Hyeun Hwan An

The growth mechanism for silicon oxide nanowires synthesized from an Au nanoparticle/polyimide/Si thin film stack

During pyrolysis of polyimide (PI) thin film, amorphous silicon oxide nanowires (SiO(x)NWs) were produced on a large scale through heat treatment of an Au nanoparticle/PI/Si thin film stack at 1000 °C. It was shown that carbonization of the PI film preceded the nucleation of the SiO(x)NWs. The formation of the SiO(x)NWs was sustained by the oxygen derived from carbonization of the polyimide thin film while Si was provided from the substrate. Au nanoparticles promoted the SiO(x)NW growth by inducing localized melting of the Si substrate and by catalyzing the nanowire growth.

Direct deposition of size-tunable Au nanoparticles on silicon oxide nanowires

Silicon oxide nanowires (SiO(X)NWs) was decorated with 3- to 6-nm-sized Au nanoparticles by direct deposition of Au on the nanowire surface without any surface pretreatment. The nanowire surface was uniformly coated with Au nanoparticles with a density of 2x10(12) particles cm(-2). It was found that the Au nanoparticles grown on the SiO(X)NWs was much finer in size compared to the Au nanoparticles deposited on a flat SiO(2) substrate because of the high curvature of the nanowires and abundant surface defects resulting from the oxygen non-stoichiometry. Moreover, the particle size as well as the particle density can be tuned at will by simply altering the growth condition of the SiO(X)NWs. The tuning of the Au particle provides an opportunity to tailor the Au nanoparticles for desired applications.

Enhanced photoluminescence of silicon oxide nanowires brought by prolonged thermal treatment during growth

Silicon oxide nanowires synthesized during carbonization of polyimide thin film on a silicon substrate exhibited marked enhancement in photoluminescence (PL) at 420 nm by prolonging the growth period. Maximum intensity was recorded when the nanowire diameter coarsened from 70 to 165 nm by extending the growth period from 1 to 3 h. The enhancement was attributed to the increase in concentration of neutral oxygen vacancies on the surface of the nanowires. It was also demonstrated that the PL peak can be shifted to 600 nm while maintaining the enhanced intensity by postannealing the nanowires in a reducing atmosphere.

Interaction of a solid supported liquid-crystalline phospholipid membrane with physical vapor deposited metal atoms

A metal (Ag, Au, Pd, Sn, Bi, In, Co, Al) layer directly deposited onto a liquid-crystalline lipid membrane resulted in different morphologies, ranging from a monolayer of discrete particles to a continuous film, depending on the metals oxidation tendency.

Effect of temperature and humidity on coarsening behavior of Au nanoparticles embedded in liquid crystalline lipid membrane.

Coarsening behavior of the Au nanoparticles produced by thermal evaporation of Au onto a liquid crystalline lipid (1,2-dioleoyl-3-trimethylammonium-propane, DOTAP) membrane was investigated by subjecting the nanoparticle-embedded DOTAP membrane to two different annealing conditions (at 100 °C under no humidity and at 20 °C and 80% relative humidity). Although the coarsening rate was relatively slow because of the low temperature (from 5.6 nm in the as-deposited state to ~7 nm after 30 h), it was identified that at 100 °C without humidity the Au nanoparticles resulted in shape refinement whereas the high humidity at 20 °C induced self-organization of the nanoparticles into a monolayer. It was also found that annealing in both cases tended to segregate the lipid molecules from the nanoparticle array and forced the nanoparticles into a tighter area. In the case of the high-humidity sample, the lipid segregation eventually led to extensive coalescence of the Au nanoparticles.

Structure of solid-supported lipid membrane probed by noble metal nanoparticle deposition.

Direct deposition of a noble metal layer onto a solid-supported membrane was proposed as an indirect microscopy tool to visually observe different lipid phases that may develop in the lipid membrane. The method relied on the different permeability of the lipid membrane towards the incident atoms during deposition. Liquid state or structural defects such as phase boundaries, step ledges in a multi-lamellar stack, and pores permitted the metal atoms to penetrate and nucleate inside the membrane whereas rigid gel state was relatively impermeable to the incident atoms, thus enabling visualization of liquid phase or structural defects inside the gel state. Based on the proposed method, we demonstrated the phase states resulting from thermotropic transitions of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), dioleoylphosphatidylethanolamine (DOPE)/DPPC mixture, and 1,2-dioleoyl-3-trimethylammonium propane (DOTAP). Although the proposed method does not allow in-situ observation of equilibrium states, the method should be an excellent complementary tool for visualizing the lipid phases as the method can resolve fine structural details (up to tens of nanometer scale) as seen in the DPPC membrane while providing macroscopic images (up to several micrometers).

Structure and magnetic properties of low-temperature annealed Ni-Mn-Al alloys

Off-stoichiometric Ni2Mn2−xAlx (x = 0.5–1.2) and Ni2+y(Mn1.3Al0.7)1−y/2, (y = 0.04–0.12) alloys were prepared to locate the composition range which is likely to form the ferromagnetic ordered L21 phase through low-temperature thermal annealing. In the Ni2Mn2−xAlx (x = 0.5–1.2) series, the alloy partially transformed to the ferromagnetic phase in the narrow composition range of x = 0.8–1.1. For the Ni-enriched composition series, Ni2+y(Mn1.3Al0.7)1−y/2, (y = 0.04–0.12), magnetic ordering was not observed and the magnetic susceptibility proportionally decreased with the Ni concentration. It was concluded that moving away from stoichiometric composition would likely increase the disordering of Mn and Al site occupation and is not conducive to achieving ferromagnetism in Ni2MnAl.

Formation of Ag nanostrings induced by lyotropic liquid-crystalline phospholipid multilayer.

Morphological variation of the Ag nanoparticles embedded in a lyotropic phospholipid (1,2-dioleoyl-sn-glycero-3-phosphoethanolamine, DOPE) membrane during hydration was investigated. Hydration at 5 °C resulted in transformation of the Ag nanoparticles into a bundle of Ag nanostrings as the Ag nanoparticles conformed to the H(II) phase of the DOPE molecules. Above 30 °C, the nanoparticles quickly coarsened into large polygonal-shaped particles since high mobility of the lipid molecules overwhelmed the tendency for the Ag nanoparticles to order. The result provided an insight into the long-term stability of nanoparticles trapped in different lipid membranes depending on the structural ordering of the molecules.

Molecular dynamics simulation of interlayer water embedded in phospholipid bilayer.

1,2-dioleoyl-sn-glycero-3-phosphocholine lipid bilayer with a thin layer of water molecules inserted in the hydrophobic region was simulated at 300K to observe the pore structure formation during escape of the water molecules from the hydrophobic region. The transformation of the water slab into a cylindrical droplet in the hydrophobic region, which locally deformed the lipid monolayer, was prerequisite to the pore formation. If the thickness of the interlayer water was increased beyond a critical value, the local deformation was suppressed as such deformation would rupture the lipid bilayer. Hence, it was demonstrated that the pore structure formation or local permeability of the lipid membrane is closely related to the rigidity of the lipid membrane.

Stabilization of Solid-Supported Phospholipid Multilayer against Water by Gramicidin Addition

It was demonstrated that hydrophobicity of solid supported planar dipalmitoyl phosphatidylcholine (DPPC) phospholipid multilayer can be greatly increased by incorporating a transmembrane protein, gramicidin, into the DPPC membrane. The contact angle of deionized water droplet on the gramicidin-modified DPPC membrane increased from 0° (complete wetting) without gramicidin to 55° after adding 15 mol % gramicidin. The increased hydrophobicity of the gramicidin-modified DPPC membrane allowed the membrane to remain stable at the air/water interface as well as underwater. The Au nanoparticles deposited on the gramicidin-modified DPPC membrane reproduced the characteristic surface plasmon resonance peak after being kept underwater or in phosphate-buffered saline solution for 5 days, attesting to the membrane stability in an aqueous environment. The enhanced underwater stability of the lipid multilayer substantially broadens the potential application of the lipid multilayer which includes biosensing, enzymatic fuel cell, surface enhanced Raman spectroscopy substrate.