Research Highlights 2015

Fast and long-range triplet exciton diffusion in metal–organic frameworks for photon upconversion at ultralow excitation power
2015-NatureMater-Monguzzi

By exploiting metal-organic frameworks (MOF) crystalline systems, we maximized the efficiency of sensitized upconversion under very weak excitation intensities. This breakthrough was based on combining donor/acceptor dyes integration into a solid upconverter and precise control of the acceptor spatial organization in crystalline MOFs, which led to fast and long-range triplet exciton diffusion. By taking advantage of the enhanced exciton migration among fluorophores that are regularly aligned with spatially controlled reciprocal orientations, we achieved a quantitative triplet-triplet annihilation in the solid state, producing high energy up-converted photons even under excitation intensities much lower than solar irradiance.

P. Mahato, A. Monguzzi, N. Yanai, T. Yamada, N. Kimizuka, Fast and long-range triplet exciton diffusion in metal–organic frameworks for photon upconversion at ultralow excitation powerNature Materials 14, 924–930 (2015)

Highly efficient large-area colourless luminescent solar concentrators using heavy-metal-free colloidal quantum dots
2015-NatureNanotech-Meinardi

Luminescent solar concentrators (LSCs) serving as semitransparent photovoltaic windows could become an important element in net zero energy consumption buildings of the future. Colloidal quantum dots are promising materials for LSCs as they can be engineered to provide the large Stokes shift necessary for suppressing reabsorption losses in large-area devices. Here, we use quantum dots of ternary I–III–VI2 semiconductors to realize the first large-area LSC free of toxic elements- By incorporating CuInSexS2–x quantum dots into photo-polymerized poly(lauryl methacrylate), we obtain freestanding, colorless slabs that introduce no distortion to perceived colors and are thus well suited for the realization of photovoltaic windows. We achieve a record optical power efficiency of 3.2%.

F. Meinardi, H. McDaniel, F. Carulli, A. Colombo, K. A. Velizhanin, N. S. Makarov, R. Simonutti, V. I. Klimov and S. Brovelli, Highly efficient large-area colourless luminescent solar concentrators using heavy-metal-free colloidal quantum dotsNature Nanotechnology 10, 178, (2015)

Lateral Manipulation of Atomic Vacancies in Ultrathin Insulating Films
2015-ACSNano-Pacchioni

During the last 20 years, using scanning tunneling microscopy (STM) and atomic force microscopy, scientists have achieved vertical and lateral repositioning of individual atoms on and in different types of surfaces. It is therefore surprising that controlled repositioning of virtual atoms, i.e., atomic vacancies, across atomic lattices has not yet been achieved experimentally. Here we use STM at liquid helium temperature (4.5 K) to create individual Cl vacancies and subsequently to laterally manipulate them across the surface of ultrathin sodium chloride films. Our findings are corroborated by density functional theory calculations and STM image simulations.

Z. Li, H-Y T. Chen, K. Schouteden, K. Lauwaet, E. Janssens, C. Van Haesendonck, G. Pacchioni, and P. Lievens, Lateral Manipulation of Atomic Vacancies in Ultrathin Insulating FilmsACS Nano 9, 5318 (2015)

Photoinduced reversible phase change in porous molecular crystals based on star-shaped azobenzene tetramers
2015-NatureChem-Comotti

Shape-persistent azobenzene tetramers form porous molecular crystals in their E-configuration and their porosity can be tuned by changing the peripheral substitutents on the molecule. Efficient E→Z photoisomerization of the azobenzene units takes place in the solid state and converts the crystals into a non-porous amorphous phase. Crystallinity and porosity are restored upon Z→E isomerization promoted by visible light irradiation or heating. We demonstrate that the photoisomerization enables reversible on/off switching of optical properties, such as birefringence, and of the capture of carbon dioxide from the gas phase. The linear design, the structural versatility and the synthetic accessibility make this new family of materials extremely interesting for technological applications.

M. Baroncini, S. d’Agostino, G. Bergamini, P. Ceroni, A. Comotti, P. Sozzani, I. Bassanetti, F. Grepioni, T. M. Hernandez, S. Silvi, M. Venturi, A. Credi, Photoinduced reversible phase change in porous molecular crystals based on star-shaped azobenzene tetramersNature Chem. 7, 634 (2015)

High-Efficiency All-Solution-Processed Light-Emitting Diodes Based on Anisotropic Colloidal Heterostructures with Polar Polymer Injecting Layers
2015-NanoLett-Brovelli

Despite over twenty years of research since the initial demonstration of QD-LEDs in 1994, the external quantum efficiency (EQE) of LEDs based on colloidal nanostructures sandwiched between semiconducting polymers has never surpassed ~0.3-0.4%, and record devices embedding vacuum-deposited small organic molecules have reached EQEs just above 3%.
Here, we outperform all previous approaches using both solution-based and thermally evaporated organic interlayers by synthesizing polyelectrolytic conjugated polymers soluble in non-solvents for the QD emitters, that can be spin-coated without causing damage to the active underlayers. Using this approach, in combination with anisotropic CdSe/CdS hetero-nanostructures, we obtain all-solution-based, roll-off-free QD-LEDs with high brightness (>1200 cd/m2) and EQE=6,1%, which is about twenty times higher than the previous record QD-LEDs with polymer interlayers and exceeds by ~200% top performing QD-LEDs embedding vacuum deposited organic molecules

A. Castelli, F. Meinardi, M. Pasini, F. Galeotti, V. Pinchetti, M. Lorenzon, L. Manna, I. Moreels, U. Giovanella, and S. Brovelli, High-Efficiency All-Solution-Processed Light-Emitting Diodes Based on Anisotropic Colloidal Heterostructures with Polar Polymer Injecting LayersNano Lett. 15, 5455 (2015) - ACS Editors' Choice, Most read article in 2015

InAs/GaAs Sharply Defined Axial Heterostructures in Self-Assisted Nanowires
2015-NanoLett-Miglio

We present the fabrication of axial InAs/GaAs nanowire heterostructures on silicon with atomically sharp interfaces by molecular beam epitaxy. Our method exploits the crystallization at low temperature, by As supply, of In droplets deposited on the top of GaAs NWs grown by the self-assisted (self-catalyzed) mode. Extensive characterization based on transmission electron microscopy sets an upper limit for the InAs/GaAs interface thickness within few bilayers (≤1.5 nm).
Importantly, we obtained such a result in a fully catalyst-free growth mode, therefore eliminating the longstanding problem of gold incorporation into the wires. Hence, the proposed procedure first gives the possibility to produce high-quality wires for laser or transport devices, whose performances are presently degraded by unwanted metal incorporation caused by Au-seeded VLS at high concentrations.

D. Scarpellini, C. Somaschini, A. Fedorov, S. Bietti, C. Frigeri, V. Grillo, L. Esposito, M. Salvalaglio, A. Marzegalli, F. Montalenti, E. Bonera, P.G. Medaglia, and S. Sanguinetti, InAs/GaAs Sharply Defined Axial Heterostructures in Self-Assisted NanowiresNano Lett. 15, 3677 (2015)

Efficient Broadband Triplet–Triplet Annihilation-Assisted Photon Upconversion at Subsolar Irradiance in Fully Organic Systems
2015-AdvFuncMater-Meinardi

The latest trend in solar cell technology is to develop photon managing processes that adapt the solar emission to the spectral range at which the devices show the largest intrinsic efficiency. Triplet–triplet annihilation assisted photon upconversion (sTTA-UC) is currently the most promising process to blue-shift sub-bandgap photons at solar irradiance. Here we demonstrate how to obtain broadband sTTA-UC at sub-solar irradiance, by enhancing the system’s light-harvesting ability by way of an ad-hoc synthesized family of chromophores with complementary absorption properties. The overall absorptance is boosted, doubling the number of upconverted photons and reducing the irradiance required to achieve the maximum upconversion yield. An unprecedented yield of ≈10% is obtained under broadband air mass 1.5 conditions.

A. Monguzzi, S. M. Borisov, J. Pedrini, I. Klimant, M. Salvalaggio, P. Biagini, F. Melchiorre, C. Lelii, F. Meinardi, Efficient Broadband Triplet–Triplet Annihilation-Assisted Photon Upconversion at Subsolar Irradiance in Fully Organic SystemsAdvanced Functional Materials 25, 5617–5624 (2015)

Dipolar Rotors Orderly Aligned in Mesoporous Fluoro-organosilica Architectures
2015-AngewChem-Sozzani

New mesoporous covalent frameworks, based on hybrid fluoro-organosilicas, were prepared to realize a periodic architecture of fast molecular rotors containing dynamic dipoles in their structure. The mobile elements, designed on the basis of fluorinated p-divinylbenzene moieties, were integrated into the robust covalent structure through siloxane bonds, and showed not only the rapid dynamics of the aromatic rings (ca. 108 Hz at 325 K), as detected by solid-state NMR, but also a dielectric response typical of a fast dipole reorientation under the stimuli of an applied electric field. Furthermore, the mesochannels are open and accessible to diffusing-in gas molecules, and rotor mobility could be individually regulated by I2 vapors. The iodine enters the channels of the periodic structure and reacts with the pivotal double bonds of the divinyl-fluoro-phenylene rotors, affecting their motion and the dielectric properties.

S. Bracco, M. Beretta, A. Cattaneo, A. Comotti, A. Falqui, K. Zhao, C. Rogers, P. Sozzani, Dipolar Rotors Orderly Aligned in Mesoporous Fluoro-organosilica ArchitecturesAngew. Chem. Int. Ed. 54, 4681 (2015). VIP article and Cover Picture of the Issue.