Most recent publications
 | Viet-Anh Ha, Feliciano Giustino High-throughput screening of 2D materials identifies p-type monolayer WS2 as potential ultra-high mobility semiconductor Journal Article In: npj Computational Materials, vol. 10, iss. 1, pp. 229, 2024. @article{Ha2024,2D semiconductors offer a promising pathway to replace silicon in next-generation electronics. Among their many advantages, 2D materials possess atomically-sharp surfaces and enable scaling the channel thickness down to the monolayer limit. However, these materials exhibit comparatively lower charge carrier mobility and higher contact resistance than 3D semiconductors, making it challenging to realize high-performance devices at scale. In this work, we search for high-mobility 2D materials by combining a high-throughput screening strategy with state-of-the-art calculations based on the ab initio Boltzmann transport equation. Our analysis singles out a known transition metal dichalcogenide, monolayer WS2, as the most promising 2D semiconductor, with the potential to reach ultra-high room-temperature hole mobilities in excess of 1300 cm2/Vs should Ohmic contacts and low defect densities be achieved. Our work also highlights the importance of performing full-blown ab initio transport calculations to achieve predictive accuracy, including spin–orbital couplings, quasiparticle corrections, dipole and quadrupole long-range electron–phonon interactions, as well as scattering by point defects and extended defects. |
 | Bruno Cucco, Joshua Leveillee, Viet-Anh Ha, Jacky Even, Mikaël Kepenekian, Feliciano Giustino, George Volonakis Intrinsic Limits of Charge Carrier Mobilities in Layered Halide Perovskites Journal Article In: PRX Energy, vol. 3, iss. 2, pp. 023012, 2024. @article{Cucco2024,Layered halide perovskites have emerged as potential alternatives to three-dimensional (3D) halide perovskites due to their improved stability and larger material phase space, allowing fine tuning of structural, electronic, and optical properties. However, their charge carrier mobilities are significantly smaller than those of 3D halide perovskites, which has a considerable impact on their application in optoelectronic devices. Here, we employ state-of-the-art ab initio approaches to unveil the electron-phonon mechanisms responsible for the diminished transport properties of layered halide perovskites. Starting from a prototypical 𝐴𝑀𝑋3 halide perovskite, we model the case of 𝑛=1 and 𝑛=2 layered structures and compare their electronic and transport properties to the 3D reference. The electronic and phononic properties are investigated within density functional theory (DFT) and density functional perturbation theory (DFPT), while transport properties are obtained via the ab initio Boltzmann transport equation. The vibrational modes contributing to charge carrier scattering are investigated and associated with polar-phonon scattering mechanisms arising from the long-range Fröhlich coupling and deformation-potential scattering processes. Our investigation reveals that the lower mobilities in layered systems primarily originate from the increased electronic density of states at the vicinity of the band edges, while the electron-phonon coupling strength remains similar. Such an increase is caused by the dimensionality reduction and the break in octahedra connectivity along the stacking direction. Our findings provide a fundamental understanding of the electron-phonon coupling mechanisms in layered perovskites and highlight the intrinsic limitations of the charge carrier transport in these materials. |
 | Jon Lafuente-Bartolome, Chao Lian, Feliciano Giustino Topological polarons in halide perovskites Journal Article In: Proceedings of the National Academy of Sciences, vol. 121, iss. 21, pp. e2318151121, 2024. @article{Lafuente-Bartolome2024,Halide perovskites emerged as a revolutionary family of high-quality semiconductors for solar energy harvesting and energy-efficient lighting. There is mounting evidence that the exceptional optoelectronic properties of these materials could stem from unconventional electron–phonon couplings, and it has been suggested that the formation of polarons and self-trapped excitons could be key to understanding such properties. By performing first-principles simulations across the length scales, here we show that halide perovskites harbor a uniquely rich variety of polaronic species, including small polarons, large polarons, and charge density waves, and we explain a variety of experimental observations. We find that these emergent quasiparticles support topologically nontrivial phonon fields with quantized topological charge, making them nonmagnetic analog of the helical Bloch points found in magnetic skyrmion lattices. |
 | Sabyasachi Tiwari, Emmanouil Kioupakis, José Menendez, Feliciano Giustino Unified theory of optical absorption and luminescence including both direct and phonon-assisted processes Journal Article In: Physical Review B, vol. 109, iss. 19, pp. 195127, 2024. @article{Tiwari2024,Most semiconductors and insulators exhibit indirect band gaps, but no theory is currently available to calculate light absorption and emission spectra of these systems over a wide spectral range with predictive accuracy. The standard textbook theory of indirect absorption becomes ill-defined and yields infinite absorption strength when a photon can promote both direct and phonon-assisted transitions. As a result, state-of-the-art ab initio methods for calculating optical spectra of solids are unable to describe direct and phonon-assisted transitions on the same footing. Here, we develop a rigorous first-principles approach that overcomes this limitation by including electron-phonon correlations via many-body quasidegenerate perturbation theory. Our present formalism enables accurate calculations of the optical spectra of materials with direct, indirect, and quasidirect band gaps, and reduces to the standard theories of direct-only absorption and indirect-only absorption in the appropriate limits. We demonstrate this methodology by investigating the optical absorption spectra of silicon, germanium, gallium arsenide, and diamond. In all cases, we obtain spectra in excellent agreement with experiments. As a more ambitious test, we investigate the temperature-dependent photoluminescence of germanium, and we obtain quantitative agreement with experiments. |
 | Zhenbang Dai, Chao Lian, Jon Lafuente-Bartolome, Feliciano Giustino Excitonic polarons and self-trapped excitons from first-principles exciton-phonon couplings Journal Article In: Physical Review Letters, vol. 132, iss. 3, pp. 036902, 2024. @article{Dai2024b,Excitons consist of electrons and holes held together by their attractive Coulomb interaction. Although excitons are neutral excitations, spatial fluctuations in their charge density couple with the ions of the crystal lattice. This coupling can lower the exciton energy and lead to the formation of a localized excitonic polaron or even a self-trapped exciton in the presence of strong exciton-phonon interactions. Here, we develop a theoretical and computational approach to compute excitonic polarons and self-trapped excitons from first principles. Our methodology combines the many-body Bethe-Salpeter approach with density-functional perturbation theory and does not require explicit supercell calculations. As a proof of concept, we demonstrate our method for a compound of the halide perovskite family. |
 | Yanxing Li, Fan Zhang, Viet-Anh Ha, Yu-Chuan Lin, Chengye Dong, Qiang Gao, Zhida Liu, Xiaohui Liu, Sae Hee Ryu, Hyunsue Kim, Chris Jozwiak, Aaron Bostwick, Kenji Watanabe, Takashi Taniguchi, Bishoy Kousa, Xiaoqin Li, Eli Rotenberg, Eslam Khalaf, Joshua A. Robinson, Feliciano Giustino & Chih-Kang Shih Tuning commensurability in twisted van der Waals bilayers Journal Article In: Nature, vol. 625, pp. 494–499, 2024. @article{Li2024,Moiré superlattices based on van der Waals bilayers created at small twist angles lead to a long wavelength pattern with approximate translational symmetry. At large twist angles (θt), moiré patterns are, in general, incommensurate except for a few discrete angles. Here we show that large-angle twisted bilayers offer distinctly different platforms. More specifically, by using twisted tungsten diselenide bilayers, we create the incommensurate dodecagon quasicrystals at θt = 30° and the commensurate moiré crystals at θt = 21.8° and 38.2°. Valley-resolved scanning tunnelling spectroscopy shows disparate behaviours between moiré crystals (with translational symmetry) and quasicrystals (with broken translational symmetry). In particular, the K valley shows rich electronic structures exemplified by the formation of mini-gaps near the valence band maximum. These discoveries demonstrate that bilayers with large twist angles offer a design platform to explore moiré physics beyond those formed with small twist angles. |