Substrate Effects on the Transient Chemiresistive Gas Sensing Performance of Monolayer Graphene

Monolayer graphene, a single layer of carbon atoms connected in a honeycomb lattice, is incredibly sensitive to its surroundings—especially gases. That is why it’s been a promising material for making light weight, low power gas sensors. However, due to its mechanical fragility in suspended form, graphene is typically deployed on various substrates. The nature of the substrate—including its material composition, surface structure, and electrochemical interaction with graphene—can significantly affect the electronic and sensing behavior of the graphene film.

In this study, researchers investigated how four different substrates—silicon (Si), silicon dioxide on silicon (SiO₂/Si), hexagonal boron nitride on silicon (hBN/Si), and hBN on SiO₂/Si—affect the gas sensing behavior of graphene. One key mechanism is doping, where the substrate adds or removes electrons from the graphene layer, shifting its electrical characteristics.

To avoid the usual complications caused by direct electrical contact (like mechanical damage or increased contact resistance), the study used a non-contact eddy current method. This technique uses changing magnetic fields to measure graphene’s sheet conductance without physically touching it—ideal for delicate materials like monolayer graphene. It also allows for consistent, averaged measurements over a relatively large area, avoiding local variability.

The experimental results demonstrate a notable variation in graphene's conductivity upon exposure to oxygen, depending on the supporting substrate. For example, on SiO₂/Si, oxygen reduced graphene’s conductivity (as expected for a p-type doping response), but on hBN/SiO₂/Si, the effect was reversed. These contrasting behaviors suggest that the substrates may dope the graphene differently—either adding electrons (n-type) or taking them away (p-type)—which may completely change how the graphene responds to gas molecules.

In addition to the direction of the conductivity change, the study also measured the magnitude and speed of graphene’s conductance change when exposed to oxygen. Substrates like plain silicon produce stronger but slower responses, while hBN/Si made graphene respond faster but with less sensitivity. These variations come from the way doping affects both the initial charge carrier density and how oxygen molecules interact with the graphene surface.

Interestingly, the researchers found that hBN layers tend to n-dope the graphene, counteracting the natural p-doping effect of silicon or SiO₂. This opens up opportunities to tune the graphene properties not just through materials engineering, but also by smart substrate selection. Moreover, the use of non-contact eddy current measurement method offers a valuable tool for rapidly assessing the electrical uniformity and quality of 2D materials, without the risk of damaging them during testing.

This study underscores the critical role of substrate selection in optimizing graphene-based gas sensors—it influences graphene through doping, shaping both its sensing response and its electrical performance. Non-contact eddy current tests not only uncovered these substrate effects with precision, but may also serve as a fast, non-destructive method for quality assessment in the manufacturing of low-dimensional materials. These findings provide dual value: practical insights for sensor design and a possible path forward for improved quality control in 2D materials production.