Chemically-sensitive interfacial force microscopy: Contact potential measurements of self-assembling monolayer films

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We report contact potential difference (CPD) measurements of n-alkanethiol self-assembling monolayers (SAMs) adsorbed to Au substrates using an organomercaptan-modified Au probe. We perform experiments by applying a triangular-sweep voltage between the sample and probe while measuring the resulting electrostatic force. The interfacial force microscope permits us to keep the probe/sample distance rigidly fixed, which allows us to directly measure theelectrostatic forceat aconstant interfacialseparation. TheCPDisdetermined by measuring the applied potential necessary to null the electric field and eliminate the interfacial force between the two We show that CPD values obtained using Au probes modified with methyl-terminated SAMs are stable and reproducible, whereas identical unmodified probes yield highly variable data. Our experimentally determined CPD values are in qualitative agreement with calculated CPDs for several different w-terminated SAMs. We report contact potential difference (CPD) measurements of n-alkanethiol self-assembling monolayers (SAMs)'J adsorbed to Au substrates using an organomercaptan-modified Au probe. Our results demonstrate for the first time the feasibility and importance of controlling the chemical properties of force microscope probe Moreover, control over the chemical characteristics of the probe provides a basis for distinguishing between chemically distinct surface features with nanometer resolution in force microscopy. We show that CPD values obtained using Au probes modified with methyl-terminated SAMs arestable and reproducible, whereas identical unmodified probes yield highly variable data. Our experimentally determined CPD values are in qualitative agreement with calculated CPDs for several different w-terminated SAMs. We have previously used the interfacial force microscope (IFM)' to obtain detailed information about the mechanical properties of methyl-terminated n-alkanethiol SAMs adsorbed to Au substrates. In the present work, we use the IFM to determine local CPDs that arise between an organomercaptanmodified Au probe and various SAM-modified Au substrates (Scheme I)? TheCPDisdefinedas theworkfunctiondifference between the modified Au surfaces, which we view as tightly bounddipolesheets that shifttbe workfunctionsofeachmodified Ausurface.lQ For example, the work functionis increased relative to bare Au for dipoles having their negative end pointing away from the modified Au surface. We perform experiments by applying a triangular-sweepvoltage between the sampleand probe, whichis heldat ground, whilemeasuring theresultingelectrostatic force. Unless otherwise noted all CPD measurements were obtained in air. The forcefeedback capability of the IFM is used tokeeptheprobe/sampledistancerigidlyfixed,whichallows us to directly measure the electrostatic force at a constant interfacial separation. The CPD is determined by measuring the applied potential necessary to null the electric field and eliminate the interfacial force between the two surfaces. Figure 1 shows forcevs substrate potential data obtained using a HS(CHZ)&H3-mdified Au probe. In these plots, negative forces are attractive, and the shape is parabolic since the electrostatic forceis proportional to thesquareof the biasvoltage. The solid line represents a second-order polynomial fit of the