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International audience; We describe the fabrication of Ge nanowires during a single co-deposition step of Ge and Mn at high temperature. In these experimental conditions, a phase separation occurs and two different phases Ge and Ge1 − xMnx are formed with Ge1 − xMnx in the shape of small clusters distributed randomly in the Ge matrix. Because of the high deposition temperature, a new Ge1 − xMnx phase with low eutectic point is stabilized; this phase is different from the one (commonly Ge3Mn5) stabilized at lower temperature. During the growth process at 350 °C, the crystalline clusters remain solid but they are highly mobile and can float at the surface, serving as seeds to direct the growth of crystalline Ge nanowires from the vapor. The sketch steps of NWs formation are first the phase separation with formation of specific Ge1 − xMnx critical nuclei with low eutectic point and second the growth of Ge NWs directed by the Ge1 − xMnx solid cluster seeds. Ge NWs growth is forced along particular crystalline axis by the cluster seeds that lower the interfacial energy Ge/Ge1 − xMnx and the energy formation of the germanium crystal stabilizes the cluster position at the tip of the NWs. The density of NWs can be tuned by varying the nominal Mn concentration since this density is related to the number of clusters with the specific Ge1 − xMnx phase (with low eutectic point). The single step MBE process presented here has the main advantage to fully avoid any incorporation of unintentional impurity into Ge nanowires (apart from Mn atoms) and could be applied to several other systems. This work also provides new insights into the vapor–solid–solid growth mechanisms of Ge NWs.

International audience; X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) are complementary techniques whose combination is a powerful tool of investigation of matter under extreme conditions (high pressure, high temperature). The standard setup at the ODE beamline (Soleil Synchrotron) has been modified in order to allow the performance of simultaneous XAS/XRD measurements on the same sample. These techniques, together with Raman spectroscopy measurements, were applied to the study of pressure-induced phase transitions of amorphous silicon-germanium alloy (a-Six Ge1 −x , with x = 0.75) using a membrane diamond anvil cell as the pressure device.

International audience; We present spectacular evidence of metallization of amorphous Ge films under high pressure using optical micrographs and Raman-scattering and x-ray-absorption spectroscopies. This transformation is identified as a low-density amorphous LDA to high-density amorphous HDA transition, relevant to a large class of systems including amorphous ice, semiconductors, and oxides. We have discovered that this transition initiates at the surface of the LDA sample, characterized intrinsically by a thickness dependent density of voids. Contrary to the case of transitions involving stable crystalline solid phases, our observations show that pressure-induced phase transitions in inhomogeneous amorphous samples are morphology driven and are favored for lower defect void densities. The metal disordered phase is observed first at 8 GPa for a lower density of voids, transforming to a metastable ordered phase upon depressurization. The local HDA structure is characterized by an increase in the first-neighbor coordination number, average distance, and variance.