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Emergence of superconductivity in (NH3)yMxMoSe2 (M: Li, Na and K)

  • Scientific Reports 6, Article number: 29292 (2016)
  • doi:10.1038/srep29292
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Abstract

We report syntheses of new superconducting metal-doped MoSe2 materials (MxMoSe2). The superconducting MxMoSe2 samples were prepared using a liquid NH3 technique, and can be represented as ‘(NH3)yMxMoSe2’. The Tcs of these materials were approximately 5.0 K, independent of x and the specific metal atom. X-ray diffraction patterns of (NH3)yNaxMoSe2 were recorded using polycrystalline powders. An increase in lattice constant c showed that the Na atom was intercalated between MoSe2 layers. The x-independence of c was observed in (NH3)yNaxMoSe2, indicating the formation of a stoichiometric compound in the entire x range, which is consistent with the x-independence of Tc. A metallic edge of the Fermi level was observed in the photoemission spectrum at 30 K, demonstrating its metallic character in the normal state. Doping of MoSe2 with Li and K also yielded superconductivity. Thus, MoSe2 is a promising material for designing new superconductors, as are other transition metal dichalcogenides.

Introduction

Searching for new superconducting materials is one of the most challenging and exciting areas of research. During the past decade, iron pnictides (FeAs) and chalcogenides (FeSe) have attracted much attention, not only from researchers interested in developing new superconductors, but also physicists who are interested in the mechanism of superconductivity1,2,3,4. Recently, syntheses of metal-intercalated systems of FeSe using a liquid NH3 technique have been extensively studied because various superconductors with high superconducting transition temperatures (Tcs) have been discovered5,6,7,8; the highest Tcs are 46 K at ambient pressure5 and 49 K at high pressure9. The pressure-induced enhancement of Tc has also been confirmed for non-NH3 KxFeSe10. Thus a layered compound like FeSe is a promising material platform for investigating high-Tc superconductors.

The Mo dichalcogenide family has also attracted much attention because of the emergence of its unique physical properties11,12 and potential use in high-speed transistors13,14. Electrostatic electron-doping of MoS2 has produced superconductivity with a Tc as high as 10.8 K 11. The plot of Tc versus the accumulated two-dimensional (2D) electron density n2D showed a dome-shaped curve, i.e., the Tc was tuned by the extent of electrostatic electron-doping. The maximum Tc was 10.8 K at 1.2 × 1014 cm−2. Also, a signature of 2D superconductivity was observed in electrostatically electron-accumulated MoS211. The chemical doping of MoS2 with alkali and alkaline-earth metal atoms15,16 provided superconductivity with Tcs lower than the maximum Tc of electrostatically electron-accumulated MoS211,12. The chemical doping of MoS2 was achieved using the liquid NH3 technique, and many superconducting materials have been produced.

Very recently, electron-doping of MoSe2 was achieved by the electrostatic method17, and the Tc was precisely tuned in the same manner as in MoS2. In the case of MoSe2, only a Sr atom was intercalated, and MoSe2 then showed a Tc as high as 5.0 K15. This sample was prepared using the liquid NH3 technique, and the chemical composition of SrxMoSe2 can be expressed as ‘(NH3)ySrxMoSe2’, where the nominal x was 0.2. The shielding fraction of (NH3)ySr0.2MoSe2 was 60%.

Here, we report syntheses of MxMoSe2 samples (M: Li, K and Na) using the liquid NH3 technique. In this study, Li, Na, K and Sr atoms were intercalated into MoSe2 solids (only Sr-intercalation had previously been reported)15. Single-crystal-like agglomerations of (NH3)yMxMoSe2 (M: Li, Na, K and Sr) were produced. Na-intercalation in (NH3)yNaxMoSe2 was indicated by its synchrotron powder X-ray diffraction (XRD) pattern. Energy dispersive X-ray spectroscopy (EDX) showed its chemical composition, and the amount of NH3 was also determined from the mass difference before and after reaction. The superconducting parameters were determined from the magnetic field (H) dependence of magnetization (M). The photoemission spectrum at 30 K showed a clear edge on the Fermi level, indicating metallic behavior in the normal state.

Results

Crystal structure of (NH3)yNaxMoSe2

Single crystals of pristine MoSe2 were prepared using the annealing technique; details are described in the Methods section. A photograph of a pure MoSe2 sample is shown in Figure S1a. A single-crystal structure analysis was produced using a piece of MoSe2 (or single crystal) separated from a MoSe2 agglomeration prepared in this study (Figure S1a); it is unclear whether an entire agglomeration is a single crystal or consists of multiple single crystals. A reasonable residual-factor (R) could be obtained in this analysis (R = 2.4% and weighted R (wR) = 4.6%). Only one phase of MoSe2 was included in the single crystal, and it was confirmed that no other phase such as Mo3Se4 was included. The structure of the MoSe2 single crystal was hexagonal (space group: No. 194, P63/mmc). The lattice constants were a = 3.289(7) Å and c = 12.96(3) Å, which are consistent with those (a = 3.283 Å and c = 12.918 Å) reported previously for pristine MoSe218. Crystallographic data are listed in Table S1. As seen from the magnetic susceptibility M/H (emu g−1 = cm3 g−1) shown in Figure S2, no superconductivity was observed in any precursor MoSe2 sample, implying no contamination with superconducting Mo3Se4. The chemical composition of one MoSe2 agglomeration was determined to be ‘MoSe1.9(2)’ from the EDX spectrum (Figure S3). These analyses also show that the precursor material was not superconducting Mo3Se419, i.e., it was non-superconducting MoSe2. The EDX spectra, magnetic susceptibilities and single-crystal analyses guaranteed that all MoSe2 agglomerations used for metal-intercalation throughout this study were in fact substantially ‘MoSe2’.

Metal-doped MoSe2 samples were prepared using the liquid NH3 technique. The experimental details are described in the Methods section. Here, it is worth noting that instead of a polycrystalline powder, in this study, an agglomeration of MoSe2 was used as the starting material for metal-intercalation. This is based on the successful synthesis of metal-doped FeSe from an agglomeration of FeSe20.

A photograph of (NH3)yNa0.5MoSe2 prepared using the liquid NH3 method is shown in Fig. 1a; the stoichiometry of Na (x = 0.5) is an experimental nominal value. The (NH3)yNa0.5MoSe2 samples (agglomerations) look like single crystals. The EDX spectrum for (NH3)yNa0.5MoSe2 is shown in Figure S4, which shows that the (NH3)yNa0.5MoSe2 sample is (NH3)0.4(1)Na0.41(1)MoSe2.04(1). The amount of NH3, y = 0.4(1), was determined from the mass difference before and after the reaction that used liquid NH3. These results indicate that NH3 was included in this material, and the amount of Na is reasonably consistent with the experimental nominal value. Here, we must consider the exact chemical structure and appropriate representation of NH3, i.e., which form exists in the MoSe2 solid? Does it exist as molecular NH3, or does it take some other form such as a metal-coordinated amide? To determine the exact chemical formula, neutron diffraction may be required. Throughout this paper the simple chemical formula, (NH3)yMxMoSe2, is used for convenience because the exact chemical form of NH3 is unclear.

Figure 1
Figure 1

(a) Photograph of (NH3)yNa0.5MoSe2 agglomerations. (b) Powder XRD pattern of (NH3)yNa0.5MoSe2 using synchrotron radiation. ‘x’ marks correspond to the experimental XRD pattern. Red and green lines refer to calculated patterns (Le Bail fitting) and background, respectively. Ticks refer to the peak positions predicted. In (b), two phases ((NH3)yNaxMoSe2 and MoSe2) are used in Le Bail fitting. The M/HT plots in ZFC and FC modes for the (NH3)yNa0.5MoSe2 sample providing the XRD pattern (b) are shown in the inset of (b). (c) Schematic representation of possible (NH3)yNa0.5MoSe2 structure; the structure was drawn based on the atomic coordinates shown in Table S2. As described in the text, this structure may be reasonable if the Na is located in the space between MoSe2 layers, a possibility supported by the expansion of (c).

The structure of (NH3)yNa0.5MoSe2 (0.5 is a nominal experimental value) was examined as a typical example using single-crystal XRD data collected at room temperature. As seen in Figure S1b, the XRD Bragg spots are quite diffuse, indicating a very disordered crystal. Because of the diffuse spots, a definitive structural analysis could not be performed.

To confirm whether the Na atom is located midway in the space between MoSe2 layers, the powder XRD pattern of (NH3)yNa0.5MoSe2 was measured with synchrotron radiation (λ = 0.4137(1) Å). The XRD pattern is shown in Fig. 1b together with the pattern calculated based on Le Bail fitting. The Le Bail fitting was performed for two phases under the space group of P63/mmc. The sample was prepared from Na and MoSe2 using the liquid NH3 technique, and ground up for the acquisition of a powder XRD pattern. The a and c of the main phase were determined to be 3.541(2) and 14.810(4) Å, respectively, while those of the minor phase were 3.2615(1) and 12.8133(5) Å. The minor phase can be assigned to pure MoSe2, the lattice constants of which are consistent with the values (a = 3.289(7) Å and c = 12.96(3) Å) determined for pure MoSe2 single crystal in this study. As seen from Fig. 1b, the peak-intensity of 002 peaks for non-doped (minor) and Na-doped MoSe2 (major) observed at angles below 2θ = 5° were virtually the same, indicating that the fractions were almost equivalent. No other phase (such as metal-doped Mo3Se4) was found, which is reasonable because the precursor material before metal-doping was demonstrated to be MoSe2.

The c of 14.810 Å of the main phase is larger by 1.85 Å than that of pure MoSe2 (12.96(3) Å), indicating that the Na is located in the space between MoSe2 layers. The a value also increased to 3.541(2) Å from 3.289(7) Å, but the expansion (Δa = 0.252 Å) is too small to be attributed to the intercalation of Na into the MoSe2 layer. As discussed later, the intercalation of Na at a 2a site, i.e., the space between MoSe2 layers, seems to be the most reasonable explanation of the observed changes. The R and weighted pattern R (wRp) were 3.2 and 4.8% in the Le Bail fitting, respectively, which are reasonable values that confirm the Le Bail analysis. The structure suggested is shown in Fig. 1c; in this structure, NH3 is not shown. A more precise crystal structure that includes NH3 must be determined using high-quality (NH3)yNa0.5MoSe2 single crystals that yield sharp Bragg spots. This study is now in progress.

In this study, we tried to perform Rietveld refinement based on the model listed in Table S2 of Supplementary Information; the atomic coordinates listed in Table S2 were obtained by a structural analysis based on single-crystal X-ray data, but a reasonable R factor could not be obtained in the analysis because of the diffuse Bragg spots collected from the single crystal (Figure S1b). The complete Rietveld refinement could not be achieved using the above model, so it was not possible to determine the exact location of the Na atom. However, the large expansion of c suggests that Na is located in the space between MoSe2 layers. If this is the case, the location of Na at a 2a site may be reasonable because of the presence of a large space around the 2a site. A possible crystal structure of (NH3)yNaxMoSe2 is shown in Fig. 1c.

Characterization of superconductivity in (NH3)yNaxMoSe2

Figure 2a shows the M/H – temperature (T) curves in zero field cooling (ZFC) and field-cooling (FC) modes for (NH3)0.4(1)Na0.41(1)MoSe2.04(1). The Tconset and Tc were 6.0 and 5.0 K, respectively, for (NH3)0.4(1)Na0.41(1)MoSe2.04(1); the Tc was determined from the crossing point of the extrapolation of the normal state and the drop of the M/HT curve in ZFC mode, as seen from the inset in Fig. 2a. Here, it may be necessary to briefly comment on a small slow decrease in M/H below Tconset (Fig. 2a). The inhomogeneous Na-doping of MoSe2 may be suggested as its origin. However, as described later, the different x values in (NH3)yNaxMoSe2 did not provide different Tc or Tconset values, which means that the inhomogeneous Na-doping cannot explain the slow decrease. The second possibility is that the (NH3)yNaxMoSe2 agglomerations shown in Fig. 1a are not single crystals but aggregates of polycrystalline grains because the small size of superconducting grains often results in such a decrease. These possibilities are fully explored later.

Figure 2
Figure 2

(a) M/H vs. T plots of the (NH3)yNa0.5MoSe2 agglomerations in ZFC and FC modes (H = 10 Oe). Inset in (a) shows the method used to determine Tc. (b) MH curve measured at 2 K for the (NH3)yNa0.5MoSe2 agglomerations. In the inset of (b), the expanded MH curve is shown together with the fitted line. The chemical composition of (NH3)yNa0.5MoSe2 used in (a,b) was determined to be (NH3)0.4(1)Na0.41(1)MoSe2.04(1) (see text). (c) x-dependence of Tc and Tconset in (NH3)yNaxMoSe2; x was evaluated from the EDX. In (c) the shielding fraction is evaluated using the ρ determined using each chemical stoichiometry for (NH3)yNaxMoSe2; y is assumed to be 0.4. The inset of (c) shows how to determine the Tc and Tconset.

The shielding fraction at 2.5 K was 100% for (NH3)0.4(1)Na0.41(1)MoSe2.04(1); the shielding fraction was evaluated using the density (ρ = 5.64 g cm−3) determined from the above chemical stoichiometry and lattice constants shown in the previous section. Here it should be noted that the above sample was made by Na-doping of an agglomeration of MoSe2. As a reference, the M/HT plot of the (NH3)yNa0.5MoSe2 sample prepared by Na-doping of polycrystalline MoSe2 powder is shown in Figure S5 of Supplementary Information. The Tc and Tconset (Figure S5) were the same as those (Fig. 2a) of a sample prepared by Na-doping of a MoSe2 agglomeration, but the shielding fraction was less than 1% at 2.5 K. The behavior of the M/HT plot below Tconset (Figure S5) was also the same as that shown in Fig. 2a. These results may show that effective Na-doping can be performed on these agglomerations of MoSe2. Moreover, we suggest that the above small fraction (<1%) may originate in a limiting thickness of superconductivity, i.e., a thin superconducting area formed by metal-doping using polycrystalline MoSe2 powder. Therefore, throughout this paper, all studies were performed using the samples prepared by metal-doping of agglomerations of MoSe2.

Finally, we comment briefly on the Meissner fraction of (NH3)0.4(1)Na0.41(1)MoSe2.04(1) at 2.5 K (shielding fraction = 100% at 2.5 K (Fig. 2a)). The Meissner fraction was approximately 6.7% at 2.5 K which was evaluated from the M/HT plot in FC mode (Fig. 2a), indicating a small size for superconducting grains. Therefore, this single-crystal like (NH3)0.4(1)Na0.41(1)MoSe2.04(1) may actually consist of polycrystalline superconducting grains, as previously suggested based on the slow drop observed in the M/HT plot below Tconset (Fig. 2a). However, some of (NH3)yNaxMoSe2 samples showed a Meissner fraction of more than 20%. Figure S6 shows M/HT plots of (NH3)yNa0.5MoSe2 exhibiting a Meissner fraction of 25%.

Figure 2b shows the MH curve at 2 K for (NH3)0.4(1)Na0.41(1)MoSe2.04(1), which exhibits a clear diamond-like shape. The lower critical field Hc1 was determined to be 18 Oe from the expanded MH curve (inset of Fig. 2b). It was concluded from the MH curve (Fig. 2b) that the upper critical field, Hc2, was > 0.3 T, indicating a type-II superconductor. Figure S7 shows M/HT plots at different H’s, and the HT phase diagram (Figure S7) was constructed from the Tconset at each H; the fitted curve indicates the Hc2 at each temperature. The positive curvature seen in Figure S7 is similar to the behavior of (NH3)yKxMoS2 reported recently21. The Hc2 at 0 K, Hc2(0), was evaluated to be 2.4 T. However, the data of the Hc2Tc plot are confined near Tc. Therefore, the Hc2 is shown just for reference. We determined the London penetration depth, λ, to be 520 nm, from Hc1. The shape of the sample was assumed to be isotropic because the measurements of MH (2 K) and M/HT at different H’s was performed using more than one agglomerations.

Figure 2c shows the x dependence of Tc in (NH3)yNaxMoSe2. The x value was determined from the EDX spectrum, and the x refers to the statistically averaged value with a small error bar falling within the range of the circle (Fig. 2c); the EDX was measured for several areas in one sample. The Tc was almost constant (~5 K) with an x-range of 0.4–1. The shielding fraction was higher than 35% in all samples. For the discussion, we plotted Tconset − x in Fig. 2c again because the previous reports on metal-doped MoS2 and MoSe2 show the Tconset. The Tconset was also constant (~6 K) in the x-range of 0.4–1. Therefore, we cannot point to an x-dependence of superconductivity in (NH3)yNaxMoSe2. Finally, we must comment that the maximum x is 1.0 in (NH3)yNaxMoSe2 if the Na occupies only a 2a site in the P63/mmc lattice, as described in the subsequent section. To sum up, it must be stressed that the x range must be 0–1 in (NH3)yNaxMoSe2. A list of typical superconducting samples is shown in Table 1.

Table 1: List of representative samples prepared in this study.

Electronic structure of (NH3)yNaxMoSe2

The photoemission spectrum of a single-crystal-like agglomeration of (NH3)yNa0.5MoSe2 measured at 30 K is shown in Fig. 3a; the spectrum was recorded at the Γ point using the Xe-Iα resonance line (8.44 eV). The photoemission intensity was observed on the Fermi level, i.e., the metallic edge was clearly recorded. This shows that (NH3)yNa0.5MoSe2 is metallic in the normal state, and the superconducting transition of (NH3)yNa0.5MoSe2 emerges from the metallic state. The evaluation of the superconducting gap in (NH3)yNa0.5MoSe2 has not yet been done due to the limited resolution of 15 meV in the photoelectron spectrometer, so this is future work. While the metallic edge was clearly observed in the normal state by Xe-Iα light, no signature of the metallic edge was obtained when changing Xe-Iα to the He-Iα resonance line (21.2 eV). We note that the surface of the (NH3)yNaxMoSe2 single crystal may be oxidized, as the photoemission spectrum using the Xe-Iα resonance line provides more bulk-sensitive results than He-Iα. The successful observation of the metallic edge at the Γ point is fully treated in the Discussion section.

Figure 3
Figure 3

(a) Photoemission spectrum of (NH3)yNa0.5MoSe2. M/H versus T plots of (b) (NH3)yLi0.5MoSe2 and (c) (NH3)yK0.5MoSe2 agglomerations, respectively, in ZFC and FC modes (H = 10 Oe). (d) Plot of Tconset vs. rion in (NH3)yMxMoSe2 and (NH3)yMxMoS2. Circles and diamonds refer to (NH3)yMxMoS2 and (NH3)yMxMoSe2, respectively. The plot is based on the data collected in this study (diamonds) and those in Refs 15 and 16 (circles).

Superconductivity in other metal-intercalated MoSe2

Figure 3b,c show the M/HT curves for (NH3)yLi0.5MoSe2 and (NH3)yK0.5MoSe2, in ZFC and FC modes. The Tconset and Tc were 6.5 and 5.0 K, respectively, for (NH3)yLi0.5MoSe2, and were 7.5 and 5.3 K for (NH3)yK0.5MoSe2. The shielding fraction at 2.5 K was 21% for (NH3)yLi0.5MoSe2, and 10.5% for (NH3)yK0.5MoSe2. These shielding fractions were roughly estimated using the ρ ( = 6.99 g cm−3) of MoSe2 because the exact ρ could not be determined for (NH3)yLi0.5MoSe2 and (NH3)yK0.5MoSe owing to the absence of structural data (lattice constants). Therefore, the values may be slightly overestimated, but the shielding fraction suggests that the superconducting phases can be formed by intercalating alkali metal atoms other than Na. The Tconsets of these materials were higher than the 6 K of (NH3)yNa0.5MoSe2. However, the Tc was almost the same for three (NH3)yMxMoSe2’s. Furthermore, we synthesized the superconducting (NH3)ySrxMoSe2 (nominal x = 0.2), which showed a Tc (Tconset) as high as 4.8 K (7.0 K) (M/HT plots not shown); the Tc was the same as that reported previously15. The shielding fraction was ~2.5% at 2.5 K which is lower than those of alkali-metal-doped MoSe2.

In the case of (NH3)yMxMoS2, the Tconset generally increases with an increase in c15, and it increases with the ionic radius (rion) of the intercalant. However, the Tconset of (NH3)yLixMoS2 deviates from this pattern15. The Tconset vs. rion for (NH3)yMxMoSe2 (M: Li, Na, Sr and K) is plotted in Fig. 3d, together with that of (NH3)yMxMoS2 reported previously15,16. Similar behavior is seen in the plots of Tconset − rion of (NH3)yMxMoSe2 and (NH3)yMxMoS2. The Tconset of (NH3)yLixMoSe2 deviates from the suggested relationship, as does that of (NH3)yLixMoS215. We briefly tried to synthesize (NH3)yMxMoSe2 (M: Rb, Cs, Ca, Ba, Sr and Yb) as well as (NH3)yLi0.5MoSe2, (NH3)yNa0.5MoSe2 and (NH3)yK0.5MoSe2. At the present stage, their superconductivity has not yet been observed, except for (NH3)ySrxMoSe2 which was previously reported15.

Discussion

Very recently, Shi et al. succeeded in achieving superconductivity through electrostatic electron-doping of MoSe217. The maximum Tc of MoSe2 reaches 7.1 K at n2D = 1.69 × 1014 cm−2, and the Tc can be tuned by the accumulated electron density. The maximum Tc is lower than the 10.8 K of MoS211 and the n2D is higher than the 1.2 × 1014 cm−2 of MoS211. For MoSe2, a dome-like phase diagram of Tc vs. n2D has not yet been observed because the number of metal-doped MoSe2 superconductors discovered is still small, i.e., a Tc in the n2D-range (>1.69 × 1014 cm−2), which will be achieved by chemical electron-doping, has not yet been plotted.

A fresh Tc − n2D diagram (Fig. 4a) was prepared using the Tc − n2D plot (electrostatic electron-doping) reported by Shi et al.17 and the Tc − n2D plot (chemical electron-doping) for (NH3)yMxMoSe2 samples produced in this study. Here, it should be noted that the 3D electron density, n3D, evaluated from the x and lattice volume in (NH3)yNaxMoSe2 was translated to 2D electron density n2D by assuming the thickness of the channel region to be one layer ( = c/2); the electron concentration donated from a metal atom to the MoSe2 layer was evaluated assuming that an alkali (alkali-earth) metal atom can donate only one (two) electron, i.e., complex processes such as back-electron transfer to NH3 were not considered. This is the same method used for the estimation of the Tc − n2D plot for metal-doped MoS217. In the phase diagram, the Tcs of (NH3)yLi0.5MoSe2, (NH3)yK0.5MoSe2 and (NH3)ySr0.261(1)MoSe2 are also plotted for reference, although the x is an experimental nominal value except in (NH3)ySr0.261(1)MoSe2. Consequently, a dome-like phase diagram was suggested in the same manner as MoS211, but a continuous change of Tc was not obtained in the high n2D range because of the almost identical Tc in metal-doped MoSe2 prepared in this study (Fig. 4a).

Figure 4
Figure 4

(a) Phase diagram of electron-accumulated MoSe2. This phase diagram is based on the Tconset (diamonds) of (NH3)yMxMoSe2 (this work) and those (circles) of electrostatically electron-accumulated MoSe2 recently reported by Shi et al.17 ‘(NH3)y’ is omitted in the formulas identifying differently M-intercalated (NH3)yMxMoSe2. (b) XRD patterns of (NH3)yNaxMoSe2 samples with different x; each x was determined from the EDX spectrum. The peaks at 2θ = 6.1°, 5.4° and 5.1° correspond to 002 peaks due to non-doped MoSe2, (NH3)yNaxMoSe2 and another (NH3)yNaxMoSe2 phases, respectively. (c) x-dependence of c for the above three phases. The c values do not change with x.

As described in the Results section, the Tconset increases with increasing rion (Fig. 3d). This behavior is contrary to that of (NH3)yMxFeSe, in which the Tconset is inversely proportional to the rion7. In the case of (NH3)yMxFeSe, the Tc is closely associated with the FeSe plane spacing ( = c/2)7,8,9, and elements with a smaller rion produced larger FeSe plane spacings. This strange behavior can be explained by the fact that the crystal structure differs (off-center or on-center structures) depending on the rion of the intercalated element8, so that (NH3)yLixFeSe, with an off-center structure, provides a larger FeSe plane spacing and high Tc (~44 K)5,8. If the Tc (or Tconset) also depends on the MoSe2 plane spacing in (NH3)yMxMoSe2, the graph shown in Fig. 3d implies that an increase in the rion of the intercalant directly affects the MoSe2 plane spacing. Actually, the deviation of Tconset of (NH3)yLi0.5MoSe2 and (NH3)yLi0.5MoS2 from the Tconset − rion curve drawn in the graph shown in Fig. 3d may imply that (NH3)yLixMoSe2 adopts a different structure from that (see Fig. 2c) determined for (NH3)yNaxMoSe2. In other words, we expect a different location for the Li atom in (NH3)yLixMoSe2 than that of the Na atom (probably 2a site), as found in (NH3)yLixFeSe6,8. To sum up, we must discuss the superconductivity of (NH3)yMxMoSe2 in the light of two variables, n2D and MoSe2 plane spacing (or two dimensionality). This makes it difficult to observe a dome-like Tc − n2D phase diagram, as seen from Fig. 4a.

As described in the Results section (Fig. 2c), no x-dependence of Tc (or Tconset) was observed in (NH3)yNaxMoSe2. Here, it is very interesting and significant to investigate whether the lattice constants (a,c) change with the x value in (NH3)yNaxMoSe2. Figure 4b shows the expanded X-ray diffraction patterns (2θ = 4.0–8.0°), indicating that the 002 peaks due to doped and non-doped phases are observed at the constant 2θ values although the peak intensity due to the doped phase increases monotonically with increasing x in the x-range of 0.35 to 0.86. From this result, it was found that the c does not change with x, suggesting that the stoichiometric (NH3)yNaxMoSe2 is formed regardless of any increase in x. In other words, the chemical stoichiometry of (NH3)yNaxMoSe2 does not change even when x increases, and only the fraction of the non-doped phase decreases. Such behavior was recently observed in (NH3)yKxMoS221, in which the K0.4MoS2 (2H structure) and K1.0MoS2 (1T and 1T’ structure) are formed in low and high K concentrations, respectively. The constant Tc may be reasonably explained by the scenario that the stoichiometric (NH3)yNaxMoSe2 compound (or the chemical compound with fixed x and y) is formed in the entire x range, i.e., the stoichiometric x value in (NH3)yNaxMoSe2 does not change with increasing x as determined from EDX; the EDX estimates the x value including non-intercalated Na atoms. This scenario corresponds to the third possibility described in the Results section.

As seen from Fig. 4b, at higher x values than 0.7, a new peak was observed, indicating the presence of a new c-expanded phase. Figure 4c shows the x-dependence of c in (NH3)yNaxMoSe2. From this graph, three different c values are found, due to (1) non-doped pure MoSe2, (2) a Na-doped MoSe2 phase, and (3) another Na-doped MoSe2 phase with a larger MoSe2 spacing. Since the Tc did not change in the entire x-range regardless of the formation of phase (3), it was unclear whether phase (3) is a new superconducting phase. To sum up, when x increases, two different Na-doped MoSe2 phases with certain chemical stoichiometry seem to be formed in (NH3)yNaxMoSe2. Further study is necessary to clarify the exact stoichiometry of their phases.

Finally, it is necessary to comment on the observation of a metallic edge on the Fermi level in the photoelectron spectrum measured at the Γ point. The band dispersion in bulk crystals of pure MoSe2 shows an indirect band gap (Γ – (ΓK))22, where (ΓK) means an intermediate state between Γ and K. However, the band dispersion in a single layer of MoSe2 shows a direct band gap (K – K)22. Therefore, a metallic edge for (NH3)yNaxMoSe2 should be observed at the (ΓK) point for MoSe2 crystal if we assume a rigid-band picture of band dispersion. Furthermore, even if we assume a single-layer like MoSe2 accompanied by expansion of the spacing between MoSe2 layers due to Na-intercalation, a metallic edge must be observed at the K point. Therefore, a metallic edge should not be observed at the Γ point. Nevertheless, a metallic edge was clearly observed in the photoemission spectrum (Fig. 3a). Relevant to this question, it can be observed that the photoemission spectrum must detect all band dispersion of (NH3)yNa0.5MoSe2 since the single crystal of MoSe2 must be disordered to possess different crystal alignments. In other words, the photoemission spectrum of a polycrystalline-like (NH3)yMxMoSe2 granule is recorded in Fig. 3a. This interpretation is reasonable since some disorder in the crystal is suggested by the XRD pattern shown in Figure S1b.

Methods

Sample preparation and characterization

Single crystals of MoSe2 were formed from a polycrystalline powder MoSe2 sample by physical vapor transport using a furnace with different temperature zones23; the powder MoSe2 sample was prepared by annealing stoichiometric amounts of Mo and Se at 800 °C for 3 days and 1000 °C for 4 days, according to a procedure reported elsewhere23. To form single crystals of MoSe2, TeCl4 was mixed with a MoSe2 sample as a transport material, the powder MoSe2 sample was set in the 1000 °C source area, and MoSe2 single crystals were collected in the low-temperature zone at 900 °C. Here we have used the term ‘MoSe2 single crystal’, but actually it is unclear whether the entirety of an agglomeration consists of one single crystal. Therefore, instead of the term ‘single crystal’, it may be valid to use the term ‘agglomeration of MoSe2’.

The samples of (NH3)yMxMoSe2 (M: Na, Li and K) were synthesized by the liquid NH3 technique as follows: (1) stoichiometric amounts of MoSe2 agglomerations and an alkali metal were placed in a glass tube, and then NH3 gas was condensed in the tube. (2) The metal dissolved in the liquid NH3 at −60 °C, and the solution (colored blue) was kept below −50 °C for 6 days. (3) When the color disappeared, the NH3 was removed by dynamical pumping at room temperature. The same method was used for Sr-intercalation in MoSe2.

The DC magnetic susceptibility (M/H) of all samples was measured using a SQUID magnetometer (Quantum Design MPMS2). The single-crystal XRD patterns of the samples were measured with a Rigaku Saturn 724 diffractometer with a Mo Kα source (wavelength λ = 0.71078 Å). The powder XRD patterns of (NH3)yNa0.5MoSe2 and (NH3)yNaxMoSe2 (x = 0–1) were obtained using synchrotron radiation (λ = 0.4137(1) Å) from the BL10XU beamline and (λ = 0.6887 Å) from the BL12B2 beamline, respectively, of the Spring-8 in Japan; the incident beam was focused by a stacked compound X-ray refractive lens. The samples were introduced into quartz tubes in an Ar-filled glove box for M/H measurements, or into capillaries for XRD; the quartz tubes were pumped and sealed under vacuum, while the capillaries were sealed under Ar atmosphere. The EDX was obtained with an EDX spectrometer equipped with a scanning electron microscope (SEM) (KEYENCE VE-9800 - EDAX Genesis XM2), and the photoemission spectrum with a SCIENTAOMICRON R4000 analyzer and a discharge lamp (SPECS). The Fermi level of the sample was referenced to that of gold, which was in electrical contact with the sample. The sample was cleaved in the ultrahigh-vacuum chamber for the measurement of photoemission spectrum. The photoemission spectrum was measured in an ultrahigh vacuum of ~5 × 10−9 Pa.

Additional Information

How to cite this article: Miao, X. et al. Emergence of superconductivity in (NH3)yMxMoSe2 (M: Li, Na and K). Sci. Rep. 6, 29292; doi: 10.1038/srep29292 (2016).

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Acknowledgements

This study was partly supported by Grants-in-aid (22244045, 24654105, 26105004) from MEXT, by the LEMSUPER project (JST-EU Superconductor Project) of the Japan Science and Technology Agency (JST), and by the Program for Promoting the Enhancement of Research Universities. The synchrotron XRD measurement was performed at BL10XU of Spring-8 under proposal (2015A1513).

Author information

Affiliations

  1. Research Laboratory for Surface Science, Okayama University, Okayama 700-8530, Japan

    • Xiao Miao
    • , Saki Nishiyama
    • , Lu Zheng
    • , Hidenori Goto
    • , Ritsuko Eguchi
    • , Kensei Terashima
    • , Takayoshi Yokoya
    •  & Yoshihiro Kubozono
  2. Advanced Science Research Centre, Okayama University, Okayama 700-8530, Japan

    • Hiromi Ota
  3. Department of Physics, Okayama University, Okayama 700-8530, Japan

    • Takashi Kambe
  4. Research Centre of New Functional Materials for Energy Production, Storage and Transport, Okayama University, Okayama 700-8530, Japan

    • Takayoshi Yokoya
    •  & Yoshihiro Kubozono
  5. Centre for Science and Technology under Extreme Conditions, Osaka University, Osaka 560-8531, Japan

    • Huyen T. L. Nguyen
    •  & Tomoko Kagayama
  6. SPring-8/JASRI, Hyogo 679-5198, Japan

    • Naohisa Hirao
    •  & Yasuo Ohishi
  7. National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan

    • Hirofumi Ishii
    •  & Yen-Fa Liao

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Contributions

Y.K. designed this research project and supervised experiments. X.M., S.N. and L.Z. synthesized and characterized MoSe2 and (NH3)yMxMoSe2 samples. H.T.L.N., T.K. (Osaka Univ.), N.H., Y.O., H.I. and Y.-F.L. measured the powder XRD pattern at Spring-8. X.M. and L.Z. analyzed powder XRD data. H.O. measured and analyzed the single-crystal XRD data. K.T. and T.Y. measured photoemission spectra at low temperatures. Y.K. prepared the paper with the help of X.M., H.G., R.E., T.K. (Okayama Univ.) and T.Y.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Yoshihiro Kubozono.

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