Research Highlights 2018

Excitonic pathway to photoinduced magnetism in colloidal nanocrystals with nonmagnetic dopants
Colloidal nanocrystals with nonmagnetic dopants

Electronic doping of colloidal semiconductor nanostructures holds promise for future device concepts in optoelectronic and spin-based technologies. In this work, we combine optical, spectro-electrochemical and magnetic spectroscopies to show that the exciton recombination process in nanocrystals doped with nonmagnetic silver atoms is inextricably tied to photoinduced magnetism. As a result, Ag-doped
nanocrystals exhibit optically-activated paramagnetic properties and concomitant sp–d exchange interactions between excitons and Ag dopants. These results represent the first demonstration that optically switchable diluted magnetic semiconductor nanomaterials can be obtained by exploiting the excitonic processes involving nominally nonmagnetic impurities.

Pinchetti, V.; Di, Q; Lorenzon, M.; Camellini, A.; Fasoli, M.; Zavelani-Rossi, M.; Meinardi, F.; Zhang, J.; Crooker, S.A.; Brovelli, S., Excitonic pathway to photoinduced magnetism in colloidal nanocrystals with nonmagnetic dopants. NATURE NANOTECHNOLOGY 13, 145 (2018).

Highly Fluorescent Metal−Organic-Framework Nanocomposites for Photonic Applications
Scheme of the process and a TEM image of the studied nanoMOFs

Metal−organic frameworks (MOFs) are porous hybrid materials built up from organic ligands coordinated to metal ions or clusters by means of self-assembly strategies which allow to manipulate both the composition and ligands arrangement in order to control their optical and energy-transport properties. Therefore, optimized MOFs nanocrystals (nanoMOFs) potentially represent the next generation of luminescent materials with features like those of their inorganic predecessors, that is, the colloidal semiconductor quantum dots. The peculiarity of the nanoMOFs is the possibility to pack the ligand chromophores close enough to allow a fast exciton diffusion but sufficiently far from each other preventing the aggregation-induced effects of the organic crystals. In particular, the formation of strongly coupled dimers or excimers is avoided, thus preserving the optical features of the isolated molecule. However, nano-MOFs have a very small fluorescence quantum yield (QY). In order to overcome this limitation and achieve highly emitting systems, we analyzed the fluorescence process in blue emitting nano-MOFs and modeled the diffusion and quenching mechanism of photogenerated singlet excitons. Our results demonstrate that the excitons quenching in nano-MOFs is mainly due to the presence of surface-located, nonradiative recombination centers. In analogy with their inorganic counterparts, we found that the passivation of the nano-MOF surfaces is a straightforward method to enhance the emission efficiency. By embedding the
nanocrystals in an inert polymeric host, we observed a +200% increment of the fluorescence QY, thus recovering the emission properties of the isolated ligand in solution.

Monguzzi, A; Ballabio, M; Yanai, N; Kimizuka, N; Fazzi, D; Campione, M; Meinardi, F. Highly Fluorescent Metal−Organic-Framework Nanocomposites for Photonic Applications. NANO LETTERS, 18, 528 (2018).

Water-Assisted Hole Trapping at Highly Curved Surface of Nano-TiO₂ Photocatalyst
Adsorbed water on TiO₂ spherical nanoparticles

Heterogeneous photocatalysis is vital in solving energy and environmental issues that this society is confronted with. Although photocatalysts are often operated in the presence of water, it has not been yet clarified how the interaction with water itself affects charge dynamics in photocatalysts. Using water-coverage-controlled steady and transient infrared absorption spectroscopy and large-model (⁓800 atoms) ab initio calculations, we clarify that water enhances hole trapping at the surface of TiO₂ nanospheres but not of well-faceted nanoparticles. This water-assisted effect unique to the nanospheres originates from water adsorption as a ligand at a low-coordinated Ti−OH site or through robust hydrogen bonding directly to the terminal OH at the highly curved nanosphere surface. Thus, the interaction with water at the surface of nanospheres can promote photocatalytic reactions of both oxidation and reduction by elongating photogenerated carrier lifetimes. This morphology-dependent water-assisted effect provides a novel and rational basis for designing and engineering nanophotocatalyst morphology to improve photocatalytic performances.

Shirai, K.; Fazio, G.; Sugimoto, T.; Selli, D.; Ferraro, L.; Watanabe, K.; Haruta, M.; Ohtani, B.; Kurata, H.; Di Valentin, C.; Matsumoto, Y.
Water-Assisted Hole Trapping at Highly Curved Surface of Nano-TiO₂ Photocatalyst. JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 140, 1415 (2018).

High-Yield Fabrication of Entangled Photon Emitters for Hybrid Quantum Networking Using High-Temperature Droplet Epitaxy
Quantum dots grown by droplet epitaxy

Basso Basset, F; Bietti, S; Reindl, M; Esposito, L; Fedorov, A; Huber, D; Rastelli, A; Bonera, E; Trotta, R; Sanguinetti, S. High-Yield Fabrication of Entangled Photon Emitters for Hybrid Quantum Networking Using High-Temperature Droplet Epitaxy. NANO LETTERS 18, 505-512 (2018).

Dye-Sensitized Photocatalytic Hydrogen Generation: Efficiency Enhancement by Organic Photosensitizer–Coadsorbent Intermolecular Interaction
Molecular structure of the sensitizers and coadsorbents

In dye-sensitized photocatalytic (PC) hydrogen generation the role of molecular antennas to optimize light harvesting is strategic. In this work we have organized self-assembled layers on the catalyst surface to control the efficiency of the electron injection from the dye excited state and minimize self-quenching. We have achieved this result by modifying the molecular structure of the dye with a sugar terminal, able to afford strong intermolecular bonds with a sugar-based spacer.
Dye-sensitized photocatalytic H2 generation has been investigated using a metal-free phenothiazine-based donor−acceptor sensitizer (PTZ-GLU) in combination with coadsorbents. The coadsorption of the PTZ-GLU dye, functionalized with a glucose end-group, in combination with a glucose-based coadsorbent, aff orded improved photocatalytic activity compared to the absence of coadsorbents, to the use of a conventional (chenodeoxycholic acid) coadsorbent, or by replacing the dye glucose functionality with an alkyl chain. The results suggest the strategic role of directional intermolecular dye− coadsorbent interactions on the semiconductor surface, as confi rmed by first principles computational modeling, which likely suppressed detrimental recombination processes.

Manfredi, N., Monai, M., Montini, T., Peri, F., De Angelis, F., Fornasiero, P. & Abbotto, A. Dye-Sensitized Photocatalytic Hydrogen Generation: Efficiency Enhancement by Organic Photosensitizer–Coadsorbent Intermolecular Interaction. ACS ENERGY LETTERS 3, 85 (2018).

Self-Assembled pH-Sensitive Fluoromagnetic Nanotubes as Archetype System for Multimodal Imaging of Brain Cancer
Brain magnetic resonance in vivo imaging, SEM images of fluoromags and pH sensitive cancer cell optical imaging

Fluoromagnetic systems are recognized as an emerging class of materials with great potential in the biomedical field. Here, it is shown how to fabricate fluoromagnetic nanotubes that can serve as multimodal probes for the imaging and targeting of brain cancer. An ionic self-assembly strategy is used to functionalize the surface of synthetic chrysotile nanotubes with pH-sensitive fluorescent chromophores and ferromagnetic nanoparticles. The acquired magnetic properties permit their use as contrast agent for magnetic resonance imaging, and enable the tracking of tumor cell migration and infiltration responsible for metastatic growth and disease recurrence. Their organic component, changing its fluorescence attitude as a function of local pH, targets the cancer distinctive acidity, and allows localizing and monitoring the tumor occurrence and progression by mapping the acidic spatial distribution within biopsy tissues.
The fluoromagnetic properties of nanotubes are preserved from the in vitro to the in vivo condition and they show the ability to migrate across the blood brain barrier, thus spontaneously reaching the brain tumor after injection. The simplicity of the synthesis route of these geomimetic nanomaterials combined with their demonstrated affinity with the in vivo condition strongly highlights their potential for developing effective functional materials for multimodal theranostics of brain cancer.

Villa, C; Campione, M; Santiago-González, B; Alessandrini, F; Erratico, S; Zucca, I; Bruzzone, M. G; Forzenigo, L; Malatesta, P; Mauri, M; Trombetta, E; Brovelli, S; Torrente, Y; Meinardi, F; Monguzzi A. Self-Assembled pH-Sensitive Fluoromagnetic Nanotubes as Archetype System for Multimodal Imaging of Brain Cancer. ADVANCED FUNCTIONAL MATERIALS 28, 1707582 (2018).