Scanning Kelvin Probe Microscopy: Challenges and Perspectives towards Increased Application on Biomaterials and Biological Samples

Mapping elemental solutes at sub-picogram levels during aqueous corrosion of Al alloys using diffusive gradients in thin films (DGT) with LA-ICP-MS

A novel approach using diffusive gradients in thin films (DGT) with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for two-dimensional mapping of elemental solute release at sub-picogram levels during aqueous corrosion of Al alloys is presented. Evaluation of different DGT gels with mixed micro-sized binding phases (polyacrylamide-Chelex-Metsorb, polyurethane (PU)-Chelex-Metsorb, PU-Chelex-Zr(OH)4) demonstrated the superior performance of PU gels due to their tear-proof handling, low shrinkage, and compliance with green chemistry. DGT devices containing PU-Chelex-Zr(OH)4 gels, which have not been characterized for Al sampling before, showed quantitative uptake of Al, Zn, and Cu solutes over time (t = 4-48 h) with higher Al capacity (ΓDGT = 6.25 µg cm-2) than different gels. Application of PU-Chelex-Zr(OH)4 gels on a high-strength Al-Cu alloy (Al2219) exposed to NaCl (w = 1.5%, pH = 4.5, T = 21 °C) for 15 min in a novel piston-type configuration revealed reproducible patterns of Al and Zn co-solubilization with a spatial expansion ranging between 50 and 1000 µm. This observation, together with complementary solid-state data from secondary electron microscopy with energy-dispersive X-ray spectroscopy, showed the presence of localized pitting corrosion at the material surface. Detection limits for total solute masses of Al, Zn, and Cu were ≤0.72 pg, ≤8.38 pg, and ≤0.12 pg, respectively, for an area of 0.01 mm2, demonstrating the method's unique capability to localize and quantify corrosion processes at ultra-trace levels and high resolution. Our study advances the assessment of Al alloy degradation in aqueous environments, supporting the design of corrosion-resistant materials for fostering technological safety and sustainability.

Kelvin probe force microscopy on patterned large-area biofunctionalized surfaces: a reliable ultrasensitive platform for biomarker detection

Kelvin probe force microscopy (KPFM) allows the detection of single binding events between immunoglobulins (IgM, IgG) and their cognate antibodies (anti-IgM, anti-IgG). Here an insight into the reliability and robustness of the methodology is provided. Our method is based on imaging the surface potential shift occurring on a dense layer of ∼5 × 107 antibodies physisorbed on a 50 μm × 90 μm area when assayed with increasing concentrations of antigens in phosphate buffer saline (PBS) standard solutions, in air and at a fixed scanning location. A comprehensive investigation of the influence of the main experimental parameters that may interfere with the outcomes of KPFM immune-assay is provided, showing the robustness and reliability of our approach. The data are supported also by a thorough polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS) analysis of the physisorbed biolayer, in the spectral region of the amide I, amide II and amide A bands. Our findings demonstrate that a 10 min incubation in 500 μL PBS encompassing ≈ 30 antigens (100 zM) triggers an extended surface potential shift that involves the whole investigated area. Such a shift quickly saturates at increasing ligand concentration, showing that the developed sensing platform works as an OFF/ON detector, capable of assessing the presence of a few specific biomarkers in a given assay volume. The reliability of the developed methodology KPFM is an important asset in single molecule detections at a wide electrode interface.

Droplet epitaxy of InGaN quantum dots on Si (111) by plasma-assisted molecular beam epitaxy

The droplet epitaxy of indium gallium nitride quantum dots (InGaN QDs), the formation of In-Ga alloy droplets in ultra-high vacuum and then surface nitridation by plasma treatment, is firstly investigated by using plasma-assisted molecular beam epitaxy. During the droplet epitaxy process, in-situ reflection high energy electron diffraction patterns performs the amorphous In-Ga alloy droplets transform to polycrystalline InGaN QDs, which are also confirmed by the characterizations of transmission electron microscopy and X-ray photoelectron spectroscopy. The substrate temperature, In-Ga droplet deposition time, and duration of nitridation are set as parameters to study the growth mechanism of InGaN QDs on Si. Self-assembled InGaN QDs with a density of 1.33 × 1011 cm-2 and an average size of 13.3 ± 3 nm can be obtained at the growth temperature of 350 °C. The photoluminescence emissions of uncapped InGaN QDs in wavelength of the visible red (715 nm) and infrared region (795 and 857 nm) are observed. The formation of high-indium composition of InGaN QDs via droplet epitaxy technique could be applied in long wavelength optoelectronic devices.

Boosting the Photoelectrochemical Performance of Au/ZnO Nanorods by Co-Occurring Gradient Doping and Surface Plasmon Modification

Band bending modification of metal/semiconductor hybrid nanostructures requires low-cost and effective designs in photoelectrochemical (PEC) water splitting. To this end, it is evinced that gradient doping of Au nanoparticles (NPs) inwards the ZnO nanorods (NRs) through thermal treatment facilitated faster transport of the photo-induced charge carriers. Systematic PEC measurements show that the resulting gradient Au-doped ZnO NRs yielded a photocurrent density of 0.009 mA/cm² at 1.1 V (vs. NHE), which is 2.5-fold and 8-fold improved compared to those of Au-sensitized ZnO and the as-prepared ZnO NRs, respectively. The IPCE and ABPE efficiency tests confirmed the boosted photoresponse of gradient Au-incorporated ZnO NRs, particularly in the visible spectrum due to the synergistic surface plasmonic effect of Au NPs. A gradient Au dopant profile promoted the separation and transfer of the photo-induced charge carriers at the electrolyte interface via more upward band bending according to the elaborated electrochemical impedance spectroscopy and Kelvin probe force microscopy analyses. Therefore, this research presents an economical and facile strategy for preparing gradient plasmonic noble NP-incorporated semiconductor NRs, which have excellent potential in energy conversion and storage technologies.

Molecular Doping of CVD-Graphene Surfaces by Perfluoroalkyl-Substituted Perylene Diimides Derivatives

Non-covalent π-π and dipolar interactions with small aromatic molecules have been widely demonstrated to be a valid option to tune graphene work functions without adding extrinsic scattering centers for charge carriers. In this work, we investigated the interaction between a CVD-graphene monolayer and a thermally evaporated sub-monolayer and the following few-layer thin films of similar perylene diimide derivatives: PDI8-CN2 and PDIF-CN2. The molecular influence on the graphene work function was estimated by XPS and UPS analysis and by investigating the surface potentials via scanning Kelvin probe force microscopy. The perfluorinated decoration and the steric interaction in the early stages of the film growth determined a positive work function shift as high as 0.7 eV in the case of PDIF-CN2, with respect to the value of 4.41 eV for the intrinsic graphene. Our results unambiguously highlight the absence of valence band shifts in the UPS analysis, indicating the prevalence of dipolar interactions between the graphene surface and the organic species enhanced by the presence of the fluorine-enriched moieties.

Application of Poly-L-Lysine for Tailoring Graphene Oxide Mediated Contact Formation between Lithium Titanium Oxide LTO Surfaces for Batteries

When producing stable electrodes, polymeric binders are highly functional materials that are effective in dispersing lithium-based oxides such as Li₄Ti₅O₁₂ (LTO) and carbon-based materials and establishing the conductivity of the multiphase composites. Nowadays, binders such as polyvinylidene fluoride (PVDF) are used, requiring dedicated recycling strategies due to their low biodegradability and use of toxic solvents to dissolve it. Better structuring of the carbon layers and a low amount of binder could reduce the number of inactive materials in the electrode. In this study, we use computational and experimental methods to explore the use of the poly amino acid poly-L-lysine (PLL) as a novel biodegradable binder that is placed directly between nanostructured LTO and reduced graphene oxide. Density functional theory (DFT) calculations allowed us to determine that the (111) surface is the most stable LTO surface exposed to lysine. We performed Kubo-Greenwood electrical conductivity (KGEC) calculations to determine the electrical conductivity values for the hybrid LTO-lysine-rGO system. We found that the presence of the lysine-based binder at the interface increased the conductivity of the interface by four-fold relative to LTO-rGO in a lysine monolayer configuration, while two-stack lysine molecules resulted in 0.3-fold (in the plane orientation) and 0.26-fold (out of plane orientation) increases. These outcomes suggest that monolayers of lysine would specifically favor the conductivity. Experimentally, the assembly of graphene oxide on poly-L-lysine-TiO₂ with sputter-deposited titania as a smooth and hydrophilic model substrate was investigated using a layer-by-layer (LBL) approach to realize the required composite morphology. Characterization techniques such as X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), Kelvin probe force microscopy (KPFM), scanning electron microscopy (SEM) were used to characterize the formed layers. Our experimental results show that thin layers of rGO were assembled on the TiO₂ using PLL. Furthermore, the PLL adsorbates decrease the work function difference between the rGO- and the non-rGO-coated surface and increased the specific discharge capacity of the LTO-rGO composite material. Further experimental studies are necessary to determine the influence of the PLL for aspects such as the solid electrolyte interface, dendrite formation, and crack formation.

Dependence of Interfacial Adhesion between Substances on Their Electron Work Functions

In this article, we demonstrate the dependence of the adhesive force (F_Ad) between two different substances on their electron work functions (EWF or φ) without atomic diffusion involved. The adhesive forces between Si₃N₄ and a number of metals were measured using an atomic force microscope. It is shown that the larger the difference in φ between the two substances in contact, the larger the F_Ad. F_Ad is also influenced by the electron freedom and density (related to the charge availability). An analytical model is proposed to elucidate the underlying mechanism and quantify the adhesive interaction.

Photolon Nanoporous Photoactive Material with Antibacterial Activity and Label-Free Noncontact Method for Free Radical Detection

The worldwide increase in bacterial resistance and healthcare-associated bacterial infections pose a serious threat to human health. The antimicrobial photodynamic method reveals the opportunity for a new therapeutic approach that is based on the limited delivery of photosensitizer from the material surface. Nanoporous inorganic–organic composites were obtained by entrapment of photosensitizer Photolon in polysiloxanes that was prepared by the sol–gel method. The material was characterized by its porosity, optical properties (fluorescence and absorbance), and laser-induced antimicrobial activity against Staphylococcus epidermidis, Staphylococcus aureus, Pseudomonas aeruginosa, and Escherichia coli. The permanent encapsulation of Photolon in the silica coating and the antimicrobial efficiency was confirmed by confocal microscope and digital holotomography. The generation of free radicals from nanoporous surfaces was proved by scanning Kelvin probe microscopy. For the first time, it was confirmed that Kelvin probe microscopy can be a label-free, noncontact alternative to other conventional methods based on fluorescence or chemiluminescence probes, etc. It was confirmed that the proposed photoactive coating enables the antibacterial photodynamic effect based on free radicals released from the surface of the coating. The highest bactericidal efficiency of the proposed coating was 87.16%. This coating can selectively limit the multiplication of bacterial cells, while protecting the environment and reducing the risk of surface contamination.

Developments and Ongoing Challenges for Analysis of Surface-Bound Proteins

Proteins at surfaces and interfaces play important roles in the function and performance of materials in applications ranging from diagnostic assays to biomedical devices. To improve the performance of these materials, detailed molecular structure (conformation and orientation) along with the identity and concentrations of the surface-bound proteins on those materials must be determined. This article describes radiolabeling, surface plasmon resonance, quartz crystal microbalance with dissipation, X-ray photoelectron spectroscopy, secondary ion mass spectrometry, sum frequency generation spectroscopy, and computational techniques along with the information each technique provides for characterizing protein films. A multitechnique approach using both experimental and computation methods is required for these investigations. Although it is now possible to gain much insight into the structure of surface-bound proteins, it is still not possible to obtain the same level of structural detail about proteins on surfaces as can be obtained about proteins in crystals and solutions, especially for large, complex proteins. However, recent results have shown it is possible to obtain detailed structural information (e.g., backbone and side chain orientation) about small peptides (5-20 amino sequences) on surfaces. Current studies are extending these investigations to small proteins such as protein G B1 (∼6 kDa). Approaches for furthering the capabilities for characterizing the molecular structure of surface-bound proteins are proposed.

A study of dopant incorporation in Te-doped GaAsSb nanowires using a combination of XPS/UPS, and C-AFM/SKPM

We report the first study on doping assessment in Te-doped GaAsSb nanowires (NWs) with variation in Gallium Telluride (GaTe) cell temperature, using X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), conductive-atomic force microscopy (C-AFM), and scanning Kelvin probe microscopy (SKPM). The NWs were grown using Ga-assisted molecular beam epitaxy with a GaTe captive source as the dopant cell. Te-incorporation in the NWs was associated with a positive shift in the binding energy of the 3d shells of the core constituent elements in doped NWs in the XPS spectra, a lowering of the work function in doped NWs relative to undoped ones from UPS spectra, a significantly higher photoresponse in C-AFM and an increase in surface potential of doped NWs observed in SKPM relative to undoped ones. The carrier concentration of Te-doped GaAsSb NWs determined from UPS spectra are found to be consistent with the values obtained from simulated I–V characteristics. Thus, these surface analytical tools, XPS/UPS and C-AFM/SKPM, that do not require any sample preparation are found to be powerful characterization techniques to analyze the dopant incorporation and carrier density in homogeneously doped NWs.

Bulk chemical composition contrast from attractive forces in AFM force spectroscopy

A key application of atomic force microscopy (AFM) is the measurement of physical properties at sub-micrometer resolution. Methods such as force-distance curves (FDCs) or dynamic variants (such as intermodulation AFM (ImAFM)) are able to measure mechanical properties (such as the local stiffness, k r) of nanoscopic heterogeneous materials. For a complete structure-property correlation, these mechanical measurements are considered to lack the ability to identify the chemical structure of the materials. In this study, the measured attractive force, F attr, acting between the AFM tip and the sample is shown to be an independent measurement for the local chemical composition and hence a complete structure-property correlation can be obtained. A proof of concept is provided by two model samples comprised of (1) epoxy/polycarbonate and (2) epoxy/boehmite. The preparation of the model samples allowed for the assignment of material phases based on AFM topography. Additional chemical characterization on the nanoscale is performed by an AFM/infrared-spectroscopy hybrid method. Mechanical properties (k r) and attractive forces (F attr) are calculated and a structure-property correlation is obtained by a manual principle component analysis (mPCA) from a k r/F attr diagram. A third sample comprised of (3) epoxy/polycarbonate/boehmite is measured by ImAFM. The measurement of a 2 × 2 µm cross section yields 128 × 128 force curves which are successfully evaluated by a k r/F attr diagram and the nanoscopic heterogeneity of the sample is determined.

Strain-Work Function Relationship in Single-Crystal Tetracene

Understanding the impact of strain on organic semiconductors is important for the development of electronic devices and sensors that are subject to environmental changes and mechanical stimuli; it is also important for understanding the fundamental mechanisms of charge trapping. Following our previous study on the strain effects in rubrene, we present here only the second example of the strain-work function relationship in an organic semiconductor; in this case, the benchmark material tetracene. Thin, platelike single crystals of tetracene with large (001) facets were laminated onto silicon and rubber substrates having significantly different coefficients of thermal expansion; mechanical strain in tetracene was subsequently induced by varying the temperature of the assembly. Tensile and compressive strains parallel to the (001) major facet were measured by grazing incidence X-ray diffraction, and the corresponding shifts in the electronic work functions were recorded via scanning Kelvin probe microscopy (SKPM). The work function of the tetracene (001) crystal surface directly correlated with the net mechanical strain and increased by ∼100 meV for in-plane tensile strains of 0.1% and decreased by approximately the same amount for in-plane compressive strains of -0.1%. This work provides evidence of the general and important impact of strain on the electrical properties of van der Waals bonded crystalline organic semiconductors and thereby supports the hypothesis that heterogeneous strains, for example in thin films, can be a major source of static electronic disorder.

High-Mobility Hydrogenated Fluorine-Doped Indium Oxide Film for Passivating Contacts c-Si Solar Cells

Broadband transparent conductive oxide layers with high electron mobility (μ_e) are essential to further enhance crystalline silicon (c-Si) solar cell performances. Although metallic cation-doped In₂O₃ thin films with high μ_e (>60 cm² V^-1 s^-1) have been extensively investigated, the research regarding anion doping is still under development. In particular, fluorine-doped indium oxide (IFO) shows promising optoelectrical properties; however, they have not been tested on c-Si solar cells with passivating contacts. Here, we investigate the properties of hydrogenated IFO (IFO:H) films processed at low substrate temperature and power density by varying the water vapor pressure during deposition. The optimized IFO:H shows a remarkably high μ_e of 87 cm² V^-1 s^-1, a carrier density of 1.2 × 10²⁰ cm^-3, and resistivity of 6.2 × 10^-4 Ω cm. Then, we analyzed the compositional, structural, and optoelectrical properties of the optimal IFO:H film. The high quality of the layer was confirmed by the low Urbach energy of 197 meV, compared to 444 meV obtained on the reference indium tin oxide. We implemented IFO:H into different front/back-contacted solar cells with passivating contacts processed at high and low temperatures, obtaining a significant short-circuit current gain of 1.53 mA cm^-2. The best solar cell shows a conversion efficiency of 21.1%.

One-step microchip for DNA fluorescent labeling

In this study, we propose a microchip that is sequentially capable of fluorescently staining and washing DNAs. The main advantage of this microchip is that it allows for one-step preparation of small amounts of solution without degrading microscopic bio-objects such as the DNAs, cells, and biomolecules to be stained. The microchip consists of two inlets, the main channel, staining zone, washing zone, and one outlet, and was processed using a femtosecond laser system. High molecular transport of rhodamine B to deionized water was observed in the performance test of the microchip. Results revealed that the one-step procedure of on-chip DNA staining and washing was excellent compared to the conventional staining method. The one-step preparation of stained and washed DNAs through the microchip will be useful for preparing small volumes of experimental samples.

Lab on a tip: Applications of functional atomic force microscopy for the study of electrical properties in biology

Electrical properties, such as charge propagation, dielectrics, surface potentials, conductivity, and piezoelectricity, play crucial roles in biomolecules, biomembranes, cells, tissues, and other biological samples. However, characterizing these electrical properties in delicate biosamples is challenging. Atomic Force Microscopy (AFM), the so called "Lab on a Tip" is a powerful and multifunctional approach to quantitatively study the electrical properties of biological samples at the nanometer level. Herein, the principles, theories, and achievements of various modes of AFM in this area have been reviewed and summarized. STATEMENT OF SIGNIFICANCE: Electrical properties such as dielectric and piezoelectric forces, charge propagation behaviors play important structural and functional roles in biosystems from the single molecule level, to cells and tissues. Atomic force microscopy (AFM) has emerged as an ideal toolkit to study electrical property of biology. Herein, the basic principles of AFM are described. We then discuss the multiple modes of AFM to study the electrical properties of biological samples, including Electrostatic Force Microscopy (EFM), Kelvin Probe Force Microscopy (KPFM), Conductive Atomic Force Microscopy (CAFM), Piezoresponse Force Microscopy (PFM) and Scanning ElectroChemical Microscopy (SECM). Finally, the outlook, prospects, and challenges of the various AFM modes when studying the electrical behaviour of the samples are discussed.

Insights into Nano-Scale Physical and Mechanical Properties of Epoxy/Boehmite Nanocomposite Using Different AFM Modes

Understanding the interaction between nanoparticles and the matrix and the properties of interphase is crucial to predict the macroscopic properties of a nanocomposite system. Here, we investigate the interaction between boehmite nanoparticles (BNPs) and epoxy using different atomic force microscopy (AFM) approaches. We demonstrate benefits of using multifrequency intermodulation AFM (ImAFM) to obtain information about conservative, dissipative and van der Waals tip-surface forces and probing local properties of nanoparticles, matrix and the interphase. We utilize scanning kelvin probe microscopy (SKPM) to probe surface potential as a tool to visualize material contrast with a physical parameter, which is independent from the mechanics of the surface. Combining the information from ImAFM stiffness and SKPM surface potential results in a precise characterization of interfacial region, demonstrating that the interphase is softer than epoxy and boehmite nanoparticles. Further, we investigated the effect of boehmite nanoparticles on the bulk properties of epoxy matrix. ImAFM stiffness maps revealed the significant stiffening effect of boehmite nanoparticles on anhydride-cured epoxy matrix. The energy dissipation of epoxy matrix locally measured by ImAFM shows a considerable increase compared to that of neat epoxy. These measurements suggest a substantial alteration of epoxy structure induced by the presence of boehmite.