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Nanocrystals with rationally tailored morphologies and properties are a thriving research field. The phase-change material (PCM) germanium telluride (GeTe) has recently been synthesized in a plethora of nanocrystalline forms and morphologies, but a mechanistic understanding of these experimental findings has been largely missing so far. Here, we present comprehensive dispersion-corrected density functional theory (DFT-D) simulations of the low-index crystal facets of GeTe and how they interact with relevant ligand molecules. These results enable a systematic understanding of experimental findings regarding GeTe nanocrystals and clarify the origin of the broad range of particle shapes observed. This study also affords semiquantitative signposts assisting future synthetic studies of nanocrystalline PCMs. [ABSTRACT FROM AUTHOR]

Phase-change materials (PCMs) are widely used for data storage and in other functional devices. Despite their seemingly simple compositions, these materials exhibit intriguing microscopic complexity and a portfolio of interesting properties. In this Feature Article, it is shown that structural and electronic peculiarities on the atomic scale are key determinants for the technological success of PCMs. Particular emphasis is put on the interplay of different experimental and theoretical methods, on the bonding nature of crystalline and amorphous PCMs, and on the role of surfaces and nanostructures. Then, unconventional transport properties of the crystalline phases are highlighted, both with regard to electrical and heat conduction. Finally, perspectives and future directions are drawn: for finding new PCMs based on microscopic understanding, and also for new applications of these materials in emerging fields. [ABSTRACT FROM AUTHOR]

Despite its simple chemical constitution and unparalleled technological importance, the phase-change material germanium telluride (GeTe) still poses fundamental questions. In particular, the bonding mechanisms in amorphous GeTe have remained elusive to date, owing to the lack of suitable bond-analysis tools. Herein, we introduce a bonding indicator for amorphous structures, dubbed 'bond-weighted distribution function' (BWDF), and we apply this method to amorphous GeTe. The results underline a peculiar role of homopolar GeGe bonds, which locally stabilize tetrahedral fragments but not the global network. This atom-resolved (i.e., chemical) perspective has implications for the stability of amorphous 'zero bits' and thus for the technologically relevant resistance-drift phenomenon. [ABSTRACT FROM AUTHOR]

Germaniumtelluride (GeTe) is an iconic functional material, bothin itself and as the parent compound for a range of ternary phase-changedata-storage alloys. Long taken to be a “simple” ABcompound, crystalline GeTe is today known to contain a large numberof germanium vacancies which directly affect the material’smacroscopic properties. Here, we use atomistic simulations to elucidatelocal mechanisms behind the motion of Ge atoms (and thus, vacancydiffusion) in crystalline GeTe. Transition pathways are computed usingthe nudged elastic band (NEB) approach at the gradient-corrected levelof density-functional theory (GGA-DFT), both for the idealized rhombohedral(R3m) crystal and a number of defectiveconfigurations. Besides obvious structural arguments (i.e., beyonda simple rigid-sphere model), the diffusion barriers show a delicatedependence on the material’s electronic structure. The latteris controlled by vacancy formation, Sb adatoms, and charge injection,all of which is discussed in a unified framework. [ABSTRACT FROM AUTHOR]

Studies of supercooled liquid phase‐change materials are important for the development of phase‐change memory and neuromorphic computing devices. Herein, a machine‐learning (ML)‐based interatomic potential for Ge2Sb2Te5 (GST) to conduct large‐scale molecular dynamics simulations of liquid and supercooled liquid GST is used. A pronounced effect of the thermostat parameters on the simulation results is demonstrated, and it is shown how using a Langevin thermostat with optimized damping values can lead to excellent agreement with reference ab initio molecular dynamics (AIMD) simulations. Structural and dynamical analyses are presented, including the studies of radial and angular distributions, homopolar bonds, and the temperature‐dependent diffusivity. Herein, the usefulness of ML‐driven molecular dynamics for further studies of supercooled liquid GST, with length and timescales far exceeding those that are accessible to AIMD is demonstrated. [ABSTRACT FROM AUTHOR]

The layered crystal structure of Cr2Ge2Te6 shows ferromagnetic ordering at the two‐dimensional limit, which holds promise for spintronic applications. However, external voltage pulses can trigger amorphization of the material in nanoscale electronic devices, and it is unclear whether the loss of structural ordering leads to a change in magnetic properties. Here, it is demonstrated that Cr2Ge2Te6 preserves the spin‐polarized nature in the amorphous phase, but undergoes a magnetic transition to a spin glass state below 20 K. Quantum‐mechanical computations reveal the microscopic origin of this transition in spin configuration: it is due to strong distortions of the CrTeCr bonds, connecting chromium‐centered octahedra, and to the overall increase in disorder upon amorphization. The tunable magnetic properties of Cr2Ge2Te6 can be exploited for multifunctional, magnetic phase‐change devices that switch between crystalline and amorphous states. [ABSTRACT FROM AUTHOR]

Phase‐change materials (PCMs) are an intriguing class of materials that enable non‐volatile data storage and other technological applications. To gain a thorough understanding of PCMs, a variety of advanced experimental techniques is routinely used, but theory and computer simulation have long been of central importance as well. In their Review @ RRL (article no. 1800579), Konze et al. describe the role that orbital‐based chemical‐bonding analyses have played in PCM research: they can be used to identify stabilizing ("bonding") and de‐stabilizing ("antibonding") interactions between atoms using local‐orbital basis sets or (nowadays) a projection from plane‐wave DFT output. Besides a concise overview of the methodology and a survey of applications to PCMs, Konze et al. also highlight recent applications in other materials classes, aiming to make new links to the PCM field—and so to enable new microscopic insight into the crystalline and amorphous structures of these important materials. [ABSTRACT FROM AUTHOR]

The atomic‐scale structures of chalcogenide phase‐change materials (PCMs) are directly relevant for macroscopic properties and practical applications. In PCMs and throughout materials science, quantum‐mechanically based atomistic simulations and chemical‐bonding analyses are increasingly helping to understand structures and properties of solids. Here, new insights into PCMs are highlighted that have recently been obtained from orbital‐based bonding indicators—in particular, from crystal orbital Hamilton population (COHP) analysis. Applications of these methods in other areas of solid‐state and materials chemistry are also discussed, from classical to emerging topics, which may have useful lessons for PCM research in store. It is hoped that this overview will inspire research in the field and enable new chemical insight into structures and properties of PCMs. The role of chemical‐bonding analyses for phase‐change materials research is highlighted in this Review. The focus is on energy‐resolved, orbital‐based bonding indicators: in particular, the crystal orbital Hamilton population (COHP) technique. This work surveys recent applications in the field and in neighboring areas, aiming to enable new chemical insight into the crystalline and amorphous structures of these important materials. [ABSTRACT FROM AUTHOR]

Tellurium-based phase-change materials (PCMs) enable applications from optical and electronic data storage to thermoelectrics and plasmonics, which all demand precise control of electronic properties. These materials contain an unusually large number of vacancies: "stoichiometric" ones that stem from the chemical composition and "excess" vacancies that act like classical dopants. Here we show how both types of vacancies can be controlled independently in the solid solution Sn(Sb1-xBix)2Te4. We vary x in small steps over the entire compositional range and show that this has a profound effect on the material's electronic nature: remarkably, we observe a change from p- to n-type conduction at x ≈ 0.7, solely controlled by composition. Our findings lead to a new compositionally (that is, chemically) tunable materials platform that enables precise control of electrical properties. [ABSTRACT FROM AUTHOR]

Chalcogenide phase-change materials (PCMs) are regarded as the leading candidate for storage-class non-volatile memory and neuro-inspired computing. Recently, using the TiTe 2 /Sb 2 Te 3 material combination, a new framework – phase-change heterostructure (PCH), has been developed and proved to effectively suppress the noise and drift in electrical resistance upon memory programming, largely reducing the inter-device variability. However, the atomic-scale structural and chemical nature of PCH remains to be fully understood. In this work, we carry out thorough ab initio simulations to assess the bonding characteristics of the PCH. We show that the TiTe 2 crystalline nanolayers do not chemically interact with the surrounding Sb 2 Te 3 , and are stabilized by strong covalent and electrostatic Ti–Te interactions, which create a prohibitively high barrier for atomic migration along the pulsing direction. We also find significant contrast in computed dielectric functions in the PCH, suggesting possible optical applications of this class of devices. With the more confined space and therefore constrained phase transition compared to traditional PCM devices, the recently introduced class of PCH-based devices may lead to improvements in phase-change photonic and optoelectronic applications with much lower stochasticity during programming. [ABSTRACT FROM AUTHOR]