Dispersion-free nanoprinting

Illustration of the µDP process. Molecules attached to the PDMS stamp replace AD molecules on the gold substrate at the point of contact. Figure reprinted in part with permission from ref. 1. Copyright 2005 American Chemical Society.

Scientists working at the Pennsylvania State University have devised a highly accurate nanofabrication process that is both greater in accuracy than its predecessors and increases the number of materials that can be printed. Microcontact Printing (µCP) is a method whereby micrometre and sub-micrometre patterns of material can be deposited directly onto the surface of a substrate without the need for artificially clean environments. Subsequent diffusion of this material along the face of the substrate however, can lead to blurring of the intended image. In Nano Letters, Paul Weiss and his team outline a modified version of this fabrication technique that they term microdisplacement printing (µDP). By first coating the entire surface of the substrate with a weakly bound material, they demonstrate that diffusion effects can be avoided, greatly increasing the potential of this 'printing' process.

Self-assembled monolayers (SAMs) consist of a single layer of molecules bonded to a surface. Many complex uses have been proposed for SAM devices fashioned using the µCP technique, from biological sensors to microelectronic circuits. The µCP process involves 'molecularly inking' an appropriate stamp, which is then used to apply material directly to the substrate surface. Lateral diffusion of the ink molecules can cause the required pattern to be significantly distorted, possibly ruining its usefulness. This diffusion effect is worsened if there are blank spaces left on the surface of the substrate where no ink molecules were applied.
The µDP technique proposed by Weiss et al. greatly reduces the lateral diffusion of molecules by ensuring that no blank spaces exist on the target surface. This is achieved by coating the blank substrate with a material called adamantanethiolate (AD), which weakly bonds to it forming a monolayer. The process of stamping the image onto the surface is then performed as usual. Providing that the ink material bonds more strongly to the substrate than the AD, it will replace the AD molecules at the points of contact, creating patterned SAMs. By carefully choosing the order in which layers are added, complex structures can be made by taking advantage of the relative bonding strengths of various materials to the substrate. This process has the added advantage of dispensing with the solvent exposure stage needed to fill the blank spaces left by the µCP method.
In addition to the greater accuracy associated with the µDP process due to reduced diffusion of the monolayer molecules, the Penn State team believes that they have increased the number of molecules that can be used to create SAM structures. They argue that molecules previously 'too mobile' to pattern will be constrained using the new technique. It is also expected that the use of competitive adsorption to pattern SAMs can be extended to other fabrication techniques.
In supporting documentation that accompanies the paper, a comparison is made between the patterns obtained using the traditional µCP method and the new technique. It is found that molecules deposited using the older process diffuse across the substrate until the desired pattern is unrecognisable. In contrast, the patterns produced using the µDP method are found to be stable and retain the required structure.
In order to investigate the efficiency of the molecular substitution process, Weiss and co-workers examine the effect of changing the concentration of the ink and the stamping duration. This was done using an unpatterned block inked with a chemical called 1-decanethiol (C10). It was found that the displacement of the AD occurs in distinct patches presumably at points of defect in the AD coverage which grow in number and in size as the concentration of C10 and stamping time are increased. This information regarding the 'kinetics' of the substitution process can be used to determine appropriate conditions for use with various stamping patterns.

SOURCE: Nanozone News