RHEED Studies on Structures of Rh and Rh-Au Bimetallic Nanoclusters on Thin Film Al2O3/NiAl

Please use this identifier to cite or link to this item: http://ir.lib.ncu.edu.tw/handle/987654321/59196

Title:	RHEED Studies on Structures of Rh and Rh-Au Bimetallic Nanoclusters on Thin Film Al2O3/NiAl
Authors:	徐柏瑋;HSU,PO-WEI
Contributors:	物理學系
Keywords:	奈米團簇;銠;高能電子繞射儀;nanocluster;Rh;RHEED
Date:	2013-01-28
Issue Date:	2013-03-25 16:06:07 (UTC+8)
Publisher:	國立中央大學
Abstract:	Rh 金屬、Au 金屬及Rh-Au 合金的奈米團簇利用蒸鍍的方法成長在氧化薄膜 θ-Al2O3/NiAl(100) 上，利用高能電子繞射儀(reflection high energy electron diffraction, RHEED)來研究。我們發現Rh 金屬及Rh-Au 合金的奈米團簇有很好的排列行為而且結構及晶格間距受到底層氧化鋁的影響，經由高能電子繞射儀的結構研究，我們發現Rh 金屬及Rh-Au 合金的奈米團簇是 fcc 的結構並且沿著平行 Al2O3(100) 表面以 (100) 面方向成長，其Rh 金屬及Rh-Au 合金的奈米團簇的 [110] 方向平行於 Al2O3(100) 的 [010] 方向，這是個理想的成長方式，Rh 金屬及Rh-Au 合金的奈米團簇 (100) 面和表面的氧化鋁結構互相吻合。而Rh 金屬奈米團簇(3.90 – 4.04 ?)的晶格常數相對於 fcc 結構的 Rh 塊材(3.80 ?)較為膨脹，這樣會使Rh 金屬奈米團簇的 (100) 面有比較好的晶格去吻合表面的氧化鋁。晶格常數會因鍍量及退火溫度而有所下降。在鍍量超過1.9 ML時退火到800 K 以上後會觀察到modulation spots的產生，這樣的情形意味著表面結構不平整，氧化鋁以及鎳鋁合金的結構被Rh金屬奈米團簇所改變。此外我們依然可以觀測到Rh的繞射點，這也表是表面上還存有Rh金屬奈米團簇。Au金屬奈米團簇是fcc的結構且沿著平行Al2O3(100) 表面以 (111) 或者 (100) 面方向成長。Au金屬奈米團簇(4.2 ?)的晶格常數相對於fcc結構的Au塊材(4.02 ?)較為膨脹3 %。在退火超過730 K後，Au金屬奈米團簇會傾向以Au(001)[110]\|\|Al2O3(100)[010]的方向生長。在退火到900 K時繞射圖形會變得較模糊，這是由於部分Au擴散到氧化鋁之下。合金奈米團簇的結構與Rh 金屬奈米團簇相同，並且合金奈米團簇的晶格常數較接近純Rh金屬奈米團簇的晶格常數而非Au金屬奈米團簇的晶格常數，因此我們認為Rh-Au 合金的奈米團簇中的結構主要是由Rh 金屬主導，在鍍量超過1.95 ML (Rh:Au = 0.95:1)將樣品退火到800 K以上後，繞射圖中會出現modulation spots但是由合金形成的繞射點卻不明顯，可知氧化鋁及鎳鋁合金的結構被合金團簇改變，也可以得知只殘存少量合金奈米團簇在表面所以沒有明顯的合金繞射點存在。The Rh, Au and Rh-Au bimetallic nanoclusters grown from vapor deposition on thin film θ-Al2O3/NiAl(100) have been studied by reflection high energy electron diffraction (RHEED). The results show that the Rh and Rh-Au bimetallic nanoclusters are highly crystalline and their structures and lattice constant are significantly affected by the oxide substrate. Structural analysis based on the RHEED patterns indicates that Rh and Rh-Au bimetallic nanoclusters have a fcc phase and grow with their (001) facets parallel to the θ-Al2O3(100) surface, and with [110] axis along the [010] direction of the θ-Al2O3(100). It is an optimal growth as the fcc (001) facets match better with the oxide surface. The lattice constant of the Rh nanoclusters (3.90 – 4.04 ?) is expanded relative to that of fcc bulk Rh (3.80 ?). The lattice constant decreases with the coverages and annealing temperature. Annealing the Rh nanoclusters at coverage higher than 1.9 ML to the temperature higher than 800 K leads to the ordered intensity modulation spots. The modulation spots imply the substrate become rough. The Al2O3 and the NiAl are structurally modified by Rh nanoclusters. Nevertheless, the reciprocal lattice spots of Rh nanoclusters are still observed in the RHEED patterns, indicating some of Rh nanoclusters remain on the substrate. The Au nanoclusters grown onto the oxide surface are structurally ordered, having a fcc phase and growing with their facets either (111) or (001) parallel to the Al2O3(100) surface at room temperature. The lattice constant of the Au nanoclusters is expanded by about 3 % (4.2 ?) relative to that of fcc bulk Au (4.08 ?). After annealing above 730 K, the Au nanoclusters grow in a preferred orientation, Au(001)[110]\|\|Al2O3(100)[010]. The diffraction pattern becomes fainter at higher annealing temperature (900 K), as some of Au diffuses into the substrate. The structure of the bimetallic nanoclusters is identical to that of the Rh nanoclusters. The lattice constant of the bimetallic nanoclusters is closer to that of the pure Rh nanocluster, rather than that of the pureivAu nanoclusters. The structure of Rh-Au bimetallic nanoclusters is thus dominated by Rh. Annealing the sample above 800 K, the RHEED patterns show modulation spots but no obvious diffraction spots from the bimetallic nanoclusters. The Al2O3 and the NiAl are structurally modified by Rh-Au bimetallic nanoclusters. There are just few bimetallic nanoclusters remaining on the substrate, so no obvious diffraction spots from the bimetallic nanoclusters are exhibited.
Appears in Collections:	[Graduate Institute of Physics] Electronic Thesis & Dissertation

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