An official website of the United States government
Spatial confinement of charge carriers in nanosize semiconductor quantum dots (QDs) results in highly tunable, size-dependent optoelectronic properties that can be utilized in various commercial applications. Although in such nanostructures, non-stoichiometry is frequently encountered using conventional synthesis techniques, it is not often addressed or considered. Here, we perform ab initio molecular dynamics simulations on non-stoichiometric CdSe clusters to study the phonon-mediated charge carrier relaxation dynamics. We model cation-rich and anion-rich QDs passivated with monocharged neutralizing ligands of different sizes. Our studies confirm the presence of localized trap states at the valence band edge in only anion-rich QDs due to the presence of undercoordinated exposed surface Se atoms. Noteworthily, these localized states disappear when using bulkier ligands. Calculations reveal that the size of the ligands controls the crystal vibrations and electron–phonon coupling, while ligand coordination number affects the electronic structure. For a particular non-stoichiometric CdSe QD, a change of a ligand can either increase or decrease the total electron relaxation time compared to that of stoichiometric QDs. Our results emphasize the importance of ligand engineering in non-stoichiometric QDs for photoinduced dynamics and guide future work for the implementation of improved materials for optoelectronic devices.
more »
« less
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.
more »
« less
- Award ID(s):
- 2107237
- PAR ID:
- 10514294
- Publisher / Repository:
- American Chemical Society
- Date Published:
- Journal Name:
- Journal of the American Chemical Society
- Volume:
- 145
- Issue:
- 50
- ISSN:
- 0002-7863
- Page Range / eLocation ID:
- 27480 to 27492
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
In nanoscale chemistry, magic-sized clusters (MSCs) stand out for their precise atomic configurations and privileged stability, offering unprecedented insights into the atomic-level structure of ligand-capped nanocrystals and a gateway to new synthesis and functionality. This article explores our efforts to shed light on the structure and reactivity of II-VI and III-V semiconductor MSCs. We have specifically been interested in the synthesis, isolation, and characterization of MSCs implicated as key intermediates in the synthesis of semiconductor quantum dots. Our exploration into their synthesis, structure, transformation, and reactivity provides a roadmap to expand the scope of accessible semiconductor clusters with diverse structures and properties. It paves the way for tailor-made nanomaterials with unprecedented atom-level control. In these studies, atomic level structure has been deduced through advanced characterization methods, including single-crystal and powder X-ray diffraction, complemented by pair distribution function analysis, nuclear magnetic resonance spectroscopy, and vibrational spectroscopy. We have identified two distinct families of CdSe MSCs with zincblende and wurtzite-like structures. We have also characterized two members of the wurtzite-like family of InP clusters and a related InAs cluster. Our research has revealed intriguing structural homologies between II-VI and III-V MSCs. These findings contribute to our fundamental understanding of semiconductor MSCs and hint at broader implications for phase control at the nanoscale and the synthesis of novel nanomaterials. We have also explored three distinct pathways of cluster reactivity, including cluster interconversion mediated by controlling the chemical potential of the reaction environment, both seeded and single source precursor growth mechanisms to convert MSCs into larger nanostructures, and cation exchange to access new cluster compositions that are precursors to nanocrystals that may be challenging or impossible to access from traditional bottom-up nucleation and growth. Together with the collective efforts of other researchers in the field of semiconductor cluster chemistry, our work establishes a strong foundation for predicting and controlling the form and function of semiconductor MSCs. By highlighting the role of surface chemistry, stoichiometry, and dopant incorporation in determining cluster properties, our work opens exciting possibilities for the design and synthesis of new materials. The insights gained through these efforts could significantly impact the future of nanotechnology, particularly in areas like photonics, electronics, and catalysis.more » « less
-
Investigation of charge transfer in quantum dot (QD) systems is an area of great interest. Specifically, the relationship between capping ligand and rate of charge transfer has been studied as a means to optimize these materials. To investigate the role of ligand interaction on the QD surface for electron transfer, we designed and synthesized a series of ligands containing an electron accepting moiety, naphthalene bisimide (NBI). These ligands differ in their steric bulk: as one allows for π–π stacking between the NBI moieties at high surface coverages, while the other does not, allowing for a direct comparison of these effects. Once grafted onto QDs, these hybrid materials were studied using UV-Vis, fluorescence, and transient absorption spectroscopy. Interestingly, the sample with the fastest electron transfer was not the sample with the most NBI π–π stacking, it was instead where these ligands were mixed amongst oleic acid, breaking up H-aggregates between the NBI groups.more » « less
-
Abstract Propylene epoxidation in the presence of oxygen and hydrogen were measured for a series of Au/TS‐1 catalysts prepared by a modified incipient wetness impregnation (mIWI) method. This method enables precise control of the Au : Ti ratio in the Au/TS‐1 catalysts. The optimized Au/TS‐1 catalyst exhibited 12 % propylene conversion, 87 % PO selectivity, and 25 % hydrogen efficiency. The particle size of gold nanoparticles prepared by the modified IWI was between 2 and 3 nm, as demonstrated by XRD patterns, STEM images, and X‐ray absorption spectroscopy at the Au L3edge. XPS spectra showed that the surface species on the catalysts were similar. UV‐Vis spectra suggested that in the modified IWI method, the chlorine ligands in Au(Cl)4−were replaced by hydroxyl groups, which contributes to form small gold nanoparticles. Kinetic studies showed that the active sites of Au(mIWI)/TS‐1 are similar to the Au(DP)/TS‐1 prepared by deposition precipitation.more » « less
-
Effective sensitization of triplet states is essential to many applications, including triplet–triplet annihilation based photon upconversion schemes. This work demonstrates successful triplet sensitization of a CdSe quantum dot (QD)–bound oligothiophene carboxylic acid (T6). Transient absorption spectroscopy provides direct evidence of Dexter-type triplet energy transfer from the QD to the acceptor without populating the singlet excited state or charge transfer intermediates. Analysis of T6 concentration dependent triplet formation kinetics shows that the intrinsic triplet energy transfer rate in 1 : 1 QD–T6 complexes is 0.077 ns −1 and the apparent transfer rate and efficiency can be improved by increasing the acceptor binding strength. This work demonstrates a new class of triplet acceptor molecules for QD-based upconversion systems that are more stable and tunable than the extensively studied polyacenes.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1021/jacs.3c08897
