Name: nanoscale characterization of liquid-solid interfaces by coupling cryo-focused ion beam milling with scanning electron microscopy and spectroscopy
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Description: Watch this Scientific Journal Video about Nanoscale Characterization of Liquid-Solid Interfaces by Coupling Cryo-Focused Ion Beam Milling with Scanning Electron Microscopy and Spectroscopy at JoVE.com

We greatly acknowledge the contributions by Shuang-Yan Lang and H&#233;ctor D. Abru&#241;a who provided samples for our research. This work was supported by the National Science Foundation (NSF) (DMR-1654596) and made use of the Cornell Center for Materials Research Facilities supported by the NSF under Award Number DMR-1719875.

1. Prepare the sample and transfer into the SEM chamber
<ol>
	<li>Set up the microscope
	<ol>
		<li>For systems that convert between room temperature and cryogenic equipment, install the cryo-SEM stage and anticontaminator according to the equipment manufacturer&#8217;s instructions and evacuate the SEM chamber.</li>
		<li>Adjust the gas injection system (GIS) platinum source so that when inserted it sits approximately 5 mm further away from the sample surface compared to typical room temperature experiments. This posi...

<ol>
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The cryogenic preparation method described here is important and must be done correctly for the chemistry and morphology to be preserved8. The foremost concern is freezing the sample quickly since this is what allows the liquid to be vitrified8. If the sample cools too slowly, liquids may crystalize resulting in a change in morphology6. To prevent crystallization, slush nitrogen is used in this procedure, as it reduces the Leidenfrost effect and acce...

Interfaces between solids and liquids play a vital role in the function of energy materials such as batteries, fuel cells, and supercapacitors1,2,3. While characterizing the chemistry and morphology of these interfaces could play a central role in improving functional devices, doing so has presented a substantial challenge1,3,4. Liquids are incompatible with the high vacuum environments needed for many common characterization techniques, such as x-ray photoemission spectroscopy, scanning electron microscopy (SEM) and transmission electron microscopy2. Historically, the solution has been to remove the liquid from the device, but this comes at the expense of potentially damaging delicate structures at the interface2,4 or modifying morphology3. In the case of batteries, especially those which employ highly reactive alkali metals, this physical damage is compounded by chemical degradation upon exposure to air5.
This paper describes cryo-SEM and focused ion beam (FIB) as a method for preserving and characterizing solid-liquid interfaces. Similar methods have been shown to preserve the structure of cells in biological samples6,7,8, energy devices5,9,10,11,12 and nanoscale corrosion reactions13,14,15. The crux of the technique is to vitrify the sample via plunge freezing in slush nitrogen prior to transfer into the microscope where it is placed onto a cryogenically cooled stage. Vitrification stabilizes the liquid in the vacuum of the microscope while avoiding the structural deformations associated with crystallization6,8. Once in the microscope, a dual beam system allows nanoscale imaging with the electron beam, and preparation of cross-sections with the focused ion beam. Lastly, chemical characterization is enabled via Energy Dispersive X-ray (EDX) mapping. Altogether, cryo-SEM/FIB can preserve the native structure of a solid-liquid interface, create cross-sections, and provide both chemical and morphological characterization.
In addition to providing a general workflow for cryo-SEM and EDX mapping, this paper will describe a number of methods to mitigate artifacts from milling and imaging. Often vitrified liquids are delicate and insulating, making them prone to charging as well as beam damage8. While a number of techniques have been established to reduce these unwanted effects in specimens at room-temperature16,17,18, several have been modified for cryogenic applications. In particular, this procedure details application of conductive coatings, first a gold-palladium alloy, followed by a thicker platinum layer. Additionally, instructions are provided to help users identify charging when it occurs and adjust the electron beam conditions to mitigate the accumulation of charge. Lastly, although beam damage has many characteristics in common with charging, the two can occur independent of one another16, and guidelines are provided for minimizing beam damage during the steps where it is most likely.
While dual-beam SEM/FIB is not the only electron microscopy tool to have been adapted for cryogenic operation, it is particularly well-suited for this work. Often realistic devices like a battery are on the scale of several centimeters in size, while many of the features of interest are on the order of microns to nanometers, and the most meaningful information can be contained in the cross-section of the interface4,5,19. Although techniques like Scanning Transmission Electron Microscopy (STEM) combined with Electron Energy Loss Spectroscopy (EELS) enable imaging and chemical mapping down to the atomic scale, they require extensive preparation to make the sample sufficiently thin to be electron transparent, dramatically limiting throughput3,4,19,20,21,22. Cryo-SEM, by contrast, allows for the rapid probing of interfaces in macroscopic devices, such as the anode of a lithium metal battery coin cell, albeit at a lower resolution of tens of nanometers. Ideally, a combined approach that leverages the advantages of both techniques is applied. Here, we focus on higher throughput cryogenic FIB/SEM techniques.
Lithium metal batteries were used as the primary test case for this work, and they demonstrate the broad utility of cryo-SEM techniques: they feature delicate structures of scientific interest4,5,9,10,11,12, have broadly varying chemistry to be revealed via EDX2, and cryogenic techniques are required to preserve the reactive lithium5,21. In particular, the uneven lithium deposits known as dendrites, as well as the interfaces with the liquid electrolyte are preserved and can be imaged and mapped with EDX4,5,12. Additionally, lithium typically would oxidize during preparation and form an alloy with gallium during milling, but the preserved electrolyte prevents oxidation and cryogenic temperatures mitigate reactions with gallium5. Many other systems (energy devices especially) feature similarly delicate structures, complex chemistries and reactive materials, so the success of cryo-SEM on the study of lithium metal batteries can be considered a promising indication that it is suitable for other materials as well.
The protocol uses a dual-beam FIB/SEM system fitted with a cryogenic stage, a cryogenic preparation chamber and a cryogenic transfer system, as detailed in the&#160;Table of Materials. For preparing the cryo-immobilized samples there is a workstation with a &#34;slush pot,&#34; which is a foam insulated pot that sits in a vacuum chamber in the station. The foam insulated dual pot slusher contains a primary nitrogen chamber and a secondary chamber which surrounds the former and reduces boiling in the main part of the pot. Once filled with nitrogen, a lid is placed over the pot and the whole system can be evacuated to form slush nitrogen. A transfer system featuring a small vacuum chamber is used to transfer the sample under vacuum to the preparation or &#34;prep&#34; chamber of the microscope. In the prep chamber the sample can be kept at -175 &#176;C and sputter coated with a conductive layer, such as a gold-palladium alloy. Both the prep chamber and the SEM chamber feature a cryogenically cooled stage for holding the sample, and an anticontaminator to adsorb contaminants and to prevent ice buildup on the specimen. The whole system is cooled with nitrogen gas that flows through a heat exchanger submerged in liquid nitrogen, and then through the two cryo-stages and two anticontaminators of the system.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>INCA EDS</td>
			<td>Oxford instruments</td>
			<td></td>
			<td>Control software for X-max 80</td>
		</tr>
		<tr>
			<td>PP3010T Cryo-preparation system</td>
			<td>Quorum Technologies, Inc.</td>
			<td></td>
			<td>FIB/SEM cryogenic preparation system. Includes pumping station, transfer rod system, preparation (prep) chamber, cryogenic stages, sample shuttles&#160;</td>
		</tr>
		<tr>
			<td>Strata 400 DualBeam System&#160;</td>
			<td>FEI Co. (now Thermo Fisher Scientific)</td>
			<td></td>
			<td>Dual beam FIB/SEM</td>
		</tr>
		<tr>
			<td>X-Max 80</td>
			<td>Oxford Instruments</td>
			<td></td>
			<td>80mm2 EDX detector</td>
		</tr>
		<tr>
			<td>xT Microscope Control</td>
			<td>FEI Co. (now Thermo Fisher Scientific)</td>
			<td></td>
			<td>Software for controlling FEI Strata&#160;</td>
		</tr>
	</tbody>
</table>

nanoscale characterization of liquid-solid interfaces by coupling cryo-focused ion beam milling with scanning electron microscopy and spectroscopy

Physical and chemical processes at solid-liquid interfaces play a crucial role in many natural and technological phenomena, including catalysis, solar energy and fuel generation, and electrochemical energy storage. Nanoscale characterization of such interfaces has recently been achieved using cryogenic electron microscopy, thereby providing a new path to advancing our fundamental understanding of interface processes.
This contribution provides a practical guide to mapping the structure and chemistry of solid-liquid interfaces in materials and devices using an integrated cryogenic electron microscopy approach. In this approach, we pair cryogenic sample preparation which allows stabilization of solid-liquid interfaces with cryogenic focused ion beam (cryo-FIB) milling to create cross-sections through these complex buried structures. Cryogenic scanning electron microscopy (cryo-SEM) techniques performed in a dual-beam FIB/SEM enable direct imaging as well as chemical mapping at the nanoscale. We discuss practical challenges, strategies to overcome them, as well as protocols for obtaining optimal results. While we focus in our discussion on interfaces in energy storage devices, the methods outlined are broadly applicable to a range of fields where solid-liquid interface play a key role.

This method has been developed on a dual FIB/SEM system equipped with a commercially available cryogenic stage, anticontaminator, and preparation chamber. For details, see the table of materials. We have primarily tested this method on lithium metal batteries with a number of different electrolytes, but the method is applicable to any solid-liquid interface that will endure the amount of dose applied during EDX mapping.
Figure 1&#160;illustrates the various compon...

Watch this Scientific Journal Video about Nanoscale Characterization of Liquid-Solid Interfaces by Coupling Cryo-Focused Ion Beam Milling with Scanning Electron Microscopy and Spectroscopy at JoVE.com

Nanoscale Characterization of Liquid-Solid Interfaces by Coupling Cryo-Focused Ion Beam Milling with Scanning Electron Microscopy and Spectroscopy

Cryogenic Focused Ion Beam (FIB) and Scanning Electron Microscopy (SEM) techniques can provide key insights into the chemistry and morphology of intact solid-liquid interfaces. Methods for preparing high quality Energy Dispersive X-ray (EDX) spectroscopic maps of such interfaces are detailed, with a focus on energy storage devices.

Physical and chemical processes at solid-liquid interfaces play a crucial role in many natural and technological phenomena, including catalysis, solar ...

The cryo-SEM and FIB method can be used to study solid-liquid interfaces and biological samples while preserving the samples native structure. The main advantage of this technique is that the cryo-SEM allows the user to quickly probe the interface of macroscopic devices like coin cell battery electrodes with tens of nanometers resolution. Begin by installing a cryo-SEM stage and an anticontaminator.Evacuate the SEM chamber and adjust the gas injection system, GIS, platinum source so that when inserted, the source will sit approximately five millimeters away from the sample surface. Set the GIS temperature to 28 degrees Celsius and open the shutter to vent the system for 30 seconds to clear out any excess material. Then, allow the SEM chamber to evacuate for a minimum of eight hours.At the end of the evacuation period, set the microscope and prep stages to minus 175 degrees Celsius and set the anticontaminator to minus 192 degrees Celsius. To vitrify the sample, sequentially fill the main volume of the nitrogen dual-pot slusher and the surrounding volume with liquid nitrogen until the liquid nitrogen stops bubbling. Seal the filled slusher with the lid and initiate the slush pump.When the liquid nitrogen starts solidifying, begin venting the slush pot. Once the pressure is high enough to allow the pot to be opened, quickly but gently place the sample in the nitrogen. When the boiling has ceased around the sample, use a pre-cooled transfer rod to transfer the sample to the vacuum chamber of a pre-cooled SEM shuttle just before the nitrogen starts to freeze.Quickly transfer the shuttle to the airlock of the prep chamber and pump on the transfer system. If desired, sputter five to 10 nanometers of a gold-palladium layer onto the sample surface to mitigate charging. Then, transfer the sample shuttle as quickly and smoothly as possible onto the cooled microscope stage.For sample surface imaging, first image the sample at a 100 X magnification. Next, bring the sample to an approximately eucentric height and acquire a second low-magnification image. Select a sacrificial test region within the vitrified liquid and identify any potential issues that may be present due to beam damage or charging.Search the sample for the regions-of-interest. When a region has been identified, tilt the sample so that the surface is normal to the direction of the platinum GIS needle and insert the GIS needle. Warm the surface to 28 degrees Celsius and open the valve for approximately 2.5 minutes before retracting the source.Tilt the sample shuttle toward the focused ion beam source and expose the organo-metallic platinum to a 30-kilovolt ion beam at 2.8 nanoamps and an 800 X magnification for 30 seconds. Then, image the sample surface with the electron beam to verify that the surface is smooth and lacks any signs of charging. To prepare a cross-section, first use the ion beam at 30 kilovolts and a lower bulk milling current of approximately 2.8 nanoamps to acquire a snapshot of the sample surface.Identify the feature of interest and measure out the rough placement of the cross-section. To create a side window for the x-rays, draw a regular cross section rotated 90 degrees relative to where the trench will be and place the side window with one edge roughly flush with the desired final cross section. Resize the rotated pattern to maximize the number of x-rays to exit the cross section surface.Use a high current to create a regular cross-section just large enough to reveal the feature of interest and use the ion beam at 30 kilovolts and the current of interest to acquire snapshot of the sample surface. Identify the feature of interest and finalize the placement of the trench. The trench should extend past either side of the feature of interest by a few microns.Confirm that there is one micrometer of material between the edge of the trench and the desired final cross-section and use the milling application to set the Z-depth to two micrometer, regularly pausing the milling process to image the cross-section with the electron beam as necessary. When the trench is much deeper than the feature of interest, note the amount of time needed to create the rough trench to guide the depth. To create a final, clean cross-section, lower the ion beam current to approximately 0.92 nanoamps and image the sample surface.After verifying the location of the feature of interest, use the focused ion beam software to draw a cleaning cross-section and overlap the cleaning window with the pre-made trench by at least one micrometer to help mitigate the re-deposition. Then, use the time needed to create the trench to set the Z-depth value. For EDX mapping, select the appropriate beam conditions for the sample and orient the sample to maximize the x-ray counts.Insert the EDX detector and set the appropriate process time. In the detector software, open the Microscope Setup and start the electron beam image. Click Hit Record to measure the count rate and the dead time.If the dead time needs to be adjusted, change the EDX time constant. Once the detector conditions have been established, collect the electron beam image and open Image Setup to select the bit depth and image resolution. Select the x-ray map resolution, spectrum range, number of channels, and map dwell time.The energy range can be as low as the beam energy used. Then, in the EDX software, select the area to map over. When the map is complete, save the map as a data cube.These images of bare lithium foil milled at 25 and minus 165 degrees Celsius highlight how cooling to cryogenic temperatures can help preserve samples during focused ion beam milling. For EDX experiments, the focused ion beam milling geometry should be optimized and the position of the EDX detector should be taken into account. Here, the difference between a well-and a poorly-prepared cryo-immobilized sample, both using the lithium metal battery as an example, can be observed.Although both samples were nominally prepared according to the same procedure, a brief exposure to air most likely resulted in the surface reactions observed in the poorly-prepared sample. Mapping a lithium deposit in 1, 3-dioxolane, 1, 2-dimethoxyethane with non-optimal conditions results in contrast variations, likely an indication of an initially well preserved interface that is lost due to radiation damage during mapping. In contrast, this map of dead lithium embedded in vitrified electrolyte and the lithium substrate beneath was performed at two kilovolts and 0.84 nanoamps, preserving the sample surface morphology.Although some damage is still visible after mapping, the extent of the damage is substantially reduced. In this analysis, EDX mapping was used to locate iron oxide nanoparticles grown in a silica hydrogel. Large field-of-view scans allowed the identification of regions-of-interest, while more localized scans were used for site-specific milling.Sample charging can be detrimental to the success of this procedure. Remember to lower beam currents and dwell times as necessary to limit the effects of charging. After this, a cryo-FIB lift-out can be performed to prepare a site-specific lamella for TEM analysis.Samples can be imaged at sub-angstrom resolution and map the chemical distribution using EELS and EDX in a TEM instrument.

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The biological uptake of plutonium (Pu) in aquatic ecosystems is of particular concern since it is an alpha-particle emitter with long half-life which can ...

Ohmic Contact Fabrication Using a Focused-ion Beam Technique and Electrical Characterization for Layer Semiconductor Nanostructures

Layer semiconductors with easily processed two-dimensional (2D) structures exhibit indirect-to-direct bandgap transitions and superior transistor ...

In Depth Analyses of LEDs by a Combination of X-ray Computed Tomography (CT) and Light Microscopy (LM) Correlated with Scanning Electron Microscopy (SEM)

In Depth Analyses of LEDs by a Combination of X-ray Computed Tomography CT and Light Microscopy LM Correlated with Scanning Electron Microscopy SEM

In failure analysis, device characterization and reverse engineering of light emitting diodes (LEDs), and similar electronic components of ...

Comprehensive Characterization of Extended Defects in Semiconductor Materials by a Scanning Electron Microscope

Extended defects such as dislocations and grain boundaries have a strong influence on the performance of microelectronic devices and on other applications ...

Studying Dynamic Processes of Nano-sized Objects in Liquid using Scanning Transmission Electron Microscopy

Samples fully embedded in liquid can be studied at a nanoscale spatial resolution with Scanning Transmission Electron Microscopy (STEM) using a ...

Precision Milling of Carbon Nanotube Forests Using Low Pressure Scanning Electron Microscopy

A nanoscale fabrication technique appropriate for milling carbon nanotube (CNT) forests is described. The technique utilizes an environmental scanning ...

A Novel Method for In Situ Electromechanical Characterization of Nanoscale Specimens

A Novel Method for In Situ Electromechanical Characterization of Nanoscale Specimens

Electrically assisted deformation (EAD) is increasingly being used to improve the formability of metals during processes such as sheet metal rolling and ...

Focused Ion Beam Fabrication of LiPON-based Solid-state Lithium-ion Nanobatteries for In Situ Testing

Focused Ion Beam Fabrication of LiPON-based Solid-state Lithium-ion Nanobatteries for In Situ Testing

Solid-state electrolytes are a promising replacement for current organic liquid electrolytes, enabling higher energy densities and improved safety of ...

Single-Digit Nanometer Electron-Beam Lithography with an Aberration-Corrected Scanning Transmission Electron Microscope

We demonstrate extension of electron-beam lithography using conventional resists and pattern transfer processes to single-digit nanometer dimensions by ...