Kelvin Probe Force Microscopy Measurements Research Articles

Nanoscale quantification of charge injection and transportation process in Si-nanocrystal based sandwiched structure

Si nanocrystals are formed by using KrF pulsed laser crystallization of an amorphous SiC/ultrathin amorphous Si/amorphous SiC sandwiched structure. Electrons and holes are injected into Si nanocrystals via a biased conductive AFM tip and the carrier decay and transportation behaviours at the nanoscale are studied by joint characterization techniques of Kelvin probe force microscopy (KPFM) and conductive atomic force microscopy (CAFM). Quantification of the surface charge density is realized by solving the Poisson equation based on KPFM measurements. Besides, the asymmetric barrier height for electrons and holes is considered to play a dominant role in controlling the charge retention and transportation characteristics. The methodology developed in this work is promising for studying the charge injection and transportation process in other materials and structures at the nanoscale.

Investigation of Kelvin probe force microscopy efficiency for the detection of hydrogen ingress by cathodic charging in an aluminium alloy

Detecting and locating absorbed hydrogen in aluminium alloys is necessary for evaluating the contribution of hydrogen embrittlement to the degradation of the mechanical properties for corroded or cathodically hydrogen-charged samples. The capability of Kelvin probe force microscopy (KFM) to overcome this issue was demonstrated. Aluminium alloy samples were hydrogenated by cathodic polarisation in molten salts (KHSO4/NaHSO4.H2O). The presence of absorbed hydrogen was revealed; the affected zone depth was measured by secondary ion mass spectroscopy (SIMS) analyses and KFM measurements.

Practical aspects of single-pass scan Kelvin probe force microscopy

The single-pass scan Kelvin probe force microscopy (KPFM) in ambient condition has a few advantages over the dual-pass lift-up scan KPFM. For example, its spatial resolution is expected to be higher; and its topographical errors caused by electrostatic forces are minimized because electrostatic forces are actively suppressed during the simultaneous topographical and KPFM measurement. Because single-pass scan KPFM in ambient condition is relatively new, it received little attention in the literature so far. In this article, we discuss several major practical aspects of single-pass scan KPFM especially in ambient condition. First, we define the resolution using a point spread function. With this definition, we analyze the relation between the resolution and the scanning parameters such as tip apex radius and tip-surface distance. We further study the accuracy of KPFM based on the point spread function. Then, we analyze the sensitivity of KPFM under different operation modes. Finally, we investigate the crosstalk between the topographical image and the surface potential image and demonstrate the practical ways to minimize the crosstalk. These discussions not only help us to understand the single-pass scan KPFM but also provide practical guidance in using single-pass scan KPFM.

Focus on materials

Focus on materials

The Effects of Aggregation on Electronic and Optical Properties of Oligothiophene Particles

Solution processing of oligothiophene molecules is shown to produce a range of particles with distinct morphologies. Once isolated on a substrate, the optical and electronic properties of individual particles were studied. From polarized scanning confocal microscopy experiments, distinct particles that are identifiable by shape were shown to have similar emission spectra except in regard to the 0-0 vibronic band intensity. This suppression of the 0-0 vibronic band correlates to the amount of energetic disorder present in a weakly coupled H-aggregate. The studied particles ranged from moderate to almost complete suppression of the 0-0 vibronic band when compared to the emission spectrum of the isolated molecule in solution. All particles were found to have a high degree of geometric order (molecular alignment) as observed from the fluorescence dichroism (FD) values of around 0.7-0.8 for all the studied morphologies. The structural and electronic properties of the particles were investigated with Kelvin probe force microscopy (KPFM) to measure the local contact potential (LCP) difference, a quantity that is closely related to the differences in intermolecular charge distribution between the oligothiophene particles. The LCP was found to vary by as much as 70 mV between different oligothiophene particles and a trend was observed that correlated the LCP changes with the amount of energetic disorder present, as signified by the suppression of the 0-0 vibronic peak in the emission spectra. Combined polarized scanning confocal microscopy studies, along with KPFM measurements, help to provide fundamental insights into the role of morphology, molecular packing, and intermolecular charge distributions in oligiothiophene particles.

Identification of nanoscale structure and morphology reconstruction in oxidized a-SiC:H thin films

Oxidation behavior of a-SiC:H layers deposited by radio-frequency magnetron sputtering technique was examined by Kelvin probe force microscopy (KPFM) in combination with scanning electron microscopy, Fourier-transform infra-red spectroscopy and submicron selected area Raman scattering spectroscopy. Partially oxidized a-SiC:H samples (oxidation at 600°C in oxygen) were examined to clarify mechanism of the oxidation process. Nanoscale and microscale morphological defects (pits) with dimension of about 50nm and several microns respectively have appeared after thermal treatment. KPFM measurements exhibited the surface potential of the material in micro pits is significantly smaller in comparison with surrounding material. Submicron RS measurements indicates formation of graphite-like nano-inclusions in the pit defects. We conclude that initial stage of oxidation process in a-SiC:H films takes place not homogeneously throughout the layer but it is initiated in local nanoscale regions followed by spreading over all layer.

Surface potential measurement of organic thin film on metal electrodes by dynamic force microscopy using a piezoelectric cantilever

We describe applications of a cantilever with a lead zirconate titanate (PZT) piezoelectric film as self-sensing to dynamic force microscopy (DFM) combined with Kelvin probe force microscopy (KFM). We adopted a frequency modulation (FM) detection method not only to stabilize the imaging conditions in our DFM but also to enhance the sensitivity for the detection of electrostatic forces in KFM measurement. We deposited Alq3 [tris (8-hydroxyquinolinato) aluminum] thin films and aluminum (Al) electrode patterns on an indium tin oxide (ITO)/glass substrate by vacuum evaporation using shadow masks. The surface structures and local surface potential of Alq3 films on metals were investigated using our DFM/KFM instrument to study the local electrical properties at the molecule–metal interface. The photosensitive organic material sample can be in a completely dark environment because no optics are required for cantilever deflection sensing in our experimental setup.

The role of the cantilever in Kelvin probe force microscopy measurements

The role of the cantilever in quantitative Kelvin probe force microscopy (KPFM) is rigorously analyzed. We use the boundary element method to calculate the point spread function of the measuring probe: Tip and cantilever. The calculations show that the cantilever has a very strong effect on the absolute value of the measured contact potential difference even under ultra-high vacuum conditions, and we demonstrate a good agreement between our model and KPFM measurements in ultra-high vacuum of NaCl monolayers grown on Cu(111). The effect of the oscillating cantilever shape on the KPFM resolution and sensitivity has been calculated and found to be relatively small.

Investigation of InN mole fraction fluctuation in InGaN films grown by RF‐MBE

AbstractWe use cathodoluminescence (CL) mapping and Kelvin probe force microscopy (KFM) to investigate fluctuations of the InN mole fraction in InGaN films grown by radio‐frequency plasma‐assisted molecular beam epitaxy. Both techniques reveal that InGaN films on GaN templates with scratches exhibit clear composition fluctuations. CL spectra of InGaN obtained in scanning mode have two peaks. Monochromatic CL maps at these two peaks reveal a clear contrast between flat regions and scratches. The shorter‐wavelength CL peak is from scratched regions and the longer‐wavelength peak is from flat regions. These results reveal that less In is incorporated in scratched regions than in flat regions. KFM measurements reveal that scratched and flat regions have different surface potentials. Experiments on a series of InGaN films with InN mole fractions that span the entire composition range confirm that the surface potential of InGaN depends on the In content. This demonstrates that KFM is useful for studying microscale fluctuations in the InN mole fraction in InGaN. A possible mechanism for this fluctuation is discussed. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electric field concentration in the vicinity of the interface between anode and degraded BaTiO3-based ceramics in multilayer ceramic capacitor

The electric field distribution of degraded dielectric layers in multilayer ceramic capacitors (MLCCs) was investigated by Kelvin probe force microscopy (KFM) to clarify the insulation degradation mechanism in MLCCs. For the degraded dielectric layers, the electric field was found to be concentrated near the anodes. This concentration easily moved to the opposite side with a reversal of the applied voltage of 5 V (13 kV/cm) during KFM measurement at room temperature. On the other hand, electric field concentrations and electric field distributions did not change in fresh MLCCs, indicating that the electric field concentrations easily transfer near higher-potential interfaces between electrodes and ceramics only in degraded MLCCs. These facts suggest that Schottky barriers would be formed in degraded MLCCs. The KFM technique discussed in this work is a very useful tool for measuring the surface potential and helps clarify the local electric field concentration near the electrodes.

AFM tip characterization by Kelvin probe force microscopy

Reliable determination of the surface potential with spatial resolution is key for understanding complex interfaces that range from nanostructured surfaces to molecular systems to biological membranes. In this context, Kelvin probe force microscopy (KPFM) has become the atomic force microscope (AFM) method of choice for mapping the local electrostatic surface potential as it changes laterally due to variations in the surface work function or surface charge distribution. For reliable KPFM measurements, the influence of the tip on the measured electrostatic surface potential has to be understood. We show here that the mean Kelvin voltage can be used for a straightforward characterization of the electrostatic signature of neutral, charged and polar tips, the starting point for quantitative measurements and for tip-charge control for AFM manipulation experiments. This is proven on thin MgO(001) islands supported on Ag(001) and is supported by theoretical modeling, which shows that single ions or dipoles at the tip apex dominate the mean Kelvin voltage.

Novel Anthracene Materials for Applications in Lithography and Reversible Photoswitching by Light and Air

Herein we demonstrate how the photoreaction between anthracenes and singlet oxygen ((1)O(2)) is employed for applications either as photoswitch or as photoresist. Thin films of the diaryl-alkyl anthracene 1 and the analogous oligomeric species 2 were irradiated under photomasks to generate pattern structures composed of 1/1-O(2) and 2/2-O(2). Kelvin probe force microscopy (KPFM) provided a powerful and nondestructive method to image the pattern information. The following studies based on AFM, KPFM and contact angle measurements unfold that the two species 1 and 2 underwent different progressions after the imaging step. Degrading is observed for the monomeric compound 1 and the pattern eventually becomes recognizable in topography. In the oxidized state (1-O(2)) the monomeric species remains physically stable. In consequence, the unreacted portion is removable and the remaining oxygenated form 1-O(2) is sufficiently stable to protect an underlying substrate (e.g., silver) from etching. Thus, the system 1/1-O(2) operates as photoresist. On the other hand, both states of the oligomer 2 remain stable. The film is stable up to temperatures >120 degrees C required to erase the pattern within acceptable time by cycloreversion. Anthracene 2 therefore acts as erasable and rewritable photochromic switch. The different behavior between 1 and 2 is explained by phase transitions which cause crystallization and finally ablation. Such transitions affect only the monomeric system 1/1-O(2) and not the oligomeric system 2/2-O(2). In conclusion, we designed two very similar materials based on diarylanthracenes, which can act either as a photoresist or as a rewritable photochromic switch.

Deconvolution of Kelvin probe force microscopy measurements—methodology and application

Kelvin probe force microscopy (KPFM) is a method to detect the surface potential of micro- and nanostructured samples using a common atomic force microscope (AFM). The electrostatic force has a very long range compared to other surface forces. By using AFM systems under ambient conditions, KPFM measurements are performed using a non-contact regime at surface distances greater than 10 nm. This paper deals with a method to deconvolve the measured KPFM data with the objective to increase the lateral resolution. The KPFM signal is a convolution of an effective surface potential and a microscopic intrinsic point spread function, which allows the restoration of the measured data by linear deconvolution. In contrast to other papers [4], we have developed a new method to use the measured AFM tip shape as a basis to construct the point spread function. The linear shift-invariant channel is introduced as a signal formation model and a Wiener-supported deconvolution algorithm is applied to the measured data. The new method was demonstrated on a nanoscale test stripe pattern for lateral resolution and calibration of length scales (BAM-L200) manufactured by the Federal Institute for Materials Research and Testing, Germany. For the first time, a two-dimensional deconvolution of the KPFM data was able to be demonstrated. An increase in the lateral resolution compared to Strassburg et al (2005 Rev. Sci. Instrum. 76 083705) was accomplished. The results demonstrate the necessity of deconvolving the virtually topography-free probe data under ambient conditions.

Surface potential of n- and p-type GaN measured by Kelvin force microscopy

n - and p-type GaN epitaxial layers grown by metal-organic chemical vapor deposition with different doping levels have been characterized by Kelvin probe force microscopy (KFM). To investigate the surface states of GaN beyond instrumental and environmental fluctuations, a KFM calibration procedure using a gold-plated Ohmic contact as a reference has been introduced, and the reproducibility of the KFM measurements has been evaluated. Results show that the Fermi level is pinned for n- and p-type GaN over the available doping ranges, and found 1.34±0.15eV below the conduction band and 1.59±0.18eV above the valence band, respectively.

Electrical investigation of V-defects in GaN using Kelvin probe and conductive atomic force microscopy

We report on the electrical characterization of V-defects in GaN-based heterostructures via Kelvin probe force microscopy (KPFM) and conductive atomic force microscopy (C-AFM). The KPFM measurements show for n- and p-doped GaN top layers an increase in the work function within the V-defects. Surprisingly, an increase in the current flow within the V-defects is found by C-AFM in case of the n-doped structure, while current flow into the V-defect is suppressed for the p-doped structure. For a consistent explanation of these results we suggest a model, which is based on an increase in the electron affinity of the {10−11}-surfaces within the V-defects as compared to the planar (0001)-surface.

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 noncontact 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 counterelectrode and the second term depicts the influence of the Madelung surface potential on the mesoscopic part of the tip, independent of its microscopic structure. The former has the lateral periodicity of the Madelung surface potential, whereas the latter only acts as a static component, which shifts the total force. These short-range electrostatic forces are in the range of 10 pN. Beyond the dielectric properties of the crystal, a major effect, which is responsible for the atomic contrast of the KPFM signal, is the ionic polarization of the sample due to the influence of the tip/counterelectrode capacitor. 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, and 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.

Microscopic investigation of InGaN/GaN heterostructure laser diode degradation using Kelvin probe force microscopy

We report on Kelvin probe force microscopy (KPFM) measurements on fresh and artificially aged InGaN/GaN laser test structures. In the case of an unbiased laser diode, a comparison of the surface potential between a fresh and a stressed laser diode shows a pronounced modification of the laser facet due to the aging process. Performing KPFM measurements under forward bias, a correlation between the macroscopic I–V characteristics and the microscopic voltage drop across the heterostructure layer sequence is found. This clearly demonstrates the potential of KPFM for investigating InGaN/GaN laser diode degradation.

Metal nanostructures assembled at semiconductor surfaces studied with high resolutionscanning probes

Dynamic force microscopy (DFM), Kelvin probe force microscopy (KPFM) andscanning tunnelling microscopy/spectroscopy (STM/STS) have been used to studyepitaxial nanostructures formed as a result of submonolayer deposition of gold onreconstructed InSb, InAs, and GaAs(001) surfaces. It has been found that bothtopography and composition of the structures are strongly dependent on the substratetemperature during evaporation. In the particular case of Au deposited on a c(8 × 2) InSb(001) surface at 300 K, the nanostructures have predominantly the form ofrectangular islands with edges oriented along [110] and [] directions. Increasing the substrate temperature up to 600 K during deposition resultsin formation of narrow, long structures (nanowires) with an average length of800 nm. The nanowires are created along the [110] direction of the reconstructedInSb(001) surface. High resolution KPFM and STS measurements indicate thatnanowires of different heights may have different chemical composition and electronicstructure.

Surface Potential Switching by Metal Ion Complexation/Decomplexation Using Bipyridinethiolate Monolayers on Gold

Surface potential switching on gold(111) surfaces is induced by complexation/decomplexation reactions of a bipyridine (BP) derivative and palladium(II) chloride, as observed by Kelvin probe force microscopy (KFM). On the basis of the theoretical predictions, a 4-(5-phenylethynyl-2,2'-bipyridine-5'-yl-ethynyl)benzenethiol (PhBP) derivative was synthesized and used as an active monolayer to catch transition metal ions. By using the microcontact printing (CP) technique, micron-size patterned PhBP monolayers, which act as effective hosts to coordinate palladium(II) chloride, were prepared on gold(111) surfaces. The KFM signal decreases by complexation of the Pd(II) chloride in PhBP monolayers and is recovered by removal of Pd ions using an ethylenediamine solution, as confirmed by X-ray photoelectron spectroscopy. This process is reversible, indicating that the surface potential switching is realized by complexation/decomplexation of Pd(II). A CP PhBP monolayer, when it detects the target palladium ion, shows sensitivity for the picomolar level detection judged from surface potential changes in KFM measurements. The dipole moment estimated by the surface potentials is much smaller than the calculated value, indicating that mechanisms for the reduction of the surface dipole moment exist in real monolayers prepared by the CP method.

Electrical charge trapping at defects on the Si(111)7×7 surface

We report on a direct measurement of electron trapping at defects on the Si(111)7×7 surface, by combining Kelvin probe force microscopy (KPFM) and scanning tunneling microscopy (STM) measurements. One-dimensional defects of atomic steps and two-dimensional defects of disordered domains were found on the surface. STM reveals that the disordered domain is located in the intersection area between three 7×7 domains. KPFM measurement shows that electrons are trapped at both the atomic steps and the disordered domains, and this electron trapping gives rise to a larger local work function on the defect region than on the defect-free 7×7 regions.