Z.N. Gu

Temperature dependence of the Raman spectra of single-wall carbon nanotubes

Raman spectra of single-wall carbon nanotubes (SWCNTs) were measured at different temperatures by varying the incident laser power. The elevated temperature of the SWCNTs and multiwall carbon nanotubes (MWCNTs) is confirmed to be due to the presence of impurities, defects, and disorder. The temperature coefficient of the frequency of the C–C stretching mode E2g (GM) and that of the radial breathing mode in the SWCNT were determined to be ∼−0.038 and ∼−0.013 cm−1/K, respectively. It is found that the temperature coefficient of the GM in the SWCNT is larger than that of the MWCNT, highly oriented pyrolytic graphite, and the graphite. This is attributed to the structural characteristic of the SWCNT—a single tubular carbon sheet with smaller diameter.

Structure modification of single-wall carbon nanotubes

Various purification processes were applied to single-wall carbon nanotubes synthesized by metal catalyzed laser ablation. Structure modifications introduced by these processes were investigated by high-resolution transmission electron microscopy and Raman spectroscopy. An apparent structure modification after purification was the increase of bundle size although breaking of nanotubes and a change of nanotube diameter distribution were also observed. More vigorous attacking of single-wall carbon nanotube structure was identified by a strong mixed-acid treatment.

Defects in arc-discharge-produced single-walled carbon nanotubes

High-yield single-wall carbon nanotubes (SWCNTs) have been produced in large quantity using a Y-Ni alloy composite graphite rod as anode in a dc arc discharge. Termination of the tubes was found to be the most common defect in the arc-discharge-produced SWCNTs. A comparison is made with samples produced by laser ablation. High-resolution transmission electron microscopy was used to investigate the cap structure of the terminated SWCNTs and their relatives. The cap shapes of most SWCNTs with high aspect ratios are hemispherical, while those of highly defective squat SWCNTs are polyhedral. Inclusion of carbon materials in the SWCNTs was also observed. The results provided the basic information for understanding the growth mechanism of SWCNTs.

RECTIFICATION PROPERTIES AND INTERFACE STATES OF HETEROJUNCTIONS BETWEEN SOLID C60 AND N-TYPE GAAS

Solid C60/n‐GaAs heterojunctions have been fabricated by deposition of solid C60 film on the (100)‐oriented epitaxial n‐type GaAs substrates and their electrical characteristics have been measured. The rectification ratio is greater than 106 at a bias of ±1 V. The current for a fixed forward bias is an exponential function of reciprocal temperature, from which the effective potential barrier height of the heterojunction is determined to be 0.58 eV. A trap with an energy level 0.35 eV below the conduction band of GaAs at the C60/GaAs interface has been observed by the deep level transient spectroscopy technique.

HETEROJUNCTIONS OF SOLID C70 AND CRYSTALLINE SILICON : RECTIFYING PROPERTIES AND BARRIER HEIGHTS

Heterojunctions of solid C70 and n‐ or p‐type crystalline Si have been made. Current–voltage measurements show that both C70/n‐Si and C70/p‐Si contacts are rectifying but their directions of rectification are opposite to each other. Thermal activation measurements at a fixed forward bias show an exponential dependence of current on the reciprocal of temperature, from which we determine the effective barrier height as 0.23 eV for C70/n‐Si and 0.27 eV for C70/p‐Si. Relative dielectric constant of solid C70 was determined to be 4.96 through the study of high‐frequency capacitance–voltage characteristics for Ti/C70/p‐Si structures.

Growth and structure of C60 thin films on NaCl, glass and mica substrates

Abstract We studied the growth and structure of C60 thin film condensed on NaCl, glass and mica substrates by transmission electron microscopy. Highly ordered, (111) textured and epitaxial thin films were obtained on (001) NaCl and mica respectively. Various deposition parameters including different substrate temperatures, deposition rates and film thicknesses were experimented with. The orientational order and nature of defects present in the films were assessed by electron diffraction, bright and dark field images. We ascribe the abnormal reflections in the electron diffraction patterns which had been thought by some workers to belong to the diffraction of an h.c.p. structure phase to the existence of stacking disorder in the f.c.c. structure.

Controlled synthesis and phase transformation of ferrous nanowires inside carbon nanotubes

Abstract Carbon nanostructures filled with Fe were synthesized. It was found that both the shape and the phase of the carbon nanocapsulate may be controlled during the growth, and controlled ferrous amorphous–crystalline phase transformation may be realized via electron beam irradiation. Since an electron beam may be focused into a nanoscale probe, this method may in principle be used for fabricating amorphous–crystalline heterostructures inside carbon nanotubes.

Heterojunctions of solid C60 and crystalline silicon: rectifying properties and energy-band models

Heterojunctions of undoped solid C60 and n- or p-type-doped crystalline Si have been obtained. Current-voltage measurements show that both C60/n-Si and C60/p-Si contacts are rectifying but their directions of rectification are opposite. Thermal activation measurements at a fixed forward bias show an exponential dependence of current on the reciprocal of temperature, from which we determine the effective barrier height as 0.30 eV for C60/n-Si and 0.48 eV for C60/p-Si. Using energy-band models for heterojunctions we assign values to the positions of the conduction and valence bands of the solid C60 relative to those of crystalline Si and derive the electron affinity and band gap of solid C60 film as 3.92 eV and <1.72 eV, respectively.

Study of microstructure of epitaxial fullerenes films

Abstract A comprehensive study was made on the structure of epitaxial thin films of C 60 and C 70 by means of transmission electron microscopy. Both the films show similar face-centered cubic structure and are epitaxial on (001) mica with close-packed plane parallel to the substrate surface. Two main kinds of defects-stacking faults and twins-were observed and are discussed. The effect of the remaining C 70 impurity on the crystal orientation of C 60 films was studied by comparing different samples made from high-purity fullerene and C 60 /C 70 mixtures. The results show that there is a higher density of planar defects in the films containing larger amounts of impurities: moreover, some faint anomalous reflections located at so-called 2a 0 fcc reciprocal lattice points were also detected, probably as a result of C 70 contamination. Finally, it is found that stacking disorders can be easily increased by keeping the high-quality pure C 60 film in air at room temperature for a few weeks, implying the instability of the crystal orientation of the epitaxial fullerene films.

Surface enhanced electronic transport. A new method to probe the possible interactions between C60 and non-alkali metals

Abstract By measuring the resistance in situ during the deposition of C 60 on ultrathin metal layers in a UHV system, we find the resistance decreases sharply in most cases. This enhanced electronic transport in metal—C 60 multilayers (bilayers), may be related to charge transfer from the metal to C 60 and surface bonding effects, and hence can be used as a new possible approach for probing the interfacial interactions between C 60 and metals.