Gas Sensors Flashcards
What is the challenge in gas sensing?
Sensors which are able to detect a small amount of a very specific molecule amongst a large number of other molecules.
How do most gas sensors work?
They change chemical information into a useful analytical signal i.e. capacitance change,
What is a capacitive sensor?
A sensor which measures a change in capacitance due to interaction of a molecule with the sensor surface.
What is a mass sensor?
A small cantilever which moves up/down due to the presence of a certain molecule and the force on the cantilever is proportional to the concentration of the gas.
What is a calorimetric sensor?
A sensor with hot and cold junctions which changes in temperature due to the concentration of an analyte gas.
What is a chemoresistive sensor?
Used at PTL: a solid state device nanoparticles on the surface absorb particles on their surface and react with molecules and cause a change in resistivity in the sensor which is the measured quantity i.e. the sensor response.
What dominates sensor response in an agglomerate?
Contact resistance. (not desiderable)
What dominates sensor response in aggregates?
Surface interaction. (desiderable because resistance changes are measurable).
What is the Debye length?
The penetration depth of a given charge carrier. I.e. how far the electrostatic effects of a charge carrier persist. In other words the length which is effected by surface interactions.
What are typical Debye lengths?
< 10 nm or up to 100 nm for semi-conductors.
What is a P-type vs N-type semiconductor?
An n-type semiconductor has spare electrons so adding electrons increases the conductivity.
A p-type semiconductor has electron holes so adding electrons removes the holes and thus reduces conductivity.
How are large vs small grains deliniated?
> > 2Debye length = large, ~<2Debye length = small
How can you improve sensor response?
Decrease crystal size, reduce neck size. Add chemical or electronic sensitization.
What is band bending?
Location where the electron states curve up or down near a junction. Or, a local imbalance in charge neutrality. In chemoresistive sensors, the area where the depletion layer and the bulk meet. Governed by Poisson equation qVs = kBT*ln(nb/ns)
