The conduction of a single-wall carbon nanotube depends on the pitch. If there are an integral number of carbon hexagons per pitch, then the system is periodic along the tube axis and allows "holes" (, and not "electrons",) to move inside the tube. This case accounts for a semiconducting behavior with the activation energy of the order of around 3 meV. There is a distribution of the activation energy since the pitch and the circumference can vary. Otherwise nanotubes show metallic behaviors. "Electrons" and "holes" can move in the graphene wall (two dimensions). The conduction in the wall is the same as in graphene if the finiteness of the circumference is disregarded. Cooper pairs formed by the phonon exchange attraction moving in the wall is shown to generate a temperature-independent conduction at low temperature (3-20 K).
A correlation is observed between the diameter (d) distribution of single walled carbon nanotubes and the percentages of metallic and semiconducting tubes in materials synthesized at low temperature (600 C) by plasma-assisted chemical vapor deposition. Small diameter nanotubes (average d~1.1 nm) show semiconducting-tube percentage much higher than expected for random chirality distribution. Density functional theory calculations reveal discernable differences in the cohesive energies and heat of formation energies for similar-diameter metallic, quasi-metallic and semiconducting nanotubes. Semi…
The conditions necessary for a nanotube junction connecting a metallic and semiconducting nanotube to rectify the current are theoretically investigated. A tight binding model is used for the analysis, which includes the Hartree-Fock approximation and the Green's function method. It is found that the junction has a behavior similar to the backward diode if the gate electrode is located nearby and the Fermi level of the semiconducting tube is near the gap. Such a junction would be advantageous since the required length for the rectification could be reduced.
We present an atomistic self-consistent study of the electronic and transport properties of semiconducting carbon nanotube in contact with metal electrodes of different work functions, which shows simultaneous electron and hole doping inside the nanotube junction through contact-induced charge transfer. We find that the band lineup in the nanotube bulk region is determined by the effective work function difference between the nanotube channel and source/drain electrodes, while electron transmission through the SWNT junction is affected by the local band structure modulation at the two metal-na…
We have investigated the magnetoconductance of semiconducting carbon nanotubes (CNTs) in pulsed, parallel magnetic fields up to 60 T, and report the direct observation of the predicted band-gap closure and the reopening of the gap under variation of the applied magnetic field. We also highlight the important influence of mechanical strain on the magnetoconductance of the CNTs.
We study theoretically the interactions of excitonic states with surface electromagnetic modes of small-diameter (~1 nm) semiconducting single-walled carbon nanotubes. We show that these interactions can result in strong exciton-surface-plasmon coupling. The exciton absorption line shape exhibits Rabi splitting ~0.1 eV as the exciton energy is tuned to the nearest interband surface plasmon resonance of the nanotube. We also show that the quantum confined Stark effect may be used as a tool to control the exciton binding energy and the nanotube band gap in carbon nanotubes in order, e.g., to bri…
In carbon nanotubes, the most abundant defects, caused for example by irradiation or chemisorption treatments, are small perturbing clusters, i.e. bi-site defects, extending over both A and B sites. The relative positions of these perturbing clusters play a crucial role in determining the electronic properties of carbon nanotubes. Using bandstructure and electronic transport calculations, we find out that in the case of armchair metallic nanotubes a band gap opens up when the clusters fulfill a certain periodicity condition. This phenomenon might be used in future nanoelectronic devices in whi…
We report a numerical study of the tunnel conductance through the Schottky barrier at the contact between a semiconducting carbon nanotube and a metal electrode. In a planar gate model the asymmetry between the p--doped and the n--doped region is shown to depend mainly on the difference between the electrode Fermi level and the band gap of carbon nanotubes. We quantitatively show how the gate/nanotube distance is important to get large on--off ratios. We explain the bend of the current versus gate voltage as the transition from a thermal--activation region to a tunneling region. A good agreeme…
The electronic Raman scattering (ERS) features of single-walled carbon nanotubes (SWNTs) can reveal a wealth of information about their electronic structures, but have previously been thought to appear exclusively in metallic (M-) but not in semiconducting (S-) SWNTs. We report the experimental observation of the ERS features with an accuracy of 1 meV in suspended S-SWNTs, the processes of which are accomplished via the available high-energy electron-hole pairs. The ERS features can facilitate further systematic studies on the properties of SWNT, both metallic and semiconducting, with defined …
Ultrafast photocurrent experiments are performed on semiconducting, single-walled carbon nanotubes under a resonant optical excitation of their subbands. The photogenerated excitons are dissociated at large electric fields and the resulting transport of the charge carriers turns out to be ballistic. Thermionic emission processes to the contacts dominate the photocurrent. The charge current without laser excitation is well described by a Fowler-Nordheim tunneling. The time-averaged photocurrent changes polarity as soon as sufficient charge carriers are injected from the contacts, which can be e…
We consider a semiconducting carbon nanotube (CNT) laying on a ferromagnetic insulating sub-strate with one end depassing the substrate and suspended over a metallic gate. We assume that the polarised substrate induces an exchange interaction acting as a local magnetic field for the electrons in the non-suspended CNT side. Generalizing the approach of I. Snyman and Yu.V. Nazarov [Phys. Rev. Lett. 108, 076805 (2012)] we show that one can generate electrostatically a tun-able spin-polarized polaronic state localized at the bending end of the CNT. We argue that at low temperatures manipulation an…
The thermoelectric effects in semiconducting single-walled carbon nanotubes (SWCNTs) are investigated based on the linear response theory combined with the thermal Green's function method. It is shown that the electronic states near the lowest conduction band minimum and the highest valence band maximum can be effectively described in terms of one-dimensional (1D) Dirac electrons to which a theoretical scheme is developed to describe the thermoelectric responses making it possible to study the effects of inter-band impurity scattering and in-gap states. Using the proposed scheme, the bipolar t…
We demonstrate assembly of solution processed semiconducting enriched (99%) single walled carbon nanotubes (s-SWNT) in an array with varying linear density via ac-dielectrophoresis and investigate detailed electronic transport properties of the fabricated devices. We show that (i) the quality of the alignment varies with frequency of the applied voltage and that (ii) by varying the frequency and concentration of the solution, we can control the linear density of the s-SWNTs in the array from 1/μm to 25 /μm. The maximum linear density of 25 s-SWNT /\mum reported here is the highest for any alig…
We study theoretically the charge separation in a planar array of semiconducting nanotubes (NTs) of length, 2h, placed in external electric field, F, parallel to their axes. By employing the conformal mapping, we find analytically the exact distribution of the induced charge density for arbitrary ratio of the gap, E_g, and the voltage drop, eFh, across the NT. We use the result obtained to (i) trace the narrowing of the neutral strip at center of the NT with increasing F; (ii) analyze the polarizability of the array as a function of E_g; (iii) study the field enhancement near the tips.
We derive the complete form of the van der Waals dispersion interaction between two infinitely long anisotropic semiconducting/insulating thin cylinders at all separations. The derivation is based on the general theory of dispersion interactions between anisotropic media as formulated in [J. N. Munday, D. Iannuzzi, Yu. S. Barash and F. Capasso, {\sl Phys. Rev. A} {\bf 71}, 042102 (2005)]. This formulation is then used to calculate the dispersion interactions between a pair of single walled carbon nanotubes at all separations and all angles. Non-retarded and retarded forms of the interactions a…
Zero bias conductance per spin of nanotube double junction (NTDJ) is investigated theoretically using the tight binding model, unrestricted Hartree-Fock approximation and non-equilibrium Green's functions. NTDJ consists of two metallic nanotubes joined by a piece of semiconducting nanotube, with the transition between the nanotubes made up of sets of 5 and 7 member carbon rings. A quantum well forms in the central semiconducting NT region, bounded by Schottky barriers. Spin current occurs when Coulomb interactions raise the spin degeneracy of resonant levels in the quantum well. As long as an …
High-performance solution-processed short-channel carbon nanotube (CNT) thin film transistors (TFTs) are fabricated using densely aligned arrays of metallic CNTs (m-CNTs) as source and drain electrodes, and aligned arrays of semiconducting enriched CNTs (s-CNTs) as channel material. The electrical transport measurements at room temperature show that the m-CNT contacted s-CNT array devices with a 2 um channel length perform superiorly to those of control Pd contacted s-CNT devices. The m-CNT contacted devices exhibit a maximum (average) on-conductance of 36.5 uS (19.2 uS), transconductance of 2…
Electronic properties of metal-finite semiconducting carbon nanotube interfaces are studied as a function of the nanotube length using a self-consistent tight-binding theory. We find that the shape of the potential barrier depends on the long-range tail of the charge transfer, leading to an injection barrier thickness comparable to half of the nanotube length until the nanotube reaches the bulk limit. The conductance of the nanotube junction shows a transition from tunneling to thermally-activated transport with increasing nanotube length.
Blends of carbon nanotubes with conjugated polymer and fullerene derivatives are complex nanocomposite systems, which have recently attracted a great research interest for their photovoltaic ability. Therefore, gaining a better understanding of the excitonic dynamics in such materials can be important to boost the efficiency of excitonic solar cells. Here, we studied the photophysics of a ternary system in which the polymer PTB7 and the fullerene derivative PCBM are integrated with (6,5) SWNTs. We highlight the contribution of SWNTs in the exciton dissociation and in the charge transfer proces…
We have theoretically investigated the electromechanical properties of a freely suspended carbon nanotube that is connected to a constant-current source and subjected to an external magnetic field. We show that self-excitation of mechanical vibrations of the nanotube can occur if the magnetic field $H$ exceeds a dissipation-dependent critical value $H_{c}$, which we find to be of the order of 10-100 mT for realistic parameters. The instability develops into a stationary regime characterized by time periodic oscillations in the fundamental bending mode amplitude. We find that for nanotubes with…
We address the effects of disorder on the conducting properties of metal and semiconducting carbon nanotubes. Experimentally, the mean free path is found to be much larger in metallic tubes than in doped semiconducting tubes. We show that this result can be understood theoretically if the disorder potential is long-ranged. The effects of a pseudospin index that describes the internal sublattice structure of the states lead to a suppression of scattering in metallic tubes, but not in semiconducting tubes. This conclusion is supported by tight-binding calculations.
A theoretical model for the calculation of the phonon-drag thermopower, $S^{g}$, in degenerately doped semiconducting single-wall carbon nanotubes (SWCNTs) is proposed. Detailed calculations of $S^{g}$ are performed as a function of temperature, tube radius and position of the Fermi level. We derive a simple analytical expression for $S^{g}$ that can be utilized to determine the free carrier density in doped nanotubes. At low temperatures $S^{g}$ shows an activated behavior characteristic of the one-dimensional (1D) character of carriers. Screening effects are taken into account and it is foun…
The remarkable electrical and optical properties or single-walled carbon nanotubes (SWNT) allowed for engineering device prototypes showing great potential for applications such as photodectors and solar cells. However, any path towards industrial maturity requires a detailed understanding of the fundamental mechanisms governing the process of photocurrent generation. Here, we present scanning photocurrent microscopy measurements on a double-gated suspended semiconducting SWNT and show that both photovoltaic and photothermal mechanisms are relevant for the interpretation of the photocurrent. W…
We report strategies of achieving both high assembly yield of carbon nanotubes at selected position of the circuit via dielectrophoresis (DEP) and field effect transistor (FET) yield using semiconducting enriched single walled carbon nanotube (s-SWNT) aqueous solution. When the DEP parameters were optimized for the assembly of individual s-SWNT, 97% of the devices show FET behavior with a maximum mobility of 210 cm2/Vs, on-off current ratio ~ 106 and on conductance up to 3 μS, however with an assembly yield of only 33%. As the DEP parameters were optimized so that 1-5 s-SWNTs are connected per…
Electroabsorption spectroscopy of well-identified index-defined semiconducting carbon nanotubes is reported. The measurement of high definition electroabsorption spectra allows direct indexation with unique nanotube chirality. Results show that at least for a limited range of diameters, electroabsorption is directly proportional to the exciton binding energy of nanotubes. Electroabsorption is a powerful technique which directly probes into carbon nanotube excitonic states, and may become a useful tool for in situ study of excitons in future nanotube-based photonic devices such as electroabsorp…
We show that the absorption spectrum in semiconducting nanotubes can be determined using the bosonization technique combined with mean-field theory and a harmonic approximation. Our results indicate that a multiple band semiconducting nanotube reduces to a system of weakly coupled harmonic oscillators. Additionally, the quasiparticle nature of the electron and hole that comprise an optical exciton emerges naturally from the bosonized model.
Engineering effective p-wave superconductors hosting Majorana quasiparticles (MQPs) is nowadays of particular interest, also in view of the possible utilization of MQPs in fault-tolerant topological quantum computation. In quasi one-dimensional systems, the parameter space for topological superconductivity is significantly reduced by the coupling between transverse modes. Together with the requirement of achieving the topological phase under experimentally feasible conditions, this strongly restricts in practice the choice of systems which can host MQPs. Here we demonstrate that semiconducting…
We present an atomistic self-consistent tight-binding study of the electronic and transport properties of metal-semiconducting carbon nanotube interfaces as a function of the nanotube channel length when the end of the nanotube wire is buried inside the electrodes. We show that the lineup of the nanotube band structure relative to the metal Fermi-level depends strongly on the metal work function but weakly on the details of the interface. We analyze the length-dependent transport characteristics, which predicts a transition from tunneling to thermally-activated transport with increasing nanotu…
We found a giant Seebeck effect in semiconducting single-wall carbon nanotube (SWCNT) films, which exhibited a performance comparable to that of commercial Bi2Te3 alloys. Carrier doping of semiconducting SWCNT films further improved the thermoelectric performance. These results were reproduced well by first-principles transport simulations based on a simple SWCNT junction model. These findings suggest strategies that pave the way for emerging printed, all-carbon, flexible thermoelectric devices.
In high magnetic fields, the exciton absorption spectrum of a semiconducting single-walled carbon nanotube splits as a result of Aharonov-Bohm magnetic flux. A magnetic field of 370 T, generated by the electro-magnetic flux compression destructive pulsed magnet-coil technique, was applied to single-chirality semiconducting carbon nanotubes. Using streak spectroscopy, we demonstrated the separation of the independent band-edge exciton states at the K and K' points of the Brillouin zone after the mixing of the dark and bright states above 150 T. These results enable a quantitative discussion of …
In this work, we have examined how the multi-vacancy defects induced in the horizontal direction change the energetics and the electronic structure of semiconducting Single-Walled Carbon Nanotubes (SWCNTs). The electronic structure of SWCNTs is computed for each deformed configuration by means of real space, Order(N) Tight Binding Molecular Dynamic (O(N) TBMD) simulations. Energy band gap is obtained in real space through the behavior of electronic density of states (eDOS) near the Fermi level. Vacancies can effectively change the energetics and hence the electronic structure of SWCNTs. In thi…
We study theoretically the interactions of excitonic states with surface electromagnetic modes of small-diameter (~1 nm) semiconducting single-walled carbon nanotubes. We show that these interactions can result in strong exciton-surface-plasmon coupling. The exciton absorption lineshapes exhibit the line (Rabi) splitting $~0.1-0.3$ eV as the exciton energy is tuned to the nearest interband surface plasmon resonance of the nanotube. We expect this effect to open a path to new optoelectronic device applications of semiconducting carbon nanotubes.
We calculate the thermoelectric power (or thermopower) of many semiconducting single wall carbon nanotubes (s-SWNTs) within a diameter range 0.5-1.5 nm by using the Boltzmann transport formalism combined with an extended tight-binding model. We find that the thermopower of s-SWNTs increases as the tube diameter decreases. For some s-SWNTs with diameters less than 0.6 nm, the thermopower can reach a value larger than 2000 μV/K at room temperature, which is about 6 to 10 times larger than that found in commonly used thermoelectric materials. The large thermopower values may be attributed to the …
At reduced dimensionality, Coulomb interactions play a crucial role in determining device properties. While such interactions within the same carbon nanotube have been shown to have unexpected properties, device integration and multi-nanotube devices require the consideration of inter-nanotube interactions. We present calculations of the characteristics of planar carbon nanotube transistors including interactions between semiconducting nanotubes and between semiconducting and metallic nanotubes. The results indicate that inter-tube interactions affect both the channel behavior and the contacts…
Semiconducting single-wall carbon nanotubes are classified into two types by means of orbital angular momentum of valley state, which is useful to study their low energy electronic properties in finite-length. The classification is given by an integer $d$, which is the greatest common divisor of two integers $n$ and $m$ specifying the chirality of nanotubes, by analyzing cutting lines. For the case that $d$ is equal to or greater than four, two lowest subbands from two valleys have different angular momenta with respect to the nanotube axis. Reflecting the decoupling of two valleys, discrete e…
In-situ Raman experiments together with transport measurements have been carried out on carbon nanotubes as a function of gate voltage. In metallic tubes, a large increase in the Raman frequency of the $G^-$ band, accompanied by a substantial decrease of its line-width, is observed with electron or hole doping. In addition, we see an increase in Raman frequency of the $G^+$ band in semiconducting tubes. These results are quantitatively explained using ab-initio calculations that take into account effects beyond the adiabatic approximation. Our results imply that Raman spectroscopy can be used …
We investigate theoretically the switching characteristics of semiconducting carbon nanotubes connected to gold electrodes under an external (gate) electric field. We find that the external introduction of holes is necessary to account for the experimental observations. We identify metal-induced-gap states (MIGS) at the contacts and find that the MIGS of an undoped tube would not significantly affect the switching behavior, even for very short tube lengths. We also explore the miniaturization limits of nanotube transistors, and, on the basis of their switching ratio, we conclude that transisto…
Semiconducting carbon nanotubes under high electric field stress (~10 V/um) display a striking, exponential current increase due to avalanche generation of free electrons and holes. Unlike in other materials, the avalanche process in such 1D quantum wires involves access to the third sub-band, is insensitive to temperature, but strongly dependent on diameter ~exp(-1/d^2). Comparison with a theoretical model yields a novel approach to obtain the inelastic optical phonon emission length, L_OP,ems ~ 15d nm. The combined results underscore the importance of multi-band transport in 1D molecular wir…
We investigate the electronic transport properties of semiconducting ($m$,$n$) carbon nanotubes (CNTs) on the mesoscopic length scale with arbitrarily distributed realistic defects. The study is done by performing quantum transport calculations based on recursive Green's function techniques and an underlying density-functional-based tight-binding model for the description of the electronic structure. Zigzag CNTs as well as chiral CNTs of different diameter are considered. Different defects are exemplarily represented by monovacancies and divacancies. We show the energy-dependent transmission a…
We report optical separation of metallic and semiconducting single walled carbon nanotubes of specific diameters from well-dispersed, stable solution. This phenomenon of optical separation was studied using lamps of different frequencies across UV, Vis and NIR spectrum. Optically aggregated SWNTs were separated from the SWNTs remaining dispersed in the solution and characterized using absorption and Raman spectroscopy. Separated SWNTs distinctly show enrichment of metallic SWNTs using UV lamp, whereas different diameter of SWNTs aggregates out depending on the frequency of illumination. Fabric…
We report specifically two things, one the phenomenon of optically induced motion in pristine single walled carbon nanotubes (SWNT) on the macro-scale and other the consequent separation of metallic and semiconducting enriched SWNT aggregates. Experiments provide direct evidence of both positive and negative photophoresis of SWNTs in solution i.e. motion away and towards light respectively. This optically induced motion was found to be dependent on frequency and intensity of light. Aggregates of pristine SWNT, moving under UV and Visible lamp were separated and characterized using absorption a…
We present a first-principles study of the electronic transport properties of micrometer long semiconducting CNTs randomly covered with carbene functional groups. Whereas prior studies suggested that metallic tubes are hardly affected by such addends, we show here that the conductance of semiconducting tubes with standard diameter is on the contrary severely damaged. The configurational averaged conductance as a function of tube diameter and with a coverage of up to one hundred functional groups is extracted. Our results indicate that the search for a conductance-preserving covalent functional…
We investigate electronic transport properties of Schottky-barrier field-effect transistors (FET) based on double-walled carbon nanotubes (DWNT) with a semiconducting outer shell and a metallic inner one. These kind of DWNT-FET show asymmetries of the $I$-$V$ characteristics and threshold voltages due to the electron-hole asymmetry of the Schottky barrier. The presence of the metallic inner shell induces a large effective band gap, which is one order of magnitude larger than that due to the semiconducting shell alone of a single-walled carbon nanotube FET.
How carbon nanotubes behave in an external electric field? What will be the relation between the intensity of the electric field and the tube's deformation? What are the geometry effects on the response of carbon nanotubes to electric fields? To answer these questions, we have developed a new combined computational technique to study electrostatic field induced deformations of carbon nanotubes. In this work, we find that the deflection angle of cantilevered semiconducting single-walled carbon nanotubes is proportional to the square of the electric field strength, and the tubes can be most bent…
We observe current rectification in a molecular diode consisting of a semiconducting single-wall carbon nanotube and an impurity. One half of the nanotube has no impurity, and it has a current-voltage (I-V) charcteristic of a typical semiconducting nanotube. The other half of the nanotube has the impurity on it, and its I-V characteristic is that of a diode. Current in the nanotube diode is carried by holes transported through the molecule's one-dimensional subbands. At 77 Kelvin we observe a step-wise increase in the current through the diode as a function of gate voltage, showing that we can…
Charge transport in semiconducting single-walled nanotubes (SWNTs) with Schottky-barrier contacts has been studied at high bias. We observe nearly symmetric ambipolar transport with electron and hole currents significantly exceeding 25 micron-ampere, the reported current limit in metallic SWNTs due to optical phonon emission. Four simple models for the field-dependent velocity (ballistic, current saturation, velocity saturation, and constant mobility) are studied in the unipolar regime; the high-bias behavior is best explained by a velocity saturation model with a saturation velocity of 2 x 10…
We present new data and a compact mobility model for single-wall carbon nanotubes, with only two adjustable parameters, the elastic and inelastic collision mean free paths at 300 K. The mobility increases with diameter, decreases with temperature, and has a more complex dependence on charge density. The model and data suggest the room temperature mobility does not exceed 10,000 cm2/V.s at high carrier density (n > 0.5 nm-1) for typical single-wall nanotube diameters, due to the strong scattering effect of the second subband.
We use a simultaneous flow of ethylene and hydrogen gases to grow single wall carbon nanotubes by chemical vapor deposition. Strong coupling to the gate is inferred from transport measurements for both metallic and semiconducting tubes. At low-temperatures, our samples act as single-electron transistors where the transport mechanism is mainly governed by Coulomb blockade. The measurements reveal very rich quantized energy level spectra spanning from valence to conduction band. The Coulomb diamonds have similar addition energies on both sides of the semiconducting gap. Signatures of subbands po…
We have measured the electroluminescence and photoluminescence of (9,7) semiconducting carbon nanotube devices and demonstrate that the electroluminescence wavelength is determined by the nanotube's chiral index (n,m). The devices were fabricated on Si3N4 membranes by dielectrophoretic assembly of tubes from monochiral dispersion. Electrically driven (9,7) devices exhibit a single Lorentzian shaped emission peak at 825 nm in the visible part of the spectrum. The emission could be assigned to the excitonic E22 interband transition by comparison of the electroluminescence spectra with correspond…
We investigate the thermal conductivity of four types of deformed carbon nanotubes by using the nonequilibrium molecular dynamics method. It is reported that various deformations have different influence on the thermal properties of carbon nanotubes. For the bending carbon nanotubes, the thermal conductivity is independent on the bending angle. However, the thermal conductivity increases lightly with XY-distortion and decreases rapidly with Z-distortion. The thermal conductivity does not change with the screw ratio before the breaking of carbon nanotubes but decreases sharply after the critica…
We have theoretically investigated thermoelectric (TE) effects of narrow-gap single-walled carbon nanotubes (SWCNTs) with randomly substituted nitrogen (N) impurities, i.e., N-substituted (20,0) SWCNTs with a band gap of 0.497 eV. For such a narrow-gap system, the thermal excitation from the valence band to the conduction band contributes to its TE properties even at the room temperature. In this study, the N-impurity bands are treated with both conduction and valence bands taken into account self-consistently. We found the optimal N concentration per unit cell, $c_{\rm opt}$, which gives the …
We use an Atomic Force Microscope (AFM) tip to locally probe the electronic properties of semiconducting carbon nanotube transistors. A gold-coated AFM tip serves as a voltage or current probe in three-probe measurement setup. Using the tip as a movable current probe, we investigate the scaling of the device properties with channel length. Using the tip as a voltage probe, we study the properties of the contacts. We find that Au makes an excellent contact in the p-region, with no Schottky barrier. In the n-region large contact resistances were found which dominate the transport properties.
We show that quantum confinement in the valence and conduction bands of semiconducting single-walled carbon nanotubes can be engineered by means of artificial defects. This ability holds potential for designing future nanotube-based quantum devices such as electrically driven room-temperature single-photon sources emitting at telecom-wavelength. Using Ar$^{+}$ and N$^{+}$ ion-induced defects, intrananotube quantum dots with sub-10 nm lateral sizes are created, giving rise to quantized electronic bound states with level spacings of the order of 100 meV and larger. Using low-temperature scanning…
Plasmon resonance is expected to occur in metallic and doped semiconducting carbon nanotubes in the terahertz frequency range, but its convincing identification has so far been elusive. The origin of the terahertz conductivity peak commonly observed for carbon nanotube ensembles remains controversial. Here we present results of optical, terahertz, and DC transport measurements on highly enriched metallic and semiconducting nanotube films. A broad and strong terahertz conductivity peak appears in both types of films, whose behaviors are consistent with the plasmon resonance explanation, firmly …
We examine the excitonic nature of high-lying optical transitions in single-walled carbon nanotubes by means of Rayleigh scattering spectroscopy. A careful analysis of the principal transitions of individual semiconducting and metallic nanotubes reveals that in both cases the lineshape is consistent with an excitonic model, but not one of free-carriers. For semiconducting species, side-bands are observed at ~200 meV above the third and fourth optical transitions. These features are ascribed to exciton-phonon bound states. Such side-bands are not apparent for metallic nanotubes,as expected from…
Optical and electronic phenomena in solids arise from the behaviour of electrons and holes (unoccupied states in a filled electron sea). Electron-hole symmetry can often be invoked as a simplifying description, which states that electrons with energy above the Fermi sea behave the same as holes below the Fermi energy. In semiconductors, however, electron-hole symmetry is generally absent since the energy band structure of the conduction band differs from the valence band. Here we report on measurements of the discrete, quantized-energy spectrum of electrons and holes in a semiconducting carbon…
We have used a femtosecond pump-probe impulsive Raman technique to explore the polarization dependence of coherent optical phonons in highly-purified and aligned semiconducting single-wall carbon nanotubes (SWCNTs). Coherent phonon spectra for the radial breathing modes (RBMs) exhibit a different monochromatic frequency between the film and solution samples, indicating the presence of differing exciton excitation processes. By varying the polarization of the incident pump beam on the aligned SWCNT film, we found that the anisotropy of the coherent RBM excitation depends on the laser wavelength…
The orbital and spin magnetic properties of iron inside transforming metallic and semiconducting 1D carbon nanotube hybrids are studied by means of local x-ray magnetic circular dichroism (XMCD) and bulk superconducting quantum interference device (SQUID) measurements. Nanotube hybrids are initially ferrocene filled single-walled carbon nanotubes (SWCNT) of different metallicities. After a high temperature nanochemical reaction ferrocene molecules react with each other to form iron nano clusters. We show that the ferrocenes molecular orbitals interact differently with the SWCNT of different me…
