Defect Engineering of Two-Dimensional Molybdenum Disulfide

Two-dimensional (2D) molybdenum disulfide (MoS₂) holds great promise in electronic and optoelectronic applications owing to its unique structure and intriguing properties. The intrinsic defects such as sulfur vacancies (SVs) of MoS₂ nanosheets are found to be detrimental to the device efficiency. To mitigate this problem, functionalization of 2D MoS₂ using thiols has emerged as one of the key strategies for engineering defects. Herein, we demonstrate an approach to controllably engineer the SVs of chemically exfoliated MoS₂ nanosheets using a series of substituted thiophenols in solution. The degree of functionalization can be tuned by varying the electron-withdrawing strength of substituents in thiophenols. We find that the intensity of 2LA(M) peak normalized to A_1g peak strongly correlates to the degree of functionalization. Our results provide a spectroscopic indicator to monitor and quantify the defect engineering process. This method of MoS₂ defect functionalization in solution also benefits the further exploration of defect-free MoS₂ for a wide range of applications.     

The use of magnetic fields in vertical Bridgman/Gradient Freeze-type crystal growth

This paper outlines advanced vertical Bridgman/Gradient Freeze techniques with flow control using magnetic fields developed for the growth of semiconductor crystals. Low-temperature flow modelling, as well as laboratory-scaled crystal growth under the influence of rotating, travelling, and static magnetic fields are presented. Experimental and numerical flow modelling demonstrate the potential of the magnetic fields to establish a well-defined flow for tailoring heat and mass transfer in the melt during growth. The results of the growth experiments are discussed with a focus on the influence of a rotating field on the segregation of dopants, the influence of a travelling field on the temperature field and thermal stresses, and the potential of rotating and static fields for a stabilization of the melt flow.     

Calibration of DFT Functionals for the Prediction of Fe Mössbauer Spectral Parameters in Iron-Nitrosyl and Iron-Sulfur Complexes: Accurate Geometries Prove Essential

Six popular density functionals in conjunction with the conductor-like screening (COSMO) solvation model have been used to obtain linear M?ssbauer isomer shift (IS) and quadrupole splitting (QS) parameters for a test set of 20 complexes (with 24 sites) comprised of nonheme nitrosyls (Fe-NO) and non-nitrosyl (Fe-S) complexes. For the first time in an IS analysis, the Fe electron density was calculated both directly at the nucleus, ρ(0)(N), which is the typical procedure, and on a small sphere surrounding the nucleus, ρ(0)(S), which is the new standard algorithm implemented in the ADF software package. We find that both methods yield (near) identical slopes from each linear regression analysis but are shifted with respect to ρ(0) along the x-axis. Therefore, the calculation of the Fe electron density with either method gives calibration fits with equal predictive value. Calibration parameters obtained from the complete test set for OLYP, OPBE, PW91, and BP86 yield correlation coefficients (r(2)) of approximately 0.90, indicating that the calibration fit is of good quality. However, fits obtained from B3LYP and B3LYP* with both Slater-type and Gaussian-type orbitals are generally found to be of poorer quality. For several of the complexes examined in this study, we find that B3LYP and B3LYP* give geometries that possess significantly larger deviations from the experimental structures than OLYP, OPBE, PW91 or BP86. This phenomenon is particularly true for the di- and tetranuclear Fe complexes examined in this study. Previous M?ssbauer calibration fit studies using these functionals have usually included mononuclear Fe complexes alone, where these discrepancies are less pronounced. An examination of spin expectation values reveals B3LYP and B3LYP* approach the weak-coupling limit more closely than the GGA exchange-correlation functionals. The high degree of variability in our calculated S(2) values for the Fe-NO complexes highlights their challenging electronic structure. Significant improvements to the isomer shift calibrations are obtained for B3LYP and B3LYP* when geometries obtained with the OLYP functional are used. In addition, greatly improved performance of these functionals is found if the complete test set is grouped separately into Fe-NO and Fe-S complexes. Calibration fits including only Fe-NO complexes are found to be excellent, while those containing the non-nitrosyl Fe-S complexes alone are found to demonstrate less accurate correlations. Similar trends are also found with OLYP, OPBE, PW91, and BP86. Correlations between experimental and calculated QSs were also investigated. Generally, universal and separate Fe-NO and Fe-S fit parameters obtained to determine QSs are found to be of good to excellent quality for every density functional examined, especially if [Fe(4)(NO)(4)(μ(3)-S)(4)](-) is removed from the test set.     

Cobalt–Pincer Complexes in Catalysis

Non-noble metal catalysts based on pincer type compounds are of special interest for organometallic chemistry and organic synthesis. Next to iron and manganese, currently cobalt–pincer type complexes are successfully applied in various catalytic reactions. In this review the recent progress in (de)hydrogenation, transfer hydrogenation, hydroboration and hydrosilylation as well as dehydrogenative coupling reactions using cobalt–pincer complexes is summarised.