Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Multiple Transitions of Charge Density Wave Order in Epitaxial FewLayered 1T′-VTe2 Films Tian Dai,† Songdan Kang,† Xingyuan Ma,† Shuai Dang,† Hongwei Li,† Zilin Ruan,‡ Wenqi Zhou,† Ping Hu,§ Shuwei Li,† and Shuxiang Wu*,†
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†
State Key Laboratory of Optoelectronic Materials and Technologies, School of Materials Science and Engineering, Sun Yat-Sen University, Guangzhou 510275, People’s Republic of China ‡ School of Materials Science and Engineering, Kunming University of Science and Technology, Kunming 650093, People’s Republic of China § School of Electronic Information and Electrical Engineering, Huizhou University, No.46, Yanda Road, Huizhou City 516000, People’s Republic of China S Supporting Information *
ABSTRACT: Charge density wave phase comprises a periodic modulation of the electronic charge density with a periodic distortion of the atomic lattice, and transition-metal dichalcogenides are very important subjects for the study of this distortion. In this work, large-area 1T′-vanadium telluride (VTe2) films grown by molecular beam epitaxy show the charge density wave order down to the few-layer limit. The film thickness was found to be strongly correlated with the out-of-plane vibration mode shifting from Raman scattering, and increasing film thickness leads to an insulating-metallic transition. We demonstrate a charge density wave phase transition at 175 K for 2 (ML) films and multiple phase transitions in films more than 5 ML. It was also found that the transition was accompanied with a strong out-of-plane distortion. These findings open up a new window for the search and control of collective phases of twodimensional VTe2 films.
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sition properties on film thickness is essential for any thin-film application. Different thicknesses exhibit different structural and electronic phases. Each transition involves both conduction electron and lattice degrees of freedom-large changes. For device applications, it is expected to control these phases through electrical means, but this method is difficult to achieve in bulk crystals considering the high conduction electron density, and reducing dimensionality might provide a new way for manipulating the CDW phase in 2D materials. Previous studies demonstrated the CDW phase transition in mono and few-layered MX2 (M = Nb, Ti, Ta; X = S, Se);19−28 however, there were no reports about the charge density wave of fewlayered vanadium telluride (VTe2), a member of the metallic TMDC family. There has been reported the chemical vapor deposition- (CVD) and molecular beam epitaxy (MBE)-grown multilayered 1T-VTe2,29,30 but the structural and transport properties of few-layered 1T′-VTe2 remain unknown; the central issues might be the growth and unstability of ultrathin films. Here, we report systematic studies on the phase transitions of few-layered 1T′-VTe2 films grown by MBE. High-quality films were obtained and characterized by several
INTRODUCTION Layered transition-metal dichalcogenides (TMDCs) exhibit a number of unique structural and electronic phases,1−4 and manipulating these complex states has been a significant scientific and technological purpose.5−9 Charge density wave (CDW) phase in these low-dimensional materials is a vital and alluring research area and has attracted huge interest because of its profound physical mechanism and potential application in electronic devices such as nonvolatile memory storage and oscillator.10−16 A CDW is a periodic modulation of both the crystal lattice and electron density taking place from the instability of the coupled electron lattice system. Unlike superconductivity, the change of transport properties in materials undergoing a CDW transition is often weak. However, a CDW transition often has strong optical signatures. A significant softening of the phonon mode at the CDW wave vectors occurs while the temperature approaches the critical transition temperature Tc from above, which then freezes for T < Tc, enabling us to study the CDW transition through electron transport and Raman spectroscopy. The CDW and superconducting order as well as their phase transitions could be turned in 2D ultrathin form. 2D ultrathin CDW conductors with high transition temperatures are potentially suitable for applications in ultrafast memories, oscillators, and integrated circuits.17,18 Understanding the dependence of CDW tran© XXXX American Chemical Society
Received: May 28, 2019 Revised: July 8, 2019 Published: July 10, 2019 A
DOI: 10.1021/acs.jpcc.9b05062 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C
Figure 1. Film growth, atomic structure, and characterization of VTe2 crystals. (a) RHEED patterns of bilayer VTe2 films (lower panel) grown on HOPG substrates (upper panel); the streaks of HOPG and VTe2 are marked by red and white arrows. (b) Top and side views of the atomic structure of the 1T′-VTe2 crystal. (c) AFM topography of 1 L VTe2 film; the inset indicates the interlayer thickness of 6.3 Å. (d) AFM topography of the 6 ML VTe2 film; a clear layer-by-layer growth mode was demonstrated. (e) Raman spectrum under a 633 nm laser excitation for VTe2 films with different thicknesses at room temperature. (f) Thickness-dependence of the A1g peak position and the illustration of lattice vibration modes corresponding to Raman spectrum.
confirmed by atomic force microscopy (AFM) measurement (Figure 1c,d). AFM measurements of 1 monolayer (ML) and 6 ML VTe2 films carried out in the tapping mode reveal that the nucleation process favors to take place at the step edge of the HOPG substrates where the nucleation densities have the highest value. The nucleus at adjacent steps grows simultaneously and connects with each other to form a flat surface for the triangular-shaped second layer (Figure S1). However, the line profile of the monolayer VTe2 directly on the HOPG substrates ranges from 0.9 to 1.5 nm, and this deviation could be attributed to the large surface roughness of the substrates, and the impurities between the substrate and the VTe2 layer, and cannot fully determine the thickness. Hence, the thickness was determined by the second and third layers, which is 6.3 Å (Figure 1c, inset). Raman spectroscopy in the backscattering configuration under a 633 nm laser excitation was performed to examine the lattice vibrational modes of the as-grown films. All measurements were performed in vacuum to avoid the sample oxidation. An optical microscope with a 50× objective lens was used to collect the scattered light. Figure 1e shows the Raman spectra of VTe2 samples grown on HOPG substrates with a thickness range from monolayer to 10 layers. There are two prominent Raman features, which are A1g (147 to 138 cm−1 from monolayer to 10 layers) and Eg (∼121 cm−1); the peak around 149 cm−1 only appeared with films thicker than five layers. These weak peaks around 149 and 215 cm−1 in thicker samples might be attributed to the overlap of the two phonon processes of VTe2 films like TaSe2.15 From monolayer to bulk, the A1g mode shows a red shift of ∼9 cm−1 from 1 to 10 ML (Figure 1f); however, the shift of the Eg mode is barely observable. Such a behavior could be assign to the shearing mode of Raman peaks, where the A1g mode shifted to lower energy with increasing number of layers, and the strong shift is mainly attributed to the decrease of the force constant resulting from the structure changes and the weakening of
in situ and ex situ measurements. Transport measurement and room-temperature and in situ low-temperature Raman spectroscopy studies on VTe2 films have been systematically conducted; the well-matched combination of electrical and optical measurements demonstrates the thickness-dependent CDW transition in the as-grown 1T′-VTe2 films. Temperaturedependent X-ray diffraction (XRD) was also conducted and indicated that CDW transition was accompanied with the strong out-of-plane structure change.
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RESULTS AND DISCUSSION The growing process was carried out in an ultrahigh vacuum (UHV) chamber of the MBE system. Sapphire-based graphene and highly ordered pyrolytic graphite (HOPG) were chosen as the substrates due to the hexagonal surface symmetry and flat surface. The crystallinity and surface roughness of the grown films were characterized by in situ reflection high-energy electron diffraction (RHEED), which provides abundant, significant information on film growth. Figure 1a shows the RHEED patterns of the HOPG substrate and six-layer VTe2 film, and the characteristic streaks of each are marked by arrows. The lattice constant of VTe2 films extracted from RHEED patterns is 7.22 Å. The sharp streaks of RHEED patterns indicate that the as-grown VTe2 films are highly crystalline, and the streaks remained the same when the electron beam moved around the entire film, indicating such high crystalline quality of the sample in a large scale. There are three typical phases of MX2: 2H, 1T, and 1T′. 1T-VTe2 is composed of three hexagonally atomic layers in an ABC stacking, where the V atoms are octahedrally coordinated with the Te atoms. 1T-VTe2 films undergo a lattice distortion by controlling the substrate temperature and turn into the 1T′ phase through a doubling of the periodicity in the x direction, where V atoms are dislocated and a zigzag chain in the y direction was then formed. Figure 1b presents the crystal structure of 1T′-VTe2. The layered surface topography was B
DOI: 10.1021/acs.jpcc.9b05062 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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uprising of resistance reached the testing limit at 60 K, where the films are nearly insulated. Same trend was observed in films with thickness below 5 ML, indicating a semiconducting nature of thinner VTe2 films. However, transport behaviors of thicker VTe2 films are completely different. Upon an increase in the thickness of VTe2 films to 6 ML (Figure 3b), the resistance
the interaction of VTe2 layers and substrate with increasing film thickness. This trend is opposite to 2H layered materials such as 2H-MoS2 but similar to the 1T-like structures such as Bi2Se3 and TiSe2.31 Observation of these associations between different layers could be a reliable way to identify the thickness of VTe2 thin films. X-ray photoelectron spectroscopy (XPS) measurements of VTe2 films grown on HOPG are given in Figure 2a,b, and it
Figure 3. Transport measurement of (a) 2 to 5 ML and (b) 6 to 15 ML VTe2 films grown on MoO3 substrates.
decreases with decreasing temperature, suggesting a metallic behavior of thick films. The representative current−voltage (I− V) measurement showing a well-defined Ohm contact was shown in Figure S3. At temperatures below 285 K, a sudden drop in resistance was observed, indicating a first-order transition. Another switching in resistance occurred with further cooling down to 125 K, which could be evidence of a second transition. These transitions were highly reproducible and took place in thickness ranges from 6 to 15 ML, and the first transition temperature decreases upon increasing the film thickness. The strong thickness-dependent insulating-metallic transition suggests two different patterns of CDW phases in 1T′-VTe2 films. The normal phase of VTe2 changes to the distorted phase below the transition temperature Tc, and the possible CDW phase transition can be confirmed by analyzing structural vibrations and the electrical transport characteristics. Therefore, the temperature-dependent Raman measurements on VTe2 samples with different thicknesses were performed. To avoid oxidation, VTe2 samples were sealed within a nitrogen glovebox immediately after MBE growth and then transferred for Raman test under vacuum condition. The total sample exposure time in air is less than 3 min. Figure 4 shows the corresponding Raman peaks of 2 and 6 ML films. No laser damage was observed during the entire measurements. CDW phase transition is accompanied by an increase in electronic susceptibility, which can be preserved through relatively sudden stiffening of phonons below the critical phase transition temperature, and it is observed in the bulk that this sudden stiffening is more prominent for the A1g mode than for the Eg mode. The 2 ML films have much broader A1g peak than 6 ML films, which might result from the respectively higher instability compared with few-layer films. For monolayer VTe2 (Figure S2), the Eg and A1g modes are clearly visible at room temperature, although slightly broader compared to thicker samples. However, both peaks became too broad after cooling (to be discussed), while the peaks remain detectable. It can be seen that for 2 and 6 ML films, the main features are
Figure 2. Phase-structure identification of 15 layer VTe2 films. (a, b) XPS spectra of V 2p and Te 3d orbitals. (c) UPS measurement shows a sharp metallic edge for VTe2 films, which is consistent with the 1T′ structure of VTe2. (d) XRD pattern of 15 layer-thick VTe2 epitaxial thin film grown on sapphire-based graphene.
can be seen that both V 2p and Te 3d spectra are composed of only one chemical state, V4+ and Te2−, with no metallic V bonding detected. The stoichiometry of the grown film was calculated from XPS data to have a V:Te ratio of 1:1.93. Ultraviolet photoelectron spectroscopy (UPS) (Figure 2c) shows a metallic edge of 15 ML VTe2, indicating a metallic structure of VTe2 crystal. The crystallinity of the obtained VTe2 epitaxial thin film was characterized by out-of-plane XRD measurements. Figure 2d displays the XRD pattern taken along the direction of the VTe2 films grown on sapphirebased graphene substrates, with four main diffraction peaks corresponding to the (001), (002), (003), and (004) planes of the hexagonal VTe2 crystals, providing the c-axis lattice parameter of the obtained VTe2 thin film to be 6.33 Å, which is in good agreement with the AFM topography. Thickness-dependent transport behavior was conducted to study the phase transition with temperature. CVD-grown MoO3 on silica were used as substrates; same growth condition was adopted, and the lattice constant obtained from the RHEED pattern (Figure S3) indicated the same structure as films grown on HOPG and graphene. Two-terminal dc pulsed voltages and currents are used to demonstrate resistive switching between the various phases of 1T′-VTe2 samples. Electrodes were fabricated with indium and directly contacted on the surface of the VTe2 films. The temperature resistance (R−T) curve was measured immediately after the growth process at the cooling rate of 1 K/min. It can be seen that 2 ML VTe2 films show a typical semiconducting behavior, with an abrupt increase in resistance during cooling, which suggests that a transition to a distorted phase occurred. The sudden C
DOI: 10.1021/acs.jpcc.9b05062 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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further cooling the temperature below 285 K, the peak position reverses back to a low-energy state, and then, the A1g energy rises again, which may indicate that the VTe2 film undergoes another phase transition period, where the transition becomes complete at 125 K. The same transition was also observed in the A1g intensity with temperature, further indicating another distorted phase at low temperature. The strong thickness dependence of CDW order could be attributed to the substrate effects, where the transition of few-layered VTe2 films was strongly restricted by the van der Waals interaction, and increasing film thickness releases the stress caused by these substrates, which leads to the multiple phase transitions. Importantly, the transition temperature coincides with the electrical measurement, establishing a reliable and straightforward means to experimentally determine Tc in a much more accurate way. We use temperature-dependent XRD to determine the transition process as shown in Figure 6a. The 6 ML VTe2 film
Figure 4. Temperature-dependent Raman spectra of VTe2 films with different thicknesses of (a) 2 and (b) 6 ML; the slightly red shifts of the A1g mode were observed in all samples with increasing temperature.
still the A1g and Eg peaks, and the A1g mode of all samples blue shifts with decreasing temperature. For 6 ML films, the peak around 149 cm−1 becomes barely observable when the temperature is lower than 280 K, which might be the identification of a new distorted phase and consistent with transport measurement. To experimentally determine the dependence of Tc on the thickness of VTe2 films, we evaluate the A1g peak position and intensity as a function of temperature for sheets with thicknesses of 2 and 6 ML and shown in Figure 5. For 2 Figure 6. Phase transition of the 6 ML VTe2 film. (a) Temperaturedependent XRD from 298 to 138 K. (b) Corresponding CDW phase transition process extracted from XRD and the free energy schematic of CDW evolution with temperature.
grown on graphene/Al2O3 was used to analyze the structural transition along the c axis. It should be pointed out that a slight increase in the out-of-plane parameter with decreasing thickness was observed (6.33 Å for 15 ML and 6.47 Å for 6 ML), indicating that thinning induced swelling along the c axis, which was also theoretically proposed for the NbSe2 film by the first-principles calculation.32 A newly emerged diffraction peak at 12.7° attached with the original (001) peak was observed when temperature decreases below 198 K, suggesting the existence of an additional larger crystal structure, which indicates that the low-temperature phase possesses strong lattice distortions in both in-plane and out-of-plane directions compared to the room-temperature normal phase. The phase transition process and structural change extracted from XRD are shown in Figure 6, where V and Te atoms are periodically stretched from their original positions to form an additional new layer, thus explaining the larger out-of-plane lattice constant of 6.93 Å. Same phenomenon was found in monolayer VSe2 films.33 This phase transition can be understood by a free energy, and the height of the barrier decreases with respect to the CDW energy. When the CDW state has lower energy, the normal phase becomes metastable, but the system only transits into the CDW phase when the activation barrier becomes comparable to the thermal energy. The situation is reversed when warming from the CDW phase.
Figure 5. Thickness-dependent charge density wave order of (a) 2 and (b) 6 ML VTe2 films with the corresponding Raman peak position and intensity.
ML films, the A1g mode blue shifts with decreasing temperature and remains still at 175 K. The intensity of A1g mode decreases with decreasing temperature, which could be attributed to enhanced electron−phonon interaction. The same trend is apparent from the differential temperature− resistance curve, which could be evidence of a CDW transition at 175 K. It can be seen that the A1g mode of both bilayer and 6 ML films blue shifts with decreasing temperature. For 6 ML samples, upon cooling, the A1g mode initially shifts to higher energy; such a behavior is expected due to phonon hardening with decreasing temperature, which is also indicated by the sudden decrease in resistance at this point. However, upon D
DOI: 10.1021/acs.jpcc.9b05062 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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CONCLUSIONS
In conclusion, we have demonstrated and characterized the growth of large-area monolayer to few-layer 1T′-VTe2 films on different substrates and investigated the temperature and thickness evolution through electric transport and Raman spectrum measurements. It was found that the A1g peak position could be suitable for determining the number of layers in VTe2 films. Electric transport measurement indicated that the VTe2 films with different thicknesses exhibit completely different transport behaviors. The abrupt increase in resistance suggests a CDW phase in films below 5 ML, and the sudden switching of resistance of thicker films (≥6 ML) suggests the existence of multiple charge density wave orders, where transitions were further indicated by Raman spectroscopy via the peak position and intensity changes with temperature. It was also found that the transition process was accompanied with a strong out-of-plane distortion. The obtained data can be useful for further theoretical studies of the scaling effects on the CDW phase and the proposed application of the nextgeneration devices.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Tian Dai: 0000-0002-0385-3274 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the National Nature Science Foundation of China (grant no. 11304399), Nature Science Foundation of Guangdong P rovince (grant no. 2015A030313121), and Fundamental Research Funds for Central Universities (grant no. 17lgpy02).
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METHODS MBE Growth of VTe2 Films. HOPG, graphene on Al2O3, and CVD-grown MoO3 on silica were used as substrates. Prior to the main growth, the substrates were annealed at 500 °C for 30 min to degas and then decreased down to 350 °C, where the main growth process was performed. The growth chamber is equipped with an electron beam evaporator that enables the growth of V (99.95%), and an effusion cell was used for evaporation of Te (99.9999%). Due to the small sticking coefficient of chalcogen, the Te:V flux rate requires a relatively much higher value up to 50:1, which is 280 °C for Te evaporation and 4 nA for the V flux current. The Te shutter was kept open during the entire growth while the V shutter was opened at the beginning of main growth. The growth rate of VTe2 on HOPG was determined to be ∼0.017 nm·min−1. The as-grown VTe2 films were capped with ∼5 nm amorphous Te layers at room temperature to prevent possible oxidation. Characterizations. AFM images were taken using the Bruker dimension fastscan bio AFM in tapping mode. Roomtemperature and low-temperature XRD were measured by the SmartLab XRD system immediately after growth; the samples were sealed in a nitrogen box, and the total air exposure time was less than 2 min. Raman measurements were conducted in vacuum by a Renishaw inVia Raman system with a 50× objective lens under a 633 nm laser excitation. The lowtemperature measurements were carried out in an evacuated chamber cooled by liquid nitrogen.
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Article
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.9b05062. Surface topography measured by AFM and corresponding in situ RHEED patterns at different growth periods for the growth of monolayer VTe2 thin films; temperature-dependent Raman shift of monolayer VTe2 films; and I−V curve and RHEED patterns of VTe2 grown on MoO3 (PDF) E
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