In situoff-axis electron holography of real-time dopant diffusion in GaAs nanowires.

Off-axis electron holography was used to reveal remote doping in GaAs nanowires occurring duringin situannealing in a transmission electron microscope. Dynamic changes to the electrostatic potential caused by carbon dopant diffusion upon annealing were measured across GaAs nanowires with radial p-p+ core-shell junctions. Electrostatic potential profiles were extracted from holographic phase maps and built-in potentials (Vbi) and depletion layer widths (DLWs) were estimated as function of temperature over 300-873 K. Simulations in absence of remote doping predict a significant increase ofVbiand DLWs with temperature. In contrast, we measured experimentally a nearly constantVbiand a weak increase of DLWs. Moreover, we observed the appearance of a depression in the potential profile of the core upon annealing. We attribute these deviations from the predicted behavior to carbon diffusion from the shell to the core through the nanowire sidewalls, i.e. to remote doping, becoming significant at 673 K. The DLW in the p and p+ regions are in the 10-30 nm range.

Coexistence and Coupling of Multiple Charge Orderings and Spin States in Hexagonal Ferrite.

The coupling between charge and spin orderings in strongly correlated systems plays a crucial role in fundamental physics and device applications. As a candidate of multiferroic materials, LuFe2O4 with a nominal Fe2.5+ valence state has the potential for strong charge-spin interactions; however, these interactions have not been fully understood until now. Here, combining complementary characterization methods with theoretical calculations, two types of charge orderings with distinct magnetic properties are revealed. The ground states of LuFe2O4 are decided by the parallel/antiparallel coupling of both charge and spin orderings in the adjacent FeO double layers. Whereas the ferroelectric charge ordering remains ferrimagnetic below 230 K, the antiferroelectric ordering undergoes antiferromagnetic-ferrimagnetic-paramagnetic transitions from 2 K to room temperature. This study demonstrates the unique aspects of strong spin-charge coupling within LuFe2O4. Our results shed light on the coexistence and competing nature of orderings in quantum materials.

Mean Inner Potential of Liquid Water.

Improving our experimental and theoretical knowledge of electric potentials at liquid-solid boundaries is essential to achieve a deeper understanding of the driving forces behind interfacial processes. Electron holography has proved successful in probing solid-solid interfaces but requires knowledge of the materials' mean inner potential (MIP, V_{0}), which is a fundamental bulk material property. Combining off-axis electron holography with liquid phase transmission electron microscopy (LPTEM), we provide the first quantitative MIP determination of liquid water V_{0}=+4.48±0.19 V. This value is larger than most theoretical predictions, and to explain the disagreement we assess the dominant factors needed in quantum simulations of liquid water. A precise MIP lays the foundations for nanoscale holographic potential measurements in liquids, and provides a benchmark to improve quantum mechanical descriptions of aqueous systems and their interfaces in, e.g., electrochemistry, solvation processes, and spectroscopy.

Quantification of Mixed Bloch-Néel Topological Spin Textures Stabilized by the Dzyaloshinskii-Moriya Interaction in Co/Pd Multilayers.

The three-dimensional structure of nanoscale topological spin textures stabilized by the Dzyaloshinskii-Moriya interaction is governed by the delicate competition between the exchange, demagnetization, and anisotropy energies. The quantification of such spin textures through direct experimental methods is crucial towards understanding the fundamental physics associated with their ordering, as well as their manipulation in spintronic devices. Here, we extend the Lorentz transmission electron microscopy technique to quantify mixed Bloch-Néel chiral spin textures stabilized by the Dzyaloshinskii-Moriya interaction in Co/Pd multilayers. Analysis of the observed intensities under varied imaging conditions coupled to corroborative micromagnetic simulations yields vital parameters that dictate the stability and properties of the complex spin texture, namely, the degree of mixed Bloch-Néel character, the domain wall width, the strength of the Dzyaloshinskii-Moriya interaction, and the exchange stiffness. This approach provides the necessary framework for the application of quantitative Lorentz phase microscopy to a broad array of topological spin systems.

Effect of Molecular Weight on the Feature Size in Organic Ice Resists.

The feature size of patterns obtained by electron-beam lithography (EBL) depends critically on resist properties, beam parameters, development process, and instrument limitations. Frozen layers of simple organic molecules such as n-alkanes behave as negative-tone resists for EBL. With the unique advantage of an in situ thermal treatment replacing chemical development, the entire lithographic process can be performed within a single instrument, thus removing the influence of chemical developers on the feature size. By using an environmental transmission electron microscope, we can also minimize the influence of instrumental limitations and explore the fundamental link between resist characteristics and feature size. Our results reveal that the onset dose of organic ice resists correlates with the inverse molecular weight and that in the thermal development the role of change in solubility of polymers is mirrored in a shift in the solid/vapor critical temperature of organic ices. With a 0.4 pA beam current, we obtained 4.5, 5.5, and 8.5 nm lines with frozen octane, undecane, and tetradecane, respectively, consistent with the predictions of a model we developed that links beam profile and feature size. The knowledge acquired on the response of small organic molecules to electron irradiation, combined with the flexibility and operational advantages of using them as qualified EBL resists, provides us with new opportunities for the design and production of nanodevices and broadens the reach of EBL especially toward biological applications.

Organic Ice Resists.

Electron-beam lithography (EBL) is the backbone technology for patterning nanostructures and manufacturing nanodevices. It involves processing and handling synthetic resins in several steps, each requiring optimization and dedicated instrumentation in cleanroom environments. Here, we show that simple organic molecules, e.g. alcohols, condensed to form thin-films at low temperature demonstrate resist-like capabilities for EBL applications and beyond. The entire lithographic process takes place in a single instrument, and avoids exposing  users to chemicals and the need of cleanrooms. Unlike EBL that requires large samples with optically flat surfaces, we patterned on fragile membranes only 5 nm-thin, and 2 × 2 mm2 diamond samples. We created patterns on the nanometer to sub-millimeter scale, as well as three-dimensional structures by stacking layers of frozen organic molecules. Finally, using plasma etching, the organic ice resist (OIR) patterns are used to structure the underlying material, and thus enable nanodevice fabrication.

Doping GaP Core-Shell Nanowire pn-Junctions: A Study by Off-Axis Electron Holography.

The doping process in GaP core-shell nanowire pn-junctions using different precursors is evaluated by mapping the nanowires' electrostatic potential distribution by means of off-axis electron holography. Three precursors, triethyltin (TESn), ditertiarybutylselenide, and silane are investigated for n-type doping of nanowire shells; among them, TESn is shown to be the most efficient precursor. Off-axis electron holography reveals higher electrostatic potentials in the regions of nanowire cores grown by the vapor-liquid-solid (VLS) mechanism (axial growth) than the regions grown parasitically by the vapor-solid (VS) mechanism (radial growth), attributed to different incorporation efficiency between VLS and VS of unintentional p-type carbon doping originating from the trimethylgallium precursor. This study shows that off-axis electron holography of doped nanowires is unique in terms of the ability to map the electrostatic potential and thereby the active dopant distribution with high spatial resolution.

Temporal dynamics of charge buildup in cryo-electron microscopy.

It is well known that insulating samples can accumulate electric charges from exposure to an electron beam. How the accumulation of charge affects imaging parameters and sample stability in transmission electron microscopy is poorly understood. To quantify these effects, it is important to know how the charge is distributed within the sample and how it builds up over time. In the present study, we determine the spatial distribution and temporal dynamics of charge accumulation on vitreous ice samples with embedded proteins through a combination of modeling and Fresnel diffraction experiments. Our data reveal a rapid evolution of the charge state on ice upon initial exposure to the electron beam accompanied by charge gradients at the interfaces between ice and carbon films. We demonstrate that ice film movement and charge state variations occur upon electron beam exposure and are dose-rate dependent. Both affect the image defocus through a combination of sample height changes and lensing effects. Our results may be used as a guide to improve sample preparation, data collection, and data processing for imaging of dose-sensitive samples.

Electron Beam Induced Enhancement and Suppression of Oxidation in Cu Nanoparticles in Environmental Scanning Transmission Electron Microscopy.

We have investigated the effects of high-energy electron irradiation on the oxidation of copper nanoparticles in environmental scanning transmission electron microscopy (ESTEM). The hemispherically shaped particles were oxidized in 3 mbar of O2 in a temperature range 100-200 °C. The evolution of the particles was recorded with sub-nanometer spatial resolution in situ in ESTEM. The oxidation encompasses the formation of outer and inner oxide shells on the nanoparticles, arising from the concurrent diffusion of copper and oxygen out of and into the nanoparticles, respectively. Our results reveal that the electron beam actively influences the reaction and overall accelerates the oxidation of the nanoparticles when compared to particles oxidized without exposure to the electron beam. However, the extent of this electron beam-assisted acceleration of oxidation diminishes at higher temperatures. Moreover, we observe that while oxidation through the outward diffusion of Cu+ cations is enhanced, the electron beam appears to hinder oxidation through the inward diffusion of O2- anions. Our results suggest that the impact of the high-energy electrons in ESTEM oxidation of Cu nanoparticles is mostly related to kinetic energy transfer, charging, and ionization of the gas environment, and the beam can both enhance and suppress reaction rates.

Electron-beam-induced charging of an Al2O3 nanotip studied using off-axis electron holography.

Electrostatic charging of specimens during electron, photon or ion irradiation is a complicated and poorly understood phenomenon, which can affect the acquisition and interpretation of experimental data and alter the functional properties of the constituent materials. It is usually linked to secondary electron emission, but also depends on the geometry and electrical properties of the specimen. Here, we use off-axis electron holography in the transmission electron microscope to study electron-beam-induced charging of an insulating Al2O3 nanotip on a conducting support. The measurements are performed under parallel electron illumination conditions as a function of specimen temperature, electron dose, primary electron energy and surface cleanliness. We observe a lack of reproducibility of charge density measurements after cycling the specimen temperature. Surprisingly, we find both positively and negatively charged regions in closely adjacent parts of the specimen.

Interpretation of mean free path values derived from off-axis electron holography amplitude measurements.

In this work, we have explored the factors which govern mean free path values obtained from off-axis electron holography measurements. Firstly, we explore the topic from a theoretical perspective, and show that the mean amplitude reconstructed from off-axis holograms is due to the coherent portion of the direct, central object-transmitted beam only - it is not affected by the presence or absence of other scattered beams. Secondly, we present a detailed experimental study which compares mean free path values obtained from hologram sideband, centreband, EELS, and TEM measurements as a function of optical collection angle and energy-loss-filtering. These results confirm that the coherent portion of the direct beam defines the mean amplitude, and additionally show that the coherent portion corresponds to the conventional energy-filtered signal (with threshold 5 eV in this work). Finally, we present summary measurements from a selection of different materials, and compare the results against a simple electron scattering model. This study reinforces the claim that the mean amplitude is defined by the energy-filtered direct beam, and confirms that the contributions of elastic and inelastic scattering to the total mean free path are broadly in line with theoretical expectations for these different materials. These results in aggregate indicate that neither experimental collection angles nor enhanced sensitivity to low-loss phonon scattering affect the mean amplitude signal arising from off-axis holography reconstructions, nor the associated mean free path values which are derived from this mean amplitude.

Investigation of gas-electron interactions with electron holography.

Using the combination of off-axis electron holography and environmental Transmission Electron Microscopy (TEM), an experimental setup termed 'gas electron holography', we investigate how the presence of gas in the microscope affects the spatial and phase resolution of electron holograms. The gas is introduced either by using an Environmental TEM (ETEM) or a closed-cell holder. The ETEM data on gas electron holography shows that the number of electrons reaching the detector decreases exponentially as a function of gas pressure. From this evidence, we construct a phenomenological model that describes how coherency changes as a function of gas pressure. By linking the model with the concept of inelastic scattering cross section we find that the change in the coherency of the electron beam due to the presence of gas is related to the number of gas molecules present, their atomic weight and the average energy lost due to inelastic scattering. Regarding gas electron holography with a closed cell holder, we conclude that the membranes surrounding the gas are the primary factor in determining the quality of the electron hologram, while the gas pressure inside the cell has a small impact on the spatial and phase resolution of the electron holograms.

Simulated clustering dynamics of colloidal magnetic nanoparticles.

Magnetically guided self-assembly of nanoparticles is a promising bottom-up method to fabricate novel materials and superstructures, such as, for example, magnetic nanoparticle clusters for biomedical applications. The existence of assembled structures has been verified by numerous experiments, yet a comprehensive theoretical framework to explore design possibilities and predict emerging properties is missing. Here we present a model of magnetic nanoparticle interactions built upon a Langevin dynamics algorithm to simulate the time evolution and aggregation of colloidal suspensions. We recognise three main aggregation regimes: non-aggregated, linear and clustered. Through systematic simulations we have revealed the link between single particle parameters and which aggregates are formed, both in terms of the three regimes and the chance of finding specific aggregates, which we characterise by nanoparticle arrangement and net magnetic moment. Our findings are shown to agree with past experiments and may serve as a stepping stone to guide the design and interpretation of future studies.

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.

Electron-Beam Patterning of Vapor-Deposited Solid Anisole.

The emerging ice lithography (IL) nanofabrication technology differs from conventional electron-beam lithography by working at cryogenic temperatures and using vapor-deposited organic molecules, such as solid water and alkanes, as e-beam resists. In this paper, we systematically investigate e-beam patterning of frozen anisole and assess its performance as an e-beam resist in IL. Dose curves reveal that anisole has a very low contrast of ∼1, with a very weak dependence on primary beam energy in the investigated range of 5-20 keV. The minimum line width of 60 nm is attainable at 20 keV, limited by stage vibration in our apparatus. Notably, various solid states of anisole have been observed and we can control the deposited anisole from crystalline to amorphous state by decreasing the deposition temperature. The critical temperature for forming an amorphous film is 130 K in the vacuum of a microscope chamber. Smooth patterns with a surface roughness of ∼0.7 nm are achieved in the as-deposited amorphous solid anisole. As a proof of principle of 3D fabrication, we finally fabricate nanoscale patterns on exotic silicon micropillars with a high aspect ratio using this resist.

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.

Quantitative measurement of charge accumulation along a quasi-one-dimensional W5O14 nanowire during electron field emission.

We use an electron holographic method to determine the charge distribution along a quasi-one-dimensional W5O14 nanowire during in situ field emission in a transmission electron microscope. The results show that the continuous charge distribution along the nanowire is not linear, but that there is an additional accumulation of charge at its apex. An analytical expression for this additional contribution to the charge distribution is proposed and its effect on the field enhancement factor and emission current is discussed.

A formula for the image intensity of phase objects in Zernike mode.

I present an analytical expression for the image intensity of a phase object visualized in Zernike phase contrast mode. The formula is valid for periodic and non-periodic weak and strong objects, and accounts for the effects of finite illumination. The expression provided is intended as a generalization of the standard reference formula given in the Born and Wolf [Principles of Optics, sixth ed., Pergamon Press, New York, 1980, p. 427] textbook as well as of the formalism employed to evaluate imaging doses in Zernike mode [M. Malac, M. Beleggia, R. Egerton, Y. Zhu, Ultramicroscopy 108 (2008) 126]. I illustrate the usefulness of the improved expression by means of three examples: a sinusoidal phase grating, a Gaussian object, and a phase step. The optimal Zernike phase angle yielding maximum image contrast is found to be object-dependent and not necessarily equal to pi/2. Phase plate optimization criteria are derived and presented for two of the examples considered.

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.