Direct Observation of Contact Electrification Effects at Nanoscale Using Scanning Probe Microscopy

In the last decade, contact electrification has gained attention for its potential in energy harvesting, addressing fundamental physics. Scanning probe microscopy (SPM) has evolved as a powerful platform, enabling the in situ characterization/manipulation of a sample's tribological/electrical properties at nanoscale. However, although both the sliding and tapping modes are available in the energy harvesting, the lateral sliding using contact mode SPM has predominantly been employed in triboelectric study. In this work, contact electrification on polydimethylsiloxane is investigated, using peak force tapping atomic force microscopy (PF‐AFM). As PF‐AFM quasi‐discretely offers vertical tapping motions of the probe with a regulated force amplitude, resembling a vertical‐type triboelectric nanogenerator, while minimizing lateral forces. The subsequent surface potential measurements reveal that the generated tribocharge is influenced by both the effective work function difference and energy dissipation at the interface. Furthermore, the accumulation of transferred charges is explored by measuring tip‐sample current during the PF‐AFM operation, showing the comparable results with the surface potential measurement. The results can be attributed to the contact potential difference assisted by the energy dissipation at the interface. This study offers an advanced opportunity to understand and study charge generation behavior based on surface properties without damaging the sample.


Introduction
−3] Despite the diverse demonstrations, an exact interpretation of the mechanism remains elusive. [1,4]eanwhile, in light of growing energy costs, renewable energy solutions are becoming crucial.Specifically, the energy scavenging from mechanical/tribological motions has become promising, due to its simple and straightforward architectures of triboelectric nanogenerators (TENG). [2,5]−11] Although various SPM derivatives have been developed, the SPM family can be roughly divided into the static contact mode and non-contact (or dynamic contact) mode based the driving mode of SPM tip and a feedback mechanism for adjusting the tip-sample interaction.So far, the typical study protocol for CE/TE involves combining contact mode SPM with non-contact-based Kelvin probe force microscopy (KPFM) [12,13] (see Figure S1, Supporting Information).−16] In principle, as both CE and TE are derived from the bond-breaking at the interface, there should be no difference between lateral and vertical motion. [17]he bond-breaking releasing frictional energy is thought to induce the CE/TE between the metal and polymer or between polymer materials.−20] Furthermore, even on the atomic scale surface of non-sticky semiconductors, the formation of chemical bonding through mechanical motions and subsequent The red (blue) regime represents the tip's approach to (withdrawal from) the substrate, respectively.b) Schematic of force−distance curve under PF-AFM.The red (blue) curve describes the tip's approach to (withdrawal from) the substrate, respectively.c) Topography and ΔCPD images obtained using KPFM-mode after tapping on PDMS with PF-AFM-mode under the conditions described in (a).d) Illustration of the CE process between the tip and the dielectric sample during the writing (with PF-AFM) and the subsequent reading (with KPFM): the series of energy band diagrams show the charge transfer process during the contact/discontact cycle.The colored area represents the electron-occupied states.The red "e", and blue "I", represent the transferred electron charge and its corresponding current, respectively.Detailed explanations are provided in the main body text.
bond-breaking with energy release has been considered as a primary origin or at least catalyze the energy conversion. [21]evertheless, individual case studies are still necessary due to the variability of charging effects under diverse contactseparation conditions.Also, despite the advancement in various TENG counterparts, the exclusive impact of vertical motion on surface charging remains insufficiently explored through SPM.In this study, we adjust the degree of charge generated on surface through tapping conditions of peak force atomic force microscopy (PF-AFM). [22]Moreover, the charging transfer behavior is observed in a more dynamic manner by observing the CE current during PF-AFM.Thus, we reveal that CE is influenced by both CPD and E dissp through PF-AFM combined with local current, as well as KPFM.

Results and Discussion
In the "writing mode," we generated tribocharge on PDMS film using PF-AFM without bias voltage.Then, we switched to KPFM mode to "read" the tapping-induced CPD.To facilitate the in situ PF-AFM/KPFM, we used a Pt-coated tip with an elastic constant of ≈2.8 N m −1 .PF-AFM oscillates a probe similarly to noncontact SPM but well below resonance frequency (several kilohertz).Therefore, unlike non-contact SPM relying on phase variation, PF-AFM reveals the quasi-static tip motion explicitly.The inset in Figure 1a displays a schematic plot of one cycle-force versus time, where the Z-piezo pushes the probe toward the sample until the peak force reaches a set-point for feedback.Then, the probe retracts from the sample and returns to the initial position.Indeed, Figure 1a shows representative force versus time curves of cantilever obtained during PF-AFM.Here, the regular tapping was continuously applied to the surface at 2 kHz, with a predefined feedback parameter of 20 nN for the peak force.Figure 1b shows the force−displacement curve in each cycle.Due to the tip-sample interaction variations, the curve logged for each pixel provides the spatial distribution of the local mechanical properties with the corresponding topography.The adhesion force (F a ) is defined as the minimum force point for the tip to be pulled away from the sample.Deformation (Δd) represent the total penetration depth, including both elastic and plastic deformations.Energy dissipation (E dissp ) is calculated by the hysteresis area between approaching and retracting processes.This dissipation is associated with the work and deformation carried out in both the repulsive and attractive regimes.Modulus of sample can be extracted from the slope of unloading curve. [14,23,24]n Figure 1c, after a 500 × 500 nm 2 was tapped by PF-AFM, 5 × 5 μm 2 -area was monitored with KPFM.Considering the scan velocity (500 nm −1 s) and the image resolution (256 pixels per line), and the tapping frequency of 2 kHz, this corresponds to eight taps at each pixel.Similarly, the tapping count at each pixel is controllable by tuning the scan conditions.The left-hand side of Figure 1c shows negligible tapping effects to topography.Conversely, the CPD image (right-hand side, Figure 1c) shows a clear dark contrast against the untapped surroundings, with a ≈140 mV reduction within the tapped area.We note that the difference in the CPD between the written and the unwritten regions (ΔCPD) was imaged to enhance visibility.During KPFM, since tip bias was adjusted to nullify the capacitive force generated by the effective work function disparity between tip and sample, the CPD value can be interpreted as where Φ s and Φ t stand for the work functions of tip and the sample surface, respectively.While Φ t is defined as the energy level difference from the fermi level, E F to the vacuum level, E vcc for a dielectric sample, effective Φ s (Φ s(eff) ) is implicitly inferred from the difference between the charge neutral level (E s,cnl ) and the vacuum level. [25,26]Φ t can be determined from Figure S2 (Supporting Information), utilizing Equation ( 1).The raw CPD image and its time evolution were suggested in Figure S3a,b (Supporting Information) through the continuous KPFM scanning.Additionally, ΔCPD in Figure S3a (Supporting Information) was plotted against time in Figure S3c (Supporting Information).In Figure S4 (Supporting Information), when the mechanical perturbation was no longer provided by PF-AFM, the dark contrast gradually fades over time, confirming that the dark contrast resulted from PF-AFM.The possible mechanism of the CPD contrast is illustrated in Figure 1d.Considering the negative CPD values in Figure S3a,b (Supporting Information) along with Equation (1), Φ s(eff) can be regarded as larger than Φ t .Therefore, as the tip approaches the sample, the electron transfer can be driven from tip to sample, due to Φ t −Φ s(eff) , while minimizing the difference between E F,t and E cnl,s .Therefore, unoccupied sample surface states would be filled with electrons by raising the energy level from E cnl,s up to E cnl,s + ΔE F ≈ E F,t , amounting to a total charge density of  = −N s .avg ΔE F , where N s.avg represents the mean surface states distributed within the energy level variation. [7] = −e Sequentially,  would also induce image charge on the metal tip,  t as well.Image charge can be induced on the bottom electrode, but its effect is likely negligible due to: 1) the sample thickness being ≈10 4 -times greater than the tip-sample distance (30 nm), and 2) the nullified CPD during KPFM. [7]Thus,  t is approximately equal to −.Assuming a parallel capacitor model for the tip-sample structure, as the tip detaches from the sample by the distance z, the sample's vacuum level can be elevated by ze/ 0 in reference to that of tip, due to the built-in field.Thus, ΔCPD reflects the superposition of ΔE F and the potential variation, ze/ 0 , in vacuum level.
therefore, the dark ΔCPD of Figure 1c can be interpreted as the negative charge transport from the tip to the sample due to CE (Figure 1d).Then, we evaluate how CE can be manipulated by PF-AFM.Figure 2a showcases the real-time plots of the tip force modulated by the tapping frequency and amplitude.In Figure 2b, the variations in both the tapping conditions significantly altered ΔCPD distributions.To gain better insight into the effect of tapping factors to the relaxation behavior of CE, time-dependent ∆CPD was studied (Figure 2c).The recovery trend of ∆CPD can be fitted to the stretched exponential law: where V 0 , t c , and  represent the initial amplitude, relax time constant, and stretch parameter, respectively. indicates the level of dispersion, ranging from 0 to 1.According to the literature, [10,27−30] a stretched exponential decay characterizes the extent of irregularity in various disordering systems.The fitting lines were overlaid with the corresponding data (Figure 2c), and the resulting parameters were individually plotted in the panels of Figure 2d.In the figure panels, both tapping factors significantly increase V 0 (top) and t c (middle).Furthermore, both factors led to a progressive decrease in  (bottom), which is associated with broadened energy states or barriers at the hopping sites. [29]ince no bias voltage was applied to the sample, only the tapping force derives the bond breaking, leading to the reactive dangling bonds (i.e., Si • or Si-CH 2 • ) as the tribocharge mechanism. [31,32]pecifically, stronger tapping is more likely to break or reconfigure surface bond structures, inducing greater CE or TE.
To systemically explore the tapping stress effect on CE, mechanical parameters were simultaneously characterized, while scanning the 500 × 500 nm 2 -area, with an increase in the tapping amplitude from 10 to 120 nN.The corresponding force−distance motions were plotted in Figure S4a (Supporting Information).Figure 3a shows the representative maps of topography, F adh , Δd, E dissp , in the first, second, third, and fourth-column images, respectively.The images in each row correspond to the tapping force employed.The topographic images reveal unchanged morphology regardless of tapping force.To assess the surface corrugation, we plotted the roughness parameter, R q , [33] versus tapping force, exhibiting no apparent variations (Figure S4b, Supporting Information).However, the corresponding F adh , Δd, and E dissp contrasts increased (Figure 3a). Figure 3b displays their signal distributions for different tapping forces, revealing a more pronounced blue shift and dispersion under stronger tapping.This indicates that the larger Δd stems from the breakage of more chemically attractive bonds, due to the stronger tapping force, releasing more E dissp .Right after conducting PF-AFM in Figure 3, the wider area encompassing the tapping region were explored with KPFM. Figure 4a indicates that the stronger tapping induces the higher ΔCPD over the larger area (4 × 2 μm 2 ).The representative ΔCPD profiles were plotted in the inset of Figure 4b.Utilizing Gaussian fitting, central ΔCPD distribution were plotted against tapping force (Figure 4b).The figure displays a faster ΔCPD growth at lower force, with the gradient declining as force increases, eventually reaching plateauing in a high-force regime.According to Shin et al., [17] friction can dissipate energy at the interface, developing the larger electrostatic potential, which, in turn, facilitates the thermoelectric charge transport across the interface.To assess the CE−E dissp correlation, ΔCPD was plotted against the E dissp suggested in Figure 3 for different tapping forces.Figure 4c shows the data cluster fitting with a power law (power factor ≈1.41), similar to Wolloch et al., [34] suggesting a superlinear relation between surface potential and the adhesive energy at the interface (a power factor of ≈1.5).Similarly, Li and Li [35] showed that E dissp linked to friction and F a results from the work function-induced electronic interaction between the tip and sample on a smooth surface.Considering the low R q irrespective of tapping force (Figure S4b, Supporting Information) and the high Seebeck coefficient of PDMS (≈−10 μV K −1 ), [17] the pronounced ΔCPD can stem from thermoelectric charging effect induced by the enhanced F a and E dissp in Figure 3.
To investigate more transient transport behavior during CE, a current measurement was attempted during PF-AFM.In the setup, the conductive probe connects to an ultra-low noise current preamplifier, like a typical current SPM. [14]Figure S5a (Supporting Information) shows a representative current signal on PDMS, simultaneously measured with the tip's vertical motion.In Figure S5b (Supporting Information), average current (I AVG ) was calculated by averaging the current signals acquired during a single tapping cycle.To mitigate noise, the probe repetitively scanned along a 100 nm-line at 1 Hz, allowing the determination of I AVG for a single line scan by calculating a mean value from the acquired I AVG data.Consequently, I AVG versus time curve can display the current trajectory over the elapsed time with 1 s time resolution.To compare with the previous PF-AFM−KPFM results, the corresponding probe was utilized again, and both the tapping force and frequency were adjusted to approximate the conditions.Interestingly, both Figure 5a,b demonstrate a common trend: I AVG increases progressively in a more positive direction over time in tapping frequency (Figure 5a) and tapping force (Figure 5b).As elucidated in Figure 1d, the negative CE charge created on the sample surface causes the positive current flow toward the tip (Figure 5c).The probability of bond-breakage can be further enhanced due to increased E dissp from the higher tapping force or tapping counts, like in the KPFM results.To validate the proposition, the relation between the I AVG and E dissp was evaluated during PF-AFM (16 nN-4taps).The concurrently acquired E dissp , and I AVG were plotted against time in Figure 5d,e, respectively, which analogously increase with the elapsed tapping time.Furthermore, utilizing the concurrently collected data pairs of (E dissp and I AVG ), we constructed a scatter plot where x-, and y-coordinates correspond to E dissp , and I AVG of each data point, respectively (Figure 5f).The linear trend in Figure 5f clarifies that E dissp and I AVG have a relatively strong positive correlation, with the correlation coefficient of ≈0.7.Likewise, we assessed the correlation between E dissp and contact current (I cont ; the average current over the contact regime in Figure S5, Supporting Information).Again, we found a similar positive correlation between E dissp and I cont , (Figure S6, Supporting Information).The results well confirm that the proposed scenario that CE is influenced by the work function difference, and that the process can be further enhanced by the interfacial energy dissipation.Our findings demonstrate that the amount of created tribocharge can be controlled by both the tapping factors, shedding light on the fundamental mechanisms behind CE phenomena.

Conclusion
In summary, we investigated tip-induced CE using in situ PF-AFM/KPFM.As PF-AFM employs quasi-static tapping motion with controlled normal force, minimizing lateral sliding, it is advantageous for exploring charge transfer behavior depending on tapping conditions (e.g., force or frequency).The study clarifies that CE is driven by effective work function differences at the interface and is enhanced by E dissp .Current measurements during PF-AFM not only reconfirmed CE-related charge accumulation  3).The x-and y-coordinates of each data were obtained from Figures 3a and Figure 4a respectively.The orange line fits all data with a power law with a factor of 1.41.The color code corresponds to the one used in Figure 4b.
but also revealed more transient transport behavior than static KPFM.Current measurements during PF-AFM not only reaffirm CE-related charge accumulation but also reveal more transient transport behavior than static KPFM.In our future work, we plan to expand our PF-AFM investigations to encompass a wide range of dielectric systems that may be influenced by atmospheric conditions or subjected to various chemical treatments, aiming at enhancing our comprehension of contact electrification (CE) phenomena.

Experimental Section
Sample Preparation: Polydimethylsiloxane (PDMS) thin film was utilized as a primary exemplary sample.To produce the PDMS film, PDMS elastomer and cross-linker were mixed at a weight ratio of 10:1, followed by degassing under vacuum for 30 min.From the solution, a PDMS film was fabricated by spin coating at 500 rpm for 1 min, and then cured in an oven 100 °C for 5 h.The thickness of the resulting PDMS was ≈300 μm.Then, the PDMS film was positioned on a metallic plate using silver epoxy.
Measurement Methods: All SPM measurements were performed with a commercial AFM (MultiMode 8, Bruker Corp.) in ambient air.To facilitate the PF-AFM/KPFM characterization, a Pt-coated tip was employed for the three different SPM modes.For each tip, the elastic constant was precisely calibrated using the thermal tune method. [14]For KPFM study, after completing each topography line in the non-contact SPM mode using an electrically conducting tip, the feedback loop controlling the vertical piezo was turned off.Subsequently, the tip was lifted from the surface and traced over the pre-acquired topography at a constant distance (30 nm in this study).During the "interleave" mode, a small AC bias voltage (1 V in this study) was applied to the tip.As the AC frequency was chosen to match the original resonant frequency of cantilever (≈75 kHz), the SPM tip exhibited a highly sensitive response to the surface potential configuration.Using the lock-in technique, CPD between tip and sample is estimated and a DC bias voltage is also applied to the tip, to nullify the electrostatic interaction.Immediately, the corresponding CPD value was recorded on KPFM channel.For more quantitative analysis, every time, the value of Φ t was calibrated as ≈4.9 eV by using HOPG slate (Φ HOPG = 4.6 eV) and Equation (1) in the manuscript, as shown in Figure S2 (Supporting Information).Since PF-AFM mode oscillates the cantilever far below its resonant frequency, the vertical motion of the cantilever depends on Z-piezo motion, with the feedback loop being maintained based on the peak force value.From each individual tap, peak interaction force and various mechanical properties of material can be collected.Considering the scan rate, image resolution and tapping frequency, the tapping number per each pixel was estimated.
To measure tip current during PF-AFM, the conductive probe was directly shorted to an ultra-low noise current preamplifier with 1 nA V −1current gain.To secure the current path toward the preamplifier, the tip was not only electrically isolated by insulating the contact between the tip and the tip holder, but also shielded by a metallic casing in the tip-current amplifier system.
Statistical Analysis: Each SPM image frame is usually composed of 256 × 256 pixels.To evaluate surface roughness, the root mean square average of the deviation from the mean height was calculated over the entire scan area.KPFM study provides a spatial CPD distribution for each condition.The CPD value and its error bar were estimated from the peak center, and the full-width at half-maximum, respectively, through Gaussian fitting of their histogram distributions.Analogously, various mechanical properties can be estimated from the corresponding channel images of PF-AFM.The statistical analysis and visualization of data were performed using NanoScope Analysis and Python.

Figure 1 .
Figure 1.a) Force−time plot in PF-AFM with 2 kHz−20 nN tapping; (inset) a single cycle of force and the corresponding piezo Z-position as a function of time.The red (blue) regime represents the tip's approach to (withdrawal from) the substrate, respectively.b) Schematic of force−distance curve under PF-AFM.The red (blue) curve describes the tip's approach to (withdrawal from) the substrate, respectively.c) Topography and ΔCPD images obtained using KPFM-mode after tapping on PDMS with PF-AFM-mode under the conditions described in (a).d) Illustration of the CE process between the tip and the dielectric sample during the writing (with PF-AFM) and the subsequent reading (with KPFM): the series of energy band diagrams show the charge transfer process during the contact/discontact cycle.The colored area represents the electron-occupied states.The red "e", and blue "I", represent the transferred electron charge and its corresponding current, respectively.Detailed explanations are provided in the main body text.

Figure 2 .
Figure 2. a) Force−time plot in PF-AFM with (left) 1 nN-8 taps, (middle) 1 nN-16 taps, (right) 16 nN-16 taps.b) ΔCPD image corresponding to the tapping conditions of PF-AFM.c) The ΔCPD versus elapsed time.Solid lines represent overlaid fitting results on the data points.d) (top) The initial amplitude, (middle) the time constant, and (bottom) stretched parameter were extracted from the fitting curves in (c).

Figure 3 .
Figure 3. a) (Column 1) topography, (column 2) adhesion force, (column 3) deformation, (column 4) energy dissipation deformation images of PDMS under varying the loading force.The images in each row were simultaneously acquired at a constant loading condition.Note that a common intensity scale was used for each signal channel.b) Distributions of (left) adhesion force, (middle) deformation, and (right) energy dissipation obtained from (a).

Figure 4 .
Figure 4. a) Series of ΔCPD images after PF-AFM in Figure 3.The dark region represents the triboelectric charge created by the previous tip tapping.b) Plot of tapping force versus ΔCPD from (a); (inset) representative ΔCPD profiles extracted from (a).The color code corresponds to that used in the main graph.c) Correlation between ΔCPD and E dissp (E dissp values extracted from Figure3).The x-and y-coordinates of each data were obtained from Figures 3a and Figure4arespectively.The orange line fits all data with a power law with a factor of 1.41.The color code corresponds to the one used in Figure4b.

Figure 5 .
Figure 5. a) Current−time plots at 16 nN-tapping force with varying tapping frequencies.b) Current−time plots corresponding to different tapping force, under a constant frequency of 16 taps per pixel; X-offsets are applied to three curves for enhanced differentiation.c) Illustration of the CE current between the tip and the dielectric sample surface during the writing and the simultaneous current reading; the energy diagrams below demonstrate the charge transfer during CE process.The red "e", and blue "I", represent the transferred electron charge and its corresponding current, respectively.d) Energy dissipation−time plot.e) CE current−time plot; (d) and (e) are simultaneously recorded during PF-AFM.f) Dissipation−current correlation constructed form (d) and (e).