J. Y. Dai

The Hong Kong Polytechnic University, Hong Kong, Hong Kong

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Publications (126)307.08 Total impact

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    ABSTRACT: The microscopic mechanisms for higher thermoelectric performance of cost competitive rock salt compound Bi2SeS2 were investigated. A low doping of Cu as an n-type dopant was conducted in order to optimize the band structure and improve the electrical conductivity. It was revealed that this compound exhibits a Seebeck coefficient higher than 300 μV K−1, which sustains above 100 μV K−1 even with Cu doping, leading to a higher power factor. The microstructural characterizations revealed nano-scale Bi2S3 precipitates in the CuxBi2SeS2 matrix, beneficial to the lower lattice thermal conductivity that is insensitive to the Cu doping. A thermoelectric figure-of-merit factor ZT of ~0.7 at 450 °C in accompanying with the power factor of ∼5.36 μW cm−1 K−2 was obtained under the optimized doping level, enabling this environmentally friendly compound interesting for thermoelectric power generation applications.
    Nano Energy 01/2015; · 10.21 Impact Factor
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    ABSTRACT: High-frequency focused intravascular ultrasonic probes were fabricated in this study using dimple technique based on PMN-PT single crystal and lead-free KNN-KBT-Mn ceramic. The center frequency, bandwidth, and insertion loss of the PMN-PT transducer were 34MHz, 75%, and 22.9dB, respectively. For the lead-free probe, the center frequency, bandwidth, and insertion loss were found to be 40MHz, 72%, and 28.8dB, respectively. The ultrasonic images of wire phantom and vessels with good resolution were obtained to evaluate the transducer performance. The -6dB axial and lateral resolutions of the PMN-PT probe were determined to be 58μm and 131μm, respectively. For the lead-free probe, the axial and lateral resolutions were found to be 44μm and 125μm, respectively. These results suggest that the mechanical dimpling technique has good potential in preparing focused transducers for intravascular ultrasound applications.
    Ultrasonics 07/2014; · 1.81 Impact Factor
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    ABSTRACT: A nano-floating gate memory capacitor consisting of a stack of 3 nm-thick HfAlOx tunneling layer, self-organized Ag nanocrystals (NCs), and a 6 nm-thick HfAlOx control layer, has been fabricated on compressively strained p-type Si83Ge17/Si(100) substrates by radio-frequency magnetron sputtering. The Ag-NCs with a size of 5-8 nm and a density of 5.7 x 10(12)/cm(2) are well dispersed in the amorphous HfAlOx matrix. Counter clockwise hysteresis capacitance-voltage curve with a memory window of similar to 2 V, corresponding to a charge storage density of about 1.3 x 10(13) electrons/cm(2), is observed in this memory capacitor. The accumulation capacitance of this memory capacitor has no obvious decrease during electrical stressing process within a period of 10(4) s, but the memory window gradually becomes narrower, and only 54% stored charges are retained in the Ag-NCs after 10(5) s stressing. Defect-enhanced Poole-Frenkel tunneling is found to be responsible for the degradation of memory properties.
    Thin Solid Films 07/2014; 564:674-679. · 1.87 Impact Factor
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    ABSTRACT: Epitaxial (Ba1-xSrx)(Zr0.1Ti0.9)O3 (BSZT, x = 0 – 0.45) thin films were deposited on (LaAlO3)0.3(Sr2AlTaO6)0.35 (LSAT) substrates by pulsed laser deposition. The experimental results demonstrate that the structural, dielectric, and ferroelectric properties of the BSZT thin films were greatly dependent on the strontium content. The BSZT thin films transformed from tetragonal to cubic phase when x ≥ 0.35 at room temperature. The Curie temperature and room-temperature remnant polarization decrease with increasing strontium concentration. The optimal dielectric properties were found in (Ba0.55Sr0.45)(Zr0.1Ti0.9)O3 thin films which is in paraelectric state, having tunability of 47% and loss tangent of 0.0338 under an electric field of 20 MV/m at 1 MHz. This suggests that BSZT thin film is a promising candidate for tunable microwave device applications.
    Journal of Applied Physics 06/2014; 115(23):234102-234102-7. · 2.19 Impact Factor
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    ABSTRACT: (Ba1−x,Srx)(Zr0.1,Ti0.9)O3 (BSZT) ceramics with x = 0, 0.05, 0.15, 0.25, 0.35 and 0.45 were prepared by conventional solid state reaction method. The structural characterization with X-ray diffraction and scanning electron microscopy indicate a monotonical drop in lattice constants and grain size with the increase of Sr concentration. Consequently, the Curie temperature and remnant polarization of the ceramics exhibit a strong compositional dependence. A linear relationship between the Curie temperature and Sr concentration is revealed. At x = 0.45, the BSZT ceramics show substantially high tunability of over 55 % under 20 kV/cm dc electric field with very low dielectric loss value of 0.0025 at room temperature, suggesting the BSZT ceramics could be a promising alternative to traditional (Ba,Sr) TiO3 ferroelectrics for developing high frequency tunable dielectric devices.
    Journal of Materials Science Materials in Electronics 06/2014; 25(6). · 1.97 Impact Factor
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    ABSTRACT: The relationship between the bipolar resistive switching and the polarization reversal is investigated at various temperatures in the Au/BiFeO3/SrRuO3 structure. It is found that the polarization-induced barrier variation in the Au/BiFeO3 and BiFeO3/SrRuO3 junctions decreases with decreasing temperature. This explains why the resistance-switching ratio decreases with decreasing temperature below 323 K and gives evidence that the polarization modulates the resistance state of the Au/BiFeO3/SrRuO3 structure. Besides, the oxygen vacancies migration and/or the carrier trapping/detrapping mechanisms are also suggested to play a very important role in the resistive switching behavior in this structure as the temperature goes above 323 K.
    Applied Physics Letters 04/2014; 104(14):143503-143503-5. · 3.52 Impact Factor
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    ABSTRACT: A systematic study on laser micro-machining of PMN-PT single crystals has been performed. The PMN-PT single crystal has been micromachined using a 355 nm Nd:YAG laser. The kerf profile and cutting quality of the laser micro-machined PMN-PT single crystal were studied as functions of the laser process parameters. It was found that the direct proportional relationship between the depth of the kerf and the laser power provides an effective reference to determine the depth required to be ablated. It is significant for fabricating micro-devices using the single crystal by laser micro-machining. Besides the physical behavior, the piezoresponse of the PMN-PT single crystal has also been investigated by PFM. The results reveal that the laser micro-machining would not affect the macroscopic piezoelectric performance of the PMN-PT single crystal. The present work shows the feasibility of micro-patterning the PMN-PT single crystal into any geometry using a Nd:YAG laser.
    Materials Research Bulletin 09/2013; 48(9):3420–3423. · 1.97 Impact Factor
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    ABSTRACT: Tuning transport properties of two-dimensional electron gas (2DEG) at the LaAlO3/SrTiO3 interface by surface modification is of great interest due to its rich physics and potential application in polar liquid or gas sensors. From first-principle calculations, we found that the adsorption of a thin layer of polar water molecules on the surface of LaAlO3 can remarkably enhance the carrier density of the interfacial 2DEG by over at least 50% which is qualitatively consistent with reported experimental result. This result sheds light on the charge coupling between surface polar molecules and interfacial 2DEG and suggests that sensor devices may be built using the hetero-oxide interfacial 2DEG.
    Solid State Communications 09/2013; · 1.70 Impact Factor
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    Journal of Applied Physics 07/2013; · 2.19 Impact Factor
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    ABSTRACT: Ag nanoparticle (NP) embedded BaTiO3 (BTO) thin films on SrRuO3-coated SrTiO3 (STO) substrates are prepared by the integrated nanocluster beam deposition and laser-molecular beam epitaxy. Enhanced resistive switching, up to an ON/OFF ration of 104, has been achieved at low switching voltage (less than 1 V) without a forming voltage. These characteristics make such nanocomposite film very promising for application of low voltage non-volatile random access memory. The enhanced resistive switching effect may be attributed to the charge storage effect of the Ag nanoparticles and easy formation of Ag filament inside the BTO film.
    Journal of Applied Physics 07/2013; 114(2). · 2.19 Impact Factor
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    ABSTRACT: The electrical properties and Raman spectroscopy of [001]-oriented pure lead-free Na0.5Bi0.5TiO3 (NBT) and manganese-doped NBT single crystals have been investigated. With the effect of Mn-doping, the remnant polarization P r has a great enhancement from 15.2 μC/cm2 to 30.0 μC/cm2, and the pyroelectric coefficient is improved significantly from 178 μC/m2K to 241 μC/m2K. The Mn-doping has further altered the domain structure of the crystals, as revealed by piezoresponse force microscopy study. The Raman studies suggest that the substitution effect of Mn ions on B-site cations of the perovskite structure results in enhancement of the performance of the Mn-doped NBT single crystals.
    Integrated Ferroelectrics 01/2013; 141(1). · 0.37 Impact Factor
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    ABSTRACT: Lead magnesium niobate–lead titanate (PMN–PT) single crystal has ultra-high piezoelectric properties compared with conventional piezoelectric materials and is suitable for high-performance ultrasonic transducer application. In this paper, an improved process has been used for fabricating an annular array ultrasonic transducer using PMN-PT single crystal. A 35 MHz annular array with 8 elements has been built. The bandwidth of the array elements were around 45%. At the center frequency, the insertion loss ranged from −21dB to −28dB. The electrical cross-talk between the adjacent elements was in a range of −24dB to −31dB.
    Integrated Ferroelectrics 12/2012; 139(1). · 0.37 Impact Factor
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    ABSTRACT: A novel type of half-concaved LiNbO(3) plate with domain inversion layer has been proposed for fabricating high-frequency broadband ultrasonic transducers. Two opposite ferroelectric polarization layers with a curved boundary are presented after heat treatment in the half-concaved LiNbO(3) plate with a total thickness of 110 μm. Characterization of the transducers illustrates that, without a matching layer, the self-focusing transducer with the half-concaved LiNbO(3) plate has achieved 123% bandwidth at the center frequency of 60 MHz, which is a significant improvement over the planar inversion layer transducer.
    The Review of scientific instruments 11/2012; 83(11):114903. · 1.58 Impact Factor
  • X. S. Gao, J. M. Liu, K. Au, J. Y. Dai
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    ABSTRACT: In this work, Ag nanoisland electrodes (nanoelectrodes) have been deposited on top of ultrathin ferroelectric BaTiO3 (BTO) films to form a nanoscale metal-ferroelectric-metal tunnel junction by integrating growth techniques of nanocluster beam source and laser-molecular beam epitaxy. The ultrathin BTO films (∼3 nm thick) exhibit both apparent ferroelectric polarization reversal and ferroelectric tunneling related resistive switching behaviors. The introducing of Ag nanoislands (∼20 nm in diameter) as top electrode substantially enhances the tunneling current and alters the symmetry of I-V hysteresis curves. The enhanced tunneling current is likely due to the reduction in tunneling barrier height and an increase in effective tunneling area by Ag nano-electrodes, while the improved symmetric in I-V curve may be attributed to the variation of electrode-oxide contact geometry.
    Applied Physics Letters 10/2012; 101(14). · 3.52 Impact Factor
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    ABSTRACT: A high-frequency (∼60 MHz) ultrasonic transducer with a [001]-oriented 0.27Pb(In1/2Nb1/2)O3–0.45Pb(Mg1/3Nb2/3)O3–0.28PbTiO3 (PIN–PMN–PT) piezoelectric single crystal as an active element has been fabricated and characterized. The poled PIN–PMN–PT single crystal has thickness mode electromechanical coefficient k t of 0.56 and piezoelectric constant d 33 of 1550 pC/N. The −6 dB bandwidth of the transducer is 73 % and the insertion loss at its centre frequency is −20 dB. With the study as a function of temperature, the PIN–PMN–PT transducer shows better thermal stability than the binary single crystal transducer. Furthermore, the transducer was evaluated using a 30-μm aluminum wire phantom image, in which the −6 dB axial and lateral resolutions are found to be 26 μm and 127 μm, respectively. Ultrasonic images of fish eyes were obtained with the 60-MHz transducer. It is shown that the high-sensitivity transducer can produce the images with high signal-to-noise ratio and good contrast.
    Applied Physics A 09/2012; 108(4). · 1.69 Impact Factor
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    ABSTRACT: C-axis orientated LuFe2O4 thin films on (001) sapphire substrates are epitaxially deposited by pulsed-laser deposition. Temperature-dependent resistance characterization reveals the ferrimagnetic transition at 237 K and charge-ordering transition at 340 K in the film. Importantly, the dielectric constant of the film can be significantly changed by both electric and magnetic fields. The dielectric tunability reaches 35% when an electric field of 5 V is applied, while this value reduces to 20% and 15%, respectively, when a magnetic field of 0.83 T is applied perpendicular and parallel to the film normal direction. This suggests a magnetically controlled dielectric tunability and strong magnetoelectric coupling, and is therefore promising for tunable device applications in film form.
    Thin Solid Films 08/2012; 520(20):6446–6449. · 1.87 Impact Factor
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    ABSTRACT: High frequency (∼30MHz and ∼80MHz) focusing ultrasound transducers were fabricated using a PMN-0.28PT single crystal by a mechanical dimpling technique. The dimpled single crystal was used as an active element for the focusing transducer. Compared with a plane transducer, the focusing transducer fabricated with a dimpled active element exhibits much broader bandwidth and higher sensitivity. Besides, a high quality image can be obtained by the 30MHz focusing transducer, in which the -6dB axial and lateral resolution is 27μm and 139μm, respectively. These results prove that the dimpling technique is capable to fabricate the high frequency focusing transducers with excellent performance for imaging applications.
    Ultrasonics 07/2012; · 2.03 Impact Factor
  • K Au, D F Li, N Y Chan, J Y Dai
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    ABSTRACT: Application of LaAlO(3) /SrTiO(3) interfacial two-dimensional electron gas (2DEG) as a polar molecule sensor is demonstrated. The sensing mechanism is due to the formation of a metal-semiconductor Schottky junction when the center of LaAlO(3) surface is covered with polar molecules such as water. These results also evidence the existence of a polarization field inside the LaAlO(3) layer.
    Advanced Materials 04/2012; 24(19):2598-602. · 15.41 Impact Factor
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    ABSTRACT: Ferroelectric and piezoelectric properties of layered Lu2Fe2.1Mn0.9O7 ceramic have been investigated by temperature-dependent piezoresponse force microcopy (PFM) and dielectric measurements. The PFM results illustrate the existence of piezoelectric properties and ferroelectric phase transition temperature of about 90 °C at which the piezoelectricity vanishes. This transition temperature is manifested by the appearance of a shoulder at about 90 °C in the temperature-dependent dielectric curve of the poled sample. A macro-to-micro domain transition is proposed to explain this ferroelectric phase transition. Besides, the ferroelectric properties of the Lu2Fe2.1Mn0.9O7 ceramic are further demonstrated by the polarization-electric field hysteresis (P–E) loop measurement.
    Materials Letters 02/2012; 68:54–56. · 2.27 Impact Factor
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    Integrated Ferroelectrics 01/2012; 134(1):13-15. · 0.37 Impact Factor

Publication Stats

2k Citations
307.08 Total Impact Points


  • 2002–2014
    • The Hong Kong Polytechnic University
      • Department of Applied Physics
      Hong Kong, Hong Kong
  • 2012
    • Tsinghua University
      Peping, Beijing, China
  • 2010
    • Northwest University
      Evanston, Illinois, United States
  • 2008
    • Jadavpur University
      • Department of Electronics and Telecommunication Engineering
      Calcutta, Bengal, India
  • 2007
    • The University of Hong Kong
      Hong Kong, Hong Kong
  • 2006
    • Shanghai Institute of Microsystem And Information Technology
      Shanghai, Shanghai Shi, China
  • 2005
    • The Chinese University of Hong Kong
      • Department of Physics
      Hong Kong, Hong Kong
  • 1996–2003
    • Northwestern University
      • • Department of Materials Science and Engineering
      • • Center for AIDS Research
      Evanston, Illinois, United States