Monolithic and Flexible ZnS/SnO2 Ultraviolet Photodetectors with Lateral Graphene Electrodes

A continuing trend of miniaturized and flexible electronics/optoelectronic calls for novel device architectures made by compatible fabrication techniques. However, traditional layer‐to‐layer structures cannot satisfy such a need. Herein, a novel monolithic optoelectronic device fabricated by a mask‐free laser direct writing method is demonstrated in which in situ laser induced graphene‐like materials are employed as lateral electrodes for flexible ZnS/SnO₂ ultraviolet photodetectors. Specifically, a ZnS/SnO₂ thin film comprised of heterogeneous ZnS/SnO₂ nanoparticles is first coated on polyimide (PI) sheets by a solution process. Then, CO₂ laser irradiation ablates designed areas of the ZnS/SnO₂ thin film and converts the underneath PI into highly conductive graphene as the lateral electrodes for the monolithic photodetectors. This in situ growth method provides good interfaces between the graphene electrodes and the semiconducting ZnS/SnO₂ resulting in high optoelectronic performance. The lateral electrode structure reduces total thickness of the devices, thus minimizing the strain and improving flexibility of the photodetectors. The demonstrated lithography‐free monolithic fabrication is a simple and cost‐effective method, showing a great potential for developement into roll‐to‐roll manufacturing of flexible electronics.     

The Art of Masonry

Brick masonry is not only a question of building physics, building construction and durability, i.e. sustainability, but has much to do with aesthetics and making sense, which may have been forgotten in modern architecture with its absolute lack of décor. Masonry as an art form has been scarce since the era of brick expressionism. Recently, there have been some exceptions – and thoroughly welcome. The ”Fjordenhus“ in Vejle, Denmark, is an astonishing example.     

On nonlinear evolution of convective flow in an active mushy layer

We consider the nonlinear effect of convective flow in a horizontal mushy layer during solidification. The mushy layer that we consider has a permeable mush–liquid interface and is treated as an active porous medium with variable permeability. The nonlinear partial differential equations involved in this system are conservation equations for flow momentum, mass, temperature, and concentration. Numerical solutions to the resulting weakly nonlinear equations are obtained using a fourth-order Runge–Kutta integration scheme via a shooting technique. An evolution equation of Landau type is derived in terms of linear and first-order solutions by introducing an adjoint operator. The Landau constant is calculated for both cases: constant permeability and variable permeability. The analysis reveals that there is a slow transition of the flow to a steady state with smaller amplitude for an active mushy layer.     

A four‐node corotational quadrilateral elastoplastic shell element using vectorial rotational variables

A four‐node corotational quadrilateral elastoplastic shell element is presented. The local coordinate system of the element is defined by the two bisectors of the diagonal vectors generated from the four corner nodes and their cross product. This local coordinate system rotates rigidly with the element but does not deform with the element. As a result, the element rigid‐body rotations are excluded in calculating the local nodal variables from the global nodal variables. The two smallest components of each nodal orientation vector are defined as rotational variables, leading to the desired additive property for all nodal variables in a nonlinear incremental solution procedure. Different from other existing corotational finite‐element formulations, the resulting element tangent stiffness matrix is symmetric owing to the commutativity of the local nodal variables in calculating the second derivative of strains with respect to these variables. For elastoplastic analyses, the Maxwell–Huber–Hencky–von Mises yield criterion is employed, together with the backward‐Euler return‐mapping method, for the evaluation of the elastoplastic stress state; the consistent tangent modulus matrix is derived. To eliminate locking problems, we use the assumed strain method. Several elastic patch tests and elastoplastic plate/shell problems undergoing large deformation are solved to demonstrate the computational efficiency and accuracy of the proposed formulation. Copyright © 2013 John Wiley & Sons, Ltd.     

Exciton storage by Mn(2+) in colloidal Mn(2+)-doped CdSe quantum dots

Colloidal Mn (2+)-doped CdSe quantum dots showing long excitonic photoluminescence decay times of up to tau exc = 15 mus at temperatures over 100 K are described. These decay times exceed those of undoped CdSe quantum dots by approximately 10 (3) and are shown to arise from the creation of excitons by back energy transfer from excited Mn (2+) dopant ions. A kinetic model describing thermal equilibrium between Mn (2+ 4)T 1 and CdSe excitonic excited states reproduces the experimental observations and reveals that, for some quantum dots, excitons can emit with near unity probability despite being approximately 100 meV above the Mn (2+ 4)T 1 state. The effect of Mn (2+) doping on CdSe quantum dot luminescence at high temperatures is thus completely opposite from that at low temperatures described previously.