Yan Wang1†, Stanislas Von Euw1†, Francisco M. Fernandes1, Sophie Cassaignon1, Mohamed Selmane2,
Guillaume Laurent1, Gérard Pehau-Arnaudet3, Cristina Coelho2, Laure Bonhomme-Coury1,
Marie-Madeleine Giraud-Guille1, Florence Babonneau1, Thierry Azaïs1* and Nadine Nassif1*
It is well known that organic molecules from the vertebrate extracellular matrix of calcifying tissues are essential in structuring the apatite mineral. Here, we show that water also plays a structuring role. By using solid-state nuclear magnetic resonance, wide-angle X-ray scattering and
cryogenic transmission electron microscopy to characterize the structure and organization of crystalline and biomimetic apatite nanoparticles as well as intact bone samples, we demonstrate that water orients apatite crystals through an amorphous calcium phosphate-like layer that coats
the crystalline core of bone apatite. This disordered layer is reminiscent of those found around the crystalline core of calcified biominerals in various natural composite materials in vivo. This work provides an extended local model of bone biomineralization. in vivo. This work provides an extended local model of bone biomineralization.
Potassium-induced surface modification of Cu(In,Ga)Se2 thin films for high-efficiency solar cells
Adrian Chirila˘1*†, Patrick Reinhard1†, Fabian Pianezzi1, Patrick Bloesch1, Alexander R. Uhl1,
Carolin Fella1, Lukas Kranz1, Debora Keller1, Christina Gretener1, Harald Hagendorfer1, Dominik
Jaeger2, Rolf Erni3, Shiro Nishiwaki1, Stephan Buecheler1 and Ayodhya N. Tiwari1
Thin-film photovoltaic devices based on chalcopyrite Cu(In,Ga)Se2 (CIGS) absorber layers show excellent light-to- power conversion efficiencies exceeding 20% (refs 1,2). This high performance level requires a small amount of alkaline metals incorporated into the CIGS layer, naturally
provided by soda lime glass substrates used for processing of champion devices3 . The use of flexible substrates requires distinct incorporation of the alkaline metals, and so far mainly Na was believed to be the most favourable element, whereas other alkaline metals have resulted in
significantly inferior device performance4,5 . Here we present a new sequential post-deposition treatment of the CIGS layer with sodium and potassium fluoride that enables fabrication of flexible photovoltaic devices with a remarkable conversion efficiency due to modified interface
properties and mitigation of optical losses in the CdS buffer layer. The described treatment leads to a significant depletion of Cu and Ga concentrations in the CIGS near-surface region and enables a significant thickness reduction of the CdS buffer layer without the commonly observed losses in
photovoltaic parameters6 . Ion exchange processes, well known in other research areas7–13 , are proposed as underlying mechanisms responsible for the changes in chemical composition of the deposited CIGS layer and interface properties of the heterojunction.
Topological surface state in the Kondo insulator
samarium hexaboride
D. J.
Kim*, J. Xia and Z. Fisk
Topological invariants of electron wavefunctions in condensed matter reveal many intriguing phenomena1,2 . A notable exam- ple is provided by topological insulators, which are characterized by an insulating bulk coexisting with a metallic boundary state3,4 . Although there has been intense interest in Bi-based topological insulators5,6 , their behaviour is complicated by the presence of a considerable residual bulk conductivity7–10 . Theories predict11,12 that the Kondo
insulator system SmB6 , which is known to undergo a transition from a Kondo lattice metal to a small-gap insulator state with decreasing tem- perature, could be a topological insulator.
Although the insulating bulk and metallic surface separation has been demonstrated in recent transport measurements13–15 , these have not demonstrated the topologically protected nature of the metallic surface state. Here we report thickness-dependent transport measurements on doped SmB6 ,
and show that magnetic and non-magnetic doping results in contrasting behaviour that supports the conclusion that SmB6 shows behaviour that supports the conclus
virtually no residual bulk conductivity.
