Graphite Target Research Articles

Photovoltaic characteristics of boron doped amorphous carbon films deposited by pulsed laser deposition using graphite target

This paper reports on the successful deposition of boron (B) doped carbon films (p-C(B)) and fabrication of p-C(B)/n-Si solar cells by pulsed laser deposition (PLD) technique at room temperature using graphite target. The B content in the film was determined by X-ray photoelectron spectroscopy (XPS) to be in the range of 0.2-1.7 atomic percentage. The photovoltaic values of the device, a maximum open circuit voltage, V oc = 250 m V and short circuit current density, J sc = 2.113 mA/cm 2 were obtained, when exposed to AM 1.5 illumination (100 mW/cm 2 , 25°C). The maximum energy conversion efficiency was found tentatively to be about, η = 0.2%, together with the fill factor, FF = 45%. In this paper, the dependence of the B content on electrical and optical properties of the p-C(B) films and the photovoltaic characteristic of the p-C(B)/n-Si structure photovoltaic solar cells are discussed.

Visible Raman spectroscopy of carbon films synthesized by ion-plasma sputtering of graphite

Abstract

Large electron screening effect in different environments

Electron screening effect was studied on different hydrogen containing targets with the 7 Li, 11 B and 19 F ion beams. Results show large electron screening potentials strongly dependent on the proton number Z of the projectile. The largest ever measured screening potential with the value about a factor of 50 above the calculations from the model in adiabatic limit was observed in the graphite target containing hydrogen as an impurity.

Photoluminescence Response in Carbon Films Deposited by Pulsed Laser Deposition onto GaAs Substrates at Low Vacuum

Carbon films were deposited onto GaAs substrates by pulsed laser deposition at low vacuum (10–15 mTorr) from a graphite target. Films were prepared at different number of pulses (1500 to 6000) with fixed fluence (32 J/cm2), target-to-substrate distance, and pulse frequency using a Q:Switched Nd:YAG laser at 1064 nm operating at a frequency of 10 Hz and producing burst-mode pulses with total duration per shot of 49 ns. Films were characterized by optical microscopy, atomic force microscopy, laser induced breakdown spectroscopy, X-ray diffraction, and photoluminescence spectroscopy. Deposited films were visually smooth and adherent but on the other hand evidence of splashing was observed in all the films. Thickness varied linearly with the number of pulses from 8 to 42 μm with maximum height differences around 700 nm. Hexagonal and orthorhombic carbon was found in all the films and there was no evidence of nitrogen or oxygen incorporation during ablation process. Broad photoluminescence bands were observed and, particularly, emission peaks at 475–480 nm, 540–550 nm, 590 nm, and 625 nm. Bands tend to shift to lower wavelength with film thickness, suggesting that luminescence comes from splashed nanostructures influenced by the semiconducting substrate. This particular substrate effect is vanished as thickness of the films increases.

Spectroscopy and Formation of Carbon Nitride by Pulse Laser Ablation in Liquid of Graphite Target

In this work carbon nitride was synthesized by pulsed laser ablation of graphite in ammonium solution. Fourier Transform Infrared Spectroscopy (FTIR), UV-Visible Spectrophotometer and Transmission Electron Microscopy (TEM) were used to study the bonding, absorption, size and morphology of the carbon nitride nanoparticles. The FTIR absorption peak at 2121.6, 1631.7, 1384 cm-1 stretching vibration bond, it is inferred that the C ≡ N, C = N and C-N, respectively. Bonds suggested the formation carbon nitride nanoparticles. UV absorption peaks coincide with the electronic transitions corresponding to formation carbon nitride C3N4 and monocyanopolyynes. The TEM image showed the aggregation of the carbon nitride nanoparticles with size ranges from (4 to 83.3 nm); the Leaf- like structure is shown in the structure of carbon nitride suspension.

Influence of C dopant on the structure, mechanical and tribological properties of r.f.-sputtered MoS2/a-C composite films

In present work, MoS2/a-C composite films were synthesized by r.f. sputtering of MoS2 and graphite targets, and the influence of C dopant on the structure, morphology, mechanical and the tribological properties of composite films in vacuum, ambient air and mutually alternated atmospheres were discussed in detail. Increasing the sputtering power applied on graphite target from 0 to 400W, the film hardness increases significantly by one order of magnitude, and the optimized lubricant performance were obtained at 300W. After sliding tests the morphology and structure of tribofilm and debris were investigated by using optical spectroscopy and Raman spectroscopy. The results reveal that the adaptive release of non-lubricant phase from tribofilm is a critical issue for determining the tribological performance of MoS2/a-C composite films in different sliding atmospheres. The more exhaustive release of non-lubricant phase, the lower friction of rubbed counterparts can be obtained. More interestingly, the lubricant mechanism of tribofilms formed in alternated atmospheres is partially affected by their initial composition established at the beginning of sliding.

Evaluation of different catalysts and development of a new catalyst for the reduction of [formula omitted] to graphite

We present an evaluation of different catalysts, methods and parameters for the catalytic reduction of CO2 for 14C-AMS graphite targets for radiocarbon measurement at the Physical Institute of the University Erlangen-Nuremberg in Germany. Currently 10μm iron powder is used as a catalyst to reduce carbon dioxide to graphite following our standard protocol. An optimal catalyst should provide a short reduction time, no additional fractionation effects, a good reduction yield, lack sintering and should not contain any amounts of both “dead” and “modern” carbon. Seven purchasable catalysts have been evaluated regarding this criteria. Further, the method using zinc and titanium hydride instead of hydrogen gas and the water-trap has been applied to our system and the effect of the water-trap temperature on the reduction time has been tested. A self-made catalyst was produced using aluminum oxide impregnated with iron nitrate. The results of this study indicate three good catalysts and show the success of the applied methods on standard samples like Ox II and Alfa graphite.

Fabrication of diamond-like carbon films using short-pulse HiPIMS

Abstract Diamond-like carbon (DLC) films were fabricated by sputtering a graphite target using a short-pulse-operated high-power impulse magnetron sputtering (HiPIMS) technology. The pulse duration was 7 μs with a maximum source voltage of − 2400 V, which efficiently facilitated the ionization of the sputtered carbon species. In order to show the effectiveness of the short pulse operation, the fabricated DLC films were investigated by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and spectroscopic ellipsometry. XPS spectra were curve-fitted using three Gaussian functions of sp3-C, sp2-C and C–O bonding. The information from the Raman and XPS spectra showed that the sp3 fraction in the DLC films fabricated using a pulse duration of 7 μs was larger than that obtained using a pulse duration of 50 μs. It was found that the sp3/sp2 ratio was larger than 40%, which was approximately 1.5-fold compared to that obtained using a pulse duration of 50 μs. A high sp3 fraction was realized due to application of high voltage without arc generation transferred from a magnetron sputtering (MS) glow discharge. To obtain the high sp3 fraction in the fabricated DLC films, a parametric survey was performed, in which the background gas pressure, bias voltage applied to the substrate, power-source voltage and the distance between the target and the substrate were varied. There were optimum deposition parameters for lowering the D-band and G-band intensity ratio (ID/IG) in Raman spectra. The optimum source voltage was in the range of − 1200 to − 1600 V. For voltages higher than − 1600 V, energetic carbon and argon ions may deteriorate the film properties. The hardness of the fabricated DLC films at 7-μs-pulse-width and − 1200 V source-voltage were obtained as 37 GPa measured with a nano-indenter. The hardness decreased to a range from 20 to 27 GPa with increasing the pulse width in range from 40 to 60 μs.

Molecular screening in nuclear reactions

The dependence of electron screening in nuclear reactions on projectile or target atomic number has been studied by bombarding different hydrogen-containing targets with beams of $^{7}\mathrm{Li}$, $^{11}\mathrm{B}$, and $^{19}\mathrm{F}$. The largest electron screening potentials were obtained in a graphite target containing hydrogen as an impurity. Some measured potentials are almost two orders of magnitude above the theoretical predictions. To explain the measurements, a new concept of electron screening is introduced.

Исследование нанокомпозитных пленок с матрицеобразующим углеродом методом зонда кельвина

Nanocomposite TiC/a-C and diamond-like carbon a-C films obtained by the vacuum ionplasma method have been studied by Kelvin Probe force microscopy. Films of thickness 200 nm were deposited on silicon wafers. The structure and composition of nanocomposite TiC/a-C films were varied by deposition conditions. It is shown that the average surface potential av SP V depends on the pulse frequency of graphite target sputtering during film deposition. The amount of sp bonded carbon on the film surface increases with scale up frequency. The av SP V of TiC/a-C nanocomposite films depends on the phase composition (Ti, TiC, a-C) and their ratio. The numerically calculated volume fractions of the phases in the TiC/a-C films correlate with av SP V .

Combustion kinetics of laser irradiated porous graphite from imaging Fourier transform spectroscopy

The combustion plumes arising from laser-irradiated graphite targets were investigated experimentally using hyperspectral, imaging Fourier transform spectroscopy (IFTS). Porous graphite targets were irradiated with a 1.07 µm, 20-kW ytterbium fiber laser at irradiances of 0.25–4 kW/cm2. Emissive plumes from the oxidation of graphite in air were monitored using a mid-wave infrared imaging Fourier-transform spectrometer with spatial resolution of 0.52 mm per pixel. Strong spectral emission of CO and CO2 were observed in the infrared between 1900 and 2400 cm−1 with an instrument spectral resolution of 2 cm−1. A homogeneous single-layer plume, line-by-line radiative transfer model (LBLRTM) and two band models (EM2C and RADCAL) were applied to estimate spatial maps of temperature and column densities of CO and CO2 with a temporal resolution of 0.47 s per hyperspectral image. Steady surface temperatures of 1800–2900 K are achieved after ∼1 min for irradiances of 0.25–1.0 kW/cm2. A stable, gas phase combustion layer extends from 4 to 12 mm from the surface, with buoyancy driving a gas flow of ∼8 m/s. Plume extent and intensity is greater for the larger porosity (6 mm particle size) samples. Steady-state gas temperatures exceed surface temperature by up to 400 K. Column densities for CO and CO2 of up to 1018 molec/cm2 were observed. The CO/CO2 concentration ratio peaks at 2500 K. The initial rise with temperature is consistent with effective activation energies of 149–111 kJ/mol at distances between 0.72 mm and 3 mm, respectively.

Positive and negative singly charged ion production of a laser induced plasma using a capillary graphite target.

A new type of laser ion source is being developed aiming at the production of positive and negative singly charged ions using a capillary graphite target structure. The initial results of the laser plasma produced inside of the 10 mm diameter conduit indicated the formation of the secondary charged particle production inside the target. A high speed camera clearly recorded the plasma plume expansion inside the target. The time-of-flight spectrum of the laser produced plasma in vacuum showed that the signal of the positive ions formed two peaks as the laser power density exceeded 10 GW/cm(2). The addition of neutral gas to the system produced a signal corresponding to negative ions after the positive signal.

Effect of nitrogen surrounding gas and plasma assistance on nitrogen incorporation in a-C:N films by femtosecond pulsed laser deposition

In the context of nitrogen-rich amorphous carbon thin films ultrafast pulsed laser deposition from graphite targets in inert nitrogen or nitrogen plasma ambient, this study assesses the correlation between the ablation plume composition and dynamics and the thin films contents and structures. The use of both optical emission spectroscopy and spectrally resolved 2D imaging, coupled with intensified CCD temporal resolution, allows to precisely follow such species of the plume as CN and C2 molecules, from their apparition to their deposition on the substrate. The results show that carbon–nitrogen bonding arises at the early time of expansion with little changes in quantity thereafter. The key role of the DC-bias is in lowering the molecular weight of the ambient gas, thus easing molecules way toward the target and interfering with the chemical reaction for CN generation. Depending on the ambient pressure, these processes will have drastically different effects on the thin films properties and contents. This work thus explains the origin of high nitrogen contents in a-C:N thin films obtained using DC-bias, and proposes an easy in situ optical observation-based way to predict and look for the best conditions to maximize those contents in future work.

Study on re-sputtering during CNx film deposition through spectroscopic diagnostics of plasma

A nitrogen-carbon plasma was generated during the deposition of carbon nitride (CNx) thin films by pulsed laser ablation of a graphite target in a discharge nitrogen plasma, and the optical emission of the generated nitrogen-carbon plasma was measured for the diagnostics of the plasma and the characterization of the process of CNx film deposition. The nitrogen-carbon plasma was recognized to contain various species including nitrogen molecules and molecular ions excited in the ambient N2 gas, carbon atoms and atomic ions ablated from the graphite target and CN radicals. The temporal evolution and spatial distribution of the CN emission and their dependence on the substrate bias voltage show two groups of CN radicals flying in opposite directions. One represents the CN radicals formed as the products of the reactions occurring in the nitrogen-carbon plasma, revealing the reactive deposition of CNx film due to the reactive expansion of the ablation carbon plasma in the discharge nitrogen plasma and the effective formation of gaseous CN radicals as precursors for CNx film growth. The other one represents the CN radicals re-sputtered from the growing CNx film by energetic plasma species, evidencing the re-sputtering of the growing film accompanying film growth. And, the re-sputtering presents ion-induced sputtering features.

Investigation of the spall strength of graphite using nano- and picosecond laser pulses

Spallation phenomena in graphite targets were investigated experimentally under nano- and picosecond shock-wave action at laser facilities “Kamerton-T” (GPI RAS) and PHELIX (GSI). In the range of strain rates of 1 to 10 μs−1 at the first time, data of dynamic tensile strength of the material were obtained. At maximal realized strain rate of 14 μs−1, the spall strength value 2.1 GPa has been achieved that is 64% of the theoretical ultimate tensile strength of the graphite. Spallation was observed not only on the backside of the target, but also on its front (irradiated) surface. The morphology of the front and rear surfaces of the targets was studied using the optical and scanning electron microscopy. The structure of the graphite in irradiated area on the facial side as well as in the spallation zone on the rear side of the target was investigated by Raman scattering method. A comparison of the dynamic strength of the graphite with the dynamic strength of a synthetic diamond is done.

INVITED] Control of femtosecond pulsed laser ablation and deposition by temporal pulse shaping

This study explores the effects of temporal laser pulse shaping on femtosecond pulsed laser deposition (PLD). The potential of laser pulses temporally tailored on ultrafast time scales is used to control the expansion and the excitation degree of ablation products including atomic species and nanoparticles. The ablation plume generated by temporally shaped femtosecond pulsed laser ablation of aluminum and graphite targets is studied by in situ optical diagnostic methods. Taking advantage of automated pulse shaping techniques, an adaptive procedure based on spectroscopic feedback regulates the irradiance for the enhancement of typical plasma features.Thin films elaborated by unshaped femtosecond laser pulses and by optimized sequence indicate that the nanoparticles generation efficiency is strongly influenced by the temporal shaping of the laser irradiation. The ablation processes leading either to the generation of the nanoparticles either to the formation of plasma can be favored by using a temporal shaping of the laser pulse. Insights are given on the possibility to control the quantity of the nanoparticles.The temporal laser pulse shaping is shown also to strongly modify the laser-induced plasma contents and kinetics for graphite ablation. Temporal pulse shaping proves its capability to reduce the number of slow radicals while increasing the proportion of monomers, with the addition of ionized species in front of the plume. This modification of the composition and kinetics of plumes in graphite ablation using temporal laser pulse shaping is discussed in terms of modification of the structural properties of deposited Diamond-Like Carbon films (DLC). This gives rise to a better understanding of the growth processes involved in femtosecond-PLD and picosecond-PLD of DLC suggesting the importance of neutral C atoms, which are responsible for the subplantation process.

Hydrogenation effects on carrier transport in boron-doped ultrananocrystalline diamond/amorphous carbon films prepared by coaxial arc plasma deposition

Boron-doped ultrananocrystalline diamond/hydrogenated amorphous carbon composite (UNCD/a-C:H) films were deposited by coaxial arc plasma deposition with a boron-blended graphite target at a base pressure of <10−3 Pa and at hydrogen pressures of ≤53.3 Pa. The hydrogenation effects on the electrical properties of the films were investigated in terms of chemical bonding. Hydrogen-scattering spectrometry showed that the maximum hydrogen content was 35 at. % for the film produced at 53.3-Pa hydrogen pressure. The Fourier-transform infrared spectra showed strong absorptions by sp3 C–H bonds, which were specific to the UNCD/a-C:H, and can be attributed to hydrogen atoms terminating the dangling bonds at ultrananocrystalline diamond grain boundaries. Temperature-dependence of the electrical conductivity showed that the films changed from semimetallic to semiconducting with increasing hydrogen pressure, i.e., with enhanced hydrogenation, probably due to hydrogenation suppressing the formation of graphitic bonds, which are a source of carriers. Carrier transport in semiconducting hydrogenated films can be explained by a variable-range hopping model. The rectifying action of heterojunctions comprising the hydrogenated films and n-type Si substrates implies carrier transport in tunneling.

Nanocrystalline diamond films prepared by pulsed electron beam ablation on different substrates

Abstract

Influence of composition on the wear properties of boron carbonitride (BCN) coatings deposited by high power impulse magnetron sputtering

Abstract We investigate boron-carbon-nitride (BCN) coatings deposited with high power impulse magnetron sputtering (HiPIMS) technology and conventional pulsed DC sputtering for their application as wear resistant coatings. Especially for BCN coatings, HiPIMS qualifies as a promising deposition technology as the short pulses with very high power density result in a high ionization degree of the plasma species, allowing a manipulation of the film structure and properties. Both, carbon and boron nitride can hybridize in sp2 or sp3 configuration or a mixture of both, so that coating features such as hardness and coefficient of friction can be tailored to meet the requirements of special applications. We studied the influence of different carbon sources and deposition modes on the composition and tribological properties of BCN films, including microhardness, friction coefficient, and thermal stability. Additionally, we investigated thermal degradation mechanisms by interpreting X-ray photoelectron spectra (XPS). We observed that the application of either pulsed DC or HiPIMS pulse mode has a significant influence on the microhardness and thermal stability of the coatings, in which HiPIMS mode generally provides higher hardness and better thermal stability. Furthermore, we found that samples co-sputtered from B4C and graphite target show superior hardness and thermal stability compared to those sputtered reactively with acetylene.

Characterization of physical and biomedical properties of nitrogenated diamond-like carbon films coated on polytetrafluoroethylene substrates

A technique to coat hydrogen-free diamond-like carbon (DLC) films on polytetrafluoroethylene (PTFE) substrates has been developed by sputtering of a negatively biased graphite target in a mixture of argon and nitrogen plasma. The coated films were characterized by various methods to investigate their chemical, electronic features, and particularly their biomedical properties. DLC films produced by this method have up to 20% sp3 carbon bonds depending on the nitrogen concentration in the plasma. Raman spectroscopy revealed that, bond-disorder increases with nitrogen doping. The average grain size of DLC decreases in the nitrogen doped samples by almost 30%. The roughness of the uncoated PTFE substrate surfaces decreased dramatically from 660nm to 170nm after DLC coating. However, the nitrogen contents in the plasma have little effects on the roughness, the cluster size, and shapes. Electronic band gap of the samples decreases with adding nitrogen from ~2eV in nitrogen-free samples to ~1eV in nitrogenated samples. Lower adhesion and aggregation of platelets on PTFE surfaces coated with DLC-10% nitrogen and DLC-20% nitrogen have been observed while there is greater adhesion of platelets on DLC-30% nitrogen and DLC-40% nitrogen.