Experimental and Simulation Investigation of the Nanoscale Charge Diffusion Process on a Dielectric Surface: Effects of Relative Humidity

Abstract

Electrostatic charge generation and diffusion on the nanoscale were studied by atomic force microscopy and Kelvin probe microscopy. The charge diffusion coefficients were obtained by matching experimental results with numerical solutions of the diffusion equation. The results found that the relative humidity variations could significantly alter both the charge generation and diffusion processes. For the charge generation, the increase in relative humidity led to a decrease in transferred charge amount between the contact surfaces owing to the presence of the absorbed water film on the tip–sample interface. For the charge diffusion, the increase in relative humidity could accelerate the charge diffusion process laterally, and the diffusion coefficient of higher relative humidity was 3–5 orders of magnitude larger than those under dry air and N2. It was proved that the charge diffusion process for the positive charge was not necessarily faster than that for the negative one. In addition, the contribution from atmospheric water molecules to the surface charge diffusion was distinguished from that of absorbed water films by calculating the net loss of surface charge, and this effect was found to be more obvious under higher relative humidity. The dominant mechanism for the charge diffusion was discussed, and we argued that the relative humidity could be the main reason, and probably the only reason, for the charge diffusion and decay on the dielectric surfaces.

KPFM Study on the Charge Sign Reversal during Nanoscale Triboelectric Charging Induced by a Worn Tip and Sample Pretreatment

“…This trend could be further verified by Figure e,f, where the CPD of the charged area was approximately 150 mV lower than the baseline under a 10 nN applied force, while it reached approximately 700 mV when the applied force was higher than 200 nN. This observation is consistent with other research studies regarding the effect of applied force on the triboelectric charging, ,, which is a load-dependent process wherein a higher applied force could enhance the charge transfer at the interface. A well-acknowledged explanation for this phenomenon is that the contact surface area could be increased with increasing applied force, resulting in more chances for the electrons to flow from the tip to the surface.…”

“…It is also worth noting that the surface potential of the uncharged area for different applied forces was different from each other. This is because the surface state of the SiO 2 surface was slightly altered by the charges that were initially absorbed, induced, or trapped by the defect on the surface . Therefore, we subtracted the baseline of the cross-sectional profile along the dashed line in Figure a and the corresponding lines in Figure b–d, and obtained the surface potential profiles under different applied forces, as shown in Figure e.…”

“…Given that the conductive layer of the tip was only tens of nanometers in thickness, the damaged tip will inevitably cause the silicon substrate to be exposed. However, it is believed that the damaged tip was still applicable for the KPFM measurement since only the contact part of the tip was worn out and the rest is still conductive, as discussed in our previous work . In the amplitude-modulation (AM) mode of KPFM, the whole tip will contribute to the surface potential measurement; thus, the slight deformation on the top of the tip has little effect on the surface potential mapping.…”

“…It was believed that the charge transfer at the contact interface could have originated from electron transfer, − ion transfer, − or material transfer . Recently, the contribution of electron transfer to contact electrification was distinguished from that of ion transfer using the theory of electron thermionic emission, and it was found that both electron transfer and ion transfer could be responsible for the charge transfer during contact electrification. , Previous investigations have proved that the polarity and magnitude of the transferred charge could be altered by applied forces, , friction cycles, , applied electric field, ,, oxygen atmosphere, relative humidity, and temperature . In some extreme cases, charge sign reversal during triboelectric charging could result from a slight change in friction velocity …”

“…Combining with contact-mode atomic force microscopy (AFM), the electrostatic charge could be deposited on the solid surface and then be measured by KPFM. This well-established method has been proved to be an effective way to quantitively study the triboelectric charging at the nanoscale. ,,,− It was reported that the increase in the applied load and the number of friction cycles could enhance the charge transfer in the tip–sample interface. , Other studies found that the SiO 2 surface could be either negatively or positively charged when triboelectric charging with the original tip and damaged tip, respectively . However, the previous study only focuses on the tip deformation, while the influence of surface pretreatment on the charge sign reversal during triboelectric charging has been inadequately investigated.…”