Kezhi Zheng

Temperature sensor based on the UV upconversion luminescence of Gd3+ in Yb3+–Tm3+–Gd3+ codoped NaLuF4 microcrystals

Optical temperature sensing characteristics based on the ultraviolet (UV) upconversion luminescence (UCL) of Gd3+ ions are reported here for the first time. Under 980 nm excitation, the temperature dependence five-photon UV UCL from the 6PJ and 6IJ levels of Gd3+ ions in NaLuF4:Yb3+, Tm3+, Gd3+ microcrystals were investigated systematically. The fluorescence intensity ratios (FIR) of two pairs of thermally coupled levels (6P5/2, 6P7/2 and 6I9/2, 6I7/2) were studied as a function of temperature in a range of 298–523 K. The maximum sensor sensitivities were found to be about 0.0004 K−1 (333 K) and 0.0029 K−1 (298 K) by exploiting the UC emissions from the 6PJ and 6IJ levels, respectively. This suggests that the Gd3+-based UCL materials are promising prototypes for application as multi-mode probes for use in bio-separation, MRI imaging, optical thermometers, etc.

Ultraviolet upconversion fluorescence from 6 D J of Gd 3+ induced by 980 nm excitation

Ultraviolet upconversion emissions of 246.2 and 252.8 nm from (6)D(J) levels of Gd(3+) ions were observed in GdF(3): 10% Yb(3+), 0.7% Tm(3+) nanocrystals under 980 nm excitation from a laser diode. The (6)D(J) levels of Gd(3+) ions can be efficiently populated by energy transfer processes of Yb-->Tm-->Gd and Yb-->Gd. A six-photon upconversion process was confirmed by the dependence of 252.8 nm emission intensity on the pumping power. The upconversion mechanism in the six-photon process was discussed based on excited state absorption of Gd(3+) ions, cross relaxation energy transfer between two excited Gd(3+) ions, and energy transfer between Gd(3+) and Yb(3+) or Tm(3+) ions.

Five-photon UV upconversion emissions of Er 3+ for temperature sensing

Under 980 nm excitation, the temperature dependence of five-photon UV (256 and 276 nm) upconversion luminescence in Yb³⁺-Er³⁺ codoped β-NaLuF₄ nanocrystals was studied from 303 K to 523 K. The ⁴D(7/2) and ⁴G(9/2) levels of Er³⁺ are confirmed to be thermally coupled levels. They are the highest energy states for optical thermometry known so far. By using fluorescence intensity ratio technique, optical temperature sensing characteristics based on the ⁴D(7/2)/⁴G(9/2) → ⁴I(15/2) transitions of Er³⁺ were reported here for the first time. The obtained sensitivity of this UV-based sensor is higher than that of green-based optical thermometer in low temperature range.

Ultraviolet upconversion fluorescence of Er3+ induced by 1560 nm laser excitation.

We observed multiphoton upconversion (UC) emissions from the high-energy states ((2)I(11/2), (4)D(7/2), (4)G(9/2), (2)D(5/2), (2)K(13/2), and (2)P(3/2)) of Er(3+) ions in Yb(3+)-Er(3+) codoped beta-NaYF(4) microcrystals under 1560 nm excitation. Analysis indicated that the highest-order emissions came from 11-photon UC processes. In the codoped system, the energy transfer (ET) from Er(3+)((4)I(11/2)) to Yb(3+)((2)F(5/2)) and subsequently the back ET from Yb(3+) to Er(3+) played important roles in populating these high-energy states of Yb(3+) and Er(3+). Several different UC processes work simultaneously to populate excited Er(3+) in this codoped sample.

Power switched multiphoton upconversion emissions of Er 3+ in Yb 3+ /Er 3+ codoped β-NaYF 4 microcrystals induced by 980 nm excitation

Multiphoton upconversion (UC) emissions from the high-energy states ((4)G(9/2), (4)G(7/2), (2)K(13/2), and (2)P(3/2)) of Er(3+) ions were observed under 980 nm excitation. These high-energy excited states were populated by a five-photon or a four-photon UC process conditionally, which depended on the near-infrared (NIR) pump density. Experiments exhibited that the power dependence originated from the varied populating routes of intermediated (4)S(3/2) and (4)F(9/2) of Er(3+) under different NIR pump power. A mechanism of the power density-dependent multiphoton UC processes was proposed based on experimental data and analysis.

Ultraviolet and violet upconversion fluorescence of europium (III) doped in YF(3) nanocrystals.

Under 980 nm excitation, Eu(3+)/Tm(3+)/Yb(3+) tri-doped YF(3) nanocrystals emitted ultraviolet and visible fluorescence. Besides the upconversion emissions from Tm(3+) ions, not only were the unusual (5)D(2)-->(7)F(3), (5)D(3)-->(7)F(J) emissions of Eu(3+) observed, but also the ultraviolet upconversion luminescence of Eu(3+) from (5)H(3-7), (5)G(2-6), (5)L(6)-->(7)F(0) transitions were recorded. This unique optical property was attributed to the bridging function of Tm(3+) ions in populating high-energy states of Eu(3+) ions.

The synthesis and ultraviolet photoluminescence of 6H-SiC nanowires by microwave method

In this paper, 6H–SiC nanowires have been largely synthesized by a novel and low-cost microwave method with the presence of nano-Al powders. Structural, morphological and elemental analysis revealed that the products consisted of Al-doped 6H–SiC nanowires with a diameter of 5–200 nm and a length of tens to hundreds of micrometres. Some unique properties are found in the Raman and photoluminescence (PL) spectra of the 6H–SiC nanowires, based on which a possible emission mechanism is also discussed. The PL spectrum shows clear evidence of the quantum confinement of 6H–SiC nanowires with the emission above the energy gap.

Controllable synthesis and size-dependent upconversion luminescence properties of Lu2O3:Yb3+/Er3+ nanospheres

Lu2O3:Yb3+/Er3+ nanocrystals with various sizes and shapes (nano-aggregates, sub-micrometer wires, and nanospheres) have been synthesized by the soft chemistry coprecipitation route. By regulating the reactant ratio of rare earth to urea precipitant ([RE3+]/(NH2)2CO), uniform spherical Lu2O3:Yb3+/Er3+ nanoparticles with the sizes of 45, 100, 165, 200, and 250 nm were obtained in the experiments. The phases, morphologies, as well as the luminescence properties of as-prepared samples were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), and upconversion luminescence (UCL) spectra, respectively. Under the excitation of a 980 nm diode laser, the Lu2O3:Yb3+/Er3+ samples showed size-dependent upconversion luminescence property. From the results of Fourier transform infrared (FTIR) spectra and dynamical analysis, it was confirmed that the surface effect is dominant in influencing the UCL of Er3+ in the spherical Lu2O3:Yb3+/Er3+ samples. By coating Lu2O3:Yb3+/Er3+ nanospheres with different shell layers, UCLs of Er3+ were changed greatly in these core–shell samples. The possible physical mechanism involved in size/surface-dependent upconversion processes was discussed in detail.

A strategy for enhancing the sensitivity of optical thermometers in β-NaLuF4:Yb3+/Er3+ nanocrystals

A strategy for enhancing the sensitivity of optical thermometers is developed herein by using non-thermally coupled levels of Er3+. Under the excitation of a 980 nm laser, the temperature dependence of 244 nm and 256 nm upconversion luminescences (UCLs) of Er3+ was studied. The corresponding 2I11/2 and 4D7/2 levels were confirmed to be non-thermally coupled levels. By using the fluorescence intensity ratio (FIR) technique and by investigating different thermal population behaviors of 2I11/2 and 4D7/2 levels, the optical temperature sensing performance based on the non-thermally coupled levels of Er3+ was fulfilled here for the first time. The obtained maximum sensor sensitivity is 0.106 K−1 at 525 K, which is much higher than those of all other RE3+ doped optical thermometers using the thermally coupled level-based FIR technique. This suggests that the use of the FIR from neighboring non-thermally coupled levels of RE3+ is a promising approach for enhancing the sensor sensitivity of optical thermometers.

NIR to VUV: Seven-Photon Upconversion Emissions from Gd(3+) Ions in Fluoride Nanocrystals.

Here we show that a near-infrared (NIR) diode laser is capable of generating vacuum ultraviolet (VUV) emissions in fluoride nanocrystals through photon upconversion (UC) processes. By using Yb(3+) and Tm(3+) as sensitizers, we successfully obtained the VUV photons with the energy exceeding 6 eV in YF3: Yb, Tm, and Gd nanocrystals. The seven photon UC fluorescence from the (6)GJ → (8)S7/2 transitions of Gd(3+) ions and the possible VUV UC mechanism were reported along with the calculation of the branching ratio under different pumping power excitation. Practically, it offers a promising solution for VUV light generation without cryogens and expensive instrumentations. Fundamentally, the extremely high-order UC processes will intrigue great interest in exploring unusual high-energy radiative transitions in rare earth ions.