Diamond is considered as a very promising material for the development of devices for radiation detection. Unlike other conventional photoconductive detectors diamond-based devices should provide high discrimination between UV and visible radiation. In this work we present the electro-optical properties of devices based on randomly oriented diamond films, synthesized in a microwave plasma enhanced chemical vapor deposition reactor. A comparative study on devices with coplanar interdigitated Cr/Au electrodes (with different interelectrode pitches) made of films grown simultaneously on intrinsic and p-doped silicon (100) substrates has been performed. The chemical-structural, morphological, electrical and optical properties of ROD films have been studied. In particular, the optical response has been measured in air using a Xe flash lamp coupled with an optical quartz fiber and a properly tailored front-end electronics based on a charge sensitive amplifier. Experimental results gave indications on how the device performances are dependent on the two types of employed substrates.

Different aspects concerning the use of nanocrystalline diamond (NCD) film, ascoating for biomedical prostheses, is discussed. An overview is done on diamondimplementation in prostheses, on the NCD mechanical properties and on thetechnological aspects concerning the NCD growth process i.e. Microwave PlasmaEnhanced Chemical Vapor deposition. Then, the attention is focused on a possibleimprovement of NCD growth on titanium (Ti) substrate. Further, a theoreticalstudy by finite element method is discussed in order to model the adhesionproperties of a NCD layer on Ti and Ti/Titanium Carbide (TiC) substrates. The goalof the proposed work is to provide a study about the use of thin NCD coating on Tibased prostheses. The function of the NCD coating on Ti material is to improve theimplanted prosthesis with a long duration time, thus decreasing the total costsand the invasive surgery treatments.

Photodetectors based on polycrystalline diamond (PCD) films are of great interest to many researches for the attractive electronic, mechanical, optical and thermal properties. PCD films are grown using the Microwave Plasma Enhanced Chemical Vapor Deposition (MWPECVD) method. First, we characterized films by means of structural and morphological analysis (Raman spectroscopy and scanning electron microscopy), then we evaporated a pattern of coplanar interdigitated Cr/Au contacts with an inter-electrode spacing of 100 um in order to perform the electrical characterization.We carried out measurements of dark current and impedance spectroscopy to investigate the film properties and conduction mechanisms of films and the effects of post-growth treatments. Finally we developed acharge sensing pre-amplifier to read-out the signal produced by UV photons in the detector.

Diamond films exhibit interesting properties, such as a wide bandgap (5.5 eV) and very low electron affinity which favours the electron photoemission. Diamond based cold photocathode is well known and demonstrated, due also to its chemical stability and mechanical robustness.In this work, a comparative study on photoemission of nanocrystalline diamond (NCD) film and nanodiamond (ND) layers obtained by microwave plasma enhanced chemical vapour deposition and pulsed spray technique, respectively, is presented and discussed evidencing advantages and drawbacks. The first technique permits to obtain thin NCD films at high deposition temperature (650-900 °C) with excellent homogeneity and adhesion; whereas the second one allows to deposit thin ND layer at a temperature of 120 °C with poor adhesion and low uniformity. Both NCD film and ND layers have been characterized by Raman spectroscopy, atomic force microscopy (AFM) and photoemission measurements.

In this paper, we present an investigation on two types of nanodiamond (ND) powders with average size of particles around 250 nm and having different sp(2) (graphite phase) and sp(3) (diamond phase) carbon contents. The ND surface modification is carried out by physical methods i.e. treatments in H-2 microwave plasma. The quantum efficiency (QE) of photocathodes and the stability of aqueous dispersions are assessed by photoemission and zeta potential (ZP) measurements, respectively.

This review surveys the recent developments in the plasma deposition of polycrystalline diamond (PCD) films from highly diluted (1% CH4 in H-2) gas mixture and superhydrophobic fluorocarbon films from C2F4 gas. Specifically, the pulsed plasmas are also used and examined at different duty cycles and pulse periods. Emphasis is given to the role of pulsed plasmas with respect to continuous ones in controlling the gas surface interaction and the growth chemistry and in determining the material properties. The obtained materials have been characterised using a wide range of methods: scanning electron microscopy, atomic force microscopy, Raman spectroscopy, X-ray diffractometry, spectroscopic ellipsometry, water contact angle, X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. Power modulation affects the PCD film morphology but deteriorates their optical and structural properties: the refractive index and the quality factor of continuous films are better than pulsed ones. As for fluorocarbon films, the control of both the plasma phase and the film deposited is improved under pulsed conditions. The pulsed plasmas, contrary to the continuous ones, allow to tune the CF2 growth precursor and correspondingly the composition, structure and morphology of the film, exhibiting a superwater repellent surface with contact angles of up to 170 degrees.

Over the last two decades the diamond with the unique combination of its extreme properties has provoked widespread scientific interest and stimulated a variety of technological applications. Besides to be the hardest and the best conductor of heat among the known materials, it has also got a very high chemical inertness and resistance to the high energy particles/radiation. Nowadays, the synthetic diamond is produced from two of the earth's most common materials: graphite or methane, by utilizing the HPHT (High Pressure 5-15 GPa, High Temperature 1500-3500 °C) technique, that imitates the formation conditions of natural diamond or the CVD (Chemical Vapor Deposition) technique, respectively. The advantage of CVD technique is the low values of the pressure and temperature below the atmospheric pressure and 1200 °C, respectively. The CVD process as its name implies, involves a gas-phase chemical reaction occurring above a solid surface, which causes direct deposition of diamond film onto the surface from activated carbon radicals (CH3, CH2, CH, C, C2) of methane. Gas phase activation can be achieved by several different routes like:i) combustion flame (oxyacetylene or plasma thorces); ii) thermal (hot filament CVD);iii) plasma (DC, RF, microwave, etc).The CVD technique assisted by microwave plasma is the most used and the most investigated. The involved processes are complex (dissociation, excitation, ionization and so on of CH4 and H2 gases) and can occur in different types of reactors: NIRIM, ASTEX e Bell Jar.In the present contribution, polycrystalline diamond films have been produced from CH4-H2 plasmas in a MWPECVD (Micro-Wave Plasma Enhanced CVD) apparatus including an ASTEX-type reactor. Generally the CH4-H2 plasmas are "black boxes" that can be highlighted by a parametric study and diagnostic techniques: of the plasma phase by the OES (Optical Emission Spectroscopy) and of the plasma-surface interface by PI (Pyrometric Interferometry). This last allows to monitor in-situ, in continuous and in real time the deposition rate and the temperature. The following MWPECVD process parameters have been studied: the power (1000-1800 Watt), the gas mixture composition (CH4 percentage 0.5-2%), the substrate temperature (740-950 °C) and the pressure (96-133 hPa). The films have been deposited on pieces of polished p-doped silicon (100) substrates, 500 ?m thick and with an area 2.6x2.6 cm2. Prior to the deposition the Si substrates have ultrasonically been treated ex-situ for one hour in an ethanol suspension of diamond powder (40-60 µm). Such treatment is aimed at the increase of the nucleation density to promote the deposition process. The chemical composition, structure and morphology of the obtained material have been characterized using the following techniques: Raman spectroscopy, XRD (X-ray diffraction) and SEM (Scanning Electron Microscopy).In particular, the influence of the different process parameters on the deposition rate,

Diamond exhibits unique physical properties, such as extreme mechanical hardness, highest known thermal conductivity, broad optical transparency, chemical inertness and biocompatibility. The interest towards diamond has led to spectacular advancements in the development of techniques, e.g., microwave plasma enhanced chemical vapor deposition (MWPECVD), for growing films on various substrates. Specifically in the last years, nanocrystalline diamond (NCD) films have attracted great interest as smooth materials and are exploited in several applications, such as coatings, seals, biomedical devices, biosensors, field-emission cathodes and displays, MEMS/NEMS devices and switches for wireless communications.Thick (around 3 ?m) and thin (48-310 nm) NCD films have been produced from Ar-rich CH4/Ar/H2 (1/89/10 %) and H2-rich CH4/H2 (1/99 %) microwave plasmas, respectively. During the deposition the working pressure and microwave power were 140 mbar and 950 Watt for CH4/Ar/H2 plasmas, and 50 mbar and 1000 Watt for CH4/H2 plasmas. In the experiments the total flow rate (?tot, 100sccm) was held constant. The deposition rate and the nucleation enhancement have been monitored in-situ and in real-time by pyrometric and laser reflectance interferometry for micrometer and nanometer thick films.The thick NCD films were obtained with and without an initial buffer layer (BL). The BL is easily obtained under typical microcrystalline diamond growth conditions (CH4/H2 mixtures). The effect of the deposition temperature (TD, 630-900°C) was investigated on the deposition rate, the morphology, the surface roughness and the bonding characteristics of the films grown with and without BL. The advantages of the BL are visible to the naked eye for the extraordinary uniformity, continuity and smoothness (roughness around 50 nm) of the NCD films. On the contrary, the films grown without BL appear inhomogeneous, non-continuous and rough (100-190 nm) with surface and interfacial pinholes and big spherical agglomerates.The thin NCD films were grown on Si substrates treated by two different methods, i.e. ultrasonic agitation in a suspension of diamond powders of 40-60 ?m or combitanorial approach in a suspension of mixed diamond powders of 250nm and 40-60 ?m. The process time of deposition was varied from 7 to 66 min by keeping constant the deposition temperature around 815 °C. The nanometer thick films with roughness of about 20 nm were easily obtained when the Si surface is treated by the combinatorial approach.The present experimental results show that the buffer layer procedure allows a good preservation of the surface of treated Si substrate and the combinatorial approach promotes effectively the seeding of the Si surface.

In this work we have fabricated and measured the frequency response of a diamond-like carbon (DLC) antenna based on a monopole layout. DLC has been deposited on a silicon substrate at room temperatures by sputtering a graphite target with Ar gas. The antenna presents a low reflectivity at 5.47 GHz suggesting its use in super high frequency (SHF, from 3 to 30 GHz) wireless communication systems, including aerospace transmissions. A good agreement between measured and numerical S-11 response has been found.

Diamond is an extremely interesting material for photoemission applications, due to the negative electron affinity of its surface, which can be obtained after suitable treatments. In the present work two sets of polycrystalline diamond films, characterized by different thicknesses and deposition conditions, are analyzed. In particular, in the examined films the relationship among the grain size, the amount of non-diamond carbon (sp2) located at the grain boundaries and their efficiency as photocathodes has been found and carefully investigated. The photoemission yield in the UV range is evaluated for all the samples, before and after hydrogenation, and after air exposure. The crucial parameter for the photocathode performances has been found not to be the film thickness, but the properties of polycrystalline diamond films, tunable with the plasma modulation and the methane percentage in the gas mixture.

Chemical, structural, morphological and micro-/macro-electrical properties of undoped and nitrogen-(N-)doped diamond films are determined by X-ray photoelectron spectroscopy, Raman and photoluminescence spectroscopies, field emission scanning electron microscopy, atomic force microscopy, scanning capacitance microscopy (SCM) and two points technique for I-V characteristics, respectively. The characterization results are very useful to examine and understand the relationship among these properties. The effect of the nitrogen incorporation in diamond films is investigated through the evolution of the chemical, structural, morphological and topographical features and of the electrical behavior. The distribution of the electrical current is first assessed at millimeter scale on the surface of diamond films and then at micrometer scale on small regions in order to establish the sites where the carriers preferentially move. Specifically, the SCM images indicate a non-uniform distribution of carriers on the morphological structures mainly located along the grain boundaries. A good agreement is found by comparing the electrical currents at the micro- and macro-scale. This work aims to highlight phenomena such as photo- and thermionic emission from N-doped diamond useful for microelectronic engineering. (C) 2017 Elsevier B.V. All rights reserved.

The deposition of as-received nanodiamond (ND) particles on silicon substrate was performed by the pulsed spray technique, using a dispersion of 250nm ND in 1,2-dichloroethane. A set of samples was sprayed by varying the number of pulses from 1 to 500. The morphology of the samples was characterized and monitored by means of optical, atomic force, and confocal microscopies. At a low number of pulses, sparse diamond particles were observed, whereas at a high number of pulses dense/quasi-continuous ND layers were formed. The electrical conductivity measurements of surface silicon substrate evidenced a remarkable change for the presence of ND particles. This behavior is also found by theoretical simulations (finite element method). Finally, a comparison between the electrical resistances measured on these samples versus the pulse number and the inverse current density calculated as a function of the number of ND particles, showed a good agreement. The experimental results highlighted an increase of the electrical current by using a number of pulses <100, whereas the simulation results proved the enhancement of current density and its surface rectification by employing a specific number of particles. The current increased by increasing the temperature and during the heating-cooling cycles hysteresis was observed. (a) Scheme of the sprayed ND particles on silicon substrate, (b) 3D AFM image 5×5?m² of 10 pulses sample, (c) trends of measured R and calculated 1/J.

This work deals with and proposes a simple and compact diagnostic method able to characterize the interaction between microwave and plasma without the necessity of using an external diagnostic tool. The interaction between 2.45 GHz microwave and plasma, in a typical ASTeX-type reactor, is investigated from experimental and numerical view points. The experiments are performed by considering plasmas of three different gas mixtures: H-2, CH4-H-2 and CH4-H-2-N-2. The two latter are used to deposit synthetic undoped and n-doped diamond films. The experimental setup equipped with a matching network enables the measurements of very low reflected power. The reflected powers show ripples due to the mismatching between wave and plasma impedance. Specifically, the three types of plasma exhibit reflected power values related to the variation of electron-neutral collision frequency among the species by changing the gas mixture. The different gas mixtures studied are also useful to test the sensitivity of the reflected power measurements to the change of plasma composition. By means of a numerical model, only the interaction of microwave and H-2 plasma is examined allowing the estimation of plasma and matching network impedances and of reflected power that is found about eighteen times higher than that measured. (C) 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

Nanodiamond (ND) layers on silicon substrate are deposited by the pulsed spray technique starting from nanoparticles of about 250 nm dispersed in 1,2-dichloroethane solvent. The aim of this letter is to investigate the quantum efficiency (QE) of photocathodes based on ND particles in the vacuum ultraviolet spectral range. Various ND layers are examined employing as-received and hydrogenated nanoparticles. As expected, the hydrogen plasma treatment improves strongly the photoemission of the layer giving a QE of 22% at 146 nm. Indeed, this efficiency value is achieved only if the particles are treated in H-2 microwave plasma before the growth of the sprayed layer rather than to hydrogenate the already formed one. These QE values are higher than those of photocathodes based on plasma chemical vapor deposition diamond films, but with the advantage of being much stable, too. The highest QE values are explained to be due to the intrinsic chemical and structural features of utilized ND particles. (C) 2016 AIP Publishing LLC.

The mechanical properties of polycrystalline diamond coatings with thickness varying from 0.92 to 44.65 um have been analysed. The tested samples have been grown on silicon substrates via microwave plasma enhanced chemical vapour deposition from highly diluted gas mixtures CH4-H2 (1% CH4 in H2). Reliable hardness and elastic modulus values have been assessed on lightly polished surface of polycrystalline diamond films.The effect of the coating thickness on mechanical, morphological and chemical-structural properties is presented and discussed. In particular, the hardness increases from a value of about 52 to 95 GPa and the elastic modulus from 438 to 768 GPa by varying the coating thickness from 0.92 to 4.85 um, while the values closer to those of natural diamond (H = 103 GPa and E = 1200 GPa) are reached for thicker films (> 5 um). Additionally, the different thickness of the diamond coatings permits to select the significance of results and to highlight when the soft silicon substrate may affect the measured mechanical data. Thus, the nanoindentation experiments were made within the range from 0.65% to 10% of the film thickness by varying the maximum load from 3 to 80 mN.

A relevant and still unsolved issue in the characterization of diamond coatings deposited on ceramic materials such as hexagonal boron nitride (h-BN) is to enhance the resistance of the substrate to erosion by Kr+ or Xe+ ions generated in the plasma propulsion systems. In this work, diamond films were grown by microwave plasma enhanced chemical vapour deposition on h-BN substrates untreated, for the first time, and pre-treated for short (31-65 min) and long (285-296 min) process times. The morphology of diamond films was analysed by scanning electron microscopy, and atomic force microscopy, and their chemistry and structure by Raman spectroscopy. Microscopy analysis revealed that non-continuous (at short process time) and continuous (at long process time) films were formed, respectively, on both untreated and pre-treated h-BN substrates. In particular, diamond films grown on untreated h-BN substrates exhibited roughness values higher than those of h-BN substrates pre-treated by a conventional ultrasonic method.

The self-assembly of pillar-like structures in nanodiamond (ND) layers was obtained by means of the pulsed spray technique. This technique enabled to deposit ND layers directly on silicon substrate using a dispersion of as-received 250 nm nanocrystals. The chemical and structural properties of the ND layer were determined by Raman and photoluminescence spectroscopies and the morphological features were measured by confocal and atomic force microscopies. Self-assembled pillar-like structures were observed by both microscopies. The local electrical voltage and current of an isolated ND pillar were studied by scanning Kelvin probe microscopy and scanning capacitance microscopy (SCM), respectively. The electrical data obtained showed that the single pillar features an increase of voltage and a decrease of current. A theoretical model based on the finite element method confirmed the current behavior measured by SCM.

Diamond coating has been proposed in different patents where prosthetic joints integrate diamond-coated load bearing surfaces in order to reduce friction and to increase the useful life of the joint [1], or where diamond coatings replace plastic (such as Ultra High Molecular Weight Poly-Ethylene)/ceramic (such as Zirc-alloy) materials suitable for ginglymous, enarthrodial implants or digital joints [2]. Concerning femoral prostheses, diamond coated silicon nitride has been considered as total hip replacement material [3]. Moreover, wear-debris-induced periprosthetic bone loss and aseptic loosening are the main long-term problems for total hip replacements, and amorphous diamond (a-D) coatings could reduce the wear debris for articulating surface [4]. In alternative, multilayered nanocrystalline diamond coatings have been studied in order to improve incredible hardness and excellent wear-resistance for articulating surfaces of structural implant devices [5]. Recently, diamond like carbon (DLC) have been considered about reducing friction and wear of orthopaedic implants [6]. In this direction nano- and ultranano-crystalline diamond (NCD and UNCD) and polished poly-crystalline diamond coatings exhibiting very low roughness, could bring to better mechanical properties if compared with DLC/a-D ones. Results of recent studies on NCD coatings applied to medical implants, has shown the high diamond biocompability and positive bioactivity [7]. We propose in this work to investigate and to fabricate tailored biocompatible NCD coatings addressed to low roughness (and consequent low friction) behavior. The used technology will be based on MicroWave Plasma Enhanced Chemical Vapour Deposition (MWPECVD) technique [8], able to improve superior hardness and Young's modulus for low temperature NCD [9]. The optimized approach will be focused on the grain surface characterization and on the mechanical properties of NCD coatings grown at different deposition temperature values (650-884°C). The surface morphology will be studied by Atomic Force Microscopy (AFM) technique, and the chemical-structural features of all coatings will be analyzed by Raman spectroscopy. Some important mechanical aspects will be discussed in details.

Diamond exhibits unique physical properties, such as extreme mechanical hardness, highest known thermal conductivity, broad optical transparency, chemical inertness and biocompatibility. The interest towards diamond has led to spectacular advancements in the development of techniques, e.g., microwave plasma enhanced chemical vapor deposition (MWPECVD), for growing thin films on various substrates. Specifically in the last years, nanocrystalline diamond (NCD) films have attracted great interest as smooth materials and are exploited in several applications, such as coatings, seals, biomedical devices, biosensors, field-emission cathodes and displays, MEMS/NEMS devices and switches for wireless communications.Since the pioneering work of Gruen [D.M.Gruen, Annu.Rev.Mater.Sci. 29 (1999) 211], argon-rich CH4-Ar-H2 gas mixtures represent an innovative and simple way to produce NCD films by MWPECVD. However, to get stable plasmas at low H2 mixture percentage, the standard Astex-type reactors must be modified, as suggested by the same Gruen and other research groups [K.Teii and T.Ikeda, Appl.Phys.Lett. 90 (2007) 111504]. In the present work, we have developed a new procedure to deposit NCD films on pre-treated silicon substrates without modifying the Astex-type reactor and using CH4-Ar-H2 (1-89-10%) gas mixture. In our reactor the 10% of H2 in the mixture assures plasma stability conditions. The key step of the process is the growth of an initial buffer layer, on which the NCD deposition takes place. The buffer layer is easily obtained under typical microcrystalline diamond growth conditions (CH4-H2 mixtures). The effect of the substrate temperature was investigated on the film deposition rate and on the morphology, surface roughness and bonding characteristics of films grown with and without buffer layer. The advantages of the buffer layer are soon visible to the naked eye for the extraordinary uniformity, continuity and smoothness of the NCD films. On the contrary, the films grown without buffer layer are inhomogeneous, non-continuous and rough with surface and interfacial pinholes and big spherical agglomerates.

Nanodiamond (ND) layers are produced by the pulsed spray technique on a heated silicon substrate. By varying the number of pulses it was possible to obtain a quasi-continuous layer on the substrate surface. The use of atomic force and confocal microscopies allowed to evidence the presence of self-assembled pillar structures. The pillar formation is due to the evaporation of 1,2-dichlorethane solvent in which 250 nm nanodiamonds were dispersed and its mechanism is based on the coffee stain effect as described in detail in ref. 1. Simple bio-chips on glass and silicon substrates can be fabricated spraying ND spots arranged in an array configuration (see Figure 1a) through a mask, and are suitable for bio-applications. Specifically, the presence of nitrogen vacancy (N-V) color center in ND particles is also promising as bio-markers2. An example of spot is illustrated in Figure 1b where pillars are present (see the zoomed area of Figure 1b). Additionally, the control of the pillar position represents an important challenge for micro-functionalised emitters arranged in a chip matrix suitable for bio-screening.

Nanocrystalline diamond (NCD) films with and without a diamond buffer layer (BL) have been grown on p-type silicon substrates by microwave plasma enhanced chemical vapor deposition technique at different values of deposition temperature (652-884°C). The photo- and thermionic electron emission properties of NCD films have been investigated, illustrated and explained by analyzing the surface morphology and the grain shape determined by atomic force microscopy, the chemical-structural properties by Raman spectroscopy and nanocrystallites size by X-ray diffraction. The NCD films with BL grown at the highest deposition temperature have shown the highest photo- and thermionic emission currents.

Highly efficient MWPECVD diamond based photocathodes have been demonstrated to be more stable than conventional materials such as CsI in UV detection. Poly- and nano-crystalline diamond films are still investigated and tested. Many research groups have found that photoemission properties are function of grain size, surface morphology and presence of defects (sp2 carbon bonding, carbon-hydrogen bond) within the grain boundaries [1]. One of the issues of diamond application in photocathodes, if compared with the CsI, is the very high temperature used during its growth, thus restricting the application fields. In this work, we present a study of photocathodes based on diamond layer deposited at low temperature by spray technique [1]. Diamond powders, with different grain sizes, were dispersed in the non-polar 1,2-dichloroethane (DCE) solvent by sonication for 30 minutes and successively sprayed on the substrates.The sprayed diamond films have been characterized by Raman spectroscopy, Atomic Force Microscopy and Photoemission measurements.Quantum efficiency (QE) measurements of the photocathodes have been assessed at normal incidence in reflective mode in the UV spectral range (150-210 nm) by means of a 30 W deuterium lamp (Mc Pherson TM) under vacuum. The photocathode was mounted in a multi-wire proportional chamber, not operating in electron multiplication mode. The absolute QE was evaluated by means of a NIST calibrated standard photodiode.The results have shown a QE dependence on the grain sizes and properties of the starting diamond powders. Moreover, the photoemissive properties of the low temperature (100 °C) sprayed diamonds are comparable with MWPECVD ones, these last produced at high temperature (>= 700 °C).

Generally materials, having a low/negative electron affinity (NEA) and/or a low work function which facilitate the electron escape from their surfaces, exhibit good photo- and thermo-emission properties. From this point of view, natural and synthetic diamond is an interesting material because possesses a low EA that becomes negative when the surface is treated in H2 plasma. Moreover, as-grown diamond films produced by microwave plasma enhanced chemical vapour deposition and starting from CH4:H2 (1:99%) gas mixture exhibit hydrogenated surfaces, suitable for photocathodes, neutralizer cathode, energy converters and so on. Very recently the authors of this contribution have patented a new methodology (easy, inexpensive and working at temperature of 120°C) to fabricate layers of nanodiamond (ND) particles. In the present work, the photoemission behaviour of ND layers produced by this new methodology i.e. the pulsed spray technique is presented. Commercial (Diamonds & Tools srl) ND particles, 250 nm in size, were utilized. Layers of untreated and treated (in pure H2 or N2 plasmas) NDs were produced. The photoemission quantum efficiency (QE), a merit figure for photocathode applications, was assessed in the UV range (140-210 nm), showing a QE enhancement for both hydrogenated and nitrogenated ND particles.

Diamond is an extremely interesting material for photoemission applications, due to the negative electron affinity which can be obtained after suitable surface treatments. In the present work, two sets of polycrystalline diamond films, characterized by dif-ferent thickness and deposition conditions, are ana-lyzed. In particular, the relationship among the grain size, the amount of non-diamond carbon (sp2) located at the grain boundaries and the film sensitivity as a photocathode has been found and carefully investi-gated. The photoemission yield in the UV range has been evaluated for all the samples, before and after hydrogenation process, and after air exposure. The critical parameter for the photocathode performances has been found not to be the film thickness, but the properties of polycrystalline diamond films, tunable with the plasma modulation and the methane percent-age in the gas mixture.

Optical emission spectroscopy has been used to characterize diamond deposition microwave chemical vapour deposition (MWCVD) plasmas operating at high power density. Electron temperature has been deduced from H atom emission lines while H-atom mole fraction variations have been estimated using actinometry technique, for a wide range of working conditions: pressure 25-400 hPa and MW power 600-4000 W. An increase of the pressure from 14 hPa to 400 hPa with a simultaneous increase in power causes an electron temperature decrease from 17,000 K to 10,000 K and a H atom mole fraction increase from 0.1 to up to 0.6. This last value however must be considered as an upper estimate due to some assumptions made as well as experimental uncertainties.

Diamond powders of various size ranging from few nanometers to tens of micrometers arecommonly used to treat the silicon substrate in order to enhance the nucleation process beforethe growth of thin diamond films by chemical vapor deposition (CVD) techniques[1 and refs.therein]. Recently a great attention is paid to nanodiamond (ND) particles which include stablenitrogen-vacancy (N-V) color centers [2]. In this work we propose the spray technique [3] todirectly deposit natural ND layers on silicon substrate using particles of 250 nm. ND particles weredispersed in the apolar solvent 1, 2 - dichloroethano (DCE) by sonication for 30 minutes. Then thedispersion was sprayed on the Si substrate, obtaining the highest ND density in the middle of it.The obtained ND films were analyzed by Raman spectroscopy, atomic force microscopy (AFM) and3D confocal microscopy. The first technique allows the measurement of the chemical andstructural composition and the photoluminescent properties, whereas the other ones measure thetopography and morphology of the layers.A careful morphological analysis showed the existence of pillar-like self-assembled structuresdistributed in an irregular way. The highest pillar density was found far from the center of thesample, where the ND layer is non-uniform. The evolution of the structures were well observed bythe 3D image analysis performed by confocal microscopy and AFM. The studyon the formationmechanisms of ND self-assembled structures will be presented and discussed.

Pillar-like structures of nanodiamonds on a silicon substrate are self-Assembled for the first time by a pulsed spray technique. This technique allows us to deposit nanodiamond layers by using high quality nanocrystals of 250 nm dispersed in 1,2-dichloroethane (DCE) solvent. The analysis of 2D/3D confocal and atomic force microscopy images evidences the presence of self-Assembled pillar-like structures distributed in an irregular way. The proposed method is simple, easy and cheap, and does not require complex growth processes or structured materials, ideal for upscaling toward industrial biochip implementation and photonic applications. The suggested formation mechanisms of self-Assembly are based on the so-called coffee stain effect, i.e., on the time evolution of DCE evaporation.

The self-assembly of pillar-like structures in nanodiamond (ND) layers has been obtained for the first time by pulsed spray technique [1]. This technique has enabled to directly deposit ND layers on silicon substrate using natural nanocrystals of 250 nm. ND particles were dispersed in the apolar solvent 1, 2 - dichloroethane (DCE) by sonication for 30 minutes, then the dispersion was sprayed on the substrate. Various samples were sprayed at different number of pulses, ranging from 1 to 500. The obtained ND layers were analyzed by Raman spectroscopy, atomic force microscopy (AFM), 3D confocal microscopy, and contact angle measurements. The Raman spectroscopy allowed the measurement of the chemical and structural composition and the photoluminescent properties, the microscopic techniques measured the topography and morphology of the layers and contact angle measurements established the hydrophilic/hydrophobic nature of substrate/ND layers surfaces. A careful morphological analysis evidenced the existence of self-assembled pillar-like structures. The formation mechanisms of self-assembly, based on the so-called coffee stain effect, i.e. on the time evolution of DCE evaporation will be presented and discussed. Theoretical electrical aspects of the single pillar will also be discussed.

Il diamante è considerato un materiale molto promettente per lo sviluppo di dispositivi per la rive-lazione di radiazione UV grazie alle sue peculiari pro-prietà chimiche e fisiche (in particolare ottiche ed elettriche). In questo lavoro sono presentati i risultati ottenuti su dispositivi basati su film di diamante poli-cristallino ROD (Random Oriented Diamond), pro-dotti con la tecnica MWPECVD (MicroWawe Plasma Enhanced Chemical Vapour Deposition) su substrati di silicio intrinseco e drogato-p (100). Sui dispositivi realizzati è stato eseguito uno studio comparativo in funzione del tipo di substrato e del diverso passo tra gli elettrodi di Cr/Au complanari ed interdigitati. So-no state quindi determinate le proprietà chimico-strutturali, morfologiche, elettriche ed ottiche dei film. In particolare, la risposta ottica è stata misurata in aria, utilizzando una lampada impulsata allo Xe accoppiata con una fibra ottica di quarzo, filtri inter-ferenziali per selezionare la lunghezza d"onda ed un"elettronica di misura di read-out basata su un am-plificatore sensibile alla carica a basso rumore. I ri-sultati sperimentali evidenziano una dipendenza delle prestazioni del dispositivo dal tipo di substrato utiliz-zato e dalla geometria dei contatti interdigitali.

Thick (around 3 um) and thin (48-310 nm) nanocrystalline diamond (NCD) films have been produced from Ar-rich CH4/Ar/H2 (1/89/10 %) and H2-rich CH4/H2 (1/99 %) microwave plasmas, respectively.The deposition rate and the nucleation enhancement have been monitored in situ and in real time by pyrometric and laser reflectance interferometry for micrometer- and nanometer-thick films. For thick films, an improvement of the NCD films' smoothness has been obtained by a buffer layer between the films and the treated Si substrate. For thin films, a combinatorial approach, i.e., a treatment of the Si substrate in a suspension of mixed diamond powders of 250 nm and 40-60 um, has been utilized. The present experimental results show that the buffer layer procedure allows good preservation of the surface of the treated Si substrate and the combinatorial approach promotes effectively the seeding of the Si surface.

Undoped and in particular Nitrogen-doped (N-doped) diamond films are interesting materials for photo- and thermo-emission applications such as photocathodes, cold cathodes in vacuum microelectronics, cathode neutralizers in thrusters for space propulsions and energy converters in solar concentrating systems. In the present contribution, these films were produced by microwave plasma enhanced chemical vapour deposition technique starting from CH4-H2 gas mixture and adding variable nitrogen amount from 0 to 6 %. The deposition rate of films, monitored in-situ by pyrometric and laser interferometries, changes drastically with the increasing nitrogen addition. The chemical-structural, morphological, electrical and photoemissive properties of films were determined by XPS, Raman and photoluminescence spectroscopies, atomic force microscopy, two points technique for I-V characteristics and photoemission measurements, respectively. The examined films exhibit an evolution of structural-chemical features, and of electrical and photoemissive properties as a function of the nitrogen added to the gas mixture. Specifically the photoemission quantum efficiency, a merit figure for photocathodes, was assessed in the UV range (140-250 nm) for all the samples before and after microwave plasma treatments in H2 or N2 gases, and after successive air exposure.

The effects of pulsed microwave discharges on the deposition and properties of a set of polycrystalline diamond films are investigated by varying the duty cycle at a fixed pulse frequency and keeping constant the peak microwave power at 1250 W, the substrate temperature and the final film thickness. The deposition polycrystalline diamond films obtained from highly diluted CH(4) (1% CH(4) in H(2)) gas mixtures was monitored by pyrometric interferometry technique. This analysis evidences that, in order to obtain the same thickness, the nucleation/deposition times and the process rates increase and decrease, respectively, by decreasing the pulse duration. Moreover, the influence of the variations of duty cycle on the deposition rates, the surface morphology, the optical properties (refractive index and extinction coefficient) and the crystallite orientations of the polycrystalline diamond films is investigated.

Acoustical shock waves (Mach number < 2) generated in situ by spark gap are propagated in weakly ionized dc discharges working at low pressure (399 Pa) and containing either Ar or N2 gas. The electrical characterization and the laser deflection technique are used to measure the characteristics of dc discharge (such as voltage, resistance and power of discharge) and the structure and velocity of shock wave, respectively. The results stress the importance of atomic and molecular nature of the gases in affecting the power deposition and the shock wave properties.

Nanocrystalline diamond (NCD) coatings with thickness of about 3 um were grown on silicon substrates at four deposition temperatures ranging from 653 to 884 degrees C in CH4/H2/Ar microwave plasmas. The morphology, structure, chemical composition and mechanical and surface properties were studied by means of Atomic Force Microscopy (AFM), X-Ray Diffraction (XRD), Raman spectroscopy, nanoindentation and Water Contact Angle (WCA) techniques. The different deposition temperatures used enabled to modulate the chemical, structural and mechanical NCD properties, in particular the grain size and the shape. The characterization measurements revealed a relatively smooth surface morphology with a variable grain size, which affected the incorporated hydrogen amount and the sp(2) carbon content, and, as a consequence, the mechanical properties. Specifically, the hydrogen content decreased by increasing the grain size, whereas the sp(2) carbon content increased. The highest values of hardness (121 +/- 25 GPa) and elastic modulus (1036 +/- 163 GPa) were achieved in NCD film grown at the lowest value of deposition temperature, which favored the formation of elongated nanocrystallites characterized by improved hydrophobic surface properties. (C) 2014 Elsevier B.V. All rights reserved.

The planned presentation will illustrate and explain the photoemissive and thermionic properties of NCD films on the basis of morphology, determined by atomic force microscopy, and chemical-structural characteristics by Ramanspectroscopy.

CVD diamond represents a very attractive material for the fabrication of devices based onthermionic emission, due to the possibility to achieve a very low or even negative electron affinity(NEA) by a hydrogen surface termination.We present here a preliminary study on thermionic conversion from diamond thin films grown ondifferent substrates, ranging from silicon to engineered ceramic materials. In particular, amongthem, HfC-based ceramics have been selected for its integration with diamond in concentrated solarsystems. CVD polycrystalline and nanocrystalline diamond films were deposited by MW-CVD(MicroWave-Chemical Vapour Deposition) by varying also their doping.Thermionic performance was evaluated in an ultra-high-vacuum (pressure <10-8 Torr)characterization setup able to accurately control the emitter and collector temperatures.Experimental activity has been carried out to integrate the HfC-diamond system in a more complexconversion module that was tested in a concentrating solar system to verify the conversionperformance under operating conditions. These results are finally reported.

Polycrystalline diamond films with a thickness of about 2 mu m were deposited by chemical vapour deposition on silicon substrates in continuous and pulsed wave regimes with duty cycle between 25% and 100%. The thermoluminescent behaviour of these films was analysed in the temperature range 323-723 K after beta irradiation with doses in the range 7-107 Gy. All the films exhibit a dosimetric peak centred at about 592 K, showing a good linearity in the whole investigated dose range. The thermoluminescence analyses show that the intensity of the dosimetric peak is maximum for the continuous wave film, while it decreases for the pulsed wave samples. The variation of the crystalline quality and the purity of the films with the employed duty cycle, investigated via micro-Raman spectroscopy and room-temperature photoluminescence, indicates that the continuous wave (duty cycle = 100%) film has the best quality, corresponding to the highest thermoluminescence efficiency.

In Ar-rich Ar-H2-CH4 gas mixture the presence of H2 is found to be beneficial to the plasma stability. On the other hand, too high H2 percentages lead to materials showing a high surface roughness. In the present work, diamond films were grown on p-type Si (100) substrates screening different quantities of H2. The plasma phase and plasma-substrate interface were investigated by in-situ optical emission spectroscopy and pyrometric interferometry to determine the behavior of emitting species and the deposition rates, respectively.The obtained films were characterized by Raman micro-spectroscopy, AFM and SEM techniques. For H2 percentages between 6.3 and 10%, the structure and morphology are characteristic of nanocrystalline films, affording low roughness values when a buffer layer was grown between the diamond coating and the treated silicon surface.

Photocathodes working in reflection-mode are made of rich-diamond (R-D) and rich-graphite (R-G) nanodiamond (ND) layers, deposited on different conductive substrates by means of the pulsed spray technique at low deposition temperatures (120 and 150 C-omicron) and starting from two types of ND particles. The two powders with an average grain size of 250 nm have variable sp(2) (graphite phase) and spa (diamond phase) hybridized carbon contents, as assessed by Raman spectroscopy and transmission electron microscopy. The ND particles are employed as-received or treated in H-2 microwave plasmas.

Method for the production of high efficiency photocathodes for ultraviolet based on nanodiamonds