Letter pubs.acs.org/NanoLett
Water-Gel for Gating Graphene Transistors Beom Joon Kim,† Soong Ho Um,†,‡ Woo Chul Song,‡ Yong Ho Kim,†,§ Moon Sung Kang,*,∥ and Jeong Ho Cho*,†,‡ †
SKKU Advanced Institute of Nanotechnology (SAINT), ‡School of Chemical Engineering, and §Department of Chemistry, Sungkyunkwan University, Suwon 440-746, Korea ∥ Department of Chemical Engineering, Soongsil University, Seoul 156-743, Korea S Supporting Information *
ABSTRACT: Water, the primary electrolyte in biology, attracts significant interest as an electrolyte-type dielectric material for transistors compatible with biological systems. Unfortunately, the fluidic nature and low ionic conductivity of water prevents its practical usage in such applications. Here, we describe the development of a solid state, megahertzoperating, water-based gate dielectric system for operating graphene transistors. The new electrolyte systems were prepared by dissolving metal-substituted DNA polyelectrolytes into water. The addition of these biocompatible polyelectrolytes induced hydrogelation to provide solid-state integrity to the system. They also enhanced the ionic conductivities of the electrolytes, which in turn led to the quick formation of an electric double layer at the graphene/electrolyte interface that is beneficial for modulating currents in graphene transistors at high frequencies. At the optimized conditions, the Na-DNA watergel-gated flexible transistors and inverters were operated at frequencies above 1 MHz and 100 kHz, respectively. KEYWORDS: Graphene transistor, water-gel, metal-substituted DNA, biocompatibility, high frequency operation
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the electrolyte for graphene gating would be ideal since water is the basic and natural electrolyte in biological systems.19−21 Furthermore, it is also abundant, inexpensive, and environmentally friendly. However, water alone exhibits unfavorable physical characteristics that are detrimental for practical, bioelectronic applications. First of all, the fluidic nature of the material prevents the preparation of practical, solid-state devices using water. Water also has a relatively low ionic conductivity compared to other common electrolytes, which leads to poor dynamic performances of devices equipped with water as the sole electrolyte. Accordingly, complements are necessary to fully demonstrate the great promise and practicality of water-gated, graphene-based electronic devices. In this Letter, we report a method that can alleviate the critical issues concerning water-gating graphene-based transistors. The core of the method is straightforward; we inserted a biocompatible polyelectrolyte into a water gate dielectric that can induce gelation to form a solid-state water-gel (often termed hydrogel) and enhance the ionic conductivity of the system. Specifically, metal-substituted salmon sperm DNAs (MDNA) with varying types of metal ions (where M = Na, Mg, Ca, Fe, and Zn) were used as the polyelectrolyte additives. The influence of M-DNA on the electric and mechanical properties of the aqueous gate dielectric as well as the resulting
tilizing electrolytes for modulating the current density of graphene is a highly promising approach toward achieving scientific and practical advances in graphene-based electronics. The formation of highly capacitive electric double layers (EDL)1−3 at the electrolyte/graphene interface leads to the induction of very high two-dimensional carrier densities (>1014 carriers/cm2),4−7 which are hardly achievable using conventional oxide gate dielectrics. This leads, for example, to carrier density- and carrier type-dependent responses in the Raman spectra and to the observation of two-dimensional Bloch−Grüneisen transport in graphene.6,8 The high capacitance of electrolytes also leads to a high transconductance of graphene at low operation voltages (
