[Paper Review] Ultra-Cold Cryogenic TEM with Liquid Helium and High Stability

Emily Rennich, Suk Hyun Sung+5 more|arXiv (Cornell University)|Feb 1, 2024

Advanced Materials Characterization Techniques4 citations

TL;DR

This paper presents an ultra-cold cryogenic TEM specimen holder that enables continuous liquid helium cooling with sub-25 K base temperature and ±2 mK thermal stability over hours, achieving atomic resolution in aberration-corrected TEM. The system uses a vacuum-insulated helium transfer line, a copper heat exchanger, and dual-stage vibration-damping bellows to decouple mechanical noise, enabling long-duration, high-stability experiments at cryogenic temperatures for quantum materials and beam-sensitive specimens.

ABSTRACT

Cryogenic transmission electron microscopy has revolutionized structural biology and materials science, but achieving temperatures below the boiling point of liquid nitrogen remains a long-standing aspiration. We introduce an ultra-cold liquid helium transmission electron microscope specimen holder, featuring continuous cryogen flow and vibration decoupling. This instrument is compatible with modern aberration-corrected microscopes and achieves sub-25 K base temperature, ${\pm}$2 mK thermal stability over many hours, and atomic resolution--setting the stage for a new era of cryogenic electron microscopy.

Motivation & Objective

• To overcome the limitations of existing cryogenic TEM holders that cannot achieve stable ultra-cold temperatures below 100 K.
• To address thermal and mechanical instabilities caused by rapid cryogen evaporation in dewar-based systems.
• To enable long-duration experiments at millikelvin temperature stability using continuous liquid helium flow.
• To develop a vibration-damped, cryogen-agnostic specimen holder compatible with modern aberration-corrected electron microscopes.
• To facilitate the study of quantum materials and beam-sensitive biological specimens at temperatures unattainable with liquid nitrogen.

Proposed method

• A side-entry specimen holder with a vacuum-insulated, radiation-shielded liquid helium transfer line delivers continuous cryogen flow to a copper heat exchanger.
• The specimen is thermally coupled to the heat exchanger via highly conductive axial components, maintaining temperatures below 25 K.
• Two pairs of flexible, ultra-high vacuum edge-welded bellows with 50A durometer rubber blocks decouple the specimen from vibrations in the transfer line and goniometer.
• Temperature is controlled via adjustable helium flow and localized heating, enabling stable operation from room temperature down to 11 K with ±2 mK stability.
• A calibrated silicon diode sensor measures temperature at the specimen tip, with a ~15 K temperature gradient between heat exchanger and specimen.
• The system is cryogen-agnostic, allowing operation with liquid nitrogen above 110 K, extending usability and reducing helium consumption.

Figure 1: Ultra-cold cryogenic TEM imaging at atomic resolution with mK temperature stability. a),b Atomic resolution image of gold at 31 K, and 115 K, respectively. The top insets represent a zoom-in into the TEM images. Bottom insets show the Fourier transform of the atomically resolved TEM images

Experimental results

Research questions

• RQ1 Can a cryogenic TEM specimen holder achieve sub-25 K base temperature with millikelvin-level thermal stability over extended durations?
• RQ2 How can mechanical vibrations from cryogen flow and environmental sources be effectively suppressed to maintain atomic-resolution imaging?
• RQ3 Can continuous liquid helium flow be implemented in modern aberration-corrected TEMs without compromising electron optics or stability?
• RQ4 What is the performance of the holder in resolving quantum phase transitions in materials like 2H-NbSe2 at temperatures below 33 K?
• RQ5 Can long-duration experiments such as ptychography and tomography be enabled by stable, low-temperature operation with minimal cryogen consumption?

Key findings

• The specimen holder achieved a base temperature of 23 K at the specimen tip, maintained for over 4 hours with continuous liquid helium flow.
• Thermal stability of ±1.4 mK was measured at 11 K and ±2.0 mK at 110 K over 4-hour data collection periods.
• Atomic-resolution imaging of gold was demonstrated at 31 K, with clear Fourier transform features confirming atomic lattice resolution.
• The emergence of charge density wave superlattice peaks at 1/3 reciprocal lattice units was observed below 33 K in 2H-NbSe2, confirming a structural phase transition.
• Millikelvin temperature stability was maintained for over 10 hours at 110 K, demonstrating long hold times limited only by helium supply.
• The system consumes less than 2 liters of helium per hour, enabling extended experiments with standard 30–1000 liter tanks.

Figure 2: The emergence of a charge density wave superlattice in 2H-NbSe 2 below 33 K. a) TEM electron diffraction pattern captured at 300 K shows the room temperature phase containing only Bragg peaks corresponding to a c) normal crystal lattice. b) At temperatures below 33 K superlattice peaks (bl