Doping transition of doped ZnO nanorods measured by Kelvin probe force microscopy

We have investigated the doping transition of one-dimensional (1-D) doped-ZnO nanorods with Kelvin probe force microscopy (KPFM). Vertically aligned (undoped, As-doped, and undoped/As-doped homo-junction) ZnO nanorods were grown on Si (111) substrates without any catalyst by vapor phase transport. Individual ZnO nanorods are removed from the substrates and transferred onto thin Au films grown on Si substrates. The morphology and surface potentials of the nanorods were measured simultaneously by the KPFM. For the homo-junction nanorods with ~ 250 nm in diameter, the KPFM image shows localization of the doping transition along the nanorods. The measured Kelvin signal (surface potential) across the junction induces the work function difference between the undoped and the As-doped region of ~ 85 meV. Also, the work function of As-doped nanorods is ~ 95 meV higher than that of intrinsically undoped nanorods grown in similar conditions. These consistent results indicate that the KPFM is reliable to determine the localization of the doping transition in 1-D structures.     

Open-loop band excitation Kelvin probe force microscopy

A multidimensional scanning probe microscopy approach for quantitative, cross-talk free mapping of surface electrostatic properties is demonstrated. Open-loop band excitation Kelvin probe force microscopy (OL BE KPFM) probes the full response-frequency-potential surface at each pixel at standard imaging rates. The subsequent analysis reconstructs work function, tip-surface capacitance gradient and resonant frequency maps, obviating feedback-related artifacts. OL BE KPFM imaging is demonstrated for several materials systems with topographic, potential and combined contrast. This approach combines the features of both frequency and amplitude KPFM and allows complete decoupling of topographic and voltage contributions to the KPFM signal.     

High spatial resolution Kelvin probe force microscopy with coaxial probes

Kelvin probe force microscopy (KPFM) is a widely used technique to measure the local contact potential difference (CPD) between an AFM probe and the sample surface via the electrostatic force. The spatial resolution of KPFM is intrinsically limited by the long range of the electrostatic interaction, which includes contributions from the macroscopic cantilever and the conical tip. Here, we present coaxial AFM probes in which the cantilever and cone are shielded by a conducting shell, confining the tip-sample electrostatic interaction to a small region near the end of the tip. We have developed a technique to measure the true CPD despite the presence of the shell electrode. We find that the behavior of these probes agrees with an electrostatic model of the force, and we observe a factor of five improvement in spatial resolution relative to unshielded probes. Our discussion centers on KPFM, but the field confinement offered by these probes may improve any variant of electrostatic force microscopy.     

Force gradient sensitive detection in lift-mode Kelvin probe force microscopy

We demonstrate frequency modulation Kelvin probe force microscopy operated in lift-mode under ambient conditions. Frequency modulation detection is sensitive to force gradients rather than forces as in the commonly used amplitude modulation technique. As a result there is less influence from electric fields originating from the tip's cone and cantilever, and the recorded surface potential does not suffer from the large lateral averaging observed in amplitude modulated Kelvin probe force microscopy. The frequency modulation technique further shows a reduced dependence on the lift-height and the frequency shift can be used to map the second order derivative of the tip-sample capacitance which gives high resolution material contrast of dielectric sample properties. The sequential nature of the lift-mode technique overcomes various problems of single-scan techniques, where crosstalk between the Kelvin probe and topography feedbacks often impair the correct interpretation of the recorded data in terms of quantitative electric surface potentials.     

Studying electric field profiles in GaAs-based detector structures by Kelvin probe force microscopy

The method of scanning Kelvin probe force microscopy has been used to study the electric field distribution in GaAs-based p ⁺-π-n-n ⁺ detector structures. In the active layer volume, two maxima in the field strength profiles have been found, which are localized in the regions of p ⁺-π and π-n junctions. A volt-age drop on the π-n junction expands the region of collection of nonequilibrium holes, thus increasing the charge collection efficiency for the absorption of γ photons with an energy of 59.5 keV.     

Kelvin probe force microscopy on surfaces of UHV cleaved ionic crystals

Force spectroscopy and Kelvin probe force microscopy (KPFM) measurements taken on (001) surfaces of UHV cleaved NaCl, KCl and MgO are presented for the first time. With the help of force spectroscopy we show first that the charging of (001) surfaces of alkali halide crystals, which generally occurs after UHV cleavage, vanishes after a couple of days due to their sufficiently high ionic conductivity at room temperature. KPFM images of these (001) surfaces show that the surface potential is not uniform but exhibits variations of up to 1?V at a nanometre length scale. Variations on terraces as well as a strong contrast at step edges can be observed, of which the latter is probably due to trapped charges. On MgO(001), we observe strong changes in the surface potential, especially at previously reported adstructures. These changes explain why imaging MgO(001) is difficult.     

Interface dipole formation of different ZnPcCl(8) phases on Ag(111) observed by Kelvin probe force microscopy

Recently, we investigated the adsorption of octachloro zinc phthalocyanine (ZnPcCl(8)) on Ag(111) by scanning tunneling microscopy. Compared to the standard phthalocyanine, halogenated phthalocyanine molecules show a much more complex binding behavior, which results in the formation of three different structural phases. These phases follow from the ordering process with the formation of 8, 4 and 0 intermolecular hydrogen-halogen bonds (Abel et al 2006 ChemPhysChem?7 82). In the present work we investigate these phases by Kelvin probe force microscopy in order to quantitatively deduce the electric interface barrier of the first monolayer. Our measurements reveal that the binding behavior does not only affect the structural ordering but also the interface dipole formation, which leads to different work functions. The fact that we observe interface barriers of opposite signs between ordered and disordered molecular layers underlines the importance of exactly knowing the molecular arrangement at the interface when assembling organic molecule devices.     

High-resolution imaging of molecular and nanoparticles assemblies with Kelvin force microscopy

High-resolution studies of self-assemblies of semifluorinated alkanes molecules F12H8 and F14H20 [FnHm = CF3(CF2)n(CH2)mCH3], and CdTe particles were performed with single-pass Kelvin force microscopy. Surface potential contrast, which is related to the strength and orientation of molecular dipoles, empowers the characterization of self-organized structures. Lamellar structures, ribbons and toroids of F14H20 and F12H8 were observed on graphite and the differences of surface potential were interpreted in terms orientation of -CH2-CF2- dipoles. A gradual sublimation of F12H8 molecules allowed a visualization of top and bottom molecular layers on the substrate. Prior to the sublimation a part of lamellae of the bottom layers was transformed into the ribbons. The surface potential data suggest that this transition proceeds with the reorientation of the molecular chains from the horizontal to vertical direction. Self-assembly of CdTe nanoparticles into nanowires was monitored upon drying on mica. The process is accompanied by drastic changes of surface potential. The formed nanowires exhibit strong positive surface potential that assumes a structural transition with a formation of strong dipole moment in these self-assemblies.     

Direct observation of minority carrier leakage in operating laser diodes by Kelvin probe force microscopy

A new approach to investigating the leakage of nonequilibrium holes and electrons from the active region of a semiconductor laser diode is proposed. According to this, the scanning Kelvin probe force microscopy is used to measure averaged local changes in the contact potential difference on the surface of laser mirrors of a device operating at a pulsed bias voltage. It is shown that the measured signal level is determined by the degree of charge exchange between the slow surface states and nonequilibrium minority carriers, the concentration of which is directly related to the leakage current.     

Local voltage drop in a single functionalized graphene sheet characterized by Kelvin probe force microscopy

We studied the local voltage drop in functionalized graphene sheets of subμm size under external bias conditions by Kelvin probe force microscopy. Using this noninvasive experimental approach, we measured ohmic current-voltage characteristics and an intrinsic conductivity of about 3.7 × 10(5) S/m corresponding to a sheet resistance of 2.7 kΩ/sq under ambient conditions for graphene produced via thermal reduction of graphite oxide. The contact resistivity between functionalized graphene and metal electrode was found to be <6.3 × 10(-7) Ωcm(2).     

Label-free and high-resolution protein/DNA nanoarray analysis using Kelvin probe force microscopy

Using the scanning probe technique known as Kelvin probe force microscopy it is possible to successfully devise a sensor for charged biomolecules. The Kelvin probe force microscope is a tool for measuring local variations in surface potential across a substrate of interest. Because many biological molecules have a native state that includes the presence of charge centres (such as the negatively charged backbone of DNA), the formation of highly specific complexes between biomolecules will often be accompanied by local changes in charge density. By spatially resolving this variation in surface potential it is possible to measure the presence of a specific bound target biomolecule on a surface without the aid of special chemistries or any form of labelling. The Kelvin probe force microscope presented here is based on an atomic force microscopy nanoprobe offering high resolution (<10 nm), sensitivity (<50 nM) and speed (>1,100 microm s(-1)), and the ability to resolve as few as three nucleotide mismatches.     

The Kelvin probe method for work function topographies: technical problems and solutions

The study of the physical properties (roughness, optical and electrical properties) of an entire surface or portions of this surface may be done by positioning a probe or using a beam of dimensions smaller than the studied area in front of the surface. The scanning method, often used in microscopy, LEED, etc., is however difficult to set up for measuring work function inhomogeneities by the Kelvin probe method: to be useful, the measurements must be made at a constant probe sample distance.We present here an original apparatus, based on a feedback loop, using the Kelvin probe, which permits us to work at a constant probe sample distance and, therefore, with a constant accuracy. Work function topographies are possible on all surfaces with an area greater than a few mm². It is also possible to find the roughness of the surface, the probe sample distance and to work in varying temperature. Results concerning semiconductor studies are given.     

Probing CO on a rutile TiO2(110)surface using atomic force microscopy and Kelvin probe force microscopy

Probing CO at a specific site on a metal oxide surface is essential for characterizing various applications such as CO oxidation,hydrogenation,and water-gas shi...     

Three-dimensional observation of nanoscale ferroelectric domains using scanning nonlinear dielectric microscopy with electric field correction by Kelvin probe force microscopy

An advanced technique for the measurement of three-dimensional ferroelectric domain structure is described. Scanning nonlinear dielectric microscopy is used to measure the polarization components both perpendicular and parallel to the specimen surface. A nanoscale electric field correction is devised and performed using Kelvin probe force microscopy to allow more precise measurement of the nanoscale polarization component parallel to the specimen surface. Using this electric field correction, three-dimensional imaging of the ferroelectric polarization orientation is demonstrated.     

Surface photovoltage analysis of thin CdS layers on polycrystalline chalcopyrite absorber layers by Kelvin probe force microscopy

An extensive Kelvin probe force microscopy study in an ultrahigh vacuum has been undertaken to examine the influence of growth modifications of a few nm thick CdS buffer layers in thin film chalcopyrite solar cells. In regions around the grain boundaries of the polycrystalline Cu(In,Ga)Se(2) substrate a lowering of the work function extending to about 200?nm away from this vertical interface was observed. This electrical inhomogeneity depends strongly on the Cu(In,Ga)Se(2) surface condition and is interpreted by a diffusion process along the substrate grain boundaries. Our results contribute to the understanding of the crucial role of the several nm thick CdS layer for improving the photovoltaic performance of chalcopyrite thin film solar?cells.     

Kelvin probe force microscopy study on conjugated polymer/fullerene bulk heterojunction organic solar cells

We conducted a comprehensive Kelvin probe force microscopy (KPFM) study on a classical organic solar cell system consisting of MDMO-PPV/PCBM blends. The KPFM method yields the information of topography and local work function at the nanometer scale. Experiments were performed either in the dark or under cw laser illumination at 442 nm. We identified distinct differences in the energetics on the surface of chlorobenzene and toluene cast blend films. Together with high-resolution scanning electron microscopy (SEM) experiments we were able to interpret the KPFM results and to draw some conclusions for the electron transport toward the cathode in the solar cell configuration. The results suggest that surfaces of toluene cast films exhibit a morphologically controlled hindrance for electron propagation toward the cathode, which is usually evaporated on top of the films in the solar cell device configuration.     

Magnetic force microscopy of superparamagnetic nanoparticles

The use of magnetic force microscopy (MFM) to detect probe-sample interactions from superparamagnetic nanoparticles in vitro in ambient atmospheric conditions is reported here. By using both magnetic and nonmagnetic probes in dynamic lift-mode imaging and by controlling the direction and magnitude of the external magnetic field applied to the samples, it is possible to detect and identify the presence of superparamagnetic nanoparticles. The experimental results shown here are in agreement with the estimated sensitivity of the MFM technique. The potential and challenges for localizing nanoscale magnetic domains in biological samples is discussed.     

Polarizability of G4-DNA observed by electrostatic force microscopy measurements

G4-DNA, a quadruple helical motif of stacked guanine tetrads, is stiffer and more resistant to surface forces than double-stranded DNA (dsDNA), yet it enables self-assembly. Therefore, it is more likely to enable charge transport upon deposition on hard supports. We report clear evidence of polarizability of long G4-DNA molecules measured by electrostatic force microscopy, while coadsorbed dsDNA molecules on mica are electrically silent. This is another sign that G4-DNA is potentially better than dsDNA as a conducting molecular wire.     

Nanoscale surface photovoltage of organic semiconductors with two pass Kelvin probe microscopy

Kelvin probe microscopy implemented with controlled sample illumination is used to study nanoscale surface photovoltage effects. With this objective a two trace method, where each scanning line is measured with and without external illumination, is proposed. This methodology allows a direct comparison of the contact potential images acquired in darkness and under illumination and, therefore, the surface photovoltage is simply inferred. Combined with an appropriate data analysis, the temporal and spatial evolution of reversible and irreversible photo-induced processes can be obtained. The potential and versatility of this technique is applied to MEH-PPV thin films. Photo-physical phenomena such as the mesoscale polymer electronic light-induced response as well as the local nanoscale electro-optical properties are studied.