Nature of the Second Optical Transition in PbSe Nanocrystals

Abstract

The second peak in the optical absorption spectrum of PbSe nanocrystals is arguably the most discussed optical transition in semiconductor nanocrystals. Ten years of scientific debate have produced many theoretical and experimental claims for the assignment of this feature as the 1P e1P h as well as the 1S h,e1P e,h transitions. We studied the nature of this absorption feature by pump-probe spectroscopy, exactly controlling the occupation of the states involved, and present conclusive evidence that the optical transition involves neither 1S e nor 1S h states. This suggests that it is the 1P h1P e transition that gives rise to the second peak in the absorption spectrum of PbSe nanocrystals.

Role of PbSe Structural Stabilization in Photovoltaic Cells

“…For maximum size focusing and minimal Ostwald ripening, the optimum time for terminating the reaction was 5-6 min after CdCl 2 injection (Figure 1 a). This protocol produced NCs with well-defi ned 1st, 2nd, and 3rd excitonic transitions [63][64][65] (Figure S1, Supporting Information). To eliminate the effects of size distribution when comparing passivated and unpassivated samples, we ensured that both NC samples have the same HWHMs (Table S1, Supporting Information).…”

Hyperspectral Probing of Exciton dynamics and Multiplication in PbSe Nanocrystals

“…These features evolve within picoseconds as the nascent excitons cool. The changes observed consist mainly of a disappearance of the broad absorption background, leaving behind the excess transmission feature at the band edge, as well as a zero integral peak shift signature associated with the second absorption [5]. After this initial evolution, at much longer delay times, a gradual decay (~ 100 ps) is observed.…”

Probing formally forbidden optical transitions in PbSe nanocrystals by time- and energy-resolved transient absorption spectroscopy

“…The populations of the 1S h 1S e , the 1S1P, and the 1P h 1P e exciton states, denoted by n ss , n sp , and n pp , respectively, decay as follows: Here ␥ is the rate at which a 1P state decays to a 1S state and is the rate for Auger recombination of multiple 1S h 1S e excitons; for the present argument it is not relevant whether Auger recombination occurs with the second or third power of the exciton density ͑the above expression displays a third power dependence͒. 32 Independently of the occurrence of cross transitions, 14 the 1S1P bleach produced in the by these populations can be written as follows:…”

“…14 Oleic acid capped PbSe nanocrystals were synthesized and purified following the recipe of Talapin and Murray. 17 The NCs were dispersed in tetrachloroethylene for the measurements.…”