Systematic investigations of the electron, phonon and elastic properties of monolayer M2C (M = V, Nb, Ta) by first-principles calculations

Endogenous Nb2CTx/Nb2O5 Schottky heterostructures for superior lithium-ion storage

Schottky heterostructures have significant advantages for exciting charge transfer kinetics at material interfaces. In this work, endogenous Nb2CTx/Nb2O5 Schottky heterostructures with a large active surface area were constructed using an in-situ architectural strategy. The semiconductor Nb2O5 has a low work function, and during the construction of Nb2CTx/Nb2O5 Schottky heterostructures, there was an interfacial electron transfer, which resulted in a built-in electric field. The electrochemical reaction kinetics of Nb2CTx/Nb2O5 Schottky heterostructures were enhanced due to the rapid transfer of charge driven by the electric field. The Nb2CTx/Nb2O5 Schottky heterostructures have a large active surface area, which contributes to excellent electrolyte diffusion kinetics. Therefore, Nb2CTx/Nb2O5 Schottky heterostructures have excellent lithium-ion storage capacity with 575 mAh/g after 200 cycles at 0.10 A/g, and 290 mAh/g after 1000 cycles at 2.00 A/g, without capacity fading. Furthermore, in-situ X-ray diffraction and ex-situ X-ray photoelectron spectroscopy analyses reveal the mechanisms for structure evolution and lithium-ion storage optimization of Nb2CTx/Nb2O5 Schottky heterostructures during the electrochemical reaction. The construction of Schottky heterostructures with excited charge transport kinetics provides a novel idea for optimizing the lithium-ion storage activity of MXenes materials.

Thermal Properties of MXenes and Relevant Applications

The properties and applications of MXenes (a family of layered transition metal carbides, nitrides, and carbonitrides) have aroused enormous research interests for a decade since the successful synthesis of few-layer transition metal carbides in 2011. Though MXenes, as the building blocks, have already been applied in various fields (such as wearable electronics) owing to the distinctive optical, mechanical and electrical properties, their thermal stability and intrinsic thermal properties were less thoroughly investigated compared to other characteristics in early reports. The pioneering theoretical prediction of the thermoelectric nature of MXenes was performed in 2013 while the first experiment-based report concerning the degradation behavior of the 2D structure at elevated temperatures in a controlled atmosphere was published in 2015, followed by numerous discoveries regarding the thermal properties of MXenes. Herein, after a brief description of the synthesis, this Review summarized the latest insights into the thermal stability and thermophysical properties of MXenes, and further associated these unique properties with relevant applications by multiple examples. Finally, current hurdles and challenges in this field were provided along with some advices on potential research directions in the future.

First-Principles Estimation of Core Level Shifts for Hf, Ta, W, and Re

A simple first-principles approach is used to estimate the core level shifts observed in X-ray photoelectron spectroscopy for the 4f electrons of Hf, Ta, W, and Re; these elements were selected because their 4f levels are relatively close to the Fermi energy. The approach is first tested by modeling the surface core level shifts of low-index surfaces of the four elemental metals, followed by its application to the well-studied material TaSe2 in the commensurate charge density wave (CDW) phase, where agreement with experimental data is found to be good, showing that this approach can yield insights into modifications of the CDW. Finally, unterminated surface core level shifts in the hypothetical MXene Ta3C2 are modeled, and the potential of XPS for the investigation of the surface termination of MXenes is demonstrated.

Giant two-photon absorption in MXene quantum dots

Looking for materials with compelling nonlinear optical (NLO) response is of great importance for next-generation nonlinear nanophotonics. We demonstrate an escalated two-photon absorption (TPA) in ultrasmall niobium carbide quantum dots (Nb₂C QDs) that is induced by a two-even-parity states transition. The TPA response of Nb₂C QDs was observed in the near-infrared band of 1064-1550 nm. Surprisingly, at 1064 nm, Nb₂C QDs shows an enhanced TPA response than other wavelengths with a nonlinear absorption coefficient up to a value of 0.52 ± 0.05 cm/GW. Additionally, the nonlinear optical response of Nb₂C changes to saturable absorption when the incident wavelength is between 400-800 nm wavelength. Density functional theory (DFT) validates that TPA, induced by two even-parity states transition, breaks the forbidden single-photon transition, enabling a tremendous TPA response in Nb₂C QDs at 1064 nm. It offers the possibility of manipulating the NLO response of Nb₂C via morphology or surface termination.

2D MXenes: Tunable Mechanical and Tribological Properties

2D transition metal carbides, nitrides, and carbonitrides, known as MXenes, were discovered in 2011 and have grown to prominence in energy storage, catalysis, electromagnetic interference shielding, wireless communications, electronic, sensors, and environmental and biomedical applications. In addition to their high electrical conductivity and electrochemically active behavior, MXenes' mechanical properties, flexibility, and strong adhesion properties play crucial roles in almost all of these growing applications. Although these properties prove to be critical in MXenes' impressive performance, the mechanical and tribological understanding of MXenes, as well as their relation to the synthesis process, is yet to be fully explored. Here, a fundamental overview of MXenes' mechanical and tribological properties is provided and the effects of MXenes' compositions, synthesis, and processing steps on these properties are discussed. Additionally, a critical perspective of the compositional control of MXenes for innovative structural, low-friction, and low-wear performance in current and upcoming applications of MXenes is provided. It is established here that the fundamental understanding of MXenes' mechanical and tribological behavior is essential for their quickly growing applications.

Niobium Carbide MXenes with Broad-Band Nonlinear Optical Response and Ultrafast Carrier Dynamics

Exploring the nonlinear photonics of emerging promising two-dimensional (2D) materials like MXenes will boost the development of broad-band optoelectronic and photonic applications. In this paper, the broad-band nonlinear optical response and the excited-carrier dynamics of an emerging MXene, Nb₂C, are systematically investigated for the wavelength range of visible to the near-infrared band. The obtained nonlinear optical response shows a wavelength and excitation intensity dependence. The imaginary part of the third-order nonlinear optical susceptibility Imχ⁽³⁾ and figure of merit were found to be -1.4 × 10^-10 esu and 7.5 × 10^-12 esu cm, respectively. The interesting nonlinear absorption response inversion properties (e.g., a shift from saturable absorption to two-photon absorption) of Nb₂C nanosheets in the near-infrared promise possible important applications in nonlinear photonics, such as an optical switch. We also demonstrate that the wavelength-dependent relaxation times consist of two different relaxation components, that is, time constants in which one is hundreds of femtoseconds and the other is several picoseconds. Our results indicate promising potential in near-infrared nanophotonic applications of 2D Nb₂C and offer a promising candidate for 2D-material-based nanophotonic devices and beyond.

2 D MXene-based Energy Storage Materials: Interfacial Structure Design and Functionalization

Transition metal carbides and/or nitrides (MXenes), a burgeoning group of 2 D layer-structure compounds, have multiple merits, such as high electrical conductivity, tunable layer structure, small band gap, and functionalized redox-active surface, and are receiving significant attention as one of the most promising class of energy storage materials. The synthesis methods, structural configuration, and surface chemistry of MXenes directly influence their performance. This Minireview focuses on interfacial structure design and functionalization of MXenes and MXene-based energy storage materials and the effect of structural configuration and surface chemistry on their electrochemical performance. Additionally, the structure-property relationships between interfacial structure, functional group, interlayer spacing, and the corresponding energy storage performance are summarized in detail. Finally, light is shed on the perspectives for the future research on advanced MXene-based energy storage materials including scientific and technical challenges.