publications

2024

1. Small

   Palladium-Doped Cs2AgBiBr6 with 1300 nm Near-Infrared Photoresponse

   Hongwei Lei, Utkarsh Singh, Fuxiang Ji, Tinghao Lin, and 12 more authors

   Small, 2024

   Abs DOI

   Lead-free halide double perovskite (HDP) Cs2AgBiBr6 has set a benchmark for research in HDP photoelectric applications due to its attractive optoelectronic properties. However, its narrow absorption range is a key limitation of this material. Herein, a novel dopant, palladium (Pd), is doped into Cs2AgBiBr6 and significantly extends the absorption to ≈1400 nm. Pd2+ ions are partially doped in the host lattice, most probably replacing Ag atoms and introducing a sub-bandgap state within the host bandgap, as indicated by the combination of spectroscopical measurements and theoretical calculations. Importantly, this sub-bandgap state extends the photoresponse of Cs2AgBiBr6 up to the NIR-II region of 1300 nm, setting a new record for HDPs. This work demonstrates a novel and efficient dopant for HDPs and highlights the effectiveness of employing a sub-bandgap to broaden the absorption of HDPs, shedding new light on tailoring large bandgap HDPs for NIR optoelectronic applications.

2. JPCC

   Understanding Antiferromagnetic Coupling in Lead-Free Halide Double Perovskite Semiconductors

   Kunpot Mopoung, Weihua Ning, Muyi Zhang, Fuxiang Ji, and 11 more authors

   The Journal of Physical Chemistry C, 2024

   DOI
3. Nat. Photonics

   Bright and stable near-infrared lead-free perovskite light-emitting diodes

   Fanglong Yuan, Giulia Folpini, Tianjun Liu, Utkarsh Singh, and 8 more authors

   Nature Photonics, Feb 2024

   Abs DOI

   Long-wavelength near-infrared light-emitting diodes (NIR LEDs) with peak emission wavelengths beyond 900\thinspacenm are of critical importance for various applications including night vision, biomedical imaging, sensing and optical communications. However, the low radiance and poor operational stability of state-of-the-art long-wavelength NIR LEDs based on soft materials remain the most critical factors limiting their practical applications. Here we develop NIR LEDs emitting beyond 900\thinspacenm with improved performance through the rational manipulation of p doping in all-inorganic tin perovskites (CsSnI3) by retarding and controlling the crystallization process of perovskite precursors in tin-rich conditions. The resulting NIR LEDs exhibit a peak emission wavelength at 948\thinspacenm, high radiance of 226\thinspaceW\thinspacesr−1\thinspacem−2 and long operational half-lifetime of 39.5\thinspaceh at a high constant current density of 100\thinspacemA\thinspacecm−2. Our demonstration of efficient and stable NIR LEDs operating at high current densities may also open up new opportunities towards electrically pumped lasers.

2023

1. PhysRev

   Exploring magnetism of lead-free halide double perovskites: A high-throughput first-principles study

   Utkarsh Singh, Johan Klarbring, Igor A. Abrikosov, and Sergei I. Simak

   Phys. Rev. Mater., Nov 2023

   Abs DOI

   We have performed a comprehensive, first-principles high-throughput study of the magnetic properties of halide double perovskites, Cs2⁢BB′⁢Cl6, with magnetic ions occupying one or both B and B′ sites. Our findings indicate a general tendency for these materials to exhibit antiferromagnetic ordering with low Néel temperatures. At the same time, we reveal a few potential candidates that predicted to be ferromagnetic with relatively high Curie temperatures. Achieving ferromagnetic coupling might be feasible via simultaneously alloying at B and B′ sites with magnetic 3d and nonmagnetic 5d ions. With this approach, we discover that Cs2⁢HgCrCl6, Cs2⁢AgNiCl6, and Cs2⁢AuNiCl6 have high Curie temperatures relative to their peers, with the latter two exhibiting half metallic behavior. Further, this study illuminates the underpinning mechanism of magnetic exchange interactions in halide double perovskites, enabling a deeper understanding of their magnetic behavior. Our findings, especially the discovery of the compounds with robust half metallic properties and high Curie temperatures, hold promise for potential applications in the field of spintronics.

2. PhysRev

   Electronic structure of the magnetic halide double perovskites Cs2Ag(Na/Fe)Cl6 from first principles

   Johan Klarbring, Utkarsh Singh, Sergei I. Simak, and Igor A. Abrikosov

   Phys. Rev. Mater., Apr 2023

   Abs DOI

   A family of magnetic halide double perovskites (HDPs) have recently attracted attention due to their potential to broaden application areas of halide double perovskites into, e.g., spintronics. Up to date the theoretical modeling of these systems have relied on primitive approximations to the density functional theory (DFT). In this paper, we study structural, electronic and magnetic properties of the Fe3+-containing HDPs Cs2⁢AgFeCl6 and Cs2⁢NaFeCl6 using a combination of more advanced DFT-based methods, including DFT+𝑈, hybrid-DFT, and treatments of various magnetic states. We examine the effect of varying the effective Hubbard parameter, 𝑈eff, in DFT+𝑈 and the mixing-parameter, 𝛼, in hybrid DFT on the electronic structure and structural properties. Our results reveal a set of localized Fe⁡(𝑑) states that are highly sensitive to these parameters. Cs2⁢AgFeCl6 and Cs2⁢NaFeCl6 are both antiferromagnets with Neél temperatures well below room temperature and are thus in their paramagnetic (PM) state at the external conditions relevant to most applications. Therefore, we have examined the effect of disordered magnetism on the electronic structure of these systems and find that while Cs2⁢NaFeCl6 is largely unaffected, Cs2⁢AgFeCl6 shows significant renormalization of its electronic band structure.

2022

1. PhysRev

   Low lattice thermal conductivity in alkali metal based Heusler alloys

   Utkarsh Singh, Sapna Singh, Mohd Zeeshan, Jeroen van den Brink, and 1 more author

   Phys. Rev. Mater., Dec 2022

   Abs DOI

   On the basis of purely their electronic properties, semiconducting alkali and alkaline earth metal based Heusler alloys have been identified as potentially promising thermoelectrics. Their thermal transport properties are decisive and help to further gauge this potential. This motivates us to systematically investigate lattice properties of three eight valence electron Heusler alloys, namely, Li2⁢NaSb, Na2⁢KSb, and K2⁢CsSb, using first-principles approaches. On the basis of both qualitative and quantitative descriptors (assessing covalent bonding, the atomic displacement parameter, the phonon density of states, the Grüneisen parameter, the phonon group velocity, and the phonon lifetime) we analyze in detail the lattice degrees of freedom and their contribution to thermal transport. Out of the three materials, Na2⁢KSb and K2⁢CsSb exhibit surprisingly low lattice thermal conductivity, which we can attribute to the anharmonic rattling of the loosely bound alkali atom in the crystal lattice. K2⁢CsSb has a lattice thermal conductivity two times lower than that of SnSe, a material with a record figure of merit.

2019

1. IOP

   First-principles investigations of orthorhombic-cubic phase transition and its effect on thermoelectric properties in cobalt-based ternary alloys

   Sapna Singh, Mohd Zeeshan, Utkarsh Singh, Jeroen van den Brink, and 1 more author

   Journal of Physics: Condensed Matter, Nov 2019

   Abs DOI

   We screened six cobalt-based 18-VEC systems CoVSi, CoNbSi, CoTaSi (Si-group) and CoVGe, CoNbGe, CoTaGe (Ge-group) by the first-principles approach, with the motivation of stabilizing these orthorhombic phases into the cubic symmetry—favorable for thermoelectrics. Remarkably, it was found that the Ge-group is energetically more favorable in the cubic symmetry than the hitherto orthorhombic phase. We account the cubic ground state of the Si-group to the interplay of internal pressure and covalent interactions. The principle of reducing covalent interactions will provide insight and could be vital in speeding the search of missing cubic half-Heusler alloys. Meanwhile, the calculated transport properties of all the systems on p -type doping, except CoVSi, are more promising than the well-known CoTiSb. We also provide conservative estimates of the figure of merit, exceeding the CoTiSb. Based on our findings, we suggest possible new phases of ternary compounds for thermoelectric applications.