Surface enhanced Raman spectroscopy (SERS) has been intensively investigated during the past decades for its enormous electromagnetic field enhancement near the nanoscale metallic surfaces. Chemical enhancement of SERS, however, remains rather elusive despite intensive research efforts, mainly due to the relatively complex enhancing factors and inconsistent experimental results. To study details of chemical enhancement mechanism, we prepared various low dimensional semiconductor substrates such as ZnO and GaN that were fabricated via metal organic chemical vapor deposition process. We used three kinds of molecules (4-MPY, 4-MBA, 4-ATP) as analytes to measure SERS spectra under non-plasmonic conditions to understand charge transfer mechanisms between a substrate and analyte molecules leading to chemical enhancement. We observed that there is a preferential route for charge transfer responsible for chemical enhancement, that is, there exists a dominant enhancement process in non-plasmonic SERS. To further confirm our idea of charge transfer mechanism, we used a combination of 2-dimensional transition metal dichalcogenide substrates and analyte molecules. We also observed significant enhancement of Raman signal from molecules adsorbed on 2-dimensional transition metal dichalcogenide surface that is completely consistent with our previous results. We also discuss crucial factors for increasing enhancement factors for chemical enhancement without involving plasmonic resonance.
Understanding the photodegradation mechanism of photoactive materials is critical for enhancing the long-term stability of organic photovoltaic cells (OPVs). However, definitive photodegradation mechanisms have not been reported yet. Here we report the comprehensive understanding of the photodegradation mechanism of the PTB7-Th polymer film. UV/vis absorption and photoluminescence spectra show that the π-conjugated backbone and the intermolecular π–π interactions are simultaneously broken under sunlight in air. Raman spectra reveal that the initial photooxidation begins at the thiophene ring in the benzo[1,2-b;3,3-b]dithiophene (BDT) unit, followed by the ring-opening of the thiophene and the break of the π-conjugated system. Infrared spectra indicate that −S–CO and −COOH groups are formed as a result of the photooxidation. On the basis of these observations, we propose that the thiophene ring in the BDT unit reacts with oxygen to generate the BDT–O2 adduct, which then produces thioester and carboxylic acid. The reaction sites in the BDT unit are consistent with the electrophilic attack positions of oxygen predicted by condensed Fukui functions. Furthermore, the DFT calculated spectrum of the proposed oxidation product agrees well with all the spectroscopic observations. Conclusively, the present work provides an important clue for understanding photodegradation of OPV materials.
Vertically-oriented two-dimensional (2D) tungsten disulfide (WS2) nanosheets were successfully grown on a Si substrate at a temperature range between and 550 °C via the direct chemical reaction between WCl6 and S in the gas phase. The growth process was carefully optimized by adjusting temperature, the locations of reactants and substrate, and carrier gas flow. Additionally, vertically-oriented 2D WS2 nanosheets with a few layers were tested as a surface-enhanced Raman scattering substrate for detecting rhodamine 6G (R6G) molecules where enhancement occurs from chemical enhancement by charge transfer transition from semiconductor). Raman spectra of R6G molecules adsorbed on vertically-oriented 2D WS2 nanosheets exhibited strong Raman enhancement effects up to 9.2 times greater than that on the exfoliated WS2 monolayer flake sample. From our results, we suggest that the WS2 nanosheets can be an effective surface-enhanced Raman scattering substrate for detecting target molecules.
Organic solar cells (OSCs) demonstrating high power conversion efficiencies have been mostly fabricated using halogenated solvents, which are highly toxic and harmful to humans and the environment. Recently, non-halogenated solvents have emerged as a potential alternative. However, there has been limited success in attaining an optimal morphology when non-halogenated solvents (typically o-xylene (XY)) were used. To address this issue, we studied the dependence of the photovoltaic properties of all-polymer solar cells (APSCs) on various high-boiling-point non-halogenated additives. We synthesized PTB7-Th and PNDI2HD-T polymers that are soluble in XY and fabricated PTB7-Th:PNDI2HD-T-based APSCs using XY with five additives: 1,2,4-trimethylbenzene (TMB), indane (IN), tetralin (TN), diphenyl ether (DPE), and dibenzyl ether (DBE). The photovoltaic performance was determined in the following order: XY + IN < XY + TMB < XY + DBE ≤ XY only < XY + DPE < XY + TN. Interestingly, all APSCs processed with an XY solvent system had better photovoltaic properties than APSCs processed with chloroform solution containing 1,8-diiodooctane (CF + DIO). The key reasons for these differences were unraveled using transient photovoltage and two-dimensional grazing incidence X-ray diffraction experiments. The charge lifetimes of APSCs based on XY + TN and XY + DPE were the longest, and their long lifetime was strongly associated with the polymer blend film morphology; the polymer domain sizes were in the nanoscale range, and the blend film surfaces were smoother, as the PTB7-Th polymer domains assumed an untangled, evenly distributed, and internetworked morphology. Our results demonstrate that the use of an additive with an optimal boiling point facilitates the development of polymer blends with a favorable morphology and can contribute to the widespread use of eco-friendly APSCs.
SUMMARYRecently, molten salt has received attention as a promising coolant for advanced nuclear reactors, especially for fluoride salt-cooled high-temperature reactor. The heat transfer characteristics of molten salt provide great advantages for application as a primary coolant, because of its superior performance in terms of sustainability, economics, safety, and reliability compared with gas coolant. However, understanding the thermal-hydraulic characteristics of molten salts by experimental method is difficult because of its high-temperature corrosion and toxicity issue. Therefore, oil fluids were introduced as simulants for studying the heat transfer phenomena of high Pr (Prandtl number) molten salts. In this study, a scaleddown experiment using simulant oil was conducted, and scaling laws were applied to investigate a single-phase natural circulation, which is important in nuclear reactors as a part of their passive safety. DOWTHERM RP (Diaryl Alkyl) was considered as a heat transfer simulant in this study because it matches the relevant dimensionless numbers (Prandtl number, Ra, Grashof number, Reynolds number, etc.) with those of molten salt. Prior to the experiment, the thermophysical properties of both the liquid and vapor phases of DOWTHERM oils were implemented into thermal-hydraulic system analysis code or multi-dimensional analysis of reactor safety code, to enable simulation and further study of the molten salts. Then, natural circulation experiments were conducted with the scaled rectangular loop, to establish similarity and experimental feasibility. For the validation, two different codes (multi-dimensional analysis of reactor safety and computational fluid dynamics (CFD) were used to simulate the same natural circulation loop. From the experimental data, new heat transfer correlation for a single-phase natural convection was developed, and the existing heat transfer correlations were compared. Copyright
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