Molecular semiconductor-doped insulator (MSDI) heterojunctions as new transducers for chemical sensors

Abstract

This article describes a new principle of transduction involving an heterojunction between a Molecular Semiconductor and a Doped Insulator (MSDI). Herein, we report on an MSDI-based sensor featuring an heterojunction between a lutetium bisphthalocyanine (LuPc2), which acts as Molecular Semiconductor (MS) and a thin film of Doped Insulator (DI) made of substituted or fluorinated copper phthalocyanine (CuFnPc, where n = 0, 8, 16). Previously, we reported the peculiar effect of the heterojunction on the MSDI's electronic behavior, suggesting this device as a new kind of transducer for gas chemosensing. Indeed, of particular significance was the key role of modulator played by the nature of the doped insulator sub-layer. While the MS thin film remains the only layer of the sensor exposed to gas atmosphere, the DI's ability to tune the electronic characteristics of the organic heterojunction allows it to drastically affect the nature of the effective charge carriers. In particular, an increase in fluorination of the doped insulator can cause an inversion of the LuPc 2 response toward electron accepting (ozone, ppb level) or donating (ammonia, ppm level) gases. The present work focuses on the structural, electronic and electrical properties of the MSDI heterojunction, which have been studied by UV-vis spectroscopy, atomic force microscopy, current-voltage measurements and chemical doping, in order to shed some light on this phenomenon. The unique ambipolar nature of LuPc 2 is suggested to be the main property responsible for the MSDI's unique behavior.

Tuning of organic heterojunction conductivity by the substituents’ electronic effects in phthalocyanines for ambipolar gas sensors

“…There is no additional band at higher wavelengths, indicating the absence of significant charge transfer between the two layers, as described previously for Cu(F 16 Pc)/LuPc 2 heterostructure [67]. The Raman spectra of the bilayer heterostructures and individual phthalocyanines in them coated on glass substrate were recorded between 500 and 1700 cm -1 .…”

Photon assisted-inversion of majority charge carriers in molecular semiconductor-based organic heterojunctions

“…The apparent energy barrier U th , deduced by extrapolating the tangent to the curve at high bias to abscissa is estimated as 6.6 V and 6. in agreement with the large difference in the energies of frontiers orbitals of LuPc 2 and Cu(F 8 Pc), which makes difficult for charges hopping from LuPc 2 orbitals to Cu(F 8 Pc) at the interface. 46 On the other hand, LUMO level of Cu(F 16 Pc) aligns well with the SOMO of LuPc 2 , facilitating easier charges hopping at the interface. Notably, in both heterojunctions, U th decreases upon light exposure, indicating that charge injection from electrode to interface becomes easier.…”

Ammonia and Humidity Sensing by Phthalocyanine–Corrole Complex Heterostructure Devices

“…The optical absorption spectra confirmed that after deposition, the devices contain both materials ( Table 1 ). Both heterostructures exhibit the same peak at 669 nm, which belongs to LuPc 2 and corresponds to its Q-band ( Figure 4 ) [ 36 ]. The bands at 605 and 604 nm correspond to the so-called “blue vibration band” of LuPc 2 .…”