Kinetics of Self-Assembled Monolayer Growth Explored via Submonolayer Coverage of Incomplete Films

Abstract

We have studied the growth kinetics of self-assembled monolayers of octadecylphosphonic acid on mica by examining films removed from solution before completion. Atomic force microscope (AFM) images of the quenched films showed islands approximately 2 nm high indicating a growth mechanism consisting of nucleation, growth, coalescence, etc. of dense submonolayer islands. This was consistent with previous in situ AFM studies. Infrared spectroscopy data were consistent with well-ordered alkyl chains and indicated only one type of chain conformation, implying that the areas between islands were bare, not covered with loosely packed disordered molecules. The surface coverage of the submonolayer islands was extracted from AFM images as a function of immersion time for solution concentrations ranging from 0.02 to 2 mM. These data were compared to two common models for adsorption kineticsdiffusion-limited kinetics and adsorption-limited (Langmuir) kinetics. The previously reported quasi-Langmuir−Blodgett deposition process occurring during removal was also taken into account in this modeling. The data were completely inconsistent with the adsorption-limited kinetics and were in reasonable agreement with the functional form for diffusion-limited kinetics. However, the diffusion parameters extracted from the latter fits were much smaller than expected for molecular diffusion in solution and did not scale correctly with solution concentration. We conclude that the island growth is limited by a process that obeys diffusion-like kinetics but not by actual solution diffusion.

In Situ Study of Model Organic Friction Modifiers Using Liquid Cell AFM: Self-Assembly of Octadecylamine

“…Apart from the above, Schwartz et al has used in situ AFM to study alkylphosphonic acid on mica [36][37][38] but work on most other amphiphiles has employed ex situ studies, and includes study of films formed by fatty acids [39][40][41][42], fatty alcohols [43][44][45] and fatty amines [46] [47]. The application of AFM to study SAMs for nano-lubrication has been reviewed by Cheng and Hu [48].…”

“…The islands were irregular in shape, contained pinhole inclusions and had diameter from 1 to 3 µm and a height of 1.5 nm. This suggests a monolayer of octadecylamine where molecules are tilted from the surface normal by 51°, assuming that the length of one octadecylamine molecule is 2.4 nm [47].…”

Dynamic scaling of the submonolayer island size distribution during self-assembled monolayer growth

“…A number of studies, encompassing a variety of different molecule/substrate systems, indicate that the mechanism of formation involves molecular adsorption from solution, followed by aggregation into densely packed submonolayer islands on the substrate surface. [2][3][4][5][6][7][8][9][10] Our recent work 9 exploited the analogy with ultrahigh vacuum vapor phase thin-film growth ͓molecularbeam epitaxy ͑MBE͔͒, 11 demonstrating that kinetic theories of two-dimensional ͑2D͒ cluster growth, [12][13][14] developed to explain MBE, quantitatively described the nucleation and growth kinetics of SAM islands in the early stages of film formation. Although the chemistry involved in SAM growth and MBE growth is clearly very different, the early stages of growth involve the same processes-submonolayer island nucleation and growth-and in both cases these processes are controlled by two competing rates: a deposition rate from the third dimension F and a surface diffusion rate D. In previous work 9 we studied the nucleation and growth kinetics in the early stages of growth.…”

Effect of Reaction Temperature on Growth of Organosilane Self-Assembled Monolayers

“…The formation mechanism and kinetics have been investigated using various experimental techniques. 13,[21][22][23][24][25][26][27][28][29] Octadecyltrichlorosilane (OTS) is the most common organosilane used for the formation of SAM. OTS molecules form large dendrite domains under certain conditions and grow two dimensionally on the SiO 2 surface.…”