Dynamics of optical vortices in 2D materials

Optical vortices in planar geometries are a universal wave phenomenon, where electromagnetic waves possess topologically protected integer values of orbital angular momentum (OAM). The conservation of OAM governs their dynamics, including their rules of creation and annihilation. However, such dynamics remained so far beyond experimental reach. Here, we present a first observation of creation and annihilation of optical vortex pairs. The vortices conserve their combined OAM during pair creation/annihilation events and determine the field profile throughout their motion between these events. We utilize free electrons in an ultrafast transmission electron microscope to probe the vortices, which appear in the form of phonon polaritons in the 2D material hexagonal boron nitride. These results provide the first observation of optical vortices in any 2D material, which were predicted but never observed. Our findings promote future investigation of vortices in 2D materials and their use for chiral plasmonics, toward the control of selection rules in light-matter interactions and the creation of optical simulators of phase transitions in condensed matter physics.

High-efficiency 2D nanosheet exfoliation by a solid suspension-improving method

Two-dimensional (2D) materials with mono or few layers have wide application prospects, including electronic, optoelectronic, and interface functional coatings in addition to energy conversion and storage applications. However, the exfoliation of such materials is still challenging due to their low yield, high cost, and poor ecological safety in preparation. Herein, a safe and efficient solid suspension-improving method was proposed to exfoliate hexagonal boron nitride nanosheets (hBNNSs) in a large yield. The method entails adding a permeation barrier layer in the solvothermal kettle, thus prolonging the contact time between the solvent and hexagonal boron nitride (hBN) nanosheetand improving the stripping efficiency without the need for mechanical agitation. In addition, the proposed method selectively utilizes a matching solvent that can reduce the stripping energy of the material and employs a high-temperature steam shearing process. Compared with other methods, the exfoliating yield of hBNNSs is up to 42.3% at 150°C for 12 h, and the strategy is applicable to other 2D materials. In application, the ionic conductivity of a PEO/hBNNSs composite electrolytes reached 2.18×10−4 S cm−1 at 60°C. Overall, a versatile and effective method for stripping 2D materials in addition to a new safe energy management strategy were provided.

Photovoltage Oscillations in Encapsulated Graphene

We theoretically analyze the rise of photovoltage oscillations in hexagonal boron-nitride (h-BN) encapsulated monolayer graphene (h-BN/graphene/h-BN) when irradiated with terahertz radiation. We use an extension of the radiation-driven electron orbit model, successfully applied to study the oscillations obtained in irradiated magnetotransport of GaAs/AlGaAs heterostructures. The extension takes mainly into account that now the carriers are massive Dirac fermions. Our simulations reveal that the photovoltage in these graphene systems presents important oscillations similar to the ones of irradiated magnetoresistance in semiconductor platforms but in the terahertz range. We also obtain that these oscillations are clearly affected by the voltages applied to the sandwiched graphene: a vertical gate voltage between the two hBN layers and an external positive voltage applied to one of the sample sides. The former steers the carrier effective mass and the latter the photovoltage intensity and the oscillations amplitude. The frequency dependence of the photo-oscillations is also investigated.

Artificially created interfacial states enabled van der Waals heterostructure memory device

Two-dimensional (2D) interface plays a predominate role in determining the performance of a device that is configured as a van der Waals heterostructure (vdWH). Intensive efforts have been devoted to suppressing the emergence of interfacial states during vdWH stacking process, which facilitates the charge interaction and transfer between the heterostructure layers. However, the effective generation and modulation of the vdWH interfacial states could give rise to a new design and architecture of 2D functional devices. Here, we report a 2D non-volatile vdWH memory device enabled by the artificially created interfacial states between hexagonal boron nitride (hBN) and molybdenum ditelluride (MoTe2). The memory originates from the microscopically coupled optical and electrical responses of the vdWH, with the high reliability reflected by its long data retention time over 10^4 s and large write-erase cyclic number exceeding 100. Moreover, the storage currents in the memory can be precisely controlled by the writing and erasing gates, demonstrating the tunability of its storage states. The vdWH memory also exhibits excellent robustness with wide temperature endurance window from 100 K to 380 K, illustrating its potential application in harsh environment. Our findings promise interfacial-states engineering as a powerful approach to realize high performance vdWH memory device, which opens up new opportunities for its application in 2D electronics and optoelectronics.

Analogous Atomic and Electronic Properties between V N and V N C B Defects in Hexagonal Boron Nitride

We investigate defect properties in hexagonal boron nitride (hBN) which is attracting much attention as a single photon emitter. Using first-principles calculations, we find that nitrogen-vacancy defect V N has a lower energy structure in C 1 h symmetry in 1− charge state than the previously known D 3 h symmetry structure. Noting that carbon has one more valence electron than boron species, our finding naturally points to the correspondence between V N and V N C B defects with one charge state difference between them, which is indeed confirmed by the similarity of atomic symmetries, density of states, and excitation energies. Since V N C B is considered as a promising candidate for the source of single photon emission, our study suggests V N as another important candidate worth attention, with its simpler form without the incorporation of foreign elements into the host material.

Corrosion and Wear Properties of h-BN Modified TC4 Titanium Alloy Micro-Arc Oxide Coatings

In this study, ceramic coatings were prepared on the surface of TC4 titanium alloy by micro-arc oxidation (MAO). The morphology, element distribution and phase composition of MAO coatings were analyzed by SEM, EDS, XRD and other analytical methods. The effect of hexagonal boron nitride(h-BN) doping on wear resistance and corrosion resistance of micro-arc oxidation layer was studied. The results show that the coating is mainly composed of rutile TiO2, anatase TiO2 and a small amount of h-BN. Furthermore, the composite coating containing h-BN was less porous than particle-free coating. The test results show that h-BN doping slightly affects the hardness of the MAO coating, and it is helpful in improving the thickness, corrosion resistance and wear resistance of the coatings. When the amount of h-BN is 3 g/L, the corrosion current density of the coating is the smallest; When the addition of h-BN is 1.5 g/L, the friction coefficient of the coating is the smallest. The wear mechanism was adhesive wear, accompanied by slight abrasive wear.