Improvement of Wrinkles in Roll-to-Roll Microwave Plasma CVD Graphene
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Improvement of Wrinkles in Roll-to-Roll Microwave Plasma CVD Graphene Takatoshi Yamada1 ,2, a, Nayuta Shimada2, Kazuki Uekusa2 and Masataka Hasegawa1 ,2 National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba 305-8565, Japan. 2 Technology Research Association for Single Wall Carbon Nanotube (TASC), 1-1-1 Higashi, Tsukuba 305-8565, Japan. a [email protected]
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ABSTRACT Wrinkle-less graphene films are obtained through roll-to-roll microwave plasma chemical vapor deposition by using flexible copper/polyimide (Cu/PI) webs. Raman spectra suggests that the average domain size of the obtained graphene on the flexible Cu/PI is almost the same compared to the graphene on a Cu web that includes wrinkles. Also, by utilizing the flexible Cu/PI webs, the compressive strains decreased. The sheet resistances of graphene deposited on the Cu/PI are (1~5)×104Ω, which is two orders of magnitude lower than those of graphene deposited on the Cu webs. Our results suggest that the controlling the expansion of web material an important technology to improve graphene transparent conductive properties. INTRODUCTION Transparent conductive films are key components for modern electronics and optoelectronics applications such as displays, solar cells and light emitting devices. Although most of the transparent conductive films used in commercial applications consists of indium tin oxide (In2O3:Sn, ITO), it is difficult to use ITO for the flexible applications. Therefore, alternative solutions must be explored in order to fabricate flexible transparent conductive films. Graphene is considered to be one of the most appropriate materials for this use [1-4], because of its high carrier mobility, high optical transmittance and high thermal conductivity. Most of the reported properties of graphene were characterized using small pieces obtained by a mechanical exfoliation technique. In order to use graphene as transparent conductive films, high quality graphene films on large area are obtained by thermal chemical vapor deposition (CVD) [5-7]. The thermal CVD process requires temperature of over 800 ºC and few tens of minutes to form single layer graphene on copper (Cu) surface. Such high temperature results in a formation of unintentional wrinkles due to thermal expansion of the Cu foil [7]. In our previous studies [8-10], roll-to-roll microwave plasma CVD processes at low temperature were proposed for high throughput and continuous deposition process of graphene. However, wrinkles were formed during the roll-to-roll microwave plasma CVD process regardless of low temperature about 300 ºC (Fig.1). This is because the Cu web was stretched during roll-to-roll process and then the stress was released after process, which resulted as wrinkles in the direction of the web flow since Cu is a non-elastic material. The formed wrinkles induced defects and compressive strains [8-10]. In addition, both the defects and the compressive strains increased the sheet resistances of the roll-to-roll microwave plasma CVD
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