Ultrathin Carbon Films Research Articles

Preparation and characterization of ultrathin free-standing carbon films

Ultrathin free-standing carbon films with thicknesses of ∼8–14 nm were prepared by peeling from glass after pyrolization of the polyfurfuryl alcohol precursor at 700 °C. The carbon films were characterized by using optical profilometry, confocal Raman spectroscopy, transmission electron microscopy, UV-vis spectroscopy, and a two-point probe I - V station measurement. The results confirmed the formation of ultrathin free-standing carbon films with a polyaromatic domain structure. Those films had up to a 90% transmittance with respect to air and had an ohmic behavior. In this work, we also suggest that the most likely peeling mechanism of carbon films results from glass corrosion.

Enhanced microwave shielding effectiveness of ultrathin pyrolytic carbon films

Electromagnetic properties of pyrolytic carbon (PyC) films with thickness from 5 to 241 nm are studied experimentally and numerically at 28 GHz. We observe experimentally that PyC films are capable to absorb up to 50% of microwave power in the Ka-band. By using boundary conditions in the rectangular waveguide, we demonstrate theoretically that 50% of microwave power can be absorbed in a conductive film with thickness much smaller than the skin depth. The results of modelling for PyC films on silica substrate are in excellent coincidence with the experimental data.

The multilayered structure of ultrathin amorphous carbon films synthesized by filtered cathodic vacuum arc deposition

Abstract

Si seed layer thickness effect on the structure of ultrathin tetrahedral amorphous carbon films

Tetrahedral amorphous carbon (ta-C) film is a promising materials serving as a protective coating layer due to its high sp3 content and its thickness up to 1–2nm. In order to improve the adhesion of ta-C on metal, a Si film is usually pre-coated as seed layer. However, the Si film will increase the total thickness of protective layer. How to reduce the thickness of Si film is a requirement of industry applications. Here, we have studied the Si film thickness effect on the structure of ultrathin ta-C film. In our experiment, the thickness of Si film varies from 5 to 30Å while the thickness of ta-C film is kept at about 2nm. The Raman result shows that the sp3 fraction of ta-C film with pre-coated 5Å Si film is a little lower than that of ta-C film without Si pre-coated. When the Si film becomes thicker from 5 to 30Å, the sp3 fraction of Si/ta-C composite films is almost no changed. The XPS analysis shows that a part of Si bonding changes to SiC bonding which can enhance the adhesion of ta-C on metal. It also confirms that the SiC bonding fraction of Si film has no change when the thickness of Si film increases from 5 to 30Å, which is consistent with the Raman analysis. Thus, an indication is that the fundamental limitation thickness of Si seed layer can be reduced to 5Å.

Transport and electromagnetic properties of ultrathin pyrolytic carbon films

We experimentally investigated the electrical and electromagnetic (EM) properties of pyrolytic carbon (PyC) ultrathin films synthesized on a quartz substrate by chemical vapor deposition at 1100°C using low pressure CH4∶H2 gas mixture as carbon source. PyC films consist of randomly oriented and intertwined graphene ribbons, which have a typical size of a few nanometers. We discovered that the manufactured PyC films of 35-nm thickness provided remarkably high attenuation caused by absorption of 37% to 24% of incident microwave power. The temperature dependence of PyC’s direct-current (DC) conductivity represents typical behavior for disordered systems. Being semitransparent in visible and infrared spectral range and highly conductive at room temperature, PyC films emerge as a promising material for manufacturing ultrathin microwave (e.g., Ka band) coatings to be used in aerospace applications.

Structural stability of hydrogenated amorphous carbon overcoats used in heat-assisted magnetic recording investigated by rapid thermal annealing

Ultrathin amorphous carbon (a-C) films are extensively used as protective overcoats of magnetic recording media. Increasing demands for even higher storage densities have necessitated the development of new storage technologies, such as heat-assisted magnetic recording (HAMR), which uses laser-assisted heating to record data on high-stability media that can store single bits in extremely small areas (∼1 Tbit/in.2). Because HAMR relies on locally changing the coercivity of the magnetic medium by raising the temperature above the Curie temperature for data to be stored by the magnetic write field, it raises a concern about the structural stability of the ultrathin a-C film. In this study, rapid thermal annealing (RTA) experiments were performed to examine the thermal stability of ultrathin hydrogenated amorphous carbon (a-C:H) films deposited by plasma-enhanced chemical vapor deposition. Structural changes in the a-C:H films caused by RTA were investigated by x-ray photoelectron spectroscopy, Raman spectroscopy, x-ray reflectivity, and conductive atomic force microscopy. The results show that the films exhibit thermal stability up to a maximum temperature in the range of 400–450 °C. Heating above this critical temperature leads to hydrogen depletion and sp2 clustering. The critical temperature determined by the results of this study represents an upper bound of the temperature rise due to laser heating in HAMR hard-disk drives and the Curie temperature of magnetic materials used in HAMR hard disks.

Ultrathin carbon film from carbonization of spin-cast polyacrylonitrile film

Here we present ultrathin carbon films derived from polyacrylonitrile precursor film. Subsequent carbonization was investigated to examine the optical and electrical properties of resulting carbon films. The precursor PAN solution was carefully prepared and heated in order to have thin and uniform PAN film, and X-ray reflectivity of the precursor film and the carbon film shows uniformity. X-ray diffraction pattern of the carbon film indicated that the obtained carbon films possess low crystallinity. It was also found that the thickness for ultrathin carbon film from PAN layer showed counterbalanced optical and electrical performance in the range of experiment.

Direct deposition of semitransparent conducting pyrolytic carbon films

Ultrathin pyrolytic carbon films were synthesized in a single step on dielectric substrates, using a methane-based chemical vapor deposition technique. The developed technique enables the production of wafer scale transparent conductive films suitable for applications in optics and optoelectronics.

Incidence Angle Effect of Energetic Carbon Ions on Deposition Rate, Topography, and Structure of Ultrathin Amorphous Carbon Films Deposited by Filtered Cathodic Vacuum Arc

The effect of the incidence angle of energetic carbon ions on the thickness, topography, and structure of ultrathin amorphous carbon ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</i> -C) films synthesized by filtered cathodic vacuum arc (FCVA) was examined in the context of numerical and experimental results. The thickness of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</i> -C films deposited at different incidence angles was investigated in the light of Monte Carlo simulations, and the calculated depth profiles were compared with those obtained from high-resolution transmission electron microscopy (TEM). The topography and structure of the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</i> -C films were studied by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS), respectively. The film thickness decreased with the increase of the incidence angle, while the surface roughness increased and the content of tetrahedral carbon hybridization ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sp</i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ) decreased significantly with the increase of the incidence angle above 45°, measured from the surface normal. TEM, AFM, and XPS results indicate that the smoothest and thinnest <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</i> -C films with the highest content of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sp</i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> carbon bonding were produced for an incidence angle of 45°. The findings of this study have direct implications in ultrahigh-density magnetic recording, where ultrathin and smooth <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</i> -C films with high <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sp</i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> contents are of critical importance.

Ultrathin Carbon Films Prepared by Negative Cluster-Beam Technology

We develop a miniaturized chamber installed on a tandetron accelerator into which negative ions of small carbon clusters are transported. Negative clusters C−1-C−10 are obtained with beam currents of 1–104 nA at energies of 10–20 keV. C−2 beams of 0.2 μA are used to directly deposit carbon films on SiO2/Si substrates. Formation of ultrathin carbon films are demonstrated by Raman scattering, which reveals the evolution of the graphitic peak (1550 cm−2) with deposition time.

Site‐specific fabrication of graphitic microporous carbon terminated with ordered multilayer graphene walls

AbstractThe site‐specific fabrication of metal‐incorporated graphitic microporous carbon terminated by highly ordered multilayer graphene walls via an ambient‐temperature vacuum‐based process is presented for the first time. Sputtering of Cr nano‐particles on the ultrathin amorphous carbon film manifests a dual effect of activation via dry‐etching by the sputtering plasma and of ‘knock‐on' inelastic collision between nanoparticles and C atoms for structural ordering. This novel and simple method is very useful for fabricating high surface area carbon nanostructures for hydrogen storage and clean energy applications. (© 2012 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Mechanical measurements of ultra-thin amorphous carbon membranes using scanning atomic force microscopy

The elastic modulus of ultra-thin amorphous carbon films was investigated by integrating atomic force microscopy (AFM) imaging in contact mode with finite element analysis (FEA). Carbon films with thicknesses of ∼10nm and less were deposited on mica by electron beam evaporation and transferred onto perforated substrates for mechanical characterization. The deformation of these ultra-thin membranes was measured by recording topography images at different normal loads using contact mode AFM. The obtained force-distance relationship at the center of membranes was analyzed to evaluate both the Young’s modulus and pre-stress by FEA. From these measurements, Young’s moduli of 178.9±32.3, 193.4±20.0, and 211.1±44.9GPa were obtained for 3.7±0.08, 6.8±0.12, and 10.4±0.17nm thick membranes, respectively. Raman spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy were used for characterizing the chemical and structural properties of the films, including the content of sp2 and sp3 hybridized carbon atoms.

Functionalized Ultrathin Carbon Films for Imaging Low-Abundance Biological Complexes

By single particle electron cryo-microscopy (cryoEM), structures of low-abundance biological complexes can be obtained if the surface density of the complexes on the EM grids can be made sufficiently high for data collection. In order to prevent most of the molecules from being lost during specimen preparation, new methods are needed to retain them selectively to the surface. Such methods will permit cryoEM imaging of macromolecules that are available at subnanogram levels. Available methods along this line of thinking all add significant biomass to the background and are not so simple or robust for routine operations. We have developed a novel “bottom-up” approach to selectively anchor proteins of interest onto the surface of nanometer-thick carbon films that are routinely used for cryoEM imaging. We show that by chemically modifying the carbon films, specific ligands can be introduced onto carbon surfaces in order to enrich complexes bearing appropriate functional groups. In particular, we have been developing the chemical procedures that present on the carbon films specific ligands for His-tagged, GST-tagged and oligonucleotide-binding complexes as well as for immunoglobulin molecules. Meanwhile, we are applying this technology to the structural study of important low-abundance biological complexes by single particle EM. Our results demonstrate that the surface-engineered carbon films should have general applicability to a wide range of biophysical and biomedical problems.

Electrical conductivity and optical properties of thin carbon films grown from ethanol vapor

Transparent, ultrathin carbon films have been grown through the pyrolysis of ethanol vapor at a reduced pressure on copper substrates at temperatures from 600 to 950°C. The electrical conductivity of the films increases with deposition temperature. Depending on deposition temperature, ethanol vapor pyrolysis may follow different mechanisms and the carbon deposition process has different key features, which influence the properties of the films. In the range 600–750°C, ethanol vapor pyrolysis is a catalytic process, which results in selective growth of a thin carbon film with an optical transmittance of ∼95% only on the copper surface. At higher temperatures, carbon deposition is nonselective, and the resultant films are darker. The carbon deposition mechanism is discussed in relation to the ethanol vapor pyrolysis temperature. The present results suggest that carbon deposition from ethanol vapor is a promising approach to producing transparent, conductive carbon films.

Thickness dependent electronic structure of ultra-thin tetrahedral amorphous carbon (ta-C) films

Microstructural properties of ultrathin (1–10nm) tetrahedral amorphous carbon (ta-C) films are investigated by Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy, X-ray Photoelectron Spectroscopy, Raman spectroscopy and Atomic Force Microscopy (AFM). The CK-edge NEXAFS spectra of 1nm ta-C films provided evidence of surface defects (C―H bonds) which rapidly diminish with increasing film thickness. A critical thickness for stabilization of largely sp3 matrix structure distorted by sp2 sites is observed via the change of π*C═C peak behavior. Meanwhile, an increase in the film thickness promotes an enhancement in sp3 content, the film roughness remains nearly constant as probed by spectroscopic techniques and AFM, respectively. The effect of thickness on local bonding states of ultrathin ta-C films proves to be the limiting factor for their potential use in magnetic and optical storage devices.

Optimization of ultrathin carbon film coated silver nanoshell for biomedical applications in vivo

The extinction spectroscopy and near-field enhancement of dielectric-silver nanoshell coated by tetrahedral amorphous carbon [ta-C] layer (DSC) have been calculated by using Mie theory. With decreasing the Ag layer thickness, the localized surface plasmon resonance (LSPR) of DSC nanoshell moves from the visible region into the near-infrared region and the corresponding local field factor (LFF) increases first and then decreases. In addition, the increase of ta-C shell thickness leads to red-shift of LSPR and the decrease of LFF in DSC nanoshell. We further find that the increase of the dielectric constant for the outer shell can induce a significant enhancement of near-field. Based on the simulation analysis, we show that the DSC nanoshell can provide strong near-field in near-infrared region and may be suitable for the biomedical applications in vivo.

Carbon-induced extreme ultraviolet reflectance loss characterized using visible-light ellipsometry

Carbon deposition on extreme ultraviolet (EUV) optics was observed due to photon-induced dissociation of hydrocarbons in a EUV lithography environment. The reflectance loss of the multilayer mirror is determined by the carbon layer thickness and density. To study the influence of various forms of carbon, EUV-induced carbon, hot filament and e-beam evaporated carbon were deposited on EUV multilayer mirrors. Spectroscopic ellipsometry was used to determine the carbon layer thickness and the optical constants ranging from ultraviolet to near infrared. The carbon density (and thus reflectance loss) was determined from the optical constants using both Bruggeman's effective medium approximation and the Clausius–Mosotti equation. Both approaches result in a similar EUV reflectance loss, with an accuracy of about 4%. The application of this process to ultrathin carbon films is further discussed.

Thermal Stability of Ultrathin Amorphous Carbon Films for Energy-Assisted Magnetic Recording

Energy-assisted magnetic recording (EAMR) uses a laser-optical system integrated into the magnetic head to heat locally a fine-grained material of high magnetic anisotropy energy density above its Curie temperature, and store single bits in very small areas without being limited by the superparamagnetic effect. However, localized laser heating may affect the thermal stability of the carbon overcoat of the hard disk. To examine the effect of laser heating on the overcoat thermal stability, ultrathin amorphous carbon (a-C) films of similar thickness (~ 3.6 nm) synthesized by filtered cathodic vacuum arc (FCVA) and chemical vapor deposition (CVD) were subjected to repetitive heating under different laser powers. Carbon hybridization and surface roughness of the a-C films were examined by Raman spectroscopy and atomic force microscopy, respectively. For the laser power range studied (150-300 mW), a-C films produced by the FCVA technique demonstrated superior thermal stability than CVD films of similar thickness. To investigate the possibility of further reducing the magnetic spacing, thinner (~ 0.9 nm) a-C films deposited by the FCVA method were subjected to the same laser heating conditions. Although the thermal stability of the FCVA-synthesized a-C films exhibited thickness dependence, even the thinner (~ 0.9 nm) FCVA film demonstrated higher thermal stability than the much thicker (~ 3.6 nm) CVD film. The results of this study illustrate the high potential of FCVA as a coating method for EAMR.

Silicon nitride gradient film as the underlayer of ultra-thin tetrahedral amorphous carbon overcoat for magnetic recording slider

There are higher technical requirements for protection overcoat of magnetic recording slider used in high-density storage fields for the future. In this study, silicon nitride (Si-N) composition-gradient films were firstly prepared by nitriding of silicon thin films pre-sputtered on silicon wafers and magnetic recording sliders, using microwave electron cyclotron resonance plasma source. The ultra-thin tetrahedral amorphous carbon films were then deposited on the Si-N films by filtered cathodic vacuum arc method. Compared with amorphous carbon overcoats with conventional silicon underlayers, the overcoats with Si-N underlayers obtained by plasma nitriding especially at the substrate bias of −150 V, can provide better corrosion protection for high-density magnetic recording sliders.

Synthesis of ordered mesoporous carbons by a soft-templating method

Ordered mesoporous carbons were synthesized by using an organic-organic self-assembly (soft-templating) method using resorcinol (R) / formaldehyde (F) and triblock copolymer Pluronic F127. A hexagonal mesostructure with channel like pores (COU-1) with a pore size of 4.7–5.8 nm and a 3D wormhole-like mesopore structure (COU-2) with a pore size of 4.8 nm were synthesized using precursor solutions with different mass ratios of F127/RF resin. The mesoporous carbons were activated by using KOH to improve their porosity. The KOH-activated COU-2 carbon showed superior capacitance compared to the COU-2 carbon and a commercial microporous carbon. Ultrathin carbon films with a monolayer of uniform mesopores were also synthesized on a silicon substrate by contacting a triblock copolymer film with a benzyl alcohol vapor followed by carbonization.