| We firstly report the growth of various low-dimensional zinc(Zn)nanocrystals on silicone oil surface by thermal evaporation,which is catalyst-free and achieved at room temperature.The morphology characteristics,size distributions and microstructures of the low-dimensional Zn nanocrystals are investigated detailedly.The dependence of the one-or two-dimensional growth tendency of the Zn nanocrystals on the deposition rate,and the relationship of the morphology characteristics of the Zn crystal mounds with the step-edge barriers are preliminarily analyzed.Besides,by using computer simulation method and establishing differential equation model,we further discuss the growth mechanism of the low-dimensional Zn nanocrystals on the liquid surfaces.The main conclusions are listed as follows:a)Zn atoms are evaporated onto both the clean ground glass surface and the silicone oil surface at room temperature by thermal evaporation under high vacuum.In our experiments,catalyst is not required.We find that the deposited Zn atoms tend to form hexagonal nanoplates on the clean ground glass surface.It is quite unexpected,however,that the one-dimensional,plate-shaped and mound-like Zn nanocrystals can be prepared on the silicone oil surface simultaneously.b)By the scanning electron microscope(SEM)measurements,we find that the one-dimensional Zn nanocrystals on the silicone oil surface include nanowires,nanobelts,kinked nanorods,needle-like nanorods,straight nanorods,etc.It should be mentioned that,however,most of them are straight nanorods.Their length and width distributions may be fitted by the lognormal distributions with the most probable length and width range of 250-500 nm and 20-40 nm,respectively.The transmission electron microscope(TEM)measurements show the straight Zn nanorods are single crystal rods with the[0002]growth direction,which is the preferential growth direction of the hexagonal close-packed(hcp)crystals.The formation of the one-dimensional Zn nanocrystals may be attributed to the isotropic and quasi-free sustained liquid surface effects,and the preferential growth direction characteristic of the hcp crystals.c)Based on the Monte Carlo method,a new simulation model on a square lattice substrate is established to simulate the one-dimensional growth mechanism of the crystals with the preferential growth direction characteristic.The morphology of the crystals obtained from the simulation is straight rod with uniform width along the growth direction.Both the length and width distributions of the crystal rods are in concordance with the lognormal distribution.All simulation results are in good agreement with the experiments.d)Various Zn nanoplates with quadrangular,pentagonal and hexagonal morphologies,etc.,together with the one-dimensional Zn nanocrystals,may appear on the silicone oil surface.The normal direction of the Zn nanoplates is[0001]direction of the Zn crystal.For the low-dimensional Zn nanocrystals prepared by the deposition rates of 0.01 and 0.10 nm/s,their aspect ratio distributions may be fitted by the lognormal and logistic distributions with the largest aspect ratio of 159 and 52,respectively.The result shows that a lower deposition rate is more beneficial to the growth of the one-dimensional Zn nanocrystals.The preferential growth directions of the Zn nanocystals compete to grow with different growth priorities.The experimental results show that the priority ratio of two main preferential growth directions of the Zn nanocystals is ??°-101.The growth priority difference is not so prominent that various Zn nanocrystals may be prepared on the silicone oil surface simultaneously.e)Based on the results above,the growth mechanism of the one-and two-dimensional Zn nanocrystals on the silicone oil surface is proposed as follows:(i)the deposited Zn atoms nucleate randomly and then form seed crystals on the isotropic and quasi-free sustained liquid substrate;(ii)the seed crystals diffuse randomly on the silicone oil surface and the surrounding atoms diffusing stochasticly in the effective diffusion area<?r2>aggregate to the seed crystals;(iii)the aggregated atoms may diffuse along the seed crystal surface until the lowest surface energy is reached,i.e.,the morphologies of the Zn nanocrystals are determined by the competitive growth among the preferential growth directions.According to the growth mechanism above,a differential equation model is established to explain the growth processes of the one-and two-dimensional Zn nanocrystals on the liquid surface.f)In addition to the one-dimensional and plate-shaped Zn nanocrystals,the Zn crystal mounds with the average step height of 16 nm are obtained on the silicone oil surface simultaneously.The competitive growth among the preferential growth directions of the Zn crystals and the effect of the step-edge barriers result in the formation of the Zn crystal mounds.The dissertation contains five chapters,which are organized as follows:In chapter 1,the preparation methods and growth mechanisms of the low-dimensional nanostructures based on solid or liquid substrates are reviewed,and the recent studies on the one-dimensional nanocrystals are emphasized.In chapter 2,an interesting phenomenon that one-dimensional Zn nanocrystals are firstly fabricated on the silicone oil surface by thermal evaporation is reported.The experiments are catalyst-free and performed at room temperature.The morphology characteristics,size distributions and microstructures of the.one-dimensional Zn nanocrystals are investigated detailedly.The growth mechanism of the one-dimensional Zn nanocrystals is discussed,and then the one-dimensional growth phenomenon of the crystals with the preferential growth directions is verified by using computer simulation method.In chapter 3,in contrast to the situation on clean ground glass surface,various low-dimensional Zn nanocrystals are simultaneously prepared on silicone oil surface by thermal evaporation.The morphology characteristics and microstructures of the low-dimensional Zn nanocrystals are investigated detailedly.The dependence of the growth tendency of the low-dimensional Zn nanocrystals on the deposition rate is studied according to the aspect ratio distributions of the one-and two-dimensional Zn nanocrystals prepared at different deposition rates.The Zn crystal mounds are observed on the silicone oil surface together with the one-and two-dimensional Zn nanocrystals,and the relationship of the morphology characteristics of the Zn crystal mounds with the step-edge barriers is preliminarily analyzed.The growth mechanism of the low-dimensional Zn nanocrystals on the silicone oil surface is further discussed.In chapter 4,according to the experimental results,a new differential equation model is established to explain the growth processes of the one-dimensional and plate-shaped Zn nanocrystals on the liquid surface,such as the dependence of the length and width of the low-dimensional Zn nanocrystals on the deposition time at every growth period etc.In chapter 5,based on the experimental findings and theoretical analysis,main conclusions and prospects are presented. |
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