To make the specific features of nanoerystalline silicon (ne-Si) cold cathode clear, its emission characteristics are investigated in low vacuum and atmospheric pressure, The experimental nc-Si diode is composed of a thin Au film, a nanocrystallized polysilicon layer, an n-type silicon wafer, and a back contact. It is shown that the ballistic electron emission mode of the nc-Si device is kept alive until the vacuum pressure is increased to about 10 Pa, and that a significant current signal is detected at the collector even in air. The current observed in air can be interpreted as a result of energetic electron attachment into oxygen molecules in proximity to the front Surface. The effectiveness of this emitter for the use in air is applied to surface charging of an insulating polymer. The experimental results demonstrate that the surface of a polymer film located in air at a distance of 1 mm from the device surface is quickly charged up to a certain negative potential determined from the collector voltage. The surface-emitting nc-Si ballistic emitter is useful not only in vacuum, but also in air and gas ambient for various applications. (c) 2005 American Vacuum Society.