Nanotechnology-Enabled Sensors

2.3 Physical Effects Employed for Signal Transduction 31
2.3.4 Electroluminescence Effect
Electroluminescence occurs when a material emits light as a result of an
electrical current flowing through it, or when subjected to an electrical potential.
It is used in the conversion of electrical energy into radiant energy.
There are two methods of producing electroluminescence. Firstly, it can
occur when a current passes through boundary of highly doped junctions
(such as p-n junctions of semiconductor materials). Electrons can recombine
with holes, causing them to fall into a lower energy level and release
energy in the form of photons. Such a device is called light-emitting diode
(LED) and its layout is shown in Fig. 2.12.
Electroluminescent devices can be implemented in spectroscopy and integrated
sensors. Many new disposable sensors with the light intensity as
the measure of a target analyte concentration or a physical change make
use of them. This effect is an integrated part of many electrochemical sensing
system (electrochemical sensing templates will be presented in Chap.
3). When an electron is generated in an electrochemical interaction it can
transformed into a photon via the usage of an electroluminescent device.
Consequently, this irradiation can be detected with a photodiode or photo
transistor. The use of optical reading reduces the electronic noise and also
it is compatible with many standard optical sensing systems.
The wavelength of the emitted light is determined by the bandgap energy
of the materials forming the junction. A flow of a current does not
guarantee electroluminescence. For example, in diodes based on indirect
bandgap materials such as silicon, the recombination of electrons and holes
is non-radiative and there is no light emission. Materials used in LEDs
must have a direct bandgap. Those comprised of group III and V elements
of the periodic table are most common used in the fabrication of LEDs.
These include GaAs and GaP. The bandgap of these materials, and hence
emission wavelength, can be tailored through the addition of impurities.
For instance, LEDs made solely from GaP emit green light at 555 nm.
However, nitrogen-doped GaP emits at yellow-green light (565 nm), and
ZnO-doped GaP emits red light (700 nm).