Publications

Coherent Erbium Spin Defects in Colloidal Nanocrystal Hosts

We demonstrate nearly a microsecond of spin coherence in Er³⁺ ions doped in cerium dioxide nanocrystal hosts, despite a large gyromagnetic ratio and nanometric proximity of the spin defect to the nanocrystal surface. The long spin coherence is enabled by reducing the dopant density below the instantaneous diffusion limit in a nuclear spin-free host material, reaching the limit of a single erbium spin defect per nanocrystal. We observe a large Orbach energy in a highly symmetric cubic site, further protecting the coherence in a qubit that would otherwise rapidly decohere. Spatially correlated electron spectroscopy measurements reveal the presence of Ce³⁺ at the nanocrystal surface, which likely acts as extraneous paramagnetic spin noise. Even with these factors, defect-embedded nanocrystal hosts show tremendous promise for quantum sensing and quantum communication applications, with multiple avenues, including core-shell fabrication, redox tuning of oxygen vacancies, and organic surfactant modification, available to further enhance their spin coherence and functionality in the future.

Size-Controlled Model Co Nanoparticle Catalysts for CO₂ Hydrogenation: Synthesis, Characterization, and Catalytic Reactions

Model cobalt catalysts for CO(2) hydrogenation were prepared using colloidal chemistry. The turnover frequency at 6 bar and at 200-300 °C increased with cobalt nanoparticle size from 3 to 10 nm. It was demonstrated that near monodisperse nanoparticles in the size range of 3-10 nm could be generated without using trioctylphosphine oxide, a capping ligand that we demonstrate results in phosphorus being present on the metal surface and poisoning catalyst activity in our application.

Highly Luminescent Nanocrystals From Removal of Impurity Atoms Residual From Ion‐Exchange Synthesis

Nonmonotonic Size Dependence in the Hole Mobility of Methoxide-Stabilized PbSe Quantum Dot Solids

We present a facile procedure to fabricate p-type PbSe-based quantum dot solids with mobilities as large as 0.3 cm(2) V(-1)s(-1). Upon partial ligand exchange of oleate-capped PbSe quantum dots with the methoxide ion, we observe a pronounced red shift in the excitonic transition in conjunction with a large increase in conductivity. We show that there is little correlation between these two phenomena and that the electronic coupling energy in PbSe quantum dot solids is much smaller than often assumed. However, we observe for the first time a nonmonotonic size dependence of the hole mobility, illustrating that coupling can nonetheless be dominant in determining the transport characteristics. We attribute these effects to a decrease in charging energy and interparticle spacing, leading to enhanced electronic coupling on one hand and enhanced dipole interactions on the other hand, which is held responsible for the majority of the red shift.

Continuous Distribution of Emission States from Single CdSe/ZnS Quantum Dots

The photoluminescence dynamics of colloidal CdSe/ZnS/streptavidin quantum dots were studied using time-resolved single-molecule spectroscopy. Statistical tests of the photon-counting data suggested that the simple "on/off" discrete state model is inconsistent with experimental results. Instead, a continuous emission state distribution model was found to be more appropriate. Autocorrelation analysis of lifetime and intensity fluctuations showed a nonlinear correlation between them. These results were consistent with the model that charged quantum dots were also emissive, and that time-dependent charge migration gave rise to the observed photoluminescence dynamics.

A Comparison of Pressure-Induced Structural Transformations in CdSe, InP, and Si Nanocrystals

Sorting Fluorescent Nanocrystals with DNA

Semiconductor nanocrystals with narrow and tunable fluorescence are covalently linked to oligonucleotides. These biocompounds retain the properties of both nanocrystals and DNA. Therefore, different sequences of DNA can be coded with nanocrystals and still preserve their ability to hybridize to their complements. We report the case where four different sequences of DNA are linked to four nanocrystal samples having different colors of emission in the range of 530-640 nm. When the DNA-nanocrystal conjugates are mixed together, it is possible to sort each type of nanoparticle by using hybridization on a defined micrometer-size surface containing the complementary oligonucleotide. Detection of sorting requires only a single excitation source and an epifluorescence microscope. The possibility of directing fluorescent nanocrystals toward specific biological targets and detecting them, combined with their superior photostability compared to organic dyes, opens the way to improved biolabeling experiments, such as gene mapping on a nanometer scale or multicolor microarray analysis.

High-Temperature Microfluidic Synthesis of CdSe Nanocrystals in Nanoliter Droplets

The high-temperature synthesis of CdSe nanocrystals in nanoliter-volume droplets flowing in a perfluorinated carrier fluid through a microfabricated reactor is presented. A flow-focusing nanojet structure with a step increase in channel height reproducibly generated octadecene droplets in Fomblin Y 06/6 perfluorinated polyether at capillary numbers up to 0.81 and with a droplet:carrier fluid viscosity ratio of 0.035. Cadmium and selenium precursors flowing in octadecene droplets through a high-temperature (240-300 degrees C) glass microreactor produced high-quality CdSe nanocrystals, as verified by optical spectroscopy and transmission electron microscopy. Isolating the reaction solution in droplets prevented particle deposition and hydrodynamic dispersion, allowing the reproducible synthesis of nanocrystals at three different temperatures and four different residence times in the span of 4 h. Our synthesis of a wide range of nanocrystals at high temperatures, high capillary numbers, and low viscosity ratio illustrates the general utility of droplet-based microfluidic reactors to encapsulate nanoliter volumes of organic or aqueous solutions and to precisely control chemical or biochemical reactions.

Electrochemical Methanation of Carbon Dioxide with Highly Dispersed Copper Nanocatalysts

Although the vast majority of fuels and hydrocarbon products are presently derived from petroleum, there is immense interest in the development of alternate routes for synthesizing these products by hydrogenating carbon oxygenates. Electrochemical methods of reducing carbon dioxide could serve as a method of storing electrical energy derived from intermittent sources like solar and wind if efficient catalysts with high hydrocarbon selectivity are developed. Although metals in the form of foils are increasingly well-characterized as electrocatalysts for carbon dioxide reduction, the activity and stability of their nanoscale counterparts remain poorly understood. We present an understanding of the electrochemical conditions and catalyst architectures that afford control over the selectivity of copper nanoparticles for electrochemical methanation. Highly dispersed copper nanoparticles supported on glassy carbon exhibit enhanced Faradaic efficiencies for methanation compared to copper foils. The improved hydrocarbon selectivity for the copper nanoparticles is due to an underlying difference in the mechanism by which electrochemical carbon dioxide reduction proceeds on the nanoparticle surface. Our understanding of highly dispersed copper nanoparticles for electrochemical methanation is a first step towards their incorporation into membrane-electrode assemblies in practical electrolyzers.

Quantifying Photochemical Propulsion in Light-Powered Janus Micromotors

Janus particles reveal transient propulsion forces with a median of 4.5-6.2 pN lasting on average 21-41 ms and reaching up to 20 pN under optical confinement. Simulations incorporating these transient forces reproduce the observed trajectories, confirming their role in driving active motion. Functionalization with long DNA polymers further enhances directional motion by reducing rotational diffusion. These results establish a single-particle framework for quantifying active forces in photocatalytic Janus particles and offer design principles for light-powered micromotors.

Temperature-Dependent Hole Transfer from Photoexcited Quantum Dots to Molecular Species: Evidence for Trap-Mediated Transfer

The effect of temperature on the rate of hole transfer from photoexcited quantum dots (QDs) is investigated by measuring the driving force dependence of the charge transfer rate for different sized QDs across a range of temperatures from 78 to 300 K. Spherical CdSe/CdS core/shell QDs were used with a series of ferrocene-derived molecular hole acceptors with an 800 meV range in electrochemical potential. Time-resolved photoluminescence measurements and photoluminescence quantum yield measurements in an integrating sphere were both performed from 78 to 300 K to obtain temperature-dependent rates for a series of driving forces as dictated by the nature of the molecular acceptor. For both QD sizes studied and all ligands, the Arrhenius plot of hole transfer exhibited an activated (linear) regime at higher temperatures and a temperature-independent regime at low temperatures. The extracted activation energies in the high-temperature regime were consistent across all ligands for a given QD size. This observation is not consistent with direct charge transfer from the QD valence band to the ferrocene acceptor. Instead, a model in which charge transfer is mediated by a shallow and reversible trap more accurately fits the experimental results. Implications for this observed trap-mediated transfer are discussed including as a strategy to more efficiently extract charge from QDs.

Microfabricated liquid chamber utilizing solvent-drying for in-situ TEM imaging of nanoparticle self-assembly

This work presents a microfabricated liquid-sample chamber for real-time TEM (Transmission Electron Microscopy) of nanoscale processes driven by liquid evaporation. Previous liquid-microchambers for TEM hold liquid samples in fully-closed (vacuum-tight) micro/nano-scale structures, thus evaporation-driven nanoscale phenomena couldn't be studied. The present work uses intended leakage/failure in bonding and e-beam induced heating in order to generate solvent-drying during TEM imaging, and the captured real-time nanometer-scale movies visualize critical steps in the self-assembly process of 2D nanoparticle arrays including the two-step crystallization process.

Redox Mediated Control of Electrochemical Potential in Liquid Cell Electron Microscopy

Liquid cell electron microscopy enables the study of nanoscale transformations in solvents with high spatial and temporal resolution, but for the technique to achieve its potential requires a new level of control over the reactivity caused by radical generation under electron beam irradiation. An understanding of how to control electron-solvent interactions is needed to further advance the study of structural dynamics for complex materials at the nanoscale. We developed an approach that scavenges radicals with redox species that form well-defined redox couples and control the electrochemical potential <i>in situ</i>. This approach enables the observation of electrochemical structural dynamics at near-atomic resolution with precise control of the liquid environment. Analysis of nanocrystal etching trajectories indicates that this approach can be generalized to several chemical systems. The ability to simultaneously observe heterogeneous reactions at near-atomic resolution and precisely control the electrochemical potential enables the fundamental study of complex nanoscale dynamics with unprecedented detail.

Nanocrystal Templating of Silica Mesopores with Tunable Pore Sizes

Metallic nanoparticles (platinum and gold) encapsulated in mesoporous silica (SBA-15) were prepared in the same solution by a novel two-step method. Characterization by X-ray scattering and electron microscopy consistently shows that the metal nanoparticles were homogeneously incorporated in the mesopores (retaining their size and morphology), even when the nanocrystal diameter exceeds the normal mesopore diameter. The nanoparticles nucleated the expansion of the mesopore channels in the 92−116 Å range so they could accommodate the metal particles. This expansion occurs in the concentration range of 1−103 nanoparticles per 103 mesopore channels. This effect can be used to tune the pore size.

Single-particle mapping of nonequilibrium nanocrystal transformations

Watching it all fall apart The control of the shape and size of metal nanoparticles can be very sensitive to the growth conditions of the particles. Ye et al. studied the reverse process: They tracked the dissolution of gold nanoparticles in a redox environment inside a liquid cell within an electron microscope, controlling the particle dissolution with the electron beam. Tracking short-lived particle shapes revealed structures of greater or lesser stability. The findings suggest kinetic routes to particle sizes and shapes that would otherwise be difficult to generate. Science , this issue p. 874

The Brain Activity Map

Researchers propose building technologies to enable comprehensive mapping of neural circuit activity to understand brain function and disease.

Transition from Isolated to Collective Modes in Plasmonic Oligomers

We demonstrate the transition from isolated to collective optical modes in plasmonic oligomers. Specifically, we investigate the resonant behavior of planar plasmonic hexamers and heptamers with gradually decreasing the interparticle gap separation. A pronounced Fano resonance is observed in the plasmonic heptamer for separations smaller than 60 nm. The spectral characteristics change drastically upon removal of the central nanoparticle. Our work paves the road toward complex hierarchical plasmonic oligmers with tailored optical properties.

Luminescent nanocrystal stress gauge

Microscale mechanical forces can determine important outcomes ranging from the site of material fracture to stem cell fate. However, local stresses in a vast majority of systems cannot be measured due to the limitations of current techniques. In this work, we present the design and implementation of the CdSe-CdS core-shell tetrapod nanocrystal, a local stress sensor with bright luminescence readout. We calibrate the tetrapod luminescence response to stress and use the luminescence signal to report the spatial distribution of local stresses in single polyester fibers under uniaxial strain. The bright stress-dependent emission of the tetrapod, its nanoscale size, and its colloidal nature provide a unique tool that may be incorporated into a variety of micromechanical systems including materials and biological samples to quantify local stresses with high spatial resolution.