Spatially Controlled Phase Transition in MoTe2 Driven by Focused Ion Beam Irradiations.

Phase transitions play an important role in tuning the physical properties of two-dimensional (2D) materials as well as developing their high-performance device applications. Here, we reported the observation of a phase transition in few-layered MoTe2 flakes by the irradiation of gallium (Ga+) ions using a focused ion beam (FIB) system. The semiconducting 2H phase of MoTe2 can be controllably converted to the metallic 1T'-like phase via Te defect engineering during irradiations. By taking advantage of the nanometer-sized Ga+ ion probe proved by FIB, in-plane 1T'-2H homojunctions of MoTe2 at submicrometer scale can be fabricated. Furthermore, we demonstrate the improvement of device performance (on-state current over 2 orders of magnitude higher) in MoTe2 transistors using the patterned 1T'-like phase regions as contact electrodes. Our study provides a new strategy to drive the phase transitions in MoTe2, tune their properties, and develop high-performance devices, which also extends the applications of FIB technology in 2D materials and their devices.

Enhancement and modulation of valley polarization in Janus CrSSe with internal and external electric fields.

The valley polarization, induced by the magnetic proximity effect, in monolayer transition metal dichalcogenides (TMDCs), has attracted significant attention due to the intriguing fundamental physics. However, the enhancement and modulation of valley polarization for real device applications is still a challenge. Here, using first-principles calculations we investigate the valley polarization properties of monolayer TMDCs CrS2 and CrSe2 and how to enhance the valley polarization by constructing Janus CrSSe (with an internal electric field) and modulate the polarization in CrSSe by applying external electric fields. Janus CrSSe exhibits inversion symmetry breaking, internal electric field, spin-orbit coupling, and compelling spin-valley coupling. A magnetic substrate of the MnO2 monolayer can induce a modest magnetic moment in CrSe2, CrSe2, and CrSSe. Notably, the Janus structure with an internal electric field has a much larger valley p compared with its non-Janus counterparts. Moreover, the strength of valley polarization can be further modulated by applying external electric fields. These findings suggest that Janus materials hold promise for designing and developing advanced valleytronic devices.

A broad-spectrum gas sensor based on correlated two-dimensional electron gas.

Designing a broad-spectrum gas sensor capable of identifying gas components in complex environments, such as mixed atmospheres or extreme temperatures, is a significant concern for various technologies, including energy, geological science, and planetary exploration. The main challenge lies in finding materials that exhibit high chemical stability and wide working temperature range. Materials that amplify signals through non-chemical methods could open up new sensing avenues. Here, we present the discovery of a broad-spectrum gas sensor utilizing correlated two-dimensional electron gas at a delta-doped LaAlO3/SrTiO3 interface with LaFeO3. Our study reveals that a back-gating on this two-dimensional electron gas can induce a non-volatile metal to insulator transition, which consequently can activate the two-dimensional electron gas to sensitively and quantitatively probe very broad gas species, no matter whether they are polar, non-polar, or inert gases. Different gas species cause resistance change at their sublimation or boiling temperature and a well-defined phase transition angle can quantitatively determine their partial pressures. Such unique correlated two-dimensional electron gas sensor is not affected by gas mixtures and maintains a wide operating temperature range. Furthermore, its readout is a simple measurement of electric resistance change, thus providing a very low-cost and high-efficient broad-spectrum sensing technique.

Laser irradiation induced structural transformation in layered transition metal trichalcogenide nanoflakes.

Laser irradiation is a powerful tool in inducing changes in lattice structures and properties of two-dimensional (2D) materials through processes such as heating, bleaching, catalysis, etc. However, the underlying mechanisms of such transformations vary dramatically in different 2D materials. Here, we report the structural transformation of layered titanium trisulfide (TiS3) to titanium disulfide (TiS2) after irradiation. We systematically characterized the dependence of the transformation on laser power, flake thickness, irradiation time, and vacuum conditions using microscopic and spectroscopic methods. The underlying mechanism is confirmed as the heat-induced materials decomposition, a process that also occurs in many other transition metal trichalcogenide materials. Furthermore, we demonstrate that this spatial-resolved method also enables the creation of in-plane TiS3-TiS2 heterostructures. Our study identifies a new family of 2D materials that undergo a structural transformation after laser irradiation and enriches the methods available for developing new prototypes of low-dimensional devices in the future.

The Antiparallel Coiled-Coil Domain Allows Multiple Forward Step Sizes of Myosin X.

Myosin X forms an antiparallel dimer and moves processively on actin bundles. How the antiparallel dimer affects the stepping mechanism of myosin X remains elusive. Here, we generated several chimeras using domains of myosin V and X and performed single-molecule motility assays. We found that the chimera containing the motor domain from myosin V and the lever arm and antiparallel coiled-coil domain from myosin X has multiple forward step sizes and moves processively, similar to full-length myosin X. The chimera containing the motor domain and lever arm from myosin X and the parallel coiled-coil from myosin V takes steps of ∼40 nm at lower ATP concentrations but was nonprocessive at higher ATP concentrations. Furthermore, mutant myosin X with four mutations in the antiparallel coiled-coil domain failed to dimerize and was nonprocessive. These results imply that the antiparallel coiled-coil domain is necessary for multiple forward step sizes of myosin X.

Quasi-One-Dimensional ZrS3 Nanoflakes for Broadband and Polarized Photodetection with High Tuning Flexibility.

Two-dimensional (2D) layered materials with low crystal symmetries have exhibited unique anisotropic physical properties. Here, we report systematic studies on the photoresponse of field effect transistors (FETs) fabricated using quasi-one-dimensional ZrS3 nanoflakes. The as-fabricated phototransistors exhibit a broadband photocurrent response from ultraviolet to visible regions, where the responsivity and detectivity can be enhanced via additional gate voltages. Furthermore, benefiting from the strong in-plane anisotropy of ZrS3, we observe a gate-voltage and illumination wavelength-dependent polarized photocurrent response, while its sub-millisecond-time response speed is also polarization-dependent. Our results demonstrate the flexible tunability of photodetectors based on anisotropic layered semiconductors, which substantially broadens the application of low symmetry layered materials in polarization-sensitive optoelectronic devices.

Few-Layered MnAl2S4 Dielectrics for High-Performance van der Waals Stacked Transistors.

The gate dielectric layer is an important component in building a field-effect transistor. Here, we report the synthesis of a layered rhombohedral-structured MnAl2S4 crystal, which can be mechanically exfoliated down to the monolayer limit. The dielectric properties of few-layered MnAl2S4 flakes are systematically investigated, whereby they exhibit a relative dielectric constant of over 6 and an electric breakdown field of around 3.9 MV/cm. The atomically smooth thin MnAl2S4 flakes are then applied as a dielectric top gate layer to realize a two-dimensional van der Waals stacked field-effect transistor, which uses MoS2 as a channel material. The fabricated transistor can be operated at a small drain-source voltage of 0.1 V and gate voltages within ranges of ±2 V, which exhibit a large on-off ratio over 107 at 0.5 V and a low subthreshold swing value of 80 mV/dec. Our work demonstrates that the few-layered MnAl2S4 can work as a dielectric layer to realize high-performance two-dimensional transistors, and thus broadens the research on high-κ 2D materials and may provide new opportunities in developing low-dimensional electronic devices with a low power consumption in the future.

Nonvolatile Ferroelectric Memory with Lateral ß/α/ß In2Se3 Heterojunctions.

The electric dipole locking effect observed in van der Waals (vdW) ferroelectric α-In2Se3 has resulted in a surge of applied research in electronics with nonvolatile functionality. However, ferroelectric tunnel junctions with advantages of lower power consumption and faster writing/reading operations have not been realized in α-In2Se3. Here, we demonstrate the tunneling electroresistance effect in a lateral ß/α/ß In2Se3 heterojunction built by local laser irradiation. Switchable in-plane polarizations of the vdW ferroelectric control the tunneling conductance of the heterojunction device by 4000% of magnitude. The electronic logic bit can be represented and stored with different orientations of electric dipoles. This prototype enables a new approach to rewritable nonvolatile memory with in-plane ferroelectricity in vdW 2D materials.

Multistep nucleation visualized during solid-state crystallization.

Mechanisms of nucleation have been debated for more than a century, despite successes of classical nucleation theory. The nucleation process has been recently argued as involving a nonclassical mechanism (the "two-step" mechanism) in which an intermediate step occurs before the formation of a nascent ordered phase. However, a thorough understanding of this mechanism, in terms of both microscopic kinetics and thermodynamics, remains experimentally challenging. Here, in situ observations using transmission electron microscopy on a solid-state nucleation case indicate that early-stage crystallization can follow the non-classical pathway, yet proceed via a more complex manner in which multiple metastable states precede the emergence of a stable nucleus. The intermediate steps were sequentially isolated as spinodal decomposition of amorphous precursor, mass transport and structural oscillations between crystalline and amorphous states. Our experimental and theoretical analyses support the idea that the energetic favorability is the driving force for the observed sequence of events. Due to the broad applicability of solid-state crystallization, the findings of this study offer new insights into modern nucleation theory and a potential avenue for materials design.

Silicon nitride waveguides with directly grown WS2 for efficient second-harmonic generation.

Different functions can be directly realized by silicon (Si) in integrated electronic circuits. Although Si and silicon nitride (Si3N4) photonics have shown great potential in integrated optoelectronic devices, different functions, such as light generation, transparency for guided light, and light detection, cannot be simultaneously achieved only by Si or Si3N4. Second-order nonlinearity is another optical property they do not possess due to their centrosymmetric properties. Several kinds of 2D materials emerged recently and were transferred to specified photonic devices aimed at improving their nonlinear performance. However, the transferring methods are time-consuming, unable to achieve large-scale production, and will inevitably cause materials damage and introduce impurities at the interface. Herein, we demonstrate the direct growth of large-area homogeneous monolayer WS2via a physical vapor deposition method onto Si3N4 waveguides. The WS2 growth can be controlled mainly along the Si3N4 waveguides and the waveguides show an obvious enhancement of second-harmonic generation with the elongated WS2 coverage. The direct growth of WS2 endows Si3N4 integrated photonics with new nonlinear optical properties. As an alternative method of transferring 2D materials, the method we present here is compatible with large-scale integrated photonic fabrication, which lays the foundation for on-chip integrated optical fabrication and applications.

Visualization of Bubble Nucleation and Growth Confined in 2D Flakes.

The nucleation and growth of bubbles within a solid matrix is a ubiquitous phenomenon that affects many natural and synthetic processes. However, such a bubbling process is almost "invisible" to common characterization methods because it has an intrinsically multiphased nature and occurs on very short time/length scales. Using in situ transmission electron microscopy to explore the decomposition of a solid precursor that emits gaseous byproducts, the direct observation of a complete nanoscale bubbling process confined in ultrathin 2D flakes is presented here. This result suggests a three-step pathway for bubble formation in the confined environment: void formation via spinodal decomposition, bubble nucleation from the spherization of voids, and bubble growth by coalescence. Furthermore, the systematic kinetics analysis based on COMSOL simulations shows that bubble growth is actually achieved by developing metastable or unstable necks between neighboring bubbles before coalescing into one. This thorough understanding of the bubbling mechanism in a confined geometry has implications for refining modern nucleation theories and controlling bubble-related processes in the fabrication of advanced materials (i.e., topological porous materials).

Atomic Steps Induce the Aligned Growth of Ice Crystals on Graphite Surfaces.

Heterogeneous ice nucleation on atmospheric aerosols strongly affects the earth's climate, and at the microscopic level, surface-irregularity-induced ice crystallization behaviors are common but crucial. Because of the lack of visual evidence and effective experimental methods, the mechanism of atomic-structure-dependent ice formation on aerosol surfaces is poorly understood. Here we chose highly oriented pyrolytic graphite (HOPG) to represent soot (a primary aerosol), and environmental scanning electron microscopy (ESEM) was performed for in situ observations of ice formation. We found that hexagonal ice crystals show an aligned growth pattern via a two-stage pathway with one a axis coinciding with the direction of atomic step edges on the HOPG surface. Additionally, the ice crystals grow at a noticeably higher speed along this direction. This study reveals the role of atomic surface defects in heterogeneous ice nucleation and may pave the way to control icing-related processes in practical applications.

Photoflexoelectric effect in halide perovskites.

Harvesting environmental energy to generate electricity is a key scientific and technological endeavour of our time. Photovoltaic conversion and electromechanical transduction are two common energy-harvesting mechanisms based on, respectively, semiconducting junctions and piezoelectric insulators. However, the different material families on which these transduction phenomena are based complicate their integration into single devices. Here we demonstrate that halide perovskites, a family of highly efficient photovoltaic materials1-3, display a photoflexoelectric effect whereby, under a combination of illumination and oscillation driven by a piezoelectric actuator, they generate orders of magnitude higher flexoelectricity than in the dark. We also show that photoflexoelectricity is not exclusive to halides but a general property of semiconductors that potentially enables simultaneous electromechanical and photovoltaic transduction and harvesting in unison from multiple energy inputs.

MoS2 Memtransistors Fabricated by Localized Helium Ion Beam Irradiation.

Two-dimensional (2D) layered semiconductors have recently emerged as attractive building blocks for next-generation low-power nonvolatile memories. However, challenges remain in the controllable fabrication of bipolar resistive switching circuit components from these materials. Here, the experimental realization of lateral memtransistors from monolayer single-crystal molybdenum disulfide (MoS2) utilizing a focused helium ion beam is reported. Site-specific irradiation with the focused probe of a helium ion microscope creates a nanometer-scale defect-rich region, bisecting the MoS2 lattice. The reversible drift of these defects in the applied electric field modulates the resistance of the channel, enabling versatile memristive functionality. The device can reliably retain its resistance ratios and set/reset biases for 1180 switching cycles. Long-term potentiation and depression with sharp habituation are demonstrated. This work establishes the feasibility of ion irradiation for controllable fabrication of 2D memristive devices with promising key performance parameters, such as low power consumption. The applicability of these devices for synaptic emulation may address the demands of future neuromorphic architectures.

Bone wax migrates to the orbit in a patient with a frontal sinus abnormality: a case report.

BACKGROUND: Bone wax is the most widely used hemostatic bone sealant because of its availability, ease of use, immediate action, and minimal adverse effects. Several complications have been reported to be associated with the use of bone wax, such as infection, osteohypertrophy, pain, granuloma formation, allergic reaction, and thrombosis. Here, we present a rare complication, namely, bone wax migration, which developed after a craniotomy on a patient who had a frontal sinus abnormality. CASE PRESENTATION: A 51-year-old woman complained of pain and swelling in her left eye accompanied by difficulty opening the left eyelid after undergoing a craniotomy. An examination revealed left eye proptosis with ptosis, eyelid swelling, and increases in intraorbital pressure and intraocular pressure (IOP). According to a CT and an MRI of the orbit, we found that the intraoperative bone wax had migrated to the orbit, thereby causing compression. We also found that the basal frontal sinus of the patient was congenitally defective, which may have induced the migration of the bone wax. Given that the patient recently underwent a craniotomy and given the risks associated with orbital surgery, she refused to undergo a surgery to remove the bone wax. Thus, the patient was administered mannitol intravenously daily, accompanied by topical Timolol, to reduce the intraorbital pressure and IOP. This treatment led to a gradual decrease in IOP and intraorbital pressure, and these parameters remained stable after treatment ended. During the 6-month follow-up, the best corrected visual acuity improved, and ptosis and restricted eye movements also improved significantly. CONCLUSIONS: We report a case of bone wax migration that developed after a craniotomy on a patient who had a congenital defect in the basal frontal sinus. Extra caution should be taken when using bone wax, and a comprehensive understanding of the patient's intracranial anatomy is important for decreasing the incidence of bone wax migration. Additionally, when a patient presents with symptoms of ocular compression, bone wax migration should be considered in addition to typical radiological changes.

Oxide-mediated recovery of field-effect mobility in plasma-treated MoS2.

Precise tunability of electronic properties of two-dimensional (2D) nanomaterials is a key goal of current research in this field of materials science. Chemical modification of layered transition metal dichalcogenides leads to the creation of heterostructures of low-dimensional variants of these materials. In particular, the effect of oxygen-containing plasma treatment on molybdenum disulfide (MoS2) has long been thought to be detrimental to the electrical performance of the material. We show that the mobility and conductivity of MoS2 can be precisely controlled and improved by systematic exposure to oxygen/argon plasma and characterize the material using advanced spectroscopy and microscopy. Through complementary theoretical modeling, which confirms conductivity enhancement, we infer the role of a transient 2D substoichiometric phase of molybdenum trioxide (2D-MoO x ) in modulating the electronic behavior of the material. Deduction of the beneficial role of MoO x will serve to open the field to new approaches with regard to the tunability of 2D semiconductors by their low-dimensional oxides in nano-modified heterostructures.

Programmable graphene doping via electron beam irradiation.

Graphene is a promising candidate to succeed silicon based devices, and the conventional strategies for fabrication and testing of graphene-based electronics often utilise an electron beam. Here, we report on a systematic study of the effect of electron beam exposure on graphene devices. We realise reversible doping of on-chip graphene using a focused electron beam. Our results demonstrate site-specific control of carrier type and concentration achievable by modulating the charge distribution in the substrate. The effect of substrate-embedded charges on carrier mobility and conductivity of graphene is studied, with a dielectric screening model proposed to explain the effective n-type and p-type doping produced at different beam energies. Multiple logic operations are thus implemented in a single graphene sheet by using site-specific e-beam irradiation. We extend the phenomenon to MoS2, generalising it to conductive two-dimensional materials. Our results are of importance to imaging, in situ characterisation and lithographic techniques employed to investigate 2D materials.

Precise milling of nano-gap chains in graphene with a focused helium ion beam.

A focused helium ion beam was used to introduce nano-sized gap chains in graphene. The effect of beam scanning strategies in the fabrication of the nano-gap chains was investigated. The tuning of graphene conductivity has been achieved by modulating the magnitude and uniformity of the ion dose and hence the morphology of the nano-gap chains. A model based on the site-specific and dose-dependent conductivity was built to understand the tuning of the conductivity, taking into account the nanoscale non-uniformity of irradiation.

Sub-5 nm graphene nanopore fabrication by nitrogen ion etching induced by a low-energy electron beam.

A flexible and efficient method to fabricate nanopores in graphene has been developed. A focused, low-energy (5 keV) electron beam was used to locally activate etching of a graphene surface in a low pressure (0.3 Pa) N2 environment. Nanopores with sub-5 nm diameters were fabricated. The lattice structure of the graphene was observed to recover within 20 nm of the nanopore edge. Nanopore growth rates were investigated systematically. The effects of nitrogen pressure, electron beam dwell time and beam current were characterised in order to understand the etching mechanism and enable optimisation of the etching parameters. A model was developed which describes how the diffusion of ionised nitrogen affects the nanopore growth rate. Etching of other two-dimensional materials was attempted as demonstrated with MoS2. The lack of etching observed supports our model of a chemical reaction-based mechanism. The understanding of the etching mechanism will allow more materials to be etched by selection of an appropriate ion species.

Quantitative secondary electron imaging for work function extraction at atomic level and layer identification of graphene.

Two-dimensional (2D) materials usually have a layer-dependent work function, which require fast and accurate detection for the evaluation of their device performance. A detection technique with high throughput and high spatial resolution has not yet been explored. Using a scanning electron microscope, we have developed and implemented a quantitative analytical technique which allows effective extraction of the work function of graphene. This technique uses the secondary electron contrast and has nanometre-resolved layer information. The measurement of few-layer graphene flakes shows the variation of work function between graphene layers with a precision of less than 10 meV. It is expected that this technique will prove extremely useful for researchers in a broad range of fields due to its revolutionary throughput and accuracy.