Applications of single-walled carbon nanotubes and type-II quantum dots in photovoltaics and passive mode-locking saturable absorbers

Abstract

Using single-molecule confocal imaging techniques combined with time-correlated single-photon counting we investigated the electron transfer (ET) rates to the single-walled carbon nanotubes from various types of semiconductor hetero-nanocrystals of type-I or type-II band alignment. We observed significantly larger ET rate for type-II ZnSe/CdS dot-in-rod nanostructures as compared to type-I spherical CdSe/ZnS core/shell quantum-dots, and to CdSe/CdS dot-in-rod structures. We demonstrated that such rapid ET dynamics can compete with both Auger and radiative recombination processes, leading to potentially more effective photovoltaic operation. In another work, we used aligned single-walled carbon nanotubes as saturable absorbers for ps laser pulse generation. Using the vertical evaporation technique we fabricated saturable absorbers by transferring the water-soluble single wall carbon nanotubes onto a hydrophilic quartz substrate. The fast recovery times of the absorber were measured to be 136 fs and 790 fs. The modulation depth of the absorber was about 1.5%. Passive mode-locked Nd: GdVO4 laser using such an absorber was demonstrated. The continuous wave mode-locked pulses with the pulse duration of 12.4 ps and the repetition of 120 MHz were achieved. The maximum average output power of the mode-locked laser is 2.4 W at the pump power of 13 W. Such a kind of absorbers has potential to be put into practical use.