Jaegeun Noh

Synthesis of electrically conductive and superparamagnetic monodispersed iron oxide-conjugated polymer composite nanoparticles by in situ chemical oxidative polymerization

Core-shell nanocomposites composed of iron oxide (Fe3O4) nanoparticles and conjugated polymer, poly(3, 4-ethylenedioxythiophene) (PEDOT), were successfully synthesized from a simple and inexpensive in situ chemical oxidative polymerization of EDOT with Fe3O4 nanoparticles in the micellar solution of lignosulfonic acid (LSA) which serves as both a surfactant and a dopant. These nanocomposites (Fe3O4-PEDOT) were subsequently characterized for morphological, crystalline, structural, electrical and magnetic properties by high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), four-probe meter and superconductor quantum interference device (SQUID), respectively. Results show that the nanocomposites have a spherical core-shell shape, are approximately 10 nm in size and are superparamagnetic with good magnetic saturation and good electrical conductivities. Existence of Fe3O4 in the nanocomposites was confirmed by using Energy dispersive X-ray photoelectron spectroscopy (EDAX) and X-ray photoelectron microscopy (XPS). We also investigated a possible formation mechanism of the core-shell nanocomposites, and the effect of Fe3O4 nanoparticles on the electro-magnetic properties of the nanocomposites. Such novel conducting and superparamagnetic composite nanomaterials can be applied to sensors, magnetic data storage, electro-magnetic resonance wave absorption, etc.

Formation and Superlattice of Long-Range-Ordered Self-Assembled Monolayers of Pentafluorobenzenethiols on Au(111)

The formation and surface structure of pentafluorobenzenethiol (PFBT) self-assembled monolayers (SAMs) on Au(111) formed under various experimental conditions were examined by means of scanning tunneling microscopy (STM). Although it is well known that PFBT molecules on metal surfaces do not form ordered SAMs, we clearly revealed for the first time that the adsorption of PFBT on Au(111) at 75 degrees C for 2 h yields long-range, well-ordered self-assembled monolayers having a (2 x 5 square root(13))R30 degrees superlattice. Our results will provide new insight into controlling the structural order of PFBT SAMs, which will be very useful in precisely tailoring the interface properties of metal surfaces in electronic devices.

Self-assembled monolayers of benzenethiol and benzenemethanethiol on Au(1 1 1): Influence of an alkyl spacer on the structure and thermal desorption behavior

The surface structures, adsorption conditions, and thermal desorption behaviors of benzenethiol (BT) and benzenemethanethiol (BMT) self-assembled monolayers (SAMs) on Au(111) were examined by means of scanning tunneling microscopy (STM), X-ray photoelectron microscopy (XPS), and thermal desorption spectroscopy to understand the effects of the alkyl spacer between the phenyl group and the sulfur atom. Although XPS spectral shapes in the S 2p region for both SAMs are similar, the surface structures and thermal desorption behaviors differ significantly. BT SAMs on Au(111) were composed of disordered phases, whereas BMT SAMs have well-ordered phases containing vacancy islands. The strong desorption peak for parent mass species (m/z=110,C(6)H(5)SH(+)) was observed in BT SAMs at about 500K, whereas no desorption peak (m/z=124,C(6)H(5)CH(2)SH(+)) was observed from BMT SAMs. Interestingly, the dominant TD peak for the benzyl fragments (m/z=91,C(6)H(5)CH(2)(+)) formed via C-S bond cleavage was observed in BMT SAMs at around 400K. From this study, we clearly revealed that the small modification in chemical structure by inserting a methylene spacer between the phenyl group and the sulfur atom affects 2D SAM structures, adsorption conditions, and thermal desorption behaviors and stability. The results obtained here will be very useful in designing and fabricating aromatic thiol SAMs for further applications.

Formation of large ordered domains in benzenethiol self-assembled monolayers on Au(111) observed by scanning tunneling microscopy

Benzenethiol (BT) self-assembled monolayers (SAMs) on Au(111) were prepared as a function of solution temperature after immersion in a 1mM ethanol solution for 1 day. The surface structures of BT SAMs were examined by means of scanning tunneling microscopy (STM). Although BT molecules usually form disordered SAMs containing the Au adatom islands at room temperature, we found that they formed two-dimensional ordered SAMs containing a large size domain at a high solution temperature of 50 or 75 degrees C. High-resolution STM imaging revealed that BT SAMs on Au(111) formed at 50 degrees C have a (2x3 radical2)R23 degrees packing structure. From our STM study, we revealed that two-dimensional ordered BT SAMs on Au(111) can be obtained by changing the solution temperature.

Two-dimensional ordering of benzenethiol self-assembled monolayers guided by displacement of cyclohexanethiols on Au(111)

Although the adsorption of benzenethiols (BT) on Au(111) usually leads to the formation of disordered phases, we demonstrate here that the displacement of preadsorbed cyclohexanethiol self-assembled monolayers (SAMs) on Au(111) by BT molecules can be a successful approach to obtain two-dimensional BT SAMs with long-range ordered domains.

Different Adsorption States between Thiophene and α-Bithiophene Thin Films Prepared by Self-Assembly Method

Using X-ray photoelectron spectroscopy (XPS) and thermal desorption spectroscopy (TDS), we investigated adsorption states of thiophene (1T) and α-bithiophene (2T) on Au(111) prepared by a self-assembly method. The S2p XP spectrum showed that 1T molecules were chemisorbed on the Au substrate, suggesting the formation of a self-assembled monolayer, while the interaction of 2T with Au was very weak. TDS results showed that 1T molecules were desorbed with dissociation by heating because of their strong interaction with the Au substrate. On the other hand, 2T molecules were directly desorbed from the Au substrate, suggesting that no chemical reaction occurs by heating. These results suggest that the adsorption state of the thiophenes depends on the unit number of the thiophene oligomer.

Comparison of near-infrared and Raman spectroscopy for the determination of the density of polyethylene pellets

In this study, we compare near-infrared (NIR) and Raman spectroscopy for the determination of the density of linear low density polyethylene (PE) (in a pellet form). As generally known, Raman spectral features are more selective than those of NIR for most chemical samples. NIR spectroscopy has been more extensively used for the quantitative analysis of polymers, but Raman spectroscopy is the better choice as long as the problem of reproducibility of Raman measurements (especially for solid samples), mostly resulting from insufficient sample representation due to probing only localized chemical information and the sensitivity of sample placement with regard to the focal plane, can be overcome. To improve sample representation and reproducibility of Raman measurements, we have employed the wide area illumination (WAI) Raman scheme, capable of illuminating a laser onto a large sample area (28.3 mm(2)) for Raman spectral collection (a 6-mm laser spot with a focal length of 248 mm). Diffuse reflectance NIR spectra of PE pellets were collected using a sample moving system which allowed for the scanning of large areas. The prediction error was 0.0008 g cm(-3) for Raman spectroscopy and 0.0011 g cm(-3) for NIR spectroscopy. The harmonization of inherently selective Raman features and a reproducible spectral collection with correct sample representations using the WAI scheme led to an accurate determination of the density of the PE pellets.

Spontaneous desorption and phase transitions of self-assembled alkanethiol and alicyclic thiol monolayers chemisorbed on Au(111) in ultrahigh vacuum at room temperature.

Scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) were used to examine the surface structure and adsorption conditions of hexanethiol (HT) and cyclohexanethiol (CHT) self-assembled monolayers (SAMs) on Au(111) as a function of storage period in ultrahigh vacuum (UHV) conditions of 3×10(-7) Pa at room temperature (RT). STM imaging revealed that after storage for 7 days, HT SAMs underwent phase transitions from c(4×2) phase to low coverage 4×√3 phase. This transition is due to a structural rearrangement of hexanethiolates that results from the spontaneous desorption of chemisorbed HT molecules on Au(111) surface. XPS measurements showed approximately 28% reduction in sulfur coverage, which indicates desorption of hexanethiolates from the surfaces. Contrary to HT SAMs, the structural order of CHT SAMs with (5×2√3)R35° phase completely disappeared after storage for 3 or 7 days. XPS results show desorption of more than 80% of the cyclohexanethiolates, even after storage for 3 days. We found that spontaneous desorption of CHT molecules on Au(111) in UHV at RT occurred quickly, whereas spontaneous desorption of HT molecules was much slower. Thermal desorption spectroscopy (TDS) results suggest CHT SAMs in UHV at RT can desorb more efficiently than HT SAMs due to formation of thiol desorption fragments that result from chemical reactions between surface hydrogen atoms and thiolates on Au(111) surfaces. This study clearly demonstrated that organic thiols chemisorbed on gold surfaces are desorbed spontaneously in UHV at RT and van der Waals interactions play an important role in determining the structural stability of thiolate SAMs in UHV.

Growth Processes and Control of Two-Dimensional Structure of Carboxylic Acid-Terminated Self-Assembled Monolayers on Au(111)

The growth processes and solvent effects on the two-dimensional structure of self-assembled monolayers (SAMs) formed by 6-mercaptohexanoic acid (MHA) on Au(111) were examined by scanning tunneling microscopy (STM) and contact angle (CA) measurements. The STM study revealed for the first time that the striped phases of 6-mercaptohexanoic acid (MHA) on Au(111) are preferentially formed near the step edges of gold terraces in the initial stage of SAM growth. In addition, it was found that MHA SAMs prepared from a polar aprotic solvent form a more well-ordered structure and a better hydrophilic surface than those prepared from a polar protic solvent. From this study, we clearly demonstrated that the properties of solvents are very important factors in controlling the two-dimensional structure of carboxylic acid-terminated SAMs.

Fast displacement and structural transition of cyclohexanethiol self-assembled monolayers by octanethiols on Au (111).

Displacement of cyclohexanethiols (CHTs) self-assembled monolayers (SAMs) by octanethiols on Au (111) (OTs) was examined by scanning tunneling microscopy (STM) and contact angle measurements. We revealed that the fast displacement of CHT by OT takes place within a few minutes, and then displacement proceeds slowly to form the closely packed OT SAMs. The main driving force for the unusual fast displacement is due to the large increase of chemical interactions between the sulfur and gold atoms, and the van der Waals interactions between alkyl chains after displacement of CHT by OT. STM imaging clearly demonstrated the structural transitions from the (5 x 2 square root 10)R48 degrees structure of CHT SAMs to the (square root 3 x square root 3)R30 degrees or c(4 x 2) structures of OT SAMs via an intermediate phase that were often observed during alkanethiol SAM growth on gold. In this study, we found that CHT SAMs can be used as a new transient layer for the fabrication of nanostructures on a surface.