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Micro- or nano-diodes suspended in water can propel controllably under alternating-current fields or light without the need for fuels such as hydrogen peroxide or urea. This makes them promising candidates for miniature motors in biomedical and other applications. However, the mechanisms underlying their propulsion remain unclear. This study investigates the propulsion of diodes floating in an aqueous solution at the millimeter scale, which facilitates observation of motion, allowing direct correlation with electrical measurements of device properties. We find that the diode’s propulsion is driven by forward current under an alternating-current field and by photocurrent under illumination. The velocity of propulsion scales linearly with the net current, with the rectified or photogenerated current creating an imbalance of ions at the ends of the diodes. This, in turn, generates an electric field that induces electrophoretic flow around the diode and propels the diode. Additionally, we assess the velocity of diodes intentionally damaged by high reverse bias and find that it decreases significantly because of the reduced difference between forward and reverse currents. These results suggest potential uses of diode propulsion for characterizing and separating bottom-up-grown nano-/micro- diodes based on their reverse-saturation current, as well as nanomotors formed from multiple-junction nanowire diodes that can self-propel in water under light. 

Doped semiconductor nanowires are emerging as next-generation electronic colloidal materials, and the efficient manipulation of such nanostructures is crucial for technological applications. In fluid suspension, pn nanowires (pn NWs), unlike homogeneous nanowires, have a permanent dipole, and thus, experience a torque under an external DC field that orients the nanowire with its n-type end in the direction of the field. Here, we quantitatively measure the permanent dipoles of various Si nanowire pn diodes and investigate their origin. By comparing the dipoles of pn NWs of different lengths and radii, we show that the permanent dipole originates from non-uniform surface-charge distributions, rather than the internal charges at the p–n junction as was previously proposed. This understanding of the mechanism for pn NWs orientation has relevance to the manipulation, assembly, characterization, and separation of nanowire electronics by electric fields. 

Abstract 2D layered nanomaterials have attracted considerable attention for their potential for highly efficient separations, among other applications. Here, a 2D lamellar membrane synthesized using hexagonal boron nitride nanoflakes (h‐BNF) for highly efficient ion separation is reported. The ion‐rejection performance and the water permeance of the membrane as a function of the ionic radius, ion valance, and solution pH are investigated. The nonfunctionalized h‐BNF membranes show excellent ion rejection for small sized salt ions as well as for anionic dyes (>97%) while maintaining a high water permeability, ≈1.0 × 10⁻³L m m⁻²h⁻¹bar⁻¹). Experiments show that the ion‐rejection performance of the membrane can be tuned by changing the solution pH. The results also suggest that the rejection is influenced by the ionic size and the electrostatic repulsion between fixed negative charges on the BN surface and the mobile ions, and is consistent with the Donnan equilibrium model. These simple‐to‐fabricate h‐BNF membranes show a unique combination of excellent ion selectivity and high permeability compared to other 2D membranes.