Nanocontact Electrification: Patterned Surface Charges Affecting Adhesion, Transfer, and Printing

Abstract

Contact electrification creates an invisible mark, overlooked and often undetected by conventional surface spectroscopic measurements. It impacts our daily lives macroscopically during electrostatic discharge and is equally relevant on the nanoscale in areas such as soft lithography, transfer, and printing. This report describes a new conceptual approach to studying and utilizing contact electrification beyond prior surface force apparatus and point-contact implementations. Instead of a single point contact, our process studies nanocontact electrification that occurs between multiple nanocontacts of different sizes and shapes that can be formed using flexible materials, in particular, surface-functionalized poly(dimethylsiloxane) (PDMS) stamps and other common dielectrics (PMMA, SU-8, PS, PAA, and SiO(2)). Upon the formation of conformal contacts and forced delamination, contacted regions become charged, which is directly observed using Kelvin probe force microscopy revealing images of charge with sub-100-nm lateral resolution. The experiments reveal chemically driven interfacial proton exchange as the dominant charging mechanism for the materials that have been investigated so far. The recorded levels of uncompensated charges approach the theoretical limit that is set by the dielectric breakdown strength of the air gap that forms as the surfaces are delaminated. The macroscopic presence of the charges is recorded using force-distance curve measurements involving a balance and a micromanipulator to control the distance between the delaminated objects. Coulomb attraction between the delaminated surfaces reaches 150 N/m(2). At such a magnitude, the force finds many applications. We demonstrate the utility of printed charges in the fields of (i) nanoxerography and (ii) nanotransfer printing whereby the smallest objects are ∼10 nm in diameter and the largest objects are in the millimeter to centimeter range. The printed charges are also shown to affect the electronic properties of contacted surfaces. For example, in the case of a silicon-on-insulator field effect transistors are in contact with PDMS and subsequent delamination leads to threshold voltage shifts that exceed 500 mV.

The Influence of Distant Substrates on the Outcome of Contact Electrification

“…[7g] Tw ocommon assumptions in these and many other [1a,b,d, 8] studies have been that 1) CE is asurface phenomenon with all the relevant processes limited to within, at most, 60 nm from the insulatorssurface [9] and 2) the maximal magnitude of the developed charges is limited by the dielectric breakdown of the gas between the charging surfaces.H owever,i ts hould be remembered that contact-charged interfaces also produce electric fields and can induce image charges far away from the interfacial region. As aconsequence of this feedback, the magnitudes of the net charges measured on the polymers depend on the conductivity of substrates that were never in contact with the chargeseparating interface.A sw es how,t hese effects can be attributed to the locations and dynamics of the discharge of gas not between the contact-charging surfaces-which, as mentioned above is well documented [10] -but between the distant substrate and these surfaces.Wealso demonstrate how the discharges can be eliminated without adjusting the dielectric strength of the gas by insulating the substrates edges.Overall, these findings extend our understanding of CE beyond the properties of the contacting materials and can help systematize [11] the study of this interesting and important phenomenon. [6] When, however, the substrates are not grounded, new channels of charge dissipation can become operative on longer timescales.H erein, we show how such dissipation of induced/image charges "feeds back" and modifies the distribution of surface charges on the initially electrified polymers.…”

Tribological interaction between polytetrafluoroethylene and silicon oxide surfaces

“…We have also carried out measurements on the PTFE transfer to silicon oxide surfaces which have been treated with aqueous acidic and basic solutions before the transfer. Although such treatments were reported to cause drastic triboelectrical changes of the silicon oxide surfaces, 16 we were not able observe any significant difference between the acid or base treated and untreated samples. In light of these new findings, additional experiments are definitely needed to clarify and further our understanding about the nature of the forces contributing to the mechanism(s) of this material transfer both from the PTFE side and also related to the surface.…”

“…Silicon/silicon oxide substrates were chosen since they provide very smooth surfaces, and also for ease of modification of their acid-base character by aqueous solutions. 16 It is expected the results can also be extended to other similar and technologically important substrates like glass, quartz, Al 2 O 3 , TiO 2 , Si 3 N 4 , TiN, ITO, etc., for other applications.…”

“…[7][8][9][10] Although many other studies on PTFE transfer to another solid surface have been published, most have focused on improvement of the physical properties of surfaces, i.e., achieving lower friction coefficients and wear rates. [11][12][13][14][15][16][17][18][19] However, the nature of those interactions at the atomic scale has not been completely understood. To this end, applying computational chemistry methods as a tool for analyzing the mechanism of friction and transfer film formation has recently been reported.…”

Base-treated polydimethylsiloxane surfaces as enhanced triboelectric nanogenerators

“…The fresh PDMS surface consists of mainly Si-CH 3 bonds with some Si-O and Si-OH bonds [27]. When the fresh PDMS is exposed to UVO, the Si-CH 3 bonds are broken and converted to Si-O, Si-OH, and Si-COOH bonds to form a mildly base and polar surface [27]. When the UVO-treated PDMS further sprinkled with NaOH solution, the Si-OH bonds are changed to Si-O bonds.…”