Doping is a fundamental process to tune the semiconductor's electrical properties on demand^[1]. Since the invention of silicon electronics a century ago, the pursuit of nanoscale doping has greatly advanced integrated circuits, underpinning modern vibrant electronics. Single ion implantation (SII)^[2]—as the premier nanoscale doping technique for inorganic semiconductors, has not only revolutionized nanomanufacturing of the logic chips but also continued to allow the creation of new functionalities. However, this successful approach is not viable for soft material systems, as high-energy ion bombardment typically results in irreversible damage to the polymers^[3]. Considering the rich history and long-term potential of doping, developing similar methodologies that are suitable for polymeric semiconductors could trigger new opportunities in the flexible electronics industry.

We dedicated to using molecule doping to enhance polymeric semiconductor devices for a long time, but the lack of high-resolution doping methods hampered us from investigating their charge transport at the nanoscale. Fortunately, while I was deeply engaged in electrochemical doping, I discovered that this process involves ion electromigration under an electric field, which is very analogous to conventional ion implantation doping but with ultralow energy. Under this situation, an idea appears in my mind, i.e., if it is possible to use the electrochemical principle to develop a “nanoconfined electrochemical ion implantation-like doping technique” for soft polymers?

We started this work full of enthusiasm, but the results were far from expectations. This deviation puzzled us for quite a long time until we turned to re-think the intrinsic nature of the electrochemical process and seek more inspiration from conventional SII doping. We found that the low resolution of electrochemical doping should arise from the global ion migration in electrolytes, which is limited by the large size of the counter electrode (CE) and the severe fringing field. Inspired by the SII doping where ion beams are physically confined by a pierced tip, we, therefore, hypothesize that similarly focused counterion beams might be achieved by creating intangible nanostencil via scaling CE to the nanoscale and simultaneously linearly sharpening the fringing field. Keeping these ideas in mind, we finally choose a nanosized conductive-AFM tip as the CE and reshape the fringing field by manipulating the glass transition of the electrolyte as the fringing effect is highly associated with ion dynamics.

Hard work always pays off for those who persevere! After months of experiments, we are very happy to see that nanoscale doping of polymers can be realized by incorporating a glassy electrolyte composed of room-temperature ionic liquids and high glass-transition temperature insulating polymers. By modulating glass transition temperature (T_g) above 100 °C, we achieved a record resolution of 56 nm with the lowest lateral-extended doping length down to 9.3 nm, which represents the state-of-art value. This result quite excited me, but further challenge comes one after another. For example, what’s the mechanism behind electrolyte glassy transition? if this concept is general to other material systems? whether this technique have significant implications in organic electronics? and if this method could be scalable. We did solve these questions one by one, but exploring them at the nanoscale requires quite meticulous experiments. With several years of effort, we revealed that the glassy electrolyte which shows decoupled ion transport from segmental relaxation of the insulating polymers, could substantially contribute to the localized electric field, and associated anisotropic ion electromigration. We further summarized a universal exponential dependence of the resolution on the temperature difference (T_g−T) and created a phase diagram that can be used to depict the doping resolution for almost infinite polymeric semiconductors. We also demonstrate its implications in a range of polymer electronic devices, including a 200% performance-enhanced organic transistor and a lateral p-n diode with seamless junction widths of sub-100 nm. Moreover, by combining the tip array, the scalability was further demonstrated with doping arrays of 2×2 cm² achieved within one minute. We believe that this nanoconfined electrochemical ion implantation-like doping represents a conceptual and technological advancement, and would serve as a powerful tool for exploring nanoscale polymeric optoelectronics

For more details, please refer to our paper “Nanoscale Doping of Polymeric Semiconductors with Confined Electrochemical Ion Implantation” in Nature Nanotechnology (2024). https://rdcu.be/dFpbZ

1 Yamashita, Y.; Tsurumi, J.; Ohno, M.; Fujimoto, R.; Kumagai, S.; Kurosawa, T.; Okamoto, T.; Takeya, J.; Watanabe, S. Efficient molecular doping of polymeric semiconductors driven by anion exchange. Nature, 2019, 572, 634-638.

2 Koenraad, P. M.; Flatte, M. E. Single dopants in semiconductors. Nat. Mater., 2011, 10, 91-100.

3 Popok, V. N. Ion implantation of polymers: Formation of nanoparticle materials. Rev. Adv. Mater. Sci., 2012, 30, 1-26.