|Title||Covalently bound organic monolayers on hydrogen-terminated silicon surfaces = covalent gebonden organische monolagen op waterstof-getermineerde siliciumoppervlakken|
|Source||Wageningen University. Promotor(en): E.J.R. Sudhölter; J.T. Zuilhof. - S.l. : S.n. - ISBN 9789058083845 - 192|
|Publication type||Dissertation, internally prepared|
|Keyword(s)||silicium - unimoleculaire films - interfase - halfgeleiders - oppervlakteverschijnselen - preparaten - silicon - unimolecular films - interphase - semiconductors - surface phenomena - preparations|
|Categories||Colloid and Surface Chemistry|
Monolayers of 1-alkenes and 1-alkynes can be prepared on hydrogen-terminated Si(100) and Si(111) surfaces by a reaction between the organic compound and the Si surface. This reaction, which is schematically depicted below, results in the formation of densely packed, covalently bound, and well-ordered monolayers of the organic compound on the hydrogen-terminated Si surface, that inhibit the oxidation of the underlying Si surface.
In this research, several aspects of this surface modification have been investigated. The scope of the reaction has been explored by using a variety of functionalized and nonfunctionalized alkenes. The results show that many functional groups can be used, provided that: a) the functional group is properly protected, and b) the formation of well-ordered monolayers is not disturbed by too much steric hindrance between these functional groups, once they are in the monolayer. A very interesting property of the resulting functionalized monolayers is that the functional groups can be deprotected and/or further modified, without damaging the monolayer or the underlying Si substrate. This gives access to functionalized monolayers that so far could not be prepared by other methods.
The method for the preparation of the monolayers has been improved by showing that the reaction can also be done using solutions of the 1-alkenes and 1-alkynes in aromatic solvents. The best solvent was found to be mesitylene (1,3,5-trimethylbenzene). In this solvent monolayers are formed that are at least as good as those prepared using neat 1-alkenes/1-alkynes, even at concentrations as low as 0.1 M. This is an important improvement, as it considerably reduces the amount of 1-alkene/alkyne needed in the surface modification.
In the case of the hydrogen-terminated Si(100) surface, there are two hydrogen atoms on each Si surface atom. Upon reaction of this surface with a 1-alkyne, not just one, but two covalent Si-C bonds are formed per organic molecule, as had been demonstrated by a combination of IR spectroscopy, X-ray reflectivity measurements, and quantum chemical calculations. This type of reactivity has so far not been observed for 1-alkynes on other H-terminated (crystalline) Si surfaces.
To get more insight in the structure of the monolayers on a molecular level, they have been investigated by molecular modeling simulations. Large modified Si surfaces, with >30 alkyl chains attached to the Si surface, were investigated, using the approach of two-dimensionally repeating boxes. Calculations without this repeating box approach failed completely, as did calculations using small boxes (<30 alkyl chains). The results show that: a) there is a good correlation between the structure as observed in the simulations and the structure as deduced from a combination of experimental data, and b) that the currently obtained substitution percentage of the Si-H for Si-alkyl groups is close to the maximum substitution percentage that can be reached. This latter conclusion shows that the obtained monolayers are (almost) as densely packed as possible, as desired.
The possibility to use the monolayers for silicon surface passivation has been investigated, determining the effective lifetimes of the minority charge carriers in p-type Si wafers modified with 1-alkenes. The passivating properties of the monolayers are found to be comparable to those of HF and iodine/ethanol solutions, two methods commonly used in semiconductor technology, but the monolayer-modified surfaces are far more stable than these two systems. This shows that these monolayers provide an interesting alternative for Si surface passivation.