R. L. Simpson

The thermal stability of diamond-like carbon

Abstract Diamond-like carbon (DLC) is a potential, low-cost substitute for diamond in certain applications, but little is known of the temperature range over which its desirable properties are retained. We have investigated the stability of DLC films at elevated temperature and high humidity using Raman spectroscopy, Auger electron spectroscopy and thermal desorption analysis. Exposure to boiling water and a hot (225 °C), humid environment does not appear to affect the DLC structure. Thermal desorption analysis detected the onset of hydrogen evolution from DLC in vacuum at 260 °C. Raman spectra show the conversion from DLC to nano-crystalline graphite (“glassy” carbon) beginning at 300 °C in ambient air. Auger spectroscopy confirms the conversion of sp 3 -bonded carbon to sp 2 -bonded carbon above 300 °C. Conversion to nano-crystalline graphite is complete by 450–600 °C in air. The structure and properties of DLC films are expected to be retained up to temperatures of at least 260 °C.

Luminescent properties of solution-grown ZnO nanorods

The optical properties of solution-grown ZnO nanorods were investigated using photoluminescence and cathodoluminescence. The as-grown nanorods displayed a broad yellow-orange sub-band-gap luminescence and a small near-band-gap emission peak. The sub-band-gap luminescence can only be observed when exciting above band gap. Scanning cathodoluminescence experiments showed that the width of the sub-band-gap luminescence is not due to an ensemble effect. Upon reduction, the sub-band-gap luminescence disappeared and the near-band-gap emission increased. Compared to ZnO powders that are stoichiometric and oxygen deficient, we conclude that the yellow-orange sub-band-gap luminescence most likely arises from bulk defects that are associated with excess oxygen.

Energy transfer and relaxation in europium-activated Y2O3 after excitation by ultraviolet photons

We present persistence data from the 5D manifolds of europium (3+)-activated Y2O3, after excitation by ultraviolet photons. For europium concentrations below 1 at. %, the persistence is largely consistent with multiphonon relaxation. For europium concentrations at and above 1 at. %, the persistence shows evidence for energy transfer interactions between europium activators. Interactions involving one activator in an upper (5D3, 5D2 or 5D1) manifold and another in a ground state (7F) manifold affect the kinetics of the relaxation of the upper 5D manifolds but do not degrade phosphor efficiency. Interactions involving two activators in excited manifolds ultimately dissipate, by phonon emission, excitation that might be emitted as photons. The interactions involving two activators in excited manifolds appear to be related to both concentration quenching and the reduction of phosphor efficiency at high excitation density.

Ultrahard carbon nanocomposite films

Modest thermal annealing to 600 C of diamondlike amorphous-carbon (a-C) films grown at room temperature results in the formation of carbon nanocomposites with hardness similar to diamond. These nanocomposite films consist of nanometer-sized regions of high density a-C embedded in an a-C matrix with a reduced density of 5--10%. The authors report on the evolution of density and bonding topologies as a function of annealing temperature. Despite a decrease in density, film hardness actually increases {approximately} 15% due to the development of the nanocomposite structure.

Preparation of diamondlike carbon films by high‐intensity pulsed‐ion‐beam deposition

Diamondlike carbon (DLC) films were prepared by high‐intensity pulsed‐ion‐beam ablation of graphite targets. A 350 keV, 35 kA, 400 ns beam, consisting primarily of hydrogen, carbon, and oxygen ions was focused onto a graphite target at a fluence of 15–45 J/cm2. Amorphous carbon films were deposited at up to 30 nm per pulse, corresponding to an instantaneous deposition rate greater than 1 mm/s. Electrical resistivities were between 1 and 1000 Ω cm. Raman spectra indicate that diamondlike carbon is present in most of the films. Electron‐energy‐loss spectroscopy indicates significant amounts of sp3‐bonded carbon, consistent with the presence of DLC. Scanning electron microscopy showed most films contain 100 nm features, but micron size particles were deposited as well. Initial tests revealed favorable electron field‐emission behavior.

Porous silicon photoluminescence: Implications from in situ studies

Photoluminescence and Raman measurements have been performed on an anodized silicon surface in an HF/ethanol anodization solution and after replacement of this solution with water. Immediately after anodization and while resident in HF/ethanol, the porous silicon produced does not exhibit intense photoluminescence. Intense photoluminescence develops spontaneously in HF/ethanol after 18–24 h or with replacement of the HF/ethanol with water. The results are consistent with a quantum confinement mechanism in which electron‐hole pair migration to traps and nonradiative recombination dominates the de‐excitation pathways until silicon nanocrystals are physically separated and energetically decoupled by hydrofluoric acid etching or surface oxidation. Raman spectra show the development of nanometer‐size silicon crystals concurrent with intense photoluminescence. Illumination of the silicon surface during anodization tends to inhibit the formation of nanocrystalline silicon, but even very thin layers (tens of nano...

Residual Stress and Raman Spectra of Laser Deposited Highly Tetrahedral-Coordinated Amorphous Carbon Films

We are studying carbon thin films by using a pulsed excimer laser to ablate pyrolytic graphite targets to form highly tetrahedral coordinated amorphous carbon (at-C) films. These films have been grown on room temperature p-type Si (100) substrates without the intentional incorporation of hydrogen. In order to understand and optimize the growth of at-C films, parametric studies of the growth parameters have been performed. We have also introduced various background gases (H 2 , N 2 and A r ) and varied the background gas pressure during deposition. The residual compressive stress levels in the films have been measured and correlated to changes in the Raman spectra of the at-C band near 1565 cm −1 . The residual compressive stress falls with gas pressure, indicating a decreasing atomic sp 3 -bonded carbon fraction. We find that reactive gases such as hydrogen and nitrogen significantly alter the Raman spectra at higher pressures. These effects are due to a combination of chemical incorporation of nitrogen and hydrogen into the film as well as collisional cooling of the ablation plume. In contrast, films grown in non-reactive Ar background gases show much less dramatic changes in the Raman spectra at similar pressures.

Mechanisms Affecting Emission in Rare-Earth-Activated Phosphors

The relatively poor efficiency of phosphor materials in cathodoluminescence with low accelerating voltages is a major concern in the design of field emission flat panel displays operated below 5 kV. The authors research on rare-earth-activated phosphors indicates that mechanisms involving interactions of excited activators have a significant impact on phosphor efficiency. Persistence measurements in photoluminescence (PL) and cathodoluminescence (CL) show significant deviations from the sequential relaxation model. This model assumes that higher excited manifolds in an activator de-excite primarily by phonon-mediated sequential relaxation to lower energy manifolds in the same activator ion. In addition to sequential relaxation, there appears to be strong coupling between activators, which results in energy transfer interactions. Some of these interactions negatively impact phosphor efficiency by nonradiatively de-exciting activators. Increasing activator concentration enhances these interactions. The net effect is a significant degradation in phosphor efficiency at useful activator concentrations, which is exaggerated when low-energy electron beams are used to excite the emission.

Characterization of Carbon Nitride Films Produced by Pulsed Laser Deposition

Carbon Nitride (CN{sub x}) films have been grown by ion-assisted pulsed-laser deposition (IAPLD). Graphite targets were laser ablated while bombarding the substrate with ions from a broad-beam Kaufman-type ion source. Ion voltage, current density, substrate temperature, and feed gas composition (N{sub 2} in Ar) were varied. Resultant films were characterized by Raman. Fourier transform infrared (FTIR), and Rutherford back scattering (RBS) spectroscopy. Samples with {approximately} 30% N/C ratio have been fabricated. The corresponding Raman and FTIR spectra indicate that nitrogen is incorporated into the samples by insertion into sp{sup 2}-bonded structures. A low level of C{identical_to}N triple bonds is also found. As the ion current and voltage are increased with a pure Ar ion beam, Raman peaks associated with nanocrystalline graphite appear in the spectra. Adding low levels of nitrogen to the ion beam first reduces the Raman intensity in the vicinity of the graphite disorder peak without adding detectable amounts of nitrogen to the films (as measured by RBS). At higher nitrogen levels in the ion beam, significant amounts of nitrogen are incorporated into the samples, and the magnitude of the ``disorder`` peak increases. By increasing the temperature of the substrate during deposition, the broad peak due mainly to sp{sup 2}-bonded C-N in the FTIR spectra is shifted to lower wavenumber. This could be interpreted as evidence of single-bonded C-N; however, it is more likely that the character of the sp{sup 2} bonding is changing.

Raman study of lead zirconate titanate under uniaxial stress

The authors used micro-Raman spectroscopy to monitor the ferroelectric (FE) to antiferroelectric (AFE) phase transition in PZT ceramic bars during the application of uniaxial stress. They designed and constructed a simple loading device, which can apply sufficient uniaxial force to transform reasonably large ceramic bars while being small enough to fit on the mechanical stage of the microscope used for Raman analysis. Raman spectra of individual grains in ceramic PZT bars were obtained as the stress on the bar was increased in increments. At the same time gauges attached to the PZT bar recorded axial and lateral strains induced by the applied stress. The Raman spectra were used to calculate an FE coordinate, which is related to the fraction of FE phase present. The authors present data showing changes in the FE coordinates of individual PZT grains and correlate these changes to stress-strain data, which plot the macroscopic evolution of the FE-to-AFE transformation. Their data indicates that the FE-to-AFE transformation does not occur simultaneously for all PZT grains but that grains react individually to local conditions.