Search

In this work, graphene field effect transistors (FETs) were fabricated on a trench structure made by carbonized poly(methylmethacrylate) to modify the graphene surface. The trench-structured devices showed different characteristics depending on the channel orientation and the pitch size of the trenches as well as channel area in the FETs. Periodic corrugations and barriers of suspended graphene on the trench structure were measured by atomic force microscopy and electrostatic force microscopy. Regular barriers of 160 mV were observed for the trench structure with graphene. To confirm the transfer mechanism in the FETs depending on the channel orientation, the ratio of experimental mobility (3.6–3.74) was extracted from the current–voltage characteristics using equivalent circuit simulation. It is shown that the number of barriers increases as the pitch size decreases because the number of corrugations increases from different trench pitches. The noise for the 140 nm pitch trench is 1 order of magnitude higher than that for the 200 nm pitch trench.

The thermoelectric power (TEP) of single walled carbon nanotube (SWCNT) thin films in pure metallic SWCNT (m-SWCNT) and pure semiconducting SWCNT (s-SWCNT) networks as well as in m- and s-SWCNT mixtures is investigated. The TEP measured on the pure s-SWCNT film (≈88 μV/K) was found to be almost 7 times higher than that of the m-SWCNTs (≈13 μV/K). Moreover, a quasilinear increase of TEP of the mixed SWCNT networks was observed as the fraction of s-SWCNTs is increased. The experimentally determined relationship between TEP and the fraction of s-SWCNTs in the mixture allows fast and simple quantitative analysis of the s:m ratio in any as-prepared heterogeneous SWCNT network. Furthermore, a semiempirical model analyzing the effect of the intertube junctions is proposed and applied to describe the thermoelectric behavior of the prepared SWCNT networks. The results of calculations match well with the experimental data and clearly demonstrate that the measured TEP of thin SWCNT films is principally controlled by the intertube junctions. The fundamental role of junctions in generating thermoelectric power is not limited to only SWCNT networks as discovered here, but also could be applied to systems where nanoparticles/nanotube form percolating paths in thin films and composite materials.

Motivated by numerous recent reports indicating attractive properties of composite materials of carbon nanotubes (CNTs) and liquid crystals (LCs) and a lack of research aimed at optimizing such composites, the process of dispersing CNTs in thermotropic LCs is systematically studied. LC hosts can perform comparably or even better than the best known organic solvents for CNTs such as N‐methyl pyrrolidone (NMP), provided that the dispersion process and choice of LC material are optimized. The chemical structure of the molecules in the LC is very important; variations in core as well as in terminal alkyl chain influence the result. Several observations moreover indicate that the anisotropic nematic phase, aligning the nanotubes in the matrix, per se stabilizes the dispersion compared to a host that is isotropic and thus yields random tube orientation. The chemical and physical phenomena governing the preparation of the dispersion and its stability are identified, taking into account enthalpic, entropic, as well as kinetic factors. This allows a guideline on how to best design and prepare CNT–LC composites to be sketched, following which tailored development of new LCs may take the advanced functional material that CNT–LC composites comprise to the stage of commercial application.

We investigated the electrochemical properties of LiCoO2 composites with single walled carbon nanotubes (SWNTs) in order to apply them as cathode material in Li-ion batteries. Purified SWNTs, produced by catalytic conversion of high-pressure CO (HiPco) have been selected as conductive additives. Reference composites based on the carbon black have also been prepared and evaluated. We found out that the 0.5 weight % of Hipco tubes can sufficiently replace 10 weight % of carbon black in the composite considering the internal resistance of the assembled electrochemical cell as well as the battery performance. Our experiments provide the proof of principle that inorganic particles of LiCoO2 in a composite with conducting SWNTs can be electrically wired in a network for efficient electronic and ionic transport in the Li-ion battery.

No Abstract

Dysprosium als Abrissbirne: Mithilfe von aberrationskorrigierter Transmissionselektronenmikroskopie lassen sich chemische Reaktionen auf der atomaren Ebene beobachten. Die Bildung von Dysprosiumclustern im Inneren von Kohlenstoffnanoröhren führt zum Bruch der Nanoröhrenwand und zum Verschluss der Bruchstelle (siehe Schema). Diese Reaktionsfolge ließ sich in situ visualisieren.

Carbon Vulcan XC72 is still considered as the standard catalyst support in gas diffusion electrodes for low temperature polymer fuel cell applications. Because of its relatively low specific area and poor corrosion resistance, alternative materials such as carbon nanotubes (CNTs) are currently the focus of numerous investigations. This work reports on the test of single-walled carbon nanotubes (SWCNTSs) as Pt and PtRu catalyst support in the direct methanol fuel cell (DMFC) anode. After the treatment of the CNTs in air at or in a high concentrated nitric acid solution, power densities of 152 and have been reached under DMFC conditions at and . The anodic and cathodic Pt loadings amounted to . The performance of the membrane electrode assembly (MEA) with a PtRu-SWCNT anode was about 10-15% higher than that of the MEA with a conventional as-prepared PtRu-Vulcan XC72 system. This was principally ascribed to higher catalyst utilization, lower impedance, and methanol crossover of the carbon-nanotube-based electrodes compared to those of the carbon-Vulcan-based one.

Oriented circular dichroism (OCD) was used to characterize and compare in a quantitative manner the secondary structure and concentration dependent realignment of the antimicrobial peptides PGLa and MSI-103, and of the structurally related cell-penetrating peptide MAP in aligned phospholipid bilayers. All these peptides adopt an amphiphilic α-helical conformation, and from solid-state NMR analysis they are known to bind to membranes in two distinct orientations depending on their concentration. At low peptide/lipid (P/L) ratio the helices are aligned parallel to membrane surface (S-state), but with increasing concentration they realign to a tilted orientation (T-state), getting immersed into the membrane with an oblique angle supposedly as a result of dimer-formation. In macroscopically aligned liquid crystalline 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine bilayers the two limiting states are represented by distinct OCD spectra, and all spectra at intermediate peptide concentrations can be described by a linear combination of these two line shapes. The corresponding fraction of molecules occupying the T-state was determined by fitting the intermediate spectra with a superposition of the two extreme line shapes. By plotting this fraction versus 1/(P/L), the threshold P/L* ratio for realignment was extracted for each of the three related peptides. Despite their structural similarity distinctly different thresholds were obtained, namely for MSI-103 realignment starts already at a low P/L of ∼1:236, for a MAP derivative (using a nonaggregating analog containing a D-amino acid) the transition begins at P/L ∼1:156, whereas PGLa needs the highest concentration to flip into T-state at P/L ∼1:85. Analysis of the original MAP sequence (containing only L-amino acids) gave OCD spectra compatible with β-pleated conformation, suggesting that this peptide starts to aggregate with increasing concentration, unlike the other helical peptides. All these changes in peptide conformation and membrane alignment observed here by OCD seem to be functionally relevant, as they can be correlated with the membrane perturbing activities of the three antimicrobial and cell-penetrating sequences.

Raman and in situ Raman spectroelectrochemical studies of Dy3N@C80@SWCNT peapods have been carried out for the first time. The formation of peapods by the encapsulation of gaseous Dy3N@C80has been confirmed by HR‐TEM microscopy and by the successful transformation of Dy3N@C80@SWCNT into double‐walled carbon nanotubes. The Raman spectra of the endohedral fullerene cluster changed dramatically in the interior of the single‐walled carbon nanotube (SWCNT). The electrochemical charging of the peapod indicates a slight reversible attenuation of the Raman intensities of fullerene features during anodic doping. The results support the assignment of Raman bands to the Dy3N@C80moiety inside a SWCNT.

The temperature-dependence of the atomic bond-lengths and -angles of the unit cell of crystalline tellurium nanocylinders (a few m in length and some 100 nm in diameter) have been investigated by X-ray diffraction in the range from 370 K to 100 K. With decrease in temperature, significant deviations from the established temperature-dependence of the bulk tellurium lattice parameters are observed. The observations in the temperature expansion are attributed to the nano-size and symmetry of the tellurium nanocylinders. The study demonstrates that a qualitative change in the material's properties is achieved when approaching the nano-scale. This opens the road for the possibility of deliberate tailoring of materials properties by proper choice of size and symmetry for thermo-mechanical applications.

Vanadium oxides, such as V2O5, are promising for lithium-ion batteries, catalysis, electrochromic devices and sensors. Vanadium oxides were proposed more than a decade ago for another redox-dependent application: the direct conversion of electrical energy to mechanical energy in actuators (artificial muscles). Although related conducting polymer and carbon nanotube actuators have been demonstrated, electromechanical actuators based on vanadium oxides have not be realized. V2O5nanofibres and nanotubes provide the potential advantages of low-cost synthesis by sol–gel routes and high charging capacity and long cycle life. Here, we demonstrate electromechanical actuation for obtained high modulus V2O5sheets comprising entangled V2O5nanofibres. The high surface area of these V2O5sheets facilitates electrochemical charge injection and intercalation that causes the electromechanical actuation. We show that the V2O5sheets provide high Young's modulus, high actuator-generated stress, and high actuator stroke at low applied voltage.

Photoluminescence, photoconductivity and resonant Raman measurements are performed to investigate the influence of high intensity illumination on the properties of Fullerenes. A highly nonlinear dependence of the luminescence emission efficiency and lifetime is observed on increasing the intensity. This nonlinear increase is associated with a dramatic shift to the red of the emission maximum. The photoconductive response of the fullerenes is also seen to increase nonlinearly with input intensity.Temperature dependent measurements indicate that the nonlinear processes are associated with an insulator-metal phase transition in the material. The transition is reversible and the observed photophysical changes coincide with a reversible shifting of the characteristic fullerene Raman lines to lower energies.

Nonlinearities photoluminescence, photoconductivity and resonant Raman measurements of fullerenes at high excitation densities are described and the observations are discussed in terms of an optically induced insulator-metal transition in fullerene crystals. The intensity dependence of the transient photoconductive response is reported to show similar nonlinear behaviour. At low intensities, the response decays exponentially within a time of 15nsec. At increased intensities, a second, delayed component emerges and evolves nonlinearly into a long lived component of lifetime 100-200nsec. The system is fitted with a simple three level model in accordance with a Mott-like transition.

We report Raman measurements on the photo- induced disassociation via a metastable excited state intermediate of the photoproduct formed from the photochemical degradation of C60. At low intensities, the intrinsic 1468cm-1 C60 mode softens to 1459cm-1 which is stable below an apparent threshold intensity above which it shifts abruptly to 1463cm-1. This mode is stable with increasing intensities. Upon a return to low intensities, the 1468cm-1 intrinsic C60 line reappears. The positioning of the Raman line at 1463cm-1 is associated with an excited state intermediate, observable only at high excitation densities.

Electrical conductivity and thermopower of two types of highly conductive polyacetylene are presented. The anisotropy of the charge transport properties is discussed in consideration of an inhomogeneous structure of polyacetylene.

Not Available

Not Available

Broadband electroluminescence emission from fullerene crystals is reported and described. The spectral distribution is comparable to that of the photoluminescence at high excitation densities. The emission intensity is nonlinearly dependent on the current. The response of the crystal to the application of an alternating current is investigated and the frequency dependence of the emission intensity is described. The observed high frequency cut-off behaviour cannot be mimicked by a simple equivalent circuit and so the system is modelled using a rate equation model to describe the state of the system. Fits of the model to the observed behaviour provide rate constants which compare favourably to those reported for excited state decay in fullerenes.

Review: 51 refs.

We investigated the electrochemical properties of LiCoO2 composites with single walled carbon nanotubes (SWNTs) in order to apply them as cathode material in Li-ion batteries. Purified SWNTs, produced by catalytic conversion of high-pressure CO (HiPco) have been selected as conductive additives. Reference composites based on the carbon black have also been prepared and evaluated. We found out that the 0.5 weight % of Hipco tubes can sufficiently replace 10 weight % of carbon black in the composite considering the internal resistance of the assembled electrochemical cell as well as the battery performance. Our experiments provide the proof of principle that inorganic particles of LiCoO2 in a composite with conducting SWNTs can be electrically wired in a network for efficient electronic and ionic transport in the Li-ion battery.

Carbon Vulcan XC72 is still considered as the standard catalyst support in gas diffusion electrodes for low temperature polymer fuel cell applications. Because of its relatively low specific area and poor corrosion resistance, alternative materials such as carbon nanotubes (CNTs) are currently the focus of numerous investigations. This work reports on the test of single-walled carbon nanotubes (SWCNTSs) as Pt and PtRu catalyst support in the direct methanol fuel cell (DMFC) anode. After the treatment of the CNTs in air at 300°Cor in a high concentrated nitric acid solution, power densities of 152 and 157mWcm−2have been reached under DMFC conditions at 2barabsand 80°C. The anodic and cathodic Pt loadings amounted to 1mgcm−2. The performance of the membrane electrode assembly (MEA) with a PtRu–SWCNT anode was about 10–15% higher than that of the MEA with a conventional as-prepared PtRu–Vulcan XC72 system. This was principally ascribed to higher catalyst utilization, lower impedance, and methanol crossover of the carbon-nanotube-based electrodes compared to those of the carbon-Vulcan-based one.

no abstract

The surface potentials of three kinds of ?-conjugated Langmuir-Blodgget (LB) films (octasubstituted palladiumphthalocyanine (PcPd), perylene-tetra-carboxyldiimide derivative (PTCDI-SPent and PTCDI-OEt)) deposited on Au electrodes were measured using a Kelvin-probe method in a dark vacuum vessel at various temperatures. It was suggested that excess electronic charges were transferred from the electrodes into these LB films until thermodynamic equilibrium was established at the metalo-interface and resulted in the formation of an electrostatic layer on the order of several nano meters. The interfacial electrostatic potential formed at the interface between PcPd and PTCDI-SPent layers was also measured at various temperatures. It was found that electrons were transferred from PcPd to PTCDI-SPent layers at the interface.

Es wird über erste Versuche berichtet, mit InSb-Dioden Kernteilchen zu zählen. Bei 75 °K wurden die Impulse von α-Strahlen und Protonen untersucht. Daraus wurde die effektive Paarerzeugungsenergie zu 2,5 eV bestimmt.