Fabrication Of Cu Microstutures And Self-Assembly Of ZnO Nanorods And Au Nanoparticles Patterns

The fabrication of micro/nanostructures places an important role in the micro/nanotechnology,which is the basis of the application of micro/nanodevice.To develop a simple method to fabricate micro/nanostructures in a low cost and large scale has attracted extensive interest.Nanomaterials are the focus of much attention due to their astonishing physicochemical properties and numerous potential applications in micro/nanotechnology.The properties of nanomaterials are dependent on not only the size,but also the structure and their arrangement.Thus,the most important and challenging part for the application of nanomaterials to the devices is how to organize them with precise control of their position and structureIn this thesis,the "bottom to up" approaches to fabricate metal microstructures and to fabricate patterns of nanomaterials have been investigated by Electrochemical Wet Stamping Technique(E-WETS) and Reaction-Diffusion technique using the agarose gel as a stamp.The thesis is divided into five chapters and organized as follows:Chap.1 introduces the fabrication of metal microstrutures in industry and the controllable self assembling technique in industry.The cases of synthesis of ZnO nanorods and self assembly of Au nanoparticles are taken for examples.Chap.2 introduces the experimental instruments and the details.The Water-Vapor Condensation method is introduced to characterize the ITO substrate modified with amino groups and the results is related those results from the measurement of XPS.Chap.3 addresses the new method to directly fabricate Cu microstructures with a high resolution of 0.6%on n-Si(111) using E-WETS technique.Chap.4 and Chap.5 focus on the hydrothermal synthesis of ZnO nanorods and self assembly of Au nanoparticles,respectively.The selective growth of ZnO nanorods on Au micropatterned substrate is dependent on the concentration of HMT and Zn(NO₃)₂.The self assembly of Au nanoparticles shows an edge-enhanced density on the border of chemically modified and unmodified NH₂-ITO substrate.Chap.6 introduces the outlook of the agarose gel stamp and self-assembly nanomaterials.The main research contents and achievements are listed as follows:1.Cu microstructures were selectively deposited on n-Si(111) by combining micropatterned agarose gel stamp with electrochemical deposition.A high replication resolution(0.6%),Very smooth Cu microstructures were achived and the thickness was adjusted by optimizing the component of electrolyte and the electrodeposition potential.The investigation of Cu²⁺diffusion in micropatterned agarose gel indicates that the uiform growth rate of copper thickness,which was attributed to the constant diffusion rate of Cu²⁺.2.Based on different nucleation rates of ZnO on Au,ITO and Si surface in the hydrothermal process,ZnO nanorods pattern has been fabricated on a Au micropatterned substrate without using SAMs or ZnO seed layer.The nanorod patterns were characterized by XRD and PL.The investigation of the field emission properties of the array of ZnO nanorods also shows a good field enhancement behavior.The turn-on field andβvalue was measured 6.0 V/μm and 1027.The nucleation rate was governed by surface-free energy,morphology and roughness of the substrate.The contact angle measurement was carried out on the bare ITO surface and sputtering Au film respectively to determine surface energy and indicate that ZnO preferred to nucleate on low-energy surface of Au surface rather than ITO.Surface roughness or the morphology of the Au patterned ITO substrate was also characterized by AFM.ITO surface has an irregular domain structure with ravine,while Au surface shows a regular grain structure which could provide more nuclei sites and promote the hetero-nucleation of ZnO nanorods from aqueous.SEM images show that ZnO nanorods with a smaller diameter(80～100 run) grow on the Au film.The density of ZnO nanorodes on Au is much higher than those on bare ITO substrate with an average diameter of 200～400nm.Thus,we can conclude that the nucleation rate of ZnO follows the order as:Au>ITO>>SiConcentration profile of precursors on slow nucleation region was calculated to demonstrate the mode of selective growth of ZnO nanorods on Au micropatterned ITO substrate.It was found that the selectivity of ZnO nanorods growth on Au micropatterned ITO substrate can be delicately adjusted by simple variation of the concentration of reactant,the size of Au array and the potential of electrochemical deposition.Since the rate of ZnO nucleation on Au island is faster than ITO surface, as soon as the nucleation begins at rapid nucleating region,mass transport to the growing ZnO depletes the solution over slowly nucleating regions(ITO) to the point below the supersaturation concentration.Nucleation does not occur on the slowly nucleating part(ITO) where the effective concentration is below supersaturation(Css). ZnO nanorods were thus well confined on Au regions.Thus the selectivity of ZnO nanorods growth shows three types growth model:completely selective growth, partially selective growth and nonselective growth corresponding to comparatively low concentration(25mM),medium concentration(50mM) and high concentration (100 mM).However,the nucleation rate of ZnO on Au is significantly faster than Si. ZnO exclusively grow on Au surface,even in a high concentration(100 mM) in our experiment),Using the electrochemical deposition method,the selectivity of ZnO nanorods growth is adjustable since the applied potential promotes the nucleation rate.When a potential of-0.7 V(vs.SCE) was applied on the Au micropatterned ITO substrate in a solution of 50 mM Zn(NO₃)₂+ 50 mM HMT,which is suitable for obtaining excellent ZnO nanorods arrays using hydrothermal synthesis method,ZnO nanorods grow on both Au and ITO surface and show a nonselective growth model.However,well aligned ZnO nanorods array was obtained by applying a potential of-0.55 V(vs.SCE) in a solution of low concentration(e.g.25 mM Zn(NO₃)₂+ 25 mM HMT) where ZnO nanorods show nonselective growth model in hydrothermal synthesis method.3.Edge density enhancement of Au nanoparticles in the self-assembling process was obtained by chemical derivation of -NH₂ functionalized ITO surface using micropatterned agarose gel stamp full of HNO₂(line pattern).The density of Au nanoparticles was higher on the border of contact area of the agarose full of 0.2 M HNO₂ and the substrate where -NH2 groups were modified.The density enhancement was in linear relationship with time less than 20 min and the highest density is around 900 particles per micrometers.The reason for the increased Au nanoparticles density near the border is not clear and still needs further investigation.Although the product of chemical derivation of -NH₂ is unknown by using agarose stamp,XPS shows the -NH₂ group has been altered after the reaction with HNO₂ and the difference between the modified and unmodified surface can be obviously characterized in water-vapor-condensation experiment.According to 'cylindrical droplet' evaporation model,the enhancement of edge density can be explained by the transport mechanism of HNO₂ in cubic agarose stamp. The model is suitable for the agarose with a high aspect ratio(1000:1).