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Disorder Enhanced Superconductivity toward TaS2 Monolayer Jing Peng,† Zhi Yu,† Jiajing Wu,† Yuan Zhou, Yuqiao Guo, Zejun Li, Jiyin Zhao, Changzheng Wu,* and Yi Xie
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Hefei National Laboratory for Physical Sciences at the Microscale, CAS Center for Excellence in Nanoscience, Hefei Science Center of Chinese Academy of Science (CAS), and CAS Key Laboratory of Mechanical Behavior and Design of Materials, University of Science & Technology of China, Hefei 230026, PR China S Supporting Information *
ABSTRACT: Appearance of disorder is commonly known as detrimental to two-dimensional (2D) superconductivity, and typically results in decrement of the critical transition temperature (Tc). Herein, an anomalous enhancement of superconductivity was observed in TaS2 monolayer with function of disorder induced by structural defect. Owing to controlled pore density by acid concentration during chemical exfoliation, the disorder level in TaS2 framework can be effectively regulated. Dome-shaped behavior was uncovered in disorder dependence of superconductivity toward the chemically functionalized TaS2 monolayers, with Tc enhanced from 2.89 to 3.61 K when below critical disorder level. The disorder-engineered Tc enhancement, which distinctly differs from monotonic decrement in deposited 2D superconductors, can be ascribed to the increment of carrier density induced by Ta atom absence. The exotic superconducting enhancement would give help to deeply understand the correlation between superconductivity and disorder in 2D system. KEYWORDS: two-dimensional superconductivity, disorder, 2H-TaS2, structural defect, carrier density
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ternary compounds holds promise for building a clear platform.15−18 Their freestanding 2D feature with homogeneous single-layer structure protects them from perturbation by substrate and thickness. Through introducing intrinsic structural or chemical defects, the 2D structure can be readily modulated and even generate different magnetic and electronic properties such as ferromagnetism and metal−insulator transition.19−21 The framework also emerges to exotic 2D superconducting phenomena when contrast to deposited films, and a different view of QPT diagram is provided in 2D superconducting NbSe2 and ZrNCl due to the incorporation of the quantum metallic state.15,18 Therefore, it is very attractive to investigate in-depth understanding of the emergent phases and correlation with internal disorder on the natural 2D superconducting structure. Nevertheless, an inevitable yet urgent challenge arises in how to control the intrinsic disorder within 2D lattice framework, which impedes the investigation. In this regard, chemical route toward defect engineering would be a promising way to investigate the impact of disorder to superconductivity.
n two-dimensional (2D) superconducting systems, disorder, which can be morphological or chemical, is always unwanted since superconductivity would be suppressed and even disappear as disorder increases.1−7 It is considered that the disorder can renormalize electron−electron interactions and enhance the Coulomb repulsion for pair-breaking effects, resulting in preclusion of phase coherence and thus fragility of 2D superconductivity.1,3,8 As a result, the competition between attractive electron−electron interaction and enhanced repulsion from disorder emerges to quantum phase transition (QPT) between superconducting and insulating state in 2D structure, with the critical transition temperature (Tc) typically decreasing by disorder.2,9−11 The detrimental effect has been widely observed both in amorphous and highly crystalline 2D superconductors, including various deposited metal, oxide and nitride films.7,12−14 In particular, the superconductivity in granular In/InOx and bismuth films was found to vanish as the normalstate resistance larger than quantum resistance h/4e2.1,2 Notably, these systems cannot survive without substrate, and the approach to 2D limit is thinning down the film thickness. This concomitantly increases disorder from substrate and thickness, and thus generates additional impact to superconductivity by the extrinsic disorder beyond 2D framework. The emergence of natural 2D superconducting materials, including transition metal dichalcogenides (TMDs) and some © XXXX American Chemical Society
Received: June 21, 2018 Accepted: August 20, 2018 Published: August 20, 2018 A
DOI: 10.1021/acsnano.8b04718 ACS Nano XXXX, XXX, XXX−XXX
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ACS Nano Herein, an anomalous disorder-enhanced of superconductivity emerges in chemically functionalized 2H-TaS2 monolayer, of which the disorder level can be well monitored by structural defect. Through modulating hydrion (H+) concentration, TaS2 monolayer with controlled structural defect was achieved. In particular, we uncovered a dome-shaped superconducting behavior with the function of defect in TaS2 monolayer, with Tc varying from 2.89 to 3.61 K. In contrast to monotonic decrement in conventional 2D superconductors, the Tc in TaS2 monolayer exhibits positive correlation with disorder when below critical defect level. Hall measurements revealed that the Tc enhancement can be ascribed to the increment of carrier density induced by Ta atom defect, which leads to enlarged density of state at Fermi energy. As disorder becomes strong enough, decrement of transition temperature emerges, which can be attributed to enhanced Coulomb repulsion by structural defect. The superconducting modulation by disorder would help to investigate the interplay of disorder and superconductivity in 2D framework.
Figure 1. Characterization of TaS2 monolayer by acid-assisted exfoliation. (A) HAADF image of as-exfoliated TaS2 monolayer by 1.0 M H+. (B) AFM and corresponding height profile of TaS2 monolayers.
subnanopores formed and embedded in the monolayer TaS2 lattice matrix by the acid-assisted exfoliation route (Figure 1A). Since the very pore-forming process can be well controlled by acid concentration, it can serve as an effective tool to control disorder level. Indeed, the structural disorder by H + manipulation was confirmed and revealed by corresponding fast Fourier transform (FFT) of atomic images. From Figures 2A−C and S3, the FFT image consists of sharp spots in TaS2
RESULTS AND DISCUSSION As a typical layered material with trigonal prismatic coordination of metal atoms, 2H-TaS2 is documented that superconductivity and charge density wave (CDW) coexist in bulk phase, indicating strong correlation between electron and phonon in the shaping of its physics.22−24 Previous reports pointed out the superconducting transition temperature of TaS2 bulk can be enhanced by either intercalation of molecular or chemical doping of metal atoms.23,25−27 Thinning down the thickness of TaS2 bulk to few-layer or single-layer was also found to enhance Tc, whereas it is still unknown the mechanism behind enhancement.28−30 For 2D materials with atomic thickness, it is known that the large specific surface area of the nanosheets always results in high sensibility of electronic configuration to structural variation, and thereby surface chemical modification could provide an effective way to engineer their intrinsic physical properties.31,32 Previous works indicated that lithium intercalation and exfoliation is an efficient way to synthesize high-quality TMD monolayer, and their lattice or electronic structure can be modulated by lithium content or reaction additive.21,33,34 In particular, when the lithium intercalated 2H-TaS2 single crystal is treated by acid before exfoliation, TaS2 monolayers with different pore density can be achieved,34 providing opportunity to investigate the correlation between superconductivity and disorder in single-layer TaS2 with natural 2D structure. Through acid-assisted exfoliation, large-sized 2H-TaS2 monolayers were prepared here. Scanning transmission electron microscopy (STEM) was performed to identify their microscopic structure. As displayed in Figures 1A and S1, typical hexagonal atomic arrangement was found in high-angle annular dark field (HAADF) images toward as-exfoliated TaS2 nanosheet assisted by 1.0 M H+, indicating 2H metallic phase remaining during exfoliation process. Atomic force microscopy (AFM) was further performed to characterize the thickness of nanosheets dropped on Si/SiO2 substrate, which was displayed in Figure 1B. The topographic height of ∼0.86 nm was verified by the large-area scanning AFM on a representative TaS2 nanosheet, demonstrating single-layer structure with several tens microns of lateral size. Raman spectra and energydispersive X-ray spectroscopy (EDS) further demonstrated the as-synthesized nanosheet is TaS2 with monolayer structure, which were shown in Figure S2 and Table S1. Notably,
Figure 2. Disorder strength measurement of TaS2 monolayer. FFT of atomic images for TaS2 monolayers treated with (A) 0.1 M, (B) 1.0 M and (C) 10.0 M H+, respectively. (D) Pore density of TaS2 monolayers exfoliated with different c(H+).
monolayer exfoliated by 0.1 M H+, whereas the spots become blurrier as H+ concentration (c(H+)) increase. When c(H+) further increasing and reaching to 10.0 M, fairly blurred spots comprise the FFT image. The gradually blurred spots in FFT images visualize the disorder strength in TaS2 monolayer, verifying enhanced disorder level by increasing c(H+). Furthermore, Figure 2D reveals that pore density exhibits a positively correlation with c(H+). The quantitative relationship further demonstrated the disorder level can be significantly varied by different c(H+). The modulated vacancy density thus B
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Figure 3. Superconducting properties of TaS2 monolayer. (A) Optical image of a typical PMMA protected TaS2 monolayer device. (B) Temperature dependence of resistance for bulk 2H-TaS2 and monolayer treated by 1.0 M H+. Inset: superconductivity of bulk and monolayer, Rn is the resistance at 5 K, and the dash line is fitted by AL model. (C) Sheet resistance of monolayer TaS2 treated by 1.0 M H+, as a function of temperature under perpendicular magnetic fields from 0 to 2 T. (D) Temperature dependence of the out-of-plane critical field for TaS2 monolayer exfoliated by 1.0 M H+. The dash line is fitted by the standard linearized GL theory.
Figure 4. Superconducting properties of TaS2 monolayer treated with gradient c(H+). Temperature dependence of (A) Sheet resistance and (B) Normalized resistance for TaS2 monolayers with treated c(H+) ranging from 0.1 to 10.0 M. (C) The Tc of TaS2 monolayers plotted with c(H+). For c(H+) less than 2.0 M, the Tc is extracted by fitting of AL model. For c(H+) more than 2.0 M, the Tc is the onset temperature of superconductivity.
much higher than that reported in bulk (0.8 K).36 It is worth noting that when TaS2 bulk is mechanically exfoliated into single-layer or inserted molecular to gigantically expanse interlayer distance (up to 60 Å) so as to weaken the interaction between adjacent layers, the superconducting transition temperature can be enhanced, despite the mechanism is still ongoing debate.37,38 Similarly, the discovery of superconductivity in chemically synthesized TaS2 monolayer verifies the existence of superconductivity in individual TaS2 monolayer even with considerable structural disorder. To further study the superconductivity in TaS2 monolayer, zoomin on the resistive transition under perpendicular magnetic fields from 0 to 2 T was conducted. As displayed in Figure 3C, the superconducting state is entirely quenched until 2 T, demonstrating the superconductivity of TaS2 monolayer is robust against magnetic field. Temperature dependence of the
propels TaS2 monolayer as a favorable platform to study its superconductivity affected by facilely controlled disorder strength. Given the fact that metallic TMDs with atomic thickness are usually unstable in ambient condition probably due to the surface passivation or oxidation,15,28 TaS2 monolayers were capped with poly(methyl methacrylate) (PMMA) film before measurement, which was shown in Figure 3A. To check for the presence of superconducting properties in our as-fabricated monolayer devices, magneto-electrical transport measurement was conducted in a Physical Property Measurement System. From Figure 3B, temperature dependence of resistance for TaS2 monolayer exfoliated by 1.0 M H+ was tested; as temperature cooled, an evident and sharp superconducting transition with Tc of 3.59 K emerges (fitted by Aslamazov− Larkin (AL) model),35 of which the transition temperature is C
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Figure 5. Carrier density and related chemical component by function of c(H+). (A) Carrier density of TaS2 monolayer with different c(H+). (B) Ta content in TaxS2 with different c(H+).
out-of-plane critical field for TaS2 monolayer was plotted and fitted with the standard linearized Ginzburg−Landau (GL) theory4 in Figure 3D, and the GL coherence length (ξ) of 13.2 nm and out-of-plane critical field of 1.91 T was extracted for TaS2 monolayer treated with 1.0 M H+. The coherence length is much shorter than that of bilayer or multilayer,29,30 indicating its monolayer feature. While the structural defect is arbitrary distributed in TaS2 matrix with considerable density, the obtained Tc of 3.59 K is higher than that achieved by mechanical exfoliation (3.40 K by AL model and ∼3.3 K for temperature at 90% of normal state resistances).29,30 The Tc enhancement in disordered 2D system indicates that structural disorder in TaS2 monolayer should be taken into consideration for playing significant roles. To explore the correlation between superconductivity and disorder in functionalized monolayer framework, the temperature dependence of resistance for six kinds of TaS 2 monolayers was tested as shown in Figures 4A,B and S4, of which the c(H+) ranging from 0.1 to 10.0 M. On cooling, all of the samples emerge to superconducting transition, despite the resistances of TaS2 monolayers with 5.0 and 10.0 M H+ treated does not drop to zero at 2 K. Clearly, the Tc (fitted by AL model in Figure S5) varying with c(H+) was shown in Figure 4C. Before 1.0 M H+ treated, an exotic increment of Tc occurs with the disorder level becoming stronger, and then Tc reaches a maximum value of 3.61 K at 1.0 M H+, after which the Tc turns decreasing. Therefore, through controlling structural disorder by treated H+, the Tc of TaS2 monolayer exhibits a dome-shaped behavior and can be significantly enhanced from 2.89 to 3.61 K at relatively low disorder level. Prominently, in contrast to conventional 2D superconductors with monotonic decrement by disorder, the Tc in TaS2 monolayer exhibits anomalous positive correlation when disorder below critical level with treated c(H+) of 2.0 M. It is worth noting that the dome-shaped superconductivity was also observed in systems including cuprates6 and TiSe2 or MoS2 by gate regulation,39,40 and carrier was expected to play an essential role in monitoring the superconducting behavior. Thus, carrier density is considered to have important impact on the disorder engineered superconductivity in TaS 2 monolayer. To check it out, carrier density from Hall measurement toward TaS2 monolayers with different c(H+) treated was performed. As seen in Figures 5A and S6, monotonous enhancement of carrier density can be found in TaS2 monolayer when increasing c(H+). Importantly, although 2H-TaS2 is strongly correlated metal with multiband, a singleband model can describe the electronic feature below CDW temperature.41 For one-band model in 2D metal, owing to the
increment of 2D carrier density, the electron−phonon interaction is enhanced and thus enlarges the density of state at Fermi energy, which contributes to the Tc enhancement by disorder at relatively low level. Therefore, the disorder engineered superconductivity implies that carrier density could also play essential role in accounting for Tc enhancement of TaS2 monolayer when thinned down from bulk. The increment of carrier density can be explained by Ta atom defect modulated by acid, as confirmed by high resolution X-ray photoelectron spectroscopy (XPS) shown in Figures 5B and S7; The Ta/S ratio decreases as c(H+) increases due to the etching of acid. Since the TaS2 monolayer is hole-type metal, which is demonstrated by the positive RH in Figure S6, the increment of hole density by Ta atom defect results in Tc enhancement. However, when Ta atom defect level becomes strong enough, long-ranged Coulomb repulsion is enhanced and responsible for pair-breaking effects, resulting in subsequently decrement of transition temperature after crossing peak. Indeed, the up-warp of temperature dependence of resistance curve during the low-temperature region in TaS2 monolayers with 5.0 and 10.0 M H+ emerges and reveals the occurrence of insulating transition from metallic state (Figure S8). The transition is widely observed in low-dimensional systems affected by strong disorder due to 2D weak localization effect,4,8 which was further verified by linear logarithmic temperature dependence of resistance and negative magnetoresistance at low temperature region in Figure S9, thereby indicating that evident disorder level already exists and results in Tc decrement in TaS2 systems with relatively strong structural disorder. Therefore, when c(H+) is less than 1.0 M, the Tc of TaS2 monolayer reveals significant enhancement; whereas Tc decrement occurs as c(H+) further increasing. We further consider the impact of pore size and CDW order to Tc in Figures S10−S12. However, no CDW transition is found in these series of TaS2 monolayers, as shown in Figure S12. Therefore, CDW order is unlikely to have pronounced impact in tuning Tc between TaS2 monolayers.
CONCLUSIONS In conclusion, we have reported a disorder enhanced superconductivity in 2H-TaS2 monolayer. Through acidassisted exfoliation, TaS2 monolayer with controlled structural disorder level was achieved by modulating c(H+). In our case, the sheet was uncovered to be superconducting with a highest transition temperature reaching to 3.61 K, demonstrating superconducting case of individual single-layer TaS2 via chemical synthesis. Furthermore, dome-shaped superconductD
DOI: 10.1021/acsnano.8b04718 ACS Nano XXXX, XXX, XXX−XXX
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ing behavior with function of disorder was found in the chemically functionalized TaS2 monolayer, of which the carrier density can be modulated by varying disorder level. The increased carrier density leads to enhanced electron−phonon interaction with enlarged density of state at Fermi energy, which contributes to the Tc enhancement of superconductivity in relatively low disorder strength. The disorder modulated superconductivity would help the in-depth understanding of 2D superconductivity in strongly correlated system.
Changzheng Wu: 0000-0002-4416-6358 Yi Xie: 0000-0002-1416-5557 Author Contributions †
J.P., Z.Y., and J.W. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS This work was financially supported by the National Basic Research Program of China (2015CB932302), the National Natural Science Foundation of China (91745113, J1030412), National Program for Support of Top-notch Young Professionals, the Fundamental Research Funds for the Central Universities (No. WK 2060190084) and the Major/Innovative Program of Development Foundation of Hefei Center for Physical Science and Technology (Grant No. 2016FXZY001). We appreciate the support from the USTC Center for Micro and Nanoscale Research and Fabrication, and also thank the staff at BL11U (National Synchrotron Radiation Laboratory in Hefei, China) for providing the beam time and helpful discussion.
METHODS Preparation of 2H-TaS2 Monolayers. 2H-TaS2 monolayers with subnanopore structure were obtained by acid-assisted exfoliation. 2HTaS2 single crystals mixed with 1.6 M n-BuLi were sealed in a quartzlined autoclave filled with argon and then kept at 100 °C for 12 h. The intercalated products were washed by n-hexane several times and dried by argon gas. The dried products were first pretreated with H2SO4 for 1 min. Then the remnant acid was removed and deionized water was added to exfoliate the products into monolayers by manual shaking. These monolayers were purified by centrifuged at 1000 and 8000 rpm for 10 min, respectively. Device Fabrication and Transport Measurement. 2H-TaS2 monolayers was dropped onto Si/SiO2 substrate and Cr (10 nm)/Ti (50 nm) contacts were defined using standard electron beam lithography followed by thermal evaporation. After then, PMMA film was coated onto the devices to protect them from passivation by air. The transport measurements of more than 18 samples were carried out in Physical Property Measurement System from 300 K down to 2 K with excitation current limited to 1 μA in order to avoid the heating effects. Hall measurement was acquired at 6−100 K under magnetic field ranging from −10 000 to 10 000 Oe. To calculate the total carrier density n2D, high-temperature Hall coefficient (RH) at 100 K was used.42 RH can provide a good approximation for estimating the carrier density by using the equation below
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n2D = 1/eRH where e is the elementary charge. Other Characterizations. The optical images were collected on Olympus BX51M. AFM was conducted by AFM (Bruker, Demension Icon) using contact mode. STEM-HAADF was measured on JEOL JEM-ARM200F microscope with spherical aberration correction. High-resolution XPS measurements were carried out by using an ESCALAB 250 high-performance electron spectrometer using monochromatized Al Kα (hν = 1486.7 eV) as the excitation source and at the Catalysis and Surface Science Endstation at the BL11U beamline in the National Synchrotron Radiation Laboratory (NSRL) in Hefei, China. The pore density is quantified by counting pore number in atomic images within 20 nm × 20 nm areas, and it is calculated by pore number/4, with the units of 100 nm−2.
ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b04718. Detailed experimental synthesis procedures and supplementary characterizations including HAADF, temperature dependence of sheet resistance, AL fitting, Hall coefficient, XPS raw data, weak localization discussion, the impact of pore size and CDW to superconductivity (PDF)
AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. E
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DOI: 10.1021/acsnano.8b04718 ACS Nano XXXX, XXX, XXX−XXX
