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Luo X.,Beijing Normal University | Li J.,Beijing Normal University | Li C.,Beijing Normal University | Heng L.,Hong Kong University of Science and Technology | And 6 more authors.
Advanced Materials | Year: 2011

The thermochromic and mechanochromic fluorescence of diphenyldibenzofulvenes is investigated. Emission is boosted and blue-shifted upon crystallization. Yellow emissive crystals of the material transform to green fluorescent crystals upon heating before melting. Reversible switching of the emission color and efficiency are achieved by repeated amorphization and crystallization of dye molecules by a pure thermal process or grinding-heating cycles. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Chen S.,Hong Kong University of Science and Technology | Chen S.,State Key Laboratory of Molecular Neuroscience | Liu J.,Institute of Molecular Functional Materials | Liu J.,State Key Laboratory of Molecular Neuroscience | And 26 more authors.
Chemical Science | Year: 2012

In this work, a red-emissive zwitterionic hemicyanine dye, named TPE-Cy, containing tetraphenylethene (TPE) and N-alkylated indolium is designed and synthesized. TPE-Cy inherits the aggregation-induced emission (AIE) feature of TPE and displays a large Stokes shift (>185 nm), overcoming the limitations of the concentration-quenching effect and small Stokes shift (from a few to 20 nm) encountered by conventional cyanine dyes. By taking advantage of the photophysical AIE property and chemical reactivity towards OH -/H +, TPE-Cy is able to sense pH in a broad range (the broadest to date) by showing different emission colors and intensities: strong to moderate red emission at pH 5-7, weak to no emission at pH 7-10, and no emission to strong blue emission at pH 10-14. The acid/base-switched red/blue emission transition is reversible and can be repeated for many cycles. By means of NMR and HRMS analyses, we have drawn a mechanistic picture at molecular level to illustrate how this dye works as a pH-sensitive fluorescent probe. © 2012 The Royal Society of Chemistry.

Liu J.,Hong Kong University of Science and Technology | Liu J.,State Key Laboratory of Molecular Neuroscience HKUST | Liu J.,Fok Ying Tung Research Institute | Su H.,HKUST | And 18 more authors.
Chemical Science | Year: 2012

In this contribution, we conceptually present a new avenue to construction of molecular functional materials with high performance of circularly polarised luminescence (CPL) in the condensed phase. A molecule (1) containing luminogenic silole and chiral sugar moieties was synthesized and thoroughly characterized. In a solution of 1, no circular dichroism (CD) and fluorescence emission are observed, but upon molecular aggregation, both the CD and fluorescence are simultaneously turned on, showing aggregation-induced CD (AICD) and emission (AIE) effects. The AICD effect is supported by the fact that the molecules readily assemble into right-handed helical nanoribbons and superhelical ropes when aggregated. The AIE effect boosts the fluorescence quantum efficiency (Φ F) by 136 fold (Φ F, ∼0.6% in the solution versus ∼81.3% in the solid state), which surmounts the serious limitations of aggregation-caused quenching effect encountered by conventional luminescent materials. Time-resolved fluorescence study and theoretical calculation from first principles conclude that restriction of the low-frequency intramolecular motions is responsible for the AIE effect. The helical assemblies of 1 prefer to emit right-handed circularly polarised light and display large CPL dissymmetry factors (g em), whose absolute values are in the range of 0.08-0.32 and are two orders of magnitude higher than those of commonly reported organic materials. We demonstrate for the first time the use of a Teflon-based microfluidic technique for fabrication of the fluorescent pattern. This shows the highest g em of -0.32 possibly due to the enhanced assembling order in the confined microchannel environment. The CPL performance was preserved after more than half year storage under ambient conditions, revealing the excellent spectral stability. Computational simulation was performed to interpret how the molecules in the aggregates interact with each other at the molecular level. Our designed molecule represents the desired molecular functional material for generating efficient CPL in the solid state, and the current study shows the best results among the reported organic conjugated molecular systems in terms of emission efficiency, dissymmetry factor, and spectral stability. © 2012 The Royal Society of Chemistry.

Yuan W.Z.,Hong Kong University of Science and Technology | Yuan W.Z.,Fok Ying Tung Research Institute | Lu P.,Hong Kong University of Science and Technology | Lu P.,Fok Ying Tung Research Institute | And 11 more authors.
Advanced Materials | Year: 2010

(Figure Presented) Efficient solid-state emitters developed by a new approach are described. While emission from triphenylamine (TPA) and its dimer (DTPA) is weakened by aggregation, attaching tetraphenylethene (TPE) units to the amines boosts their emission efficiencies up to 100% in the aggregate state. without harming their hole-transport properties. The resultant 3TPETPA and 4TPEDTPA luminogens show excellent electroluminescence performance. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA.

Chen Y.,Hong Kong University of Science and Technology | Zhang J.,Hong Kong University of Science and Technology | Wu K.,Fok Ying Tung Research Institute | Wu K.,National Sun Yat - sen University | Zhang Q.,Hong Kong University of Science and Technology
IEEE Transactions on Parallel and Distributed Systems | Year: 2014

To accommodate the soaring mobile broadband traffic, the Federal Communications Commission (FCC) in the U.S. sets out to retrieve under-utilized spectrum (e.g., TV Whitespace) and lay the groundwork for spectrum redistribution. Auction is an efficient way to allocate resources to those who value them the most. The large pool of spectrums to be released, especially the ones in TV Whitespace, consist of wide-range frequencies. Apart from spatial reuse, spectrum heterogeneity imposes new challenges for spectrum auction design: 1) Wireless service providers with different targeted cell coverages have different spectrum frequency preferences; 2) interference relationship is frequency-dependent due to frequency-selective signal fading. Unfortunately, existing spectrum auction mechanisms either assume spectrum valuation is homogeneous or use homogeneous interference graph to group buyers who can reuse the same spectrum. In this paper, we propose TAMES, an auction framework for heterogeneous spectrum transaction. We consider a multi-seller-multi-buyer double auction, in which every buyer submits a bid, consisting of the spectrum demand and a bidding profile of prices for spectrums contributed by all sellers. A novel buyer grouping approach is proposed to tackle the problem of heterogeneous interference graph. TAMES is proved to be truthful as well as individually rational. The simulation results show that TAMES significantly improves spectrum utilization, sellers' revenue and buyers' utility by making smart use of spectrum heterogeneity, while keeping low running time comparable with existing auction mechanisms. Moreover, via simulation, we show how to help buyers obtain continuous spectrums which further improves buyers' satisfaction. © 2014 IEEE.

Li H.,Hong Kong University of Science and Technology | Wu K.,Shenzhen University | Wu K.,Fok Ying Tung Research Institute | Zhang Q.,Hong Kong University of Science and Technology | Ni L.M.,Hong Kong University of Science and Technology
IEEE Transactions on Parallel and Distributed Systems | Year: 2014

Improving channel utilization is a well-known issue in wireless networks. In traditional point-to-point wireless communication, significant efforts had been made by the existing studies on enhancing the utilization of the channel access time. However, in the emerging wireless network using MU-MIMO, considering only the time domain in channel utilization is not sufficient. As multiple transmitters are allowed to transmit packets simultaneously to the same AP, allowing more antennas at AP would lead to higher channel utilization. Thus, the channel utilization in MU-MIMO should consider both time and spatial domains, i.e., the channel access time and the antenna usage, which have not been considered in the existing methods. In this paper, we point out that the fundamental problem is lacking of the antenna information of contention nodes in channel contention. To address this issue, we propose a new MAC-PHY architecture design, CUTS, to allow distributed nodes effectively contend for the channel and utilize the channel in both domains. Particularly, CUTS adopts interference nulling to attach the antenna information in channel contention. Meanwhile, techniques such as channel contention in frequency domain and ACK in frequency domain using self-jamming are adopted. Through the software defined radio-based real experiments and extensive simulations, we demonstrate the feasibility of our design and illustrate that CUTS provides better channel utilization with the gain over IEEE 802.11 reaching up to 470 percent. © 1990-2012 IEEE.

Qian Y.,Hong Kong University of Science and Technology | Deng Y.,South China University of Technology | Deng Y.,Fok Ying Tung Research Institute | Shi Z.,Hong Kong University of Science and Technology | And 8 more authors.
Electrochemistry Communications | Year: 2013

Spinel LiMn1.5Ni0.5O4 (LMNO) sub-micrometer-sized spheres were prepared in one step using porous MnCO 3 microspheres as a self-supporting template. The LMNO spheres were characterized by XRD, SEM and TEM. The obtained LMNO was in a well-crystallized cubic phase with little impurity. It showed high rate capability and excellent cycling stability as a cathode material for lithium ion batteries. Initial discharge capacities of 122.0, 120.3, 119.5 and 112.1 mAh g- 1 were found at 2, 3, 5 and 10C respectively with 1C being 146.7 mA g- 1. Their capacity retentions are 92, 91, 82 and 79% respectively after 500 cycles. The as-prepared LMNO is a promising cathode material for high power and long-life lithium ion batteries. © 2012 Elsevier B.V. All rights reserved.

Guo X.,HKUST | Guo X.,Singapore Management University | Zhang D.,Shenzhen University | Wu K.,Shenzhen University | And 2 more authors.
IEEE Transactions on Parallel and Distributed Systems | Year: 2014

Radio frequency (RF) based technologies play an important role in indoor localization, since Radio Signal Strength (RSS) can be easily measured by various wireless devices without additional cost. Among these, radio map based technologies (also referred as fingerprinting technologies) are attractive due to high accuracy and easy deployment. However, these technologies have not been extensively applied on real environment for two fatal limitations. First, it is hard to localize multiple objects. When the number of target objects is unknown, constructing a radio map of multiple objects is almost impossible. Second, environment changes will generate different multipath signals and severely disturb the RSS measurement, making laborious retraining inevitable. Motivated by these, in this paper, we propose a novel approach, called Line-of-sight radio map matching, which only reserves the LOS signal among nodes. It leverages frequency diversity to eliminate the multipath behavior, making RSS more reliable than before. We implement our system MODLoc based on TelosB sensor nodes and commercial 802.11 NICs with Channel State Information (CSI) as well. Through extensive experiments, it shows that the accuracy does not decrease when localizing multiple targets in a dynamic environment. Our work outperforms the traditional methods by about 60 percent. More importantly, no calibration is required in such environment. Furthermore, our approach presents attractive flexibility, making it more appropriate for general RF-based localization studies than just the radio map based localization. © 2014 IEEE.

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