Unique Bond Breaking in Crystalline Phase Change Materials and the Quest for Metavalent Bonding

Abstract

Laser-assisted field evaporation is studied in a large number of compounds, including amorphous and crystalline phase change materials employing atom probe tomography. This study reveals significant differences in field evaporation between amorphous and crystalline phase change materials. High probabilities for multiple events with more than a single ion detected per laser pulse are only found for crystalline phase change materials. The specifics of this unusual field evaporation are unlike any other mechanism shown previously to lead to high probabilities of multiple events. On the contrary, amorphous phase change materials as well as other covalently bonded compounds and metals possess much lower probabilities for multiple events. Hence, laser-assisted field evaporation in amorphous and crystalline phase change materials reveals striking differences in bond rupture. This is indicative for pronounced differences in bonding. These findings imply that the bonding mechanism in crystalline phase change materials differs substantially from conventional bonding mechanisms such as metallic, ionic, and covalent bonding. Instead, the data reported here confirm a recently developed conjecture, namely that metavalent bonding is a novel bonding mechanism besides those mentioned previously.

Chalcogenide Thermoelectrics Empowered by an Unconventional Bonding Mechanism

“…Figure a shows that with increasing the content of GeSe, the ternary alloy undergoes a phase transition from rhombohedral to hexagonal at x > 0.65, while the pure GeSe phase is orthorhombic. The phase transition from rhombohedral to hexagonal is accompanied also by a change of chemical bonding from metavalent to covalent as confirmed by previous studies . The power factor of GeSe is negligible due to its ultralow carrier concentration.…”

“…However, in some cases, one single pulse induces the evaporation of multiple ions, which is termed as multiple events. The ratio between multiple events and total events is called the “probability of multiple events.” By analyzing more than 50 different compounds, Zhu et al found that the “probability of multiple events” for metavalently bonded materials is larger than 50%, while the value for all covalent and metallic materials studied is lower than 40% with the same measurement parameters, Figure . The chalcogenides which show superior performance as thermoelectric materials, do not only show an exceptional property portfolio, they are also characterized by an unconventional bond breaking mechanism.…”

“…Indeed, a study of bond breaking using atom probe tomography supports the idea of an exceptional bonding mechanism. To this end, laser‐assisted atom probe tomography has been utilized to study bond rupture . In an ideal case, the atoms on the specimen's surface will be ionized and evaporated successively triggered by the laser pulse.…”

“…There is thus apparently a correlation between the thermoelectric performance and the bond breaking as measured by the atomprobe, as well as the material properties (see Table S1 (Supporting Information) and ref. for details).…”

Advanced Optical Programming of Individual Meta‐Atoms Beyond the Effective Medium Approach

“…Tuning MSs on a meta‐atom or sub‐meta‐atom scale becomes possible when utilizing phase‐change materials (PCMs). These materials are commonly used for nonvolatile optical and electrical data storage because of the huge property contrast between their amorphous (A) and crystalline (C) phases resulting from a unique bonding mechanism . This makes them ideally suited for photonic applications ranging from integrated optical memories over color displays to active MSs .…”