Femtosecond Time-Resolved Spectroscopic Investigation of the Opto-Electrochemical Properties of Novel Nanomaterials

The ability to slow the relaxation rate of hot carriers in quantum-confined materials provides a means to enhance photocurrents. Among the different materials considered for photovoltaic applications, Cu₂S (chalcocite) has been a candidate of choice as a heterojuction cell. The ability of the copper sulfide system to form different compositions and their appropriate bandgaps for solar spectrum absorption inspired us to synthesize and characterize other different copper sulfides in nanoscale.

In this study, Cu_1.8S quantum dots (QDs) were prepared by a single-source-precursor type method and investigated in the light of opto-electronic applications. With femtosecond time-resolved transient absorption measurements, the electron relaxation as well as their trapping dynamics could be evaluated. The measurements reveal that the largest and the smallest QD samples prepared exhibit the longest mobility lifetimes, and that the electron-hole relaxation dynamics is strongly dependent on the occurrence of trapping sites. Based on the argument of optical response, it appears that the largest prepared Cu_1.8S QDs with band gap energy of 2.35 eV are preferred candidates for opto-electronic device fabrication.

Different Cu_xS (x = 1, 1.8, 2) nanocrystals have been successfully prepared via an environmental benign sonoelectrochemical method. TEM, XRD and XPS analysis provided proofs of the control over crystal structure and composition. Electron and hole cooling, and carrier trapping dynamics have been investigated by time-resolved femtosecond pump-probe spectroscopy. Upon 150 fs laser pulse excitation, fast intraband relaxation occurred within 1 ps. On the time scale of 40 to 450 ps, the excited electrons got trapped to different trapping sites. An extended carrier recombination time (> 3 ns) has been observed for these Cu ₂S nanocrystals, which has not been reported in the literature, yet. This suggests potential applicability for photovoltaic and photocatalytical applications.

In addition, femtosecond pump-probe absorption spectroscopy is used to investigate the role of Er³⁺dopants in the early relaxation pathways of photoexcited Si nanocrystals. The fate of photoexcited electrons in three different Si nanostructures was studied and correlated with the effect of Er-doping.