Graphene Suspended on Silicon Nanowire Arrays for Enhanced Gas Sensing
Authors: Panteli Christoforos , Sydoruk Oleksiy , Fobelets Kristel 
Published: February 2020

Graphene is attractive for gas sensing due to its large surface area to volume ratio. Gas molecules adsorbed on the surface of graphene change its electrical conductivity, which can be measured as a resistance change as a function of time. Normally, only one side of the graphene is exposed to the gas [1]. Exposing both sides for molecule adsorption should increase its sensitivity as the effective surface area is increased. Graphene can be suspended on mesas of a patterned substrate [2]. The maximum distance across which graphene can be suspended without substantial deformation is 100 μm [2]. Contacts need to be defined aligned to the mesa structures to avoid breaking the graphene layer.
We suspend graphene over a large area (~cm2), namely on top of a of silicon nanowire array (SiNWA) [3]. This approach removes the need for aligned contacts. In addition we found that it increases the sensitivity to gas molecules dramatically. We present results of the conductivity variation of suspended graphene on Si NWAs in response to NH3 molecules evaporating from 1 ml of aqueous NH4OH in a glass flask [4] in a closed container.
The SiNWA was fabricated using metal assisted chemical etching (MACE) [5]. A p-type Si substrate, with resistivity ρ = 1-5 Ω cm, was processed in three consecutive steps. First was the nucleation of Ag nanoparticles (NPs) on the surface using 0.06M AgNO3:28.2M HF:H2O for 10 min. Subsequently, the NWs were etched in 0.6M H2O2:28.2M HF:H2O for 10 min. Finally, the Ag NPs were removed in 67% HNO3:H2O.
CVD grown, multilayer graphene on Ni was transferred to the SiNWA via a PMMA support layer. To ensure good adhesion between graphene and SiNWA, the PMMA was dissolved in vapour rather than liquid acetone [3]. The SEM images (see Figure) show that graphene is suspended on the SiNWA over a large area.
The resistivity of the graphene measured with a four-point-probe (4PP) set-up was ρ = 10-3 Ωcm. The SiNWA support was found to be non-conductive in the direction parallel to the substrate.
To compare the influence of the increase in effective surface area, NH3 adsorption measurements were carried using both graphene-on-SiNWA (GrNW) and graphene-on-SiO2 (GrOx). In the figure, the resistivity is plotted as a function of time while NH3 was evaporating from NH4OH. All measurements were carried out at room temperature. Graphene behaves as a p-type semiconductor, due to ambient H2O and O2 influencing electron and hole mobilities [6]. Since, NH3 molecules are electron donors [7], the conductivity decreases when NH3 molecules adsorb onto graphene. When the current saturates, the NH4OH was removed and the lid of the container was opened. The reaction time of GrNW to NH3 is 5 times larger than for GrOx. There is a 30% increase in resistivity for GrNW compared to only 6% for GrOx, showing a clear increase in sensitivity of the suspended graphene layer. Both samples recover slowly to the initial resistivity value when NH4OH is removed. GrNW needs more time to saturate and recover, probably due to NH3 molecules entrapment in the SiNWA. Results from further experiments with different NWA densities will also be presented at the conference to analyse this phenomenon.

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Figure 1