NOVELTY - The method comprises exposing an array of structures to a gas or vapor containing hydrogen atom and/or hydrogen molecule at a gas temperature of greater than or equal to 400 degrees C for 30 minutes, allowing the array to react with the gas so that one of the structures containing copper oxide is reduced to pure copper with a diameter of 20-100 nm, providing a substrate of copper having an exposed surface, heating the substrate to a temperature of greater than or equal to 400 degrees C for greater than or equal to 30 minutes, and allowing (13) formation of nano structures containing copper (I) oxide and copper (II) oxide on the substrate with a diameter of 20-100 nm.    USE - The method is useful for fabricating small diameter nanowires of metallic copper, where the nano wires are useful as vertical interconnects in a circuit of a probe for an atomic force microscope, a scanning tunneling microscope, a photo detector and a solar cell (all claimed).    ADVANTAGE - The method allows the reliable fabrication of the nanowires with reduced diameter (20 nm) and with high thermal and electrical conductivity and without any cracks.    DETAILED DESCRIPTION - The method comprises exposing an array of structures to a gas or vapor containing hydrogen atom and/or hydrogen molecule at a gas temperature of greater than or equal to 400 degrees C for 30 minutes, allowing the array to react with the gas so that one of the structures containing copper oxide is reduced to pure copper with a diameter of 20-100 nm, providing a substrate of copper having an exposed surface, heating the substrate to a temperature of greater than or equal to 400 degrees C for greater than or equal to 30 minutes, allowing (13) formation of nano structures containing copper (I) oxide and copper (II) oxide on the substrate with a diameter of 20-100 nm, and orienting the nano structures in the array in a vertical direction that is perpendicular to the surface of the substrate. The structures have a diameter of 40 nm, before the exposing step. The substrate is heated to a plateau temperature of 400 degrees C for a temperature ramp-up time of 60 minutes, where the plateau temperature is constant. The method further comprises permitting the temperature of the array to decrease towards a room temperature at a temperature reduction rate that is equal to a cooling rate in ambient air so that one of the nano structures does not develop a crack, orienting the nano structures in the array in a lateral direction that is parallel to the surface of the substrate, and growing the nano structures with a length of less than 25 nm. The step of allowing the array to react with the gas includes removing oxygen atom and oxygen molecule from the structures.    DESCRIPTION OF DRAWING(S) - The figure shows a flow chart of a method for fabricating small diameter nanowires.    Providing bulk cooper wire (11)    Thermally oxidizing wire (12)    Allowing formation of nanostructures of copper oxide on the wire (13)    Exposing nanostructures to hydrogen gas (14)    Cooling nanostructures slowly to room temperature. (15)