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Direct write with a liquid precursor using an ion beam in situ, allows fabrication of nanostructures with higher purity than using gas phase deposition. Specifically, positively charged helium ions, when compared to electrons, localize the reaction zone to a single-digit nanometer scale. However, to control the interaction of the ion beam with the liquid precursor, as well as enable single digit fabrication, a comprehensive understanding of the radiolytic process, and the role of secondary electrons has to be developed. Here, we demonstrate an approach for directly writing platinum nanostructures from aqueous solution using a helium ion microscope, and discuss possible mechanisms for the beam-induced particle growth in the framework of Born-Oppenheimer and real-time electron dynamics models. We illustrate the nanoparticle nucleation and growth parameters through data analysis of in situ acquired movie data, and correlate these results to a fully encompassing, time-dependent, quantum dynamical simulation that takes into account both quantum and classical interactions. Finally, sub-15 nm resolution platinum structures generated in liquid are demonstrated.

Rapid advances in nanoscience rely on continuous improvements of material manipulation at near-atomic scales. Currently, the workhorse of nanofabrication is resist-based lithography and its various derivatives. However, the use of local electron, ion, and physical probe methods is expanding, driven largely by the need for fabrication without the multistep preparation processes that can result in contamination from resists and solvents. Furthermore, probe-based methods extend beyond nanofabrication to nanomanipulation and to imaging which are all vital for a rapid transition to the prototyping and testing of devices. In this work we study helium ion interactions with the surface of bulk copper indium thiophosphate CuM(III)P2X6 (M = Cr, In; X= S, Se), a novel layered 2D material, with a Helium Ion Microscope (HIM). Using this technique, we are able to control ferrielectric domains and grow conical nanostructures with enhanced conductivity whose material volumes scale with the beam dosage. Compared to the copper indium thiophosphate (CITP) from which they grow, the nanostructures are oxygen rich, sulfur poor, and with virtually unchanged copper concentration as confirmed by energy-dispersive X-ray spectroscopy (EDX). Scanning electron microscopy (SEM) imaging contrast as well as scanning microwave microscopy (SMM) measurements suggest enhanced conductivity in the formed particles, whereas atomic force microscopy (AFM) measurements indicate that the produced structures have lower dissipation and are softer as compared to the CITP.

Serial block-face electron microscopy (SBEM) provides nanoscale 3D ultrastructure of embedded and stained cells and tissues in volumes of up to 107 µm3. In SBEM, electrons with 1–3 keV energies are incident on a specimen block, from which backscattered electron (BSE) images are collected with x, y resolution of 5–10 nm in the block-face plane, and successive layers are removed by an in situ ultramicrotome. Spatial resolution along the z-direction, however, is limited to around 25 nm by the minimum cutting thickness. To improve the z-resolution, we have extracted depth information from BSE images acquired at dual primary beam energies, using Monte Carlo simulations of electron scattering. The relationship between depth of stain and ratio of dual-energy BSE intensities enables us to determine 3D structure with a ×2 improvement in z-resolution. We demonstrate the technique by sub-slice imaging of hepatocyte membranes in liver tissue.

“High resolution SEM imaging” refers to the capability of discerning fine-scale spatial features of a specimen. Such features may be free-standing objects or structures embedded in a matrix. The definition of “fine-scale” depends on the application, which may involve sub-nanometer features in the most extreme cases. The most important factor determining the limit of spatial resolution is the footprint of the incident beam as it enters the specimen. Depending on the level of performance of the electron optics, the limiting beam diameter can be as small as 1 nm or even finer. However, the ultimate resolution performance is likely to be substantially poorer than the beam footprint and will be determined by one or more of several additional factors: (1) delocalization of the imaging signal, which consists of secondary electrons and/or backscattered electrons, due to the physics of the beam electron specimen interactions; (2) constraints imposed on the beam size needed to satisfy the Threshold Equation to establish the visibility for the contrast produced by the features of interest; (3) mechanical stability of the SEM; (4) mechanical stability of the specimen mounting; (5) the vacuum environment and specimen cleanliness necessary to avoid contamination of the specimen; (6) degradation of the specimen due to radiation damage; and (7) stray electromagnetic fields in the SEM environment. Recognizing these factors and minimizing or eliminating their impact is critical to achieving optimum high resolution imaging performance. Because achieving satisfactory high resolution SEM often involves operating at the performance limit of the instrument as well as the technique, the experience may vary from one specimen type to another, with different limiting factors manifesting themselves in different situations. Most importantly, because of the limitations on feature visibility imposed by the Threshold Current/Contrast Equation, for a given choice of operating conditions, there will always be a level of feature contrast below which specimen features will not be visible. Thus, there is always a possible “now you see it, now you don’t” experience lurking when we seek to operate at the limit of the SEM performance envelope.

A k-ratio is the ratio of a pair of characteristic X-ray line intensities, I, measured under similar experimental conditions for the unknown (unk) and standard (std): $$ k={I}_{unk}/{I}_{std} $$ The measured intensities can be associated with a single characteristic X-ray line (as is typically the case for wavelength spectrometers) or associated with a family of characteristic X-ray lines (as is typically the case for energy dispersive spectrometers.) The numerator of the k-ratio is typically the intensity measured from an unknown sample and the denominator is typically the intensity measured from a standard material—a material of known composition.

The incident beam energy, E0, is the parameter that determines which characteristic X-rays can be excited: the beam energy must exceed the critical excitation energy, Ec, for an atomic shell to initiate ionization and subsequent emission of characteristic X-rays. This dependence is parameterized with the “overvoltage” U0, defined as $$ {U}_0={E}_0/{E}_c $$ U0 must exceed unity for X-ray emission. The intensity, Ich, of characteristic X-ray generation follows an exponential relation: $$ {I}_{ch}={i}_Ba{\left({U}_0-1\right)}^n $$ where iB is the beam current, a and n are constants, with 1.5 ≤ n ≤ 2.

As illustrated in Fig. 16.1, the physical basis of energy dispersive X-ray spectrometry (EDS) with a semiconductor detector begins with photoelectric absorption of an X-ray photon in the active volume of the semiconductor (Si). The entire energy of the photon is transferred to a bound inner shell atomic electron, which is ejected with kinetic energy equal to the photon energy minus the shell ionization energy (binding energy), 1.838 keV for the Si K-shell and 0.098 keV for the Si L-shell. The ejected photoelectron undergoes inelastic scattering within the Si crystal. One of the consequences of the energy loss is the promotion of bound outer shell valence electrons to the conduction band of the semiconductor, leaving behind positively charged “holes” in the valence band. In the conduction band, the free electrons can move in response to a potential applied between the entrance surface electrode and the back surface electrode across the thickness of the Si crystal, while the positive holes in the conduction band drift in the opposite direction, resulting in the collection of electrons at the anode on the back surface of the EDS detector. This charge generation process requires approximately 3.6 eV per electron hole pair, so that the number of charge carriers is proportional to the original photon energy, Ep: $$ n={E}_p/3.6\ eV $$ For a Mn K-L3 photon with an energy of 5.895 keV, approximately 1638 electron–hole pairs are created, comprising a charge of 2.6 × 10−16 coulombs. Because the detector can respond to any photon energy from a threshold of approximately 50 eV to 30 keV or more, the process has been named “energy dispersive,” although in the spectrometry sense there is no actual dispersion such as occurs in a diffraction element spectrometer.

When studying the topographic features of a specimen, the microscopist has several useful software tools available. Qualitative stereomicroscopy provides a composite view from two images of the same area, prepared with different tilts relative to the optic axis, that gives a visual sensation of the specimen topography, as shown for a fractured galena crystal using the anaglyph method in Fig. 15.1 (software: Anaglyph Maker). The “3D Viewer” plugin in ImageJ-Fiji can take the same members of the stereo pair and render the three-dimensional surface, as shown in Fig. 15.2, which can then be rotated to “view” the surface from different orientations (Fig. 15.3).

Information in SEM images about specimen properties is conveyed when contrast in the backscattered and/or secondary electron signals is created by differences in the interaction of the beam electrons between a specimen feature and its surroundings. The resulting differences in the backscattered and secondary electron signals (S) convey information about specimen properties through a variety of contrast mechanisms. Contrast (Ctr) is defined as $$ {C}_{\mathrm{tr}}=\left({S}_{\mathrm{max}}-{S}_{\mathrm{min}}\right)/{S}_{\mathrm{max}} $$ where is Smax is the larger of the signals. By this definition, 0 ≤ Ctr ≤ 1.

«Trace analysis” refers to the measurement of constituents presents at low fractional levels. For SEM/EDS the following arbitrary but practical definitions have been chosen to designate various constituent classes according to these mass concentration (C) ranges

Before attempting automatic or manual peak identification, it is critical that the EDS system be properly calibrated to ensure that accurate energy values are measured for the characteristic X-ray peaks. Follow the vendor’s recommended procedure to rigorously establish the calibration. The calibration procedure typically involves measuring a known material such as copper that provides characteristic X-ray peaks at low photon energy (e.g., Cu L3-M5 at 0.928 keV) and at high photon energy (Cu K-L3 at 8.040 keV). Alternatively, a composite aluminum-copper target (e.g., a copper penny partially wrapped in aluminum foil and continuously scanned so as to excite both Al and Cu) can be used to provide the Al K-L3 (1.487 keV) as the low energy peak and Cu K-L3 for the high energy peak. After calibration, peaks occurring within this energy range (e.g., Ti K-L3 at 4.508 keV and Fe K-L3 at 6.400 keV) should be measured to confirm linearity. A well-calibrated EDS should produce measured photon energies within ±2.5 eV of the ideal value. Low photon energy peaks below 1 keV photon energy should also be measured, for example, O K (e.g., from MgO) and C K. For some EDS systems, non-linearity may be encountered in the low photon energy range. Figure 18.1 shows an EDS spectrum for CaCO3 in which the O K peak at 0.523 keV is found at the correct energy, but the C K peak at 0.282 keV shows a significant deviation below the correct energy due to non-linear response in this range.

The conventional SEM must operate with a pressure in the sample chamber below ~10−4 Pa (~10−6 torr), a condition determined by the need to satisfy four key instrumental operating conditions

Background: As part of a study into the in-service failure of the bearing surface of a large water pump, characterization was requested of the hard-facing alloy, which was observed to have separated from the stainless steel substrate, causing the failure.

Electron beams have made possible the development of the versatile, high performance electron microscopes described in the earlier chapters of this book. Techniques for the generation and application of electron beams are now well documented and understood, and a wide variety of images and data can be produced using readily available instruments. While the scanning electron microscope (SEM) is the most widely used tool for high performance imaging and microanalysis, it is not the only option and may not even always be the best instrument to choose to solve a particular problem. In this chapter we will discuss how, by replacing the beam of electrons with a beam of ions, it is possible to produce a high performance microscope which resembles an SEM in many respects and shares some of its capabilities but which also offers additional and important modes of operation.

By selecting the operating parameters of the SEM electron gun, lenses, and apertures, the microscopist controls the characteristics of the focused beam that reaches the specimen surface: energy (typically selected in the range 0.1–30 keV), diameter (0.5 nm to 1 μm or larger), beam current (1 pA to 1 μA), and convergence angle (semi-cone angle 0.001–0.05 rad). In a conventional high vacuum SEM (typically with the column and specimen chamber pressures reduced below 10−3 Pa), the residual atom density is so low that the beam electrons are statistically unlikely to encounter any atoms of the residual gas along the flight path from the electron source to the specimen, a distance of approximately 25 cm.

While amorphous substances such as glass are encountered both in natural and artificial materials, most inorganic materials are found to be crystalline on some scale, ranging from sub-nanometer to centimeter or larger. A crystal consists of a regular arrangement of atoms, the so-called «unit cell,» which is repeated in a two- or three-dimensional pattern. In the previous discussion of electron beam–specimen interactions, the crystal structure of the target was not considered as a variable in the electron range equation or in the Monte Carlo electron trajectory simulation. To a first order, the crystal structure does not have a strong effect on the electron–specimen interactions. However, through the phenomenon of channeling of charged particles through the crystal lattice, crystal orientation can cause small perturbations in the total electron backscattering coefficient that can be utilized to image crystallographic microstructure through the mechanism designated «electron channeling contrast,» also referred to as «orientation contrast» (Newbury et al. 1986). The characteristics of a crystal (e.g., interplanar angles and spacings) and its relative orientation can be determined through diffraction of the high-energy backscattered electrons (BSE) to form «electron backscatter diffraction patterns (EBSD).

Software is an essential tool for the scanning electron microscopist and X-ray microanalyst (SEMXM). In the past, software was an important optional means of augmenting the electron microscope and X-ray spectrometer, permitting powerful additional analysis and enabling new characterization methods that were not possible with bare instrumentation. Today, however, it is simply not possible to function as an SEMXM practitioner without using at least a minimal amount of software. A graphical user interface (GUI) is an integral part of how the operator controls the hardware on most modern microscopes, and in some cases it is the only interface. Even many seemingly analog controls such as focus knobs, magnification knobs, or stigmators are actually digital interfaces mounted on hand-panel controllers that connect to the microscope control computer via a USB interface.

Reading about a new subject is good but there is nothing like doing to reinforce understanding. With this in mind, the authors of this textbook have designed a number of practical exercises that reinforce the book’s subject matter. Some of these exercises can be performed with software you have available to you—either instrument vendor software or a spreadsheet like MS Excel or LibreOffice/OpenOffice Calc. Other exercises require functionality which may not be present in all instrument vendor’s software. Regardless, it is much easier to explain an exercise when everyone is working with the same tools.

The use of focused ion beams (FIB) in the field of electron microscopy for the preparation of site specific samples and for imaging has become very common. Site specific sample preparation of cross-section samples is probably the most common use of the focused ion beam tools, although there are uses for imaging with secondary electrons produced by the ion beam. These tools are generally referred to as FIB tools, but this name covers a large range of actual tools. There are single beam FIB tools which consist of the FIB column on a chamber and also the FIB/SEM platforms that include both a FIB column for sample preparation and an SEM column for observing the sample during preparation and for analyzing the sample post-preparation using all of the imaging modalities and analytical tools available on a standard SEM column. A vast majority of the FIB tools presently in use are equipped with liquid metal ion sources (LMIS) and the most common ion species used is Ga. Recent developments have produced plasma sources for high current ion beams. The gas field ion source (GFIS) is discussed in module 31 on helium ion microscopy in this book.

This module discusses the procedure used to perform a rigorous quantitative elemental microanalysis by SEM/EDS following the k-ratio/matrix correction protocol using the NIST DTSA-II software engine for bulk specimens. Bulk specimens have dimensions that are sufficiently large to contain the full range of the direct electron-excited X-ray production (typically 0.5–10 μm) as well as the range of secondary X-ray fluorescence induced by the propagation of the characteristic and continuum X-rays (typically 10–100 μm).

A conducting or semiconducting specimen must maintain good contact with electrical ground to dissipate the injected beam current. Without such an electrical path, even a highly conducting specimen such as a metal will show charging artifacts, in the extreme case acting as an electron mirror and reflecting the beam off the specimen. A typical strategy is to use an adhesive such as double-sided conducting tape to both grip the specimen to a support, for example, a stub or a planchet, as well as to make the necessary electrical path connection. Note that some adhesives may only be suitable for low magnification (scanned field dimensions greater than 100 × 100 μm, nominally less than 1,000× magnification) and intermediate magnification (scanned field dimensions between 100 μm x 100 μm, nominally less than 1,000X magnification and 10 μm × 10 μm, nominally less than 10,000× magnification) due to dimensional changes which may occur as the adhesive outgases in the SEM leading to image instability such as drift. Good practice is to adequately outgas the mounted specimen in the SEM airlock or a separate vacuum system to minimize contamination in the SEM as well as to minimize further dimensional shrinkage. Note that some adhesive media are also subject to dimensional change due to electron radiation damage during imaging, which can also lead to image drift.