Nanoscale Desorption of Self Assembled Monolayers

The resolution of the patterning method is usually determined by two major factors: the size of the proximal probe and the thickness of patterning materials. Thus, the highest resolution can be achieved by patterning monolayer materials such as self-assembled monolayer. Figure 12A shows the basic mechanism of SAM patterning process under ambient conditions via conducting SPM probes. At high humidity (Figure 12 A (b)), SAM layer is stripped off by the electrochemical reaction in water meniscus, while it remains without a water meniscus at low humidity (Figure 12 A (a)). Here, the water meniscus also plays a role as an electrochemical reaction chamber like nano-oxidation processes while in a little bit different manner. Crooks et al. proposed that

Figure 12. (A) Schematic diagram depicting nanoscale desorption process of SAM (a) with and (b) without water meniscus. With a water meniscus, the electrochemical reaction causes the removal of the SAM layer. However, the reaction does not occur without a water meniscus at low humidity. (B) The SAM in the square region is removed utilizing a STM probe. The experiment is performed in air with 75% relative humidity and 3 V tip bias. Adapted with permission from [87] J . Phys. Chem. 100 (1996) 11086.

Figure 12. (A) Schematic diagram depicting nanoscale desorption process of SAM (a) with and (b) without water meniscus. With a water meniscus, the electrochemical reaction causes the removal of the SAM layer. However, the reaction does not occur without a water meniscus at low humidity. (B) The SAM in the square region is removed utilizing a STM probe. The experiment is performed in air with 75% relative humidity and 3 V tip bias. Adapted with permission from [87] J . Phys. Chem. 100 (1996) 11086.

the current flow in the water meniscus induce the oxidation process and remove the SAM molecules from the surface [87, 88]. Example reaction for alkanethiol SAMs on Au is:

CH3(CH2)nS - Au + 2H2O ^ Au + CH3(CH2)nSO2H + 3e- + 3H+

Since a water meniscus works as a reaction chamber, the patterning speed strongly depends on the relative humidity. Figure 12B shows the result after stripping off the SAM in a nanoscale square region. The stripped part can be later chemically etched to transfer the pattern to the substrate.

Different self-assembled monolayers have been patterned using a conducting SPM probes on various surfaces such as Au, GaAs, Si, SiO2 etc [89—95].

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