Raman Observation of Carbon Nanotubes
Raman observation of Carbon Nanotubes bridged between two electrodes
Spatial resolution of 350nm-exceeds the conventional Raman microscope's common sense
The image above is a Raman image of Carbon Nanotubes(CNTs) bridged between two electrodes. The substrate material-SiO2 and G-band of CNTs are shown in red and green respectively. And electrodes are shown in black. It captures the distribution of detailed CNT by a high spatial resolution of 350nm that exceeds the conventional Raman microscope's common sense.
About the sample used for measurement
The right hand side images show schematics of the sample used for measurement. SiO2 film formed on the wafer, where Ti,Au/Cr, Al pattern electrodes are being fabricated. Distance between electrodes(L) and width of the electrode(W) are both 4μm. CNT is bridged between the Au/Cr electrodes by dripping the supernatant fluid of commercial CNT dispersion which treats ultrasonic cleaning and centrifugation and then applying 5V potential.
*This sample was provided by Associate Professor Masamichi Yoshimura of Toyota Technological Institute.
A bird eye view(left) and a cross section(right) of electrode
Observation of high wavenumber region(near D-band and G-band)
The graph below shows the averaged Raman spectrum in the selected rectangular region (the green frame in the bottom-right image). In the spectra of the green frame, both G-band(1590cm-1), which is common to graphite, and D-band(1350cm-1) resulted from point defects and crystal edge defects can be seen.
Averaged Raman spectrum in the selected rectangular region (the green frame in the bottom-right image)
Observation of low wavenumber region(near RBM)
The graph below shows the averaged Raman spectra in each selected rectangular region (blue frame, light blue frame and pink frame). The typical Raman peaks of CNT- peaks of Radial Breathing Mode(RBM) can be seen at different positions each region. In each peak, blue, light blue, pink colors are assigned to show their distribution as the image right below. In addition to each area enclosed by the frame, it can be seen that around the middle of the image CNT,which have the pink color assigned RBM peaks, are distributed.
Averaged Raman spectra in each selected rectangular region (blue frame, light blue frame and pink frame)
(Reference)About the Raman spectrum of Carbon nanotube
CNT is a graphene that rounded to a cylindrical nanostructure. The structure is distinguished by the diameter and the chirality. It is possible to become the metal or a semiconductor by the difference of the 3D structure.
When Raman of CNT is observed, the energy of incident light resonates with the absorption of light, compared with usual Raman scattering, about 1000 times intensity of Raman scattering can be seen (resonance Raman effects). In the Raman spectrum of CNT obtained by this resonance Raman effects, it is well known that many characteristic Raman peaks appear. By analyzing these peaks, you will find out the diameter, the chirality and the defects of CNT.
Raman spectra of Cabon Nanotube
G-band(G+,G-)
It is a Raman peak common to graphite material that appears near 1590cm-1. It corresponds to the vibrational mode in the graphitized plane and shown by the strong intensity when LO mode (Longitudinal Optic mode) and TO mode (Transverse Optic mode) are degenerated.
Because CNT is a rounded graphene, the degeneracy of LO and TO is lifted, the G-band splits into G+ near 1590cm-1 and G- in lower wavenumber than G+. The position in which G- appears depends on its property, the metallic nanotube or the semiconductor nanotube. G-band of the metallic nanotube shifts greatly from G+ and appears near 1550cm-1 compared with the semiconductor nanotube
D-band
It is a Raman peak that originates in the defects and appears near 1350cm-1. It is shown by the strong intensity when there're a lot of point defects and crystal edge defects in the nanotube and graphite. The G/D ratio is the relative intensity ratio between G-band of the nanotube and D-band, which originates in defects. It is used as an index that shows the quality of the nanotube.
Radial Breathing Mode(RBM)
It is an extremely important Raman peak that originates in the diameter and the chirality of CNT that appears in the low wavenumber area of 100~300cm-1. For the isolation nanotube
D(nm)=248/ν ν:Raman shift(cm-1)
Diameter(D) can be evaluated by the equation above.
While the wavelength of the exciting light is changed for the sample that contains various diameter and the chiralities, different kinds of CNT may show the resonance Raman effects, therefore the peak of RBM appears at a different position.
Other Raman peaks
G'-band: It is an overtone of D-band that appears near 2700cm-1. Regardless of crystalline, it always shown by the same strong intensity with G-band's.
D*-band: It is related to the phonon density of state that appears near 1605cm-1. Although it is related to crystalline as well as D-band's and the intensity grows when the crystal with a lot of defects, it is smaller than D-band's.
