Room temperature rubbing for few-layer two-dimensional thin flakes directly on flexible polymer substrates

OPEN SUBJECT AREAS: TWO-DIMENSIONAL MATERIALS DESIGN, SYNTHESIS AND PROCESSING SURFACES, INTERFACES AND THIN FILMS GRAPHENE Received 7 August 2013 Accepted 2 September 2013 Published 18 September 2013 Room temperature rubbing for few-layer two-dimensional thin flakes directly on flexible polymer substrates Yan Yu, Shenglin Jiang, Wenli Zhou, Xiangshui Miao, Yike Zeng, Guangzu Zhang & Sisi Liu School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan, 430074, PR China. The functional layers of few-layer two-dimensional (2-D) thin flakes on flexible polymers for stretchable applications have attracted much interest. However, most fabrication methods are ‘‘indirect’’ processes that require transfer steps. Moreover, previously reported ‘‘transfer-free’’ methods are only suitable for graphene and not for other few-layer 2-D thin flakes. Here, a friction based room temperature rubbing method is proposed for fabricating different types of few-layer 2-D thin flakes (graphene, hexagonal boron nitride (h-BN), molybdenum disulphide (MoS2), and tungsten disulphide (WS2)) on flexible polymer substrates. Commercial 2-D raw materials (graphite, h-BN, MoS2, and WS2) that contain thousands of atom layers were used. After several minutes, different types of few-layer 2-D thin flakes were fabricated directly on the flexible polymer substrates by rubbing procedures at room temperature and without any transfer step. These few-layer 2-D thin flakes strongly adhere to the flexible polymer substrates. This strong adhesion is beneficial for future applications. Correspondence and requests for materials should be addressed to S.J. (jslhust@hotmail. com) R ecent developments in electronics, photonics, and mechanics have increased the demand for mono- and few-layer two-dimensional (2-D) thin flakes1,2, such as graphene1–4, hexagonal boron nitride (h-BN)5–8, molybdenum disulphide (MoS2)6–12, and tungsten disulphide (WS2)6–11. The novel properties13–19 of these materials enable their use for many applications1–12,20–23. Within this broad range of applications, stretchable films that are composed of mono- and few-layer 2-D thin flake functional layers on flexible polymers have attracted interest22,24,25. Several methods, including liquid-phase exfoliation, chemical vapour deposition (CVD), and chemical approaches, have been studied for fabricating mono- and few-layer 2-D thin flakes on polymer substrates26–35. However, these approaches are ‘‘indirect’’ fabrication approaches26. After mono- and few-layer 2-D thin flakes are prepared in liquids27–29 or on templates30–32, an additional step is required to ‘‘transfer’’ the 2-D thin flakes to the polymer substrate26,33. Thus, recently developed ‘‘transfer-free’’ processes for attaching graphene and its 2-D analogues on polymer substrates are important26,33–35. However, these ‘‘transfer-free’’ methods have only been applied to graphene. In addition, these methods are unsuitable for use with other 2-D thin flakes. Furthermore, most of these methods require processing temperatures that are greater than 150uC, which common polymers cannot withstand. Here, a room temperature rubbing method that is based on the friction and self-lubricating properties of the 2D layered materials is proposed for the transfer-free fabrication of few-layer 2-D thin flakes on polymer substrates. Different types of few-layer 2-D thin flakes were directly fabricated on flexible polymer substrates with commercial two-dimensional raw materials (composed of thousands graphite, h-BN, MoS2, and WS2 atomic layers) after performing rubbing procedures for several minutes at room temperature with no transfer step. Results Advantages of rubbing method. The experiment details are given in Figure 1 and the Methods section. This rubbing method has the following important advantages: (1) producing a variety of few-layer 2-D thin flakes, (2) the rapid fabrication procedures (minutes) at room temperature, (3) the direct fabrication on flexible polymer substrates without any transfer step, (4) low raw material costs, and (5) a strong surface adhesion between the few-layer 2-D thin flakes and the flexible polymer substrates. Characterization results of the few-layer 2-D thin flakes. Commercial 2-D raw materials with thousands of atom layers were used as starting materials (see FSEM images of raw materials in the Supplementary SCIENTIFIC REPORTS | 3 : 2697 | DOI: 10.1038/srep02697 1 www.nature.com/scientificreports Figure 1 | Diagram of the rubbing steps: (a) Step-1, Sandpaper Rubbing Step. (b) Step-2, Smoothing Step. (c) Step-3, Soft Contact Rubbing Step. Information). Because poly(ethylene terephthalate) (PET) films are very smooth (see the FSEM and AFM images of the raw PET in the Supplementary Information), they were used as flexible polymer substrates for field emission scanning electron microscopy (FSEM), atomic force microscopy (AFM), and other optical characterisations. In addition, polyvinylchloride (PVC) films were used as a flexible polymer substrate for Raman characterisation because the Raman peak positions of the PVC do not interfere with the graphene, MoS2, and WS2 peak positions. Figure 2a shows that the 514 nm peak in the Raman spectra changes, with a clear red shift of 2D from approximately 2720 cm21 for the raw graphite powders to approximately 2690 cm21 for the few-layer graphene. In addition, the G/2D intensity ratio decreased significantly from the raw graphite powders to the few-layer graphene. When comparing these results with previous findings36–38, the Raman spectra of graphene suggested that five carbon atom layers occurred in the few layers of graphene on the PVC substrate. Furthermore, the polycrystalline features that were revealed by the D peak originated from the polycrystalline raw graphite powders (see the D peak of the raw graphite powders in Figure 2a and the FSEM image of the raw graphite powders in the Supplementary Information). Figure 2b contains an AFM image of the few-layer graphene on the PET substrate. The thickness of the graphene layer was approximately 1.15 nm. According to previous research37,39, graphene monolayers on substrates are approximately 0.65 , 0.95 nm thick. In addition, each additional carbon atom layer is approximately 0.34 nm thick. Thus, based on the calculated graphene thickness on the PET substrate, 5 or fewer carbon atom layers occurred. The AFM results are consistent with the Raman spectra results. SCIENTIFIC REPORTS | 3 : 2697 | DOI: 10.1038/srep02697 Figure 2c shows an AFM image of few-layer h-BN on a PET substrate. The thickness of this layer is approximately 1.70 nm. Based on previous results5,40, a monolayer of h-BN on a substrate is approximately 0.60 , 0.90 nm thick. In addition, each additional hBN atom layer is approximately 0.333 nm thick. Thus, the calculated thickness of few-layer h-BN on the PET substrate corresponded with to 5 or fewer atom layers. In addition to the (...truncated)