Bright fluorescence from individual single-walled carbon nanotubes

Single-walled carbon nanotubes (SWNTs) have unique photophysical properties but low fluorescence efficiency. We have found significant increases in the fluorescence efficiency of individual DNA-wrapped SWNTs upon addition of reducing agents, including dithiothreitol, Trolox, and β-mercaptoethanol. Brightening was reversible upon removal of the reducing molecules, suggesting that a transient reduction of defect sites on the SWNT sidewall causes the effect. These results imply that SWNTs are intrinsically bright emitters and that their poor emission arises from defective nanotubes.     

Polarized light microscopy and spectroscopy of individual single-walled carbon nanotubes

Polarized light microscopy (PLM) is used to image individual single-walled carbon nanotubes (SWNTs) suspended in air across a slit opening. The imaging contrast relies on the strong optical anisotropy typical of SWNTs. We combine PLM with a tunable light source to enable hyperspectral excitation spectroscopy and nanotube chirality assignment. Comparison with fluorescence microscopy and spectroscopy confirms the assignment made with PLM. This represents a versatile new approach to imaging SWNTs and related structures.      

Electrically driven gallium movement in carbon nanotubes

Electrically driven gallium movement in carbon nanotubes is discussed. A higher current (~15 mA) makes the gallium migrate sharply toward the anode, which increases its mass transport speed with time in the range of 0 to more than 10.345 fg s(-1). In contrast, a lower current (~2 mA) only drives gallium to contact the anode, which decreases the resistance of the nanocomposite sharply, from 2.564 kΩ to 0.4 Ω. These results are valuable for designing electrically driven nanomass delivery and nanoswitches, respectively.     

Phonon-assisted electron emission from individual carbon nanotubes

A question of how electrons can escape from one-atom-thick surfaces has seldom been studied and is still not properly answered. Herein, lateral electron emission from a one-atom-thick surface is thoroughly studied for the first time. We study electron emission from side surface of individual electrically biased carbon nanotubes (CNTs) both experimentally and theoretically and discover a new electron emission mechanism named phonon-assisted electron emission. A kinetic model based on coupled Boltzmann equations of electrons and optical phonons is proposed and well describes experimentally measured lateral electron emission from CNTs. It is shown that the electrons moving along a biased CNT can overflow from the one-atom-thick surface due to the absorption of hot forward-scattering optical phonons. A low working voltage, high emission density, and side emission character make phonon-assisted electron emission primarily promising in electron source applications.     

Temperature sensing using individual single-walled carbon nanotubes

Molecular dynamics simulations and quantum transport theory are employed to study the temperature-dependent electrical properties of individual (12,0) zigzag and (5,5) armchair carbon nanotubes deposited on silicon substrates. The results demonstrate that the magnitude of the leakage current depends on the length of the nanotube. Furthermore, the leakage current is generated periodically along the length of the nanotube. Finally, the results indicate that given an appropriate value of the applied bias voltage, the induced current varies linearly with the temperature over specific temperature ranges. As a result, the temperature can be inversely derived from the measured current signal. Overall, the results show that the (12,0) zigzag and (5,5) armchair carbon nanotubes are suitable for temperature sensing applications over temperature ranges of 200-420?K and 300-440?K, respectively.     

Reflectance spectra of individual single-walled carbon nanotubes

We report backscattering spectroscopic measurements on individual single-walled carbon nanotubes (SWNTs). The reflectance spectra show geometry-dependent resonant peaks corresponding to optical transitions between Van Hove singularities in the SWNTs' joint density of states. All nanotubes display certain colours as their reflectance spectra demonstrate strong energy dependence. This approach was proved to be an effective tool for probing the geometric structures and optical properties of individual SWNTs.     

Absorption spectroscopy of individual single-walled carbon nanotubes

Current methods for producing single-walled carbon nanotubes (SWNTs) lead to heterogeneous samples containing mixtures of metallic and semiconducting species with a variety of lengths and defects. Optical detection at the single nanotube level should thus offer the possibility to examine these heterogeneities provided that both SWNT species are equally well detected. Here, we used photothermal heterodyne detection to record absorption images and spectra of individual SWNTs. Because this photothermal method relies only on light absorption, it readily detects metallic nanotubes as well as the emissive semiconducting species. The first and second optical transitions in individual semiconducting nanotubes have been probed. Comparison between the emission and absorption spectra of the lowest-lying optical transition reveal mainly small Stokes shifts. Side bands in the near-infrared absorption spectra are observed and assigned to exciton-phonon bound states. No such sidebands are detected around the lowest transition of metallic nanotubes.     

Electrically excited, localized infrared emission from single carbon nanotubes

Carbon nanotube field-effect transistors (CNTFETs) produce band gap derived infrared emission under both ambipolar and unipolar transport conditions. We demonstrate here that heterogeneities/defects in the local environment of a CNTFET perturb the local potentials and, as a result, the characteristic bias dependent motion of the ambipolar light emission. Such defects can also introduce localized infrared emission due to impact excitation by carriers accelerated by a voltage drop at the defect. The correlation of the change in the motion of the ambipolarlight emission and of the stationary electroluminescence with the electrical characteristics of the CNTFETs shows that stationaryelectroluminescence can identify "environmental defects" in carbon nanotubes and help evaluate their influence on electrical transport and device operation. A number of different defects are studied involving local dielectric environment changes (partially polymer-covered nanotubes), nanotube-nanotube contacts in looped nanotubes, and nanotube segments close to the electronic contacts. Random defects due to local charging are also observed.     

Structure-dependent fluorescence efficiencies of individual single-walled carbon nanotubes

Single-nanotube photometry was used to measure the product of absorption cross section and fluorescence quantum yield for 12 (n,m) structural species of semiconducting single-walled carbon nanotubes in aqueous SDBS suspension. These products ranged from 1.7 to 4.5 x 10(-19) cm(2)/C atom, generally increasing with optical band gap as described by the energy gap law. The findings suggest fluorescent quantum yields of approximately 8% for the brightest, (10,2) species and introduce the empirical calibration factors needed to deduce quantitative (n,m) distributions from bulk fluorimetric intensities.     

On the electron-phonon coupling of individual single-walled carbon nanotubes

We show that the phonon coupling to the electronic system in individual metallic single-walled carbon nanotubes is not due to coupling to low-energy plasmons. The evidence stems from the measured Raman-Stokes G-mode, which for metallic and semiconducting tubes could be fitted well by the superposition of only two Lorentzian lines associated with vibrational modes along the nanotube axis and the nanotube circumference. In the case of metallic tubes the lower-energy G mode is significantly broadened, however maintaining the Lorentzian line shape, in contrast to the theoretically expected asymmetric Breit-Wigner-Fano line shape from phonon-plasmon coupling. The results were obtained by studying 25 individual metallic and semiconducting single-walled carbon nanotubes with atomic force microscopy, electron transport measurements, and resonant Raman spectroscopy.     

Synthesis of single-walled carbon nanotubes by induction thermal plasma

The production of high quality single-walled carbon nanotubes (SWCNTs) on a bulk scale has been an issue of considerable interest. Recently, it has been demonstrated that high quality SWCNTs can be continuously synthesized on large scale by using induction thermal plasma technology. In this process, the high energy density of the thermal plasma is employed to generate dense vapor-phase precursors for the synthesis of SWCNTs. With the current reactor system, a carbon soot product which contains approximately 40 wt% of SWCNTs can be continuously synthesized at the high production rate of ∼100 g/h. In this article, our recent research efforts to achieve major advances in this technology are presented. Firstly, the processing parameters involved are examined systematically in order to evaluate their individual influences on the SWCNT synthesis. Based on these results, the appropriate operating conditions of the induction thermal plasma process for an effective synthesis of SWCNTs are discussed. A characterization study has also been performed on the SWCNTs produced under the optimum processing conditions. Finally, a mathematical model of the process currently under development is described. The model will help us to better understand the synthesis of SWCNTs in the induction plasma process.      

Visualization of individual single-walled carbon nanotubes by fluorescent polymer wrapping

Manipulating optical properties of single-walled nanotubes (SWNTs) is necessary for the development of nanoscale optical devices and probes for biomedical research. In life sciences it will make possible the direct observation of SWNTs inside living cells using optical microscopes. In the nanotechnology field it will enable the development of nanosensors with fluorescent reporting. However, the direct fluorescent labeling of SWNTs is obstructed by their strong light quenching qualities. Besides, chemical functionalization of SWNTs needed for the covalent attachment of fluorescent dyes could change favorable properties of nanotubes. Here we report that optical properties of SWNTs can be manipulated without their covalent modification by wrapping them with fluorescently labeled polymer poly(vinylpyrrolidone) (PVP-1300). Fluorescent PVP-1300 forms a monomolecular approximately 2.5 nm thick layer coiling around individual SWNTs and nanotube bundles. PVP casing is fluorescent although it is only several nanometers thick. This makes individual SWNTs observable by a fluorescent microscope. The spare polymer strands left over after wrapping around the relatively shorter nanotubes form junctions between SWNTs tying them together into new configurations, primarily Y- and psi-type junctions. The ability to use a single fluorescent polymer strand to fasten nanotubes together can be useful in assembly of nanotube-made devices. In PVP-covered SWNTs multiple fluorophores are attached to each single nanotube making them unique composite fluorophores attractive as parts of biological fluorescent probes and in the development of the new materials in photonics and nanotechnology.     

High frequency performance of individual and arrays of single-walled carbon nanotubes

We have studied the high frequency performance limits of single-walled carbon nanotube (SWNT) transistors in the diffusive transport regime limited by the acoustic phonon scattering. The relativistic band structure of single-walled carbon nanotubes combined with the acoustic phonon scattering provides an analytical model for the charge transport of the radio frequency transistors. We were able to obtain the intrinsic high frequency performance such as the cut-off frequency and the linearity of the SWNT transistors. We have extended our model to include transistors based on arrays of SWNTs. The effect of electrostatic screening in a dense array of SWNTs on the cut-off frequency is studied.     

Energy loss of the electron system in individual single-walled carbon nanotubes

We characterize the energy loss of the nonequilibrium electron system in individual metallic single-walled carbon nanotubes at low temperature. Using Johnson noise thermometry, we demonstrate that, for a nanotube with Ohmic contacts, the dc resistance at finite bias current directly reflects the average electron temperature. This enables a straightforward determination of the thermal conductance associated with cooling of the nanotube electron system. In analyzing the temperature- and length-dependence of the thermal conductance, we consider contributions from acoustic phonon emission, optical phonon emission, and hot electron outdiffusion.     

Identifying individual single-walled and double-walled carbon nanotubes by atomic force microscopy

We show that the number of concentric graphene cylinders forming a carbon nanotube can be found by squeezing the tube between an atomic force microscope tip and a silicon substrate. The compressed height of a single-walled nanotube (double-walled nanotube) is approximately two (four) times the interlayer spacing of graphite. Measured compression forces are consistent with the predicted bending modulus of graphene and provide a mechanical signature for identifying individual single-walled and double-walled nanotubes.     

Versatile visualization of individual single-walled carbon nanotubes with near-infrared fluorescence microscopy

Fluorescence microscopy in the near-infrared between 950 and 1600 nm has been developed as a novel method to image and study single-walled carbon nanotubes (SWNTs) in a variety of environments. Intrinsic photoluminescence of disaggregated pristine SWNTs was excited by a diode laser and detected with a two-dimensional InGaAs photodiode array. Individual nanotubes were visualized with a spatial resolution of ca. 1 microm and characterized with polarization measurements and emission spectroscopy. Spatially resolved emission spectra allowed (n,m) identification of single nanotubes and revealed small environmentally induced spectral shifts between segments of long tubes. Nanotube motions in aqueous surfactant were visualized with a time resolution of 50 ms and used to estimate the diffusion coefficient.     

Cutting single-walled carbon nanotubes

A two-step process is utilized for cutting single-walled carbon nanotubes (SWNTs). The first step requires the breakage of carbon-carbon bonds in the lattice while the second step is aimed at etching at these damage sites to create short, cut nanotubes. To achieve monodisperse lengths from any cutting strategy requires control of both steps. Room-temperature piranha and ammonium persulfate solutions have shown the ability to exploit the damage sites and etch SWNTs in a controlled manner. Despite the aggressive nature of these oxidizing solutions, the etch rate for SWNTs is relatively slow and almost no new sidewall damage is introduced. Carbon-carbon bond breakage can be introduced through fluorination to ～C(2)F, and subsequent etching using piranha solutions has been shown to be very effective in cutting nanotubes. The final average length of the nanotubes is approximately?100?nm with carbon yields as high as 70-80%.     

Nanoplasmonic colloidal suspensions for the enhancement of the luminescent emission from single-walled carbon nanotubes

Aiming to enhance the luminescence yield of carbon nanotubes, we introduce a new class of hybrid nanoplasmonic colloidal systems （π-hybrids）. Nanotubes dispersed in gold nanorod colloidal suspensions yield hybrid structures exhibiting enhanced luminescence up to a factor of 20. The novelty of the proposed enhancement mechanism relies on including metal proximity effects in addition to its localized surface plasmons. This simple, robust and flexible technique enhances the luminescence of nanotubes with chiralities whose enhancement has never reported before, for example the （8,4） tube.     

Controlled switching of optical emission energies in semiconducting single-walled carbon nanotubes

We present scanning photoluminescence (PL) microscopy of freely suspended single-walled carbon nanotubes grown by chemically assisted vapor deposition (CVD) across micron-sized open apertures. Scans of the PL emission versus excitation position show unusual "holes"having subwavelength spatial features associated with abrupt blue shifts of the emission energy. By varying the excitation polarization, energy, intensity, and position, we demonstrate that optical switching in some nanotubes is controllable in a highly nonlinear manner by adjusting the nonequilibrium carrier density in the nanotube. Technologically important attributes include large spectral contrast between on/off states at room temperature, a dramatic response to small changes in light intensity near threshold, and the possibility that electrical charge injection could also be used to control emission energies.     

In situ raman measurements of suspended individual single-walled carbon nanotubes under strain

We present a technique for in situ Raman measurements of suspended individual single-walled carbon nanotubes (SWNTs) under strain. We observe a strong change in the radial breathing mode intensity with increasing strain as the nanotube moves out of (or into) resonance, and for strain greater than approximately 2%, there is a clear irreversible upshift in the G-mode frequencies accompanied by an increase in intensity of a broad peak at a position associated with the D mode. For lower strain, the G-mode peaks (A1, E1, and E2) do not change significantly in position but change in relative intensity.