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Optical TEM Specimen Holder


for use. Since certain stainless steels are magnetic and could potentially have interfered with imaging, we chose to fabricate our holder bodies with brass and titanium. Te flanges at the rear handle of the holder (purchased off-the-shelf) were made of stainless steel. One important note is that since brass is an alloy of copper and zinc, and zinc outgasses at high temperatures (several hundred degrees Celsius), it is important not to heat a specimen holder made of brass to avoid coating the inside of the specimen chamber with zinc. Other considerations. Modular design: We chose to


implement a modular design, where the holder is made up of a few sections that can be easily assembled and disassembled (Figure 4). Te advantage of such a design is that specific parts such as the rear handle or tip can be easily interchanged, and we deemed this important in a research project where our needs and application might change every few months. TEM grid clamp: Traditional FEI (a company acquired


by Termo Fisher Scientific) single-tilt holders have a spring- loaded clamp away from the tip of the holder. Since the optical fiber occupies the space right before the grid, we designed a screw-on clamp on the side closer to the tip (Figure 5). Care must be taken to ensure that the screw does not protrude out of the top or bottom of the holder tip.


Results and Discussion We fabricated two optical holders—one with a brass body


and 3-axis external micrometer stage, and another with a titanium body and fiber positioning screws at the tip (Figure 6). Te brass holder was machined at Excel CNC Machining Inc., San Jose, CA, and the titanium holder was machined at the Stanford Termosciences Machine Shop, Stanford University. Both holders were vacuum-tested using a Hummingbird Scientific (Lacey, WA) high-vacuum leak test station and were found to be leakproof. Following vacuum testing, they were inserted into an FEI Tecnai TEM and tested for X-ray safety by Stanford Environment Health & Safety and were found to be safe for use. We also characterized the laser spot size on the sample.


Figure 7 shows the laser spot (using a 405 nm laser) from a single- mode fiber on a piece of filter paper, used in place of a TEM grid. Te line profile of the laser intensity shows the spot size to be about 25 μm. With a laser power of 10 mW, this yields an optical flux of greater than 107


W/m2 , which was our target flux.


Figure 6: (top) Brass optical holder with a 3-axis micrometer stage at the rear handle and (bottom) titanium optical holder with fiber positioning screws at the tip.


We also tested the TEM holders to see if they impacted


the resolution in HRTEM. Figure 8 shows a GaAs cross section (sample courtesy of Chen Shang, Bowers group, UCSB) imaged at 300 keV with its corresponding fast Fourier transform (FFT). In the FFT, diffraction spots are visible beyond 10 nm-1 demonstrating sub-Angstrom lattice fringe resolution.


,


Conclusion In summary, we designed and constructed two TEM


specimen holders with optical fiber feedthroughs to enable in situ photoexcitation of materials inside TEMs. We constructed two different motion control mechanisms to enable precise positioning of the laser spot on the TEM grid. Care was taken during materials selection to ensure vacuum compatibility and X-ray safety, and these were tested extensively upon fabrication of the holders. We found that the holders were sufficiently stable in the TEM to enable atomic-resolution imaging. Our TEM holders are currently supporting proof-of-


concept PAMELA-TEM experiments. In addition to this, we envision them being used to study various kinds of light matter interactions at the atomic scale, such as in situ photocatalysis and light-driven phase transformations. We are now developing TEM holders with electrical bias and light input capabilities, with the additional possibility of cryogenic cooling. We believe that the capability to fabricate TEM specimen holders quickly at an affordable cost will expedite research and enable new possibilities in electron microscopy.


Acknowledgements Te authors would like to acknowledge and thank


Figure 4: Modular design of the specimen holder; (top) disassembled and (bottom) assembled.


2021 September • www.microscopy-today.com


Els Kok (Termo Fisher Scientific, Te Netherlands), Kate Marusak (Protochips, Cary, NC), and Brad Takasuka (Silicon Valley Peripherals, San Jose, CA) for helpful discussions. Te


43


Figure 5: Top view of the tip of the holder showing the design of the TEM grid clamp.


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