Scanning tunneling spectroscopy is used to investigate single InAs nanocrystals, 20--70 {} in diameter, in a highly asymmetric double barrier tunnel junction configuration. The I-V characteristics reflect contributions of both single-electron charging and the atomiclike level structure of the quantum dots. The spectra are simulated and well described within the framework of the ``orthodox model'' for single-electron tunneling. The peaks in the tunneling spectra display a systematic broadening with the reduction of dot diameter, from 40 to 150 meV over the studied quantum dot size range. This is assigned to a decreased electron dwell time on the dot, due to reduction of the barrier height, induced by the blueshift of the quantum-confined levels. The distribution of peak spacings within charging multiplets in the tunneling spectra is found to be Gaussian, resembling observations on metallic quantum dots.
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Millo et al. (2000) studied this question.