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The electronic and transport properties of MoS2 combined with thiol-based molecules: A first principles study
Journal Pre-proofs Research paper The electronic and transport properties of MoS2 combined with thiol-based molecules: A first principles study Jie Sun, Jiancai Leng PII: DOI: Reference:
S0009-2614(19)30927-3 https://doi.org/10.1016/j.cplett.2019.136946 CPLETT 136946
To appear in:
Chemical Physics Letters
Received Date: Revised Date: Accepted Date:
25 August 2019 20 October 2019 6 November 2019
Please cite this article as: J. Sun, J. Leng, The electronic and transport properties of MoS2 combined with thiolbased molecules: A first principles study, Chemical Physics Letters (2019), doi: https://doi.org/10.1016/j.cplett. 2019.136946
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
The electronic and transport properties of MoS2 combined with thiol-based molecules: A first principles study Jie Sun, Jiancai Leng*
School of Electronic and Information Engineering (Department of Physics), Qilu University of Technology (Shandong Academy of Sciences), 250353 Jinan, Shandong, P. R. China
*Corresponding authors: [email protected]
Abstract Using first-principles calculations, we investigate the electronic and transport properties of MoS2 combined with thiol-based molecules. The calculated results indicate alkane thiol molecule with anchor group -NH2 is more likely to be captured by S vacancy. Charges transfer from the molecule to MoS2, leading the Fermi level of MoS2 closes to its conduction band. The defect states vanish in the repaired MoS2, nevertheless, the unexpected extra flat bands dominated by the molecular states arise. Electron transmission spectrums calculations show that the thiol-based molecules can significantly degrade the transmission ability of MoS2.
Keywords: MoS2; thiol molecules; sulphur vacancy, electronic structure; electronic transport
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1. Introduction Two-dimensional (2D) MoS2 has been intensively studied due to its unique properties and great potential applications in electronics and optoelectronics [1]. MoS2 based transistors have exhibited large current on/off ratios and ultralow standby power dissipation [2]. However, the ultralow carrier mobility largely impedes its further development compared with other familiar two-dimensional materials such as graphene and black phosphorus [3,4]. Besides its intrinsic limit, the sulfur vacancies play an important part in decreasing the carrier mobility of MoS2, which act as the major traps or scattering centers of charge carrier [5,6]. In addition, the existences of sulfur vacancies in MoS2 lattice also reduce the mechanical strength and stiffness [7], suppress the thermal conductivity [8] and enhance the interface Fermi level pinning [9]. Therefore, eliminating sulfur vacancies is imperative to improve the performance of MoS2 based electronic device. Apart from improving growth technics, atomic or molecular healing has been verified an effective way to remove S vacancies [10]. Oxygen passivation has been proved as effectively means healing the electronic structure of the material [11], changing the vacancies from harmful carrier-traps to electronically benign sites [12] and mediated modulating the trion and excitons in MoS2 [13]. Besides the oxygen passivation, the H2O and TFSI molecules are also effective candidate for healing S vacancies, which can enhance the photoluminescence in MoS2 [14-17]. In addition to the above valid strategies, thiol chemistry has been explored for S vacancies repaired and surface functionalization [18-20]. Yu et al. develop a facile low-temperature thiol chemistry route to repair the sulfur vacancies and improve the interface, resulting in significant reduction of the charged impurities and traps [21]. Ding et al. observed the enhanced photoluminescence response and decreased active sites for hydrogen evolution catalysis in thiol functionalized MoS2 [22]. Sim et al. demonstrated that the electrical properties of MoS2 can be systematically engineered by exploiting the tight binding between the thiol-based molecules and S vacancies [23]. Bertolazzi et al. show the performances of defective MoS2 field effect transistors (FETs) can be largely 2
improved by exposing the devices to vapors of short linear thiolated molecules [24]. Despite thiol-based molecules are promising candidates for S vacancies healing, there is still lack of the systematic theoretical study of their effect on the electronic and transport properties of MoS2. In this paper, thiol-based molecules with various functional groups are involved. The possibilities of these molecules formed stable configurations with defective MoS2 are checked and the charge transfer mechanism between the conjunct molecules and MoS2 is revealed. A two-probe system is constructed to examine the response of electron transport properties of MoS2 combined with thiol-based molecules. Moreover, the dissociation of thiol-based molecules from MoS2 surface is also explored.
2. Computational details The geometry structure relaxation and electronic structure calculations are carried out based on density functional theory (DFT) implemented in the Vienna ab initio simulation package (VASP) [25]. The electron–ion interaction is described with the projector-augmented wave (PAW) method. The exchange–correlation part of the electron–electron interaction is obtained by using generalized gradient approximation in its Perdew, Burke, and Ernzerhof form (GGA-PBE) [26]. The plane-wave energy cutoff is set to 400 eV. The Monkhorst-Pack k-points of 4×6×1 are applied for sampling in the first Brillouin zone during geometry optimizations. In order to avoid the interaction among periodic images, a vacuum layer larger than 15 Å is added. Geometry structures are fully relaxed until energy and force are converged to 10-5 eV and 0.02 eVÅ-1, respectively. The nudged elastic band method is used to find the transition states and the reaction pathways between two stable states [27]. The transmission spectra calculations are carried out in the TRANSIESTA program [28]. Double-ζ plus polarization (DZP) basis set is employed. The k-point grids of 1×50×30 are used for the transmission spectra calculations. The basal plane of MoS2 is constructed using a rectangle supercell with the dimension of 22.04 Å× 12.73 Å. A single S atom is removed to simulate the MoS2 with S vacancy (SV). Three alkane thiol molecules with different anchor groups, 3
1,5-pentanedithiol (PDT), 5-aminopentane-1-thiol (APT), 5-nitropentane-1-thiol (NPT) and one pyridine thiol 5-(trifluoromethyl) pyridine-2-thiol (TPT), are respectively connected with defected MoS2 to simulate its repaired condition (Figure 1(a)-(d)).
3. Results and discussion As the four hybrid systems are fully optimized, it is found that the S atom filling the vacancy is slightly dragged from the basal plane of MoS2.The distance between molecule and MoS2 is represented by the C-S bond length. The four distances show similar values and in the range of 1.86-1.89 Å (Table 1).
Figure 1. The geometry structure of MoS2 connects with thiol-based molecules, (a) 1,5-pentanedithiol (PDT), (b) 5-aminopentane-1-thiol (APT), (c) 5-nitropentane-1-thiol (NPT) (d) 5-(trifluoromethyl) pyridine-2-thiol (TPT). Table 1. Formation energies (Ef) of the molecule combine with defected MoS2, the distance (D) between molecule section and MoS2 and the charge transfer value (Q) from molecule to MoS2.
System
Ef (eV)
D (Å)
Q (e)
PDT APT NPT TPT
-0.800 -0.842 -0.777 -0.332
1.890 1.888 1.891 1.867
0.171 0.172 0.136 0.055
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In order to evaluate the possibility of the thiol-containing molecules captured by S vacancy, the formation energy of the molecule combining with defected MoS2 is obtained according to
E f = Emol + MoS2 + µ H − Emol − EMoS2 where Emol, EMoS2 and Emol+MoS2 are the total energy of the molecule, the MoS2 with single S vacancy and the hybrid system, respectively. µH is chemical potentials of the hydrogen atom, which is given as the single atom energy in H2. The calculated results are listed in Table 1. We can see that three alkane thiol molecules have lower formation energy compared with pyridine thiol molecule. This is probably due to the existence of repulsive interaction between the π electrons in pz orbital of pyridine and the electrons of S atom, which enlarges the formation energy. As for three alkane thiol molecules, we can see the formation energy of APT with -NH2 group is lower than that of PDT with -SH group and NPT with NO2 group. This indicates the APT molecule can be highly effortless to heal the S vacancy. In addition, electrons accumulation is found in MoS2 based on Bader charge analysis, which indicates the electrons transfer from molecule to MoS2. The charge transfer value of PDT, APT, NPT and TPT system are 0.171 e, 0.172 e, 0.136 e and 0.055 e, respectively. We note that TPT possesses less charge transfer owing to its relative weak coupling between the conjunct molecule and MoS2 in four systems. Moreover, the charge transfer value can be modulated by the anchor group. More charge transfer in PDT and APT systems is on account of the stronger electron donation ability of anchor group -SH and -NH2. Furthermore, charges transfers in hybrid systems are expected to have impact on the electronic properties of MoS2.
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Figure 2. The electronic band structure and the corresponding density of states (DOS) of (a) pristine MoS2, (b) MoS2 with S vacancy (SV), (c) PDT, (d) APT, (e) NPT, (f) TPT. The total DOS are projected on MoS2 (Mo_4d and S_3p orbital) and conjunct molecules, respectively. (g)-(h) Isosurfaces of band-decomposed charge density of VBM and CBM of PDT (marked as A and B in its band structure) at Γ point.
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Our calculated band structure of pure monolayer MoS2 possesses a direct band gap ~1.64 eV (Figure 2 (a)), which is consistent with previous studies [29]. From the projected density of states (PDOS), we can see its valence band maximum (VBM) and conduction band minimum (CBM) are dominated by the hybridization of Mo_4d and S_3p orbitals. The occurrence of S vacancy introduces flat bands that decrease the band gap to ~1.13 eV (Figure 2 (b)). It is clear see that these flat bands mainly consist of localized Mo_4d orbitals, which play the role of the scattering center and suppress the electron transmission. As S vacancy is repaired, the Fermi level of each hybrid system shifts upwards and closes to the conduction band, resulting in n type doping of MoS2 (Figure 2 (c)-(f)). The doping concentration of electrons ne is proportional to exp(∆EF), the shift Fermi level ∆EF of PDT, APT, NPT and TPT are 1.23 eV, 1.24 eV, 1.14 eV and 1.10 eV, respectively, that is ne(APT)>ne(PDT)>ne(NPT)>ne(TPT), which accord well with the Bader charge analysis. Despite the flat bands caused by defect states vanish and the intrinsic band structure of MoS2 is recovered, the unexpected new flat bands arise in PDT and APT. We note that the flat bands significantly dominate their VBM, which probably decrease the carrier mobility of MoS2. The PDOS indicate these flat bands in PDT and APT are mainly caused by the conjunct molecules. In order to further depict the composition of these flat bands, we plot the isosurfaces of band-decomposed charge density of VBM and CBM of PDT at Γ point (Figure 2 (g)). It clearly shows the states of VBM of PDT localized at the top of the conjunct molecule. However, these flat bands are not found in NPT and TPT system, which is due to the conjunct molecules only contributing to the deep level doping. Besides, the existence of thiol molecules lead the states of CBM of each system become more localized. This states localization is mainly induced by the reduction of the states near the conjunct molecules (Figure 2 (h)).
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Figure 3. The VBM and CBM of each configuration refer to the vacuum level. The Fermi level of Ti and NbS2 is marked as red and blue line, respectively.
Apart from electronic structure, schottky barrier height between MoS2 and metallic electrode is another big issue confronting MoS2 based devices. Here, the band edge of each configuration with respect to the vacuum level is revealed in order to check how the thiol-containing molecules affect the band alignment between MoS2 and metallic electrode (Figure 3). Here, two metallic electrodes are involved, Ti and H-NbS2, which have been reported that can yield lower schottky barrier height with MoS2, respectively. The calculated work function of Ti and NbS2 is 4.3 eV and 6.0 eV, respectively, according well with previous reports [30,31]. The schottky barrier height between Ti and MoS2 is defined as the energy difference between the Fermi level of Ti and CBM of MoS2, φe = ECBM − EF . And the schottky barrier height between NbS2 and MoS2 is defined as the energy difference between the Fermi level of NbS2 and VBM of MoS2, φh = EF − EVBM .Φe and Φh are the barrier heights for electrons and holes, respectively. When Φe or Φh ≤0, the junction is considered as forming ohmic contact. The schottky barrier height for MoS2, SV, PDT, APT, NPT and TPT with Ti is 0.08 eV, -0.47 eV, 0.10 eV, 0.11 eV, 0.01 eV, and -0.04 eV, respectively. It is obvious that the Φe is large negative value when Ti contacts MoS2 with S vacancy, indicating the existence of S vacancy decreases the barrier height that will lead to ohmic contact with Ti electrode. As S vacancies are repaired, the schottky barrier height of four 8
hybrid systems is comparable to that of pure MoS2. Moreover, the TPT also can form nearly ohmic contact with Ti electrode. As for NbS2, the schottky barrier height for MoS2, SV, PDT, APT, NPT and TPT is -0.11 eV, -0.09 eV, -0.19 eV, -0.24 eV, -0.08 eV, and -0.10 eV, respectively. Not large Φh variations are primarily due to the little energy difference in VBM of each configuration. This may also indicate the VBM of
MoS2 is not sensitive to vacancy or chemical doping.
Figure 4. (a) Two-probe systems where semi-infinite left and right electrode regions are in contact with the central scattering region. (b) Transmission spectra of each configuration.
In the following part, we turn to the transmission ability of the electrons across
MoS2 with vacancy and thiol-containing molecules. The electron transmission spectrum is obtained by using a two-probe system as shown in Figure 4 (a), which contains a scattering region connecting with two electrodes. The calculated results are shown in Figure 4(b). We can see that there is a transmission gap around the Fermi
level of MoS2 with a width ~1.51 eV due to its semiconductor nature. As occurrence of S vacancy, it is found that the transmission gap is negligible increase (~1.53 eV). However, the transmission ability is obviously decreased in the region of E> 0.7 eV
and E<-1.5 eV. 9
After the vacancy is repaired, the conjunct thiol-containing molecules are found not only enlarge the transmission gap but also severely decrease the transmission ability compared with both MoS2 and MoS2 with S vacancy. The decrease transmission ability is in accordance with the previous observations [32]. As for four hybrid systems, PDT, APT, NPT and TPT have nearly the same outline in the transmission spectrum. However, their transmission abilities perform discrepant due
to the anchor group effect. For example, the transmission ability in the region of -1.8 eV PDT= APT>NPT. Since the thiol-based molecules are not benefit for the electron transmission, their dissociation from the hybrid system is
important for improving the transmission ability of MoS2.
Figure 5. (a) The dissociation energy of the conjunct molecule. (b) Minimal energy paths of the conjunct molecule diffusion on MoS2 surface through the transition states.
In order to evaluate the possibility of the conjunct molecules dissociated from
MoS2 surface, minimum energy paths for the molecule diffusion are calculated. In the diffusion process, two diffusion paths are considered. One is the molecule vertical moving a distance (~5 Å) from the initial position (Figure 5 (a)); another is the
molecule horizontal migration between two S top positions via bridge positions (Figure 5(b)). In the first path, the total energy is increased as the moving distance enlarged and there is no transition state. The energy differences between the initial
and final states for PDT, APT, NPT and TPT system are 0.46 eV, 0.50 eV, 0.45 eV and 0.40 eV, respectively, which correspond to the dissociation energy of the C-S bond connecting the molecule and MoS2. We can see that the pyridine thiol molecule is easier to leave from MoS2 surface compared with alkane thiol molecules. In the second path as shown in Figure 5 (b), the corresponding structures are the reactant, 10
transition state, and product. The calculated energy barrier of PDT, APT, NPT and TPT are 0.40 eV, 0.44 eV, 0.37 eV and 0.39 eV, respectively. We note that dissociation energy of each configuration is larger than their diffusion barrier, indicating the conjunct molecules will be likely to migrate on the MoS2 surface before leaving. We make an assumption to use heating strategy to dissociate the conjunct molecules, the ultimate temperature is evaluated on the basis of the classical energy equipartition theorem [33], E=
i k BT 2
where i is the degree of freedom of the molecule, kB is the Boltzmann constant and T is the temperature. Here, the molecule is regarded as rigid, including three translational, one rotational and one vibration degree of freedom. The calculated dissociation temperatures are 2135 K, 2320 K, 2088 K and 1856 K for PDT, APT, NPT and TPT system, respectively. These temperatures are smaller than the previous predicting melting point of MoS2 (3700 K) [34], which ensures this strategy can be realized in the experiment.
4. Conclusions In summary, we have systematically studied the electronic and transport properties of the repaired MoS2 with thiol-based molecules. The calculated results indicate the possibilities of the thiol-based molecules captured by S vacancy depend on the anchor group of thiol molecules. The Bader charge analysis shows the n-type doping of MoS2 as the molecules chemisorb on its surface. Further band structure investigations declare that the occurrence of flat bands arise from molecule dominate the VBM in PDT and APT, and these flat bands are not found in NPT and TPT. It is also found the hybrid systems possess similar schottky barrier heights with pure MoS2 when they contact with metallic electrodes Ti and NbS2. The calculated transmission spectra show that the transmission ability of MoS2 is largely decreased by the conjunct molecules. The dissociation energies of different thiol-containing molecules leaving from MoS2 are 0.4 ~ 0. 5 eV and the corresponding desorption temperatures are evaluated based on energy equipartition theorem.
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Acknowledgements We acknowledge the Nature Science Foundation of Shandong Province (Grant No. ZR2018BA031) and (Grant No. ZR2019MA037).
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The transmission ability of repaired MoS2 with thiol-based molecule is decreased.
.The chemisorbed thiol-based molecules result in the n-type doping of MoS2. .Unexpected new flat bands dominated by the thiol-based molecules arise in repaired MoS2. .The transmission ability of MoS2 is largely decreased by the conjunct molecules. .The desorption energies of thiol-based molecules are evaluated 0.4~0. 5 eV. We declare that no potential conflict of interest related to this paper exists.
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S0009-2614(19)30927-3 https://doi.org/10.1016/j.cplett.2019.136946 CPLETT 136946
To appear in:
Chemical Physics Letters
Received Date: Revised Date: Accepted Date:
25 August 2019 20 October 2019 6 November 2019
Please cite this article as: J. Sun, J. Leng, The electronic and transport properties of MoS2 combined with thiolbased molecules: A first principles study, Chemical Physics Letters (2019), doi: https://doi.org/10.1016/j.cplett. 2019.136946
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
The electronic and transport properties of MoS2 combined with thiol-based molecules: A first principles study Jie Sun, Jiancai Leng*
School of Electronic and Information Engineering (Department of Physics), Qilu University of Technology (Shandong Academy of Sciences), 250353 Jinan, Shandong, P. R. China
*Corresponding authors: [email protected]
Abstract Using first-principles calculations, we investigate the electronic and transport properties of MoS2 combined with thiol-based molecules. The calculated results indicate alkane thiol molecule with anchor group -NH2 is more likely to be captured by S vacancy. Charges transfer from the molecule to MoS2, leading the Fermi level of MoS2 closes to its conduction band. The defect states vanish in the repaired MoS2, nevertheless, the unexpected extra flat bands dominated by the molecular states arise. Electron transmission spectrums calculations show that the thiol-based molecules can significantly degrade the transmission ability of MoS2.
Keywords: MoS2; thiol molecules; sulphur vacancy, electronic structure; electronic transport
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1. Introduction Two-dimensional (2D) MoS2 has been intensively studied due to its unique properties and great potential applications in electronics and optoelectronics [1]. MoS2 based transistors have exhibited large current on/off ratios and ultralow standby power dissipation [2]. However, the ultralow carrier mobility largely impedes its further development compared with other familiar two-dimensional materials such as graphene and black phosphorus [3,4]. Besides its intrinsic limit, the sulfur vacancies play an important part in decreasing the carrier mobility of MoS2, which act as the major traps or scattering centers of charge carrier [5,6]. In addition, the existences of sulfur vacancies in MoS2 lattice also reduce the mechanical strength and stiffness [7], suppress the thermal conductivity [8] and enhance the interface Fermi level pinning [9]. Therefore, eliminating sulfur vacancies is imperative to improve the performance of MoS2 based electronic device. Apart from improving growth technics, atomic or molecular healing has been verified an effective way to remove S vacancies [10]. Oxygen passivation has been proved as effectively means healing the electronic structure of the material [11], changing the vacancies from harmful carrier-traps to electronically benign sites [12] and mediated modulating the trion and excitons in MoS2 [13]. Besides the oxygen passivation, the H2O and TFSI molecules are also effective candidate for healing S vacancies, which can enhance the photoluminescence in MoS2 [14-17]. In addition to the above valid strategies, thiol chemistry has been explored for S vacancies repaired and surface functionalization [18-20]. Yu et al. develop a facile low-temperature thiol chemistry route to repair the sulfur vacancies and improve the interface, resulting in significant reduction of the charged impurities and traps [21]. Ding et al. observed the enhanced photoluminescence response and decreased active sites for hydrogen evolution catalysis in thiol functionalized MoS2 [22]. Sim et al. demonstrated that the electrical properties of MoS2 can be systematically engineered by exploiting the tight binding between the thiol-based molecules and S vacancies [23]. Bertolazzi et al. show the performances of defective MoS2 field effect transistors (FETs) can be largely 2
improved by exposing the devices to vapors of short linear thiolated molecules [24]. Despite thiol-based molecules are promising candidates for S vacancies healing, there is still lack of the systematic theoretical study of their effect on the electronic and transport properties of MoS2. In this paper, thiol-based molecules with various functional groups are involved. The possibilities of these molecules formed stable configurations with defective MoS2 are checked and the charge transfer mechanism between the conjunct molecules and MoS2 is revealed. A two-probe system is constructed to examine the response of electron transport properties of MoS2 combined with thiol-based molecules. Moreover, the dissociation of thiol-based molecules from MoS2 surface is also explored.
2. Computational details The geometry structure relaxation and electronic structure calculations are carried out based on density functional theory (DFT) implemented in the Vienna ab initio simulation package (VASP) [25]. The electron–ion interaction is described with the projector-augmented wave (PAW) method. The exchange–correlation part of the electron–electron interaction is obtained by using generalized gradient approximation in its Perdew, Burke, and Ernzerhof form (GGA-PBE) [26]. The plane-wave energy cutoff is set to 400 eV. The Monkhorst-Pack k-points of 4×6×1 are applied for sampling in the first Brillouin zone during geometry optimizations. In order to avoid the interaction among periodic images, a vacuum layer larger than 15 Å is added. Geometry structures are fully relaxed until energy and force are converged to 10-5 eV and 0.02 eVÅ-1, respectively. The nudged elastic band method is used to find the transition states and the reaction pathways between two stable states [27]. The transmission spectra calculations are carried out in the TRANSIESTA program [28]. Double-ζ plus polarization (DZP) basis set is employed. The k-point grids of 1×50×30 are used for the transmission spectra calculations. The basal plane of MoS2 is constructed using a rectangle supercell with the dimension of 22.04 Å× 12.73 Å. A single S atom is removed to simulate the MoS2 with S vacancy (SV). Three alkane thiol molecules with different anchor groups, 3
1,5-pentanedithiol (PDT), 5-aminopentane-1-thiol (APT), 5-nitropentane-1-thiol (NPT) and one pyridine thiol 5-(trifluoromethyl) pyridine-2-thiol (TPT), are respectively connected with defected MoS2 to simulate its repaired condition (Figure 1(a)-(d)).
3. Results and discussion As the four hybrid systems are fully optimized, it is found that the S atom filling the vacancy is slightly dragged from the basal plane of MoS2.The distance between molecule and MoS2 is represented by the C-S bond length. The four distances show similar values and in the range of 1.86-1.89 Å (Table 1).
Figure 1. The geometry structure of MoS2 connects with thiol-based molecules, (a) 1,5-pentanedithiol (PDT), (b) 5-aminopentane-1-thiol (APT), (c) 5-nitropentane-1-thiol (NPT) (d) 5-(trifluoromethyl) pyridine-2-thiol (TPT). Table 1. Formation energies (Ef) of the molecule combine with defected MoS2, the distance (D) between molecule section and MoS2 and the charge transfer value (Q) from molecule to MoS2.
System
Ef (eV)
D (Å)
Q (e)
PDT APT NPT TPT
-0.800 -0.842 -0.777 -0.332
1.890 1.888 1.891 1.867
0.171 0.172 0.136 0.055
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In order to evaluate the possibility of the thiol-containing molecules captured by S vacancy, the formation energy of the molecule combining with defected MoS2 is obtained according to
E f = Emol + MoS2 + µ H − Emol − EMoS2 where Emol, EMoS2 and Emol+MoS2 are the total energy of the molecule, the MoS2 with single S vacancy and the hybrid system, respectively. µH is chemical potentials of the hydrogen atom, which is given as the single atom energy in H2. The calculated results are listed in Table 1. We can see that three alkane thiol molecules have lower formation energy compared with pyridine thiol molecule. This is probably due to the existence of repulsive interaction between the π electrons in pz orbital of pyridine and the electrons of S atom, which enlarges the formation energy. As for three alkane thiol molecules, we can see the formation energy of APT with -NH2 group is lower than that of PDT with -SH group and NPT with NO2 group. This indicates the APT molecule can be highly effortless to heal the S vacancy. In addition, electrons accumulation is found in MoS2 based on Bader charge analysis, which indicates the electrons transfer from molecule to MoS2. The charge transfer value of PDT, APT, NPT and TPT system are 0.171 e, 0.172 e, 0.136 e and 0.055 e, respectively. We note that TPT possesses less charge transfer owing to its relative weak coupling between the conjunct molecule and MoS2 in four systems. Moreover, the charge transfer value can be modulated by the anchor group. More charge transfer in PDT and APT systems is on account of the stronger electron donation ability of anchor group -SH and -NH2. Furthermore, charges transfers in hybrid systems are expected to have impact on the electronic properties of MoS2.
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Figure 2. The electronic band structure and the corresponding density of states (DOS) of (a) pristine MoS2, (b) MoS2 with S vacancy (SV), (c) PDT, (d) APT, (e) NPT, (f) TPT. The total DOS are projected on MoS2 (Mo_4d and S_3p orbital) and conjunct molecules, respectively. (g)-(h) Isosurfaces of band-decomposed charge density of VBM and CBM of PDT (marked as A and B in its band structure) at Γ point.
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Our calculated band structure of pure monolayer MoS2 possesses a direct band gap ~1.64 eV (Figure 2 (a)), which is consistent with previous studies [29]. From the projected density of states (PDOS), we can see its valence band maximum (VBM) and conduction band minimum (CBM) are dominated by the hybridization of Mo_4d and S_3p orbitals. The occurrence of S vacancy introduces flat bands that decrease the band gap to ~1.13 eV (Figure 2 (b)). It is clear see that these flat bands mainly consist of localized Mo_4d orbitals, which play the role of the scattering center and suppress the electron transmission. As S vacancy is repaired, the Fermi level of each hybrid system shifts upwards and closes to the conduction band, resulting in n type doping of MoS2 (Figure 2 (c)-(f)). The doping concentration of electrons ne is proportional to exp(∆EF), the shift Fermi level ∆EF of PDT, APT, NPT and TPT are 1.23 eV, 1.24 eV, 1.14 eV and 1.10 eV, respectively, that is ne(APT)>ne(PDT)>ne(NPT)>ne(TPT), which accord well with the Bader charge analysis. Despite the flat bands caused by defect states vanish and the intrinsic band structure of MoS2 is recovered, the unexpected new flat bands arise in PDT and APT. We note that the flat bands significantly dominate their VBM, which probably decrease the carrier mobility of MoS2. The PDOS indicate these flat bands in PDT and APT are mainly caused by the conjunct molecules. In order to further depict the composition of these flat bands, we plot the isosurfaces of band-decomposed charge density of VBM and CBM of PDT at Γ point (Figure 2 (g)). It clearly shows the states of VBM of PDT localized at the top of the conjunct molecule. However, these flat bands are not found in NPT and TPT system, which is due to the conjunct molecules only contributing to the deep level doping. Besides, the existence of thiol molecules lead the states of CBM of each system become more localized. This states localization is mainly induced by the reduction of the states near the conjunct molecules (Figure 2 (h)).
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Figure 3. The VBM and CBM of each configuration refer to the vacuum level. The Fermi level of Ti and NbS2 is marked as red and blue line, respectively.
Apart from electronic structure, schottky barrier height between MoS2 and metallic electrode is another big issue confronting MoS2 based devices. Here, the band edge of each configuration with respect to the vacuum level is revealed in order to check how the thiol-containing molecules affect the band alignment between MoS2 and metallic electrode (Figure 3). Here, two metallic electrodes are involved, Ti and H-NbS2, which have been reported that can yield lower schottky barrier height with MoS2, respectively. The calculated work function of Ti and NbS2 is 4.3 eV and 6.0 eV, respectively, according well with previous reports [30,31]. The schottky barrier height between Ti and MoS2 is defined as the energy difference between the Fermi level of Ti and CBM of MoS2, φe = ECBM − EF . And the schottky barrier height between NbS2 and MoS2 is defined as the energy difference between the Fermi level of NbS2 and VBM of MoS2, φh = EF − EVBM .Φe and Φh are the barrier heights for electrons and holes, respectively. When Φe or Φh ≤0, the junction is considered as forming ohmic contact. The schottky barrier height for MoS2, SV, PDT, APT, NPT and TPT with Ti is 0.08 eV, -0.47 eV, 0.10 eV, 0.11 eV, 0.01 eV, and -0.04 eV, respectively. It is obvious that the Φe is large negative value when Ti contacts MoS2 with S vacancy, indicating the existence of S vacancy decreases the barrier height that will lead to ohmic contact with Ti electrode. As S vacancies are repaired, the schottky barrier height of four 8
hybrid systems is comparable to that of pure MoS2. Moreover, the TPT also can form nearly ohmic contact with Ti electrode. As for NbS2, the schottky barrier height for MoS2, SV, PDT, APT, NPT and TPT is -0.11 eV, -0.09 eV, -0.19 eV, -0.24 eV, -0.08 eV, and -0.10 eV, respectively. Not large Φh variations are primarily due to the little energy difference in VBM of each configuration. This may also indicate the VBM of
MoS2 is not sensitive to vacancy or chemical doping.
Figure 4. (a) Two-probe systems where semi-infinite left and right electrode regions are in contact with the central scattering region. (b) Transmission spectra of each configuration.
In the following part, we turn to the transmission ability of the electrons across
MoS2 with vacancy and thiol-containing molecules. The electron transmission spectrum is obtained by using a two-probe system as shown in Figure 4 (a), which contains a scattering region connecting with two electrodes. The calculated results are shown in Figure 4(b). We can see that there is a transmission gap around the Fermi
level of MoS2 with a width ~1.51 eV due to its semiconductor nature. As occurrence of S vacancy, it is found that the transmission gap is negligible increase (~1.53 eV). However, the transmission ability is obviously decreased in the region of E> 0.7 eV
and E<-1.5 eV. 9
After the vacancy is repaired, the conjunct thiol-containing molecules are found not only enlarge the transmission gap but also severely decrease the transmission ability compared with both MoS2 and MoS2 with S vacancy. The decrease transmission ability is in accordance with the previous observations [32]. As for four hybrid systems, PDT, APT, NPT and TPT have nearly the same outline in the transmission spectrum. However, their transmission abilities perform discrepant due
to the anchor group effect. For example, the transmission ability in the region of -1.8 eV
important for improving the transmission ability of MoS2.
Figure 5. (a) The dissociation energy of the conjunct molecule. (b) Minimal energy paths of the conjunct molecule diffusion on MoS2 surface through the transition states.
In order to evaluate the possibility of the conjunct molecules dissociated from
MoS2 surface, minimum energy paths for the molecule diffusion are calculated. In the diffusion process, two diffusion paths are considered. One is the molecule vertical moving a distance (~5 Å) from the initial position (Figure 5 (a)); another is the
molecule horizontal migration between two S top positions via bridge positions (Figure 5(b)). In the first path, the total energy is increased as the moving distance enlarged and there is no transition state. The energy differences between the initial
and final states for PDT, APT, NPT and TPT system are 0.46 eV, 0.50 eV, 0.45 eV and 0.40 eV, respectively, which correspond to the dissociation energy of the C-S bond connecting the molecule and MoS2. We can see that the pyridine thiol molecule is easier to leave from MoS2 surface compared with alkane thiol molecules. In the second path as shown in Figure 5 (b), the corresponding structures are the reactant, 10
transition state, and product. The calculated energy barrier of PDT, APT, NPT and TPT are 0.40 eV, 0.44 eV, 0.37 eV and 0.39 eV, respectively. We note that dissociation energy of each configuration is larger than their diffusion barrier, indicating the conjunct molecules will be likely to migrate on the MoS2 surface before leaving. We make an assumption to use heating strategy to dissociate the conjunct molecules, the ultimate temperature is evaluated on the basis of the classical energy equipartition theorem [33], E=
i k BT 2
where i is the degree of freedom of the molecule, kB is the Boltzmann constant and T is the temperature. Here, the molecule is regarded as rigid, including three translational, one rotational and one vibration degree of freedom. The calculated dissociation temperatures are 2135 K, 2320 K, 2088 K and 1856 K for PDT, APT, NPT and TPT system, respectively. These temperatures are smaller than the previous predicting melting point of MoS2 (3700 K) [34], which ensures this strategy can be realized in the experiment.
4. Conclusions In summary, we have systematically studied the electronic and transport properties of the repaired MoS2 with thiol-based molecules. The calculated results indicate the possibilities of the thiol-based molecules captured by S vacancy depend on the anchor group of thiol molecules. The Bader charge analysis shows the n-type doping of MoS2 as the molecules chemisorb on its surface. Further band structure investigations declare that the occurrence of flat bands arise from molecule dominate the VBM in PDT and APT, and these flat bands are not found in NPT and TPT. It is also found the hybrid systems possess similar schottky barrier heights with pure MoS2 when they contact with metallic electrodes Ti and NbS2. The calculated transmission spectra show that the transmission ability of MoS2 is largely decreased by the conjunct molecules. The dissociation energies of different thiol-containing molecules leaving from MoS2 are 0.4 ~ 0. 5 eV and the corresponding desorption temperatures are evaluated based on energy equipartition theorem.
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Acknowledgements We acknowledge the Nature Science Foundation of Shandong Province (Grant No. ZR2018BA031) and (Grant No. ZR2019MA037).
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The transmission ability of repaired MoS2 with thiol-based molecule is decreased.
.The chemisorbed thiol-based molecules result in the n-type doping of MoS2. .Unexpected new flat bands dominated by the thiol-based molecules arise in repaired MoS2. .The transmission ability of MoS2 is largely decreased by the conjunct molecules. .The desorption energies of thiol-based molecules are evaluated 0.4~0. 5 eV. We declare that no potential conflict of interest related to this paper exists.
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