Kazuhiro Yanagi

Imaging the dynamic behaviour of individual retinal chromophores confined inside carbon nanotubes

Retinal is the molecule found in photoreceptor cells that undergoes a change in shape when it absorbs light. Specifically, the cis/trans isomerization of a carbon-carbon double bond in this chromophore sets in motion the chain of biochemical processes responsible for vision. Here, we obtain atomically resolved images of individual structural isomers of the retinal chromophore attached to C60 molecules and study their dynamic behaviour inside a confined space--that is, inside single-walled carbon nanotubes--using high-resolution transmission electron microscopy (HR-TEM). Sequential HR-TEM images with sub-second time resolution directly reveal the isomerization between the cis and all-trans forms of retinal, as well as conformational changes and volume-conserving effects. This work opens up the possibility of investigating in vitro the biological activities of these photoresponsive molecules on an individual basis, and the molecular imaging technique described here is a general one that can be applied to a wide range of systems.

Second Order Nonlinear Optical Properties of the Single Crystal of N-Benzyl 2-methyl-4-nitroaniline: Anomalous Enhancement of the d333 Component and Its Possible Origin

A single crystal of N-benzyl 2-methyl-4-nitroaniline (BNA) whose dimensions were 8×10 mm (diameter × length) was grown using a vertical Bridgman method with sufficient optical transparency for spectroscopic use. Polarized reflection spectra of the crystal were measured in order to determine its linear optical parameters (refractive indices and extinction coefficients). The relative magnitudes of the d-tensor components of the crystal were determined to be d333=(17±9)×d322, d322>d311, d311~0 from polarization dependencies of the second harmonic generation. These results are in disagreement with previously reported theoretical studies, in which the effects of intermolecular interactions were completely neglected. Here, we have included these intermolecular interactions in the calculations assuming the presence of supra-molecular clusters. We conclude that the anomalous enhancement of d333 is due to the interactions along the direction of the hydrogen bonds in the crystal and the resonance effect.

Ink-Jet Printing of a Single-Walled Carbon Nanotube Thin Film Transistor

Single-walled carbon nanotube (SWCNT) thin-film transistors (TFTs) were fabricated using ink-jet printing. We printed both thick source–drain electrodes and thin active semiconducting films using N,N-dimethylformamide (DMF)-based SWCNT dispersion. Despite the presence of metallic SWCNTs, the device exhibited field-effect behavior, with an effective mobility of 2.99 cm2 V-1 s-1 and an on/off current ratio of up to 75. The method used in this study is promising for the fabrication of large-scale high-performance SWCNT-TFTs.

Vibrational Analysis of Organic Molecules Encapsulated in Carbon Nanotubes by Tip-Enhanced Raman Spectroscopy

Tip-enhanced Raman spectroscopy revealed the nanoscale chemical properties of organic molecules encapsulated in single-wall carbon nanotubes (SWNTs). Our approach is based on an enhanced electric field near a laser-irradiated metal tip functioning as a Raman excitation source. The enhanced field can successfully act on encapsulated molecules through the walls of the SWNTs to extract molecular vibrational information. β-carotene, which exhibits several active Raman modes under visible light illumination, was used as the encapsulated molecule. Tip-enhanced Raman spectra measured at seven different positions on SWNT bundles showed that β-carotene molecules inside the tubes were not uniformly distributed. We also found that the filling rate and peak position of the radial breathing mode of the SWNTs are linearly correlated.

Non-volatile Resistance Switching using Single-Wall Carbon Nanotube Encapsulating Fullerene Molecules

A resistance switching effect was found for a nanogap junction that has electrodes composed of single-wall carbon nanotubes (SWCNTs) that encapsulate fullerenes. A clear negative differential resistance effect and repeated on–off cycles were observed in the current–voltage characteristics of the nanogap junctions. The results suggest that the resistance switch effect is due to gap size changes that result in the migration of fullerene molecules. This implied that the electrode areas of a resistance switch could be miniaturized to true nanoscale size.

Effective Separation of Carbon Nanotubes and Metal Particles from Pristine Raw Soot by Ultracentrifugation

A quick and effective method for the separation of single-wall carbon nanotubes (SWCNTs) from impurity particles has been demonstrated. High-purity SWCNTs and impurities, such as metal particles and amorphous carbons, were separated by ultracentrifugation from raw soot containing SWCNTs. The purity of each component was evaluated by scanning electron microscopy, thermogravimetric analysis, and Raman spectroscopy. The advantage of this method is that the impurity particles can be collected as residual soot in the ultracentrifugation process without any loss, while the impurity particles are more or less chemically modified or disappeared in previous purification protocols. The present technique can provide suitable samples for the research of both SWCNTs and impurity particles, particularly for nano-risk assessment.

Incident light polarization dependence of terahertz emission spectrum of crystalline 4‐N,N-dimethylamino-4′‐N′-methyl-stilbazolium tosylate

In this work we have measured the dependence of terahertz emission via optical rectification from the 4‐N,N-dimethylamino-4′‐N′-methyl-stilbazolium tosylate crystal as a function of incident light polarization. Many structures, which varied depending on the polarization of the incident pulse, were observed in the frequency spectra. The observed structures can be explained by incorporating the refractive index dispersion, the response function, and the birefringence properties of the crystal into the mathematical model used to reconstruct the experimental data. Our results show that careful attention must be paid to these three properties when enhancing nonlinear susceptibility during the fabrication of efficient terahertz emitters.

Encapsulation of Carotenoids in Single-wall Carbon Nanotubes and their Optical Properties

In order to develop next generation technologies such as all-optical switching devices, data processing using an optical-neural network, etc., it is necessary to develop materials which have the following three features; (1) large third-order optical nonlinearity, (2) ultrafast optical response, and (3) sufficient robustness for device applications. π-conjugated molecules have the potential to fulfill all the three requirement as they satisfy the first and second requirements. However, π-conjugated polyene molecules are not stable enough for use as nonlinear optical device materials. They easily degrade under ambient conditions when reacting with radical species (e.g., singlet oxygen), and also trans-to-cis isomerization occurs under illumination of light or as a result of heat treatment. Encapsulation of organic molecules inside single-wall carbon nanotubes (SWCNTs) has recently been reported. Air stable n-type SWCNT was produced by encapsulating tetrathiafulvalene (TTF) or tetramethyltetraselenafulvalene (TMTSF) inside the tubes. Takenobu et al. proposed that organic molecules inside the tubes should be protected from any external reactive species by the surrounding tube wall. Therefore we expected the degradation problem of π-conjugated molecules might be overcome by encapsulating them inside carbon nanotubes. Carotenoids are very important natural pigments in plants and animals, and they have been extensively used as model systems for the study of π-conjugated polyene molecules. Figure 1a shows the chemical structure of βcarotene. We recently succeeded to encapsulate βcarotene in SWCNT (Figure 1b shows a schematic illustration of β-carotene inside SWCNT). The physical properties and filling rate of β-carotene inside SWCNT were evaluated from the results of Raman and optical absorption measurements. (Figure 2 shows the Raman spectra of SWCNTs before and after the encapsulation procedure of β-carotene.) We found that the stability of βcarotene to the UV light is highly improved by encapsulation in SWCNT. The presence of a surrounding tube wall would protect β-carotene from the attack of radical species existing outside the wall, and isomerization would be prohibited since there would not be enough space for β-carotene to change its conformation inside the nanotube. It is reported that the encapsulation of polyacetylene will change the density of state of SWCNT. Therefore we expect that the encapsulation of polyene molecules inside SWCNTs can change physical properties of SWCNT as well as those of encapsulated molecules. Therefore the encapsulation technique presented here will contribute to various applications of SWCNTs since it can be used not only to stabilize polyene molecules but also to optimize the electronic and optical properties of SWCNTs. We found that several other carotenoids, lycopene and astaxantin, can be also encapsulated in SWCNTs with a method similar to that used in β-carotene. Thus we expect that functionalized carotenoids would also be able to enter easily into SWCNT. As with functionalized fullerene peapods, simple chemical transformation of the organic groups would enable us to introduce different chemical functionalities into the cavity of SWCNTs.

Ultrafast dynamics of light-harvesting function of β-carotene in carbon nanotube

Ultrafast dynamics of β-carotene encapsulated in single-walled carbon nanotubes (SWCNTs) was investigated by femtosecond absorption spectroscopy. Energy transfer from the excited states of β-carotene to SWCNTs (light-harvesting function) has been observed.

Coherent Spectroscopy of Carotenoid and Bacteriochlorophyll

Nonlinear Optical Responses Of A β-Carotene Homologue Having A Conjugation-Double Bond N = 15 And Bacteriochlorophyll A (Bchl A) Were Investigated In Order To Clarify The Dissipation Processes Of Excess Energy. The Experimental Results Were Explained By A Theoretical Model Calculation. We Have Determined The Spectral Density Which Is The Most Important Parameter To Calculate Optical Signals. We Found That The Linear Absorption Spectrum Can Be Fairly Well Reproduced When The Vibronic Oscillation Modes Of The Solvent Together Are Properly Taken Into Consideration. In Case Of The β;-Carotene Homologue, The Nonlinear Optical Response Can Be Well Reproduced By Properly Taking The S0, S1, And S2 Electronic Levels Into Consideration. In Case Of Bchl A, Our Computer Simulation Suggests The Involvement Of The Higher-Order Interaction, Such As The Two-Photon Process. The Role Of Coherence And The Efficient Energy Transfer In The Light-Harvesting Antenna Complexes Are Discussed.