Evaporated Carbon Films Research Articles

The surface of evaporated carbon films is an insulating, high-bandgap material

The electrical conductance and the optical density of evaporated carbon films are measured as a function of the thickness of the films. The resulting data show that up to a thickness of approximately 4 nm, carbon films are optically transparent and electrically insulating. The same data also suggest that this insulating character persists near to the surface when the overall thickness is further increased. Since a support film with poor surface conductivity is undesirable for many applications in electron microscopy, we suggest that the usefulness of evaporated carbon films in electron microscopy might be further improved by doping or by other methods that improve the electrical conductivity near the surface.

Effects of bonding on the energy distribution of electrons scattered elastically at high momentum transfer

High-resolution measurements of 40-keV electrons scattered over $44.3\ifmmode^\circ\else\textdegree\fi{}$ from evaporated carbon films are presented. The observed width of the energy distribution of electrons scattered from carbon is significantly larger than the experimental energy resolution, and its position is shifted to lower energy. Measurements were done for transmission and reflection geometries for thin films with thicknesses varying from $90\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ to $1400\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. The observed peak shape is largely independent of the thickness and measurement geometry. The peak shape deviates from Gaussian in all cases, in a way consistent with theories that describe these processes beyond the impulse approximation. The energy shift of the carbon peak is measured by evaporating a small amount of Au on these films. Separation of the Au and C peak is somewhat smaller than calculated assuming scattering from free C and Au atoms, but independent of measurement geometry. Finally spectra were measured from highly oriented pyrolytic graphite (HOPG) films. Now different widths are observed in reflection geometry and transmission geometry. This is attributed to the anisotropy of the motion of the C atoms in HOPG. Also the $\mathrm{Au}\text{\ensuremath{-}}\mathrm{C}$ separation is slightly orientation dependent for HOPG. All observations agree at least semiquantitatively with neutron Compton scattering results, a related scattering experiment that studies neutron-atom collisions at similar momentum transfers.

The characterisation of e-beam evaporated and magnetron sputtered carbon films fabricated for atomic oxygen sensors

Carbon coatings, for atomic oxygen sensor applications, have been deposited onto alumina substrates and subjected to atomic oxygen and various heat treatments. The effect of these treatments has been studied. Carbon was fabricated using two methods: electron-beam (e-beam) evaporation and dc-magnetron sputtering. The carbon films were exposed to 5 eV atomic oxygen (AO) fluences ranging from 0.5×10 19 to 7.9×10 20 atoms cm −2 and their resultant characteristics investigated. E-beam evaporated carbon films were found to be graphitic in content and conducting. The sputtered carbon films were insulating and their resistivity remained high even after high-temperature annealing. From Raman spectroscopy it was possible to determine the I D/ I G ratio in the carbon from the intensity of the characteristic D and G peaks. After AO exposure, Raman spectroscopy shows the appearance of a characteristic D peak in the e-beam evaporated carbon films, implying the formation of sp 3 clusters. For the magnetron sputtered films, the D peak broadened, the broadening being proportional to ordering in the films. A plot of the G peak position with temperature shows the trend in graphitisation as temperature is increased. From XPS analysis, there was an increase in the surface concentration of the O1s peak at the expense of the C1s peak, signifying that the carbon reacts with oxygen atoms. The reaction of carbon to oxygen is not a physical process, but involves the removal of carbon by a chemical process of CO or CO 2, which is lost to the surrounding environment.

Characterization of electron beam evaporated carbon films and compound formation on titanium and silicon

The formation of carbon-based mixed materials is unavoidable on the plasma-facing components (e.g. first wall and divertor) of fusion devices when carbon is used together with other materials. On the surfaces of these components very different conditions with respect to particle and energy impact occur. To predict the mixed material formation under these conditions the precise knowledge of the fundamental mechanisms governing these interactions is essential. In this paper we present the results of carbon interaction with titanium and silicon, as model substances for metallic and covalent carbides, during thermal treatment. To perform basic studies of the reactions of carbon with different elements, thin carbon films are produced by electron beam evaporation on the different substrates under UHV conditions. All measurements for chemical analysis are performed using x-ray photoelectron spectroscopy (XPS). We discuss first the properties of the deposited carbon films. The carbon films are characterized on inert gold surfaces and are compared to bulk graphite. Annealing of the carbon films up to 970 K leads to a transition from a disordered carbon network into a graphitic structure. Preparation of carbon films at room temperature on titanium or silicon leads to a limited carbide formation at the carbon/substrate interface. Carbon deposited in excess of several monolayers is present in elementary form. Annealing of the samples leads to complete carbidization consuming the available carbon in both cases. Titanium reacts to TiC and additional substoichiometric carbide, silicon forms SiC with exact stoichiometry.

Sprayed and Thermally Evaporated Carbon Films as Anodes for Lithium Ion Batteries

Sprayed and Thermally Evaporated Carbon Films as Anodes for Lithium Ion Batteries

The Accurate Control of the Thickness of Evaporated Carbon Films

Abstract There are two approaches to determining the thickness of a carbon film evaporated onto a specimen. One approach is to do the evaporation first and to measure the result afterwards. There are several ways to do this. The second approach is to put down a determined amount of carbon in the first place. This clearly has advantages in many cases. The method described here is of this second kind. The thickness of the carbon coating is not measured, but by a predetermined recipe, the amount put on is controlled at the time of evaporation, In principle, the thickness could be measured during the evaporation with a quartz-crystal thickness monitor. However, we find that the monitor we have does not work well with carbon.

Carbon nanotubes and other graphitic structures as contaminants on evaporated carbon films

Evaporated carbon support films for transmission electron microscopy (TEM) often contain fullerene‐like structures such as nanotubes and nanoparticles. This could cause serious confusion in TEM studies of fullerene‐related carbons. Other, poorly graphitized, particles are also frequently seen.

Surface and bulk properties of electron beam evaporated carbon films

Amorphous carbon films deposited by electron beam evaporation onto glass and oxidized Si(111) substrates have been investigated by local probe methods, spectroscopic techniques and electrical measurements. Clusters ∼ 15 Å in diameter are visible in the topography imaged by scanning tunneling microscopy. The microscopic potential distribution mapped by scanning tunneling polcntiomciry is characterized by a disturbed lateral potential drop in the submicrometcr scale. X-ray diffraction analysis reveals the amorphous structure of the evaporated film. In the Raman spectrum two broad lines at 1585 cm −1 and at 1375 cm −1 are observed. We obtained an optical absorption coefficient α greater than 10 5 cm −1, and a small optical gap of about 0.2 eV. The electrical resistance at low temperatures can be fitted to the form R α exp( T 0/ T) ½. Our experimental results indicate a structure consisting of very small graphite (sp 2) clusters with an incomplete trigonally atomic arrangement. The changed bonding configuration between the clusters may include sp 3 sites.

Diamond growth on hard carbon films

A three-step process for enhancing the nucleation density of diamond on unscratched silicon substrates using microwave-plasma-enhanced chemical vapor deposition is described. Hard diamond-like carbon (dlc) films with thickness in the range 5–100 nm were first formed on smooth (100) silicon surfaces using a vacuum-arc plasma deposition technique at room temperature. The dlc-coated silicon substrates were subsequently subjected to a low-temperature pretreatment with a methane-rich plasma for an hour before switching to the diamond nucleation conditions. To investigate the effect of the dlc film structure on diamond nucleation, experiments with silicon substrates coated with evaporated carbon films approximately 50–100 nm thick were also performed under similar pretreatment and diamond nucleation conditions. Diamond films with nucleation density about 2 × 10 8 cm −2 were obtained with pretreated dlc films, depending on the film thickness and the pretreatment time, whereas in the absence of the pretreatment and/or with evaporated carbon films the diamond nucleation density was less than 10 4 cm −2. The significant enhancement in diamond nucleation density obtained with pretreated dlc films is attributed to the inherently high etching resistance of the films resulting from the high fraction of sp 3 bonds. The actual nucleation sites might be very fine carbon or, possibly, SiC particles produced by etching back the dlc film.

Electronic and atomic structure of evaporated carbon films

Carbon films have been prepared at different substrate temperatures (25–800°C) by evaporating carbon from a graphite source with an electron beam evaporation device. The films have been analyzed by in situ electron spectroscopy (UPS, XPS and EELS), ex situ scanning tunneling microscopy (STM) and Raman spectroscopy. The UPS valence band and EELS spectra reveal a gradual transition from a disordered carbon film at room temperature deposition to a polycrystalline graphite-like film at 800°C substrate temperature deposition, in good agreement with the results obtained from Raman spectroscopy. STM images of films deposited at 25°C also exhibit a rather disordered carbon network, whereas with increasing substrate temperature, graphite nanocrystallites with sizes of about 20 nm are formed. A correlation between these different characterization techniques is performed.

Influence of different correction models on investigations of carbonaceous insulating films by electron beam X-ray microanalysis

Normally quantitative investigations of insulators by electron beam X-ray microanalysis are made possible by coating special samples with thin evaporated carbon films. This, however, will fail if the samples themselves contain carbon as polysilastyrene {[(CH3)2Si]x (CH3)C6H5Si}n, which is used for the preparation of silicon carbide. Samples of polysilastyrene, together with reference layers (pure silicon) were investigated using different equipment (JEOL, CAMECA) and with varying correction procedures (ZAF, PAP). Compared with the results of reference measurements (elementary analysis, NMR) it appears that the PAP correction procedure is better suited for solving this problem than the classical ZAF correction procedure.

Diamond nucleation on unscratched silicon substrates coated with various non-diamond carbon films by microwave plasma-enhanced chemical vapor deposition

The efficacy of various non-diamond carbon films as precursors for diamond nucleation on unscratched silicon substrates was investigated with a conventional microwave plasma-enhanced chemical vapor deposition system. Silicon substrates were partially coated with various carbonaceous substances such as clusters consisting of a mixture of C60 and C70, evaporated films of carbon and pure C70, and hard carbon produced by a vacuum are deposition technique. For comparison, diamond nucleation on silicon substrates coated with submicrometer-sized diamond particles and uncoated smooth silicon surfaces was also examined under similar conditions. Except for evaporated carbon films, significantly higher diamond nucleation densities were obtained by subjecting the carbon-coated substrates to a low-temperature high-methane concentration hydrogen plasma treatment prior to diamond nucleation. The highest nucleation density (∼3 × 108 cm−2) was obtained with hard carbon films. Scanning electron microscopy and Raman spectroscopy demonstrated that the diamond nucleation density increased with the film thickness and etching resistance. The higher diamond nucleation density obtained with the vacuum are-deposited carbon films may be attributed to the inherent high etching resistance, presumably resulting from the high content of sp3 atomic bonds. Microscopy observations suggested that diamond nucleation in the presence of non-diamond carbon deposits resulted from carbon layers generated under the pretreatment conditions.

Specimen flatness of glucose-embedded biological materials for electron crystallography is affected significantly by the choice of carbon evaporation stock

Imperfect specimen flatness can be a significant limitation in the application of electron crystallography to high-resolution structure analysis of biological macromolecules. We now report that the choice of solid carbon stock that is used to make evaporated carbon films can have a very great effect on the preparation of flat specimens of glucose-embedded purple membrane. The degree of purity of the carbon does not seem to be the controlling factor, and other likely factors such as the type of mica used as a substrate, the evaporation apparatus used (and its limiting vacuum), and the use of a continuous versus an interrupted evaporation protocol do not have a discernible influence. The physical or chemical basis for the observed differences in specimen flatness is still unknown; however, the important conclusion that we can communicate at this point is that the choice of evaporating material does have a major effect on the flatness of purple membrane, the specimen used here. The implication is that different sources of carbon stock should be tried whenever difficulty is encountered in the preparation of suitably flat specimens of biological macromolecules.

Characterization of microtubules by dark-field STEM and replication

As part of a study on protein interactions involved in microtubule (MT)-based transport, we used the VG HB501 field-emission STEM to obtain low-dose dark-field mass maps of isolated, taxol-stabilized MTs and correlated these micrographs with detailed stereo images from replicas of the same MTs. This approach promises to be useful for determining how protein motors interact with MTs. MTs prepared from bovine and squid brain tubulin were purified and free from microtubule-associated proteins (MAPs). These MTs (0.1-1 mg/ml tubulin) were adsorbed to 3-nm evaporated carbon films supported over Formvar nets on 600-m copper grids. Following adsorption, the grids were washed twice in buffer and then in either distilled water or in isotonic or hypotonic ammonium acetate, blotted, and plunge-frozen in ethane/propane cryogen(ca.-185 C). After cryotransfer into the STEM, specimens were freeze-dried and recooled toca.-160 C for low-dose (<3000 e/nm2) dark-field mapping. The molecular weights per unit length of MT were determined relative to tobacco mosaic virus standards from elastic scattering intensities. Parallel grids were freeze-dried and rotary shadowed with Pt/C at 14°.

Potassium promotion of carbon reactivity: 104-fold increase of oxidation rate in O2

We have investigated, experimentally and theoretically, the oxidation rate of UHV evaporated carbon films with added surface deposits of potassium. The oxidation rate in O2 increases faster than linearly with increasing K coverage. A monolayer of K increases the rate by a factor ∼104, in comparison with pure carbon. A theoretical calculation assumes that O2 dissociation is the rate limiting step. Based on a charge transfer mechanism, this calculation reproduces the order of magnitude of the rate increase.

Strengths of evaporated carbon films

We have tested the capability of unsupported carbon films to separate regions at different pressures. We find that a well-mounted foil behaves as if it had the yield tensile strength of bulk graphite (5.0 × 10 8 dyn/cm 2), and that the median breaking strength is somewhat less than half of this value.

Semiconducting amorphous carbon films and carbon-single-crystal silicon heterojunctions

Recent studies have shown good potential for evaporated carbon films in memory and switching devices. This trend can be attributed to the behaviour of carbon films as amorphous semiconductors. Experiments were carried out to study the electron diffraction patterns of the films as well as the variation of the electrical conductivity with temperature. The results clearly prove the amorphous nature of carbon films. Heterojunctions were prepared by depositing carbon onto single-crystal silicon. These heterojunctions showed a photovoltaic output of about 280 mV (under Air Mass 1 conditions) and a rectification ratio of the order of hundreds.

X-ray emission cross sections for carbon bombarded with 4- to 40-MeV C, N, and O ions

Cross sections for production of target-atom carbon $K$ x rays in thin evaporated carbon films are determined from spectra obtained using a double-focusing soft-x-ray spectrometer with calibrated transmission. A maximum value of 2.3 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}19}$ ${\mathrm{cm}}^{2}$ is found for 18-MeV oxygen ions. A maximum ionization cross section of 4.8 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}17}$ ${\mathrm{cm}}^{2}$ is estimated from the data. Cross sections for x rays on both sides of the carbon $K$ edge are determined. Nitrogen ions produce more $L$-shell ionization at low energies than carbon or oxygen ions, both of which create very few $K{L}^{2}$ satellite x rays below about 9 MeV.

Reaction of thermal atomic hydrogen with carbon

The reaction of thermal atomic hydrogen, produced at the surface of a hot tungsten filament, with evaporated carbon films leads to the production of a number of gaseous products. At least 12 have been identified with the largest number of these being cyclic compounds consistent with the structure of graphite. Methane, the major gas phase product (74% partial pressure), is produced with a stoichiometric reaction probability of 0.002. Although the relative reaction rates decrease rapidly with increasing product mass, the total rate of carbon transport is 60% greater than the rate of methane production. Activation energies obtained for 10 of the products by varying the carbon temperature are the same within experimental error. These features are discussed in terms of a highly simplified model of the reaction with the actual reaction following a complicated branching free radical mechanism.

Optical properties of arc-evaporated carbon films between 0.6 and 3.8 eV

Ellipsometric measurements have been performed at room temperature on arc-evaporated carbon films between 0.6 and 3.8 eV. Previously published data on evaporated carbon films are reviewed and a comparison made between the present data and those for glassy carbon over the same energy range.