Subsurface imaging of carbon nanotube networks in polymers with DC-biased multifrequency dynamic atomic force microscopy

Abstract

The characterization of dispersion and connectivity of carbon nanotube (CNT) networks inside polymers is of great interest in polymer nanocomposites in new material systems, organic photovoltaics, and in electrodes for batteries and supercapacitors. We focus on a technique using amplitude modulation atomic force microscopy (AM-AFM) in the attractive regime of operation, using both single and dual mode excitation, which upon the application of a DC tip bias voltage allows, via the phase channel, the in situ, nanoscale, subsurface imaging of CNT networks dispersed in a polymer matrix at depths of 10-100 nm. We present an in-depth study of the origins of phase contrast in this technique and demonstrate that an electrical energy dissipation mechanism in the Coulomb attractive regime is key to the formation of the phase contrast which maps the spatial variations in the local capacitance and resistance due to the CNT network. We also note that dual frequency excitation can, under some conditions, improve the contrast for such samples. These methods open up the possibility for DC-biased amplitude modulation AFM to be used for mapping the variations in local capacitance and resistance in nanocomposites with conducting networks.

Visualizing the Path of DNA through Proteins Using DREEM Imaging

“…We adapted and extended the dual-frequency single-pass techniques that take advantage of the resonance properties of the cantilever (Glatzel, 2003; Kikukawa et al, 1996; Leung et al, 2010; Stark et al, 2007; Thompson et al, 2013; Ziegler et al, 2007). To simultaneously obtain topographic and DREEM images, we mechanically vibrate the cantilever near the fundamental resonance (ω 1 ), as is done in standard repulsive intermittent contact mode topographic imaging, while applying a static and a modulated bias voltage (V DC and V AC , respectively) to the tip at the first overtone (ω 2 ) to monitor the surface electrical properties (Figure 1) (Stark et al, 2007).…”

“…Because there is no feedback at the first overtone, the DREEM amplitude and phase signals depend on both the strength of the electrostatic force and force gradient, including the static force gradient ($F_{DC}^{\prime }$) (Supplemental Information) (Cleveland et al, 1998; Rodríguez and García, 2004; Tamayo, 2005; Thompson et al, 2013). In addition, other forces may contribute to the signal at ω 2 if they are not canceled by the feedback at the fundamental frequency (Cleveland et al, 1998; Martínez and García, 2006; Martínez et al, 2008; Rodríguez and García, 2004; Tamayo, 2005; Thompson et al, 2013). Generally, the phase image produces higher contrast due to the nonlinear dependence of the phase on the force gradient and energy dissipation ( φ ω 2 depends on the arcsine of the force gradient and the energy dissipation) (Cleveland et al, 1998; Rodríguez and García, 2004; Tamayo, 2005).…”

“…Third, the contribution of the electrostatic interaction between the cantilever and the sample to the electrostatic force is minimized at ω 2 , thereby enhancing spatial resolution in the DREEM image (Ding et al, 2009). Fourth, higher eigenmodes provide enhanced phase contrast compared to the fundamental mode of tip oscillation for both AFM and EFM imaging (Martínez et al, 2008; Stark et al, 1999; Thompson et al, 2013). …”

“…In KPFM, a feedback loop is used to adjust V DC such that it compensates for Δϕ TS , thereby nullifying F ω and generating a potential map of the surface; whereas, in EFM, there is no feedback voltage, and although EFM does not measure surface potential, images of the electrostatic properties of the surface are produced by monitoring the amplitude and/or phase of the induced vibration. Dual-frequency single-pass techniques, where the topography and the surface electrical potential are monitored simultaneously have the highest sensitivity (Barth et al, 2011; Glatzel, 2003; Leung et al, 2010; Thompson et al, 2013). In fact, dual-frequency KPFM has been used to obtain images of DNA (Leung et al, 2010) and transcription complexes (Mikamo-Satoh et al, 2009); however, no details about the DNA in the transcription complexes were revealed.…”

Exfoliated graphite flakes as soft-electrodes for precisely contacting nanoobjects

“…Besides, there are no traces of contamination or degradation along it. Figure 2b is the same region as in figure 2a but now a tip-sample bias gate of 4 V is applied to enhance the contrast of the nanotube [21,22]. Figure 2c shows a zoom-in image of the green square drawn in figure 2a.…”