Atomic Force Microscopy Measurements Research Articles

Linear and nonlinear optical parameters of biodegradable chitosan/polyvinyl alcohol/sodium montmorillonite nanocomposite films for potential optoelectronic applications

Innovations in sophisticated optoelectronic devices have increased interest in high-refractive index polymers. Herein, we report innovative nanocomposite films with high linear and nonlinear refractive indices prepared by casting chitosan (Cs) with polyvinyl alcohol (PVA) (50:50 wt%) along with different concentrations (10–50 wt%) of sodium montmorillonite (NaMMT) nanoclay. The refractive indices in addition to other optical parameters of homopolymers and hybrid materials were investigated by UV–Vis. spectroscopy and optical modeling to assess their potential applications in optics. Besides, the structure, morphology, and thermal stability of the prepared films were investigated by a multitude of experimental techniques including X-ray diffraction (XRD), attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), scanning electron microscopy (SEM), atomic force microscopy (AFM), and thermogravimetric analysis (TGA/DTG). The ATR-FTIR, XRD, SEM, and AFM measurements confirmed the complete exfoliation of NaMMT nanolayers in the Cs/PVA matrix. The TGA/DTG revealed an increase in the thermal stability of Cs/PVA film with increasing clay content. The UV–Vis. measurements revealed a decrease in the optical energy gap (Eg) and a substantial increase in the linear (nD) and nonlinear (n2) refractive indices as clay content increased. Additionally, the nanohybrids displayed low UV transmission and reflected about 80 % of UV rays, making them excellent candidates for UV protection. For the first time, the dissipation factor (tanδ) in the UV/Vis. region has been calculated and fitted with the Drude-Lorentz model to predict the plasma frequency (ωp), resonance frequency (ω0), and electron lifetime (τ) of pristine polymers and nanocomposites.

Atomic Layer Etching of GaN and AlGaN Using CH4/H2, H2 and HCl Chemistry

Modern microelectronic components for high-power and high-frequency applications require reliability and reproducibility of the production processes. Atomic layer etching (ALE) represents a key technology for achieving these requirements.An ALE sequence is characterized by a chemical modification step that affects only the top atomic layer of the surface (adhesion) and an ion assisted non-reactive inert gas etching step that removes only this chemically modified area due to creation of volatile byproducts. These two steps are separated by inert gas purge. This grants the etching of single atomic layers. The etching step can be triggered by thermal energy or kinetic energy by impinging particles (e.g. ion bombardment by a plasma). The low energy required for ALE guarantees low-damage etching.In literature ALE processes using Cl2 are often reported. In this work, the results of plasma-enhanced ALE of GaN and AlGaN using different chemistries of CH4/H2, H2 and HCl as etch gases are presented. The experiments were performed with a highly customized etch facility equipped with a capacitively coupled plasma source (CCP, 60 MHz) and a substrate bias generator (RF, 2 MHz). The investigations were carried out on epitaxially deposited GaN on sapphire with masks of protective resist and SiO2 as well as GaN-AlGaN heterostructures with patterned SiO2 maskings. For process analysis optical emission spectroscopy (OES) was carried out. The etching rate and roughness were determined by ellipsometry and atomic force microscopy (AFM) as well as scanning electron microscopy (SEM).Observations by OES indicate a successful excitation and dissociation of the etching molecules. With rising plasma powers physical sputtering effects were revealed. At the beginning of the process development continuous etching processes were examined. With CH4/H2 chemistry using GaN samples it was found that the etching behavior is strongly influenced by the type of masking material. Protective resist resulted in a low etch rate of 7.5 nm/min and significant damage of the masking material. The cause of this behavior is assumed to be redeposition of material by interaction of methane, resist, and plasma. With the change to a hard mask of SiO2 etch rates of 45 to 72 nm/min were achieved, increasing with RF-power and CH4/H2 ratio. Adding methane led to detrimental side effects such as dendrite growth and wall formation on the transient region of unmasked to former masked sample areas. In the further development of the ALE process an etch chemistry using only hydrogen as the chemical etchant was used resulting in an etch per cycle (EPC) of about 3 Å for GaN samples. Further investigations of ALE with hydrogen etch chemistry on GaN-AlGaN samples revealed that AlGaN, in contrast to pure GaN, could not be etched successfully with this chemistry.Substituting CH4/H2 or H2 to HCl enables the etch of AlGaN. The investigations carried out indicate that this etching process is also a self-limiting ALE. AFM measurements revealed an EPC of 1.2 Å for lower RF-powers (50 W). Increasing the RF power to 150 W only slightly enhance the EPC to 2.0 Å. The surface roughness tends to decrease with rising RF-power from 50 to 300 W.

Effects of Temperature and Humidity on Energy Dissipation between Human Corneocytes and Nanoasperity Sliding Contacts.

Human haptic perception relies on the ability of sensory receptors underneath the skin corneocyte layer to sense external load, where adhesion and friction play an essential role in nanoscale solid-solid contact. Energy dissipation present at the surface interface due to the change of separation distance during sliding contact was uncovered, but the energy dissipation of human finger skin cell-nanoprobe contact under humidity and temperature conditions has not been investigated yet. In this paper, the energy dissipation of skin corneocyte-nanoprobe interface under variation of both humidity, 0.05-80%RH, and temperature ranging from 25 to 40 °C is directly measured by atomic force microscopy (AFM). Analytical models of dissipation energy for this nanomaterial interface mechanism are developed, and the results are compared to the measured values. AFM measurements of dissipation energy reveal that the amount of dissipated energy caused by water meniscus stretching monotonically increases with humidity and temperature, resulting in adhesion and friction decreases. The purposed analytical model represents that dissipation energy trend.

Perovskite Solar Cell on Stainless Steel Substrate over 10% Efficiency for Building-Integrated Photovoltaics

This study investigated the integration of perovskite solar cells (PSCs) on stainless steel (SS) substrates for application in building-integrated photovoltaics (BIPV). Using advanced atomic force microscopy measurements, we confirmed that enhanced substrate roughness increased the reflectance along an interface. Consequently, a remarkable final efficiency of 11.9% was achieved. Notably, PSCs, known for their exceptional efficiency of 26.1%, can overcome the inherent efficiency limitations of SS-based thin-film solar cells. In this study, a PSC with an efficiency of 14% was fabricated on a flexible SS substrate. This study is a significant step towards advancing sustainable energy solutions for BIPV applications. The global shift towards renewable energy sources has catalyzed intensive research and development efforts, rendering the exploration of alternative materials and manufacturing processes a priority. The success of PSCs on SS substrates underscores their promise to achieve a balance between efficiency and versatility in BIPV solutions. Moreover, our findings reveal that controlling the substrate surface characteristics can significantly enhance the performance of PSCs, offering a pathway toward greater energy efficiency and sustainability in the construction industry.

Probing chiral discrimination in biological systems using atomic force microscopy: The role of van der Waals and exchange interactions

We analyze from a theoretical perspective recent experiments where chiral discrimination in biological systems was established using Atomic Force Microscopy (AFM). Even though intermolecular forces involved in AFM measurements have different origins, i.e., electrostatic, bonding, exchange, and multipole interactions, the key molecular forces involved in enantiospecific biorecognition are electronic spin exchange and van der Waals (vdW) dispersion forces, which are sensitive to spin–orbit interaction (SOI) and space-inversion symmetry breaking in chiral molecules. The vdW contribution to chiral discrimination emerges from the inclusion of SOI and spin fluctuations due to the chiral-induced selectivity effect, a result we have recently demonstrated theoretically. Considering these two enantiospecific contributions, we show that the AFM results regarding chiral recognition can be understood in terms of a simple physical model that describes the different adhesion forces associated with different electron spin polarization generated in the (DD), (LL), and (DL) enantiomeric pairs, as arising from the spin part of the exchange and vdW contributions. The model can successfully produce physically reasonable parameters accounting for the vdW and exchange interaction strength, accounting for the chiral discrimination effect. This fact has profound implications in biorecognition where the relevant intermolecular interactions in the intermediate-distance regime are clearly connected to vdW forces.

Nanoscale one-dimensional close packing of interfacial alkali ions driven by water-mediated attraction.

The permeability and selectivity of biological and artificial ion channels correlate with the specific hydration structure of single ions. However, fundamental understanding of the effect of ion-ion interaction remains elusive. Here, via non-contact atomic force microscopy measurements, we demonstrate that hydrated alkali metal cations (Na+ and K+) at charged surfaces could come into close contact with each other through partial dehydration and water rearrangement processes, forming one-dimensional chain structures. We prove that the interplay at the nanoscale between the water-ion and water-water interaction can lead to an effective ion-ion attraction overcoming the ionic Coulomb repulsion. The tendency for different ions to become closely packed follows the sequence K+ > Na+ > Li+, which is attributed to their different dehydration energies and charge densities. This work highlights the key role of water molecules in prompting close packing and concerted movement of ions at charged surfaces, which may provide new insights into the mechanism of ion transport under atomic confinement.

Unraveling the formation dynamics of metallic femtosecond laser induced periodic surface structures

Femtosecond laser surface processing (FLSP) is an emerging fabrication technique to efficiently control the surface morphology of many types of materials including metals. However, the theoretical understanding of the FLSP formation dynamics is not a trivial task, since it involves the interaction of various physical processes (electromagnetic, thermal, fluid dynamics) and remains relatively unexplored. In this work, we tackle this problem and present rigorous theoretical results relevant to low-fluence FLSP that accurately match the outcomes of an experimental campaign focused on the formation dynamics of laser induced periodic surface structures (LIPSS) on stainless steel. More specifically, the topography and maximum depth of LIPSS trenches are theoretically and experimentally investigated as a function of the number of laser pulses. Moreover, precise LIPSS morphology measurements are performed using atomic force microscopy (AFM). The proposed comprehensive simulation study is based on two-temperature model (TTM) non-equilibrium thermal simulations coupled with fluid dynamic computations to capture the melting metal phase occurring during FLSP. Our rigorous simulation results are found to be in excellent agreement with the AFM measurements. The presented theoretical framework to model FLSP under low-fluence femtosecond laser pulses will be beneficial to various emerging applications of LIPSS on metallic surfaces, such as cooling high-powered laser diodes and controlling the thermal emission or absorption of metals.

Silica nanoparticle-membrane interactions investigated by atomic force microscopy in an organic solvent

Ion-exchange membranes have been widely used not only in water but also in organic solvents for both scientific exploration and industrial applications. Atomic force microscopy (AFM) is a promising analytical technique to determine membrane functions in both water and organic solvents. Nevertheless, the use of organic solvents frequently causes significant contamination and mechanical drift in AFM imaging investigations. Additionally, the rapid evaporation of organic solvents imposes the limitation of measurement time. Thus, a force curve array measurement requiring a longer time is challenging in organic solvents. Herein, we propose a widely adaptable holder design that facilitates stable AFM measurements in organic solvents for a broad range of AFM systems. Using the designed AFM holder, we investigated silica-membrane interactions in 2-propanol. Our AFM evaluation provides important information on nanoparticle removal with ion-exchange membranes in 2-propanol. The concept of widely adaptable holder design should contribute to the understanding of nanoparticle-membrane interactions and the development of various functional membranes used in organic solvents.

Study on Selective Adsorption Behavior and Mechanism of Quartz and Magnesite with a New Biodegradable Collector

Research on the efficient flotation desilication of low-grade magnesite is of great significance for the sustainable development of magnesium resources. Traditional collectors usually have some disadvantages, such as poor selectivity, severe environmental pollution, and weak water solubility. To strengthen the desilication flotation process of magnesite ore, the biodegradable surfactant, cocamidopropyl amine oxide (CPAO), was first utilized as the collector for the separation of the magnesite and quartz. The selective adsorption behavior and mechanism of the quartz and magnesite with the CPAO as the collector were studied through the micro-flotation experiments of the single mineral and the artificially mixed mineral, contact angle and atomic force microscopy (AFM) measurements, fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses. The flotation results indicated that the CPAO showed good selectivity and could effectively separate magnesite and quartz. When the concentration of the CPAO was 10.0 mg/L in the natural pulp pH (about 7.2), the concentrates with 97.67% MgO recovery and 45.62% MgO grade were obtained. The contact angle and AFM measurements indicated that the CPAO could selectively adsorb on the quartz surface rather than the magnesite surface to improve the interface difference between them, especially its surface hydrophobicity. The results of the FTIR and XPS analyses indicated that the CPAO is selectively adsorbed on the surface of the quartz, mainly through electrostatic interaction and hydrogen bonding. In conclusion, the CPAO had good selectivity and great potential as an effective collector in the reverse flotation desilication progress of magnesite.

Combining atomic force microscopy and nanoindentation helps characterizing in-situ mechanical properties of organic matter in shale

The quantification of mechanical properties of organic matter in shale is of significance for the fine prediction and characterization of shale reservoir's mechanical properties. Due to the micron-sized and dispersed distribution of organic matter particles in shale, the accurate evaluation of the actual mechanical response remains challenging. This work focuses on shale from Wufeng-Longmaxi Formation, which is the main shale gas exploration and development formation in China. A method based on atomic force microscopy (AFM) with an optical microscope (i.e., in-situ AFM technique) is presented to locate the organic matter in-situ and then visualize and quantify its mechanical properties using AFM Young's modulus mapping. The merits and limitations for determining the mechanical properties of organic matter in shale between the AFM and the more conventional nanoindentation technique are discussed. Results show that combining in-situ nanoindentation and in-situ AFM mapping provides more accurate description of the mechanical properties of organic matter in shale than traditional grid indentation methods with low spatial resolution. The Young's moduli of organic matter calculated from nanoindentation are around twice smaller than those obtained from AFM measurements mainly because the elasto-plastic deformation zone of organic matter in nanoindentation tests is larger and can be additionally affected by the presence of inorganic particles and/or larger micro-pores in organic matter. The Young's modulus and hardness of graptolite in the shale obtained by nanoindentation are slightly larger than those of solid bitumen at the same thermal maturity. Both in-situ AFM and in-situ nanoindentation results show that the mechanical strength of organic matter increases with increasing maturity. Overall, the presented approach shows a great potential for accurate and in-situ measurement of the mechanical properties of organic matter in shale at the nanoscale, which may be beneficial to the development of rock mechanical models for the accurate evaluation of the actual mechanical properties of shale.

Investigation of the incorporation of C60 into PC61BM to enhance the photovoltaic performance of inverted-type perovskite solar cells based on MAPbI3

Perovskite Solar Cells (PSCs) have managed to significantly capture attention by achieving efficiency values of 25.6% in a remarkably short period of around ten years. Each layer within the device plays a crucial role in the overall device efficiency when it comes to PSC production. PC61BM, a derivative of fullerene, is one of the most commonly used electron-transport layers (ETLs) in inverted-type PSCs. In this study, the improvement of the ETL was aimed by incorporating C60 into PC61BM, and the effects of the doped ETL on MAPbI3-based inverted-type PSCs were investigated. For inverted type PSCs which are fabricated under high humidity (40-60%) and room conditions (~25 °C), the power conversion efficiencies (PCEs) have boosted from 11.54% (for undoped PC61BM) to 13.40% (for C60-doped PC61BM). To comprehend the sources of improvement in the fabricated devices, a series of characterizations were carried out, including Current Density-Voltage (J-V), Hysteresis Factor (HF), Scanning Electron Microscope (SEM), and Atomic Force Microscope (AFM) measurements.

A multi-resistance wide-range calibration sample for conductive probe atomic force microscopy measurements.

Measuring resistances at the nanoscale has attracted recent attention for developing microelectronic components, memory devices, molecular electronics, and two-dimensional materials. Despite the decisive contribution of scanning probe microscopy in imaging resistance and current variations, measurements have remained restricted to qualitative comparisons. Reference resistance calibration samples are key to advancing the research-to-manufacturing process of nanoscale devices and materials through calibrated, reliable, and comparable measurements. No such calibration reference samples have been proposed so far. In this work, we demonstrate the development of a multi-resistance reference sample for calibrating resistance measurements in conductive probe atomic force microscopy (C-AFM) covering the range from 100 Ω to 100 GΩ. We present a comprehensive protocol for in situ calibration of the whole measurement circuit encompassing the tip, the current sensing device, and the system controller. Furthermore, we show that our developed resistance reference enables the calibration of C-AFM with a combined relative uncertainty (given at one standard deviation) lower than 2.5% over an extended range from 10 kΩ to 100 GΩ and lower than 1% for a reduced range from 1 MΩ to 50 GΩ. Our findings break through the long-standing bottleneck in C-AFM measurements, providing a universal means for adopting calibrated resistance measurements at the nanoscale in the industrial and academic research and development sectors.

Structural, optical, electrical and gas sensor properties of Fe-doped CdO thin films synthesized by spray pyrolsis

In this study, the structural, optical, electrical and gas sensor properties of Fe-doped CdO thin films synthesized using the spray pyrolysis method were investigated as a function of Fe doping. The characterizations of the thin films were made by using X-ray diffraction (XRD), X-ray photoelectron (XPS), ultraviolet–visible (UV-VIS), field emission-scanning electron microscope (FE-SEM), atomic force microscope (AFM) and Hall measurements. In addition, the response values of the films for H2 gas were determined by a current-sensitive gas sensor measurement system. XRD analyzes revealed that the produced films were in polycrystalline cubic structure and their unit cell volumes were reduced by Fe doping. The mean particle size and mean dislocation density of Fe doped samples were obtained as 11.99 nm and 6.98 × 1015 (line/m2), respectively. XPS results showed that the experimentally obtained doping ratios of the produced films were very close to the planned doping ratios and the formation of CdxFe(1-x)-O2 and CdxFe(1-x)-O3 on the surfaces of the samples decreased with the increment of Fe doping. UV-VIS measurements demostrated that the band gap is generally reduced due to doping, in line with the Moss-Burstein effect. According to Hall results, the mobility of the samples increases with increasing doping. SEM and AFM analyzes revealed that Fe doping to CdO causes nanoroads formation on thin film surfaces, and nanoroads disappear in 4 % and 5 % doped films. In addition to the specified characterizations, how these changes in the surface morphology of the films with Fe doping affect the H2 gas responses of the samples is also explained in the study.

Extraction, purification and characterization of amylose from sago and corn: Morphological, structural and molecular comparison

In the present work, a comprehensive study was carried out to better understand the molecular characteristics of amylose extracted from sago starch, using butanol as the extraction solvent. The sago derived amylose was compared with amylose extracted from corn starch and both characterized through different techniques, i.e. size exclusion chromatography, X-ray diffraction (XRD), Fourier transform infrared spectroscopy, Raman spectroscopy, Scanning electron microscopy, Atomic force microscopy and Zeta potential measurements. The purity of the amylose extracted from sago and corn was 99.20 % and 93.46 %, respectively. From XRD results, it was revealed that sago amylose had more crystallinity with high thermal stability compared to corn amylose. Based on Raman spectra, single and double helices formed in both extracted amyloses, but due to their intrinsic differences, the intensities associated with these helices varied for sago and corn amylose. Purified amyloses were shown to have two different forms of spherulite morphology: torus and spherical shapes with varying degrees of roughness. Our findings demonstrated that sago starch is a novel and low-cost source for supplying amylose, a promising polymer for different applications.

Solvent Impact on Langmuir and Langmuir–Schaefer Films of Soluble Main‐Chain Poly(fullerene)s Based on C60

Understanding the morphology and electronic properties of poly(fullerene)s is crucial for the development of new organic devices. This work addresses the fabrication and characterization of Langmuir–Schaefer (LS) films of poly(fullerene)s based on C60 with short (HSS8) and long (HSS16) sidechains, solubilized in chloroform or xylene. In addition, density functional theory (DFT) calculations are used to optimize the molecular geometries, determine energies, and investigate the influence of solvent applied. Depending on the organic solvent, floating material isotherms indicate the formation of disordered aggregates in the aqueous subphase. The influence of solvent in LS films is also evidenced by way of atomic force microscopy (AFM), UV‐vis, and cyclic voltammetry (CV) measurements. From DFT calculations, the arms of the poly(fullerene)s start to extend from an initial position of surrounding the fullerene sphere. In AFM measurements, depending on the organic solvent, the roughness significantly reduces, while the homogeneity is much higher. In UV‐vis and CV measurements, the propensity to form aggregates depends mainly on the polarization of the solvent and is directly related to the maximum absorption, oxidation, and reduction peaks. From the optical bandgap energy values, the poly(fullerene)s studied here present high potential for application in organic electronic devices.

The role of gellan gum in the selective flotation separation of fluorite from calcite: An experimental and molecular dynamics simulation study

Fluorite as the main source of fluorine generally coexists with calcite. The highly similar floatabilities and physicochemical properties make it tough to separate these two minerals. Herein, gellan gum (GG) was used as depressant to separate fluorite from calcite for the first time. Several experiments and tests were utilized to explain the inhibiting performance and mechanism of GG. Flotation experiments illustrated that GG displayed an outstanding depression performance for the calcite flotation, whereas it hardly depressed fluorite. Zeta potential analyses, surfaces wettability measurements and Fourier transform infrared (FTIR) indicated that GG strongly adhered onto calcite and prevented the NaOL from adhering on it. Adsorption amount measurements indicated that the GG adhered onto calcite with a higher amount compared to fluorite. X-ray photoelectron spectroscopy (XPS) tests pointed out that GG adsorbed onto calcite and fluorite surfaces by strong chemisorption and weak physisorption, respectively. Atomic force microscopy (AFM) measurements implied visually that GG adsorbed denser and stronger on the calcite than fluorite. Molecular dynamics (MD) simulations demonstrated that compared with fluorite, more GG adsorbed on the calcite surface due to the reaction between hydroxyl groups and the mineral surface, as evidenced by higher diffusion coefficient for water (Dwater) value, smaller interaction distance, and higher relative concentration of GG on the calcite surface. Therefore, GG could be employed as a promising flotation depressant in separating fluorite from calcite.

Effect of out of plane orientation on polarization second harmonic generation of single collagen fibrils.

Second harmonic generation (SHG) microscopy has emerged as a powerful technique for visualizing collagen organization within tissues. Amongst the many advantages of SHG is its sensitivity to collagen nanoscale organization, and its presumed sensitivity to the relative out of plane polarity of fibrils. Recent results have shown that circular dichroism SHG (CD-SHG), a technique that has been commonly assumed to reveal the relative out of plane polarity of collagen fibrils, is actually insensitive to changes in fibril polarity. However, results from another research group seem to contradict this conclusion. Both previous results have been based on SHG imaging of collagen fibrils within tissues, therefore, to gain a definitive understanding of the sensitivity of SHG to relative out of plane polarity, the results from individual fibrils are desirable. Here we present polarization resolved SHG microscopy (PSHG) data from individual collagen fibrils oriented out of the image plane by buckling on an elastic substrate. We show through correlation with atomic force microscopy measurements that SHG intensity can be used to estimate the out of plane angle of individual fibrils. We then compare the sensitivity of two PSHG techniques, CD-SHG and polarization-in, polarization-out SHG (PIPO-SHG), to the relative out of plane polarity of individual fibrils. We find that for single fibrils CD-SHG is insensitive to relative out of polarity and we also demonstrate the first direct experimental confirmation that PIPO-SHG reveals the relative out of plane polarity of individual collagen fibrils.

Chirality Versus Symmetry: Electron's Spin Selectivity in Nonpolar Chiral Lead-Bromide Perovskites.

In the last decade, chirality-induced spin selectivity (CISS), the spin-selective electron transport through chiral molecules, has been described in a large range of materials, from insulators to superconductors. Because more experimental studies are desired for the theoretical understanding of the CISS effect, chiral metal-halide semiconductors may contribute to the field thanks to their chiroptical and spintronic properties. In this regard, this work uses new chiral organic cations S-HP1A and R-HP1A (HP1A = 2-hydroxy-propyl-1-ammonium) to prepare 2D chiral halide perovskites (HPs) which crystallize in the enantiomorphic space groups P43 21 2 and P41 21 2, respectively. The fourfold symmetry induces antiferroelectricity along the stacking axis which, combined to incomplete Rashba-like splitting in each individual 2D polar layer, results in rare spin textures in the band structure. As revealed by magnetic conductive-probe atomic force microscopy (AFM) measurements, these materials show CISS effect with partial spin polarization (SP; ±40-45%). This incomplete effect is efficient enough to drive a chiro-spintronic device as demonstrated by the fabrication of spin valve devices with magnetoresistance (MR) responses up to 250 K. Therefore, these stable lead-bromide HP materials not only represent interesting candidates for spintronic applications but also reveal the importance of polar symmetry-breaking topology for spin selectivity.

Mechanism analysis of Lignin's effect on Asphalt's resistance to moisture damage

The present study investigates the mechanism by which lignin enhances the moisture resistance of asphalt materials, employing a combination of atomic force microscopy (AFM) and molecular simulations. The research aims to understand the changes in surface morphology and adhesion properties of asphalt upon immersion, and elucidate the role of lignin modification in mitigating the reduction of adhesive forces. AFM measurements reveal that lignin-modified asphalt exhibits higher adhesion force after moisture damage, possibly attributable to less nanostructures on the surface of lignin modified asphalt. Molecular dynamics simulations are employed to study the binding energies between lignin and different asphalt constituents. The results demonstrate that lignin exhibits the strongest binding affinity with asphaltene molecules, and also displays significant binding affinity with a specific saturated molecule. Furthermore, diffusion of water molecules on asphalt surfaces is investigated, revealing that lignin-modified asphalt exhibits lower water molecule diffusion coefficients and reduced binding energies. Finally, a spherical water molecule model is employed to analyze the wetting behavior on lignin-modified asphalt surfaces, illustrating the dual mechanism of lignin molecules resisting water molecules and inhibiting the binding of polar molecules within asphalt to water molecules. The findings of this research provide insights into the enhancement of moisture resistance in asphalt materials through lignin modification. The combination of AFM and molecular simulations offers a comprehensive understanding of the morphological and adhesive changes at the nanoscale, as well as the fundamental molecular interactions involved. The results contribute to the development of strategies for designing and engineering asphalt materials with improved resistance to moisture damage.

Thermoresponsive Helical Dendronized Poly(phenylacetylene)s: Remarkable Stabilization of Their Helicity via Photo-Dimerization of the Dendritic Pendants.

Dynamic helical polymers can change their helicity according to external stimuli due to the low helix-inversion barriers, while helicity stabilization for polymers is important for applications in chiral recognition or chiral separations. Here, we present a convenient methodology to stabilize dynamic helical conformations of polymers through intramolecular cross-linking. Thermoresponsive dendronized poly(phenylacetylene)s (PPAs) carrying 3-fold dendritic oligoethylene glycol pendants containing cinnamate moieties were synthesized. These polymers exhibit typical features of dynamic helical structures in different solvents, that is, racemic contracted conformations in less polar organic solvents and predominantly one-handed stretched helical conformations in highly polar solvents. This dynamic helicity can be enhanced through selective solvation by increasing the polarity of the organic solvents or simply via their thermally mediated dehydration in water. However, through photocycloaddition of the cinnamate moieties between the neighboring pendants via UV irradiation, these dendronized PPAs adopt stable helical conformations either below or above their phase transition temperatures in water, and their helical conformations can even be retained in less polar organic solvents. Spectroscopic and atomic force microscopy measurements demonstrate that photocycloaddition between the cinnamate moieties occurs on the individual molecular level, and this is found to be helpful in restraining the photodegradation of the PPA backbones. Molecular dynamics simulations reveal that the spatial orientation of the pendants along the rigid polyene backbone is crucial for the photodimerization of cinnamates within one helix pitch.