Quantum and electrochemical interplays in hydrogenated graphene

Abstract

The design of electrochemically gated graphene field-effect transistors for detecting charged species in real time, greatly depends on our ability to understand and maintain a low level of electrochemical current. Here, we exploit the interplay between the electrical in-plane transport and the electrochemical activity of graphene. We found that the addition of one H-sp³ defect per hundred thousand carbon atoms reduces the electron transfer rate of the graphene basal plane by more than five times while preserving its excellent carrier mobility. Remarkably, the quantum capacitance provides insight into the changes of the electronic structure of graphene upon hydrogenation, which predicts well the suppression of the electrochemical activity based on the non-adiabatic theory of electron transfer. Thus, our work unravels the interplay between the quantum transport and electrochemical kinetics of graphene and suggests hydrogenated graphene as a potent material for sensing applications with performances going beyond previously reported graphene transistor-based sensors.

Fig. 1

Raman characterization of hydrogenated graphene. a Averaged Raman spectra of CVD graphene on a Si/SiO₂ substrate after 0-60 s of H₂ plasma (10 W, 1.0 mbar). b The intensity ratio I(D)/I(D′) after 30 s and 60 s of hydrogenation. c The intensity ratio I(2D)/I(G) for hydrogenation times ranging from 0 to 60 s. d The intensity ratio I(D)/I(G) and the derived defect density n_D, plotted vs the hydrogenation time. The error bars include results from both exfoliated and CVD graphene. e The FWHM of the 2D, G, and D peaks vs the hydrogenation time. The spectra are recorded using a 2.33 eV (532 nm) laser excitation. The error bars in b–e are the standard deviation of experimental values

Fig. 2

Transport characteristic and quantum capacitance of CVD graphene upon hydrogenation. a Illustration of the field effect transistor setup fabricated from CVD graphene. b Room temperature conductance (G) plots as a function of the gate voltage (V_g) showing the p-doping effect upon hydrogenation from 0 to 30 s. The gray dashed line is a guide-to-the-eye, highlighting the sublinear behavior of the G(V_g) curves. c The shifts of the charge neutrality point (CNP) upon hydrogenation. d The carrier mobility of graphene, µ, vs the hydrogenation time. e Quantum capacitance C_q of graphene measured as a function of V_ch for 0–30 s of hydrogenation. f Impurity density n_imp vs hydrogenation time. The electrolyte solution is 0.1 M KCl with 10 mM Tris (pH 8). The error bars in d, f are the standard deviation of experimental values

Fig. 3

Electrochemical behavior of CVD graphene upon hydrogenation. a Cyclic voltammograms (CVs) obtained on graphene after 0–30 s of hydrogenation at a scan rate of 100 mV s⁻¹. b Current density vs scan rate for untreated graphene shown in a. c  The electron transfer rate k⁰ vs hydrogenation time from 0 to 30 s. d The averaged total capacitance C_ave− tot vs hydrogenation time from 0 to 30 s. e CV curves obtained on graphene after 0–13 s of Ar treatment at a scan rate of 100 mV s⁻¹. f $k_{\mathrm{Ar}}^0$ vs argon plasma treating time from 0 to 13 s. The aqueous electrolyte solution contains 0.1 M KCl supplemented with 10 mM Tris at pH 8. The redox probe employed is 1 mM hexaammineruthenium (II)/hexaammineruthenium (III) chloride. The error bars in c, d, f are the standard deviation of experimental values

Fig. 4

Quantum and electrochemical interplays in hydrogenated graphene. a The dependence of n_imp on n_D. b Correlations of μ and k⁰ with n_D, respectively. c The minimum conductivity (G_min) vs n_D. d The correspondence between ADOS and k⁰ as a function of the hydrogenation time. The purple region represents the cleaning-dominated regime and the blue region represents the regime where the chemical modification dominates. e The relative variations of Δμ/μ_untre correlating with $\Delta k^0∕k_{\mathrm{untre}}^0$ according to the corresponding hydrogenation time. The subscript "untre" represents the untreated graphene. The error bars are the standard deviation of experimental values