Synthesis,Characterization And Growth Mechanism Of ZnO Hierarchical Nanostructures

Nanostructures of zinc oxide have attracted much attention in recent decades because of their novel and unique properties and a wide range of potential applications in solar cells,sensors,photocatalysts,nano-photonics and nano-electronics,nanogenerators and so on.These properties sensitively depend on the morphology of grown nanostructures.For example,hexagonal ZnO nanorods can be applied as whispering gallery resonators,where the coupling between the resonant modes and free excitons depends on the cross-sectional radius.It is therefore essential to understand and control the growth of nanostructures.Depending on growth conditions,morphology of freely grown ZnO varies from nanorods to nanobelt,nanoring,and nanotetrapod,etc.Despite of extensive ingenious studies on ZnO nanostructures thus far,research about ZnO hierarchical nanostructures remains little.Especially in most past reports,catalysts were commonly introduced in the growth of hierarchical nanostructures of ZnO,this would make the research on the growth mechanism become more complex and difficult.In this thesis,taking the hierarchical nanostructures of ZnO fabricated without employing any metal catalysts for example,we have researched the remarkable contribution of stacking-fault-induced repeated polytypism on the formation of a unique hierarchical nanostructure;The controllable changes of the tip topography of ZnO nanorods controlled by the partial pressure of oxygen;and the external appearance smooth-rough alternating along the growth direction induced by puffing phenomenon,etc.The research enriched the morphologies of the hierarchical nanostructures.Our work indicates that intrinsic structural defects(such as stacking-fault)and external growing conditions(such as partial pressure of oxygen,gas flow field distribution)and so on play an important role for the morphology of hierarchical nanostructures.The studies demonstrated that part hierarchical nanostructures have very important potential applications in resonant waveguide,emission,etc.These studies promoted the further research on growth mechanism and physical properties of hierarchical nanostructures.The main research parts are included as follows:1.For the first time we demonstrate the remarkable contribution of stacking-fault-induced repeated polytypism on the formation of a unique hierarchical nanostructure,and the windmill-like hierarchical nanostructure of ZnO has been obtained.Numerical calculation indicates that this hierarchical nanostructure could orient propagation of the normal incident light.The windmill-like hierarchical nanostructure of ZnO has been grown by the catalyst-free evaporation oxidation deposition method at 550℃.This hierarchical nanostructure of ZnO possesses a hexagonal central trunk decorated with thin blades,growing along the[0001]direction.The HRTEM of the ZnO hierarchical demonstrates that there exists a lot of stacking-fault at the main trunk along the[0001]direction and repeated polytypism.Accumulation of stacking faults induced repeated polytypism.HRTEM and cathodoluminescence indicates that this stunning growth behavior is induced by the remarkable contribution of stacking-fault-induced repeated polytypism along the[0001]direction of the main trunk,and the blades are epitaxially nucleated on the main trunk assisted by reentrant-corner-mediated nucleation.These findings shown here pinpoint a new route in self-assembling of hierarchical nanostructures,and show a possible way to engineer nanostructures by controlling the density of planar defects in crystallization.As an effort to explore the application potentials of the novel hierarchical nanostructures,the optical properties have been numerically studied.Numerical calculation indicates this hierarchical nanostructure can be used as a resonant cavity,the normal incident light is coupled into the trunk and a portion of the incident energy is transported along the trunk.The unique hierarchical nanostructure self-assembled may act as a resonant waveguide for light transmission,and hence possess significant potential applications in photonics and energy harvesting.Finally,we would like to point out that despite our experiment focus on ZnO in this paper,the revealed growth mechanism is not limited to ZnO only.2.At the topography of ZnO nanorods,for the first time we demonstrate the growth mechanism that the density of the boundle of tines could be controlled through the second nucleation growth.With control by the partial pressure of oxygen,the boundle of tines at the tip of ZnO nanorods could gradually decrease.The ZnO hierarchical nanorods with much nanoneedle have the largest emission efficiency.The self-organization hierarchical structures of ZnO nanorods are fabricated on silicon(100)substrate by the catalyst-free evaporation oxidation deposition method.The topography of the tip of ZnO hierarchical nanorods sensitively depends on the partial pressure of O2.The driving force for needle growth in our system is controlled by the partial pressure of oxygen.After the initial growth of ZnO nano-prisms for 60min,the flux of O2 is cut off while the flux of N2 is set at different values to control density and length of tines.Experimental results show when nitrogen flux is increased,the number of tines nucleated atop of the nanorod gradually decreases.According to the classic nucleation theory,the nucleus density increases with increasing ratio of the deposition rate to the surface diffusion rate.Therefore,when oxygen supply is stopped,the higher nucleus density contributes to the generation of a number of tines on the facets atop of the prism.As the flux of nitrogen increases,the partial pressure of ZnO/O2 decreases,and deposition rate decreases too.Therefore,nucleation density also decreases,so filaments become less on the top.So filaments become less on the top.In CL spectra,the green light emission is particularly strong,indicating that oxygen vacancy and other structure defects in ZnO hierarchical nanorods are a large quantity.The field-emission properties of ordered ZnO hierarchical nanorods with different morphologies were investigated in detail.Among them,the ZnO hierarchical nanorods with many nanoneedles have the lowest turn-on field,highest current density,and the largest emission efficiency.This implies that the unique morphology reported here is inspiring in exploring electron-emission devices.3.By the Zn evaporation oxidation deposition method we the first time synthesized a single crystalline of node-like-nanorod structure of ZnO,which is novel and unique nanostructure,smooth-rough alternation in side surface induced by puffing phenomenon.This provides a new way for the growth and regulation of ZnO nanorods resonance cavity.The unique node-like nanostructure of ZnO single-crystalline nanorods are grown on Si(100)substrate by evaporation oxidation deposition with zinc powder.No catalyst has been introduced in our experiments.Through SEM and XRD characterization methods we found that the external appearance of node-like-nanorod is smooth-rough alternation along the growth direction[0001].HRTEM indicates this nanostructure is a single crystalline,and node-like shape is due to the surface alternately roughing.CL illustrates that at smooth and rough area,both spectra are similar with each other,and this indicates the phase structure and defects are the same at both areas.It is also found that the emergence and growth of this nanostructure had very close relationship with gas flow field distribution inside the quartz tube.Combined with experimental results,analysis found that the possible reason for formation of smooth-rough-induced nodular structure was the puffing phenomenon in the quartz tube.The puffing phenomenon induced by local pressure change caused driving force at the growth front alternating change,and finally resulted in nodular nanostructure.This provides a new way for the growth and regulation of ZnO nanorods resonance cavity.