- 學位論文
氧化亞銅奈米晶體其具晶面效應的導電性質及金-氧化亞銅、金@銀-氧化亞銅核殼奈米晶體其具晶面效應的光學性質
Facet-Dependent Electrical Conductivity Properties of Cu2O Crystals and Facet-Dependent Optical Properties of Au–Cu2O and Au@Ag–Cu2O Core–Shell Nanocrystals
摘要
觀察其不同晶面氧化亞銅具有晶面效應之導電性，藉由測量單顆氧化亞銅晶體，了解產生在半導體材料之不同晶面上的導電性結果。我們發現到八面體的氧化亞銅有最佳的導電性，立方體在提高電壓時其導電度會增高，菱形十二面體沒有導電性。導電度的差異，其歸因於不同晶面表面有著不同程度的能帶彎曲。當探針接觸小斜方截半立方體的兩個不同晶面時，會有整流效應產生，證明單一多面體奈米晶體有潛力作為功能性電子元件，連接氧化亞銅之兩晶面，藉由改變電流流經的方向，形成通路或斷路來建構電晶體及電開關。透過理論計算，氧化亞銅三個不同晶面（111）、（100）和（110）的能量狀態密度（DOS）圖得到三個晶面之能帶結構為金屬、半金屬、及半導體的結果，理論計算結果與實驗結果相互吻合。計算不同晶面的氧化亞銅其表面原子不同層數所構成的能量狀態密度圖，決定了其表面厚度所反映出的晶面效應，總結這些結果，得到晶面層厚皆小於1.5奈米，氧化亞銅的晶面效應應會產生在所有尺寸的晶體中。 在金屬‒氧化亞銅核殼結構其具晶面效應的光學性質研究中，首先是先合成「金‒氧化亞銅核殼奈米晶體」，利用35奈米的金八面體作為核，合成立方體、八面體和菱形十二面體的金‒氧化亞銅核殼結構且具序列大小變化，利用這些奈米晶體來研究其在光學上的晶面效應。並且完成金‒氧化亞銅核殼菱形十二面體的結構鑑定。金‒氧化亞銅核殼菱形十二面體、八面體和立方體其金核之表面電漿共振吸收鋒波長位於694、721與741奈米。值得注意，改變氧化物殼層的形狀，就可以產生47奈米的差異。而當殼層的厚度改變，表面電漿共振吸收鋒仍固定在同一波長上。當合成出混有截邊截角八面體及截角菱形十二面體兩種形狀的產物時，其金的表面電漿共振吸收鋒在710奈米。說明吸收鋒位置與不同氧化亞銅晶面比例有關連。此外，準備以58、65、68與73奈米的金八面體來合成金‒氧化亞銅核殼八面體，用上述的奈米晶體來觀察因殼層厚度改變其金的表面電漿共振吸收鋒位置的變化。使用尺寸大的金核所合成的核殼八面體，其金的表面電漿共振吸收鋒位置較尺寸小的金核所合成的核殼八面體吸收鋒位置略微紅位移。 核殼結構其具晶面效應的光學性質研究中，再來是合成出「金@銀‒氧化亞銅核殼奈米晶體」，以42奈米的金‒銀核殼奈米立方體作為核，合成截半立方體、截角八面體和菱形十二面體的金@銀‒氧化亞銅核殼結構且具序列大小變化，利用這些奈米晶體來研究其在光學上的晶面效應。另外，以38和50奈米的金‒銀核殼奈米立方體作為核，合成截半立方體與截角八面體。金‒銀核之表面電漿共振吸收鋒位置產生260‒280奈米的紅位移，較金‒氧化亞銅奈米粒子所產生的紅位移大。改變氧化物殼層的形狀，從菱形十二面體到截半立方體其表面電漿共振吸收鋒位置就可以產生55奈米的差異。氧化亞銅殼層，在光學上具有相同的晶面效應 本論文，增強了我們對於晶面如何能強烈地影響氧化亞銅的導電性和光學性質。在其他研究中，我們還發現，光催化、有機催化及熱傳遞也都具有晶面效應。關於晶面效應在氧化亞銅及其他半導體材料的研究越來越多，人們會逐漸了解並接受半導體材料確實在各種方面性質具有晶面效應。
並列摘要
In Chapter 1, we examined the facet-dependent electrical conductivity properties of single Cu2O crystals. We showed that a Cu2O octahedron is highly conductive, a cube is moderately conductive, and a rhombic dodecahedron is non-conductive. The conductivity differences are ascribed to the presence of a thin surface layer having different degrees of band bending. When electrical connection was made on two different facets of a rhombicuboctahedron, a diode-like response was obtained, demonstrating the potential of using single polyhedral nanocrystals as functional electronic components. Density of state (DOS) plots for three layers of Cu2O (111), (100), and (110) planes show respective metallic, semimetal, and semiconducting band structures. By examining DOS plots for varying number of planes, the surface layer thicknesses responsible for the facet-dependent electrical properties of Cu2O crystals have been determined to be below 1.5 nm for these facets. In Chapter 2, we have synthesized Au‒Cu2O core‒shell nanocubes, octahedra, and rhombic dodecahedra with three different sizes for each particle shape by using 35 nm octahedral gold cores to investigate their facet-dependent optical properties. Structural characterization of the Au‒Cu2O rhombic dodecahedra has been extensively performed. The surface plasmon resonance (SPR) absorption band arising from the gold cores is centered at 694, 721, and 741 nm for the Au‒Cu2O rhombic dodecahedra, octahedra, and nanocubes, respectively. Remarkably, tuning the oxide shell surface can produce a band position difference as large as 47 nm. The SPR band position is fixed despite large changes in the shell thickness. Cu2O shells also exhibit facet-dependent optical properties. A mixed sample of edge- and corner-truncated octahedra and truncated rhombic dodecahedra having significant {110} facets gives a gold SPR band at 710 nm, showing the band position is highly linked to the proportions of different Cu2O surfaces exposed. In addition, different-sized Au‒Cu2O core‒shell octahedra with 58, 65, 68, and 73 nm octahedral gold cores were prepared to clearly show the transition from the shell thickness-independent gold SPR peak for octahedra with smaller gold cores to a progressive red-shift of the band with increasing shell thickness in octahedra with larger gold cores. In Chapter 3, we have synthesized Au@Ag‒Cu2O core‒shell rhombic dodecahedra, truncated octahedra and cuboctahedra with different sizes for each shape by using 42 nm cubic Au‒Ag cores for facet-dependent optical property examination. Truncated octahedra and cuboctahedra with tunable sizes were also prepared using 38 and 50 nm cubic Au‒Ag cores. The magnitudes of Au‒Ag SPR band red-shift at 260‒280 nm are much larger than those seen in Au‒Cu2O nanocrystals. By tuning the Cu2O shell shape from rhombic dodecahedral to cuboctahedral structures, the SPR absorption band separation can reach 55 nm. Facet-dependent optical absorption effect from the Cu2O shells can also be identified. This thesis enhances our understanding of how crystal facets can strongly affect the electrical conductivity and optical absorption of Cu2O nanocrystals. In other studies, we have also shown that photocatalysis and organocatalysis, and more recently heat transmission, are also facet-dependent for Cu2O nanocrystals. As more demonstrations of facet-dependent effects are reported for Cu2O and other semiconductor materials, people would gradually realize and accept that many, if not all, semiconductor materials really possess various facet-dependent properties.