We report the synthesis of nanocrystals with an optical feature in the THz range. To do so, we develop a new synthetic procedure for the growth of HgTe, HgSe, and HgS nanocrystals, with strong size tunability from 5 to 200 nm. This is used to tune the absorption of the nanocrystals all over the infrared range up to terahertz (from 2 to 65 μm for absorption peak and even 200 μm for cutoff wavelength). The interest for this procedure is not limited to large sizes since for small objects we demonstrate low aggregation and good shape control (i.e., spherical object) while using nonexpansive and simple mercury halogenide precursors. By integrating these nanocrystals into an electrolyte-gated transistor, we evidence a change of carrier density from p-doped to n-doped as the confinement is vanishing.

Self-doped colloidal quantum dots (CQDs) attract a strong interest for the design of a new generation of low-cost infrared (IR) optoelectronic devices because of their tunable intraband absorption feature in the mid-IR region. However, very little remains known about their electronic structure which combines confinement and an inverted band structure, complicating the design of optimized devices. We use a combination of IR spectroscopy and photoemission to determine the absolute energy levels of HgSe CQDs with various sizes and surface chemistries. We demonstrate that the filling of the CQD states ranges from 2 electrons per CQD at small sizes (<5 nm) to more than 18 electrons per CQD at large sizes (≈20 nm). HgSe CQDs are also an interesting platform to observe vanishing confinement in colloidal nanoparticles. We present lines of evidence for a semiconductor-to-metal transition at the CQD level, through temperature-dependent absorption and transport measurements. In contrast with bulk systems, the transition is the result of the vanishing confinement rather than the increase of the doping level.

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−xSex, Sb2Te3(1−x)Se3x, and Bi2−2xSb2xSe3, 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.

Chalcogenide phase-change materials (PCMs) exhibit optical phonons at terahertz (THz) frequencies, which can be used for studying basic properties of the phase transition and which lead to a strong dielectric contrast that could be exploited for THz photonics applications. Here, we demonstrate that the phonons of PCMs can be studied by frequency-tunable THz scattering-type scanning near-field optical microscopy (s-SNOM). Specifically, we perform spectroscopic THz nanoimaging of a PCM sample comprising amorphous and crystalline phases. We observe phonon signatures, yielding strong s-SNOM signals and, most important, clear spectral differences between the amorphous and crystalline PCM, which allows for distinguishing the PCM phases with high confidence on the nanoscale. We also found that the spectral signature can be enhanced, regarding both signal strength and spectral contrast, by increasing the radius of the probing tip. From a general perspective, our results establish THz s-SNOM for nanoscale structural and chemical mapping based on local phonon spectroscopy.

Optical conductivity measurements on a BaCoS 2 single crystal unveil an unusual linear behavior over a broad spectral range. In the paramagnetic phase above 300 K, the spectrum shows no gap, which contradicts the previously proposed scenario of a charge-transfer Mott insulator. Ab initio dynamical mean field theory calculations including a retarded Hubbard interaction explain the data in terms of an incipient opening of a Co(3d)-S(3p) charge-transfer gap concomitant to incoherent charge transport driven by electronic correlations. These results point to a non-Fermi liquid scenario with Hund's metal properties in the paramagnetic state, which arises from an incipient Mott phase destabilized by low-energy charge fluctuations across the vanishing 3d-3p charge-transfer gap.

We performed optical studies on CaFeAsF single crystals, a parent compound of the 1111-type iron-based superconductors that undergoes a structural phase transition from tetragonal to orthorhombic at T s = 121 K and a magnetic one to a spin density wave (SDW) state at T N = 110 K. In the low-temperature optical conductivity spectrum, after the subtraction of a narrow Drude peak, we observe a pronounced singularity around 300 cm (2017)]. In support of this interpretation, we show that for the latter system this singular feature disappears rapidly upon electron and hole doping, as expected if it arises from a van Hove singularity in between two Dirac cones. Finally, we show that one of the infrared-active phonon modes (the Fe-As mode at 250 cm −1 ) develops a strongly asymmetric line shape in the SDW state and note that this behavior can be explained in terms of a strong coupling with the Dirac fermions.

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