Abstract
Carrier multiplication in nanostructures promises great improvements in a number of widely used technologies, among others photodetectors and solar cells. The decade since its discovery was ridden with fierce discussions about its true existence, magnitude, and mechanism. Here, we introduce a novel, purely spectroscopic approach for investigation of carrier multiplication in nanocrystals. Applying this method to silicon nanocrystals in an oxide matrix, we obtain an unambiguous spectral signature of the carrier multiplication process and reveal details of its size-dependent characteristics-energy threshold and efficiency. The proposed method is generally applicable and suitable for both solid state and colloidal samples, as well as for a great variety of different materials.
Highlights
Carrier multiplication in nanostructures promises great improvements in a number of widely used technologies, among others photodetectors and solar cells
While direct bandgap quantum dots allow for multiple-exciton generation and radiative biexciton recombination competing with non-radiative processes[40], indirect bandgap materials like silicon, due to long exciton lifetimes, do not exhibit increased emission through multiple-exciton generation within the same nanocrystal; it has been shown[32] that sufficiently high excess energy in one nanocrystal can undergo an ultrafast transfer to a neighboring nanocrystal, where another exciton is created
The new approach proposed here builds upon a presumption that, since Carrier multiplication (CM) involves exciton generation, its energy threshold should be correlated to the bandgap value, and to the nanocrystal size
Summary
Carrier multiplication in nanostructures promises great improvements in a number of widely used technologies, among others photodetectors and solar cells. We introduce a novel, purely spectroscopic approach for investigation of carrier multiplication in nanocrystals Applying this method to silicon nanocrystals in an oxide matrix, we obtain an unambiguous spectral signature of the carrier multiplication process and reveal details of its size-dependent characteristics-energy threshold and efficiency. We explicitly demonstrate that the threshold energy and possibly the efficiency of CM are functions of the bandgap, and the size of nanocrystals, within a particular ensemble These essential insights into the CM process are directly relevant to its physical mechanism and represent a unique advantage of the spectroscopic method developed in this study. The method itself is of general character and can be applied to any ensemble of nanocrystals, which is not monodisperse, and where emission due to multiple excitons is possible[39]
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