1. Preparation of Siox and Hfo2 Surface for Contact and Friction Experiments
1. Preparation of SiOx and HfO2 surface for contact and friction experiments
The preparation of SiOx substrates: After being ultrasonicated in ethanol and Milli-Q water for 5 min stepwise, the p-doped Si(111) wafer was cleaned in piranha solution (H2SO4/H2O2 = 7/3, v/v) at 130 °C for 30 min. This wafer was then rinsed with Milli-Q water again, and blown dried with high-purity nitrogen. Subsequently, the wafer was dipped in 5% HF solution for 10 seconds and rinsed with Milli-Q water. At last, the wafer was mounted in the furnace at 1000 °C for 2.5 h to form the SiOxtop layer.
The preparation of HfO2 substrates:After being ultrasonicated in ethanol and Milli-Q water for 5 min and then rinsed with ethanol,the p-doped Si(111) wafer wasblown dried with high-purity nitrogen. 30 nm HfO2 film was deposited by atomic layer deposition (ALD) at 90 °C using a Cambridge Technology Savannah 100 apparatus.
All chemicals used are of analytical grade. Water of Milli-Q quality obtained from the commercially available water purification equipment from Millipore (Bedford, MA) was used throughout the experiment.
2. Contact and friction experiment, KFM imaging
The contact experiment was performed by pressing a new AFM tip with ~0.5nN contact force on the substrate for 5 seconds. After withdrawing the tip with the speed of about 20 m/s and switching to KFM mode, the contact area was then characterized by KFM.The friction experiment was performedin two different ways,using “vector scan” macro commands to control the tip to scan across the defined lines with different length for relative low friction speed experiments and using the tip to scan over the defined areas (typically 3 μm 3 μm) under contact mode for relative high friction speed experiments.The potential image was then recorded together with the topographical image by KFM.
In active traceKFM mode, SI-DF3-A KFM tip (Seiko Instrument Inc., spring constant k=1.4 Nm-1) was used for KFM imaging. Topographic and potential images were acquired simultaneously in KFM mode. In this mode, the line scan for topography measurement and potential measurement can be completely separated, and it is possible to make a setup for the distance between the point of potential measurement and the point of topography measurement. The vibration for topography measurement (r) was cut during the potential measurement, and the potential measurement was performed only by using the vibration of ac frequency (). For the topography imaging, the tip was vibrating at its resonance frequencyr(~23 kHz) with the tip bias of zero.For the potential distribution measurement, the cantilever was moved at trace distance z parallel to the substrate plane.An ac voltage of 10V at frequency (~21 kHz), which was slightly lower than r, was applied between the tip and the substrate. At the same time, a feedback dc voltage was applied to the tip to minimize the electrostatic force. If the tip and substrate were at the same dc voltage, there was no force on the cantilever at ω and the cantilever amplitude went to zero. Local surface potential was determined by adjusting the dc voltage on the tip until the oscillation amplitude became zero and the tip voltage was the same as the surfacepotential.
3. Topographic and potential images of contact and friction experiments
Fig. 1. (a) and (b) are the topographic image and potential image of contact experiment, respectively. (c) is the cross-section profile of the line shown in (b). The contact experiment was performed by pressing a new AFM tip with ~0.5 nN loaded force on the substrate for 5 seconds and then withdrawing the tip with the speed of 20 m/s. (d) and (e) are the topographic image and potential image of friction experiment, respectively. (f) is the cross-section profile of the line shown in (e). The friction experiment was performed by using a new tip to scan across the substrate at a speed of 5 nm/s. The electric quantities in the charge pattern of (b) and (e) are ~220 e and ~750 e, respectively, estimated by our method.
Fig. 2. The topographic images obtained by using different tips on SiOx substrates. (a) CSG11 tip, (b) NSG11 tip. The corresponding potential images of the same area are shown in fig. 2(a) and 2(b). These figures illustrate that there is no materials transfer or topographic change of surface in friction process, even with the very high loaded force.
4. Results of LDA calculation
Results of LDA calculation reveals the change of electronic levels in SiOx with the pressure. (a) Energy bands of the p-doped silicon AFM tips with native silicon oxide layer of several nanometers. The black lines indicate band structure without applying tensile force and the red lines indicate band structure with 1010Pa tensile force. (b) Energy bands of the p-doped silicon substrate with thermal oxide layer of ~500 nm. The black lines indicate band structure without applying tensile force and the red lines indicate band structure with 1010Pa tensile force. ~0.1 eV energy shift is observed in this system.
5. Interpretation of our calculation
According to equation (2), the sum of the hi can be written as
(3)
If the tip is right above point j on the substrate, we note hij as the weighting factor to measure the contribution of local real potential Vi at point i on the substrate to the measured potential VDCj. In our experiment, the tip-substrate distance z is fixed at 100 nm. Consequently, hij only has relation with the distance of point i and point j. The sum total of the measured potential can be expressed as
(4)
where VDCj is the measured potential when the tip is over point j on the substrate and Vi is the real local potential of point i on the substrate.
In fact, one cannot talk about capacitance of a “point”. The subscript i runs over positions where the measurements took place; and the capacitance at a position refers to the capacitance of a small area at that position. In this case, the function h is a constant when the tip is above any position of the substrate. Here we considered the surface as 256×256 electrodes due to the resolution of the KFM measurement. The area of each electrode is 62.5 nm × 62.5nm. In this case, the substrate is not an infinite plate; however, the discussion above still holds in this finite case if we consider the finite substrate as a part of an infinite plate where the surface potential is always zero besides the measured site. The area which has the largest contribution to the measured potential is the area just below the tip. It implies that the areas which have larger distance from the tip have smaller weighting factors thus smaller contributions to the measured potential. According to equation (1), if VDC equals to zero, then the actual potential below the tip is also zero. For our KFM measurement, the tip-sample system can be considered as 256×256 capacitors, each is composed of the tip, air and charged area just below the tip.
In cone model, the structure of the tip is considered as a cone whose apex forms part of a sphere. The derivative of the capacitance of such a tip-sample structure can be expressed as
(5)
where
in addition, is the half angle of the cone representing the tip, ht is the cone height, and 0 is the permittivity of vacuum. Here, the half angle of the KFM tip (SI-DF3-A tip) is about 25, and the cone height is about 60 nm, which is consistent with estimates from scanning electronic microscope (SEM) measurement. The trace distance z=100 nm, then the capacitance is calculated to be 8.9610-18 F.
6.The influence of relative humidity on nanotriboelectrification
The influence of relative humidity on nanotriboelectrification.Charge patterns decay away with time. Potential images of the same friction location after (a) 1min, (b) 1h, (c) 8h and (d) 1day at 25%relative humidity.