  Graphene is a promising material for sensing applications because of its
large specific surface area and low noise. In many applications, graphene will
inevitably be in contact with oxygen since it is the second most abundant gas
in the atmosphere. Therefore, it is of interest to understand how this gas
affects the sensor properties. In this work, the effect of oxygen on the
low-frequency noise of suspended graphene is demonstrated. Devices with
suspended graphene nanoribbons with a width (W) and length (L) of 200 nm were
fabricated. The resistance as a function of time was measured in a vacuum and
pure oxygen atmosphere through an ac lock-in method. After signal processing
with wavelet denoising and analysis, it is demonstrated that oxygen causes
random telegraphy signal (RTS) in the millisecond scale, with an average dwell
time of 2.9 milliseconds in the high-resistance state, and 2 milliseconds in
the low-resistance state. It is also shown that this RTS occurs only at some
periods, which indicates that, upon adsorption, the molecules take some time
until they find the most energetically favorable adsorption state. Also, a
slow-down in the RTS time constants is observed, which infers that less active
sites are available as time goes on because of oxygen adsorption. Therefore, it
is very important to consider these effects to guarantee high sensitivity and
high durability for graphene-based sensors that will be exposed to oxygen
during their lifetime.
