EPL335

Study of Raman scattering in Carbon-nanotubes

Presented by : Ajay Singh (2010PH10821)

Graphene is an allotrope of carbon whose structure is a single planer Sheet of sp2 bonded  carbon atoms, that are densely packed in a honeycomb crystal lattice.

Graphene Carbon-nanotube

• Layers of graphite wrapped into cylinders of few nanometers in diameters

• Depending on how the 2D graphene sheet “rolled up ” three types of CNT : armchair , zigzag and chiral.

• SWNT (Diameter 1-2 nm & length 100µm) • MWNT ( interlayer distance in multi-

walled 0.34 nm & outer diameter 5-15 nm)

Raman Scattering

• A small portion of the scattered radiation has frequencies different from that of the incident beam, inelastic scattering.

• Wavenumber shift , the difference in wavenumbers (cm-1) between the observed radiation and that of the source.

• Stoke scattering and Anti stock scattering.• First order Raman scattering and second order Raman scattering.

First order Raman scattering Second order Raman scattering Resonance Raman scattering Surface enhanced Raman scattering

HER = Hamiltonian for electron-radiation interactionHEL = Hamiltonian for electron-phonon/lattice interaction

(b)

In second-order Raman scattering process, an incident photon excites the lattice from an initial state to a virtual state. The lattice emits a scattered photon by making a transition to a final state mediated by two phonons. The second-order Raman process involves either the creation of a phonon and the annihilation of another , or two successive first-order processes of Fig.2.14 (b).

Resonance Raman scatteringIf the wavelength of the exciting laser coincides with an electronic absorption of a molecule, the intensity of Raman- active vibrations associated with the absorbing chromophore are enhanced by a factor of 102 to 104. Thus the resonance Raman technique is used for providing both structural and electronic insight into species of interest. Surface Enhanced Raman Scattering (SERS) The Raman scattering from a compound (or ion) adsorbed on or even within a few Angstroms of a structured metal surface can be 103 to 106x greater than in solution. This surface-enhanced Raman scattering is strongest on silver, but is observable on gold and copper as well. SERS arises from two mechanisms:• The first is an enhanced electromagnetic field produced at the surface of the metal. When the wavelength of the incident light is close to the plasma wavelength of the metal, conduction electrons in the metal surface are excited into an extended surface electronic excited state called a surface plasmon resonance. Vibrational modes normal to the surface are most strongly enhanced.• The second mode of enhancement is by the formation of a charge-transfer complex between the surface and analyte molecule. The electronic transitions of many charge transfer complexes are in the visible, so that resonance enhancement occurs.Molecules with lone pair electrons or pi clouds show the strongest SERS. SERS is commonly employed to study monolayers of materials adsorbed on metals, including electrodes.

Raman spectroscopy• A spectroscopic technique used to observe vibrational ,

rotational and other low-frequency modes in a system.

• A laser beam is used to irradiate a spot on the sample under investigation.

• The scattered radiation produced by the Raman effect contains information about the energies of molecular vibrations and rotations, and these depend on the particular atoms or ions that comprise the molecule, the chemical bonds connect them, the symmetry of their molecule structure, and the physico-chemical environment where they reside.

• Wave number displacement (ΔV) of Raman lines is independent of the frequency of the exciting line.

Raman spectra of a MWCNT

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Raman shift (cmˉ¹)

Intensity

(a.u.)

Raman spectra of MWCNT measured with 514.5-nm excitation

1580(cmˉ¹)

1350(cmˉ¹)

2698(cmˉ¹)

Second order Raman peak(depends on laser excitation)

Defect induced Raman peak

Order of defects can be calculated ID/IG.

2D mode enhances due to double resonance effect.

Raman spectra of 1-LG

 

G-band for highly ordered pyrolytic graphite (HOPG), MWNT bundles, one isolated semiconducting SWNT and one isolated metallic SWNT. The multi-peak G-band feature is not clear for MWNTs due to the large tube size.

Raman signal from three isolated semiconducting and three isolated metallic SWNTs showing the G-and D-band profiles. SWNTs in good resonance (strong signal with low signal to noise ratio) show practically no D-band.

A. Jorio, M. A. Pimenta, A. G. S. Filho, R. Saito, G. Dresselhaus, and M. S. Dresselhaus, New J. Phys., 2003, 5, 139.

Phonon dispersion of sp² Carbon

phonon dispersion relation of graphene showing the LO, iTO, oTO, LA, iTA, and oTA phonon branches.

Raman spectrum of a graphene edge, showing the main Raman features, the D, G and G’ bands taken with a laser excitation energy of 2.41 eV.

Dresselhaus, M.S., Jorio, a. & Saito, R. Characterizing Graphene, Graphite, and Carbon Nanotubes by Raman Spectroscopy. Annual Review of Condensed Matter Physics1, 089-108 (2010).

http://en.wikipedia.org/wiki/Graphinehttp://epsc.wustl.edu/haskin-group/Raman/faqs.htmhttp://en.wikipedia.org/wiki/Raman_spectroscopyhttp://en.wikipedia.org/wiki/Carbon_nanotubehttp://scientificentrepreneur.wordpress.com/2011/12/21/raman-spectroscopy-in-graphene-and-nanoribbons/http://www.sciencedirect.com/science/article/pii/S0370157309000520

Reference links:

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