The challenge: seeing atoms without freezing the budget
Optical-coupled scanning probe microscopes (OC-SPM) can visualize individual atoms and probe the electronic and chemical properties at the ångström scale. But to achieve the stability needed for such precision characterization, these instruments usually need to be cooled to just a few degrees above absolute zero—traditionally using liquid helium.
Liquid helium is a resource plagued by high prices and supply chain volatility. The conventional low-temperature OC-SPM relies on the dewar-type cryostat, which contains several liters of liquid helium in a huge Dewar tank. However, the liquid helium is continuously boiling off, forcing the experiments to halt every few days for refills. This creates a critical barrier for complex studies requiring weeks of uninterrupted stability.
"Our previous system could only run for about two days before the helium ran out. It's like you have to restart a movie every few minutes—you lose continuity and miss important details," said Professor Shijing Tan from the University of Science and Technology of China. "Worse still, each refill requires more than 10 liters of liquid helium, representing a significant operating cost."
A clever workaround: remote liquefaction cryogen-free system
Previously, Huan's group from the Institute of Physics, Chinese Academy of Sciences, had developed a remote liquefaction cryogen-free SPM system. Instead of using liquid helium, they use a closed-cycle cryocooler to liquefy helium to superfluid state in a separate chamber, then deliver it to the microscope through a flexible transfer line. The exhausted helium gas is pumped back and re-liquefied, creating a continuous loop. Now, they collaborate with Professor Tan to extend this technology to the OC-SPM.
This "remote liquefaction" scheme solves three problems at once. First of all, it eliminates the need for constant liquid helium refills—the system can run for months without interruption. Moreover, it isolates the microscope from the mechanical vibrations of the cryocooler, which would otherwise blur atomic-scale images. Last but not least, the compact architecture releases sufficient spatial clearance for optical elements and extensive setups.
"Conventional cryocoolers mounted atop introduce annoying vibration to high-resolution SPM," explained Qing Huan from the Institute of Physics, Chinese Academy of Sciences. "By separating the cooling source from the microscope, we get the best of both worlds: continuous cooling without the vibration noise."
Performance that matches liquid helium
The system achieves a stable base temperature below 3 K and a tunneling current noise level under 20 fA/Hz1/2—comparable to the best liquid-helium-based systems. Additionally, it supports multiple imaging modes:
- Scanning tunneling microscopy (STM) for visualizing atomic structures
- Atomic force microscopy (AFM) for visualizing the bonds and atomic structures
- Scanning tunneling spectroscopy (STS) for resolving surface electronic properties
- Tip-enhanced Raman spectroscopy (TERS) for identifying chemical fingerprints
- Scanning tunneling luminescence (STML) for studying light emission from the tip and electro-optical properties from the sample
Seeing chemistry at the single-molecule level
To demonstrate the system's capabilities, the team studied silver phthalocyanine (AgPc) molecules on a silver surface—when the metal-free phthalocyanine was annealed on the silver surface, the self-metalation reaction happened, forming AgPc molecules.
Using the microscope's multimodal capabilities, they:
- Resolved the molecule's atomic structure with STM and AFM
- Mapped its electronic orbitals with STS
- Identified its chemical fingerprint with TERS, revealing different vibrational modes at different parts of the molecule
- Achieved spatial resolution below 1 nanometer in chemical imaging
"The same molecule, the same spot, multiple techniques—that's the power of this system," said Huan. "Those experiments that previously took months to complete intermittently can now be performed uninterruptedly, making the data more convincing."
Why it matters
This technology makes atomic-scale chemical imaging more accessible and sustainable. Laboratories can now run continuous experiments for weeks or months, enabling studies of slow processes like catalytic reactions, surface diffusion, and molecular self-assembly that were previously impractical.
It also eliminates dependence on liquid helium, whose price has tripled in recent years due to global supply shortages. For countries without domestic helium production, this is a game-changer.
What's next
The team plans to integrate the system with even more optical techniques and higher magnetic fields, opening new possibilities for studying quantum materials and single-molecule chemistry. They also see potential for commercializing the technology to make advanced microscopy more widely available.
