Wall-selective probing of double-walled carbon nanotubes using covalent functionalization

Click-Functionalization of Silanized Carbon Nanotubes: From Inorganic Heterostructures to Biosensing Nanohybrids

Here we present an approach to functionalize silanized single-walled carbon nanotubes (SWNTs) through copper-free click chemistry for the assembly of inorganic and biological nanohybrids. The nanotube functionalization route involves silanization and strain-promoted azide-alkyne cycloaddition reactions (SPACC). This was characterized by X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy and Fourier transform infra-red spectroscopy. Silane-azide-functionalized SWNTs were immobilized from solution onto patterned substrates through dielectrophoresis (DEP). We demonstrate the general applicability of our strategy for the functionalization of SWNTs with metal nanoparticles (gold nanoparticles), fluorescent dyes (Alexa Fluor 647) and biomolecules (aptamers). In this regard, dopamine-binding aptamers were conjugated to the functionalized SWNTs to perform real-time detection of dopamine at different concentrations. Additionally, the chemical route is shown to selectively functionalize individual nanotubes grown on the surface of silicon substrates, contributing towards future nano electronic device applications.

Hydrogels from a Self-Assembling Tripeptide and Carbon Nanotubes (CNTs): Comparison between Single-Walled and Double-Walled CNTs

Supramolecular hydrogels obtained from the self-organization of simple peptides, such as tripeptides, are attractive soft materials. Their viscoelastic properties can be enhanced through the inclusion of carbon nanomaterials (CNMs), although their presence can also hinder self-assembly, thus requiring investigation of the compatibility of CNMs with peptide supramolecular organization. In this work, we compared single-walled carbon nanotubes (SWCNTs) and double-walled carbon nanotubes (DWCNTs) as nanostructured additives for a tripeptide hydrogel, revealing superior performance by the latter. Several spectroscopic techniques, as well as thermogravimetric analyses, microscopy, and rheology data, provide details to elucidate the structure and behavior of nanocomposite hydrogels of this kind.

TEGylated Double-Walled Carbon Nanotubes as Platforms to Engineer Neuronal Networks

In the past two decades, important results have been obtained on the application of carbon nanotubes (CNTs) as components of smart interfaces promoting neuronal growth and differentiation. Different forms of CNTs have been employed as scaffolds, including raw CNTs and functionalized CNTs, characterized by a different number of walls, mainly single-walled CNTs (SWCNTs) or multiwalled CNTs (MWCNTs). However, double-walled carbon nanotubes (DWCNTs), which present interesting electronic and transport properties, have barely been studied in the field. Apart from the electrical conductivity, the morphology, shape, porosity, and corresponding mechanical properties of the scaffold material are important parameters when dealing with neuronal cells. Thus, the presence of open porous and interconnected networks is essential for cell growth and differentiation. Here, we present an easy methodology to prepare porous self-standing and electrically conductive DWCNT-based scaffolds and study the growth of neuro/glial networks and their synaptic activity. A cross-linking approach with triethylene glycol (TEG) derivatives is applied to improve the tensile performance of the scaffolds while neuronal growth and differentiation are promoted. By testing different DWCNT-based constructs, we confirm that the manufactured structures guarantee a biocompatible scaffold, while favoring the design of artificial networks with high complexity.

Parallel Field-Effect Nanosensors Detect Trace Biomarkers Rapidly at Physiological High-Ionic-Strength Conditions

Sensitivity and speed of detection are contradicting demands that profoundly impact the electrical sensing of molecular biomarkers. Although single-molecule sensitivity can now be achieved with single-nanotube field-effect transistors, these tiny sensors, with a diameter less than 1 nm, may take hours to days to capture the molecular target at trace concentrations. Here, we show that this sensitivity-speed challenge can be addressed using covalently functionalized double-wall CNTs that form many individualized, parallel pathways between two electrodes. Each carrier that travels across the electrodes is forced to take one of these pathways that are fully gated chemically by the target-probe binding events. This sensor design allows us to electrically detect Lyme disease oligonucleotide biomarkers directly at the physiological high-salt concentrations, simultaneously achieving both ultrahigh sensitivity (as low as 1 fM) and detection speed (<15 s). This unexpectedly simple strategy may open opportunities for sensor designs to broadly achieve instant detection of trace biomarkers and real-time probing of biomolecular functions directly at their physiological states.

Combined spin filtering actions in hybrid magnetic junctions based on organic chains covalently attached to graphene

We present a bias-controlled spin-filtering mechanism in spin-valves including a hybrid organic chain/graphene interface. Wet growth conditions of oligomeric molecular chains would usually lead, during standard CMOS-compatible fabrication processes, to the quenching of spintronics properties of metallic spin sources due to oxidation. We demonstrate by X-ray photoelectron spectroscopy that the use of a protective graphene layer fully preserves the metallic character of the ferromagnetic surface and thus its capability to deliver spin polarized currents. We focus here on a small aromatic chain of controllable lengths, formed by nitrobenzene monomers and derived from the commercial 4-nitrobenzene diazonium tetrafluoroborate, covalently attached to the graphene passivated spin sources thanks to electroreduction. A unique bias dependent switch of the spin signal is then observed in complete spin valve devices, from minority to majority spin carriers filtering. First-principles calculations are used to highlight the key role played by the spin-dependent hybridization of electronic states present at the different interfaces. Our work is a first step towards the exploration of spin transport using different functional molecular chains. It opens the perspective of atomic tailoring of magnetic junction devices towards spin and quantum transport control, thanks to the flexibility of ambient electrochemical surface functionalization processes.

Interlayer Interactions in 1D Van der Waals Moiré Superlattices

Studying two-dimensional (2D) van der Waals (vdW) moiré superlattices and their interlayer interactions have received surging attention after recent discoveries of many new phases of matter that are highly tunable. Different atomistic registry between layers forming the inner and outer nanotubes can also form one-dimensional (1D) vdW moiré superlattices. In this review, experimental observations and theoretical perspectives related to interlayer interactions in 1D vdW moiré superlattices are summarized. The discussion focuses on double-walled carbon nanotubes (DWNTs), a model 1D vdW moiré system, and the authors highlight the new optical features emerging from the non-trivial strong interlayer coupling effect and the unique physics in 1D DWNTs. Future directions and questions in probing the intriguing physical phenomena in 1D vdW moiré superlattices such as, correlated physics in different 1D moiré systems beyond DWNTs are proposed and discussed.

A Meta-Analysis of Conductive and Strong Carbon Nanotube Materials

A study of 1304 data points collated over 266 papers statistically evaluates the relationships between carbon nanotube (CNT) material characteristics, including: electrical, mechanical, and thermal properties; ampacity; density; purity; microstructure alignment; molecular dimensions and graphitic perfection; and doping. Compared to conductive polymers and graphitic intercalation compounds, which have exceeded the electrical conductivity of copper, CNT materials are currently one-sixth of copper's conductivity, mechanically on-par with synthetic or carbon fibers, and exceed all the other materials in terms of a multifunctional metric. Doped, aligned few-wall CNTs (FWCNTs) are the most superior CNT category; from this, the acid-spun fiber subset are the most conductive, and the subset of fibers directly spun from floating catalyst chemical vapor deposition are strongest on a weight basis. The thermal conductivity of multiwall CNT material rivals that of FWCNT materials. Ampacity follows a diameter-dependent power-law from nanometer to millimeter scales. Undoped, aligned FWCNT material reaches the intrinsic conductivity of CNT bundles and single-crystal graphite, illustrating an intrinsic limit requiring doping for copper-level conductivities. Comparing an assembly of CNTs (forming mesoscopic bundles, then macroscopic material) to an assembly of graphene (forming single-crystal graphite crystallites, then carbon fiber), the ≈1 µm room-temperature, phonon-limited mean-free-path shared between graphene, metallic CNTs, and activated semiconducting CNTs is highlighted, deemphasizing all metallic helicities for CNT power transmission applications.

Mechanical Characterization of Multiwalled Carbon Nanotubes: Numerical Simulation Study

The elastic properties of armchair and zigzag multiwalled carbon nanotubes were investigated under tensile, bending, and torsion loading conditions. A simplified finite element model of the multiwalled carbon nanotubes, without taking into account the van der Waals interactions between layers, was used to assess their tensile, bending, and torsional rigidities and, subsequently, Young's and shear moduli. Relationships between the tensile rigidity and the squares of the diameters of the outer and inner layers in multiwalled carbon nanotubes, and between the bending and torsional rigidities with the fourth powers of the diameters of the outer and inner layers, were established. These relationships result in two consistent methods, one for assessment to the Young's modulus of armchair and zigzag multiwalled carbon nanotubes, based on tensile and bending rigidities, and the other to evaluate shear modulus using tensile, bending, and torsional rigidities. This study provides a benchmark regarding the determination of the mechanical properties of nonchiral multiwalled carbon nanotubes by nanoscale continuum modeling approach.

Giant spin signals in chemically functionalized multiwall carbon nanotubes

Transporting quantum information such as the spin information over micrometric or even millimetric distances is a strong requirement for the next-generation electronic circuits such as low-voltage spin-logic devices. This crucial step of transportation remains delicate in nontopologically protected systems because of the volatile nature of spin states. Here, a beneficial combination of different phenomena is used to approach this sought-after milestone for the beyond-Complementary Metal Oxide Semiconductor (CMOS) technology roadmap. First, a strongly spin-polarized charge current is injected using highly spin-polarized hybridized states emerging at the complex ferromagnetic metal/molecule interfaces. Second, the spin information is brought toward the conducting inner shells of a multiwall carbon nanotube used as a confined nanoguide benefiting from both weak spin-orbit and hyperfine interactions. The spin information is finally electrically converted because of a strong magnetoresistive effect. The experimental results are also supported by calculations qualitatively revealing exceptional spin transport properties of this system.

Synthesis of outer tube-selectively nitrogen-doped double-walled carbon nanotubes by nitrogen plasma treatment

We demonstrate selective nitrogen doping on the outer tubes of double walled carbon nanotubes by nitrogen plasma treatment. Such selectivity improves the chemical activity of the outer tubes, including their wettability and oxygen reduction reaction properties, while retaining the structural, optical, and electrical properties of the inner tubes.

Inner- and outer-wall sorting of double-walled carbon nanotubes

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.

Light Control of Charge Transfer and Excitonic Transitions in a Carbon Nanotube/Porphyrin Hybrid

Carbon nanotube-chromophore hybrids are promising building blocks in order to obtain a controlled electro-optical transduction effect at the single nano-object level. In this work, a strong spectral selectivity of the electronic and the phononic response of a chromophore-coated single nanotube transistor is observed for which standard photogating cannot account. This paper investigates how light irradiation strongly modifies the coupling between molecules and nanotube within the hybrid by means of combined Raman diffusion and electron transport measurements. Moreover, a nonconventional Raman enhancement effect is observed when light irradiation is on the absorption range of the grafted molecule. Finally, this paper shows how the dynamics of single electron tunneling in the device at low temperature is strongly modified by molecular photoexcitation. Both effects will be discussed in terms of photoinduced excitons coupled to electronic levels.

Laser Lithography of a Tube-in-a-Tube Nanostructure

Carbon nanotubes hold vast potential for device innovations because their optical and electronic properties can be synthetically tailored at a length scale unattainable by lithographic techniques. However, lithographic patterning of carbon nanotubes with electronic-type control remains one of the major problems for the integration of these nanomaterials for practical device applications. In this work, we propose a laser lithography method for direct-write patterning of devices on thin films of outer wall selectively functionalized double-walled carbon nanotubes (Tube^2). This method is enabled by the reversible removal of surface functional groups with a laser tuned into resonance with the inner tube of Tube^2. We show that it is possible to directly create patterned dot arrays and conductive pathways and circuits on insulating Tube^2 thin films by tuning the resonance of the direct-writing laser with the electronic type of the inner tube (i.e., metallic or semiconducting). The successful patterning was unambiguously confirmed with in situ Raman spectral imaging and electrical characterization. This work suggests the possibility of developing a nanostructure-specific nanofabrication technology reminiscent of thermal printing.

Chemical Gating of a Synthetic Tube-in-a-Tube Semiconductor

A critical challenge to translating field effect transistors into biochemical sensor platforms is the requirement of a gate electrode, which imposes restrictions on sensor device architectures and results in added expense, poorer scalability, and electrical noise. Here we show that it is possible to eliminate the need of the physical gate electrode and dielectrics altogether using a synthetic tube-in-a-tube (Tube^∧2) semiconductor. Composed of a semiconducting single-walled carbon nanotube nested in a charged, impermeable covalent functional shell, Tube^∧2 allows the semiconducting conduction pathway to be modulated solely by surface functional groups in a chemically gated-all-around configuration. The removal of physical gates significantly simplifies the device architecture and enables photolithography-free, highly scalable fabrication of transistor sensors in nonconventional configurations that are otherwise impossible. We show that concomitant FET sensitivity and single-mismatch selectivity can be achieved with Tube^∧2 even in a two-terminal, thin film transistor device configuration that is as simple as a chemiresistor. Miniaturized two-terminal field effect point sensors can also be fabricated, using a straightforward dice-and-dip procedure, for the detection of tuberculosis biomarkers.

Separation of double-wall carbon nanotubes by electronic type and diameter

We introduce a new procedure for the efficient isolation and subsequent separation of double-wall carbon nanotubes (DWCNTs). A simplified, rate zonal ultracentrifugation (RZU) process is first applied to obtain samples of highly-enriched DWCNTs from a raw carbon nanotube material that has both single- and double-wall carbon nanotubes. Using this purified DWCNT suspension, we demonstrate for the first time that DWCNTs can be further processed using aqueous two-phase extraction (ATPE) for sequential separation by electronic structure and diameter. Additionally, we introduce analytical ultracentrifugation (AUC) as a new method for DWCNT characterization to assess DWCNT purity in separated samples. Results from AUC analysis are utilized to compare two DWCNT separation schemes. We find that RZU processing followed by sequential bandgap and diameter sorting via ATPE provides samples of highest DWCNT enrichment, whereas single-step redox sorting of the same raw material through ATPE yields SWCNT/DWCNT mixtures of similar diameter and electronic character. The presented methods offer significant advancement in DWCNT processing and separation while also providing a promising alternative for DWCNT sample analysis.

Chemical Control and Spectral Fingerprints of Electronic Coupling in Carbon Nanostructures

The optical and electronic properties of atomically thin materials such as single-walled carbon nanotubes and graphene are sensitively influenced by substrates, the degree of aggregation, and the chemical environment. However, it has been experimentally challenging to determine the origin and quantify these effects. Here we use time-dependent density-functional-theory calculations to simulate these properties for well-defined molecular systems. We investigate a series of core-shell structures containing C₆₀ enclosed in progressively larger carbon shells and their perhydrogenated or perfluorinated derivatives. Our calculations reveal strong electronic coupling effects that depend sensitively on the interparticle distance and on the surface chemistry. In many of these systems we predict considerable orbital mixing and charge transfer between the C₆₀ core and the enclosing shell. We predict that chemical functionalization of the shell can modulate the electronic coupling to the point where the core and shell are completely decoupled into two electronically independent chemical systems. Additionally, we predict that the C₆₀ core will oscillate within the confining shell, at a frequency directly related to the strength of the electronic coupling. This low-frequency motion should be experimentally detectable in the IR region.

Double-walled carbon nanotube processing

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.

Sorting of Double-Walled Carbon Nanotubes According to Their Outer Wall Electronic Type via a Gel Permeation Method

In this work, we demonstrate the application of the gel permeation technique to the sorting of double-walled carbon nanotubes (DWCNTs) according to their outer wall electronic type. Our method uses Sephacryl S-200 gel and yields sorted fractions of DWCNTs with impurities removed and highly enriched in nanotubes with either metallic (M) or semiconducting (S) outer walls. The prepared fractions are fully characterized using optical absorption spectroscopy, transmission electron microscopy, and atomic force microscopy, and the entire procedure is monitored in real time using process Raman analysis. The sorted DWCNTs are then integrated into single nanotube field effect transistors, allowing detailed electronic measurement of the transconductance properties of the four unique inner@outer wall combinations of S@S, S@M, M@S, and M@M.

Probing differential optical and coverage behavior in nanotube-nanocrystal heterostructures synthesized by covalent versus non-covalent approaches

Double-walled carbon nanotube (DWNT)-CdSe heterostructures with the individual nanoscale building blocks linked together by 4-aminothiophenol (4-ATP) have been successfully synthesized using two different and complementary routes, i.e. covalent attachment and non-covalent π-π stacking. Specifically, using a number of characterization methods, we have probed the effects of these differential synthetic coupling approaches on the resulting CdSe quantum dot (QD) coverage on the underlying nanotube template as well as the degree of charge transfer between the CdSe QDs and the DWNTs. In general, based on microscopy and spectroscopy data collectively, we noted that heterostructures generated by non-covalent π-π stacking interactions evinced not only higher QD coverage density but also possibly more efficient charge transfer behavior as compared with their counterparts produced using covalent linker-mediated protocols.

Graft-induced midgap states in functionalized carbon nanotubes

Covalent addition of functional groups onto carbon nanotubes is known to generate lattice point defects that disrupt the electronic wave function, resulting namely in a reduction of their optical response and electrical conductance. Here, conductance measurements combined with numerical simulations are used to unambiguously identify the presence of graft-induced midgap states in the electronic structure of covalently functionalized semiconducting carbon nanotubes. The main experimental evidence is an increase of the conductance in the OFF-state after covalent addition of 4-bromophenyl grafts on many single- and double-walled individual nanotubes, the effect of which is fully suppressed after thermodesorption of the adducts. The graft-induced current leakage is thermally activated and can reach several orders of magnitude above its highly insulating pristine-state level. Ab initio simulations of various configurations of functionalized nanotubes corroborate the presence of these midgap states and show their localization around the addends. Moreover, the electronic density of these localized states exhibits an extended hydrogenoid profile along the nanotube axis, providing access for long-range coupling between the grafts. We argue that covalent nanotube chemistry is a powerful tool to prepare and control midgap electronic states on nanotubes for enabling further studies of the intriguing properties of interacting 1D localized states.

Selective breakdown of metallic pathways in double-walled carbon nanotube networks

Covalently functionalized, semiconducting double-walled carbon nanotubes exhibit remarkable properties and can outperform their single-walled carbon nanotube counterparts. In order to harness their potential for electronic applications, metallic double-walled carbon nanotubes must be separated from the semiconductors. However, the inner wall is inaccessible to current separation techniques which rely on the surface properties. Here, the first approach to address this challenge through electrical breakdown of metallic double-walled carbon nanotubes, both inner and outer walls, within networks of mixed electronic types is described. The intact semiconductors demonstrate a ∼62% retention of the ON-state conductance in thin film transistors in response to covalent functionalization. The selective elimination of the metallic pathways improves the ON/OFF ratio, by more than 360 times, to as high as 40 700, while simultaneously retaining high ON-state conductance.

Double-wall carbon nanotube-porphyrin supramolecular hybrid: synthesis and photophysical studies

Double-wall carbon nanotubes (DWCNTs) with pyridyl units covalently attached to the external wall through isoxazolino linkers and carboxylic groups that have been esterified by pentyl chains are synthesized. The properties of these modified DWCNTs are then compared with an analogous sample based on single-wall carbon nanotubes (SWCNTs). Raman spectroscopy shows the presence of characteristic radial breathing mode vibrations, confirming that the samples partly retain the integrity of the nanotubes in the case of DWCNTs, including the internal and external nanotubes. Quantification of the pyridyl content for both samples (DWCNT and SWCNT derivatives) is based on X-ray photoelectron spectroscopy and thermogravimetric profiles, showing very similar substituent load. Both pyridyl-containing nanotubes (DWCNTs and SWCNTs) form a complex with zinc porphyrin (ZnP), as evidenced by the presence of two isosbestic points in the absorption spectra of the porphyrin upon addition of the pyridyl-functionalized nanotubes. Supramolecular complexes based on pyridyl-substituted DWCNTs and SWCNTs quench the emission and the triplet excited state identically, through an energy-transfer mechanism based on pre-assembly of the ground state. Thus, the presence of the intact inner wall in DWCNTs does not influence the quenching behavior, with respect to SWCNTs, for energy-transfer quenching with excited ZnP. These results sharply contrast with previous ones referring to electron-transfer quenching, in which the double-wall morphology of the nanotubes has been shown to considerably reduce the lifetime of charge separation, owing to faster electron mobility in DWCNTs compared to SWCNTs.

Physical removal of metallic carbon nanotubes from nanotube network devices using a thermal and fluidic process

Electronic and optoelectronic devices based on thin films of carbon nanotubes are currently limited by the presence of metallic nanotubes. Here we present a novel approach based on nanotube alkyl functionalization to physically remove the metallic nanotubes from such network devices. The process relies on preferential thermal desorption of the alkyls from the semiconducting nanotubes and the subsequent dissolution and selective removal of the metallic nanotubes in chloroform. The approach is versatile and is applied to devices post-fabrication.

Photochemical evidence of electronic interwall communication in double-wall carbon nanotubes

Single- and double-wall carbon nanotubes (CNTs) having dimethylanilino (DMA) units covalently attached to the external graphene wall have been prepared by the reaction of the dimethylaminophenylnitronium ion with the corresponding CNT. The samples have been characterized by Raman and XPS spectroscopies, thermogravimetry, and high-resolution transmission electron microscopy in which the integrity of the single or double wall of the CNT and the percentage of substitution (one dimethylanilino group every 45 carbons of the wall for the single- and double-wall samples) has been determined. Nanosecond laser flash photolysis has shown the generation of transients that has been derived from the charge transfer between the dimethylanilino (as the electron donor) to the CNT graphene wall (as the electron acceptor). Importantly, the lifetime of the double-wall CNT is much shorter than that monitored for the single-wall CNT. Shorter-lived transients were also observed for the pentyl-esterified functionalized double-wall CNT with respect to the single-wall analogue in the presence of hole (CH(3)OH) and electron quenchers (O(2), N(2)O), which has led to the conclusion that the inner, intact graphene wall that is present in double-wall CNT increases the charge mobility significantly, favoring charge recombination processes. Considering the importance that charge mobility has in microelectronics, our finding suggests that double-wall CNT or two-layer graphene may be more appropriate to develop devices needing fast charge mobility.

Experimental evidence of a mechanical coupling between layers in an individual double-walled carbon nanotube

We perform transmission electron microscopy, electron diffraction, and Raman scattering experiments on an individual suspended double-walled carbon nanotube (DWCNT). The first two techniques allow the unambiguous determination of the DWCNT structure: (12,8)@(16,14). However, the low-frequency features in the Raman spectra cannot be connected to the derived layer diameters d by means of the 1/d power law, widely used for the diameter dependence of the radial-breathing mode of single-walled nanotubes. We discuss this disagreement in terms of mechanical coupling between the layers of the DWCNT, which results in collective vibrational modes. Theoretical predictions for the breathing-like modes of the DWCNT, originating from the radial-breathing modes of the layers, are in a very good agreement with the observed Raman spectra. Moreover, the mechanical coupling qualitatively explains the observation of Raman lines of breathing-like modes, whenever only one of the layers is in resonance with the laser energy.