Visible light is emitted when electrons (holes) are injected into a smaple from the tip of the scanning tunneling microscope (STM). By analyzing the spectra of the emitted light, one can not only determined the surface geometry by usual STM imaging, but also learn the electronic and optical properties of specific individual nanostructures. This technique has been applied to investigate the electronic transitions of individual protrusions of porous Si and semiconductor quantum wells of AlGaAs/GaAs. The usefulness, limitations, and future expectations of this novel technique are discussed.
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