Session V6: Focus Session: Carbon Nanotube Optics I: Spectroscopy and Excitons

High Throughput Optical Imaging and Spectroscopy of Individual Carbon Nanotubes

Laser-based imaging and characterization of individual carbon nanotubes provides a number of significant advantages over other imaging techniques, including its high throughput, spectral characterization capability, and relatively simple sample preparation. We recently reported a novel on-chip Rayleigh imaging technique using widefield laser illumination to measure optical scattering from individual single-walled carbon nanotubes (SWNTs) on a solid substrate with high spatial and spectral resolution. This method accurately measures the resonance energies and diameters for a large number of SWNTs in parallel. This technique can be used for fast mapping of key SWNT parameters, including the electronic-types and chiral indices for individual SWNTs, position and frequency of chirality-changing events, and intertube interactions in both bundled and distant SWNTs. Further Rayleigh characterization showed that SWNTs can form ideal optical wires. Interestingly, 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 and other intrinsic properties. Moreover, the nanotubes display a uniform peak optical conductivity, suggesting universal behavior similar to that observed in nanotube conductance. Finally, two other high-throughput optical imaging techniques, widefield Raman imaging and confocal absorption microscopy, and their applications in nanotube imaging will be discussed.   

Plasmon Nanooptics with Pristine and Hybrid Nanotube Systems

In general, plasmons cannot be excited by light in optical absorption since they are longitudinal excitations while photons are transverse. In small-diameter ($\sim $1 nm) semiconducting carbon nanotubes (CNs), light polarized along the CN axis excites excitons which, in turn, can couple to the nearest (same-band) interband plasmons [1,2]. Both of these collective excitations originate from the same electronic transitions and, therefore, occur at the same (low) energies $\sim $1 eV, as opposed to bulk semiconductors where they are separated by tens of eVs. They do have different physical nature and properties. Their coexistence at the same energies in CNs is a unique feature of confined quasi-1D systems where the transverse electronic motion is quantized to form 1D bands and the longitudinal one is continuous. We discuss how low-energy interband plasmon excitations can efficiently mediate enhanced electromagnetic absorption in pristine semiconducting CNs and bipartite entanglement in hybrid metallic CN systems. We develop a theory for (non-linear) optical monitoring and control of the phenomena above.\\[4pt] [1] I.V.Bondarev, JCTN7, 1673(2010).\\[0pt] [2] I.V.Bondarev, L.M. Woods, and K. Tatur, PRB80, 085407(2009).   

Photoluminescence from suspended individual $^{13}$C-enriched nanotubes

We investigate isotope effects on the electronic structure of single-walled carbon nanotubes by photoluminescence microscopy. In order to suspend nanotubes for luminescence measurements, trenches are formed on SiO$_2$/Si substrates by electron beam lithography and dry etching processes. No-flow chemical vapor deposition is used to grow carbon nanotubes with small amounts of isotopically enriched ethanol with 99\% $^{13}$C. Optical measurements are done in air at room temperature using a laser scanning confocal microscope with a wavelength tunable Ti:sapphire laser as an excitation source. We have successfully identified suspended $^{13}$C-enriched nanotubes by photoluminescence imaging and assigned their chirality with excitation spectroscopy.   

Circular dichroism in air-suspended single-walled carbon nanotubes

Chiral carbon nanotubes lack inversion symmetry and are expected to show circular dichroism. We have investigated the helicity dependence of absorption in air-suspended single-walled carbon nanotubes by using photoluminescence intensity for detection. Patterned chemical vapor deposition is used to grow carbon nanotubes over trenches etched on SiO$_{2}$/Si substrates. We identified suspended carbon nanotubes by taking photoluminescence images with a home-built laser-scanning confocal microscope and characterized them by excitation spectroscopy. A quarter-wave plate is inserted in the excitation laser path, and the photoluminescence intensity is measured as a function of the angle of the wave plate. We observe differences in the luminescence intensity between left- and right-circularly polarized light.   

Visible fluorescence from 5-Angstrom single-wall carbon nanotubes

We report the observation of visible fluorescence from the ultrathin single-wall carbon nanotubes (SWCNTs) with diameters of less than 5-Angstrom. Such ultrathin nanotubes were prepared by extracting the inner shells of double-wall carbon nanotubes using ultrasonication [1]. The extracted sample shows two visible photoluminescence (PL) peaks at 700 and 720 nm under light excitation at 410 and 540 nm, respectively. These peaks can be assigned, respectively, as the PL of (4,3) and (5,3) SWCNTs by comparison with the experimental Kataura plot proposed by Weisman et al. [2]. The present findings provide an important insight for the studies of the structural stability and electric structure of ultrathin SWCNTs. [1] Y. Miyata et al. ACS Nano. 4, 5807 (2010), [2] R. Weisman et. al., Nano Lett. 3, 1235 (2003).   

Symmetry-breaking of carbon nanotubes vibrational modes induced by transversal deformations

In this work we combine an atomic force microscope (AFM) with a setup to perform confocal Raman spectroscopy to follow, \textit{in situ}, the evolution of the G-band feature of a Single Wall Carbon Nanotube (SWNT) with transversal pressure applied to the nanotube via the AFM probe. We observe a previously elusive and fundamental symmetry-breaking effect of the totally symmetric tangential optical TO modes in the G-band feature which exhibits two distinct Raman active modes with an anomalous frequency behavior with increasing applied pressure, while the totally symmetric longitudinal optical LO component remains unaltered. We propose a simple analytical model based on a mass-spring ring system, which satisfactory explains the main observed effects and shows that the pressure effects change with tube flattening.   

Non-Radiative Exciton Decay in Single-Walled Carbon Nanotubes

We report on the exciton dynamics for an ensemble of individual, suspended (6,5) single-walled carbon nanotubes via single color $E_{22}$ pump-probe spectroscopy for a wide range of pump fluences. The calculated initial exciton population ranges from $\sim$ 5 to 120 excitons per $\sim$ 725 nm long nanotube, putting the high fluence experiment well into the nonlinear regime. The pump-probe data is not well described by multi-exponential decay or by power law behavior for all fluences. We have developed a single model that describes all data, ranging over two decades of pump fluence and three decades of delay times. The signal decay at low fluence is dominated by a stretched exponential that is consistent with the distribution of relaxation rates resulting from diffusion-limited contact quenching for a nanotube ensemble. The change in dynamics as a function of increasing pump intensity is attributed to exciton-exciton Auger de-excitation in the $E_{11}$ subband and, to a lesser extent, in the $E_{22}$ subband. The initial sub-picosecond decay of the observed response is attributed to $E_{22}$ excitons rapidly acquiring non-zero momentum while remaining in the $E_{22}$ subband.   

Fine structure of exciton levels in carbon nanotubes: A semianalytical approach

We propose a new approach [1] toward excitons in carbon nanotubes whereby the matrix elements of the electron-hole Coulomb interaction are expanded into a series over the nanotube's one-dimensional reciprocal lattice vectors. We show that only a few terms of this expansion give a non-vanishing contribution to the Coulomb matrix elements. The proposed approach allows one to single out Fourier components of the Coulomb potential responsible for the intervalley coupling and formation of the exciton fine structure for each particular nanotube chirality. \\[4pt] [1] S.V. Goupalov, Phys. Rev. B 84, 125407 (2011).   

Dynamical crossovers, universality and long-range interactions of excitons on carbon nanotubes

Simple microscopic interactions in non-equilibrium systems give rise to complex emergent macroscopic phenomena. There has been much theoretical work to understand dynamics of different systems, and equilibrium concepts of scaling, criticality and universality have proved useful. However there is a noted lack of experimental studies. Here we show that exciton reactions on carbon nanotubes display the rich kinetics of the prototypical 1D coalescence reaction A+A-$>$A. An Auger-like exciton interaction\footnote{Y.-Z. Maet al, Phys Rev Lett 94, 157402 (2005).} and anomalous kinetics\footnote{R. M. Russo et al. Phys Rev B 74, 041405 (2006).} have already been reported. Here we demonstrate the existence of four distinct dynamical regimes: (1) early dynamics determined by spatial ordering of excitons due to Pauli repulsion at high concentrations; (2) a classical mean-field region with exciton population n decaying as $t^{-1}$; (3) a self-organized critical state with anomalous reaction kinetics limited by diffusion and characterized by $n\sim t^{-1/2}$, which we show to be universal with respect to the initial population; and (4) an exponential approach towards an absorbing state corresponding to one exciton per nanotube. The abrupt crossover between regimes indicates a long-range exciton interaction, which introduces a non-scaling dimension that breaks universality at intermediate length-scales.   

Exciton transport and exchange self-energy in semiconducting carbon nanotubes

We present direct measurements of $S_1$ exciton transport in (6,5) carbon nanotubes. Exciton diffusion lengths associated with end quenching, photoluminescence lifetimes, and homogeneous emission linewidths provide a basis for determining an intrinsic diffusion constant of 5 cm$^2$s$^{-1}$ within the dispersion of light. Exciton diffusion is modeled in terms of an anomalous dispersion within a marginal Fermi liquid description of the exciton exchange self-energy and acoustic phonon scattering.   

Quantum Interference Between the Third and Fourth Excitonic States in Semiconducting Carbon Nanotubes

We exploit an energy level cross-over effect\footnote{H\'aroz,~E.~H. \emph{et al.}; \emph{Phys. Rev. B.} \textbf{2008}, \emph{77}, 125405} to probe quantum interference in the resonance Raman response from carbon nanotube samples highly enriched in the single semiconducting chiralities of (8,6), (9,4), and (10,5). UV Raman excitation profiles of G-band spectra reveal unambiguous signatures of interference between the third and fourth excitonic states (E33 and E44). Both constructive and destructive responses are observed and lead to anomalous intensity ratios in the LO and TO modes. Especially large anomalies for the (10,5) structure result from nearly identical energies found for the two Eii transitions. The interference patterns demonstrate that the sign of the exciton-phonon coupling matrix elements changes for the LO mode between the two electronic states, and remains the same for the TO mode. Significant non-Condon contributions to the Raman response are also found.\footnote{Duque,~J.~G. \emph{et al.}; \emph{ACS Nano} \textbf{2011}, \emph{5}, 5233--41}   

New Fundamental Optical Behaviors of Single-Wall Carbon Nanotubes at Cryogenic Temperatures: Closer to their Intrinsic Behavior

Development of single walled carbon nanotube (SWNT) materials for optoelectronics and nanophotonics has been especially challenging in that SWNT optical properties are highly sensitive to environmental interactions, which can be particularly severe in composite matrices. Here, we present for the first time an innovative approach to obtain highly photoluminescent (PL) solid-state SWNT-nanocomposites, which provides access to novel photophysical properties. Strongly blue-shifted spectral features, and significant increase ($\sim $ 3x) in PL intensities at croyogenic temp in comparison to room tem or previous reports. This difference can be understood as arising from a significantly slower relaxation of excitons from bright to dark states in our SWNTs, as a result of much weaker interaction with the environment. That is, the bright/dark exciton distribution is highly non-thermal, even at the lowest temperatures. In our SWNT-nanocomposites, environmental interactions are minimized, thus bright excitons \textit{cannot} relax efficiently to the dark state, causing a highly non-equilibrium exciton distribution and a correspondingly large PL intensity, even at low temperatures.   

Coherent anti-stokes spectroscopy as a probe of chemical disorder in isolated carbon nanotubes

We use a third-order coherent anti-stokes (CAS) optical technique to study chemical disorder in individual carbon nanotubes. The CAS response is highly sensitive to this disorder, to the extent that a few chemical defects can appreciably decrease the overall signal. The experiments are performed on individual single- and multi-walled carbon nanotubes (SWNTs and MWNTs) connected in a transistor geometry and subjected to varying degrees of controlled, electrochemical oxidation. The overall CAS intensity can be used to probe the extent of chemical modification, and inhomogeneities along a nanotube resolve local coherent electron density fluctuations. We find that the CAS signal is also strongly affected by substrate interactions: aligned SWNTs grown on single crystal quartz are quenched compared to SWNTs on fused quartz. Finally, CAS spectroscopy on individual SWNTs and MWNTs using picosecond pulses resolves the third-order vibrational signal component at the G-band frequency. The ratio of electronic to vibrational CAS signal components is diameter dependent and in small diameter SWNTs the vibrational component is dominated by the electronic CAS signal. However, in MWNTs, this technique is a first step toward chemically sensitive CARS imaging on a single nanotube level.