Atomic Force Microscopy Measurements Research Articles

Native Silicon Oxide Properties Determined by Doping.

The physico-chemical properties of native oxide layers, spontaneously forming on crystalline Si wafers in air, can be strictly correlated to the dopant type and doping level. In particular, our investigations focused on oxide layers formed upon air exposure in a clean room after Si wafer production, with dopant concentration levels from ≈1013 to ≈1019 cm-3. In order to determine these correlations, we studied the surface, the oxide bulk, and its interface with Si. The surface was investigated using the contact angle, thermal desorption, and atomic force microscopy measurements which provided information on surface energy, cleanliness, and morphology, respectively. Thickness was measured with ellipsometry and chemical composition with X-ray photoemission spectroscopy. Electrostatic charges within the oxide layer and at the Si interface were studied with Kelvin probe microscopy. Some properties such as thickness, showed an abrupt change, while others, including silanol concentration and Si intermediate-oxidation states, presented maxima at a critical doping concentration of ≈2.1 × 1015 cm-3. Additionally, two electrostatic contributions were found to originate from silanols present on the surface and the net charge distributed within the oxide layer. Lastly, surface roughness was also found to depend upon dopant concentration, showing a minimum at the same critical dopant concentration. These findings were reproduced for oxide layers regrown in a clean room after chemical etching of the native ones.

Structural properties of aqueous grown polydopamine thin films determined by neutron reflectometry

In this work, neutron reflectometry (NR) studies of the bio-mimetic polymer, polydopamine (PDA), deposited from differing initial concentrations of precursor material dopamine hydrochloride for a range of polymerization times, is reported. PDA can form a complex and highly versatile polymer film, with many structure studies having been performed previously, but a comprehensive structural determination of PDA by NR is lacking in the literature. It was found that simple box models were incapable of fully explaining the observed data, necessitating the use of a composite model consisting of the weighted average of both the 1 and 2 box models to fully capture the heterogeneous nature of the PDA film structure. Confocal laser scanning microscopy (CLSM) and atomic force microscopy (AFM) were performed to capture the surface structure and relative mechanical difference between the separate domains of the PDA. The CLSM results provide evidence that the PDA domains are larger than the coherent scattering length of a neutron used in these measurements, 10μm, supporting the need for a composite model. The AFM measurements show complex structure below the coherence length provide physical justification for the two box model. It was determined that film structure and quality are both heavily impacted by the initial concentration of dopamine hydrochloride and the polymerization time, giving confidence to the highly customizable nature of PDA as an adhesion promoting interface treatment in composite systems, such as plastic bonded explosives (PBX).

Unraveling the Relevance of Electron and Hole Transfer in Lead Halide Perovskite Nanocrystals on Current Conduction.

Optimization of perovskite-based optoelectronic performance demands prudent engineering in the device architecture with facile transport of generated charge carriers. Herein, we explore the charge transfer (CT) kinetics in perovskite nanocrystals (PNCs), CsPbBr3, with two redox-active quinones, menadione (MD) and anthraquinone (AQ), and its alteration in halide exchanged CsPbCl3. With a series of spectroscopic and microscopic measurements, we infer that both electron and hole transfer (ET-HT) prevail in CsPbCl3 with quinones, resulting in a faster CT, while ET predominates for CsPbBr3. Furthermore, current-sensing atomic force microscopy measurements demonstrate that the conductance across a metal-PNC-metal nanojunction is improved in the presence of quinones. The contributions of ET and HT to current conduction across PNCs are well supported and validated by theoretical calculations of the density of states. These outcomes convey a new perspective on the relevance of ET and HT in the optimal current conduction and optoelectronic device engineering of perovskites.

Acute brain slice elastic modulus decreases over time

A common benchmark in the brain tissue mechanics literature is that the properties of acute brain slices should be measured within 8 h of the experimental animal being sacrificed. The core assumption is that—since there is no substantial protein degradation during this time—there will be no change to elastic modulus. This assumption overlooks the possibility of other effects (such as osmotic swelling) that may influence the mechanical properties of the tissue. To achieve consistent and accurate analysis of brain mechanics, it is important to account for or mitigate these effects. Using atomic force microscopy (AFM), tissue hydration and volume measurements, we find that acute brain slices in oxygenated artificial cerebrospinal fluid (aCSF) with a standard osmolarity of 300 mOsm/l experience rapid swelling, softening, and increases in hydration within the first 2 hours after slicing. Reductions in elastic modulus can be partly mitigated by addition of chondroitinase ABC enzyme (CHABC). Increasing aCSF osmolarity to 400 mOsm/l does not prevent softening but may hasten equilibration of samples to a point where measurements of relative elastic modulus are consistent across experiments.

Heterocyclic group end-capped polyethylene glycol: Synthesis and used as corrosion inhibitors for mild steel in HCl medium

In this work, two categories of water-soluble polymeric corrosion inhibitors (CIs) are contributed. These CIs are composed by polyethylene glycol (PEG) middle segment and oleic imidazoline (or thiophene) end groups, respectively labeled as OIM-PEG-OIM and Th-PEG-Th. Two facile synthetic strategies involve epoxy-amino reaction and thiol-acrylate Michael addition were employed. Both Fourier transform infrared (FTIR) spectroscopy and nuclear magnetic resonance (NMR) spectroscopy verified that a series of target products were successfully obtained. The solution transmittance measurements indicated that with the increase of the PEG segment length, the water solubility of CIs enhanced. And these obtained CIs exhibit somewhat surface activity and could reduce the solution surface tensions to a certain extent. The inhibition efficiencies of these CIs for mild steel in HCl medium were evaluated by weight loss measurements, potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS). It was found that both OIM-PEG-OIM and Th-PEG-Th show an improved corrosion inhibition efficiency as compared to PEG. And as compared to heterocyclic compounds, introduction of PEG segment showed a negligible influence on their performances. Both CIs can form chemisorption on the metal surface, and the adsorption process follow Langmuir adsorption mechanism. OIM-PEG-OIM exhibit superior inhibition performance and stability as compared to Th-PEG-Th, which can be attributed to the fact that OIM-PEG-OIM bearing two long hydrophobic alkyl end groups. Scanning electron microscope (SEM) observations as well as atomic force microscopy (AFM) measurements also confirmed both OIM-PEG-OIM and Th-PEG-Th could serve as water-soluble CIs for mild steel.

Oil–wall interaction induced droplet deformation dynamics in a capillary tube

Displacement of residual oil droplet in high water-cut oilfields can be optimized through the understanding of droplet deformation dynamics. The current studies demonstrated that the changing of capillary number or the radius ratio of droplet and tube can control the droplet deformation, but neglects the intricate interaction between the wall and droplet. Due to the complex physicochemical properties of the reservoir wall and crude oil, the interaction forces between them are very rich and unoverlooked. The current contact angle and atomic force microscope measurement are insufficient in accurately describing the oil–wall interaction forces system, resulting in discrepancies of droplet deformation dynamics with the actual reservoir conditions. We used the extended Derjaguin–Landau–Verwey–Overbeek (EDLVO) theory to establish a system of oil–wall interaction forces. The deformation dynamics of high-viscosity droplets (plug and slug droplets) and low-viscosity droplets (tail-concaved droplets) in a capillary tube were studied under various EDLVO forces values. The results showed that: (1) for high-viscosity droplets, the influence of the EDLVO forces differs significantly between plug and slug droplets, with a greater tendency to suppress the deformation of slug droplets. (2) For low-viscosity droplets, the EDLVO forces cause an increased degree of droplet concavity and hinder the reconnection of the droplet tail. These findings significantly contribute to our understanding of droplet deformation dynamics within reservoir pores and hold great importance for optimizing droplet control strategies in enhance oil recovery processes.

SiC Doping Impact during Conducting AFM under Ambient Atmosphere.

The characterization of silicon carbide (SiC) by specific electrical atomic force microscopy (AFM) modes is highly appreciated for revealing its structure and properties at a nanoscale. However, during the conductive AFM (C-AFM) measurements, the strong electric field that builds up around and below the AFM conductive tip in ambient atmosphere may lead to a direct anodic oxidation of the SiC surface due to the formation of a water nanomeniscus. In this paper, the underlying effects of the anodization are experimentally investigated for SiC multilayers with different doping levels by studying gradual SiC epitaxial-doped layers with nitrogen (N) from 5 × 1017 to 1019 at/cm3. The presence of the water nanomeniscus is probed by the AFM and analyzed with the force-distance curve when a negative bias is applied to the AFM tip. From the water meniscus breakup distance measured without and with polarization, the water meniscus volume is increased by a factor of three under polarization. AFM experimental results are supported by electrostatic modeling to study oxide growth. By taking into account the presence of the water nanomeniscus, the surface oxide layer and the SiC doping level, a 2D-axisymmetric finite element model is developed to calculate the electric field distribution nearby the tip contact and the current distributions at the nanocontact. The results demonstrate that the anodization occurred for the conductive regime in which the current depends strongly to the doping; its threshold value is 7 × 1018 at/cm3 for anodization. Finally, the characterization of a classical planar SiC-MOSFET by C-AFM is examined. Results reveal the local oxidation mechanism of the SiC material at the surface of the MOSFET structure. AFM topographies after successive C-AFM measurements show that the local oxide created by anodization is located on both sides of the MOS channel; these areas are the locations of the highly n-type-doped zones. A selective wet chemical etching confirms that the oxide induced by local anodic oxidation is a SiOCH layer.

A Next-Generation qPlus-Sensor-Based AFM Setup: Resolving Archaeal S-Layer Protein Structures in Air and Liquid.

Surface-layer (S-layer) proteins form the outermost envelope in many bacteria and most archaea and arrange in two-dimensional quasicrystalline structures via self-assembly. We investigated S-layer proteins extracted from the archaeon Pyrobaculum aerophilium with a qPlus sensor-based atomic force microscope (AFM) in both liquid and ambient conditions and compared it to transmission electron microscopy (TEM) images under vacuum conditions. For AFM scanning, a next-generation liquid cell and a new protocol for creating long and sharp sapphire tips was introduced. Initial AFM images showed only layers of residual detergent molecules (sodium dodecyl sulfate, SDS), which are used to isolate the S-layer proteins from the cells. SDS was not visible in the TEM images, requiring more thorough sample preparation for AFM measurements. These improvements allowed us to resolve the crystallike structure of the S-layer samples with frequency-modulation AFM in both air and liquid.

Scalable High-Mobility Graphene/hBN Heterostructures.

Graphene-hexagonal boron nitride (hBN) scalable heterostructures are pivotal for the development of graphene-based high-tech applications. In this work, we demonstrate the realization of high-quality graphene-hBN heterostructures entirely obtained with scalable approaches. hBN continuous films were grown via ion beam-assisted physical vapor deposition directly on commercially available SiO2/Si and used as receiving substrates for graphene single-crystal matrixes grown by chemical vapor deposition on copper. The structural, chemical, and electronic properties of the heterostructure were investigated by atomic force microscopy, Raman spectroscopy, and electrical transport measurements. We demonstrate graphene carrier mobilities exceeding 10,000 cm2/Vs in ambient conditions, 30% higher than those directly measured on SiO2/Si. We prove the scalability of our approach by measuring more than 100 transfer length method devices over a centimeter scale, which present an average carrier mobility of 7500 ± 850 cm2/Vs. The reported high-quality all-scalable heterostructures are of relevance for the development of graphene-based high-performing electronic and optoelectronic applications.

One Step Closer to Coatings Applications Utilizing Self-Stratification: Effect of Rheology Modifiers.

Self-stratification of model blends of colloidal spheres has recently been demonstrated as a method to form multifunctional coatings in a single pass. However, practical coating formulations are complex fluids with upward of 15 components. Here, we investigate the influence of three different rheology modifiers (RMs) on the stratification of a 10 wt % 7:3 w:w blend of 270 and 96 nm anionic latex particles that do not stratify without RM. However, addition of a high molar mass polysaccharide thickener, xanthan gum, raises the viscosity and corresponding Péclet number enough to achieve small-on-top stratification as demonstrated by atomic force microscopy (AFM) measurements. Importantly, this was possible due to minimal particle-rheology modifier interactions, as demonstrated by the bulk rheology. In contrast, Carbopol 940, a microgel-based RM, was unable to achieve small-on-top stratification despite a comparable increase in viscosity. Instead, pH-dependent interactions with latex particles lead to either laterally segregated structures at pH 3 or a surface enrichment of large particles at pH 8. Strong RM-particle interactions are also observed when the triblock associative RM HEUR10kC12 is used. Here, small-on-top, large-enhanced, and randomly mixed structures were observed at respectively 0.01, 0.1, and 1 wt % HEUR10kC12. Combining rheology, dynamic light scattering, and AFM results allows the mechanisms behind the nonmonotonic stratification in the presence of associative RMs to be elucidated. Our results highlight that stratification can be predicted and controlled for RMs with weak particle interactions, while a strong RM-particle interaction may afford a wider range of stratified structures. This takes a step toward successfully harnessing stratification in coatings formulations.

Spin selection in atomic-level chiral metal oxide for photocatalysis

The spin degree of freedom is an important and intrinsic parameter in boosting carrier dynamics and surface reaction kinetics of photocatalysis. Here we show that chiral structure in ZnO can induce spin selectivity effect to promote photocatalytic performance. The ZnO crystals synthesized using chiral methionine molecules as symmetry-breaking agents show hierarchical chirality. Magnetic circular dichroism spectroscopic and magnetic conductive-probe atomic force microscopic measurements demonstrate that chiral structure acts as spin filters and induces spin polarization in photoinduced carriers. The polarized carriers not only possess the prolonged carrier lifetime, but also increase the triplet species instead of singlet byproducts during reaction. Accordingly, the left- and right-hand chiral ZnO exhibit 2.0- and 1.9-times higher activity in photocatalytic O2 production and 2.5- and 2.0-times higher activities in contaminant photodegradation, respectively, compared with achiral ZnO. This work provides a feasible strategy to manipulate the spin properties in metal oxides for electron spin-related redox catalysis.

Biomass‐functionalized graphene oxide toward waterborne polyurethane composite with enhanced flame retardancy and hydrophobicity

AbstractIn this study, phytic acid‐polydopamine functionalized GO (GO@PA@PDA) was first synthesized. The DOPO derivatives, hydroxyl‐terminated polydimethylsiloxane (PDMS) and GO@PA@PDA were incorporated into waterborne polyurethane (WPU) by in situ polymerization. The chemical structure of GO@PA@PDA was characterized by Fourier transform infrared spectroscopy, Raman spectroscopy, X‐ray diffraction, ultraviolet–visible (UV–vis) spectroscopy and scanning electron microscopy. A cone calorimeter test and a universal testing machine investigated the flame retardancy and mechanical properties of GO@PA@PDA modified waterborne polyurethane (GPD‐WPU). The results showed that the incorporation of GO@PA@PDA into WPU significantly enhanced the flame retardancy by reducing the heat release rate, total heat release, smoke production rate, and total smoke production. Furthermore, the mechanical properties of the GPD‐WPU composite films increased with the dosage of GO@PA@PDA from 0 to 2.0 wt%, and decreased slightly afterward. The contact angle and atomic force microscope measurements showed that the addition of GO@PA@PDA increased the surface roughness of GPD‐WPU composite film, thereby improving the hydrophobicity of GPD‐WPU composite films. Afterward, GPD‐WPU as a finishing agent was coated on polyester fabric. The experimental results of limiting oxygen index (LOI) and the vertical burning test indicated that when the dosage of GO@PA@PDA was 2.5 wt% in GPD‐WPU, after‐flame time, after‐glow time, damage length and LOI value of the coated polyester fabric were 2.9 s, 0 s, 6.8 cm, and 28.6%, respectively.

A Comparative Analysis on the Crystalline Behavior, Topographical, Photoluminescence, and Leakage Current Characteristics of BiFeO3 Nanopowders and Thin Films

AbstractThin films and nanocrystalline powders of BiFeO3 (BFO) are prepared from pulsed laser deposition (PLD) and sol–gel process, respectively. Their grain sizes, microstrain, morphological features, leakage current characteristics, and photoluminescence (PL) emission behavior are investigated and compared with each other. X‐ray diffraction (XRD) technique has been utilized to compute the grain sizes and crystalline strain values for the thin films and nanopowders of BFO through Williamson–Hall (W–H) plots. XRD and Raman peaks of the deposited thin film and synthesized nanopowders of BFO confirm the formation of rhombohedral unit cell character and its vibrational structure. Topographical analysis of nanopowders and thin films of BFO from scanning electron microscopy (SEM) and atomic force microscopy (AFM) measurements reveals the spherical morphology of grown particles. Suppression of emission peaks, as noticed from PL spectra for BFO nanopowders and thin film, indicates the possible electron defect level transitions due to oxygen vacancies. Leakage current density studies suggest the detectable flow of current in both study of specimen.

Optical properties of polyaniline/modified graphene oxide nanocomposites

In this research, the complex refractive index of polyaniline (PANI)/hexamethylene diisocyanate-modified graphene oxide (HDI-GO) nanocomposites has been obtained. The PANI/HDI-GO nanocomposites were prepared via in situ polymerization of aniline monomer in the presence of HDI-GO nanofillers, and their crystalline structure and surface morphology were investigated via X-ray diffraction (XRD) and atomic force microscopy (AFM) measurements. Then, the experimental reflectance and transmittance for pure PANI and PANI/HDI-GO nanocomposite films with HDI-GO contents of 0.5, 1 and 2 wt% deposited on glass substrates were measured using UV-Vis spectrophotometry within the spectral region 300–900 nm. Then, the refractive index and extinction coefficient have been obtained using a novel method developed by the authors. The model is based on the generalized Scattering Matrix Method, which is used to obtain the theoretical values for the thin film/substrate reflectance and transmittance. Root mean square errors obtained are systematically lower than 0.21%, which corroborates the excellent accuracy of the model. The proposed method does not require sophisticated equipment, just a spectrophotometer, to obtain the complex refractive index of a thin film from transmittance and reflectance measurements.

Studies of temperature-dependent Raman spectra of thin PtSe2 layers on Al2O3 substrate

The results of temperature-dependent Raman spectroscopy studies of thin layers of PtSe2 (1–10 monolayers) deposited on an Al2O3 substrate are disscused in this paper. A redshift of the vibrational Raman modes (Eg and A1g) is observed when the thickness of PtSe2 and temperature increase. The results allow for determining the thickness of the PtSe2 layer deposited on the Al2O3 substrate by analysing the Raman shift of the PtSe2 modes (Eg and A1g) and the screening effects on the surface vibration mode (A1g). The other original result is the determination of the stresses and doping effects in PtSe2 for the considered range of layer thicknesses by analyses of the correlative plot. Finally, the thermal dependences of the Raman spectra are discussed regarding Raman shifts and intensity. The atomic force microscopy measurements show the presence of residual contamination with surface densities varying between samples.

The performance and mechanism for photocatalytic degradation of methylene blue catalyzed by ultrathin NiAl-layered double hydroxides

Ultrathin materials are widely used in various photocatalytic reactions due to their large specific surface area, multi-surface active sites, easy carrier separation and transport. In this work, ultrathin NiAl-layered double hydroxides (U-LDHs) nanosheets were prepared by a urea-assisted strategy for photocatalytic degradation of methylene blue. Atomic force microscopy (AFM) measurements show that the thickness of U-LDHs nanosheet is generally between 3 and 8 nm, with a wide pore size distribution and a high specific surface area (109 m2/g). U-LDHs nanosheets can remove 92.42% of methylene blue after 90 min light irradiation, which is nearly 40% higher than that of bulk NiAl-LDHs (B-LDHs). In addition, combined with photoelectric performance characterization and density functional theory calculation, it is confirmed that the U-LDHs nanosheets are much better to the separation and transfer of photogenerated carriers compared with the bulk LDHs. Besides, U-LDHs is conducive to adsorb oxygen to generate superoxide radicals, promoting the progress of photocatalytic reactions and significantly improving photodegradation performance.

Low-temperature, single-source, chemical vapor deposition of molybdenum nitride thin films

The 1,4-di-tert-butyl-1,3-diazabutadiene adduct of bis(tert-butylimido)dichloro-molybdenum(VI), (tBuN)2MoCl2⋅dad, was used as a single-source precursor for the chemical vapor deposition of molybdenum nitride from 350–600 °C. Deposition at 400 °C had a growth rate of 55 nm h−1 and was comprised of a mixture of Mo2N and MoN, based on x-ray photoelectron spectroscopy and grazing-incidence x-ray diffraction results. The films are essentially featureless and are as smooth as the underlying substrate, based on atomic force microscopy measurements. Because the depositions could be carried out at a low temperature, there was minimal carbon (1.4%) inclusion in the film as shown by XPS.

Anisotropy of the Tribological Performance of Periodically Oxidated Laser-Induced Periodic Surface Structures

Laser-induced periodic surface structures (LIPSS) enable advanced surface functionalization with broad applications in various fields such as micro- and nanoelectronics, medicine, microbiology, tribology, anti-icing systems, and more. This study demonstrates the possibility of achieving anisotropy in the tribological behavior of C45-grade steel structured by nanosecond laser radiation using the LIPSS method. The lateral surface of the steel roller was irradiated with a pulsed Nd:YAG laser at an optimum intensity I = 870 MW/cm2 for the formation of LIPSS. Two sets of samples were formed with LIPSS that were perpendicular and parallel to the roller’s rotational motion direction. The Raman intensity maps revealed that the LIPSS structure consisted of periodically arranged oxides at the top of hills. At the same time, the valleys of the LIPSS structures were almost not oxidized. These results correlated well with scanning electron microscopy energy dispersive X-ray spectroscopy mapping and atomic force microscopy measurements. A comparison of Raman and X-ray photoelectron spectroscopy spectra revealed that both the magnetite phase and traces of the hematite phase were present on the surface of the samples. Tribological tests were performed in two cycles with periodic changes in the normal clamping force and sliding speed. It was found that the LIPSS structures which were formed perpendicularly to the sliding direction on the roller had a significantly greater impact on the friction processes. Structures oriented perpendicular to the direction of motion had a positive influence on reducing the energy consumption of a friction process as well as increasing the wear resistance compared to LIPSS formed parallel to the direction of motion or ones having a non-texturized surface. Laser texturing to produce LIPSS perpendicular to the direction of motion could be recommended for friction pairs operating under low-load conditions.

Simultaneous detection of force and tunneling current in electrolyte solution by using qPlus sensor.

We have developed a sensor for simultaneous measurement of atomic force microscopy (AFM) and scanning tunneling microscopy (STM) under liquid environments. The sensor, which is based on the qPlus sensor, is equipped with an insulated conductive tip. Owing to its electrical insulation except for the tip apex, the developed sensor enabled simultaneous detection of tip-sample interaction force and tunneling current, suppressing the Faradaic leakage current. As a fundamental demonstration, we performed simultaneous AFM/STM imaging in an electrolyte solution by using the developed sensor.

Multiscale Characterization of Rejuvenated RAP Binders

Abstract Reclaimed asphalt pavement (RAP) is increasingly being utilized in asphalt pavements because of the high cost and scarcity of virgin asphalt materials. However, because RAP is a stiff material, it requires careful engineering to prevent cracking. Recycling agents have shown the ability to partially restore the properties of RAP binders if applied in a proper type and dose. Many studies investigated the impact of recycling agents when added to blends of virgin and RAP binders, but only a few investigated their effectiveness when applied to only RAP binders. The aim of this paper is to investigate the effect of various types and doses of recycling agents on the properties of RAP binders. Rheological properties were measured using a dynamic shear rheometer (DSR). Fourier-transform infrared spectroscopy was utilized to measure the degree of oxidation of the rejuvenated RAP binders. Furthermore, atomic force microscopy (AFM) tests were conducted to measure the morphology and modulus of rejuvenated RAP binders at the nanoscale. Finally, AFM measurements were used to calculate a rejuvenation index (RI), which was developed in an earlier study by the authors to assess the effectiveness of recycling agents in resisting the adverse effects of aging. DSR results demonstrated that the inclusion of recycling agents lowered the stiffness of the RAP binders. AFM morphology images revealed the presence of a large number and varying sizes of phase associations in the RAP binders. These phase associations became more pronounced with aging. Some recycling agents partially reversed the aging effect by reducing the area occupied by these associations. Nonetheless, after long-term aging, the effect of recycling agents diminished. The results of this study revealed that characterizing rejuvenated RAP binders at both the microscale and macroscale levels can indicate the effectiveness of recycling agents and aid in the selection of type or dose of recycling agent.