We demonstrate postsynthetic modification of CsPbBr nanocrystals by a thiocyanate salt treatment. This treatment improves the quantum yield of both freshly synthesized (PLQY ≈ 90%) and aged nanocrystals (PLQY ≈ 70%) to within measurement error (2-3%) of unity, while simultaneously maintaining the shape, size, and colloidal stability. Additionally, the luminescence decay kinetics transform from multiexponential decays typical of nanocrystalline semiconductors with a distribution of trap sites, to a monoexponential decay, typical of single energy level emitters. Thiocyanate only needs to access a limited number of CsPbBr nanocrystal surface sites, likely representing under-coordinated lead atoms on the surface, in order to have this effect.

Controlling the structure of colloidal nanocrystals (NCs) is key to the generation of their complex functionality. This requires an understanding of the NC surface at the atomic level. The structure of colloidal PbS-NC passivated with oleic acid has been studied theoretically and experimentally. We show the existence of surface OH-groups, which play a key role in stabilizing the PbS(111) facets, consistent with x-ray photoelectron spectroscopy as well as other spectroscopic and chemical experiments. The role of water in the synthesis process is also revealed. Our model, along with the existing observations of NC surface termination and passivation by ligands, helps to explain and predict the properties of NCs and their assemblies. The structure of the interior of nanocrystals (NCs) can be determined quite accurately by crystallography, but the structure of their surfaces cannot be obtained from this analysis due to the complexity of organic capping ligands (1-6). However, the NC surface structure controls the growth and solubility, and strongly influences the physical and chemical properties (7-12). We take advantage of improved insights of the mechanisms of nanocrystal growth (3-6), combined with extensive ab initio total energy calculations, to create a testable model for the atomic surface passivation structure of the exposed facets of a PbS-NC. The model makes specific, and at first surprising, predictions about the surface-bound species that were subsequently verified experimentally. PbS is ideal for this study because of the high symmetry of its rocksalt structure and its propensity to form NCs with well-defined (111) and (001) facets (13, 14). PbS-NCs with controlled size and shape can be produced from a PbO lead precursor, hexamethyldisilathiane (TMS2S) sulfur precursor, and oleic acid that binds to exposed Pb atoms to stabilize the surface (1-6). We performed ab initio electronic structure calculations on relevant subsystems and on the reaction steps involved in the synthetic process, including: studies of immediate precursors; fate of by-products of the initial decomposition step; NC-ligand interactions; and ligand-ligand and ligand-solvent interactions. Our methods (15) are density functional theory (DFT) in the generalized gradient approximation (GGA). The GGA is not accurate enough to describe the van der Waals (vdW) interactions, and the entropic contributions in the solvent are prohibitively time-consuming at present to be calculated directly. However, we have taken advantage of likely cancellations of these terms when comparing different systems. We found the following features in our theoretically derived model: (1) The Pb:S atomic ratio is roughly 1.2:1 (Pbexcess:S~0.2:1) for ~5 nm NCs; (2) The ratio between the number of surface oleate molecules and the number of excess Pb atoms is ~1:1; (3) The ligands can be easily removed from the (001) surface; (4) The average (111)/(001) surface to center distance ratio of the truncated octahedral shape is about 0.82:1; (5) OH-groups are present on the NC ...

Lanthanide-doped nanocrystals are of particular interest for the research community not only due to their ability to shape light by downshifting, quantum cutting, and upconversion but also because novel optical properties can be found by the precise engineering of core-shell nanocrystals. Because of the large surface area-to-volume ratio of nanocrystals, the luminescence is typically suppressed by surface quenching. Here, we demonstrate a mechanism that exploits surface quenching processes to improve the luminescence of our core-shell lanthanide-doped nanocrystals. By carefully tuning the shell thickness of inert β-NaLuF around β-NaYF nanocrystals doped with Yb and Er, we unravel the relationship between quantum yield and shell thickness, and quantify surface quenching rates for the relevant Er and Yb energy levels. This enhanced understanding of the system's dynamics allowed us to design nanocrystals with a surface quenching-assisted mechanism for bright NIR to NIR downshifting with a distinctive efficiency peak for an optimized shell thickness.

A variety of optical applications rely on the absorption and reemission of light. The quantum yield of this process often plays an essential role. When the quantum yield deviates from unity by significantly less than 1%, applications such as luminescent concentrators and optical refrigerators become possible. To evaluate such high performance, we develop a measurement technique for luminescence efficiency with sufficient accuracy below one part per thousand. Photothermal threshold quantum yield is based on the quantization of light to minimize overall measurement uncertainty. This technique is used to guide a procedure capable of making ensembles of near-unity emitting cadmium selenide/cadmium sulfide (CdSe/CdS) core-shell quantum dots. We obtain a photothermal threshold quantum yield luminescence efficiency of 99.6 ± 0.2%, indicating nearly complete suppression of nonradiative decay channels.

We modify the fundamental electronic properties of metallic (1T phase) nanosheets of molybdenum disulfide (MoS) through covalent chemical functionalization, and thereby directly influence the kinetics of the hydrogen evolution reaction (HER), surface energetics, and stability. Chemically exfoliated, metallic MoS nanosheets are functionalized with organic phenyl rings containing electron donating or withdrawing groups. We find that MoS functionalized with the most electron donating functional group (p-(CHCH)NPh-MoS) is the most efficient catalyst for HER in this series, with initial activity that is slightly worse compared to the pristine metallic phase of MoS. The p-(CHCH)NPh-MoS is more stable than unfunctionalized metallic MoS and outperforms unfunctionalized metallic MoS for continuous H evolution within 10 min under the same conditions. With regards to the entire studied series, the overpotential and Tafel slope for catalytic HER are both directly correlated with the electron donating strength of the functional group. The results are consistent with a mechanism involving ground-state electron donation or withdrawal to/from the MoS nanosheets, which modifies the electron transfer kinetics and catalytic activity of the MoS nanosheet. The functional groups preserve the metallic nature of the MoS nanosheets, inhibiting conversion to the thermodynamically stable semiconducting state (2H) when mildly annealed in a nitrogen atmosphere. We propose that the electron density and, therefore, reactivity of the MoS nanosheets are controlled by the attached functional groups. Functionalizing nanosheets of MoS and other transition metal dichalcogenides provides a synthetic chemical route for controlling the electronic properties and stability within the traditionally thermally unstable metallic state.

Ion-exchange transformations allow access to nanocrystalline materials with compositions that are inaccessible via direct synthetic routes. However, additional mechanistic insight into the processes that govern these reactions is needed. We present evidence for the presence of two distinct mechanisms of exchange during anion exchange in CsPbX3 nanocrystals (NCs), ranging in size from 6.5 to 11.5 nm, for transformations from CsPbBr3 to CsPbCl3 or CsPbI3. These NCs exhibit bright luminescence throughout the exchange, allowing their optical properties to be observed in real time, in situ. The iodine exchange presents surface-reaction-limited exchanges allowing all anionic sites within the NC to appear chemically identical, whereas the chlorine exchange presents diffusion-limited exchanges proceeding through a more complicated exchange mechanism. Our results represent the first steps toward developing a microkinetic description of the anion exchange, with implications not only for understanding the lead halide perovskites but also for nanoscale ion exchange in general.

We utilize CdSe/CdS seeded nanorods as a tunable lumophore for luminescent concentration. Transfer-printed, ultrathin crystalline Si solar cells are embedded directly into the luminescent concentrator, allowing the study of luminescent concentrators with an area over 5000 times the area of the solar cell. By increasing the size of the CdS rod with respect to the luminescent CdSe seed, the reabsorption of propagating photons is dramatically reduced. At long luminescence propagation distances, this reduced reabsorption can overcome the diminished quantum yield inherent to the larger semiconductor structures, which is studied with lifetime spectroscopy. A Monte Carlo ray tracing model is developed to explain the performance of the luminescent concentrator and is then used as a design tool to determine the effect of luminescence trapping on the concentration of light using both CdSe/CdS nanorods and a model organic dye. We design an efficient luminescence trapping structure that should allow the luminescent concentrator based on CdSe/CdS nanorods to operate in the high-concentration regime.

Luminescent solar concentrators doped with CdSe/CdS quantum dots provide a potentially low-cost and high-performance alternative to costly highband-gap III−V semiconductor materials to serve as a top junction in multijunction photovoltaic devices for efficient utilization of blue photons. In this study, a photonic mirror was coupled with such a luminescent waveguide to form an optical cavity where emitted luminescence was trapped omnidirectionally. By mitigating escape cone and scattering losses, 82% of luminesced photons travel the length of the waveguide, creating a concentration ratio of 30.3 for blue photons in a waveguide with a geometric gain of 61. Further, we study the photon transport inside the luminescent waveguide, showing unimpeded photon collection across the entire length of the waveguide. L uminescent solar concentrators 1−4 (LSCs) have been studied extensively for the last three decades as low-cost alternatives to single-and multijunction photovoltaic (PV) devices. As silicon prices have fallen, it has become increasingly clear that future solar panels will need to have both low cost and high efficiency. One promising strategy for achieving a higher efficiency is to use different parts of the solar spectrum in photovoltaic materials with varying band gaps to minimize losses associated with carrier thermalization and incomplete photon absorption. For these multijunction (MJ) PV devices, there is a strong need for developing low-cost, high-band-gap solar cells for efficient utilization of the high-energy part of the solar spectrum. A luminescent solar concentrator could provide exactly this function, serving as the top junction in a multijunction architecture by converting blue photons into guided luminescence. Due to the concentration effect, only small amounts of high-performing but expensive III−V photovoltaic materials are needed to collect the light from an inexpensive luminescent waveguide. Such a device requires high concentration factors to reduce the cost of the III−V photovoltaic material. High concentration also allows the Stokes shift of the lumophore to be recovered in the operating voltage of the photovoltaic cell.The concentration factor and collection efficiency achieved by LSCs to date have been limited due to parasitic losses such as nonunity quantum yields of the lumophores, imperfect light trapping within the waveguide, and reabsorption and scattering of propagating photons. 5 Previous studies have sought to solve each of these parasitic losses individually, resulting in modest performance improvements. 6−15 Here we achieve a luminescent concentration ratio greater than 30 with an optical efficiency of 82% for blue photons by simultaneously addressing the materials and optical challenges of the LSC system. These concentration ratios are achieved through the combination of designer quantum dot lumophores and photonic mirrors, and microscale silicon photovoltaic cells are used to detect the concentration of light in the waveguide. To the best of our knowledge, this is the highest ...