arXiv subjects

Mauricio E. Calvo

Publications and source records attributed to Mauricio E. Calvo.

5 recordsLinked to original sources

arXiv2026

Beyond Panchromatic Absorption: Deciphering the Excited-State Maze from Light Absorption to Photocatalysis in Dye-Sensitized MOFs

Metal--organic frameworks (MOFs) are promising photocatalysts whose visible-light absorption can be extended through linker functionalization; however, a red-shifted absorption edge does not guarantee enhanced efficiency. Here, we establish a multi-spectroscopic framework to decipher the photophysical fate of photoexcited states in diazo-sensitized UiO-66 using 2D photoluminescence (PL/PLE) mapping and wavelength-resolved continuous-wave X-band photo-EPR. By correlating visible absorption with photo-EPR and PLE action spectra, we distinguish a \textit{productive red shift} from a \textit{non-productive emissive red shift}. In highly active UiO-66-Anisole (97\% activity relative to \ch{TiO2}), photo-EPR tracks the new absorption band, confirming that excitation populates a charge-transfer pathway yielding persistent, spin-separated states. Conversely, poorly active UiO-66-$β$-naphthol (4\%) exhibits an extended visible absorption tracked by PLE but not photo-EPR. This reflects excitation trapping in a localized state caused by an \textit{ortho}-$\mathrm{OH}$ group forming a rigid intramolecular hydrogen bond, which locks the keto-hydrazone tautomer and disrupts the conjugated azo bridge. A new optical overlap descriptor ($S_{\mathrm{Exc}}$) quantitatively captures this trade-off across the series. We demonstrate that photosensitizer design must suppress rigid tautomeric traps and target specific charge-transfer manifolds ($λ\le 500$~nm) rather than merely maximizing apparent panchromatic absorption breadth.

cond-mat.mtrl-sci↗

arXiv2024

Photoinduced Spin Centers in Photocatalytic Metal-Organic Framework UiO-66

Metal-Organic frameworks (MOFs) are promising candidates for advanced photocatalytically active materials. These porous crystalline compounds have large active surface areas and structural tunability and are thus highly competitive with oxides, the well-established material class for photocatalysis. However, due to their complex organic and coordination chemistry composition, photophysical mechanisms involved in the photocatalytic processes in MOFs are still not well understood. Employing electron paramagnetic resonance (EPR) spectroscopy and time-resolved photoluminescence spectroscopy (trPL), the fundamental processes of electron and hole generation are investigated, as well as capture events that lead to the formation of various radical species in UiO-66, an archetypical MOF photocatalyst. A manifold of photoinduced electron spin centers is detected, which is subsequently analyzed and identified with the help of density-functional theory (DFT) calculations. Under UV illumination, the symmetry, g-tensors and lifetimes of three distinct contributions are revealed: a surface O2-radical, a light-induced electron-hole pair, and a triplet exciton. Notably, the latter was found to emit (delayed) fluorescence. Our findings provide new insights into the photoinduced charge transfer processes, which are the basis of photocatalytic activity in UiO-66. This sets the stage for further studies on photogenerated spin centers in this and similar MOF materials.

cond-mat.mtrl-sci↗

arXiv2024

Intense and Stable Blue Light Emission from CsPbBr$_3$/Cs$_4$PbBr$_6$ Heterostructures Embedded in Transparent Nanoporous Films

Lead halide perovskite nanocrystals are attractive for light emitting devices both as electroluminescent and color converting materials, since they combine intense and narrow emissions with good charge injection and transport properties. However, most perovskite nanocrystals shine at green and red wavelengths, the observation of intense and stable blue emission still being a challenging target. In this work, we report a method to attain intense and enduring blue emission (470-480 nm), with a photoluminescence quantum yield (PLQY) of 40%, originated from very small CsPbBr$_3$ nanocrystals (diameter<3nm) formed by controllably exposing Cs$_4$PbBr$_6$ to humidity. This process is mediated by the void network of a mesoporous transparent scaffold in which the zero-dimensional (0D) Cs$_4$PbBr$_6$ lattice is embedded, which allows the fine control over water adsorption and condensation that determines the optimization of the synthetic procedure and, eventually, the nanocrystal size. By temperature dependent photoemission analysis of samples with different [CsPbBr$_3$]/[Cs$_4$PbBr$_6$] volume ratios, we show that the bright blue emission observed results from the efficient charge transfer to the CsPbBr$_3$ inclusions from the Cs$_4$PbBr$_6$ host. Our approach provides a means to attain highly efficient transparent blue light emitting films that complete the palette offered by perovskite nanocrystals for lighting and display applications.

physics.app-ph↗

arXiv2023

Effect of connectivity on the carrier transport and recombination dynamics of perovskite quantum dot networks

Quantum dot (QD) solids are being widely exploited as a solution-processable technology to develop photovoltaic, light-emission, and photo-detection devices. Charge transport in these materials is the result of a compromise between confinement at the individual QD level and electronic coupling among the different nanocrystals in the ensemble. While this is commonly achieved by ligand engineering in colloidal-based systems, ligand-free QD assemblies have recently emerged as an exciting alternative where nanostructures can be directly grown into porous matrices with optical quality as well as control over their connectivity and hence charge transport properties. In this context, we present a complete photophysical study comprising fluence and temperature-dependent time-resolved spectroscopy to study carrier dynamics in ligand-free QD networks with gradually varying degrees of interconnectivity, which we achieve by changing the average distance between the QDs. Analysis of the photoluminescence and absorption properties of the QD assemblies, involving both static and time-resolved measurements, allows us to identify the weight of the different recombination mechanisms, both radiative and non-radiative, as a function of QD connectivity. We propose a picture where carrier diffusion, which is needed for any optoelectronic application and implies inter-particle transport, gives rise to the exposure of carriers to a larger defect landscape than in the case of isolated QDs. The use of a broad range of fluences permits extracting valuable information for applications demanding either low or high carrier injection levels and highlighting the relevance of a judicious design to balance recombination and diffusion.

cond-mat.mes-hall↗

arXiv2023

Exciton-carrier coupling in a metal halide perovskite nanocrystal assembly probed by two-dimensional coherent spectroscopy

The surface chemistry and inter-connectivity within perovskite nanocrystals play a critical role in determining the electronic interactions. They manifest in the Coulomb screening of electron-hole correlations and the carrier relaxation dynamics, among other many-body processes. Here, we characterize the coupling between the exciton and free carrier states close to the band-edge in a ligand-free formamidinium lead bromide nanocrystal assembly via two-dimensional coherent spectroscopy. The optical signatures observed in this work show: (i) a nonlinear spectral lineshape reminiscent of Fano-like interference that evidences the coupling between discrete electronic states and a continuum, (ii) symmetric excited state absorption cross-peaks that suggest the existence of a coupled exciton-carrier excited state, and (iii) ultrafast carrier thermalization and exciton formation. Our results highlight the presence of coherent coupling between exciton and free carriers, particularly in the sub-100 femtosecond timescales.

cond-mat.mtrl-sci↗