Electron Acceleration Voltage Research Articles

Synthesis of inter-[60]fullerene conjugates with inherent chirality

Coalescence of [60]fullerenes potentially produces hypothetical nanocarbon assemblies with non-naturally occurring topologies. Since the discovery of [60]fullerene in 1985, coalesced [60]fullerene oligomers have only been observed as transient species by transmission electron microscopy during an oligomerization process under a high electron acceleration voltage. Herein, we showcase the rational synthesis of covalent assemblies consisting of inherently chiral open-[60]fullerenes. The crystallographic analyses unveiled double-caged structures of non-conjugated and conjugated inter-[60]fullerene hybrids, in which the two [60]fullerene cages are bounds to each other through a covalent linkage. The former one further assembles via a heterochiral recognition so that four carbon cages are arranged in a tetrahedral manner both in solution and solid state. Reflecting radially-conjugated double π-surface nature, the inter-[60]fullerene conjugate exhibits strong electronic communication in its reduced states, intense absorption behavior, and chiroptical activity with a dissymmetry factor of 0.21 (at 674 nm) which breaks the record for known chiral organic molecules.

A novel ground-potential monochromator design

An electron monochromator design is presented as an instrumental development for electron energy loss spectroscopy (EELS) and imaging in (scanning) transmission electron microscopy ((S)TEM). The main purpose of this development is enhancing the energy resolving power in spectroscopy and filtering. In addition, it helps reducing the effect of the objective lens’ chromatic aberration Cc in imaging and therefore, enhancing the spatial resolving power of electron microscopes. General estimates for the performance of a monochromator in energy distribution and the resulting usable beam currents are given. The special monochromator design presented is a ground-potential monochromator based on magnetic sector fields. The monochromator generates a spatially and angular un-dispersed spot and has no mechanically actuated parts in the filter sections. The optics can be operated at electron acceleration voltages from 30kV to 300kV and shows an energy resolving power of better than 2⋅10−7 relative to the primary electron energy. The actual device is designed to be retro-fittable to microscopes from various manufacturers.

Electron-Enhanced Atomic Layer Deposition of Titanium Nitride Films Using an Ammonia Reactive Background Gas

Electron-enhanced atomic layer deposition (EE-ALD) of titanium nitride (TiN) films was achieved using sequential exposures of tetrakis(dimethylamido)titanium (TDMAT) and low energy electrons in the presence of a continuous NH3 reactive background gas (RBG). Performing EE-ALD concurrently with a RBG is a new ALD film growth technique. The TiN EE-ALD was performed utilizing a hollow cathode plasma electron source (HC-PES). The HC-PES can deliver a high electron flux into background gases at pressures up to several mTorr. The TiN EE-ALD was conducted at temperatures of 30–70 °C using an electron acceleration voltage of 100 V and a NH3 pressure of ∼1 mTorr. The incident electron flux promotes electron stimulated desorption (ESD) and facilitates rapid nucleation and low temperature film growth. The TiN EE-ALD film growth was achieved on a variety of substrates including a native oxide on silicon, a SiO2 thermal oxide, and in situ silicon nitride films grown using electron-enhanced chemical vapor deposition (EE-CVD). Growth rates of 0.75 to 1.8 Å per cycle were measured using in situ four-wavelength ellipsometry for different TDMAT precursor exposures. Ex situ X-ray photoelectron spectroscopy (XPS) studies indicated that the TiN films were high purity and slightly nitrogen-rich. The in situ ellipsometry also measured low resistivities of ∼120 μΩ cm for the TiN films with thicknesses of ≥60 Å. These low resistivities were confirmed by ex situ four-point probe measurements and ex situ spectroscopic ellipsometry. X-ray diffraction investigations determined that the TiN EE-ALD films were crystalline. X-ray reflectivity studies also indicated that the thin TiN films had densities similar to bulk films. The high quality of the TiN EE-ALD films is attributed to the NH3 RBG. Interaction between the low energy electrons and the NH3 RBG is believed to form •NH2 and •H radical species that react with the surface during EE-ALD and improve film purity. The reactive •NH2 and •H species likely lead to nitridation and carbon removal from the films. RBGs greatly expand the possibilities for tuning film composition and properties during EE-ALD. In addition, TiN EE-ALD was accomplished on insulating substrates such as an SiO2 thermal oxide. The TiN EE-ALD is believed to be possible on insulating substrates because the secondary electron yield for electron energies of ∼100 eV is greater than unity.

Optimal acceleration voltage for near-atomic resolution imaging of layer-stacked 2D polymer thin films

Despite superb instrumental resolution in modern transmission electron microscopes (TEM), high-resolution imaging of organic two-dimensional (2D) materials is a formidable task. Here, we present that the appropriate selection of the incident electron energy plays a crucial role in reducing the gap between achievable resolution in the image and the instrumental limit. Among a broad range of electron acceleration voltages (300 kV, 200 kV, 120 kV, and 80 kV) tested, we found that the highest resolution in the HRTEM image is achieved at 120 kV, which is 1.9 Å. In two imine-based 2D polymer thin films, unexpected molecular interstitial defects were unraveled. Their structural nature is identified with the aid of quantum mechanical calculations. Furthermore, the increased image resolution and enhanced image contrast at 120 kV enabled the detection of functional groups at the pore interfaces. The experimental setup has also been employed for an amorphous organic 2D material.

Excited state modulation of C70 dimerization in a carbon nanotube under a variable electron acceleration voltage

Cinematographic recording of chemical reactions with transmission electron microscopy provides information unavailable by any other analytical methods. Studies have thus far remained mostly phenomenological, lacking information on the reactive species involved. To gain insight into the nature of the reactive species, we need to obtain kinetic information under various temperatures and variable acceleration voltages, i.e., electronic energy supply. We studied the mechanism of [2 + 2] dimerization of [70] fullerene in a carbon nanotube as an example. We describe herein a statistical analysis of individual reaction events of the dimerization that revealed dose-dependent first-order kinetics and voltage-dependent crossover from a singlet to a triplet mechanism, as highlighted by the pre-exponential factor (the frequency of excitation) that is a million times larger for the singlet reaction than for the triplet one. Comparison with the results of a recent study of [60] fullerene dimerization lets us propose that electron-impact excitation of the carbon nanotube is the first step, followed by energy transfer to fullerene molecules and their dimerization via an excited state. The results show that a variable-voltage kinetic study is indispensable for discussing the mechanism of chemical transformations under electron microscopic observation.

Determination of geometrical structure of CCl3+ by trapped-ion electron diffraction

A trapped-ion electron diffraction (TIED) apparatus has been developed. By sufficient suppression of the background signals originating from electrons scattered by residual gases and those originating from the back-scattered and secondary electrons generated at a Faraday cup, the gas electron diffraction pattern of CCl3+, generated through the dissociative ionization of CCl4 by near-IR ultrashort intense laser pulses, is recorded using the TIED apparatus at the electron acceleration voltage of 18.8 keV. The determined internuclear distances, r(C-Cl) = 1.66(2) Å and r(Cl⋯Cl) = 2.87(2) Å, which are consistent with theoretically calculated values, confirm that CCl3+ takes a planar geometry.

Carrier Diffusion in GaN : A Cathodoluminescence Study. III. Nature of Nonradiative Recombination at Threading Dislocations

We investigate the impact of threading dislocations with an edge component ($a$ or $a+c$ type) on carrier recombination and diffusion in $\mathrm{Ga}\mathrm{N}$(0001) layers close to the surface as well as in the bulk. To this end, we utilize cathodoluminescence imaging of the top surface of a $\mathrm{Ga}\mathrm{N}$(0001) layer with a deeply buried (In,$\mathrm{Ga}$)N quantum well. Varying the acceleration voltage of the primary electrons and comparing the signal from the layer and the quantum well enables us to probe carrier recombination at depths ranging from the close vicinity of the surface to the position of the quantum well. Our experiments are accompanied by fully three-dimensional Monte Carlo simulations of carrier drift, diffusion, and recombination in the presence of the surface, the quantum well, and the dislocation, taking into account the dislocation strain field and the resulting piezoelectric field at the dislocation outcrop. Near the surface, this field establishes an exciton dead zone around the dislocation, the extent of which is not related to the carrier diffusion length. However, reliable values of the carrier diffusion length can be extracted from the dipolelike energy shift observed in hyperspectral cathodoluminescence maps recorded around the dislocation outcrop at low acceleration voltages. For high acceleration voltages, allowing us to probe a depth where carrier recombination is unaffected by surface effects, we observe a much stronger contrast than expected from the piezoelectric field alone. This finding provides unambiguous experimental evidence for the strong nonradiative activity of edge threading dislocations in bulk $\mathrm{Ga}\mathrm{N}$ and hence also in buried heterostructures.

Reducing electron beam damage through alternative STEM scanning strategies, Part I: Experimental findings

The highly energetic electrons in a transmission electron microscope (TEM) can alter or even completely destroy the structure of samples before sufficient information can be obtained. This is especially problematic in the case of zeolites, organic and biological materials. As this effect depends on both the electron beam and the sample and can involve multiple damage pathways, its study remained difficult and is plagued with irreproducibility issues, circumstantial evidence, rumors, and a general lack of solid data. Here we take on the experimental challenge to investigate the role of the STEM scan pattern on the damage behavior of a commercially available zeolite sample with the clear aim to make our observations as reproducible as possible. We make use of a freely programmable scan engine that gives full control over the tempospatial distribution of the electron probe on the sample and we use its flexibility to obtain multiple repeated experiments under identical conditions comparing the difference in beam damage between a conventional raster scan pattern and a newly proposed interleaved scan pattern that provides exactly the same dose and dose rate and visits exactly the same scan points. We observe a significant difference in beam damage for both patterns with up to 11 % reduction in damage (measured from mass loss). These observations demonstrate without doubt that electron dose, dose rate and acceleration voltage are not the only parameters affecting beam damage in (S)TEM experiments and invite the community to rethink beam damage as an unavoidable consequence of applied electron dose.

Extending Cryo-EM to Nonaqueous Liquid Systems.

ConspectusCryogenic-temperature transmission electron microscopy (cryo-TEM) of aqueous systems has become a widely used methodology, especially in the study of biological systems and synthetic aqueous systems, such as amphiphile and polymer solutions. Cryogenic-temperature scanning electron microscopy (cryo-SEM), while not as widely used as cryo-TEM, is also found in many laboratories of basic and applied research. The application of these methodologies, referred to collectively as cryogenic-temperature electron microscopy (cryo-EM) for direct nanostructural studies of nonaqueous liquid systems is much more limited, although such systems are important in basic research and are found in a very large spectrum of commercial applications. The study of nonaqueous liquid systems by cryo-EM poses many technical challenges. Specimen preparation under controlled conditions of air saturation around the specimen cannot be performed by the currently available commercial system, and the most effective cryogen, freezing ethane, cannot be used for most such liquid systems. Imaging is often complicated by low micrograph contrast and high sensitivity of the specimens to the electron beam.At the beginning of this Account, we describe the basic principles of cryo-EM, emphasizing factors that are essential for successful direct imaging by cryo-TEM and cryo-SEM. We discuss the peculiarities of nonaqueous liquid nanostructured systems when studied with these methodologies and how the technical difficulties in imaging nonaqueous systems, from oil-based to strong acid-based liquids, have been overcome, and the applicability of cryo-TEM and cryo-SEM has been expanded in recent years. Modern cryo-EM has been advanced by a number of instrumental developments, which we describe. In the TEM, these include improved electron field emission guns (FEGs) and microscope optics, the Volta phase plate to enhance image contrast by converting phase differences to amplitude differences without the loss of resolution by an objective lens strong underfocus, and highly sensitive image cameras that allow the recording of TEM images with minimal electron exposure. In the SEM, we take advantage of improved FEGs that allow imaging at a low (around 1 kV) electron acceleration voltage that is essential for high-resolution imaging and for avoiding specimen charging of uncoated nonconductive specimens, better optics, and a variety of sensitive detectors that have considerably improved resolution and, under the proper conditions, give excellent contrast even between elements quite close on the periodic table of the elements, such as the most important oxygen and carbon atoms.Finally we present and analyze several examples from our recent studies, which illustrate the issues presented above, including the remarkable progress made in recent years in this field and the strength and applicability of cryo-EM methodologies.

Non-Destructive Imaging on Synthesised Nanoparticles.

Our recently developed non-destructive imaging technique was applied for the characterisation of nanoparticles synthesised by X-ray radiolysis and the sol-gel method. The interfacial conditions between the nanoparticles and the substrates were observed by subtracting images taken by scanning electron microscopy at controlled electron acceleration voltages to allow backscattered electrons to be generated predominantly below and above the interfaces. The interfacial adhesion was found to be dependent on the solution pH used for the particle synthesis or particle suspension preparation, proving the change in the particle formation/deposition processes with pH as anticipated and agreed with the prediction based on the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. We found that our imaging technique was useful for the characterisation of interfaces hidden by nanoparticles to reveal the formation/deposition mechanism and can be extended to the other types of interfaces.

Investigations of internal stresses in high-voltage devices with deep trenches

Deep trenches, as essential elements of silicon chips used in electronic high-power and high-frequency devices, are known as starting points for dislocation generation under the influence of internal mechanical stresses resulting mainly from the difference in the thermal expansion coefficients between silicon and silicon dioxide. Since the electrical insulation of the devices requires a sequence of mechanical, chemical, and high-temperature processes during the preparation of the deep trenches, including the formation of an amorphous SiO2 edge layer, the emergence of the internal stresses is hardly avoidable. The method of cross correlation backscattered electron diffraction in the scanning electron microscope is used here to quantitatively determine the magnitude and local distribution of internal stresses in silicon around the deep trenches after four different process steps. For this purpose, Kikuchi diffraction images are recorded of the wafer cross section areas along lines perpendicular and parallel to the deep trenches. After Fourier transformation, these images are cross correlated with the Fourier transform of the diffraction image from a stressfree reference sample site. The well-established numerical evaluation of cross correlation functions provides the complete distortion tensor for each measuring point of the line scan, from which the stress tensor can be calculated using Hooke's law. It is found that the in-plane normal stress component σ11 perpendicular to the long edges of the deep trench is larger than the other stress components. That means it essentially determines the magnitude of the von-Mises stress, which was determined as a general stress indicator for all measuring points, too. A characteristic feature is the local distribution of the stress component σ11 with maximum tensile stresses of some hundred megapascals at transition between Si and amorphous SiO2 on the long edges of the deep trench, and with even higher maximum compressive stresses immediately below the bottom of the deep trench. At a distance of about 2 μm from the edges of a single deep trench, all stress components decrease to negligibly small values so that steep stress gradients occur. The range and distribution of tensile and compressive stresses are in accordance with finite element simulations; however, the measured stresses are higher than expected for all investigated states so that dislocation formation seems to be possible. The influence of the electron acceleration voltage on the determination of the internal stresses is discussed as well.

Tunable free-electron X-ray radiation from van der Waals materials

Tunable sources of X-ray radiation are widely used for imaging and spectroscopy in fundamental science, medicine and industry. The growing demand for highly tunable, high-brightness laboratory-scale X-ray sources motivates research into new fundamental mechanisms of X-ray generation. Here, we demonstrate the ability of van der Waals materials to serve as a platform for tunable X-ray generation when irradiated by moderately relativistic electrons available, for example, from a transmission electron microscope. The radiation spectrum can be precisely controlled by tuning the acceleration voltage of the incident electrons, as well as by our proposed approach: adjusting the lattice structure of the van der Waals material. We present experimental results for both methods, observing the energy tunability of X-ray radiation from the van der Waals materials WSe2, CrPS4, MnPS3, FePS3, CoPS3 and NiPS3. Our findings demonstrate the concept of material design at the atomic level, using van der Waals heterostructures and other atomic superlattices, for exploring novel phenomena of X-ray physics. Tunable X-ray generation, from ultrathin van der Waals materials impacted by relativistic electrons, is demonstrated.

Voltage Contrast in Scanning Electron Microscopy to Distinguish Conducting Ag Nanowire Networks from Nonconducting Ag Nanowire Networks.

A study of the electrical properties of metallic nanowires requires a clear analysis of conductive networks. In this study, we demonstrated that the conducting networks of Ag nanowires (AgNW) could be visually observed by examination of the voltage contrast of the scanning electron microscopy (SEM) images, which was caused by the differences in the degrees of charging of AgNWs. When AgNWs dispersed on a quartz glass were irradiated by primary electrons, the substrate became negatively charged. This induced positive charges on the AgNWs in contact with the electrodes. As a result, AgNW networks connected to electrodes appeared dark in the SEM image, while the isolated AgNWs appeared brighter. By varying the acceleration voltage of the primary electrons, the extent of charging could be controlled, which, in turn, enabled the observation of the voltage contrast of AgNWs. Using the voltage contrast of SEM images, we could visually distinguish the AgNW networks having an electrical connection with the electrode from the ones that were not connected to the electrode.

Uma proposta de baixo custo para experimentos com raios catódicos

Este trabalho apresenta uma proposta de adaptação de um dos experimentos sobre a natureza dos raios catódicos realizado ao final do século XIX, especificamente, aquele apresentado em livros didáticos e de divulgação científica como “o experimento de Thomson” para o cálculo da relação carga/massa do elétron. Nossa proposta utiliza materiais com custos inferiores aos de kits de experimentos disponíveis no mercado internacional e apresenta uma descrição pormenorizada da sua construção e operação, concorrendo, em vários aspectos, como solução para alguns dos motivos relacionados à pouca utilização dos experimentos nas aulas de ciências: o custo elevado dos materiais e dos equipamentos, a falta de laboratórios e a falta de habilidade dos professores. A elaboração do aparato experimental é ainda acompanhada do desenvolvimento matemático necessário à sua utilização e da indicação dos vários fenômenos físicos que podem ser tratados paralelamente à sua implementação. Com a adaptação experimental, fomos capazes de medir e calcular o campo magnético a que os elétrons são submetidos no interior de um tubo de raios catódicos, a diferença de potencial entre catodo e anodo no tubo (tensão de aceleração dos elétrons), a deflexão magnética que os elétrons sofrem, e, com uma boa aproximação, a velocidade de translação dos elétrons e a relação carga/massa dessas partículas.

Chromatic- and geometric-aberration-corrected TEM imaging at 80 kV and 20 kV

The interest in light-element nanomaterials stimulated the development of atomic-resolution low-voltage transmission electron microscopy. While geometric-aberration correction made high-resolution imaging at around 80-kV electron acceleration voltage possible, imaging at substantially lower voltages requested the development of chromatic-aberration correction in addition. Recently, the SALVE instrument was introduced with the capability of atomic resolution imaging at electron acceleration voltages from 80 kV down to 20 kV. Here, we show on the example of imaging graphene that at these electron energies, the residual geometric aberrations reintroduce contrast oscillations and delocalization. Also, we show an atomically resolved image of a single vacancy in graphene with a pronounced Jahn-Teller distortion.

Estimation of the thickness of graphite nanofilm on a silicon substrate by using energy dispersive X-ray analysis data

Energy-dispersive X-ray spectroscopy is applied to analyze composition and thickness of graphite nanofilm on a substrate. We estimate the thickness of graphite nanofilm on a silicon substrate using a simple equation proposed in this work. The equation is based on data of the energy-dispersive microanalysis and not contains fitting parameters. The condition of applicability of the formula is a low value of the electron acceleration voltage at which the depth of electron beam penetration not exceeds a silicon oxide layer thickness. The number of graphite nanofilm layers estimated from the calculated thickness is reasonably confirmed by Raman spectroscopy data. It is shown, that the number of graphite nanofilm layers can be determined by carbon atomic content in at %.

B12-P-14Direct observation of concentration-dependent growth and dissolution of silver nanoparticles using in-situ liquid transmission electron microscopy

Recently, much interest has been increased to develop in-situ TEM technique using various TEM holders. These techniques allow us to measure various materials properties while observing microstructure at high magnification. There is category of in-situ TEM that allows us to control the environment also called environmental transmission electron microscopy (E-TEM). E-TEM is a powerful technique to observe gas-solid or liquid-solid reactions in real time. In this study, we fabricated a window type environmental cell TEM holder and applied it to in-situ observation of decomposition of ZnO nanoparticles. The environmental cell TEM holder uses two silicon nitride membrane window TEM grids that are commercially available from Ted-Pella Inc.[1]. These windows should be thin enough to be electron transparent and thick enough to withstand pressure difference between inside and outside of the cell. The window grids were placed in a custom-made TEM holder between top and bottom o-ring that seal vacuum from column. By using custom made TEM holder, decomposition of ZnO nanoparticle have been observed. A JEOL 3010 TEM was used for the observation, which operated at 300kV electron acceleration voltage. ZnO nanoparticles mixed with DI water are decomposed under high-keV electron beam irradiation. ZnO nanoparticles are decomposed quite rapidly regardless of size of them when focused electron beam was irradiated. This observation allows a discussion of electron beam irradiation effect on the ZnO nanoparticles with DI water.

Stabilization of Gyrotron Frequency by PID Feedback Control on the Acceleration Voltage

The results of frequency stabilization by proportional-integral-derivative (PID) feedback control of acceleration voltage in the 460-GHz Gyrotron FU CW GVI (the official name in Osaka University is Gyrotron FU CW GOI) are presented. The experiment was organized on the basis of the frequency modulation by modulation of acceleration voltage of beam electrons. The frequency stabilization during 10 h experiment was better than 10−6, which is compared with the results of the frequency deviation in free-running gyrotron operation.

Layered structure of MoS2 investigated using electron energy loss spectroscopy

Transition metal dichalcogenide MoS2 has attracted significant interest for its unique electronic, optical and catalytic properties. Layered crystalline MoS2 can be mechanically exfoliated into single monolayers. With the advancement of microscopic and analytical techniques, significant insight knowledge has been obtained on the structure and properties of two-dimensional (2D) materials. In this paper, the authors have carried out high resolution transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) to investigate MoS2 layers transferred on silicon substrate. Due to the weak nature of van der Waal forces between the S–S layers, stability of layers poses a challenge at high electron acceleration voltages. Dark field STEM (DF-STEM) and EELS, used on 11 layers of MoS2, reveal an interlayer spacing of ~6.25Å, which is consistent with the reported crystal structure of MoS2.

High-Speed Frequency Modulation of a 460-GHz Gyrotron for Enhancement of 700-MHz DNP-NMR Spectroscopy

The high-speed frequency modulation of a 460-GHz Gyrotron FU CW GVI (the official name in Osaka University is Gyrotron FU CW GOI) was achieved by modulation of acceleration voltage of beam electrons. The modulation speed f m can be increased up to 10 kHz without decreasing the modulation amplitude δf of frequency. The amplitude δf was increased almost linearly with the modulation amplitude of acceleration voltage ΔV a. At the ΔV a = 1 kV, frequency spectrum width df was 50 MHz in the case of f m < 10 kHz. The frequency modulation was observed as both the variation of the IF frequency in the heterodyne detection system measured by a high-speed oscilloscope and the widths of frequency spectra df measured on a frequency spectrum analyzer. Both results well agree reasonably. When f m exceeds 10 kHz, the amplitude δf is decreased gradually with increasing f m because of the degradation of the used amplifier in response for high-speed modulation. The experiment was performed successfully for both a sinusoidal wave and triangle wave modulations. We can use the high-speed frequency modulation for increasing the enhancement factor of the dynamic nuclear polarization (DNP)-enhanced nuclear magnetic resonance (NMR) spectroscopy, which is one of effective and attractive methods for the high-frequency DNP-NMR spectroscopy, for example, at 700 MHz. Because the sensitivity of NMR is inversely proportional to the frequency, high-speed frequency modulation can compensate the decreasing the enhancement factor in the high-frequency DNP-NMR spectroscopy and keep the factor at high value. In addition, the high-speed frequency modulation is useful for frequency stabilization by a PID control of an acceleration voltage by feeding back of the fluctuation of frequency. The frequency stabilization in long time is also useful for application of a DNP-NMR spectroscopy to the analysis of complicated protein molecules.