Semiconductor Nanowire

perovskite_1    CsPbBr3 NW laser    Screen Shot 2017-03-21 at 11.25.51 PM    2D NS

The term ‘nanowire’ is generally used to describe a large aspect ratio nanostructure with a diameter between 1-100 nm.  Both descriptors are pertinent to the physical and technological significance of nanowires.  Firstly, the diameter puts the radial dimension of these structures at or below the characteristic length scale of various interesting and fundamental solid-state phenomena: the exciton Bohr radius, wavelength of light, phonon mean free path, critical size of magnetic domains, exciton diffusion length, and others.  As a result, many physical properties of semiconductors are significantly altered within the confines of the nanowire surfaces. In addition, their large surface-to-volume ratio allows for distinct structural and chemical behavior, as well as greater chemical reactivity. This two-dimensional confinement endows nanowires with unique properties that stray from those of their corresponding bulk material.  Secondly, the large aspect ratio of nanowires imitates their technological application. The one unconstrained dimension can direct the conduction of quantum particles such as electrons, phonons, and photons.  As a result, nanowires provide a convenient platform through which researchers may study confined transport phenomena. This control over various forms of energy transport makes nanowires ideal materials for advanced solid-state devices, especially when combined with the fact that nanowire lengths are normally sufficient to interface with top-down fabrication processes such as photolithography.

For decades, nanowire research has exclusively focused on traditional covalent solids including group-IV, III-V, and II-VI materials featuring four-fold coordination and covalent sp3 bonding. The recent renaissance of halide perovskites has raised the prospects of semiconductor materials with ionic bonding, featuring more complex crystal structures and unique properties of interest for future energy conversion and microelectronics applications. These ionic crystals are distinguished by lower cohesive energies compared with traditional covalent semiconductors (2-3 eV per atom for typical ionic crystals and halide perovskites, but > 4 eV per atom for covalent semiconductors such as Si). Because of this low cohesive energy, the chemical bonds within these new semiconductors can be readily reconfigured (i.e., bond breaking and formation with low energy input). The materials consequently are responsive towards environmental stimuli, such as temperature, strain, chemical environment, laser irradiation, and electric field. Balancing ionic and covalent bonding in complex nanostructure motifs creates new opportunities to advance the science of semiconductor nanowires with properties and processing pathways unattainable in conventional covalent solids.

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