28 Ultra-Violet Visible Spectroscopy

 Contents of the Unit

1.      Introduction

2.      UV-visible spectrophotometer

3.      Principle for UV-Vis-IR

4.      Optical band gap measurement

5.     Instrument details

6.     Components: UV Source , Visible Light Source, Cuvettes, Detectors, Charge-Coupled Devices (CCDs)

Learning Objectives

• UV-Vsible spectrometer
• Principle of UV-Visible spectrophotometer
• Optical band gap measurement
• Instrument details with each component details

1.    Introduction

Over the years, spectroscopy has evolved as a potential tool for experiments and analyses conducted in research laboratories and industries. This technique is essentially considered by analysts as an apparent solution. The objective should also be to use these spectroscopic techniques in control and industrial laboratories and to develop fully recognised spectroscopic techniques. Spectroscopy involves investigating the interaction of electromagnetic field with matter. Spectroscopic data is represented as an emission spectrum of wavelength or frequency dependent response. Spectroscopy can be broadly classified into two categories – (a) techniques based on energy transfer between photon and sample, and (b) reflections, refraction, diffraction, dispersion, or scattering from the sample altering the amplitude, phase angle, polarization, or direction of propagation of the electromagnetic radiation.

2. UV-Visible spectrometer

Ultraviolet-Visible Spectroscopy refers to the absorption spectroscopy in the ultraviolet-visible spectral region wherein, the spectrophotometer quantifies the electronic transitions made by the sample. It is an efficient technique to determine the optical properties of any sample which includes film thickness, refractive index, band gap, color and the reflectance properties in general. In the present work, optical properties of the prepared thin films were determined using a double beam UV–Vis spectrophotometer (Make: Perkin Elmer: Lambda35) over a wavelength range of 190–1100 nm. The spectrophotometer is computer interfaced so that all the parameters like light source, change over position, filter selection, slit-width, and wavelength can be automatically controlled.

3.    Principle for UV-Vis-IR

Ultraviolet (UV) spectroscopy is an important physical tool which exploits light in ultraviolet, visible, and near infrared range of electromagnetic spectrum. Beer-Lambert law establishes a linear relationship between absorbance, concentration of absorbers (or absorbing species) in the solution and the path length. Therefore, UV-Vis spectroscopy can be employed for determining the concentration of the absorbing species, for a fixed path length [1]. This is a very simple, versatile, fast, accurate and cost-effective technique. Instrument employed for ultraviolet−visible (or UV-Vis) spectroscopy is called UV−Vis−NIR Spectrophotometer. This can be used to analyze liquids, gases and solids by using radiative energy corresponding to far and near ultraviolet (UV), visible (Vis) and near infrared (NIR) regions of electromagnetic spectrum. Consequently, predetermined wavelengths in these regions have been defined as: UV: 300 – 400 nm; Vis: 400 – 765 nm; and NIR: 765 – 3200 nm.

2.1 Principle: A light beam is passed through an object and wavelength of the light reaching the detector is measured. The measured wavelength provides important information about chemical structure and number of molecules (present in intensity of the measured signal). Thus, both quantitative and qualitative information can be gathered. Information may be obtained as transmittance, absorbance or reflectance of radiation in 160 to 3500 nm wavelength range [2,3]. The absorption of incident energy promotes electrons to excited states or the anti-bonding orbitals.

For this transfer to occur, photon energy must match the energy needed by electron to be promoted to next higher energy state. This process forms the basic operating principle of absorption spectroscopy. Potentially, there may be three types of ground state orbitals involved:

1.  σ (bonding) molecular orbital

2.  π (bonding) molecular orbital

3.  n (non-bonding) atomic orbital

Besides, the anti-bonding orbitals are:

i.        σ* (sigma star) orbital

ii.      π* (pi star) orbital

A transition involving excitation of an electron from s bonding orbital to σ anti-bonding orbital is called σ to σ* transition. Likewise, π to π* represents the excitation of an electron of a lone pair (non-bonding electron pair) to an antibonding π orbital. Electronic transitions occurring due to absorption of UV and visible light are:

σ to σ*;

n to σ*;

n to π*;

π to π*.

The transitions s to σ* and n to σ* involve higher energies and thus usually occur in far UV region or weakly in 180 to 240 nm region. Thus, saturated groups do not show strong absorption in UV region. Molecules with unsaturated centres undergo n to π* and π to π* transitions; these transitions involve lesser energies and thus occur at longer wavelengths than transitions to σ* anti-bonding orbitals.

4. Optical band gap measurement

Optical band gap refers to the minimum photon energy required to transfer the electron from the top of the valence band to the bottom of the conduction band. It is one of the major factors determining the conductivity of a given sample. The band gap of the deposited thin films was calculated from the values of absorption coefficient plotted as a function of wavelength

where, t is the film thickness and T is the transmittance. For a direct band gap material, relationship between absorption coefficient and energy band gap can be expressed as follows:

where, hν is the incident photon energy and E_g is the direct optical band gap of the sample. A graph (Tauc plot) between (αhν)² and hν is plotted and the band gap is estimated by extrapolating the linear part of the Tauc plot on the energy axis.

5. Instrument details: The basic components of a spectrometer include: light source (UV and visible), monochromator (wavelength selector), sample stage, and detector. A tungsten filament, continuous over UV region is generally used as light source. Detector is usually a photodiode or CCD. Photodiodes go with monochromators to filter light of a particular wavelength, to be fed to the detector. While monitoring the absorbance in UV spectrum, the visible lamp must be turned off, and vice-versa. Figure 2 includes schematic UV−Vis−NIR Spectrometer.

2.     Components

1.   UV Source

The power of radiating source should not vary in its operating wavelength range. Continuous UV spectrum is produced by electrically exciting deuterium or hydrogen at low pressures. The mechanism for generation of UV light includes creating an excited molecular species, that breaks into two atomic species and a UV photon. The emission wavelengths of both deuterium and hydrogen lamps are in 160 to 375 nm range. The material of the cuvettes needs to selected such that it does not absorb the light incident, because this will result in errors in obtained absorption spectrum. Thus, quartz is usually used.

2.   Visible Light Source

Tungsten filament lamp is used as visible light source. This lamp can produce light in 350 to 2500 nm wavelength range. In a tungsten filament lamp, energy emitted is proportional to the fourth power of the operating voltage. Thus, in order to get stable emission, a highly stable voltage must be applied to the lamp. The stability of voltage is ensured by using electronic voltage regulators or constant-voltage transformers.Tungsten/halogen lamps include small quantities of iodine embedded within a quartz ‘envelope’, which also contains the tungsten filament. The iodine reacts with gaseous tungsten, formed by sublimation, and produces a volatile compound WI2. As WI2 molecules hit the filament, they decompose, and redeposit tungsten back on the filament. The tungsten/halogen lamps usually have lifetime twice to the conventional tungsten filament lamp. Tungsten/halogen lamps are used in modern spectrophotometers owing to their high efficiency, and their output extends to UV region as well.

3.   Cuvettes

Monochromator source is used; before reaching sample, light is divided in two parts of similar intensity with a half-mirror splitter. One part (or sample beam), travels via the cuvette having the solution of material to be examined in transparent solvent. Second beam, or reference beam, travels via similar cuvette having only solvent. Reference and sample solution containers have to be transparent towards passing beam.

4.    Detectors

Detector detects intensity of light transmitted by cuvettes and sends this data to a meter to record and display the values. Electronic detectors calculate and compare the intensities of light beams. Several UV−Vis spectrophotometers have two detectors – a phototube and a photomultiplier tube, and reference and sample beams are monitored simultaneously. The photomultiplier tube is the extensively used detector in UV-Vis instruments. It includes a photoemissive cathode (electrons are emitted from the cathode when photons strike it), several dynodes (a dynode emits multiple electrons when one electron strikes it) and an anode. The incident photon, after entering the tube, strikes the cathode. The cathode then emits multiple electrons, which are then accelerated towards the first dynode (whose potential is 90V more positive than cathode). The electrons strike the first dynode, leading to the emission of several electrons for each incident electron. These electrons are then accelerated towards the second dynode, to produce more electrons which are accelerated towards dynode three and so on. All the electrons are eventually collected at the anode. By this time, each original photon has produced 106 – 107 electrons. The resulting current is amplified and measured. Photomultipliers are highly sensitive towards UV and visible radiations and have fast response times. However, photomultipliers are used only at low power radiation as high power light may damage them.

The linear photodiode array is an example of a multichannel photon detector. These detectors can simultaneously measure all elements of a beam of dispersed radiation. A linear photodiode array consists of several small silicon photodiodes created on a single silicon chip. The number of photodiodes can vary between 64 to 4096 sensor elements on a chip, however, 1024 photodiodes is most common. For each diode, there is also a storage capacitor and a switch. The individual diode-capacitor circuits can be sequentially scanned.

Charge-Coupled Devices (CCDs) are like diode array detectors, but instead of diodes, they consist of an array of photocapacitors. Reference beam intensity, should suffer little or no absorption, and termed I0 whereas that of sample beam is called I. The spectrophotometer automatically examines all wavelength components in a short time. This technique is good to evaluate the concentration as well as molecular structure or structural changes. It may also be used to examine the vibrational and conformational energy levels alterations before and after an interaction with a substrate, or a molecule.

5. Summary

• UV-Vsible spectrometer
• Principle of UV-Visible spectrophotometer
• Optical band gap measurement
• Instrument details with each component details

 REFERENCES

1. Skoog, Douglas A.; Holler, F. James; Crouch, Stanley R. (2007). Principles of Instrumental Analysis (6th ed.). Belmont, CA: Thomson Brooks/Cole. pp. 169–173.
2. Metha, Akul (13 Dec 2011). “Principle”. PharmaXChange.info.
3. Metha, Akul (22 Apr 2012). “Derivation of Beer-Lambert Law”. PharmaXChange.info.
4. Misra, Prabhakar; Dubinskii, Mark, eds. (2002). Ultraviolet Spectroscopy and UV Lasers. New York: Marcel Dekker.
5. Metha, Akul (14 May 2012). “Limitations and Deviations of Beer-Lambert Law”.
6. Ansell, S.; Tromp, R. H.; Neilson, G. W. (1995). “The solute and aquaion structure in a concentrated aqueous solution of copper(II) chloride”. J. Phys.: Condens. Matter. 7 (8): 1513–1524.
7. “Spectroscopic thin film thickness measurement system for semiconductor industries”, Horie, M.; Fujiwara, N.; Kokubo, M.; Kondo, N., Proceedings of Instrumentation and Measurement Technology Conference, Hamamatsu, Japan, 1994.
8. Sooväli, L.; Rõõm, E.-I.; Kütt, A.; et al. (2006). “Uncertainty sources in UV-Vis spectrophotometric measurement”. Accreditation and Quality Assurance. 11: 246–255.