Mechanically exfoliated low-layered [Ca$_2$CoO$_3$]$_{0.62}$[CoO$_2$]: A single-crystalline p-type transparent conducting oxide

TL;DR

This study demonstrates the fabrication of low-layered, single-crystalline [Ca$_2$CoO$_3$]$_{0.62}$[CoO$_2$] with enhanced transparency and electrical properties, highlighting its potential as a multifunctional transparent thermoelectric oxide.

Contribution

It introduces a mechanical tape-peeling method to produce low-layered, single-crystalline [Ca$_2$CoO$_3$]$_{0.62}$[CoO$_2$], improving transparency and electrical performance over previous polycrystalline and film forms.

Findings

Reduced electrical resistivity in single-crystalline samples.

Higher figure of merit compared to previous reports.

Thickness of low-layered samples is several hundred nanometers.

Abstract

Transparent conducting oxides (TCOs) are essential components of optoelectronic devices and various materials have been explored for highly efficient TCOs having a combination of high transmittance and low sheet resistance. Here, we focus on a misfit thermoelectric oxide [Ca$_2$CoO$_3$]$_{0.62}$[CoO$_2$] and fabricate the transparent low-layered crystals by a mechanical tape-peeling method using the single-crystalline samples. From the transmittance measurement, we find that the thickness of low-layered samples is several orders of hundred nanometers, which is comparable with the estimation from the scanning electron microscopy images. Compared to the previous results on the polycrystalline and $c$-axis oriented transparent films, the electrical resistivity is reduced owing to the single-crystalline nature. The figure of merit for the transparent conducting materials in the present low-layered samples is then evaluated to be higher than the values in the previous reports. The present results on the low-layered single-crystalline [Ca$_2$CoO$_3$]$_{0.62}$[CoO$_2$] may offer a unique class of multi-functional transparent thermoelectric oxides.