Hole Mobility in Nanocrystal Solids as a Function of Constituent Nanocrystal Size

Solids of semiconductor nanocrystals (NCs) are semiconductors in which the band gap can be controlled by changing the size of the constituent NCs. To date, nontrivial dependencies of the carrier mobility on the NC size have been reported. We use the time-of-flight (TOF) technique to measure the carrier mobility as a function of the NC size and find that the hole mobility of the NC solid increases dramatically with decreasing NC radius. We show that this result is in agreement with an analytic model for carrier mobility in NC solids. We further implement Monte Carlo simulations to aid in understanding the transient measurements in the context of models of dispersive transport. This work highlights that changing NC size in a device has important implications for charge transport

Tuning Electron–Phonon Interactions in Nanocrystals through Surface Termination

We perform ab initio molecular dynamics on experimentally relevant-sized lead sulfide (PbS) nanocrystals (NCs) constructed with thiol or Cl, Br, and I anion surfaces to determine their vibrational and dynamic electronic structure. We show that electron–phonon interactions can explain the large thermal broadening and fast carrier cooling rates experimentally observed in Pb–chalcogenide NCs. Furthermore, our simulations reveal that electron–phonon interactions are suppressed in halide-terminated NCs due to reduction of both the thermal displacement of surface atoms and the spatial overlap of the charge carriers with these large atomic vibrations. This work shows how surface engineering, guided by simulations, can be used to systematically control carrier dynamics

Tuning Electron–Phonon Interactions in Nanocrystals through Surface Termination

We perform ab initio molecular dynamics on experimentally relevant-sized lead sulfide (PbS) nanocrystals (NCs) constructed with thiol or Cl, Br, and I anion surfaces to determine their vibrational and dynamic electronic structure. We show that electron–phonon interactions can explain the large thermal broadening and fast carrier cooling rates experimentally observed in Pb–chalcogenide NCs. Furthermore, our simulations reveal that electron–phonon interactions are suppressed in halide-terminated NCs due to reduction of both the thermal displacement of surface atoms and the spatial overlap of the charge carriers with these large atomic vibrations. This work shows how surface engineering, guided by simulations, can be used to systematically control carrier dynamics

Measuring the Electronic Structure of Nanocrystal Thin Films Using Energy-Resolved Electrochemical Impedance Spectroscopy

Use of nanocrystal thin films as active layers in optoelectronic devices requires tailoring of their electronic band structure. Here, we demonstrate energy-resolved electrochemical impedance spectroscopy (ER-EIS) as a method to quantify the electronic structure in nanocrystal thin films. This technique is particularly well-suited for nanocrystal-based thin films as it allows for in situ assessment of electronic structure during solution-based deposition of the thin film. Using well-studied lead sulfide nanocrystals as an example, we show that ER-EIS can be used to probe the energy position and number density of defect or dopant states as well as the modification of energy levels in nanocrystal solids that results through the exchange of surface ligands. This work highlights that ER-EIS is a sensitive and fast method to measure the electronic structure of nanocrystal thin films and enables their optimization in optoelectronic devices

Tuning Electron–Phonon Interactions in Nanocrystals through Surface Termination

We perform ab initio molecular dynamics on experimentally relevant-sized lead sulfide (PbS) nanocrystals (NCs) constructed with thiol or Cl, Br, and I anion surfaces to determine their vibrational and dynamic electronic structure. We show that electron–phonon interactions can explain the large thermal broadening and fast carrier cooling rates experimentally observed in Pb–chalcogenide NCs. Furthermore, our simulations reveal that electron–phonon interactions are suppressed in halide-terminated NCs due to reduction of both the thermal displacement of surface atoms and the spatial overlap of the charge carriers with these large atomic vibrations. This work shows how surface engineering, guided by simulations, can be used to systematically control carrier dynamics

In Situ Monitoring of Cation-Exchange Reaction Shell Growth on Nanocrystals

We demonstrate how in situ monitoring of the photoluminescence during shell growth around colloidal nanocrystals (NCs) can be used to develop a detailed and quantitative model for this process. We apply it here to study cation-exchange based growth of ZnS on a Cu–In–Se NC to form Cu–In–Se/ZnSe_1–<i>x</i>S_<i>x</i> alloyed NCs. We determine that this process begins with the Zn precursor binding to the outer layer of the NC followed by diffusion of Zn cations into successive atomic monolayers of the NC. At temperatures below 100 °C, Zn cations can only diffuse into the outermost atomic monolayer of the Cu–In–Se NCs. At growth temperatures above 100 °C, the second monolayer also becomes thermally accessible and can be filled with Zn cations. Our results provide an understanding of cation-exchange shell growth at the atomic level via optical analysis. The approach and mathematical model described here can be applied to other core/shell nanostructures and allows selection of optimal synthesis conditions to achieve desired core/shell design for a specific application

Upscaling Colloidal Nanocrystal Hot-Injection Syntheses via Reactor Underpressure

We report an approach to linearly upscale hot-injection syntheses of colloidal nanocrystals by applying mild underpressure to the flask reactor prior to the injection such that rapid addition of large volumes of the precursor is facilitated. We apply this underpressure-assisted approach to successfully upscale synthetic protocols for metallic (Sn) and semiconductor (PbS, CsPbBr₃, and Cu₃In₅Se₉) nanocrystals by 1–2 orders of magnitude to obtain tens of grams of nanocrystals per synthesis. Here, we provide the technical details of how to perform underpressure-assisted upscaling and demonstrate that nanocrystal quality is maintained for the large-batch syntheses by characterizing the size, size distribution, composition, optical properties, and ligand coverage of the nanocrystals for both small- and large-scale syntheses. This work shows that fast addition of large injection volumes does not intrinsically limit upscaling of hot-injection-based colloidal syntheses

Upscaling Colloidal Nanocrystal Hot-Injection Syntheses via Reactor Underpressure

We report an approach to linearly upscale hot-injection syntheses of colloidal nanocrystals by applying mild underpressure to the flask reactor prior to the injection such that rapid addition of large volumes of the precursor is facilitated. We apply this underpressure-assisted approach to successfully upscale synthetic protocols for metallic (Sn) and semiconductor (PbS, CsPbBr₃, and Cu₃In₅Se₉) nanocrystals by 1–2 orders of magnitude to obtain tens of grams of nanocrystals per synthesis. Here, we provide the technical details of how to perform underpressure-assisted upscaling and demonstrate that nanocrystal quality is maintained for the large-batch syntheses by characterizing the size, size distribution, composition, optical properties, and ligand coverage of the nanocrystals for both small- and large-scale syntheses. This work shows that fast addition of large injection volumes does not intrinsically limit upscaling of hot-injection-based colloidal syntheses

Enhanced Charge Transport Kinetics in Anisotropic, Stratified Photoanodes

The kinetics of charge transport in mesoporous photoanodes strongly constrains the design and power conversion efficiencies of dye sensitized solar cells (DSSCs). Here, we report a stratified photoanode design with enhanced kinetics achieved through the incorporation of a fast charge transport intermediary between the titania and charge collector. Proof of concept photoanodes demonstrate that the inclusion of the intermediary not only enhances effective diffusion coefficients but also significantly suppresses charge recombination, leading to diffusion lengths two orders of magnitude greater than in standard mesoporous titania photoanodes. The intermediary concept holds promise for higher-efficiency DSSCs

Measuring the Vibrational Density of States of Nanocrystal-Based Thin Films with Inelastic X‑ray Scattering

Knowledge of the vibrational structure of a semiconductor is essential for explaining its optical and electronic properties and enabling optimized materials selection for optoelectronic devices. However, measurement of the vibrational density of states of nanomaterials is challenging. Here, using the example of colloidal nanocrystals (quantum dots), we show that the vibrational density of states of nanomaterials can be accurately and efficiently measured with inelastic X-ray scattering (IXS). Using IXS, we report the first experimental measurements of the vibrational density of states for lead sulfide nanocrystals with different halide-ion terminations and for CsPbBr₃ perovskite nanocrystals. IXS findings are supported with <i>ab initio</i> molecular dynamics simulations, which provide insight into the origin of the measured vibrational structure and the effect of nanocrystal surface. Our findings highlight the advantages of IXS compared to other methods for measuring the vibrational density of states of nanocrystals such as inelastic neutron scattering and Raman scattering