Charge-transfer dynamics in van der Waals heterojunctions formed by thiophene-based semiconductor polymers and exfoliated franckeite investigated from resonantly core-excited electrons.

Organic/inorganic van der Waals heterojunctions formed by a combination of 2D materials with semiconductor polymer films enable the fabrication of new device architectures that are interesting for electronic and optoelectronic applications. Here, we investigated the charge-transfer dynamics at the interface between 2D layered franckeite (Fr) and two thiophene-based conjugated polymers (PFO-DBT and P3HT) from the resonantly core-excited electron. The unoccupied electronic states of PFO-DBT/Fr and P3HT/Fr heterojunctions were studied using near-edge X-ray absorption fine structure (NEXAFS) and resonant Auger (RAS) synchrotron-based spectroscopies. We found evidence of ultrafast (subfemtosecond charge-transfer times) interfacial electron delocalization pathways from specific electronic states. For the interface between the PFO-DBT polymer and exfoliated franckeite, the most efficient interfacial electron delocalization pathways were found through π*(S-N) and π*(S-C) electronic states corresponding to the benzothiadiazole and thiophene units. On the other hand, for the P3HT polymer, we found that electrons excited to π-π* and S1s-π*(C-C) electronic states of the P3HT polymer are the most affected by the presence of exfoliated franckeite and consequently are the main interfacial electron-transfer pathways in this heterojunction. Our results have important implications in understanding how ultrafast electron delocalization is taking place in organic/inorganic van der Waals heterojunctions, which is relevant information in designing new devices involving these systems.

Screening effect of CVD graphene on the surface free energy of substrates.

The wettability of graphene has been a topic under constant discussion in the literature since 2012. In this work we measured the contact angle (CA) of six different types of substrates (glass, quartz, Si3N4, Si/SiO2, sapphire and Si) with varying dielectric constants and surface roughnesses in order to calculate the surface free energy of graphene films to evaluate how the wetting properties of graphene-coated substrates are changed according to the underlying substrate. We used a residual-free transfer process to remove the high-quality graphene (CVD-Gr) grown onto copper foil. Afterwards, we performed an inert thermal treatment (Ar, at 300 °C for 30 minutes) to remove airborne contaminants from the graphene surface and evaluate the roughness of substrates by atomic force microscopy, the advancing and receding contact angles of two liquids (water and ethylene glycol), hysteresis, and surface free energy (polar and dispersive components) calculations. The presence of high-quality monolayer graphene (free of any air contaminants, polymer residues, etc.) led to a common wettability behaviour for all coated surfaces, regardless of the nature of the underlying substrate. This result can be understood in terms of the screening of van der Waals and dipole interactions by the electrons in graphene.

Species selective charge transfer dynamics in a P3HT/MoS2 van der Waals heterojunction: fluorescence lifetime microscopy and core hole clock spectroscopy approaches.

Hybrid van der Waals heterojunctions based on organic polymers and 2D materials have emerged as a promising solution for developing more efficient optoelectronic devices. Herein, we investigated the charge transfer (CT) dynamics at the interface of the poly[3-hexylthiophene-2,5-diyl] (P3HT) organic polymer and a MoS2 monolayer. A global picture of the charge transfer dynamics of a P3HT/MoS2/SiO2 heterojunction was elucidated from photoluminescence (PL) spectroscopy and the fluorescence lifetime decay profile. Rapid interfacial charge transfer between P3HT and MoS2 was indicated by strong PL quenching and a reduction in the average fluorescence lifetime (τav) of the P3HT/MoS2/SiO2 heterojunction. The role of specific electronic states in the interfacial CT process was investigated by applying the core hole clock approach. CT times (τCT) on femtosecond and sub-femtosecond timescales were estimated using the S1s core-hole lifetime as the internal clock. Sub-femtosecond CT was observed for electrons excited to S3pz (0.34 fs) electronic states of MoS2 and to π* (C-C) (0.45 fs) electronic states of P3HT in the P3HT/MoS2/SiO2 heterojunction. These fast bidirectional CT processes result from strong coupling between these two electronic states in the P3HT/MoS2/SiO2 heterostructure. However, the reduction of the τCT values in the heterojunction compared with those of the isolated films shows that interfacial CT from the P3HT species to MoS2 is more efficient. Interfacial CT was not observed for electrons excited to electronic states S3px,y (MoS2) and σ* (S-C) (P3HT). We conclude that the π* (C-C) electronic state of the P3HT species is the main pathway for interfacial ultrafast CT in a P3HT/MoS2/SiO2 heterojunction.