Christian Loppacher

Analytical approach to the local contact potential difference on (001) ionic surfaces: Implications for Kelvin probe force microscopy

An analytical model of the electrostatic force between the tip of a non-contact Atomic Force Microscope (nc-AFM) and the (001) surface of an ionic crystal is reported. The model is able to account for the atomic contrast of the local contact potential difference (CPD) observed while nc-AFM-based Kelvin Probe Force Microscopy (KPFM) experiments. With the goal in mind to put in evidence this short-range electrostatic force, the Madelung potential arising at the surface of the ionic crystal is primarily derived. The expression of the force which is deduced can be split into two major contributions: the first stands for the coupling between the microscopic structure of the tip apex and the capacitor formed between the tip, the ionic crystal and the counter-electrode; the second term depicts the influence of the Madelung surface potential on the mesoscopic part of the tip, independently from its microscopic structure. These short-range electrostatic forces are in the range of ten pico-Newtons. When explicitly considering the crystal polarization, an analytical expression of the bias voltage to be applied on the tip to compensate for the local CPD, i.e. to cancel the short-range electrostatic force, is derived. The compensated CPD has the lateral periodicity of the Madelung surface potential. However, the strong dependence on the tip geometry, the applied modulation voltage as well as the tip-sample distance, which can even lead to an overestimation of the real surface potential, makes quantitative KPFM measurements of the local CPD extremely difficult.

Understanding the atomic-scale contrast in Kelvin probe force microscopy.

A numerical analysis of the origin of the atomic-scale contrast in Kelvin probe force microscopy is presented. Atomistic simulations of the tip-sample interaction force field have been combined with a noncontact atomic force microscope simulator including a Kelvin module. The implementation mimics recent experimental results on the (001) surface of a bulk alkali halide crystal for which simultaneous atomic-scale topographical and contact potential difference contrasts were reported. The local contact potential difference does reflect the periodicity of the ionic crystal, but not the magnitude of its Madelung surface potential. The imaging mechanism relies on the induced polarization of the ions at the tip-surface interface owing to the modulation of the applied bias voltage. Our findings are in excellent agreement with previous theoretical expectations and experimental observations.

Kelvin probe force microscopy of alkali chloride thin films on Au(111)

Alkali chloride thin films on Au(111) are investigated under ultrahigh vacuum conditions at room temperature by noncontact atomic force microscopy in combination with Kelvin probe force microscopy. Sample preparation is carried out in situ and optimized in order to achieve a sub-monolayer coverage showing extended alkali chloride island formation. The local surface potential for LiCl, NaCl, KCl and RbCl thin films on Au(111) was directly determined and compared to values of the bare Au(111) substrate. Hence, the absolute change in the local work function ?? was evaluated. We observe a linear dependence between ?? and the ionic radius of the mentioned alkali chloride thin films.

FM demodulated Kelvin probe force microscopy for surface photovoltage tracking

The surface photovoltage (SPV) of a structured semiconductor surface is deduced via detection of the contact potential difference measured with Kelvin probe force microscopy (KPFM). Our setup is based on a quantitative KPFM method complemented with modulated laser illumination in order to measure SPV. A high lateral resolution and quantitative values for the SPV are obtained by operating the KPFM in the so-called frequency modulation (FM) mode which is advantageous compared to the amplitude sensitive (AM) mode as proven by our simulations. In contrast to similar studies based on scanning tunnelling microscopy, KPFM offers the clear advantage that there is virtually no electric DC field between tip and sample and, therefore, the SPV is not affected by the presence of the tip.

Ordered growth and local workfunction measurements of tris(8-hydroxyquinoline) aluminium on ultrathin KBr films

In this work we investigate the growth of tris(8-hydroxyquinoline) aluminium (Alq(3)) on single-crystal Ag(111) substrates partially covered by an ultrathin KBr film. Noncontact atomic force microscopy is used to determine the molecular ordering of 0.8 monolayer Alq(3) evaporated onto these substrates. The simultaneous measurement of the local surface potential by means of Kelvin probe force microscopy yields the local workfunction difference between the pure Ag(111) surface and the one covered by an ultrathin KBr film, by pure Alq(3), or by both (KBr|Alq(3)). The molecular ordering and the interface dipole formation are discussed with respect to experiments described in the literature in which electron diffraction and photoelectron spectroscopy were used, respectively.

Dipole-driven self-organization of zwitterionic molecules on alkali halide surfaces.

Summary We investigated the adsorption of 4-methoxy-4′-(3-sulfonatopropyl)stilbazolium (MSPS) on different ionic (001) crystal surfaces by means of noncontact atomic force microscopy. MSPS is a zwitterionic molecule with a strong electric dipole moment. When deposited onto the substrates at room temperature, MSPS diffuses to step edges and defect sites and forms disordered assemblies of molecules. Subsequent annealing induces two different processes: First, at high coverage, the molecules assemble into a well-organized quadratic lattice, which is perfectly aligned with the <110> directions of the substrate surface (i.e., rows of equal charges) and which produces a Moiré pattern due to coincidences with the substrate lattice constant. Second, at low coverage, we observe step edges decorated with MSPS molecules that run along the <110> direction. These polar steps most probably minimize the surface energy as they counterbalance the molecular dipole by presenting oppositely charged ions on the rearranged step edge.

Electrothermally driven high-frequency piezoresistive SiC cantilevers for dynamic atomic force microscopy

Cantilevers with resonance frequency ranging from 1 MHz to 100 MHz have been developed for dynamic atomic force microscopy. These sensors are fabricated from 3C-SiC epilayers grown on Si(100) substrates by low pressure chemical vapor deposition. They use an on-chip method both for driving and sensing the displacement of the cantilever. A first gold metallic loop deposited on top of the cantilever is used to drive its oscillation by electrothermal actuation. The sensing of this oscillation is performed by monitoring the resistance of a second Au loop. This metallic piezoresistive detection method has distinct advantages relative to more common semiconductor-based schemes. The optimization, design, fabrication, and characteristics of these cantilevers are discussed.

Probing polarization and dielectric function of molecules with higher order harmonics in scattering–near-field scanning optical microscopy

The idealized system of an atomically flat metallic surface [highly oriented pyrolytic graphite (HOPG)] and an organic monolayer (porphyrin) was used to determine whether the dielectric function and associated properties of thin films can be accessed with scanning–near-field scanning optical microscopy (s-NSOM). Here, we demonstrate the use of harmonics up to fourth order and the polarization dependence of incident light to probe dielectric properties on idealized samples of monolayers of organic molecules on atomically smooth substrates. An analytical treatment of light/sample interaction using the s-NSOM tip was developed in order to quantify the dielectric properties. The theoretical analysis and numerical modeling, as well as experimental data, demonstrate that higher order harmonic scattering can be used to extract the dielectric properties of materials with tens of nanometer spatial resolution. To date, the third harmonic provides the best lateral resolution(∼50 nm) and dielectric constant contrast ...

Novel diazosulfonate terpolymers for the preparation of structured functionalized surfaces: Synthesis and characterization

We report the synthesis and characterization of novel diazosulfonate copolymers and terpolymers by free-radical polymerization for the preparation of ultrathin films. Such films were covalently linked to silicon and glass substrates after a spin-coating and annealing process using 3-(trimethoxysilyl)propyl methacrylate as the adhesive co-monomer. The polymers were characterized by NMR and IR spectroscopy and ellipsometry, and contact-angle measurements were used to analyze the film properties. The light-sensitive but thermally stable polymer films can be structured successfully by UV light down to the micrometer scale. The patterns written into the diazofulfonate film were produced patterns written into the diazosulfonate film were produced by a UV laser without a mask. The photolysis in solution and in film was examined by UV spectroscopy.

Nanoscale nondestructive electric field probing in ferroelectrics, organic molecular films and near-field optical nanodevices

Inspecting and tuning electric fields on the nanometer scale offers a great potential in overcoming limitations inherent in assembling nanostructures. Both optical and electronic devices may be improved in performance provided that a quantitative knowledge on the strength and orientation of local (stray) fields is gained. Here we present nanoscale investigations of functional surfaces probing the surface potential and electronic properties of ferroelectric and ultra thin organic films. We developed methodologies that are able to non-invasively track the electric field both above and below interfaces, thus providing insight also into the sample. Hence, interface dipole formation and interface charging directly shows up in potential changes revealing the donor/acceptor characteristics of molecules, as well as the surface charge screening in ferroelectrics. Such inspections are possible using conventional scanning force microscopy operated in sophisticated modes measuring the electrostatic force or the inverse piezoelectric effect. Finally, electric fields are also probed in the optical regime using near-field optical methods. Examples are shown where the strength and frequency of surace plasmon resonances become tunable due to simple nanostructuring of metallic thin films.