Xiao-Min Yang

Structural investigation of Langmuir-Blodgett monolayers of L-α-dipalmitoylphosphatidylcholine by atomic force microscopy

Atomic force microscopy (AFM) was applied to observe the formation of solid phase domains from liquid phase regions on compression of Langmuir-Blodgett (LB) monolayers of L-α-dipalmitoylphosphatidylcholine (L-α-DPPC). The essential features in monolayers are remarkably similar to those observed by the fluorescence technique. However, many more detailed structural features are directly observed which cannot be resolved by fluorescence microscopy. Besides, AFM images clearly reveal that the chiral domain morphologies, which are directly related to the enantiomorphic configuration of the lipids composing the monolayer, remain intact upon transfer to solid substrates. More importantly, the observation of these rotative solid phase grains nearby chiral domains leads us to believe that there exist chiral nearest neighbor forces between chiral molecules, which can produce strongly chiral domain shapes. Finally, the influence of the compression rate on shape changes as well as microstructure within a single domain was also investigated using AFM.

In-situ observation of the electrochemical Hg/Au amalgam process on an Au electrode surface by atomic force microscopy

Abstract Atomic force microscopy (AFM) has been applied to investigate in situ the Hg/Au amalgam process by depositing the excess Hg atoms on an Au electrode surface in an acidic solution, which has led to form an Au/Hg alloy layer. We found that the formation of alloy layer proceeded through an island-growth mechanism and the growth of these islands is always fed by dissolution of nearby gold clusters. Based on AFM observations, a simple model of the Hg/Au amalgam process is proposed.

Observation of chiral domain morphology in a phospholipid Langmuir-Blodgett monolayer by atomic force microscopy

Abstract Using atomic force microscopy, we directly observe monolayer Langmuir-Blodgett (LB) films of dipalmitoylphosphatidylcholine (DPPC) deposited onto a solid substrate. Under specified deposition conditions, we find that the lipid monolayers form two-dimensional chiral morphologies, most with 2-,3-, 4- or 6-fold rotation symmetry, which is directly related to the enantiomorphic configuration of the lipids composing the monolayer. In addition, the high resolution AFM images also reveal the fine structure of the domains in the two-phase coexistence region as well as within a single sollid phase domain, which cannot be identified by optical microscopic techniques.

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.

In-situ atomic force microscope observation of stripping of mercury from HgAu alloy films in acidic media

We report the first in-situ atomic force microscope (AFM) observation of the stripping process of mercury occurring in HgAu alloy films. AFM images revealed that stripping preferentially occurred at the edges of defect sites during the initial stages and this process directly led the electrode surface to roughen with a multiple pit-like structure on nanometer scale. The formation mechanism of such structure is also discussed.

Morphologies of phase-separated monolayers investigated by atomic force microscopy

Abstract The miscibility behavior and surface morphology of a mixed monolayer, stearic acid (SA) with 5,10,15-triphenyl-20-(4-DL-phenylalanylamindo)phenyl porphyrin (TPPP), were studied by analyzing the surface pressure vs. molecular area isotherms and atomic force microscopy (AFM) images of the transferred monolayer films. The mixed monolayer is a two-phase system with a liquid-condensed phase consisting of SA and a liquid-expanded phase consisting of TPPP with a trace of SA. The different domain structures of SA sensitive to surface pressure, compression rate, annealing, etc. were also studied with AFM in detail.

Liquid crystal alignment on polymer Langmuir-Blodgett films: the side chain effects

Abstract By virtue of the flexibility of the Langmuir-Blodgett (LB) technique we prepare LB films of polyamic acid alkylamine salt with different structures and use them as alignment layers for liquid crystals. The alignment ability depends upon the controllability of the structure. Also the side chain effects are discussed.

Molecular images of phospholipid Langmuir-Blodgett films by atomic force microscopy

Abstract Langmuir-Blodgett films of dimyristoylphosphatidylcholine (DMPC) have been studied on both macroscopic and microscopic scales by atomic force microscopy in the LE-LC coexisting phase (liquid expanded and liquid condensed phase). Macroscopic scale images show that Langmuir-Blodgett films of DMPC have many similar domain structures as observed in fatty acids Langmuir-Blodgett films. In addition, it is observed that some phospholipid molecules crystalize out. A molecularly resolved image was only obtained in LC phase while in the LE phase, no molecularly resolved image was obtained because the films were destroyed by the tip interaction.

Observation of elongated and aligned polymer chains in a two-dimensional system by atomic force microscopy

Abstract Elongational effects on polymer chains in the polyamic acid alkylanine salt monolayer on the air-water interface are studied. Polymer chains elongated by a flow transferred onto graphite are observed by atomic force microscopy. We found that when the polymers are compressed to a condensed state by the elongational flow their chains are aligned in the flow direction.

Atomic force microscopy studies of domain structures in phase-separated monolayers

Abstract Domain structures were studied with atomic force microscopy (AFM) in binary phase-separated monolayer films composed of 5, 10, 15-triphenyl-20-(4-dl-α-phenylalanylamindo) phenyl porphyrin (TPPP) and one of a series of fatty acids which are arachidic acid (AA), palmitic acid (PA), and lauric acid (LA). The liquid-condensed (LC) domain structures of AA and PA were observed in their corresponding mixed monolayers. However, instead of the fatty acid domain, a liquid-expanded (LE) domain structure of TPPP appears in the mixed monolayer of LA/TPPP.