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Thermal and concentrated solar solid-state converters are devices with no moving parts, corresponding to long lifetimes, limited necessity of maintenance, and scalability. Among the solid-state converters, the thermionic-based devices are attracting an increasing interest in the specific growing sector of energy conversion performed at high-temperature. During the last 10 years, hybrid thermionic-based concepts, conceived to cover operating temperatures up to 2000 °C, have been intensively developed. In this review, the thermionic-thermoelectric, photon-enhanced thermionic emission, thermionic-photovoltaic energy converters are extensively discussed. The design and development processes as well as the tailoring of the properties of nanostructured materials performed by the authors are comprehensively described and compared with the advances achieved by the international scientific community. [ABSTRACT FROM AUTHOR]

The response of polycrystalline diamond detectors to low-energy x-ray beams is presented. Diamond metal/semiconductor/metal vertical structures have been realized to investigate the influence of the electric field in the dark and under x-ray irradiation. Nonlinear contributions to the current, compatible with field-assisted thermal ionization of trapped charges, have been evidenced, as well as the presence of defect-related electronic states acting as traps has been confirmed by subband-gap photoconductivity measurements. Moreover, correlations have been evidenced among irradiation time, variation of the current in the dark, and photoconductive response of the devices to the x-ray beam. An evaluation of the minority-carrier mobility-lifetime μτ product, at the end of the activation process, has led to a value of 9.2×10-7 cm2/V. Two orders of magnitude linear response to the beam intensity variations have been evaluated after removal of the bremsstrahlung radiation contribution on a 43-μm-thick device. [ABSTRACT FROM AUTHOR]

The transport properties of a "thin" polycrystalline diamond film are analyzed after the sample exposure to 8.06-keV x-ray radiation. Structure and morphology of the as-grown film have been evaluated by Raman, x-ray diffraction, and scanning electron microscopy techniques. The transport properties have been investigated by measuring dark current-voltage characteristics in the temperature range of 60 to 360 K. Ohmic transport has been evidenced on the as-grown film up to 1.16×10[SUP5]V/cm. After irradiation, nonlinear contributions to the dark current have been evidenced and related to field-assisted thermal ionization of traps. Below 200 K, hopping mechanisms have been observed. Correlations have been found among x-ray irradiation, density of traps involved in the transport processes, and the nonhomogeneous nature of the sample. A simple model of the grain boundary structure is proposed to explain the observations. [ABSTRACT FROM AUTHOR]

Thin films of various borides, nitrides, and barium fluorides were tentatively deposited by pulsed laser deposition and by magnetron sputtering in order to develop the components of thermionic‐photovoltaic devices for the high‐temperature thermal to electrical conversion by solid state. To improve the device performance, the materials characterized by a low work function were selected. In the present work, the chemical composition and work function of obtained films were investigated by X‐ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy techniques. The values of work function were determined from the cut‐off in the He I valence band spectra. Different films were compared and estimated on the basis of X‐ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy results. [ABSTRACT FROM AUTHOR]

A prototype Single crystal Diamond Detector (SDD) was installed at the Joint European Torus (JET) in 2013 along an oblique line of sight and demonstrated the possibility to carry out neutron spectroscopy measurements with good energy resolution and detector stability in discharges heated by neutral beam injection and radio-frequency waves. Starting from these positive results, within the Vertical Neutron Spectrometer project of the Joint European Torus, we have developed a pixelated instrument consisting of a matrix of 12 independent SDDs, called the Diamond Vertical Neutron Spectrometer (DVNS), which boosts the detection efficiency of a single SDD by an order of magnitude. In this paper we describe the main features of the DVNS, including the detector design, energy resolution, and data acquisition system for on-line processing. Preliminary spectroscopy measurements of 2.5 MeV neutrons from the present deuterium plasma at JET are finally presented. [ABSTRACT FROM AUTHOR]

A buried boron (B) doped layer has been fabricated by ion implantation into the bulk structure of a chemical vapor deposition (CVD) diamond film engineered to act as a photo-thermionic cathode for high-temperature solar cells. The boron layer implantation is a fundamental step in order to obtain an efficient diamond-based solar cell. Implantation of boron ions has been performed by fixing the ion dose to a value of 1 \times 10^15 at/cm2, while varying the ion beam kinetic energy from 40 to 250 keV which localizes the doped layer at different depths (from tens to a few hundreds of nanometers) below the absorbing surface of the cathode. Characterization of the optical and photoelectronic properties of the different implanted layers has been carried out so as to evaluate their effectiveness within the cathode structure. An ion beam kinetic energy of 40 keV is found to significantly increase the quantum efficiency of the CVD diamond plate. This will be useful for the development of the diamond-based photo-thermionic cathode. [ABSTRACT FROM PUBLISHER]

Single crystal Diamond Detectors (SDD) are being increasingly exploited for neutron diagnostics in high power fusion devices, given their significant radiation hardness and high energy resolution capabilities. The geometrical efficiency of SDDs is limited by the size of commercially available crystals, which is often smaller than the dimension of neutron beams along collimated lines of sight in tokamak devices. In this work, we present the design and fabrication of a 14 MeV neutron spectrometer consisting of 12 diamond pixels arranged in a matrix, so to achieve an improved geometrical efficiency. Each pixel is equipped with an independent high voltage supply and read-out electronics optimized to combine high energy resolution and fast signals (<30 ns), which are essential to enable high counting rate (>1 MHz) spectroscopy. The response function of a prototype SDD to 14 MeV neutrons has been measured at the Frascati Neutron Generator by observation of the 8.3 MeV peak from the 12C(n, α)9 Be reaction occurring between neutrons and C nuclei in the detector. The measured energy resolution (2.5% FWHM) meets the requirements for neutron spectroscopy applications in deuterium-tritium plasmas. [ABSTRACT FROM AUTHOR]

Recently, carbon spectroscopy became very important due to the growing interest in the applications of new carbon allotropes, especially of 2D structures such as graphene and nanotubes. It is well known that the hybridization of carbon electrons defines most of the properties of these materials. Among the analytical techniques used for the identification of carbon allotropes is the spectroscopy of carbon Auger line, which involves the transitions of valence band electrons and can be excited by electron beam (AES) or X-ray photons (XAES). From the shape of C KVV spectrum, it is possible not only to recognize the cases of pure sp2 and sp3 configurations but also to quantify their ratio in amorphous carbon films. In present work, an experimental study of three main allotropes of carbon: graphite, diamond, and graphene is reported. The average thickness of graphene was evaluated from the ratio of C 1s and substrate photoelectron peaks, registered at grazing angle. Carbon KVV spectra were investigated by using XAES and AES techniques. Obtained results were compared with the ones acquired for reference samples of graphite and monocrystalline diamond. In addition, some samples of hydrogenated diamond were analyzed. It was demonstrated that the comparison of C KVV spectra excited by electrons and X-rays can be used for identification of graphene. Diamond-like C KVV spectrum, observed in XAES experiments, is a very promising fingerprint of graphene. Obtained results are also compared with valence band spectra of graphene, diamonds and graphite that were acquired by using ultraviolet photoemission spectroscopy. [ABSTRACT FROM AUTHOR]

The measurement of the density of occupied states as a function of the applied electric field, performed on single-crystal chemical vapour deposition diamond by x-ray modulated photocurrent technique, is reported. Two regimes of non-linear charge transport were observed: a classical Frenkel-Poole (FP) process at high electric fields (>6800 V/cm), and a radiation-assisted transport mechanism at intermediate electric fields (2000 to 6800 V/cm), consisting of a double-step process in which the direct re-emission into the extended band occurs following multiple photo-induced FP-like hopping transitions. [ABSTRACT FROM AUTHOR]

X-ray dosimeters were developed by tailoring novel injecting diamond-like-carbon/Pt/Au contacts on single-crystal high-purity diamond films. A dark resistivity of (\5.6 \pm \0.1) \times \10^14\ \Omega \cdot \cm and no appreciable presence of deep traps in the bandgap confirmed a very low defect density in the diamond films. The dosimeters resulted to be primingless (i.e., no need of preactivation) and characterized by high sensitivity [( \58.20 \pm \3.26) \times \10^-3\ \C \cdot \Gy^-1 \cdot \cm^-3] and linear response to X-ray dose rate, produced by a molybdenum target. Transient X-ray modulated analysis allowed the determination of fast-trap influence and the estimation of very fast response times (\sim\!\! \10^-3\ \s), at electric fields \geq \3 \times \10^3\ \V/cm. [ABSTRACT FROM AUTHOR]

Three metals (Ag, Au, and Ti) and a carbide (WC) were studied to verify their suitability to act as high-temperature electric contacts for photon-enhanced thermionic emission cathodes based on an AlGaInP/GaInP heterostructure. Transfer length method measurements, performed in the temperature range 25-350°C, showed that the lowest specific contact resistivity values were found for Au (4.0 x 10-4 Ω cm-2 @350°C) and Ti (1.2 x 10-3 Ω cm-2 @350°C). A further selection was made on the basis of film adhesion tests performed after the thermal cycles, in which Ti demonstrated an excellent mechanical robustness. [ABSTRACT FROM AUTHOR]

Hydrogen terminated diamond is a promising material for metal semiconductor field effect transistor (MESFETs) fabrication and research effort is focused on diamond quality improvement. In this reported work, the focus of attention is on device layout and structure to improve RF performances, designed on the basis of diamond physical properties, taking into account the characterisation of previously fabricated devices. Significant increase of current gain cutoff frequency fT, from 3.4 to 6.7 GHz, has been obtained, while maximum oscillation frequency fMAX is raised from 10.4 to 11.8 GHz. [ABSTRACT FROM AUTHOR]

We have correlated the surface conductivity of hydrogen-terminated diamond to the electronic structure in the Fermi region. Significant density of electronic states (DOS) in proximity of the Fermi edge has been measured by photoelectron spectroscopy (PES) on surfaces exposed to air, corresponding to a p-type electric conductive regime, while upon annealing a depletion of the DOS has been achieved, resembling the diamond insulating state. The surface and subsurface electronic structure has been determined, exploiting the different probing depths of PES applied in a photon energy range between 7 and 31 eV. Ab initio density functional calculations including surface charge depletion and band-bending effects favorably compare with electronic states measured by angular-resolved photoelectron spectroscopy. Such states are organized in the energy-momentum space in a twofold structure: one, bulk-derived, band disperses in the G-X direction with an average hole effective mass of (0.43±0.02)m0, where m0 is the bare electron mass; a second flatter band, with an effective mass of (2.2±0.9)m0, proves that a hole gas confined in the topmost layers is responsible for the conductivity of the (2×1) hydrogen-terminated diamond (100) surface. [ABSTRACT FROM AUTHOR]

We report the discovery of a nearly-free electronlike resonant state for a β-Si3N4(0001)-8×8 layer grown on Si(111), as observed by angle-resolved photoemission and scanning tunneling spectroscopy. Comparison with measurements performed on a Si(111)-7×7 surface helped us in the identification of the band. It is found that this parabolic state is degenerate with surface projected bulk bands of the Si(111) substrate. [ABSTRACT FROM AUTHOR]