Two-Dimensional 2M-WS2 Nanolayers for Superconductivity.

Recently, a newly discovered VIB group transition metal dichalcogenide (TMD) material, 2M-WS2, has attracted extensive attention due to its interesting physical properties such as topological superconductivity, nodeless superconductivity, and anisotropic Majorana bound states. However, the techniques to grow high-quality 2M-WS2 bulk crystals and the study of their physical properties at the nanometer scale are still limited. In this work, we report a new route to grow high-quality 2M-WS2 single crystals and the observation of superconductivity in its thin layers. The crystal structure of the as-grown 2M-WS2 crystals was determined by X-ray diffraction (XRD) and scanning tunneling microscopy (STM). The chemical composition of the 2M-WS2 crystals was determined by energy dispersive X-ray spectroscopy (EDS) analysis. At 77 K, we observed the spatial variation of the local tunneling conductance (dI/dV) of the 2M-WS2 thin flakes by scanning tunneling spectroscopy (STS). Our low temperature transport measurements demonstrate clear signatures of superconductivity of a 25 nm-thick 2M-WS2 flake with a critical temperature (T C) of ∼8.5 K and an upper critical field of ∼2.5 T at T = 1.5 K. Our work may pave new opportunities in studying the topological superconductivity at the atomic scale in simple 2D TMD materials.

Introduction

The topological superconductor (TSC) has been recognized as a promising candidate for making future topological quantum computers that can revolutionize current computing technology. The TSCs possess a full superconducting gap but gapless excitation at the surface (edge), sharing similar features of Majorana fermions initially predicted in particle physics.[1,2] In recent years, searching for condensed matter systems that support topological superconductivity has been a popular and important research direction in the condensed matter community. So far, topological superconductivity has been theoretically proposed or experimentally explored in two categories of materials: (1) intrinsic TSCs—single phase materials, such as Sr2RuO4,[3−6] CuBi2Se3,[7−9] and Sn1–InTe,[10−12] and ion-based superconductors Fe(Te, Se)[13−16] and (2) artificially engineered TSCs—heterostructures utilizing the superconducting proximity effect,[17−20] such as semiconductor nanowires (InSb),[21−23] magnetic wires (Fe),[24] topological insulators (Bi2Te3),[20] and quantum anomalous Hall insulators [(Cr0.12Bi0.26Sb0.62)2Te3][25] in direct contact with s-wave superconductors. However, the study of topological superconductivity in TSCs, in spite of the considerable progress in the above-mentioned systems, is still in its infancy. For future nanodevice application purposes, it would be ideal to realize topological superconductivity at high temperatures and atomic scales in TSCs with a much simpler composition. Thus, searching for TSCs in simple two-dimensional (2D) compounds has become a priority research topic. A notable recent achievement is the observation of the coexistence of topological surface states and superconductivity in a 2D binary transition metal dichalcogenide (TMD) 2M-WS2 with a critical temperature TC ≈ 8.9 K.[26−29] Here, 2M stands for a monoclinic structure with two layers per unit cell (Figure ). Thus, 2M-WS2 provides a new platform for exploring topological superconductivity/Majorana particles at the 2D limit. However, current research on this material is very limited, and there are only few publications on this topic thus far.[26−29] Therefore, it is critical for developing new synthesis methods to grow high-quality 2M-WS2 samples and study their physical properties at the atomic scale.

Figure 1

2M-WS2 crystal and structure. (a) Schematic of [WS6] structure. (b) Side view of the lattice structure of 2M-WS2, (c) top view of the lattice structure of 2M-WS2 with a W–W zigzag structure along the a direction. (d) Image of the as-grown 2M-WS2 crystals.

In this paper, we report a new method to grow high-quality 2M-WS2 crystals and the observation of superconductivity in its thin layers. The crystal structure of the as-grown 2M-WS2 bulk crystals was characterized using X-ray powder diffraction (XRD). The chemical composition of the 2M-WS2 was determined by energy dispersive X-ray spectroscopy (EDS) analysis. Atomically resolved lattice structures were obtained by scanning tunneling microscopy (STM) at 77 K. The spatial variation of the local tunneling conductance (dI/dV) of 2M-WS2 was clearly revealed by scanning tunneling spectroscopy (STS) measurements. To measure the superconductivity, the 2M-WS2 thin layers were exfoliated from the bulk crystal and isolated on Si/SiO2 substrates with prefabricated Hall-bar shaped Au electrodes. The thicknesses of the 2M-WS2 thin layers were measured using atomic force microscopy (AFM). The electrical transport properties of the 2M-WS2 thin layers were studied in a variable temperature insert with a base temperature of 1.5 K and under a magnetic field perpendicular to the current applied. A clear signature of superconductivity in a 25 nm-thick 2M-WS2 flake was observed with a transition temperature of ∼8.5 K at zero magnetic field and an upper critical field of ∼2.5 T at 1.5 K.

Results and Discussion

Different from the commonly existing crystal structures (such as 2H and 1T types) of WS2, the building block of the 2M-WS2 crystal is a distorted octahedral of [WS6] as shown in Figure a. The S–W–S monolayer shows the 1T′ type structure with a W–W zigzag chain in the a direction as shown in Figure c. Furthermore, it is suggested that the 2M-WS2 single crystal normally grows along the b direction (Figure c), and monolayers stack along the c direction through translational operation[27] (Figure b). To determine the crystal structure of the synthesized WS2 crystals, we performed the powder XRD measurements and the corresponding result is shown in Figure . We found that the powder XRD pattern of the 2M-WS2 bulk samples matches well with the simulated one from the single crystal structure of 2M-WS2 (red lines in Figure ), indicating the high-quality of our 2M-WS2 crystals. The lattice parameters of the as-grown 2M-WS2 crystals calculated based on the XRD data are a = 3.23 Å, b = 5.71 Å, c = 12.87 Å, α = γ = 90°, and β = 112.94°. We note that the powder XRD data here were collected from a few clusters of the synthesized samples, and the small unmatched peak around 39.4° can be assigned to the 2H phase WS2. Furthermore, the composition of 2M-WS2 was analyzed on both its bulk and thin flakes (Figure S1) by EDS. Figure S2a shows the representative result of the 2M-WS2 thin flakes (inset of Figure S1 and inset of Figure S2a) which were prepared by exfoliating from its bulk crystals using a “scotch tape” method.[30] The EDS results show no detectable potassium or chromium elements under the detection limit. We note that potassium can be clearly detected in the K0.7WS2 bulk crystals (Figure S2b), suggesting that the potassium cations have been deintercalated from 2M-WS2. Quantitative analysis shows that the atomic ratio of W/S is very close to 0.5, confirming the right composition of our samples. Since the source materials used to grow 2M-WS2 do not contain any carbon or oxygen, the detected carbon element shown in Figure S2a is most likely from the carbon tape used to hold the samples, whereas the small amount of oxygen was possibly detected from the environment or the surface oxide layer after exposing in air.

Figure 2

The powder XRD pattern of 2M-WS2 crystals, where the red lines were simulated from its single crystal structure.

We further characterized the crystal structure and electronic properties of the 2M-WS2 thin flakes using STM/S at 77 K. We know that it is always challenging to locate the thin flakes of the target 2D materials for STM/S measurements. To overcome this challenge, we developed an Au (5 nm)/SiO2 (300 nm)/Si substrate with a series of concentric gold rings (with a thickness of ∼15 nm) on the top surface, which can guide us to easily find the target 2M-WS2 thin flakes for the following STM/S measurements (Figure a). The 2M-WS2 thin flakes exfoliated from the bulk crystal were isolated on a polydimethylsiloxane (PDMS) stamp. Then, with the PDMS stamp, the 2M-WS2 flakes were transferred to the center of the Au rings on the substrate. Figure a shows a representative optical image of the 2M-WS2 thin flakes on our designed substrate, where the dark blue areas represent the 2M-WS2 thin flakes, located at the center region of the concentric Au rings. This unique sample preparation method for STM/S measurements not only ensures the conductivity between the sample and the substrate but also enables quickly locating the targeted 2D thin flakes.

Figure 3

STM/S measurements on a 2M-WS2 thin layer. (a) An optical image of the prepared 2M-WS2 thin layers on a designed Au (5 nm)/SiO2 (300 nm)/Si substrate with a series of concentric Au rings. (b) An atomically resolved STM topography image taken from the 2M-WS2 thin layer. Image size: 10 nm × 10 nm. (c) The corresponding dI/dV mapping taken from the same location as shown in (b). Scanning condition: Vbias = −1 V, Itunneling = 200 pA. (d) dI/dV point spectra measured on the sample at locations A and B in (c). (e) dI/dV point spectrum measured on the sample at locations 1 and 2 in (c). All the measurements were performed at 77 K.

Figure b,c shows the atomic resolution topography and the corresponding dI/dV mapping of a 2M-WS2 thin flake shown in Figure a, respectively. In Figure b,c, the atomically resolved topography and dI/dV images agree with lattice structures of 2M-WS2 and are in agreement with the previous works.[26,27]Figure d shows the dI/dV point spectra taken from the locations marked by A (a zigzag region with the bright contrast) and B (a gap region with the dark contrast) in Figure c. Note that the “zigzag” and “gap” regions denoted in Figure d represent the W–W zigzag pattern region and the region in-between the W–W zigzag patterns, respectively, as seen from the crystal structures (Figure c). The dI/dV point spectra shown in Figure d are averaged over 15 spectra taken at the same locations (Figure S3). The two gapless spectra are qualitatively the same with a slight difference in the spectral weight, leading to the observable dI/dV contrast seen in Figure c. Furthermore, both the dI/dV point spectra exhibit a hump-like feature near a sample bias voltage of +0.5 V. This feature is consistent with the first-principles calculated band structure,[26] where the bottom of a dispersive band near the Y point in the Brillouin zone has an energy of ∼0.3–0.4 eV above Fermi energy. The bottom or the top of a parabolic dispersive band is where one may find high density of states; hence, a hump is expected in the dI/dV spectrum. Interestingly, a brighter region with a lateral size of ∼2 nm at the center of the topography image (blue dash-dot circle in Figure b) is observed. It appears as a low contrast region in dI/dV mapping (Figure c). As there is no observable adatoms or missing atoms in the topography (Figure b) and dI/dV mapping (Figure c), we speculate that such a contrast difference might be due to the possible residual potassium or defects underneath the topmost atomic layers. While the wide sample bias voltage range (±1 V) dI/dV spectra are similar to the ones shown in Figure d, the dI/dV spectra with a smaller sample bias voltage range (±100 meV) measured at locations marked by 1 (near the bright contrast region) and 2 (far away from the bright contrast region) in Figure c exhibit small shifts with respect to each other, as shown in Figure e. The bias of the minimum, nonzero signal in each spectrum is determined by a simple parabolic function fitting: −19 meV (point 2) and −9 meV (point 1). Since point 2 is away from the high contrast region in topography, it can be considered as pristine 2M-WS2. Then, the spectrum taken at point 1 can be seen shifted toward the positive bias direction compared with pristine 2M-WS2, indicating a relative hole-doping or less electron doping compared with pristine 2M-WS2. This result suggests that the doping effect is less likely associated with the potassium residual, as it is expected to provide electron doping compared to the pristine 2M-WS2 region. Though we cannot further determine the type of defects, the observed high contrast region is likely related to defects underneath the topmost layer. At last, 2M-WS2 is relatively stable against air exposure. We note that the total air exposure time of the prepared 2M-WS2 thin flakes is less than 1 min and the sample used in the STM/S measurements did not go through any further treatment under ultrahigh vacuum (such as thermal annealing for degassing). Our atomically resolved STM image confirms that the as-grown 2M-WS2 sample in our work is of high quality and relatively stable compared to other air sensitive 2D superconductors.

To study the superconductivity of the pristine 2M-WS2 thin layers, we have developed techniques to fabricate the high quality nanodevices. To avoid the possible degradation of the 2M-WS2 thin layers during the normal nanofabrication process, predefined Hall-bar (Au) electrodes (Figure a) were made on bare SiO2/Si substrates using the standard electron beam lithography and e-beam metal deposition. Once the substrates with the Hall-bar-shaped Au electrodes were made, the 2M-WS2 thin layers exfoliated from the bulk crystals were isolated on a PDMS stamp. The thickness of the 2M-WS2 layer can be determined through the layer dependent contrast under an optical microscope (Figure S1). Then, a thin and uniform flake was selected and transferred on top of the premade Hall-bar Au electrodes using the dry transfer technique (Figure a). After removing the PDMS stamp, the thickness of the 2M-WS2 thin layer was further measured using AFM (Figure c). Finally, a thin h-BN layer was transferred on top of the 2M-WS2 thin layer to prevent any possible surface oxidation after exposure to air. The optical image of the final device of a 25 nm-thick 2M-WS2 layer is shown in Figure b. We note that the whole device fabrication process was performed in a glove box filled with high purity Ar gas with the oxygen (O2) and water (H2O) concentrations less than 0.1 ppm. Figure c is the corresponding AFM image of the device taken before the h-BN layer transfer. From the height profile (Figure d) taken along the white line (AB) in Figure c, we can determine that the thickness of the 2M-WS2 thin layer on this device is ∼25 nm.

Figure 4

Device fabrication and layout. (a) Schematic illustration of the device fabrication procedure using the dry transfer technique. The 2M-WS2 flake exfoliated on a PDMS stamp was transferred on the pre-made Hall-bar pattern with gold electrodes. The device was eventually covered by a thin h-BN flake. (b) An optical image of the fabricated device. (c) The corresponding AFM image of the 2M-WS2 flake on top of the predefined Au Hall-bar electrodes. (d) Line profile along the white line (AB) shown in (c). The thickness of the 2M-WS2 flake is around 25 nm.

We further studied the electrical transport properties of the 2M-WS2 thin flake device shown in Figure b. The corresponding transport results are summarized in Figure . The room temperature longitudinal resistance (R) is around 37 Ω. As the sample temperature drops, the resistance also decreases, indicating a metallic behavior of the 2M-WS2 sample. When the sample temperature reaches ∼8.5 K (critical temperature TC, where the sample resistance reaches 75% of its normal state resistance), the sample resistance suddenly drops to zero within a narrow temperature window of less than 0.6 K, demonstrating a clear signature of the superconducting state (Figure a). We note that the calculated residual resistivity ratio, RRR = R300K/R8.5K, of the thin flake is ∼30, which is comparable to the 24 nm thick flake reported previously,[29] indicating that the quality of our bulk sample is at the same level as that of the reported bulk crystals.[26] We further studied the temperature dependence of the longitudinal resistance R at different out-of-plane magnetic fields (μ0H⊥) as shown in Figure b. As expected, when the out-of-plane magnetic field is increased, the transition temperature TC gradually decreases (Figure b). The superconductivity disappears when the magnetic field is at 4 T, suggesting that the upper critical magnetic field μ0HC2 is within 2–4 T because 2M-WS2 is a type-II superconductor.[26] We note that a shoulder is observed near the transition region of the R versus the T curve (red color in Figure b) at the zero magnetic field. The shoulder disappears when the out-of-plane magnetic field is equal to or larger than 0.2 T. Although many factors, such as surface oxidation and nonuniform thickness, may cause the effect, the exact reasons are still elusive. We further plotted the reduced critical field, , as a function of the reduced temperature T/TC, as shown in Figure c. To analyze the possible pairing symmetry of the superconductivity observed in our sample, we fitted the hc2* dependence using the model for the orbital limited s-wave superconductors (red dashed line).[31] Our data seem to deviate from the standard spin-singlet behavior, suggesting a possible different paring symmetry (such as the topological superconducting states) of the observed superconductivity in 2M-WS2. We further analyzed our data using the one-band Werthamer Helfand and Hohenberg (WHH) model (blue solid line), .[31,32] We can extract the upper critical field at 0 K (μ0HC2(0)) to be ∼5.09 T. Obviously, μ0HC2(0) is lower than the Pauli paramagnetic limit of μ0Hp = 1.84 TC = 15.64 T for TC = 8.5 K, suggesting no Pauli pair breaking in our samples. We further note that the quasilinear temperature dependence of the upper critical field (inset of Figure c) has been observed in a few material systems, such as the pressurized Bi2Se3[33] and 2M-WSe2,[32] YPtBi,[34] and UBe13,[35] possibly arising from an unconventional superconducting state or a multiband Fermi surface topology. To further clarify the possible unconventional superconductivity in 2M-WS2 thin flakes, more detailed studies are required, such as the angle and temperature dependences of the upper critical field μ0Hc2 and the specific heat.

Figure 5

Superconductivity of the 2M-WS2 thin flake. (a) Temperature dependence of the longitudinal resistance R measured on a 25 nm thick 2M-WS2 flake at zero magnetic field, indicating a transition temperature of ∼8.5 K. The inset shows the data near the transition temperature. (b) Temperature dependences of the longitudinal resistance R under different out-of-plane magnetic fields. (c) Reduced upper critical field as a function of the reduced temperature T/TC. The inset is the upper critical field μ0HC2 as a function of temperature T, where the black dashed line is a linear fit. (d) Out-of-plane magnetic field dependence of the longitudinal resistance measured at the base temperature T = 1.5 K.

We also studied the influence of the out-of-plane magnetic field on the longitudinal resistance R at the base temperature T = 1.5 K, as shown in Figure d. We see that the superconductivity (zero resistance) state starts to disappear when the out-of-plane magnetic field is around 2.5 T, suggesting that the upper critical magnetic field μ0HC2 of the 25 nm-thick 2M-WS2 flake is ∼2.5 T. We note here using our transport technique, the lower critical magnetic field of the 2M-WS2 flake cannot be determined. We notice that the previously reported critical temperature of the 2M-WS2 samples with a thickness of 90 or 120 nm was around 8.8 K.[26] In this study, the critical temperature of the 25 nm-thick 2M-WS2 is around 8.5 K, which is consistent with that of a 24 nm-thick flake,[29] indicating a weak layer-dependent TC of the 2M-WS2 layers within the previously reported thickness range of 120–24 nm. However, the layer dependence of the TC below 24 nm is still elusive due to the difficulties in preparing the atomic thin 2M-WS2 layers.

Conclusions

In conclusion, we have successfully developed a new route to grow high-quality 2M-WS2 single crystals by a direct combination of the elements followed by heating in a thermal gradient at elevated temperatures. The 2M type crystal structure was determined by XRD and the atomically resolved STM measurements. At 77 K, we observed the spatial variation of the local tunneling conductance of the 2M-WS2 thin flake by STS which is not reported previously. We further developed a dry transfer technique to fabricate the high-quality 2M-WS2 thin layer-based nanodevices. Our transport measurements demonstrate a clear signature of the superconductivity observed in a 25 nm-thick 2M-WS2 flake. The critical temperature is found to be ∼8.5 K, and the upper critical field is ∼2.5 T. Thus, our work demonstrates a new route to grow a high-quality and binary TMD superconductor, 2M-WS2, and opens up new opportunities in studying the topological superconductivity in simple 2D TMD materials.

Experimental Section

The direct growth of the 2M-WS2 crystal is difficult. A precursor of K0.7WS2 is essential for obtaining the 2M phase of WS2. Compared with previously reported growth methods[26−29] of K0.7WS2 crystals, we have used different source materials and developed different growth procedures/conditions in our growth. Specifically, we first synthesized the precursor K0.7WS2 crystals, followed by deintercalation of potassium (K) cations to obtain 2M-WS2 crystals. The K0.7WS2 bulk crystals were prepared using a direct combination of the elements followed by heating in a thermal gradient at elevated temperatures. Stoichiometric quantities of sulfur (S) (0.64 g), tungsten (W) powder (1.85 g), and potassium (K) turnings (0.28 g) were placed into a 1 cm × 25 cm Pyrex tube in a glovebox filled with high purity argon gas. The tube was then evacuated (2 Pa), sealed, and placed in an oven at 100 °C. As the tube warms up, slow diffusion of potassium into the sulfur occurred with subsequent reaction. After 20 h, the tube was cooled down and then vigorously shaken to mix the potassium sulfide formed with residual tungsten and sulfur. The tube was then returned to the furnace and heated up to 200 °C for 24 h, followed by 50 °C incremental increases in temperature every 20 h until a temperature of 450 °C was reached. The obtained K0.7WS2 was then grounded into powders with a diameter ∼0.05 mm in an argon atmosphere and sealed in a 1 cm × 15 cm fused silica tube under a pressure of 2 Pa. Then, the tube was placed in a furnace at 850 °C with a 3 °C/cm gradient across the diameter of the tube and a 7.5 °C/cm gradient down the tube axis. After 60 h, the tube was cooled down at 100 °C/h to the ambient temperature. The final product consists of a black micaceous crystalline mass with numerous individual crystals reaching diminutions up to 1 × 2 × 0.01 mm3. Then, the as-grown K0.7WS2 bulk crystals were oxidized chemically using K2Cr2O7 (0.01 mol L–1) in aqueous H2SO4 of 50 mL with a concentration of 0.02 mol L–1 for 1 h at room temperature. Then, the crystals were washed in distilled water several times and followed by vacuum drying for more than 24 h at room temperature. The final obtained product of the 2M-WS2 bulk crystals is shown in Figure d.

The crystal structure of the as-grown 2M-WS2 bulk crystals was characterized by X-ray diffraction (XRD) on a Rigaku X-ray diffractometer with Cu Kα (λ = 154.059 pm) radiation. The chemical composition of the 2M-WS2 bulk crystals and its thin layers and the corresponding molar ratio of the W and S elements were analyzed by the energy dispersive spectroscopy (EDS) equipped in a scanning electron microscope at room temperature. We further characterized the atomic lattice structure and electronic properties of the 2M-WS2 thin layers using STM and spectroscopy (STM/S) in Omicron low-temperature STM at a base pressure of low 10–10 mbar and T = 77 K. The electrical transport properties of the 2M-WS2 thin flakes with Hall-bar shaped Au electrodes were systematically studied in an Oxford TeslatronPT system with a base temperature of T = 1.5 K and a perpendicular magnetic field up to 12 T.