Doping Of Single-walled Carbon Nanotubes Research Articles

Electrochemical doping of single-walled carbon nanotubes in double layer capacitors studied by in situ Raman spectroscopy

The electrochemical doping of single-walled carbon nanotubes (SWCNTs) in 1 M Et 4NBF 4 in acetonitrile was investigated by in situ Raman spectroscopy. The capacitance was determined to be 82 F/g for the positive and 71 F/g for the negative SWCNT electrode, respectively, which approaches the typical values for microporous activated carbons used in supercapacitors. The changes in the Raman intensities and shifts of the D and G + bands as well as of the radial breathing modes (RBMs) during electron and hole injection were studied as a function of the electrode potential. For the D and G + bands, hole doping leads to strong upshifts which can be attributed to a stiffening of C–C bonds and the corresponding phonon modes. Electron doping results in much less pronounced changes in the band positions. The intensity attenuation of the RBM bands was found to be markedly different for semi-conducting and metallic SWCNTs, whereby sufficiently high doping leads to a loss of Raman intensity due to bleaching of electronic transitions. The main RBM bands upshift upon both electron and hole doping, which is attributed to changes in the chemical environment of individual SWCNTs upon charging and discharging of the electrochemical double layer within SWCNT bundles.

Boron and Nitrogen Doped Single walled Carbon Nanotubes as Possible Dilute Magnetic Semiconductors

The structure of single walled armchair and zig-zag carbon nanotubes having 70 atoms and two carbons replaced by boron or nitrogen is obtained at minium energy using HF/6-31G* molecular orbital theory. The calculations show that the ground state of the zig-zag tubes is a triplet state while for the armchair tubes it is a singlet. In the zig-zag tubes the density of states at the Fermi level is greater for the spin down states compared to the spin up state indicating that the doped tubes could be ferromagnetic.

Improvement of Upper Critical Field and Critical Current Density in Single Walled CNT Doped ${\hbox{MgB}}_{2}/{\hbox{Fe}}$ Wires

We evaluated the doping effect of single walled carbon nanotubes (SWCNT) on the phase formation, microstructure, critical current density (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ), and upper critical field (H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c2</sub> ) of MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> superconductor. All samples were sintered at temperatures ranging from 650 to 1000degC for 30 min. A typical high sintering temperature (900degC) sample was composed of well-consolidated MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> grain of 100 to 200 nm and nanosized particles, resulting in enhancement of J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> and H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c2</sub> . This indicates that lattice defects and microstructural change have occurred as a result of carbon (C) coming from SWCNT. On the other hand, a sample sintered at low temperature (700degC) contained relatively large grains. Specifically, SWCNT doped MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> wire sintered at 900degC exhibited excellent J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> > 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> Acm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> up to 8 T at 4.2 K. This result indicates that flux pinning for sample produced at a high sintering temperature are enhanced by SWCNT doping. SWCNT is one of the promising C sources for MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> superconductor with excellent J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> .

Surface potential analyses of single-walled carbon nanotube/metal interfaces

The band structure of a single-walled carbon nanotube (SWNT)/metal system, including a shift in the vacuum level which appears at the SWNT/metal interface, was characterized by using a Kelvin-probe force microscopy method. In the SWNT/gold system, the shift (Δ) of the vacuum level was observed to be +5.2meV, which is one order of magnitude smaller than previously observed. When we carried out carrier doping of the SWNTs, the observed Δ was downshifted from +5.2to−52meV, which introduced some upshift in the band structure of the doped SWNTs. This change induced easier hole injection, which coincides with the field effect transistor characteristics of undoped and doped SWNT transistors. A similar analysis was also carried out for a SWNT/ferromagnet system.

Cover Picture: Bilayer Organic–Inorganic Gate Dielectrics for High‐Performance, Low‐Voltage, Single‐Walled Carbon Nanotube Thin‐Film Transistors, Complementary Logic Gates, and p–n Diodes on Plastic Substrates (Adv. Funct. Mater. 18/2006)

AbstractLow‐voltage, hysteresis‐free, flexible thin‐film‐type electronic systems based on networks of single‐walled carbon nanotubes and bilayer organic–inorganic nanodielectrics are detailed in work by Rogers and co‐workers reported on p. 2355. The cover image shows a schematic array of such thin‐film transistors (TFTs) on a plastic substrate. The structure of the bilayer nanodielectric, which consists of a film of HfO2 formed by atomic layer deposition and an ultrathin layer of epoxy formed by spin‐casting, is also illustrated schematically.High‐capacitance bilayer dielectrics based on atomic‐layer‐deposited HfO2 and spin‐cast epoxy are used with networks of single‐walled carbon nanotubes (SWNTs) to enable low‐voltage, hysteresis‐free, and high‐performance thin‐film transistors (TFTs) on silicon and flexible plastic substrates. These HfO2–epoxy dielectrics exhibit excellent properties including mechanical flexibility, large capacitance (up to ca. 330 nF cm–2), and low leakage current (ca. 10–8 A cm–2); their low‐temperature (ca. 150 °C) deposition makes them compatible with a range of plastic substrates. Analysis and measurements of these dielectrics as gate insulators in SWNT TFTs illustrate several attractive characteristics for this application. Their compatibility with polymers used for charge‐transfer doping of SWNTs is also demonstrated through the fabrication of n‐channel SWNT TFTs, low‐voltage p–n diodes, and complementary logic gates.

Bilayer Organic–Inorganic Gate Dielectrics for High‐Performance, Low‐Voltage, Single‐Walled Carbon Nanotube Thin‐Film Transistors, Complementary Logic Gates, and p–n Diodes on Plastic Substrates

AbstractHigh‐capacitance bilayer dielectrics based on atomic‐layer‐deposited HfO2 and spin‐cast epoxy are used with networks of single‐walled carbon nanotubes (SWNTs) to enable low‐voltage, hysteresis‐free, and high‐performance thin‐film transistors (TFTs) on silicon and flexible plastic substrates. These HfO2–epoxy dielectrics exhibit excellent properties including mechanical flexibility, large capacitance (up to ca. 330 nF cm–2), and low leakage current (ca. 10–8 A cm–2); their low‐temperature (ca. 150 °C) deposition makes them compatible with a range of plastic substrates. Analysis and measurements of these dielectrics as gate insulators in SWNT TFTs illustrate several attractive characteristics for this application. Their compatibility with polymers used for charge‐transfer doping of SWNTs is also demonstrated through the fabrication of n‐channel SWNT TFTs, low‐voltage p–n diodes, and complementary logic gates.

The effects of nitrogen and boron doping on the optical emission and diameters of single-walled carbon nanotubes

Using TEM, absorption and photoluminescence-excitation spectroscopy we have shown that nitrogen and nitrogen/boron doping of single-walled carbon nanotubes produce significant changes in both the optical properties and the diameter distribution of nanotubes produced by the arc-discharge method. Smaller diameter tubes are preferentially formed in the presence of boron. In addition the presence of nitrogen is found to significantly affect the emission properties of the nanotube ensemble, causing a shift in the dominant emission to lower energies, possibly due to changes in the bundling structure of the nanotubes in solution, but only very small changes are observed in the emission energies for individual nanotubes.

High-temperature electrical transport properties of buckypapers composed of doped single-walled carbon nanotubes

Data on temperature-dependent electrical resistance of buckypaper flakes are presented in this paper. The buckypapers are composed of ropes of aligned single-walled carbon nanotubes doped with HNO 3, which are treated as mixed systems with their properties being dependent on the treatment performed. The measurements cover rather wide temperature range from 300 up to 900 K. In case of untreated samples, curves with two well-defined activation energies are seen, which are discussed in terms of different DC conductivity mechanisms, with a great attention paid to the parallel metal-semiconductor system. In turn, in heat-treated samples the resistance is found nearly temperature-independent except for the significant peak centered at about 600–650 K. Observed characteristics are also fitted using the parallel model, although with a less accuracy suggesting influence of another conductivity mechanisms. At any rate, the resistance peak is possibly related to the metal/non-metal transition observed in disordered solids.

Chemical Reaction of Nitric Oxides with the 5-1DB Defect of the Single-Walled Carbon Nanotube

In this work, we applied a two-layered ONIOM (B3LYP/6-31G(d):UFF) method to study the reaction of nitric oxides with a 5-1DB defect on the sidewall of the single-walled carbon nanotube (SWCNT). We have chosen a suitable ONIOM model for the calculation of the SWCNT based on the analyses of the frontier molecular orbitals, local density of states, and natural bond orbitals. Our calculations clearly indicate that the 5-1DB defect is the chemically active center of the SWCNT. In the reaction of nitric oxides with the defected SWCNT, the 5-1DB defect site can capture a nitrogen atom from nitric oxides, yielding the N-substitutionally doped SWCNT. We have explored the reaction pathway in detail. Our work verifies the chemical reactivity of the 5-1DB defects of the SWCNTs, indicates that the 5-1DB defect is a possible site for the functionalization of the SWCNTs, and demonstrates a possible way to fabricate position controllable substitutionally doped SWCNTs with a low doping concentration under mild conditions via some simple chemical reactions.

Li-inserted carbon nanotube Raman scattering

The possibility of inserting lithium into single wall carbon nanotube bundles during the growth process is analyzed in this work by using the Raman technique as probe. The nanotubes were prepared by the arc discharge method by using catalysts prepared by mixing compounds containing lithium and as their counterpart, a similar mixture without this alkali-metal. The two pair of samples studied in this work were obtained with the following catalysts: (i) Li2CO3/NiO/CoO and NiO/CoO; or (ii) LiCo0.5Ni0.5O2 and Ni/Co. Raman spectra reveal that the tangential bands profiles of the samples prepared with the catalyst containing lithium is considerably modified in both cases. In the case of the carbon nanotubes obtained using the LiCo0.5Ni0.5O2 catalyst a down shift and severe broadening are observed in addition. Comparison of our results with those published previously for alkali-metals doped single wall carbon nanotubes allowed to conclude that lithium incorporation, actually, occurs during the growth process.

Controllable p and n doping of single-walled carbon nanotubes by encapsulation of organic molecules and fullerene: A theoretical investigation

We report theoretical investigations on the changes in optical and electronic properties of single-walled carbon nanotubes (SWNTs) induced by chemical doping with organic molecules or fullerene C60. It is found that doping alters the electronic and optical properties of parent SWNTs and both p- and n-type doping can be realized on SWNTs by encapsulating organic molecules which have large electron affinities or small ionization energies. The doping-induced optical features and additions of donor∕acceptor states to the density of states provide compelling evidence that the standard rigid-band model breaks down and the band-structure changes play an important role on the solid state properties of doped SWNTs.

Metal-insulator transition in doped single-wall carbon nanotubes

We find strong evidence for a metal-insulator sMId transition in macroscopic single wall carbon nanotube sSWNTd conductors. This is revealed by systematic measurements of resistivity and transverse magnetoresistance sMRd in the ranges 1.9‐300 K and 0‐9 Tesla, as a function of p-type redox doping. Strongly H2SO4-doped samples exhibit small negative MR, and the resistivity is low and only weakly temperaturedependent. Stepwise dedoping by annealing in vacuum induces a MI transition. Critical behavior is observed near the transition, with rsTd obeying a power-law temperature dependence, rsTd ~ T ˛b . In the insulating regime shigh annealing temperaturesd, the rsTd behavior ranges from Mott-like three-dimensional s3Dd variable-range hopping sVRHd, rsTd ~ expfsT0 / Td ˛1/4 g, to Coulomb-gap sCGVRHd behavior, rsTd ~ expfs˛T0 / Td ˛1/2 g. Concurrently, MRsBd becomes positive for large B, exhibiting a minimum at magnetic field Bmin. The temperature dependence of Bmin can be characterized by BminsTd = Bcs 1˛ T/ Tcd for a large number of samples prepared by different methods. Below a sample-dependent crossover temperature Tc, MRsBd is positive for all B. The observed changes in transport properties are explained by the effect of doping on semiconducting SWNTs and tube-tube coupling.

Nitrogen Doping of Single-Walled Carbon Nanotube by Using Mass-Separated Low-Energy Ion Beams

Mass-separated nitrogen ions with the mass number of 14 were irradiated to the single-walled carbon nanotubes (SWCNTs) under an ultra high-vacuum pressure of 10-7 Pa for the purpose of achieving nitrogen doping in nanotubes. The incident angle of the ion beam was normal to the target nanotube, and the ion beam energy was 30 eV, which was close to the displacement energy of graphite. The dependence of the structure of SWCNTs on the ion dose was investigated. The ion dose ranged from 2.8×1014 to 2.2×1016 ions/cm2. The nitrogen ions are incorporated into graphite sheets of SWCNTs after irradiation at 2.8×1014 ions/cm2. The graphite structure is distorted and many defects are induced in the nanotube by the nitrogen incorporation. The structure is changed to amorphous after irradiation at 2.2×1016 ions/cm2. The nitrogen ions with the ion energy of 25 eV are irradiated to the field effect transistor device with the nanotube channel. The n-type characteristic appears upon ion irradiation, and the device exhibits ambipolar behavior.

Bandgap-selective chemical doping of semiconducting single-walled carbon nanotubes

Bandgap-selective p doping of single-walled carbon nanotubes (SWCNTs) in aqueoussurfactant suspensions is reported. Nanotubes with narrower bandgaps are preferentiallydoped by hexacyanoferrate (III), as observed by optical absorption. The bandgapabsorption characteristics, which are controlled by the chemical potential, can bereversibly adjusted by controlling the oxidation potential of the dopants throughthe pH value. The origin of the selective modification of bandgap absorption iselectron transfer between the carbon nanotubes and the hexacyanoferrate (III),which preferentially depopulates the valence band in narrower gap nanotubes.

Synthesis and characterization of SWNT-heavy alkali metal intercalation compounds, effect of host SWNTs materials

Singlewall carbon nanotubes (SWNTs) produced by electric-arc and laser ablation methods were characterized by X-ray diffraction before and after the reaction with alkali metals (M=K, Rb, and Cs). Reaction with annealed SWNTs gave MC 8 composition at saturation. The alkali metal lattice showed short range order incommensurate with graphene cylinders of SWNTs. X-ray diffractogram simulations have enabled the study of the influence of SWNTs structure on that of intercalation compounds. Chemically-purified bundles, constituted of open SWNTs, can be intercalated inside and between the tubes forming disordered structures. Annealed or pristine bundles were intercalated only between the tubes leading to short or long range ordered structure depending on host crystallinity and alkali metal (K, Rb or Cs). The expansion of the 2D SWNTs lattice after intercalation is comparable to graphite intercalation compounds. Some 2D arrangements of SWNTs and K atoms are proposed and discussed to reproduce XRD results. 13C NMR and ESR studies of annealed doped SWNTs emphasize the fact that the intercalation compounds of SWNTs are metallic.

Quantum interference induced by nonmagnetic impurities in a single-walled carbon nanotube

The scanning tunnelling spectroscopy of a doped single-walled armchair carbon nanotube has been investigated by using the $\ensuremath{\pi}$-electron tight-binding model. The spatial oscillations arising from quantum interference between the forward and backward electron waves are found in the local density of states. These oscillations are strongly dependent on the geometric arrangement of the relationship between the doped nonmagnetic impurities. Three typical oscillations are predicted and their characteristic features have been studied in detail. It is found that the symmetry and parity of the quasibound states induced by impurities are important to distinguish the oscillation types.

Formation and structural observation of cesium encapsulated single-walled carbon nanotubes.

Cesium encapsulation inside single-walled carbon nanotubes (SWNTs) is for the first time realized by ion irradiation of SWNTs immersed in a magnetized alkali-metal plasma, the configuration of which is confirmed to comprise three varieties by field emission type transmission electron microscopy (FE-TEM) and scanning TEM (STEM) observation.

Resonance Raman Scattering of Br2 Doped Single-Walled Carbon Nanotube Bundles

We have measured Raman spectra of bromine doped single-walled carbon nanotubes (SWNTs) using various laser lines to clarify the electronic states of the doped SWNT. In the case of evacuated sample after full doping, two breathing mode peaks were observed simultaneously by visible laser excitations. We assigned the higher frequency peak to the doped SWNT bundles, and the other peak to the undoped portions in the sample. Intensity ratio between them decreased with decreasing excitation energy, and in the infrared region, the breathing mode band of the doped bundle was not observed. These results can be explained by a simple rigid band model.

Position of K Atoms in Doped Single-Walled Carbon Nanotube Crystals

Recent experiments by Lee et al. [Nature (London) 363, 255 (1997)] show that doping carbon single-wall nanotube (SWNT) ropes with K, Rb, or Br2 leads to metallic conductivity, but the structure and properties are not known. We used molecular dynamics to predict structures and properties which should help motivate and interpret experiments on SWNT/K. We find the optimum stoichiometry to be KC16 if the K cannot penetrate the tubes and K1C10 ( K5exoK3endoC80, 3 within the tube) if they can. We predict the optimum structure and the associated powder-diffraction x-ray pattern expected for KnC80 from n = 0–10 (optimum is n = 5). The Young's modulus per tube along the tube axis varies from 640 to 525 GPa for n = 0 to 5.

Conductivity enhancement in single-walled carbon nanotube bundles doped with K and Br

Single-walled carbon nanotubes (SWNTs), prepared by metal-catalysed laser ablation of graphite, form close-packed bundles or ‘ropes;1. These rope crystallites exhibit metallic behaviour above 50K (ref. 2), and individual tubes behave as molecular wires, exhibiting quantum effects at low temperatures3,4. They offer an all-carbon host lattice that, by analogy with graphite5 and solid C60 (ref. 6), might form intercalation compounds with interesting electronic properties, such as enhanced electrical conductivity and superconductivity. Multi-walled nanotube materials have been doped with alkali metals7 and FeCl3 (ref. 8). Here we report the doping of bulk samples of SWNTs by vapour-phase reactions with bromine and potassium—a prototypical electron acceptor and donor respectively. Doping decreases the resistivity at 300K by up to a factor of 30, and enlarges the region where the temperature coefficient of resistance is positive (the signature of metallic behaviour). These results suggest that doped SWNTs represent a new family of synthetic metals.