In this study, we employ a combination of various in-situ surface analysis techniques to investigate the thermally induced degradation processes in MAPbI3 perovskite solar cells (PeSCs) as a function of temperature under air-free conditions (no moisture and oxygen). Through a comprehensive approach that combines in-situ grazing-incidence wide-angle X-ray diffraction (GIWAXD) and high-resolution X-ray photoelectron spectroscopy (HR-XPS) measurements, we confirm that the surface structure of MAPbI3 perovskite film changes to an intermediate phase and decomposes to CH3I, NH3, and PbI2 after both a short (20 min) exposure to heat stress at 100 °C and a long exposure (>1 hour) at 80 °C. Moreover, we observe clearly the changes in the orientation of CH3NH3
+ organic cations with respect to the substrate in the intermediate phase, which might be linked directly to the thermal degradation processes in MAPbI3 perovskites. These results provide important progress towards improved understanding of the thermal degradation mechanisms in perovskite materials and will facilitate improvements in the design and fabrication of perovskite solar cells with better thermal stability.
Graphene
nanoribbons (GNRs) have recently emerged as alternative
2D semiconductors owing to their fascinating electronic properties
that include tunable band gaps and high charge-carrier mobilities.
Identifying the atomic-scale edge structures of GNRs through structural
investigations is very important to fully understand the electronic
properties of these materials. Herein, we report an atomic-scale analysis
of GNRs using simulated X-ray photoelectron spectroscopy (XPS) and
Raman spectroscopy. Tetracene with zigzag edges and chrysene with
armchair edges were selected as initial model structures, and their
XPS and Raman spectra were analyzed. Structurally expanded nanoribbons
based on tetracene and chrysene, in which zigzag and armchair edges
were combined in various ratios, were then simulated. The edge structures
of chain-shaped nanoribbons composed only of either zigzag edges or
armchair edges were distinguishable by XPS and Raman spectroscopy,
depending on the edge type. It was also possible to distinguish planar
nanoribbons consisting of both zigzag and armchair edges with zigzag/armchair
ratios of 4:1 or 1:4, indicating that it is possible to analyze normally
synthesized GNRs because their zigzag to armchair edge ratios are
usually greater than 4 or less than 0.25. Our study on the precise
identification of GNR edge structures by XPS and Raman spectroscopy
provides the groundwork for the analysis of GNRs.
We report a spectroscopic indicator showing the bending of a chemical vapor deposition (CVD) graphene monolayer on Cu foil or an arbitrary substrate after transfer. Using a Au nanoparticle (NP)-graphene monolayer-Au thin film (TF) junction system, the Radial Breathing-Like Mode (RBLM) Raman signal from the sandwiched graphene monolayer is evidently observed by employing a local z-polarized incident field formed at the Au NP-Au TF junction. We also utilized the RBLM intensity as a quantitative tool with a wide dynamic range (∼300%) compared to the 2D peak width (∼35%) for determining the relative degree of bending on the Au TF substrate. The RBLM signal from the CVD graphene monolayer is anticipated to be used as a valuable marker in exploring out-of-plane directional properties.
The tungsten oxide covered tungsten (W) tip of a scanning tunneling microscope was found to act as a catalyst to catalyze the S-H dissociative adsorption of phenylthiol and 1-octanethiol molecules onto a Ge(100) surface. By varying the distance between the tip and the surface, the area of the tip-catalyzed adsorption could be controlled. We have found that the thiol headgroup is the critical functional group for this catalysis and the catalytic material is the tungsten oxide layer of the tip. This local tip-catalyzed adsorption may be used in positive lithographic methods to produce nanoscale patterning on semiconductor substrates.
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