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.

Colloidal, Room-Temperature Growth of Metal Oxide Shells on InP Quantum Dots.

We demonstrate colloidal, layer-by-layer growth of metal oxide shells on InP quantum dots (QDs) at room temperature. We show with computational modeling that native InP QD surface oxides give rise to nonradiative pathways due to the presence of surface-localized dark states near the band edges. Replacing surface indium with zinc to form a ZnO shell results in reduced nonradiative decay and a density of states at the valence band edge that resembles defect-free, stoichiometric InP. We then developed a synthetic strategy using stoichiometric amounts of common atomic layer deposition precursors in alternating cycles to achieve layer-by-layer growth. Metal-oxide-shelled InP QDs show bulk and local structural perturbations as determined by X-ray diffraction and extended X-ray absorption fine structure spectroscopy. Upon growing ZnSe shells of varying thickness on the oxide-shelled QDs, we observe increased photoluminescence (PL) quantum yields and narrowing of the emission linewidths that we attribute to decreased ion diffusion to the shell, as supported by phosphorus X-ray emission spectroscopy. These results present a versatile strategy to control QD interfaces for novel heterostructure design by leveraging surface oxides. This work also contributes to our understanding of the connections between structural complexity and PL properties in technologically relevant colloidal optoelectronic materials.

Synthesis and Single Crystal X-ray Diffraction Structure of an Indium Arsenide Nanocluster.

The discovery of magic-sized clusters as intermediates in the synthesis of colloidal quantum dots has allowed for insight into formation pathways and provided atomically precise molecular platforms for studying the structure and surface chemistry of those materials. The synthesis of monodisperse InAs quantum dots has been developed through the use of indium carboxylate and As(SiMe3)3 as precursors and documented to proceed through the formation of magic-sized intermediates. Herein, we report the synthesis, isolation, and single-crystal X-ray diffraction structure of an InAs nanocluster that is ubiquitous across reports of InAs quantum dot synthesis. The structure, In26As18(O2CR)24(PR'3)3, differs substantially from previously reported semiconductor nanocluster structures even within the III-V family. However, it can be structurally linked to III-V and II-VI cluster structures through the anion sublattice. Further analysis using variable temperature absorbance spectroscopy and support from computation deepen our understanding of the reported structure and InAs nanomaterials as a whole.

Electronic Structures and Spectroscopic Signatures of Noble-Gas-Doped Nanodiamonds.

Fluorescent nanodiamonds, that is, those containing optically active defects, have attracted interest for their ability to be used as qubits; for in vivo imaging; and as sensors for spin, stress, and temperature. One of the most commonly studied nanodiamond color centers is the nitrogen vacancy. However, there is strong interest in discovering other impurity centers that provide localized midband gap transitions. Noble gas atoms have garnered attention since they have been discovered within nanodiamonds produced through high-pressure-high-temperature laser-heated diamond anvil cell synthesis methods, where they are commonly used as hydrostatic pressure media. Noble gas atoms that exist in macrosized natural or synthetic diamonds have been shown to be able to form color centers. This research uses ab initio density functional theory and cluster models to systematically study the localized electronic structure for group VIII impurities of nanodiamond, including helium, neon, argon, krypton, and xenon. An in-depth examination of the interaction between the noble gas atom and diamond lattice has been carried out. The changes to the vibrational and UV/vis absorption spectra have been analyzed. It was determined that the energetically preferred geometry is dependent on the atom size. Most noble gas defects are stabilized within the nanodiamond lattice and exist in tetrahedral interstitial positions, except for the largest noble gas studied in this work, Xe, which was determined to prefer a substitutional configuration. Both Kr and Xe are expected to be able to manifest visible/near-IR optical responses when included in the diamond lattice.

Defect-Induced Magnetic Skyrmion in a Two-Dimensional Chromium Triiodide Monolayer.

Chromium iodide monolayers, which have different magnetic properties in comparison to the bulk chromium iodide, have been shown to form skyrmionic states in applied electromagnetic fields or in Janus-layer devices. In this work, we demonstrate that spin-canted solutions can be induced into monolayer chromium iodide by select substitution of iodide atoms with isovalent impurities. Several concentrations and spatial configurations of halide substitutional defects are selected to probe the coupling between the local defect-induced geometric distortions and orientation of chromium magnetic moments. This work provides atomic-level insight into how atomically precise chemical doping can be used to create and control complex magnetic patterns in chromium iodide layers and lays out the foundation for investigating the field- and geometric-dependent magnetic properties in similar two-dimensional materials.

Simulating Magnetic Circular Dichroism Spectra with Real-Time Time-Dependent Density Functional Theory in Gauge Including Atomic Orbitals.

Magnetic circular dichroism (MCD) spectra are able to provide insights into the geometric, electronic, and magnetic properties of chemical systems. However, they can be challenging to understand and simulate given the need to simultaneously treat both the finite magnetic and optical fields. Thus, efficient simulations are desired to understand the spectra and resolve the molecular electronic states. Real-time dynamics are used widely in the simulation of electronic spectroscopies such as absorption as well as electronic circular dichroism, but simulating MCD with real-time dynamics is technically and theoretically challenging. In this work, we introduce a real-time dynamics-based ab initio method with a nonperturbative treatment of a static magnetic field with London orbitals for simulating the MCD spectra of closed shell systems. Effects of a magnetic field are included variationally in the spin-free nonrelativistic Hamiltonian. Real-time time-dependent density functional theory dynamics are then performed, from which we compute the response function in the presence of the external magnetic field, giving the MCD spectrum. The method developed in this paper is applied to simulate the MCD spectra for pyrimidine, pyrazine, and 1,4-naphthoquinone. Results are discussed and compared to the experiment.

Carboxylate Anchors Act as Exciton Reporters in 1.3 nm Indium Phosphide Nanoclusters.

Developing interfacial probes of ligand-nanocluster interactions is crucial for understanding and tailoring the optoelectronic properties of these emerging nanomaterials. Using transient IR spectroscopy, we demonstrate that ligand vibrational modes of oleate-capped 1.3 nm InP nanoclusters report on the photogenerated exciton. The exciton induces an intensity change in the asymmetric carboxylate stretching mode by 57% while generating no appreciable shift in frequency. Thus, the observed difference signal is attributed to an exciton-induced change in the dipole magnitude of the asymmetric carboxylate stretching mode. Additionally, the transient IR data reveal that the infrared dipole change is dependent on the geometry of the ligand bound to the nanocluster. The experimental results are interpreted using TDDFT calculations, which identify how the spatial dependence of an exciton-induced electron density shift affects the vibrational motion of the carboxylate anchors. More broadly, this work demonstrates transient IR spectroscopy as a useful method for characterizing ligand-nanocluster coupling interactions.