| 摘要: | 氧化鋁薄膜的結構和生長模式在鎳鋁合金(100)上，已經完整的被高能電子繞射儀器、低能電子繞射儀器、歐傑電子能譜儀和掃瞄穿遂式顯微鏡所研究。 氧化鋁薄膜的有序結構的生長方式，是經由氣相氧化於鎳鋁合金(100)表面上隨著a 軸和b 軸平行於[0-10]和[001]方向，可找到氧化鋁的(2×1)單胞。但是，三維奈米尺度NiAl(100)的突起物，氧化鋁和NiAl(100)的奈米粒子以偶合的形式在NiAl(100)表面上呈現；這個跡象清楚地由高能電子繞射影像中，由特別的反射穿透點在表面的反射柱上可以得知。另外，在NiAl(100)表面上的NiAl(100) 奈米粒子成長平行於(011)平面，而且(011)平面的法向量[100]平行於表面結構的[0-11] 方向。從掃瞄穿遂式顯微鏡中獲得的資料，配合上歐傑電子能譜和熱脫附實驗可以暗示出生長在鎳鋁合金上的有序結構氧化鋁會在加熱過程當中分解。另ㄧ方面，在NiAl(100)表面上，利用電化學氧化所形成的氧化鋁(2×1)厚度，接近單層氧化鋁，而且電化學氧化所形成的氧化鋁，比氣態氧化更能得到較大區域的有序結構。 此外，利用蒸鍍吸附的方式，使鈷的奈米粒子生長在氧化鋁薄膜上的行為，已經在高能電子繞射、掃瞄穿遂式電子顯微鏡和歐傑電子能譜中被研究。這個結果表示：當加熱鈷的奈米粒子時，其平均直徑分別為 2.4，3.4，5.8nm 和其平均直徑所對應高度為0.7,1.5,1.5nm。鈷的奈米粒子結構分析，可以在高能電子影像中得知，雖然這些奈米粒子有不同大小的尺寸，但是卻有相同形式的fcc 和朝著本身(001)面成形且平行鎳鋁合金(100)結構表面，這個(001)面的[110]軸平行NiAl(100)表面的[0-10]方向。最適合鈷奈米粒子的生長，是鈷的fcc(001)面，即使鈷的晶格與氧化鋁表面上的晶格有些許的不同，但是較能配合氧化鋁表面的長方形網狀結構。由於鈷奈米粒子為了配合氧化鋁表面結構和界面上晶格常數不同所形成的應力，因此鈷奈米粒子的晶格常數比fcc 的單晶晶格常數增加4-5％。影響鈷的fcc 結構排列，是來自於界面的交互作用力(晶格排列方式)和鈷(Co)-fcc 奈米粒子的較小表面自由能。 The growth and structures of aluminum oxides on NiAl(100) have been investigated by RHEED(reflection high energy diffraction) ,complemented by LEED (low energy electron diffraction) ,AES (Auger electron spectroscopy) and STM (scanning tunneling microscopy). Crystalline θ-Al2O3 phase grows through gas-phase oxidation on the NiAl(100) substrate with its a and b-axes parallel to [0-10] and [001] direction of the substrate,respectively,forming a (2 × 1) unit cell.Whilst,three-dimensional nano-sized NiAl(100) protrusions and Al2O3,NiAl (011)clusters were found to co-exit at the surface,evidenced by extraordinary transmission spots superposed to the substrate reflection rods in the RHEED patterns,particularly,the NiAl (011) clusters develop with their (011) plane parallel to the NiAl(100) surface,and [100] axis parallel to the [0-10] direction of the substrate.STM observation combined with information from AES and TPD (temperature programmed desorption) suggest the formation of these 3D structures is closely associated with partial decomposition of the crystalline oxide during annealing.On the other hand,smoother (2×1) oxide islands with thickness close to a complete monolayer of θ-Al2O3 can be formed on NiAl(100) by electro-oxidation,in contrast with the large crystalline films formed by gas-oxidation. Moreover,the Co nanoclusters grown from vapour deposition on thin film Al2O3/NiAl(100) have been studied by reflection high energy electron diffraction (RHEED),scanning tunneling microscopy (STM) and Auger electron spectroscopy (AES).The results show that the annealed Co nanoclusters,with mean diameters of 2.4 ,3.4 ,5.8 nm and heights of 0.7 ,1.5 ,1.5 nm respectively,are highly crystalline.Structural analysis based on the RHEED patterns indicate that these Co clusters,although with different sizes,have an fcc phase and grow with their (001) facet parallel to the NiAl(100).It is optimal growth for the Co clusters,as the Co fcc (001) facet is to match better with the rectangular oxygen mesh of the oxide surface even though there is an appreciable lattice mismatch.The lattice constant of the Co clusters is consequently expanded by 4-5 ％ in comparison with that of fcc Co bulk,in order to match better with the oxide surface and minimize the interface strain.The preferential formation of the Co fcc phase is also driven by such significant interfacial interaction and perhaps a relatively smaller surface free energy of the Co-fcc clusters. |
