Single-exciton optical gain in semiconductor nanocrystals

Abstract

Nanocrystal quantum dots have favourable light-emitting properties. They show photoluminescence with high quantum yields, and their emission colours depend on the nanocrystal size--owing to the quantum-confinement effect--and are therefore tunable. However, nanocrystals are difficult to use in optical amplification and lasing. Because of an almost exact balance between absorption and stimulated emission in nanoparticles excited with single electron-hole pairs (excitons), optical gain can only occur in nanocrystals that contain at least two excitons. A complication associated with this multiexcitonic nature of light amplification is fast optical-gain decay induced by non-radiative Auger recombination, a process in which one exciton recombines by transferring its energy to another. Here we demonstrate a practical approach for obtaining optical gain in the single-exciton regime that eliminates the problem of Auger decay. Specifically, we develop core/shell hetero-nanocrystals engineered in such a way as to spatially separate electrons and holes between the core and the shell (type-II heterostructures). The resulting imbalance between negative and positive charges produces a strong local electric field, which induces a giant ( approximately 100 meV or greater) transient Stark shift of the absorption spectrum with respect to the luminescence line of singly excited nanocrystals. This effect breaks the exact balance between absorption and stimulated emission, and allows us to demonstrate optical amplification due to single excitons.

Nearly Temperature‐Independent Threshold for Amplified Spontaneous Emission in Colloidal CdSe/CdS Quantum Dot‐in‐Rods

“…[ 9 ] Consequently, gain may occur near the singleexciton regime, further suppressing Auger recombination. [ 10 ] In this letter, we demonstrate an important prerequisite toward the realization of solution processed effi cient laser devices: a nearly temperature-independent ASE in a closepacked thin fi lm of colloidal Qdots. So far, only Kazes et al [ 11 ] have reported on such behavior in colloidal nanocrystals.…”

Colloidal synthesis and optical properties of type-II CdSe–CdTe and inverted CdTe–CdSe core–wing heteronanoplatelets

“…Such NCs exhibit optical properties different from those for type-I materials such as suppressed blinking and Auger recombination 11,12 and increased multiexciton generation rates. 13 This, in turn, allows lowering lasing thresholds, 14 improving the photovoltaic device performance 10,15 and utilization of these heterostructures in photocatalytic 16 and biomedical applications. 17 Generally, photovoltaic and photocatalytic applications require directional charge separation which could not be realized in the case of core-shell QDs.…”

Excited-State Dynamics in Colloidal Semiconductor Nanocrystals

“…c In the biexciton state, the recombination energy of an electronhole pair can be transferred either to the additional hole (the ''positive trion pathway''; to the left) or to the additional electron (the ''negative trion pathway''; to the right) [201] illumination NCs charge up intermittently and seemingly randomly, leading to a phenomenon known as PL intermittency or ''blinking'' (see Sect. 3.4 below) [195,196], while for applications such as lasing high excitation intensities are a necessity [14,197,198]. As a result, the possibility of Auger quenching cannot always be avoided.…”

“…Since the pioneering work of Brus, Ekimov, and many others in the early 1980-1990s [1][2][3][4][5][6][7][8][9][10][11][12][13], the study of semiconductor nanocrystals (NCs) has developed into a mature, dynamic and multidisciplinary research field, which attracts increasing attention worldwide, both for its fundamental challenges and its potential for a number of technologies (light emitting devices, solar cells, luminescent solar concentrators, optoelectronics, sensing, thermoelectrics, biomedical applications, catalysis) [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32]. Colloidal semiconductor NCs are particularly attractive, since they consist of an inorganic core that is coated with a stabilizing layer of (usually) organic ligand molecules.…”