Research

What happens to the ~1023 interacting electrons in a material after they are excited with sunlight? How can we use this rich yet complicated atomistic picture to predict the performance of real-world optoelectronic devices?

Our group studies this class of problems from a parameter-free, or ab initio, perspective – we develop high-performance computational methods and quantum many-body formalisms to understand how real materials react upon electric and optical excitations. We are interested in studying fundamental aspects of these excitations – their lifetimes, dynamics, and stability/binding energies – and how they can be engineered in novel materials, such as nanostructured and low-dimensional systems. Our ultimate goal is to use insights from our atomistic calculations to rationally design new materials with applications in energy research, renewables, and quantum technologies.

Below are some recent research interests in these areas.

Electronic and optical properties of twisted 2D materials

When a pair of 2D materials is vertically stacked with a twist angle, a moiré pattern emerges – a repeating motif with a larger length scale. Unusual quantum phases, such as superconductivity and magnetism, and technologically useful optical properties for quantum technologies, such as single-photon emission behavior, can be engineered by simply twisting two otherwise trivial monolayers. Such dramatic effects arise from the correlations when electrons localize along the moiré pattern. Our group aims at understanding such properties atomistically and search for new twisted materials with properties that are unheard of.

Relevant publications:

  • O. Karni*, E. Barré*, V. Pareek*, J. D. Georgaras*, M. K. L. Man*, C. Sahoo*, D. R. Bacon, X. Zhu, H. B. Ribeiro, A. L. O’Beirne, J. Hu, A. Al-Mahboob, M. M. M. Abdelrasoul, N. S. Chan, A. Karmakar, A. J. Winchester, B. Kim, K. Watanabe, T. Taniguchi, K. Barmak, J. Madéo, F. H. da Jornada, T. F. Heinz, K. M. Dani, Moiré-localized interlayer exciton wavefunctions captured by imaging its electron and hole constituents, arXiv:2108.01933 (2021). [Accepted, Nature]

Exciton dynamics, dissociation, and interaction

See PRL 119, 267401 (2017). Image by Florian Brown-Altvater.

While available first-principles quantum many-body formalisms allow us to accurately predict the absorption spectrum of a wide variety of functional materials, we have limited predictive theoretical and computational tools to quantify how these excitons will subsequently diffuse, recombine, or break down into free carriers. This represents a valuable research opportunity, as these processes involving dynamics of excitons are central to the design of next-generation solar-cell devices and systems for artificial photosynthesis. Our group is actively working in these problems by developing new theories based on interacting Green’s-function formalisms and high-performance computer codes.

We have recently shown how to understand the phenomenon of singlet fission in organic crystals – wherein an initial exciton of singlet spin can decay into two lower-energy excitons – a process that has been proposed as a route to design solar cells with efficiencies higher than the Shockley–Queisser limit of ~34%. Our calculations show the microscopic origin behind singlet-fission in organic crystals such as pentacene, and simple paradigms such as intra- and intercell symmetry can help in the design of new singlet-fission materials. We are currently using our unique expertise to study a range of phenomena involving the dynamics of excitons in real materials, from their diffusivity to their breakdown into free carriers.

Related publication:

  • A. Altman, S. Refaely-Abramson, F. H. da Jornada, Identifying Hidden Intracell Symmetries in Molecular Crystals and their Impact for Multiexciton Generation, J. Phys. Chem. Lett. 13, 747 (2022).
  • S. Refaely-Abramson★, F. H. da Jornada★, S. G. Louie, J. B. Neaton, S. G. Louie, Origins of singlet fission in solid pentacene from an ab initio Green’s-function approach. Physical Review Letters 119, 267401 (2017). [link]

Tuning many-body interactions in novel low-dimensional materials via proximity effects

See Nano Lett. 17, 4706 (2017).

One of the hallmarks of low-dimensional systems, such as monolayer transition metal dichalcogenide and phosphorene, is that the electronic screening is weak in these materials, which leads to enhanced many-electron interactions, and for instance, strongly bound excitons. Our group, among others, have realized that this electronic screening can be dramatically modified by changing the environment around the material, such as by encapsulating the low-dimensional material around high-κ dielectrics. We have shown that this principle can be exploited to create novel devices, such as a rectifier heterostructure from a single quasi-2D material by utilizing two different supporting substrates.

Our group has pioneered new computational approaches to efficiently understand the role of external substrates in the electronic and optical properties of novel materials. More recently, we are also looking into other ways of how proximity effects, such as twisting and stacking, can lead to novel excitation properties in low-dimensional materials.

Related publications:

  • Z.-F. Liu, F. H. da Jornada, S. G. Louie, J. B. Neaton, Speeding up GW-Based Energy Level Alignment Calculations for Molecule-Metal Interfaces Using a Substrate Screening Approach. Journal of Chemical Theory and Computation 15, 7 (2019). [link]
  • I. Utama, H. Kleemann, W. Zhao, C. S. Ong, F. H. da Jornada, D. Y. Qiu, H. Cai, H. Li, R. Kou, S. Zhao, S. Wang, K. Watanabe, T. Taniguchi, S. Tongay, A. Zettl, S. G. Louie, F. Wang, Dielectric-Defined Lateral Heterojunction in a Monolayer Semiconductor. Nature Electronics 2, 60 (2019). [link]
  • D. Y. Qiu, F. H. da Jornada, S. G. Louie, Environmental Screening Effects in 2D Materials: Renormalization of the Bandgap, Electronic Structure, and Optical Spectra of Few-Layer Black Phosphorus. Nano Letters 17, 4706 (2017). [link]
  • F. H. da Jornada★, D. Y. Qiu★, S. G. Louie, Nonuniform sampling schemes of the Brillouin zone for many-electron perturbation-theory calculations in reduced dimensionality. Physical Review B 95, 035109 (2017). [link]
  • A. J. Bradley★, M. M. Ugeda★, F. H. da Jornada★, D. Y. Qiu, W. Ruan, Y. Zhang, S. Wickenburg, A. Riss, J. Lu, S.-W. Mo, Z. Hussain, Z.-X. Shen, S. G. Louie, M. F. Crommie, Probing the Role of Interlayer Coupling and Coulomb Interactions on Electronic Structure in Few-Layer MoSe2 Nanostructures. Nano Letters 15, 2594 (2015). [link]
  • M. M. Ugeda, A. J. Bradley, S.-F. Shi, F. H. da Jornada, Y. Zhang, D. Y. Qiu, W. Ruan, S.-K. Mo, Z. Hussain, Z.-X. Shen, F. Wang, S. G Louie, M. F. Crommie, Giant bandgap renormalization and excitonic effects in a monolayer transition metal dichalcogenide semiconductor. Nature Materials 13, 1091 (2014). [link]

Beyond weakly interacting quasiparticles and excitons

Higher-order multiparticle excitations such as trions and biexcitons recently gained experimental interest due to their relatively large binding energy in low-dimensional materials. Besides their fundamental relevance for the characterization of novel materials, especially under strong illumination, these excitations play an important role in determining the efficiency of exciton diffusion and dissociation. For low-dimensional materials, in particular, multiparticle excitations such as biexcitons can also be exploited for other interesting application in quantum information and single-photon emitters.

Our group is marrying techniques from quantum field theory with massively parallel computer calculations to study these excitations without fitting parameters from experiments. We seek to understand the microscopic origin of these excitations and how they can be tuned for novel applications.

Relevant publications:

  • F. H. da Jornada, A. Cepellotti, S. G. Louie, First-principles Green’s function formalism for multiparticle excitations in solids. To be submitted.