Plasmon-mediated scanning tunneling microscopy induced luminescence in CdSe/ZnCdS quantum dots

We report scanning tunneling microscopy-induced luminescence (STML) from CdSe/ZnCdS colloidal quantum dots (QDs) deposited on Au substrates by immersion. Localized electron injection through the tunneling junction enables site-specific optical excitation and photon emission. Light emission from the QDs is observed only when using STM tips that exhibit strong plasmonic emission on the gold surface. Spectral analysis reveals two distinct features: a plasmonic peak at approximately 1.8 eV, which shifts with the applied bias, and an excitonic peak at 1.96 eV, which is bias-independent and consistent with photoluminescence measurements. The bias-dependent tunneling-induced luminescence shows exciton quenching and plasmon enhancement at higher voltages, and exclusively plasmonic emission below the quantum cutoff required for excitonic recombination. The excitation is spatially confined to a nanometer-scale region of the tunnel junction, enabling locally selective optical excitation, and the tunneling-induced quantum yield was ≈ 6.4 × 10⁻⁷ photons per electron. Scanning tunneling spectroscopy measurements indicate an electronic gap of roughly 2.3 eV, compatible with the ZnCdS shell and surface ligands. These results demonstrate the ability of STML to probe charge injection and light-emission pathways in colloidal quantum emitters with nanometer-scale spatial selectivity, as well as the challenges associated with achieving full single-QD spectroscopy using this approach.