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Abstract

A careful engineering of the central metal coordination spheres provides adducts with excellent properties for application as precursors in vapor phase and solution processes. The family of precursors under study concerns the fluorinated metal-organic β-diketonates of alkaline, alkaline-earth and rare-earth metals adducted with a polyether, with general formula M(hfa)n·L (M = Ca, Na, Y, Yb, Er, Tm; Hhfa = 1,1,1,5,5,5 hexafluoroacetylacetone, L = diglyme or tetraglyme). Mass transport properties, such as volatility and thermal stability, of these adducts have been deeply analyzed through thermogravimetric analysis and differential scanning calorimetric measurements. These properties are rationalized in relation to the metal coordination sphere in the precursors and their applications. In particular, the precursors under focus have been applied to metal organic chemical vapor deposition and a combined sol–gel/spin-coating approach. Both methods allow us to obtain selectively and reproducibly CaF2and NaYF4phases with several combinations of lanthanide doping ions, using a proper mixture of fluorinated precursors. A careful optimization of both synthetic strategies is the key point for the production of different lanthanide-doped binary and multicomponent fluoride films, i.e., CaF2:Yb3+,Er3+; CaF2:Yb3+,Tm3+; CaF2:Yb3+,Er3+,Tm3+and NaYF4:Yb3+,Er3+; NaYF4:Yb3+,Tm3+, with suitable morphologies, compositions and crystalline structures. The films show very promising upconversion properties, thus pointing to their appealing applications in photovoltaic systems and white light emission devices.

Octahedral crystals of Fe(III)-based Mil-101 and amino-functionalized Mil-101 are directly grown on Si(100) using easy solution routes under mild conditions. Images from microscopy analyses show that samples consist of a layer of isolated octahedral crystals in which the [111]-direction develops vertically with respect to the Si(100) substrate. Structural and chemical characterizations indicate that crystals consist of Mil-101(Fe) with the following stoichiometry: Fe3(O)Cl-(DMF)2(Terepht)3(DMF = dimethylformamide and Terepht = terephthalate or aminoterephthalate). Both Si-grown Mil-101(Fe) and NH2-Mil-101(Fe) layers are capable of adsorbing two aromatic gases with different polarities, toluene and nitrobenzene. Their characteristic thermal desorption profiles depend on the amino-functionalization of the terephthalate ligand as well as on the mutual interactions between the two guests when adsorbed from a mixture. It is also shown that, by adapting suitable experimental conditions, Mil-101(Fe) layers are capable of separating a toluene/nitrobenzene gas mixture through selective thermal desorption. A model based on the evaluation of the involved host–guest interactions through the density functional theory (DFT) approach is proposed to explain the observed selective desorption and to account for the key parameters involved in the desorption process of aromatic vapors.

In this work, we present all-oxide p-n junction core-shell nanowires (NWs) as fast and stable self-powered photodetectors. Hydrothermally grown n-type ZnO NWs were conformal covered by different thicknesses (up to 420 nm) of p-type copper oxide layers through metalorganic chemical vapor deposition (MOCVD). The ZnO NWs exhibit a single crystalline Wurtzite structure, preferentially grown along the [002] direction, and energy gap Eg= 3.24 eV. Depending on the deposition temperature, the copper oxide shell exhibits either a crystalline cubic structure of pure Cu2O phase (MOCVD at 250 °C) or a cubic structure of Cu2O with the presence of CuO phase impurities (MOCVD at 300 °C), with energy gap of 2.48 eV. The electrical measurements indicate the formation of a p-n junction after the deposition of the copper oxide layer. The core-shell photodetectors present a photoresponsivity at 0 V bias voltage up to 7.7 µA/W and time response ≤ 0.09 s, the fastest ever reported for oxide photodetectors in the visible range, and among the fastest including photodetectors with response limited to the UV region. The bare ZnO NWs have slow photoresponsivity, without recovery after the end of photo-stimulation. The fast time response for the core-shell structures is due to the presence of the p-n junctions, which enables fast exciton separation and charge extraction. Additionally, the suitable electronic structure of the ZnO-Cu2O heterojunction enables self-powering of the device at 0 V bias voltage. These results represent a significant advancement in the development of low-cost, high efficiency and self-powered photodetectors, highlighting the need of fine tuning the morphology, composition and electronic properties of p-n junctions to maximize device performances.