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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 ultrafast monitoring of deoxyribonucleic acid (DNA) dynamic structural changes is an emerging and rapidly growing research topic in biotechnology. The existing optical spectroscopy used to identify different dynamical DNA structures lacks quick response while requiring large consumption of samples and bulky instrumental facilities. It is highly demanded to develop an ultrafast technique that monitors DNA structural changes with the external stimulus or cancer‐related disease scenarios. Here, we demonstrate a novel photonic integrated graphene‐optofluidic device to monitor DNA structural changes with the ultrafast response time. Our approach is featured with an effective and straightforward design of decoding the electronic structure change of graphene induced by its interactions with DNAs in different conformations using ultrafast nanosecond pulse laser and achieving refractive index sensitivity of ~3 × 10−5 RIU. This innovative technique for the first time allows us to perform ultrafast monitoring of the conformational changes of special DNA molecules structures, including G‐quadruplex formation by K+ ions and i‐motif formation by the low pH stimulus. The graphene‐optofluidic device as presented here provides a new class of label‐free, ultrafast, ultrasensitive, compact, and cost‐effective optical biosensors for medical and healthcare applications.

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...

The kagome lattice, whose electronic valence band (VB) structure includes two Dirac bands and one flat band, offers a rich space to realise tuneable topological and strongly correlated electronic phases in two-dimensional (2D) and layered materials. While strong electron correlations have been observed in inorganic kagome crystals, they remain elusive in organic systems. The latter offer versatile synthesis protocols via molecular self-assembly and metal-ligand coordination. Here, we report the synthesis of a 2D metal-organic framework (MOF) where di-cyano-anthracene (DCA) molecules form a kagome arrangement via coordination with copper (Cu) atoms on a silver surface [Ag(111)]. Low-temperature scanning tunnelling spectroscopy revealed Kondo screening--by the Ag(111) conduction electrons--of magnetic moments localised at Cu and DCA sites of the MOF. Density functional theory and mean-field Hubbard modelling show that these local moments emerge from strong interactions between kagome MOF electrons, enabling organic 2D materials as viable platforms for strongly correlated nanoelectronics and spintronics., Main text: 32 pages, 5 figures. Supplementary Information: 51 pages, 20 figures

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.

We use angle-resolved photoemission spectroscopy to investigate the electronic structure of bilayer graphene at high n-doping and extreme displacement fields, created by intercalating epitaxial monolayer graphene on silicon carbide with magnesium to form quasi-freestanding bilayer graphene on magnesium-terminated silicon carbide. Angle-resolved photoemission spectroscopy reveals that upon magnesium intercalation, the single massless Dirac band of epitaxial monolayer graphene is transformed into the characteristic massive double-band Dirac spectrum of quasi-freestanding bilayer graphene. Analysis of the spectrum using a simple tight binding model indicates that magnesium intercalation results in an n-type doping of 2.1 $\times$ 10$^{14}$ cm$^{-2}$, creates an extremely high displacement field of 2.6 V/nm, opening a considerable gap of 0.36 eV at the Dirac point. This is further confirmed by density-functional theory calculations for quasi-freestanding bilayer graphene on magnesium-terminated silicon carbide, which show a similar doping level, displacement field and bandgap. Finally, magnesium-intercalated samples are surprisingly robust to ambient conditions; no significant changes in the electronic structure are observed after 30 minutes exposure in air., 46 pages including supporting information, 4 figures in the main paper and 9 figures in the 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.

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 edge states are of particular importance to understand fundamental properties of finite two-dimensional (2D) crystals. Based on first-principles calculations, we investigated on the bare zigzag boron nitride nanoribbons (zzBNNRs) with different spin-polarized states well localized at and extended along their edges. Our calculations examined the edge stress, which is sensitively dependent on the magnetic edge states, for either B-terminated edge or N-terminated edge. Moreover, we revealed that different magnetic configurations lead to a rich spectrum of electronic behaviors at edges. Using an uniaxial tensile strain, we proposed the magnetic phase transitions and thereby obtained the metallic to half-metallic (or reverse) phase transitions at edges. It suggests zzBNNR as a promising candidate for potential applications of non-metal spintronic devices.

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., G.Á.-P. acknowledges support through the Severo Ochoa Program from the Government of the Principality of Asturias (PA20-PF-BP19-053). P.A.-G. and J.D. acknowledge support from the European Research Council under starting grant no. 715496, 2DNANOPTICA. Q.B. acknowledges support from the Australian Research Council (ARC, FT150100450 and IH150100006). Q.B. and Q.O. acknowledge support from the Australian Research Council Centre of Excellence in Future Low-Energy Electronics Technologies (FLEET) (project number: CE170100039).

The intercalation of epitaxial graphene on SiC(0001) with Ca has been studied extensively, yet precisely where the Ca resides remains elusive. Furthermore, the intercalation of Mg underneath epitaxial graphene on SiC(0001) has not been reported. Here, we use low energy electron diffraction, x-ray photoelectron spectroscopy, secondary electron cut-off photoemission and scanning tunneling microscopy to elucidate the physical and electronic structure of both Ca- and Mg-intercalated epitaxial graphene on 6H-SiC(0001). We find that Ca intercalates underneath the buffer layer and bonds to the Si-terminated SiC surface, breaking the C-Si bonds of the buffer layer i.e. 'freestanding' the buffer layer to form Ca-intercalated quasi-freestanding bilayer graphene (Ca-QFSBLG). The situation is similar for the Mg-intercalation of epitaxial graphene on SiC(0001), where an ordered Mg-terminated reconstruction at the SiC surface and Mg bonds to the Si-terminated SiC surface are formed, resulting in Mg-intercalated quasi-freestanding bilayer graphene (Mg-QFSBLG). Ca-intercalation underneath the buffer layer has not been considered in previous studies of Ca-intercalated epitaxial graphene. Furthermore, we find no evidence that either Ca or Mg intercalates between graphene layers. However, we do find that both Ca-QFSBLG and Mg-QFSBLG exhibit very low workfunctions of 3.68 and 3.78 eV, respectively, indicating high n-type doping. Upon exposure to ambient conditions, we find Ca-QFSBLG degrades rapidly, whereas Mg-QFSBLG remains remarkably stable., 58 pages, 10 figures, 4 tables. Revised text and figures

Layered indium selenides ($In_{2}Se_{3}$) have recently been discovered to host robust out-of-plane and in-plane ferroelectricity in the $\alpha$ and $\beta$' phases, respectively. In this work, we utilise angle-resolved photoelectron spectroscopy to directly measure the electronic bandstructure of $\beta '-In_{2}Se_{3}$, and compare to hybrid density functional theory (DFT) calculations. In agreement with DFT, we find the band structure is highly two-dimensional, with negligible dispersion along the c-axis. Due to n-type doping we are able to observe the conduction band minima, and directly measure the minimum indirect (0.97 eV) and direct (1.46 eV) bandgaps. We find the Fermi surface in the conduction band is characterized by anisotropic electron pockets with sharp in-plane dispersion about the $\overline{M}$ points, yielding effective masses of 0.21 $m_{0}$ along $\overline{KM}$ and 0.33 $m_{0}$ along $\overline{\Gamma M}$. The measured band structure is well supported by hybrid density functional theory calculations. The highly two-dimensional (2D) bandstructure with moderate bandgap and small effective mass suggest that $\beta'-In_{2}Se_{3}$ is a potentially useful new van der Waals semiconductor. This together with its ferroelectricity makes it a viable material for high-mobility ferroelectric-photovoltaic devices, with applications in non-volatile memory switching and renewable energy technologies., Comment: 19 pages, 4 + 1 figures; typos corrected;added references; updated figures & discussion to reflect changes in model

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.

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