Diverse sensor responses from two functionalized tris(phthalocyaninato)europium ambipolar semiconductors towards three oxidative and reductive gases

Abstract

Ambipolar tris(phthalocyaninato) rare earth semiconductors, Eu₂[Pc(OC₈H₁₇)₈]₂[Pc(OCH₂CF₃)₈] (1) and Eu₂{Pc[(OC₂H₄)₃OCH₃]₈}₂[Pc(OCH₂CF₃)₈] (2), are synthesized and fabricated into well-organized films via a Langmuir-Shäfer (LS) method. Driven by intermolecular π–π stacking, as well as hydrophilic–hydrophobic interactions, 1 and 2 aggregated in different modes, H for 1 and J for 2, in LS films. Consequently, higher conductivity and carrier mobilities for electrons and holes in the LS films of 1 are achieved compared to those of 2, with gate-induced sheet electron and hole densities (n_e and n_h) of 2.48 × 10¹² and 8.31 × 10⁹ cm⁻² for 1, and 1.77 × 10¹² and 8.01 × 10¹⁰ cm⁻² for 2, respectively. Comparative studies on the gas sensing behavior of the LS films of 1 and 2 revealed that, towards oxidative NO₂, both 1 and 2 show n-type sensing behavior, with a higher sensitivity of 2 (54.6% ppm⁻¹) than 1 (12.5% ppm⁻¹) in the 0.10–0.85 ppm range, which can be attributed to the lower n_e of 2. Unexpectedly, towards reductive NH₃ (3.0–7.0 ppm) and H₂S (5.0–9.0 ppm), only ambipolar 1 exhibits current responses, determined by the lower n_h of 1. More importantly, the current of 1 responds reversely towards NH₃ and H₂S (decreases vs. increases) even though they are both reductive gases. This is because weakly reductive NH₃ reduces n_h while strongly reductive H₂S increases n_e. By combining the distinct response patterns of ambipolar 1 and 2, these three toxic gases can be distinguished unambiguously from each other, showing the potential for these two molecules in efficient chemiresistor sensor arrays.