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Lead halide perovskite nanocrystals, such as CsPbBr₃, exhibit efficient photoluminescence (PL) up-conversion, also referred to as anti-Stokes photoluminescence (ASPL). This is a phenomenon where irradiating nanocrystals up to 100 meV below gap results in higher energy band edge emission. Most surprising is that ASPL efficiencies approach unity and involve single-photon interactions with multiple phonons. This is unexpected given the statistically disfavored nature of multiple-phonon absorption. Here, in this study, we report and rationalize near-unity anti-Stokes photoluminescence efficiencies in CsPbBr₃ nanocrystals and attribute them to resonant multiple-phonon absorption by polarons. The theory explains paradoxically large efficiencies for intrinsically disfavored, multiple-phonon-assisted ASPLmore » in nanocrystals. Moreover, the developed microscopic mechanism has immediate and important implications for applications of ASPL toward condensed phase optical refrigeration.« less

Label-free, bond-selective imaging offers new opportunities for fundamental and applied studies in chemistry, biology, and materials science. Preventing its broader application to investigating spectrally congested specimens are issues related to low sensitivity as well as low spatial and temporal resolution. Here, we demonstrate a widefield, mid-infrared (MIR) photothermal imaging technique, called widefield Infrared Photothermal Heterodyne imaging (wIR-PHI), that massively parallelizes acquisition of MIR absorption data through use of a high-speed complementary metal-oxide-semiconductor camera. wIR-PHI possesses notable features that include: spatial resolution significantly below the MIR diffraction limit, hyperspectral imaging capabilities, high sensitivity, and ~100 ns temporal resolution. Here, the firstmore » two features are highlighted by hyperspectral imaging of proximally close poly(methyl methacrylate) (PMMA) and polystyrene (PS) nanoparticles where clear, bond-specific imaging of nanoparticles, separated by less than the MIR diffraction limit, is demonstrated. Sensitivity is highlighted by imaging individual PMMA and PS nanoparticles with radii between r = 97-556 nm. This leads to a current, peak absorption cross-section limit-of-detection of σ_abs = 1.9× 10^–16 cm². wIR-PHI's 100 ns temporal resolution is simultaneously demonstrated by observing the decay of photothermal contrast on individual nanoparticles on a ~200-6000 ns timescale. In whole, wIR-PHI’s dramatic increase in acquisition speed opens opportunities for future MIR kinetic imaging and spectroscopic studies of important chemical, biological, and material processes.« less

Key features of syntheses, involving the quaternary ammonium passivation of CsPbBr₃ nanocrystals (NCs), include stable, reproducible and large (often near-unity) emission quantum yields (QYs). The archetypical example involves didodecyl dimethyl ammonium (DDDMA⁺) passivated CsPbBr₃ NCs where robust QYs stem from interactions between DDDMA⁺ and NC surfaces. Despite widespread adoption of this synthesis, specific ligand-NC surface interactions responsible for large DDDMA⁺-passivated NC QYs have not been fully established. Multidimensional nuclear magnetic resonance experiments now reveal a new DDDMA⁺-NC surface interaction, beyond established “tightly-bound” DDDMA⁺ interactions, which strongly affects observed emission QYs. Depending upon the existence of this new DDDMA⁺ coordination, NCmore » QYs vary broadly between 60% and 85%. More importantly, these measurements reveal surface passivation through unexpected didodecyl ammonium (DDA⁺) that works in concert with DDDMA⁺, to produce near unity (i.e. >90%) QYs.« less

Photoinduced halide segregation in mixed halide hybrid perovskites [i.e., APb(I_1–xBr_x)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. In this work, 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 examinationmore » 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.« less

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 absorptionmore » 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. Overall, the results point to an origin for ΔE_s, intrinsic to the size-dependent electronic properties of individual perovskite NCs.« less