High quality single crystals of Cs$_2$Au$_2$X$_6$ (X=Br,I) were grown using a ternary self-flux method. Structural refinements based on single crystal X-ray diffraction measurements show that both materials have a distorted perovskite structure belonging to the I4/mmm space group with full site occupancy. Transport measurements reveal a large bandgap of 550 $\pm$ 100 meV for Cs$_2$Au$_2$I$_6$ and 520 $\pm$ 80 meV for Cs$_2$Au$_2$Br$_6$. Initial attempts at chemical substitution are described.
We performed ab initio simulations aimed at identifying the atomistic and electronic structure origin of the high valence and conduction band, and band gap tunability of halide perovskites. We found that the two key ingredients are the overlap between bivalent cation and halide atomic orbitals, and the electronic charge of the bottom of the conduction band (CBM) state on the Sn or Pb atoms. In particular, we found that lower gaps are associated to higher negative antibonding overlap, and higher CBM charge on the bivalent cation. Both overlap and CBM charge on Sn/Pb can be tuned by the chemical…
The emergence of methyl-ammonium lead halide (MAPbX3) perovskites motivates the identification of unique properties giving rise to exceptional bulk transport properties, and identifying future materials with similar properties. Here, we propose that this "defect tolerance" emerges from fundamental electronic structure properties, including the orbital character of the conduction and valence band extrema, the effective masses, and the static dielectric constant. We use MaterialsProject.org searches and detailed electronic-structure calculations to demonstrate these properties in other materials…
New varieties of quasi-one-dimensional halogen (X)-bridged transition-metal (M) complexes, (C_8_H_6_N_4_)[Pt(C_2_H_8_N_2_)X]_2_X(ClO_4_)_3_\cdotH_2_O (X=Br,Cl) and (C_10_H_8_N_2_)[Pt(C_4_H_13_N_3_)Br]_2_Br_4_\cdot2H_2_O, comprising two-leg ladders of mixed-valent platinum ions, are described in terms of a multiband extended Peierls-Hubbard Hamiltonian. The polarized optical conductivity spectra are theoretically reproduced and the ground-state valence distributions are reasonably determined. The latter variety, whose interchain valence arrangement is out of phase, is reminiscent of conventiona…
The discovery of double perovskites A2B(I)B(III)X6 (A=monovalent cation; B(I)/B(III) = metal cation; X = halogen) as Pb(II)-free alternatives has attracted widespread attention, making it possible to introduce d-block metal cations (e.g., d0 and d10) into halide perovskites. However, at present, there are quite limited insights into the underlying bonding orbitals for d-block metal cation-based halide perovskites. Here, we present an orbital engineering strategy to construct two-dimensional (2D) electronic structures in three-dimensional (3D) halide perovskites by rationally controlling the d …
The paper presents the results of measurements of XPS valence band spectra of SiO2/MAPbI3 hybrid perovskites subjected to irradiation with visible light and annealing at an exposure of 0-1000 hours. It is found from XPS survey spectra that in both cases (irradiation and annealing) a decrease in the I:Pb ratio is observed with aging time, which unambiguously indicates PbI2 phase separation as a photo and thermal product of degradation. The comparison of the XPS valence band spectra of irradiated and annealed perovskites with density functional theory calculations of the MAPbI3 and PbI2 compound…
In this study, different electronic structure evolutions of perovskite single crystals are found via angle-resolved photoelectron spectroscopy (ARPES): (i) unchanged top valence band (VB) dispersions under different temperatures can be found in the CH3NH3PbI3, (ii) phase transitions induced the evolution of top VB dispersions, and even a top VB splitting with Rashba effects can be observed in the CH3NH3PbBr3. Combined with low-energy electron diffraction (LEED), metastable atom electron spectroscopy (MAES), and DFT calculation, we confirm different band structure evolutions observed in these t…
We report a comprehensive study of aqueous halide adsorption on nanoparticles of gold and palladium that addresses several limitations hampering the use of atomistic modeling as a tool for understanding and improving wet-chemical synthesis and related applications. A combination of thermodynamic modeling with density functional theory (DFT) calculations and experimental data is used to predict equilibrium shapes of halide-covered nanoparticles as a function of the chemical environment. To ensure realistic and experimentally relevant results, we account for solvent effects and include a large s…
Solar cells based on a light absorbing layer of the organometal halide perovskite CH$_3$NH$_3$PbI$_3$ have recently reached 15% conversion efficiency, though how these materials work remains largely unknown. We analyse the electronic structure and optical properties within the quasiparticle self-consistent GW approximation. While this compound bears some similarity to conventional sp semiconductors, it also displays unique features. Quasiparticle self-consistency is essential for an accurate description of the band structure: bandgaps are much larger than what is predicted by the local density…
Distorted polyhedral architecture enabled high thermoelectric performance of columnar double halide perovskites Cs2AgPdCl5 and Cs2AgPtCl5
Jamia Millia Islamia（贾米阿米利娅伊斯兰大学）；Govt Naveen College（政府纳文学院）；Indian Institute of Technology, Jodhpur（印度理工学院乔德普尔分校）；University of Ulsan（蔚山大学）；Mody University of Science and Technology（莫迪科学与技术大学）
How unconventional oxidation state Au$^{2+}$ is stabilized in halide perovskite Cs$_4$Au$_3$Cl$_{12}$: a first-principles study of its polaron crystal nature
Formation of self-trapped holes (STH) in a comprehensive list of scintillator materials, including halides and chalcogenides, are studied using an accurate and computationally efficient first-principles method, the polaron self-interaction correction (pSIC). The key characteristics of small hole polarons, including their geometries, energies and degree of localization, are found vastly different from halides to oxides to systems with open-shell cations. Nevertheless, we find a universal linear relation between the energy gap separating the bound hole level from the valence band maximum and the…
Single-orbital tight-binding model for chiral one-dimensional hybrid organic-inorganic lead halide perovskites
Waseda Institute for Advanced Study, Waseda University（早稻田大学先进研究院）；Institute for Materials Research, Tohoku University（东北大学物质材料研究所学）；Department of Chemistry and Biochemistry, School of Advanced Science and Engineering, Waseda University（早稻田大学理工学术院化学·生物化学系）；Department of Applied Physics, Tokyo University of Science（东京理科大学应用物理系）
The charge, spin, and composition degrees of freedom in high-entropy alloy endow it with tunable valence and spin states, infinite combinations and excellent mechanical performance. Meanwhile, the stacking, interlayer, and angle degrees of freedom in van der Waals material bring it with exceptional features and technological applications. Integration of these two distinct material categories while keeping their merits would be tempting. Based on this heuristic thinking, we design and explore a new range of materials (i.e., dichalcogenides, halides and phosphorus trisulfides) with multiple meta…
Understanding the type, formation energy and capture cross section of defects is one of the challenges in the field of organometallic halide perovskite (OMHP) devices. Currently, such understanding is limited, restricting the power conversion efficiencies of OMHPs solar cells from reaching their Shockley Queisser limit. Here, we report on deep level (DL) defects and their effect on free charge transport properties of single crystalline methylammonium lead bromide perovskite (MAPB). In order to determine DL activation energy and capture cross section we used photo-Hall effect spectroscopy (PHES…
We demonstrate the critical role of surface recombination in mixed-cation, mixed-halide perovskite, FA0.83Cs0.17Pb(I0.85Br0.15)3. By passivating non-radiative defects with the polymerizable Lewis base (3-aminopropyl)trimethoxysilane (APTMS) we transform these thin films. We demonstrate average minority carrier lifetimes > 4 μs, nearly single exponential monomolecular PL decays, and concomitantly high external photoluminescence quantum efficiencies (>20%, corresponding to ~97% of the maximum theoretical quasi-Fermi-level splitting) at low excitation fluence. We confirm both the composition and …
High-entropy oxides (HEOs) are a new class of materials that are promising for a wide range of applications. Designing HEOs needs to consider both geometric compatibility and electrical equilibrium. However, there is currently no available method to systematically consider these two factors when selecting constituent materials for making HEOs. Here we propose a two-step strategy, where a HEO system to be explored is first partitioned into multiple subsystems based on the valence combinations of substituted cations; the geometric compatibility is then considered in selecting suitable substitute…
We report a new family of ternary 111 hexagonal LnAuSb (Ln = La-Nd, Sm) compounds that, with a 19 valence electron count, has one extra electron compared to all other known LnAuZ compound. The "19th" electron is accommodated by Au-Au bonding between the layers; this Au-Au interaction drives the phases to crystallize in the YPtAs-type structure rather than the more common LiGaGe-type. This is critical, as the YPtAs structure type has the symmetry-allowed band crossing necessary for the formation of Dirac semimetals. Band structure, density of stats, and crystal orbital calculations confirm this…
Stability and toxicity issues with the hybrid lead iodide perovskite MAPbI$_3$ necessitate a hunt for potential alternatives. Here, we shed light on promising photovoltaic properties of gold mixed-valence halide perovskites Cs$_2$Au$_2$X$_6$ (X = I, Br, Cl). They satisfy fundamental requirements such as nontoxicity, better stability, a band gap in the visible range, and a low excitonic binding energy. Our study shows a favorable electronic structure, resulting in a high optical-transition strength, and thus a sharp rise in the absorption spectrum near the band gap. This, in turn, yields a very…
We present a theoretical study of the conductance of atomic junctions comprising single noble gas atoms (He, Ne, Ar, Kr, and Xe) coupled to gold electrodes. The aim is to elucidate how the presence of noble gas atoms affects the electronic transport through metallic atomic-size contacts. Our analysis, based on density functional theory and including van der Waals interactions, shows that for the lightest elements (He and Ne) no significant current flows through the noble gas atoms and their effect is to reduce the conductance of the junctions by screening the interaction between the gold elect…
The lead halide perovskites demonstrate huge potential for optoelectronic applications, high energy radiation detectors, light emitting devices and solar energy harvesting. Those materials exhibit strong spin-orbit coupling enabling efficient optical orientation of carrier spins in perovskite-based devices with performance controlled by a magnetic field. Perovskites are promising for spintronics due to substantial bulk and structure inversion asymmetry, however, their spin properties are not studied in detail. Here we show that elaborated time-resolved spectroscopy involving strong magnetic fi…
