לוח שנה משולב

One-dimensional materials represent an attractive class of nanostructures, mainly because of their geometry which inherently implies applications such as electrodes for sensing purposes or conduction channels in nanoscale electronics. Different mechanisms may be utilized to prepare nanowires, e.g. stress-driven one-dimensional diffusion, metal-catalyzed growth (VLS) etc. The most important role in identifying and description of the growth mechanisms is played by real-time microscopies, mostly TEM. However, although very powerfull in terms of image resolution, TEM is also limited in use, especially because of very strict sample geometry requirements. In this seminar talk, I will present our real-time in-situ scanning electron microscopy experiments of nanowire growth. Two different material systems will be presented – germanium nanowires catalyzed by Au nanoparticles and WOx nanowires. As for the latter case, our experiments reveal a very unusual oxidation mechanism of tungsten disulfide nanotubes, resulting in tungsten oxide nanowire formation. The talk will summarize studies on quasi-1D systems conducted at IPE and CEITEC BUT.

A Brownian motor rectifies thermal noise and creates useful work. Here we address how this machine can emerge without predefined energy minimum in a system out of thermal equilibrium. Intuitively, Brownian motor as any artificial or biological machine should degrade with time. I will show that on contrary, a system with multiple degrees of freedom out of thermal equilibrium can be stable at a state that generates useful work.

It is demonstrated with the help of ab initio analysis of a modified Feynman-Smoluchowski ratchet with two degrees of freedom. Out of thermal equilibrium, an environment imposes effective mechanical forces on nano-fabricated devices as well as on microscopic chemical or biological systems. Thus out of thermal equilibrium environment can enforce a specific steady state on the system by creating effective potentials in otherwise homogeneous configuration space.

I present an ab initio path from the elastic scattering of a single gas particle by a mechanical system to the transition rate probability between the states of the system with multiple degrees of freedom, together with the corresponding Masters-Boltzmann equation and the average velocities of the system’s degrees of freedom as functions of the macroscopic parameters of the out-of-equilibrium environment. It results in Onsager relations that include the influence of the different degrees of freedom on each other.

An interesting finding is that some of these forces persist even in a single temperature environment if the thermodynamic limit does not hold. In addition, the spatial asymmetry of the system’s stable state, together with the corresponding directed motion, may possess preferred chiral symmetry.

Simple 2-dimensional cut and fold patterns can be transformed into 3-dimensional shapes upon stretch-ing. We use this simple approach to develop mechanical metamaterials with several interesting proper-ties and applications. I will describe ways of tuning properties via geometric structure, and discuss ex-amples of how this can be used to achieve superior performance in mechanics, photonics, electronics, sensors, and other areas.

References:
“Dynamic kirigami structures for integrated solar tracking.” Nature Comm. 6, 8092 (2015)
“A kirigami approach to engineering elasticity in nanocomposites through patterned defects.” Na-ture Mater., 14 (2015) 785
“An Electric Eel-Inspired Artificial Soft Power Source from Stacked Hydrogels.” Nature, 552 (2017) 214