Solvent Effect on Optical Limiting Properties of Single-Walled Carbon Nanotube Dispersions

Single-walled carbon nanotubes (SWNTs) were dispersed in N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF), and N,N-dimethylacetamide (DMA), respectively. The nonlinear optical properties of SWNT dispersions were studied using the open aperture Z-scan technique at 532 nm. The nonlinear extinction coefficients strongly increase with increasing SWNT concentration. In the three dispersions, the DMF dispersions show the strongest nonlinear extinction effect. In conjunction with this, the optical limiting performance is also superior for the DMF dispersions. Compared with DMF and DMA, NMP has a much better debundling effect for SWNTs; however, the optical limiting properties of the NMP dispersions is inferior. The SWNT dispersions seem to attenuate the intense light more effectively, as is shown by the stronger optical limiting response compared to the zinc phthalocyanine solution, at the same concentration. The static light scattering experiment revealed that the DMF dispersions have the largest average bundle size. The main origin of the optical limiting effect of the SWNT dispersions is due to the solvent and/or carbon vapor bubble-induced nonlinear scattering. Our results show that the average bundle size of SWNTs in combination with the physical properties of the solvent dominate the nonlinear extinction and optical limiting properties of SWNT dispersions

Control of Optical Limiting of Carbon Nanotube Dispersions by Changing Solvent Parameters

Nonlinear optical and optical limiting properties of a range of single-walled carbon nanotube dispersions prepared in N-methyl-2-pyrrolidinone (NMP) were studied using the open aperture Z-scan technique at 532 nm. As the appropriate thermodynamic properties of the solvents are much more important than the bundle size of nanotubes for improving the optical limiting performance, the solvent parameters were controlled by either changing the temperature of the dispersions or blending a secondary solvent. While the optical limiting performance can be varied freely by increasing or decreasing the temperature from room temperature to 100 °C, the reduction of temperature below the freezing point of NMP and then down as far as −80 °C has little influence on the limiting performance. As a result of adding a small amount of organic solvent into the NMP dispersions, the nonlinear optical responses were enhanced significantly due to the reduction of surface tension and other parameters. By contrast, the addition of water leads to a decrease in the optical limiting response. Nanotube dispersions in water/surfactant exhibit a similar limiting performance to the nanotubes in NMP. Our results reveal that the optical limiting performance of the nanotube dispersions can be engineered by adjusting the solvent properties. Because the carbon nanotube dispersions are typical of the thermally induced light scattering dominated optical limiting materials, we believe the conclusions fit not only the nanotubes but also other nanomaterials with the similar limiting mechanism

Synthesis, Characterization, and Optoelectronic Properties of a Novel Polyfluorene/Poly(<i>p</i>-Phenylenevinylene) Copolymer

A novel conjugated polyfluorene/poly(p-phenylenevinylene) copolymer containing the pendant bis(4-alkoxyphenyl) groups in the C-9 position of every alternating fluorene unit has been synthesized and well structurally characterized. The photoluminescence spectrum of this polymer exhibits strong concentration and excitation wavelength dependence in solution. The excited triplet-state maximum of polymer occurs in the region of 460−540 nm with a lifetime of 65.8 μs. This copolymer displays a minor positive nonlinear absorption at the focus of the laser irradiation, suggesting possible reverse saturable absorption. The stable electroluminescent spectrum of the polymer light-emitting diode device based on this copolymer (device configuration, indium−tin oxide/Au/copolymer/LiF/Al) was obtained with a peak wavelength of 515 nm. The bright-green emission observed over the whole active area of the copolymer closely resembles the photoluminescence of the most concentrated solution (0.5 M) used. This suggests that chain stacking in the solid state is responsible for the observed green electroluminescence

Spontaneous Debundling of Single-Walled Carbon Nanotubes in DNA-Based Dispersions

Natural salmon testes DNA has been used to disperse single-walled carbon nanotubes (SWNTs) in water. It has been found that the primary factor controlling the nanotube bundle size distribution in the dispersion is the nanotube concentration. As measured by AFM, the mean bundle diameter tends to decrease with decreasing concentration. The number fraction of individual nanotubes increases with decreasing concentration. At low nanotube concentrations, number fractions of up to 83% individual SWNTs, equating to a mass fraction of 6.2%, have been obtained. Both the absolute number density and mass per volume of individual nanotubes initially increased with decreasing concentration, displaying a peak at ∼0.027 mg/mL. This concentration thus yields the largest quantities of individually dispersed SWNTs. The AFM data for populations of individual nanotubes was confirmed by infrared photoluminescence spectroscopy. The photoluminescence intensity increased with decreasing concentration, indicating extensive debundling. The concentration dependence of the luminescence intensity matched well to the AFM data on the number density of individual nanotubes. More importantly, it was found that, once initially dispersed, spontaneous debundling occurs upon dilution without the need for sonication. This implies that DNA−SWNT hybrids exist in water as a solution rather than a dispersion. The effects of dilution have been compared to the results obtained by ultracentrifuging the samples, showing dilution methods to be a viable and cost-effective alternative to ultracentrifugation. It was found that even after 4 h of ultracentrifugation at 122 000g, bundles with diameters of up to 4 nm remained in solution. The bundle diameter distribution after ultracentrifugation was very similar to the equilibrium distribution for the appropriate concentration after dilution, showing ultracentrifugation to be equivalent to dilution

Selective Positioning and Density Control of Nanotubes within a Polymer Thin Film

We introduce a completely new and innovative method of producing polymer/nanotube composites where the density and position of the nanotubes within the composite can be controlled. Carbon nanotubes are grown from organometallic micropatterns. These periodic nanotube arrays are then incorporated into a polymer matrix by depositing a curable polymer film on the as-grown tubes. This controlled method of producing free-standing nanotube/polymer composite films represents a more efficient method of combining these materials for potential flexible electronic applications in an inexpensive and scalable manner

A Generic Organometallic Approach toward Ultra-Strong Carbon Nanotube Polymer Composites

Multiwalled carbon nanotubes have been functionalized using n-butyllithium and then covalently bonded to a chlorinated polypropylene. The following addition of the polymer-grafted nanotubes to the chlorinated polypropylene polymer matrix resulted in significant increase of mechanical properties. As nanotube content is increased to 0.6 vol %, Young's Modulus increased by a factor of 3, while both the tensile strength and the toughness increased by factors of 3.8 and 4, respectively. This covalent functionalization enables us to get an efficient dispersion and excellent interfacial stress transfer, potentially leading to new ultra-strong polymer composite materials

Ordered DNA Wrapping Switches on Luminescence in Single-Walled Nanotube Dispersions

An extensive study of the time dependence of DNA wrapping in single-walled nanotube (SWNT) dispersions has been carried out, revealing a number of unusual phenomena. SWNTs were dispersed in water with salmon testes DNA and monitored over a three-month period. Between 20 and 50 days after the sample was first prepared, the SWNT photoluminescence (PL) intensity was observed to increase by a factor of 50. This increase was accompanied by a considerable sharpening of the van Hove absorption peaks. High-resolution transmission electron microscopy (HRTEM) images showed the progressive formation of a coating of DNA on the walls of the nanotubes over the three-month period. HRTEM and circular dichroism spectroscopy studies showed that the improvement in both the NIR PL intensity and the van Hove absorption peaks coincided with the completion of a monolayer coating of DNA on the SWNT walls. HRTEM images clearly showed the DNA wrapping helically around the SWNTs in a surprisingly ordered fashion. We suggest that the initial quenching of NIR photoluminescence and broadening of absorption peaks is related to the presence of protonated surface oxides on the nanotubes. The presence of an ordered DNA coating on the nanotube walls mediates both deprotonation and removal of the surface oxides. An extensive DNA coating is required to substantially restore the photoluminescence, and thus, the luminescence switch-on and subsequent saturation indicate the completion of the DNA-wrapping process. The temperature dependence of the PL switch-on, and thus of the wrapping process, was investigated by measuring as functions of temperature both the time before PL switch-on and the time required for the PL intensity to saturate. This allowed the calculation of the activation energies for both the process preceding PL switch-on and the process limiting the rise of PL intensity, which were found to be 31 and 41 kJ mol−1, respectively. The associated entropies of activation were −263 and −225 J mol−1 K−1, respectively. These negative activation entropies suggest that the rate-limiting step is characterized by a change in the system from a less-ordered to a more-ordered state, consistent with the formation of an ordered DNA coating

Large Populations of Individual Nanotubes in Surfactant-Based Dispersions without the Need for Ultracentrifugation

Stable dispersions of single-walled carbon nanotubes have been produced using the surfactant sodium dodecylbenzene sulfonate (SDBS). Atomic force microscopy analysis shows that, on dilution of these dispersions, the nanotubes exfoliate from bundles, resulting in a concentration-dependent bundle diameter distribution which saturates at Drms ≈ 2 nm for concentrations, CNT < 0.05 mg/mL. The total bundle number density increases with concentration, saturating at ∼6 bundles per μm3 for CNT > 0.05 mg/mL. As the concentration is reduced the number fraction of individual nanotubes grows, approaching 50% at low concentration. In addition, partial concentrations of individual SWNTs approaching 0.01 mg/mL can be realized. These values are far superior to those for solvent dispersions due to repulsion stabilization of the surfactant-coated nanotubes. These methods facilitate the preparation of high-quality nanotube dispersions without the need for ultracentrifugation, thus significantly increasing the yield of dispersed nanotubes

Versatile Solution Phase Triangular Silver Nanoplates for Highly Sensitive Plasmon Resonance Sensing

Solution phase triangular silver nanoplates (TSNP) with versatile tunability throughout the visible−NIR wavelengths are presented as highly sensitive localized surface plasmon refractive index sensors. A range of 20 TSNP solutions with edge lengths ranging from 11 to 200 nm and aspect ratios from 2 to 13 have been studied comprehensively using AFM, TEM, and UV−vis−NIR spectroscopy. Studies of the localized surface plasmon resonance (LSPR) peak’s sensitivity to refractive index changes are performed using a simple sucrose concentration method whereby the surrounding refractive index can solely be changed without variation in any other parameter. The dependence of the TSNP localized surface plasmon resonance (LSPR) peak wavelength λmax and its bulk refractive index sensitivity on the nanoplate’s structure is determined. LSPR sensitivities are observed to increase linearly with λmax up to 800 nm, with the values lying within the upper limit theoretically predicted for optimal sensitivity, notwithstanding any diminution due to ensemble averaging. A nonlinear increase in sensitivity is apparent at wavelengths within the NIR region with values reaching 1096 nm·RIU−1 at λmax 1093 nm. Theoretical studies performed using a simple aspect ratio dependent approximation method and discrete dipole approximation methods confirm the dependence of the LSPR bulk refractive index sensitivity upon the TSNP aspect ratio measured experimentally. These studies highlight the importance of this key parameter in acquiring such high sensitivities and promote these TSNP sols for sensing applications at appropriate wavelengths for biological samples

Characterization of Covalent Functionalized Carbon Nanotubes

The characterization of chemically modified carbon nanotubes has been achieved using 13C nuclear magnetic resonance (NMR) spectroscopy. Amino-functionalized multiwall carbon nanotubes (MWNT−NH2) were reacted with a 13C terephthalic acid. Covalent attachment of the acid to the nanotubes is confirmed by 13C NMR and 2D NMR through the presence of amide grouping resonances