Abstract
Polar materials display a series of interesting and widely exploited properties owing to the inherent coupling between their fixed electric dipole and any action that involves a change in their charge distribution. Among these properties are piezoelectricity, ferroelectricity, pyroelectricity, and the bulk photovoltaic effect. Here we report the observation of a related property in this series, where an external electric field applied parallel or anti-parallel to the polar axis of a crystal leads to an increase or decrease in its second-order nonlinear optical response, respectively. This property of electric-field-modulated second-harmonic generation (EFM-SHG) is observed here in nanowires of the polar crystal ZnO, and is exploited as an analytical tool to directly determine by optical means the absolute direction of their polarity, which in turn provides important information about their epitaxy and growth mechanism. EFM-SHG may be observed in any type of polar nanostructures and used to map the absolute polarity of materials at the nanoscale.
Highlights
Polar materials display a series of interesting and widely exploited properties owing to the inherent coupling between their fixed electric dipole and any action that involves a change in their charge distribution
Polarity was found to influence chemical reactivity[3,4], doping and impurity incorporation[5,6,7], surface states and bandbending, which can strongly affect the nature of metallic contacts in functional systems, reflectivity[8] and the performance of devices based on polar orientations, such as LEDs and high-mobility transistors[1] (HEMTs)
Determination of the polarity of nanowires has been achieved by advanced transmission electron microscopy (TEM) methods, such as high-angle annular dark field (HAADF) and annular bright field (ABF) imaging along the nanowires[14,15]
Summary
Polar materials display a series of interesting and widely exploited properties owing to the inherent coupling between their fixed electric dipole and any action that involves a change in their charge distribution. We report the observation of a related property in this series, where an external electric field applied parallel or anti-parallel to the polar axis of a crystal leads to an increase or decrease in its second-order nonlinear optical response, respectively This property of electric-field-modulated second-harmonic generation (EFM-SHG) is observed here in nanowires of the polar crystal ZnO, and is exploited as an analytical tool to directly determine by optical means the absolute direction of their polarity, which in turn provides important information about their epitaxy and growth mechanism. Crystals with polar orientation can grow along either the [0001] or [0001] directions, termed metal-polar or anion-polar, respectively, which have a strong influence on both their chemical and optoelectronic properties[1] This effect is especially significant in 2D and 1D nanostructures due to their large surface-to-volume ratio. Surface-guided growth has been shown to enable the production of planar nanowires with a large variety of controlled crystallographic orientations, including zincblende (ZB)- and wurtzite (WZ)-
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