Didier Perrodin

Real-time Crystal Growth Visualization and Quantification by Energy-Resolved Neutron Imaging

Energy-resolved neutron imaging is investigated as a real-time diagnostic tool for visualization and in-situ measurements of “blind” processes. This technique is demonstrated for the Bridgman-type crystal growth enabling remote and direct measurements of growth parameters crucial for process optimization. The location and shape of the interface between liquid and solid phases are monitored in real-time, concurrently with the measurement of elemental distribution within the growth volume and with the identification of structural features with a ~100 μm spatial resolution. Such diagnostics can substantially reduce the development time between exploratory small scale growth of new materials and their subsequent commercial production. This technique is widely applicable and is not limited to crystal growth processes.

In situ diagnostics of the crystal-growth process through neutron imaging: application to scintillators

The unique possibilities enabled by neutron imaging for in situ remote diagnostics of microstructural characteristics during crystal growth are demonstrated, even when the materials and surrounding structures are opaque to other more conventional interrogation techniques. Neutron radiography is implemented to image remotely the uniformity of elemental distribution (e.g. dopant concentration) during crystal growth, the location of the liquid/solid interface and the presence of macroscopic crystal defects (e.g. cracks), all with a temporal resolution of 5–7 s.

Determination of the structural phase and octahedral rotation angle in halide perovskites

A key to the unique combination of electronic and optical properties in halide perovskite materials lies in their rich structural complexity. However, their radiation sensitive nature limits nanoscale structural characterization requiring dose efficient microscopic techniques in order to determine their structures precisely. In this work, we determine the space-group and directly image the Br halide sites of CsPbBr3, a promising material for optoelectronic applications. Based on the symmetry of high-order Laue zone reflections of convergent-beam electron diffraction, we identify the tetragonal (I4/mcm) structural phase of CsPbBr3 at cryogenic temperature. Electron ptychography provides a highly sensitive phase contrast measurement of the halide positions under low electron-dose conditions, enabling imaging of the elongated Br sites originating from the out-of-phase octahedral rotation viewed along the [001] direction of I4/mcm persisting at room temperature. The measurement of these features and compariso...

Unraveling the Crystal Structure of All-Inorganic Halide Perovskites using CBED and Electron Ptychography

The unique electronic properties observed in halide perovskites originate in their rich structural complexity that allows compatibility with a variety of structural motifs and compounds. Adjustments of the corner-connected BX6 octahedral network in the ABX3 structure promote a wide range of optical and electronic properties [1]. For this reason, methods to precisely identify the local symmetry are necessary to unambiguously distinguish different possible structural phases. Here, we determine the crystallography of all-inorganic CsPbBr3-xClx perovskite single crystals, grown via the BridgmanStockbarger method, by exploring information contained in electron diffraction patterns. Due to the material’s sensitivity to the electron beam, precise atomistic studies of halide perovskites by transmission electron microscopy (TEM), scanning TEM (STEM) and electron diffraction are relatively underdeveloped. Consequently, X-ray diffraction has been the technique of choice to assign crystal structures, requiring a careful study of the splitting of certain lines and of the presence of superlattice reflections. Effects due to small atomic shifts can easily be missed in X-ray diffraction, which accounts for reports of conflicting structures.

Symmetry group determination and direct imaging of all-inorganic halide perovskites CsPbBr3−xClx

The unique properties observed in halide perovskites originate in their rich structural complexity that allows compatibility with a variety of structural motifs and compounds. Adjustments of the corner-connected BX6 octahedral network in the ABX3 structure promote a wide range of optical and electronic properties [1]. For this reason, methods to precisely identify the local symmetry are necessary to unambiguously distinguish different possible structural phases. Here, we determine the crystallography of all-inorganic CsPbBr1-xClx perovskite single crystals, grown via the Bridgman-Stockbarger method, by exploring information contained in electron diffraction patterns. Due to the material’s sensitivity to the electron beam, precise atomistic studies of halide perovskites by transmission electron microscopy (TEM), scanning TEM (STEM) and electron diffraction are relatively underdeveloped. Consequently, X-ray diffraction has been the technique of choice to assign crystal structures, requiring a careful study of the splitting of certain lines and of the presence of superlattice reflections. Effects due to small atomic shifts can easily be missed in X-ray diffraction, which accounts for reports of incorrect structures. In this work, we use convergent-beam electron diffraction (CBED) acquired at low-temperature (~70K) to identify the symmetry of CsPbBr3-xClx single crystals with x = 0, 1 and 3. High-symmetry CBED pattern containing information on high-order Laue zones (HOLZ), in which the intensities of certain reflections are quite sensitive to small rotations of the octahedral unit, reveal the whole crystal symmetry and positions of Br/Cl anions. Local imaging of the projected atomistic distribution in these crystals is achieved by simultaneous atomic-resolution electron ptychography (a dose efficient diffractive imaging technique [2]) and Z-contrast, using high-speed direct electron detector. A phase transformation dependency on the halide anion is observed as function of composition x. A tetragonal I4/mcm structure is assigned to CsPbBr3 whereas CsPbCl3 is found to be orthorhombic. [3]

Pulling of 3 mm diameter AlSb rods by micro-pulling down method

We designed and supplied special crucibles for AlSb material. Thermal insulation and limitation of Sb losses were our first work. The protection of the growth environment was also one of our priority to avoid any pollution of the Fibercryst {mu}PD facility. When this work was achieved, the next step was the calibration of the heating power for these new crucibles. Then, it was the definition of single crystal growth conditions that oriented our research. Following our proposal, many growths attempts were performed. We started from Al & Sb pure powder or from LBNL AlSb crystal as expected. We used different crucibles and different seeds.