Session X6: Carbon Nanotube Optics II: Absorption and Raman Spectroscopy

Measurement of Absolute Absorption Cross-section of Individual Carbon Nanotubes on a Substrate

The absorption cross-section is one of the central parameters that determine the efficiency of most optical and optoelectronics processes in single-walled carbon nanotubes (SWNT), including photoluminescence, photodetection, and photovoltaic energy conversion. While absorption measurements on SWNT ensembles in solution provide a reliable estimate, the absolute absorption cross-secions of individual carbon nanotubes on and off resonance have not been reported. Here, we measure the absorption cross-section of SWNTs on chip with spatial resolution using lock-in technique with a spatial modulation of a focused laser spot near SWNTs. We measure the absorption cross-section of SWNTs near resonance to be on the order of 10$^{-17}$ cm$^2$ per carbon atom, which is consistent with on-chip Rayleigh measurements as well as recent time-resolved photoluminescence measurements. Since our measurement is performed on SWNTs on chip, it can be directly applied to various optoelectronic devices made with SWNTs, thus allowing quantitative analysis of the fundamental performance limits of SWNT photovoltaic devices.   

Measurement of Absorption Cross-section for Single-walled Carbon Nanotubes

Optical absorption is the most fundamental optical property. Quantitative knowledge of absorption cross-section provides valuable information on the material electronic structure, and is necessary for evaluating quantum efficiency of other important optical processes, such as photoluminescence and photocurrent generation. However, absorption measurement of individual single-walled carbon nanotubes(SWCNTs) is quite challenging. Here we used an efficient interferometric method to obtain the absorption cross-section of individual chirality-defined SWCNTs. We will discuss how the absorption cross-section varies in different carbon nanotubes.   

Enhancement of photoluminescence from single-walled carbon nanotubes by photonic crystal microcavities

Single-walled carbon nanotubes are bright nanoscale emitters, while photonic crystal microcavities offer the possibility for efficient optical coupling at the nanoscale because of their small mode volumes and high quality factors. Here we report on the enhancement of photoluminescence from single-walled carbon nanotubes by L3 cavities in hexagonal lattice photonic crystals. Free-standing photonic-crystal membranes are fabricated from silicon-on-insulator substrates, and micelle-encapsulated carbon nanotubes are dispersed on the devices. We observe sharp peaks with a typical spectral width of 0.4 nm which corresponds to a quality factor of $\sim$3000. As the peaks appear at wavelengths longer than those of Si photoluminescence, they are attributed to carbon nanotube emission coupled to the microcavity modes. We find that the photoluminescence intensity is enhanced by more than a factor of four compared to luminescence from an unpatterned area.   

Can we look into carbon nanotubes by infrared light?

Individual molecules filled into carbon nanotubes exhibit Raman activity but very weak, if any, infrared absorption. We will present infrared (transmission and ATR), Raman and transmission electron microscopy data of various filled nanotubes (sorted by diameter and metallicity; encapsulating organometallic, aromatic and fullerene-based molecules) to illustrate this puzzling behavior. In the infrared spectra of double-walled carbon nanotubes, however, vibrational signatures of the inner and outer tubes are clearly discernible. A strong proof for this assignment is the shift of the inner-tube modes with $^{13}$C isotope content in samples where the inner tube is enriched with $^{13}$C.   

Intersubband Edge Singularity in Metallic Nanotubes

Tunneling density of states of both the massless and massive (gapped) particles in metallic carbon nanotubes is known to have anomalous energy dependence. This is the result of coupling to multiple low-energy bosonic excitation (plasmons). For both kinds of particles the ensuing effect is the suppression of the density of states by electron-electron interactions. We demonstrate that the optical absorption between gapless and gapped states is affected by the many-body effects in the opposite way. The absorption probability is enhanced compared with the non-interacting value and develops a power-law frequency dependence, with the -0.2 exponent for typical nanotubes.   

Indication of Long Lived States in Decay Associated Spectra of Single-Walled Carbon Nanotubes

The direct bandgap nature of Single-Walled Carbon Nanotubes(SWCNTs) along with the quantized energy level structure due to reduced dimensionality makes them useful elements in chip-based photonic devices for sensing and communications. However, their fundamental linear and nonlinear optical properties remain poorly understood. Using various nonlinear optical spectroscopy techniques with micelle encapsulated SWCNTs, we have measured carrier dynamics at both the picosecond, and tens of nanosecond timescale. We measure a fast 20ps timescale decay that agrees well with the lifetime of the lowest excited state measured before for such samples but surprisingly we also obtain an optical double-resonance signal on the slow timescale (~10nsec). Such slow timescale signals due to artifacts such as thermal effects have been ruled out. Decay associated spectra shows striking differences between the spectral lineshapes arising from the fast and slow components of the nonlinear optical signal which might indicate the creation of a long-lived state by the pump pulse that changes the subsequent probe spectra. This indicates the possibility of the presence of either a trap state or possibly a more complex energy level structure for the SWCNT involving the presence of a metastable state.   

Asymmetric resonance Raman excitation profiles and violation of the Condon approximation in single-wall carbon nanotubes

DNA wrapping-based ion exchange chromatography and density gradient ultracentrifugation provide nanotube samples highly enriched in single chiralities. We present resonance Raman excitation profiles for the G-band of several single chirality semiconducting and metallic species. The expected incoming and outgoing resonance peaks are observed in the profiles, but contrary to long-held assumptions, the outgoing resonance is always significantly weaker than the ingoing resonance peak. This strong asymmetry in the profiles arises from a violation of the Condon approximation [1]. Results will be discussed in the context of theoretical models that suggest significant coordinate dependence in the transition dipole (non-Condon effects). The generality of the behavior across semiconducting and metallic types, nanotube family, phonon mode, and Eii will be demonstrated. \\[4pt] [1] J. Duque et. al., ACS Nano, 5, 5233 (2011).   

Raman Spectroscopy characterization of individual triple-walled carbon nanotubes

The characterization of individual triple-walled carbon nanotubes (TWCNT) was studied in detail by using Raman spectroscopy resonant signals taken with various laser excitation energies. TWCNTs are in fact an assembly of three concentric weakly coupled single-walled carbon nanotubes (SWCNTs) or, equivalently, a double-walled carbon nanotube (DWCNT) concentric to an external SWCNT. An isolated TWCNT consists of an inner, middle and outer tube, each of which can be either metallic or semiconducting. All of the eight possible combinations provide a multitude of information about the electrical and optical properties. Among numerous applications, TWCNTs offer an ideal structure to study and understand how an interacting medium influences the properties of both, SWCNTs and DWCNTs structures. The measured spectra show exceptionally distinctive radial breathing modes, G-, G'-band and further modes of the three concentric tubes. All of the Raman three bands described above are very sensitive to changes in the structure of each tube. By following the spectral changes of these frequency modes, we can extract important information about, for example, the respective tube distances, inter-tube interaction in TWCNTs systems and its consequences on their related SWCNTs and DWCNTs counterparts.   

Resonance Raman Spectroscopy of Separated Single-Wall Carbon Nanotube

The heterogeneity of single-wall carbon nanotubes (SWCNTs) produced by typical techniques complicates characterization and presents a barrier for technological applications. Improvements in separation and purification techniques enable detailed studies of specific nanotube properties by providing samples of unique chirality, length, metallicity, bundling, and interior filling. We report resonant Raman spectroscopy (RRS) measurements on these samples over a wide range of excitation wavelengths using a series of discrete and continuously tunable laser sources coupled to a triple-grating spectrometer. RRS of these homogeneous samples reveals unique spectral features and affords interpretation of intrinsic nanotube optical properties. Of particular interest are the G-band of chirally-pure armchair metallic SWCNTS and shifts of the radial breathing mode and excitation energy with water filling. Additionally, we will compare Raman results with other optical characterization techniques.   

Optical properties of single-walled carbon nanotube aerogels

A network of connected single-walled carbon nanotubes (SWNT) is created by a novel DNA-protein complex directed assembly. Due to a point-like nature of connectors, the SWNT aerogel represents a network of self-suspended nanotubes with a record ultra-low density of less 0.75 mg/cm$^{3}$. The assembly method and low density enables a direct comparison of optical properties of nanotubes in solvent and air to surfactant solubilized nanotubes. Optical properties of SWNT gels are investigated using optical absorption, photoluminescence and Raman spectroscopy. Gelled nanotubes in water and in the low population regime behave similar to solubilized nanotubes. In contrast, photoluminescence of SWNT aerogels exhibit nonlinear effects and a phonon-induced broadening. In addition, aerogels show a previously unobserved photoluminescence peak at 1.3 eV that corresponds to a phonon-assisted recombination of photoexcited charges. Raman spectra of carbon nanotube aerogels display narrow peaks due to the phonon decoupling of suspended SWNTs in air and a redistribution of G phonon population due to nonlinear effects.   

Coupled Radial Breathing Oscillation in Double-Walled Carbon Nanotubes

Double-walled carbon nanotubes (DWNTs) provide a model system for quantitative study of electronic and vibrational couplings in the nanoscale. Here we investigate coupled radial breathing mode (RBM) oscillations in structurally defined DWNTs by combining electron diffraction, Rayleigh scattering, and Raman scattering spectroscopy on the same individual nanotubes. We find that the two RBM oscillations in DWNTs are strongly coupled, with vibration energies significantly higher than those of constituent inner- and outer-wall nanotubes. In addition, the oscillation strength of these two coupled modes shows an interesting quantum interference behavior between the inner- and outer-wall electronic resonance channels.   

Observation of Optical Phonon Emission Threshold in the Current-Voltage Characteristics of Suspended Carbon Nanotubes

Electrically-heated suspended, nearly defect-free, carbon nanotubes (CNTs) exhibiting negative differential conductance in the high bias regime experience a sudden drop in current (or ``kink''). The bias voltage at the kink ($V_{kink})$ is found to depend strongly on the applied gate voltage, substrate temperature, pressure, and gas environment. After subtracting the voltage drop across the contacts, however, the kink bias voltages converge around 0.2V, independent of gate voltage and gas environment. Due to the ballistic nature of these defect free carbon nanotubes, this bias voltage of 0.2V corresponds to the threshold energy of optical phonon emission. This phenomenon is corroborated by simultaneously monitoring the Raman spectra of these nanotubes as a function of bias voltage. At the kink bias voltage, the $G $band Raman modes experience a sudden downshift, further indicating threshold optical phonon emission. A Landauer model is used to fit these kinks in various gas environments where the kink is modeled as a sudden change in the optical phonon lifetime, which corresponds to a change in the non-equilibrium factor that describes the existence of hot phonons in the system.   

The Electronic and Vibrational Properties of Carbon Nanorings

[n]-Cycloparaphenylenes ([n]-CPPs), carbon nanorings with n benzene units reminiscent of a single unit of a (n, n) armchair carbon nanotubes, show unusual optical behaviors: the absorption maximum is almost size independent at 340 nm, while the lower energy fluorescence exhibits multiple peaks with strong size dependence, opposite of quantum confinement behavior [1]. The energy levels for CPPs with various sizes are calculated using time-dependent density functional theory (TDDFT) and compared to the experimental results. Moreover, we calculate the Raman modes of various [n]-CPPs and propose that the multiple peaks observed in the fluorescence spectra could be due to the electron-phonon coupling effects. We also consider possible excitonic effects in these carbon nanorings, and discuss their similarities as well as differences to carbon nanotubes. [1] T. Iwamoto, Y. Watanabe, Y. Sakamoto, T. Suzuki, S. Yamago. \textit{J. Am. Chem. Soc.}, 133, 2011;