CHAPTER SEVEN

Design, Fabrication, and Applications of In Situ Fluid Cell TEM Dongsheng Li*,1, Michael H. Nielsen†,{, James J. De Yoreo*,1

*Physical Sciences Division, Pacific Northwest National Laboratory, Richland, Washington, USA † Department of Materials Science and Engineering, University of California, Berkeley, California, USA { Materials Science Division, Lawrence Berkeley National Lab, Berkeley, California, USA 1 Corresponding author: e-mail address: [email protected]; [email protected]

Contents 1. Introduction 2. Fluid Cell Design 3. Cell Fabrication and Assembly 4. TEM Operation 5. Effects of the Electron Beam on Reactions in the Fluid Cell 6. Conclusions Acknowledgments References

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Abstract In situ fluid cell TEM is a powerful new tool for understanding dynamic processes during liquid phase chemical reactions, including mineral formation. This technique, which operates in the high vacuum of a TEM chamber, provides information on crystal structure, phase, morphology, size, aggregation/segregation, and crystal growth mechanisms in real time. In situ TEM records both crystal structure and morphology at spatial resolutions down to the atomic level with high temporal resolution of up to 106 s per image, giving it distinct advantages over other in situ techniques such as optical microscopy, AFM, or X-ray scattering or diffraction. This chapter addresses the design, fabrication, and assembly of TEM fluid cells and applications of fluid cell TEM to understanding mechanisms of mineralization.

1. INTRODUCTION In situ techniques are useful for understanding the kinetics and mechanisms of chemical reactions, crystal nucleation and growth, development of morphology, structural transitions, changes in chemical composition and Methods in Enzymology, Volume 532 ISSN 0076-6879 http://dx.doi.org/10.1016/B978-0-12-416617-2.00007-2

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electronic structure, and the formation of defect structures such as vacancies, dislocations, and kinks. In situ experiments provide information that may be missed via ex situ observations, such as a transient step in a reaction process. Moreover, it is often difficult or impossible to infer mechanistic information about material formation processes simply from the final composition, structure, or morphology. Thus, the inherent lack of temporal resolution in ex situ experiments limits their contribution towards understanding dynamic processes in materials. As a consequence, many characterization tools, including optical microscopy, atomic force microscopy (AFM), scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction, and transmission electron microscopy (TEM), have been modified to deliver in situ monitoring of chemical reactions, crystal growth, mechanical properties, phase transformation, and other dynamic processes (Brennan, Fuoss, Kahn, & Kisker, 1990; Hu et al., 2012; Land, DeYoreo, & Lee, 1997; Teng, Dove, Orme, & De Yoreo, 1998; Todorov, Martins, & Viana, 2013; Tsuchiya, Taniwatari, Uomi, Kawano, & Ono, 1993). Each of these in situ methods has its advantages and disadvantages. In situ optical microscopy and spectroscopy provide information on particle size, composition, phase, and morphology during chemical reactions with a resolution in the range of tens to hundreds of nanometer. In situ AFM is useful for imaging particle size and morphological changes in liquids at single digit nanometer lateral resolution and provides 3D surface profiles during reactions with subangstrom resolution. Thus, in situ AFM has been a valuable tool for investigating the nucleation and growth of biomineral phases. However, AFM has three drawbacks. First, conventional AFM requires a minute or two to acquire an image, though the recent development of high-speed AFM promises to reduce that time to a second or less per image in both air and fluids (Ando et al., 2001; Schitter et al., 2007), albeit with some loss of resolution. The second drawback of AFM is that it generally cannot provide crystal structure or phase information. Finally, the processes of interest must occur on a fixed substrate, which can either influence or be incompatible with the process of interest. In situ STM can probe particle and domain sizes and morphology with 0.1 nm lateral resolution and 0.01 nm height resolution while providing information on electronic structure, but it cannot be used on insulating materials, which largely eliminates its utility in biomineralization studies. In situ XPS directly measures changes in elemental composition, chemical state, and electronic state of the elements in the upper 1–10 nm of the surface

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of materials during reactions. However, XPS has traditionally required ultrahigh vacuum (UHV) conditions, and although there are now liquid cells for XPS, the technique has been mostly used for solid-state reactions and has seen only a few applications to studies of biomineralization (Lee, Han, Willey, Nielsen, et al., 2013; Lee, Han, Willey, Wang, et al., 2007). In situ X-ray diffraction is a fast and accurate tool for identifying the structure, phases, and sizes of crystals but requires a large sample volume and provides little information on morphology or small-scale variations in structure. In situ TEM has a number of attributes that make it particularly useful for investigating mineralization. First, it not only provides information on crystal size, phase, and morphology but also can probe transient and dynamic processes like nucleation and growth, morphological evolution, aggregation, segregation, and phase transformation with atomic spatial resolution at video rates. Moreover, the development of dynamic TEM in which the normal thermionic or field emission source is replaced by a photoemission source promises to deliver a temporal resolution of 106 s (Armstrong et al., 2007). Unlike AFM, TEM operation requires UHV conditions (108 to 6 10 Torr). Thus, early applications were carried out in vacuum. In 1986–1998, in situ TEM was used to observe the atomic mechanisms of phase transformation associated with dislocations and stacking faults (Parker, Sigmon, & Sinclair, 1986), interphase boundary dynamics (Howe et al., 1998), nanocrystallite nucleation, and coarsening (Li-Chi & Risbud, 1994) in real time at elevated temperatures by video recording of images. In 1998, Ross from IBM introduced gas phase to in situ TEM experiments and reported real-time imaging of Ge island growth in a UHV TEM equipped with chemical vapor deposition (Ross, Tersoff, Reuter, Legoues, & Tromp, 1998). The extension of TEM to liquid environments came in 2003 when Ross reported a real-time TEM study of the growth of Cu clusters in liquid phase (Williamson, Tromp, Vereecken, Hull, & Ross, 2003). In this work, a sealed fluid cell compatible with UHV was made from Si wafers. The cell had a thin layer of liquid sandwiched between two ultrathin Si3N4 membranes, through which the electron beam passed. This fluid cell was later modified to include electrochemical control (Radisic, Ross, & Searson, 2006; Radisic, Vereecken, Hannon, Searson, & Ross, 2006). Imaging of cells (de Jonge, Peckys, Kremers, & Piston, 2009) and, later, protein structures (Evans et al., 2012; Mirsaidov, Zheng, Casana, & Matsudaira, 2012) was

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reported using similar cell designs. Atomic resolution TEM imaging in fluid cells of this type was reported for the study of oriented attachment between ferrihydrite (Li, Nielsen, Lee, Frandsen, Banfield, & De Yoreo, 2012) and Pt3Fe (Liao, Cui, Whitelam, & Zheng, 2012) nanoparticles. In 2012, a new type of liquid cell based on entrapment of a very thin-liquid film between layers of graphene was introduced (Jong Min et al., 2012). This cell also provided atomic-level resolution imaging. However, image resolution in both types of TEM fluid cells is primarily limited by the thickness of the liquid layer, and, to date, precise control over that thickness has been difficult. Therefore, atomic resolution has not been obtained reproducibly. Since development of the first liquid cell for TEM, cell designs have been modified to include electrochemical and heating control, as well as singlechannel and two-channel flow. With the growing demand for in situ TEM, many manufactures have developed a wide range of TEM holders for various purposes, such as heating, electrical biasing, magnetizing, and fluid flow. Here, we will discuss the design and fabrication of the basic fluid cell made from Si wafers, the addition of electrochemical control and heating, and the application to studies of mineralization reactions.

2. FLUID CELL DESIGN Most silicon-based fluid cell designs are based on Ross’s work at IBM. A schematic side-view diagram of the basic design for an experimental cell used by Li et al. (2012) and Nielsen, Lee, Hu, Han, and De Yoreo (2012) is presented in Fig. 7.1. Each cell is hermetically sealed to isolate the solution from the high vacuum environment of the TEM chamber. The cell is mainly constructed by gluing together two pieces of Si (Fig. 7.1A). The cell contains two solution reservoirs (Fig. 7.1G), which are composed of hollow Si towers (Fig. 7.1D) sealed by glass caps (Fig. 7.1E) to provide large volumes of excess solution that ensure the hydration of the imaging area. In between the two reservoirs, there is an electron transparent window (Fig. 7.1F) constructed from two Si3N4 membranes separated by a spacer (Fig. 7.1C), which provides a gap for a thin layer of solution. In principle, the distance between the two Si3N4 membranes is determined by the thickness of the spacer. This spacer is typically 200–500 nm in thickness but can be increased to accommodate larger biological samples or decreased to achieve the highest possible resolution. However, in reality, the solution thickness under the electron beam can be as large as a many micrometers due to an outward bowing

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Figure 7.1 Schematic cross section of the TEM fluid cell. Components: A, Si wafer with; B, 50–100 nm Si3N4 coating; C, 200–500 nm metal or Si3N4 spacer; D, Si tower; E, glass cap; F, electron transparent Si3N4 window; G, solution reservoir (Li et al., 2012).

of the Si3N4 membranes or very close to zero due to an inward bowing of the membranes. This basic fluid cell design can be modified to provide electrochemical and temperature control to initiate crystal nucleation or other reactions in the microscope. The system consists of a hermetically sealed cell on a custom TEM stage containing electrical feed-throughs for connection to external electronics and is compatible with commercial electron microscopes. Electrochemical control is enabled by incorporation of three electrodes: the reference, working, and counter electrodes as shown in Fig. 7.2a (A, B, and C, respectively). The external portion of the working electrode extends to the edge of the Si bottom chip, which is connected via the electrode feedthrough, and the internal portion extends over the electron transparent Si3N4 window. This design ensures that electrochemically induced processes of interest occur within the field of view of the microscope. In some cases, the effects of an applied potential can be reversed by inverting or removing the voltage, which enables one to reversibly grow and dissolve an inorganic phase and thus image nucleation repeatedly in a single experiment (Radisic, Ross, & Searson, 2006; Radisic, Vereecken, Hannon, et al., 2006; Radisic, Vereecken, Searson, & Ross, 2006). The Au film can also be used for studies of biological materials by providing a platform upon which to deposit self-assembled monolayers of organothiol monomers, which can be used to immobilize biomolecules. For cell heating and temperature control, a platinum resistive heater 100 mm wide by 100–150 nm thick was deposited on the backside of the

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Figure 7.2 Sketches of basic design for electrochemical and thermal control and a photo of the fluid cell holder and fluid cell. (a) Patterning of Au electrodes for electrochemical control showing: reference electrode (A), working electrode (B), and counter electrode (C); (b) Pt resistor pattern for thermal control; (c) photograph of electrochemical cell on custom-made fluid stage made by Hummingbird Scientific (Nielsen et al., 2012).

bottom silicon wafer (Fig. 7.2b). Platinum, copper, and nickel are known to have a unique and repeatable resistance versus temperature relationship over an element-dependent temperature range. Platinum was chosen due to its chemical inertness in air and because it has the most stable resistance– temperature relationship over the largest temperature range. Application of a voltage across the Pt resister results in a temperature rise due to Joule heating. By measuring the change of the resistance while applying this voltage, the temperature can be calculated, monitored, and therefore controlled. Thus, the Pt resistive element serves as both heater and thermometer, though it is important to note that, because the Pt film is on the outside of the lower wafer, it may not give an accurate reading of the temperature in the liquid between the membranes should there be substantial heating by the beam. The resistance and temperature follow a linear relationship between 0 and 100  C given by: RðtÞ  Rð0Þð1 þ aT Þ

ð7:1Þ

where R(T) is the resistance at temperature T in  C, R(0) is the resistance at 0  C, and a is a constant called the temperature coefficient. For pure Pt, a equals 0.003925 1 C between 0 and 100  C. Depending on the Pt purity, the value of a can vary. The value of a for the Pt resistor of the cell is calibrated via a benchtop experiment. This Pt heater design enables

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temperature control between room temperature and about 100  C, which is a temperature range well suited to the study of reactions in aqueous solutions. Because each cell is constructed for an individual set of experiments, additional functionality can be included via modification of the core design. For example, choice of the thickness of the cell for a given model of TEM determines whether chemical mapping via energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy are possible. Recently, an alternative design based on graphene liquid cells (GLCs) was introduced ( Jong Min et al., 2012). A graphene layer is grown on a copper foil substrate via chemical vapor deposition and then directly transferred onto a gold TEM mesh with amorphous carbon film support. The reaction solution is pipetted directly onto two graphene-coated TEM grids facing in opposite directions. Upon wetting, one of the graphene membranes detaches from its associated TEM grid. Due to van der Waals interaction between graphene sheets, the liquid droplets of various thicknesses from 6 to 200 nm can be securely trapped between the double-membrane pocket. While this cell design may give thin liquid layers more consistently, reaction volumes are highly constrained and, unlike, Si-based fluid cells where routine lithographic patterning, Si etching, e-beam evaporating, and liftoff processes make incorporating temperature, electrochemical, flow, or other environmental controls fairly straightforward (Li et al., 2012; Yu, Liu, & Yang, 2013) expansion of the GLC design presents a formidable challenge.

3. CELL FABRICATION AND ASSEMBLY Fluid cells are fabricated using 4 in. silicon wafers 300 mm in thickness. Low-stress silicon nitride membranes 50–100 nm thick are grown on both sides of silicon wafers (e.g., B in Fig. 7.1). Lithographic patterning and KOH etching of these wafers are then used to fabricate arrays of top and bottom halves—or “chips”—of the fluid cell (e.g., A in Fig. 7.1), the solution reservoir access ports on the top Si chips (port below D in Fig. 7.1), and the free-standing Si3N4 membranes that provide the electron transparent windows (e.g., F in Fig. 7.1) on the top and bottom chips. The window size is set as desired. The original design utilized windows that were 200 mm  1000 mm, where the larger dimension was incorporated to accommodate line of sight from the window to an EDS detector. However, due to substantial issues with bowing, the window dimensions were reduced to 50 mm by 50 mm. An alternative design with windows that are narrower,

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but longer, for example, 25 mm  100 mm, and oriented in orthogonal directions on the two wafers would further stiffen the windows while increasing the ease of alignment during assembly. The Si towers are fabricated from Si wafers by lithographic patterning and deep dry Si etching. The lower wafer is coated with a rectangular ring of Si3N4 or a metal that is inert to the reactants to be used in the experiments in order to provide a gap between the upper and lower chips that accommodates the solution layer (C in Fig. 7.1). This spacer is deposited by e-beam evaporation or plasma-enhanced chemical vapor deposition. Different thicknesses of the spacer can be chosen according to experimental needs. The Au electrodes and Pt resistive heaters used for electrochemical and temperature control, as described earlier, are deposited onto the bottom Si chips by lithographic patterning and e-beam evaporative deposition followed by a liftoff process. The bottom and top chips of the liquid cell are plasma cleaned for 1–3 min to make the Si3N4 membrane window hydrophilic for aqueous reaction solutions and then glued together using epoxy (M-Bond 610, SPI supplies Inc.) around the outer edges of the wafers after alignment of the bottom and top windows. The glue is cured at a temperature of 100–150  C, which is reached using a slow ramp upward of approximately 20  C/h from room temperature. The towers that serve as solution reservoirs are aligned with the reservoir access ports on the top chip and glued on using the same epoxy. Others have reported sealing static fluid cells by plasma-activated wafer bonding of the bottom and top chips and by using O-rings for the solution reservoirs (Grogan & Bau, 2010).

4. TEM OPERATION To perform an experiment, an aliquot of a few microliters of solution is loaded into one of the two reservoirs. After the solution is drawn between the two Si3N4 membranes via capillary action, the second reservoir is also filled with the solution. The two reservoirs are then covered with glass caps (E in Fig. 7.1) and sealed using UV curable glue (Norland Opticure 63). The sealed cell-containing reaction solution is then mounted onto the TEM sample holder (a portion of a custom-made holder from Hummingbird Scientific is shown in Fig. 7.2c). The holder is put into a prepump stage to test for leakage before being put into the TEM. In our research, imaging is conducted with a JEOL 2100F TEM at an accelerating voltage of 200 kV. The overall thickness of the cell exceeds the thickness of normal foils or grids.

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However, in this microscope, the holder still can be tilted by 20 in the alpha direction. Video is recorded with the VirtualDub software from the live images in Digital Micrograph (Gatan). We have carried out fluid cell TEM experiments with this combination of cell, holder, and microscope using a variety of solution-based systems, in which the chemical reactions were trigged by electron beam energy, electrochemical control, and cell heating. The electron beam in TEM is known to carry enough energy to damage a sample through heating, electrostatic charging, ionization damage, displacement damage, and hydrocarbon contamination (de Jonge et al., 2009). An electron beam of 200–300 keV has enough energy to trigger some chemical reactions (Li et al., 2012) or reduce metal ions to metal (Zheng, Smith, Young-wook, Kisielowski, Dahmen, & Alivisatos, 2009). Beaminduced heating is due to inelastic scattering between the incoming electrons and particles. The energy transferred in the process ends up as heat, resulting in a locally higher temperature within the specimen. Although the temperature of the sample holder rises only a few K under the electron beam, depending on the material’s thermal conductivity, it is not unusual for the electron beam to melt common thin metal samples under high electron fluxes (Egerton, Li, & Malac, 2004). While beam-induced effects are a problem in many TEM experiments at high incident beam currents, with in situ TEM experiments, we have found we can take advantage of beam-induced initiation of reactions. For example, in the system iron (III) chloride in a potassium dihydrogen phosphate solution, we showed that by exposing the solution to an electron beam for a period of a few seconds to minutes, previously precipitated akaganeite nanorods dissolved and ferrihydrite nanoparticles were produced in their place (Li et al., 2012). We were then able to examine the dynamics of ferrihydrite nanoparticle interaction and attachment. Figure 7.3A presents a sequence of in situ TEM images at atomic resolution showing the details of the ferrihydrite nanoparticle attachment process. This work highlights the unique advantage of in situ TEM in terms of spatial and temporal resolution. Here, it provided information about the dynamics of the direction-specific attachment process—a process that takes place within a few seconds—at lattice resolution. A similar study was performed on metallic nanoparticles using GLCs. As shown in Fig. 7.3B, the thinness of the GLCs enabled atomic resolution of Pt nanoparticle fusion. For both of the GLC- and Si chip-based cells, precise control over the thickness of the fluid layer is critical for reproducible imaging. We noted earlier that the spacers used to fabricate the silicon-based cells are

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Figure 7.3 Examples of fluid cell TEM images of nanoparticle aggregation and attachment from two different fluid cell designs and an illustration of a graphene liquid cell (GLC). (A) Sequence of in situ TEM images with atomic resolution showing the details of the ferrihydrite nanoparticle attachment process. The dashed lines indicate edge dislocations formed at the moment of attachment. These rapidly translate laterally across the boundary and are expelled, leaving behind a defect-free interface. (B) An illustration (top right) of a GLC encapsulating growth solution and TEM stills (bottom right) from a movie of Pt nanocrystal growth via coalescence (Jong Min et al., 2012). All scale bars are 2 nm.

200–500 nm in thickness. Based on our experience and that of others, atomic resolution cannot be obtained unless the fluid thickness is about 100 nm or less. Yet the images in Fig. 7.3 reveal the atomic details of the attachment process. This apparent contradiction is likely due to the fact that the even a slight bowing of the Si3N4 membranes can substantially alter the spacing between the windows. In this case, the atomic resolution indicates

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an inward bending of the two windows. In contrast, we find that when the windows bend outward from each other, the liquid layer under the electron beam can be up to a couple of microns in thickness, resulting in poor resolution. A number of design and fabrication features have been implemented to producing thinner liquid layers. Patterns of SiO2 nanopillars were fabricated and deposited on the bottom chip to define the minimum thickness of the liquid layer (Grogan & Bau, 2010). A plasma wafer-bonding method was used to create a liquid layer of 100 nm (Creemer et al., 2008). However, even with these approaches, the outward bending of the Si3N4 membrane often makes the upper limit of the thickness difficult to control. Consequently, efforts to develop procedures for reproducibly fabricating Si-based cells with ultrathin windows and precisely defined gap sizes below 100 nm are ongoing and appear to offer the greatest promise for widespread application of the technique. Electrochemical control can be very useful in studies of mineralization, because it can be used to alter the pH of the solution, which typically results in changes in mineral solubility. When an electrical bias (1.2 V) is applied to the working electrode of a fluid cell filled with an aqueous solution, the reduction of dissolved molecular oxygen in the solution produces hydroxide ions at the metal/liquid interface according to the following reactions (Tlili, Benamor, Gabrielli, Perrot, & Tribollet, 2003):

This reaction then increases the pH locally near the surface of the electrode. In a solution of a mineral phase like CaCO3, this in turn results in a local decrease in solubility. As long as the calcium and carbonate concentrations are chosen appropriately, the solution can begin in an undersaturated state and become supersaturated only when the voltage is turned on and then only near the electrode. By placing the electrode on the window, this electrochemical trigger ensures that nucleation occurs where and when it can be imaged. We have demonstrated that this approach works in benchtop tests (Nielsen et al., 2012); however, we have not yet implemented this in the TEM. Moreover, we have found that even in the absence of a voltage, the electron beam itself induces nucleation and growth of CaCO3 on the

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Au electrode, giving a clear indication that the beam causes dissociation of H2O. Other applications of electrochemical control over reactions during in situ TEM have been reported. Here, we consider two examples. The first comes from the Ross group’s research on electrochemical nucleation and growth of Cu nanoparticles on Au (Radisic, Vereecken, Searson, et al., 2006) (Fig. 7.4A–E). The high spatial and temporal resolution of in situ

Figure 7.4 Sequences of TEM images from in situ video showing electrochemical nucleation and growth of Cu nanoclusters on Au (Radisic, Vereecken, Hannon, et al., 2006).

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TEM enabled collection of data on the formation and growth of individual Cu nanoclusters, leading to a significant revision of conventional models. The improved understanding provided a more quantitative approach to the electrochemical fabrication of nanoscale structures. The second example comes from the work of Huang et al. (2010) who used this approach to study the failure mechanism during battery charging and discharging by driving electrochemical lithiation of a single SnO2 nanowire electrode (Huang et al., 2010). Figure 7.5 presents a time-lapse sequence showing the morphological evolution of the nanowire during charging at –3.5 V against a LiCoO2 cathode (Huang et al., 2010). Upon charging, a reaction front along the nanowire caused the nanowire to swell, elongate, and spiral. Because lithiation-induced volume expansion,

Figure 7.5 Sequences of TEM images from in situ video showing morphological evolution of a SnO2 nanowire anode during charging (Huang et al., 2010).

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plasticity, and pulverization of electrode materials are the key mechanical factors that affect the performance and lifetime of high-capacity anodes in lithium-ion batteries, real-time observations of these mechanical effects should provide key mechanistic insights for the design of advanced batteries. These examples demonstrate the power of in situ electrochemical control for understanding reaction, nucleation, and growth kinetics in a wide range of materials, including insulating crystals, thin metal films, and semiconducting nanowires. Numerous chemical reactions cannot be triggered electrochemically or by the e-beam, but can be successfully driven thermally. Heating control has been used for some time in the study of solid-state phases and phase transformations (Parker et al., 1986), interphase boundary dynamics (Howe et al., 1998), and nanocrystallite nucleation and coarsening in vapor (Li-Chi & Risbud, 1994). The biggest obstacle to the use of controlled heating with liquid cells is leakage due to increased pressure inside the cell. Even though the cell and heater detailed earlier can easily reach 100  C, in practice, we have found that the temperature must be kept below 60  C when filled with an aqueous solution in order to avoid leakage out of the cell and into the TEM column. Research into different methods for sealing the cell is under way to extend this upper limit on temperature.

5. EFFECTS OF THE ELECTRON BEAM ON REACTIONS IN THE FLUID CELL Whether the electron beam serves as an aid to studying reactions in situ or is only a source for imaging, the potential for deleterious effects is substantial. Most of the effects seen with dry samples (Egerton et al., 2004) are also possible in solution. These include heating, electrostatic charging, ionization damage (radiolysis), displacement damage, sputtering, and hydrocarbon contamination. The extent of radiation damage is proportional to the electron dose, regardless of beam diameter. However, the magnitude of heating and sputtering are likely to be greatly reduced by the presence of the surrounding fluid. On the other hand, the interaction of the high-energy electron beam with the water (if in aqueous solution) will lead to production of free radicals and radiolysis of water produces H, H2, OH, H2O2, and hydrated electrons (Garrett et al., 2005). The presence of these radiolysis products and their effect on both pH and solute speciation probably explains the many observations of nucleation, growth, and dissolution induced by the beam. In heating experiments, these effects may be enhanced due to the high

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temperature. In each case, the only strategy used to date to minimize the damage is to reduce the electron dose by either limiting the time of exposure to the beam through beam shuttering or lowering of the beam current.

6. CONCLUSIONS With the development of in situ TEM techniques, real-time imaging of various kinds of chemical reactions, morphologic development, crystal growth, and chemical and electronic structure development in gas-, liquid-, and solid-state phases is feasible (de Jonge & Ross, 2011). The advantage of in situ TEM over other in situ techniques, such as optical microscopy, AFM, or X-ray scattering or diffraction, is that it records both structure and morphology at spatial resolutions down to the atomic level with high temporal resolution of up to 106 s per image. Thus, fluid cell in situ TEM is a powerful new tool for understanding dynamic processes during liquid phase chemical reactions, including mineral formation. Si- and graphene-based liquid cells have been developed and used to study the mechanisms of nanocrystal nucleation and growth, as well as their aggregation via oriented attachment. The thickness of the liquid layer is the key factor in determining the spatial resolution, with reported thicknesses of Si-based fluid cells ranging from near zero up to a couple of micrometers and those of GLCs ranging from 6 to 200 nm. For both the GLC- and Si-based cells, precise control over that thickness has been difficult. While GLCs provide a narrow thickness range, the advantage of Si-based fluid cells is that incorporating temperature, electrochemical, flow, or other environmental controls is fairly straightforward through routine Si wafer fabrication processes. Electrochemical control has been developed to study electrochemically induced nucleation and growth, as well as failure mechanisms during electrode charging and discharging. Heating control is, in principle, straightforward to implement for liquid phase reactions that require high temperature. However, in practice, leakage has been a problem during heating of siliconbased cells due to the method by which cells are sealed. Modifications of the cell design and methods of sealing are under study to avoid leakage. Reproducible control of the liquid layer thickness in the 10–100 nm range is critical for maximizing the impact of this technique in applications where atomic resolution is important, such as the study of crystal structure development during nucleation. Development of methods for fabricating

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Si-based cells with ultrathin windows and precisely defined gap sizes in this range is currently under way.

ACKNOWLEDGMENTS This research was supported by the US Department of Energy, Office of Basic Energy Sciences (OBES), and by LBNL under contract no. DE-AC02-05CH11231. Development of the TEM fluid cell was supported by the OBES, Division of Chemical, Biological and Geological Sciences; analysis of iron oxide formation was supported by the OBES, Division of Materials Science and Engineering; and cell fabrication and TEM analysis were performed at the Molecular Foundry, LLNL, which is supported by the OBES, Scientific User Facilities Division. M. H. N. acknowledges government support under and awarded by the Department of Defense, the Air Force Office of Scientific Research, and a National Defense Science and Engineering Graduate Fellowship, 32 CFR 168a.

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Design, fabrication, and applications of in situ fluid cell TEM.

In situ fluid cell TEM is a powerful new tool for understanding dynamic processes during liquid phase chemical reactions, including mineral formation...
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