Elsevier

Chemical Physics Letters

Volume 689, 1 December 2017, Pages 206-211
Chemical Physics Letters

Research paper
Effects of chemical and physical defects on the humidity sensitivity of graphene surface

https://doi.org/10.1016/j.cplett.2017.10.028Get rights and content

Highlights

  • The effect of humidity on the electrical properties of the graphene was reported in view of chemical and physical defects.

  • The physical defects of the graphene by reactive ion etching give negligible effect on the humidity sensing of the graphene.

  • The graphene chemically defected with thinner or smaller area of the PMMA coating has the improved humidity sensitivity.

Abstract

We investigate the effect of the chemical and physical defects on the humidity sensitivity of graphene. For this we apply reactive ion etching for physical defects and the poly(methyl methacrylate) (PMMA) coating for chemical defects to the CVD graphene. The tendency of humidity sensing is hardly found by the physical defects while the distinct changes are observed with chemical defects by control of the thickness and the coverage area of the PMMA on the graphene surface. The graphenes covered with thinner or smaller area of PMMA show an enhanced humidity sensitivity, indicating the possibility of H2O sensing.

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).

References (35)

  • F. Yavari et al.

    Tunable bandgap in graphene by the controlled adsorption of water molecules

    Small

    (2010)
  • O.C. Compton et al.

    Graphene oxide, highly reduced graphene oxide, and graphene: versatile building blocks for carbon-based materials

    Small

    (2010)
  • R.R. Nair et al.

    Unimpeded permeation of water through helium-leak-tight graphene-based membranes

    Science

    (2012)
  • K.S. Kim et al.

    Large-scale pattern growth of graphene films for stretchable transparent electrodes

    Nature

    (2009)
  • S. Bae et al.

    Roll-to-roll production of 30-inch graphene films for transparent electrodes

    Nat. Nanotechnol.

    (2010)
  • A.D. Smith et al.

    Resistive graphene humidity sensors with rapid and direct electrical readout

    Nanoscale

    (2015)
  • C. Meng-Chu et al.

    Fabrication of humidity sensor based on bilayer graphene

    IEEE Electron Dev. Lett.

    (2014)
  • Cited by (3)

    • Metallurgical graphene under different gas atmospheres and UV radiation for gas-sensing applications

      2020, Sensors and Actuators, A: Physical
      Citation Excerpt :

      Due to the manufacturing process and transfer operation, real-life graphene contains point defects, multi-layer areas, cracks or process residues [2,19,20]. All of these imperfections influence the electrical properties of graphene, which leads to change in the overall response of the sensing element [21]. The goal of the following paper was to establish the change in electrical resistance of High Strength Metallurgical Graphene (HSMG®) [22,23] under different gas atmospheres and UV radiation.

    1

    These authors contributed equally to this work.

    View full text