Ultrafast Electron Trapping at the Surface of Semiconductor Nanocrystals: Excitonic and Biexcitonic Processes

Abstract

Aging of semiconductor nanocrystals (NCs) is well-known to attenuate the spontaneous photoluminescence from the band edge excitonic state by introduction of nonradiative trap states formed at the NC surface. In order to explore charge carrier dynamics dictated by the surface of the NC, femtosecond pump/probe spectroscopic experiments are performed on freshly synthesized and aged CdTe NCs. These experiments reveal fast electron trapping for aged CdTe NCs from the single excitonic state (X). Pump fluence dependence with excitonic state-resolved optical pumping enables directly populating the biexcitonic state (XX), which produces further accelerated electron trapping rates. This increase in electron trapping rate triggers coherent acoustic phonons by virtue of the ultrafast impulsive time scale of the surface trapping process. The observed trapping rates are discussed in terms of electron transfer theory.

A microscopic picture of surface charge trapping in semiconductor nanocrystals

“…The spectral distribution gives info mation on t e occu ence, population and dept s of t e su face t aps." 18 Findings from conductance spectroscopy 24 and cyclic voltammetry 25 experiments have been cited in support of this model, and the model proposed by Weller has been adopted by others. 26 In the 1990s, t eo etical wo by W aley's g oup found t at an intense surface state with high oscillator strength could occur within the band gap of a CdSe nanocluster.…”

“…Considerable recent work has suggested that the NCs within an ensemble are far from homogeneous in their decay pathways. 23,[25][26][27] Thus, we propose that the percentage of NCs within the ensemble which emit in the first place is a temperature-dependent quantity, n(T).…”

“…In order to extend the cold electron transfer model to include hot exciton effects, the rates of hot exciton relaxation and hot exciton surface trapping must be known. Our prior works have produced measurements of both the cooling rate, k relax (E), 6,7,16,17 as well as the surface trapping rate, k trap (E) 6,[23][24][25][26] , as a function of energy. These results indicate that the ratio of cooling (4.0 ps -1 ) to trapping (0.65 ps -1 ) is ~6 for the 2S state, while it grows to ~10 for higher states.…”

“…These results indicate that the ratio of cooling (4.0 ps -1 ) to trapping (0.65 ps -1 ) is ~6 for the 2S state, while it grows to ~10 for higher states. [23][24][25][26] Drawing from experimental values, these rates are used to perform a semi-empirical calculation, Fig. 5.4b.…”

Ultrafast Optical Spectroscopy Techniques Applied to Colloidal Nanocrystals

“…[66,129] Engineering of surface traps to control recombination rate is critical for the design of sensitive and fast photoconductive detectors, while suppres- properties. [3,124,129,[132][133][134][135][136][137] Apart from an early report in Bi 2 S 3 nanocrystals with band gap at 1.7 eV, [138] to the best of our knowledge, no systematic investigation of the excited-state dynamics in bismuth sulfide has been reported until now. shows the presence of rod shaped particles with average sizes of 29 nm (14% polydispersity) and of 14 nm (19% polydispersity).…”