Atomic Force Microscopy Phase Images Research Articles

Atomic force microscopy phase imaging of epitaxial graphene films

Dynamic mode atomic force microscopy phase imaging is known to produce distinct contrast between graphene areas of different atomic thickness. But the intrinsic complexity of the processes controlling the tip motion and the phase angle shift excludes its use as an independent technique for a quantitative type of analysis. By investigating the relationship between the phase shift, the tip-surface interaction, and the thickness of the epitaxial graphene areas grown on silicon carbide, we shed light on the origin of such phase contrast, and on the complex energy dissipation processes underlying phase imaging. In particular, we study the behavior of phase shift and energy dissipation when imaging the interfacial buffer layer, single-layer, and bilayer graphene regions as a function of the tip-surface separation and the interaction forces. Finally, we compare these results with those obtained on differently-grown quasi free standing single- and bilayer graphene samples.

Synthesis, Characterization and Application of Chitosan Coated Magnetite Nanoparticles in Drug Delivery

Magnetite nanoparticles were used extensively for various applications. In the present study, magnetite nanoparticles were synthesized and characterized by atomic force microscopy (AFM). AFM images showed that the obtained particles were perfectly spherical. Functionality is afforded to these magnetite nanoparticles by adding biocompatible polymer chitosan during the synthesis. AFM phase image clearly showed that the magnetite core is encapsulated with the polymeric shell. Fourier-transform infrared spectroscopy (FTIR) study showed the chitosan absorption on Fe2O3 nanoparticle surface. The drug sulphamethoxazole was loaded over magnetite nanoparticles and the encapsulation efficiency of drug was calculated at different concentrations of magnetite. The encapsulation efficiency increases with increase in the concentration of magnetite. Thus, an attempt was made in synthesizing drug loaded biopolymer magnetite nanoparticles suitable for targeted drug delivery.

Dependence of fractal characteristics on the scan size of atomic force microscopy (AFM) phase imaging of aluminum thin films

In this article, data demonstrating the effect of scan size on the fractal characteristics of phase images obtained via atomic force microscopy (AFM) are presented. The AFM phase imaging was conducted on thin aluminum films deposited on stainless steel substrates by radiofrequency (RF) magnetron sputtering at power of 150 W for 2 h and at room temperature. The AFM phase images were obtained at three different scan sizes (1 × 1 μm, 3 × 3 μm, 30 × 30 μm) at a constant scan rate and proportional-integral-derivative (PID) control. The raw and analyzed data of the AFM visuals (micrographs) are presented in this article. The data on power spectral density functions (PDSF), fractal dimensions (D) and texture features obtained through image analyses at different scan sizes are also presented. The article also contains data (PSDF and fractal dimensions) comparing the fractal features obtained from the phase and height AFM micrographs at large scan size (30 × 30 μm). The data can be used in optimization of AFM measurement for industrial quality control.

Diversity of physical properties of bacterial extracellular membrane vesicles revealed through atomic force microscopy phase imaging.

Bacteria release nanometer-scale extracellular membrane vesicles (MVs) to mediate a variety of biological processes. We analyzed individual MVs under physiological conditions by phase imaging of high-speed atomic force microscopy to assess the physiological heterogeneity of MVs isolated from bacterial cultures. Phase imaging makes it possible to map the physical properties of an individual, fragile MV in an isolated MV population containing a broad variety of vesicle diameters, from 20 to 150 nm. We also developed a method for quantitatively comparing the physical properties of MVs among samples. This allowed for the comparison of the physical properties of MVs isolated from different bacterial species. We compared bacterial MVs isolated from four bacterial species and artificially synthesized liposomes. We demonstrate that each bacterial species generates physically heterogeneous types of MVs, unlike the physical homogeneity displayed by liposomes. These results indicate that the physical heterogeneity of bacterial MVs is mainly caused by compositional differences mediated through biological phenomena and could be unique to each species. We provide a new methodology using phase imaging that would pave the way for single-vesicle analysis of extracellular vesicles of a broad size range.

1-nm-Wide Hydrated Dipole Arrays Regulate AuNW Assembly on Striped Monolayers in Nonpolar Solvent

Summary We show that striped phases of horizontally oriented phospholipids presenting 1-nm-wide orientable dipole arrays can order and straighten flexible 2-nm-diameter gold nanowires (AuNWs) with lengths (up to 1 μm) that greatly exceed the template pitch (∼7.5 nm). AuNW ordering can extend over areas > 100 μm2. Whereas wires bundle in solution, with contact between ligand shells, wires adsorbing to the template separate to much larger center-to-center distances (e.g., 12–30 nm) near multiples of the template pitch, consistent with repulsive interactions between wires emerging during interactions with dipole arrays in the template. AuNW assembly was carried out in cyclohexane, a nonpolar solvent. Interestingly, we found that wire ordering was contingent on the extent of phospholipid headgroup hydration and the presence of excess oleylamine, which assembled into hemicylindrical micelles around the hydrated headgroups. Together, these components generated a protected polar environment that enabled the phospholipid headgroups to function collectively to regulate AuNW assembly.

Determination of the Thin-Film Structure of Zwitterion-Doped Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate): A Neutron Reflectivity Study.

Doping poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is known to improve its conductivity; however, little is known about the thin-film structure of PEDOT:PSS when doped with an asymmetrically charged dopant. In this study, PEDOT:PSS was doped with different concentrations of the zwitterion 3-( N, N dimethylmyristylammonio)propanesulfonate (DYMAP), and its effect on the bulk structure of the films was characterized by neutron reflectivity. The results show that at a low doping concentration, the film separates into a quasi-bilayer structure with lower roughness (10%), increased thickness (18%), and lower electrical conductivity compared to the undoped sample. However, when the doping concentration increases, the film forms into a homogeneous layer and experiences an enhanced conductivity by more than an order of magnitude, a 20% smoother surface, and a 60% thickness increase relative to the pristine sample. Atomic force microscopy (AFM) and profilometry measurements confirmed these findings, and the AFM height and phase images showed the gradually increasing presence of DYMAP on the film surface as a function of the concentration. Neutron reflectivity also showed that the quasi-bilayer structure of the lowest concentration-doped PEDOT:PSS is separated by a graded rather than a well-defined interface. Our findings provide an understanding of the layer structure modification for doped PEDOT:PSS films which should prove important for device applications.

Self-assembly of copper-free maltoheptaose-block-polystyrene nanostructured thin films in real and reciprocal space

The promising carbohydrate-based block copolymer maltoheptaose-block-polystyrene (MH-b-PS) has been used for high-performance memory transistors and next generation nanolithography. In order to realize the potential of MH-b-PS especially in microelectronic applications, we firstly improved its synthetic method for obtaining large amount of copper-free MH-b-PS. The main improvement relies on the removal of the residual copper catalyst by using a chelating resin. Then, the microphase separation of copper-free MH-b-PS in both thin film and bulk states under different solvent vapor annealing conditions were investigated comprehensively and compared with our previous report by using both real-space and reciprocal-space techniques. A phase transition of MH-b-PS from hexagonal close-packed horizontal cylinders to face-centered cubic were observed when increasing the amount of tetrahydrofuran in the mixture annealing solvent of tetrahydrofuran and H2O. More details about self-assembled MH-b-PS nanostructures were analyzed by comparing grazing incidence small angle X-ray scattering patterns with corresponding atomic force microscopy phase images.

Structure of Mixed Acid/Decyl Monolayers Grafted on Oxide-Free Si(111) Surfaces.

The structure of mixed acid/decyl monolayers (MLs) grafted on oxide-free Si(111) surfaces by photochemical hydrosilylation in a mixture of neat undecylenic acid and 1-decene is studied in detail. After appropriate surface cleaning of the as-grafted surfaces, atomic force microscopy (AFM) (topography and phase imaging) and calibrated FTIR analysis demonstrate that a mixed monolayer is formed, free of residue. When the acid-molecule fraction (ΓSOL) is >0.1, mixed MLs are homogeneous on the scale of observations and they are only slightly enriched in acid chains with respect to the solution. Conversely, when ΓSOL < 0.1, the acid chain fraction within the ML becomes quasi-independent of the solution composition and may become much larger than ΓSOL. In addition, dark domains are observed in AFM phase images. Correlations between the characteristic parameters of νCO IR bands and AFM phase images suggest a strong phase separation of acid and alkyl chains at various length scales. A model involving a structuration of the grafting solution is proposed to explain observations.

Vitamin C/Stearic Acid Hybrid Monolayer Adsorption at Air-Water and Air-Solid Interfaces.

Because of the antioxidant activity of vitamin C (Vit C) polar heads, they can be used as a protective agent for fatty acids. Hence, the study on the growth of Vit C/stearic acid (SA) mixed binary films at air–water interface (known as Langmuir monolayer) and air–solid interface (known as Langmuir–Blodgett films) is of paramount interest. Although Vit C is situated at subsurface beneath SA molecules and interacts via hydrogen bonding between the hydroxyl groups of Vit C and SA, several Vit C molecules may infiltrate within SA two-dimensional matrix at the air–water interface. The increased mole fraction of Vit C (0.125–0.5) and the reduction of temperature (from 22 to 10 °C) of the subphase water result in an increase in the amount of adsorbed Vit C at the air–water interface. The surface pressure (π)–area (A) isotherms illustrate that such inclusion of Vit C provokes a spreading out of Vit C/SA binary monolayers, which leads to an alteration of different physicochemical parameters such as elasticity, Gibbs free energy of mixing, enthalpy, entropy, interaction energy parameter, and activity coefficient. However, being polar in nature, the transfer of pure Vit C on substrates gets affected. It can be transferred onto substrate by mixing suitably with SA as confirmed by infrared spectra. Their structures, extracted X-ray reflectivity, and atomic force microscopy (topography and phase imaging) are found to be strongly dependent on the nature of the substrate (hydrophilic and hydrophobic).

Lamellipod Reconstruction by Three-Dimensional Reflection Interference Contrast Nanoscopy (3D-RICN).

There are very few techniques to reconstruct the shape of a cell at nanometric resolution, and those that exist are almost exclusively based on fluorescence, implying limitations due to staining constraints and artifacts. Reflection interference contrast microscopy (RICM), a label-free technique, permits the measurement of nanometric distances between refractive objects. However, its quantitative application to cells has been largely limited due to the complex interferometric pattern caused by multiple reflections on internal or thin structures like lamellipodia. Here we introduce 3D reflection interference contrast nanoscopy, 3D-RICN, which combines information from multiple illumination wavelengths and aperture angles to characterize the lamellipodial region of an adherent cell in terms of its distance from the surface and its thickness. We validate this new method by comparing data obtained on fixed cells imaged with atomic force microscopy and quantitative phase imaging. We show that as expected, cells adhering to micropatterns exhibit a radial symmetry for the lamellipodial thickness. We demonstrate that the substrate-lamellipod distance may be as high as 100 nm. We also show how the method applies to living cells, opening the way for label-free dynamical study of cell structures with nanometric resolution.

Nano-optical imaging of monolayer MoSe2-WSe2 lateral heterostructure with subwavelength domains

Atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDs) are the materials of recent interest to study the spatial confinement of charge carriers, photons, and phonons. Heterostructures based on TMD monolayers, especially composed of Mo and W, form type-II band alignment, and hence, the optically excited carriers can be easily separated for applications pertaining to optoelectronics. Mapping the spatially confined carriers or photons in lateral heterostructures with nanoscale resolution as well as their recombination behavior at the heterointerfaces is necessary for the effective use of 2D materials in optoelectronic devices. Near-field (NF) optical microscopy has been used as a viable route to understand the nanoscale material properties below the diffraction limit. The authors performed tip-enhanced photoluminescence (TEPL) imaging with a spatial resolution of 40 nm of multijunction monolayer MoSe2-WSe2 lateral heterostructures with subwavelength domains grown by chemical vapor deposition. Monolayer MoSe2 and WSe2 domains were identified by atomic force microscopy (AFM) through the topography and phase mapping. Far-field (FF) and NF techniques were used for the optical imaging of the WSe2 ↔ MoSe2 multijunction heterostructure correlated with AFM phase imaging. Near-field TEPL imaging was able to successfully distinguish the presence of distinct crystalline boundaries across the WSe2 ↔ MoSe2 interfaces in 2D lateral heterostructures with a higher spatial resolution, as compared to the far-field imaging, which failed to resolve the interfaces on one of the crystal sides due to the asymmetric FF excitation.

Superhydrophilic/superhydrophobic surfaces fabricated by spark‐coating

In this study, the authors researched the preparations of superhydrophilic/superhydrophobic surfaces on commercial cup stock polyethylene coated papers by using sparked aluminum nanoparticles deposited on substrates through a sparking process. In this stage, the surface was porous and showed superhydrophilic properties. The samples were then annealed in air at various temperatures and some transformed to superhydrophobicity. It is well known that a suitable roughness in combination with low surface energy has been required to obtain superhydrophobic surfaces. Therefore, it is believed that during annealing process, when polyethylene is diffused from the substrate through the nanoparticle films and the superhydrophobic characteristics were created. The scanning electron microscope images showed that the film surfaces had a fluffy structure for both the as‐deposited and the annealed samples. However, the atomic force microscopy phase images showed completely different surface properties. Moreover, the X‐ray photoelectron spectroscopy spectra showed different surface chemical compositions. The experimental results revealed that the working temperature to produce superhydrophobic surfaces depended on the sparked film thickness. Furthermore, in order to prove the assumption explained above, glass and poly (methyl methacrylate) were also used as substrates.

Using Atomic Force Microscopy to Probe Crystallization in Atactic Polystyrenes

AbstractThe use of atomic force microscopy phase imaging to study crystallization in near oligomeric atactic polystyrenes is demonstrated. By considering different thermal histories, the distribution of polymerization index in the samples can be changed, and how this effects the resulting crystallinity can be measured. Different thermal treatments in the nucleation of crystals and the effect on the resulting measured crystal formation are also examined.

Characterization of titanyl phthalocyanine (TiOPc) thin films by microscopic and spectroscopic method

The titanyl phthalocyanine (TiOPc) thin film deposited on glass, silicon and gold substrate have been studied using Raman spectroscopy, atomic force microscopy (AFM), absorption and profilometry measurements. The TiOPc thin layers have been deposited at room temperature by the quasi-molecular beam evaporation technique. The Raman spectra have been recorded using micro Raman system equipped with a confocal microscope. Using surface Raman mapping techni que with polarized Raman spectra the polymorphic forms of the TiOPc thin films distribution have been obtained. The AFM height and phase image were examined in order to find surface features and morphology of the thin films. Additionally to compare experimental results, structure optimization and vibrational spectra calculation of single TiOPc molecule were performed using DFT calculations. The received results showed that the parameters like polymorphic form, grain size, roughness of the surface in TiOPc thin films can well characterize the obtained organic thin films structures in terms of their use in optoelectronics and photovoltaics devices.

Synthesis and healing properties of poly(arylether sulfone)–poly(alkylthioether) multiblock copolymers containing disulfide bonds

ABSTRACTThermoplastic elastomers composed of soft and hard segments are important elastic and processable synthetic polymers. The microphase‐separated soft domains show low glass transition temperature and possess sufficient chain mobility at room temperature. In this study, we report the synthesis and healing properties of multiblock copolymers containing disulfide bonds as dynamic covalent bonds. The multiblock copolymers composed of poly(arylether sulfone) and poly(alkylthioether) segments were synthesized by oxidative coupling polymerization of the corresponding thiol‐terminated oligomers. Atomic force microscopy phase images, differential scanning calorimetry, and dynamic mechanical analysis curves indicated the microphase‐separated morphology of the multiblock copolymer. Self‐healing properties of the polymer were evaluated by changes in the elongation at break of the cut/adhered samples. The elongation recovery increased with UV irradiation time, and the multiblock copolymer showed a 93% recovery after UV irradiation for 5 h. The healing efficiency induced by UV irradiation, determined by subtracting the recovery without UV irradiation, was calculated to be 51%. According to the UV spectra and solubility changes after UV irradiation, the main healing factor in this study was the crosslinking reactions caused by thiyl radicals generated from UV irradiation instead of disulfide exchange reactions. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 3545–3553

Piezoelectric immunochip coated with thin films of bacterial cellulose nanocrystals for dengue detection

Low-cost piezoelectric devices, such as simple frequency monitoring quartz crystal microbalance (QCM) devices, have good clinical utility as fast diagnostic tools for the detection of several diseases. However, unspecific antigen recognition, poor molecular probe adsorption and the need for sample dilution are still common drawbacks that hinder their use in routine diagnosis. In this work, piezoelectric sensors were previously coated with thin films of bacterial cellulose nanocrystals (CN) to provide a more sensitive and adapted interface for the attachment of monoclonal immunoglobulin G (IgGNS1) and to favor specific detection of non-structural protein 1 (NS1) of dengue fever. The assembly of the immunochip surface was analyzed by atomic force microscopy (AFM) and the NS1 detection was followed by quartz crystal microbalance with (QCM-D) and without energy dissipation monitoring (QCM). The CN surface was able to immobilize 2.30±0.5mgm−2 of IgGNS1, as confirmed by AFM topography and phase images along with QCM-D. The system was able to detect the NS1 protein in serum with only 10-fold dilution in the range of 0.01–10µgmL−1 by both QCM and QCM-D. The limits of detection of the two devices were 0.1μgmL−1 for QCM-D and 0.32μgmL−1 for QCM. As a result, QCM-D and QCM apparatuses can be used to follow NS1 recognition and have good potential for more sensitive, fast and/or less expensive diagnostic assays for dengue.

Poly(ethylene) glycol-modified, amphiphilic, siloxane–polyurethane coatings and their performance as fouling-release surfaces

Amphiphilic siloxane–polyurethane (AmSiPU) coatings were prepared using a series of polyisocyanate prepolymers modified with polydimethyl siloxane (PDMS) and poly(ethylene glycol) (PEG). Fouling-release performance of the AmSiPU coatings was evaluated through laboratory biological assays using several representative marine organisms. First, polyisocyanate prepolymers with compositional variation in PDMS and PEG were synthesized and characterized using Fourier transform infrared spectroscopy (FTIR) and isocyanate titrations. Then, the prepolymers were incorporated into coatings. Surface wettability of the coatings was evaluated using contact angle and surface energy measurements. Coatings’ surfaces were also characterized using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). ATR-FTIR and XPS experiments revealed that both PDMS and PEG moieties were present on the surface suggesting amphiphilic character. AFM phase images show microphase separation. AmSiPU coatings show excellent fouling-release performance toward bacteria (Cellulophaga lytica), the diatoms (Navicula incerta), and the green algae (Ulva linza), demonstrating comparable or superior performance to many commercial amphiphilic fouling-release coatings. Despite the incorporation of hydrophilic PEG, AmSiPU coatings show good macrofouling release which is often challenging with amphiphilic coating systems. AmSiPU coatings are a nontoxic and tough fouling-release solution with comparable performance to benchmarks in the fouling-release coatings market.

Substantially reinforcing plant oil-based materials via cycloaliphatic epoxy with double bond-bridged structure

A diepoxide ((E)-1,2-di(7- oxabicyclo [4.1.0]heptan-3-yl)ethane, named DCE), possessing two cyclohexane rings bridged by a double bond was obtained via self-metathesis approach. This unique epoxy structure shows great potential in substantially overcoming the poor performances of plant oil materials in thermal and mechanical aspects. The molecular structure of DCE is more compact and regular than the commercial 3,4-epoxycyclohexylmethyl-3,4- epoxycyclohexanecarboxylate (ECHM) and was evidenced by the small-/wide-angle X-ray scattering. After conetwork was built between epoxidized soybean oil (ESO) and DCE, stiff crosslinks were created through the double bonds of DCE, which restricts the backbone rotation of ESO (P-ESO) networks. DCE-rich domains with dimensions ranging from 0.5 to 2 μm were observed with the increase of DCE, revealed by atomic force microscopy (AFM) phase image and voltage-distance curve. These DCE-rich domains act as micro-scale reinforcements homogenously dispersed in the ESO matrix, effectively sustaining the applied stress and preventing the chain break from high force loading. Compared to P-ESO, the co-polymerized ESO-DCE at 30 wt % DCE loading showed tensile strength of 62 MPa and glass transition temperature of 163 °C that are much higher than 4 MPa and 10 °C for P-ESO, respectively. ECHM also showed significant reinforcement effects on the thermal and mechanical properties of ESO, but DCE's reinforcement effects is doubled compared to ECHM.

Facile fabrication of superhydrophobic paper with improved physical strength by a novel layer-by-layer assembly of polyelectrolytes and lignosulfonates-amine

Interest in superhydrophobic paper is growing, and efforts have been made to prepare superhydrophobic paper with an improved physical strength property. This study reported a facile method for fabricating a superhydrophobic paper with enhanced physical strength through layer-by-layer self-assembly of poly(allylamine hydrochloride) (PAH) and lignosulfonates-amine (LSA) on cellulose fiber surfaces, followed by a heat treatment at 160 °C for 30 min. The formation of PAH/LSA multilayers on cellulose fiber surfaces was confirmed by X-ray photoelectron spectroscopy (XPS), zeta potential measurement and atomic force microscopy (AFM). The XPS results showed that the contents of the two characteristic elements (i.e., Cl and S) increased with increasing bilayer numbers. The zeta potential of modified fibers was regularly inversed after each layer deposition. The AFM phase image revealed that the surface root-mean-square roughness of modified cellulose increased along with the increasing bilayer number. Moreover, the wetting properties and physical strength of handsheets prepared from modified cellulose fibers with and without heat treatment were tested, respectively. The result showed that the water contact angle was higher when LSA was in the outermost layer than when PAH was in the outermost layer, and the heat treatment improved the hydrophobicity for all the handsheets. The non-heat-treated handsheet prepared from (PAH/LSA)5 multilayer-modified fibers gave a water contact angle of 120.3o, and then after heat treatment, the contact angle reached up to 151.7o, indicating that high temperature treatment induced the most hydrophobic long chain groups in LSA macromolecules orientated to the multilayer-air interface. The tensile strength properties of both non-heat-treated and heat-treated handsheets made from PAH/LSA-modified fibers were significantly enhanced. When a (PAH/LSA)5 multilayer was deposited on the cellulose fiber surface, the tensile strength increased by 33.9 % for non-heat-treated handsheets and by 57.8 % for heat-treated handsheets compared to that of handsheets made from original fibers, respectively.

Influence of the active layer nanomorphology on device performance for ternary PbSxSe1−x quantum dots based solution-processed infrared photodetector

In this paper, the influence of the active layer nanomorphology on device performance for ternary PbSxSe1−x quantum dot-based solution-processed infrared photodetector is presented. Firstly, ternary PbSxSe1−x quantum dots (QDs) in various chemical composition were synthesized and the bandgap of the ternary PbSxSe1−x QDs can be controlled by the component ratio of S/(S + Se), and then field-effect transistor (FET) based photodetectors Au/PbS0.4Se0.6:P3HT/PMMA/Al, in which ternary PbS0.4Se0.6 QDs doped with poly(3-hexylthiophene) (P3HT) act as the active layer and poly(methyl methacrylate) (PMMA) as the dielectric layer, were presented. By changing the weight ratio of P3HT to PbS0.4Se0.6 QDs (K = MP3HT:MQDs) in dichlorobenzene solution, we found that the device with K = 2:1 shows optimal electrical property in dark; however, the device with K = 1:2 demonstrated optimal performance under illumination, showing a maximum responsivity and specific detectivity of 55.98 mA W−1 and 1.02 × 1010 Jones, respectively, at low VDS = −10 V and VG = 3 V under 980 nm laser with an illumination intensity of 0.1 mW cm−2. By measuring the atomic force microscopy phase images of PbS0.4Se0.6:P3HT films in different weight ratio K, our experimental data show that the active layer nanomorphology has a great influence on the device performance. Also, it provides an easy way to fabricate high performance solution-processed infrared photodetector.