Electrochemical Synthesis of Iron-Based Superconductor FeSe Films

Abstract

The superconducting FeSe films were successfully fabricated using the electrochemical synthesis. The composition ratio of Fe and Se can be controlled by the electric potential and pH value. We found that the FeSe films deposited at the electric potential -1.75 V and pH 2.3 show the superconducting transition at 3.5 K. The establishment of this electrochemical synthesis technique will provide many advantages for application.Since the discovery of superconductivity in LaFeAsO 1-x F x 1 , several types of iron-based superconductors have been discovered. [2][3][4][5][6][7] Among them, FeSe with T c ∼8 K is quite promising for practical applications because of the simplest structure and less toxicity compared to other As-based compounds. Furthermore, the superconducting transition temperature T c dramatically increases up to 37 K under high pressure. The electrochemical deposition has further advantages to fabricate FeSe films because of the inexpensive equipment, quick synthesis, and ease of deposition of large area at room temperature. There are some reports about the challenge for the electrochemical deposition of iron chalcogenide. However, no one has succeeded to obtain superconducting FeSe films. [15][16][17][18][19] Here, we show a synthesis of superconducting FeSe films using the electrochemical method.Electrochemical depositions were performed by a three-electrode method.Cathode, anode and reference electrode were Pt plate, Fe plate and Ag/AgCl electrode, respectively. In order to prepare electrolyte, we dissolved 0.01 mol/l FeSO 4 •7H 2 O and 0.005 mol/l SeO 2 into distilled water. The HNO 3 was used as pH adjustor. We performed cyclic voltammetry (CV) in a range of the electric potential between -0.4 and -2.5 V.The films were characterized by x-ray diffraction with Cu-Kα radiation using the 2θ-θ method. The actual composition of the films was investigated by the energy dispersive x-ray spectrometry (EDX). The temperature dependence of magnetization was measured using a superconducting quantum interface device (SQUID) magnetometer with an applied field of 5 Oe. The electric potential dependence of the composition ratio of Fe and Se estimated by EDX measurements is shown in Fig. 2. At the electric potential of -1.00 V, Se was dominantly detected. At the electric potential between -1.50 and -2.50 V, Se and Fe composition ratio was estimated to be around 60 and 40 %. We found that the Se composition is slightly higher than that of Fe at the electric potentials between -1.50 and -2.50 V.The XRD patterns for the samples obtained at the electric potential of -1.00 and -2.50 V are summarized in Fig. 3. At the electric potential of -1.0 V, only hexagonal Se peaks were observed, which is consistent with EDX results. In contrast, we found that peaks identified as tetragonal FeSe were appeared in the samples at -1.5, -1.75 and -2.00V. However, the peaks are noticeably broadened because of Se-rich composition. SinceFeSe phase is stable in the slightly Fe-rich condition 4, 20 , we tried to control the Fe/Se rati...

Superconductivity in Fe-chalcogenides

“…Thin films were synthesized with other approaches such as metal-organic chemical vapor deposition (MOCVD) [29,30], molecular beam epitaxy (MBE) [31], and electrochemical synthesis [32]. In general it is found that β-FeSe 1-x Te x can be easily grown in a wide temperature range (250°C-600°C) on various substrates such as MgO, LaAlO 3 , SrTiO 3 , Si, SiO x /Si, GaAs, R-Al 2 O 3 , CaF2, and LSAT.…”

Superconducting properties of iron chalcogenide thin films

“…Recently, very thick (thickness 100 µm) superconducting films of FeSe (T onset c = 8 K) were successfully deposited by electrochemical route [31].…”

“…It was shown by scanning tunneling microscopy (STM) that this new kind of substrate can yield strain-free FeSe films. Very recently, Demura et al [31] demonstrated the growth of 100 µm thick films on metallic tapes by electrochemical route. In their preliminary work, the composition ratio of Fe and Se was controlled by electric potential ( figure 12) and pH value (figure 13).…”

Controllable Phase Transition for Layered β-FeSe Superconductor Synthesized by Solution Chemistry

“…Moreover, NiAs-type FeSe with ferrimagnetism often coexists and contaminates the superconducting β-FeSe in the products of the solid-state synthesis . Chemical methods such as metal organic chemical vapor deposition and electrochemical deposition have been applied to grow β-FeSe thin films − and nanorod arrays . Superconducting β-FeSe nanocables, nanorods, nanoislands, and ultrathin films with several unit cells thickness on SrTiO 3 substrate provided possibilities for the fundamental understanding of reduced dimensionality on the enhanced T C .…”