Handbook of Carbon Nanomaterials

The following sections are included:

• Contents
• Preface
• A Lifetime in Carbon: A Personal Remembrance of Mildred S. Dresselhaus
• A Scientific Remembrance of Mildred S. Dresselhaus
• List of Contributors
• List of Abbreviations and Symbols

This chapter presents an introductory tutorial overview of single-wall carbon nanotube (SWCNT) spectroscopy at optical wavelengths. Topics are treated assuming that the reader has little or no specialized background in order to prepare students and new investigators to access the deeper presentations found in later chapters of this two-volume set. We first describe the physical structures of SWCNTs, along with the electronic band properties arising from those structures. The simplest model for electronic interband transitions is then introduced as a framework for understanding experimental results. The next sections deal with the three most widely applied optical spectroscopies that involve those nanotube electronic transitions: photoluminescence, visible and near-infrared optical absorption, and resonance Raman scattering. A brief discussion follows on the excitonic nature of nanotube excited states and the spectral and photophysical signatures of excitons. Some other optical methods used in basic nanotube research are then presented. These are magneto-optical, Rayleigh scattering, and ultrafast time-resolved spectroscopies. We close by describing a sampling of the uses of carbon nanotube optical properties in some emerging biomedical and engineering applications.

Here, we present a metrological perspective on SWCNT optics, covering only the most essential photophysics, discussing sample properties and conditions that affect optical characterization, and the current metrological application of the most important optical characterization methods. We give a perspective on the continued development of nanotube metrology. Standards meetings, technical specifications, and other standard documents and protocols, as well as SWCNT reference materials are reviewed.

This chapter presents two aspects of recent progress in the optical spectroscopy of individual carbon nanotubes. First, the authors describe the development of a new polarization-based technique that enables high throughput imaging and spectroscopy of individual nanotubes on substrates. An application of this method for characterizing as-grown single-wall carbon nanotube samples is also presented. In the second part, the authors review recent studies of Luttinger liquid quantum plasmons in individual metallic carbon nanotubes using near-field infrared nanoscopy. Such Luttinger liquid plasmons exhibit remarkable spatial confinement and high quality factors as compared with conventional plasmonic structures.

This chapter summarizes the application of Raman spectroscopy to study individual carbon nanotubes (CNTs) under various perturbations of electric field, mechanical strain, and temperature. The insight gained in these experiments can be largely attributed to Millie Dresslhaus’ early work on Raman spectroscopy of CNTs. These include studies of thermal transport, electron–phonon scattering, non-adiabatic effects, and mechanical and electromechanical properties of suspended CNTs. A discussion of light detection and emission using suspended CNTs is also presented.

Our understanding of the electronic and vibronic properties of carbon nanomaterials is largely based on the results obtained by optical techniques. Raman and photoluminescence spectroscopy, in particular, have played a vital role for the structural identification and characterization of graphitic materials. Tip-enhanced near-field probing takes these optical techniques to the nanometer length scale by providing highly resolved spectroscopic images. In this chapter, we first outline the physical principles of the field and the resulting signal enhancement generated by sharp metal tips and discuss the symmetry aspects for Raman scattering. The strongly confined optical fields involved in tip enhancement are seen to influence light–matter interactions, for example, with respect to selection rules, decay rates and the spatial coherence of emitted radiation. Tip enhancement thus does not just give images with high spatial resolution, but also provides access to sample properties inaccessible with conventional techniques. We then review representative applications of tip-enhanced near-field optical spectroscopy and microscopy to 1D single-wall carbon nanotubes (SWCNTs) and linear carbon chains, and 2D graphene, which illustrate the enormous potential of the technique.

Carbon nanotubes represent a paradigm system for the study of phenomena arising from dimensionality-enhanced Coulomb interactions in the presence of reduced screening. Strong one-dimensional confinement of band-edge electrons and holes in semiconducting single-wall carbon nanotubes with exceptionally short screening lengths gives rise to dramatically enhanced electron-hole binding with respective exciton binding energies exceeding those in conventional bulk semiconductors by a factor of hundred. In consequence, the elementary optical interband transitions of single-wall carbon nanotubes are dominated by excitons in a wide range of temperatures. At elevated temperatures, nanotube excitons are highly mobile and explore diffusively the cylindrical honeycomb carbon lattice. At cryogenic temperatures, however, new phenomena arise from exciton localization by disorder. This chapter highlights some of the rich phenomena that emerge from exciton localization in cryogenic nanotubes with consequences for potential applications of carbon nanotubes in quantum optical technologies.

Exciton physics in individual air-suspended single-wall carbon nanotubes is investigated, where we take advantage of photonic and optoelectronic device structures to manipulate and probe excitonic properties. Automated photoluminescence microscopy systems are used to perform chirality-on-demand measurements. We discuss room-temperature photon antibunching, which results from long exciton diffusion lengths and efficient exciton–exciton annihilation. Exciton–carrier interactions are studied using field-effect devices, where gate voltages allow for modulation of photoluminescence intensities and generation of trions. Photoconductivity spectroscopy reveals field-induced phenomena such as spontaneous exciton dissociation, Stark effect, and activation of dark states. Electroluminescence from split-gate devices exhibit cold exciton recombination. Quantum electrodynamical effects are investigated using silicon photonic structures including microdisk resonators and nanobeam cavities. We also discuss optical bistability and circular dichroism that arise from mechanisms unique to nanotubes.

The following sections are included:

• Contents
• Preface
• A Lifetime in Carbon: A Personal Remembrance of Mildred S. Dresselhaus
• A Scientific Remembrance of Mildred S. Dresselhaus
• List of Contributors
• List of Abbreviations and Symbols

A brief review is given of effects of environment and collapsing on electronic states and optical properties of carbon nanotubes from a theoretical point of view. They are discussed in the lowest-order k · p scheme with higher-order corrections when necessary. Topics include effects of dielectric material surrounding nanotubes or inserted into nanotubes, effects of interwall screening in double-wall nanotubes, and effects of interwall coupling in collapsed nanotubes.

Features of Raman scattering spectroscopy as a practical characterization tool of single-wall carbon nanotube (SWCNT) samples are reviewed. After the basic introduction of G-band, D-band, radial breathing mode (RBM), and 2D-band (G′-band) peaks, chirality dependent G⁺, G⁻ features, doping effect, and resonance energy profiles are reviewed. The Kataura plot—optical transition energy vs. RBM Raman shift—is an essential tool for the practical analysis of SWCNT samples. Available experimental data and correlation are summarized in the form of a Kataura plot. The Raman shift and transition energy shift are compared for different SWCNT morphologies such as directly grown samples on substrates, vertically aligned samples, horizontally aligned samples, suspended samples, and micelle-wrapped chirality-selected samples. A detailed discussion of the environmental effect on the Kataura plot follows. Finally, the temperature dependence of Raman features is discussed.

The goal of this chapter is to review the main information derived from Raman spectroscopy on individual suspended (free-standing) chirality-identified single-wall carbon nanotubes (SWCNTs) and double-wall carbon nanotubes (DWCNTs) with a special focus on the characteristics of their radial breathing modes and G modes, including their resonance conditions. For SWCNTs, the different relationships between the radial breathing mode frequency and the inverse of the diameter illustrate the high sensitivity of individual suspended SWCNTs to their environmental conditions. The intrinsic profiles of the optical longitudinal (LO) and transverse (TO) G modes are unambiguously identified both for metallic and semiconducting chiral and achiral SWCNTs, and the diameter dependence of the LO and TO frequencies are established. In DWCNTs, the intertube coupling, originating from the van der Waals interaction between the inner and outer tubes, plays an important role in determining the features/characteristics of the collective radial breathing-like modes and G modes.

Results of my close collaboration with Professor Mildred S. Dresselhaus and Professor Gene Dresselhaus on graphene and carbon nanotubes are overviewed. From the ∼200 publications we coauthored between 1991 and 2017, here we mainly discuss the established facts of resonance Raman spectroscopy of graphene and carbon nanotubes. To highlight some of the specific current problems of Raman spectroscopy, the title of each section is given as a question regarding an aspect of Raman spectroscopy, which can also be useful for understanding Raman spectroscopy of other low-dimensional materials. Instead of attempting to provide a detailed account of each fact, an extensive reference list is provided for the reader to obtain the current understanding of specific problems. We conclude by providing future prospects for Raman spectroscopy studies of nanotubes and 2D materials.

Low-level covalent functionalization of single-wall carbon nanotubes introduces structural defects that are accompanied by new photoluminescent electronic states. Localization of excitons at the defect sites gives rise to a wide range of associated new photophysical phenomena and optical functionality to probe, understand, and harness. Here, we review recent progress in chemistry, electronic structure, spectroscopy, and quantum light emission properties of these intriguing defects and provide some perspective on potential future areas for research in this rapidly developing area of nanotube studies. Current approaches to chemical introduction of photoluminescent defects and synthetic modification of molecular dopants for tuning of emission properties are described. Spectroscopic probes paired with quantum chemistry modeling of the defect states are outlined as an efficient route toward understanding the defect electronic structure. Exciton localization at defects and its consequences for photoluminescence response are also discussed. Such spatial localization of the electronic wavefunction is especially important for introducing new functionality, including expanded possibilities for imaging and sensing applications. We focus on localization as a route for achieving room-temperature single photon emission, important for optical-based quantum information processing. Conditions required for quantum light emission, its demonstration at room temperature, and tunability to telecom wavelengths are shown. We conclude with a discussion of some critical challenges that remain for understanding and controlling covalent defects in nanotube materials in the areas of chemistry, photophysics, quantum optics, and electro-optic device integration.

Controlling charges in semiconductors is key for many of the device technologies that enable the modern information and communication age. Doping by chemical or physical means is intended to provide mobile charge carriers which facilitate crucial semiconductor device functionality. This chapter discusses some of the aspects related to the fabrication and optical characterization of reduced or oxidized semiconducting single-wall carbon nanotubes (s-SWCNTs) for the purpose of doping. We focus on redox- and electro-chemical modification to illustrate how far our current understanding allows not only a qualitative but also a quantitative spectroscopic assessment of impurity concentrations. Somewhat surprisingly, we find that commonly used n- or p-doping schemes appear to lead to inhomogeneous carrier distributions with some excess electrons or holes immobilized by deep Coulomb traps. Such carrier localization is believed to be symptomatic of the weak screening and pronounced susceptibility to environmental perturbations of low-dimensional materials such as SWCNTs. These findings may also have broader implications for the fabrication of functional semiconductor devices from nanoscale materials.

A host of both natural processes and man-made technologies rely upon the flow of electronic charge to transport and inter-convert energy over widely varying time scales. Semiconductor-based technologies rely upon careful control over both the ground-state densities of electrons or holes (in n- or p-type semiconductors, respectively) and the dynamic processes that occur for minority and/or majority charge carriers in the excited state. Single-walled carbon nanotubes (SWCNTs) have now been demonstrated as useful elements in a wide range of energy conversion (and other) technologies, including photovoltaics, thermoelectrics, digital logic, and quantum information processing. Elucidating the fundamental mechanisms at play in these technologies hinges upon a deep understanding of the energies and densities of energetic species (e.g. charge carriers, excitons, phonons) and the dynamic processes that define the interconversion between these species when the SWCNTs are exposed to perturbations (e.g. illumination, heat, bias). As such, a large body of work has been devoted to spectroscopic measurements of electrons and holes in SWCNTs over a broad range of energy and timescales. In this chapter, we discuss some critical aspects of using spectroscopic techniques to probe charge carriers in both the ground and excited state of SWCNTs, with a particular focus on recent experiments involving highly enriched semiconducting SWCNTs.

Optical phenomena arising from elementary excitations in carbon nanotubes (CNTs) have been extensively studied as central issues. The charged exciton (or trion), which is a bound state composed of two electrons (two holes) and a hole (a electron) by the Coulomb interaction, plays a dominant role in the optical phenomena of carrier-doped CNTs. New absorption and luminescence peaks appear in carrier-doped CNTs, which come from positive and negative trions. In CNTs, trions with a large binding energy of several tenths of a meV are stable, even at room temperature. Herein, we also describe the 0D nature of trions formed by chemical doping of 1D CNTs. Moreover, the much longer effective radiative lifetime of trions compared with neutral excitons is attributed to the spin configuration. This knowledge of trions constitutes an important advance in our understanding of the photophysics of CNTs.

The following section is included:

• Index