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<jats:title>Abstract</jats:title> <jats:p>The possibility of almost linear tuning of the band gap and of the electrical and optical properties in monolayers (MLs) of semiconducting transition metal dichalcogenide (S-TMD) alloys opens up the way to fabricate materials with on-demand characteristics. By making use of photoluminescence spectroscopy, we investigate optical properties of WSSe MLs with a S/Se ratio of 57/43 deposited on SiO$_2$/Si substrate and encapsulated in hexagonal BN flakes. Similarly to the <jats:italic>"parent"</jats:italic> WS<jats:sub>2</jats:sub> and WSe<jats:sub>2</jats:sub> MLs, we assign the WSSe MLs to the ML family with the dark ground exciton state. We find that, in addition to the neutral bright A exciton line, three observed emission lines are associated with negatively charged excitons. The application of in-plane and out-of-plane magnetic fields allows us to assign undeniably the bright and dark (spin- and momentum-forbidden) negative trions as well as the phonon replica of the dark spin-forbidden complex. Furthermore, the existence of the single photon emitters in the WSSe ML is also demonstrated, thus prompting the opportunity to enlarge the wavelength range for potential future quantum applications of S-TMDs.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Transition metal phosphorus trichalcogenides retain spin-charge coupling and lattice vibrations in different layers, which are useful for spintronic and optoelectronic devices. The phonon, magnons and excitonic properties of 2D ternary nickel-phosphorus trisulfides (NiPS3) are investigated using Raman spectroscopy and photoluminescence (PL) study. With magnetic exchange interaction, an exotic phonon scattering degenerates the optical phonons into in-plane Ag and Bg modes. We have observed eight Raman modes with two acoustic anisotropic magnon modes (M1, M2) below the critical temperature for co-(XX), while only M1 at cross (XY) polarizations. The M1 mode is coupled with phonon Bg mode that can survive after transition temperature. The phonon and magnon modes soften with temperature variation, which is attributed to anharmonic phonon-phonon coupling and interlayer forces. The polarized Raman shows the 2-fold and 4-fold symmetry orientation of phonon and magnon modes, respectively, which exhibits strong in-plane anisotropic phonon/magnon. The PL spectra revealed the existence of bound excitonic features and ensemble emitters in NiPS3. The robust interlayer excitation and structural stability further revealed the optothermal properties. Moreover, the fabricated field-effect transistor (FET) on NiPS3 reveals p-type semiconducting nature with an ON/OFF ratio~5×106 and mobility of~16.34 cm2V−1s−1, while the rectification ratio indicates their diode characteristics. Similarly, the photocurrent is enhanced by changing the wavelength of light, which shows the potential for optoelectronics. The strong spin-charge interaction provides new insights into these materials' magneto-optical and thermal properties for memory devices.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present a systematic investigation of the edge states of two-dimensional α-bismuthene (α-Bi) structures self-assembled on HOPG substrates, using scanning tunnelling microscopy and scanning tunnelling spectroscopy. The measurements are carried out for 3ML, 5ML and 7ML thick Bi structures. Our spectroscopy studies reveal clear features at the edges of the 5ML and 7ML thick structures, and the positions of the edge states (ESs) coincide with the topographical step edges. In contrast, in 3ML structures the ESs appear to be absent and instead new states are sometimes observed, far from the topographical edge. These states are associated with a moiré pattern and result from strain-induced modulation of the topology. Our observations demonstrate the impact on the edge states of coupling to adjacent structures. </jats:p>

<jats:title>Abstract</jats:title> <jats:p>Two-dimensional group-IV monochalcogenides such as GeS, GeSe, SnS, and SnSe were theoretically predicted as multiferroic materials with two or more ferroic properties. However, most of their bulk crystals are stacked layer by layer with an antiferroelectric manner, which lose the the macroscopic in-plane ferroelectricity. In this work, we studied SnS in which the layers are stacked in a ferroelectric manner both experimentally and theoretically. We utilized polarization-resolved second harmonic generation (SHG) microscopy to investigate numerous flakes of ferroelectric SnS few layers on mica substrates. We found the SHG polar patterns dramatically varied in the range of 800 nm and 1000 nm due to the frequency-dependent SHG susceptibilities. First-principles calculations have been performed to study the frequency-dependent and layer-dependent SHG susceptibilities in the ferroelectric SnS with AA and AC stacking orders. The variation trend of calculated SHG polar patterns as a function of frequency agrees well with that of the experimental results. Since polarization-resolved SHG is a noncontact and nondestructive technique to determine the crystal orientation, understanding of its properties is important, especially for monitoring the transition of different ferroic phases.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Van der Waals (vdW) magnets are appealing candidates for realising spintronic devices that exploit current control of magnetization (e.g. switching or domain wall motion), but so far experimental demonstrations have been sparse, in part because of challenges associated with imaging the magnetization in these systems. Widefield nitrogen-vacancy (NV) microscopy allows rapid, quantitative magnetic imaging across entire vdW flakes, ideal for capturing changes in the micromagnetic structure due to an electric current. Here we use a widefield NV microscope to study the effect of current injection in thin flakes (∽10 nm) of the vdW ferromagnet Fe3GeTe2 (FGT). We first observe current-reduced coercivity on an individual domain level, where current injection in FGT causes substantial reduction in the magnetic field required to locally reverse the magnetisation. We then explore the possibility of current-induced domain-wall motion, and provide preliminary evidence for such a motion under relatively low current densities, suggesting the existence of strong current-induced torques in our devices. Our results illustrate the applicability of widefield NV microscopy to imaging spintronic phenomena in vdW magnets, highlight the possibility of efficient magnetization control by direct current injection without assistance from an adjacent conductor, and motivate further investigations of the effect of currents in FGT and other vdW magnets.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Modification of the structure and morphology of MXene electrodes and the formulation of the electrolytes used in their supercapacitor configurations are significant factors affecting the performance of electrochemical devices. In this study, we investigated the electrochemical performance and ion dynamics of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [EmimTFSI], ionic liquid in the presence of acetonitrile (ACN) at different concentrations in Ti3C2Tx MXene supercapacitor. We found an optimum concentration of ACN, at which more cations from the ionic liquid attach to the MXene electrode surface, providing higher electrochemical performance. This higher capacitance is also associated with increased microscopic dynamics of the cation away from the pore wall. These findings give a guideline to optimize the performance of MXene-based supercapacitors using organic solvents-ionic liquid-based electrolyte systems.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Orthogonal α-MoO<jats:sub>3</jats:sub> is one of the most common and air-stable compounds of molybdenum, holding the merits of wide bandgap, Van der Waals (vdW) structure, biaxial symmetry and recently discovered hyperbolic topological transitions, which has drawn significant attention in developing novel nanophotonic and optoelectronic devices. Herein the broadband optical anisotropy, one of the most fundamental physical characteristics of α-MoO<jats:sub>3</jats:sub> crystal, was systematically investigated using a combination of spectroscopic ellipsometry (SE) and reflectance difference spectroscopy (RDS). The centimeter-level high-quality α-MoO<jats:sub>3</jats:sub> crystal was grown by modified physical vapor deposition. The optical refractive indices along three crystalline axes were precisely determined by SE in the broad spectral range (400-1600 nm), and then the in-plane and out-plane birefringence was analyzed. Both the intrinsic and resonant cavity modulated optical anisotropy of α-MoO<jats:sub>3</jats:sub> was studied by polarization-resolved RDS, from which we find the physical origins of linear dichroism are dominated by electronical transitions along the c-axis. Furthermore, the external photonic cavity of SiO<jats:sub>2</jats:sub> enables enhanced sensitivity to view electronical transitions and a high modulation ratio of optical anisotropy reached 30, which provides new opportunities to tune optical anisotropy for polarized photonic devices. Our results can help understand the physical origin of the highly optical anisotropy of α-MoO<jats:sub>3</jats:sub> and establish an effective metrological tool to study other types of vdW crystals.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>In the past decade, flexible strain sensors have attracted much attention in the fields of health care, soft robots and other flexible electronics due to their unique flexibility, high stability, and strong mechanical properties. To further meet the requirements of the excellent performance for electronic equipment, carbon-based conductive sensitive materials have become one of the first choice for the preparation of flexible strain sensors due to their excellent electrical conductivity, mechanical properties, and high compatibility. Herein, based on different strain behaviors, this paper analyzes the working mechanism of tensile and compressive strain sensors, focusing on the latest research progress of carbon-based conductive materials in strain sensors with different dimensions. The applications of carbon-based sensitive materials with multifunctional strain sensing in the areas of physiological information detection, human motion, human-machine interaction, and visual display have also been summarized. Furthermore, it has been attempted to discuss the current challenges of carbon-based strain sensors as well as the prospect of flexible strain sensors. This review is aimed to provide appropriate references for further exploitation of multi-functional flexible carbon-based strain sensors.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Graphene and many 2D carbon allotropes are good support materials for single-atom catalysts (SACs) and have been successfully applied to many catalytic reactions. Herein, based on the egg tray graphene (ETG), a carbon allotrope constructed in our previous report, we designed ETG and three N-doped ETG supported Pd SACs, Pd@ETG-Nx (x=0~3), for dehydrogenation of formic acid (HCOOH) by density functional theory. Our calculations show that ETG is easier for N doping than graphene, and Pd single atom can be stably adsorbed on the ETG with different N doping concentrations. Major pathways of formic acid dehydrogenation and dehydration were identified. We found that HCOOH dehydrogenation proceeds along the COOH-mediated pathway on each catalyst. With the increased N content in the substrate, the activity and H2 selectivity of Pd SACs are greatly improved. Especially, among these four SACs, Pd@ETG-N3 shows the best catalytic performance, which is even better than Pd(111). Furthermore, electronic analysis was made to reveal the metal-support interactions and the origin of the activity trend of Pd@ETG-Nx. Our study reveals the unique potential of carbon allotropes in catalyst design, and provides theoretical insights for rational design of efficient catalysts by adjusting the support and the coordination environment.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Two-dimensional transition-metal carbides/nitrides (MXenes) have been intensively investigated as electrode materials for electrochemical energy storage devices, such as batteries and supercapacitors, due to their high capacitance, high-rate capability, and good cycle stability. Although MXenes possess various surface termination groups (e.g., –O, –OH, –F, –Cl, and –Br) that directly interact with adsorbed cations to exhibit charge transfer, the influence of each surface termination group on the electrochemical properties of MXene remains elusive because of difficulty in achieving exclusively modified termination. Herein, we report the electrochemical properties of MXenes with surface termination groups controlled by using fluorine-based aqueous solutions and molten salts as etchants. In aqueous electrolytes, Ti<jats:sub>3</jats:sub>C<jats:sub>2</jats:sub>Cl<jats:italic> <jats:sub>x</jats:sub> </jats:italic> and Ti<jats:sub>3</jats:sub>C<jats:sub>2</jats:sub>Br<jats:italic> <jats:sub>x</jats:sub> </jats:italic> synthesized using molten salts show no electrochemical activity in contrast to Ti<jats:sub>3</jats:sub>C<jats:sub>2</jats:sub>T<jats:italic> <jats:sub>x</jats:sub> </jats:italic> (T = O, OH, F, and Cl). Meanwhile, in a nonaqueous electrolyte, Ti<jats:sub>3</jats:sub>C<jats:sub>2</jats:sub>Cl<jats:italic> <jats:sub>x</jats:sub> </jats:italic> and Ti<jats:sub>3</jats:sub>C<jats:sub>2</jats:sub>Br<jats:italic> <jats:sub>x</jats:sub> </jats:italic> undergo amorphization upon the initial lithiation. These results suggest that the –O, –OH, and –F terminations play an important role in the electrochemical properties of MXene electrodes.</jats:p>