Spectroscopic Observation and Modeling of Photonic Modes in CeO2 Nanostructures.

Photonic modes in dielectric nanostructures, e.g., wide gap semiconductor like CeO2 (ceria), have the potential for various applications such as information transmission and sensing technology. To fully understand the properties of such phenomenon at the nanoscale, electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope was employed to detect and explore photonic modes in well-defined ceria nanocubes. To facilitate the interpretation of the observations, EELS simulations were performed with finite-element methods. The simulations allow the electric and magnetic field distributions associated with different modes to be determined. A simple analytical eigenfunction model was also used to estimate the energy of the photonic modes. In addition, by comparing various spectra taken at different location relative to the cube, the effect of the surrounding environment on the modes could be sensed. This work gives a high-resolution description of the photonic modes' properties in nanostructures, while demonstrating the advantage of EELS in characterizing optical phenomena locally.

Exploring Blob Detection to Determine Atomic Column Positions and Intensities in Time-Resolved TEM Images with Ultra-Low Signal-to-Noise.

Spatially resolved in situ transmission electron microscopy (TEM), equipped with direct electron detection systems, is a suitable technique to record information about the atom-scale dynamics with millisecond temporal resolution from materials. However, characterizing dynamics or fluxional behavior requires processing short time exposure images which usually have severely degraded signal-to-noise ratios. The poor signal-to-noise associated with high temporal resolution makes it challenging to determine the position and intensity of atomic columns in materials undergoing structural dynamics. To address this challenge, we propose a noise-robust, processing approach based on blob detection, which has been previously established for identifying objects in images in the community of computer vision. In particular, a blob detection algorithm has been tailored to deal with noisy TEM image series from nanoparticle systems. In the presence of high noise content, our blob detection approach is demonstrated to outperform the results of other algorithms, enabling the determination of atomic column position and its intensity with a higher degree of precision.

Role of Convergence and Collection Angles in the Excitation of Long- and Short-Wavelength Phonons with Vibrational Electron Energy-Loss Spectroscopy.

Current generation electron monochromators employed as attachments to scanning transmission electron microscopes (STEM) offer the ability to obtain vibrational information from materials using electron energy-loss spectroscopy (EELS). We show here that in crystals, long- and short-wavelength phonon modes can be probed simultaneously with on-axis vibrational STEM EELS. The long-wavelength phonons are probed via dipole scattering, while the short-wavelength modes are probed via impact scattering of the incident electrons. The localized character of the short-wavelength modes is demonstrated by scanning the electron beam across the edge of a hexagonal boron nitride nanoparticle. It is found that employing convergence angles that encompass multiple Brillouin zone boundaries enhances the short-wavelength phonon contribution to the vibrational energy-loss spectrum much more than that achieved by employing collection angles that encompass multiple Brillouin zone boundaries. Probing short-wavelength phonons at high spatial resolution with on-axis vibrational STEM EELS will help develop a fundamental connection between vibrational excitations and bonding arrangements at atomic-scale heterogeneities in materials.

Vibrational spectroscopy in the electron microscope.

Vibrational spectroscopies using infrared radiation, Raman scattering, neutrons, low-energy electrons and inelastic electron tunnelling are powerful techniques that can analyse bonding arrangements, identify chemical compounds and probe many other important properties of materials. The spatial resolution of these spectroscopies is typically one micrometre or more, although it can reach a few tens of nanometres or even a few ångströms when enhanced by the presence of a sharp metallic tip. If vibrational spectroscopy could be combined with the spatial resolution and flexibility of the transmission electron microscope, it would open up the study of vibrational modes in many different types of nanostructures. Unfortunately, the energy resolution of electron energy loss spectroscopy performed in the electron microscope has until now been too poor to allow such a combination. Recent developments that have improved the attainable energy resolution of electron energy loss spectroscopy in a scanning transmission electron microscope to around ten millielectronvolts now allow vibrational spectroscopy to be carried out in the electron microscope. Here we describe the innovations responsible for the progress, and present examples of applications in inorganic and organic materials, including the detection of hydrogen. We also demonstrate that the vibrational signal has both high- and low-spatial-resolution components, that the first component can be used to map vibrational features at nanometre-level resolution, and that the second component can be used for analysis carried out with the beam positioned just outside the sample--that is, for 'aloof' spectroscopy that largely avoids radiation damage.

Properties of Dipole-Mode Vibrational Energy Losses Recorded From a TEM Specimen.

The authors discuss the dipole vibrational modes that predominate in the energy-loss spectra of ionic materials below 1 eV, concentrating on thin-film specimens of typical transmission electron microscopy (TEM) thickness. The thickness dependence of the intensity is shown to be a useful guide to the bulk or surface character of vibrational peaks. The lateral and depth resolution of the energy-loss signal is investigated with the aid of finite-element calculations.

An Open-Cell Environmental Transmission Electron Microscopy Technique for In Situ Characterization of Samples in Aqueous Liquid Solutions.

The desire to image specimens in liquids has led to the development of open-cell and closed-cell techniques in transmission electron microscopy (TEM). The closed-cell approach is currently more common in TEM and has yielded new insights into a number of biological and materials processes in liquid environments. The open-cell approach, which requires an environmental TEM (ETEM), is technically challenging but may be advantageous in certain circumstances due to fewer restrictions on specimen and detector geometry. Here, we demonstrate a novel approach to open-cell liquid TEM, in which we use salt particles to facilitate the in situ formation of droplets of aqueous solution that envelope specimen particles coloaded with the salt. This is achieved by controlling sample temperature between 1 and 10°C and introducing water vapor to the ETEM chamber above the critical pressure for the formation of liquid water on the salt particles. Our use of in situ hydration enables specimens to be loaded into a microscope in a dry state using standard 3 mm TEM grids, allowing specimens to be prepared using trivial sample preparation techniques. Our future aim will be to combine this technique with an in situ light source to study photocorrosion in aqueous environments.

Approaches to Exploring Spatio-Temporal Surface Dynamics in Nanoparticles with In Situ Transmission Electron Microscopy.

Many nanoparticles in fields such as heterogeneous catalysis undergo surface structural fluctuations during chemical reactions, which may control functionality. These dynamic structural changes may be ideally investigated with time-resolved in situ electron microscopy. We have explored approaches for extracting quantitative information from large time-resolved image data sets with a low signal to noise recorded with a direct electron detector on an aberration-corrected transmission electron microscope. We focus on quantitatively characterizing beam-induced dynamic structural rearrangements taking place on the surface of CeO2 (ceria). A 2D Gaussian fitting procedure is employed to determine the position and occupancy of each atomic column in the nanoparticle with a temporal resolution of 2.5 ms and a spatial precision of 0.25 Å. Local rapid lattice expansions/contractions and atomic migration were revealed to occur on the (100) surface, whereas (111) surfaces were relatively stable throughout the experiment. The application of this methodology to other materials will provide new insights into the behavior of nanoparticle surface reconstructions that were previously inaccessible using other methods, which will have important consequences for the understanding of dynamic structure-property relationships.

Atomic-Scale Observations of Catalyst Structures under Reaction Conditions and during Catalysis.

Heterogeneous catalysis is a chemical process performed at a solid-gas or solid-liquid interface. Direct participation of catalyst atoms in this chemical process determines the significance of the surface structure of a catalyst in a fundamental understanding of such a chemical process at a molecular level. High-pressure scanning tunneling microscopy (HP-STM) and environmental transmission electron microscopy (ETEM) have been used to observe catalyst structure in the last few decades. In this review, instrumentation for the two in situ/operando techniques and scientific findings on catalyst structures under reaction conditions and during catalysis are discussed with the following objectives: (1) to present the fundamental aspects of in situ/operando studies of catalysts; (2) to interpret the observed restructurings of catalyst and evolution of catalyst structures; (3) to explore how HP-STM and ETEM can be synergistically used to reveal structural details under reaction conditions and during catalysis; and (4) to discuss the future challenges and prospects of atomic-scale observation of catalysts in understanding of heterogeneous catalysis. This Review focuses on the development of HP-STM and ETEM, the in situ/operando characterizations of catalyst structures with them, and the integration of the two structural analytical techniques for fundamentally understanding catalysis.

Advanced and In Situ Analytical Methods for Solar Fuel Materials.

In situ and operando techniques can play important roles in the development of better performing photoelectrodes, photocatalysts, and electrocatalysts by helping to elucidate crucial intermediates and mechanistic steps. The development of high throughput screening methods has also accelerated the evaluation of relevant photoelectrochemical and electrochemical properties for new solar fuel materials. In this chapter, several in situ and high throughput characterization tools are discussed in detail along with their impact on our understanding of solar fuel materials.

Analysis of catalytic gas products using electron energy-loss spectroscopy and residual gas analysis for operando transmission electron microscopy.

Operando transmission electron microscopy (TEM) of catalytic reactions requires that the gas composition inside the TEM be known during the in situ reaction. Two techniques for measuring gas composition inside the environmental TEM are described and compared here. First, electron energy-loss spectroscopy, both in the low-loss and core-loss regions of the spectrum was utilized. The data were quantified using a linear combination of reference spectra from individual gasses to fit a mixture spectrum. Mass spectrometry using a residual gas analyzer was also used to quantify the gas inside the environmental cell. Both electron energy-loss spectroscopy and residual gas analysis were applied simultaneously to a known 50/50 mixture of CO and CO2, so the results from the two techniques could be compared and evaluated. An operando TEM experiment was performed using a Ru catalyst supported on silica spheres and loaded into the TEM on a specially developed porous pellet TEM sample. Both techniques were used to monitor the conversion of CO to CO2 over the catalyst, while simultaneous atomic resolution imaging of the catalyst was performed.

Derivation of optical properties of carbonaceous aerosols by monochromated electron energy-loss spectroscopy.

Monochromated electron energy-loss spectroscopy (EELS) is employed to determine the optical properties of carbonaceous aerosols from the infrared to the ultraviolet region of the spectrum. It is essential to determine their optical properties to understand their accurate contribution to radiative forcing for climate change. The influence of surface and interface plasmon effects on the accuracy of dielectric data determined from EELS is discussed. Our measurements show that the standard thin film formulation of Kramers-Kronig analysis can be employed to make accurate determination of the dielectric function for carbonaceous particles down to about 40 nm in size. The complex refractive indices of graphitic and amorphous carbon spherules found in the atmosphere were determined over the wavelength range 200-1,200 nm. The graphitic carbon was strongly absorbing black carbon, whereas the amorphous carbon shows a more weakly absorbing brown carbon profile. The EELS approach provides an important tool for exploring the variation in optical properties of atmospheric carbon.