Kelvin Probe Force Microscopy Images Research Articles

Extended Work Function Shift of Large‐Area Biofunctionalized Surfaces Triggered by a Few Single‐Molecule Affinity Binding Events

AbstractFew binding events are here shown to elicit an extended work function change in a large‐area Au‐surface biofunctionalized with ≈108 capturing antibodies. This is demonstrated by Kelvin probe force microscopy (KPFM), imaging a ≈105 µm2 wide Au‐electrodes covered by a dense layer (≈104 µm−2) of physisorbed anti‐immunoglobulin‐M (anti‐IgM). A 10 min incubation in 100 µL phosphate buffer saline solution encompassing ≈10 IgM antigens (10−19 mole L−1  102 × 10−21 m) produces a work function shift ΔW ≈ –60 meV. KPFM images prove that this shift involves the whole inspected area. Notably, no work function change occurs upon incubation in highly concentrated (3 × 10−15 m) nonbinding IgG solutions. The ΔW measured by KPFM is in quantitative agreement with the threshold voltage shift of an electrolyte‐gated single‐molecule large‐area transistor (SiMoT). The findings provide direct experimental evidence for the SiMoT ultrahigh sensitivity, by imaging the extensive shift of the gate work function, likely arising from collective surface phenomena, elicited by single‐molecule binding events.

Nanotribological properties and scratch resistance of MoS2 bilayer on a SiO2/Si substrate

The tribological properties and scratch resistance of MoS2 bilayer deposited on SiO2/Si substrates prepared via chemical vapor deposition are investigated. Friction force microscopy (FFM) is employed to investigate the friction and wear properties of the MoS2 bilayer at the nanoscale by applying a normal load ranging from 200 to 1,000 nN. Scratch resistance is measured using the scratch mode in FFM based on a linearly increasing load from 100 to 1,000 nN. Kelvin probe force microscopy (KPFM) is performed to locally measure the surface potential in the tested surface to qualitatively measure the wear/removal of MoS2 layers and identify critical loads associated with the individual failures of the top and bottom layers. The analysis of the contact potential difference values as well as that of KPFM, friction, and height images show that the wear/removal of the top and bottom layers in the MoS2 bilayer system occurred consecutively. The FFM and KPFM results show that the top MoS2 layer begins to degrade at the end of the low friction stage, followed by the bottom layer, thereby resulting in a transitional friction stage owing to the direct contact between the diamond tip and SiO2 substrate. In the stable third stage, the transfer of lubricious MoS2 debris to the tip apex results in contact between the MoS2-transferred tip and SiO2. Nanoscratch test results show two ranges of critical loads, which correspond to the sequential removal of the top and bottom layers.

Twisted graphene in graphite: Impact on surface potential and chemical stability

Highly-oriented pyrolytic graphite (HOPG), i.e., the 3D stack of sp2-hybridized carbon sheets, is an attractive material thanks to its high electrical conductivity, chemical inertness, thermal stability, atomic-scale flatness, and ease of exfoliation. Despite an apparently ideal and uniform material, freshly cleaved HOPG shows domains in Kelvin probe force microscopy (KPFM) with surface potential contrast over 30 mV. We systematically investigated these domains using an integrated approach, including time-dependent KPFM and hyperspectral Raman imaging. The observed time-evolving domains are attributed to locally different hydrocarbon adsorption from the environment, driven by structural defects likely related to rotational mismatch, i.e., twisted layers. These defects affect the interlayer coupling between topmost graphene and the underlying layers. Our hypothesis was supported by Raman spectroscopy results, showing domains with G peak shifts and 2D line shape compatible with bilayer graphene. We attribute the selective sensitivity of our Raman spectroscopy results to the top graphene layers as resonances due to van Hove singularities. Our results show that the chemical and electrical properties of HOPG are far more complex than what is generally believed due to the broken symmetry at the top surface, giving rise to graphene bilayer-like behavior.

Interfacial Potentials in a Thin Film All-Solid-State Li-Ion Battery Probed By in Operando KPFM

Designing smaller, safer, cheaper, and more stable batteries necessitates thorough understanding of the electrochemical processes governing their operation at multiple length scales. In all-solid-state power sources the electrolyte is no-flammable and can be made ultimately thin, making them safer and more compact. Yet, experimental evidence suggests that the multiple internal interfaces in their layered structure give rise to high impedances, limiting their performance. Studying these nanoscopic interfaces requires microscopic tools. Here, we employ in operando Kelvin Probe Force Microscopy (KPFM) to quantitatively measure the potential distribution in a solid-state Li-ion battery as a function of its state of charge. The battery was fabricated by sequentially depositing thin layers of Pt (110-130 nm), LiCoO2 (280-420 nm), LIPON (1100-1200 nm), Si (50-240 nm) Cu or Pt (150-200 nm) onto a Si/SiO2 wafer (oxide thickness 100 nm). The fabricated battery was cleaved in an Ar atmosphere to expose the stacked layers, mounted on a holder, wired, and safely transferred without exposing to air into a dual-beam instrument that combines a scanning electron microscope (SEM), a Ga-ion focused ion beam (FIB) and an atomic force microscope (AFM) in one vacuum chamber (residual pressure of 10-4 Pa). The stacked battery was milled to expose a cross-section of the layers, and imaged using SEM and KPFM, while cycling the battery. The acquired potential maps reveal a highly non-uniform interelectrode potential distribution, with most of the potential drop occurring at the electrolyte-Si anode interface in the pristine battery. During the first charge, the potential distribution gradually changes, revealing complex polarization within the LIPON layer due to Li-ion redistribution. Cycling the battery at high rate significantly decreased its capacity, although the capacity loss can be recovered. KPFM imaging allowed the detection of the interface responsible for this capacity loss. Li distribution in the battery was also measured using Neutron Depth Profiling as a function of the state of charge. The acquired data was compared to first principles calculations shedding light onto the interfacial Li-ion transport in the battery and its reversibility.ES acknowledges support under the Cooperative Research Agreement between the University of Maryland and the National Institute of Standards and Technology Center for Nanoscale Science and Technology, Award 70NANB14H209, through the University of Maryland.

Stable and Highly Flexible Perovskite Solar Cells with Power Conversion Efficiency Approaching 20% by Elastic Grain Boundary Encapsulation

Here, we show that flexible perovskite solar cells (PSCs) with high operational stability and power conversion efficiency (PCE) approaching 20% were achieved by elastic grain boundary (GB) encapsul...

Effects of subsurface charge on surface defect and adsorbate of rutile TiO<sub>2 </sub>(110)

<sec>Transition-metal-oxide as a typical model surface for investigating the catalytic mechanism has been widely studied. Over the past years, the TiO<sub>2</sub> properties have been reported. It is commonly accepted that the catalytic activity of reduced TiO<sub>2</sub> is related to its defects, with the accompanying excess electrons leading to n-type conductivity. It is realized that subsurface charge is of key importance for the redox chemistry of TiO<sub>2</sub> (110).</sec><sec>Subsurface charge is explored by atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM). Subsurface charge exerts an additional attractive force on the scanning AFM tip, resulting in the relative retraction of tip motion in order to keep a constant frequency shift. As a result, the subsurface charged region is imaged as protrusion in an AFM topographic image. The height of bright hillock is mainly distributed in three different ranges, which means that the subsurface charges are at three different subsurface layers. The AFM results show such subsurface charges repel the electropositive oxygen vacancy, hydrogen atoms and step edges. It is obvious that there is not only an O<sub>v</sub> depletion zone but also the subsurface charge free region in the proximity of the <inline-formula><tex-math id="Z-20201022132953">\begin{document}$\left\langle {001} \right\rangle $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20200773_Z-20201022132953.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20200773_Z-20201022132953.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="Z-20201022132947">\begin{document}$\left\langle {1\bar 11} \right\rangle$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20200773_Z-20201022132947.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20200773_Z-20201022132947.png"/></alternatives></inline-formula> step edge.</sec><sec>The KPFM image indicates that the subsurface charges are the positive charges. which is consistent with common sense. After oxygen exposure, it is found that the oxygen adatom is electronegative, but it is absent in the vicinity of positive subsurface charges. Irrespective of adsorbate being electropositive or electronegative, an adsorbate-free zone generally exists in the proximity of the charged region. Obviously, the present study is expected to provide some insights into clarifying the nature of subsurface charge and improving catalytic design.</sec>

Ion‐Induced Defects in Graphite: A Combined Kelvin Probe and Raman Microscopy Investigation

Carbon nanomaterials are important for future sensors and electronics. Defects determine the material properties, therefore, it is critical to find new ways to investigate defects at the nanoscale. In this context, Raman spectroscopy (RS) is the tool of choice to study defects in carbon nanomaterials. On the other hand, Kelvin probe force microscopy (KPFM) provides structural and surface potential information at the nanoscale. Here, the authors demonstrate the synergistic application of these methods in the investigation of ion‐beam‐induced defects in graphite. KPFM and RS imaging are used for visualizing ion‐induced defects in a wide range of ion doses from 1010 to 1016 ions cm−2. For the lower range of ion dose, the authors find that RS provides image contrast for the different defect regions in graphite up to a dose of 5 × 1013 ions cm−2. For higher doses, the sp2 carbon concentration becomes so small that the Raman spectra get dominated by broad amorphous carbon bands. For this dose range, the KPFM contrast allows the defective regions to be differentiated. This contrast in KPFM originates from sp3 carbons that act as charge traps. The results show that KPFM and Raman microscopy make a powerful and necessary combination for studying the spatial distribution of defects in carbon.

Surface potential extraction from electrostatic and Kelvin-probe force microscopy images

A comprehensive comparison study of electrostatic force microscopy (EFM) and Kelvin probe force microscopy (KPFM) is conducted in this manuscript. First, it is theoretically demonstrated that for metallic or semiconductor samples, both the EFM and KPFM signals are a convolution of the sample surface potential with their respective transfer functions. Then, an equivalent point-mass model describing cantilever deflection under distributed loads is developed to reevaluate the cantilever influence on detection signals, and it is shown that the cantilever has no influence on the EFM signal, while it will affect the KPFM signal intensity but not change the resolution. Finally, EFM and KPFM experiments are carried out, and the surface potential is extracted from the EFM and KPFM images by deconvolution processing, respectively. The extracted potential intensity is well consistent with each other and the detection resolution also complies with the theoretical analysis. Our work is helpful to perform a quantitative analysis of EFM and KPFM signals, and the developed point-mass model can also be used for other cantilever beam deflection problems.

Force-gradient sensitive Kelvin probe force microscopy by dissipative electrostatic force modulation

We report a Kelvin probe force microscopy (KPFM) implementation using the dissipation signal of a frequency modulation atomic force microscopy that is capable of detecting the gradient of electrostatic force rather than electrostatic force. It features a simple implementation and faster scanning as it requires no low frequency modulation. We show that applying a coherent ac voltage with two times the cantilever oscillation frequency induces the dissipation signal proportional to the electrostatic force gradient which depends on the effective dc bias voltage including the contact potential difference. We demonstrate the KPFM images of a MoS2 flake taken with the present method are in quantitative agreement with those taken with the frequency modulated Kelvin probe force microscopy technique.

Reconstruction of Kelvin probe force microscopy image with experimentally calibrated point spread function.

A Kelvin probe force microscopy (KPFM) image is sometimes difficult to interpret because it is a blurred representation of the true surface potential (SP) distribution of the materials under test. The reason for the blurring is that KPFM relies on the detection of electrostatic force, which is a long-range force compared to other surface forces. Usually, KPFM imaging model is described as the convolution of the true SP distribution of the sample with an intrinsic point spread function (PSF) of the measurement system. To restore the true SP signals from the blurred ones, the intrinsic PSF of the system is needed. In this work, we present a way to experimentally calibrate the PSF of the KPFM system. Taking the actual probe shape and experimental parameters into consideration, this calibration method leads to a more accurate PSF than the ones obtained from simulations. Moreover, a nonlinear reconstruction algorithm based on total variation (TV) regularization is applied to KPFM measurement to reverse the blurring caused by PSF during KPFM imaging process; as a result, noises are reduced and the fidelity of SP signals is improved.

Piezoresponse force microscopy studies of domains in PbFe1/2Nb1/2O3 ceramics

ABSTRACTLi-doped PbFe1/2Nb1/2O3 ferroelectric ceramics was investigated by the Piezoresponse Force Microscopy (PFM) and Kelvin Probe Force Microscopy (KPFM) methods. Areas with principally different piezoresponse were observed at the PFM images. The stripe-like structures were revealed, which seem to be either 110±4° or 67±4° ferroelectric domains. The electric potential contours of the KPFM images demonstrate a significant discrepancy with the surface topography, which may be valuable for characterization of the samples.

Preventing probe induced topography correlated artifacts in Kelvin Probe Force Microscopy

Kelvin Probe Force Microscopy (KPFM) on samples with rough surface topography can be hindered by topography correlated artifacts. We show that, with the proper experimental configuration and using homogeneously metal coated probes, we are able to obtain amplitude modulation (AM) KPFM results on a gold coated sample with rough topography that are free from such artifacts. By inducing tip inhomogeneity through contact with the sample, clear potential variations appear in the KPFM image, which correlate with the surface topography and, thus, are probe induced artifacts. We find that switching to frequency modulation (FM) KPFM with such altered probes does not remove these artifacts. We also find that the induced tip inhomogeneity causes a lift height dependence of the KPFM measurement, which can therefore be used as a check for the presence of probe induced topography correlated artifacts. We attribute the observed effects to a work function difference between the tip and the rest of the probe and describe a model for such inhomogeneous probes that predicts lift height dependence and topography correlated artifacts for both AM and FM-KPFM methods. This work demonstrates that using a probe with a homogeneous work function and preventing tip changes is essential for KPFM on non-flat samples. From the three investigated probe coatings, PtIr, Au and TiN, the latter appears to be the most suitable, because of its better resistance against coating damage.

Atomic-Scale Imaging of the Surface Dipole Distribution of Stepped Surfaces.

Stepped well-ordered semiconductor surfaces are important as nanotemplates for the fabrication of 1D nanostructures. Therefore, a detailed understanding of the underlying stepped substrates is crucial for advances in this field. Although measurements of step edges are challenging for scanning force microscopy (SFM), here we present simultaneous atomically resolved SFM and Kelvin probe force microscopy (KPFM) images of a silicon vicinal surface. We find that the local contact potential difference is large at the bottom of the steps and at the restatoms on the terraces, whereas it drops at the upper part of the steps and at the adatoms on the terraces. For the interpretation of the data we performed density functional theory (DFT) calculations of the surface dipole distribution. The DFT images accurately reproduce the experiments even without including the tip in the calculations. This underlines that the high-resolution KPFM images are closely related to intrinsic properties of the surface and not only to tip-surface interactions.

Kelvin probe force microscopy of DNA-capped nanoparticles for single-nucleotide polymorphism detection.

Kelvin probe force microscopy (KPFM) is a robust toolkit for profiling the surface potential (SP) of biomolecular interactions between DNAs and/or proteins at the single molecule level. However, it has often suffered from background noise and low throughput due to instrumental or environmental constraints, which is regarded as limiting KPFM applications for detection of minute changes in the molecular structures such as single-nucleotide polymorphism (SNP). Here, we show KPFM imaging of DNA-capped nanoparticles (DCNP) that enables SNP detection of the BRCA1 gene owing to sterically well-adjusted DNA-DNA interactions that take place within the confined spaces of DCNP. The average SP values of DCNP interacting with BRCA1 SNP were found to be lower than the DCNP reacting with normal (non-mutant) BRCA1 gene. We also demonstrate that SP characteristics of DCNP with different substrates (e.g., Au, Si, SiO2, and Fe) provide us with a chance to attenuate or augment the SP signal of DCNP without additional enhancement of instrumentation capabilities.

Carrier density distribution in silicon nanowires investigated by scanning thermal microscopy and Kelvin probe force microscopy

The use of scanning thermal microscopy (SThM) and Kelvin probe force microscopy (KPFM) to investigate silicon nanowires (SiNWs) is presented. SThM allows imaging of temperature distribution at the nanoscale, while KPFM images the potential distribution with AFM-related ultra-high spatial resolution. Both techniques are therefore suitable for imaging the resistance distribution. We show results of experimental examination of dual channel n-type SiNWs with channel width of 100nm, while the channel was open and current was flowing through the SiNW. To investigate the carrier distribution in the SiNWs we performed SThM and KPFM scans. The SThM results showed non-symmetrical temperature distribution along the SiNWs with temperature maximum shifted towards the contact of higher potential. These results corresponded to those expressed by the distribution of potential gradient along the SiNWs, obtained using the KPFM method. Consequently, non-uniform distribution of resistance was shown, being a result of non-uniform carrier density distribution in the structure and showing the pinch-off effect. Last but not least, the results were also compared with results of finite-element method modeling.

(Invited) Electronic Scattering in One-Dimensional Carbon Nanotubes

A quasi-ballistic, semi-metallic carbon nanotube is a nearly ideal conductor for investigating scattering in the one-dimensional (1D) limit and for comparing experiment with theoretical predictions. Recently, we have developed high resolution Kelvin Probe force microscopy (KPFM) methods of characterizing transport in single nanotubes. KPFM images of potential gradients uniquely reveal the bias- and temperature-dependent mechanisms of scattering in nanotube devices, and clearly discriminate between contact effects and homogenous, diffusive scattering. Using this data, a comprehensive, quantitative model has been developed that includes intrinsic energy-relaxation processes, extrinsic interactions with the surrounding environment, and the effects of point defects. The new results show that the literature grossly underestimates the mean free path for optical phonon emission. Defect scattering follows a mechanism similar to Poole-Frenkel conduction but with modifications accounting for the 1D geometry. The Poole-Frenkel mechanism, which normally applies to 2D interfaces, has never been observed in the limit of a single defect limit, and its application to point defect scattering in 1D is wholly unanticipated in the theoretical literature. This presentation will summarize these findings and their implications for nanotube devices as sensors or digital electronics.

Quantitative 3D-KPFM imaging with simultaneous electrostatic force and force gradient detection

Kelvin probe force microscopy (KPFM) is a powerful characterization technique for imaging local electrochemical and electrostatic potential distributions and has been applied across a broad range of materials and devices. Proper interpretation of the local KPFM data can be complicated, however, by convolution of the true surface potential under the tip with additional contributions due to long range capacitive coupling between the probe (e.g. cantilever, cone, tip apex) and the sample under test. In this work, band excitation (BE)-KPFM is used to negate such effects. In contrast to traditional single frequency KPFM, multifrequency BE-KPFM is shown to afford dual sensitivity to both the electrostatic force and the force gradient detection, analogous to simultaneous amplitude modulated and frequency modulated KPFM imaging. BE-KPFM is demonstrated on a Pt/Au/SiOx test structure and electrostatic force gradient detection is found to lead to an improved lateral resolution compared to electrostatic force detection. Finally, a 3D-KPFM imaging technique is developed. Force volume (FV) BE-KPFM allows the tip–sample distance dependence of the electrostatic interactions (force and force gradient) to be recorded at each point across the sample surface. As such, FVBE-KPFM provides a much needed pathway towards complete tip–sample capacitive de-convolution in KPFM measurements and will enable quantitative surface potential measurements with nanoscale resolution.

Nanoscale Investigation of Grain Growth in RF-Sputtered Indium Tin Oxide Thin Films by Scanning Probe Microscopy

This work studied the electronic characteristics of the grains and grain boundaries of indium tin oxide (ITO) thin films using electrostatic and Kelvin probe force microscopy. Two types of ITO films were compared, deposited using radiofrequency magnetron sputtering in pure argon or 99% argon + 1% oxygen, respectively. The average grain size and surface roughness increased with substrate temperature for the films deposited in pure argon. With the addition of 1% oxygen, the increase in the grain size was inhibited above 150°C, which was suggested to be due to passivation of the grains by the excess oxygen. Electrostatic force microscopy and Kelvin probe force microscopy (KPFM) images confirmed that the grain growth was defect mediated and occurred at defective interfaces at high temperatures. Films deposited at room temperature with 1% oxygen showed crystalline nature, while films deposited with pure argon at room temperature were amorphous as observed from KPFM images. The potential drop across the grain and grain boundary was determined by taking surface potential line profiles to evaluate the electronic properties.

Cross-talk artefacts in Kelvin probe force microscopy imaging: A comprehensive study

We provide in this article a comprehensive study of the role of ac cross-talk effects in Kelvin Probe Force Microscopy (KPFM), and their consequences onto KPFM imaging. The dependence of KPFM signals upon internal parameters such as the cantilever excitation frequency and the projection angle of the KPFM feedback loop is reviewed, and compared with an analytical model. We show that ac cross-talks affect the measured KPFM signals as a function of the tip-substrate distance, and thus hamper the measurement of three-dimensional KPFM signals. The influence of ac cross-talks is also demonstrated onto KPFM images, in the form of topography footprints onto KPFM images, especially in the constant distance (lift) imaging mode. Our analysis is applied to unambiguously probe charging effects in tobacco mosaic viruses (TMVs) in ambient air. TMVs are demonstrated to be electrically neutral when deposited on silicon dioxide surfaces, but inhomogeneously negatively charged when deposited on a gold surface.

In Situ KPFM Imaging of Local Photovoltaic Characteristics of Structured Organic Photovoltaic Devices

Here, we discuss the local photovoltaic characteristics of a structured bulk heterojunction, organic photovoltaic devices fabricated with a liquid carbazole, and a fullerene derivative based on analysis by scanning kelvin probe force microscopy (KPFM). Periodic photopolymerization induced by an interference pattern from two laser beams formed surface relief gratings (SRG) in the structured films. The surface potential distribution in the SRGs indicates the formation of donor and acceptor spatial distribution. Under illumination, the surface potential reversibly changed because of the generation of fullerene anions and hole transport from the films to substrates, which indicates that we successfully imaged the local photovoltaic characteristics of the structured photovoltaic devices. Using atomic force microscopy, we confirmed the formation of the SRG because of the material migration to the photopolymerized region of the films, which was induced by light exposure through photomasks. The structuring technique allows for the direct fabrication and the control of donor and acceptor spatial distribution in organic photonic and electronic devices with minimized material consumption. This in situ KPFM technique is indispensable to the fabrication of nanoscale electron donor and electron acceptor spatial distribution in the devices.