Amorphous Hydrogenated Carbon Layers Research Articles

Thin amorphous hydrogenated carbon layers on polylactic acid and polyhydroxybutyrate biopolymers - influence of substrate chemistry on interlayer formation

Polymers are often modified by plasma treatment to adapt them to new applications. During the deposition of an amorphous hydrogenated (a-C:H) layer, a mixed phase (interlayer) of the polymer substrate and the deposited layer often forms. The interlayer is responsible among others for a good adhesion between the layer and the substrate. In this study, amorphous hydrogenated carbon films were deposited on the structurally similar biopolymers polylactic acid (PLA) and polyhydroxybutyrate (PHB) using radio frequency plasma enhanced chemical vapor deposition (RF-PECVD). In an industrially established process, layer thicknesses of up to 50 nm were produced in 10 nm steps. The surface morphology of each layer was investigated by ex situ atomic force microscopy. Chemical composition was explored first with surface sensitive synchrotron X-ray-based techniques such as X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure. Subsequently, IR measurements were performed for a more detailed analysis of the functional groups contained. Based on the data obtained (e.g. sp2/sp3 ratio, grain formation on the surface), it was possible to determine the transition of polymer to a-C:H layer and therefore the thickness of the interlayer dimension formed during coating. For PLA, the thickness is about 40 nm, for PHB 20 nm.

Controlling the sp2/sp3 ratio of amorphous hydrogenated carbon layers (a-C:H) on polylactide acid (PLA) at constant distance but angle-dependent deposition

The biopolymer polylactide acid (PLA) is suitable as an eco-friendly substitute for conventional polymers, but polymer surfaces often do not meet industrial requirements and thus severely limit applications. Surface adhesion, substrate related decomposition and barrier permeability can be modified with plasma-deposited thin amorphous hydrogenated carbon (a-C:H) coatings. Their properties depend significantly on the sp2/sp3 ratio, here to be controlled by changing the coating geometry between sample and plasma source. A 50 µm PLA film was a-C:H coated at different angle settings (30° intervals) for 2.5, 5.0, 7.5 and 10.0 min to analyze angle-dependent layer growth. Surface morphology was recorded by scanning electron and atomic force microscopy, and the surface free energy (SFE) and contact angle were determined. Modified barrier properties such as the water vapor transmission rate were investigated and showed a reduction in permeability of up to 47% for defined coating geometries and times. In addition, investigations of the top a-C:H layers by synchrotron assisted X-ray techniques revealed angle-dependent changes in the sp2/sp3 binding ratio. The angle-dependent coating not only changes the chemical composition of the layer, but also improves the barrier properties and extends the SFE of PLA by an increased polar fraction.

The Growth Behavior of Amorphous Hydrogenated Carbon a-C:H Layers on Industrial Polycarbonates—A Weak Interlayer and a Distinct Dehydrogenation Zone

Polycarbonate (PC) is a material that is used in many areas: automotive, aerospace engineering and data storage industries. Its hardness is of particular importance, but some applications are affected by its low wettability or scratch susceptibility. This can be changed either by blending with other polymers, or by surface modifications, such as the application of an amorphous hydrogenated carbon layer (a-C:H). In this study, individual a-C:H layers of different thicknesses (10–2000 nm) were deposited on PC by RF PECVD. Both the layer morphology with AFM and SEM and the bonding states of the carbon on the surface with synchrotron-assisted XPS and NEXAFS were studied. The aim was to investigate the coatability of PC and the stability of the a-C:H. Special attention was paid to the interlayer region from 0 to 100 nm, since this is responsible for the layer to base material bonding, and to the zone of dehydrogenation (from about 1000 nm), since this changes the surface composition considerably. For PC, the interlayer was relatively small with a thickness of only 20 nm. Additionally, a correlation was found between the evolving grain structure and the development of the C‒H peak according to NEXAFS C K-edge measurements.

Effect of Cellulose Nanocrystals on the Coating of Chitosan Nanocomposite Film Using Plasma-Mediated Deposition of Amorphous Hydrogenated Carbon (a–C:H) Layers

The substitution of petroleum-based polymers with naturally derived biopolymers may be a good alternative for the conservation of natural fossil resources and the alleviation of pollution and waste disposal problems. However, in order to be used in a wide range of applications, some biopolymers’ properties should be enhanced. In this study, biocompatible, non-toxic, and biodegradable chitosan (CS) film and CS reinforced with 10 wt% of cellulose nanocrystals (CN–CS) were coated with amorphous hydrogenated carbon layers (a–C:H) of different thickness. To investigate the effect of the nano-reinforcement on the a–C:H layer applied, mild radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) was used to coat the CS and its CN–CS bio-nanocomposite film. Both the surface characteristics and the chemical composition were analyzed. The surface morphology and wettability were examined by ex-situ atomic force microscopy (AFM) and contact angle measurements (CA), respectively. Hereby, the relationship between sp2/sp3 ratios on a macroscopic scale was also evaluated. For the investigation of the chemical composition, the surface sensitive synchrotron X-ray radiation techniques near edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS) as well as diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) were used.

Changing Contents of Carbon Hybridizations in Amorphous Hydrogenated Carbon Layers (a-C:H) on Sustainable Polyhydroxybutyrate (PHB) Exhibit a Significant Deterioration in Stability, Depending on Thickness

PHB is a biodegradable polymer based on renewable raw materials that could replace synthetic polymers in many applications. A big advantage is the resulting reduction of the waste problem, as well as the conservation of fossil resources. To arrange it for various applications, the surface is arranged by plasma-enhanced chemical vapor deposition (PECVD) with amorphous hydrogenated carbon layers (a-C:H). Here, on a 50 µm thick PHB-foil, a-C:H layers of different thicknesses (0–500 nm) were deposited in 50 nm steps. Surface topography was investigated by scanning electron microscopy (SEM), chemical composition by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and wettability checked by contact angle. In addition, layers were examined by synchrotron supported X-ray photoelectron spectroscopy (XPS) and near edge X-ray absorption fine structure (NEXAFS), which revealed thickness dependent changes of the sp2/sp3 ratio. With increasing thickness, even the topography changes show internal, stress-induced phenomena. The results obtained provide a more detailed understanding of the predominantly inorganic a-C:H coatings on (bio)polymers via in situ growth.

Correlation Between 3-D Surface Topography and Different Deposition Times of Engineered Ni@a-C:H Thin Films

This study is aimed at analyzing the surface micromorphology of amorphous hydrogenated carbon layers containing nickel nanoparticles (Ni-NPs@a-C:H) within their structure, which were sputtered by co-deposition of RF-Sputtering and RF-PECVD method from acetylene gas and nickel target on glass substrates. Prepared films were then used as research material following their deposition time at three different times of 3, 7 and 13 minutes. Series of height samples were taken with the help of atomic force microscopy (AFM) operating in a non-contact mode and examined in order to determine their surface engineering characteristics. The analysis of 3-D surface texture helps to understand their essential characteristics and their implications as well as graphicalmodels and their implementation in computer simulation.

Microstructure of nickel nanoparticles embedded in carbon films: case study on annealing effect by micromorphology analysis

The presented study is aimed at analyzing the surface texture of amorphous hydrogenated carbon layers containing nickel nanoparticles (Ni‐NPs@a‐C:H) within their structure, which were deposited by Radio Frequency (RF) sputtering and RF‐Plasma Enhanced Chemical Vapor Deposition (RF‐PECVD) methods on glass substrates. Prepared films were then used as research material following their annealing at two different temperatures of 250 °C and 350 °C in an inert argon atmosphere. Series of height samples were taken with the help of atomic force microscopy (AFM) operating in a non‐contact mode and examined in order to determine their fractal characteristics. Raw AFM data were first plane‐fitted to remove the surface bow exhibiting the so‐called residual surface, and then numerically processed to calculate the Areal Autocorrelation Function (AACF), which was later used to compute the Structure Function (SF). The log–log plots of the latter served for calculation of fractal properties of surfaces under investigation, including fractal dimension D, and pseudo‐topothesy K. The analysis of 3‐D surface texture helps to understand their essential characteristics and their implications as well as graphical models and their implementation in computer simulation. Copyright © 2016 John Wiley & Sons, Ltd.

Surface effects on the diagnostic of carbon/nitrogen low-pressure plasmas studied by differentially pumped mass spectrometry.

In this work, the characterization of the species produced in reactive plasmas by differentially pumped mass spectrometry is addressed. A H2/CH4/N2 mixture (90 : 5 : 5) was fed into a direct current glow discharge and analysed by conventional and cryo-trap assisted mass spectrometry. The gaseous mixture was chosen because of its particular relevance in the inhibition of tritium-rich carbon film deposition in fusion plasmas (scavenger technique) and in the deposition of amorphous hydrogenated carbon films by plasma-assisted chemical vapour deposition. Important changes in the composition of the detected species upon surface modification of the reactor walls (stainless steel or covered by an amorphous hydrogenated carbon layer) or in the way they are sampled (length and spatial configuration of the stainless steel duct) were detected. They are analysed in terms of radical formation and recombination on the reactor walls or into the sampling duct, thus providing some insight into the underlying chemistry. In general, when the reactor walls are covered by an amorphous hydrogenated carbon layer, more hydrocarbons are produced, but the radical production is lower and seem to be less reactive than in stainless steel. Also, two sources of oxygen contamination in the plasma have been identified, from the native oxide layer in stainless steel and from unintended water contamination in the chamber, which modify considerably the detected species.

Stability of a-C:N:H Layers Deposited by RF Plasma Enhanced CVD

A series of amorphous hydrogenated carbon layers doped with nitrogen (a-C:N:H) was deposited on Si (001). The synthesis was performed from gaseous N2/CH4 mixture using PE CVD (RF CVD technique; 13,56 MHz). An influence of the processing conditions on layer-growth rate was analysed. Thickness of the layers deposited during 1 hour at various temperatures, pressures and RF powers were taken as a basis. It has been proved that the substrate temperature is a key parameter for the layer formation. Temperature rise results in the deposition rate decrease. This unfavourable effect may be reduced by application of increased gas pressure and/or higher plasma RF generator power. At optimal conditions (46 oC; 0,8 Tr; 60 W) the deposition rate reaches up to 600 nm/hour. FT-IR spectra of the layers were measured within 1250 - 4000 cm-1 and discussed with regard to the atomic structure. The intensities of the characteristic absorption bands were compared. The results show that the layers have various N/C ratios according to the applied processing conditions.

Plasma deposition of carbon layer: Correlations between plasma parameters, film structure and properties

The links between plasma parameters, discharge, film structure and properties are not really yet understood for amorphous hydrogenated carbon layers (a-C:H) plasma deposition. Here, a-C:H layers are deposited in a dual radio-frequency–multipolar microwave plasma excited by distributed electron cyclotron resonance reactor at low CH 4 pressure. This study deals with the plasma analysis, the film characterization and with plasma parameters effect on a-C:H films deposition, structure and properties. The discharge analysis shows that CH 4 is decomposed in CH y1 + H radicals and that C 2H 2 y2 are present in the discharge. As at low pressure, recombination can only take place on the surface, C 2 H 2 y2 desorbs from the surface. Moreover, C 2 species are observed attributed to C 2 H 2 y2 dissociation. The evolution of film composition with plasma power shows that the proportion of sp 2 CC decreases in contrast with those of CH bonds which increases. From these observations, a phenomenological model for a-C:H deposition can be proposed. Finally, properties are correlated with the film structure and the effect of MW plasma power can then be given.

Chemical sputtering of carbon by nitrogen ions

Chemical sputtering of amorphous hydrogenated carbon layers by nitrogen molecular ions was studied as a function of the ion energy in the range from 30to900eV. The sputtering yield shows only a very weak variation with energy in the range from 900 down to 50eV. For lower energies it decreases significantly. This behavior is interpreted as an indication of chemical sputtering.

Deposition of amorphous hydrogenated carbon films due to hydrocarbon molecule ion-beam bombardment

Carbon deposition and hydrogen codeposition are investigated as functions of ion energy, fluence and target temperature at normal incidence on silicon and graphite by bombardment with mass selected CH 2 +, CH 3 +, CH 4 + and CD 4 + molecule beams. An amorphous hydrogenated carbon layer (a-C:H) is formed in a thickness range of 40 to 130 nm by CH 3 + and CH 2 + bombardment up to a fluence of 3 × 10 18/cm 2. The deposition process, the re-erosion phenomenon and the H C ratio of the a-C:H films are studied between 300 and 1100 K in the ion energy range from 0.15 to 3 keV by means of ion beam analysis and Auger Electron Spectroscopy (AES). The experimental results are compared with TRIDYN computer simulation and previous experimental results of carbon sputtering by atomic H + and C + beams in order to get a better understanding of the interaction between hydrocarbon ions and the carbon-based wall materials in fusion devices.

Optical absorption of amorphous hydrogenated carbon thin films

Amorphous hydrogenated carbon layers were deposited from ethylene in a capacitively coupled r.f.-discharge reactor under different self-bias voltages and by addition of different amounts of phosphine as an assumed “doping” gas. The optical constants of the thin films were determined by analysing reflection, transmission and photothermal deflection measurements. Accordingly, the absorption behaviour below an absorption coefficient α of 10 4 cm −1 can be fitted very well by a simple power law. Extrapolating the absorption to lower excitation energies suggests an increase of the density of states at the Fermi level accompanying the increase of the electrical conductivity with increasing self-bias voltage or phosphine concentration. No hint of any “doping” effect is found.

Deposition and hydrogen content of carbon films grown by CH+3 ion-beam bombardment

Carbon deposition and hydrogen codeposition is investigated as a function of ion energy, fluence, and target temperature at normal incidence by bombardment of silicon and pyrolitic graphite substrates with mass selected CH+3 molecules. An amorphous hydrogenated carbon layer (a-C:H) is formed in a thickness range of 40–130 nm at a fluence of 3×1018/cm2. The deposition process, the re-erosion phenomenon, the hydrogen content, and the H/C ratios of the carbon films are studied between 300 and 1000 K in the ion energy range from 150 eV to 3 keV. The experimental results are compared with those of TRIDYN computer simulations and previous experimental results of carbon sputtering by atomic H+ and C+ beams in order to obtain a better understanding of the interaction between hydrocarbon ions and the carbon-based wall materials in fusion devices.

A novel setup for spectroscopic ellipsometry using an acousto-optic tuneable filter

A rotating-analyzer ellipsometer for fast measurements at multiple wavelengths as well as for spectroscopic measurements has been developed. The most important novelty of the setup is the use of an acousto-optic tuneable filter (AOTF) as dispersing element. This offers advantages with respect to the speed of measurement, the adjustment of the intensity of the light, the use of lock-in techniques, and the stability and size of the setup. Advantages and limitations of the use of AOTF’s in ellipsometry as well as possible changes in the ellipsometric setup are discussed. The ellipsometer has been mounted on an electron cyclotron resonance plasma deposition chamber. As an example, the deposition of amorphous hydrogenated carbon layers has been studied in situ and are compared to ex situ measurements by variable angle of incidence spectroscopic ellipsometry of the same samples.

The effect of nitrogenation on the electrical properties of amorphous hydrogenated carbon layers

The electrical properties of nitrogen-containing amorphous hydrogenated carbon layers are investigated. The nitrogen concentration was between 0 and 6 at%. The electrical conductivity is found to increase with the nitrogen content. The temperature dependent conductivity in the temperature range between 150 and 350 K can be well fitted by a semi-empirically derived equation which considers the conductivity as a superposition of two hopping mechanisms with different activation energies. Together with experimental results on the optical properties and the mass density, these data allowed to propose a structural model which explains the observed effects in terms of familiar a-C:H cluster models. Die elektrischen Eigenschaften stickstoffhaltiger amorpher hydrierter Kohlenstoffschichten werden untersucht. Die Stickstoffkonzentration liegt zwischen 0 und 6 At%. Die elektrische Leitfahigkeit steigt mit dem Stickstoffgehalt. Die temperaturabhangige Leitfahigkeit im Temperaturbereich von 150 bis 350 K last sich gut mit einer semiempirisch erhaltenen Formel fitten, die die Leitfahigkeit der Schichten als Superposition zweier Hopping-Leitfahigkeitsmechanismen mit verschiedenen Aktivierungsenergien beschreibt. Im Zusammenhang mit experimentellen Ergebnissen aus optischen Untersuchungen und Massedichtebestimmungen erlauben die erhaltenen Daten, ein Strukturmodell aufzustellen, das die beobachteten Effekte auf der Basis bekannter a-C:H-Clustermodelle beschreibt.

The effect of nitrogenation on the IR optical constants of amorphous hydrogenated carbon layers

Nitrogenated amorphous carbon layers have been deposited by a dc plasma technique on silicon substrates. The optical constants of the film material have been determined from spectrophotometric measurements in the MIR/NIR spectral ranges. Special attention was paid to the parameters of the exponential absorption tail. The layers have been found to have IR refractive indices around 1.9. They are MIR transparent while showing considerable absorption in the NIR. Therefore, they may be of interest for the design of spectrally selective solar absorber coatings.

Optical performance of amorphous hydrogenated carbon layers: range of optical constants achievable

The optical properties of differently deposited amorphous carbon layers have been investigated by spectrophotometric means from the middle infrared up to the visible spectral region. The optical constants of the layers could be calculated by a numerical procedure that combines features of single-wavelength methods and multiwavelength methods. Depending on the deposition technique applied, the optical constants of the layers have been found to vary over a wide range. Thus, refractive index values could be established between 1.6 and 2.9, while IR-absorption coefficients vary between 20/cm and 40,000/cm. The optical performance of the layers is thus nonuniform and may be influenced by the selection of suitable deposition parameters. Due to the potentially wide range of optical constants of amorphous carbon, various possibilities exist for the application as an optical thin film material. Applications such as selective absorber coatings for solar physical purposes are discussed as well as mechanically and chemically resistant antireflection coatings on high-refractive index semiconducting materials (silicon, germanium).

Electrical and photoelectrical characteristics of heterostructures with amorphous carbon films

Abstract Electrical and photoelectrical characteristics of heterostructures fabricated by ion-plasma deposition of thin amorphous carbon films onto silicon substrates were studied. In particular, current-voltage, capacitance-voltage and charge transient characteristics were measured at different illumination intensities. A charge memory effect, resulting in a frequency dispersion of electrical and photoelectrical characteristics was observed, and its dependence on the structural parameters of the films were investigated. Multilayer structures were prepared using thin (less than or equal to 5 nm) amorphous hydrogenated carbon layers with two values of forbidden gap (mobility gap), the narrow gap layers being wells and the wider gap layers being barriers. Current-voltage and capacitance-voltage characteristics of these structures revealed some features which are related to the confinement of carriers in the potential wells and their resonant tunneling through barriers. Possible applications of the structures as photosensors and dynamical memory elements were demonstrated.

Optical and electrical properties of amorphous hydrogenated carbon layers deposited by dc plasma decomposition

The optical properties of hard dc plasma deposited amorphous carbon layers have been investigated from the middle IR up to the visible spectral region. Refractive indices and absorption coefficient behaviour have been determined by means of spectral transmittance and reflectance measurements. The layers show a transparency window in the middle IR in the wavenumber range 1700 cm −1 < v<2800 cm −1. The refractive indices in the “window” region are between 2.1 and 2.3 with weak dispersion. IR-absorption losses have been discussed in terms of a dispersion model, considering the absorption detected to be caused by a superposition of Lorentz-lines and an Urbach-part. Planar and transverse conductivities of the layers have been determined at room temperature. In addition, the conductivity of representative samples has been investigated in dependence of the temperature in the range of 80 K up to 350 K. A correlation between the thus-obtained activation energies for thermally-activated hopping in the band tails and the Urbach parameters of the layers could be established.