Yuen Hong Tsang

Stretchable all-solid-state supercapacitor with wavy shaped polyaniline/graphene electrode

A stretchable electronic device can retain its functionalities during high-level mechanical deformation, and stimulates the applications in the field of wearable and bio-implantable electronics. Efficient energy storage devices are an indispensable component in stretchable electronic systems. To integrate power supplies together with electronic devices that are mechanically flexible and stretchable, we demonstrate a new kind of stretchable all-solid-state supercapacitor, which consists of two slightly separated polyaniline/graphene electrodes in a wavy shape, with a phosphoric acid/polyvinyl alcohol gel as the solid-state electrolyte and separator. The as-fabricated wavy shaped supercapacitor was encapsulated in an elastomeric substrate which can be stretched to a large extent without mechanical degradation. The supercapacitor exhibited a maximum specific capacitance of 261 F g−1. Electrochemical cycling testing with the supercapacitor showed 89% capacitance retention over 1000 charge–discharge cycles at a current density of 1 mA cm−2. The bending and stretching tests showed that the supercapacitor maintained high mechanical strength and high capacitance simultaneously, even under a strain of 30%. This stretchable all-solid-state supercapacitor shows great potential as an energy storage device for stretchable electronic systems.

Superbroadband near-IR photoluminescence from Pr3+-doped fluorotellurite glasses.

Praseodymium(Pr3+)-doped fluorotellurite glasses were synthesized and broadband photoluminescence (PL) covering a wavelength range from 1.30 to 1.67 μm was observed under both 488 and 590 nm wavelength excitations. The broadband PL emission is mainly due to the radiative transition from the manifolds Pr3+: 1D2 to 1G4. The PL line-shape, band width, and lifetime were modified by the Pr3+ dopant concentration, and a quantum efficiency as high as 73.7% was achieved with Pr3+ dopant in a low concentration of 0.05 mol%. The good spectroscopic properties were also predicted by the Judd-Ofelt analysis, which indicates a stronger asymmetry and covalent bonding between the Pr3+ sites and the matrix lifgand field. The large stimulated emission cross-section, long measured lifetime, and broad emission bandwidth confirm the potential of the Pr3+-singly doped fluorotellurite glass as broadband luminescence sources for the broadband near-infrared optical amplifications and tunable lasers.

Controllable Growth of Large-Size Crystalline MoS2 and Resist-Free Transfer Assisted with a Cu Thin Film.

Two-dimensional MoS2 is a promising material for future nanoelectronics and optoelectronics. It has remained a great challenge to grow large-size crystalline and high surface coverage monolayer MoS2. In this work, we investigate the controllable growth of monolayer MoS2 evolving from triangular flakes to continuous thin films by optimizing the concentration of gaseous MoS2, which has been shown a both thermodynamic and kinetic growth factor. A single-crystal monolayer MoS2 larger than 300 μm was successfully grown by suppressing the nuclei density and supplying sufficient source. Furthermore, we present a facile process of transferring the centimeter scale MoS2 assisted with a copper thin film. Our results show the absence of observable residues or wrinkles after we transfer MoS2 from the growth substrates onto flat substrates using this technique, which can be further extended to transfer other two-dimensional layered materials.

Efficient 2.96 μm dysprosium-doped fluoride fibre laser pumped with a Nd:YAG laser operating at 1.3 μm

A quasi-continuous wave Dy3+-doped ZBLAN fibre laser pumped by a ~1.3 μm Nd:YAG laser and operating at 2.96 μm with an emission linewidth of ~14 nm (FWHM) has been demonstrated. The 6H15/2 → 6H9/2 , 6F11/2 absorption band of Dy3+-doped ZBLAN centred at 1.3 μm has been used to pump the 6H13/2 → 6H15/2 laser transition. For a 60 cm fibre length, a threshold of 0.5 W and a slope efficiency of ~20% with respect to the absorbed pump power was measured. The experimental slope efficiency was ~45% of the Stokes efficiency limit. The high efficiency relates to low pump ESA losses and an optimised output coupling as compared with previous demonstrations.

High-responsivity UV-Vis Photodetector Based on Transferable WS2 Film Deposited by Magnetron Sputtering.

The two-dimensional layered semiconducting tungsten disulfide (WS2) film exhibits great promising prospects in the photoelectrical applications because of its unique photoelectrical conversion property. Herein, in this paper, we report the simple and scalable fabrication of homogeneous, large-size and transferable WS2 films with tens-of-nanometers thickness through magnetron sputtering and post annealing process. The produced WS2 films with low resistance (4.2 kΩ) are used to fabricate broadband sensitive photodetectors in the ultraviolet to visible region. The photodetectors exhibit excellent photoresponse properties, with a high responsivity of 53.3 A/W and a high detectivity of 1.22 × 1011 Jones at 365 nm. The strategy reported paves new way towards the large scale growth of transferable high quality, uniform WS2 films for various important applications including high performance photodetectors, solar cell, photoelectrochemical cell and so on.

Constructing Interfacial Energy Transfer for Photon Up- and Down-Conversion from Lanthanides in a Core-Shell Nanostructure.

We report a new mechanistic strategy for controlling and modifying the photon emission of lanthanides in a core-shell nanostructure by using interfacial energy transfer. By taking advantage of this mechanism with Gd(3+) as the energy donor, we have realized efficient up- and down-converted emissions from a series of lanthanide emitters (Eu(3+) , Tb(3+) , Dy(3+) , and Sm(3+) ) in these core-shell nanoparticles, which do not need a migratory host sublattice. Moreover, we have demonstrated that the Gd(3+) -mediated interfacial energy transfer, in contrast to energy migration, is the leading process contributing to the photon emission of lanthanide dopants for the NaGdF4 @NaGdF4 core-shell system. Our finding suggests a new direction for research into better control of energy transfer at the nanometer length scale, which would help to stimulate new concepts for designing and improving photon emission of the lanthanide-based luminescent materials.

Core–shell nanoarchitecture: a strategy to significantly enhance white-light upconversion of lanthanide-doped nanoparticles

We report that the upconversion white-light luminescence in ultrasmall CaF2:Yb/Tm/Ho nanoparticles has been enhanced by more than one order of magnitude through growing the Yb3+-containing NaYF4 shell. Such white-light enhancement achieved in the ultrasmall (sub-10 nm) core–shell nanocrystals is attributed to the surface protection and the efficient energy transfer from the active shell which not only protects the lanthanide emitters located in the core especially those near the surface from quenchers at the surface (e.g., defect, organic ligands, etc.) but also absorbs more excitation energy due to the additionally incorporated Yb3+ in the shell. The result suggests an effective approach to significantly improve the white-light emission of lanthanide-doped nanocrystals which show great potential in diverse industrial as well as newly emerging biomedical applications.

Graphene Oxide Absorbers for Watt-Level High-Power Passive Mode-Locked Nd:GdVO

A novel graphene-oxide-based saturable absorber with 12% modulation depth, which is fabricated by the novel vertical evaporation technique, can be used to passively mode locked the Nd:GdVO4 ultrafast laser system. The maximum average output power measured is 1.1 W and it is operating at TEM00 mode. The measured pulse duration and repetition rate are 4.5 ps and 70 MHz, respectively. This low-cost, broadband graphene-oxide-based saturable absorber can potentially be used practically in the high-power mode-locking laser system.

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By use of CW diode laser stacked arrays, side-pumping Q-switched composite ceramic Nd:YAG rod laser based on a type II KTP crystal intracavity frequency-doubled, a high power high stability green laser has been demonstrated. Average output power of 104 W is obtained at a repetition rate of 10.6 kHz with a diode-to-green optical conversion efficiency of 10.9%. For the average output power of about 100 W, the measured pulse width is 132 ns with power fluctuation of less than 0.2%. The experimental results show that the green laser system using this novel ceramic Nd:YAG offers better laser performance and output stability than the traditional single Nd:YAG crystal green laser system with the same operating conditions and experimental configuration.

Laser Operating at 1

We report the first demonstration of superbroadband emission extending from 1.30 to 1.68 μm in praseodymium(Pr(3+))-erbium(Er(3+)) codoped fluorotellurite glasses under 488 nm excitation. This superbroad near-infrared emission is contributed by the Pr(3+): (1)D(2)→(1)G(4) and Er(3+): (4)I(13/2)→(4)I(15/2) transitions which lead to emissions located at 1.48 and 1.53 μm, respectively. The quenching of the Pr(3+) emission resulted from the cross relaxation [(1)D(2), (3)H(4)]→[(1)G(4), (3)F(3,4)] was effectively compensated by the codoping of Er(3+). The results suggest that, other than the heavy-metal and transition-metal elements of active bismuth (Bi), nickel (Ni), chromium (Cr), etc., Pr(3+)-Er(3+) codoped system is a promising alternative for the broadband near-infrared emission covering the expanded low-loss window.