Size-dependent photoluminescence Stokes shifts (ΔE s) universally exist in CsPbX3 (X = Cl–, Br–, or I–) perovskite nanocrystals (NCs). ΔE s values, which range from ∼15 to 100 meV for NCs with average edge lengths (l) from approximately 13 to 3 nm, are halide-dependent such that ΔE s(CsPbI3) > ΔE s(CsPbBr3) ≳ ΔE s(CsPbCl3). Observed size-dependent Stokes shifts are not artifacts of ensemble size distributions as demonstrated through measurements of single CsPbBr3 NC Stokes shifts (⟨ΔE s⟩ = 42 ± 5 meV), which are in near quantitative agreement with associated ensemble (l = 6.8 ± 0.8 nm) ΔE s values (ΔE s ≈ 50 meV). Transient differential absorption measurements additionally illustrate no significant spectral dynamics on the picosecond time scale that would contribute to ΔE s. This excludes polaron formation as being responsible for ΔE s. Altogether, the results point to an origin for ΔE s, intrinsic to the size-dependent electronic properties of individual perovskite NCs.
Photoinduced halide segregation in mixed halide hybrid perovskites [i.e., APb(I1– xBrx)3] represents an intrinsic instability that impedes their commercialization. Resolving this issue requires developing a microscopic understanding of the phenomenon. Key to this is distinguishing existing models of halide photosegregation by comparing their corresponding predictions to experiment. Here, we test the temperature dependency of predicted perovskite terminal stoichiometries (x terminal), following photosegregation, in single and double cation mixed halide perovskites. Our results show a general temperature invariance of x terminal. This largely supports the idea that band gap difference between parent and halide-segregated perovskite phases drives photosegregation. Beyond this, careful examination of temperature- and halide composition-dependent emission energies suggests an alternative explanation for the enhanced photostability of mixed cation/mixed halide perovskites, linked to band gap energy differences between parent and phase-segregated perovskite phases. Together, our results make inroads in clarifying the microscopic origin of mixed halide perovskite photosegregation and pave the way toward approaches that stabilize their use in applications.
Laser cooling in semiconductors has recently been demonstrated in cadmium sulfide nanobelts (NBs) as well as in organic–inorganic lead halide perovskites. Cooling by as much as 40 K has been shown in CdS nanobelts and by as much as 58 K in hybrid perovskite films. This suggests that further progress in semiconductor-based optical refrigeration can ultimately lead to solid state cryocoolers capable of achieving sub 10 K temperatures. In CdS, highly efficient photoluminescence (PL) up-conversion has been attributed to efficient exciton–longitudinal optical (LO) phonon coupling. However, the nature of its efficient anti-Stokes emission has not been established. Consequently, developing a detailed understanding about the mechanism leading to efficient PL up-conversion in CdS NBs is essential to furthering the nascent field of semiconductor laser cooling. In this study, we describe a detailed investigation of anti-Stokes photoluminescence (ASPL) in CdS nanobelts. Temperature- and frequency-dependent band edge emission and ASPL spectroscopies conducted on individual belts as well as ensembles suggest that CdS ASPL is defect-mediated via the involvement of donor–acceptor states.
Limited approaches exist for imaging and recording spectra of individual nanostructures in the midinfrared region. Here we use infrared photothermal heterodyne imaging (IR-PHI) to interrogate single, high aspect ratio Au nanowires (NWs). Spectra recorded between 2,800 and 4,000 cm−1 for 2.5–3.9-μm-long NWs reveal a series of resonances due to the Fabry–Pérot modes of the NWs. Crucially, IR-PHI images show structure that reflects the spatial distribution of the NW absorption, and allow the resonances to be assigned to the m = 3 and m = 4 Fabry–Pérot modes. This far-field optical measurement has been used to image the mode structure of plasmon resonances in metal nanostructures, and is made possible by the superresolution capabilities of IR-PHI. The linewidths in the NW spectra range from 35 to 75 meV and, in several cases, are significantly below the limiting values predicted by the bulk Au Drude damping parameter. These linewidths imply long dephasing times, and are attributed to reduction in both radiation damping and resistive heating effects in the NWs. Compared to previous imaging studies of NW Fabry–Pérot modes using electron microscopy or near-field optical scanning techniques, IR-PHI experiments are performed under ambient conditions, enabling detailed studies of how the environment affects mid-IR plasmons.
Accurate measurements of semiconductor nanocrystal (NC) emission quantum yields (QYs) are critical to condensed phase optical refrigeration. Of particular relevance to measuring NC QYs is a longstanding debate as to whether an excitation energy-dependent (EED) QY exists. Various reports indicate existence of NC EED QYs, suggesting that the phenomenon is linked to specific ensemble properties. We therefore investigate here the existence of EED QYs in two NC systems (CsPbBr3 and CdSe) that are possible candidates for use in optical refrigeration. The influence of NC size, size-distribution, surface ligand, and as-made emission QYs are investigated. Existence of EED QYs is assessed using two approaches (an absolute approach using an integrating sphere and a relative approach involving excitation spectroscopy). Altogether, our results show no evidence of EED QYs across samples. This suggests that parameters beyond those mentioned above are responsible for observations of NC EED QYs.
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