Suppression of Magnetoresistance in Thin WTe2 Flakes by Surface

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Suppression of Magnetoresistance in Thin WTe Flakes by Surface Oxidation John M. Woods, Jie Shen, Piranavan Kumaravadivel, Yuan Pang, Yujun Xie, Grace A. Pan, Min Li, Eric I. Altman, Li Lu, and Judy J. Cha ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b04934 • Publication Date (Web): 20 Jun 2017 Downloaded from http://pubs.acs.org on June 27, 2017

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ACS Applied Materials & Interfaces

Suppression of Magnetoresistance in Thin WTe2 Flakes by Surface Oxidation

John M. Woods1,2,†, Jie Shen1,2,†,‡, Piranavan Kumaravadivel1,2, Yuan Pang3, Yujun Xie1,2,4, Grace A. Pan5, Min Li6, Eric I. Altman4,7, Li Lu3, Judy J. Cha1,2,4,* 1

Department of Mechanical Engineering and Materials Sciences, Yale University, New

Haven, CT 06511, USA 2

Energy Sciences Institute, Yale West Campus, West Haven, CT 06516, USA

3

Institute of Physics, Chinese Academy of Sciences, Beijing, China

4

Center for Research on Interface Structures and Phenomena, Yale University, New

Haven, CT 06511, USA 5

Department of Physics, Yale University, New Haven, CT 06511, USA

6

Materials Characterization Core, Yale West Campus, West Haven, CT 06516, USA

7

Department of Chemical & Environmental Engineering, Yale University, New Haven,

CT 06520, USA *Corresponding author. E-mail: [email protected] Telephone number: +1 (203) 737-7293, Fax number: (203) 432-6775

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† These authors contributed equally to this work. ‡ present address: Delft University of Technology, GA 2600 Delft, The Netherlands

KEYWORDS: Tungsten ditelluride, Two-dimensional materials, layered materials, Magnetoresistance, Fermi level pinning

ABSTRACT:

Recent renewed interest in layered transition metal dichalcogenides stems from the exotic electronic phases predicted and observed in the single- and few-layer limit. Realizing these electronic phases requires preserving the desired transport properties down to a monolayer, which is challenging.

Surface oxides are known to impart Fermi level

pinning or degrade the mobility on a number of different systems, including transition metal dichalcogenides and black phosphorus.

Semimetallic WTe2 exhibits large

magnetoresistance due to electron-hole compensation, thus Fermi level pinning in thin WTe2 flakes could break the electron-hole balance and suppress the large magnetoresistance. We show that WTe2 develops a ~ 2 nm-thick amorphous surface oxide, which shifts the Fermi level by ~ 300 meV at the WTe2 surface. We also observe a dramatic suppression of the magnetoresistance for thin flakes. However, due to the semimetallic nature of WTe2, the effects of Fermi level pinning are well screened and are not the dominant cause for the suppression of magnetoresistance, supported by fitting a two-band model to the transport data, which showed the electron and hole carrier densities are balanced down to ~ 13 nm.

However the fitting shows a significant

decrease of the mobilities of both electrons and holes. We attribute this to the disorder

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introduced by the amorphous surface oxide layer. Thus, the decrease of mobility is the dominant factor in the suppression of magnetoresistance for thin WTe2 flakes. Our study highlights the critical need to investigate often unanticipated and sometimes unavoidable extrinsic surface effects on the transport properties of layered dichalcogenides and other 2D materials.

INTRODUCTION:

Exotic electronic phases found in layered transition metal dichalcogenides (TMDCs), such as charge density waves,1-3 topological surface states,4 and 2D superconductivity,2, 5 make TMDCs promising for a wide range of novel thin film electronics. In the single layer limit, additional electronic phases such as the quantum spin Hall state6 are predicted, which can be harnessed for spintronic and low-dissipation electronic applications.

Recent progress in the ability to build heterostructures by stacking

chalcogenide single layers with different electronic phases offers exciting opportunities for realizing interface- and proximity-induced electronic phases not possible in bulk forms.7

However, a critical requirement for device applications is to retain these

predicted electronic phases and transport properties in the single layer limit. Often, this is not the case, as demonstrated by the presence of a 2D electron gas with low mobility at the surface of topological insulators such as Bi2Se3 and by surface oxidation of many 2D materials.8-10 Surface oxidation often has a detrimental effect on electrical performance, but the reason for this performance loss depends on the system. Surface oxidation can decrease the mobility through increased scattering as observed in the Si-SiO2 interface



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and in black phosphorous,11-13 but surface oxidation is also well known to cause Fermi level pinning in a variety of materials including Ge nanowires,14 GaN films,15 and Bi2Se3.9 After understanding the mechanism of performance loss, strategies to mitigate the adverse effects of surface oxidation can be developed.

Encapsulating layered

materials using boron nitride can be effective in preserving transport properties as shown for graphene,16, 17 but is still limited to individual nanoscale devices. Systematic studies of thickness-dependent transport properties of TMDCs are crucial for implementing practical applications based on these materials.

The magnetoresistance (MR) is an interesting property that varies across the range of 2D materials. MR is small for WS2, a TMDC, and for Mo2TiC2Tx and Ti3C2Tx, two MXenes.18, 19 Black phosphorus has a large non-saturating MR, but the MR is linear in high magnetic fields.20 The TMDC WTe2 is a candidate for a type II Weyl semimetal21 and has recently received much attention largely due to its large non-saturating quadratic MR.22 The MR of WTe2 has been ascribed to the nearly perfect, but not exact, electron and hole charge compensation,22 further supported by band structure calculations,23 angle-resolved photoemission spectroscopy (ARPES) measurements,24 and analysis of Shubnikov-de Hass oscillations,25-27 and temperature dependent Hall resistance measurements.28 A classical two-band model is sufficient to describe the observed large MR despite the fact that the actual band structures are more complicated with four Fermi pockets along the Γ-Χ axis in the 1st Brillouin zone.27, 29 In addition, monolayer WTe2 in the 1T’ phase is predicted to exhibit the quantum spin Hall effect.6 Further theoretical calculation of layered WTe2 predicts that the extremely large MR, and therefore the near-


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perfect charge compensation and high mobility, should be preserved down to the monolayer limit.23 The actual preservation of these electronic properties for fabricated devices of monolayer WTe2 is vital in harnessing the exotic transport behavior.6

In contrast to these theoretical predictions, here we show that the MR of thin, mechanically exfoliated WTe2 flakes decreases significantly compared to that of a bulk crystal. This is in agreement with the recent report by L. Wang et al.,30 which also showed dramatic suppression of the MR in WTe2 thin flakes down to 6 layers. The suppression of the MR was attributed to decreased mobility and increased disorder due to surface degradation,30 but a microscopic origin was not investigated. Here, we reveal the microscopic origin for MR suppression by correlating transport measurements with the microstructure of WTe2 flakes from 145 nm down to 2.5 nm (~ 3 layers). We find that the microscopic source is a ~ 2 nm amorphous surface oxide layer. The work function of the oxide layer is found to be ~300 meV less than that of clean WTe2, suggesting a potential charge imbalance between the electron and hole carriers of WTe2 at the WTe2 surface due to downward band bending by the Fermi level pinning, which would lead to suppression of MR, similar to what has been observed in doped WTe2.31 However, due to the limited spatial extent of the band bending, estimated to be ~ 1 nm by Poisson’s equation, the Fermi level pinning is not the dominant cause behind the degradation of MR. A much larger factor is the increased disorder induced by the surface oxide layer, reflected in the transport data as a decrease in mobility and weak anti-localization.

RESULTS AND DISCUSSION:



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Electronic transport measurements of WTe2 thin flakes, mechanically exfoliated from bulk crystals (Supplementary Figure S1-3), were taken from 300 K to 2 K with magnetic fields up to 9 T. Figure 1(a)-(e) show the temperature-dependent longitudinal resistance, Rxx(T), with and without a 9 T magnetic field perpendicularly applied to the basal plane of WTe2. In the absence of a magnetic field, Rxx(T) shows the expected semi-metallic behavior with the residual resistance ratio (RRR) defined as Rxx(300K)/Rxx(1.8K). When field-cooled at 9 T, Rxx increases at low temperature due to the emergence of the large MR, with Ton defined as the temperature at which Rxx starts to increase. Both Ton and RRR start to decrease significantly for flakes thinner than ~ 40 nm (Figure 1(f)). Figure 1(g)-(k) show the corresponding MR curves measured at 1.8 K with MR defined as [R(B)-R(0)]/R(0). With decreasing WTe2 thickness, MR decreases linearly (Figure 1(l)). These observations clearly indicate that the transport properties of WTe2 degrade with decreasing WTe2 thickness.  (nh µ h2 − ne µe2 ) B + µ h2 µ e2 ( nh − ne ) B 3  = ρ  xy  2 e ( nh µ h + ne µe ) + ( nh − ne ) 2 µ h2 µe2 B 2      (1)  2 2 2   ρ = (nh µ h + ne µ e ) + ( nh µ h µe + ne µe µ h ) B   xx e ( n µ + n µ )2 + ( n − n ) 2 µ 2 µ 2 B 2   e e h e h e   h h  

Strong correlations between the carrier doping and the MR31 as well as the crystal quality and MR32 have been shown using WTe2 bulk crystals. Thus, the observed MR suppression suggests either degradation of crystal quality or changes in transport property such as the carrier density and mobility for thin WTe2 flakes. We measure the Hall resistance Rxy(B) of the flakes from 145 nm down to 2.5 nm to find the origin of the MR suppression (Figure 2 and Supplementary Figure S4). For all thicknesses, strong nonlinearity is observed in Rxy, suggesting that at least two carrier types contribute to the Rxy 6 ACS Paragon Plus Environment

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curves. In WTe2 the Fermi level cuts through the conduction and valence bands,22 so both electron and hole carriers are expected to contribute to transport. Therefore, we use the classical two-band model for resistivity (ρxx and ρxy in equation (1))33 to fit the Rxx and Rxy curves, with the appropriate geometrical factors considered, assigning one band to electrons and the other to holes. Our transport model does not incorporate differing Hall and transport scattering rates that are necessary to understand some systems, such as Ce-115 based compounds,34 because the classical model considers these equivalently and has been sufficient to explain the MR by charge compensation.22-24, 28, 30 Supplementary Figure S5 shows the fitted curves, which overlap well with the measured data down to the 13 nm flake.

Figure 2(f) shows the electron and hole carrier densities, ne and nh, obtained from the fit as a function of WTe2 thickness. Figure 2(g) shows the corresponding mobility values, µe and µh. The error of these fit values is presented in Supplementary Figure S6. Figure 2(h) shows the charge carrier density ratio, ne/nh. We see that as thickness decreases the mobility and the total charge density decrease, but the ratio of the electron to hole density remains close to one down to 13 nm. The small deviations from perfect balance are minor factors for the magnitude of the observed MR. For the 145 nm flake we would only expect a 2.5% increase of the magnitude of MR at B=9 T if we forced ne=nh. The two-band model does not describe well the transport data of the 6.3 nm and 2.5 nm flakes because of the weak anti-localization that starts to dominate in these thin flakes (Figure 1(k) and Supplementary Figure S5(j)). Temperature-dependent MR and Rxy studies for 115 nm and 13 nm WTe2 flakes are shown in Supplementary Figure S7. Based on the fit



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results using the two-band model, we attribute the suppression of MR with decreasing WTe2 flake thickness (Figure 1(l)) to the decrease in electron and hole mobility values, and not to the charge imbalance as the electron and hole carrier densities are nearly equal down to the 13 nm WTe2 flake.

To underscore the importance of careful fitting of data, an erroneous fit of the two-band model to the Rxy curve alone is shown in Supplementary Figure S8. The results from this improper fit indicate charge imbalance consistent with the potential effects of Fermi-level pinning on thin WTe2 flakes, but the parameters determined from this limited fit do not replicate the experimentally observed MR (Supplementary Figure S8(c)). Only when fitting to the two-band model was carried out with proper considerations, we see that the charge compensation is stable, while the mobility drops for thin WTe2 flakes (Figure 2(fh)).

The dramatic decrease in the mobility values suggests presence of a 2D impurity band with very low mobility that induces strong scatter in thin flakes. The effect of the 2D impurity band to the transport properties of WTe2 thin flakes is studied by analyzing Shubnikov-de Hass (SdH) oscillations as a function of WTe2 flake thickness. Figure 3(a) shows Rxx(B) plotted in B2, which clearly shows the quadratic dependence of MR on B, as expected from Equation (1) for ρxx when nh ≈ ne, and also shows the suppression of MR with decreasing thickness. From the second derivative of Rxx(B), d2Rxx/dB2 (Figure 3(b)), SdH oscillations are observed down to the 13 nm thick sample, indicating the presence of high mobility carriers. More importantly, the field BQ at which the SdH


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oscillations start to emerge does not shift much (SdH oscillations are plotted as a function of 1/B in Supplementary Figure S9). With thickness down to 13 nm, BQ ~4.7 T, and BQ slightly decreases for the thicker 115 nm and 145 nm flakes. Using the condition µSdH·BQ~1 we obtain a roughly constant mobility for the highest mobility carriers. This is in contrast to the sharply decreasing electron and hole aggregate mobilities obtained from the Rxx and Rxy curves (Figure 2(g)). The average mobility obtained from the Lorentz law, MR ≈ ( µ ave ⋅ B ) , is plotted together with µSdH, for comparison, as a function of 2

WTe2 thickness (Figure 3(c)).

The systematic decrease in µave suggests increased

scattering in thinner WTe2 flakes due to the 2D impurity band near the surface. The role of the 2D impurity band on µave is further substantiated from the change in conductivity (and corresponding resistivity) with decreasing thickness (Figure 3(d)).

To reveal the microscopic origin of the 2D impurity band, we examine a WTe2 flake by cross-sectional transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS). Figure 4(a) shows the cross-sectional TEM image of a WTe2 flake transferred onto a Si/SiO2 substrate, in which WTe2 layers are clearly resolved (dotted red box).

Below the crystalline WTe2,

we observe two different amorphous layers with a distinct contrast in intensity.

The

bottom layer of lighter contrast corresponds to the SiO2 substrate, confirmed by EDX. Chemical analysis of the darker amorphous layer (dotted green box) indicates the presence of oxygen, tellurium, and tungsten (Figure 4(b)), whereas the oxygen peak is absent in the crystalline WTe2 region. Thus, we conclude that a ~ 2 nm thick oxide layer forms on WTe2 flakes.

Oxide formation can be detected within a few hours by



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comparing XPS spectra of WTe2 flakes, freshly cleaved/cleaned and after being left in air for 3.5 hours (Figure 4(c,d)). The clean WTe2 flakes were obtained by sputtering the surface of freshly cleaved WTe2 flakes for ~2 min with Ar at 5 keV, which completely removed the oxide layer. Pronounced W-Ox and Te-Ox peaks are observed after 3.5 hours in air, which were not present in the freshly cleaved and sputtered sample. This is evidence that WTe2 can degrade in air even faster than previously reported (1 day exposure),35 and that both W and Te participate in the oxidation of WTe2 flakes. The XPS data corroborates the observation that the amorphous layer observed in the crosssectional TEM image is indeed a surface oxide of WTe2.

The surface oxide layer induces disorder that manifests itself as a 2D impurity band with low mobility, leading to the suppression of MR for thinner flakes. In addition, the surface oxide layer can induce downward band bending near the surface of WTe2 by pinning the Fermi level. This was measured by ultraviolet photoelectron spectroscopy (UPS). Figure 4(e) shows the secondary edge of clean (black) and air-exposed (red) WTe2 flakes. After exposure to air, the secondary edge increases by about 300meV, which indicates a corresponding decrease in the work function from 4.5 eV to 4.2 eV. For the WTe2 near the surface oxide, the bands are bend (Figure 4(f)). Using Poisson’s equation and the dielectric constant of WTe236 we determine the spatial extent of the band bending to be about 1 nm. This limited spatial extent of the band bending explains why the charge compensation holds down to a 13 nm WTe2 flake. Full UPS profiles and details of Poisson’s equation calculations are shown in Supplementary Figure S10. The absence of any gate-dependence in field effect transistor measurements (Supplementary


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Figure S11) further corroborates the screened nature of the Fermi level pinning. Additionally, the ~ 2 nm thick oxide layer induces strong disorder for very thin WTe2 flakes so that a localization effect should be discernible. This is indeed the case for the ~ 3 layer WTe2 flake where a pronounced weak antilocalization (WAL) signal is observed due to the induced disorder (Figure 1(k)). Disorder is known to cause weak localization (WL) in materials such as graphene,37 and WAL has been observed as a result of scattering in other systems.38, 39 Moreover, WAL is expected due to the strong spin-orbit coupling in WTe2.26, 29 We fit the two-dimensional Hikami-Larkin-Nagaoka localization model40 to the observed WAL and obtain a Bϕ of 0.074T for fixed BSO above 3.5 T (Supplementary Figure S12).

CONCLUSION: Transport degradation due to surface oxidation is a major issue that affects most of the transition metal dichalcogenide family and other 2D materials.

In the topological

insulators Bi2Se3 and Bi2Te3, a disordered 2D gas at the surface complicates the analysis of the topological surface states.8, 9 In black phosphorus, which possesses high mobility and anisotropic optical properties due to its crystal structure, fast surface oxidation significantly degrades device performance.41,

42

Here, the observed degradation in

transport properties of thin WTe2 flakes is due to disorder induced by surface oxidation, leading to suppression of the large MR in WTe2. The revelation that the main effect of the surface oxide is not charge doping, indicates that future studies investigating the Weyl state of WTe221 will need to protect the surface states by preventing scattering into the bulk. In addition, our finding suggests that the predicted quantum spin Hall state may not



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be observed in unprotected single layer WTe2 unless an effective encapsulation layer such as boron nitride is used.

MATERIALS AND METHODS: WTe2 synthesis. WTe2 crystals were grown by a chemical vapor transport (CVT) method. 0.55 g of a WTe2 source (American Elements, 99.999%) and 80 mg of an I2 transport agent were placed in a quartz tube sealed at one end. The tube was purged with argon gas multiple times, and sealed with a base pressure below 50 mTorr. The sealed quartz vessel was placed in a two zone furnace. Over the course of 6 hours, the ‘hot’ end with the source powder was ramped up to 950 ⁰C and the ‘cold’ end was ramped up to 800 ⁰C. The furnace was held at these temperatures for 3 days. Upon cooling, WTe2 crystals formed at the ‘cold’ end of the tube. These bulk crystals were then mechanically exfoliated on 285 nm Si/SiO2 substrates using the standard tape method to obtain thin WTe2 flakes. Device fabrication and transport measurements.

Devices were fabricated by

standard e-beam lithography using a Vistec EBPG 5000+. 5/200 nm thick Cr/Au electrical contacts were deposited by thermal evaporation (MBraun MB-EcoVap). The low temperature and magnetic field transport measurements were performed soon after the lift-off process, using a Quantum Design Dynacool physical property measurement system equipped with a 9 T magnet and at a base temperature of 1.8 K. The Rxx and Rxy data (and corresponding resistivity data) were measured using pseudo-Hall-bar electrode geometry. We fit the symmetrized data to the two-band conduction model by minimizing the squared error between the data and the fit. Instead of only fitting ρxx or ρxy to the two-


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band model, we fit the sum, ρxx + ρxy, using equation (1). Three parameters (ne, nh, and µe) were free to vary, and the fourth, µh, was determined from the other three based on the value of ρxx (B=0 T). Initial parameters were order of magnitude estimates from results in Ali et al and Lv et al.22, 23 The thickness of the WTe2 flakes was measured by a Bruker Fastscan AFM directly after the transport measurements to minimize the exposure of flakes to air. Structure and chemical characterizations.

The morphology and chemical

composition were characterized by TEM/STEM (FEI Technai Osiris200kV). Crosssectional TEM samples were prepared by focused ion beam (Zeiss Crossbeam 540) at Carl Zeiss Microscopy, LLC. Planar samples were obtained by sonicating WTe2 flakes in methanol and drop-casting onto a TEM grid. XPS and UPS (He I illumination, E=21.22eV) measurements (PHI 5000 Versa Probe II) were conducted to determine the chemical and electrical nature of WTe2 before and after exposure to air. For clean WTe2 flakes for the UPS measurements, the freshly cleaved flakes were sputtered for ~2 minutes using an Ar source at 4 keV. Author Info Address correspondence to [email protected] Author Contributions: The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Conflict of Interest: The authors declare no competing financial interest. Acknowledgement. This work was supported by Department of Energy, Basic Energy Science, Award No. DE-SC0014476.

J. S. was partially supported by NSF DMR



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1402600. J. M. W. was partially supported by NSF EFMA 1542815. P.K. was supported by DOE DE-SC0014476. Characterization facilities used in this work were supported by the Yale Institute for Nanoscience and Quantum Engineering (YINQE), MRSEC DMR 1119826,

and

Yale

West

Campus

Materials

Characterization

Core.

Raman

characterization was conducted at the Yale Institute for the Preservation of Cultural Heritage. Supporting Information Available: further TEM, AFM & Raman, XRD, further transport data, model fit, fit error, temperature-dependent transport data, Rxy only fit, The observed Shubnikov-de Hass oscillations plotted as 1/B, UPS, gated transport, and WAL fit. This material is available free of charge via the Internet at http://pubs.acs.org.

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22. Ali, M. N.; Xiong, J.; Flynn, S.; Tao, J.; Gibson, Q. D.; Schoop, L. M.; Liang, T.; Haldolaarachchige, N.; Hirschberger, M.; Ong, N. P.; Cava, R. J., Large, Non-Saturating Magnetoresistance in WTe2. Nature 2014, 514, 205-208. 23. Lv, H. Y.; Lu, W. J.; Shao, D. F.; Liu, Y.; Tan, S. G.; Sun, Y. P., Perfect Charge Compensation in WTe2 for the Extraordinary Magnetoresistance: From Bulk to Monolayer. Europhys. Lett. 2015, 110, 37004. 24. Pletikosić, I.; Ali, M. N.; Fedorov, A. V.; Cava, R. J.; Valla, T., Electronic Structure Basis for the Extraordinary Magnetoresistance in WTe2. Phys. Rev. Lett. 2014, 113, 216601. 25. Cai, P. L.; Hu, J.; He, L. P.; Pan, J.; Hong, X. C.; Zhang, Z.; Zhang, J.; Wei, J.; Mao, Z. Q.; Li, S. Y., Drastic Pressure Effect on the Extremely Large Magnetoresistance in WTe2 Quantum Oscillation Study. Phys. Rev. Lett. 2015, 115, 057202. 26. Rhodes, D.; Das, S.; Zhang, Q. R.; Zeng, B.; Pradhan, N. R.; Kikugawa, N.; Manousakis, E.; Balicas, L., Role of Spin-Orbit coupling and Evolution of the Electronic Structure of WTe2 Under an External Magnetic Field. Phys. Rev. B 2015, 92, 125152. 27. Zhu, Z.; Lin, X.; Liu, J.; Fauqué, B.; Tao, Q.; Yang, C.; Shi, Y. G.; Behnia, K., Quantum Oscillations, Thermoelectric Coefficients, and the Fermi Surface of Semimetallic WTe2. Phys. Rev. Lett. 2015, 114, 176601. 28. Luo, Y.; Li, H.; Dai, Y. M.; Miao, H.; Shi, Y. G.; Ding, H.; Taylor, A. J.; Yarotski, D. A.; Prasankumar, R. P.; Thompson, J. D., Hall Effect in the Extremely Large Magnetoresistance Semimetal WTe2. Appl. Phys. Lett. 2015, 107, 182411. 29. Jiang, J.; Tang, F.; Pan, X. C.; Liu, H. M.; Niu, X. H.; Wang, Y. X.; Xu, D. F.; Yang, H. F.; Xie, B. P.; Song, F. Q.; Dudin, P.; Kim, T. K.; Hoesch, M.; Das, P. K.; Vobornik, I.; Wan, X. G.; Feng, D. L., Signature of Strong Spin-Orbital Coupling in the Large Nonsaturating Magnetoresistance Material WTe2. Phys. Rev. Lett. 2015, 115, 166601. 30. Wang, L.; Gutierrez-Lezama, I.; Barreteau, C.; Ubrig, N.; Giannini, E.; Morpurgo, A. F., Tuning Magnetotransport in a Compensated Semimetal at the Atomic Scale. Nat. Commun. 2015, 6, 8892. 31. S. Flynn; M. Ali; Cava, R. J., The Effect of Dopants on the Magnetoresistance of WTe2. arXiv:1506.07069 2015. 32. Ali, M. N.; Schoop, L.; Xiong, J.; Flynn, S.; Gibson, Q.; Hirschberger, M.; Ong, N. P.; Cava, R. J., Correlation of Crystal Quality and Extreme Magnetoresistance of WTe2. Europhys. Lett. 2015, 110, 67002. 33. Murzin, S. S.; Dorozhkin, S. I.; Landwehr, G.; Gossard, A. C., Effect of HoleHole Scattering on the Conductivity of the Two-Component 2D Hole Gas in GaAs/(AlGa)As heterostructures. J. Exper. Theor. Phys. Lett. 67, 113-119. 34. Nair, S.; Nicklas, M.; Steglich, F.; Sarrao, J. L.; Thompson, J. D.; Schofield, A. J.; Wirth, S., Precursor State to Superconductivity in CeIrIn5: Unusual Scaling of Magnetotransport. Phys. Rev. B 2009, 79, 094501. 35. Lee, C. H.; Silva, E. C.; Calderin, L.; Nguyen, M. A. T.; Hollander, M. J.; Bersch, B.; Mallouk, T. E.; Robinson, J. A., Tungsten Ditelluride: a Layered Semimetal. Sci. Rep. 2015, 5, 10013. 36. Kumar, A.; Ahluwalia, P. K., Tunable Dielectric Response of Transition Metals Dichalcogenides MX2 (M=Mo, W; X=S, Se, Te): Effect of Quantum Confinement. Physica B 2012, 407, 4627-4634. 16 ACS Paragon Plus Environment

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Figure 1. Thickness dependent transport properties of WTe2 films: Ton, RRR and MR. (a)-(e) R-T curves at 0 T and 9 T for samples with different thicknesses. (f) The Ton (marked in (a-e) with black arrow) and RRR extracted from these curves are plotted against thickness. We see that values for both Ton and RRR systematically decrease for decreasing thickness. The Ton for the 2.5 nm flake is influenced by the presence of WAL, so it is marked with an open square in (f). (g)-(k) the corresponding MR curves of the samples on the upper panel at T = 1.8 K. MR at 9 T as a function of thickness is summarized in (l), mirroring the effects seen on Ton and RRR; the magnitude of the MR decreases dramatically as thinner WTe2 flakes are measured. For the thinnest flake (device #5, 2.5 nm thickness) WAL is observed in the resistance vs. magnetic field plot.


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Figure 2. Thickness dependent mobility and carrier density from a two-band model. (a)-(e) Rxy curves measured at 1.8 K for samples with different thicknesses. (f) The carrier densities and (g) mobilities of the electrons (solid squares) and holes (open squares) from the two band model fits of the Rxy and MR curves summarized as a function of thickness down to 13 nm. Comparison of the data to the fit is shown in Supplementary Figure S5. The fits for the 6.3 nm and 2.5 nm flakes are not shown due to the addition of WAL behavior in the Rxx data. (h) Ratio of the electron carrier density, ne, to the hole carrier density, nh, as a function of thickness. Carrier compensation holds down to the 13 nm flake.



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Figure 3. MR versus B2 and SdH oscillations for WTe2 flakes of different thicknesses. (a) MRs measured at T= 1.8 K plotted as a function of B2. The curves are linear as expected from the classical Lorentz law for MR.

(b) SdH oscillations observed in

samples thicker than 13 nm disappear in thinner samples. SdH oscillations are plotted as 1/B in Supplementary Figure S9. (c) The average mobility calculated from MR (µave) and the mobility of the fastest carriers calculated based on the onset of the SdH oscillations (µSdH) plotted together as a function of thickness. (d) The thickness dependence of the conductivity and resistivity of samples is consistent with that of µave.


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Figure 4.

STEM characterization, X-ray photoelectron spectroscopy (XPS) and

ultraviolet photoelectron spectroscopy (UPS). (a) Cross-sectional TEM image of WTe2. The inset shows the corresponding STEM annular dark field image. (b) EDX spectra of red and green dashed-line regions of (a). Oxygen is present only in the green region indicating this is an amorphous tungsten-tellurium oxide layer. This amorphous oxide layer is also present on the top surface of the flake. The bottom-most brighter amorphous layer is SiO2 (c,d) XPS profiles for W (c) and Te (d) on clean flakes (black) and flakes 21 ACS Paragon Plus Environment


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with 3.5 hours of exposure to air (red). The presence of Te-Ox and W-Ox peaks after 3.5 hours of air exposure indicates formation of an oxide layer on the surface of the WTe2 flakes. (e) UPS profiles of the secondary edge for clean (black) and 3.5 hours air exposed (red) WTe2 flakes. The location of the secondary edge increases by about 300 meV, indicating a corresponding work function, φ, decrease of about 300 meV. (f) Schematic of Fermi level pinning between the surface oxide (left) and WTe2 flake (right). The Fermi level is marked by a red dashed line. The valence and conduction bands are shown on right.

Black dashed lines track the location of the top/bottom of the

valence/conduction bands in the proximity of the oxide interface.


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