Research paperEffects of chemical and physical defects on the humidity sensitivity of graphene surface
Graphical abstract
Introduction
As one of the crystalline carbon structures, graphene is well known for its various kinds of applications [1], [2], [3], [4]. Among them, one of the representative applications is the gas sensors. Electrical properties graphene are effected by a few molecules adsorbed on the surface of graphene. [5]. Among them, H2O molecules can be readily doped on graphene surfaces [6], [7], implying that graphene could be used as a promising material in humidity sensing applications, but, it is still challenging to achieve high sensitivity for water molecule. In some reports, graphene derivatives like graphene oxide (GO) and reduced graphene oxide (rGO) had been chosen for the sensing material for humidity change [8], [9]. GO and rGO are hydrophilic due to oxygen functional groups [10], [11], so it is possible to obtain high humidity sensitivity. However, the structure of the GO layer on the substrate, is layer by layer structure of GO flakes [8], [9], so it is hard to apply photolithography process and GO itself can have degradation in humid air because of its high solubility in water.
The graphene grown by chemical vapor deposition (CVD) is favored in most of the graphene devices, because it is easy to obtain the mono layer of graphene in a large scale [12], [13], so it is useful to apply mass production. There are already some reports about the humidity sensor using CVD graphene [14], [15]. Despite these advantages of the CVD graphene, there are some factors to carefully consider when we fabricate the humidity sensor using graphene. The first thing is the polymer residue on the graphene. Wet transfer method are a frequently used method for transferring the CVD graphene [4], [12], [16], but after transferring graphene by the this method, the PMMA residues remain on the surface of graphene [16]. However, completely removing the polymer residues is impossible [16], and not only the PMMA residues have a doping effect for the graphene [17], but also PMMA residues on the graphene can adsorb water molecules [18], [19] so that the electrical properties of graphene can be effected. The second thing is that the CVD graphene intrinsically have various kinds of defects on it [20], [21]. The defects on the graphene effects the adsorption of several gas molecules including water molecules [20], [22], [23], [24], [25], [26]. Therefore, it is necessary to consider the amount of PMMA residues and defects the on the graphene when making the humidity sensor using graphene or the graphene devices under humidity changing conditions.
In this study, we report the effect of the physical and chemical defects on the CVD graphene for humidity sensing. The physical defects on the graphene were created by reactive ion etching (RIE). To tailor the chemical defects on the graphene, we covered the PMMA on the graphene surface with different coverage areas and thicknesses using spin coating. To examine the humidity sensitivity, the resistance changes of the modified graphene were measured under the various humidity environment. Effects of the physical and chemical defects were demonstrated in terms of the enhancement of the humidity sensitivity of the graphene.
Section snippets
Experimental section
To study the effect of physical defects on humidity sensitivity of graphene, defects in the graphene samples were controlled using a plasma etching process. The fabrication process of the graphene having defects is depicted in Fig. 1(a). CVD graphene was transferred onto a SiO2/Si substrate by a wet transfer method and a Cr/Au electrode (5 nm/30 nm thick) was deposited using an E-beam evaporator. The size of the graphene sample exposed to air was 7 mm × 10 mm. Subsequently, the graphene sample
Results and discussions
With RIE method, a two-dimensional (2D) mapping indicating the intensity ratio of D and G bands in the Raman spectra is shown in Fig. 2(a)–(d). The size of the mapping area is 30 μm × 30 μm, and the number of measurement points is 30 × 30. The D/G intensity ratio is increased with an increase in the etching time, and the physically-created defects are uniformly distributed on the graphene surfaces. In this paper, the physically-created defects of the graphene means the empty spaces of the
Conclusions
In conclusion, we investigated the humidity sensitivity of modified graphene by creating defects. We described that the physical defects on the graphene by RIE method had negligible effects on the humidity sensing ability of graphene. The correlations between the amount of defects and the sensitivity of NO2 and NH3 are clear from the previous studies. Unlike the graphene’s sensitivity of NO2 and NH3, the graphene have relatively low sensitivity for water molecules. Additionally, the defects
Acknowledgement
This work was supported by Center for Advanced Soft Electronics, ERC project (2016R1A5A1010148) and Basic Science Research Program funded by the Korea government (MSIP) (2015R1A2A2A01004751, 2015R1A2A1A01005931).
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These authors contributed equally to this work.