Structure of organosilane-based self-assembled monolayers

Formation of self-assembled monolayers (SAMs) on material substrates has now become a routine way to tailor the surfaces properties of materials. The most commonly used molecules forming SAMs are those bearing either i) a mercapto- group that can be terminally attached to gold or other noble metal surfaces, or ii) a chloro- or alkoxy- silane moiety capable of reacting with a hydroxy terminus on the surface. The application of the latter family of compounds is particularly useful for modifying the surfaces of materials that are covered with a thin (several Angstroms) native oxide layer. These molecules (R*) bonded to a functionalized Si group exist either in the form of chlorosilanes, e.g., mono- (R*-SiR2Cl), di- (R*-SiRCl2) or tri- ((R*-SiCl3)) chlorosilanes, or alkoxysilanes, e.g., mono- ((R*-Si(OR)2Cl)), di- ((R*-Si(OR)Cl2)) or tri- ((R*-Si(OR)3)) alkoxysilanes, where R is an alkyl. SAMs based on these compounds can be utilized to tailor surface properties of materials, making them suitable for biocompatibility, biosensors, the reduction of corrosion rates, pattern creation (lithographic processes), friction reduction (useful in microelectro-mechanical systems, MEMS, technology), foul resistance, modification of membrane properties, and changes in hydrophobicity.

The organosilane molecules attach to hydrophilic substrates terminated with surface-bound -OH groups. The general scheme illustrating the attachment of trichlorosilanes is shown on the right.

schematic illustrating the attachment of trichlorosilanes

We have recently conducted a comprehensive study of the formation and structure of semifluorinated (SF) mono- (F3C(CF2)8(CH2)2Si(CH3)2Cl, m-F8H2), di- (F3C(CF2)8(CH2)2Si(CH3)Cl2, d-F8H2), and tri- (F3C(CF2)8(CH2)2SiCl3, t-F8H2) organosilanes. The respective SAMs were formed by vapor deposition of the molecules on flat silica-covered substrates. Ellipsometry, cOntact angle, and near-edge x-ray absorption fine structure (NEXAFS) spectroscopy experiments were utilized to determine the SAM thickness, degree of packing, and the molecular orientation of the molecules in the SAMs, respectively.

Carbon K-edge PEY NEXAFS spectra from t-F8H2 collected at various sample orientations with respect to the X-ray beam, θ, ranging from 20 deg (glancing geometry) to 90 deg (normal geometry). The pre-edge and post-edge in each spectrum has been normalized to 0 and 1, respectively. The arrows in the figure indicate the positions of the 1s->s* transitions associated with the C-F (E=292.0 eV) and C-C (E=295.3 eV) bonds.

ASM NEXAFS Spectra

Our results indicate that the kinetics of SF SAM formation during vapor deposition is faster that that of normal organosilane hydrocarbons. We speculate that this faster kinetics is a consequence of stronger intermolecular interaction between the F8H2 moieties, relative to simple hydrocarbons. Second, results of NEXAFS spectroscopy experiments reveal that the molecular organization in F8H2-SAMs depends critically on the bonding environment around the silicon group. The average tilt angles (from the sample normal) of the fluorocarbon part, F(CF<2)8-, of t-F8H2, d-F8H2, and m-F8H2 measured by NEXAFS are 10±2, 35±2, and 45±3 deg, respectively. We argued that the increase of the tilt angle is associated with the steric hindrance of the methyl groups attached to silicon close to the bonding substrate. We showed that the molecular orientation elucidated from the NEXAFS spectroscopy measurements can be used to estimate the grafting densities of the F8H2 molecules on the substrates. Analysis using a simple one-dimensional geometric model revealed that the grafting density of m-F8H2 was approximately one half of that corresponding to the t-F8H2 SAM. We have also demonstrated the self-consistency of these results. Specifically, we have shown that the grafting densities of F8H2 determined from the one-dimensional chain tilt model are in accord with those that can be inferred from the relative edge-jumps in the fluorine K-edge NEXAFS spectra.

Schematic illustrating the molecular organization of t-F8H2 (top) and m-F8H2 (bottom) on silica substrates.

Schematic illustrating the molecular organization of t-F8H2 (top) and m-F8H2 (bottom) on silica substrates. Also shown is the geometry used to evaluate the distance between two neighboring F8H2 molecules, s, from the SF mesogen average tilt angle, <τF>, and the mesogen diameter, d.

SAM Thickness a)
(nm)
F> b)
(deg)
ѕF8H2 c)
(nm-2)
m-F8H2 0.9 45±3 1.59
d-F8H2 1.4 35±2 2.14
t-F8H2 1.65 10±2 3.09
A) Thickness measured by VASE (assuming nF8H2=1.38)
B) Average tilt of the F(CF2)8-measured by NEXAFS
C) Grafting density of F8H2 calculated from <τF>