Open Porosity and Pore Size Distribution of Mesoporous Silica Films Investigated by Positron Annihilation Lifetime Spectroscopy and Ellipsometric Porosimetry

Tunable mesoporous silica films were prepared though a sol-gel process directed by the self-assembly of various triblock copolymers. Positron annihilation γ-ray energy spectroscopy and positron annihilation lifetime spectroscopy (PALS) based on intense pulsed slow positron beams as well as ellipsometric porosimetry (EP) combined with heptane adsorption were utilized to characterize the open porosity/interconnectivity and pore size distribution for the prepared films. The consistency between the open porosities was examined by the variations of orthopositronium (o-Ps) 3γ annihilation fractions and the total adsorbed volumes of heptane. The average pore sizes deduced by PALS from the longest-lived o-Ps lifetimes are in good agreement with those by EP on the basis of the Barrett–Joyner–Halenda model, as indicated by a well fitted line of slope k = 1. The results indicate that the EP combined with heptane adsorption is a useful method with high sensitivity for calibrating the mesopore size in highly interconnected mesoporous films, whereas PALS is a novel, complementary tool for characterizing both closed and open pores in them.

Analytical Study of Porous Organosilicate Glass Films Prepared from Mixtures of 1,3,5- and 1,3-Alkoxysilylbenzenes

Organosilicate glass (OSG)-based porous low dielectric constant (low-k) films with different molar ratios of 1,3,5-tris(triethoxysilyl)benzene to 1,3-bis(triethoxysilyl)benzene bridging organic groups (1:3 and 1:7) were spin-on deposited, followed by a soft bake in air and N₂ at 150 °C and hard bake in air and N₂ at 400 °C. Non-ionic template (Brij^®30) concentrations were varied from 0 to 41 wt% to control the porosity of the films. The chemical composition of the matrix of the films was evaluated and discussed with the shrinkage of the film during the curing, refractive indices, mechanical properties, k-values, porosity and pore structure. The chemical composition of the film cured in both air and N₂-containing ambient were evaluated and compared. The benzene bridging groups containing films change their porosity (0 to 43%) but keep the pore size constant and equal to 0.81 nm when porosity is lower than 30%. The k-value decreases with increasing porosity, as expected. The films containing benzene bridge have higher a Young’s modulus than plasma-enhanced chemical vapor deposition (PECVD) methyl-terminated low-k films with the same porosity and show good hydrophobic properties after a hard bake and close to the values reported for 1,4-benzene-bridged films. The fabricated films show good stability after a long time of storage. However, the improvement of mechanical properties was lower than the values predicted by the published literature data. It was concluded that the concentration of 1,3,5-benzene bridges was below the stiffness threshold required for significant improvement of the mechanical properties. The films show UV-induced luminescence with a photon energy of 3.6 to 4.3 eV. The luminescence is related to the presence of oxygen-deficient-type defects or their combination with organic residues. The most intensive luminescence is observed in as-deposited and soft bake samples, then the intensity is reduced after a hard bake. It is assumed that the oxygen-deficient centers form because of the presence of Si–OC₂H₅ groups in the films and the concentration of these centers reduces when all these groups completely transformed into siloxane (Si–O–Si).

Effects of Methyl Terminal and Carbon Bridging Groups Ratio on Critical Properties of Porous Organosilicate-Glass Films

Organosilicate glass-based porous low dielectic constant films with different ratios of terminal methyl to bridging organic (methylene, ethylene and 1,4-phenylene) groups are spin-on deposited by using a mixture of alkylenesiloxane with organic bridges and methyltrimethoxysilane, followed by soft baking at 120-200 °C and curing at 430 °C. The films' porosity was controlled by using sacrificial template Brij^® L4. Changes of the films' refractive indices, mechanical properties, k-values, porosity and pore structure versus chemical composition of the film's matrix are evaluated and compared with methyl-terminated low-k materials. The chemical resistance of the films to annealing in oxygen-containing atmosphere is evaluated by using density functional theory (DFT). It is found that the introduction of bridging groups changes their porosity and pore structure, increases Young's modulus, but the improvement of mechanical properties happens simultaneously with the increase in the refractive index and k-value. The 1,4-phenylene bridging groups have the strongest impact on the films' properties. Mechanisms of oxidative degradation of carbon bridges are studied and it is shown that 1,4-phenylene-bridged films have the highest stability. Methylene- and ethylene-bridged films are less stable but methylene-bridged films show slightly higher stability than ethylene-bridged films.

Structural Characterization of Mesoporous Thin Film Architectures: A Tutorial Overview

Mesoporous thin film architectures are an important class of materials that exhibit unique properties, which include high surface area, versatile surface functionalization, and bicontinuous percolation paths through a broad library of pore arrangements on the 10 nm length scale. Although porosimetry of bulk materials via sorption techniques is common practice, the characterization of thin mesoporous films with small sample volumes remains a challenge. A range of techniques are geared toward providing information over pore morphology, pore size distribution, surface area and overall porosity, but none of them offers a holistic evaluation and results are at times inconsistent. In this work, we present a tutorial overview for the reliable structural characterization of mesoporous films. Three model samples with variable pore size and porosity prepared by block copolymer (BCP) coassembly serve for a rational comparison. Various techniques are assessed side-by-side, including scanning electron microscopy (SEM), atomic force microscopy (AFM), grazing incidence small-angle X-ray scattering (GISAXS), and ellipsometric porosimetry (EP). We critically discuss advantages and limitations of each technique and provide guidelines for reliable implementation.

Photocatalytic Template Removal by Non-Ozone-Generating UV Irradiation for the Fabrication of Well-Defined Mesoporous Inorganic Coatings

The processing of mesoporous inorganic coatings typically requires a high-temperature calcination step to remove organic precursors that are essential during the material assembly. Lowering the fabrication energy costs and cutting back on the necessary resources would provide a greater scope for the deployment in applications such as architectural glass, optical components, photovoltaic cells, and energy storage, as well as further compatibilize substrates with low temperature stability. Organic removal methods based on UV-ozone treatment are increasing in popularity, but concerns remain regarding large-scale ozone generation and usage of mercury-containing UV lamps. To this end, we present a method that relies on non-ozone-generating UV radiation at 254 nm (UV₂₅₄) and incorporation of small amounts of photocatalytic material in the formulation, here demonstrated with TiO₂ nanocrystals. At concentrations as low as 5 wt % relative to the main inorganic aluminosilicate material, the TiO₂ nanocrystals catalyze a "cold combustion" of the organic components under UV₂₅₄ irradiation to reveal a porous inorganic network. Using block copolymer-based co-assembly in conjunction with photocatalytic template removal, we produce well-defined mesoporous inorganic thin films with controlled porosity and refractive index values, where the required processing time is governed by the amount of TiO₂ loading. This approach provides an inexpensive, flexible, and environmentally friendly alternative to traditional organic removal techniques, such as UV-ozone degradation and thermal calcination.

Robust Operation of Mesoporous Antireflective Coatings under Variable Ambient Conditions

Generating mesoporous films with adequate film thickness and refractive index is a common method to achieve amplitude and phase matching in low-cost interference-based antireflective coatings (ARCs). For high-surface-energy materials, pores on the 2-50 nm (i.e., on the subwavelength scale) are subject to capillary condensation by surrounding gas phase water molecules, which hampers their functioning. In this work, we examine the effect of relative humidity on mesoporous ARCs and present a simple method for the preparation of ARCs with robust operation under variable conditions. The materials route is based on the generation of well-defined porous aluminosilicate networks by block copolymer co-assembly with poly(isobutylene)- block-poly(ethylene oxide) and postsynthesis grafting of trichloro(octyl)silane molecules to the pore walls. The functionalized films exhibited a maximum transmittance value of 99.8%, with an average transmittance of 99.1% in the visible wavelength range from 400 to 700 nm. Crucially, the antireflection performance was maintained at high humidity values, with an average transmittance decrease of only 0.2% and maximum values maintained at 99.7%. This compared to maximum and average losses of 3.6 and 2.7%, respectively, for nonfunctionalized reference samples. The ARCs were shown to retain their optical properties within 50 humidity cycles, indicating long-term stability against fluctuating environmental conditions.

Low-Temperature Plasma-Assisted Atomic Layer Deposition of Silicon Nitride Moisture Permeation Barrier Layers

Encapsulation of organic (opto-)electronic devices, such as organic light-emitting diodes (OLEDs), photovoltaic cells, and field-effect transistors, is required to minimize device degradation induced by moisture and oxygen ingress. SiNx moisture permeation barriers have been fabricated using a very recently developed low-temperature plasma-assisted atomic layer deposition (ALD) approach, consisting of half-reactions of the substrate with the precursor SiH2(NH(t)Bu)2 and with N2-fed plasma. The deposited films have been characterized in terms of their refractive index and chemical composition by spectroscopic ellipsometry (SE), X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared spectroscopy (FTIR). The SiNx thin-film refractive index ranges from 1.80 to 1.90 for films deposited at 80 °C up to 200 °C, respectively, and the C, O, and H impurity levels decrease when the deposition temperature increases. The relative open porosity content of the layers has been studied by means of multisolvent ellipsometric porosimetry (EP), adopting three solvents with different kinetic diameters: water (∼0.3 nm), ethanol (∼0.4 nm), and toluene (∼0.6 nm). Irrespective of the deposition temperature, and hence the impurity content in the SiNx films, no uptake of any adsorptive has been observed, pointing to the absence of open pores larger than 0.3 nm in diameter. Instead, multilayer development has been observed, leading to type II isotherms that, according to the IUPAC classification, are characteristic of nonporous layers. The calcium test has been performed in a climate chamber at 20 °C and 50% relative humidity to determine the intrinsic water vapor transmission rate (WVTR) of SiNx barriers deposited at 120 °C. Intrinsic WVTR values in the range of 10(-6) g/m2/day indicate excellent barrier properties for ALD SiNx layers as thin as 10 nm, competing with that of state-of-the-art plasma-enhanced chemical vapor-deposited SiNx layers of a few hundred nanometers in thickness.