Rucheng Dai

Thickness Dispersion of Surface Plasmon of Ag Nano-thin Films: Determination by Ellipsometry Iterated with Transmittance Method

Effective optical constants of Ag thin films are precisely determined with effective thickness simultaneously by using an ellipsometry iterated with transmittance method. Unlike the bulk optical constants in Paliks database the effective optical constants of ultrathin Ag films are found to strongly depend on the thickness. According to the optical data two branches of thickness dispersion of surface plasmon energy are derived and agreed with theoretical predication. The thickness dispersion of bulk plasmon is also observed. The influence of substrate on surface plasmon is verified for the first time by using ellipsometry. The thickness dependent effective energy loss function is thus obtained based on this optical method for Ag ultrathin films. This method is also applicable to other ultrathin films and can be used to establish an effective optical database for ultrathin films.

Raman scattering and photoluminescence investigation of YBO3:Eu3+ under high temperature and high pressure

Raman scattering, X-ray diffraction and photoluminescence spectra of YBO3:Eu3+ are measured at high temperatures up to 1323 K. Results show that a temperature-induced phase transition from the LT to the HT phase starts at about 1273 K, then the HT phase transforms into the LT phase on cooling to 873 K. A theoretical model based on a luminescent dynamic process describes the temperature-dependent emission intensity well. In this model, the contributions from thermal activation, phonon-assisted absorption, and nonradiative energy transfer are analyzed in detail. In addition, we report the structure and optical behavior of YBO3:Eu3+ under high pressure by means of Raman scattering and photoluminescence measurements. When pressure is increased from zero to 25 GPa, Raman and photoluminescence measurements show that the structure of YBO3:Eu3+ remains stable and no pressure-driven phase transition occurs throughout the entire pressure range of these experiments. In addition, the mode-Gruneisen parameters and thermal expansion coefficient are calculated.

Upconversion luminescence of NaYF4:Yb,Er nanocrystals with high uniformity

Abstract The NaYF4:Yb,Er nanocrystals were synthesized via the thermal decomposition of metal oleate precursors. The nanocrystals in hexagonal structure were highly uniform and in size of 25 nm. The bright upconversion luminescence was observed under the excitation of 980 nm laser and the upconversion emission spectra were investigated at different pump powers. The emission intensity ratio of red light to green light linearly increased with pump power increasing. This result indicated that there existed a large threshold power of saturation pump for the first excitation state in NaYF4:Yb,Er nanocrystals comparing to that in bulk material.

Photoluminescence study of SiO2 coated Eu3+:Y2O3 core-shells under high pressure

Abstract Uniform core-shell Eu 3+ :Y 2 O 3 /SiO 2 spheres were synthesized via precipitation and the Stober method. The structural transition of core-shell Eu 3+ :Y 2 O 3 /SiO 2 was studied by using high pressure photoluminescence spectra. With pressure increasing, the emission intensities of 5 D 0 → 7 F 0,1,2 transitions of Eu 3+ ions decreased and the transition lines showed a red shift. The relative luminescence intensity ratio of 5 D 0 → 7 F 2 to 5 D 0 → 7 F 1 transitions decreased with increasing pressure, indicating lowering asymmetry around Eu 3+ ions. During compression, structural transformation for cores in the present core-shell Eu 3+ :Y 2 O 3 /SiO 2 sample from cubic to monoclinic took place at 7.5 GPa, and then the monoclinic structure turned into hexagonal above 15.2 GPa. After the pressure was released, the hexagonal structure transformed back to monoclinic and the monoclinic structure was kept stable to ambient pressure.

Pressure and temperature dependent up-conversion properties of Yb3+-Er3+ co-doped BaBi4Ti4O15 ferroelectric ceramics

Abstract Yb 3+ and Er 3+ co-doped BaBi 4 Ti 4 O 15 (BBT) ceramic samples showed brighter up-conversion photoluminescence (UC-PL) under excitation of 980 nm. The monotonous increase of fluorescence intensity ratio (FIR) from 525 to 550 nm with temperature showed that this material could be used for temperature sensing with the maximum sensitivity to be 0.0046 K −1 and the energy difference was 700 cm −1 . Moreover, the sudden change of red and green emissions around 400 °C might imply a phase transition. With increasing pressure up to 4 GPa, the PL intensity decreased but was still strong enough. These results illustrated the wide applications of BBT in high temperature and high pressure conditions.

Laser effects on phase transition for cubic Sb2O3 microcrystals under high pressure

The effects of laser irradiation and high pressure on the phase transition of cubic phase antimony trioxide microcrystals have been investigated. In situ Raman spectroscopy was performed at high pressure up to 45 GPa in a diamond anvil cell. The results indicated that two pressure-induced isostructural phase transitions occurred at 4.5 GPa and 11.3 GPa and were independent of laser irradiation. The Sb2O3 sample started transforming into a high density amorphous phase at 25 GPa and completed at more than 35 GPa. After irradiation for 7 minutes by 514.5 nm laser with 8 mW, the required pressure of the complete phase transition reduced to 27 GPa. On releasing pressure to ambient pressure, the samples reverse partly to the cubic phase, which takes days for the complete transition. This transition duration was reduced to seconds after irradiation by laser. It was also found that the residual amorphous phase at ambient pressure was of low density and medium-range order, which was not similar to the one that was observed at more than 25 GPa. The mechanism of laser irradiation boosting up the process of phase transitions was studied. Combining the cut-off wavelength of laser irradiation and absorption spectra, the photothermal effect induced via laser irradiation was found to dominate phase transition.

White light emission of Eu3+/Ag co-doped Y2Si2O7

Abstract The Eu 3+ /Ag co-doped rare earth disilicate Y 2 Si 2 O 7 microcrystal was synthesized by sol-gel method. Through controlling the thermal treatment process of Y 2 Si 2 O 7 :Eu 3+ /Ag precursor, various phases (amorphous, α, β, γ, δ) were prepared. White light emission was observed under UV light excitation in the samples heavily doped with Ag. The white light was realized by combining the intense red emission of Eu 3+ , the green emission attributed to the very small molecule-like, non-plasmonic Ag particles (ML-Ag-particles), and the blue emission due to Ag ions. Results demonstrated that Eu 3+ /Ag co-doped Y 2 Si 2 O 7 microcrystal could be potentially applied as white light emission phosphors for UV LED chips.

Structure transformation and remarkable site-distribution modulation of Eu3+ ions in CaMoO4 : Eu3+ nanocrystals under high pressure

High-pressure behaviors of scheelite-type CaMoO4 : Eu3+ nanocrystals with an average size of 30 nm have been investigated by using Raman and luminescence spectroscopy at pressures of up to 21 GPa in a diamond anvil cell (DAC). Under the loading of pressure, the softening of the external T(Bg) mode before 10.8 GPa and the appearance of new Raman peaks suggest a phase transformation around this pressure from scheelite to fergusonite structure. When the pressure is released, the scheelite phase recovers due to the small difference in bond strength between scheelite and fergusonite structures. The transformation pressure is slightly enhanced in nanosized CaMoO4 : Eu3+ as compared to the bulk due to the difference in surface energy between the two samples of different sizes. Moreover, it is identified that the Eu3+ ions occupy both the bulk and the surface sites in CaMoO4 : Eu3+ nanocrystals at ambient pressure by the site-selective excitation, emission and lifetime spectra. As an effective site probe, the red-to-orange luminescence intensity ratio of Eu3+ ions, I(5D0 → 7F2)/I(5D0 → 7F1), is found to exhibit remarkable changes with pressure, which indicates a large variation of the distribution and the local symmetry of Eu3+ ions.

Size dependence of up-conversion luminescence of NaYF4:Yb3+/Tm3+

Abstract Tm3+/Yb3+ codoped NaYF4 microcrystals were synthesized using a hydrothermal method. The bright upconversion light was observed under 980 nm excitation. The upconversion luminescence was systematically investigated at different Yb3+ concentrations and different reaction temperatures and time. The sample with 60% Yb3+ concentration and reacting at 180 °C for 24 h possessed the highest luminescent efficiency. The higher luminescent efficiency was contributed to a large surface area. The large surface area induced the large vibration mode by absorbed H2O and CO2. The larger vibration mode could enhance the energy transfer efficiency from the excited Yb3+ to Tm3+ by the process of phonon assisted energy transfer.

Anharmonic effects in single-walled carbon nanotubes

We have investigated the temperature and excitation energy dependence of Raman spectra of single-wall carbon nanotubes (SWCNTs). The measured Raman shifts for D and G bands of SWCNTs show a softening behavior upon increasing temperature (83–733 K). A theoretical model is developed to study the frequency shifts for these two Raman modes as a function of temperature; the calculated results agree well with the experimental data. The calculation indicates that the virtual optical-phonon couplings dominate contributions to the temperature variance of frequency shifts for D and G bands of SWCNTs while the pure-volume effect is very weak. Measurements of peak positions and linewidths of D and G bands are also carried out at three different excitation wavelengths (325, 514.5 and 785 nm). It is found that not only does the peak position of the D band exhibit a linear increase, but also the linewidth of the G band does upon increasing excitation energy. In addition, we also measure the relative intensity ratios of the D band to the G band (ID/IG) with temperature and excitation energy in this work.