Luminescence and energy transfer excitation of infrared colloidal semiconductor nanocrystals

This work advances the field of near-infrared optoelectronic integration. Strategies were developed to integrate near-infrared colloidal PbS nanocrystals with two different signal-processing materials: solution-processible semiconducting polymers, and silicon---the canonical platform for computation. In addition, a method was devised to improve the performance of these nanocrystals in solid films.; In this work, the photoluminescence quantum efficiency (PLQE) of nanocrystals in solid films was shown to increase by an order of magnitude from that of conventional nanocrystal films by dispersing the nanocrystals in solid matrices to avoid the formation of ordered close-packed nanocrystal domains in films. When MEH-PPV polymer films are used as the matrices, the PLQE of nanocrystals reaches 12%. Because the MEH-PPV polymer is semiconducting and can be used as the active layer of optoelectronic devices, this result benefits directly the performance of nanocrystal-based light-emitting devices.; To evaluate the effectiveness of nanocrystal integration with signal-processing materials, the mechanism-independent normalized photoluminescence excitation technique was invented to quantify the overall efficiency of excitation energy transfer between materials having different bandgaps. When nanocrystals are dispersed into solution-processible MEH-PPV polymer films, 20% of the excitation energy absorbed by the polymer is transferred to the oleate-capped PbS nanocrystals. When the oleate ligands are replaced with shorter octylamine ligands, the transfer efficiency is increased to 60%. Since the polymer is solution-processible and semiconducting, its efficient energy coupling with the nanocrystals enables low-cost printable optoelectronic circuits active in the near-infrared range. Moreover, when a thin layer of butylamine-capped PbS nanocrystals is deposited on silicon-rich silicon nitride, 60% of the excitation energy from the nitride is transferred non-radiatively to the nanocrystals. Since the fabrication process of silicon-rich silicon nitride is compatible with that of standard silicon integrated circuits, the strong coupling of silicon-rich silicon nitride with PbS nanocrystals leads to low-cost integration of near-infrared photonic functionality with the signal-processing capability of silicon.; This work presents novel strategies to augment the performance of nanocrystal-based near-infrared devices. It opens up avenues to integrate economically efficient infrared photonic materials with signal-processing materials. It paves new routes to converge communications and computing.