Effect of the chemical functionalization on charge transport in carbon nanotubes at the mesoscopic scale

Conductivity vs functionalization in single-walled carbon nanotube films

Diazo functionalization is a chemical method that changes the conductance of metallic single-walled carbon nanotubes (SWCNTs) by disrupting the C–C double bonds. Its application to native mixtures of metallic and semiconducting SWCNTs is a promising way of large-scale production of semiconducting SWCNTs for use in electronics. This has been well studied on isolated SWCNTs, but the implications on the conductivity of SWCNT materials are still unclear. Here, we study the conductivity of such functionalized SWCNT films with a progressively decreased metallic/semiconducting ratio in a wide range of temperatures (4–300 K) to unravel the charge transport mechanisms of metallic and semiconducting SWCNT subnetworks to show how these components participate in the total conductivity of the films. At low functionalization degree (below 0.2 mol%), the conductivity is dominated by a subnetwork of metallic SWCNTs through two parallel mechanisms: a Luttinger liquid mechanism and a Variable Range Hopping process. Higher functionalization (over 0.4 mol%) destroys the Luttinger liquid mechanism, and a second parallel Variable Range Hopping process arises, attributed to the conduction through the semiconducting SWCNTs. At these high functionalization degrees, the SWCNT film behaves as a material with the desired semiconducting properties.

Graphical abstract

We studied the conductivity of chemically functionalized Single Walled Carbon Nanotube films with a progressively decreased metallic/semiconducting ratio in a wide range of temperatures (4–300 K) to unravel the charge transport mechanisms of metallic and semiconducting SWCNT subnetworks to show how these components participate in the total conductivity of the films.

Computational study on noncovalent interactions between (n, n) single-walled carbon nanotubes and simple lignin model-compounds

Composites of carbon nanotubes (CNTs) and lignin are promising and potentially cheap precursors of-to this day-expensive carbon fibers. Since the control of the CNT-lignin interface is crucial to maximize fiber performance, it is imperative to understand the fundamental noncovalent interactions between lignin and CNT. In the present study a density functional theory study is conducted to investigate the fundamental noncovalent interaction strength between metallic (n, n) single-walled CNT (SWCNT) and simple lignin model molecules. In particular, the respective adsorption energies are used to gauge the strength of interaction classes (ππ interaction, CHπ hydrogen bonding and OH-related hydrogen bonding. From the data, substituent-dependent interaction trends as well as class- and curvature-dependent interaction trends are derived. Overall, we find that most of the interaction strength trends appear to be strongly influenced by geometry: flat orientation of the test molecules relative to the (n, n) SWCNT surface and small (n, n) SWCNT curvature-that is, large diameter enhances the CHπ and ππ interactions.

Covalent Functionalization of Single-Walled Carbon Nanotubes by the Bingel Reaction for Building Charge-Transfer Complexes

Functionalization of nanotubes with donor and acceptor partners by the Bingel reaction leads to the formation of charge-transfer dyads, which can operate in organic photovoltaic devices. In this work, we theoretically examine the mechanism of the Bingel reaction for the (6,5)-chiral, (5,5)-armchair, and (9,0)-zigzag single-walled carbon nanotubes (SWCNTs), and demonstrate that the reaction is regioselective and takes place at the perpendicular position of (6,5)- and (5,5)-SWCNTs, and the oblique position of (9,0)-SWCNT. Further, we design computationally the donor-acceptor complexes based on (6,5)-SWCNT coupled with partners of different electronic nature. Analysis of their excited states reveals that efficient photoinduced charge transfer can be achieved in the complexes with π-extended analogue of tetrathiafulvalene (exTTF), zinc tetraphenylporphyrin (ZnTPP), and tetracyanoanthraquinodimethane (TCAQ). The solvent can significantly affect the population of the charge-separated states. Our calculations show that electron transfer (ET) occurs in the normal Marcus regime on a sub-nanosecond time scale in the complexes with exTTF and ZnTPP, and in the inverted Marcus regime on a picosecond time scale in the case of the TCAQ derivative. The ET rate is found to be not very sensitive to the degree of functionalization of the nanotube.

Quantum localization and electronic transport in covalently functionalized carbon nanotubes

Carbon nanotubes are of central importance for applications in nano-electronics thanks to their exceptional transport properties. They can be used as sensors, for example in biological applications, provided that they are functionalized to detect specific molecules. Due to their one-dimensional geometry the carbon nanotubes are very sensitive to the phenomenon of Anderson localization and it is therefore essential to know how the functionalization modifies their conduction properties and if they remain good conductors. Here we present a study of the quantum localization induced by functionalization in metallic single walled carbon nanotubes (SWCNT) with circumferences up to 15 nm. We consider resonant and non-resonant adsorbates that represent two types of covalently functionalized groups with strong and moderate scattering properties. The present study provides a detailed analysis of the localization behaviour and shows that the localization length can decrease down to 20-50 nm at concentrations of about 1 percent of adsorbates. On this basis we discuss the possible electronic transport mechanisms which can be either metallic like or insulating like with variable range hopping.

Fluorescent Ultrashort Nanotubes from Defect-Induced Chemical Cutting

Ultrashort single-walled carbon nanotubes (SWCNTs) that fluoresce brightly in the shortwave infrared could open exciting opportunities in high-resolution bioimaging and sensing. However, this material remains largely unexplored due to the synthetic challenge. Here, we describe a high-yield synthesis of fluorescent ultrashort nanotubes based on a fundamentally new understanding of defect-induced chemical etching of SWCNTs. We first implant fluorescent sp3 quantum defects along the nanotube sidewalls and then oxidatively cut the nanotubes into ultrashort pieces using hydrogen peroxide. This simple two-step process leads to the synthesis of fluorescent ultrashort nanotubes with a narrow length distribution (38 ± 18 nm) and a yield as high as 57%. Despite their ultrashort length, the cut SWCNTs fluoresce brightly in the shortwave infrared at wavelengths characteristic of the sp3 defects, which provides a spectral fingerprint allowing us to uncover new insights into this defect-induced cutting process. Quantum chemical computations suggest that this etching reaction occurs selectively at the defect sites where hydroxyl radicals (•OH) attack the surrounding electron-rich carbon atoms. This work reveals fundamental insights into defect chemistry and makes fluorescent ultrashort nanotubes synthetically accessible for both basic and applied studies of this largely unexplored but rich class of synthetic molecular nanostructures.

A tool box to ascertain the nature of doping and photoresponse in single-walled carbon nanotubes

The effect of doping on the electronic properties in bulk single-walled carbon nanotube (SWCNT) samples is studied for the first time using a new in situ Raman spectroelectrochemical method, and further verified by DFT calculations and photoresponse. We use p-/n-doped SWCNTs prepared by diazonium reactions as a versatile chemical strategy to control the SWCNT behavior. The measured and calculated data testify an acceptor effect of 4-aminobenzenesulfonic acid (p-doping), and a donor effect (n-doping) in the case of benzyl alcohol. In addition, pristine and covalently functionalized SWCNTs were used for the preparation of photoactive film electrodes. The photocathodic current in the photoelectrochemical cell is consistently modulated by the doping group. These results validate the in situ Raman spectroelectrochemistry as a unique tool box for predicting the electronic properties of functionalized SWCNTs in the form of thin films and their operational functionality in thin film devices for future optoelectronic applications.

Electronic transport properties of MoS2 nanoribbons embedded in butadiene solvent

Transition metal dichalcogenides (TMDCs) are promising materials for applications in nanoelectronics and correlated fields, where their metallic edge states play a fundamental role in the electronic transport.

Carbon Nanotubes Covalently Attached to Functionalized Surfaces Directly through the Carbon Cage

The covalent attachment of nonfunctionalized and carboxylic acid-functionalized carbon nanotubes to amine-terminated organic monolayers on gold and silicon surfaces is investigated. It is well established that the condensation reaction between a carboxylic acid and an amine is a viable method to anchor carbon nanotubes to solid substrates. The work presented here shows that the presence of the carboxylic group on the nanotube is not required for attachment to occur, as direct attachment via the substrate amine and the nanotube cage can take place. Scanning and transmission electron microscopy and atomic force microscopy confirm the presence of carbon nanotubes in intimate contact with the surface. X-ray photoelectron spectroscopy is utilized to compare the surface chemistry of the functionalized and nonfunctionalized carbon nanotubes and is supported by a computational investigation. Ion fragments attributed to the direct attachment between the surface and carbon nanotube cage are detected by time-of-flight secondary ion mass spectrometry. The overall attachment scheme is evaluated and can be further used on multiple carbonaceous materials attached to solid substrates.

The morphological effect on electronic structure and electrical transport properties of one-dimensional carbon nanostructures

This work reveals a relationship of morphological structure, electronic structure and electrical transport properties in carbon nanomaterials.

Single-Molecule Reaction Chemistry in Patterned Nanowells

A new approach to synthetic chemistry is performed in ultraminiaturized, nanofabricated reaction chambers. Using lithographically defined nanowells, we achieve single-point covalent chemistry on hundreds of individual carbon nanotube transistors, providing robust statistics and unprecedented spatial resolution in adduct position. Each device acts as a sensor to detect, in real-time and through quantized changes in conductance, single-point functionalization of the nanotube as well as consecutive chemical reactions, molecular interactions, and molecular conformational changes occurring on the resulting single-molecule probe. In particular, we use a set of sequential bioconjugation reactions to tether a single-strand of DNA to the device and record its repeated, reversible folding into a G-quadruplex structure. The stable covalent tether allows us to measure the same molecule in different solutions, revealing the characteristic increased stability of the G-quadruplex structure in the presence of potassium ions (K(+)) versus sodium ions (Na(+)). Nanowell-confined reaction chemistry on carbon nanotube devices offers a versatile method to isolate and monitor individual molecules during successive chemical reactions over an extended period of time.

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.

Electronic and Quantum Transport Properties of Atomically Identified Si Point Defects in Graphene

We report high-resolution scanning transmission electron microscopy images displaying a range of inclusions of isolated silicon atoms at the edges and inner zones of graphene layers. Whereas the incorporation of Si atoms to a graphene armchair edge involves no reconstruction of the neighboring carbon atoms, the inclusion of a Si atom to a zigzag graphene edge entails the formation of five-membered carbon rings. In all the observed atomic edge terminations, a Si atom is found bridging two C atoms in a 2-fold coordinated configuration. The atomic-scale observations are underpinned by first-principles calculations of the electronic and quantum transport properties of the structural anomalies. Experimental estimations of Si-doped graphene band gaps realized by means of transport measurements may be affected by a low doping rate of 2-fold coordinated Si atoms at the graphene edges, and 4-fold coordinated at inner zones due to the apparition of mobility gaps.

STUDY OF THE NEAR INFRARED-MEDIATED HEATING OF DISPERSIONS OF PROTEIN-COATED PRISTINE AND CARBOXYLATED SINGLE-WALLED CARBON NANOTUBES

Previously, we demonstrated the selective NIR-mediated ablation of tumor cells in vitro using pristine single-walled carbon nanotubes (SWNTs) with adsorbed tumor-targeting ligands and carboxylated SWNTs with covalently-attached ligands. The covalent approach is advantageous in ensuring that protein ligands remain associated with the NIR-absorbing SWNTs in biological matrices and the noncovalent approach has the advantage of enabling SWNT functionalization without perturbation of the SWNT lattice and photothermal properties. Herein, we compare the ability of moderately-carboxylated (~4 at.% carboxylic acid groups) and pristine SWNT materials to absorb 808 nm radiation and convert it to heat. Under conditions of a constant 808 nm laser power density, the approach involved measuring the temperature of aqueous dispersions of protein-coated SWNTs as a function of the irradiation time. Nearly identical temperature profiles were observed for dispersions of moderately-carboxylated and pristine SWNTs possessing matched 808 nm optical densities and equivalent concentrations of carbonaceous species (i.e., SWNTs and amorphous carbon impurities). The results indicate that the amount of carbonaceous species in purified dispersions of protein-coated SWNTs is more important for converting absorbed 808 nm radiation into heat than whether or not the SWNTs were moderately carboxylated, and that moderately-carboxylated SWNTs could be the SWNT-material of choice for the targeted photothermal ablation of tumor cells.

Improving the electrical conductivity of carbon nanotube networks: a first-principles study

We address the issue of the low electrical conductivity observed in carbon nanotube networks using first-principles calculations of the structure, stability, and ballistic transport of different nanotube junctions. We first study covalent linkers, using the nitrene-pyrazine case as a model for conductance-preserving [2 + 1] cycloadditions, and discuss the reasons for their poor performance. We then characterize the role of transition-metal adsorbates in improving mechanical coupling and electrical tunneling between the tubes. We show that the strong hybridization between the transition-metal d orbitals with the π orbitals of the nanotube can provide an excellent electrical bridge for nanotube-nanotube junctions. This effect is maximized in the case of nitrogen-doped nanotubes, thanks to the strong mechanical coupling between the tubes mediated by a single transition metal adatom. Our results suggest effective strategies to optimize the performance of carbon nanotube networks.

Confined propagation of covalent chemical reactions on single-walled carbon nanotubes

Covalent chemistry typically occurs randomly on the graphene lattice of a carbon nanotube because electrons are delocalized over thousands of atomic sites, and rapidly destroys the electrical and optical properties of the nanotube. Here we show that the Billups-Birch reductive alkylation, a variant of the nearly century-old Birch reduction, occurs on single-walled carbon nanotubes by defect activation and propagates exclusively from sp(3) defect sites, with an estimated probability more than 1,300 times higher than otherwise random bonding to the 'π-electron sea'. This mechanism quickly leads to confinement of the reaction fronts in the tubular direction. The confinement gives rise to a series of interesting phenomena, including clustered distributions of the functional groups and a constant propagation rate of 18 ± 6  nm per reaction cycle that allows straightforward control of the spatial pattern of functional groups on the nanometre length scale.

One-dimensional extended lines of divacancy defects in graphene

Since the outstanding transport properties of graphene originate from its specific structure, modification at the atomic level of the graphene lattice is needed in order to change its electronic properties. Thus, topological defects play an important role in graphene and related structures. In this work, one-dimensional (1D) arrangement of topological defects in graphene are investigated within a density functional theory framework. These 1D extended lines of pentagons, heptagons and octagons are found to arise either from the reconstruction of divacancies, or from the epitaxial growth of graphene. The energetic stability and the electronic structure of different ideal extended lines of defects are calculated using a first-principles approach. Ab initio scanning tunneling microscopy (STM) images are predicted and compared to recent experiments on epitaxial graphene. Finally, local density of states and quantum transport calculations reveal that these extended lines of defects behave as quasi-1D metallic wires, suggesting their possible role as reactive tracks to anchor molecules or atoms for chemical or sensing applications.

Switchable conductance in functionalized carbon nanotubes via reversible sidewall bond cleavage

We propose several covalent functionalizations for carbon nanotubes that display switchable on/off conductance in metallic tubes. The switching action is achieved by reversible control of bond-cleavage chemistry in [1 + 2] cycloadditions via the sp(3) ⇌ sp(2) rehybridization that it induces; this leads to remarkable changes of conductance even at very low degrees of functionalization. Reversible bond-cleavage chemistry is achieved by identifying addends that provide optimal compensation between the bond-preserving through-space π orbital interactions with the tube against the bond-breaking strain energy of the cyclopropane moiety. Several strategies for real-time control, based on redox or hydrolysis reactions, cis-trans isomerization or excited-state proton transfer are proposed. Such designer functional groups would allow for the first time direct control of the electrical properties of metallic carbon nanotubes, with extensive applications in nanoscale devices.

Critical comparison of electrode models in density functional theory based quantum transport calculations

We study the performance of two different electrode models in quantum transport calculations based on density functional theory: parametrized Bethe lattices and quasi-one-dimensional wires or nanowires. A detailed account of implementation details in both the cases is given. From the systematic study of nanocontacts made of representative metallic elements, we can conclude that the parametrized electrode models represent an excellent compromise between computational cost and electronic structure definition as long as the aim is to compare with experiments where the precise atomic structure of the electrodes is not relevant or defined with precision. The results obtained using parametrized Bethe lattices are essentially similar to the ones obtained with quasi-one-dimensional electrodes for large enough cross-sections of these, adding a natural smearing to the transmission curves that mimics the true nature of polycrystalline electrodes. The latter are more demanding from the computational point of view, but present the advantage of expanding the range of applicability of transport calculations to situations where the electrodes have a well-defined atomic structure, as is the case for carbon nanotubes, graphene nanoribbons, or semiconducting nanowires. All the analysis is done with the help of codes developed by the authors which can be found in the quantum transport toolbox ALACANT and are publicly available.

Gold-Substrate-Enhanced Scanning Electron Microscopy of Functionalized Single-Wall Carbon Nanotubes

Functionalized regions of a single-wall carbon nanotube were resolved by scanning electron microscopy at 1 kV when the functionalized nanotube was placed on a gold substrate. Beam energy and substrate dependence studies suggest that the sharp imaging contrast arises from an increase in the yield of secondary electrons as compared to gold due to covalent modification of the nanotube. Using this surprisingly simple technique, it becomes possible to rapidly map surface functionalization on individual carbon nanotubes with a spatial resolution better than 10 nm. This new functionalization imaging technique may facilitate spatial control of surface chemistry and defect engineering in carbon nanomaterials.

Phosphorus and phosphorus-nitrogen doped carbon nanotubes for ultrasensitive and selective molecular detection

A first-principles approach is used to establish that substitutional phosphorus atoms within carbon nanotubes strongly modify the chemical properties of the surface, thus creating highly localized sites with specific affinity towards acceptor molecules. Phosphorus-nitrogen co-dopants within the tubes have a similar effect for acceptor molecules, but the P-N bond can also accept charge, resulting in affinity towards donor molecules. This molecular selectivity is illustrated in CO and NH3 adsorbed on PN-doped nanotubes, O2 on P-doped nanotubes, and NO2 and SO2 on both P- and PN-doped nanotubes. The adsorption of different chemical species onto the doped nanotubes modifies the dopant-induced localized states, which subsequently alter the electronic conductance. Although SO2 and CO adsorptions cause minor shifts in electronic conductance, NH3, NO2, and O2 adsorptions induce the suppression of a conductance dip. Conversely, the adsorption of NO2 on PN-doped nanotubes is accompanied with the appearance of an additional dip in conductance, correlated with a shift of the existing ones. Overall these changes in electric conductance provide an efficient way to detect selectively the presence of specific molecules. Additionally, the high oxidation potential of the P-doped nanotubes makes them good candidates for electrode materials in hydrogen fuel cells.

Chemically reactive species remain alive inside carbon nanotubes: a density functional theory study

The behavior of alkyl guest radicals inside carbon nanotube hosts with different diameters is analyzed using density functional theory (DFT) calculations. Here the inner alkyl radicals are assumed to be formed by decomposition of their precursors, which had been incorporated into the tubes. DFT calculations show that inner alkyl radicals prefer to exist separately from the nanotube wall (separate form) rather than forming an inner covalent bond with the wall (bound form). Keeping a radical apart from the inner wall is more likely for a more bulky radical inside a smaller diameter tube. A key to the preference for the separate forms over the bound forms is that the bound forms gain a weak attraction due to the formation of a bond with the inner wall. The weak attraction, ascribed to the inertness of the inner surface, is counteracted by destabilization due to deformations of a tube and radical induced by guest-host coupling. The energy balance argument illuminates that the inertness of the inner wall makes an alkyl radical species remain alive inside a tube and retain its reactivity. These findings can help us to understand experimental results where chemical reactions inside a tube proceed after guests are activated.

First-principles study of He effects in a bcc Fe grain boundary: site preference, segregation and theoretical tensile strength

We perform a first-principles calculation to investigate the effects of He in an Fe Σ5(310)/[001] grain boundary (GB) with the SIESTA code, for which the reliability of the pseudopotential and the basis set are systematically tested. We calculate the formation and segregation energies for different substitutional and interstitial cases in order to determine the site preference and the segregation properties of He in the Fe GB. It is demonstrated that the He segregation either breaks (substitution) or weakens (interstitial) the surrounding interfacial Fe-Fe bonds, leading to the GB tensile strength reduction.

Properties, synthesis, and growth mechanisms of carbon nanotubes with special focus on thermal chemical vapor deposition

Carbon nanotubes (CNTs) have been extensively investigated in the last decade because their superior properties could benefit many applications. However, CNTs have not yet made a major leap into industry, especially for electronic devices, because of fabrication challenges. This review provides an overview of state-of-the-art of CNT synthesis techniques and illustrates their major technical difficulties. It also charts possible in situ analyses and new reactor designs that might enable commercialization. After a brief description of the CNT properties and of the various techniques used to synthesize substrate-free CNTs, the bulk of this review analyzes chemical vapor deposition (CVD). This technique receives special attention since it allows CNTs to be grown in predefined locations, provides a certain degree of control of the types of CNTs grown, and may have the highest chance to succeed commercially. Understanding the primary growth mechanisms at play during CVD is critical for controlling the properties of the CNTs grown and remains the major hurdle to overcome. Various factors that influence CNT growth receive a special focus: choice of catalyst and substrate materials, source gases, and process parameters. This review illustrates important considerations for in situ characterization and new reactor designs that may enable researchers to better understand the physical growth mechanisms and to optimize the synthesis of CNTs, thus contributing to make carbon nanotubes a manufacturing reality.

Quantum transport in graphene nanoribbons: effects of edge reconstruction and chemical reactivity

We present first-principles transport calculations of graphene nanoribbons with chemically reconstructed edge profiles. Depending on the geometry of the defect and the degree of hydrogenation, spectacularly different transport mechanisms are obtained. In the case of monohydrogenated pentagon (heptagon) defects, an effective acceptor (donor) character results in strong electron-hole conductance asymmetry. In contrast, weak backscattering is obtained for defects that preserve the benzenoid structure of graphene. Based on a tight-binding model derived from ab initio calculations, evidence for large conductance scaling fluctuations are found in disordered ribbons with lengths up to the micrometer scale.

Chemical functionalization effects on armchair graphene nanoribbon transport

We report first-principles transport calculations in chemically functionalized graphene nanoribbons. The effect of the joint attachment of hydroxyl and hydrogen groups on the graphene surface is investigated as a function of defect location and coverage density. The chemical bonding of a single defect pair (C-OH and C-H) is shown to considerably alter the conduction capability of ribbon channels, similarly to an sp(3) type of defect. With transport calculations in disordered ribbons with lengths up to the micrometer scale, the elastic mean free paths and conduction regimes are analyzed. Even in the low grafting density limit, transport properties are found to be severely damaged by the functionalization, indicating a strong tendency toward an insulating regime.