Slow Electron Cooling in Colloidal Quantum Dots

Hot electrons in semiconductors lose their energy very quickly ( within picoseconds) to lattice vibrations. Slowing this energy loss could prove useful for more efficient photovoltaic or infrared devices. With their well- separated electronic states, quantum dots should display slow relaxation, but other mechanisms have made it difficult to observe. We report slow intraband relaxation (> 1 nanosecond) in colloidal quantum dots. The small cadmium selenide ( CdSe) dots, with an intraband energy separation of similar to 0.25 electron volts, are capped by an epitaxial zinc selenide ( ZnSe) shell. The shell is terminated by a CdSe passivating layer to remove electron traps and is covered by ligands of low infrared absorbance ( alkane thiols) at the intraband energy. We found that relaxation is markedly slowed with increasing ZnSe shell thickness.