Engineering of TMDC-OSC hybrid interfaces: the thermodynamics of unitary and mixed acene monolayers on MoS2

Abstract

Hybrid systems of two-dimensional (2D) materials such as transition metal dichalcogenides (TMDCs) and organic semiconductors (OSCs) have become subject of great interest for future device architectures. Although OSC-TMDC hybrid systems have been used in first device demonstrations, the precise preparation of ultra-thin OSC films on TMDCs has not been addressed. Due to the weak van der Waals interaction between TMDCs and OSCs, this requires precise knowledge of the thermodynamics at hand. Here, we use temperature-programmed desorption (TPD) and Monte Carlo (MC) simulations of TPD traces to characterize the desorption kinetics of pentacene (PEN) and perfluoropentacene (PFP) on MoS₂ as a model system for OSCs on TMDCs. We show that the monolayers of PEN and PFP are thermally stabilized compared to their multilayers, which allows preparation of nominal monolayers by selective desorption of multilayers. This stabilization is, however, caused by entropy due to a high molecular mobility rather than an enhanced molecule-substrate bond. Consequently, the nominal monolayers are not densely packed films. Molecular mobility can be suppressed in mixed monolayers of PEN and PFP that, due to intermolecular attraction, form highly ordered films as shown by scanning tunneling microscopy. Although this reduces the entropic stabilization, the intermolecular attraction further stabilizes mixed films.

Fig. 1. Temperature-dependent C 1s-NEXAFS spectra (φ = 55°) of (a) PEN and (b) PFP films on MoS₂ (initial film thickness 0.5 nm; green spectra: multilayers on SiO₂ with nominal thickness of 30 nm recorded at 300 K). The dashed lines mark the C 1s absorption edges. Panels (c) and (d) show C 1s-NEXAFS dichroism measurements of the PEN and PFP films, respectively, after annealing to 410 K.

Fig. 2. Series of TPD traces for different film thicknesses of (a) PEN and (b) PFP on MoS₂. The nominal monolayers are marked with bold lines. Panels (c) and (d) depict monolayer traces of PEN and PFP, respectively, prepared with three different methods: (i) deposition of 3 Å (nominal monolayer) at 270 K, (ii) deposition of 8 Å at elevated temperature (PEN: 390 K; PFP: 400 K) and (iii) deposition of 8 Å at 270 K and subsequent annealing for 1 min (PEN: 390 K; PFP: 400 K), dashed lines indicate the desorbing multilayers upon annealing. Afterwards, the sample is cooled to 270 K and then heated again to record the TPD trace (solid line).

Fig. 3. (a) and (b) TPD traces of PEN and PFP, respectively, on MoS₂ for different initial (sub-) monolayer coverages Θ₀. (c) and (d) MC simulations of TPD traces for PEN, respectively, for the same initial coverages as in (a) and (b). The inset shows the intermolecular interaction energy V as a function of the coverage Θ for PEN (blue) and PFP (green) obtained from the MC TPD simulations (circles) with exponential fits (lines). (e) and (f) Coverage-dependent desorption energies E_d (black) and pre-exponential factors ν (red) of PEN and PFP, respectively, on MoS₂.

Fig. 4. (a) Quasi-simultaneously recorded TPD traces of 0.6 ML PEN and 0.8 ML PFP in a mixed film (filled traces) and unitary films of 0.4 ML PEN (blue line) and 0.7 ML PFP (green line) on MoS₂. (b) Simulated TPD traces of a randomized mixture of 0.5 ML PEN and 0.5 ML PFP (dots), 0.5 ML PEN (blue line) and 0.5 ML PFP (green line). (c) STM micrograph (−2.5 V, 250 pA, 110 K) of an intermixed monolayer of PEN and PFP on MoS₂ with a linescan. The inset illustrates the alternating molecular structure. The substrate azimuth was determined from low-energy electron diffraction. (d) Illustration of the intermixed monolayer. (e) STM micrograph (−3.76 V, 120 pA, 110 K) of 8 Å PEN on MoS₂ after annealing to 390 K for 1 min. (f) Illustration of a highly mobile (sub-) monolayer of PEN.

Fig. 5. Illustration of electrostatic intermolecular interactions of PEN in a multilayer film with a herringbone packing motif (a), in a (sub-) monolayer film of PEN (b) and a mixed (sub-) monolayer film of PEN and PFP (c), shown in a side view along the long molecular axis. The molecular quadrupole moments are indicated by negative (blue) and positive (orange) point charges.