Siegmar Roth

One-dimensional metals : conjugated polymers, organic crystals, carbon nanotubes

1. Introduction.2. One-dimensional Substances.3. One-dimensional Solid-State Physics.4. Electron-Phonon Coupling, Peierls Transition.5. Conducting Polymers: Solitons and Polarons.6. Conducting Polymers: Conductivity.7. Superconductivity.8. Charge Density Waves.9. Molecular-scale Electronics.10. Molecular Materials for Electronics.11. Applications.12. Finally.Index.

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.

Modification of Electrical Properties of Graphene by Substrate-Induced Nanomodulation

A periodically modulated graphene (PMG) generated by nanopatterned surfaces is reported to profoundly modify the intrinsic electronic properties of graphene. The temperature dependence of the sheet resistivity and gate response measurements clearly show a semiconductor-like behavior. Raman spectroscopy reveals significant shifts of the G and the 2D modes induced by the interaction with the underlying grid-like nanostructure. The influence of the periodic, alternating contact with the substrate surface was studied in terms of strain caused by bending of graphene and doping through chemical interactions with underlying substrate atoms. Electronic structure calculations performed on a model of PMG reveals that it is possible to tune a band gap within 0.14-0.19 eV by considering both the periodic mechanical bending and the surface coordination chemistry. Therefore, the PMG can be regarded as a further step toward band gap engineering of graphene devices.

Field emission characteristics of point emitters fabricated by a multiwalled carbon nanotube yarn

We fabricated point emitters using a multiwalled carbon nanotube (MWCNT) yarn which was treated by ethylene glycol. The point emitter showed a very high emission current of 3.01 mA (current density of 1.1 x 10(8) A cm(-2)) and good emission stability of over 20 h. We attributed the excellent field emission properties to a large field enhancement factor caused by the large aspect ratio of the sharp tip of the point emitter and the tight bonding of neighboring MWCNTs due to the ethylene glycol treatment. We investigated the field enhancement factor according to the gap between the anode and the emitter tip at a macroscopic gap regime. The measured field enhancement factor of the MWCNT point emitter was in good agreement with theoretical models.

Effect of Single-Walled Carbon Nanotubes as Conductive Additives on the Performance of LiCoO2-Based Electrodes

Single-walled carbon nanotubes (SWNTs) and carbon black (CB) were used as conductive additives in lithium-ion batteries. Composites containing nanostructured LiCoO 2 and carbon additives were applied as positive electrodes in coin-type electrochemical cells with Li metal as a counter electrode. The conductive SWNTs with their wirelike shape and high aspect ratio are proved to have significant impact on the electrochemical performance of the electrode. The electrode composite containing 0.5 wt % of SWNTs has an internal resistance comparable to that of the 10 wt % of carbon black. Whereas the discharge capacity of the electrode containing 0.5 wt % of CB drops to 0 when increasing the rate to 5C, the capacity of the electrode containing the same weight fraction of SWNTs retains 67% of its initial capacity even at 18C. Cycling performance measured up to 23 cycles demonstrates that the SWNTs at a low concentration are as efficient in the capacity retention as the 10 wt % of carbon black. We interpret the SWNT-induced electrode performance by formation of a flexible, electrically wired network of conducting SWNTs in close contact with LiCoO 2 nano-particles, which accelerates the exchange of the Li ions and allows a rapid transfer of electrons throughout the electrode.

Electron beam tuning of carrier concentrations in oxide nanowires

In spite of the attractive electrical properties of metal oxide nanowires, it is difficult to tune their surface states, notably the ionic adsorbents and oxygen vacancies, both of which can cause instability, degradation, and the irreproducibility or unrepeatable changes of the electrical characteristics. In order to control the surface states of the nanowires, electron beams were locally irradiated onto the channels of metal oxide nanowire field effect transistors. This high energy electron beam irradiation changed the electrical properties of the individual metal oxide nanowires, due to the removal of the negative adsorbents (O2-, O-). The detachment of the ionic adsorbents changes the charge states of the nanowires, resulting in the enhancement of the electrical conductance in n-type nanowires (ZnO, SnO2) and the degradation of the conductance in p-type nanowires (CuO). By investigating the changes in the electrical properties of nanowire devices in air or vacuum, with or without exposure to electron b...

Electrical characteristics of carbon nanotube network fabricated by a simple transfer method

We demonstrated a simple transfer method enables to fabricate the single-walled carbon nanotube (SWCNT) film on the plastic substrate. SWCNT network was separated from the initial substrate and transferred onto another substrate using the nitric acid. We also found that electrical conductivity of transferred film was improved. The sheet resistance of SWCNT films was changed from a 150∼300 Ω/sq to a 80∼150 Ω/sq after transferring the SWCNT film in the range of 70∼80% transmittance. This is contributed to the densification of the SWCNT network. During the transfer process, the nitric acid fills the voids between the tubes and form homogeneous interfaces among the tubes, liquid and air. Pressure differences at the interface exert bending moments to the tubes and induce deformation. By the way, deformation leads to reduce the distance between the tubes and it is stuck together due to the van der Waals forces. We suggest that this transfer method provides electrically improved SWCNT films as well as easy, contrallable and inexpensive way to fabricate. This work can open up new possibility for the flexible electronics.

Synthesis of Carbon Nanotubes

A general overview is given over the most common methods to synthesize single-walled and multi-walled carbon nanotubes. In particular carbon arc synthesis, laser ablation synthesis, chemical vapor deposition, and carbon monoxide disproportionation are discussed. A special section is devoted to the role of the catalyst and some ideas on the growth mechanism are presented.