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.
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.
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.