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Register a new userThe mechanism of ion induced amorphization in Si
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Some results on damage build up in, and amorphization of, Si, induced by 25-30 keV Al$_5^-$, Si$_5^-$ and Cs$^-$ ions, at room temperature, are reported. We show that at low energy, amorphization is a nucleation and growth process, based on the direct impact mechanism. With an Avrami exponent $sim 1.6$, the growth towards amorphization seems to be diffusion limited. A transition to a completely amorphized state is indicated at a dose exceeding 17 eV/atom, which is higher than 6-12 eV/atom as predicted by simulations. The observed higher threshold could be due to temperature effects although an underestimation of keV-energy recoils, in simulation, may not be ruled out.
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We propose that the driving force of an ultrafast crystalline-to-amorphous transition in phase-change memory alloys are strained bonds existing in the (metastable) crystalline phase. For the prototypical example of GST, we demonstrate that upon breaking of long Ge-Te bond by photoexcitation Ge ion shot from an octahedral crystalline to a tetrahedral amorphous position by the uncompensated force of strained short bonds. Subsequent lattice relaxation stabilizes the tetrahedral surroundings of the Ge atoms and ensures the long-term stability of the optically induced phase.
It has recently been shown that amorphization and melting of ice were intimately linked. In this letter, we infer from molecular dynamics simulations on the SiO2 system that the extension of the quartz melting line in the metastable pressure-temperature domain is the pressure-induced amorphization line. It seems therefore likely that melting is the physical phenomenon responsible for pressure induced amorphization. Moreover, we show that the structure of a pressure glass is similar to that of a very rapidly (1e+13 to 1e+14 kelvins per second) quenched thermal glass.
We observe that pressure-induced amorphization of Ga2SeTe2 (a III-VI semiconductor) is directly influenced by the periodicity of its intrinsic defect structures. Specimens with periodic and semi-periodic two-dimensional vacancy structures become amorphous around 10-11 GPa in contrast to those with aperiodic structures, which amorphize around 7-8 GPa. The result is a notable instance of altering material phase-change properties via rearrangement of stoichiometric vacancies as opposed to adjusting their concentrations. Based on our experimental findings, we posit that periodic two-dimensional vacancy structures in Ga2SeTe2 provide an energetically preferred crystal lattice that is less prone to collapse under applied pressure. This is corroborated through first-principles electronic structure calculations, which demonstrate that the energy stability of III-VI structures under hydrostatic pressure is highly dependent on the configuration of intrinsic vacancies.
The structural phase transitions of single crystal TiO2-B nanoribbons were investigated in-situ at high-pressure using the synchrotron X-ray diffraction and the Raman scattering. Our results have shown a pressure-induced amorphization (PIA) occurred in TiO2-B nanoribbons upon compression, resulting in a high density amorphous (HDA) form related to the baddeleyite structure. Upon decompression, the HDA form transforms to a low density amorphous (LDA) form while the samples still maintain their pristine nanoribbon shape. HRTEM imaging reveals that the LDA phase has an {alpha}-PbO2 structure with short range order. We propose a homogeneous nucleation mechanism to explain the pressure-induced amorphous phase transitions in the TiO2-B nanoribbons. Our study demonstrates for the first time that PIA and polyamorphism occurred in the one-dimensional (1D) TiO2 nanomaterials and provides a new method for preparing 1D amorphous nanomaterials from crystalline nanomaterials.
Conversion reaction is one of the most important chemical processes in energy storage such as lithium ion batteries. While it is generally assumed that the conversion reaction is initiated by ion intercalation into the electrode material, solid evidence of intercalation and the subsequent transition mechanism to conversion remain elusive. Here, using well-defined WO3 single crystalline thin films grown on Nb doped SrTiO3(001) as a model electrode, we elucidate the conversion reaction mechanisms during Li+, Na+ and Ca2+ insertion into WO3 by in situ transmission electron microscopy studies. Intercalation reactions are explicitly revealed for all ion insertions. With corroboration from first principle molecular simulations, it is found that, beyond intercalation, ion-oxygen bonding destabilize the W framework, which gradually collapses to pseudo-amorphous structure. In addition, we show the interfacial tensile strain imposed by the SrTiO3 substrate can preserve the structure of an ultra-thin layer of WO3, offering a possible engineering solution to improve the cyclability of electrode materials. This study provides a detailed atomistic picture on the conversion-type electrodes in secondary ion batteries.
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