Deep ultra-violet plasmonics: exploiting momentum-resolved electron energy loss spectroscopy to probe germanium.

Germanium is typically used for solid-state electronics, fiber-optics, and infrared applications, due to its semiconducting behavior at optical and infrared wavelengths. In contrast, here we show that the germanium displays metallic nature and supports propagating surface plasmons in the deep ultraviolet (DUV) wavelengths, that is typically not possible to achieve with conventional plasmonic metals such as gold, silver, and aluminum. We measure the photonic band spectrum and distinguish the plasmonic excitation modes: bulk plasmons, surface plasmons, and Cherenkov radiation using a momentum-resolved electron energy loss spectroscopy. The observed spectrum is validated through the macroscopic electrodynamic electron energy loss theory and first-principles density functional theory calculations. In the DUV regime, intraband transitions of valence electrons dominate over the interband transitions, resulting in the observed highly dispersive surface plasmons. We further employ these surface plasmons in germanium to design a DUV radiation source based on the Smith-Purcell effect. Our work opens a new frontier of DUV plasmonics to enable the development of DUV devices such as metasurfaces, detectors, and light sources based on plasmonic germanium thin films.

Chromosome inner structure investigation by electron tomography and electron diffraction in a transmission electron microscope.

Our understanding of the inner structure of metaphase chromosomes remains inconclusive despite intensive studies using multiple imaging techniques. Transmission electron microscopy has been extensively used to visualize chromosome ultrastructure. This review summarizes recent results obtained using two transmission electron microscopy-based techniques: electron tomography and electron diffraction. Electron tomography allows advanced three-dimensional imaging of chromosomes, while electron diffraction detects the presence of periodic structures within chromosomes. The combination of these two techniques provides results contributing to the understanding of local structural organization of chromatin fibers within chromosomes.

High-Energy Electron Scattering in Thick Samples Evaluated by Bright-Field Transmission Electron Microscopy, Energy-Filtering Transmission Electron Microscopy, and Electron Tomography.

Energy-filtering transmission electron microscopy (TEM) and bright-field TEM can be used to extract local sample thickness $t$ and to generate two-dimensional sample thickness maps. Electron tomography can be used to accurately verify the local $t$. The relations of log-ratio of zero-loss filtered energy-filtering TEM beam intensity ($I_{{\rm ZLP}}$) and unfiltered beam intensity ($I_{\rm u}$) versus sample thickness $t$ were measured for five values of collection angle in a microscope equipped with an energy filter. Furthermore, log-ratio of the incident (primary) beam intensity ($I_{\rm p}$) and the transmitted beam $I_{{\rm tr}}$ versus $t$ in bright-field TEM was measured utilizing a camera before the energy filter. The measurements were performed on a multilayer sample containing eight materials and thickness $t$ up to 800 nm. Local thickness $t$ was verified by electron tomography. The following results are reported:• The maximum thickness $t_{{\rm max}}$ yielding a linear relation of log-ratio, $\ln ( {I_{\rm u}}/{I_{{\rm ZLP}}})$ and $\ln ( {I_{\rm p}}/{I_{{\rm tr}}} )$, versus $t$.• Inelastic mean free path ($\lambda _{{\rm in}}$) for five values of collection angle.• Total mean free path ($\lambda _{{\rm total}}$) of electrons excluded by an angle-limiting aperture.• $\lambda _{{\rm in}}$ and $\lambda _{{\rm total}}$ are evaluated for the eight materials with atomic number from $\approx$10 to 79.The results can be utilized as a guide for upper limit of $t$ evaluation in energy-filtering TEM and bright-field TEM and for optimizing electron tomography experiments.

Higher-Order Structure of Human Chromosomes Observed by Electron Diffraction and Electron Tomography.

It is well known that two DNA molecules are wrapped around histone octamers and folded together to form a single chromosome. However, the nucleosome fiber folding within a chromosome remains an enigma, and the higher-order structure of chromosomes also is not understood. In this study, we employed electron diffraction which provides a noninvasive analysis to characterize the internal structure of chromosomes. The results revealed the presence of structures with 100­200 nm periodic features directionally perpendicular to the chromosome axis in unlabeled isolated human chromosomes. We also visualized the 100­200 nm periodic features perpendicular to the chromosome axis in an isolated chromosome whose DNA molecules were specifically labeled with OsO4 using electron tomography in 300 keV and 1 MeV transmission electron microscopes.

Fast electrons interacting with a natural hyperbolic medium: bismuth telluride.

Fast electrons interacting with matter have been instrumental for probing bulk and surface photonic excitations including Cherenkov radiation and plasmons. Additionally, fast electrons are ideal to investigate unique bulk and longitudinal photonic modes in hyperbolic materials at large wavevectors difficult to probe optically. Here, we use momentum-resolved electron energy loss spectroscopy (k-EELS) to perform the first experimental demonstration of high-k modes and hyperbolic Cherenkov radiation in the natural hyperbolic material Bi2Te3. This work establishes Bi2Te3 as one of the few viable natural hyperbolic materials in the visible and paves the way for k-EELS as a fundamental tool to probe hyperbolic media.

Sample thickness affects contrast and measured shape in TEM images and in electron tomograms.

We investigated the effect of nanoparticle (NP) image broadening and its contrast change dependence on a support matrix thickness in a transmission electron microscope (TEM). We measured the effect of NP size and atomic number on its image broadening. Based on the experimental TEM images we generated tomograms of NPs on four types of support matrix. The measured shape aspect ratio of the NPs in such tomograms depends on the geometry of the support matrix. For example, the aspect ratio of 6 nm NP placed on a thin film with window-frame support is 1.14, while the aspect ratio of 6 nm NP on a rod-shaped support with 910 nm diameter is 1.67 in a tomogram.

Origin of spurious intensity in vacuum near sample edge in bright field TEM images.

Bright-field transmission electron microscope (BFTEM) images exhibit spurious image intensity in the vacuum near the sample edge. The spurious intensity gradually decreases with increasing distance from the sample edge. By taking into account angular and energy loss distribution of the scattered electrons and lens aberrations, the origin of the spurious intensity of BFTEM images can be explained. The spurious intensity extent and magnitude can be significantly reduced by using either electron energy filtering or a small collection semiangle.

Higher-order structure of barley chromosomes observed by electron tomography.

The higher order structure of the metaphase chromosome has been an enigma for over a century and several different models have been presented based on results obtained by a variety of techniques. Some disagreements in the results between methods have possibly arisen from artifacts caused during sample preparation such as staining and dehydration. Therefore, we treated barley chromosomes with ionic liquid to minimize the effects of dehydration. We also observed chromosomes on a film with holes to keep pristine chromosome structure from being flattened as seen when placed on a continuous support film. A chromosome placed over a hole in a thin carbon film was mounted on a tomography holder, and its structure was observed in three dimensions (3D) using electron tomography. We found that there are periodic structures with 300-400 nm pitch along the axis in barley chromosomes. The pitch sizes are larger than those observed in human chromosomes.

NanoMi: An open source electron microscope hardware and software platform.

We outline a public license (open source) electron microscopy platform, referred to as NanoMi. NanoMi offers a modular, flexible electron microscope platform that can be utilized for a variety of applications, such as microscopy education and development of proof-of-principle experiments, and can be used to complement an existing experimental apparatus. All components are ultra-high vacuum compatible and the electron optics elements are independent from the vacuum envelope. The individual optical components are mounted on a 127 mm (5-inch) diameter half-pipe, allowing customizing of electron optics for a variety of purposes. The target capabilities include SEM, TEM, scanning TEM (STEM), and electron diffraction (ED) at up to 50 keV incident electron energy. The intended image resolution in SEM, TEM and STEM modes is ≈ 10 nm. We describe the existing components and the interfaces among components that ensure their compatibility and interchangeability. The paper provides a resource for those who consider building or utilizing their own NanoMi.

Phase plates in the transmission electron microscope: operating principles and applications.

In this paper, we review the current state of phase plate imaging in a transmission electron microscope. We focus especially on the hole-free phase plate design, also referred to as the Volta phase plate. We discuss the implementation, operating principles and applications of phase plate imaging. We provide an imaging theory that accounts for inelastic scattering in both the sample and in the hole-free phase plate.

Nanoparticle size and 3D shape measurement by electron tomography: An Inter-Laboratory Comparison.

Electron tomography (ET) has been used for quantitative measurement of shape and size of objects in three dimensions (3D) for many years. However, systematic investigation of repeatability and reproducibility of ET has not been evaluated in detail. To assess the reproducibility and repeatability of a protocol for measuring size and three-dimensional (3D) shape parameters for nanoparticles (NPs) by ET, an inter-laboratory comparison (ILC) has been performed. The ILC included six laboratories and six instruments models from three instrument manufacturers following a standard measurement protocol. A technical specification describing the normative steps of the protocol is published by the International Standards Organization (ISO). Gold NPs with 30 nm nominal diameter contained within a rod-shaped carbon support were measured. The use of a rod-shaped sample support eliminated the missing wedge effect in the experimental tilt series of projected images for improved quantification. A total of 443 NPs were initially measured by NRC-NANO and then 115 out of the 443 NPs were measured by five other labs to compare measurands such as the Volume (V), maximum Feret diameter (Fmax), minimum Feret diameter (Fmin), volume-equivalent diameter (Deq) and aspect ratio (Frat) of the NPs. The results of the five labs were compared with the results obtained at NRC-NANO. The maximum disagreement in measurements of Fmin and Fmax obtained by the participating labs did not exceed 7 %. The measured Deq was between 27.5 nm and 30.3 nm in agreement with the NP manufacturer's specification (28 nm-32 nm). In addition to the above, the influence of the missing wedge effect and beam-induced NP movement was quantified based on the differences of the results between labs.

Water-soluble J-type rosette nanotubes with giant molar ellipticity.

A new self-assembling tricyclic module (×K1) featuring the Watson-Crick H-bonding arrays of guanine and cytosine fused to an internal pyridine ring was synthesized. When dissolved in water at room temperature, this module rapidly self-assembles into hexameric rosettes, which then stack to form J-type rosette nanotubes (RNTs) with increased inner/outer diameters and the largest molar ellipticity ever reported (4 × 10(6) deg·M(-1)·m(-1)). Using a combination of imaging and spectroscopic techniques we established the structure of ×K1-RNT and have shown that the extended π system of the self-assembling module resulted in a new family of J-type RNTs with enhanced intermodular electronic communication.

Taking a little off the top: nanorod array morphology and growth studied by focused ion beam tomography.

The high surface area, large aspect ratio, and porous nature of nanorod arrays make them excellent foundation materials for many devices. Of the many synthesis techniques for forming nanorods, glancing angle deposition (GLAD) offers one of the more straightforward and flexible methods for ensuring control of alignment, porosity, and architecture of the nanorods. Here we demonstrate the first use of a dual-beam (focused ion beam (FIB) combined with scanning electron microscopy (SEM)) instrument to section and image the internal morphology of a nanorod array fabricated using the GLAD technique. We have used the FIB-SEM to reconstruct the 3D composition of TiO(2) nanorods, allowing us to visualize for the first time the core structures of many potential devices. We have also been able to probe the relationship between critical parameters such as diameter (w(act)), internanorod spacing (ν(act)), center-to-center spacing (c(act)), and nanorod population density (d(act)) and the depth of the nanocolumn (t) for a single homogeneous structure. A continuous data set was obtained from a single 5-µm-thick GLAD film, avoiding the artifacts arising from the analysis of the top surfaces of multiple samples of varying thicknesses. An analysis of the acquired sectioned data has allowed us to determine that the critical nanocolumn parameters follow a power-law scaling trend with w(act) = 9.4t(0.35) nm, ν(act) = 15.2t(0.25) nm, c(act) = 24.8t(0.31) nm, and d(act) = 3402t(-0.65) columns µm(-2). Using the FIB/SEM images acquired for the TiO(2) nanorods, we have also investigated the evolution of individual nanocolumns and have observed that bifurcation and branching play a significant role in the extinction or survival of these nanorods. These findings will allow for the optimization of nanorod properties for device applications. Also, the FIB sectioning and reconstruction process developed here will permit for the investigation of nanorod arrays formed from a range of synthesis techniques and materials.

Toward the quantitative the interpretation of hole-free phase plate images in a transmission electron microscope.

We present progress toward the quantitative interpretation of phase contrast images obtained using a hole-free phase plate (HFPP) in a transmission electron microscope (TEM). We consider a sinusoidal phase grating test object composed of ~5 nm deep groves in a ~13 nm thick amorphous silicon membrane. The periodic grating splits the beam current into direct beam and diffracted side beams in the focal plane of the imaging lens, where the HFPP is located. The physical separation between the beams allows for a detailed study of the HFPP phase shift evolution and its effect on image contrast. The residual phase shift of the electron beam footprint on the phase plate was measured by electron holography and used as input to image simulations that were compared to experimental data. Our results confirm that phase contrast is established by the phase difference between the direct and side beams, which we can estimate by fitting the image contrast evolution in time with an analytical formula describing the image intensity of a sinusoidal strong phase object. We also observed contrast reversal and frequency doubling of the grating image with time, which we interpret as the phase contrast arising from the interference between side beams becoming dominant. Another observation is the lateral displacement of the image fringes, which can be accounted for by a phase difference between the side beams.

Charging of electron beam irradiated amorphous carbon thin films at liquid nitrogen temperature.

We studied the charging behavior of an amorphous carbon thin film kept at liquid-nitrogen temperature under focused electron-beam irradiation. Negative charging of the thin film is observed. The charging is attributed to a local change in the work function of the thin film induced by electron-stimulated desorption similar to the working principle of the hole free phase plate in its Volta potential implementation at elevated temperature. The negative bias of the irradiated film arises from the electron beam induced desorption of water molecules from the carbon film surface. The lack of positive charging, which is expected for non-conductive materials, is explained by a sufficient electrical conductivity of the carbon thin film even at liquid-nitrogen temperature as proven by multi-probe scanning tunneling microscopy and spectroscopy measurements.

Parameters affecting the accuracy of nanoparticle shape and size measurement in 3D.

While electron tomography can be used to visualize objects at nanoscale, it is difficult to perform reproducible quantitative measurements. Here we measure the shape and size of nanoparticles (NPs) in three dimensions (3D) using electron tomography. We evaluated the accuracy of maximum Feret diameter (Feretmax), minimum Feret diameter (Feretmini) and volume of NPs measurements from reconstructed 3D images which were obtained from data acquired with varied electron dose. We perform both simulations and experiment to clarify what factors effect on the accuracy of the NP shape measurement. Based on the results, suitable reconstruction methods and threshold for binarization were evaluated. We also report comparison results obtained on exactly the same samples in two different laboratories.

Hole free phase plate tomography for materials sciences samples.

We report, for the first time, the three dimensional reconstruction (3D) of a transistor from a microprocessor chip and roughness of molecular electronic junction obtained by electron tomography with Hole Free Phase Plate (HFPP) imaging. The HFPP appears to enhance contrast between inorganic materials and also increase the visibility of interfaces between different materials. We demonstrate that the degree of enhancement varies depending on material and thickness of the samples using experimental and simulation data.

Imaging of radiation-sensitive samples in transmission electron microscopes equipped with Zernike phase plates.

We have optimized a bright-field transmission electron microscope for imaging of high-resolution radiation-sensitive materials by calculating the imaging dose n(0) needed to obtain a signal-to-noise ratio (SNR)=5. Installing a Zernike phase plate (ZP) decreases the dose needed to detect single atoms by as much as a factor of two at 300 kV. For imaging larger objects, such as Gaussian objects with full-width at half-maximum larger than 0.15 nm, ZP appears more efficient in reducing the imaging dose than correcting for spherical aberration. The imaging dose n(0) does not decrease with extending of chromatic resolution limit by reducing chromatic aberration, using high accelerating potential (U(0)=300 kV), because the image contrast increases slower than the reciprocal of detection radius. However, reducing chromatic aberration would allow accelerating potential to be reduced leading to imaging doses below 10 e(-)/A(2) for a single iodine atom when a CS-corrector and a ZP are used together. Our simulations indicate that, in addition to microscope hardware, optimization is heavily dependent on the nature of the specimen under investigation.

Low-dose performance of parallel-beam nanodiffraction.

We evaluate the low-dose performance of parallel nano-beam diffraction (NBD) in the transmission electron microscope as a method for characterizing radiation sensitive materials at low electron irradiation dose. A criterion, analogous to Rose's, is established for detecting a diffraction spot with desired signal-to-noise ratio. Our experimental data show that a dose substantially lower than in high-resolution bright-field imaging is sufficient to determine structure and orientation of individual nanoscale objects embedded in amorphous matrix. In an instrument equipped with a cold field-emission gun it is possible to form a probe with sub-3 nm diameter and sub-0.3 mrad convergence angle with sufficient beam current to record a diffraction pattern with less than 0.2 s acquisition time. The interpretation of NBD patterns is identical to that of selected area diffraction patterns. We illustrate the physical principles underlying good low-dose performance of NBD by means of a phase grating. The electron irradiation dose needed to detect a diffraction peak in NBD is found proportional to 1/N2, where N is the number of lattice planes contributing to the peak.