2.6 Electrochemistry of Thin Film Superconductor Electrodes

While barriers to acquiring good electrochemistry on ceramic superconductors have been overcome,¹⁸ the electrochemical response of redox active solution species using bare YBa₂Cu₃O_7-x thin film electrodes has been problematic.^{21, 22}  Redox active assemblies anchored on the surface, ^37-40 on the other hand, have successfully been used to obtain faradaic electron transfer reactions.  One advantage that bulk YBa₂Cu₃O_7-x electrodes have over thin films is that they can be mechanically resurfaced just prior to electrochemical measurements in an inert atmosphere.  This provides a pristine surface, free of insulating corrosion products.  Unfortunately, there are no methods currently available for mechanically resurfacing YBa₂Cu₃O_7-x thin films to a pristine state, though polishing and cleaving of microband edge electrodes have proved successful.^{13, 14}  Chemical etching of YBa₂Cu₃O_7-x thin films with bromine / ethanol solutions has been employed,⁴¹ but limitations of controlling the etching rate and non-uniform etching prevent the formation of a clean uniform surface. 

The electrolyte tetraethylammonium tetrafluoroborate, Et₄NBF₄, (Aldrich) was recrystallized from ethyl acetate / ethanol and vacuum dried overnight prior to use.  The compound 7,7’,8,8’-tetracyanoquinodimethane, TCNQ, (Aldrich) was used as the redox couple.  Acetonitrile was distilled with P₂O₅ under N₂ and passed over activated alumina in an inert atmosphere before use.  Here, 0.1 M Et₄NBF₄ in CH₃CN was used as electrolyte solution.   Solutions containing 1mM TCNQ were also made from the electrolyte solution.  Cyclic voltammetry was accomplished using an EG&G PAR 273 potentiostat.  All electrochemical measurements were carried out in an inert atmosphere glovebox.  Thin films were transferred from the laser ablation chamber to an inert atmosphere with less than 2-3 minutes atmospheric exposure to prevent surface corrosion. 

Investigations targeted YBa₂Cu₃O_7-x thin films since these are the most commonly researched high-temperature superconducting thin film materials.  The orientation of the thin films can be manipulated by careful control of the deposition temperature.³⁴  At a temperature of 750^oC, deposition of the c-axis orientation is preferred.  The a-axis orientation of YBa₂Cu₃O_7-x is favored when the deposition temperature is 630^oC.  At temperatures between 630^oC and 750^oC, a mixture of both orientations is obtained.  The choice of substrates also plays a factor in the determination of orientation.³⁴  For c-axis orientation, the lattice mismatches for MgO and LaAlO₃ are 2.00% and 2.62%, respectively.  While the lattice mismatches for a-axis oriented thin films on MgO and LaAlO₃ are 8.41% and 0.81%, respectively.  There are two primary conduction pathways in the YBa₂Cu₃O_7-x structure.  The first conduction pathway is aligned within the a-b plane through the copper-oxide sheets.  The second pathway is along the copper

Illustration 2.6: Structural schematic for YBa₂Cu₃O_7-z orientations a) a-axis b) c-axis (side view).  Arrows indicate the locations where electronic conductivity is thought to be highest within the lattice.          oxide chains. 

Illustrations 2.5(a) and 2.6(b) show that the a-axis oriented thin film positions its copper-oxide sheets perpendicular to the electrode interface, whereas the c-axis oriented thin film, shown in Illustrations 2.5(b) and 2.6(b), has its copper-oxide sheets parallel to the interface.   Perpendicular alignment of the copper-oxide sheets at the surface of the electrode may provide active sites near the electrode’s surface which promotes electron transfer to the solution-dissolved redox species.