Advanced microanalytical techniques such as high-resolution transmission electron microscopy (HRTEM), atom probe tomography (APT), and synchrotron-based scanning transmission X-ray microscopy (STXM) enable one to characterize the structure and chemical and isotopic compositions of natural materials down towards the atomic scale. Dual focused ion beam-scanning electron microscopy (FIB-SEM) is a powerful tool for site-specific sample preparation and subsequent analysis by TEM, APT, and STXM to the highest energy and spatial resolutions. FIB-SEM also works as a stand-alone technique for three-dimensional (3D) tomography. In this review, we will outline the principles and challenges when using FIB-SEM for the advanced characterization of natural materials in the Earth and Planetary Sciences. More specifically, we aim to highlight the state-of-the-art applications of FIB-SEM using examples including (a) traditional FIB ultrathin sample preparation of small particles in the study of space weathering of lunar soil grains, (b) migration of Pb isotopes in zircons by FIB-based APT, (c) coordinated synchrotron-based STXM characterization of extraterrestrial organic material in carbonaceous chondrite, and finally (d) FIB-based 3D tomography of oil shale pores by slice and view methods. Dual beam FIB-SEM is a powerful analytical platform, the scope of which, for technological development and adaptation, is vast and exciting in the field of Earth and Planetary Sciences. For example, dual beam FIB-SEM will be a vital technique for the characterization of fine-grained asteroid and lunar samples returned to the Earth in the near future.
Most physical, chemical, and biological processes on Earth involve the interaction of naturally occurring materials at the macroscopic, submicron to nanoscopic scale. Beyond the Earth, unique astrophysical processes are recorded in planetary materials by, for instance, high pressure impact-related minerals [
In recent decades, advances in microbeam analytical technology, such as micro-Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), atom-probe tomography (APT), and scanning transmission X-ray microscopy (STXM), enable one to accurately determine the morphology, crystal structure, and elemental, organic functional chemical and isotopic compositions of natural and synthetic materials at the micro-, nano-, or even the atomic scale [
The concept of the focused ion beam technique using a liquid Ga ion source was originally developed by Seliger and Fleming in 1974 [
Since then, FIB-SEM has become a diverse multifunctional tool for revealing structural, elemental, and isotopic chemical information in natural materials down to the nanometer scale. However, for their effective characterization, analytical methods require adaptation when using the dual beam FIB-SEM system. This review addresses the advanced use of FIB-SEM for a range of application in the natural sciences: from TEM analysis of small lunar soil grains, APT analysis of Pb isotopes in Earth-based zircons, and 3D slice and view of Earth-based shale kerogen to the characterization of extraterrestrial organics by coordinated synchrotron-based STXM-TEM. In addition, we address the key adaptations required in the FIB-SEM analytical methodologies for these different applications in Earth and Planetary Sciences. Finally, we assess the future development of the dual beam FIB-SEM technique.
A schematic of the FIB-SEM system is illustrated in Figure
Schematic illustration of the FIB-SEM system. GIS: gas injection system.
The interaction between Ga+ ions and the target material can offer imaging, milling, and deposition. These are the generic processes performed in the operation of the dual FIB-SEM system. Specific adaptations to the FIB sample preparation method however need to be tailored for the preparation of samples specific to particular analytical techniques (Section
Similar to a typical SEM, the incident Ga+ beam can produce high-resolution secondary electron (SE) images with enhanced contrast of a sample surface, as illustrated in Figure
Principle of FIB (a) imaging, (b) milling, and (c) deposition (modified from [
Samples can be microscopically removed (also called sputtering) by ion-atom collisions [
Unlike milling, the FIB-SEM system can offer precise, localized deposition of materials by using a gas injection system (GIS). In this process, the metal-containing organic compound is heated to a gas that flows out of a narrow tube above the sample surface. When ions or electrons scan a selected region, the metallic components of the precursor will be deposited on the surface of substrate [
Energy-dispersive X-ray spectroscopy (EDS) is the most wildly used method for measuring major elements (0.1 at %) in a sample, and its elemental detection range is 4Be~92 U [
It is well known that TEM has the capability to image the structure and chemistry of materials at the nanometer scale. Spherical aberration (Cs) correction techniques can extend TEM imaging to the sub-Angstrom scale. The high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and electron energy loss spectroscopy with the energy resolution better than 10 meV have been used in condensed matter physics and material science [
Prior to the use of FIB-SEM, TEM foils of terrestrial rocks and extraterrestrial materials were prepared using ultramicrotome and argon ion milling. The ultramicrotome is widely used in the preparation of biological samples. Examples of its application are for the characterization of acid mine drainage sediments [
The FIB and TEM techniques has been a routine analytical technique for terrestrial samples, such as ore minerals, natural diamonds, high-pressure experiment products, and microfossils [
The FIB-SEM is also readily used for the preparation of TEM samples in the study of extraterrestrial materials, such as presolar grains, high pressure mineral phases, and other meteorite components [
Microstructure and elemental mapping of a lunar pyroxene grain (Px) and adhering particles. (a) HAADF-STEM image of the sample. Platinum has filled between fragments and the pyroxene grain. Feldspathic fragment (Fel) and a small ilmenite (Ilm) grain were also found on the surface of pyroxene. (b) High magnification HAADF image of the Mg-Fe silicate fragment, showing the space weathered features. Dark and light particles are observed. (c–i) EDS element mapping of Si, Fe, Al, Ca, Mg, and O, suggesting that the dark particles in (b) are silicon oxide and the light particles are nanophase Fe. Reprinted with permission from Wiley and modified from [
It should be noted that the high-energy ion beam will damage the sample surface, forming an amorphous layer. Previous studies have shown that accelerating voltage is a major factor affecting the thickness of amorphous layer [
Atom probe tomography is a kind of field ion microscopy (FIM). A pulsed voltage is applied to the needle-shaped tip sample, breaking atomic bonds at the surface. The evaporated ions are projected onto a position-sensitive detector by a strong electrostatic field. The measurement of the ion flight times, from laser pulse to detector impact, allows the ion identities to be determined by time-of-flight mass spectrometry [
Needle-shaped specimen prepared by FIB-SEM technique.
When applied to geological samples, which are commonly insulating, APT is usually operated in a “pulsed laser” mode that promotes field evaporation [
Zircon is one of the best minerals for U-Pb dating due to its high physical and chemical stability. The oxygen isotopic composition of ancient zircons is useful for inferring the formation time of the hydrosphere and the habitability of the early Earth [
APT images of Y and Pb clusters in the 4.4-Gyr-oldzircon (reprinted with permission from Springer Nature [
Synchrotron-based Scanning Transmission X-ray Microscopy (STXM) takes images of X-ray transparent samples via the raster of a stage behind a fixed highly focused X-ray beam down to the nanometer scale [
Organic matter is found in a wide range of planetary materials such as chondrites, interplanetary dust particles (IDPs), Martian meteorites, micrometeorites, and comets. The morphology, molecular composition, and distribution of OM can better constrain its hydrothermal evolution in the early solar system [
Analysis of organic matter in carbonaceous chondrites by FIB-STXM-TEM. (a) Showing the preparation of FIB foil from a chip of chondrite matrix. (b) A STXM map showing the distribution of a spectral population characteristic of aromatic-carbonyl-carboxyl/ester bearing macromolecular insoluble organic matter (bright blue features). (c) XANES spectra of organic particles 1 and 2 in (b) and diffuse spectra (fainter blue). (d) The coordinated STEM dark field image of (b) displaying the setting of the organic particles (dark grey). Note the TEM-BF images of particles 1 and 2. Particle 1 occurs as beads, and particle 2 is a vein filling the matrix. Reprinted with permission from Wiley and modified from [
The microscopic features of rocks in 2D can be well analyzed using scanning electron microscopy. However, 3D is crucial for understanding the true distribution of phases in samples. In the dual beam system, the Ga+ beam can be used for serial milling of the sample, and at the same time, a sequence of SEM cross-sectional images can be obtained. Then, segmentation and 3D visualization of the sample can be rendered using commercial software (e.g., Avizo) [
Taking oil shale as an example, characterization and modeling of the pores’ shape and connectivity provide essential information on the permeability and gas accumulation space [
A typical 3D structure of shale is shown in Figure
A typical 3D structure of shale sample from Hunan Province, China. (a, b) Three-phase segmentation of mineral (blue), organic matter (green), and pore (red). (c, d) The visualization of segmented pores and organic matter.
The volume of FIB 3D tomography is usually about ~1000
TEM for small particles, APT, STXM, and 3D tomography all have to apply the FIB technique in different ways for the successful preparation of samples and the minimization of sample damage. Ultimately, FIB-SEM is a partially destructive technique and the interaction of Ga+ ions on various materials can alter their properties. It is therefore vital to prepare samples using methods minimizing the alteration of the material to be characterized. This is arguably even more important for labile, soft, and low Z number materials such as organic material that are susceptible to the remobilization of their functional groups by Ga+ interactions. Here, we summarize variations on the conditions and give specific suggestions required for the different applications using FIB for either TEM, APT, 3D tomography, or organic-based STXM analyses (Table
FIB as an independent instrument or combined with other techniques.
Synergistic techniques | Features | Target materials | Conditions and suggestions | References |
---|---|---|---|---|
Stand-alone FIB-SEM | Cross-section imaging or 3D tomography integrated with multiple detectors | Clay minerals and oil shales (this study) | (1) Medium current to balance milling speed and minimized curtain effect | [ |
FIB and TEM | Microstructural and crystallographic characterization | Earth & planetary materials such as ore minerals, high pressure phases and extraterrestrial materials | (1) Stepping down the polishing currents to 10~50 pA/5 kV or even lower | [ |
FIB and APT | 3D chemical and isotopic information | U-Th-Pb isotope systems & trace element compositions in zircon, monazite etc. | (1) Pt/Au-coated as protective layer—evaporation field of carbon is too high | [ |
FIB and synchrotron techniques | Elemental mapping | Terrestrial shale kerogen organic carbon & extraterrestrial organic matter in planetary materials, e.g., carbonaceous chondrites. | (1) None carbon capping. Ideally no EXPOXY embedding | [ |
As discussed above, although the FIB-SEM dual-beam system has been essential and widely used on natural materials, there are still some technical limitations: (1) high-energy Ga+ ion beam implantation causes damage during sample preparation and (2) cross-section milling in FIB-SEM 3D tomography often has the “curtain” effect, which affects the imaging resolution and causes reconstruction artifacts [
Higher ion beam currents for improved signal/noise and higher spatial resolutions are common goals in microscopy techniques. As mentioned above, most FIB-SEM ion beam systems use a liquid Ga ion source. The ion beam current of commercial FIB system is less than 100 nA, and the best resolution is about 2.5 nm at 30 kV, as shown in Table
Overview of ion beam current and resolution in commercial FIB systems.
FIB | Beam current | Resolution |
---|---|---|
Ga | 0.1-100 nA | ~3 nm |
He | 0.1-100 pA | ~0.5 nm |
Ne | 0.1-100 pA | ~1.9 nm |
Xe | 0.1-2 | <20 nm |
Xe+ plasma FIB microscopy (PFIB) has a slightly lower resolution, but its beam current is about 20 times as large as the Ga ion beam. Higher beam current means higher sputtering rates and larger analytical volumes at the same time. This PFIB system has been employed in semiconductor industry for milling materials at hundreds of micron length scale. In addition, previous studies have shown that the depth of damage caused by the Xe+ plasma FIB is 20~40% less than that measured from a Ga+ FIB-prepared specimen [
Compared with using FIB-SEM as a stand-alone instrument, synergistic techniques show unique advantages that enable us to acquire more information from one sample and unravel the complex natural processes. This has been used to reveal shock-induced trace element segregation [
Additionally, with the use of the sample preparation technique of FIB-SEM system, mass spectroscopy techniques such as thermal ionization mass spectrometry (TIMS) and quadrupole inductively coupled plasma mass spectrometry (Q-ICP-MS) could offer some level of spatial resolution that was not typically available [
FIB-SEM is a powerful tool for site-specific sample preparation of TEM ultrathin foils and APT needle-shaped samples of natural and synthetic materials and also can work as a stand-alone technique for three-dimensional (3D) tomography. This technique has been widely used to study the microstructure, chemical and isotopic compositions of terrestrial rocks, extraterrestrial materials, paleontology, and marine sciences from the micron scale down to the single-atom resolution, through combining with multiple detectors and other in situ analytical instruments, including TEM, APT, and synchrotron-based STXM as examples FIB methodologies need to be tailored for different purposes, whether it is small particle analysis, atomic probe tomography, synchrotron-based scanning transmission X-ray microscopy, or 3D FIB-SEM tomography Multiple ion beam microscopy and some new technologies that have emerged recently are expected to be used to study the Earth and extraterrestrial materials for better revealing complex natural processes
The authors declare that they have no conflict of interest.
We are grateful to Jinhua Li and Yangting Lin for their suggestions on the article. The authors would like to thank Zhang Yuxing of Multi-Scale Imaging and Characterization Laboratory and Han Chen of Nanjing University of Science and Technology for their help in 3D tomography and APT experiment, respectively. This work was supported by the Instrument Function Developing Project of the Chinese Academy of Sciences (IGG201901).