Luiz Fernando Zagonel

Due to the mechanical and inertness properties of the e-Fe2-3N phase, its formation as a
compact monolayer is most wanted in plasma surface treatments of steels. This phase can be
obtained by the inclusion of carbon species in the plasma. In this work, we present a
systematic study of the carbon influence on the compound layer in an AISI H13 tool steel
by pulsed plasma nitrocarburizing process with different gaseous ratios (0%
[CH4]/
[N2 þ CH4 þ H2]
4%). The plasma treatment was carried out for 5 h at 575 8C. The microstructure and phase composition of the modified layers were studied by scanning electron
microscopy and X-ray diffraction, respectively. X-Ray photoelectron spectroscopy was used to
measure the relative concentration of carbon and nitrogen on the surface. The hardening
profile induced by the nitrocarburized process is also reported.

The microstructural development in H13 tool steel upon nitriding by an ion beam process was investigated. The nitriding experiments were performed at a relatively low temperature of ,400uC and at constant ion beam energy (400 eV) of different doses in a high vacuum preparation chamber; the ion source was fed with high purity nitrogen gas. The specimens were characterised by X-ray photoelectron spectroscopy, electron probe microanalysis, scanning and transmission electron microscopy, and grazing incidence and Bragg–Brentano X-ray diffractometry. In particular, the influence of the nitrogen surface concentration on the development of the nitrogen concentration depth profile and the possible precipitation of alloying element nitrides were discussed.

The nitrocarburization of the AISI-H13 tool steel by ion beam assisted deposition is reported. In this technique, a carbon film is continuously deposited over the sample by the ion beam sputtering of a carbon target while a second ion source is used to bombard the sample with low energy nitrogen ions. The results show that the presence of carbon has an important impact on the crystalline and microstructural properties of the material without modification of the case depth.

In this paper we report the effect of hydrogen on the structural properties of AISI-H13 steel nitrogen-implanted samples in low oxygen partial pressure atmosphere. The samples were implanted in a high vacuum chamber by using a broad ion beam source. The H 2 + /N 2 + ion composition of the beam was varied and the surface composition studied in situ by photoemission electron spectroscopy (XPS). The samples were also ex situ analyzed by X-ray diffraction and scanning electron microscopy (SEM), including energy-dispersive spectroscopy measurements. It was found that hydrogen has the effect of modifying the amount of retained nitrogen at the surfaces. This result shows that hydrogen plays a role beyond the well-established effect of oxygen etching in industrial machines where vacuum is relatively less well controlled. Finally, an optimum concentration of 20–40% [H 2 ]/[H 2 +N 2 ] ion beam composition was determined to obtain maximum nitrogen incorporation on the metal surface.

We report on a detailed study of the intensity dependent optical properties of individual GaN/AlN quantum disks (QDisks) embedded into GaN nanowires (NW). The structural and optical properties of the QDisks were probed by high spatial resolution cathodoluminescence (CL) in a scanning transmission electron microscope (STEM). By exciting the QDisks with a nanometric electron beam at currents spanning over three orders of magnitude, strong nonlinearities (energy shifts) in the light emission are observed. In particular, we find that the amount of energy shift depends on the emission rate and on the QDisk morphology (size, position along the NW and shell thickness). For thick QDisks (>4 nm), the QDisk emission energy is observed to blueshift with the increase of the emission intensity. This is interpreted as a consequence of the increase of carriers density excited by the incident electron beam inside the QDisks, which screens the internal electric field and thus reduces the quantum confined Stark effect (QCSE) present in these QDisks. For thinner QDisks (<3 nm), the blueshift is almost absent in agreement with the negligible QCSE at such sizes. For QDisks of intermediate sizes there exists a current threshold above which the energy shifts, marking the transition from unscreened to partially screened QCSE. From the threshold value we estimate the lifetime in the unscreened regime. These observations suggest that, counterintuitively, electrons of high energy can behave ultimately as single electron-hole pair generators. In addition, when we increase the current from 1 to 10 pA the light emission efficiency drops by more than one order of magnitude. This reduction of the emission efficiency is a manifestation of the " efficiency droop " as observed in nitride-based 2D light emitting diodes, a phenomenon tentatively attributed to the Auger effect.

Cathodoluminescence (CL) is a powerful tool for the investigation of optical properties of materials. In recent years, its combination with scanning transmission electron microscopy (STEM) has demonstrated great success in unveiling new physics in the field of plasmonics and quantum emitters. Most of these results were not imaginable even twenty years ago, due to conceptual and technical limitations. The purpose of this review is to present the recent advances that broke these limitations, and the new possibilities offered by the modern STEM-CL technique. We first introduce the different STEM-CL operating modes and the technical specificities in STEM-CL instrumentation. Two main classes of optical excitations, namely the coherent one (typically plasmons) and the incoherent one (typically light emission from quantum emitters) are investigated with STEM-CL. For these two main classes, we describe both the physics of light production under electron beam irradiation and the physical basis for interpreting STEM-CL experiments. We then compare STEM-CL with its better known sister techniques: scanning electron microscope CL, photoluminescence, and electron energy-loss spectroscopy. We finish by comprehensively reviewing recent STEM-CL applications.

ABSTRACT The optical properties of metal nanoparticles are governed by their surface plasmon (SP) modes, which are resonant electromagnetic fields associated to the collective oscillations of their conduction electrons along the boundaries. Both the resonant energies and the electric field patterns associated to these SP modes strongly depend on numerous parameters, such as the material itself, its dielectric environment, and the size and shape of the nano-object. They therefore require spectroscopic techniques with sub-wavelength spatial resolution in order to be completely understood. Over the last few years, the highly focused electron probe of a Scanning Transmission Electron Microscope (STEM), acting as a virtual white light photon source, has successfully demonstrated its potential ability to map SP modes in nanoparticles with nanometer spatial resolution. Indeed, the nanometric analogue to extinction spectroscopy, the Electron Energy Loss Spectroscopy (EELS), has been of great interest for studying SP resonances in highly symmetric nano-objects [1], thanks to a relatively high signal to noise ratio. In parallel, cathodoluminescence (CL) has proved to be very powerful when applied in analyzing the light emission induced from SP in nanoparticles with a unique spectral resolution [2]. Moreover, both extinction and emission mappings may provide complementary as well as precious information for the physics of SP when compared using these two techniques on a single nano-object. Figure 1 shows a STEM High Angle Angular Dark Field (HAADF) image of a gold (bright) nanoprism relying on a (dark) carbon grid, together with EELS and CL spectra for a given probe position at the top tip of the nanoprism. In EELS mode, the fast electron beam is spectrally analyzed through transmission of the sample, giving access to Electron Energy Loss (EEL) probability spectra (in blue). As the incident electron beam acts as a white light source, all types of excitations are probed, including &amp;amp;quot;bright&amp;amp;quot; as well as &amp;amp;quot;dark&amp;amp;quot; SP modes. In CL mode, light coming from deexcitation of the locally excited radiative SP modes is dispersed to obtain Electron Induced Radiation Emission (EIRE) probability spectra (in red). As can be seen, CL displays clear redshift as compared to EELS, mostly due to retardation effects [3]. Hyperspectral imaging data is obtained by raster scanning the incident electron probe over the sample to investigate the related SP modes. Figure 2 shows spatially resolved EEL and EIRE maps, filtered at the relevant peak wavelengths. This evidences the dipolar character of the mode, similar to dipolar SP modes previously observed by EELS for silver nanoprisms [1]. Furthermore, it is now well established that these maps display the spatial variations of a quantity which is related to the total [4] (for EELS) and radiative [5] (for EIRE) Electromagnetic Local Density of States (EMLDOS). As the contribution of several SP modes may blur the EMLDOS patterns, we fit on each pixel a Gaussian function around the peak positions to obtain the amplitude weight of the dipolar mode. Results are given on figure 3, demonstrating the coherent character of the mode over the three tips, despite a maximum in photon emission at the upper tip. By comparing both spatially resolved EELS and CL on a single nano-object, we therefore gain insight on its complete optical properties at the nanoscale. This includes discerning its &amp;amp;quot;dark&amp;amp;quot; and &amp;amp;quot;bright&amp;amp;quot; modes, and comparing emission and absorption properties. [6] [6] This work is partially supported by the Centre National de la Recherche Scientifique and the Délégation Générale de l&amp;amp;#39;Armement. Figure 1. Left: HAADF image of a gold nanoprism relying on a carbon grid. Right: EEL (blue) and EIRE (red) spectra obtained at a fixed probe position indicated by the white square.

The influence of nano-structure and composition of the substrate on the properties of carbon nanotubes (CNTs) is presented. The samples are obtained following a sequential in situ deposition routine. First, TiN x O y films are grown on a crystalline silicon substrate. Immediately, dispersed nickel catalyst particles are deposited on the film. The non-stoichiometric TiN x O y films and Ni particles are grown by ion beam sputtering of Ti and Ni targets, respectively. Soon after that, the CNTs are grown by feeding acetylene gas into the chamber and maintaining the substrate at 973 K. In situ x-ray photoelectron spectroscopy allows compositional and structural analysis in all the stages of the sample growth process. The CNTs are further studied by scanning and transmission electron microscopy techniques, showing different population densities, sizes and diameters as a function of the oxygen content in the TiN x O y films. The results show that oxygen influences the surface diffusion mob...

ABSTRACT Acquisition scheme for spectrum imaging. On the left, in the microscope (STEM) two kinds of detectors are used: a high-angle annular dark-field (HAADF) detector and a CCD camera attached at the exit of an optical spectrometer. Upon the electron beam scanning on the sample, the former will produce an image while the latter, having collected a spectrum at each pixel of such image, will provide a spectral image. On the right, one HAADF image is shown together with an illustrative view of a spectrum image. In the latter, the light emission is shown as an opaque volume. Within this volume, it is possible to distinguish an isolated emission of the left.

ABSTRACT Fast electron based spectroscopies are often loosely compared to light scattering. By performing Electron Energy Loss Spectroscopy and Cathodoluminescence on single metallic nanoobjects, we show that these techniques are nanometric probes of extinction and scattering.

ABSTRACT The optical properties of metal nanoparticles are governed by their surface plasmon (SP) modes, which are resonant electromagnetic fields associated to the collective oscillations of their conduction electrons along the boundaries. Both the resonant energies and the electric field patterns associated to these SP modes strongly depend on numerous parameters, such as the material itself, its dielectric environment, and the size and shape of the nano-object. They therefore require spectroscopic techniques with sub-wavelength spatial resolution in order to be completely understood. Over the last few years, the highly focused electron probe of a Scanning Transmission Electron Microscope (STEM), acting as a virtual white light photon source, has successfully demonstrated its potential ability to map SP modes in nanoparticles with nanometer spatial resolution. Indeed, the nanometric analogue to extinction spectroscopy, the Electron Energy Loss Spectroscopy (EELS), has been of great interest for studying SP resonances in highly symmetric nano-objects [1], thanks to a relatively high signal to noise ratio. In parallel, cathodoluminescence (CL) has proved to be very powerful when applied in analyzing the light emission induced from SP in nanoparticles with a unique spectral resolution [2]. Moreover, both extinction and emission mappings may provide complementary as well as precious information for the physics of SP when compared using these two techniques on a single nano-object. Figure 1 shows a STEM High Angle Angular Dark Field (HAADF) image of a gold (bright) nanoprism relying on a (dark) carbon grid, together with EELS and CL spectra for a given probe position at the top tip of the nanoprism. In EELS mode, the fast electron beam is spectrally analyzed through transmission of the sample, giving access to Electron Energy Loss (EEL) probability spectra (in blue). As the incident electron beam acts as a white light source, all types of excitations are probed, including &amp;amp;quot;bright&amp;amp;quot; as well as &amp;amp;quot;dark&amp;amp;quot; SP modes. In CL mode, light coming from deexcitation of the locally excited radiative SP modes is dispersed to obtain Electron Induced Radiation Emission (EIRE) probability spectra (in red). As can be seen, CL displays clear redshift as compared to EELS, mostly due to retardation effects [3]. Hyperspectral imaging data is obtained by raster scanning the incident electron probe over the sample to investigate the related SP modes. Figure 2 shows spatially resolved EEL and EIRE maps, filtered at the relevant peak wavelengths. This evidences the dipolar character of the mode, similar to dipolar SP modes previously observed by EELS for silver nanoprisms [1]. Furthermore, it is now well established that these maps display the spatial variations of a quantity which is related to the total [4] (for EELS) and radiative [5] (for EIRE) Electromagnetic Local Density of States (EMLDOS). As the contribution of several SP modes may blur the EMLDOS patterns, we fit on each pixel a Gaussian function around the peak positions to obtain the amplitude weight of the dipolar mode. Results are given on figure 3, demonstrating the coherent character of the mode over the three tips, despite a maximum in photon emission at the upper tip. By comparing both spatially resolved EELS and CL on a single nano-object, we therefore gain insight on its complete optical properties at the nanoscale. This includes discerning its &amp;amp;quot;dark&amp;amp;quot; and &amp;amp;quot;bright&amp;amp;quot; modes, and comparing emission and absorption properties. [6] [6] This work is partially supported by the Centre National de la Recherche Scientifique and the Délégation Générale de l&amp;amp;#39;Armement. Figure 1. Left: HAADF image of a gold nanoprism relying on a carbon grid. Right: EEL (blue) and EIRE (red) spectra obtained at a fixed probe position indicated by the white square.

ABSTRACT Understanding and controlling the growth, usually performed by Vapour-Liquid- Solid method, of silicon nanowires with grown metal catalysts is crucial for optimized morphologies, surface and electronic properties of these nanostructures regarding their future applications. One of the issues is the suppression of the tapering effect resulting in a cone-shaped wire, and arising from the diffusion, during growth, of the metal catalyst along the sidewall. Indirect evidences for such a phenomenon and for the presence of gold on the sidewall have been proposed previously [1], before a qualitative and direct identification using SEM-EDX and TEM was reported for technologically relevant Si NWs [2]. However, such techniques cannot provide quantitative and surface-sensitive information relevant to the diffusion process, and moreover can be extremely localized, preventing to obtain a global picture of the surface chemistry at the scale of the wire.

Plasmon modes of the exact same individual gold nanoprisms are investigated through combined nano-meter-resolved electron energy-loss spectroscopy (EELS) and cathodoluminescence (CL) measurements. We show that CL only probes the radiative modes, in contrast to EELS, which additionally reveals dark modes. The combination of both techniques on the same particles thus provides complementary information and also demonstrates that although the radiative modes give rise to very similar spatial distributions when probed by EELS or CL, their resonant energies appear to be different. We trace this phenomenon back to plasmon dissipation, which affects in different ways the plasmon signatures probed by these techniques. Our experiments are in agreement with electromagnetic numerical simulations and can be further interpreted within the framework of a quasistatic analytical model. We therefore demonstrate that CL and EELS are closely related to optical scattering and extinction, respectively, with the addition of nanometer spatial resolution.

Over the past ten years, Scanning Transmission Electron Microscopes (STEM) fitted with
Electron Energy Loss Spectroscopy (EELS) and/or Cathodoluminescence (CL) spectroscopy
have demonstrated to be essential tools for probing the optical properties of nano-objects
at sub-wavelength scales. Thanks to the possibility of measuring them at a nanometer scale
in parallel to the determination of the structure and morphology of the object of interest,
new challenging experimental and theoretical horizons have been unveiled. As regards
optical properties of metallic nanoparticles, surface plasmons have been mapped at a scale
unimaginable only a few years ago, while the relationship between the energy levels and
the size of semiconducting nanostructures a few atomic layers thick could directly be
measured. This paper reviews some of these highly stimulating recent developments.

The production of hydrogen from water using only a catalyst and solar energy is one of the most
challenging and promising outlets for the generation of clean and renewable energy. Semiconductor
photocatalysts for solar hydrogen production by water photolysis must employ stable, non-toxic,
abundant and inexpensive visible-light absorbers capable of harvesting light photons with adequate
potential to reduce water. Here, we show that a-Fe2O3 can meet these requirements by means of using
hydrothermally prepared nanorings. These iron oxide nanoring photocatalysts proved capable of
producing hydrogen efficiently without application of an external bias. In addition, Co(OH)2
nanoparticles were shown to be efficient co-catalysts on the nanoring surface by improving the
efficiency of hydrogen generation. Both nanoparticle-coated and uncoated nanorings displayed
superior photocatalytic activity for hydrogen evolution when compared with TiO2 nanoparticles,
showing themselves to be promising materials for water-splitting using only solar light.