Atomic-layered hexagonal boron nitride (hBN) is expected to be the best two-dimensional (2D) anti-oxidation layer on metals for its incomparable impermeability, insulativity, and stability, as well as the progressive bottom-up growth techniques to ensure fast coating on metal surface in large area. However, its real anti-oxidation ability in practice is found to be unsatisfactory and nonuniform, and the main obstacle to achieving ideal anti-oxidation performance lies in unclear anti-oxidation behavior at special interface between 2D hBN and three-dimensional (3D) metals. Herein, system of monolayer hBN grown on copper (Cu) foils with various lattice orientations was grown to investigate the anti-oxidation behavior of different interlayer configurations. By using structural characterizations together with analysis of topography, we surprisingly found that stronger interlayer coupling led to worse anti-oxidation performance owing to fast diffusion of O₂ through higher hBN corrugations generated at the commensurate hBN/Cu(111) configuration. In view of this, we developed the approach of cyclic reannealing that can effectively flatten corrugations and steps, and therefore improve the anti-oxidation performance to a great extent. This work provides a more in-depth understanding of anti-oxidation behavior of 2D materials grown on 3D metals, and a practical method to pave the way for its large-scale applications in future.

The state-of-the-art semiconductor industry is built on the successful production of silicon ingot with extreme purity as high as 99.999999999%, or the so-called "eleven nines". The coming high-end applications of graphene in electronics and optoelectronics will inevitably need defect-free pure graphene as well. Due to its two-dimensional (2D) characteristics, graphene restricts all the defects on its surface and has the opportunity to eliminate all kinds of defects, i.e., line defects at grain boundaries and point or dot defects in grains, and produce intrinsically pure graphene. In the past decade, epitaxy growth has been adopted to grow graphene by seamlessly stitching of aligned grains and the line defects at grain boundaries were eliminated finally. However, as for the equally common dot and point defects in graphene grain, there are rare ways to detect or reduce them with high throughput and efficiency. Here, we report a methodology to realize the production of ultrapure graphene grown on copper by eliminating both the dot and point defects in graphene grains. The dot defects, proved to be caused by the silica particles shedding from quartz tube during the high-temperature growth, were excluded by a designed heat-resisting box to prevent the deposition of particles on the copper surface. The point defects were optically visualized by a mild-oxidation-assisted method and further reduced by etching-regrowth process to an ultralow level of less than 1/1, 000 μm². Our work points out an avenue for the production of intrinsically pure graphene and thus lays the foundation for the large-scale graphene applications at the integrated-circuit level.

Strong geometrical confinement and reduced dielectric screening of two-dimensional (2D) materials leads to strong Coulomb interaction and eventually give rise to extraordinary excitonic effects, which dominates the optical and optoelectronic properties. For nonlinear 2D photonic or optoelectronic applications, excitonic effects have been proved effective to tune the light-matter interaction strength. However, the modulation of excitonic effects on the other aspect of nonlinear response, i.e., polarization dependence, has not been fully explored yet. Here we report the first systemic study on the modulation of excitonic effects on the polarization dependence of second and third harmonic generation (SHG and THG) in strained monolayer WS₂ by varying excitation wavelength. We demonstrated that polarization-dependent THG patterns undergo a giant evolution near two-photon excitonic resonance, where the long-axis of the parallel component (originally parallel to the strain direction) has a 90° flip when the excitation wavelength increases. In striking contrast, no apparent variation of polarization-dependent SHG patterns occurs at either two- or three-photon excitonic resonance conditions. Our results open a new avenue to modulate the anisotropic nonlinear optical response of 2D materials through effective control of excitonic resonance states, and thus open opportunity for new designs and applications in nonlinear optoelectronic 2D devices.

The continuous pursuit of miniaturization in the electronics and optoelectronics industry demands all device components with smaller size and higher performance, in which thin metal film is one heart material as conductive electrodes. However, conventional metal films are typically polycrystalline with random domain orientations and various grain boundaries, which greatly degrade their mechanical, thermal and electrical properties. Hence, it is highly demanded to produce single-crystal metal films with epitaxy in an appealing route. Traditional epitaxy on non-metal single-crystal substrates has difficulty in exfoliating away due to the formation of chemical bonds. Newly developed epitaxy on single-crystal graphene enables the easy exfoliation of epilayers but the annealing temperature must be high (typical 500–1, 000 ℃ and out of the tolerant range of integrated circuit technology) due to the relative weak interfacial interactions. Here we demonstrate the facile production of 6-inch transferable high-quality Pd(111) films on single-crystal hybrid graphene/Cu(111) substrate with CMOS-compatible annealing temperature of 150 ℃ only. The interfacial interaction between Pd and hybrid graphene/Cu(111) substrate is strong enough to enable the low-temperature epitaxy of Pd(111) films and weak enough to facilitate the easy film release from substrate. The obtained Pd(111) films possess superior properties to polycrystalline ones with ~ 0.25 eV higher work function and almost half sheet resistance. This technique is proved to be applicable to other metals, such as Au and Ag. As the single-crystal graphene/Cu(111) substrates are obtained from industrial Cu foils and accessible in meter scale, our work will promote the massive applications of large-area high-quality metal films in the development of next-generation electronic and optoelectronic devices.

Beyond graphene, two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted significant attention owing to their potential in next-generation nanoelectronics and optoelectronics. Nevertheless, grain boundaries are ubiquitous in large-area as-grown TMD materials and would significantly affect their band structure, electrical transport, and optical properties. Therefore, the characterization of grain boundaries is essential for engineering the properties and optimizing the growth in TMD materials. Although the existence of boundaries can be measured using scanning tunneling microscopy, transmission electron microscopy, or nonlinear optical microscopy, a universal, convenient, and accurate method to detect boundaries with a twist angle over a large scale is still lacking. Herein, we report a high-throughput method using mild hot H₂O etching to visualize grain boundaries of TMDs under an optical microscope, while ensuring that the method is nearly noninvasive to grain domains. This technique utilizes the reactivity difference between stable grain domains and defective grain boundaries and the mild etching capacity of hot water vapor. As grain boundaries of two domains with twist angles have defective lines, this method enables to visualize all types of grain boundaries unambiguously. Moreover, the characterization is based on an optical microscope and therefore naturally of a large scale. We further demonstrate the successful application of this method to other TMD materials such as MoS₂ and WSe₂. Our technique facilitates the large-area characterization of grain boundaries and will accelerate the controllable growth of large single-crystal TMDs.