Photoacoustic and Luminescence Spectra of CdS Fine Particles

Some size effects of the relaxation mechanism of optically excited electrons in CdS fine particles were investigated by means of photoacoustic and luminescence spectroscopies. The intensity of green luminescence (∼2.47 eV) decreased and its peak position shifted to the lower-energy side upon decreasing the particle size. The impurity luminescence which appears in the longer-wavelength region, red emission (∼1.7 eV), did not vary as much upon decreasing the particle size to ∼40 nm in diameter. The absorption spectrum in the intrinsic energy region for a small-size particle, which was determined from the photoacoustic effect, formed a long tailing towards the lower-energy side. The non-radiative recombinalion rate, thermal relaxation rate, increases with decreasing particle size. These phenomena were interpreted to originate from the increment of the ratio of the surface to volume in small-size particles.