Scientists are often intrigued by the properties shown by the nanoparticles and nanostructures. Although, microscopic methods like SPMs and electron microscopes paved the way to observe nanomaterials at the nanoscale, most of the chemical, structural and optical properties cannot be characterized by these instruments. Fortunately, spectroscopic characterization techniques can be used to investigate these properties of the nanomaterials. This page is dedicated to introduce you to some of the most frequently used spectroscopic techniques in the field of nanotechnology.
By definition, spectroscopy is the study of interaction between the electromagnetic radiation with matter. Electromagnatic radiation can be classified in to eight broad categories based on the frequency. The categories are, gamma, X-ray, Ultraviolet, visible, infrared, terahertz, microwave radiation and radio waves. Each of these radiation types can interact with the matter in number of different ways. These interactions can only be converted to spectroscopic technique if one of the key wave properties like its energy, velocity, amplitude, frequency, phase angle, polarization, and direction of propagation get altered during the interaction. Following image shows the electromagnetic spectrum and the atomic or molecular level transition caused by each category of electromagnetic radiation.
Spectroscopy can be categorized in to two broad classes based on energy transfer conditions. In the first category, an effective energy transfer between the photon and the material will take place. In the second category, there is no effective transfer of energy between the photon and the material, yet a change of amplitude, phase angle, polarization or direction of wave propagation will take place. These changes are typically caused by reflection, scattering or diffraction.
In this page we will look at five main spectroscopic techniques and their applications in nanotechnology characterizations.
Ultraviolet-Visible spectroscopy (UV-Vis)
UV-Vis consist of a source, optical bench, sample and reference beam lines, monochromator(device that outputs a narrow wavelength beam) and a detector. Reference beam travels through the spectrophotometer without interacting with the sample. Sample beam is exposed to the sample and the wavelength of the exposed light is changed across the desired range. Energy is absorbed when the wavelength reaches to the energy level that promotes the excitation of the electrons to higher molecular orbital. Detector records the ratio between the sample and the reference beam. When the sample is absorbing the energy, transmission intensity of the sample beam gets lower than that of the reference beam. The main output of the UV-Vis is the absorbance spectrum; absorbance of UV-Vis radiation with respect to the wavelength.
The UV-Vis is especially useful when characterizing the color properties of the materials. In metallic nanoparticles unique color absorption properties can be seen due to plasmonic absorbance phenomenon. These nanoparticles start to absorb the light when the natural frequency of the electron cloud matches with the incoming electromagnetic radiation, producing a color. UV-vis can help scientists to explore exact wavelengths that visible light absorption take place.
Apart from color characterization, UV-Vis is also important in analyzing concentration of a light absorbing material. It’s known that the concentration of color producing material dispersed/dissolved in a solvent is proportional to the absorption, as presented by the Beer–Lambert law. Hence, UV-vis can be used to quantitative characterization of concentration which is quite useful in areas such as, sorption, diffusion and release studies in nanotechnology research.
When light collide with a material most of the photons are deflected off thel without any change of energy. This phenomenon is referred to as elastic scattering (Rayleigh scattering) where the scattered light has the same wavelength as the incident light. However, C.V Raman, an Indian scientist first discovered that very small percentage of the scattered light is inelastically scattered with a net change to the energy due to interaction with the incoming radiation and the vibrational energy levels of the molecules in the sample. Inelastically scatted photons can gain or lose energy. Therefore, Raman spectrum is given as a shift from the incoming frequency and the Rayleigh band locate at 0 cm-1. Plotted in this scale, energy shifts corresponds to vibrations of the various functional groups and can be used as a signature spectrum for a given material.
Many nanomaterials have strong intra and inter bond vibrations that can be characterized using Raman spectroscopy. This technique has found lot of popularity among the scientist who work with carbon based nanomaterials like graphene and carbon nanotubes. Even for the same material, spectral frequencies and shapes can differ according to the material quality and material dimensions giving vital clues about the material.
Because Raman spectrum serves as a fingerprint for a given material, selective identification of the unknown samples is possible by matching spectra with the database. Raman spectroscopy is mostly used in qualitative purposes. However, quantitative analysis is also possible as the area under the spectral curve is proportional to the concentration.
Furrier transformed infrared spectroscopy (FT-IR)
Identification of the functional groups present in a nanomaterial is a frequent requirement in nanoscience and nanotechnology research. Among other tools, FT-IR has found much popularity among researches due to its versatility, relative ease of use and ability to use as a quantification tool.
Atoms in a chemical bonds constantly vibrate. This vibration can be analogue to a system with two masses attached to a spring. The vibration frequency depend upon the weight of the masses and the spring constant of the connecting spring. In the same way, depending on the masses of the atoms that contributes to a bond and cohesiveness of the bond, frequency differ. Since bonds have atoms with different shapes and sizes and different strength, each combination of atoms in an each type of bond has a unique harmonic frequency. This natural frequency lies in the range of infrared region and therefore a spectroscopic method that use IR can be devised to analyze bond vibrations.
When the IR radiation with the same harmonic frequency of the bond shines upon the bond. The bond vibration is amplified by increased transfer of energy from the IR radiation. When range of IR frequencies given to the material, it only absorb IR frequencies that corresponds to the natural frequencies of the bonds that exist in the sample. Others are not absorbed and can be analyzed using an Infrared spectrometer, which tells you the frequencies that are absorbed by the sample. This provides important information about the functional groups present in the sample. This is exactly what FT-IR does.
As FT-IR can be used to get information about functional groups present in nanomaterials. This is particularly useful in cases such as when one attempts to surface modify nanomaterials to increase affinity, reactivity or compatibility. Analyzing the FT-IR of a nanomaterial would tell you what groups present and then appropriate surface modification strategy be decided based on the groups present. Further, it can also be useful in characterizing the surface modification has taken place, as new groups should emerge if the reaction is successful.
Similar to Raman spectroscopy, FT-IR also can be used for both quantitative and qualitative purposes. The area under the curve of IR absorption spectrum of The FT-IR spectra is proportional to the concentration of the chemical bond present. FT-IR spectrum of a material, also serves as a fingerprint curve which can be useful in identifying unknown samples.
X-rays reside in higher frequency part of electromagnetic spectrum and their frequency is only lower than that of gamma rays. X-rays are produced by bombarding an accelerated electron beam in to a metal target. These X-rays can be used to probe in to atomic scale, primarily through a mechanism call diffraction. Diffraction is a phenomenon that can be seen when electromagnetic radiation encounters an obstacle in its pathway. This is characterized by the bending of the light around the edge of the obstacle. The diffraction depend on relative size of the wavelength of the radiation to the size of the opening made by the obstacle. If the opening is larger than the wavelength of the electromagnetic radiation, only weak diffraction can be observed. However, if the opening size is equal or closer to the wavelength of the opening significant diffraction can be seen. The reason that the X-ray are used to analyze diffraction patterns made by materials is that their wavelength is in range of atomic scale. Hence, X-ray give rise to significant diffraction particularly with materials that has ordered atomic structure.
There are two types of solid materials: amorphous and crystalline. Amorphous materials have random atomic structure and no order or pattern can be seen in the atomic level. One good example for amorphous material is glass. Crystalline materials have rather well organized atomic structure. These materials have a very precise order in the atomic arrangement, that through a repetition of a smaller unit, whole structure can be explained. This smallest, unit that can describe the whole crystal is referred to as the unit cell.
When X-rays encounters an atomic layer in the crystal lattice, general scattering take place. However, these scattered waves weakens during the travel as destructive interference takes place with other scattered rays. The diffraction occurs when the scattered X-rays from one atomic layer is in phase with the scattered X-rays from other planes, which enhances the wave fronts through constructive interference. The condition for this is given by Bragg’s law of diffraction. Using this low, atomic layer distances can be calculated. X-ray diffractometer: the instrument used to characterize diffraction outputs a graph showing the intensity of diffracted X-rays with respective to the angle of incidence of X-ray beam with the sample. Diffraction pattern is useful in determining atomic distances between different atomic layers that exist in the crystal. It also serves as a fingerprint for a material, so that an unknown sample can be matched with a database to find the exact match that describe the material.
There are number of crystalline nanoparticles that can be characterized with X-ray diffraction. They include, metals, metal oxides, semiconductors, layered nanomaterials, crystalline polymers, etc.
In the case of nanomaterials, diffraction peaks appear much broader compared to their bulk counterparts. Unlike, in bulk materials where atomic structures and atomic layer distances are well defined, nanomaterials show less defined structure, which make the peaks much broader. This broadening can be used to calculate the average crystallite (smaller crystals that make up a bigger particle) size. It can also be used to understand crystal orientation and growth direction.
Dynamic light scattering
Light scattering take place when propagating light wave is deflected from an irregularity in the propagation media such as interfaces or particles. When light is scattered change in the direction of wave propagation takes place. Scattering intensity will depend on the wavelength of the scattered light and the size of the object that scatters light. Bigger the object, bigger the scattering intensity and higher the wavelength lower the intensity of light scattered.
Light scattering by small particles such as nanoparticles can be used to calculate the particle size of the nanoparticles and their size distribution. This method is referred to as dynamic light scattering or photon correlation spectroscopy.
In this method, particles suspended in a solvent is taken for the analysis. These particles undergo Brownian motion due to constant collision of solvent particles with them. These molecules are moving themselves due to their thermal energy. When this suspension is shined with a laser, fluctuations can be seen due to light scattering by particles that enter in to the laser path. The rate of fluctuation of scattered light intensity is dependent upon the size of the particles. Smaller particles show, very high rate of fluctuation as they are pushed further by the bombarding solvent molecules compared to bigger ones. Rate of these fluctuations can be used to calculate the velocity of the Brownian motion and particle size can be obtained using Stokes-Einstein relationship.
DLS is a relatively easy and non-invasive method of obtaining information about particle size and size distribution of nanoparticle dispersion. This technique is widely used in nano colloid characterizations. DSL outputs a profile of scattering intensity with respect to particle size.