Shell Thickness-Dependent Tunable Threshold Voltage Single Quantum Dot Rectification Diode

Ambient atmosphere single colloidal quantum dot (QD) rectifying diode with tunable threshold voltage has been fabricated by using a type-II heterojunction core/shell structure with a device geometry ITO/ZnO/QDs. Specifically, in our work we have used ZnTe/CdS core/shell QDs in which hole wave function strongly confined to the core, whereas the lowest-lying conduction band state resides in the shell. Current–voltage (I–V) characterization of this device has been done using an ambient atmosphere scanning tunneling microscope. The scanning tunneling spectra (STS) shows high rectification with a ratio of 10³. The rectification is found to arise because of the bias-dependent band alignment of ZnO/QDs heterojunction and the effect of shell of each QD that presents a barrier for hole tunneling into the substrate. This barrier is overcome by the externally applied bias. This mechanism is distinct from the rectification observed in conventional p–n junction diodes. In particular, we find that even for QDs with optical band gaps of ∼1 eV, the threshold voltage may be tuned from 1 to 3 V by regulating shell thickness.