Fabrication of surfaces with nanoislands of chemical functionality by the phase separation of self-assembling monolayers on silicon

In this paper we outline a method to prepare surfaces with nanoisland-sized domains of one terminal chemical functionality in a matrix of a second by using self-assembling monolayers (SAMs). We deposit a SAM solution consisting of two different alkylsilane amphiphiles (CH3(CH2)(17)SiCl3 (OTS) and NH2C6H4-Si(OMe)(3) (APhMS)) which phase separate on the oxide surface of silicon due to the stronger cohesive interaction of OTS and the exclusive hydrogen bonding of APhMS. We vary the deposition conditions-solvent polarity and composition ratio at constant total concentration-to control the surface morphology (i.e., which surfactant occupies the island and which the matrix phase), island size, and density. We change the solvent polarity by either changing solvents (using, in order of increasing polarity, carbon tetrachloride, toluene, chloroform, and tetrahydrofuran) or changing the water content when chloroform is used. Composition ratios are varied depositing from chloroform. We find that the island morphology is determined by which surfactant has the higher concentration on the surface; islands are formed only from the minor component although area fractions are not equivalent to composition fractions in the depositing solvent. For 1:1 composition ratios, the more polar the solvent or the higher the water content in the solvent, the more affinity the solvent has for the amine, and islands of the amine are formed. In chloroform, composition ratios strongly favoring either amphiphile result in continuous phases of that amphiphile, while 1: 1 ratios can, depending on the water content, result in either amine (high water content) or OTS (low water content) islands. Under similar small fractional surface coverages of the minor phase (i.e., less than 10%), when OTS forms the island phase, the islands tend to be larger and less dense, while when the amine forms the island phase, the islands are smaller and more finely dispersed. In the latter case we achieve finely distributed islands 25-50 nm in diameter, and O(30) islands/mum(2). Surfaces chemically patterned with nanosized islands are of broad technological interest because they can be used to localize adsorption by functionalizing the islands to bind individual molecules or nanosized particles and designing the matrix to be inert to their adsorption. Previous nanometer patterning techniques using SAMs utilize the tip of a scanning probe microscope to write directly or alter a preexisting SAM; these methods pattern only planar areas, on the order of tens or hundreds of microns squared in area. Our nanometer-scale patterning can pattern large as well as curved areas.