ABSTRACT
Doping is usually the first step to tailor thermoelectrics. It enables precise control of the charge-carrier concentration and concomitant transport properties. Doping should also turn GeSe, which features an intrinsically a low carrier concentration, into a competitive thermoelectric. Yet, elemental doping fails to improve the carrier concentration. In contrast, alloying with Ag-V-VI2 compounds causes a remarkable enhancement of thermoelectric performance. This advance is closely related to a transition in the bonding mechanism, as evidenced by sudden changes in the optical dielectric constant ε∞ , the Born effective charge, the maximum of the optical absorption ε2 (ω), and the bond-breaking behavior. These property changes are indicative of the formation of metavalent bonding (MVB), leading to an octahedral-like atomic arrangement. MVB is accompanied by a thermoelectric-favorable band structure featuring anisotropic bands with small effective masses and a large degeneracy. A quantum-mechanical map, which distinguishes different types of chemical bonding, reveals that orthorhombic GeSe employs covalent bonding, while rhombohedral and cubic GeSe utilize MVB. The transition from covalent to MVB goes along with a pronounced improvement in thermoelectric performance. The failure or success of different dopants can be explained by this concept, which redefines doping rules and provides a "treasure map" to tailor p-bonded chalcogenides.
ABSTRACT
Low-cost, easily integrable photodetectors (PDs) for silicon (Si) photonics are still a bottleneck for photonic-integrated circuits (PICs), especially for wavelengths above 1.8 µm. Multilayered platinum diselenide (PtSe2) is a semi-metallic two-dimensional (2D) material that can be synthesized below 450 °C. We integrate PtSe2-based PDs directly by conformal growth on Si waveguides. The PDs operate at 1550 nm wavelength with a maximum responsivity of 11 mA/W and response times below 8.4 µs. Fourier-transform IR spectroscopy in the wavelength range from 1.25 to 28 µm indicates the suitability of PtSe2 for PDs far into the IR wavelength range. Our PtSe2 PDs integrated by direct growth outperform PtSe2 PDs manufactured by standard 2D layer transfer. The combination of IR responsivity, chemical stability, selective and conformal growth at low temperatures, and the potential for high carrier mobility makes PtSe2 an attractive 2D material for optoelectronics and PICs.
ABSTRACT
The chemical bond is one of the most powerful, yet much debated concepts in chemistry, explaining property trends in solids. Recently, a novel type of chemical bonding was identified in several higher chalcogenides, characterized by a unique property portfolio, unconventional bond breaking, and sharing of about one electron between adjacent atoms. This metavalent bond is a fundamental type of bonding in solids, besides covalent, ionic, and metallic bonding, raising the pertinent question as to whether there is a well-defined transition between metavalent and covalent bonds. Here, three different pseudo-binary lines, namely, GeTe1- x Sex , Sb2 Te3(1- x ) Se3 x , and Bi2-2 x Sb2 x Se3 , are studied, and a sudden change in several properties, including optical absorption ε2 (ω), optical dielectric constant ε∞ , Born effective charge Z*, electrical conductivity, as well as bond breaking behavior for a critical Se or Sb concentration, is evidenced. These findings provide a blueprint to experimentally explore the influence of metavalent bonding on attractive properties of phase-change materials and thermoelectrics. Particularly important is its impact on optical properties, which can be tailored by the amount of electrons shared between adjacent atoms. This correlation can be used to design optoelectronic materials and to explore systematic changes in chemical bonding with stoichiometry and atomic arrangement.
ABSTRACT
The high dielectric optical contrast between the amorphous and crystalline structural phases of non-volatile phase-change materials (PCMs) provides a promising route towards tuneable nanophotonic devices. Here, we employ the next-generation PCM In3SbTe2 (IST) whose optical properties change from dielectric to metallic upon crystallization in the whole infrared spectral range. This distinguishes IST as a switchable infrared plasmonic PCM and enables a programmable nanophotonics material platform. We show how resonant metallic nanostructures can be directly written, modified and erased on and below the meta-atom level in an IST thin film by a pulsed switching laser, facilitating direct laser writing lithography without need for cumbersome multi-step nanofabrication. With this technology, we demonstrate large resonance shifts of nanoantennas of more than 4 µm, a tuneable mid-infrared absorber with nearly 90% absorptance as well as screening and nanoscale "soldering" of metallic nanoantennas. Our concepts can empower improved designs of programmable nanophotonic devices for telecommunications, (bio)sensing and infrared optics, e.g. programmable infrared detectors, emitters and reconfigurable holograms.
