Azobenzene Adsorption on the MoS2 (0001) Surface: A Density

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Azobenzene Adsorption on the MoS2 (0001) Surface: A Density Functional Investigation within van der Waals Corrections Luís Cabral, Fernando Pereira Sabino, Matheus Paes Lima, Gilmar Eugenio Marques, Victor Lopez-Richard, and Juarez L. F. Da Silva J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 24 Jul 2018 Downloaded from http://pubs.acs.org on July 25, 2018

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The Journal of Physical Chemistry

Azobenzene Adsorption on the MoS2 (0001) Surface: A Density Functional Investigation within van der Waals Corrections L. Cabral,† Fernando P. Sabino,‡ Matheus P. Lima,† Gilmar E. Marques,† Victor Lopez-Richard,† and Juarez L. F. Da Silva∗,¶ †Department of Physics, Federal University of São Carlos, 13565 − 905, São Carlos, SP, Brazil ‡São Carlos Institute of Physics, University of São Paulo, 13560 − 970, São Carlos, SP, Brazil ¶São Carlos Institute of Chemistry, University of São Paulo, PO Box 780, 13560 − 970, São Carlos, SP, Brazil E-mail: juarez [email protected]

Phone: +55 16 3373 9930

Abstract The interest in light sensitive organic molecules, such as azobenzene, has increased due to their ability to functionalize two-dimensional layered systems and to engineer their electronic structure. In this work, we explore the azobenzene trans- and cis-isomers adsorption on molybdenum disulfide MoS2 (0001) layer employing the density functional theory (DFT) within van der Waals corrections to the semilocal exchange-correlation functional. We found that the aromatic rings of the azobenzene lay parallel to the surface (2 in the trans-isomer and 1 in the cis-isomer), which contributes to increase the configuration stability by van der Waals interactions. Furthermore, we found a relatively large work function change upon adsorption due to the electron density rearrangement, and hence, it might affect the electronic transport properties within the single MoS2 (0001) layer. We observed an increasing in the relative DFT+vdW total energy among the azobenzene isomers (trans- and cis-) from 0.51 eV in gas-phase (trans-isomer has the lowest energy) to 0.81 eV for azobenzene supported on the MoS2 (0001) surface, which can be explained by the contact of the two rings of the trans-isomer directly to the surface. Thus, the binding of azobenzene on the single MoS2 (0001) layer affects the isomerization process due to the relative energy increase, and this, in turn, influences the transport properties of the single MoS2 (0001) layer due to the changes in the electrostatic potential.

I

Introduction

The emergence of ultra thin semiconductor systems, such as transition-metal dichalcogenides (TMDs), have paved the way for electronic two-dimensional (2D) architectures of new device concepts, and sensors. 1,2 The 2D-TMD materials have been intensively investigated due to the wide range of possible applications, 3,4 which can be extended by the combination of different 2D-TMDs in van der Waals hetero-structures. 5,6 For

example, the control of the number of layers linked by van der Waals interactions 7 and the incidence of electromagnetic fields allows to manipulate their electronic properties. 8 The 2D-TMDs composed by MQ2 layers, where M is a transition metal atom, such as Mo, and Q is a chalcogen atom (S, Se, Te), have attracted great interest. 9 In particular, molybdenum disulfide, MoS2 , 10 has been used in several applications, such as photovoltaics, 11 lubricants, 12 and etc. These systems are flexible, optically active, and their properties can be tuned through composition variation, or stacking, but also by the character of their substrates 13 or the surrounding chemical environment. 14 In that respect, progress has been attained by combining diverse atomic layers and organic interfaces that allows foreseen the functionalisation of 2D devices and their integration at nanoscales. A wide variety of these organicinorganic hybrids has been studied and the reported results have evidenced not just how the film morphologies can be altered, 15 but also their quantum efficiency, 16 and their transport and thermal properties. 17 The integration of organic molecules and two-dimensional layers of TMDs, such as MoS2 , has made these structures attractive for new hybrid solar cells proposals 18 or phototransistor architectures. 19 In the case of photosensitive switches, the use of photochromic azobenzene molecules as a bulding block in these hybrid architectures is very promising. 19 The photoisomerization of azobenzene has already been successfully used for the modulation of biological processes. 20,21 The reversibility of the cis-trans isomerization in the azobenzene groups has also allowed promoting strategies for wavelength-selective control of a wide variety of chemical, 22 mechanical 23 , and electrical properties 24 of their host systems. Azobenzene is an organic molecule formed by the functional group azo, i.e., – N – N – , 25 and two aromatic benzene rings, as represented in Figure 1. Two isomers have been reported so far for azobenzene, namely, the trans-azobenzene with planar structure, and a second isomer, known as cis-, where the two benzene rings are not in the

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same plan. 21 The transition between the isomers was reported by Hartley, 26 noting that the process cis-trans can occur in the dark, while the inverse isomerization, i.e., trans-cis, is only feasible under the ultraviolet light incidence. 27 As azobenzene directly photoisomerizes in gas-phase, 26,28 this transformation can be quenched when the molecule is placed on metal surfaces, 29,30 which boosted investigations exploring the reversible azobenzene photo-switch. 31 Previous ab initio investigations characterized the potential energy curves of azobenzene isomers and their intermediate configurations. 32 Comstock et. al. studied the influence of the Au(111) surface in the azobenzene photo-switching. 33 The optical properties of the MoS2 mono- and multi-layers exfoliated on Au surfaces functionalized with the azobenzene molecule was also reported. 34 In turn, the magnetic effect of graphene doped by Co in the trans- cis-azobenzene reversibly was investigated by Nurbawono and Zhang, 35 while the influence of graphene in the optical excitation of azobenzene was analyzed by Fu et. al.. 36 In this work, we report the characterization of the adsorption properties of azobenzene conformations on the molybdenum disulfide MoS2 (0001) surface. Our calculations are based on the density functional theory (DFT) within the van der Waals (vdW) correction to improve the description of the weak molecule-surface interactions. We found that the trans-isomer in gas-phase and the conformations located horizontally on the substrate are the lowest energy configurations. Adsorption energies and work functions are also analyzed. In addition, we were able to determine the mechanism of the charge flow between the molecule and the surface within the effective Bader charge concept.

II Theoretical Approach Computational Details

and

A Total Energy Calculations Our total energy calculations are based on DFT 37,38 within the semilocal Perdew–Burke–Ernzerhof 39 (PBE) formulation for the exchange-correlation energy functional. To improve the description of the weak vdW azobenzene-MoS2 (0001) interactions, we employed the vdW D3 correction proposed by S. Grimme, namely, the DFT-D3 framework. 40 For comparison, we performed also DFT-PBE calculations without the vdW correction. In the DFT-D3 framework, as implemented in the Vienna ab initio simulation package DFT+vdW , (VASP), 41,42 the vdW DFT corrected total energy, Etot is obtained by the sum of the vdW energy D3 correction, D3 , and the DFT-PBE total energy, E DFT-PBE , namely, Edisp tot DFT+vdW DFT-PBE D3 Etot = Etot + Edisp ,

(1)

where ij

D3 Edisp =−

ij M M C C 1 XXX fd,6 66 + fd,8 88 . 2 i=1 j=1 L Ri j,L R i j,L

(2)

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The sums are over all atoms M and all translations of the unit cell L = (l1 , l2 , l3 ), while the dispersion coefficients, ij ij namely, C6 and C8 , play a crucial role in the magnitude of the vdW interactions. The damping functions, fd,6 and fd,8 , are employed to avoid singularities for small inter-atomic distances, fd,n =

sn ij

1 + 6(Ri j /( sR,n R0 ))−αn

,

(3)

where the sR,n parameters are the scaling factors of the cutoff ij ij ij radii, and R0 = (C8 /C6 )1/2 . The αn parameters are fixed and depends on the selected exchange-correlation functional. Further details on the DFT+vdW framework can be found elsewhere. 40–43 The Kohn–Sham (KS) equations were solved using the projected augmented-wave (PAW) method 44,45 as implemented in VASP, 46,47 version 5.4.1. We employed the PAW projectors provided within VASP, namely, Mo (5s1 4d5 ), S (3s2 3p4 ), C (2s2 2p2 ), N (2s2 2p3 ) and H (1s1 ), where the valence electrons are indicated in parentheses. The equilibrium volume of the bulk MoS2 in the hexagonal P63 /mmc crystal structure was obtained by minimization of the stress tensor and atomic forces using a plane-wave cutoff energy of 689 eV, while for the total energy calculations for all systems (geometric optimizations, density of states, etc), we employed a cutoff energy of 473 eV, which is 12.5 % higher than the largest recommended PAW cutoff energy for the mentioned chemical species. The differences among the cutoff energies is due to the slow (faster) convergence of the stress tensor (total energy) with the number of plane-waves. For the Brillouin zone integration, we employed a kmesh of 8 × 8 × 2 for the bulk MoS2 in the primitive unit cell, while a k-mesh of 8 × 8 × 1 was used for a single MoS2 (0001) monolayer using a 1 × 1 surface unit cell and a vacuum thickness of about 15 Å. For the density of states, we increased the k-point mesh twice for both systems. For the azobenzene/MoS2 (0001) calculations, we employed the repeated slab geometry using a hexagonal 8 × 8 unit cell, one single MoS2 (0001) layer, and 25 Å for the vacuum region due to the large size of the azobenzene molecule, e.g., the trans-azobenzene has a length of 11.26 Å. For convergence tests, we employed also a 4 × 4 and 6 × 6 surface unit cells, and three MoS2 (0001) layers for the slab. For those calculations, we employed only the Γ-point for the Brillouin zone integration due to the larger size of the surface unit cell. For azobenzene in gas-phase, we employed an orthorhombic 3 box with 24 × 25 × 26 Å , which yields a minimum distance between the azobenzene molecules of about 15 Å, and hence, the Γ-point was also used due to the lack of dispersion in the electronic states. We employed a Gaussian smearing of 0.010 eV for all calculations, except for azobenzene in gasphase, where we used 0.0010 eV, which is required to avoid fractional occupation of the electronic states. For the azobenzene/MoS2 (0001) calculations, we considered the adsorption on only one side of the layer, and hence, dipole corrections were employed for all calculations. 48,49 All structures reached the equilibrium geometry once the atomic forces on every atom are smaller


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Figure 1 Optimized atomic configurations for the trans- and cis-azobenzene in gas-phase (left side). The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are indicated for both configurations (right side). −1

than 0.010 eV Å . For the analysis of the electron density, we employed the Bader charge analysis, where the grid-points were increased by a factor of three for the fast Fourier transform (FFT), which plays a crucial role to obtain well converged effective Bader charges on each atom.

B Azobenzene/MoS2 (0001) Azobenzene is composed by two benzene rings linked by the – Cb – N – N – Cb – chain, where the Cb atoms belong to the benzene rings and the double – N – N – bonds play a crucial role in the azobenzene configurations. 25,50 Azobenzene adopts two configurations, namely, a ground state planar structure, in which the two benzene rings and the – Cb – N – N – Cb – chain are in the same plane (trans-azobenzene). However, under ultraviolet light, 28 mechanical stress, or even electrostatic stimulation, 51 the trans- configuration changes to the cis-azobenzene, where the two benzene rings change their orientation with respect to each other due to the change of the – Cb – N – N – Cb – torsion angle. 50 The reverse isomerization, cis- to trans-, can occur spontaneously under dark conditions. 21 With the aim to understand the role of the azobenene-MoS2 (0001) interaction in the relative stability of both azobenzene isomers, which can play a crucial role in the electronic switch properties, we considered both configurations for our adsorption studies and several adsorption sites, which are shown in the Supporting Information.

III

Results

A Azobenzene in Gas-Phase According to our calculations, we found that the trans-isomer is 0.51 eV lower in energy than the cis-isomer, which is in excellent agreement with experimental results obtained under dark conditions, i.e., 0.52 eV. 28 The optimized gas-phase

structures for the trans- and cis-azobenzene are depicted in Figure 1, where the structural differences can be easily seen, in particular, the dihedral angle for – Cb – N – N – Cb – are 180◦ (trans-) and 10.8◦ (cis-), which are in good agreement with experimental results. 28 From the KS-orbital analysis, we found that the highest occupied molecular orbital (HOMO) is mainly localized on the – Cb – N – N – Cb – fragment in the trans- configuration, while it extends to the benzene rings in the cis-azobenzene, and hence, it indicates a clear difference among both isomers. In contrast, no substancial differences was found in the lowest unoccupied molecular orbital (LUMO) between both molecules. To improve our understanding of the dihedral angle, we Bader , calculated the effective Bader charge, QB e f f = Zval − Q where Zval is the number of valence electrons of the respective chemical species. As expected, the nitrogen atoms have an effective negative charge, which can be explained by the higher electronegativity of N (3.04) compared with C (2.55) atoms. Furthermore, the magnitude of the effective charge on the N atoms is larger for the trans-isomer, i.e., −0.28 e (trans) and −0.23 e (cis-). This is due to the enhanced Coulomb repulsion between the N atoms, and hence, we expect a larger bond length between the N – N atoms in the trans-azobenzene, which is in fact confirmed by our results, e.g., N – N = 1.26 Å (trans-) and 1.25 Å (cis-). Although the difference is small, it correlates with the nature of the interactions in both isomers.

B Bulk and Layered MoS2 Systems The equilibrium lattice constants, a0 and c0 , for the bulk MoS2 in the hexagonal P63 /mmc structure are 3.16 Å and 12.35 Å, respectively, which are consistent with the experimental results, a0 = 3.16 Å and c0 = 12.29 Å, 52 i.e., corresponding to deviations smaller than 0.49 %. For comparison, DFT-PBE yields c0 = 13.45 Å, i.e., a relative error of 9.39 %, and hence, the vdW D3 correction plays a crucial role to reduce the error from 9.39 % to 0.49 %. From the Bader charge analysis, we obtained an effective cationic



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Figure 2 Band structure and local density of states for MoS2 in the bulk phase (left panel) and for the single MoS2 (0001) layer (right panel) calculated with the DFT-PBE+D3 framework. The Brillouin zone is depicted on the top of band structure with the k-path showed in green lines.

charge on the Mo atoms of 0.74 e, and hence, an anionic charge of −0.37 e on the S atoms, which changes to −0.36 e in the single MoS2 (0001) layer using the same a0 parameter as in the bulk structure, i.e., a tiny change of 0.01 e due to the vdW interactions between the MoS2 layers. The band structure and density of states (DOS) for the MoS2 bulk and single MoS(0001) layer are shown in 2, where the valence band maximum (VBM) is located at the Γ-point for MoS2 bulk, while the conduction band minimum (CBM) is located between the Γ- and K-points, i.e., an indirect band gap of 1.07 eV, which is 17.05 % (1.29 eV) smaller than the experimental result, 53 which is a well known problem in DFTPBE, which cannot be corrected by the addition of the vdW correction as the D3 framework does not affect the electron density directly, but only indirectly due to the changes in the geometric parameters. From the local DOS, the valence band is composed mainly by the Mo and S states and the VBM, in particular, is mostly derived from Mo states. Compared with MoS2 bulk, the single layer DOS is shifted up in energy, which is related to the confinement of the electronic states within the layer.

C Azobenzene Adsorption on the MoS2 (0001) We analyzed the cis- and trans-azobenzene configurations adsorbed on several positions on the MoS2 (0001) surface, namely, perpendicular and planar orientations in different positions on the surface. All calculated configurations are shown in the Supporting Information, while the lowest energy configurations for both isomers on the MoS2 (0001) layer (top and side views) are depicted in Figure 3. Below, we will discuss the structural, energetics, and electronic properties for the lowest energy configurations, while the same parameters are summarized in the Supporting Information for the high energy configurations.

Structural Configurations: In the lowest energy configurations shown in Figure 3, the benzene rings are parallel to the MoS2 (0001) surface, which is expected to enhance the vdW and Coulomb interactions. In particular, the planar trans- structure contains two benzene rings horizontally placed on the surface, while only one benzene ring is parallel to the surface in the cis-isomer. As mentioned above, the binding energy among the N, C, and H atoms is given by the strong covalent and ionic chemical interactions, and hence, expressively larger than the van der Waals interaction with the MoS2 (0001) surface. Thus, we do not expect considerable changes in the conformation of the azobenzene isomers, which are supported by our DFT-D3 calculations. Both molecules (trans- and cis-isomers) and monolayer MoS2 (0001) structures are composed by hexagonal rings, however, there is no alignment of these hexagonal rings upon the adsorption, i.e., both rings are horizontally displaced from one another. We found a distance of 3.26 Å (3.23 Å) between the hexagonal rings of the cis-isomer (trans-) and the MoS2 (0001) surface, which increases to 3.63 Å (3.79 Å) using DFT-PBE. In addition, as the conformation of the molecule on the monolayer changes, the charge redistribution in the geometry also varies, as depicted at the bottom of Figure 3. However, the effective Bader charge of the lowest energy configurations, depicted at the upper part of Figure 3, is very similar to the cis- and trans- isomers in gas-phase, represented in the left part of Figure 1, indicating the weak magnitude of the vdW forces. Adsorption energy: The adsorption energy, Ead , which measures the magnitude of the azobenzene-substrate interaction, was calculated using the following equation, AZO/MoS2 (0001)

Ead = Etot


MoS2 (0001)

AZO − Etot − Etot

,

(4)

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Figure 3 Charge density analysis. (a) Top and side views of the lowest energy configurations for the cis- and trans-azobenzene adsorbed on the MoS2 (0001) surface. The effective Bader charges are indicated in the top view, while the vertical distance between the surface and the closest atom is depicted in the side view. (b) The electron density differences induced by the azobenzene adsorption are indicated using a three-dimensional representation. (c) Average electron density difference along the direction perpendicular to the surface. The position of the cis- (red) and the trans- (blue) isomers (closest atom to the surface), as well as the Mo-layer atoms (green), are indicated by the dashed lines. AZO/MoS (0001)

2 where Etot is the total energy of the AZO and azobenzene/MoS2 (0001) configuration, while Etot

MoS (0001)

Etot 2 are the total energies of the azobenzene and single MoS2 (0001) monolayer, respectively. For both azobenzene conformations, we employed the total energy AZO . Thus, the of the respective isomers to calculate Etot adsorption energy focuses only on the interaction energy, neglecting the energy differences due to the geometry variation of the isomers. We found Ead = −0.98 eV (−0.12 eV using PBE) and −0.68 eV (−0.13 eV using PBE) for the trans- and cisisomers, respectively. Thus, trans- binds 0.30 eV stronger than cis-azobenzene on the MoS2 (0001) surface using the DFT-D3, which is expected due to the larger contact of the trans-isomer with the MoS2 (0001) surface, i.e., the vdW interaction and Coulomb forces are stronger in the trans- than in cis-configuration on the layer. The azobenzene rings are in direct contact with the surface and the interaction increases for the two parallel benzene rings of the trans- in comparison with the one horizontal ring of the cis-geometry. Therefore, the vdW D3 correction plays a crucial role in the magnitude of the adsorption energy and in the lowest energy configuration. The perpendicular orientation of the trans-isomer, where

the molecule interacts with a single H atom, increases the DFT-D3 total energy by 0.78 eV, indicating that the parallel orientation of the benzene rings is preferable. On the MoS2 (0001) surface, the trans-isomer is 0.81 eV lower in energy than the cis-azobenzene, whereas this difference is 0.51 eV in the gas-phase. Thus, the interaction with the surface enhances the energy difference between the transand cis-isomers, however, we should keep in mind that those relative total energies are not the energy barriers for the transcis switch. The isomerization switching under light irradiation depends on the azobenzene electronic states, which can be affected by the interaction with the surface. As mentioned above, we found an energy difference between the trans- and cis-isomers supported on the MoS2 (0001) surface of 0.81 eV, which is near to the adsorption energy calculated above, i.e., −0.98 eV, however, it does not indicate light-induced molecular desorption along the trans-cis switch, which is supported by experimental observations, e.g., azobenzene on Au(111) surface. 33 Work Function: The work function, Φ, is the energy required to remove one electron from the system, given


The Journal of Physical Chemistry by Φ = Ves − EF , where Ves and EF are the electrostatic potential at infinite (middle of the vacuum region) and the Fermi energy, respectively. For the clean MoS2 (0001) surface, we obtained a work function of 5.88 eV, which is reduced upon the adsorption of the azobenzene molecules. In the lowest energy configurations, we obtained work functions of 5.41 eV (∆Φ = 0.47 eV) and 5.24 eV (∆Φ = 0.64 eV) for the trans- and cis-isomers on the surface, respectively. Those results were obtained using the 8 × 8 surface unit cell, while smaller surface unit cells, e.g., 4 × 4 and 6 × 6, yields work functions than differ by less than 0.05 eV, i.e., orders of magnitude smaller than the work function changes. Thus, there is a clear difference in the magnitude of the work function changes due to the different orientation of the trans- and cis-isomers, and hence, the obtained work function reduction can be explained as follows: (i) there is an electron density rearrangement, and as a result, there is an effective electron density flow towards to the MoS2 (0001) surface, which is supported by our analysis indicated in Figure 3 for both isomers; (ii) the difference between the trans- and cisisomers work functions of 0.17 eV is due to the orientation of the cis-isomer, in which one of the benzene rings is nearly perpendicular to the surface. Below, we will discuss our charge transfer analysis, which can helps to clarify these trends. Charge Transfer Analyses: To obtain a better understanding of the substrate work function change upon the adsorption of azobenzene, we performed a charge density analyses using the Bader charge concept, where we Bader . calculated the effective Bader charge, QB e f f = Zval − Q QBader

Zval is the number of valence electrons and is the Bader charge, respectively. Furthermore, we calculated the electron density difference, ∆ρ = ρmol/sub − ρ sub − ρmol , and the averaged xy-density along the z-direction. Theses analyses were performed for the lowest energy configurations and the results are shown in Figure 3. The effective Bader charges on the chemical species in the trans- and cis-isomers are nearly the same as in the azobenzene in gas-phase, which indicates that the effective charges are not affected by the weak van der Waals interactions. Although the average charges are nearly the same for both isomers, there are differences in the local charges, which can be seen from the electron density difference analyses, in particular, the three-dimensional plot and the averaged xy-density along the z-direction, shown in Figure 3, which indicates larger local effects derived from the trans-isomer. It can be explained by the closer approach to the surface, which enhances the interactions. Thus, the larger work function change for the cis-isomer can be explained by the orientation of the second benzene ring, which is nearly perpendicular to the surface and not by the electron density changes. Density of States: To improve our understanding of the electronic levels of the azobenzene molecule on the MoS2 (0001) surface, we calculated the local density of states (LDOS) for the trans- and cis-isomers on the single

2.00 1.50 1.00 0.50 0.00 1.50

Mo

S

1.00 Local Density of States (State/eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.50 0.00 1.50 H

1.00

trans cis

0.50 0.00 1.50 C

1.00 0.50 0.00 1.50

N

1.00 0.50 0.00 -5

-4

-3

-2

-1 0 1 Energy (eV)

2

3

4

5

Figure 4 Local density of states of the trans- and cis-azobenzene isomers on the top of a single MoS2 (0001) layer. The electronic states of the semiconductor are smoothly displaced from the Fermi energy, where the azobenzene levels enter inside the gap energy of the MoS2 (0001).

MoS2 (0001) layer, depicted in Figure 4. In the LDOS of the trans- and cis-configurations on the MoS2 (0001) layer, the HOMO states of the single MoS2 (0001) layer is displaced from the Fermi energy by 0.5 eV, which is mostly composed by Mo d- and S p-states, while the LUMO is composed by Mo d- and S p-states. The states of the azobenzene are displaced from each other, where the highest contribution is given by the trans-isomer on the surface. Thus, as the states of the semiconductor are smoothly displaced from the Fermi energy, the electronic levels of the azobenzene enters inside the gap region, changing the electronic properties of the carriers near the Fermi energy. This is an interesting fact to study in the modulation of the electronic transport and its switching under light incidence. As depicted in Figure 4, the calculated band gap of the single MoS2 (0001) layer is 1.42 eV. A ruling factor in the manipulation of the band gap energy is the effect of the confinement profile through different number of stacking layers. Thus, increasing the number of layers, the electronic properties tend to the MoS2 bulk phase. It is important to note that, although the lowest energy configuration shows the higher work function and an adsorption site of preference, the electronic properties of the MoS2 (0001) are smoothly disturbed by the interaction with the azobenzene isomers. The molecule on the MoS2 (0001) does not practically change the electronic properties of the semiconductor.


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IV

Summary

In this work, we performed a DFT-PBE+D3 investigation of the adsorption properties of the azobenzene isomers on the MoS2 (0001) surface. Furthermore, for comparison, DFTPBE calculations were also performed. The aromatic rings of azobenzene lay parallel to the surface, e.g., two rings for transand only one for the cis-isomer. The relative total energy among the trans- and cis-isomers in gas-phase, 0.51 eV, increases to 0.81 eV upon the adsorption on the MoS2 (0001) surface. Thus, the adsorption on the 2D substrate might affect the conversion trans-cis process. The electronic properties of the semiconductor layer are locally disturbed by the presence of the azobenzene molecules, which introduce changes in the substrate work function and also in the behavior of the confined carriers of the semiconductor. Therefore, we expect that the prospects for introducing photo-responsive units in electronics and optoelectronics make this study useful.

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V

Supporting Information

The following additional data are shown in the Supporting Information: all calculated configurations for azobenzene supported on the MoS2 (0001) surface; geometric and energetic parameters; convergence tests using the different surface unit cells.

VI

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Acknowledgements

The authors thanks the Brazilian Agencies CAPES (Coordenaça˜ o de Aperfeiçoamento de Pessoal de N´ıvel Superior), CNPq (Conselho Nacional de Desenvolvimento Cient´ıfico e Tecnológico) and FAPESP (Fundaça˜ o de Amparo a` Pesquisa do Estado de São Paulo), grants 2014/19142 − 2, 2014/02112 − 3, for the financial support.

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Azobenzene Adsorption on the MoS2 (0001) Surface: A Density

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