Synthesis, optical properties and ultrafast electronic relaxation of layered semiconductor nanoparticles: Lead iodide, bismuth iodide, bismuth sulfide

The first direct observation of the electronic relaxation dynamics in PbI2, BiI3 and Bi2S3 nanoparticles using femtosecond transient spectroscopy has been reported. The nanoparticles were synthesized in polar and nonpolar solvents, polymer matrices and inverse micelles using colloidal chemistry methods and in thin films. The particle sizes were determined using TEM which provided direct evidence of photodegradation of nanoparticles. The ground state electronic absorption spectra of PbI 2 and BiI3 colloidal nanoparticles show well-resolved peaks in the near UV and visible region. Using a particle in an anisotropic box model the blue shift and increase in absorption of these peaks with simultaneous decrease in particle size have been explained. The UV-visible absorption spectra of Bi2S3 colloidal nanoparticles have very different features compared to those of PbI2 and BiI3 nanoparticles. Similarities and differences found among these nanoparticle systems, PbI 2, BiI3 and Bi2S3, in terms of optical properties and femtosecond electronic relaxation dynamics have been discussed. For PbI2, BiI3 and Bi2S3 with excitation at 390 nm and probing in the near infrared, the electronic relaxation dynamics were directly monitored and found to be sensitive to solvent and insensitive to particle size. For both PbI2 and BiI3 there appeared to be an oscillatory feature at early times with a period changing with solvent but not with particle size. However, for BiI3, the oscillation periods were slightly shorter and overall relaxation was somewhat faster than that in PbI2. The possibilities of the origin of this feature have been addressed. For Bi2S3 system, the early time dynamics did not show any resolvable oscillation. It could be due to the fact that the origin of oscillations in PbI2 and BiI3 nanoparticle system is connected to layered iodide structure and thus is not found in layered sulfide structure or the feature was not observed due to overall fast relaxation in Bi2S3 nanoparticle system. The dynamics observed were somewhat dependent on the probe wavelength and independent of the excitation intensity. These results suggest that the surface plays a major role in the electronic relaxation processes of colloidal solutions of layered semiconductor nanoparticles, such as, PbI2, BiI3 and Bi2S3.