Nanotechnology-Enabled Sensors

2.3 Physical Effects Employed for Signal Transduction 23
ing nanomaterials exhibit a charge depletion layer, which extends a few
nanometers. This extension of the depletion region changes when exposed
to irradiation. Depending on their dimensions and the amount of doping,
photosensitive devices may become completely depleted of charge when
irradiated. For instance, the photocurrent resulting from the interaction of
UV light and semiconducting GaN-nanowires is seen in Fig. 2.6, where a
distinct dependence on the nanowires diameter is observed. 6
The response of photoconductive devices may also be tuned by varying
the composition and dimensions of the utilized nanomaterials. This is seen
in devices based on CdS and CdSe nanoparticles and nanostructured thin
films for applications such as Tera Hertz (THz) signal monitoring. 7,8 The
size-dependent transient photoconductivity of CdSe nanoparticles using
time-resolved THz spectroscopy (TRTS) is shown in Fig. 2.7, which reveals
the response time is reduced to less than 5 ps when the nanoparticle
sizes are reduced to approximately 3.5 nm.
Fig. 2.6 Photocurrent with UV illumination of approximately 15 W/cm 2 versus diameter.
The kink in the fitting curve at 85 nm indicates the critical diameter where
the surface depletion layer just completely depletes the nanowire. For smaller diameters
the photocurrent shows an exponential decrease, for larger diameters the
photocurrent is proportional to the diameter. Reprinted with permission from the
American Chemical Society publications. 6