Mechanical and electrical properties of CdTe tetrapods studied by atomic force microscopy

The mechanical and electrical properties of CdTe tetrapod-shaped nanocrystals have been studied with atomic force microscopy. Tapping mode images of tetrapods deposited on silicon wafers revealed that they contact the surface with three of its arms. The length of these arms was found to be 130+/-10 nm. A large fraction of the tetrapods had a shortened vertical arm as a result of fracture during sample preparation. Fracture also occurs when the applied load is a few nanonewtons. Compression experiments with the atomic force microscope tip indicate that tetrapods with the shortened vertical arm deform elastically when the applied force was less than 50 nN. Above 90 nN additional fracture events occurred that further shortened the vertical arm. Loads above 130 nN produced irreversible damage to the other arms as well. Current-voltage characteristics of tetrapods deposited on gold revealed a semiconducting behavior with a current gap of approximately 2 eV at low loads (<50 nN) and a narrowing to about 1 eV at loads between 60 and 110 nN. Atomistic force field calculations of the deformation suggest that the ends of the tetrapod arms are stuck during compression so that the deformations are due to bending modes. Empirical pseudopotential calculation of the electron states indicates that the reduction of the current gap is due to electrostatic effects, rather than strain deformation effects inside the tetrapod.     

Probing small unilamellar EggPC vesicles on mica surface by atomic force microscopy

Sonicated small unilamellar egg yolk phosphatidylcholine (EggPC) vesicles were investigated using atomic force microscopy (AFM) imaging and force measurements. Three different topographies (convex, planar, and concave shape) of the EggPC vesicles on the mica surface were observed by tapping mode in fluid, respectively. It was found that the topography change of the vesicles could be attributed to the interaction force between the AFM tip and vesicles. Force curves between an AFM tip and an unruptured vesicle were obtained in contact mode. During approach, two breaks corresponding to the abrupt penetration of upper and lower bilayer of vesicle were exhibited in the force curve. Both breaks spanned a distance of around 4 nm close to the EggPC bilayer thickness. Based on Hertz analysis of AFM approach force curves, the Young's modulus (E) and the bending modulus (kc) for pure EggPC vesicles were measured to be (1.97 +/- 0.75) x 10(6)Pa and (0.21 +/- 0.08) x 10(-19)J, respectively. The results show that the AFM can be used to obtain good images of intact and deformed vesicles by tapping mode, as well as to probe the integrity and bilayer structure of the vesicles. AFM force curve compare favorably with other methods to measure mechanical properties of soft samples with higher spatial resolution.     

Mechanical properties of cellulose nanomaterials studied by contact resonance atomic force microscopy

Quantification of the mechanical properties of cellulose nanomaterials is key to the development of new cellulose nanomaterial based products. Using contact resonance atomic force microscopy we measured and mapped the transverse elastic modulus of three types of cellulosic nanoparticles: tunicate cellulose nanocrystals, wood cellulose nanocrystals, and wood cellulose nanofibrils. These modulus values were calculated with different contact mechanics models exploring the effects of cellulose geometry and thickness on the interpretation of the data. While intra-particle variations in modulus are detected, we did not observe a measureable difference in modulus between the three types of cellulose particles. Improved practices and experimental complications for the characterization of cellulosic nanomaterials with atomic force microscopy are discussed.     

Mechanical properties of polyelectrolyte-filled multilayer microcapsules studied by atomic force and confocal microscopy

By using a combination of atomic force and confocal microscopy, we explore the deformation properties of multilayer microcapsules filled with a solution of strong polyelectrolyte. Encapsulation of polyelectrolyte was performed by regulation of the multilayer shell permeability in water-acetone solutions. The "filled"capsules prepared by this method were found to be stiffer than "hollow" ones, which reflects the contribution of the excess osmotic pressure to the capsule stiffness. The force-deformation curves contain three distinct regimes of reversible, partially reversible, and irreversible deformations depending on the degree of compression. The analysis of the shape of compressed capsules and of the inner polyelectrolyte spacial distribution allowed one to relate the deformation regimes to the permeability of the multilayer shells for water and inner polyelectrolyte at different stage of compression.     

Temperature-dependent formation of octadecylsiloxane self-assembled monolayers on mica as studied by atomic force microscopy

We have investigated the growth of octadecylsiloxane (ODS) self-assembled monolayers on mica. Freshly cleaved muscovite mica and octadecyltrichlorosilane (OTS) dissolved in toluene (c = 1.0 mmol/L) have been used as substrate and precursor, respectively. The water content of the adsorption solution was between 14.6 and 16.6 mmol/L. Adsorption experiments were carried out in a temperature range between 5 and 45 degrees C, and the obtained submonolayer ODS films were characterized with atomic force microscopy (AFM). Besides the morphology of the films, also information on the surface coverage has been obtained by quantitative evaluation of the AFM images. Depending on the temperature, evidence for both ordered and disordered expanded ODS phases has been found. The pronounced maximum in surface coverage--in contrast to adsorption on silicon substrates--at a temperature of about 27 degrees C and the different morphology of the submonolayer films as compared to silicon substrates could be explained in terms of a deposition, diffusion, and aggregation (DDA) model.     

Investigation of surface changes on mica induced by atomic force microscopy imaging under liquids

Surface changes on muscovite mica induced by tip-surface interactions in atomic force microscopy (AFM) experiments under liquids are described. Investigations have been performed with AFM operated both in contact mode (CM-AFM) and in tapping mode (TM-AFM). Additionally, force-distance measurements have been carried out. In contrast to CM-AFM pronounced surface changes can be observed in TM-AFM experiments. However, TM-AFM images of areas previously scanned in contact mode show that imaging in contact mode changes the surface, too. An evaluation of force-distance measurements reveals that these changes depend on the adhesive interaction between tip and sample, which in turn strongly depends on the surrounding medium. The artefact can be avoided by changing the pH-value of the medium or by working with mixtures of ethanol and water. This greatly enhances the applicability of TM-AFM for in-situ investigation of surface processes on mica, which is a frequently used substrate for many technological and biological applications.     

Formation of aminosilane-functionalized mica for atomic force microscopy imaging of DNA

Factors affecting the functionalization of mica with aminosilanes, in particular, aminopropyltriethoxysilane (APTES-mica), formed from the vapor phase have been systematically studied. The relative humidity (RH) during vapor deposition has been varied, and postdeposition treatment through baking has been used, as well as the comparison of mono and trifunctionality, to investigate how optimal surfaces for AFM imaging of DNA are formed. It is found that the stability of the APTES layers is a consequence of lateral polymerization and not covalent attachment to the mica substrate. At low RH (<25%), DNA adopts an open, well-resolved conformation, whereas at >25% RH, DNA surface-induced condensation occurs. Contact mode AFM scratching experiments show that two main structures of the silane layer exist at different humidity: a monolayer exists at RH < 25%, and a bilayer structure exists at RH > 25%. Finally, structural changes that these two layer types undergo after baking at 150 degrees C were investigated by AFM and X-ray photoelectron spectroscopy (XPS), and these now prevented DNA from binding to the APTES-mica, except in the presence of Mg(II) ions.     

Effect of tip size on force measurement in atomic force microscopy

An atomic force microscope (AFM) has been used to study solvation forces at the solid-liquid interface between highly oriented pyrolytic graphite (HOPG) and the liquids octamethylcyclotetrasiloxane (OMCTS), n-hexadecane (n-C16H34), and n-dodecanol (n-C11H23CH2OH). Oscillatory solvation forces (F) are observed for various measured tip radii (Rtip=15-100 nm). It is found that the normalized force data, F/Rtip, differ between AFM tips with a clear trend of decreasing F/Rtip with increasing Rtip.     

Controlled rearrangement of adsorbed undecanol films on mica surfaces induced by an atomic force microscopy tip

An undecanol film adsorbed on a mica surface was found to rearrange and spread in a position-controlled way induced by a tapping mode atomic force microscopy (AFM) probe. AFM images of varying scanning times showed that before forming an ordered monolayer the undecanol molecules were adsorbed on the mica surface in the disordered and disorganized status. With the proceeding of scanning, these undecanol molecules gradually formed an ordered and flat film. Such behavior was caused by the formation of a stable film and had never been reported for other alcohols.     

Generation of nanostructures of mica supported lysozyme molecules in aqueous solution by atomic force microscopy

Nanostructures of lysozyme molecules adsorbed to mica were generated by the tip of an atomic force microscope in contact, tapping, and force-distance mode in aqueous solution. In contact mode at high ionic strength and adjusted lysozyme concentration a monolayer of defined pattern and orientation could be formed by the scan process of the tip. A lysozyme monolayer with minimal pattern size of about 60 nm was achieved by line scan. At larger loading forces besides a monolayer also 3D-aggregates of lysozyme molecules could be generated. In force-distance mode the volume of 3D-aggregates grows with increasing generation time, lysozyme concentration in the bulk phase, loading force, and frequency of up- and down-movement of the substrate toward the fixed cantilever. In tapping mode 3D-aggregates could be generated as well. It is postulated that reduction of electrostatic interaction between the oppositely charged lysozyme molecules and mica surface by sufficient high ionic strength is essential for monolayer formation. It is discussed that for the underlying mechanism of monolayer generation in contact mode lysozyme molecules of the bulk phase adsorb to the tip, become pulled off and attach to the mica surface by the scan process of the tip.     

Nanomanipulation of extended single-DNA molecules on modified mica surfaces using the atomic force microscopy

The technique of nanometer scale manipulation is very important in constructing nano-structures and nano-devices. By using atomic force microscope, three kinds of controllable manipulation on single-DNA molecules were introduced in the paper. DNA molecules deposited and extended on modified mica surface were first imaged by tapping mode, then cutting, bending, and pushing were respectively performed on single-DNA molecules. The results of the manipulation depend on the interaction between tip and DNA as well as between substrate and DNA.     

Adhesive properties of Staphylococcus epidermidis probed by atomic force microscopy

Mapping of the surface properties of Staphylococcus epidermidis and of biofilm forming bacteria in general is a key to understand their functions, particularly their adhesive properties. To gain a comprehensive view of the structural and chemical properties of S. epidermidis, four different strains (biofilm positive and biofilm negative strains) were analyzed using in situ atomic force microscopy (AFM). Force measurements performed using bare hydrophilic silicon nitride tips disclosed similar adhesive properties for each strain. However, use of hydrophobic tips showed that hydrophobic forces are not the driving forces for adhesion of the four strains. Rather, the observation of sawtooth force-distance patterns on the surface of biofilm positive strains documents the presence of modular proteins such as Aap that may mediate cell adhesion. Treatment of two biofilm positive strains with two chemical inhibitor compounds leads to a loss of adhesion, suggesting that AFM could be a valuable tool to screen for anti-adhesion molecules.     

Mechanical characterization of polymeric thin films by atomic force microscopy based techniques

Polymeric thin films have been awakening continuous and growing interest for application in nanotechnology. For such applications, the assessment of their (nano)mechanical properties is a key issue, since they may dramatically vary between the bulk and the thin film state, even for the same polymer. Therefore, techniques are required for the in situ characterization of mechanical properties of thin films that must be nondestructive or only minimally destructive. Also, they must also be able to probe nanometer-thick ultrathin films and layers and capable of imaging the mechanical properties of the sample with nanometer lateral resolution, since, for instance, at these scales blends or copolymers are not uniform, their phases being separated. Atomic force microscopy (AFM) has been proposed as a tool for the development of a number of techniques that match such requirements. In this review, we describe the state of the art of the main AFM-based methods for qualitative and quantitative single-point measurements and imaging of mechanical properties of polymeric thin films, illustrating their specific merits and limitations.     

Direct observation of surfactant aggregate behavior on a mica surface using high-speed atomic force microscopy

The processes of dynamic aggregate transformations of micelles at a mica surface were observed using high-speed atomic force microscopy. We showed that salt concentration affects not only the equilibrium state adsorbed structure but also the time required to reach the equilibrium state on the mica surface.     

Conformational change in an isolated single synthetic polymer chain on a mica surface observed by atomic force microscopy

The random coil conformation of an isolated conventional synthetic polymer chain was clearly imaged by atomic force microscopy (AFM). The sample used was a poly(styrene)-block-poly(methyl methacrylate) diblock copolymer. A very dilute solution of the copolymer with benzene was spread on a water surface. The structure thus formed on water was subsequently transferred and deposited onto mica at various surface pressures and observed under AFM. The AFM images obtained with films deposited at a low surface pressure (<0.1 mN/m) showed a single polystyrene (PS) block chain aggregated into a single PS particle with a single poly(methyl methacrylate) (PMMA) block chain emanating from the particle. Immediately after the deposition, the single PMMA block chain aggregated to form a condensed monolayer around the polystyrene particles. However, after exposing the deposited film to highly humid air for 1 day, the PMMA chains spread out so that the single PMMA block chain could be identified as a random coil on the substrate. The thin water layer formed on the mica substrate in humid air may enable the PMMA block chain to be mobilized on the substrate, leading to the conformational rearrangement from the condensed monolayer conformation to an expanded and elongated coil. The elongation of the PMMA chain was highly sensitive to the humidity; the maximum elongation was obtained at 79% relative humidity. The elongation was a slow process and took about 20 h.     

An atomic force microscopy study of asphaltenes on mica surfaces. Influence of added resins and demulsifiers

 Monolayers of asphaltene and resins on the water surface have been transferred at a surface pressure of 10 mN/m onto mica substrates using the Langmuir–Blodgett technique. Atomic force microscopy (AFM) has been used to examine the topography of these layers. Monolayers consisting of pure asphaltene fractions provide a rigid film with a close-packed structure, while the resins build up a continuous open network. Mixed films of these two fractions show that a gradual increase in resin concentration leads to an opening of the rigid asphaltene structure towards a more resin like configuration. Increased aggregation when the two heavy functions are present in one film is seen as larger individual units in the AFM pictures. Addition of high-molecular-weight demulsifiers/inhibitors results in the same kind of influence on the asphaltene film as seen with the resins. Received: 30 April 1999 Accepted: 29 November 1999