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An important advancement towards optical communication on a chip would be the development of integratable, nanoscale photonic emitters with tailored optical properties. Here we demonstrate the use of carbon nanotubes as electrically driven high-speed emitters in combination with a nanophotonic cavity that allows for exceptionally narrow linewidths. The one-dimensional photonic crystal cavities are shown to spectrally select desired emission wavelengths, enhance intensity and efficiently couple light into the underlying photonic network with high reproducibility. Under pulsed voltage excitation, we realize on-chip modulation rates in the GHz range, compatible with active photonic networks. Because the linewidth of the molecular emitter is determined by the quality factor of the photonic crystal, our approach effectively eliminates linewidth broadening due to temperature, surface interaction and hot-carrier injection. Carbon nanotubes in a nanocavity offer a route to narrow-linewidth on-chip light emitters.

Traditional silicon solar cells extract holes and achieve interface passivation with the use of a boron dopant and dielectric thin films such as silicon oxide or hydrogenated amorphous silicon. Without these two key components, few technologies have realized power conversion efficiencies above 20%. Here, a carbon nanotube ink is spin coated directly onto a silicon wafer to serve simultaneously as a hole extraction layer, but also to passivate interfacial defects. This enables a low‐cost fabrication process that is absent of vacuum equipment and high‐temperatures. Power conversion efficiencies of 21.4% on an device area of 4.8 cm$^{2}$ and 20% on an industrial size (245.71 cm$^{2}$) wafer are obtained. Additionally, the high quality of this passivated carrier selective contact affords a fill factor of 82%, which is a record for silicon solar cells with dopant‐free contacts. The combination of low‐dimensional materials with an organic passivation is a new strategy to high performance photovoltaics.

Double walled carbon nanotubes (DWCNTs) are an intriguing example of a one-dimensional Moiré system, because they differ in diameter and chirality of the inner and outer tubes. For many years, DWCNTs have been described as two weakly coupled single walled carbon nanotubes (SWCNTs) without much electronic interaction. Advances in theoretical modeling, however, predicted diverse coupling possibilities when considering the electronic interactions between twisted layers of low dimensional materials. This coupling can induce the formation flat bands and strongly perturb the electronic states. Experimentally, this should manifest in a reduced bandgap in DWCNTs and a red-shift of the optical transition energies. Chasing this coupling effect, we used resonant Raman spectroscopy to measure the transition energies of semiconducting inner tubes in DWCNTs. The transition energies are indeed reduced, with varying red-shifts for the same inner wall chirality (50 and 150 meV). The shift depends on the electronic type of the host tube that we assign from based on the Radial breathing mode frequencies. The coupling increases from weak to strong by tightening intratube spacing in agreement with theoretical predictions. We discuss our observations in light of joint and localized vibrational and optical excitations in the two tubes making up the DWCNT.

Single-walled carbon nanotubes as emerging quantum-light sources may fill a technological gap in silicon photonics due to their potential use as near-infrared, electrically-driven, classical or non-classical emitters. Unlike in photoluminescence, where nanotubes are excited with light, electrical excitation of single-tubes is challenging and heavily influenced by device fabrication, architecture and biasing conditions. Here we present electroluminescence spectroscopy data of ultra-short channel devices made from (9,8) carbon nanotubes emitting in the telecom band. Emissions are stable under current biasing and no quenching is observed down to 10 nm gap size. Low-temperature electroluminescence spectroscopy data also reported exhibits cold emission and linewidths down to 2 meV at 4 K. Electroluminescence excitation maps give evidence that carrier recombination is the mechanism for light generation in short channels. Excitonic and trionic emissions can be switched on and off by gate voltage and corresponding emission efficiency maps were compiled. Insights are gained into the influence of acoustic phonons on the linewidth, absence of intensity saturation and exciton-exciton annihilation, environmental effects like dielectric screening and strain on the emission wavelength, and conditions to suppress hysteresis and establish optimum operation conditions.

An aqueous two-phase extraction (ATPE) technique capable of separating small-diameter single-walled carbon nanotubes in one, two, or at the most three steps is presented. Separation is performed in the well-studied two-phase system containing polyethylene glycol and dextran, but it is achieved without changing the global concentration or ratio of cosurfactants. Instead, the technique is reliant upon the different surfactant shell around each nanotube diameter at a fixed surfactant concentration. The methodology to obtain a single set of surfactant conditions is provided, and strategies to optimize these for other diameter regimes are discussed. In total, 11 different chiralities in the diameter range 0.69-0.91 nm are separated. These include semiconducting and both armchair and nonarmchair metallic nanotube species. Titration of cosurfactant suspensions reveal separation to be driven by the pH of the suspension with each ( n, m) species partitioning at a fixed pH. This allows for an ( n, m) separation approach to be presented that is as simple as pipetting known volumes of acid into the ATPE system.

Carbon nanotubes are one-dimensional nanoscale systems with strongly pronounced chirality-dependent optical properties with multiple excitonic transitions. We investigate the high-energy $G$ mode of semiconducting single-walled nanotubes of different chiralities at first excitonic transition by applying resonant Raman spectroscopy. The $G$ mode intensity dependence on excitation energy yielded asymmetric resonance Raman profiles similar to ones we reported for the second excitonic transition. We find the scattering efficiency to be strongest at the incoming Raman resonance. Still, the degree of asymmetry is different for the first and second transitions and the first transition profiles provide a narrower line shape due to longer exciton lifetimes. The overall scattering efficiency is up to a factor of 25 times more intense at first excitonic transition, compared to the second transition. The fifth-order perturbation theory, with implemented phonon scattering pathways between excitonic states, excellently reproduced experimental data.

Single walled carbon nanotube thin films are fabricated by solution shearing from high concentration sodium nanotubide polyelectrolyte inks. The solutions are produced by simple stirring of the nanotubes with elemental sodium in dimethylacetamide, and the nanotubes are thus not subject to any sonication-induced damage. At such elevated concentrations (∼4 mg mL(-1)), the solutions exist in the liquid crystal phase and during deposition this order is transferred to the films, which are well aligned in the direction of shear with a 2D nematic order parameter of ∼0.7 determined by polarized absorption measurements. Compared to similarly formed films made from superacids, the polyelectrolyte films contain smaller bundles and a much narrower distribution of bundle diameters. After p-doping with an organic oxidizer, the films exhibit a very high DC electrical to optical conductivity ratio of σ(DC)/σ(OP) ∼ 35, corresponding to a calculated DC conductivity of over 7000 S cm(-1). When very thin (T550 ∼ 96%), smooth (RMS roughness, R(q) ∼ 2.2 nm), and highly aligned films made via this new route are used as the front electrodes of carbon nanotube-silicon solar cells, the power conversion efficiency is almost an order of magnitude greater than that obtained when using the much rougher (R(q) ∼ 20-30 nm) and less conductive (peak σ(DC)/σ(OP) ∼ 2.5) films formed by common vacuum filtration of the same starting material, and having the same transmittance.

Purpose – The purpose of this study was to provide an empirical investigation into the relationship between employee orientation and both financial performance and leverage. Design/methodology/approach – The selected sample set consisted of German publicly listed and unlisted companies identified as a “Top Employer” by the Top Employers Institute for the period 2008-2010. The relationship between employee orientation and both financial performance and leverage was then examined for this sample set for the period 2007 and 2011, with the rating “Top Employer Germany” used as a proxy measure of employee orientation. Findings – The findings show a strong correlation with trade-off, traditional human relations and managerial opportunism theory. It is shown that investments in employee-orientated activities, such as career opportunities, and secondary benefits and work–life balance, lead to reductions in financial performance, which in turn leads to higher levels of employee orientation. Furthermore, no statistically significant relationship between the level of employee orientation and company leverage was found which is in disagreement with stakeholder capital structure theories which propose that there is a negative relationship, where highly leveraged companies tend to invest less in employee-orientated activities which in turn lead to higher levels of leverage. Originality/value – This is one of the first studies to provide an empirical investigation into the relationship between the level of employee orientation and both financial performance and corporate leverage. Most previous studies have focused on either financial performance or leverage. Furthermore, this is one of the first studies which has its geographical focus on Continental Europe. Most previous studies focused on the Anglo-American corporate environment.

Single-walled carbon nanotubes (SWCNTs) have been the focus of intense research, and the body of literature continues to grow exponentially, despite more than two decades having passed since the first reports. As well as extensive studies of the fundamental properties, this has seen SWCNTs used in a plethora of applications as far ranging as microelectronics, energy storage, solar cells, and sensors, to cancer treatment, drug delivery, and neuronal interfaces. On the other hand, the properties and applications of double-walled carbon nanotubes (DWCNTs) have remained relatively under-explored. This is despite DWCNTs not only sharing many of the same unique characteristics of their single-walled counterparts, but also possessing an additional suite of potentially advantageous properties arising due to the presence of the second wall and the often complex inter-wall interactions that arise. For example, it is envisaged that the outer wall can be selectively functionalized whilst still leaving the inner wall in its pristine state and available for signal transduction. A similar situation arises in DWCNT field effect transistors (FETs), where the outer wall can provide a convenient degree of chemical shielding of the inner wall from the external environment, allowing the excellent transconductance properties of the pristine nanotubes to be more fully exploited. Additionally, DWCNTs should also offer unique opportunities to further the fundamental understanding of the inter-wall interactions within and between carbon nanotubes. However, the realization of these goals has so far been limited by the same challenge experienced by the SWCNT field until recent years, namely, the inherent heterogeneity of raw, as-produced DWCNT material. As such, there is now an emerging field of research regarding DWCNT processing that focuses on the preparation of material of defined length, diameter and electronic type, and which is rapidly building upon the experience gained by the broader SWCNT community. This review describes the background of the field, summarizing some relevant theory and the available synthesis and purification routes; then provides a thorough synopsis of the current state-of-the-art in DWCNT sorting methodologies, outlines contemporary challenges in the field, and discusses the outlook for various potential applications of the resulting material.

Applications of single-walled carbon nanotubes (SWCNTs) are hampered by the mixtures of metallic and semiconducting SWCNTs that are present in commercial samples. Separation of SWCNTs according to electronic type is therefore extremely important. Recently, the selective interaction between the conjugated polymer, poly(9,9-di-n-dodecyl-fluorenyl-2,7-diyl) and semiconducting SWCNTs has been reported. However, the mechanism responsible for this selectivity is poorly understood. To determine whether this polymer is only selective for semiconducting SWCNTs, we exposed it to mixtures of metallic and semiconducting SWCNTs in different ratios. We found that the polymer is indeed selective for semiconducting SWCNTs in toluene, but only when the starting ratio is below 67:33 metallic:semiconducting. When the starting ratio is increased to 67:33 or higher, the amount of metallic SWCNTs dispersed dramatically increases. If the solvent is changed to THF, the threshold ratio at which metallic SWCNTs begin to be dispersed is much lower. This indicates that the polymer exhibits a preference for interaction with semiconducting SWCNTs, but is not precluded from interaction with metallic SWCNTs if exposed to a high enough concentration.

The dependence of the anti-Stokes Raman intensity on the excitation laser energy in carbon nanotubes is studied by resonant Raman spectroscopy. The complete resonant anti-Stokes and Stokes Raman profiles of the high-energy longitudinal phonon (${G}^{+}$) are obtained for (8,3), (7,5), (6,4), and (6,5) single chirality enriched samples. A high asymmetry between the intensity of the incoming and outgoing resonance is observed in the resonant Raman profiles. In contrast to Stokes scattering, anti-Stokes scattering is more intense at the outgoing resonance then at the incoming resonance. The resonance profiles are explained by a Raman process that includes the phonon-mediated interactions with the dark excitonic state. The chirality dependence of the Raman profiles is due to the variation in the exciton-phonon matrix elements, in agreement with tight-binding calculations. Based on the asymmetric Raman profiles we present the resonance factors for the Stokes/anti-Stokes ratios in carbon nanotubes.

Graphitization of polymers is an effective way to synthesize nanocrystalline graphene on different substrates with tunable shape, thickness and properties. The catalyst free synthesis results in crystallite sizes on the order of a few nanometers, significantly smaller than commonly prepared polycrystalline graphene. Even though this method provides the flexibility of graphitizing polymer films on different substrates, substrate free graphitization of freestanding polymer layers has not been studied yet. We report for the first time the thermally induced graphitization and domain growth of free-standing nanocrystalline graphene thin films using in situ TEM techniques. High resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED) and electron energy loss spectroscopy (EELS) techniques were used to analyze the graphitization and the evolution of nanocrystalline domains at different temperatures by characterizing the crystallinity and domain size, further supported by ex situ Raman spectroscopy. The graphitization was comparable to the substrate supported heating and the temperature dependence of graphitization was analyzed. In addition, the in situ analysis of the graphitization enabled direct imaging of some of the growth processes taking place at different temperatures.

Double-walled carbon nanotubes (DWCNTs) consist of two coaxially aligned single-walled carbon nanotubes (SWCNTs), and previous sorting methods only achieved outer-wall electronic-type selectivity. Here, a separation technique capable of sorting DWCNTs by semiconducting (S) or metallic (M) inner- and outer-wall electronic type is presented. Electronic coupling between the inner and outer wall is used to alter the surfactant coating around each of the DWCNT types, and aqueous gel permeation is used to separate them. Aqueous methods are used to remove SWCNT species from the raw material and prepare enriched DWCNT fractions. The enriched DWCNT fractions are then transferred into either chlorobenzene or toluene using the copolymer PFO–BPy to yield the four inner@outer combinations of M@M, M@S, S@M and S@S. The high purity of the resulting fractions is verified by absorption measurements, transmission electron microscopy, atomic force microscopy, resonance Raman mapping and high-density field-effect transistor devices. For the first time, four different double-walled carbon nanotubes were sorted via aqueous gel permeation according to the electronic coupling between the inner and outer wall.

In the past, the application of carbon nanotube-silicon solar cell technology to industry has been limited by the use of a metallic frame to define an active area in the middle of a silicon wafer. Here, industry standard device geometries are fabricated with a front and back-junction design which allow for the entire wafer to be used as the active area. These are enabled by the use of an intermixed Nafion layer which simultaneously acts as a passivation, antireflective, and physical blocking layer as well as a nanotube dopant. This leads to the formation of a hybrid nanotube/Nafion passivated charge selective contact, and solar cells with active areas of 1–16 cm$^{2}$ are fabricated. Record maximum power conversion efficiencies of 15.2% and 18.9% are reported for front and back-junction devices for 1 and 3 cm$^{2}$ active areas, respectively. By placing the nanotube film on the rear of the device in a back-junction architecture, many of the design-related challenges for carbon nanotube silicon solar cells are addressed and their future applications to industrialized processes are discussed.

Photocurrent spectroscopy has recently been used by several research groups to study the light–matter interaction in carbon nanotubes. To achieve this, long-channel devices with symmetric metallic contacts are typically used, where photocurrent of opposite sign is generated at the two contacts. However, in short-channel devices (

Dielectrophoresis (DEP) is an established method for the integration of solution-processed single-walled carbon nanotubes into nanoscale device structures. However, this method is less effective for small-diameter semiconducting nanotubes and leads to an enrichment of metallic tubes in multi-tube devices. In this work, we present the first results of a novel light-assisted DEP technique, which enhances the deposition of (6,5) semiconducting carbon nanotubes. Transistors fabricated with this technique show higher on/off ratios compared to transistors that were fabricated by conventional DEP. We explain this effect by an enhanced polarizability of the irradiated semiconducting nanotubes and discuss spontaneous and field-driven exciton dissociation as the underlying mechanism. Strategies to further improve the effect are also proposed.

In this report we demonstrate the separation of raw carbon nanotube material into fractions of double-walled (DWCNTs) and single-walled carbon nanotubes (SWCNTs). Our method utilizes size exclusion chromatography with Sephacryl gel S-200 and yielded two distinct fractions of single- and double-walled nanotubes with average diameters of 0.93 ± 0.03 and 1.64 ± 0.15 nm, respectively. The presented technique is easily scalable and offers an alternative to traditional density gradient ultracentrifugation methods. CNT fractions were characterized by atomic force microscopy and Raman and absorption spectroscopy as well as transmission electron microscopy.

This study takes a sample set of German publicly listed and unlisted companies between 2007 and 2011 and investigates the relationship between gender diverse corporate boards and the level of employee-orientation. From this work it is suggested that employee-orientation on the supervisory board has the potential to enhance the level of employee-orientation, in particular investments in corporate culture and secondary benefits and work-life-balance.

Research into silicon solar cells that use a thin film of single walled carbon nanotubes as the front electrode is an important area of increasing research activity. This paper provides the first ever direct performance comparison between devices fabricated with either semiconducting, metallic or mixed nanotubes to probe the effect of the semiconducting/metallic nature, or 'metallicity', of the nanotube film on solar cell performance and properties. HiPCO nanotube material sorted using the gel chromatography technique is highly purified in either metallic or semiconducting nanotube species. The solar cells fabricated with the metallic nanotubes greatly outperform their semiconducting or mixed counterparts. The operating mechanisms underlying this observation and its implications in regards to current understanding are discussed in light of recent literature. Dramatic increases in performance as well as substantial changes in the effect of metallicity due to subsequent hole doping of the sorted nanotube films are also demonstrated.[GRAPHICS].Using highly pure semiconducting and metallic carbon nanotubes, film metallicity is shown to be a vital factor in the performance of carbon nanotube-silicon solar cells. (C) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

An aqueous two-phase extraction (ATPE) technique capable of separating small diameter single walled carbon nanotubes in 1, 2 or at the most 3 steps will be presented. Separation is performed in the well-studied two-phase system containing polyethylene glycol (PEG) and dextran but it is achieved without changing the initial concentration or ratio of co-surfactants. Instead, the technique is reliant upon the unique surfactant shell around each nanotube diameter at a fixed surfactant concentration. The methodology to optimize the surfactant to a single set of conditions for the diameter range (0.69 - 0.91 nm) is provided and 11 different chiralities with a purity of 73 – 97% are isolated. These include semiconducting and both armchair and non-armchair metallic nanotube species. Titration of co-surfactant suspensions reveal separation to be driven by the pH of the suspension with each (n,m) species partitioning over a unique pH range. This allows for an (n,m) separation approach to be presented that is as simple as pipetting known volumes of acid into the ATPE system.

Separation of single-wall carbon nanotube (SWCNT) populations to completely specified physical structures is a primary milestone necessary for enabling SWCNT-based technologies. Aqueous two-phase extraction (ATPE), in which the spontaneous phase separation of an aqueous polymer mixture is utilized to spatially separate solutes on the basis of their chemical potential difference for the two phases, is a promising technology for achieving this milestone. However, due to the expanded number of (n,m) species with increasing nanotube diameter, and the increased difficulty in optical characterization of aqueous-dispersed samples, isolation of larger diameter SWCNTs, > ~1.4 nm in diameter, has continued to be a difficult proposition. Advances in methodology of separation and sample preparation are now overcoming both these barriers in our laboratories. In this work I will detail improved separation procedures utilizing alkane-filled SWCNT populations for the isolation of individual SWCNT (n,m) structures from > 1.5 nm average diameter SWCNT dispersions. These method developments can readily isolate multiple large diameter (n,m) species, and are enabling new scientific and device application work. The enhanced separation methods, isolated (n,m) species, and applications of those populations will be presented.

A gel permeation chromatography system is used to separate aqueous sodium dodecyl sulfate suspensions of single-walled carbon nanotubes (SWCNTs). This automated procedure requires no precentrifugation, is scalable, and is found to yield monochiral SWCNT fractions of semiconducting SWCNTs with a purity of 61-95%. Unsorted and resulting monochiral fractions are characterized using optical absorption and photoluminescence spectroscopy.

Poly(ethylene glycol) (PEG) is one of the most extensively studied antifouling coatings due to its ability to reduce protein adsorption and improve biocompatibility. Although the use of PEG for antifouling coatings is well established, the stability and density of PEG layers are often inadequate to provide optimum antifouling properties. To improve on these shortcomings, we employed the stepwise construction of PEG layers onto a silicon surface. Acetylene- terminated alkyl monolayers were attached to nonoxidized crystalline silicon surfaces via a one-step hydrosilylation procedure with 1,8-nonadiyne. The acetylene-terminated surfaces were functionalized via a copper-catalyzed azide − alkyne cycloaddition (CuAAC) reaction of the surface-bound alkynes with an azide to produce an amine terminated layer. The amine terminated layer was then further conjugated with PEG to produce an antifouling surface. The antifouling surface properties were investigated by testing adsorption of human serum albumin (HSA) and lysozyme (Lys) onto PEG layers from phosphate buffer solutions. Detailed characterization of protein fouling was carried out with X-ray photoelectron spectroscopy (XPS) and time-of- flight secondary ion mass spectrometry (ToF-SIMS) combined with principal component analysis (PCA). The results revealed no fouling of albumin onto PEG coatings whereas the smaller protein lysozyme adsorbed to a very low extent. Refereed/Peer-reviewed

The mechanism of action of nanotube-silicon heterojunction solar cells is under discussion with literature reports suggesting either p-n or Schottky junction characteristics. The crux of the issue is whether the nanotubes contribute to the observed photocurrent or not. In order to further understand the mechanism of action of these solar cells, devices were fabricated using nanotubes sorted by (n,m) species, so that the excitonic transition is well defined and is outside the range of absorption of silicon and such that any contribution to the photocurrent from the nanotubes should be easily resolved from that of the silicon by analysis of the photocurrent spectrum. The devices exhibited the photocurrent spectra of silicon only, indicating that the nanotubes do not contribute to the photocurrent. However, by changing the back contact electrode material, results were obtained that appear to show such a contribution.

Carbon nanotube (CNT) electrodes for electrochemistry were fabricated from single- and double-walled carbon nanotubes. The electrodes were subsequently covalently loaded with a ferrocene modified α-aminoisobutyric acid peptide, and the electron transfer (ET) capabilities were probed with cyclic voltammetry. The CNT electrodes comprised of double walled CNTs (DWCNTs) demonstrated significantly higher peak current compared to their single walled counterparts (SWCNTs). This is attributed to a higher loading of the ferrocene modified peptide to the outer wall of the nanotube, through the presence of a larger number of defects sites within the sp 2 carbon lattice for the DWCNTs. This higher loading was achieved without compromising the ET rate, indicating that DWCNTs may offer a useful alternative to SWCNTs in future electrochemical sensors and biosensors.

A simple method for producing patterned forests of multiwalled carbon nanotubes (MWCNTs) is described. An aqueous metal salt solution is spin-coated onto a substrate patterned with photoresist by standard methods. The photoresist is removed by acetone washing leaving the acetone-insoluble catalyst pattern on the substrate. Dense forests of vertically aligned (VA) MWCNTs are grown on the patterned catalyst layers by chemical vapour deposition. The procedures have been demonstrated by growing MWCNT forests on two substrates: silicon and conducting graphitic carbon films. The forests adhere strongly to the substrates and when grown directly on carbon film, offer a simple method of preparing MWCNT electrodes.

The length of single-walled carbon nanotubes (SWCNTs) is an important metric for the integration of SWCNTs into devices and for the performance of SWCNT-based electronic or optoelectronic applications. In this work we propose a rather simple method based on electric-field induced differential absorption spectroscopy (EFIDAS) to measure the chiral-index-resolved average length of SWCNTs in dispersions. The method takes advantage of the electric-field induced length-dependent dipole moment of nanotubes and has been verified and calibrated by atomic force microscopy. This method not only provides a low cost, in-situ approach for length measurements of SWCNTs in dispersion, but due to the sensitivity of the method to the SWCNT chiral index, the chiral index dependent average length of fractions obtained by chromatographic sorting can also be derived. Also, the determination of the chiral-index resolved length distribution seems to be possible using this method., 36 pages, 17 figures

An experimental study on the interaction between the top and bottom layer of a chemically functionalized graphene bilayer by mild oxygen plasma is reported. Structural, chemical, and electrical properties are monitored using Raman spectroscopy, transport measurements, conductive atomic force microscopy and X-ray photoelectron spectroscopy. Single- and double-sided chemical functionalization are found to give very different results: single-sided modified bilayers show relatively high mobility (200-600 cm V s at room temperature) and a stable structure with a limited amount of defects, even after long plasma treatment (>60 s). This is attributed to preferential modification and limited coverage of the top layer during plasma exposure, while the bottom layer remains almost unperturbed. This could eventually lead to decoupling between top and bottom layers. Double-sided chemical functionalization leads to a structure containing a high concentration of defects, very similar to graphene oxide. This opens the possibility to use plasma treatment not only for etching and patterning of graphene, but also to make heterostructures (through single-sided modification of bilayers) for sensors and transistors and new graphene-derivatives materials (through double-sided modification). Single- and double-sided chemical functionalization of bilayer graphene is performed using mild oxygen plasma. A single-sided modified bilayer shows relatively high mobility and a stable structure with a limited amount of defects, even after long plasma treatment. Double-sided chemical functionalization leads to a structure containing a high concentration of defects, very similar to graphene oxide.

Due to the high cost of silicon photovoltaics there is currently great interest in finding alternative semiconductor materials for light harvesting devices. Single-walled carbon nanotubes are an allotrope of carbon with unique electrical and optical properties and are promising as future photovoltaic materials. It is thus important to investigate the methods of exploiting their properties in photovoltaic devices. In addition to already extensive research using carbon nanotubes in organic photovoltaics and photoelectrochemical cells, another way to do this is to combine them with a relatively well understood model semiconductor such as silicon. Nanotube-silicon heterojunction solar cells are a recent photovoltaic architecture with demonstrated power conversion efficiencies of up to ∼14% that may in part exploit the photoactivity of carbon nanotubes.

Electrochemical electrodes incorporating double- and single-walled carbon nanotubes (CNTs) were fabricated on cysteamine modified flat gold substrates. Through covalent coupling of the amine end groups with carboxyl functionalized CNTs, a dense forest of vertically aligned CNTs was produced. To these a 30 nm thick insulating polystyrene layer was spin coated, resulting in exposure of the uppermost carbon nanotube ends. The electrochemical performance of each electrode was then determined using the redox probe ruthenium hexaamine. Once surrounded by polymer, the double-walled CNTs (DWCNTs) showed an improved electron transfer rate, compared to the single-walled electrode. This improvement was attributed to the protection of the electronic properties of the inner wall of the DWCNT during the chemical modification and suggests that DWCNTs may offer a useful alternative to SWCNTs in future electrochemical sensors and biosensors. Keywords: Double-walled carbon nanotubes, Cyclic volammetry, Gold electrode

Double-walled carbon nanotubes (DWCNTs) were selectively functionalised by treatment with concentrated nitric and sulphuric acid, resulting in carboxylated outer and pristine inner tube constituents. The functionalised DWCNTs were then incorporated into two types of pre-existing carbon nanotube (CNT) electrode platforms, and the performance of each was compared to single-walled carbon nanotubes (SWCNTs). To make the CNT electrode platforms DWCNTs were covalently bound to fluorinated tin oxide glass (FTO) or electrografted aminophenyl tether layers on silicon. The performance of single- compared to double-walled CNTs on FTO or silicon supported electrodes was then determined through electrochemical methods, using the redox probes, ferrocene and ruthenium hexaamine, respectively. The DWCNTs showed an improved heterogeneous rate constant. This improvement was attributed to the protection of the electronic properties of the inner wall of the DWCNT during the chemical modification and suggests that DWCNTs may offer a useful alternative to SWCNTs in future electronic devices.

The photovoltaic properties of a new working electrode for dye sensitised solar cells, consisting of networks of covalently bound single walled carbon nanotubes on indium tin oxide, have been investigated. Following covalent sensitisation of the carbon nanotube networks with a ruthenium dye an appreciable cathodic photocurrent is measured upon illumination with simulated sunlight. Significant increases in photocurrent density are observed by building up sequential layers of carbon nanotube cross-linked with ethylenediamine to form a three-dimensional dye sensitised single walled carbon nanotube network. Such electrodes are promising for the future fabrication of low cost, minimal material use solar cells.

Double walled carbon nanotubes (DWCNTs) consist of two coaxially aligned single walled carbon nanotubes (SWCNTs) and previous sorting methods only achieved outer-wall electronic type selectivity. Here a separation technique capable of sorting DWCNTs by semiconducting (S) or metallic (M) inner- and outer- wall electronic type is presented. Electronic coupling between the inner- and outer- wall is used to alter the surfactant coating around each of the DWCNT types and aqueous gel permeation is used to separate them. Aqueous methods are used to remove SWCNT species from the raw material and prepare enriched DWCNT fractions. The enriched DWCNT fractions are then transferred into either chlorobenzene or toluene using the copolymer PFO-BPy to yield the four inner@outer combinations of M@M, M@S, S@M and S@S. The high-purity of the resulting fractions is verified by absorption measurements, TEM, AFM, resonance Raman mapping and high-density field effect transistor devices. [1] Han Li, Georgy Gordeev, Soeren Wasserroth, Venkata Sai Kiran Chakravadhanula, Shyam Kumar Chethala Neelakandhan, Frank Hennrich, Ado Jorio, Stephanie Reich, Ralph Krupke, Benjamin Flavel, Nature Nanotechnology (2017) DOI: 10.1038/nnano.2017.207 [2] Katherine Moore, Moritz Pfohl, Daniel Tune, Frank Hennrich, Simone Dehm, Venkata Sai Kiran Chakravadhanula, Christian Kuebel, Ralph Krupke, Benjamin Flavel, ACS Nano (2015) DOI: 10.1021/nn506869h [3] Katherine Moore, Moritz Pfohl, Frank Hennrich, Venkata Sai Kiran Chakravadhanula, Christian Kuebel, Manfred Kappes, Joe Shapter, Ralph Krupke, Benjamin Flavel, ACS Nano (2014) DOI: 10.1021/nn500756a

Diazonium ion chemistry has been used to electrochemically graft aminophenyl layers onto p-type silicon (1 0 0) substrates. A condensation reaction was used to immobilise single-walled carbon nanotubes with high carboxylic acid functionality directly to this layer. Electrochemical monitoring of the aminophenyl groups confirmed the formation of an amide linkage between the single-walled carbon nanotubes and the aminophenyl layer. The carbon nanotube electrode showed high stability and good electrochemical performance in aqueous solution. At moderate scan rates the Ru(NH3)6+3/+2 couple exhibited quasi-reversible electron transfer kinetics with a standard heterogenous rate constant of 1.2 × 10−3 cm s−1 at the covalently-linked carbon nanotube surface. The electrode thus combines the advantages of a silicon substrate for easy integration into sophisticated electrical and electronic devices, carbon nanotubes for desirable electrochemical properties, and stability in aqueous medium for future applications in environmental sensing.

Forests of vertically aligned carbon nanotubes (VACNTs) have been chemically assembled on carbon surfaces. The structures show excellent stability over a wide potential range and are resistant to degradation from sonication in acid, base, and organic solvent. Acid-treated single-walled carbon nanotubes (SWCNTs) were assembled on amine-terminated tether layers covalently attached to pyrolyzed photoresist films. Tether layers were electrografted to the carbon substrate by reduction of the p-aminobenzenediazonium cation and oxidation of ethylenediamine. The amine-modified surfaces were incubated with cut SWCNTs in the presence of N,N'-dicyclohexylcarbodiimide (DCC), giving forests of vertically aligned carbon nanotubes (VACNTs). The SWCNT assemblies were characterized by scanning electron microscopy, atomic force microscopy, and electrochemistry. Under conditions where the tether layers slow electron transfer between solution-based redox probes and the underlying electrode, the assembly of VACNTs on the tether layer dramatically increases the electron-transfer rate at the surface. The grafting procedure, and hence the preparation of VACNTs, is applicable to a wide range of materials including metals and semiconductors.

The reaction of gold substrates with p-nitrobenzene diazonium tetrafluoroborate (NBD) in 0.1 M H(2)SO(4) at open-circuit potential (OCP) is demonstrated to proceed by electron transfer from gold to the NBD cation. Electrochemical, atomic force microscopy, and X-ray photoelectron spectroscopy analyses reveal the formation of multilayer films with the same composition as electrografted films. The film growth characteristics (surface concentration and film thickness vs time) also follow those observed during electrografting, consistent with electron transfer from the substrate to the diazonium cation. The OCP of the gold substrate increases during the period of film growth ( approximately 60 min) and then decreases to close to its initial value. The increase corresponds to accumulation of positive charge as electrons are transferred to NBD; the discharge process is tentatively attributed to slow oxidation of adventitious impurities in the reaction solution. Films formed at OCP or by electrografting from aqueous acid solution are markedly less stable to sonication in acetonitrile than are those electrografted from acetonitrile. Increased amounts of physisorbed material in films prepared in aqueous media or bonding of aryl groups to different gold sites in the two media are tentatively proposed to account for the different stabilities.

A monolayer of –NH2 terminated 3-aminopropyltriethoxysilane (APS) was self-assembled onto a p-type silicon (1 0 0) substrate. This amine terminated silane monolayer provided an electrostatic point of attachment for citrate stabilised gold colloid nanoparticles, which act as ‘seed’ particles for the electroless deposition of gold, creating an electrolessly deposited gold layer on silicon. A –NH2 terminated cysteamine monolayer was then deposited onto the gold layer and carbon nanotubes, with high carboxylic acid functionality, were immobilised via a condensation reaction. A redox active molecule ferrocenemethanol was then chemically attached to the immobilised carbon nanotubes. These nanostructures were used as working electrodes in cyclic voltammetry to observe the oxidation and reduction of ferrocene. Important electrochemical parameters such as electrode kinetics, electron transfer rate and surface concentration of the redox active molecules were obtained, providing information on the ability of electroless plated gold surfaces to act as supports for carbon nanotube-based electrodes. This information has also provided insights into the behaviour of vertically aligned carbon nanotubes immobilised on nanoscale gold wires, which have been previously fabricated using atomic force microscopy.

Vertically-aligned single-walled carbon nanotubes (SWCNTs) (VACNTs) are becoming increasingly recognized due to their fast electron transfer rates. However, the chemistry available for further functionalizing these electrodes is limited. Here we describe a new approach to the fabrication of VACNTs. SWCNTs were covalently attached to a p-type silicon (100) (Si) wafer surface using a thioester linkage in which the nanotubes were firstly acid treated and then, in the presence of a hydroxylated Si wafer surface, reacted with phosphorus pentasulfide (a mild electrophilic catalyst). The novel nanostructure was characterized using atomic force microscopy (AFM) showing vertical alignment with FTIR spectroscopy indicating pendant thiocarboxylic acid groups for further reaction. In addition, electrochemical properties using cyclic voltammetry indicate that the electrodes have excellent electrochemical properties with an electron transfer rate of 2.98 × 10- 3 cm s- 1. © 2009 Elsevier B.V. All rights reserved. Refereed/Peer-reviewed

Atomic force anodisation lithography has been used to create patterned silicon oxide nanostructures on a p-type silicon (100) substrate. Modification of these oxide nanostructures either by acid treatment or self assembly of 3-aminopropyltriethoxysilane yields a chemically heterogeneous surface containing hydrophilic and hydrophobic regions allowing the site-selective deposition of thin films of conductive polyaniline. Carbon nanotubes, with high carboxylic acid functionality, were immobilised on the patterned hydrophilic regions using condensation reactions. Due to the hydrophobic nature of the nanotube sidewalls polyaniline is found to adsorb to the carbon nanotube structure and has allowed the creation of patterned carbon nanotube–polyaniline composites. Such composite materials may prove to be useful in the field of molecular electronics. These two approaches allow the construction of what are basically positive and negative resists using polymers.

The adhesion of gold nanoparticles either electrostatically or chemically attached to a substrate has been probed using AFM operating in force spectroscopy mode. A monolayer of –NH2 terminated 3-aminopropyltriethoxysilane or –SH terminated 3-mercaptopropyltrimethoxysilane was self-assembled onto a p-type silicon (100) substrate. Each silane monolayer provided the point of attachment for citrate stabilised gold colloid nanoparticles. In the case of the –NH2 terminated layer gold colloid assembly was driven by the electrostatic attraction between the negative, citrate-capped, gold nanoparticles and a partially protonated amine layer. In the case of the –SH terminated regions, well-known gold–thiol chemistry was used to chemically attach the nanoparticles. An atomic force microscope tip was chemically modified with 3-mercaptopropyltrimethoxysilane and scanned across each surface, where the cantilever deflection was measured at each x, y pixel of the image to create an array of adhesion force curves. This has allowed an unprecedented nanoscale characterisation of the adhesion force central to two common surface attachment methods of gold colloid nanoparticles, providing useful insights into the stability of nanoscale constructs.

Previously we have used atomic force anodisation lithography, with a self-assembled monolayer of hexadecyltrichlorosilane as a resist, to pattern silicon oxide nanostructures onto a p-type silicon (1 0 0) substrate. A condensation reaction was used to immobilise carbon nanotubes with high carboxylic acid functionality directly to the silicon oxide. A further condensation reaction using this surface attached the molecule ferrocenemethanol to the bound nanotubes. These new nanostructures were used as electrodes to observe the oxidation and reduction of ferrocene. However, because the small currents measured are near the detection limits of the electrochemical system used, important electrode kinetics could not to be obtained. A scribing approach made larger regions of oxidised silicon leading to the creation of larger scale patterned arrangements of carbon nanotubes allowing measurement of important electrochemical parameters such as electrode kinetics, electron transfer rates and surface concentration of redox molecules. Knowledge of these characteristics has provided insights into the behaviour of the microelectrodes created using atomic force microscopy.

Arrays of aligned single‐walled carbon nanotubes (SWCNTs) have been attached to a Si (100) surface using two approaches. The first method uses a self‐assembled ethyl undecanoate monolayer (SWCNTs‐SAM structure), while the second sees the nanotubes attached directly to an ultrathin SiO2 layer (SWCNTs‐SiO2 structure). The optical and electrochemical properties of two nanotube arrays were compared by fluorescence spectroscopy and cyclic voltammetry. The results show that the SWCNTs‐SiO2 structure has strong coupling between the attached SWCNTs and the silicon substrate, but the coupling gets smaller with increasing size of attached SWCNT bundles. The SWCNTs‐SAM structure shows poor conductivity and electrochemical reversibility. The SWCNTs‐SiO2 structure does appear to be a very useful substrate for use in further applications.

We combine on-chip single-walled carbon nanotubes (SWNTs) emitters with directional coupling devices as fundamental building blocks for carbon photonic systems. These devices are essential for studying the emission properties of SWNTs in the few photon regime for future applications in on-chip quantum photonics. The combination of SWNTs with on-chip beam splitters herein provides the basis for correlation measurements as necessary for nanoscale source characterization. The employed fabrication methods are fully scalable and thus allow for implementing a multitude of functional and active circuits in a single fabrication run. Our metallic SWNT emitters are broadband and cover both visible and near-infrared wavelengths, thus holding promise for emerging hybrid optoelectronic devices with fast reconfiguration times.

Open Access Article. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence., We show that the application of lateral shear force on a randomly oriented thin film of carbon nanotubes, in the dry state, causes significant reordering of the nanotubes at the film surface. This new technique of dry shear aligning is applicable to carbon nanotube thin films produced by many of the established methods.

Recent results in the field of carbon nanotube–silicon solar cells have suggested that the best performance is obtained when the nanotube film provides good coverage of the silicon surface and when the nanotubes in the film are aligned parallel to the surface. The recently developed process of dry shear aligning – in which shear force is applied to the surface of carbon nanotube thin films in the dry state, has been shown to yield nanotube films that are very flat and in which the surface nanotubes are very well aligned in the direction of shear. It is thus reasonable to expect that nanotube films subjected to dry shear aligning should outperform otherwise identical films formed by other processes. In this work, the fabrication and characterisation of carbon nanotube–silicon solar cells using such films is reported, and the photovoltaic performance of devices produced with and without dry shear aligning is compared.

A monolayer of methyl terminated hexadecyltrichlorosilane was self-assembled onto a p-type silicon (1 0 0) substrate to provide a resist for electrochemical anodisation with an atomic force microscope cantilever. Through precise control of the cantilever’s position on the surface and the applied bias voltage, a variety of different surface architectures have been fabricated on the substrate. Single-walled carbon nanotubes (SWCNT), with high carboxylic acid functionality, have been immobilised to these etched regions using a condensation reaction. Highly selective condensation has been shown to be possible, both directly onto etched silicon regions as well as with the use of amine terminated 3-aminopropyltriethoxysilane as a molecular anchor. This has enabled the controlled attachment of nanotubes on nanoscale features.

Graphene is of increasing interest for optoelectronic applications exploiting light detection, light emission and light modulation. Intrinsically, the light-matter interaction in graphene is of a broadband type. However, by integrating graphene into optical micro-cavities narrow-band light emitters and detectors have also been demonstrated. These devices benefit from the transparency, conductivity and processability of the atomically thin material. To this end, we explore in this work the feasibility of replacing graphene with nanocrystalline graphene, a material which can be grown on dielectric surfaces without catalyst by graphitization of polymeric films. We have studied the formation of nanocrystalline graphene on various substrates and under different graphitization conditions. The samples were characterized by resistance, optical transmission, Raman and x-ray photoelectron spectroscopy, atomic force microscopy and electron microscopy measurements. The conducting and transparent wafer-scale material with nanometer grain size was also patterned and integrated into devices for studying light-matter interaction. The measurements show that nanocrystalline graphene can be exploited as an incandescent emitter and bolometric detector similar to crystalline graphene. Moreover the material exhibits piezoresistive behavior which makes nanocrystalline graphene interesting for transparent strain sensors.

Single-walled carbon nanotubes (SWCNTs) have been the focus of intense research, and the body of literature continues to grow exponentially, despite more than two decades having passed since the first reports. As well as extensive studies of the fundamental properties, this has seen SWCNTs used in a plethora of applications as far ranging as microelectronics, energy storage, solar cells, and sensors, to cancer treatment, drug delivery, and neuronal interfaces. On the other hand, the properties and applications of double-walled carbon nanotubes (DWCNTs) have remained relatively under-explored. This is despite DWCNTs not only sharing many of the same unique characteristics of their single-walled counterparts, but also possessing an additional suite of potentially advantageous properties arising due to the presence of the second wall and the often complex inter-wall interactions that arise. For example, it is envisaged that the outer wall can be selectively functionalized whilst still leaving the inner wall in its pristine state and available for signal transduction. A similar situation arises in DWCNT field effect transistors (FETs), where the outer wall can provide a convenient degree of chemical shielding of the inner wall from the external environment, allowing the excellent transconductance properties of the pristine nanotubes to be more fully exploited. Additionally, DWCNTs should also offer unique opportunities to further the fundamental understanding of the inter-wall interactions within and between carbon nanotubes. However, the realization of these goals has so far been limited by the same challenge experienced by the SWCNT field until recent years, namely, the inherent heterogeneity of raw, as-produced DWCNT material. As such, there is now an emerging field of research regarding DWCNT processing that focuses on the preparation of material of defined length, diameter and electronic type, and which is rapidly building upon the experience gained by the broader SWCNT community. This review describes the background of the field, summarizing some relevant theory and the available synthesis and purification routes; then provides a thorough synopsis of the current state-of-the-art in DWCNT sorting methodologies, outlines contemporary challenges in the field, and discusses the outlook for various potential applications of the resulting material.