We managed to show the very first tracking of excitons at conditions below sunlight; first as a proof of concept on photovoltaic materials, showing the full absence of non-linear annihilation; next a real application on a light-harvesting membrane. This achievement is crucial for any success in this project. Brinatti Vazquez et al., Structured Excitation Energy Transfer: Tracking Exciton Diffusion below Sunlight Intensity", ACS Photonics 11 (3), 1318-1326 (2024).
Using Photoelectrochemical Two-Dimensional Electronic Spectroscopy we recorded the first 2D spectra on plant Photosystem I, providing a 2D-map of the first ps, López-Ortiz et al., Photoelectrochemical Two-Dimensional Electronic Spectroscopy (PEC2DES) of Photosystem I: Charge Separation Dynamics Hidden in a Multichromophoric Landscape, ACS Appl. Mater. Interfaces, 16 (33), 43451 – 43461 (2024).
The photocurrent detected 2DES turned out to compensate terms in its nonlinear response, moreover multi-chromophoric ground state bleach dominated over stimulated emissions. We explained these unexpected contrast issues and proposed effected compressed sensing for effective data collection. Bolzonello et al., Nonlinear Optical Spectroscopy of Molecular Assemblies: What Is Gained and Lost in Action Detection?', J. Phys. Chem. Lett. 14 (50), 11438 – 11446 (2023); Bolzonello et al., Fisher information for smart sampling in time-domain spectroscopy, J. Chem. Phys., 160, 214110 (2024).
To pursue the photocurrent spectroscopy on 2D -material substrate, we engineered dual action spectroscopy, a photocurrent- and luminescence-detected Fourier-transform excitation spectroscopy scheme, to microscopically map the energy landscape of WSe2. As a test, we addressed WSe2 for which the bright excitons naturally dominate the luminescence response, while dark excitons dominate the current response. This groundwork provides the basis for a new, current-detected approach to study the dynamics of dark exciton states across different materials incl. biomembranes. NanoLett. 25, 7658−7664 (2025); Nature Commun. 16:5184 (2025).
Interestingly, beyond the proposed research, using a novel SPAD camera we succeeded in superior spatio-temporal exciton tracking, at even lower fluence. An important achievement for further success of this project. Diana Dall’Aglio et al., Spatio-temporal exciton tracking with a SPAD camera, ACS Photonics, 12, 1291−1299 (2025). The SPAD camera allows to combine dedicated encoded excitation patterns, with super-resolution and ps time resolution. We are pursuing this direction.
In 2024 we could for the first time find an optimum in the exciton diffusion length of LH2 monolayers (~45 nm) at a critical concentration, not fully packed compared to crystal layer, while with enough proximity to have lifetime reduced from 1 ns to 300 ps. We believe these conditions are very close to the packing in the natural membrane. Moreover, the transport is recorded at sunlight illumination conditions. Still, AFM data so far revealed rather disordered layers, also with the LH2 random top-down and down-top. In collaboration with Sheffield group, but also in house Liguori group we are now (June 2025) working on higher sample control, with defined orientation and packing order.
