Graphene has received enormous attention from both academia and industry owing to its excellent electrical, optical, chemical, and mechanical properties. A combination of properties makes graphene a strong candidate for the next generation of flexible and transparent conductive electrode (TCE) materials, but in practice the sheet resistance of a single, as-grown layer of graphene is still not sufficient for this purpose. One of the methods for creating highly conductive graphene films is to increase the charge carrier density through doping.

Here, we propose a novel method of graphene doping based on atomic layer deposition (ALD). Ideally, adsorption of precursor should be suppressed on clean surface of graphene since the surface is inert without dangling bonds and chemically active functional group. Defects (e.g., grain boundaries, wrinkles, cracks, holes and residual material) work as selective nucleation sites. Ru is selected due to it having a well-established precursor and recipe and the difference in work function between graphene and Ru can also allow for work function tuning and carrier generation.

The sheet resistance of graphene synthesized by chemical vapor deposition is found to be significantly reduced by the selective atomic layer deposition (ALD) of Ru onto defect sites such as wrinkles and grain boundaries. With 200 ALD cycles, the sheet resistance is reduced from ~500 Ω/sq to <50 Ω/sq and the p-type carrier density is drastically increased from 10¹³ cm^-2 to 10¹⁵ cm^-2. At the same time, the carrier mobility is reduced from ~670 cm²V^-1s^-1 to less than 100 cm²V^-1s^-1. This doping of graphene proved to be very stable, with the electrical properties remaining unchanged over 8 weeks of measurement. The selective deposition of Ru on defect sites also makes it possible to obtain a graphene film that is both highly transparent and electrically conductive (e.g., a sheet resistance of 125 Ω/sq with 92 % optical transmittance at 550 nm). The increase in the work function of graphene from 4.6 to 5.1 eV with Ru ALD provides clear evidence of hole carrier generation as a result of an electronic interaction between graphene and Ru. This demonstration of strong and stable doping of graphene with a high optical transmittance is therefore expected to contribute to the use of graphene as a TCE, and in other fields.

Figure 1. Ru ALD on graphene. (a) TEM images of graphene after 20 cycles of Ru ALD. (b) Sheet resistance of graphene as a function of the number of ALD cycles. Inset of (b) shows sheet resistance as a function of time.

Reference

Minsu Kim et al, ACS Appl. Mater. Interfaces, Article ASAP