Porphyrin-Based MOF Thin Film on Transparent Conducting Oxide: Investigation of Growth, Porosity and Photoelectrochemical Properties.

Synthesizing metal-organic frameworks (MOFs) composites with a controlled morphology is an important requirement to access materials of desired patterning and composition. Since the last decade, MOF growth from sacrificial metal oxide layer is increasingly developed as it represents an efficient pathway to functionalize a large number of substrates. In this study, porphyrin-based Al-PMOF thin films were grown on conductive transparent oxide substrates from sacrificial layers of ALD-deposited alumina oxide. The control of the solvent composition and the number of atomic layer deposition (ALD) cycles allow us to tune the crystallinity, morphology and thickness of the produced thin films. Photophysical studies evidence that Al-PMOF thin films present light absorption and emission properties governed by the porphyrinic linker, without any quenching upon increasing the film thickness. Al-PMOF thin films obtained through this methodology present a remarkably high optical quality both in terms of transparency and coverage. The porosity of the samples is demonstrated by ellipsometry and used for Zn(II) insertion inside the MOF thin film. The multifunctional transparent, porous and luminescent thin film grown on fluorine-doped tin oxide (FTO) is used as an electrode capable of photoinduced charge separation upon simulated sunlight irradiation.

Vapor-Phase Infiltration inside a Microporous Porphyrinic Metal-Organic Framework for Postsynthesis Modification.

Vapor-phase infiltration (VPI), a technique derived from atomic layer deposition (ALD) and based on sequential self-limiting chemistry, is used to modify the stable microporous porphyrin-based metal-organic framework (MOF) MIL-173(Zr). VPI is an appealing approach to modifying MOFs by inserting reactants with atomic precision. The microporous nature and chemical stability of MIL-173 enable postsynthesis modification by VPI without MOF degradation even with extremely reactive precursors such as trimethylaluminum (TMA) and diethylzinc (DEZ). VPI proceeds through the diffusion of gaseous organometallic reactants TMA and DEZ inside the microporous framework, where they react with two kinds of chemical sites offered by the porphyrinic linker (phenolic and pyrrolic functions in the porphyrin core), without altering the crystallinity and permanent porosity of the MOF. 27Al NMR, UV-vis absorption, and IR spectroscopies are used to further characterize the modified material. Physisorption of both precursors is computationally simulated by grand canonical Monte Carlo methods and outlines the preferential adsorption sites. The impact of temperature, number of VPI cycles, and pulse length are investigated and show that aluminum and zinc are introduced in a saturating manner inside the MOF on both available reactive sites. The porosity prerequisite is outlined for VPI, which is proven to be much more effective than classical solution-based methods because it is solventless and fast, prevents workup steps, and allows reactions not possible by the classical solution approach.

Gas sensing properties and p-type response of ALD TiO2 coated carbon nanotubes.

Amorphous titanium dioxide-coated carbon nanotubes (CNTs) were prepared by atomic layer deposition (ALD) and investigated as sensing layers for resistive NO2 and O2 gas sensors. By varying ALD process conditions and CNT structure, heterostructures with different metal oxide grain size, morphology and coating thickness were synthesized. Higher responses were observed with homogeneous and continuous 5.5 nm thick films onto CNTs at an operating temperature of 150 °C, while CNTs decorated with either discontinuous film or TiO2 nanoparticles showed a weak response close to the one of device made of bare CNTs. An unexpected p-type behavior in presence of the target gas was also noticed, independently of the metal oxide morphology and thickness. Based on previous works, hypotheses were made in order to explain the p-type behavior of TiO2/CNT sensors.

Investigating the vapour phase synthesis of copper terephthalate metal organic framework thin films by atomic/molecular layer deposition.

Solventless synthesis and processing of Metal Organic Frameworks (MOFs) is critical to implement these materials in applied technologies. Vapour phase synthesis of MOF thin films is particularly suitable for such applications, but challenging compared to the conventional solution based methods. It is therefore compelling to advance and widen the vapour phase synthesis of MOF thin films. Crystalline copper terephthalate MOF thin films are grown in the vapour phase by means of atomic and molecular layer deposition (ALD/MLD) on different kinds of substrates. Expanding from the pioneering work, the formation of the 3D phase is clearly evidenced for the first time and the adaptability of the process to several kinds of substrates is revealed. A directional film growth is observed at the early stage of the ALD/MLD process leading to oriented MOF crystallites on a surface, when isotropical growth proceeds with the increasing number of ALD/MLD cycles. Notably, this study primarily demonstrates a heteroepitaxial growth achievable in the vapour phase by using DMOF-1 single crystals as the starting surface with a lattice matching topology. Such an approach offers an appealing pathway to develop MOF on MOF superlattice materials in the vapour phase.

CoFe2O4-TiO2 and CoFe2O4-ZnO thin film nanostructures elaborated from colloidal chemistry and atomic layer deposition.

CoFe(2)O(4)-TiO(2) and CoFe(2)O(4)-ZnO nanoparticles/film composites were prepared from directed assembly of colloidal CoFe(2)O(4) in a Langmuir-Blodgett monolayer and atomic layer deposition (ALD) of an oxide (TiO(2) or ZnO). The combination of these two methods permits the use of well-defined nanoparticles from colloidal chemistry, their assembly on a large scale, and the control over the interface between a ferrimagnetic material (CoFe(2)O(4)) and a semiconductor (TiO(2) or ZnO). Using this approach, architectures can be assembled with a precise control from the Angstrom scale (ALD) to the micrometer scale (Langmuir-Blodgett film). The resulting heterostructures present well-calibrated thicknesses. Electron microscopy and magnetic measurement studies give evidence that the size of the nanoparticles and their intrinsic magnetic properties are not altered by the various steps involved in the synthesis process. Therefore, the approach is suitable to obtain a layered composite with a quasi-monodisperse layer of ferrimagnetic nanoparticles embedded in an ultrathin film of semiconducting material.

High-quality photonic crystals with a nearly complete band gap obtained by direct inversion of woodpile templates with titanium dioxide.

Photonic crystal materials are based on a periodic modulation of the dielectric constant on length scales comparable to the wavelength of light. These materials can exhibit photonic band gaps; frequency regions for which the propagation of electromagnetic radiation is forbidden due to the depletion of the density of states. In order to exhibit a full band gap, 3D PCs must present a threshold refractive index contrast that depends on the crystal structure. In the case of the so-called woodpile photonic crystals this threshold is comparably low, approximately 1.9 for the direct structure. Therefore direct or inverted woodpiles made of high refractive index materials like silicon, germanium or titanium dioxide are sought after. Here we show that, by combining multiphoton lithography and atomic layer deposition, we can achieve a direct inversion of polymer templates into TiO2 based photonic crystals. The obtained structures show remarkable optical properties in the near-infrared region with almost perfect specular reflectance, a transmission dip close to the detection limit and a Bragg length comparable to the lattice constant.

Atomic layer deposition of nanostructured materials for energy and environmental applications.

Atomic layer deposition (ALD) is a thin film technology that in the past two decades rapidly developed from a niche technology to an established method. It proved to be a key technology for the surface modification and the fabrication of complex nanostructured materials. In this Progress Report, after a short introduction to ALD and its chemistry, the versatility of the technique for the fabrication of novel functional materials will be discussed. Selected examples, focused on its use for the engineering of nanostructures targeting applications in energy conversion and storage, and on environmental issues, will be discussed. Finally, the challenges that ALD is now facing in terms of materials fabrication and processing will be also tackled.

Nonaqueous sol-gel chemistry applied to atomic layer deposition: tuning of photonic band gap properties of silica opals.

Combining both electromagnetic simulations and experiments, it is shown that the photonic pseudo band gap (PPBG) exhibited by a silica opal can be fully controlled by Atomic Layer Deposition (ALD) of titania into the pores of the silica spheres constituting the opal. Different types of opals were assembled by the Langmuir-Blodgett technique: homogeneous closed packed structures set up of, respectively, 260 and 285 nm silica spheres, as well as opal heterostructures consisting of a monolayer of 430 nm silica spheres embedded within 10 layers of 280 nm silica spheres. For the stepwise infiltration of the opals with titania, titanium isopropoxide and acetic acid were used as metal and oxygen sources, in accordance with a recently published non-aqueous approach to ALD. A shift of the direct PPBG, its disappearance, and the subsequent appearance and shifting of the inverse PPBG are observed as the opal is progressively filled. The close agreement between simulated and experimental results is striking, and promising in terms of predicting the properties of advanced photonic materials. Moreover, this work demonstrates that the ALD process is rather robust and can be applied to the coating of complex nanostructures.