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 Nb
2AlC-MAX using a wet electrochemical route under well-optimized conditions to obtain fine 2D-Nb
2C 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 Nb
2AlC-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 Nb
2C 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 Nb
2C. The present report provides a pathway to deeply understanding the existence of superconductivity-like diamagnetic behavior in Nb
2C MXene, which is useful for future magnetic applications.Transition metal carbides (TMCs) have recently emerged as competent members among the family of two-dimensional (2D) materials, owing to their promising applications.In two-dimensional (2D) materials, magnetism at the nanoscale is at the forefront of many cutting-edge technological applications, such as spintronic devices. Naguib
et al. have synthesized a new class of two-dimensional materials, known as MXenes (M
nX
n+1), discovered in 2011, giving a possibility of magnetism in such 2D materials and their promising uses in spintronic devices.
1 These 2D layer structured early transition metal carbides and/or nitrides are known as MXenes, so named to indicate their structural similarities with graphene.
2 MXenes are derived from 3D MAX phases (space group
P6
3/
mmc) in which “M” is an early transition metal,
e.g., Ti, Ta, V,
etc.; “A” is mainly a group 13 or group 14 element (group III-A or IV-A),
e.g., Si, Al; “X” is a carbide, nitride or can be both; and
n = 1, 2, 3 represents the number of layers, forming 211, 312 and 413 phases.
3–5 Over the past decade, this new material has gained enormous attention, thus developing an entirely new research field to disclose the properties of the 2D state of this material. The materials in the 2D regime own a cluster of astonishing physical properties as compared to the 3D nature, but intrinsic two-dimensional magnetism has proved to be challenging. As 2D semiconductors have revamped the field of electronics, similarly, magnetism in 2D materials could remodel spintronic devices that can employ a spin degree of freedom.
6,7Nb
2C MXene was first synthesized by Naguib
et al., but they just reported its electrochemical activity as a promising electrode material.
4 Further work has been reported in Nb
2C with various biomedical applications, energy storage, supercapacitors, and nanoelectronics.
9–19 As far as the magnetism in such 2D MXenes is concerned, it remains less investigated, and this research void needs to be filled. Recently, Babar
et al. reported the observation of superconductivity in as-prepared powdered Nb
2C for the first time, with the highest onset transition temperature
Tc,onset = 12.5 K among the MXene family. However, the authors did not discuss the magnetic nature of the parent Nb
2AlC MAX itself and did not reason for the presence of unusual magnetic effects in MXene.
8 MXenes are favorable members of 2D magnetism, and different magnetic natures are computationally predicted in various carbide and nitride MXenes.
7,20 The existence of novel room-temperature ferromagnetic order in doped MXene and the coexistence of different magnetic phases in MXene, along with experimental evidence, indicate its potential of hosting diverse magnetic natures.
21,22 Considerable research has been focused on these 2D structures due to their importance and favorable applications, such as spintronics. MXene could provide a vast platform for exploring the magnetic properties and is one of the best candidates that can host superconductivity as compared to other members of the 2D family. Experimental studies are generally dependent upon numerous variables, thus affecting the research pace. However, density functional theory-based first principles calculation and theoretical simulations are a successful way to examine and foresee the properties of low-dimensional materials. This provoked us to theoretically explore superconductivity in Nb
2C and their validation through superconductivity measurements of experimentally synthesized Nb
2C MXene. In this work, we report a systematic etching mechanism of Nb
2C MXene to obtain fine 2D sheets
via a chemical etching route using hydrofluoric (HF) acid. Structural and morphological studies using the X-ray diffraction technique (XRD), scanning electron microscopy (SEM) and elemental analysis by energy-dispersive X-ray spectroscopy (EDX) show the effective removal of Al from the parent 3D-Nb
2AlC MAX, thus revealing an accordion-like sheet structure. Optical analysis indicates a significant reduction in bandgap after chemical etching. Magnetic properties were carried out to observe the signatures of superconductivity (a perfect diamagnetic state, negative magnetic moment) and its magnetic nature at room temperature. To study the magnetic nature of as-prepared powder-form Nb
2C, density functional theory (DFT) first principles calculation was carried out through Wein2K using spin-polarized generalized gradient approximation (sp-GGA). The magnetic moment of the compound is calculated to be −0.00485, which although but small is important, as the value is negative, which is an indication of the presence of diamagnetism in Nb
2C. Here, the detailed chemical etching process, magnetic properties of Nb
2AlC MAX and its effect on magnetic phase of Nb
2C MXene, and the density functional theory calculation are reported, which were not discussed by Babar
et al. in
ref. 8.
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