Thermal stability and surface modifications of detonation diamond nanoparticles studied with X-ray photoelectron spectroscopy

Detonation nanodiamond (ND) particles were dispersed on silicon nitride (SiN_x) coated sc-Si substrates by spin-coating technique. Their surface density was in the 10¹⁰–10¹¹ cm^?2 range. Thermal stability and surface modifications of ND particles were studied by combined use of X-ray Photoelectron Spectroscopy (XPS) and Field Emission Gun Scanning Electron Microscopy (FEG SEM). Different oxygen-containing functional groups could be identified by XPS and their evolution versus UHV annealing temperature (400–1085 °C) could be monitored in situ. The increase of annealing temperature led to a decrease of oxygen bound to carbon. In particular, functional groups where carbon was bound to oxygen via one σ bond (C–OH, C–O–C) started decomposing first. At 970 °C carbon–oxygen components decreased further. However, the sp²/sp³ carbon ratio did not increase, thus confirming that the graphitization of ND requires higher temperatures. XPS analyses also revealed that no interaction of ND particles with the silicon nitride substrate occurred at temperatures up to about 1000 °C. However, at 1050 °C silicon nitride coated substrates started showing patch-like damaged areas attributable to interaction of silicon nitride with the underlying substrate. Nevertheless ND particles were preserved in undamaged areas, with surface densities exceeding 10¹⁰ cm^?2. These nanoparticles acted as sp³-carbon seeds in a subsequent 15 min Chemical Vapour Deposition run that allowed growing a 60–80 nm diamond film. Our previous study on Si(100) showed that detonation ND particles reacted with silicon between 800 and 900 °C and, as a consequence, no diamond film could be grown after Chemical Vapour Deposition (CVD). These findings demonstrated that the use of a thin silicon nitride buffer layer is preferable insofar as the growth of thin diamond films on silicon devices via nanoseeding is concerned.     

An efficient purification method for detonation nanodiamonds

This article reports the purification process of detonation soot to obtain pure nanodiamond powder. Nanodiamonds are synthesized by detonation using a high explosive mixture composed of trinitrotoluene and hexogen. The detonation of the charge leads to a powder containing nanodiamonds as well as metallic impurities and sp² carbon species. Further, to remove metallic particles, an unusual acidic treatment (hydrofluoric/nitric acids; i.e. fluorinated aqua regia) was set up. To eliminate sp² carbon species such as graphite and amorphous carbon, a thermal oxidation treatment was performed at 420 °C under air in a furnace during several hours. Transmission Electronic Microscopy, Raman spectroscopy, X-ray diffraction and Thermo-Gravimetric Analysis showed that this purification process is very efficient. From TGA measurements, a model of the carbon grain combustion was developed by considering graphitic shells surrounding the nanodiamond particles, and was used to demonstrate that the selective oxidation of graphite was experimentally realistic. Moreover, another model was set up from specific area measurements to evaluate the thickness of the functional groups surrounding the nanodiamonds after the oxidation of sp² carbonaceous species. The treatment described herein was achieved on several tens of grams of product and could be easily adapted to the industrial scale.     

Synthesis and electron field emission properties of nanodiamond films

Effects of CH₄/H₂ ratio and bias voltage of the microwave plasma-enhanced chemical vapor deposition (MPE-CVD) process on the nucleation behavior and associated characteristics of nanodiamonds were investigated. While the scanning electron microscopy (SEM) microstructure and Raman crystal structure of the films insignificantly vary with CH₄/H₂ ratio and bias voltage, electron field emission properties of the materials markedly change with these deposition parameters. The predominating factor modifying the electron field emission properties of the nanodiamond films is presumed to be the increase in the proportion of sp²-bonded grain boundaries when the grain size of the nanodiamond films decreases. Between these two major factors, the bias voltage shows more prominent effects on modifying the granular structure of the nanodiamonds than the CH₄/H₂ ratio does. The best electron field emission properties attainable are J_e=500 μA/cm² at 20 V/μm and E₀=8.5 V/μm.     

Clean graphene surface through high temperature annealing

The cleanliness of the surface of graphene is important for its proper functioning in devices and sensors. Impurities including residual poly(methyl methacrylate) (PMMA) and hydrocarbon contaminants can alter its electronic and chemical properties. In this study, we used two surface-sensitive techniques, X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS), to monitor the chemical composition of the surface of graphene after washing it with acetone and annealing at high temperatures. The concentration of residual PMMA and hydrocarbon contaminants decreased as the annealing temperature increased. The atomic ratio of sp³ carbons to sp² carbons of a clean graphene surface determined using XPS can be used to estimate the amounts of sp³ defects in graphene. ToF-SIMS spectra indicate that residual PMMA was removed from the surface of graphene at 400 °C, while hydrocarbon contaminants required a higher temperature of 500 °C to remove. In ToF-SIMS spectra obtained at 500 °C, the characteristic ions for graphene, which are related to cleavage of ring structure, include C_x⁺ (x = 1, 2, 3…), C_xH⁺ and C_xH₂⁺ as well as C_x⁻ and C_xH⁻. For the first time, we developed a process to produce a very clean graphene surface which was verified by ToF-SIMS and XPS analyses.     

Substrate temperature effects on the microhardness and adhesion of diamond-like thin films

Diamond-like films were deposited on silicon substrates by r.f. plasma-enhanced chemical vapor deposition from gas methane. In this study, the substrate temperature, T_S, was varied in a wide range from 20 to 370°C while maintaining fixed other important process parameters such as r.f. power (70 W) or pressure (2.5 Pa). The increase of T_S causes an increase of the sp²/(sp²+sp³) bonded carbon ratio and a decrease of the hydrogen content. These changes produce a great modification of the mechanical properties: microhardness, friction coefficient and adhesion. The variations of mechanical properties with T_S correlate well with the sp²/(sp²+sp³) bonded carbon ratio and the hydrogen content in the films showing a gradual transformation of the diamond-like structure into a more sp²-rich one.     

Microstructure and optical band gap control of DLC film deposited by pulsed discharge plasma CVD

DLC films were deposited on silicon and quartz glass substrates by pulsed discharge plasma chemical vapor deposition (CVD), where the plasma was generated by pulsed DC discharge in H₂–CH₄ gas mixture at about 90 Torr in pressure, and the substrates were located near the plasma. The repetition frequency and duty ratio of the pulse were 800 Hz and 20%, respectively. When CH₄ / (CH₄ + H₂) ratio, i.e. methane concentration (Cm), increased from 3 to 40%, C₂ species in the plasma was increased, and corresponding to the increase of C₂, deposition rate of the film was increased from about 0.2 to 2.4 μm/h. The absorption peaks of sp³C–H and sp²C–H structures were observed in the FT-IR spectra, and the peak of sp²C–H structure was increased with increasing Cm, showing that sp² to sp³ bonding ratio was increased when Cm was increased. Corresponding to these structural changes due to the increase of Cm, optical band gap (Eg) was decreased from 3 to 0.5 eV continuously when Cm was increased from 3 to 40%.     

The effect of CNT content on the surface and mechanical properties of CNTs doped diamond like carbon films

The carbon nanotubes (CNTs) doped diamond like carbon films were carried out by spinning coating multi-walled carbon nanotubes (CNTs) on silicon covered with diamond like carbon films via PECVD with C₂H₂ and H₂. The results show that the I_D/I_G and sp²/sp³ ratios are proportional to the CNT contents. For wettability and hydrogen content, the increase of CNT content results in more hydrophobic and less hydrogen for CNT doped DLC films. As for mechanical properties, the hardness and elastic modulus increases linearly with increasing CNT content. The residual stress is reduced for increasing CNT content. As for the surface property, the friction coefficient is reduced for higher CNT content. For CNT doped DLC films, the inclusion of horizontal CNT into DLC films increases the hardness, elastic modulus and reduces the hydrogen content, friction coefficient and residual stress. Like the light element and metal doping, the CNT doping has effects on the surface and mechanical properties on DLC which might be useful to specific application.     

Removal of Boron from Molten Si and Si-Cu Using Ammonia Containing Gas

All the reported thermodynamics analysis implied that ammonia is a promising reagent for removing boron from silicon, but efforts to employ it in silicon refining have failed in practice. As such, there are few reports detailing this process. In this study, this process was analyzed experimentally. Various concentrations of ammonia were introduced to remove boron from silicon in the temperature range of 1200–1550 ^°C with a total pressure of 1 atm. Boron containing nitrides precipitates were detected in the furnace tube. The effect of ammonia content in the feeding gas was explored. It implied that higher ammonia partial pressures promote the boron removal. The experimental results have suggested that ammonia could remove boron from silicon in the form of volatiles, such as BH_x (x = 1, 2, 3) and B₃ H ₆ N ₃, in practice. The reaction-rate constant was limited to 10^?6– 10^?7 m/s in pure ammonia at 1450 ^°C. Moreover, a higher ammonia flow rate resulted in lower boron removal ratio. It was indicated that the rate determining steps of boron removal and silicon loss in this process were the chemical reaction at the surface of the melt and the transport of ammonia from gas phase to the surface, respectively. The relationship of the boron-removal rate with temperature followed a “V”-shaped curve, which implied the limit of thermodynamic factors at high temperature and the limit of kinetic factors at temperatures lower than 1300 ^°C. Based on the analysis results, temperature-programed reaction was designed to promote the boron-removal efficiency doubled. Cu was used to decrease the liquidus temperature of Si based alloy in the process. As a result, more than 80% boron in Si-Cu alloy could be removed.     

Structural and mechanical properties of facing-target sputtered amorphous CNx films

Structural and mechanical properties of carbon nitride films, deposited using a DC facing-target reactive sputtering system at various N₂ fractions (P_N) in the gas mixture, were studied systematically. XPS analyses indicate that N concentration is not directly proportional to P_N, and it rises quickly to a saturation value of ∼ 33 at.% at a P_N of 20%. The ratio of N–C(sp²)/N–C(sp³) increases with the rise of P_N from 0% to 20%, and then decreases with further rising P_N. However, the number and size of disordered sp²-hybridized C clusters continue to increase over the whole range of P_N, which is consistent with the Raman and high-resolution transmission electron microscopy measurements. Nanoindenter measurements show that the hardness of the films continuously decreases from ∼ 17.5 to ∼ 5.6 GPa with the increasing P_N from 0% to 100%, due to the conversion from sp³ C to sp² C and the clustering of sp² C structure.     

Influence of the boron doping level on the electrochemical oxidation of the azo dyes at Si/BDD thin film electrodes

In this study the efficiency of electrochemical oxidation of aromatic pollutants, such as reactive dyes, at boron-doped diamond on silicon (Si/BDD) electrodes was investigated. The level of [B]/[C] ratio which is effective for the degradation and mineralization of selected aromatic pollutants, and the impact of [B]/[C] ratio on the crystalline structure, layer conductivity and relative sp³/sp² coefficient of a BDD electrode were also studied. The thin film microcrystalline electrodes have been deposited on highly doped silicon substrates via MW PE CVD. Si/BDD electrodes were synthesized for different [B]/[C] ratios of the gas phase. Mechanical and chemical stability of the electrodes was achieved for the microcrystalline layer with relatively high sp³/sp² band ratio. Layer morphology and crystallite size distribution were analyzed by SEM. The resistivity of BDD electrodes was studied using four-point probe measurements. The relative sp³/sp² band ratios were determined by deconvolution of Raman and X-ray photoelectron spectra. The efficiency of degradation and mineralization of the reactive azo dye rubin F-2B was estimated based on the absorbance measurements at 545 nm. The influence of commonly used electrolytes NaCl and Na₂SO₄ on the dye removal efficiency was also investigated. The results suggest that, in general, the oxidation occurs indirectly at the anode through generation of hydroxyl radicals •OH, which react with the dye in a very fast and non-selective manner. In NaCl electrolyte the dye was also decomposed by more selective, active chlorine species (Cl₂, HOCl). However the efficiency of this process in BDD depended on the electrode's doping level. Higher amounts of dopant on the surface of BDD resulted in the higher efficiency of dye removal in both electrolytes.     

Inhibiting the sp2 carbon deposition by adjunction of sulphurous species in refractory ceramics subjected to CO and H2 reducing atmosphere

The formation of sp² carbon by the Boudouard reaction significantly damages the refractory ceramics. Sulphur is an efficient way to prevent the carbon deposition catalysed by Fe₃C, in the presence of H₂. Thermogravimetric analysis was carried out on Fe₂O₃ samples exposed to a CO/H₂ gas mixture at 600 °C. Solid sulphur was mixed with Fe₂O₃ powder or continually added in the form of gas into the CO/H₂ reducing gas. The samples were characterised by X-ray diffraction, Raman spectroscopy, SEM and TEM. The addition of 100 ppm of sulphur species in the gas prevents the formation of carbon. The mechanism that governs the inhibition of the reaction is proposed, in which the formation of a thin protective FeS layer (0.5–1 nm) is involved. This study paves the way to an effective solution to inhibit the sp² carbon deposition in the refractories by poisoning the Fe₃C catalyst with sulphur.     

Fabrication of nitrogen-incorporated nanodiamond ultra-microelectrode array for Dopamine detection

Well established silicon microfabrication technology and PECVD nanodiamond growth process enabled us to fabricate an ultra-microelectrode array (UMEA) for biosensing applications. The UMEA consists of 2500 nanodiamond elements in a square array surrounded by a layer of thermally grown SiO₂ on a highly doped silicon substrate. Fe(CN)₆^3?/4? redox couple was used for electrochemical characterization of the UMEA using cyclic voltammetry and gave us a steady state response consistent with hemispherical diffusion limited mass transport mechanism. Using the nanodiamond UMEA, we were also able to detect different concentrations of Dopamine in phosphate buffered saline (pH 7.4) without any surface functionalization. The cyclic voltammograms show a steady state response and a linear relationship between the limiting current and Dopamine concentration.     

Bacterial adhesion on silicon-doped diamond-like carbon films

In this paper the surface properties of silicon-doped diamond-like carbon films with various Si contents on 316 stainless steel substrate by a magnetron sputtering technique were investigated. X-ray photoelectron spectroscopy was applied to determine the surface chemical composition of the films. Atomic force microscopy was used for the determination of surface roughness and topography. The sp² contents in the films were determined with Auger electron spectroscopy, which were 67.1%, 34.2% and 25.0% for silicon contents 1%, 2% and 3.8%. The sp³/sp² ratio increases with increasing the silicon contents in the films. Contact angles of three test liquids on the films were obtained with a Dataphysics OCA-20 contact angle analyzer. Surface free energies of the films and their dispersive and polar components were calculated using van Oss acid–base approach. Staphylococcus aureus was used for bacterial adhesion test. The experimental results showed that bacterial adhesion decreased with increasing the silicon content or with increasing sp³/sp² ratio in the films.     

Deposition of hydrogenated amorphous carbon films with enhanced sp3-C bonding on nanocrystalline palladium interlayer

The present study deals with the deposition of hydrogenated amorphous carbon (a-C:H) films on Si (100) substrates with and without an interlayer of nanocrystalline palladium (nc-Pd) on them, by high-voltage electro-dissociation of N,N-dimethyl formamide (DMF). Significant improvement in the sp³ carbon content has been observed for a-C:H films grown on nc-Pd interlayer as revealed by Fourier transformed infrared (FTIR), Raman, X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) spectroscopic techniques. It is inferred that H₂ activation on palladium sites leads to the stabilization of sp³-C bonding, thereby improving the quality of the deposits grown on them.     

Correlation of film structure and molecular oxygen reduction at nitrogen doped amorphous carbon thin film electrochemical electrodes

Nitrogen doped amorphous carbon (a-C:N) thin film electrodes with a range of film structures have been deposited using a filtered cathodic vacuum arc system. The correlation between film structure and electro-reduction of molecular oxygen in aqueous media at the electrodes has been explored. In aqueous 0.1 M NaOH, dioxygen reduction is inhibited at all the a-C:N electrodes compared with that at glassy carbon electrodes. The potential of the dioxygen reduction current peak shifts negatively at a-C:N electrodes as the sp³ C fraction in the a-C:N materials increases, while the current peak height decreases simultaneously. The a-C:N electrodes possess high sensitivity for investigating the mechanism of dioxygen reduction. It was found that the catalytic H₂O₂ reduction to H₂O on carbon materials is attributed to oxygen species at sp² C sites.     

Synthesis of 9H-carbazole-9-carbothioic methacrylic thioanhydride,electropolymerization, characterization and supercapacitor applications

A novel organic molecule of 9H-carbazole-9-carbothioic methacrylic thioanhydride (CzCS₂metac) was synthesized by incorporating CS₂ and methacrylate groups into the carbazole monomer structure. CzCS₂metac was characterized by FTIR, ¹H-NMR and ¹³C-NMR spectroscopy. CzCS₂metac was electropolymerized in 0.1 M tetraethylammonium tetrafluoroborate (TEABF₄)/acetonitrile (CH₃CN) on glassy carbon electrode (GCE). The characterization of the electrocoated P(CzCS₂metac)/CFME thin film was studied by various techniques, such as cyclic voltammetry, scanning electron microscopy–energy-dispersive X-ray analysis and electrochemical impedance spectroscopy. The specific capacitance (C _sp) of P(CzCS₂metac)/MWCNT/GCE in the scan rate of 20 mV s^?1 (C _sp = 38.48 F g^?1 from area formula, C _sp = 38.52 F g^?1 from charge formula) was increased ~15.66 and ~15.64 times in area and charge formulas compared to P(CzCS₂metac)/GCE (C _sp = 2.46 F g^?1 from area and charge formulas). The same results were also obtained from Nyquist graphs. The specific capacitance value of composite film (C _sp = 1.09 × 10^?3 F) is ~15.66 times higher than the polymer film (C _sp = 6.92 × 10^?5 F). The composite film may be used as supercapacitor electrode material in energy storage devices.     

Evaluation of citric acid added cleaning solution for removal of metallic contaminants on Si wafer surface

We have investigated cleaning solutions based on citric acid (CA) to remove metallic contaminants from the silicon wafer surface. Silicon wafers were intentionally contaminated with Fe, Ca, Zn, Na, Al and Cu standard solution by spin coating method and cleaned in various CA-added cleaning solutions. The concentration of metallic contaminants on the silicon wafer surface before and after cleaning was analyzed by vapor phase decomposition/inductively coupled plasma-mass spectrometry (VPD/ICP-MS). And the surface micro-roughness was also measured by atomic force microscopy (AFM) to evaluate the effect of cleaning solutions. It was found that acidic CA/H₂O solution has the ability to remove metallic contaminants from silicon surfaces. Fe, Ca, Zn and Na on silicon surface were decreased from the order of 10¹² atoms/cm² to the order of 10⁹ atoms/cm² even at low CA concentration, low temperature of CA solution and with short immersion time. CA was also effective in alkali cleaning solution. Fe, Ca, Zn, Na and Cu were reduced down to the order of 10⁹ atoms/cm² in CA added with NH₄OH/H₂O₂/H₂O solution without degradation of surface micro-roughness.     

Nanodiamond synthesis by pulsed laser ablation in liquids

Pulsed laser ablation in liquids (PLAL) has become an attractive method for the synthesis of nanodiamond. This work deals with the growth kinetics study of structures of nano-diamonds embedded in sp² carbon synthetized by this method. The plasma created by the laser pulse has been monitored by time resolved spectroscopy to analyze the evolution of the plume and therefore the transient species created. Typical C₂ vibrational bands appear, as well as a continuous spectrum due to various phenomena. The study of both the background and the vibrational features gives information on the reaction kinetics and on the plasma density. The presence of nanodiamonds has been confirmed by Raman spectroscopy as well as TEM analysis.     

Possibility of graphene growth by close space sublimation

Carbon films on the Si/SiO₂ substrate are fabricated using modified method of close space sublimation at atmospheric pressure. The film properties have been characterized by micro-Raman and X-ray photoelectron spectroscopy and monochromatic ellipsometry methods. Ellipsometrical measurements demonstrated an increase of the silicon oxide film thickness in the course of manufacturing process. The XPS survey spectra of the as-prepared samples indicate that the main elements in the near-surface region are carbon, silicon, and oxygen. The narrow-scan spectra of C1s, Si2p, O1s regions indicate that silicon and oxygen are mainly in the SiO _x (x ≈ 2) oxide form, whereas the main component of C1s spectrum at 284.4 eV comes from the sp²-hybridized carbon phase. Micro-Raman spectra confirmed the formation of graphene films with the number of layers that depended on the distance between the graphite source and substrate.     

Vibrational studies of microcrystalline diamond and diamond-like carbon by high resolution electron energy loss spectroscopy

High resolution electron energy loss spectroscopy (HREELS) has been applied to investigate the vibrational states of microcrystalline diamond and diamond-like carbon (DLC) films prepared in a low pressure inductively coupled plasma. The CO additive to a CH₄/H₂ plasma exhibits different phonon density of states. Without CO additive, the HREELS spectrum shows a faint peak at ∼1500 cm⁻¹ due to CC stretching vibration of sp² bonds, indicating that the sample is mainly composed of DLC. On the other hand, the HREELS profiles show a peak at ∼1100 cm⁻¹ assigned to CC stretching vibration of sp³ sites with CO additive. The intensity of the peak becomes strong and a shoulder centered at ∼700 cm⁻¹ corresponding to the bending vibration of sp³ bonded carbons appears with increasing CO additive. It consequently implies that the CO additive brings about the decrease of the fraction of sp² bonded carbons in the resultant films, and it is qualitatively in agreement with the previous characterizations by Raman spectroscopy, transmission electron microscopy, and reflection high energy electron diffraction.