Low-force spectroscopy on graphene membranes by scanning tunneling microscopy

Area of Science:

• Materials Science
• Condensed Matter Physics
• Nanotechnology

Background:

• Atomically flat two-dimensional (2D) materials offer high mechanical flexibility, making them promising for ultrathin membranes.
• Investigating the mechanical properties of these delicate 2D materials at the nanoscale presents significant challenges for traditional microscopy.
• Graphene, a prominent 2D material, requires advanced techniques to probe its mechanical behavior without causing damage.

Purpose of the Study:

• To introduce a novel, non-indenting method for mechanical characterization of freestanding graphene membranes.
• To enable precise, low-force spectroscopy on micrometer-sized 2D materials using Scanning Tunneling Microscopy (STM).
• To establish a continuous stress-strain measurement capability for ultrathin membranes.

Main Methods:

• Utilizing ultra-high vacuum Scanning Tunneling Microscopy (STM) for measurements.
• Employing quasi-static voltage ramps with active feedback at low tunneling currents.
• Leveraging attractive electrostatic forces between the STM tip and the graphene membrane to create a bulge-test scenario.
• Simultaneously measuring loading force and membrane deflection with sub-nanometer resolution.

Main Results:

• Successfully established a bulge-test scenario on freestanding graphene membranes using STM.
• Achieved continuous measurement of the stress-strain relation with applied electrostatic forces typically below 1 nN.
• Determined the two-dimensional elastic modulus (E2D) of single-layer graphene membranes to be 220 N m⁻¹ at 0.1% strain.

Conclusions:

• The developed STM-based method provides a convenient and simple approach for non-contact mechanical testing of 2D materials.
• This technique allows for precise characterization of mechanical properties like the elastic modulus without damaging the sample.
• The findings demonstrate the potential of STM for detailed investigation of the mechanical behavior of flexible, atomically thin membranes.