Evidence of Band-Edge Hole Levels Inversion in Spherical CuInS₂ Quantum Dots

Abstract

CuInS (CIS) quantum dots (QDs) have emerged as one of the most promising candidates for application in a number of new technologies, mostly due to their heavy-metal-free composition and their unique optical properties. Among those, the large Stokes shift and the long-lived excited state are the most striking ones. Although these properties are important, the physical mechanism that originates them is still under debate. Here, we use two-photon absorption spectroscopy and ultrafast dynamics studies to investigate the physical origin of those phenomena. From the two-photon absorption spectroscopy, we observe yet another unique property of CIS QDs, a two-photon absorption transition below the one-photon absorption band edge, which has never been observed before for any other semiconductor nanostructure. This originates from the inversion of the 1S and 1P hole level order at the top of the valence band and results in a blue-shift of the experimentally measured one photon absorption edge by nearly 100 to 200 meV. However, this shift is not large enough to account for the Stokes shift observed, 200-500 meV. Consequently, despite the existence of the below band gap optical transition, photoluminescence in CIS QDs must originate from trap sites. These conclusions are reinforced by the multiexciton dynamics studies. From those, we demonstrate that biexciton Auger recombination behaves similarly to negative trion dynamics on these nanomaterials, which suggests that the trap state is an electron donating site.

Optoelectronic Properties of Ternary I–III–VI2 Semiconductor Nanocrystals: Bright Prospects with Elusive Origins

“…53 Further experimental support for predictions i–iii has been recently given by the observation of two-photon absorption transitions below the one-photon absorption band edge of spherical chalcopyrite CuInS 2 /ZnS core/shell NCs (core diameter ranging from 2.1 to 3.1 nm) and by the ability of the model to account for both the two-photon and one-photon transitions. 58 Prediction iv is consistent with the long lifetimes experimentally observed for CuInS 2 NCs, but not with the multiexponential character of the PL decay curves. Moreover, there is no experimental evidence supporting the notion of longer radiative lifetimes for smaller NCs.…”

“…Interestingly, the emission bandwidths and the global Stokes shifts have been reported to be independent of the size, shape, and crystal structure of the CuInS 2 NCs. 53,58 In contrast, the band-edge absorption and the emission peak shift to higher energies with decreasing NC size, following a 1/ d -dependence for both chalcopyrite ( a 0 = 4.1 nm) and wurtzite CuInS 2 NCs. 53 The size-dependence is weaker for wurtzite CuInS 2 because of the larger electron and hole effective masses of this material (viz., m e = 0.173 m 0 and m h = 2.181 m 0 in bulk while those for chalcopyrite CuInS 2 are m e = 0.16 m 0 and m h = 1.3 m 0 in bulk).…”

Giant Stokes Shifts in AgInS2 Nanocrystals with Trapped Charge Carriers

“…20 The most striking characteristic feature observed experimentally in these materials is the giant Stokes shift. Depending on the nanocrystal size and composition, this shift in CuInS 2 nanocrystals may vary from 200 to 500 meV 13,[21][22][23] , while AgInS 2 nanocrystals demonstrate even larger shifts between 300 and 1000 meV. 10,12,14,24,25 Giant Stokes shifts open a possibility to achieve better efficiency in several technologies of light emission, including LEDs, 26 solar cells, 26,27 and reabsorption-free luminescent solar concentrators.…”

Stoichiometry-controllable optical defects in CuxIn2−xSy quantum dots for energy harvesting

“…19,20 Band-edge emission can only occur in defect-free structures, which have not been synthesized to date. 13,50 Notably, hole localization for CuIn'' is similar to the self-trapped exciton model, and even though we expect band-edge absorption (hυx,a) to occur in conjunction with defect absorption (hυCu,a), an electron-phonon coupled excited-state nuclear reorganization similar to Jahn-Teller distorted Cu 2+ is required to transiently stabilize the photogenerated hole. 11,12,20,51 We therefore calculate the Franck-Condon shift (δ 0 ) for anti-site defects (CuIn''  CuIn' in Fig.…”

“…[7][8][9][10][11][12][13][14][15][16][17][18][19] For example, CIS QDs have larger Stokes shifts (270-750 meV), broader absorption and ensemble photoluminescence (PL) emission spectra (>300 meV), and longer radiative lifetimes (100-500 ns) than II-VI QDs. [7][8][9][10][11][12][13][14][15][16][17][18][19] This is generally attributed to radiative recombination between a delocalized conduction band electron and a hole localized on a Cu atom via either native defects, [7][8][9][10][13][14][15][16][17][18][19] or self-trapped excitons. 9,[11][12]14 For defect emission, an interruption in the crystal lattice from an "out of place" Cu ion is expected to lead to an intragap hole localized state.…”