Ultrananocrystalline Diamond Research Articles

Cryogenic operation of planar ultrananocrystalline diamond field emission source in SRF injector

Here, we report a demonstration of electron beam generation in a superconducting radio frequency (SRF) Teraelectronvolt Energy Superconducting Linear Accelerator 1.3 GHz gun equipped with a field emission cathode and operated at 2 K. The cathode is a submicrometer film of nitrogen-incorporated ultrananocrystalline diamond [(N)UNCD] deposited atop a Nb RRR300 cathode plug that is 4 mm in diameter. Measurements of the output current showed that it increased exponentially as a function of the gun gradient. Our results demonstrate a feasible path toward simplified fully cryogenic SRF injector technology. At a maximal gradient of 0.9 MV/m, a current density of 22 μA/cm2 was obtained [calculated as a 2.75 μA current over the total cathode surface area of π(2 mm)2]. One important finding is that the electron emitter made of (N)UNCD, a material long been known as a highly efficient field emission material, demonstrated a record low turn-on gradient of 0.6 MV/m. A hypothesis explaining this behavior is proposed.

Ultrananocrystalline Diamond Nanowires: Fabrication, Characterization, and Sensor Applications.

The aim of this review is to provide a survey of the recent advances and the main remaining challenges related to the ultrananocrystalline diamond (UNCD) nanowires and other nanostructures which exhibit excellent capability as the core components for many diverse novel sensing devices, due to the unique material properties and geometry advantages. The boron or nitrogen doping introduced in the gas phase during deposition promotes p-type or n-type conductivity. With the establishment of the UNCD nanofabrication techniques, more and more nanostructure-based devices are being explored in measuring basic physical and chemical parameters via classic and quantum methods, as exemplified by gas sensors, ultraviolet photodetectors, piezoresistance effect-based devices, biological applications and biosensors, and nitrogen-vacancy color center-based magnetic field quantum sensors. Highlighted finally are some of the remaining challenges and the future outlook in this area.

Nitrogen doped ultrananocrystalline diamond conductive layer grown on InGaN-based light-emitting diodes using nanopattern enhanced nucleation

This study grew nitrogen doped ultrananocrystalline diamond (N-UNCD) conductive layer on InGaN-based light emitting diodes (LEDs) using the concave nanopattern (CNP) enhanced nucleation. The low nucleation density of the N-UNCD on bare LEDs (Br-LED) resulted in an island morphology. The nucleation density of the N-UNCD increased significantly from 1.8 × 108 cm−2 for Br-LED to 3.6 × 109 cm−2 for LEDs with a CNP density of 6.1 × 109 cm−2 (CNP-LED). The N-UNCD preferred to nucleate inside the CNP during the initial growth stage. The N-UNCD islands then merged and grew laterally to form a continuous thin film within a thickness of 300 nm. The N-UNCD/CNP-LED exhibited a stable electroluminescence peak wavelength of ~447.1 nm in the injection current range of 10–100 mA. The decrease in the compressive stress due to removing the p-GaN layer (i.e. the CNP structure) resulted in a quantum confined Stark effect (QCSE) mitigation in the multi-quantum wells (MQWs). In addition, the lower turn on voltage caused a lower electric field in the MQWs and diminished the screen of the QCSE. The N-UNCD prepared by the proposed nucleation technique demonstrated a promising conductive layer for InGaN-based LEDs.

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.

Enhanced electrochemical capacitance of nitrogen-doped ultrananocrystalline diamond through oxygen treatment

The electrochemical capacitance of nitrogen-doped ultrananocrystalline diamond (N-UNCD) can be dramatically increased by treating the surface with an RF-oxygen plasma. Such treated surfaces display excellent properties for use as electrodes in neural stimulation and recording. In the present work, we elucidate the origins of this phenomenon by investigating the effects of different methods of oxygen termination. We found that the increase in electrochemical capacitance is dependent on the details of the method used for oxygen termination. Whilst N-UNCD subjected to UV/ozone treatment, oxygen plasma treatment, and furnace annealing in oxygen gas all displayed increased surface capacitance, the highest capacitance was exhibited by the oxygen annealed sample, with which we achieved ~ 3 orders of magnitude increase in the electrochemical capacitance as compared to the as-grown sample. The maximum recorded capacitance was 3746 ± 132 µF cm−2, which is substantially greater than previously reported N-UNCD electrodes’ electrochemical capacitance (1070 µF cm−2, W. Tong. et al, 2016). Our findings point to the presence of sub-surface solid state capacitance which contributes significantly to the observed electrochemical capacitance of the oxygen terminated N-UNCD electrodes. When combined with the favourable biocompatibility and inertness of the N-UNCD, our approach may provide a route towards the development of advanced neural sensing and stimulating electrodes.

Dynamic graphitization of ultra-nano-crystalline diamond and its effects on material resistivity

Detailed structural and electrical properties of ultra-nano-crystalline diamond (UNCD) films grown in H2/CH4/N2 plasma were systematically studied as a function of deposition temperature (Td) and nitrogen content (% N2) to thoroughly evaluate their effects on resistivity. It was found that even the films grown with no nitrogen in the synthetic gas mixture could be made as conductive as 10−2 Ω cm. The overall resistivity of all the films was tunable over 4 orders of magnitude through varying growth parameters. On a set of 27 samples, Raman spectroscopy and scanning electron microscopy show a progressive and highly reproducible material phase transformation from ultra-nano-crystalline diamond to nano-crystalline graphite as deposition temperature increases. The rate of this transformation is heavily dependent on the N2 content estimated by secondary ion mass spectroscopy. The addition of nitrogen greatly increases the amount of sp2 bonded carbon in the films, thus enhancing the physical connectivity in the grain boundary (GB) network that has high electronic density of states. However, the addition of nitrogen greatly slows down crystallization of the sp2 phase in the GBs compromising electron transport. Therefore, the proper balance between GB connectivity and crystallinity is the key in resistivity engineering of UNCD.

Tribological performance and wear mechanism of smooth ultrananocrystalline diamond films

• HFCVD is used to synthesize UD-MCD, UD-UNCD, UD-MCD/UD-UNCD, BD-MCD, BD-UNCD, BD-MCD/BD-UNCD films on WC-Co substrates. • Growth mechanism of UNCD with high carbon concentration and low gas pressure in hydrogen-rich chemistry are illustrated. • Undoped and boron doped UNCD coatings are firstly applied on WC-Co micro drills to improve the surface smoothness. • Both boron doping and middle layer MCD contribute to the strong adhesive level of UD-MCD/UD-UNCD and BD-MCD/BD-UNCD. • The UD-MCD/UD-UNCD, BD-UNCD and BD-MCD/BD-UNCD coated PCB micro drills show superior cutting performances. 5G printed circuit boards (PCBs), composed of glass fiber, epoxy resin and ceramic, are a kind of difficult-to-machine material. The manufacture of massive high-quality micro holes on 5G PCBs raise high requirements on the tool life and machining precision of PCB micro drills. Chemical vapor deposition (CVD) diamond coatings can be used as effective protective films on PCB micro drills. The surface smoothness and coating-substrate adhesion of diamond coated micro drills are two important factors affecting the drilling performances. Different from previously reported microcrystalline diamond (MCD) or nanocrystalline diamond (NCD) coating types on cutting tools, undoped (UD-) and boron doped (BD-) ultrananocrystalline diamond (UNCD) coatings with higher surface smoothness are firstly applied on PCB micro drills to reduce the friction and cutting resistance and improve the cutting performances while machining PCBs. Moreover, different from the usual argon-rich gas atmosphere for UNCD synthesis, the synthesis mechanisms of UD-UNCD and BD-UNCD with hydrogen-rich atmosphere adopting HFCVD method are clarified. Sufficiently high renucleation rate of diamond crystallites, long mean free path of reactive precursor species and high removal rate of non-diamond carbon phases lead to the successful synthesis of UNCD films. UDMCD, UD-UNCD, UD-MCD/UD-UNCD, BD-MCD, BD-UNCD, and BD-MCD/BD-UNCD, were synthesized on PCB micro drills using HFCVD. The tribological performances and wear mechanisms of these different diamond coatings were studied systematically. The dominant wear mechanism causing tool wear and fracture of diamond coated micro drills in high-speed machining PCBs is abrasive wear. The degree of abrasive wear while machining PCBs are mainly determined by the surface smoothness, surface roughness and coefficient of friction of diamond coatings. The top layer UNCD with tiny grain size leads to the improvement of surface smoothness and tribological properties, which enhance the chip evacuation capacity and reduce the tool wear. Besides, both boron doping and middle layer MCD can contribute to the strong coating-substrate adhesion. Therefore, BD-MCD/BD-UNCD coated micro drill exhibits the best cutting performances while machining PCBs.

Evidence of hydrogen termination at grain boundaries in ultrananocrystalline diamond/hydrogenated amorphous carbon composite thin films synthesized via coaxial arc plasma

Ultrananocrystalline diamond/hydrogenated amorphous carbon composite thin films consist of three different components: ultrananocrystalline diamond crystallites, hydrogenated amorphous carbon, and grain boundaries between them. Since grain boundaries contain many dangling bonds and unsaturated bonds, they would be a cause of carrier trap center degrading device performance in possible applications such as UV photodetectors. We experimentally demonstrate that hydrogen atoms preferentially incorporate at grain boundaries and terminate dangling bonds by means of several spectroscopic techniques. XPS measurements cannot detect quantitative transitions of sp2- and sp3-hybridized carbons in the films, resulting in 55%–59% of sp3 content. On the other hand, Fourier transform infrared spectroscopy and near-edge x-ray absorption fine structure exhibit some variations of the amounts of certain carbon hybridization for sure. The former confirms the transformation from sp2 to sp3 hydrocarbons by ∼10% by additional hydrogenation, and the latter represents chemical configuration changes from π* C≡C and π* C=C to σ* C—H as well as more σ* C—C. These results can be evidence of localized hydrogen at grain boundaries, which plays a part in terminating dangling bonds and unsaturated bonds, and they are correlated with the optical and electrical properties of the films investigated in some previous research studies. Our spectroscopic studies on the hydrogenation effects combined with the discussion on the optical and electrical characteristics confirm that the hydrogenation can be an effective tool for the enhancement of photovoltaic performance in the above sensing applications.

Demonstration of nitrogen-incorporated ultrananocrystalline diamond photocathodes in a RF gun environment

Quantum efficiency (QE), intrinsic emittance, and robustness are the three most important figures of merit for photocathodes, the first two determine the ultimate achievable brightness of an electron beam, and the third one directly correlates with the complications of a beamline design. Nitrogen-incorporated ultrananocrystalline diamond [(N)UNCD] materials are promising candidates for photocathode applications due to their remarkable electron emission performance as well as the moderate vacuum requirement. Two (N)UNCD photocathodes have been characterized in a realistic RF gun environment with the nicely balanced performance of all three figures of merit. The QE of the first (N)UNCD cathode (stored in air for two years before the test) was found to be 3.8 × 10−4 using a 262 nm UV laser and a cathode surface field of 30 MV/m. It was found that the QE of the second (N)UNCD sample (grown days before the test) was nearly the same and, therefore, demonstrates the exceptional environmental tolerance of the material. The intrinsic emittance of (N)UNCD was measured to be 1.00 μm/mm.

Diamond-Coated Plasma Probes for Hot and Hazardous Plasmas.

Plasma probes are simple and inexpensive diagnostic tools for fast measurements of relevant plasma parameters. While in earlier times being employed mainly in relatively cold laboratory plasmas, plasma probes are now routinely used even in toroidal magnetic fusion experiments, albeit only in the edge region, i.e., the so-called scrape-off layer (SOL), where temperature and density of the plasma are lower. To further avoid overheating and other damages, in medium-size tokamak (MST) probes are inserted only momentarily by probe manipulators, with usually no more than a 0.1 s per insertion during an average MST discharge of a few seconds. However, in such hot and high-density plasmas, their usage is limited due to the strong particle fluxes onto the probes and their casing which can damage the probes by sputtering and heating and by possible chemical reactions between plasma particles and the probe material. In an attempt to make probes more resilient against these detrimental effects, we tested two graphite probe heads (i.e., probe casings with probes inserted) coated with a layer of electrically isolating ultra-nano-crystalline diamond (UNCD) in the edge plasma region of the Experimental Advanced Superconducting Tokamak (EAST) in Hefei, People’s Republic of China. The probe heads, equipped with various graphite probe pins, were inserted frequently even into the deep SOL up to a distance of 15 mm inside the last closed flux surface (LCFS) in low- and high-confinement regimes (L-mode and H-mode). Here, we concentrate on results most relevant for the ability to protect the graphite probe casings by UNCD against harmful effects from the plasma. We found that the UNCD coating also prevented almost completely the sputtering of graphite from the probe casings and thereby the subsequent risk of re-deposition on the boron nitride isolations between probe pins and probe casings by a layer of conductive graphite. After numerous insertions into the SOL, first signs of detachment of the UNCD layer were noticed.

Determination of sp2 and sp3 phase fractions on the surface of diamond films from C1s, valence band X-ray photoelectron spectra and CKVV X-ray-excited Auger spectra

The practice of applying the methods for determining the relative contents of carbon atoms in sp2 and sp3 hybridization states from the profiles of C1s X-ray photoelectron and carbon KVV (CKVV) Auger spectra is analyzed. Difficulties in using C1s and CKVV spectra for determining the sp2/sp3 ratios are discussed. A method is proposed for evaluating the fractions of sp2 and sp3 phases based on the fitting of the valence band X-ray photoelectron spectra by superposition of valence band spectra of samples with carbon atoms in pure sp2 and sp3 states. Relative fractions of sp2 and sp3 phases on the virgin surfaces of two ultrananocrystalline diamond films, and on their surfaces subjected to friction, were determined using C1s spectra, CKVV Auger spectra, and valence band spectra. All three methods gave the results that are consistent for virgin film surfaces with accuracy of 2%, and for the surfaces of films subjected to friction, 15%. In contrast to the ultrananocrystalline films, three XPS-based methods applied to a microcrystalline diamond film yielded significantly different sp2/sp3 ratios. The reasons for this discrepancy are shown to lie in different analysis depths of these methods, and in different morphology of the film surfaces.

Laminin coated diamond electrodes for neural stimulation

The performance of many implantable neural stimulation devices is degraded due to the loss of neurons around the electrodes by the body's natural biological responses to a foreign material. Coating of electrodes with biomolecules such as extracellular matrix proteins is one potential route to suppress the adverse responses that lead to loss of implant functionality. Concurrently, however, the electrochemical performance of the stimulating electrode must remain optimal to continue to safely provide sufficient charge for neural stimulation. We have previously found that oxygen plasma treated nitrogen included ultrananocrystalline diamond coated platinum electrodes exhibit superior charge injection capacity and electrochemical stability for neural stimulation (Sikder et al., 2019). To fabricate bioactive diamond electrodes, in this work, laminin, an extracellular matrix protein known to be involved in inter-neuron adhesion and recognition, was used as an example biomolecule. Here, laminin was covalently coupled to diamond electrodes. Electrochemical analysis found that the covalently coupled films were robust and resulted in minimal change to the charge injection capacity of diamond electrodes. The successful binding of laminin and its biological activity was further confirmed using primary rat cortical neuron cultures, and the coated electrodes showed enhanced cell attachment densities and neurite outgrowth. The method proposed in this work is versatile and adaptable to many other biomolecules for producing bioactive diamond electrodes, which are expected to show reduced the inflammatory responses in vivo.

Submicrometer ultrananocrystalline diamond films processed in oxygen and hydrogen plasma and analyzed by UV-vis spectroscopy: Thickness and optical constant results

Results of UV-vis spectroscopy (spectrophotometry) of highly conductive submicrometer nitrogen-incorporated ultrananocrystalline diamond, (N)UNCD, processed in technologically important oxygen and hydrogen plasmas are presented for the spectral range of 200–1200 nm. The (N)UNCD films feature high contrast interference. As-grown and etched samples were analyzed using a simple practical analytical optics methodology that allowed for accurate evaluation of film thicknesses and (N)UNCD etching rates. The obtained results were cross-validated using scanning electron microscopy. Reflection simulations based on the Fresnel equations and using the optical constants of the Si substrate provided for evaluation of spectral dependencies of the refractive index and extinction coefficient of (N)UNCD. It was found that the presence of grain boundary sp2 phase causes noticeable extinction (zero for diamond) and slightly changes the refractive index, making its behavior nonmonotonic.

Impedance spectroscopy analysis of n-type (nitrogen-doped) ultrananocrystalline diamond/p-type Si heterojunction diodes

Heterojunction diodes are constructed by growing n-type (nitrogen-doped) ultrananocrystalline diamond (UNCD)/hydrogenated amorphous carbon (a-C:H) composite films onto p-type Si substrates in nitrogen and hydrogen mixed-gases by coaxial arc plasma deposition. The fabricated heterojunctions are analyzed regarding their film morphology and electrical properties. The complex structure of UNCD/a-C:H films, which consists of nano-sized-sp3 diamond grains and sp2-grain boundaries (GBs), makes it difficult to separate their contribution to electrical conductivity of the films using conventional characterization methods. In this paper we characterize the n-type UNCD/p-type Si heterojunction diode by employing impedance spectroscopy method, which can isolate the differing components that contribute to the overall conductivity of the film. The impedance spectroscopy is measured in the frequency range of 100 Hz to 2 MHz, with AC small signal superimpose on DC bias voltage in the range of 0–1 V. The influence of the bias on UNCD grains and GBs contribution to resistance and capacitance of UNCD/a-C:H film and n-UNCD/p-Si heterojunction is investigated by equivalent circuit model using fitting of the impedance data. The results revealed that the electrical conductivity is mainly controlled by the GBs rather than the UNCD grains. Furthermore, the extracted value of dielectric constant of N-doped UNCD/a-C:H film is found to be comparable with that of microcrystalline diamond, which indicates the capacitance contribution in the device is mainly originated from the UNCD grains. This study demonstrates capability of impedance spectroscopy to provide an obvious separated contribution of sp3 and sp2 bonded carbons to the electrical conductivity in their coexistence materials.

Integration of Ultrananocrystalline Diamond (UNCD)-Coatings on Chemical-Mechanical Surface Nano-structured (CMNS) Titanium-Based Dental Implants

This paper focuses on describing the integration of ultrananocrystalline diamond (UNCD) coating on pure titanium-based dental implants (DIs) integrated with the surface pre-treatment by chemical-mechanical nano-structuring (CMNS) process. The combination of the UNCD coating with the CMNS metal surface treatment provides a transformational process to produce a new generation of metallic implants. CMNS promotes a uniform and dense titanium oxide interface and UNCD enables higher resistance to chemical-induced corrosion by oral fluids and enhanced bone attachment due to superior bone cell growth on C atoms (element of life in human DNA and cell). The main focus of the presented research is to establish the preliminary studies on the integration of the UNCD coating process on CMNS treated dental implants to promote corrosion resistance and biocompatibility. It is demonstrated that the CMNS process in the presence of an oxidizer (1M to be optimal) induces a tailored interface to promote UNCD coating capability through effective interface passivation leading to uniform surface coverage. The final implant product is observed to have improved corrosion potential and enhanced hydrophobicity indicating better biocompatibility providing the basis for a new generation of superior DIs. The findings can further be extended to the hip, knee, and other orthopedic metallic implants, which require major performance improvements, particularly in reducing or eliminating in-vivo body fluid-induced chemical corrosion.

In vivo feasibility of epiretinal stimulation using ultrananocrystalline diamond electrodes

Objective. Due to their increased proximity to retinal ganglion cells (RGCs), epiretinal visual prostheses present the opportunity for eliciting phosphenes with low thresholds through direct RGC activation. This study characterised the in vivo performance of a novel prototype monolithic epiretinal prosthesis, containing Nitrogen incorporated ultrananocrystalline (N-UNCD) diamond electrodes. Approach. A prototype implant containing up to twenty-five 120 × 120 µm N-UNCD electrodes was implanted into 16 anaesthetised cats and attached to the retina either using a single tack or via magnetic coupling with a suprachoroidally placed magnet. Multiunit responses to retinal stimulation using charge-balanced biphasic current pulses were recorded acutely in the visual cortex using a multichannel planar array. Several stimulus parameters were varied including; the stimulating electrode, stimulus polarity, phase duration, return configuration and the number of electrodes stimulated simultaneously. Main results. The rigid nature of the device and its form factor necessitated complex surgical procedures. Surgeries were considered successful in 10/16 animals and cortical responses to single electrode stimulation obtained in eight animals. Clinical imaging and histological outcomes showed severe retinal trauma caused by the device in situ in many instances. Cortical measures were found to significantly depend on the surgical outcomes of individual experiments, phase duration, return configuration and the number of electrodes stimulated simultaneously, but not stimulus polarity. Cortical thresholds were also found to increase over time within an experiment. Significance. The study successfully demonstrated that an epiretinal prosthesis containing diamond electrodes could produce cortical activity with high precision, albeit only in a small number of cases. Both surgical approaches were highly challenging in terms of reliable and consistent attachment to and stabilisation against the retina, and often resulted in severe retinal trauma. There are key challenges (device form factor and attachment technique) to be resolved for such a device to progress towards clinical application, as current surgical techniques are unable to address these issues.

Diode Parameters and Equivalent Electrical Circuit Model of n-Type Silicon/B-Doped p-Type Ultrananocrystalline Diamond Heterojunctions Manufactured Through Coaxial Arc Plasma Deposition.

Coaxial arc plasma deposition (CAPD) was employed to manufacture n-type silicon/boron-doped p-type ultrananocrystalline diamond heterojunctions. Measurement and analysis of their dark current density-voltage curve were carried out at room temperature in order to calculate the requisite junction parameters using the Cheung and Norde approaches. For the calculation based on the Cheung approach, the series resistance (Rs), ideality factor (n) and barrier height (Φb) were 4.58 kΩ, 2.82 and 0.75 eV, respectively. The values of Rs and Φb were in agreement with those calculated using the Norde approach. Their characteristics for alternative current impedance at different frequency values were measured and analyzed as a function of the voltage (V) values ranging from 0 V to 0.5 V. Appearance of the real (Z') and imaginary (Z″) characteristics for all V values presented single semicircles. The centers of the semicircular curves were below the Z' axis and the diameter of the semicircles decreased with increments of the V value. The proper equivalent electrical circuit model for the manufactured heterojunction behavior was comprised of Rs combined with the parallel circuit of resistance and constant phase element.

Diode parameters and ultraviolet light detection characteristics of n-type silicon/p-type nanocrystalline diamond heterojunctions at different temperatures

n-Type silicon (Si)/p-type boron (B)-doped ultrananocrystalline diamond (UNCD) heterojunctions were manufactured through coaxial arc plasma deposition, and were examined in terms of the diode parameters and ultraviolet (UV) photodetection at different temperatures. The structure of the deposited films was examined by Raman spectroscopy and a field emission scanning electron microscope. The Raman spectra revealed two wide peaks centered at the positions of 1345 cm−1 and 1585 cm−1 that represented the D and G peaks, respectively. At 300 K, the current density-voltage characteristics showed a considerable high leakage current, which was reduced over two orders of magnitude at 150 K. The ideality factor (n) increased from 2.82 at 300 K to 6.37 at 150 K. In parallel, the barrier height was reduced from 0.75 eV at 300 K to 0.43 eV at 150 K. Through Norde’s approach, the series resistance values were found to be 4.33 kΩ at 300 K and 277.52 kΩ at 150 K, which increased by decreasing the temperature because of the increased n and the lack of carrier mobility in the B-doped UNCD/a-C:H films. Although a small UV response was observed at 300 K, the photocurrent at 150 K was over two orders of magnitude above the dark current at zero voltage. The detectivity values at 0 V were 2.37 × 109 Hz1/2 cm/W and 1.34 × 1010 Hz1/2 cm/W at 300 K and 150 K, respectively. This research proposed a comprehensive study on the n-type Si/p-type UNCD heterojunctions as candidate photodiodes for UV photodetection applications.

Microstructure and electronic properties of ultra-nano-crystalline-diamond thin films

Ultra-nano-crystalline diamond (UNCD) thin films with average thickness ∼200 nm, were grown on n-type mirror polished silicon (100) substrates using microwave plasma enhanced chemical vapour deposition system in different gas (H2 - N2 - Ar - CH4) composition plasma atmospheres at 1200 W (2.45 GHz) and in a pressure of 120 Torr with plasma-temperature ∼475 °C. Raman spectroscopy was used for microstructural study and nano-indentation was used for Hardness/Young’s modulus study; whereas X-ray absorption near edge structure, X-ray photoelectron and ultraviolet photoemission spectroscopies were used for electronic structure of UNCD thin films. The hardness of the films is found to be ∼30 GPa, Young’s modulus ∼300 GPa and induced electron field emission, the turn on electric field, ETOE = 11 V/μm. All results show that the UNCD could be useful for different industrial semiconductor/optoelectronic devices and as flexible materials for thin film coating technology.

Deposition of mirror-like surface finish ultrananocrystalline diamond films on tungsten carbide by optimizing the substrate pretreatment

In this study, we successfully fabricated ultrananocrystalline diamond (UNCD) films on tungsten carbide with mirror-like surface finish (Ra < 40 nm), strong adhesion, and exceptional tribological performance by adopting the optimized two-step pretreatment process. It was found that the ultrasonication in Murakami reagent will substantially increase the substrate surface roughness even with short duration, which is less controllable compared with that of immersion in Murakami reagent. Besides, the surface roughness is strongly affected by acid etching when the substrate is firstly ultrasonicated in Murakami reagent. However, it is much less influenced for the immersion pretreatment. With the UNCD film, a decrease in the surface roughness up to 72% can be realized compared with the as-pretreated substrate. The Rockwell indentation tests demonstrate that the adhesive strength of the UNCD film deposited with the optimized pretreatment is comparable to that of MCD and NCD films as reported in other studies. Ball-on-plate tribotests with Si3N4 counterpart balls reveal that the coefficient of friction (COF) of the UNCD film is 0.07–0.12, which is 76.6% and 41.6% lower compared with that of the MCD and NCD film. The wear rate of the UCND film is 0.708–1.844 × 10−7 mm3/(N·m), close to that of the MCD and NCD film. The results reported in study provide significant insights into the scientific and industrial research of diamond-coated WC-Co cutting tools.