A nanotube assembly including a nanotube layer, a first layer and a second layer. The nanotube layer comprises a vertically aligned nanotube array. The nanotube array includes a plurality of nanotubes. The first layer of a first conductive material is disposed on one surface of the nanotube layer. The second layer of a second conductive material is disposed on an opposite surface of the nanotube layer. The nanotube of the nanotube layer includes a first end against the first layer and a second end against the second layer. The resistance from the first end to the first layer is lower than a resistance from the second end to the second layer. One or more nano-particles are placed within the nanotube. At least one of the nano-particles is electrically charged, and can move along the nanotube under influence of an electric field.