Facile Synthetic Route to Atomically Thin Conductive Wires from Single-Species Molecules in One-Dimensionally Confined Space: Doped Conjugated Polymers inside Single-Walled Carbon Nanotubes.

A facile synthetic method for doped conjugated molecules by a heating process is demonstrated. Br-terminated terthiophene precursors are encapsulated in single-walled carbon nanotubes by a vapor-phase reaction, and additional heat treatment promotes the thermal condensation of the precursors. Transmission electron microscopy observations and optical measurements show the successful synthesis of sexithiophenes and their doping (oxidation) by Br dopants generated by the condensation reaction. This study provides a new strategy for the synthesis of the doped conjugated polymers from single-species molecules by only a heating process.

Assessment of tumor cells in a mouse model of diffuse infiltrative glioma by Raman spectroscopy.

Glioma of infiltrative nature is challenging for surgeons to achieve tumor-specific and maximal resection. Raman spectroscopy provides structural information on the targeted materials as vibrational shifts. We utilized Raman spectroscopy to distinguish invasive tumors from normal tissues. Spectra obtained from replication-competent avian sarcoma-(RCAS-) based infiltrative glioma cells and glioma tissues (resembling low-grade human glioma) were compared with those obtained from normal mouse astrocytes and normal tissues. In cell analysis, the spectra at 950-1000, 1030, 1050-1100, 1120-1130, 1120-1200, 1200-1300, 1300-1350, and 1450 cm(-1) were significantly higher in infiltrative glioma cells than in normal astrocytes. In brain tissue analysis, the spectra at 1030, 1050-1100, and 1200-1300 cm(-1) were significantly higher in infiltrative glioma tissues than in normal brain tissues. These spectra reflect the structures of proteins, lipids, and DNA content. The sensitivity and specificity to predict glioma cells by distinguishing normal cells were 98.3% and 75.0%, respectively. Principal component analysis elucidated the significance of spectral difference between tumor tissues and normal tissues. It is possible to distinguish invasive tumors from normal tissues by using Raman spectroscopy.

Development of an environmental high-voltage electron microscope for reaction science.

Environmental transmission electron microscopy and ultra-high resolution electron microscopic observation using aberration correctors have recently emerged as topics of great interest. The former method is an extension of the so-called in situ electron microscopy that has been performed since the 1970s. Current research in this area has been focusing on dynamic observation with atomic resolution under gaseous atmospheres and in liquids. Since 2007, Nagoya University has been developing a new 1-MV high voltage (scanning) transmission electron microscope that can be used to observe nanomaterials under conditions that include the presence of gases, liquids and illuminating lights, and it can be also used to perform mechanical operations to nanometre-sized areas as well as electron tomography and elemental analysis by electron energy loss spectroscopy. The new instrument has been used to image and analyse various types of samples including biological ones.

Ultrafast energy transfer of one-dimensional excitons between carbon nanotubes: a femtosecond time-resolved luminescence study.

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.

Influence of surface roughness on field emission of electrons from carbon nanotube films.

Electron field emission properties of a nanotube film are influenced not only by a field enhancement factor of carbon nanotubes (CNTs) beta(CNT) but also by that of film morphology beta(P). A simple method to separate beta(P), and beta(CNT) is proposed by using their different dependences on the cathode-anode distance. Analyses conducted for CNT emitters with rough surface exhibit that beta(P) ranges from 1 to approximately 4.5. The separated beta(CNT) values are in good agreement with the CNT geometries observed by a scanning electron microscope. Variation in beta(P)-values is ascribed to the surface roughness of the CNT films.

Empirical expression for the emission site density of nanotube film emitters.

The influence of surface roughness on emission site density (ESD) is investigated for nanotube film emitters. An empirical expression for film emitters, [Formula: see text], where B is the intercept in the modified Fowler-Nordheim (FN) plots, A(CNT) the emission area of one site, and C related to the work function, is derived from the FN theory and experimentally proved. This expression effectively excludes the influence of electric field strength, being different from the conventional method of counting site numbers from emission patterns. By using this equation and emission patterns, variation in ESD, induced by the change in cathode-anode distance, is confirmed for film emitters with rough surfaces. This variation is ascribed to the change in screen-effect strength among emission sites. It is also found that the smaller gaps produce the higher emission uniformity and site densities for rough-surface film emitters.

Relation between peak structures of loss functions of single double-walled carbon nanotubes and interband transition energies.

Electron energy-loss spectra of single double-walled carbon nanotubes (DWCNTs) were compared with calculated joint density of states (jDOSs) obtained by a simple tight-binding (STB) and an extended tight-binding (ETB) method. From the comparisons, interband transition energies of ETB calculations show better agreement with peak positions of the experimental spectra than those of STB results. From a further comparison among calculated jDOS, real and imaginary parts of a dielectric function and a loss function Im[-1/epsilon], it was confirmed that the peak energies in a spectrum of single DWCNTs are almost equal to those of the optical absorption spectrum epsilon(2).

High energy-resolution electron energy-loss spectroscopy study of the electric structure of double-walled carbon nanotubes.

Electron energy-loss spectra were obtained from two double-walled carbon nanotubes (DWCNTs) with an energy resolution of 85 meV. The spectra showed multiple peak structures between 2 and 3 eV. However, peak positions are different for these two DWCNTs. The chiral indices of CNT layers of the two DWCNTs were determined to be (29,4)(in) (17,8)(out) and (46,6)(out) (26,21)(in), respectively, by comparing experimental electron diffraction patterns with simulated ones. The spectra were also compared with simulated joint density of states, which were derived from the determined chiral indices. It was confirmed that the peak structures in the spectra are due to interband transitions intrinsic for tubular structures of graphitic sheets.

Carbon nanotube field emitter.

Recently, carbon nanotubes (CNTs), possessing excellent properties as field emitters, are attracting considerable attention as electron emitters of a cold cathode. In this review article, field emission phenomena of carbon nanotubes with various morphologies and surfaces (clean surface or adsorbed molecules on it) revealed by field emission microscopy are first described. Then, the main subject of this article, application of CNTs as electron sources in display devices is reviewed. Other electric devices utilizing CNT-field emitters are also presented.

Field emission from multiwall carbon nanotubes in controlled ambient gases, H2, CO, N2 and O2.

Adsorption and desorption on clean pentagons at a tip of multiwall carbon nanotube (MWNT) have been investigated by field emission microscopy (FEM) in an atmosphere of various gases, i.e., hydrogen, carbon monoxide, nitrogen and oxygen. A MWNT with clean surface which is obtained by heat treatment gives FEM patterns consisting of six bright pentagonal rings. Adsorbates are recognized as bright spots in the FEM pattern. They reside preferentially on the pentagonal sites where the strong electric field is concentrated, and bring about stepwise increase in the emission current. Heat treatment of the MWNT emitter at about 1300K allows adsorbates to desorb. After the desorption of hydrogen and nitrogen, the original clean surface with pentagons is recovered, while the tip structure is destroyed after the desorption of oxygen.

Energy distributions of field emitted electrons from a multi-wall carbon nanotube.

Field emission energy distributions of electrons from one of the six pentagons located at the end of a multi-wall carbon nanotube have been measured by means of a high-resolution cylindrical energy analyzer. In a clean pentagon, the sub-peak was obtained at about 500 meV below the main peak, exhibiting a shift with increasing applied voltage. For electrons emitted from an adsorbate onto the pentagon, no fine structure was observed in the spectra. The broadening of the leading edge was also observed for both clean and adsorbed pentagon, indicating the field penetration into the nanotube due to its semimetallic nature. The full-width at half-maximum was 280 meV at the applied voltage of 660 V and increased linearly with applied voltage.