Anti‐Stokes Raman spectroscopy as a method to identify the metallic and semiconducting configurations of double‐walled carbon nanotubes

Although Raman spectra reveal, as a signature of double‐walled carbon nanotubes (DWCNTs), two radial breathing mode (RBM) lines associated with the inner and outer tubes, the specification of their nature as metallic or semiconducting remains a topic for debate. Investigating the spectral range of the RBM lines, we present a new procedure of the indexing of the semiconducting or metallic nature of the inner and outer shell that forms the DWCNT. The procedure exploits the difference between the intensities of recorded anti‐Stokes Raman spectrum and the anti‐Stokes spectrum calculated by applying the Boltzmann formulae to the recorded Stokes spectrum. The results indicate that the two spectra do not coincide with what should happen in a normal Raman process, namely, that there are RBM lines of the same intensity in both spectra, as well as RBM lines of higher intensity that are observed in the calculated spectrum. This discrepancy results from the surface‐enhanced Raman scattering mechanism that operates differently on metallic or semiconducting nanotubes. In this context, the analysis of the RBM spectrum can reveal pairs of lines associated with the inner/outer shell structure of DWCNT, and when the intensities between the recorded and calculated spectra coincide, the nanotube is metallic; otherwise, the nanotube is semiconducting. Copyright © 2014 John Wiley & Sons, Ltd.     

Electrical properties of transparent carbon nanotube networks prepared through different techniques

Ultra‐thin, optically transparent and electrically conducting films of pure carbon nanotubes (CNTs) are widely studied thanks to their promise for broad applications. In the present work, we study and compare different deposition techniques for the production of these networks: dip‐coating, spray‐coating, vacuum filtration and electrophoretic deposition on a quartz glass using single‐walled carbon nanotubes (SWCNTs) produced by the HiPCo method. In order to optimize the networks, besides the various deposition techniques we also investigate how the optical and electrical properties vary if the networks are fabricated from different CNTs, all synthesized by the CVD method: SWCNTs, DWCNTs and MWCNTs. As the main criteria for evaluating the quality of these CNT networks we measure the electrical surface resistance at a certain optical transmittance and correlate it to the morphology (homogeneity and roughness) of the networks. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Surface‐enhanced Raman scattering of pyridine‐functionalized multi‐walled carbon nanotubes

Carbon nanotubes (CNTs) have attracted great attention for their potential use in many applications because of their intrinsic properties. The importance other than the impact of the application of CNT‐embedded membranes in the area of water technology development is immense. In this context, the identification and quantification of CNTs in aqueous resources during relevant water purification processes can be proven of high significance. Surface‐enhanced Raman scattering (SERS) potentially has the sensitivity required for trace analysis and has been previously used for CNT identification on solid substrates. A thorough study for the identification and quantification of small concentrations of multi‐walled CNTs (MWCNTs) in water suspensions via SERS has been performed. The functionalization of MWCNTs with pyridine groups seems to favor the surface enhancement of relevant Raman signal. This study constitutes the first step of a work in progress for the characterization of CNTs at quite low concentration range by SERS in any water suspension. It is based on an ex ante functionalization of the CNTs by pyridine, demonstrating the potential of the method. Our long‐term aim is its general application built, however, in an ex post relevant functionalization of the CNTs in any aqueous solution. Copyright © 2014 John Wiley & Sons, Ltd.     

Hot filament effects on CVD of carbon nanotubes

We grew vertically aligned CNTs via HFCVD using mixtures of methane and hydrogen as feedstock, and investigated the dependence of CNT growth on feedstock composition, filament temperature, and filament types. At the filament temperature of 2050 °C tungsten filaments were more efficient for CNT growth than tantalum ones, and higher CNT growth rates were observed when tungsten filaments were operated at 1900 °C. Regardless of filament temperatures and types, monotonic increase in growth rate of vertically aligned CNTs was observed as we increased the methane concentration in the feedstock. In‐situ investigation of feedstock dissociation revealed the generation of various radical species, and, moreover, a strong correlation between CNT growth rates and relative mole fractions of single‐carbon radicals. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Analysis of the number of hydrogen bond groups of a multiwalled carbon nanotube probe tip for chemical force microscopy

In this paper, we describe a statistical method of quantification of the number of functional groups at the contact area of a probe tip for atomic force microscopy from the result of repetitive pull-off force measurements. We have investigated laboratory-made carbon nanotube (CNT) probe tips to apply them for chemical force microscopy because limited number of functional groups at the tip-end is expected. Using a CNT tip, we conducted repetitive pull-off force measurements against a self-assembled monolayer terminated with carboxyl group and analyzed them in terms of the number of hydrogen bond groups at the CNT tip. The elementary hydrogen bond rupture force quantum in n-decane medium was estimated to be 84.2 ± 0.5 pN in the present system. Thus it was revealed that only a couple of hydrogen bond groups of the CNT tip were participating in hydrogen bonding with the sample on an average in this experimental system.     

Doped single‐walled carbon nanotubes and low‐mobility graphene: impact of disorder and dopants on electronic magnetotransport

The impact of disorder introduced by intentional and unintentional (environmental) dopants on the charge transport has been investigated in boron‐ and nitrogen‐doped single‐walled carbon nanotubes, and in low‐mobility monolayer graphene. For doped single‐walled nanotubes a theoretically not predicted plateau‐like region and onsets of oscillatory behaviour in the conductance for boron‐ and nitrogen‐doped nanotubes were observed, respectively. The oscillatory behaviour suggests interplay between the dopants and the helical movement of charges along the tube due to the magnetic field. For low‐mobility graphene the simultaneous appearance of the filling‐factor 2 and 3 plateau was observed in the Quantum‐Hall regime. This is attributed to an electron‐spin–isospin interaction mediated through the high disorder. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Characterisation of carbonaceous materials using Raman spectroscopy: a comparison of carbon nanotube filters,single‐ and multi‐walled nanotubes,graphitised porous carbon and graphite

Multi‐walled carbon nanotube (MWCNT) filters have been recently synthesised which have specific molecular filtering capabilities and good mechanical strength. Optical and scanning electron microscopy (SEM) reveals the formation of highly aligned arrays of bundles of carbon nanotubes having lengths up to 500 µm. The Raman spectra of this material along with four other carbonaceous materials, commercially available single‐walled carbon nanotubes (SWCNTs) and MWCNTs, graphitised porous carbon (Carbotrap) and graphite have been recorded using two‐excitation wavelengths, 532 and 785 nm, and analysed for band positions and shape with special emphasis paid to the D‐, G‐ and G′‐bands. A major difference between the different MWCNT varieties analysed is that G‐bands in the MWCNT filters exhibit almost no dispersion, whereas the other MWCNTs show a noticeable dispersive behaviour with a change in the excitation wavelength. Spectral features similar to those of the MWCNT filter varieties were observed for the Carbotrap material. From the line shape analysis, the intensity ratio, I_D/I_G, of the more ordered MWCNT filter material using the integral G‐band turns out to be two times lower than that of the less ordered MWCNT filter product at both excitation wavelengths. This parameter can, therefore, be used as a measure of the degree of MWCNT alignment in filter varieties, which is well supported also by our SEM study. Copyright © 2008 John Wiley & Sons, Ltd.     

Novel post‐process for the passivation of a CMOS biosensor

Sensors, which are designed and fabricated in complementary metal oxide semiconductor (CMOS) technology, have become increasingly important in the field of bioelectronics. The standardized industry processes enable a fast, cheap, and reliable fabrication of biosensor devices with integrated addressing and processing units. However, the interfacing of such chips with a liquid environment has been a challenge in recent years. Especially for interfacing living cells with CMOS biosensors different elaborate post‐processes have been proposed. In this article we describe a novel and single step passivation of a CMOS biosensor using a bio‐compatible high‐permittivity thin film, which can be directly applied to the top aluminium layer of a CMOS process. The aluminium oxide and hafnium oxide multi‐layer thin films were prepared using atomic layer deposition at low process temperatures. Electrical I –V and capacitance measurements as well as electrochemical leakage current measurements were performed on films grown on aluminium bottom electrodes. The films showed a very low leakage current and were stable up to 6 V at a thickness of just 50 nm. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

ZnSe‐based conductive nanotemplates for nanofabrication

We show that anodic etching of n‐type ZnSe crystalline substrates leads to the formation of pores which, after nucleation at surface defects, prove to follow the current lines, exhibiting multiplication until the front of the porous network covers the whole available space. No growth of crystallographically oriented pores has been observed in ZnSe. The formation of multilayer porous structures is realized, including layers subjected to successive porosification at two different length scales. The electrochemical pulsed deposition of arrays of Pt nanotubes in the porous ZnSe matrix is demonstrated. The obtained results show that porous ZnSe structures are promising for use as conductive and optically transparent nanotemplates for nanofabrication, in particular for the important application of metal nanotubes. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electron spin resonance investigation of ultra-small double walled carbon nanotubes embedded in zeolite nanochannels

We report on the low temperature electron spin resonance (ESR) properties of ultra-small (0.45?nm) double walled carbon nanotubes (DWCNTs) embedded in zeolite nanochannels. An isotropic ESR signal is observed at g(c)?=?2.002?77 with the spin density (S?=?1/2)?～?10(19)?g(-1), which is suggested to originate from the carbon related point defects in the DWCNTs. Measurements of the ESR line width and signal intensity as a function of temperature indicate that the spins are of a localized nature as opposed to the conduction type electrons observed in large diameter CNTs. The results are consistent with the suggestion that electrons are trapped at interstitial defects. The observed linear frequency dependence of the ESR line width of embedded DWCNTs points to 'strain' as the prime source of broadening. By contrast, the study of free standing DWCNTs shows the presence of a distinctly superlinear frequency dependence of the signal width at low temperatures. The possible origin of the frequency dependence is discussed.     

Unusual role of the substrate in droplet‐induced GaAs/AlGaAs quantum‐dot pairs

The structural complexity of GaAs quantum‐dot pairs has been revealed by cross‐sectional transmission electron microscopy. As a result of high‐temperature droplet epitaxy, the AlGaAs substrate beneath the quantum‐dot pairs is no longer immobile and its reconstruction is observed to define the crystallization of gallium droplets under an arsenic flux. The GaAs quantum‐dot pairs are immersed into the substrate and further confined by the re‐distributed AlGaAs materials above the substrate plane. There are two underlying mechanisms responsible for the final nanostructure configuration, melt‐back etching by the gallium droplets and preferential crystallization of gallium around reconstructed sidewalls. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A sagittally focusing double‐multilayer monochromator for ultrafast X‐ray imaging applications

The development of a sagittally focusing double‐multilayer monochromator is reported, which produces a spatially extended wide‐bandpass X‐ray beam from an intense synchrotron bending‐magnet source at the Advanced Photon Source, for ultrafast X‐ray radiography and tomography applications. This monochromator consists of two W/B₄C multilayers with a 25 Å period coated on Si single‐crystal substrates. The second multilayer is mounted on a sagittally focusing bender, which can dynamically change the bending radius of the multilayer in order to condense and focus the beam to various points along the beamline. With this new apparatus, it becomes possible to adjust the X‐ray beam size to best match the area detector size and the object size to facilitate more efficient data collection using ultrafast X‐ray radiography and tomography.     

Ultra‐light carbon nanotube sponge as an efficient electromagnetic shielding material in the GHz range

CVD synthesised CNT flexible sponge with density lower than 0.02 g cm^–3 has been found to serve as high performance EMI shielding material without the aid of any polymer infiltration or impregnation. Due to its extreme lightweight, the specific SE of the CNT‐sponge was found to be as high as 1100 dB cm³ g^–1, having a total SE above 20 dB in the whole 1–18 GHz range, and being able to shield by absorption. The material is the best of our knowledge this specific SE value appears to be the highest reported hitherto. Improved EM absorbers should fulfil the synergic requirements of being low reflective and highly absorptive. In our CNT‐sponges this condition is not satisfied because, although their net absorption ability is strongly remarkable, their high electrical conductivity favours the wave to be reflected at the input interface. Therefore, this sponge material would have a great potential for microwave‐frequency applications that need negligible reflection and great absorption when combined in a multilayered structure that could prevent the wave to be reflected at the input interface. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Comparing the weak and strong gate‐coupling regimes for nanotube and graphene transistors

We present an experimental and theoretical comparison of the weak and strong gate‐coupling regimes that arise for carbon nanotube (CNT) and graphene field‐effect transistors (FETs) in back‐gated and liquid‐gated configuration, respectively. We find that whereas the back‐gate efficiency is suppressed for a liquid‐gated CNT FET, the back gate is still effective in case of a liquid‐gated graphene FET. We calculate the gate‐induced Fermi‐level shifts and induced charge densities. In both strong and weak coupling regimes, nonlinearities occur in the gate dependence of these parameters, which can significantly influence the electronic transport. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Interaction mechanism between serine functional groups and single‐walled carbon nanotubes

The adsorption of serine (Ser) on the (8, 8) and (10, 0) single‐walled carbon nanotubes (CNTs) was studied by density‐functional tight‐binding calculations. For Ser, the two most stable configurations were chosen to research the interactions with the CNT. It found that the most stable Ser/(8, 8) and Ser/(10, 0) complexes have similar structures, in which the amino group, carboxyl, and side chain of serine directly interact with the CNT. The binding energies, charge transfer properties, the shortest distance (d₁) between the H atom and the corresponding benzene ring, distance (d₂) between the H atom and the center of benzene ring (HCB), and the angle (α) between the HCB line and the corresponding benzene ring plane were analyzed to explain the interactions. Because of the interaction, the ?CH of the main chain runs away from the surface of CNT, and the angles between the ?C?H bond of the main chain and the carboxyl, the amino group, and the side chain of the Ser become small. The strain energies and changes of angles and dihedral angles of the serine after adsorption were analyzed to illustrate the deformation. The interactions of Ser with the CNT were further illustrated by calculating the molecular orbitals and the partial density of states of the stable complexes. We further compared the binding energies of armchair (n, n) and zigzag (n, 0) CNTs to investigate the diameter dependence of binding energies. Copyright © 2015 John Wiley & Sons, Ltd.     

Comparison of different types of polypyrimidine/CNTs/Pt hybrids in fuel cell catalysis

In this paper, we mainly studied the preparation of platinum-containing composite materials with carbon nanotubes wrapped by polypyrimidine-conjugated polymers and the performance of the composites. The polymer-based carbon nanotubes/Pt catalysts were prepared successfully and confirmed by infrared spectroscopy, XPS, XRD, and TEM images. The performance of polypyrimidine/multi-walled carbon nanotubes (MWCNTs)/Pt and polypyrimidine/double-walled carbon nanotubes (DWCNTs)/Pt was compared with the polypyrimidine/single-walled carbon nanotubes (SWCNTs)/Pt. The amount of the loaded Pt on the polypyrimidine/MWCNTs and polypyrimidine/DWCNTs was calculated to be 50.5 wt% and 52.7 wt% respectively. The effective specific surface area of the polypyrimidine/MWCNTs/Pt (45.7 m²/g) and polypyrimidine/DWCNTs/Pt (42.47 m²/g) was observed by electrochemical cyclic voltammetry. These studies strongly imply that the MWCNTs were better candidates than DWCNTs and SWCNTs in the application of polypyrimidine/CNT materials as catalyst for fuel cells.     

Superior capacitive characteristics of RuO2 nanorods grown on carbon nanotubes

Carbon nanotubes (CNTs) were used as the electric double layer capacitor (EDLC) material and were synthesized by using thermal chemical vapor deposition (TCVD). To enhance the EDLC capacity, the ruthenium dioxide (RuO₂) nanorods were grown on CNTs by using metal organic chemical vapor deposition (MOCVD). The synthesized CNTs were the principal part and template, and the RuO₂ nanorods were grown outwardly from CNTs. The increase of effective specific area between electrode and electrolyte played an important role in enhancing the capacitance. Different concentrations of KOH were used as electrolyte to measure the capacitance to find the variation of capacitance. Moreover, the RuO₂/CNT composites demonstrated a stable cycle life. The results showed that the RuO₂/CNT composites were a promising supercapacitor device material.     

Inhomogeneity of a highly efficient InGaN based blue LED studied by three‐dimensional atom probe tomography

The InGaN based multiple quantum well (MQW) structure in a commercially available white light emitting diode (LED) was studied by transmission electron microscopy (TEM) and three‐dimensional atom probe tomography (APT). The average In mole fraction by three‐dimensional (3D) APT was found to be about 18% in the InGaN well which is consistent with the secondary ion mass spectrometry (SIMS) analysis. The In distribution in the InGaN well layer was analyzed by the iso curve mapping of 3D APT and found to be non‐uniform in the InGaN active layer. In clustering or In rich regions in the range of 2–3 nm size were found, in contrast to recent reports. Our results thus indicate that In clustering is essential for high‐brightness InGaN based LEDs. We have also observed a discontinuity in the range of 50–100. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Coherence and wavefront characterization of Si‐111 monochromators using double‐grating interferometry

A study of the coherence and wavefront properties of a pseudo‐channel‐cut monochromator in comparison with a double‐crystal monochromator is presented. Using a double‐grating interferometer designed for the hard X‐ray regime, the complex coherence factor was measured and the wavefront distortions at the sample position were analyzed. A transverse coherence length was found in the vertical direction that was a factor of two larger for the channel‐cut monochromator owing to its higher mechanical stability. The wavefront distortions after different optical elements in the beam, such as monochromators and mirrors, were also quantified. This work is particularly relevant for coherent diffraction imaging experiments with synchrotron sources.     

Atomic structure of the non‐polar GaN($ \bar 2 $110) surface by cross‐sectional scanning tunneling microscopy

The ( 110) plane of gallium nitride, exposed by cleaving a GaN single crystal under ultra‐high vacuum conditions, has been atomically resolved for the first time, using cross‐sectional scanning tunneling microscopy. The spatial period length supports a (1 × 1) unit mesh size, i.e., the absence of a reconstruction. The contrast observed in the experimental data is well explained by the atomic arrangement expected for a truncated‐bulk structure. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)