Electronic properties of doped molecular thin films measured by Kelvin probe force microscopy

We report on high-resolution electronic measurements of doped organic thin-film transistors using Kelvin probe force microscopy. Measurements conducted on field effect transistors made of N,NI-diphenyl-N,NI-bis(1-naphthyl)-1,1I-biphenyl-4,4I-diamine p-doped with tetrafluoro-tetracyanoquinodimethane have allowed us to determine the rich structure of the doping-induced density of states. In addition, the doping process changes only slightly the Fermi energy position with respect to the highest occupied molecular orbital level center. The moderate change is explained by two counter-acting effects on the Fermi energy position: the doping-induced additional charge and the broadening of the density of states.     

Delamination of organic coating on carbon steel studied by scanning Kelvin probe force microscopy

Corrosion‐induced delamination of an epoxy coating on the AISI/SAE 1045 carbon steel was studied under a humid atmospheric condition (temperature of 25 °C, 1 standard atmospheric pressure, relative humidity of 90%) by the technique of scanning Kelvin probe force microscopy (SKPFM). Surface‐polished 1045 samples were first cold‐coated with the epoxy and then subject to the atmospheric corrosion under the specified condition. At predetermined time intervals, surface Volta potential differences of the samples were measured using the SKPFM over the dry surface of epoxy coating. The map of Volta potential differences demonstrated high contrasts among three characteristic zones: intact steel‐epoxy interface, delaminated interface, and interface with active corrosion, which was then linked to the actual corrosion potential of the steel (measured using a potentiostat with respect to a saturated calomel electrode) based on a rigorous calibration procedure. It was found that the SKPFM was able to provide direct and nondestructive detection of early active corrosion and coating delamination on steels at a submicroscopic resolution, which outperformed the conventional electrochemical techniques for the same purposes. Copyright © 2012 John Wiley & Sons, Ltd.     

The relationship between nanoscale architecture and function in photovoltaic multichromophoric arrays as visualized by Kelvin probe force microscopy

The physicochemical properties of organic (multi)component films for optoelectronic applications depend on both the mesoscopic and nanoscale architectures within the semiconducting material. Two main classes of semiconducting materials are commonly used: polymers and (liquid) crystals of small aromatic molecules. Whereas polymers (e.g., polyphenylenevinylenes and polythiophenes) are easy to process in solution in thin and uniform layers, small molecules can form highly defined (liquid) crystals featuring high charge mobilities. Herein, we combine the two material types by employing structurally well-defined polyisocyanopeptide polymers as scaffolds to precisely arrange thousands of electron-accepting molecules, namely, perylenebis(dicarboximides) (PDIs), in defined chromophoric wires with lengths of hundreds of nanometers. The polymer backbone enforces high control over the spatial location of PDI dyes, favoring both enhanced exciton and charge transfer. When blended with an electron-donor system such as regioregular poly(3-hexylthiophene), this polymeric PDI shows a relative improvement in charge generation and diffusion with respect to monomeric, aggregated PDI. In order to correlate this enhanced behavior with respect to the architecture, atomic force microscopy investigations on the mixtures were carried out. These studies revealed that the two polymers form interpenetrated bundles having a nanophase-segregated character and featuring a high density of contact points between the two different phases. In order to visualize the relationship between the architecture and the photovoltaic efficiency, Kelvin probe force microscopy measurements were carried out on submonolayer-thick films. This technique allowed for the first time the direct visualization of the photovoltaic activity occurring in such a nanoscale phase-segregated ultrathin film with true nanoscale spatial resolution, thus making possible a study of the correlation between function and architecture with nanoscale resolution.     

Determination of quasi-Fermi levels across illuminated organic donor/acceptor heterojunctions by Kelvin probe force microscopy

Using Kelvin probe force microscopy (KFM), we have measured the electrochemical potentials across indium tin oxide/organic and donor/acceptor heterojunctions in the dark and under illumination with white light. We have found that the photovoltage generated across these heterojunctions is strongly correlated with the difference between the respective HOMO and LUMO levels of the donor and acceptor and also very closely approximates measured open-circuit voltages in completed solar cells. These results imply that KFM tracks the Fermi level positions within the donor and acceptor layers under photoexcitation. Overall, these results demonstrate the utility of KFM for understanding potential profiles across active layers in planar-heterojunction organic solar cells.     

Growth and structure of water on SiO2 films on Si Investigated by Kelvin probe microscopy and in Situ X-ray spectroscopies

The growth of water on thin SiO2 films on Si wafers at vapor pressures between 1.5 and 4 Torr and temperatures between -10 and 21 degrees C has been studied in situ using Kelvin probe microscopy and X-ray photoemission and absorption spectroscopies. From 0 to 75% relative humidity (RH), water adsorbs forming a uniform film 4-5 layers thick. The surface potential increases in that RH range by about 400 mV and remains constant upon further increase of the RH. Above 75% RH, the water film grows rapidly, reaching 6-7 monolayers at around 90% RH and forming a macroscopic drop near 100%. The O K-edge near-edge X-ray absorption spectrum around 75% RH is similar to that of liquid water (imperfect H-bonding coordination) at temperatures above 0 degrees C and is ice-like below 0 degrees C.     

Interaction forces between metal-chelating lipid monolayers measured by colloidal probe atomic force microscopy

Surface forces between LB films of metal-chelating lipids in water have been studied using colloidal probe atomic force microscopy. The LB films of an amphiphile functionalized by the iminodiacetic acid group were prepared on hydrophobic glass substrates. The electric double layer repulsion operated between these LB film surfaces changed depending on pH reflecting the different protonation states of the iminodiacetic acid groups. The titration curve of the iminodiacetic acid monolayer was obtained from the force profiles. The Cu²⁺ complexation process was also monitored by measuring the force profiles at various Cu²⁺ ion concentrations.     

Scanning Kelvin probe study of photolabile silane surface modification of indium tin oxide

The scanning Kelvin nanoprobe (SKN) is an exquisitely sensitive device capable of detecting subtle changes in work function associated with alteration of surface chemistry and interfacial dipole. This instrument is highly versatile and has notably been recently used for (i) the investigation of biological interactions occurring at the interface of multiplexed microarrayed platforms and (ii) the characterization of high work function materials for application in molecular optoelectronics. Herein, we further implement the SKN to characterize, along with angle‐resolved X‐ray photoelectron spectroscopy and contact angle goniometry, the surface modification of indium tin oxide substrates with photopatternable silane adlayers. These molecular films are constructed in a straightforward and economical manner from alkyltrichlorosilane surface‐modifying molecules that possess a distal, UV‐photolabile o‐nitrobenzyl moiety. Employing a photomask, we were able to selectively pattern regions of the photoreactive silane adlayer and confirm the corresponding changes in surface potential through contact potential difference measurements. Copyright © 2013 John Wiley & Sons, Ltd.     

Label-free detection of the aptamer binding on protein patterns using Kelvin probe force microscopy (KPFM)

Anti-lysozyme aptamers are found to preferentially bind to the edge of a tightly packed lysozyme pattern. Such edge-binding is due to the better accessibility and flexibility of the edge lysozyme molecules. Kelvin probe force microscopy (KPFM) was used to study the aptamer–lysozyme binding. Our results show that KPFM is capable of detecting the aptamer–protein binding down to the 30 nm scale. The surface potential of the aptamer–lysozyme complex is approximately 12 mV lower than that of the lysozyme. The surface potential images of the aptamer-bound lysozyme patterns have the characteristic shoulder steps around the pattern edge, which is much wider than that of a clean lysozyme pattern. These results demonstrate the potentials of KPFM as a label-free method for the detection of protein–DNA interactions. Figure Aptamers preferentially bind on the edge of a protein pattern as revealed by Kelvin force microscopy.

Effect of rubbing load on nanoscale charging characteristics of human hair characterized by AFM based Kelvin probe

The manageability and feel of human hair is significantly affected by its surface charge. Understanding and developing ways to control charge build up is hence highly beneficial. Previous studies have looked at static charging characteristics of hair on a macroscale. In this study, static charging characteristics of hair are studied on the nanoscale with an AFM. Hair is charged by rubbing a control area on its surface with an AFM tip, to which a small voltage bias is applied. The resulting charge distribution is characterized by measuring the surface potential of the control area in situ with AFM based Kelvin probe microscopy. The rubbing load is progressively increased, and the effect of this increase on the charge build up is assessed. Virgin, damaged and conditioner treated hair samples are studied for a better understanding of charge build up and dissipation. Relevant mechanisms are discussed.     

Photoinduced surface potential change of bacteriorhodopsin mutant D96N measured by scanning surface potential microscopy

We report the direct measurement of photoinduced surface potential differences of wild-type (WT) and mutant D96N bacteriorhodopsin (BR) membranes at pH 7 and 10.5. Atomic force microscopy (AFM) and scanning surface potential microscopy (SSPM) were used to measure the BR membrane with the extracellular side facing up. We present AFM and SSPM images of WT and mutant D96N in which the light-dark transition occurred in the mid-scan of a single BR membrane. Photosteady-state populations of the M state were generated to facilitate measurement in each sample. The photoinduced surface potential of D96N is 63 mV (peak to valley) at pH 10.5 and is 48 mV at pH 7. The photoinduced surface potential of WT is 37 mV at pH 10.5 and approximately 0 at pH 7. Signal magnitudes are proportional to the amount of M produced at each pH. The results indicated that the surface potentials were generated by photoformation of surface charges on the extracellular side of the membrane. Higher surface potential correlated with a longer lifetime of the charges. A mechanistic basis for these signals is proposed, and it is concluded that they represent a steady-state measurement of the B2 photovoltage.     

The surface oxidation potential of melanosomes measured by free electron laser-photoelectron emission microscopy

A technique for measuring the photoionization spectrum and the photoelectron emission threshold of a microscopic structured material is presented. The theoretical underpinning of the experiment and the accuracy of the measurements are discussed. The technique is applied to titanium silicide nanostructures and melanosomes isolated from human hair, human and bovine retinal pigment epithelium cells, and the ink sac of Sepia officinalis. A common photothreshold of 4.5 +/- 0.2 eV is found for this set of melanosomes and is attributed to the photoionization of the eumelanin pigment. The relationship between the photoionization threshold and the electrochemical potential referenced to the normal hydrogen electrode is used to quantify the surface oxidation potential of the melanosome. The developed technique is used to examine the effect of iron chelation on the surface oxidation potential of Sepia melanosomes. The surface oxidation potential is insensitive to bound Fe(III) up to saturation, suggesting that the metal is bound to the interior of the granule. This result is discussed in relation to the age-dependent accumulation of iron in human melanosomes in both the eye and brain.     

Raman imaging and Kelvin probe microscopy for the examination of the heterogeneity of doping in polycrystalline boron-doped diamond electrodes

The issue of the heterogeneity of boron doping in microcrystalline diamond films was addressed by four different methods: micro-Raman spectroscopy and Raman imaging, Kelvin probe force microscopy, conducting atomic force microscopy, and scanning electrochemical microscopy. The samples were commercially available films from Windsor Scientific, with an average boron concentration of about 5 x 10(20) cm(-3). In agreement with previous works, all of the methods showed that the boron uptake was nonuniform across the surface of the electrode. Two different types of regions were evidenced, with metallic or semiconducting properties that were characterized with different types of Raman spectra. The line shape of these spectra was strongly dependent on the excitation wavelength. Local variations in electroactivity were evidenced by the SECM curves, which are related to the electronic properties of the individual grains, which, in turn, are governed by the boron content of the individual crystallites. In this study, two different micro-Raman imaging techniques were used that reveal the grain structure of the films: the images constructed from the diamond line intensity perfectly reproduced the optical image obtained by illuminating the sample in reflection. The method also allows detection of the presence of nondiamond carbon, especially in the metallic parts of the samples. Other spectral features (intensity of the boron-related broad lines, as well as the frequency and width of the diamond line) were used to construct images. In every case, the grain structure of the film was revealed, as well as twinning within individual crystallites. All approaches revealed that no enhanced doping or boron depletion occurred at the grain boundaries.     

Components of the measured surface charge of low temperature ceramic oxides

This paper contains an analysis of the interfacial behaviour of samples of monoclinic zirconia. The oxide powder was obtained by calcining a hydrothermal precursor at 200° C. Results of surface charge determinations are elaborated to yield total capacitance values which, in turn, are analyzed on the basis of Parsons and Zobel (P-Z) plots (R. Parsons and F.G.R. Zobel, J. Electroanal. Chem., 9 (1965) 333). The aptitude of selecting either a planar or a spherical geometry for the diffuse double layer is discussed. Analysis of the results in P-Z coordinates indicates that samples subjected to the “routine” overnight conditioning in the solution present only effects due to simple diffuse double layer charge compensation. In the case, instead, of samples conditioned for a longer time (143 h), different components of the surface charge become apparent. Phenomena related to bulk penetration of the base electrolyte are suggested to occur in the latter case.     

Ferrocenylundecanethiol self-assembled monolayer charging correlates with negative differential resistance measured by conducting probe atomic force microscopy

Electrical and mechanical properties of metal-molecule-metal junctions formed between Au-supported self-assembled monolayers (SAMs) of electroactive 11-ferrocenylundecanethiol (FcC(11)SH) and a Pt-coated atomic force microscope (AFM) tip have been measured using a conducting probe (CP) AFM in insulating alkane solution. Simultaneous and independent measurements of currents and bias-dependent adhesion forces under different applied tip biases between the conductive AFM probe and the FcC(11)SH SAMs revealed reversible peak-shaped current-voltage (I-V) characteristics and correlated maxima in the potential-dependent adhesion force. Trapped positive charges in the molecular junction correlate with high conduction in a feature showing negative differential resistance. Similar measurements on an electropassive 1-octanethiol SAM did not show any peaks in either adhesion force or I-V curves. A mechanism involving two-step resonant hole transfer through the occupied molecular orbitals (MOs) of ferrocene end groups via sequential oxidation and subsequent reduction, where a hole is trapped by the phonon relaxation, is proposed to explain the observed current-force correlation. These results suggest a new approach to probe charge-transfer involving electroactive groups on the nanoscale by measuring the adhesion forces as a function of applied bias in an electrolyte-free environment.     

The dynamic nature of contact angles as measured by atomic force microscopy

Atomic force microscopy appears to be a useful tool for determining the contact angle for small particles. It is shown in this paper that the contact angle of a spherical polyethylene particle changes with the speed of the AFM piezoelectric translator. Such dynamic behavior of the contact angle and other uncertainties such as the position of the three-phase contact on the particle surface during bubble-particle interaction make it difficult to decide whether or not the AFM single-particle contact angle can be used to describe the hydrophobic state of the particle surface.     

Effect of low-molecular-weight organic anions on surface charge of variable charge soils

Low-molecular-weight (LMW) organic acids exist widely in soils and have been implicated in many soil processes. In the present paper, the effect of the anions of four low-molecular-weight organic acids on the surface charge of three variable charge soils was investigated. The results showed that the presence of organic anions led to an increase in negative charge and a decrease in positive charge. Positive charges decreased to a larger extent than negative charges. The effect of different anions on surface charge followed the order citrate > malate > oxalate > acetate. For hyper-rhodic ferrasol and rhodic ferrasol, the change of positive charge decreased with the increase in pH, while that of negative charge increased with the increase in pH. Among different soils the extent of change in surface charge was related to their iron oxides content. When free iron oxides were removed from the soil, the effect of organic anions on surface charge decreased sharply. These findings may be of practical significance for variable charge soils low in nutrient-retaining capacity by increasing the retaining capacity for cations such as potassium and calcium considerably but decreasing that for anions such as nitrate remarkably.     

Correlation between desorption force measured by atomic force microscopy and adsorption free energy measured by surface plasmon resonance spectroscopy for peptide-surface interactions

Surface plasmon resonance (SPR) spectroscopy is a useful technique for thermodynamically characterizing peptide-surface interactions; however, its usefulness is limited to the types of surfaces that can readily be formed as thin layers on the nanometer scale on metallic biosensor substrates. Atomic force microscopy (AFM), on the other hand, can be used with any microscopically flat surface, thus making it more versatile for studying peptide-surface interactions. AFM, however, has the drawback of data interpretation due to questions regarding peptide-to-probe-tip density. This problem could be overcome if results from a standardized AFM method could be correlated with SPR results for a similar set of peptide-surface interactions so that AFM studies using the standardized method could be extended to characterize peptide-surface interactions for surfaces that are not amenable for characterization by SPR. In this article, we present the development and application of an AFM method to measure adsorption forces for host-guest peptides sequence on surfaces consisting of alkanethiol self-assembled monolayers (SAMs) with different functionality. The results from these studies show that a linear correlation exists between these data and the adsorption free energy (ΔG(o)(ads)) values associated with a similar set of peptide-surface systems available from SPR measurements. These methods will be extremely useful to characterize thermodynamically the adsorption behavior for peptides on a much broader range of surfaces than can be used with SPR to provide information related to understanding protein adsorption behavior to these surfaces and to provide an experimental database that can be used for the evaluation, modification, and validation of force field parameters that are needed to represent protein adsorption behavior accurately for molecular simulations.     

Nanoscale structural and electronic properties of ultrathin blends of two polyaromatic molecules: a Kelvin probe force microscopy investigation.

We describe a Kelvin Probe Force Microscopy (KPFM) study on the morphological and electronic properties of complex mono and bi-molecular ultrathin films self-assembled on mica. These architectures are made up from an electron-donor (D), a synthetic all-benzenoid polycyclic aromatic hydrocarbon, and an electron-acceptor (A), perylene-bis-dicarboximide. The former molecule self-assembles into fibers in single component films, while the latter molecule forms discontinuous layers. Taking advantage of the different solubility and self-organizing properties of the A and D molecules, multicomponent ultrathin films characterized by nanoscale phase segregated fibers of D embedded in a discontinuous layer of A are formed. The direct estimation of the surface potential, and consequently the local workfunction from KPFM images allow a comparison of the local electronic properties of the blend with those of the monocomponent films. A change in the average workfunction values of the A and D nanostructures in the blend occurs which is primarily caused by the intimate contact between the two components and the molecular order within the nanostructure self-assembled at the surface. Additional roles can be ascribed to the molecular packing density, to the presence of defects in the film, to the different conformation of the aliphatic peripheral chains that might cover the conjugated core and to the long-range nature of the electrostatic interactions employed to map the surface by KPFM limiting the spatial and potential resolution. The local workfunction studies of heterojunctions can be of help to tune the electronic properties of active multicomponent films, which is crucial for the fabrication of efficient organic electronic devices as solar cells.