Two-dimensional (2D) transition metal dichalcogenides (TMDs) recently emerged as novel materials displaying a wide variety of physico-chemical properties that render them unique scaffolds for high-performance (opto)electronics. The controlled physisorption of molecules on the TMD surface is a viable approach to tune their optical and electronic properties. Solvents, made of small aromatic molecules, are frequently employed for the cleaning of the 2D materials or as “dispersant” for their chemical functionalization with larger (macro)molecules, without considering their potential key effect in locally modifying the characteristics of 2D materials. In this work, we demonstrate how the electronic and optical properties of mechanically exfoliated monolayer of MoS2 and WSe2 are modified when physically interacting with small aromatic molecules of common solvents. Low-temperature photoluminescence (PL) spectra recorded at 78 K revealed that physisorbed benzene derivatives could modulate the charge carrier density in monolayer TMDs, hence affecting the switching between neutral exciton and trion (charged exciton). By combining experimental evidences with DFT calculations, we confirm that charge transfer doping on TMDs depends not only on difference in chemical potential between molecules and 2D materials, but also on the thermodynamic stability of physisorption. Our results provide unambiguous evidence of the great potential of optical and electrical tuning of monolayer MoS2 and WSe2 by physisorption of small aromatic solvent molecules, which is highly relevant both for fundamental studies and more device applied purposes.

The rapid increase in charge carrier mobility in organic field-effect transistors (OFETs) in the past few years, with a number of reports >10 cm² V⁻¹ s⁻¹, calls for a simultaneous improvement in charge injection at the electrode–semiconductor interface. Chemical modification of the electrodes with self-assembled monolayers (SAMs) allows the optimization of three key properties for lowering the contact resistance, thus fine-tuning the charge injection into OFET channels: the electrode work function, the surface energy of the modified electrodes and tunnelling resistance of the SAM. Understanding of the interplay of these properties is of vital importance for organic device design. In this paper, we report a model study based on the modulation of all three of these properties via chemisorption of fluorinated mono- or biphenylthiol molecules (PFBT and PF2BT, respectively) onto gold electrodes. Density functional theory simulations confirm the higher work function of the PFBT monolayers compared to PF2BT and provide evidence that this work function difference is entirely due to differences in the bond dipole to the gold surface. This observation is of importance for the development of future SAM molecules both for organic electronics and across the field of surface chemistry. Incorporation of these SAM-modified Au surfaces as the source and drain electrodes of an OFET with prototypical polymer semiconductors exhibiting different transport levels makes it possible to unravel the role of energetic alignment as well as surface energy and tunnelling resistance on the device performance. Interestingly, our results show that it is not always the high work function PFBT-modified electrodes that give the lowest contact resistance.

Responsive monolayers are key building blocks for future applications in organic and molecular electronics in particular because they hold potential for tuning the physico-chemical properties of interfaces, including their energetics. Here we study a photochromic SAM based on a conjugated azobenzene derivative and its influence on the gold work function (Φ_Au) when chemisorbed on its surface. In particular we show that the Φ_Au can be modulated with external stimuli by controlling the azobenzenetrans/cisisomerization process. This phenomenon is characterized experimentally by four different techniques, kelvin probe, kelvin probe force microscopy, electroabsorption spectroscopy and ultraviolet photoelectron spectroscopy. The use of different techniques implies exposing the SAM to different measurement conditions and different preparation methods, which, remarkably, do not alter the observed work function change (Φ_trans–Φ_cis). Theoretical calculations provided a complementary insight crucial to attain a deeper knowledge on the origin of the work function photo-modulation.