Controllable and Gradient Wettability of Bilayer Two-Dimensional Materials Regulated by Interlayer Distance

Surfaces with controllable and gradient wettability often require an elaborate design of the microstructure or its response under electrical, thermal, optical, pH, and other stimuli. Generally, the wettability change under these physical or chemical effects relies on a complex mechanism that is difficult to be quantitatively described. In this study, an online controlling strategy for surface wettability and the corresponding theoretical model are put forward based on a bilayer graphene-like atomic structure. Molecular dynamics results indicate that the surface wettability varies toward hydrophilicity after sticking a bottom material regardless of its wettability. But such an influence becomes weak with increasing interlayer distance, and the overall wettability approaches that of the upper layer material gradually. This variation is elucidated by the increase of the work of adhesion, providing new insight into the wetting transparency of graphene. A theoretical model of the governing relationship is established based on the work of adhesion, which correlates the overall surface wettability with the interlayer distance and the wettabilities of individual materials. Moreover, a surface with a uniform wettability gradient is achieved by inclining the bottom material. The spontaneous and steady motion of droplets can be induced by this gradient wettability. The relevant speedup behavior is evaluated through a theoretical model considering the varying interlayer distance, which reveals the critical role of the lower layer. This study proposes a novel strategy for controllable wetting and relevant gradient surfaces using prevailing two-dimensional materials, paving new routes to many applications such as microfluidic chips, virus diagnosis, and intelligent sensors.

Concurrent fast growth of sub-centimeter single-crystal graphene with controlled nucleation density in a confined channel

The synthesis of large-domain-sized graphene requires a low nucleation density, which inevitably leads to a reduced growth rate. To achieve both a large domain size and high growth rate, we designed a simple channel structure that allowed us to control the nucleation density by tuning the flow dynamics and by introducing an additional catalyst inside to control the growth kinetics at the same time. The designed channel structure plays three roles in the growth of graphene: (1) it retains oxygen to passivate the active nucleation sites; (2) it restricts the mass transfer of CH₄ to control the supersaturation for nucleation; and (3) it provides additional catalytic sites for the decomposition of CH₄ to boost the graphene growth rate. Our strategy allowed the successful preparation of sub-centimeter-domain-sized graphene in 1 h with an average growth rate of 70 μm min^-1, and with a hole mobility of 5500 cm² V^-1 S^-1, which is sufficient for practical applications. Our method paves the way for the large-scale production of single-crystal graphene or other 2D materials at a highly efficient level.

Breakdown of self-limiting growth on oxidized copper substrates: a facile method for large-size high-quality bi- and trilayer graphene synthesis

Copper nanoparticles induced by oxidation can be utilized to tune the dispersion and size of bi- and trilayer graphene grains.