Physical Mechanics Study Of Fabrication And Properties Of Low-dimensional Materials

Two-dimensional materials are those with thickness down to one or a few atomic layers.As typical ones,van der Waals materials like graphene,hexagonal boron nitride(h-BN)and transition-metal dichalcogenides have attracted intense research interest.Due to the quantum confinement effect arisen from the atomic thickness,those materials have exhibited novel and extraordinary properties.Nowadays,large-scale preparation of high-quality two-dimensional crystals are mainly using chemical vapor deposition.Here,we utilized experimental tools to adjust and analyze the process of chemical vapor deposition growth with respect to domains,involving the crystallographic orientation,morphology evolution and large single crystal growth.The growth mechanism could be further understood with computational simulation based on first-principle caculations and phase filed model.The physical and mechanical properties of synthesized low-dimensional materials and their one-dimensional derivatives are investigated,forward realizing potential applications.The main results are concluded as below:1)Oriented expitaxy of h-BN domains on Cu.Through chemical vapor deposition growth,the crystallographic orientation dependence between h-BN and metal substrate is manifested.Here,we show that triangular h-BN domains share a unique orientation on one-fold symmetric Cu(102)or(103)during chemical vapor deposition in sharp contrast with that grown on some other Cu facets.Our density functional theory calculations revealed that the van der Waals energy become lowest when one zigzag edge of the h-BN domain is perpendicular to the symmetry axis of the Cu(102)or(103),guiding the growth orientation of h-BN.Our finding sets up a promising strategy on synthesizing polar two-dimensional materials with unique orientation on nonpolar substrate in large area.2)Growth of h-BN domains on Cu-Si Alloy.Silicon-assisted chemical vapor deposition of h-BN is proposed to realize millimeter-scale growth of single crystal and wide-range control over domain morphologies.Here,we successfully reduced the nucleation density of h-BN on Cu by over two orders of magnitude simply via introducing a small amount of silicon,giving rise to large triangular domains with maximum 0.25 mm lateral size.Moreover,the domain morphologies could be modified from needles,tree patterns,and leaf darts to triangles through controlling the growth temperature.The presence of silicon altered the growth mechanism from attachment-limited mode to the diffusion-limited mode,leading to dendrite domains that were rarely observed on pure Cu.Phase-field model was utilized to reveal the growing dynamics regrading to B-N diffusion,desorption,flux and reactivity variables,and explain the morphology evolution.Our work sheds lights on the h-BN growth towards large single crystals and morphology probabilities.3)Fabrication and properties of two-dimensional molybdenum oxides.The chemical vapor deposition growth of two-dimensional molybdenum dioxide is successfully employed.The ultra-thin molybdenum dioxide nanosheets are grown on Si O₂/Si substrate utilizing molybdenum trioxide and sublimated sulfur as precursors.The thicknesses of these obtained nanosheets show notable dependence on the baking temperature of the sulfur precursor.At sulfur temperature of 90oC,the obtained nanosheets can be 5.5 nm thin,more than one order of magnitude thinner than that previously reported,in a narrow scatter ranging from 5.5–11.5 nm.Two-probe electrical measurements show that the as-prepared ultrathin nanosheets preserve a high electrical conductivity of 3600 S/cm with thermal stability up to 200oC.Above 250oC,metallic molybdenum dioxide nanosheets are oxidized into insulating molybdenum trioxide flakes in air.Particularly,after annealing in the hydrogen atmosphere with the Pt catalyst,the molybdenum dioxide nanosheets endure obvious color change.4)Aligned Nickel nanowires towords highly stretchable electrode.Based on the unidirectional alignment technique of nickel nanowires with the assistance of magnetic field,a super-stretchable metal network of high conductivity is constructed.The as-prepared film can withstand 300%deformation applied perpendicular to the nanowire-aligned direction,owing to the development of the two-dimensional percolating nanowire network into a three-dimensional network.In contrast to conventional reduced film conductivity under large tension,which is usually associated with unrecoverable crackers and deformations,our film conductivity can increase with the applied large strain,which benefits from the enhanced contacts of the twisted junctions due to the strain induced traction among the nanowires.The developed super-stretchable film with unprecedented performance in this work have implication in the facile fabrication of super-stretchable electrodes with durable performance.