Session C31: Carbon Nanotubes and Related Materials: Physical and Chemical Propertes II

Temperature-Induced Density Control of CVD Grown Horizontally Aligned Single-Walled Carbon Nanotubes

We report an exponential dependence of the density of CVD-grown horizontally aligned single-walled carbon nanotube (SWNT) on the growth temperature. The SWNT arrays, which are grown on ST-cut quartz with Fe catalysts patterned 100 µm apart, have a density of 1/ µm at 885o C and demonstrate a density drop of one decade per 28oC thereafter. This enables a way to make parallelizable arrays of individual SWNT devices. In line with previous studies, We also find the SWNT diameter to increase with temperature, from 0.8 nm at 850oC to 1.8 nm at 920oC. Furthermore, we demonstrate that the SWNT diameter is correlated with the catalyst particle size, which increases with temperature.   

Ballistic and resonant negative photocurrents in single carbon nanotubes

We present ultrafast photocurrent experiments on semiconducting, single-walled carbon nanotubes under a resonant optical excitation of their subbands. We demonstrate that a ballistic transport of the photogenerated charge carriers can be achieved. Moreover, thermionic emission processes to the contacts dominate the photocurrent. In contrast, the charge current without laser excitation is well described by a Fowler-Nordheim tunneling. The time-averaged photocurrent changes polarity as soon as sufficient charge carriers are injected from the contacts, which can be explained by an effective population inversion in the optically pumped subbands.   

Near-field optical study of Luttinger liquid plasmons in single walled carbon nanotubes.

Quantum-confined electrons in one dimension (1D) behave as Luttinger liquid, which features charge spin separation and other intriguing properties starkly different from Fermi liquid. Single walled carbon nanotubes (SWNTs), with their extraordinary one-dimensional quantum confinement, provide the ideal platform to explore such Luttinger-liquid plasmons. We continued our efforts on using near-field optical microscopy to probe the Luttinger liquid plasmons in SWNTs. Our systematic study on plasmons in SWNTs provides more fundamental insight into the Luttinger liquid physics in 1D carbon nanotubes. This understanding is not only of fundamental interests, but is also important for nanophotonic and nanoplasmonic applications based on carbon nanotubes.   

Inversion of light polarization for absorption in doped carbon nanotubes

A carbon nanotube (CNT) exhibits absorption peaks of light with linear polarization parallel to the tube's axis, but not for the polarization perpendicular to it. We proposed that in a previous paper[1] the polarization dependence changes with charge doping; when a CNT is heavily doped, many of the absorption peaks of parallel polarized light disappear due to the Pauli exclusion principle, and this causes an absorption peak of a perpendicularly polarized light to appear in the near-infrared region, which accounts for experimental results. However, because this theoretical conclusion was derived using the Drude model for the dynamical conductivity which takes into account intraband electron-hole pairs only, we could not get quantitative estimation regarding doping and chirality dependencies of the absorption spectrum. In this presentation we elucidate these dependencies using Kubo formula that includes both intra and interband electron-hole pairs. Some interesting interplay between them is found and sizable chirality dependence is observed. [1] K. Sasaki et al., Applied Physics Letters 108, 163109 (2016).   

Anomalous depolarization effect in metallic nanotubes

Optical absorption in metallic nanotubes for the transitions between gapless and gapped subbands requires a transverse polarization of the external field. Accordingly, the magnitude of the absorption is determined by the depolarization effect which stems from the redistribution of conduction electrons around the circumference of the nanotube. Crudely, the electric field inside the nanotube might be determined by approximating the nanotube as a solid cylinder with some effective dielectric permittivity. We demonstrate that this intuitive picture does not adequately describe optical absorption: the depolarization effect is determined by many-body correlations within the gapless subbands, which dramatically modifies the spectral dependence near the absorption threshold.   

Simulations of resonant Raman response in bundles of semiconductor carbon nanotubes

This work is motivated by experimental study of resonant Raman response associated with E$_{\mathrm{22}}$ exciton state coupled to G$^{\mathrm{+}}$-mode vibrational mode in bundles of (6,5) semiconductor carbon nanotubes. In order to provide an insight into 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.   

Resonance Raman Spectroscopy of Chirality Enriched Semiconducting Carbon Nanotubes

Relative intensities of resonant Raman RBM and G modes of 11 chirality-enriched SWCNT species were established under second-order excitation. Results demonstrate an under-recognized complexity in evaluation of Raman spectra for assignment of (n,m) population distributions. Strong chiral angle and mod dependencies affect the intensity ratio of RBM/G modes and can result in misleading interpretations. We report 5 new (n,m) values for chirality-dependent G$^+$ and G$^-$ Raman peak positions and intensity ratios, extending the available data to cover smaller diameters down to (5,4). The Raman spectral library sufficiently decouples G peaks from multiple species and enables fundamental characterization in mixed chirality samples. Our results on dispersive properties of the D modes will also be discussed. Probing defects is crucial to evaluate SWCNT quality and to understand the photophysics behind defect-induced optoelectronic features. Using high-quality, chirality-enriched semiconducting SWCNTs and tunable lasers, our results show a non-dispersive D band throughout the resonant window within the same (n,m). Our results were validated by multiple (n,m) samples and intentional covalent surface functionalization generating D peaks with increased intensity, which remain non-dispersive.   

Investigating fluorescent single-walled carbon nanotube quantum defects using Single Particle Fluorescent Microscope

It has been shown previously that functionalizing carbon nanotubes under controlled procedure strengthens their photoluminescence (PL) properties significantly due to the formation of defect states. However, the effect of this functionalization on the molecular level has not been shown yet. In this work, we skip the ensemble averaging method and probe individual SWCNT PL characteristic alteration due to presence of one to few fluorescent defects to understand physics behind this phenomenon better. Long SWCNTs are functionalized covalently with 4-nitrobenzenediazonium tetrafluoroborate to form different densities of defect quantum states, and increase their photon-conversion efficiency. Near-infrared fluorescence microscopy between 950 and 1600 nm, and emission spectroscopy between 800-1600 are used to image and characterize the brightened individuated SWCNTs that are excited by a diode laser. We show that the effect of new quantum states on emission properties of the individual SWCNTs depends on the chirality of the tube and its defect density. Also, the efficiency of functionalization in different concentration of nitrobenzenediazonium tetrafluoroborate is reported.   

Photoluminescence from oxygen-doped single-walled carbon nanotubes modified by dielectric metasurfaces

Oxygen dopants in single-walled carbon nanotubes (SWCNTs) have recently been discovered as a novel single photon source enabling single photon generation up to room temperature in the telecom wavelength range.[1] While they are promising for quantum information processing, it is fundamentally important to be able to manipulate their photoluminescence (PL) properties. All-dielectric metasurfaces made from arrays of high index nanoparticles have emerged as an attractive alternative to plasmonic metasurfaces due to their support of both electric and magnetic modes.[2] Their low intrinsic losses at optical frequencies compared to that of plasmonic nanostructures provide a novel setting for tailoring emission from quantum emitters. We couple PL from single oxygen dopants in SWCNTs to the magnetic mode of silicon metasurfaces. Aside from the observation of a PL enhancement due to the Purcell effect, more interestingly, we find that the presence of the silicon metasurfaces significantly modifies the PL polarization of the dopants, which we attribute to near-field polarization modification caused by the silicon metasurfaces. Our finding presents dielectric metasurfaces as potential building blocks of photonic circuits for controlling PL intensity and polarization of single photon sources. 1. X. Ma et al. Nat. Nanotechnol. 2015, 10, 671-675. 2. I. Staude et al. ACS Nano 2013, 7, 7824-7832.   

Spectral tuning of optical coupling between air-mode nanobeam cavities and individual carbon nanotubes

Air-mode nanobeam cavities allow for high efficiency coupling to air-suspended carbon nanotubes due to their unique mode profile that has large electric fields in air\footnote{R. Miura, S. Imamura, R. Ohta, A. Ishii, X. Liu, T. Shimada, S. Iwamoto, Y. Arakawa, and Y. K. Kato, {\it Nature Commun.} {\bf 5}, 5580 (2014).}. Here we utilize heating-induced energy shift of carbon nanotube emission\footnote{P. Finnie, Y. Homma, and J. Lefebvre, {\it Phys. Rev. Lett.} {\bf 94}, 247401 (2005).} to investigate the cavity quantum electrodynamics effects. In particular, we use laser-induced heating which causes a large blue-shift of the nanotube photoluminescence as the excitation power is increased. Combined with a slight red-shift of the cavity mode at high powers, detuning of nanotube emission from the cavity can be controlled. We estimate the spontaneous emission coupling factor $\beta$ at different spectral overlaps and find an increase of $\beta$ factor at small detunings, which is consistent with Purcell enhancement of nanotube emission.   

Doping and Electronic Properties of Boron-Nitride Nanotubes

We study the electronic structure of boron-nitride (BN) nanotubes in the framework of the density-functional theory. The generalized Bloch theorem for one-dimensional helical-symmetry systems is used for chiral nanotubes. It is shown that, in the case of thin nanotubes, the width of the fundamental gap depends on not only the diameter but also the chiral angle of nanotubes. We next study the electronic properties of substitutionally C-doped BN Nanotubes. In the case of the C-doped flat BN sheet, the impurity-induced states have been found to be rather deep [1]. Interestingly, in the case of BN nantoubes the ionization energy of the donor state is found to depend on the diameter, showing that the importance of the curvature for the electronic transport properties of sp$^2$ BN nanostructured materials. We also discuss the multiply C doped BN nanotubes with C at both B and N sites. The system shows a rich variety of electronic properties depending not only the geometry of BN nanotubes but also the relative positions of the doped sites. [1] Y. Fujimoto and S. Saito, Phys. Rev. B 93 (2016) 045402.   

Characterization and Mechanism Studies of Carbon incorporation into Al alloys

The incorporation of C nanostructures into Al alloys, such as Al 6061 and Al 1350, has the potential to further improve the mechanical, electrical and anti-corrosion properties of these alloys. We report on an electrocharging-assisted method to incorporate up to 10.0 wt.{\%} C into the crystal structure of Al alloys to form a new material ``Al Covetics''. In this method, a DC current is applied to molten Al metal containing activated C particles. The current facilitates ionization of the C atoms followed by polymerization of the C structures and formation of graphitic chains and ribbons along preferred directions of the Al lattice. Raman mapping results indicate uniform C distribution over the surface in the Covetics. XPS carbon peak decompositions show \textit{sp2} and \textit{sp3} bonding from C structures and carbide bonding from Al-C bonds. TEM EELS spectra present a sharp C-K edge at 284eV, which further confirms the presence of \textit{sp2} bonding in Covetics. The possible mechanism of Covetics conversion is similar to ``electromigration in a plasma''. The current attracts charged Al and C atoms and the reaction takes place. The dependence of the mechanical, electrical and structural properties of Al covetics on carbon content from 3 to 10 wt. {\%} will be presented.