Modulating Carrier Density and Transport Properties of MoS2 by

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Modulating Carrier Density and Transport Properties of MoS2 by Organic Molecular Doping and Defect Engineering Yongqing Cai, Hangbo Zhou, Gang Zhang, and Yong-Wei Zhang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b03539 • Publication Date (Web): 16 Nov 2016 Downloaded from http://pubs.acs.org on November 18, 2016

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Chemistry of Materials

Modulating Carrier Density and Transport Properties of MoS2 by Organic Molecular Doping and Defect Engineering Yongqing Cai, Hangbo Zhou, Gang Zhang* and Yong-Wei Zhang† Institute of High Performance Computing, A*STAR, Singapore 138632

Abstract Using first-principles calculations, we investigate the effect of molecular doping and sulfur vacancy on the electronic properties and charge modulation of monolayer MoS2. It is found that tetrathiafulvalene (TTF) and dimethyl-p-phenylene diamine (DMPD) molecules

are

effective

donors,

while

tetracyanoethylene

(TCNE)

and

tetracyanoquinodimethane (TCNQ) are effective acceptors, and all these molecules are able to shift the work function of MoS2. For MoS2 containing sulfur vacancies, these molecules are able to change the position of the defect levels within the band gap and modulate the carrier density around the defect center. Charge transfer analysis shows that TCNE and TCNQ induce a free-carrier depletion of the defect states, which is beneficial for the suppression of the non-radiative trionic decay and a higher excitonic efficiency due to a decrease in the screening of excitons. Furthermore, the effects of molecular adsorption on Seebeck coefficient of MoS2 are also explored. Our work suggests that an enhanced excitonic efficiency of MoS2 may be achieved via proper defect engineering and molecular doping arising from the charge density modulation and charge screening.

* †

[email protected] [email protected] 1

ACS Paragon Plus Environment


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INTRODUCTION Charge transfer between organic molecules and two-dimensional (2D) materials is of great importance for constructing high-performance 2D materials-based nanoelectronics.1-3 Initially, graphene was considered as an ideal system for hosting organic molecules with promoted solar energy utilization due to its ultrafast carrier mobility, efficient electron-hole separation and tunability of the population and polarity of charge carriers.4,5 More recently, MoS2, another 2D material in the family of transition metal dichalcogenides (TMDs),6,7 has gained great interests due to its potential applications in transistor, optoelectronics and catalysis.8,9 In principle, 2D semiconducting MoS2 should be highly suitable for hosting organic molecule to achieve the modulation of electronic properties due to the following reasons: 1) The partially occupied Mo 4d1 manifolds render great possibility of transferring electrons/holes. 2) In contrast to the C-C hompolar bonds in graphene, the heteropolar Mo-S bonds of monolayer MoS2 are able to facilitate the adsorption of functionalized molecules due to the larger dispersive interaction. Indeed, interactions between molecules and semiconducting 2D materials have been studied before. For example, control of charge carriers and optical emission by hybrid 2D sheets of MoS210,11 and WS212,13 with chemical functional groups were investigated. Proof-of-concept devices, which took advantage of the strong molecule-MoS2 interaction, were fabricated to achieve highly effective gas sensors and separations.14,15 Physisorbed or chemisorbed molecular groups were shown to lead to efficient exfoliation16 and significant modification of the surface properties of 2


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MoS2 layers.17 In addition, due to the atomically thin thickness, the accumulated or depleted charges transferred between the molecules and MoS2 were demonstrated to result in a remarkably high sheet density.3,18,19 Last but not least, properly chemical functionalization of TMDs can lead to tunable photoluminescence20-25 and enhanced structural stability of the sheets.26 Point defects such as vacancies are quite popular in 2D TMD materials, which are likely to be unintentionally introduced during fabrication and applications.27-30 It is known that defective states, which are formed within the bandgap, are able to dramatically affect electronic mobilities31 and excitons.32,33 Besides, such states are often associated with dangling bonds, which may serve as the active sites for chemical adsorption of various species.34-38 In particular, the interplay between the localized defective states and molecular orbitals may lead to new optical excitations and modulation of electron-hole recombination.39-43 A recent study showed that passivation of MoS2 surface with organic superacid greatly suppressed the defect-mediated

nonradiative

recombination

and

led

to

near-unity

photoluminescence.44 In addition, resonance of these defective states with orbital states of functionalized molecules or atoms was found to enable the modulation of carrier content and polarity in host 2D materials.38,45 It is noted, however, that the combined effect of doping molecules and sulfur vacancy on the charge transfer and electronic properties of monolayer MoS2 are still largely unexplored. MoS2 46,47 and TMDs hybrids48-51 were recently shown to be promising materials for thermoelectric applications, due to their relatively low thermal conductivity52,53 3



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and high carrier mobility.54 Since defects and chemical functionalization are able to simultaneously create additional electronic states in the bandgap,55 proper engineering of these two factors may lead to a change in thermoelectric efficiency. However, the effect of molecular doping and point defects on the Seebeck coefficient of MoS2 remains unexplored. In this work, we perform first-principles calculations to analyze the charge transfer and electronic structure of molecule-doped MoS2, focusing on the combined effect of molecular doping and defect engineering. We consider four different molecules: dimethyl-p-phenylene diamine (DMPD), tetrathiafulvalene (TTF), tetracyanoethylene (TCNE) and tetracyanoquinodimethane (TCNQ), which have been widely used in doping graphene to tailor the conducting polarity and carrier density.56-59 Our work here shows that the presence of the sulfur vacancy (VS) can greatly modify the orbital alignment and charge transfer between the functionalized molecules and MoS2, which are critical for MoS2-based electronic and optical applications. It is suggested that these molecularly gated or defect-engineered MoS2 are highly promising for thermoelectric applications due to non-reduced Seebeck coefficient and simultaneously improved charge transport via the in-gap states related to either the intrinsic defects or the external functionalized molecules.

COMPUTATIONAL DETAILS First-principles spin-polarized calculations are performed by using the Vienna Ab initio simulation package (VASP) package.60 The weak Van der Waals (vdW) 4


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interactions between the organic molecules and the monolayer MoS2 substrate are calculated with the Becke88 optimization (optB88) functional together with a kinetic energy cutoff of 400 eV. The relaxed lattice constant of MoS2 is 3.183 Å. Low-coverage and high-coverage adsorptions are realized by putting one molecule in the MoS2 supercell containing 90 and 36 atoms (Mo and S), respectively. A vacuum layer with a thickness of 15 Å is inserted to avoid the spurious interactions between image layers. The first Brillouin zone is sampled with a 3×3×1 Monkhorst-Pack grid. All atomic structures are fully relaxed until the force on each atom is smaller than 0.01 eV/Å. The adsorption energy (Ead) is calculated via Ead=EMol+MoS2-EMol-EMoS2, where EMol, EMoS2 and EMol+MoS2 are the energies of the molecule, the MoS2 sheet and the molecule adsorbed on MoS2, respectively.

RESULTS and DISCUSSION

Low-coverage adsorption of molecules on pristine MoS2.

We first investigate the

adsorption for each molecule by identifying the lowest energy configuration. For the low-coverage adsorption, several high-symmetry adsorption sites are considered and shown in Fig. 1. For DMPD, the most stable adsorption is at site 4, where the cyano groups align parallel to the armchair direction of MoS2 and the benzene ring center locates directly above the S atom. The value of Ead is found to be -0.99 eV. Adsorptions on other sites all have slightly higher (less than 0.15 eV) energy except the site 5 adsorption which is the least stable, with a higher Ead of 0.25 eV (see Table 5



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S1). Since the adsorption configuration at site 5 is just a shift of the molecule from that at site 4 along the armchair direction, this suggests that the DMPD molecule at finite temperature is less likely to move along the armchair direction. Instead, rotation and translation along the zigzag direction are likely to be dynamically more frequent. For the TTF molecule, the lowest energy configuration corresponds to the adsorption at site 7 with the Ead of -1.11 eV, where the molecule aligns along the zigzag direction with the three carbon dimers in the molecule sitting directly above the S atom. For the adsorption of the TCNE molecule, the most stable configuration is found to be site 6 with the Ead of -0.79 eV, where the center of the molecule coincides with the center of hexagon of MoS2 and the central carbon dimer aligns along the zigzag direction. Concerning the TCNQ molecule, which has four cyano groups at the two ends of TCNE, it has exactly the same configuration of the ground state (site 2 with the Ead of -0.92 eV) with the TCNE molecule if the central C6 ring of TCNQ is neglected. In the ground states of both molecules, the four cyano groups nearly overlap with the S-Mo bonds. Due to the strong electronegativity of cyano groups, TCNE and TCNQ molecules act as effective acceptor with drawing electrons from graphene56,57 and MoS2 (see below). The obtained alignment between the molecules and MoS2 by increasing the overlapping between cyano groups and S-Mo bonds promotes the charge transfer and enhances the binding strength. Maximizing the overlapping between the accepting centers in the molecule and the S-Mo bonds in MoS2 may also allow a favorable adsorption and a high doping efficiency. Figure 2 shows the density of states (DOS) of the MoS2 upon the adsorption of 6


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the above mentioned molecules in the lowest energy configurations. For all the molecules, there is no spin splitting in the DOS, indicating the absence of magnetism in these doped systems. For DMPD and TTF, their highest occupied molecular orbital (HOMO) locates around 0.32 eV below the conduction band minimum (CBM), signifying n-type doping of the MoS2. In contrast, for TCNE and TCNQ, their lowest unoccupied molecular orbital (LUMO) locates around 0.3 and 0.43 eV above the valence band maximum (VBM), respectively. These empty levels could accept excited electrons, thus simultaneously creating holes in the MoS2. Therefore, TCNE and TCNQ serve as an effective molecular acceptor, leading to p-type doping in MoS2. To estimate the exact amount of charge transfer between these molecular dopants and the MoS2, we perform the analysis of differential charge density ∆ρ(r) in these hybrid systems. For DMPD and TTF, the isosurface plot of ∆ρ(r) is plotted in Fig. 3. The green (red) denotes depletion (accumulation) of electrons in the molecules and the MoS2. It is seen that for these two molecules, the electrons are transferred from the HOMO of the molecules. The transferred electrons are mainly distributed directly in the S atoms and partly in the Mo atoms of the contacting region of the underlying MoS2.The total amount of charge transfer from the molecule up to z point is

calculated using ∆ = ∆′ dz′ , where ∆ρ(z’) is the plane-averaged differential charge density along the normal direction to the basal plane of MoS2 by integrating ∆ρ(r) within the x-y plane at the z’. For DMPD, the amount of donated electrons to the MoS2 is about 0.13 e per molecule, and for TTF, it is around 0.14 e 7



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per molecule. Our predicted donor character of TTF is consistent with the experimental results of TTF-doped MoS2 by using Raman spectrum and images by Kelvin probe force microscopy.61,62 Concerning TCNQ and TCNE as shown in Fig. 4, an opposite flow of electrons is observed in comparison with the above two molecular donors. The total amounts of electrons received from the MoS2 are 0.03 and 0.02 e per TCNQ and TCNE, respectively, indicating that they are weak acceptors. The isosurface plots show that the donated electrons from the MoS2 are mainly located in the dicyano groups in the two ends of each molecule, reflecting that the cyano groups are the functional accepting groups. In contrast, the central C=C group in TCNQ and the benzene rings in TCNE show a clear loss of electrons, while the underlying MoS2 directly below these groups undergoes a clear increase in electron density. This suggests that the two acceptors induce strong charge redistribution in the underlying MoS2 layer, which may alter the transport behavior of carriers and optical performance of the host MoS2.

High-coverage adsorption of molecules on pristine MoS2. We also examine the high-coverage adsorption of these molecules on the MoS2 surface by reducing the size of the super cell. Similar to the aforementioned low-coverage adsorption case, we have considered several adsorption sites for the high-coverage adsorption case, from which we select the lowest-energy configuration for further analysis. In addition to the molecule-MoS2 interaction in aforementioned low-coverage cases, the inter-molecular interaction becomes important in this high-coverage adsorption and 8


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induces a different adsorbing structure (Fig. 5a). The interaction between the molecule and MoS2 becomes weaker with a smaller magnitude of Ead due to the energy penalty of repulsive interaction between molecules. For instance, the Ead of DMPD changes from -0.99 eV (low-coverage adsorption) to -0.72 eV (high-coverage adsorption). Figure 5b shows the band structures of the high-coverage molecularly adsorbed MoS2, where the HOMO of the molecular donors (DMPD and TTF) and the LUMO of the molecular acceptors (TCNE and TCNQ) are located within the band gap of MoS2. Due to the inter-molecular interaction, the molecular levels shift upward compared with the low-coverage cases (see Fig. 2): the HOMO of DMPD/TTF now is located at 0.16 and 0.24 eV below the CBM, respectively, whereas the LUMO of TCNE/TCNQ is located at 0.38 and 0.72 eV above the VBM (Fig. 5b), respectively. This indicates that the molecular coverage may affect the in-gap levels, enabling the modulation of the optical properties of the doped system. For the high-coverage adsorption, the trend of charge transfer is the same as the low-coverage cases (Table 1). However, the amount of the charge transfer per molecule becomes smaller in the case of high-coverage adsorption (see the dashed ∆Q curves in Figs. 3 and 4). However, owing to the increased number of molecules per unit area, an overall higher charge transfer is obtained in this high-coverage adsorption. A consequence of this charge transfer is the modification of the work function of MoS2. Owing to the presence of a dipole layer on the surface,63 functionalization with donor molecules (DMPD and TTF) leads to a reduction of the work function; whereas that with acceptor molecules leads to an increase in the work 9



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function of pristine MoS2 (5.48 eV), as shown in Figure 5(c).

Adsorption of molecules above sulfur vacancy of MoS2. We next examine the electronic properties of functionalized molecules adsorbing on the VS, which is the most popular defect in MoS2. For each molecule, various possible configurations with different relative alignments with the VS core are examined, and the most stable structure is selected for analyzing electronic structure. The band structure in Fig. 6 shows that the presence of single VS induces two localized states in the band gap: the fully occupied A1 state around 0.01 eV above the valence band and the fully doubly degenerate unoccupied E state around 0.58 eV below the conduction band. The A1 state mainly consists of the Mo dxy and dyz orbitals while the E state largely involves orbitals of S atoms. As shown in Fig.6, the defect states are strongly renormalized upon adsorbing the molecules. For instance, upon the adsorption of DMPD, both the A1 and E states associated with the VS become spin-split, which is normally an indicator of the change in occupation of the energy levels. The A1 state shifts upward to 0.09 and 0.12 eV above the VBM for spin up and down states, respectively, compared with the bare vacancy case. Similarly, the double degeneracy of E state is lifted in the range from 0.32 to 0.51 eV below the CBM. Interestingly, the HOMO drops dramatically from 0.16 eV below the CBM in the defect-free case to 0.76 eV (spin up) and 0.58 eV (spin down) below the CBM. For TTF molecules, the molecular donor level is also spin-split, but the degree of split is much smaller than that of the DMPD molecule, and the spin-up and spin-down 10


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components of HOMO locate at 0.65 and 0.59 eV below the CBM, respectively. Different from the DMPD case, no spin splitting is found in the vacancy related states despite the fact that the degeneracy of the E state is lifted into two states locating at 0.35 and 0.46 eV below the CBM. The A1 state shifts upward in comparison with the case without molecule adsorption, and locates at 0.09 eV above the VBM. Clearly, the adsorption of donor molecules above the VS center induces an upward shift of the defective E and A1 states. This upshift originates from the electron transfer from the molecules, which increases the on-site energy of these localized levels. In contrast, the HOMO states of the donor molecules significantly shift downward from the conduction bands compared with the defect-free case. For the DMPD and TTF molecules, the charge redistribution between the molecule and the vacancy induces local magnetic moments. For the DMPD doped case, the spin density is distributed both at the VS center and also at the molecule, whereas for the TTF doped case, the spin density is only distributed at the TTF molecule (see Figure S1 in Supporting Information). For the TCNE and TCNQ molecules, their adsorption on the VS center triggers a relatively smaller effect on the E and A1 states than that for the TTF and DMPD molecules. The E state is split slightly into two states locating at 0.46 and 0.50 eV below the CBM. The A1 state locates at around 0.06 eV above the VBM for both molecules. The energy level of the LUMO state locates at 0.34 eV (TCNE) and 0.44 eV (TCNQ) above the VBM, shifting downward with respect to the adsorption on perfect MoS2. 11



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The alignment of HOMO and VS related states with the valance and conduction bands of the host MoS2 can be more clearly appreciated in the DOS plot. For DMPD and TTF molecules, the downward shift of the HOMO states can be inferred from the comparison between Fig.7 (a) and (d) and Fig.2 (the DOS of the perfect cases). By examining the VS effect on the charge transfer, we find that upon adsorbing the molecules, the charge carriers around the VS center are strongly redistributed (Fig. 7b and e). Compared with the adsorption on perfect MoS2, the molecule triggers more significant charge modulation on the bottom sulfur layer. Integration of plane-averaged differential charge density shows that 0.32 electrons per molecule are transferred from the DMPD to the defected MoS2, double of that for the adsorption on perfect MoS2 (0.13 e). Similarly, an enhanced electron transfer at the defect center is also observed in the TTF adsorption as shown in Fig. 7e and f. Owing to the increased electron accumulation at the VS due to the adsorption of DMPD and TTF, defect-related bound excitons may become more evident, and thus a promoted screening and lower exciton efficiency are expected in these molecularly doped MoS2. Hence, these donor molecules may affect the optical properties and exciton efficiency. For TCNE and TCNQ molecules, the LUMO level aligns closely to the fully occupied A1 level (see the band structure in Fig. 6 and DOS in Fig.8a and d). However, the charge transfer from the A1 state to the molecules is small as there is only tiny charge depletion in the central Mo layer (Fig. 8b and e). This is likely due to the small overlap between the A1 state and LUMO since the A1 state mainly consists of the Mo d orbital, which is spatially distributed relatively far from the molecule. The charge 12


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transfer analysis shows that both molecules accept electrons from the VS center, amounting to 0.05 and 0.02 e per molecule for TCNE and TCNQ, respectively, which are close to the values for their adsorptions on perfect MoS2. Owing to the presence of TCNE and TCNQ molecules, the electron density in MoS2 is reduced, leading to a weakened screening of excitons and suppressed non-radiative trionic decay. As a result, an enhanced exciton efficiency is expected. In microelectronics and nanoelectronics, accurately adjusting the carrier concentration is a key factor to achieve novel functionalities and performances of electronic devices64. A recent study showed that an increase in the electron doping density was able to significantly reduce the Schottky barrier height in few-layer WS2 field-effect transistors65. However, for 2D materials in general, their ultrathin channels pose great difficulties to achieve controllable and stable doping66. In the present work, we have demonstrated that physical adsorption of organic molecules is able to achieve a high carrier concentration. For example, MoS2 with a high coverage adsorption of DMPD molecules exhibits a carrier concentration of about 1013 cm-2, which is close to that observed in MoS2 with chemically doped chloride molecules65. Remarkably, the carrier density of MoS2 can be further increased by introducing sulfur vacancies. Thus, physical doping with organic molecules presents a promising route to adjust the carrier density of MoS2-based devices.

Effect of vacancy and molecular doping on Seebeck coefficient. In a thermoelectric material, a temperature difference ∆T across the material is able to cause the carriers 13



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to flow within the material, leading to an electromotive force V. Seebeck coefficient, which is defined as S = V / ∆ T , is an important factor that directly influences the thermoelectric performance of the material, that is, its ability in energy conversion, such as waste heat harvesting 67. In this section, we consider the Seebeck coefficient of molecularly doped MoS2. The Seebeck coefficient can be evaluated from the transport coefficients as:

=

(1)

where the transport coefficients can be obtained by solving the Boltzmann transport equation (BTE). Using a constant relaxation time approximation68, the transport coefficients can be written as:

= − ! − " #$

(2)

where is the group velocity of the electrons in the direction of $, ! is the energy of the electrons depending on $, #$ is the density of states in k-space such that # = 2/2' , where a factor of 2 arises from spin-degeneracy, and ( is the )

Fermi-distribution of electrons ( = *+,- .). By assuming that the group velocity of

electrons depends only on its energy but not the effective mass = /∗ , we can convert the -integration to energy integration: 1

= − /∗ ! ! ! − " 3!

(3)

where 3! is the density of states in the energy space. Note that Eq.(3) has been widely used to estimate the transport coefficients68. For a semiconductor with a considerable band gap such as MoS2, the group velocity for conduction electrons is proportional to its distance to !456 and that for valence electrons is proportional to 14


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its distance from !756 . By separating the contribution from electrons and holes respectively, we have: =

| 9:; | /∗

(4)

Modulating Carrier Density and Transport Properties of MoS2 by

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