Color imaging with a low temperature scanning tunneling microscope

We report on an improved optical design for detecting light emitted from a scanning tunneling microscope (STM). Using a charge coupled device camera and a grating spectrometer a photon detection efficiency of ≈2.5% at 550 nm is achieved and count rates of up to ${\text{5×10}}^4\hspace{0.17em}\mathrm{counts/nA/s}$ are observed on a noble metal surface and a W tip. Statistically significant spectra from noble metal surfaces are detected in tens of milliseconds. Thus, new modes of measurement become available, which encompass spectroscopic imaging (acquisition of fluorescence spectra at each point of a STM image), and excitation spectroscopy (acquisition of fluorescence spectra while varying the tip–sample bias). Spectroscopic imaging is used to observe gradual changes of the emission spectra as the STM tip approaches a monoatomic step of Ag(111) on a nanometer scale. Excitation spectroscopy with high resolution in both wavelength and bias voltage is demonstrated for a Ag(111) surface.