Selective Area Band Engineering of Graphene using Cobalt-Mediated Oxidation

This study reports a scalable and economical method to open a band gap in single layer graphene by deposition of cobalt metal on its surface using physical vapor deposition in high vacuum. At low cobalt thickness, clusters form at impurity sites on the graphene without etching or damaging the graphene. When exposed to oxygen at room temperature, oxygen functional groups form in proportion to the cobalt thickness that modify the graphene band structure. Cobalt/Graphene resulting from this treatment can support a band gap of 0.30 eV, while remaining largely undamaged to preserve its structural and electrical properties. A mechanism of cobalt-mediated band opening is proposed as a two-step process starting with charge transfer from metal to graphene, followed by formation of oxides where cobalt has been deposited. Contributions from the formation of both CoO and oxygen functional groups on graphene affect the electronic structure to open a band gap. This study demonstrates that cobalt-mediated oxidation is a viable method to introduce a band gap into graphene at room temperature that could be applicable in electronics applications

Impact of sputtered ZnO interfacial layer on the S-curve in conjugated polymer/fullerene based-inverted organic solar cells

The impact of crystalline structure changes of sputtered ZnO interfacial layer on performances of inverted organic solar cells (OSCs) has been investigated. We find that the structural modification of the ZnO cathode interfacial layer, obtained by thermal annealing, plays a crucial role in the origin and solving of the S-curve in conjugated polymer/fullerene photovoltaics. Our results show that the crystallization (i.e. crystallites size) of poly(3hexylthiophene) (P3HT) evolves as a function of that of ZnO according to the annealing temperature. This evolution can directly impact the interfacial orientation and organization of the chains of P3HT at the ZnO buried interface. Such an ordered profile favors the vertical phase segregation and raises the carrier mobility, which explains the disappearance of the S-shape observed in current density-voltage device characteristics for annealing temperatures above 200 degrees C. These results adequately address recent research and provide an important insight into the interfacial layers of inverted OSCs. (C) 2014 Elsevier B.V. All rights reserved

Efficient energy transfer from ZnO to Nd3+ ions in Nd-doped ZnO films deposited by magnetron reactive sputtering

In this paper, a detailed study of the luminescent properties of Nd3+ ions in sputtered ZnO thin films is reported for the first time. Experimental evidence is provided showing that Nd is inserted and optically active in the ZnO matrix. Despite the small amount (<2%) of rare earth in these thin ZnO films, intense luminescence signals have been collected, indicating efficient infrared emission of Nd3+ in ZnO. Direct excitation of Nd3+ ions in the ZnO matrix was possible, suggesting that most of the Nd atoms are in the 3+ form at all deposition temperatures. Moreover, intense Nd3+ emission has been recorded also when the host was excited, indicating that an efficient energy transfer occurs from ZnO to Nd ions. Both the transfer efficiency and the Nd3+ concentration seem to depend on the deposition temperature. In particular, indirect excitation of the sample deposited at 400 degrees C generates a richer emission pattern compared to lower temperatures. The careful analysis of the luminescence data indicated that the new pattern comes from Nd sites that cannot be efficiently directly excited, but that are characterized by intense emission under indirect excitation of the host. The possible transfer mechanisms leading to this behavior will be outlined

Photon management properties of rare-earth (Nd,Yb,Sm)-doped CeO2 films prepared by pulsed laser deposition

CeO2 is a promising material for applications in optoelectronics and photovoltaics due to its large band gap and values of the refractive index and lattice parameters, which are suitable for silicon-based devices. In this study, we show that trivalent Sm, Nd and Yb ions can be successfully inserted and optically activated in CeO2 films grown at a relatively low deposition temperature (400 [degree]C), which is compatible with inorganic photovoltaics. CeO2 thin films can therefore be efficiently functionalized with photon-management properties by doping with trivalent rare earth (RE) ions. Structural and optical analyses provide details of the electronic level structure of the films and of their energy transfer mechanisms. In particular, we give evidence of the existence of an absorption band centered at 350 nm from which energy transfer to rare earth ions occurs. The transfer mechanisms can be completely explained only by considering the spontaneous migration of Ce3+ ions in CeO2 at a short distance from the RE3+ ions. The strong absorption cross section of the f-d transitions in Ce3+ ions efficiently intercepts the UV photons of the solar spectrum and therefore strongly increases the potential of these layers as downshifters and downconverters