We have measured the scanning tunneling microscope (STM) light emission spectrum of a single molecule of rhodamine 6G (R6G) adsorbed on highly oriented pyrolytic graphite (HOPG). Since the HOPG substrate radiates no STM light, we have succeeded in observing the spectrum radiated by R6G alone. The spectrum agrees well with the photoluminescence spectrum of R6G on HOPG with the exception of two structures that may arise from a triplet state whose transition is forbidden in photoluminescence. Based on this agreement, we have determined the STM light emission mechanism of adsorbed R6G.
We have measured the spectra of visible light emitted from the individual structures of porous Si (PS) below the probe tip of the scanning tunneling microscope (STM), and found that the peak energy of the emission spectrum shifts with the size of nanometer-scale structures on the PS surface. Samples with a PS layer ∼50 nm thick were formed by anodic etching of p+Si(100) substrates (∼0.005 Ω cm). The STM images show that protrusions whose dimensions are 3–10 nm in diameter are distributed on the PS surface. The peak energy of the STM light emission spectrum shifts from ∼1.7 to ∼2.1 eV as the diameter of the structure below the STM tip decreases from ∼9 to ∼3 nm. The measured peak shift with the size of the structure is consistent with the shift of the energy gap predicted on the basis of a quantum confinement model.
Measurement of the light emission spectrum from a scanning tunneling microscope (STM) requires a long exposure time due to its extremely low intensity, and thermal drift of the tip during the exposure time limits the spatial resolution. To improve the resolution, a computer controlled servomechanism that locks the STM tip over a target position has been developed. We have measured the light emission spectra from individual nanometer scale structures on an evaporated Au film with and without this mechanism, and demonstrated the effectiveness of the servomechanism.
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