Showing papers by "Tsinghua University published in 2008"

Flexible Graphene Films via the Filtration of Water-Soluble Noncovalent Functionalized Graphene Sheets

Abstract: Flexible graphene films were prepared by the filtration of water-soluble noncovalently functionalized graphene sheets with pyrenebutyrate. The work presented here will not only open a new way for preparing water-soluble graphene dispersions but also provide a general route for fabricating conducting films based on graphene.

Effects of particle size, particle/matrix interface adhesion and particle loading on mechanical properties of particulate–polymer composites

Abstract: There have been a number of review papers on layered silicate and carbon nanotube reinforced polymer nanocomposites, in which the fillers have high aspect ratios. Particulate–polymer nanocomposites containing fillers with small aspect ratios are also an important class of polymer composites. However, they have been apparently overlooked. Thus, in this paper, detailed discussions on the effects of particle size, particle/matrix interface adhesion and particle loading on the stiffness, strength and toughness of such particulate–polymer composites are reviewed. To develop high performance particulate composites, it is necessary to have some basic understanding of the stiffening, strengthening and toughening mechanisms of these composites. A critical evaluation of published experimental results in comparison with theoretical models is given.

The LHCb detector at the LHC

Abstract: The LHCb experiment is dedicated to precision measurements of CP violation and rare decays of B hadrons at the Large Hadron Collider (LHC) at CERN (Geneva). The initial configuration and expected performance of the detector and associated systems, as established by test beam measurements and simulation studies, is described.

ArnetMiner: extraction and mining of academic social networks

Abstract: This paper addresses several key issues in the ArnetMiner system, which aims at extracting and mining academic social networks. Specifically, the system focuses on: 1) Extracting researcher profiles automatically from the Web; 2) Integrating the publication data into the network from existing digital libraries; 3) Modeling the entire academic network; and 4) Providing search services for the academic network. So far, 448,470 researcher profiles have been extracted using a unified tagging approach. We integrate publications from online Web databases and propose a probabilistic framework to deal with the name ambiguity problem. Furthermore, we propose a unified modeling approach to simultaneously model topical aspects of papers, authors, and publication venues. Search services such as expertise search and people association search have been provided based on the modeling results. In this paper, we describe the architecture and main features of the system. We also present the empirical evaluation of the proposed methods.

Superhydrophobic surfaces: from structural control to functional application

Abstract: A superhydrophobic surface is a surface with a water contact angle close to or higher than 150°. In this feature article, we review the historical and present research on superhydrophobic surfaces, including the characterization of superhydrophobicity, different ways to fabricate rough surfaces, and low-surface-energy modifications on inorganic and organic rough surfaces. It is the combination of surface roughness and low-surface-energy modification that leads to superhydrophobicity. Notably, research on superhydrophobic surfaces has not only fundamental interest but various possible functional applications in micro- and nano-materials and devices.

Dcell: a scalable and fault-tolerant network structure for data centers

Abstract: A fundamental challenge in data center networking is how to efficiently interconnect an exponentially increasing number of servers. This paper presents DCell, a novel network structure that has many desirable features for data center networking. DCell is a recursively defined structure, in which a high-level DCell is constructed from many low-level DCells and DCells at the same level are fully connected with one another. DCell scales doubly exponentially as the node degree increases. DCell is fault tolerant since it does not have single point of failure and its distributed fault-tolerant routing protocol performs near shortest-path routing even in the presence of severe link or node failures. DCell also provides higher network capacity than the traditional tree-based structure for various types of services. Furthermore, DCell can be incrementally expanded and a partial DCell provides the same appealing features. Results from theoretical analysis, simulations, and experiments show that DCell is a viable interconnection structure for data centers.

Selective Synthesis of Co3O4 Nanocrystal with Different Shape and Crystal Plane Effect on Catalytic Property for Methane Combustion

Abstract: We have succeeded in synthesizing Co(3)O(4) nanosheets, nanobelts, and nanocubes with a hydrothermal process of cobalt hydroxide precursor and subsequent direct thermal decomposition. The predominantly exposed planes are {112}, {011}, and {001}, respectively. The methane combustion catalytic activity order of crystal planes follows {112} > {011} >> {001}. The selective synthesis of transition metal oxides with uniform and different reactive crystal planes under nanoscale conditions is expected to bring up new opportunities for design, tuning, and control of chemical activity, specificity, and selectivity.

Perspectives of microbial oils for biodiesel production.

Abstract: Biodiesel has become more attractive recently because of its environmental benefits, and the fact that it is made from renewable resources. Generally speaking, biodiesel is prepared through transesterification of vegetable oils or animal fats with short chain alcohols. However, the lack of oil feedstocks limits the large-scale development of biodiesel to some extent. Recently, much attention has been paid to the development of microbial, oils and it has been found that many microorganisms, such as algae, yeast, bacteria, and fungi, have the ability to accumulate oils under some special cultivation conditions. Compared to other plant oils, microbial oils have many advantages, such as short life cycle, less labor required, less affection by venue, season and climate, and easier to scale up. With the rapid expansion of biodiesel, microbial oils might become one of potential oil feedstocks for biodiesel production in the future, though there are many works associated with microorganisms producing oils need to be carried out further. This review is covering the related research about different oleaginous microorganisms producing oils, and the prospects of such microbial oils used for biodiesel production are also discussed.

Protein production and purification.

Abstract: In selecting a method to produce a recombinant protein, a researcher is faced with a bewildering array of choices as to where to start. To facilitate decision-making, we describe a consensus 'what to try first' strategy based on our collective analysis of the expression and purification of over 10,000 different proteins. This review presents methods that could be applied at the outset of any project, a prioritized list of alternate strategies and a list of pitfalls that trip many new investigators.

Efficient TiO2 Photocatalysts from Surface Hybridization of TiO2 Particles with Graphite-like Carbon

Abstract: Surface hybridization of TiO2 with graphite-like carbon layers of a few molecular layers thickness yields efficient photocatalysts. Photoelectrochemical measurements confirm an electronic interaction between TiO2 and the graphite-like carbon. A TiO2 photocatalyst with a carbon shell of three molecular layers thickness (∼1 nm) shows the highest photocatalytic activity which is about two times higher than that of Degussa P25 TiO2 under UV light irradiation. The mechanism of the enhanced photocatalytic activity under UV irradiation is based on the high migration efficiency of photoinduced electrons at the graphite-like carbon/TiO2 interface, which is due to the electronic interaction between both materials. In addition, a high activity under visible light irradiation is observed after graphite-like carbon hybridization. TiO2's response is extended into the visible range of the solar spectrum due to the electronic coupling of π states of the graphite-like carbon and conduction band states of TiO2.

Label Propagation through Linear Neighborhoods

Abstract: In many practical data mining applications such as text classification, unlabeled training examples are readily available, but labeled ones are fairly expensive to obtain. Therefore, semi supervised learning algorithms have aroused considerable interests from the data mining and machine learning fields. In recent years, graph-based semi supervised learning has been becoming one of the most active research areas in the semi supervised learning community. In this paper, a novel graph-based semi supervised learning approach is proposed based on a linear neighborhood model, which assumes that each data point can be linearly reconstructed from its neighborhood. Our algorithm, named linear neighborhood propagation (LNP), can propagate the labels from the labeled points to the whole data set using these linear neighborhoods with sufficient smoothness. A theoretical analysis of the properties of LNP is presented in this paper. Furthermore, we also derive an easy way to extend LNP to out-of-sample data. Promising experimental results are presented for synthetic data, digit, and text classification tasks.

International Stock Return Comovements

Abstract: We examine international stock return comovements using country-industry and country-style portfolios. We first establish that parsimonious risk-based factor models capture the covariance structure of the data better than the popular Heston-Rouwenhorst (1994) model. We then establish the following stylized facts regarding stock return comovements. First, we do not find evidence for an upward trend in return correlations, excpet for the European stock markets. Second, the increasing imporatnce of industry factors relative to country factors was a short-lived, temporary phenomenon. Third, we find no evidence for a trend in idiosyncratic risk in any of the countries we examine.

Materials and noncoplanar mesh designs for integrated circuits with linear elastic responses to extreme mechanical deformations

Abstract: Electronic systems that offer elastic mechanical responses to high-strain deformations are of growing interest because of their ability to enable new biomedical devices and other applications whose requirements are impossible to satisfy with conventional wafer-based technologies or even with those that offer simple bendability. This article introduces materials and mechanical design strategies for classes of electronic circuits that offer extremely high stretchability, enabling them to accommodate even demanding configurations such as corkscrew twists with tight pitch (e.g., 90° in ≈1 cm) and linear stretching to “rubber-band” levels of strain (e.g., up to ≈140%). The use of single crystalline silicon nanomaterials for the semiconductor provides performance in stretchable complementary metal-oxide-semiconductor (CMOS) integrated circuits approaching that of conventional devices with comparable feature sizes formed on silicon wafers. Comprehensive theoretical studies of the mechanics reveal the way in which the structural designs enable these extreme mechanical properties without fracturing the intrinsically brittle active materials or even inducing significant changes in their electrical properties. The results, as demonstrated through electrical measurements of arrays of transistors, CMOS inverters, ring oscillators, and differential amplifiers, suggest a valuable route to high-performance stretchable electronics.

Network-based global inference of human disease genes

Abstract: Deciphering the genetic basis of human diseases is an important goal of biomedical research. On the basis of the assumption that phenotypically similar diseases are caused by functionally related genes, we propose a computational framework that integrates human protein-protein interactions, disease phenotype similarities, and known gene-phenotype associations to capture the complex relationships between phenotypes and genotypes. We develop a tool named CIPHER to predict and prioritize disease genes, and we show that the global concordance between the human protein network and the phenotype network reliably predicts disease genes. Our method is applicable to genetically uncharacterized phenotypes, effective in the genome-wide scan of disease genes, and also extendable to explore gene cooperativity in complex diseases. The predicted genetic landscape of over 1000 human phenotypes, which reveals the global modular organization of phenotype-genotype relationships. The genome-wide prioritization of candidate genes for over 5000 human phenotypes, including those with under-characterized disease loci or even those lacking known association, is publicly released to facilitate future discovery of disease genes.

Quantum spin Hall effect in inverted type-II semiconductors.

Abstract: The quantum spin Hall (QSH) state is a topologically nontrivial state of quantum matter which preserves time-reversal symmetry; it has an energy gap in the bulk, but topologically robust gapless states at the edge. Recently, this novel effect has been predicted and observed in HgTe quantum wells and in this Letter we predict a similar effect arising in Type-II semiconductor quantum wells made from InAs/GaSb/AlSb. The quantum well exhibits an "inverted" phase similar to HgTe/CdTe quantum wells, which is a QSH state when the Fermi level lies inside the gap. Due to the asymmetric structure of this quantum well, the effects of inversion symmetry breaking are essential. Remarkably, the topological quantum phase transition between the conventional insulating state and the quantum spin Hall state can be continuously tuned by the gate voltage, enabling quantitative investigation of this novel phase transition.

Ferromagnetism and possible application in spintronics of transition-metal-doped ZnO films

Abstract: This review article first presents a summary of current understanding of the magnetic properties and intrinsic ferromagnetism of transition-metal (TM)-doped ZnO films, which are typical diluted magnetic oxides used in spintronics. The local structure and magnetic behavior of TM-doped ZnO are strongly sensitive to the preparation parameters. In the second part, we discuss in detail the effects of doping elements and concentrations, oxygen partial pressure, substrate and its orientation and temperature, deposition rate, post-annealing temperature and co-doping on the local structure and subsequent ferromagnetic ordering of TM-doped ZnO. It is unambiguously demonstrated that room-temperature ferromagnetism is strongly correlated with structural defects, and the carriers involved in carrier-mediated exchange are by-products of defects created in ZnO. The third part focuses on recent progress in TM-doped ZnO-based spintronics, such as magnetic tunnel junctions and spin field-effect transistors, which provide a route for spin injection from TM-doped ZnO to ZnO. Thus, TM-doped ZnO materials are useful spin sources for spintronics.

Fuzzy Process, Hybrid Process and Uncertain Process

Abstract: This paper flrst reviews difierent types of uncertainty. In order to construct fuzzy counterparts of Brownian motion and stochastic calculus, this paper proposes some basic concepts of fuzzy process, including fuzzy calculus and fuzzy difierential equation. Those new concepts are also extended to hybrid process and uncertain process. A basic stock model is presented, thus opening up a way to fuzzy flnancial mathematics.

The Quantum Spin Hall Effect: Theory and Experiment

Abstract: The search for topologically non-trivial states of matter has become an important goal for condensed matter physics. Recently, a new class of topological insulators has been proposed. These topological insulators have an insulating gap in the bulk, but have topologically protected edge states due to the time reversal symmetry. In two dimensions the helical edge states give rise to the quantum spin Hall (QSH) effect, in the absence of any external magnetic field. Here we review a recent theory which predicts that the QSH state can be realized in HgTe/CdTe semiconductor quantum wells (QWs). By varying the thickness of the QW, the band structure changes from a normal to an “inverted” type at a critical thickness d c . We present an analytical solution of the helical edge states and explicitly demonstrate their topological stability. We also review the recent experimental observation of the QSH state in HgTe/(Hg,Cd)Te QWs. We review both the fabrication of the sample and the experimental setup. For thin QWs w...

Effect of Phase Structure of MnO2 Nanorod Catalyst on the Activity for CO Oxidation

Abstract: The α-, β-, γ-, and δ-MnO2 nanorods were synthesized by the hydrothermal method. Their catalytic properties for CO oxidation were evaluated, and the effects of phase structures on the activities of the MnO2 nanorods were investigated. The activities of the catalysts decreased in the order of α- ≈ δ- > γ- > β-MnO2. The mechanism of CO oxidation over the MnO2 nanorods was suggested as follows. The adsorbed CO was oxidized by the lattice oxygen, and the MnO2 nanorods were partly reduced to Mn2O3 and Mn3O4. Then, Mn2O3 and Mn3O4 were oxidized to MnO2 by gaseous oxygen. CO chemisorption, the Mn−O bond strength of the MnO2, and the transformation of intermediate oxides Mn2O3 and Mn3O4 into MnO2 can significantly influence the activity of the MnO2 nanorods. The activity for CO oxidation was mainly predominated by the crystal phase and channel structure of the MnO2 nanorods.

Tuning Photoelectrochemical Performances of Ag−TiO2 Nanocomposites via Reduction/Oxidation of Ag

Abstract: The effects of chemical states of Ag on the photoelectrochemical (PEC) properties of Ag−TiO2 composites were investigated with Ag(0)−TiO2 and Ag(I)−TiO2 prepared by photoreduction-thermal treatment (PRT) method. The comparison of photoaction spectra of Ag(0)−TiO2 and Ag(I)−TiO2 showed that only the Ag(0) containing samples had notable photocurrent under visible light (in the range of 400−800 nm), which was attributed to the highly dispersed Ag(0), according to the DRS, XRD and XPS measurements. During the photocurrent spectra measurements of Ag(0)−TiO2, it was demonstrated that Ag(0) was photoexcited because of plasma resonance in the visible light region, and charge separation was accomplished by the transport of photoexcited electrons from Ag(0) to the TiO2 conduction band with the simultaneous formation of Ag(I), which could be partially reduced to the initial active Ag(0) state under the following UV light irradiation. Actually, it was the interconversion of Ag(0) and Ag(I) during the alternating irra...

An Exploration of Chinese EFL Learners' Unwillingness to Communicate and Foreign Language Anxiety

Abstract: This article reports the results of a study of the unwillingness to communicate, and anxiety of Chinese learners of English as a foreign language (EFL) in English language classrooms. A 70-item survey of 547 first-year undergraduate non-English majors revealed that (a) Most of the students were willing to participate in interpersonal conversations, but many of them did not like to risk using/speaking English in class; (b) more than one third of the students felt anxious in their English language classrooms, and they feared being negatively evaluated and were apprehensive about public speaking and tests; (c) their unwillingness to communicate and their foreign language anxiety correlated significantly with each other and with their self-rated English proficiency and access to English; and (d) many of the variables of interest were good predictors of the students' unwillingness to communicate and of their foreign language anxiety, which were also powerful predictors for each other.

High-density fermentation of microalga Chlorella protothecoides in bioreactor for microbio-diesel production.

Abstract: Agal-fermentation-based microbio-diesel production was realized through high-cell-density fermentation of Chlorella protothecoides and efficient transesterification process. Cell density achieved was 16.8 g l(-1) in 184 h and 51.2 g l(-1) in 167 h in a 5-l bioreactor by performing preliminary and improved fed-batch culture strategy, respectively. The lipid content was 57.8, 55.2, and 50.3% of cell dry weight from batch, primary, and improved fed-batch culture in 5-l bioreactor. Transesterification was catalyzed by immobilized lipase, and the conversion rate reached up to 98%. The properties of biodiesel from Chlorella were comparable to conventional diesel fuel and comply with US standard for Biodiesel. In a word, the approach including high-density fermentation of Chlorella and enzymatic transesterification process were set up and proved to be a promising alternative for biodiesel production.

Real-time KD-tree construction on graphics hardware

Abstract: We present an algorithm for constructing kd-trees on GPUs. This algorithm achieves real-time performance by exploiting the GPU's streaming architecture at all stages of kd-tree construction. Unlike previous parallel kd-tree algorithms, our method builds tree nodes completely in BFS (breadth-first search) order. We also develop a special strategy for large nodes at upper tree levels so as to further exploit the fine-grained parallelism of GPUs. For these nodes, we parallelize the computation over all geometric primitives instead of nodes at each level. Finally, in order to maintain kd-tree quality, we introduce novel schemes for fast evaluation of node split costs.As far as we know, ours is the first real-time kd-tree algorithm on the GPU. The kd-trees built by our algorithm are of comparable quality as those constructed by off-line CPU algorithms. In terms of speed, our algorithm is significantly faster than well-optimized single-core CPU algorithms and competitive with multi-core CPU algorithms. Our algorithm provides a general way for handling dynamic scenes on the GPU. We demonstrate the potential of our algorithm in applications involving dynamic scenes, including GPU ray tracing, interactive photon mapping, and point cloud modeling.