Liquid Helium TEM Sample Holder with High Stability and Long Hold Times

Here, we introduce a novel ultra-low-temperature TEM sample holder compatible with liquid helium. The holder, which can be loaded into modern S/TEM microscopes, achieves sub-35 K base temperatures with ±2 mK thermal stability continuously over +8-hour periods. A charge density wave transition in 2H-NbSe2 with a transition temperature of ∼35 K was detected, highlighting the capability of the instrument for the study of phase transitions in quantum materials. Despite S/TEM’s incredible potential for the exploration of quantum materials or radiation-sensitive matter, shortcomings in current ultra-low-temperature designs make these materials difficult or impossible to study. Rapid advancement of cryogenic TEM techniques has occurred over the past 10 years, and atomic-resolution STEM study of low-temperature phenomena has been demonstrated with success at liquid nitrogen (LN2) temperature [1, 2], and even near liquid helium (LHe) temperature [3]. Common cryogenic instrumentation includes cartridge-based microscopes with built-in cryogenic capabilities, superconducting lenses [4, 5], or low-temperature side-entry sample holders [6]. Cartridge-based LN2 microscopes are ideal for many life-sciences applications but lack capabilities suitable for materials science [7]. Meanwhile, low-temperature side-entry holders suffer from severe mechanical and thermal instabilities as well as very short hold-times due to rapid evaporation of the cryogen from attached lightweight dewars. LHe is much more volatile than LN2, exacerbating all the issues found in current side-entry holders. A modular solution that enables stable, low-temperature imaging in modern TEMs is required for widespread investigation of quantum and radiation-sensitive materials. Here, we design and develop an ultra-low-temperature TEM sample holder that achieves high stability and long holding times. To demonstrate ultra-low temperatures within a modern TEM, measurements of 2H-NbSe2 are performed using a double-corrected JEOL 3100R05 operating at 300 kV. NbSe2 is a layered transition metal dichalcogenide exhibiting both charge density wave (CDW) ordering below 35 K and superconductivity below 7 K [8]. CDW formation is characterized by the appearance of superlattice peaks in reciprocal space. Fig. 1a shows an electron diffraction pattern at 300 K from a thin exfoliated flake. Bragg peaks with the symmetry of the crystal are prominent. At temperatures below 35 K achieved by our holder, we observe the formation of additional superlattice peaks (Fig. 1b, circles) indicating the emergence of the CDW. The peaks are located at 1/3 reciprocal lattice units. In addition to low base temperatures, this LHe instrument allows a high level of temperature control without compromising thermal stability. By adjusting the flow of helium or applying careful heating, any temperature between room and base temperature can be reached with a stability of ±2 mK or better. This capability has been demonstrated, reaching ±1.4 mK and ±2.0 mK stability at 11 K and 110 K, respectively (see Fig. 2). Here, the temperature stability is held over four hours; however, hold times exceeding eight hours can be achieved. These long hold times present a substantial opportunity for measurements that require long experimental durations including spectroscopy, tomography, and 4D STEM. The ability to easily reach intermediate temperatures also affords a great deal of flexibility: materials can now be reliably characterized at any desired temperature, not just those near the boiling point of cryogenic liquids [9]. The emergence of CDW superlattice peaks in 2H-NbSe2 below 35 K, shown in reciprocal space. a. Diffraction pattern captured at 300 K shows the room-temperature phase. b. Diffraction pattern captured below 35 K shows emergent superlattice peaks indicated by white circles. Temperature fluctuation distribution (top) over a period of four hours (bottom). All measurements were taken using a calibrated Si diode sensor on the cold finger. a. A temperature stability of ±1.4 mK was measured at a base temperature of 11 K. b. A temperature stability of ±2.0 mK was measured at an intermediate temperature of 110 K. Target temperature and high temperature stability were held for many hours.