While solid-state materials are commonly classified as covalent, ionic, or metallic, there are cases that defy these iconic bonding mechanisms. Phase-change materials (PCMs) for data storage are a prominent example: they have been claimed to show "resonant bonding," but a clear definition of this mechanism has been lacking. Here, it is shown that these solids are fundamentally different from resonant bonding in the π-orbital systems of benzene and graphene, based on first-principles data for vibrational, optical, and polarizability properties. It is shown that PCMs and related materials exhibit a unique mechanism between covalent and metallic bonding. It is suggested that these materials be called "incipient metals," and their bonding nature "metavalent". Data for a diverse set of 58 materials show that metavalent bonding is not just a superposition of covalent and metallic cases, but instead gives rise to a unique and anomalous set of physical properties. This allows the derivation of a characteristic fingerprint of metavalent bonding, composed of five individual components and firmly rooted in physical properties. These findings are expected to accelerate the discovery and design of functional materials with attractive properties and applications, including nonvolatile memories, thermoelectrics, photonics, and quantum materials.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/adma.201803777

Incipient Metals: Functional Materials with a Unique Bonding Mechanism.

A 2D map is created for solid-state materials based on a quantum-mechanical description of electron sharing and electron transfer. This map intuitively identifies the fundamental nature of ionic, metallic, and covalent bonding in a range of elements and binary compounds; furthermore, it highlights a distinct region for a mechanism recently termed "metavalent" bonding. Then, it is shown how this materials map can be extended in the third dimension by including physical properties of application interest. Finally, it is shown how the map coordinates yield new insight into the nature of the Peierls distortion in phase-change materials and thermoelectrics. These findings and conceptual approaches provide a novel avenue to tailor material properties.

https://onlinelibrary.wiley.com/doi/epdf/10.1002/adma.201806280

A Quantum‐Mechanical Map for Bonding and Properties in Solids

Driven by the ability to harvest waste heat into reusable electricity and the exclusive role of serving as the power generator for deep spacecraft, intensive endeavors are dedicated to enhancing the thermoelectric performance of ecofriendly materials. Herein, the most recent progress in superhigh-performance GeTe-based thermoelectric materials is reviewed with a focus on the crystal structures, phase transitions, resonant bondings, multiple valance bands, and phonon dispersions. These features diversify the degrees of freedom to tune the transport properties of electrons and phonons for GeTe. On the basis of the optimized carrier concentration, strategies of alignment of multiple valence bands and density-of-state resonant distortion are employed to further enhance the thermoelectric performance of GeTe-based materials. To decrease the thermal conductivity, methods of strengthening intrinsic phonon-phonon interactions and introducing various lattice imperfections as scattering centers are highlighted. An overview of thermoelectric devices assembled from GeTe-based thermoelectric materials is then presented. In conclusion, possible future directions for developing GeTe in thermoelectric applications are proposed. The achieved high thermoelectric performance in GeTe-based thermoelectric materials with rationally established strategies can act as a reference for broader materials to tailor their thermoelectric performance.

Thermoelectric GeTe with Diverse Degrees of Freedom Having Secured Superhigh Performance.

The silicon-photomultiplier (SiPM) is becoming the device of choice for different applications, for example in fast timing like in time of flight positron emission tomography (TOF-PET) and in high energy physics (HEP). It is also becoming a choice in many single-photon or few-photon based applications, like for spectroscopy, quantum experiments and distance measurements (LIDAR). In order to fully benefit from the good performance of the SiPM, in particular its sensitivity, the dynamic range and its intrinsically fast timing properties it is necessary to understand, quantitatively describe and simulate the various parameters concerned. These analyses consider the structure and the electrical model of a single photon avalanche diode (SPAD), i.e. the SiPM microcell, and the integration in an array, i.e. the SiPM. Additionally, for several applications a more phenomenological and complete view on SiPMs has to be done, e.g. photon detection efficiency, single photon time resolution, SiPM signal response, gain fluctuation, dark count rate, afterpulse, prompt and delayed optical crosstalk. These quantities of SiPMs can strongly influence the time and energy resolution, for example in PET and HEP. Having a complete overview on all of these parameters allows to draw conclusions on how best performances can be achieved for the various needs of different applications.

http://cds.cern.ch/record/2692477/files/10.1016_j.nima.2018.11.118.pdf

Understanding and simulating SiPMs

https://onlinelibrary.wiley.com/doi/pdf/10.1002/adfm.201904862

Chalcogenide Thermoelectrics Empowered by an Unconventional Bonding Mechanism

Thermoelectric materials provide a challenge for materials design, since they require optimization of apparently conflicting properties. The resulting complexity has favored trial-and-error approaches over the development of simple and predictive design rules. In this work, the thermoelectric performance of IV-VI chalcogenides on the tie line between GeSe and GeTe is investigated. From a combination of optical reflectivity and electrical transport measurements, it is experimentally proved that the outstanding performance of IV-VI compounds with octahedral-like coordination is due to the anisotropy of the effective mass tensor of the relevant charge carriers. Such an anisotropy enables the simultaneous realization of high Seebeck coefficients, due to a large density-of-states effective mass, and high electrical conductivity, caused by a small conductivity effective mass. This behavior is associated to a unique bonding mechanism by means of a tight-binding model, which relates band structure and bond energies; tuning the latter enables tailoring of the effective mass tensor. The model thus provides atomistic design rules for thermoelectric chalcogenides.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/adma.201801787

Thermoelectric Performance of IV–VI Compounds with Octahedral-Like Coordination: A Chemical-Bonding Perspective

Silicon Photomultipliers (SiPMs) are versatile and sensitive photon detectors that experience a fast growing variety of use in particle physics and related fields of application. These photo detectors have a very promising photon detection efficiency and are therefore interesting for very low light flux applications such as scintillation and fluorescence light detection. As a semiconductor device the SiPM׳s gain and time response strongly depend on the operating temperature and voltage. Thus they have to be understood for a proper use of the SiPM. Therefore, accurate electrical simulations of the SiPM׳s behavior involving electrical readout and front-end electronics help to improve the design of experimental setups, since several different designs can be tested and simulated with a manageable amount of effort. To perform these simulations, a detailed equivalent circuit of the SiPM has to be used containing a set of well-defined parameters. For this purpose, SPICE simulations of SiPMs and readout electronics have been performed. These simulations utilize an improved SiPM model consisting of resistors, capacitances and inductances. The SiPM parameters for these simulations have been determined by measuring the impedance over a wide frequency range while applying a DC voltage in forward direction and various DC voltages from zero up to the SiPM breakdown voltage in order to determine the behavior under operating conditions. The impedance measurements, the electrical model and the resulting simulations are presented. The impact of different setups and the electrical properties of the SiPM is discussed.

Daniel Führen

Papers

Thermoelectric Performance of IV–VI Compounds with Octahedral-Like Coordination: A Chemical-Bonding Perspective

Electrical characterization and simulation of SiPMs