Exciton energy transfer-assisted photoluminescence brightening from freestanding single-walled carbon nanotube bundles.
ABSTRACT Photoluminescence (PL) brightening is clearly observed through the direct morphology transition from isolated to thin bundled vertically- and individually freestanding single-walled carbon nanotubes (SWNTs). On the basis of the precise spectra analysis and equation-based estimation of the PL time trace, the origin of the PL brightening is consistently explained in terms of the exciton energy transfer through the tube bundles. The PL brightening is also revealed to obviously depend on SWNT diameters. Only the small-diameter rich sample can realize the PL brightening, which can be explained by the different concentrations of metallic SWNTs causing a PL quenching. Since it can be possible to fabricate brightly illuminating nanotubes on various kinds of substrates, the bundle engineering with freestanding nanotubes is expected to be a potential candidate for realizing the nanotube-based PL device fabrication.
- [Show abstract] [Hide abstract]
ABSTRACT: Excitation energy transfer has long been an intriguing subject in the fields of photoscience and materials science. Along with the recent progress of photovoltaics, photocatalysis, and photosensors using nanoscale materials, excitation energy transfer between a donor and an acceptor at a short distance (≤1-10 nm) is of growing importance in both fundamental research and technological applications. This Perspective highlights our recent studies on exciton energy transfer between carbon nanotubes with interwall (surface-to-surface) distances of less than ∼1 nm, which are equivalent to or shorter than the size of one-dimensional excitons in carbon nanotubes. We show exciton energy transfer in bundles of single-walled carbon nanotubes with the interwall distances of ∼0.34 and 0.9 nm (center-to-center distances ∼1.3-1.4 and 1.9 nm). For the interwall distance of ∼0.34 nm (center-to-center distance ∼1.3-1.4 nm), the transfer rate per tube from a semiconducting tube to adjacent semiconducting tubes is (1.8-1.9) × 10(12) s(-1), and that to adjacent metallic tubes is 1.1 × 10(12) s(-1). For the interwall distance of ∼0.9 nm (center-to-center distance ∼1.9 nm), the transfer rate per tube from a semiconducting tube to adjacent semiconducting tubes is 2.7 × 10(11) s(-1). These transfer rates are much lower than those predicted by the Förster model calculation based on a point dipole approximation, indicating the failure of the conventional Förster model calculations. In double-walled carbon nanotubes, which are equivalent to ideal nanoscale coaxial cylinders, we show exciton energy transfer from the inner to the outer tubes. The transfer rate between the inner and the outer tubes with an interwall distance of ∼0.38 nm is 6.6 × 10(12) s(-1). Our findings provide an insight into the energy transfer mechanisms of one-dimensional excitons.Physical Chemistry Chemical Physics 11/2011; 14(3):1070-84. · 3.83 Impact Factor
- [Show abstract] [Hide abstract]
ABSTRACT: The integrity of the serotonin (5-HT) system is essential to normal respiratory and thermoregulatory control. Male and female transgenic mice lacking central 5-HT neurons (Lmx1b(f/f/p) mice) show a 50% reduction in the hypercapnic ventilatory response and insufficient heat generation when cooled (Hodges and Richerson, 2008a; Hodges et al., 2008b). Lmx1b(f/f/p) mice also show reduced body temperatures (T(body)) and O(2) consumption [Formula: see text] , and breathe less at rest and during hypoxia and hypercapnia when measured below thermoneutrality (24 °C), suggesting a role for 5-HT neurons in integrating ventilatory, thermal and metabolic control. Here, the hypothesis that Pet-1 null mice, which retain 30% of central 5-HT neurons, will demonstrate similar deficits in temperature and ventilatory control was tested. Pet-1 null mice had fewer medullary tryptophan hydroxylase-immunoreactive (TPH(+)) neurons compared to wild type (WT) mice, particularly in the midline raphé. Female (but not male) Pet-1 null mice had lower baseline ventilation (V(E)), breathing frequency (f), [Formula: see text] and T(body) relative to female WT mice (P < 0.05). In addition, V(E) and [Formula: see text] were decreased in male and female Pet-1 null mice during hypoxia and hypercapnia (P < 0.05), but only male Pet-1 null mice showed a significant deficit in the hypercapnic ventilatory response when expressed as % of control (P < 0.05). Finally, male and female Pet-1 null mice showed significant decreases in T(body) when externally cooled to 4 °C. These data demonstrate that a moderate loss of 5-HT neurons leads to a modest attenuation of mechanisms defending body temperature, and that there are gender differences in the contributions of 5-HT neurons to ventilatory and thermoregulatory control.Respiratory Physiology & Neurobiology 03/2011; 177(2):133-40. · 2.05 Impact Factor
- [Show abstract] [Hide abstract]
ABSTRACT: We investigate the exciton energy transfer (ET) in nanoassemblies (nanotube based aggregates) formed by polymer wrapped single-walled carbon nanotubes (SWNTs) using photoluminescence (PL) spectroscopy and simulation. The distinctive feature of this study is the gradual growth of such nanostructures in aqueous medium induced by increasing the concentration of porphyrin molecules stitching nanotube-polymer complexes in densely packed assemblies. Experimental dependencies of PL intensity on the porphyrin concentration for different types of semiconducting SWNTs demonstrate step-like behavior controlled by the amount of bound nanotubes and are in good agreement with the simulating model. The simulation algorithm determines the criterion of the aggregate formation depending on the number of porphyrin molecules per tube and the cascade exciton energy transfer between neighboring semiconducting nanotubes of different chiralities. Aggregates of small sizes (up to six-eight individual tubes) contain mostly semiconducting species, while aggregates of a larger size (up to several tens of tubes) incorporate metallic SWNTs, inducing strong PL quenching. From the fitting procedure, an ET rate of 0.6 × 10(10) s(-1) has been determined which is consistent with the center to center distance (∼2.3 nm) between adjacent tubes separated by polymer and porphyrin molecules. The threshold of the dimer formation corresponds to one porphyrin molecule per ∼20 nm of tube lengths that was supported by molecular dynamics simulation. These findings provide insight into the ET mechanism in SWNT nanoassemblies of variable sizes, which can be gradually controlled by the external factor (the concentration of porphyrin molecules).Physical Chemistry Chemical Physics 04/2014; · 3.83 Impact Factor