Two-dimensional functional molecular nanoarchitectures – Complementary investigations with scanning tunneling microscopy and X-ray spectroscopy

Functional molecular nanoarchitectures (FMNs) are highly relevant for the development of future nanotechnology devices. Profound knowledge about the atomically controlled construction of such nanoscale assemblies is an indispensable requirement to render the implementation of such components into a real product successful. For exploiting their full potential the architectures’ functionalities have to be characterized in detail including the ways to tailor them. In recent years a plethora of sophisticated constructs were fabricated touching a wide range of research topics.     

Electrically controlled waveguide mode in LC layer for fiber optic applications

A new electrically controlled waveguide mode for optical beams propagating in the plane of a liquid crystal (LC) layer is described. The mode is based on the total internal reflection of light beams from two parallel boundaries, which separates the regions of a liquid crystal cell with different director orientations and refractive indices. In experiments electric field was applied to the initially planar layer to produce the channel with a homeotropic orientation surrounded by the planar regions. It results in focusing and waveguide propagation of a light beam emitted by an optical fiber inserted in a liquid crystal cell. Scattering of light by thermal LC director fluctuations is used to visualize light beams propagation. The obtained waveguide mode can be used for an elaboration of electrically controlled LC switches with low operating voltages. The prototype of such device made with usage of a photolithography and a photo alignment technique is described. The possible ways of optimization of LC-switch are considered.     

A simple proof for the constructions of optical priority queues

Constructions of optical queues by optical Switches and fiber Delay Lines (SDL) have received a lot of attention lately. In this short paper, we provide a simple proof for the construction of a priority queue with a switch and fiber delay lines in Sarwate and Anantharam (Queueing Syst. Theory Appl. 53, 115–125, 2006). Our proof not only gives the insights needed to understand why such a construction works, but also leads to a more general result that recovers the result in Sarwate and Anantharam (Queueing Syst. Theory Appl. 53, 115–125, 2006) as a special case. This research was supported in part by the National Science Council, Taiwan, ROC, under Contract NSC-93-2213-E-007-040, Contract NSC-93-2213-E-007-095, Contract NSC-94-2213-E-007-046, and the Program for Promoting Academic Excellence of Universities NSC 94-2752-E-007-002-PAE.     

Integrated Electro-Optic Switches Based on Ferroelectric Liquid Crystal Waveguides

Two integrated electro-optic switches based on smectic C * ferroelectric liquid crystal (FLC) planar waveguides oriented in the bookshelf geometry have been designed and realized. In order to test the first switch, a particular waveguide structure consisting of a three-stage device, having a thin FLC film middle stage and two glass waveguides as other stages, has been designed and realized. The second one is realized by a planar waveguide with a smectic C * ferroelectric liquid crystal overlayer. In this article the design, the realization, and the experimental characterization of electro-optical switches are reported. The electro-optical behavior and the response time of both devices have been studied for different configurations for both TE and TM polarization. Our main aim was to demonstrate the feasibility of an integrated electro-optic device based on a FLC waveguide, and our experimental results provide a number of interesting indications about device optimization and practical applications.     

Dynamic Reconfigurable DNA Nanostructures,Networks and Materials

DNA nanotechnology relies on the structural and functional information encoded in nucleic acids. Specifically, the sequence-guided reconfiguration of nucleic acids by auxiliary triggers provides a means to develop DNA switches, machines and stimuli-responsive materials. The present Review addresses recent advances in the construction and applications of dynamic reconfigurable DNA nanostructures, networks and materials. Dynamic transformations proceeding within engineered origami frames or between origami tiles, and the triggered dynamic reconfiguration of scaled supramolecular origami structures are addressed. The use of origami frameworks to assemble dynamic chiroplasmonic optical devices and to operate switchable chemical processes are discussed. Also, the dynamic operation of DNA networks is addressed, and the design of “smart” stimuli-responsive all-DNA materials and their applications are introduced. Future perspectives and applications of dynamic reconfigurable DNA nanostructures are presented.     

Molecular Lanthanide Switches for Magnetism and Photoluminescence

Solvation of [(CNT)Ln(η⁸-COT)] (Ln=La, Ce, Nd, Tb, Er; CNT=cyclononatetraenyl, i.e., C₉H₉⁻; COT=cyclooctatetraendiid, i.e., C₈H₈²⁻) complexes with tetrahydrofuran (THF) gives rise to neutral [(η⁴-CNT)Ln(thf)₂(η⁸-COT)] (Ln=La, Ce) and ionic [Ln(thf)_x(η⁸-COT)][CNT] (x=4 (Ce, Nd, Tb), 3 (Er)) species in a solid-to-solid transformation. Due to the severe distortion of the ligand sphere upon solvation, these species act as switchable luminophores and single-molecule magnets. The desolvation of the coordinated solvents can be triggered by applying a dynamic vacuum, as well as a temperature gradient stimulus. Raman spectroscopic investigations revealed fast and fully reversible solvation and desolvation processes. Moreover, we also show that a Nd:YAG laser can induce the necessary temperature gradient for a self-sufficient switching process of the Ce(III) analogue in a spatially resolved manner.     

Electrically Switchable Persistent Spin Texture in a Two-Dimensional Hybrid Perovskite Ferroelectric

As a momentum-independent spin configuration, persistent spin texture (PST) could avoid spin relaxation and play an advantageous role in spin lifetime. Nevertheless, manipulation of PST is a challenge due to the limited materials and ambiguous structure–property relationships. Herein, we present electrically switchable PST in a new 2D perovskite ferroelectric, (PA)₂CsPb₂Br₇ (where PA is n-pentylammonium), which has a high Curie temperature of 349 K, evident spontaneous polarization (3.2 μC cm⁻²) and a low coercive electric field of 5.3 kV cm⁻¹. The combination of symmetry-breaking in ferroelectrics and effective spin-orbit field facilitates intrinsic PST in the bulk and monolayer structure models. Strikingly, the directions of spin texture are reversible by switching the spontaneous electric polarization. This electric switching behavior relates to the tilting of PbBr₆ octahedra and the reorientation of organic PA⁺ cations. Our studies on ferroelectric PST of 2D hybrid perovskites afford a platform for electrical spin texture manipulation.     

Graph Optimization Approaches for Minimal Rerouting in Symmetric Three Stage Clos Networks

We consider routing in symmetrical three stage Clos networks. Especially we search for the routing of an additional connection that requires the least rearrangements, i.e. the minimal number of changes of already routed connections. We describe polynomial methods, based on matchings and edge colorings. The basic idea is to swap colors along alternating paths. The paths need to be maximal, and the shortest of these maximal paths is chosen, since it minimizes the rerouting that needs to be done. Computational tests confirm the efficiency of the approach.     

含二噻吩乙烯的非线性光学开关分子的新设计方案研究

本文提出了一种利用二噻吩乙烯来设计光致二阶非线性光学开关分子的新方案.与传统设计方案不同之处在于,新方案中,二噻吩乙烯主要作为一个得失电子能力可以改变的基团处于化合物的端基上.通过设计一组结构简单的模型化合物,并利用密度泛函理论计算它们的一阶超极化率β,我们实验了新方案的效果.计算结果表明,使用适当的取代基,能使设计的分子的光异构体之间的β比值达到10倍左右,差值超过100×10-30esu,使得这组模型化合物的二阶非线性光学开关能力不逊色于一些遵循传统设计方案的复杂金属有机化合物.