sp3 content of mass-selected ion-beam-deposited carbon films determined by inelastic and elastic electron scattering

Background optimization of powder electron diffraction for implementation of the e-PDF technique and study of the local structure of iron oxide nanocrystals

The local structural characterization of iron oxide nanoparticles is explored using a total scattering analysis method known as pair distribution function (PDF) (also known as reduced density function) analysis. The PDF profiles are derived from background-corrected powder electron diffraction patterns (the e-PDF technique). Due to the strong Coulombic interaction between the electron beam and the sample, electron diffraction generally leads to multiple scattering, causing redistribution of intensities towards higher scattering angles and an increased background in the diffraction profile. In addition to this, the electron-specimen interaction gives rise to an undesirable inelastic scattering signal that contributes primarily to the background. The present work demonstrates the efficacy of a pre-treatment of the underlying complex background function, which is a combination of both incoherent multiple and inelastic scatterings that cannot be identical for different electron beam energies. Therefore, two different background subtraction approaches are proposed for the electron diffraction patterns acquired at 80 kV and 300 kV beam energies. From the least-square refinement (small-box modelling), both approaches are found to be very promising, leading to a successful implementation of the e-PDF technique to study the local structure of the considered nanomaterial.

Structure and Characterization of Vacuum Arc Deposited Carbon Films—A Critical Overview

This critical overview analyzes the relations between deposition conditions and structure for hydrogen-free carbon films, prepared by vacuum arc deposition. The manifold of film structures can be roughly divided into graphitic, nanostructured and amorphous films. Their detailed characterization uses advantageously sp3 fraction, density, Raman peak ratio and the mechanical properties (Young’s modulus and hardness). Vacuum arc deposition is based on energetic beams of carbon ions, where the film growth is mainly determined by ion energy and surface temperature. Both parameters can be clearly defined in the case of energy-selected carbon ion deposition, which thus represents a suitable reference method. In the case of vacuum arc deposition, the relation of the external controllable parameters (especially bias voltage and bulk temperature) with the internal growth conditions is more complex, e.g., due to the broad energy distribution, due to the varying “natural” ion energy and due to the surface heating by the ion bombardment. Nevertheless, some general trends of the structural development can be extracted. They are critically discussed and summarized in a hypothetical structural phase diagram in the energy-temperature plane.

Surface Structure of Aerobically Oxidized Diamond Nanocrystals

We investigate the aerobic oxidation of high-pressure, high-temperature nanodiamonds (5-50 nm dimensions) using a combination of carbon and oxygen K-edge X-ray absorption, wavelength-dependent X-ray photoelectron, and vibrational spectroscopies. Oxidation at 575 °C for 2 h eliminates graphitic carbon contamination (>98%) and produces nanocrystals with hydroxyl functionalized surfaces as well as a minor component (<5%) of carboxylic anhydrides. The low graphitic carbon content and the high crystallinity of HPHT are evident from Raman spectra acquired using visible wavelength excitation (λ_excit = 633 nm) as well as carbon K-edge X-ray absorption spectra where the signature of a core-hole exciton is observed. Both spectroscopic features are similar to those of chemical vapor deposited (CVD) diamond but differ significantly from the spectra of detonation nanodiamond. The importance of these findings to the functionalization of nanodiamond surfaces for biological labeling applications is discussed.

Amorphous Diamond Films Deposited by Pulsed-Laser Ablation: the Optimum Carbon-Ion Kinetic Energy and Effects of Laser Wavelength

A systematic study has been made of changes in the bonding and optical properties of hydrogen-free tetrahedral amorphous carbon (ta-C) films, as a function of the kinetic energy of the incident carbon ions measured under film-deposition conditions. Ion probe measurements of the carbon ion kinetic energies produced by ArF and KrF laser ablation of graphite are compared under identical beam-focusing conditions. Much higher C+ kinetic energies are produced by ArF-laser ablation than by KrF for any given fluence and spot size. Electron energy loss spectroscopy and scanning ellipsometry measurements of the sp3 bonding fraction, plasmon energy, and optical properties reveal a well-defined optimum kinetic energy of 90 eV to deposit ta-C films having the largest sp3 fraction and the widest optical (Tauc) energy gap (equivalent to minimum near-gap optical absorption). Tapping-mode atomic force microscope measurements show that films deposited at near-optimum kinetic energy are extremely smooth, with rms roughness of only ~ 1 Å over distances of several hundred nm, and are relatively free of particulates.

Oriented Graphitic Carbon Film Grown by Mass-Selected Ion Beam Deposition at Elevated Temperatures

Mass-selected ion-beam deposition using 120 eV C+ ions has been used to grow a carbon film on a Si substrate held at 200° C. The structure of the film has been characterized by transmission electron microscopy and electron energy loss spectroscopy. The film is graphitic and highly oriented with the c-axis lying parallel to the substrate. Moreover, the film is under significant biaxial stress such that the graphitic layer spacing is reduced by 4% from that of ambient pressure graphite. This oriented structure evolves due to the mobility of the carbon atoms at 200 °C. The material is sufficiently crystalline on the nanometer scale so as to produce Bragg diffraction discs in a convergent beam electron diffraction pattern using a 2.5 nm probe.

Control of sp2/sp3 Carbon Ratio and Surface Chemistry of Nanodiamond Powders by Selective Oxidation in Air

The presence of large amounts of nondiamond carbon in detonation-synthesized nanodiamond (ND) severely limits applications of this exciting nanomaterial. We report on a simple and environmentally friendly route involving oxidation in air to selectively remove sp(2)-bonded carbon from ND. Thermogravimetric analysis and in situ Raman spectroscopy shows that sp(2) and sp(3) carbon species oxidize with different rates at 375-450 degrees C and reveals a narrow temperature range of 400-430 degrees C in which the oxidation of sp(2)-bonded carbon occurs with no or minimal loss of diamond. X-ray absorption near-edge structure spectroscopy detects an increase of up to 2 orders of magnitude in the sp(3)/sp(2) ratio after oxidation. The content of up to 96% of sp(3)-bonded carbon in the oxidized samples is comparable to that found in microcrystalline diamond and is unprecedented for ND powders. Transmission electron microscopy and Fourier transform infrared spectroscopy studies show high purity 5-nm ND particles covered by oxygen-containing surface functional groups. The surface functionalization can be controlled by subsequent treatments (e.g., hydrogenization). In contrast to current purification techniques, the air oxidation process does not require the use of toxic or aggressive chemicals, catalysts, or inhibitors and opens avenues for numerous new applications of nanodiamond.