Observations on the strong photochromic effect of crystalline TiO 2 quantum dots (mean size ≈ 4 nm) are presented. The synthesized quantum dots consist of irregularly shaped anatase TiO 2 nanoparticles (NPs) and are dispersed in butanol (8% by mass). Obtained NPs exhibit a dramatic photoresponse to UV light, enabling effective transmittance modulation in a broad wavelength range extending from the visible to near-infrared region, and even the thermal black body radiation regime beyond 10 μm. The exceptional photoresponse is attributed to hole-scavenging by butanol, TiO 2 self-reduction, injection of electrons to the conduction band, and consequent localized surface plasmon resonances in NPs. The observed optical effect is reversible, and the initial high transmittance state can be restored simply by exposing the NPs to air. The applied NP synthesis route is economic and can be easily scaled for applications such as smart window technologies.

This work concerns a novel red emitting Mn 4+ doped phosphor showing an extraordinarily high thermal quenching temperature. The composition Y 2 Mg 3 Ge 3 O 12 :Mn 4+ ,Li + exhibits efficient red photoluminescence peaking at 658 nm, which can be assigned to the 2 E g → 4 A 2g intraconfigurational transition of Mn 4+ ([Ar]3d 3 configuration) located at the octahedral site in the garnet structure. Photoluminescence properties, such as temperature dependence of the luminescence intensity and luminescence lifetime are presented. Additionally, the band structure of the undoped host material were treated with Density Functional Theory (DFT) and evaluated with UV-reflectance spectroscopy. [1][2][3][4][5] However, this approach suffers from major shortcomings such as cold white light and a low color rendering index (CRI), which results from the lack of red light. Therefore, red phosphors with outstanding luminescent performance and strong absorption in the near UV to blue spectral range are highly demanded. Compared with the established commercial red phosphors, for example, (Ca,Sr)AlSiN 3 :Eu 2+ and (Ca,Sr,Ba) 2 Si 5 N 8 :Eu 2+ , which typically possess a broad emission band, Mn 4+ activated oxide and fluoride phosphors exhibit an emission spectrum consisting of narrow lines in the red region.6-14 Moreover, the preparation of nitride phosphors requires harsh conditions, i.e. high temperature and pressure, which to a tremendous extent, increase the cost and difficulty of preparation. 15,16 Additionally, rare earth elements as raw materials are too expensive for producing respective phosphors at a low cost-price. Recently, Mn 4+ doped fluoride red emitting phosphors have attracted much attention because of the simple and efficient fabrication process, i.e. they can be prepared at room temperature or slightly above (20-80• C) by cation exchange reaction. 17 However, these types of materials have a huge drawback because toxic hydrofluoric acid is needed for their synthesis. Additionally, Mn 4+ doped fluorides are rather sensitive toward high temperature, high humidity, and large excitation densities provided by LEDs. [18][19][20] In general, Mn 4+ doped oxide phosphors were prepared using conventional oxides, nitrates, and perchlorates as starting materials. All these disadvantages could be overcome by using an oxide type host material, if one is able to find a Mn 4+ doped oxide host with a rather narrow emission band peaking at approximately 630 nm. 24-26 Accordingly, research activity and commercial interest is highly focused on Mn 4+ based phosphors presently. These type of materials can emit in the desired red spectral region with a high quantum yield and thus meet the efficiency and color quality demands of the next generation warm white pcLEDs. It is well known, that the Mg 14 Ge 5 O 24 :Mn 4+ phosphor (also known as Mg-fluorogermanate due to the presence of some MgF 2 as a flux) has extraordinary good properties in terms of the thermal quenching of the luminescence (T 1/2 > 700 K).27 Due to this circumstance, the au...

a b s t r a c tLuminescence properties and electronic excitations of d-and Q-phases of Al 2 O 3 nano-powders, prepared by plasma processing technique, were studied using time-resolved luminescence spectroscopy under vacuum ultraviolet excitation. The fundamental absorption edge was found at 7.5 eV, which is 1.5 eV less than that of a-Al 2 O 3 in agreement with theoretical predictions for transition alumina. The broad-band emissions with maxima at 5.5 and 4.6 eV are assigned to radiative decay of the intrinsic excitations. Emission of transition metal impurities was found near 1.75 eV. The emission bands peaked at 3.1 and 2.6 eV are assigned to F þ and F centres, respectively.

No abstract

No abstract