Size Effect Of Raman Scattering In Single-walled Carbon Nanotubes

Single-walled carbon nanotubes(SWCNTs)are ideal one-dimensional(1D)materials,which have the significant academic value and wide application foreground.Raman spectroscopy is a powerful tool for the characterization of SWCNTs.However,due to the harsh resonance condition between the laser energy and optical transitions in SWCNTs,the majority of individual SWCNTs cannot be detected using several different wavelength lasers.In this thesis,Gold nanoparticles(Au NPs)were in-situ coupled with straight and defect-free individual SWCNTs,Raman scattering of individual SWCNTs at different length scales(1 μm,10 nm and sub nanometer)were intensively studied for breaking the limit of selection rules.Raman responses from different length scales reveal the size effect of Raman scattering in SWCNTs.We provided a method for improving the tube collimation in horizontal ultralong SWCNT arrays by rapidly changing the temperature of catalyst.Then we prepared horizontal ultralong SWCNT arrays with high collimation via this method.Individual ultralong SWCNTs with straight profile are suitable for studying Raman scattering of SWCNTs.Using an improved gas flow-directed chemical vapor deposition of ultralong SWCNTs,we solved the problem of catalyst deactivation caused by ultralow carbon source feeding in laminar flow-directed growth process.This method can increase the density of the array and is beneficial to the growth of SWCNTs with small diameters.The length of ultralong SWCNTs in the prepared horizontal arrays is more than 5 mm,the density of ultralong SWCNTs is about 100 per mm.The average diameter of ultralong SWCNTs is 1.53 ± 0.36 nm.By analyzing the radial breathing mode(RBM),G mode and G’mode excited by 5 different lasers in a same individual SWCNT,we determined the(n,m)of individual ultralong SWCNTs.Focused Ga3+ beam was used to in-situ introduce defect in defect-free individual SWCNTs with known(n,m).The relationship between the frequency of defect induced D mode and nanotube diameter was accurately studied.Moreover,the splitting of G mode induced by defect was also found.Individual ultralong SWCNTs with straight profile were used as sensitive probes to investigate local electromagnetic(EM)field.Surface-enhanced Raman scattering contribution from different hotspots with a space about 10 nm can be distinguished.By assembling Au NPs around individual ultralong SWCNTs,SWCNT/Au NPs coupling systems are constructed.Under optimized conditions(Au NPs with a diameter of 30 nm),Raman enhancement with a factor more than 104 can be found.By studying the polarization dependence of Raman scattering intensity,SERS contributions from different hotspots with a space of 7.5 nm were distinguished,and the enhancement factor in the center of nanogap is about 103 multiple of the enhancement factor on the boundary of the nanogap.Owing to the hexagonal rotational symmetry of the Au NP cluster,polarization-insensitive Raman scattering was also found in this system,the polarization grade of G mode is below 0.33.SERS of defect-free individual ultralong SWCNTs with straight profile at the length scales of sub nanometer was intensively studied.The defect-free D-mode of an individual SWNT induced by gradient-field is found for the first time.The defect-free D-mode and defect induced D-mode are at the same frequency,but their scattering process are different.The intensity of G mode is proportional to the electric field intensity of the 4 order,however,the intensity of defect-free D mode is proportional to the electric field intensity of the 8 order.When the light is confined at atomic scale,gradient-field Raman scattering becomes significant and dipole-forbidden phonon modes can be activated by quadrupole Raman tensor variation,indicating the breakdown of the Raman selection rules.We also found RBM which does not satisfy the conventional resonance condition,and this can be attributed to selection rule breakdown in plasmon-induced electronic excitation and the resonance window broadening.