Lead-halide perovskite nanocrystals are promising candidates for semiconductor laser cooling due to their near-unity photoluminescence quantum yields and efficient photon/phonon up-conversion process. This unexpected, efficient sub-gap energy up-conversion implies an unexpectedly strong electron-phonon interaction in perovskite nanocrystals. However, the underlying mechanism remains mostly unclear. Detailed experiments, along with theory, have now been conducted to elucidate the efficient up-conversion in CsPbBr3 NCs, utilizing a combination of techniques: photothermal absorption spectroscopy, up-conversion detuning spectroscopy, and ultrafast transient differential absorption spectroscopy.
Cesium lead bromide nanocrystals, in contrast to most other materials, exhibit near-unity photoluminescence quantum yields (PLQY). When excited below the band gap, they absorb the photons and show anti-Stokes photoluminescence (ASPL), emitting higher energy, band-gap photons. Simultaneous existence of near-unity PLQY and ASPL can be used to optically cool these materials. In this talk, I will report near-unity ASPL efficiencies in CsPbBr3 nanocrystals and attribute it to resonant multiple-phonon absorption by polarons. The theory explains paradoxically large efficiencies for intrinsically disfavored, multiple-phonon-assisted ASPL in nanocrystals.
Achieving condensed phase optical refrigeration requires near-unity emission quantum yields (QYs). Colloidal CsPbBr3 nanocrystals (NCs) are promising candidates in this respect given near unity QY-values, achieved by post-synthetic surface treatment with quaternary ammonium bromide ligands. The origin of these QY enhancements, however, is not understood. Systematic nuclear magnetic resonance studies of the organic ligand passivation of near unity QY CsPbBr3 NC surfaces are therefore conducted to better reveal their surface-ligand interactions.
CsPbBr3 perovskite nanocrystals have been identified as a potential medium to realize condensed phase optical refrigeration. This is due to its near unity emission quantum yields and efficient anti-Stokes photoluminescence (ASPL). Despite much work on CsPbBr3’s optical response, the origin of its efficient ASPL remains unclear. We conduct detailed optical spectroscopy measurements in conjunction with theory to establish mechanistic insights into CsPbBr3’s up-conversion process. Experimental techniques utilized include: temperature-dependent and detuning energy-dependent ASPL measurements, temperature-tunable photothermal heterodyne absorption spectroscopy, and ultrafast transient differential absorption (TDA) spectroscopy.
Infrared photothermal heterodyne imaging (IR-PHI) is an ultrasensitive technique capable of achieving super-resolution chemical and morphological characterization of specimens via absorption of mid-infrared light. While early iterations of IR-PHI have involved point-by-point raster-scanning, here, we introduce a widefield modality to IR-PHI that utilizes ns-timescale infrared pump pulses synchronized to an ultrafast complementary metal-oxide-semiconductor camera to parallelize data acquisition. A 300-fold decrease in image acquisition time is realized, falling from 20 minutes to four seconds.
Semiconductor nanocrystals (NCs) are potential materials for verifiable demonstrations of semiconductor-based laser cooling. The key feature that makes NCs appealing for laser cooling is their near unity emission quantum yields (QYs). An unresolved issue regarding NC QYs, however, centers on the existence of an excitation energy dependent (EED) QY. Here, we study EED QYs on three NC systems, aimed at demonstrating NC-based laser cooling (CsPbBr3, CsPbI3, and CdSe/CdS core/shell NCs). We evaluate the impact of EED QYs using two approaches. The first involves direct QY measurements using an integrating sphere. The second entails photoluminescence excitation spectroscopy where changes to NC QYs with excitation energy can be assessed qualitatively.
Establishing the optical refrigeration of semiconductors remains a longstanding goal due to potential applications in optoelectronics. Apart from stringent materials requirements, required to realize condensed phase laser cooling, namely the need to have near unity emission quantum yields, a practical challenge involves accurately measuring specimen temperatures in a non-contact fashion. Common all-optical approaches developed in response to this need include: pump– probe luminescence thermometry (PPLT) and differential luminescence thermometry (DLT). In this study, we compare and contrast PPLT and DLT to a newly developed up-conversion emission thermometry to establish the most robust approach for measuring semiconductor nanocrystal (NC) temperatures. Using high external quantum efficiency CdSe/CdS core/shell NCs, we reveal that up-conversion emission thermometry possesses higher accuracy than either PPLT or DLT. Up-conversion emission thermometry can also be used on specimens such as CsPbBr3 NCs with temperature-insensitive band gaps.
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