Chunshan Shi

Color-control of long-lasting phosphorescence (LLP) through rare earth ion-doped cadmium metasilicate phosphors

We report on a novel luminescent phenomenon in one-component cadmium metasilicate phosphors doped with various rare earth ions. After irradiation by a 254 nm ultraviolet lamp, these phosphors emit bright long-lasting phosphorescence (LLP), and their phosphorescence can be seen with the naked eye (0.32 × 10−3 cd m−2) in the darkroom clearly for 2–5 hours even after the irradiation light sources have been removed. Furthermore, color-control of the LLP is also available in these cadmium metasilicate luminescent materials. By selecting appropriate rare earth ions single or double doped into cadmium metasilicate, optional three-dimensional image patterns emitting LLP in various colors, covering most of the visible light region from short wavelength indigo blue to long wavelength deep red, can be formed under the same wavelength ultraviolet light excitation. X-Ray diffraction (XRD), photoluminescence (PL), long-lasting phosphorescence (LLP), and thermostimulated luminescence (TSL) spectra are used to characterize the synthesized phosphors. PL and LLP spectra reveal that there are two different kinds of LLP emissions in RE3+-doped CdSiO3 (the ∼420 nm self-trapped excitation (STE) emission and the characteristic rare earth ions emissions). It is found that these two kinds of emissions are the origin of the multi-colored LLP phenomenon in these phosphors. The finding has enlarged the family of long-lasting phosphors available, and offers a promising approach for controlling the LLP color of other materials.

Synthesis of barium fluoride nanoparticles from microemulsion

The cetyltrimethylammonium bromide (CTAB)/2-octanol/water microemulsion system was used to synthesize barium fluoride nanoparticles. X-ray powder diffraction (XRD) analysis showed that the products were single phase. The results of scanning electron microscopy and calculations using the Scherrer equation from the line widths of the XRD have been used to estimate the average particle sizes of the powder products. The results showed that the nanoparticle size was affected by water content and surfactant (CTAB) concentration. As water content decreases from 14.2 to 9.47% (w/w), the particle size decreases from 75 to 40 nm. In addition, increasing the reaction times from 5 to 120 min increases the particle size from 75 to 150 nm, and increasing the amount of surfactant decreases the size of the particle. Luminescence spectra of the BaF2:Ce nanoparticles are also discussed.

Eu2 + -Doped High-Temperature Phase Ca3SiO4Cl2 A Yellowish Orange Phosphor for White Light-Emitting Diodes

In this paper, we synthesized a novel yellowish orange phosphor, Eu 2 + -doped high-temperature phase (HTP) Ca 3 SiO 4 Cl 2 at 1020°C. The primary crystal structure of HTP-Ca 3 SiO 4 Cl 2 is determined as monoclinic structure with a space group of 14 P21/c. The excitation spectrum of Eu 2 + -doped HTP-Ca 3 SiO 4 Cl 2 exhibits a broad-band between 250 and 500 nm with three peaks of around 327, 371, and 426 nm, which are coupled well with the near-UV (n-UV) or deep blue emission of GaN-based light-emitting diodes (LEDs). Its emission spectrum shows a single intense broad emission band centered at 572 nm. The measured chromaticity of yellowish orange luminescence is x = 0.52, y = 0.45. Eu 2 + -doped HTP-Ca 3 SiO 4 Cl 2 would be suitable for yellowish orange phosphor of white LED because HTP-Ca 3 SiO 4 Cl 2 :Eu 2 + is excellent in the excitation spectral profile and the chromaticity.

Synthesis of the complex fluoride LiBaF3 and optical spectroscopy properties of LiBaF3 : M(M = Eu,Ce) through a solvothermal process

The complex fluoride LiBaF3 and LiBaF3:M(M = Eu, Ce) is solvothermally synthesized at 180 degreesC and characterized by means of X-ray powder diffraction, scanning electron microscopy, thermogravimetric analysis and infrared spectroscopy. In the solvothermal process, the solvents, molar ratios of initial mixtures and reaction temperature play important roles in the formation of products. The excitation and emission spectra of the LiBaF3:M(M= Eu,Ce) have been measured by fluorescence spectrophotometer. In the LiBaF3: Eu emission spectra, there is one sharp line emission located at 360 nm arising from f --> f transition of Eu2+ in the host lattice, and typical doublet 5d-4f emission of Ce3+ in LiBaF3 powder is shown.

Enhanced photoluminescence from porous silicon by hydrogen-plasma etching

Porous silicon (PS) was etched by hydrogen plasma. On the surface a large number of silicon nanocone arrays and nanocrystallites were formed. It is found that the photoluminescence of the H-etched porous silicon is highly enhanced. Correspondingly, three emission centers including red, green, and blue emissions are shown to contribute to the enhanced photoluminescence of the H-etched PS, which originate from the recombination of trapped electrons with free holes due to SiO bonding at the surface of the silicon nanocrystallites, the quantum size confinement effect, and oxygen vacancy in the surface SiO2 layer, respectively. In particular, the increase of SiOx(x<2) formed on the surface of the H-etched porous silicon plays a very important role in enhancing the photoluminescence properties.

Enhanced electron field emission from oriented AlN films

(002) and (100) oriented AlN films were deposited on silicon substrates and tungsten tips by radio frequency magnetron reactive sputtering. The electron field emission (FE) properties of (002) and (100) oriented AlN films were investigated and found to be significantly different in emission threshold and current density. The threshold electric field was only 0.14V∕μm for (002) oriented AlN film—far lower than the threshold of 1.13V∕μm for (100) oriented AlN film on tungsten tips. Maximum FE currents of 183 and 27μA were obtained for (002) and (100) oriented AlN films on tungsten tips, respectively. Comparative analysis showed that the absence of linear relation in Fowler-Nordheim plots could be attributed to the high current density integrated over the emitting areas. The excellent FE property of (002) oriented AlN film can be attributed to its vertically oriented grains and the broad distribution of defect-related subbands within its band gap as analyzed by photoluminescence spectra from (002) and (100) ...

Optical spectroscopy properties of KMgF3 : Eu2+ nanocrystals and powder synthesized by microemulsion and solvothermal process

Nanocrystals and powders of KMgF3 doped with Eu2+ were synthesized by the microemulsion method and the solvothermal process, respectively. The emission and excitation spectra of KMgF3:Eu2+ phosphors were measured and compared with those of the samples synthesized through a solid. state reaction, Bridgman-Stockbarger method, and mild hydrothermal technique. The KMgF3: Eu2+ samples synthesized by means of the microemulsion method and the solvothermal process show only a sharp emission peak located at 360 nm, in the emission spectra, which arises from the f -> f(P-6(1/2)-> S-8(1/2)) transition of Eu2+. The broad emission bands appear at 420 nm,,which arises from Eu2+ <- O2- cannot be observed(in the mild hydrothermal and single crystal samples, the emission peak at 420 nm besides the emission of Eu2+ at 360 nm is observed). In the excitation spectrum of the KMgF3: Eu2+ samples synthesized by the microemulsion method and the solvothermal process, the excitation peaks show an intensive blue shift. The blue shift can he attributed to the lower oxygenic content in the KMgF3: Eu2+ samples synthesized by the microemulsion method and the solvothermal process.

Upconversion of Y 2 O 2 S : Tm , Mg, Ti Phosphor and Its Long-lasting Phosphorescence

We report the upconversion luminescence of Tm 3 + in Y 2 O 2 S:Tm, Mg, Ti phosphor. The upconversion luminescence of Y 2 O 2 S:Tm, Mg, Ti phosphor and its unusually long-lasting phosphorescence properties have been discussed.Two upconversion emission bands located at 466 and 545 nm from Tm 3 + in Y 2 O 2 S:Tm, Mg, Ti phosphor are observed after near infrared light (λ = 798 nm) or red light (λ = 648 nm) excitation. The afterglow light of Y 2 O 2 S:Tm, Mg, Ti phosphor is so strong that it can be seen by the naked eye for above an hour. The possible mechanism of the unusual upconversion long-lasting phosphorescence is also discussed.

Spectroscopic studies on charge transfer from Ce3+, to Yb3+ in KMgF3 : Ce, Yb nanocrystals

Nanocrystals of KMgF3 single-doped and codoped with Ce3+ or/and Yb3+ were synthesized separately by the microemulsion method. The X-ray diffraction(XRD) patterns were indexed to show that the KMgF, crystal system was unchanged. The fluorescent spectra of KMgF3:Ce, Yb polycrystal powders were studied and compared with those of the Ce, Yb doped KMgF3 crystals produced using the high-temperature solid phase method. The diffuse reflection spectra and infrared. emission of KMgF3:Ce, Yb were investigated. From the results, the authors could confirm that there were charge transfer processes from Ce3+ to Yb3+ in both KMgF3: Ce,Yb nanocrystals and polycrystal powders.

Local field emission characteristic of individual AIN cone fabricated by focused ion beam

In summary, an individual AlN cone with high aspect ratio was formed by FIB technique. And a nature field emission property of individual AlN cone was measured in a dual probe SEM system. The results indicated that as formed single AlN cone with high aspect ratio exhibits a good field emission ability without any field shielding effect although only has a tiny emission area. Compared with a single Si cone fabricated by the same method, a single AlN cone has a better electron emission ability and hence a good promising candidate of point electron source candidate for the application of vacuum electronic device field.