Spontaneous “Phase Separation” of Patterned Binary Alkanethiol Mixtures

Abstract

This article describes novel phase-separation behavior by a binary mixture of alkanethiols when deposited onto a gold surface using micro- and nanodeposition tools, such as microcontact printing (muCP) and dip-pen nanolithography (DPN). This behavior is significantly different than that observed in the bulk. We demonstrate this behavior using three model compounds: 16-mercaptohexadecanoic acid (MHA), 1-octadecanethiol (ODT), and CF3(CF2)11(CH2)2SH (PFT). The identity of the resulting segregated structure is confirmed by lateral force microscopy (LFM) and by selective metal-organic coordination chemistry. Importantly, this phenomenon can be exploited to print sub-100 nm wide alkanethiol lines via conventional muCP and to form sub-15 nm features using DPN, which is below the ultimate resolution of both these techniques. We also demonstrate that these nano-patterned materials can serve as templates for constructing more complex architectures.

Fabrication of Micropatterned Stimulus‐Responsive Polymer‐Brush ‘Anemone’

“…Because polymer-brush growth can only occur in areas that contain the initiator thiol, and since the feature diameter of the polymer brushes matched that of the patterned thiol, our observations lend evidence to the existence of a microphase separation between the two thiol species, in which the initiator thiol, the more hydrophilic of the two thiols with a contact angle of 73 AE 38, was concentrated in the center of the patterned features. This preferential deposition agrees with the observations by Salaita, [19] who showed that in DPN of binary alkanethiol mixtures with matched alkyl chain lengths the more hydrophilic thiol deposits first. Furthermore, the work by Cooper and Leggett [17] showed that hydrogen bonding between terminal functional groups (such as -OH or -COOH) on alkene thiols contributes strongly to SAM stability.…”

“…Although the spontaneous, stochastic phase separation of binary coadsorbed alkanethiols on the nanoscale has been reported, [13][14][15][16][17][18] there is only one report on their ordered nano-and microscale phase separation that occurs during dippen nanolithography (DPN) and microcontact printing. [19] This report atributes the phase separation of a mixture of acidterminated and methyl-terminated alkanethiols on gold surfaces during DPN to 1) attractive enthalpic interactions between the polar headgroups of mercaptohexadecanoic acid (MHA), outweighing the van der Waals stabilization of the methylene groups common to all alkane thiols, and 2) the kinetics of ink-transfer, due to the higher solubility and diffusion of MHA in water compared to in octadecane thiol (ODT). [19] Our approach focuses on a similar microphase separation of a binary mixture of alkanethiols during mCP, to ultimately yield initiator-gradient patterns that can be amplified into annular, stimulus-responsive poly(N-isopropylacrylamide) (PNIPAAM) brush microstructures by Si-ATRP.…”

“…[19] This report atributes the phase separation of a mixture of acidterminated and methyl-terminated alkanethiols on gold surfaces during DPN to 1) attractive enthalpic interactions between the polar headgroups of mercaptohexadecanoic acid (MHA), outweighing the van der Waals stabilization of the methylene groups common to all alkane thiols, and 2) the kinetics of ink-transfer, due to the higher solubility and diffusion of MHA in water compared to in octadecane thiol (ODT). [19] Our approach focuses on a similar microphase separation of a binary mixture of alkanethiols during mCP, to ultimately yield initiator-gradient patterns that can be amplified into annular, stimulus-responsive poly(N-isopropylacrylamide) (PNIPAAM) brush microstructures by Si-ATRP. We show that our PNIPAAM-brush microstructures can potentially be used in microfluidic systems to capture, dock, and release microparticles in predefined locations, analogous to sea anemones capturing their prey.…”

“…Although we show in Step A that the thiol initiator is concentrated at the center of the stamp features and that UDT is concentrated at the periphery, we concede that the initial phase separation may alternatively occur during the ink-transfer step. [19] Upon contact between the stamp and the substrate, the polar thiol initiator is, compared with the transfer of the hydrophobic UDT, preferentially transferred to the gold substrate. This results in a circular initiator pattern on the gold substrate with a feature diameter that is significantly smaller than the nominal diameter of the stamp (Step B).…”

Hybrid approaches to nanometer-scale patterning: Exploiting tailored intermolecular interactions

“…This displacement process has been exploited to improve existing chemical patterning strategies (vide infra). Even small multicomponent patches of molecules (*15 nm) can be made to separate on the nanoscale (Salaita et al 2005). In many cases, phase separation can be controlled by the selection of the molecules and processing conditions, which determine the dynamics of the structures created (Bumm et al 1999, Smith et al 2004.…”