Xiyou Li

High-performance ambipolar responses to oxidizing NO2 and reducing NH3 based on the self-assembled film of an amphiphilic tris(phthalocyaninato) europium complex

A heteroleptic amphiphilic tris(phthalocyaninato) europium triple-decker complex, (Pc)Eu{Pc[(OC2H4)3OCH3]8}Eu{Pc[(OC2H4)3OCH3]8} (1), has been designed and synthesized successfully. The hydrophilic polyoxyethylene substituents attached onto the periphery of two phthalocyanine rings in the sandwich-type phthalocyaninato triple-decker not only increase the solubility and improve the film-forming ability, but also importantly ensure the suitable HOMO and LUMO energy levels and thus successfully realize amphiphilic ambipolar organic semiconductors. The solution-processed thin film of the complex is prepared by a simple and low-cost quasi-Langmuir–Shafer (QLS) method. Importantly, within the dynamic exposure period of 30 s, highly sensitive, stable, reproducible n-type response to electron-accepting gas NO2 in the range of 50–500 ppb and p-type response to electron-donating gas NH3 in 5–25 ppm range, have been first revealed, based on the QLS film of 1 at room temperature, depending on the optimized molecular packing in J-aggregation mode with a large specific surface area and good film conductivity. Furthermore, the responses of the QLS film are all linearly correlated with either NO2 or NH3 with good sensitivity of 0.06% ppb−1 and 0.17% ppm−1, respectively, indicating the great potential of phthalocyanine-based rare earth triple-decker complexes in the field of chemical sensors. The present result provides a new strategy to obtain high-performance room-temperature gas sensors by the molecular design combined with a low-cost solution-based method.

The lower rather than higher density charge carrier determines the NH3-sensing nature and sensitivity of ambipolar organic semiconductors

Despite the extensive studies and great application potentials, the sensing nature of ambipolar organic semiconductor gas sensors still remains unclarified, unlike their inorganic counterparts. Herein, different numbers of thiophenoxy groups are introduced into the phthalocyanine periphery of bis(phthalocyaninato) rare earth semiconductors to continuously tune their HOMO and LUMO energies, resulting in the ambipolar M[Pc(SPh)8]2 [M = Eu (1), Ho (2)] and p-type M(Pc)[Pc(SPh)8] [M = Eu (3), Ho (4)]. An OFET in combination with direct I–V measurements over the devices from the self-assembled nanostructures of 1–4 revealed the original electron and hole densities (ne and nh) of 3.6 × 1015 and 3.6 × 1018 cm−3 for ambipolar 1, 9.8 × 1016 and 6.0 × 1020 cm−3 for ambipolar 2, and the original hole density (nh) of 2.8 × 1017 and 2.4 × 1017 cm−3 for 3 and 4, respectively. The comparative studies on the sensing behavior of the self-assembled nanostructures of 1–4 revealed that, towards reducing gas NH3, the ambipolar 1 and 2 show an n-type sensing behavior, with the response nature determined by the lower ne rather than higher nh. Meanwhile, the NH3 sensor from 1 with much lower ne than 2 displays higher sensitivity. Nevertheless, also towards NH3, 3 and 4 exhibit a p-type response, with the lower carrier density device 4 showing higher sensitivity. Consequently, the originally lower density carrier (hole vs. electron) with a faster charge transporting speed in the ambipolar semiconducting layer determines not only the gas sensing response nature but also the sensitivity. This is also true for the p-type organic semiconductor in terms of the gas sensing sensitivity.