Deposition of organosilane-based self-assembled monolayers from carbon-dioxide

Recent advances in nanotechnology calling for decreases in the feature sizes of nanodevices and microelectronic components to the nanometer scale has made the need for a new class of interfacial modifiers with sizes comparable to the dimension of a single molecule very eminent. Self-assembled monolayers (SAMs) represent a class of materials that enables the fabrication of surfaces with nanometer thick and structurally well-defined characteristics. The aim of our work was to investigate the potential of depositing SAMs from carbon dioxide (CO2), thus exploring the possibility of replacing environmentally harmful vapor and organic media. By combining the power of “classical” analytical techniques (ellipsometry and contact angle) and non-traditional tools (near-edge x-ray absorption fine structure spectroscopy) we were able to probe the complete kinetics of SAM formation from CO2 on silica substrates.

image of C02 chamber

different angle of C02 chamber

Photographs of the CO2 deposition apparatus built in the Genzer lab. The chamber consists of the solution chamber (SC) and the deposition chamber (DC). Liquid CO2 is generated by compressing gaseous CO2 (CO2 source) by pressure generator (H). The flow of the liquid CO2 between the solution and deposition chambers is controlled by the micropump (MP). The temperature in each chamber is adjusted indepedently by using the temperature controllers (TM1 and TM2). The pressure is monitored by the pressure meters (PM1 and PM2).

Most of the current SAM technologies rely on either vapor or organic solvent based deposition techniques. We recently studied the formation and properties of SAMs prepared by depositing semifluorinated and hydrocarbon trichlorosilane precursors, F(CF2)8(CH2)2SiCl3 (F8H2) and H(CH2)18SiCl3 (H18), respectively, from vapor, organic solvent, and liquid CO2. In addition to the obvious environmental benefits, CO2 has many possible technological advantages. The diffusion coefficients of low molecular weight SAM precursors in CO2 are about an order of magnitude larger than those in typical organic solvents and/or water. The low viscosity of CO2 should also decrease the amount of time necessary for adsorption and reaction of the SAM precursor with the surface. Carbon dioxide has an additional advantage over typical organic solvents in that separation and recovery of many coating compounds is accomplished by a pressure decrease.

graphs illustrating ellipsometric thickness

Ellipsometric thickness (a) and average tilt angle (b) of the molecules in F8H2-SAM (circles) and H18-SAM (squares) as a function of the deposition time from liquid CO2 mixtures. The lines are mean to guide the eye. The inset shows schematically the geometry of the NEXAFS experiment.

A battery of experimental probes was utilized to probe the formation of the F8H2- and H18-SAMs on flat silica-covered surfaces. Contact angle and ellipsometry measurements of the SAMs deposition kinetics were utilized to get insights about the mechanisms of the SAM formation. We showed that while a diffusion-limited model was not capable of describing the experimental data, an adsorption-limited model captured the major features of the adsorption kinetics quite well. We applied the results of the adsorption-limited model to conclude that although the SAM formation kinetics from CO2 were slower than that from vapor and faster than that from organic medium and that the overall SAM formation was fastest from CO2 because of higher precursor concentrations. Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy was used to monitor the ordering kinetics of the F8H2 and H18 molecules in their corresponding SAMs. Our NEXAFS data showed that the F8H2 molecules adsorbed initially from CO2 without any molecular order in the monolayer. As more F8H2 molecules partitioned at the silicon oxide surface, they started to organize and orient. The deposition kinetics and molecular behavior of the H18 moieties in the SAMs was found to be different. After a brief exposure (< 2 seconds) of the silica substrate to the H18/CO2 solution, the molecules adsorbed and formed an organized monolayer. Similar to the case of the semifluorinated species, the order in the H18-SAM increased with increasing time and saturates after ~5 minutes exposure to H18/CO2 solution. We attributed the difference in the orientation kinetics to the different solubilities of F8H2 and H18 in CO2 and the intermolecular forces between the organosilane molecules.