Subsurface imaging of rigid particles buried in a polymer matrix based on atomic force microscopy mechanical sensing
Introduction
Non-destructive subsurface imaging at the nanoscale resolution is of significant importance in numerous research fields [1]. For instance, volume organization of nanofillers in a polymer matrix is closely relevant with the functionality of composite materials [2]. Therefore, visualizing subsurface dispersion of the buried nanofillers is highly demanded [3]. Similarly, detecting nanomaterials distribution within a complex biological system is of great interest in biomedicine and nanotoxicity [4]. Identification and diagnosis of underneath defects including voids and cracks can help to enhance the yield limit in micro-and-nano device manufacturing and integration [5]. In all these practical situations, the nano-objects of concern are covered by a medium that conceals their relevant characteristics, which challenge the non-destructive high-resolution imaging.
Currently available tools and techniques for nanoscale non-destructive subsurface imaging are quite limited. Owing to its ultra-high resolution and flexibility, atomic force microscopy (AFM) has drawn considerable interests to fulfill this demand. However, conventional AFM is mainly sensitive to surface or near-surface properties and only allows the shallowly buried nanostructures to be detected [6]. To enhance the subsurface imaging ability, an external wave or force field that can pass through the surrounding medium and interact with the inside nano-objects should be coupled. Following this general principle, several AFM operation modes have been developed to reveal the embedded nanostructures. Incorporating various electromagnetic waves into the tip-sample junction can enable subsurface imaging capability. For example, backscattered infrared light was integrated to visualize gold nanodisks covered by a silicon oxide layer [7] and scanning microwave microscopy was applied to detect in-depth oxygen dissolved in metals [8]. Launching an acoustic wave from either beneath the sample or the tip, subsurface nanostructures and defects were detectable by monitoring the disturbed cantilever oscillations [9], [10], [11], [12], [13], [14], [15], [16]. Heterodyne detection for a sample excited with ultrasonic vibration was also proposed to resolve local stiffness differences [17]. Following this approach, alternatives including scanning ultrasound holography [18,19] and mode-synthesizing AFM [20,21] were further developed. Nanoparticles in macrophages [22] and defects in semiconductor interconnects [23] were then non-destructively probed from amplitude and phase signals at the differential frequency. However, a comprehensive mechanism of the image contrast is not yet available [18,[24], [25], [26]]. In case of acoustic AFM whose tip is in contact with the sample, the combined contributions from both contact mechanics and wave propagation could play a critical role in forming the subsurface image contrast [27].
In addition to the waves, numerous tip-sample interaction force fields are feasible for subsurface imaging. When the medium and the embedded nanomaterials have obvious differences in certain physical properties, the nano-objects are detectable by characterizing the corresponding features. The force fields could be electric [28], [29], [30], magnetic, thermal, mechanical [31,32]. Among them, subsurface detection based on mechanical characterization has a prominent advantage of no special requirement for the buried materials and the probe, that is, being magnetic, conductive, etc. The presence of nanomaterials having different elasticity as compared with the medium will change the effective local modulus or the contact stiffness and they can be subsequently sensed. Some AFM techniques, which allow simultaneous topography acquisition and mechanical characterization, have been applied for subsurface imaging. To enhance the mechanical sensitivity as compared with conventional AFM tapping mode, detection of higher harmonics was employed where the dynamic responses at an integer multiple of the fundamental resonance frequency were analyzed [33]. Multifrequency methods such as bimodal and triple-frequency AFMs were verified to have the capability of characterizing mechanical properties [34] and imaging shallow subsurface nanoparticles [35]. Several eigenmodes of the cantilever are excited simultaneously in multifrequency AFM [36]. Another approach to determine the local stiffness quantitatively is contact resonance (CR) method where the CR frequency of a specified eigenmode is tracked during scanning. Such an approach enables accurate measurement of sample modulus [37,38]. In addition to mechanical characterization, CR-AFM was also exploited to probe buried nanoparticles [39,40], nanotubes [31], cavities [41], and atomic structures in two-dimensional materials and heterostructures [42].
Despite all these rapid progresses, the subsurface detection capability such as spatial resolution and detectable depth limit has not been completely understood. Here, we compared several methods based on mechanical sensing, namely CR, bimodal and harmonic AFMs. With experiments on microparticles embedded in a polymer matrix, finite element analyses (FEA) of local elastic modulus of the composite and theoretical calculations of cantilever oscillations, the maximum detectable subsurface depths by the three methods were evaluated and compared. These studies are toward the main purposes of understanding the subsurface imaging mechanism via AFM mechanical sensing modes and evaluating the corresponding depth limit quantitatively.
Section snippets
Experimental setup
The experiments were performed on a commercial AFM (MFP-3D Origin, Asylum Research). The Multi75Al-G cantilevers (Budget Sensors, Bulgaria) were used. The spring constants of the cantilevers were determined to be typically 2.69 N/m by using the thermal calibration and the first two natural frequencies were respectively 66.1 kHz and 430.7 kHz. The sample was prepared by dispersing SiO2 microparticles into a polydimethylsiloxane (PDMS) matrix. The particle diameter was 5.0 ± 0.125 μm by scanning
Results and discussion
Typical comparisons of CR, bimodal and harmonic AFM subsurface imaging on the SiO2 microparticles buried in a PDMS matrix are presented in Fig. 2. The spring constant of the cantilever was calibrated to be 2.69 N/m. The first two free resonance frequencies were 66.1 kHz and 430.7 kHz. The corresponding quality factors were respectively 173 and 470. In both bimodal imaging and harmonic imaging, the free amplitude was 133.9 nm and the ratio of feedback amplitude to free amplitude was set to 0.9.
Conclusion
In summary, subsurface AFM nano-imaging based on linear nanomechanical sensing was investigated. Three methods, namely CR, bimodal and harmonic AFMs were compared. By embedding microscale SiO2 particles in a PDMS matrix as a reference sample, the detectable depth limit was determined from mapping the apparent particle diameters in the subsurface images into a simple geometrical model. Results demonstrated that the depth limit from high to low is CR-AFM, bimodal AFM and harmonic AFM. In CR-AFM,
Acknowledgements
This work was supported by the National Natural Science Foundation of China (no. 51675504) and Anhui Major Scientific Instruments Development Project(no. 1704c0402198).
References (56)
- et al.
Eur. Polym. J.
(2014) - et al.
Ultramicroscopy
(2011) - et al.
Ultramicroscopy
(2013) - et al.
Sens. Actuators A: Phys.
(2017) - et al.
J. Phys. Condens. Matter
(2017) - et al.
Energy Env. Sci.
(2011) - et al.
Nanotechnology
(2014) - et al.
Sci. Rep.
(2013) - et al.
ACS Nano
(2010) - et al.
Opt. Express
(2012)
Nanoscale
Jpn. J. Appl. Phys.
Appl. Phys. Lett.
Nanotechnology
J. Appl. Phys.
ACS Nano
Microsc. Res. Tech.
J. Phys. D. Appl. Phys.
Science
IEEE Trans. Nanotechnol.
Nat. Nanotechnol.
Appl. Phys. Lett.
Appl. Phys. Lett.
Appl. Phys. Lett.
Appl. Phys. Lett.
J. Appl. Phys.
Nanotechnology
Nanoscale
Cited by (8)
Tomographic imaging using conductive atomic force microscopy
2022, Materials CharacterizationCitation Excerpt :Sub-surface imaging capabilities of nanostructured materials have been explored widely via scanning probe microscopy modes, such as contact resonance atomic force microscopy (CR-AFM) [1], Kelvin probe force microscopy (KPFM) [2], scanning thermal noise microscopy [3], and scanning dielectric microscopy [4].
Best practices and recommendations for accurate nanomechanical characterization of heterogeneous polymer systems with atomic force microscopy
2021, Progress in Polymer ScienceCitation Excerpt :It was observed that the mechanical properties and conductivity were through application of a compressive force to the surface of the nanocomposite films, which was attributed to compaction of the brick-and-mortar structure of the composite [371]. The high sensitivity of CR AFM to changes in contact stiffness [372] has resulted in frequent application of CR to detecting buried nanoparticles [225-227,373] and imaging of subsurface features buried under a layer of polymer, an example of which is provided in Fig. 23(b). Imaging of sub-surface features with DART-CR AFM was applied to Au circuits buried in PMMA, with defects detected in the printed circuit as small as 100 nm, the smallest feature size tested. [374]
Molecular dynamics simulation of bimodal atomic force microscopy
2020, UltramicroscopyCitation Excerpt :At present, the most promising approaches to obtain the topography and properties involve the excitation and the detection of several frequencies of the tip's oscillation, which is known as multi-frequency AFM [4,5]. There are a variety of modes in multi-frequency AFM, including bimodel AFM [6], trimodal AFM [7], band excitation mode AFM [8], harmonic AFM [9], etc. As the most commonly used mode, the bimodal AFM offers a straightforward approach to separate topography from other interactions, and could detect the mechanical, electrical and magnetic properties [10–17].
Imaging beyond the surface region: Probing hidden materials via atomic force microscopy
2023, Science AdvancesResearch Progress on Multimodal Atomic Force Microscopy
2022, Guti Lixue Xuebao/Acta Mechanica Solida SinicaCharacterization of Cell Response on Patterned Stiffness Substrate by AFAM
2022, 2022 IEEE International Conference on Manipulation, Manufacturing and Measurement on the Nanoscale, 3M-NANO 2022 - Proceedings