本研究以陽極化技術(anodizing)結合異向性蝕刻，於鋅箔上製作微米六重對稱尖錐結構作為基材，以水溶液與熱氧化法成長多層氧化鋅奈米結構，探討各種參數對多層結構成長之影響，並研究成長機制與結晶學之關係，最後量測此特殊多層結構作為場發射元件陰極之電子場發射特性。此外，結合陽極化與奈米壓印技術，於鋅箔表面製作規則陣列微米結構，探討不同圖案轉移對陽極化之影響；於阻劑上壓印5μm凸點圖案，再施以陽極化反應後可獲得四重對稱結構。

金屬鋅箔於電解質(NH4Cl/H2O2溶液)中，經施加電流密度55mA/cm2進行陽極化反應1分鐘後，獲得鋅的尖錐。此微米結構為單晶且具有六重對稱性，係結晶面選擇性蝕刻所獲得，由TEM分析此尖錐沿 方向成長，圍成該尖錐之刻面則為{1-100}，其場發射起始電場為33.9V/μm。

以鋅的尖錐作為基材，利用水溶液法成長奈米結構，可生成鋅/氧化鋅核殼多層結構，此多層陣列可自行組裝為六重對稱；形成機制為尖錐鋅於表面氧化形成氧化鋅薄膜，接著由氧化鋅薄膜表面成長奈米氧化鋅結構。在不同酸鹼度的水溶液中，隨pH值增加氧化鋅越易沿c軸成長形成一維氧化鋅結構，致使氧化鋅奈米結構由二維板狀逐漸變為一維線狀形態；所成長多層結構，由不規則組織變為六重對稱結構。由於水溶液溶蝕與奈米棒成長交互作用，隨著反應時間與溫度增加，溶蝕現象於尖錐六重對稱角落更加顯著，多層六重對稱則轉變為放射狀結構。而直接由多晶鋅箔成長一維奈米結構，其場發射性質起始電場為24.8 V/μm，與多層結構場發射性起始電場為13.0 V/μm相比較；多層結構為立體陣列，比表面積較大，發射尖端也較多，經幾何結構因數計算(β)，一維奈米結構幾何結構因數為126，多層結構陣列則為647。由起始電場與幾何結構因數計算，顯示具有較大發射尖端面積的三維多層結構，其場發射性質較佳。

本研究中，由鋅尖錐直接熱氧化所獲得的多層結構陣列，不但為立體結構並同時擁有對稱性。由於鋅尖錐的特殊表面形態，在熱氧化過程中，引發氧化物/金屬界面間的應力，為釋放應力奈米氧化鋅鬚晶由應力集中處成核成長；由於成長應力關係，此奈米氧化鋅視活化能為103.1kJ/mol，較低於一般低溫製程。隨著熱氧化溫度的增加，於單晶鋅尖錐表面所成長奈米結構的長寬比隨之增加，當熱氧化溫度增加375oC時，奈米結構長寬比達最大值(~71)，因而產生遮蔽效應，單位面積多層三維結構密度為0.05 tips/μm2達最小值，其場發射性質可獲得最低起始電場為8.5 V/μm，最高結構因數值為3490。由穿透式電子顯微鏡觀察結果，水溶液與熱氧化所成長獲得核殼多層結構為：微米鋅尖錐/氧化鋅薄膜/氧化鋅奈米結構組成。在不同成長方法下，於鋅尖錐表面成長氧化鋅結構，其成長特性有所不同分別沿[0001]與[11-20]方向成長。進一步分析奈米氧化鋅與尖錐/氧化鋅薄膜間結晶學關係分別為：[2-1-10]奈米氧化鋅//[0001]氧化鋅薄膜與[-12-13]奈米氧化鋅//[-12-13]鋅的尖錐。

結合高分子阻劑的壓印製程與陽極化技術，使用圖案為5μm之凸點模具於鋅箔上的PMMA製作凹點圖案；經陽極化後，凹點處的鋅箔溶蝕，於鋅箔上呈現四重對稱的鋅角錐形態，與未經圖案轉移之鋅箔陽極化所獲得的六重對稱單晶鋅尖錐相比較，整體均勻性較佳，不論角錐直徑與高度皆較尖錐形態均勻。於鋅角錐表面，以水溶液與熱氧化法所成長之奈米結構，因角錐尺寸較大，所獲得之三維結構並未具對稱性；在相同製程條件下，於單晶鋅尖錐成長奈米氧化鋅，則形成鋅/氧化鋅核殼對稱多層結構，由TEM分析瞭解其對稱性主要來源為中心鋅尖錐。而高分子阻劑圖案轉移，可於鋅箔表面形成阻擋層，使得異向性溶解特性由圖案所侷限，因而獲得與自然異向性溶解不同的金屬鋅微米結構，與一般尖錐成長奈米結構不同。

A highly ordered array of sixfold and single-crystalline Zn microtips is obtained by a selective anisotropic etching process during the anodization of Zn foil. The microtip arrays are used as substrates for aqueous chemical growth and annealing for preparation ZnO nanostructures. The different experimental parameters for branch growth have been discussed, and the crystallographic relationships between Zn core and ZnO nanostructures have been illustrated. Finally, the field emission property of such Zn microtips and Zn/ZnO hierarchical structures has also been described.

Sixfold single crystalline Zn microtips are obtained using an electrolyte NH4Cl/H2O2 at a current density of 55 mA/cm2 by selective anisotropic etching of crystallographic planes with different dissolution rates. From the results of TEM analysis, the single crystalline Zn microtips, formed on the Zn foil after anodic dissolution treatment, are enclosed by {1-100}and are preferentially oriented along the c-axis, and a turn-on field of 33.9 V/μm is obtained in FE measurement.

Zn/ZnO self-assembled core-shelled hierarchical structures have been fabricated by a simple aqueous chemical growth method on Zn foil substrate. With variation of the solution pH values, the nanostructures grow vigorously; however, with increasing the pH value, they grow preferentially in the longitudinal direction. By optimizing the synthesis precursor composition, well-defined 3D hierarchical arrays can be produced. Owing to the phenomenon of simultaneous growth of nanostructures and dissolution of Zn by the solution erosion, a radial hierarchical array is obtained by extending reaction time and temperature. It is observed that the turn-on electric field for ZnO nanowires grown from Zn foil is 24.8V/μm. The field emission characteristics are enhanced for the novel nanoarchitectures, and exhibit a reduced turn-on field of 13.0 V/μm owing to the much larger efficient emission area of the Zn/ZnO branched structures. The enhancement factor for the one-dimensional ZnO nanowires and the Zn/ZnO hierarchical structures are 126 and 647, respectively. While the larger enhancement factor interprets that the branched hierarchical structures have much larger efficient emission areas and a more proper geometric arrangement to emit electrons.

In the present study, a facile annealing route and the related mechanism for forming sixfold core-shell hierarchical structures with well-defined anisotropic 3-dimensional arrays have been described. The regularity and symmetry of hierarchical structures prepared by annealing Zn microtips are superior to the random nanostructure arrays formed in general vapor system reported in the literatures. Owing to the distinct stress distribution on the topography of microtips during annealing, the mechanism for growing branched ZnO nanowhiskers is related to the relaxation of stress. At the annealing temperature of 375oC, the largest aspect ratio (~71) of branched nanostructures and the smallest density (0.05tips/μm2) of hierarchical arrays can be obtained. For field emission characteristics, a better turn-on electric field (8.5V/μm) and a larger enhancement factor 3490 are obtained for branched nanostructures. From the TEM analyses, the Zn/ZnO core-shelled hierarchical structures are composed of Zn microtips, ZnO thin films, and ZnO nanostructures. The growth characteristics for ZnO nanostructures obtained by aqueous chemical and direct oxidation are different, where the grow along the [0001] and [11-20] axes, respectively. However, the relationship of crystallography among Zn core, ZnO films and branched ZnO under solution and oxidation methods are [2-1-10]ZnO branches//[0001]ZnO films and[-12-13]ZnO branches //[-12-13]Zn microtips, respectively.

The patterned single-crystalline Zn microtips are fabricated on the surface of Zn foil by combining both nanoimprinting and anodizing technique. The patterned Zn microtip arrays with specific pitch and four-fold symmetry are fabricated by imprinting PMMA resist on Zn surface before anodizing. After nanoimprinting procedure, both the diameter and height of the fourfold Zn micro-horns obtained after anodization are uniform. However, the hierarchical structures obtained by aqueous chemical growth and annealing cannot have symmetrical patterns, which differ from the results with sixflod symmetrical microtips arrays.

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