Subsurface Imaging of the Cores of Polymer-Encapsulated Cobalt

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Subsurface Imaging of the Cores of Polymer Encapsulated Cobalt Nanoparticles Using Force Modulation Microscopy Stephen M. Deese, Lauren E. Englade-Franklin, Lawrence J. Hill, Jeffrey Pyun, Julia Y. Chan, and Jayne C. Garno J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07994 • Publication Date (Web): 03 Oct 2017 Downloaded from http://pubs.acs.org on October 7, 2017

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The Journal of Physical Chemistry

Subsurface Imaging of the Cores of Polymer Encapsulated Cobalt Nanoparticles Using Force Modulation Microscopy

Stephen M. Deese,1 Lauren E. Englade-Franklin,2 Lawrence J. Hill,3 Jeffrey Pyun4 Julia Y. Chan5 and Jayne C. Garno1*

1

Department of Chemistry, Louisiana State University, Baton Rouge, LA 70803 2

3

4

5

Department of Chemistry, Nunez Community College, Chalmette, LA 70043

Department of Chemistry, Western Kentucky University, Bowling Green, KY 42101

Department of Chemistry and Biochemistry, University of Arizona, Tucson, AZ 85721

Department of Chemistry & Biochemistry, Univ. of Texas at Dallas, Richardson, TX 75080

*corresponding author:

Jayne C. Garno, Phone: 225-578-8942 E-mail: [email protected]

*address:

Chemistry Department Louisiana State University 232 Choppin Hall Baton Rouge, LA 70803 FAX: 225-578-3458

ACS Paragon Plus Environment


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ABSTRACT Force modulation microscopy (FMM) is a mode of scanning probe microscopy that can be used to visualize changes of tip-sample interactions for hard and soft areas of samples such as polymers and organic thin films. In designed experiments, polystyrene-encapsulated cobalt nanoparticles were imaged with FMM using a home-built sample stage for sample actuation. Regions of the outer polymer coating and the inner cobalt nanoparticle were resolved with high resolution. Using FMM, differences in the elastic and viscoelastic properties of the nanoparticles were visualized with nanoscale resolution by monitoring the return amplitude and phase signals as the AFM tip is scanned over areas of a sample. Regions of the sample with greater elasticity and viscoelasticity generate a weaker signal relative to harder areas because more of the energy associated with the cantilever oscillation is dissipated by the material. Areas with greater elasticity will tend to absorb more of the energy of the cantilever causing the amplitude of the oscillation to be dampened. Conversely, harder areas, having a lower elasticity, will cause the tip to oscillate closer to the input driving amplitude of the piezoceramic. The polymer encapsulated nanoparticles were patterned using two-particle lithography to prevent aggregation of the nanoparticles.

KEYWORDS Atomic force microscopy, nanoparticles, force modulation microscopy, core-shell nanoparticles, nanofabrication


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INTRODUCTION Force Modulation Microscopy (FMM) is an advanced imaging mode of atomic force microscopy (AFM) that is coupled with contact-mode AFM, and is used to investigate surface elasticity and viscoelastic response at the nanoscale. The AFM instrument is operated in continuous contact mode for FMM, however the sample is driven to vibrate by a piezoactuator as the tip is rastered across the surface. Changes in surface elasticity are measured by monitoring the change between the driving amplitude and return signal. Softer areas dampen the oscillation amplitude of the cantilever by a greater amount relative to harder areas. The phase signal provides information of the viscoelastic properties of the surface according to changes in the phase lag of the return signal.1 Complex changes in modulation amplitude, phase lag, and frequency shifts enable nanoscale resolution for the detection of elastic and viscoelastic surface properties.2 The AFM can be applied to probe surface structures at the submicrometer scale with exquisite resolution.3 Studies with AFM enable collection of data at the nanoscale that can quantify surface roughness,4 packing density,5 magnetic properties,6 as well as the local density of states.5 Characterization and quantification of ferroelectric and magnetic properties as well as measurements of the polarizability and compressibility of nanoscopic materials has made AFM characterizations a valuable tool for understanding the nature of surfaces.6-8 Scaling down from the bulk regime to the nanoscale reveals differences in mechanical properties and overall characteristics of materials. Further information of material properties can be obtained with dynamic modes of AFM, in which the tip or sample are caused to vibrate.9, 10



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Using FMM, the elastic and viscoelastic properties of samples have been investigated for polymers,11-14 organic thin films,15 and single crystals.16 To image samples with FMM, a designed sample stage was constructed which contains a piezoactuator in the middle of the stage directly beneath the sample. For sample evaluation, the AFM is operated in contact mode while the cantilever is raster scanned across the surface. An AC current supplied to the piezoelectric actuator causes the sample to vibrate at a desired frequency specifically chosen by the operator so that the frequency and magnitude of vibration is precisely controlled by the input AC signal. Changes in amplitude and phase depend on the driving frequency selected. The contrast of the constructed image is dependent upon differences in mechanical properties of the surface of the material. Characterizations such as UV-Vis, XPS, and NMR are not able to differentiate between the types of materials within polymer encapsulated nanoparticles. Force modulation microscopy enables elucidation of the thickness of the polymer coating as well as the inner core nanoparticle. Recent breakthroughs in surface science have enabled unprecedented resolution and complex measurements at the nanoscale using AFM for detecting heterogeneities, cracks and defects below the surface.17 Subsurface imaging of glass nanoparticles embedded within a polymer film was reported using triple-frequency AFM; a variation of the amplitude-modulation mode.18 An imaging strategy was used for increasing tip-sample indentation by using a higher eigenmode of operation for compositional mapping at a depth of tens of nanometers. The subsurface morphology and periodicity of pairs of silicon nanowires was characterized using bimodal and trimodal amplitude modulation AFM, for nanostructures buried under a polymer film, 70 nm in thickness.19 Multifrequency AFM imaging of buried nanostructures was found to depend on the softness of the interface. Subsurface, depth-resolved imaging of polymer films (20 nm thickness) were obtained with amplitude modulation AFM using maps of tip indentation to


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obtain depth information.20 Amplitude modulation (AM-AFM), also known as tapping mode AFM is configured for intermittent contact with the surface as the tip is applied and removed from the sample with selected parameters of amplitude and frequency of the tapping motion of the probe.21, 22 Subsurface imaging of carbon nanotubes inside polymers was accomplished with AM-AFM by applying a DC bias to the tip for dual mode imaging.23 A mechanism of electrical energy dissipation in the Coulomb attractive regime was proposed for mapping spatial variations in the local capacitance and resistance of carbon nanotubes. Defects that were buried in stiff substrate materials at depths ranging from 180 to 900 nm were detected using atomic force acoustic microscopy (AFAM), which incorporates a broadband ultrasonic transducer to induce out of plane surface vibrations.24 Gold nanoparticles buried within a spin-coated polymer film (900 nm thickness) were detected with AFAM by Kimura et al.25 Subsurface imaging of magnetite nanoparticles within microglial cell structures was accomplished with contact resonance AFM (CR-AFM) by Reggente et al.26 Subsurface features can alter the tip-sample contact stiffness, which will influence cantilever oscillation. By extracting the amplitude, phase, and frequencies of the CR-AFM spectra, information of subsurface cavity structures was obtained from images which were not visible in the topography image.27 Gold nanoparticles buried under a polymer film (900 nm in thickness) were resolved using UAFM, FMM and heterodyne force microscopy (HFM) imaging by Kimura et al.28 The parameters of drive frequencies and detection frequencies were summarized, comparing example images with each operating mode. A method of ultrasonic force microscopy (UFM) was developed for subsurface imaging of highly oriented pyrolytic graphite (HOPG), in which the samples were driven to vibrate at ultrasonic frequencies that are much higher than the resonant frequency of the cantilever.29



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Subsurface imaging of samples prepared from thin flakes of graphite and MoS2 placed on a film of cyclic olefin copolymer were studied with UFM, to enable detection of buried interfaces at depths of tens of nanometers.30 A comparison of parameters for subsurface imaging using contact resonance AFM and ultrasonic AFM was done for samples with cavity structures on a polymethylmethacrylate (PMMA) substrate covered by flakes of HOPG.27 Theoretical analysis of the mechanisms for contrast with UAFM was reported by Sharahi et al. showing that results depend on the combination of two contrast mechanisms: the scattering of ultrasonic waves and the stiffness at the tip–sample contact.31 Elucidation of cores versus the shell of encapsulated nanoparticles using AFM imaging modes is useful for the future development of multicomponent nanoscale systems. The softer polystyrene outer coating has greater elasticity and viscoelasticity than that of the inner metal core (cobalt) to produce differences in the elastic response of amplitude and phase channels. For this report, the subsurface features of polystyrene encapsulated cobalt nanoparticles were revealed with high resolution using FMM. Particle lithography was used to prepare samples, to control the arrangement and surface density of the sample for AFM imaging. Using particle lithography, the encapsulated nanoparticles were segregated across the surface to enable close-up evaluation of the fine details of sample morphology.32, 33 EXPERIMENTAL METHODS Materials and Reagents. The polystyrene encapsulated cobalt nanoparticles were synthesized as previously reported.39 Silicon wafers doped with boron of 5 × 5 mm2 dimension were obtained from Ted Pella Inc., Redford, California, and were used as substrates. The substrates were cleaned in Piranha solution containing sulfuric acid (96% EMD Chemical Inc., Gibbstown, NJ) and hydrogen peroxide (30%, Sigma-Aldrich) at a ratio of 3:1 (v/v). Piranha is a strong oxidizing


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agent and must be handled with care. After 1.5 h submerged in Piranha solution, substrates were rinsed with deionized water (Milli-Q, Millipore, Bedford, MA) and dried with argon. Monodisperse latex mesospheres were purchased from Thermo Fisher Scientific, Waltham, Massachusetts. The latex spheres were washed three times with deionized water by centrifugation in water to remove any residual surfactants or charge stabilizers. Centrifuging the suspension for 15 min at 14000 rpm produced a pellet which was then resuspended in deionized water. Sample Preparation. Particle lithography was used to prepare samples. A sample of 500 nm latex spheres was suspended in high purity deionized water. Next, 15 µL drop of the suspension was deposited onto the silica substrates and left to dry for 24 h under ambient conditions. The polymer encapsulated cobalt nanoparticles (0.5 mg/mL) were suspended in a solution of methylene chloride. A drop of 20 µL was deposited onto the previously prepared substrate with a mask of latex spheres. The sample was dried under ambient conditions for 24 h and then imaged with AFM. Atomic force microscopy. A model 5500 scanning probe microscope (Agilent Technologies, Chandler, AZ) equipped with PicoView v1.12 software was used for AFM characterizations. The system was operated with an open-loop feedback for continuous scanning in contact mode. A homebuilt sample stage containing a piezoceramic actuator in the center was used to modulate the sample for FMM experiments.40 Data was acquired with Picoscan v5.3.3 software and the digital images were processed with Gwyddion (version 2.31) open source software supported by the Czech Metrology Institute.41 A nonmagnetic silicon nitride cantilever from Bruker with a spring constant of 0.01 N m-1 was used for tapping mode and FMM imaging (Veeco Probes, Santa Barbara, CA).



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RESULTS AND DISCUSSION A sample of polystyrene encapsulated cobalt nanoparticles was characterized using tapping mode AFM to provide a frame of reference for studies with force modulation AFM. Clusters of cobalt nanoparticle deposited on a silicon substrate are shown in Figure 1. The nanoparticles tended to form chains and aggregates, with a few individual nanoparticles scattered across the surface. A relatively uniform size and shape of the nanoparticles is evident, as shown with a representative topography frame (Figure 1a). The shapes and arrangement of nanoparticles can also be viewed with the corresponding trace and retrace phase images presented in Figures 1b and 1c. Phase images provide the best contrast of the fine details of morphology and are highly sensitive to surface imperfections.34 There are no distinguishable differences between the trace and retrace images, and the color contrast of the nanoparticles is

Figure 1. Polystyrene encapsulated cobalt nanoparticles imaged with tapping mode AFM; (a) topography frame; (b) simultaneously acquired trace and (c) re-trace phase images. homogeneous. The encapsulated nanoparticles have a regular, spherical shape, and appear to be uniformly coated with polystyrene.



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The size of the encapsulated cobalt nanoparticles measured 25 nm ± 2.8 nm obtained by averaging 100 height profiles. Measurements of height were evaluated since the dimensions in the z direction are not distorted by the shape of the AFM tip. The geometry of the apex of the probe can distort the sizes that are measured for lateral measurements of the width of nanoparticles. The size of the nanoparticles ranged from 16 to 32 nm, and diameters between 2526 nm were most prevalent as revealed in Figure 2.

Figure 2. Size distribution of polystyrene encapsulated cobalt nanoparticles acquired from AFM height profiles (n = 100). To prevent aggregation of the nanoparticles into tight clusters, particle lithography35, 36 was used to control the sample arrangement. To accomplish particle lithography, a solution of nanoparticles was added to a substrate that was coated with a surface mask of larger latex spheres. The mask was prepared by depositing a drop of latex mesospheres on the substrate which was then dried under ambient conditions. After adding encapsulated nanoparticles, the mesospheres were subsequently removed with scotch tape as previously described.37,


38

The

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method of using scotch tape to remove the latex spheres of the surface mask was applied to avoid disturbing the underlying rings of nanoparticles which can be washed away with solvents. A sample prepared with particle lithography was imaged to visualize how the nanoparticles pack together to form rings. Nanopatterned rings of polystyrene encapsulated cobalt nanoparticles were imaged using FMM to visualize and map sample elasticity as shown in Figure 3. The periodicity of the nanorings measured 500 nm matching the diameters of the latex mask. For this experiment, the AC current that was applied to the piezoceramic in the sample stage was deactivated midway through the scan to evaluate the FMM set-up. For the topography frame, (Figure 3a) the size of the nanoparticles remains the same between the top half of the image when the sample is being oscillated compared to the bottom half of the image when the AFM is operating in contact mode without sample oscillation. The topograph in Figure 3a is not affected when the sample modulation was interrupted, the top and bottom areas of the scan are indistinguishable. However, the FMM amplitude and phase images reveal clear changes when

Figure 3. Ring nanopatterns of polymer encapsulated cobalt nanoparticles imaged with FMM. (a) Topography image acquired using FMM; concurrently acquired (b) amplitude and (c) phase images; (d) resonance spectrum when the tip is placed in contact with the sample. the field is applied or discontinued (Figures 3b and 3c). The lower half of the images show no features of the sample, whereas the top half of the scan exhibits details of the sample.



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The FMM amplitude channel furnishes information about the elastic response of the sample as in the example of Figure 3b. Areas with lighter contrast reveal the size and shape of the cobalt core of the nanoparticles which is harder as compared to softer areas of the surface and outer polystyrene layer. When imaging in FMM, the elasticity of the sample is related to the change in the deflection of the cantilever as the AFM tip scans across the surface in contact with the vibrating sample. Softer areas of the sample are compressed to a greater extent and thus absorb more of the energy of the oscillating cantilever which dampens the amplitude signal. The FMM phase shift between the driving signal and AC deflection signal is used to map the viscoelastic properties of the sample. The resolution and color contrast of images is determined by the selected feedback parameters and is based on the type of sample imaged as well as the resonance frequency selected for imaging. In this example, the FMM phase channel (Figure 3c) exhibits better resolved images than the FMM amplitude frame (Figure 3b). A resonance spectrum with the drive activated was obtained when the AFM tip was placed in contact with the sample, as shown in Figure 3d. The AC drive was set to 5% and a frequency of 205 kHz was chosen for imaging parameters with FMM. Other resonance peaks including the ones at 200 kHz and 220 kHz were evaluated and showed morphology details in the amplitude and phase images, however 205 kHz provided the best details and contrast and therefore was chosen as the imaging frequency for FMM studies. Prominent peaks were not observed at frequencies below 180 kHz or above 300 kHz for this experiment. A magnified view of the sample is presented in Figure 4 showing an area where a few individual nanoparticles were randomly distributed in between the rings. The ring nanopatterns of polymer encapsulated cobalt nanoparticles were imaged with FMM at a frequency of 205.7 kHz and 5% drive frequency (Figure 4). Details of the nanoparticle size, shape and arrangement


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are viewed in the topography images in Figures 4a and 4d. A few individual nanoparticles are evident in between the ring nanopatterns. The encapsulated nanoparticles are not strongly bound to the substrate, and with sufficient force can be pushed around on the surface. An advantage of FMM imaging in comparison to standard contact mode AFM is that less force can be applied to the sample, to minimize surface damage. Soft commercial cantilevers with small spring constants (0.01 N m-1) were used for FMM studies.

Figure 4. Metal cores of polymer encapsulated cobalt nanoparticles revealed with FMM to enable subsurface imaging. (a) Topography, (b) amplitude, and (c) phase images and subsequent enlarged views of six nanoparticles; (d) topography, (e) amplitude, and (f) phase frames. For FMM, subsurface imaging is detected with the amplitude images (Figures 4b and 4e) that are derived from tip-sample interactions in which softer areas of the sample absorb a greater amount



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of kinetic energy from the oscillating tip-sample causing the AC deflection signal to dampen with respect to the driving signal. A response in amplitude is attributable to the interaction between the tip and a hard surface caused by a greater deflection in the oscillation of the cantilever. For this experiment, there are three distinct regions of contrast that are revealed. The brightest areas are detected for the hard metal cores of the nanoparticles, the softest regions are shown with dark contrast for the surrounding polymer shell. An intermediate color is observed for areas of the substrate in between the nanoparticles. Measurements of the lateral dimensions of the outer shell of the nanoparticles are not accurate due to a convolution of the geometry of the AFM probe. The shape of the AFM tip is profiled rather than the nanoparticle shape for such very small surface features. Information of surface viscoelasticity is provided for the FMM phase images in Figures 4c and 4f. The phase shift is related to how the tip-sample interactions alter the resonance oscillation of the tip through the time it takes to reestablish an equilibrium position. For FMM experiments with encapsulated metal nanoparticles, not only can information of surface morphology can be acquired for the topography frames, views of what is under the surface can be obtained with optimized parameters. Within the FMM amplitude and phase frames, a surrounding shell of polymer provides a softer surface region that is mapped out surrounding the metal cores to demonstrate subsurface imaging. There are three distinct regions of contrast in the amplitude images, the areas of the substrate located between the nanoparticles, as well as the areas of the encapsulated cobalt nanoparticles and the polystyrene shell. The bright or dark contrast do not necessarily correspond to regions of higher/lower stiffness because the elastic response is sensitive to the frequency selected for characterizations. The reversal in contrast has been reported previously by Jourdan et al.15


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A control experiment was done to acquire FMM images of polystyrene nanoparticles that did not contain a metal core. A comparison of a latex nanoparticle without a metal core was imaged using the same AFM probe and operational parameters that were used to characterize a polymer-encapsulated cobalt nanoparticle (Figure 5). The frequency that was chosen for each experiment was obtained by taking a resonance sweep of the tip in contact with the sample and selecting the most prominent peak, which was chosen at 205 kHz as used previously for Figures 3 and 4. The same AFM cantilever was used in both experiments to minimize potential differences in experimental parameters. A representative image from the control experiment for a single polystyrene nanosphere ~45 nm in diameter within a 300 × 300 nm2 area is presented in Figures 5a-5c. A slightly smaller, individual polystyrene encapsulated cobalt nanoparticle ~30 nm in diameter is shown in Figures 5d-5f within an area of 200 × 200 nm2.



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Figure 5. Comparison of FMM images of a polystyrene nanoparticle versus a polystyrene encapsulated cobalt nanoparticle. Corresponding (a) topography, (b) FMM amplitude, and (c) FMM phase image of an individual polystyrene nanoparticle. Images of an encapsulated cobalt nanoparticle (d) topography, (e) FMM amplitude and (f) FMM phase frames. The topography frames obtained with FMM provide views of the shape, size and surface morphology of an individual nanoparticle (Figures 5a and 5d). The topography images are comparable to those acquired with contact mode. The FMM amplitude and phase images are shown for an isolated polystyrene nanoparticle in Figures 5b and 5c, respectively. The frames reveal interesting details of the surface of the nanoparticle, with a mostly circular region surrounding the nanoparticle. As the tip is scanned across the nanoparticle, the motion of the tip is influenced by the edges of the nanoparticle. However, the bright region of a metal nanoparticle core is not detected for the control sample in Figures 5b and 5c. The nanoparticle is comprised of a single elastic domain and has a higher elasticity and viscoelasticity than the silica surface. The amplitude and phase images of the polymer encapsulated cobalt nanoparticle (Figures 5e and 5f,


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respectively) reveal distinct differences in the elastic and viscoelastic response of the probe to enable subsurface images of the inner cobalt core and surrounding polymer shell. Comparing the top row of images of the control sample to the bottom row of images of the encapsulated metal nanoparticle, the subsurface imaging capabilities of FMM are evident. Even with a high magnification view of a single nanoparticle the differences in elastic response can be sensitively detected with the FMM mode of imaging. The mechanism for subsurface imaging with FMM is based on mechanical differences in tip-sample interactions. The mechanical actuation of the sample at selected frequencies determines the resolution for mapping materials. The stiffness of the sample, the tip-sample interaction, the amount of force applied to the sample by the tip and the energy dissipation by the material are factors to be considered for FMM imaging. CONCLUSIONS Force modulation microscopy was successfully used to accomplish subsurface imaging of polymer encapsulated cobalt nanoparticles with nanometer resolution. Although the nanoparticles are ferromagnetic and have a tendency to aggregate when dried on surfaces, we were able to apply particle lithography to control the arrangement of nanoparticles on the surface. A sample with a ring arrangement of nanoparticles was evaluated with FMM to reveal differences in elastic response for the polymer coating and inner metal core. The FMM mode of scanning probe microscopy has proven to be valuable in the characterization of polymers and nanoparticles. In our studies, The FMM mode has enabled direct visualization between an outer organic layer and an inner metal core that is not possible with other characterizations. Future studies using FMM with nanoparticles will address analysis of the changes in thickness of polymer coatings as a function of reaction variables such as time, temperature, and



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concentration. The thickness of the outer organic coating of nanoparticles is important in determining the impact that polymers have on catalytic and other properties of encapsulated nanoparticles. ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the National Science Foundation (DMR-0906873), and the American Chemical Society Petroleum Research Fund (New Directions 52305-ND).


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REFERENCES 1.

Radmacher, M.; Tillmann, R. W.; Gaub, H. E. Imaging viscoelasticity by force modulation with the atomic force microscope. Biophys. J. 1993, 64, 735.

2.

Maivald, P.; Butt, H. J.; Gould, S. A. C.; Prater, C. B.; Drake, B.; Gurley, J. A.; Elings, V. B.; Hansma, P. K. Using force modulation to image surface elasticities with the atomic force microscope. Nanotechnology 1991, 2, 103.

3.

Binnig, G.; Quate, C. F.; Gerber, C. Atomic Force Microscope. Phys. Rev. Lett. 1986, 56, 930.

4.

Poon, C. Y.; Bhushan, B. Comparison of surface roughness measurements by stylus profiler, AFM and non-contact optical profiler. Wear 1995, 190, 76.

5.

Mikulski, P. T.; Harrison, J. A. Packing-density effects on the friction of n-Alkane monolayers. J. Am. Chem. Soc. 2001, 123, 6873.

6.

Martin, Y.; Wickramasinghe, H. K. Magnetic imaging by ‘‘force microscopy’’ with 1000 Å resolution. Appl. Phys. Lett. 1987, 50, 1455.

7.

Gruverman, A.; Kolosov, O.; Hatano, J.; Takahashi, K.; Tokumoto, H. Domain structure and polarization reversal in ferroelectrics studied by atomic force microscopy. J. Vac. Soc. Technol. B 1995, 13, 1095.

8.

Vesenka, J.; Manne, S.; Giberson, R.; Marsh, T.; Henderson, E. Colloidal gold particles as an incompressible atomic force microscope imaging standard for assessing the compressibility of biomolecules. Biophys. J. 1993, 65, 992.

9.

Garcia, R.; Perez, R. Dynamic atomic force microscopy methods. Surf. Sci. Reports 2002, 47, 197.

10.

Voigtlaender, B. Chapter 14: Amplitude modulation (AM) mode in dynamic atomic force microscopy. In Scanning Probe Microscopy, Atomic Force Microscopy and Scanning Tunneling Microscopy, Springer: 2015.

11.

Akari, S. O.; vanderVegte, E. W.; Grim, P. C. M.; Belder, G. F.; Koutsos, V.; Brinke, G. t.; Hadziioannou, G. Imaging of single polymer chains based on their elasticity. Appl. Phys. Lett. 1994, 65, 1915.

12.

Tomasetti, E.; Legras, R.; Nysten, B. Quantitative approach towards the measurement of polypropylene/(ethylene-propylene) copolymer blends surface elastic properties by AFM. Nanotechnology 1998, 9, 305.

13.

Yamamoto, S.-i.; Yamada, H. Interpretation of direct and indirect force modulation methods using polymer films. Langmuir 1997, 13, 4861.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 29

14.

Galuska, A. A.; Poulter, R. R.; McElrath, K. O. Force modulation AFM of elastomer blends: Morphology, fillers and cross-linking. Surf. Interface Anal. 1997, 25, 418.

15.

Jourdan, J. S.; Cruchon-Dupeyrat, S. J.; Huan, Y.; Kuo, P. K.; Liu, G. Y. Imaging nanoscopic elasticity of thin film materials by Atomic Force Microscopy: Effects of force modulation frequency and amplitude. Langmuir 1999, 15, 6495.

16.

Kuniko, K.; Kei, K.; Hirofumi, Y.; Kazumi, M. Submolecular resolution viscoelastic imaging of a poly(p-toluene-sulfonate) single crystal using force modulation microscopy. Nanotechnology 2008, 19, 065701.

17.

Soliman, M.; Ding, Y.; Tetard, L. Nanoscale subsurface imaging. J. Phys.: Condens. Matter 2017, 29, 173001.

18.

Ebeling, D.; Eslami, B.; Solares, S. D. J. Visualizing the subsurface of soft matter: simultaneous topographical imaging, depth modulation, and compositional mapping with triple frequency Atomic Force Microscopy. ACS Nano 2013, 7, 10387.

19.

Perrino, A. P.; Ryu, Y. K.; Amo, C. A.; Morales, M. P.; Garcia, R. Subsurface imaging of silicon nanowire circuits and iron oxide nanoparticles with sub-10 nm spatial resolution. Nanotechnology 2016, 27, 275703 (10pp).

20.

Spitzner, E.-C.; Riesch, C.; Magerle, R. Subsurface imaging of soft polymeric materials with nanoscale resolution. ACS Nano 2011, 5, 315.

21.

Fukuma, T.; Kobayashi, K.; Kazumi, M.; Yamada, H. True molecular resolution in liquid by frequency-modulation atomic force microscopy. Appl. Phys. Lett. 2005, 86, 193108.

22.

Martinez, N. F.; Garcia, R. Measuring phase shifts and energy dissipation with amplitude modulation atomic force microscopy. Nanotechnology 2006, 17, S167.

23.

Thompson, H. T.; Barroso-Bujans, F.; Herrero, J. G.; Reifenberger, R.; Raman, A. Subsurface imaging of carbon nanotube networks in polymers with DC-biased multifrequency dynamic atomic force microscopy. Nanotechnology 2013, 24, 135701.

24.

Striegler, A.; Koehler, B.; Bendjus, B.; Roellig, M.; Kopycinska-Mueller, M.; Meyendorf, N. Detection of buried reference structures by use of atomic force acoustic microscopy. Ultramicroscopy 2011, 111, 1405.

25.

Kimura, K.; Kobayashi, K.; Yao, A.; Yamada, H. Visualization of subsurface nanoparticles in a polymer matrix using resonance tracking atomic force acoustic microscopy and contact resonance spectroscopy. Nanotechnology 2016, 27, 415707.

26.

Reggente, M.; Passeri, D.; Angeloni, L.; Scaramuzzo, F. A.; Barteri, M.; DeAngelis, F.; Persiconi, I.; DeStefanog, M. E.; Rossi, M. Detection of stiff nanoparticles within cellular structures by contact resonance atomic force microscopy subsurface nanomechanical imaging. Nanoscale 5671, 9, 5671.


Page 21 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


27.

Ma, C.; Chen, Y.; Arnold, W.; Chu, J. Detection of subsurface cavity structures using contact-resonance atomic force microscopy. J. Appl. Phys. 2017, 121, 154301 (1.

28.

Kimura, K.; Kobayashi, K.; Matsushige, K.; Yamada, H. Imaging of Au nanoparticles deeply buried in polymer matrix by various atomic force microscopy techniques. Ultramicroscopy 2013, 133, 41.

29.

Yamanaka, K.; Ogiso, H.; Kolosov, O. Ultrasonic force microscopy for nanometer resolution subsurface imaging. Appl. Phys. Lett. 1994, 64, 178.

30.

Dinelli, F.; Pingue, P.; Kay, N. D.; Kolosov, O. V. Subsurface imaging of twodimensional materials at the nanoscale. Nanotechnology 2017, 28, 085706.

31.

Sharahi, H. J.; Shekhawat, G.; Dravid, V.; Park, S.; Egbertsa, P.; Kim, S. Contrast mechanisms on nanoscale subsurface imaging in ultrasonic AFM: scattering of ultrasonic waves and contact stiffness of the tip–sample. Nanoscale 2017, 9, 2330.

32.

Lewandowski, B. R.; Kelley, A. T.; Singleton, R.; Li, J.-R.; Lowry, M.; Warner, I. M.; Garno, J. C. Nanostructures of cysteine-coated CdS nanoparticles produced with “twoparticle” lithography. J. Phys. Chem. C 2009, 113, 5933.

33.

Lusker, K. L.; Li, J.-R.; Garno, J. C. Nanostructures of functionalized gold nanoparticles prepared by particle lithography with organosilanes. Langmuir 2011, 27, 13269.

34.

Magonov, S. N.; Reneker, D. H. Characterization of polymer surfaces with atomic force microscopy. Annu. Rev. Mater. Sci. 1997, 27, 175.

35.

Saner, C. K.; Lusker, K. L.; LeJeune, Z. M.; Serem, W. K.; Garno, J. C. Self-assembly of octadecyltrichlorosilane: Surface structures formed using different protocols of particle lithography. Beilstein J. Nanotechnol. 2012, 3, 114.

36.

Li, J.-R.; Lusker, K. L.; Yu, J. J.; Garno, J. C. Engineering the spatial selectivity of surfaces at the nanoscale by patterning organosilane self-assembled monolayers via particle lithography. ACS Nano 2009, 3, 2023.

37.

Chen, J.; Liao, W.-S.; Chen, X.; Yang, T.; Wark, S. E.; Son, D. H.; Batteas, J. D.; Cremer, P. S. Evaporation-Induced Assembly of Quantum Dots into Nanorings. ACS Nano 2009, 3, 173.

38.

Mullen, T. J.; Zhang, M.; Feng, W.; El-khouri, R. J.; Sun, L.-D.; Yan, C.-H.; Patten, T. E.; Liu, G.-Y. Fabrication and characterization of rare-earth-doped nanostructures on surfaces. ACS Nano 2011, 5, 6539.

39.

Bull, M. M.; Chung, W. J.; Anderson, S. R.; Kim, S.-j.; Shim, I.-B.; Paik, H.-j.; Pyun, J. Synthesis of ferromagnetic polymer coated nanoparticles on multi-gram scale with tunable particle size. J. Mater. Chem. 2010, 20, 6023.



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40.

Lu, L.; Xu, S.; Zhang, D.; Garno, J. C. Sample stage designed for force modulation microscopy using a tip-mounted AFM scanner. Analyst 2016, 141, 1753.

41.

Necas, D.; Klapetek, P. Gwyddion: an open-source software for SPM data analysis. Cent. Eur. J. Phys. 2012, 10, 181.


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TABLE OF CONTENTS GRAPHIC



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Figure 1 299x118mm (96 x 96 DPI)


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Table of Contents Graphic 307x53mm (96 x 96 DPI)


Subsurface Imaging of the Cores of Polymer-Encapsulated Cobalt

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