And finally, we will look at how the chemical nature and
interactions in graphene can be used in chemical and gas sensing.
Few years after the discovery of graphene by Geim and Novoselov,
they demonstrated that gas molecules can absorb on and
desorb from graphene and create a change in it's whole resistivity
even at room temperature due to graphene exceptionally low noise.
This change in resistivity is so sensitive, that it can detect
concentrations of nitric oxide as low as one part per billion as can be seen here.
You can see examples of changes in whole resistivity observed near the charge
neutrality point during absorption of strongly diluted NO2.
The blue curve and
its desorbtion at vacuum at 50 degrees C, which is the red curve.
The green curve is a reference.
The grid lines correspond to changes in the sigma xy
caused by adding one electron charge.
Next, we see the statistical distribution of step heights in this device
without it's exposure to NO2 or in helium, and during a slow desorption of NO2.
Since then, owing to a large surface area and electron mobility at room temperature.
Graphene and graphene derivatives have been successfully used to
fabricate sensing devices for the detection of a wide range of
gas molecules such as ammonia, hydrogen, water vapor,
hydrogen sulfide, carbon dioxide, carbon monoxide,
dinitrotoluene, alcohols, organic vapors such as toluene,
chloroform, tetrahydrofuran, acetonitrile, etc.
Gas molecules, such as carbon monoxide, nitrogen oxides, and ammonia are of
great practical interests for industrial, environmental, and medical applications.
NO2 is possibly the most studied one as it is a decomposition
product of many explosive formulations, and is also very useful for
monitoring pollutants emitted by industrial processes.
Graphene based gas senses are based on conductivity
fluctuation produced by electron transfer from gas molecules towards
the graphene such as the case of ammonia, carbon monoxide, or
ethanol, or in the other direction from graphene towards the gas molecules.
causing whole enrichment in the graphene such as NO2 or H2O.
Compared to other sensing materials, graphene is more sensitive
because of the extraordinary mobility of carriers at room temperature and
it's intrinsic low noise.
However, graphene oxide and reduced graphene oxide are electrochemically
more active as compared to pristine graphene owing to the oxygen containing
reactive sites at the edges and in the basal plane.
In addition, graphene oxide suspensions are much easier to
process into a device and to further control of the reduction degrees possible,
by using different chemical agents and
temperature conditions, which produces reduced graphene oxide.
Showing much higher sensitivity
due to its higher conductivity as compared to graphene oxide.
In this example, we will look at an ultra fast and sensitive room temperature
ammonia gas sensor based on the reduction of graphene oxide by pyrrole.
Here, we see a schematic diagram of graphene sensors
based on reduced graphene oxide, and the current voltage or
IV curve of a sensing device based on reduced graphene oxide.
We also see the scanning electron micrograph image
of a sensing device composed of reduced graphene oxide platelets
that bridge neighbouring gold fingers.
An enlarged image of the reduced graphene oxide membrane
formed between the electrodes.
It exhibits excellent response and sensitivity and selectivity to ammonia
gas, and sensors with resistance change as high as 2.4%.
And response time as fast as 1.4 seconds can be achieved when
the concentration of the ammonia gas is as low as one parts per billion.
Furthermore, these RGO sensors can rapidly recover to that initial state with
infrared illumination and show excellent repeatability and selectivity to ammonia.
Gas absorption on to graphene and graphene derivatives have been
extensively investigated through theoretical and computational models.
These works reveal that graphene-based gas senses could be dramatically improved
by introducing an appropriate dopant or defect into the graphene.
For example, it has been demonstrated that reduced graphene oxide with
sulphur phenol could improve the recovery performance of reduced graphene oxide for
the detection of NO2.
Related to this, a flexible NO2 sensor based on sulphonated
reduced graphene oxide decorated with silver nanoparticles was demonstrated.
Here we see a photograph of a printed silver reduced graphene oxide sensor and
the mechanical bend test conditions on the left.
And on the right we see the response of the sensor
as a function of time in various concentrations of NO2 gas.
The silver sulphonated reduced graphene oxide inks prepared by
a simple wet chemical pot using graphene oxide, sulfanilic acid and
silver nitrate would printed on to polyimide substrate.
Using a Graveot printing technique with high sensitivity and
fast response recovery time for NO2 even after 1,000 bending cycles.
This sensor has exhibited a high sensitivity of 50 parts per million and
fast response recovery at room temperature.
Both of the order of seconds which is crucial since most sensing devices
have a recovery time relatively long, ranging from tens of minutes to hours.
Graphene can be doped with phosphorous which is a more electron-rich
atom compared to carbon and can modify the chemical properties of
graphene like nitrogen to produce p-type semiconducting behaviour and
act as an active site for ammonia absorption.
And finally one of the main challenges faced in graphene gas sensors is
the ability to detect different gasses simultaneously using the same
sensing device.
This might be possible by functionalizing the surface of graphene,
as it has been demonstrated.
And graphene devices exposed to various vapors induce
very clear peaks in the low-frequency noise response.
The difference in the frequency is not sufficiently understood,
but it allows in combination with other sensing parameters,
the identification of different gases, such as tetrahydrofuran,
methanol, acetonitrile, and chloroform with same graphene transistor.
On the left, we see the SEM mage of a backgated graphene device.
And on the right, we see the noise-spectral density
multiplied by the frequency, versus the frequency for
the device in open air and, under the influence of different vapors.
Different vapors induce noise with difference characteristic frequencies.
The difference in the frequency is sufficient for reliable identification
of different types of gasses with the same graphene transistor.
For comparison, the pure one over f noise dependence is also indicated.