M. P. Siegal

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.

Thermal stability of amorphous carbon films grown by pulsed laser deposition

The thermal stability in vacuum of amorphous tetrahedrally coordinated carbon (a‐tC) films grown on Si has been assessed by in situ Raman spectroscopy. Films were grown in vacuum on room‐temperature substrates using laser fluences of 12, 22, and 45 J/cm2 and in a background gas of either hydrogen or nitrogen using a laser fluence of 45 J/cm2. The films grown in vacuum at high fluence (≳20J/cm2) show little change in the a‐tC Raman spectra with temperature up to 800 °C. Above this temperature the films convert to glassy carbon (nanocrystalline graphite). Samples grown in vacuum at lower fluence or in a background gas (H2 or N2) at high fluence are not nearly as stable. For all samples, the Raman signal from the Si substrate (observed through the a‐tC film) decreases in intensity with annealing temperature indicating that the transparency of the a‐tC films is decreasing with temperature. These changes in transparency begin at much lower temperatures (∼200 °C) than the changes in the a‐tC Raman band shape an...

Electron field emission from amorphous tetrahedrally bonded carbon films

Electron field emission from two amorphous, tetrahedrally bonded diamondlike carbon films, one with (a‐tC:N), and a second without nitrogen doping (a‐tC), prepared by pulsed laser deposition has been investigated using a scanning probe apparatus with micrometer spatial resolution. Electric fields of 100 V/μm (180 V/μm) were required to initiate emission from our a‐tC:N (a‐tC) films; however, once emission was established at a particular location, electrons could be drawn at average fields as low as 10 V/μm (60 V/μm) from the same region. The initiation of emission was concomitant with electrical discharges which were observed by video techniques. These discharges left craters with micrometer dimensions on the surfaces of otherwise smooth films.

All solution-chemistry approach for YBa2Cu3O7−δ-coated conductors

A need exists for low-cost coated-conductor fabrication methods for applications in magnet and electric-power technologies. We demonstrate high-critical current density (Jc) YBa2Cu3O7−δ (YBCO) films grown on Nb-doped SrTiO3 (Nb:STO) buffered Ni(100) tapes. All buffer and superconductor layers are deposited using solution chemistry. A 50 nm thick Nb:STO seed layer on Ni(100) acts as a template for the growth of subsequent thicker layers of Nb:STO. Nb doping improves the electrical conductivity and oxygen diffusion barrier properties of STO. YBCO grows heteroepitaxially directly on this buffer layer, resulting in a transport Jc(77 K)=1.3 MA/cm2.

Amorphous-tetrahedral diamondlike carbon layered structures resulting from film growth energetics

High-resolution transmission electron microscopy (HRTEM) shows that amorphous-tetrahedral diamondlike carbon (a-tC) films grown by pulsed-laser deposition on Si(100) consist of three-to-four layers, depending on the growth energetics. We estimate the density of each layer using both HRTEM image contrast and Rutherford backscattering spectrometry. The first carbon layer and final surface layer have relatively low density. The bulk of the film between these two layers has higher density. For films grown under the most energetic conditions, there exists a superdense a-tC layer between the interface and bulk layers. The density of all four layers, and the thickness of the surface and interfacial layers, correlate well with the energetics of the depositing carbon species.

Precise control of multiwall carbon nanotube diameters using thermal chemical vapor deposition

We grow multiwall carbon nanotube (CNT) films using thermal chemical vapor deposition at atmospheric pressure using a mixture of acetylene and nitrogen from a 4-nm-thick Ni film catalyst. CNTs are characterized using electron microscopy and Rutherford backscattering spectrometry. CNTs grown with this method are extremely uniform in diameter, both throughout the sample and within the lengths of individual tubes. Nanotube outer diameters, ranging from 5–350 nm, and the total deposition of carbon material, increase exponentially with growth temperature from 630 °C–790 °C.

Electron emission induced modifications in amorphous tetrahedral diamondlike carbon

The cold-cathode electron emission properties of amorphous tetrahedral diamondlike carbon are promising for flat-panel display and vacuum microelectronics technologies. The onset of electron emission is, typically, preceded by “conditioning” where the material is stressed by an applied electric field. To simulate conditioning and assess its effect, we combined the spatially localized field and current of a scanning tunneling microscope tip with high-spatial-resolution characterization. Scanning force microscopy shows that conditioning alters surface morphology and electronic structure. Spatially resolved electron-energy-loss spectroscopy indicates that the predominant bonding configuration changes from predominantly fourfold to threefold coordination.

Thick sol-gel derived YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// films

YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// films have been prepared by spin-coating solutions of metal acetates dissolved in trifluoroacetic acid/methanol and trifluoroacetic acid/isopropanol/1,3 propanediol onto LaAlO/sub 3/[100] substrates. By using a rapid, low pO/sub 2/ pyrolysis process, high-quality diol films (0.25 /spl mu/m thick) with J/sub c/ values as high as 14 MA/cm/sup 2/ and 2 MA/cm/sup 2/ at 7 K and 77 K, respectively, were fabricated in one tenth the time, compared to conventional processing schemes. The effect of multicoating, in order to reach YBCO film thicknesses of 1.5 /spl mu/m, on the J/sub c/ was also studied.

Synthesis and properties of Tl–Ba–Ca–Cu–O superconductors

We review the synthesis methods and properties of single crystal, powder and thin film TlBaCaCuO high-temperature superconducting (Tl-HTS) materials. With transition temperatures {ge}100K for several compounds, Tl-HTS materials present real opportunities for applications above 77 K. Experiments using (1) single crystals: determined precise structural parameters and identified the complex Tl{sup 1+}{minus}Tl{sup 3+} equilibrium model; (2) powders: studied the complex thermodynamic phase diagram; and (3) epitaxial films: have studied fundamental properties such as electron pair symmetry and the effect of controlled extrinsic defects on flux pinning strength, as well as providing the large-area surfaces required for device applications. {copyright} {ital 1997 Materials Research Society.}

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.