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A Study of Specific Factors Affecting Molecular Self-Assembly on GaAs (001) Substrate

Lu, Hao

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The major goal of this thesis is to gain a better understanding of the molecular self-assembly on GaAs, which can be useful for design of functional self-assembled monolayers (SAMs) on this technologically important semiconductor substrate. For this purpose, high quality SAMs of non-substituted alkanethiols and some specially designed molecules, including 4,4´-terphenyl-substituted alkanethiols, -(4´-methyl-biphenyl-4-yl)-alkanethiols, partially fluorinated alkanethiols (PFAT), and dihexadecyl diselenide have been prepared on GaAs (001) through the optimization and careful control of the rigorous experimental conditions. These SAMs were investigated in detail by a combination of advanced surface characterization techniques, providing a deep insight into the molecular organization and properties of these films. In the first place, pronounced chain length effect for all studied SAM systems on GaAs was elucidated, viz. deterioration of the film quality occurs with decreasing length of the molecular chain, accompanied by a partial oxidation of the GaAs substrate, due to the less effective protection property. In the second place, by using the series of the 4,4´-terphenyl-substituted alkanethiols and -(4´-methyl-biphenyl-4-yl)-alkanethiols, the existence of so called bending potential at the headgroup-substrate interface in the SAM/GaAs(001) system with a preferable GaAs-S-C angle of ~104° was demonstrated. For the above SAMs, this potential plays the dominant role in the balance of the structure-building forces, mediating the odd-even behavior in the molecular orientation and packing density. This result suggests that the bending potential should always be taken into account at the design of functional molecular films on GaAs substrate. The influence of the above factors, viz. the chain length effect and the bending potential, were additionally studied by the example of PFAT SAMs with variable length of the fluorocarbon chain, viz. CF3(CF2)n-1(CH2)11SH (denoted as FnH11SH, n = 6, 8, and 10). To better understand the structure and organization of these films, the corresponding study of the reference PFAT/Au system was performed first, resulting in a variety of valuable general findings regarding the balance of the structure-building interactions in complex monomolecular films. As for the PFAT/GaAs system, the respective SAMs were found to be highly ordered and densely packed, and consequently able to protect the GaAs surface from the oxidation. This protection depended on the chain length and was less effective for the films with shorter fluorocarbon chain due to their lower quality. Indeed, with decreasing length of the fluorocarbon segment, progressive deterioration of the orientation order accompanied by a slight decrease in the packing density was observed in the fluorocarbon part. In contrast, the hydrocarbon segments in FnH11SH/GaAs exhibited similar orientation, with the average tilt angles close to the optimum one determined by the bending potential. This underlines once more the important role of bending potential in the balance of structure-building interactions in aliphatic SAMs on GaAs.

Item Type: Dissertation
Supervisor: Zharnikov, Prof. Dr. Michael
Date of thesis defense: 8 November 2013
Date Deposited: 14 Nov 2013 12:49
Date: 2013
Faculties / Institutes: Fakultät für Chemie und Geowissenschaften > Institute of Physical Chemistry
Subjects: 540 Chemistry and allied sciences
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