Publicações

 

Para uma lista atualizada de publicações visite minha página no ResearcherID. Alternativamente, veja as publicações listadas no meu Currículo Lattes.

Alguns manuscritos de artigos que publiquei podem ser obtidos na minha página pessoal.

 

Veja abixo algumas publicações selecionadas.

 

Figure 1-b

Nanometer-scale monitoring of quantum-confined Stark effect and emission efficiency droop in multiple GaN/AlN quantum disks in nanowires

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 (<3nm), 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.
L. F. Zagonel, L. H. G. Tizei, G. Z. Vitiello, G. Jacopin, L. Rigutti, M. Tchernycheva, F. H. Julien, R. Songmuang, T. Ostasevicius, F. de la Peña, C. Ducati, P. A. Midgley, and M. Kociak
Phys. Rev. B 93, 205410 – Published 6 May 2016.

pdf Ver texto completo em PDF.

Nanoscale EELS-HRTEM

Photocatalytic hydrogen production of Co(OH)2 nanoparticle-coated α-Fe2O3 nanorings

Reference: Nanoscale, 2013,5, 9310-9316.

Authors: Heberton Wender, Renato V. Gonçalves, Carlos Sato B. Dias, Maximiliano J. M. Zapata, Luiz F. Zagonel, Edielma C. Mendonça, Sérgio R. Teixeira and Flávio Garcia

 Color QDiscs

Seeing and measuring in colours: Electron microscopy and spectroscopies applied to nano-optics

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.
Reference: Comptes Rendus Physique 15 (2014) 158–175.
Authors: Mathieu Kociak, Odile Stéphan, Alexandre Gloter, Luiz F. Zagonel, Luiz H. G. Tizei, Marcel Tencé, Katia March, Jean Denis Blazit, Zackaria Mahfoud, Arthur Losquin, Sophie Meuret, Christian Colliex

DOI: 10.1016/j.crhy.2013.10.003

 

NL TOC figure

 

 

 

Nanometer Scale Spectral Imaging of Quantum Emitters in Nanowires and Its Correlation to Their Atomically Resolved Structure

We report the spectral imaging in the UV to visible range with nanometer scale resolution of closely packed GaN/AlN quantum disks in individual nanowires using an improved custom-made cathodoluminescence system. We demonstrate the possibility to measure full spectral features of individual quantum emitters as small as 1 nm and separated from each other by only a few nanometers and the ability to correlate their optical properties to their size, measured with atomic resolution. The direct correlation between the quantum disk size and emission wavelength provides evidence of the quantum confined Stark effect leading to an emission below the bulk GaN band gap for disks thicker than 2.6 nm. With the help of simulations, we show that the internal electric field in the studied quantum disks is smaller than what is expected in the quantum well case. We show evidence of a clear dispersion of the emission wavelengths of different quantum disks of identical size but different positions along the wire. This dispersion is systematically correlated to a change of the diameter of the AlN shell coating the wire and is thus attributed to the related strain variations along the wire. The present work opens the way both to fundamental studies of quantum confinement in closely packed quantum emitters and to characterizations of optoelectronic devices presenting carrier localization on the nanometer scale.

Reference: Nano Letters 2011, 11, 568–573.


Authors: Luiz Fernando Zagonel, Stefano Mazzucco, Marcel Tencé, Katia March, Romain Bernard, Benoît Laslier, Gwenole Jacopin, Maria Tchernycheva, Lorenzo Rigutti, Francois H. Julien, Rudeesun Songmuang, and Mathieu Kociak.

doi:10.1021/nl103549t.

pdf  Ver texto completo em PDF  (cópia da versão publicada no Arxiv).

 CL diagram

Visualising highly localised luminescence in GaN/AlN heterostructures in nanowires

 

The optical properties of a stack of GaN/AlN quantum discs (QDiscs) in a GaN nanowire have been studied by spatially resolved cathodoluminescence (CL) at the nanoscale (nanoCL) using a Scanning Transmission Electron Microscope (STEM) operating in spectrum imaging mode. For the electron beam excitation in the QDisc region, the luminescence signal is highly localized with spatial extension as low as 5 nm due to the high band gap difference between GaN and AlN. This allows for the discrimination between the emission of neighbouring QDiscs and for evidencing the presence of lateral inclusions, about 3 nm thick and 20 nm long rods (quantum rods, QRods), grown unintentionally on the nanowire sidewalls. These structures, also observed by STEM dark-field imaging, are proven to be optically active in nanoCL, emitting at similar, but usually shorter, wavelengths with respect to most QDiscs.

Rerence: Nanotechnology 23 (2012) 455205.

Authors: L. F. Zagonel, L. Rigutti, M. Tchernycheva, G. Jacopin, R. Songmuang, M. Kociak

pdf  Ver texto completo em PDF (cópia da versão publicada no Arxiv).

 

InGaN

Projected spectrum image shows the light emisstion spectra along the NW.  Only in 2 InGaN inclutions, high luminescence is observed. 

Growth mechanism and properties of InGaN insertions in GaN nanowires

We demonstrate the strong influence of strain on the morphology and In content of InGaN insertions in GaN nanowires, in agreement with theoretical predictions which establish that InGaN island nucleation on GaN nanowires may be energetically favorable, depending on In content and nanowire diameter. EDX analyses reveal In inhomogeneities between the successive dots but also along the growth direction within each dot, which is attributed to compositional pulling. Nanometer-resolved cathodoluminescence on single nanowires allowed us to probe the luminescence of single dots, revealing enhanced luminescence from the high In content top part with respect to the lower In content dot base.
Reference:  Nanotechnology 23 (2012) 135703 (6pp).
 
Authors: G Tourbot, C Bougerol, F Glas, L F Zagonel, Z Mahfoud, S Meuret, P Gilet, M Kociak, B Gayraland and B Daudin.
 Wire

 Single-Wire Light-Emitting Diodes Based on GaN Wires Containing Both Polar and Nonpolar InGaN/GaN Quantum Wells

Single-wire light-emitting diodes based on radial p–i–n multi quantum well (QW) junctions have been realized from GaN wires grown by catalyst-free metal organic vapor phase epitaxy. The Inx Ga1Àx N/GaN undoped QW system is coated over both the nonpolar lateral sidewalls and on the polar upper surface. Cathodo- and electroluminescence (EL) experiments provide evidence that the polar QWs emit in the visible spectral range at systematically lower energy than the nonpolar QWs. The EL of the polar or nonpolar QWs can be selectively activated by varying the sample temperature and current injection level.

Reference: Applied Physics Express 5 (2012) 014101.

Authors: Gwenole Jacopin, Andre De Luna Bugallo, Pierre Lavenus, Lorenzo Rigutti, Francois H. Julien, Luiz F. Zagonel, Mathieu Kociak, Christophe Durand, Damien Salomon, Xiao Jun Chen, Joel Eymery, and Maria Tchernycheva.
DOI: 10.1143/APEX.5.014101  

 

 TOC NL2010

 Ultraviolet Photodetector Based on GaN/AlN  Quantum Disks in a Single Nanowire

We report the demonstration of single-nanowire photodetectors relying on carrier generation in GaN/AlN QDiscs. Two nanowire samples containing QDiscs of different thicknesses are analyzed and compared to a reference binary n-i-n GaN nanowire sample. The responsivity of a single wire QDisc detector is as high as 2 × 10 3 A/W at λ ) 300 nm at room temperature. We show that the insertion of an axial heterostructure drastically reduces the dark current with respect to the binary nanowires and enhances the photosensitivity factor (i.e., the ratio between the photocurrent and the dark current) up to 5 × 10 2 for an incoming light intensity of 5 mW/cm2. Photocurrent spectroscopy allows identifcation of the spectral contribution related to carriers generated within large QDiscs, which lies below the GaN band gap due to the quantum confined Stark effect.

Reference: Nano Letters 2010, 10, 2939.

Authors: L. Rigutti, M. Tchernycheva, A. De Luna Bugallo, G. Jacopin, F. H. Julien, L. F. Zagonel, K. March, O. Stephan, M. Kociak, and R. Songmuang

 

 WorkFunction

Orientation-dependent work function of in situ annealed strontium titanate (SrTiO3)

We have used energy-filtered x-ray photoelectron emission microscopy (XPEEM) and synchrotron radiation to measure the grain orientation dependence of the work function of a sintered niobium-doped strontium titanate (SrTiO3) ceramic. A significant spread in work function values is found. Grain orientation and surface reducing/oxidizing conditions are the main factors in determining the work function. Energy-filtered XPEEM looks ideally suited for analysis of other technologically interesting polycrystalline samples. The mean work function value is 4.26 eV with a standard deviation of 0.06 eV. The grain orientation is determined by electron backscattered diffraction and correlated with the XPEEM work functions. A good linear correlation is obtained between WF and the polar EBSD angle θ . In particular, the work functions determined for the three principal crystal faces [100], [110] and [111] are 4.13, 4.32 and 4.34 eV, respectively.

Reference: J. Phys.: Condens. Matter 21 (2009) 314013.
Authors:L F Zagonel, M Bäurer, A Bailly, O Renault, M Hoffmann, S-J Shih, D Cockayne and N Barrett
DOI: 10.1088/0953-8984/21/31/314013

pdf  Ver texto completo em PDF.

 STOcomp

Orientation-dependent surface composition ofin situ annealed strontium titanate

The surface composition of polycrystalline niobium-doped strontium titanate (SrTiO3: Nb) is studied using X-ray photoelectron emission microscopy (XPEEM) for many grain orientations in order to characterise the surface chemistry with high spatial resolution. The surface sensitivity is maximised by the use of soft X-ray synchrotron radiation (SR). The grain orientation is determined by electron backscattering diffraction (EBSD). Stereographic plots are used to show the correlation between surface composition and orientation for several grains. Predominant surface terminations are assigned to major orientations.Reference: Surface and Interface Analysis 2008, 40, 1709–1712.
Authors: Luiz F. Zagonel, Nicholas Barrett, Olivier Renault, Aude Bailly, Michael Bäurer, Michael Hoffmann, Shao-Ju Shih, David Cockayne.

 SiliconSuboxides

Full field chemical imaging of buried native sub-oxide layers on doped silicon patterns

Fully energy-filtered X-ray photoelectron emission microscopy is used to analyze the spatial distribution of the silicon sub-oxide structure at the SiO2/Si interface as a function of underlying doping pattern. Using a spectroscopic pixel-by-pixel curve fitting analysis, we obtain the sub-oxide binding energy and intensity distributions over the full field of view. Binding energy maps for each oxidation state are obtained with a spatial resolution of 120 nm. Within the framework of a five-layer model, the experimental data are used to obtain quantitative maps of the sub-oxide layer thickness and also their spatial distribution over the p–n junctions. Variations in the sub-oxide thicknesses are found to be linked to the level and type of doping. The procedure, which takes into account instrumental artefacts, enables the quantitative analysis of the full 3D dataset.

Reference: Surface Science 604 (2010) 1628–1636
Authors: F. de la Peña, N. Barrett, L.F. Zagonel, M. Walls, O. Renault

DOI: 10.1016/j.susc.2010.06.006

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