a, Thermal DPN, which uses a heated AFM cantilever whose tip is coated with a solid 'ink'. When the tip is hot enough, the ink melts and flows onto the substrate. No deposition occurs when the tip is cold, so imaging without any unintended deposition is possible. b, Electro pen nanolithography on an octadecyltrichlorosilane (OTS)-coated surface. The terminal methyl group of the OTS is converted to a reactive COOH-terminated surface (OTSox) by applying a voltage between the conducting AFM tip and the conducting silicon substrate in a humid environment. 'Ink' molecules are delivered from the 'inked tip' to the reactive OTSox surface, forming a second layer in the same sweep. No second layer is formed on the methyl-terminated regions. c, Writing mechanism of the Nano Fountain Pen. Molecular ink fed from the reservoir forms a liquid–air interface at the annular aperture of the volcano tip. Molecules are transferred by diffusion from the interface to a substrate and a water meniscus is formed by capillary condensation.