Electrochemical/Chemical Synthesis
of Size-Tunable CdS/S Core-Shell Nanocrystals
Principle Scientist: Sasha Gorer
S. Gorer, J.A. Ganske, J.C. Hemminger, and R.M. Penner, The Size-Selective Electrochemical/Chemical Synthesis of Sulfur-Passivated Cadmium Sulfide Nanocrystals on Graphite, J. Am. Chem. Soc., 120 (1998) 9584.[PDF]
In earlier work, we showed that CdS nanocrystals could be synthesized from cadmium metal nanocrystallites (NCs) by oxidation of these to Cd(OH)2 and subsequent exposure to basic, aqueous S2- (the top route shown below). The optical properties of the CdS prepared using this approach, however, was far from ideal.
Recent work has demonstrated the feasibility of converting Cd(OH)2 nanocrystals into CdS by exposure to gas phase H2S at 300oC (the bottom route shown above). The advantage of this method is that following coversion of the nanocrystals to CdS, H2S continues to selectively decompose on these particles and an elemental sulfur shell is thereby formed. This sulfur shell scavenges the dangling bonds thereby eliminating the trap state emission from the PL spectrum of these particles.
As in previous synthesis of CdS from aqueous S2-, transmission electron microscopy (TEM) and selected area electron diffraction (SAED) data show that the synthesis of CdS NCs proceeded on a particle-by-particle basis such that the particle size, and monodispersity of the CdS core were directly related to that of the cadmium metal precursor particles electrodeposited in the first step of the synthesis. The CdS cores of these particles were found by electron diffraction to be epitaxially aligned with the hexagonal periodicity of the graphite surface and oriented with the c-axis of the wurtzite unit cell perpendicular to the surface (as shown schematically above).
The low temperature photoluminescence (PL) spectra for CdS nanocrystals without the sulfur capping layer were dominated by broad trap state emission peaks. In contrast, the PL spectra for sulfur passivated CdS NCs (shown above) were characterized by a prominent exciton emission band and much weaker trap state emission peaks. As the radius of the CdS core was reduced from 50Å to 17Å, the energy of the exciton emission peak shifted from the macroscopic value of 2.56 eV to 3.1 eV in excellent agreement with the predictions of the Coulomb corrected, effective mass model (see below).
