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Li-Jun Wan

Publications and source records attributed to Li-Jun Wan.

At least 37 records · Page 2Linked to original sources
PubMed2005

Self-assembly of PcOC8 and its sandwich lanthanide complex Pr(PcOC8)(2) with oligo(phenylene-ethynylene) molecules.

Self-assemblies of octakis(octyloxy) phthalocyanine (PcOC8) and its sandwich lanthanide complex, substituted praseodymium bis(phthalocyanine) (Pr(PcOC8)(2)), with oligo(phenylene-ethynylene) (OPE) have been investigated by scanning tunneling microscopy (STM) on a highly oriented pyrolytic graphite (HOPG) surface. The assemblies were prepared by dissolving the molecules in phenyloctane solution. It was found that both PcOC8 and Pr(PcOC8)(2) can form 4-fold or 6-fold symmetrical adlayers on HOPG. The intramolecular structure of Pr(PcOC8)(2) molecule was revealed by a high-resolution STM image. When OPE molecules are added into phenyloctane solvent, Pr(PcOC8)(2) molecules prefer to form an ordered adlayer at the top of the OPE adlayer, while PcOC8 molecules adsorb on HOPG surface directly and form separated domains with OPE. These results may be helpful to construct surface assemblies and develop molecular electronic devices in the future.

Graphite↗
PubMed2005

Two-dimensional assemblies of banana-shaped liquid crystal molecules on HOPG surface.

Two-dimensional assemblies of a series of banana-shaped liquid crystal molecules, 1,3-phenylene bis[4-(4-n-alkylphenyliminomethyl) benzoates] (P-n-PIMB, n = 12, 14, 18), are investigated on a highly oriented pyrolytic graphite (HOPG) surface by using scanning tunneling microscopy (STM) at a submolecular level in an ambient environment. Two types of molecular arrangements are observed in these self-assemblies with different stripe widths that resulted from the organization of the alkyl chains. The polarization of the P-n-PIMB monolayers is exactly compensated by the adjacent lamellae with opposite bent direction. Furthermore, a bilayer structure is found in the assembly of P-18-PIMB molecules. In comparison with a gemini surfactant molecule, 4,4'-di[4-(octadecylmethylamino)styryl]-1,1'-pentamethylenebispyridinium dibromide (OPD), P-n-PIMB molecules appear in more complicated packing arrangements. The results in this paper demonstrate the effects of the molecular structures on the adlayer arrangement and will be helpful in fabricating two-dimensional molecular structures on solid surfaces.

Journal Article↗
PubMed2005

Light-induced structural transformation in self-assembled monolayer of 4-(amyloxy)cinnamic acid investigated with scanning tunneling microscopy.

UV light irradiation effect on the structural transformation in a self-assembled monolayer of 4-(amyloxy)cinnamic acid (AOCA) on Au(111) has been investigated by using electrochemical scanning tunneling microscopy (ECSTM), cyclic voltammetry, and infrared (IR) spectroscopy. A well-defined 4-(amyloxy)cinnamic acid adlayer with a (4 x 11) symmetry was first prepared on Au(111). After UV-light irradiation onto the adlayer, a new adlayer is observed with different molecular arrangement and a symmetry of (5 x 8). On the basis of the results from high-resolution STM image and photochemical reaction, a dimerizaion of AOCA molecules in the adlayer with structural transformation is concluded. Schematic models have been proposed for the unirradiated and irradiated adlayers, respectively. The direct evidence at molecular level about photodimerization of cinnamic acid on metal substrate is presented.

Letter↗
PubMed2005

Study of fibrinogen adsorption on self-assembled monolayers on Au(111) by atomic force microscopy.

The adsorption of plasma protein fibrinogen on the self-assembled monolayers (SAMs) of n-hexadecyl mercaptan and citrate was investigated with atomic force microscopy (AFM). On the basis of the preparation of these two flat SAMs on Au(111), high-resolution AFM images of bovine fibrinogen were obtained with different protein concentrations. The results indicated that the surface chemical composition significantly affected the adsorption behavior of fibrinogen. Since fibrinogen plays a key role in the regulation of both haemostasis and thrombosis, high-resolution AFM imaging on SAMs is expected to be an effective approach to study the haemocompatibility of materials with different surface chemistry.

Adsorption↗
PubMed2005

Ni-Pt multilayered nanowire arrays with enhanced coercivity and high remanence ratio.

Highly ordered Ni-Pt multilayered nanowire arrays have been fabricated using a porous anodic aluminum oxide (AAO) template by pulse electrodeposition. The cylindrical Ni nanoparticles with different lengths and diameters in these arrays were characterized by transmission electron microscope (TEM) and alternating-gradient magnetometer (AGM) measurements. Magnetization measurements revealed that an array of such nanowires with 20-nm diameters has an enhanced coercivity (ca. 1169 Oe) and a high remanence ratio (ca. 0.96).

Journal Article↗
PubMed2005

Gold hollow nanospheres: tunable surface plasmon resonance controlled by interior-cavity sizes.

Uniform gold hollow nanospheres with tunable interior-cavity sizes were fabricated by using Co nanoparticles as sacrificial templates and varying the stoichiometric ratio of starting material HAuCl4 over the reductants. The formation of these hollow nanostructures is attributed to two subsequent reduction reactions: the initial reduction of HAuCl4 by Co nanoparticles, followed by the reduction by NaBH4. In addition, a thick layer of silica was successfully coated onto the gold hollow nanospheres. These nanostructures are extensively characterized by TEM, XRD, HRTEM, SEM, electron diffraction, energy-dispersive X-ray analysis, and UV-visible absorption spectroscopy. It is evident that the SPR peak locations corresponding to these hollow nanospheres are shifted over a region of more than 100 nm wavelength due to changes of shell thickness, which make these optically active nanostructures of great interest in both fundamental research and practical applications.

Journal Article↗
PubMed2005

Direct evidence of molecular aggregation and degradation mechanism of organic light-emitting diodes under joule heating: an STM and photoluminescence study.

The Joule heating effect on electroluminescent efficiency is important in the degradation origin of organic light-emitting diodes (OLED). Scanning tunneling microscopy (STM) and photoluminescence (PL) measurements were performed on the guest molecule BT (1,4-bis(benzothiazole-vinyl) benzene), host molecule TPBI (2, 2',2' '-(1,3,5-phenylene)tris-[1-phenyl-1H-benzimidazole]), and their mixture deposited on an HOPG surface to study the OLED degradation mechanism due to thermal heating. At room temperature, BT and TPBI in the mixed layer show good compatibility and high PL intensity, but at higher temperatures, they show phase separation and aggregation into their own domains and a concomitant decrease in PL intensity. The PL intensity loss suggests ineffective energy transfer from TPBI to BT due to phase separation, which may cause OLED degradation. Scanning tunneling spectroscopy (STS) results show that the band gaps of TPBI and BT remain unchanged with the annealing temperature, suggesting that the heat-induced decay of OLED is related to the interfacial structural change rather than the respective molecular band gap. The results provide direct evidence showing how the molecular structures of the mixed layer vary and affect the PL intensity due to temperature.

Journal Article↗
PubMed2005

Mesoscopic self-organization of a self-assembled supramolecular rectangle on highly oriented pyrolytic graphite and Au(111) surfaces.

A self-assembled supramolecular metallacyclic rectangle was investigated with scanning tunneling microscopy on highly oriented pyrolytic graphite and Au(111) surfaces. The rectangles spontaneously adsorb on both surfaces and self-organize into well ordered adlayers. On highly oriented pyrolytic graphite, the long edge of the rectangle stands on the surface, forming a 2D molecular network. In contrast, the face of the rectangle lays flat on the Au(111) surface, forming linear chains. The structures and intramolecular features obtained through high-resolution scanning tunneling microscopy imaging are discussed.

Gold↗
PubMed2004

Adsorption and coordination of tartaric acid enantiomers on Cu(111) in aqueous solution.

Chiral modifiers have gained much attention because they can induce high enantioselectivity on reactive metal surface in heterogeneous enantioselective catalysis. The high enantioselectivity is attributed to that the chirality of modifiers is bestowed onto the metal surface upon adsorption. Much study on the adsorption of modifiers on metal surface has been performed in an ultrahigh vacuum. In this paper, the adsorption of tartaric acid on Cu(111) has been studied by electrochemical scanning tunneling microscopy (STM) in aqueous solution. It is found that (R,R)-tartaric acid and (S,S)-tartaric acid can form a well-ordered adlayer on the Cu(111) surface with a (4 x 4) symmetry. A dimeric structure is proposed in the temporary model from STM observation.

Letter↗
PubMed2004

Controllable AuPt bimetallic hollow nanostructures.

We describe a facile procedure for one step, large-scale synthesis of AuPt bimetallic hollow tube-like 1-D nanomaterials and hollow nanospheres, which can be easily manipulated by merely changing the concentration of citric acid.

Journal Article↗
PubMed2004

Direct STM investigation of cinchona alkaloid adsorption on Cu(III).

Scanning tunneling microscopy (STM) combined with cyclic voltammetry has been employed to investigate the adsorption of cinchonine on Cu(111). Similar to cinchonidine, cinchonine forms a long-range ordered adlayer with (4 x 4) symmetry on the substrate. The structural details on molecular adsorption were obtained by high-resolution STM images. On the basis of the previous results and obtained STM images, the quinoline ring is proposed to lie parallel to Cu(111) and serve as an anchoring group. The chiral quinuclidine moiety extends out of the surface to facilitate the interaction with the prochiral reactants.

Adsorption↗
PubMed2004

Absolute configuration of monodentate phosphine ligand enantiomers on Cu(111).

Scanning tunneling microscopy (STM) has been employed to investigate the chirality of monophosphine compounds that are highly efficient chiral ligands in transition-metal-catalyzed organic transformations. The absolute configuration of 1-(2-diphenyphosphino-1-naphthyl)isoquinoline enantiomers with axial chirality was discriminated directly by the "marker" group, PPh(2) substitutes. Although the two enantiomer molecules adsorb on a Cu(111) surface and form well-defined (4 x 4) structures, the positions of PPh(2) substitutes in the molecular adlayers are different. The mirror symmetry between two adlayers is demonstrated. On the basis of STM results, structural models are proposed to interpret the chiral adsorption. The results presented here supply a straightforward method for axial chirality analysis in adsorbed adlayers by STM.

Journal Article↗
PubMed2004

Electrochemical scanning tunneling microscope imaging of self-assembled monolayer of double-stranded DNA on Au(111).

Self-assembled monolayer of a double-stranded DNA, which was formed by a 20-nt 5'-thiol-modified oligonucleotide at the interface of Au(111)/solution, has been imaged and studied with electrochemical scanning tunneling microscopy (ECSTM). The results showed that the preferred directions of the adsorbed DNA were <110> and <112>. The measured width of the DNA stripes was 0.95 +/- 0.02 nm, which was consistent with the width of dsDNA base pairs. The contour of DNA in the STM image consisted of a series of blobs, which were due to the anchoring of the bases to the substrate along the dsDNA chain. The imaging of a large-scale array of DNA lying flat on the substrate offers a convenient method for the further application of high-resolution STM to the study of the interaction between DNA and a variety of molecules.

Base Composition↗
PubMed2004

Electronic characteristics of Au-mercaptohexadecanoic acid-Au junction in a capillary.

The electrical property of a self-assembled monolayer (SAM) of mercaptohexadecanoic acid (HS-C15H30-COOH) has been investigated with a tunneling junction confined in a capillary. This capillary method can shun the interference of vacuum gap. The contact area can be determined according to the diameter of capillary. The nonlinear current-voltage curve and dl/dV curve are measured in the potential range of +/-0.8 V. The slight asymmetry of I-V curve and dl/dV curve is attributed to the different coupling between the metal and the end-groups of molecule. This method will supply a simple way to measure the property of single molecules and monolayers.

Capillary Action↗