SIZE-SELECTIVE AND EPITAXIAL ELECTROCHEMICAL CHEMICAL SYNTHESIS OF SULFUR-PASSIVATED CADMIUM-SULFIDE NANOCRYSTALS ON GRAPHITE/

Cadmium metal nanocrystallites (NCs), prepared on graphite surfaces by electrochemical deposition, are employed as precursors to synthesize core-shell nanoparticles consisting of a crystalline cadmium sulfide ( CdS) core and a sulfur or polysulfide shell. Core-shell NCs having a l arge CdS core (radii R-Cds > 40 Angstrom) were prepared by exposing el ectrodeposited cadmium particles (R-Cd > 25 Angstrom) to H2S at 300 de grees C, whereas nanoparticles having a smaller CdS core (down to 17 A ngstrom) were obtained from cadmium precursor particles via a Cd(OH)(2 ) intermediate. For both large-core and small-core CdS nanoparticles, the addition of the sulfur capping layer (ranging in thickness from 5 to 30 Angstrom) occurred during exposure to H2S st 300 degrees C. Tran smission electron microscopy (TEM) and selected area electron diffract ion (SAED) data show that the synthesis of CdS NCs proceeded on a part icle-by-particle basis such that the particle size and monodispersity of the CdS core were directly related to those of the cadmium metal pr ecursor particles electrodeposited in the first step of the synthesis. The CdS cores of these particles were found by electron diffraction t o be epitaxially aligned with the hexagonal periodicity of the graphit e surface and oriented with the c-axis of the wurtzite unit cell perpe ndicular to the surface. The low-temperature photoluminescence (PL) sp ectra for CdS nanocrystals without the sulfur capping layer were domin ated by broad trap state emission peaks. In contrast, the PL spectra f or sulfur-passivated CdS NCs were characterized by a prominent exciton emission band and much weaker trap state emission peaks. As the radiu s of the CdS core was reduced from 50 to 17 Angstrom, the energy of th e exciton emission peak shifted from the macroscopic value of 2.56 to 3.1 eV in excellent agreement with the predictions of the Coulomb-corr ected, effective mass model.