Br Vacancies Research Articles

Bromine Vacancy Redistribution and Metallic‐Ion‐Migration‐Induced Air‐Stable Resistive Switching Behavior in All‐Inorganic Perovskite CsPbBr3 Film‐Based Memory Device

AbstractAll‐inorganic halide perovskites have attracted a great deal of attention for applications in resistive switching (RS) memory devices due to their superior stability compared to organic–inorganic hybrid halide perovskites. RS memory devices utilizing air‐stable all‐inorganic halide perovskite cesium lead bromide (CsPbBr3) film as the switching layer, which are successfully prepared by spin coating at low temperature, are demonstrated. Memory devices based on CsPbBr3 film exhibit typical reproducible bipolar RS behavior and superior switching characteristics, including the high ON/OFF ratio (≈104), long data retention (>5 × 104 s), and environmental stability. In addition, multilevel storage capability can be achieved through controlling the different compliance currents. The formation and rupture of bromine (Br) vacancy conducting filaments (CFs) is proposed to explain the switching behavior in the Pt‐anode‐based memory devices, which is verified by XPS depth‐profiling analysis. Moreover, the coexistence of Br vacancies and Ag metallic CFs is suggested to be responsible for the switching behavior in Ag‐anode based device. These results demonstrate that the all‐inorganic halide perovskite CsPbBr3 film will be the promising switching material for nonvolatile memory devices.

Revealing the Origin of Luminescence Center in 0D Cs4PbBr6 Perovskite

Zero dimensional perovskite Cs4PbBr6 has attracted considerable attention recently not only because of its highly efficient green photoluminescence (PL), but also its two highly debated opposing mechanisms of the luminescence: embedded CsPbBr3 nanocrystals versus intrinsic Br vacancy states. After a brief discussion on the root cause of the controversy, we provide sensitive but non-invasive methods that can not only directly correlate luminescence with the underlying structure, but also distinguish point defects from embedded nanostructures. We first synthesized both emissive and non-emissive Cs4PbBr6 crystals, obtained the complete Raman spectrum of Cs4PbBr6 and assigned all Raman bands based on density functional theory simulations. We then used correlated Raman-PL as a passive structure-property method to identify the difference between emissive and non-emissive Cs4PbBr6 crystals and revealed the existence of CsPbBr3 nanocrystals in emissive Cs4PbBr6. We finally employed a diamond anvil cell to probe the response of luminescence centers to hydrostatic pressure. The observations of fast red-shifting, diminishing and eventual disappearance of both green emission and Raman below Cs4PbBr6 phase transition pressure of ~3 GPa is compatible with CsPbBr3 nanocrystal inclusions as green PL emitters and cannot be explained by Br vacancies. The resolution of this long-lasting controversy paves the way for further device applications of low dimensional perovskites, and our comprehensive optical technique integrating structure-property with dynamic pressure response is generic and can be applied to other emerging optical materials to understand the nature of their luminescent centers.

Mechanistic Insight into Surface Defect Control in Perovskite Nanocrystals: Ligands Terminate the Valence Transition from Pb2+ to Metallic Pb0.

Organolead halide perovskite nanocrystals (NCs) have emerged as promising materials for various optoelectronic applications. However, their practical applications have been limited due to low structural integrity and poor luminescence stability associated with fast attachment-detachment dynamics of surface capping molecules during postprocessing. At present, a framework for understanding how the functional additives interact with surface moieties of organolead halide perovskites is not available. Methylammonium lead bromide NCs without surfactants on their surface provide an ideal system to investigate the direct interactions of the perovskite with functional molecules. When the oleic acid is used in a combination with n-octylamine, its contribution to surface passivation is significantly increased by protonating the alkyl amine to the corresponding ammonium ion. Our results demonstrate that the Br vacancies at thenonpassivated surface result in a reduction of Pb2+ to Pb0 by trapping electrons generated from the exciton dissociation, which provides a main pathway for exciton trapping.

Light assisted multilevel resistive switching memory devices based on all-inorganic perovskite quantum dots

All-inorganic perovskite quantum dots (APQDs) have emerged as excellent materials which have been widely used in numerous micro-nano photoelectric devices. However, resistive random access memory (RRAM) devices based on APQDs are relatively scarce. In this work, RRAM based on CsPbBr3 APQDs prepared by the solution processed method was fabricated at room temperature. The sandwich structure memory device shows high reproducibility, good data retention ability, and light assisted multilevel storage capability. The resistance ratio (ON/OFF) of the RRAM device between the high resistance state and the low resistance state reaches almost 107. Additionally, the device exhibits high performances under low power consumption—low reading voltage (−0.3 V) and operation voltage (−2.4 V/1.55 V). It is suggested that the connection and rupture of conducting filaments, which are formed by Br vacancies under an electric field, are responsible for the resistive switching effect. Our work provides an opportunity to develop the next generation high-performance and stable nonvolatile memory devices.

Defining the composition and electronic structure of large-scale and single-crystalline like Cs2AgBiBr6 films fabricated by capillary-assisted dip-coating method

Owning the merits of both lead-free and air-stable, the double-perovskite Cs2AgBiBr6 has attracted increasing attention, but suffers low visible-light absorption coefficient due to its large indirect bandgap. Moreover, the electronic structure of its synthesized films has not been explored clearly yet. In this work, we developed a general and promising method to fabricate continuous, uniform and highly orientated Cs2AgBiBr6 films in large scale on various substrates through capillary-assisted dip-coating method. Strikingly, those optimized films are single crystalline verified by φ-scan XRD. Its electronic structure was carefully studied independently by multi-photo-physical characterizations. Its bandgap can be tuned from 2.65 to 2.25 eV by changing the substrate temperature in growth from 40 to 160 °C. Essentially, their work-function (WF) was determined at −5.01 eV and WF-VBM is around 2 eV. This novel band structure with typical n-type characteristic, was further confirmed by DFT calculations, which reveals that the Cs interstitials and Br vacancies derived deep defect levels were fixed around its Fermi level, closer to the conduction band. This conclusion is different from its widely accepted p-type feature, but definitely deepens our understanding of this material and inspires us to find more valuable strategies of modulating its band structure and optoelectronic properties.

Halogen Migration in Hybrid Perovskites: The Organic Cation Matters.

In hybrid perovskite materials, the organic cations are one of the key structural components used to tune the electronic and optical properties of this promising class of materials. Here, we studied the strong impact of organic cations, methylammonium (MA) and formamidinium (FA), on halogen vacancy and interstitial migration, as well as surface degradation in cubic-phase MAPbBr3 and FAPbBr3 crystals using density functional theory calculations. We found Br vacancies and interstitials have much lower formation energies and higher density in MAPbBr3 in comparison to FAPbBr3 crystals. Moreover, the transition energy barrier for Br migration through vacancies within the bulk phase is lower in MAPbBr3 than in FAPbBr3. We also found that FAPbBr3 has a much higher rotation barrier of the organic cation than MAPbBr3, which points to a much stronger H-bonding with Br in the former case. Our results show that incorporating organic cations with the appropriate structure, shape, and strong H-bonding capabilities in hybrid perovskite crystals is very beneficial for suppressing ion migration and thus further improving the performance of hybrid perovskite-based devices.

All-Ambient Processed Binary CsPbBr3-CsPb2Br5 Perovskites with Synergistic Enhancement for High-Efficiency Cs-Pb-Br-Based Solar Cells.

All-inorganic CsPbBr3 perovskite solar cells display outstanding stability toward moisture, light soaking, and thermal stressing, demonstrating great potential in tandem solar cells and toward commercialization. Unfortunately, it is still challenging to prepare high-performance CsPbBr3 films at moderate temperatures. Herein, a uniform, compact CsPbBr3 film was fabricated using its quantum dot (QD)-based ink precursor. The film was then treated using thiocyanate ethyl acetate (EA) solution in all-ambient conditions to produce a superior CsPbBr3-CsPb2Br5 composite film with a larger grain size and minimal defects. The achievement was attributed to the surface dissolution and recrystallization of the existing SCN- and EA. More specifically, the SCN- ions were first absorbed on the Pb atoms, leading to the dissolution and stripping of Cs+ and Br- ions from the CsPbBr3 QDs. On the other hand, the EA solution enhances the diffusion dynamics of surface atoms and the surfactant species. It is found that a small amount of CsPb2Br5 in the composite film gives the best surface passivation, while the Br-rich surface decreases Br vacancies (VBr) for a prolonged carrier lifetime. As a result, the fabricated device gives a higher solar cell efficiency of 6.81% with an outstanding long-term stability.

Hydrothermal growth and scintillation properties of γ-CuBr single crystals

A fast scintillation crystal, γ-CuBr, was successfully grown by hydrothermal method with NH4Br mineralizer. The element mole ratio of the Cu/Br crystal was about 0.91 caused by the crystal grown in a rich bromine environment. Under ultraviolet excitation, the crystal exhibited an intense 420 nm emission, a shoulder emission at 428 nm and a weak broadband emission at 560 nm, which could be related to free exciton recombination, Cu and Br vacancies, respectively. Furthermore, the X-ray excited optical luminescence of the crystal was first reported, and a strong 430 nm emission was observed. The analysis of luminescence decay kinetics revealed that the 430 nm emission is composed of two components with an average decay time of 36.9 ns. It means that the γ-CuBr crystal is a promising candidate for applications in fast radiation detection.

Atomistic Origins of Surface Defects in CH3NH3PbBr3 Perovskite and Their Electronic Structures

The inherent instability of CH3NH3PbX3 remains a major technical barrier for the industrial applications of perovskite materials. Recently, the most stable surface structures of CH3NH3PbX3 have been successfully characterized by using density functional theory (DFT) calculations together with the high-resolution scanning tunneling microscopy (STM) results. The two coexisting phases of the perovskite surfaces have been ascribed to the alternate orientation of the methylammonium (MA) cations. Notably, similar surface defect images (a dark depression at the sites of X atoms) have been observed on surfaces produced with various experimental methods. As such, these defects are expected to be intrinsic to the perovskite crystals and may play an important role in the structural decomposition of perovskite materials. Understanding the nature of such defects should provide some useful information toward understanding the instability of perovskite materials. Thus, we investigate the chemical identity of the surface defects systematically with first-principles density functional theory calculations and STM simulations. The calculated STM images of the Br and Br-MA vacancies are both in good agreement with the experimental measurements. In vacuum conditions, the formation energy of Br-MA is 0.43 eV less than the Br vacancy. In the presence of solvation effects, however, the formation energy of a Br vacancy becomes 0.42 eV lower than the Br-MA vacancy. In addition, at the vacancy sites, the adsorption energies of water, oxygen, and acetonitrile molecules are significantly higher than those on the pristine surfaces. This clearly demonstrated that the structural decomposition of perovskites are much easier to start from these vacancy sites than the pristine surfaces. Combining DFT calculations and STM simulations, this work reveals the chemical identities of the intrinsic defects in the CH3NH3PbX3 perovskite crystals and their effects on the stability of perovskite materials.

Bismuth chalcohalides and oxyhalides as optoelectronic materials

Several Tl and Pb based halides and chalcohalides have recently been discovered as promising optoelectronic materials [i.e., photovoltaic (PV) and gamma-ray detection materials]. Efficient carrier transport in these materials is attributed partly to the special chemistry of $n{s}^{2}$ ions (e.g., $\mathrm{T}{\mathrm{l}}^{+}, \mathrm{P}{\mathrm{b}}^{2+}$, and $\mathrm{B}{\mathrm{i}}^{3+}$). However, the toxicity of Tl and Pb is challenging to the development and the wide use of Tl and Pb based materials. In this paper, we investigate materials that contain $\mathrm{B}{\mathrm{i}}^{3+}$, which is also an $n{s}^{2}$ ion. By combining Bi halides with Bi chalcogenides or oxides, the resulting ternary compounds exhibit a wide range of band gaps, offering opportunities in various optoelectronic applications. Density functional calculations of electronic structure, dielectric properties, optical properties, and defect properties are performed on selected $\mathrm{B}{\mathrm{i}}^{3+}$ based chalcohalides and oxyhalides, i.e., BiSeBr, BiSI, BiSeI, and BiOBr. We propose different applications for these Bi compounds based on calculated properties, i.e., $n$-BiSeBr, $p$-BiSI, and $p$-BiSeI as PV materials, BiSeBr and BiSI as room-temperature radiation detection materials, and BiOBr as a $p$-type transparent conducting material. BiSeBr, BiSI, and BiSeBr have chain structures while BiOBr has a layered structure. However, in BiSI, BiSeI, and BiOBr, significant valence-band dispersion is found in the directions perpendicular to the atomic chain or layer because the valence-band edge states are dominated by the halogen states that have strong interchain or interlayer coupling. We find significantly enhanced Born effective charges and anomalously large static dielectric constants of the Bi compounds, which should reduce carrier scattering and trapping and promote efficient carrier transport in these materials. The strong screening and the small anion coordination numbers in Bi chalcohalides should lead to weak potentials for electron localization at anion vacancies. Defect calculations indeed show that the anion vacancies (Se and Br vacancies) in BiSeBr are shallow, which is beneficial to efficient electron transport.

Effects of <formula formulatype="inline"> <tex Notation="TeX">${\hbox {Na}}^ + $</tex></formula>, <formula formulatype="inline"><tex Notation="TeX">${\hbox {Mg}}^{2+} $</tex> </formula>, <formula formulatype="inline"><tex Notation="TeX">${\hbox {Ca}}^{2+} $</tex></formula>, <formula formulatype="inline"><tex Notation="TeX">${\hbox {Sr}}^{2 +} $</tex></formula> and <formula formulatype="inline"><tex Notation="TeX">${\hbox {Ba}}^{2 +} $</tex> </formula> Doping on the Scintillation Properties of <formula formulatype="inline"><tex Notation="TeX">${\hbox {CeBr}}_3$</tex> </formula>

The effects of ${\hbox {Na}}^ + $ , ${\hbox {Mg}}^{2 +} $ , ${\hbox {Ca}}^{2 +} $ , ${\hbox {Sr}}^{2 +} $ and ${\hbox {Ba}}^{2 +} $ doping on the scintillation properties of ${\hbox {CeBr}}_3$ are evaluated in this paper. ${\hbox {Ca}}^{2 +} $ and ${\hbox {Sr}}^{2 +} $ doped ${\hbox {CeBr}}_3$ show different scintillation properties as compared to undoped ${\hbox {CeBr}}_3$ , i.e. an additional perturbed ${\hbox {Ce}}^{3 +} $ site with emission maxima red-shifted by 30 nm compared to unperturbed ${\hbox {Ce}}^{3 +} $ sites as evidenced by X-ray excited emission and photoluminescence measurements. Also a nearly constant light yield at temperatures below 300 K, an increased decay time and the appearance of multiple glow peaks in thermoluminescence measurements are observed for Ca and Sr doped ${\hbox {CeBr}}_3$ . These effects on the scintillation properties of ${\hbox {CeBr}}_3$ are explained using a single trap model where Br vacancies, formed as charge compensation for the aliovalent dopants, act as electron traps.

Structure and properties of electronic and hole centers in CsBr from theoretical calculations

The electronic structure, geometry, diffusion barriers and optical properties of fundamental defects of CsBr are calculated using hybrid functional DFT and TD-DFT methods. The B3LYP functional with a modified exchange contribution has been used in an embedded cluster scheme to model the structure and spectroscopic properties of the self-trapped triplet exciton, interstitial Br atoms and ions, self-trapped holes and Br vacancies. The calculated migration barriers and positions of maxima of optical absorption bands are in good agreement with experiment, justifying the obtained defect geometries. The off-center triplet exciton luminescence energy is also accurately calculated.

Origin of resolution enhancement by co-doping of scintillators: Insight from electronic structure calculations

It was recently shown that the energy resolution of Ce-doped LaBr3 scintillator radiation detectors can be crucially improved by co-doping with Sr, Ca, or Ba. Here, we outline a mechanism for this enhancement on the basis of electronic structure calculations. We show that (i) Br vacancies are the primary electron traps during the initial stage of thermalization of hot carriers, prior to hole capture by Ce dopants; (ii) isolated Br vacancies are associated with deep levels; (iii) Sr doping increases the Br vacancy concentration by several orders of magnitude; (iv) SrLa binds to VBr resulting in a stable neutral complex; and (v) association with Sr causes the deep vacancy level to move toward the conduction band edge. The latter is essential for reducing the effective carrier density available for Auger quenching during thermalization of hot carriers. Subsequent de-trapping of electrons from SrLa–VBr complexes can activate Ce dopants that have previously captured a hole leading to luminescence. This mechanism implies an overall reduction of Auger quenching of free carriers, which is expected to improve the linearity of the photon light yield with respect to the energy of incident electron or photon.

Oxygen deficiency and excess of rutile titania (1 1 0) surfaces analyzed by ion scattering coupled with elastic recoil detection

Oxygen deficiency and excess of rutile titania (TiO 2) surfaces are important factors for catalytic activities of metal nano-particles on the TiO 2 supports. Medium energy ion scattering (MEIS; 80 keV He +) coupled with elastic recoil detection analysis (ERD; 150 keV Ne +) can determine the numbers of bridging O (O br) vacancies (V O) and excess O atoms adsorbed on the 5-fold Ti rows of TiO 2(1 1 0) surfaces. The amounts of V O and adsorbed O were derived by H 2O and 18O 2 exposure followed by ERD and MEIS analyses, respectively. The present analysis revealed that only about a half of V O are filled and a comparable amount of O atoms are adsorbed on the reduced TiO 2(1 1 0) surface after exposure to O 2 (1000 L; 1 L = 1 × 10 −6 Torr s) at room temperature (RT). We also detected the adsorbed O for the hydroxylated TiO 2(1 1 0) after 18O 2 exposure at RT. Finally, it is shown that the O adsorbed on the Ti rows reacts with CO probably to form CO 2 at RT. Based on the results obtained here, we clarify the reason why only a half of V O are filled by exposing reduced surface to O 2 at RT and what is the primary source of subsurface excess electronic charge, which acts as a leading part of the surface electrochemistry and gives the defect state in the band gap seen in the valence band spectra for reduced and hydroxylated TiO 2(1 1 0) surfaces.

The Defect and Transport Properties of Donor Doped Single Crystal TlBr

Thallium bromide (TlBr) is attractive for high energy radiation detection, given its large molecular weight and wide energy bandgap. However, TlBr exhibits levels of ionic conductivity that can lead to an undesirable leakage, or dark current, thereby reducing sensor performance. To investigate the role of dopants in controlling the ionic conductivity, single crystals of TlBr were grown using zone refining and/or vertical Bridgman methods with controlled levels of donor (Pb) dopants. Their electrical properties were examined as a function of temperature with frequency dependent impedance spectroscopy. A Schottky-based defect equilibria model was fitted to the resulting conductivity data, and enthalpies of Schottky defect formation , cation migration , and anion migration were extracted. Br vacancies were found to posses about 5 orders of magnitude higher mobility than that of Tl vacancies at .

First-principles study of the Cl and Br adsorbed Si(100) surfaces

We have studied the atomic and electronic structure of the Cl and Br adsorbed Si(100) surfaces using density-functional theory calculations. We find that the Cl- and Br-terminated $\mathrm{Si}(100)\ensuremath{-}(2\ifmmode\times\else\texttimes\fi{}1)$ surfaces show many adsorbate-induced surface electronic features, the energy levels and charge characters of which explain well the electronic origin of the Cl and Br hoppings observed in recent scanning-tunneling-microscopy (STM) experiments. The atomic structures and simulated STM images of Cl and Br vacancies are also examined. The vacancy structures underlying experimental STM images are identified, and the experimental bias-voltage dependence of Cl and Br vacancy images is reproduced in our simulations.