Session P28: Carbon Nanotubes: Optical Properties

Empirical Study of the $\pi $ electron Plasmon Energy Dependence on the Bundling/De-bundling Effect in Single Wall Carbon Nanotubes

Many researchers have done detailed studies on optical, thermal and electronic properties of SWNTs. But, very few studies have been done on sub-nanometer SWNTs. Here we studied collective electron excitations in this quasi 1D system. At high excitation energies, broad absorption peak is observed which is attributed to the $\pi $ plasmon (5-7 eV). We used UV-Vis NIR spectroscopy to determine the energy of the $\pi $ plasmons in sub nanometer diameter SWNTs (0.4 nm to 0.9 nm). SWNTs form bundles due to van der Waal forces and this bundling influences their electronic structure. It is known that SWNTs wrapped with a surfactant can be isolated with long centrifugation. The hydrodynamic sizes of the dispersed SWNTs at different centrifugation times were determined by using the Dynamic Light Scattering technique. Systematic studies have been done on the dependence of the $\pi $ plasmon energy on the nanotube bundle diameter. The energy of the $\pi $ plasmon was found to vary with the bundle diameter and the energy to be given by the relation E= (-0.022 eV)*ln(d/1 nm)+5.34 eV. We have done similar studies on HiPCo and Carbolex SWNTs and the empirical relation obtained is consistent with the results above.   

Simultaneous measurement of length, concentration and brightness of single-walled carbon nanotubes with fluorescence correlation spectroscopy

We report on the application of fluorescence correlation spectroscopy (FCS) to simultaneously measure the brightness, concentration, and length of single-walled carbon nanotubes (SWCNTs). The technique relies on the intrinsic bandgap luminescence of (6,5) chirality semiconducting SWCNTs in the near infra-red (NIR) range and does not require sample labeling. The nanotubes used in this study have been dispersed in solution of single stranded DNA and length fractionated via size exclusion chromatography. The SWCNT length measured by FCS was in excellent agreement with more traditional techniques - polarized dynamic light scattering (DLS) and atomic force microscopy (AFM). The apparent nanotube brightness is shown to grow linearly with the mean nanotube length, having a zero intensity cut-off at 110 nm, implying an exciton diffusion length of 55 nm for SWCNTs dispersed in sodium deoxycholate.   

Absolute Rayleigh Intensity and Uniform Optical Conductivity in Carbon Nanotubes

We used a novel on-chip Rayleigh imaging technique to measure the absolute intensity of Rayleigh scattering of single-walled carbon nanotubes. The spatial distribution of the radiation scattered by the nanotubes is determined by their shape, but the intensity and spectrum of the scattered radiation are determined by exciton dynamics, quantum-dot-like optical resonances and other intrinsic properties. Moreover, the nanotubes display a uniform peak optical conductivity $\sim $8 e\^{}2/ h, which we derive using an exciton model, suggesting universal behaviour similar to that observed in nanotube conductance. We further demonstrate a radiative coupling between two distant nanotubes, with potential applications in metamaterials and optical antennae. This is in contrast to the optical properties of metal nanostructures and show that nanotubes can form ideal optical wires.   

Dielectric screening dependence of excitonic transition energies in single-wall carbon nanotubes

The measured optical transition energies E$_{ii}$ of single-wall carbon nanotubes are compared with bright exciton energy calculations. The E$_{ii}$ differences between experiment and theory are minimized by considering first, a diameter/chiral angle-dependent dielectric constant and second, a diameter/exciton size-dependent dielectric constant (k). In our description, k is composed of the screening contributions from the tube, represented by k$_{tube,}$ and from the environment, represented by k$_{env}$. We discuss the main aspects of each approach and show that in the first case, different k dependencies are obtained for (E$^{S}_{11}$, E$^{S}_{22}$, E$^{M}_{11})$ relative to (E$^{S}_{33}$, E$^{S}_{44})$ which is understood as follows: A changing environment changes the k diameter dependence for (E$^{S}_{11}$, E$^{S}_{22}$, E$^{M}_{11})$, but for (E$^{S}_{33}$, E$^{S}_{44})$ the environmental effects are minimal. We show that in order to achieve a single dependence for all E$_{ii}$, the exciton's size should be taken into account, as considered in the second approach. The resulting calculated exciton energies reproduce experimental E$_{ii}$ values within $\vert $50$\vert $ meV for a diameter range (0.7$<$ dt $<$3.8 nm) and 1.2 $<$ Eii $<$2.7 eV, thus providing a theoretical justification for E$_{ii}$ and important insights into the dielectric screening in one-dimensional structures.   

Temperature-Dependent Maximum Density of 1D Excitons in Carbon Nanotubes

Previous studies have shown that an upper limit exists on the density of 1D excitons in single-walled carbon nanotubes (SWNTs) due to very efficient exciton-exciton annihilation (EEA). A recent theoretical study based on a dark-bright two-band exciton model predicts that there is a temperature at which the achievable exciton density will be maximized, surpassing the room-temperature upper limit. Therefore, we performed temperature-dependent (300 K to 11 K) photoluminescence (PL) on HiPco SWNTs embedded in an i-carrageenan matrix under high resonant excitation. To achieve high densities, we used pump fluences up to $\sim $ 10$^{14}$ photons/cm$^{2}$, utilizing intense fs pulses from a wavelength-tunable optical parametric amplifier. We found that for each temperature the PL intensity saturates as a function of pump fluence and the saturation intensity increases from 300 K to a moderate temperature around 100-150 K. Below that critical temperature, the PL intensity decreases with decreasing temperature. Within the framework of diffusion-limited EEA, we successfully estimated the upper limit of the density of 1D excitons in SWNTs as a function of temperature and chirality   

Experimental Kataura Plot in Single-walled Carbon Nanotubes

Single-walled carbon nanotubes (SWNTs) comprise a family of more than 400 structures characterized by different chiral indices n-m, each having a distinct electronic structure that can be either metallic or semiconducting. An outstanding question is how the physical properties, such as optical transitions, vary with the exact nanotube structures. By combining TEM diffraction and Rayleigh scattering spectroscopy on the same individual nanotubes, we determine independently the chiral indices and optical transitions of over 170 single-walled nanotubes. These data permit us to create an experimental Kataura plot for single-walled carbon nanotubes.   

Optical Properties of Empty and Water-Filled Single-Wall Carbon Nanotubes

The necessity for separation of single-wall carbon nanotube (SWCNT) populations to achieve desired properties presents a major technical barrier for the development of SWCNT-based applications, and has been the focus of significant academic and industrial research. Recent advances include the separation of SWCNT populations by diameter through buoyancy differences. Here we report on the optical spectroscopic properties of large diameter SWCNTs synthesized by laser ablation and electric arc methods and then separated by centrifugation to produced isolated bands of empty and water-filled nanotubes. This separation is consistent across multiple nanotube populations dispersed from different source material. Optical absorption, near-infrared fluorescence, and Raman spectroscopic measurements of the resulting empty and filled fractions reveal that water filling leads to systematic changes in the optical properties. Specifically, the peak locations in absorbance and fluorescence display red-shifts with the presence of water in the nanotube cavity and a hardening of the Raman radial breathing modes. The presence of water in the SWCNT interior is found to facilitate the subsequent separation into sub-populations of metallic and semiconducting SWCNTs.   

Rapid widefield Raman imaging of individual carbon nanotubes

Raman spectroscopy is a powerful tool for characterizing carbon nanotubes. Confocal micro-Raman imaging can provide detailed spatial and spectral information about individual nanotubes, but this technique is often time-consuming. We present a widefield Raman microscope capable of rapid and large-area imaging of carbon nanotube samples. Thanks to a widefield excitation geometry, a high-power excitation laser (3W in our experiment) can be used without causing thermal damage to nanotubes, which dramatically shortens image acquisition time ($\sim$20sec for G- band for a 60$\mu$m field of view). With a custom-built tunable bandpass filter, our widefield Raman images let us quantitatively compare the D, G and 2D-band intensities of many nanotubes with different known resonant energies, diameters, and metallicities, while providing the Raman scattering cross-section length for individual nanotubes. This technique allows Raman-based spatially resolved investigation of dynamic processes in nanotubes for the first time, which we demonstrate by real-time imaging of the oxidation of nanotubes at high temperatures.   

Absence of Broad G$^{-}$ Feature in Resonant Raman Spectra of Armchair Carbon Nanotubes

Unlike the radial breathing mode in carbon nanotubes (CNTs), the G-band Raman feature does not display a strong frequency dependence on nanotube structure. The appearance of a broad G$^ {-}$ peak in CNT Raman spectra has been attributed to numerous phenomena including the presence of metallic nanotubes, although a consensus has yet to be achieved amongst researchers. Here, we present resonant Raman measurements on macroscopic ensembles enriched in armchair CNTs produced by density gradient ultracentrifugation. Our G-band data clearly show that the broad, lower-frequency G$^{-}$ mode is absent for armchair structures, in contrast with recent theoretical and experimental results, and only occurs with resonance of non- armchair metals. This forms a generalized correlation between G-band lineshape and nanotube structure due to the sampling of a large number of nanotubes of several armchair species.   

Infrared active vibrations in carbon nanotubes

The method of choice for the study of vibrational modes of carbon nanotubes has been almost exclusively Raman spectroscopy. Although calculations predict also infrared-active modes in nanotubes, so far only very few experimental results have been published. We conducted a systematic investigation of the infrared transmission of various types of single- and double-walled carbon nanotubes. Experiments were done on self-supporting transparent films in order to avoid perturbation from substrates. We find weak but reproducible vibrational peaks in the infrared spectrum. Their frequency scales with the diameter of the tubes, indicating their intrinsic character. Furthermore, on doping, some of the peaks change from Lorentzian to Fano-like character. This change can be explained by coupling of the tube vibrations to the conduction electrons introduced by doping. Finally, in double-walled nanotubes peaks typical of both outer and inner tubes can be distinguished. The vibrations of the inner tubes occur at the same frequency as those of single-walled tubes with the same diameter.   

Far Infrared Optical Studies of Single and Double Walled Carbon Nanotubes

Variable temperature far infrared absorption measurements were carried out for single walled and double walled carbon nanotubes samples (SWCNT and DWCNT) encased in a polymer matrix to investigate the effects of temperature and surface interaction on the low frequency phonons associated with the circumferential vibrations. At a temperature where kBT is significantly lower than the phonon energy, the broad absorption features as observed at room temperature become well resolved phonon transitions. For a DWCNT sample whose inner tubes have a similar diameter distribution as the SWCNT sample studied, a series of sharp features were observed at room temperature at similar positions as for the SWCNT samples studied. The narrow linewidth is attributed to the fact that the inner tubes are isolated from the polymer matrix and from the weak inter-tubule interactions. First principles calculations are carried out to understand the pertinent interactions and the temperature-dependent effects.   

Influence of Defects and Doping on Optical Phonon Dynamics in Carbon Nanotubes

The relaxation of electronic excitations induced by high bias or photoexcitation occurs primarily through optical phonon emission. Optical phonon relaxation may be affected by metallic/semiconducting character of carbon nanotubes, defect concentration, as well as doping. Changes in carbon nanotube G-band optical phonon population and pure dephasing lifetimes with doping and defects are described. Time-resolved incoherent anti-Stokes Raman spectroscopy is used to directly measure phonon decay lifetime, T$_{1}$, while total dephasing rate is inferred from static Raman linewidths. Defect concentration is varied by sample annealing and covalent functionalization showing increasing disorder reduces T$_{1}$ as well as overall dephasing time, T$_{2}$, with an even greater dependence. Samples with different metallic and semiconducting contribution have similar lifetimes, T$_{1}\sim $ 1.2 $\pm $ 0.1 ps in the no defect limit. Doping is shown to increase G-band linewidth, and therefore overall dephasing rate, for semiconducting nanotubes while leaving T$_{1}$ unaffected.   

Ultrafast Terahertz Probes of Individualized, Chirality-Enriched Single-Walled Carbon Nanotubes

Singled-walled carbon nanotubes (SWNTs) represent a model system to systematically investigate correlated charge excitation in 1-D limits. One of the most outstanding issues both in fundamental nanotube physics and for their technological development is to detect and understand optically-forbidden, dark collective states. Thus far supporting evidence of dark states has been demonstrated in static magneto-optics and light scattering. However, the unique internal transitions from dark excitonic ground states and their dynamic evolution remain highly elusive. We report our investigation of this problem using optical-pump, terahertz probe spectroscopy of individualized, (6,5) and (7,5) SWNTs. We measure transient THz conductivity from 1-15 meV at low temperature down to 4K with resonant and off-resonant excitation at the $E_{22}$ transitions of (6,5) and (7,5) nanotubes. The intra-excitonic spectroscopy with THz pulses represents a fundamentally different spectroscopy tools to study dark excitons and shine new lights on the nature of excitonic ground states.   

Nonlinear Optical Response of Individual Carbon Nanotubes

Single walled carbon nanotubes (SWCNTs) are low dimensional conductors with unique nonlinear electro-optic properties. To investigate these properties we study the third-order, coherent anti-stokes (CAS) response of electrically connected individual SWCNTs on quartz substrates, using a four-wave-mixing (FWM) technique with femtosecond laser pulses. Because the CAS response is primarily electronic in nature [1], the signal from metallic SWCNTs is much stronger than from semiconducting ones. CAS easily distinguishes between the two types, as well as between semiconducting SWCNTs doped to be conductive or insulating. Furthermore, the CAS signal is sensitive to excitation resonances, the same effect that allows SWCNT fingerprinting by photoluminescence and Raman techniques. In addition to the strong electronic signal, we can also resolve a vibrational signal component at the G-band frequency, which suggests a method for studying chemical bond vibrations with this coherent technique. The good spatial resolution and high signal-to-noise achieved with femtosecond laser pulses provides opportunities for time-resolved optical measurements of SWCNT excitation dynamics. Funded by NSF Center for Chemistry at the Space-Time Limit at UCI (CHE-0847097). \\[4pt] [1] H. Kim et al, Nano Lett. 9 2991-2995 (2009)   

Second Harmonic Generation in Highly Aligned Carbon Nanotubes on GaAs

Optical properties of carbon nanotubes (CNTs) have been extensively investigated during the last decade, and much basic knowledge has been accumulated on how light emission, scattering, and absorption occur in CNTs. However, their nonlinear optical properties remain largely unexplored, except for theoretical studies predicting highly chirality-selective nonlinear optical processes. In particular, all chiral nanotubes are expected to possess finite second-order nonlinear susceptibilities due to the lack of inversion symmetry. Here, we have observed second harmonic generation (SHG) from samples consisting of highly aligned CNTs on GaAs with linearly-polarized intense mid-infrared femtosecond radiation. SHG is expected from both the CNTs and the substrate, thus the contribution of the CNTs to the overall enhanced SHG signal will be obtained by factoring out contributions from the GaAs substrate, which include any anisotropic effects (absorption, polarization) from the CNTs on the fundamental light incident in the GaAs substrate. We performed detailed angular dependent measurements to separate the two contributions, based on the crystal symmetries of the two. The results will be shown as a function of laser power and wavelength, and discussed in light of CNT band structure.