Showing papers by "Qunyang Li published in 2019"

Epitaxial growth of a 100-square-centimetre single-crystal hexagonal boron nitride monolayer on copper.

Abstract: The development of two-dimensional (2D) materials has opened up possibilities for their application in electronics, optoelectronics and photovoltaics, because they can provide devices with smaller size, higher speed and additional functionalities compared with conventional silicon-based devices1. The ability to grow large, high-quality single crystals for 2D components—that is, conductors, semiconductors and insulators—is essential for the industrial application of 2D devices2–4. Atom-layered hexagonal boron nitride (hBN), with its excellent stability, flat surface and large bandgap, has been reported to be the best 2D insulator5–12. However, the size of 2D hBN single crystals is typically limited to less than one millimetre13–18, mainly because of difficulties in the growth of such crystals; these include excessive nucleation, which precludes growth from a single nucleus to large single crystals, and the threefold symmetry of the hBN lattice, which leads to antiparallel domains and twin boundaries on most substrates19. Here we report the epitaxial growth of a 100-square-centimetre single-crystal hBN monolayer on a low-symmetry Cu (110) vicinal surface, obtained by annealing an industrial copper foil. Structural characterizations and theoretical calculations indicate that epitaxial growth was achieved by the coupling of Cu step edges with hBN zigzag edges, which breaks the equivalence of antiparallel hBN domains, enabling unidirectional domain alignment better than 99 per cent. The growth kinetics, unidirectional alignment and seamless stitching of the hBN domains are unambiguously demonstrated using centimetre- to atomic-scale characterization techniques. Our findings are expected to facilitate the wide application of 2D devices and lead to the epitaxial growth of broad non-centrosymmetric 2D materials, such as various transition-metal dichalcogenides20–23, to produce large single crystals. The epitaxial growth of large single-crystal hexagonal boron nitride monolayers on low-symmetry copper foils is demonstrated.

Spontaneous droplets gyrating via asymmetric self-splitting on heterogeneous surfaces

Abstract: Droplet impacting and bouncing off solid surface plays a vital role in various biological/physiological processes and engineering applications. However, due to a lack of accurate control of force transmission, the maneuver of the droplet movement and energy conversion is rather primitive. Here we show that the translational motion of an impacting droplet can be converted to gyration, with a maximum rotational speed exceeding 7300 revolutions per minute, through heterogeneous surface wettability regulation. The gyration behavior is enabled by the synergetic effect of the asymmetric pinning forces originated from surface heterogeneity and the excess surface energy of the spreading droplet after impact. The findings open a promising avenue for delicate control of liquid motion as well as actuating of solids.

Deep neural network method for predicting the mechanical properties of composites

Abstract: Determining the macroscopic mechanical properties of composites with complex microstructures is a key issue in many of their applications. In this Letter, a machine learning-based approach is proposed to predict the effective elastic properties of composites with arbitrary shapes and distributions of inclusions. Using several data sets generated from the finite element method, a convolutional neural network method is developed to predict the effective Young's modulus and Poisson's ratio of composites directly from a window of their microstructural image. Through numerical experiments, we demonstrate that the trained network can efficiently provide an accurate mapping between the effective mechanical property and the microstructures of composites with complex structures. This study paves a way for characterizing heterogeneous materials in big data-driven material design.

Tuning friction to a superlubric state via in-plane straining

Abstract: Controlling, and in many cases minimizing, friction is a goal that has long been pursued in history. From the classic Amontons-Coulomb law to the recent nanoscale experiments, the steady-state friction is found to be an inherent property of a sliding interface, which typically cannot be altered on demand. In this work, we show that the friction on a graphene sheet can be tuned reversibly by simple mechanical straining. In particular, by applying a tensile strain (up to 0.60%), we are able to achieve a superlubric state (coefficient of friction nearly 0.001) on a suspended graphene. Our atomistic simulations together with atomically resolved friction images reveal that the in-plane strain effectively modulates the flexibility of graphene. Consequently, the local pinning capability of the contact interface is changed, resulting in the unusual strain-dependent frictional behavior. This work demonstrates that the deformability of atomic-scale structures can provide an additional channel of regulating the friction of contact interfaces involving configurationally flexible materials.

Modeling Atomic-Scale Electrical Contact Quality Across Two-Dimensional Interfaces.

Abstract: Contacting interfaces with physical isolation and weak interactions usually act as barriers for electrical conduction. The electrical contact conductance across interfaces has long been correlated ...

Impacts of the substrate stiffness on the anti-wear performance of graphene

Abstract: Owing to its excellent mechanical and tribological properties, graphene has been proposed to be a promising atomically-thin solid lubricant for engineering applications. However, as a typical two-dimensional (2D) material, graphene has an exceptionally high surface-to-volume ratio and is very susceptible to the surrounding environments. By performing nanoscale scratch tests on graphene deposited on four different substrates, we have shown that the anti-wear performance of graphene, characterized by the maximum load carrying capacity, is not an intrinsic material property. Instead, its value is significantly affected by the stiffness the substrates: Stiffer substrate typically results in a higher load carrying capacity. As revealed by finite element simulations, stiffer substrate can effectively share the normal load and reduce the in-plane stress of graphene by limiting graphene deformation, which enhances the overall load carrying capacity. In addition to the load sharing mechanism, the experimental results also suggest that the frictional shear stress during scratch tests may facilitate wear of graphene by lowering its equivalent strength. The deformation mechanism of graphene/substrate systems revealed in this work provides guidelines for optimizing the mechanical performance of 2D materials for a wide range of tribological applications.

Rate‐Dependent Decohesion Modes in Graphene‐Sandwiched Interfaces

Abstract: Practical industrial mass production of macrosized graphene has been realized with the development of the chemical vapor deposition (CVD) method, envisioning a wide range of potential applications in microelectronic devices.[1–3] However, as a key step in the fabrication process, the successful transfer of highquality macrosized graphene to a specific target substrate of these devices continues to be a challenge. Compared with the typical transfer method of graphene, which often involves wet chemical etching, the etching-free mechanical dry-transfer process is fast, renewable, cost-competitive, and environmentally friendly.[4,5] The adhesion energy of graphene to various substrate materials, as a key parameter characterizing the mechanical resistance to delamination of the graphene/substrate interface, has been shown to be a critical factor determining the quality of the transferred graphene and the performance of graphene-based devices.[5] Therefore, the development of an appropriate method for measuring the adhesion energy of the graphene/substrate interface is essential for large-scale fabrication and device applications of graphene. In the last decade, significant progress has been made in experimental investigations characterizing the interfacial properties of graphene using the shear-lag method,[6–10] the blister tests,[11–16] the double cantilever beam (DCB) fracture tests,[5,17,18] and nanoindentation methods.[19,20] Shearlag methods are often employed to study sliding and shear interactions of the graphene/substrate interfaces, whereas the adhesion energy is typically measured using the DCB and blister tests. For experimental investigation of the adhesive interactions, Koenig et al.[11] performed a pressurized blister test by creating a pressure difference across the graphene membrane to directly measure the adhesion energy between graphene and a silicon-oxide substrate and obtained a value of Γg/Si = 0.45 ± 0.02 J m−2, whereas Wang et al.[12] used the same method to obtain a smaller value of Γg/Si = 0.19 ± 0.02 J m−2. Recently, Xin et al.[13] measured the adhesion energy of the as-grown graphene on copper foil of different roughness using a blister test and obtained values Mechanical dry transfer of large-area graphene is increasingly applied in fabrication of graphene-based electronic devices, and adhesion energy of graphene/substrate interface is a key factor affecting reliability and performance of these devices. Herein, the adhesion energy of a graphene/poly(ethylene terephthalate) (PET) interface is measured by widely adopted double cantilever beam (DCB) fracture tests. Results show that the apparent adhesion energy of sandwiched interface is highly rate-dependent. When separation rate increases from 20 to 150 μm s−1, apparent adhesion energy increases by an order of magnitude. By examining fractured interfaces after DCB tests with micro-Raman spectroscopy, the graphene is found to be fractured and transferred in fragments, with residual tensile strain up to 3% for high separation rates. The results are contrary to earlier reports, where higher separation rate in dry-transfer process would typically enhance the dry transfer of graphene, resulting in better integrity and performance. Based on Raman spectroscopy measurements, three distinct decohesion modes are identified for PET-/ graphene-/adhesive-sandwiched interface, which consistently explain the rate-dependent apparent adhesion energy. The complicated decohesion modes also suggest that an optimal separation rate should be used to properly measure the adhesion energy and improve the dry-transfer technique of graphene with minimum damage and residual strain.