(Invited) Organization in Molecular Layers Covalently Attached to Oxide-Free Silicon Surfaces

The grafting of molecular layers on  oxide-free silicon surfaces is an approach widely used for preventing  surface oxidation and/or for the construction of functional surfaces with  specific properties. In particular, this strategy is often used for the passivation/functionalization of  porous silicon. If the large specific surface area makes porous silicon especially interesting for applications, the porous structure makes delicate and sometimes difficult the use of surface techniques for the characterization of surface properties. The molecular organization within the layer on the pore’s wall remains an open issue despite the fact that this aspect is of prime importance for the final properties of the system. In the present work, we have investigated the properties of organic monolayers grafted onto model Si(111) surfaces in order to gain generic knowledge on the molecular physico-chemical processes governing the molecular organization, which should also apply to more complex, real systems such as porous silicon.

The model system consisting of mixed alkyl/carboxy-alkyl layers grafted on atomically flat Si(111) surfaces has been carefully studied using Atomic Force Microscopy (AFM) in dynamic mode, and quantitative Fourier-Transform InfraRed (FTIR) spectroscopy. The mixed layers were produced by direct, UV-activated reaction of decene/undecylenic acid mixtures on hydrogenated silicon surfaces.  Different concentrations of undecylenic acid in decene (from 0.01 to 100% by vol) have been investigated in order to change the composition of the grafted layer. The composition of the mixed layers has been determined by quantitative infrared spectroscopy. For concentrated (above 20%) acid solutions, the fraction of acid molecules in the grafted layer is found to be larger than in solution, in agreement with previous reports [1]. Surprisingly, this trend is much amplified at lower acid concentration, with an acid content of ~30% weakly dependent on the grafting solution composition for acid contents in the range 0.1% to 10%.

The surfaces were characterized by AFM imaging in non-contact mode. When the acid fraction in the monolayer exceeds ~40%, phase images, which are sensitive to tip-surface interactions, are featureless, suggesting a uniform spreading of acid chains. A phase separation is found at large scale (a few µm-size  domains) on molecular layers obtained from grafting solutions with an acid content varying from 0.1% to 10%. This chemical contrast is consistent with acid-rich domains. However, the domain coverage turns out to be much too low in order to account for the total acid content of the molecular layer, as stated by FTIR analysis. For molecular layers obtained from grafting solutions with a very low acid content (0.01%), nanometric (typical size below 10 nm) domains are also revealed. In this case, the acid amount derived from the domain coverage is in agreement with the acid content of the layer determined by FTIR spectroscopy.

All these observations reveal that the mixed alkyl/carboxy-alkyl layers formed on Si(111) surfaces exhibit a multi-scale inhomogeneous structure, characterized by a phase separation down to the nanometer scale. This picture can be made slightly more precise by complementary spectroscopic investigations. All the observations can be rationalized by the formation of supramolecular assemblies in the grafting solution and their transfer to the surface during the grafting process.  A consistent picture will be presented and the applicability of the results to real systems like porous silicon will be discussed.

[1] A. Faucheux et al., Langmuir 22 (2006) 153.