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Electronic and transport properties of 2H1−x1Tx MoS2 hybrid structure: A first-principle study
Physica E 91 (2017) 178–184
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Physica E journal homepage: www.elsevier.com/locate/physe
Electronic and transport properties of 2H1−x1Tx MoS2 hybrid structure: A first-principle study ⁎
Jie Suna, Na Lina, , Cheng Tanga, Haoyuan Wanga, Hao Renb, Xian Zhaoa,
MARK
⁎
a
State Key Laboratory of Crystal Materials, Shandong University, 250100 Jinan, Shandong, PR China State Key Laboratory of Heavy Oil Processing & Center for Bioengineering and Biotechnology, China University of Petroleum (East China), 266580 Qingdao, PR China b
A R T I C L E I N F O
A BS T RAC T
Keywords: First principles MoS2 hybrid Band structure Transmission spectra BN encapsulation
The impure 1T phase (mixed with 2H phase) MoS2 electrodes adopted in 2H phase MoS2-based Field effect transistors (FETs) has been demonstrated efficiently decreasing the contact resistance in recent experiment. Here, first principles calculations are carried out to reveal the electronic and transport properties of the hybrid electrode 2H1−x1Tx MoS2 with different 1T compositions. Our calculated results show that the stability of hybrid 2H1−x1Tx MoS2 system is decreased as increasing the concentration of 1T phase. The band gaps are largely reduced as introducing 1T phase to 2H phase MoS2. The occurrence of flat band around the Fermi level of the hybrid 2H1−x1Tx MoS2 system is not beneficial for the electronic transport. Different transmission performances of each configuration indicate that transmission probabilities are not only subjected to the concentration of 1T phase, but also depending on the specific arrangement of the two phase structure. It is found that binding energies of O2 molecule adsorbed on the phase boundaries of the hybrid 2H1−x1Tx MoS2 is enlarged compared with that on pure 2H and 1T MoS2, accompanying with charge transfer from the substrate to adsorbate. To protect the hybrid 2H1−x1Tx MoS2 state from molecule adsorption, we predict that BN sheet can be used as an effective encapsulating material.
1. Introduction Two-dimensional (2D) MoS2 is one of the most intensively studied transition metal dichalcogenides (TMDs) from both theories and experiments in recent years [1–4]. Several different polymorphs of MoS2 have been reported, including semiconducting 2H phase and metallic 1T/1T′ phase [5–7]. Thermodynamically, MoS2 prefers to the 2H phase [8], in which the molybdenum atoms are trigonal prismatically coordinated by sulfur atoms. The metallic 1T/1T′ phase of MoS2 differs from 2H MoS2 in that the coordination of molybdenum by sulfur atoms is octahedral (twisted octahedral in the 1T′ variant) [9]. Phase transition from 2H to 1T/1T′ has been realized by Li intercalation [10,11] or using an electron beam [12]. In addition, 1T phase is found to transform into the distorted structure (1T′ phase) spontaneously [13]. Due to the occurrence of a moderate band gap, 2H MoS2 is expected to be a potential candidate for future nanodevice. Field-effect transistors (FETs) based on ultrathin 2H MoS2 channels have attracted great attention due to their exhibition of large on/off ratios and near theoretical subthreshold swing values [1,14–16]. Despite the merit of
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2H MoS2 seems promising, the relative low and deviated carrier motilities (vary from 1 to 400 cm2 V−1 S−1) arising from the nature of defects [17,18], the chemical stability in ambient conditions [19] and contact with metal electrode [20–23] still limit its practical application. Especially the last aspect always generates large contact resistance owing to the formation of a Schottky barrier for electron injection [24]. Further efforts [25,26] have been made in order to achieve highperformance of 2H MoS2 based electronics with ideal contact electrodes, which consist of trap-free interfaces, and possess low contact resistance, and are ohmic. Recently, a new strategy based on phase engineering has been realized to improve performance of MoS2 device with contact resistances of 200–300 Ω at zero gate bias [27,28]. The metallic 1T phase was locally patterned on semiconducting 2H MoS2 nanosheets and used as electrodes. However, we note that the concentration of the 1T phase in the converted region is obtained only 60–70% by X-ray photo electron spectroscopy (XPS) analysis, namely, 2H and 1T phase coexist in this electrode region. Therefore, the unclear arrangement and different concentration of two phases probably result in different transport behaviors of 2H MoS2 nanosheets. Moreover, early experi-
Corresponding authors. E-mail addresses: [email protected] (N. Lin), [email protected] (X. Zhao).
http://dx.doi.org/10.1016/j.physe.2017.04.026 Received 30 November 2016; Received in revised form 27 February 2017; Accepted 28 April 2017 Available online 29 April 2017 1386-9477/ © 2017 Elsevier B.V. All rights reserved.
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Fig. 1. Top and side view of 2H1−x1Tx MoS2 hybrid structure with different 1T concentrations.
structure. 2. Computational details The geometry optimizations and electronic structure calculations were carried out using first-principles methods based on density functional theory (DFT) as implemented in the Vienna ab initio simulation package (VASP) [31]. The generalized gradient approximation (GGA) functional of Perdew, Burke and Ernzerhof (PBE) [32] describing the exchange and correlation interactions, together with the projector-augmented wave (PAW) method, were adopted. A cutoff energy of 400 eV was chosen for the plane-wave basis set. The Brillouin zone integration was sampled by a 4×6×1 Monkhorst-Pack mesh. The convergence threshold was set as 10−5 eV in energy and 0.02 eV/Å in force. The long-range interaction is considered by the van der Waals (vdW) correction proposed by Grimme (DFT-D2) [33]. A vacuum layer at least 15 Å was used to avoid interaction between neighboring images. The transmission spectra calculations were carried out in the TRANSIESTA program [34]. Single-ζ plus polarization (SZP) basis set was employed. The k-point grids of 1×100×100 were used for the transmission spectra calculations. Binding energy (Eb) was defined as the energy of the combined system minus the total energy of individual systems. Therefore, a more negative binding energy represents the combined system more stable. Charge transfer between two component systems was obtained by Bader analysis [35].
Fig. 2. Total energies of 2H1−x1Tx MoS2 hybrid systems with different 1T concentrations. The energy of 2H MoS2 has been set to zero and normalized to 1 unit cell.
ment has demonstrated that the single layer MoS2 obtained via Li intercalation exhibits a mixed phase structure and the phase composition strongly influences the electronic properties of the material [29]. Besides, the nature of this hybrid region under ambient conditions is still unknown, especially in the boundaries of two phases stitched together, since previous investigations have revealed the grain boundaries of graphene show enhanced chemical reactivity [30]. Based on the unexplored issues mentioned above, we construct a hybrid structure 2H1−x1Tx MoS2 with different concentration of 1T phase. The electronic structure and charge distribution of the hybrid structure are investigated. Besides, we use this hybrid structure as electrodes to study the electronic transport properties of 2H MoS2 nanosheets. Moreover, the behavior of oxygen molecule adsorbed on the hybrid structure is checked to clarify the environment effect and BN nanosheet is verified a good protecting material for this hybrid
3. Results and discussion At the beginning of our calculations, a rectangular unit cell was optimized with dimensions 3.18 Å×5.51 Å of 2H MoS2 and 3.17 Å×5.49 Å of 1T MoS2, respectively. The hybrid structures 179
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Fig. 3. (a)-(g) Band structures of 2H1−x1Tx MoS2 hybrid systems with different 1T concentrations. (h) Brillouin-zone and special points for 2H1−x1Tx MoS2 supercell electronic bands.
the most stable configuration among them. The pure 1T MoS2 has the least stable ground-state with the energy about 1.68 eV per unit cell higher than pure 2H MoS2. This result is consistent with previous theoretical studies [36]. The stability of the hybrid structure is decreased as increasing the concentration of 1T phase. Apart from the concentration correlation, the stability of the hybrid structure is depended on the specific arrangement of two phases. It is worth to note that the hybrid structure 2H0.51T0.5 Ⅲ, intrinsically a heterojunction, has the total energy close to the low 1T concentration hybrid structure 2H0.751T0.25 and shows more stability compared with 2H0.51T0.5ⅠandⅡ. We have carefully checked its structure and found that an embossment around the boundary of two phases stitch leading to a fluctuant structure, which may well release the in-plane strain of the heterojunction and reduce the total energy. In the following part, we pay our attention to the electronic band structures of the hybrid systems (Fig. 3). We can see that the pure 2H MoS2 possesses a band gap ~1.67 eV, which accords well with previous theoretical results [37], performing a semiconductor nature, while the pure 1T MoS2 is metallic. The discrepancies in electronic properties of two phases are mainly due to the different localization behavior of the d-band of transition metal [9,38]. Previous photoluminescence from chemically exfoliated MoS2 has revealed the phase composition
2H1−x1Tx MoS2 with different 1T MoS2 concentrations (x=0.25, 0.5, 0.75) were constructed by using a 4×4 2H MoS2 supercell, then, directly replace one, two and three 2×2 2H MoS2 supercells by 1T MoS2 due to the little lattice mismatch between two phases. Note the supercell lattice constant is constrained and only atomic positions are relaxed, which induce ~1–3% strain in each hybrid structures, however, this strain has negligible effect on the electronic properties of hybrid systems (more details see Supporting information Section 1). Here, x=0 and x=1 respectively correspond to the pure 2H and 1T MoS2. For x=0.5, we note the two 2×2 1T MoS2 supercell can be patterned in three different parts of 4×4 2H MoS2 supercell signed as 2H0.51T0.5 (Ⅰ,Ⅱ, Ⅲ). The fully relaxed hybrid structures are shown in Fig. 1. It can be seen that the 1T phase in both 2H0.51T0.5 and 2H0.251T0.75 have transformed to distorted structure (1T′ phase). This phenomenon was confirmed by previously prediction that the 1T lattice transformed into the distorted 1T′ MoS2 form after cell relaxation with no barrier [13]. No structure transformations found in other hybrid structures. Moreover, the presence of phase boundaries is expected to play an important role in the stabilization of the hybrid structure as well as charge transport across the sheet. In order to evaluate their stability, the total energy of each configuration is shown in Fig. 2. We find that the pure 2H MoS2 is
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Fig. 4. Isosurfaces of band-decomposed charge density of valence band maximum and conduction band minimum of 2H1−x1Tx MoS2 hybrid systems with different 1T concentrations.
hybrid structure by phase engineered is expected to be useful for optoelectronic applications. Besides the band gap variations, the presence of some flat bands near the Fermi level is observed in the hybrid structure. For example, both the valance and conduction bands close to the Fermi level in 2H0.751T0.25 are quiet flat in the whole region along high symmetry points. The flat conduction band near the Fermi level exists in the 2H0.51T0.5ⅠandⅡ. For 2H0.51T0.5 Ⅲ and 2H0.251T0.75, the flat valance and conduction bands only arises in the region along Y-S and X-Γ line. The flat band regime usually corresponds to the large effective mass and low mobility of carriers, which is not benefit for the transport properties of the hybrid structure. In order to gain further insight into the band states lying close to
strongly affects the electronic properties of the mixed phase structure material [29]. As the 1T phase MoS2 is introduced into 2H phase MoS2, the band gap of the hybrid structure is decreased compared with that of pure 2H phase MoS2. We note that the band gap is 0.47 eV and 0.05 eV for the hybrid structure with 1T concentration of 0.25 and 0.75, respectively. It is initially anticipated that larger 1T concentration will result in smaller band gap of the hybrid structure. However, the band gap of the hybrid structure is not solely depend on the proportion of 1T phase, but also relies on the specific arrangement of two phases. We note the band gap of 2H0.51T0.5Ⅰis 0.63 V, which is larger than that of 2H0.751T0.25 and 2H0.51T0.5Ⅱ(0.29 eV). As for 2H0.51T0.5 Ⅲ, there is a partially occupied band across the Fermi level showing metallic characteristic of this hybrid structure. This band gap regulation of 181
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the Fermi level, the band-decomposed electron densities of the valence band maximum (VBM) and conduction band minimum (CBM) of each configuration are represented in Fig. 4. It is clear that the charge density of the VBM and CBM is uniformly spread over the entire supercell for pure 2H MoS2, while that of 2H0.751T0.25 and 2H0.51T0.5Ⅱis mainly localized on the part of the 1T region. As for 2H0.51T0.5Ⅰ, we note that the VBM is mainly contributed by 1T phase and CBM is depended on both two phases. Edge localized states, similar to that in the zigzag-edged graphene nanoribbons, are found in VBM and CBM of 2H0.51T0.5 Ⅲ. For 2H0.251T0.75, the charge density of VBM is also confined in the edge of two phases and extended to part of 1T phase, whereas, that of its CBM is mainly distributed on the centre of 1T region. The band alignment between 2H MoS2 and the hybrid electrode has been evaluated as shown in Fig. 5. It is noted that the Fermi level of 1T MoS2 is more close to the CBM of 2H MoS2, indicating the n-type Schottky contact between two materials. The conduction band alignment between two materials is 0.62 eV. As for the hybrid systems, we note 2H0.751T0.25, 2H0.51T0.5 Ⅰ and Ⅱ still maintain n-type Schottky contact with 2H MoS2 and their conduction band alignment is 0.53 eV, 0.43 eV and 0.64 eV, respectively. However, for 2H0.51T0.5 Ⅲ and 2H0.251T0.75 forming heterostructures with MoS2, we note their valence band alignment is 0.5 eV and 0.46 eV, respectively, which is smaller than the conduction band alignment, showing p-type Schottky contact. Here, we also evaluate the band alignment between 2H MoS2 and typical Au metal electrode with the value 0.8 eV, which is larger than that of the hybrid systems. This indicates the hybrid electrode may be superior to metal ones. In the following part, we turn to investigate the electronic transport properties using it as electrode. In the experiments, the FET device based on 1T/2H/1T MoS2 nanosheet performs decreased contact resistance compared with Au/2H/Au device. We note the concentration of the 1T phase in the converted region is only 60–70%, besides, the specific stitch between the 1T phase and 2H phase MoS2 is also unknown. Therefore, it is significant to clarify the nature of 2H MoS2 device using impure 1T MoS2 as electrode with different concentrations. Here, a two-probe system, as shown in Fig. 6(a), including three part, left electrode, right electrode and the central scattering region, was employed to simulate the electronic transport properties (all transport models are shown in Supporting information Section 2). The calculated transmission spectra of all configurations at zero bias are shown in Fig. 6(b). Not surprisingly, the transmission probabilities depend strongly on the concentration of the 1T MoS2 and the specific arrangement of 2H/1T hybrid structure. A zero transmission region can be clearly seen around the Fermi level of pristine 2H MoS2, and beyond this region, the transmission probabilities are produced by the available conductance channels of various energy bands. Noteworthy, in contrast to the pristine 2H MoS2, the transmission probabilities of 2H0.751T0.25 are severely suppressed in both valence and conduction band regions. This reduced transmission ability will lead to the reduction of passing current. As for the 1T concentration of 0.5, we notice that new transmission signals in the region about −1.0~ −0.6 eV arise in 2H0.51T0.5Ⅰand 2H0.51T0.5Ⅱ, which decreases the zero transmission gap and increases the electron transport channels. However, the transmission probabilities of 2H0.51T0.5 Ⅲ
Fig. 5. Band alignment between 2H MoS2 and hybrid electrode.
Fig. 6. (a) Top view (upper panel) and side view (lower panel) of the schematic model of two-probe system. (b) Zero-bias transmission spectra of 2H1−x1Tx MoS2 hybrid systems with different 1T concentrations.
Table 1 Binding energy (Eb) and charge transfer (ΔQ(e)) for O2 adsorption and BN encapsulation on 2H1−x1Tx MoS2 hybrid systems. A positive (negative) sign in ΔQ corresponds to electron transfer from the substrate (adsorbate) to the adsorbate (substrate).
O2 BN
Eb(eV) ΔQ(e) Eb(eV) ΔQ(e)
2H
2H0.751T0.25
2H0.51T0.5 Ⅰ
2H0.51T0.5 Ⅱ
2H0.51T0.5 Ⅲ
2H0.751T0.25
1T
−0.109 0.0604 −4.558 −0.0997
−0.150 0.128 −4.355 −0.1083
−0.124 0.124 −4.405 −0.0954
−0.139 0.157 −4.042 −0.107
−0.499 0.231 −4.013 −0.116
−0.129 0.107 −4.218 −0.1024
−0.068 0.271 −4.324 −0.1315
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Fig. 7. Electronic band structure and ELF map of (scale bar is from 0.1 to 0.9) of BN/2H1−x1Tx MoS2 systems. The band structures projected on two components are signed as green and red symbols, respectively.
been studied and the calculated binding energies are shown in Table 1. At initial, we obtain the most stable configurations of O2 molecule adsorbed on pure 2H and 1T MoS2 with binding energies of −0.109 eV and −0.06 eV, respectively (more details see supporting information Section 3). Then, we randomly place the O2 molecule on the phase boundaries of the hybrid structures. Enlarged binding energies of O2 molecule adsorbed on the hybrid structure surface are observed compared with that on pure 2H and 1T MoS2. Especially the case of O2 binding with 2H0.51T0.5 Ⅲ MoS2 surface, which possesses the largest binding energy of −0.499 eV, is expected to be easily influenced and not survival in the ambient conditions. In addition, Bader analysis indicates the charge transfer from the substrate to the adsorbate. Since the environment can affect the hybrid structure 2H1−x1Tx
is largely impeded probably due to the existence of edge localized states hindering the electrons transport. The degraded transmission ability is also found in 2H0.251T0.75. As for pure 1T electrode case, the transmission ability is slightly decreased due to the occurrence of the phase boundaries as electron scattering centers. It should be pointed that the effect of contact with metal is not involved in our simulations. The pure 1T phase MoS2 is metallic, which have little contact resistance with metal electrode than that of 2H MoS2. That is, the conversion to the pure 1T phase may greatly enhance the performance of device. Since the hybrid structure 2H1−x1Tx MoS2 plays an important role in the electronic transport of the FET device, it is necessary to check its chemical active in the ambient environment for practical applications. Adsorption of oxygen molecule on the hybrid structure surface has 183
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Center in Tianjin-TianHe-1(A) and the Norwegian Programme for Supercomputing.
MoS2, it is necessary to search an encapsulation material for electronics applications of the hybrid structure. Here, thinner 2D BN nanosheet is selected as encapsulating material, since it has been proved to be a good protecting material for other 2D materials such as graphene [39] and phosphorene [40]. Our calculated results have indeed verified that BN can form a weak binding with 2H1−x1Tx MoS2. In the optimized structures of BN/2H1−x1Tx MoS2 systems (Supporting information Section 4), it is clear to see that BN layers in some cases such as BN/ 2H0.51T0.5 Ⅲ and BN/2H0.251T0.75 become fluctuant, in order to avoid strong interaction with hybrid structure surface. The BN covered pristine 2H MoS2, possesses a binding energy of −4.558 eV per supercell (about 0.142 eV per Mo atom) (Table 1), which is larger than that covered the hybrid structures with the binding energy −4.013 eV~ −4.405 eV (about 0.125–0.137 eV per Mo atom). Therefore, interactions between BN and the hybrid structures are very weak. The band structures of BN/2H1−x1Tx MoS2 systems (Fig. 7) are fully consistent with the above weak interaction picture. The projected band structures individually on BN and 2H1−x1Tx MoS2 indicate that the band structures of two systems is almost a rigid combination. Due to the large band gap of BN layer, there are almost no BN states hybridized with 2H1−x1Tx MoS2 states around the Fermi level. The band structure of 2H1−x1Tx MoS2 preserves well before and after encapsulated with BN layer, also indicating little charge transfer between two components. Bader analysis shows a charge transfer from BN to 2H1−x1Tx MoS2 surface only ~0.003 electron per Mo atom (Table 1). Electron-localization function (ELF) [41] maps also support that 2H1−x1Tx MoS2 systems encounter negligible interaction with BN sheets. Therefore, BN encapsulation does not affect the electronic properties of 2H1−x1Tx MoS2 hybrid structure, and the transport properties will be well kept in BN/2H1−x1Tx MoS2.
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4. Conclusions In summary, we have systematically investigated the electronic and transport properties of 2H1−x1Tx MoS2 hybrid structure. It is found that the band gaps of the hybrid systems are decreased compared with pure 2H MoS2. The localized states around the Femi level arise accompany with introducing 1T phase MoS2 to 2H phase, which will not contribute to the electronic transport. The band alignment calculations indicate the hybrid electrode may be superior to metal ones. The calculated transmission spectra show that the transmission probabilities not only depend on the concentration of 1T phase, but also rely on the specific arrangement of the two phases. Binding energies of O2 covered on hybrid structure surfaces are larger than those on pure 2H and 1T MoS2. Therefore, in order to make it realize practical application, protection material is needed. BN nanosheet has been found weak bind with the 2H1−x1Tx MoS2 hybrid structures and the states of each configuration of 2H1−x1Tx MoS2 hybrid systems remain intact, which indicate BN is a good protecting material. Our calculated results are expected to be useful for designing practical electronic devices based on MoS2 materials at realistic nanometre scale. Acknowledgements We acknowledge the National Natural Science Foundation of China (Grant nos. 21573129 and 21403300), the National Nature Science Foundation of Shandong Province (Grant no. ZR2015BQ001), and the General Financial Grant from the China Postdoctoral Science Foundation (Grant no. 2013M531595). The authors also acknowledge a generous grant of computer time from the National Supercomputer
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Physica E journal homepage: www.elsevier.com/locate/physe
Electronic and transport properties of 2H1−x1Tx MoS2 hybrid structure: A first-principle study ⁎
Jie Suna, Na Lina, , Cheng Tanga, Haoyuan Wanga, Hao Renb, Xian Zhaoa,
MARK
⁎
a
State Key Laboratory of Crystal Materials, Shandong University, 250100 Jinan, Shandong, PR China State Key Laboratory of Heavy Oil Processing & Center for Bioengineering and Biotechnology, China University of Petroleum (East China), 266580 Qingdao, PR China b
A R T I C L E I N F O
A BS T RAC T
Keywords: First principles MoS2 hybrid Band structure Transmission spectra BN encapsulation
The impure 1T phase (mixed with 2H phase) MoS2 electrodes adopted in 2H phase MoS2-based Field effect transistors (FETs) has been demonstrated efficiently decreasing the contact resistance in recent experiment. Here, first principles calculations are carried out to reveal the electronic and transport properties of the hybrid electrode 2H1−x1Tx MoS2 with different 1T compositions. Our calculated results show that the stability of hybrid 2H1−x1Tx MoS2 system is decreased as increasing the concentration of 1T phase. The band gaps are largely reduced as introducing 1T phase to 2H phase MoS2. The occurrence of flat band around the Fermi level of the hybrid 2H1−x1Tx MoS2 system is not beneficial for the electronic transport. Different transmission performances of each configuration indicate that transmission probabilities are not only subjected to the concentration of 1T phase, but also depending on the specific arrangement of the two phase structure. It is found that binding energies of O2 molecule adsorbed on the phase boundaries of the hybrid 2H1−x1Tx MoS2 is enlarged compared with that on pure 2H and 1T MoS2, accompanying with charge transfer from the substrate to adsorbate. To protect the hybrid 2H1−x1Tx MoS2 state from molecule adsorption, we predict that BN sheet can be used as an effective encapsulating material.
1. Introduction Two-dimensional (2D) MoS2 is one of the most intensively studied transition metal dichalcogenides (TMDs) from both theories and experiments in recent years [1–4]. Several different polymorphs of MoS2 have been reported, including semiconducting 2H phase and metallic 1T/1T′ phase [5–7]. Thermodynamically, MoS2 prefers to the 2H phase [8], in which the molybdenum atoms are trigonal prismatically coordinated by sulfur atoms. The metallic 1T/1T′ phase of MoS2 differs from 2H MoS2 in that the coordination of molybdenum by sulfur atoms is octahedral (twisted octahedral in the 1T′ variant) [9]. Phase transition from 2H to 1T/1T′ has been realized by Li intercalation [10,11] or using an electron beam [12]. In addition, 1T phase is found to transform into the distorted structure (1T′ phase) spontaneously [13]. Due to the occurrence of a moderate band gap, 2H MoS2 is expected to be a potential candidate for future nanodevice. Field-effect transistors (FETs) based on ultrathin 2H MoS2 channels have attracted great attention due to their exhibition of large on/off ratios and near theoretical subthreshold swing values [1,14–16]. Despite the merit of
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2H MoS2 seems promising, the relative low and deviated carrier motilities (vary from 1 to 400 cm2 V−1 S−1) arising from the nature of defects [17,18], the chemical stability in ambient conditions [19] and contact with metal electrode [20–23] still limit its practical application. Especially the last aspect always generates large contact resistance owing to the formation of a Schottky barrier for electron injection [24]. Further efforts [25,26] have been made in order to achieve highperformance of 2H MoS2 based electronics with ideal contact electrodes, which consist of trap-free interfaces, and possess low contact resistance, and are ohmic. Recently, a new strategy based on phase engineering has been realized to improve performance of MoS2 device with contact resistances of 200–300 Ω at zero gate bias [27,28]. The metallic 1T phase was locally patterned on semiconducting 2H MoS2 nanosheets and used as electrodes. However, we note that the concentration of the 1T phase in the converted region is obtained only 60–70% by X-ray photo electron spectroscopy (XPS) analysis, namely, 2H and 1T phase coexist in this electrode region. Therefore, the unclear arrangement and different concentration of two phases probably result in different transport behaviors of 2H MoS2 nanosheets. Moreover, early experi-
Corresponding authors. E-mail addresses: [email protected] (N. Lin), [email protected] (X. Zhao).
http://dx.doi.org/10.1016/j.physe.2017.04.026 Received 30 November 2016; Received in revised form 27 February 2017; Accepted 28 April 2017 Available online 29 April 2017 1386-9477/ © 2017 Elsevier B.V. All rights reserved.
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Fig. 1. Top and side view of 2H1−x1Tx MoS2 hybrid structure with different 1T concentrations.
structure. 2. Computational details The geometry optimizations and electronic structure calculations were carried out using first-principles methods based on density functional theory (DFT) as implemented in the Vienna ab initio simulation package (VASP) [31]. The generalized gradient approximation (GGA) functional of Perdew, Burke and Ernzerhof (PBE) [32] describing the exchange and correlation interactions, together with the projector-augmented wave (PAW) method, were adopted. A cutoff energy of 400 eV was chosen for the plane-wave basis set. The Brillouin zone integration was sampled by a 4×6×1 Monkhorst-Pack mesh. The convergence threshold was set as 10−5 eV in energy and 0.02 eV/Å in force. The long-range interaction is considered by the van der Waals (vdW) correction proposed by Grimme (DFT-D2) [33]. A vacuum layer at least 15 Å was used to avoid interaction between neighboring images. The transmission spectra calculations were carried out in the TRANSIESTA program [34]. Single-ζ plus polarization (SZP) basis set was employed. The k-point grids of 1×100×100 were used for the transmission spectra calculations. Binding energy (Eb) was defined as the energy of the combined system minus the total energy of individual systems. Therefore, a more negative binding energy represents the combined system more stable. Charge transfer between two component systems was obtained by Bader analysis [35].
Fig. 2. Total energies of 2H1−x1Tx MoS2 hybrid systems with different 1T concentrations. The energy of 2H MoS2 has been set to zero and normalized to 1 unit cell.
ment has demonstrated that the single layer MoS2 obtained via Li intercalation exhibits a mixed phase structure and the phase composition strongly influences the electronic properties of the material [29]. Besides, the nature of this hybrid region under ambient conditions is still unknown, especially in the boundaries of two phases stitched together, since previous investigations have revealed the grain boundaries of graphene show enhanced chemical reactivity [30]. Based on the unexplored issues mentioned above, we construct a hybrid structure 2H1−x1Tx MoS2 with different concentration of 1T phase. The electronic structure and charge distribution of the hybrid structure are investigated. Besides, we use this hybrid structure as electrodes to study the electronic transport properties of 2H MoS2 nanosheets. Moreover, the behavior of oxygen molecule adsorbed on the hybrid structure is checked to clarify the environment effect and BN nanosheet is verified a good protecting material for this hybrid
3. Results and discussion At the beginning of our calculations, a rectangular unit cell was optimized with dimensions 3.18 Å×5.51 Å of 2H MoS2 and 3.17 Å×5.49 Å of 1T MoS2, respectively. The hybrid structures 179
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Fig. 3. (a)-(g) Band structures of 2H1−x1Tx MoS2 hybrid systems with different 1T concentrations. (h) Brillouin-zone and special points for 2H1−x1Tx MoS2 supercell electronic bands.
the most stable configuration among them. The pure 1T MoS2 has the least stable ground-state with the energy about 1.68 eV per unit cell higher than pure 2H MoS2. This result is consistent with previous theoretical studies [36]. The stability of the hybrid structure is decreased as increasing the concentration of 1T phase. Apart from the concentration correlation, the stability of the hybrid structure is depended on the specific arrangement of two phases. It is worth to note that the hybrid structure 2H0.51T0.5 Ⅲ, intrinsically a heterojunction, has the total energy close to the low 1T concentration hybrid structure 2H0.751T0.25 and shows more stability compared with 2H0.51T0.5ⅠandⅡ. We have carefully checked its structure and found that an embossment around the boundary of two phases stitch leading to a fluctuant structure, which may well release the in-plane strain of the heterojunction and reduce the total energy. In the following part, we pay our attention to the electronic band structures of the hybrid systems (Fig. 3). We can see that the pure 2H MoS2 possesses a band gap ~1.67 eV, which accords well with previous theoretical results [37], performing a semiconductor nature, while the pure 1T MoS2 is metallic. The discrepancies in electronic properties of two phases are mainly due to the different localization behavior of the d-band of transition metal [9,38]. Previous photoluminescence from chemically exfoliated MoS2 has revealed the phase composition
2H1−x1Tx MoS2 with different 1T MoS2 concentrations (x=0.25, 0.5, 0.75) were constructed by using a 4×4 2H MoS2 supercell, then, directly replace one, two and three 2×2 2H MoS2 supercells by 1T MoS2 due to the little lattice mismatch between two phases. Note the supercell lattice constant is constrained and only atomic positions are relaxed, which induce ~1–3% strain in each hybrid structures, however, this strain has negligible effect on the electronic properties of hybrid systems (more details see Supporting information Section 1). Here, x=0 and x=1 respectively correspond to the pure 2H and 1T MoS2. For x=0.5, we note the two 2×2 1T MoS2 supercell can be patterned in three different parts of 4×4 2H MoS2 supercell signed as 2H0.51T0.5 (Ⅰ,Ⅱ, Ⅲ). The fully relaxed hybrid structures are shown in Fig. 1. It can be seen that the 1T phase in both 2H0.51T0.5 and 2H0.251T0.75 have transformed to distorted structure (1T′ phase). This phenomenon was confirmed by previously prediction that the 1T lattice transformed into the distorted 1T′ MoS2 form after cell relaxation with no barrier [13]. No structure transformations found in other hybrid structures. Moreover, the presence of phase boundaries is expected to play an important role in the stabilization of the hybrid structure as well as charge transport across the sheet. In order to evaluate their stability, the total energy of each configuration is shown in Fig. 2. We find that the pure 2H MoS2 is
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Fig. 4. Isosurfaces of band-decomposed charge density of valence band maximum and conduction band minimum of 2H1−x1Tx MoS2 hybrid systems with different 1T concentrations.
hybrid structure by phase engineered is expected to be useful for optoelectronic applications. Besides the band gap variations, the presence of some flat bands near the Fermi level is observed in the hybrid structure. For example, both the valance and conduction bands close to the Fermi level in 2H0.751T0.25 are quiet flat in the whole region along high symmetry points. The flat conduction band near the Fermi level exists in the 2H0.51T0.5ⅠandⅡ. For 2H0.51T0.5 Ⅲ and 2H0.251T0.75, the flat valance and conduction bands only arises in the region along Y-S and X-Γ line. The flat band regime usually corresponds to the large effective mass and low mobility of carriers, which is not benefit for the transport properties of the hybrid structure. In order to gain further insight into the band states lying close to
strongly affects the electronic properties of the mixed phase structure material [29]. As the 1T phase MoS2 is introduced into 2H phase MoS2, the band gap of the hybrid structure is decreased compared with that of pure 2H phase MoS2. We note that the band gap is 0.47 eV and 0.05 eV for the hybrid structure with 1T concentration of 0.25 and 0.75, respectively. It is initially anticipated that larger 1T concentration will result in smaller band gap of the hybrid structure. However, the band gap of the hybrid structure is not solely depend on the proportion of 1T phase, but also relies on the specific arrangement of two phases. We note the band gap of 2H0.51T0.5Ⅰis 0.63 V, which is larger than that of 2H0.751T0.25 and 2H0.51T0.5Ⅱ(0.29 eV). As for 2H0.51T0.5 Ⅲ, there is a partially occupied band across the Fermi level showing metallic characteristic of this hybrid structure. This band gap regulation of 181
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the Fermi level, the band-decomposed electron densities of the valence band maximum (VBM) and conduction band minimum (CBM) of each configuration are represented in Fig. 4. It is clear that the charge density of the VBM and CBM is uniformly spread over the entire supercell for pure 2H MoS2, while that of 2H0.751T0.25 and 2H0.51T0.5Ⅱis mainly localized on the part of the 1T region. As for 2H0.51T0.5Ⅰ, we note that the VBM is mainly contributed by 1T phase and CBM is depended on both two phases. Edge localized states, similar to that in the zigzag-edged graphene nanoribbons, are found in VBM and CBM of 2H0.51T0.5 Ⅲ. For 2H0.251T0.75, the charge density of VBM is also confined in the edge of two phases and extended to part of 1T phase, whereas, that of its CBM is mainly distributed on the centre of 1T region. The band alignment between 2H MoS2 and the hybrid electrode has been evaluated as shown in Fig. 5. It is noted that the Fermi level of 1T MoS2 is more close to the CBM of 2H MoS2, indicating the n-type Schottky contact between two materials. The conduction band alignment between two materials is 0.62 eV. As for the hybrid systems, we note 2H0.751T0.25, 2H0.51T0.5 Ⅰ and Ⅱ still maintain n-type Schottky contact with 2H MoS2 and their conduction band alignment is 0.53 eV, 0.43 eV and 0.64 eV, respectively. However, for 2H0.51T0.5 Ⅲ and 2H0.251T0.75 forming heterostructures with MoS2, we note their valence band alignment is 0.5 eV and 0.46 eV, respectively, which is smaller than the conduction band alignment, showing p-type Schottky contact. Here, we also evaluate the band alignment between 2H MoS2 and typical Au metal electrode with the value 0.8 eV, which is larger than that of the hybrid systems. This indicates the hybrid electrode may be superior to metal ones. In the following part, we turn to investigate the electronic transport properties using it as electrode. In the experiments, the FET device based on 1T/2H/1T MoS2 nanosheet performs decreased contact resistance compared with Au/2H/Au device. We note the concentration of the 1T phase in the converted region is only 60–70%, besides, the specific stitch between the 1T phase and 2H phase MoS2 is also unknown. Therefore, it is significant to clarify the nature of 2H MoS2 device using impure 1T MoS2 as electrode with different concentrations. Here, a two-probe system, as shown in Fig. 6(a), including three part, left electrode, right electrode and the central scattering region, was employed to simulate the electronic transport properties (all transport models are shown in Supporting information Section 2). The calculated transmission spectra of all configurations at zero bias are shown in Fig. 6(b). Not surprisingly, the transmission probabilities depend strongly on the concentration of the 1T MoS2 and the specific arrangement of 2H/1T hybrid structure. A zero transmission region can be clearly seen around the Fermi level of pristine 2H MoS2, and beyond this region, the transmission probabilities are produced by the available conductance channels of various energy bands. Noteworthy, in contrast to the pristine 2H MoS2, the transmission probabilities of 2H0.751T0.25 are severely suppressed in both valence and conduction band regions. This reduced transmission ability will lead to the reduction of passing current. As for the 1T concentration of 0.5, we notice that new transmission signals in the region about −1.0~ −0.6 eV arise in 2H0.51T0.5Ⅰand 2H0.51T0.5Ⅱ, which decreases the zero transmission gap and increases the electron transport channels. However, the transmission probabilities of 2H0.51T0.5 Ⅲ
Fig. 5. Band alignment between 2H MoS2 and hybrid electrode.
Fig. 6. (a) Top view (upper panel) and side view (lower panel) of the schematic model of two-probe system. (b) Zero-bias transmission spectra of 2H1−x1Tx MoS2 hybrid systems with different 1T concentrations.
Table 1 Binding energy (Eb) and charge transfer (ΔQ(e)) for O2 adsorption and BN encapsulation on 2H1−x1Tx MoS2 hybrid systems. A positive (negative) sign in ΔQ corresponds to electron transfer from the substrate (adsorbate) to the adsorbate (substrate).
O2 BN
Eb(eV) ΔQ(e) Eb(eV) ΔQ(e)
2H
2H0.751T0.25
2H0.51T0.5 Ⅰ
2H0.51T0.5 Ⅱ
2H0.51T0.5 Ⅲ
2H0.751T0.25
1T
−0.109 0.0604 −4.558 −0.0997
−0.150 0.128 −4.355 −0.1083
−0.124 0.124 −4.405 −0.0954
−0.139 0.157 −4.042 −0.107
−0.499 0.231 −4.013 −0.116
−0.129 0.107 −4.218 −0.1024
−0.068 0.271 −4.324 −0.1315
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Fig. 7. Electronic band structure and ELF map of (scale bar is from 0.1 to 0.9) of BN/2H1−x1Tx MoS2 systems. The band structures projected on two components are signed as green and red symbols, respectively.
been studied and the calculated binding energies are shown in Table 1. At initial, we obtain the most stable configurations of O2 molecule adsorbed on pure 2H and 1T MoS2 with binding energies of −0.109 eV and −0.06 eV, respectively (more details see supporting information Section 3). Then, we randomly place the O2 molecule on the phase boundaries of the hybrid structures. Enlarged binding energies of O2 molecule adsorbed on the hybrid structure surface are observed compared with that on pure 2H and 1T MoS2. Especially the case of O2 binding with 2H0.51T0.5 Ⅲ MoS2 surface, which possesses the largest binding energy of −0.499 eV, is expected to be easily influenced and not survival in the ambient conditions. In addition, Bader analysis indicates the charge transfer from the substrate to the adsorbate. Since the environment can affect the hybrid structure 2H1−x1Tx
is largely impeded probably due to the existence of edge localized states hindering the electrons transport. The degraded transmission ability is also found in 2H0.251T0.75. As for pure 1T electrode case, the transmission ability is slightly decreased due to the occurrence of the phase boundaries as electron scattering centers. It should be pointed that the effect of contact with metal is not involved in our simulations. The pure 1T phase MoS2 is metallic, which have little contact resistance with metal electrode than that of 2H MoS2. That is, the conversion to the pure 1T phase may greatly enhance the performance of device. Since the hybrid structure 2H1−x1Tx MoS2 plays an important role in the electronic transport of the FET device, it is necessary to check its chemical active in the ambient environment for practical applications. Adsorption of oxygen molecule on the hybrid structure surface has 183
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Center in Tianjin-TianHe-1(A) and the Norwegian Programme for Supercomputing.
MoS2, it is necessary to search an encapsulation material for electronics applications of the hybrid structure. Here, thinner 2D BN nanosheet is selected as encapsulating material, since it has been proved to be a good protecting material for other 2D materials such as graphene [39] and phosphorene [40]. Our calculated results have indeed verified that BN can form a weak binding with 2H1−x1Tx MoS2. In the optimized structures of BN/2H1−x1Tx MoS2 systems (Supporting information Section 4), it is clear to see that BN layers in some cases such as BN/ 2H0.51T0.5 Ⅲ and BN/2H0.251T0.75 become fluctuant, in order to avoid strong interaction with hybrid structure surface. The BN covered pristine 2H MoS2, possesses a binding energy of −4.558 eV per supercell (about 0.142 eV per Mo atom) (Table 1), which is larger than that covered the hybrid structures with the binding energy −4.013 eV~ −4.405 eV (about 0.125–0.137 eV per Mo atom). Therefore, interactions between BN and the hybrid structures are very weak. The band structures of BN/2H1−x1Tx MoS2 systems (Fig. 7) are fully consistent with the above weak interaction picture. The projected band structures individually on BN and 2H1−x1Tx MoS2 indicate that the band structures of two systems is almost a rigid combination. Due to the large band gap of BN layer, there are almost no BN states hybridized with 2H1−x1Tx MoS2 states around the Fermi level. The band structure of 2H1−x1Tx MoS2 preserves well before and after encapsulated with BN layer, also indicating little charge transfer between two components. Bader analysis shows a charge transfer from BN to 2H1−x1Tx MoS2 surface only ~0.003 electron per Mo atom (Table 1). Electron-localization function (ELF) [41] maps also support that 2H1−x1Tx MoS2 systems encounter negligible interaction with BN sheets. Therefore, BN encapsulation does not affect the electronic properties of 2H1−x1Tx MoS2 hybrid structure, and the transport properties will be well kept in BN/2H1−x1Tx MoS2.
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4. Conclusions In summary, we have systematically investigated the electronic and transport properties of 2H1−x1Tx MoS2 hybrid structure. It is found that the band gaps of the hybrid systems are decreased compared with pure 2H MoS2. The localized states around the Femi level arise accompany with introducing 1T phase MoS2 to 2H phase, which will not contribute to the electronic transport. The band alignment calculations indicate the hybrid electrode may be superior to metal ones. The calculated transmission spectra show that the transmission probabilities not only depend on the concentration of 1T phase, but also rely on the specific arrangement of the two phases. Binding energies of O2 covered on hybrid structure surfaces are larger than those on pure 2H and 1T MoS2. Therefore, in order to make it realize practical application, protection material is needed. BN nanosheet has been found weak bind with the 2H1−x1Tx MoS2 hybrid structures and the states of each configuration of 2H1−x1Tx MoS2 hybrid systems remain intact, which indicate BN is a good protecting material. Our calculated results are expected to be useful for designing practical electronic devices based on MoS2 materials at realistic nanometre scale. Acknowledgements We acknowledge the National Natural Science Foundation of China (Grant nos. 21573129 and 21403300), the National Nature Science Foundation of Shandong Province (Grant no. ZR2015BQ001), and the General Financial Grant from the China Postdoctoral Science Foundation (Grant no. 2013M531595). The authors also acknowledge a generous grant of computer time from the National Supercomputer
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