Diamond nucleation and growth on zeolites

In this work, we report the use of zeolites as substrates for the deposition of porous diamond films. Films were deposited in a hot-filament chemical vapor deposition (HFCVD) apparatus. The HFCVD system was fed with a mixture of methane (0.8%) with the balance being hydrogen. A series of depositions were done in the pressure range 20–120 Torr and at substrate temperature 880 °C. The morphologies of the as-deposited films were analyzed by scanning electron microscopy and show isolated diamond grains in the initial nucleation stages, which develop into a microporous film in the next stage and form a continuous film after long time deposition. Raman spectroscopy was used to investigate the crystal morphology, structure and non-diamond impurities in the films deposited at various growth conditions. The nature of the hydrogen bonding with sp³ and sp² network and the quantitative analysis were done by Fourier transform infrared spectroscopy.     

Transmission electron microscopy study of diamond nucleation and growth on smooth silicon surfaces coated with a thin amorphous carbon film

Amorphous carbon (a-C) films with high contents of tetrahedral carbon bonding (sp³) were synthesized on smooth Si(100) surfaces by cathodic arc deposition. Before diamond growth, the a-C films were pretreated with a low-temperature methane-rich hydrogen plasma in a microwave plasma-enhanced chemical vapor deposition system. The evolution of the morphology and microstructure of the a-C films during the pretreatment and subsequent diamond nucleation and initial growth stages was investigated by high-resolution transmission electron microscopy (TEM). Carbon-rich clusters with a density of ∼10¹⁰ cm⁻² were found on pretreated a-C film surfaces. The clusters comprised an a-C phase rich in sp³ carbon bonds with a high density of randomly oriented nanocrystallites and exhibited a high etching resistance to hydrogen plasma. Selected area diffraction patterns and associated dark-field TEM images of the residual clusters revealed diamond fingerprints in the nanocrystallites, which played the role of diamond nucleation sites. The presence of non-diamond fingerprints indicated the formation of Si–C-rich species at C/Si interfaces. The predominantly spherulitic growth of the clusters without apparent changes in density yielded numerous high surface free energy diamond nucleation sites. The rapid evolution of crystallographic facets in the clusters observed under diamond growth conditions suggested that the enhancement of diamond nucleation and growth resulted from the existing nanocrystallites and the crystallization of the a-C phase caused by the stabilization of sp³ carbon bonds by atomic hydrogen. The significant increase of the diamond nucleation density and growth is interpreted in terms of a simple three-step process which is in accord with the experimental observations.     

Molecular dynamics simulation of shear-induced graphitization of amorphous carbon films

The shear-induced graphitization of amorphous carbon (a-C) films in sliding contact with a diamond counterface is investigated by molecular dynamics (MD) simulations. The gradual formation of a graphene-like sp² dominant layer on the a-C film surface is observed after steady-state sliding has been achieved, which provides direct evidence for the experimental observations of friction induced graphitization of a-C film. After the graphitized layer is formed, the relative sliding occurs between the graphitized atomic layers. During the shearing process, the biaxial stress in the graphitized layer experiences a transition from highly compressive (42 GPa) to tensile (−3 GPa). It is the relaxation of the local biaxial stress that leads to the sp³-to-sp² structural transformation.     

Electroless oxidation of boron-doped diamond surfaces: comparison between four oxidizing agents; Ce, MnO4, H2O2 and S2O8

The electrochemical properties of diamond are very sensitive to the surface terminations. It is still a challenge to successfully produce well-defined “C-O” functions. In this paper, we describe and compare the oxidation of as-grown polycrystalline boron-doped diamond (BDD) films using four different oxidizing agents in aqueous media: Ce⁴⁺, MnO₄⁻, H₂O₂ and S₂O₈²⁻. The different treatments lead to the formation of oxygenated functions at the diamond surface, mainly singly oxidized “C-O” groups such as “C-OH” or “C-O-C”. Processes with Ce⁴⁺ and MnO₄⁻ seem to be particularly interesting as they both lead to the creation of a high amount of oxygenated functions and an improvement of the charge transfer at BDD surfaces.     

Improved surface coverage of an optical fibre with nanocrystalline diamond by the application of dip-coating seeding

Growth processes of diamond thin films on the fused silica optical fibres (10 cm in length) were investigated at various temperatures. Fused silica pre-treatment by dip-coating in a dispersion consisting of detonation nanodiamond (DND) in dimethyl sulfoxide (DMSO) with polyvinyl alcohol (PVA) was applied. Nanocrystalline diamond (NCD) films were deposited on the fibres using the microwave plasma assisted chemical vapour deposition (MW PA CVD) method. The longitudinal variation of NCD morphology, structure and optical parameters were specifically investigated. The evolution of the film morphology and film thickness along the fibre length was studied using scanning electron microscopy (SEM). The chemical composition of the NCD film was examined with micro-Raman Spectroscopy. The sp³/sp² band ratio was calculated using the Raman spectra deconvolution method. An approximately 5 cm-long homogeneous diamond film has been obtained on the surface of the fibre sample. Thickness, roughness and optical properties of NCD films in the VIS–NIR range were investigated on the reference quartz slides using spectroscopic ellipsometry. The samples exhibited relatively low deviations of refractive index (2.3 ± 0.25) and extinction coefficient (0.05 ± 0.02) along the length of 5 cm, as estimated at a wavelength of 550 nm. In order to show the effectiveness of deposition process on optical fibres, diamond films were also grown on the fibre with induced long-period grating (LPG). The results of transmission measurements demonstrated that an LPG with diamond overlay exhibits the appropriate dependency on the optical properties of external medium. Thus, the deposition process has a negligible effect on the fibre transmission properties.     

Influence of nucleation density on film quality,growth rate and morphology of thick CVD diamond films

The optimum growth parameters of our 5 kW microwave plasma CVD reactor were obtained using CH₄/H₂/O₂ plasma and high quality transparent films can be produced reproducibly. Among the films prepared in this system, the film of best quality has very smooth crystalline facets free of second nucleation and the full width at half maximum (FWHM) of the diamond Raman peak is 2.2 cm⁻¹, as narrow as that of IIa natural diamond. For this study, diamond films were grown on silicon substrates with low (10⁴–10⁵ cm⁻²) and high nucleation densities (>10¹⁰ cm⁻²), respectively. From the same growth run, a highly 〈110〉 textured 300 μm thick white diamond film with a growth rate of 2.4 μm/h was obtained from high nucleation densities (>10¹⁰ cm⁻²), and a white diamond film of 370 μm in thickness with a higher growth rate of 3 μm/h was obtained from low nucleation densities (5×10⁴–10⁵ cm⁻²) too. The effect of nucleation density on film quality, growth rate, texture and morphology was studied and the mechanism was discussed. Our results suggest that under suitable growth conditions, nucleation density has little effect on film quality and low nucleation density results in higher growth rate than high nucleation density due to less intense grain growth competition.     

Structure of the silver containing diamond like carbon films: Study by multiwavelength Raman spectroscopy and XRD

In the present study structure of silver containing diamond like carbon (DLC:Ag) films deposited by reactive magnetron sputtering was investigated by X-ray diffractometry (XRD) and multiwavelength Raman spectroscopy. In the case of the DLC:Ag films containing low amount of silver, crystalline silver oxide prevails over silver. While at higher Ag atomic concentrations formation of the silver crystallites of the different orientations was observed. Surface enhanced Raman scattering (SERS) effect was detected for high Ag content in the films. For UV excited Raman spectra sp³ bonded carbon related Raman scattering T peak at ~ 1060 cm^− 1 was detected only for the films with the highest amount of silver (34.3 at.%). The dependence of the Raman scattering spectra parameters such as position of the G peak, G peak full width at half maximum (FWHM(G)), D/G peak area ratio on Ag atomic concentration in DLC:Ag film as well as Raman scattering spectra excitation wavelength were studied. The dependence on Ag amount in film was more pronounced in the case of the Raman scattering spectra excited by higher wavelength laser beam, while in the case of the spectra excited by 325 nm and 442 nm laser beams only weak dependence (or no dependence) was observed. Overall tendency of the decrease of the dispersion of the G peak with the increase of Ag atomic concentration was found. Thus sp³/sp² bond ratio in DLC:Ag film decreased with the increase of Ag atomic concentration in the films.     

Controllable electrode activities of nano-carbon films while maintaining surface flatness by electrochemical pretreatment

A carbon film consisting nanocrystallites with mixed sp² and sp³ bonds formed by using the electron cyclotron resonance (ECR) sputtering method was studied with respect to the changes in characteristics caused by electrochemical pretreatment (ECP). Unlike glassy carbon, our sputtered nanocrystalline carbon film deposited at an acceleration voltage of 75 V (ECR-75 nano-carbon film) largely retained its surface flatness after the ECP. This robust surface could be caused by an increase of 42% in the sp³ carbon realized by increasing the acceleration voltage during sputtering. The electrode activity of ECR-75 nano-carbon film was improved for surface sensitive species including Fe^3+/2+ unlike the boron doped diamond (BDD) electrode. This is because a sufficient quantity of surface sp² bonds remained and because the introduction of surface oxygen-containing groups is more efficient than with the BDD electrode. With pretreated ECR-75 nano-carbon film, the peak potential of glutathione was reduced solely due to the increase in the surface hydrophilicity with a sufficient quantity of surface sp² bonds, thus achieving the lowest detection limit (0.4 μM) ever obtained with carbon electrodes. We also achieved the stable measurement of 30 μM of serotonin (20 times) without the electrode surface fouling found with other electrodes.     

Investigation of the initial growth of ultrananocrystalline diamond films by multiwavelength Raman spectroscopy

The initial growth phase of ultrananocrystalline diamond/amorphous carbon nanocomposite films (UNCD/a-C) has been investigated by scanning electron microscopy, atomic force microscopy and especially Raman spectroscopy. As due to resonance effects Raman spectra of carbon materials strongly depend on the excitation wavelength, a multiwavelength analysis has been performed with λ_exc ranging from the UV region (325 nm) over the visible range (488 and 514 nm) to the IR region (785 nm). In addition, a set of measurements has been performed with a confocal Raman microscope, i.e. depth resolved, with a wavelength of 532 nm. The samples investigated were deposited with constant parameters, the deposition time being the only parameter varied, resulting in film thicknesses from 100 to 500 nm. It turned out that the diamond fraction and also the grain boundary material do not vary during that stage whereas there are slight but distinct changes of the nature of the amorphous matrix which reflect, among others, in a shift of the graphite-related G band to higher wavenumbers and in an increase of the ratio of D and G bands with increasing film thickness. These changes are discussed in terms of the above mentioned resonance effects; the major changes are a transition of hydrogen containing sp² chains to hydrogen-free condensed sp² rings when the material is no longer in the surface region of the films but becomes incorporated within the film bulk.     

High-quality diamond film deposition on a titanium substrate using the hot-filament chemical vapor deposition method

Diamond film on titanium substrate has become extremely attractive because of the combined properties of these two unique materials. Diamond film can effectively improve the properties of Ti for applications as aerospace and biomedical materials, as well as electrodes. This study focuses on the effects of process parameters, including gas composition, substrate temperature, gas flow rate and reactor pressure on diamond growth on Ti substrates using the hot-filament chemical vapor deposition (HFCVD) method. The nucleation density, nuclei size as well as the diamond purity and growth tendency indices were used to quantify these effects. The crystal morphology of the material was examined with scanning electron microscopy (SEM). Micro-Raman spectroscopy provided information on the quality of the diamond films. The growth tendency of TiC and diamond film was determined by X-ray diffraction analysis. The optimal conditions were found to be: CH₄:H₂ = 1%, gas flow rate = 300 sccm, substrate temperature T_sub = 750 °C, reaction pressure = 40 mbar. Under these conditions, high-quality diamond film was deposited on Ti with a growth rate of 0.4 μm/h and sp² carbon impurity content of 1.6%.     

Effect of surface treatments on the electronic properties of ultra-nanocrystalline diamond films

We present the soft x-ray spectroscopic study of the ultra-nanocrystalline diamond (UNCD) films with different surface treatments. The samples were prepared by means of microwave plasma enhanced chemical vapor deposition (MPECVD) and the different surface treatments are applied to alter their field emission properties. The electronic properties were subsequently examined by the soft x-ray absorption and x-ray emission spectroscopy at carbon 1s threshold. From the experimental results, there is no significant variation in electronic structure of oxygen- and hydrogen-plasma treated UNCD films. On the other hand, the biased treated UNCD film shows more remarkable change on the sp² and sp³ states. The formation of sp² bonding and the reduction of sp³ bonding are the consequence of the improved electron field emission properties.     

Hydrogen incorporation,bonding and stability in nanocrystalline diamond films

The hydrogen concentration in hot filament and microwave plasma CVD nanocrystalline diamond films is analysed by secondary ion mass spectrometry and compared to the film grain size. The surface and bulk film carbon bonds are analysed respectively by X-ray photoelectron spectroscopy (XPS) and ultra-violet Raman spectroscopy. XPS results show the presence of the hydrogenated p-type surface conductive layer. The respective intensities of the 1332 cm^− 1 diamond peak, of the G and D bands related to sp² phases, and of the 3000 cm^− 1 CH_x stretching mode band, are compared on Raman spectra. The samples are submitted to thermal annealing under ultra-high vacuum in order to get hydrogen out-diffusion. XPS analysis shows the surface desorption of hydrogen. Thermal annealing modifies the sp² phase structure as hydrogen out diffuses.     

Growth,electronic properties and applications of nanodiamond

Nanodiamond or nanocrystalline diamond is a broad term used to describe a plethora of materials. It is generally accepted that nanocrystalline diamond (NCD) consists of facets less than 100 nm in size, whereas a second term “ultrananocrystalline diamond” (UNCD) has been coined to describe material with grain sizes less than 10 nm. These differences in morphology originate in the growth process. Conventional hydrogen rich gas phases produce facetted diamond with grain size proportional to film thickness and low sp² content. If these films are thin the grains can be less than 100 nm and hence NCD. By starving the plasma of hydrogen, the reduction in etching of sp² can lead to re-nucleation. At the extreme this results in very small grain sizes of around 3–5 nm, UNCD.The electronic properties of these two materials are vastly different. NCD is basically very thin microcrystalline diamond and thus can be doped with boron. It is intrinsically transparent, with absorption increasing with doping level. UNCD is highly absorbing due to its higher sp² content, and exhibits a reduced bandgap due to disorder. By adding nitrogen to the gas phase, the density of states within the bandgap increases and ultimately metallic conductivity can be achieved. This conductivity is n-type but not doping.     

Electrochemical characteristics of lithium metal anodes with diamond like carbon film coating layer

To overcome the poor electrochemical characteristics of lithium metal anodes due to the dendrite formations, diamond like carbon (DLC) films were deposited onto the surface of lithium metal by radio frequency-plasma enhanced chemical vapor deposition (CVD) technique using acetylene gas as carbon precursor. The substrate temperature was selected as the main experimental parameter to control the bonding characteristic (sp²/sp³ ratio) of the films. The presence of diamond like structures was confirmed by Raman and Fourier transform infra red spectroscopy. The DLC coated lithium metal was then characterized as an anode material for lithium secondary batteries. The results showed that the DLC coated lithium metal anodes exhibited better electrochemical characteristics in terms of higher specific capacity and smaller interfacial impedance. These improved characteristics were attributed to the presence of DLC film coating which might suppress the dendrite's formation by protecting the lithium metal surface from the direct contact with the electrolyte.     

Thickness-dependent crystal orientation in poly(trimethylene 2,6-naphthalate) films studied with GIWAXD and RA-FTIR methods

The thickness dependence of the crystal orientation of poly(trimethylene 2,6-naphthalate) (PTN) films was clearly demonstrated using the methods of two-dimensional grazing incidence wide angle X-ray diffraction (2D GIWAXD) and grazing incidence reflection absorption FTIR (RA-FTIR) spectroscopy. The 2D GIWAXD results showed that for films thicker than 200 nm, the “c” axis (main chain direction) and “b” axis of crystal unit cell are almost parallel to the sample surface, whereas for thin films the “c” axis is preferentially perpendicular to the film plane in the crystalline phase of isothermally crystallized PTN films. The anisotropic orientation of the naphthalene rings in the isothermally crystallized PTN film was also confirmed. By analyzing the relative absorbance of the parallel band (1602 cm⁻¹) to the one of perpendicular band (917 cm⁻¹), the thickness dependence of the crystal orientation suggested by the GIWAXD results was also confirmed. Furthermore, the naphthalene rings in the isothermally crystallized thick films were found to lie flat on the film plane. The chain orientations derived from the GIWAXD and RA-FTIR results in this work were found to be consistent with the “flat-on” and “edge-on” lamellar orientation for the thin and thick films, respectively, which has previously been reported in many polymer systems.     

Preparation of diamond whiskers using Ar/O2 plasma etching

Radio frequency (RF) plasma etching of chemical vapor deposition (CVD) diamond film has been investigated in Ar/O₂ plasmas, with an emphasis to elucidate the effects of reacting gas on the fabrication of diamond whiskers. Diamond whiskers were formed on diamond films pre-coated with Al. It was found that diamond whiskers preferentially formed at the diamond grain boundaries. The densities of diamond whiskers increased with O₂ / Ar ratio. Whiskers obtained in pure O₂ plasma etching were 50 nm in diameter and 1 μm in height. The etching rate was increased by mixing Ar with appropriate volume of O₂. Al coated on the diamond surface reacted with O₂ to form Al₂O₃, serving as mask to restrain the etching underneath. Raman spectroscopy measurement confirmed that the whiskers kept sp³ diamond bonding structure after RF plasma etching. The field emission characteristics of the whiskers were also inspected.     

Growth and characterization of diamond micro and nano crystals obtained using different methane concentration in argon-rich gas mixture

Diamond with different grain sizes and nanographite films were grown on silicon and diamond substrate using 90 vol.% argon in hydrogen and methane gas mixtures by hot filament chemical vapor deposition method (HFCVD). In current study, the methane volume concentration was varied from 0.125 to 2 vol. % in order to estimate its effect on film morphology. The substrate temperature was varied from 550 to 850 °C by external heating independently of other CVD parameters, in order to estimate the activation energy. Characterization techniques have involved Raman spectroscopy, high resolution X-ray difractometry and scanning electron microscopy. The CHEMKIN computer package has also been used to simulate the experiments. The results obtained here indicate a single mechanism for diamond growth but with a high competition with sp² phase's growth.     

Quantification of sliding-induced phase transformation in N3FC diamond-like carbon films

We investigate magnetron-sputtered “N3FC” diamond-like carbon films at the nanoscale using an in situ transmission electron microscopy sliding experiment. We measure the sliding-induced changes in sp³- and sp²-hybridized carbon using electron energy loss spectroscopy, taking into account the effect of the electron beam. The rate of the phase transformation from sp³ to sp² bonding is quantified as being between 0.009% and 0.018% volume transformed per sliding pass.     

Field emission from nano-cluster carbon films

Field emission has been reported to occur at much lower fields in carbon based thin film systems than from any other material systems. The emission has been shown to depend on the various material parameters, but whichever carbon based system is used, it is found that emission occurs at localised sites rather than uniformly over the entire surface. Carbon films with mixed sp³/sp² bonding, like nanocrystalline diamond and nanocluster graphitic films emit at lower fields with a higher emission site density than single-phase films. The sp² cluster size in any carbon film can be altered during deposition, but it is easier to control nanocluster size by post-deposition annealing. Annealing increases the sp² cluster size embedded in a sp³ matrix until the sp³ matrix disappears completely and the film transforms into nanocrystalline graphite. To distinguish the effects of the sp² cluster size from other material parameters, a series of different carbon films were annealed post-deposition and the sp² cluster size was measured using visible Raman. Field emission was then measured at a vacuum of 10⁻⁸ mbar on all films using a parallel plate configuration. It was found that the field emission for all films tested depended upon the clustering of the sp² phase and this effect dominates the effects of the other parameters, such as chemical composition, surface termination, sp³ content or conductivity. The optimum size of the sp² was of the order of 1 nm for all systems tested. We believe that field emission occurs form the localised conducting, predominantly sp² bonded regions, which provideds the large field enhancement required for effective emission.     

Surface composition,bonding, and morphology in the nucleation and growth of ultra-thin,high quality nanocrystalline diamond films

The morphology, composition, and bonding character (carbon hybridization state) of continuous, ultra-thin (thickness ∼ 60 nm) nanocrystalline diamond (NCD) membranes are reported. NCD films were deposited on a silicon substrate that was pretreated using an optimized, two-step seeding process. The surface after each of the two steps, the as-grown NCD topside and the NCD underside (revealed by etching away the silicon substrate) is examined by X-ray PhotoElectron Emission spectroMicroscopy (X-PEEM) combined with X-ray absorption near edge structure (XANES) spectroscopy, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). The first step in the seeding process, a short exposure to a hydrocarbon plasma, induces the formation of SiC at the diamond/Si interface along with a thin, uniform layer of hydrogenated, amorphous carbon on top. This amorphous carbon layer allows for a uniform, dense layer of nanodiamond seed particles to be spread over the substrate in the second step. This facilitates the growth of a homogeneous, continuous, smooth, and highly sp³-bonded NCD film. We show for the first time that the underside of this film possesses atomic-scale smoothness (RMS roughness: 0.3 nm) and > 98% diamond content, demonstrating the effectiveness of the two-step seeding method for diamond film nucleation.