Scanning probe microscopy (SPM) may be the best way to image the surfaces with ultimate near-atomic lateral resolution, but its ability to look under the immediate sample surface is inevitably limited. At the same time, ultrasound is well known for its ability to penetrate objects and imaging internal body organs and defects in semiconductor wafers on millimiter to tens of micrometer length scales. Whereas Ultrasonic and Heterodyne Force Microscopies (UFM and HFM) [1,2] and other ultrasonic SPM methods [1,2] use ultrasound to achieve nanoscale resolution down to 10-9 m while preserving subsurface imaging capabilities [3-5], questions still remain on
a) how deep one can observe the nanoscale features using these methods,
b) what is the achievable lateral resolution for subsurface features, and
c) what are the physical mechanisms and the role of phase and amplitude detection in subsurface UFM/HFM imaging.
Moreover, the true subsurface imaging in solid state nanostructures composed of stiff materials has yet to be reliably demonstrated, and misconceptions exist as to how the propagation of ultrasonic waves with wavelength 105 -106 times larger than the imaged features can contribute to subsurface imaging.
Here we use UFM images to produce unambiguous subsurface images with 5 nm lateral resolution of internal morphology of high stiffness solid state nanostructures - iii-v semiconductor quantum dots hidden under atomically flat capping layer. We then explore stacks of atomically layered two-dimensional (2D) materials such as graphene, MoS2, Bi2Se3 of varied thickness using wide range of ultrasonic frequencies from kHz to several MHz. This reveals effects of residual stresses in supported graphene layers, and explores nanomechanical behaviour of few layer graphene, MoS2 and Bi2Se3 films as well as visualizes nanoelectromechanical phenomena in these 2D materials [6]. By directly observing the transition of for few layer graphene sheets deformation from plate to stretched membrane behaviour, we create nanoscale maps of shell instability, providing insight to the stresses in the free standing 2D films.
Finally, by analysing the UFM and HFM imaging process, we show that subsurface imaging mechanisms in both are indeed linked to the elastic field produced by the indention of dynamically stiffened cantilever-tip system coupled with the detection of vibrations via nonlinear tip-surface interactions, with phase information providing much less significant contribution. Further expansion of this methodology, challenges and potential applications are also
[1] Kolosov, O. V. and Yamanaka, K. JJAP, 32, (8A), L1095-8, (1993).
[2] M T Cuberes et al J. Phys. D: Appl. Phys. 33 2347, (2000).
[3] Yamanaka, K., H. Ogiso and O. Kolosov, APL 64(2) :178-80, (1994);
[4] Diebold, A. C., Science 310 (5745): 61-62, (2005).
[5] Tetard, L. et al, Nature Nanotechnology 5 (2), 105-9 (2010).
[6] Kay, N. D. et al, Nano Letters 14, 3400-4 (2014).