One-step and controllable bipolar doping of reduced graphene oxide using TMAH as reducing agent and doping source for field effect transistors Firoz Khan, Seong-Ho Baek, Jae Hyun Kim * Division of Nano and Energy Convergence Research, Daegu Gyeongbuk Institute of Science & Technology (DGIST), 50-1 Sang-Ri, Hyeonpung-Myeon, Dalseong-gun, Daegu 711-873, Republic of Korea article info Article history: Received 5 October 2015 Received in revised form 14 January 2016 Accepted 16 January 2016 Available online 18 January 2016 abstract Simultaneous reduction and doping of the graphene oxide (GO) is an important issue for low temper- ature processed flexible electronic devices. A low temperature method for reduction and ambipolar doping has been developed which yield the doped reduced GO with wide range of work function with a mass production using tetra-methyl ammonium hydroxide (TMAH). The doping type of obtained reduced GO is tuned with TMAH concentration. XPS analysis revealed that the graphitic N is converted to oxidized N with increase of TMAH concentration. The work function is tuned via wide range variation in the carrier concentration in neutral (rGO-A, 4.46 eV), n-type (rGO-B, 3.90 eV) and p-type (rGO-C, 5.29 eV) regimes. The obtained Dirac voltages of field effect devices are 1 V, 31 V and þ35 V with active layer of rGO-A, rGO-B and rGO-C, respectively. The n-type doping is due to incorporation of graphitic N, whereas, oxidized N acts as electron withdrawing group which causes p-type doping. © 2016 Elsevier Ltd. All rights reserved. 1. Introduction Beyond the graphene, the graphene oxide (GO) and reduced graphene oxide (rGO) have emerged as alternative materials. They possess sizable band gaps [1,2], exhibit large carrier mobilities [3,4], and emit light in the visible and infrared regions [5]. The synthesis of GO involves low cost chemical process that results in scalable and solution processable materials [6]. Moreover, the 2D electronic structure can be chemically tailored via sp 3 -sp 2 inter conversion [6]. In the past few years, several methods such as chemical reduction [7], thermal reduction [8], photo-reduction [9], micro- mechanical exfoliation [10], pyrolysis [11], and chemical vapour deposition (CVD) [12] have been developed to produce rGOs. After reduction of GO, it possess some of favourable properties including reasonable mechanical strength [13], optical transparency [14], good conductivity [15], and catalytic properties [16] for their favourable applications in nanoelectromechanical sensors, solar cells, high frequency electronic devices and energy storage devices, respectively. Several reducing agents namely, hydrazine [17], hy- drazine derivatives [18], sodium hydride [19], sodium borohydride [20], hydrogen iodide [21], aluminium iodide with alcohol [22], and bacteria respiration [23] have been used to reduce GO via chemical route. The chemical doping of rGO is equally important to modulate the electrical properties (tune the work function, WF) for electronic device applications. Several methods i.e., thermal doping of foreign atoms (B and N), self-assembled monolayer (SAM), and chemical doping have been proposed to tune the WF of graphene [24e27]. There are very few reports [28e30] available in the literature that describes the simultaneous doping and reduction of GO. However, none of the above methods provides sufficient tuning of WF of rGO via tailoring the doping levels over a wide range of carrier con- centration from n-to p-type regimes [31]. Wei et al. [28] have synthesized in-situ N-doped multi-layer graphene using CVD by adding ammonia. Wang et al. [29] doped graphene nano-ribbons via electrical joule heating in ammonia. Moreover, simultaneous nitrogen doping and thermal reduction of GO has been done by Li et al. [30] via annealing the GO in ammonia. Best of our knowledge, to date none of the low temperature reduction method is reported which can tune the WF of rGO in a wide range of carrier concen- trations. Generally, flexible organic electronic device or polymer fuel cell based on rGO requires green approach at low processing temperature. Therefore, it is important to develop a low * Corresponding author. E-mail address: jaehyun@dgist.ac.kr (J.H. Kim). Contents lists available at ScienceDirect Carbon journal homepage: www.elsevier.com/locate/carbon http://dx.doi.org/10.1016/j.carbon.2016.01.064 0008-6223/© 2016 Elsevier Ltd. All rights reserved. Carbon 100 (2016) 608e616