Unconventional Charge-Density-Wave Transition in Monolayer 1T

Unconventional Charge-Density-Wave Transition in Monolayer 1T‑TiSe2

Katsuaki Sugawara,*,† Yuki Nakata,‡ Ryota Shimizu,† Patrick Han,† Taro Hitosugi,† Takafumi Sato,‡ and Takashi Takahashi†,‡ †

WPI Research Center, Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan Department of Physics, Tohoku University, Sendai 980-8578, Japan

‡

ABSTRACT: Reducing the dimension in materials sometimes leads to unexpected discovery of exotic and/or pronounced physical properties such as quantum Hall effect in graphene and high-temperature superconductivity in iron-chalcogenide atomically thin films. Transition-metal dichalcogenides (TMDs) provide a fertile ground for studying the interplay between dimensionality and electronic properties, since they exhibit a variety of electronic phases like semiconducting, superconducting, and charge-densitywave (CDW) states. Among TMDs, bulk 1T-TiSe2 has been a target of intensive studies due to its unusual CDW properties with the periodic lattice distortions characterized by the three-dimensional (3D) commensurate wave vector. Clarifying the ground states of its two-dimensional (2D) counterpart is of great importance not only to pin down the origin of CDW, but also to find unconventional physical properties characteristic of atomic-layer materials. Here, we show the first experimental evidence for the realization of 2D CDW phase without Fermi-surface nesting in monolayer 1T-TiSe2. Our angle-resolved photoemission spectroscopy (ARPES) signifies an electron pocket at the Brillouin-zone corner above the CDW-transition temperature (TCDW ∼ 200 K), while, below TCDW, an additional electron pocket and replica bands appear at the Brillouin-zone center and corner, respectively, due to the back-folding of bands by the 2 × 2 superstructure potential. Similarity in the spectral signatures to bulk 1T-TiSe2 implies a common driving force of CDW, i.e., exciton condensation, whereas the larger energy gap below TCDW in monolayer 1T-TiSe2 suggests enhancement of electron−hole coupling upon reducing dimensionality. The present result lays the foundation for the electronic-structure engineering based with atomic-layer TMDs. KEYWORDS: transition-metal dichalchogenides, 1T-TiSe2, charge density wave, angle-resolved photoemission spectroscopy, scanning tunneling microscopy, electronic states The Jahn−Teller mechanism4 is based on the band-structure calculations in which the lowest lying Ti 3d band in the 1T crystal structure has a slightly lower energy than the 2H counterpart, leading to the CDW transition due to the local coordination change from 1T to 2H.5−8 On the other hand, the excitonic-condensate mechanism9−13 relies on both the semimetallic band structure with a small indirect band gap and the direct Coulomb interaction between holes at the Γ point and electrons at the L point. This leads to the semimetal− semiconductor transition due to the exciton condensation accompanied by the structural transition. One effective approach to pin down the driving force of CDW in 1T-TiSe2 is to reduce the dimensionality down to ultrathin 2D limit, i.e., monolayer TiSe2, and find a possible link between the electronic states and the CDW properties. However, only a

L

ayered transition-metal dichalchogenides (TMDs) MX2 (M = transition metal, X = chalcogen) have been a target of intensive studies for more than four decades since they show a variety of physical properties such as superconductivity and charge density wave (CDW).1 Among TMDs, 1T-TiSe2 with the octahedral crystal structure (space group D3d) has attracted particular attention since it undergoes a semimetal to commensurate CDW phase transition at around 200 K.2 The CDW in bulk 1T-TiSe2 is characterized by the 3D commensurate (2 × 2 × 2) wave vector, which connects the Γ (kz = 0) and L points (kz = π/c, where c is the c-axis length) of the hexagonal Brillouin zone (BZ). Early studies suggested that the CDW arises from the conventional Fermi-surface (FS) nesting which connects the Se 4p hole pocket at the Γ point and the Ti 3d electron pocket at the L point.2,3 Recent studies, however, have proposed various different CDW scenarios such as the band-type Jahn−Teller mechanism4−8 and the excitoniccondensate mechanism,9−13 because of the fact that the FSs at both Γ and L points have a 3D spherical shape unfavorable for the nesting. © XXXX American Chemical Society

Received: October 26, 2015 Accepted: December 1, 2015

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from bilayer graphene and underlaid buffer layer, respectively.19 When we evaporate Ti on bilayer graphene in Se atmosphere, the RHEED image exhibits an additional 1 × 1 streak pattern [Figure 1d], similarly to monolayer WSe2 on bilayer graphene.20 As seen in Figure 1e, the scanning-tunnelingmicroscopy (STM) image signifies the formation of TiSe2 islands on bilayer graphene, whose height (∼6.5 Å) is similar to the previous report on monolayer TiSe2 [ref 15] and slightly larger than the c-axis unit-cell length of bulk 1T-TiSe2 (∼6.0 Å). Figure 1f shows the STM image at T = 6 K zoomed in the region indicated by square in Figure 1e. One can recognize a regular 1 × 1 pattern showing some defects (black dots),21 together with a periodic honeycomb-like pattern corresponding to the 2 × 2 superstructure, as in a cleaved surface of bulk 1TTiSe2.21 The Fourier-transformation image [inset to Figure 1f] also signifies the 2 × 2 spots besides the 1 × 1 spots.15,22 These results indicate that a high-quality monolayer 1T-TiSe2 film with a periodic lattice distortion at low temperature is successfully fabricated on bilayer graphene. Panels a and b of Figures 2 display a side-by-side comparison of the valence-band ARPES intensity in monolayer 1T-TiSe2

limited number of such studies on 1T-TiSe2 and related 2D TMDs have been reported,14−18 and the electronic structure in monolayer 1T-TiSe2 as well as the origin of CDW has remained elusive. In this paper, we report ARPES study on monolayer 1TTiSe2 film grown epitaxially on bilayer graphene on SiC(0001). We found a marked similarity in the temperature variation of band structure near the Fermi level (EF) between monolayer and bulk 1T-TiSe2, in particular, regarding the appearance of replica bands at the M point and the gradual energy shift of the Se 4p bands at the Γ point. The present ARPES result strongly suggests the excitonic-condensate mechanism for the CDW transition in monolayer 1T-TiSe2.

RESULTS AND DISCUSSION At first we present the fabrication and characterization of monolayer 1T-TiSe2. Panels a and b of Figure 1 show the crystal structure and Brillouin zone, respectively. Figure 1c shows the reflection high-energy electron diffraction (RHEED) pattern of pristine bilayer graphene grown on SiC(0001). We clearly observe the 1 × 1 and 6√3 × 6√3R30° spots arising

Figure 2. (a and b) ARPES intensity plots of monolayer 1T-TiSe2 measured along the Γ-M cut at room temperature and T = 40 K, respectively, with the He−Iα line (hν = 21.218 eV). (c and d) ARPES-intensity mapping at EF at room temperature and T = 40 K, respectively. The intensity is obtained by integrating the energy distribution curves (EDC) within ±50 meV of EF. Solid and broken white lines indicate the BZ boundaries of (1 × 1) and (2 × 2) superstructure, respectively.

plotted as a function of wave vector and binding energy (EB) at room temperature (RT) and T = 40 K, respectively, measured along the Γ−M cut with the He−Iα line (hν = 21.218 eV). We clearly observe several dispersive bands at both temperatures, for example, two holelike bands with the Se 4p character near EF at the Γ point,9,11,23−26 called here Se1 and Se2, respectively. As also seen in Figure 2a, there exists a weak but finite spectral weight around the M point, which is better resolved in the ARPES-intensity mapping in the vicinity of EF [Figure 2c]. This suggests the existence of a shallow electron pocket at the M point, as detailed later. Absence of a hole pocket at Γ, unlike the first-principles band calculation of monolayer 1T-TiSe2,23

Figure 1. (a) Schematic view of crystal structure in monolayer 1TTiSe2. (b) 2D BZs of 1T-TiSe2 in the normal phase (solid line) and the 2 × 2 CDW phase (broken line). (c and d) RHEED patterns of bilayer graphene and monolayer 1T-TiSe2, respectively, obtained along the [11̅00] direction of the SiC substrate. (e) Constantcurrent STM images (87 × 87 nm2, sample bias voltage Vs = +2.0 V, and set-point tunneling current It = 100 pA) and height profiles (green line) along the cut indicated by blue line. (f) Expanded STM image (8.7 × 8.7 nm2, Vs = −200 mV, It = 100 pA) in the region indicated by square in (e). Inset shows a Fourier-transform image of (f). B

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Figure 3. (a−d) ARPES-intensity plots around (a and b) Γ point and (c and d) M point, measured at (a and c) room temperature and (b and d) T = 40 K. Se1, Se2, X1, X2, and folded M electron bands are also indicated. (e) Comparison of the EDC between Γ and M points at T = 40 K. The spectra are normalized with respect to the background at the binding energy higher than the Se2 (and X2) peak. Weaker Se2 intensity compared to that of X1 is mainly due to overlapping of the tail of a strong peak at EF only for the spectrum at the M point.

suggests that the present 1T-TiSe2 sample is slightly electrondoped by the Se deficiency in the film and/or the charge transfer from the substrate. While the band calculations suggested the semimetallic nature of monolayer 1T-TiSe2 with negative band gap of ∼0.2 eV between the holelike band at Γ (Γ-hole) and the electronlike band at M (M-electron),23 the present ARPES data rather support the semiconducting nature of the stoichiometric sample at room temperature with a slightly positive or almost zero band gap, as inferred from the similar energy position of the top of Γ-hole bands and the bottom of M-electron bands [this is better visible in Figure 3a,c]. Such a positive band gap is also reported in previous ARPES experiments of lightly (1−4%) Cu-intercalated 1TTiSe2,25,26 and has been discussed in terms of possible CDW fluctuations at room temperature.9,11,12 As shown in Figure 2b, one can see a drastic change in the band structure at T = 40 K. In particular, the shallow electronlike band near EF becomes more prominent, and two holelike bands, called X1 and X2, newly appear around the M point. To see more clearly the electronic states in the vicinity of EF, we show in Figure 3 the near-EF band dispersion around the Γ and M points at room temperature and T = 40 K. A direct comparison of the band structure at the Γ point in Figure 3a,b reveals that the top of bands Se1 and Se2 at T = 40 K (0.15 and 0.32 eV, respectively) is shifted downward by 0.08 eV relative to that at room temperature. The energy position of bands Se1 and Se2 is almost identical to that of new holelike bands X1 and X2 in Figure 3d, as visible from a direct comparison of the energy distribution curves (EDCs) between the Γ and M points in Figure 3e. This unambiguously indicates that bands X1 and X2 are a replica of bands Se1 and Se2. As shown in Figure 3b, a weak but finite spectral weight is observed at the Γ point at T = 40 K; this is also attributed to the replica of the M-electron pocket. Since these observations closely resemble the temperature evolution of bands in bulk 1T-TiSe2,8,9,11,25,26 the present ARPES result strongly suggests that the observed drastic reconstruction of the electronic structure at T = 40 K is due to the CDW transition with commensurate 2 × 2 periodic lattice distortion, consistent with the STM observation in Figure 1f. To clarify the evolution of CDW as a function of temperature, we show in panels a and b of Figure 4 the temperature dependence of EDCs at Γ and M points, respectively. As shown in Figure 4a, the Se 4p peaks (Se1, Se2) at the Γ point located at 0.15 and 0.32 eV are stationary in the temperature range of T = 220−300 K, while they gradually move toward higher EB below T = 180 K upon decreasing

Figure 4. (a and b) Temperature dependence of EDCs at Γ and M points, respectively. (c and d) Temperature dependence of the peak positions at Γ and M points, respectively.

temperature. At the M point [Figure 4b], two peaks X1 and X2 suddenly appear at around T = 180−220 K, and these peaks are shifted toward higher EB on decreasing temperature with enhancing their spectral weight. Extracted peak positions for bands Se1, Se2 in Figure 4c exhibit a characteristic upturn below ∼200 K. This temperature coincides well with the temperature at which bands X1 and X2 emerge, as displayed in Figure 4d. These results indicate that the CDW transition temperature of monolayer 1T-TiSe2 on bilayer graphene is ∼200 K, similar to the value of bulk 1T-TiSe2 (202 K) [ref 2] but lower than that of exfoliated films with less than 100 nm thickness (∼240 K).14 The lower CDW temperature in the present monolayer 1T-TiSe2 compared to the exfoliated film may be due to the electron-doped nature of our epitaxial film as C

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ACS Nano inferred from the gradual suppression of TCDW upon electron doping in Cu-intercalated 1T-TiSe2.27 Having established the CDW origin at low temperature in monolayer 1T-TiSe2, a next issue is to find the driving force of CDW. First possibility is the FS nesting whose CDW vector spans the Γ-hole and M-electron pockets.23 However, this possibility is excluded since the nesting condition is ill defined due to the slightly electron-doped nature of our sample and resultant absence of the hole pocket at the Γ point. Second possibility is the structural transition derived from the Jahn− Teller-type lattice distortion.4−8 The good agreement in the band dispersions below TCDW between the ARPES experiment and the band calculations which incorporates the band folding due to the Jahn−Teller effect7 is in favor of this scenario. However, since the Jahn−Teller mechanism is based on the negative band gap above TCDW,6,7 this scenario looks incompatible with the ARPES observation in Figure 3a,c where the band edges for Γ-hole and M-electron are located at EB = 0.10−0.15 eV. The last possibility is the exciton condensation accompanied by the 2 × 2 periodic lattice distortion.9−13 Since the observed band dispersions around the Γ and M points above and below TCDW as well as the positive or nearly zero-gap nature of the bands are consistent with this model that accompanies the band folding and the positive band gap,11−13 the excitonic condensate could be a likely cause of the CDW in monolayer 1T-TiSe2. It is also noted that the observed intensity of replica bands at the M point (X1 and X2) is stronger than the original band at the Γ point (Se1 and Se2) as shown in Figure 3e, in contrast to a simple expectation from the conventional Fermi-surface-nesting or Jahn−Teller scenario in which the spectral intensity of replica bands is weaker than that of the original bands. Such intensity behavior in monolayer 1TTiSe2 resembles that of bulk pristine 1T-TiSe2 and Cu-doped 1T-TiSe2 where the exciton condensation was suggested (refs9−1325, 26,). Besides the similarity in the overall band structure between monolayer and bulk 1T-TiSe2, we also found a quantitative difference in the gap parameter. We have estimated the band gap (excitonic gap) Δ of monolayer 1T-TiSe2 in the CDW phase to be 180 meV, from the energy difference between the top of the holelike band at the Γ point and the bottom of the electronlike band at the M point. Intriguingly, this value is much larger than that of bulk (110 meV).12 Accordingly, the coupling value 2Δ/kBTCDW for the monolayer sample is ∼20, about twice as large as that of bulk (∼12.5 [ref 12]). This suggests that electrons and holes in 2D case are more strongly coupled with each other than in the 3D counterpart. In other words, a finite out-of-plane component in the CDW vector may reduce the electron−hole-coupling strength, and the 2D excitonic phase is likely more stable than the 3D counterpart. Considering together the electron-doped nature of our monolayer sample as well as the fact that the electron doping monotonically reduces TCDW (ref 27), the stronger electron− hole coupling in the monolayer sample would be consistent with the previous report in exfoliated 1T-TiSe2 film (likely nondoped sample) which suggested an enhancement of TCDW upon reducing the film thickness.14 The present ARPES study has suggested that the CDW ground state is common for both bulk and monolayer 1T-TiSe2 despite the lack of interlayer interaction in monolayer 1T-TiSe2. This suggests that physical properties inherent to the bulk crystal can be transferred to the atomic layer 2D counterpart in some TMDs, thanks to the weak van der Waals type coupling

between adjacent layers. This would be useful for realizing ultrathin electronic devices (e.g., optical switch and memory28) when one intends to utilize exotic physical properties of bulk into ultrathin devices.

CONCLUSION We performed ARPES study on monolayer 1T-TiSe2 grown on bilayer graphene, and found that it undergoes a commensurate 2 × 2 CDW transition below TCDW ∼ 200 K. We observed the band folding at the M point and the energy shift of the Se 4p bands as a function of temperature below TCDW. These ARPES observations provide a clear evidence for the occurrence of 2D CDW without FS nesting in monolayer 1T-TiSe2. The present result provides a pathway toward the band-structure engineering of atomic-layer TMDs. METHODS A monolayer 1T-TiSe2 film was grown on bilayer graphene by the molecular-beam-epitaxy (MBE) method in an ultrahigh vacuum of 3 × 10−10 Torr. Bilayer graphene was prepared by annealing an n-type Si rich 6H-SiC(0001) single crystal wafer19 by resistive heating at 1100 °C in a high vacuum better than 1.0 × 10−9 Torr for 20 min. Monolayer 1T-TiSe2 was grown by evaporating Ti on the bilayergraphene/SiC substrate in Se atmosphere. The substrate was kept at ∼450 °C during the sample growth. The as-grown film was annealed at ∼400 °C for 30 min, and was transferred to the ARPESmeasurement chamber without breaking vacuum. The growth process was monitored by reflection high-energy electron diffraction (RHEED). The custom-made STM system was operated at T = 6 K under ultrahigh vacuum below 2 × 10−10 Torr.29,30 ARPES measurements were carried out using a MBS-A1 electron-energy analyzer with a high-flux helium discharge lamp and a toroidal grating monochromator at Tohoku University. The energy and angular resolutions were set at 16 meV and 0.2°, respectively. The Fermi level (EF) of samples was referenced to that of a gold film deposited onto the sample substrate.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Author Contributions

K.S. carried out the fabrication of thin films and their characterizations. K.S. and Y.N. performed the ARPES measurements. R.S., P.H., and T.H. performed the STM experiments. K.S., T.S., and T.T. wrote the manuscript. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We thank H. Kimizuka, K. Yamada, Y. Tanaka, and S. Souma for their help in ARPES experiments. This work was supported by the JSPS (KAKENHI 15H02105), MEXT (Grant-in-Aid for Scientific Research on Innovative Areas “Science of Atomic Layers” and “Topological Materials Science”), and the fund from World Premier International Research Center, Advanced Institute for Materials Research. REFERENCES (1) Chhowalla, M.; Shin, H. S.; Eda, G.; Li, L. − J.; Loh, K. P.; Zhang, H. The Chemistry of Two-Dimensional Layered Transition Metal Dichalcogenide Nanosheets. Nat. Chem. 2013, 5, 263−275. (2) Di Salvo, F. J.; Moncton, D. E.; Waszczak, J. V. Electronic Properties and Superlattice Formation in The Semimetal TiSe2. Phys. Rev. B 1976, 14, 4321−4328. D

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1T-TiSe2 in The Presence of Single Atom Defects. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 92, 081101. (22) Ishioka, J.; Liu, Y. H.; Shimatake, K.; Kurosawa, T.; Ichimura, K.; Toda, Y.; Oda, M.; Tanda, S. Chiral Charge-Density Waves. Phys. Rev. Lett. 2010, 105, 176401. (23) Fang, C. M.; de Groot, R. A.; Haas, C. Bulk and Surface Electronic Structure of 1T-TiS2 and 1T-TiSe2. Phys. Rev. B: Condens. Matter Mater. Phys. 1997, 56, 4455−4463. (24) Rohwer, T.; Hellmann, S.; Wiesenmayer, M.; Sohrt, C.; Stange, A.; Slomski, B.; Carr, A.; Liu, Y.; Avila, L. M.; Kalläne, M.; et al. Collapse of Long-Range Charge Order Tracked by Time-Resolved Photoemission at High Momenta. Nature 2011, 471, 490−493. (25) Qian, D.; Hsieh, D.; Wray, L.; Morosan, E.; Wang, N. L.; Xia, Y.; Cava, R. J.; Hasan, M. Z. Emergence of Fermi Pockets in A New Excitonic Charge-Density-Wave Melted Superconductor. Phys. Rev. Lett. 2007, 98, 117007. (26) Zhao, J. F.; Ou, H. W.; Wu, G.; Xie, B. P.; Zhang, Y.; Shen, D. W.; Wei, J.; Yang, L. X.; Dong, J. K.; Arita, M.; et al. Evolution of The Electronic Structure of 1T-CuxTiSe2. Phys. Rev. Lett. 2007, 99, 146401. (27) Morosan, E.; Zandbergen, H. W.; Dennis, B. S.; Bos, J. W. G.; Onose, Y.; Klimczuk, T.; Ramirez, A. P.; Ong, N. P.; Cava, R. J. Superconductivity in CuxTiSe2. Nat. Phys. 2006, 2, 544−550. (28) Ogawa, N.; Miyano, K. Charge-Density Wave as An ElectroOptical Switch and Memory. Appl. Phys. Lett. 2002, 80, 3225. (29) Hanaguri, T. Development of High-Field STM and Its Application to The Study on Magnetically-Tuned Criticality in Sr3Ru2O7. J. Phys.: Conf. Ser. 2006, 51, 514−521. (30) Iwaya, K.; Shimizu, R.; Teramura, A.; Sasaki, S.; Itagaki, T.; Hitosugi, T. Design of An Effective Vibration Isolation System for Measurements Sensitive to Low-Frequency Vibrations. J. Vac. Sci. Technol., A 2012, 30, 063201.

(3) Zunger, A.; Freeman, A. J. Band Structure and Lattice Instability of TiSe2. Phys. Rev. B: Condens. Matter Mater. Phys. 1978, 17, 1839− 1842. (4) Hughes, H. P. Structural Distortion in TiSe2, and Related Materials-a Possible Jahn-Teller Effect? J. Phys. C: Solid State Phys. 1977, 10, L319−L323. (5) Whangbo, M. − H.; Canadell, E. Analogies Between The Concepts of Molecular Chemistry and Solid-State Physics Concerning Structural Instabilities. Electronic Origin of the Structural Modulations in Layered Transition-Metal Dichalcogenides. J. Am. Chem. Soc. 1992, 114, 9587−9600. (6) Yoshida, Y.; Motizuki, K. Electron Lattice Interaction and Lattice Instability of 1T-TiSe2. J. Phys. Soc. Jpn. 1980, 49, 898−905. (7) Suzuki, N.; Yamamoto, A.; Motizuki, K. Microscopic Theory of The CDW State of 1T-TiSe2. J. Phys. Soc. Jpn. 1985, 54, 4668−4679. (8) Rossnagel, K.; Kipp, L.; Skibowski, M. Charge-Density-Wave Phase Transition in 1T-TiSe2: Excitonic Insulator Versus Band-Type Jahn-Teller Mechanism. Phys. Rev. B: Condens. Matter Mater. Phys. 2002, 65, 235101. (9) Cercellier, H.; Monney, C.; Clerc, F.; Battaglia, C.; Despont, L.; Garnier, M. G.; Beck, H.; Aebi, P.; Patthey, L.; Berger, H.; et al. Evidence for An Excitonic Insulator Phase in 1T-TiSe2. Phys. Rev. Lett. 2007, 99, 146403. (10) Monney, C.; Battaglia, C.; Cercellier, H.; Aebi, P.; Beck, H. Exciton Condensation Driving the Periodic Lattice Distortion of 1TTiSe2. Phys. Rev. Lett. 2011, 106, 106404. (11) Monney, C.; Cercellier, H.; Clerc, F.; Battaglia, C.; Schwier, E. F.; Didiot, C.; Garnier, M. G.; Beck, H.; Aebi, P.; Berger, H.; et al. Spontaneous Exciton Condensation in 1T-TiSe2: BCS-Like Approach. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 79, 045116. (12) Monney, C.; Schwier, E. F.; Garnier, M. G.; Mariotti, N.; Didiot, C.; Beck, H.; Aebi, P.; Cercellier, H.; Marcus, J.; Battaglia, C.; et al. Temperature-Dependent Photoemission on 1T-TiSe2: Interpretation within The Exciton Condensate Phase Model. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 81, 155104. (13) Monney, G.; Monney, C.; Hildbrand, B.; Aebi, P.; Beck, H. Impact of Electron-Hole Correlations on The 1T-TiSe2 Electronic Structure. Phys. Rev. Lett. 2015, 114, 086402. (14) Goli, P.; Khan, J.; Wickramaratne, D.; Lake, R. K.; Balandin, A. Charge Density Waves in Exfoliated Films of Van Der Waals Materials: Evolution of Raman Spectrum in TiSe2. Nano Lett. 2012, 12, 5941− 5945. (15) Peng, J.-P.; Guan, J.-Q.; Zhang, H. − M.; Song, C. − L.; Wang, L.; He, K.; Xue, Q. − K.; Ma, X. − C. Molecular Beam Epitaxy Growth and Scanning Tunneling Microscopy Study of TiSe2 Ultrathin Films. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 91, 121113. (16) Samnakay, R.; Wickramaratne, D.; Pope, T. R.; Lake, R. K.; Salguero, T. T.; Balandin, A. A. Zone-Folded Phonons and The Commensurate-Incommensurate Charge-Density-Wave Transition in 1T-TaSe2 Thin Films. Nano Lett. 2015, 15, 2965−2973. (17) Renteria, J.; Samnakay, R.; Jiang, C.; Pope, T. R.; Goli, P.; Yan, Z.; Wickramaratne, D.; Salguero, T. T.; Khitun, A. G.; Lake, R. K.; et al. All-Metallic Electrically Gated 2H-TaSe2 Thin-Film Switches and Logic Circuits. J. Appl. Phys. 2014, 115, 034305. (18) Khan, J.; Nolen, C. M.; Teweldebrhan, D.; Wickramaratne, D.; Lake, R. K.; Balandin, A. A. Anomalous Electron Transport in BackGated Field-Effect Transistors with TiTe2 Semimetal Thin-Film Channels. Appl. Phys. Lett. 2012, 100, 043109. (19) Sugawara, K.; Sato, T.; Kanetani, K.; Takahashi, T. Semiconductor−Metal Transition and Band-Gap Tuning in Quasi-FreeStanding Epitaxial Bilayer Graphene on SiC. J. Phys. Soc. Jpn. 2011, 80, 024705. (20) Sugawara, K.; Sato, T.; Tanaka, Y.; Souma, S.; Takahashi, T. Spin- and Valley-Coupled Electronic States in Monolayer WSe2 on Bilayer Graphene. Appl. Phys. Lett. 2015, 107, 071601. (21) Novello, A. M.; Hildebrand, B.; Scarfato, A.; Didiot, C.; Monney, G.; Ubaldini, A.; Berger, H.; Bowler, D. R.; Aebi, P.; Renner, Ch. Scanning Tunneling Microscopy of The Charge Density Wave in E

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