Direct measurement of the polarized optical absorption cross section of single-wall carbon nanotubes

Numerical Analysis for Light Absorption Spectra of the Base of DNA-Wrapped Single-Walled Carbon Nanotubes

This study numerically demonstrates the light absorption spectra of each base of DNA-wrapped single-walled carbon nanotubes (SWCNTs). Previous experimental and theoretical studies show that the optical properties of these composites are different from the bare SWCNTs. In this work, we investigated the bases of DNA that influence optical properties. To obtain stable molecular states for studying optical properties, molecular dynamics calculations were performed. Additionally, light absorption spectra in the ultraviolet-to-near-infrared region of one type of base-wrapped (e.g., adenine-, thymine-, cytosine-, or guanine-wrapped) SWCNTs were investigated by utilizing the semi-empirical molecular orbital theory using SCIGRESS commercial software. This method can significantly reduce the calculation time compared to the ab initio molecular orbital method, making the handling of composites of bases and SWCNTs possible. We found that the largest peaks appear at a wavelength of around 300 nm for all the composites. Furthermore, we found that the light absorption spectra above 570 nm are strongly influenced by adenine and cytosine. Thus, our computational results provide insight into the optical properties and the effects of base–SWCNTs that are difficult to investigate experimentally under the influence of solvents and various molecules.

Interlayer Interactions in 1D Van der Waals Moiré Superlattices

Studying two-dimensional (2D) van der Waals (vdW) moiré superlattices and their interlayer interactions have received surging attention after recent discoveries of many new phases of matter that are highly tunable. Different atomistic registry between layers forming the inner and outer nanotubes can also form one-dimensional (1D) vdW moiré superlattices. In this review, experimental observations and theoretical perspectives related to interlayer interactions in 1D vdW moiré superlattices are summarized. The discussion focuses on double-walled carbon nanotubes (DWNTs), a model 1D vdW moiré system, and the authors highlight the new optical features emerging from the non-trivial strong interlayer coupling effect and the unique physics in 1D DWNTs. Future directions and questions in probing the intriguing physical phenomena in 1D vdW moiré superlattices such as, correlated physics in different 1D moiré systems beyond DWNTs are proposed and discussed.

Horizontal Single-Walled Carbon Nanotube Arrays: Controlled Synthesis, Characterizations, and Applications

Single-walled carbon nanotubes (SWNTs) emerge as a promising material to advance carbon nanoelectronics. However, synthesizing or assembling pure metallic/semiconducting SWNTs required for interconnects/integrated circuits, respectively, by a conventional chemical vapor deposition method or by an assembly technique remains challenging. Recent studies have shown significant scientific breakthroughs in controlled SWNT synthesis/assembly and applications in scaled field effect transistors, which are a critical component in functional nanodevices, thereby rendering the horizontal SWNT array an important candidate for innovating nanotechnology. This review provides a comprehensive analysis of the controlled synthesis, surface assembly, characterization techniques, and potential applications of horizontally aligned SWNT arrays. This review begins with the discussion of synthesis of horizontally aligned SWNTs with regulated direction, density, structure, and theoretical models applied to understand the growth results. Several traditional procedures applied for assembling SWNTs on target surface are also briefly discussed. It then discusses the techniques adopted to characterize SWNTs, ranging from electron/probe microscopy to various optical spectroscopy methods. Prototype applications based on the horizontally aligned SWNTs, such as interconnects, field effect transistors, integrated circuits, and even computers, are subsequently described. Finally, this review concludes with challenges and a brief outlook of the future development in this research field.

Highly conductive and transparent coatings from flow-aligned silver nanowires with large electrical and optical anisotropy

Conductive and transparent coatings consisting of silver nanowires (AgNWs) are promising candidates for emerging flexible electronics applications. Coatings of aligned AgNWs offer unusual electronic and optical anisotropies, with potential for use in micro-circuits, antennas, and polarization sensors. Here we explore a microfluidics setup and flow-induced alignment mechanisms to create centimeter-scale highly conductive coatings of aligned AgNWs with order parameters reaching 0.84, leading to large electrical and optical anisotropies. By varying flow rates, we establish the relationship between the shear rate and the alignment and investigate possible alignment mechanisms. The angle-dependent sheet resistance of the aligned AgNW networks exhibits an electronic transport anisotropy of ∼10× while maintaining low resistivity (<50 Ω sq^-1) in all directions. When illuminated, the aligned AgNW coatings exhibit angle- and polarization-dependent colors, and the polarized reflection anisotropy can be as large as 25. This large optical anisotropy is due to a combination of alignment, polarization response, and angle-dependent scattering of the aligned AgNWs.

Measurement of complex optical susceptibility for individual carbon nanotubes by elliptically polarized light excitation

The complex optical susceptibility is the most fundamental parameter characterizing light-matter interactions and determining optical applications in any material. In one-dimensional (1D) materials, all conventional techniques to measure the complex susceptibility become invalid. Here we report a methodology to measure the complex optical susceptibility of individual 1D materials by an elliptical-polarization-based optical homodyne detection. This method is based on the accurate manipulation of interference between incident left- (right-) handed elliptically polarized light and the scattering light, which results in the opposite (same) contribution of the real and imaginary susceptibility in two sets of spectra. We successfully demonstrate its application in determining complex susceptibility of individual chirality-defined carbon nanotubes in a broad optical spectral range (1.6-2.7 eV) and under different environments (suspended and in device). This full characterization of the complex optical responses should accelerate applications of various 1D nanomaterials in future photonic, optoelectronic, photovoltaic, and bio-imaging devices.

High-Throughput Optical Imaging and Spectroscopy of One-Dimensional Materials

Direct visualization of one-dimensional (1D) materials under an optical microscope in ambient conditions is of great significance for their characterizations and applications. However, it is full of challenges to achieve such goal due to their relative small size (ca. 1 nm in diameter) in the optical-diffraction-limited laser spot (ca. 1 μm in diameter). In this Concept article, we introduce a polarization-based optical homodyne detection method that can be used as a general strategy to obtain high-throughput, real-time, optical imaging and in situ spectroscopy of polarization-inhomogeneous 1D materials. We will use carbon nanotubes (CNTs) as an example to demonstrate the applications of such characterization with respect to the absorption signal of individual nanotubes, real-time imaging of individual nanotubes in devices, and statistical structure information of nanotube arrays.

Solitary Oxygen Dopant Emission from Carbon Nanotubes Modified by Dielectric Metasurfaces

All-dielectric metasurfaces made from arrays of high index nanoresonators supporting strong magnetic dipole modes have emerged as a low-loss alternative to plasmonic metasurfaces. Here we use oxygen-doped single-walled carbon nanotubes (SWCNTs) as quantum emitters and couple them to silicon metasurfaces to study effects of the magnetic dipole modes of the constituent nanoresonators on the photoluminescence (PL) of individual SWCNTs. We find that when in resonance, the magnetic mode of the silicon nanoresonators can lead to a moderate average PL enhancement of 0.8-4.0 of the SWCNTs, accompanied by an average increase in the radiative decay rate by a factor of 1.5-3.0. More interestingly, single dopant polarization experiments show an anomalous photoluminescence polarization rotation by coupling individual SWCNTs to silicon nanoresonators. Numerical simulations indicate that this is caused by modification of near-field polarization distribution at certain areas in the proximity of the silicon nanoresonators at the excitation wavelength, thus presenting an approach to control emission polarization. These findings indicate silicon nanoresonators as potential building blocks of quantum photonic circuits capable of manipulating PL intensity and polarization of single photon sources.

Thermomechanical properties of PMMA and modified SWCNT composites

It is well known that the addition of carbon nanotubes (CNTs) can strongly affect the thermomechanical and electrical properties of the polymer into which they are dispersed. The common solvent mixing dispersion method of functionalized CNTs and polymer composites can improve thermal, mechanical, and electrical properties. In this study, functionalized single-walled CNTs (COOH-SWCNTs) and poly(methyl methacrylate) were used to fabricate the polymer nanocomposites using a common solvent dispersion mixing method. The homogenous dispersion of COOH-SWCNTs in the composites resulted in improved thermomechanical properties of these composites; this was analyzed using scanning electron microscopy.

(n,m)-Specific Absorption Cross Sections of Single-Walled Carbon Nanotubes Measured by Variance Spectroscopy

A new method based on variance spectroscopy has enabled the determination of absolute absorption cross sections for the first electronic transition of 12 (n,m) structural species of semiconducting single-walled carbon nanotubes (SWCNTs). Spectrally resolved measurements of fluorescence variance in dilute bulk samples provided particle number concentrations of specific SWCNT species. These values were converted to carbon concentrations and correlated with resonant components in the absorbance spectrum to deduce (n,m)-specific absorption cross sections (absorptivities) for nanotubes ranging in diameter from 0.69 to 1.03 nm. The measured cross sections per atom tend to vary inversely with nanotube diameter and are slightly greater for structures of mod 1 type than for mod 2. Directly measured and extrapolated values are now available to support quantitative analysis of SWCNT samples through absorption spectroscopy.

Photodetection and Photoswitch Based On Polarized Optical Response of Macroscopically Aligned Carbon Nanotubes

Light polarization is extensively applied in optical detection, industry processing and telecommunication. Although aligned carbon nanotube naturally suppresses the transmittance of light polarized parallel to its axial direction, there is little application regarding the photodetection of carbon nanotube based on this anisotropic interaction with linearly polarized light. Here, we report a photodetection device realized by aligned carbon nanotube. Because of the different absorption behavior of polarized light with respect to polarization angles, such device delivers an explicit response to specific light wavelength regardless of its intensity. Furthermore, combining both experimental and mathematical analysis, we found that the light absorption of different wavelength causes characteristic thermoelectric voltage generation, which makes aligned carbon nanotube promising in optical detection. This work can also be utilized directly in developing new types of photoswitch that features a broad spectrum application from near-ultraviolet to intermediate infrared with easy integration into practical electric devices, for instance, a "wavelength lock".

High visible-light photochemical activity of titania decorated on single-wall carbon nanotube aerogels

Titania decorated on single-wall carbon nanotube aerogels degraded organic dyes under visible-light irradiation at ultrahigh rates.

Preformed nanoporous carbon nanotube scaffold-based multifunctional polymer composites

Multifunctional polymer nanocomposites that simultaneously possess high modulus and strength, high thermal stability, novel optical responses, and high electrical and thermal conductivity have been actively researched. Carbon nanotubes are considered an ideal additive for composites because of their superlative physical, electronic and optical properties. While nanotubes have successfully added electrical conductivity, thermal stability, and novel optical responses to polymers, mechanical reinforcements, although substantial, have been well below any theoretical estimations. Here, we integrated preformed hydrogels and aerogels of individually dispersed nanotubes with polymer to increase elastic modulus of composites according to Halpin-Tsai model up to at least 25 vol % of nanotubes. Our solution-based fabrication method allowed us to create bulk composites with tunable form-factors, and with polymers that were incompatible with nanotubes. Further, in this approach, nanotubes were not covalently linked among themselves and to the polymer, so intrinsic optical, electrical, and thermal properties of nanotubes could be exploited. The optically active nanotubes, for example, added a strain-dependent, spatially resolved fluorescence to these composites. Finally, the nanoporous nanotube networks suppressed the polymer glass transition and extended the mechanical integrity of polymer well above its polymer melting point, and both the nanotubes and polymer remained thermally stable above their decomposition temperatures.

Highly conductive single-walled carbon nanotube thin film preparation by direct alignment on substrates from water dispersions

A safe, scalable method for producing highly conductive aligned films of single-walled carbon nanotubes (SWNTs) from water suspensions is presented. While microfluidic assembly of SWNTs has received significant attention, achieving desirable SWNT dispersion and morphology in fluids without an insulating surfactant or toxic superacid is challenging. We present a method that uniquely produces a noncorrosive ink that can be directly applied to a device in situ, which is different from previous fabrication techniques. Functionalized SWNTs (f-SWNTs) are dispersed in an aqueous urea solution to leverage binding between the amine group of urea and the carboxylic acid group of f-SWNTs and obtain urea-SWNT. Compared with SWNTs dispersed using conventional methods (e.g., superacid and surfactants), the dispersed urea-SWNT aggregates have a higher aspect ratio with a rodlike morphology as measured by light scattering. The Mayer rod technique is used to prepare urea-SWNT, highly aligned films (two-dimensional nematic order parameter of 0.6, 5 μm spot size, via polarized Raman) with resistance values as low as 15-1700 Ω/sq in a transmittance range of 2-80% at 550 nm. These values compete with the best literature values for conductivity of SWNT-enabled thin films. The findings offer promising opportunities for industrial applications relying on highly conductive thin SWNT films.

A reference material of single-walled carbon nanotubes: quantitative chirality assessment using optical absorption spectroscopy

A quantitative chirality assessment of a SWCNT reference material is presented by using an enhanced method for absorption spectrum analysis.

Anisotropic terahertz response of stretch-aligned composite films based on carbon nanotube–SiC hybrid structures

Anisotropic terahertz response of stretch-aligned composite films based on carbon nanotube–SiC hybrid structure was investigated.

Characterizing the chiral index of a single-walled carbon nanotube

The properties of single-walled carbon nanotubes (SWCNTs) mainly depend on their geometry. However, there are still formidable difficulties to determine the chirality of SWCNTs accurately. In this review, some efficient methods to characterize the chiral indices of SWCNTs are illuminated. These methods are divided into imaging techniques and spectroscopy techniques. With these methods, diameter, helix angle, and energy states can be measured. Generally speaking, imaging techniques have a higher accuracy and universality, but are time-consuming with regard to the sample preparation and characterization. The spectroscopy techniques are very simple and fast in operation, but these techniques can be applied only to the particular structure of the sample. Here, the principles and operations of each method are introduced, and a comprehensive understanding of each technique, including their advantages and disadvantages, is given. Advanced applications of some methods are also discussed. The aim of this review is to help readers to choose methods with the appropriate accuracy and time complexity and, furthermore, to put forward an idea to find new methods for chirality characterization.

Photothermal Microscopy of Nonluminescent Single Particles Enabled by Optical Microresonators

A powerful new paradigm for single-particle microscopy on nonluminescent targets is reported using ultrahigh-quality factor optical microresonators as the critical detecting element. The approach is photothermal in nature as the microresonators are used to detect heat dissipated from individual photoexcited nano-objects. The method potentially satisfies an outstanding need for single-particle microscopy on nonluminescent objects of increasingly smaller absorption cross section. Simultaneously, our approach couples the sensitivity of label-free detection using optical microresonators with a means of deriving chemical information on the target species, a significant benefit. As a demonstration, individual nonphotoluminescent multiwalled carbon nanotubes are spatially mapped, and the per-atom absorption cross section is determined. Finite-element simulations are employed to model the relevant thermal processes and elucidate the sensing mechanism. Finally, a direct pathway to the extension of this new technique to molecules is laid out, leading to a potent new method of performing measurements on individual molecules.

Systematic determination of absolute absorption cross-section of individual carbon nanotubes

Optical absorption is the most fundamental optical property characterizing light-matter interactions in materials and can be most readily compared with theoretical predictions. However, determination of optical absorption cross-section of individual nanostructures is experimentally challenging due to the small extinction signal using conventional transmission measurements. Recently, dramatic increase of optical contrast from individual carbon nanotubes has been successfully achieved with a polarization-based homodyne microscope, where the scattered light wave from the nanostructure interferes with the optimized reference signal (the reflected/transmitted light). Here we demonstrate high-sensitivity absorption spectroscopy for individual single-walled carbon nanotubes by combining the polarization-based homodyne technique with broadband supercontinuum excitation in transmission configuration. To our knowledge, this is the first time that high-throughput and quantitative determination of nanotube absorption cross-section over broad spectral range at the single-tube level was performed for more than 50 individual chirality-defined single-walled nanotubes. Our data reveal chirality-dependent behaviors of exciton resonances in carbon nanotubes, where the exciton oscillator strength exhibits a universal scaling law with the nanotube diameter and the transition order. The exciton linewidth (characterizing the exciton lifetime) varies strongly in different nanotubes, and on average it increases linearly with the transition energy. In addition, we establish an empirical formula by extrapolating our data to predict the absorption cross-section spectrum for any given nanotube. The quantitative information of absorption cross-section in a broad spectral range and all nanotube species not only provides new insight into the unique photophysics in one-dimensional carbon nanotubes, but also enables absolute determination of optical quantum efficiencies in important photoluminescence and photovoltaic processes.

Shape-dependent dispersion and alignment of nonaggregating plasmonic gold nanoparticles in lyotropic and thermotropic liquid crystals

We use both lyotropic liquid crystals composed of prolate micelles and thermotropic liquid crystals made of rod-like molecules to uniformly disperse and unidirectionally align relatively large gold nanorods and other complex-shaped nanoparticles at high concentrations. We show that some of these ensuing self-assembled orientationally ordered soft matter systems exhibit polarization-dependent plasmonic properties with strongly pronounced molar extinction exceeding that previously achieved in self-assembled composites. The long-range unidirectional alignment of gold nanorods is mediated mainly by anisotropic surface anchoring interactions at the surfaces of gold nanoparticles. Polarization-sensitive absorption, scattering, and extinction are used to characterize orientations of nanorods and other nanoparticles. The experimentally measured unique optical properties of these composites, which stem from the collective plasmonic effect of the gold nanorods with long-range order in a liquid crystal matrix, are reproduced in computer simulations. A simple phenomenological model based on anisotropic surface interaction explains the alignment of gold nanorods dispersed in liquid crystals and the physical underpinnings behind our observations.

Photothermoelectric effect in suspended semiconducting carbon nanotubes

We have performed scanning photocurrent microscopy measurements of field-effect transistors (FETs) made from individual ultraclean suspended carbon nanotubes (CNTs). We investigate the spatial-dependence, polarization-dependence, and gate-dependence of photocurrent and photovoltage in this system. While previous studies of surface-bound CNT FET devices have identified the photovoltaic effect as the primary mechanism of photocurrent generation, our measurements show that photothermoelectric phenomena play a critical role in the optoelectronic properties of suspended CNT FETs. We have quantified the photothermoelectric mechanisms and identified regimes where they overwhelm the photovoltaic mechanism.

High-throughput optical imaging and spectroscopy of individual carbon nanotubes in devices

Single-walled carbon nanotubes are uniquely identified by a pair of chirality indices (n,m), which dictate the physical structures and electronic properties of each species. Carbon nanotube research is currently facing two outstanding challenges: achieving chirality-controlled growth and understanding chirality-dependent device physics. Addressing these challenges requires, respectively, high-throughput determination of the nanotube chirality distribution on growth substrates and in situ characterization of the nanotube electronic structure in operating devices. Direct optical imaging and spectroscopy techniques are well suited for both goals, but their implementation at the single nanotube level has remained a challenge due to the small nanotube signal and unavoidable environment background. Here, we report high-throughput real-time optical imaging and broadband in situ spectroscopy of individual carbon nanotubes on various substrates and in field-effect transistor devices using polarization-based microscopy combined with supercontinuum laser illumination. Our technique enables the complete chirality profiling of hundreds of individual carbon nanotubes, both semiconducting and metallic, on a growth substrate. In devices, we observe that high-order nanotube optical resonances are dramatically broadened by electrostatic doping, an unexpected behaviour that points to strong interband electron-electron scattering processes that could dominate ultrafast dynamics of excited states in carbon nanotubes.

Chirality dependence of the absorption cross section of carbon nanotubes

The variation of the optical absorption of carbon nanotubes with their geometry has been a long-standing question at the heart of both metrological and applicative issues, in particular because optical spectroscopy is one of the primary tools for the assessment of the chiral species abundance of samples. Here, we tackle the chirality dependence of the optical absorption with an original method involving ultraefficient energy transfer in porphyrin-nanotube compounds that allows uniform photoexcitation of all chiral species. We measure the absolute absorption cross section of a wide range of semiconducting nanotubes at their S22 transition and show that it varies by up to a factor of 2.2 with the chiral angle, with type I nanotubes showing a larger absorption. In contrast, the luminescence quantum yield remains almost constant.

Quantum efficiency and capture cross section of first and second excitonic transitions of single-walled carbon nanotubes measured through photoconductivity

Comparing photoconductivity measurements, using p-n diodes formed along individual single-walled carbon nanotubes (SWNT), with modeling results, allows determination of the quantum efficiency, optical capture cross section, and oscillator strength of the first (E11) and second (E22) excitonic transitions of SWNTs. This is in the infrared region of the spectrum, where little experimental work on SWNT optical absorption has been reported to date. We estimate quantum efficiency (η) ~1-5% and provide a correlation of η, capture cross section, and oscillator strength for E11 and E22 with nanotube diameter. This study uses the spectral weight of the exciton resonances as the determining parameter in optical measurements.

Metrological Investigation of the (6,5) Carbon Nanotube Absorption Cross Section

Using single-nanotube absorption microscopy, we measured the absorption cross section of (6,5) carbon nanotubes at their second-order optical transition. We obtained a value of 3.2 × 10(-17) cm(2)/C atom with a precision of 15% and an accuracy below 20%. This constitutes the first metrological investigation of the absorption cross section of chirality-identified nanotubes. Correlative absorption-luminescence microscopies performed on long nanotubes reveal a direct manifestation of exciton diffusion in the nanotube.

Carbon nanotube photoelectronic and photovoltaic devices and their applications in infrared detection

Semiconducting carbon nanotubes (CNTs) are direct bandgap materials with outstanding electronic and optoelectronic properties and have been investigated for various electronic and optoelectronic device applications, such as light-emitting diodes, photodetectors and photovoltaic cells. Here, a brief review of the various types of CNT diodes is presented, with a focus on one particular type of CNT diodes fabricated via a doping-free process. Their application for constructing high-performance optoelectronic and photovoltaic devices is also discussed, as well as the newly discovered photovoltage multiplication effect in CNTs and its application in improving the efficiency of CNT-based infrared detector.

Ultrafast spectral migration of photoluminescence in graphene oxide

We use subpicosecond time-resolved photoluminescence measurements to study the nature of photoluminescence in graphene oxide and reduced graphene oxide. Our data indicate that, in contrast to prior suggestions, the photoluminescence spectra of graphene oxide and reduced graphene oxide are inhomogeneously broadened. We observe substantial energy redistribution and relaxation among the emitting states within the first few picoseconds, leading to a progressive red shift of the emission spectrum. Blue shifts that arise in time-integrated spectra upon photothermal reduction are easily understood within this dynamical context without invoking a modified distribution of dipole-coupled states. Rather, reduction increases the nonradiative electron-hole recombination rate and curtails the red-shifting process, which is consistent with an increase in quenching through the introduction of larger and/or more numerous sp(2) clusters. Polarization memory measurements show energetic signatures of electron-hole correlations, established on a subpicosecond time scale and developing little thereafter.

Exciton states and optical properties of carbon nanotubes

Exciton states and related optical properties of a single-walled carbon nanotube are reviewed, primarily from a theoretical viewpoint. The energies and wavefunctions of excitons are discussed using a screened Hartree-Fock approximation with an effective-mass or k·p approximation. The close relationship between a long-range electron-hole exchange interaction and a depolarization effect is clarified. I discuss optical properties including the radiative lifetime of excitons, absorption spectra and radiation force. To describe these properties in a unified scheme, a self-consistent method is introduced for calculating the scattering light and induced current density due to excitons. I also briefly review experimental results on the Aharonov-Bohm effect in excitons and quasi-dark excitons excited by light polarized perpendicular to the tube axis.

Probing optical transitions in individual carbon nanotubes using polarized photocurrent spectroscopy

Carbon nanotubes show vast potential to be used as building blocks for photodetection applications. However, measurements of fundamental optical properties, such as the absorption coefficient and the dielectric constant, have not been accurately performed on a single pristine carbon nanotube. Here we show polarization-dependent photocurrent spectroscopy, performed on a p-n junction in a single suspended semiconducting carbon nanotube. We observe an enhanced absorption in the carbon nanotube optical resonances, and an external quantum efficiency of 12.3% and 8.7% was deduced for the E11 and E22 transitions, respectively. By studying the polarization dependence of the photocurrent, a dielectric constant of 3.6 ± 0.2 was experimentally determined for this semiconducting carbon nanotube.

Optoelectronic properties of single-wall carbon nanotubes

Single-wall carbon nanotubes (SWCNTs), with their uniquely simple crystal structures and chirality-dependent electronic and vibrational states, provide an ideal laboratory for the exploration of novel 1D physics, as well as quantum engineered architectures for applications in optoelectronics. This article provides an overview of recent progress in optical studies of SWCNTs. In particular, recent progress in post-growth separation methods allows different species of SWCNTs to be sorted out in bulk quantities according to their diameters, chiralities, and electronic types, enabling studies of (n,m)-dependent properties using standard macroscopic characterization measurements. Here, a review is presented of recent optical studies of samples enriched in 'armchair' (n = m) species, which are truly metallic nanotubes but show excitonic interband absorption. Furthermore, it is shown that intense ultrashort optical pulses can induce ultrafast bandgap oscillations in SWCNTs, via the generation of coherent phonons, which in turn modulate the transmission of a delayed probe pulse. Combined with pulse-shaping techniques, coherent phonon spectroscopy provides a powerful method for studying exciton-phonon coupling in SWCNTs in a chirality-selective manner. Finally, some of the basic properties of highly aligned SWCNT films are highlighted, which are particularly well-suited for optoelectronic applications including terahertz polarizers with nearly perfect extinction ratios and broadband photodetectors.

Plasmonic complex fluids of nematiclike and helicoidal self-assemblies of gold nanorods with a negative order parameter

We describe a soft matter system of self-organized oblate micelles and plasmonic gold nanorods that exhibit a negative orientational order parameter. Because of anisotropic surface anchoring interactions, colloidal gold nanorods tend to align perpendicular to the director describing the average orientation of normals to the discoidal micelles. Helicoidal structures of highly concentrated nanorods with a negative order parameter are realized by adding a chiral additive and are further controlled by means of confinement and mechanical stress. Polarization-sensitive absorption, scattering, and two-photon luminescence are used to characterize orientations and spatial distributions of nanorods. Self-alignment and effective-medium optical properties of these hybrid inorganic-organic complex fluids match predictions of a simple model based on anisotropic surface anchoring interactions of nanorods with the structured host medium.

Evidence for long-lived, optically generated quenchers of excitons in single-walled carbon nanotubes

The nonlinear dependence of near-infrared photoluminescence (PL) emission on excitation intensity has been measured for individual nanotubes representing six different (n,m) species. Significant deviations from linearity are observed for intensities as low as ~100 W/cm(2), and an approximate inverse correlation is found between nonlinearity and PL action cross section (brightness). A model in which all PL nonlinearity arises from exciton-exciton annihilation is insufficient to account for the experimental data using realistic parameters. It is proposed that additional nonlinear quenching arises from photoinduced quenching states or species with longer lifetimes than emissive excitons. Evidence is also found for metastable photogenerated PL quenchers with lifetimes up to 20 s.

A reel-wound carbon nanotube polarizer for terahertz frequencies

Utilizing highly oriented multiwalled carbon nanotube aerogel sheets, we fabricated micrometer-thick freestanding carbon nanotube (CNT) polarizers. Simple winding of nanotube sheets on a U-shaped polyethylene reel enabled rapid and reliable polarizer fabrication, bypassing lithography or chemical etching processes. With the remarkable extinction ratio reaching ∼37 dB in the broad spectral range from 0.1 to 2.0 THz, combined with the extraordinary gravimetric mechanical strength of CNTs, and the dispersionless character of freestanding sheets, the commercialization prospects for our CNT terahertz polarizers appear attractive.

Absorption cross section and interfacial thermal conductance from an individual optically excited single-walled carbon nanotube

The heat generation and dissipation of an individual optically excited metallic single-walled carbon nanotube is characterized using a thermal sensor thin film of Al(0.94)Ga(0.06)N embedded with Er(3+). The absorption cross section from an individual SWCNT excited at 532 nm is revealed from the steady-state temperature of the thermal sensor film. A maximum temperature of 4.3 K is observed when the SWCNT is excited with parallel polarization and an average intensity of 7 × 10(10) W/m(2). From this temperature measurement, we determine an absorption cross section for the SWCNT of 9.4 × 10(-17) m(2)/μm using parallel polarization and 2.4 × 10(-17) m(2)/μm for perpendicular polarization. We establish a temperature difference between the SWCNT and the substrate of 315 K by converting the G band shift of the SWCNT into the local SWCNT temperature and scaling the measured film temperature to the local non-resolution-limited temperature rise. From the temperature difference and heat flux, we deduce a value of 6.6 MW/m(2)·K for the thermal interfacial conductance of a SWCNT sitting on a thin film of amorphous Al(0.94)Ga(0.06)N.

Quantification of uptake and localization of bovine serum albumin-stabilized single-wall carbon nanotubes in different human cell types

Single-wall carbon nanotubes (SWCNTs) possess many unique, inherent properties that make them attractive materials for application in medical and biological technologies. Development of concentrated SWCNT dispersions of isolated nanotubes that retain SWCNTs' inherent properties with minimal negative cellular effects is essential to fully realize the potential of SWCNTs in biotechnology. It is shown that bovine serum albumin (BSA), a common and well-characterized model blood serum protein, can individually disperse SWCNTs at concentrations of up to 0.3 mg mL(-1) while retaining SWCNTs' optical properties. Uptake into human mesenchymal stem cells (hMSC) and HeLa cells is quantified, revealing strikingly high concentrations of 86 ± 33 × 10(6) and 21 ± 13 × 10(6) SWCNTs per cell, respectively, without any apparent acute deleterious cellular effects. Through high-resolution confocal Raman spectroscopy and imaging, it is established that SWCNT-BSAs are preferentially localized intracellularly, especially in the cytoplasm of both hMSCs and HeLa cells. The uptake and localization results demonstrate the efficacy of BSA as a biocompatible dispersant and a mediator of bioactivity. BSA is widely available and inexpensive, which make these concentrated, highly-dispersed, noncovalently modified SWCNT-BSAs suitable for the development of SWCNT-based biotechnologies.

Significant FRET between SWNT/DNA and rare earth ions: a signature of their spatial correlations

Significant acceleration of the photoluminescence (PL) decay rate was observed in water solutions of two rare earth ions (REIs), Tb and Eu. We propose that the time-resolved PL spectroscopy data are explained by a fluorescence resonance energy transfer (FRET) between the REIs. FRET was directly confirmed by detecting the induced PL of the energy acceptor, Eu ion, under the PL excitation of the donor ion, Tb, with FRET efficiency reaching 7% in the most saturated solution, where the distance between the unlike REIs is the shortest. Using this as a calibration experiment, a comparable FRET was measured in the mixed solution of REIs with single-wall nanotubes (SWNTs) wrapped with DNA. From the FRET efficiency of 10% and 7% for Tb and Eu, respectively, the characteristic distance between the REI and SWNT/DNA was obtained as 15.9 ± 1.3 Å, suggesting that the complexes are formed because of Coulomb attraction between the REI and the ionized phosphate groups of the DNA.

Chemical approaches towards single-species single-walled carbon nanotubes

Small variations in diameter and chirality could bring striking changes in the electronic and optical properties of single-walled carbon nanotubes (SWCNTs). Therefore, SWCNTs of a specific diameter/chirality are required for many applications. In this review we provide an overview of the recent progress in various chemical approaches towards producing specific nanotubes. Issues regarding the structure of SWCNTs, characterization tools and various separation techniques are presented in this article. The benefits and limits of current chemical approaches are discussed and the perspectives of emerging strategies for achieving identical single-walled carbon nanotubes are highlighted.

Multiple exciton generation in single-walled carbon nanotubes

Upon absorption of single photons, multiple excitons were generated and detected in semiconducting single-walled carbon nanotubes (SWNTs) using transient absorption spectroscopy. For (6,5) SWNTs, absorption of single photons with energies corresponding to three times the SWNT energy gap results in an exciton generation efficiency of 130% per photon. Our results suggest that the multiple exciton generation threshold in SWNTs can be close to the limit defined by energy conservation.

Saturation of the photoluminescence at few-exciton levels in a single-walled carbon nanotube under ultrafast excitation

Single air-suspended carbon nanotubes (length 2-5 microm) exhibit high optical quantum efficiency (7-20%) for low intensity resonant pumping. Under ultrafast excitation (150 fs), emission dramatically saturates at very low exciton numbers (2-6), which is attributed to highly efficient exciton-exciton annihilation over micron-length scales. Similar saturation behavior for 4 ps pulse excitation shows nonlinear absorption is not a contributing factor. The absorption cross sections (0.6-1.8x10(-17) cm2/atom) are determined by fitting to a stochastic model for exciton dynamics.

Multimodal, nanoscale, hyperspectral imaging demonstrated on heterostructures of quantum dots and DNA-wrapped single-wall carbon nanotubes

A multimodality imaging technique integrating atomic force, polarized Raman, and fluorescence lifetime microscopies, together with 2D autocorrelation image analysis is applied to the study of a mesoscopic heterostructure of nanoscale materials. This approach enables simultaneous measurement of fluorescence emission and Raman shifts from a quantum dot (QD)-single-wall carbon nanotube (SWCNT) complex. Nanoscale physical and optoelectronic characteristics are observed including local QD concentrations, orientation-dependent polarization anisotropy of the SWCNT Raman intensities, and charge transfer from photoexcited QDs to covalently conjugated SWCNTs. Our measurement approach bridges the properties observed in bulk and single nanotube studies. This methodology provides fundamental understanding of the charge and energy transfer between nanoscale materials in an assembly.

Carbon nanotube terahertz polarizer

We describe a film of highly aligned single-walled carbon nanotubes that acts as an excellent terahertz linear polarizer. There is virtually no attenuation (strong absorption) when the terahertz polarization is perpendicular (parallel) to the nanotube axis. From the data, the reduced linear dichrosim was calculated to be 3, corresponding to a nematic order parameter of 1, which demonstrates nearly perfect alignment as well as intrinsically anisotropic terahertz response of single-walled carbon nanotubes in the film.

Imaging the electrical conductance of individual carbon nanotubes with photothermal current microscopy

The one-dimensional structure of carbon nanotubes leads to a variety of remarkable optical and electrical properties that could be used to develop novel devices. Recently, the electrical conductance of nanotubes has been shown to decrease under optically induced heating by an amount proportional to the temperature change. Here, we show that this decrease is also proportional to the initial nanotube conductance, and make use of this effect to develop a new electrical characterization tool for nanotubes. By scanning the focal spot of a laser across the surface of a device through which current is simultaneously measured, we can construct spatially resolved conductance images of both single and arrayed nanotube transistors. We can also directly image the gate control of these devices. Our results establish photothermal current microscopy as an important addition to the existing suite of characterization techniques for carbon nanotubes and other linear nanostructures.