Nanotechnology-Enabled Sensors

212 Chapter 5: Characterization Techniques for Nanomaterials
plotted against wavelength or frequency. From this spectrum, materials can
be identified and quantified.
The measured spectrum strongly depends on the ambient environment
and the presence of other molecules nearby. Therefore, electromagnetic
spectroscopy techniques can be directly employed for sensing applications,
with the intensity of electromagnetic waves absorbed or emitted, or the shift
in absorption/emission wavelengths as the output parameters. Electromagnetic
spectroscopy also provides a contactless way to probe materials.
Electromagnetic radiation consists of mutually perpendicular propagating
electric and magnetic waves, with particle-like (photon) properties.
The relationships between frequency f, wavelength λ, and energy of the
particle E, are given by:
c = λf, (5.1)
E = hf, (5.2)
where h is Planck’s constant (6.626 × 10 –34 J·s), and c is the speed of light.
In free space, c has the value of 2.998×10 8 m/s, whereas in other media, its
value is adjusted to c/n, where n is the refractive index of the medium.
The information that can be obtained from a material strongly depends
on the wavelengths of electromagnetic wave that is applied. From Table
5.1 it is observed that different wavelengths of incident waves are responsible
for the observation of various phenomena. For example, microwaves
stimulate rotations of molecules; infrared waves stimulates vibrations of
higher energy orbitals; visible and ultraviolet waves promote electrons to
higher energy orbitals; and X-rays and short wavelengths break chemical
bonds and ionize molecules and can even damage living tissue.
When the near ultraviolet, visible, and near/mid infrared regions of the
electromagnetic spectrum are utilized, the electromagnetic spectroscopy
techniques are referred to as spectrophotometry. In these techniques, the
wavelengths of the incident electromagnetic waves are generally scanned
across a range to produce an absorption or emission spectrum. Analogous
phenomena occur in the X-ray, microwave, radio, and other regions of the
electromagnetic spectrum and these will be discussed later in this chapter.