We introduce a general surface passivation mechanism for cesium lead halide perovskite materials (CsPbX 3 , X = Cl, Br, I) that is supported by a combined experimental and theoretical study of the nanocrystal surface chemistry. A variety of spectroscopic methods are employed together with ab initio calculations to identify surface halide vacancies as the predominant source of charge trapping. The number of surface traps per nanocrystal is quantified by 1 H NMR spectroscopy, and that number is consistent with a simple trapping model in which surface halide vacancies create deleterious under-coordinated lead atoms. These halide vacancies exhibit trapping behavior that differs between CsPbCl 3 , CsPbBr 3 , and CsPbI 3. Ab initio calculations suggest that introduction of anionic X-type ligands can produce trap-free bandgaps by altering the energetics of lead-based defect levels. General rules for selecting effective passivating ligand pairs are introduced by considering established principles of coordination chemistry. Introducing softer, anionic, X-type Lewis bases that target under-coordinated lead atoms results in absolute quantum yields approaching unity and monoexponential luminescence decay kinetics, thereby indicating full trap passivation. This work provides a systematic framework for preparing highly luminescent CsPbX 3 nanocrystals with variable compositions and dimensionalities, thereby improving fundamental understanding of these materials and informing future synthetic and post-synthetic efforts towards trap-free CsPbX 3 nanocrystals.

Perovskites are processed from solution; understanding the influence of solution composition on crystallization and degradation is critical to their success.

The vapor-liquid-solid (VLS) mechanism is widely used for the synthesis of semiconductor nanowires (NWs), yet several aspects of the mechanism are not fully understood. Here, we present comprehensive experimental measurements on the growth rate of Au-catalyzed Si NWs over a range of temperatures (365-480 °C), diameters (30-200 nm), and pressures (0.1-1.6 Torr SiH4). We develop a kinetic model of VLS growth that includes (1) Si incorporation into the liquid Au-Si catalyst, (2) Si evaporation from the catalyst surface, and (3) Si crystallization at the catalyst-NW interface. This simple model quantitatively explains growth rate data collected over more than 65 distinct synthetic conditions. Surprisingly, upon increasing the temperature and/or pressure, the analysis reveals an abrupt transition from a diameter-independent growth rate that is limited by incorporation to a diameter-dependent growth rate that is limited by crystallization. The identification of two distinct growth regimes provides insight into the synthetic conditions needed for specific NW-based technologies, and our kinetic model provides a straightforward framework for understanding VLS growth with a range of metal catalysts and semiconductor materials.

Carrier recombination is a crucial process governing the optical properties of a semiconductor. Although various theoretical approaches have been utilized to describe carrier behaviors, a quantitative understanding of the impact of defects and interfaces in low dimensional semiconductor systems is still elusive. Here we develop a model system consisting of chemically tunable, highly luminescent halide perovskite nanocrystals to illustrate the role of carrier diffusion and material dimensionality on the carrier recombination kinetics and luminescence efficiency. Our advanced synthetic methods provide a well-controlled colloidal system consisting of nanocrystals with different aspect ratios, halide compositions, and surface conditions. Using this system, we reveal the scaling laws of photoluminescence quantum yield and radiative lifetime with respect to the aspect ratio of nanocrystals. The scaling laws derived herein are not only a phenomenological observation but proved a powerful tool disentangling the carrier dynamics of microscopic systems in a quantitative and interpretable manner. The investigation of our model system and theoretical formulation bring to light the dimensionality as a hidden constraint on carrier dynamics and identify the diffusion length as an important parameter that distinguishes nanoscale and macroscale carrier behaviors. The conceptual distinction in carrier dynamics in different dimensionality regimes informs new design rules for optical devices where complex microstructures are involved.