Zhongfei Mu

Enhanced red emission in ZnMoO4?:?Eu3+ by charge compensation

Eu3+ doped ZnMoO4 with charge compensation were synthesized by solid-state reactions. The phosphors with different charge compensation approaches, and , were studied in detail. All of the phosphors can be effectively excited by 396 and 467 nm light, and exhibit superior red emission around 614 nm to ZnMoO4 : Eu3+ without charge compensation, which implies that efficient charge compensation can promote the luminescence of Eu3+ in ZnMoO4. The influence of sintering temperature on the luminescent properties of phosphors with different charge compensation approaches is also discussed. The result indicates that appropriate charge compensation can not only enhance the relative intensity but can also improve the colour purity of the red phosphor.

Multifunctional near-infrared emitting Cr3+-doped Mg4Ga8Ge2O20 particles with long persistent and photostimulated persistent luminescence, and photochromic properties

Here we report a series of multifunctional Cr3+-doped magnesium gallogermanate particles. Several minutes of ultraviolet (UV) light excitation can lead to strong near-infrared (NIR) long persistent luminescence (LPL) at 600–850 nm with a long lasting time of at least 25 h. After the disappearance of LPL, NIR photostimulated persistent luminescence (PSPL) can be rejuvenated repeatedly several times by an external photostimulus (visible or NIR light). Meanwhile, the material shows photochromic (PC) properties, i.e., the surface color can change reversibly between white/pale green and rosybrown with high fatigue resistance by alternating UV and visible light irradiation. When the temperature is increased to 300 °C, the induced rosybrown surface color can also be bleached. This new multifunctional material has potential applications in a wide range of fields, such as in vivo bio-imaging, optical information write-in and read-out media, erasable optical memory media and optical sensors. The continuous trap distribution and modulation were characterized. The optimum trap depth was determined to be 0.51–0.86 eV. In addition, we gained insight into the mechanism of multifunctional properties and the interconnection between them.

Reversible colorless-cyan photochromism in Eu2+-doped Sr3YNa(PO4)3F powders

Photochromic materials have attracted increasing interest as optical switches and erasable optical memory media. Here, we report on a novel reversible colorless-cyan photochromic powder material Sr3YNa(PO4)3F:Eu2+. The photochromic properties including coloring and bleaching are characterized by reflectance spectra after the powder is irradiated by different wavelengths of light for different irradiation times or processed by thermal treatment. The results show that it can be colored by short wavelength light (200–320 nm) and bleached by longer wavelength light (320–800 nm) or thermal treatment at 240 °C. The optimal doping concentration of Eu2+ is determined to be about 0.5 mol%. It shows high fatigue resistance in photochromism performance. Electrons in 4f ground levels excited by short wavelength irradiation to higher 5d states of Eu2+ and then captured by traps are responsible for the coloring process. Correspondingly, the release of trapped electrons from two kinds of traps with different depths causes the initially fast and later slow bleaching processes. The critical role of charge carrier motions between two different traps is discussed. The colorless-cyan photochromism can be explained semi-quantitatively on the basis of obtained results. A tentative model was proposed to illustrate the PC mechanism.

Larger acoustic band gaps obtained by configurations of rods in two-dimensional phononic crystals

A method is introduced to optimize the acoustic band structures of two-dimensional phononic crystals (PCs). The acoustic band gap (ABG) between any two bands could be maximized by introducing an additional rod at a suitable position in a unit cell. The results also show that the symmetry reduction of the system is more efficient in creating the ABGs except for the first one. This additional rod method will be useful in designing the ABGs of PCs.

Preparation and characterization of a long persistent phosphor Na 2 Ca 3 Si 2 O 8 :Ce 3+

A novel host lattice Na2Ca3Si2O8 was used for synthesizing long persistent phosphors for the first time. A blue-emitting long persistent phosphor was prepared successfully via a traditional high temperature solid-state reaction method. The phase structure was checked by XRD. The photoluminescence and persistent luminescence decay properties of Ce3+-doped samples were studied systematically. The defects acting as traps were investigated by thermoluminescence. It demonstrated that the doping Ce3+ ions into the Na2Ca3Si2O8 host not only largely enriched the intrinsic traps but also introduced abundant new extrinsic traps which play a determining role in the generation of persistent luminescence. The origin of the persistent luminescence was analyzed and a related mechanism was systematically discussed based on a schematic diagram as well.

Luminescent properties of a green long persistent phosphor Li 2 MgGeO 4 :Mn 2+

A novel rare-earth ion free phosphor, Mn2+-activated Li2MgGeO4, was prepared by a high temperature solid-state reaction method. The phase structure was identified by XRD. We reported on its photoluminescence and long persistent luminescence properties. The green emission with a broad band centered at 525 nm corresponds to the 4T1(4G)–6A1(6S) transition of the Mn2+ ions. After irradiation by 254 nm UV light, green long persistent luminescence can be observed and last at least 5 h. Thermoluminescence was studied and the long persistent luminescence mechanism was also discussed in detail.

Effect of alkali metal ions co-doping on the structure and luminescent properties of phosphor Zn3(BO3)2:Eu3+

A series of the Zn3(BO3)2:Eu3+ without or with alkali metal ions doping at a low sintering temperature were synthesized by the solid-state reaction method. The XRD pattern shows that all samples exhibit Zn3(BO3)2 crystalline phase. The samples co-doped with alkali metal ions have better crystallinity compared with the un-compensated ones. The different charge compensation approaches have no influence on the shape and position of the emission and excitation spectra. However, the luminescent intensity of samples has been obviously enhanced with different alkali metal ions co-doping. The introduction of Li+ can increase the red emission of Eu3+ compared with the others. Thus, the volume compensation and the equilibrium of mole number can be taken into consideration by charge compensated (CC) approaches.

Tunable Polarity Behavior and High-Performance Photosensitive Characteristics in Schottky-Barrier Field-Effect Transistors Based on Multilayer WS2

Schottky-barrier field-effect transistors (SBFETs) based on multilayer WS2 with Au as drain/source contacts are fabricated in this paper. Interestingly, the novel polarity behavior of the WS2 SBFETs can be modulated by drain bias, ranging from p-type to ambipolar and finally to n-type conductivity, due to the transition of band structures and Schottky-barrier heights under different drain and gate biases. The electron mobility and the on/off ratio of electron current can reach as high as 23.4 cm2/(V s) and 8.5 × 107, respectively. Moreover, the WS2 SBFET possesses high-performance photosensitive characteristics with response time of 40 ms, photoresponsivity of 12.4 A/W, external quantum efficiency of 2420%, and photodetectivity as high as 9.28 × 1011 cm Hz1/2/W. In conclusion, the excellent performance of the WS2 SBFETs may pave the way for next-generation electronic and photoelectronic devices.

Adjusting the structure and luminescence properties of Sr2−x Ba x MgAl22O36:Eu2+ phosphors by Sr:Ba ratio

Europium ion (Eu2+ )-doped phosphors exhibit adjustable photoluminescence due to the sensitivity of their luminescence to the local environment. It is of great significance to adjust the luminescence of Eu2+ by changing their local environment in the host. In this work, we investigated the effect of strontium/barium (Sr:Ba) ratio on the structure and luminescence properties of Sr2-x Bax MgAl22 O36 :Eu2+ phosphors. Our investigation indicates that with the decrease of Sr:Ba ratio, the matrix lattice gradually expands and the peak wavelength for the luminescence of Eu2+ presents an obvious blue shift. The occupancy of Eu2+ was analyzed and the reason for the blue shift was explained. Thermal stability for the luminescence of Eu2+ can also be adjusted by changing the Sr:Ba ratio. This work has a positive effect on the regulation of the emission of phosphors and the improvement of thermal stability, which will promote the application of Sr2-x Bax MgAl22 O36 :Eu2+ phosphors in the field of white light emitting diodes.

Reversible white-brown photochromism in a self-activated long-persistent phosphor Mg 2 SnO 4

Recently, considerable attention has been paid to photochromic (PC) materials owing to its great potential application in various fields, such as rewritable copy papers, erasable optical memory media, smartwindows, sensors, photoswitches and so on. Up until now, most of PC materials come from organics. However, we report that a self-activated LAG phosphor possesses PC property based on non-doped Mg2SnO4 synthesized via a traditional solid-state reaction method. The photoluminescence and long afterglow (LAG) properties were investigated. Interestingly, the white surface color can be colored into brown by ultraviolet-light (UV) irradiation. After visible light irradiation or heat-treatment, the colored Mg2SnO4 can be bleached into white again. The reversible white-brown PC properties of Mg2SnO4 were characterized by diffuse reflectance spectra. Based on the observed phenomena and the obtained experimental results, a tentative model was constructed to illustrate the LAG and PC mechanism.