Growth and Atomic‐Scale Characterization of Ultrathin Silica and Germania Films: The Crucial Role of the Metal Support

The present review reports on the preparation and atomic‐scale characterization of the thinnest possible films of the glass‐forming materials silica and germania. To this end state‐of‐the‐art surface science techniques, in particular scanning probe microscopy, and density functional theory calculations have been employed. The investigated films range from monolayer to bilayer coverage where both, the crystalline and the amorphous films, contain characteristic XO4 (X=Si,Ge) building blocks. A side‐by‐side comparison of silica and germania monolayer, zigzag phase and bilayer films supported on Mo(112), Ru(0001), Pt(111), and Au(111) leads to a more general comprehension of the network structure of glass former materials. This allows us to understand the crucial role of the metal support for the pathway from crystalline to amorphous ultrathin film growth

Preparation of Silica Films on Ru 0001 a LEEM PEEM Study

We use an aberration corrected spectro-microscope, the low energy electron microscope/photoelectron emission microscope (LEEM/PEEM) SMART, to follow the preparation and structure of a bilayer silica film on Ru(0001) as a function of temperature and oxidation conditions. This allows us to analyze the growth process at different length scales in order to judge on the overall quality and the morphology of the film. It is found that the film growth occurs in a crystalline and a vitreous phase as previously discovered using scanning tunneling microscopy. However, the present experiment allows an analysis on the sub-micron level to gain insight into the growth process at a mesoscopic scale. We find that the fully oxidized film can be prepared but that this film contains holes. These are unavoidable and are important to consider, if one wants to use the films for ensemble averaging experiments to investigate migration and reaction of molecules between the silica film and the Ru(0001) substrate

Formation and Evolution of Ultrathin Silica Polymorphs on Ru 0001 Studied With Combined in Situ, Real Time Methods

Silica mono- and bilayer films on Ru(0001) can be physisorbed or chemisorbed, with ordered or vitreous structures, depending on the particular preparation procedures applied. Using the SMART spectro-microscope at BESSY-II with its capabilities for µ-spectroscopy, µ-diffraction, and LEEM imaging with lateral resolution below 5 nm, in situ and in real time and applied to identical areas, we have investigated the formation of these layers, defined and characterized their properties and their connected morphology, and followed their evolution. Two distinct chemisorbed monolayers and three bilayers (physisorbed crystalline and vitreous, and chemisorbed zigzag phases), and some transitions between them, have been studied. We found that, apart from the deposited silicon amount, the most important parameter for steering the evolution to a particular well-defined layer is the oxygen content at the Ru interface. Nucleation and growth of all layers are homogeneous on the scale of our resolution, leading to rather small domains (20 – 40 nm), mostly of the same phase, separated by defect lines. We discuss these and other basic findings in context and point out open questions. We also offer alternative recipes for the preparation of some phases, to obtain more homogeneous layers on a mesoscopic scale

Endocannabinoid Regulation in Human Endometrium Across the Menstrual Cycle

Humans produce endogenous cannabinoids (endocannabinoids), a group of molecules that activate the same receptors as tetrahydrocannabinol. Endocannabinoids play important roles in reproduction in multiple species, but data in human endometrium are limited. Because endocannabinoids such as anandamide (AEA) and 2-arachidonoyl glycerol (2-AG) often act within tissues as paracrine factors, their effects can be modulated by changes in expression of locally produced synthetic and degradative/oxidative enzymes. The objective of this study was to localize and quantify expression of these key synthetic and degradative/oxidative enzymes for AEA and 2-AG in human endometrium throughout the menstrual cycle. Key synthetic enzymes include N-arachidonyl-phosphatidylethanolamine phospholipase-D (NAPE-PLD), diacylglycerol-lipase a (DAGL-α, and DAGL-β. Key degradative enzymes include fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL); cyclooxygenase 2 (COX2) is an oxidative enzyme. Endometrial samples were collected in 49 regularly cycling, normal women. Protein localization and expression were achieved by immunohistochemistry and messenger RNA (mRNA) expression by real-time reverse transcriptase polymerase chain reaction. No significant cycle-dependent mRNA expression was observed except that of COX2 (P = .002), which demonstrated maximum expression in the proliferative phase. During the secretory phase, NAPE-PLD protein had increased expression in luminal (P = .001), stromal (P = .007), and glandular (P = .04) epithelia, while FAAH had increased glandular (P = .009) and luminal (P = .01) expression. Increased expression in glandular epithelia was identified for MAGL (P = .03). The COX2 had increased luminal expression during the early secretory phase (P < .0001). In conclusion, maximal expression of degradatory/oxidative enzymes in the secretory phase may foster decreased endocannabinoid tone during implantation

Growth and Atomic Scale Characterization of Ultrathin Silica and Germania Films The Crucial Role of the Metal Support

The present review reports on the preparation and atomic‐scale characterization of the thinnest possible films of the glass‐forming materials silica and germania. To this end state‐of‐the‐art surface science techniques, in particular scanning probe microscopy, and density functional theory calculations have been employed. The investigated films range from monolayer to bilayer coverage where both, the crystalline and the amorphous films, contain characteristic XO4 (X=Si,Ge) building blocks. A side‐by‐side comparison of silica and germania monolayer, zigzag phase and bilayer films supported on Mo(112), Ru(0001), Pt(111), and Au(111) leads to a more general comprehension of the network structure of glass former materials. This allows us to understand the crucial role of the metal support for the pathway from crystalline to amorphous ultrathin film growth

Spatially resolved insight into the chemical and electronic structure of solution processed perovskites why to not worry about pin holes

The unprecedented speed at which the performance of solar cells based on solution-processed perovskite thin films has increased, in some ways, appears to violate conventional understanding of device optimization. The relatively poor coverage of the TiO₂ electron transport layer by the absorber should cause shunting of the cell. This, however, is not the case. In this paper, it is attempted to explain this “discrepancy.” Insights into coverage, morphology, local elemental composition, and spatially resolved electronic structure of CH₃NH₃PbI_(3-x)Cl_x perovskite absorbers wet-chemically deposited on planar compact TiO₂ electron transport material (ETM) are revealed. Microscopy images indicate an incomplete coverage of the ETM. Depending on the degree of coverage, a variation in iodine oxidation and metallic lead formation is found. With the electronic structure of the absorber and the ETM established experimentally and taking literature on the commonly used hole transport material spiro-MeOTAD into account, it is revealed that excellent charge selectivity occurs at the interfaces between the absorber and both the hole and electron transport layers. It can also be surmised that, crucially, any direct interface between the TiO₂ and spiro-MeOTAD would be characterized by a large recombination barrier preventing shunts; to some extent minimizing the negative effects of absorber pinholes

A Two Dimensional Zigzag Silica Polymorph on a Metal Support

We present a new polymorph of the two-dimensional (2D) silica film with a characteristic ‘zigzag’ line structure and a rectangular unit cell which forms on a Ru(0001) metal substrate. This new silica polymorph may allow for important insights into growth modes and transformations of 2D silica films as a model system for the study of glass transitions. Based on scanning tunneling microscopy, low energy electron diffraction, infrared reflection absorption spectroscopy, and X-ray photoelectron spectroscopy measurements on the one hand, and density functional theory calculations on the other, a structural model for the ‘zigzag’ polymorph is proposed. In comparison to established monolayer and bilayer silica, this ‘zigzag’ structure system has intermediate characteristics in terms of coupling to the substrate and stoichiometry. The silica ‘zigzag’ phase is transformed upon reoxidation at higher annealing temperature into a SiO₂ silica bilayer film which is chemically decoupled from the substrate