This tutorial review focuses on recent work addressing the properties and potential of semiconductor nanowires as building blocks for photovoltaic devices based on investigations at the single nanowire level. Two central nanowire motifs involving p-i-n dopant modulation in axial and coaxial geometries serve as platforms for fundamental studies. Research illustrating the synthesis of these structural motifs will be reviewed first, followed by an examination of recent studies of single axial and coaxial p-i-n silicon nanowire solar cells. Finally, challenges and opportunities for improving efficiency enabled by controlled synthesis of more complex nanowire structures will be discussed, as will their potential applications as power sources for emerging nanoelectronic devices.
This paper presents a short synthesis of new analogs of N-acetylneuraminic acid (Neu5Ac) varied structurally at C-5. The synthetic strategy includes indium-mediated coupling reactions between ethyl 2-(bromomethyl)acrylate and N-derivatized mannosamines, and the ozonolysis of the resulting enoates. The main advantage of this indium-mediated allylation for the synthesis of neuraminic acids comes from the efficient, stereoselective C-C bond formation, which affords predominantly the correct diastereomer having a threo relationship between the newly generated hydroxyl group and the C-2 amide group of mannosamine. By this approach, Neu5Boc (4a), Neu5Gly (4b), Neu5(6-NHCbz)hexanoyl (4c), and Neu5(1-naphthyl)acetyl (4d) were prepared in three steps (overall approximately 50%). In addition, several N-substituted neuraminic acids were synthesized by N-acylation of the amino functionality of neuraminic acid (5b), which was obtained by deprotecting the N-Boc group of Neu5Boc (4a). These analogs include Neu5BrAc (6a), Neu5acryloyl (6b), Neu5benzoyl (6c) and Neu5benzoyl-4-benzoyl (6d). The N-acylation method is especially suited for synthesis of neuraminic acids bearing substituents that can not tolerate ozonolysis or that are unstable (photo)chemically. Finally, we illustrate the utility of synthetic neuraminic acids by converting 4c to a derivative of 2-deoxy-2,3-didehydroneuraminic acid (8c), a precursor to inhibitors of neuraminidases.
Departures of the geometry of the middle surface of a thin shell from the perfect shape have long been regarded as the most deleterious imperfections responsible for reducing a shell’s buckling capacity. Here, systematic simulations are conducted for both spherical and cylindrical metal shells whereby, in the first step, dimple-shaped dents are created by indenting a perfect shell into the plastic range. Then, in the second step, buckling of the dented shell is analyzed, under external pressure for the spherical shells and in axial compression for the cylindrical shells. Three distinct buckling analyses are carried out: (1) elastic buckling accounting only for the geometry of the dent, (2) elastic buckling accounting for both dent geometry and residual stresses, and (3) a full elastic–plastic buckling analysis accounting for both the dent geometry and residual stresses. The analyses reveal the relative importance of the geometry and the residual stress associated with the dent, and they also provide a clear indicator of whether plasticity is important in establishing the buckling load of the dented shells.
Scanning tunneling microscopy (STM) and atomic force microscopy (AFM) have been used to characterize the structural and electronic properties of single-crystal MoS2, nickel-substituted MoS2 (Ni x Mo1−x S2), and chalcogenide-substituted MoS2 (MoS2−x Ch x , Ch ≡ Se, Te) at the atomic level. Images of Ni0.1Mo0.9S2 demonstrate that nickel substitution causes localized changes in the electronic states, although the structure of the surface sulfur layer is unchanged compared with MoS2. Investigations of MoS1.75Se0.25 also show that within the detection limits of AFM selenium substitution does not perturb the sulfur surface structure, and STM data further indicate that the substituted selenium is electronically delocalized. In contrast, AFM studies of MoS1.75Te0.25 show that tellurium substitution produces atomic-sized structural protrusions that may modify significantly the tribological properties of MoS2. In addition, we demonstrate that material wear can be characterized on an atomic scale by AFM. These studies indicate that the microscopic origin of material wear as well as the local structure and electronic properties should be considered to develop further models of friction and wear in metal dichalcogenide materials.
No abstract is provided for this article.
We study the effect of time‐variable normal stress perturbations on a creeping fault which satisfies a velocity‐weakening rate‐ and state‐dependent friction law and is slipping at constant speed. We use the spring‐block model and include the effect of inertia. To account for the variable normal stress, we use the description introduced by Linker and Dieterich [1992], which links normal stress fluctuations to changes of the state variable. We consider periodic perturbations of the normal stress in time (as caused, for instance, by tides) and compare the behavior for two commonly used friction laws (the “slip” and the “ageing” laws). Their mechanical response is shown to be significantly different for normal stress fluctuations. It could be used to probe these two laws during laboratory friction experiments. We show that there is a resonance phenomenon, involving strong amplification of the shear and velocity response of the interface, when the spring stiffness is modestly above its critical value (or when, at a given stiffness, the normal stress is modestly below its critical value). We show that such an amplification is also observed when periodic fluctuations of the shear loading are considered, making the resonance phenomenon a general feature of the response of a near‐critical creeping surface to periodic fluctuations of the external loading. Analytical solutions are based on a linear expansion for low amplitude of normal or shear stress variations and are in very good agreement with numerical solutions. A method to find the evolution of friction in the case of an arbitrary perturbation of the normal stress is also presented. The results show that a creeping fault may be destabilized and enter a stick‐slip regime owing to small normal stress oscillations. This may also account for a mechanism for the generation of “creep bursts.” However, these phenomena require very specific parameter ranges to excite the resonance, which may not be met very generally in nature. This study illustrates the importance of the normal stress fluctuations on stable sliding and suggests further friction laboratory experiments.
