Publications

2022

• Electron Dynamics in Hybrid Perovskites Reveal the Role of Organic Cations on the Screening of Local Charges
  
  • Cherasse Marie
  • Dong Jingwei
  • Trippé-Allard Gaëlle
  • Deleporte Emmanuelle
  • Garrot Damien
  • Maehrlein Sebastian
  • Wolf Martin
  • Chen Zhesheng
  • Papalazarou Evangelos
  • Marsi Marino
  • Rueff Jean-Pascal
  • Taleb-Ibrahimi Amina
  • Perfetti Luca
  Nano Letters, American Chemical Society, 2022, 22 (5), pp.2065-2069. The large tolerance of hybrid perovksites to the trapping of electrons by defects is a key asset in photovoltaic applications. Here, the ionic surface terminations of CH3NH3PbI3 are employed as a testbed to study the effect of electrostatic fields on the dynamics of excited carriers. We characterize the transition across the tetragonal to orthorhombic phase. The observed type II band offset and drift of the excited electrons highlight the important role that organic cations have on the screening of local electrostatic fields. When the orientation of organic cations is frozen in the orthorhombic phase, the positively charged termination induces a massive accumulation of excited electrons at the surface of the sample. Conversely, no electron accumulation is observed in the tetragonal phase. We conclude that the local fields cannot penetrate in the sample when the polarizability of freely moving cations boosts the dielectric constant up to ϵ = 120. (10.1021/acs.nanolett.2c00001)
  DOI : 10.1021/acs.nanolett.2c00001
• Reply to "Comment on 'Understanding first order Raman spectra of boron carbides across the homogeneity range
  
  • Roma Guido
  • Jay Antoine
  • Vast Nathalie
  • Hardouin Duparc Olivier
  • Gutierrez Gaelle
  Physical Review Materials, American Physical Society, 2022, 6 (1), pp.016602. In response to Werheit's Comment, we first discuss the following three points: (i) Attribution of the Raman bands with the help of calculations based on the density functional perturbation theory (DFPT), (ii) similarity of the Raman spectrum of boron carbide to the Raman spectrum of α rhombohedral boron, and (iii) dependence of the experimental Raman spectrum on the frequency of the excitation laser. Central to the point at stake, we also present a Raman spectrum computed for the first time on a 405-atom unit-cell of boron carbide, where the simulation cell accounts for the substitutional disorder of the polar carbon atom in the six atomic positions that are equivalent in the trigonal symmetry.We thus obtain a theoretical spectrum whose agreement with experiment is unprecedented in boron carbide, all of the features being present, with however negligible intensities for the doublet at 270/320 cm-1. We argue that the doublet intensity seen in experiments is not purely vibrational ---as computed at mechanical equilibrium--- and that FT-Raman spectroscopy taken at 1.06 μm lacks the accuracy that is necessary to study vibrational modes of boron carbides. (10.1103/PhysRevMaterials.6.016602)
  DOI : 10.1103/PhysRevMaterials.6.016602
• Combining two-photon photoemission and transient absorption spectroscopy to resolve hot carrier cooling in 2D perovskite single crystals: the effect of surface layer
  
  • Lin Weihua
  • Liang Mingli
  • Niu Yuran
  • Chen Zhesheng
  • Cherasse Marie
  • Meng Jie
  • Zou Xianshao
  • Zhao Qian
  • Geng Huifang
  • Papalazarou Evangelos
  • Marsi Marino
  • Perfetti Luca
  • Canton Sophie
  • Zheng Kaibo
  • Pullerits Tönu
  Journal of Materials Chemistry C, Royal Society of Chemistry, 2022, 10 (44), pp.16751-16760. We investigate HC cooling in two-dimensional perovskite single-crystals by applying two complementary ultrafast techniques and observe spatial sensitivity of cooling. (10.1039/D2TC03111F)
  DOI : 10.1039/D2TC03111F
• Molecular dynamics between amorphous and crystalline phases of e-beam irradiated piezoelectric PVDF thin films employing solid-state NMR spectroscopy
  
  • Potrzebowska Natalia
  • Cavani Olivier
  • Kazmierski Slawomir
  • Wegrowe Jean-Eric
  • Potrzebowski Marek
  • Clochard Marie-Claude
  Polymer Degradation and Stability, Elsevier, 2022, 195, pp.109786. (10.1016/j.polymdegradstab.2021.109786)
  DOI : 10.1016/j.polymdegradstab.2021.109786
• On-line optical absorption of electron-irradiated yttria-stabilized zirconia
  
  • Costantini Jean-Marc
  • Cavani Olivier
  • Boizot Bruno
  Journal of Physics and Chemistry of Solids, Elsevier, 2022, 169, pp.110853. The kinetics of color-center formation and decay in electron-irradiated cubic yttria-stabilized zirconia (ZrO$_2$: Y$^{3+}$) is followed by on-line UV-visible absorption spectroscopy. Growth and time decay of absorption spectra was measured upon electron irradiation and subsequent beam shut-off for energies of 0.8, 1.0, 1.75, and 2.5 MeV. For a high beam current intensity, spectra are fitted by two broad absorption bands centered at 3.0 eV and 3.8 eV. Such bands are assigned to the so-called T-center, i. e. Zr$^{3+}$ ions in a trigonal point symmetry, which is produced by ionization processes either by photon irradiation or charged particle irradiation. A red-shift of absorption spectra is observed for lower current intensities regardless of electron energy. Such a modification is attributed to a change in the local environment of Zr$^{3+}$ ions due to the formation of neighboring oxygen vacancies by elastic collisions, depending on the electron energy and enhanced by a higher flux. A common behavior of increase of the differential absorbance to similar saturation values after accumulation of the irradiation dose and time-decays to similar non-zero asymptotic values are observed, regardless of the electron energy. However, the rise time and lifetime deduced from growth and decay curves of the absorbance are depending on the electron energy. The increase and saturation of the T-center growth rate with electron energy is attributed to competitive channels of hole trapping on the oxygen and zirconium vacancies induced by elastic collisions. The present on-line experiments reveal the complex process of point-defect generation resulting from the interplay of displacement damage and electronic excitations.
• Visualizing the effects of plasma-generated H atoms <i>in situ</i> in a transmission electron microscope
  
  • Maurice Jean-Luc
  • Bulkin Pavel
  • Ngo Éric
  • Wang Weixi
  • Foldyna Martin
  • Florea Ileana
  • Roca I Cabarrocas Pere
  • Béjaud Romuald
  • Hardouin Duparc Olivier
  European Physical Journal: Applied Physics, EDP Sciences, 2022, 97, pp.7. The radicals and atoms generated by a plasma have the effect, among others, of changing the surface energies of materials, which allows one to prepare nano-objects that would not stabilise in other conditions. This is the case of the Sn catalysed silicon nanowires (NWs) we present in this paper: without plasma, the liquid Sn at the top of NWs is unstable (because Sn naturally wets the Si) so that no growth is allowed, while in presence of the H atoms generated by the plasma, the balance of surface energies is drastically changed; the Sn droplet stabilises and can be used efficiently by the vapour-liquid-solid (VLS) mechanism of growth. Thus, if one wants to study the growth mechanisms of such NWs in situ in the transmission electron microscope (TEM), one has to adapt a plasma system on the TEM. This is precisely what was done at École polytechnique on the NanoMAX environmental TEM. The paper reports on the plasma effects, on the catalyst and on NW growth, recorded in situ in real time, at atomic resolution. The results are discussed in the light of density functional calculations of bare and hydrogenated Si surface energies. (10.1051/epjap/2022210276)
  DOI : 10.1051/epjap/2022210276
• Defects Characterization of HgCdTe and CdZnTe Compounds by Positron Annihilation Spectroscopy
  
  • Léger Valentin
  • Desgardin Pierre
  • Destefanis Vincent
  • Botsoa Jacques
  • Patriarche Gilles
  • Barthe Marie-France
  • Corbel Catherine
  • Rubaldo Laurent
  Journal of Electronic Materials, Institute of Electrical and Electronics Engineers, 2022, 51 (9), pp.4659-4665. Infrared cooled photodetectors must operate at higher temperatures to reduce their size, weight and power consumption (SWaP context). Their stability and image quality are then challenged by extra electrical activity of crystal defects. Knowledge of defect populations is mandatory to improve the material quality of the epitaxial Hg1−xCdxTe (MCT) active layer and the Cd1−xZnxTe (CZT) substrate. Positron annihilation spectroscopy with a slow positron beam was used to study near-surface open-volume defects profiles. Low- and high-momentum fractions (S,W) were used to characterize the Doppler broadening of the 511 keV electron-positron pair annihilation-line as a function of the positron implantation energy E. The results show that three regions can be identified beneath the surface of the as-grown non-optimized MCT layer. The quasi-linear relationship between the annihilation characteristics in the regions suggests that the defect populations mainly correspond to the same open-volume defect in different concentrations. The probed defect is thought to be related to the mercury vacancy. This hypothesis is discussed in an original way with near-surface elemental profiles using scanning transmission electron spectroscopy combined to energy dispersive x-ray spectroscopy (STEM-EDX). Afterwards, this approach is extended to CZT substrates showing that surface and bulk properties of those fabricated by LYNRED tend to match those that are state-of-the-art. A common open-volume defect is probed, in concentration estimated by Hall effect around 1015 cm−3 and thought to be related to the cadmium vacancy. (10.1007/s11664-022-09801-6)
  DOI : 10.1007/s11664-022-09801-6