UV photopatterning is a well-known method for the creation of patterned alkanethiolate SAM surfaces on metal substrates. Previous experiments have demonstrated that the results of UV photopatterning are dependent on the length of the methylene chain and the terminal group interactions.
Figure 1: A schematic diagram of the UV-photopatterning process of SAMs on Au. In step 1, UV light is shown through a mask onto the SAM#1 surface. In the areas exposed to UV light SAM#1 is photooxidized. In step 2 a second SAM (SAM#2) is adsorbed on the surface, forming a patterned surface (step 3). If IR light is present, SAMs shorter than 14 carbons melt and are displaced by the adsorption of the second SAM, leading to pattern degradation. When an IR filter is employed, SAMs with as few as 10 carbon atoms remain stable on the surface and can be photopatterned.
We have shown that UV photopatterning is also strongly dependent on the wavelengths of light that reach the sample (Figure 1). When an unfiltered arc lamp is employed, IR light falls on the sample and causes considerable sample heating. Methyl-terminated SAMs with fewer than 14 carbons in the methylene chain melt into a disordered state and so can be easily displaced by a second SAM. This leads to significant pattern degradation or erosion. SAMs with more than 14 carbons undergo a transition to an incommensurate phase but remain stable, and the pattern is retained. When IR light is filtered out, different behavior is observed. Methyl-terminated SAMs with 10 or more carbons remain ordered under these conditions. Interactions between terminal groups, such as hydrogen-bonding, provide extra stabilization energy and so patterns with some shorter carbon chain SAMs may also be stable.
Figure 2: Negative ion TOF SIMS images centered at m/z = 17 (OH-) and m/z = 285 (SC18H37-) for MHA/ODT and ODT/MHA on GaAs. Area of analysis: 500 x 500 μm2, 128 x 128 pixels2.
We have demonstrated that alkanethiolate SAMs adsorbed on GaAs can also be UV photopatterned. In this case, both the SAM and the GaAs substrate photooxidize upon exposure to UV light in the presence of oxygen. We remove the oxidized GaAs by etching the patterned surface using 30% ammonium hydroxide prior to immersion in the second SAM solution. We have also investigated the reaction pathways involved in the photooxidation of alkanethiolate SAMs on GaAs (001). As in the photooxidation of SAMs on metals, the rate determining step is the penetration of oxygen to the S/substrate interface. The reaction pathways involved change as the length of the methylene chain increases. For alkanethiols with fewer than 16 carbons the reaction mechanism is initially similar to that observed for SAMs adsorbed on Au and Ag: the thiol group oxidizes to form a sulfonate. Subsequently, the alkylsulfonate either continues to oxidize, forming an alkylsulfate, or decomposes. For a longer chain alkanethiol, octadecanethiol, the reaction proceeds via the simultaneous photooxidation of the alkanethiol to alkylsulfonate and alkylsulfates and the decomposition of the alkanethiol.