Furthermore, the growth mechanism for the 2D single crystals is presented and the following application in electronics, optics and antioxidation coatings are also discussed. Then, other typical 2D materials (including graphene, h-BN, and TMDs) are given in terms of the unique feature under the guideline of the multi-seed growth approach. In this review, we present the detailed cases of growing the representative monocrystalline 2D materials via the single-seed CVD method as well as show its advantages and disadvantages in shaping 2D materials. Currently, the latter is recognized as a more effective method to meet the demand of industrial production, whereas the oriented domains can seamlessly merge into a continuous single-crystal film in a short time. Two types of strategies have been developed: one is single-seed method, which focuses on the ultimate control of the density of nucleation into only one nucleus and the other is a multi-seed approach, which concentrates on the precise engineering of orientation of nuclei into a uniform alignment. Recently, chemical vapor deposition (CVD) shows great promise for the synthesis of high-quality 2D materials owing to high controllability, high scalability and ultra-low cost. In a 3-D crystal, such as a real one, each ball would also touch three on the plane above, and three on the plane below.Single-crystal 2D materials have attracted a boom of scientific and technological activities. Note that in this 2-D model, each ball touches six others. This pattern is typical of hexagonal close-packed and cubic close-packed lattices. In metals, close-packing of atoms is a very common structure. The balls within a grain arrange themselves into close-packed planes. The reason for this involves entropy: at all finite temperatures, there will be some disorder in the crystal. Note that diffusion occurs mainly near the top of the balls: those towards the middle and bottom do not easily move, as the photographs show.Įven in a single crystal, or large-grained sample, there are still vacancies, as the shot model shows. With great care, it may be possible to create a single crystal, as all the balls form a single pattern. Note the presence of vacancies in the structure. The following image sequence shows the behaviour of the shot model as it is rearranged by tapping, starting from a polycrystalline state with many small grains and ending with much larger grains.
Occasionally, the "diffusion" process may cause two grains to join together, or for some grains to "grow". This is similar to diffusion, in which case the tapping is analogous to thermal activation. Tapping of the model causes minor rearrangements of the balls, especially at the top of the "solid" region. In some places the balls form close-packed regions.
One or two balls may be suspended above the main body by electrostatic forces: this is comparable to the vapour found above the crystal. The balls form a liquid-like structure.Īs the model is tilted towards the vertical, the balls pack closely together. When the shot model is held horizontally, so that the balls flow freely, the resulting structure is similar to a liquid. They tend to behave like the atoms in a crystal, and can show the same kind of defects. The model consists of many small ball bearings trapped in a single layer between two transparent plates. We can use a "shot model" to get a picture of crystal defects. Crystal defects are important in determining many material properties, such as the rate of atomic diffusion and mechanical strength. Grain boundaries in polycrystals can be considered as two-dimensional defects in the perfect crystal lattice. The structure contains defects such as vacancies, where an atom is missing altogether, and dislocations, where the perfection of the structure is disrupted along a line. Within a single crystal or grain, the crystal structure is not perfect.
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