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Conformational arrangements within nanostructures play a crucial role in shaping the overall configuration and determining the properties, for example in covalent/metal organic frameworks. In on-surface synthesis, conformational diversity often leads to uncontrollable or disordered structures. Therefore, the exploration of controlling and directing the conformational arrangements is significant in achieving desired nanoarchitectures. Herein, a conformationally flexible precursor 2,4,6-tris(3-bromophenyl)−1,3,5-triazine is employed, and a random phase consisting of C 3h and C s conformers is firstly obtained after deposition of the precursor on Cu(111) at room temperature to 365 K. At low coverage (0.01 ML) selenium doping, we achieve the selectivity of the C 3h conformer and improve the nanopore structural homogeneity. The ordered two-dimensional metal organic nanostructure can be fulfilled by selenium doping from room temperature to 365 K. The formation of the conformationally flexible precursor on Cu(111) is explored through the combination of high-resolution scanning tunneling microscopy and non-contact atomic force microscopy. The regulation of energy diagrams in the absence or presence of the Se atom is revealed by density functional theory calculations. These results can enrich the on-surface synthesis toolbox of conformationally flexible precursors, for the design of complex nanoarchitectures, and for future development of engineered nanomaterials. Research into the control of conformational arrangements is of great importance for achieving bespoke nanoarchitectures. Here, the authors achieve topology selectivity of a conformationally flexible precursor by Se doping.

The availability of an ever‐expanding portfolio of 2D materials with rich internal degrees of freedom (spin, excitonic, valley, sublattice, and layer pseudospin) together with the unique ability to tailor heterostructures made layer by layer in a precisely chosen stacking sequence and relative crystallographic alignments, offers an unprecedented platform for realizing materials by design. However, the breadth of multi‐dimensional parameter space and massive data sets involved is emblematic of complex, resource‐intensive experimentation, which not only challenges the current state of the art but also renders exhaustive sampling untenable. To this end, machine learning, a very powerful data‐driven approach and subset of artificial intelligence, is a potential game‐changer, enabling a cheaper – yet more efficient – alternative to traditional computational strategies. It is also a new paradigm for autonomous experimentation for accelerated discovery and machine‐assisted design of functional 2D materials and heterostructures. Here, the study reviews the recent progress and challenges of such endeavors, and highlight various emerging opportunities in this frontier research area. The family of 2D materials is an unprecedented platform for materials by design, thanks to their ever‐expanding material portfolio with rich internal degrees of freedom. The study provides a comprehensive overview of the recent progress, challenges and emerging opportunities in a frontier research area that exploits machine learning—a very powerful data‐driven approach and subset of artificial intelligence—for 2D materials.

Phase engineering of two-dimensional transition metal dichalcogenides (2D-TMDs) offers opportunities for exploring unique phase-specific properties and achieving new desired functionalities. Here, we report a phase-selective in-plane heteroepitaxial method to grow semiconducting H-phase CrSe 2 . The lattice-matched MoSe 2 nanoribbons are utilized as the in-plane heteroepitaxial template to seed the growth of H-phase CrSe 2 with the formation of MoSe 2 -CrSe 2 heterostructures. Scanning tunneling microscopy and non-contact atomic force microscopy studies reveal the atomically sharp heterostructure interfaces and the characteristic defects of mirror twin boundaries emerging in the H-phase CrSe 2 monolayers. The type-I straddling band alignments with band bending at the heterostructure interfaces are directly visualized with atomic precision. The mirror twin boundaries in the H-phase CrSe 2 exhibit the Tomonaga-Luttinger liquid behavior in the confined one-dimensional electronic system. Our work provides a promising strategy for phase engineering of 2D TMDs, thereby promoting the property research and device applications of specific phases. Phase engineering of 2D transition metal dichalcogenides enables the investigation of emerging physical properties. Here, the authors report a phase selective in-plane heteroepitaxial method to grow semiconducting H-phase CrSe 2 thin films from MoSe 2 nanoribbons, showing Tomonaga-Luttinger liquid behaviour in the CrSe 2 mirror twin boundaries.

The kagome lattice is a versatile platform for investigating correlated electronic states. However, its realization in two-dimensional (2D) semiconductors for tunable device applications is still challenging. An alternative strategy to create kagome-like bands is to realize a coloring-triangle (CT) lattice in semiconductors through a distortion of a modified triangular lattice. Here, we report the observation of low-energy kagome-like bands in a semiconducting 2D transition metal chalcogenide—Cr 8 Se 12 with a thickness of 7 atomic layers—which exhibits a CT lattice and a bandgap of 0.8 eV. The Cr-deficient layer beneath the topmost Se-full layer is partially occupied with 2/3 occupancy, yielding a √3 × √3 Cr honeycomb network. Angle-resolved photoemission spectroscopy measurements and first-principles investigations reveal the surface kagome-like bands near the valence band maximum, which are attributed to topmost Se p z orbitals modulated by the honeycomb Cr. The search for low-dimensional condensed matter platforms that host correlated electronic states has recently revolved around kagome lattices. Here, the authors evidence kagome-like bands in a 2D coloring-triangle lattice with a semiconducting bandgap.

Exciton is a bosonic quasiparticle consisting of a pair of electron and hole, with promising potentials for optoelectronic device applications, such as exciton transistors, photodetectors and light emitting devices. However, the charge-neutral nature of excitons renders them challenging to manipulate using electronics. Here we present the generation of trions, a form of charged excitons, together with enhanced exciton resonance in monolayer WSe 2 . The excitation of the trion quasiparticles is achieved by the hot carrier transport from the integrated gold plasmonic nanocavity, formed by embedding monolayer WSe 2 between gold nanoparticles and a gold film. The nanocavity-induced negatively charged trions provide a promising route for the manipulation of excitons, essential for the construction of all-exciton information processing circuits.

Impressive properties arise from the atomically thin nature of transition metal dichalcogenide two-dimensional materials. However, being atomically thin limits their optical absorption or emission. Hence, enhancing their photoluminescence by plasmonic nanostructures is critical for integrating these materials in optoelectronic and photonic devices. Typical photoluminescence enhancement from transition metal dichalcogenides is 100-fold, with recent enhancement of 1,000-fold achieved by simultaneously enhancing absorption, emission and directionality of the system. By suspending WSe 2 flakes onto sub-20-nm-wide trenches in gold substrate, we report a giant photoluminescence enhancement of ∼20,000-fold. It is attributed to an enhanced absorption of the pump laser due to the lateral gap plasmons confined in the trenches and the enhanced Purcell factor by the plasmonic nanostructure. This work demonstrates the feasibility of giant photoluminescence enhancement in WSe 2 with judiciously designed plasmonic nanostructures and paves a way towards the implementation of plasmon-enhanced transition metal dichalcogenide photodetectors, sensors and emitters. Two-dimensional materials have excellent electrical properties, but poor luminescence limits their application in optoelectronics. Here, the authors demonstrate a plasmon-induced 20,000-fold enhancement in photoluminescence from tungsten diselenide suspended across a nanometre-scale gap.

The field of two‐dimensional (2D) magnets has expanded rapidly during the past several years since the first demonstration of intrinsic 2D magnetism in atomically thin CrI3 and Cr2Ge2Te6 in 2017. 2D transition metal chalcogenides (TMCs), a class of strongly correlated materials, have exhibited a wide variety of novel electronic and optical properties, and more recently magnetism. Here, we review recent experimental progress achieved in the growth of 2D magnetic TMC materials using chemical vapor deposition and molecular beam epitaxy methods. Outstanding examples include the demonstration of room temperature intrinsic and extrinsic ferromagnetism in monolayer VSe2, MnSe2, Cr3Te4, V‐doped WSe2, and so on. A brief discussion on the origin of the exotic magnetic properties and emergent phenomena is also presented. Finally, we summarize the remaining challenges and future perspective on the development of 2D magnetic materials for next‐generation spintronic devices. The realization of long‐range magnetic ordering in the two‐dimensional (2D) limit opens up opportunities to explore 2D magnetic states and potential applications in 2D spintronics. Here, we review recent experimental progress achieved in the 2D transition metal chalcogenides with intrinsic and extrinsic magnetism.

Complex correlated states emerging from many-body interactions between quasiparticles (electrons, excitons and phonons) are at the core of condensed matter physics and material science. In low-dimensional materials, quantum confinement affects the electronic, and subsequently, optical properties for these correlated states. Here, by combining photoluminescence, optical reflection measurements and ab initio theoretical calculations, we demonstrate an unconventional excitonic state and its bound phonon sideband in layered silicon diphosphide (SiP 2 ), where the bound electron–hole pair is composed of electrons confined within one-dimensional phosphorus–phosphorus chains and holes extended in two-dimensional SiP 2 layers. The excitonic state and emergent phonon sideband show linear dichroism and large energy redshifts with increasing temperature. Our ab initio many-body calculations confirm that the observed phonon sideband results from the correlated interaction between excitons and optical phonons. With these results, we propose layered SiP 2 as a platform for the study of excitonic physics and many-particle effects. Distinct electronic and optical properties emerge from quantum confinement in low-dimensional materials. Here, combining optical characterization and ab initio calculations, the authors report an unconventional excitonic state and bound phonon sideband in layered silicon diphosphide.

Polymorphism of two-dimensional transition metal dichalcogenides such as molybdenum disulfide (MoS 2 ) exhibit fascinating optical and transport properties. Here, we observe a tunable inverted gap (~0.50 eV) and a fundamental gap (~0.10 eV) in quasimetallic monolayer MoS 2 . Using spectral-weight transfer analysis, we find that the inverted gap is attributed to the strong charge–lattice coupling in two-dimensional transition metal dichalcogenides (2D-TMDs). A comprehensive experimental study, supported by theoretical calculations, is conducted to understand the transition of monolayer MoS 2 on gold film from trigonal semiconducting 1H phase to the distorted octahedral quasimetallic 1T’ phase. We clarify that electron doping from gold, facilitated by interfacial tensile strain, is the key mechanism leading to its 1H–1T’ phase transition, thus resulting in the formation of the inverted gap. Our result shows the importance of charge–lattice coupling to the intrinsic properties of the inverted gap and polymorphism of MoS 2 , thereby unlocking new possibilities for 2D-TMD-based device fabrication. MoS 2 exhibits multiple electronic properties associated with different crystal structures. Here, the authors observe inverted and fundamental gaps through a designed annealing-based strategy, to induce a semiconductor-to-metal phase transition in monolayer-MoS 2 on Au, facilitated by interfacial strain and electron transfer from Au to MoS 2 .

Atomically thin tungsten disulfide (WS 2 ) has attracted much attention in recent years due its indirect-to-direct band gap transition, band gap tunability and giant spin splitting. However, the fabrication of atomically thin WS 2 remains largely underdeveloped in comparison to its structural analogue MoS 2 . Here we report the direct fabrication of highly crystalline few-layer WS 2 on silver substrates by pulse laser deposition at the relatively low temperature of 450 °C. The growth takes places by conventional epitaxy, through the in-situ formation of nearly lattice-matching Ag 2 S on the silver surface. Intriguingly, it was observed that the resulting film was composed of not only the usual semiconducting 2H-WS 2 structure but also the less common metallic 1T-WS 2 . Modifications of the synthesis parameters allow for control over the crystalline quality, film thickness and crystal phase composition of the resulting WS 2 film.