Scanning Force Microscopy Measurements Research Articles

Effect of Ag doping on the structural, electrical and optical properties of ZnO grown by MOCVD at different substrate temperatures

ZnO films and nanostructures were deposited on Si substrates by MOCVD using single source solid state zinc acetylacetonate (Zn(AA)) precursor. Doping by silver was realized in-situ via adding 1 and 10 wt. % of Ag acetylacetonate (Ag(AA)) to zinc precursor. Influence of Ag on the microstructure, electrical and optical properties of ZnO at temperature range 220–550 °C was studied by scanning, transmission electron and Kelvin probe force microscopy, photoluminescence and four-point probe electrical measurements. Ag doping affects the ZnO microstructure via changing the nucleation mode into heterogeneous and thus transforming the polycrystalline films into a matrix of highly c-axis textured hexagonally faceted nanorods. Increase of the work function value from 4.45 to 4.75 eV was observed with Ag content increase, which is attributed to Ag behaviour as a donor impurity. It was observed, that near-band edge emission of ZnO NS was enhanced with Ag doping as a result of quenching deep-level emission. Upon high doping of ZnO by Ag it tends to promote the formation of basal plane stacking faults defect, as it was observed by HR TEM and PL study in the case of 10 wt.% of Ag. Based on the results obtained, it is suggested that NS deposition at lower temperatures (220–300 °C) is more favorable for p-type doping of ZnO.

Spin Lifetimes Exceeding 12 ns in Graphene Nonlocal Spin Valve Devices.

We show spin lifetimes of 12.6 ns and spin diffusion lengths as long as 30.5 μm in single layer graphene nonlocal spin transport devices at room temperature. This is accomplished by the fabrication of Co/MgO-electrodes on a Si/SiO2 substrate and the subsequent dry transfer of a graphene-hBN-stack on top of this electrode structure where a large hBN flake is needed in order to diminish the ingress of solvents along the hBN-to-substrate interface. Interestingly, long spin lifetimes are observed despite the fact that both conductive scanning force microscopy and contact resistance measurements reveal the existence of conducting pinholes throughout the MgO spin injection/detection barriers. Compared to previous devices, we observe an enhancement of the spin lifetime in single layer graphene by a factor of 6. We demonstrate that the spin lifetime does not depend on the contact resistance area products when comparing all bottom-up devices indicating that spin absorption at the contacts is not the predominant source for spin dephasing.

Mobility of a supported lipid bilayer on dispersed single‐walled carbon nanotubes

The mobility of lipid bilayers supported on single‐walled carbon nanotube (SWCNT) networks on mica has been investigated by fluorescence recovery after photobleaching (FRAP). The carbon nanotubes have been dispersed by two different bile salt derivatives (sodium taurodeoxycholate (TDOC) and sodium cholate (SC)) as well as by single‐stranded deoxyribonucleic acid (ssDNA) oligonucleotides. Whereas the diffusion coefficient of the supported lipid bilayer (SLB) is not influenced by TDOC‐dispersed SWCNTs, for SC‐ and ssDNA‐dispersed SWCNTs, it is reduced and becomes SWCNT concentration dependent. Scanning force microscopy (SFM) measurements reveal that the lipid bilayer is not affected, when spanning over TDOC‐dispersed SWCNTs, whereas SC‐ and ssDNA‐dispersed SWCNTs are only bridged by a lipid monolayer acting as diffusion barrier. The effect is more pronounced for ssDNA‐dispersed SWCNTs.Schematic viewgraph of lipid bilayer supported on a SWCNT network on mica.

Copper(ii) and zinc(ii) dependent effects on Aβ42 aggregation: a CD, Th-T and SFM study

Aβ aggregation is a central event in Alzheimer's disease (AD). In vitro evidence indicates that Aβ aggregation and fibrillogenesis are significantly influenced by the employed experimental conditions. Indeed, although it is widely established that metal ions, such as copper and zinc, have significant effects on the Aβ aggregation process, their actual role in Aβ fibrillogenesis is still debated. In this work the effects of a molar excess of zinc(II) and/or copper(II) ions on the Aβ42 aggregation process and the morphology of the resultant aggregates have been compared in samples exhibiting different initial conformations. CD spectroscopy, Th-T-induced fluorescence and Scanning Force Microscopy (SFM) measurements indicated that both metal ions accelerate the aggregation process, yet significantly affect the morphology of aggregates. In particular, copper(II) ions were the most effective in promoting non-fibrillar, amorphous Aβ aggregates. These results further support the hypothesis that an altered distribution of metal ions in neurons might drive alternative Aβ aggregation pathways.

Synthesis and characterization of cationic silsesquioxane hybrids by hydrolytic condensation of triethoxysilane derived from 2-(dimethylamino)ethyl acrylate

A new family of silsesquioxane hybrids was synthesized by hydrolytic condensation of a triethoxysilane precursor, R-Si(OCH2CH3)3, R = -CH2CH2CH2N[CH2CH2COOCH2CH2N(CH3)2]2, derived from 2-(dimethylamino)ethyl acrylate. Condensation of the triethoxysilane precursor proceeded as a homogeneous system in methanol in the presence of aqueous HF solution (3.2 %) to afford the water-soluble silsesquioxane hybrid having a high density of chemically bonded peripheral tertiary amino groups on the outermost surface, as confirmed by nuclear magnetic resonance and Fourier transform infrared analyses. The relatively low polydispersity (M w/M n = 1.33) and a reasonable molecular weight (M n = 2700), corresponding to species having 6–12 silicon atoms, were confirmed by size exclusion chromatography. The size of the silsesquioxane hybrid (1.7 nm) was also determined by X-ray diffraction. Co-condensation of tetraethoxysilane (TEOS) with the triethoxysilane precursor was carried out under different feed ratios, and water-soluble products were obtained in the cases of TEOS molar ratio up to 40 %. Quaternization reaction of the tertiary amine-containing hybrids with methyl iodide led to cationic silsesquioxane hybrids containing quaternized amine functionalities, which showed good solubility in polar solvents. Scanning force microscopy measurements indicated the formation of the cationic silsesquioxane hybrids having relatively narrow size distribution with average particle diameter (about 2.0 nm) without aggregation.

Interface Induced Crystal Structures of Dioctyl-Terthiophene Thin Films

Temperature dependent structural and morphological investigations on semiconducting dioctyl-terthiophene (DOTT) thin films prepared on silica surfaces reveals the coexistence of surface induce order and distinct crystalline/liquid crystalline bulk polymorphs. X-ray diffraction and scanning force microscopy measurements indicate that at room temperature two polymorphs are present: the surface induced phase grows directly on the silica interface and the bulk phase on top. At elevated temperatures the long-range order gradually decreases, and the crystal G (340 K), smectic F (348 K), and smectic C (360 K) phases are observed. Indexation of diffraction peaks reveals that an up-right standing conformation of DOTT molecules is present within all phases. A temperature stable interfacial layer close to the silica-DOTT interface acts as template for the formation of the different phases. Rapid cooling of the DOTT sample from the smectic C phase to room temperature results in freezing into a metastable crystalline state with an intermediated unit cell between the room temperature crystalline phase and the smectic C phase. The understanding of such interfacial induced phases in thin semiconducting liquid crystal films allows tuning of crystallographic and therefore physical properties within organic thin films.

Formation of metallic colloids in CaF2 by intense ultraviolet light

Highest purity CaF2 single crystals are irreversibly modified when irradiated with millions of pulses of 193 nm light at fluences of 120 mJ/cm2. Mie theory explains the observed haze by attributing the wavelength dependent extinction and the ratio between absorption and scattering to metallic colloids with radii in the range of 20 to 30 nm and a fractional volume of up to 2.8·10-7. Non-contact scanning force microscopy (NC-AFM) measurements performed on a surface produced by in-vacuo cleavage reveals that laser irradiation additionally produces a 104 times higher volume density of colloids with a radius of 1 to 2 nm.

Human Serum Albumin Nanotubes with Esterase Activity

A nanocylindrical wall structure was obtained by layer-by-layer (LbL) assembly of poly-L-arginine (PLA) and human serum albumin (HSA) and characterized by scanning electron microscopy (SEM), scanning force microscopy (SFM), and cryogenic transmission electron microscopy (cryo-TEM). SEM and SFM measurements of a lyophilized powder of (PLA/HSA)(3) nanotubes yielded images of round, chimney-like architectures with approximately 100 nm wall thickness. Cryo-TEM images of the hydrated sample revealed that the tube walls are composed of densely packed HSA molecules. Moreover, when small-angle X-ray scattering was used to characterize the individual PLA and HSA components in aqueous solutions, maximum diameters of approximately 28 nm and 8 nm were obtained, respectively. These values indicate the minimum thickness of wall layers consisting of PLA and HSA. It can also be concluded from SEM as well as from cryo-TEM images that the protein cylinders are considerably swollen in the presence of water. Furthermore, HSA retains esterase activity if assembled in nanotubes, as indicated by measurements of para-nitrophenyl acetate hydrolysis under semi-physiological conditions (pH 7.4, 22 °C). The enzyme activity parameters (Michaelis constant, K(m), and catalytic constant, k(cat)) were comparable to those of free HSA.

Supramolecular Order of Solution‐Processed Perylenediimide Thin Films: High‐Performance Small‐Channel n‐Type Organic Transistors

AbstractN,N′‐1H,1H‐perfluorobutyl dicyanoperylenecarboxydiimide (PDIF‐CN2), a soluble and air stable n‐type molecule, undergoes significant reorganization upon thermal annealing after solution deposition on several substrates with different surface energies. Interestingly, this system exhibits an exceptional edge‐on orientation regardless of the substrate chemistry. This preferential orientation is rationalized in terms of strong intermolecular interactions between the PDIF‐CN2 molecules. The presence of a pronounced π–π stacking is confirmed by combining near‐edge X‐ray absorption fine structure spectroscopy (NEXAFS), dynamic scanning force microscopy (SFM) and surface energy measurements. The remarkable charge carrier mobility measured in field‐effect transistors, using both bottom‐ and top‐contact (bottom‐gate) configurations, underlines the importance of strong intermolecular interactions for the realization of high performing devices.

Multi-scale electrical response of silicon nitride/multi-walled carbon nanotubes composites

Dense silicon nitride (Si 3N 4) composites with various amounts (0–8.6 vol%) of multi-walled carbon nanotubes (MWCNTs) are electrically characterised by combining macroscopic dc–ac and nanoscale conductive scanning force microscopy (C-SFM) measurements. In this way, a coherent picture of the dominant charge transport mechanisms in Si 3N 4/MWCNTs composites is presented. A raise of more than 10 orders of magnitude in the electrical dc conductivity compared to the blank specimen is measured for MWCNTs contents above 0.9 vol%. Semiconductor and metallic-like behaviours are observed depending on both the temperature and the MWCNTs content. Macroscopic measurements are further supported at the nanoscale by means of C-SFM. The metallic-type conduction is associated to charge transporting along the nanotube shells, whereas the semiconductor behaviour is linked to hopping conduction across nanotube–nanotube contacts and across intrinsic defect clusters within the nanotubes.

Impact of electrostatic forces in contact-mode scanning force microscopy

In this contribution we address the question to what extent surface charges affect contact-mode scanning force microscopy measurements. We therefore designed samples where we could generate localized electric-field distributions near the surface as and when required. We performed a series of experiments where we varied the load of the tip, the stiffness of the cantilever and the hardness of the sample surface. It turned out that only for soft cantilevers could an electrostatic interaction between tip and surface charges be detected, irrespective of the surface properties, i.e., basically regardless its hardness. We explain these results through a model based on the alteration of the tip-sample potential by the additional electric field between charged tip and surface charges.

Absence of self-heated bistable resistivity inLa0.7Sr0.3MnO3films up to high current densities

The $E(J)$ characteristics of well-characterized fully strained epitaxial ${\text{La}}_{0.7}{\text{Sr}}_{0.3}{\text{MnO}}_{3}$ films have been investigated up to high current densities $(J\ensuremath{\sim}{10}^{4}--{10}^{5}\text{ }\text{A}/{\text{cm}}^{2})$. Different electric contact resistances and different current path configurations have been used to sort out the role of Joule self-heating from contact heating in the transport properties. It is demonstrated through macroscopic transport and conductive scanning force microscopy measurements that contact heating may lead to nonlinear and irreversible $E(J)$ characteristics when high contact resistances are used. Low dissipative contact power measurements are crucial to define the upper $J$ limits to keep linear and reversible $E(J)$ characteristics. We demonstrate that Joule self-heating in ${\text{La}}_{0.7}{\text{Sr}}_{0.3}{\text{MnO}}_{3}$ thin films only induces moderate warming at high temperatures while achieving bistable resistivity at low temperatures would require the use of very high current densities $(J\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}{10}^{5}\text{ }\text{A}/{\text{cm}}^{2})$.

Synthesis of polydimethylsiloxane‐containing block copolymers via reversible addition fragmentation chain transfer (RAFT) polymerization

AbstractLow polydispersity polydimethylsiloxane (PDMS) was end functionalized with a reversible addition fragmentation chain transfer (RAFT) agent by the esterification of hydroxyl terminated PDMS with a carboxylic acid functional RAFT agent. These PDMS‐RAFT agents were able to control the free radical polymerization of styrene and substituted styrene monomers to produce PDMS‐containing block copolymers with low polydispersities and targeted molecular weights. A thin film of polydimethylsiloxane‐block‐polystyrene was prepared by spin coating and exhibited a microphase separated morphology from scanning force microscopy measurements. Controlled swelling of these films in solvent vapor produced morphologies with significant long‐range order. This synthetic route will allow the straightforward production of PDMS‐containing block copolymer libraries that will be useful for investigating their thin film morphological behavior, which has applications in the templating of nanostructured materials.© 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Calcium Ions as “Miscibility Switch”: Colocalization of Surfactant Protein B with Anionic Lipids under Absolute Calcium Free Conditions

One of the main determinants of lung surfactant function is the complex interplay between its protein and lipid components. The lipid specificity of surfactant protein B (SP-B), however, and the protein's ability to selectively squeeze out lipids, has remained contradictory. In this work we present, for the first time to our knowledge, by means of time-of-flight secondary ion mass spectrometry chemical imaging, a direct evidence for colocalization of SP-B as well as its model peptide KL4 with negatively charged dipalmitoylphosphatidylglycerol under absolute calcium free conditions. Our results prove that protein/lipid localization depends on the miscibility of all surfactant components, which itself is influenced by subphase ionic conditions. In contrast to our earlier studies reporting SP-B/KL4 colocalization with zwitterionic dipalmitoylphosphatidylcholine, in the presence of even the smallest traces of calcium, we finally evidence an apparent reversal of protein/lipid mixing behavior upon calcium removal with ethylene diamine tetraacetic acid. In addition, scanning force microscopy measurements reveal that by depleting the subphase from calcium ions the protrusion formation ability of SP-B or KL4 is not hampered. However, in the case of KL4, distinct differences in protrusion morphology and height are visible. Our results support the idea that calcium ions act as a “miscibility switch” in surfactant model systems and probably are one of the major factors steering lipid/protein mixing behavior as well as influencing the protein's protrusion formation ability.

Advanced three-dimensional scan methods in the nanopositioning and nanomeasuring machine

The nanopositioning and nanomeasuring machine developed at the Ilmenau University of Technology was originally designed for surface measurements within a measuring volume of 25 mm × 25 mm × 5 mm. The interferometric length measuring and drive systems make it possible to move the stage with a resolution of 0.1 nm and a positioning uncertainty of less than 10 nm in all three axes. Various measuring tasks are possible depending on the installed probe system. Most of the sensors utilized are one-dimensional surface probes; however, some tasks require measuring sidewalls and other three-dimensional features. A new control system, based on the I++ DME specification, was implemented in the device. The I++ DME scan functions were improved and special scan functions added to allow advanced three-dimensional scan methods, further fulfilling the demands of scanning force microscopy and micro-coordinate measurements. This work gives an overview of these new functions and the application of them for several different measurements.

Synthesis and characterization of water-soluble SiO 1.5/TiO 2 hybrid nanoparticles by hydrolytic co-condensation of triethoxysilane containing hydroxyl groups

Novel R–SiO 1.5/TiO 2 hybrid nanoparticles were synthesized by hydrolytic co-condensation of titanium alkoxides (Ti(OR′) 4, R′ = ethyl, isopropyl, and butyl) with a triethoxysilane precursor, R–Si(OCH 2CH 3) 3, R = –CH 2CH 2CH 2N(CH 2CH 2COOCH 2CH 2OH) 2, derived from 2-hydroxyethyl acrylate. Co-condensation of a titanium alkoxide with the triethoxysilane precursor was investigated at different feed ratios, suggesting that water-soluble nanoparticles were obtained only at less than 30% of Ti(OEt) 4 molar ratio in the feed. In contrast, the co-condensation of titanium tetraisopropoxide, Ti(O iPr) 4, with the triethoxysilane precursor in the presence of acetylacetone proceeded as a homogeneous system until 70% of Ti(O iPr) 4 molar ratio to afford water-soluble organic–inorganic hybrid nanoparticles containing titania–silica mixed oxides, as confirmed by NMR, FT-IR, elemental and ICP analyses. Scanning force microscopy (SFM) measurements of the product prepared at Ti(O iPr) 4/triethoxysilane = 50/50 mol% with acetylacetone indicated the formation of the nanoparticles having relatively narrow size distribution with average particle diameter less than 2.0 nm without aggregation. The refractive index of the hybrid nanoparticle was 1.571. The isolated nanoparticles distributed homogeneously were visualized by transmission electron microscopy (TEM), and the size of the hybrid nanoparticle (1.9 nm) was determined by X-ray diffraction (XRD).

Influence of salt on assembly and compression of PDADMAC/PSSMA polyelectrolyte multilayers

Influence of NaCl and NaBr and their concentration on assembly of polyelectrolyte multilayers and their compression behavior was studied using poly(diallyldimethylammonium chloride) (PDADMAC) and poly(4-styrenesulfonic acid-co-maleic acid, 1 : 1 SS:MA) sodium salt (PSSMA 1 : 1) as the building blocks. Stepwise growth of the PSSMA/PDADMAC multilayers was characterized by a quartz crystal microbalance with dissipation (QCM-D), revealing faster and slower growth of the layer thickness in higher concentration of NaCl and NaBr solutions, respectively. Moreover, QCM-D, ellipsometry and scanning force microscopy (SFM) measurements demonstrated that the multilayers were highly hydrated with a factor of 3-9 in a wet state depending on the type and concentration of the salts used during the assembly. The type and concentration of the salts used during the assembly also influenced greatly the compression ratio of the polyelectrolyte multilayers, i.e. the compression ratio increased along with the increase of NaCl concentration, whereas it was constant when NaBr was used. The friction force of the PEI(PSSMA/PDADMAC)(7) multilayers measured by SFM showed that the force peaks gradually shifted from low to high scanning frequency along with the increase of NaCl concentration, illustrating the gradual softening of the multilayers. By contrast, the peak positions in the friction force curves for all the multilayers assembled in NaBr solution kept unchanged regardless of the NaBr concentration. Contrast experiments further revealed that the compression ratios, masses in wet and wet thicknesses of the multilayers alternatingly treated in Cl(-) and in Br(-) solutions were all reversible.

Specific Features of Defect Structure and Dynamics in the Cylinder Phase of Block Copolymers

We present a systematic study of defects in thin films of cylinder-forming block copolymers upon long-term thermal or solvent annealing. In particular, we consider in detail the peculiarities of both classical and specific topological defects, and conclude that there is a strong "defect structure-chain mobility" relationship in block copolymers. In the systems studied, representative defect configurations provide connectivity of the minority phase in the form of dislocations with a closed cylinder end or classical disclinations with incorporated alternative, nonbulk structures with planar symmetry. In solvent-annealed films with enhanced chain mobility, the neck defects (bridges between parallel cylinders) were observed. This type of nonsingular defect has not been identified in block copolymer systems before. We argue that topological arguments and 2D defect representation, sufficient for lamellar systems, are not sufficient to determine the stability and mobility of defects in the cylindrical phase. In-situ scanning force microscopy measurements are compared with the simulations based on the dynamic self-consistent mean field theory. The close match between experimental measurements and simulation results suggests that the lateral defect motion is diffusion-driven. In addition, 3D simulations demonstrated that the bottom (wetting) layer is only weakly involved into the structure ordering at the free surface. Finally, the morphological evolution is considered with the focus on the motion and interaction of the representative defect configurations.

Neural network approximation of tip-abrasion effects in AFM imaging

The abrasion (wear) of tips used in scanning force microscopy (SFM) directly influences SFM image quality and is therefore of great relevance to quantitative SFM measurements. The increasing implementation of automated SFM measurement schemes has become a strong driving force for increasing efforts towards the prediction of tip wear, as it needs to be ensured that the probe is exchanged before a level of tip wear is reached that adversely affects the measurement quality. In this paper, we describe the identification of tip abrasion in a system of SFM measurements. We attempt to model the tip-abrasion process as a concatenation of a mapping from the measured AFM data to a regression vector and a nonlinear mapping from the regressor space to the output space. The mapping is formed as a basis function expansion. Feedforward neural networks are used to approximate this mapping. The one-hidden layer network gave a good quality of fit for the training and test sets for the tip-abrasion system. We illustrate our method with AFM measurements of both fine periodic structures and randomly oriented sharp features and compare our neural network results with those obtained using other methods.

SFM studies of carbon ion induced damages in plasmid DNA

In this study we present for the first time detailed scanning force microscopy (SFM) investigations of carbon ion induced damages in plasmid DNA in order to obtain information about the biological effectiveness of particle radiation. For this purpose, we have combined SFM and gel electrophoresis measurements in a dose range between D = 0 Gy and 5000 Gy. After irradiation with C ions, the percentage of double-strand breaks (DSBs) increases drastically, i.e. from initially 0% for D = 0 Gy to 38% for D = 5000 Gy. Increasing the dose over the total range is accompanied by a shortening of the average fragment length from L = 1100 nm to L = 575 nm. In addition to our experiments, the average numbers of induced DSBs per irradiated plasmid and per broken plasmid have been calculated from the SFM measurements. The most important among the numerous results is that a significant amount of plasmids has suffered more than two DSBs for all applied doses, indicating multiple DSBs. The number of DSBs per broken plasmid increases from approximately 1.7 after irradiation with a dose of D = 250 Gy to 3.2 after exposure to the highest dose of D = 5000 Gy. The results provide experimental data for the spatially correlated production of DSBs after carbon irradiation, that are relevant to the understanding of its biological effectiveness.