Tunability of transport properties in semi-exfoliated AgxCoO2−δ (0.4 < x < 1) crystals

Abstract

Transparent semiconductors continue to be of high interest for a variety of electronic applications with the ability to tune transport properties being highly sought-after. Herein, we develop novel Ag_xCoO_2−δ delafossite materials with silver content being varied for the first time. The silver content is controlled by the ion-exchange temperature used during synthesis with single-crystal NaCoO₂ precursors. By contrast, materials made from polycrystalline precursors show a constant silver content (x = 0.8) regardless of ion-exchange temperature. Importantly, we observe for the first time that the single-crystal precursors in fact separate into many sheets during ion-exchange resulting in partial exfoliation, and this occurs even at low ion-exchange temperatures. The materials were characterized further with X-ray diffraction and X-ray photoemission spectroscopy, both of which were consistent with the Ag content varying both at the surface and the bulk of the material. The results also show a high oxygen deficiency at least at the surface. The impact of varying silver and oxygen content of the crystals was further explored with hall measurements with higher silver content yielding a higher charge carrier (hole) concentration. The samples made from single-crystals showed higher electronic conductivities, hole densities and mobilities than those made from powders. The hole concentration was found to be further tunable by varying the oxygen content of the material by heating in an oxygen atmosphere. The role of silver vacancies in creating holes was confirmed by DFT calculations. Surprisingly, the calculations also indicated that oxygen deficiency results in a band rearrangement that also provides accessible holes at the Fermi level. This study illustrates the benefit that optimized synthesis conditions can have in tuning overall composition to yield optimal transport properties in delafossites. This work also helps elucidate the consequences of competing cationic and anionic off-stoichiometries.