Meissner to ferromagnetic phase transition in La-decorated functionalized Nb2C MXene: an experimental and computational analysis.

This work reports experimental and computational magnetic phase transition from superconducting-diamagnet to ferromagnet in lanthanum (La)-doped functionalized Nb2C MXene. Co-precipitation method is used to synthesize La-doped Nb2C 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 Nb2C 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 Nb2C 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 Nb2C MXene while inducing FM nature. This work provides an interesting superconducting-diamagnetic to FM transition with a possibility of its implementation in 2D spintronics.

Volatile and Nonvolatile Resistive Switching Coexistence in Conductive Point Hexagonal Boron Nitride Monolayer.

Recently, we demonstrated the nonvolatile resistive switching effects of metal-insulator-metal (MIM) atomristor structures based on two-dimensional (2D) monolayers. However, there are many remaining combinations between 2D monolayers and metal electrodes; hence, there is a need to further explore 2D resistance switching devices from material selections to future perspectives. This study investigated the volatile and nonvolatile switching coexistence of monolayer hexagonal boron nitride (h-BN) atomristors using top and bottom silver (Ag) metal electrodes. Utilizing an h-BN monolayer and Ag electrodes, we found that the transition between volatile and nonvolatile switching is attributed to the thickness/stiffness of chain-like conductive bridges between h-BN and Ag surfaces based on the current compliance and atomristor area. Computations indicate a "weak" bridge is responsible for volatile switching, while a "strong" bridge is formed for nonvolatile switching. The current compliance determines the number of Ag atoms that undergo dissociation at the electrode, while the atomristor area determines the degree of electric field localization that forms more stable conductive bridges. The findings of this study suggest that the h-BN atomristor using Ag electrodes shows promise as a potential solution to integrate both volatile neurons and nonvolatile synapses in a single neuromorphic crossbar array structure through electrical and dimensional designs.

A comprehensive investigation of MoO3 based resistive random access memory.

The bipolar resistive switching of molybdenum oxide is deliberated while molybdenum and nickel are used as bottom and top electrodes, respectively, to present a device with resistive random access memory (RRAM) characteristics. For the trilayered structure, the SET voltage lies around 3.3 V and RESET voltage is observed to be in the -2.3 V to -2.7 V range. The conduction mechanism has been observed and revealed for the Metal-Insulator-Metal (MIM) structure which is a space-charge-limited current mechanism that follows both ohmic conduction and Child's law. Furthermore, a theoretical study has been performed by using density functional theory (DFT) to evaluate the resistance switching role of molybdenum oxide (MoO3). The structure has been studied with oxygen vacancy sites induced into the system which shows the reduction in bandgap, whereas an indirect bandgap of 1.9 eV and a direct bandgap of 3.1 eV are calculated for molybdenum oxide. Conclusively, the formation of a conduction filament which is fundamental for resistive switching has been explained through band structure and density of states per eV for oxygen vacancy structures of molybdenum oxide. The current work presents an in-depth understanding of the resistive switching mechanism involved in MoO3 based resistive random access memory devices for future data storage applications.

Magnetic phase transition from paramagnetic in Nb2AlC-MAX to superconductivity-like diamagnetic in Nb2C-MXene: an experimental and computational analysis.

Transition metal carbides (TMCs) have recently emerged as competent members among the family of two-dimensional (2D) materials, owing to their promising applications. There are many promising applications of MXenes; however, their magnetic properties lack a wide margin, both experimentally as well as theoretically, which needs to be investigated for potential use in spintronics. In this study, we carried out a comprehensive etching process via selective extraction of Al layers from Nb2AlC-MAX using a wet electrochemical route under well-optimized conditions to obtain fine 2D-Nb2C MXene sheets. Structural analysis using X-ray diffraction (XRD) confirms the effective removal of Al followed by confirmation of a 2D layered structure from morphological analysis using scanning electron microscopy (SEM). Zero-field-cooled (ZFC) and field-cooled (FC) measurements of MAX and MXene at different field strengths were performed using a superconducting quantum interference device (SQUID). Magnetic measurements reveal the paramagnetic nature of Nb2AlC-MAX measured under 5 mT; however, this changes to a clear superconductor-like diamagnetic behavior with a shift of the magnetization from positive to negative values at low temperatures when measured under 5 mT and 10 mT for Nb2C MXene. The diamagnetism, however, is changed to paramagnetism at 100 mT, which shows the existence of critical fields known typically for a type-II superconductor. To gain an insight into this unusual behavior in MXene, density functional theory (DFT) first-principles calculation was also performed in Wein2K software using spin-polarized generalized gradient approximation (sp-GGA). The magnetic moment of the compound is calculated to be negative, which corresponds well with the experimental finding and suggests that the negative magnetic moment originated from the d-orbital of Nb2C. The present report provides a pathway to deeply understanding the existence of superconductivity-like diamagnetic behavior in Nb2C MXene, which is useful for future magnetic applications.

Nb-Doped MXene With Enhanced Energy Storage Capacity and Stability.

MXenes present unique features as materials for energy storage; however, limited interlayer distance, and structural stability with ongoing cycling limit their applications. Here, we have developed a unique method involving incorporating Nb atoms into MXene (Ti3C2) to enhance its ability to achieve higher ionic storage and longer stability. Computational analysis using density functional theory was performed that explained the material structure, electronic structure, band structure, and density of states in atomistic detail. Nb-doped MXene showed a good charge storage capacity of 442.7 F/g, which makes it applicable in a supercapacitor. X-ray diffraction (XRD) indicated c-lattice parameter enhancement after Nb-doping in MXene (from 19.2A° to 23.4A°), which showed the effect of the introduction of an element with a larger ionic radius (Nb). Also, the bandgap changes from 0.9 eV for pristine MXene to 0.1 eV for Nb-doped MXene, which indicates that the latter has the signature of increased conductivity due to more metallic nature, in support of the experimental results. This work presents not only the effect of doping in MXene but also helps to explain the phenomena involved in changes in physical parameters, advancing the field of energy storage based on 2D materials.