Nanoscale electrodeposition of metals and semiconductors from ionic liquids

Abstract

The electrocrystallization of Ni, Co and their respective alloys with Al and the electrodeposition of Ge on Au(1 1 1) and Si(1 1 1):H have been studied in the underpotential (UPD) and overpotential (OPD) range. To this end, in situ electrochemical STM and STS measurements have been performed in ionic electrolytes, the room temperature molten salts or ionic liquids AlCl 3–[BMIm] +Cl − and [BMIm] +PF 6 −. The larger electrochemical windows of these ionic electrolytes in comparison to aqueous media enables the investigation of electrodeposition of these elements and compounds on a nanometer scale. We present and compare recent results of 2D and 3D phase formation of Ni and Co electrodeposition. Clear differences are observed in the 2D phase formation—Ni monolayer vs. Co island formation—which are discussed in the light of the distinct values of the interfacial free energies of these two metals. In the overpotential (OPD) range, Ni deposition proceeds by a columnar growth of 3D Ni clusters along step edges, whereas Co clusters grow homogeneously at potentials below −0.17 V vs. Co/Co(II). Electrodeposition of Ni x Al 1− x and Co x Al 1− x is found to be very similar. In both cases, codeposition starts at a potential clearly positive of the Al/Al(III)-Nernst potential and with increasing Al content smaller grains are observed. The composition of the respective alloy clusters has been probed in situ by STS spectra and it is found that the effective tunneling barriers at different potentials E scale with the cluster composition determined from independent conventional electrochemical and spectroscopic measurements. Finally, we report first investigations of electrodeposition of ultrathin Ge films with varying thickness on Au(1 1 1) and Si(1 1 1):H. Probing the electronic structure of these films by in situ STS spectra a metal–semiconductor transition is indicated with increasing film thickness above 2–3 nm. This is discussed in comparison with ultrathin Ge films grown by expitaxial vapour deposition.