Lattice constants of Langmuir-Blodgett films measured by atomic force microscopy

Langmuir-Blodgett (LB) films are examples of soft organic and related biological samples. Therefore it is essential to make sure that the mechanical scanning process does not disturb the specimen. We describe atomic force microscopy measurements of the lattice constants of Cd arachidate LB films. A lattice constant d_hk is revealed correctly by scanning parallel to the corresponding lattice line [hk]. Scanning in deviating directions enlarges the lattice spacing. The phenomena are explained with a simple model. Previous studies where such artefacts have not been reported are discussed.     

Domain structures of phospholipid monolayer Langmuir-Blodgett films determined by atomic force microscopy

Atomic Force Microscopy (AFM) has been used to show the formation of solid-phase domains from fluid-phase domains on compression of DiPalmitoyl-PhosphatidylCholine (DPPC) monolayer Langmuir-Blodgett (LB) films. The chiral structures on the solid substrates were observed for the first time. By applying the friction force technique, we were able to distinguish the different regions of LB films according to their elastic properties. The influence of rates of compression on the domain shape as well as the microstructure within the domain were also studied.     

Observation of phase separation of phospholipids in mixed monolayer Langmuir-Blodgett films by atomic force microscopy

We report here the first AFM studies of phase-separated monolayer Langmuir-Blodgett films composed of a mixture of L-α-dipalmitoylphosphocholine (DPPC) and peptide bond bridged dl-α-phenylalanine-tetraphenylporphyrin (PTPP) under different surface pressures. The shapes of DPPC domains as well as microstructures within domains were also directly observed by AFM.     

Thermal transitions of polymers measured by atomic force microscopy

A new method has been developed to measure thermal transitions by atomic force microscopy in the non-contact mode, using it as a dynamic mechanical analyser on a local scale. In this method the cantilever is oscillating above the polymer surface and the resonance frequency is measured as a function of the temperature. Thermal transitions of a polymer are clearly visible as a change in the characteristic-frequency behaviour of the cantilever. This paper introduces a simple model to explain the response of the cantilever caused by the transitions in the polymer and the related form of the frequency/temperature curves. This new technique adds a new dimension to the standard thermal analysis techniques, with which the thermal transitions of different polymer phases can be resolved individually for polymer blends or copolymers, for example in structured multiphase polymers. Received: 1 November 2001 / Accepted: 7 November 2001 / Published online: 23 January 2002     

Investigation of organic films by atomic force microscopy: Structural,nanotribological and electrical properties

Atomic force microscopy (AFM) has found its applications in a wide range of research fields. In this review, we show by examples that atomic force microscopy is a powerful technique to investigate structural, mechanical and electrical properties of organic films. We start with an introduction of AFM instrumentation highlighting AFM developments that are of direct relevance to organic films. Next, we review AFM studies on organic films according to their preparation methods: self-assembly, the Langmuir–Blodgett technique, solution preparation, and thermal evaporation. In the discussion on self-assembled monolayers, we focus on aspects such as structural evolution, load-induced molecular tilting, annealing, and incorporation of conjugated groups. For solution prepared organic films, we stress annealing-induced structural evolution as well as the effects of phase separation/segregation. We also briefly summarize the progress of AFM investigation on Langmuir–Blodgett films and thermally evaporated organic films. We conclude the review by providing some thoughts for future exploration. In particular, atomic force microscopy combined with ultra-flat coplanar nano-electrodes provides a promising platform to isolate single or a small number of molecular features (e.g. vacancies, defects, grain boundaries) in organic films as well as to identify the role of these features at the nanometer scale.     

Strain relaxation at cleaved surfaces studied by atomic force microscopy

Atomic force microscopy (AFM) in air is used to study the (110) cleaved surface of strained (100) In_xGa_1-xAs/ InP heterostructures for different compositions and thicknesses of the ternary compound layers. We find that the elastic strain relaxation induces a surface undulation of a few ? amplitude, even for very small misfits, provided the layers are thick enough. Using finite-element calculations of the strain relaxation near the cleaved edge, we reproduce quantitatively the AFM observations for compressive- as well as for tensile-strained layers with an accuracy better than 0.1 nm. This demonstrates the ability of AFM to quantify strain distributions by making use of surface profile measurements. Received: 9 November 1998 / Accepted: 11 March 1999 / Published online: 7 July 1999     

Elastic modulus of suspended purple membrane measured by atomic force microscopy

We have probed the mechanical properties of purple membrane (PM) in a physiological environment using the atomic force microscope (AFM). By suspending PM over nano-trenches, the elastic properties of PM can be evaluated free from the interaction with the substrate. Force-displacement curves were obtained on the suspended membrane and the data was compared to that of a simple model of a thin film over a trench. By fitting the data to the model, the elastic modulus of PM was estimated to be 8 MPa. When the membrane is repeatedly indented, we observed a change in the force-distance data consistent with damage to the two-dimensional crystal of PM. In this paper we demonstrate that the AFM allows us to evaluate the mechanics of biological membranes in their native conditions.     

Compliance profile of the human cornea as measured by atomic force microscopy

The ability to accurately determine the elastic modulus of each layer of the human cornea is a crucial step in the design of better corneal prosthetics. In addition, knowledge of the elastic modulus will allow design of substrates with relevant mechanical properties for in vitro investigations of cellular behavior. Previously, we have reported elastic modulus values for the anterior basement membrane and Descemet's membrane of the human cornea, the surfaces in contact with the epithelial and endothelial cells, respectively. We have completed the compliance profile of the stromal elements of the human cornea by obtaining elastic modulus values for Bowman's layer and the anterior stroma. Atomic force microscopy (AFM) was used to determine the elastic modulus, which is a measure of the tissue stiffness and is inversely proportional to the compliance. The elastic response of the tissue allows analysis with the Hertz equation, a model that provides a relationship between the indentation force and depth and is a function of the tip radius and the modulus of the substrate. The elastic modulus values for each layer of the cornea are: 7.5±4.2 kPa (anterior basement membrane), 109.8±13.2 kPa (Bowman's layer), 33.1±6.1 kPa (anterior stroma), and 50±17.8 kPa (Descemet's membrane). These results indicate that the biophysical properties, including elastic modulus, of each layer of the human cornea are unique and may play a role in the maintenance of homeostasis as well as in the response to therapeutic agents and disease states. The data will also inform the design and fabrication of improved corneal prosthetics.     

Silver-island films probed by hyper-Rayleigh scattering and atomic force microscopy

Incoherent second-harmonic generation, or hyper-Rayleigh scattering (HRS), and atomic force microscopy (AFM) are proposed as a combined probe of nonlinear optical and structural properties of silver-island films. HRS and linear (Rayleigh) scattering indicatrices are measured. The correlation function, deduced from the HRS indicatrices and characterising spatial fluctuations of the total polarisation at second-harmonic frequency, has a length scale significantly larger than that of the correlation functions that are obtained from the AFM data and characterise spatial fluctuations of quadratic optical susceptibilities of the dipole and quadrupole types. This difference is interpreted as indicating that the HRS indicatrix shape is determined by the long-range fluctuations of the local-field factors. Received: 16 October 2001 / Revised version: 16 April 2002 / Published online: 6 June 2002     

自组装硫醇分子膜电输运特性的导电原子力显微镜研究

在Au(111)表面自组装制备了不同链长的烷烃硫醇分子膜，并利用导电原子力显微镜研究了 自组装分子膜的输运特性随外加压力的变化.结果发现分子膜的电流随压力的增加而增大， 其变化特征可以较好地用Hertz模型描述.在相同压力和电压下，通过分子膜的电流随分子链 长的增加呈指数衰减，其衰减因子先随压力的增加而减小，后逐渐趋于稳定.此外，长链分 子自组装膜的电流随压力的变化比短链分子膜更为明显.分析表明，自组装硫醇分子膜输运 特征的压力依赖性主要源于电荷在分子膜中的链间隧穿过程. 关键词： 分子自组装 输运特性 原子力显微镜     

Analysis of the structure of block copolymer films by atomic force microscopy

The structure of thin films of the polysterene-polymethylacrylate-polysterene triblock copolymer was studied. Universal algorithms to analyze atomic-force-microscopy images of thin block-copolymer films were developed.     

Investigation of heteroepitaxial diamond films by atomic force and scanning tunneling microscopy

We report on Atomic Force Microscopy (AFM) and Scanning Tunneling Microscopy (STM) investigations on chemical vapour deposited heteroepitaxial diamond films. Besides the good macroscopic crystal morphology a statistical tilt up to ±5.2° of the oriented crystallites has been found relative to the silicon substrates. By optimizing the process conditions, however, the crystal tilt of the films can be reduced, resulting in an improved film perfection. On crystallite (001)-surfaces a substructure of growth facets or islands has been found and high resolution STM images have established a 2×1 surface reconstruction on these growth facets. AFM and SEM were applied to study the morphology of diamond nuclei initially grown on the silicon substrate. Strong island like (Volmer-Weber) growth has been found, with a nucleus height to diameter ratio of 1:1. While the islands are growing in size with respect to time of nucleation, its aspect ratio does not change, due to the high surface free energy of the diamond relative to silicon.     

Enhanced dielectric constant resolution of thin insulating films by electrostatic force microscopy

Electrostatic force microscopy has been shown to be a useful tool to determine the dielectric constant of insulating films of nanometer thicknesses that play a key role in many electrical, optical and biological phenomena. Previous approaches have made use of simple analytical formulas to analyze the experimental data for thin insulating films deposited directly on a metallic substrate. Here we show that the sensitivity of the EFM signal to changes in the dielectric constant of the thin film can be enhanced by using dielectric substrates with low dielectric constants. We present detailed numerical calculations of the tip-sample electrostatic interaction in the following setup: an insulating thin film, a dielectric substrate (or spacing layer) of known low dielectric constant and a metallic electrode. The EFM sensitivity to the dielectric constant increases with the thickness of the spacing layer and saturates for thicknesses above 100-300 nm, when it is close to that of an infinite medium.     

Investigation of ripple structures of thin polycrystalline Co films by magnetic force microscopy

Received: 14 August 1997/Accepted: 14 August 1997     

Growth of thin polymer films containing side-chain azo-dye analyzed by atomic force microscopy

Thin films containing both the azo-dye disperse red 1 (DR1) and the poly(methylmetacrylate) (PMMA) or the poly(phenylenevinylene) (PPV) are deposited by Langmuir-Blodgett and spin coating techniques on various substrates. The morphology, surface structure and growth pattern of the azo-dye-polymers were studied by atomic force microscopy. Nucleation and growth processes are responsible for the generation of different structures. The spin coated films were additionally oriented using the electric field poling method (corona poling). The film homogeneity is improved by this method in all cases. Due to the polar character of DR1 molecules, rod-like sub-structures are observed after the corona poling process in DR1-MMA films. The indium tin oxide surface enhances reorientation of the azo-dye-polymer molecules during corona poling and results in a significant reduction of the surface roughness.     

Frictional and atomic-scale study of C60 thin films by scanning force microscopy

Scanning force microscopy (SFM) was employed to characterize C₆₀ island films in an ultra-high vacuum (UHV). The initial growth stage of C₆₀ on NaCl cleavage faces and nanotribological properties of this solid lubricant are investigated. In comparison to the NaCl(001) face, higher friction is measured on the C₆₀ islands, resulting in a ratio of friction of 13 for NaClC₆₀. The friction coefficient of the (111) oriented C₆₀ island is determined to be 0.15±0.05. High-resolution SFM images reveal the hexagonal lattice of the unreconstructed (111) top surfaces and the overgrowth relationships of the C₆₀ islands.     

Surface potential images of polycrystalline organic semiconductors obtained by Kelvin probe force microscopy

P-type copper phthalocyanine (CuPc) and n-type hexadecafluorophthalocyanina-tocopper (F₁₆CuPc) polycrystalline films were investigated by Kelvin probe force microscopy (KPFM). Topographic and corresponding surface potential images are obtained simultaneously. Surface potential images are related with the local work function of crystalline facets and potential barriers at the grain boundaries (GBs) in organic semiconductors. Based on the spatial distribution of surface potential at GBs, donor- and acceptor-like trapping states in the grain boundaries (GBs) of p-CuPc and n-F₁₆CuPc films are confirmed respectively. In view of spatial energy spectrum in micro-scale provided by KPFM, it is going to be a powerful tool to characterize the local electronic properties of organic semiconductors.