J. Garnaes

Surface Order and Stability of Langmuir-Blodgett Films

Angstrom-resolution atomic force microscope images of Langmuir-Blodgett monolayers and multilayers of cadmium arachidate in air and under water show a dramatic change from a disordered arrangement to a crystalline lattice by the addition or removal of a single layer of molecules. The disordered surface is less stable than the ordered one to mechanical stresses such as atomic force microscopy tip forces or at the air-water contact line during contact angle measurements. The difference in the degree of order in the alkyl chains is attributed to the strong attractive interaction between headgroups in the presence of the divalent cation.

Nanoscale lithography on Langmuir–Blodgett films of behinic acid

By increasing the force when scanning with an atomic force microscope, it was possible to scratch holes in a monolayer of behinic acid. Patterns with a line width of 50 to 90 nm were ‘‘written’’ with an applied force of ≊70 nN within a few minutes. The measured depth was consistent with the height of a monolayer of behinic acid with tilted chains (≊2 nm). Ten hours later, the pattern was imaged again and no change was found when an applied force of ≊10 nN was used. Contrary to other monolayers, which often have hole defects, the behinic acid monolayer was free of such defects and seemed, in particular, well suited for nanoscale lithography. Based on several different images, a model of how the tip removes the monolayer was formulated.

Applications of atomic force microscopy to structural characterization of organic thin films

Abstract The atomic force microscope (AFM) has created exciting new possibilities for imaging thin organic films under ambient conditions at length scales ranging from tens of microns to the sub-molecular scale. We present images of thin organic films prepared by the Langmuir-Blodgett (LB) and self-assembly (SA) techniques that demonstrate the possibilities and limitations of the AFM. Atomic force microscope images of LB films show that manganese arachidate (MnA2) monolayers are short-range ordered and lead stearate (PbSt2) monolayers are long-range ordered on crystalline mica substrates, but disordered on amorphous oxidized silicon substrates. The lattice structures of PbSt2 and MnA2 monolayers on mica were previously unknown and have larger lattice parameters and molecular areas than do multilayer films of the same materials, indicating the strong interactions with the larger mica lattice. Multilayer films of PbSt2, cadmium arachidate (CdA2), and MnA2, have centered rectangular “herringbone” lattices on both silicon and mica substrates. After sufficient layers, the effect of the mica substrate is eliminated and the lattice parameters and area per molecule of films deposited on mica relax to those of multilayer films on amorphous oxidized silicon. This limiting area per molecule correlates well with the degree of ionic versus covalent bonding as estimated by the Pauling electronegativity, with barium arachidate (BaA2) > MnA2 > CdA2 > PbSt2. For BaA2 and MnA2 the increased molecular area is sufficient to induce a tilt in the molecular packing. The lattice parameters, symmetry, and area per molecule are independent of the length of the alkane chain of the fatty acid for all cations and substrates examined. AFM images also show that self-assembled monolayers of octadecyltrichlorosilane (OTS) form on mica by nucleating isolated, self-similar domains. With increasing coverage, the fractal dimension of the growing domains evolves from 1.6 to 1.8. At higher coverage, continued growth is limited by adsorption from solution.

Langmuir-Blodgett Films of a Functionalized Molecule with Cross-Sectional Mismatch Between Head and Tail

A functionalized surfactant has been investigated as floating monolayers by synchrotron x-ray diffraction and as bilayers transferred to solid supports by the Langmuir-Blodgett technique through atomic force microscopy. The transfer process is accompanied by an increase of the unit cell area (about 17 percent) and by an increase of the average domain diameter of nanometer-scale domains (about three times). The unit cell area of the floating monolayer corresponds to close packing of the head groups and a noncharacteristic packing of the tifted alkyl chains. The larger unit cell area of the bilayer film is consistent with a particular ordered packing of the alkyl chains, leaving free space for the head groups.

Nano scale defects in langmuir-blodgett film observed by atomic force microscopy

Abstract In scientific and technical applications, Langmuir-Blodgett films may be useful as nonlinear optical systems, insulating or patterning layers in microelectronics, model systems for two-dimensional phases and templates for protein crystallization. These applications are based on the assumption that a defect-free layered film structure of oriented amphiphilic molecules exists. We here present an atomic force microscopy study of nano scale defects in a typical Langmuir-Blodgett film of fatty acid with hydrocarbon chain length from C 16 to C 22 , prepared under conditions often described in the literature. On the surfaces of all films, steps due to the layered structure were found and for a hydrocarbon chain length of C 20 and C 22 a surprising number of holes, approximately one monolayer deep, were identified. Molecular resolution showed that the different layers either had the same lattice orientation or a relative orientation close to 0° or 60°. Four observed grain boundaries had a relative orientation close to 60° and the lattice structure was preserved to within less than 1 nm of the grain boundaries. A typical domain size was about 2 μm. All these departures from two-dimensional periodicity may have an important bearing on applications that rely on perfect crystallinity.

Crystallization process of Langmuir–Blodgett films of octadecylthiobenzoquinone

Atomic force microscopy images of the electron acceptor 2‐octadecylthio‐1,4‐benzoquinone prepared as 2–4 Y‐type Langmuir–Blodgett films revealed the formation of nanoscale crystals one week after deposition. The crystals have a layered structure with a total height from 3 to 15 nm and they each covered an area of <1 μm2 on the substrate. In the plane of a layer, the molecules formed a two molecular rectangular unit cell with sides of 0.51±0.01 nm and 1.35±0.03 nm. The measured step height of a layer was 3.6±0.2 nm and gives a tilt of the alkyl chains of 54±3° relative to the normal to the interface between layers. The packing density of 0.34±0.1 nm2 per molecules in the plane of a layer is significantly different from the packing density on the water subphase during deposition and the packing density found on the flat Langmuir–Blodgett bilayer of about 0.26 nm2. This means that a transition from a two‐dimensional packing to a three‐dimensional bulk crystal has taken place. This transition is more easily i...

Structural studies of Langmuir and Langmuir-Blodgett films of functionalized surfactants

Abstract Structural studies of Langmuir and Langmuir-Blodgett films of functionalized surfactants show that the organization of surfactants is strongly dependent on cross-sectional match between the functionalized headgroup and the film-forming alkyl chain parts of the molecules. The organization of a particular functionalized surfactant with a slight cross-sectional mismatch between head and tail has been investigated by synchrotron radiation in floating monolayers and by atomic force microscopy (AFM) in Langmuir-Blodgett films. The transfer process was accompanied by an increase of the oblique unit cell area by 17% and by an increase of the average diameter of perfectly ordered domains from 3 nm to approximately 10 nm. We have calculated a number of likely packing patterns of alkyl chains and the lattice parameters of the Langmuir-Blodgett films are found to be consistent with one of these packings. For comparison, the results of preliminary studies on a functionalized surfactant with a severe cross-sectional mismatch between head and tail are presented, indicating that the tail is bent around the head group thereby preventing efficient packing of the alkyl chains.