Hydrogen fuel cells have attracted much attentions as future power sources because of their lower environmental load than fossil fuel. However, applications of hydrogen-based systems involve risk of explosion due to its low explosion limit [1]. Therefore, high sensitive and portable hydrogen sensor is required for practical applications of safe hydrogen-based energy systems. As a candidate of these hydrogen sensors, Pd-decorated graphene have been investigated [2,3]. Thanks to high reactivity of Pd nanoparticles to hydrogen and surface to volume ratio of two-dimensionally layered graphene, graphene decorated with Pd nanoparticles can be used as a highly sensitive hydrogen sensor. There are many reports [2,3] on the resistive change of Pd-decorated graphene in hydrogen atmosphere. Some of them demonstrated that carrier concentration in graphene is changed under H₂ ambient [3]. However, physical origins of the resistive change have not been fully understood. In this work, physical sensing mechanisms of Pd-decorated graphene transistors are investigated by means of Hall effect characterization in hydrogen/nitrogen atmosphere.

Graphene sensor decorated with Pd nanoparticles was fabricated. Graphene films grown by chemical vapor deposition (CVD) were transferred to 90-nm-thick SiO₂ on p⁺-Si substrates. Then, graphene was patterned into Hall bar structure using photo-lithography followed by oxygen plasma etching. The Cr/Au (10 nm / 100 nm) electrodes were deposited at terminals as well as back-side using electron-beam evaporation. Finally, 1-nm-thick Pd deposition was followed by 400 ℃ annealing in N₂ for 2 hours to form Pd nanoparticles. The Hall effects were measured at constant drain voltage (V_D) of 100 mV and magnetic field (B) of 0.43 T using a neodymium magnet under the sensor chip. Two sensors (devices A and B) fabricated through the same processes were characterized. The channel length/width of device A and B were 400/20 μm and 300/50 μm, respectively. In order to investigate carrier concentration and Hall mobility in hydrogen/nitrogen atmosphere, 1000 ppm hydrogen (nitrogen balance) and nitrogen gases were introduced at the flow rate of 500 mL/min onto the sensor devices.

We investigated sensing performance of Pd-decorated graphene against H₂. From 0 to 3 min and from 6 to 9 min, sensors were under inert (N₂) ambient. Whereas, from 3 to 6 min, 1000 ppm H₂ was introduced. Resistance increase was observed under 1000 ppm H₂. V_G dependence of carrier concentration (N_s) was observed through Hall effect measurement. Because of the positive-shift of the Dirac point, N_s decreases as V_G increases. The changes of N_s and Hall mobility (μ_H) during 3-min 1000 ppm H₂ ambient were summarized in Table I. Note that V_G of 0 and 40 V correspond to the larger and smaller N_s, owing to the shift of the Dirac point, respectively.

First, we will discuss N_s change under H₂ ambient. For all devices and applied V_G, N_s decreases under H₂ ambient. In other words, I_D-V_G characteristics of graphene shift in the negative direction in the amount of −ΔV_G. When |V_G−V_dirac| is large, the ratio of N_s change ΔN_s/N_s should be equal to −ΔV_G/V_G, where V_dirac is the V_G corresponding to the Dirac point. ΔN_s/N_s observed in device A can be understood using this relationship. Simple calculation shows |ΔN_s/N_s| should be less than |ΔV_G/V_G|, when V_G is close to V_dirac. Therefore, we consider that V_dirac of device B is smaller than that of device A. Next, Hall mobility change (Δμ_H) will be discussed. As shown in Table I, μ_H also changes under H₂ ambient. Assume that effective mass of hole is proportional to root of N_s [4] and that carrier scattering mechanisms are the same in H₂ ambient as in N₂ ambient. Then, it is shown that Δμ_H/μ_H is −ΔN_s/2N_s. As shown in Table I, the expected relationship were qualitatively observed. However, Δμ_H/μ_H is always smaller than −ΔN_s/2N_s. This is due to the increased Coulomb scattering induced by the hydrogen ions, which constitute the origin of negative shift of I_D-V_G characteristics under H₂ ambient.

We have shown that not only carrier concentrations but also carrier mobility is greatly changed under H₂ ambient. The physical mechanisms of mobility change is considered to be due to carrier-concentration-dependent effective mass and increased Coulomb scattering by hydrogen ions.

[1] J. Villatoro et al., Sensors and Actuators B, 110, 23-27, 2005.

[2] R.Kumar et al., Sensors and Actuators B, 209, 919-926, 2015.

[3] W.Wu et al., Sensors and Actuators B, 150, 296-300, 2010.

[4] K.S.Novoselov et al., Nature, 438, 197-200, 2005.