In this Letter, we describe a technique for dramatically improving the size monodispersity for cadmium sulfide/sulfur core/shell nanocrystals (henceforth CdS/S NCs) prepared on graphite surfaces using the Electrochemical/Chemical (E/C) method. Improved size monodispersity was achieved by depositing cadmium precursor particles using a train of 8-12 ms deposition voltage pulses separated by 1-5 second " mixing " segments during which growth is suspended instead of a single, longer deposition pulse (as shown below).

The effect of these shorter deposition pulses and mixing segments is to diffusionally decouple the growth of densely nucleated regions on the graphite surface from sparsely nucleated regions. The size monodispersity of cadmium NCs should be improved as a consequence, and this narrowing of the size distribution should translate into a narrowing of the PL emission lines for CdS/S NCs synthesized using the multipulse approach. This hypothesis is supported by the PL spectra shown below. CdS/S NCs synthesized from cadmium NCs deposited using a single plating pulse exhibit PL emission line widths of 125-180 meV whereas ensembles of approx. 300,000 Cd/S NCs nanocrystals synthesized using the "multipulse" approach exhibit PL emission linewidths of 15-35 meV.

As shown in below, the radius of the CdS core can be adjusted by increasing or decreasing the Cd^o plating pulse duration, while maintaining the decoupling time at 1 s. For example, the CdS/S NCs probed in the blue PL spectrum were synthesized using 10 x 8 ms Cd^o plating pulses. This spectrum exhibits a 150 meV blue shift (characteristic of a core radius of 39 �) and a linewidth of just 15 meV. Increasing the pulse duration to 12 ms yields the CdS/S NC sample probed in the green PL spectrum with a blueshift of just 58 meV (corresponding to a CdS core radius of 61 �), and a linewidth of 35 meV.