Colloidal nanocrystals of metals, semiconductors, and other functional materials can self-assemble into long-range ordered crystalline and quasicrystalline phases, but insulating organic surface ...ligands prevent the development of collective electronic states in ordered nanocrystal assemblies. We reversibly self-assembled colloidal nanocrystals of gold, platinum, nickel, lead sulfide, and lead selenide with conductive inorganic ligands into supercrystals exhibiting optical and electronic properties consistent with strong electronic coupling between the constituent nanocrystals. The phase behavior of charge-stabilized nanocrystals can be rationalized and navigated with phase diagrams computed for particles interacting through short-range attractive potentials. By finely tuning interparticle interactions, the assembly was directed either through one-step nucleation or nonclassical two-step nucleation pathways. In the latter case, the nucleation was preceded by the formation of two metastable colloidal fluids.
Improving charge mobility in quantum dot (QD) films is important for the performance of photodetectors, solar cells and light-emitting diodes. However, these applications also require preservation of ...well defined QD electronic states and optical transitions. Here, we present HgTe QD films that show high mobility for charges transported through discrete QD states. A hybrid surface passivation process efficiently eliminates surface states, provides tunable air-stable n and p doping and enables hysteresis-free filling of QD states evidenced by strong conductance modulation. QD films dried at room temperature without any post-treatments exhibit mobility up to μ ~ 8 cm
V
s
at a low carrier density of less than one electron per QD, band-like behaviour down to 77 K, and similar drift and Hall mobilities at all temperatures. This unprecedented set of electronic properties raises important questions about the delocalization and hopping mechanisms for transport in QD solids, and introduces opportunities for improving QD technologies.
The ensemble emission spectra of colloidal InP quantum dots are broader than achievable spectra of cadmium- and lead-based quantum dots, despite similar single-particle line widths and significant ...efforts invested in the improvement of synthetic protocols. We seek to explain the origin of persistently broad ensemble emission spectra of colloidal InP quantum dots by investigating the nature of the electronic states responsible for luminescence. We identify a correlation between red-shifted emission spectra and anomalous broadening of the excitation spectra of luminescent InP colloids, suggesting a trap-associated emission pathway in highly emissive core–shell quantum dots. Time-resolved pump–probe experiments find that electrons are largely untrapped on photoluminescence relevant time scales pointing to emission from recombination of localized holes with free electrons. Two-dimensional electronic spectroscopy on InP quantum dots reveals multiple emissive states and increased electron–phonon coupling associated with hole localization. These localized hole states near the valence band edge are hypothesized to arise from incomplete surface passivation and structural disorder associated with lattice defects. We confirm the presence and effect of lattice disorder by X-ray absorption spectroscopy and Raman scattering measurements. Participation of localized electronic states that are associated with various classes of lattice defects gives rise to phonon-coupled defect related emission. These findings explain the origins of the persistently broad emission spectra of colloidal InP quantum dots and suggest future strategies to narrow ensemble emission lines comparable to what is observed for cadmium-based materials.
Leveraging triplet excitons in semiconductor quantum dots (QDs) in concert with surface-anchored molecules to produce long-lifetime thermally activated delayed photoluminescence (TADPL) continues to ...emerge as a promising technology in diverse areas including photochemical catalysis and light generation. All QDs presently used to generate TADPL in QD/molecule constructs contain toxic metals including Cd(II) and Pb(II), ultimately limiting potential real-world applications. Here, we report newly conceived blue-emitting TADPL-producing nanomaterials featuring InP QDs interfaced with 1- and 2-naphthoic acid (1-NA and 2-NA) ligands. These constitutional isomers feature similar triplet energies but disparate triplet lifetimes, translating into InP-based TADPL processes displaying two distinct average lifetime ranges upon cooling from 293 to 193 K. The time constants fall between 4.4 and 59.2 μs in the 2-NA-decorated InP QDs while further expanding between 84.2 and 733.2 μs in the corresponding 1-NA-ligated InP materials, representing a 167-fold time window. The resulting long-lived excited states enabled facile bimolecular triplet sensitization of 1O2 phosphorescence in the near-IR and promoted sensitized triplet–triplet annihilation photochemistry in 2,5-diphenyloxazole. We speculate that the discovery of new nanomaterials exhibiting TADPL lies on the horizon as myriad QDs can be readily derivatized using isomers of numerous classes of surface-anchoring chromophores yielding precisely regulated photophysical properties.
HgTe colloidal quantum dots (QDs) are of interest because quantum confinement of semimetallic bulk HgTe allows one to synthetically control the bandgap throughout the infrared. Here, we synthesize ...highly monodisperse HgTe QDs and tune their doping both chemically and electrochemically. The monodispersity of the QDs was evaluated using small-angle X-ray scattering (SAXS) and suggests a diameter distribution of ∼10% across multiple batches of different sizes. Electron-doped HgTe QDs display an intraband absorbance and bleaching of the first two excitonic features. We see splitting of the intraband peaks corresponding to electronic transitions from the occupied 1Se state to a series of nondegenerate 1Pe states. Spectroelectrochemical studies reveal that the degree of splitting and relative intensity of the intraband features remain constant across doping levels up to two electrons per QD. Theoretical modeling suggests that the splitting of the 1Pe level arises from spin–orbit coupling and reduced QD symmetry. The fine structure of the intraband transitions is observed in the ensemble studies due to the size uniformity of the as-synthesized QDs and strong spin–orbit coupling inherent to HgTe.
Improved mid-infrared photoconductors based on colloidal HgTe quantum dots are realized using a hybrid ligand exchange and polar phase transfer. The doping can also be controlled n and p by adjusting ...the HgCl2 concentration in the ligand exchange process. We compare the photoconductive properties with the prior “solid-state ligand exchange” using ethanedithiol, and we find that the new process affords a ∼100-fold increase of the electron and hole mobility, a ∼100-fold increase in responsivity, and a ∼10-fold increase in detectivity. These photodetector improvements are primarily attributed to the increase in mobility (μ) because the optical properties are mostly unchanged. We show that the specific detectivity (D*) of a photoconductive device is expected to scale as μ . The application potential is further verified by long-term device stability.
The goal of this work is to determine the kinetic factors that govern isovalent cation exchange in III–V colloidal quantum dots using molten salts as the solvent and cation source. We focus on the ...reactions of InP + GaI3→ In1–x Ga x P and InAs + GaI3→ In1–x Ga x As to create technologically important ternary III–V phases. We find that the molten salt reaction medium causes the transformation of nearly spherical InP nanocrystals to tetrahedron-shaped In1–x Ga x P nanocrystals. Furthermore, we determine that the activation energy for the cation exchange reaction is 0.9 eV for incorporation of Ga into InP and 1.2 eV for incorporation of Ga into InAs, both much lower than the measured values in bulk semiconductors. Next, we use powder XRD simulations to constrain our understanding of the structure of the In1–x Ga x P nanocrystals. Together our results reveal several important features of molten salt-mediated cation exchange and provide guidance for future development of these materials.
Semiconductors are commonly divided into materials with
direct
or indirect band gaps based on the relative positions of the top of
the valence band and the bottom of the conduction band in crystal
...momentum (
k
) space. It has, however, been debated if
k
is a useful quantum number to describe the band structure
in quantum-confined nanocrystalline systems, which blur the distinction
between direct and indirect gap semiconductors. In bulk III–V
semiconductor alloys like In
1–
x
Ga
x
P, the band structure can be tuned
continuously from the direct- to indirect-gap by changing the value
of
x
. The effect of strong quantum confinement on
the direct-to-indirect transition in this system has yet to be established
because high-quality colloidal nanocrystal samples have remained inaccessible.
Herein, we report one of the first systematic studies of ternary III–V
nanocrystals by utilizing an optimized molten-salt In-to-Ga cation
exchange protocol to yield bright In
1–
x
Ga
x
P/ZnS core–shell particles
with photoluminescence quantum yields exceeding 80%. We performed
two-dimensional solid-state NMR studies to assess the alloy homogeneity
and the extent of surface oxidation in In
1–
x
Ga
x
P cores. The radiative decay
lifetime for In
1–
x
Ga
x
P/ZnS monotonically increases with higher gallium
content. Transient absorption studies on In
1–
x
Ga
x
P/ZnS nanocrystals demonstrate
signatures of direct- and indirect-like behavior based on the presence
or absence, respectively, of excitonic bleach features. Atomistic
electronic structure calculations based on the semi-empirical pseudopotential
model are used to calculate absorption spectra and radiative lifetimes
and evaluate band-edge degeneracy; the resulting calculated electronic
properties are consistent with experimental observations. By studying
photoluminescence characteristics at elevated temperatures, we demonstrate
that a reduced lattice mismatch at the III–V/II–VI core–shell
interface can enhance the thermal stability of emission. These insights
establish cation exchange in molten inorganic salts as a viable synthetic
route to nontoxic, high-quality In
1–
x
Ga
x
P/ZnS QD emitters with desirable
optoelectronic properties.
Semiconductors are commonly divided into materials with direct or indirect band gaps based on the relative positions of the top of the valence band and the bottom of the conduction band in crystal ...momentum (
) space. It has, however, been debated if
is a useful quantum number to describe the band structure in quantum-confined nanocrystalline systems, which blur the distinction between direct and indirect gap semiconductors. In bulk III-V semiconductor alloys like In
Ga
P, the band structure can be tuned continuously from the direct- to indirect-gap by changing the value of
. The effect of strong quantum confinement on the direct-to-indirect transition in this system has yet to be established because high-quality colloidal nanocrystal samples have remained inaccessible. Herein, we report one of the first systematic studies of ternary III-V nanocrystals by utilizing an optimized molten-salt In-to-Ga cation exchange protocol to yield bright In
Ga
P/ZnS core-shell particles with photoluminescence quantum yields exceeding 80%. We performed two-dimensional solid-state NMR studies to assess the alloy homogeneity and the extent of surface oxidation in In
Ga
P cores. The radiative decay lifetime for In
Ga
P/ZnS monotonically increases with higher gallium content. Transient absorption studies on In
Ga
P/ZnS nanocrystals demonstrate signatures of direct- and indirect-like behavior based on the presence or absence, respectively, of excitonic bleach features. Atomistic electronic structure calculations based on the semi-empirical pseudopotential model are used to calculate absorption spectra and radiative lifetimes and evaluate band-edge degeneracy; the resulting calculated electronic properties are consistent with experimental observations. By studying photoluminescence characteristics at elevated temperatures, we demonstrate that a reduced lattice mismatch at the III-V/II-VI core-shell interface can enhance the thermal stability of emission. These insights establish cation exchange in molten inorganic salts as a viable synthetic route to nontoxic, high-quality In
Ga
P/ZnS QD emitters with desirable optoelectronic properties.
Autism spectrum disorder (ASD).
2020.
The Autism and Developmental Disabilities Monitoring (ADDM) Network is an active surveillance program that provides estimates of the prevalence of ASD among ...children aged 8 years. In 2020, there were 11 ADDM Network sites across the United States (Arizona, Arkansas, California, Georgia, Maryland, Minnesota, Missouri, New Jersey, Tennessee, Utah, and Wisconsin). To ascertain ASD among children aged 8 years, ADDM Network staff review and abstract developmental evaluations and records from community medical and educational service providers. A child met the case definition if their record documented 1) an ASD diagnostic statement in an evaluation, 2) a classification of ASD in special education, or 3) an ASD International Classification of Diseases (ICD) code.
For 2020, across all 11 ADDM sites, ASD prevalence per 1,000 children aged 8 years ranged from 23.1 in Maryland to 44.9 in California. The overall ASD prevalence was 27.6 per 1,000 (one in 36) children aged 8 years and was 3.8 times as prevalent among boys as among girls (43.0 versus 11.4). Overall, ASD prevalence was lower among non-Hispanic White children (24.3) and children of two or more races (22.9) than among non-Hispanic Black or African American (Black), Hispanic, and non-Hispanic Asian or Pacific Islander (A/PI) children (29.3, 31.6, and 33.4 respectively). ASD prevalence among non-Hispanic American Indian or Alaska Native (AI/AN) children (26.5) was similar to that of other racial and ethnic groups. ASD prevalence was associated with lower household income at three sites, with no association at the other sites.Across sites, the ASD prevalence per 1,000 children aged 8 years based exclusively on documented ASD diagnostic statements was 20.6 (range = 17.1 in Wisconsin to 35.4 in California). Of the 6,245 children who met the ASD case definition, 74.7% had a documented diagnostic statement of ASD, 65.2% had a documented ASD special education classification, 71.6% had a documented ASD ICD code, and 37.4% had all three types of ASD indicators. The median age of earliest known ASD diagnosis was 49 months and ranged from 36 months in California to 59 months in Minnesota.Among the 4,165 (66.7%) children with ASD with information on cognitive ability, 37.9% were classified as having an intellectual disability. Intellectual disability was present among 50.8% of Black, 41.5% of A/PI, 37.8% of two or more races, 34.9% of Hispanic, 34.8% of AI/AN, and 31.8% of White children with ASD. Overall, children with intellectual disability had earlier median ages of ASD diagnosis (43 months) than those without intellectual disability (53 months).
For 2020, one in 36 children aged 8 years (approximately 4% of boys and 1% of girls) was estimated to have ASD. These estimates are higher than previous ADDM Network estimates during 2000-2018. For the first time among children aged 8 years, the prevalence of ASD was lower among White children than among other racial and ethnic groups, reversing the direction of racial and ethnic differences in ASD prevalence observed in the past. Black children with ASD were still more likely than White children with ASD to have a co-occurring intellectual disability.
The continued increase among children identified with ASD, particularly among non-White children and girls, highlights the need for enhanced infrastructure to provide equitable diagnostic, treatment, and support services for all children with ASD. Similar to previous reporting periods, findings varied considerably across network sites, indicating the need for additional research to understand the nature of such differences and potentially apply successful identification strategies across states.
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