Thermochemical nanopatterning of organic semiconductors Oliver Fenwick 1,2 , Laurent Bozec 1,3 , Dan Credgington 1,2 , Azzedine Hammiche 4 , Giovanni Mattia Lazzerini 5 , Yaron R. Silberberg 1 and Franco Cacialli 1,2 * Patterning of semiconducting polymers on surfaces is important for various applications in nanoelectronics and nanophotonics. However, many of the approaches to nanolithography that are used to pattern inorganic materials are too harsh for organic semi- conductors, so research has focused on optical patterning 1–3 and various soft lithographies 4 . Surprisingly little attention has been paid to thermal 5 , thermomechanical 6,7 and thermochemi- cal 8–13 patterning. Here, we demonstrate thermochemical nano- patterning of poly( p-phenylene vinylene), a widely used electroluminescent polymer 14 , by a scanning probe. We produce patterned structures with dimensions below 28 nm, although the tip of the probe has a diameter of 5 mm, and achieve write speeds of 100 mms 21 . Experiments show that a resolution of 28 nm is possible when the tip–sample contact region has dimen- sions of 100 nm and, on the basis of finite-element modelling, we predict that the resolution could be improved by using a thinner resist layer and an optimized probe. Thermochemical lithography offers a versatile, reliable and general nanopatterning technique because a large number of optical materials, including many commercial crosslinker additives and photoresists, rely on chemical mechanisms that can also be thermally activated 8,15,16 . Since its use in the first polymer light-emitting diodes (LEDs) 14 , poly( p-phenylene vinylene) (PPV) has been one of the most widely studied of a class of materials, with applications ranging from polymer LEDs to transistors 17 and photovoltaic cells 18,19 . These con- jugated polymers attract commercial interest because of the possi- bility of incorporating them into flexible devices and for the relative ease of solution processing by methods such as inkjet print- ing 17 that are synonymous with cheap fabrication over large areas. PPV can be used in either its unsubstituted insoluble form, prepared by pyrolysis of a soluble precursor 14 , or as a functionalized derivative with side-groups conferring solubility 20 . The unsubstituted PPV precursors can be converted in situ either thermally 14 or optically 2 . We have previously used scanning near-field optical lithography for nano-patterning of PPV structures from a precursor polymer, poly( p-xylene tetrahydrothiophenium chloride) (PXT), including the fabrication of quasi-periodic two-dimensional structures with potential for photonic applications 2 . Insolubilization of PXT follows ultraviolet (UV) exposure and allows the patterned film to be developed in methanol. A further vacuum baking step (200 8C) 21 ensures complete conversion to PPV. However, it would seem feasible to bypass the UV insolubilization step if the precursor could be heated locally. For our experiments we used a Wollaston wire probe 22,23 (Fig. 1), consisting of a silver wire with a 5-mm-diameter platinum–rhodium core. This has been etched to leave a 50-mm-long section of the core exposed and the wire is shaped to form a resistively heated probe that is mounted on an atomic force microscope (AFM) and scanned at a constant temperature to locally insolubilize a PXT film. Thermochemical pat- terning of a sacrificial resist has previously only been demonstrated at a resolution of 400 nm (ref. 8). More recently, thermochemical alteration of crosslinked films of hydrophobic polymers, rendering them hydrophilic, has been reported at high write speeds (.1 mm s 21 ) on the nanoscale 9,10 . Here, we show that soft, lumi- nescent and semiconducting materials can be directly thermo- chemically patterned with nanoscale resolution. Furthermore, we present isolated negative tone lithographic features in PPV, display- ing improvements by more than an order of magnitude in resolution and by three orders of magnitude in write speed for resist-type thermochemical patterning, opening up a range of appli- cations for this technique. Grids formed by lines of PPV written with a periodicity of 2 mm are shown in Fig. 2a,b. These demonstrate the capability for scanning thermochemical lithography as an effective patterning technique for these materials that gives reproducibility over the full 100  100 mm 2 scan area of the AFM at nanoscale resolutions. Furthermore, the associated confocal luminescence images confirm that the resulting structures are photoluminescent. An investigation of a number of lines drawn over a range of scan speeds (Fig. 2d,e) shows that the width of the lines decreases rapidly as a function of the scan speed, Wollaston wire (75 μm diameter, silver coated) Platinum–rhodium core (~5 μm diameter) Fast-scan direction Spin-coated sample n S + n Cl – S ~200 °C HCl + Figure 1 | Thermochemical nanopatterning. Schematic of the Wollaston wire probe (a resistively heated wire), which is connected to an external temperature control circuit. This probe is mounted on an atomic force microscope (AFM) and is scanned in contact with our precursor polymer films at a constant temperature. The precursor polymer films are deposited on fused silica substrates by spin-coating. The inset shows the thermal conversion route of the precursor polymer, poly( p-xylene tetrahydrothiophenium chloride) (PXT), to fully conjugated PPV, which occurs optimally at 200 8C. 1 London Centre for Nanotechnology, 17-19 Gordon Street, London WC1H 0AH, UK, 2 Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK, 3 UCL Eastman Dental Institute, University College London, 256 Gray’s Inn Road, London WC1X 8LD, UK, 4 Department of Physics, Lancaster University, Lancaster, LA1 4YB, UK, 5 Dipartimento di Ingegneria dell’Informazione, Universita ` di Pisa, Via G. Caruso 16, 56122, Pisa, Italy. *e-mail: f.cacialli@ucl.ac.uk LETTERS PUBLISHED ONLINE: 6 SEPTEMBER 2009 | DOI: 10.1038/NNANO.2009.254 NATURE NANOTECHNOLOGY | VOL 4 | OCTOBER 2009 | www.nature.com/naturenanotechnology 664 © 2009 Macmillan Publishers Limited. All rights reserved.