Chemical bonding present in crystalline solids has a significant impact on how heat moves through a lattice, and with the right chemical tuning, one can achieve extremely low thermal conductivity. The desire for intrinsically low lattice thermal conductivity (κ lat ) has gained widespread attention in thermoelectrics, in refractories, and nowadays in photovoltaics and optoelectronics. Here we have synthesized a high-quality crystalline ingot of cubic metal halide CuBiI 4 and explored its chemical bonding and thermal transport properties. It exhibits an intrinsically ultralow κ lat of ∼0.34−0.28 W m −1 K −1 in the temperature range 4−423 K with an Umklapp crystalline peak of 1.82 W m −1 K −1 at 20 K, which is surprisingly lower than other copper-based halide or chalcogenide materials. The crystal orbital Hamilton population analysis shows that antibonding states generated just below the Fermi level (E f ), which arise from robust copper 3d and iodine 5p interactions, cause copper−iodide bond weakening, which leads to reduction of the elastic moduli and softens the lattice, finally to produce extremely low κ lat in CuBiI 4 . The chemical bonding hierarchy with mixed covalent and ionic interactions present in the complex crystal structure generates significant lattice anharmonicity and a low participation ratio in low-lying optical phonon modes originating mostly from localized copper−iodide bond vibrations. We have obtained experimental evidence of these low-lying modes by low-temperature specific heat capacity measurement as well as Raman spectroscopy. The presence of strong p−d antibonding interactions between copper and iodine leads to anharmonic soft crystal lattice which gives rise to low-energy localized optical phonon bands, suppressing the heat-carrying acoustic phonons to steer intrinsically ultralow κ lat in CuBiI 4 .

Cu–Sn-based sulfides are earth-abundant and nontoxic compounds of special interest for low-cost energy harvesting applications. In the present work, we have investigated the effect of grain size on the thermoelectric properties of Cu2SnS3 (CTS). Three dense CTS samples with nanometric grains were produced by mechanical alloying combined with spark plasma sintering, preserving the small size of crystalline domains to 12, 25, and 37 nm, respectively. The experimental results show that the Seebeck coefficient (S) and electrical resistivity (ρ) decrease with decreasing domain sizes, while the thermal conductivity (κ) increases. A smaller domain size correlates with a lower resistivity and a degenerate semiconductor-like behavior due to higher carrier concentration. At the same time, our synthesis method leads to materials with very low lattice thermal conductivity, thanks to the nanometric size of grains and structural disorder. As a result, the sample with the smallest grain size exhibits the highest zT of ∼0.4 at 650 K. First-principles density functional theory (DFT) simulations on various CTS crystallite surfaces revealed localized states near the Fermi level and the absence of band gap, indicating the metallic nature of the surfaces. Various CTS systems were tested by DFT, showing the following order of increasing formation energy: stoichiometric CTS, Cu vacancy, Cu-rich, Sn vacancy, and Sn-rich.

Two-dimensional (2D) layered Ruddlesden–Popper (RP) phases of halide perovskite offer exotic properties and interesting structure, which make them suitable candidates for solar photovoltaics, light emitting diodes (LEDs), and photodetector applications. Simple and scaled-up synthesis, chemical transformations, doping, and stability are the important steps toward the applications. Herein, all-inorganic RP phase of Cs2PbI2Cl2 was synthesized via a facile hot-injection method using benzoyl halides as halide sources. Different morphologies in the form of 2D nanoplates (NPLs) and small nanocrystals (NCs) were obtained by changing the concentration of capping agents (i.e., oleic acid and oleylamine) in solution. The excitonic absorption peak appeared for NPLs and NCs, which is the characteristic feature of 2D halide perovskites. Further, the scalable quantity (∼1 g) of bulk powder and micrometer-sized particles of Cs2PbI2Cl2 were synthesized via liquid assisted mechanochemical grinding and antisolvent method, respectively. We have performed post-synthetic chemical transformation to synthesize three-dimensional (3D) CsPbBr3 disk-shaped particles and zero-dimensional (0D) Cs4PbCl6 NCs from the presynthesized RP Cs2PbI2Cl2 NCs in solution and studied their optical properties. Finally, doping of Mn2+ was carried out in Cs2PbI2Cl2 NCs, which demonstrated a typical feature of Mn2+ dopant emission along with host emission properties. Low-temperature (77 K) photoluminescence (PL) spectra reveal red-shifted and line-width broadening emission along with longer PL lifetime for both undoped and Mn-doped NCs compared to room temperature PL. Further, the temperature-dependent PL spectra and thermogravimetric analysis (TGA) revealed excellent thermal stability of Cs2PbI2Cl2. This work offers an insight for exploration of the synthesis process, post-synthetic chemical transformation, and dopant insertion in all-inorganic 2D RP perovskites, which is an important step forward for application. Demonstrations of various simple syntheses of both the nanophase and bulk phase, structural transformation, and detailed optical properties of doped and undoped RP perovskite halide nanostructures unfold innovative opportunities for applicability in optoelectronics such as in solar cell and photodetectors.

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Ordered self-assembly of small organic molecules may induce novel properties in a supramolecular arrangement and can act as advance functional materials. This paper discusses the development of a new stimuli-responsive dipeptide hydrogelator containing l -phenylalanine and α-aminoisobutyric acid (Aib). The dipeptide Boc-Phe-Aib-OH, on addition with three equivalent of sodium hydroxide and water, transformed into a robust hydrogel. The transparent hydrogel is self-healing in nature. The instant gelation is highly selective toward sodium hydroxide and does not need any sonication or heating–cooling cycle. The thixotropic nature of the gel has been confirmed by rheological step-strain experiments at room temperature. Moreover, in the oil–water mixture, the compound exhibits phase-selective gelation. When the gel cylinder is cut into pieces, it does not conduct electricity, but once self-healing occurs, it conducts electricity. The diffusion of rhodamine 6G through the hydrogel indicates the dynamic nature. The hydrogel is highly sensitive toward HCl, that is, in the presence of HCl vapor, the gel becomes deformed.

Two‐dimensional (2D) lead‐free halide perovskites have generated enormous perception in the field of optoelectronics due to their fascinating optical properties. However, an in‐depth understanding on their shape‐controlled charge‐carrier recombination dynamics is still lacking, which could be resolved by exploring the photoluminescence (PL) blinking behaviour at the single‐particle level. Herein, we demonstrate, for the first time, the synthesis of nanocrystals (NCs) and 2D nanosheets (NSs) of layered mixed halide, Cs3Bi2I6Cl3, by solution‐based method. We applied fluorescence microscopy and super‐resolution optical imaging at single‐particle level to investigate their morphology‐dependent PL properties. Narrow emission line widths and passivation of non‐radiative defects were evidenced for 2D layered nanostructures, whereas the activation of shallow trap states was recognized at 77 K. Interestingly, individual NCs were found to display temporal intermittency (blinking) in PL emission. On the other hand, NS showed temporal PL intensity fluctuations within localized domains of the crystal. In addition, super‐resolution optical image of the NS from localization‐based method showed spatial inhomogeneity of the PL intensity within perovskite crystal.

Heterostructures of inorganic halide perovskites with mixed-dimensional inorganic nanomaterials have shown great potential not only in the field of optoelectronic energy devices and photocatalysis but also for improving our fundamental understanding of the charge transfer across the heterostructure interface. Herein, we present for the first time the heterostructure integration of the CsPbBr3 nanocrystal with an N-doped carbon dot. We explore the photoluminescence (PL) and photoconductivity of the heterostructure of CsPbBr3 nanocrystals and N-doped carbon dots. PL quenching of CsPbBr3 nanocrystals with the addition of N-doped carbon dots was observed. The photoexcited electrons from the conduction band of CsPbBr3 are trapped in the N-acceptor state of N-doped carbon dots, and the charge transfer occurs via quasi type II-like electronic band alignment. The charge transfer in the halide perovskite-based heterostructure should motivate further research into the new heterostructure synthesis with perovskites and the fundamental understanding of the mechanism of charge/energy transfer across the heterostructure interface.

Donor-doped TiO 2 ceramics are promising high-temperature oxide thermoelectrics. Highly dense (1 − x)TiO 2 −xNb 2 O 5 (0.005 ≤ x ≤ 0.06) ceramics were prepared by a single-step, mixed-oxide route under reducing conditions. The microstructures contained polygonal-shaped grains with uniform grain size distributions. Subgrain structures were formed in samples with low Nb contents by the interlacing of rutile and higher-order Magneĺi phases, reflecting the high density of shear planes and oxygen vacancies. Samples prepared with a higher Nb content showed no subgrain structures but high densities of planar defects and lower concentrations of oxygen vacancies. Through optimizing the concentration of point defects and line defects, the carrier concentration and electrical conductivity were enhanced, yielding a much improved power factor of 5.3 × 10 −4 W m −1 K −2 at 823 K; lattice thermal conductivity was significantly reduced by enhanced phonon scattering. A low, temperature-stable thermal conductivity of 2.6 W m −1 K −1 was achieved, leading to a ZT value of 0.17 at 873 K for compositions with x = 0.06, the highest ZT value reported for single Nb-doped TiO 2 ceramics without the use of spark plasma sintering (SPS). We demonstrate the control of the thermoelectric properties of Nbdoped TiO 2 ceramics through the development of balanced defect structures, which could guide the development of future oxide thermoelectric materials.