Session F27: Carbon Nanotube & Related Materials: Optical & Other Properties

Terahertz Science and Technology of Macroscopically Aligned Carbon Nanotube Films

One of the outstanding challenges in nanotechnology is how to assemble individual nano-objects into macroscopic architectures while preserving their extraordinary properties. For example, the one-dimensional character of electrons in individual carbon nanotubes leads to extremely anisotropic transport, optical, and magnetic phenomena, but their macroscopic manifestations have been limited. Here, we describe methods for preparing macroscopic films, sheets, and fibers of highly aligned carbon nanotubes and their applications to basic and applied terahertz studies. Sufficiently thick films act as ideal terahertz polarizers, and appropriately doped films operate as polarization-sensitive, flexible, powerless, and ultra-broadband detectors. Together with recently developed chirality enrichment methods, these developments will ultimately allow us to study dynamic conductivities of interacting one-dimensional electrons in macroscopic single crystals of single-chirality single-wall carbon nanotubes.   

Photocurrent generation efficiency of a carbon nanotube pn junction

Carrier multiplication effects can enhance the quantum efficiency of photovoltaic devices. For example, quantum dot solar cells have demonstrated photon-to-electron conversion efficiencies greater than 100{\%} when photon energies exceed twice the band gap. Carbon nanotube photodiodes exhibit carrier multiplication effects (Gabor et al, Science 2009), but the quantum efficiency of such photodiodes has not previously been characterized. We have reproduced the carrier multiplication phenomena in individual CNT pn junctions and investigated the conditions under which it occurs. We will present early results quantifying the internal quantum efficiency of the process.   

Excitonic Contribution to Near-Field Enhancements of a Carbon Nanotube Antenna

Complexes containing rare earth ions (REI) and single-walled carbon nanotubes (SWNT) show promise to be utilized for the optical sensing of biomolecules. We theoretically study the near-field electromagnetic effects in such a system using a propagating polariton model for the SWNT antenna. To be in resonance with the REI transitions that are in the NIR/optical range, excitonic transitions in the SWNT must be included in the model. We calculate measurable field enhancements in the vicinity of the SWNT antenna that lead to an increase of the REI photoluminescence.   

Electroluminescence from individual air-suspended carbon nanotubes within split-gate structures

Electrically induced light emission from chirality-identified single-walled carbon nanotubes are investigated by utilizing split-gate field-effect devices fabricated on silicon-on-insulator substrates. We begin by etching trenches through the top silicon layer into the buried oxide, and the silicon layer is thermally oxidized for use as local gates~[1]. We partially remove the oxide and form gate electrodes, then contacts for nanotubes are deposited on both sides of the trench. Catalyst particles are placed on the contacts, and nanotubes are grown over the trench by chemical vapor deposition. We use photoluminescence microscopy to locate the nanotubes and perform excitation spectroscopy to identify their chirality. Gate-induced photoluminescence quenching is used to confirm carrier doping~[2], and electroluminescence intensity is investigated as a function of the split-gate and bias voltages. \\[4pt][1] M. Jiang, Y. Kumamoto, A. Ishii, M. Yoshida, T. Shimada, Y. K. Kato, Nature Commun. 6, 6335 (2015). \\[0pt][2] S. Yasukochi, T. Murai, S. Moritsubo, T. Shimada, S. Chiashi, S. Maruyama, Y. K. Kato, Phys. Rev. B 84, 121409(R) (2011).   

Theory of quadruple plasmon in doped carbon nanotubes

A single-wall carbon nanotube possesses two different types of plasmons specified by wavenumbers in the azimuthal and axial directions. In this presentation we show that the azimuthal plasmons consist of underdamped oscillations forming electric dipoles inside a nanotube and overdamped oscillations forming magnetic dipoles. These, originating from the surface plasmons of graphene, are of prime importance in the optical properties of doped "metallic" tubes, such as depolarization effect and relaxation of photo-excited carriers. The axial plasmons also consist of underdamped and overdamped oscillations which are inherent in the cylindrical waveguide-structures of nanotubes and relevant to optics and transport. We discuss the exact configurations of the electromagnetic fields in connection with Aharonov-Bohm effect and point out a possibility of the generation of transient energy band gaps in metallic nanotubes.   

Electrical Activation of Dark Excitonic States in Carbon Nanotubes

Electrical activation of optical transitions to parity-forbidden dark excitonic states in individual carbon nanotubes is reported. We examine electric field effects on various excitonic states by simultaneously measuring both photocurrent and photoluminescence. As the applied field increases, we observe an emergence of new absorption peaks in the excitation spectra. From the diameter dependence of the energy separation between the new peaks and the ground state of $E_{11}$ excitons, we attribute the peaks to the dark excited states which became optically active due to the applied field. A simple field-induced exciton dissociation model is introduced to explain the photocurrent threshold fields, and the edge of the $E_{11}$ continuum states have been identified using this model.   

Multi-Excitonic Emission from Solitary Dopant States of Carbon Nanotubes.

Oxygen doping of single wall carbon nanotubes (SWCNTs) has been rapidly emerging as an effective mean for introduction of new functionalities. Recently, through demonstration of fluctuation free, room temperature single photon generation$^{1}$, we established these states as a new type of solid-state two level atom with potentials in quantum information technologies. This study further showed that while some doped tubes were characterized with a near complete photon antibunching, significant numbers of doped tubes exhibit some degree of photon bunching indicating that they emit more than one photon in one excitation cycle. Here in this work, by separating slow and fast photons in the time domain, we show for the first time that the multiple photon emissions originated from higher order multi-exciton states of solitary dopants. We also show that such multi-exciton states can allow emission of photon pairs with efficiency as high as 20-30{\%} of single exciton emission. With this work, we bring out multi-excitonic processes of the solitary dopant states as a new area to be explored for potential applications in lasing, entangled photon generation and carrier multiplication. $^{1}$ X. Ma et al., Nature Nanotech. \textbf{10}, 671 (2015)   

Gate-voltage induced trions in suspended carbon nanotubes

We observe trion emission from suspended carbon nanotubes where carriers are introduced electrostatically using field-effect transistor structures [1]. The trion peak emerges below the $E_{11}$ emission energy at gate voltages that coincide with the onset of bright exciton quenching. By investigating nanotubes with various chiralities, we verify that the energy separation between the bright exciton peak and the trion peak becomes smaller for larger diameter tubes. Trion binding energies that are significantly larger compared to surfactant-wrapped carbon nanotubes are obtained, and the difference is attributed to the reduced dielectric screening in suspended tubes. \\[4pt][1] M. Yoshida, A. Popert, and Y. K. Kato, arXiv:1510.08996.   

New Feature Observed in the Raman Resonance Excitation Profiles of $(6,5)$-Enriched, Selectively Bundled SWCNTs

Understanding the photophysics of exciton behavior in single wall carbon nanotube (SWCNT) bundles remains important for opto-electronic device applications. We report resonance Raman spectroscopy (RRS) measurements on $(6,5)$-enriched SWCNTs, dispersed in aqueous solutions and separated using density gradient ultracentrifugation into fractions of increasing bundling. Near-IR to UV absorption spectroscopy shows a redshift and broadening of the main excitonic transitions with increasing bundling. A continuously tunable dye laser coupled to a triple-grating spectrometer affords measurement of Raman resonance excitation profiles (REPs) over a range of wavelengths covering the $(6,5)$-$E_{22}$ range (505 to 585)\,nm. REPs of both the radial breathing mode (RBM) and G-band reveal a redshifting and broadening of the $(6,5)\ E_{22}$ transition energy with increasing bundling. Additionally, we observe an unexpected peak in the REP of bundled SWCNTs, which is shifted lower in energy than the main $E_{22}$ and is anomalously narrow. We compare these observations to a theoretical model that examines the origin of this peak in relation to bundle polarization-enhanced exciton response.   

Simulations of resonant Raman response in bundles of semiconductor carbon nanotubes.

This work is motivated by an experimental study of resonant Raman spectroscopy under E22 excitation, which shows a new, sharp feature associated with bundling in (6,5) semiconductor carbon nanotubes. In order to provide an insight into the experimental data, we model Raman excitation spectra using our modified discrete dipole approximation (DDA) method. The calculations account for the exciton states polarized along and across the nanotube axis that are characterized by a small energy splitting. Strong polarization of the nanotubes forming the bundle results in the exciton state mixing whose spectroscopic signatures such as peaks positions, line widths, and depolarization ratio are calculated and compared to the experiment. Furthermore, the effects of the energy and structural disorder, as well as structural defects within the bundle are also examined and compared with the experimental data.   

Improvement of polypyrrole nanowire devices by plasmonic space charge generation: high photocurrent and wide spectral response by Ag nanoparticle decoration

In this study, improvement of the opto-electronic properties of non-single crystallized nanowire devices with space charges generated by localized surface plasmon resonance (LSPR) is demonstrated. The photocurrent and spectral response of single polypyrrole (PPy) nanowire (NW) devices are increased by electrostatically attached Ag nanoparticles (Ag NPs). The photocurrent density is remarkably improved, up to 25.3 times, by the Ag NP decoration onto the PPy NW (PPy$_{\mathrm{AgNPs}}$ NW) under blue light illumination. In addition, the PPy$_{\mathrm{AgNPs}}$ NW shows a photocurrent decay time twice that of PPy NW, as well as an improved spectral response of the photocurrent. The improved photocurrent efficiency, decay time, and spectral response resulted from the space charges generated by the LSPR of Ag NPs. Furthermore, the increasing exponent ($m)$ of the photocurrent ($J_{\mathrm{PC}}\sim V_{m})$ and finite-differential time domain (FDTD) simulation straightforwardly indicate relatively large plasmonic space charge generation.   

Optical Properties of waste derived carbon dots

Carbon dots (CDs) have been extensively examined recently, mainly due to their luminescence and excitation wavelength dependent emission behavior. These dots can be derived from a variety of carbonaceous sources. Some of the possible sources are carbonaceous waste materials. Although it is possible to synthesize CDs using waste and their applications in light source, few steps such as to purification of starting material and removal of other impurities during solvothermal processing can enhance the performance of CDs and associated devices. Our primary results suggest that carbonaceous waste in liquid form is easy to process. In contrast the solid carbonaceous wastes are relatively difficult to process, but their availability is higher. In this regard, a detailed study has been performed to formulate the appropriate processing parameters for best performing CDs.   

C$_{8}$-structured carbon quantum dots: Synthesis, blue and green double luminescence, and origins of surface defects

Carbon quantum dots (CQDs) have attracted great attention in the past few years due to their low cytotoxicity, exploited various synthesis methods, unexampled abundance of raw materials on earth, and robust near-infrared to near-UV luminescence. Carbon nanoparticles have applications in biological labeling, delivery of drugs and biological molecules into cells, and light emitting diodes and lasing. CQDs generally exist as nanodiamonds or graphite quantum dots according to previous research reports. In this study, we report the first synthesis of the third-allotrope CQDs through carbonization of sucrose and study their luminescence properties. These CQDs have a body-centered cubic structure and each lattice point is composed of eight atoms which form a sub-cube (so called C$_{8}$ crystal structure). High-resolution transmission electron microscopy and X-ray diffraction confirm the C$_{8}$ structure of the synthesized carbon nanocrystallites with an average size of 2 nm. The C$_{8}$ CQDs exhibit double-band luminescence with two peaks centered at around 432 and 520 nm. The study based on the photoluminescence, UV–Vis absorption, Fourier-transform infrared, and X-ray photoelectron spectroscopies reveals that the green emission originates from the C=O related surface defect.