Impact of Hydrogen on Carrier Mobility and Concentration in Graphene Decorated with Pd Nanoparticles
Authors: Goto Akihito , Takeuchi Go , Yamachi Ryosuke , Tanaka Takahisa , Takahashi Tsunaki , Uchida Ken 
Published: February 2020

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.

Figure 1