Lancaster University | Department of Physics
Prof. Oleg Kolosov
Merging acoustic and atomic force microscopies to probe physical properties of surfaces and buried layers with nm length and ns time resolution
Abstract
Perhaps two most famous inventions of Calvin Quate are Scanning Acoustic Microscopy (SAM) and Atomic Force Microscopy (AFM). Here we explore how by combining these two became possible to expand SAM ability to map surface and subsurface properties of materials and devices, from polymers to semiconductors, ceramics and two-dimensional heterostructures, to the nanoscale realm of AFM.
While multiple approaches were initially used for to combined these two, the key for successful marrying of SAM and AFM lied in the two fundamental features of AFM probe. First, it is the unique properties of the AFM force sensitive cantilever - a highly dispersive media, that is extremely stiff at high frequencies, and at the same time very soft and, hence, force sensitive at low frequencies. Second was the universally highly nonlinear atomic force vs tip-surface distance dependence that allowed detecting the high frequency minute tip-surface distance modulation (~1 nm) as the additional “ultrasonic” force. These two formed the principles of Ultrasonic Force [1] and Heterodyne Force [2] Microscopies (UFM/HFM) where ultrasonic frequency (a few MHz to sub-GHz) oscillation resulted in the modulation of the tip-surface distance independently of the sample stiffness allowing probing with the same setup both soft (polymer, biomaterials) and very stiff (ceramic, semiconductors, metals, 2D materials, etc) with the compliant, low force constant cantilever. Not surprisingly, as for part of the ultrasonic cycle the AFM tip loses the contact with the surface, the friction was also vanishing making UFM and HFM very gentle probing (e.g. mapping latex particles adhered on mica surface) while still capable of mechanical map of the stiff materials at the same time. By using amplitude modulation (AM) of the HF oscillation at low frequency (~a few kHz) where the flexible AFM cantilever is highly force sensitive and detecting such “ultrasonic force” at AM frequency, it is possible to map both surface and subsurface features of soft and stiff materials from polymers to semiconductor heterostructures to 2D materials.
By using the simultaneous ultrasonic excitation of the sample and ultrasonic or electrical excitation of the cantilever at the adjacent high frequencies f1, f2 and detecting the amplitude and phase of the “ultrasonic” force at the low difference frequency (f2-f1), using HFM approach, it is possible to reveal resonance and relaxation phenomena (mechanical, electrical, ferroelectric switching etc) in AFM with the nm length and ns time scale, making two wonderful inventions of Cal Quate – SAM and AFM opening multiple new avenues in the nanoscience research.
REFERENCES.
- Yamanaka, Ogiso and Kolosov, https://doi.org/10.1063/1.111524
- Cuberes, Assender, Briggs and Kolosov https://iopscience.iop.org/article/10.1088/0022-3727/33/19/301
- Dinelli, Pingue, Kay and Kolosov, https://iopscience.iop.org/article/10.1088/1361-6528/aa55e2