Weikang Wu

Liquid-Liquid Phase Transition in Nanoconfined Silicon Carbide

We report theoretical evidence of a liquid-liquid phase transition (LLPT) in liquid silicon carbide under nanoslit confinement. The LLPT is characterized by layering transitions induced by confinement and pressure, accompanying the rapid change in density. During the layering transition, the proportional distribution of tetracoordinated and pentacoordinated structures exhibits remarkable change. The tricoordinated structures lead to the microphase separation between silicon (with the dominant tricoordinated, tetracoordinated, and pentacoordinated structures) and carbon (with the dominant tricoordinated structures) in the layer close to the walls. A strong layer separation between silicon atoms and carbon atoms is induced by strong wall-liquid forces. Importantly, the pressure confinement phase diagram with negative slopes for LLPT lines indicates that, under high pressure, the LLPT is mainly confinement-induced, but under low pressure, it becomes dominantly pressure-induced.

Coalescence behavior of liquid immiscible metal drops in two-wall confinement

Molecular dynamics (MD) simulations are performed to investigate the coalescence of the liquid Al and Pb drops in the graphene (G) walls and pillared-graphene (PG) walls. The confining walls can affect the coalescence dynamics of two adjacent films by restricting the movement of one of the metal drops; however, the coalescence behavior is different in the G-walls and the PG-walls. Two un-contacted films can still merge into one bigger drop because of the restricting effect of the walls, in which the movement of the Pb drop plays a predominant role. The coalescence time decreases with the decrease of the confining space. Our findings demonstrate that the coalescence dynamics can be controlled by tuning the confining space or wall surface.

Wettability and Coalescence of Cu Droplets Subjected to Two-Wall Confinement

Controlling droplet dynamics via wettability or movement at the nanoscale is a significant goal of nanotechnology. By performing molecular dynamics simulations, we study the wettability and spontaneous coalescence of Cu droplets confined in two carbon walls. We first focus on one drop in the two-wall confinement to reveal confinement effects on wettability and detaching behavior of metallic droplets. Results show that Cu droplets finally display three states: non-detachment, semi-detachment and full detachment, depending on the height of confined space. The contact angle ranges from 125° to 177°, and the contact area radius ranges from 12 to ~80 Å. The moving time of the detached droplet in the full detachment state shows a linear relationship with the height of confined space. Further investigations into two drops subjected to confinement show that the droplets, initially distant from each other, spontaneously coalesce into a larger droplet by detachment. The coalescing time and final position of the merged droplet are precisely controlled by tailoring surface structures of the carbon walls, the height of the confined space or a combination of these approaches. These findings could provide an effective method to control the droplet dynamics by confinement.

Electronic transport properties of ultra-thin Ni and Ni–C nanowires

The structures and electronic transport properties of ultra-thin Ni and Ni-C nanowires obtained from carbon nanotube (CNT) templates are theoretically investigated. C atoms tend to locate at the central positions of nanowires and are surrounded by Ni atoms. Spin polarization at the Fermi level is not responsible for the spin filtration of these nanowires. Increasing C concentration can improve the resistance of nanowires by abating the number of electronic transmission channels and the coupling of electron orbitals between Ni atoms. Moreover, with the increase of diameter, the conductance of these nanowires increases as well. This study is helpful for guiding the synthesis of nanowires with desired applications.

Structural evolution of a Si melt in nanoscale confined space

Molecular dynamics (MD) simulations are performed to systematically study the structural evolution of a Si melt confined in nanoscale space. The freezing Si structure at 300 K is stratification which is composed of a stable crystalline shell and a metastable glassy core. Due to the spatial restriction effect, the confined structure consists of higher-coordinated clusters compared to the bulk Si. It is revealed that the statistical average of the ordered shell and the disordered core gives rise to the split of the second peak of the pair distribution function curves of the Si melt. Moreover, increasing the cavity size is detrimental to the stability of the layered configuration of the confined melt and increasing the cooling rate mainly influences the arrangement of Si atoms adjacent to the SWCNT wall. Interestingly, we also find that the cylindric cavity is more beneficial than the square one in inducing the formation of long-range crystalline order in nanoscale space.

Structural and Dynamical Properties of Metallic Glassy Films

In this chapter, a series of molecular dynamics simulations have been carried out to explore structural and dynamical features of monatomic liquid metallic films during rapid cooling. Results show a semi‐ordered inhomogeneous morphology containing crystal‐like and disordered regions. The icosahedron contributes to nucleation through the synergy with other short‐range ordered structures and participates in crystal growth via assimilation, but the pinning effect should be overcome. The second‐peak splitting in pair correlation functions is found as the result of a statistical average of crystal‐ like and disordered structural regions, not just the amorphous structure. The splitting can be viewed as a prototype of crystal‐like peaks exhibiting distorted and vestigial features. Besides, we use the parameter P(a, τ, ν) for predicting both local structural order and motion propensity. The fraction of crystalline clusters follows a negative power‐law scaling with the cooling rate increasing, which is the inverse of P(a, τ, ν).