We report the electrical properties of cadmium sulfide (CdS) nanoclusters investigated by fabricating bulk-nano (bn) junctions of CdS. These diodes exhibited the characteristic tunneling peaks in the reverse biasing region, which were used to estimate the cluster sizes, and are in agreement with those estimated from the X-ray diffraction (XRD) experiments. On forward biasing, the bn junction showed a step-like behavior, which could be a direct demonstration of the quantum size effects (QSE) exhibited by these clusters. This effect can be assigned either to the enhancement of the collective Coulomb blockade phenomenon occurring in the quantum dots or to the collective tunneling of electrons through the dots. (C) 2002 Elsevier Science B.V. All rights reserved.