Active control of surface forces via nanopore structures Free

Metallic nanostructures are of great interests due to their excellent optical and electrical properties.¹ Among them, three dimensional metallic porous nanostructures are particularly interesting due to their unique properties and numerous applications.² Highly ordered pore arrays of anodic aluminum oxide (AAO) have been used as a template, and the process of making them is generally simple.³ Using AAO structures one can easily fabricate metallic nanopore structures (MNS)⁴ for making nanosensors, including surface enhanced Raman spectroscopy (SERS), and gas detecting sensors.⁵ 

The durability of today's nanodevices has been a bottleneck for further development due to their unexpected interfacial failures. To eliminate such interface failures, the tribological properties of nanostructured devices are needed for design and fabrication.^6,7 The atomic force microscope (AFM) is commonly used for measuring tribological properties such as adhesion and friction at the nanometer scale.⁸ 

The effect of electrical potential on surface morphology and friction has been reported in various materials.^9–11 To date, however, the tribological performance of nanodevices under the influence of an applied electrical potential has not been well-studied. An understanding of the effects of electrical potential on the tribological performance at the nanoscale is important for the design and reliability of nanoelectromechanical systems (NEMS)/MEMS devices.

In the present letter, we study the effects of an electrical potential on the tribological properties of nickel-based MNS (fabricated via AAO) using an AFM. Nickel is interesting to study because doping nickel could increase the piezoelectricity of certain materials due to being natural magnet.¹² Nickel based materials with nanopore structures have been reported to be used in nanosensors and energy devices, such as electrodes for batteries and capacitors, among others.¹³ 

Preparation of the MNS: Using AAO structures as a template, the MNS was fabricated by evaporating nickel as a metal source on AAO structures.⁴ The average thickness of AAO template is 250 nm and that of MNS, which is located on the AAO, is 300 nm. The MNS pore diameter is controlled by changing the pore diameter of AAO, which in this work varied from 150 nm to 350 nm (zero nm pole implies dense surface with no pores).

Electrical-tribology study using an AFM: To study the effects of electrical potential on the tribological performance, an electrical circuit configuration was built and attached to the AFM, as illustrated in Figure 1. This electrical circuit configuration includes both a multimeter (Wavetek Meterman 35XP) and an ampere meter (keithley 486 picoammeter) for checking the magnitude of the electrical potential and current signal, it also contains a DC type adjustable power source that can apply an electrical potential to the MNS during scanning. The AFM probe tips were made of silicon nitride (Si₃N₄). One AFM tip which was for contact mode tribology test has a tip diameter of 950 nm and another AFM tip which was for non-contact mode image scan has a tip diameter of 30 nm. During scanning, the electrical potential applied was from 0 mV to 800 mV. AFM test was done by repeating over 15 times. The pull-off force was measured using the AFM under an applied electrical potential. Using force-distance measurements from AFM and knowing the spring constant and deflection distance from the initial set point of the cantilever, one can readily calculate the pull-off force at a point of contact of the AFM tip and the sample. The friction properties of the sample are measured using the lateral force microscope (LFM) mode of the AFM.

Morphology: Figure 2 depicts AFM height images of the MNS under various applied electrical potentials from 0 to 800 mV. As shown in the figure, the higher regions (peaks) become larger as the applied electrical potential increases. The lower regions (valleys), on the other hand, become deeper with increased electrical potential. The circle in the figure represents a unit wall of the MNS. It is seen to expand with increasing electrical potential. Figure 3 shows MNS profiles (corresponding to white lines in Figure 2) are changed with the electrical potential applied in comparison 0 mV electrical potential with 800 mV applied condition. As a result, the morphology of the MNS was expanded with the increase in electrical potential. To analyze the AFM height image change, alteration of over 350 nm height regions were examined with applying electrical potential. Figure 4 shows a percentage change of over 350 nm regions in certain areas of MNS height image when the electrical potential increases. The percentage of over 350 nm height region was increased by increasing electrical potential. This result indicates that the surface morphology of MNS is influenced by the electrical potential. In one of our previous studies, we have reported expansion of a piezoelectric polymer (PVDF in that case) under an applied voltage.¹⁶ It is interesting that the current letter showed similar effects on Ni-containing MNS. Figure 5 shows the corresponding AFM phase images of the MNS. On the same scale bar, the contrast of MNS phase images change with electrical potential that was varied from 0 to 800 mV. The brighter region (phase-shift signal: over 4000 mV) becomes larger with the increase in potential. The AFM phase images clearly show that surface property of MNS is affected by applied electrical potential and it can be related to tribological behavior of MNS. These height and phase image alterations indicate the change of contact area that is related to friction and adhesion. This will be discussed next.

Pull-off force and friction: Using the force-distance AFM mode, we conducted MNS pull-off force which is represented as a characteristic parameter of the adhesion and friction experiments, and the results are shown in Figure 6. It is possible to identify that the pull-off force was reduced due to the existence of the pores. The white bar is that of the “flat” (no pores) surface that exhibits the highest pull-off force values. The lowest values of pull-off force are obtained from the sample with the largest pores of 350 nm, showing an 88.32% reduction in pull-off force, in comparison with the flat surface without the applied electrical potential. It is known that the pull-off force is affected by the real contact area. A contact radius a, which is based on the Hertz contact theory,¹⁴ between the AFM tip and the flat (no pores) nickel surface is represented by the following equation:

where P is the applied load, R is the radius of tip, and E^* is the effective modulus expressed by

where both E₁ and E₂ are modulii of elastic, and υ₁ and υ₂ are Poisson's ratios. Those parameters (1 and 2) represent tip and surface. The Hertzian contact area A is assuming infinitely smooth surfaces.

When the AFM tip comes in touch with the MNS, the Hertzian contact area is set by the surface porosity of MNS.⁷ Basically, the contact area is decreased with the increase in surface porosity based on the Hertzian contact. The inter-pore distance of MNS is about 500 nm, the surface porosity for flat surface (no pore) is 0, for 150 nm pore is 8.16%, for 350 nm pores is 44.43%. An equivalent contact area A_eq is represented by

where P.O. is surface porosity. The equivalent contact area is reduced by pore size enlarging which means that the surface porosity increasing without electrical potential applied (0 mV). When an electrical potential is applied, the magnitude of pull-off force of MNS is gradually decreased, in Figure 6. According to Eq. (1), the contact area is decreasing with increasing effective modulus. From AFM phase image results, the brighter region which represents even higher stiffness or elastic modulus¹⁵ of MNS gradually increased with increasing electrical potential. This means effective modulus between the MNS and AFM tip is increasing. The contact area between AFM tip and MNS decreases with increasing electrical potential due to the stiffness of the MNS increasing. On the other hand, the equivalent contact area increases with decreasing pore size due to the porosity decreased, according to Eq. (4). Figure 2 shows that the pore size is decreased by applying electrical potential due to the unit wall expanded.

For the discussion of MNS contact area, the increasing stiffness conflicts with the porosity decreasing under same electrical potential increased condition. Porosity-decreased works to increase the contact area, however, stiffness-increased works to decrease contact area. Overall, pull-off force decreases with increasing electrical potential. This means that stiffness factor more intensively affects to change the equivalent contact area of MNS than porosity factor. This result proves that the pull-off force could be changed by applying electrical potential into the MNS.

According to Figure 6, the flat sample has the highest pull-off force. This indicates the more active interactions between AFM tip and the surface itself. Without any porous, the contact between the AFM tip and the surface is expected to be larger hence the high force. Friction experiments were conducted using the AFM with LFM mode and results are shown in Figure 7. The flat surface (no pore) with much smoother surface profile shows lower electrical volt, i.e., friction volt than MNSs. The friction volt, overall, is decreased with the increase in electrical potential. The frictional performance which is determined by LFM mode depends on the degree of the lateral twist of an AFM tip. Surface morphology and contact area of MNS are critical parameters to determine the friction performance of MNS with AFM. According to results of height images, the edge of the surface became much blunter rather than sharp with applying electrical potential due to the swelling of surface (inlet image of Figure 3). In this case, AFM tip can be less twisted than before applied electrical potential into the MNS. Moreover, contact area decreasing at interface between AFM tip and MNS with increasing electrical potential could contribute to the decrease in friction. The big error bars of each data point is due to the relative size comparison between the AFM tip and sample surface. The variation of the surface with pores will cause the high variation of the tip-surface interactions. This is correlated to our analysis in contact area.

On average, the pull-off force was non-linearly decreasing with increasing electrical potential. This is because the pore size of MNS is not the same in the scanning area. Basically, the pull-off force experiments were performed by single point of contact of the tip and target. The result of pull-off force which depends on localized pore size could show nonlinear behavior with applying electrical potential due to the lack of uniformity about pore size in MNS. In comparison, friction test is relatively insensitive to localized pore size, because the friction result stems from scanning large area of surface. The conflict interaction by electrical potential increasing between porosity and stiffness is also caused that tribological behavior of the MNS decreased, nonlinearly. It is possible to indicate how many percent of tribological property such as pull-off force and friction signal changed in case of the MNS which pore size is 150 nm and 350 nm. Based on 0 mV applied condition, the pull-off force decreased at 800 mV applied condition. In case of 150 nm pore size sample, the friction signal and pull-off force were decreased by 28.82% and 24.71%, respectively. About 350 nm pore size sample, the friction signal and pull-off force were decreased by 43.41% and 67.70%, respectively. Reduction gap appears even larger in 350 nm pore size than 150 nm pore size. This result can reveal that degree of contact area alteration depends on the pore size of MNS.

The results of pull-off force and friction performance in MNS clearly indicate that applying electrical potential into the MNS can change the surface shape, surface morphology, and material property such as stiffness which are important parameter to determine tribological performance of MNS. We can identify that the reason of triobolgical behavior changing about MNS is attributed the surface condition alteration by applying electrical potential.

In conclusion, effects of applied electrical potential on surface morphology and tribological performance of MNS were performed using an AFM. Samples of various pore sizes, 0 nm (flat), 150 nm, and 350 nm were studied. Results showed that nickel based MNS expanded with increase in electrical potential. The tribological performance such as friction and pull-off force of the MNS was affected by an applying electrical potential. Both friction and the pull-off force were decreased with increasing electrical potential. This is because mainly of the decreasing of the surface area. Subsequently, the friction signal and pull-off force were decreased. The nickel based MNS enables the control of friction and pull-off force performance, it is beneficial for design of NEMS.