Vibrational properties of monodispersed CdS nanoparticles immersed in a matrix constituted of SnO₂ nanostructured thin films

Cadmium sulfide (CdS) nanoparticles immersed in a tin oxide (SnO₂) matrix were deposited on glass substrates by r.f. sputtering. X-ray diffractograms of the CdS-SnO₂ thin films revealed the growth of CdS quantum dots (QD's) in the hexagonal wurtzite phase while SnO₂ nanoparticles crystallized in the tetragonal phase. The radius of CdS QD's lies in the 2.0-8.5 nm interval, and the energy band gap in the 2.5-3.1 eV range. Transmission electron microscopy images supported the particle size and the spherical aspect of the CdS QD. Raman spectra showed the presence of the A₁(LO), E₂(LO), and A₁(TO) vibrational modes of CdS at the Γ point of the first Brillouin zone. The energy-position of the A₁(LO) band showed that this mode hardens as the radius (R) decreases. This unexpected phononic behavior in CdS nanoparticles is due to the uniaxial compressive stress, as a consequence of the SnO₂ capping. The E₂(LO) and A₁(TO) modes hardened when R decreased, in agreement with dispersion curves when the translational symmetry of the lattice is broken for low-dimensional systems. (copyright 2017 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)