Ravi Mohan Prasad

Synthesis, Characterization, Electronic and Gas‐Sensing Properties towards H2 and CO of Transparent, Large‐Area, Low‐Layer Graphene

Low-layered, transparent graphene is accessible by a chemical vapor deposition (CVD) technique on a Ni-catalyst layer, which is deposited on a <100> silicon substrate. The number of graphene layers on the substrate is controlled by the grain boundaries in the Ni-catalyst layer and can be studied by micro Raman analysis. Electrical studies showed a sheet resistance (R(sheet)) of approximately 1435 Ω per □, a contact resistance (R(c)) of about 127 Ω, and a specific contact resistance (R(sc)) of approximately 2.8×10(-4)  Ω cm(2) for the CVD graphene samples. Transistor output characteristics for the graphene sample demonstrated linear current/voltage behavior. A current versus voltage (I(ds)-V(ds)) plot clearly indicates a p-conducting characteristic of the synthesized graphene. Gas-sensor measurements revealed a high sensor activity of the low-layer graphene material towards H(2) and CO. At 300 °C, a sensor response of approximately 29 towards low H(2) concentrations (1 vol %) was observed, which is by a factor of four higher than recently reported.

Response of Gallium Nitride Chemiresistors to Carbon Monoxide is Due to Oxygen Contamination

We report on the influence of oxygen impurities on the gas sensing properties of gallium nitride (GaN) chemiresistors. As shown by XRD, elemental analysis, and TEM characterization, surface oxidation of GaN-for example, upon contact to ambient air atmosphere-creates an oxidative amorphous layer which provides the sites for the sensing toward CO. Treating this powder under dry ammonia at 800 °C converts the oxide layer in nitride, and consequently the sensing performance toward CO is dramatically reduced for ammonia treated GaN gas sensors. Hence the response of GaN sensors to CO is caused by oxygen in the form of amorphous surface oxide or oxynitride.

Sensing in harsh conditions: How to protect SnO 2 sensing layer

The detection of low CO amounts in fuel cell conditions - high hydrogen background in almost the absence of oxygen - is a challenging problem for SnO2 based gas sensors. SnO2 - the most used material for metal-oxide based gas sensors - is not stable in hydrogen-rich atmospheres at high temperatures being reduced to metallic tin. To prevent SnO2 reduction, a SnO2 based sensing layer, previously deposited by screen-printing onto an alumina substrate, was coated with the additional microporous ceramic Si-B-C-N layer. The ceramic coating prevents the reduction of SnO2 and allow for the diffusion of the target gas molecules towards the SnO2 layer for their detection, acting as a filter. The Si-B-C-N coated SnO2 sensors show stable sensor signals towards hydrogen and carbon monoxide in the absence of oxygen whereas the unprotected SnO2 sample becomes reduced over time.