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The presence of the van der Waals gap in layered materials creates a wealth of intriguing phenomena different to their counterparts in conventional materials. For example, pressurization can generate a large anisotropic lattice shrinkage along the stacking orientation and/or a significant interlayer sliding, and many of the exotic pressure-dependent properties derive from these mechanisms. Here we report a giant piezoresistivity in pressurized β′-In2Se3. Upon compression, a six-orders-of-magnitude drop of electrical resistivity is obtained below 1.2 GPa in β′-In2Se3 flakes, yielding a giant piezoresistive gauge πp of −5.33 GPa−1. Simultaneously, the sample undergoes a semiconductor-to-semimetal transition without a structural phase transition. Surprisingly, linear dichroism study and theoretical first principles modelling show that these phenomena arise not due to shrinkage or sliding at the van der Waals gap, but rather are dominated by the layer-dependent atomic motions inside the quintuple layer, mainly from the shifting of middle Se atoms to their high-symmetric location. The atomic motions link to both the band structure modulation and the in-plane ferroelectric dipoles. Our work not only provides a prominent piezoresistive material but also points out the importance of intralayer atomic motions beyond van der Waals gap.

Intrinsic magnetic topological insulators offer low disorder and large magnetic bandgaps for robust magnetic topological phases operating at higher temperatures. By controlling the layer thickness, emergent phenomena such as the Quantum Anomalous Hall (QAH) effect and axion insulator phases have been realised. These observations occur at temperatures significantly lower than the Neel temperature of bulk MnBi2Te4, and measurement of the magnetic energy gap at the Dirac point in ultra-thin MnBi2Te4 has yet to be achieved. Critical to achieving the promise of this system is a direct measurement of the layer-dependent energy gap and verifying whether the gap is magnetic in the QAH phase. Here we utilise temperature dependent angle-resolved photoemission spectroscopy to study epitaxial ultra-thin MnBi2Te4. We directly observe a layer dependent crossover from a 2D ferromagnetic insulator with a bandgap greater than 780 meV in one septuple layer (1 SL) to a QAH insulator with a large energy gap (>100 meV) at 8 K in 3 and 5 SL MnBi2Te4. The QAH gap is confirmed to be magnetic in origin, as it abruptly diminishes with increasing temperature above 8 K. The direct observation of a large magnetic energy gap in the QAH phase of few-SL MnBi2Te4 is promising for further increasing the operating temperature of QAH materials.

The presence of the van der Waals (vdW) gap in layered materials creates a wealth of intriguing phenomena different to their counterparts in conventional materials. For example, pressurization can generate a large anisotropic lattice shrinkage along the vdW stacking orientation and/or a significant interlayer sliding, and many of the exotic pressure-dependent properties derive from these mechanisms. Here we report a giant piezoresistivity in pressurized \(\beta \text{’}\)-In2Se3. Upon compression, a six-orders-of-magnitude drop of electrical resistivity is obtained below 1.2 GPa in \(\beta \text{’}\)-In2Se3 flakes, yielding a giant piezoresistive gauge \({\pi }_{P}\) of -5.33 GPa− 1. Simultaneously, the sample undergoes a semiconductor-to-semimetal transition without a structural phase transition. Surprisingly, linear dichroism study and theoretical first principles modelling show that these phenomena arise not due to shrinkage or sliding at the vdW gap, but rather are dominated by the layer-dependent atomic motions inside the quintuple layer, mainly from the shifting of middle Se atoms to their high-symmetric location. Our work not only provides a prominent piezoresistive material but also points out the importance of intralayer atomic motions beyond vdW gap.

Excellent light-matter interaction and a wide range of thickness-tunable bandgaps in layered vdW materials coupled by the facile fabrication of heterostructures have enabled several avenues for optoelectronic applications. Realization of high photoresponsivity at fast switching speeds is a critical challenge for 2D optoelectronics to enable high-performance photodetection for optical communication. Moving away from conventional type-II heterostructure pn junctions towards a WSe

Quantum dots (QD) with electric-field-controlled charge state are promising for electronics applications, e.g., digital information storage, single-electron transistors, and quantum computing. Inor...

Intrinsic magnetic topological insulators offer low disorder and large magnetic band gaps for robust magnetic topological phases operating at higher temperatures. By controlling the layer thickness, emergent phenomena such as the quantum anomalous Hall (QAH) effect and axion insulator phases have been realized. These observations occur at temperatures significantly lower than the Néel temperature of bulk MnBi

Magnetic Weyl semimetals with spontaneously broken time-reversal symmetry exhibit a large intrinsic anomalous Hall effect originating from the Berry curvature. To employ this large Hall current for room temperature topo-spintronics applications, it is necessary to fabricate these materials as thin or ultrathin films. Here, we experimentally demonstrate that Weyl semimetal Co2MnGa thin films (20–50 nm) show a large anomalous Hall angle ~11.4% at low temperature and ~9.7% at room temperature, which can be ascribed to the non-trivial topology of the band structure with large intrinsic Berry curvature. However, the anomalous Hall angle decreases significantly with thicknesses below 20 nm, which band structure calculations confirm is due to the reduction of the majority spin contribution to the Berry curvature. Our results suggest that Co2MnGa is an excellent material to realize room temperature topo-spintronics applications; however, the significant thickness dependence of the Berry curvature has important implications for thin-film device design.

Layered two-dimensional van der Waals (vdW) semiconductors and their heterostructures have been shown to exhibit positive photoconductance (PPC) in many studies. A few recent reports have demonstrated negative photoconductance (NPC) as well that can enable broadband photodetection besides multi-level optoelectronic logic and memory. Controllable and reversible switching between PPC and NPC is a key requirement for these applications. This report demonstrates visible-to-near infrared wavelength-driven NPC and PPC, along with reversible switching between the two, in an air stable, high mobility, broadband black phosphorus (BP) field effect transistor (FET) covered with a few layer MoS$_2$ flake. The crossover switching wavelength can be tuned by varying the MoS$_2$ bandgap through its flake thickness and the NPC and PPC photoresponsivities can be modulated using electrostatic gating as well as laser power. Recombination-driven NPC and PPC allows for reversible switching at reasonable time scales of a few seconds. Further, gate voltage-dependent negative persistent photoconductance enables synaptic behavior that is well-suited for optosynaptic applications., Comment: Main Manuscript and Supporting Information

Molybdenum based compounds are an emerging class of non-metallic catalytic materials for the hydrogen evolution reaction (HER) in acidic media. However, most of them lose considerable catalytic performance and exhibit poor long-term stability in alkaline media. Here, planar molybdenum oxide, with high alkaline stability and ordered intracrystalline pores, is developed as the HER candidate. The pores with diameters in the order of ∼5–7 A are HER-active, and appear after an NH4+ doping-driven phase transition from the orthorhombic to hexagonal phase. Such a unique structure facilitates the diffusion of ionic entities and water molecules to the HER sites and helps in the removal of gaseous products, therefore improving the surface active area and reaction kinetics. These intracrystalline pores also reduce the long term stress on electrodes. The corresponding HER activity is extremely stable for >40 h in an alkaline medium at an overpotential of 138 mV with a Tafel slope of 50 mV dec−1. Such properties offer a superior combination compared to those of other reported molybdenum based nanostructures, hence providing a great opportunity for developing high-performance alkaline non-metal HER catalysts.

The application of ultrathin two-dimensional (2D) perovskites in near-infrared and visible-range optoelectronics has been limited owing to their inherent wide bandgaps, large excitonic binding energies and low optical absorption at higher wavelengths. Here, we show that by tailoring interfacial band alignments via conjugation with low-dimensional materials like monolayer transition metal dichalcogenides (TMD), the functionalities of 2D perovskites can be extended to diverse, visible-range photophysical applications. Based on the choice of individual constituents in the 2D perovskite/TMD heterostructures, our first principles calculations demonstrate widely tunable type-II band gaps, carrier effective masses and band offsets to enable an effective separation of photogenerated excitons for enhanced photodetection and photovoltaic applications. In addition, we show the possibilities of achieving a type-I band alignment for recombination based light emitters as well as a type-III configuration for tunnelling devices. Further, we evaluate the effect of strain on the electronic properties of the heterostructures to show a significant strain tolerance, making them prospective candidates in flexible photosensors., Comment: 38 pages, main manuscript and supporting information

Supramolecular chemistry protocols applied on surfaces offer compelling avenues for atomic-scale control over organic-inorganic interface structures. In this approach, adsorbate-surface interactions and two-dimensional confinement can lead to morphologies and properties that differ dramatically from those achieved via conventional synthetic approaches. Here, we describe the bottom-up, on-surface synthesis of one-dimensional coordination nanostructures based on an iron (Fe)-terpyridine (tpy) interaction borrowed from functional metal-organic complexes used in photovoltaic and catalytic applications. Thermally activated diffusion of sequentially deposited ligands and metal atoms and intraligand conformational changes lead to Fe-tpy coordination and formation of these nanochains. We used low-temperature scanning tunneling microscopy and density functional theory to elucidate the atomic-scale morphology of the system, suggesting a linear tri-Fe linkage between facing, coplanar tpy groups. Scanning tunneling spectroscopy reveals the highest occupied orbitals, with dominant contributions from states located at the Fe node, and ligand states that mostly contribute to the lowest unoccupied orbitals. This electronic structure yields potential for hosting photoinduced metal-to-ligand charge transfer in the visible/near-infrared. The formation of this unusual tpy/tri-Fe/tpy coordination motif has not been observed for wet chemistry synthetic methods and is mediated by the bottom-up on-surface approach used here, offering pathways to engineer the optoelectronic properties and reactivity of metal-organic nanostructures.

The emergence of graphene in recent years provides exciting avenues for achieving fast, reliable DNA/RNA sensing and sequencing. Here we explore the possibility of enhancing electronic fingerprints of nucleobases adsorbed on graphene by tuning the surface coverage and modifying molecular dipoles using first-principles calculations. We demonstrate that intermolecular interactions have a strong influence on the adsorption geometry and the electronic structure of the nucleobases, resulting in tilted configurations and a considerable modification of their electronic fingerprints in graphene. Our analysis reveals that the molecular dipole of the nucleobase molecules plays a dominant role in the electronic structure of graphene-nucleobase systems, inducing significant changes in the work functions and energy level alignments at the interface. These results highlight tunable control of the measured molecular signals in graphene by optimizing the surface contact between nucleobases and graphene. Our findings have important implications for identification and understanding of molecular fingerprints of DNA/RNA nucleobases in graphene-based sensing and sequencing methods.

Phonon polaritons (PhPs) have attracted significant interest in the nano-optics communities because of their nanoscale confinement and long lifetimes. Although PhP modification by changing the local dielectric environment has been reported, controlled manipulation of PhPs by direct modification of the polaritonic material itself has remained elusive. Here, chemical switching of PhPs in α-MoO3 is achieved by engineering the α-MoO3 crystal through hydrogen intercalation. The intercalation process is non-volatile and recoverable, allowing reversible switching of PhPs while maintaining the long lifetimes. Precise control of the intercalation parameters enables analysis of the intermediate states, in which the needle-like hydrogenated nanostructures functioning as in-plane antennas effectively reflect and launch PhPs and form well-aligned cavities. We further achieve spatially controlled switching of PhPs in selective regions, leading to in-plane heterostructures with various geometries. The intercalation strategy introduced here opens a relatively non-destructive avenue connecting infrared nanophotonics, reconfigurable flat metasurfaces and van der Waals crystals., Phonon polaritons hold promises for nanophotonic applications but external control of phonon polaritons remains challenging. Here, the authors achieve reversible and non-volatile switching of phonon polariton by modifying crystal structure and lattice vibrations via hydrogenation.

Topological materials host robust surface states that could form the basis for future electronic devices. As such states have spins that are locked to the momentum, they are of particular interest for spintronic applications. Understanding spin textures of the surface states of topologically nontrivial materials, and being able to manipulate their polarization, is therefore essential if they are to be utilized in future technologies. Here we use first-principles calculations to show that pyrite-type crystals OsX2 (X = Se, Te) are a class of topological materials that can host surface states with spin polarization that can be either in-plane or out-of-plane. We show that the formation of low-energy states with symmetry-protected energy- and direction-dependent spin textures on the (001) surface of these materials is a consequence of a transformation from a topologically trivial to nontrivial state, induced by spin orbit interactions. The unconventional spin textures of these surface states feature an in-plane to out-of-plane spin polarization transition in the momentum space protected by local symmetries. Moreover, the surface spin direction and magnitude can be selectively filtered in specific energy ranges. Our demonstration of a new class of topological materials with controllable spin textures provides a platform for experimentalists to detect and exploit unconventional surface spin textures in future spin-based nanoelectronic devices.

An excellent infrared window material should have both high transmittance and good mechanical strength to adapt to applications in harsh environments. In this report, we use first-principles calculations to investigate the effect of oxygen alloying on the mechanical and optical properties of the well-used infrared window material ZnS. We have found that with increasing oxygen content, the mechanical properties including elastic constants, bulk modulus, shear modulus, Young’s modulus of ZnS1−xOx compounds are improved compared to the pristine phase. Meanwhile, the infrared optical transmittance of the system is retained with changing oxygen compositions. We have also considered the influence of oxygen/sulfur vacancy defects in the system. We have observed that the optical transmittance of the system is degraded upon induction of vacancies. These results have revealed that the mechanical properties of optical materials can be effectively enhanced with appropriate alloying, but there are also challenges in practice due to the presence of defects. Our investigations can guide the design of these infrared optical materials in special applications such as aircraft and space systems.

Motivated by the fascinating properties of both two-dimensional transition metal dichalcogenide quantum dots (TMD QDs) and Janus TMD monolayers, we theoretically explore the equilibrium structures of free-standing Janus MoSSe QDs in which atomic asymmetry of chalcogen is introduced. Two distinct types of spontaneous curling are observed by molecular dynamics simulations, and the curling behavior depends on the size of QD. The bowl-like (tube-like) curling occurs in relatively small (large) MoSSe QDs with different shapes (hexagon and triangle) and edge types (zigzag and armchair). The transition between these two curling types occurs at the sizes of around 10 nm and 13 nm for hexagonal and triangular shapes, respectively. By applying equivalent misfit strains into two adjacent sublayers, finite element analysis reproduces similar curling behavior. This confirms the relaxation of intrinsic strain in Janus structure acting as the predominant driving force of spontaneous curling. In addition, the curvatures of Janus TMD QDs increase from MoSSe to MoSeTe to MoSTe, indicating the positive correlation between the curling and misfit.

π-Complexation triggered Lewis acid-base interactions between open metal sites (OMS) of metal-organic frameworks (MOFs), and π-e(-) rich adsorptive benzene (Bz) is exploited to establish M-MOF-74 as the best Bz-selective MOF sorbent, marking the first report of utilizing OMS behind benzene/cyclohexane separation; a key advance from the energy-economy standpoint of industrial separation.

Herein we describe a large-scale, non-injection “one-pot” batch method for producing large quantities of novel colloidal Ag–Ag8GeS6 heteronanostructures. Using a suite of analytical techniques, including high resolution TEM, HAADF-STEM, XEDS mapping, and XRD, the formation mechanism of the nanostructures is elucidated. The formation is discovered to occur in three stages comprising nucleation, phase separation of the metal and semiconductor components, and the final segregation of the metal and semiconductor components to form the Janus nanostructure. The high ionic mobility and chemical reactivity of Ag enables the self-regulated formation of Janus nanostructures in optimized one-pot reactions – a phenomenon that is almost unique to silver-based systems. As such, silver-based systems are ideal candidates to study the formation of Janus nanostructures.

Non-covalent functionalization via physisorption of organic molecules provides a scalable approach for modifying the electronic structure of graphene while preserving its excellent carrier mobilities. Here we investigated the physisorption of long-chain acenes, namely, hexacene and its fluorinated derivative perfluorohexacene, on bilayer graphene for tunable graphene devices using first principles methods. We find that the adsorption of these molecules leads to the formation of localized states in the electronic structure of graphene close to its Fermi level, which could be readily tuned by an external electric field. The electric field not only creates a variable band gap as large as 250 meV in bilayer graphene, but also strongly influences the charge redistribution within the molecule-graphene system. This charge redistribution is found to be weak enough not to induce strong surface doping, but strong enough to help preserve the electronic states near the Dirac point of graphene., 17 pages, 7 figures, supporting information

In order to overcome the low energy transfer efficiency of the conventional FRET (Forster resonance energy transfer) system, a novel spectra-matching and distance-controllable CIS@ZnS-SQ FRET assembly has been prepared via ultrasonic self-assembly method, by using the synthesized visible CIS@ZnS type I core-shell quantum dots as energy donor and the near infrared SQ dyes as acceptor. Through controllable synthesis of quantum dots, the absorption and fluorescence performance of FRET system were adjusted by the size of CIS@ZnS, while the distance of energy donor-acceptor and the non-valid charge recombination in the FRET system were controlled by the wide-band shell of quantum dots. The excitons transfer and recombination kinetics in CIS@ZnS-SQ assembly were investigated by the pump-probe femtosecond ultrafast transient absorption measurements, with which results in the FRET-type energy transfer mechanism: CIS*+SQ→CIS+SQ* has been proven and a high energy transfer rate of about 5.0×10 s has been gained between CIS@ZnS and SQ. The excitons’ lifetime and FRET energy transfer efficiency were calculated from the fluorescence decay kinetic curves tested by time-resolved fluorescence measurements. The results show that the energy transfer in CIS@ZnS-SQ depends on the size of CIS@ZnS quantum dots. As the size of CIS@ZnS (mainly refers to the ZnS shell thickness) increases from 2.1±0.4 nm to 2.9±0.4 nm, 4.1±0.3 nm, 5.4±0.5 nm and 7.2±0.5 nm, the fluorescence quantum yield of CIS@ZnS improves from 5.4% to 26%, 33%, 38% and 43.3% as well as the distance between CIS@ZnS and SQ (energy donor and acceptor) increases gradually, which makes the FRET energy transfer efficiency (ηFRET) first rise and then decline. As a result, an optimal ηFRET value of 62.8% was gained in the FRET assembly when the reaction time of ZnS shell was 20 min. This research will have a promising theoretical and practical value for the development of the panchromatic and high-efficiency solar cells.