical and X-ray light curves showed very different behavior, and no significant correlation or simple relationship could be found. These results appear difficult to reconcile with previous reports of correlations between X-ray and optical variations and of measurable lags within the optical band for some other Seyfert 1 galaxies. These results also present serious problems for "reprocessing" models in which the X-ray source heats a stratified accretion disk, which then reemits in the optical/ultraviolet: the synchronous variations within the optical would suggest that the emitting region is [?]0.3 lt-day across, while the lack of correlation between X-ray and optical variations would indicate, in the context of this model, that any reprocessing region must be [?]1 lt-day in size. It may be possible to resolve this conflict by invoking anisotropic emission or special geometry, but the most natural explanation appears to be that the bulk of the optical luminosity is generated by some mechanism other than reprocessing.

During 1998 April 13-16, the bright, strongly variable Seyfert 1 galaxy NGC 3516 was monitored almost continuously with HSTfor 10.3 hr at ultraviolet wavelengths and 2.8 days at optical wavelengths, and simultaneous RXTEand ASCAmonitoring covered the same period. The X-ray fluxes were strongly variable with the soft (0.5-2 keV) X-rays showing stronger variations (~65% peak to peak) than the hard (2-10 keV) X-rays (~50% peak to peak). The optical continuum showed much smaller but still highly significant variations: a slow ~2.5% rise followed by a faster ~3.5% decline. The short ultraviolet observation did not show significant variability.

We present Hubble Space Telescope UV/optical spectrophotometry and ground-based optical spectrophotometry of the brightest knots in the emission-line regions of the Seyfert galaxies NGC 5643 and NGC 5728. These objects have ionization cones, radio and soft X-ray emission, and evidence of nuclear outflows and winds, suggesting that shocks may be associated with the emission-line gas. Comparison of the UV/optical data with grids of ionization-bounded and mixed ionization- and matter-bounded photoionization models, and shock models with and without ionized precursors, are used to determine whether radiative shocks that photoionize the shock precursor material ("autoionizing shocks") are a significant excitation mechanism for the emission-line gas in these objects. Although the UV/optical spectra studied here are generally compatible with both photoionization and shock + precursor models, an unambiguous shock + precursor signature is not present. This suggests that mechanical energy input may not play the same dominant role in exciting these objects with weak radio jets as appears to be the case in strong radio jet sources such as M87.