Entangled Nanoparticles: Discovery by Visualization in 4D Electron Microscopy

Particle interactions are fundamental to our understanding of nanomaterials and biological assemblies. Here, we report on the visualization of entangled particles, separated by as large as 70 nm, and the discovery of channels in their near-fields. For silver nanoparticles, the induced field of each particle extends to 50–100 nm, but when particles are brought close in separation we observe channels as narrow as 6 nm, a width that is 2 orders of magnitude smaller than the incident field wavelength. The channels’ directions can be controlled by the polarization of the incident field, particle size, and separation. For this direct visualization of these nanoscopic near-fields, the high spatial, temporal, and energy resolutions needed were hitherto not possible without the methodology given here. This methodology, we anticipate, paves the way for further fundamental studies of particle entanglement and for possible applications spanning materials and macromolecular assemblies

Graphene-layered steps and their fields visualized by 4D electron microscopy

Enhanced image contrast has been seen at graphene-layered steps a few nanometers in height by means of photon-induced near-field electron microscopy (PINEM) using synchronous femtosecond pulses of light and electrons. The observed steps are formed by the edges of graphene strips lying on the surface of a graphene substrate, where the strips are hundreds of nanometers in width and many micrometers in length. PINEM measurements reflect the interaction of imaging electrons and induced (near) electric fields at the steps, and this leads to a much higher contrast than that achieved in bright-field transmission electron microscopy imaging of the same strips. Theory and numerical simulations support the experimental PINEM findings and elucidate the nature of the electric field at the steps formed by the graphene layers. These results extend the range of applications of the experimental PINEM methodology, which has previously been demonstrated for spherical, cylindrical, and triangular nanostructures, to shapes of high aspect ratio (rectangular strips), as well as into the regime of atomic layer thicknesses

Constructing quantum dots sensitized TiO2 nanotube p-n heterojunction for photoelectrochemical hydrogen generation

Photoelectrochemical (PEC) water splitting is a promising approach to convert solar radiation into hydrogen (H2) as a clean fuel. The PEC device performance depends on the light harvesting efficiency of the photoanode and the carrier dynamics (i.e. separation/transport rate) at the photoanode/electrolyte interface. Herein, we report a photoanode architecture consisting of self-organized TiO2 nanotubes (NTs) sensitized by CdS/CdSe quantum dots (QDs) and treated with a Cu-based solution to create a p-n heterojunction. Our results demonstrate that the TiO2 NTs/QDs PEC device yields a photocurrent density of 4.18 mA.cm−2 at 0.5 V vs RHE, which is 51 times higher than the device based on TiO2 NTs only (i.e., 0.08 mA.cm−2) and 7 times compared to TiO2/QDs nanoparticles (NPs) (i.e. 0.45 mA.cm−2) under one sun illumination. The p-type CuSe coating over the TiO2/QDs NTs photoanodes forms a p-n heterojunction that improves the carrier dynamics. The resulting PEC device shows a 13% improvement in the photocurrent density. In addition, employing longer TiO2 NTs improves the device's stability. Our results offer a simple and scalable method for the design and optimization of the photoanodes to enhance the performance of PEC and other optoelectronic devices

(Invited) Nanohybrids for Manipulating Solar Energy

With unique physical and chemical properties, and high potential for many important applications, nanomaterials have attracted extensive attention in the past two decades. For instance, due to their unique, size- and shape-tunable surface plasmon resonance, plasmonic nanostructures have recently been explored for increasing the efficiency of solar cells and photocatalysis. Essentially they enhance and/or broaden solar photon harvesting via improved light scattering, strong near field effect and/or hot electron injection. Combination of different nanomaterials into a single architecture leads to greatly enhanced properties, or even better, promising, multifunctional nanomaterials. In this talk, I will present our recent work on the synthesis of hybrid nanomaterials (the assemblies of different types of nanoscale materials) as well as their applications in smart windows, solar cells, solar fuel, photocatalysis, etc. [1-8]. Rational design of hybrid nanomaterials, which is the key to maximize the benefits from respective nano-components, is highlighted. References: 1. Am. Chem. Soc., 2013, 135, 9616; 2. Adv. Energy Mater. 2018, 1703658; 3. Adv. Funct. Mater. 2018, 1706235; 4. ACS Catalysis, 2017, 7, 6225; 5. Adv. Funct. Mater, 2015, 25, 2950; 6. Adv. Funct. Mater. 2015, 25, 6650; 7. Adv. Funct. Mater., 2012, 22, 3914; 8. Adv. Mater., 2012, 24, 6289. </jats:p

4D Nanoscale Diffraction Observed by Convergent-Beam Ultrafast Electron Microscopy

Converging on Dynamics Electron diffraction is a versatile technique for discerning atomic-level structure, but the data emerge averaged over the micron–scale area sampled by the electrons, and so blur local distinctions in systems that aren't strictly periodic. A recent approach to minimizing this problem has been to focus the electron beam impinging on the sample. Yurtsever and Zewail (p. 708 ) have now applied convergent focusing to an ultrafast electron diffraction apparatus and were thus able to resolve picosecond structural dynamics in local regions tens of nanometers across. The technique was used to probe heterogeneous temperature changes in laser-heated silicon. </jats:p

Direct Visualization of Near-Fields in Nanoplasmonics and Nanophotonics

Electric fields of nanoscale particles are fundamental to our understanding of nanoplasmonics and nanophotonics. Success has been made in developing methods to probe the effect of their presence, but it remains difficult to directly image optically induced electric fields at the nanoscale and especially when ensembles of particles are involved. Here, using ultrafast electron microscopy, we report the space-time visualization of photon-induced electric fields for ensembles of silver nanoparticles having different sizes, shapes, and separations. The high-field-of-view measurements enable parallel processing of many particles in the ensemble with high throughput of information. Directly in the image, the evanescent fields are observed and visualized when the particles are polarized with the optical excitation. Because the particle size is smaller than the wavelength of light, the near-fields are those of nanoplasmonics and are precursors of far-field nanophotonics. The reported results pave the way for quantitative studies of fields in ensembles of complex morphologies with the nanoparticles being embedded or interfacial

4D Nanoscale Diffraction Observed by Convergent-Beam Ultrafast Electron Microscopy

Diffraction with focused electron probes is among the most powerful tools for the study of time-averaged nanoscale structures in condensed matter. Here, we report four-dimensional (4D) nanoscale diffraction, probing specific site dynamics with 10 orders of magnitude improvement in time resolution, in convergent-beam ultrafast electron microscopy (CB-UEM). As an application, we measured the change of diffraction intensities in laser-heated crystalline silicon as a function of time and fluence. The structural dynamics (change in 7.3 ± 3.5 picoseconds), the temperatures (up to 366 kelvin), and the amplitudes of atomic vibrations (up to 0.084 angstroms) are determined for atoms strictly localized within the confined probe area (10 to 300 nanometers in diameter). We anticipate a broad range of applications for CB-UEM and its variants, especially in the studies of single particles and heterogeneous structures