Lifan Shen

Dynamic colour and utilizable white fluorescence from Eu/Tb ions codoped lithium-yttrium-aluminium-silicate glasses

A group of dynamic-colour white fluorescences with various colour temperatures that can be applied to circadian lighting are achieved in Eu/Tb-codoped lithium-yttrium-aluminium-silicate (LYAS) glasses, which can be attributed to the simultaneous generation of three primary colours emitting from Eu3+ (red), Eu2+ (blue) and Tb3+ (green) by varying the ultraviolet (UV) radiation wavelength. Fluorescence colour coordinates pass through the whole white region of the CIE x, y chromaticity diagram when the UV excitation wavelength is increased from 300 to 370 nm. A favourable white light with colour coordinates (0.338, 0.298) close to the equal energy white is obtained under 360 nm excitation. These results indicate that the Eu/Tb-codoped LYAS glasses are a promising candidate to develop white lighting devices under the excitation of commercial UV light-emitting diodes, and a smart lighting system based on rare-earth doped glasses will be a potential illumination source offering controllability of the colour temperature that can adjust to specific environments and requirements, and benefit human health, well-being and productivity.

Quantitative characterization on multichannel transition emissions originating from 3P0 and 1D2 levels of Pr3+ in fluorotellurite glasses

Multichannel transition emissions in Pr3+-doped fluorotellurite (4BaF2?4AlF3?16BaO?6La2O3?70TeO2, BABLT) glasses have been quantitatively characterized. Broadband ?1.48??m near-infrared (NIR) emission due to 1D2???1G4 transition covering a 1.28?1.68??m wavelength region possesses a full-width at half-maximum of 120?nm, along with the spontaneous emission probability (Aij) and maximum stimulated emission cross-section (?em) are calculated to be 693?s?1 and 8.97???10?21?cm2, respectively. Intense visible fluorescence originating from 3P0 and 1D2 levels has been observed and the radiant flux for the visible fluorescence of Pr3+ is solved to be 299??W under commercial blue LED excitation. Quantitative characterization reveals that Pr3+-doped BABLT glasses exhibit a quantum yield (QY) as high as 14.9%, which is 3.1% higher than that in Pr3+-doped heavy metal germanium tellurite glasses. High QY and broadband ?1.48??m emission illustrate great advantages of Pr3+-doped BABLT glasses as fibre luminescence sources in photodynamic therapy treatment and gain media for tunable lasers and NIR optical amplifiers.

Polymer-confined colloidal monolayer: a reusable soft photomask for rapid wafer-scale nanopatterning.

We demonstrate the repeated utilization of self-assembled colloidal spheres for rapid nanopattern generations. Highly ordered micro-/nanosphere arrays were interlinked and confined by a soft transparent polymer (polydimethylsiloxane, PDMS), which can be used as light-focusing elements/photomasks for area-selective exposures of photoresist in contact. Because of the stiffness of the colloidal spheres, the photomasks do not encounter feature-deformation problems, enabling reliable production of highly uniform patterns over large areas. The geometrical feature of the patterns, including the size, pitch, and even the shape, can be finely tuned by adjusting the mask design and exposure time. The obtained patterns could be used as deposition or etching mask, allowing easy pattern transfer for various applications.

Broadband fluorescence emission of Eu 3+ doped germanotellurite glasses for fiber-based irradiation light sources

Eu3+ doped fiber-based germanotellurite (NZPGT) glasses with medium-low maximum phonon energy of 782 cm−1 have been fabricated and characterized. Judd-Ofelt intensity parameters Ω2 (6.25 × 10−20 cm2) and Ω4 (1.77 × 10−20 cm2) indicate a high asymmetrical and covalent environment around Eu3+ in the optical glasses. The spontaneous emission probability of the dominant transition 5D0→7F2 peaking at 612.5 nm and the corresponding maximum stimulated emission cross-section were derived to be 445.7 s−1 and 2.05 × 10−21 cm2, respectively, confirming the effectiveness of the red fluorescence emission. The quantum yield was derived to be 12% under 391 nm LED excitation, and approximately 88% photons have been demonstrated in wavelength range of 600−720 nm, indicating that Eu3+ doped NZPGT glasses under proper excitation conditions are promising optical materials for fiber-based irradiation light sources that are competent to activate diverse photodynamic therapy photosensitizers.

Nd 3+ -doped ion-exchanged aluminum germanate glass channel waveguide

K+–Na+ ion-exchanged channel waveguide has been fabricated in Nd3+ doped aluminum germanate (NMAG) glasses with potential photosensitive property. The channel waveguide exhibits single mode at 1.3μm and the mode field diameters were measured to be horizontally 10.1μm and vertically 5.3μm, respectively. Amplified spontaneous emissions (ASE) of 905, 1060, 1334 and 1816nm originating from the 4F3/2 level were recorded under ~800nm diode laser pumping and the maximum stimulated emission cross-sections for the 4F3/2→4I11/2 and 4F3/2→4I13/2 transitions are derived to be 21.5 × 10−21 and 7.6 × 10−21cm2, respectively. In addition, with 71.8% quantum efficiency and the largest emission intensity among various Nd3+ doping cases, 2wt% Nd2O3 is considered as the optimized doping concentration for the compact channel waveguide. The ion-exchanged Nd3+-doped NMAG glass channel waveguides offer favorable prospects for the development of optical waveguide amplifiers, broadband light sources and infrared UV-written grating waveguide lasers.

Recent developments in III–V semiconducting nanowires for high-performance photodetectors

Recently, high-performance III–V semiconductor nanowires (NWs) have been extensively explored as promising active material candidates for high-sensitivity and broad-spectrum photodetectors operating at room temperature, which would have potential application in integrated optoelectronic devices for photovoltaics, radiation imaging, sensing, and information communications and processing. In this review, we present a comprehensive overview of the recent advances in the study of III–V NW-based photodetectors operating in the UV, visible and infrared spectral range, starting from the cost-effective synthesis and assembly of III–V NWs, the precise manipulation of their optoelectronic properties, and all the way to their different types of utilizations in photodetection and solar cells. Specifically, the synthesis methods and growth mechanisms of typical III–V NWs are discussed, emphasizing the low-cost solid-source chemical vapor deposition (CVD) technique, which has been widely employed in the growth of various III–V NW materials due to the setup simplicity as well as the excellent process controllability. The optoelectronic properties of these synthesized NWs, such as light absorption and photoconductive gain, are then investigated accordingly. Effective light absorption and extremely high photoconductive gain have been demonstrated, confirming their suitability as active components for photodetector applications. By utilizing the recently developed contact printing method, high-performance III–V NW solar cells can also be easily integrated on the glass and plastic substrates for transparent and flexible photovoltaics, which explicitly indicates the outstanding versatility and promising perspective of these NW Schottky photovoltaics for next-generation smart solar energy harvesting devices. Towards the end, we also discuss the progress made and the challenges in the research of high-performance photodetectors and photovoltaic devices based on III–V NWs.

Complementary Metal Oxide Semiconductor-Compatible, High-Mobility, ⟨111⟩-Oriented GaSb Nanowires Enabled by Vapor–Solid–Solid Chemical Vapor Deposition

Using CMOS-compatible Pd catalysts, we demonstrated the formation of high-mobility ⟨111⟩-oriented GaSb nanowires (NWs) via vapor-solid-solid (VSS) growth by surfactant-assisted chemical vapor deposition through a complementary experimental and theoretical approach. In contrast to NWs formed by the conventional vapor-liquid-solid (VLS) mechanism, cylindrical-shaped Pd5Ga4 catalytic seeds were present in our Pd-catalyzed VSS-NWs. As solid catalysts, stoichiometric Pd5Ga4 was found to have the lowest crystal surface energy and thus giving rise to a minimal surface diffusion as well as an optimal in-plane interface orientation at the seed/NW interface for efficient epitaxial NW nucleation. These VSS characteristics led to the growth of slender NWs with diameters down to 26.9 ± 3.5 nm. Over 95% high crystalline quality NWs were grown in ⟨111⟩ orientation for a wide diameter range of between 10 and 70 nm. Back-gated field-effect transistors (FETs) fabricated using the Pd-catalyzed GaSb NWs exhibit a superior peak hole mobility of ∼330 cm2 V-1 s-1, close to the mobility limit for a NW channel diameter of ∼30 nm with a free carrier concentration of ∼1018 cm-3. This suggests that the NWs have excellent homogeneity in phase purity, growth orientation, surface morphology and electrical characteristics. Contact printing process was also used to fabricate large-scale assembly of Pd-catalyzed GaSb NW parallel arrays, confirming the potential constructions and applications of these high-performance electronic devices.

Visible photon multiplication in Ce3+–Tb3+ doped borate glasses for enhanced solar cells

Visible photon multiplication is exposed in the Ce3+–Tb3+ doped alkaline-earth borate (LKZBSB) glass system. Efficient green and blue fluorescences originate from Tb3+ and Ce3+ emitting centres, respectively. Evaluation of absolute spectral parameters reveals that the quantum yield of Tb3+ single doped LKZBSB glasses is ~8% under UVA radiation. Furthermore, with the introduction of Ce3+ into the Tb3+ doping system, the effective excitation wavelength range and the emission intensity of Tb3+ in LKZBSB glasses are remarkably expanded and improved by a maximum sensitization factor of ~52 in the UVB spectral region. These results demonstrate that the Ce3+–Tb3+ doped LKZBSB glass system has promising potential as an efficient UV → Visible radiation conversion layer for the enhancement of solar cell efficiency, including cells employed in outer space.

Diameter Dependence of Planar Defects in InP Nanowires

In this work, extensive characterization and complementary theoretical analysis have been carried out on Au-catalyzed InP nanowires in order to understand the planar defect formation as a function of nanowire diameter. From the detailed transmission electron microscopic measurements, the density of stacking faults and twin defects are found to monotonically decrease as the nanowire diameter is decreased to 10 nm, and the chemical analysis clearly indicates the drastic impact of In catalytic supersaturation in Au nanoparticles on the minimized planar defect formation in miniaturized nanowires. Specifically, during the chemical vapor deposition of InP nanowires, a significant amount of planar defects is created when the catalyst seed sizes are increased with the lower degree of In supersaturation as dictated by the Gibbs-Thomson effect, and an insufficient In diffusion (or Au-rich enhancement) would lead to a reduced and non-uniform In precipitation at the NW growing interface. The results presented here provide an insight into the fabrication of “bottom-up” InP NWs with minimized defect concentration which are suitable for various device applications.

High-Performance Wrap-Gated InGaAs Nanowire Field-Effect Transistors with Sputtered Dielectrics.

Although wrap-gated nanowire field-effect-transistors (NWFETs) have been explored as an ideal electronic device geometry for low-power and high-frequency applications, further performance enhancement and practical implementation are still suffering from electron scattering on nanowire surface/interface traps between the nanowire channel and gate dielectric as well as the complicated device fabrication scheme. Here, we report the development of high-performance wrap-gated InGaAs NWFETs using conventional sputtered Al2O3 layers as gate dielectrics, instead of the typically employed atomic layer deposited counterparts. Importantly, the surface chemical passivation of NW channels performed right before the dielectric deposition is found to significantly alleviate plasma induced defect traps on the NW channel. Utilizing this passivation, the wrap-gated device exhibits superior electrical performances: a high ION/IOFF ratio of ~2 × 106, an extremely low sub-threshold slope of 80 mV/decade and a peak field-effect electron mobility of ~1600 cm2/(Vs) at VDS = 0.1 V at room temperature, in which these values are even better than the ones of state-of-the-art NWFETs reported so far. By combining sputtering and pre-deposition chemical passivation to achieve high-quality gate dielectrics for wrap-gated NWFETs, the superior gate coupling and electrical performances have been achieved, confirming the effectiveness of our hybrid approach for future advanced electronic devices.