functionalized surfaces on porous silica by atomic layer ... - Aaltodoc

Figure 1. Schematic presentation of the reaction sequences when alkoxysilanes are deposited
on silica in an aqueous solvent after Vansant et al. 1
Controlled deposition of surface-saturated molecular layers from the gas phase onto the
substrate surface is achieved by vaporizing the precursor molecules and allowing them to
react with the substrate under completely dry conditions. The presence of water can be
excluded in the gas phase because no solvents are needed and the precursor molecules, which
may be possibly hydrolyzed, cannot be vaporized at low deposition temperatures. When
alkoxysilanes (or aminoalkoxysilanes) are used as precursors, the hydrolysis of alkoxy
groups, hydrogen bonding of hydrolyzed alkoxy groups, and further condensation reactions
before deposition can all be avoided (Figure 2). 1 Thus, the chemical interactions of the
precursor molecules with the reactive surface species result in direct bond formation and the
deposition of a single surface-saturated molecular layer in a reproducible manner. However,
total surface density of (amino)propylsilyl groups on the surface is lower than in multilayers
formed under aqueous conditions. The gas-phase deposition of a single surface-saturated
molecular layer can be performed by various chemical vapor deposition (CVD) techniques
but especially by ALD which also enables a simple control of the surface density and the film
thickness, as will be discussed in Section 2.2. ALD is not only closely related to CVD, but
also to grafting. 8
13