Letter pubs.acs.org/NanoLett
Gate-Optimized Thermoelectric Power Factor in Ultrathin WSe2 Single Crystals Masaro Yoshida,† Takahiko Iizuka,† Yu Saito,† Masaru Onga,† Ryuji Suzuki,† Yijin Zhang,† Yoshihiro Iwasa,*,†,‡ and Sunao Shimizu‡ †
Quantum-Phase Electronics Center (QPEC) and Department of Applied Physics, the University of Tokyo, Tokyo 113-8656, Japan RIKEN Center for Emergent Matter Science (CEMS), Wako 351-0198, Japan
‡
S Supporting Information *
ABSTRACT: We report an electric field tuning of the thermopower in ultrathin WSe2 single crystals over a wide range of carrier concentration by using electric double-layer (EDL) technique. We succeeded in the optimization of power factor not only in the hole but also in the electron side, which has never been chemically accessed. The maximized values of power factor are one-order larger than that obtained by changing chemical composition, reflecting the clean nature of electrostatic doping. KEYWORDS: Thermopower, thermoelectric power factor, on-chip thermometer, electric double-layer transistor (EDLT), tungsten diselenide (WSe2)
T
doping in TMDs, the thermoelectric characterization is impossible due to the extreme sensitivity to air. On the other hand, the field effect carrier doping is an effective method to achieve the fine and continuous tuning of the carrier concentration, which can be scanned by sweeping gate voltage (VG).10 In addition, we can modulate the carrier density over a wide range up to 1014 cm−2 by employing electric double-layer transistor (EDLT) using ionic liquids as gate dielectrics.11 The ionic gating is versatile. We can realize the self-assembled channel/dielectric interface by putting the ionic liquid on the TMD film, and electrons and holes can be always injected to the channel material in principle. Because heavily doped semiconductors usually show the optimized carrier concentration between 1019 and 1021 cm−3,7 the carrier concentration realized in EDLTs of ultrathin TMD films12,13 likely covers the area of the optimal power factor S2σ. As a typical semiconducting TMD, we focus on tungsten diselenide (WSe2).14−17 The layered structure shown in Figure 1a enables us to obtain ultrathin WSe2 single crystals by mechanical exfoliation of bulk single crystals with the Scotch tape. We transferred the ultrathin single crystals onto a doped silicon wafer covered with a layer of thermally grown silicon dioxide (SiO2) and patterned four-probe electrodes with an electron-beam lithography technique, followed by the vacuum deposition of titanium (5 nm) and gold (60 nm). Sequentially, we deposited titanium (5 nm) and insulating SiO2 (30 nm) to
wo-dimensional (2D) materials based on transition metal dichalcogenides (TMDs) are one of the central materials of nanoscience because of its variety of novel properties and device functionalities.1−3 Recently, a large thermopower value was recorded in a monolayer molybdenum disulfide (MoS2),4,5 posing potential importance of the thermoelectric properties6,7 of TMDs. The 2D electronic structure and the multivalley bands in TMDs, which are the basis for valleytronics, are both known to enhance the thermoelectric performance. In fact, the thermoelectric properties of hole-doped tungsten diselenide (WSe2) have been systematically investigated by using bulk polycrystals.8,9 To reveal the potential of TMDs as thermoelectric materials, we need to maximize the thermoelectric figure of merit (ZT) or power factor. The figure of merit is given by ZT = (S2σ/κ)T, where S is thermopower or Seebeck coefficient, σ is electrical conductivity, κ is thermal conductivity, and T is temperature. It is known that the power factor, S2σ, can be another index to be optimized. Because the parameters including S, σ, and κ are functions of carrier concentration and are interrelated to each other, we need to control the carrier concentration precisely to optimize ZT or S2σ. The carrier concentration can be modulated by chemical substitution or intercalation. However, such chemical doping methods lack the precise tunability, and we may miss the optimum carrier concentration. Also, the chemical process can induce significant disorder in the system, which hides the intrinsic carrier density dependence of the thermoelectric properties. Moreover the chemical approach is not always effective. For instance, although the intercalation of alkali metals is known to be a powerful means for electron © XXXX American Chemical Society
Received: January 7, 2016 Revised: February 2, 2016
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DOI: 10.1021/acs.nanolett.6b00075 Nano Lett. XXXX, XXX, XXX−XXX
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To obtain the capacitance of the EDL, we performed a Hall effect measurement in another WSe2 EDLT so that we calculated the sheet carrier density (n2D) in the WSe2 single crystal. We applied VG at 220 K under high vacuum condition and measured the Hall effect at 170 K. The transverse resistance (Rxy) was linearly dependent on the magnetic field (B), directly giving n2D as n2D = (eRxy/B)−1, where e is elementary charge. Figure 1d shows the σ2D and n2D as a function of VG. In Figure 1d, the n2D was proportional to |VG − Vth| in both positive and negative VG, and the slopes of the linear fits yielded the capacitance of EDL (C) to be 5.8 and 7.1 μF/cm2 for electrons and holes, respectively. Using the values of the C, we obtained the field effect mobility of 72 and 1.8 × 102 cm2/(V s) for electrons and holes, respectively. For the measurement of thermopower of WSe2, we introduced on-chip microheaters and microthermometers to the EDLT of the trilayer WSe2 single crystal. Figure 2a is the
Figure 1. (a) The crystal structure of the trilayer WSe2 where the planes of tungsten (W) atoms are surrounded by selenium (Se) atoms in a hexagonal arrangement. (b−d) The VG dependence of (b) sourcedrain current (IDS), (c) IG, and (d) σ2D in the EDLT of a trilayer WSe2 single crystal measured at T = 300 K under high vacuum condition. (e) σ2D and n2D as a function of VG measured in another WSe2 EDLT at T = 170 K.
avoid the occurrence of electrochemical process in the interface between the electrodes and ionic liquid. We covered both the gate electrode and the channel material WSe2 with an organic ionic liquid, N,N-diethyl-N-(2-methoxyethyl)-N-methylammonium bis-trifluoromethylsulfonyl)-imide (DEME-TFSI), so that the EDLT configuration was finalized. Figure 1b shows the transfer curve in the EDLT of an ultrathin WSe2 single crystal, which we confirmed to be a trilayer both by atomic force microscopy and by optical contrast derived from the optical microscope images18 (see Figure S1 in the Supporting Information). We measured the transfer characteristic with the excitation voltage VDS = 0.01 V at 300 K under a vacuum of 10−4 Pa. The ambipolar transistor operation indicates that the application of VG shifts the Fermi level across the band gap, and that electrons and holes are accumulated in the ultrathin WSe2 single crystal by applying positive and negative VG, respectively. Figure 1c displays the simultaneously measured VG dependence of gate electrode current (IG). The measured IG is mainly the displacement current, and the finite values of IG imply the charging of the capacitors formed at the channel/liquid interface. Figure 1d shows the VG dependence of sheet conductance (σ2D). As indicated by black broken lines in Figure 1d, we extrapolated the σ2D−VG curve to zero so that the threshold voltage (Vth) was found to be 0.84 and −1.35 V for n- and p-type conduction, respectively.
Figure 2. (a) Schematic of a thermoelectric micro-EDLT. The thermal gradient on the trilayer WSe2 single crystal is realized with the righthand Joule heater by applying VHeat. The four-lead electrodes, Th1 and Th2, are used for thermometry to measure the ΔT across the WSe2 single crystal and are also employed to quantify the ΔV. In the fourterminal electrical conductivity measurement, Th1 and Th2 are utilized as the current drain and source, respectively, and the electrodes between Th1 and Th2 as voltage probes. (b−e) The time evolution of (b) VG, (c) VHeat, (d) ΔT across the WSe2 single crystal in response to the stepwise change of VHeat, and (e) ΔV measured just before VHeat is changed. (f) ΔV as a function of ΔT measured at VG = 1.92 V. The dashed and solid lines represent the increase and decrease of VHeat, respectively.
optical micrograph of a thermoelectric EDLT device. We fabricated a Joule heater close to the WSe2 single crystal so that the temperature gradient was generated across the crystal by applying voltage (VHeat) on the heater electrode. On the crystal, we prepared two four-lead electrodes, Th1 and Th2, which worked as on-chip thermometers to quantify the temperature B
DOI: 10.1021/acs.nanolett.6b00075 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters gradient (ΔT). These electrodes were also used to measure the thermoelectric voltage drop (ΔV). In the four-terminal electrical transport measurement, Th1 and Th2 served as current drain and source and the electrodes between Th1 and Th2 served as voltage probes. Such a thermopower measurement technique using microfabricated electrodes was developed initially in field-effect transistors with solid gate dielectric in carbon nanotube, graphene, and nanowires.19−23 The concept of thermoelectric EDLT device is recently emerging,24−27 and this Letter is the first report of introducing microheaters and microthermometers to EDLTs for TMD single crystals. We performed measurement of gate-controlled thermopower, whose schematic procedure is depicted in Figure 2b−e. As shown in Figure 2b, we changed the VG in steps of 20 mV. At each VG, first we applied drain voltage (VDS) to measure the four-terminal σ2D (results shown in Figure 1c). After releasing VDS, we applied VHeat on the heater electrode in steps of 0.5 V at every 0.3 s, which is shown in Figure 2c. Prior to the gatecontrolled thermoelectric measurement, we calibrated the thermometers and established the linear relation between the ΔT and VHeat2 (see Figure S2g in the Supporting Information). In response to the stepwise change of VHeat, ΔT (Figure 2d) and the corresponding ΔV (Figure 2e) are generated. Here, we need to check whether ΔT and ΔV is promptly realized against the change of VHeat. Figure 2f is an example data measured at VG = 1.92 V, where we obtained a linear relation between the ΔV and ΔT. In Figure 2f, the linear relation upon decreasing VHeat is in good agreement with that on increasing VHeat, reflecting the immediate response of ΔT and ΔV to the stepwise change of VHeat. The slope of the linear fit to these data sets provides the Seebeck coefficient (S = −ΔV/ΔT). It takes only 6 s to obtain σ2D and S at each VG. Figure 3a,b summarizes the gate dependence of σ2D and S at temperature T = 300 K. In Figure 3a,b, we excluded the sets of data points whose linear fit to ΔV/ΔT, as in Figure 2f, was less than 90% accurate. In accordance with the ambipolar gate dependence of σ2D, the sign of S showed a clear reversal as displayed in Figure 3b: the positive and negative values of S indicate that holes and electrons are the majority charge carriers, respectively. The measured gate voltage dependence of σ2D and S enables us to calculate the power factor (S2σ) as S2σ = S2σ2D/t*, where σ is conductivity and t* is the thickness of the conductive channel. Here we need to estimate t*, and we refer to the firstprinciple calculations on the field-effect transistor of a trilayer WSe2,28 which takes the in-plane band dispersion into account. In the present experiment, the accumulated carrier concentration was n2D ≤ 4.2 × 1013 cm−2 for electrons and n2D ≤ 2.9 × 1013 cm−2 for holes, respectively. In these carrier density regions, Brumme et al. showed that 90% of the accumulated carriers are distributed in the first two layers. We therefore assumed that for both holes and electrons, t* is the thickness of the bilayer, 1.3 nm.29 Figure 3c is the S2σ versus VG curve, exhibiting a dome structure in both types of carriers. Here, it should be stressed that the gate-controlled surface carrier doping enables us to optimize the power factor in n-type WSe2, which has never been accessed by chemical doping. The maximum values of S2σ were 32 and 37 μW/K2cm for n- and p-type conduction, respectively. These optimized values are comparable to that of the famous thermoelectric material bismuth telluride (Bi2Te3), which is about 50 μW/K2cm.6
Figure 3. (a−c) The VG dependence of (a) σ2D, (b) S, and (c) S2σ at T = 300 K, where we assumed that the thickness of conductive channel is that of bilayer. The orange (dark blue) and red (cyan) circles correspond to the data points measured upon increasing and decreasing absolute value of positive (negative) VG, respectively. The arrows indicate the direction of VG scan.
To clarify the doping effect on the thermoelectric properties, we compare the dependences of electric transport and thermoelectric properties on carrier density (n3D) in WSe2 obtained by electrostatic and chemical doping, which are summarized in Figure 4a−c. There are comprehensive researches on W1−xTaxSe2 by Brixner8 and Hicks,30 and W1−xTaxSe1.6Te0.4 by Kriener,9 where the Ta atoms work as hole dopants. Brixner measured both σ and S, whereas he performed Hall effect measurement only for the compound with x = 0.01. On the other hand, Hicks measured both electrical conductivity and Hall coefficient for all the compounds but did not performed the thermoelectric measurement. Therefore, in Figure 4a−c, we assume that n3D of W1−xTaxSe2 synthesized by Brixner is equal to the nominal Tadoping concentration x except for the compound with x = 0.01, whereas n3D of W 0.99Ta 0.01Se 2 synthesized by Brixner, W1−xTaxSe2 by Hicks, and W1−xTaxSe1.6Te0.4 are the literature values obtained by Hall effect measurement. On the other hand, in the case of WSe2 EDLT we first calculated the n2D as n2D = C(VG − Vth)/e, where C are 5.8 and 7.1 μF/cm2 for electrons and holes, respectively, and the Vth for electron and hole side are 0.84 and −1.35 V, respectively. We then obtained n3D from the relation of n3D = n2D/t*. Figure 4a is the carrier concentration (n3D) dependence of the conductivity (σ), showing that the conductivity for the electrostatic doping of both electrons and holes is noticeably higher than that for the chemical hole doping. The upshift of the σ−n3D curves for the electrostatic doping comes from the difference in mobility (μ), because the conductivity is given by σ = n3Deμ. In the present work, we used a mechanically exfoliated single crystal and modulated the carrier density by field effect doping with minimal disorder, leading to a relatively high field effect mobility of 1.8 × 102 and 72 cm2/(V s) for C
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the relatively high mobility of the WSe2 EDLT in the low carrier density region (Figure 4a), which is attributed to the single crystal materials used. Also it is shown in Figure 4c that the difference in doping method and crystallinity affects the optimized carrier concentration (nopt) to give the peak in S2σ: nopt in WSe2 EDLT is 9.5 × 1019 and 1.1 × 1020 cm−3 for holes and electrons, respectively, whereas nopt is about 5 × 1020 cm−3 in Ta-doped systems. In Ta-doped WSe2, the extrinsic suppression of mobility results in the significant reduction of S2σ in the low carrier density region, accompanied by the apparent increase of nopt. In conclusion, we fabricated a thermoelectric EDLT microdevice on a trilayer WSe2 single crystal and measured the gate voltage dependence of σ2D and S simultaneously in both the n- and p-type regions. We succeeded in the optimization of S2σ by fine-tuning of carrier density. The ntype doping, which is impossible without using field-effect transistor, was revealed to yield larger |S| at given carrier concentrations than the p-type doping. The enhancement of |S| by electron doping indicates the larger DOS in the conduction band. The comparison to chemically doped WSe2 systems disclosed that the relatively high S2σ obtained in the WSe2 EDLT comes from the clean nature of field effect doping. Recently the thermoelectric properties of 2D materials are intensively cultivated by field effect doping5,20−22,31−33 (see Supporting Information Table S5). The present results revealed that the ionic gating is a powerful way to optimize thermoelectric properties in 2D materials, also giving new insights into their peculiar electronic band structures.
Figure 4. (a−c) Evolutions of (a) σ, (b) absolute values of |S|, and (c) S2σ as a function of n3D. The red and cyan circles are the values of nand p-type WSe2 realized by field effect doping at T = 300 K, respectively. The arrows indicate the direction of VG scan. The green diamonds represent the literature values of W0.99Ta0.01Se2 (ref 8). The green triangles also represent the literature values of WxTa1−xSe2 (ref 8), where we assumed that n3D is equal to the nominal Ta-doping concentration x. The green circles and squares represent the literature values of WxTa1−xSe2 (ref 30) and W1−xTaxSe1.6Te0.4 (ref 9), respectively.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b00075. Optical contrast for ultrathin WSe2 flakes, details of the thermoelectric measurement, carrier concentration dependence of Seebeck coefficient in other devices, possible interpretation of the observed |S|-n3D relation based on a band calculation, and thermoelectric properties in gated 2D materials. (PDF)
holes and electrons, respectively. In contrast, the Ta-doped WSe2 compounds are polycrystals with mobility around 3−15 cm2/(V s).8,9,30 The low mobility reflects the scattering by the Ta dopants as well as by the grain boundary, both of which are absent in the WSe2 EDLT. Thus, the difference in mobility between Ta-doped and gated WSe2 systems is extrinsic. However, the difference in mobility between positive and negative gating in the EDLT may be related to the band structure of WSe2, which is also reported previously.17 Figure 4b is the evolution of the absolute values of |S| as a function of n3D. It was revealed that the n-type WSe2, which had never been reported, shows larger |S| at a given carrier concentration than the conventional p-type WSe2. The similar behavior was observed in another device (see Supporting Information Figure S3). The enhancement of |S| by electron doping indicates the larger density of states (DOS) in the conduction band (see Supporting Information Figure S4). On the other hand, the |S|−n3D curves obtained by negative gating and Ta-doping basically follow the same trend: the |S| monotonically increases as n3D is reduced. Assuming the constant carrier scattering time, S basically depends only on DOS and n3D, and it is reasonable that both electrostatic and chemical doping show such a similar relation between |S| and n3D. Figure 4c shows the S2σ versus n3D curves, which exhibits that the optimized value of S2σ obtained by gating is one-order of magnitude larger than that of chemically doped WSe2. The dramatic enhancement of S2σ in WSe2 EDLT is understood by
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
M.Y., T.I., and Y.J.Z. fabricated the devices. R.S. grew the bulk single crystals. T.I. and M.Y. constructed the thermoelectric measurement system and carried out the thermoelectric measurement. Y.S. performed Hall effect measurement. M.O. contributed to the characterization of the ultrathin exfoliated single crystals. M.Y., T.I., Y.I., and S.S. planned and supervised the study. M.Y., Y.I., and S.S. wrote the manuscript. M.Y. and T.I. contributed equally to this work. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by Grants-in-Aid for Scientific Research (Grant No. 25000003 and No. 26820298) by the Japan Society for the Promotion of Science (JSPS). Y.I. was supported by the Strategic International Collaborative Research D
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(20) Zuev, Y. M.; Chang, W.; Kim, P. Thermoelectric and magnetothermoelectric transport measurements of graphene. Phys. Rev. Lett. 2009, 102, 096807. (21) Wei, P.; Bao, W.; Pu, Y.; Lau, C. N.; Shi, J. Anomalous thermoelectric transport of Dirac particles in graphene. Phys. Rev. Lett. 2009, 102, 166808. (22) Checkelsky, J. G.; Ong, N. P. Thermopower and Nernst effect in graphene in a magnetic field. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 80, 081413. (23) Boukai, A. I.; Bunimovich, Y.; Tahir-Kheli, J.; Yu, J.-K.; Goddard III, W. A. G.; Heath, J. R. Silicon nanowires as efficient thermoelectric materials. Nature 2008, 451, 168−171. (24) Takayanagi, R.; Fujii, T.; Asamitsu, A. Control of thermoelectric properties of ZnO using electric double-layer transistor structure. Jpn. J. Appl. Phys. 2014, 53, 111101. (25) Yanagi, K.; Kanda, S.; Oshima, Y.; Kitamura, Y.; Kawai, H.; Yamamoto, T.; Takenobu, T.; Nakai, Y.; Maniwa, Y. Tuning of the thermoelectric properties of one-dimensional material networks by electric double layer techniques using ionic liquids. Nano Lett. 2014, 14, 6437−6442. (26) Svensson, S. F.; Burke, A. M.; Carrad, D. J.; Leijnse, M.; Linke, H.; Micolich, A. P. Using polymer electrolyte gates to set-and-freeze threshold voltage and local potential in nanowire-based devices and thermoelectrics. Adv. Funct. Mater. 2015, 25, 255−262. (27) Shimizu, S.; Ono, S.; Hatano, T.; Iwasa, Y.; Tokura, Y. Enhanced cryogenic thermopower in SrTiO3 by ionic gating. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 92, 165304. (28) Brumme, T.; Calandra, M.; Mauri, F. First-principle theory of field-effect doping in transition-metal-dichalcogenides: Structural properties, electronic structure, Hall coefficient, and electrical conductivity. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 91, 155436. (29) Podberezskaya, N. V.; Magarill, S. A.; Rervukhina, N. V.; Borisov, S. V. Crystal chemistry of dichalcogenides MX2. J. Struct. Chem. 2001, 42, 654−681. (30) Hicks, W. T. Semiconducting behavior of substituted tungsten diselenide and its analogues. J. Electrochem. Soc. 1964, 111, 1058− 1065. (31) Kim, D.; Syers, P.; Butch, N. P.; Paglione, J.; Fuhrer, M. S. Ambipolar surface state thermoelectric power of topological insulator Bi2Se3. Nano Lett. 2014, 14, 1701−1706. (32) Kayyalha, M.; Shi, L.; Chen, Y. P. Gate-tunable and thicknessdependent electronic and thermoelectric transport in few-layer MoS2. 2015, arXiv:1505.05891. arXiv.org e-Print archive. http://arxiv.org/ abs/1505.05891 (accessed Feb. 4, 2015). (33) Hippalgaonkar, K.; Wang, Y.; Ye, Y.; Zhu, H.; Wang, Y.; Moore J.; Zhang, X. Record high thermoelectric powerfactor in single and few-layer MoS2. 2015, arXiv:1505.06779. arXiv.org e-Print archive. http://arxiv.org/abs/1505.06779 (accessed Feb. 4, 2015).
Program (SICORP-LEMSUPER) of the Japan Science and Technology Agency. M.Y., Y.S., R.S., and Y.J.Z. were supported by JSPS through a research fellowship for young scientists. R.S. was supported by the Leading Graduate Program of Materials Education for future leaders in Research, Industry, and Technology (MERIT). M.O. and Y.J.Z. were supported by the Advanced Leading Graduate Course for Photon Science (ALPS).
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