Understanding the nature of surface states and their exchange gaps in magnetic topological insulator MnBi₂Te₄ (MBT) thin films is crucial for achieving robust topological Chern and axion insulating phases where the quantum anomalous Hall effect and topological magneto-electric effect can be realized. Here we focus on some rather unexplored features of possible surface reconstructions of interstitial-2H and peripheral-2H types, which are likely to occur in experiments. Using first-principles calculations together with molecular dynamics simulations accelerated by a machine learning force field, we demonstrate that interstitial-2H and peripheral-2H type atomic reconstructions play a crucial role in modifying the exchange gap and surface characteristics of MBT thin films, alongside previously reported factors such as surface magnetism, stacking configurations, and native defects. Moreover, these surface reconstructions have important implications for the topological indices and the nature of quasi-one-dimensional side-wall edge states dominating quantum transport. Specifically, the calculation of the energy landscape and barriers for the proposed surface reconstructions indicates that the interstitial-2H reconstruction is thermodynamically more stable than the peripheral-2H reconstruction. The latter case is also hypothesized as providing a plausible explanation for the Rashba surface states observed in angle-resolved photoemission spectroscopy measurements. Our analysis provides a theoretical framework to elucidate the nature and effect of reconstructions in MnBi₂Te₄ thin films, with predictions for the experimental realization of different topological phases.

Enabling highly efficient photocatalytic hydrogen production from solar-driven water splitting is of immense potential and environmental significance. However, the crucial issue of the low utilization efficiency of photogenerated charges in most photocatalysts, such as polymeric graphitic carbon nitride, g-C₃N₄ (CN), hampers the overall photocatalytic activity and hinders practical applications. To surmount this parasitic phenomenon, we develop a heterojunction-based strategy that improves the charge separation efficiency in CN. The heterostructure is constructed between thermally exfoliated CN and liquid phase exfoliated Bi₂O₂Se (BOS) via a solution-phase, electrostatically driven self-assembly process. The properly aligned band positions between the two components create a built-in electric field, which endows the composite with an enhanced charge separation efficiency. The optimized Pt-deposited heterostructure photocatalyst exhibits a hydrogen production rate of 6481 μmol h⁻¹ g⁻¹, and an apparent quantum efficiency of 11.65% at 420 nm, compared to those of Pt-deposited ECN (4595 μmol h⁻¹ g⁻¹, 6.64 %). We validate the efficient charge separation effect and the prolonged lifetime of photogenerated carriers in the heterostructure using a series of comprehensive characterizations across multiple timescales, thus, elucidating the origin of the observed photocatalytic activity. This demonstration offers valuable insights into improving the utilization efficiency of photogenerated charges for photocatalysis by heterostructure engineering with materials of distinct electronic configurations.

We report on the experimental realization of Pb_1-xSn_x Te pentagonal nanowires (NWs) with [110] orientation using molecular beam epitaxy techniques. Using first-principles calculations, we investigate the structural stability of NWs of SnTe and PbTe in three different structural phases: cubic, pentagonal with [001] orientation and pentagonal with [110] orientation. Within a semiclassical approach, we show that the interplay between ionic and covalent bonds favors the formation of pentagonal NWs. Additionally, we find that this pentagonal structure is more likely to occur in tellurides than in selenides. The disclination and twin boundary cause the electronic states originating from the NW core region to generate a conducting band connecting the valence and conduction bands, creating a symmetry-enforced metallic phase. The metallic core band has opposite slopes in the cases of Sn and Te twin boundaries, while the bands from the shell are insulating. We finally study the electronic and topological properties of pentagonal NWs unveiling their potential as a new platform for higher-order topology and fractional charge. These pentagonal NWs represent a unique case of intrinsic core-shell one-dimensional nanostructures with distinct structural, electronic and topological properties between the core and the shell region. (a) Scanning transmission electron microscopy image of a pentagonal nanowire; the inset shows the disclination and core chain (CC). The red bands from the core connect the valence and conduction bands for (b) cation and (c) anion twin-boundaries.

Two-dimensional (2D) materials with intrinsic antiferromagnetic (AFM) order provide a unique avenue to harness both charge and spin degrees of freedom for practical spintronics applications. Here, by using ab initio electronic structure calculations, the interplay of discrete crystal symmetries (such as inversion (P ), time-reversal (T ), or combined PT symmetry) of 2D semiconducting AFM manganese selenide (MnSe) and external electric field along with graphene proximity is investigated. We show that both an external electric field and graphene proximity can independently break otherwise conserved combined PT symmetry in 2D MnSe, resulting in large and tunable spin-splittings in both valence and conduction bands, and provide electrical control over a wide energy range. We further propose a current-in-plane electronic device consisting of semiconducting 2D MnSe as a channel material and graphene as a metal contact which preserves not only these features but additionally provides a mechanism to further tune metal-semiconductor contact characteristics such as Schottky barrier height leading to an Ohmic contact. Our results provide a comprehensive insight into the electrical control of the charge and spin degrees of freedom in 2D AFM MnSe.

Two-dimensional (2D) transition metal dichalcogenide (TMD) layers are highly promising as field-effect transistor (FET) channels in the atomic-scale limit. However, accomplishing this superiority in scaled-up FETs remains challenging due to their van der Waals (vdW) bonding nature with respect to conventional metal electrodes. Herein, we report a scalable approach to fabricate centimeter-scale all-2D FET arrays of platinum diselenide (PtSe2) with in-plane platinum ditelluride (PtTe2) edge contacts, mitigating the aforementioned challenges. We realized a reversible transition between semiconducting PtSe2 and metallic PtTe2 via a low-temperature anion exchange reaction compatible with the back-end-of-line (BEOL) processes. All-2D PtSe2 FETs seamlessly edge-contacted with transited metallic PtTe2 exhibited significant performance improvements compared to those with surface-contacted gold electrodes, e.g., an increase of carrier mobility and on/off ratio by over an order of magnitude, achieving a maximum hole mobility of similar to 50.30 cm(2) V-1 s(-1) at room temperature. This study opens up new opportunities toward atomically thin 2D-TMD-based circuitries with extraordinary functionalities.

To bridge the technology gap between IC-level and board-level fabrications, a fully additive selective metallization has already been demonstrated in the literature. In this article, the surface characterization of each step involved in the fabrication process is outlined with bulk metallization of the surface. This production technique has used polyurethane as epoxy resin and proprietary grafting chemistry to functionalize the surface with covalent bonds on an FR-4 base substrate. The surface was then metalized using an electroless copper (Cu) bath. This sequential growth of layers on top of each other using an actinic laser beam and palladium (Pd) ions to deposit Cu is analyzed. State-of-the-art material characterization techniques were employed to investigate process mechanism at the interfaces. Density functional theory calculations were performed to validate the experimental evidence of covalent bonding of the layers. This manufacturing approach is capable of adding metallic layers in a selective manner to the printed circuit boards at considerably lower temperatures. A complete analysis of the process using bulk deposition of the materials is illustrated in this work.

A direct band gap that solely exists in monolayer semiconducting transition metal dichalcogenides (TMDs) endows strong photoluminescence (PL) features, whereas multilayer TMD structures exhibit quenched PL due to the direct-to-indirect band gap transition. We demonstrate multi-layer TMD (such as MoS2 and WS2) nanoscrolls with a preserved direct band gap fabricated by an effective and facile method of solvent-driven self-assembly. The resultant multi-layer nanoscrolls, exhibiting up to 11 times higher PL intensity than the remanent monolayer, are carefully characterized using PL spectroscopy. Significantly enlarged interlayer distances and modulated interlayer coupling in the fabricated nanostructures are unveiled by cross-sectional scanning transmission electron microscopy, atomic force microscopy, and Raman spectroscopy. The preservation of direct band gap features is further evidenced by density functional theory calculations using the simplified bilayer model with an experimentally obtained 15 & ANGS; interlayer distance. The modulation of the PL intensity as an indicator of the band gap crossover in the TMD nanoscrolls is demonstrated by removing the acetone molecules trapped inside the interlayer space. The general applicability of the method presents an opportunity for large-scale fabrication of a plethora of multilayer TMD nanoscrolls with direct band gaps.

We reveal giant proximity-induced magnetism and valley-polarization effects in Janus Pt dichalcogenides (such as SPtSe), when bound to the europium oxide (EuO) substrate. Using first-principles simulations, it is surprisingly found that the charge redistribution, resulting from proximity with EuO, leads to the formation of two K and K' valleys in the conduction bands. Each of these valleys displays its own spin polarization and a specific spin texture dictated by broken inversion and time-reversal symmetries, and valley-exchange and Rashba splittings as large as hundreds of meV. This provides a platform for exploring spin-valley physics in low-dimensional semiconductors, with potential spin transport mechanisms such as spin-orbit torques much more resilient to disorder and temperature effects.

We present first-principles results on the structural, electronic, and magnetic properties of a new family of twodimensional antiferromagnetic (AFM) manganese chalcogenides, namely, monolayer MnX and Janus XMnY (X, Y = S, Se, Te), among which monolayer MnSe was recently synthesized in experiments [Aapro et al., ACS Nano 15, 13794 (2021)]. By carrying out calculations of the phonon dispersion and ab initio molecular dynamics simulations, we first confirmed that these systems, characterized by an unconventional strongly-coupled-bilayer atomic structure [consisting of Mn atoms buckled to chalcogens forming top and bottom ferromagnetic (FM) planes with antiparallel spin orientation], are dynamically and thermally stable. The analysis of the magnetic properties shows that these materials have robust AFM order, retaining a much lower energy than the FM state even under strain. Our electronic structure calculations reveal that pristine MnX and their Janus counterparts are indirect-gap semiconductors, covering a wide energy range and displaying tunable band gaps by the application of biaxial tensile and compressive strain. Interestingly, owing to the absence of inversion and time-reversal symmetry, and the presence of an asymmetrical potential in the out-of-plane direction, Janus XMnY become spin-split gapped systems, presenting a rich physics yet to be explored. Our findings provide insights into this physics and highlight the potential for these two-dimensional manganese chalcogenides in AFM spintronics.

van der Waals (vdW) crystals with unparalleled electromechanical properties have been explored for transformative devices. Currently, the availability of 2D vdW crystals is rather limited in nature as they are only obtained from certain mother crystals with intrinsically possessed layered crystallinity and anisotropic molecular bonding. Recent efforts to transform conventionally non-vdW three-dimensional (3D) crystals into ultrathin 2D-like structures have seen rapid developments to explore device building blocks of unique form factors. Herein, we explore a "peel-and-stick" approach, where a nonlayered 3D platinum sulfide (PtS) crystal, traditionally known as a cooperate mineral material, is transformed into a freestanding 2D-like membrane for electromechanical applications. The ultrathin (???10 nm) 3D PtS films grown on large-area (>cm2) silicon dioxide/silicon (SiO₂/Si) wafers are precisely "peeled" inside water retaining desired geometries via a capillary-force-driven surface wettability control. Subsequently, they are "sticked" on strain-engineered patterned substrates presenting prominent semiconducting properties, i.e., p-type transport with an optical band gap of ∼1.24 eV. A variety of mechanically deformable strain-invariant electronic devices have been demonstrated by this peel-and-stick method, including biaxially stretchable photodetectors and respiratory sensing face masks. This study offers new opportunities of 2D-like nonlayered semiconducting crystals for emerging mechanically reconfigurable and stretchable device technologies.