The scanning tunneling microscope (STM) uses the concept of quantum tunneling to directly image atoms. A STM can be used to study the three-dimensional structure of a sample, by scanning the surface with a sharp, metal, conductive tip close to the sample. Such an environment is conducive to quantum tunneling: a quantum mechanical effect that occurs when electrons move through a barrier due to their wave-like properties. Tunneling depends on the thickness of the barrier; the Schrödinger equation gives the probability that a particle will be detected on the far side and, for a sufficiently thin barrier, predicts some electrons will cross it. This creates a current across the tunnel. The number of electrons that tunnel is dependent on the thickness of the barrier, therefore the current through the barrier also depends on this thickness. The distance between the tip and the sample affects the current measured by the tip. The tip is formed by a single atom that slowly moves across the surface at a distance of one atomic diameter. By observing the current, the distance can be kept fairly constant, allowing the tip to move up and down according to the structure of the sample.