Coulomb-blockade oscillation in CdS, ZnS and CdS/ZnS core-shell quantum dots

Abstract

CdS, ZnS and core–shell CdS/ZnS quantum dots (QDs) with different ratio concentration of core to shell (1:1, 1:2, 1:3, 1:4, 1:5) were synthesized and their structural, optical and electrical properties were studied with respect to the increase shell thickness. XRD exhibits the wurtzite structure of CdS QDs along with the cubic trace of ZnS. HRTEM images exhibit spherical shaped particle morphology for both CdS and CdS/ZnS core–shell QDs. The optical properties show a defect control mechanism in carrier transport. Enhancing shell thickness introduces higher defect density, defect induced crystal strain and charge separation which leads to quantum mechanical tunneling in determination of conduction mechanism through QDs. The current-voltage (I-V) characteristics of the QD devices shows an oscillatory behavior which may be attributed to coulomb-blockade oscillations because of quantum structure of QDs. Room temperature quantum oscillation behavior has been studied systematically for the synthesized semiconductor core/shell QDs which is the major finding of the present work. Mullen’s two tunnel junction model has been used for the QD devices, which is in agreement with the observed coulomb oscillations for R1 ≪ R2 and C1 ≪ C2, originated from defect induced surface and interface of core/shell structure. The present experimental results may contribute towards study and development of futuristic quantum oscillatory nanoscale devices.