Visualization of Au Nanoparticles Buried in a Polymer Matrix by Scanning Thermal Noise Microscopy

Atsushi Yao,Kei Kobayashi,Kuniko Kimura,Hirofumi Yamada,Shunta Nosaka

doi:10.1038/srep42718

Atsushi Yao, Kei Kobayashi + Show 3 more

Open Access

https://doi.org/10.1038/srep42718

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Abstract

Several researchers have recently demonstrated visualization of subsurface features with a nanometer-scale resolution using various imaging schemes based on atomic force microscopy. Since all these subsurface imaging techniques require excitation of the oscillation of the cantilever and/or sample surface, it has been difficult to identify a key imaging mechanism. Here we demonstrate visualization of Au nanoparticles buried 300 nm into a polymer matrix by measurement of the thermal noise spectrum of a microcantilever with a tip in contact to the polymer surface. We show that the subsurface Au nanoparticles are detected as the variation in the contact stiffness and damping reflecting the viscoelastic properties of the polymer surface. The variation in the contact stiffness well agrees with the effective stiffness of a simple one-dimensional model, which is consistent with the fact that the maximum depth range of the technique is far beyond the extent of the contact stress field.

Highlights

One of the major imaging schemes is to excite two piezoelectric actuators located at the cantilever base and the bottom of the sample at two different frequencies and detect the flexural oscillation of the cantilever at the beat frequency, which is caused by the nonlinear tip-sample interaction[3,4,5,8,9,10,11]
We found that the contact resonance spectrum was affected by the Au nanoparticle underneath, and we concluded that the variation in the contact stiffness and damping was playing a major role in making subsurface contrasts in the atomic force acoustic microscopy (AFAM) images, while the tip-sample nonlinearity does not seem to significantly contribute[22]
We quantitatively evaluated the differences in the contact stiffness and damping of the polymer surface areas with and without the Au nanoparticle underneath using a linear spring dashpot model, and discuss the imaging mechanisms by scanning thermal noise microscopy (STNM) as well as those by other schemes

Summary

Introduction

One of the major imaging schemes is to excite two piezoelectric actuators located at the cantilever base and the bottom of the sample at two different frequencies and detect the flexural oscillation of the cantilever at the beat frequency, which is caused by the nonlinear tip-sample interaction[3,4,5,8,9,10,11]. We recently measured the contact resonance spectra of the cantilever while the tip was scanned over the surface by sweeping the frequency of the sample excitation at each pixel.

Results

Conclusion