Dip-pen nanolithography of electrical contacts to organic nanostructures [electronic resource]
Keywords: Chemical Engineering.

The continuous drive towards nanoelectronics prompts development of alternative semiconductors that are compatible with silicon technology platforms and provide higher carrier mobilities than Si. Carbon nanotubes and their planar counterpart, graphene, hold promising potential as these next-generation materials. Furthermore, organic semiconductors such as pi-conjugated hydrocarbons may enable circuitry on substrates other than Si, filling a niche range of applications as flexible displays and chemical sensors. Engineering of these organic-based devices, however, requires further fundamental studies on their charge transport properties and correlating these properties to their structural order. Conventional methods of fabricating nanoscale devices required for such studies, e.g., isolated carbon nanotubes and graphene sheets, involve electron-beam lithography (EBL). This work focuses on developing dip-pen nanolithography (DPN) as an alternative method for patterning electrical contacts in these nanoscale devices. In DPN a scanning probe tip is coated with a molecular ink and acts as a nanoscale quill to directly write patterns on substrates, thereby forming functional nanostructures. The potential advantages of using DPN as a nanoscale patterning alternative to EBL include mild processing conditions (lack of electron beam irradiation) and ease of use and accessibility of AFM systems. Two schemes for DPN-patterning of electrical contacts were explored: 1) a direct patterning method in which DPN was used to deposit Au nanoparticles (NPs) that would eventually be chemically grown into conductive leads, and 2) an indirect patterning method in which DPN was used to deposit an alkanethiol on Au, acting as a mask against subsequent Au etching to reveal electrical contacts. In the first scheme, a procedure was developed for increasing the loading of Au NPs onto AFM tips to prolong patterning life, and surface interactions, relative humidity and writing speed were controlled to determine an optimal range of conditions for deposition. Results indicate that a highly hydrophilic surface is required for Au NP patterning, unless covalent binding can occur between the Au and substrate surface. This stringent surface chemistry requirement renders direct Au NP deposition unsuitable for making electrical contacts to arbitrary nanostructures and led us to pursue the second scheme for DPN patterning. In this indirect patterning scheme, proof-of-concept was demonstrated by creating single-walled carbon nanotube (SWNT) devices. Electrical contacts to individual and small bundle SWNTs were masked by an alkanethiol that was deposited via DPN on a thin film of Au evaporated onto spin-cast, non-percolating, and highly isolated SWNTs. A wet Au etching step was used to form the individual devices. The electrical contacts were comparable in conductivity to those made with EBL. Furthermore, the electrical characteristics for three different single-walled carbon nanotube devices -- semi-metallic, semiconducting and metallic -- were analyzed and indicated performance consistent with literature reports for isolated, solution-processed SWNT devices fabricated via EBL. Raman analysis on representative devices corroborates the results from AFM imaging and electrical testing. These optimized fabrication conditions were then applied toward making electrical contacts to graphene sheets and organic semiconducting grains.