Exploring a novel class of Janus MXenes by first principles calculations: structural, electronic and magnetic properties of Sc2CXT, X = O, F, OH; T = C, S, N

The already intriguing electronic and optical properties of the MXene Sc₂C family can be further tuned through a wide range of possible functionalizations. Here, by means of density functional theory, we show that the 36 possible elements of the Janus MXT (M: Sc₂C, X: O, F, OH, T: C, N, S) family, built by considering the four possible structural models (i) FCC, (ii) HCP, (iii) FCC + HCP, and (iv) HCP + FCC, are all potentially stable. The analysis of their mechanical properties shows the excellent mechanical flexibility of functionalized MXenes (f-MXenes) under large strain, making them more suitable for applications where stress could be an issue. Interestingly, while Sc₂C presents a metallic character, Sc₂COS, Sc₂CFN and Sc₂COHN are found to be semiconductors with bandgaps of 2.5 eV (indirect), 1.67 eV (indirect) and 1.1 eV (direct), respectively, which presents promising applications for nano- and optoelectronics. Moreover, Sc₂CFC presents a ferromagnetic ground state with the 2 × 2 × 1 supercell magnetic moment of 3.99 μ_B, while the ground state of Sc₂COHC might be antiferromagnetic with a magnetic moment of 3.98 μ_B, depending on the environment. Remarkably, the band structures of Sc₂CFC and Sc₂COHC present a half-metallic character with an HSE06 fundamental band gap of 0.60 eV and 0.48 eV, respectively. Our results confirm the extraordinary potential of the Janus MXT (M: Sc₂C, X: O, F, OH, T: C, N, S) family for novel applications in 2D nano-,opto- and spintronics.

Mini-review of interesting properties in Mn2CoAl bulk and films

Heusler compounds exhibit many interesting properties, such as high thermopower, magnetocaloric properties, and even topological insulator states. Heusler Mn₂CoAl alloy has been experimentally and theoretically proposed as a promising spin-gapless semiconductor with novel electronic, magnetic, spintronic, transport, and topological properties. Furthermore, the spin-gapless semiconducting-like behaviors are also predicted in Mn₂CoAl films by measuring the transport and magnetic properties. This mini-review systematically summarizes the interesting properties of Mn₂CoAl bulk and Mn₂CoAl-based films. This mini-review is hoped to guide further experimental investigations and applications in the particular scientific community.

Optoelectronic and photocatalytic applications of hBP-XMY (M = Mo, W; (X ≠ Y) = S, Se, Te) van der Waals heterostructures

Stacking of layers via weak van der Waals interactions is an important technique for tuning the physical properties and designing viable electronic products. Using first-principles calculations, the geometry, electronic structure, and optical and photocatalytic performance of novel vdW heterostructures based on hexagonal boron phosphide (hBP) and Janus (XMY (M = Mo, W; (X ≠ Y) = S, Se, Te)) monolayers are investigated. Favorable (dynamically and energetically) stacking patterns of two different models of hBP-XMY heterostructures are presented with an alternative order of chalcogen atoms at opposite surfaces in SMSe. A direct type-II band alignment is obtained in both models of hBP-SMoSe, hBP-SWSe and hBP-SeWTe, while the rest are type-II indirect bandgap semiconductors. The Bader charge, and planer-averaged and plane-averaged charge density differences are investigated, which show that hBP donates electrons to the SMoSe and SWSe layer in the hBP-SMoSe and hBP-SWSe vdW heterostructure, while in the case of the hBP-SMoTe (hBP-SWTe) and hBP-SeMoTe (hBP-SeWTe) vdW heterostructures, the transfer of electrons is observed from SMoTe (SWTe) and SeMoTe (SeWTe) to hBP. The imaginary part of the dielectric function shows that the lowest energy transitions are dominated by excitons with a systematic red shift for heavier chalcogen atoms. Furthermore, the photocatalytic performance indicates that the hBP-XMY (M = Mo, W; (X ≠ Y) = S, Se, Te) vdW heterostructures in model-I are suitable for water splitting at pH = 0.

Two-Dimensional Hydroxyl-Functionalized and Carbon-Deficient Scandium Carbide, ScC xOH, a Direct Band Gap Semiconductor

Two-dimensional (2D) materials have attracted intense attention in nanoscience and nanotechnology due to their outstanding properties. Among these materials, the emerging family of 2D transition metal carbides, carbonitrides, and nitrides (referred to as MXenes) stands out because of the vast available chemical space for tuning materials chemistry and surface termination, offering opportunities for property tailoring. Specifically, semiconducting properties are needed to enable utilization in optoelectronics, but direct band gaps are experimentally challenging to achieve in these 2D carbides. Here, we demonstrate the fabrication of 2D hydroxyl-functionalized and carbon-deficient scandium carbide, namely, ScC _xOH, by selective etching of a layered parent ScAl₃C₃ compound. The 2D configuration is determined as a direct band gap semiconductor, with an experimentally measured band gap approximated at 2.5 eV. Furthermore, this ScC _xOH-based device exhibits excellent photoresponse in the ultraviolet-visible light region (responsivity of 0.125 A/W at 360 nm/10 V, and quantum efficiency of 43%). Thus, this 2D ScC _xOH direct band gap semiconductor may find applications in visible light detectors, photocatalytic chemistry, and optoelectronic devices.

MXene/Graphene Heterostructures as High-Performance Electrodes for Li-Ion Batteries

Recently, MXene/graphene heterostructures have been successfully fabricated and found to exhibit outstanding performance as electrodes for Li-ion batteries. However, insights into the mechanism behind the encouraging experimental results are missing. We use first-principles calculations to systematically investigate the electrochemical properties of MXene/graphene heterostructures, choosing Ti₂CX₂ (X = F, O, and OH) as representative MXenes. Our calculations disclose that the presence of graphene not only avoids restacking effects of MXene layers but also enhances the electric conductivity, Li adsorption strength (while maintaining a high Li mobility), and mechanical stiffness. These favorable attributes collectively lead to the excellent performance of MXene/graphene electrodes observed experimentally. While the Ti₂CO₂/graphene heterostructure is proposed to be the most promising candidate within the studied materials, the developed comprehensive understanding is of significance also for the future rational design of MXene-based electrodes.

First-principles study on the electrical and thermal properties of the semiconducting Sc3(CN)F2 MXene

The two-dimensional materials MXenes have recently attracted interest for their excellent performance from diverse perspectives indicated by experiments and theoretical calculations. For the application of MXenes in electronic devices, the exploration of semiconducting MXenes arouses particular interest. In this work, despite the metallic properties of Sc₃C₂F₂ and Sc₃N₂F₂, we find that Sc₃(CN)F₂ is a semiconductor with an indirect band gap of 1.18 eV, which is an expansion of the semiconducting family members of MXene. Using first-principles calculations, the electrical and thermal properties of the semiconducting Sc₃(CN)F₂ MXene are studied. The electron mobilities are determined to possess strong anisotropy, while the hole mobilities show isotropy, i.e. 1.348 × 10³ cm² V⁻¹ s⁻¹ along x, 0.319 × 10³ cm² V⁻¹ s⁻¹ along the y directions for electron mobilities, and 0.517 × 10³ cm² V⁻¹ s⁻¹ along x, 0.540 × 10³ cm² V⁻¹ s⁻¹ along the y directions for hole mobilities. The room-temperature thermal conductivity along the Γ → M direction is determined to be 123–283 W m⁻¹ K⁻¹ with a flake length of 1–100 μm. Besides, Sc₃(CN)F₂ presents a relatively high specific heat of 547 J kg⁻¹ K⁻¹ and a low thermal expansion coefficient of 8.703 × 10⁻⁶ K⁻¹. Our findings suggest that the Sc₃(CN)F₂ MXene might be a candidate material in the design and application of 2D nanoelectronic devices.

The two-dimensional semiconducting Sc₃(CN)F₂ MXene presents relatively high carrier mobilities, specific heat and low thermal expansion coefficient from DFT calculations, and produces a good application prospect for nanoelectronic devices.