Daisheng Zhang

Greatly enhanced size-tunable ultraviolet upconversion luminescence of monodisperse β-NaYF4:Yb,Tm nanocrystals

Monodisperse β-NaYF4:Yb,Tm nanocrystals (nanospheres and nanoplates) with a uniform size were successfully synthesized. By tuning the ratios of solvent, reaction temperature, and reaction time, we manipulated their phase, shape, and size in the range of 30–400 nm. As a chelating agent and shape modifier, oleic acid (OA) was introduced into the reaction mixtures and played a key role in fine-tuning the nanocrystals. Possible mechanisms were proposed for forming the products with various architectures. Spectral analysis showed that the β-NaYF4:Yb,Tm nanocrystals were excellent materials for intense ultraviolet and blue upconversion luminescence. Here, we demonstrate that, under 980 nm excitation, the intense 5-photon upconversion fluorescence (290 nm and 345 nm) from the 1I6 level of Tm3+ ions is much stronger than the 4-photon upconversion fluorescence (361 nm and 451 nm) from the 1D2 level and the 3-photon upconversion fluorescence (474 nm) from the 1G4 level. The analysis of the temporal evolution of UC luminescence suggests that long lifetimes benefit the intense ultraviolet upconversion luminescence.

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.

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.

Synthesis and upconversion luminescence properties of NaYF4:Yb3+/Er3+ microspheres

Abstract Cubic NaYF 4 :Yb 3+ (20%)/Er 3+ (1%) microspheres were synthesized by EDTA-assisted hydrothermal method. Under 980 nm excitation, ultraviolet ( 4 G 11/2 → 4 I 15/2 ), violet ( 2 H 9/2 → 4 I 15/2 ), green ( 4 F 7/2 → 4 I 15/2 , 2 H 11/2 → 4 I 15/2 , and 4 S 3/2 → 4 I 15/2 ), and red ( 4 F 9/2 → 4 I 15/2 ) upconversion fluorescence were observed. The number of laser photons absorbed in one upconversion excitation process, n , was determined to be 3.89, 1.61, 2.55, and 1.09 for the ultraviolet, violet, green, and red emissions, respectively. Obviously, n =3.89 indicated that a four-photon process was involved in populating the 4 G 11/2 state, and n =2.55 indicated that a three-photon process was involved in populating the 4 F 7/2 / 2 H 11/2 / 4 S 3/2 levels. For the violet and red emissions, the population of the states 2 H 9/2 and 4 F 9/2 separately came from three-photon and two-photon processes. The decrease of n was well explained by the mechanism of competition between linear decay and upconversion processes for the depletion of the intermediate excited states.

Enhanced ultraviolet upconversion in YF3:Yb3+/Tm3+ nanocrystals

Abstract Unusual intense infrared-to-ultraviolet upconversion luminescence was observed in YF 3 :Yb 3+ (20%)/Tm 3+ (1%) nanocrystals under 980 nm excitation. The intense ultraviolet emissions ( 1 I 6 → 3 H 6 , 1 I 6 → 3 F 4 , and 1 D 2 → 3 H 6 ) were affirmed arising from the excitation processes of five-photon and four-photon. In comparison with the bulk sample with the same chemical compositions, ultraviolet upconversion luminescence of the nanocrystals was markedly enhanced. Spectral analysis indicated that the enhancement was attributed to the decrease of Judd-Ofelt parameter Ω 2 , which precluded the transition rate from 3 F 2 to 3 F 4 , enhanced the energy transfer process and populated the 1 D 2 level: 3 F 2 → 3 H 6 (Tm 3+ ): 3 H 4 → 1 D 2 (Tm 3+ ).