Transparent ultrananocrystalline diamond films on quartz substrate

Highly transparent ultrananocrystalline diamond (UNCD) films were deposited on quartz substrates using microwave plasma enhanced chemical vapor deposition (MPECVD) method. Low temperature growth of high quality transparent UNCD films was achieved by without heating the substrates prior to the deposition. Additionally, a new method to grow NCD and microcrystalline diamond (MCD) films on quartz substrates has been proposed. Field emission scanning electron microscopy (FESEM) and Raman spectroscopy were used to analyze the surface and structural properties of the films. The surface morphology of UNCD film shows very smooth surface characteristics. The transparent property studies of UNCD film on quartz substrate showed 90% transmittance in the near IR region. The transparent and dielectric properties of UNCD, NCD, and MCD films on quartz substrates were compared and reported.     

Formation of ultrananocrystalline diamond films with nitrogen addition

Nitrogen-doped ultrananocrystalline diamond (UNCD) films have been prepared by the microwave plasma jet chemical vapor deposition system (MPJCVD) using a gas mixture of Ar-1%CH₄-10%H₂ and addition of 0.5-7% nitrogen. This growth process by MPJCVD with 10% hydrogen addition that yields UNCD films compared with those UNCD films produced by MPCVD with a high Ar/CH₄ ratio due to the focused microwave plasma jet greatly enhanced the enough dissociation of react gases and formed C₂ species with an energetic state at lower argon concentration. The surface morphologies were changed drastically from continuous to rough granular surface with increasing the nitrogen content due to the great rise of CN species in the plasma. The width of grain boundaries composed of sp²-bonded carbon increased with increasing nitrogen content in the films. Moreover, the seldom defects in the UNCD films induced by the addition of nitrogen in the plasma were identified and investigated by using a scanning transmission electron microscope (STEM). The highest nitrogen-doped benefit with a N/C atomic ratio of 3.25% in UNCD films was reached by addition of only 3% N₂ in plasma (Ar-1%CH₄-10%H₂-3%N₂), showing the MPJCVD can greatly reduce the used amount of nitrogen in the synthesis of nitrogen-doped UNCD films.     

Charge carrier transport anisotropy in ultrananocrystalline diamond films

The optoelectronic properties of ultrananocrystalline diamond films (UNCD) grown using N₂ = 0 and 5% in the deposition gas mixture, are investigated by transient photocurrent measurements under nanosecond light pulses, both in planar and sandwich contact arrangements.Independent of contact configuration and N₂% value, very similar characteristic times in the 6-7 ns range are detected in the nanosecond range, reflecting a homogeneous distribution of states responsible for such decay times. On a longer time scale, nitrogen addition appears to slow down carrier transport promoting trapping and detrapping processes responsible for single and two power law photocurrent decays in films deposited using N₂ = 5% for sandwich and planar contact arrangements, respectively. Such a result suggests a nitrogen induced transport anisotropy tentatively related to structural modifications occurring at relatively low N₂%.     

Electrical properties of ultrananocrystalline diamond/amorphous carbon nanocomposite films

The electrical surface properties of ultrananocrystalline diamond/amorphous carbon composite films have been investigated by four-point probe I/V and Hall measurements, whereas impedance spectroscopy has been used to establish the electrical bulk properties of the films. It turned out that the surface is p-type conductive with a resistivity of 0.14 Ω cm and a sheet carrier concentration of 7.6 × 10¹³ cm^?2. The bulk resistivity is higher by almost seven orders of magnitude (1.3 × 10⁶ Ω cm). The bulk conduction is thermally activated with an apparent activation energy of 0.17 eV. From Cole–Cole plots of the impedance spectra it can be concluded that there are three different contributions to the bulk conductivity. In order to try to identify these three components contributing to the electrical bulk conduction, Raman spectra have been recorded at five different wavelengths from the IR to UV region. These measurements showed that the UNCD/a-C films consist of at least three components: diamond nanocrystallites, an amorphous carbon matrix, and trans-polyacetylene-like structures probably at the interface between these two.     

Optical properties of ultrananocrystalline diamond/amorphous carbon composite films prepared by pulsed laser deposition

Ultrananocrystalline diamond (UNCD)/amorphous carbon (a-C) composite thin films were grown in ambient hydrogen by pulsed laser deposition using a graphite target, and their optical properties were determined by optical absorption spectroscopy and Raman scattering spectroscopy. Three optical bandgaps exist. Two bandgaps are indirect and their values were estimated to be 1.0 eV and 5.4 eV; these bandgaps correspond to the a-C surrounding the UNCDs and the UNCDs respectively. The third bandgap is direct and has a value of 2.2 eV, which significantly contributes to a large absorption coefficient, (10⁶ cm^− 1 at 3.0 eV). Possible origins of the third bandgaps are the grain boundaries (GBs) between the UNCDs and the a-C since they are specific to the UNCD/a-C composite films. The infrared absorption spectrum and the Raman scattering spectrum revealed the incorporation of hydrogen in the GBs. The hydrogen incorporated in the GBs might also have some influence on the appearance of the direct bandgap and its value.     

Grazing-incidence small-angle X-ray scattering study on ultrananocrystalline diamond films

In this work, an ultrananocrystalline diamond film was studied with grazing-incidence small-angle X-ray scattering (GISAXS) to determine the diamond grain size and average distance of the grains with a non-destructive method and with excellent sampling statistics. The measured 2D GISAXS patterns were modelled with the assumption of monodisperse spheres. The best fits were obtained with the "buried layer" model where the spheres are correlated within the film plane. This correlation was approximated with a two-dimensional Percus–Yevick structure factor. The average diamond grain size of D = 8.0–8.5 nm and a centre-to-centre distance of the grains with 10.4–11.9 nm agrees well with transmission electron microscopy results of comparable samples.     

Vibrational properties of nitrogen-doped ultrananocrystalline diamond films grown by microwave plasma CVD

Ultrananocrystalline diamond (UNCD) films grown in an argon-rich Ar/CH₄/H₂ microwave plasma with nitrogen gas added in amounts of 0%–20% were studied by Raman spectroscopy with multiple excitation wavelengths in the range of 244–647 nm and by optical absorption in UV–visible. The Raman spectra have demonstrated the presence of diamond, amorphous carbon and polyacetylene in the UNCD films. Analysis of vibrational and optical properties of amorphous carbon phase proves that nitrogen stimulates the transition from amorphous carbon into an ordered graphite-like structure with narrowed optical band gap, which is supposed to be responsible for the high electrical conductivity of the N-doped UNCD.     

Wettability and protein adsorption on ultrananocrystalline diamond/amorphous carbon composite films

Ultrananocrystalline diamond/amorphous carbon (UNCD/a-C) composite films have been prepared by microwave plasma chemical vapour deposition (MWCVD) from 17% CH₄/N₂ mixtures and modified with O₂ and CHF₃ plasmas, which changed the surface termination from hydrogen to oxygen and fluorine, respectively. X-ray photoelectron spectroscopy (XPS) showed that successful oxidation and fluorination of the UNCD surface has been achieved with surface O or F concentrations of ca. 12 at.%. None of the plasma modification processes led to a change of the film topography as studied by atomic force microscopy (AFM); for all samples the rms roughness was in the range of 10–12 nm. The UNCD/a-C films with different terminations were characterized by contact angle measurements with water, formamide and benzyl alcohol; from the results obtained the surface energy was calculated. The adsorption of albumin and fibrinogen to the different UNCD/a-C samples was assessed by an inverted enzyme-linked immunosorbent assay (ELISA). The determined albumin/fibrinogen ratios, which could be used to evaluate the tendency of thrombus formation, are correlated with the surface properties of as-deposited and modified UNCD/a-C films.     

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.     

Plasma post-treatment process for enhancing electron field emission properties of ultrananocrystalline diamond films

This paper demonstrated the plasma post-treatment (ppt) process for modifying the granular structure of ultrananocrystalline diamond (UNCD) films so as to improve their electron field emission (EFE) properties. The ppt-processed UNCD films exhibited improved EFE properties as turn-on field of E₀ = 7.0 V/μm (J_e = 0.8 mA/cm² at 17.8 V/μm). TEM investigation revealed that the prime factor, which enhanced the EFE properties of the UCND films, is the induction of nano-graphitic clusters due to the ppt-process. However, for achieving such a goal, the granular structure of the primary UNCD layer has to be relatively open. That is, the size of grains should be sufficiently small and the grain boundaries should be of considerable thickness, containing abundant hydro-carbon species. Such a simple and robust process for synthesizing conductive UNCD films is especially useful for practical applications.     

Piezoresistivity of n-type conductive ultrananocrystalline diamond

Recent developments of a piezoresistive sensor prototype based on n-type conductive ultrananocrystalline diamond (UNCD) are presented. Samples were deposited using hot filament chemical vapor deposition (HFCVD) technique, with a gas mixture of H₂, CH₄ and NH₃, and were structured using multiple photolithographic and etching processes. Under controlled deposition parameters, UNCD thin films with n-type electrical conductivity at room temperature (5 × 10^− 3 – 5 × 10¹ S/cm) could be grown. Respective piezoresistive response of such films was analyzed and the gauge factor was evaluated in both transverse and longitudinal arrangements, also as a function of temperature from 25 °C up to 300 °C. Moreover, the gauge factor of piezoresistors with various sheet resistance values and test structure geometries was evaluated. The highest measured gauge factor was 9.54 ± 0.32 at room temperature for a longitudinally arranged piezoresistor with a sheet resistance of about 30 kΩ/square. This gauge factor is well comparable to that of p-type boron doped diamond; however, with a much better temperature independency at elevated temperatures compared to the boron-doped diamond and silicon. To our best knowledge, this is the first report on piezoresistive characteristics of n-type UNCD films.     

Optical emission spectroscopy of deposition process of ultrananocrystalline diamond/hydrogenated amorphous carbon composite films by using a coaxial arc plasma gun

Deposition processes of ultrananocrystalline diamond/hydrogenated amorphous carbon composite (UNCD/a-C:H) films in a hydrogen atmosphere using a coaxial arc plasma gun were spectroscopically investigated with an intensified charge-coupled device (ICCD) camera equipped with narrow-bandpass filters. Strong emissions lasted for more than 100 μs. From the emission time and deposition rate, a supersaturated condition should be strongly realized. This might contribute to the formation of UNCD crystallites. A UNCD crystallite size was estimated to be 2.3 nm from an X-ray diffraction peak. The extremely small value implies that nuclei are repeatedly generated without the subsequent growth. A probable reason is as follows: since the number of carbon species caused using the arc plasma gun is so large as compared to that of hydrogen atoms, which generate from ambient hydrogen molecules by the collision with carbon species, the hydrogen atoms do not effectively contribute to the UNCD crystallite formation. This consideration does not contradict the fact that a coaxial arc plasma gun makes possible UNCD crystallite formation even in vacuum.     

Plasma amination of ultrananocrystalline diamond/amorphous carbon composite films for the attachment of biomolecules

Ultrananocrystalline diamond/amorphous carbon nanocomposite films (UNCD/a-C) have been deposited by microwave plasma chemical vapour deposition at 600 °C from 17% CH₄/N₂ mixtures. The as-grown films turned out to be hydrogen terminated and very stable. Photochemical amination of H-terminated diamond is a well-established route to attach functional groups to such surfaces for applications in biosensors. Here we report on experiments to aminate UNCD surfaces directly by exposure to ammonia plasmas. Thereafter the surfaces were reacted with the heterobifunctional crosslinker molecule SSMCC bearing a N-hydroxysuccinimide (NHS) ester group which should react with the surface NH₂ groups. By means of X-ray photoelectron spectroscopy (XPS), contact angle measurements and fluorescence microscopy it is shown that both steps, plasma amination and SSMCC attachment lead to the desired aims. On the other hand, experiments to attach a thiol-bearing fluorescein molecule directly to H-terminated UNCD films turned out to be partially successful although according to literature such a reaction should be very unlikely.     

Interpretation of the Raman spectra of ultrananocrystalline diamond

It has long been known that by slightly altering the deposition conditions for diamond in plasma-enhanced chemical vapor deposition (PECVD), a transition from a microcrystalline to a nanocrystalline diamond morphology can be affected. The method of this transition, however, is not clear. This work investigates that transition by using transmission electron microscopy (TEM), scanning electron microscopy (SEM), and Raman spectroscopy. These experiments show that far from being a continuous transition, there is competitive growth between microcrystalline and nanocrystalline diamonds. Additionally, this work confirms the interpretation that certain peaks in the Raman spectrum previously attributed to “nanocrystalline diamond” are indeed due to the presence of hydrogen at the grain boundaries. For ultrananocrystalline diamond (UNCD) films, we verify that none of the spectral features observed using visible Raman spectroscopy can be attributed to sp³-bonded carbon, although the sample is composed of ∼95% sp³-bonded carbon. Thus, the Raman signal in UNCD can be considered to be solely due to the disordered sp²-bonded carbon at the grain boundaries.     

Surface properties of differently prepared ultrananocrystalline diamond surfaces

Ultrananocrystalline diamond/amorphous carbon composite films have been deposited by microwave plasma chemical vapour deposition from 17% CH₄/N₂ mixtures at 600 °C. Thereafter the films were subjected to various treatments (plasma processes, UV/O₃ exposure) to obtain hydrogen, oxygen, and fluorine terminated surfaces, which then have been characterized with respect to their composition, roughness, wettability, and other properties. Among others, it will be shown that H- and F-terminated surfaces are very stable even if exposed for long time to air, while O-terminated ones are prone to contaminations. H- and O-termination can be patterned by applying the UV/O₃ treatment through a mask. Finally, it will be shown that a non-fouling poly(ethylene glycol) layer can be grafted directly on oxygen terminated surfaces by an atom transfer radical polymerization process using α-bromoisobutyryl bromide as an initiator.     

Synthesis of highly transparent ultrananocrystalline diamond films from a low-pressure,low-temperature focused microwave plasma jet

This paper describes a new low-temperature process underlying the synthesis of highly transparent ultrananocrystalline diamond [UNCD] films by low-pressure and unheated microwave plasma jet-enhanced chemical vapor deposition with Ar-1%CH₄-10%H₂ gas chemistry. The unique low-pressure/low-temperature [LPLT] plasma jet-enhanced growth even with added H₂ and unheated substrates yields UNCD films similar to those prepared by plasma-enhanced growth without addition of H₂ and heating procedure. This is due to the focused plasma jet which effectively compensated for the sluggish kinetics associated with LPLT growth. The effects of pressure on UNCD film synthesis from the microwave plasma jet were systematically investigated. The results indicated that the substrate temperature, grain size, surface roughness, and sp ³ carbon content in the films decreased with decreasing pressure. The reason is due to the great reduction of H _α emission to lower the etching of sp ² carbon phase, resulting from the increase of mean free path with decreasing pressure. We have demonstrated that the transition from nanocrystalline (80 nm) to ultrananocrystalline (3 to 5 nm) diamond films grown via microwave Ar-1%CH₄-10%H₂ plasma jets could be controlled by changing the pressure from 100 to 30 Torr. The 250-nm-thick UNCD film was synthesized on glass substrates (glass transition temperature [T _g] 557°C) using the unique LPLT (30 Torr/460°C) microwave plasma jet, which produced UNCD films with a high sp ³ carbon content (95.65%) and offered high optical transmittance (approximately 86% at 700 nm).     

A stable suspension of single ultrananocrystalline diamond particles

As the result of successful disintegration of tight aggregates in detonation nanodiamond by stirred-media milling with microbeads, stable colloid of nanodiamond particles with a mean core size of 4 nm is obtained for the first time, but the colloid is colored deep black. X-ray diffraction, Raman scattering, HRTEM, UV–vis absorption spectra and viscosity data were used to characterize the colloid. It was suggested that the reason for the unexpected black color of the suspension is a result of graphitic partial surface (π-electrons formation of 4 nm diamond particles) induced by strong collision with beads during milling process. π-electrons are a reason of double electric layer formation and high viscosity of the suspension. Theoretical estimations fitted experimental data.     

Influence of substrate temperature on formation of ultrananocrystalline diamond films deposited by HFCVD argon-rich gas mixture

The influence of the substrate temperature on the formation of ultrananocrystalline diamond (UNCD) thin films, prepared by an argon-based hot filament chemical vapor deposition (HFCVD), is discussed in this work. The gas mixture used for diamond growth was 1 vol.% methane, 9 vol.% hydrogen and 90 vol.% argon at a total flow rate of 200 sccm and at a total pressure of 30 Torr. The substrate temperature range was from 550 to 850 °C at deposition time of 8 h. Mass growth rate was determined at different deposition temperatures. The activation energy for UNCD growth, determined from the Arrhenius plot, was lower (5.7 kcal/mol) than the values found for standard diamond deposition (around 11 kcal/mol). In this work, we suggest that the activation energy was lower because the growth of these films occurs at conditions that there is a high growth competition between diamond phase and sp² phases. To support this hypothesis, systematic characterization studies based on Raman scattering spectroscopy, high-resolution X-ray diffractometry and high-resolution scanning electron microscopy were performed.     

Low temperature electron spin resonance investigation of ultrananocrystalline diamond films as a function of nitrogen content

We have investigated the room and low temperature ESR signals in a series of ultrananocrystalline diamond films grown with different N₂ enrichments in the gas phase. We have found that exchange interaction is playing the major role in spin–spin interaction and determines the linewidths. The relatively high values of spin–lattice relaxation and other features of the ESR signals show that the ESR active centers are sitting in the grain boundaries' regions.     

Improvement of field emission performance on nitrogen ion implanted ultrananocrystalline diamond films through visualization of structure modifications

The relationship between the electron field emission properties and structure of ultra-nanocrystalline diamond (UNCD) films implanted by nitrogen ions or carbon ions was investigated. The electron field emission properties of nitrogen-implanted UNCD films and carbon-implanted UNCD films were pronouncedly improved with respect to those of as-grown UNCD films, that is, the turn-on field decreased from 23.2 V/μm to 12.5 V/μm and the electron field emission current density increased from 10E−5 mA/cm² to 1 × 10E−2 mA/cm². The formation of a graphitic phase in the nitrogen-implanted UNCD films was demonstrated by Raman microscopy and cross-sectional high-resolution transmission electron microscopy. The possible mechanism is presumed to be that the nitrogen ion irradiation induces the structure modification (converting sp³-bonded carbons into sp²-bonded ones) in UNCD films.