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Due to the graphene superior electrical mechanical and optical properties, graphene is particularly advantageous for flexible transparent conductive electrode applications. However, its sheet resistance still higher than indium tin oxide has been a critical problem. Increasing of graphene layers is well known and basic method for sheet resistance reduction. But, this enhancement was not fully explained in terms of carrier density and mobility which are the elements of the sheet resistance. Because of the graphene electrical property is strongly influenced by surface condition, each graphene layer in the multilayer might be has different electrical property. Therefore, simple measuring the sheet resistance gives insufficient information about the origin of sheet resistance decreasing. By understanding the number of layer and surface condition effect on carrier density and mobility, we can get the potential pathway to reduce the sheet resistance. So, separated measurement of electrical property is required. In this work, enhancement of electrical properties with increasing the number of graphene layer is systemically investigated by Hall measurement which can divide sheet resistance into the carrier mobility and density. In order to get multilayer graphene with the different interface condition, two different layer stacking methods were introduced. Single layer graphenes were synthesized by chemical vapor deposition and stacked the graphene with and without benzimidazole which strongly p-dopes the graphene. 1 to 4 layers of graphene were stacked and analyzed. The sheet resistance of inter-layer undoped and doped graphene are reduced from ~700 to ~220 Ω/sq and ~280 to ~80 Ω/sq, respectively. Both sheet resistances are inversely proportional to the number of layers. However, carrier mobility and density show clearly different characteristics. Regardless of the number of graphene layers, carrier density of inter-layer undoped graphene is maintained in the range of ~8 x 1012 /cm2, but it is linearly increased from ~1.8 x 1013 to ~6.2 x 1013 /cm2 when inter-layer was chemically doped. The carrier mobility of inter-layer doped graphene remained same in 1200 cm2/Vs but, that of undoped graphene is increased from 1000 to 3000 cm2/Vs. Because the major carrier generation of undoped graphene occurred only on the top of the layer, it has constant carrier density regardless of the number of graphene layer. Compare to the top layer, the layers inside undoped graphene have a much smaller carrier density it leads less charge carrier interaction and high carrier mobility. On the other hands, inter-layer doped graphene has uniform carrier density in inside each layer. So, it result the uniform carrier mobility and linearly increasing carrier density depending on the number of graphene layers. In addition, by stacking the various coverage of graphene which are the 16, 38, 84 and 100 % onto the full coverage monolayer graphene, we checked the partial coverage additional layer effect even without the second layer conducting path. The sheet resistance is gradually decreases from ~700 to ~350 Ω/sq by increasing the second layer coverage. The carrier density show uniform value of ~8 x 1012 /cm2 in all coverage, but mobility linearly increases from 1040 to 2288 cm2/Vs. Increased mobility leads the sheet resistance decreases. We find out that the origin of sheet resistance reduction in the multilayer graphene is different depending on the surface condition. This demonstration would be very useful for the use of graphene as a flexible transparent conductive electrode, and in other electrical applications.

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 13 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 Figure 1

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