Ligand Steric Profile Tunes the Reactivity of Indium Phosphide Clusters.

Indium phosphide quantum dots have become an industrially relevant material for solid-state lighting and wide color gamut displays. The synthesis of indium phosphide quantum dots from indium carboxylates and tris(trimethylsilyl)phosphine (P(SiMe3)3) is understood to proceed through the formation of magic-sized clusters, with In37P20(O2CR)51 being the key isolable intermediate. The reactivity of the In37P20(O2CR)51 cluster is a vital parameter in controlling the conversion to quantum dots. Herein, we report structural perturbations of In37P20(O2CR)51 clusters induced by tuning the steric properties of a series of substituted phenylacetate ligands. This approach allows for control over reactivity with P(SiMe3)3, where meta-substituents enhance the susceptibility to ligand displacement, and para-substituents hinder phosphine diffusion to the core. Thermolysis studies show that with complete cluster dissolution, steric profile can modulate the nucleation period, resulting in a nanocrystal size dependence on ligand steric profile. The enhanced stability from ligand engineering also allows for the isolation and structural characterization by single-crystal X-ray diffraction of a new III-V magic-sized cluster with the formula In26P13(O2CR)39. This intermediate precedes the In37P20(O2CR)51 cluster on the InP QD reaction coordinate. The physical and electronic structure of this cluster are analyzed, providing new insight into previously unrecognized relationships between II-VI and III-V materials and the discrete growth of III-V cluster intermediates.

Navigating the Potential Energy Surface of CdSe Magic-Sized Clusters: Synthesis and Interconversion of Atomically Precise Nanocrystal Polymorphs.

Magic-sized clusters (MSCs) are kinetically stable, atomically precise intermediates along the quantum dot (QD) reaction potential energy surface. Literature precedent establishes two classes of cadmium selenide MSCs with QD-like inorganic cores: one class is proposed to be cation-rich with a zincblende crystal structure, while the other is proposed to be stoichiometric with a "wurtzite-like" core. However, the wide range of synthetic protocols used to access MSCs has made direct comparisons of their structure and surface chemistry difficult. Furthermore, the physical and chemical relationships between MSC polymorphs are yet to be established. Here, we demonstrate that both cation-rich and stoichiometric CdSe MSCs can be synthesized from identical reagents and can be interconverted through the addition of either excess cadmium or selenium precursor. The structural and compositional differences between these two polymorphs are contrasted using a combination of 1H NMR spectroscopy, X-ray diffraction (XRD), pair distribution function (PDF) analysis, inductively coupled plasma optical emission spectroscopy, and UV-vis transient absorption spectroscopy. The subsequent polymorph interconversion reactions are monitored by UV-vis absorption spectroscopy, with evidence for an altered cluster atomic structure observed by powder XRD and PDF analysis. This work helps to simplify the complex picture of the CdSe nanocrystal landscape and provides a method to explore structure-property relationships in colloidal semiconductors through atomically precise synthesis.

Comparison of Phonon Damping Behavior in Quantum Dots Capped with Organic and Inorganic Ligands.

Surface ligand modification of colloidal semiconductor nanocrystals has been widely used as a means of controlling photoexcited-state generation, relaxation, and coupling to the environment. While progress has been made in understanding how surface ligand modification affects the behavior of electronic states, less is known about the influence of surface ligand modification on phonon behavior, which impacts relaxation dynamics and transport phenomena. In this work, we compare the dynamics of optical and acoustic phonons in CdTe quantum dots (QDs), CdTe/CdSe core/shell QDs capped with octadecylphosphonic acid ligands, and CdTe QDs capped with Se2- to ascertain how ligand exchange from native aliphatic ligands to single-atom Se2- ligands affects phonon behavior. We use transient absorption spectroscopy and observe modulations in the kinetics of excited-state decay due to QD lattice vibrations from both optical and acoustic phonons, which we describe using the damped oscillator model. The longitudinal optical phonons have similar frequencies and damping behavior in all three samples. In contrast, the longitudinal acoustic phonon mode in the Se2--capped CdTe QDs is severely damped, much more so than in CdTe and CdTe/CdSe QDs capped with the native aliphatic ligands. We attribute these differences in the acoustic phonon behavior to the differences in how the QD dissipates vibrational energy to its surroundings as a function of ligand identity. Our results indicate that these inorganic surface-capping ligands enhance not only the electronic but also the mechanical coupling of nanocrystals with their environment.

Materials Properties and Solvated Electron Dynamics of Isolated Nanoparticles and Nanodroplets Probed with Ultrafast Extreme Ultraviolet Beams.

We present ultrafast photoemission measurements of isolated nanoparticles in vacuum using extreme ultraviolet (EUV) light produced through high harmonic generation. Surface-selective static EUV photoemission measurements were performed on nanoparticles with a wide array of compositions, ranging from ionic crystals to nanodroplets of organic material. We find that the total photoelectron yield varies greatly with nanoparticle composition and provides insight into material properties such as the electron mean free path and effective mass. Additionally, we conduct time-resolved photoelectron yield measurements of isolated oleylamine nanodroplets, observing that EUV photons can create solvated electrons in liquid nanodroplets. Using photoemission from a time-delayed 790 nm pulse, we observe that a solvated electron is produced in an excited state and subsequently relaxes to its ground state with a lifetime of 151 ± 31 fs. This work demonstrates that femotosecond EUV photoemission is a versatile surface-sensitive probe of the properties and ultrafast dynamics of isolated nanoparticles.

Solvents effects on charge transfer from quantum dots.

To predict and understand the performance of nanodevices in different environments, the influence of the solvent must be explicitly understood. In this Communication, this important but largely unexplored question is addressed through a comparison of quantum dot charge transfer processes occurring in both liquid phase and in vacuum. By comparing solution phase transient absorption spectroscopy and gas-phase photoelectron spectroscopy, we show that hexane, a common nonpolar solvent for quantum dots, has negligible influence on charge transfer dynamics. Our experimental results, supported by insights from theory, indicate that the reorganization energy of nonpolar solvents plays a minimal role in the energy landscape of charge transfer in quantum dot devices. Thus, this study demonstrates that measurements conducted in nonpolar solvents can indeed provide insight into nanodevice performance in a wide variety of environments.

Mapping nanoscale absorption of femtosecond laser pulses using plasma explosion imaging.

We make direct observations of localized light absorption in a single nanostructure irradiated by a strong femtosecond laser field, by developing and applying a technique that we refer to as plasma explosion imaging. By imaging the photoion momentum distribution resulting from plasma formation in a laser-irradiated nanostructure, we map the spatial location of the highly localized plasma and thereby image the nanoscale light absorption. Our method probes individual, isolated nanoparticles in vacuum, which allows us to observe how small variations in the composition, shape, and orientation of the nanostructures lead to vastly different light absorption. Here, we study four different nanoparticle samples with overall dimensions of ∼100 nm and find that each sample exhibits distinct light absorption mechanisms despite their similar size. Specifically, we observe subwavelength focusing in single NaCl crystals, symmetric absorption in TiO2 aggregates, surface enhancement in dielectric particles containing a single gold nanoparticle, and interparticle hot spots in dielectric particles containing multiple smaller gold nanoparticles. These observations demonstrate how plasma explosion imaging directly reveals the diverse ways in which nanoparticles respond to strong laser fields, a process that is notoriously challenging to model because of the rapid evolution of materials properties that takes place on the femtosecond time scale as a solid nanostructure is transformed into a dense plasma.

Photoelectron spectroscopy of CdSe nanocrystals in the gas phase: a direct measure of the evanescent electron wave function of quantum dots.

We present the first photoelectron spectroscopy measurements of quantum dots (semiconductor nanocrystals) in the gas phase. By coupling a nanoparticle aerosol source to a femtosecond velocity map imaging photoelectron spectrometer, we apply robust gas-phase photoelectron spectroscopy techniques to colloidal quantum dots, which typically must be studied in a liquid solvent or while bound to a surface. Working with a flowing aerosol of quantum dots offers the additional advantages of providing fresh nanoparticles for each laser shot and removing perturbations from bonding with a surface or interactions with the solvent. In this work, we perform a two-photon photoionization experiment to show that the photoelectron yield per exciton depends on the physical size of the quantum dot, increasing for smaller dots. Next, using effective mass modeling we show that the extent to which the electron wave function of the exciton extends from the quantum dot, the so-called "evanescent electron wavefunction", increases as the size of the quantum dot decreases. We show that the photoelectron yield is dominated by the evanescent electron density due to quantum confinement effects, the difference in the density of states inside and outside of the quantum dots, and the angle-dependent transmission probability of electrons through the surface of the quantum dot. Therefore, the photoelectron yield directly reflects the fraction of evanescent electron wave function that extends outside of the quantum dot. This work shows that gas-phase photoelectron spectroscopy is a robust and general probe of the electronic structure of quantum dots, enabling the first direct measurements of the evanescent exciton wave function.

(Ga1-xZnx)(N1-xOx) nanocrystals: visible absorbers with tunable composition and absorption spectra.

Bulk oxy(nitride) (Ga(1-x)Zn(x))(N(1-x)O(x)) is a promising photocatalyst for water splitting under visible illumination. To realize its solar harvesting potential, it is desirable to minimize its band gap through synthetic control of the value of x. Furthermore, improved photochemical quantum yields may be achievable with nanocrystalline forms of this material. We report the synthesis, structural, and optical characterization of nanocrystals of (Ga(1-x)Zn(x))(N(1-x)O(x)) with the values of x tunable from 0.30 to 0.87. Band gaps decreased from 2.7 to 2.2 eV over this composition range, which corresponded to a 260% increase in the fraction of solar photons that could be absorbed by the material. We achieved nanoscale morphology and compositional control by employing mixtures of ZnGa(2)O(4) and ZnO nanocrystals as synthetic precursors that could be converted to (Ga(1-x)Zn(x))(N(1-x)O(x)) under NH(3). The high quality of the resulting nanocrystals is encouraging for achieving photochemical water-splitting rates that are competitive with internal carrier recombination pathways.

Recent Progress in Photocatalysis Mediated by Colloidal II-VI Nanocrystals.

The use of photoexcited electrons and holes in semiconductor nanocrystals as reduction and oxidation reagents is an intriguing way of harvesting photon energy to drive chemical reactions. This review focuses on recent research efforts to understand and control the photocatalytic processes mediated by colloidal II-VI nanocrystalline materials, such as cadmium and zinc chalcogenides. First, we highlight how nanocrystal properties govern the rates and efficiencies of charge-transfer processes relevant to photocatalysis. We then describe the use of nanocrystal catalyst heterostructures for fuel-forming reactions, most commonly H2 generation. Finally, we review the use of nanocrystal photocatalysis as a synthetic tool for metal-semiconductor nano-heterostructures.