Bi-rich Conditions Research Articles

Investigation of CsBr:BiBr3 precursor ratio concentration on cesium bismuth bromide perovskite formation

The effect of precursor ratios on the formation of Cs3Bi2Br9 via a re-precipitation synthesis method has been studied thoroughly. X-ray diffraction analysis showed that the precursor ratio was critical for the Cs3Bi2Br9 phase formation. The Cs-rich or Bi-rich precursor ratio resulted in diverse perovskite structure formation of Cs3BiBr6 and Cs3Bi2Br6 with micron size platelet and semi-micron size crystal morphology respectively. Unreacted BiBr3 precursor remains in the supernatant solution under Bi-rich conditions. The synthesized powder dispersed in toluene shows the degradation of the Cs3Bi2Br9 phase at Cs-rich conditions. Photoluminescence quantum yield (PLQY) of 0.73 % was identified in Cs3Bi2Br9 with a large contribution to the exciton-related recombination process. A precursor ratio CsBr:BiBr3 of 3.0:2.0 is proposed to form a single-phase Cs3Bi2Br9 structure, as the deviation in precursor ratio would promote the Cs3BiBr6 structure growth and affect the material’s optical performance.

Evolution of atomic structure and electronic transport properties in n-type Bi2Te3 films via Bi2 planar defects

Bi2Te3 films always exhibit n-type transport characteristics even under the Bi-rich condition, which, however, was not clarified clearly. Herein, by virtue of advanced techniques such as scanning tunneling microscopy, angle-resolved photoelectron spectroscopy, scanning transmission electron microscopy, and x-ray photoelectron spectroscopy, we are able to identify the structural evolution on the atomic scale for Bi-rich Bi2Te3 films. The excess of Bi content will lead to the formation of p-type BiTe antisite defects; however, there is a doping limit for the excess of Bi to form BiTe antisites. Beyond this limit, the excess of Bi will form the n-type Bi2 planar defects in the van der Waals gap, the excellent electron donors, which can enhance the electron density by over one order of magnitude and up to the 1021 cm−3 range for Bi-rich Bi2Te3 films. Benefiting from the remarkable increase in the electron density and the suppression of carrier intrinsic excitations, Bi2Te3 films with Bi2 planar defects possess a much improved thermoelectric power factor, with a maximum value of 1.4 mW m−1 K−2 at 450 K, showing about 130% enhancement compared to that of the film without Bi2 intercalations. The discovery opens a new avenue to improve the thermoelectric properties of Bi2Te3 films utilizing the Bi2 planar defects.

Simulation studies to quantify the impacts of point defects: An investigation of Cs2AgBiBr6 perovskite solar devices utilizing ZnO and Cu2O as the charge transport layers

In this investigation, we have applied SCAPS and wxAMPS to simulate defects and probe a photovoltaic device utilizing Cs2AgBiBr6 as the active photovoltaic layer and ZnO and Cu2O as the electron transport layer (ETL) and hole transport layer (HTL) respectively. At the Cs2AgBiBr6 bulk we find that with increasing defect density, each defect level has increasing impact on all device performance parameters. At a given defect density however, we find that that deeper defects have more profound impacts on Jsc and FF, and minimal effects on Voc. Specific to the Cs2AgBiBr6 structure, we have investigated VAg (shallow defect), VBi (deep defect) and Bri (quasi-deep defect). Our results provide insight into the growth conditions of Cs2AgBiBr6, with a need to have both Br-poor and Bi-rich conditions, and a preference for the latter over the former to suppress the deeper defect. Exploring the performance kinetics at the ZnO/Cs2AgBiBr6 and Cs2AgBiBr6/Cu2O interfaces due to defect type, location and density, we showcase a remarkably stable behavior in both Voc and Jsc across both interfaces. We attribute this to much higher charge mobilities in the ZnO and Cu2O compared to the Cs2AgBiBr6 layer combined with similar defect densities across the layers, leading to effective charge extraction and minimal charge recombination.

First-Principles Calculation of Defect Formation and Positron Annihilation States in Bi<sub>2</sub>Te<sub>3</sub>

Defect formation energy in Bi2Te3 thermoelectric material was calculated using a first principles approach based on the Density Functional Theory (DFT). For vacancy-type defect, the Te1 vacancy (VTe1) is the most stable defect with low formation energy in both Bi-rich and Te-rich conditions, which indicates that the Te1 vacancies have higher probability to be formed. For antisite defects, the formation energy of BiTe1 is much lower than that of BiTe2 in Bi-rich condition, while in Te-rich condition it is beneficial for TeBi with lower formation energy. Positron wave function distribution and positron lifetimes of different annihilation states in Bi2Te3 were also calculated using the atomic superposition (ATSUP) method. The positron bulk lifetime in Bi2Te3 is about 231 ps, and for the neutral vacancy-type defects without relaxation, the positron lifetimes of VBi, VTe1 and VTe2 are 275 ps, 295 ps and 269 ps, respectively.

Communication: Surface stability and topological surface states of cleaved Bi2Se3: First-principles studies.

Through systematic first principles calculations within the nonlocal van der Waals functional, we investigated the structure stability and topological surface state properties of various surface cleaves in the topological insulator Bi2Se3 Our results reveal that under Bi-rich conditions, the Bi bilayer-covered surface and the Se-terminated surface with a Bi bilayer under the first quintuple layer are even more stable than the generic Bi2Se3. The surface state bands are changed by different surface terminations, causing the formation of new Dirac states. We may distinguish various surfaces by using scanning tunneling microscopy experimentally.

Origin of the Bismuth-Induced Decohesion of Nickel and Copper Grain Boundaries

Ductile metals such as Ni and Cu can become brittle when certain impurities (e.g., Bi) diffuse and segregate into their grain boundaries (GBs). Using first-principles calculations, we investigate the microscopic origin of the Bi-induced loss of cohesion of Ni and Cu GBs. We find that the Bi bilayer interfacial phase is the most stable impurity phase under the Bi-rich condition, while the Bi monolayer phase is a metastable phase regardless of the value of the Bi chemical potential. Our finding is consistent with the recent experimental observation for Ni GBs [Luo et al. Science 333, 1730 (2011)]. The electric polarization effect of the Bi bilayer substantially enhances the strength of the Bi-metal interfacial bonds, stabilizing the bilayer phase over other phases. The Bi-Bi interlayer bonding is significantly weakened in the GBs, leading to a factor of 20 to 50 decrease in the GB cohesion, which has strong implications for the understanding of Bi-induced intergranular fracture of Ni and Cu polycrystals.

First-principles study of native point defects in Bi2Se3

Using first-principles method within the framework of the density functional theory, we study the influence of native point defect on the structural and electronic properties of Bi2Se3. Se vacancy in Bi2Se3 is a double donor, and Bi vacancy is a triple acceptor. Se antisite (SeBi) is always an active donor in the system because its donor level (ɛ(+1/0)) enters into the conduction band. Interestingly, Bi antisite (BiSe1) in Bi2Se3 is an amphoteric dopant, acting as a donor when μe < 0.119 eV (the material is typical p-type) and as an acceptor when μe > 0.251 eV (the material is typical n-type). The formation energies under different growth environments (such as Bi-rich or Se-rich) indicate that under Se-rich condition, SeBi is the most stable native defect independent of electron chemical potential μe. Under Bi-rich condition, Se vacancy is the most stable native defect except for under the growth window as μe > 0.262 eV (the material is typical n-type) and ΔμSe < −0.459 eV (Bi-rich), under such growth window BiSe1 carrying one negative charge is the most stable one.

Obtaining an intermediate band photovoltaic material through the Bi insertion in CdTe

Defect interaction can take place in CdTe under Te and Bi rich conditions. We demonstrate in this work through first principles calculations, that this phenomenon allows a Jahn Teller distortion to form an isolated half-filled intermediate band in the host semiconductor band-gap. This delocalized energy band supports the experimental deep level reported in the host band-gap of CdTe at a low bismuth concentration. Furthermore, the calculated optical absorption of CdTe:Bi in this work shows a significant subband-gap absorption that also supports the enhancement of the optical absorption found in the previous experimental results.

Native defects in tetradymite Bi2(TexSe3−x) topological insulators

Formation energies of native defects in Bi${}_{2}$(Te${}_{x}$Se${}_{3\ensuremath{-}x}$), with comparison to ideal Bi${}_{2}$Te${}_{2}$S, are calculated in density-functional theory to assess transport properties. Bi${}_{2}$Se${}_{3}$ is found to be $n$ type for both Bi- and Se-rich growth conditions, while Bi${}_{2}$Te${}_{3}$ changes from $n$ to $p$ type going from Te- to Bi-rich conditions, as observed. Bi${}_{2}$Te${}_{2}$Se and Bi${}_{2}$Te${}_{2}$S are generally $n$ type, explaining observed heavily doped $n$-type behavior in most samples. A (0/\ensuremath{-}) transition level at 16 meV above valence-band maximum for Bi on Te antisites in Bi${}_{2}$Te${}_{2}$Se is related to the observed thermally active transport gap causing a $p$-to-$n$ transition at low temperature. Bi${}_{2}$(Te${}_{x}$Se${}_{3\ensuremath{-}x}$) with $x>2$ are predicted to have high bulk resistivity due to effective carrier compensation when approaching the $n$-to-$p$ crossover. Predicted behaviors are confirmed from characterization of our grown single crystals.

First principles investigation of the effects of Bi vacancy on the magnetic, conductive and electrochemical properties of BiF3

The structural, electronic, magnetic and electrochemical properties of BiF3 with various concentration of Bi vacancy have been investigated by first principles calculations. The calculated Bi vacancy formation energy in BiF3 indicates that Bi vacancy is easier to fabricate under F-rich condition than under Bi-rich condition. Bi3F12 has non-polarized character, while Bi15F48 and Bi31F96 have polarized character. Magnetism in Bi1−xF3 mainly origins from the contribution of F 2p orbits. Besides, introducing Bi vacancy can also improve the conductive and electrochemical properties of BiF3.

Growth of high Bi concentration GaAs1−xBix by molecular beam epitaxy

The incorporation of Bi is investigated in the molecular beam epitaxy growth of GaAs1−xBix. Bi content increases rapidly as the As2:Ga flux ratio is lowered to 0.5 and then saturates for lower flux ratios. Growth under Ga and Bi rich conditions shows that Bi content increases strongly with decreasing temperature. A model is proposed where Bi from a wetting layer incorporates through attachment to Ga-terminated surface sites. The weak Ga-Bi bond can be broken thermally, ejecting Bi back into the wetting layer. Highly crystalline films with up to 22% Bi were grown at temperatures as low as 200 °C.

Intrinsic defect in BiNbO4: A density functional theory study

Formation energies, transition energy levels, and electronic properties of various intrinsic defects in BiNbO4 systems are studied based on the first-principles density-functional theory. Our results indicate that the acceptor defects form easier than donor defects under O rich condition, while it is opposite under Bi rich condition. Under O-rich condition, Bi vacancies (Bivac) leading to p-type conductivity are the dominant intrinsic defects, whereas O vacancies (Ovac) inducing moderate n-type conductivity are the dominant intrinsic defects under Bi-rich condition. Among these intrinsic defects, Ovac is a deep donor; to the contrary, Bivac is found to be a shallow acceptor which is benefit to the separation and migration of the photogenerated carriers. Consequently, the BiNbO4 with Bivac under O-rich growth condition should be of better photocatalytic performance.

First principles study on the structural, magnetic and electronic properties of Te-doped BiF3

The structural, magnetic and electronic properties of Te-doped BiF3 were investigated by first-principles calculations in this paper. We focused on TexBi1−xF3 systems, in which x is equal to 0, 3.125atom%, 6.25atom%, 12.5atom% and 25atom% of Te doping concentration, respectively. The formation energy analysis indicates that the difficulty of Te doping BiF3 decreases with the increase of Te doping concentration, and Te doping BiF3 is much easier in the F-rich conditions than in the Bi-rich conditions. Besides, the calculated cohesive energy of Te doping BiF3 shows that the stability of Te doped BiF3 becomes worse with the increase of Te doping concentration. What is more, Te doped BiF3 shows obvious magnetic character. As for electronic properties of Te-doped BiF3, the results show that the band gap of TexBi1−xF3 decreases with the increase of Te doping concentration. For TeBi3F12, TeBi7F24 and TeBi15F48, Fermi level shifts into the spin-up conductivity band while spin-down energy band has a band gap at Fermi level, and thus the compounds exhibit half metallic characters with good conductivity when Te atoms substitute partly Bi atoms.

First-principles density functional theory study of native point defects in Bi2Te3

We present a first-principles study of the native point defects in the thermoelectric material Bi${}_{2}$Te${}_{3}$. Calculated formation energies of defects and electronic densities of states were analyzed in detail. The most prominent native point defects considered are vacancies and antisite defects on the Bi, Te1, and Te2 sublattices of the Bi${}_{2}$Te${}_{3}$ structure. Vacancies on all three sublattices are found to have much higher formation energies than antisite defects. The most dominant antisite defects are found to be Bi${}_{\mathrm{Te}1}$ at Bi-rich conditions, and Te${}_{\mathrm{Bi}}$ at Te-rich conditions. These lead to the formation of resonant defect states at the top of the valence band and bottom of the conduction band, respectively. Hence they are expected to impact charge and energy transport in a profound way. Furthermore antisite defect pairs tend to form at nearest-neighbor distances, and lead to substantial changes in the electronic structure and hence in the thermoelectric properties of Bi${}_{2}$Te${}_{3}$.