Letter Cite This: Nano Lett. XXXX, XXX, XXX−XXX
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Strongly Compressed Few-Layered SnSe2 Films Grown on a SrTiO3 Substrate: The Coexistence of Charge Ordering and Enhanced Interfacial Superconductivity Zhibin Shao,†,∇ Zhen-Guo Fu,‡,∇ Shaojian Li,†,∇ Yan Cao,† Qi Bian,† Haigen Sun,† Zongyuan Zhang,† Habakubaho Gedeon,† Xin Zhang,† Lijun Liu,† Zhengwang Cheng,† Fawei Zheng,‡ Ping Zhang,*,‡ and Minghu Pan*,† Downloaded via IDAHO STATE UNIV on July 17, 2019 at 20:30:48 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
†
School of Physics, Huazhong University of Science and Technology, Wuhan 430074, China Institute of Applied Physics and Computational Mathematics, Beijing 100088, China
‡
S Supporting Information *
ABSTRACT: High pressure has been demonstrated to be a powerful approach of producing novel condensed-matter states, particularly in tuning the superconducting transition temperature (Tc) of the superconductivity in a clean fashion without involving the complexity of chemical doping. However, the challenge of high-pressure experiment hinders further in-depth research for underlying mechanisms. Here, we have successfully synthesized continuous layer-controllable SnSe2 films on SrTiO3 substrate using molecular beam epitaxy. By means of scanning tunneling microscopy/spectroscopy (STM/S) and Raman spectroscopy, we found that the strong compressive strain is intrinsically built in few-layers films, with a largest equivalent pressure up to 23 GPa in the monolayer. Upon this, unusual 2 × 2 charge ordering is induced at the occupied states in the monolayer, accompanied by prominent decrease in the density of states (DOS) near the Fermi energy (EF), resembling the gap states of CDW reported in transition metal dichalcogenide (TMD) materials. Subsequently, the coexistence of charge ordering and the interfacial superconductivity is observed in bilayer films as a result of releasing the compressive strain. In conjunction with spatially resolved spectroscopic study and first-principles calculation, we find that the enhanced interfacial superconductivity with an estimated Tc of 8.3 K is observed only in the 1 × 1 region. Such superconductivity can be ascribed to a combined effect of interfacial charge transfer and compressive strain, which leads to a considerable downshift of the conduction band minimum and an increase in the DOS at EF. Our results provide an attractive platform for further in-depth investigation of compressioninduced charge ordering (monolayer) and the interplay between charge ordering and superconductivity (bilayer). Meanwhile, it has opened up a pathway to prepare strongly compressed two-dimensional materials by growing onto a SrTiO3 substrate, which is promising to induce superconductivity with a higher Tc. KEYWORDS: scanning tunneling microscopy/spectroscopy, molecular beam epitaxy, SnSe2/SrTiO3, compressive strain, charge ordering, enhanced interface superconductivity
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INTRODUCTION
recent study of bulk SnSe2 reports a periodic lattice distortion at high pressure (>17 GPa), in analogy to the behavior of CDW transition. 23 However, because of the stringent conditions of high-pressured experiments, further in-depth research for underlying mechanisms using other experimental methods is hindered. Thus, the searching for the synthesis of strongly compressed 2D materials by heterostructure engineering is urgently demanded.
Charge density wave (CDW), as a ground state, is displayed as a periodic modulation of charge density below a critical temperature TCDW, resulting in the opening of electronic band gap at EF. Because the electronic properties of materials can be tuned dramatically by CDW phase transition, it is promising to fabricate novel electronic devices, such as oscillators,1 for nonvolatile memory storage.2,3 Over the past decade, the experimental and theoretical studies for two-dimensional (2D) CDW materials have mainly focused on layered transitionmetal dichalcogenides MX2 (M = Ta, V, Nb, Ti and X = S, Se, or Te).4−22 Generally, the reduced dimension and applied tensile strain are beneficial to the formation of CDW.16,19−21 A © XXXX American Chemical Society
Received: April 30, 2019 Revised: June 18, 2019 Published: July 9, 2019 A
DOI: 10.1021/acs.nanolett.9b01766 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 1. (a) Side and top views of 1T-SnSe2 structure. The interlayer distance of SnSe2 is 0.62 nm, while the lattice constant of a unit cell (bounded by the orange diamond) is 0.38 nm. (b) Topographic image of monolayer SnSe2 film grown on SrTiO3(001) (120 nm × 120 nm, Vb = 2 V, It = 20 pA). (c) Line profile along the orange arrow shown in panel (b). The distance between the substrate and the monolayer films is ∼0.79 nm, while the interlayer thickness between the monolayer and the second-layer films is 0.68 nm. (d−g) Atomically resolved images acquired on monolayer and bilayer SnSe2 films at positive and negative bias (It = 150 pA, size: 10 nm × 10 nm for panel (a) and 7 nm × 7 nm for panels (b)− (d). The panels on the right-hand side are the FFT images of atomically resolved images at negative bias. The 1 × 1 atom structure and 2 × 2 charge ordering are denoted by orange and green arrows, respectively.
detailed spectroscopic survey reveals an interfacial superconductivity that exists only in the 1 × 1 region with an estimated critical temperature (Tc) of 8.3 K. Such an enhanced Tc value, which is higher than previous reports on SnSe2 crystal or the SnSe2/graphene heterostructure, is mainly ascribed to a combined effect of charge injection and compressive strain induced by the SrTiO3 substrate, which is supported by firstprinciples calculation. Therefore, the SnSe2/SrTiO3 heterostructure prepared here offers an alternative platform to investigate novel charge ordering as well as the relationship between charge ordering and interfacial superconductivity without the limitation of high-pressure experiment.
More importantly, superconductivity can be implemented in some 2D CDW materials by applied pressure or intercalation with metal atoms;18,24−28 however, the relationship between CDW states and superconductivity remains controversial so far. Recently, interface superconductivity has been observed in the SnSe2/graphene heterostructure, induced by interfacial charge transfer.44 Considering that the SrTiO3 substrate can provide more charge injection29−32 and induce compressed epitaxial strain in thin films,33 the heterostructure of few-layers SnSe2 on the SrTiO3 substrate becomes much more promising as a platform to investigate the interplay between CDW and superconductivity. In this work, we have successfully grown strongly compressed few-layers SnSe2 on a SrTiO3 substrate via molecular beam epitaxy, and is further investigated by scanning tunneling microscopy/spectroscopy (STM/S) and Raman spectra. Upon this strong compressive strain, 2 × 2 charge ordering is induced in the monolayer film; meanwhile, an electronic band gap opens nea rthe Fermi energy (EF). As the compressive strain releases in the bilayer, the coexistence of charge ordering and 1 × 1 normal lattice region is observed. Subsequently,
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RESULTS Charge Ordering in Strongly Compressed SnSe2 Films Revealed by STM and Raman Characterization. As schematically shown in Figure 1a, the layered semiconductor SnSe2 has a CdI2 structure (the so-called “1T phase”), which consists of a trilayer structure with a layer of Sn atoms sandwiched between two layers of Se atoms. The SnSe2 B
DOI: 10.1021/acs.nanolett.9b01766 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 2. (a) Thickness dependence of the lattice constant and equivalent pressure. (b) Room-temperature Raman spectra measured on SnSe2 films with different thicknesses. Inset in panel (b) schematically shows the atomic displacements of the chalcogen atoms for corresponding Ramanactive vibrational modes.
crystal has a unit-cell lattice constant of 0.38 nm, while the interaction between sandwich layers is a weak van der Waals type of interaction, with an interlayer thickness of 0.62 nm.34,35 Figure 1b shows a topographic image of single-layer SnSe2 film. Similar to layered PdTe 2 and FeSe films grown on SrTiO3(001),33,36 here, SnSe2 thin films also follow a layerby-layer growth mode, which allows us to grow continuous SnSe2 films with full coverage. Some bright spots that are visible above the surface of the films are redundant single-layer islands. The line profile measurement presented in Figure 1c shows a distance of 0.79 nm between the substrate and the monolayer films, which is obviously larger than the thickness of a freestanding unit cell of SnSe2 crystal, because of the interaction between the substrate and the single-layer thin films. This phenomenon is also often observed in epitaxial thin films by MBE technology.37 Moreover, an interlayer height between films is ∼0.68 nm, which is slightly larger than the bulk value, suggesting a possible strain in SnSe2 films. As the coverage increases, the layer-by-layer growth mode continues and perfect layer-controllable SnSe2 films can be obtained. Both of the atomically resolved images of monolayer and bilayer films acquired at unoccupied states (Figures 1d and 1f) show a hexagonal lattice, corresponding to the top layer of Se atoms. For monolayer films, a lattice constant of 3.55 Å, denoted by a white rhombus, is smaller than the bulk value (3.8 Å). As the film thickness increases to bilayer, the lattice constant increases to 3.62 Å. The smaller lattice constants, in conjunction with the larger interlayer height, reveals a compressive epitaxial strain in SnSe2 films, which may arise from a lattice mismatch with SrTiO3 substrate. In contrast with that acquired at unoccupied states, a charge ordering with 2 × 2 periodicity, in addition to a 1 × 1 structure, was revealed by both atomically resolved images taken at occupied states and their Fast Fourier transition (FFT) patterns, as shown in Figures 1c and 1g, respectively. This 2 × 2 charge ordering dominates in the monolayer, but exhibits a short-range distribution in the bilayer. For bilayer films, the area with 2 × 2 charge ordering decreases obviously and a phase
separation can be observed. The different bias images of Figure 1g are shown in Figure S6 in the Supporting Information. The 2 × 2 charge ordering is only observed in negative bias and does not change as a function of energies, which eliminates a dispersive quasiparticle interference signal38 as the cause of our observation. When the thickness increases to 3 monolayers (ML), the lattice constant becomes even larger (3.68 Å), closer to the bulk value. More importantly, such charge ordering totally disappears in 3 ML, exhibiting a regular 1 × 1 hexagonal lattice at either unoccupied states or occupied states (Figure S1 in the Supporting Information). Such evolution of charge ordering with the thickness suggests that the appearance of 2 × 2 charge ordering is closely related with compressive strain in SnSe2 films. As the compressive strain releases with thickness increasing, the coverage of charge ordering decreases and finally disappear at 3 ML. The relationship between 2 × 2 charge ordering and compressive strain can be further illustrated by our STM observation in bilayer films. As we know, a local compressive strain field in a 2D film can be relieved by inducing a structural buckling, which lifts the surface atoms upward.39,40 As expected, the 2 × 2 charge ordering in Figure 1g mainly appears at the dark (lower) regions, as marked by a white dashed line, because of stronger compressive strain. For the case in the monolayer, the strongest compressive strain evidenced by the measured lattice constant leads to the dominance of the 2 × 2 charge-ordering region, while the inhomogeneous distribution of the strain field gives rise to the irregular distribution of 2 × 2 charge-ordering region and the appearance of a few areas with a 1 × 1 hexagonal lattice is the result of much less strain. Recent study reveals a periodic lattice distortion appearing at pressures of >18 GPa in bulk SnSe2 crystal, which is likely related to CDW states. Moreover, high-pressure X-ray diffraction (XRD) measurement also shows a strong in-plane lattice compression. To estimate the intensity of compression in few-layers SnSe2 films qualitatively, we compare lattice parameters under pressure in ref 23 with that acquired by our STM measurement. As shown in Figure 2a, we present the lattice C
DOI: 10.1021/acs.nanolett.9b01766 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 3. (a) Tunneling spectra measured on 2 × 2 charge-ordering regions of few-layers SnSe2 films. The triangles mark the energy position of CBM, and the black arrows denote the energy position of the DOS decrease near EF. Set point: Vb = 0.3 V, It = 200 pA, and the magnitude of the bias modulation for the lock-in technique is 8 mV. Inset shows the scheme of interfacial charge transfer for the SnSe2/SrTiO3 heterostructure. (b) Rectangular supercell band structures of monolayer SnSe2 without substrate (left panel) and strained lattice with the SrTiO3 substrate (right panel). Red and blue dots in the panel on the right-hand side denote the weight of the electronic wave function on the cation Sn and the anion Se respectively, while the contribution of SrTiO3 substrate is represented by black solid lines. Detailed information on the calculation can be found in Figure S4 in the Supporting Information.
decreases to ∼100 mV for the bilayer, matching well with the suppression of charge ordering in thick films. Downshift of the Conduction Band Minimum (CBM) Induced by Interfacial Charge Transfer and Compressive Strain. From the first-principles calculation of layerdependent properties of SnSe2,46 it is found to be an indirect band gap semiconductor, with a maximal band gap of 1.64 eV for the monolayer and a minimal band gap of 1.07 eV for the bulk. Such a result is distinct from our experimental observation. In our results (Figure 3a), an obvious layer dependence of the location of the conduction band minimum (CBM) marked by triangles indicates a downward band bending of the SnSe2 bands. Inspired by the study of SnSe2/ graphene,47 one reason could be attributed to interfacial charge transfer stemming from the SrTiO3 substrate. Bulk SnSe2 has an exceptionally large work function (5.3 eV)48 among metal dichalcogenide semiconductors, while Nb-doped SrTiO3 has a lower work function of ∼ 4.4−4.8 eV.49 Thus, the energy of the CBM in SnSe2 is lower than the EF value of the SrTiO3 substrate, leading to a considerable amount of electrons transferring from the SrTiO3 substrate to the SnSe2 films. Compared to the case of SnSe2 grown on graphene, in which the location of CBM is approximately −130 mV for the monolayer,47 the SrTiO3 substrate can provide stronger charge transfer in our case (evidenced by a CBM located at −240 mV for the monolayer). Furthermore, a graphene substrate has almost a same work function (4.4 eV), meaning that such a huge charge transfer cannot be simply attributed to the work function difference between the SnSe 2 and the SrTiO3 substrate. Two-dimensional electron gas, induced by the oxygen vacancies in the topmost TiO2 layer,31,32 can be formed on the surface of SrTiO3(001) substrate after high-temperature annealing. We perform a controlled experiment on the SrTiO3(111) substrate to explore the origin of the charge transfer. As shown in Figure S3, the dI/dV spectra measured on approximately freestanding SnSe2 films on SrTiO3(111)
constant and the equivalent pressure, as a function of thickness. Surprisingly, the equivalent pressure is as large as 23 GPa for monolayer SnSe2 on SrTiO3, but rapidly decreases to 16 GPa for the bilayer and 11 GPa for 3 ML films, indicating a critical pressure of charge-order transition between 11 GPa and 16 GPa (by rough estimation). Raman characterization is also performed on SnSe2 thin films under ambient conditions, as shown in Figure 2b. The 20 ML SnSe2 films exhibit two characteristic intralayer vibrational modes E1g and A1g, observed at ∼112.2 and 185.7 cm−1, resembling the bulk phonon modes.41,42 However, the characteristic in-plane E1g mode is suppressed in 3 ML, which can be attributed to inplane strong compression that resulted from the underlying SrTiO3 substrate.43 Consequently, Raman measurement also provides an evidence for compressed few-layers SnSe2 films. Unfortunately, monolayer and bilayer films are degraded quickly during measurement, because of exposure to air or heating by laser, and only the signal from the substrate can be detected. Existence of Prominent DOS Decreases near EF in the Region of Charge Ordering. Next, we perform tunneling spectroscopic measurements on few-layers SnSe2 films (Figure 3a) and then the obtained dI/dV spectra show a notable feature: dI/dV spectra acquired within charge-ordering regions show prominent decreases in the density of states (DOS) near EF. However, this decrease feature smears out within the 1 × 1 regions in 2 ML (Figure S2 in the Supporting Information). Moreover, such DOS decreases were not observed in approximately freestanding SnSe2 films either grown on graphene or on SrTiO3(111) (Figure S3 in the Supporting Information), and also disappears at 3 ML, where compressive strain is much weaker than the monolayer and bilayer films. Consequently, we suggest that this DOS decrease mainly originates from the formation of charge ordering, resembling the CDW gap states, as reported in transition metal dichalcogenide (TMD) materials.8,11,13,15,17,44,45 The gap of the monolayer has a size of ∼160 mV and subsequently D
DOI: 10.1021/acs.nanolett.9b01766 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 4. (a, b) Tunneling spectra acquired on different regions of monolayer films. (c) A series of dI/dV curves obtained from a line spectroscopic survey along the black arrowed line in the inset image. The 2 × 2 charge-ordering region is marked by a white dashed line in the inset. (d, e) Typical dI/dV spectra acquired on different regions of bilayer films. Set point: Vb = 10 mV, It = 300 pA, and the magnitude of the bias modulation for the lock-in technique is 0.5 mV. All the spectra are acquired at 0.4 K.
Figure 5. (a) Temperature and (b) magnetic field dependence of normalized dI/dV spectra acquired on the 1 × 1 region of bilayer films. Set point: Vb = 10 mV, It = 300 pA, the magnitude of the bias modulation for the lock-in technique is 0.5 mV. (c) Temperature and (d) applied field dependence of normalized zero-bias conductance extracted from panels (a) and (b). The deduced Tc and Hc values of 8.3 K and 5 T, respectively, can be obtained.
E
DOI: 10.1021/acs.nanolett.9b01766 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters show that the DOS at EF is empty, even for the monolayer, and the CBM shifts upward as the thickness increases, showing that interfacial charge transfer still exists but is much weaker than that on SrTiO3(001). On the other hand, compressive strain can also lead to a downshift of the conduction band, even resulting in metal−insulator transition.24 To explore those effects for SnSe2 films, we next perform first-principles calculation on simulative experimental situation. Figure 3b shows the band structure of monolayer SnSe2 films without (left panel) and with the SrTiO3 substrate (right panel). By comparing the results of different lattice constants, we find that strong compressive strain can lead to considerable movement of the band structure toward EF. However, different from our experimental observation, there is still a small gap across the Fermi level. Considering that the PBE method used in first-principles calculation usually underestimates the gap size of the semiconductor, CBM should be located at a higher energy. Thus, an SrTiO3 substrate indeed provides charge transfer into SnSe2 films, resulting in a nonzero DOS at EF, which is critically important for the emerging superconductivity in bilayer SnSe2 film. Subsequently, the substrate can further provide charge transfer into SnSe2 films, resulting in a nonzero DOS at EF, which is critically important for the emerging superconductivity in bilayer SnSe2 film. In conjunction with the calculated results, such a considerable downshift of CBM and the nonzero DOS for monolayer and bilayer films grown on SrTiO3(001) is most likely caused by the combined effect of interfacial charge transfer and compressive strain in the films. Enhanced Interfacial Superconductivity Observed in the 1 × 1 Region. A recent study has reported an interface superconductivity with a U-shaped spectrum in the SnSe2/ graphene heterostructure,47 which solely originates from interface charge transfer. It is worthwhile to determine whether a higher-Tc interfacial superconductivity exists in SnSe2/ SrTiO3(001) because of a stronger charge injection. We first perform a small-range tunneling spectra investigation on the monolayer. dI/dV spectra acquired at the regions of charge ordering exhibit a pair of asymmetrically weak peaks located outside a shallow gap at EF (Figure 4a), whereas a few spectra with a gap value of 6 meV (defined as half of the distance between two sharp peaks) after normalization is occasionally observed in the 1 × 1 region (Figure 4b). To investigate the evolution of gap feature between two regions, we focus on bilayer SnSe2 films, because of apparent phase separation. Spatially resolved dI/dV spectra measured across two phases (Figure 4c) show a variation that the coherent peaks and zerobias DOS decrease are obviously smeared out in chargeordering regions, exhibiting a higher zero-bias DOS. However, the spectra acquired on the 1 × 1 region show a well-defined V-shaped gap with a pair of sharp and symmetric coherent peaks located at approximately ±4 mV (Figure 4e), which usually is seen as a signature of superconductivity. Even at 0.4 K, no coherence peaks are observed (Figure 4d) in the 2 × 2 charge-ordering region. Thus, we confirm that the superconductivity exists only in the 1 × 1 region. To verify the characteristic of this gap, the temperature and magnetic field dependence of the dI/dV spectra are subsequently obtained and show the suppression of a coherent peak and the increase of zero bias conductance at higher temperature and magnetic field, in agreement with the behavior of superconductivity. To estimate the superconducting transition temperature (Tc) and critical field (Hc), the
normalized zero bias conductance values are extracted from Figures 5a and 5b. A rough linear fitting is performed, and the deduced Tc and Hc values of ∼8.3 K and 5 T, respectively, are obtained (Figures 5c and 5d). Here, compared with SnSe2/ graphene (Tc ≈ 4.84 K), the superconducting transition temperature has been improved. Not only charge transfer but also compressive strain can lead to considerable energy shifts of band structure toward EF, increasing the DOS at EF. Thus, we believe that the enhanced superconductivity should be a combined effect of interfacial charge transfer and lattice mismatch induced compressive strain. The possibly underlying enhanced electron−phonon coupling from SrTiO3 may also play a role in this superconducting enhancement, but no relevant evidence have been observed so far. Besides, the extremely large critical magnetic field is naturally reminiscent of our previous work of Sn islands grown on a SrTiO3 substrate, in which superconducting gap can even persist up to 8 T, because of the size effect and decreased electron meanfree path.50 The existence of apparent phase separation in bilayer films can induce strong electron scattering, resulting in the formation of a V-shaped superconducting gap instead of a U-shape, while reduce the electron mean-free path. A small electron mean-free path can decrease the superconducting coherence length ξ and increase the effective penetration length λ, leading to a much higher magnetic field, which is needed to destroy superconductivity.
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DISCUSSION The high Tc superconductivity in FeSe/SrTiO3 is mainly attributed to the consequence of combined effect of enhanced e-p coupling and interfacial charge transfer. In the case of SnSe2/SrTiO3, the charge transfer behavior is also observed and considered to be an essential factor for the emergence of interfacial superconductivity. As for the enhanced e-p coupling from the SrTiO3 substrate, the interfacial phonon mode is not observed here, perhaps because of the weak van der Waals interaction between the SnSe2 films and the SrTiO3 substrate. Besides, the effect of strain for superconducting enhancement in FeSe/SrTiO3 is still under debate. Generally, tensile strain favors the superconductivity in iron-based superconductors by suppressing the antiferromagnetic spin density wave.51 Here, compressive strain enhances superconductivity in SnSe2/ SrTiO3 in a distinctly different way by increasing the DOS at the Fermi level. Although an enhanced interfacial superconductivity is indeed observed, vortex imaging, which is more critical evidence for superconductivity, is still missing, because of the limited area of superconducting regions. On the other hand, the relationship between charge ordering and superconductivity is also unclear. Previous studies in TMD materials argue that a pseudogap, bearing a resemblance to the feature found in the small-range spectra of charge-ordering regions here, may be the precursor to the superconducting gap.17,22 Thus, further electronic doping experiments (e.g., depositing potassium element) to carefully investigate the evolution of either the morphology or tunneling spectrum of charge ordering are desired. Furthermore, compared with √3 × √3 periodic lattice distortion in pressured bulk SnSe2, different periodicity of charge ordering is perhaps caused by electron doping.52 We expect that future ARPES and transport measurement based on epitaxial SnSe2 films on SrTiO3 will show whether the charge ordering observed here is related to a true CDW quantum state. F
DOI: 10.1021/acs.nanolett.9b01766 Nano Lett. XXXX, XXX, XXX−XXX
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CONCLUSION
ASSOCIATED CONTENT
S Supporting Information *
In summary, strongly compressed SnSe2 films have been successfully synthesized. Compressive-strain-induced 2 × 2 charge ordering is observed in the first two layers. Accompanied by the formation of charge ordering, a prominent DOS decrease is observed at EF, showing a similarity with the gap states of CDW in TMD materials. As the compressive strain is released, the coverage of charge ordering decreases and a 1 × 1 normal lattice appears at residual areas, exhibiting an apparent phase separation in the bilayer. In contrast to charge-ordering regions with no superconductivity, the 1 × 1 region exhibits an enhanced interface superconductivity with a higher Tc of 8.3 K, compared with SnSe2/graphene, which is attributed to stronger interfacial charge transfer and compressive strain. Importantly, our results provide a new platform for further experimental and theoretical investigation, which also reveals a promising pathway to realize strong strain and charge injection in 2D materials, which can enhance or even induce superconductivity.
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Letter
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.9b01766.
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Supplementary figures (Figures S1−S4) (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (P. Zhang). *E-mail:
[email protected] (M. Pan). ORCID
Minghu Pan: 0000-0002-1520-209X Author Contributions
Z.B.S. and S.J.L. performed the MBE and STM experiments. Y.C. and Q.B. performed the Raman measurement. Z.B.S. and M.P. analyzed the STM data. Z.G.F., F.W.Z., and P.Z. performed DFT calculations. All authors discussed the results. Z.B.S. and M.P. wrote the paper with help from all authors. Author Contributions ∇
METHODS
These authors contributed equally to this work.
Notes
Sample Synthesis. The preparation of the substrate and the growth of SnSe2 thin films are performed in a standard molecular beam epitaxy (MBE) system with a base pressure of 2 × 10−10 Torr. The Nb-doped (001)-oriented single-crystal SrTiO3 substrate is heated to 1100 °C for 1 h in an ultrahigh vacuum (UHV) MBE chamber to obtain a clean and uniform surface. High-purity Sn (99.99%) and Se sources (99.99%) are coevaporated onto the SrTiO3 substrate at 215 °C from standard Knudsen cells. A Se-rich atmosphere with a Sn/Se flux ratio of 1:30 is used during the epitaxial growth process. After the evaporation, the sample is annealed at the growth temperature for 40 min. A higher post-annealing temperature (e.g., above 240 °C) leads to the desorption of Se atoms and the formation of defects. Subsequently, the sample is transferred into low-temperature scanning tunneling microscopy (Unisoku-1300) for in situ characterization. First-Principles Calculations. Our first-principles calculations were performed by employing the Vienna ab initio simulation package (VASP).53 We adopted the Perdew− Burke−Ernzerhof (PBE) form of generalized gradient approximation (GGA)54 for the exchange-correlation functional. The projected augmented wave (PAW) method was used to describe the electron−ion interactions.55 A plane wave cutoff energy of 450 eV was chosen. All atoms were fully relaxed until the Hellmann−Feynman forces on them were
