Zheng Hairong

Surface enhanced fluorescence by porous alumina with nanohole arrays

The fluorescence enhancement of Rhodamine 6G (Rh6G) fluorophore in the close vicinity of porous alumina film with ordered nanohole arrays is investigated. Experimental observations show that the nonmetallic substrate with hole arrays enhances the fluorescence intensity. By comparing the fluorescence emissions that are excited with 325 nm and 532 nm, better fluorescence enhancement is obtained with excitation at a shorter wavelength. The study suggests that higher fluorescence excitation efficiency due to the energy transfer from oxygen vacancies to Rh6G fluorophore molecules is responsible for better fluorescence enhancement. The contribution of the scattering of nanohole arrays to the fluorescence enhancement is also proposed based on the intensity increase and reduced lifetime when the energy transfer from oxygen vacancy is absent. The result of the current study is useful for developing non-metal substrates in the study of spectroscopic enhancement, and is expected to advance the applications of porous alumina to microanalysis.

Spectroscopic study of thulium doped transparent glass ceramics

A systematic investigation on fluorescence spectroscopy of trivalent thulium doped in oxyfluoride glass ceramics containing LaF3 nanocrystals has been carried out in a spectral range from 400 to 900 nm under the direct excitation of 1D2 level at a low temperature. Specific optical transitions related to the fluorescence emissions are studied based on experimental measurements in frequency and time domain. Fluorescence emissions from the ions in crystal phase are distinguished from what in glass phase and their spectroscopic properties are explored. The dynamical process shows that the temporal decay of fluorescence emission consists of two parts: a rapid decay from the ions in glass phase and a slower decay from the ions in crystal phase.

Synthesis of Ag-SiO 2 composite nanospheres and their catalytic activity

A simple, mild, and time-saving method is employed to synthesize Ag-SiO2 composite nanospheres with Ag nanoparticles uniformly distributed on the surface of SiO2 nanoparticles. The chemical elements and the morphology of Ag-SiO2 composite nanospheres were analyzed with transmission electron microscopy (TEM), X-ray power diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). On the surface of Ag-SiO2 composite nanospheres, silane coupling agent (KH-550) is introduced as an intermediary to connect the surfaces of SiO2 nanospheres and Ag nanoparticles, which is also helpful for avoiding the aggregation of Ag nanoparticles. It is found that Ag-SiO2 composite nanospheres have very good catalytic properties for the reduction of organic dyes, which may have potential application in wastewater treatment.

Up-conversion photoluminescence emissions of CaMoO 4 :Pr 3+ /Yb 3+ powder

Abstract CaMoO4:Pr3+/Yb3+ powder was successfully synthesized by a facile hydrothermal method. X-ray diffraction (XRD) patterns of samples confirmed tetragonal structure and morphology and sizes were confirmed by scanning electron microscopy (SEM) analyses. Particles consisted of regular micro-spheres with uniform sizes, the diameter of each sphere lay in the range of 3 to 4 μm. The up-conversion photoluminescence emission and its concentration dependence were investigated under infrared excitation at 980 nm. All the UC micro-particles exhibited the typical blue, green and red emissions. Dominant blue emissions originated from 3P0→3H4 and intense red emissions originated from 3P0→3F2 transitions, and they both belonged to two-photon excitation processes in CaMoO4: Yb3+/Pr3+ powder. The optimum doping concentrations of Pr3+ and Yb3+ for the highest UC luminescence were 0.1 mol.% and 16 mol.%, respectively. The possible up-conversion mechanisms were discussed in detail. It was found that the UC emission could be well controlled from blue to green to white color by adjusting the concentration of Pr3+ ions in CaMoO4:Pr3+/Yb3+ microcrystal. So it is a candidate material for solid-state lasers, biological imaging, solar cells, and optical communications.

Spectroscopic exploration of growth mechanism of LaF 3 nanocrystals embedded in glass ceramics under restricted conditions

Eu3+ doped 45SiO2-25Al2O3-18Na2O-9.5LaF3-2.5EuF3 glass was successfully prepared via solid-state reaction, and the corresponding glass ceramics showing an excellent optical property was also synthesized by annealing the prepared glass at 700℃. The glass ceramics was characterized by XRD, TEM and spectroscopic routes. The results indicated that LaF3 nanocrystals with size about 25 nm were embedded uniformly into the annealed glass. The Eu3+ ions in glass phase and crystal phase local environment are excited selectively using the spectroscopic strategy. The difference of emission spectra of Eu3+ in the glass ceramics, nanocrystals and glass matrices was compared in detail. We made up the characteristic spectrum of Eu3+ in glass ceramics using the spectra of Eu3+ in nanocrystals and glass matrices and subsequently inferred the radio of rare-earth ions located in two kinds of local ambiences including glass phase and crystal phase ambience. The effects of annealing temperatures and heavy metallic ions in starting materials on the growth of LaF3 nanocrystals are explored as well.

Influence of Ag film on upconversion luminescence enhancement of a single β-NaYF 4 :Yb 3+ /Er 3+ microcrystal particle

Rare-earth-doped upconversion (UC) is a nonlinear optical phenomenon in which the successive absorption of two or more low-energy photons via intermediate energy states leads to high-energy light emission (anti-Stokes emission). It endows their unique optical properties including low auto-fluorescence, high signal to noise ratio, a large anti-Stokes shift, and low toxicity. Therefore, rare-earth-doped UC materials have been widely used in photovoltaics, color displays, optical storages, biological imaging, and solar cells. However, the UC luminescence efficiency is much lower and largely limited the practical applications. The noble metal nanostructures as promised enhanced substrates could efficiently improve the intensity of UC luminescence and expand its applications. In this work, Yb3+and Er3+ co-doped β-NaYF4 microcrystals are synthesized via a simple and facile hydrothermal method with assistance of sodium citrate to control the crystal growth. Then, a Ag films/β-NaYF4:Yb3+/Er3+ hybrid structure that the β-NaYF4:Yb3+/Er3+ microcrystals were deposited on silver films with different thickness is constructed. The crystal phase and the morphology of the β-NaYF4:Yb3+/Er3+ microcrystals are analyzed with X-ray power diffraction (XRD), scanning electron microscope (SEM), and transmission electron microscopy (TEM). According to the results of XRD, SEM, and TEM, the pure hexagonal phased NaYF4:Yb3+/Er3+ are arc-shaped hexagonal plate microcrystals with smooth surface. The average overall dimension of each microcrystal is about 5.0 µm in diameter. The UC luminescence property of Ag film/β-NaYF4:Yb3+/Er3+ hybrid structure is investigated carefully by a confocal setup equipped with an optical microscope. When 980 nm continuous wave (CW) laser is focused on the central of a single microcrystal particle deposited on silver films with different thickness, the intensity of blue (2H9/2→4I15/2), green (4S3/2→4I15/2 and 2H11/2→4I15/2), and red (4F9/2→4I15/2) emission bands increases with increasing the Ag film thickness. The maximum enhancement factor (EF) for each emission band occurs at the Ag film thickness of 100 nm. But we realize that the intensity EF of each emission band is different, the EF for the green emission is greater than the blue and red one. A comprehensive analysis, based on the rate equations and the numerical calculation | E |2, reveals that the optical reflection and local electric field enhancement effect of the Ag film play a role in the UC luminescence enhancement for the single β-NaYF4:Yb3+/Er3+ microcrystal. This finding breaks the limitation of conventional core-shell system, and it also provides a unique platform for micro-optoelectronic devices, solar energy conversion, optical sensors, and biological applications.

Extraordinary optical transmission properties of multilayer metallic slit arrays

In this paper, multilayer metallic slit arrays structure is proposed and its transmission properties are investigated using the finite element method. Results show that, compared with monolayer metallic slit arrays structure, multilayer metallic slit arrays can achieve enhanced electric fields in the dielectric layer and there are more transmission peaks in the spectra. In addition, the effects of the thickness and the position of the dielectric layer and the period of the arrays on the transmission property are also studied. These results would be helpful for tuning transmission peak and understanding the mechanism of EOT phenomena of multilayer metallic slits.

Spectroscopic study of local thermal effect in transparent glass ceramics containing nanoparticles

Local thermal effect influencing the fluorescence of triply ionized rare earth ions doped in nanocrystals is studied with laser spectroscopy and theory of thermal transportation for transparent oxyfluoride glass ceramics containing nanocrystals. The result shows that the local temperature of the nanocrystals embedded in glass matrices is much higher than the environmental temperature of the sample. It is suggested that the temperature-dependent thermal energy induced by the light absorption must be considered when the theory of thermal transportation is applied to the study of local thermal effect.