Ryongjin Kim

Intense ultraviolet upconversion luminescence from hexagonal NaYF 4 :Yb 3+ /Tm 3+ microcrystals

Under 980 nm excitation, unusual 3P2-->3H6 (approximately 264 nm) and 3P2-->3F4 (approximately 309 nm) emissions from Tm3+ ions were observed in hexagonal NaYF4:Yb3+ (20%)/Tm3+ (1.5%) microcrystals. In comparison with the strong emissions from 1D2 and 1I6, the emissions from 1G4 and 3H4 almost vanished due to the efficient cross-relaxation of 1G4 + 3H4-->3F4 + 1D2(Tm3+). Double logarithmic plots of the upconversion emission intensity versus the excitation power are neither straight lines nor typical saturation curves. Theoretical analysis indicated that the complicated dependent relationships were mainly caused by phonon-assisted energy transfers and nonradiative relaxation.

Ultraviolet upconversion emissions of Gd3

Under 980 nm excitation, upconversion (UC) emissions in the UV range of 270-320 nm were observed in nanocrystals Y(0.795-x)Gd(x)Yb(0.2)Tm(0.005)F(3) (x=0, 0.1, 0.2, 0.5, and 0.795), which were synthesized through a hydrothermal method. These UC emissions can be assigned to the transitions of (6)I(J), (6)P(J)-->(8)S(7/2)(Gd(3+)), and (3)P(0)/(1)I(6)-->(3)H(6) (Tm(3+)). The energy transfer from Tm(3+) to Gd(3+) plays a crucial role in populating the excited states of Gd(3+). The shortest wavelength of upconverted emission converted from the infrared region was demonstrated here.

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 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.