  We report an optically gated transistor composed of CdSe nanocrystals (NCs),
sensitized with the dye Zinc beta-tetraaminophthalocyanine for operation in the
first telecom window. This device shows a high ON/OFF ratio of six orders of
magnitude in the red spectral region and an unprecedented 4.5 orders of
magnitude at 847 nm. By transient absorption spectroscopy, we reveal that this
unexpected infrared sensitivity is due to electron transfer from the dye to the
CdSe NCs within 5 ps. We show by time-resolved photocurrent measurements that
this enables fast rise times during near-infrared optical gating of 74 ns.
Electronic coupling and accelerated non-radiative recombination of charge
carriers at the interface between the dye and the CdSe NCs are further
corroborated by steady-state and time-resolved photoluminescence measurements.
Field-effect transistor measurements indicate that the increase in photocurrent
upon laser illumination is mainly due to the increase in carrier concentration
while the mobility remains unchanged. Our results illustrate that organic dyes
as ligands for NCs invoke new optoelectronic functionalities, such as fast
optical gating at sub-bandgap optical excitation energies.
