Publications

The motion of nanoparticles near surfaces is of fundamental importance in physics, biology, and chemistry. Liquid cell transmission electron microscopy (LCTEM) is a promising technique for studying motion of nanoparticles with high spatial resolution. Yet, the lack of understanding of how the electron beam of the microscope affects the particle motion has held back advancement in using LCTEM for in situ single nanoparticle and macromolecule tracking at interfaces. Here, we experimentally studied the motion of a model system of gold nanoparticles dispersed in water and moving adjacent to the silicon nitride membrane of a commercial liquid cell in a broad range of electron beam dose rates. We find that the nanoparticles exhibit anomalous diffusive behavior modulated by the electron beam dose rate. We characterized the anomalous diffusion of nanoparticles in LCTEM using a convolutional deep neural network model and canonical statistical tests. The results demonstrate that the nanoparticle motion is governed by fractional Brownian motion at low dose rates, resembling diffusion in a viscoelastic medium, and continuous time random walk at high dose rates, resembling diffusion on an energy landscape with pinning sites. Both behaviors can be explained by the presence of silanol molecular species on the surface of the silicon nitride membrane and the ionic species in solution formed by radiolysis of water in presence of the electron beam.

The ability to predict and control the formation of bimetallic heterogeneous nanocrystals is desirable for many applications in plasmonics and catalysis. Here, we report the synthesis and characterization of stable, monodisperse, and solution-processed Cu-Ag bimetallic nanoparticles with specific but unusual elemental arrangements that are consistent with a recently developed thermodynamic model. Using air-free scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy, the distribution of Cu and Ag positions was unambiguously identified within individual nanocrystals (NCs), leading to the discovery of a Cu-Ag nanocrescent shape. A simple yet versatile thermodynamic model was applied to illustrate how the interplay between surface and interface energies determines the particle morphology. It is found that there exists a range of surface-to-interface energy ratios under which crescent-shaped nanocrystals are the thermodynamically favored products, with the morphology tunable by adjusting the Ag content. We further show the conversion of Cu-Ag nanocrescents into Ag@Cu₂O upon mild oxidation, whereas fully core-shell Cu@Ag NCs are robust against oxidation up to 100 °C. The plasmonic and interband absorptions of Cu-Ag NCs depend on the composition and the degree of Cu oxidation, which may find application in light-driven catalysis.

Significance Solar-powered chemical production from CO 2 promises to alleviate petrochemical consumption. Hybrid systems of an inorganic semiconductor light harvester and a microbial catalyst offer a viable way forward. Whereas a number of such systems have been described, the semiconductor-to-bacterium electron transfer mechanism remains largely unknown, limiting rational approaches to improving their performance. In this work, we look at how a semiconductor nanoparticle-sensitized bacterium transforms CO 2 and sunlight into acetic acid, a known precursor for fuels, food, pharmaceuticals, and polymers. Using time-resolved spectroscopy and biochemical analysis, we conclude that multiple pathways facilitate electron and light energy transfer from semiconductor to bacterium. This foundational study enables future investigation, understanding, and improvement of complex biotic–abiotic hybrid systems.

Spezifische, unregelmäßige Anordnungen von Gold-Nanokristallen mit Durchmessern von 5 und 10 nm lassen sich gezielt mit Hilfe doppelsträngiger DNA als Templat herstellen (siehe Bild; A′ und B′ bezeichnen die zu den Sequenzen A bzw. B komplementären Oligonucleotidsequenzen). Die hier beschriebenen Methoden sollten sich auf Nanokristalle aus den verschiedensten Materialien anwenden lassen.

We review recent advances in the development of colloidal fluorescent semiconductor nanocrystals (a class of quantum dots) for biological labeling. Although some of the photophysical properties of nanocrystals are not fully understood and are still actively investigated, researchers have begun developing bioconjugation schemes and applying such probes to biological assays. Nanocrystals possess several qualities that make them very attractive for fluorescent tagging: broad excitation spectrum, narrow emission spectrum, precise tunability of their emission peak, longer fluorescence lifetime than organic fluorophores and negligible photobleaching. On the down side, their emission is strongly intermittent (“blinking”) and their size is relatively large for many biological uses. We describe how to take advantage of nanocrystals' spectral properties to increase the resolution of fluorescence microscopy measurements down to the nanometer level. We also show how their long fluorescence lifetime can be used to observe molecules and organelles in living cells without interference from background autofluorescence, a pre-requisite for single molecule detectability. Finally, their availability in multicolor species and their single molecule sensitivity open up interesting possibilities for genomics applications.

The optical, electronic, and catalytic properties of nanocrystals (NCs) are often determined by their size, shape, and materials. Understanding the underlying mechanisms of shape-controlled synthesis and transformation is crucial for revealing fundamental reaction kinetics, enabling the design of more precisely controlled materials. Liquid cell transmission electron microscopy (LCTEM) enables the observation of individual NC growth and dissolution with millisecond time resolution and subnanometer space resolution. In this study, we harnessed the chemical environment to analyze the single-particle etching trajectories of well-faceted copper arsenide (Cu₃As) nanocubes. Distinct kinetically controlled dissolution trajectories are identified by adjusting the chemical reactions of Cu or As through pH, coordination, concentration, and oxidative species. These LCTEM observations illustrate how the nanoscale shape transformations in binary semiconductors can be controlled through manipulation of cationic and anionic reactivity within a highly reactive nonequilibrium radiolysis liquid environment.

Thiol-modified single stranded oligonucleotides of different lengths (8 to 135 bases) were attached to the surface of 10 nm diameter Au nanocrystals with different DNA/Au ratios (1,2, saturation). The electrophoretic mobility of these conjugates was determined on 2% agarose gels, and the effective diameter of the DNA/Au conjugates was determined. This diameter depends on the conformation of the oligonucleotides adsorbed on the Au surface. For low surface coverage, nonspecific wrapping of the DNA around the nanoparticles was observed. For high surface coverage, short oligonucleotides were shown to be oriented perpendicular to the surface and fully stretched. For high surface coverage and long oligonucleotides, the inner part close to the Au surface was determined to be fully stretched and pointed perpendicular to the surface, whereas the outer part adopts random coil shape.

Lead halide perovskite nanocrystals demonstrate efficient up-conversion, although the precise mechanism remains a subject of active research. This study utilizes steady-state and time-resolved spectroscopy methods to unravel the mechanism driving the up-conversion process in CsPbBr3 nanocrystals. Employing above- and below-gap photoluminescence measurements, we extract a distinct phonon mode with an energy of ~7 meV and identify the Pb-Br-Pb bending mode as the phonon involved in the up-conversion process. This result was corroborated by Raman spectroscopy. We confirm an up-conversion efficiency reaching up to 75%. Transient absorption measurements under conditions of sub-gap excitation also unexpectedly reveal coherent phonons for the subset of nanocrystals undergoing up-conversion. This coherence implies that the up-conversion and subsequent relaxation is accompanied by a synchronized and phased lattice motion. This study reveals that efficient up-conversion in CsPbBr3 nanocrystals is powered by a unique interplay between the soft lattice structure, phonons, and excited states dynamics.

We study the formation of bismuth oxide hollow nanoparticles by the Kirkendall effect using liquid cell transmission electron microscopy (TEM). Rich dynamics of bismuth diffusion through the bismuth oxide shell have been captured in situ. The diffusion coefficient of bismuth through bismuth oxide shell is 3-4 orders of magnitude higher than that of bulk. Observation reveals that defects, temperature, sizes of the particles, and so forth can affect the diffusion of reactive species and modify the kinetics of the hollowing process.

Luminescent solar concentrators doped with CdSe/CdS quantum dots provide a potentially low-cost and high-performance alternative to costly high-band-gap III–V semiconductor materials to serve as a top junction in multijunction photovoltaic devices for efficient utilization of blue photons. In this study, a photonic mirror was coupled with such a luminescent waveguide to form an optical cavity where emitted luminescence was trapped omnidirectionally. By mitigating escape cone and scattering losses, 82% of luminesced photons travel the length of the waveguide, creating a concentration ratio of 30.3 for blue photons in a waveguide with a geometric gain of 61. Further, we study the photon transport inside the luminescent waveguide, showing unimpeded photon collection across the entire length of the waveguide.