Research highlights 2014

Large-area luminescent solar concentrators based on ‘Stokes-shift-engineered’ nanocrystals in a mass-polymerized PMMA matrix
2014-NaturePhotonics-Brovelli

Luminescent solar concentrators (LSCs) are cost-effective complements to semiconductor photovoltaic (PV) systems that can both boost the power output of standalone solar cells and allow for integration of PV-active architectural elements into buildings in the form of, for example, semi-transparent PV windows. In this work, scientists of our Department (Meinardi, Colombo, Simonutti, Lorenzon, Beverina, and Brovelli) in collaboration with the Los Alamos National Laboratory in New Mexico (USA) used “Stokes-shift-engineered” CdSe/CdS core/shell colloidal quantum dots (QDs) with especially thick shells to realize the first LSCs without any losses to re-absorption for optical paths as long as tens of centimeters.
These novel devices (see picture) were obtained using high-optical-quality QD polymethylmethacrylate (PMMA) composites fabricated by a modified version of a common industrial cell-casting synthesis adjusted in such a way as to minimize harmful interactions of the QDs with radical polymerization initiators. The use of this new fairly gentle procedure together with the robustness of thick–shell QDs, in which emitting core-localized excitons are isolated from the environment, preserved the QD light emitting properties completely intact upon incorporation into PMMA. The studies of these prototype LSC devices yield optical quantum efficiencies of more than 10% per absorbed photon. Following its publication in Nature Photonics, this study was highlighted by over 200 newspapers and web-magazines worldwide, as well as in dedicated TV and radio interviews.

Meinardi, F; Colombo, A; Velizhanin, KA;  Simonutti, R; Lorenzon, M; Beverina, L; Viswanatha, R; Klimov, VI; Brovelli, S, Large-area luminescent solar concentrators based on ‘Stokes-shift-engineered’ nanocrystals in a mass-polymerized PMMA matrixNature Photonics 8, 392 (2014)

Post-deposition Activation of Latent Hydrogen-Bonding: A New Paradigm for Enhancing the Performances of Bulk Heterojunction Solar Cells
2014-AdvFuncMater-Brovelli

This study, published as cover article in Advanced Functional Materials, demonstrated a new paradigm for fine tuning of the nanoscale connectivity of organic bulk heterojunction solar cells based on the post-deposition exploitation of latent hydrogen bonding. Through this approach,  scientists of our Department (Bruni, Sassi, Ruffo, Meinardi, Beverina, and Brovelli), in collaboration with the ISMac-CNR Institute in Milano, achieved over 20 fold enhancement of the device power conversion efficiency. The strategy can be readily extended to many conjugated organic systems for achieving highly efficient small-molecule solar cells.

Bruni, F; Sassi, M; Campione, M; Giovanella, U; Ruffo, R; Luzzati, S; Meinardi, F; Beverina, L; Brovelli, S, Post-deposition Activation of Latent Hydrogen-Bonding: A New Paradigm for Enhancing the Performances of Bulk Heterojunction Solar CellsAdv. Func. Mater. 24, 7410 (2014)

Plastic and Elastic Strain Fields in GaAs/Si Core-Shell Nanowires
2014-NanoLett-Miglio

Thanks to their unique morphology, nanowires have enabled integration of materials in a way that was not possible before with thin film technology. In turn, this opens new avenues for applications in the areas of energy harvesting, electronics, and optoelectronics. This is particularly true for axial heterostructures, while core–shell systems are limited by the appearance of strain-induced dislocations.
Even more challenging is the detection and understanding of these defects. We combine geometrical phase analysis with finite element strain simulations to quantify and determine the origin of the lattice distortion in core–shell nanowire structures. Such combination provides a powerful insight in the origin and characteristics of edge dislocations in such systems and quantifies their impact with the strain field map. We apply the method to heterostructures presenting single and mixed crystalline phase. Mixing crystalline phases along a nanowire turns out to be beneficial for reducing strain in mismatched core–shell structures.

Conesa-Boj, S; Boioli, F; Russo-Averchi, E; Dunand, S; Heiss, M; Ruffer, D; Wyrsch, N; Ballif, C; Miglio, L; Morral, AF, Plastic and Elastic Strain Fields in GaAs/Si Core-Shell NanowiresNano Lett. 14, 1859 (2014)

Diffraction of quantum dots reveals nanoscale ultrafast energy localization
2014-NanoLett-Sanguinetti

Unlike in bulk materials, energy transport in low-dimensional and nano-scale systems may be governed by a coherent ‘ballistic’ behavior of lattice vibrations, the phonons. If dominant, such behavior would determine the mechanism for transport and relaxation in various energy-conversion applications. In the study published in Nano Letters by the Nobel Prize in Chemistry Ahmed H. Zewail (CalTech) in collaboration with the group of Prof. Stefano Sanguinetti (our department), who developed the materials used in this study, the lattice dynamics in nano-scale quantum dots of gallium arsenide was studied in time and space using ultrafast electron diffraction. When the dot size is smaller than the inelastic phonon mean-free path, the energy remains localized in high-energy acoustic modes which travel coherently within the dot. As the dot size increases, an energy dissipation toward low-energy phonons takes place, and the transport becomes diffusive. These results are fundamental for the understanding of energy conversion in nano-scale materials, and for the control of properties involving thermal conductivity and optical design. See also: "Quantum-dot per la generazione elettrica del futuro", Smart City, Voci e luoghi dell'innovazione (http://www.radio24.ilsole24ore.com/programma/smart-city/mercoledi-dicemb...)

Vanacore, G; Hu, J; Liang, W; Bietti, S; Sanguinetti, S; Zewail, A, Diffraction of quantum dots reveals nanoscale ultrafast energy localizationNano Lett. 14, 6148 (2014)

Molecular Rotors in Porous Organic Frameworks
2014-AngewChem-Sozzani

Porous organic frameworks exhibit ultra-fast molecular rotors (108 Hz at RT) in their architectures, resulting in a dynamic material whose motion can be frozen or released at will. In fact, the rotational motion can be actively regulated in response to guests. As the temperature is increased, the rotors spin ever faster, approaching free-rotational diffusion at 550 K. The unusual combination of remarkable nanoporosity with fast dynamics is intriguing for engineering oscillating dipoles and producing responsive materials with switchable ferroelectricity, and for applications spanning from sensors to actuators, which capture and release chemicals on command.

Comotti, A; Bracco, S;Ben, T; Qiu, S; Sozzani, P, Molecular Rotors in Porous Organic FrameworksAngew. Chem. 53, 1043 (2014)

Theoretical studies on anatase and less common TiO2 phases: Bulk, surfaces, and nanomaterials
2014-ChemRev-DiValentin

Titanium dioxide is a very popular semiconducting oxides which is applied in a wide variety of technological fields. In this review we present a survey of the theoretical studies on the stability, reactivity, electronic and structural properties of anatase and other less common TiO2 phases. Since sensitization and nanostructuring play a crucial role in most of the application, the review spans from bulk to surfaces, to two-dimensional systems, to nanoparticles. The evolution of modern computing has largely expanded the scope of first-principles calculations which now provide an accurate description of systems of few nanometers size.

De Angelis, F; Di Valentin, C; Fantacci, S; Vittadini, A; Selloni, A, Theoretical studies on anatase and less common TiO2 phases: Bulk, surfaces, and nanomaterialsChem. Rev. 114, 9708 (2014)

Spectroscopic properties of doped and defective semiconducting oxides from hybrid density functional calculations
2014-AccChemRes-DiValentin

Very rarely metal oxides are used in their pure and fully stoichiometric form. On the contrary, in most of the applications, ranging from catalysis to electronic devices, they are either doped or made defective (e.g. oxygen deficient) because interesting chemical, electronic, optical and magnetic properties arise only when foreign components or defects are introduced in the lattice. For this reason an enormous effort is devoted to the understanding of the physical and chemical properties of doped oxides. This review shows how the direct comparison between spectroscopic experimental and computational data allow one to acquire a deep understanding and control of these complex systems.

Di Valentin, C; Pacchioni, G, Spectroscopic properties of doped and defective semiconducting oxides from hybrid density functional calculationsAcc. Chem. Res. 47, 3233 (2014)

π-conjugated zwitterions as paradigm of donor-acceptor building blocks in organic-based materials
2014-AccChemRes-Beverina

The very peculiar characteristics of zwitterions, as well as a clear and unambiguous definition, have been overlooked in past literature. However, these compounds are particularly important in view of the impact they have had in the recent past and will likely continue to have in the future as components of performing functional organic and hybrid materials. In this Account, authors primarily aim to define critically important organic concepts of zwitterions regarding both their design and nomenclature. 

Beverina, L; Pagani, G, π-conjugated zwitterions as paradigm of donor-acceptor building blocks in organic-based materials, Acc. Chem. Res. 47, 319 (2014)