News Archive

2022

• Prashun visits NUS, NTU, and A*STAR in Singapore
  Prashun visited collaborator Pieremanuele Canepa (NUS) and gave departmental seminars at NUS, NTU, and A*STAR during his week-long trip to Singapore in Dec 2022. Picture: graduate students and postdocs from Kedar Hippalgaonkar’s research group at NTU.

• New paper: Upper-bound Energy Minimization to Search for Stable Functional Materials with Graph Neural Networks
  How do we predict stable structures from unrelaxed geometries using GNN? We propose an upper-bound energy minimization approach in our study funded by ARPA-E DIFFERENTIATE program. This study was in collaboration with Jeff Law and Peter St. John at NREL. Paper published as a gold open access article in JACS Au. 

• Announcement: Prashun receives the 2022 Young Investigator Award from the International Thermoelectric Society
  From ITS website: “The Young Investigator Award for Excellence in Research in Thermoelectrics was initiated in 2007. The Award goes to a young investigator holding a researcher/engineer position in either: academia, industry, government lab or scientific institute… This award is to be given to a young researcher who has exhibited a record of excellent original work and significant results in the field of thermoelectrics…”. Award Announcement

• New paper: Local Chemical Origin of Ferroelectric Behavior in Wurtzite Nitrides
  (Al,Sc)N is the first known wurtzite ferroelectric. We show that ferroelectric behavior in wurtzite nitrides has local chemical origin rather than extended structural origin. The coercive field decreases with Sc content due to the more ionic Sc-N bonds that locally drives the AlN structure closer to the layered hexagonal phase – the transition state along the switching pathway. This work was done in collaboration with experimental partners at NREL and Colorado School of Mines. Paper published in Journal of Materials Chemistry C (open access).

• New paper: Y₂Te₃: A New n-type Thermoelectric Material
  We introducing rocksalt-like, ordered vacancy Y₂Te₃ – a new thermoelectric materials with low lattice thermal conductivity, high valley degeneracy, and n-type doping. This work was done in collaboration with Jeff Snyder’s group at the Northwestern University, and is published in ACS Applied Materials & Interfaces.

• New paper: Role of Electronic Passivation in Stabilizing the Li-Li_xPO_yN_z Solid-Electrolyte Interphase
  In this study with Pieremanuele Canepa’s group at NUS (Singapore), we used defect calculations to understand the role of electronic passivation in stabilizing the Li-LiPON solid-electrolyte interphase (SEI). We find that the stoichiometrically-abundant SEI phase, Li₂O, which accumulates adjacent to the Li anode surface, is an electronic insulator. The Li₂O layer provides the electronic insulation that prevents further decomposition of LiPON, after a self-limiting thin SEI layer is formed. Published in PRX Energy (open access).

• New paper: Controlling Defects to Achieve Reproducibly High Ionic Conductivity in Na₃SbS₄ Solid Electrolyte
  In this joint study with Saneyuki Ohno at Kyushu University (Japan), we show that the ionic conductivity depends on the “thermodynamic state” of the solid electrolyte (or any material, for that matter) due to the inherent dependence of the defect concentrations on the elemental chemical potentials. Here, the thermodynamic state is set by the chemical potentials of the constituent elements within the phase stability region. We also show that the “state” can be carefully controlled by pinning the growth conditions to the corners of the phase stability region where the degrees of freedom (Gibbs phase rule) is reduced to zero. In case of Na₃SbS₄, this is achieved in the three-phase equilibrium regions. Preprint on ChemRxiv and published in Chemistry of Materials.

• Presentation: Discovery of Low-temperature Thermoelectric Materials
  Undergraduate researcher, Adam Carranco, worked with our group as part of the year-long Mines Undergraduate Research Fellowship (MURF) program. His work focused on the computational discovery of new low-temperature thermoelectric materials for cooling applications. He presented his work at the campus-wide MURF symposium held in April 2022. Adam is headed to NREL this summer to participate in the prestigious SULI internship program!

• Editorial: Emerging Earth-abundant Solar Absorbers
  In this ACS Energy Letters editorial, we highlight some recent work on emerging earth-abundant solar absorbers, including those that were presented at the European MRS Fall 2022 – Symposium F.  The editorial piece is co-authored by the symposium organizers, Byungha Shin (KAIST, Korea), Robert Hoye (Imperial College, London), and Robert Palgrave (UCL, London).

• Review paper: Designing Defects and Diffusion through Substitutions in Metal Halide Solid Electrolytes
  We review the effects of aliovalent and isovalent substitutions on the ion transport, electrochemical stability, and environmental tolerance of Li₃MX₆ halide solid electrolytes. As an Editor’s choice article, it is available as open access in the J. Electrochemical Society.

• New paper: Computational Design of Thermoelectric Alloys Through Optimization of Transport and Dopability
  We develop a computational workflow to optimize the electronic, thermal, and doping properties of thermoelectric alloys. We demonstrate this framework to optimize (Ba,Sr)₂CdP₂ Zintl phase alloys.  Experiments by Svilen Bobev‘s team at the University of Delaware. Preprint on ChemRxiv and published in Materials Horizons.

2021

• New paper: Predicting Energy and Stability of Known and Hypothetical Crystal Structures Using Graph Neural Network
  Discovery of new materials in unexplored chemical spaces necessitates calculating total energy quickly and with sufficient accuracy. ML models that provide such a capability for both ground-state (GS) and higher-energy (HE) structures are instrumental in accelerating the screening for new materials over vast chemical spaces. We developed a unique graph neural network model to accurately predict the total energy of both GS and HE hypothetical structures. This work is a collaboration with NREL. Preprint on ChemRxiv and published in Patterns. Datasets, codes, and pre-trained models on GitHub.

 

• New paper: Defect Chemistry and Doping of BiCuSeO
  We explore the possibility of n-type doping of BiCuSeO, an otherwise well-known p-type mid-temperature thermoelectric material. We find that BiCuSeO can be doped n-type with halogens (Cl, Br) under Cu-rich growth conditions. Preprint on ChemRxiv. and published in the Journal of Materials Chemistry A.

 

• New paper: Devil is in the Defects: Electronic Conductivity in Solid Electrolytes
  In our first collaboration with Pieremanuele Canepa (National University of Singapore), we used first-principles computations to estimate the electronic conductivity in three topical solid electrolytes. Findings suggest that native defects in sulfide electrolytes may be the source of bulk electronic conductivity. Rigorous and systematic experimental protocols should be developed to measure electronic conductivity. Preprint on ChemRxiv and published in Chemistry of Materials.

 

• New paper: Charting Lattice Thermal Conductivity for Inorganic Crystals and Discovering Rare Earth Chalcogenides for Thermoelectrics 
  In this collaboration led by Jeff Grossman’s group at MIT, graph neural networks (GNN) and random forest approaches were combined to chart the structural chemistry of lattice thermal conductivity. Using the GNN model, the lattice thermal conductivity of all ordered inorganic materials in ICSD were predicted and a new rare-earth chalcogenide thermoelectric material was discovered with zT exceeding 1.0. Published in Energy and Environmental Science.

 

2020

• Editorial: Early Career Researchers Present Their Latest Work at the Virtual Conference on Thermoelectrics 2020
  An editorial in ACS Applied Energy Materials highlighting the key takeaways from organizing VCT2020 as well as the latest developments in the field of thermoelectrics that were presented during the three-day virtual conference in July 2020.

 

• New paper: New n-type Zintl Phases for Thermoelectrics: Discovery, Structural Characterization, and Band Engineering of the Compounds A₂CdP₂ (A = Sr, Ba, Eu)
  We report the discovery and band engineering of new Zintl phases A₂CdP₂ (A = Sr, Ba, Eu), in collaboration with Svilen Bobev‘s team at the University of Delaware. The computational work was led by Jiaxing Qu. Preprint on ChemRxiv and published in Chemistry of Materials.

 

• New paper: Doping by Design: Finding New n-type Dopable ABX₄ Phases for Thermoelectrics
  In this study led by Jiaxing Qu, we demonstrate “doping by design” by performing chemical substitutions in suitably chosen prototype structures. We report the discovery of 7 as-yet-unknown ABX₄ Zintl phases  (in the prototypical KGaSb₄ structure) that n-type dopable, a rarity among Zintl compounds. Preprint on ChemRxiv and published in the Journal of Materials Chemistry A. 

 

• Virtual Conference on Thermoelectrics 2020
  P. G. is co-organizing the first virtual conference on thermoelectrics to be held on July 21-23, 2020. VCT 2020 is an online event to highlight the work of early-career researchers in the field on thermoelectrics. This virtual conference will feature 300+ talks from US/Canada, Europe, and Asia/Australia. ACS Applied Energy Materials has partnered with us for VCT 2020.

 

• New paper: Computational Discovery of Promising New n-type Dopable ABX Zintl Thermoelectric Materials
  With under exploration comes opportunities. We use a chemical replacements in structure prototype (CRISP) approach to discover new, unreported Zintl phases for thermoelectrics. This was a collaboration with a team from Berkeley National Lab (LBL). Published in Materials Horizons. A preprint is available on ChemRxiv.

 

• Recognition: Outstanding reviewer for the Journal of Materials Chemistry A 2019
  P. G. recognized as one of the 25 outstanding reviewers for the Journal of Materials Chemistry A based on the number, timeliness, and quality of the reports completed in 2019. The Royal Society of Chemistry published a blog post and editorial highlighting the outstanding reviewers.

 

• New paper: Discovery of n-type Zintl Phases RbAlSb4, RbGaSb4, CsAlSb4, and CsGaSb4
  Looking beyond known materials, we report the discovery of n-type Zintl phases for thermoelectric applications. We using first-principles calculations to understand their defect chemistry and also report new Zintl phases for further exploration. Published in ACS Applied Energy Materials.

 

• New paper: Defect Chemistry of Disordered Solid-State Electrolyte Li₁₀GeP₂S₁₂
  We modeled the defect chemistry of a complex disordered solid-state electrolyte LGPS. Preprint on ChemRxiv. Published in the Journal of Materials Chemistry A.

 

2019

• New paper: Comment on “Understanding the Intrinsic p-Type Behavior and Phase Stability of Thermoelectric α-Mg₃Sb₂” 
  We tested and clarified some recent claims in the literature regarding Mg complexes in this material. Preprint on ChemRxiv. Published in ACS Applied Energy Materials.

 

• Featured on cover: A computational survey of semiconductors for power electronics
  Cover image of Energy and Environmental Science

 

• New paper: A computational survey of semiconductors for power electronics
  Power electronics are ubiquitous, from consumer electronics to solar/wind energy conversion to large-scale industrial motors. Si-based devices dominate the market while GaN, SiC, and  Ga₂O₃ are emerging materials but suffer from issues related to non-negligible defect densities, high synthesis costs, or poor thermal properties. In this work, we report a computational survey of semiconductors to identify novel materials for future power electronics. Preprint on ChemRxiv. Published in Energy & Environmental Science.

 

• New paper: A simple chemical guide for finding n-type dopable Zintl pnictide thermoelectric materials
  Zintl phases are among the best-performing thermoelectric materials; however, almost all known Zintl thermoelectric materials are persistently p-type because of self-doping. Interestingly, computational searches have have revealed many Zintl phases that are expected to be promising thermoelectric materials if doped n-type. There is an increasing interest in discovering novel n-type Zintl thermoelectric materials. In this work, we present a simple, yet effective, chemical guide for finding n-type dopable Zintl pnictides and perform a large-scale search to identify candidate materials. Preprint on ChemRxiv. Accepted in Journal of Materials Chemistry A.

 

• Recognition: Outstanding reviewer for the Journal of Materials Chemistry A 2018
  P. G. recognized as one of the 25 outstanding reviewers for the Journal of Materials Chemistry A based on the number, timeliness, and quality of the reports completed in 2018.  The Royal Society of Chemistry published a blog post and editorial highlighting these 25 outstanding reviewers.

 

• Conference: Twitter poster conference organized by the Royal Society of Chemistry
  P. G. participated in the virtual poster conference on Twitter organized by the Royal Society of Chemistry (RSC) on March 5, 2019. The poster describes the model for predicting anisotropic lattice thermal conductivity and the ongoing efforts to build a database (Anisotropy Atlas) of anisotropic transport properties. Download hi-res poster.

 

• New paper: rapid prediction of anisotropic lattice thermal conductivity: application to layered materials
  Thermal transport is an important consideration in many applications from thermoelectrics to optoelectronics. In anisotropic materials such as those with layered motifs, lattice thermal conductivity (k_L) can vary significantly with the direction of heat transport. We have developed a semi-empirical model to predict the direction-dependent k_L. The inputs to the model are obtained from simple DFT calculations. We demonstrate the utility of this model by performing a large-scale study of the anisotropy in k_L of 2261 layered materials. Preprint on ChemRxiv. Accepted in Chemistry of Materials.

 

 

2018

• New paper: combined experimental and computational phase boundary mapping of Co₄Sn₆Te₆
  In this paper published in the J. Materials Chemistry A, we utilize complementary experimental and theoretical tools to understand the complex phase stability and defect chemistry of the ternary Co₄Sn₆Te₆ for thermoelectric applications. Defect calculations suggest that p-type doping of Co₄Sn₆Te₆ can be readily tuned but n-type doping is expected to be challenging.

 

• New paper: large piezoelectric response of van der Waals layered solids
  Motivated by the notion that elastically soft materials could exhibit large piezoelectric responses, we performed a broad search of 869 binary and ternary van der Waals (vdW) bonded, quasi-2D ionic solids to identify novel piezoelectric materials. Our findings are reported in a paper in the J. Materials Chemistry C. In this paper, we also demonstrate that the large piezoelectric responses in these materials are  coupled to axial or shear deformations of the inter-layer, weak vdW bonds.

 

• Invited talk at the International Conference on Ternary and Multinary Compounds (ICTMC) in Boulder, CO
  P. G. gave an invited talk on structural and chemical features promoting defect tolerance in the semiconductors. Slides can be found here.

 

• New paper: structural and chemical features giving rise to defect tolerance of semiconductors
  In this paper published in Chemistry of Materials, we demonstrate the role of orbital chemistry and crystal structure in defect tolerance of semiconductors. We outline chemical and structural features, in addition to the heuristic guidelines (partially-oxidized cation, large dielectric constant, low effective masses), that give rise to defect tolerance in binary semiconductors.

 

• New paper: investigation of n-type doping strategies for Mg₃Sb₂
  In our paper in JMCA, we use first-principles defect calculations to computationally investigate various n-type doping strategies for the thermoelectric material Mg₃Sb₂. In addition to the successful anion-site doping with Se and Te, we explore cation-site and interstitial doping. We identify La, Sc, and Y as effective cation-site n-type dopants; calculated electron concentrations exceed those achieved with Se and Te doping. Interstitial doping is largely ineffective for n-type doping of Mg₃Sb₂.

 

• New paper: identification of ionic layered materials and anisotropy in elastic properties of layered materials
  Well-known layered materials such as graphite and transition metal dichalcogenides have weak van der Waals (vdW) interaction between the layers. There exists another class of layered materials with stronger interlayer bonding, termed as ionic layered materials. Examples include 1-2-2 Zintl thermoelectrics (e.g. CaZn₂Sb₂) and superconducting La₂CuO₄. In our work published in JMCA, we identify 1500+ ternary ionic layered materials  from the ICSD and conduct a broad assessment of their elastic anisotropy; we find them to be more isotropic compared to vdW layered materials.

 

• Talks at the European Materials Research Society (eMRS) and International Conference on Thermoelectrics (ICT)
  P. G. gave a talk at eMRS in Strasbourg, France, on the underlying structural and chemical features giving rise to defect tolerance of semiconductors. At the ICT in Caen, France, P. G. shared new findings from a computational investigation of the various n-type doping strategies for the thermoelectric materials Mg₃Sb₂.

 

• Invited talk at the Kyushu University in Fukuoka, Japan
  P. G. gave an invited talk on computationally guided materials discovery at the Kyushu University in Fukuoka, Japan on Feb 1, 2018. The theory symposium was held in conjunction with the I²CNER (International Institute of Carbon-Neutral Energy Research) annual symposium.

 

• Invited talk at Panasonic Research and Development, Osaka, Japan
  P. G. shared his research on thermoelectric materials with research and development team at the Panasonic headquarters in Osaka, Japan.

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2017

• New paper: phase boundary mapping to obtain n-type Mg₃Sb₂-based thermoelectrics
  Recent reports of high figure of merit in n-type Mg₃Sb₂ has renewed interest in this material, which has been historically synthesized as a p-type material. In our recent article published in Joule, we explore the defect chemistry of Mg₃Sb₂ both theoretically and experimentally to explain why there are two distinct thermodynamic states of Mg₃Sb₂ and why only one of these states allows n-type doping. This work highlights the importance of exploring the multiple thermodynamic states via phase boundary mapping in a nominally single-phase semiconductor.
  Preview of this work: A Hidden Dimension to Explore New Thermoelectrics

 

• New paper: thermoelectrics review published in Nature Reviews Materials
  Our review article on computationally guided discovery of new thermoelectric materials was published in Nature Reviews Materials! The discovery of new thermoelectric materials is challenging owing to the diversity of the chemical space and to the serial nature of experimental work. This Review highlights the recent progress in computationally guided discovery of thermoelectric materials and identifies the key outstanding challenges.
  Cover page: our review article was featured on the cover page of Nature Reviews Materials!

 

• Invited talk and tutorial at the International Materials Research Congress in Cancun, Mexico.
  P. G. gave an invited talk titled “Computationally Guided Discovery of Chalcogenide Thermoelectrics” at the International Materials Research Congress in Cancun, Mexico. P.G., along with Stephan Lany (NREL) and Rafael Jaramillo (MIT) also conducted a tutorial on “Chalcogenide Electronic Materials Research” at IMRC on August 20, 2017. The tutorial slides can be downloaded at this link.

 

• New paper: suppression of Lorenz number via intervalley scattering
  Traditionally, methods for thermal conductivity reduction in thermoelectric materials have focused on the lattice component. However, in an optimized thermoelectric material, the electronic component is also a significant contribution. We demonstrated that the Lorenz number (L) can be suppressed to a fraction of the Sommerfeld value by using energy filtering, which can be realized through intervalley scattering. In real materials, the shape of the density of states (DOS) provides a signature for such behavior. Using a descriptor of the DOS shape, we performed a high-throughput search of ~1300 materials from the TEDesignLab to identify candidates, where potentially low L can be realized. See article published in J. Materials Chemistry A.

 

• New paper: defect-tolerant materials exhibiting carrier lifetimes of the order of nanoseconds
  In collaboration with Tonio Buonassisi’s group at MIT,  we tested the efficacy of our previously-proposed screening criteria for identifying “defect-tolerant” semiconductors. Six materials were reported with minority carrier lifetimes exceeding 1 ns. Through a combined experimental and theoretical approach, we were able to understand the nuances of the defect chemistry that prevents these materials from achieving lifetimes in the order of ms. This work was published in Chemistry of Materials.

 

• Talk on n-type Zintl thermoelectrics at the European Materials Research Society in Strasbourg, France.
  P. G. gave a talk titled “Quest for n-type Zintl Thermoelectrics” in Symposium H at the European Materials Research Society (E-MRS) in beautiful Strasbourg, France. The results presented in this talk is the subject of a forthcoming paper where we have developed a simple chemical descriptor to identify new n-type Zintl pnictides for thermoelectric applications.

 

• New paper: high thermoelectric performance in n-type Zintls
  An overwhelming majority of Zintl pnictide thermoelectrics are hole-doped i.e. p-type materials. Our predictions suggest that n-type Zintl pnictides have the potential to exhibit high thermoelectric performance. We have recently reported two new n-type Zintl pnictides, KAlSb₄ and KGaSb₄, with zT ~ 1. First-principles calculations reveal the unique defect chemistry in these materials that allow them to be n-type dopable. These findings were published in the J. Materials Chemistry A and Chemistry of Materials.