Interaction between pentacene molecules and monolayer transition metal dichalcogenides

Using first-principles calculations based on density-functional theory, we investigated the adsorption of pentacene molecules on monolayer two-dimensional transition metal dichalcogenides (TMD). We considered the four most popular TMDs, namely, MoS$_2$, MoSe$_2$, WS$_2$ and WSe$_2$, and we examined the structural and electronic properties of pentacene/TMD systems. We discuss how monolayer pentacene interacts with the TMDs, and how this interaction affects the charge transfer and work function of the heterostructure. We also analyse the type of band alignment formed in the heterostructure and how it is affected by molecule-molecule and molecule-substrate interactions. Such analysis is valuable since pentacene/TMD heterostructures are considered to be promising for application in flexible, thin and lightweight photovoltaics and photodetectors.


Introduction
Monolayer two-dimensional transition metal dichalcogenides (TMD) such as MoS 2 , MoSe 2 , WS 2 and WSe 2 have emerged in the past decade as promising materials for a variety of applications, ranging from photovoltaics [1] and photodetectors [2] to gas sensors [3] and DNA sequencing [4].In bulk crystals, TMDs form layered structures where the layers are held together via van der Waals (vdW) forces.When exfoliated or synthesized in two-dimensional (2D) form, the monolayers have no dangling bonds, making it easy to combine them with other systems forming vdW heterostructures.
Due to the non-directional nature of vdW forces, 2D materials can be combined with materials of different dimensionalities such as quantum dots [5] and nanotubes [6], which may result in heterostructures with better properties and functionalities than the individual components.
Organic molecules are particularly interesting materials to combine with 2D systems.The large library of known molecules, which includes donors and acceptors as well as excellent absorbers and photo-and thermoresponsive molecules, offers a wide variety of systems that can be employed to enhance the properties and modify the functionalities of 2D materials; for instance, the adsorption of F 4 TCNQ and PTCDA molecules has been found to turn monolayer MoS 2 into a ptype semiconductor [7] and to enhance its photoluminescence intensity [8,9].Additionally, both 2D and organic materials are flexible, thin and lightweight systems, which makes organic/2D heterostructures especially attractive for wearable and portable applications.Pentacene (PEN) is one of the most popular organic materials, largely investigated for optoelectronic and photovoltaic applications due to its high carrier mobility [10], intense photoluminescence [11], excellent photosensitivity [12] and strong absorption in the visible range of the solar spectrum [13].).We considered one monolayer molecular coverage and we examined the structural and electronic properties of pentacene/TMD heterostructures.We examined the interaction between monolayer pentacene and TMD, and how this interaction affects the charge transfer, work function and band alignment of the pentacene/TMD heterostructures.
Projector augmented wave (PAW) [23] pseudopotentials [24]  This agrees with recent experimental [16] and theoretical [15] work reporting that pentacene molecules in the monolayer regime lie flat on the MoS 2 surface; in particular, the theoretical work based on DFT calculations for pentacene/MoS 2 revealed that vertical adsorption (with the long axis of pentacene lying parallel to MoS 2 ) is 0.6 eV higher in energy than the horizontal orientation.
Therefore, herein we will only focus on horizontal adsorption.
We    [15].We note that the distances for Se-systems (MoSe 2 and WSe 2 ) are 0.09 Å larger than those for S-systems, even though the adsorption energies are also larger for those systems (as will be discussed in the next paragraph).This difference, which has also been observed in other organic/TMD heterostructures [29], may be due to the larger vdW radius of Se atom (1.90 Å [30]) when compared to S atoms (1.73 Å [30]).We also observe that the geometries:  that the formation of hybrid pentacene/TMD is energetically favourable.As can be seen in Table 1, single-layer pentacene molecules are expected to favourably adsorb on all four TMDs, with adsorption energies between 1.38 and 1.46 eV for top-Ch adsorption sites, which is slightly smaller than the value (1.6 eV) calculated in Ref. [15] for pentacene/MoS 2 .We note that pentacene molecules bind more strongly with Sesystems when compared with the S-counterparts, and with W-systems when compared with Mosystems.Further analysis on the electronic properties and charge density of the PEN/TMD heterostructures will try to shine a light on the reason for that.molecule-molecule interaction.We also observe small values for the energy associated with the deformation of the molecules and the substrates.

From the results displayed in
In fact, we did not observe any bending of the molecule under the adsorption process and the C-C bond length in pentacene changes by less than 0.1% in all four heterostructures when compared with isolated pentacene.For the TMDs, we observed small contractions (< 0.1%) in the bond lengths between transition metal and chalcogen atoms, mostly in the region where the pentacene is adsorbed.
In addition to the structural properties, we also investigated the electronic properties of pentacene/TMD heterostructures, considering single-layer pentacene on top-Ch adsorption sites for all the four TMDs.This helps to explain why the interaction between pentacene and the Se-system is stronger than that of pentacene and Ssystems.
The lowest unoccupied molecular orbital (LUMO) of pentacene is located above the conduction band minimum (CBM) of MoS We have not considered the effect of spin-orbit coupling (SOC) in our calculations here.We do not expect significant changes in the adsorption energies and geometries, but for the electronic properties, SOC will cause splits in the topmost valence bands as well as in the lowest conduction bands of the TMDs, in particular for WS 2 and WSe 2 .We have computed the shift of the VBM and CBM of the TMDs (see Table S4 have examined five adsorption sites, as shown in Fig.1, based on the position of the central ring of the pentacene molecule: two bridge sites: bridge-A (Fig.1(a)) and bridge-B (Fig.1(b)), where the central ring of pentacene lies over a bond between the transition metal (Mo or W) and the chalcogen atoms (S or Se); hollow site (Fig.1(c)), where the central ring of pentacene is on top of a hexagon in the TMD cell; top-TM (Fig.1(d)), where the central ring of pentacene is on top of a transition metal (Mo or W) atom; and top-Ch (Fig.1(e)), where the central ring of pentacene is on top of chalcogen (S or Se) atom.For the bridge sites, we considered two configurations: bridge-A (Fig.1(a)) and bridge-B (Fig.1(b)), which resulted in the atoms of the molecule being located in different sites on the TMD.After geometry optimization (in which the internal coordinates of both moleculeand TMD were allowed to relax), top-Ch was found to be the most favourable adsorption site for all of the TMDs, followed very close by the bridge B configuration, which is less than 6 meV higher in energy (see TableS2 in the Supplementary Material).The reason why top-Ch and bridge-B adsorption sites are more favourable may be due to the fact that in such configurations there are more C atoms from pentacene sitting on hollow sites of the TMD and less C atoms sitting on top of S/Se sites, which reduces the steric repulsion between pentacene C atoms and TMD chalcogen atoms.The other adsorption sites are between 24 and 83 meV higher in energy than top-Ch configuration (see less favourable adsorption sites (namely, hollow, bridge-B, and top-TM) have larger adsorption distances (between 0.06 and 0.08 Åas listed in Table S3 of the Supplementary Material) than those found for top-Ch configuration; this is also a result of the steric repulsion between the C atoms of pentacene and the chalcogen atoms of the TMD, which causes larger adsorption distances and is expected to be more significant in hollow, bridge-A and top-TM configurations where several pentacene C atoms are located on top of the TMD chalcogen atoms.Adsorption energies (E ads ) of pentacene/TMD systems were calculated as the difference between the total energy of the combined system (E PEN/TMD ) and the total energies of the isolated systems (E relax TMD and E iso−relax PEN ) in their relaxed E relax TMD was computed considering the TMD systems in a 7 × 4 supercell with their geometries optimized in the absence of the pentacene molecules.E iso−relax PEN was obtained considering one single pentacene molecule in a cubic supercell with lateral dimension of 48 Å and allowing all the atomic positions to relax.This way of calculating the adsorption energy ensures that E ads includes contributions from molecule-molecule interactions, deformation of the TMD systems, deformation of the pentacene molecules, in addition to molecule-substrate interaction.A negative value for E ads indicates

Figure 1 :
Figure 1: Ball and stick representation of the adsorption sites examined for the adsorption of pentacene molecule (horizontally oriented) on 2D monolayer TMD: (a) bridge-A, (b) bridge-B, (c) hollow, (d) top-TM, and (e) top-Ch.The adsorption sites were named based on the position of the central ring of the pentacene molecule.

Figure 2 displays
the density of states (DOS) of pentacene/TMD heterostructures (considering the most favourable adsorption site, namely, top-Ch), clearly showing that the highest occupied molecular orbital (HOMO) of pentacene is located within the band gap of the 2D TMDs, closer to the valence band maximum (VBM) of the selenide systems, MoSe 2 (Fig. 2(b)) and WSe 2 (Fig. 2(d)) when compared to the sulfide systems, MoS 2 (Fig. 2(a)) and WS 2 (Fig. 2(c)).

CBM, facilitating charge
transfer between these systems and potentially restoring the type-II band alignment.Finally, we examine the charge transfer between pentacene molecules and the 2D TMD systems.Charge density difference between the heterostructure and isolated systems (Fig. 3), plotted on a plane cutting the long axis of the pentacene molecule and perpendicular to both molecule and TMD monolayer, shows characteristics of a Pauli repulsion pillow effect for all the systems: the overlap between the electronic clouds of the molecule and the TMD causes the charge to be pushed back into the TMD and around the edges of the molecules-

Figure 2 : 4 Conclusions
Figure 2: Total and partial density of states (DOS) of (a) PEN/MoS 2 , (b) PEN/MoSe 2 , (c) PEN/WS 2 and (d) PEN/WSe 2 heterostructures.The Fermi level of the heterostructure is indicated by a vertical dashed line.HOMO and LUMO of pentacene are highlighted in each plot.

Figure 3 :
Figure 3: Charge density difference between the heterostructures ((a) PEN/MoS 2 , (b) PEN/MoSe 2 , (c) PEN/WS 2 and (d) PEN/WSe 2 ) and the isolated systems plotted on a plane perpendicular to both the pentacene molecule and the 2D TMD monolayer, cutting through the long axis of the molecule.Regions in blue and red represent depletion and accumulation of charge, respectively.

Figure 4 :
Figure 4: Charge density difference integrated along the horizontal direction of the 2D plots shown in Fig. 3 plotted along the z direction.Horizontal dashed lines show the positions of the pentacene molecule, and the top/bottom chalcogen layer and the transition-metal layer of the TMD monolayer.

Table S2
[16]he Supplementary Material).This small difference in energy among the different adsorption sites indicate that the molecules may be highly mobile in the single layer regime, as has also been suggested in Ref.[16].The pentacene molecule was found to lie flat in all four TMDs,

Table 1 :
Adsorption energies (E ads ) and adsorption distances (d) of pentacene/TMD heterostructures (with pentacene adsorbed on top-Ch configuration) obtained with (PBE+vdW) and without (PBE only) including vdW correction methods.E ads was computed using in Eq. 1. d was obtained as the distance between the centre of masses of the pentacene molecule and the top

Table 2 :
Contributions to the adsorption energy from molecule-molecule interaction, moleculesubstrate interaction, deformation of the molecule and deformation of the substrate.Energies are given in eV.PEN/MoS 2 PEN-MoSe 2 PEN-WS 2 PEN-WSe 2 2 , MoSe 2 and WS 2 , indicating that PEN/MoS 2 , PEN/MoSe 2 and PEN/WS 2 form staggered type-II heterostructures; however, pentacene's LUMO has lower energy than the CBM of