Realizing p-Type MoS2 with Enhanced Thermoelectric Performance

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Realizing p-type MoS with Enhanced Thermoelectric Performance by Embedding VMoS Nanoinclusions 2

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Shuang Kong, Tianmin Wu, Wei Zhuang, Peng Jiang, and Xinhe Bao J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b06379 • Publication Date (Web): 21 Aug 2017 Downloaded from http://pubs.acs.org on August 22, 2017

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The Journal of Physical Chemistry

Realizing p-type MoS2 with Enhanced Thermoelectric Performance by Embedding VMo2S4 Nanoinclusions Shuang Kong,‡ ‡

a,c

Tianmin Wu,‡ ‡b,d Wei Zhuang,*b Peng Jiang*a and Xinhe

Bao*a a

State Key Laboratory of Catalysis, CAS Center for Excellence in Nanoscience,

Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, Liaoning 116023, China b

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the

Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China c

University of Chinese Academy of Sciences, Beijing, 100049, China

d

Department of Chemical Physics, University of Science and Technology of China,

Hefei, Anhui 230026, China

Abstract Two-dimensional transition-metal dichalcogenide semiconductors (TMDCs) such as MoS2 are attracting increasing interest as thermoelectric materials owing to their abundance, nontoxicity and promising performance. Recently, we have successfully developed n-type MoS2 thermoelectric material via oxygen doping. Nevertheless, an efficient thermoelectric module requires both n-type and p-type materials with similar

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compatibility factors. Here, we present a facile approach to obtain a p-type MoS2 thermoelectric material with a maximum figure of merit of 0.18 through the introduction of VMo2S4 as a second phase by vanadium doping. VMo2S4 nanoinclusions, confirmed by X-ray powder diffraction (XRD) and transmission electron microscopy (TEM) measurements, not only improve the electrical conductivity by simultaneously increasing the carrier concentration and the mobility, but also result in the reduction of lattice thermal conductivity by enhancing the interface phonon scattering. Our studies not only shed a new light towards improving thermoelectric performance of TMDCs by a facile elemental doping strategy, but also pave the way towards thermoelectric devices based on TMDCs.

1. INTRODUCTION Thermoelectric materials, which can directly convert thermal energy to electrical energy, have been investigated over the past several decades for the growing energy demand. The conversion efficiency of a thermoelectric device is determined by the figure of merit (), = ( ⁄ ), where is the Seebeck coefficient, is the electrical conductivity, is the total thermal conductivity consisting of electron contribution ( ) and lattice contribution ( ), is the absolute temperature. is termed as the power factor ().1-3 The record of value has been constantly broken in recent decades, due to the development of novel concepts and mechanisms,

including

“phonon-glass/electron-crystal”,4

nanostructuring,5-7

electronic resonant doping8 and energy filtering effect.9-11 Besides high performance, the thermoelectric technology for large-scale application requires materials to be 2 ACS Paragon Plus Environment

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economic and environmental friendly. However, most of thermoelectric materials that possess excellent values contain highly toxic and rare elements,2,12 such as Bi, Pb and Te.5,13,14 Therefore, searching for novel thermoelectric materials based on nontoxic and abundant elements is of great interest. MoS2, a layered transition metal dichalcogenide, has attracted attention as a promising thermoelectric material. Theoretical studies have predicted the high value of MoS2 nanoribbons (=3), which can be essentially attributed to its high electrical conductivity due to the small gap caused by the strong edge reconstruction.15 The thermal conductivity of MoS2 along the cross-plane direction is 2.0±0.3 W m-1 K-1, comparable with that of the traditional thermoelectric materials.16 Furthermore, a large Seebeck coefficient (around 600 µV K-1) has been reported in bulk MoS2.17,18 Nevertheless, development of intrinsic MoS2 as a thermoelectric material is hampered by its small power factor due to the poor electrical conductivity. Therefore, a major challenge is to improve the electrical conductivity by breaking the trade-off between σ and . Recently, we have demonstrated that, by adopting a facile oxygen doping strategy, n-type MoS2 thermoelectric material can be obtained with greatly enhanced up to 50 times.19 Nevertheless, in order to make a thermoelectric module, both n-type and p-type thermoelectric legs are needed. Therefore, achieving p-type MoS2 thermoelectric material is imperative to facilitate their application as thermoelectric materials. Currently, modulation of carrier type of layered transition-metal dichalcogenide semiconductors (TMDCs) has be achieved by means of surface 3 ACS Paragon Plus Environment


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functionalization,20-22 electrostatic FET gating23 and elemental doping.24-29 Among these methods, elemental doping is the most promising and facile strategy for large-scale applications. Herein we demonstrate that p-type MoS2 thermoelectric material can be successfully obtained by doping vanadium (V), which has one less valence electron than molybdenum (Mo) and can create the acceptor states at the valence band maximum (VBM) of MoS2. We systematically investigate the influence of vanadium doping amount on the thermoelectric performance, and identify that the existence of a VMo2S4 phase enhances the electrical conductivity of MoS2 by increasing both the carrier concentration and mobility. Furthermore, the thermal conductivity is suppressed by the enhanced boundary scattering at the VMo2S4/MoS2 interface. As a result, the peak ZT reaches 0.18 at 1000 K. We further evaluate the anisotropic thermoelectric properties of textured samples with layered structures. 2. EXPERIMENTAL SECTION 2.1 Sample synthesis. We prepared a series MoS2-x%V samples with doping concentration x = 0, 1, 2, 5, 10 (mole percent). Briefly, the MoS2 powder (Sigma Aldrich) and V powder (Alfa Aesar) were weighted in specific ratios and grounded using an agate mortar for 30 min. Then about 8 g of the resulting powder mixture was loaded into graphite die and consolidated into dense pellets (the densities were higher than 95% of the theoretical one) using the Spark Plasma Sintering technique (Labox-650F, Japan). A pressure of 50 MPa was applied and the temperature was increased to 1573 K at a rate of about 60 ºC/min in vacuum with a dwelling time of 30 min. The pellets were approximately 4 ACS Paragon Plus Environment

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12.7 mm in diameter and 10 mm in thickness, allowing all the different characterization techniques along both directions to be carried out on the same pellet. To investigate the anisotropy of thermoelectric performance, we cut and polished the spark plasma sintering cylinder-shaped sample along both parallel (cross-plane direction, labelled as //) and perpendicular (in-plane direction, labelled as ⊥) to the pressing direction (Figure S1).

2.2 Characterization The phase structures of all samples were analyzed by X-ray diffraction (XRD), performed on a Rigaku D/Max-2500 diffractometer with a Cu Kα radiation source (λ = 1.5418 Å) at 40 KV and 200 mA at a scan rate of 5°/min. Microstructures for the bulk samples sintered by SPS were investigated by a QUANTA 200 FEG scanning electron microscope (SEM) at 20 KV. An energy dispersive X-ray spectrometer (EDX) integrated on SEM was used to investigate the composition and distribution of the elements. Transmission electron microscopy (TEM) was performed on a FET Tecnai F30 microscope at an accelerating voltage of 300 KV. The Hall carrier concentration and mobility was measured by a commercial instrument (HL5500PC) based on van der Pauw’s method with a magnetic field of 0.51 T. The thermoelectric properties of samples MoS2-x%V (x = 0, 1, 2, 5, 10) were measured within the temperature range of 300 K-1000 K. The Seebeck coefficient and electrical conductivity were measured simultaneously using an ULVAC-RIKO ZEM-3 system under partial helium pressure. The thermal conductivity is calculated according to the equation = . The density is estimated by the Archimedes method. The 5 ACS Paragon Plus Environment


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thermal diffusivity was measured by a flash diffusivity technique on Netzsch LFA 457 instrument. The specific heat capacity (Figure S2) was determined by Simultaneous Thermogravimetry-Differential Scanning Calorimetry (Netzsch STA 449). 2.3 Theoretical calculations First-principles calculations in this work were conducted using density functional theory (DFT) method implemented in the Vienna ab initio simulation package (VASP).30 The projector-augmented-wave (PAW) pseudopotentials and generalized gradient approximation of Perdew-Burke-Ernzerhof (PBE) for exchange and correlation are adopted in our simulations.31 The lattice vectors and atomic positions of VMo2S4 (space group: Cc) are fully optimized. The optimized lattice vectors of monoclinic VMo2S4 (a = 12.03 Å, b = 6.34 Å, c = 12.83 Å, and β = 114.5°) are well consistent with experimentally measured values (a ≈ 11.87 Å, b ≈ 6.49 Å, c ≈ 12.82 Å, and β ≈ 114.5°).32 A cutoff of 500 eV is used for the plane-wave expansion of the wave function to converge the relevant quantities. All atomic coordinates are relaxed until the forces on the atoms have declined to 0.001 eV Å−1, enforcing a total energy convergence criterion of 1×10−7 eV. For Brillouin zone (BZ) intergrations, a Monkhorst-Pack k-point mesh schemes with 7×11×7 is adopted for VMo2S4. 3. RESULTS AND DISCUSSIONS 3.1 Phase and microstructure X-ray powder diffraction (XRD) patterns of MoS2 samples with different vanadium doping amount are presented in Figure 1a and Figure 1b. For the pristine MoS2, all 6 ACS Paragon Plus Environment

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diffraction peaks well match with the record in the JCPDS card no. 37-1492, indicating that the sintered MoS2 sample has a single-phase hexagonal lattice structure. Comparing to the pristine MoS2, additional peaks at 15.2°, 16.3° and 17° appear in the XRD patterns for the V containing samples, which can be assigned to a VMo2S4 phase. The formation of VMo2S4 phase is due to the reaction between V and MoS2 matrix.33 It can be clearly observed that the peak intensity of VMo2S4 phase gradually increases with increasing V content, indicating that the addition of V results in the formation of VMo2S4. The existence of the VMo2S4 phase is further verified by high-resolution transmission electron microscopy (HR-TEM) images. Figure 1c shows the TEM images of MoS2-5%V sample, which exhibits the best thermoelectric performance. The left panel is the low-magnification image and the right panel is the corresponding high-magnification image of the yellow circle area. The lattice fringe spacing of about 0.52 nm fits well with the plane (101) of VMo2S4, which is consistent with the XRD results. The sheet-like morphology of MoS2 remains as the HR-TEM image indicates some folded edges exhibiting parallel lines corresponding to the different layers of MoS2. To check the possibility of substitutional vanadium, we refined the lattice parameters of all the samples from the XRD patterns. The values are listed in Table S1. The difference of the lattice parameters between all the samples is rather small and random, which excludes the picture of substitution of Mo by V and is consistent with the scenario of formation of VMo2S4 phase.



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Figure 1. X-ray powder diffraction (XRD) patterns of samples MoS2-x%V (x = 0, 1, 2, 5, 10) both in (a) parallel direction and (b) perpendicular direction. The blue filled circles indicate VMo2S4 phase. Right panels correspond to the enlarged view in the 14.5°-18° regions. (c) Transmission electron microscopy images of sample MoS2-5%V. Left panel is the low magnification image; right panel is the high magnification image of the yellow circle area.


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We further compared XRD patterns of both the planes parallel and perpendicular to the pressing direction. As shown in Figure 1a and Figure 1b, for the plane perpendicular to the pressing direction, (00l) peaks are strongly enhanced while other peaks are suppressed, indicating significant grain orientation anisotropy. The degree of (00l) preferred orientation termed as f was evaluated with Lotgering’s method.34 f is defined as:

= ,

( ) , ( )

= ∑

( )

= ∑ ( )

where and are the ratios of the relative intensities of all (00l) reflections to those of all (hkl) reflections for random and oriented samples, respectively. f=0 stands for non-oriented polycrystalline sample and f=1 stands for single crystal. The f values of MoS2-x%V (x = 0, 1, 2, 5, 10) obtained using the above equations are 0.80, 0.77, 0.74, 0.75 and 0.68, respectively, which suggests that the c-axis of sample grains is preferentially oriented parallel to the pressing direction. Consistent with the XRD results, the cross-section SEM images (Figure 2) also demonstrate that the MoS2 sheets are stacked almost layer-by-layer along the pressing direction. Similar oriented structures have been observed for other layered thermoelectric materials prepared by SPS method.35-38 Due to the textured structure, the thermoelectric performance will be expected to exhibit anisotropy, which will be discussed in detail as follows.



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Figure 2. SEM images of the sections (a) parallel and (b) perpendicular to the pressing direction for the SPS sintered MoS2-5%V sample.

3.2 Electrical properties

Figure 3. Temperature dependence of the electrical conductivity of MoS2-x%V (x = 0, 1, 2, 5, 10) samples both in the (a) parallel and (b) perpendicular directions.

The electrical conductivities of samples MoS2-x%V (x = 0, 1, 2, 5 10) between 300 K and 1000 K were shown in Figure 3a and Figure 3b. All samples show a


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semiconducting transport behavior as the electrical conductivity increases with increasing temperature. For the pristine MoS2, the electrical conductivity is poor (0.2 S m-1 parallel to the pressing direction and 1.7 S m-1 perpendicular to the pressing direction at room temperature), which is consistent with previous report.18 With V doping, electrical conductivity is improved dramatically. For example, the electrical conductivity along parallel direction at room temperature reaches to 6173 S m-1 with 10 % V doping.

Figure 4. (a) The crystal structure of VMo2S4. (b) The electronic band structure along the high symmetry point and density of state (DOS) of VMo2S4.

The improvement on the electrical conductivity is closely related to the formation of VMo2S4 phase. Unlike pristine MoS2, which is a semiconductor, VMo2S4 exhibit metallic properties, as shown in the electronic band structure from the first principle calculations (Figure 4). Therefore, incorporation of these metallic nanoinclusions greatly enhances the electrical conductivity of semiconducting MoS2 matrix. To further elucidate the modification of the electrical conductivity over doping, the carrier concentration and mobility of all the samples at room temperature were investigated (Figure 5). The carrier concentration of pristine MoS2 is rather low, only on the order of ~1016 to ~1017 cm-3. The introduction of V significantly increases the 11 ACS Paragon Plus Environment


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carrier concentration. For sample MoS2-5%V with the best thermoelectric performance, the carrier concentration rises to ~1019 cm-3, which is consistent with the fact that the carrier concentration of good thermoelectric materials is usually on the order of ~1019 to ~1021 cm-3. (Ref. 2) Interestingly, the carrier mobilities of V-doped samples increase (Figure 5) with doping. For the pristine MoS2, the mobility is very small (0.259 cm2 V-1 s-1 along the parallel direction and 0.877 cm2 V-1 s-1 along the perpendicular direction). For MoS2-1%V and MoS2-2%V samples, the mobilities increase to 1.68 cm2 V-1 s-1 and 2.29 cm2 V-1 s-1 along the parallel direction, while along the perpendicular direction the mobilities increase to 10.5 cm2 V-1 s-1 and 10.8 cm2 V-1 s-1. The improvement of mobility with doping contradicts with the traditional transport scenario, in which the carrier mobility usually decreases with doping due to the increased impurity scattering.39 Recently, similar abnormal phenomena have been observed in several other thermoelectric systems and different scenarios have been proposed to explain this elusive behavior, such as modulation doping,40,41 tuning the carrier scattering mechanism42 and improving the grain boundary connectivity by a second phase.43-47 In modulation doping picture, the charge carriers transfer from the doped grains to the un-doped grains, which enhances the mobility in comparison to uniform doping samples, but the mobility is still lower than the un-doped samples.40,41 Shuai et al.42 reported that, by co-doping with Te and Nb, the carrier scattering mechanism of Mg3Sb2-based materials can be tuned from ionization scattering to mixed scattering (between ionization and acoustic phonon scattering), which can improve the mobility 12 ACS Paragon Plus Environment

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from 19 cm2 V-1 s-1 to 77 cm2 V-1 s-1. It is worth noting that, in the Mg3Sb2 case, the carrier concentration decreases after doping, wheareas, in our V-doped MoS2 samples, the carrier concentration increases with doping. Therefore, the simultaneously increase of carrier concentration and mobility in our system cannot be explained by the above two scenarios. One reasonable explanation is the improved grain boundary electrical connectivity by a second phase with good conductivity and large mobility. For example, it was reported that AgSbTe2 and Bi0.4Sb1.6Te3 nanoinclusions can enhance the connection between the Yb0.2Co4Sb12 grains in the composite, which increased both the carrier concentration and the mobility.45,47 In our system, VMo2S4, a good electrical conductor (Figure 4), might act as the carrier conduction channel between grain boundaries of MoS2, which leads to the increase of both the mobility and the carrier concentration. With further addition of V (5% and 10%), the mobility starts to decrease possibly due to the significantly increased interface scattering between MoS2 and VMo2S4.44

Figure 5. Hall carrier concentration and mobility increase as a function of vanadium amount at 300 K along (a) parallel direction (b) and perpendicular direction.



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Figure 6a and Figure 6b show the temperature dependence of Seebeck coefficients for MoS2 bulk samples with different V amount. The positive sign of Seebeck coefficients for all V-containing samples indicates the p-type electrical transport properties. Consistent with our recent work,19 the Seebeck coefficient along each direction of pristine MoS2 is negative at low temperature range. As temperature increases, the Seebeck coefficient eventually changes sign. The sign reversal of Seebeck coefficient for both parallel and perpendicular directions implies that the thermally excited hole carriers tend to dominate transport and change the character of semiconductor from n-type to p-type at high temperature. Furthermore, we noticed that there is some discrepancy on the absolute Seebeck coefficient for MoS2 bulk samples between this paper and the previous report,19 which may be caused by the uncontrolled trace impurities in different batch of MoS2 we used.

Figure 6. Temperature dependence of (a, b) the Seebeck coefficient, and (c, d) the power factor of MoS2-x%V (x = 0, 1, 2, 5, 10) samples both in the (left column) parallel and (right column) perpendicular directions. 14 ACS Paragon Plus Environment

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The power factors, which can be calculated from the Seebeck coefficients and electrical conductivities, are shown in Figure 6c and Figure 6d. For the pristine MoS2 bulk sample, the maximum power factor is only 1.88 µW m-1 K-2 along parallel direction and 1.72 µW m-1 K-2 along perpendicular direction. The values of power factor for all V containing samples are much higher than the pristine MoS2 over the whole temperature range due to the combination of increased electrical conductivity and improved Seebeck coefficient. For sample MoS2-5%V, the peak power factor reaches 438.95 µW m-1 K-2 at 1000 K along parallel direction. 3.3 Thermal conductivity We next examine the temperature dependence of the total thermal conductivity

( ), which is shown in Figure 7a and Figure 7b. Room temperature

of the

initial MoS2 bulk sample is 6.67 W m-1 K-1 along the parallel direction and 40.78 W m-1 K-1 along the perpendicular direction, which are comparable with the literature values for bulk MoS2 with 85-110 W m-1 K-1 for the in-plane thermal conductivity and 2.0-2.3 W m-1 K-1 for the cross-plane thermal conductivity.48-50 After adding 5 mol% V，

decreases to 4.16 W m-1 K-1 along the parallel direction and 21.78 W m-1 K-1

along the perpendicular direction. The minimum

for sample MoS2-5%V is 2.44

W m-1 K-1 at 1000 K along the parallel direction. The total thermal conductivity consists of electric thermal conductivity ( ) and lattice thermal conductivity

( ). is estimated by = ! based on the Wiedemann-Franz law, where ! is Lorenz number varying from 1.5×10-8 W Ω K-2 (used for the pristine MoS2) for nondegenerated semiconductors to 2.44 × 10-8 W Ω K-2 (used for V containing 15 ACS Paragon Plus Environment


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samples) for strongly degenerated semiconductors.51 (Figure 7c and Figure 7d) is obtained by subtracting (Figure S3) from . We found that for all samples dominates the thermal conductivity. The temperature dependence of can be fitted to ~ 1⁄ $ .52 The δ value is shown in Table S2. It is known that for pure Umklapp scattering process, the value of δ is 1. For the pristine MoS2 bulk sample along the parallel direction, δ=0.81 which indicates that the dominant scattering mechanism in MoS2 is the U-process.52 With increasing V doping amount, the value of δ decreases (MoS2-1%V-//: 0.78, MoS2-2%V-//: 0.71, MoS2-5%V-//: 0.58, MoS2-10%V-//: 0.41), so the main scattering mechanism changes to impurity scattering due to the presence of the second phase VMo2S4. Furthermore, the monoclinic VMo2S4 nanoinclusions represent a significantly different crystal structure (space group: Cc) with the hexagonal host matrix 2H-MoS2 (P63/mmc), thus the obvious lattice mismatch and difference in lattice vibration between them could induce the strong scattering of long wavelength phonons at the VMo2S4/MoS2 interface.11 Therefore, the embedded second phase of VMo2S4 into the host MoS2 could significantly enhance the phonon scattering and reduce the lattice thermal conductivity.


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Figure 7. Temperature dependence of (a, b) the total thermal conductivity with error bars and (c, d) the lattice thermal conductivity of MoS2-x%V samples (x = 0, 1, 2, 5, 10) both in the (left column) parallel and (right column) perpendicular directions.

It is noteworthy that the for sample MoS2-10%V shows an upturn comparing to MoS2-5%V sample. One possible explanation is the aggregation of VMo2S4 demonstrated by Figure S4. The aggregation can be found in the V containing samples like the insert of Figure S4a, which is confirmed to be VMo2S4 phase by Energy dispersive X-ray (EDX) spectroscopy analysis. For sample MoS2-10%V, there are more aggregations of VMo2S4 than sample MoS2-5%V (Figure S4b and Figure S4c), which can weaken the extra phonon scattering.53 Furthermore, we find that at higher temperatures,

of MoS2-10%V sample (Figure 7a) is even higher than the

pristine MoS2. Comparing Figure 7a and Figure 7c, we attributed this phenomenon to the larger (Figure S3) of MoS2-10%V at higher temperatures. 3.4 Thermoelectric figure of merit %& The figure of merit for samples MoS2-x%V (x = 0, 1, 2, 5, 10) is calculated and shown in Figure 8. The addition of V in the MoS2 matrix results in the dramatically enhanced of values over the full temperature ranges. It is demonstrated that much larger values are obtained in the parallel direction, which originates from the oriented layered structure of the samples. The optimum proportion of V is 5% and the maximum value in MoS2-5%V reaches to 0.18 at 1000 K. 17 ACS Paragon Plus Environment


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Figure 8. Temperature dependence of thermoelectric figure of merit () for samples MoS2-x%V (x = 0, 1, 2, 5, 10) both along (a) parallel and (b) perpendicular directions.

4. CONCLUSION We have realized p-type MoS2 with a value of 0.18 at 1000 K by doping vanadium. The electrical conductivity is dramatically increased due to the increase of the carrier concentration and mobility by embedding metallic VMo2S4 nanoinclusions into the host semiconducting MoS2 matrix. The introduction of interfaces between VMo2S4 and MoS2 leads to the reduction of thermal conductivity by enhancing phonon scattering. By combining p-type MoS2 reported herein and n-type MoS2 in our previous work19, a prototype thermoelectric module is under developing. We believe that further increase of the figure of merit for MoS2 will be realized by preparing nanostructured MoS2 materials with a lower thermal conductivity.54

ASSOCIATED CONTENT Supporting Information


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Figure S1: Typical spark plasma sintering pellet and samples for thermoelectric measurement used in the study; Figure S2: The specific heat capacity of all the samples; Figure S3: Electronic thermal conductivity of all the samples; Figure S4: SEM image of the aggregation of VMo2S4 phase; Table S1: The values of lattice parameters of MoS2-x%V (x = 0, 1, 2, 5, 10); Table S2: The value of δ ( = 1⁄ $ ) of all the samples.

AUTHOR INFORMATION Corresponding Authors *Wei Zhuang: Email: [email protected] *Peng Jiang: Email: [email protected] *Xinhe Bao: Email: [email protected] Author Contributions P. J. and X. H. B. conceived the research and coordinated the collaborations. K. S. carried out the experimental work. T. M. W. performed the calculations under the guidance of Z. W. K. S. and P. J. analyzed the data and wrote the paper with input from other authors. ‡These authors contributed equally.

ACKNOWLEDGMENT We thank Can Li for providing the access to Hall measurement instrument (HL5500PC) and thank Dr. Mingrun Li for the HR-TEM characterization. P. J. thanks financial support from National Natural Science Foundation of China (NSFC) (Grant: 51290272) and Dalian Institute of Chemical Physics (Grant: DICP ZZBS201608). W. 19 ACS Paragon Plus Environment


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Z. acknowleges the "Strategic Priority Research Program" of the Chinese Academy of Sciences XDB10040304 and XDB20000000 for the financial support.

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TOC Graphic


Realizing p-Type MoS2 with Enhanced Thermoelectric Performance

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