Hydrothermal synthesis is described of layered lithium iron selenide hydroxides Li(1-x)Fe(x)(OH)Fe(1-y)Se (x ∼ 0.2; 0.02 < y < 0.15) with a wide range of iron site vacancy concentrations in the iron selenide layers. This iron vacancy concentration is revealed as the only significant compositional variable and as the key parameter controlling the crystal structure and the electronic properties. Single crystal X-ray diffraction, neutron powder diffraction, and X-ray absorption spectroscopy measurements are used to demonstrate that superconductivity at temperatures as high as 40 K is observed in the hydrothermally synthesized samples when the iron vacancy concentration is low (y < 0.05) and when the iron oxidation state is reduced slightly below +2, while samples with a higher vacancy concentration and a correspondingly higher iron oxidation state are not superconducting. The importance of combining a low iron oxidation state with a low vacancy concentration in the iron selenide layers is emphasized by the demonstration that reductive postsynthetic lithiation of the samples turns on superconductivity with critical temperatures exceeding 40 K by displacing iron atoms from the Li(1-x)Fe(x)(OH) reservoir layer to fill vacancies in the selenide layer.

Attention on semiconductor nanocrystals have been largely focused because of their unique optical and electrical properties, which can be applied as light absorber and luminophore. However, the band gap and structure engineering of nanomaterials is not so easy because of their finite size. Here we demonstrate an approach for preparing ternary AgInS2 (AIS), quaternary AgZnInS (AZIS), AgInS2/ZnS and AgZnInS/ZnS nanocompounds based on cation exchange. First, pristine Ag2S quantum dots (QDs) with different sizes were synthesized in one-pot, followed by the partial cation exchange between In(3+) and Ag(+). Changing the initial ratio of In(3+) to Ag(+), reaction time and temperature can control the components of the obtained AIS QDs. Under the optimized conditions, AIS QDs were obtained for the first time with a cation disordered cubic phase and high photoluminescence (PL) quantum yield (QY) up to 32% in aqueous solution, demonstrating the great potential of cation exchange in the synthesis for nanocrystals with excellent optical properties. Sequentially, Zn(2+) ions were incorporated in situ through a second exchange of Zn(2+) to Ag(+)/In(3+), leading to distinct results under different reaction temperature. Addition of Zn(2+) precursor at room temperature produced AIS/ZnS core/shell NCs with successively enhancement of QY, while subsequent heating could obtain AZIS homogeneous alloy QDs with a successively blue-shift of PL emission. This allow us to tune the PL emission of the products from 483 to 675 nm and fabricate the chemically stable QDs core/ZnS shell structure. Based on the above results, a mechanism about the cation exchange for the ternary nanocrystals of different structures was proposed that the balance between cation exchange and diffusion is the key factor of controlling the band gap and structure of the final products. Furthermore, photostability and in vitro experiment demonstrated quite low cytotoxicity and remarkably promising applications in the field of clinical diagnosis.

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Planar heterojunction perovskite solar cells (PSCs) provide great potential for fabricating high‐efficiency, low‐cost, large‐area, and flexible photovoltaic devices. In planar PSCs, a perovskite absorber is sandwiched between hole and electron transport materials. The charge‐transporting interlayers play an important role in enhancing charge extraction, transport, and collection. Organic interlayers including small molecules and polymers offer great advantages for their tunable chemical/electronic structures and low‐temperature solution processibility. Here, recent progress of organic interlayers in planar heterojunction PSCs is discussed, and the effect of chemical structures on device performance is also illuminated. Finally, the main challenges in developing planar heterojunction PSCs based on organic interlayers are identified, and strategies for enhancing the device performance are also proposed.

Here we report the first in-situ high pressure (up to ~ 50 GPa) Hall effect measurements on single crystal black phosphorus. We find a strong correlation between the sign of the Hall coefficient, an indicator of the dominant carrier type, and the superconducting transition temperature (T C). Importantly, we find a change from electron-dominant to hole-dominant carriers in the simple cubic phase of phosphorus at a pressure of ~17.2 GPa, providing an explanation for the puzzling valley it displays in its superconducting T C vs. pressure phase diagram. Our results reveal that hole-carriers play an important role in developing superconductivity in elemental phosphorus, and the valley in T C at 18.8 GPa is associated with a Lifshitz transition.

Here we report the observation of pressure-induced melting of antiferromagnetic (AFM) order and emergence of a new quantum state in the honeycomb-lattice halide -RuCl 3 , a candidate compound in the proximity of quantum spin liquid state. Our high-pressure heat capacity measurements demonstrate that the AFM order smoothly melts away at a critical pressure (P C ) of 0.7 GPa. Intriguingly, the AFM transition temperature displays an increase upon applying pressure below the P C , in stark contrast to usual phase diagrams, for example in pressurized parent compounds of unconventional superconductors. Furthermore, in the high-pressure phase an unusual steady of magnetoresistence is observed. These observations suggest that the high-pressure phase is in an exotic gapped quantum state which is robust against pressure up to ~140 GPa.

Isotropic negative area compressibility, which is very rare, is observed in KBBF and the related mechanism is investigated by combined high-pressure X-ray diffraction (XRD) experiments and first-principles calculations. The strong mechanical anisotropy leads to a large Poisson's ratio and high figure of merit for the acoustic-optics effect, giving KBBF potential applications as smart strain converters and deep-ultraviolet (DUV) acoustic-optic devices.

Negative linear compressibility (NLC) is a rare and counterintuitive phenomenon because materials with this property would expand along one specific direction when uniformly compressed. NLC materials have a broad range of potential applications in designing pressure sensors, artificial muscles, and so on. Designing and searching for systems with NLC is desired and crucial for material and compression science. Herein, with the help of high-pressure X-ray diffraction measurements and density functional theory calculations, we find that the 2D layered Co(SCN)(pyrazine) exhibits NLC with a new mechanism: layer sliding. When compressed, the ab planes slide along the a axis, leading to the decrease of lattice parameter β, which results in the NLC effect along principal axis X (≈ -0.84a - 0.55c). The layer sliding mechanism opens exciting opportunities for seeking, designing, and synthesizing new classes of materials with anomalous mechanical properties in monoclinic layered or other related systems.