Charectization Of Carbon Nanotubes Based On Spectroscopic Techniques & Their Optical Properties

 

Charectization of carbon nanotubes based on spectroscopic techniques & their optical properties.

The optical properties of carbon nanotubes refers to the absorption, photoluminescence, and Raman spectroscopy of carbon nanotubes. Because spectroscopic techniques  allow quick and reliable characterization of “nanotube quality” in terms of non-tubular carbon content, structure (chirality) of the produced nanotubes, and structural defects. Those features determine nearly any other properties such as optical, mechanical, and electrical properties.

Carbon nanotubes are unique “one dimensional systems” which can be achieved by rolling graphene sheet.This rolling can be done at different angles and curvatures resulting in different nanotube properties. The diameter canvaries in the range 0.4–40 nm, but the length nanotube is about ~10,000 times reaching 4 cm. Thus the nanotube  have aspect ratio, (i.e  the length-to-diameter ratio) as high as 28,000,000:1,which is unequalled by any other material.As a result, all the properties of the carbon nanotubes relative to those of conventional semiconductors are extremely anisotropic and tunable.

 The tunability of properties is most useful in optics and photonics.

Carbon nanotubes are of three types

1.zig-zag

2.armchair

3.chiral

Following are the optical methods of charecterization ogf CNTs:

1. Optical absorption

Optical absorption in carbon nanotubes differs from absorption in conventional 3D materials by presence of sharp peaks (1D nanotubes) instead of an absorption threshold followed by an absorption increase (most 3D solids). Absorption in nanotubes originates from electronic transitions from the v2 to c2 (energy E22) or v1 to c1 (E11) levels, etc. The transitions are relatively sharp and can be used to identify nanotube types.

Interactions between nanotubes, such as bundling, broaden optical lines. While bundling strongly affects photoluminescence, it has much weaker effect on optical absorption and Raman scattering.

Optical absorption is routinely used to justify the  quality of the carbon nanotube powders. The spectrum is analyzed in terms of intensities of nanotube-related peaks, background and pi-carbon peak; the latter two mostly originate from non-nanotube carbon.
2.Carbon nanotubes as a black body

An ideal black body should have emissivity or absorbance of 1.0, which is difficult to attain in practice, especially in a wide spectral range. Vertically aligned “forests” of single-wall carbon nanotubes can have absorbances of 0.98–0.99 from the far-ultraviolet (200 nm) to far-infrared (200 μm) wavelengths. By coating Super black( a chemically etched nickel-phosphorus) the absorption of 1.0 can be achieved.

These SWNT forests (buckypaper) were grown by the super-growth CVD method to about 10 μm height. Two factors could contribute to strong light absorption by these structures: (i) a distribution of CNT chiralities resulted in various bandgaps for individual CNTs. Thus a compound material was formed with broadband absorption. (ii) Light might be trapped in those forests due to multiple reflections.

3.Luminescence

 

The Photoluminescence map of single-wall carbon nanotubes.  Can be help ful in identifying the nanotube semiconducting nanotubes with indices (n,m). The PL measurements do not detect other nanotubes with  indices n = m or mExcitation mechanism.Hence Photoluminescence (PL) is one of the important tools for nanotube characterization.

The excitation  mechnism of PL

 The excitation  mechnism of PL occurs as follows: an electron in a nanotube absorbs excitation light via S22 transition, creating an electron-hole pair (exciton). Both electron and hole rapidly relax (via phonon-assisted processes) from c2 to c1 and from v2 to v1 states, respectively. Then they recombine through a c1 − v1 transition resulting in light emission.

No excitonic luminescence can be produced in metallic tubes — electron can be excited, thus resulting in optical absorption, but the hole is immediately filled by another electron out of many available in metal. Therefore no exciton is produced .

 

properties

1.Photoluminescence, optical absorption and Raman scattering from SWCNT is linearly polarized along the tube axis..

2.PL is quick: relaxation typically occurs within 100 picoseconds.

     3.PL efficiency is usually low (~0.01%).

     4. The spectral range of PL is rather wide. Emission wavelength can vary between 0.8and 2.1 micrometers depending on the nanotube structure.

     5. Interaction between nanotubes or between nanotube and another material (e.g., substrate) quenches PL.Hence, PL is not observed in multi-wall carbon nanotubes.

6. PL from double-wall carbon nanotubes strongly dependsthe method of preparation.     For  eg;CVD grown DWCNTs show emission both from inner and outer shells. Position of the (S22, S11) PL peaks depends slightly (within 2%) on the nanotube environment (air, dispersant, etc.). However, the shift depends on the (n, m) index, and thus the whole PL map not only shifts, but also warps upon changing the CNT medium.

Applications of photoluminecense

PL is widely used to deduce (n, m) indexes: first nanotubes are isolated (dispersed) using an appropriate chemical agent (“dispersant”) to reduce the intertube quenching. Then PL is measured, scanning both the excitation and emission energies and thereby producing a PL map. The ovals in the map define (S22, S11) pairs, which unique identify (n, m) index of a tube. The data of Weisman and Bachillo are conventionally used for the identification.

Sensitization

Optical properties, including the PL efficiency, can be modified by encapsulating organic dyes (carotene, lycopene, etc.) inside the tubes. Efficient energy transfer occurs between the encapsulated dye and nanotube — light is efficiently absorbed by the dye and without significant loss is transferred to the SWCNT. Thus potentially, optical properties of a carbon nanotube can be controlled by encapsulating certain molecule inside it.

Cathodoluminescence

Cathodoluminescence (CL) — light emission excited by electron beam — is a process commonly observed in TV screens. An electron beam can be finely focused and scanned across the studied material. This technique is widely used to study defects in semiconductors and nanostructures with nanometer-scale spatial resolution. It would be beneficial to apply this technique to carbon nanotubes. However, no reliable CL, i.e. sharp peaks assignable to certain (n, m) indexes, has been detected from carbon nanotubes yet.

Electroluminescence

If appropriate electrical contacts are attached to a nanotube, electron-hole pairs (excitons) can be generated by injecting electrons and holes from the contacts. Subsequent exciton recombination results in electroluminescence (EL). Electroluminescent devices have been produced from single nanotubes.

 Raman spectrum of single-wall carbon nanotubes

Raman spectroscopy has good spatial resolution (~0.5 micrometers) and sensitivity  to single nanotubes; it requires minimum sample preparation and is rather informative. Raman spectroscopy is the most popular technique of carbon nanotube characterization. Raman scattering in SWCNTs is resonant, i.e., only those tubes are probed which have one of the bandgaps equal to the exciting laser energy. Similar to photoluminescence mapping, the energy of the excitation light can be scanned in Raman measurements, thus producing Raman maps. Those maps also contain oval-shaped features uniquely identifying (n, m) indexes. Contrary to PL, Raman mapping detects not only semiconducting but also metallic tubes, and it is less sensitive to nanotube bundling than PL.

 

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Anti-Stokes scattering

All the above Raman modes can be observed both as Stokes and anti-Stokes scattering. As mentioned above, Raman scattering from CNTs is resonant in nature, i.e. only tubes whose band gap energy is similar to the laser energy are excited. The difference between those two energies, and thus the band gap of individual tubes, can be estimated from the intensity ratio of the Stokes/anti-Stokes line. This estimate however relies on the temperature factor (Boltzmann factor), which is often miscalculated – focused laser beam is used in the measurement, which can locally heat the nanotubes without changing the overall temperature of the studied sample.

 

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G mode:

G mode (gG from graphite)corresponds to planar vibrations of carbon atoms and is present in most graphite-like materials. G band in SWCNT is shifted to lower frequencies relative to graphite (1580 cm−1) and is split into several peaks. The splitting pattern and intensity depend on the tube structure and excitation energy; they can be used, though with much lower accuracy compared to RBM mode, to estimate the tube diameter and whether the tube is metallic or semiconducting

Rayleigh scattering

Carbon nanotubes have very largeaspect ratio, i.e., their length is much larger than their diameter. Consequently, as expected from the classical electromagnetic theory, elastic light scattering (or Rayleigh scattering) by straight CNTs has anisotropic angular dependence, and from its spectrum, the band gaps of individual nanotubes can be deduced.

References

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12. ^ *Openov, Leonid A.; Elesin, Vladimir F. (1998). “Prismane C8: A new form of carbon?”. JETP Letters 68: 726. doi:10.1134/1.567936. http://www.jetpletters.ac.ru/ps/969/article_14784.pdf. 
13. External links
14. http://www.dendritics.com/scales/c-allotropes.asp

http://cst-www.nrl.navy.mil/lattice/struk/carbon.html

diamond 3D animation.

RABIYA TANVEER.                                                               

LECTURER IN PHYSICS

CHAITANYA DEGREE AND P.G COLLEGE

HNK,WARANGAL,INDIA.

AFFILIATION:

1.NANO SCIENCE & TECHNOLOGY CONSORTIUM,

NOIDA,UP.INDIA.

2.PHOTONICS 21,EUROPEAN TECHNOLOGY PLATFORM. EMAIL:munaizag@gmail.com                      

 

 

 

About the Author

lecturer in physics & electronics dept. of physics & electronics, chaitanya degree & p.g college, kishan pura ,hanamkonda, warangal.A.P.

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