Showing papers by "Nanyang Technological University published in 2010"

Simultaneous phase and size control of upconversion nanocrystals through lanthanide doping

Abstract: Doping is a widely applied technological process in materials science that involves incorporating atoms or ions of appropriate elements into host lattices to yield hybrid materials with desirable properties and functions. For nanocrystalline materials, doping is of fundamental importance in stabilizing a specific crystallographic phase, modifying electronic properties, modulating magnetism as well as tuning emission properties. Here we describe a material system in which doping influences the growth process to give simultaneous control over the crystallographic phase, size and optical emission properties of the resulting nanocrystals. We show that NaYF(4) nanocrystals can be rationally tuned in size (down to ten nanometres), phase (cubic or hexagonal) and upconversion emission colour (green to blue) through use of trivalent lanthanide dopant ions introduced at precisely defined concentrations. We use first-principles calculations to confirm that the influence of lanthanide doping on crystal phase and size arises from a strong dependence on the size and dipole polarizability of the substitutional dopant ion. Our results suggest that the doping-induced structural and size transition, demonstrated here in NaYF(4) upconversion nanocrystals, could be extended to other lanthanide-doped nanocrystal systems for applications ranging from luminescent biological labels to volumetric three-dimensional displays.

Constructing Hierarchical Spheres from Large Ultrathin Anatase TiO2 Nanosheets with Nearly 100% Exposed (001) Facets for Fast Reversible Lithium Storage

Abstract: Synthesis of nanocrystals with exposed high-energy facets is a well-known challenge in many fields of science and technology. The higher reactivity of these facets simultaneously makes them desirable catalysts for sluggish chemical reactions and leads to their small populations in an equilibrated crystal. Using anatase TiO2 as an example, we demonstrate a facile approach for creating high-surface-area stable nanosheets comprising nearly 100% exposed (001) facets. Our approach relies on spontaneous assembly of the nanosheets into three-dimensional hierarchical spheres, which stabilizes them from collapse. We show that the high surface density of exposed TiO2 (001) facets leads to fast lithium insertion/deinsertion processes in batteries that mimic features seen in high-power electrochemical capacitors.

The Loss of Species: Mangrove Extinction Risk and Geographic Areas of Global Concern

Abstract: Mangrove species are uniquely adapted to tropical and subtropical coasts, and although relatively low in number of species, mangrove forests provide at least US $1.6 billion each year in ecosystem services and support coastal livelihoods worldwide. Globally, mangrove areas are declining rapidly as they are cleared for coastal development and aquaculture and logged for timber and fuel production. Little is known about the effects of mangrove area loss on individual mangrove species and local or regional populations. To address this gap, species-specific information on global distribution, population status, life history traits, and major threats were compiled for each of the 70 known species of mangroves. Each species' probability of extinction was assessed under the Categories and Criteria of the IUCN Red List of Threatened Species. Eleven of the 70 mangrove species (16%) are at elevated threat of extinction. Particular areas of geographical concern include the Atlantic and Pacific coasts of Central America, where as many as 40% of mangroves species present are threatened with extinction. Across the globe, mangrove species found primarily in the high intertidal and upstream estuarine zones, which often have specific freshwater requirements and patchy distributions, are the most threatened because they are often the first cleared for development of aquaculture and agriculture. The loss of mangrove species will have devastating economic and environmental consequences for coastal communities, especially in those areas with low mangrove diversity and high mangrove area or species loss. Several species at high risk of extinction may disappear well before the next decade if existing protective measures are not enforced.

Synthesis of Few-Layer Hexagonal Boron Nitride Thin Film by Chemical Vapor Deposition

Abstract: In this contribution we demonstrate a method of synthesizing a hexagonal boron nitride (h-BN) thin film by ambient pressure chemical vapor deposition on polycrystalline Ni films. Depending on the growth conditions, the thickness of the obtained h-BN film is between ∼5 and 50 nm. The h-BN grows continuously on the entire Ni surface and the region with uniform thickness can be up to 20 μm in lateral size which is only limited by the size of the Ni single crystal grains. The hexagonal structure was confirmed by both electron and X-ray diffraction. X-ray photoelectron spectroscopy shows the B/N atomic ratio to be 1:1.12. A large optical band gap (5.92 eV) was obtained from the photoabsorption spectra which suggest the potential usage of this h-BN film in optoelectronic devices.

Graphene for electrochemical sensing and biosensing

Abstract: Graphene has proved to be an excellent nanomaterial for applications in electrochemistry. We review progress in constructing high-performance electrochemical sensors and biosensors. We also discuss: different routes for graphene fabrication; graphene-modified electrodes and graphene-composite electrodes for sensing, including those based on ionic liquids; incorporation of biorecognition elements into graphene-based electrodes; and, graphene-supported electrocatalytic nanoparticle-based electrochemical sensors and biosensors.

Graphene-based nanomaterials and their electrochemistry

Abstract: Graphene-based nanomaterials are in the forefront of chemical research. This tutorial review provides an introduction to their electrochemistry, its fundamentals and applications. Selected examples of applications in energy storage and sensing are presented. The synthetic methods for preparing graphenes as well as their materials chemistry are thoroughly discussed, as they have a profound influence on the electronic and electrochemical behavior of graphene-related nanomaterials. Inherent electrochemistry and spectroelectrochemistry of graphene nanomaterials is discussed thoroughly. Important application in sensing and energy storage areas are highlighted.

Cloud Computing: a Perspective Study

Abstract: The Cloud computing emerges as a new computing paradigm which aims to provide reliable, customized and QoS guaranteed dynamic computing environments for end-users. In this paper, we study the Cloud computing paradigm from various aspects, such as definitions, distinct features, and enabling technologies. This paper brings an introductional review on the Cloud computing and provides the state-of-the-art of Cloud computing technologies.

Transposable elements have rewired the core regulatory network of human embryonic stem cells

Abstract: Detection of new genomic control elements is critical in understanding transcriptional regulatory networks in their entirety. We studied the genome-wide binding locations of three key regulatory proteins (POU5F1, also known as OCT4; NANOG; and CTCF) in human and mouse embryonic stem cells. In contrast to CTCF, we found that the binding profiles of OCT4 and NANOG are markedly different, with only approximately 5% of the regions being homologously occupied. We show that transposable elements contributed up to 25% of the bound sites in humans and mice and have wired new genes into the core regulatory network of embryonic stem cells. These data indicate that species-specific transposable elements have substantially altered the transcriptional circuitry of pluripotent stem cells.

Self-Assembling TiO2 Nanorods on Large Graphene Oxide Sheets at a Two-Phase Interface and Their Anti-Recombination in Photocatalytic Applications

Abstract: TiO 2 nanorods are self-assembled on the graphene oxide (GO) sheets at the water/toluene interface. The self-assembled GO–TiO 2 nanorod composites (GO–TiO 2 NRCs) can be dispersed in water. The effective anchoring of TiO 2 nanorods on the whole GO sheets is confi rmed by transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform IR spectroscopy (FTIR), and thermogravimetric analysis (TGA). The signifi cant increase of photocatalytic activity is confi rmed by the degradation of methylene blue (MB) under UV light irridiation. The large enhancement of photocatalytic activity is caused by the effective charge anti-recombination and the effective absorption of MB on GO. The effective charge transfer from TiO 2 to GO sheets is confi rmed by the signifi cant photoluminescence quenching of TiO 2 nanorods, which can effectively prevent the charge recombination during photocatalytic process. The effective absorption of MB on GO is confi rmed by the UV-vis spectra. The degradation rate of MB in the second cycle is faster than that in the fi rst cycle because of the reduction of GO under UV light irradiation.

Probing layer number and stacking order of few-layer graphene by Raman spectroscopy.

Abstract: Graphene is a two-dimensional material defined as a planar honeycomb lattice of close-packed carbon atoms, where the electrons exhibit a linear dispersion near Dirac K points and behave as massless Dirac fermions. However, the valence and conduction bands in an AB stacked graphene bilayer split into two parabolic branches near the K point originating from the interaction of p electrons, and the electrons are hence described by massive Dirac fermions. Moreover, a graphene bilayer is a tunable-gap semiconductor under electric-field biasing. With a further increase in the number of layers along with AB stacking, the electronic structure reveals stepwise variations that eventually approach that of the three-dimensional counterpart. Considering the close relation between the electronic properties and layer number of few-layer graphene (FLG), the ability to accurately determine the layer number and correlating this with the electronic structure is a prerequisite in understanding the evolution of the electronic properties from twoto threedimensional graphitic materials. In addition to graphene layers with AB stacking, FLG with arbitrary stacking (Figure 1) is considered to possess distinct properties arising from its different crystalline structure and p electron interactions. Experimentally, it has been observed that the electroand magnetotransport properties for folded graphene sheets are different to thoseofAB stackedbilayers. Furthermore,FLG grown on SiC, Ni, and Ru also have non-AB stacking order. Therefore, elucidating the detailed character-

A survey of graph edit distance

Abstract: Inexact graph matching has been one of the significant research foci in the area of pattern analysis. As an important way to measure the similarity between pairwise graphs error-tolerantly, graph edit distance (GED) is the base of inexact graph matching. The research advance of GED is surveyed in order to provide a review of the existing literatures and offer some insights into the studies of GED. Since graphs may be attributed or non-attributed and the definition of costs for edit operations is various, the existing GED algorithms are categorized according to these two factors and described in detail. After these algorithms are analyzed and their limitations are identified, several promising directions for further research are proposed.

Layered graphene/quantum dots for photovoltaic devices.

Abstract: To meet the increasing demand of clean energy the harvesting of electricity from solar incident photons with high efficiency at economically viable cost is needed. Quantum dot (QD) based solar cells are poised to play a leading role in this revolution owing to their potential in exceeding the Shockley–Queissar limit, their size-tuned optical response, and their efficient multiple carrier generation. 6] A major challenge in developing high-performance QD solar cells is the effective separation of photogenerated electron–hole pairs and the transfer of the electrons to the electrode. Strategies that have been tried include the introduction of nanomaterials with a suitable band energy as efficient acceptors. Carbon, an environmentally friendly and inexpensive material, exists in a variety of nanostructures ranging from insulator/semiconducting diamond to metallic/semimetallic graphite, conducting/semiconducting fullerenes, and single-walled carbon nanotubes (SWNTs), 10] and recently has been widely used in QD solar cells. Particularly, SWNTs 12] and stacked-cup carbon nanotubes have been used as efficient acceptors to enhance photoinduced charge transfer for improved performance because of their unique one-dimensional nanostructure and appropriate band energy. However, the efficiency of carbon nanomaterial based QD solar cells reported so far is still low (incident photon-to-charge-carrier conversion efficiency (IPCE) 5 % and photocurrent response 0.4 mAcm 2 under light illumination of 100 mWcm ), which is still some distance from the requirement for the next generation of solar cells. Graphene, a new class of two-dimensional carbon material with single-atom-thick layer features different from balllike C60 and one-dimensional carbon nanotubes, has attracted attention in recent years. As a result of its high specific surface area for a large interface, high mobility up to 10000 cm V 1 s , and tunable band gap, graphene should be a very promising electron acceptor in photovoltaic devices. In this work, a novel layered nanofilm of graphene/QDs was constructed from all aqueous solutions to fabricate a photovoltaic device using graphene as acceptor, demonstrating the best performance (IPCE of 16% and photoresponse of 1.08 mAcm 2 under light illumination of 100 mWcm ) in all reported carbon/QD solar cells. For a better understanding of the mechanism of the graphene in improving the performance of the device, the graphene/QDs and SWNT/QDs photovoltaic devices are compared. The fabrication of the layered graphene/QDs device is shown schematically in Figure 1. Chemically reduced graphene was used not only because of its unique properties

Multi-layered surgical prosthesis

Abstract: The invention relates to prostheses having a multi-layered sheet structure comprising at least two continuous polymer film layers. Also disclosed are methods of manufacturing the prostheses, as well as methods of treating a patient by implanting them into a patient. The prostheses are used in hernia repair, the repair of anatomical defects of the abdominal wall, diaphragm and chest wall, correction of defects in- the genitourinary system, and repair of traumatically damaged organs such as the spleen, liver or kidney.

Propping through related party transactions

Abstract: Based on a sample of Chinese listed firms from 1998 through 2002, this paper documents that listed firms prop up earnings by using abnormal related sales to their controlling owners. Such related sales propping is more prevalent among state-owned firms and in regions with weaker economic institutions. We also find that these abnormal related sales are not entirely accrual-based but can be cash-based as well, and they serve as a substitute rather than complement to accruals management for meeting earnings targets. Since these abnormal related sales can be cash-based, there is significant cash transfer via related lending from listed firms back to controlling owners after the propping. However, no cash transfer via related lending is found to be associated with accruals earnings management.

Toward Broadband Vibration-based Energy Harvesting

Abstract: The dramatic reduction in power consumption of current integrated circuits has evoked great research interests in harvesting ambient energy, such as vibrations, as a potential power supply for electronic devices to avoid battery replacement. Currently, most vibration-based energy harvesters are designed as linear resonators to achieve optimal performance by matching their resonance frequencies with the ambient excitation frequencies a priori. However, a slight shift of the excitation frequency will cause a dramatic reduction in performance. Unfortunately, in the vast majority of practical cases, the ambient vibrations are frequency-varying or totally random with energy distributed over a wide frequency spectrum. Hence, developing techniques to increase the bandwidth of vibration-based energy harvesters has become the next important problem in energy harvesting. This article reviews the advances made in the past few years on this issue. The broadband vibration-based energy harvesting solutions, covering re...

Electrochemical deposition of ZnO nanorods on transparent reduced graphene oxide electrodes for hybrid solar cells.

Abstract: Monocrystalline ZnO nanorods (NRs) with high donor concentration are electrochemically deposited on highly conductive reduced graphene oxide (rGO) films on quartz. The film thickness, optical transmittance, sheet resistance, and roughness of rGO films are systematically studied. The obtained ZnO NRs on rGO films are characterized by X-ray diffraction, transmission electron microscopy, photoluminescence, and Raman spectra. As a proof-of-concept application, the obtained ZnO NRs on rGO are used to fabricate inorganic-organic hybrid solar cells with layered structure of quartz/rGO/ZnO NR/poly(3-hexylthiophene)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (P3HT/PEDOT:PSS)/Au. The observed power conversion efficiency (PCE, eta), approximately 0.31%, is higher than that reported in previous solar cells by using graphene films as electrodes. These results clearly demonstrate that rGO films with a higher conductivity have a smaller work function and show a better performance in the fabricated solar cells.

Augmented Lagrangian Method, Dual Methods, and Split Bregman Iteration for ROF, Vectorial TV, and High Order Models

Abstract: In image processing, the Rudin-Osher-Fatemi (ROF) model [L. Rudin, S. Osher, and E. Fatemi, Phys. D, 60 (1992), pp. 259-268] based on total variation (TV) minimization has proven to be very useful. So far many researchers have contributed to designing fast numerical schemes and overcoming the nondifferentiability of the model. Methods considered to be particularly efficient for the ROF model include the Chan-Golub-Mulet (CGM) primal-dual method [T.F. Chan, G.H. Golub, and P. Mulet, SIAM J. Sci. Comput., 20 (1999), pp. 1964-1977], Chambolle's dual method [A. Chambolle, J. Math. Imaging Vis., 20 (2004), pp. 89-97], the splitting and quadratic penalty-based method [Y. Wang, J. Yang, W. Yin, and Y. Zhang, SIAM J. Imaging Sci., 1 (2008), pp. 248-272], and the split Bregman iteration [T. Goldstein and S. Osher, SIAM J. Imaging Sci., 2 (2009), pp. 323-343], as well as the augmented Lagrangian method [X.C. Tai and C. Wu, Lecture Notes in Comput. Sci. 5567, Springer-Verlag, Berlin, 2009, pp. 502-513]. In this paper, we first review the augmented Lagrangian method for the ROF model and then provide some convergence analysis and extensions to vectorial TV and high order models. All the algorithms and analysis will be presented in the discrete setting, which is much clearer for practical implementation than the continuous setting as in Tai and Wu, above. We also present, in the discrete setting, the connections between the augmented Lagrangian method, the dual methods, and the split Bregman iteration. Using our extensions and observations, we can easily figure out CGM and the split Bregman iteration for vectorial TV and high order models, which, to the best of our knowledge, have not been presented in the literature. Numerical examples demonstrate the efficiency and accuracy of our method, especially in the image deblurring case.

Hysteresis of electronic transport in graphene transistors.

Abstract: Graphene field effect transistors commonly comprise graphene flakes lying on SiO2 surfaces. The gate-voltage dependent conductance shows hysteresis depending on the gate sweeping rate/range. It is shown here that the transistors exhibit two different kinds of hysteresis in their electrical characteristics. Charge transfer causes a positive shift in the gate voltage of the minimum conductance, while capacitive gating can cause the negative shift of conductance with respect to gate voltage. The positive hysteretic phenomena decay with an increase of the number of layers in graphene flakes. Self-heating in a helium atmosphere significantly removes adsorbates and reduces positive hysteresis. We also observed negative hysteresis in graphene devices at low temperature. It is also found that an ice layer on/under graphene has a much stronger dipole moment than a water layer does. Mobile ions in the electrolyte gate and a polarity switch in the ferroelectric gate could also cause negative hysteresis in graphene t...

Organic photovoltaic devices using highly flexible reduced graphene oxide films as transparent electrodes.

Abstract: The chemically reduced graphene oxide (rGO) was transferred onto polyethylene terephthalate (PET) substrates and then used as transparent and conductive electrodes for flexible organic photovoltaic (OPV) devices. The performance of the OPV devices mainly depends on the charge transport efficiency through rGO electrodes when the optical transmittance of rGO is above 65%. However, if the transmittance of rGO is less than 65%, the performance of the OPV device is dominated by the light transmission efficiency, that is, the transparency of rGO films. After the tensile strain (∼2.9%) was applied on the fabricated OPV device, it can sustain a thousand cycles of bending. Our work demonstrates the highly flexible property of rGO films, which provide the potential applications in flexible optoelectronics.

Centimeter-Long and Large-Scale Micropatterns of Reduced Graphene Oxide Films: Fabrication and Sensing Applications

Abstract: Recently, the field-effect transistors (FETs) with graphene as the conducting channels have been used as a promising chemical and biological sensors. However, the lack of low cost and reliable and large-scale preparation of graphene films limits their applications. In this contribution, we report the fabrication of centimeter-long, ultrathin (1-3 nm), and electrically continuous micropatterns of highly uniform parallel arrays of reduced graphene oxide (rGO) films on various substrates including the flexible polyethylene terephthalate (PET) films by using the micromolding in capillary method. Compared to other methods for the fabrication of graphene patterns, our method is fast, facile, and substrate independent. In addition, we demonstrate that the nanoelectronic FETs based on our rGO patterns are able to label-freely detect the hormonal catecholamine molecules and their dynamic secretion from living cells.

Switched Inductor Z-Source Inverter

Abstract: On the basis of the classical Z-source inverter, this paper presents a developed impedance-type power inverter that is termed the switched inductor (SL) Z-source inverter. To enlarge voltage adjustability, the proposed inverter employs a unique SL impedance network to couple the main circuit and the power source. Compared with the classical Z-source inverter, the proposed inverter increases the voltage boost inversion ability significantly. Only a very short shoot-through zero state is required to obtain high voltage conversion ratios, which is beneficial for improving the output power quality of the main circuit. In addition, the voltage buck inversion ability is also provided in the proposed inverter for those applications that need low ac voltages. Similar to the classical Z-source inverter, the proposed concepts of SL Z-source inverter can be applied to various applications of dc-ac, ac-ac, dc-dc, and ac-dc power conversion. A detailed topology analysis and a generalized discussion are given. Both simulation and experimental results verify the analytical results.