Koushik Biswas

Energy transport and scintillation of cerium doped elpasolite Cs2LiYCl6: Hybrid functional calculations

Elpasolites are a large family of halides which have recently attracted considerable interest for their potential applications in room-temperature radiation detection. Cs2LiYCl6 is one of the most widely studied elpasolite scintillators. In this paper, we will show hybrid density functional calculations on electronic structure, energetics of small electron and hole polarons and self-trapped excitons, and the excitation of luminescence centers (Ce impurities) in Cs2LiYCl6. The results provide important understanding in energy transport and scintillation mechanisms in Cs2LiYCl6 and rare-earth elpasolites in general.

DX-like centers in NaI:Tl upon aliovalent codoping

Aliovalent doping has been recently shown to remarkably improve energy resolution in some halide scintillators. Based on first-principles calculations we report on the formation of DX-like centers in a well-known scintillator material, Tl-doped NaI (NaI:Tl), when codoped with Ca or Ba. Our calculations indicate a net binding energy favoring formation of the defect complex (TlNa−+CaNa+) involving a new cation-cation bond, instead of the isolated substitutional defects. The pair has properties of a deep DX-like acceptor complex. Doping with the aliovalent anion impurity Te is also found to induce deep centers, which can act as effective electron or hole traps. The hole trapped as TeI0 involves large lattice relaxation of the Te and an adjacent iodine, consistent with extrinsic self-trapping of the hole. Thus, in contrast to the positive effect achieved by aliovalent co-doping of the rare-earth tri-halides LaBr3:Ce and CeBr3:Ca as reported recently, co-doping with donor-like cations Ca, Ba, or the acceptor-l...

Exploring Polaronic, Excitonic Structures and Luminescence in Cs4PbBr6/CsPbBr3

Among the important family of halide perovskites, one particular case of all-inorganic, 0-D Cs4PbBr6 and 3-D CsPbBr3-based nanostructures and thin films is witnessing intense activity due to ultrafast luminescence with high quantum yield. To understand their emissive behavior, we use hybrid density functional calculations to first compare the ground-state electronic structure of the two prospective compounds. The dispersive band edges of CsPbBr3 do not support self-trapped carriers, which agrees with reports of weak exciton binding energy and high photocurrent. The larger gap 0-D material Cs4PbBr6, however, reveals polaronic and excitonic features. We show that those lattice-coupled carriers are likely responsible for observed ultraviolet emission around ∼375 nm, reported in bulk Cs4PbBr6 and Cs4PbBr6/CsPbBr3 composites. Ionization potential calculations and estimates of type-I band alignment support the notion of quantum confinement leading to fast, green emission from CsPbBr3 nanostructures embedded in Cs4PbBr6.

Bismuth and antimony-based oxyhalides and chalcohalides as potential optoelectronic materials

In the last decade the ns2 cations (e.g., Pb2+ and Sn2+)-based halides have emerged as one of the most exciting new classes of optoelectronic materials, as exemplified by for instance hybrid perovskite solar absorbers. These materials not only exhibit unprecedented performance in some cases, but they also appear to break new ground with their unexpected properties, such as extreme tolerance to defects. However, because of the relatively recent emergence of this class of materials, there remain many yet to be fully explored compounds. Here, we assess a series of bismuth/antimony oxyhalides and chalcohalides using consistent first principles methods to ascertain their properties and obtain trends. Based on these calculations, we identify a subset consisting of three types of compounds that may be promising as solar absorbers, transparent conductors, and radiation detectors. Their electronic structure, connection to the crystal geometry, and impact on band-edge dispersion and carrier effective mass are discussed.Optoelectronics: new kids in townDetailed first-principles calculations reveal the potential of bismuth-based and antimony-based chalcohalides and oxyhalides for optoelectronics applications. The presence of ions with outer electron configuration of ns2 in halides has rendered them very promising for applications, like solar cells. In this work, collaborators from Jilin University, University of Missouri, and Arkansas State University have used density functional theory to study the properties of several chalcohalides and oxyhalides containing bismuth or antimony ns2 cations. It turns out that certain bismuth-based chalcohalides are promising for solar cells applications and room-temperature radiation detectors, with bandgaps in the range 1.5–2 eV. Some oxyhalides, on the other hand, with bandgaps above 3 eV are hole-conducting, which makes them suitable for transparent conducting materials, if they can be doped. This work underlines that further experimental work is needed to fully assess the potential of this class of materials for optoelectronics applications.

Search for improved-performance scintillator candidates among the electronic structures of mixed halides

The application of advanced theory and modeling techniques has become an essential component to understand material properties and hasten the design and discovery of new ones. This is true for diverse applications. Therefore, current efforts aimed towards finding new scintillator materials are also aligned with this general predictive approach. The need for large scale deployment of efficient radiation detectors requires discovery and development of high-performance, yet low-cost, scintillators. While Tl-doped NaI and CsI are still some of the widely used scintillators, there are promising new developments, for example, Eu-doped SrI2 and Ce-doped LaBr3. The newer candidates have excellent light yield and good energy resolution, but challenges persist in the growth of large single crystals. We will discuss a theoretical basis for anticipating improved proportionality as well as light yield in solid solutions of certain systems, particularly alkali iodides, based on considerations of hot-electron group velocity and thermalization. Solid solutions based on NaI and similar alkali halides are attractive to consider in more detail because the end point compositions are inexpensive and easy to grow. If some of this quality can be preserved while reaping improved light yield and possibly improved proportionality of the mixture, the goal of better performance at the low price of NaI:Tl might be attainable by such a route. Within this context, we will discuss a density functional theory (DFT) based study of two prototype systems: mixed anion NaIxBr1-x and mixed cation NaxK1-xI. Results obtained from these two prototype candidates will lead to further targeted theoretical and experimental search and discovery of new scintillator hosts.

InSe: a two-dimensional material with strong interlayer coupling

Atomically thin, two-dimensional (2D) indium selenide (InSe) has attracted considerable attention due to the large tunability in the band gap (from 1.4 to 2.6 eV) and high carrier mobility. The intriguingly high dependence of the band gap on layer thickness may lead to novel device applications, although its origin remains poorly understood, and is generally attributed to the quantum confinement effect. In this work, we demonstrate via first-principles calculations that strong interlayer coupling may be mainly responsible for this phenomenon, especially in the fewer-layer region, and it could also be an essential factor influencing other material properties of β-InSe and γ-InSe. The existence of strong interlayer coupling manifests itself in three aspects: (i) indirect-to-direct band gap transitions with increasing layer thickness; (ii) fan-like frequency diagrams of the shear and breathing modes of few-layer flakes; and (iii) strong layer-dependent carrier mobilities. Our results indicate that multiple-layer InSe may be deserving of attention from FET-based technologies and may also be an ideal system to study interlayer coupling, possibly inherent in other 2D materials.

Emerging New Pseudobinary and Ternary Halides as Scintillators for Radiation Detection

Recently there has been a discernible shift from simple binary halide scintillators (e.g., NaI, CsI) toward host compounds that are structurally and electronically more complex. Besides SrI₂ and LaBr₃, several pseudobinary, ternary and quaternary halides have emerged as promising scintillators for radiation detection. Here, we survey our recent first-principles based computational studies of different hosts belonging to a class of mixed halides or distinct stoichiometric compounds. The mixed halides are comprised of simple binary end members, NaI or CsI, that are known scintillators. The ternary compounds belong to a family of iodides of the type AB₂I₅ or ABI₃, where the A and B cations are alkali and alkaline-earth metals, respectively. These are usually activated by Eu²⁺. We will consider Eu-dopant behavior in these compounds before delving into a set of ns² containing ternaries. They are analogous to the AB₂I₅ group of materials, except that the ns² ion is part of the crystal framework, replacing the alkali “A” ion, e.g., InBa₂I₅ or TlBa₂I₅. Interestingly, we predict Eu²⁺ activation will be rendered ineffective in these ns² compounds, caused by changes in the valence and conduction band edges. However, the possibility of fast electron capture at ns² sites and the prospect of self-activated scintillation could be interesting for detector applications.

Ramifications of codoping SrI2:Eu with isovalent and aliovalent impurities

Eu2+ doped SrI2 is an important scintillator having applications in the field of radiation detection. Codoping techniques are often useful to improve the electronic response of such insulators. Using first-principles based approach, we report on the properties of SrI2:Eu and the influence of codoping with aliovalent (Na, Cs) and isovalent (Mg, Ca, Ba, and Sn) impurities. These codopants do not preferably bind with Eu and are expected to remain as isolated impurities in the SrI2 host. As isolated defects they display amphoteric behavior having, in most cases, significant ionization energies of the donor and acceptor levels. Furthermore, the acceptor states of Na, Cs, and Mg can bind with I-vacancy forming charge compensated donor-acceptor pairs. Such pairs may also bind additional holes or electrons similar to the isolated defects. Lack of deep-to-shallow behavior upon codoping and its ramifications will be discussed.