Compressibility Studies of Copper Selenides Obtained by Cation Exchange Reaction Revealing the New CsCl Phase.

In this study, we conducted a high-pressure investigation of Cu2-xSe nanostructures with pyramid- and plate-like morphologies, created through cation exchange from zinc-blende CdSe nanocrystals and wurtzite CdSe nanoplatelets respectively. Using a diamond anvil cell setup at the APS synchrotron, we observed the phase transitions in the Cu2-xSe nanostructures up to 40 GPa, identifying a novel CsCl-type lattice with Pm3̅m symmetry above 4 GPa. This CsCl-type structure, previously unreported in copper selenides, was partially retained after decompression. Our results indicate that the initial crystalline structure of CdSe does not affect the stability of Cu2-xSe nanostructures formed via cation exchange. Both morphologies of Cu2-xSe sintered under compression, potentially contributing to the stabilization of the high-pressure phase through interfacial defects. These findings are significant for discovering new phases with potential applications in future technologies.

Isolation of cubic Si3P4 in the form of nanocrystals.

This article describes an approach for synthesizing silicon phosphide nanoparticles with a defective zinc blende structure under mild conditions through thermal annealing of hydrogenated silicon nanoparticles with red phosphorus. The synthesized Si3P4 nanoparticles were analyzed using FTIR, XRD, electron diffraction, EDX, TEM, Raman spectroscopy, X-ray fluorescence spectrometry, and UV-vis spectrophotometry. For the isolated cubic Si3P4 phase, a cell parameter of a = 5.04 Å was determined, and the bandgap was estimated to be equal to 1.25 eV. Because of the nanoscale dimensions of the obtained Si3P4 nanoparticles, the product may exhibit several exceptional properties as a precursor for diffusion doping of wafers and as anode material for Li-ion batteries. A similar method with a hydrogenation step offers the possibility to obtain other compounds, such as silicon selenides, arsenides, and sulfides.

A Comparative Study of the Band-Edge Exciton Fine Structure in Zinc Blende and Wurtzite CdSe Nanocrystals.

In this paper, we studied the role of the crystal structure in spheroidal CdSe nanocrystals on the band-edge exciton fine structure. Ensembles of zinc blende and wurtzite CdSe nanocrystals are investigated experimentally by two optical techniques: fluorescence line narrowing (FLN) and time-resolved photoluminescence. We argue that the zero-phonon line evaluated by the FLN technique gives the ensemble-averaged energy splitting between the lowest bright and dark exciton states, while the activation energy from the temperature-dependent photoluminescence decay is smaller and corresponds to the energy of an acoustic phonon. The energy splittings between the bright and dark exciton states determined using the FLN technique are found to be the same for zinc blende and wurtzite CdSe nanocrystals. Within the effective mass approximation, we develop a theoretical model considering the following factors: (i) influence of the nanocrystal shape on the bright-dark exciton splitting and the oscillator strength of the bright exciton, and (ii) shape dispersion in the ensemble of the nanocrystals. We show that these two factors result in similar calculated zero-phonon lines in zinc blende and wurtzite CdSe nanocrystals. The account of the nanocrystals shape dispersion allows us to evaluate the linewidth of the zero-phonon line.

Positive longitudinal magnetoconductivity induced by chiral magnetic effect in mercury selenide.

A negative longitudinal magnetoresistance without any sign of saturation was found in a non-centrosymmetric Weyl semimetal (WSM) candidate mercury selenide in an electron concentration range of 5.5 × 1015-1.7 × 1017cm-3and a temperature range of 0.33-150 K. The magnitude of the effect varies with a sample from≈10% up to≈30% in a magnetic field of 12 T atT= 150 K. Moreover, the positive contribution to magnetoconductivity has a characteristic quadratic dependence on the magnetic field, increasing with a charged center concentration atT= 150 K. The most likely explanation for the discovered longitudinal magnetoconductivity feature lies in the chiral magnetic effect, which is inherent to WSMs. The role of the Dyakonov-Perel mechanism in inter-nodal spin relaxation is discussed in regard to HgSe.

Fully Active Bimetallic Phosphide Zn0.5Ge0.5P: A Novel High-Performance Anode for Na-Ion Batteries Coupled with Diglyme-Based Electrolyte.

Metal phosphides are promising candidates for sodium-ion battery (SIB) anode owing to their large capacities with suitable redox potential, while the reversibility and rate performances are limited due to some electrochemically inactive transition-metal components and sluggish reaction kinetics. Here, we report a fully active bimetallic phosphide Zn0.5Ge0.5P anode and its composite (Zn0.5Ge0.5P-C) with excellent performance attributed to the Zn, Ge, and P components exerting their respective Na-storage merit in a cation-disordered structure. During Na insertion, Zn0.5Ge0.5P undergoes an alloying-type reaction, along with the generation of NaP, Na3P, NaGe, and NaZn13 phases, and the uniform distribution of these phases ensures the electrochemical reversibility during desodiation. Based on this reaction mechanism, excellent electrochemical properties such as a high reversible capacity of 595 mAh g-1 and an ultrafast charge-discharge capability of 377.8 mAh g-1 at 50C for 500 stable cycles were achieved within the Zn0.5Ge0.5P-C composite in a diglyme-based electrolyte. This work reveals the Na-storage reaction mechanism within Zn0.5Ge0.5P and offers a new perspective on designing high-performance anodes.

Experimental and Theoretical Study of Stable and Metastable Phases in Sputtered CuInS2.

The chalcopyrite Cu(In,Ga)S2 has gained renewed interest in recent years due to the potential application in tandem solar cells. In this contribution, a combined theoretical and experimental approach is applied to investigate stable and metastable phases forming in CuInS2 (CIS) thin films. Ab initio calculations are performed to obtain formation energies, X-ray diffraction (XRD) patterns, and Raman spectra of CIS polytypes and related compounds. Multiple CIS structures with zinc-blende and wurtzite-derived lattices are identified and their XRD/Raman patterns are shown to contain overlapping features, which could lead to misidentification. Thin films with compositions from Cu-rich to Cu-poor are synthesized via a two-step approach based on sputtering from binary targets followed by high-temperature sulfurization. It is discovered that several CIS polymorphs are formed when growing the material with this approach. In the Cu-poor material, wurtzite CIS is observed for the first time in sputtered thin films along with chalcopyrite CIS and CuAu-ordered CIS. Once the wurtzite CIS phase has formed, it is difficult to convert into the stable chalcopyrite polymorph. CuIn5 S8 and NaInS2 accommodating In-excess are found alongside the CIS polymorphs. It is argued that the metastable polymorphs are stabilized by off-stoichiometry of the precursors, hence tight composition control is required.

Ground State Properties of the Wide Band Gap Semiconductor Beryllium Sulfide (BeS).

We report the results from self-consistent calculations of electronic, transport, and bulk properties of beryllium sulfide (BeS) in the zinc-blende phase, and employed an ab-initio local density approximation (LDA) potential and the linear combination of atomic orbitals (LCAO). We obtained the ground state properties of zb-BeS with the Bagayoko, Zhao, and Williams (BZW) computational method, as enhanced by Ekuma and Franklin (BZW-EF). Our findings include the electronic energy bands, the total (DOS) and partial (pDOS) densities of states, electron and hole effective masses, the equilibrium lattice constant, and the bulk modulus. The calculated band structure clearly shows that zb-BeS has an indirect energy band gap of 5.436 eV, from Γ to a point between Γ and X, for an experimental lattice constant of 4.863 Å. This is in excellent agreement with the experiment, unlike the findings of more than 15 previous density functional theory (DFT) calculations that did not perform the generalized minimization of the energy functional, required by the second DFT theorem, which is inherent to the implementation of our BZW-EF method.

In situ phase transformation of polytypic zinc-blende/wurtzite copper indium sulfide via a facile polyol method to boost visible-light photocatalytic performance.

A zinc-blende/wurtzite (ZB/WZ) copper indium sulfide (CuInS2/CIS) polymorph with high visible-light absorption ability and high charge separation rate was developed by using a facile polyol method. Results showed that when thioacetamide served as a sulfur precursor, the crystalline phase of CIS was zinc-blende. Meanwhile, when thiourea served as a sulfur precursor, the crystalline phase of CIS was wurtzite, which exhibited good photocatalytic acid red 1 (AR1) dye decolorization efficiency. When the precursor/ethylene glycol ratio was 1/50-7/50, the AR1 decolorization efficiency followed the order: T-5-CIS > T-7-CIS > T-3-CIS > T-1-CIS, and the TOC removal efficiency of T-5-CIS was 45.7%. The PL and EIS analyses indicated that T-5-CIS showed the highest charge separation rate. Mott-Schottky analysis demonstrated that the remarkably enhanced photocatalytic decolorization rate was ascribed to the stronger reduction potential of CIS with the mixed ZB/WZ phases and the redox potential difference between the ZB and WZ phases, leading to a good oxidation ability and charge separation. The results indicated that O2- was the main reactive specie in this study, and this study provided a potential photocatalyst in the treatment of dye wastewater.

Effect of the Uniaxial Compression on the GaAs Nanowire Solar Cell.

Research regarding ways to increase solar cell efficiency is in high demand. Mechanical deformation of a nanowire (NW) solar cell can improve its efficiency. Here, the effect of uniaxial compression on GaAs nanowire solar cells was studied via conductive atomic force microscopy (C-AFM) supported by numerical simulation. C-AFM I-V curves were measured for wurtzite p-GaAs NW grown on p-Si substrate. Numerical simulations were performed considering piezoresistance and piezoelectric effects. Solar cell efficiency reduction of 50% under a -0.5% strain was observed. The analysis demonstrated the presence of an additional fixed electrical charge at the NW/substrate interface, which was induced due to mismatch between the crystal lattices, thereby affecting the efficiency. Additionally, numerical simulations regarding the p-n GaAs NW solar cell under uniaxial compression were performed, showing that solar efficiency could be controlled by mechanical deformation and configuration of the wurtzite and zinc blende p-n segments in the NW. The relative solar efficiency was shown to be increased by 6.3% under -0.75% uniaxial compression. These findings demonstrate a way to increase efficiency of GaAs NW-based solar cells via uniaxial mechanical compression.

Synthesis of Anisotropic ZnSe Nanorods with Zinc Blende Crystal Structure.

Compared with the well-explored cadmium-based one-dimensional nanorods (NRs), it is still a challenge to produce heavy-metal-free II-VI semiconductor analogues with a controlled size, shape, and crystal structure. Herein, a synthetic strategy towards ZnSe NRs with a zinc blende crystal structure is presented, where use of the anisotropic nuclei produced via a high-temperature selenium injection favors anisotropic growth. Elongated ZnSe NRs were produced from anisotropic ZnSe nuclei, while spherical ZnSe nanocrystals were obtained starting from isotropic nuclei. The different free energy at (111) and (220) planes in anisotropic ZnSe nuclei induces the anisotropic growth of (111) plane for ZnSe NRs. Proper choice of the capping ligand (1-dodecanethiol) has an important implication for the formation of anisotropic ZnSe nuclei and also allows the control of the diameter of the final ZnSe NRs by limiting the growth of the (220) crystal plane of anisotropic ZnSe nuclei.