  The resistance of chemically synthesized polypyrrole (PPy) thin films is
investigated as a function of the pressure of various gases as well as of the
film thickness. A physical, piezoresistive response is found to coexist with a
chemical response if the gas is chemically active, like, e.g., oxygen. The
piezoresistance is studied separately by exposing the films to the chemically
inert gases such as nitrogen and argon. We observe that the character of the
piezoresistive response is a function not only of the film thickness, but also
of the pressure. Films of a thickness below 70 nm show a decreasing resistance
as pressure is applied, while for thicker films, the piezoresistance is
positive. Moreover, in some films of thickness of about 70 nm, the
piezoresistive response changes from negative to positive as the gas pressure
is increased above 500 mbars. This behavior is interpreted in terms of a total
piezoresistance which is composed of a surface and a bulk component, each of
which contributes in a characteristic way. These results suggest that in
polypyrrole, chemical sensing and piezoresistivity can coexist, which needs to
be kept in mind when interpreting resistive responses of such sensors.
