Raman spectroscopy, which uses the inelastic scattering of light falling on a material, is used for the characterization of materials (including nanostructures) because it is a non-destructive, fast, efficient, and simple technique.
Carbon-based materials, for example, can be characterized using Raman spectroscopy to indicate the formation of nanostructures. Zhu et al., 2012 for example, showed hollow carbon nanopolyhedrons synthesized at low temperature are characterized by the presence of a D band at 1328 cm−1. This was attributed to the presence of defects and disorder in the carbonaceous materials. They were also characterized by a G band at 1579 cm−1, which is assigned to C–C stretching.
One parameter used in the Zhu study to evaluate the degree of disorder is the ID/IG ratio (where ID and IG are the intensities of the D and G bands, respectively). In the case of the nanopolyhedrons, the ID/IG ratio is 1.15, which indicates the absence of long-range order and confirms the formation of the nanostructures.
Molecular structure of a nanotube (Image credit: Raimundo79/Shutterstock)
One of the more popular materials in nanoscience are carbon nanotubes, due to their electrical and mechanical properties and chemical stability. Using Raman spectroscopy, it is possible to determine the nanotube diameters (Graupner, 2007), nanotube-nanotube interactions (Rao et al., 2001), and thermal conductivity. The analyses are performed considering the position, intensity, and full width at half-maximum of the peaks of the nanotubes.
Carbon nanotubes can be single-walled (SWCNTs) or multiwalled (MWCNTs). The determination of thermal conductivity is performed by analyzing the displacement of the G-band position, which is attributed to the thermal expansion and weakening of the C–C bond (Kim et al., 2011). It is also possible to characterize carbon nanotube structures or composites with hybrid characteristics, and Raman spectroscopy is useful to evaluate the potential chemical or physical interactions between the materials.
Nanowires, Nanocombs and Nanobelts
Kim et al., 2012 demonstrates Raman spectroscopy could also be used to characterize different nanostructures made from the same compound, for example, zinc sulfide (ZnS). Different ZnS nanostructures, such as nanowires, nanocombs, and nanobelts were studied and characterized using Raman Spectroscopy.
The Raman spectroscopy analysis in the Kim et al., 2012 study indicated that the signal/noise ratio for the nanowires was lower when they were irradiated with 2.71 eV, and they also showed more bands between 200 and 300 cm−1. The peak at 521 cm−1 is due to the Si substrate, and the disappearance of this peak in the other spectra is due to the dense growth of nanocombs and nanobelts. The wave number change from 350.2 to 349.2 cm−1 during the formation of the nanobelts indicates the generation of elastic tension.
Other nanomaterial properties can be studied using Raman spectroscopy, such as the mechanical properties of silicon nanowires (SiNWs) (Khachadorian et al., 2011) and the size distribution of nanocrystals. Georgescu et al., 2012 worked with titanium oxide aero-gel (TiO2). Their calculations were performed using the peak position and the full width at half-maximum for three main bands at 144, 398, and 638 cm−1.
The band at 144 cm−1 was more susceptible to changes in the sample’s crystalline structure, and the band at 398 cm−1 was more sensitive to the variations in the size of the nanocrystals; shifting to shorter wavelengths as the nanocrystals’ size increased. Regarding the full width at half-maximum, the most sensitive band is the one at 638 cm−1, in which the width tends to decrease with an increase in the nanocrystal size.
Wu et al., 2010 characterized nanocrystals of CeO2 with different shapes: nanorods, nanocubes, and nanooctahedra. The presence of defects on the surfaces of nanomaterials makes them capable of adsorbing O2, and it is possible to monitor the occurrence of this gas using Raman spectroscopy. Raman spectroscopy can be used for determination of phase transitions in carbon nanotubes (Liu et al., 2012a, 2012b) and to investigate the crystallinity of nanomaterials and nanostructured materials.
Zucolotto et al., 2006 in their research showed Raman Spectroscopy could be used to assess the doping of conducting polymers. They investigated thin films of polyaniline (PaNI) with three phthalocyanines, iron (FeTsPc), nickel (NiTsPc), and copper (CuTsPc), grown using the LbL technique. The Raman spectra showed that the primary doping was due to protonation and the secondary doping was due to structural and conformational changes in the PaNI chain. The interactions between the phthalocyanine −SO3 groups and PaNI NH groups allow the growth of the LbL films.
The similarity between the spectra indicated the phthalocyanine did not induce the secondary doping. The work of Silva et al., 2012 used Raman spectroscopy on hybrid thin films of a semiconductor oxide (hexaniobate nanospirals), PaNI. It indicated that the semiconductor oxide could induce secondary doping in PaNI to further improve its conductive property.
The photodegradation of Congo Red (CR) can also be studied using Raman Spectroscopy. CR was adsorbed onto the last layer of thin films with titanium dioxide nanoparticles (TiO2) and polyelectrolytes, such as Pah and sodium polystyrene sulfonate (SPS), with Pah/PSS/TiO2(PSS/TiO2) 5 architecture (Sansiviero et al., 2011). The film was irradiated with UV light for 24 h, and Raman spectra were obtained, which revealed the emergence of new bands attributed to the cr oxidation process. The CR characteristic bands at 1595 cm−1 (phenyl ring), 1457 cm−1 (C═C stretching), and 1155 cm−1 appeared with low intensity after the irradiation, which indicated photodegradation and photoisomerization.
In addition, Raman spectra of commercially purchased TiO2 in the anatase phase and of the LbL film Pah/PSS/TiO2(PSS/TiO2) 5 were obtained. The results indicated that the bands present in the film are broader and shifted to the red relative to pure TiO2. This phenomenon is attributed to a disruption in the dynamic selection rule of phonons, which is attributed to ordered systems.
Surface Enhanced Raman Scattering (SERS)
Nanostructured materials show comparative advantages in efficiency and stability when compared to specific properties related to the bulk of the material. Raman scattering is not particularly destructive, but it has a small cross-section compared to the absorption and emission processes, which complicates the characterization of dilute solutions or nanosized structures such as ultrathin films.
However, these systems can be mixed with metallic nanostructures, that is, solutions diluted in metal colloids and thin films deposited on roughened metal surfaces, to enhance the Raman signal by a factor of up to 107. This spectroscopy technique is known as SERS.
The grounds and selection rules of SERS spectroscopy, in addition to the preparation of metallic nanoparticles, have been widely described (Aroca, 2006; Ru and Etchegoin, 2009). Jensen et al. (2008) published a review in which they mention four enhancement mechanisms (cheM, resonance, cT, IN) based on resonant and non-resonant processes.
The development of a wide range of nanostructured materials requires control strategies for the chemical surface and nanoparticle synthesis in the search for specific properties (adsorption, electron transfer, stability, etc.). Therefore, a broad range of nano-structured materials can be characterized using SERS spectroscopy when they are:
- Mixed with metal nanoparticles
- Deposited on roughened metal surfaces
- Covered with evaporated metal films
- Dipped in metal ion solutions to induce chemical reduction with laser excitatiom
In this section, SERS characterization is addressed with a focus on the molecular identification of analytes on metal nanostructures or incorporated in nanostructured materials in sensor units, electronic circuits, biological materials, and historical heritage.
Characterization of Paintings and Textiles of Historical Value
An alternative to characterizing historical paintings and textiles is to identify the dyes present in the sample using metal nanoparticles to generate an amplification of the Raman scattering (SERS) and suppression of the pigment fluorescence. SERS identification of flavonoids in textiles (Jurasekova et al., 2008) and of various dyes, such as alizarin, purpurin, carminic acid, hematoxylin, fisetin, quercitrin, quercetin, rutin, and morin, has been conducted (Leona et al., 2006).
Brosseau et al. (2009) conducted a comparative SERS study of a painting by artist Mary Cassatt (Pastel Studio: “Sketch of Margaret Sloane, Looking right”) and the pastels she used (Boston Museum of Fine arts).
Characterization of Biological Materials
The preparation of reproducible and organized nanostructures is essential for obtaining materials with light scattering properties. Some naturally acquired materials have impressive optical effects. Butterflies use several layers of cuticles and air to produce the blue color on their wings, and some insects use matrices of elements to reduce the reflectivity of the compounds in their eyes (Vukusic and Sambles, 2003).
Tan et al. (2012) used the wings of Lepidoptera (butterflies and moths) as organized substrates and made an electrolytic coating with Cu (10 min) to obtain a structure suitable for amplification of the Raman signal (SERS). The substrates coated with Cu were tested with rhodamine 6G, and a reproducible SERS spectrum was obtained at a concentration of 10−5 M. The intensity of the band at 1650 cm−1 was used to compare the different nanostructures.