$\r{A}$-Indentation for non-destructive elastic moduli measurements of supported ultra-hard ultra-thin films and nanostructures

TL;DR

This paper introduces a novel modulated e5-indentation technique that enables non-destructive, sub-e5 depth measurements of ultra-thin and ultra-stiff films and 2D materials with atomic-scale resolution using standard AFM equipment.

Contribution

The authors develop and demonstrate a new e5-indentation method that achieves sub-e5 depth resolution for ultra-thin films and 2D materials, expanding the capabilities of nanoindentation.

Findings

Achieves indentation depths as small as 0.3 e5.

Provides indentation resolution better than 0.05 e5.

Enables non-destructive elastic modulus measurements of ultra-stiff films.

Abstract

During conventional nanoindentation measurements, the indentation depths are usually larger than 1-10 nm, which hinders the ability to study ultra-thin films ($<$ 10 nm) and supported atomically thin two-dimensional (2D) materials. Here, we discuss the development of modulated \r{A}-indentation to achieve sub-\r{A} indentation depths during force-indentation measurements while also imaging materials with nanoscale resolution. Modulated nanoindentation (MoNI) was originally invented to measure the radial elasticity of multi-walled nanotubes. Now, by using extremely small amplitude oscillations ($\ll$ 1 \r{A}) at high frequency, and stiff cantilevers, we show how modulated nano/\r{A}-indentation (MoNI/\r{A}I) enables non-destructive measurements of the contact stiffness and indentation modulus of ultra-thin ultra-stiff films, including CVD diamond films (modulus $\sim$ 1000 GPa), as well as the transverse modulus of 2D materials. Our analysis demonstrates that in presence of a standard laboratory noise floor, the signal to noise ratio of MoNI/\r{A}I implemented with a commercial atomic force microscope (AFM) is such that a dynamic range of 80 dB $-$ achievable with commercial Lock-in amplifiers $-$ is sufficient to observe superior indentation curves, having indentation depths as small as 0.3 \r{A}, resolution in indentation $<$ 0.05 \r{A}, and in normal load $<$ 0.5 nN. Being implemented on a standard AFM, this method has the potential for a broad applicability.