A new way to synthesize superconducting metal-intercalated C60 and FeSe

Doping with the optimum concentration of carriers (electrons or holes) can modify the physical properties of materials. Therefore, improved ways to achieve carrier doping have been pursued extensively for more than 50 years. Metal-intercalation is one of the most important techniques for electron doping of organic / inorganic solids, and has produced superconductors from insulators and metallic solids. The most successful examples are metal-intercalated graphite and C60 superconductors. Metal intercalation has been performed using solid-reaction and liquid solvent techniques. However, precise control of the quantity of intercalants in the target solids can be difficult to achieve using these methods, as that quantity depends largely on the initial conditions. Here we report an electrochemical method for metal-intercalation, and demonstrate the preparation of superconductors using organic and inorganic materials (C60 and FeSe). The metal atoms are effectively intercalated into the spaces in C60 and FeSe solids by supplying an electric current between electrodes in a solvent that includes electrolytes. The recorded superconducting transition temperatures, Tc’s, were the same as those of metal-intercalated C60 and FeSe prepared using solid-reaction or liquid solvent techniques. This technique may open a new avenue in the search for organic / inorganic superconductors.

organic materials involves complex accumulation processes. From this experiment, we obtained a hint about how to accumulate carriers in solids and to modify their electronic structure. That is, the electrochemical reaction may be an effective way to induce novel physical properties.
This paper discusses how we applied the electrochemical approach to C 60 and FeSe solids to accumulate electrons, i.e. to intercalate metal atoms. This leads to electron accumulation in the C 60 molecules or FeSe layers. As a result, superconducting K 3 C 60 and K-or Na-doped FeSe could be generated.

Results
Fabrication and characterisation of C 60 superconductor. Figure 1a,b show a diagram and photo of the electrochemical reaction cell. The working electrode (WE) consists of either K x C 60 or C 60 , while the counter electrode (CE) is sodium (Na) or potassium (K) metal. We applied a constant voltage between the CE and WE during the electrochemical reaction. Firstly, we performed the electrochemical intercalation of K atoms in K x C 60 that was synthesized by the usual solid-state reaction method 13 , because metallic/superconducting K x C 60 should operate effectively as a good WE. Secondly, we tried to directly intercalate K atoms into semiconducting non-doped C 60 . Figure 2a,b show the temperature dependence of magnetic susceptibility (M/H -T, M: magnetization, H: applied magnetic field, T: temperature) for three samples (pre-KC 60 , EC-KC 60 and EC-C 60 ); "pre-KC 60 " refers to K-intercalated C 60 prepared by the solid-state reaction, "EC-KC 60 " refers to the sample synthesized electrochemically from pre-KC 60 , and "EC-C 60 " refers to the sample synthesized electrochemically from C 60 . For electrochemical intercalation of the pre-KC 60 pellet sample, a voltage of − 5 V was applied to the CE for 4 h. As seen from Fig. 2a, the superconducting volume faction was clearly increased from 1% (pre-KC 60 ) to 3% (EC-KC 60 ) through the electrochemical intercalation of K atoms. The temperature dependence of magnetic susceptibility measured under the zero-field-cooled (ZFC) protocol for EC-KC 60 shows a clear drop at ~18 K, showing that the onset superconducting transition temperature, T C , is consistent with that, 19.3 K, of the K 3 C 60 superconductor 14 and that, 18.5K, of pre-KC 60 . Figure 2b shows the M/H -T plot under field-cooled (FC) and ZFC protocols for the EC-C 60 sample. In this case, we used the pellet of PVDF/C 60 /CB mixture for the WE because the conductivity must be increased, where the C 60 powder and carbon black powder (CB) were mixed with polyvinylidene difluoride (PVDF) as a binder. We applied a voltage of − 3 V for 8 h. The T C in the EC-C 60 sample was found to be ~17.5 K.
To see the bias voltage dependence of the superconductivity, we applied different bias voltages between the CE and WE. Figure 2c shows the bias voltage dependence of magnetic susceptibility for the semiconducting C 60 sample (EC-C 60 ). In this experiment, we used reaction times of 24 h. After applying a bias voltage of − 3 V for 24 h, the temperature dependence of magnetic susceptibility was measured (see red solid circle (− 3 V)), and then a bias voltage of − 4 V was applied for 24 h and the magnetic susceptibility was measured again (blue solid circle). The experimental procedure was repeated up to − 5 V (green solid circle). During the measurement of temperature dependence of magnetic susceptibility, no bias voltage was applied. The superconducting volume fraction increases with increasing bias voltage, as seen from Fig. 2c. As seen from the inset of Fig. 2c, the superconducting volume fraction increased linearly with increasing bias voltage. The number of K ions dissolved in the electrolytic solution would be expected to increase with an increasing bias voltage, leading to the observed increase in the metal-intercalated (or superconducting) domain in the WE. Thus, an electrochemical reaction has intercalated K atoms into C 60 to produce the superconducting phase.
To investigate carrier donation to a C 60 molecule, a Raman scattering experiment was carried out at room temperature. It is well established that the electron-filling of the t 1u -orbital of C 60 can be clarified by the negative shift of the centro-symmetric Ag(2) mode of the C 60 molecule (1469 cm −1 for pure C 60 ) 15 . Figure 2d shows Raman spectra of both pristine C 60 and the EC-C 60 sample. Ag(2) peak was recorded for the EC-C 60 at 1450 cm −1 , where EC-C 60 refers to the sample after the bias-voltage application of − 5 V (Fig. 2c). This result implies that the valence of EC-C 60 is 3.1, because a negative shift by 6 cm −1 per 1 electron-donation on C 60 should be observed. The value of 3.1 is consistent with the valence of the C 60 molecule in superconducting metal-doped C 60 (M 3 C 60 ; M = alkali metal atom). As the Raman scattering experiment probes the electronic state around the surface, the doped main phase there would be M 3 C 60 . This experiment suggests that no other phase exists with a metal concentration greater than 3, i.e., M 4 C 60 or M 6 C 60 . Thus, superconducting K 3 C 60 was prepared by this electrochemical reaction. This is the first preparation of superconducting metal-doped C 60 using any technique other than solid-state or liquid NH 3 chemical reactions One of distinct advantages of the electrochemical technique is to charge or discharge carriers by applying the reversible bias voltage. We demonstrates reversibility of superconductivity in C 60 by the application of bias voltage with different polarity (see Fig. 3). After the appearance of superconductivity due to the negative bias voltage, the application of positive bias voltage leads to a reduction or disappearance of superconducting fraction. This can repeat itself completely. As the superconductivity should be founded on the K-metal intercalation into the parent C 60 , the reduction of superconductivity implies a de-intercalation of K-metal. Accordingly, the intercalation / de-intercalation of K-metal atoms can be reversibly controlled by the bias voltage.
Fabrication and characterisation of FeSe superconductors. We applied electrochemical intercalation to the layered inorganic material, FeSe, the T C of which is 8 K 16 . It is well known that alkali and alkali-earth metal intercalation in the FeSe solids can lead to higher T C 's than that, 8 K, of pristine FeSe. A maximum T C of 46 K is currently obtained in ammoniated Na-doped FeSe 17 . Here, in addition to the metal atoms, NH 3 is incorporated between the FeSe layers. It is proposed that the T C is closely correlated with the FeSe interlayer distance, d 18 , which implies that an increase in two-dimensionality (2D) produces a higher T C . Recent theoretical study of HfNCl suggests that the improvement of Fermi-surface-nesting increases spin-fluctuation to strengthen pair coupling 19 . This scenario may be applied to FeSe and similar van der Waals materials. Thus, the intercalation of metal atoms into FeSe layers is very attractive for the realization of high-T C superconductors. Furthermore, the parent material, FeSe, has a simple structure consisting solely of conducting layers that are coupled through van der Waals interaction, i.e., a very flexible structure. Therefore, the effective intercalation of metal atoms into the FeSe system by the electrochemical method was judged to be achievable. In the experiment for K-intercalation, the FeSe pellet sample was used as the WE, and K metals were used as the CE. In the case for Na-intercalation, we used the PVDF/FeSe mixture spread on an Al-foil as the WE. A bias voltage of − 3 V was applied for 24 hours. Figure 4a shows the M/H -T plot in the ZFC protocol for a pristine FeSe sample (black dot), electrochemically K-intercalated FeSe (blue dot) and electrochemically Na-intercalated FeSe (red dot), respectively, which are denoted "FeSe", "EC(K)-FeSe" and "EC(Na)-FeSe", respectively. For the EC(K)-FeSe, clear drops in the M/H-T plot are found at 9 K and 29 K. The 9 K superconducting transition is due to the residual non-intercalated FeSe phase, while the 29 K superconducting transition is consistent with that, T C = 30 K 20 , of K x FeSe prepared by the solid-state reaction and that, T C = 31 K 18,21 , of (NH 3 ) y K x FeSe prepared by the liquid NH 3 technique, respectively. These results support the intercalation of K atoms into FeSe layers. Here, we must ask whether the electrolyte is also incorporated in the space between FeSe layers together with K atoms. This is fully discussed in the subsequent section.
The superconducting volume fraction for EC(K)-FeSe is estimated to be 11% at 10 K. Accordingly, we have succeeded in synthesizing a K-doped FeSe phase using electrochemical intercalation. The magnetic susceptibility of the EC(Na)-FeSe sample is also shown in Fig. 4a. The superconducting volume fraction for EC(Na)-FeSe is estimated to be 2% at 10 K. The temperature dependence of magnetic susceptibility of EC(Na)-FeSe shows a T C as high as 30 K, which is lower than that, T C = 46 K 17 , previously reported for Na x Fe 2 Se 2 , and is close to that, T C = 32 K 22 , of the low-T C phase of (NH 3 ) y Na x FeSe found recently.

x-ray diffraction and composition of electrochemically K-intercalated FeSe superconductor.
To confirm the intercalation of metal into solid FeSe, we performed x-ray diffraction measurements with synchrotron radiation (KEK-PF, BL-8B). Figure 3b shows the x-ray diffraction pattern for EC(K)-FeSe powder measured with λ = 1.0004(8) Å. As shown in Fig. 4b, the (002) peak characteristic of the ThCr 2 Si 2 -type structure (body-centred tetragonal: space group No.139) was clearly observed in EC(K)-FeSe; the (002) peak is not observed in pure FeSe (see Fig. 3b). The existence of the (002) peak is attributed to the expansion along the c-axis due to K-intercalation between the FeSe layers. The lattice parameters for EC(K)-FeSe were determined, based on the the Le Bail fitting analysis (see in Fig. 4b). The a and c were determined to be 3.71 Å and 16.4 Å, respectively. The estimated c is much larger than that, c = 14.0367(7) Å, of K x Fe 2 Se 2 19 while it is close to that, c = 16.16(5) Å, of (NH 3 ) y KFe 2-δ Se 2 21 . These results suggested that not only K atoms but also electrolyte may have been incorporated in the FeSe solid.
To confirm the exact intercalation of both K atoms and electrolyte molecules, we used energy dispersive x-ray spectroscopy (EDX) on the EC(K)-FeSe and EC(Na)-FeSe samples (see Fig. 4c). From the EDX spectra shown in Fig. 4c, the composition of the EC(K)-FeSe was estimated to be K 0.19 Fe 0.96 Se 1 . Here, only the peaks of Fe, Se and K are observed, suggesting no incorporation of electrolyte such as ClO 4 . Therefore, in spite of the expansion of c (the FeSe interlayer distance), the electrolyte molecule was not incorporated in the EC(K)-FeSe. It is of interest that electrochemical K-intercalation increases the FeSe layer separation without incorporating any electrolyte molecules (or any solvent). In the case of EC(Na)-FeSe, the peaks of Fe, Se and Na are observed, suggesting the Na-intercalation into FeSe.

Discussion
We have demonstrated the electrochemical synthesis of superconducting phases of C 60 and FeSe by intercalating alkali metal atoms into solid C 60 and FeSe. Thus, the electrochemical approach is an effective way to produce superconducting phases.
The superconducting volume fraction of the electrochemically synthesized C 60 and FeSe tends toward saturation as the reaction time increases. Raman scattering from the C 60 indicates that the superconducting K 3 C 60 phase is formed at the surface. The formation of the stable superconducting phase at the surface may prevent the metal atoms from further intercalating into the sample, thus causing the observed saturation of the superconducting volume fraction. This is likely to be an important consideration to realize higher superconducting volume fraction when attempting to achieve uniform penetration of the metal atoms into the bulk region.
Electrochemically Na-intercalated FeSe (EC(Na)-FeSe) shows a T C as high as 30 K, which is close to that of the low-T C phase of (NH 3 ) y Na x FeSe (T C = 32 K 22 ). (NH 3 ) y Na x FeSe has multiple superconducting phases depending on the concentrations of Na and NH 3 . Guo et al. reported three superconducting phases of (NH 3 ) y Na x FeSe: an NH 3 -poor phase (T C = 45 K), an NH 3 -rich phase (T C = 42 K), and an NH 3 -free phase (T C = 37 K) 23 . Very recently, Zheng et al. reported the presence of an NH 3 -poor low-T C phase (T C = 32 K), which was formed in a low concentration of Na 24 . This electrochemical synthesis produced the 30 K superconducting phase, whose T C is close to that of the NH 3 -poor low-T C phase (T C = 32 K) 24 . Therefore, several T C phases besides the low-T C phase can be produced by adjusting the concentration of Na by varying the reaction time and/or initial concentration of Na during the electrochemical process. The precise control of the dopant concentration in M x FeSe will be a future issue. The metal intercalation in the electrochemical process appears to be similar to the redox reaction in Li-ion batteries. Raman scattering of the electrochemically prepared superconducting C 60 proved the donation of electrons to the t 1u -levels of C 60 . As seen from Fig. 1c, the reduction of C 60 and FeSe (or formation of the anions) during the electrochemical process involves the intercalation of K + as the counter ion, since K + is the sole cation in the electrolyte. In fact, K atoms are detected in the EDX spectra for the electrochemically prepared FeSe (EC(K)-FeSe) (see Fig. 4c).
The efficiency of normal metal-intercalation procedures such as high-temperature annealing and liquid solvent methods should be correlated with the ionization potential of metal atoms and the electron affinity of the target materials. Lower ionization potential of the dopant and higher electron affinity of the host material could be expected to produce effective metal intercalation in these reaction processes. On the other hand, in the electrochemical reaction, applying a positive voltage to the CE (K metal) forces the removal of electrons from K metal, as shown in Fig. 1c, i.e., K metal in the CE is oxidized to supply K + to the electrolyte solution. Electrons are supplied to C 60 solid (WE) through the electric circuit (see Fig. 1c). The K + migrates from the CE to the WE in the electrolyte solution so that K + is intercalated into the C 60 and FeSe solids. Therefore, the concentration of K + is maintained at a constant level in the electrolyte solution until the K metal is exhausted. As a result, the electrochemical reactions of C 60 and FeSe can be expressed as ' + + ↔ ' , respectively. Our experiments suggest the practical utility of the electrochemical reaction for the intercalation of metal atoms into C 60 and FeSe. The electrochemical technique has the advantage of controlling the metal-concentration by the bias voltage. The reversible bias voltage yields the intercalation / de-intercalation of metal atoms, leading to appearance / disappearance of superconductivity. Intercalation of alkali-earth atom should be more effective because they can donate twice as many electrons as alkali metal atoms. In fact, Mg intercalation into a layered nitride (ZrNCl) using the electrochemical reaction produced a new superconducting phase 25 . Therefore, this technique may enable the control of the Fermi level by donating electrons to target materials. Layered materials with large interstitial sites are promising candidates for producing novel superconductors using the electrochemical process.

Methods
Electrochemical intercalation. Figure 1 shows a diagram and photo of the electrochemical cell used in this experiment. The electrochemical reaction for metal intercalation was performed using three electrodes; CE, reference electrode (RE) and WE. The CE is formed with alkali metal, which is an intercalant. The RE refers to the standard voltage of a potentiostat as shown below, and is composed of the same metal as the CE. The WE consists of the target material for the metal intercalation, C 60 or FeSe, which is connected by a Pt wire. The electrolyte for K intercalation was made by dissolving KClO 4 in polyethylene glycol (PEG) (KClO 4 : PEG = 1: 20), while the electrolyte for Na intercalation was made by dissolving NaCl in PEG (NaCl: PEG = 1: 20). A potentiostat (HA151 and/or HAL3001, Hokuto Denko Co. Ltd.) was used to apply a constant voltage between CE and WE. The voltage was varied from 0 to -5 V during the electrochemical reaction. The electrochemical cell (see Fig. 1) was placed in an incubator set at 30 °C. The detail of the electrochemical reaction is described in the Results section. After the electrochemical reaction, the sample prepared was introduced into a measurement cell in an Ar-filled glove box (O 2 , H 2 O < 0.1 ppm) for the measurement of the M/H.

Measurements of physical properties.
The M/H values of the sample were measured by a squid magnetometer (MPMS2 or MPSM3, Quantum Design Co. Ltd.). The T C value was estimated from the onset temperature. To determine the valence of C 60 , Raman scattering experiments were performed using a Raman spectrometer (NRS-5000, JASCO Co. Ltd). We recorded the synchrotron radiation x-ray powder diffraction pattern at the BL-8B in KEK-PF, Japan; the wavelength of the x-ray beam was 1.0004(8) Å. The quantitative composition of the K-doped FeSe sample was analysed by electron dispersive x-ray (EDX) spectrometer equipped with a scanning microscope (SEM-EDAX Genesis XM 2 ) (VE-9800SP, Keyence Co. Ltd).