Thin liquid films studied by atomic force microscopy

Since its invention twenty years ago the atomic force microscope (AFM) has become one of the most important instruments in colloid and interface science. The ability of tracing force profiles between single particles or particles and flats in liquid environment makes it a tool-of-choice for investigating thin liquid films. In this paper we review experimental work on confined Newtonian and non-Newtonian liquids using the AFM.     

Simultaneous polarographic determination of cadmium and tellurium in electro-deposited cadmium telluride thin films

A polarographic method has been developed for the simultaneous determination of cadmium and tellurium in thin-film cadmium telluride. The procedure involves dissolution of the film with concentrated nitric acid, which is subsequently removed by evaporation. The Cd(II) and Te(IV) waves are well separated at pH 10, but sufficient ammonia must be present to prevent the precipitation of cadmium hydroxide.     

Halo-like structures studied by atomic force microscopy

Nanometer-sized clusters of copper have been produced in a hollow cathode sputtering source and deposited on SiO_x. Halo-like structures consisting of micrometer sized protrusions in the silicon oxide surface surrounded by thin rings of smaller particles are observed. The area in between seems to be depleted of particles. We propose that the halo-like structures are a result of electrostatic forces acting between the incoming charged clusters and charged regions on the surface. A simple computer simulation supports this suggestion.     

Anion intercalation into highly oriented pyrolytic graphite studied by electrochemical atomic force microscopy

Knowledge of the dimensional changes occurring during electrochemical processes is fundamental for understanding of the electrochemical intercalation/insertion mechanism and for evaluation of potential application in electrochemical devices. We studied a highly oriented pyrolitic graphite (HOPG) electrode in perchloric acid, as a model to elucidate the mechanism of electrochemical anion intercalation in graphite. The aim of the work is the local and time dependent investigation of dimensional changes of the host material during electrochemical intercalation processes on the nanometer scale. We used atomic force microscopy (AFM), combined with cyclic voltammetry, as the in situ tool of analysis during intercalation and deintercalation of perchlorate anions. According to the AFM measurements, the HOPG interlayer spacing increases by 32% in agreement with the formation of stage IV of graphite intercalation compounds, when perchlorate anions intercalate. In addition, the local aspect of the process has been demonstrated by revealing coexisting regions with different kinetics for intercalation and deintercalation processes.     

In-situ investigations on the SILAR-growth of ZnS films as studied by tapping mode atomic force microscopy

Tapping mode atomic force microscopy (TM-AFM) has been successfully used for in-situ imaging of the deposition of ZnS films with the successive ionic layer adsorption and reaction (SILAR) method. The films were deposited in-situ using the commercial TM-AFM liquid cell as a flow-through reactor. The potential of TM-AFM has been used to study the growth of ZnS on different substrates up to 50 SILAR cycles. Reactants and rinsing water were alternately exchanged in the cell by a computer controlled valve system. In comparison to earlier work performed with the conventional AFM operated in contact mode, imaging artefacts introduced by lateral shear forces can be largely eliminated with TM-AFM. On glass the roughness is observed to decrease initially until typical island formation takes place at a larger number of deposition cycles. On mica island formation can be observed right from the beginning of the process and the roughness increases with increasing number of deposition cycles.     

Confined polymer melts studied by atomic force microscopy

Using an atomic force microscope (AFM) the interaction between an AFM tip and different planar solid surfaces have been measured across a long-chain poly(dimethyl siloxane) (PDMS, M_W = 18,000 g/mol), a short-chain PDMS (M_W = 4200 g/mol), a poly(ethylmethyl siloxane) (PEMS, M_W = 16,800 g/mol), and a diblock copolymer consisting of one PDMS and one PEMS block (PDMS-b-PEMS, M_W = 15,100 g/mol). The interaction changed significantly during the first 10 h after immersing the solids in the polymer melt. This demonstrates that the time scale of structural changes at a solid surface is much slower than in the bulk. On mica and silicon oxide both polymers formed an immobilized “pinned” layer beyond which a monotonically decaying repulsive force was observed. Attractive forces were observed with short-chain PDMS on silicon oxide and PEMS on mica and silicon oxide. On the basal plane of graphite PEMS caused a stable, exponentially decaying oscillatory force.     

Electro-nanopatterning of surface relief gratings on azobenzene layer-by-layer ultrathin films by current-sensing atomic force microscopy

The electro-nanopatterning and mechanism of pattern formation in azobenzene-containing layer-by-layer (LbL) ultrathin films is described using surface probe microscopy techniques. First, arrays of nanodots were patterned on these films to investigate applied time at constant voltage bias dependence in electro-nanopatterning. The anisotropic mass transport and polar alignment of the azobenzene-containing films were observed after applying the electric field and heating the sample locally with the cantilever tip. On the basis of this novel phenomenon, small-sized surface relief gratings (SRG)s and their alignment were fabricated and observed by current-sensing atomic force microscopy. The rate of mass transport for the polymer is mainly controlled by the applied time at constant voltage bias between the cantilever and the electrode/substrate.     

Surface organization of hyperbranched polymer molecules,as studied by atomic force microscopy

The recent emergence of hyperbranched polymers has opened the door for the design of a large variety of novel, well‐controlled chain architectures. For instance, «comb‐like» and “dendritic‐like” polymers can be obtained from hyperbranched poly(chloroethyl vinyl ether)‐g‐polystyrene (PCEVE‐g‐PS) copolymers, with excellent control over the dimensions of the polystyrene lateral branches and the PCEVE backbone. In this work, the nanometer scale organization of these materials is studied by means of Tapping Mode Atomic Force Microscopy. We focus on the influence of the intrinsic molecular architecture of the hyperbranched PCEVE‐g‐PS on the organization of the material. In the case of thin deposits, we observe a layer‐by‐layer organization. On the free surface, it is possible to image single polymer molecules and to analyze their size in terms of polymer molecular weight. In most cases, the molecules are found to adopt an extended conformation and to form lamellar arrangements. We observe that the degree of lateral ordering of these molecules strongly depends on their intrinsic architecture.     

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.     

Influence of single-wall carbon nanotubes on Langmuir–Blodgett films of ferroelectric liquid crystals as studied by atomic force microscopy

Langmuir–Blodgett films of ferroelectric liquid crystal (LC) doped with low concentration of single-wall carbon nanotubes have been prepared and characterised. Pressure–area isotherms show that the films are stable and have good spreading properties. The interaction between nanotubes and LC molecules in the monolayer was increased during barrier compression, resulting in increased surface pressure. We observed phase change with increasing nanotube concentration in ferroelectric LC matrix. Atomic force microscopy profiles indicate uniform deposition of material on single crystal silicon wafer.     

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.     

SEBS aggregate patterning at a surface studied by atomic force microscopy

The morphologies of films spin coated from dilute block copolymer solution onto a mica substrate were studied by atomic force microscopy (AFM). Variables of interest were the polymer concentration, solvent, heating temperature, aging, and ultrasonic effect. It is shown that the solution concentration is the predominant factor in determining the shape of the aggregates displayed from spheres and rods to irregular patches with increasing concentration. The solubility parameter of the solvent plays an important role in modifying the distribution and the size of clusters at the surface. The structures of the aggregates at the surface are metastable, which could evolve with temperature from rodlike aggregates into regular stripes when annealed at a temperature higher than the order-disorder transition temperature of SEBS, whereas those in solution could evolve with aging and ultrasonic treatment into a more stable network structure.     

Nanotopographical changes on graphite tube surfaces in electrothermal atomic absorption spectrometry experiments as studied by atomic force microscopy

Atomic force microscope images of surfaces of graphite tubes used in chromium determinations in electrothermal atomic absorption spectrometry (ET-AAS) reveal severe topographical modifications on both the micrometer and the nanometer scale after only a few atomisation cycles. A previously undescribed nanostructure has been found on unused and also on used graphite tubes. During the first atomisation cycles the protrusion size distribution is found to become more uniform, which is in agreement with the observed increase in reproducibility of ET-AAS measurements after some 10 to 20 analysis cycles. The homogenisation of the graphite surface is interpreted as the initial phase of a secondary coating.     

Surfactant exudation in the presence of a coalescing aid in latex films studied by atomic force microscopy1

We report atomic force microscopy images of surfactant (SDS) exudation in PBMA latex films, in the presence and the absence of a coalescing aid (Texanol^?, TPM). The exudates appear as hilly islets, and at times as mountains, at the film surface. Their size and number increase upon annealing above the glass-transition temperature of the latex polymer. TPM was found to be a strong promoter of surfactant exudation at the air-polymer interface. In the absence of TPM, annealing the films for several hours at 70°C led to very little migration of surfactant to the surface at most sites in the film. When the films with structures of SDS on their surface were immersed in water, these structures disappeared. Pores, ranging in size from tens to hundreds of nm in diameter, were clearly visible in the surface of the films. These films dry from the edges of the film inward, with a propagation front concentrating the water-soluble species into a turbid, moist region in the center. At this site, the rate at which the surfactant comes to the surface is enormously enhanced over that at other sites in the film. This is likely due to the high concentration of surfactant in this region, transported there by the drying process. © 1995 John Wiley & Sons, Inc.     

Adhesion forces between protein layers studied by means of atomic force microscopy

Adhesion forces between different protein layers adsorbed on different substrates in aqueous media have been measured by means of an atomic force microscope using the colloid probe technique. The effects of the loading force, the salt concentration and pH of the medium, and the electrolyte type on the strength, the pull-off distance, and the separation energy of such adhesion forces have been analyzed in depth. Two very different proteins (bovine serum albumin and apoferritin) and two dissimilar substrates (silica and polystyrene) were used in the experiments. The results clearly point out a very important contribution of the electrostatic interactions in the adhesion between protein layers.     

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.     

Microcontact printed diaphorase monolayer on glass characterized by atomic force microscopy and scanning electrochemical microscopy

Micropatterns of diaphorase (Dp) were fabricated on glass substrates by the microcontact printing (μCP) method and characterized with atomic force microscopy (AFM) and scanning electrochemical microscopy (SECM). AFM images of the printed samples revealed that the mean height of the Dp patterns was 3–5 nm, indicating the formation of a monolayer pattern. The Dp molecules on the surface organized themselves into two-dimensional arrays. We used two kinds of inking solutions: Dp–phosphate buffer solution (PBS) (pH 7.0) and Dp–PBS (pH 7.0) with glutaraldehyde (GA, 1% v/v) as a cross-linking reagent. Although the AFM imaging showed high-quality Dp monolayer patterns in both cases, SECM measurements indicated that the enzymatic activity of Dp was almost lost when Dp–PBS with GA was used as the inking solution, whereas clear enzymatic activity was found when Dp–PBS was used.