Tuning the Electrical Transport Properties of Multilayered Molybdenum

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Tuning the Electrical Transport Properties of Multi-Layered Molybdenum Disulfide Nanosheets by Intercalating Phosphorus Lijuan Ye, Shijian Chen, Wanjun Li, Mingyu Pi, Tianli Wu, and Dingke Zhang J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 07 Apr 2015 Downloaded from http://pubs.acs.org on April 7, 2015

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The Journal of Physical Chemistry

Tuning the Electrical Transport Properties of Multi-layered Molybdenum Disulfide Nanosheets by Intercalating Phosphorus Lijuan Ye,1 Shijian Chen,1, * Wanjun Li,1 Mingyu Pi,1 Tianli Wu,1 and Dingke Zhang2 1

School of Physics, Chongqing University, Shapingba, Chongqing 401331, China

2

College of Physics and Electronic Engineering, Chongqing Normal University, Shapingba, Chongqing 401331, China

*

Corresponding author: Dr. Shijian CHEN, Tel.: +86 23 65678362. E-mail: [email protected]

1

ACS Paragon Plus Environment


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Abstract We report on the demonstration of tuning the electrical transport properties of MoS2 nanosheets by intercalating phosphorus (P). The P doped MoS2 nanosheets were synthesized by a facile hydrothermal method. The structures and electrical properties of P doped MoS2 nanosheets were systematically investigated by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectrometer, transmission electron microscopy, Raman spectra, adsorption spectra and Hall measurements. The results indicate that the stacking of (002) plane in multi-layered MoS2 nanosheets is inhibited and the interlayer spacing is enlarged with introduce of P atoms. Both experimental results and theoretical calculations indicate that P atoms are much easier to intercalate into the interlayers of MoS2, compared with substitution of Mo and S, which significantly affects the vibrational modes of Raman spectra. Furthermore, due to the extra electrons introduced by intercalating P atoms, the conductivity of MoS2 could be gradually modulated from p type to n-type by increasing the content of intercalated P. This demonstration of tuning the electrical transport properties of MoS2 could help design the electrical and opto-electronic devices based on layered metal dichalcogenides.

Keywords: P-intercalated MoS2, Electrical transport properties, First principle calculations, hydrothermal method

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1. Introduction Similar to graphene, two-dimensional (2D) nanomaterials such as transition metal dichalcogenides (TMD) have attracted considerable attention due to their unique physical, chemical, and structural properties as well as their great potential for applications.1-3 Among these TMD materials, 2D layered molybdenum disulfide (MoS2) is a newly popular nanomaterial. Structurally, the bulk MoS2 can be regarded as the order stacking of sandwich-like 2D S-Mo-S layers. Within the layers, Mo and S atoms form hexagonal lattices in separate planes by strongly ionic covalent bond, with each Mo atom coordinated by six nearest-neighboring S atoms in the trigonal prismatic geometry.4 Each S-Mo-S layer loosely couples to one another by Van der Waals interactions, resulting in the stacking of S-Mo-S layers along the c axis. The weak interlayer interactions allow MoS2 to undergo a transition from bulk to 2D configuration, accompanying with the evolution of fundamental properties. For instance, the field-effect transistor based on single-layer MoS2 exhibits a high channel mobility (approximately 200 cm2V−1s−1) and current on/off ratio (1×108).5 The multilayers MoS2 based phototransistors exhibit a broad spectral response from ultraviolet to infrared.6 Strong photoluminescence could be detected in single- and few-layered MoS2 nanosheets.7,

8

Meanwhile, the remarkable electronic properties

support MoS2 nanomaterials to be used in field of sensors

9, 11

, and photocatalyst for

hydrogen production 12, 13 and degradation of pollutants 14 etc. As well-known, moreover, doping is an effective approach for tuning the band structures and electrical properties of materials. Therefore, the electrical properties of 3



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MoS2 may then be further tuned by doping strategies. Although many relevant MoS 2 characteristics have been determined, a little is known about the effects of doping categories on electrical characteristics in MoS2 system. Kapildeb Dolui etc.15 employed density functional theory to systematically study the electronic properties of doped MoS2 monolayers, where a variety of dopants are incorporated via S or Mo substitution or as adsorbates. It is found that Nb is identified as a suitable p-type dopant, while Re is the donor with the lowest activation energy. Relevant experimental reports have exemplified this conclusion and investigated the corresponding properties.16-21 S substitution with non-metals (F, Cl, Br, I) and Mo substitution with transition metals (Ru, Rh, Pd, Ag and Cd) create deep donor levels inside the band-gap of the MoS2 monolayer so that the n-type doping does not possibly happen.15 The simulation of doping by adatoms in monolayer MoS2 suggests 1st (Li, Na, and K) column atoms are potentially effective donor dopants, while the 7 th (F, Cl, Br) column atoms appear not to be effective dopants due to the deep level traps.22 The adsorption of molecular ions can lead to both n- and p-type conductivity but depends on the charge polarity of the adsorbed species.15 The above findings are all the results of theoretical simulating calculations. Rare experimental work has been reported. In addition, when phosphorus (P), a group V element, is used to replace S atoms, P could shift the Fermi level energy into the MoS2 valence band, making the system p type.15 It is well known that P element owns several valence states, including positive and negative valence states. Thus, as a dopant, P may substitute the Mo or S sites. Furthermore, due the weak interlayer interactions, the P atoms will probably be 4


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introduced between the layers of MoS2 through intercalating. From this view, it is important to know how the P dopant behaves in the MoS2 and how the impurity P affects the electrical properties of MoS2. To our best knowledge, however, such relevant works have not been reported in the literature up to now. In this paper, we first report the one-step direct synthesis of P doped MoS2 nanosheets with a facile hydrothermal approach and discuss the effects of P doping on the structures, optical, and electrical properties of MoS2. Combining theoretical calculations and experimental results, we demonstrate that the electrical transport properties of MoS2 nanosheets can be modulated by intercalating P.

2. EXPERIMENTAL AND THEORETICAL METHODS 2.1 Experimental Methods and Characterization Techniques. Our growth process for phosphorus-doped MoS2 is based on a facile hydrothermal method: The mixtures of ammonium molybdate ((NH4)6Mo7O24·4H2O, 1.2 g), elemental sulfur (S, 0.435 g) powders, nontoxic red phosphorus (P) powders with various nominal mole ratios of P/S (0, 8, 16 and 24%) and 12 ml hydrazine monohydrate (N2H4·H2O, 85%) were added into the four containers, respectively. These containers were filled with 20 ml distilled water to 70% of the total volume and sonicated for 30 minutes before they were transferred into the stainless steel autoclaves and maintained at 180 °C in an air oven for 50 h. After the hydrothermal reaction was completed, the autoclaves were cooled to room temperature naturally. The final reaction products were centrifuged, and repeatedly washed with distilled 5



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water, diluted hydrochloric acid (HCl) and ethanol to remove the residual ions. The black precipitates were collected and dried in the vacuum at 60 °C for 30 h, named as P0, P8, P16 and P24 for convenience, respectively. The crystal structures of the samples were obtained by PANalytical X’Pert Powder diffractometer with Cu Kα radiation (1.54 Å). Diffraction angle (2-theta) ranges from 10° to 80° and the scanning step is 0.026°. The field emission scanning electron microscopy (FE-SEM, JEOL JSM-7800F) with an energy dispersive X-ray spectrometer (EDS) and a transmission electron microscopy (TEM, FEI Tecnai G2 20) was employed for the morphology, size, and structure observation of samples. The adsorption spectra of samples were acquired by UV-Vis-NIR spectrophotometer (UV-4100). Raman spectra were acquired in the back scattering geometry using a JY-HR800 Raman spectrometer by exciting the samples with 532 nm solid-state laser. The laser power was reduced to few milliwatts. The electrical properties were obtained by the Van der Pauw configuration in a Hall-effect measurement system (Ecopia HMS-3000) at room temperature (RT). The powder samples were first pressed into pellets with ~ 0.25 mm thickness and 1cm2 in area and Indium/Gallium alloy was used for electrode by point contact.

2.2 Calculation Model and Methods. We employed density functional theory (DFT) using the Vienna Ab-initio Simulation package (VASP) to investigate the atomic and electronic structures of MoS2.23 In the simulations, electron exchange and correlation were described using the generalized gradient approximation (GGA) of the Perdew-Burke-Ernzerhof (PBE) form.24 Core and valence electrons were 6


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treated using the projector-augmented wave potentials supplied by VASP. The simulations for bulk MoS2 substituted and intercalated by P atom were performed within the supercell approach. The supercell of MoS2 crystalline is consisted of 4 × 4× 3 MoS2 unit cells. One S or Mo atom is replaced by a P atom in the cases of substituted MoS2, while a P atom is put between two S-Mo-S layers in the case of intercalated MoS2.

3. RESULTS AND DISCUSSION

Figure 1. XRD patterns of undoped MoS2 (P0) and P-doped MoS2 (P8, P16 and P24) samples. The (002) peak becomes weak and broad with the increase of P content, suggesting the dopant P suppresses the stacking of S-Mo-S layers effectively.

First, X-ray diffraction (XRD) measurements were used to investigate the structural properties of the sample P0, P8, P16 and P24. As shown in Figure 1, the XRD 7



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patterns only show three typical diffraction peaks located at 2θ = 13.94°, 32.75° and 57.27° due to the poor crystal quality from hydrothermal method, which corresponding to (002), (100) and (110) planes of 2H-MoS2 (JCPDS card No.37-1492, a = 3.16 Å, c = 12.29 Å), respectively. It is known that (002) peak is corresponding to the c-plane of MoS2 and can reflect the ordered stacking of S-Mo-S sandwich layers. As observed in other reports, a little low angle shift happens to the (002) peak of sample P0, which is attributed to the lattice expansion when the crystal defects or strains exist owing to curvature of the layers.25-29 The relative strong (002) peak for sample P0 indicates relatively well-stacked layered structure for undoped MoS2. However, the (002) peak becomes weak and broad with the increase of P content, suggesting that the crystal quality is degenerated and the stacking of S-Mo-S sandwich layers suffers destruction. The similar variation of (002) peak has also appeared in MoS2/C system 30, which raises from C materials inserted into the MoS2 interlayers to form a new sandwich structure. One the other hand, the (100) and (110) peaks, which show obviously asymmetric due to the random displacement of S-Mo-S layers with respect to one another, correspond the in-plane crystalline quality.31, 32 Compared with (002) peak, these two peaks change little in intensity with the increase of P content, which indicates that P dopants do barely affect the in-plane crystalline of MoS2. The above results suggest that P atoms may have intercalated into the interlayers of MoS2, rather than incorporated into crystal lattice sites of MoS2, which effectively inhibits the ordered growth of MoS2 crystals along the c-axis during the hydrothermal growth. It should be noted that any peaks related to impurities were not 8


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observed, indicating that all the samples are pure MoS2 phase.

Figure 2. The HRTEM images of sample P0 (a), P8 (b) and P24 (c). The scale bar is 5 nm. The inset is the corresponding LRTEM images with scale bar of 100 nm. (d) EDX spectra of P-doped MoS2. Since the P intercalating destroys the ordered stacking of (002) planes in MoS 2, the interlayer spacing of S-Mo-S layers may be affected. In order to further understand the effect of P intercalating on the structural properties and morphologies of MoS2, the high resolution transmission electron microscopy (HRTEM) images were measured, as shown in Figure 2. The low resolution transmission electron microscopy (LRTEM) images in the insets show that the samples are all composed of wrinkled and uneven thin sheets. Sample P24 shows smaller sheet-like structure, compared with the sample P0. Lattice fringes observed clearly in the HRTEM images suggest that the nanosheets of all the samples are mostly composed of multiple 9



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S-Mo-S layers. For sample P0, the interlayer spacing of (002) plane is homogeneous and the average value is about 0.66 nm, which is in good agreement with the reported value

33

. For the sample P8, the interlayer spacing is still homogeneous and without

obvious variation, even though P element is detected in the energy dispersive X-ray (EDX) spectra (Figure 2d). For the sample P24, however, the interlayer spacing of (002) plane is obviously enlarged. The large variation of interlayer spacing of (002) might be attributed to the intercalated P atoms into the MoS2 interlayers, which is consistent with the above XRD results. Although impurity peaks that arise from elementary substance P, phosphorus oxides or other phosphorus compounds have not been observed in the XRD patterns, P element has been detected in the EDX spectra, which implies that P has been doped into MoS2 during hydrothermal processing. However, only 1.99, 2.49 and 3.20 at.% P is detected in sample P8, P16 and P24, respectively, which corresponds 2.03, 2.55 and 3.30 at.% of ratios of P/MoS2, indicating that the effective doping level is very low. Raman spectroscopy was also utilized to investigate the effects of P intercalating 1 on the structural properties of MoS2. From Figure 3a, two typical Raman peaks (𝐸2𝑔

and 𝐴1𝑔 ) of MoS2 are observed around 378 and 402 cm-1, respectively. As well 1 known, the 𝐸2𝑔 mode is the in-plane vibration due to the relative intra-plane motion

of the Mo and two S atoms of S-Mo-S layer in the MoS2, while the 𝐴1𝑔 mode is the out-of-plane vibration due to the relative inter-plane motion of the two S atoms. Usually, Raman spectroscopy can be used to quantitatively identify the layers of 1 MoS2 by the frequency difference between 𝐴1𝑔 and 𝐸2𝑔 peaks.34, 35 However, it is

10


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only suitable when the layer is less than or equal to four layers (24.3 cm-1 of frequency difference).36 In our experiment, the frequency differences between A1g and E12g peaks of all the samples are evaluated among 24.3 to 26.0 cm-1, which just indicates the layer number is more than four and can be classified as multilayers, as observed in the TEM graphs. It should be noted that the intensities of two modes show much different dependence on P intercalating content. It can be seen that the 1 intensity of 𝐴1𝑔 peak decreases dramatically and 𝐸2𝑔 peak, on the contrary, shows

increase of intensity with the increasing P intercalating content. The relative ratio of 1 E2g and A1g peak intensities becomes larger with the increasing P intercalating

content, and which can be used as an indicator of doping levels, as reported in the recent literature 37. 1 Furthermore, the 𝐸2𝑔 peak of P-doped MoS2 shows apparent asymmetric

distribution. A closer look in Figure 3b reveals such asymmetric peak can be split into two peaks at around 370 and 378 cm-1, respectively. The peak at 378 cm-1 should be 1 assigned to 𝐸2𝑔 mode since it locates at the same wavenumber for all the samples.

However, the peak located at 370 cm-1 is absent in pure MoS2 sample and increases in intensity significantly with the increase of P content. Therefore, this newly emerged mode should be related to the dopant P (we refer it as E P ).

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1 Figure 3. (a) Typical Raman spectra of samples. (b) Peak fitting on asymmetric 𝐸2𝑔

peak of samples. The red and green lines are the fitting curves. After P intercalating, the newly fitted peak around 370 cm-1 is denoted by 𝐸 𝑃 . (c) The overlapped atom structures of MoS2 before and after P intercalating. This configuration in which P atom locates below the Mo-ring of Mo atomic layer has the lowest energy and is most stable. The red ball represents the dopant P atom. The green and blue balls represent 12


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the S and Mo atoms after P intercalating, respectively.

To further understand the doping behavior of P atom in the MoS2, we performed first-principle density functional (DFT) calculations

38, 39

. The formation energies and

atomic structures for three types (i.e. intercalated P (Pi), substituted Mo (PMo) and substituted S (PS)) of P doping in MoS2 were calculated. First, we calculated the formation energy of P dopant in MoS2, which is defined by 𝐸𝑓 = 𝐸Tot − 𝐸Mo𝑆2 − ∑𝑖 𝑛𝑖 𝜇𝑖

(1)

where 𝐸Tot and 𝐸Mo𝑆2 are the total energies of the supercell with and without the impurity, respectively. 𝑛𝑖 and 𝜇𝑖 are the number and the chemical potential of the atoms added to (positive 𝑛𝑖 ), or taken from (negative 𝑛𝑖 ) the bulk reference supercell in order to create the defect, respectively. Here, 𝜇𝑃 , 𝜇Mo , and 𝜇𝑆 are defined as the total energy per atom in the bulk red phosphorus (P), bulk Mo, and bulk S, respectively. Our calculation results show that the formation energies of Pi, PMo, and PS defects are 0.20, 0.48, and 1.24 eV, respectively. Compared with PMo and PS defects, the Pi defects possess the lowest formation energy, which implies that P atoms are much easier to intercalate into the interlayers of MoS2, rather than substitute Mo or S atoms. Combining with experimental results and theoretical calculations, it can be concluded that P atoms have mainly intercalated into the interlayers of MoS 2 in this experiment. Meanwhile, the intercalating of dopant P in MoS2 causes the structural distortion locally (as shown in Figure 3c), and consequently results in the changes of vibrational 1 modes in Raman spectra (both E2g and A1g modes) with P intercalating. One can

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see that an up/down movement happens to both S and Mo atoms near the intercalated P atom after fully relaxation. This movement enlarges the interlayer spacing of MoS 2 locally and changes the in-plane S-Mo bonds length from 2.40 Å to 2.42 Å, which, consequently, would have great effect on the vibrational behaviors in Raman spectra. One the one hand, naturally, the intercalated P atoms could inhibit the out-of-plane vibrating (𝐴1𝑔 mode) of S-Mo-S layers, leading to the degeneration of 𝐴1𝑔 peak, which is well manifested in Raman spectra, as shown in Figure 3a. The primary cause is attributed to the strong electron-phonon coupling of 𝐴1𝑔 mode due to its sensitivity to electron doping. The fact that 𝐴1𝑔 peak broadens significantly with electron doping has been observed by the Raman spectroscopy of field-effect-doped single-layer MoS2

40

, and confirmed with the first principles DFT calculations and

group symmetry theoretical arguments

40, 41

. On the other hand, the S-Mo bonds near

1 intercalated P atoms are lengthened and, correspondingly, the local 𝐸2𝑔 mode would

shift to the lower energy side. It is worth to note that this shift only happens to those 1 1 𝐸2𝑔 modes near the intercalated P atoms and the rest of 𝐸2𝑔 vibrational modes far 1 away from P atoms keep unchanged. Therefore, the 𝐸2𝑔 peaks in the Raman spectra 1 of P-doped MoS2 are composed of two peaks. One is the pristine 𝐸2𝑔 mode, and the 1 other one, accompanying with P dopant, is the red-shifted 𝐸2𝑔 (i.e.E P ) mode caused

by the intercalated P atoms. Apparently, the E P mode would enhance in intensity with the increase of P content, as observed in Raman spectra. Our argumentations based on the intercalated P atom in MoS2 successfully interpret the Raman spectra results. 14


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Figure 4a shows the optical absorption spectra of samples at room temperature. As seen, all the samples exhibit broad absorption behavior in the visible region and an absorption threshold around 780 nm (corresponding to the smallest direct transition at the point K of the Brillouin zone42). On the high energy site of the absorption threshold, four typical convoluted exciton peaks A and B, C and D of MoS2 can be observed in 700~600 nm and 500~400 nm, respectively, corresponding to the optical transitions between d-orbitals (non-bonding states of MoS2).43 For undoped MoS2 sample, A- and B-exciton peaks, separately located at 680 and 620 nm, correspond to the band edge excitons at the K and 𝐾 ′ points of the Brillouin zone.44 The D and C transitions are the interband transitions from the occupied 𝑑𝑧 2 orbit to unoccupied 𝑑𝑥𝑧,𝑦𝑧 and 𝑑𝑥𝑦,𝑥 2 −𝑦 2 orbits split by spin–orbit coupling.45 For P intercalated MoS2 samples, the intensities of these four exciton peaks present obvious change, suggesting that P intercalating could effect on the optical transitions between d-orbitals of MoS2. An additional exciton peak (~575 nm) is observed in all P intercalated MoS2. We speculate that such peak may arise from the transition from the valance band maximum to some new bands or to some higher conduction bands of MoS2 introduced by intercalated P atoms.

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Figure 4. Absorption spectra (a) and the (𝑎ℎ𝑣)1⁄2 ~ ℎ𝑣 curves (b) of sample P0, P8, P16 and P24. The evaluated band gap energy of samples is plotted in the inset. Four typical excitonic peaks A and B, C and D of MoS2 are observed in the visible region. P-intercalating introduces new adsorption peak located around 575 nm.

The band gap energy (𝐸𝑔 ) is evaluated from a general relation: 𝑎ℎ𝑣 = B(ℎ𝑣 − 𝐸𝑔 )𝑛

(1)

where B is the band edge constant, ℎ𝑣 is the incident photon energy, and the exponent n depends on the kind of optical transition. Based on the TEM analysis, the prepared MoS2 in this work is multilayer structure, which should belong to indirect band gap semiconductor. Therefore, the value of n is taken 2 in the equation (1). Figure 3b shows the (𝑎ℎ𝑣)1⁄2 of MoS2 as a function of the photon energy, and the evaluated 𝐸𝑔 values are depicted in the inset. As seen from the Figure, the evaluated 𝐸𝑔 of MoS2 is about 1.57 eV, which is between 1.20 eV (bulk) and 1.90 eV 16


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(monolayer) 46. This indicates that the prepared MoS2 in this work are all consisted of multiple stacked S-Mo-S structures, as the above TEM images shown. It is well known that doping intentionally introduces impurities into pure semiconductor and is widely used to modulate the band gap of semiconductor. This modulation of band gap, however, is highly dependent on the element types, concentrations and atomic configurations of dopants in the semiconductor. In our case of P-intercalated MoS2, the band gap only changes very little (from 1.576 to 1.567 eV) after P intercalating. The little change of band gap is reasonable since the interaction between S-Mo-S layers is the weak Van der Waals force and the intercalated P atoms would have not much effect on the electronic band structures of MoS2. An impurity band, however, is created in the band gap due to the intercalated P, which induces a band tail absorption in the low energy site of spectra and would significantly affect the electric conductivity of MoS2 system.

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Figure 5. (a) The calculated band structures of undoped MoS2 (black lines) and P-intercalated MoS2 (red lines). The black dashed line indicates the Fermi energy. (b) The density of states (DOS) of undoped MoS2 and P-intercalated MoS2. Note that the DOS are all aligned to have a common Fermi level, EF= 0.

The band structures and density of states (DOS) of undoped and P-intercalated MoS2 were calculated and shown in the Figures 5a and 5b, respectively. Obviously, an impurity band appears near the Fermi level (EF) in the band structure after P intercalating, which can be also observed in the corresponding DOS and mainly attributed to the intercalated P atom. Importantly, the EF of undoped MoS2 is close to 18


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the valence band, but after P intercalating, the EF shifts closely to the conduction band, resulting in n type conductivity. This should be attributed to the extra electrons introduced into MoS2 system by intercalating P atoms, which is directly manifested in the charge density difference between P-intercalated MoS2 and undoped MoS2 in Figure 6. One can see that electrons transfer from P to the nearby S atoms after P intercalating. By performing the Bader analysis, it is estimated that about 0.43 e- have been transferred and 4.57 e- have been remained. Those transferred electrons are loosely bounded on S atoms (as shown in the DOS of S atom in Figure 5b) and can be easily excited into conduction bands to participate in the conduction. Therefore, the intercalated P can theoretically be used to tune the conductivity of MoS2.

Figure 6. The electronic charge density difference profiles of P-intercalated MoS2. The purple, yellow and red balls represent the Mo, S and P atoms, respectively. The green color donates the isodensity of magnitude of 2.75 E-3 Å-3.

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Last, we measured the electrical transport properties of samples to test our theoretical predictions. The electrical transport measurements were carried out with the Van der Pauw configuration in a Hall-effect measurement system. As shown in Table 1, the undoped MoS2 (P0) shows p-type conduction behavior (hole concentration: 9.537×1018 cm-3) in this work. With respect to the n or p type conductivity of pure MoS2, there are no unifying views on the origins up to now. However, the previous studies found that the conduction type of undoped MoS 2 depends on the experimental details.5,

15, 47

A very recent report has declared the

undoped MoS2 could exhibit p-type conductivity in the sulfur-rich condition in the process of preparation.48 Thus, the stoichiometric ratio of S/Mo should affect the conductivity of undoped MoS2. To verify this view, the undoped MoS2 samples have been intendedly synthesized with nonstoichiometric ratio of S/Mo. The results were shown in Table 1 and the samples with rich Mo exhibit n type, otherwise, exhibit p type. Experimentally, the stoichiometric ratio of S/Mo was proved to influence the conduction types of MoS2. In addition, it can be seen that P intercalating has great effect on the electrical transport properties of MoS2. With the introduction of dopant P, the p type conductivity of MoS2 becomes weak (see sample P8), and the conduction type exhibits a conversion from p-type to n-type with the increase of P intercalating content. It can be seen the n-type conductivity can be improved with further increase of P intercalating content. Obviously, the dominant contribution of intercalated P atom is to provide extra electrons to MoS2 system, which is consistent with the above 20


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calculated results. Precisely because of the introduction of the intercalated P atoms, the conductivity of MoS2 is gradually modulated in the conversion from p type to n-type. Therefore, P-intercalating can be also used to intentionally realize the n-type conductivity of MoS2, except Re-doping, which suggests that such P-doping strategy could be used to tune the electrical properties of MoS2 and help to design the electrical and opto-electronic devices based on layered metal dichalcogenides.

Table 1. Electrical Properties of MoS2 Samples with Various Nominal P Intercalating Contents (0%, 8%, 16%, 24%) and with Various Nonstoichiometric Ratios of Sulfur Powders and Molybdate (2.2, 2, 1.8 and 1.6) Samples

Nominal mole ratios of S/Mo

Carrier concentration (cm-3)

Resistivity (Ω.cm)

Mobility (cm2V-1s-1)

Conduction types

P0 P8 P16 P24

2.0 2.0 2.0 2.0

9.537×1018 1.665×1017 -4.380 ×1016 -1.853×1018

4.628×100 3.117×103 3.023×102 2.374×101

1.41×10-1 1.20×10-2 4.71×10-1 1.42×10-1

p p n n

P0-1 P0-2 P0 P0-3

1.6 1.8 2.0 2.2

-1.290×1019 -1.136×1019 9.537×1018 3.663 ×1019

3.004×100 1.925×100 4.628×100 5.517×100

1.83×10-1 2.51×10-1 1.41×10-1 3.09×10-2

n n p p

4. CONCLUSIONS In conclusion, P-intercalated MoS2 were successfully synthesized by a facial hydrothermal route. XRD and TEM indicate that as-synthesized MoS2 are multilayer nanosheets, but the stacking of MoS2 (002) plane is inhibited and its interlayer spacing is enlarged with the introduction of P atoms. Both experimental results and theoretical calculations demonstrate that dopant P atoms trend to occupy the space of interlayers of MoS2 instead of lattice sites. Consequently, the intercalated P causes the 21



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local change of atomic structure of MoS2, where significantly affects the vibrational modes of Raman spectra. In addition, the DFT calculations predicate that the dominant contribution of intercalated P atoms is to provide extra electrons to MoS2 system, which is tested by the Hall measurements. Lastly, the conductivity of MoS2 can be gradually modulated in the conversion from p type to n-type by intercalating P atoms. Therefore, such P-doping strategy can be used to tune the electrical transport properties of MoS2 system, which could help in designing the electrical and opto-electronic devices based on layered metal dichalcogenides.

Acknowledgements This work is supported by the National Natural Science Foundation of China (NSFC) (grants 11304406 and 61307035).

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Tuning the Electrical Transport Properties of Multilayered Molybdenum

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