Ultrananocrystalline Diamond Films Research Articles

Maximizing visible Raman resolution of nanodiamond grains fabricated by coaxial arc plasma deposition through oxygen plasma etching optimization

Among the nondestructive carbon material characterization tools, the prominence of visible Raman spectroscopy has surged remarkably for many years due to its ability to explore a diverse array of carbon bonding configurations. However, to fully unlock the distinctive features concealed within carbon composite materials, additional specimen treatments or precise spectroscope calibrations are necessary. In the same regard, the tiny diamond grain size (5–10 nm) and the pronounced amount of sp2 carbon in the ultrananocrystalline diamond film represent major challenges in visible light excitation. In this work, we employ calibrated oxygen plasma reactive ion etching conditions to manifest the nanodiamond visible Raman signature from ultrananocrystalline diamond/amorphous carbon composite (UNCD/a‐C) films fabricated by coaxial arc plasma deposition. Upon plasma etching, the broad defect band in the visible Raman spectra converged toward the diamond characteristic Raman peak at 1332 cm−1. A detailed explanation of band components of the Raman spectra is extracted through peak fitting procedures. The results of Raman spectroscopy are further correlated with the electrical characteristics of the nitrogen‐doped UNCD/a‐C films due to the optimized oxygen plasma etching processes.

CH4/(Ar–H2) plasma post-treatments produce nano-diamond aggregation and improvement in field emission properties of ultrananocrystalline diamond films

CH4/(Ar–H2) plasma post-treatments produce nano-diamond aggregation and improvement in field emission properties of ultrananocrystalline diamond films

Noninvasive Salivary Sensor Based on Ferrocene/ZnO/Nitrogen-Incorporated Nanodiamond/Si Heterojunction Nanostructures for Glucose Sensing in Neutral Conditions

The development of a simple and convenient noninvasive salivary glucose sensor to replace the needle-prick blood collection process can not only enhance early detection and diagnosis but also reduce excess medical care costs due to delayed treatment. This study presents a noninvasive sensor based on a ferrocene/ZnO/nitrogen-incorporated nanodiamond/Si heterojunction structure to measure glucose in saliva. The sensing electrode based on a heterojunction structure composed of ferrocene (FC), Bidens-like zinc oxide (b-ZnO), and nitrogen-incorporated ultra-nanocrystalline diamond (NUNCD) films, which is a kind of nitrogen-incorporated nanodiamond, was constructed on a silicon (Si) substrate. Herein, FC and b-ZnO act as a mediator and a specific surface area promoter, respectively, and NUNCD provides a good nucleation site and conducting path. Glucose oxidase (GOx) was introduced into the system to play the role of enzyme to catalyze the reaction and increase the electrode selectivity. The FC/b-ZnO/NUNCD/Si sensor exhibited superior performance in glucose measurements within the linear ranges (1) 20–100 μM (R2 = 0.9717) with a sensitivity of 1125.18 μA/(mM cm2) and (2) 20–600 μM (R2 = 0.9926) with a sensitivity of 804.11 μA/(mM cm2). Additionally, the sensor has a sensitivity of 1012.35 μA/(mM cm2) within the range 50–400 μM (R2 = 0.9912) for artificial saliva.

Plasma Post‐Treatment Process‐Induced Grain Coalescence to Improve the Electron Field‐Emission Properties of Ultrananocrystalline Diamond Films

Plasma post‐treatment (PPT) process is an effective way to improve the electron field‐emission (EFE) properties of microwave plasma chemical vapor deposited ultrananocrystalline diamond (MP‐UNCD) films, while its effects on hot‐filament chemical vapor deposited UNCD (HF‐UNCD) films were not confirmed. Due to monodispersed Ta atoms existing in HF‐UNCD films, PPT process shows different effects on the structure evolution and EFE properties of HF‐UNCD films and MP‐UNCD films. CH4/N2 PPT process make diamond grains change from small spherical particles to large plate‐like aggregates with lots of stacking faults in HF‐UNCD films. This is ascribed to both the CN species in CH4/N2 plasma, which induces anisotropic growth of diamond and the monodispersed Ta atoms resulted in transition of graphite to diamond, which connect the neighboring nanosized diamond grains to a larger one and produce lots of stacking faults. The charges are easier to accumulate and emitted in the edges of the plate‐like diamond and stacking faults have lower negative electron affinity. These lower the turn‐on field from 26.3 to 12.7 V μm−1. It is different from that of MP‐UNCD films, in which only anisotropic growth occurs and forms needle‐like diamond due to CN species.

Toward ultralow friction and wear of ultrananocrystalline diamond film by formation of graphene nanosheets with a Cu catalyst

The present work has realized ultralow friction and wear of ultrananocrystalline diamond (UNCD) film through in-situ formation of graphene nanosheets by introducing Cu catalyst. UNCD film was first coated with a Cu layer by magnetron sputtering method and then annealed with the protection of Ar. After that, some thin and wrinkled graphene-like nanosheets were in-suit formed in UNCD film, which avoided the contamination of graphene during transfer process and enabled better adhesion between graphene and UNCD film. A comparative study of tribological behavior of UNCD, Cu/UNCD and annealed Cu/UNCD films were performed against Si3N4 ball on a ball-on-disk tribometer. The results showed that graphene nanosheets over UNCD film enhanced its tribological performance evidently. The annealed Cu/UNCD film exhibited ultralow friction coefficient of 0.025 and a kind of zero wear as the wear track is higher than the film surface. The findings in this study present a novel and effective method to enhance the lubricity of any engineering UNCD film.

Viability and proliferation of A549 cell line on the surface of micro-, nano- and ultrananocrystalline diamond films grown by HFCVD with tailored gases

This article describes key material science/technology issues to implement polycrystalline diamond scaffolds to enable processes for biological cells growth relevant for using cells grown in the laboratory for the treatment of human biological conditions. Issues investigated include 1. Synthesis/characterization of microcrystalline diamond (MCD), nanocrystalline diamond (NCD) and transformational ultrananocrystalline diamond (UNCD) coating-based scaffolds. Diamond films were grown on silicon substrates using the hot filament chemical vapor deposition (HFCVD) technique, by which filaments heated to ∼2,300 °C induce cracking of CH4 molecules into C atoms/CHx (x = 1, 2, 3) radicals, growing diamond films via chemical reaction on the substrates’ surface. MCD and NCD + films were grown flowing the H2/CH4 gas mixture. NCD- and UNCD films were grown using the Ar/CH4 gas mixture plus H2 flux (73.5%, 49%, and 9.8%), which for high fluxes, induced increased the concentration of H-containing trans-polyacetylene (T-PA) molecules in UNCD films’ grain boundaries, impacting biological performance. 2. Studies of viability and proliferation of human lung carcinoma cell line (A549) grown on surfaces of MCD, NCD, and UNCD films, using 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide (MTT) assay, which showed no significant difference in cell proliferation among the MCD, NCD, and UNCD films.

Growth of Nitrogen Incorporated Ultrananocrystalline Diamond Coating on Graphite by Hot Filament Chemical Vapor Deposition.

This article shows the results of experiments to grow Nitrogen incorporated ultrananocrystalline diamond (N-UNCD) films on commercial natural graphite (NG)/Cu anodes by hot chemical vapor deposition (HFCVD) using a gas mixture of Ar/CH4/N2/H2. The experiments focused on studying the effect of the pressure in the HFCVD chamber, filament-substrate distance, and temperature of the substrate. It was found that a substrate distance of 3.0 cm and a substrate temperature of 575 C were optimal to grow N-UNCD film on the graphite surface as determined by Raman spectroscopy, SEM, and TEM imaging. XPS analysis shows N incorporation through the film. Subsequently, the substrate surface temperature was increased using a heater, while keeping the substrate-filament distance constant at 3.0 cm. In this case, Raman spectra and SEM images of the substrate surface showed a major composition of graphite in the film as the substrate-surface temperature increased. Finally, the process pressure was increased to 10 Torr where it was seen that the growth of N-UNCD film occurred at 2.0 cm at a substrate temperature of 675 C. These results suggest that as the process pressure increases a smaller substrate-filament distance and consequently a higher substrate surface temperature can still enable the N-UNCD film growth by HFCVD. This effect is explained by a mean free path analysis of the main precursors H2 and CH3 molecules traveling from the filament to the surface of the substrate The potential impact of the process developed to grow electrically conductive N-UNCD films using the relatively low-cost HFCVD process is that this process can be used to grow N-UNCD films on commercial NG/Cu anodes for Li-ion batteries (LIBs), to enable longer stable capacity energy vs. charge/discharge cycles.

Human Exhalation CO2 Sensor Based on the PEI-PEG/ZnO/NUNCD/Si Heterojunction Electrode.

Gas sensors based on semiconductors have outstanding sensitivity compared with the oxide-based devices; however, the high operation temperature greatly hinders its development in practical applications. Chronic obstructive pulmonary disease (COPD) is one of the leading causes of death worldwide, and the patients with severe COPD with or without exacerbation tend to have airflow obstruction, which results in an increase of CO2 concentration and subsequent hypercapnic respiratory failure. At present, COPD detection relies on professional operation; however, the patients suffer great discomfort during the arterial blood sampling. All these facts reduce patient’s willingness to test their physical health. Thus, noninvasive monitoring of CO2 levels is crucial for the early diagnosis of high-risk COPD patients. A nitrogen-incorporated ultrananocrystalline diamond (NUNCD) film exhibits excellent properties in biosensing and polyetherimide-polyethylene glycol (PEI-PEG) polymer possesses a great capability of CO2 capturing. By incorporating NUNCD into PEI-PEG film, this work focuses on ameliorating the sensitivity and selectivity of the present semiconductor CO2 sensor. From the theoretical regression analyses of the experimental results, it is found that the excellent performance of the PEI-PEG/ZnO/NUNCD/Si electrode is contributed by two main reaction layers, the adsorption layer (PEI-PEG) and the electric transfer layer (ZnO/NUNCD). The selectivity is dominated by the PEI-PEG adsorption layer and the sensitivity is directly related to the changes in the work function of the ZnO/NUNCD interface. The high aspect ratio (>10) of the flower-like ZnO structure, growth from ZnO nanoparticles, can provide a more active adsorption area, as a result, extremely enhancing the sensitivity of the CO2 sensor.

CVD diamond-SiC composite films: Structure and electrical properties

Thin films of microcrystalline diamond-silicon carbide composite have been deposited by microwave plasma chemical vapor deposition (CVD) in methane-silane-hydrogen gas mixtures on Si substrates with an insulating polycrystalline diamond buffer layer. The structure and phase composition of the films were characterized by scanning electron microscopy, Raman and photoluminescence spectroscopy. The silicon carbide grains are identified to be cubic polytype 3C-SiC with only a minor admixture of hexagonal 6H-SiC. The measured electrical resistivity of the films is found to decrease with the temperature from ~10−1 Ω·cm at 300 K to ~10−2 Ω·cm at 460 K, with activation energy Ea = 0.20 eV. The conductivity is assumed to be realized via pathways in the SiC network immersed in isolating diamond matrix. The obtained results show that such diamond/SiC composites may be considered as an alternative to electrically conductive nitrogenated ultrananocrystalline diamond films for a number of practical applications.

Deposition of ultrananocrystalline diamond at low temperature using CO, CH4 mixed carbon source

UNCD (Ultrananocrystalline diamond) is a material with various excellent properties, but the high deposition temperature limits its application. UNCD was deposited at low temperature (700 K) using CO–CH4–H2-Ar as gas source. CO as reaction gas reduces the activation energy of CH4 system. The effects of different H2 concentrations on the morphology of diamond films were studied. The results show that ultra-nanocrystalline films can be deposited by CO–CH4–H2-Ar. The morphology of UNCD film can be observed by SEM images from dense film to needle film and then to spherical clusters. With the increase of H2 concentration, sp2 content decreased, and a-C:H appeared, and a-C:H increased first and then decreased. Analysis of OES(optical emission spectra) showed that the content of C2 under mixed carbon source was higher than that of single carbon source, and the change of CH390 was related to the formation of a-C:H. The optical properties of the film were tested, and the maximum transmittance was 88.9%.

Synthesis and Properties of Electrically Conductive/Nitrogen Grain Boundaries Incorporated Ultrananocrystalline Diamond (N-UNCD) Thin Films Grown by Microwave Plasma Chemical Vapor Deposition (MPCVD)

Research and development have been performed to investigate the effect of total pressure and microwave power on the electrical conductivity of nitrogen (N) atoms’ grain boundaries incorporated ultrananocrystalline diamond (N-UNCD) films grown by microwave plasma chemical vapor deposition (MPCVD). Insertion of N atoms into the UNCD film’s grain boundaries induces N atoms chemical reaction with C-atoms dangling bonds, resulting in release of electrons, which induce electrical conductivity. Four-point probe electrical measurements show that the highest electrically conductive N-UNCD films, produced until now, exhibit electrical resistivity of ~1 Ohm.cm, which is orders of magnitude lower than the ≥106 Ohm.cm for undoped ultrananocrystalline diamond (UNCD) films. X-ray diffraction analysis and Raman spectroscopy revealed that the growth of the N-UNCD films by MPCVD do not produce graphite phase but only crystalline nanodiamond grains. X-ray photoelectron spectroscopy (XPS) analysis confirmed the presence of nitrogen (N) in the N-UNCD films and the high conductivity (no electrical charging is observed during XPS analysis) shown in electrical measurements.

High-temperature wear behavior of micro- and ultrananocrystalline diamond films against titanium alloy

In the present study, we conducted tribological experiments to study the wear behavior of micro- (MCD) and ultrananocrystalline diamond (UNCD) films against titanium alloy at temperatures up to 700 °C. We found that at comparatively low temperatures (25–500 °C), the wear of diamond films is basically induced by the strong adhesion between diamond and titanium, which promotes the formation of Ti C bonds and detaches the carbon atoms from the diamond surface. When the temperature increases to 600 °C, oxygen starts to significantly etch the diamond films, producing gaseous CO/CO 2 and causing more loss of carbon atoms. At 700 °C, the UNCD film is systematically etched by oxygen, and the film is dominated by WO 3 , with drastically decreased wear resistance. The MCD film still maintains certain wear resistance at 700 °C, but it is also severely etched by oxygen, with substantially shrunk MCD grains. This work provides significant insights into the high-temperature wear behavior of diamond films and the application of diamond-coated cutting tools for machining titanium alloy and other high-cutting-temperature materials. • Adhesive wear and oxidation are main wear patterns of microcrystalline (MCD) and ultrananocrystalline diamond (UNCD) films • Adhesive wear can be induced by the formation of interfacial Ti-C bonds • Both MCD and UNCD films are substantially etched by oxygen when the temperature is higher than 600 °C • Drastic decrease of wear resistance occurs in UNCD films at 700 °C due to the intense oxidation of the UNCD film

Influence of surface termination of ultrananocrystalline diamond films coated on titanium on response of human osteoblast cells: A proteome study

Successful osseointegration, i.e. the fully functional connection of patient's bone and artificial implant depends on the response of the cells to the direct contact with the surface of the implant. The surface properties of the implant which trigger cell responses leading to its integration into the surrounding bone can be tailored by surface modifications or coating with thin layers. One potential material for such applications is ultrananocrystalline diamond (UNCD). It combines the exceptional mechanical properties of diamond with good biocompatibility and possibility of coating as thin uniform films on different substrates of biological interest. In the current work we firstly deposited UNCD films on titanium-coated substrates and applied oxygen or ammonia plasma to modify their surface properties. The as-grown and modified UNCD exhibited relatively smooth surfaces with topography dominated by rounded features. The modifications induced oxygen- or amino-terminated surfaces with increased hydrophilicity. In addition, the UNCD coatings exhibited very low coefficient of friction when diamond was used as a counterpart. As-grown and modified UNCD samples were applied to study the responses of human osteoblast MG63 cells triggered by surfaces with various terminations assessed by proteomic analysis. The results revealed that the coating of Ti with UNCD as well as the plasma modifications resulting in O- or NH2-terminated UNCD induced upregulation of proteins specific for cytoskeleton, cell membrane, and extracellular matrix (ECM) involved in the cell-ECM-surface interactions. Proteins from each of these groups, namely, vimentin, cadherin and fibronectin were further studied immunocytochemically and the results confirmed their increased abundance leading to improved cell-to-surface adhesion and cell-to-cell interactions. These findings demonstrate the potential of implant coating with UNCD and its surface modifications for better osseointegration and bone formation.

Direct microscopic evidence of shear induced graphitization of ultrananocrystalline diamond films

The origin of ultralow friction and high wear resistance in ultrananocrystalline diamond (UNCD) films is still under active debate because of the perplexed tribochemistry at the sliding interface. Herein, we report a comparative study on surface topography and nanoscale friction of tribofilms, in wear tracks of two sets of UNCD films having different structural characteristics. Despite both the films display ultralow coefficient of friction, the UNCD films grown under Ar atmosphere (UNCDAr) exhibit a high wear resistance while the wear rate is higher for the films grown in N2 (UNCDN). Frictional force microscopic (FFM) investigations clearly reveal the manifestation of shear induced graphitization on both the films. However, the wear track of UNCDAr films have a large network of a few layer graphene (FLG) structures over the amorphous carbon tribofilms while only isolated clusters of FLG structures are present in the wear track of UNCDN films. Here, we demonstrate the direct micro-/nanoscopic evidence for the formation of large network of ~ 0.8 - 6 nm thick FLG structures, as a consequence of shear induced graphitization and discuss their decisive role in ultralow friction and wear.

A novel growth model for depositing ultrananocrystalline diamond films in CH4/H2 chemistry

For more than twenty years, the synthesis of ultrananocrystalline diamond (UNCD) films is typically realized by the well-known Ar-rich atmosphere. In the present study, we demonstrate that the deposition of UNCD films can be achieved in CH4/H2 chemistry without Ar in a hot-filament chemical vapor deposition (HFCVD) apparatus. In this case, the high re-nucleation rate for the deposition of UNCD films is realized through the combination of high kinetic energy and high local concentration of the growth species (CHx, x = 0, 1, 2, 3). By synergistically tuning the gas pressure and carbon concentration (CH4/H2), the kinetic energy and local concentration of the growth species can be specifically regulated for adjusting the re-nucleation rate. In this paper, a novel kinetic growth model was established to interpret the UNCD growth mechanism with CH4/H2 chemistry, and the atmospheric window of the gas pressure and CH4/H2 ratio for the deposition of UNCD films was obtained. High growth rate, high surface finish and superior tribological performance of the UNCD films were demonstrated. This study provides significant insights into the UNCD growth mechanism without Ar content, and renders the gas strategy simpler, more controllable and more flexible.

Study of Atomic Hydrogen Concentration in Grain Boundaries of Polycrystalline Diamond Thin Films

This paper describes research focused on investigating the effect of hydrogen (H) atom insertion into the grain boundaries of polycrystalline diamond (PCD) films. This is required in order to understand the key morphological, chemical, physical, and electronic properties of the films. The PCD films were grown using the hot filament chemical vapor deposition (HFCVD) process, with flowing Ar gas mixed with CH4 and H2 gases to control film growth into microcrystalline diamond (MCD, 0.5–3 µm grain sizes), nanocrystalline diamond (NCD, 10–500 nm grain sizes), and ultrananocrystalline diamond (UNCD, 2–5 nm grain sizes) films depending on the Ar/CH4/H2 flow ratios. This study focused on measuring the H atom concentration of the PCD films to determine the effect on the properties indicated above. A simple model is presented, including a hypothesis that the two dangling bonds per unit cell of C atoms serve as the site of hydrogen incorporation. This correlates well with the observed concentration of H atoms in the films. Dangling bonds which are not passivated by hydrogen are postulated to form surface structures which include C double bonds. The Raman peak from these surface structures are the same as observed for transpolyacetyline (TPA). The data reveal that the concentration of H atoms at the grain boundaries is around 1.5 × 1015 atoms/cm2 regardless of grain size. Electrical current measurements, using a conductive atomic force microscopy (CAFM) technique, were performed using an MCD film, showing that the current is concentrated at the grain boundaries. Ultraviolet photo electron spectroscopy (UPS) confirmed that all the PCD films exhibited a metallic behavior. This is to be expected if the nature of grain boundaries is the same regardless of grain size.

Low-Defect Nanodiamonds and Graphene Nanoribbons Enhanced Electron Field Emission Properties in Ultrananocrystalline Diamond Films

It is known that the electron field emission (EFE) properties of ultrananocrystalline diamond (UNCD) films were usually produced by doping. Here, we performed a low-pressure annealing treatment wit...

Correlation of zeta potential and contact angle of oxygen and fluorine terminated nitrogen incorporated ultrananocrystalline diamond (N-UNCD) thin films

The surface chemistry of nitrogen incorporated ultrananocrystalline diamond (N-UNCD) films was altered by plasma treatment utilizing oxygen and fluorine plasma chemistries; the modified N-UNCD surfaces were characterized using contact angle and zeta potential measurements to give a more complete understanding of the interactions between the solid surface and the aqueous solution. The bonding character, surface composition, and morphology of the N-UNCD films before and after surface treatment were also monitored using X-ray photoelectron spectroscopy, atomic force microscopy, and Raman spectroscopy to ensure the grafting of functional groups; the contributing factor to the results was purely from the surface termination.

In-Situ Nanotribological Properties of Ultrananocrystalline Diamond Films Investigated with Ambient Pressure Atomic Force Microscopy

The relationship between nanoscale friction and the surrounding environment has long been a critical issue in the field of nanotribology. Here, we utilized ambient pressure–atomic force microscopy to investigate the effect of environmental gas on nanoscale friction of ultrananocrystalline diamond (UNCD) films. The frictional forces were measured in an atomic force microscopy (AFM) chamber in the environmental range from an ultrahigh vacuum to near ambient pressure in the presence of oxygen, nitrogen, and water. We observed that friction increased with the pressure of the oxygen responsible for the oxidation of the surface of the UNCD, while that in nitrogen gas remained unchanged. Interestingly, friction decreased in water, due to the tribochemical reaction caused by surface passivation. When two diamond materials come into contact under water conditions, the water molecules are dissociated because of normal pressure between the AFM tip and diamond surface, and the dissociative water molecule adsorption passivates the surfaces of the diamond-coated tip and UNCD, resulting in a reduction of friction force. The chemical state of the UNCD surface in various environmental conditions was confirmed using near ambient pressure X-ray photoelectron spectroscopy. This result elucidates the role of vapor-phase oxygen and water in the tribological properties of carbon-based materials.

Effects of carbon concentration and gas pressure with hydrogen-rich gas chemistry on synthesis and characterizations of HFCVD diamond films on WC-Co substrates

The study concentrates on systematic investigations on the effects of extremely high or low carbon concentration (2%–14%) and gas pressure (800 Pa–3200 Pa) on crystal structures, grain sizes, phase compositions, surface and cross-sectional morphologies, growth rates, surface roughness, mechanical and tribological performances of various polycrystalline diamond films synthesized on cemented carbide (WC-Co) substrates with hydrogen-rich/argon-lean gas mixture adopting hot filament chemical vapor deposition (HFCVD) method. The crystal structure, grain size and phase composition of various diamond films synthesized with different carbon concentration and gas pressure were analyzed, indicating that as-deposited diamond films cover microcrystalline diamond (MCD), submicrocrystalline diamond (SMCD), nanocrystalline diamond (NCD) and ultrananocrystalline diamond (UNCD). The surface and cross-sectional morphologies of these various polycrystalline diamond films were observed, and the growth rates of various diamond films were calculated. The three-dimensional morphologies and surface roughness were obtained. With increasing the carbon concentration and decreasing the gas pressure during the synthesis, the crystal size decreases so that diamond film with high smoothness, low surface roughness can be obtained. Subsequently, the synthesis mechanisms of MCD, SMCD, NCD and UNCD films with different carbon concentration and gas pressure under hydrogen-rich gas mixture adopting HFCVD were discussed in detail. The elastic modulus and nanohardness of different diamond films were measured quantitatively, and the film-substrate adhesion were evaluated qualitatively. Moreover, the tribological performances of various diamond films synthesized with different carbon concentration and gas pressure were studied by sliding against silicon nitride (Si3N4) ceramic balls. Results indicate that obtaining the dynamic equilibrium between sufficiently renucleation rate of diamond nuclei and effectively removal rate of non-diamond carbon phase by adjusting carbon and gas pressure under the hydrogen-rich gas chemistry can contribute to the synthesis of high-quality UNCD film.