New microscopy technique maps 2D material vibrations with near-nanometer precision

Researchers at the University of Maryland have developed a microscopy method that images how two-dimensional materials vibrate in response to infrared light with a spatial resolution of nearly one nanometer. The technique, called infrared torsional force microscopy (TFM-IR), builds on atomic force microscopy by rapidly twisting a microscopic tip back and forth instead of tapping it vertically. This twisting motion allows the tip to detect both vertical and horizontal expansions of a material's surface as it absorbs infrared pulses.

Standard atomic force microscopy measures surface topography by tapping a tip up and down, but it struggles to sense lateral movements caused by light absorption. In 2024, a Stanford-led team demonstrated that twisting the tip — torsional force microscopy — could reveal previously undetectable variations in twisted bilayer graphene. The Maryland group, led by physics professor Min Ouyang and graduate student Yonatan Gazit, added synchronized infrared laser pulses to this torsional approach. By tuning the pulse frequency, they can selectively probe either vertical or horizontal vibrational modes, similar to a stroboscopic effect.

The team first tested TFM-IR on mica, a mineral with well-understood chemical bonds oriented in different directions. They detected four distinct vibration patterns and distinguished horizontal from vertical movements by varying the infrared pulse rate. Measurements across a nanoscale mica bubble matched theoretical simulations of how strain distributes horizontal and vertical responses, confirming the technique's directional sensitivity.

Applying the method to twisted bilayer graphene — two carbon-atom layers stacked at a slight angle — revealed details invisible to standard torsional microscopy. While conventional TFM shows only the honeycomb lattice shape, TFM-IR mapped how individual chemical bonds within and between the graphene layers respond to infrared light. This vibrational fingerprint varies with the stacking angle, offering a new window into the microscopic origins of exotic quantum properties such as superconductivity in moiré materials.

Professor Ouyang notes the technique combines three rarely unified capabilities: nanoscale optical imaging and spectroscopy, directional resolution of anisotropic material responses, and material-specific spectroscopic fingerprints. The method operates at room temperature, which the researchers suggest could make it practical for semiconductor manufacturing, where mapping nanoscale chemical and mechanical variations might help identify chip defects or guide development of quantum sensors and photonic quantum computers.

The work was published in Nature Communications. The researchers emphasize that TFM-IR does not merely show a material's shape but identifies its composition and reveals hidden physical processes by mapping light-driven responses with near-nanometer precision.