时 间： 2018 年 1 月 26 日 9 : 30 – 11 : 30
地 点： 南五楼 612 学术报告厅
报告人： Riccardo Mazzarello 教授(W2)，德国亚琛工大
邀请人： 徐 明 教授
Phase-change materials (PCMs) are capable of switching very rapidly and reversibly between the amorphous and crystalline state at high temperature. Yet, the two phases are stable at room temperature and exhibit pronounced optical and electrical contrast. The ability of the glass to crystallize quickly at high temperature and yet to be extremely stable at low temperature has been linked to the pronounced fragility of the supercooled liquid phase of PCMs. These unique properties are exploited in rewritable optical devices (Blu-Ray Disc) and electronic non-volatile random access memories (PC-RAM), where heating is induced by laser irradiation and the Joule effect, respectively.
Remarkably, the time and length scales relevant to state-of-the-art phase-change memory cells are almost within reach of first-principles methods based on density functional theory. Here we present ab initio molecular dynamics simulations of crystallization of two prototypical PCMs, namely Ge2Sb2Te5 and Ag,In-doped Sb2Te, as well as a recently discovered ultrafast PCM, Sc0.2Sb2Te3. It is shown that high-temperatures simulations of Ge2Sb2Te5 and Ag/In-doped Sb2Te yield crystal growth velocities in good agreement with experimental data. They indicate that fast crystal growth stems from the large diffusion constants and sticking coefficients and the presence of a sharp crystalline-liquid interface. It is also discussed how metadynamics, an enhanced-sampling method capable of accelerating the occurrence of rare events, can be effectively used in combination with ab initio molecular dynamics to create crystalline nuclei within amorphous models of Ge2Sb2Te5.
In the last part of the talk, an alloying strategy to tune the crystallization kinetics of PCMs is introduced. It is shown that ab initio simulations enable efficient screening of transition-metal alloys to select those that best speed up the high-temperature nucleation rate of Sb2Te3, a parent compound of Ge2Sb2Te5. These simulations predict that Scandium-alloyed Sb2Te3 displays the fastest crystallization kinetics. Subsequent experiments on phase-change memory cells containing Sc0.2Sb2Te3 confirm these predictions by showing a record-breaking writing speed of 700 picoseconds without preprogramming.
Riccardo Mazzarello，德国亚琛工业大学W2教授、国际知名理论计算物理学家和相变存储材料专家、著名量子化学计算软件Quantum ESPRESSO的开发者之一。1994-2000年在意大利Genoa大学以最高荣誉获得学士和硕士学位，2004年在德国汉堡大学获得物理学博士学位。此后在意大利的Trieste先进科学国际学院和瑞士的苏黎世理工大学进行博士后研究。于2009年入职德国亚琛工业大学并在2016年获得终生教授职位。专注于第一性原理的方法研究和相变存储材料的研究，在相变材料的结构特征、电子结构、动力学特性等方面取得重要进展。至今已在Science, Nature Mater, Nat Comm, Adv Mater, PRL等国际知名学术期刊上发表论文60多篇，引用一万多次。 Dr. Riccardo Mazzarello is a W2 professor in RWTH Aachen University, prestigious young theoretical physicist and expert in phase-change materials, the developer of Quantum ESPRESSO. Dr. Mazzarello obtained his bachelor and master degrees with the highest honor in University of Genoa, Italy during 1994-2000 and got the doctoral degree in physics in University of Hamburg, Germany in 2004. From 2004 to 2009, he worked as postdoctoral researcher in the International School for Advanced Studies (SISSA), Trieste, Italy and ETH Zurich. He joined RWTH Aachen University in 2009 and became a tenured professor in 2016. Dr. Mazzarello has published over 60 high-profile papers, including Science, Nature Mater, Nat Comm, Adv Mater, PRL etc and has been cited over 10,000 times.