NOVELTY - An array of aligned silicon nanowires is attached to a substrate (42) at a crystalline silicon surface of the substrate. The majority of nanowires have diameters of no more than about 150 nm and are attached at least approximately perpendicular to the substrate. The crystalline silicon surface at which the array is attached is a crystal plane in an orientation other than (100) and (111).    USE - Array of aligned silicon nanowires is used in lithium-ion absorbing materials of lithium ion batteries, optoelectronic devices e.g. LEDs, photonic elements and/or waveguides, and photovoltaic or solar cell devices (all claimed). Can also be used for photodetectors, photodiodes, phototransistors, photomultipliers, and integrated optical circuits.    ADVANTAGE - The array of aligned silicon nanowires has controlled diameter and length. Formation of unintentional nanowires (grass) is eliminated, while desirable sub-100 nm nanowires are produced. Rounding of nanostructure corners and edges resulting from etching is avoided. The etching process can be extended to specified crystallographic directions. The metal film has excellent adhesiveness. The array can be used in applications where silicon will be subjected to stress or strain where the nanostructure absorbs and relaxes the stress or strain. The nanowires act as an interfacial layer between bulk silicon and another material grown on top which is not lattice-matched to it. The array has low discharge potential and high charge capacity, which makes it suitable for use in lithium ion batteries. The nanowires have the ability to form well ordered and aligned nanostructures with a great deal of control over the resulting diameter and void spacing between them. A dense array of silicon nanowires with specific control over wire diameter, doping, and crystallographic orientation is obtained.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) etched nanowire array, where 75% or more of the nanowires have a diameter below 150 nm; and    (2) etching process of silicon-containing substrate to form structures, which involves depositing nanoparticles on the surface of a silicon-containing substrate, depositing metal on top of the nanoparticles and silicon in such a way that the metal is present and touches silicon where etching is desired and is blocked from touching silicon or not present elsewhere, and contacting the metallized substrate with an etchant aqueous solution comprising 2-49 wt.% hydrofluoric acid and an oxidizing agent. The process results in nanowire arrays in which the average diameter of the nanowires is less than 125 nm.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic set up used for etching a silicon-containing substrate.    Container (40)    Silicon substrate (42)    Oxygen gas (44)    Oxygen bubbles (46,48)