Chirality-selective functionalization of semiconducting single-walled carbon nanotubes (SWCNTs) has been a difficult synthetic goal for more than a decade. Here we describe an on-demand covalent chemistry to address this intriguing challenge. Our approach involves the synthesis and isolation of a chemically inert diazoether isomer that can be switched to its reactive form in situ by modulation of the thermodynamic barrier to isomerization with pH and visible light that resonates with the optical frequency of the nanotube. We found that it is possible to completely inhibit the reaction in the absence of light, as determined by the limit of sensitive defect photoluminescence (less than 0.01% of the carbon atoms are bonded to a functional group). This optically driven diazoether chemistry makes it possible to selectively functionalize a specific SWCNT chirality within a mixture. Even for two chiralities that are nearly identical in diameter and electronic structure, (6,5)- and (7,3)-SWCNTs, we are able to activate the diazoether compound to functionalize the less reactive (7,3)-SWCNTs, driving the chemical reaction to near exclusion of the (6,5)-SWCNTs. This work opens opportunities to chemically tailor SWCNTs at the single chirality level for nanotube sorting, on-chip passivation, and nanoscale lithography.

Atom-thick materials such as single-walled carbon nanotubes (SWCNTs) and graphene exhibit ultrahigh sensitivity to chemical perturbation partly because all of the constituent atoms are surface atoms. However, low selectivity due to nonspecific binding on the graphitic surface is a challenging issue to many applications including chemical sensing. Here, we demonstrated simultaneous attainment of high sensitivity and selectivity in thin-film field effect transistors (TFTs) based on outer-wall selectively functionalized double-walled carbon nanotubes (DWCNTs). With carboxylic acid functionalized DWCNT TFTs, we obtained excellent gate modulation (on/off ratio as high as 4000) with relatively high ON currents at a CNT areal density as low as 35 ng/cm2. The devices displayed an NH3 sensitivity of 60 nM (or ∼1 ppb), which is comparable to small molecule aqueous solution detection using state-of-the-art SWCNT TFT sensors while concomitantly achieving 6000 times higher chemical selectivity toward a variety of amine-containing analyte molecules over that of other small molecules. These results highlight the potential of using covalently functionalized double-walled carbon nanotubes for simultaneous ultrahigh selective and sensitive detection of chemicals and illustrate some of the structural advantages of this double-wall materials strategy to nanoelectronics.

Single-walled carbon nanotubes (SWCNTs) hold vast potential for future electronic devices due to their outstanding properties, however covalent functionalization often destroys the intrinsic properties of SWCNTs, thus limiting their full potential. Here, we demonstrate the fabrication of a functionalized graphene/semiconducting SWCNT (T@fG) heterostructured thin film transistor as a chemical sensor. In this structural configuration, graphene acts as an atom-thick, impermeable layer that can be covalently functionalized via facile diazonium chemistry to afford a high density of surface functional groups while protecting the underlying SWCNT network from chemical modification, even during a covalent chemical reaction. As a result, the highly functionalized carbon-based hybrid structure exhibits excellent transistor properties with a carrier mobility and ON/OFF ratio as high as 64 cm2/Vs and 5400, respectively. To demonstrate its use in potential applications, T@fG thin films were fabricated as aqueous ammonium sensors exhibiting a detection limit of 0.25 μM in a millimolar ionic strength solution, which is comparable with state-of-the-art aqueous ammonium nanosensors.Download high-res image (327KB)Download full-size image

Bismuth is a lithium-ion battery anode material that can operate at an equilibrium potential higher than graphite and provide a capacity twice as high as that of Li4Ti5O12, making it intrinsically free from lithium plating that may cause catastrophic battery failure. However, the potential of bismuth is hampered by its inferior cyclability (limited to tens of cycles). Here, we propose an “ion conductive solid-state matrix” approach to address this issue. By homogeneously confining bismuth nanoparticles in a solid-state γ-Li3PO4 matrix that is electrochemically formed in situ, the resulting composite anode exhibits a reversible capacity of 280 mA hours per gram (mA h/g) at a rate of 100 mA/g and a record cyclability among bismuth-based anodes up to 500 cycles with a capacity decay rate of merely 0.071% per cycle. We further show that full-cell batteries fabricated from this composite anode and commercial LiFePO4 cathode deliver a stable cell voltage of ∼2.5 V and remarkable energy efficiency up to 86.3%, on par with practical batteries (80–90%). This work paves a way for harnessing bismuth-based battery chemistry for the design of high capacity, safer lithium-ion batteries to meet demanding applications such as electric vehicles.Keywords: battery safety; bismuth electrode; conductive matrix; Electrical energy storage; energy efficiency;

We describe the chemical creation of molecularly tunable fluorescent quantum defects in semiconducting carbon nanotubes through covalently bonded surface functional groups that are themselves nonemitting. By variation of the surface functional groups, the same carbon nanotube crystal is chemically converted to create more than 30 distinct fluorescent nanostructures with unique near-infrared photoluminescence that is molecularly specific, systematically tunable, and significantly brighter than that of the parent semiconductor. This novel exciton-tailoring chemistry readily occurs in aqueous solution and creates functional defects on the sp2 carbon lattice with highly predictable C–C bonding from virtually any iodine-containing hydrocarbon precursor. Our new ability to control nanostructure excitons through a single surface functional group opens up exciting possibilities for postsynthesis chemical engineering of carbon nanomaterials and suggests that the rational design and creation of a large variety of molecularly tunable quantum emitters—for applications ranging from in vivo bioimaging and chemical sensing to room-temperature single-photon sources—can now be anticipated.

Ordered mesoporous carbons (OMCs) are ideal host materials that can provide the desirable electrical conductivity and ion accessibility for high-capacity oxide electrode materials in lithium-ion batteries (LIBs). To this end, however, it is imperative to establish the correlations among material morphology, pore structure and electrochemical performance. Here, we fabricate an ordered mesoporous carbon nanowire (OMCNW)/Fe2O3 composite utilizing a novel soft–hard dual-template approach. The structure and electrochemical performance of OMCNW/Fe2O3 were systematically compared with single-templated OMC/Fe2O3 and carbon nanowire/Fe2O3 composites. This dual-template strategy presents synergetic effects combining the advantages of both soft and hard single-template methods. The resulting OMCNW/Fe2O3 composite enables a high pore volume, high structural stability, enhanced electrical conductivity and Li+ accessibility. These features collectively enable excellent electrochemical cyclability (1200 cycles) and a reversible Li+ storage capacity as high as 819 mA h g−1 at a current density of 0.5 A g−1. Our findings highlight the synergistic benefits of the dual-template approach to heterogeneous composites for high performance electrochemical energy storage materials.

Fluorescent defects have opened up exciting new opportunities to chemically tailor semiconducting carbon nanotubes for imaging, sensing, and photonics needs such as lasing, single photon emission, and photon upconversion. However, experimental measurements on the trap depths of these defects show a puzzling energy mismatch between the optical gap (difference in emission energies between the native exciton and defect trap states) and the thermal detrapping energy determined by application of the van ’t Hoff equation. To resolve this fundamentally important problem, here we synthetically incorporated a series of fluorescent aryl defects into semiconducting single-walled carbon nanotubes and experimentally determined their energy levels by temperature-dependent and chemically correlated evolution of exciton population and photoluminescence. We found that depending on the chemical nature and density of defects, the exciton detrapping energy is 14–77% smaller than the optical gap determined from photoluminescence. For the same type of defect, the detrapping energy increases with defect density from 76 to 131 meV for 4-nitroaryl defects in (6,5) single-walled carbon nanotubes, whereas the optical gap remains nearly unchanged (<5 meV). These experimental findings are corroborated by quantum-chemical simulations of the chemically functionalized carbon nanotubes. Our results suggest that the energy mismatch arises from vibrational reorganization due to significant deformation of the nanotube geometry upon exciton trapping at the defect site. An unexpectedly large reorganization energy (on the order of 100 meV) is found between ground and excited states of the defect tailored nanostructures. This finding reveals a molecular picture for description of these synthetic defects and suggests significant potential for tailoring the electronic properties of carbon nanostructures through chemical engineering.

Covalent chemistries have been widely used to modify carbon nanomaterials; however, they typically lack the precision and efficiency required to directly engineer their optical and electronic properties. Here, we show, for the first time, that visible light which is tuned into resonance with carbon nanotubes can be used to drive their functionalization by aryldiazonium salts. The optical excitation accelerates the reaction rate 154-fold (±13) and makes it possible to significantly improve the efficiency of covalent bonding to the sp2 carbon lattice. Control experiments suggest that the reaction is dominated by a localized photothermal effect. This light-driven reaction paves the way for precise nanochemistry that can directly tailor carbon nanomaterials at the optical and electronic levels.

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.

Silicon can store Li+ at a capacity 10 times that of graphite anodes. However, to harness this remarkable potential for electrical energy storage, one has to address the multifaceted challenge of volume change inherent to high capacity electrode materials. Here, we show that, solely by chemical tailoring of Si-carbon interface with atomic oxygen, the cycle life of Si/carbon matrix-composite electrodes can be substantially improved, by 300%, even at high mass loadings. The interface tailored electrodes simultaneously attain high areal capacity (3.86 mAh/cm2), high specific capacity (922 mAh/g based on the mass of the entire electrode), and excellent cyclability (80% retention of capacity after 160 cycles), which are among the highest reported. Even at a high rate of 1C, the areal capacity approaches 1.61 mAh/cm2 at the 500th cycle. This remarkable electrochemical performance is directly correlated with significantly improved structural and electrical interconnections throughout the entire electrode due to chemical tailoring of the Si-carbon interface with atomic oxygen. Our results demonstrate that interfacial bonding, a new dimension that has yet to be explored, can play an unexpectedly important role in addressing the multifaceted challenge of Si anodes.

We show that local pH can be optically probed through defect photoluminescence from semiconducting carbon nanotubes covalently functionalized with aminoaryl groups. Switching between protonated and deprotonated forms of the amino moiety produces an energy shift in the defect state of the functionalized nanotube by as much as 33 meV in the near-infrared region. This unexpected observation enables a new optical pH sensor that features ultrabright near-infrared II (1.1–1.4 μm) photoluminescence, a sensitivity for pH changes as small as 0.2 pH units over a wide working window that covers the entire physiologic pH range, and potentially molecular resolution. Independent of pH, this nanoprobe can simultaneously act as a nanothermometer by monitoring temperature-modulated changes in photoluminescence intensity, which follows the van’t Hoff equation. This work opens new opportunities for quantitative probing of local pH and temperature changes in complex biological systems.

We demonstrate efficient creation of defect-bound trions through chemical doping of controlled sp3 defect sites in semiconducting, single-walled carbon nanotubes. These tricarrier quasi-particles luminesce almost as brightly as their parent excitons, indicating a remarkably efficient conversion of excitons into trions. Substantial populations of trions can be generated at low excitation intensities, even months after a sample has been prepared. Photoluminescence spectroscopy reveals a trion binding energy as high as 262 meV, which is substantially larger than any previously reported values. This discovery may have important ramifications not only for studying the basic physics of trions but also for the application of these species in fields such as photonics, electronics, and bioimaging.Keywords: carbon nanomaterials; chemical functionalization; defects; exciton; photophysics; spectroscopy

High quality graphene materials that readily disperse in water or organic solvents are needed to achieve some of the most ambitious applications. However, current synthetic approaches are typically limited by irreversible structural damages, little solubility, or low scalability. Here, we describe a fundamental study of graphene chemistry and covalent functionalization patterns on sp2 carbon lattices, from which a facile, scalable synthesis of high quality graphene sheets was developed. Graphite materials were efficiently exfoliated by reductive, propagative alkylation. The exfoliated, propagatively alkylated graphene sheets (PAGenes) not only exhibited high solubility in common solvents such as chloroform, water, and N-methyl-pyrrolidone, but also showed electrical conductivity as high as 4.1 × 103 S/m, which is 5 orders of magnitude greater than those of graphene oxides. Bright blue photoluminescence, unattainable in graphene, was also observed. We attribute the rise of blue photoluminescence in PAGenes to small on-graphene sp2 domains created by the propagative covalent chemistry, which may expand from graphene edges or existing defect sites leaving sp2-hybridized patches interlaced with sp3-hybridized regions. The intact sp2 domains enable effective electrical percolation among different graphene layers affording the observed high electrical conductivity in PAGene films.Keywords: band gap engineering; chemical functionalization; electrical percolation; functionalization pattern; graphitic materials; photoluminescence;

Semiconducting single-walled carbon nanotubes (SWCNTs) are direct band gap materials in which exciton photoluminescence (PL) occurs at the same wavelength as excitation. Here, we show that propagative sidewall alkylation can induce a new PL peak in (6,5) SWCNTs red-shifted from the E11 near-infrared exciton excitation and emission by ∼140 meV. The magnitude of the red-shift is weakly dependent on the terminal functional group. This new emission peak is relatively bright even after a high degree of functionalization because the reaction occurs by propagating outward from initial defects, creating bands of functional groups while maintaining the number of effective defect sites. Density functional theory computations suggest that the covalently attached alkyl functional groups introduce a new, optically allowed, low-lying state from which this new emission may arise. This method of shifting nanotube PL away from the bare nanotube excitation may find applications in near-infrared (IR) fluorescence imaging by allowing both excitation and emission to occur in the optically transparent window for biological tissues.Keywords: bioimaging; covalent functionalization; defect; density functional theory; exciton; nanomaterials; spectroscopy;

•A Si–CNT composite yarn was designed and fabricated with high scalability.•The yarns are mechanically robust, electrically conductive, and readily weavable.•The yarn electrodes exhibit a Li-storage capacity 5 times higher than graphite.Nanomaterials have shown enormous potentials in electronics and energy technology innovations. However, processing of nanoscale materials is challenging and often has extremely limited scalability. We designed and synthesized a Si–carbon nanotube (CNT) yarn which is mechanically as strong and flexible as widely used yarns in the textile industry but simultaneously possesses high electrical conductivity. This composite yarn is readily weavable and the process is highly scalable. As an illustration of potential applications, coin-cell lithium ion batteries were fabricated using the yarns as the anode. Because of the high electrical conductivity, the yarn electrode performs without the need of a copper current collector. A Li-storage capacity of 2200 mA h/g was demonstrated, which is five times higher than graphite electrodes and higher compared with previously demonstrated energy yarns based on pseudo capacitor or ultracapacitor materials.Graphical abstract

Interfacial instability is a fundamental issue in heterostructures ranging from biomaterials to joint replacement and electronic packaging. This challenge is particularly intriguing for lithium ion battery anodes comprising silicon as the ion storage material, where ultrahigh capacity is accompanied by vast mechanical stress that threatens delamination of silicon from the current collectors at the other side of the interface. Here, we describe Si-beaded carbon nanotube (CNT) strings whose interface is controlled by chemical functionalization, producing separated amorphous Si beads threaded along mechanically robust and electrically conductive CNT. In situ transmission electron microscopy combined with atomic and continuum modeling reveal that the chemically tailored Si–C interface plays important roles in constraining the Si beads, such that they exhibit a symmetric “radial breathing” around the CNT string, remaining crack-free and electrically connected throughout lithiation–delithiation cycling. These findings provide fundamental insights in controlling nanostructured interfaces to effectively respond to demanding environments such as lithium batteries.Keywords: carbon nanotube; in situ TEM; interface; lithium ion battery; modeling; nanofabrication; propagation

Making single-walled carbon nanotubes (SWNTs) soluble in water is a challenging first step to use their remarkable electronic and optical properties in a variety of applications. We report that acyclic cucurbit[n]uril molecular containers 1 and 2 selectively solubilize small-diameter and low chiral angle SWNTs. The selectivity is tunable by increasing the concentration of the molecular containers or by adjusting the ionic strength of the solution. Even at a concentration 1000 times lower than typically required for surfactants, the molecular containers render SWNTs soluble in water. Molecular mechanics simulations suggest that these C-shaped acyclic molecules complex the SWNTs such that a large portion of nanotube sidewalls are exposed to the external environment. These “naked” nanotubes fluoresce upon patching the exposed surface with sodium dodecylbenzene sulfonate.

Double-walled carbon nanotubes are coaxial nanostructures composed of exactly two single-walled carbon nanotubes, one nested in another. This unique structure offers advantages and opportunities for extending our knowledge and application of the carbon nanomaterials family. This review seeks to comprehensively discuss the synthesis, purification and characterization methods of this novel class of carbon nanomaterials. An emphasis is placed on the double wall physics that contributes to these structures’ complex inter-wall coupling of electronic and optical properties. The debate over the inner-tube photoluminescence provides an interesting illustration of the rich photophysics and challenges associated with the myriad combinations of the inner and outerwall chiralities. Outerwall selective covalent chemistry will be discussed as a potential solution to the unattractive tradeoff between solubility and functionality that has limited some applications of single-walled carbon nanotubes. Finally, we will review the many different uses of double-walled carbon nanotubes and provide an overview of several promising research directions in this new and emerging field.

Double-walled carbon nanotubes were enriched from their single- and few-walled counterparts as well as other carbonaceous byproducts coexisting in as-synthesized materials using Billups–Birch reductive alkylcarboxylation chemistry. The enrichment was made possible due to high selectivity of the reaction towards smaller diameter, single-walled carbon nanotubes and more defective carbonaceous structures. Raman resonant scattering studies suggest that alkylcarboxylation selectively occurred on the outerwalls while the inner-tubes remained unaffected. The separated double-walled carbon nanotubes were water soluble due to outerwall alkylcarboxylation, but the Raman modes and part of the optical properties of the inner-tubes were preserved.

We demonstrate diameter-dependent, progressive alkylcarboxylation of single-walled carbon nanotubes by recycling a modified Billups–Birch reaction. The strong diameter dependence was confirmed by Raman spectroscopy. Alkylcarboxylation made SWNTs soluble in water, allowing the more readily functionalized, smaller diameter nanotubes to be enriched by water extraction.

Atom-thick materials such as single-wall carbon nanotubes (SWNTs) and graphene are prone to chemical attacks because all constituent atoms are exposed. Here we report the retention of optical and electrical properties of inner tubes in heavily functionalized double-wall carbon nanotubes (DWNTs). Correlated optical absorption spectroscopy, Raman scattering, and thin film electrical conductivity all suggest that an inner tube behaves strikingly similar to a pristine SWNT; however, because of the protection of the outer wall, the inner tube can survive aggressive chemical attacks (e.g., by diazonium chemistry) without compromising physical properties. At the saturation limit of the diazonium functionalization, an SWNT network becomes electrically insulating; in stark contrast, the double-walled structure retains ∼50% of the initial conductivity, owing to the intact inner tube pathway. These results suggest the possibility of high-performance DWNT electronic devices with important capabilities for tailored surface chemistry on the outer walls, whereas the inner tubes are chemically protected.Keywords: chemical selectivity; double-walled carbon nanotubes; electrical percolation; electrode network; electronic nanomaterials; semiconducting nanotubes; spectroscopy;

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.Keywords: carbon nanomaterials; chemical pattern; contrast mechanism; electron microscopy; functionalization imaging; materials chemistry;

The outer walls of double-walled carbon nanotubes (DWNTs) were selectively oxidized using a combination of oleum and nitric acid. Intercalation of oleum between bundled DWNTs enabled a homogeneous reaction by equally exposing all outer wall surfaces to the oxidants. At optimized reaction conditions, this double-wall chemistry enabled high water solubility through carboxylic acid functional groups introduced to the outer wall, while leaving the inner tube intact, as shown by Raman scattering and high resolution TEM. These outer wall selectively oxidized DWNTs retained electrical conductivity up to 65% better than thin films of similarly functionalized single-walled carbon nanotubes, which can be attributed to enhanced electrical percolation via the nonoxidized inner tubes.

This Nano Focus article reviews recent developments in nanofabrication based on invited talks given at the “Chemical Methods of Nanofabrication” symposium, which was organized by the authors and presented at the 237th ACS National Meeting and Exhibition as one of seven symposia within the meeting theme, “Nanoscience: Challenges for the Future”. The three-day symposium included 25 experts from academia, national laboratories, and industry from around the world, to discuss current progress and future directions in nanofabrication. We highlight several of the key results discussed and future directions and challenges in this rapidly changing field.

Atom-thick materials such as single-walled carbon nanotubes (SWCNTs) and graphene exhibit ultrahigh sensitivity to chemical perturbation partly because all of the constituent atoms are surface atoms. However, low selectivity due to nonspecific binding on the graphitic surface is a challenging issue to many applications including chemical sensing. Here, we demonstrated simultaneous attainment of high sensitivity and selectivity in thin-film field effect transistors (TFTs) based on outer-wall selectively functionalized double-walled carbon nanotubes (DWCNTs). With carboxylic acid functionalized DWCNT TFTs, we obtained excellent gate modulation (on/off ratio as high as 4000) with relatively high ON currents at a CNT areal density as low as 35 ng/cm2. The devices displayed an NH3 sensitivity of 60 nM (or ∼1 ppb), which is comparable to small molecule aqueous solution detection using state-of-the-art SWCNT TFT sensors while concomitantly achieving 6000 times higher chemical selectivity toward a variety of amine-containing analyte molecules over that of other small molecules. These results highlight the potential of using covalently functionalized double-walled carbon nanotubes for simultaneous ultrahigh selective and sensitive detection of chemicals and illustrate some of the structural advantages of this double-wall materials strategy to nanoelectronics.

Double-walled carbon nanotubes were enriched from their single- and few-walled counterparts as well as other carbonaceous byproducts coexisting in as-synthesized materials using Billups–Birch reductive alkylcarboxylation chemistry. The enrichment was made possible due to high selectivity of the reaction towards smaller diameter, single-walled carbon nanotubes and more defective carbonaceous structures. Raman resonant scattering studies suggest that alkylcarboxylation selectively occurred on the outerwalls while the inner-tubes remained unaffected. The separated double-walled carbon nanotubes were water soluble due to outerwall alkylcarboxylation, but the Raman modes and part of the optical properties of the inner-tubes were preserved.

We demonstrate diameter-dependent, progressive alkylcarboxylation of single-walled carbon nanotubes by recycling a modified Billups–Birch reaction. The strong diameter dependence was confirmed by Raman spectroscopy. Alkylcarboxylation made SWNTs soluble in water, allowing the more readily functionalized, smaller diameter nanotubes to be enriched by water extraction.