Thiol Diffusion and the Role of Humidity in “Dip Pen Nanolithography”

Abstract

The radii of octadecanethiol spots deposited by an atomic force microscope tip onto a gold surface were studied as a function of contact time and humidity. The deposition is well described by twodimensional diffusion from an annular source of constant concentration, with a surface diffusion coefficient of 8400 nm 2 s 21 , independent of humidity. Facile transfer is observed even after near continuous deposition for more than 24 h in a dry N 2 environment, indicating that a water meniscus is not required. DOI: 10.1103/PhysRevLett.88.156104 PACS numbers: 68.37.Ps, 68.43.Jk, 81.16.Nd, 81.16.Rf Since the first use of alkanethiols by Nuzzo and Allara [1] to form a self-assembled monolayer (SAM) on a gold surface, the number of applications for thiolated hydrocarbons has increased rapidly [2], ranging from resists for electronics to biosensor substrates. The major advantages offered by thiol SAMs include long-term stability, versatility in terminal functionality, and ease of use. Perhaps the most attractive feature is their utility for forming small, reproducible surface features outside of a clean room. For example, facile creation of micron-scale patterns has been demonstrated by "stamping" thiols onto a surface using a flexible polymer master, a technique known as microcontact printing (mCP) [3]. Even smaller, nanometer-scale features can be patterned from thiols using the recently developed "Dip Pen Nanolithography" (DPN) [4], where alkanethiols are "written" via transfer from an atomic force microscope (AFM) tip to a surface. The prospect of nanometer-scale control over physical dimensions combined with the flexible terminal group chemistry makes thiol patterning a critical component of many proposed nanotechnologies.Because alkanethiol patterning is emerging as a key technique for nanofabrication, it is crucial that the mechanisms of deposition -and thereby the ultimate technological potential-be understood. The most important process of the deposition may be diffusion, which controls both the extent and the quality of an alkanethiol SAM. As previously noted in a study of mCP [5], the spatial resolution of a pattern is fundamentally limited by diffusion of the thiol "ink." First, diffusion of the thiol beyond the initial contact area causes the pattern to be wider than the stamp (for mCP) or the AFM tip terminus (for DPN). Although there have been a few attempts to measure the diffusion rate of thiols on gold [5][6][7][8], these measurements have been mostly phenomenological and thus have not reported diffusion coefficients. A second, more subtle, effect of diffusion is its role in the phase transition that occurs during SAM deposition: At a critical surface concentration, the adsorbed thiols reorient from a prone to a standing orientation. This change in orientation affects physical properties of the SAM, such as friction, as well as important chemical properties, such as its ability to mask the substrate from an etchant [9].Another important issue in determining the overall potential of DPN is the role of ...

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