Xiwei Lin

Analytical electron microscopy and Raman spectroscopy studies of carbon nitride thin films

Deposition of carbon nitride thin films on Si(100) and polycrystalline Zr substrates was performed by dc magnetron sputtering of a graphite target under a pure nitrogen ambient. The resulting carbon nitride films (CNx) are primarily amorphous with a small volume fraction of nanocrystallites. Both energy‐dispersive x‐ray analysis and electron energy loss spectroscopy measurements showed that the amorphous matrix has uniform nitrogen‐to‐carbon ratios ∼0.4–0.8 depending on deposition conditions. Carbon and nitrogen K edge structures obtained from electron energy loss spectroscopy studies suggest that the amorphous carbon nitride matrix is primarily sp2 bonded. Apart from the carbon–nitrogen stretching vibration, Raman spectra of CNx films closely resemble those of diamondlike carbon films. Intensity and peak width changes of Raman features in the 1300–1600 cm−1 range due to inorganic carbon (D and G peak) under different deposition conditions are explained in terms of the extent of structural disorder in the...

Buckytubes and Derivatives: Their Growth and Implications for Buckyball Formation

Transmission electron microscopy (TEM) observations of graphite tubules (buckytubes) and their derivatives have revealed not only the previously reported buckytube geometries but also additional shapes of the buckytube derivatives. Detailed cross-sectional TEM images reveal the cylindrical cross section of buckytubes and the growth pattern of buckytubes as well as their derivatives. These observations of frozen growth stages of buckytubes and derivatives suggest a helical growth mechanism analogous to that of crystal growth via screw dislocations. The helicacy of buckytubes is analyzed by electron diffraction whereas the anisotropy of electronic structure is revealed by momentum transfer resolved electron energy loss spectrometry. Based on the TEM observations, it is proposed that buckytubes act as precursors to closed-shell fullerene (buckyball) formation and the possible steps in buckyball formation are outlined. In arc evaporation experiments in which residue rods (containing various amounts of buckytubes) were used as the starting anode for fullerene production, the amount of buckytubes in the rod was correlated with fullerene yield.

Carbon nanotubes synthesized in a hydrogen arc discharge

We have developed a novel method to synthesize ‘‘clean’’ carbon nanotubes with relatively higher yield using a hydrogen arc discharge. The quality and yield of the tubes depend sensitively on the gas pressure in the arc discharge. We have observed sharp, open‐ended nanotubes with clear lattice fringes at the edges and empty interiors. The existence of these frozen‐open‐ended tubes in the buckybundles provides evidence for an open‐ended growth model for nanotubes.

Growth and characterization of C-N thin films

Abstract Our preliminary studies showed that carbon nitride in amorphous and crystalline forms can be synthesized using reactive magnetron sputtering with substrates held at ambient temperatures. The films were evaluated by a wide range of diagnostic techniques (Auger spectroscopy, IR spectroscopy, scanning electron microscopy, energy-dispersive X-ray analysis, atomic force microscopy, transmission electron microscopy and tribo-testing). The most important finding is that we can deposit smooth, continuous C-N films on a variety of substrates. Unlike previous work, we were able to incorporate significant amounts of nitrogen into these films (N:C ratio exceeding 0.4 in the bulk). More importantly, IR studies show that nitrogen exists not merely as gaseous species but is chemically bonded to carbon in the film. Transmission electron microscopy studies showed clearly the coexistence of amorphous and crystalline C-N phases. When deposited on polycrystalline zirconium substrates, these films appear to be very adherent and wear resistant under lubricated sliding conditions. In dry sliding against 52100 steels, carbon nitride gives friction coefficients as low as 0.16, comparable with that of diamond.

Growth and characterization of buckybundles

Buckybundles (bundles of buckytubes), approximately 50 μm in diameter and 1 cm in length, have been observed in a deposited carbon rod on the cathode electrode of a dc arc. Scanning electron microscopy images clearly show an evenly spaced array of parallel buckybundles. High resolution electron microscopy reveals that each buckybundle consists of closely packed buckytubes with their axes parallel to the bundle axis. Within each bundle it is estimated that there are about 107 buckytubes with diameters in the range (20–300 A). We have measured the deposited rod growth rate as a function of the He gas pressure and have evaluated the influence of the graphite feed rod diameter on the yield of buckybundles. The magnetic susceptibilities parallel and perpendicular to the bundle axes were measured. The results show that the bundles have anisotropic diamagnetic properties.

Synthesis and tribological properties of carbon nitride as a novel superhard coating and solid lubricant

This paper reports the discovery of a new crystalline thin-film material suitable for tribological applications. Carbon nitride thin films were synthesized using DC magnetron sputtering of a high-purity graphite target in an argon/nitrogen plasma using various substrates held at ambient temperatures. These thin films consist of mixed amorphous and crystalline phases. Unlike previous work, a significant amount of nitrogen, the NIC ratio exceeding 0.4 in the bulk, was incorporated. Infra-red studies showed that nitrogen was bonded to carbon in the film. These films appeared to be very wear-resistant under lubricated sliding conditions. In dry sliding, carbon nitride gave friction coefficients as low as 0.16, comparable to diamond. Presented as a Society of Tribologists and Lubrication Engineers paper at the ASME/STLE Tribology Conference In San Diego, California, October 19–21, 1992

Large scale synthesis of single‐shell carbon nanotubes

We report on a novel technique for synthesizing large quantities of single‐shell carbon nanotubes in a dc arc with a composite anode of graphite and copper. Based on systematic studies involving a varying diameter and distribution of copper rods inside the graphite anode, we have correlated the presence of copper to the formation of monolayer buckytubes. We speculate that copper atoms in the discharge disrupt the orderly formation of multilayer buckytubes observed in earlier experiments.

The effect of hydrogen on the formation of carbon nanotubes and fullerenes

A novel method to synthesize “clean” carbon nanotubes with relatively high yield in a hydrogen arc discharge has been developed. The quality and yield of the tubes depend sensitively on the gas pressure in the arc discharge. Sharp, open-ended nanotubes with clear lattice fringes at the edges and empty interiors have been observed. The existence of these frozen-open-ended tubes as part of nanotube-bundles provides evidence for an open-ended growth model for nanotubes. Using time of flight mass spectrometry, it was found that fullerenes, such as C 60 and C 70 , are almost absent from the soot collected in the hydrogen arc discharge. The effect of hydrogen on the formation of fullerenes, both in the laboratory and in space, will be discussed.

Properties of buckytubes and derivatives

Abstract The structural, magnetic, and transport properties of bundles of buckytubes (buckybundles) have been studied. High-resolution electron microscopy (HREM) images have revealed the detailed structural properties and the growth pattern of buckytubes and their derivatives. The magnetic susceptibility of a bulk sample of buckybundles is −10.75 × 10 −6 emu/g for the magnetic field parallel to the bundle axes, which is approximately 1.1 times the perpendicular value and 30 times larger than that of C 60 . The magnetoresistance (MR) and Hall coefficient measurements on the buckybundles show a negative MR at low temperature and a positive MR at a temperature above 60 K and a nearly linear increase in conductivity with temperature. The results show that a buckybundle may best be described as a semimetal. Using a stable glow discharge, buckybundles with remarkably large diameters (up to 200 μm) have been synthesized. These bundles are evenly spaced, parallel, and occupy the entire central region of a deposited rod. HREM images revealed higher yield and improved quality buckytubes produced by this technique compared to those produced by a conventional arc discharge.

Stable glow discharge for synthesis of carbon nanotubes

The use of a stable glow discharge for the synthesis of carbon nanotubes is reported. Bundles of buckytubes are synthesized with remarkably large diameters (up to 200 μm). The bundles are evenly spaced, parallel, and occupy the entire central region of the deposited rod. High resolution electron microscopy (HREM) images of the deposited rod produced by the glow discharge revealed higher yield and improved quality buckytubes as compared to those produced by an arc discharge. The behavior of the two deposition modes (glow and arc) has been compared and their effects on the formation of buckytubes are discussed.