Richard Czerw

Identification of electron donor states in N-doped carbon nanotubes

Nitrogen-doped carbon nanotubes have been synthesized using pyrolysis and characterized by scanning tunneling spectroscopy and transmission electron microscopy. The doped nanotubes are all metallic and exhibit strong electron donor states near the Fermi level. Using tight-binding and ab initio calculations, we observe that pyridine-like N structures are responsible for the metallic behavior and the prominent features near the Fermi level. These electron rich structures are the first example of n-type nanotubes, which could pave the way to real molecular heterojunction devices.

Multilayered Carbon Nanotube/Polymer Composite Based Thermoelectric Fabrics

Thermoelectrics are materials capable of the solid-state conversion between thermal and electrical energy. Carbon nanotube/polymer composite thin films are known to exhibit thermoelectric effects, however, have a low figure of merit (ZT) of 0.02. In this work, we demonstrate individual composite films of multiwalled carbon nanotubes (MWNT)/polyvinylidene fluoride (PVDF) that are layered into multiple element modules that resemble a felt fabric. The thermoelectric voltage generated by these fabrics is the sum of contributions from each layer, resulting in increased power output. Since these fabrics have the potential to be cheaper, lighter, and more easily processed than the commonly used thermoelectric bismuth telluride, the overall performance of the fabric shows promise as a realistic alternative in a number of applications such as portable lightweight electronics.

Raman characterization of boron-doped multiwalled carbon nanotubes

We present first- and second-order Raman spectra of boron-doped multiwalled carbon nanotubes. The Raman intensities are analyzed as a function of the nominal boron concentration. The intensities of both the D mode and the high-energy mode in the first-order spectra increase with increasing boron concentration, if normalized with respect to a second-order mode. We interpret this result as an indication that the high-energy mode in carbon nanotubes is defect-induced in a similar way as the D mode. Based on this result, we provide a preliminary quantitative relation between the boron concentration and the Raman intensity ratios.

Hole blocking in carbon nanotube–polymer composite organic light-emitting diodes based on poly (m-phenylene vinylene-co-2, 5-dioctoxy-p-phenylene vinylene)

In order to investigate the role of carbon nanotubes in a polymer matrix, organic light-emitting diodes were fabricated from a polymer composite composed of poly (m-phenylene vinylene-co-2,5-dioctoxy-p-phenylene) (PmPV) and dispersed single-wall carbon nanotubes (SWNTs). Tris-(8-hydroxyquinolinolato) aluminum (Alq3) doped by Nile Red was used as an emissive material between the polymer composite and cathode. The device fabricated without SWNTs dispersed in the PmPV shows a dominant emission near red at 600 nm, which is in the range of the characteristic emission of Nile Red-doped Alq3, with a small amount of green emission from the PmPV. However, the devices fabricated with the polymer composite show an increase in the oscillator strength of the green emission with a dominant emission peak near 500 nm, the characteristic emission of PmPV. This was observed for SWNT concentrations up to 0.1 wt %. The shift in the emission indicates that the SWNTs in the PmPV matrix act as a hole-blocking material that resu...

Microscopy studies of nanotube-conjugated polymer interactions

A promising composite material based on single-walled carbon nanotubes (SWNT) and a conjugated polymer, has been developed. Characterisation of these materials, by Transmission Electron Microscopy (TEM), and both Scanning Tunneling Microscopy and Spectroscopy (STM, STS) are presented. Evidence of interaction between the SWNT and conjugated polymers is seen. Uniform coating of the nanotubes by polymer is observed.

Varying the concentration of single walled carbon nanotubes in thin film polymer composites, and its effect on thermoelectric power

Resistivity and thermoelectric power (TEP) measurements were conducted on single walled carbon nanotube (SWNT), polyvinylidene fluoride composite thin films of varying SWNT concentrations. This heterogeneous material was used in order to utilize the good electrical conductance of the nanotubes and the poor thermal conductance of the polymer to increase the figure of merit (ZT). As the nanotube weight percent decreased from 100% to 5%, the beneficial effects of the TEP increase and thermal conductivity decrease outweighed the negative effect of decreased electrical conductivity, resulting in an increase in ZT by a factor of 100.

Electronic structure of kinked multiwalled carbon nanotubes

Changes in the local density of states (LDOS) of kinked multiwalled carbon nanotubes have been studied using scanning tunneling microscopy and spectroscopy (STM)/(STS). The measured STS spectra in the inelastically kinked region show that the LDOS is asymmetric about the Fermi level, while the spectra from regions away from the kink exhibit the usual symmetric LDOS that corresponds to perfect tubes. In the kinked region, the distribution of the unoccupied states is suppressed; this suppression of the conduction band states becomes unnoticeable ∼1.66 nm from the kink center.

Large-scale synthesis of highly aligned nitrogen doped carbon nanotubes by injection chemical vapor deposition methods

In this study, we compare the effects of pyridine (C 5 H 5 N) and pyrimidine (C 4 H 4 N 2 ) precursors, using ferrocene as a metal source, in the production of nitrogen containing multiwalled carbon nanotubes. Using standard chemical vapor deposition techniques, highly aligned mats of carbon-nitrogen carbon nanotube were synthesized. The maximum nitrogen concentration in these materials is between 1% and 2% when pyridine is used as the precursor and can be increased to 3.2% when pyrimidine is used as the precursor. However, the electronic structure of both materials, as determined using scanning tunneling spectroscopy, suggests that the nitrogen is incorporated into the nanotube lattice in the same way for both precursors.

Nonlinear optical transmission in VOx nanotubes and VOx nanotube composites

Optical-limiting behavior of vanadium oxide nanotubes is characterized for the visible and infrared spectral ranges using 8 ns pulses from a Nd:YAG laser with an f/40 optical system. Vanadium oxide nanotube dispersions were investigated in both water suspensions and embedded in solid polymethyl methacrylate films. In each case, these nanotubes exhibit strong optical-limiting at 532 nm (in comparison to carbon nanotubes); however, no nonlinear behavior is observed for 1064 nm. This suggests that a two photon or excited state absorption mechanism is responsible for the observed nonlinearity.

Complex nano-assemblies of polymers and carbon nanotubes

Since the discovery of carbon nanotubes in 1991 [1], researchers have envisaged potential applications such as nanoscale electronic circuits and the construction of complex carbon-based nano-machines. Thus, the assembly of basic building blocks of complex nano-architectures, such as conjugated polymers and nanotubes, has been a driving goal of much of the nano-science community. A first step towards realizing this goal may be the attachment to, or modification by carbon nanotubes of structures such as polymers. This leads to the possibility of assembling individual polymer molecules onto carbon nanotubes with the net effect being the modification of the polymers electronic properties and structure in a predictable way. To accomplish this, clearly, a more detailed understanding of the interactions between conjugated polymers and carbon nanotubes must be sought. In this paper, we describe the assembly of the polymer, poly(m-phenylenevinylene-co-2,5-dioctoxy-p-phenylenevinylene) (PmPV), into a coating around single-walled carbon nanotubes. Using scanning tunnelling microscopy, and scanning tunnelling spectroscopy, we demonstrate that the low-energy electronic structure of the assembled material is dominated by the one-dimensional nature of the nanotube as reflected in van Hove singularities. Further, we examine the modifications to electronic structure at higher energies using spectroscopy, which suggests that the polymers electronic structure is altered by the introduction of nanotubes.