Resonant Energy Transfer in Si Nanocrystal Solids

Energy exchange between
closely packed semiconductor quantum dots
allows for long-range transfer of electronic energy and enables new
functionalities of nanostructured materials with a huge application
potential in photonics, optoelectronics, and photovoltaics. This is
illustrated by impressive advances of quantum-dot solids based on
nanocrystals (NCs) of direct bandgap materials, where this effect
has been firmly established. Regretfully, the (resonant) energy transfer
in close-packed ensembles of NCs remains elusive for siliconthe
main material for electronic and photovoltaic industries. This is
the subject of the present study in which we conclusively demonstrate
this process taking place in dense dispersions of Si NCs in an SiO2 matrix. Using samples with different NC configurations, we
can directly determine the wavelength dependent energy transfer rate
and show that it (i) can be modulated by material parameters, and
(ii) decreases with the NCs size, and thus being consistent with the
energy flow proceeding from smaller to larger NCs. This result opens
the way to new applications of Si NCs, requiring energy transport
and extraction. In particular, it forms a fundamental step toward
development of an excitonic all-Si solar cell, operating in some analogy
to polymer devices.