With the proliferation of mobile devices and wireless technologies, mobile social network systems are increasingly available. A mobile social network plays an essential role as the spread of information and influence in the form of "word-of-mouth". It is a fundamental issue to find a subset of influential individuals in a mobile social network such that targeting them initially (e.g. to adopt a new product) will maximize the spread of the influence (further adoptions of the new product). The problem of finding the most influential nodes is unfortunately NP-hard. It has been shown that a Greedy algorithm with provable approximation guarantees can give good approximation; However, it is computationally expensive, if not prohibitive, to run the greedy algorithm on a large mobile network. In this paper we propose a new algorithm called Community-based Greedy algorithm for mining top-K influential nodes. The proposed algorithm encompasses two components: 1) an algorithm for detecting communities in a social network by taking into account information diffusion; and 2) a dynamic programming algorithm for selecting communities to find influential nodes. We also provide provable approximation guarantees for our algorithm. Empirical studies on a large real-world mobile social network show that our algorithm is more than an order of magnitudes faster than the state-of-the-art Greedy algorithm for finding top-K influential nodes and the error of our approximate algorithm is small.
Dichalcogenides with the common formula MX2 are layered materials with electrical properties that range from semiconducting to superconducting. Here, we describe optimal imaging conditions for the optical detection of ultrathin, two-dimensional dichalcogenide nanocrystals containing single, double and triple layers of MoS2, WSe2 and NbSe2. A simple optical model is used to calculate the contrast for nanolayers deposited on wafers with varying thicknesses of SiO2. The model is extended for imaging using the green channel of a video camera. Using AFM and optical imaging we confirm that single layers of MoS2 and WSe2 can be detected on 90 and 270 nm SiO2 using optical means. By measuring contrast under broadband green illumination we are also able to distinguish between nanostructures containing single, double and triple layers of MoS2 and WSe2. We observe and discuss discrepancies in the case of NbSe2.
We report chemical doping (p-type) to reduce the sheet resistance of graphene films for the application of high-performance transparent conducting films. The graphene film synthesized by chemical vapor deposition was transferred to silicon oxide and quartz substrates using poly(methyl methacrylate). AuCl3 in nitromethane was used to dope the graphene films and the sheet resistance was reduced by up to 77% depending on the doping concentration. The p-type doping behavior was confirmed by characterizing the Raman G-band of the doped graphene film. Atomic force microscope and scanning electron microscope images reveal the deposition of Au particles on the film. The sizes of the Au particles are 10-100 nm. The effect of doping was also investigated by transferring the graphene films onto quartz and poly(ethylene terephthalate) substrates. The sheet resistance reached 150 Omega/sq at 87% transmittance, which is comparable to those of indium tin oxide conducting film. The doping effect was manifested only with 1-2 layer graphene but not with multi-layer graphene. This approach advances the numerous applications of graphene films as transparent conducting electrodes.
We report a change in the semimetallic nature of single-layer graphene after exposure to oxygen plasma. The resulting transition from semimetallic to semiconducting behavior appears to depend on the duration of the exposure to the plasma treatment. The observation is confirmed by electrical, photoluminescence and Raman spectroscopy measurements. We explain the opening of a bandgap in graphene in terms of functionalization of its pristine lattice with oxygen atoms. Ab initio calculations show more details about the interaction between carbon and oxygen atoms and the consequences on the optoelectronic properties, that is, on the extent of the bandgap opening upon increased functionalisation density.
This work demonstrates a large-scale batch fabrication of GaN light-emitting diodes (LEDs) with patterned multi-layer graphene (MLG) as transparent conducting electrodes. MLG films were synthesized using a chemical vapor deposition (CVD) technique on nickel films and showed typical CVD-synthesized MLG film properties, possessing a sheet resistance of similar to 620 Omega/square with a transparency of more than 85% in the 400-800 nm wavelength range. The MLG was applied as the transparent conducting electrodes of GaN-based blue LEDs, and the light output performance was compared to that of conventional GaN LEDs with indium tin oxide electrodes. Our results present a potential development toward future practical application of graphene electrodes in optoelectronic devices.
In this work graphene sheets grown by chemical vapor deposition (CVD) with controlled numbers of layers were used as transparent electrodes in organic photovoltaic (OPV) devices. It was found that for devices with pristine graphene electrodes, the power conversion efficiency (PCE) is comparable to their counterparts with indium tin oxide (ITO) electrodes. Nevertheless, the chances for failure in OPVs with pristine graphene electrodes are higher than for those with ITO electrodes, due to the surface wetting challenge between the hole-transporting layer and the graphene electrodes. Various alternative routes were investigated and it was found that AuCl3 doping on graphene can alter the graphene surface wetting properties such that a uniform coating of the hole-transporting layer can be achieved and device success rate can be increased. Furthermore, the doping both improves the conductivity and shifts the work function of the graphene electrode, resulting in improved overall PCE performance of the OPV devices. This work brings us one step further toward the future use of graphene transparent electrodes as a replacement for ITO.
We report detailed current-voltage and current-time measurements to reveal the forming and switching behaviors of Cu/Ta2O5/Pt nonvolatile resistive memory devices. The devices can be initially SET (from the OFF state to the ON state) when a low positive bias voltage is applied to the Cu electrode. This first SET operation corresponds to the first formation of a metal filament by inhomogeneous nucleation and subsequent growth of Cu on the Pt electrode, based on the migration of Cu ions in the stable Ta2O5 matrix. After the forming, the device exhibits bipolar switching behavior (SET at positive bias and RESET (from the ON state to the OFF state) at negative bias) with increasing the ON resistance from a few hundred Omega to a few k Omega. From the measurements of the temperature stability of the ON states, we concluded that the RESET process consists of the Joule-heating-assisted oxidation of Cu atoms at the thinnest part of the metal filament followed by diffusion and drift of the Cu ions under their own concentration gradient and the applied electric field, disconnecting the metal filament. With ON resistances of the order of a few k Omega, the SET and RESET operations are repeated by the inhomogeneous nucleation and the Joule-heating-assisted dissolution of a small filament on a remaining filament. This switching model is applicable to the operation of cation-migration-based resistive memories using other oxide materials.
In this paper, the resistive switching characteristics in a Cu/HfO2:Cu/Pt sandwiched structure is investigated for multilevel non-volatile memory applications. The device shows excellent resistive switching performance, including good endurance, long retention time, fast operation speed and a large storage window (R-OFF/R-ON > 10(7)). Based on the temperature-dependent test results, the formation of Cu conducting filaments is believed to be the reason for the resistance switching from the OFF state to the ON state. By integrating the resistive switching mechanism study and the device fabrication, different resistance values are achieved using different compliance currents in the program process. These resistance values can be easily distinguished in a large temperature range, and can be maintained over 10 years by extrapolating retention data at room temperature. The integrated experiment and mechanism studies set up the foundation for the development of high-performance multilevel RRAM.
Molecular-receptor-targeted imaging of folate receptor positive oral carcinoma cells using folic-acid-conjugated fluorescent Au25 nanoclusters (Au NCs) is reported. Highly fluorescent Au25 clusters were synthesized by controlled reduction of Au+ ions, stabilized in bovine serum albumin (BSA), using a green-chemical reducing agent, ascorbic acid (vitamin-C). For targeted-imaging-based detection of cancer cells, the clusters were conjugated with folic acid (FA) through amide linkage with the BSA shell. The bioconjugated clusters show excellent stability over a wide range of pH from 4 to 14 and fluorescence efficiency of similar to 5.7% at pH 7.4 in phosphate buffer saline (PBS), indicating effective protection of nanoclusters by serum albumin during the bioconjugation reaction and cell-cluster interaction. The nanoclusters were characterized for their physico-chemical properties, toxicity and cancer targeting efficacy in vitro. X-ray photoelectron spectroscopy (XPS) suggests binding energies correlating to metal Au 4f(7/2) similar to 83.97 eV and Au 4f(5/2) similar to 87.768 eV. Transmission electron microscopy and atomic force microscopy revealed the formation of individual nanoclusters of size similar to 1 nm and protein cluster aggregates of size similar to 8 nm. Photoluminescence studies show bright fluorescence with peak maximum at similar to 674 nm with the spectral profile covering the near-infrared (NIR) region, making it possible to image clusters at the 700-800 nm emission window where the tissue absorption of light is minimum. The cell viability and reactive oxygen toxicity studies indicate the non-toxic nature of the Au clusters up to relatively higher concentrations of 500 mu g ml(-1). Receptor-targeted cancer detection using Au clusters is demonstrated on FR+ve oral squamous cell carcinoma (KB) and breast adenocarcinoma cell MCF-7, where the FA-conjugated Au-25 clusters were found internalized in significantly higher concentrations compared to the negative control cell lines. This study demonstrates the potential of using non-toxic fluorescent Au nanoclusters for the targeted imaging of cancer.
Fe nanoflakes were prepared by the ball-milling technique, and then were coated with 20 nm-thick SiO2 to prepare Fe/SiO2 core-shell nanoflakes. Compared with the uncoated Fe nanoflakes, the permittivity of Fe/SiO2 nanoflakes decreases dramatically, while the permeability decreases slightly. Consequently, reflection losses exceeding -20 dB of Fe/SiO2 nanoflakes are obtained in the frequency range of 3.8-7.3 GHz for absorber thicknesses of 2.2-3.6 mm, while the reflection loss of uncoated Fe nanoflakes almost cannot reach -10 dB in the same thickness range. The enhanced microwave absorption of Fe/SiO2 nanoflakes can be attributed to the combination of the proper electromagnetic impedance match due to the decrease of permittivity and large magnetic loss due to strong and broadband natural resonance. The key to the combination is the coexistence of the nanoshell microstructure and the nanoflake morphology.
Recently, the use of gold nanoparticles as potential tumor selective radiosensitizers has been proposed as a breakthrough in radiotherapy. Experiments in living cells and in vivo have demonstrated the efficiency of the metal nanoparticles when combined with low energy x-ray radiations (below conventional 1 MeV Linac radiation). Further studies on DNA have been performed in order to better understand the fundamental processes of sensitization and to further improve the method. In this work, we propose a new strategy based on the combination of platinum nanoparticles with irradiation by fast ions effectively used in hadron therapy. It is observed in particular that nanoparticles enhance strongly lethal damage in DNA, with an efficiency factor close to 2 for double strand breaks. In order to disentangle the effect of the nano-design architecture, a comparison with the effects of dispersed metal atoms at the same concentration has been performed. It is thus shown that the sensitization in nanoparticles is enhanced due to auto-amplified electronic cascades inside the nanoparticles, which reinforces the energy deposition in the close vicinity of the metal. Finally, the combination of fast ion radiation (hadron therapy) with platinum nanoparticles should strongly improve cancer therapy protocols.
First-principles calculations are performed to study the geometry, electronic structure and magnetic properties of light non-metallic atom-doped graphene (B, N, O and F). The planar structure and the quasi-linear energy dispersion near the Dirac point remain through doping with B and N atoms, by which p-type doping and n-type doping graphene are respectively induced. A bandgap of about 0.5 eV is generated through O doping, and geometrically the O atom is also in the graphene plane. No magnetic moment is detected in B-, N- and O-doped graphene. For F doping, the F atom bonds with one of the carbon atoms close to the vacancy, with the other two carbon atoms undergoing a Jahn-Teller distortion. A weak polarized magnetic moment of 0.71 mu(B) is detected through F doping.
Carbon nanotube (CNT) based sensors are often fabricated by dispersing CNTs into different types of polymer. In this paper, a prototype carbon nanotube (CNT) yarn strain sensor with excellent repeatability and stability for in situ structural health monitoring was developed. The CNT yarn was spun directly from CNT arrays, and its electrical resistance increased linearly with tensile strain, making it an ideal strain sensor. It showed consistent piezoresistive behavior under repetitive straining and unloading, and good resistance stability at temperatures ranging from 77 to 373 K. The sensors can be easily embedded into composite structures with minimal invasiveness and weight penalty. We have also demonstrated their ability to monitor crack initiation and propagation.
Improvement of the robustness of superhydrophobic surfaces is critical in order to achieve commercial applications of these surfaces in such diverse areas as self-cleaning, water repellency and corrosion resistance. In this study, the mechanical robustness of superhydrophobic surfaces was evaluated on hierarchically structured silicon surfaces. The effect of two-scale hierarchical structures on robustness was investigated using an abrasion test and the results compared to those of superhydrophobic surfaces fabricated from polymeric materials and from silicon that contains only nanostructures. Unlike the polymeric and nanostructure-only surfaces, the hierarchical structures retained superhydrophobic behavior after mechanical abrasion.
We report a simple and continuous spinning method that combines twisting and shrinking processes to produce carbon nanotube yarns. In this method, a yarn freshly spun from a super-aligned carbon nanotube array is first twisted and then passes through a volatile solvent for shrinking. The as-produced yarn consists of densely packed carbon nanotubes, and thus has a tensile strength up to about 1 GPa. The tensile strength depends on the diameter and the twisting angle of the yarn. Different kinds of solvents, such as water, ethanol, and acetone, are used to shrink the twisted yarns, and acetone shows the best shrinking effect. The origin of the solvent shrinking effect is investigated. Our method is favorable for continuous mass production of high strength carbon nanotube yarns with a wide range of diameters, especially ultra-thin yarns.
A nanoparticle-conjugated cancer drug provides a novel strategy for cancer therapy. In this study, we manipulated nanodiamond (ND), a carbon nanomaterial, to covalently link paclitaxel for cancer drug delivery and therapy. Paclitaxel was bound to the surface of 3-5 nm sized ND through a succession of chemical modifications. The ND-paclitaxel conjugation was measured by atomic force microscope and nuclear magnetic resonance spectroscopy, and confirmed with infrared spectroscopy by the detection of deuterated paclitaxel. Treatment with 0.1-50 mu g ml(-1) ND-paclitaxel for 48 h significantly reduced the cell viability in the A549 human lung carcinoma cells. ND-paclitaxel induced both mitotic arrest and apoptosis in A549 cells. However, ND alone or denatured ND-paclitaxel (after treatment with strong alkaline solution, 1 M NaOH) did not induce the damage effects on A549 cells. ND-paclitaxel was taken into lung cancer cells in a concentration-dependent manner using flow cytometer analysis. The ND-paclitaxel particles were located in the microtubules and cytoplasm of A549 cells observed by confocal microscopy. Furthermore, ND-paclitaxel markedly blocked the tumor growth and formation of lung cancer cells in xenograft SCID mice. Together, we provide a functional covalent conjugation of ND-paclitaxel, which can be delivered into lung carcinoma cells and preserves the anticancer activities on the induction of mitotic blockage, apoptosis and anti-tumorigenesis.
We use suspended graphene electromechanical resonators to study the variation of resonant frequency as a function of temperature. Measuring the change in frequency resulting from a change in tension, from 300 to 30 K, allows us to extract information about the thermal expansion of monolayer graphene as a function of temperature, which is critical for strain engineering applications. We find that thermal expansion of graphene is negative for all temperatures between 300 and 30 K. We also study the dispersion, the variation of resonant frequency with DC gate voltage, of the electromechanical modes and find considerable tunability of resonant frequency, desirable for applications like mass sensing and RF signal processing at room temperature. With a lowering of temperature, we find that the positively dispersing electromechanical modes evolve into negatively dispersing ones. We quantitatively explain this crossover and discuss optimal electromechanical properties that are desirable for temperature-compensated sensors.
Noble metal nanoparticles exhibit sharp spectral extinction peaks at visible and near-infrared frequencies due to the resonant excitation of their free electrons, termed localized surface plasmon resonance (LSPR). Since the resonant frequency is dependent on the refractive index of the nanoparticle surroundings, LSPR can be the basis for sensing molecular interactions near the nanoparticle surface. However, previous studies have not yet determined whether the LSPR mechanism can reach the ultimate sensing limit: the detection of individual molecules. Here we demonstrate single molecule LSPR detection by monitoring antibody-antigen unbinding events through the scattering spectra of individual gold bipyramids. Both experiments and finite element simulations indicate that the unbinding of single antigen molecules results in small, discrete <0.5 nm blue-shifts of the plasmon resonance. The unbinding rate is consistent with antibody-antigen binding kinetics determined from previous ensemble experiments. According to these results, the effective refractive index of a single protein is approximately 1.54. LSPR sensing could therefore be a powerful addition to the current toolbox of single molecule detection methods since it probes interactions on long timescales and under relatively natural conditions.
The effect of average particle sizes on basic macroscopic properties and heat transfer performance of alpha-SiC/water nanofluids was investigated. The average particle sizes, calculated from the specific surface area of nanoparticles, were varied from 16 to 90 nm. Nanofluids with larger particles of the same material and volume concentration provide higher thermal conductivity and lower viscosity increases than those with smaller particles because of the smaller solid/liquid interfacial area of larger particles. It was also demonstrated that the viscosity of water-based nanofluids can be significantly decreased by pH of the suspension independently from the thermal conductivity. Heat transfer coefficients were measured and compared to the performance of base fluids as well as to nanofluids reported in the literature. Criteria for evaluation of the heat transfer performance of nanofluids are discussed and optimum directions in nanofluid development are suggested.
Miniaturization and manipulation of materials at nanometer scale are key challenges in nanoscience and nanotechnology. In membrane science and technology, the fabrication of ultra-thin polymer films (defect-free) on square meter scale with uniform thickness (5 m(3) (STP) m(-2) h(-1) bar(-1). The permeances are extremely high, because the membranes are made from a CO2 philic polymer material and they are only a few tens of nanometers thin. Thus, these thin film membranes have potential application in the treatment of large gas streams under low pressure like, e.g., carbon dioxide separation from flue gas.