Channeling Excitons to Emissive Defect Sites in Carbon Nanotube Semiconductors beyond the Dilute Regime.

The exciton photoluminescence of carbon nanotube semiconductors has been intensively exploited for bioimaging, anticounterfeiting, photodetection, and quantum information science. However, at high concentrations, photoluminescence is lost to self-quenching because of the nearly complete overlap of the absorption and emissive states (∼10 meV Stokes shift). Here we show that by introducing sparse fluorescent quantum defects via covalent chemistry, self-quenching can be efficiently bypassed by means of the new emission route. The defect photoluminescence is significantly red-shifted by 190 meV for p-nitroaryl tailored (6,5)-single-walled carbon nanotubes (SWCNTs) from the native emission of the nanotube. Notably, the defect photoluminescence is more than 34 times brighter than the native photoluminescence of unfunctionalized SWCNTs in the most concentrated nanotube solution tested (2.7 × 1014 nanotubes/mL). Moreover, we show that defect photoluminescence is more resistant to self-quenching than the native state in a dense film, which is the upper limit of concentration. Our findings open opportunities to harness nanotube excitons in highly concentrated systems for applications where photoluminescence brightness and light-collecting efficiency are mutually important.

Capillary electrophoresis of covalently functionalized single-chirality carbon nanotubes.

We demonstrate the separation of chirality-enriched single-walled carbon nanotubes (SWCNTs) by degree of surface functionalization using high-performance CE. Controlled amounts of negatively charged and positively charged functional groups were attached to the sidewall of chirality-enriched SWCNTs through covalent functionalization using 4-carboxybenzenediazonium tetrafluoroborate or 4-diazo-N,N-diethylaniline tetrafluoroborate, respectively. Surfactant- and pH-dependent studies confirmed that under conditions that minimized ionic screening effects, separation of these functionalized SWCNTs was strongly dependent on the surface charge density introduced through covalent surface chemistry. For both heterogeneous mixtures and single-chirality-enriched samples, covalently functionalized SWCNTs showed substantially increased peak width in electropherogram spectra compared to nonfunctionalized SWCNTs, which can be attributed to a distribution of surface charges along the functionalized nanotubes. Successful separation of functionalized single-chirality SWCNTs by functional density was confirmed with UV-Vis-NIR absorption and Raman scattering spectroscopies of fraction collected samples. These results suggest a high degree of structural heterogeneity in covalently functionalized SWCNTs, even for chirality-enriched samples, and show the feasibility of applying CE for high-performance separation of nanomaterials based on differences in surface functional density.

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.

Optical Excitation of Carbon Nanotubes Drives Localized Diazonium Reactions.

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 sp(2) 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.

Intensity Ratio of Resonant Raman Modes for (n,m) Enriched Semiconducting Carbon Nanotubes.

Relative intensities of resonant Raman spectral features, specifically the radial breathing mode (RBM) and G modes, of 11, chirality-enriched, single-wall carbon nanotube (SWCNT) species were established under second-order optical transition excitation. The results demonstrate an under-recognized complexity in the evaluation of Raman spectra for the assignment of (n,m) population distributions. Strong chiral angle and mod dependencies affect the intensity ratio of the RBM to G modes and can result in misleading interpretations. Furthermore, we report five additional (n,m) values for the chirality-dependent G(+) and G(-) Raman peak positions and intensity ratios; thereby extending the available data to cover more of the smaller diameter regime by including the (5,4) second-order, resonance Raman spectra. Together, the Raman spectral library is demonstrated to be sufficient for decoupling G peaks from multiple species via a spectral fitting process, and enables fundamental characterization even in mixed chiral population samples.

Brightening of carbon nanotube photoluminescence through the incorporation of sp3 defects.

Semiconducting carbon nanotubes promise a broad range of potential applications in optoelectronics and imaging, but their photon-conversion efficiency is relatively low. Quantum theory suggests that nanotube photoluminescence is intrinsically inefficient because of low-lying 'dark' exciton states. Here we demonstrate the significant brightening of nanotube photoluminescence (up to 28-fold) through the creation of an optically allowed defect state that resides below the predicted energy level of the dark excitons. Emission from this new state generates a photoluminescence peak that is red-shifted by as much as 254 meV from the nanotube's original excitonic transition. We also found that the attachment of electron-withdrawing substituents to carbon nanotubes systematically drives this defect state further down the energy ladder. Our experiments show that the material's photoluminescence quantum yield increases exponentially as a function of the shifted emission energy. This work lays the foundation for chemical control of defect quantum states in low-dimensional carbon materials.

Covalently functionalized double-walled carbon nanotubes combine high sensitivity and selectivity in the electrical detection of small molecules.

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/cm(2). The devices displayed an NH(3) 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.

Propagative Sidewall Alkylcarboxylation that Induces Red-Shifted Near-IR Photoluminescence in Single-Walled Carbon Nanotubes.

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.

Diameter-dependent, progressive alkylcarboxylation of single-walled carbon nanotubes.

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.