Research Highlights 2017
Fully Atomistic Understanding of the Electronic and Optical Properties of a Prototypical Doped Charge-Transfer Interface
The alkali-metal doping-induced changes of the structural, electronic, and optical properties of the prototypical PTCDA/Ag(111) interface is presented in this work. For doping K atoms are used, as KxPTCDA/Ag(111) has the distinct advantage of forming well-defined stoichiometric phases. K atoms are observed to both structurally and electronically reduce the organic molecule-metal bonding. To arrive at a conclusive, unambiguous, and fully atomistic understanding of the interface properties, we combined the state-of-the-art density-functional theory calculations using VASP, Quantum Espresso and Yambo with the various experimental techniques. In combination with the full structural characterization with low-energy electron diffraction of the KxPTCDA/Ag(111) interface, where for the first time the dopant atoms were visualized by scanning tunneling microscopy experiments (ACS Nano 2016, 10, 2365−2374), the present comprehensive study provides access to a fully characterized reference system for a well-defined metal-organic interface in the presence of dopant atoms, which can serve as an ideal benchmark for future research and applications.
In the figure: Structural and electronic decoupling of PTCDA from the Ag(111) surface by K doping.
Brivio, G.P., Baby, A; Fratesi, G; Gruenewald, M; Zwick, C; Otto, F; Forker, R; van Straaten, G; Franke, M; Stadtmüller, B; Kumpf, C; Fritz, T; and Zojer, E; Fully Atomistic Understanding of the Electronic and Optical Properties of a Prototypical Doped Charge-Transfer Interface, ACS Nano 11, 10495-10508 (2017).
Highly efficient luminescent solar concentrators based on earth-abundant indirect-bandgap silicon quantum dots
Building-integrated photovoltaics is gaining consensus as a renewable energy technology for producing electricity at the point of use. Luminescent solar concentrators (LSCs) could extend architectural integration to the urban environment by realizing electrode-less photovoltaic windows. Crucial for large-area LSCs is the suppression of reabsorption losses, which requires emitters with negligible overlap between their absorption and emission spectra. This work demonstrates the use of indirect-bandgap semiconductor nanostructures such as highly emissive silicon quantum dots. Silicon is non-toxic, low-cost and ultraearth-abundant, which avoids the limitations to the industrial scaling of quantum dots composed of low-abundance elements.
Suppressed reabsorption and scattering losses lead to nearly ideal LSCs with an optical efficiency of η=2.85%, matchingstate-of-the-art semi-transparent LSCs. Monte Carlo simulations indicate that optimized silicon quantum dot LSCs have aclear path to η > 5% for 1m2 devices. We are finally able to realize flexible LSCs with performances comparable to those of flat concentrators, which opens the way to a new design freedom for building-integrated photovoltaic elements.
In the figure: Visible and NIR picture of a flexible Si-based LSC under UV illumination for curved building integrated photovoltaic elements.
Meinardi F, Ehrenberg S, Dhamo L, Carulli F, Mauri M, Bruni F, Simonutti R, Kortshagen U, Highly efficient luminescent solar concentrators based on earth-abundant indirect-bandgap silicon quantum dots. NATURE PHOTONICS, 11, 177 (2017).
Nanodroplet-Containing Polymers for Efficient Low-Power Light Upconversion
Sensitized triplet–triplet-annihilation-based photon upconversion (TTA-UC) permits the conversion of light into radiation of higher energy at low intensity light, which makes it particularly suitable for solar-energy harvesting technologies. High upconversion yields can be observed for low viscosity solutions of dyes; but, in solid materials, which are better suited for integration in devices, the process is usually less efficient. We show that this problem can be solved by using transparent nanodroplet-containing poly-mers that consist of a continuous polymer matrix and a dispersed liquid phase containing the upconverting dyes. These materials can be accessed by a simple one-step procedure that involves the free-radical polymerization of a micro-emulsion of hydrophilic monomers, a lipophilic solvent, the upconverting dyes, and a surfactant. Several glassy and rubbery materials are explored and ranges of dyes that enable TTA-UC in different spectral regions are utilized. The materials display UC efficiencies approaching the performance of optimized reference solutions and grants good protection against the atmospheric oxygen.
In the figure: Sketch of the material morphology and scanning electron microscopy images of a nanodroplet-containing glassy polymer. Pictures of dye-doped nanodroplet-containing polymer glasses under laser excitation for red-to-green, red-to-blue and red-to-yellow photon upconversion.
Vadrucci R; Monguzzi, A; Saenz, F; Wilts, BD; Simon, YC; Weder C. Nanodroplet-Containing Polymers for Efficient Low-Power Light Upconversion. ADVANCED MATERIALS 29, 1702992 (2017).
Novel High Spin (S=2) States in TiO2 Anatase
Turning titanium dioxide (TiO2), a well-known white pigment, into a colorful solid, is one of the current strategies to shift the TiO2 absorption edge into the visible-light region, for the development of new advanced materials with emphasis on energetic and photochemical applications. The insertion in diamagnetic matrices of defect centers, mutually interacting by virtue of well-defined and controlled magnetic interactions, has become a highly important topic in the quest for new materials. In this work we describe the synthesis, characterization, and DFT modeling of TiO2 anatase nanoparticles featuring unprecedented magnetically active centers characterized by an S=2 spin state, originating from four ferromagnetically interacting Ti3+ centers. The observation of high spin states in TiO2 provides an interesting platform for the exploration of new spin-bearing multifunctional oxide based materials.
In the figure: (a) Side view of the supercell adopted. The light blue plane contains the four Ti3+ ions; the green planes contain the reduced Ti centers that form the more stable tetrahedral cluster. (b) Top and bottom view of the main features of the relaxed tetrahedral cluster with S=2. Distances between Ti3+ are in Å.
Chiesa, M; Livraghi, S; Giamello, E; Albanese, E; Pacchioni, G; Novel High Spin (S=2) States in TiO2 Anatase. ANGEWANDTE CHEMIE INTERNATIONAL EDITION 56, 2604 (2017).
Shape-Controlled TiO2 Nanocrystals for Na-Ion Battery Electrodes: The Role of Different Exposed Crystal Facets on the Electrochemical Properties
Rechargeable sodium-ion batteries are becoming a viable alternative to lithium-based technology in energy storage strategies, due to the wide abundance of sodium raw material. Notwithstanding the large number of research papers concerning sodium-ion battery electrodes, the development of a low-cost, well-performing anode material remains the largest obstacle to overcome. The well-known anatase, one of the allotropic forms of natural TiO2, suffers from reduced cyclability and limited power, due to kinetic drawbacks and to its poor charge transport properties. A systematic approach in the morphological tuning of the anatase nanocrystals is needed, to optimize its structural features toward the electrochemical properties. Aiming to face with these issues, we were able to obtain a fine tuning of the nanoparticle morphology and to expose the most favorable nanocrystal facets to the electrolyte and to the conductive wrapping agent (graphene), thus overcoming the intrinsic limits of anatase transport properties. The result is a TiO2-based composite electrode able to deliver an outstandingly stability over cycles (150 mA h g–1 for more than 600 cycles in the 1.5–0.1 V potential range) never achieved with such a low content of carbonaceous substrate (5%). Moreover, it has been demonstrated for the first time than these outstanding performances are not simply related to the overall surface area of the different morphologies but is directly related to the peculiar surface characteristics of the crystals.
G. Longoni, R. L. Pena Cabrera, S. Polizzi, M. D’Arienzo, C. M. Mari, Y. Cui, and R. Ruffo, Shape-Controlled TiO2 Nanocrystals for Na-Ion Battery Electrodes: The Role of Different Exposed Crystal Facets on the Electrochemical Properties. NANO LETTERS 17, 992. (2017).