Charge Density Wave Phase Transitions in Large-Scale Few-Layer 1T

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Functional Inorganic Materials and Devices

Charge Density Wave Phase Transitions in Large Scale Few-layer 1T-VTe2 Grown by Molecular Beam Epitaxy Xingyuan Ma, Tian Dai, Shuai Dang, Songdan Kang, Xuexian Chen, Wenqi Zhou, Gaili Wang, Hongwei Li, Ping Hu, Zhihao He, Yue Sun, Dan Li, Fengmei Yu, Xiang Zhou, Huanjun Chen, Xinman Chen, Shuxiang Wu, and Shuwei Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21442 • Publication Date (Web): 25 Feb 2019 Downloaded from http://pubs.acs.org on March 1, 2019

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Charge Density Wave Phase Transitions in Large Scale Few-layer 1T-VTe2 Grown by Molecular Beam Epitaxy

Xingyuan Ma,1 Tian Dai,1 Shuai Dang,1 Songdan Kang,1 Xuexian Chen,2 Wenqi Zhou,1 Gaili Wang,1 Hongwei Li,1 Ping Hu,1 Zhihao He,1 Yue Sun,1 Dan Li,1 Fengmei Yu,3 Xiang Zhou,4 Huanjun Chen,2* Xinman Chen,5 Shuxiang Wu,

1*

Shuwei Li1

1State

Key Laboratory of Optoelectronic Materials and Technologies, School of Materials Science and

Engineering, Sun Yat-Sen University, Guangzhou, 510275, People’s Republic of China 2State

Key Laboratory of Optoelectronic Materials and Technologies, Guangdong Province Key Laboratory of

Display Material and Technology, School of Electronics and Information Technology, Sun Yat-sen University, Guangzhou, 510275, People’s Republic of China 3Automation

College, Zhongkai University of Agriculture and Engineering, Guangzhou, 510225, People’s

Republic of China 4School

of Physics and Astronomy, Sun Yat-sen University Zhuhai campus, Zhuhai, 519082, People’s Republic of

China 5Laboratory

of Nanophotonic Functional Materials and Devices, Institute of Opto-electronic Materials and

Technology, South China Normal University, Guangzhou 510631, China

*E-mail:

[email protected] (Shuxiang Wu)


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*E-mail:

[email protected] (Huanjun Chen)

ABSTRACT: Charge density wave (CDW) as a novel effect in two-dimensional transition metal dichalcogenides (TMDs) has obtained a rapid rise of interest for the physical nature and potential applications in oscillator and memory devices. Here, we report var der Waals epitaxial growth of centimeter scale 1T-VTe2 thin films on mica by molecular beam epitaxy. The VTe2 thin films showed sudden resistance change at temperature of 240 K and 135 K, corresponding to two CDW phase transitions driven by temperature. Moreover, the phase transitions can be driven by electric field due to local Joule heating and the corresponding resistance states are nonvolatile and controllable, which could be applied to the memory device where the logic states can be switched by electric field. The multistage CDW phase transitions in the VTe2 thin films could be contributed to electron–phonon coupling in the two-dimensional VTe2, which is supported by twice pronounced Raman blue shifts of the vibration modes associating with in-plane phonons at CDW phase transition temperature. The results open up a new platform for understanding of microscopic physical essence and electrical control of CDW phases of TMDs, expanding the functionalities of these materials for memory applications. KEYWORDS:

Charge

density

wave,

phase

electron–phonon coupling, memory device


transition,

TMDs,

VTe2,


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INTRODUCTION Two-dimensional transition metal dichalcogenides (TMDs) are attracting great attention due to their exotic physical properties1-3, including photoelectricity,4,

5

ferromagnetism,6 quantum hall effect,7, 8 and superconductivity.9 Charge density wave (CDW) as a novel effect in two-dimensional TMDs has obtained a rapid rise of interest in the physical nature and potential applications in oscillator10,11 and memory devices.12 The CDW state is a quantum state consisting of a periodic modulation of the electronic charge density accompanied by a periodic distortion of the atomic lattice.13-16 Early works on CDW effects were performed with bulk samples, for example, TaS3 or NbSe3 crystals, which have quasi-1D crystal structures of strongly bound 1D atomic chains which are weakly bound together by van der Waals forces.17-20 In the 1D system, the transition to the CDW state is conventionally described as the result of Fermi surface nesting giving an energy gap at the Fermi energy. Although the Fermi surface nesting theory is established well in the 1D system, the question of whether this theory actually applies to two-dimensional TMDs is on debate. This theory challenged by electronic structure calculations,21 Raman spectroscopy,22 and angle-resolved photoemission spectroscopy (ARPES) studies,23 revealing some controversies over the origin of the CDW in two-dimensional TMDs. Alternative explanations, such as electron−phonon coupling as well as a saddle point (van Hove singularity) mechanism have been proposed.24 Recent optical,22 spectroscopic,25,

26

and electrical transport studies7 on single-layer NbSe2 show


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different results regarding formation of CDW. The CDW transition had been widely observed in many two-dimensional group VB dichalcogenides, such as TaS2,12 TaSe2,27 TaTe2,28 NbSe2,22 VS2,29 VSe2,4 but not that much in VTe2. As a member of group VB dichalcogenides, two-dimensional VTe2 with hexagonal lattice parameters30 of a = b = 3.64 Å and c = 6.51 Å, should also show the CDW effect due to similar lattice and electronic structure. In this work, large scale 1T-VTe2 thin films were epitaxially grown by molecular beam epitaxy (MBE) on mica with hexagonal lattice surface. Two CDW phase transitions were observed, corresponding to two resistance switching and two pronounced Raman blue shifts. Moreover, the phase transitions can be driven by electric field and the corresponding resistance states are nonvolatile and controllable. These findings suggest that the 1T-VTe2 thin films can be expected to be a novel CDW material for the application in the memory device where the logic states can be switched by electric field. RESULTS AND DISCUSSION Two-dimensional VTe2 films were epitaxially grown on insulating mica with size of 12 mm × 12 mm by MBE. The mica is an appropriate substrate for van der Waals epitaxy of TMDs materials due to atomically smooth surface with hexagonal lattice symmetry and high-temperature endurance. Moreover, the mica is insulating substrate, which is suitable for electrical measurement in comparison to typical conductive substrates for the two-dimensional materials growth, such graphene, MoS2, and gold. Therefore, the VTe2 thin films could be epitaxially grown on the hexagonal surface of



insulating mica substrate and further perform the measurement of electrical transport properties. The films growth was monitored by in situ reflection high energy electron diffraction (RHEED), which provides plenty of fundamental information on film growth such as in-plane crystallographic orientation, lattice relationship between film and substrate, surface roughness of growing film, and evolution of crystal lattice. Figure 1a shows the RHEED patterns of the hexagonally symmetric surface of insulating mica substrate along and azimuths, respectively. The patterns are streaky, demonstrating the atomically flat surface of the mica, which is essential for the two-dimensional VTe2 film growth. The RHEED patterns of the VTe2 thin film along the same azimuth of mica shown in figure 1b indicate that the VTe2 thin film is hexagonally symmetric31 and thereby the films could be epitaxially grown on mica substrate with in-plane orientation. During the growth, the streaks of mica gradually became blur and then disappeared while that of VTe2 got more clear (S1). The disappearance of the mica patterns at approximately 12 minutes revealed that the surface of mica was covered by monolayer (ML) of VTe2 film, corresponding to the growth rate of 1 ML/12 min. Additionally, it was found that the sharp RHEED patterns of VTe2 film unchanged when electron beam scanned over the whole surface with size of 12 mm × 12mm, demonstrating high crystallographic homogeneity and atomically flat surface morphology of the VTe2 thin films in millimeter scale. In-plane lattice constants of VTe2 films were calculated to be a = b ≈ 3.67 Å from the streak spacing, consistent with bulk 1T-VTe2 (a = b = 3.64 Å).30, 31 The in-plane lattice constant of VTe2 is largely different from that of mica (a = 5.21 Å), and


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therefore it is difficult for the VTe2 to be epitaxially grown on mica directly. However, if the films were grown in the way of the 3 × 3 superlattice of VTe2 on 2 × 2 superlattice of mica, lattice mismatch would be lowered to approximately 4%. According to the azimuth relationship and lattice mismatch between VTe2 and mica, it was deduced that this lattice combination enables epitaxial growth of VTe2 with the lattice relationship of VTe2 (001) ∥ mica (001) and VTe2 [100] ∥ mica [100], as illustrated in figure 1c, d. To further confirm the phase of the VTe2 films, X-Ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) were performed at room temperature. Figure 2a shows the XRD pattern of the VTe2 films grown on mica, with a peak corresponding to the (001) plane of the hexagonal 1T-VTe2 crystals.30 The c-axis lattice constant is calculated to be 6.71 Å, further confirming that the VTe2 film is grown on mica substrate with in-plane lattice relationship. Figure 2b shows Raman spectrum of VTe2 thin films with three Raman phonon modes. The intense Raman peak of VTe2 at approximately 119 cm-1 is related to the Eg mode, corresponding to in-plane vibration, and A1g mode at approximately 138 cm-1 is contributed from out-plane vibration. Weak Raman peak at 92 cm-1 could be related to the Eg phonons with contributions coming possibly from symmetry points.27, 32 Figure 2c, d show the high-resolution XPS spectra of the VTe2 thin films grown on mica. The binding energies of V-2p3/2 (512.84 eV) and V-2p1/2 (520.45 eV) are close to that in 1T-VSe2 and 1T-VS2,6,

29, 33, 34

less than that in VO2.35 The binding energies of Te-3d5/2

(572.94 eV) and Te-3d3/2 (583.31 eV) correspond to the oxidation state of Te. The



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binding energy difference (ΔE) between V-2p3/2 and Te-3d5/2 in VTe2 is 60.1 eV, which approaches to that of V and Te elementary substance (60.7 eV), indicating V-Te bonds in VTe2 are not strong due to small electronegativity difference between V and Te. Recent years have witnessed an increasing interest in exploring the CDW phase transition in TMDs. Significant achievements have also been made in exploiting applications of such two-dimension CDW materials in electronics and memory applications.10-12 With increasing interest in exploring the CDW phase transitions in TMDs, various research techniques have been developed, such as scanning tunneling microscope (STM),6, 23, 28, 36 transmission electron microscope (TEM), 37, 38 and Angle Resolved Photoemission Spectroscopy (ARPES),23,

25, 36

but traditional electrical

transport measurements still play an irreplaceable role due to the convenience.10-12, 29, 39, 40

Electrical transport properties of the VTe2 thin films were measured by two-wire

method as illustrated in figure 3a. Figure 3b shows a temperature-dependent resistivity of the VTe2 thin films with thickness of about 7 nm (about 10 MLs). The resistivity was measured by low voltage of 40mV. As the temperature decreases from 300 to 40 K, the resistivity jumps at 240K and 135K, indicating that VTe2 thin films could go through two phase transitions. It should be noted a CDW phase transition always accompanies with the increase of resistance due to change of electronic structure. The resistivity change was also observed in many two-dimensional CDW materials, such as TaS2,11, 12, 39 VSe2,29 NbSe2,22 VS2,29 and TaTe2.28 In this work, the two obvious changes at 240 K and 135 K revealed that the CDW phase transition in


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VTe2 is not straightforward from normal state to CDW state and additional state is existed between them. The multistage CDW phase transition here is similar to CDW effects in 1T-TaS2. In VTe2 thin films, however, the resistance switching ratio is very small compared to that in 1T-TaS2. In 1T-TaS2, during CDW phase transition, the band gap opens and the material transforms from metal to insulator, so that the resistance switching ratio is tremendous.2 In VTe2, the CDW phase transition might not result in an evident energy gap at the Fermi energy and the resistivity does not increase prodigiously, which is similar to, for example, TaSe2 and NbSe2. In TaTe2, there are two different CDW phases, which are (3×3) hexagonal superstructure and (3×1) stripe structure, respectively.28 The multistage CDW phase transition in 1T-VTe2 could be close to that in TaTe2 due to their similar chemical compositions. Three characteristic resistance values at approximately 2.8, 3.4, and 4.1 ohm-μm correspond to normal state, CDW-I state and CDW-II state, respectively. The origin of the little change of resistivity in normal state would be discusses later. The CDW phase transition could be driven by not only temperature but also electrical field.10-12,

20, 39

More importantly, electrical driven CDW phase transition

could be applied to high-efficiency oscillators and memristive switching devices.10-12 In the VTe2 films, we also found the typical resistivity switching could be driven by in-plane electrical field and the resistivity states are nonvolatile, which could be applied to nonvolatile memory devices. Figure 3c shows the resistivity switching at electric field of approximately 7.2 V/cm when electric field scan from 0 to 14 V/cm at temperature of 120 K, below the CDW-I-CDW-II transition temperature, while the



inverse switching could not be observed as voltage swept back to 0 V/cm. The resistance switching (from 4.1 ohm-μm to 3.4 ohm-μm) is corresponding to the transition from CDW-II state to CDW-I state. Figure 3d shows the current-voltage characteristic at temperature of 210 K, below the CDW-I-normal transition temperature. Similarly, when the electric field is swept from 0 to 8 V/cm and back to 0V/cm, an abrupt resistivity switching from 3.4 ohm-μm to 2.8 ohm-μm was observed, corresponding to the phase transition from the CDW-I state to the normal state. CDW slide could be also responsible for the resistivity change in CDW materials with increase of voltage, but the change is gradual, not abrupt, as the CDW primordially pinned by crystal lattice could be triggered by application of external field.17-20,

40

Accordingly, beyond the critical electric field, the current gradually

increase and the CDW slide conduction exhibits as a kink or elbow in the I-V relationship. Hall et al. utilized a temperature dependent investigation into CDW depinning in NbSe3 to distinguish between “CDW slide” and “CDW switch”. The CDW slide was shown to result in a kink type transition in the I−V response while the CDW switch performed as an abrupt change in resistance. In addition, the onset field of CDW slide was explored and exhibits a negative correlation with temperature. Many transitions could be driven by local Joule heating as current flows through the material.12, 41 In the VTe2 films, the sudden increase of current at critical electric field should result from CDW phase transitions rather than CDW slide. The CDW phase transition could be due to Joule heating, which increases the local temperature above the


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transition

temperature.

The

resistivity-voltage

curves

extracting

from

the

current-voltage characteristics were shown in the inset of figure 3c, d. At 120 K, before and after the transition, the I-V responses both exhibit linear behavior, which suggests that the conduction due to CDW slide does not appear. However, at the temperature of 210 K, the low-field I-V response exhibits linear behavior, while the high-field I-V response exhibits nonlinear behavior which the resistivity reduces gradually as the increase of electric field. The source of the nonlinear I-V response would be the onset of CDW slide due to charge collective transport mode. Furthermore, the slight change of resistivity at high temperature region in resistivity-temperature curve could be also caused by CDW slide where the critical field of CDW slide reduces to less than the testing voltage of 40mV (figure 3b). These results suggest that the normal state is not a really metallic state due to the existence of CDW slide. It is worth noting that the phase transitions driven by in-plane electrical field are nonvolatile and the states after transition can be preserved for at least 24 hours if the ambient temperature unchanged. At the temperature of 210 and 120 K, the CDW-I (CDW-II) state transform into the normal (CDW-I) state due to the local temperature above the transition temperature, but the normal (CDW-I) state would not recover into CDW-I (CDW-II) state as the temperature fall back. The existence of the normal state and the CDW-I state in VTe2 thin films at lower temperature induced by in-plane electric field could be interpreted in consideration of kinetics. Due to the slow kinetics at VTe2 thin films, the thin film would need more time or some stimulation to reach



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its stable state. As voltage was swept and then stopped, the local temperature initially increased and then quenched, where the VTe2 thin film changed into high energy state (normal or CDW-I state) at first but had not time to recover due to the temperature decreased quickly and the kinetics was slow (figure 4b). Extremely slow kinetics in the nano-system was proposed for the mechanism to realize the nonvolatile switching behavior in 1T-TaS2.12 In this work, the thickness of the VTe2 thin films as-grown on mica is about 7 nm, which might achieve a slow kinetics in VTe2. Furthermore, the interfacial interaction between two material combined by van der Waals bond is significant, although relatively weak for as-grown thin film.7,

8, 42

The VTe2 layer

would be pinned by mica due to the small mismatch between the 3 × 3 superlattice of VTe2 and 2 × 2 superlattice of mica, which could result in slow kinetics at such thin VTe2 films. Therefore, the normal and CDW-I state could exist at lower temperature due to the slow kinetics at the mica-pinned VTe2 thin films. Although the normal state would not recover into CDW-I at same ambient temperature, the inverse transition from the normal state to the CDW-I state could be achieved by applying in-plane electric field at lower temperature, as shown in figure 4a (orange line). The inverse transition is possibly due to the growth of CDW-I (equilibrium state at this temperature) domains when the local temperature is increased by the Joule heating owing to the current flow. Here, the local temperature might not exceed the CDW-I-normal transition temperature. In addition, it is easy to control the phase transitions between the normal state and the CDW-I state by applying in-plane electric field, while the phase transition from the CDW-I state to the


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CDW-II state were unstable, which might result from the significant interfacial interaction between VTe2 thin films and mica substrate. The nonvolatile and controllable phase transitions between the CDW-I state and the normal state driven by in-plane electric field due to the slow kinetics in the mica-pinned VTe2 thin film could be a “Writing” and “Erasing” process, presenting the potential of the two-dimensional VTe2 rewriteable memristor. Recently,

not

only

electrical

transport

measurement,

but

also

variable-temperature Raman spectroscopy is a reliable and straightforward tool to investigate the CDW phase transitions in various two-dimensional materials and explore its nature.22,

27, 29, 32, 37, 43, 44

In order to further probe the multistage CDW

phase transitions and determine the CDW phase transition temperature in VTe2, variable-temperature Raman spectroscopy measurements were also carried out on VTe2 thin films with the same thickness. Figure 5 shows the temperature dependence of the Raman spectra for VTe2 thin films, from 298 to 77K. There are two prominent peaks at around 119 cm-1 and 138 cm-1, corresponding to the Eg vibration mode and the A1g vibration mode, respectively. The two modes are associated with in-plane phonons, as show in figure 2a. During the cooling process, the peak positions of Eg and A1g shifted mildly. The peak positions of the Eg mode and A1g mode as functions of temperature are plotted in figure 5b. It is worth noting that temperature dependence of peak positions of the Eg mode and A1g mode is almost similar and the both modes show evident blueshift with reducing temperature at approximately 250K and 140K. These two featured temperatures coincide with the transition temperatures in



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resistance-temperature curve as show in figure 5b. Both Eg and A1g modes, associated with in-plane phonons show evident blue shift at the phase transition temperature, similar to phenomena in 1T-VS2, 1T-VSe2 and 1T-TaS2.28,

43

With temperature

decreasing, both Eg and A1g mode initially shifts to higher energy slowly, which is expected due to the phonon hardening with decreasing temperature. However, with further cooling below 250 K, the blue shift rate of the peak positions sudden increase, indicating the gradual formation of the CDW-I phase. The formation accompanies by the lattice distortion, and accordingly the atoms condense and then combine into a stable supercell. The interaction force between neighboring atoms would become stronger with the atoms condensing together, and simultaneously the vibration frequency of phonons should increase. Correspondingly, the Raman peak positions gradually shift to higher energy and reach maximum value. With temperature decreasing, the CDW-I phase starts to degenerates, the interaction force would decrease, resulting in lowering of vibration frequency of phonons and thereby recovery of the Raman peak positions to low energy. Once the temperature decreases to critical point, the atoms displace again, and another supercell (the CDW-II phase) gradually form and then complete. Accordingly, the interaction force and vibration frequency increase again, give rise to second pronounced blue shift of the Raman peaks. The synchronous phenomenon of the two pronounced Raman blue shift and two phase transition suggests that the electron-phonon coupling is probably an vital mechanism for CDW formation in VTe2, comparing to other mechanism, such as the Fermi surface nesting and a saddle point, which would not perform the two CDW


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phase transitions accompanied by distinct Raman mode peak shift. CONCLUSION In summary, we have demonstrated that the CDW phase transitions in MBE-grown VTe2 thin films could be driven by temperature and in-plane electrical field. It was also found nonvolatile and controllable resistance switching are possibly the consequence of the slow kinetics realized as the VTe2 thin film could be pinned by mica substrate, which could be applying to rewriteable memory device. Furthermore, the two phase transitions in VTe2 thin film driven by temperature were observed by means

of

variable-temperature

Raman

spectroscopy,

suggesting

that

the

electron-phonon coupling would play a significant role in the CDW formation. These findings demonstrate that the 1T-VTe2 thin film is an advanced CDW phase transition material for fundamental research and for the application in the next-generation nonvolatile memory device.

MATERIALS AND METHODS Film growth and stability. The synthesis of VTe2 was achieved by depositing V and Te atoms on mica (001) in ultrahigh vacuum (1×10-8 mbar). Before growth, the mica substrates (van der Waals materials) were annealed at 500 ℃ for an hour to obtain atomically smooth surface, and then the temperature decreased down to 350 ℃ for the film growth. The VTe2 thin films were grown under a Te-rich condition and in-situ monitored by reflection high-energy electron diffraction. The flux of V source was 6 nA, giving rise to a slow growth rate of 1 ML/12 min. The few-layer VTe2 thin



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films could remain stable in the dry environment for 2 hours at least, but it would be oxidized quickly in the humid environment. The resistance of the oxidized sample is much higher than that of the unoxidized sample. Raman spectroscopy. Raman spectroscopy was performed using a commercial Raman system (Renishaw inVia Reflex) under normal incidence of a helium-neon laser (λ= 514.5 nm). The laser beam was focused on the samples by a × 50 objective, the beam diameter was about 1 μm. The sample was located in a continuous-flow liquid-nitrogen cryostat where the cooling rate is 5 K/min. Before each measurements, the temperature held steady for at lest 5 min. The Raman signal was collected in reflected configuration without an extra polarizer. Electrical transport measurement. The two-terminal devices were fabricated base on the VTe2 thin films grown on mica. All the electrical transport properties were measured by 4200-SCS (Keithley) when the temperature was controlled by SHI Cryocoolers RDK-101D (Sumitomo Heavy Industries. Ltd.) under high vacuum (10-4 mbar). The electrical measurements were preformed under high vacuum, so that the oxidation of material could be excluded.

ASSOCIATED CONTENT Supporting information Figures show Reflection high-energy electron diffraction (RHEED) patterns as a function of growth time during VTe2 growth on mica, surface morphology of few-layer

VTe2

on

mica,

survey

spectrum

of


VTe2

thin

films,

and

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temperature-dependent resistivity of the VTe2 thin films in warming process.

AUTHOR INFORMATION Corresponding Authors *E-mail:

[email protected] (Shuxiang Wu)

*E-mail:

[email protected] (Huanjun Chen)

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is supported by National Nature Science Foundation of China (Grant No. 61273310, 11304399 and 11274402), Nature Science Foundation of Guangdong Province (Grant No. 2015A030313121, 2016A030310234, and S2012020011003), Fundamental Research Funds for Central Universities (Grant No. 17lgpy02), and Science and Technology Planning Project of Guangdong Province (2015A01010312).

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Figure 1. Growth of epitaxial 1T-VTe2 films. a. The RHEED patterns of the hexagonally symmetric surface of insulating mica substrate. b. The RHEED patterns of the VTe2 thin films on mica c. Side-view crystal structure of 1T-VTe2 on mica. d. Top-view crystal structure of 1T-VTe2 and mica surface with the lattice relationship of VTe2 (001)∥mica (001) and VTe2 [100]∥mica [100].


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Figure 2. a. The XRD pattern for VTe2 thin films on mica shows (001) peak of the 1T phase, calculated out the c-axis lattice constant of 6.71 Å. b. The Raman spectrum of the VTe2 films shows two active Raman phonon modes at 119 and 138 cm-1 which is similar to bulk 1T-VTe2. The inset shows A1g and Eg vibrational modes. c, d. High-resolution XPS spectra of the VTe2 films show clean doublet feature of V-2p and Te-3d peaks, respectively, indicating homogeneous phases of 1T-VTe2.



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Figure 3. Electrical transport measurements for the VTe2 films on mica. a. Schematic representation of two-terminal device. b. The temperature-dependent resistivity of the VTe2 thin films with thickness of about 7 nm exhibits two abrupt jumps at 135 K and 240 K, corresponding to two CDW phase transitions. The resistance values were measured by a 40mV impulse current. c, d. Typical current-voltage sweeps show abrupt resistance switching at temperature of 120 and 210 K, respectively, indicating the VTe2 thin films transform from CDW-II (CDW-I) state to CDW-I (normal) state driven by in-plane electric field. Insets show the corresponding resistivity-voltage curves.


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Figure 4. Voltage-driven memristive phase transition in VTe2 thin films. a. Resistivity-voltage curves measured at 160 K (orange line) and 210 K (purple line). The phase transitions between normal state and intermediate state are controllable and could be a “Writing” and “Erasing” process, presenting the potential of the VTe2 memristor. b. Schematic energy diagram of the VTe2 thin film, showing a multiminimum potential. Purple and green arrows indicate the application of electric field at high temperatures, which results in the nonvolatile phase transitions due to the local Joule heating and the slow kinetics. Orange arrow indicates the application of electric field at low temperature, which results in the growth of CDW-I domains due to the increase of local temperature.



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Figure 5. Temperature dependence of the Raman spectra of VTe2 thin films with thickness of about 7 nm. a. Variation of Raman spectrum of VTe2 films with sample temperature varying from 298 to 77 K. b. Peak position plots of the Eg and A1g mode for VTe2 thin film as functions of temperature. Two Onsets of pronounced Raman blue shifts take place at CDW-I and CDW-II phase transition temperatures.


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TOC Figure


Charge Density Wave Phase Transitions in Large-Scale Few-Layer 1T

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