Facet Engineering for Amplified Spontaneous Emission in Metal Halide Perovskite Nanocrystals

Abstract

Auger recombination and thermalization time are detrimental in reducing the gain threshold of optically pumped semiconductor nanocrystal (NC) lasers for future on-chip nanophotonic devices. Here, we report the design strategy of facet engineering to reduce the gain threshold of amplified spontaneous emission by manyfold in NCs of the same concentration and edge length. We achieved this hallmark result by controlling the Auger recombination rates dominated by processes involving NC volume and thermalization time to the emitting states by optimizing the number of facets from 6 (cube) to 12 (rhombic dodecahedron) and 26 (rhombicuboctahedrons) in CsPbBr3 NCs. For instance, we demonstrate a 2-fold reduction in Auger recombination rates and thermalization time with increased number of facets. The gain threshold can be further reduced ∼50% by decreasing the sample temperature to 4 K. Our systematic studies offer a new method to reduce the gain threshold that ultimately forms the basis of nanolasers.

Facet Engineering for Decelerated Carrier Cooling in Polyhedral Perovskite Nanocrystals

“…Inspired by the recent success of facet engineering, we have studied the HC cooling dynamics in 6-, 12-, and 26-faceted polyhedral CsPbBr 3 NCs. Interestingly, our study revealed that 12-faceted dodecahedral NCs (d-CsPbBr 3 ) and 26-faceted rhombicuboctahedral NCs (r-CsPbBr 3 ) are coveted materials characterized by polaron-mediated slower cooling (up to ∼10 times) compared to that of their cubic counterpart (c-CsPbBr 3 ) of similar size. ,− Subsequently, with the help of a molecular system, we successfully harvested HCs before their thermalization, signifying the tremendous implications of surface engineering for device applications. − …”

“…The past few years have witnessed the great promise of six-faceted ({110}, {002}) all-inorganic perovskite NCs (CsPbX 3 ; X = Br, Cl, I) in photovoltaic and optoelectronic applications. − Their unique optical properties, such as near-unity photoluminescence quantum yields (PLQYs), narrow emission spectra, tunable photoluminescence (PL) in the visible region, long carrier diffusion length, etc., are keys to their massive success in several real applications such as displays, lasers, LEDs and solar cells. − Despite the vast accomplishments, six-faceted cube NCs are not free from challenges; in fact, many of them are directly linked to the device’s performance. − One such example is the rapid cooling (∼0.5–0.7 ps) of hot carriers (HCs) in cubic nanocrystals (NCs), a significant setback toward achieving a power conversion efficiency (PCE) of a theoretically predicted value (∼66%) or at least to a Shockley–Queisser limiting value (∼33%). , Extensive efforts have been put forward in slowing down the cooling in perovskite materials to a time scale slower than the time needed for the extractions of HCs. Such attempts proposed several phenomena, including acoustical–optical phonon upconversion, hot-phonon bottleneck, large-polaron formation, Auger heating, etc., were responsible for slow cooling in perovskite materials. , For instance, Fu et al and Wang et al have observed decelerated cooling in perovskite materials exploiting the Auger heating and synergetic effect of doped alkali cations (K + , Cs + , Rb + ) reducing the phonon bottleneck threshold.…”

“…A facet engineering strategy has recently entered the limelight because of its vast potential in modulating photophysical properties of NCs. The latest reports highlighted several remarkable properties of 12- and 26-faceted CsPbBr 3 NCs that are absent in their cubic counterparts. ,− For example, Adarsh and co-workers observed a noteworthy improvement of the gain threshold of photoexcited CsPbBr 3 NCs with an increase in the number of NC facets, which is beneficial for on-chip nanophotonic applications . Ghosh et al reported a slower rate of Auger recombination in 12-faceted NCs .…”

“…NCs were synthesized using reported methods with slight modifications (see the Supporting Information). ,,,, Initially, we synthesized three types of NCs (r-/d-/c-CsPbBr 3 ) with similar edge lengths (∼17 nm) for the facet-dependent cooling study. Later we introduced smaller-sized cubes (∼8 nm edge length), intending to realize the cooling time scale as a function of particle size (Figure S2).…”

“…HRTEM images of CsPbBr 3 NCs showing ∼17 nm average edge lengths characterized by cubic (c-CsPbBr 3 ) (A), dodecahedral (d-CsPbBr 3 ) (B), and rhombicuboctahedral (r-CsPbBr 3 ) (C) morphologies. Insets show 3-D representations of particle morphologies (Reproduced from ref . Copyright 2022 American Chemical Society) correlating with our TEM images (right inset), and edge-length distribution histograms of the NCs (left inset).…”