Abstract Semiconductor molybdenum ditelluride (2H-MoTe2) possess multiple valleys in the band structure, enriching its physical properties and potentials in applications. However, the effect of multiple valleys on the mechanisms of population and relaxation of carriers and phonons remains limited, particularly due to the inadequacy of current optical probes that lack momentum sensitivity. Here, we rely on time resolved measurements of optical absorption and electron diffraction to investigate the carrier intra- and intervalley scattering and the phonon dynamics in different valleys in photoexcited few-layer 2H-MoTe2. Our experimental results are complemented by density functional theory calculations and molecular dynamics simulations. We reveal the pathways and timescales of carrier relaxation, accompanied with the emissions of optical phonons at the Brillouin zone center and acoustic phonons at the zone border. We estimate the population of different phonon modes based on the measured results, identifying quantitatively the occurrences of phonon bottleneck located in different valleys. Our technique allows constructing a comprehensive picture of the complex interactions between carriers and phonons in 2H-MoTe2 with the valley degree of freedom resolved.
Abstract Black nightshade ( Solanum nigrum, S. nigrum L.) and red nightshade ( Solanum villosum, S. villosum Mill.) are medicinal plants from the Solanaceae family that synthesize glycoalkaloids and other secondary metabolites. To recognize the potential insecticide activity of these compounds, leaf extracts (containing glycoalkaloid and methanol fractions) were tested for enzyme inhibition, antifeedant activity and toxicity. For in‐vitro glutathione S‐transferase (GST) inhibition activity, we used insecticide‐resistant Colorado potato beetle, Leptinotarsa decemlineata ( L. decemlineata ; Say) midgut and fat‐body homogenate. In‐vivo toxicity and the antifeedant activity were performed using larval bioassays. The methanol extracts had greater GST inhibitory activity compared to the glycoalkaloids, as well as greater 2nd instar larvae mortality and antifeedant activity. Furthermore, the green leaf volatile compound, cis‐hex‐3‐enyl acetate, at the concentration of 5 ppm, caused 50% mortality of 2nd instar larvae. Our findings suggest the potential usefulness of S. nigrum and S. villosum extracts to control L. decemlineata .
The physical and chemical properties of nanophase materials rely on their crystal and surface structures. Transmission electron microscopy (TEM) is a powerful and unique technique for structure characterization. The most important application of TEM is the atomic-resolution real-space imaging of nanoparticles. This article introduces the fundamentals of TEM and its applications in structural determination of shape-controlled nanocrystals and their assemblies. By forming a nanometer size electron probe, TEM is unique in identifying and quantifying the chemical and electronic structure of individual nanocrystals. Electron energy-loss spectroscopy analysis of the solid-state effects and mapping the valence states are even more attractive. In situ TEM is demonstrated for characterizing and measuring the thermodynamic, electric, and mechanical properties of individual nanostructures, from which the structure−property relationship can be registered with a specific nanoparticle/structure.
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Wurtzite structures, such as ZnO, GaN, InN and CdS, are piezoelectric semiconductor materials. A piezopotential is formed in the crystal by the piezoelectric charges created by applying a stress. The inner-crystal piezopotential can effectively tune/control the carrier separations and transport processes at the vicinity of a p–n junction or metal–semiconductor contact, which is called the piezo-phototronic effect. The presence of piezoelectric charges at the interface/junction can significantly affect the performances of photovoltaic devices, especially flexible and printed organic/inorganic solar cells fabricated using piezoelectric semiconductor nano/microwires. In this paper, the current–voltage characteristics of a solar cell have been studied theoretically and experimentally in the presence of the piezoelectric effect. The analytical results are obtained for a ZnO piezoelectric p–n junction solar cell under simplified conditions, which provide a basic physical picture for understanding the mechanism of the piezoelectric solar cell. Furthermore, the maximum output of the solar cell has been calculated numerically. Finally, the experimental results of organic solar cells support our theoretical model. Using the piezoelectric effect created by external stress, our study not only provides the first basic theoretical understanding about the piezo-phototronic effect on the characteristics of a solar cell but also assists the design for higher performance solar cells.
The nanogenerator (NG), first proposed for the purpose of self-powered nanotechnology in 2006, converts random mechanical energy into electric energy using piezoelectric zinc oxide nanowire (NW) arrays. The mechanism of the NG relies on the piezoelectric potential created in the NWs by an external strain: a dynamic straining of the NWs results in a transient flow of the electrons in the external load because of the driving force of the piezoelectric potential. The advantage of using NWs is that they can be triggered by tiny-scale physical motions. Further, the excitation frequency can range from one Hz to thousands of Hz, which makes NGs ideal for harvesting random energy in the environment. In our living environment, there exists an abundant amount of mechanical energy otherwise lost, such as light wind, body movement, muscle stretching, acoustic/ultrasonic waves, noises, mechanical vibration, and blood flow. Hence, the NW based piezoelectric NGs can provide a promising potential as energy harvester for solving the energy crisis and preserving environmental health. The objective of this chapter is to introduce the fundamentals of piezoelectric NW based NGs. We start from the synthesis of piezoelectric NWs, followed by a description of the fundamental principle of a NG based on a single NW. We then demonstrate the engineering approaches for achieving high output power by laterally and vertically integrating the contribution from thousands of NWs. Lastly, we show the prototype for self-powered systems and self-powered active sensors.
