Session V36: Focus Session: Optical Properties of Carbon Nanotubes and C60

Spectroscopy of Optical Excitations in Carbon Nanotubes

Understanding the optical spectra and electronic excited state dynamics of carbon naotubes is important both for fundamental research and a wide variety of potential applications. In this presentation, we will report the results of a systematic study on semiconducting single-walled carbon nanotubes (SWNTs) obtained by utilizing complementary femtosecond spectroscopic techniques, including fluorescence up-conversion, frequency-resolved transient absorption, and three-pulse photon echo peakshift (3PEPS) spectroscopy. Our efforts have focused on optically selective detection of the spectra and dynamics associated with structurally distinct semiconducting SWNT species. Using individual nanotube enriched micelle-dispersed SWNT preparations, in combination with resonant excitation and detection, has enabled us to independently access selected species, such as the (8,3), (6,5), (7,5), (11,0), (7,6) and (9,5) nanotubes. We will discuss the following topics: (1) the excitonic nature of the elementary excitation and its unambiguous identification from direct determination of the exciton binding energy for a selected semiconducting nanotube, the (8,3) tube; (2) the spectroscopic and dynamical signatures of exciton-exciton annihilation and its predominant role in governing ultrafast excited state relaxation; (3) the annihilation-concomitant exciton dissociation and the spectroscopic and dynamic features of the resulting electron-hole continuum; (4) timescales characterizing the ultrafast thermalization processes. In addition, we will demonstrate the power of 3PEPS spectroscopy to elucidate the spectral properties and dynamics of SWNTs. This work was supported by the NSF.   

Real time approach for non-linear optical response of nano-scale systems

We present a real-time, time-dependent density-functional approach for the calculation of the frequency-dependent linear and non-linear optical response, which is based on the approach of Tsolakidis et al.\footnote{A. Tsolakidis, D. Sanchez-Portal and R.M. Martin, Phys. Rev. B {\bf66}, 235416 (2002).} Tensor components of linear polarizabilities and first order hyper-polarizabilities are extracted by fitting net time dependent polarizations with different electric field strengths. The method is computationally efficient and can be applied to large, molecular and nano-scale systems. Results are presented for C$_{60}$ and for a number of ``push-pull" molecules. Our results for the static limit are in good agreement with other density-functional calculations.   

First-Principles study of the optical properties of BN nanotubes and h-BN

We present first-principles calculations of the effects of quasiparticle self-energy and electron-hole interaction on the optical properties of single-walled BN nanotubes (both zigzag and armchair tubes) and bulk h-BN. Excitonic effects are shown to be even more important in BN nanotubes than in carbon nanotubes, giving rise to excitons with binding energy larger than 2 eV for the zigzag (8,0) BN nanotubes. Moreover, unlike the carbon nanotubes, theory predicts that these exciton states are comprised of coherent supposition of transitions from several different subband pairs, giving rise to novel behaviors. Our calculatioins are in quantitative agreement with available experimental data. We also compare the optical properties of single-walled BN nanotubes with those of bulk h-BN. This work was supported by the NSF under Grant No. DMR04-39768, and the U.S. DOE under Contract No. DE-AC03-76SF00098. Computer time was provided by NERSC and NPACI.   

{\em Ab initio} Studies of Potassium Adsorption on Graphite and Carbon Nanotubes

We present an {\it ab initio} study of a single potassium atom adsorption on the surface of graphite and single walled carbon nanotubes using density functional theory within the pseudopotential approximation. Our study is, in part, inspired by inconsistent results reported for this system in the existing literature. The potassium adsorption energy on graphite obtained in different calculations ranges from approximately 0.4 eV to 1.75 eV. Our calculations demonstrate that the reported disagreements can be explained by electrostatic interactions rather than complex quantum factors. We illustrate this with a simple model based on a classical electrostatic interaction between the potassium atom and the graphite surface. Adsorption energies predicted by the electrostatic model are in good agreement with our {\it ab initio} calculations. In contrast, we find that the adsorption energies of potassium on carbon nanotubes are directly related to the nanotube geometry and chirality and cannot be described in terms of a simple classical model.   

Bound excitons and optical absorption spectra of (10,10) metallic single-walled carbon nanotubes

Motivated by recent theoretical prediction of bound excitons [1] in small diameter metallic single-walled carbon nanotubes, we study the optical spectra of the larger diameter (10,10) metallic tube. We use an interacting-particle Green’s function approach which includes calculations of the quasiparticle excitation spectrum (within the GW approximation for the electron self energy) and the electron-hole excitation spectrum (within the Bethe-Salpeter formalism). We show that the (10,10) tube has important excitonic effects despite being a metal, due to the quasi-one dimensional nature of the carbon nanotubes. A bound exciton with binding energy of ~60 meV is found, and the location of the excitonic peak in the optical spectrum is at 1.8 eV. [1] C. D. Spataru, S. Ismail-Beigi, L. X. Benedict, and S. G. Louie, Phys. Rev. Lett. 92, 077402 (2004).   

Photocurrent Generation in Bent Nanostructures

We study theoretically the effect of circularly-polarized electro-magnetic radiation on electrons confined in quantum rings and in bent ballistic quantum wires. The radiation couples to clockwise- or counterclockwise-propagating charge excitations in regions of the system with non-zero curvature, depending on the light polarization. This provides a transfer of the angular momentum from the radiation to the electrons. Response of the electron system to the external radiation displays a resonant behavior and can be measured through a current-induced magnetic field in quantum rings or using a standard current measurement technique in quantum wires.   

Ultrafast Carrier Relaxation Measurements of SWCNT-Doped Polymer Thin Films

The primary application of interest of single-walled carbon nanotube (SWCNT) -doped polymer thin films is to serve as a replacement of gold active regions on integrated optic surface enhanced biosensors and as potential transparent conducting polymers. In addition, the ultrafast nonlinear optical switching properties are of particular interest. We report femtosecond time-resolved measurements on SWCNT -doped polymer thin films. These films were made by spin-coating the monomer-SWCNTs suspension onto glass substrates and UV curing to initiate polymerization. The SWCNTs are predominately semiconducting. The thin film contains $\sim $ 0.4 wt {\%} of SWCNTs, with an average thickness of 7 $\mu $m. Non-degenerate pump-probe transmission experiments were performed using $\lambda _{pump }$at 400 nm and white light continuum as the probe beam generated by a modelocked Ti:Sapphire laser ($\tau _{p }$=160 fs, rep. rate 250 kHz). Preliminary results indicated two lifetimes: the fast decay of 1.4 ps and a longer relaxation time of 18 ps. Experiments are underway to study the carrier dynamics and determine the magnitude of the nonlinearity.   

Carbon nanotube field effect transistors under high magnetic fields

Magnetic field, when applied parallel to the CNT axis, alters the energy gap in the CNT electron spectrum with a period corresponding to one quantum of magnetic flux through the cross-section of the CNT. With available magnetic fields (10$^{1}$T by the order of magnitude), gap oscillations can be observed only in multi-wall CNTs with diameters larger than approx 5 nm, where effects of band structure variation are smeared out by defects and by quantum interference effects. As follows from [1], it is possible to separate effects of disorder from those of the band structure modification by studying magnetotransport in small diameter CNTs while controllably changing the position of the Fermi level of the CNT by electrostatic doping, i.e. by applying a gate voltage in the field effect transistor configuration. We have studied several samples of individual CTNs contacted by palladium electrodes placed on an oxidized heavily doped silicon substrate that served as a back-gate. We find that magnetoresistance of a CNT strongly depends on the Fermi level position with respect to the nanotube's charge neutrality point (CNP). Details and the analysis of our experimental data will be presented. [1] S. Roche, R. Saito, Phys. Rev. Lett. 87, 246803 (2001)