Scanning Probe Lithography. 3. Nanometer-Scale Electrochemical Patterning of Au and Organic Resists in the Absence of Intentionally Added Solvents or Electrolytes

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Abstract

Here we provide evidence that the principal mechanism responsible for scanning tunneling microscope (STM)-induced removal (or deposition) of material from organic thin films in air is electrochemical in nature. In experiments conducted in high-humidity (>∼70% relative humidity) ambients, patterning proceeds at biases above ∼+2.3 V because a thin layer of water adsorbed to the tip and surface establishes an ultra-thin-layer electrochemical cell. The low-energy self-assembled monolayer (SAM) restricts the dimensions of the highly resistive solution in the tip−sample gap and confines the patterning to the immediate vicinity of the tip, passivates unetched regions of the Au(111) substrate, and retards the surface mobility of Au atoms thereby stabilizing the patterns. In the absence of SAMs, patterns in nominally naked Au(111) are irreproducible and rapidly anneal to their pre-etch form. In low-humidity (<∼25% relative humidity) ambients there is insufficient water on the SAM surface to support Faradaic electrochemistry and insignificant patterning is observed at sample biases up to +5.0 V. We observed a bias threshold for patterning that is dependent on the composition of the tip−sample gap, but found that the bias threshold is essentially independent of the tunneling current. Using this scanning tunneling microscope-induced electrochemical patterning, we are able to reproducibly and selectively deposit or remove material from the surface to yield features having critical dimensions of less than 10 nm.

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