Ultrananocrystalline Diamond Research Articles

Straight imaging and mechanism behind grain boundary electron emission in Pt-doped ultrananocrystalline diamond films

A detailed scanning tunneling microscopic (STM) investigation is carried out to directly image and understand the mechanism behind enhanced conductivity and electron field emission (EFE) properties for platinum (Pt) ion doped/post-annealed ultrananocrystalline diamond (UNCD) films. Straight imaging of conducting/non-conducting sites is mapped by dynamic scanning tunneling microscopy (D–STM). Energy dissipation mapping and the local current–voltage measurements illustrate that grain boundaries are the conducting/emitting sites. Further, this fact is supported by the high resolution current imaging tunneling spectroscopy (CITS). The formation of abundant conducting sp2 nanographitic phases along the diamond grain boundaries, confirmed from transmission electron microscopic examinations, is believed to be the genuine factor that helps for the easy transport of electrons and hence enhanced the conductivity/emission properties. The fabrication of these films with high conductivity and superior EFE properties is a direct and simple approach which opens up new prospects in flat panel displays and high brightness electron sources. Moreover, we expect the energy dissipation associated with assisted tunneling process of electrons from the tip of STM to sample surface, can be a very useful technique for imaging heterostructured semiconducting surfaces due to their electrical conductivity differences in nanometer scale, which can explain many physical and chemical properties.

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 (5×10−3–5×101S/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 30kΩ/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.

Science and technology of diamond films grown on HfO2 interface layer for transformational technologies

This paper describes the science underlying the synthesis and characterization of microcrystalline diamond (MCD) to ultrananocrystalline diamond (UNCD) films on hafnium oxide (HfO2) thin films, grown on flat Si substrates and micro-pillars on Si substrates, for the first time. HfO2 is used as a novel inter-phase layer for the integration of microcrystalline (1–3μm grain size), nanocrystalline (10–200nm grain size), and ultrananocrystalline diamond (3–5nm grain size) as coatings on substrates used in transformational technologies such as silicon, oxides, and metals that need protective corrosion/mechanical abrasion resistant coatings developed in this work. Atomic layer deposition was used to grow HfO2 films with 5, 10, 30 and 100nm in thickness, while hot filament chemical vapor deposition was used to grow diamond films, respectively. High resolution transmission electron microscopy, X-ray photoelectron and Raman spectroscopies revealed the formation of an atomic scale hafnium carbide (HfC) interphase layer on the surface of the HfO2 film, which provides efficient nucleation for diamond film growth to produce tailored diamond surfaces on flat Si substrates and Si micro-pillars on flat Si substrates, for new transformational micro/nano-electronics and other high-tech technologies.

Biokinetics and tissue response to ultrananocrystalline diamond nanoparticles employed as coating for biomedical devices.

Although Ultrananocrystalline diamond (UNCD) has been proposed as a coating material for titanium biomedical implants, the biological effects and toxicity of UNCD particles that could eventually detach have not been studied to date. The biokinetics and biological effects of UNCD compared to titanium dioxide (TiO2 ) nanoparticles was evaluated in vivo using Wistar rats (n = 30) i.p. injected with TiO2 , UNCD or saline solution. After 6 months, blood, lung, liver, and kidney samples were histologically analyzed. Oxidative damage by membrane lipidperoxidation (thiobarbituric acid reactive substances-TBARS), generation of reactive oxygen species (superoxide anion- O2-), and antioxidant enzymes (superoxide dismutase-SOD, catalase-CAT) was evaluated in lung and liver. Histologic observation showed agglomerates of TiO2 or UNCD in the parenchyma of the studied organs, though there were fewer UNCD than TiO2 deposits. In addition, TiO2 caused areas compatibles with foci of necrosis in the liver and renal hyaline cylinders. Regarding UNCD, no membrane damage (TBARS) or mobilization of enzymatic antioxidants was observed either in lung or liver samples. No variations in O2- generation were observed in lung (Co: 35.1 ± 4.02 vs. UNCD: 48 ± 9.1, p > 0.05). Conversely, TiO2 exposure caused production of O2- in alveolar macrophages and consumption of catalase (p < 0.05). The studied parameters suggest that UNCD caused neither biochemical nor histological alterations, and therefore may prove useful as a surface coating for biomedical implants. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 2408-2415, 2017.

Low temperature hot filament chemical vapor deposition of Ultrananocrystalline Diamond films with tunable sheet resistance for electronic power devices

This paper describes the results from systematic experiments performed to investigate the influence of different substrate-filaments distances, surface-substrate temperatures and reaction of precursors during the growth of Ultrananocrystalline Diamond (UNCD) films via the hot filament chemical vapor deposition (HFCVD) process. The experimental results provide valuable information to understand the important role of key molecules (CHx, x=1, 2, 3) and atoms (H), resulting from the cracking of precursor molecules (CH4, H2) on the hot surface of the filaments, and the contribution of argon (Ar) inert gas atoms, all interacting on the surface of the substrates when using different filaments-substrate distances to produce films with different structures and properties. The interaction of the cracked molecular and atomic species at different filaments-substrate distances, play a critical role in the nucleation and growth of films with different structures, as observed by complementary analytical techniques. Films grown at 5, 15, and 30mm filaments-substrate distances exhibit graphite-disordered graphene phases, while those grown at 20mm distances exhibit mixed large UNCD-minor graphite mixed phases, and those grown at 25 and 35mm distances exhibit pure UNCD phase. UNCD films grown at ~600°C exhibit grain boundaries with sp2 and Trans-Polyacetylene (T-PA) dangling chemical bonds and low sheet resistance (~5Ω), while UNCD films grown at relativity low temperature (~490°C) exhibit high sheet resistance (~8MΩ) and more T-PA dangling bonds. The results presented in this paper indicate that UNCD and mixed UNCD/disordered graphene/graphite films can be grown with tailored resistivity enabling potential applications in electronic power devices.

Thermal Conductivity of Ultrananocrystalline Diamond/Nonhydrogenated Amorphous Carbon Composite Films Prepared By Coaxial Arc Plasma Deposition

Diamond possesses the highest thermal conductivity (2 × 103 Wm-1K-1) in all bulk materials, owing to low phonon scattering in diamond. Since phonon scattering is drastically enhanced by grain boundaries, the thermal conductivity of polycrystalline diamond films is strongly dependent on the grain size. The thermal conductivity of ultrananocrystalline diamond (UNCD)/hydrogenated amorphous carbon (a-C:H) composite (UNCD/a-C:H) films comprising a large number of UNCD grains with diameters of less than 10 nm and an a-C:H matrix have been prepared mainly by chemical vapor deposition (CVD). The highest thermal conductivity reported so far is 12 Wm- 1K- 1, which is comparable with that of sp3-rich non-hydrogenated amorphous carbon (a-C). It has been reported for a-C:H and UNCD/a-C:H films that the thermal conductivity is degraded by the incorporation of H atoms in the films. Recently we have realized the formation of UNCD/a-C films comprising UNCD grains and an a-C matrix, by coaxial arc plasma deposition (CAPD). CAPD does not necessarily require a hydrogen atmosphere during the deposition for the formation of diamond grains. So far, the thermal conductivity of hydrogen-free UNCD/a-C films has never been studied because H atoms are unintentionally incorporated into the films from source gases in CVD. In this study, UNCD/a-C films were deposited under a base pressure, and the thermal conductivity of the films were measured for the first time to our knowledge. UNCD/a-C films were deposited on n-Si substrates at base pressures of less than 10-4 Pa and the substrate temperature of 550 °C using coaxial arc plasma gun equipped with a graphite target. Mo films with the thickness of 80 nm were deposited on the UNCD/a-C films by sputtering for the TDTR. The thermal conductivity of the UNCD/a-C films was measured by a time-domain thermoreflactance (TDTR) technique. The thermal conductivity of the UNCD/a-C films was estimated to be 23.7 Wm- 1K- 1 by TDTR. The thermal conductivity is evidently higher than that of UNCD/a-C:H prepared by CVD. Two reasons for it are supposed. One is that an a-C in UNCD/a-C is probably advantageous for thermal conduction as compared with an a-C:H in UNCD/a-C:H, since it has been reported for a-C:H that the thermal conductivity decreases with increasing hydrogen content in a-C:H. The other is that UNCD/a-C does not contain hydrogen atoms that prevent from phonon scattering, whereas the preferential existence of hydrogen atoms at grain boundaries between UNCD grains and those between UNCD grains and an a-C:H matrix enhances phonon scattering for UNCD/a-C:H films prepared by CVD, which results in a decrease in the thermal conductivity. Further details will be presented in the conference.

Photoconduction of p-type Ultrananocrystalline Diamond/Hydrogenated Amorphous Carbon Composite Films in Metal-Semiconductor-Metal Geometry

Photovoltaics are representative renewable energy devices and the development of the next-generation photovoltaics have attracted considerable attentions. Ultrananocrystalline diamond (UNCD)/hydrogenated amorphous carbon (a-C:H) composite (UNCD/a-C:H) films, which possess large absorption coefficients of more than 106 cm−1 at photon energy range between 3 and 6 eV is one of the candidate materials to be employed as photovoltaic devices. It has been theoretically predicted that additional energy states are introduced between the bandgap of diamond owing to a large amount of GBs. Here, GBs denote interfaces between UNCD grains and those between UNCD grains and an a-C:H matrix. Doped impurity atoms are preferentially incorporated into GBs. As a result, the production of p-type and n-type conduction with enhanced electrical conductivities is realized by boron (B) and nitrogen (N) doping, respectively. Thus far, p-type UNCD/a-C:H films were deposited on n-type Si substrates, and it was confirmed that they exhibit typical rectifying behaviors. The UNCD/a-C:H films certainly act as drift layers in heterojunction diodes. Moreover, our recent studies have experimentally proved that UNCD/a-C:H can detect deep-ultraviolet (DUV) light with high external quantum efficiencies and the detectivity for 254 nm monochromatic light was enhanced at low temperatures. As for the photodetection of UNCD/a-C:H, spectral photoresponse measurements suggests that localized mid-gap states act as the generation centers of photocarriers. Although the peculiar photodetection properties might be attributable to GBs, the details were unknown. In this study, the photovoltaic characteristics of UNCD/a-C:H films are experimentally demonstrated. The photovoltaic characteristics of UNCD/a-C:H films are discussed on the basis of photoconduction spectroscopic analysis data.

Direct Determination of the Barrier Height of Au Contact on p-Type Ultrananocrystalline Diamond/Hydrogenated Amorphous Carbon Composite Films

Ultrananocrystalline diamond (UNCD)/hydrogenated amorphous carbon (a-C:H) composite (UNCD/a-C:H) films have a specific film structure, wherein a large number of diamond nanograins are embedded in an a-C:H matrix. The UNCD/a-C:H films possess large optical absorption coefficients of more than 106 cm–1 in the photon energy range between 3 and 6 eV. These optical features are profitable for the application of thin-film based solar cell and UV photodetectors. Although the Metal-Semiconductor-Metal (MSM) geometry is suitable for photodetection by using single-layered UNCD/a-C:H film, the property of film/metal interface hasn’t been investigated in detail. In this study, a direct determination of barrier height of metal contact on p-type UNCD/a-C:H film was performed using X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). As a result, it was revealed that the down bending occurs on the film/metal interface, which indicates that depletion layer might appear on the surface of p-type UNCD/a-C:H by this structure. Whereas the surface of UNCD/a-C:H films is oxidized, that band-alignment of that does not change. It is possible that that different physical phenomena from that of single crystalline diamond occurs. There are numerous dangling bonds on the surface of UNCD/a-C:H film, therefore it is supposed that the dangling bonds have greatly affected the surface state.

Effects of Deposition Temperature on Synthesis of Ultrananocrystalline Diamond/Amorphous Carbon Composite Films on Cemented Carbide Substrate By Coaxial Arc Plasma Deposition

Synthesis of hard carbon coatings on cutting tools is indispensable for machining the nonferrous materials and carbon fiber composites. Diamond, an allotrope of carbon, is classified at the top of carbon coatings for its unique characteristics, it has the highest known hardness and thus provides superior wear resistance. A diamond crystal consists of carbon atoms tetrahedrally bonded with sp3 hybrid bonds. On the basis of its crystal size, polycrystalline diamond is categorized to microcrystalline, nanocrystalline, and ultrananocrystalline diamond. Ultrananocrystalline diamond/amorphous carbon composite (UNCD/a-C) films, wherein a large number of diamond grains with diameters of less than 10 nm are embedded in an amorphous carbon matrix. A high hardness of UNCD/a-C films is attributed to the refinement of grains in nanoscale and its resultant high content of sp3-bonded carbon. To provide highly energetic carbon atoms and thus sp3-rich amorphous carbon films, cathodic arc discharge has ever been employed. In addition to the merits of cathodic arc discharge, coaxial arc plasma deposition technique (CAPD) has the following distinctive feature: a coaxial arc plasma gun is equipped with an anodic cylinder that can bunch ions ejected from a cathodic graphite rod located inside the cylinder. Owing to this structure, a supersaturated condition with highly energetic ions can be realized, which is desired for the growth of UNCD crystallites In this work, the influences of deposition temperature on growth and mechanical properties of the UNCD/a-C films were studied. The films were deposited at different temperatures on cemented carbide (WC-6wt.% Co) plate substrates by using CAPD. The hardness and elastic modulus were investigated by nanoindentation. Structural characterizations of the films have been examined by X-ray diffraction, scanning electron microscopy, hard X-ray photoelectron spectroscopy, and energy-dispersive X-ray. The revealed mechanical and structural properties will be presented.

Enabling long term monitoring of dopamine using dimensionally stable ultrananocrystalline diamond microelectrodes

Chronic dopamine (DA) monitoring is a critical enabling technology to identify the neural basis of human behavior. Carbon fiber microelectrodes (CFM), the current gold standard electrode for in vivo fast scan cyclic voltammetry (FSCV), rapidly loses sensitivity due to surface fouling during chronic neural testing. Periodic voltage excursions at elevated anodic potentials regenerate fouled CFM surfaces but they also chemically degrade the CFM surfaces. Here, we compare the dimensional stability of 150 μm boron-doped ultrananocrystalline diamond (BDUNCD) microelectrodes in 1X PBS during ‘electrochemical cleaning’ with a similar-sized CFM. Scanning electron microscopy and Raman spectroscopy confirm the exceptional dimensional stability of BDUNCD after 40 h of FSCV cycling (∼8 million cycles). The fitting of electrochemical impedance spectroscopy data to an appropriate circuit model shows a 2x increase in charge transfer resistance and an additional RC element, which suggests oxidation of BDUNCD electrode surface. This could have likely increased the DA oxidation potential by ∼34% to +308 mV. A 2x increase in BDUNCD grain capacitance and a negligible change in grain boundary impedance suggests regeneration of grains and the exposure of new grain boundaries, respectively. Overall, DA voltammogram signals were reduced by only ∼20%. In contrast, the CFM is completely etched with a ∼90% reduction in the DA signal using the same cleaning conditions. Thus, BDUNCD provides a robust electrode surface that is amenable to repeated and aggressive cleaning which could be used for chronic DA sensing.

Patterned hydrophobic and hydrophilic surfaces of ultra-smooth nanocrystalline diamond layers

In this work, we show that ultra nanocrystalline diamond (UNCD) surfaces have been modified to add them hydrophobic and hydrophilic properties. The nanocrystalline diamond films were deposited using the hot filament chemical vapor deposition (HFCVD) technique. This allows growing diamond on different substrates which can be even 3D or structured. Silicon and, for optical applications, transparent quartz glass are the preferred substrates for UNCD layers growth. Fluorine termination leads to strong hydrophobic properties as indicated by a high contact angle for water of more than 100°. Hydrogen termination shows lesser hydrophobic behavior. Hydrophilic characteristics has been realised with oxygen termination. X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray spectroscopy (EDX) measurements confirm the oxygen and fluorine- termination on the nanocrystalline diamond surface. Further, by micropatterning using photolithography, multi-terminated layers have been created with both hydrophobic and hydrophilic areas. In addition, we have shown that retermination is achieved, and the properties of the surface have been changed from hydrophobic to hydrophilic and vice versa. Micro- roughness and stress in the grown film influences slightly the wetting angle as well. The opportunity to realize local differences in hydrophobicity on nanocrystalline diamond layers, in any size or geometry, offers interesting applications for example in microbiological investigations. Multi-terminated arrays show identical surface roughness and at the same time differences in hydrophobicity. These arrays have been visualized with scanning electron microscopy (SEM) and lateral force microscopy (LFM).

Hard coating of ultrananocrystalline diamond/nonhydrogenated amorphous carbon composite films on cemented tungsten carbide by coaxial arc plasma deposition

Ultrananocrystalline diamond (UNCD)/nonhydrogenated amorphous carbon (a-C) composite (UNCD/a-C) films were deposited on cemented carbide containing Co by coaxial arc plasma deposition. With decreasing substrate temperature, the hardness was enhanced accompanied by an enhancement in the sp3/(sp2 + sp3). Energy-dispersive X-ray and secondary ion mass spectrometry spectroscopic measurements exhibited that the diffusion of Co atoms from the substrates into the films hardly occurs. The film deposited at room temperature exhibited the maximum hardness of 51.3 GPa and Young’s modulus of 520.2 GPa, which evidently indicates that graphitization induced by Co in the WC substrates, and thermal deformation from sp3 to sp2 bonding are suppressed. The hard UNCD/a-C films can be deposited at a thickness of approximately 3 μm, which is an order larger than that of comparably hard a-C films. The internal compressive stress of the 51.3-GPa film is 4.5 GPa, which is evidently smaller than that of comparably hard a-C films. This is a reason for the thick deposition. The presence of a large number of grain boundaries in the film, which is a structural specific to UNCD/a-C films, might play a role in releasing the internal stress of the films.

Optimizing growth and post treatment of diamond for high capacitance neural interfaces

Electrochemical and biological properties are two crucial criteria in the selection of the materials to be used as electrodes for neural interfaces. For neural stimulation, materials are required to exhibit high capacitance and to form intimate contact with neurons for eliciting effective neural responses at acceptably low voltages. Here we report on a new high capacitance material fabricated using nitrogen included ultrananocrystalline diamond (N-UNCD). After exposure to oxygen plasma for 3h, the activated N-UNCD exhibited extremely high electrochemical capacitance greater than 1mF/cm(2), which originates from the special hybrid sp(2)/sp(3) structure of N-UNCD. The invitro biocompatibility of the activated N-UNCD was then assessed using rat cortical neurons and surface roughness was found to be critical for healthy neuron growth, with best results observed on surfaces with a roughness of approximately 20nm. Therefore, by using oxygen plasma activated N-UNCD with appropriate surface roughness, and considering the chemical and mechanical stability of diamond, the fabricated neural interfaces are expected to exhibit high efficacy, long-term stability and a healthy neuron/electrode interface.

Diamond coated artificial cardiovascular devices.

Ultrananocrystalline diamond (UNCD), an extremely smooth, low cost diamond coating was successfully developed herein for antithrombogenic application which requires high biocompatibility, low wear, low friction, and chemical inertness. The substrate materials utilized in the Jarvik 2000 ventricular assist device (VAD), silicon carbide and titanium alloy, provide an excellent substrate match for UNCD integration. The paper addresses the development of medical-quality UNCD films to significantly improve the knowledge base regarding the defect mechanisms of UNCD films, to reduce or eliminate known wear-inducing imperfections in the film, and to thoroughly characterize and test the films as well as assembled UNCD-coated VADs. After the defect reduction and seeding experiments to improve film adhesion and coating quality, the best candidate deposition method has been down-selected for coating and assembly of VAD parts from Jarvik Heart. The coated and assembled devices have been tested with mechanical and blood-simulating fluid hydrodynamic testing at Jarvik Heart for full verification of the new coating technology. UNCD interface takes advantage of combining unmatched durability and antithrombogenicity.

Plasma surface fluorination of ultrananocrystalline diamond films

Abstract Ultrananocrystalline diamond films have been subjected to CHF3 and SF6 rf plasmas in order to achieve a fluorine terminated surface. Major parameter varied in this study was besides the nature of the process gas the exposure time. The treated surfaces have been investigated by contact angle and atomic force microscopy measurements; X-ray photoelectron spectroscopy has been used to determine the surface composition and to get insight into the bonding environments of the treated surfaces. Both plasma treatments lead to similar results. The surfaces are fluorinated by fluorine containing groups, namely CF, CF2, and CF3, with F concentrations of about 20%. This leads to high contact angles of 95–110°; the surfaces are slightly etched and became somewhat smoother. Nevertheless, there are distinct differences between the surface obtained by the CHF3 and SF6 treatments, respectively, the nature of which will be discussed in this paper, together with the underlaying mechanisms.

Long-Life Silicon/Graphite Anode with Nano Carbon Coatings for Lithium Ion Battery - Graphene Nanowalls and Nitrogen Incorporated Ultrananocrystalline Diamond

Although lithium ion batteries (LIBs) have been used in many consumer and industrial products, the energy storage capacity per volume/weight and the battery cycling lifetime have much room for further improvement. This is especially challenging for high demanding applications such as long-range electric vehicles, and smart grid renewable energy storage. Among LIB anode materials, graphite is the most common one in the market today. Many novel anode materials have been studied in pursuit of much higher battery energy storage capacity for a given volume and weight. Among them, silicon is the most attractive one which theoretically is capable of storing more than ten times of energy than that of graphite anode for the same anode weight. However, silicon suffers from extensive volume expansion and shrinkage by about 400% when it cycles through fully charged and discharged states. The volume difference leads to breakage of silicon particles and the loss of their electrical contacts to the current collector and thus its capability of further discharging and charging. On the other hand, graphite has a layered structure, which suffers from breakage between adjacent layers into smaller pieces and thus possibly loses the capability of broken apart small graphite pieces in charging and discharging. Therefore, technologies for preventing aforementioned huddles against achieving long-life and high-capacity LIBs are highly desirable. Novel electrically conductive ultrananocrystalline diamond, graphene nanowalls, and diamond-graphene hybrid nano-carbon coatings have been applied by plasma enhanced chemical vapor deposition technique to graphite and silicon based LIB anodes and achieved much longer cycling lifetime compared to those without such nano carbon coatings. This paper will present the nano-carbon coating processes, materials characterization, LIB fabrication processes, and the characterization of LIB half cells which demonstrates enhance lifetime and charge storage capacity.

Measurement of the strength and range of adhesion using atomic force microscopy

Adhesion of nanoscale contacts is important in many applications, including microelectromechanical systems, fibrillar adhesives, and atomic force microscopy (AFM). Here, we quantify the properties of the adhesive traction–separation relation between ultrananocrystalline diamond (UNCD) AFM tips and polymethyl methacrylate (PMMA) surfaces using a novel AFM-based method that combines pull-off force measurements and characterization of the 3D geometry of the AFM tip. Three AFM tips with different nanoscale geometries were characterized and used to perform pull-off force measurements. Using the pull-off force data, the measured 3D tip geometries, and an assumed form of the traction–separation relation, specifically the Dugdale or 3-9 Lennard-Jones relations, the range, strength, and work of adhesion of the UNCD–PMMA contact were determined. The assumptions in the analyses were validated via finite element modeling. Both forms of the traction–separation laws result in a work of adhesion of approximately 50 mJ/m2 and the peak adhesive stress in the Lennard Jones relation is found to be about 50% higher than that obtained for the Dugdale law.

Porous diamond foam with nanometric diamond grains using Bias Enhanced Nucleation on iridium

Porous diamond foam exhibiting a three-dimensional structure was synthesized by applying prolonged Bias Enhanced Nucleation on iridium buffer layers. The Raman spectrum of this material recorded at 325nm looks very close to the signature obtained for detonation nanodiamonds. HRTEM investigations well confirm the presence of diamond crystals with nanometric size between 2 and 6nm. All these diamond nanocrystals form a three-dimensional structure very different from ultrananocrystalline diamond (UNCD) generally embedded in a graphitic matrix. According to its high specific surface area, such a 3D material may have promising applications for sensors.

SYNTHESIS OF GRAPHENE/DIAMOND DOUBLE-LAYERED STRUCTURE FOR IMPROVING ELECTRON FIELD EMISSION PROPERTIES

Ultrananocrystalline diamond (UNCD) films on silicon were prepared by microwave plasma chemical vapor deposition (MPCVD) method using argon-rich CH4/H2/Ar plasmas. The graphene sheets synthesized by chemical vapor deposition (CVD) were successfully transferred on to the UNCD surface to fabricate electron field emission (EFE) property-enhanced graphene/UNCD films. The surface morphology, structure and composition of the graphene/UNCD double-layered structures were characterized by scanning electron microscope (SEM), atomic force microscope (AFM), Raman spectroscopy and grazing incidence X-ray diffraction (GXRD). GXRD clearly shows the characteristic diffraction peaks of both diamond and graphene. The Raman spectrum shows the characteristic band of diamond at 1332[Formula: see text]cm[Formula: see text] and D, G and 2D bands of graphene at 1360, 1550 and 2610[Formula: see text]cm[Formula: see text], respectively. The EFE behavior of the composite films can be turned on at [Formula: see text][Formula: see text]V/[Formula: see text]m, attaining a current density of 0.065[Formula: see text]mA/cm2at an applied field of 7.3[Formula: see text]V/[Formula: see text]m.

Plasma Etching Chemistry for Smoothening Ofultrananocrystalline Diamond Films

Ultrananocrystalline diamond (UNCD) is characterized by the dimensions of its ultra-nanoscale grains and is typically synthesized under high nucleation conditions using an argon-rich hydrogen/methane/argon gas mixture. UNCD films consist of crystalline grains of at least 95% sp3-bonded carbon that are 2-10 nm in size with a surface roughness ranging between 20 and 40 nm. UNCD films exhibit most of the outstanding properties of diamond such as an exceptional hardness, high mechanical strength, extremely low friction coefficient, chemical inertness, thermal stability and high electrical conductivity at room temperature when doped with nitrogen. While all these properties are intrinsic of the material, the required roughness scale of the UNCD film needs to be carefully tailored and customized on an application-by-application basis. For instance, a low roughness is key to manufacturing adequate micro-electromechanical systems (MEMS), wear-resistant low friction coatings, X-ray lithography masks and biomedical devices where stringent tolerance and topological precision are of utmost importance. Here, a new process for controlled smoothening of ultrananocrystalline diamond films using H2/O2 plasma is presented. Diamond films were exposed to oxygen, hydrogen and hydrogen/oxygen mixture (19:1) plasmas. Evolution of morphology and thickness were monitored ex-situ using scanning electron microscopy (SEM) while roughness was measured using atomic force microscopy (AFM). Neither pure oxygen nor pure hydrogen plasmas were found suitable for smoothening the films. The H2/O2 mixture plasma exposure resulted in a constant roughness decrement of 0.043 nm/min with a slow etching rate (0.07 µm/h) allowing for the control of the final film roughness without sacrificing too much of the material thickness.