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Effects of structural phase transition on thermoelectric performance in lithiumintercalated Molybdenum Disulfide (LixMoS2) Hong Kuan Ng, Anas Abutaha, Damien Voiry, Ivan Verzhbitskiy, Yongqing Cai, Gang Zhang, Yi Liu, Jing Wu, Manish (M.) Chhowalla, Goki Eda, and Kedar Hippalgaonkar ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22105 • Publication Date (Web): 27 Feb 2019 Downloaded from http://pubs.acs.org on March 3, 2019

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ACS Applied Materials & Interfaces

Effects Of Structural Phase Transition On Thermoelectric Performance In LithiumIntercalated Molybdenum Disulfide (LixMoS2) Hong Kuan Ng,§,†,‡,⊥,# Anas Abutaha,§ ,# Damien Voiry,‖ Ivan Verzhbitskiy,†,⊥ Yongqing Cai,п Gang Zhang,п Yi Liu,∥ Jing Wu,§,⊥ Manish Chhowalla,¶ Goki Eda,*,†,‡,⊥ Kedar Hippalgaonkar*,§ §Institute

of Materials Research and Engineering, #08-03, 2 Fusionopolis Way, Agency for

Science, Technology and Research, Singapore 138634 †Department ‡NUS

of Physics, National University of Singapore, Singapore 117551

Graduate School for Integrative Sciences and Engineering, National University of

Singapore, Center for Life Sciences, #05-01, 28 Medical Drive, Singapore 117456 ⊥Center

for Advanced 2D Materials and Graphene Research Center, National University of

Singapore, Singapore 117546 ‖Institut

Européen des Membranes, University of Montpelier, Montpellier, France 34095

¶Department

of Materials Science and Engineering, Rutgers - The State University of New

Jersey, 607 Taylor Road, Piscataway, NJ 08854, USA ∥Department

of Electrical and Computer Engineering, National University of Singapore,

Singapore 117583 п

Institute of High Performance Computing, 1 Fusionopolis Way, #16-16 Connexis, Agency

for Science, Technology and Research, Singapore 138632

Abstract Layered transition metal dichalcogenides (TMDCs) intercalated with alkali metals exhibit mixed metallic and semiconducting phases with variable fractions. Thermoelectric properties of such mixed-phase structure are of great interest due to potential energy filtering effect, wherein interfacial energy barriers strongly scatter cold carriers rather than hot carriers, leading to enhanced Seebeck coefficient (𝑆). Here, we study the thermoelectric properties of mixedphase LixMoS2 as a function of its phase composition tuned by in-situ thermally driven de-

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intercalation. We find that the sign of Seebeck coefficient changes from positive to negative during initial reduction of the 1T/1T’ phase fraction, indicating crossover from p- to n-type carrier conduction. These anomalous changes in Seebeck coefficient, which cannot be simply explained by the effect of de-intercalation-induced reduction in carrier density, can be attributed to the hybrid electronic property of the mixed-phase LixMoS2. Our work shows that careful phase engineering is a promising route towards achieving thermoelectric performance in TMDCs.

Keywords: Thermoelectric, de-intercalation, p-to-n transition, phase engineering, mixed phase

Introduction Ultrathin crystals of transition metal dichalcogenides (TMDCs) exhibit promising thermoelectric performance due to quantum confinement effects compared to their bulk counterparts.1,2 Thermoelectric performance is governed by the dimensionless figure of merit, 𝑍𝑇 = 𝑆2𝜎𝑇 𝜅 where 𝑆, 𝜎, 𝜅, and 𝑇 represent Seebeck coefficient, electrical conductivity, thermal conductivity and absolute temperature, respectively. Previous studies have reported large 𝑆 (as large as ~105 μV K ―1 in the insulating state) and thermoelectric power factor (𝑆2𝜎) up to 8.5 mW m ―1 K ―2 in mono- and few-layer TMDCs.3,4 Thermoelectric performance is optimizable by varying layer thickness to increase valley degeneracy, by modulating carrier concentration to enhance 𝜎,3,4 or by reducing 𝜅 through chemical doping or nanostructuring5– 7.

However, the inter-dependence between 𝑆 and 𝜎 generally makes it a challenging task to

achieve thermoelectric performance beyond a theoretical limit expected for an ordinary parabolic band two-dimensional semiconductor.1 A promising alternative to enhance thermoelectric performance in 2-dimensional (2D) TMDCs is through the energy filtering effect, wherein a system with interfacial energy barriers

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strongly scatters cold carriers rather than hot carriers, thereby enhancing 𝑆 due to increased average entropy per charge carrier together with enhanced asymmetry of density of states (DOS) around the Fermi level.8 This approach is expected to work well in 2D TMDCs with mixed metallic and semiconducting phases (e.g. those consisting of 1T’ and 2H phases) because Metal-Insulator (MI) interfaces can preferentially scatter cold carriers, leading to an anomalous enhancement in 𝑆. In addition, the metallic phases serve to boost the 𝜎, thus possibly enhancing thermoelectric performance compared to pure semiconducting or metallic phases that were previously studied. It has been shown that the phase of TMDCs can be engineered to achieve desired electronic and catalytic properties.9,10 Most group VI TMDCs such as MoS2 are thermodynamically stable in the semiconducting 2H phase (trigonal prismatically coordinated),11,12 but metallic 1T/1T’ phase (octahedrally coordinated) can be stabilized by intercalation of alkali ions (e.g. lithium, sodium, potassium),10,13 direct solution synthesis,2 doping,14,15 or strain.9 In this paper, we investigate the effect of phase transition on the thermoelectric performance as a function of the relative compositions of 1T’ and 2H phases in thin (~20 nm) LixMoS2 crystals. The evolution of the Seebeck coefficient reveals that LixMoS2 exhibits a pto n-type transition due to composition tuning from 1T’-dominant structure to 2H-dominant structure induced by in-situ thermal annealing. The thermoelectric power factor is found to decrease from 20 𝜇𝑊 𝑚 ―1 𝐾 ―2 to nearly zero with initial increase in 2H phase fraction, followed by an increase to ~27 𝜇𝑊 𝑚 ―1 𝐾 ―2 upon further annealing. This peculiar behavior cannot be explained simply by the reduction in carrier density due to de-intercalation, and it reflects the evolution of hybrid electronic structure of the system during phase transformation. Our study presents an opportunity to explore energy-dependent filtering signatures through active phase engineering towards enhancing thermoelectric performance in TMDCs.

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Results and discussion Figure 1a shows a schematic of LixMoS2 device with a four-probe geometry with intercalated Li+ ions in the interlayer van der Waals (vdW) gaps. A metal heater line, fabricated next to the device, generates temperature gradient across the sample by Joule heating for Seebeck measurements. Figure 1b shows Raman spectra of the as-exfoliated and post-annealed LixMoS2 flake, where thermal annealing was performed at 473 K in air for one hour. Characteristic Raman modes of J1, J2, and J3 are observed in as-exfoliated LixMoS2, confirming the presence of 1T’ phase in LixMoS2.16 In contrast, only Raman modes of E2g and A1g are observed in the post-annealed LixMoS2.17 The disappearance of characteristic 1T’ Raman modes after thermal annealing demonstrates clearly the restoration of 2H phase within the same LixMoS2 flake. The change in optical contrast of the same LixMoS2 flake before and after annealing (insets of Figure 1b) reflects change in its electronic structure due to the alteration in relative phase compositions. Non-uniform color contrast across the as-exfoliated LixMoS2 flake also suggests inhomogeneity of 1T’ and 2H phases. It is observed that thermal annealing of LixMoS2 in air accelerates the 1T’-to-2H phase transition most likely due to hydration of Li+ ions (Figure 1b and S1).13,18 Therefore, annealing temperature, time, and atmosphere play crucial roles in controlling the rate of phase transition. Hence, thermal annealing in this study is performed in high vacuum (~10 ―7 Torr) to achieve gradual phase transition in order to precisely monitor its impact on the thermoelectric properties.

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Figure 1. (a) Schematic diagram of the as-prepared LixMoS2 device with Li+ ions intercalated within interlayer vdW gaps. Regions near any intercalated Li+ ions possess 1T’ phase, while the rest are 2H phase MoS2. (b) Raman spectra of as-exfoliated and post-annealed LixMoS2 sample on SiO2/Si substrate indicating signature Raman modes of 1T’ phase and 2H phase MoS2, respectively. Insets depict optical images of LixMoS2 flake. Scale bar is 3 μm. (c) Seebeck coefficient of LixMoS2 throughout all annealing cycles, with an optical image of an as-prepared LixMoS2 device shown in the inset. The blue (red) shaded region corresponds to positive (negative) Seebeck coefficient. Scale bar is 5 μm.

Thermoelectric characterization of LixMoS2 device as a function of annealing cycles is shown in Figure 1c, where a direct current (DC) is applied through the heater to generate a temperature gradient (T). A positive Seebeck coefficient of +25 μV K ―1 is observed at room temperature indicating a hole conductive nature (p-type) of LixMoS2, which is a result of ambipolar conduction, consistent with the observation by Huang et al.13 This p-type conduction can be attributed to the Fermi level being deeper into the valence band of semi-metallic 1T’ LixMoS2.13,19,20 This Seebeck coefficient is significantly lower compared to that of a pure 2H phase MoS2,3,4 which could be attributed to either a low asymmetry of DOS around the Fermi level or ambipolar conduction with holes being the majority carriers, although this cannot be corroborated directly by our experiment. Moisture trapped in the LixMoS2 device is removed

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in the first two annealing cycles, resulting in a slight increase in Seebeck coefficient to +50 μV K ―1 (Figure 1c). Thereafter, the Seebeck coefficient is found to decrease until a negative Seebeck coefficient is observed after the 6th annealing cycle. A negative Seebeck coefficient indicates an electron conductive nature (n-type), and this anomalous sign crossover in Seebeck coefficient signifies a p-type to n-type (p-to-n) transition, suggesting that majority carriers become electrons instead of holes. This p-to-n transition is somewhat counterintuitive considering the reduction in electron doping induced by de-intercalation. This may be attributed to the emergence of 2H phase, which has a distinct electronic structure. The Seebeck coefficient of LixMoS2 finally saturates around ― 290 μV K ―1 after > 20 annealing cycles. We have further conducted more precise measurements around this sign change point to verify p-to-n transition. It is known that a small signal-to-noise ratio is inherent in DC measurements when the absolute Seebeck coefficient approaches values near zero around the p-to-n transition around 5th to 7th annealing cycles (See Figure S2). It has been demonstrated that alternating current (AC) measurements can overcome this issue by enhancing signal-tonoise ratio using a lock-in amplifier.3 Figure 2a illustrates the thermoelectric open-circuit voltage (𝑉OC) and its corresponding phase as a function of an AC heater current for the 5th and 7th annealing cycles. The observed sign and phase of 𝑉OC then elucidates the conduction nature present in LixMoS2. This p-to-n transition is consistent with DC measurements and also observed in two other LixMoS2 devices (See Figure S2 and S3). Figure 2b illustrates the correlation between the AC heater current and the generated thermoelectric 𝑉OC for p- (blue) and n-type (red) as a function of time.

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Figure 2. (a) AC measurements of thermoelectric open-circuit voltage (Voc) with their corresponding phase signal of LixMoS2. Blue (red) shaded regions are demarcated as p-type (n-type) conduction. A comparison between AC 𝑉OC and phase measurements after the 5th and 7th annealing cycles clearly indicates a p-to-n transition. (b) AC measurement technique: With an applied AC current of frequency 𝜔, 𝑉OC can be detected at the second harmonic frequency 2𝜔 using a lock-in amplifier. The conduction nature of the measured device can be obtained from the detected amplitude and phase.

The effect of phase transformation on thermoelectric properties of LixMoS2 is further studied by comparing the evolution of Seebeck coefficient with that of electrical conductivity, power factor, and phase fraction as shown in Figure 3. Here, the thermoelectric transport is broken down into four regimes (I, II, III, and IV) based on distinct trends as discussed below.

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Figure 3. Seebeck coefficient, electrical conductivity, and thermoelectric power factor (PF) of LixMoS2 device across all annealing cycles. The relative compositions of 1T’ phase is determined using XPS (1st and 26th annealing cycles: pre-anneal and post-anneal, respectively) as well as estimated using effective medium theory (EMT) up till the 6th annealing cycle. Colored regions demarcate four different regimes where the transport mechanism in LixMoS2 is expected to differ. Refer to Table S1 for the corresponding annealing temperatures and period.

In regime I, the small positive Seebeck coefficient (~25 μV K ―1) and large electrical conductivity (~23,000 S m ―1) reflect a metallic behavior in the 0th annealing cycle, consistent with 1T’-phase dominant LixMoS2. X-ray photoelectron spectroscopy (XPS) analysis estimates the relative composition of 1T’ phase over 2H phase to be ~57 % (Figure S4). This behavior is similar to that of in metals where carrier density is high with low asymmetry of DOS around the Fermi level.21 This implies that 1T’ phase forms a percolative path across the device channel. In this regime, the conductivity of the device remains insensitive to gate modulation ( ± 60 V) due to dominance of the metallic phase (Figure S5). The conductivity of LixMoS2 in this regime

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can be modelled using effective medium theory (EMT), where thermoelectric transport is solely determined by the effective proportion of 1T’ and 2H phases present within LixMoS2.22 We find the composition of 1T’ phase to be 58.7 % based on EMT, which is in agreement with the XPS results (Figure S6). Initial thermal annealing (1st  2nd cycle) results in a slightly increased Seebeck coefficient ( +50 μV K ―1) along with a substantially reduced electrical conductivity (~3,000 S m ―1). The relative composition of 1T’ phase in LixMoS2 is estimated to reduce from the initial ~ 57 % down to ~ 34 % from EMT calculations. This reduction of 1T’ phase decreases the global carrier density, which explains the observed trade-off trend between electrical conductivity and Seebeck coefficient.

In regime II, both Seebeck coefficient and electrical conductivity decrease with further thermal annealing (3rd  6th cycle) unlike in regime I. Reduction in Seebeck coefficient may be attributed to compensation by both types of carriers present in the material. The sign change in Seebeck coefficient (p-to-n transition) observed here can be attributed to the loss of 1T’ phase percolation and dominance of insulating 2H phase. The dramatic decrease in electrical conductivity is consistent with loss of metallic percolation.

In regime III (7th  15th cycle), LixMoS2 remains n-type with increasing magnitude of Seebeck coefficient. The increment in Seebeck coefficient cannot be fully explained by dedoping because slight increase in electrical conductivity is observed. This observed anomaly in conductivity-Seebeck trade-off is attributed to not only the 2H phase restoration but also the alteration of electronic band structure. We speculate that increase in surface defect state density due to thermally-induced defect formation (such as sulfur vacancies),23,24 or emergence of metallic edge states25 are responsible for the observed behavior, although the specific mechanism behind the simultaneous enhancement in Seebeck coefficient and electrical

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conductivity are still unclear. In this regime, relative composition of 1T’ phases estimated using EMT is inaccurate because the assumption that each phase remains electronically unaltered breaks down. Other complications include generation of local strain from multiple heating and cooling cycles.26–28

In regime IV, the magnitude of Seebeck coefficient continues to increase while electrical conductivity gradually decreases. Reduction in conductivity may be explained by reduced mobility due to substantial defect formation which is evident from the poor transistor characteristics of the devices (Figure S5). Whereas, enhancement in Seebeck coefficient may be explained by preferential scattering of cold electrons, scattering from defects introduced from deintercalation, and/or higher composition of semiconducting 2H phases. It should also be noted that these scattering mechanisms may be attributed to phase boundaries that require the presence of Mo atoms with non-deal coordination,29,30 giving rise to localized electronic states that can result in energy-dependent scattering. Finally, the Seebeck coefficient saturates at about ―290 μV K ―1 after the last (26th) annealing cycle. The magnitude of Seebeck coefficient is significantly smaller than those reported for highly crystalline 2H MoS2, suggesting that the derived 2H-MoS2 is electronically inequivalent. The relative composition of 1T’ phase remaining in this regime is estimated to be ~ 1 % from XPS analysis. The thermoelectric power factor peaks around ~27 μW m ―1K2 in this regime. However, this power factor still pales in comparison to those reported for mono-, bi- and tri-layers of pure 2H phase MoS2.3,4

In order to better understand the structural changes within LixMoS2 as a function of insitu annealing, Raman spectra of a LixMoS2 flake are acquired after three separate annealing cycles (in vacuum) at different temperatures and duration as shown in Figure 4a. Judging from

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the Raman peaks (J1, J3, E12g and A1g) across annealing cycles, a clear reduction of 1T’ phase can be observed with the simultaneous restoration of 2H phase in LixMoS2. Figure 4b displays Raman mappings of intensity ratios (A1g/J1 and E2g/J1) across the same annealing cycles as performed in Figure 4a, tagged with their corresponding regimes as in Figure 3. The increasing proportion of A1g/J1 and E2g/J1 regions inside LixMoS2 clearly indicates inhomogeneity of phase transformation induced by thermal annealing, which ascertains structural disorder as a by-product that is consistent with the suggested complications due to de-intercalation as discussed above. Moreover, intercalation and de-intercalation in MoS2 have been demonstrated to introduce structural disorder.31–33

Figure 4. (a) Raman spectra of a LixMoS2 flake on SiO2/Si substrate across three separate annealing cycles performed in vacuum. Signature 1T’ Raman modes are still observed after the 1st annealing cycle, confirming co-existence of 1T’ and 2H phases. After subsequent annealing cycles, the 1T’ phase Raman modes diminish while the 2H phase Raman modes intensify, indicating the restoration of 2H phase within the LixMoS2 flake. (b) Raman mapping of intensity ratio (A1g/J1 and E2g/J1) shows the restoration of 2H phase across the LixMoS2 flake with thermal annealing. White dotted lines outline the LixMoS2

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flake. Scale bar is 1 μm. (c) Band picture of LixMoS2 across different thermoelectric transport regimes according to Figure 3.

Figure 4c illustrates the evolution of the hybrid electronic properties of LixMoS2 in a simple band picture as we understand from the overall behaviors. In regime I, the thermoelectric properties are primarily determined by valence (hole) band of semi-metallic 1T’ phase (See Figure S7 for DFT calculations). Gradual restoration of 2H phase in regime II leads to loss of percolation and change in majority carriers. In regimes III/IV, further annealing induces structural disorder, which alters the conduction mechanism and carrier mobility in complex ways due to various competing effects arising from defects and strain.

CONCLUSIONS In conclusion, we observe a transition in conduction nature (p-to-n transition) of LixMoS2 as well as 1T’-2H phase transformation owing to thermally-induced de-intercalation of Li+ ions. This p-to-n transition is demonstrated through measurements of Seebeck coefficient using a low-noise AC measurement setup with careful control of the phase transition rate. The thermoelectric power factor is found to peak at ~ 27 μW m ―1 K ―2 after prolong annealing cycles. We demonstrate that it is possible to gain insight into the evolution of electronic and structural properties of this complex mixed-phase system through thermoelectric measurements, and conversely, thermoelectric performance can be fine-tuned by carefully engineering the phase structure of MoS2. Further studies are warranted to elucidate the exact mechanism behind active phase engineering and possible energy filtering effect, especially linking the change in band structure as a function of carrier concentration by employing experiments such as Hall measurements.

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ASSOCIATED CONTENT Supporting information is available free of charge on the ACS Publications website at DOI: Details on device fabrication and characterization (Raman spectroscopy, X-Ray Photoelectron Spectroscopy, transfer characteristics), alternating current Seebeck coefficient measurement, information on in-situ thermal annealing, as well as effective medium theory (EMT) and density functional theory (DFT) calculations.

AUTHOR INFORMATION Corresponding Authors *Email: [email protected] *Email: [email protected] Author Contributions #

H.K. N. and A. A. contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS We acknowledge financial support from the A*Star Science and Engineering Research Council (Grant No. 152 70 00015). The authors thank HL. Seng for her assistance with XPS and discussions.

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