Helen L. W. Chan

Efficient and stable perovskite solar cells prepared in ambient air irrespective of the humidity

Poor stability of organic–inorganic halide perovskite materials in humid condition has hindered the success of perovskite solar cells in real applications since controlled atmosphere is required for device fabrication and operation, and there is a lack of effective solutions to this problem until now. Here we report the use of lead (II) thiocyanate (Pb(SCN)2) precursor in preparing perovskite solar cells in ambient air. High-quality CH3NH3PbI3−x(SCN)x perovskite films can be readily prepared even when the relative humidity exceeds 70%. Under optimized processing conditions, we obtain devices with an average power conversion efficiency of 13.49% and the maximum efficiency over 15%. In comparison with typical CH3NH3PbI3-based devices, these solar cells without encapsulation show greatly improved stability in humid air, which is attributed to the incorporation of thiocyanate ions in the crystal lattice. The findings pave a way for realizing efficient and stable perovskite solar cells in ambient atmosphere.

Down- and up-conversion photoluminescence, cathodoluminescence and paramagnetic properties of NaGdF4 : Yb3+,Er3+ submicron disks assembled from primary nanocrystals

NaGdF4 : Yb3+,Er3+ submicron disks have been synthesized at a relatively low temperature in aqueous solution with citric acid as the structure-directing agent. The structure, luminescence, and magnetic properties of the synthesized materials have been characterized by a variety of techniques. The as-prepared NaGdF4 : Yb3+,Er3+ has hexagonal structure, and is mainly composed of submicron disks with a diameter of around 870 nm and a thickness of about 420 nm. Citrate groups selectively bonded to a certain crystal surface of the nanocrystals probably provide the driving force that makes primary particles assemble into submicron disks. The phase structure of these submicron disks is affected by the annealing temperature, while the disk morphology could be essentially preserved even when annealed at high temperatures. The annealed submicron disks exhibit prominent visible emission with different excitation sources, including ultraviolet light, low-voltage electron beam and near-infrared laser. In addition, these disks exhibit paramagnetic features with the mass magnetic susceptibility value of 9.82 × 10−5 emu/g·Oe at room temperature. These multifunctional disks would have potential in applications as building blocks for many functional devices such as solid-state lasers, lighting and displays, magnetic resonance imaging and so on.

The Application of Organic Electrochemical Transistors in Cell‐Based Biosensors

www.MaterialsViews.com C O M M The Application of Organic Electrochemical Transistors in Cell-Based Biosensors U N IC A By Peng Lin , Feng Yan , * Jinjiang Yu , Helen L. W. Chan , and Mo Yang IO N In the last decade, cell-based biosensors are the most studied novel technique for cell monitoring due to their simplicity, high sensitivity, and low cost. [ 1 ] The application of fi eld effect transistors on cell-based sensors has gained considerable interests in the past few years since the transistors are highly sensitive to electric signal and suitable for miniaturization and multiplexing. [ 2–8 ] Siliconbased transistors have already been used for in vitro measurements of various types of cells. [ 2–5 ] However, silicon-based transistors require access to advanced cleanroom facilities and thus are not suitable for low cost and disposable sensors. Organic thin-fi lm transistors (OTFTs) have been extensively studied for various applications, including fl exible display, organic circuits, memories, chemical sensors, and biosensors. [ 9–12 ] However, OTFTs have not been used in cell-based biosensors until now although OTFTs show many advantages over silicon-based transistors, such as fl exibility, low cost, light weight, easy fabrication, biocompatibility, and environmental friendliness. Therefore, it will be of great interest to develop OTFTs to be disposable cell-based biosensors for in vitro or in vivo measurements. [ 13 ]

Double hysteresis loop in Cu-doped K0.5Na0.5NbO3 lead-free piezoelectric ceramics

In this letter the authors report the observation of double hysteresis loops in Cu-doped K0.5Na0.5NbO3 (KNN) ceramics. Unlike other ferroelectric titanates (e.g., BaTiO3), aging is not required for the ceramic to exhibit the double-loop-like characteristics. Based on the symmetry-conforming principle of point defects, it is suggested that defect dipoles are formed by the acceptor dopant ions-Cu2+ and O2− vacancies along the polarization direction after the diffuse tetragonal-orthorhombic phase transition of the ceramic. Because of the low migration rates of defects, the defect dipoles remain in the original orientation during the P-E loop measurement, providing a restoring force to reverse the switched polarization. The defect dipoles also provide “pinning” effects in the normal piezoelectric activities. As a result, the ceramic becomes “hardened,” exhibiting an extraordinarily high mechanical quality factor (2500), while the other piezoelectric properties remain reasonably good: electromechanical couplin...

High tunability in compositionally graded epitaxial barium strontium titanate thin films by pulsed-laser deposition

Compositionally graded barium strontium titanate [(BaxSr1−x)TiO3—BST, x=0.75, 0.8, 0.9, and 1.0] thin films are fabricated by pulsed-laser deposition on a LaAlO3 substrate with (La0.7Sr0.3)MnO3 as the bottom electrode. A high dielectric permittivity and temperature characteristic without Curie–Weiss law are obtained. A tunability of over 70% is obtained at frequency of 1 MHz, which is higher than that of single BST layer with the same compositions. All the results indicate that the graded thin films have better electrical properties than a single-layer film.

Magnetic and luminescent properties of multifunctional GdF3:Eu3+ nanoparticles

Multifunctional GdF3:Eu3+ nanoparticles were synthesized using a hydrothermal method. Photoluminescent excitation and emission spectra, and lifetime were measured. The average lifetime of the nanoparticles is about 11 ms. The nanoparticle exhibits paramagnetism at both 293 and 77 K, ascribing to noninteracting localized nature of the magnetic moment in the compound. The magnetic properties of GdF3:Eu3+ is intrinsic to the Gd3+ ions, which is unaffected by the doping concentration of the Eu3+ luminescent centers. A measured magnetization of approximately 2 emu/g is close to reported values of other nanoparticles for bioseparation.

Evaluation of the material parameters of piezoelectric materials by various methods

The elastic, dielectric and piezoelectric constants of four piezoelectric materials, including polyvinylidene fluoride, vinylidene fluoride-trifluoroethylene copolymer, PZT/epoxy 1-3 composite, and lead metaniobate ceramic, have been evaluated from the impedance data using five different methods. A method described in ANSI/IEEE Std. 176-1987, though based on formulae derived for loss-less materials, is found to be applicable to materials with moderate loss. However, for high-loss materials such as polyvinylidene fluoride, the electromechanical coupling constant (/spl kappa//sub t/) obtained by the method of Std. 176 is substantially higher than the actual value. Calculations based on a piezoelectric resonance analysis program (PRAP) combine the best features of two earlier methods. In addition to the impedance at the parallel resonance frequency, impedances at two other frequencies are required for calculation. The PRAP method gives quite accurate material parameters regardless of the magnitude of the loss, but the parameters (including /spl kappa//sub t/) vary by as much as 15% depending on the choice of data. In the nonlinear regression method described in the present work, all the impedance data points around the resonance are least-squares fitted to the theoretical expression for the impedance. Besides the advantage of requiring no arbitrary choice of data, the nonlinear regression method can readily take account of the frequency dependence of the dielectric constant.

Highly sensitive dopamine biosensors based on organic electrochemical transistors

Organic electrochemical transistors (OECTs) based on poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS) with different gate electrodes, including graphite, Au and Pt electrode, etc., have been used as dopamine sensor for the first time. The sensitivity of the OECT to dopamine depends on its gate electrode and operation voltage. We find that the device with a Pt gate electrode characterized at the gate voltage of 0.6 V shows the highest sensitivity. The detection limit of the device to dopamine is lower than 5 nM, which is one order of magnitude better than a conventional electrochemical measurement with the same Pt electrode. It is expected that OECT is a good candidate for low cost and highly sensitive biosensor for the detection of dopamine.

Pyroelectric or piezoelectric compensated ferroelectric composites

The matrix and inclusions of 0–3 composites of lead titanate (PT) in polyvinylidenefluoride trifluoroethylene (P(VDF–TrFE)) have been independently poled by a special poling method. The polarization states of both constituents are investigated by the measurement of the dynamic pyroelectric coefficients of the composites in the temperature range of 20–90 °C, within which the copolymer matrix undergoes a ferroelectric-paraelectric phase change. The pyroelectric coefficients of PT and P(VDF–TrFE) have the same sign, while their piezoelectric coefficients have opposite signs. This allows the preparation of composites with enhanced pyroelectric but reduced piezoelectric activity when the matrix and inclusions are polarized in the same direction, or vice versa if the constituents are oppositely polarized. For a PT volume fraction of 27% it was possible to prepare a pyroelectric composite with vanishing piezoelectric activity or a piezoelectric composite with vanishing pyroelectric activity by poling the matrix ...

Organic phototransistor based on poly(3-hexylthiophene)/TiO2 nanoparticle composite

Light sensitive phototransistor based on the composite of poly(3-hexylthiophene) and TiO2 nanoparticles has been developed. The device shows a quick change in channel current under light exposure, which can be attributed to a positive shift of the threshold voltage, while no change in the field effect mobility and off current can be observed. The shift of the threshold voltage is induced by accumulated electrons trapped by the TiO2 nanoparticles in the channel. The photosensitivity of the device has been found to be dependent on the concentration of TiO2 nanoparticles, the incident wavelength and the voltage between the source and drain.