Saleem Ayaz Khan

Although the binary alkali metal thallides ATl with A= Li, Na, K, and Cs have been reported in literature, binary RbTl at ambient pressure is still missing. Experiments according to a 1:1 ratio of Rb:Tl, either according to Zintl’s procedure in low temperature experiments in liquid ammonia or classical solid state synthesis at high temperature, did not result in the desired product. Therefore, several ternary compositions with a mixture of K/Rb and Cs/Rb have been prepared. For K/Rb mixtures a solid solution in the KTl structure type up to a proportion of 69% rubidium could be obtained. Site occupancy preferences for rubidium on the alkali metal sites in the KTl type are obtained in experiment and supported by theory. In contrast to Rb/K mixtures being realizable in the KTl structure type, Rb/Cs mixtures did not allow for the isolation of materials according to the CsTl structure type. Instead, two new monoclinic compounds could be isolated (Cs0.82Rb0.18Tl: C2/c, a=14.4136(4) Å, b=11.1678(3) Å, c=40.8013(11) Å, β=96.353(2) °, V=6527.4(3) ų; Cs0.58Rb0.42Tl: C2/c, a=14.2610(3) Å, b=11.1116(2) Å, c=27.5589(7) Å, β=104.056(2) °, V=4236.30(17) ų). Detailed DFT calculations on both, binary and mixed cation systems were performed and support the experimental results.

β‐Si_6−zAl_zO_zN_8−z is a prominent example of systems suitable as hosts for creating materials for light‐emitting diodes (LEDs). In this work, the electronic structure of a series of semiordered and disordered β‐Si_6−zAl_zO_zN_8−z systems is investigated by means of ab initio calculations, using the FLAPW and Green function KKR methods. Finite temperature effects are included by averaging over thermodynamic configurations within the alloy analogy model. We found that the dependence of the electronic structure on the (Al,O) concentration z is similar for semiordered and disordered structures. The electronic band gap decreases with increasing z by about 1.5 eV when going from z=0 to z=2. States at the top of the valence band are mostly associated with N atoms whereas the states at the bottom of the conduction band are mostly derived from O atoms. Increasing the temperature leads to a shift of the bottom of the conduction band to lower energies. The amount of this shift increases with increasing z.

Owing to the tremendous energy storage capacity of two-dimensional transition metal carbides (MXenes), they have been efficiently utilized as a promising candidate in the field of super-capacitors. The energy storage capacity of MXenes can be further enhanced using metal dopants. Herein, we have reported the synthesis of pristine and nickel doped niobium-carbide (Nb₂C) MXenes, their computational and electrochemical properties. Upon introduction of nickel (Ni) the TDOS increases and a continuous DOS pattern is observed which indicates coupling between Ni and pristine MXene. The alterations in the DOS, predominantly in the nearby region of the Fermi level are profitable for our electrochemical applications. Additionally, the Ni-doped sample shows a significant capacitive performance of 666.67 F g⁻¹ which can be attributed to the additional active sites generated by doping with Ni. It is worth noting that doped MXenes exhibited a capacitance retention of 81% up to 10 000 cycles. The current study unveils the opportunities of using MXenes with different metal dopants and hypothesize on their performance for energy storage devices.

This work reports experimental and computational magnetic phase transition from superconducting-diamagnet to ferromagnet in lanthanum (La)-doped functionalized Nb ₂ C MXene. Co-precipitation method is used to synthesize La-doped Nb ₂ C MXene. Structure and morphology of the compound are studied through x-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy and energy dispersion spectroscopy, confirming the successful doping of La while retaining the two-dimensional (2D) structure of MXene. The magnetic properties of doped sample are studied using field-cooled and zero-field-cooled curves as well as from magnetization ( M ) versus applied magnetic field ( H ) graphs. Contrary to the superconductivity-like diamagnetic behavior in pristine Nb ₂ C MXene, the La-doped MXene converts the diamagnetism into the ferromagnetic (FM) phases at all temperatures. The ferromagnetism arises due to the pinning of magnetic spins pinned by Lanthanum itself. The computational analysis of pristine Nb ₂ C MXene confirms its diamagnetic behavior and further clarifies the role of La and functional groups (O and F) in the reduction of diamagnetic behavior in La-doped Nb ₂ C MXene while inducing FM nature. This work provides an interesting superconducting-diamagnetic to FM transition with a possibility of its implementation in 2D spintronics.

The arrangement of B atoms in a doped Si(1 1 1)- :B system was studied using a near-edge x-ray absorption fine structure (NEXAFS). Boron atoms were deposited via segregation from the bulk by flashing the sample repeatedly. The positions of B atoms are determined by comparing measured polarized (angle-dependent) NEXAFS spectra with spectra calculated for various structural models based on ab initio total energy calculations. It is found that most of boron atoms are located in sub-surface L positions, beneath a Si atom. However, depending on the preparation method a significant portion of B atoms may be located elsewhere. A possible location of these non-L -atoms is at the surface, next to those Si atoms which form the reconstruction.

Cd-doped ZnO nanosheet (ZnO NS) were investigated using a full-potential linearized augmented plane wave method within the generalized gradient approximation (GGA) to calculate the electronic structure and its optical response. The calculated band structures have shown that the Cd-doped ZnO NS is a direct band gap semiconductor at Γ with 1.50 eV band gap. The contribution of each atom/orbital were commented in light of total and partial densities of states. We also derived the optical constants (mainly the dielectric constants ε₁(0) and ε₂(0)), the absorption coefficient I(ω), refractive index n(ω), extinction coefficient k(ω), and energy-loss function L(ω). The spectrum of absorption coefficient has revealed to increase rapidly for photon energies higher than 2.5 eV. The absorption spectrum was found to be limited in energy region due to different contributions electronic transitions that occurred within ZnO NS and effect of Cd doping. Reducing the band gap of ZnO NS to low values is suitable process for light-emitting devices and solar cells applications.