鈷在矽(111)和鍺(111)表面與雙羧基紫精在銅(100)表面上的表面結構變化研究

Investigations structural transitions of the cobalt on the Si(111) and Ge(111) surfaces and dicarboxylated viologens on the Cu(100) surface

蔡松麟(Sung-Lin Tsay)

指導教授 ： 傅袓怡 ； Klaus Wandelt

國立臺灣師範大學/理學院/物理學系/博士(2009年)

摘要

此論文主要分為兩個部分，第一個部分的實驗，為鐵磁性物質鈷原子被蒸鍍在矽(111)-7×7與鍺(111)-c(2×8)表面上，隨不同加熱退火的溫度處理後，利用超高真空掃描穿隧顯微鏡(UHV-STM)與低能量電子繞射儀(LEED)來研究鈷島成長行為。第二個部分的實驗藉由電化學掃描穿隧顯微鏡(EC-STM)與循環伏安法(Cyclic Voltammetry, CV)，研究有機分子雙羧基紫精 (Dicarboxylatic Viologens)在銅(100)與HOPG表面上隨著溶液偏壓而發生相變的行為。相變會因為外在環境的影響而發生，如同利用加熱退火使得鈷島産生結構的改變，與溶液在外加偏壓下驅使Violognes分子重新排列形成新的排列。
當鈷被蒸鍍在矽(111)-7×7與鍺(111)-c(2×8)表面上，在鈷鍍量低時，表面上出現因為鈷矽與鈷鍺形成合金時所出現的缺陷，為了確認這些缺陷的確是因為鈷與矽形成反應，把鈷鍍在低溫的矽表面，在回溫過程時發現當鈷與矽形成反應時造成缺陷的出現，此反應出現在126K到130K之間。600K的加熱退火之後，會出現Co5Ge7的合金，另外鈷在鍺上有機會形成具有√13×√13 R14°週期性的表面結構，但是出現機率小，所以此結構的週期在LEED上沒有呈現出來，之前也沒有任何的研究發現。當鈷與矽或與鍺形成反應後所呈現的磁性強度遠比單純鈷還來得小，所以銀被選為緩衝層避免鈷與底下的基底反應。在鈷鍍量低時，鈷無法形成2維並且具有週期性的結構在矽(111)-√3×√3表面上，然而鈷能夠形成√13×√13 R14°與2×2的表面結構在鍺(111)-√3×√3表面上在經過加熱退火的處理後。只有當鈷鍍量大於1.8單層之後，才有辨法形成1×1的表面結構在銀/矽(111)-√3×√3表面上。因為銀/矽(111)-√3×√3面具有未滿足的電子態，所以使得鈷在低鍍量時，無法形成具有週期的表面結構，鈷傾向滿足那些未滿足的電子態來降低表面自由能而不是形成2維島，但是當鍍量增加時，鈷不受到那些未滿足的電子態的影響而形成1×1的表面結構，除此之外基底鍺也扮演著重要的角色使得鈷能夠形成具有表面結構的2維島，因為同樣是√13×√13 R14°的表面結構能夠在Si(111)-7×7與矽(111)-√3×√3表面形成，進一步發現√13×√13 R14°與2×2的表面結構跟加熱退火的溫度和鈷的鍍量有關係造成兩種結構的相變。為了避免因為銀/矽(111)-√3×√3表面上的未滿足電子態而使得鈷無法形成具有週期性的2維鈷島，約6單層鍍量的銀鍍在100K的矽(111)-7×7表面上，再經過加熱退火至300oC後，銀能夠形成平坦的表面在矽(111)-7×7表面之上，然後再鍍上鈷，發現依然無法形成具有週期性的2維鈷島，所以是否能夠形成具有週期性的2維鈷島主要是受到鍺基底的影響。
濃度0.1mM的雙羧基紫精在銅(100)表面上發現6種排列的模式，雙羧基紫精在氧化態(+2)時程現出平均分佈的點陣排列與傾斜的排列，在此氧化態時雙羧基紫精的含氮雙芽基平面平行表面，因為+2的氧化態與基底陰離子層氯/銅(100)-c(2×2)之間的作用比分子之間還來得強，在還原態(+1)時分子之間傾向以面對面的方式排列( )形成條紋的圖案，主要發現了亞穩態與鬆散的條紋排列和緊密的條紋排列與雙分子排列，這麼多的排列方式主要是因為此雙羧基紫精具有7個長度的烷基鍵，因此在空間與分子之間作用力的影響下造成了多樣的排列。更進一步發現雙層的堆積在亞穩態時就已經形成了，這是因為氮與氧之間的氫鍵作用力所造成的，所以當溶液濃度升高至1.0 mM時，分子多層堆積的成長被發現，此外也發現了不同陰離子層對分子的排列造成影響，當陰離子層為氯離子時，可以發現然亞穩態，然而當陰離子層為溴時，並沒有發現此態。

關鍵字

真空掃描電子隧道顯微鏡 ； 電化學掃描電子隧道顯微鏡 ； 鈷 ； 雙羧基紫精 ； 表面結構的變化

並列摘要

In this thesis, the investigations include two parts. The first part (Part I) is the research of the magnetic material cobalt on the Si(111) and the Ge(111) surfaces dependent on annealing temperatures studied by ultra high vacuum-scanning tunneling microscopy (UHV-STM) and low energy electron diffraction (LEED). The second part (Part II) is the research of the surface structures and phase transitions of viologens on the Cu(100) and HOPG surfaces dependent on the applied potentials studied by cyclic voltammetry (CV) and electrochemistry scanning tunneling microscopy (EC-STM). The aims of both subjects are focused on the observation of the surface structure and morphology at stable thermal/kinetics conditions.
There are two major topics in part I. The first topic describes the Co absorbed on the pure Si(111)-7×7 and Ge(111)-c(2×8) surfaces. The initial reaction of the Co with the Si substrate happens at the temperature range from 126 to 130K. The defects of the Co-Si compounds are different from the intrinsic defects of the Si(111)-7×7 surface. The Co-Si compounds also decrease the brightness of the neighboring adatoms compared to the intrinsic defects of the Si(111)-7×7 surface. Therefore, the Co on the pure Si(111)-7×7 and Ge(111)-c(2×8) surfaces forms the Co-Si and the Co-Ge compounds at room temperature appeared as the dark region of the defect-like feature. The silicon atoms can separate on top of the Si(111)-7×7 surface after annealing to 400oC, and then the Si(111)-7×7 surface structure disappears. The Co5Ge7 alloy is observed on the Co/Ge(111) surface after annealing to 600K. Further, Co atoms can form the √13×√13 R14° periodic surface structure, but the structure is unfavorably formed compared to the Co5Ge7 alloy. The compound formations of the Co-Si and the Co-Ge result in a lower magnetic property than bulk Co. Therefore, the silver buffer layer is introduced on the intermediate layer between the Co and the Si(111) and Ge(111) surfaces as described at the second topic. At low Co coverage, the Co can form periodic surface structures of the √13×√13 R14° and the 2×2 on the Ag/Ge(111)-√3×√3 surface. For the Co/Ag/Si(111) case, the Co forms a cluster shape both on the Ag/Si(111)-√3×√3 and on the flat Ag/Si(111)-1×1 surfaces at low Co coverage. Further, the average size and height of Co clusters on the flat Ag/Si(111)-1×1 surface are almost independent on annealing temperatures from room temperature to 300oC. The reasons for the Co/Ag/Si(111) surface without periodic surface structure are due to the unsaturated states on the Ag/Si(111)-√3×√3 surface and the weaker interaction of the Co with the Si(111) surface than the Ge(111) surface.
Dicarboxylated viologens mixed with a 10 mM HCl on the Cu(100) and HOPG surfaces was studied in different redox states. At the beginning, a 0.1 mM violgens on the Cu(100) surface is investigated. The dicationic viologens show the dot array and the oblique row phases. The radical viologens exhibit the metastable phases, a stripe pattern, the closed stacking stripe pattern, and a dimer phase. The stacking configuration of the dicationic viologen core plane is preferred to be face-on on the surface and that of the radical viologen is formed by π-π stacking with the neighboring viologens. Because dicarboxylated viologens bear long alkyl chains and carboxylic acid groups at the ends of the alkyl chains as illustrated by (HOOC-(CH2)7-V-(CH2)7-COOH), the complex interactions are considered to be the reason of forming various phases on the Cu(100) surface. The experiment of a 0.1 mM dicarboxylated viologens on the HOPG surface without the influence of an anion layer is to confirm the existence of a bilayer formation due to the intermolecular interaction of the hydrogen bonding. The high viologen concentration (1.0 mM) on the Cu(100) surface shows the effects of the chloride and bromide anion layers reflecting on phase transition and the multilayer growth behavior due to the hydrogen bonding interaction and the polarizability, respectively. The effect of the anion layers is consistent with by a 0.1 mM viologens mixed with a 10 mM KBr on the Cu(100) surface.

並列關鍵字

UHV-STM ； EC-STM ； Co ； Dicarboxylated viologens ； Structure transition

參考文獻

1. J.A. Kubby and J.J. Boland, Surf. Sci. Rep. 26, 61 (1996).

6. Joseph Wang, Analytical Electrochemistry, Wiley-VCH, 3st ed, 2005.

7. S. Datta and B. Das, Appl. Phys. Lett. 56, 665 (1990).

8. F. G. Monzon, M. Johnson, and M. L. Roukes, Appl. Phys. Lett. 71 , 3087 (1997).

Molenkamp, Nature 402, 787 (1999).