Cryogenic Focused Ion Beam Enables Atomic-Resolution Imaging of Local Structures in Highly Sensitive Bulk Crystals and Devices

With the development of ultralow-dose (scanning) transmission electron microscopy ((S)TEM) techniques, atomic-resolution imaging of highly sensitive nanomaterials has recently become possible. However, applying these techniques to the study of sensitive bulk materials remains challenging due to the lack of suitable specimen preparation methods. We report that cryogenic focused ion beam (cryo-FIB) can provide a solution to this challenge. We successfully extracted thin specimens from metal–organic framework (MOF) crystals and a hybrid halide perovskite single-crystal film solar cell using cryo-FIB without damaging the inherent structures. The high quality of the specimens enabled the subsequent (S)TEM and electron diffraction studies to reveal complex unknown local structures at an atomic resolution. The obtained structural information allowed us to resolve planar defects in MOF HKUST-1, three-dimensionally reconstruct a concomitant phase in MOF UiO-66, and discover a new CH3NH3PbI3 structure and locate its distribution in a single-crystal film perovskite solar cell. This proof-of-concept study demonstrates that cryo-FIB has a unique ability to handle highly sensitive materials, which can substantially expand the range of applications for electron microscopy.

The studied crystals were dispersed on a silicon wafer fixed on the cryo-stage. Electron beam-induced Pt deposition was employed to connect the selected crystal to the probe needle and later onto the TEM grid. During Pt deposition, the electron beam only irradiated the connection point and did not cause damage to the rest of the crystal. After the crystal was firmly attached to the TEM grid, the probe needle was cut off using the (Ga + ) ion beam. The orientation of the crystal relative to the TEM grid was controlled by combining the probe needle rotation, stage rotation, and stage tilt.
After the selected crystal was mounted on the TEM grid, the cryo-stage was gradually cooled to about -140C by supplying liquid N2. The stage slightly moved away from the eucentric position and the gas injection tube to ensure that the top crystal surface exposed to the ion beam could be covered by the organometallic Pt precursor. The cold surface of the crystal significantly promotes the adsorption and deposition of the precursor forming a thick organometallic layer (about 5 µm) within 2 min. The stage was then tilted by 52° to align the TEM grid with the incidence of the ion beam, which was subsequently used to cure the organometallic precursor to form a Pt-C protective layer.
After the protective layer was formed, the stage was finely tilted to perfectly align the desired direction of the crystal with respect to the incidence of the ion beam. A milling pattern was drawn around the region of interest, based on which the crystal was sectioned into a 3-µm-thick lamella using an ion beam (accelerating voltage: 30 kV, beam current: 2.5 nA) and further thinned to 1 µm with a lower beam current of 0.43 nA. Lastly, the lamella was finely milled to a thickness of less than 100 nm using an ion beam at 16 kV with a gentle beam current of 0.22 nA.
The stage was heated to 55C at a rate of 5C/min and kept for 30 min. Finally, the prepared specimen was collected from the FIB and transferred immediately to the TEM microscope for imaging.
2) Specimen preparation from bulk samples (millimeter-sized CH3NH3PbI3 crystal and solar cell) After the bulk sample (crystal or device) was placed on the cryo-stage, the cryo-stage was gradually cooled to about -140C by supplying liquid N2. The stage was slightly moved away from the eucentric height and gas injection tube to ensure that the organometallic Pt precursor could cover a large surface area. The stage was then tilted by 52° to align the specimen surface with the incidence of the (Ga + ) ion beam, which was subsequently used to cure the organometallic precursor to form a Pt-C protective layer. Then, a prism-shaped specimen (about 10 µm long, 5 µm wide, and 5 µm deep) was ablated using the ion beam (30 kV and 21 nA). The stage was tilted back to 0°, and a series of undercuts and side cuts (the U-cut) was then performed to free the "prism" specimen from the bulk sample.
The probe needle was carefully moved to touch one side of the "prism." A small region of the probe needle was milled with the ion beam to connect it with the "prism" through redeposition. Then, the "prism" specimen was cut off from the bulk sample and lifted out by the probe needle. The specimen was transferred to a TEM grid by the probe needle and mounted on the grid by ion-beam milling of the Cu skeleton through the redeposition effect. The subsequent sectioning, fine milling, and specimen transfer processes were the same as those used for micron-sized crystals (described above).

Ultralow-dose HRTEM imaging and image processing
Ultralow-dose HRTEM was performed on a Cs-corrected electron microscope (FEI Titan) operated at 300 kV. The spherical aberration was corrected to a range of ± 5 μm. Specimen searching, zone axis alignment, and prefocusing were conducted at a 13,000× magnification with a dose rate of about 0.03 e/Å 2 /s. The HRTEM images were collected using a Gatan K2 direct-detection camera in the electron-counting mode. The total electron dose used for each image was less than 15 e/Å 2 to avoid structural damage. The images were processed using CTF correction to be more directly interpretable. The detailed methods for image acquisition and processing (image alignment, determination of the absolute defocus value, and CTF correction) can be found in earlier publications. 5, 6

Ultralow-dose iDPC-STEM imaging
Ultralow-dose iDPC-STEM was performed on a double Cs-corrected electron microscope (FEI Spectra 300) operated at 300 kV. Specimen searching and zone axis alignment were conducted at a 13,000× magnification under the TEM mode. Following the alignment of the crystal zone axis, the microscope was switched to the STEM mode with a convergence semi-angle of 10.0 mrad to acquire images. The probe current was about 1 pA, and the pixel size was 0.3716 Å. The dwell time was 5 µs/pixel, and the collection angle was 4-15 mrad. A four-quadrant DF4 detector was used to produce iDPC-STEM images and an applied high-pass filter to reduce low-frequency information.

Selected-Area Electron Diffraction (SAED) mapping
SAED patterns were recorded in a raster fashion over the cryo-FIB-prepared specimen (step size: 500 nm; 15×10 steps), using an aperture of about 400 nm, and a Gatan Ultra-scan CCD camera. Each pattern had a frame size of 512×512 (binning 4) and an exposure time of 2 s. The automatic acquisition of the SAED series was achieved through GMS scripting. During the SAED mapping process, the electron dose received by the specimen was less than 1 e/Å 2 .

Three-Dimensional Electron Diffraction
The 3D ED data was collected with a step-wise rotation method, using a selected area aperture of ~400 nm, a constant stage rotation interval of 0.5°, an exposure time of 2 s for each pattern, and a Gatan Oneview camera. The rotation angle range was ±45°. Software REDp was used to reconstruct the reciprocal lattice and determine the extinction rules.