Hoi Lam Tam

Defect emissions in ZnO nanostructures

Defects in three different types of ZnO nanostructures before and after annealing under different conditions were studied. The annealing atmosphere and temperature were found to strongly affect the yellow and orange-red defect emissions, while green emission was not significantly affected by annealing. The defect emissions exhibited a strong dependence on the temperature and excitation wavelength, with some defect emissions observable only at low temperatures and for certain excitation wavelengths. The yellow emission in samples prepared by a hydrothermal method is likely due to the presence of OH groups, instead of the commonly assumed interstitial oxygen defect. The green and orange-red emissions are likely due to donor acceptor transitions involving defect complexes, which likely include zinc vacancy complexes in the case of orange-red emissions.

NiO∕ZnO light emitting diodes by solution-based growth

Heterojunction NiO∕ZnO light emitting diodes have been fabricated using low temperature solution-based growth methods. While negligible light emission has been obtained for the as-grown NiO film, devices with annealed NiO film exhibit room-temperature electroluminescence (EL), which was attributed to the detrimental effects of nickel oxide hydroxide in as-grown NiO layers. The device performance can be further modified by insertion of the organic layers between NiO and ZnO and the EL spectra exhibited dependence on the bias voltage. For higher bias voltages, strong UV-violet emission peak can be obtained in spite of the dominance of defect emission in the photoluminescence spectra.

Remanagement of Singlet and Triplet Excitons in Single‐Emissive‐Layer Hybrid White Organic Light‐Emitting Devices Using Thermally Activated Delayed Fluorescent Blue Exciplex

A high-performance hybrid white organic light-emitting device (WOLED) is demonstrated based on an efficient novel thermally activated delayed fluorescence (TADF) blue exciplex system. This device shows a low turn-on voltage of 2.5 V and maximum forward-viewing external quantum efficiency of 25.5%, which opens a new avenue for achieving high-performance hybrid WOLEDs with simple structures.

Water-Soluble Mitochondria-Specific Ytterbium Complex with Impressive NIR Emission

A water-soluble porphyrinato ytterbium complex linked with rhodamine B (Yb-2) showed mitochondria-specific subcellular localization and strong two-photon-induced NIR emissions (λ(em) = 650 nm, porphyrinate ligand π → π* transition; λ(em) = 1060 nm, Yb(III) (5)F(5/2) → (5)F(7/2) transitions; σ(2) = 375 GM in DMSO) with an impressive Yb(III) NIR emission quantum yield (1% at λ(ex) = 340 nm; 2.5% at λ(ex) = 430 nm) in aqueous solution.

White OLED with a single-component europium complex.

A new direction for white organic light-emitting devices is shown, fabricated from a novel europium complex; this single component contains a double emission center of bluish-green and red, combined to a give a pure white emission (CIE x = 0.34 and y = 0.35).

A bioaccumulative cyclometalated platinum(II) complex with two-photon-induced emission for live cell imaging

The cyclometalated platinum(II) complex [Pt(L)Cl], where HL is a new cyclometalating ligand 2-phenyl-6-(1H-pyrazol-3-yl)pyridine containing C(phenyl), N(pyridyl), and N(pyrazolyl) donor moieties, was found to possess two-photon-induced luminescent properties. The two-photon-absorption cross section of the complex in N,N-dimethylformamide at room temperature was measured to be 20.8 GM. Upon two-photon excitation at 730 nm from a Ti:sapphire laser, bright-green emission was observed. Besides its two-photon-induced luminescent properties, [Pt(L)Cl] was able to be rapidly accumulated in live HeLa and NIH3T3 cells. The two-photon-induced luminescence of the complex was retained after live cell internalization and can be observed by two-photon confocal microscopy. Its bioaccumulation properties enabled time-lapse imaging of the internalization process of the dye into living cells. Cytotoxicity of [Pt(L)Cl] to both tested cell lines was low, according to MTT assays, even at loadings as high as 20 times the dose concentration for imaging for 6 h.

Efficient three-photon excited deep blue photoluminescence and lasing of diphenylamino and 1,2,4-triazole endcapped oligofluorenes

A novel homologues series of diphenylamino and 1,2,4-triazole endcapped pi-conjugated oligofluorenes has been synthesized using palladium-catalyzed Suzuki cross-coupling of 9,9-dibutyl-7-(diphenylamino)-2-fluorenylboronic acid, and 1,2,4,-triazole-substituted oligofluorenyl halide was used as a key step. Efficient two- and three-photon excited photoluminescence and lasing were obtained by pumping with near-infrared femto-second lasers. The three-photon absorption cross-section enhances significantly up to 2.48 x 10(-78) cm(6) s(2) in the femtosecond regime with the length of the conjugation system. The emission is in the deep blue region and the best full width half-maximum (fwhm) of the three-photon lasing is narrower than 6 nm which is the narrowest ever reported. It was also shown that the nonlinear optics (NLO) effect increased parabolically with the conjugation length but without changing the emission wavelength. This demonstrates that varying the conjugation length is a very effective way to tailor an OF-based NLO device for deep blue application. Our findings open a new avenue to design highly efficient multiphoton absorption molecules for photoluminescence and lasing as well as provide a novel series of organic molecules that can be used in NLO applications and fundamental study.

GaN/ZnO nanorod light emitting diodes with different emission spectra

Light emitting diodes (LEDs) consisting of p-GaN epitaxial films and n-ZnO nanorods have been fabricated and characterized. The rectifying behavior and emission spectra were strongly dependent on the electronic properties of both GaN film and ZnO nanorods. Light emission under both forward and reverse bias was obtained in all cases, and emission spectra could be changed by annealing the ZnO nanorods. The emission spectra could be further tuned by using a GaN LED epiwafer as a substrate. Both forward and backward diode behavior has been observed and the emission spectra were significantly affected by both the properties of the GaN substrate and the annealing conditions for the ZnO nanorods.

Comparative Studies of the Cellular Uptake, Subcellular Localization, and Cytotoxic and Phototoxic Antitumor Properties of Ruthenium(II)-Porphyrin Conjugates with Different Linkers

Six water-soluble free-base porphyrin-Ru(II) conjugates, 1-3, and Zn(II) porphyrin-Ru(II) conjugates, 4-6, with different linkers between the hydrophobic porphyrin moiety and the hydrophilic Ru(II)-polypyridyl complex, have been synthesized. The linear and two-photon-induced photophysical properties of these conjugates were measured and evaluated for their potential application as dual in vitro imaging and photodynamic therapeutic (PDT) agents. Conjugates 1-3, with their high luminescence and singlet oxygen quantum yields, were selected for further study of their cellular uptake, subcellular localization, and cytotoxic and photocytotoxic (under linear and two-photon excitation) properties using HeLa cells. Conjugate 2, with its hydrophobic phenylethynyl linker, was shown to be highly promising for further development as a bifunctional probe for two-photon (NIR) induced PDT and in vitro imaging. Cellular uptake and subcellular localization properties were shown to be crucial to its PDT efficacy.

Efficient and ultraviolet durable inverted organic solar cells based on an aluminum-doped zinc oxide transparent cathode

High performance inverted bulk heterojunction organic solar cells (OSCs), based on the blend of poly[[4,8-bis[(2-ethylhexyl)oxy] benzo [1,2-b:4,5-b′] dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7): 3′H-Cyclopropa[8,25] [5,6]fullerene-C70-D5h(6)-3′-butanoicacid, 3′-phenyl-, methyl ester (PC70BM), were achieved using an aluminum-doped zinc oxide (AZO) front transparent cathode. A structurally identical PTB7:PC70BM-based OSC having an indium tin oxide (ITO) front cathode was also made for comparison studies. The surface of AZO and ITO was modified with a 10 nm thick solution-processed ZnO interlayer to facilitate the efficient electron extraction. This work yielded AZO-based OSCs with a promising power conversion efficiency of 6.15%, slightly lower than 6.57% of a control ITO-based OSC, however, a significant enhancement in the stability of AZO-based OSCs was observed under an ultraviolet (UV)-assisted acceleration aging test. The distinctive enhancement in the l...