Effect of probe and sample pre-treatment

Section 2.3.2 confirmed the susceptibility of Pt-Pt contacts to tribopolymer formation and its implications on increasing the contact resistance. We now present attempts to enhance the performance of nanoscale Pt-Pt contacts in cycling tests through two different pre-treatment methods: slide-cleaning and plasma-cleaning of the Pt-coated AFM probe and Pt-coated sample. Figure 26 illustrates the used sample pre- treatment methods. During slide-cleaning, the as-deposited AFM probe was scanned over a 2 μm x 2 μm area of the Pt-coated sample under 15 nN normal force. This mechanical sliding procedure was expected to remove any adsorbed surface

contaminants on both AFM tip and sample. AFM cycling of the Pt-coated AFM probe inside the slide-cleaned area of the Pt-coated sample, as described in section 2.3.1, was immediately started after the slide-cleaning. During plasma-cleaning, the as-deposited AFM probe and Pt-coated sample were cleaned for 2 minutes inside an oxygen plasma cleaner (Solarus™ Advanced Plasma Cleaning System, Gatan Inc., Pleasanton, CA) under an oxygen plasma (15 W, pO2 = 250 mTorr). Upon completion of the plasma cleaning, the Pt-coated AFM probe and sample were immediately loaded into the Bruker Dimension ICON AFM (Bruker Corporation, Billerica, MA) and the cycling experiment was started.

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FIGURE 26: Illustrations of sample pre-treatment methods. a) Pt-coated tip and sample tested in the as-deposited state without any pre-treatment. b) Pt-coated tip was scanned in contact mode over the sample to physically remove any contaminants. c) Oxygen plasma cleaning was employed on both, Pt-coated tip and sample, so remove any hydrocarbon contaminants.

Figure 27 shows the results of the AFM cycling lifetime tests conducted on untreated, slide-cleaned, and plasma-cleaned Pt-Pt contacts. While the untreated Pt-Pt contact degrades (increase in contact resistance by three orders of magnitude) as previously seen in section 2.3.2, both the slide-cleaned and plasma-cleaned contacts performed noticeably better. The contact resistance of the slide-cleaned Pt-Pt contact increased by approximately one order of magnitude over 107 _{cycles and the contact} resistance of the plasma-cleaned Pt-Pt contact stayed within the same order of magnitude for the duration of the test.

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FIGURE 27: Atomic force microscopy lifetime tests of Pt-Pt nanocontacts subject to different pre-treatment methods. Untreated Pt-Pt contact exhibits more than 3 orders of magnitude increase in contact resistance after cycling to 107 _{contact cycles. Slide-} cleaned contact exhibits reduced contact resistance increase, whereas plasma-cleaned contacts maintain initial contact resistance over 107 _{contact cycles.}

Figure 28 shows the adhesion force as a function of cycle count for the untreated, slide-cleaned, and plasma-cleaned Pt-Pt contacts. The adhesion force

steadily and constantly increases for the untreated and slide-cleaned contact throughout the tested 107 _{cycles, which indicates that no sudden changes in contact and/or tip} geometry occurred throughout the test. The adhesion force of the plasma-cleaned Pt-Pt contact increased equally until 5∙105 _{cycles after which two drops in adhesion force were} observed. The first drop of about 5 nN occurred at 8∙105 _{cycles and was followed by an}

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increase in adhesion 2∙105 _{later. The second drop was of 20 nN magnitude and occurred} at the end of the test, starting at 2∙106 _{cycles. The observed adhesion force drops of the} plasma-cleaned Pt-Pt contacts are not noticeably correlated with the contact resistance and suggest minor changes of the tip geometry. The changes in adhesion can also be due to chemical changes at the end of the tip (i.e., –H terminated surface vs. –OH terminated surface).

FIGURE 28: Adhesion behavior during atomic force microscopy lifetime tests of Pt-Pt nanocontacts subject to different pre-treatment methods. Adhesion force increases continuously for the untreated and slide-cleaned Pt-Pt samples. Adhesion force initially increases until 5∙105 _{cycles for the plasma-cleaned Pt-Pt sample and then becomes} unstable.

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X-ray photoelectron spectroscopy (XPS) measurements were conducted in order to identify the mechanism by which oxygen plasma cleaning reduces the susceptibility of Pt-Pt contacts to tribopolymer formation. Flat Pt surfaces, identical to those used as counter-samples in the untreated Pt-Pt cycling studies, were analyzed before and after oxygen plasma cleaning in order to identify the composition of the near surface region. Details of the XPS system used here can be found later in this thesis in chapter 3.3.2. Here, we performed angular-resolved measurements of the carbon, platinum, and oxygen signals. Angular-resolved XPS varies the incident angle of the X-ray beam to achieve increased surface sensitivity. The 0° angle corresponds to the lowest surface sensitivity and the 45° angle delivers the highest surface sensitivity. Figure 29 shows the angular-resolved XPS spectra of the C 1s peak of the as-deposited and plasma-cleaned Pt surfaces. The oxygen plasma cleaning reduces the C 1s peak by more than 65% due to its removal of adsorbed adventitious carbon. For the plasma-cleaned sample, the intensity of the C 1s peak decreases with decreasing XPS angle showing that the

carbonaceous species are located at the sample surface. These hydrocarbon containing contaminants are present due to the catalytic activity of Pt (see section 3.5 for more details).

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FIGURE 29: Angular-resolved high resolution XPS spectra of the C 1s peak. Oxygen plasma cleaning reduces the C 1s peak noticeable due to its removal of adventitious carbon. The red area in the plasma-cleaned Pt C 1s spectrum represents the as- deposited Pt C 1s spectra at 0° for reference.

Figure 30 presents the angular-resolved XPS spectra of the Pt 4f peaks of the as-deposited and plasma-cleaned Pt surfaces. Small amounts of platinum oxide are formed as a consequence of the oxygen plasma treatment, while most of the metallic platinum is maintained.

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FIGURE 30:Angular-resolved high resolution XPS spectra of the Pt 4f peaks. Oxygen plasma cleaning forms small amounts of platinum oxygen, while still maintaining most of the metallic platinum.

In summary, the oxygen plasma cleaning has two effects on as-deposited Pt surfaces: first, it removes large amounts of adsorbed adventitious carbon leading to less tribopolymer formation and thereby less contact resistance increase during cycling; second, it maintains most of the metallic platinum by only forming small amounts of high resistive platinum oxide ensuring low starting contact resistances.