Resonant tunneling in a colloidal CdS semiconductor quantum-dot single-electron transistor based on heteroepitaxial-spherical Au/Pt nanogap electrodes.

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Semiconductor quantum dots (QDs) have unique discrete energy levels determined by the particle size and material. Therefore, they have potential applications as novel optical and electronic devices. Among those, colloidal group II-VI semiconductor quantum dots stand out for their facile synthesis and band gaps aligned with the visible light spectrum. However, the electrical characterization studies of an individual quantum dot necessitate the size of nanogap electrodes being equal to the size of the quantum dot, which has conventionally been evaluated using techniques such as scanning tunneling microscopy (STM) and nanogaps fabricated by electromigration. The complexity of device fabrication has restricted research in this area. Here, we present a pioneering approach for the electrical characterization of single-QD: heteroepitaxial-spherical (HS) Au/Pt nanogap electrodes. We fabricated transistors through chemisorption, an anchoring colloidal CdS QD (3.8 nm) between the HS-Au/Pt nanogap electrodes (gap separation: 4.5 nm). The resulting device functions as a quantum-dot single-electron transistor (QD-SET), showing resonant tunneling-an inherent characteristic of the QD. A steep current increase was observed at a negative voltage, apart from the theoretical single-electron tunneling current by Coulomb blockade phenomena, which agreed with the theoretical resonant tunneling current through a discrete energy level of the QD. This underscores the promise of HS-Au/Pt nanogap electrodes in realizing single-QD devices, offering a pathway toward unlocking their full potential.