Morphology-driven gas sensing by fabricated fractals: A review.

Fractals are intriguing structures that repeat themselves at various length scales. Interestingly, fractals can also be fabricated artificially in labs under controlled growth environments and be explored for various applications. Such fractals have a repeating unit that spans in length from nano- to millimeter range. Fractals thus can be regarded as connectors that structurally bridge the gap between the nano- and the macroscopic worlds and have a hybrid structure of pores and repeating units. This article presents a comprehensive review on inorganic fabricated fractals (fab-fracs) synthesized in labs and employed as gas sensors across materials, morphologies, and gas analytes. The focus is to investigate the morphology-driven gas response of these fab-fracs and identify key parameters of fractal geometry in influencing gas response. Fab-fracs with roughened microstructure, pore-network connectivity, and fractal dimension (D) less than 2 are projected to be possessing better gas sensing capabilities. Fab-fracs with these salient features will help in designing the commercial gas sensors with better performance.

Effect of nitrogen doping on hydrogen storage capacity of palladium decorated graphene.

A high hydrogen storage capacity for palladium decorated nitrogen-doped hydrogen exfoliated graphene nanocomposite is demonstrated under moderate temperature and pressure conditions. The nitrogen doping of hydrogen exfoliated graphene is done by nitrogen plasma treatment, and palladium nanoparticles are decorated over nitrogen-doped graphene by a modified polyol reduction technique. An increase of 66% is achieved by nitrogen doping in the hydrogen uptake capacity of hydrogen exfoliated graphene at room temperature and 2 MPa pressure. A further enhancement by 124% is attained in the hydrogen uptake capacity by palladium nanoparticle (Pd NP) decoration over nitrogen-doped graphene. The high dispersion of Pd NP over nitrogen-doped graphene sheets and strengthened interaction between the nitrogen-doped graphene sheets and Pd NP catalyze the dissociation of hydrogen molecules and subsequent migration of hydrogen atoms on the doped graphene sheets. The results of a systematic study on graphene, nitrogen-doped graphene, and palladium decorated nitrogen-doped graphene nanocomposites are discussed. A nexus between the catalyst support and catalyst particles is believed to yield the high hydrogen uptake capacities obtained.

Carbon nanotube photo-thermo-mechanical actuator.

Carbon nanotubes show a remarkable tendency for direct (rapid) temperature increase of the order of hundreds of degrees when exposed to near infra-red light. The reason is local confinement of the heat wave in their 1D structure which generates rapid temperature rise. Here we demonstrate that these high temperatures can be exploited to generate large deformation and force output by anchoring the nanotubes to a substrate. We report energy density (i.e., work done per unit mass) of the nanotube actuator as approximately 4268 J/Kg which is significantly larger than piezoceramic (approximately 4.25 J/Kg), magnetostrictive (approximately 21.6 J/Kg), lead-zinc-niobate/lead-titanate single-crystals (approximately 131 J/Kg), polyvinylidene fluoride trifluoro-ethylene copolymers (approximately 160 J/Kg) and shape memory alloys (approximately 1337 J/Kg).

Mechanical characteristics of silicon nanostructures using force distance spectroscopy.

Experimental studies were undertaken to determine mechanical stiffness of Si chevron nanostructures grown by glancing angle deposition. Atomic force microscope based force-distance spectroscopy was performed on two types of chevron structures. The average stiffness of four-armed chevrons was found to be 381 +/- 16 Nm(-1), while that of five-armed chevrons was determined to be 375 +/- 23 Nm(-1). Simulations using finite element modeling were carried out to understand the mechanical characteristics of chevrons. For the nanostructures investigated in the present study, the simulation results indicate that while five-armed chevrons behave as springs, the four-armed chevrons act as cantilevers. It is shown that the position of loading point, physical dimensions and the geometry of the chevron control the overall mechanical response of chevron structures when subjected to an external load. It is proposed that by controlling the deposition parameters in glancing angle deposition, the topography of the structures and hence the position of loading points can be manipulated to generate a desirable mechanical response.