Unveiling the Mechanism of Spontaneous Nanoscroll Formation from Janus Transition Metal Dichalcogenide Nanoribbons.

Due to the atomic asymmetry, Janus transition metal dichalcogenide monolayers possess spontaneous curling and can even form one-dimensional nanoscrolls. Unveiling this spontaneous formation mechanism of nanoscrolls is of great importance for precise structural control. In this paper, we successfully simulate the process of Janus MoSSe nanoscroll formation from flat nanoribbons, based on molecular dynamics (MD) simulations with hybrid potentials. The spontaneous scrolling is purely driven by the relaxation of intrinsic strain in Janus MoSSe. The final structure of nanoscroll is strongly affected by the length of nanoribbon with a nonmonotonous relation. To further understand the mechanism, we establish a thermodynamic model to determine the inner radius of MoSSe nanoscrolls, which is shown to be related to spontaneous curvature, bending stiffness, interlayer van der Waals interaction, interlayer distance, and length of initial nanoribbon. The results correspond well with MD simulations of nanoscrolls from flat nanoribbons and the molecular static simulations of directly built nanoscrolls. Moreover, the inner radii of MoSeTe and MoSTe nanoscrolls are predicted based on the model. Our results provide insights into the Janus TMD nanoscroll formation and a pathway for controllable fabrication of nanoscrolls.

Spontaneous formation of MoS2 nanoscrolls from flat monolayers with sulfur vacancies: a molecular dynamics investigation.

The unique physical properties exhibited by one-dimensional nanoscrolls assembled from nanosheets have propelled them into the spotlight of two-dimensional materials research. However, the self-scrolling mechanism of transition metal dichalcogenides has not been unveiled with an appropriate theoretical approach. In this paper, we systematically investigate the spontaneous formation of MoS2 nanoscrolls from flat monolayers by molecular dynamics simulations based on a reactive force field. The sulfur vacancies on one side break the atomic symmetry and the reconstruction acts as the driving force for the curling of the flat nanoribbon. If sulfur vacancies are arranged in a line, clear bending angles of the nanoribbon can be obtained and the angle relies on the direction of the line vacancy. With random sulfur vacancies on the top, spontaneous curling and a time-dependent scrolling process of the nanoribbon can be observed. The interplay between dangling bonds and van der Waals (vdW) interactions plays a pivotal role in the formation process of MoS2 nanoscrolls. With an increasing density of sulfur vacancies, the curvature of the nanoscrolls increases. Meanwhile, the scrolling rate accelerates and the time required for the formation of vdW structures decreases. These results provide theoretical insights into the fabrication of nanoscrolls and pave avenues for tailoring nanoscrolls with different morphologies.

Second Harmonic Generation in Janus Transition Metal Chalcogenide Oxide Monolayers: A First-Principles Investigation.

Due to the unique optical responses induced by vertical atomic asymmetry inside a monolayer, two-dimensional Janus structures have been conceived as promising building blocks for nanoscale optical devices. In this paper, second harmonic generation (SHG) in Janus transition metal chalcogenide oxide monolayers is systematically investigated by the first-principles calculations. Second-order nonlinear susceptibilities are theoretically determined for Janus MXO (M = Mo/W, X = S/Se/Te) monolayers. The calculated values are comparable in magnitude with Janus MoSSe monolayer. X-M-O symmetry breaking leads to non-zero components in vertical direction, compared with the non-Janus structure. Focusing on the SHG induced by incident light at 1064 nm, polarization-dependent responses of six Janus MXO monolayers are demonstrated. The symmetry of p-polarization changes from six-fold to three-fold with acute incidence angle. Moreover, the effects of biaxial strain on band structures and SHG are further investigated, taking MoSO as an exemplary case. We expect these results to bring in recipes for designing nonlinear optical devices based on Janus transition metal chalcogenide oxide monolayers.

Shape and Composition Evolution in an Alloy Core-Shell Nanowire Heterostructure Induced by Adatom Diffusion.

A core-shell nanowire heterostructure is an important building block for nanowire-based optoelectronic devices. In this paper, the shape and composition evolution induced by adatom diffusion is investigated by constructing a growth model for alloy core-shell nanowire heterostructures, taking diffusion, adsorption, desorption and incorporation of adatoms into consideration. With moving boundaries accounting for sidewall growth, the transient diffusion equations are numerically solved by the finite element method. The adatom diffusions introduce the position-dependent and time-dependent adatom concentrations of components A and B. The newly grown alloy nanowire shell depends on the incorporation rates, resulting in both shape and composition evolution during growth. The results show that the morphology of nanowire shell strongly depends on the flux impingement angle. With the increase in this impingement angle, the position of the largest shell thickness on sidewall moves down to the bottom of nanowire and meanwhile, the contact angle between shell and substrate increases to an obtuse angle. Coupled with the shell shapes, the composition profiles are shown as non-uniform along both the nanowire and the shell growth directions, which can be attributed to the adatom diffusion of components A and B. The impacts of parameters on the shape and composition evolution are systematically investigated, including diffusion length, adatom lifetime and corresponding ratios between components. This kinetic model is expected to interpret the contribution of adatom diffusion in growing alloy group-IV and group III-V core-shell nanowire heterostructures.

Armchair Janus MoSSe Nanoribbon with Spontaneous Curling: A First-Principles Study.

Based on density functional theory, we theoretically investigate the electronic structures of free-standing armchair Janus MoSSe nanoribbons (A-MoSSeNR) with width up to 25.5 nm. The equilibrium structures of nanoribbons with spontaneous curling are obtained by energy minimization in molecular dynamics (MD). The curvature is 0.178 nm-1 regardless of nanoribbon width. Both finite element method and analytical solution based on continuum theory provide qualitatively consistent results for the curling behavior, reflecting that relaxation of intrinsic strain induced by the atomic asymmetry acts as the driving force. The non-edge bandgap of curled A-MoSSeNR reduces faster with the increase of width compared with planar nanoribbons. It can be observed that the real-space wave function at the non-edge VBM is localized in the central region of the curled nanoribbon. When the curvature is larger than 1.0 nm-1, both edge bandgap and non-edge bandgap shrink with the further increase of curvature. Moreover, we explore the spontaneous curling and consequent sewing process of nanoribbon to form nanotube (Z-MoSSeNT) by MD simulations. The spontaneously formed Z-MoSSeNT with 5.6 nm radius possesses the lowest energy. When radius is smaller than 0.9 nm, the bandgap of Z-MoSSeNT drops rapidly as the radius decreases. We expect the theoretical results can help build the foundation for novel nanoscale devices based on Janus TMD nanoribbons.