Nanometer-scale capillary-driven flow and molecular weight govern polymer nanostructure deposition from a heated tip

This paper investigates the physical process of polymer nanostructure deposition from a heated atomic force microscope (AFM) tip and focuses on the role of capillary-driven flow on deposited feature sizes during thermal dip-pen nanolithography. We used a heated AFM tip to deposit 50–350 nm wide poly(methyl methacrylate) nanoribbons by varying tip temperature, tip speed, and polymer molecular weight. For polymers of different molecular weights, the width of the deposited polymer nanoribbons decreases with capillary number (Ca), independent of tip temperature, tip speed, and polymer molecular weight. These results indicate that the capillary-driven flow governs polymer nanostructure deposition from a heated tip. For high molecular weight polymers deposited with feature size close to the polymer radius of gyration Rg, the molecular weight also influences the size of the deposited polymer ribbon. Using scaling arguments, we show that the feature size can be predicted by Ca and Rg. Uniform and continuous deposition occurs only when Ca << 1, confirming that the polymer flow is driven by the capillary force. The results of this study enable better control of speed and resolution at which polymer nanostructures can be fabricated using AFM.