Graphene has been extensively studied as an outstanding material for flexible electronic devices and wearable biosensors due to its excellent mechanical flexibility, high carrier mobility, high surface to volume ratio and biocompatibility. In particular, for achieving graphene field-effect transistors (GFETs) based sensors of high sensitivity and stability, the deposition of thin and uniform high-k dielectric films on graphene is important. Among the dielectric films deposited through various deposition methods, atomic layer deposited (ALD) dielectric films have advantages for depositing ultra-thin and uniform films at relatively low temperature (<300℃) due to its surface-limited reaction mechanism. However, ALD dielectric films on graphene usually show non-continuous and rough surface morphology owing to inertness of the graphene basal plane. ALD relies on the chemisorption of precursor molecules with the surface functional groups, but strong sp² carbon bonding of graphene basal plane prevents the facile nucleation of dielectric materials on the graphene surface during the ALD process. Thus, various techniques for functionalization of the graphene surface have been suggested.

In this paper, we demonstrate the use of low-cost and scalable atmospheric oxygen plasma treatment to efficiently functionalize the surface of chemical vapor deposited (CVD) graphene with minimal structural and electrical degradation. We show the deposition of ultra-thin (< 10 nm) and uniform high-k dielectric films of ALD ZrO₂ on the graphene without any seed layer after graphene functionalization through atmospheric oxygen plasma treatment. Water contact angle measurements and Raman spectroscopy show that graphene surface becomes hydrophilic due to dangling bond-type defects after atmospheric plasma treatment. Atomic force microscopy (AFM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) show that ALD ZrO₂ films on the plasma-treated graphene are deposited thin, uniform, and polycrystalline at low ALD process temperature of 150℃. It is notable that the plasma treatment affects not only the surface coverage but also the crystallinity of the ALD ZrO₂ film on graphene at low temperature. Electrical characterization also reveals that atmospheric plasma treatment generates functional groups in graphene surface by shifting the Dirac voltage (V_dirac) of GFETs to positive voltage. However, after ALD process, the thin yet pin-hole free ALD ZrO₂ films on the plasma-treated graphene reduces charge trapping sites in and around graphene, recovering the charge transport and V_dirac (~0V) in GFETs. The passivation effect of ALD is also revealed as shown by a dramatic enhancement in air stability of GFETs. The sensors based on GFETs of V_dirac close to 0V can operate at low voltages, which is beneficial for power limited portable or wearable devices. Consequently, we believe that uniform and crystalline ALD ZrO₂ film on the atmospheric plasma treated graphene will find use in flexible graphene electronics and biosensors owing to its low process temperature and capacity to improve device performance and stability.