Xingxing Zhang

An MXene/GaN self-powered ultraviolet photodiode with superhigh peak external quantum efficiency (over 99%) is reported. It is found that two synergistic effects, the robust Schottky-junction-induced built-in electric field and the photoexcited hot carriers, contribute to the high-performance photodiode. The MXene/GaN van der Waals heterojunction can be easily fabricated into ultraviolet photodiode arrays with uniform and stable photocurrent outputs.

Abstract

A self-powered, high-performance Ti₃C₂T _x MXene/GaN van der Waals heterojunction (vdWH)-based ultraviolet (UV) photodiode is reported. Such integration creates a Schottky junction depth that is larger than the UV absorption depth to sufficiently separate the photoinduced electron/hole pairs, boosting the peak internal quantum efficiency over the unity and the external quantum efficiency over 99% under weak UV light without bias. The proposed Ti₃C₂T _x /GaN vdWH UV photodiode demonstrates pronounced photoelectric performances working in self-powered mode, including a large responsivity (284 mA W⁻¹), a high specific detectivity (7.06 × 10¹³ Jones), and fast response speed (rise/decay time of 7.55 µs/1.67 ms). Furthermore, the remarkable photovoltaic behavior leads to an impressive power conversion efficiency of 7.33% under 355 nm UV light illumination. Additionally, this work presents an easy-processing spray-deposition route for the fabrication of large-area UV photodiode arrays that exhibit highly uniform cell-to-cell performance. The MXene/GaN photodiode arrays with high-efficiency and self-powered ability show high potential for many applications, such as energy-saving communication, imaging, and sensing networks.

The increasing interest in novel materials requires in-depth research on its mechanisms, structural analyses, characterization, and potential applications. The comprehensive progress of novel layered compounds synthesized via topochemical approaches is analyzed. Along with its current status from the existing literature, future research directions to address their challenges and unveil their potential are also discussed.

Abstract

The designing of novel materials is a fascinating and innovative pathway in materials science. Particularly, novel layered compounds have tremendous influence in various research fields. Advanced fundamental studies covering various aspects, including reactants and synthetic methods, are required to obtain novel layered materials with unique physical and chemical properties. Among the promising synthetic techniques, topochemical approaches have afforded the platform for widening the extent of novel 2D materials. Notably, the synthesis of binary layered materials is considered as a major scientific breakthrough after the synthesis of graphene as they exhibit a wide spectrum of material properties with varied potential applicability. In this review, a comprehensive overview of the progress in the development of metastable layered compounds is presented. The various metastable layered compounds synthesized from layered ternary bulk materials through topochemical approaches are listed, followed by the descriptions of their mechanisms, structural analyses, characterizations, and potential applications. Finally, an essential research direction concerning the synthesis of new materials is indicated, wherein the possible application of topochemical approaches in unprecedented areas is explored.

2D electrolytes are stimuli-responsive materials that possess the chemical and physical properties of 2D materials and electrolytes. These materials can undergo reversible morphological transformations according to the environmental conditions, such as pH, temperature, electric permittivity of the medium, and ionic concentration, and exhibit flexible functionalities, being promising for highly dynamic systems, such as drug-delivery applications, artificial muscles, and energy-storage systems.

Abstract

A class of compounds sharing the properties of 2D materials and electrolytes, namely 2D electrolytes is described theoretically and demonstrated experimentally. 2D electrolytes dissociate in different solvents, such as water, and become electrically charged. The chemical and physical properties of these compounds can be controlled by external factors, such as pH, temperature, electric permittivity of the medium, and ionic concentration. 2D electrolytes, in analogy with polyelectrolytes, present reversible morphological transitions from 2D to 1D, as a function of pH, due to the interplay of the elastic and Coulomb energies. Since these materials show stimuli-responsive behavior to the environmental conditions, 2D electrolytes can be considered as a novel class of smart materials that expand the functionalities of 2D materials and are promising for applications that require stimuli-responsive demeanor, such as drug delivery, artificial muscles, and energy storage.

In article number 2005735, Jiaxu Yan, Wei Huang, and co‐workers systematically review the progress of stacking engineering of transition metal dichalcogenide hetero‐bilayers: from controllable fabrication methods to routine characterization, then to the dependence of interlayer coupling on stacking configurations/angles, and lastly the current challenges and possible future strategies.

Benefiting from the diffusion of oxygen vacancies from the surface to inside, the regional dual structure with anisotropic oxygen vacancies at both the surface and in the interior of TiO₂ is obtained by constructing amorphous domains. The as-prepared sample exhibits superior catalytic activity, which can immediately degrade rhodamine B solution to colorless with a shake.

Abstract

Although oxygen vacancies (O_vs) play a critical role for many applications of metal oxides, a controllable synthetic strategy for anisotropic O_vs remains a great challenge. Here, a novel strategy is proposed to achieve the regional dual structure with anisotropic O_vs at both the surface and in the interior of TiO₂ by constructing amorphous domains. The as-prepared black TiO₂ with amorphous domains exhibits superior activity in degrading rhodamine B (RhB) solutions, which can instantly decompose RhB with just a shake. First-principle simulations reveal that subsurface O_vs in TiO₂ are energetically favored, resulting in the formation of amorphous domains in the interior region via diffusion of surface-formed O_vs into the subsurface. The stable O_v-induced amorphous domains in TiO₂ with enhanced catalytic performances provide a scalable strategy to practical O_v engineering in functional metal oxides.

The in-plane elastic constants of a few-layer suspended 2H-MoSe₂ soften about 30% while decreasing the thickness from bulk to three-layers. The results obtained employing the contactless technique indicate the so-called negative elastic size-effect.

Abstract

Few-layer van der Waals (vdW) materials have been extensively investigated in terms of their exceptional electronic, optoelectronic, optical, and thermal properties. Simultaneously, a complete evaluation of their mechanical properties remains an undeniable challenge due to the small lateral sizes of samples and the limitations of experimental tools. In particular, there is no systematic experimental study providing unambiguous evidence on whether the reduction of vdW thickness down to few layers results in elastic softening or stiffening with respect to the bulk. In this work, micro-Brillouin light scattering is employed to investigate the anisotropic elastic properties of single-crystal free-standing 2H-MoSe₂ as a function of thickness, down to three molecular layers. The so-called elastic size effect, that is, significant and systematic elastic softening of the material with decreasing numbers of layers is reported. In addition, this approach allows for a complete mechanical examination of few-layer membranes, that is, their elasticity, residual stress, and thickness, which can be easily extended to other vdW materials. The presented results shed new light on the ongoing debate on the elastic size-effect and are relevant for performance and durability of implementation of vdW materials as resonators, optoelectronic, and thermoelectric devices.

The electronic and magnetic properties of a 2D monolayer ferromagnet on a layered superconducting substrate are studied using different experimental techniques and theoretical calculations. It is confirmed that the chromium tribromide monolayer retains its ferromagnetic order and induces proximitized magnetism on the underlying superconductor niobium diselenide. The results contribute to the broader framework of exploiting proximity effects to realize novel phenomena in 2D heterostructures.

Abstract

The ability to imprint a given material property to another through a proximity effect in layered 2D materials has opened the way to the creation of designer materials. Here, molecular-beam epitaxy is used for direct synthesis of a superconductor–ferromagnet heterostructure by combining superconducting niobium diselenide (NbSe₂) with the monolayer ferromagnetic chromium tribromide (CrBr₃). Using different characterization techniques and density-functional theory calculations, it is confirmed that the CrBr₃ monolayer retains its ferromagnetic ordering with a magnetocrystalline anisotropy favoring an out-of-plane spin orientation. Low-temperature scanning tunneling microscopy measurements show a slight reduction of the superconducting gap of NbSe₂ and the formation of a vortex lattice on the CrBr₃ layer in experiments under an external magnetic field. The results contribute to the broader framework of exploiting proximity effects to realize novel phenomena in 2D heterostructures.

Topological materials host both topological (TP) and trivial (TR) surface states in their band structure, the latter being undesired for topology-driven applications. A full suppression of trivial surface states is achieved by surface reconstruction of epitaxial NbP Weyl semimetal thin films. Moreover, large shifts of the chemical potential are induced by doping, enabling Fermi-level engineering of purely topological states.

Abstract

Weyl semimetals, a class of 3D topological materials, exhibit a unique electronic structure featuring linear band crossings and disjoint surface states (Fermi-arcs). While first demonstrations of topologically driven phenomena have been realized in bulk crystals, efficient routes to control the electronic structure have remained largely unexplored. Here, a dramatic modification of the electronic structure in epitaxially grown NbP Weyl semimetal thin films is reported, using in situ surface engineering and chemical doping strategies that do not alter the NbP lattice structure and symmetry, retaining its topological nature. Through the preparation of a dangling-bond-free, P-terminated surface which manifests in a surface reconstruction, all the well-known trivial surface states of NbP are fully suppressed, resulting in a purely topological Fermi-arc dispersion. In addition, a substantial Fermi-energy shift from −0.2 to 0.3 eV across the Weyl points is achieved by surface chemical doping, unlocking access to the hitherto unexplored n-type region of the Weyl spectrum. These findings constitute a milestone toward surface-state and Fermi-level engineering of topological bands in Weyl semimetals, and, while there are still challenges in minimizing doping-driven disorder and grain boundary density in the films, they do represent a major advance to realize device heterostructures based on Weyl physics.

To epitaxially grow large area single crystals of hBN and other 2D materials with lower symmetries, using a high-index substrate is highly favorable. The above figure illustrates the alignments of hBN islands on various Cu surfaces, from which we can clearly see that the unidirectional alignment of hBN islands can be achieved on various high-index Cu surfaces.

Abstract

Recently, the successful synthesis of wafer-scale single-crystal graphene, hexagonal boron nitride (hBN), and MoS₂ on transition metal surfaces with step edges boosted the research interests in synthesizing wafer-scale 2D single crystals on high-index substrate surfaces. Here, using hBN growth on high-index Cu surfaces as an example, a systematic theoretical study to understand the epitaxial growth of 2D materials on various high-index surfaces is performed. It is revealed that hBN orientation on a high-index surface is highly dependent on the alignment of the step edges of the surface as well as the surface roughness. On an ideal high-index surface, well-aligned hBN islands can be easily achieved, whereas curved step edges on a rough surface can lead to the alignment of hBN along with different directions. This study shows that high-index surfaces with a large step density are robust for templating the epitaxial growth of 2D single crystals due to their large tolerance for surface roughness and provides a general guideline for the epitaxial growth of various 2D single crystals.

Intercalation of Ti₃C₂T_x MXene with didecyldimethyl ammonium bromide (DDAB) yields an additive for polymer transistors. Ti₃C₂T_x –DDAB boosts simultaneously the hole and electron mobility of conjugated polymers. A counterbalancing doping is revealed, featuring DDAB's n-doping effect and Ti₃C₂T_x's electron-accepting capability. Based on this intercalation engineering, complementary logic gates are demonstrated with well-centered trip points.

Abstract

MXenes are highly conductive layered materials that are attracting a great interest for high-performance opto-electronics, photonics, and energy applications.. Their non-covalent functionalization with ad hoc molecules enables the production of stable inks of 2D flakes to be processed in thin-films. Here, the formation of stable dispersions via the intercalation of Ti₃C₂T_x with didecyldimethyl ammonium bromide (DDAB) yielding Ti₃C₂T_x–DDAB, is demonstrated. Such functionalization modulates the properties of Ti₃C₂T_x, as evidenced by a 0.47 eV decrease of the work function. It is also shown that DDAB is a powerful n-dopant capable of enhancing electron mobility in conjugated polymers and 2D materials. When Ti₃C₂T_x –DDAB is blended with poly(diketopyrrolopyrrole-co-selenophene) [(PDPP–Se)], a simultaneous increase by 170% and 152% of the hole and electron field-effect mobilities, respectively, is observed, compared to the neat conjugated polymer, with values reaching 2.0 cm² V⁻¹ s⁻¹. By exploiting the balanced ambipolar transport of the Ti₃C₂T_x –DDAB/PDPP–Se hybrid, complementary metal–oxide–semiconductor (CMOS) logic gates are fabricated that display well-centered trip points and good noise margin (64.6% for inverter). The results demonstrate that intercalant engineering represents an efficient strategy to tune the electronic properties of Ti₃C₂T_x yielding functionalized MXenes for polymer transistors with unprecedented performances and functions.