零維暨一維奈米材料之光電磁特性及結構研究

Abstract

奈米所具有之量子尺寸效應、表面界面效應等特殊現象，使奈米之光電材料展現微米顆粒所不及之光電及催化特性，因顆粒尺寸減少時，對於紫外光之吸收產生藍移現象，使能隙擴大，並產生更大之氧化還原電位；此外，而粒徑減少時表面積增大，促進光電轉換，且吸附待分解物之能力增強，使光催化分解之效率亦隨之增加；另外，尺寸減少時，光激發而形成之電子電洞對擴散至表面的時間愈短，電子與電洞分離之效果愈好，進而提升光電效率。在奈米磁性材料之研究方面，高深寬比的奈米磁導線，因為可以磁操控的關係所以可以將高深寬比的銀鈷導線拉直，就因為如此可將此種導線，有高的導電異向性，因此可應用在超細間距異向性導電膜為高密度磁記憶體及覆晶構裝重要材料。本文用bottom-up之方法製作零維奈米結構(如金屬和半導體奈米粒子及自聚核/殼奈米粒子)及一維奈米結構(如奈米管、奈米線、奈米棒)，擁有許多新穎之特性及做為電子、磁性、光學及化學性質之應用。一方面研究其製程之穩定性及發展性，另一方面也發展其奈米特性進一步發展組裝新穎功能奈米材料的能力。進一步考量各重要材料介面（如金屬－半導體，有機－無機等）後衍伸出來各種豐富的功能性，將可能使許多發展中的奈米科學及科技應用產生巨大的改變，主要應用於太陽電池、光觸媒、遠紅外線放射及磁性元件等。 第二章以多組分之ZrO2, TiO2, ZnO and Al2O3為起始原料，利用雷射剝離法製作複晶相遠紅外線奈米材料。包括雷射劑量與氣氛等製程參數將影響生成之微粉大小、形態與組成。形成之複晶相奈米材料之粒徑分布成雙峰分布，約7~15 nm (70~80%) and 40~100 nm (20~30%)。於低雷射劑量時有Zn組成富集且粒徑分布窄化之趨勢，而隨雷射劑量增加，Zr組成亦隨之增加。而靶材之結構演變、及劑量為控制奈米材料演化之重要因子。 第三章以液相電位分散及金屬鹽類還原之方法合成TiO2於奈米級貴重金屬樹枝狀結構之微粒上，形成全時效型光觸媒。研究結果顯示以金屬鹽類還原法較光照還原法形成貴金屬樹枝狀結構之微粒，再利用負電位控制奈米TiO2於奈米級貴屬樹枝狀結構之微粒上為佳。針對滅菌能力、空氣及液相污染物的降解能力來評估全時效型光觸媒的去除效能，以結合分散良好之10奈米TiO2左右之奈米貴重金屬與光觸媒載體效能最佳，隨奈米金屬粒徑增加而降低其去除效能。而對次甲基藍分解效率亦可分別達到99%以上。 第四章使用次微米至奈米級孔洞的氧化鋁模板，以含NiSO4, NaH2PO2, NaC2H3O2 and Na3C6H5O7之亞磷酸鹽電解質溶液，在80–100°C與pH = 3–6條件下合成磷酸鎳奈米管陣列。奈米管具有1µm長度、200-300nm直徑與80-150nm管壁厚度。穿透式電子顯微鏡顯示奈米管由約5nm的粒子所組成。經95%N2 /5%H2 氣氛與 500°C溫度處理，奈米管陣列有200Oe的矯頑磁場與垂直奈米管方向易磁化的磁異方性。隨處理溫度升高至900°C，矯頑場與飽和磁化呈下降趨勢。 第五、六章以奈米模板(Nano template)陽極處理氧化鋁為基礎，使用電鍍生成奈米銀線陣列的方式，電鍍鈷奈米線後，因鈷金屬可帶有磁效應，利用的此性質即可製做出可以磁操控。在導電性上，可保有部分銀線的導電奈米導線，並且以AAO當過模板的作法，可製造出高深寬比的奈米導線，因為可以磁操控的關係所以可以將高深寬比的銀鈷導線拉直，就因為如此可將此種導線，有高的導電異向性，因此可應用在超細間距異向性導電膜，為高密度磁記憶體及覆晶構裝重要材料。規則的銀、銀/鎳、銀/鈷陣列利用脈衝電化學沉積在多孔的氧化鋁模板從一個水溶性Ag(NO3)2 和 Ac(NH3) 電鍍液中製作，移出模板，銀、銀/鎳、銀/鈷奈米線陣列在1.6~2.6V 和40~90℃之條件使用，將CoSO4，NiSO4, NiCl2 和 H3BO3 電鍍液，以形成殼核或多層之結構。15nm鎳薄膜匹覆在直徑200nm之銀奈米線上，形成殼核結構，依據TEM之結果顯示它是被發現已被鎳金屬匹覆之銀奈米線之的磁性性質與傳統的純銀線相互比較其磁性性質能被加強。從磁力顯微鏡影像是被觀察到銀-鎳核殼奈米線之domain 狀態。另外, 銀-鎳奈米線顯示180Oe之矯頑磁場，施加磁場平行與垂直至奈米線幾乎是獨立的。無論如何，鎳層能產生一個較大磁domain是被發現，變成平行方向超越垂直方向。 第七章以N型氧化鋅奈米線陣列與P型有機光電轉換材料結合，形成混成太陽電池元件，以鋁摻雜之方式，使氧化鋅奈米線陣列對光之吸收峰往可見光部分偏移，增加其對可見光之吸收率，並使元見之光電轉換效率、光電流和填充因子有所提昇。 第八章利用新的一維及二維奈米結構為做太陽能電池或TFT模組，通過採取便宜的鍍膜的玻璃基板，研究了於其上成長之多晶矽取代矽基板。同時，利用雷射退火，快速熔融最初玻璃基板的矽薄膜和促進它的獨特的方法在低溫之下再結晶，特別使用熱傳導層使之促進巨晶多晶矽之生長。重要製程如下： 1. 電子傳導層在玻璃基體形成。 2. 黃光微影過程被形成，造成許多熱導變化的熱傳導區域。3. 200nm非晶矽薄膜形成熱導層數之上。 4. 保溫層如SiO2為保留雷射退火溫度於矽薄膜之上形成。 5. 當雷射脈衝(excimer laser)入射此結構時， 非晶矽薄膜可瞬間地吸收激光能和變換成結晶型。 而且，利用溫度差及非晶矽對於234nm波段對於雷射波段之吸收作用，於450-475mJ//cm2雷射脈衝之下，在非晶矽薄膜層能引起和大約導致1~2μm巨大之多晶矽成長，而利用另一種複合雷射劑量的方法，將可以增加Si (111)順向優選排列取向。

Material with nanometer-scale size have large ratio of surface to bulk atoms. Large surface always gives high active behavior and changes in both physical and chemical properties. Nanomaterials such as nanoparticles, nanotubes, nanorods and nanowires having size generally smaller than 100nm exhibit superior photoelectronic and magnetic properties in various applications. Therefore, in this thesis, the studies will be focused on the synthesis, structure analysis and property characterization of zero-/one dimensional nanoscaled materials. In chapter 2, ZrO2, TiO2, ZnO and Al2O3, were chosen as raw materials to synthesize a target for laser ablation. The formed nanoparticles exhibit two kinds of particle size distribution with 7~15 nm (70~90%) and 40~100 nm (10~30%). Nanoparticles synthesized at lower fluence laser ablation are rich in Zn composition and show more narrow distribution. While increasing laser fluence, the composition of the collected nanoparticles is primarily composed of Zr. A model based on composition and morphology of both nanoparticles and target with changing laser fluence was proposed to explain the phase evolution of nanoparticles. The average far-infrared emissivity of the nanoparticles based on ZrTiO4--ZnAl2O4 system is measured to be more than 80﹪(wavelength range from 4 to 12 μm) and varies with crystal phase ratio. In chapter 3, a highly dispersed nano-TiO2/Ag catalyst is synthesized in an alkaline solution. Nearly all of the dimethy-blue target pollutant at high concentration was removed when the photoreaction was performed in a short period. This novel nano TiO2 photocatalyst exhibits excellent photocatalytic activity because it is well dispersed. Since no dispersant or organic binder was used, this synthetic process has the advantages of low cost and convenience. In chapter 4, One-dimensional nanotube arrays of nickel-phosphate have been developed by electroless deposition into sub-micro to nanometer sized pores of the porous alumina templates. The dimension of the formed nanotubes has 1μm in length, 200~300nm in diameter and 80~150nm in thickness of tube walls. Transmission electron microscopy examination of the nanotubes clearly show amorphous hallow structure with a average grain size of ~5 nm. The hysteresis loops of the nanotube arrays show a coercive field of about 200Oe under treatment in 95%N2/5%H2 atmosphere at 500 oC as the magnetic field was applied along parallel and perpendicular to tube axis. The nanotube arrays also exhibit an anisotropic magnetic property with easier saturation along the perpendicular direction. However, both coercive field and saturation of remanent magnetization of the nanotube arrays become lower while continually increasing heat treatment temperature up to 900oC. In chapter 5, ordered silver- nickel core-shell nanowire arrays were successfully fabricated by electrodeposition. The ordered silver nanowire arrays embedded in a porous alumina template were first fabricated from an aqueous solution of Ag(NO3)2 and Ac(NH3). After removing out the template, the obtained silver nanowire arrays were subsequently electrodeposited with nickel at 1.6~2.6V and 60oC using the electrolyte composed of NiSO4, NiCl2 and H3BO3. Transmission electron microscopy (TEM) observation reveals that a 15 nm thick nickel film was coated on the surface of the silver nanowires with about 200 nm in diameter. It was found that the silver nanowires with nickel coating showed enhanced magnetic properties in comparison to that of pure silver nanowires. The Magnetic Force Microscope (MFM) image of silver- nickel core-shell nanowires exhibits magnetic domain state. In addition, the hysteresis loops of the silver-nickel nanowire arrays show a coercive field of 180Oe, almost independent of the applied magnetic field parallel and perpendicular to nanowires. However, it was observed that a larger magnetic domain was found in parallel direction than that in perpendicular direction. In chapter 6, a single bath electrodeposition method was developed to integrate nanowires of Ag/Co with multi-layer structures within a commercial AAO (anodic alumina oxide) template, with a pore diameter 100~200 nm. An electrolyte system containing silver nitride and cobalt sulfide was explored by using cyclic-voltammetry and electrodeposition rate to optimize electrodeposition conditions. A designed step-wise potential and different cations ratio [Co2+] / [Ag+] were adopted for the electrodeposition. After dissolution by NaOH, Ag/Co multilayered nanowires were obtained with a composition {[Co]/[Ag80Co20]}30 identified by XRD and TEM when [Co2+] / [Ag+] = 150. By annealing at 200oC for 1hr, the uniformly structured {Co99.57/Ag100}30 nanowires were obtained. Compared with pure Co nanowire, the magnetic hysteresis loops showed manifest magnetic anisotropy for {Co99.57/Ag100}30 nanowires than that of pure Co nanowires corresponding to a change of easy axis upon magnetization. In chapter 7, The heterojunction photovoltaic devices consist of hybrid p-type organic Cu-phthalocyanine and inorganic n-type semiconductor ZnO nanostructures which include vertically aligned nanorods, randomly oriented nanorods and nanoparticles. The strong absorption of ZnO appears in 250~460nm wavelength and Cu-phthalocyanine exhibits broad absorption in 440-700nm with an absorption maximum at 630nm. The incorporation of partial Al into ZnO leads to the shift of absorb light from UV region to visible light and subsequently causes more charge generation. Charge recombination from hybrid devices of vertically aligned ZnO nanorods was more efficient than that fabricated from the other types. The maximum incident photon to electron conversion and energy conversion efficiencies under simulated sunlight AM1.5 (10mW/cm2) in aligned ZnO are 0.036mA and 1.32%, respectively. In chapter 8, a new 1 and 2 dimension nano structure for making solar cell or TFT module have been researched by adopting low cost coated glass substrate with polysilicon instead of silicon wafer. Meanwhile, laser annealing is used as the unique method to melt primal amorphous silicon thin film and promote it recrystalize under low temperature. Particularly, the new thermal conducting layers are patterned under the silicon layer to enhance the lateral giant grain growth. The important processes are represented as follows: 1. A electrical conducting layer are formed on the glass substrate. 2. Photolithographic process is executed to pattern the thermal conducting layer and form many variable thermal conducting zone. 3. An about 200nm amorphous silicon film is formed on the patterned thermal conductive layer. 4. A thermal isolation layer such as SiO2 is deposited for keeping laser annealing temperature. 5. While the pulse excimer laser is injected the structure, the amorphous silicon film can absorb laser energy instantaneously and transform it to crystalline type. Moreover, the temperature gradient could be generated on the silicon layer and cause the uniform polysilicon growth of 1~2μm giant grains under about 450-475mJ/cm2 laser fluence. A unique method of multiple laser fluence have been executed to increase the crystalline orientation of Si(111).