Exciton energy transfer in pairs of single-walled carbon nanotubes

Photoinduced Nonadiabatic Dynamics of a Single-Walled Carbon Nanotube-Porphyrin Complex

Single-walled carbon nanotubes (SWCNTs) have gained a lot of attention in the past few decades due to their promising optoelectronic properties. In addition, SWCNTs can form complexes that have good chemical stability and transport properties with other optical functional materials through noncovalent interactions. Elucidating the detailed mechanism of these complexes is of great significance for improving their optoelectronic properties. Nevertheless, simulating the photoinduced dynamics of these complexes accurately is rather challenging since they usually contain hundreds of atoms. To save computational efforts, most of the previous works have ignored the excitonic effects by employing nonadiabatic carrier (electron and hole) dynamics simulations. To properly consider the influence of excitonic effects on the photoinduced ultrafast processes of the SWCNT-tetraphenyl porphyrin (H2TPP) complex and to further improve the computational efficiency, we developed the nonadiabatic molecular dynamics (NAMD) method based on the extended tight binding-based simplified Tamm-Dancoff approximation (sTDA-xTB), which is applied to study the ultrafast photoinduced dynamics of the noncovalent SWCNT-porphyrin complex. In combination with statically electronic structure calculations, the present work successfully reveals the detailed microscopic mechanism of the ultrafast excitation energy transfer process of the complex. Upon local excitation on the H2TPP molecule, an ultrafast energy transfer process occurs from H2TPP (SWCNT-H2TPP*) to SWCNT (SWCNT*-H2TPP) within 10 fs. Then, two slower processes corresponding to the energy transfer from H2TPP to SWCNT and hole transfer from H2TPP to SWCNT take place in the 1 ps time scale. The sTDA-xTB-based electronic structure calculation and NAMD simulation results not only match the previous experimental observations from static and transient spectra but also provide more insights into the detailed information on the complex's photoinduced dynamics. Therefore, the sTDA-xTB-based NAMD method is a powerful theoretical tool for studying the ultrafast photoinduced dynamics in large extended systems with a large number of electronically excited states, which could be helpful for the subsequent design of SWCNT-based functional materials.

Unraveling the mechanism of tip-enhanced molecular energy transfer

Electronic Energy Transfer (EET) between chromophores is fundamental in many natural light-harvesting complexes, serving as a critical step for solar energy funneling in photosynthetic plants and bacteria. The complicated role of the environment in mediating this process in natural architectures has been addressed by recent scanning tunneling microscope experiments involving EET between two molecules supported on a solid substrate. These measurements demonstrated that EET in such conditions has peculiar features, such as a steep dependence on the donor-acceptor distance, reminiscent of a short-range mechanism more than of a Förster-like process. By using state of the art hybrid ab initio/electromagnetic modeling, here we provide a comprehensive theoretical analysis of tip-enhanced EET. In particular, we show that this process can be understood as a complex interplay of electromagnetic-based molecular plasmonic processes, whose result may effectively mimic short range effects. Therefore, the established identification of an exponential decay with Dexter-like effects does not hold for tip-enhanced EET, and accurate electromagnetic modeling is needed to identify the EET mechanism.

Chirality Dependence of Triplet Excitons in (6,5) and (7,5) Single-Wall Carbon Nanotubes Revealed by Optically Detected Magnetic Resonance

The excitonic structure of single-wall carbon nanotubes (SWCNTs) is chirality dependent and consists of multiple singlet and triplet excitons (TEs) of which only one singlet exciton (SE) is optically bright. In particular, the dark TEs have a large impact on the integration of SWCNTs in optoelectronic devices, where excitons are created electrically, such as in infrared light-emitting diodes, thereby strongly limiting their quantum efficiency. Here, we report the characterization of TEs in chirality-purified samples of (6,5) and (7,5) SWCNTs, either randomly oriented in a frozen solution or with in-plane preferential orientation in a film, by means of optically detected magnetic resonance (ODMR) spectroscopy. In both chiral structures, the nanotubes are shown to sustain three types of TEs. One TE exhibits axial symmetry with zero-field splitting (ZFS) parameters depending on SWCNT diameter, in good agreement with the tighter confinement expected in narrower-diameter nanotubes. The ZFS of this TE also depends on nanotube environment, pointing to slightly weaker confinement for surfactant-coated than for polymer-wrapped SWCNTs. A second TE type, with much smaller ZFS, does not show the same systematic trends with diameter and environment and has a less well-defined axial symmetry. This most likely corresponds to TEs trapped at defect sites at low temperature, as exemplified by comparing SWCNT samples from different origins and after different treatments. A third triplet has unresolved ZFS, implying it originates from weakly interacting spin pairs. Aside from the diameter dependence, ODMR thus provides insights in both the symmetry, confinement, and nature of TEs on semiconducting SWCNTs.

Deterministic transfer of optical-quality carbon nanotubes for atomically defined technology

When continued device scaling reaches the ultimate limit imposed by atoms, technology based on atomically precise structures is expected to emerge. Device fabrication will then require building blocks with identified atomic arrangements and assembly of the components without contamination. Here we report on a versatile dry transfer technique for deterministic placement of optical-quality carbon nanotubes. Single-crystalline anthracene is used as a medium which readily sublimes by mild heating, leaving behind clean nanotubes and thus enabling bright photoluminescence. We are able to position nanotubes of a desired chirality with a sub-micron accuracy under in-situ optical monitoring, thereby demonstrating deterministic coupling of a nanotube to a photonic crystal nanobeam cavity. A cross junction structure is also designed and constructed by repeating the nanotube transfer, where intertube exciton transfer is observed. Our results represent an important step towards development of devices consisting of atomically precise components and interfaces.

As device fabrication reach atomic scales, assembly of atomically defined components becomes crucial. Here, the authors demonstrate a low contamination transfer technique, using single-crystalline anthracene as medium, for placement of structure-specific carbon nanotubes with submicron accuracy.

Excitonic Energy Transfer in Heterostructures of Quasi-2D Perovskite and Monolayer WS2

Quasi-two-dimensional (2D) organic-inorganic hybrid perovskite is a re-emerging material with strongly excitonic absorption and emission properties that are attractive for photonics and optoelectronics. Here we report the experimental observation of excitonic energy transfer (ET) in van der Waals heterostructures consisting of quasi-2D hybrid perovskite (C₆H₅C₂H₄NH₃)₂PbI₄ (PEPI) and monolayer WS₂. Photoluminescence excitation spectroscopy reveals a distinct ground exciton resonance feature of perovskite, evidencing ET from perovskite to WS₂. We find unexpectedly high photoluminescence enhancement factors of up to ∼8, which cannot be explained by single-interface ET. Our analysis reveals that interlayer ET across the bulk of the layered perovskite also contributes to the large enhancement factor. Further, from the weak temperature dependence of the lower-limit ET rate, which we found to be ∼3 ns^-1, we conclude that the Förster-type mechanism is responsible.

Providing Time to Transfer: Longer Lifetimes Lead to Improved Energy Transfer in Films of Semiconducting Carbon Nanotubes

The performance of photovoltaic devices made using semiconducting carbon nanotubes is limited by the transverse exciton diffusion length, which is ultimately set by intertube energy transfer. In this paper, we study whether extending the exciton lifetime improves energy transfer, by allowing more time for exciton transfer between carbon nanotubes, and thereby device performance. To do so, we prepare nanotubes by either shear-force mixing or ultrasonication, leading to different lengths and defect densities. We create thin films that mix (6,5) and (7,5) nanotubes and quantify the relative amounts of energy transfer in them using two-dimensional white-light (2DWL) spectroscopy and photoluminescence excitation (PLE) spectroscopy. Cross-peaks appearing in 2DWL spectra and quenching of the (6,5) PLE signal upon mixing both quantify energy transfer from (6,5) to (7,5). In both spectroscopies, energy transfer between shear-force mixed tubes is ∼20% more efficient. The cross-peaks in 2DWL spectra grow in at the same rate regardless of the processing method with the all shear-force mixed sample ultimately reaching a larger cross-peak amplitude. Shear-force mixing methods instead of sonication have improved external quantum efficiency in carbon nanotube devices by 30%. The spectroscopic results observed here link energy transfer to exciton diffusion and correlate to device performance.

New Light on Molecule-Nanotube Hybrids

Optoelectronics benefits from outstanding new nanomaterials that provide emission and detection in the visible and near-infrared range, photoswitches, two level systems for single photon emission, etc. Among these, carbon nanotubes are envisioned as game changers despite difficult handling and control over chirality burdening their use. However, recent breakthroughs on hybrid carbon nanotubes have established nanotubes as pioneers for a new family of building blocks for optics and quantum optics. Functionalization of carbon nanotubes with molecules or polymers not only preserves the nanotube properties from the environment, but also promotes new performance abilities to the resulting hybrids. Photoluminescence and Raman signals are enhanced in the hybrids, which questions the nature of the electronic coupling between nanotube and molecules. Furthermore, coupling to optical cavities dramatically enhances single photon emission, which operates up to room temperature. This new light on nanotube hybrids shows their potential to push optoelectronics a step forward.

Hydroxide Ions Stabilize Open Carbon Nanotubes in Degassed Water

The main hurdle preventing the widespread use of single-walled carbon nanotubes remains the lack of methods with which to produce formulations of pristine, unshortened, unfunctionalized, individualized single-walled carbon nanotubes, thus preserving their extraordinary properties. In particular, sonication leads to shortening, which is detrimental to percolation properties (electrical, thermal, mechanical, etc.). Using reductive dissolution and transfer into degassed water, open-ended, water-filled nanotubes can be dispersed as individualized nanotubes in water-dimethyl sulfoxide mixtures, avoiding the use of sonication and surfactant. Closed nanotubes, however, aggregate immediately upon contact with water. Photoluminescence and absorption spectroscopy both point out a very high degree of individualization while retaining lengths of several microns. The resulting transparent conducting films are 1 order of magnitude more conductive than surfactant-based blanks at equal transmittance.

Evidence for Fast Interlayer Energy Transfer in MoSe2/WS2 Heterostructures

Strongly bound excitons confined in two-dimensional (2D) semiconductors are dipoles with a perfect in-plane orientation. In a vertical stack of semiconducting 2D crystals, such in-plane excitonic dipoles are expected to efficiently couple across van der Waals gap due to strong interlayer Coulomb interaction and exchange their energy. However, previous studies on heterobilayers of group 6 transition metal dichalcogenides (TMDs) found that the exciton decay dynamics is dominated by interlayer charge transfer (CT) processes. Here, we report an experimental observation of fast interlayer energy transfer (ET) in MoSe2/WS2 heterostructures using photoluminescence excitation (PLE) spectroscopy. The temperature dependence of the transfer rates suggests that the ET is Förster-type involving excitons in the WS2 layer resonantly exciting higher-order excitons in the MoSe2 layer. The estimated ET time of the order of 1 ps is among the fastest compared to those reported for other nanostructure hybrid systems such as carbon nanotube bundles. Efficient ET in these systems offers prospects for optical amplification and energy harvesting through intelligent layer engineering.

Ultrafast Exciton Hopping Observed in Bare Semiconducting Carbon Nanotube Thin Films with Two-Dimensional White-Light Spectroscopy

We observe ultrafast energy transfer between bare carbon nanotubes in a thin film using two-dimensional (2D) white-light spectroscopy. Using aqueous two-phase separation, semiconducting carbon nanotubes are purified from their metallic counterparts and condensed into a 10 nm thin film with no residual surfactant. Cross peak intensities put the time scale for energy transfer at <60 fs, and 2D anisotropy measurements determine that energy transfer is most efficient between parallel nanotubes, thus favoring directional energy flow. Lifetimes are about 300 fs. Thus, these results are in sharp contrast to thin films prepared from nanotubes that are wrapped by polymers, which exhibit picosecond energy transfer and randomize the direction of energy flow. Ultrafast energy flow and directionality are exciting properties for next-generation photovoltaics, photodetectors, and other devices.

Excitonic energy transfer in polymer wrapped carbon nanotubes in gradually grown nanoassemblies

We investigate the exciton energy transfer (ET) in nanoassemblies (nanotube based aggregates) formed by polymer wrapped single-walled carbon nanotubes (SWNTs) using photoluminescence (PL) spectroscopy and simulation. The distinctive feature of this study is the gradual growth of such nanostructures in aqueous medium induced by increasing the concentration of porphyrin molecules stitching nanotube-polymer complexes in densely packed assemblies. Experimental dependencies of PL intensity on the porphyrin concentration for different types of semiconducting SWNTs demonstrate step-like behavior controlled by the amount of bound nanotubes and are in good agreement with the simulating model. The simulation algorithm determines the criterion of the aggregate formation depending on the number of porphyrin molecules per tube and the cascade exciton energy transfer between neighboring semiconducting nanotubes of different chiralities. Aggregates of small sizes (up to six-eight individual tubes) contain mostly semiconducting species, while aggregates of a larger size (up to several tens of tubes) incorporate metallic SWNTs, inducing strong PL quenching. From the fitting procedure, an ET rate of 0.6 × 10(10) s(-1) has been determined which is consistent with the center to center distance (∼2.3 nm) between adjacent tubes separated by polymer and porphyrin molecules. The threshold of the dimer formation corresponds to one porphyrin molecule per ∼20 nm of tube lengths that was supported by molecular dynamics simulation. These findings provide insight into the ET mechanism in SWNT nanoassemblies of variable sizes, which can be gradually controlled by the external factor (the concentration of porphyrin molecules).

Near-field Raman spectroscopy of nanocarbon materials

Nanocarbon materials, including sp² hybridized two-dimensional graphene and one-dimensional carbon nanotubes, and sp¹ hybridized one-dimensional carbyne, are being considered for the next generation of integrated optoelectronic devices. The strong electron–phonon coupling present in these nanocarbon materials makes Raman spectroscopy an ideal tool to study and characterize the material and device properties. Near-field Raman spectroscopy combines non-destructive chemical, electrical, and structural specificity with nanoscale spatial resolution, making it an ideal tool for studying nanocarbon systems. Here we use near-field Raman spectroscopy to study strain, defects, and doping in different nanocarbon systems.

Photoinduced Dynamics in Carbon Nanotube Aggregates Steered by Dark Excitons

Only optically active excitons can be identified by transient absorption spectroscopy, and the actual mechanisms of exciton relaxation in nanoscale systems remain unknown as dipole-forbidden transitions and charge-transfer states are not accounted for. Focusing on interacting (6,4) and (8,4) carbon nanotubes (CNTs), we show that dark excitons largely determine the relaxation pathways for photogenerated excitons in CNT bundles. New channels appear involving asymmetric electron-hole excitations within the same CNT and charge-transfer states, in which the electron and hole are confined to separate CNTs. The energy and charge transfers are facilitated by coupling to both low- and high-frequency phonons. Radial breathing modes are particularly important because they distort the CNT geometry, induce crossings of electronic states, and modulate coupling between CNTs. The time domain simulations reported herein uncover the quantum states and phonon modes that contribute to exciton relaxation in a CNT cluster, elucidating the complete relaxation mechanism. The established role of optically dark states pertains to nonequilibrium dynamics in nanoscale materials in general.

Helical polycarbodiimide cloaking of carbon nanotubes enables inter-nanotube exciton energy transfer modulation

The use of single-walled carbon nanotubes (SWCNTs) as near-infrared optical probes and sensors require the ability to simultaneously modulate nanotube fluorescence and functionally derivatize the nanotube surface using noncovalent methods. We synthesized a small library of polycarbodiimides to noncovalently encapsulate SWCNTs with a diverse set of functional coatings, enabling their suspension in aqueous solution. These polymers, known to adopt helical conformations, exhibited ordered surface coverage on the nanotubes and allowed systematic modulation of nanotube optical properties, producing up to 12-fold differences in photoluminescence efficiency. Polymer cloaking of the fluorescent nanotubes facilitated the first instance of controllable and reversible internanotube exciton energy transfer, allowing kinetic measurements of dynamic self-assembly and disassembly.

Diffusion-assisted photoexcitation transfer in coupled semiconducting carbon nanotube thin films

We utilize femtosecond transient absorption spectroscopy to study dynamics of photoexcitation migration in films of semiconducting single-wall carbon nanotubes. Films of nanotubes in close contact enable energy migration such as needed in photovoltaic and electroluminescent devices. Two types of films composed of nanotube fibers are utilized in this study: densely packed and very porous. By comparing exciton kinetics in these films, we characterize excitation transfer between carbon nanotubes inside fibers versus between fibers. We find that intrafiber transfer takes place in both types of films, whereas interfiber transfer is greatly suppressed in the porous one. Using films with different nanotube composition, we are able to test several models of exciton transfer. The data are inconsistent with models that rely on through-space interfiber energy transfer. A model that fits the experimental results postulates that interfiber transfer occurs only at intersections between fibers, and the excitons reach the intersections by diffusing along the long-axis of the tubes. We find that time constants for the inter- and intrafiber transfers are 0.2-0.4 and 7 ps, respectively. In total, hopping between fibers accounts for about 60% of all exciton downhill transfer prior to 4 ps in the dense film. The results are discussed with regards to transmission electron micrographs of the films. This study provides a rigorous analysis of the photophysics in this new class of promising materials for photovoltaics and other technologies.

Double-wall carbon nanotubes for wide-band, ultrafast pulse generation

We demonstrate wide-band ultrafast optical pulse generation at 1, 1.5, and 2 μm using a single-polymer composite saturable absorber based on double-wall carbon nanotubes (DWNTs). The freestanding optical quality polymer composite is prepared from nanotubes dispersed in water with poly(vinyl alcohol) as the host matrix. The composite is then integrated into ytterbium-, erbium-, and thulium-doped fiber laser cavities. Using this single DWNT-polymer composite, we achieve 4.85 ps, 532 fs, and 1.6 ps mode-locked pulses at 1066, 1559, and 1883 nm, respectively, highlighting the potential of DWNTs for wide-band ultrafast photonics.

Tip-enhanced near-field optical microscopy

Tip-enhanced near-field optical microscopy (TENOM) is a scanning probe technique capable of providing a broad range of spectroscopic information on single objects and structured surfaces at nanometer spatial resolution and with highest detection sensitivity. In this review, we first illustrate the physical principle of TENOM that utilizes the antenna function of a sharp probe to efficiently couple light to excitations on nanometer length scales. We then discuss the antenna-induced enhancement of different optical sample responses including Raman scattering, fluorescence, generation of photocurrent and electroluminescence. Different experimental realizations are presented and several recent examples that demonstrate the capabilities of the technique are reviewed.

Nanotubes complexed with DNA and proteins for resistive-pulse sensing

We use a resistive-pulse technique to analyze molecular hybrids of single-wall carbon nanotubes (SWNTs) wrapped in either single-stranded DNA or protein. Electric fields confined in a glass capillary nanopore allow us to probe the physical size and surface properties of molecular hybrids at the single-molecule level. We find that the translocation duration of a macromolecular hybrid is determined by its hydrodynamic size and solution mobility. The event current reveals the effects of ion exclusion by the rod-shaped hybrids and possible effects due to temporary polarization of the SWNT core. Our results pave the way to direct sensing of small DNA or protein molecules in a large unmodified solid-state nanopore by using nanofilaments as carriers.

Carbon nanomaterials for electronics, optoelectronics, photovoltaics, and sensing

In the last three decades, zero-dimensional, one-dimensional, and two-dimensional carbon nanomaterials (i.e., fullerenes, carbon nanotubes, and graphene, respectively) have attracted significant attention from the scientific community due to their unique electronic, optical, thermal, mechanical, and chemical properties. While early work showed that these properties could enable high performance in selected applications, issues surrounding structural inhomogeneity and imprecise assembly have impeded robust and reliable implementation of carbon nanomaterials in widespread technologies. However, with recent advances in synthesis, sorting, and assembly techniques, carbon nanomaterials are experiencing renewed interest as the basis of numerous scalable technologies. Here, we present an extensive review of carbon nanomaterials in electronic, optoelectronic, photovoltaic, and sensing devices with a particular focus on the latest examples based on the highest purity samples. Specific attention is devoted to each class of carbon nanomaterial, thereby allowing comparative analysis of the suitability of fullerenes, carbon nanotubes, and graphene for each application area. In this manner, this article will provide guidance to future application developers and also articulate the remaining research challenges confronting this field.

Spectroscopic characterization of the chiral structure of individual single-walled carbon nanotubes and the edge structure of isolated graphene nanoribbons

The chiral structure of single-walled carbon nanotubes (SWNTs) and the edge structure of graphene nanoribbons (GNRs) play an important role in determining their electronic and phonon structures. Spectroscopic methods, which require simple sample preparation and cause minimal sample damage, are the most commonly utilized techniques for determining the structures of SWNTs and graphene. In this review the current status of various spectroscopic methods are presented in detail, including resonance Raman, photoluminescence (PL), and Rayleigh scattering spectroscopies, for determination of the chiral structure of individual SWNTs and the edge structure of isolated graphene, especially of graphene nanoribbons. The different photophysical processes involved in each spectroscopic method are reviewed to achieve a comprehensive understanding of the electronic and phonon properties of SWNTs and graphene. The advantages and limitations of each spectroscopic method as well as the challenges in this area are discussed.

Photoexcitation dynamics of coupled semiconducting carbon nanotube thin films

Carbon nanotubes are a promising means of capturing photons for use in solar cell devices. We time-resolved the photoexcitation dynamics of coupled, bandgap-selected, semiconducting carbon nanotubes in thin films tailored for photovoltaics. Using transient absorption spectroscopy and anisotropy measurements, we found that the photoexcitation evolves by two mechanisms with a fast and long-range component followed by a slow and short-range component. Within 300 fs of optical excitation, 20% of nanotubes transfer their photoexcitation over 5-10 nm into nearby nanotube fibers. After 3 ps, 70% of the photoexcitation resides on the smallest bandgap nanotubes. After this ultrafast process, the photoexcitation continues to transfer on a ~10 ps time scale but to predominantly aligned tubes. Ultimately the photoexcitation hops twice on average between fibers. These results are important for understanding the flow of energy and charge in coupled nanotube materials and light-harvesting devices.

Ultrafast exciton energy transfer between nanoscale coaxial cylinders: intertube transfer and luminescence quenching in double-walled carbon nanotubes

We study exciton energy transfer in double-walled carbon nanotubes using femtosecond time-resolved luminescence measurements. From direct correspondence between decay of the innertube luminescence and the rise behavior in outertube luminescence, it is found that the time constant of exciton energy transfer from the inner to the outer semiconducting tubes is ∼150 fs. This ultrafast transfer indicates that the relative intensity of steady-state luminescence from the innertubes is ∼700 times weaker than that from single-walled carbon nanotubes.

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.

Properties and application of double-walled carbon nanotubes sorted by outer-wall electronic type

Double-walled carbon nanotubes (DWNTs) can adopt four distinct permutations arising from the electronic type (metallic or semiconducting) of their inner and outer walls. This polydispersity limits the utility of DWNTs in applications such as thin film electronics. We demonstrate that density gradient ultracentrifugation can be employed to address this source of heterogeneity by producing DWNTs with well-defined outer-wall electronic types. Optical absorption measurements of sorted DWNTs reveal outer-wall purities of 96% and 98% for sorted semiconducting and metallic samples, respectively. Electrical characterization of semiconducting and metallic outer-wall DWNTs in thin film transistors directly confirms the efficacy of these separations, with semiconducting DWNT devices yielding on/off ratios 2 orders of magnitude higher than comparable metallic DWNT devices.

Exciton antennas and concentrators from core-shell and corrugated carbon nanotube filaments of homogeneous composition

There has been renewed interest in solar concentrators and optical antennas for improvements in photovoltaic energy harvesting and new optoelectronic devices. In this work, we dielectrophoretically assemble single-walled carbon nanotubes (SWNTs) of homogeneous composition into aligned filaments that can exchange excitation energy, concentrating it to the centre of core-shell structures with radial gradients in the optical bandgap. We find an unusually sharp, reversible decay in photoemission that occurs as such filaments are cycled from ambient temperature to only 357 K, attributed to the strongly temperature-dependent second-order Auger process. Core-shell structures consisting of annular shells of mostly (6,5) SWNTs (E(g)=1.21 eV) and cores with bandgaps smaller than those of the shell (E(g)=1.17 eV (7,5)-0.98 eV (8,7)) demonstrate the concentration concept: broadband absorption in the ultraviolet-near-infrared wavelength regime provides quasi-singular photoemission at the (8,7) SWNTs. This approach demonstrates the potential of specifically designed collections of nanotubes to manipulate and concentrate excitons in unique ways.

Sorting single-walled carbon nanotubes by electronic type using nonionic, biocompatible block copolymers

As-synthesized single-walled carbon nanotubes (SWNTs) typically possess a range of diameters and electronic properties. This polydispersity has hindered the development of many SWNT-based technologies and encouraged the development of postsynthetic methods for sorting SWNTs by their physical and electronic structure. Herein, we demonstrate that nonionic, biocompatible block copolymers can be used to isolate semiconducting and metallic SWNTs using density gradient ultracentrifugation. Separations conducted with different Pluronic block copolymers reveal that Pluronics with shorter hydrophobic chain lengths lead to higher purity semiconducting SWNTs, resulting in semiconducting purity levels in excess of 99% obtained for Pluronic F68. In contrast, X-shaped Tetronic block copolymers display an affinity for metallic SWNTs, yielding metallic purity levels of 74% for Tetronic 1107. These results suggest that high fidelity and high yield density gradient separations can be achieved using nonionic block copolymers with rationally designed homopolymer segments, thus generating biocompatible monodisperse SWNTs for a range of applications.

Nanoscale near-field imaging of excitons in single heterostructured nanorods

The mixed 0D-1D dimensionality of heterostructured semiconductor nanorods, resulting from the dot-in-rod architecture, raises intriguing questions concerning the location and confinement of the exciton and the origin of the fluorescence in such structures. Using apertureless near-field distance-dependent lifetime imaging together with AFM topography, we directly map the emission and determine its location with high precision along different types of nanorods. We find that the fluorescence is emanating from a sub-20 nm region, correlated to the seed location, clearly indicating exciton localization.

Ultrafast excitation energy transfer in small semiconducting carbon nanotube aggregates

We study excitation energy transfer in small aggregates of chirality enriched carbon nanotubes by transient absorption spectroscopy. Ground state photobleaching is used to monitor exciton population dynamics with sub-10 fs time resolution. Upon resonant excitation of the first exciton transition in (6,5) tubes, we find evidence for energy transfer to (7,5) tubes within our time resolution (<10 fs). Excitation in the visible spectral range, where the second excitonic transitions occur, is followed by fast intratube relaxation and subsequent energy transfer, in particular from the (8,4) tube toward other tubes, the latter process occurring in less than 10 fs.

A Comprehensive Review on Separation Methods and Techniques for Single-Walled Carbon Nanotubes

Structural control of single-walled carbon nanotubes (SWNTs) is attracting enormous interest in view of their applications to nanoelectronics and nanooptics. Actually, more than 200 papers regarding separation of SWNTs have been published since 1998. In this review, they are classified into the following five sections according to the separation methods; electrophoresis, centrifugation, chromatography, selective solubilization and selective reaction. In each method, all literature is summarized in tables showing the separated objects (metallic/semiconducting (M/S), length, diameter, (n, m) structure and/or handedness), the production process of the used SWNTs (CoMoCAT, HiPco, arc discharge and/or laser vaporization) and the employed chemicals, such as detergents and polymers. Changes in annual number of publications related to this subject are also discussed.

Excitation energy transfer from a fluorophore to single-walled carbon nanotubes

We study the process of electronic excitation energy transfer from a fluorophore to the electronic energy levels of a single-walled carbon nanotube. The matrix element for the energy transfer involves the Coulombic interaction between the transition densities on the donor and the acceptor. In the Forster approach, this is approximated as the interaction between the corresponding transition dipoles. For energy transfer from a dye to a nanotube, one can use the dipole approximation for the dye, but not for the nanotube. We have therefore calculated the rate using an approach that avoids the dipole approximation for the nanotube. We find that for the metallic nanotubes, the rate has an exponential dependence if the energy that is to be transferred, variant Planck's over 2piOmega is less than a threshold and a d(-5) dependence otherwise. The threshold is the minimum energy required for a transition other than the k(i, perpendicular)=0 and l=0 transition. Our numerical evaluation of the rate of energy transfer from the dye pyrene to a (5,5) carbon nanotube, which is metallic leads to a distance of approximately 165 A up to which energy transfer is appreciable. For the case of transfer to semiconducting carbon nanotubes, apart from the process of transfer to the electronic energy levels within the one electron picture, we also consider the possibility of energy transfer to the lowest possible excitonic state. Transfer to semiconducting carbon nanotubes is possible only if variant Planck's over 2piOmega > or = epsilon(g)-epsilon(b). The long range behavior of the rate of transfer has been found to have a d(-5) dependence if variant Planck's over 2piOmega > or = epsilon(g). But, when the emission energy of the fluorophore is in the range epsilon(g) > variant Planck's over 2piOmega > or = epsilon(g)-epsilon(b), the rate has an exponential dependence on the distance. For the case of transfer from pyrene to the semiconducting (6,4) carbon nanotube, energy transfer is found to be appreciable up to a distance of approximately 175 A.

Optical microcavity with semiconducting single-wall carbon nanotubes

We report studies of optical Fabry-Perot microcavities based on semiconducting single-wall carbon nanotubes with a quality factor of 160. We experimentally demonstrate a huge photoluminescence signal enhancement by a factor of 30 in comparison with the identical film and by a factor of 180 if compared with a thin film containing non-purified (8,7) nanotubes. Furthermore, the spectral full-width at half-maximum of the photo-induced emission is reduced down to 8 nm with very good directivity at a wavelength of about 1.3 microm. Such results prove the great potential of carbon nanotubes for photonic applications.

Do inner shells of double-walled carbon nanotubes fluoresce?

The reported fluorescence from inner shells of double-walled carbon nanotubes (DWCNTs) is an intriguing and potentially useful property. A combination of bulk and single-molecule methods was used to study the spectroscopy, chemical quenching, mechanical rigidity, abundance, density, and TEM images of the near-IR emitters in DWCNT samples. DWCNT inner shell fluorescence is found to be weaker than SWCNT fluorescence by a factor of at least 10,000. Observable near-IR emission from DWCNT samples is attributed to SWCNT impurities.

Electroluminescence from electrolyte-gated carbon nanotube field-effect transistors

We demonstrate near-infrared electroluminescence from ambipolar, electrolyte-gated arrays of highly aligned single-walled carbon nanotubes (SWNT). Using electrolytes instead of traditional oxide dielectrics in carbon nanotube field-effect transistors (FET) facilitates injection and accumulation of high densities of holes and electrons at very low gate voltages with minimal current hysteresis. We observe numerous emission spots, each corresponding to individual nanotubes in the array. The positions of these spots indicate the meeting point of the electron and hole accumulation zones determined by the applied gate and source-drain voltages. The movement of emission spots with gate voltage yields information about relative band gaps, contact resistance, defects, and interaction between carbon nanotubes within the array. Introducing thin layers of HfO(2) and TiO(2) provides a means to modify exciton screening without fundamentally changing the current-voltage characteristics or electroluminescence yield of these devices.

Coating individual single-walled carbon nanotubes with nylon 6,10 through emulsion polymerization

Solvent microenvironments are formed around individual single-walled carbon nanotubes (SWNTs) by mixing SWNT suspensions with water-immiscible organic solvents. These microenvironments are used to encapsulate the SWNTs with the monomer sebacoyl chloride. Hexamethylene diamine is then injected into the aqueous phase so the formation of nylon 6,10 is restricted to the interface between the microenvironment and water. This emulsion polymerization process results in uniform coatings of nylon 6,10 around individual SWNTs. The nylon-coated SWNTs remain dispersed in the aqueous phase and are highly luminescent at pH values ranging from 3 to 12. This emulsion polymerization method provides a general approach to coat nanotubes with various polymers.

Tip-enhanced near-field optical microscopy of carbon nanotubes

We review recent experimental studies on single-walled carbon nanotubes on substrates using tip-enhanced near-field optical microscopy (TENOM). High-resolution optical and topographic imaging with sub 15 nm spatial resolution is shown to provide novel insights into the spectroscopic properties of these nanoscale materials. In the case of semiconducting nanotubes, the simultaneous observation of Raman scattering and photoluminescence (PL) is possible, enabling a direct correlation between vibrational and electronic properties on the nanoscale. So far, applications of TENOM have focused on the spectroscopy of localized phonon modes, local band energy renormalizations induced by charge carrier doping, the environmental sensitivity of nanotube PL, and inter-nanotube energy transfer. At the end of this review we discuss the remaining limitations and challenges in this field.

Processing and properties of highly enriched double-wall carbon nanotubes

Carbon nanotubes consist of one or more concentric graphene cylinders and are under investigation for a variety of applications that make use of their excellent thermal, mechanical, electronic and optical properties. Double-wall nanotubes are ideal systems for studying the interwall interactions influencing the properties of nanotubes with two or more walls. However, current techniques to synthesize double-wall nanotubes produce unwanted single- and multiwall nanotubes. Here, we show how density gradient ultracentrifugation can be used to separate double-wall nanotubes from mixtures of single- and multiwall nanotubes through differences in their buoyant density. This technique results in samples that are highly enriched in either single- or double-wall nanotubes of similar outer wall diameter, with the double-wall nanotubes being, on average, approximately 44% longer than the single-wall nanotubes. The longer average length of the double-wall nanotubes provides distinct advantages when they are used in transparent conductors.

Thin film nanotube transistors based on self-assembled, aligned, semiconducting carbon nanotube arrays

Thin film transistors (TFTs) are now poised to revolutionize the display, sensor, and flexible electronics markets. However, there is a limited choice of channel materials compatible with low-temperature processing. This has inhibited the fabrication of high electrical performance TFTs. Single-walled carbon nanotubes (CNTs) have very high mobilities and can be solution-processed, making thin film CNT-based TFTs a natural direction for exploration. The two main challenges facing CNT-TFTs are the difficulty of placing and aligning CNTs over large areas and low on/off current ratios due to admixture of metallic nanotubes. Here, we report the self-assembly and self-alignment of CNTs from solution into micron-wide strips that form regular arrays of dense and highly aligned CNT films covering the entire chip, which is ideally suitable for device fabrication. The films are formed from pre-separated, 99% purely semiconducting CNTs and, as a result, the CNT-TFTs exhibit simultaneously high drive currents and large on/off current ratios. Moreover, they deliver strong photocurrents and are also both photo- and electroluminescent.

Hydrodynamic characterization of surfactant encapsulated carbon nanotubes using an analytical ultracentrifuge

The hydrodynamic properties of surfactant encapsulated single-walled carbon nanotubes (SWNTs) have been characterized by optically measuring their spatial and temporal redistribution in situ in an analytical ultracentrifuge. The measured redistribution profiles are fit to the Lamm equation, thus determining the sedimentation, diffusion, and hydrodynamic frictional coefficients of the surfactant encapsulated SWNTs. For sodium cholate encapsulated SWNTs, we demonstrate that the technique of analytical ultracentrifugation can be utilized to determine the linear packing density of surfactant molecules along the length of the SWNTs, 3.6 +/- 0.8 nm(-1), and the anhydrous molar volume of the surfactant molecules on the SWNT surfaces, 270 +/- 20 cm(3) mol(-1). Additionally, analytical ultracentrifugation is used to measure and compare the sedimentation rates of bundled and isolated carbon nanotubes. This study should serve as a guide for designing centrifuge-based processing procedures for preparing samples of SWNTs for a wide variety of applications and studies. Additionally, the results obtained here should aid in understanding the hydrodynamic properties of SWNTs and the interactions between SWNTs and surfactants in aqueous solution.