Ultra-Violet, Visible, and Near-Infrared Spectroscopy for the Characterization of Materials


Introduction

Ultraviolet, visible and near-infrared (UV-VIS-Near-IR) spectroscopy is a non-invasive technique to characterize the optical properties of materials. The spectrometer measures the attenuation of a beam intensity to investigate the absorption process in a material. Absorption measurements are measured through a single wavelength or an extended range of wavelengths. From the transmission curves, one can determine the short (UV/VIS) and long (near-IR) wavelength cut off. The spectrometer can be used to characterized the optical process in this range that occurs in a material, usually molecules or inorganic complexes in solution. One can determine the concentration of an analyte in solution by using Beer-Lambert Law.

Another use of the spectrometer is to investigate the wavelength dependence of absorption bands. An example is when a transition metal is doped into a glass compound eg: heavy metal fluoride glass (ZBLAN), the glass gives a different color. This is because a material only absorbs all wavelengths outside the range of the color seen optically. Co+2 ions from a small doping of cobalt chloride or cobalt oxide produces dark blue color glass as reported in the lecture notes.The exact wavelength dependence of these absorption bands can be accurately determined using a wavelength scanning spectrophotometer.

Similarly, the band gap of semi-conducting particles of materials such as cadmium sulfide or titanium dioxide (TiO2) give rise to optical transitions that can be characterized by scanning UV-VIS spectrometer. These measurements are important in the semiconductor and nanoparticle materials especially solar industries. A sample spectrum of titania (TiO2) is given below.



A diagram of band gap. Band gap is the small energy gap between their valence and conduction bands, and depend strongly on the size of the particle.



Spectrum of TiO2 from Perkin Elmer. The spectral shows a strong cut off at 410.57 nm; where the absorbance value is minimum.


Electromagnetic waves - UV, Visible and NIR ranges



An oscillating charge creates electromagnetic waves, consisting of electric and magnetic field propagating through a medium. Wavelength λ is defined as the distance between the adjacent peaks or throughs as seen in the picture above. Frequency (f or ν) is the number of wave cycles that travel past a fixed point per unit of time, and is usually given in cycles per second, or hertz (Hz). Electromagnetic waves carries energy that is proportional to the frequency as the relation:

E = h f;

where h is the Planck constant. The speed of the wave traveling in a medium is given as

v = fλ

and for vacuum or air v = c = 3 x 10 8 m/s -1.


Ultraviolet and visible radiation interacts with matter, which causes electronic transitions (promotion of electrons from the ground state to a high energy state). The ultraviolet region falls in the range between 190-380 nm, the visible region fall between 380-750 nm.

The following electronic transitions are possible: